CN111370771A - Preparation method of lithium secondary battery - Google Patents

Abstract

The invention provides a preparation method of a lithium secondary battery, which comprises a positive electrode and a negative electrode, wherein the active material of the positive electrode is LiNi_0.15Co_0.35Mn_0.5O₂The preparation method comprises the steps of placing the positive electrode and the negative electrode into a first electrolyte for pre-formation, wherein the first electrolyte contains saturated lithium carbonate and saturated lithium sulfite as additives; and obtaining the battery. According to the method, the SEI film is formed on the surface of the electrode in the pre-formation process, and the electrolyte used in the pre-formation is not the electrolyte of the finished battery, so that the influence of the additive on the later performance of the battery is not considered, the selection source of the additive is wider, and the SEI film is not formed from the electrolyte of the batteryThe lithium ions are obtained, so that irreversible capacity reduction is avoided, the obtained battery has better cycle performance, and the production cost is lower.

Description

Preparation method of lithium secondary battery

Technical Field

The invention relates to a preparation method of a lithium secondary battery, in particular to a formation method of the lithium secondary battery.

Background

Lithium ion batteries are widely used in portable electronic devices due to their high energy density, and are expected to be applied to hybrid vehicles and non-intermittent energy supply fields in the future. The additive and the formation process of the lithium ion battery have important influence on the performance of the lithium ion battery. In the prior art, in order to improve the cycle performance of the lithium ion battery, a film forming additive is mostly needed to be added, but the addition of the film forming additive firstly influences the energy density of the battery, and also influences the rate performance of the battery and the loss of charge and discharge efficiency caused by the loss of lithium ions in an electrolyte in a film forming process.

Disclosure of Invention

The invention provides a preparation method of a lithium secondary battery, which comprises a positive electrode and a negative electrode, wherein the active material of the positive electrode is LiNi_0.15Co_0.35Mn_0.5O₂The preparation method comprises the steps of placing the positive electrode and the negative electrode into a first electrolyte for pre-formation, wherein the first electrolyte contains saturated lithium carbonate and saturated lithium sulfite as additives; and after the pre-formation, taking out the anode and the cathode, assembling the anode and the cathode into a battery, injecting a second electrolyte, and performing secondary formation to obtain the battery. According to the method, the SEI film is formed on the surface of the electrode in the pre-formation process, and the electrolyte used in the pre-formation is not the electrolyte of the finished battery, so that the influence of the additive on the later performance of the battery is not considered, the selection source of the additive is wider, lithium ions are not obtained from the battery electrolyte when the SEI film is formed, the irreversible capacity is not reduced, the obtained battery has better cycle performance, and the production cost is lower.

The specific scheme is as follows:

a method of manufacturing a lithium secondary battery including a positive electrode and a negative electrode, the method comprising placing the positive electrode and the negative electrode in a first electrolyte containing saturated lithium carbonate and saturated lithium sulfite as additives for preliminary formation; and after the pre-formation, taking out the anode and the cathode, assembling the anode and the cathode into a battery, injecting a second electrolyte, sealing the opening, and performing secondary formation to obtain the battery.

Further, the method specifically comprises the following steps:

1) placing the positive electrode and the negative electrode in a first electrolyte, wherein the solvent in the first electrolyte is EC and DMC, the electrolyte lithium salt is lithium hexafluorophosphate, and the additive is saturated lithium carbonate and saturated lithium sulfite;

2) charging to a first voltage by adopting a current of 0.01-0.02C in a constant current manner; the first voltage is 2.90-2.95V;

3) charging to a second voltage by adopting a current of 0.02-0.05C in a constant current manner; the second voltage is 3.45-3.50V;

4) charging to a third voltage by adopting a current of 0.05-0.1C in a constant current manner; the third voltage is 3.75-3.80V; then charging at constant voltage under the voltage until the charging current is lower than the cut-off current;

5) taking out the positive electrode and the negative electrode, assembling the positive electrode and the negative electrode with a diaphragm to form a battery core, and putting the battery core into a battery shell;

6) injecting a second electrolyte, and sealing, wherein in the second electrolyte, electrolyte lithium salt is lithium hexafluorophosphate, additives are vinyl ethyl phosphate and trimethyl phosphate, and the content ratio of the vinyl ethyl phosphate to the trimethyl phosphate is 1.9-2.1: 1;

7) charging to a third voltage with a constant current of 0.02-0.05C;

8) then pulse charging is carried out until the charging cut-off voltage is reached, and constant voltage charging is carried out under the voltage until the charging current is lower than the cut-off current;

9) and carrying out constant-current charge-discharge circulation for a plurality of times between the charge cut-off voltage and the discharge cut-off voltage to obtain the battery.

