 | |
| Specific energy | 100–265W·h/kg(0.36–0.95 MJ/kg)[1] |
|---|---|
| Energy density | 250–670W·h/L(0.90–2.63 MJ/L)[1] |
Alithium polymer battery, or more correctly,lithium-ion polymer battery (abbreviated asLiPo,LIP,Li-poly,lithium-poly, and others), is arechargeable battery derived fromlithium-ion andlithium-metal battery technology. The primary difference is that instead of using a liquidlithium salt (such aslithium hexafluorophosphate, LiPF6) held in anorganic solvent (such asEC/DMC/DEC) as theelectrolyte, the battery uses a solid (or semi-solid)polymer electrolyte such aspolyethylene glycol (PEG),polyacrylonitrile (PAN),poly(methyl methacrylate) (PMMA) orpoly(vinylidene fluoride) (PVdF). Other terms used in the literature for this system include hybrid polymer electrolyte (HPE), where "hybrid" denotes the combination of the polymer matrix, the liquid solvent, and the salt.[2]
Polymer electrolytes can be divided into two large categories: dry solid polymer electrolytes (SPE) and gel polymer electrolytes (GPE).[3]
In comparison to liquid electrolytes and solid organic electrolytes, polymer electrolytes offer advantages such as increased resistance to variations in the volume of the electrodes throughout the charge and discharge processes, improved safety features, excellent flexibility, and processability. These batteries provide higherspecific energy than other lithium battery types.
They are used in applications whereweight is critical, such aslaptop computers,tablets,smartphones,radio-controlled aircraft, and someelectric vehicles.[4]
The dry SPE was the first used in prototype batteries, around 1978 byMichel Armand,[5][6] and 1985 by ANVAR and Elf Aquitaine of France, andHydro-Québec of Canada.[7]
Nishi mentions thatSony started research on lithium-ion cells with gelled polymer electrolytes (GPE) in 1988, before the commercialisation of the liquid-electrolyte lithium-ion cell in 1991.[8] At that time, polymer batteries were promising, and it seemed polymer electrolytes would become indispensable.[9] Eventually, this type of cell went into the market in 1998.[8] However, Scrosati argues that, in the strictest sense, gelled membranes cannot be classified as "true" polymer electrolytes but rather as hybrid systems where the liquid phases are contained within the polymer matrix.[10] Although these polymer electrolytes may be dry to the touch, they can still include 30% to 50% liquid solvent.[11]
Since 1990, several organisations, such as Mead and Valence in the United States andGS Yuasa in Japan, have developed batteries using gelled SPEs.[7]
In 1996,Bellcore in the United States announced a rechargeable lithium polymer cell using porous SPE,[7][12] which was called a "plastic" lithium-ion cell (PLiON) and subsequently commercialised in 1999.[2]
Like other lithium-ion cells, LiPos operate based on the intercalation and de-intercalation of lithium ions between a positive and a negative electrode. However, instead of a liquid electrolyte, LiPos typically use a gelled or solid polymer-based electrolyte as the conductive medium. A microporous polymer separator is used to prevent direct contact between the electrodes, while still allowing lithium-ion transport.[13]
A typical cell has four main components: a positiveelectrode, a negative electrode, a separator, and anelectrolyte. The separator itself may be apolymer, such as a microporous film ofpolyethylene (PE) orpolypropylene (PP); thus, even when the cell has a liquid electrolyte, it will still contain a "polymer" component. In addition to this, the positive electrode can be further divided into three parts: the lithium-transition-metal-oxide (such as LiCoO2 or LiMn2O4), a conductive additive, and a polymer binder ofpoly(vinylidene fluoride) (PVdF).[14][15] The negative electrode material may have the same three parts, only withcarbon replacing the lithium-metal-oxide.[14][15] The main difference between lithium-ion polymer cells and lithium-ion cells is the physical phase of the electrolyte, such that LiPo cells use dry solid, gel-like electrolytes, whereas Li-ion cells use liquid electrolytes.
Polymer electrolytes can be divided into two large categories: dry solid polymer electrolytes (SPE) and gel polymer electrolytes (GPE).[3]
Solid polymer electrolyte was initially defined as a polymer matrix swollen with lithium salts, now called dry solid polymer electrolyte.[3] Lithium salts are dissolved in the polymer matrix to provide ionic conductivity. Due to its physical phase, there is poor ion transfer, resulting in poor conductivity at room temperature. To improve the ionic conductivity at room temperature, gelled electrolyte is added resulting in the formation of GPEs. GPEs are formed by incorporating an organic liquid electrolyte in the polymer matrix. Liquid electrolyte is entrapped by a small amount of polymer network, hence the properties of GPE is characterized by properties between those of liquid and solid electrolytes.[17] The conduction mechanism is similar for liquid electrolytes and polymer gels, but GPEs have higher thermal stability and a low volatile nature which also further contribute to safety.[18]
The voltage of a single LiPo cell depends on its chemistry and varies from about 4.2 V (fully charged) to about 2.7–3.0 V (fully discharged). The nominal voltage is 3.6 or 3.7 volts (about the middle value of the highest and lowest value) for cells based on lithium-metal-oxides (such as LiCoO2). This compares to 3.6–3.8 V (charged) to 1.8–2.0 V (discharged) for those based on lithium-iron-phosphate (LiFePO4).
The exact voltage ratings should be specified in product data sheets, with the understanding that the cells should be protected by an electronic circuit that won't allow them to overcharge or over-discharge under use.
