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A lithium polymer battery, or more correctly, lithium-ion polymer battery (abbreviated as LiPo, LIP, Li-poly, lithium-poly, and others), is a rechargeable battery derived from lithium-ion and lithium-metal battery technology. The primary difference is that instead of using a liquid lithium salt (such as lithium hexafluorophosphate, LiPF6) held in an organic solvent (such as EC/DMC/DEC) as the electrolyte, the battery uses a solid (or semi-solid) polymer electrolyte such as polyethylene glycol (PEG), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA) or poly(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.
Polymer electrolytes can be divided into two large categories: dry solid polymer electrolytes (SPE) and gel polymer electrolytes (GPE).
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 higher specific energy than other lithium battery types.
They are used in applications where weight is critical, such as , tablet computer, , radio-controlled aircraft, and some .Bruno Scrosati, K. M. Abraham, Walter A. van Schalkwijk, Jusef Hassoun (ed), Lithium Batteries: Advanced Technologies and Applications, John Wiley & Sons, 2013 ,page 44
Nishi mentions that Sony 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. (2025). 9780387344447, Springer. ISBN 9780387344447 At that time, polymer batteries were promising, and it seemed polymer electrolytes would become indispensable. Eventually, this type of cell went into the market in 1998. 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. Although these polymer electrolytes may be dry to the touch, they can still include 30% to 50% liquid solvent.
Since 1990, several organisations, such as Mead and Valence in the United States and GS Yuasa in Japan, have developed batteries using gelled SPEs.
In 1996, Bellcore in the United States announced a rechargeable lithium polymer cell using porous SPE, which was called a "plastic" lithium-ion cell (PLiON) and subsequently commercialised in 1999.
Solid polymer electrolyte was initially defined as a polymer matrix swollen with lithium salts, now called dry solid polymer electrolyte. 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. 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.
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.
LiPo , 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).
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. 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.
LiPo packs also see widespread use in airsoft, where their higher discharge currents and better energy density than traditional NiMH batteries have very noticeable performance gain (higher rate of fire).
LiPo batteries are pervasive in , , subnotebook, 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's internal 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 the other 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 its battery-electric and hybrid vehicles and Kia Motors in its battery-electric Kia Soul. The Bolloré Bluecar, which is used in car-sharing schemes in several cities, also uses this type of battery.
LiPo batteries are becoming increasingly commonplace in Uninterruptible power supply (UPS) systems. They offer numerous benefits over the traditional VRLA 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. 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.
Unlike lithium-ion cylindrical and prismatic cells, with a rigid metal case, LiPo cells have a flexible, foil-type (polymer laminate) 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 and delamination and deformation is prevented, which is associated with increase of cell impedance and degradation.
Cells with solid polymer electrolytes have not been fully commercialised and are still a topic of research. Prototype cells of this type could be considered to be between a traditional lithium-ion battery (with liquid electrolyte) and a completely plastic, solid-state lithium-ion battery. The simplest approach is to use a polymer matrix, such as polyvinylidene fluoride (PVdF) or poly(acrylonitrile) (PAN), gelled with conventional salts and solvents, such as LiPF6 in EC/DMC/DEC.
Other attempts to design a polymer electrolyte cell include the use of inorganic such as 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) as a plasticizer in a microporous polymer matrix like poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methyl methacrylate) (PVDF-HFP/PMMA).
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