Further, wherein the active material of the positive electrode is LiNi_0.15Co_0.35Mn_0.5O₂And the active material of the negative electrode is silicon particles coated with carbon.

Further, the charge cut-off voltage is 4.2-4.3V; the discharge cut-off voltage is 2.7-2.8V.

Further, solid lithium carbonate and lithium sulfite are stored at the bottom of the first electrolyte.

Furthermore, the solvent in the second electrolyte is ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 1:1: 1.

Further, the content of the vinyl ethyl phosphate in the second electrolyte is 1.9-4.2% by volume; the content of trimethyl phosphate is 1-2 vol%.

Further, the lithium ion battery is prepared by the method.

The invention has the following beneficial effects:

1) the SEI film is formed on the surface of the electrode in the pre-formation process, and the electrolyte used in the pre-formation is not the electrolyte of the finished battery, so that the influence of the additive on the later performance of the battery is not considered, the selection source of the additive is wider, lithium ions are not obtained from the battery electrolyte when the SEI film is formed, the irreversible capacity is not reduced, the obtained battery has better cycle performance, and the production cost is lower;

2) and because the influence of the additive on the later performance of the battery does not need to be considered, the saturated inorganic additive of lithium carbonate and lithium sulfite is selected, a stable SEI film is formed more effectively, solid lithium carbonate and lithium sulfite are stored at the bottom of the first electrolyte, and along with the consumption of lithium ions, the solid lithium carbonate and lithium sulfite are continuously dissolved, so that the saturated states of the lithium carbonate and the lithium sulfite in the first electrolyte are maintained, and the stability of the SEI is improved.

3) And the concentration of lithium ions in the first electrolysis does not change as the preliminary formation proceeds, and the first electrolyte can be recycled.

4) Designing a pre-formation charging process aiming at the lithium carbonate and the lithium sulfite selected by the invention, adopting a stepped constant-current formation in a mode of gradually increasing current, and performing constant-voltage formation under a specific third voltage, thereby completing the pre-formation process;

5) the second electrolyte contains vinyl ethyl phosphate and trimethyl phosphate, and although the flame retardance of the trimethyl phosphate is good, the influence on the cycle performance of the battery is large, the vinyl ethyl phosphate can generate a synergistic effect with the trimethyl phosphate after being added, and experimental data show that the cycle performance of the battery can be greatly improved by adding the vinyl ethyl phosphate at a specific content.

Detailed Description

The present invention will be described in more detail below with reference to specific examples, but the scope of the present invention is not limited to these examples.

The active material of the positive electrode is LiNi_0.15Co_0.35Mn_0.5O₂The active material of the negative electrode is carbon-coated silicon particles.

Example 1

1) Placing the positive electrode and the negative electrode in a first electrolyte, wherein the solvent in the first electrolyte is EC and DMC, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, and the additive is saturated lithium carbonate and saturated lithium sulfite, wherein solid lithium carbonate and solid lithium sulfite are stored at the bottom of the first electrolyte;

2) charging to 2.90V by adopting a current of 0.01C and a constant current;

3) charging to 3.45V by adopting a current of 0.02C and constant current;

4) charging to 3.75V by adopting a current of 0.05C and constant current; then charging at constant voltage under the voltage until the charging current is lower than 0.01C;

5) taking out the positive electrode and the negative electrode, assembling the positive electrode and the negative electrode with a diaphragm to form a battery core, and putting the battery core into a battery shell;

6) injecting a second electrolyte, and sealing, wherein the solvent in the second electrolyte is ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 1:1:1, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, and the additive is vinyl ethyl phosphate and trimethyl phosphate, wherein the content of the vinyl ethyl phosphate in the second electrolyte is 1.9% by volume; the content of trimethyl phosphate was 1% by volume;

7) charging to 3.75V with a constant current of 0.02C;

8) then pulse charging is carried out to 4.2V at the pulse current of 0.02C and the pulse time of 30s at the interval of 1s, and constant-voltage charging is carried out under the voltage until the charging current is lower than 0.01C;

9) and performing constant current charge and discharge at 0.1 ℃ between 4.2V and 2.7V for 3 times to obtain the battery.

