Detailed Description
Hereinafter, embodiments of a lithium ion secondary battery, a battery device, and an electric device according to the present application are specifically disclosed with reference to the accompanying drawings. However, unnecessary detailed description may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of the actual same structure may be omitted. This is to avoid that the following description becomes unnecessarily lengthy, facilitating the understanding of those skilled in the art. Furthermore, the drawings and the following description are provided for a full understanding of the present application by those skilled in the art, and are not intended to limit the subject matter recited in the claims.
The "range" disclosed herein is defined in terms of lower and upper limits, with the given range being defined by the selection of a lower and an upper limit, the selected lower and upper limits defining the boundaries of the particular range. Ranges that are defined in this way can be inclusive or exclusive of the endpoints, and any combination can be made, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if minimum range values 1 and 2 are listed, and if maximum range values 3,4, and 5 are listed, then the following ranges are all contemplated as 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise indicated, the numerical range "a-b" represents a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed throughout, and "0-5" is simply a shorthand representation of a combination of these values. When a certain parameter is expressed as an integer of 2 or more, it is disclosed that the parameter is, for example, an integer of 2,3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or the like.
All embodiments of the application, as well as alternative embodiments, may be combined with each other to form new solutions, unless otherwise specified, and such solutions should be considered to be included in the disclosure of the application.
All technical features and optional technical features of the application may be combined with each other to form new technical solutions, unless specified otherwise, and such technical solutions should be considered as included in the disclosure of the application.
All the steps of the present application may be performed sequentially or randomly, preferably sequentially, unless otherwise specified. For example, the method comprises steps (a) and (b), meaning that the method may comprise steps (a) and (b) performed sequentially, or may comprise steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), may include steps (a), (c) and (b), may include steps (c), (a) and (b), and the like.
In the present application, the terms "plurality" and "a plurality" mean two or more.
Unless otherwise indicated, terms used in the present application have well-known meanings commonly understood by those skilled in the art.
Unless otherwise indicated, the values of the parameters mentioned in the present application may be determined by various test methods commonly used in the art, for example, may be determined according to the test methods given in the examples of the present application. The test temperature for each parameter was 25 ℃ unless otherwise indicated.
The battery referred to in the embodiments of the present application may be a single physical module including one or more lithium ion secondary batteries to provide higher voltage and capacity. For example, the battery referred to in the present application may include a lithium ion secondary battery, a battery module, a battery pack, or the like.
The lithium ion secondary battery is the smallest unit constituting the battery, which alone can realize the charge and discharge functions. The lithium ion secondary battery may have a cylindrical shape, a rectangular parallelepiped shape, or other shapes, etc., which are not limited in the embodiment of the present application. Fig. 2 shows a lithium ion secondary battery 5 having a rectangular parallelepiped structure as an example.
The lithium ion secondary battery includes an electrode assembly and an electrolyte.
The lithium ion secondary battery may further include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte. The overwrap may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The overwrap may also be a flexible package, such as a bag-type flexible package. The soft bag can be made of one or more of plastics such as polypropylene (PP), polybutylene terephthalate (PBT) and polybutylene succinate (PBS).
In some embodiments, as shown in fig. 3, the outer package may include a housing 51 and a cover 53. The housing 51 may include a bottom plate and a side plate coupled to the bottom plate, the bottom plate and the side plate enclosing to form a receiving chamber. The housing 51 has an opening communicating with the accommodation chamber, and the cover plate 53 is used to cover the opening to close the accommodation chamber. The electrode assembly 52 is packaged in the receiving chamber. The number of electrode assemblies 52 included in the lithium ion secondary battery 5 may be one or more, and may be adjusted according to the need.
The electrode assembly generally includes a positive electrode tab, which is an electrode where a reaction of absorbing or lithiating lithium ions when charged and releasing or delithiating lithium when discharged occurs, and a negative electrode tab, which is an electrode where a reaction of releasing or delithiating lithium ions when charged and absorbing or lithiating lithium when discharged occurs.
When there are a plurality of lithium ion secondary batteries, the plurality of lithium ion secondary batteries are connected in series, in parallel or in series-parallel through the converging component. In some embodiments, the battery may be a battery module, and when there are a plurality of lithium ion secondary batteries, the plurality of lithium ion secondary batteries are arranged and fixed to form one battery module. In some embodiments, the battery may be a battery pack including a case and a lithium ion secondary battery, the lithium ion secondary battery or the battery module being accommodated in the case. In some embodiments, the tank may be part of the chassis structure of the vehicle. For example, a portion of the tank may become at least a portion of a floor of the vehicle, or a portion of the tank may become at least a portion of a cross member and a side member of the vehicle.
In some embodiments, the battery may be an energy storage device. The energy storage device comprises an energy storage container, an energy storage electric cabinet and the like.
In some embodiments, the lithium ion secondary batteries may be assembled into a battery module, and the number of lithium ion secondary batteries contained in the battery module may be plural, and the specific number may be adjusted according to the application and capacity of the battery module. Fig. 4 is a schematic view of the battery module 4 as an example. As shown in fig. 4, in the battery module 4, a plurality of lithium ion secondary batteries 5 may be arranged in order along the longitudinal direction of the battery module 4. Of course, the arrangement may be performed in any other way. The plurality of lithium ion secondary batteries 5 may be further fixed by fasteners.
Alternatively, the battery module 4 may further include a case having an accommodating space in which the plurality of lithium ion secondary batteries 5 are accommodated.
In some embodiments, the battery modules may be further assembled into a battery pack, and the number of battery modules included in the battery pack may be adjusted according to the application and capacity of the battery pack.
Fig. 5 and 6 are schematic views of the battery pack 1 as an example. As shown in fig. 5 and 6, a case and a plurality of battery modules 4 disposed in the case may be included in the battery pack 1. The case includes an upper case 2 and a lower case 3, the upper case 2 being used to cover the lower case 3 and forming a closed space for accommodating the battery module 4. The plurality of battery modules 4 may be arranged in the case in any manner.
The lithium-containing transition metal phosphate material has been widely used in lithium ion batteries due to the characteristics of stable structure, good safety and long cycle life, but has the problems of low electronic conductivity and low stacking efficiency, so that the loading capacity of lithium-containing phosphate in a pole piece is difficult to further and effectively improve, and the requirement of a high-energy-density battery cannot be met.
In order to further improve the energy density of the battery and the compaction density of the pole piece, the common mode in the industry is to increase the grain size distribution in the pole piece, and in order to increase the grain size distribution, the duty ratio of large grains needs to be increased. However, studies have shown that large particle fractions in the pole pieces exceeding a certain range sacrifice the dynamic performance of the battery. How to obtain a battery with both energy density and kinetic performance is a technical problem to be solved in the art.
The first aspect of the application provides a lithium ion secondary battery, which comprises a positive electrode plate, a negative electrode plate and an electrolyte, wherein the positive electrode plate comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, at least part of the positive electrode active material comprises lithium-containing transition metal phosphate particles with carbon coating materials arranged on the surface, the area ratio of the particles with the particle size of more than or equal to 1.5 mu m in a section of the positive electrode film layer along the thickness direction of the electrode plate is more than or equal to 8.0% and less than or equal to 20.0%, and in a sphericity-like area cumulative distribution curve of the particles obtained by the section of the positive electrode film layer along the thickness direction of the electrode plate, the sphericity-like median LA50 is 0.70-0.74.
In the tangent plane of the positive electrode film layer along the thickness direction of the electrode plate, the area ratio of particles with the particle diameter of more than or equal to 1.5um is less than 8%, which means that the large particles have serious shortage of the ratio, reasonable grading is not constructed, close stacking is difficult to achieve, and high compaction density of the positive electrode plate is difficult to achieve. In the section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of particles with the particle diameter of more than or equal to 1.5um exceeds 20%, which is favorable for improving the solid density of the positive electrode film layer, but can sharply reduce the contact area of electrolyte and positive electrode active materials, increase the diffusion path length of lithium ions in the particles, cause serious polarization of the pole piece locally, increase the impedance of the battery and obviously deteriorate the dynamic performance of the battery.
The area ratio of the particles with the particle diameter of more than or equal to 1.5 mu m in the positive electrode film layer is controlled to be more than or equal to 8.0% and less than or equal to 20.0%, so that the obvious short plate effect caused by large-size particles can be reduced, the impedance of the battery is kept at a lower level, the dynamic performance of the battery is improved, and the compaction density of the pole piece is limited to be further improved. The applicant has further studied to find that by adjusting the sphericity of the particles in the film layer, the particles are easy to slip during rolling, so that the particles are mutually filled, and a positive electrode film layer with high compaction density can be obtained. When the sphericity-like median LA50 of the particles in the positive electrode film layer is smaller than 0.70, the whole irregularity degree of the particles is larger, the particles are difficult to slide, and the pole piece compaction density is further improved.
In the embodiment of the application, the area ratio of the particles with the particle diameter of more than or equal to 1.5 mu m in the positive electrode film layer is controlled to be more than or equal to 8.0% and less than or equal to 20.0%, and in a sphericity-like area cumulative distribution curve of the particles obtained by the tangent plane of the positive electrode film layer along the thickness direction of the pole piece, the sphericity-like median LA50 is 0.70-0.74, so that the short plate effect can be reduced, the impedance of the battery can be kept at a lower level, the dynamic performance of the battery can be improved, the particles are more easily slipped between the particles, the compaction density of the pole piece is optimized, and the dynamic performance and the energy density of the battery can be further realized.
In the present application, the term "particles" means particles having identifiable complete boundaries in the field of view at a magnification of, for example, 10 thousand times, and defects and scratches may be present in the interior of the particles, but the interior of the particles cannot be identified to be sufficient to divide the complete boundaries of the particles.
In some embodiments, the area ratio of the particles with the particle diameter of 1.5 μm or more in the section of the positive electrode film layer along the thickness direction of the pole piece is 8.0% or more and 20.0% or less.
In some embodiments, the area ratio of the particles with the particle diameter of 1.5 μm or more in the section of the positive electrode film layer along the thickness direction of the pole piece can be 8.0%、9.0%、9.08%、10.0%、11.0%、12.0%、12.78%、13.0%、13.23%、14.0%、14.49%、15.0%、15.41%、15.46%、16.0%、17.0%、18.0%、19.0%、19.51%、19.96%、20.0% or a numerical range between any two.
In the section of the positive electrode film layer along the thickness direction of the pole piece, the particle identification method specifically comprises the steps of cutting the positive electrode film layer along the thickness direction of the pole piece through an argon ion beam (for example, equipment model: leka EM TIC 3X CP, working voltage: 6kV, working time length: 6 h), exposing the section, and observing the section of the positive electrode film layer along the thickness direction of the pole piece by using a scanning electron microscope (for example, equipment model: hitachi SU8230, working voltage: 3kV, beam current: high, probe model: U (LA 100), working distance <5 mm). The image was collected by a field emission scanning electron microscope at a non-edge position in the section of the positive electrode film layer (after the edge of the pole piece was observed under the scanning electron microscope, the field of view was adjusted to the center portion of the sample) by a secondary electron mode, an electron image was taken at a magnification of 10k, and particles in the electron image were analyzed by ImageJ software (version 1.46r, win 64). The application method of the imageJ software comprises the steps of loading a scanning electron microscope Image to be analyzed, identifying particles by adopting Cellpose plug-in software therein, carrying out manual correction on the basis, and reading and counting data by adopting Image J, wherein the scanning electron microscope Image to be analyzed is shown in figure 1. The concrete method for identifying particles by using Cellpose plug-in software comprises the steps of setting a segmentation diameter parameter (diameter in Segmantation module) to 15 pixels, clicking run cyto3 to identify particles, and manually identifying particles which are not identified by software or are not completely identified by software or have errors in the image. Particles which are not recognized by software or are not completely recognized by software or have errors in the images mainly comprise the following steps of 1, particles which cannot be recognized or cannot be completely recognized due to oversized particles or scratches on the surfaces of the particles, 2, scratches are generated on the surfaces of the particles in the argon ion beam cutting process, the scratches are possibly misjudged as particle boundaries in the recognition process by the software so as to generate recognition errors, 3, the particles are not successfully recognized due to the fact that the particles are too small, 4, the particles are located at the edges of the field of view of an electron microscope, the inside of the particles are penetrated by the edges, the appearance of the particles is not completely displayed, and the particles are partially replaced by the whole to be recognized so as to generate recognition errors. The manual calibration of the particles which are not identified or have identification errors is carried out, and the specific process is that the edges around the scanning electron microscope are deleted, the method comprises the steps of judging whether slit scratches exist in other particles which are not recognized or have recognition errors, judging whether slit scratches exist in the particles, if the slit scratches do not exist in the particles, judging the particles to be one particle, manually marking the particles according to manually observed particle boundaries, judging whether the slit scratches penetrate through the particles in response to the fact that the slit scratches exist in the particles, judging the particles to be one particle if the slit scratches do not penetrate through the particles, manually marking the particles, judging whether the slit scratches penetrate through the particles in response to the fact that the slit scratches are linear or irregular, judging the boundary between the particles in response to the fact that the slit scratches are irregular, dividing the particles along the boundary, comparing the contrast in response to the slit scratches to be linear, judging the particles to be one particle in response to the fact that the contrast is not obvious and has no crack feeling, marking the particles as one particle in response to the contrast is strong and judging the boundary between the particles to be the particles, and marking the particles as two particles. And deleting information irrelevant to particles in the automatic image processing process after manual identification, namely finishing the judgment and identification of the particles in the picture.