LiPobattery packs, with cells connected in series and parallel, have separate pin-outs for every cell. A specialized charger may monitor the charge per cell so that all cells are brought to the same state of charge (SOC).
.png)
.jpg)
LiPo cells provide manufacturers with compelling advantages. They can easily produce batteries of almost any desired shape. For example, the space and weight requirements ofmobile devices andnotebook computers can be met. They also have a low self-discharge rate of about 5% per month.[19]
LiPo batteries are now almost ubiquitous when used to power commercial and hobby drones (unmanned aerial vehicles),radio-controlled aircraft,radio-controlled cars, and large-scale model trains, where the advantages of lower weight and increased capacity and power delivery justify the price. Test reports warn of the risk of fire when the batteries are not used per the instructions.[20] The voltage for long-time storage of LiPo battery used in the R/C model should be 3.6~3.9 V range per cell, otherwise it may cause damage to the battery.[21]
LiPo packs also see widespread use inairsoft, where their higher discharge currents and better energy density than traditionalNiMH batteries have very noticeable performance gain (higher rate of fire).[original research?]
LiPo batteries are pervasive inmobile devices,power banks,very thin laptop computers,portable media players, wireless controllers for video game consoles, wireless PC peripherals,electronic cigarettes, and other applications where small form factors are sought. The high energy density outweighs cost considerations.
The battery used to start a vehicle'sinternal combustion engine is typically 12 V or 24 V, so a portable jump starter or battery booster uses three or six LiPo batteries in series (3S1P/6S1P) to start the vehicle in an emergency instead of theother jump-start methods. The price of a lead-acid jump starter is less but they are bigger and heavier than comparable lithium batteries. So such products have mostly switched to LiPo batteries or sometimes lithium iron phosphate batteries.
Hyundai Motor Company uses LiPo batteries in some of itsbattery-electric andhybrid vehicles[22] andKia Motors in itsbattery-electric Kia Soul.[23] TheBolloré Bluecar, which is used in car-sharing schemes in several cities, also uses this type of battery.
LiPo batteries are becoming increasingly commonplace inUninterruptible power supply (UPS) systems. They offer numerous benefits over the traditionalVRLA battery, and with stability and safety improvements confidence in the technology is growing. Their power-to-size and weight ratio is seen as a major benefit in many industries requiring critical power backup, including data centers where space is often at a premium.[24] The longer cycle life, usable energy (Depth of discharge), and thermal runaway are also seen as a benefit of using Li-po batteries over VRLA batteries.
All Li-ion cells expand at high levels ofstate of charge (SOC) or overcharge due to slight vaporisation of the electrolyte. This may result indelamination and, thus, bad contact with the internal layers of the cell, which in turn diminishes the reliability and overall cycle life.[25] This is very noticeable for LiPos, which can visibly inflate due to the lack of a hard case to contain their expansion. Lithium polymer batteries' safety characteristics differ from those oflithium iron phosphate batteries.
Unlike lithium-ion cylindrical and prismatic cells, with a rigid metal case, LiPo cells have a flexible, foil-type (polymerlaminate) case, so they are relatively unconstrained. Moderate pressure on the stack of layers that compose the cell results in increased capacity retention, because the contact between the components is maximised anddelamination and deformation is prevented, which is associated with increase of cell impedance and degradation.[25][26]
A solid polymer electrolyte (SPE) is a solvent-free salt solution in a polymer medium. It may be, for example, a compound of lithium bis(fluorosulfonyl)imide (LiFSI) and high molecular weightpoly(ethylene oxide) (PEO),[27] a high molecular weightpoly(trimethylene carbonate) (PTMC),[28] polypropylene oxide (PPO), poly[bis(methoxy-ethoxy-ethoxy)phosphazene] (MEEP),etc. PEO exhibits the most promising performance as a solid solvent for lithium salts, mainly due to its flexible ethylene oxide segments and other oxygen atoms that comprise a strong donor character, readily solvating Li+ cations. PEO is also commercially available at a very reasonable cost.[3] The performance of these proposed electrolytes is usually measured in ahalf-cell configuration against an electrode of metalliclithium, making the system a "lithium-metal" cell. Still, it has also been tested with a common lithium-ion cathode material such aslithium-iron-phosphate (LiFePO4).
Cells with solid polymer electrolytes have not been fully commercialised[29] and are still a topic of research.[30] Prototype cells of this type could be considered to be between a traditionallithium-ion battery (with liquid electrolyte) and a completely plastic,solid-state lithium-ion battery.[10] The simplest approach is to use a polymer matrix, such aspolyvinylidene fluoride (PVdF) orpoly(acrylonitrile) (PAN), gelled with conventional salts and solvents, such asLiPF6 inEC/DMC/DEC.
Other attempts to design a polymer electrolyte cell include the use ofinorganicionic liquids such as 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) as a plasticizer in a microporous polymer matrix like poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methyl methacrylate) (PVDF-HFP/PMMA).[31]
{{cite book}}: CS1 maint: location missing publisher (link)Gel polymer electrolytes (GPEs) are composed of a polymer matrix that immobilizes liquid electrolyte components to form a semi-solid structure. GPEs combine the safety and mechanical advantages of solid electrolytes with the high ionic conductivity of liquid electrolytes.
I've not yet heard of a LiPo that burst into flames during storage. All of the fire incidents that I'm aware of occurred during charge or discharge of the battery. Of those cases, the majority of problems happened during charge. Of those cases, the fault usually rested with either the charger or the person who was operating the charger... but not always.