Example 2

1) Placing the positive electrode and the negative electrode in a first electrolyte, wherein the solvent in the first electrolyte is EC and DMC, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, and the additive is saturated lithium carbonate and saturated lithium sulfite, wherein solid lithium carbonate and solid lithium sulfite are stored at the bottom of the first electrolyte;

2) charging to 2.95V by adopting a current of 0.02C and constant current;

3) charging to 3.50V by adopting a current of 0.05C and constant current;

4) charging to 3.80V by adopting a current of 0.1C and a constant current; then charging at constant voltage under the voltage until the charging current is lower than 0.01C;

5) taking out the positive electrode and the negative electrode, assembling the positive electrode and the negative electrode with a diaphragm to form a battery core, and putting the battery core into a battery shell;

6) injecting a second electrolyte, and sealing, wherein the solvent in the second electrolyte is ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 1:1:1, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, the additive is vinyl ethyl phosphate and trimethyl phosphate, and the content of the vinyl ethyl phosphate in the second electrolyte is 4.2% by volume; the content of trimethyl phosphate was 2% by volume;

7) charging to 3.80V with a constant current of 0.05C;

8) then pulse charging is carried out to 4.3V at the pulse current of 0.05C and the pulse time of 60s and the interval of 3s, and constant voltage charging is carried out under the voltage until the charging current is lower than 0.01C;

9) and performing constant current charge and discharge at 0.1 ℃ between 4.3V and 2.8V for 3 times to obtain the battery.

Example 3

1) Placing the positive electrode and the negative electrode in a first electrolyte, wherein the solvent in the first electrolyte is EC and DMC, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, and the additive is saturated lithium carbonate and saturated lithium sulfite, wherein solid lithium carbonate and solid lithium sulfite are stored at the bottom of the first electrolyte;

2) charging to 2.95V by adopting a current of 0.02C and constant current;

3) charging to 3.45V by adopting a current of 0.05C and constant current;

4) charging to 3.80V by adopting a current of 0.1C and a constant current; then charging at constant voltage under the voltage until the charging current is lower than 0.01C;

5) taking out the positive electrode and the negative electrode, assembling the positive electrode and the negative electrode with a diaphragm to form a battery core, and putting the battery core into a battery shell;

6) injecting a second electrolyte, and sealing, wherein the solvent in the second electrolyte is ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 1:1:1, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, the additive is vinyl ethyl phosphate and trimethyl phosphate, and the content of the vinyl ethyl phosphate in the second electrolyte is 3% by volume; the content of trimethyl phosphate was 1.5% by volume;

7) charging to 3.80V with a constant current of 0.05C;

8) then pulse charging is carried out to 4.2V at the pulse current of 0.05C and the pulse time of 40s at the interval of 2s, and constant-voltage charging is carried out under the voltage until the charging current is lower than 0.01C;

9) and performing constant current charge and discharge at 0.1 ℃ between 4.2V and 2.7V for 3 times to obtain the battery.

Comparative example 1

1) Placing the positive electrode and the negative electrode in a first electrolyte, wherein the solvent in the first electrolyte is EC and DMC, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, and the additive is saturated lithium carbonate and saturated lithium sulfite, wherein solid lithium carbonate and solid lithium sulfite are stored at the bottom of the first electrolyte;

2) charging to 2.95V by adopting a current of 0.02C and constant current;

3) charging to 3.45V by adopting a current of 0.05C and constant current;

4) charging to 4.2V by adopting a current of 0.1C and constant current; then charging at constant voltage under the voltage until the charging current is lower than 0.01C;

5) taking out the positive electrode and the negative electrode, assembling the positive electrode and the negative electrode with a diaphragm to form a battery core, and putting the battery core into a battery shell;

6) injecting a second electrolyte, and sealing, wherein the solvent in the second electrolyte is ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 1:1:1, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, the additive is vinyl ethyl phosphate and trimethyl phosphate, and the content of the vinyl ethyl phosphate in the second electrolyte is 3% by volume; the content of trimethyl phosphate was 1.5% by volume;

7) charging to 4.2V with a current of 0.05C;

8) and performing constant current charge and discharge at 0.1 ℃ between 4.2V and 2.7V for 3 times to obtain the battery.

Comparative example 2

The second electrolyte contained no vinyl ethyl phosphate, and the other processes were the same as in example 3.

Comparative example 3

The content of vinyl ethyl phosphate in the second electrolyte is 2% by volume; the content of trimethyl phosphate was 1.5% by volume; the other processes were the same as in example 3.