In the section of the positive electrode film layer along the thickness direction of the pole piece, the area statistical mode of the particles is specifically as follows. And (3) importing the picture with the particles subjected to the judgment and identification into imageJ software for analysis, completing scale setting according to a scanning electron microscope picture, and analyzing the particle size of the particles in the picture through the analysis functions of 'Feret diameter', 'Area', 'Round' and 'Solidity'. According to the software manual (ImageJ User Guide IJ 1.46.46 r), the "Feret" parameter obtained by analysis represents the maximum spacing between all parallel lines in a two-dimensional projection of the particle, thus characterizing the particle size of the particle, and the "Area" parameter represents the pixel Area of the particle. Because the particles with the particle size smaller than 50nm have larger errors in the counting process, the particles are difficult to accurately identify, and the particle size of the conductive agent is generally smaller than 50nm, larger errors can be generated on the counting result, the particles with the particle size smaller than 50nm are not counted in the counting process of the particle size, and the particles with the particle size smaller than 50nm are deleted, wherein AR, round or Solidity are displayed as particle counting data corresponding to NaN. According to the method, in order to meet the number of samples with statistical significance, each pole piece acquires not less than 10 scanning electron microscope images with non-overlapping visual fields, the Area of not less than 5000 particles is counted, the sum of the 'Area' parameters of the particles with the particle size of more than or equal to 1.5 mu m and the sum of the 'Area' parameters of all the particles are calculated and respectively used as the Area of the particles with the particle size of more than or equal to 1.5 mu m and the total Area of the counted particles. And dividing the sum of areas of particles with the particle size of 1.5 mu m or more by the total area of the counted particles to obtain the area ratio of the particles with the particle size of 1.5 mu m or more in the tangential plane of the positive electrode film layer along the thickness direction of the pole piece.
The sectional morphology diagram of the positive electrode film layer along the thickness direction of the pole piece is shown in fig. 1, and is different from the state of the positive electrode active material in a Markov laser scattering method and the state of the positive electrode active material when the positive electrode active material is directly observed by a scanning electron microscope. Particles in the positive electrode film layer are in a good dispersion state under the action of rolling pressure, and observation of the positive electrode film layer is beneficial to effectively representing the objective condition of particle size and area ratio of the particles in the positive electrode film layer.
The positive electrode film layer is compacted in the thickness direction in the cold pressing process, so that the tangent plane of the positive electrode film layer along the thickness direction of the pole piece is more capable of reflecting the real compaction condition of the internal particles of the film layer on the space scale compared with the surface of the positive electrode film layer. In the section of the positive electrode film layer along the thickness direction of the pole piece, the area occupation ratio of the particles with the particle size of more than or equal to 1.5 mu m can intuitively reflect the proportional relation between the partial particle area and the whole particle area of the particle size section, and the distribution condition of the particles of the particle size section is reflected.
It is understood that particles in a tangential plane of the positive electrode film layer in the thickness direction of the electrode sheet, particularly particles of 50nm or more, are mainly derived from the positive electrode active material. Therefore, the embodiment of the application can accurately and objectively reflect the distribution condition of lithium-containing transition metal phosphate particles in the positive electrode film layer in the pole piece through the observation statistics of the particle area in the tangent plane of the positive electrode film layer.
In the prior art, a malvern laser diffraction method is generally adopted to count the granularity of the positive electrode active material. However, the applicant's study shows that since lithium-containing phosphates are prone to agglomeration, the particle size of the particle agglomerates thereof, as measured by the test results obtained by the malvern laser diffraction method according to the laser scattering principle, cannot truly reflect the particle size of the particles in the positive electrode active material, and cannot reflect the dispersion state of the positive electrode active material in the film layer, because the degree of dispersion of the positive electrode active material in the film layer during film forming and rolling is improved. The test result obtained by the Markov laser diffraction method is closely related to the granularity, specific surface area and agglomeration degree of the positive electrode active material, so that the particle size obtained by the Markov laser diffraction method cannot be equal to or analogized to the particle size obtained by statistics in the embodiment of the application.
The regulation of the particle size of the particles can be achieved by any process known to the person skilled in the art. By way of example, the growth rate and time of the positive electrode material are controlled by regulating the temperature and time during the preparation of the positive electrode material. The method comprises the steps of processing raw materials to a target particle size distribution range by utilizing the mechanical force action of crushing and ore grinding processes, realizing adjustment of particle size, adopting screening and grading equipment to carry out particle size separation on a particle system, obtaining the particle size ratio meeting the requirements, and regulating the residence time and the stress state of particles in the equipment by accurately controlling the feeding rate, thereby being beneficial to realizing regulation and control of the particle size of the particles.
Lithium-containing transition metal phosphates refer to phosphate materials comprising lithium elements and transition metal elements, which may be detected by any means known in the art. For example, it can be detected by X-ray diffraction (XRD) in combination with an energy spectrum analyzer.
The carbon-coated material disposed on at least a portion of the surface of the lithium-containing transition metal phosphate may be detected by any means known in the art. By way of example, characterization of lithium-containing transition metal phosphates by transmission electron microscopy and spectroscopy can be used in combination to observe carbon-coated materials disposed on at least a portion of the surface of the lithium-containing transition metal phosphates.
In some embodiments, in the cumulative distribution curve of the sphere-like area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the median LA50 of sphere-like is 0.70-0.74.
The method for testing the sphericity of the particles in the tangent plane of the positive electrode film layer along the thickness direction of the pole piece is characterized by specifically identifying the particles in the tangent plane of the positive electrode film layer by referring to the method disclosed by the application, and analyzing the morphology of the particles and the Area of the particles in the tangent plane of the positive electrode film layer along the thickness direction of the pole piece by adopting the shape description and the Area analysis functions. According to the software handbook (ImageJ User Guide IJ 1.46.46 r), the "Round" parameter obtained by analysis represents the ratio of the pixel area of the particles to the area of the circle with the fitted major diameter as the diameter, and can be used to characterize the sphericity of the particles. The closer the particle is to the sphere, the closer the ratio of the pixel area to the area of the circle having the fitted major diameter as the diameter is to 1. Thus, the sphericity of the particles is characterized by the "Round" parameter of the particles obtained by analysis.
And arranging the obtained sphericity of at least 5000 particles in order from small to large, and obtaining a sphericity area accumulation distribution curve of the particles in the positive electrode film layer by taking the sphericity as a horizontal axis and the accumulation area occupation ratio as a vertical axis. LA50 is the corresponding sphericity L value when the cumulative area of the vertical axis in the sphericity L value cumulative distribution curve is 50%.
In some embodiments, in the cumulative distribution curve of the sphere-like area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the median LA50 of the sphere-like can be selected from a value range of 0.70, 0.707, 0.71, 0.717, 0.72, 0.721, 0.723, 0.73, 0.736, 0.738, 0.74 or any two.
The regulation of the sphericity of the particles can be achieved by any process known to the person skilled in the art. As an example, the sphericity of the particles can be adjusted by grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulating, adding surfactants, etc., and adjusting the parameters of each process.
The particles with the sphericity-like median LA50 in the range are approximately spherical, and are easy to slide under the action of external force, so that the compaction density of the pole piece can be further improved, and the energy density of the battery is improved.
In some embodiments, the concentration of the spheroid (LA90-LA10）/LA50 is 0.450-0.535, optionally 0.450-0.500) in the cumulative distribution curve of spheroid area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece.
In some embodiments, in the cumulative distribution curve of the sphere-like area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the concentration of the sphere-like is selected from 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.523, 0.526, 0.528, 0.53, 0.534, 0.535 or a numerical range between any two.
In the sphericity-like area cumulative distribution curve of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the sphericity-like concentration test method is concretely as follows, and by referring to the sphericity-like test method disclosed by the application, LA90 is the L value corresponding to the case that the cumulative area of the longitudinal axis in the sphericity-like L value cumulative distribution curve is 90%, and LA10 is the L value corresponding to the case that the cumulative area of the longitudinal axis in the sphericity-like L value cumulative distribution curve is 10%. The concentration of the spheroidicity is represented by (LA90-LA10)/LA50) (LA90-LA10)/LA50 can reflect the spheroidicity of most particles and can also reflect the asymmetry and width of the spheroidicity distribution of the particles in the positive electrode film layer, the smaller the numerical value is, the more concentrated the distribution is, the higher the spheroidicity median is combined, the whole approximate sphere of the particles is reflected, the compact stacking is facilitated, the contact among the particles is increased, and the dynamic performance of the battery is further improved.
In some embodiments, in the cumulative distribution curve of the sphere-like area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the sphere-like LA10 is 0.40-0.60, and the sphere-like LA90 is 0.85-1.
In some embodiments, in the cumulative distribution curve of the sphere-like area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, LA10 of the sphere-like can be 0.40、0.41、0.42、0.43、0.44、0.45、0.46、0.47、0.48、0.49、0.50、0.507、0.509、0.51、0.512、0.514、0.516、0.52、0.524、0.526、0.53、0.54、0.55、0.56、0.57、0.58、0.59、0.60 or a numerical range between any two.
In some embodiments, in the cumulative distribution curve of the sphere-like area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, LA90 of the sphere-like can be 0.85、0.86、0.865、0.87、0.88、0.886、0.887、0.89、0.894、0.898、0.90、0.91、0.918、0.92、0.93、0.94、0.95、0.96、0.97、0.98、0.99、1 or a numerical range between any two.
The LA10、LA90 of the sphericity is in the range, which shows that most of particles have higher sphericity, extreme morphological difference is avoided, excessive gaps and stress concentration caused by bridging among the particles are avoided, regular accumulation and uniform sliding are facilitated, and the compaction density of the pole pieces is improved.
In some embodiments, the area ratio of the particles with the particle diameter of 5 μm or more in the section of the positive electrode film layer along the thickness direction of the pole piece is 0.
Research shows that the particles with the particle diameter of more than or equal to 5 mu m in the positive electrode film layer can obviously deteriorate the infiltration of electrolyte in the positive electrode film layer and the diffusion of the electrolyte in active material particles, and the area ratio of the particles with the particle diameter of more than or equal to 5 mu m is 0, so that the internal resistance of the battery is further reduced, and the dynamic performance of the battery is improved.
In some embodiments, the area ratio of the particles with the particle diameter of 1.5 μm or more and less than 5 μm in the section of the positive electrode film layer along the thickness direction of the pole piece is 9.0% -20.0%, and optionally 10.0% -20.0%.
In some embodiments, in a tangential plane of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of the particles with the particle diameter of 1.5 μm or more and less than 5 μm may be 9.0%、9.08%、10.0%、11.0%、12.0%、12.78%、13.0%、13.23%、14.0%、14.49%、15.0%、15.41%、15.46%、16.0%、17.0%、18.0%、19.0%、19.51%、19.96%、20.0% or any numerical range between the two.
In the tangential plane of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with the particle diameter of 1.5 μm or more and less than 5 μm can be tested in the manner described above. And dividing the particle area of the positive electrode film layer with the particle size of more than or equal to 1.5 mu m and less than 5 mu m in a section of the positive electrode film layer along the thickness direction of the pole piece by the total particle area of the particles, wherein the area ratio of the particles with the particle size of more than or equal to 1.5 mu m and less than 5 mu m in the section of the positive electrode film layer along the thickness direction of the pole piece is taken as the area ratio of the particles.
In the process of improving the grain size distribution and increasing the large grain size or area ratio, it is inevitable to introduce grains having a grain size of 1.5 μm or more and less than 5 μm. In the section of the positive electrode film layer along the thickness direction of the electrode plate, the area ratio of particles with the particle diameter of more than or equal to 1.5 mu m and less than 5 mu m can be further reduced, the blocking effect of large-size particles in the electrode plate on the infiltration and diffusion of electrolyte in the positive electrode film layer can be further reduced, the uniformity of the diffusion rate of lithium ions in positive electrode active material particles can be improved, the local polarization can be reduced, and the dynamic performance of the battery can be improved.
In some embodiments, the area ratio of the particles with the particle diameter of 1 μm or more and less than 1.5 μm in the section of the positive electrode film layer along the thickness direction of the pole piece is 15.0% -25.0%, and optionally 16.0% -24.0%.
In some embodiments, in a tangential plane of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of the particles with the particle diameter of 1 μm or more and less than 1.5 μm may be 15.0%、15.65%、15.91%、16.0%、17.0%、18.0%、18.33%、18.38%、19.0%、19.65%、20%、20.45%、20.87%、21%、22%、23%、23.88%、24%、25% or any numerical range between the two.
In a tangential plane of the positive electrode film layer in the thickness direction of the electrode sheet, the area ratio of particles having a particle diameter of 1 μm or more and less than 1.5 μm can be tested in the manner described above. The area ratio of the particles with the particle diameter of more than or equal to 1 mu m and less than 1.5 mu m in the section of the positive electrode film layer along the thickness direction of the pole piece is taken as the area ratio of the particles with the particle diameter of more than or equal to 1 mu m and less than 1.5 mu m in the section of the positive electrode film layer along the thickness direction of the pole piece by dividing the total area of the counted particles.
