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. 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).Īpplying pressure on LiPo cells An experimental lithium-ion polymer battery made by Lockheed-Martin for NASA LiPo battery packs, with cells connected in series and parallel, have separate pin-outs for every cell. 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 nor over-discharge under use. This compares to 3.6–3.8 V (charged) to 1.8–2.0 V (discharged) for those based on lithium-iron-phosphate (LiFePO 4). 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), where the nominal voltage is 3.6 or 3.7 volts (about the middle value of highest and lowest value) for cells based on lithium-metal-oxides (such as LiCoO 2). Main article: Lithium-ion battery § Charge and discharge 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. The negative electrode material may have the same three parts, only with carbon replacing the lithium-metal-oxide. In addition to this, the positive electrode can be further divided into three parts: the lithium-transition-metal-oxide (such as LiCoO 2 or LiMn 2O 4), a conductive additive, and a polymer binder of poly(vinylidene fluoride) (PVdF). The separator itself may be a polymer, such as a microporous film of polyethylene (PE) or polypropylene (PP) thus, even when the cell has a liquid electrolyte, it will still contain a "polymer" component. Ī typical cell has four main components: positive electrode, negative electrode, separator and electrolyte. In 1996, Bellcore in the United States announced a rechargeable lithium polymer cell using porous SPE. From 1990 several organisations like Mead and Valence in the United States and GS Yuasa in Japan developed batteries using gelled SPEs. The dry SPE was the first used in prototype batteries, around 1978 by Michel Armand, and 1985 by ANVAR and Elf Aquitaine of France, and Hydro-Québec of Canada. The solid electrolyte can typically be classified as one of three types: dry SPE, gelled SPE and porous SPE. In the 1970s the original polymer design used a solid dry polymer electrolyte resembling a plastic-like film, replacing the traditional porous separator that is soaked with electrolyte. The primary difference is that instead of using a liquid lithium-salt electrolyte (such as LiPF 6) 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). Lithium polymer cells have evolved from lithium-ion and lithium-metal batteries. After that, other packaging forms evolved, including the flat pouch format. LiPo cells follow 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. Main article: Lithium-ion battery § History
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