Comparative example 4

The content of vinyl ethyl phosphate in the second electrolyte was 4% by volume; the content of trimethyl phosphate was 1.5% by volume; the other processes were the same as in example 3.

Comparative example 5

1) Assembling the positive electrode, the negative electrode and a diaphragm into a battery core, and putting the battery core into a battery shell;

2) injecting a second electrolyte, and sealing, wherein the solvent in the second electrolyte is ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 1:1:1, the electrolyte lithium salt is 1mol/L lithium hexafluorophosphate, the additive is vinyl ethyl phosphate and trimethyl phosphate, and the content of the vinyl ethyl phosphate in the second electrolyte is 3% by volume; the content of trimethyl phosphate was 1.5% by volume;

7) charging to 3.8V with a current of 0.02C;

8) then pulse charging is carried out to 4.2V at the pulse current of 0.02C and the pulse time of 40s at the interval of 2s, and constant-voltage charging is carried out under the voltage until the charging current is lower than 0.01C;

9) and performing constant current charge and discharge at 0.1 ℃ between 4.2V and 2.7V for 3 times to obtain the battery.

Test and results

The batteries of examples 1 to 3 and comparative examples 1 to 5 were measured for the first irreversible capacity rate, and the capacity retention rate of the battery was recorded by cycling 200 times at a rate of 0.5C. The results are shown in Table 1.

TABLE 1

While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention.

Claims (8)

1. A method of manufacturing a lithium secondary battery including a positive electrode and a negative electrode, the method comprising placing the positive electrode and the negative electrode in a first electrolyte containing saturated lithium carbonate and saturated lithium sulfite as additives for preliminary formation; and after the pre-formation, taking out the anode and the cathode, assembling the anode and the cathode into a battery, injecting a second electrolyte, sealing the opening, and performing secondary formation to obtain the battery.

2. The method according to the preceding claim, which comprises:

1) placing the positive electrode and the negative electrode in a first electrolyte, wherein the solvent in the first electrolyte is EC and DMC, the electrolyte lithium salt is lithium hexafluorophosphate, and the additive is saturated lithium carbonate and saturated lithium sulfite;

2) charging to a first voltage by adopting a current of 0.01-0.02C in a constant current manner; the first voltage is 2.90-2.95V;

3) charging to a second voltage by adopting a current of 0.02-0.05C in a constant current manner; the second voltage is 3.45-3.50V;

4) charging to a third voltage by adopting a current of 0.05-0.1C in a constant current manner; the third voltage is 3.75-3.80V; then charging at constant voltage under the voltage until the charging current is lower than the cut-off current;

5) taking out the positive electrode and the negative electrode, assembling the positive electrode and the negative electrode with a diaphragm to form a battery core, and putting the battery core into a battery shell;

6) injecting a second electrolyte, and sealing, wherein in the second electrolyte, electrolyte lithium salt is lithium hexafluorophosphate, additives are vinyl ethyl phosphate and trimethyl phosphate, and the content ratio of the vinyl ethyl phosphate to the trimethyl phosphate is 1.9-2.1: 1;

7) charging to a third voltage with a constant current of 0.02-0.05C;

8) then pulse charging is carried out until the charging cut-off voltage is reached, and constant voltage charging is carried out under the voltage until the charging current is lower than the cut-off current;

9) and carrying out constant-current charge-discharge circulation for a plurality of times between the charge cut-off voltage and the discharge cut-off voltage to obtain the battery.

3. The production method according to the above claim, wherein the active material of the positive electrode is LiNi_0.15Co_0.35Mn_0.5O₂And the active material of the negative electrode is silicon particles coated with carbon.

4. The production method according to the above claim, wherein the charge cut-off voltage is 4.2 to 4.3V; the discharge cut-off voltage is 2.7-2.8V.

5. The production method according to the preceding claim, wherein solid lithium carbonate and lithium sulfite are present in the bottom of the first electrolyte solution.

6. The method of claim, wherein the solvent in the second electrolyte is ethylene carbonate, diethyl carbonate and propylene carbonate in a volume ratio of 1:1: 1.

7. The method according to the preceding claim, wherein the second electrolyte solution contains 1.9-4.2 vol.% of vinyl ethyl phosphate; the content of trimethyl phosphate is 1-2 vol%.

8. A lithium ion battery prepared by the method of any one of claims 1-7.