In the section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of particles with the particle diameter of more than or equal to 1 mu m and less than 1.5 mu m can form a sufficient supporting structure in stacking in the range, so that the overall structural strength among the particles is enhanced, meanwhile, a supporting structure is formed among smaller particles, larger gaps are filled, the void ratio among the particles is reduced, and the compaction density of the pole piece is further improved and the energy density of the battery is improved on the basis of keeping good dynamic performance of the battery.
In some embodiments, the area ratio of particles with the particle diameter of 200nm or more and less than 1500nm in a section of the positive electrode film layer along the thickness direction of the pole piece is 73.0% -80.0%, and optionally 73.0% -78.0%.
In some embodiments, in the section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of the particles with the particle diameter of 200nm or more and less than 1500nm can be selected to be 73.0%、73.01%、73.15%、74.0%、75.0%、75.16%、75.20%、75.48%、75.78%、75.88%、76.0%、77.0%、78.0%、78.69%、79.0%、80.0% or a numerical range between any two.
In the section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of particles with the particle diameter of more than or equal to 200nm and less than 1500nm can be tested by referring to the method. And dividing the sum of areas of particles with the particle size of more than or equal to 200nm and less than 1500nm by the total area of the counted particles to obtain the area ratio of the particles with the particle size of more than or equal to 200nm and less than 1500nm in the tangential plane of the positive electrode film layer along the thickness direction of the pole piece.
The area ratio of particles with the particle diameter of more than or equal to 200nm and less than 1500nm in the positive electrode film layer can further improve the uniformity of the lithium ion diffusion rate in the range, so that the dynamic performance of the lithium ion secondary battery is improved, the compaction density of the pole piece can be further improved, and the energy density of the battery is improved.
In some embodiments, the positive electrode film layer has a graphitization degree C value cumulative distribution curve obtained in a laser micro-confocal Raman spectrometer surface scanning mode, wherein the graphitization degree C value is IG/ID, wherein IG represents the G peak intensity of a Raman spectrum at 1580+/-100 cm-1, and ID represents the D peak intensity of the Raman spectrum at 1350+/-100 cm-1, and the graphitization degree C50 is 0.95-1.20, further optionally 0.98-1.15.
In the application, the graphitization degree C value of the positive electrode film layer can be obtained by a laser micro-confocal Raman spectrometer surface scanning mode. As an example, specifically, a laser micro confocal raman spectrometer (high-precision ranishao laser micro confocal raman spectrometer) is adopted, an excitation wavelength of 532nm is selected, a proper amount of positive electrode film layer is taken to sweep the surface of the positive electrode film layer or a section along the thickness direction of a pole piece, a scanning area is 45 μm×45 μm and divided into 10×10 grids, grid vertexes are used as test points, a step length is 5 μm, the number of total scanning points is 100 points, and therefore a C value of different sites and a C value accumulation distribution curve of a sweeping area are obtained.
The graphitization degree C value of the positive electrode film layer is obtained through the peak intensity ratio of G peak (G-band) and D peak (D-band) of a Raman spectrum, wherein the G peak position is 1585+/-100 cm-1 which represents a carbon sp2 hybridized structure, and the D peak position is 1350+/-50 cm-1 which represents a disordered structure, and the disorder represents an unordered arrangement mode among carbon atoms in the structure. In graphite crystals, the carbon atoms in the same layer are hybridized with sp2 to form covalent bonds, and van der Waals forces are arranged between the layers, so that carbon in a graphite structure is easy to slide. Thus, the C value may characterize the graphitization degree of the positive electrode active material. It is understood that the graphitization degree of the lithium-containing phosphate as the positive electrode active material in the present application is mainly derived from the carbon-coated material on the surface thereof. Although the carbon nanotube conductive agent rich in sp2 hybridized structure also has relatively high IG/ID, the addition of the carbon nanotube conductive agent in the positive electrode film layer is an extreme value in a Raman surface scanning test of the positive electrode film layer due to the small addition content and small pipe diameter, and the graphitization degree C50 in the positive electrode film layer cannot be influenced. Therefore, the graphitization degree of the positive electrode film layer can also be used to characterize the graphitization degree of the positive electrode active material.
The cumulative distribution curve of C values of graphitization degree means a curve obtained by arranging at least 100C values obtained in order from small to large, with graphitization degree on the horizontal axis and cumulative number ratio on the vertical axis. C50 is the corresponding C value when the cumulative number of the vertical axis in the cumulative distribution curve of the graphitization degree C value is 50 percent. Compared with a point value, the median C50 of the graphitization degree can reflect the graphitization degree of the whole particles in the positive electrode film layer, namely the easy slip degree, and compared with a mean value, the influence of an extreme value in the test process can be reduced, and the confidence of the test result is improved.
The higher the graphitization degree of carbon on the surface of the positive electrode active material is, the higher the proportion of graphite structural carbon in the positive electrode film layer is, and particles are easier to slide by means of a carbon structure with high graphitization degree in the coating material, so that the compaction density of the pole piece is improved.
The control of the graphitization degree of the active material particles can be achieved by any process known to the skilled person. By way of example, the degree of graphitization of the active material particles can be adjusted by controlling the carbon source (which may be selected as a polymeric carbon source such as PEG), sintering temperature, sintering time, sintering pressure, sintering atmosphere, and nucleation process.
In some embodiments, in the cumulative distribution curve of graphitization degree C values obtained by the positive electrode film layer in the surface scanning mode of the laser micro-confocal raman spectrometer, the median C50 of graphitization degree can be 0.95、0.96、0.97、0.98、0.984、0.99、1、1.01、1.012、1.015、1.016、1.02、1.025、1.03、1.032、1.04、1.05、1.06、1.07、1.08、1.09、1.1、1.11、1.12、1.123、1.13、1.14、1.15、1.16、1.17、1.18、1.19、1.20 or a numerical range between any two.
In some embodiments, in the roughness area cumulative distribution curve of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the median RA50 of the roughness is 0.92-0.96.
In a roughness area accumulation distribution curve of particles obtained by a section of the positive electrode film layer along the thickness direction of the pole piece, a method for testing the median RA50 of roughness is specifically characterized in that the particles in the section of the positive electrode film layer are identified by referring to the method disclosed by the application, and the morphology of the particles in the section of the positive electrode film layer along the thickness direction of the pole piece is analyzed by adopting a shape description analysis function in imageJ. According to the software manual (ImageJ User Guide IJ 1.46.46 r), the "Solidity" parameter obtained by analysis represents the ratio of the pixel area to the convex area of the particles. Thus, the roughness of the particles was characterized by the "Solidity" parameter of the particles obtained by analysis. As defined, the closer the roughness is to 1, the smoother the particles. And arranging the sphericity of the obtained at least 5000 particles in order from small to large, and obtaining a roughness accumulation distribution curve of the particles in the positive electrode film layer by taking the roughness as a horizontal axis and the accumulation area occupation ratio as a vertical axis. RA50 is the corresponding roughness R value when the cumulative area of the vertical axis in the roughness R value cumulative distribution curve is 50%.
In some embodiments, in the cumulative distribution curve of the roughness area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the median RA50 of the roughness of the particles can be selected to be 0.92, 0.93, 0.94, 0.95, 0.96 or a numerical range between any two.
The regulation of the roughness of the particles can be achieved by any process known to the person skilled in the art. As an example, particle roughness adjustment can be achieved by grinding, polishing, grinding, milligram energy, electroplating, polishing, etc. processes and adjusting parameters of each process.
The surface of the particles with the roughness of the median RA50 in the range is relatively smooth, the friction force between the particles is relatively small, the particles are easy to slide under the action of external force, the compaction density of the pole piece can be further improved, and the energy density of the battery is improved.
In some embodiments, the concentration (RA90-RA10）/RA50 is 0.06-0.15 in the cumulative distribution curve of the roughness area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece.
In some embodiments, the concentration (RA90-RA10）/RA50 may be selected from a range of values of 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, or any two of them) in the cumulative distribution curve of the roughness area of the particles obtained from the cross section of the positive electrode film layer along the thickness direction of the electrode sheet.
In the roughness area accumulation distribution curve of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the roughness concentration test method is specifically as follows, and by referring to the roughness test method disclosed by the application, RA90 is the R value corresponding to the condition that the accumulation area of the longitudinal axis in the sphericity R value accumulation distribution curve is 90%, and RA10 is the R value corresponding to the condition that the accumulation area of the longitudinal axis in the roughness R value accumulation distribution curve is 10%. The concentration of the roughness is expressed by (RA90-RA10)/RA50. The concentration value of the roughness is extremely small, which indicates that the overall roughness consistency of the particles is high, the relative sliding among the particles is facilitated, the particles are easier to form high-density accumulation in rolling on the basis of high sphericity, and the compaction density of the pole piece and the energy density of the battery are increased.
In some embodiments, the positive electrode active material includes elemental iron, and the positive electrode film layer has an iron dissolution rate of 500ppm to 2000ppm.
The iron dissolution rate of the positive electrode film layer can be tested by adopting a method known in the art, after disassembling and cleaning a pole piece from a battery, filling into a small wafer with the diameter of 14mm, taking a plurality of small wafer samples to ensure that the total mass of the samples is about 5g, adding the small wafer samples into 100.3g of ascorbic acid solution with the mass concentration of 0.3 percent (the solvent is ultrapure water), stirring for 305 minutes at the speed of 500 revolutions per minute, rapidly sucking the solution by using a 5mL needle tube, filtering the solution into a test tube by using a 0.45 mu m aperture filter head, sucking 1mL of supernatant by using a liquid-transferring gun, adding into a glass volumetric flask for dilution by 50 times, testing by using an inductively coupled plasma mass spectrometer (ICP-OES), and obtaining the concentration of iron element in the solution, and calculating the iron dissolution rate of the positive electrode film layer by the formula [ (ICP test iron element concentration x solution volume/current collector mass of the small wafer ]. 100.3 g/(pole piece mass of small wafer) current collector mass of the small wafer ] with the mass of 1 g. Preferably, the mass of the current collector of the small disc is obtained by multiplying the thickness of the small disc by the area by the density. The thickness of the small disc can be equivalent by measuring the thickness of the current collector in the uncoated region with a thickness gauge. It will be appreciated that although the coated regions will develop an extension of the current collector during compaction, resulting in a slight reduction in thickness compared to the uncoated regions, the reduction will not have an excessive impact on the test results due to the negligible magnitude of the reduction. More preferably, when the current collector is aluminum foil, the density is 2.7g/cm3.
In some embodiments, the positive electrode active material includes elemental iron, and the iron dissolution rate of the positive electrode film layer may be selected to be 500ppm、575ppm、600ppm、700ppm、800ppm、816ppm、900ppm、1000ppm、1051ppm、1062ppm、1100ppm、1187ppm、1200ppm、1300ppm、1334ppm、1400ppm、1500ppm、1600ppm、1700ppm、1761ppm、1800ppm、1894ppm、1900ppm、2000ppm or a range of values therebetween.
The iron dissolution rate of the positive electrode film layer can be regulated and controlled by a person skilled in the art through any known process. As an example, the iron dissolution rate of the positive electrode film layer is regulated and controlled by regulating and controlling the surface coating quality of the positive electrode material, the temperature, the time and the pressure in the preparation process. In addition, during the use process of the battery, the design of the battery, the content of an oxidant in electrolyte, the working temperature of the battery and the charge and discharge strength of the battery can also influence the iron dissolution rate of the positive electrode film layer.
The iron dissolution rate of the positive electrode film layer mainly comes from lithium-containing transition metal phosphate positive electrode active substances in the positive electrode film layer, the completeness and compactness of carbon coating on the surface of the positive electrode active material can be reflected from the side surface, and the lower the iron dissolution rate is, the less likely the iron ions after acid dissolution are separated out of the carbon coating material, namely the more complete and compact the carbon coating material on the surface of the positive electrode active material is. The iron dissolution rate of the positive electrode film layer is in the range, and the positive electrode film layer represents that the positive electrode active material has relatively complete and compact carbon coating material, so that the electric contact between the positive electrode active materials can be improved, the conductivity of the positive electrode active material is improved, the polarization of the positive electrode active material is reduced, and the dynamic performance of the lithium ion secondary battery is further optimized. Meanwhile, the space occupancy rate of the densely coated carbon layer is low, the particle gaps are easy to be compressed by stress in the rolling process, and the compaction density of the pole piece and the energy density of the battery can be improved at the same time.
In some embodiments, the carbon element is present in an amount of 0.8% to 1.8%, alternatively 0.90% to 1.5%, by mass based on the total mass of the positive electrode active material.
The mass ratio of the carbon element based on the total mass of the positive electrode active material may be measured by methods and apparatuses known in the art. For example, the measurement is carried out by using a Dekka HCS infrared carbon-sulfur analyzer with reference to GB/T21023-2006 method for measuring the total carbon-sulfur content of iron and steel by infrared absorption after combustion in a high-frequency induction furnace.
In some embodiments, the mass content of the carbon element may be selected to be 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8% or a numerical range therebetween, based on the total mass of the positive electrode active material.
