Chemistry Set
23 May 2009
The common features of all lithium ion cells are that, in the charged state, one electrode contains lithium, and thatcharge is transported across the cell by lithium ions. A significant advantage of lithium-based cells is their high terminal voltage and good capacity-to-weight ratio. A wide range of recipes is available for the electrode material itself, each having its characteristic terminal voltage and other properties . The other factors affecting characteristics and cost are the choice of electrolyte and of separator. Liquid electrolytes based on organic solvents and conductive lithium salts can be used, as can solid or gel film electrolytes (as in lithium-polymer, or ‘LiPo’ cells). LiPo cells have a particularly high energy density and are available in a range of shapes; however, they do not perform well at low temperatures.
The choice of separator is an important factor in both the manufacturing cost and the safety of the cells. An interesting recent development is a ceramic separator called Separion already in use in cells by German battery maker Li-Tec. The separator can withstand high temperatures, and can thus help to prevent an internal short-circuit in the cell. This is a particularly important aspect as fully-charged lithium ion cells, in contrast to NiMH cells, cannot continue to be accept current without damage, including risk of explosion. The other side of this coin is the very high efficiency of the cell (90 % to 95 %). Safer types are the lithium iron phosphate (LiFePO4) cells from A123 and GAIA, at the cost of around 10 % less energy density and terminal voltage.
The common features of all lithium ion cells are that, in the charged state, one electrode contains lithium, and thatcharge is transported across the cell by lithium ions. A significant advantage of lithium-based cells is their high terminal voltage and good capacity-to-weight ratio. A wide range of recipes is available for the electrode material itself, each having its characteristic terminal voltage and other properties . The other factors affecting characteristics and cost are the choice of electrolyte and of separator. Liquid electrolytes based on organic solvents and conductive lithium salts can be used, as can solid or gel film electrolytes (as in lithium-polymer, or ‘LiPo’ cells). LiPo cells have a particularly high energy density and are available in a range of shapes; however, they do not perform well at low temperatures.
The choice of separator is an important factor in both the manufacturing cost and the safety of the cells. An interesting recent development is a ceramic separator called Separion already in use in cells by German battery maker Li-Tec. The separator can withstand high temperatures, and can thus help to prevent an internal short-circuit in the cell. This is a particularly important aspect as fully-charged lithium ion cells, in contrast to NiMH cells, cannot continue to be accept current without damage, including risk of explosion. The other side of this coin is the very high efficiency of the cell (90 % to 95 %). Safer types are the lithium iron phosphate (LiFePO4) cells from A123 and GAIA, at the cost of around 10 % less energy density and terminal voltage.
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