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Revision as of 14:35, 13 December 2020 "Li-Po" and "LiPo" redirect here. For other uses, see Li Po (disambiguation). Lithium polymer battery Lipolybattery.jpg A lithium-ion polymer battery used to power a smartphone Specific energy 100–265 W·h/kg(0.36–0.95 MJ/kg)[citation needed] Energy density 250–730 W·h/L(0.90–2.63 MJ/L) In day to day usage, the term "lithium polymer" or "lipo" is used to describe a Lithium-ion_battery (lithium ion) cell packaged in a flattened format without a hard casing. These are also known commonly as pouch cells. Chemically, such cells use essentially the same internal components and materials as found in prismatic and cylindrical lithium ion cells.

Historically, the term Lithium Polymer was used to describe an experimental type of lithium ion cells with a solid polymer electrolyte. As of 2020, "true" Lithium Polymer cells do not exist in commercial production. [1] [2].

Contents 1 History 2 "Lithium polymer" cells in contemporary terminology 2.1 Working principle and components 2.1.1 Applying pressure on LiPo cells 2.2 Safety 2.3 Applications 2.3.1 Radio controlled equipment and aircraft 2.3.2 Personal electronics 2.3.3 Electric vehicles 3 Lithium cells with solid polymer electrolyte 3.1 Dry solid polymer electrolyte 3.2 Gelled solid polymer electrolyte 4 See also 5 References 6 External links History Main article: Lithium-ion battery § History The development of LiPo cells follows the history of lithium-ion and lithium-metal cells which underwent extensive research during the 1980s, reaching a significant milestone with Sony's first commercial cylindrical Li-ion cell in 1991. After that, other packaging forms evolved, including the flat pouch format.

"Lithium polymer" cells in contemporary terminology In common usage, the term "Lithium Polymer" or "LiPo" refers simply to cells which are packaged in soft enclosures also known as pouch cells. The term does not indicate the internal physical or electrochemical properties of the cell in any distinct sense.

Cells marketed as "lithium polymer" have liquid electrolytes very similar to those found in prismatic or cylindrical cells. Furthermore, the range of anode chemistries found in lithium polymer pouch cells is limited to, and corresponds with, the range found in cylindrical and prismatic formats [3].

The internal composition of a "lithium polymer" cell or "pouch" cell may differ from that of other cell formats in containing additional binding or gelling agents in the electrolyte to resist the delamination and swelling which affects all lithium ion cells as they age [2]. For mechanical containment, other cell formats may rely to a greater degree on their hard casing which is absent in pouch cells. When assembled as batteries, stacks of "lithium polymer" pouch cells are often compressed between structural plates to provide mechanical containment and resist delamination.

The pouch cell format can offer weight savings by doing away with the rigid casing found in other formats. This becomes particularly significant in applications where the form factor dictates a relatively large packaging surface area, as seen in cell phone batteries where a thin, flat package is desirable.

Working principle and components See:

Main article: Lithium-ion battery § Electrochemistry

Applying pressure on LiPo cells

An experimental lithium-ion polymer battery made by Lockheed-Martin for NASA Unlike lithium-ion cylindrical and prismatic cells, which have a rigid metal case, LiPo cells have a flexible, foil-type (polymer laminate) case, so they are relatively unconstrained.

Being lightweight is an advantage when the application requires minimum weight, as in the case of radio controlled aircraft. However, it has been established that 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. [4][5]

Safety Main article: Lithium-ion battery § Safety

Apple iPhone 3GS's Lithium-ion battery, which has expanded due to a short circuit failure. LiPo cells are affected by the same problems as other lithium-ion cells. This means that overcharge, over-discharge, over-temperature, short circuit, crush and nail penetration may all result in a catastrophic failure, including the pouch rupturing, the electrolyte leaking, and fire.[6]

All Li-ion cells expand at high levels of state of charge (SOC) or over-charge, due to slight vaporisation of the electrolyte. This may result in delamination, and thus bad contact of the internal layers of the cell, which in turn brings diminished reliability and overall cycle life of the cell.[4] This is very noticeable for LiPos, which can visibly inflate due to lack of a hard case to contain their expansion.

Applications Main article: Lithium-ion battery § Uses

Hexagonal lithium polymer battery for underwater vehicles made by Custom Cells Itzehoe GmbH LiPo can easily produce batteries of almost any desired shape. For example, the space and weight requirements of mobile devices and notebook computers can be met.

LiPo battery packs, with cells connected in series and parallel, have separate pin-outs for every cell. A specialized charger may monitor the charge on a per-cell basis so that all cells are brought to the same state of charge (SOC).

Radio controlled equipment and aircraft

3-Cell LiPo battery for RC models LiPo batteries are now almost ubiquitous when used to power radio-controlled aircraft, radio-controlled cars and large scale model trains, where cells are often selected for high energy density rather than long cycle life or enhanced safety characteristics.

Personal electronics LiPo batteries are pervasive in mobile 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, light form factors are sought.

Electric vehicles Hyundai Motor Company (among others) uses this type of battery in some of their hybrid vehicles,[7] as well as Kia Motors in their battery electric Kia Soul.[8] The Bolloré Bluecar, which is used in car sharing schemes in several cities, also uses this type of battery.

Lithium cells with solid polymer electrolyte As of December 2020, true lithium polymer cells with a solid polymer electrolyte are unavailable commercially, being an immature technology and the subject of active research and development stemming from the broader development of lithium-ion and lithium-metal batteries. [9].[10] 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.[11]

The primary difference is that instead of using a liquid lithium-salt electrolyte (such as LiPF6) held in an organic solvent (such as EC/DMC/DEC), the battery uses a solid polymer electrolyte (SPE) such as poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA) or poly(vinylidene fluoride) (PVdF).

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 weight poly(ethylene oxide) (PEO),[12] or a high molecular weight poly(trimethylene carbonate) (PTMC).[13]

The performance of these proposed electrolytes is usually measured in a half-cell configuration against an electrode of metallic lithium, making the system a "lithium-metal" cell, but it has also been tested with a common lithium-ion cathode material such as lithium-iron-phosphate (LiFePO4).

Solid electrolytes can typically be classified as one of three types: dry SPE, gelled SPE and porous SPE.

Dry solid polymer electrolyte Dry SPE was the first used in prototype batteries, around 1978 by Michel Armand,[14][15] and 1985 by ANVAR and Elf Aquitaine of France, and Hydro Quebec of Canada.[16]

Gelled solid polymer electrolyte From 1990 several organisations like Mead and Valence in the United States and GS Yuasa in Japan developed batteries using gelled SPEs.[16] In 1996, Bellcore in the United States announced a rechargeable lithium polymer cell using porous SPE.[16] 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.

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.[17] At that time polymer batteries were promising and it seemed polymer electrolytes would become indispensable.[18] Eventually, this type of cell went into the market in 1998.[17]

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.[11] Although these polymer electrolytes may be dry to the touch, they can still contain 30% to 50% liquid solvent.[19] In this regard, how to really define what a "polymer battery" is remains an open question.

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.[20] It was a system like this that Bellcore used to develop an early lithium-polymer cell in 1996,[21] which was called "plastic" lithium-ion cell (PLiON), and subsequently commercialised in 1999.[20]

Other attempts to design a polymer electrolyte cell include the use of inorganic ionic 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).[22]