Compared with the lithium-containing phosphate anode active material in the prior art, the anode active material has relatively low-content carbon coating, can further improve the loading capacity of the lithium-containing phosphate in the anode sheet, and improves the energy density of the lithium ion secondary battery.
In some embodiments, the positive electrode active material has a lithium iron dislocation defect concentration of 0.1% to 1.5%, alternatively 0.3% to 1.0%.
XRD data of the sample is collected by using an X-ray diffractometer, phase analysis is carried out on the sample, and CIF files of the phase obtained by an open source website are used as an initial model of a crystal structure, wherein the initial model comprises unit cell parameters, atomic positions, space occupation probability and the like. In the initial model of the crystal structure, the possible Li content at the Fe site and the possible Fe content at the Li site were set to an initial value of 0.1% in consideration of the possibility of the inversion of Fe-Li. And adopting FullProf Suite software to carry out fitting refinement on the collected XRD data, and refining parameters according to the sequence of the background parameters, the peak intensity, the unit cell parameters and the peak shape. When the fitting peak shape and the experimental peak shape are optimally provided, and Rwp is smaller than 10, the occupation probability of refined Li and Fe is obtained, and the occupation probability is used as the concentration of the lithium iron inversion defect.
In some embodiments, the lithium iron reverse defect concentration of the positive electrode active material may be selected from the range of values of 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or any two.
The control of the lithium iron inversion defect of the positive electrode active material can be realized by any known process by a person skilled in the art. As an example, the control of the dislocation defect of the lithium iron of the positive electrode active material can be achieved by controlling the sintering temperature, the sintering time, the preparation method, the raw material metering ratio and the like.
During the preparation and cycling, certain lithium vacancies are inevitably present in the crystal structure of the positive electrode active material. The lithium vacancy not only can cause ferrous ions to oxidize into ferric ions, but also can induce the ferric ions to partially migrate to lithium positions, so that a lithium iron inversion defect is formed, a one-dimensional diffusion channel of the lithium ions is blocked, and the solid phase transmission of the lithium ions is adversely affected. The anode active material provided by the embodiment of the application has low lithium iron inversion defect, is favorable for uniform transmission of lithium ions in a solid phase, and further improves the dynamic performance of the lithium ion secondary battery.
In some embodiments, the lithium-containing transition metal phosphate particles comprise a component having the general formula:
LimFexPyOjQq,
Wherein Q comprises one or more of Al, na, K, mg, cu, mn, cr, zn, pb, ca, co, ni, sr, nb, V, ti, B, S, si, N, F, cl, br, m is more than or equal to 0.8 and less than or equal to 1.15,0.9 x is less than or equal to 1,0.95 and less than or equal to x is less than or equal to 1,0.95 is less than or equal to.
In some embodiments, m may be selected from 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, or a value in the range of any two values recited above.
In some embodiments, x may be selected from 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or a value in the range of any two values recited above.
In some embodiments, y may be selected to be 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, or a value in the range consisting of any two of the above.
In some embodiments, j may be selected from a value in the range consisting of 3.5, 3.6, 3.7, 3.8, 3.9, 4, or any two of the foregoing values.
In some embodiments, q may be selected from 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a range of any two values recited above.
The proper modification element Q is selected to improve the lattice change rate of the positive electrode active material in the lithium removal process, reduce the oxygen activity of the particle surface, improve the structural stability of the material, further improve the gram capacity exertion level of the material in the circulation process and further improve the circulation stability of the lithium ion secondary battery.
In some embodiments, the positive electrode active material comprises lithium iron phosphate and one or more of doping modified material and coating modified material thereof.
In some embodiments, the positive electrode active material includes titanium element in an amount of 2000ppm to 6000ppm by mass based on the total mass of the positive electrode active material.
The type and amount of elements in the positive electrode active material may be tested by any means known in the art. By way of example, inductively coupled plasma emission spectrometry was used to perform elemental titanium and content testing with reference to appendix C of GB/T33822-2017.
The doping of titanium element in the positive electrode active material is beneficial to causing lattice distortion, reducing Li-O bond energy, improving lithium ion transmission rate and improving the dynamic performance of the lithium ion secondary battery. However, in the prior art, the doping content of titanium element in the lithium-containing phosphate often cannot exceed 3000ppm, because excessive titanium element is difficult to completely enter the lithium-containing phosphate phase, and is easy to become a harmful impurity phase remained on the surface, which has a negative effect on the battery performance.
The positive electrode active material in the embodiment of the present application has a high titanium element content, and surprisingly, the high addition amount of titanium element does not form a harmful impurity phase that adversely affects the energy density and the dynamic performance of the battery, although not clear, presumably because titanium element forms a fast ion conductor together with phosphate and other elements (e.g., lithium element), but rather exerts an improvement effect on the dynamic performance of the battery.
In some embodiments, the positive electrode active material has a powder tap density of 0.70g/cm3-1.50g/cm3, optionally 0.7-1.20g/cm3.
The powder tap density may be tested using methods known in the art. An electronic balance is opened by taking a conical bottle as a base, putting the electronic balance on the electronic balance, clearing the electronic balance, taking a tap cylinder, putting the tap cylinder on the conical bottle, weighing and recording the weight of the cylinder, opening a sample bag, stirring a sample in the sample bag for 3-5 circles by using a clean sample spoon, uniformly mixing the sample, then stably transferring the sample into the cylinder, wiping powder stained on the communication surface by using dust-free paper, putting the powder into the cleared conical bottle for weighing, sealing a bottle mouth by using a sealing film, putting the tap cylinder into a matched instrument rubber ring, ensuring that the tap cylinder is tightly attached to the rubber ring and kept perpendicular to the surface of the instrument, setting the vibration frequency on the instrument for 250 times/min, the vibration times for 5000 times, pressing a key, vibrating for 20min, then taking down the tap cylinder, irradiating the surface of the cylinder by using a flashlight, reading the highest scale V1 and the lowest scale V2 by using a visual method, taking the average value of the two, subtracting the mass m0 of the cylinder from the sample, and obtaining the powder mass ρ = m/V, and obtaining the tap density of the sample by a formula.
In some embodiments, the powder tap density of the positive electrode active material may be selected to be 0.70g/cm3、0.80g/cm3、0.90g/cm3、1.00g/cm3、1.10g/cm3、1.20g/cm3、1.30g/cm3、1.40g/cm3、1.50g/cm3 or a range of values therebetween.
In some embodiments, the positive electrode active material has a powder compaction density of 2.50g/cm3-2.70g/cm3, optionally 2.52g/cm3-2.68 g/cm3, at 3T pressure.
In the present application, the term "powder compacted density" means the density of a compact having a certain density and strength, in g/cm3, formed by increasing the contact area between particles and generating attractive force between atoms and enhancing the mechanical engagement between particles as larger voids are filled with the movement and deformation of the powder during the compression process by external force.
The powder compacted density of the positive electrode active material may be measured by methods and apparatuses known in the art. For example, reference may be made to GB/T24533-2009 for measurement with a compaction density instrument. Specifically, a certain amount of positive electrode active material was placed on a compaction dedicated mold (known as the mold diameter), with a metal disc above and below the center of the mold. The positive electrode active material is placed between metal discs, a metal cylinder is placed on the top of the positive electrode active material, a die is placed on a compaction density instrument, the bottom area of the die is 1.327cm2, the set pressure is 3T, the thickness of the positive electrode active material under the pressure of 3T can be read out on equipment, the powder compaction density of the positive electrode active material is rho=m/v, wherein v= (S×H), m is the mass of the positive electrode active material, S is the bottom area of the die, and H is the thickness of the positive electrode active material after compaction.
In some embodiments, the powder compaction density of the positive electrode active material at 3T pressure may be selected to be 2.50g/cm3、2.51g/cm3、2.52g/cm3、2.53g/cm3、2.54g/cm3、2.55g/cm3、2.56g/cm3、2.57g/cm3、2.58g/cm3、2.59g/cm3、2.60g/cm3、2.61g/cm3、2.62g/cm3、2.63g/cm3、2.64g/cm3、2.65g/cm3、2.67g/cm3、2.68g/cm3、2.69g/cm3、2.70g/cm3 or a range of values therebetween.
The positive electrode active material has low area ratio of particles with the particle diameter of more than or equal to 1.5 mu m, limited active gradation formed independently and relatively low powder tap density. However, by virtue of high sphericity, the positive electrode active materials are easy to roll under the action of external force, so that gaps are mutually filled to realize higher compaction density, and a material foundation is provided for improving the compaction density of the pole piece and preparing the lithium ion secondary battery with high energy density.
In some embodiments, the positive electrode active material has a powder resistivity of 0.5 Ω -cm to 30.0 Ω -cm, optionally 2.0 to 20.0 Ω -cm, at a pressure of 8 MPa.
The powder resistivity of the positive electrode active material may be measured using methods and apparatus known in the art. For example, reference may be made to GB/T33822-2017 for measurement using a powder resistivity meter (Suzhou lattice, ST2722 type). Specifically, a certain amount of positive electrode active material (for example, 1 g) is weighed and added into a charging cavity of a powder resistivity meter, 8 MPa pressure is applied, the positive resistivity and the reverse resistivity of the positive electrode active material are respectively tested, and the average value of the positive electrode active material and the reverse resistivity is taken as the powder resistivity of the positive electrode active material.
In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8MPa may be selected to be 0.5Ω·cm、1Ω·cm、2Ω·cm、3Ω·cm、4Ω·cm、5Ω·cm、6Ω·cm、6.3Ω·cm、7Ω·cm、8Ω·cm、9Ω·cm、10Ω·cm、15Ω·cm、20Ω·cm、25Ω·cm、30Ω·cm or a range of values therebetween.
According to the positive electrode active material, the surface of the positive electrode active material is coated with the carbon material, and the rapid conduction of electrons among particles is easily realized by means of the sp2 structure of surface carbon, so that the positive electrode active material has low powder resistivity, is beneficial to improving the solid phase transmission rate of electrons, and improves the dynamic performance of a battery.
In some embodiments, the positive electrode active material has a discharge gram capacity of 135mAh/g to 150mAh/g at a discharge rate of 1C.
In the present application, the positive electrode active material was assembled into a button cell and tested for electrical properties on a blue tester. And (3) after the constant-current charging at 1C to 3.75V is carried out within the voltage range of 2.0V-3.75V at 25+/-5 ℃, the constant-voltage charging is stopped for 5 minutes until the cut-off current is 50 mu A, and then the constant-current discharging at 1C is carried out until the voltage is 2.0V. The discharge capacity of the button cell was divided by the mass of the positive electrode active material to give the positive electrode active material a discharge gram capacity at room temperature at a discharge rate of 1C.
The preparation and test flow of the button cell are as follows: mixing 2.0g of positive electrode active material, conductive carbon black and PVDF according to the mass ratio of 0.9:0.05:0.05, adding an organic solvent NMP (N-methylpyrrolidone), fully and uniformly mixing, coating by using a 150 mu m scraper, drying at 100 ℃ for 2 hours, pressing a cold-pressed positive electrode plate with the solid density of 2.0g/cm3-2.2g/cm3, beating into a circular sheet with the diameter of 14mm by using a puncher, weighing and recording the weight, putting the weighed positive electrode plate into a vacuum drying box (105 ℃,1-12hrs, -90 kpa), putting the dried positive electrode plate into a glove box, assembling into a battery according to the sequence of a negative electrode shell-nickel net-lithium sheet-isolating film-positive electrode plate-positive electrode shell, dripping 65-87 mu L (a liquid transferring gun) electrolyte (the electrolyte is EC (ethylene carbonate) with the volume ratio of 1:1), DMC (1, 2-dimethyl carbonate) mixed solvent, placing the electrolyte LiPF6) above, placing the negative electrode plate into a groove of a sealing machine, sealing the groove with the pressure of 650kg/cm, buckling, placing the electrode plate into a dust-free tweezer, taking out, placing the electrode bags under a constant temperature condition, and placing the insulating bag into a dust-free room for testing room, and placing the dust-free forceps for 3 h.
It will be appreciated that the discharge gram capacity of the positive electrode active material can also be obtained by testing after the positive electrode sheet is obtained after all-electric disassembly and assembled into a button cell according to the method described above.
In some embodiments, the positive electrode active material may have a discharge gram capacity of 135mAh/g, 140mAh/g, 142.3mAh/g, 145mAh/g, 150mAh/g, or a range of values therebetween at a discharge rate of 1C.
The positive electrode active material has high discharge gram capacity under the 1C multiplying power, which shows that the positive electrode active material has good charge and discharge capacity and is beneficial to improving the dynamic performance of the battery.
In some embodiments, the positive electrode active material is discharged to 3.2V, wherein the discharge capacity ratio eta is more than or equal to 85%, the eta is defined as that a button cell containing the positive electrode active material is charged and discharged twice at a constant current of 0.1C in a voltage range of 2.0V-3.75V at room temperature, then is charged and discharged once at a constant current of 1C, in a charging and discharging test of the multiplying power of 1C, the capacity value of extracting and discharging voltage of 3.2V is recorded as C1, the capacity value of extracting and discharging voltage of 2.0V is C2,η=C1/C2, and the charging process comprises constant voltage charging, constant voltage of 3.75V and constant voltage cut-off current of 50 mu A.
In some embodiments, the positive electrode active material is discharged to a discharge capacity of 3.2V with a ratio η+.88%.
The η value of the positive electrode active material may be measured by methods and apparatus known in the art. As an example, first, referring to the method described above, the prepared button cell was prepared, and the electrical properties of the prepared button cell were tested on a blue tester at room temperature, specifically, the button cell was charged and discharged twice with a constant current of 0.1C at a rate of 2.0v to 3.75v, charged to a cut-off voltage at a constant voltage to a current of 50 μa, and then charged and discharged once with a constant current of 1C at a rate of 1C. In the charge-discharge test at a magnification of 1C, the capacity value discharged from 3.75V to a voltage of 3.2V is denoted as C1, the capacity value discharged from 3.75V to 2.0V is denoted as C2, and η=c1/C2.
In some embodiments, η may be selected to be 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.1%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1% or a range consisting of any two numerical intervals or a number in the range.
In some embodiments, the positive electrode active material in freshly prepared lithium ion secondary batteries is discharged to a discharge capacity of 3.2V with a ratio η+.88%. After the freshly prepared lithium ion secondary battery is charged and discharged for a period of time at a constant current with the multiplying power of 0.1C in the voltage range of 2.0V-3.75V, the discharge capacity of the positive electrode active material discharged to 3.2V is maintained to be more than or equal to 85 percent.
The high discharge capacity ratio of the positive electrode active material adopted in the lithium ion secondary battery to 3.2V means that the positive electrode active material has good dynamic performance. Meanwhile, a high η value indicates that the lithium ion secondary battery including the positive electrode active material has a high voltage when discharged to a low state of charge (SOC), which is advantageous for maintaining good power performance.
In some embodiments, in the 0.1C discharge curve of the coin cell comprising the positive electrode active material, there is a discharge plateau in the voltage range of 2.5v to 2.9 v.
The discharge plateau generally refers to a region where the voltage is relatively stable during the charge and discharge of the battery. During discharge of the battery, current flows from the battery, and the voltage of the battery initially drops, but then enters a relatively stable region, the voltage of which changes little, and this stable voltage region is called the discharge plateau.
The button cell can be formed by disassembling a positive electrode plate in a lithium ion secondary battery and combining the positive electrode plate with lithium metal. Can also be assembled and prepared by the method described above. In the present application, the positive electrode active material was assembled into a button cell and tested for electrical properties on a blue tester. In the voltage range of 2.0V-3.75V, after charging to 3.75V with 0.1C constant current, the charging is stopped for 5 minutes, the constant voltage charging is stopped to 50 mu A of cut-off current, and then the charging is stopped to 2.0V with 0.1C constant current.
The discharge curve shows that the standard charge-discharge plateau voltage for lithium-containing phosphates is typically between 3.2V and 3.65V. The button cell containing the positive electrode active material in the embodiment of the application shows a new charge-discharge platform in the voltage range of 2.5V-2.9V, which is beneficial to increasing the discharge interval of the cell and improving the energy density of the cell. At the same time, this also verifies the hypothesis that the positive electrode active material of the embodiment of the present application contains a fast ion conductor.
In some embodiments, the positive electrode film layer further includes a conductive agent, the conductive agent being present in an amount of 0.1% to 1.5% by mass based on the total mass of the positive electrode film layer.
In some embodiments, the positive electrode film layer further includes a conductive agent, the conductive agent being selected from 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or a range of values therebetween by mass based on the total mass of the positive electrode film layer.
In some embodiments, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.
The particles in the positive electrode film layer have high sphericity, so the particles can be closely stacked in the rolling process, and the particles are in good contact, so that the particles in the positive electrode film layer have good electron conductivity, the use of a conductive agent in the positive electrode film layer can be reduced or even cancelled, the loading capacity of a positive electrode active material can be further improved, and the energy density of the lithium ion secondary battery can be improved.
In some embodiments, the positive electrode film layer does not include a conductive agent.
The particles in the positive electrode film layer have extremely high electron conductivity, so that even no conductive agent is added in the positive electrode film layer, thereby being beneficial to further improving the loading capacity of the positive electrode active material and improving the energy density of the lithium ion secondary battery.
In some embodiments, the positive electrode film layer further includes a binder, the mass content of the positive electrode active material is 95.5% -99.5%, optionally 96.5% -99.5%, based on the total mass of the positive electrode film layer, and the mass content of the binder is 0.5% -3%.
In some embodiments, the binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluoroacrylate resin.
In some embodiments, the mass content of the positive electrode active material may be selected from 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.5%, or a range of values therebetween, based on the total mass of the positive electrode film layer.
In some embodiments, the mass content of the binder may be selected to be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or a range of values therebetween, based on the total mass of the positive electrode film layer.
The mass content of the positive electrode active material and the mass content of the binder are in the ranges, so that the active material load in the positive electrode film layer in unit volume can be effectively improved, the good internal binding force is maintained, the occurrence probability of the problems of powder dropping and expansion cracking is reduced, the energy density of the secondary battery is improved, and meanwhile, the safety performance is also considered.
In some embodiments, the positive electrode film layer has a single-sided density of 300mg/1540mm2-450mg/1540mm2.
In the present application, the single-sided density of the positive electrode film layer is in the meaning known in the art, and can be tested using methods known in the art. For example, a single-sided coated and cold-pressed positive electrode sheet (if a double-sided coated positive electrode sheet is used, the positive electrode film layer on one side of the positive electrode sheet can be wiped off first), a small wafer with an area of S1 is punched, and the weight of the small wafer is recorded as M1. And then wiping the positive electrode film layer of the weighed positive electrode plate, weighing the weight of the current collector, and recording as M0. One-sided surface density= (M1-M0)/S1. To ensure accuracy of test results, multiple (e.g., 10) sets of samples to be tested may be tested, and an average value calculated as the test result.
In some embodiments, the areal density of the positive electrode film layer on one side may be selected to be 300 mg/1540mm2、310 mg/1540mm2、320 mg/1540mm2、330 mg/1540mm2、340 mg/1540mm2、350 mg/1540mm2、360 mg/1540mm2、370 mg/1540mm2、380 mg/1540mm2、390 mg/1540mm2、400 mg/1540mm2、410 mg/1540mm2、420 mg/1540mm2、430 mg/1540mm2、440 mg/1540mm2、450 mg/1540mm2 or a range of values therebetween.
The positive electrode film layer having the areal density in the above-described range can contribute to an improvement in the energy density of the lithium ion secondary battery.
In some embodiments, the positive electrode film layer has a compacted density of 2.51g/cm3-2.73 g/cm3 in the fully discharged state of the lithium ion secondary battery.
In some embodiments, the positive electrode film layer has a compacted density of 2.55g/cm3-2.70g/cm3 in the fully discharged state of the lithium ion secondary battery.
In the application, the full-discharge state refers to a state that the battery is placed at 25 ℃, kept stand for 2 hours, and discharged to 2.0V at 0.1 ℃ after the battery is discharged to 2.5V at 1/3 ℃ at constant current when the temperature of the battery is kept at 25 ℃.
The compacted density of the positive electrode film layer may be tested using methods known in the art. As an example, the battery is placed in a 25 ℃ oven environment, left stand for 2 hours, after the battery temperature is kept at 25 ℃, the battery is discharged to 2.5V at a constant current of 1/3 ℃ and then discharged to 2.0V at a constant current of 0.1 ℃ and disassembled to obtain a positive electrode sheet, the residual electrolyte is treated with a dimethyl carbonate solvent, the sheet is dried and cut into small discs with an area of S to obtain a mass of W1, the thickness T1 of the positive electrode sheet is measured by using a ten-thousandth, then the positive electrode film layer of the weighed sheet is wiped off, the mass of the current collector is weighed and recorded as W2, and the thickness T2 of the current collector is measured by using a ten-thousandth, and the compacted density pd= (W1-W2）/[(T1-T2) ×s of the positive electrode film layer is obtained.
In some embodiments, the positive electrode film layer has a compacted density of 2.51g/cm3、2.52g/cm3、2.53g/cm3、2.54g/cm3、2.55g/cm3、2.56g/cm3、2.57g/cm3、2.58g/cm3、2.59g/cm3、2.60g/cm3、2.61g/cm3、2.62g/cm3、2.63g/cm3、2.64g/cm3、2.65g/cm3、2.66g/cm3、2.67g/cm3、2.68g/cm3、2.69g/cm3、2.70g/cm3、2.71g/cm3、2.72g/cm3、2.73g/cm3 or any value therebetween.
In some embodiments, the positive electrode film layer has a compacted density of 2.63g/cm3-2.85g/cm3 after treatment by a cold press process.
In some embodiments, the compacted density of the positive electrode film layer after being processed by the cold pressing process may be 2.63g/cm3、2.64g/cm3、2.65g/cm3、2.66g/cm3、2.67g/cm3、2.68g/cm3、2.69g/cm3、2.70g/cm3、2.71g/cm3、2.72g/cm3、2.73g/cm3、2.74g/cm3、2.75g/cm3、2.76g/cm3、2.77g/cm3、2.78g/cm3g/cm3、2.79g/cm3、2.80g/cm3、2.81g/cm3、2.82g/cm3、2.83g/cm3、2.84g/cm3、2.85g/cm3 or any range therebetween.
In the present application, cold pressing means that the positive electrode film layer is compacted by mechanical pressure during the battery assembly process to improve the compactness and conductivity thereof.
In some embodiments, the positive electrode film layer has a compacted density of 2.51g/cm3-2.73g/cm3 after the chemical conversion process.
In some embodiments, the compacted density of the positive electrode film layer after the chemical conversion process may be selected to be 2.51g/cm3、2.52g/cm3、2.53g/cm3、2.54g/cm3、2.55g/cm3、2.56g/cm3、2.57g/cm3、2.58g/cm3、2.59g/cm3、2.60g/cm3、2.61g/cm3、2.62g/cm3、2.63g/cm3、2.64g/cm3、2.65g/cm3、2.66g/cm3、2.67g/cm3、2.68g/cm3、2.69g/cm3、2.70g/cm3、2.71g/cm3、2.72g/cm3、2.73g/cm3 or any range therebetween.
In the present application, formation means that a stable solid electrolyte interface (SEI film) and an electrode structure are formed through an electrochemical reaction in the first charge and discharge process of a battery.
It can be understood that the compacted density of the positive electrode film layer in the full discharge state of the lithium ion secondary battery is slightly lower than that of the positive electrode film layer after cold pressing and after formation along with rebound of the pole piece during the circulation.
The compacted density of the positive electrode film layer is within the above range, which is advantageous for improving the energy density of the lithium ion secondary battery.
In some embodiments, the positive electrode film layer has a compacted density of 2.51g/cm3-2.73g/cm3, and the positive electrode film layer has a porosity of 10% -22% in a section of the positive electrode film layer along the thickness direction of the pole piece.
In some embodiments, the positive electrode film layer has a compacted density of 2.55g/cm3-2.70g/cm3, and the positive electrode film layer has a porosity of 10% -20% in a section of the positive electrode film layer along the thickness direction of the pole piece.
In some embodiments, the porosity of the positive electrode film layer in a section along the thickness direction of the electrode sheet may be selected from 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22% or a range of values therebetween.
In the section of the positive electrode film layer along the thickness direction of the pole piece, the porosity of the positive electrode film layer can be tested in the following manner. And importing the section scanning electron microscope image of the positive electrode film layer along the thickness direction of the pole piece obtained in the mode described above into imageJ software, selecting a straight line tool, marking the length of a scale in the image by using the straight line, clicking 'Analyze Set Scale', and setting scale parameters in the software according to the length of the scale in the image. Selecting a rectangular tool, selecting a picture part outside a scale Area, copying the selected Area by using Image Duplex, adjusting a picture format by using IMAGE TYPE bit, selecting Analyze Set Measurements, and selecting 5 options including ' Area ', ' Mean gray value ', ' Area Fraction ', ' Limit to Threshold ', ' Feret ' S DIAMETER ', wherein ' DECIMAL PLACES ' selects 3, sequentially selects ' Image ', ' Adjust ', ' Threshold ', and sequentially sets 0 and 100 at a ' Threshold ' frame selection position, so that pore data in the section electron microscope Image can be derived by using an analysis-Measure function. And (3) deriving by using 'Image' - 'Overlay' - 'flame' to obtain a pore picture, clicking 'Apply' in 'Threshold', clicking 'analysis' - 'Analyze Particles', and checking four columns on the left side to obtain the pore statistical data.
As shown in fig. 8, it can be understood that the "pores" in the cut surface of the positive electrode film layer are identified by the picture color difference and the threshold in the embodiment of the present application. The porosity is not the pore data obtained in the exhaust test, and is mainly used for representing the sectional area between particles in the section of the positive electrode film layer, and the method is better than an exhaust method, because the porosity obtained by the exhaust method is related to the pores between the particles and also related to the pores in the carbon layer coated on the surface of the lithium iron phosphate particles, so that the pores between the particles cannot be objectively reflected. The lower the porosity in the section of the positive electrode film layer tested by the method is, the better the grading of large, medium and small particles in the positive electrode film layer is, the high compaction density is achieved, and on the other hand, after the same grading and rolling pressure are achieved, if the porosity is low, the particles are easy to slide mutually, so that the risk of film layer overpressure and stress concentration is reduced, the demolding probability of the positive electrode film sheet in the long-cycle process is further reduced, and the long-cycle performance of the battery is improved.
In some embodiments, the positive electrode sheet comprises an undercoat layer arranged between the positive electrode film layer and the positive electrode current collector, wherein the undercoat layer comprises carbon-based particles, and the distribution density of the carbon-based particles with the particle size of more than 100nm in the undercoat layer is less than or equal to 10pcs/10 mu m.
Wherein, the carbon-based particles refer to particles with carbon elements as main components, including but not limited to conductive carbon, carbon black, and the like.
The bottom coating is beneficial to improving the conductivity and the binding force of the positive electrode film layer and the current collector, reducing the demolding of the positive electrode film layer and the current collector in the circulation process, and improving the dynamic performance of the battery. In the high-compaction-density pole piece provided by the embodiment of the application, for example, when the compaction density of the positive pole piece in a full-discharge state is more than or equal to 2.4g/cm3, the current collector is easy to damage in the high-pressure compaction process of the pole piece, large-size particles are easy to generate pits on the current collector, the distribution density of carbon-based particles with the particle size of more than 100nm in the undercoat is controlled to be less than or equal to 10pcs/10 mu m, the probability of damage of the current collector in the high-pressure compaction pole piece is reduced, and the limit compaction density of the positive pole piece is further improved.
The distribution density of the carbon-based particles with the particle size of more than 100nm in the bottom coating can be obtained by adopting the method, cutting the positive electrode film layer along the thickness direction of the pole piece by an argon ion beam, shooting a scanning electron microscope image or a microscopic image, detecting the size of the carbon particles in the bottom coating by a statistical method, counting the number of the carbon-based particles with the particle size of more than 100nm contained in the bottom coating per 10 mu m, counting not less than 5 times, and averaging.
The undercoat layer in the embodiment of the application can be realized through any known preparation process, such as sieving or centrifuging operation in advance in the preparation process of the carbon-based particles to remove large-particle carbon-based materials, so that DV50 of the carbon-based particles added in the preparation process of the undercoat layer is 20-60nm, DV90 is less than or equal to 70nm, and the carbon-based materials are mixed with a binder, stirred and coated on a current collector to obtain the undercoat layer.
In some embodiments, the positive electrode sheet has a compacted density of 2.4g/cm3 or more in the fully discharged state, and the primer layer has a single-sided thickness of 1 μm to 4 μm.
In some embodiments, the positive electrode sheet has a compacted density of 2.5g/cm3 or more in the fully discharged state, and the primer layer has a single-sided thickness of 2 μm to 4 μm.
With an increase in the compacted density of the pole piece, the larger the particle size of the lithium-containing phosphate material (e.g., greater than 1 μm) in the positive electrode film layer, the more significant the extrusion effect on the primer layer. Thus, stress concentrations tend to occur at large particle sites and even damage to the current collector through the primer layer. The thickness of the bottom coating is increased, so that the stress concentration phenomenon in the pole piece is improved, and the limit compaction density of the pole piece is further increased.
The thickness of one side of the primer layer can be tested in the following manner. The positive electrode film layer was cut through the argon ion beam in the thickness direction of the pole piece, a scanning electron microscope image was taken, the thickness of the single-sided primer layer was measured at 1m intervals in the length direction of the pole piece, and the thickness of the primer layer at 10 points was measured and then averaged. It should be noted that during the measurement of the point, abnormal points, i.e. the primer areas with a thickness of less than 50nm and a thickness of greater than 4m, need to be avoided, and these abnormal points are not statistically significant due to extreme fluctuations in the thickness of the individual areas caused by the concentrated extrusion of abnormal stresses during the compaction of the pole pieces.
In some embodiments, the positive electrode current collector has a thickness of 17 μm or less, optionally 13 μm to 15 μm.
In some embodiments, the positive electrode current collector has a thickness of 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or a range of values therebetween.
In some embodiments, the positive current collector may employ a metal foil or a composite current collector. For example, as the metal foil, aluminum foil may be used. The composite current collector may include a polymeric material base layer and a metal layer formed on at least one surface of the polymeric material base layer. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel alloy, titanium alloy, silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
In some embodiments, the negative electrode tab comprises a negative electrode current collector and a negative electrode film layer disposed on at least one side of the negative electrode current collector, the negative electrode film layer on one side having an areal density of 140mg/1540mm2-221mg/1540mm2, and/or the negative electrode film layer has a compacted density of 1.40g/cm3-1.75g/cm3.
The single-sided density and compacted density of the negative electrode film layer can be tested by methods similar to those described previously for the positive electrode film layer.
The surface density and the compaction density of the negative electrode film layer are in the above ranges, which is beneficial to improving the energy density of the lithium ion secondary battery.
In some embodiments, the negative electrode current collector may employ a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymeric material base layer and a metal layer formed on at least one surface of the polymeric material base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel alloy, titanium alloy, silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material may employ a negative electrode active material for a battery, which is well known in the art. As an example, the anode active material may include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, lithium titanate, and the like. The silicon-based material may be at least one selected from elemental silicon, silicon oxygen compounds, silicon carbon composites, silicon nitrogen composites, and silicon alloys. The tin-based material may be at least one selected from elemental tin, tin oxide, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as a battery anode active material may be used. These negative electrode active materials may be used alone or in combination of two or more.
In some embodiments, the negative electrode film layer further optionally includes a binder. The binder may be at least one selected from Styrene Butadiene Rubber (SBR), polyacrylic acid (PAA), sodium Polyacrylate (PAAs), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium Alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).
In some embodiments, the negative electrode film layer further optionally includes a conductive agent. The conductive agent is at least one selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.
In some embodiments, the negative electrode film layer may optionally further include other adjuvants, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)), and the like.
In some embodiments, the negative electrode tab may be prepared by dispersing the above components for preparing the negative electrode tab, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry, coating the negative electrode slurry on a negative electrode current collector, and performing processes such as drying, cold pressing, and the like to obtain the negative electrode tab.
In some embodiments, a lithium ion secondary battery includes an electrolyte. The electrolyte plays a role in ion conduction between the positive electrode plate and the negative electrode plate. The application is not particularly limited in the kind of electrolyte, and may be selected according to the need. For example, the electrolyte may be liquid, gel, or all solid.
In some embodiments, the electrolyte is an electrolyte. The electrolyte includes an electrolyte salt and a solvent.
In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis-fluorosulfonyl imide, lithium bis-trifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalato borate, lithium difluorodioxaato phosphate, and lithium tetrafluorooxalato phosphate.
In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methylethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1, 4-butyrolactone, sulfolane, dimethyl sulfone, methyl sulfone, and diethyl sulfone.
In some embodiments, the electrolyte further optionally includes an additive. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives capable of improving certain properties of the battery, such as additives that improve the overcharge performance of the battery, additives that improve the high or low temperature performance of the battery, and the like.
In some embodiments, a separator is further included in the lithium ion secondary battery. The type of the separator is not particularly limited, and any known porous separator having good chemical stability and mechanical stability can be used.
In some embodiments, the material of the isolating film may be at least one selected from glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The separator may be a single-layer film or a multilayer composite film, and is not particularly limited. When the separator is a multilayer composite film, the materials of the respective layers may be the same or different, and are not particularly limited.
In some embodiments, the positive electrode tab, the negative electrode tab, and the separator may be manufactured into an electrode assembly through a winding process or a lamination process.
In some embodiments, the lithium ion secondary battery may include an outer package. The outer package may be used to encapsulate the electrode assembly and electrolyte described above.
In some embodiments, the outer package of the lithium ion secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The exterior package of the secondary battery may also be a pouch type pouch, for example. The material of the flexible bag may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.
A second aspect of the present application provides a battery device, including the lithium ion secondary battery provided in the first aspect of the present application, where the battery device includes at least one of a battery module, a battery pack, and an energy storage battery.
The third aspect of the application provides an electric device comprising the lithium ion secondary battery provided in the first aspect of the application.
The fourth aspect of the application provides a preparation method of a positive electrode active material, which comprises the steps of obtaining a mixed raw material comprising a carbon source, a lithium source, an iron source and a phosphorus source, wherein the molar ratio of lithium to iron in the mixed raw material is more than or equal to 1 and less than or equal to 1.05, obtaining mixed slurry after grinding, obtaining precursor powder after drying the mixed slurry, sintering the precursor powder, obtaining the positive electrode active material after crushing, wherein the sintering comprises a first temperature and a second temperature, the second temperature of the sintering is 750-800 ℃, the crushing comprises air current crushing, the grading frequency of the air current crushing is 18-24 Hz, and the crushing air pressure is 0.45-0.65 MPa.
According to the preparation method provided by the embodiment of the application, the anode active material with large particle size and small proportion is obtained by regulating and controlling the lithium iron ratio, the sintering temperature and the crushing process, on one hand, the sphericity of particles in the anode active material is improved, the area ratio of the particles with the particle size of more than or equal to 1.5 mu m in a section along the thickness direction of the pole piece is more than or equal to 8.0% and less than or equal to 20.0%, and in the sphericity-like area cumulative distribution curve of the particles obtained in the section along the thickness direction of the pole piece, the sphericity-like median LA50 is 0.70-0.74, and the preparation of the anode film layer provides a material foundation.
In some embodiments, the iron source comprises ferrous iron, optionally one or more of ferrous oxalate, ferrous carbonate, ferrous nitrate.
In the sintering process, the ferrous iron source is decomposed preferentially to generate a large amount of ferrous oxide, and the ferrous oxide is used as a nucleation site to generate nanocrystal cores of lithium-containing transition metal phosphate. Meanwhile, the polymer carbon source has relatively low decomposition temperature, and the iron element positioned on the surface of the crystal nucleus of the nano crystal nucleus can further catalyze the decomposition of the carbon source, so that the carbon coating material on the surface of the anode active material has relatively high graphitization degree at a low sintering temperature, the resistivity of the anode active material is reduced, and the compactness and uniformity of the carbon coating material coated on the surface of the lithium-containing transition metal phosphate are improved. In addition, the uniform deposition of carbon on the surface of the lithium-containing transition metal phosphate further hinders the growth of lithium-containing transition metal phosphate grains, reducing the probability of the positive electrode active material particles growing into large particles having a particle size of more than 1.5 μm.
In some embodiments, the ferrous oxalate has a particle size D10 of 3 μm or greater, a particle size D50 of 50-80 μm, and a particle size D90 of 150 μm or less.
In the present application, the terms "D10"、"D50" and "D90" correspond to the particle sizes corresponding to the cumulative percentage of particle size distribution of the sample particle size as measured by the malvern laser scattering method reaching 10%, 50% and 90%, respectively.
The particle diameter D10 of the ferrous oxalate is controlled to be more than or equal to 3 mu m, so that the duty ratio of the ferrous oxalate particles with small particle diameter can be reduced, and the reactivity of the ferrous oxalate particles in the grinding process can be controlled. The particle size D50、D90 of the ferrous oxalate is controlled, so that the raw materials are uniformly mixed in the grinding process, the mixed slurry with uniform components and uniform particle size is obtained, and the particle size consistency of the prepared lithium-containing transition metal phosphate is improved.
In some embodiments, the ferric element is present in an amount of 0.08% by mass or less.
In some embodiments, the ferric element content may be selected from the range of values of 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08% or any combination thereof.
Controlling the mass content of ferric iron helps to improve the uniformity and consistency of the carbon-coated material. The excessive ferric iron element can consume the carbon source preferentially, so that the quality and thickness consistency of the carbon layer coated among the particles are poor, on one hand, the carbon coating material with uneven thickness can influence the compaction among the particles, and on the other hand, the local carbon deficiency can influence the lap joint of the conductive network among the particles, thereby being unfavorable for the effective improvement of the compaction density of the pole pieces and the improvement of dynamics.
In some embodiments, the lithium source comprises one or more of lithium dihydrogen phosphate, lithium carbonate, lithium acetate.
In some embodiments, the carbon source comprises a polymeric carbon source, optionally one or more of polyethylene glycol, polyvinyl alcohol.
In some embodiments, the polyethylene glycol is present in an amount of 1.0% to 4.0% by mass based on the total mass of the carbon source.
The polymer carbon source has relatively low decomposition temperature and graphitization temperature, so that the carbon coating material on the surface of the positive electrode active material can be decomposed to form a carbon layer at a low sintering temperature, the growth and sintering growth of lithium-containing transition metal phosphate grains are prevented, and the particle size of positive electrode active material particles is reduced.
Meanwhile, the polymer carbon source generally has higher molecular weight or longer molecular chain, and is easy to form a stable framework structure through crosslinking or orientation in the heat treatment process, and the ordering is reserved in the high-temperature carbonization process, so that the oriented growth of graphite crystals is facilitated; meanwhile, the winding and crosslinking among long chains are beneficial to reducing structural defects and reducing lattice disorder caused by chain breakage in the carbonization process, so that the graphitization degree is improved.
The organic molecules in the carbon source can be decomposed at high temperature to release carbon atoms, the carbon atoms can cover and fill micro gaps or defects on the surface of the active material, the surface roughness is reduced, the graphitization degree of the coating material formed by the polymer carbon source is higher, the carbon structure is compact, and the optimization of the surface roughness of the positive electrode active material is facilitated.
In some embodiments, the polyethylene glycol has a weight average molecular weight of 10000 or less.
Polyethylene glycol with weight average molecular weight less than 10000 is used, the carbon chain is shorter, and the decomposition rate during sintering is easy to control, so that the carbon coating material with proper and uniform thickness is formed.
In some embodiments, the polyethylene glycol has a moisture content of 0.5% or less.
If the polyethylene glycol has a high moisture content, the moisture may affect the decomposition process, so that the decomposition is incomplete or the decomposition rate is uneven during sintering. Excessive moisture can also lead to uneven distribution of molten polyethylene glycol during sintering, affecting the uniformity of the carbon layer, leading to unstable carbon coating materials or falling off.
In some embodiments, the water content of the polyethylene glycol may be selected to be 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any range therebetween.
In some embodiments, the polyethylene glycol has a pH of 5 to 7.
The polyethylene glycol with the pH value of 5-7 has higher stability, and can not be degraded in the mixing process due to peracid, especially under the high-temperature condition, so that the degradation is too fast, and the quality of the coating material is not affected. If the polyethylene glycol is alkaline, the stability of other components can be influenced, so that metal ions are dissolved or oxidized, and the performance of the final positive electrode active material is influenced.
In some embodiments, the phosphorus source comprises one or more of lithium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate.
In some embodiments, the lithium source and the phosphorus source may be the same species.
In some embodiments, the iron source comprises ferrous oxalate, the lithium source and the phosphorus source comprise lithium dihydrogen phosphate, and the carbon source comprises polyethylene glycol.
In some embodiments, the atomic molar ratio of lithium element to iron element in the lithium source and the iron source is from 1.00 to 1.05.
In some embodiments, the atomic molar ratio of lithium element to iron element in the lithium source and the iron source may be selected to be 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, or a range of values therebetween.
When the atomic mole ratio of the lithium element to the iron element is 1, the lithium ion battery belongs to an ideal stoichiometric ratio, can keep the optimal electrochemical performance, optimizes the reversible deintercalation capability of lithium ions in the charge and discharge process, has good crystal structure stability, prolongs the cycle life, and can reduce the occurrence probability of impurity phases. However, in practical production, in order to compensate for lithium loss during sintering, the molar ratio of lithium iron element needs to be adjusted to be slightly higher than 1.
In some embodiments, a titanium source is also included in the slurry, optionally including one or more of titanium dioxide, tetrabutyl titanate, titanium nitrate, titanic acid.
The titanium source often has lower surface activity, and the inclusion of the titanium source in the slurry can reduce the activity of the lithium-containing transition metal phosphate precursor, inhibit the growth of particles of the lithium-containing transition metal phosphate during high-temperature sintering, and enable the lithium-containing transition metal phosphate to form smaller particles during sintering.
Titanium is used as a lattice stabilizer, titanium element generally enters a lattice of lithium-containing transition metal phosphate in the form of Ti4+, and part of titanium ions can replace the positions of iron ions, so that the crystal structure is more stable, and the possibility of inversion of lithium and iron ions, particularly during high-temperature or high-current charge and discharge, is reduced.
Meanwhile, the titanium doping is beneficial to improving the sphericity of the particles and reducing the roughness of the particles, so that the overall structural stability of the material is enhanced.
In some embodiments, lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol and titanium dioxide are uniformly mixed and ground in an organic solvent to obtain a mixed raw material.
The organic solvent can effectively reduce the occurrence of side reaction and improve the purity and consistency of the material. And the organic solvent has better volatility, is easier to remove in the subsequent drying process, and can not remain in the material, so that air holes are generated in the material, and the compactness and the structural stability of the material are affected.
In some embodiments, the post-milling obtaining a mixed slurry includes at least two ball milling-demagnetizing cycles, each of which independently satisfies one or more of the following conditions:
(1) The grinding balls of the ball mill are one or more of zirconia balls, silicon nitride zirconium balls and ceramic zirconium balls,
(2) The diameter of the ball-milling grinding ball for the first time is 5mm-6mm, and the diameter of the ball-milling grinding ball for the second time is 0.5mm-0.7mm;
(3) The primary ball milling rotating speed of the ball milling is 1400rpm-1600rpm, and the secondary ball milling rotating speed is 400rpm-600rpm;
(4) The first ball milling time of the ball milling is 150-200min, and the second ball milling time is 140-180 min;
(5) The demagnetizing mode is permanent magnet deironing;
(6) The demagnetizing strength of the demagnetizing is more than or equal to 8000GS.
By combining at least two ball milling and demagnetizing, large particle materials can be rapidly processed and further refined in a shorter time. Therefore, the particle size is not uniform in the ball milling process, the agglomeration phenomenon among particles is reduced, the conductivity and the cycling stability of the battery are improved, and meanwhile, the overall production efficiency is improved while the performance of a final product is ensured.
In some embodiments, the volume distribution particle size DV50 of the particles in the mixed slurry is 1.0 μm to 4.0 μm.
In the present application, the term "DV50" refers to the particle size corresponding to a percentage of cumulative particle size distribution of the sample volume as measured by the malvern laser scattering method of up to 50%;
In some embodiments, the volume distribution particle size DV50 of the particles in the mixed slurry may be selected to be 1.0 μm, 1.5 μm,2 μm, 2.5 μm, 3 μm, 3.5 μm, 4.0 μm, or a range of values therebetween.
The volume distribution granularity DV50 of the particles in the mixed slurry can increase the activity of the particles to a certain extent in the above range, generate partial positive electrode active material particles with the particle diameter of 1-1.5 mu m at the same temperature, improve the compaction density of the pole piece and the energy density of the battery, improve the catalytic decomposition efficiency of iron elements on the surface of crystal nucleus on the carbon source, improve the coating quality of the carbon source, improve the uniformity and graphitization degree of the coating of the carbon coating material, and further improve the compaction density of the pole piece and the energy density of the battery.
In some embodiments, drying the mixed slurry to obtain a precursor powder comprises spray drying the mixed slurry to obtain a precursor powder.
In some embodiments, the sintering the precursor powder to obtain a positive electrode active material comprises at least two sinters.
In some embodiments, a first sintering of the at least two sintering satisfies one or more of the following conditions:
(1) The temperature rising rate is more than or equal to 2 ℃ per minute;
(2) The heat preservation temperature is 300-400 ℃;
(3) The heat preservation time is 2-6 h.
In some embodiments, the second sintering of the at least two sintering satisfies one or more of the following conditions:
(1) The temperature rising rate is more than or equal to 3 ℃ per minute;
(2) The heat preservation temperature is 750-800 ℃;
(3) The heat preservation time is 8-15 h.
The temperature is quickly increased to the target temperature by adopting a higher temperature increasing rate, so that the uniform growth of particles is facilitated, and the existence of the particles with the particle size of more than or equal to 1.5 mu m is reduced.
By controlling the sintering temperature in the primary sintering and secondary sintering processes, the speed of the sintering diffusion rate can be controlled. At high temperatures, the diffusion of the particle surface increases, defects in the particles are repaired, and the lattice rearranges. By recrystallization, defects on the surface of the particles are eliminated, the grain structure of the particles is more ordered, the size of the particles is gradually increased, and the smoothing of the particle surface and the promotion of the particle to spherical shape are facilitated. The sintering temperature also affects the graphitization rate of the carbon source, and kinetically, the carbon atoms gain more energy, which can overcome the original energy barrier, causing them to undergo more severe rearrangements in the lattice. The sintering time influences the proceeding degree of the reaction, the too short sintering time does not completely complete the diffusion and rearrangement of the lithium-containing transition metal phosphate and the carbon source, and the too long sintering time can cause abnormal growth of particles, coarsening of crystal grains in the particles, unstable material structure, enhanced cohesive force among the particles and agglomeration.
In some embodiments, the positive electrode active material is obtained by jet milling the product after sintering the precursor.
In some embodiments, the jet milling has a classification frequency of 18Hz to 24Hz and a milling gas pressure of 0.45MPa to 0.65MPa.
The classification frequency in jet milling refers to the operating frequency of the classification device in jet milling, and is generally related to the classification efficiency and particle size distribution of the particles. Higher classification frequencies screen the particles in the gas stream more times so that larger particles are screened out, leaving smaller particles. And the higher grading frequency can increase the collision times of the particles, so that the irregular particles are further impacted, the surfaces of the particles are smoother, and the shapes of the particles tend to be spherical.
The high air pressure can enable the particles to receive larger impact force, the collision between the particles is more severe, the surfaces of the particles can be subjected to stronger impact and abrasion, large particles can be crushed into small particles, the collision between the particles is more severe, the surface is more easily trimmed, and the sphericity and the surface flatness of the particles are improved.
However, too high classification frequency and crushing air pressure can cause further cracking and crushing after the agglomerated particles are dispersed into primary particles, affect the predetermined particle grading distribution, cause incomplete carbon coating material, show increased iron dissolution, negatively affect the sliding of particles in rolling, increase the contact and reaction of lithium-containing transition metal phosphate with external factors such as electrolyte, and the like, and are not beneficial to the maintenance of the cycle performance and the service life of batteries. Therefore, it is necessary to control the classification frequency of jet milling and the milling air pressure within a proper interval.
The fifth aspect of the application provides a preparation method of a positive electrode plate, which comprises the steps of sequentially adding a binder, a conductive agent and a positive electrode active material prepared by the preparation method of the fourth aspect, dry-mixing, adding a solvent, stirring, adjusting viscosity to obtain a shipment slurry, transferring and coating the shipment slurry to at least one side of a current collector, drying, and hot-pressing to obtain a positive electrode film layer.
In some embodiments, the stirring includes pre-stirring and main stirring for a period of 200min to 250min at a temperature of 5 ℃ to 50 ℃.
In some embodiments, the hot pressing comprises at least three hot rolls, the hot rolls are sequentially increased, the hot rolls are sequentially 20-50 tons, 50-70 tons and 70-90 tons, the hot rolls are at a temperature of 40-80 ℃, the positive electrode sheet is heated before the positive electrode sheet is compacted by the hot rolls for the first time, and the heated temperature is 40-50 ℃.
The positive electrode active material prepared by the hot pressing process and the preparation method of the fourth aspect is favorable for further reducing the section porosity of the positive electrode film layer, improving the limit compaction density of the pole piece and improving the energy density of the battery.
In addition, the application also provides an electric device which comprises at least one of the secondary battery, the battery module or the battery pack. The secondary battery, the battery module, or the battery pack may be used as a power source of the power consumption device, and may also be used as an energy storage unit of the power consumption device. The power utilization device may include mobile devices (e.g., cell phones, notebook computers, etc.), electric vehicles (e.g., electric-only vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.
As the electricity consumption device, a secondary battery, a battery module, or a battery pack may be selected according to the use requirements thereof.
Fig. 7 is an electrical device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle or the like. In order to meet the high power and high energy density requirements of the secondary battery by the power consumption device, a battery pack or a battery module may be employed.
As another example, the device may be a cell phone, tablet computer, notebook computer, or the like. The device is generally required to be light and thin, and a secondary battery can be used as a power source.
Examples
Hereinafter, embodiments of the present application are described. The following examples are illustrative only and are not to be construed as limiting the application. The examples are not to be construed as limiting the specific techniques or conditions described in the literature in this field or as per the specifications of the product. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.
Example 1
(1) Preparation of positive electrode active material
Mixing lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol and titanium dioxide in methanol uniformly, and grinding to obtain mixed raw material. Wherein, the proportion of the lithium dihydrogen phosphate and the ferrous oxalate is such that the atomic mole ratio of lithium to iron is 1.03.
Spray drying the mixed slurry to obtain dried precursor powder, wherein the dried precursor powder material has light yellow appearance and uniform color.
The precursor powder is placed in a sintering furnace, heated from 25 ℃ to 350 ℃ at 2 ℃ per min under nitrogen atmosphere and kept at the temperature for 3 hours, then heated to 770 ℃ at 5 ℃ per min and kept at the temperature for 10 hours, and cooled down after the end.
The obtained material is crushed by adopting an air flow crushing method with the classification frequency of 22Hz and the crushing air pressure of 0.55MPa, so as to obtain the carbon-coated lithium iron phosphate positive electrode active material.
The mass content of carbon element in the prepared positive electrode active material is 1.177%, the concentration of lithium iron inversion defects is 0.53%, the powder tap density is 1.19g/cm3, the powder compaction density under 3T pressure is 2.57g/cm3, the powder resistivity under 8MPa is 6.3 Ω & cm, the discharge gram capacity under 1C discharge rate is 142.3mAh/g, a discharge platform exists in the voltage range of 2.5V-2.9V, and the discharge capacity of the 3.2V discharge platform is 92.1%.
(2) Preparing a positive electrode plate:
Sequentially adding PVDF (polyvinylidene fluoride) 2.2wt%, conductive carbon black 0.8wt% and positive electrode active material 97.0wt% into dry-mixed materials, adding N-methyl pyrrolidone, stirring, adjusting viscosity to obtain shipment slurry, transferring and coating the shipment slurry onto an undercoat of a current collector aluminum foil, wherein the undercoat comprises carbon black and PVDF, the mass ratio of the carbon black to the PVDF is 1:1, the distribution density of carbon-based particles with the particle size of more than 100nm in the undercoat is less than or equal to 10pcs/10 mu m, and the thickness of the undercoat is 2 mu m. And drying and hot-pressing to obtain the positive electrode film layer with the single-side surface density of 350mg/1540mm2. Wherein the stirring comprises pre-stirring and main stirring, wherein the stirring time of the main stirring is 220min, and the temperature is 40 ℃.
The hot pressing process comprises three hot rolling processes, wherein the hot rolling pressure is sequentially increased, the hot rolling pressure is sequentially 40 tons, 55 tons and 80 tons, the hot roller temperature is 65 ℃, and the polar plate is heated to 45 ℃ before the first hot rolling compaction.
The compacted density of the pole piece is pole piece limit compacted density, the limit compacted density test method of the pole piece is shown below, and the limit compacted density of the pole piece in the embodiment is compacted density of 2.68g/cm3.
The statistics result in the section along the thickness direction of the pole piece shows that the area ratio of particles with the particle diameter of more than or equal to 1.5 mu m and less than 5 mu m in the section of the positive pole film layer is 14.49%, the area ratio of particles with the particle diameter of more than 5 mu m is 0, the area ratio of particles with the particle diameter of more than 1 mu m and less than 1.5 mu m is 19.65%, and the area ratio of particles with the particle diameter of more than or equal to 200nm and less than 1500nm is 75.16%. The iron dissolution rate of the positive electrode film layer was 1062ppm.
The median C50 of graphitization degree obtained in the laser micro-confocal raman spectrometer face scanning mode was 1.025. In the particle sphericity area accumulation distribution curve obtained by the positive electrode film layer along the section in the thickness direction of the pole piece, the sphericity median LA50 is 0.72. In the particle roughness area cumulative distribution curve obtained by the positive electrode film layer along the section in the thickness direction of the pole piece, the median RA50 of the roughness is 0.942.
(3) Preparing a negative electrode plate:
95.5 wt% of negative electrode active material (artificial graphite), 1.0% by weight of conductive agent (conductive carbon black), 2.0% by weight of binder (styrene-butadiene rubber (SBR)) and 1.5% by weight of thickener (sodium carboxymethyl cellulose (CMC)) are mixed, and deionized water is added to stir and disperse to prepare the negative electrode slurry. And then coating the negative electrode slurry on the surfaces of both sides of the Cu foil, and drying, cold pressing, slitting and tabletting after both sides are finished to prepare the negative electrode plate. The coated single side had a density of 165mg/1540mm2 and a compacted density of 1.60g/cm3.
(4) Preparation of a separator film
A polypropylene film was used as a separator.
(5) Preparation of electrolyte
In an argon atmosphere glove box (H2O<0.1ppm,O2 <0.1 ppm), uniformly mixing an organic solvent of Ethylene Carbonate (EC)/dimethyl carbonate (DMC) according to a volume ratio of 1/1, adding lithium salt LiPF6 to dissolve in the organic solvent, wherein the content of LiPF6 in the solution is 1mol/L, and uniformly stirring to obtain the electrolyte.
(6) Preparation of the battery:
And stacking the positive electrode plate, the isolating film and the negative electrode plate in sequence, wherein the isolating film can play a role of isolating the cathode and the anode, a bare cell is obtained by winding, the bare cell is placed in an outer package, electrolyte is injected, and the lithium ion battery is finally obtained through the procedures of packaging, formation, exhaust and the like.
Example 2
This preparation example is substantially the same as preparation example 1 except that in the preparation step of the positive electrode active material, the maximum temperature of the second-stage sintering process is 750 ℃.
Example 3
This preparation example is substantially the same as preparation example 1 except that the highest temperature of the second-stage sintering process in the preparation step of the positive electrode active material is 790 ℃.
Example 4
This production example is substantially the same as production example 1 except that in the production step of the positive electrode active material, lithium dihydrogen phosphate and ferrous oxalate are mixed so that the atomic molar ratio of lithium to iron is 1.01.
Example 5
This production example is substantially the same as production example 1 except that in the production step of the positive electrode active material, lithium dihydrogen phosphate and ferrous oxalate are mixed so that the atomic molar ratio of lithium to iron is 1.05.
Example 6
This preparation example is substantially the same as preparation example 1 except that in the preparation step of the positive electrode active material, lithium dihydrogen phosphate and ferrous oxalate are mixed so that the molar ratio of lithium to iron is 1.05, and the maximum sintering temperature in the second-stage sintering process is 790 ℃.
Example 7
This production example was substantially the same as production example 1 except that the pulverization frequency of jet milling was 18Hz.
Example 8
This production example was substantially the same as production example 1 except that in the production step of the positive electrode active material, the pulverizing frequency of jet milling was 24Hz.
Comparative example 1
This preparation example is substantially the same as preparation example 1 except that in the positive electrode active material preparation step, lithium dihydrogen phosphate and ferrous oxalate are mixed so that the atomic molar ratio of lithium to iron is 1.05 and the maximum sintering temperature in the second stage sintering process is 810 ℃.
Comparative example 2
This preparation example is substantially the same as preparation example 1 except that in the positive electrode active material preparation step, lithium dihydrogen phosphate and ferrous oxalate are mixed so that the atomic molar ratio of lithium to iron is 1.01, and the maximum sintering temperature in the second stage sintering process is 740 ℃.
Comparative example 3
This preparation example is substantially the same as preparation example 1, except that, in the positive electrode active material preparation step, the preparation is performed by the following method,
Mixing lithium carbonate, ferric phosphate, titanium dioxide, glucose and polyethylene glycol in water uniformly, and grinding to obtain a mixed raw material. Wherein, the ratio of the lithium carbonate to the ferric phosphate is such that the atomic mole ratio of the lithium to the ferric is 1.03:1.0.
Spray drying the mixed slurry to obtain dried precursor powder, wherein the dried precursor powder material has light yellow appearance and uniform color.
The precursor powder is placed in a sintering furnace, heated from 25 ℃ to 450 ℃ at 2 ℃ per min under nitrogen atmosphere and kept at the temperature for 3 hours, then heated to a second temperature of 780 ℃ at 5 ℃ per min and kept at the temperature for 10 hours, and cooled down after the end.
The obtained material is crushed by adopting an air flow crushing method with the classification frequency of 22Hz and the crushing air pressure of 0.55MPa, so as to obtain the carbon-coated lithium iron phosphate positive electrode active material.
Performance testing
1. Limit compaction density of pole piece
And compacting the pole piece with the double-sided coating through a roller press, testing the extensibility of the pole piece after compacting, and simultaneously evaluating the flexibility of the pole piece after compacting. By increasing the pressure of the roller press, pole pieces with different compaction densities can be obtained, the compaction density of the pole pieces is increased along with the increase of the pressure, the extensibility of the pole pieces is increased, and the flexibility of the pole pieces is reduced. The pole piece has high extensibility and is easy to warp, and the pole piece has low flexibility and is easy to be brittle. Thus, the smaller of the corresponding compacted densities when the extensibility of the pole piece is 8% or when the number of flexible folds of the pole piece is 3 is defined as the ultimate compacted density of the pole piece.
The compacted density is calculated from the mass of the positive electrode film layer/the volume of the positive electrode film layer.
The elongation test method is as follows:
The method comprises the steps of spreading pole pieces on a horizontal tabletop, cutting the pole pieces in sections, removing copper foil on the edge of each pole piece, keeping the cutting edge of each pole piece parallel to the MD direction (the direction perpendicular to a press roller) of the pole piece, ensuring that the pole piece is completely covered by a coating, measuring the length between marking points at the same position of the head and tail of the pole piece and the width of the length direction by using a steel rule, estimating and reading to 0.1mm, recording the length before compaction, and recording the length after compaction between the corresponding marking points after compaction, wherein the length after compaction (length after compaction-length before compaction)/the length before compaction is taken as the extensibility of the pole piece.
The method for testing the number of flexible folds is as follows.
Cutting the positive pole piece into a test sample with the size of 20X 100mm2, folding the positive pole piece forward, flattening the positive pole piece by using a 2kg press roller, unfolding the positive pole piece to check whether the gap has light transmission or not, folding the positive pole piece backward by using a 2kg press roller if the gap does not have light transmission, flattening the positive pole piece by using the 2kg press roller, checking the positive pole piece again by using the light, repeating the steps until the gap has light transmission, recording folding times, repeating the test for three times, and taking an average value as reference data of the flexibility of the pole piece.
2. Energy density testing
The lithium ion secondary battery was left standing at 25 ℃ for 2 hours, ensuring that the temperature of the lithium ion secondary battery was 25 ℃. After the lithium ion secondary battery was charged to a charge cutoff voltage of 3.65V at 25 ℃ at 0.33 ℃, constant voltage charging was continued at the charge cutoff voltage until the current was 0.05C, and the charge was cut off (where C represents the rated capacity of the lithium ion secondary battery). After the lithium ion secondary battery was left standing at 25 ℃ for 1h, the lithium ion secondary battery was discharged at 25 ℃ to a discharge cut-off voltage of 2.5V at 0.33 ℃, and the total discharge energy of the lithium ion secondary battery was recorded as E0.
And measuring the length, width and height of the lithium ion secondary battery, and calculating the volume value V0 = length, width and height of the lithium ion secondary battery.
Mass energy density of lithium ion secondary battery = lithium ion secondary battery discharge energy E0/lithium ion secondary battery volume V0.
3. DC impedance (DCR) testing method
At 25 ℃, charging to 3.65V at constant current of 0.33 ℃, charging to current of 0.05 ℃ at constant voltage, discharging to 20% soc at 0.33 ℃, discharging to 30s at 3C pulse after 5min of standing, charging to 40s at 3C after 40s of standing, charging to 10% soc at 0.33 ℃ after 5min of standing, discharging to 30s at 3C pulse after 5min of standing, charging to 40s at 3C after 40s of standing, charging to 50% soc after 5min of standing, discharging to 30s at 0.33C pulse after 2h of standing at-25 ℃, discharging to 2h of standing at 25 ℃, discharging to 20% soc at 0.33C after 0.33C constant current is charged to 0.05C after constant voltage after 3.65V of standing, discharging to 2h of standing at 1h of standing at-25 ℃, and discharging to 30s at 1h of standing at 1C pulse after 2h of standing at-25 ℃.
The voltage at this time was recorded before and after each pulse discharge, and DCR under different conditions was calculated, and the calculation formula was dcr= (voltage before pulse discharge after end of rest-voltage before rest after pulse discharge)/pulse current. Experimental parameters and test results
Batteries of each example and comparative example were prepared separately according to the above-described methods, and each performance parameter was measured, and the results are shown in table 1 below.
TABLE 1
Table 1, below
As can be seen from comparison of the data of the embodiment and the comparative example, in the tangent plane of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of the particles with the particle diameter of more than or equal to 1.5 μm is more than or equal to 8.0% and less than or equal to 20.0%, and in the sphere-like area cumulative distribution curve of the particles obtained by the tangent plane of the positive electrode film layer along the thickness direction of the pole piece, when the median LA50 of sphere-like is 0.70-0.74, the battery has higher positive electrode pole piece compaction density and energy density, and has good dynamic performance.
As can be seen from comparison of examples 3 and 5 with other examples, in the cumulative distribution curve of the sphericity-like area of the particles obtained by the section of the positive electrode film layer along the thickness direction of the pole piece, the concentration (LA90-LA10）/LA50 is 0.450-0.535), which is favorable for forming close packing, increasing the contact between the particles and further improving the dynamic performance of the battery.
As can be seen from a comparison between the embodiment 2 and other embodiments, in the section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of the particles with the particle diameter of 1.5 μm or more and less than 5 μm is 9.0% -20.0%, and the ratio is 10.0% -20.0%, which is beneficial to achieving both the dynamic performance and the energy density of the battery.
As can be seen from comparison of examples 3 and 6 with other examples, in the section of the positive electrode film layer along the thickness direction of the pole piece, the area ratio of the particles with the particle diameter of 1 μm or more and less than 1.5 μm is 15.0% -25.0%, and the further ratio is 16.0% -24.0%, which is beneficial to reducing the direct current internal resistance of the battery and improving the dynamic performance of the battery.
As can be seen from comparison of the embodiment 2 with other embodiments, in the section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with the particle diameter of more than or equal to 200nm and less than 1500nm is 73.0% -80.0%, and the further ratio is 73.0% -78.0%, so that the high compaction density of the positive electrode sheet can be realized, and the energy density of the battery can be further improved.
The present application is not limited to the above embodiment. The above embodiments are merely examples, and embodiments having substantially the same configuration and the same effects as those of the technical idea within the scope of the present application are included in the technical scope of the present application. Further, various modifications that can be made to the embodiments and other modes of combining some of the constituent elements in the embodiments, which are conceivable to those skilled in the art, are also included in the scope of the present application within the scope not departing from the gist of the present application.