Changes in material systems are responsible for some of the capacity enhancements, including one by Toshiba. For its negative (anode) electrode material, the company is moving from existing graphite materials to higher-capacity graphite materials, and eventually to "nanocomposite" graphite, to achieve greater packing density for the anode. At the same time, the lithium-cobalt oxide material used for the positive (cathode) electrode material is being enhanced with greater boron doping.
But Toshiba anticipates even greater capacity improvements when it re-places its lithium-cobalt oxide-based electrodes with nickel-cobalt hybrids. In the 18650 cell, the result will be a 2300-mAh cell that achieves 480 Wh/l and 195 Wh/kg. That cell is expected in the fourth quarter of this year.
Further improvements should be obtained by migrating to a positive electrode based on nickel-manganese oxide and, ultimately, an all-nickel positive electrode. The all-nickel cathode should boost cell capacity and lower cost because it will replace more expensive cobalt with an inherently safer metal.
Now the company is developing a nickel-based cell that could provide 540 Wh/l. But the operating voltage will be only 2.8 V, instead of the 3.7 V normally associated with Li-ions.
Toshiba's Li-ion technology actually contains a polymer-style electrolyte. But the company shies away from calling its cells Li-polymers because they contain trace amounts of liquid electrolyte. This reduces the cell's electrical resistance. It also improves low temperature performance, and reduces swelling.
The application of small amounts of liquid electrolyte in Li-polymer cells is considered a common practice. That may change, though, as vendors attempt to build cells with greater resistance to leakage.
Typically with Li-polymer cells, manufacturers convert the polymer from liquid into gel form, then apply it along with the separator between the positive and negative electrodes as the cell layers are wound. This approach requires that a small amount of liquid electrolyte be inserted into the electrodes to achieve the necessary electrical conductivity.
However, Sanyo has created a technique for gelling the electrolyte within the battery case. After the cell electrodes and separators are wound, a solution of Li-salt, solvent, and prepolymer material is poured into the battery case. This is then heated to form a completely gelled electrolyte. In addition to promising better leakage protection, the Sanyo polymer electrolyte is said to maintain good discharge characteristics.
Vendors also are making strides into the area of cell packaging. Although thin foil packages have been adopted in both Li-polymer and Li-ion cell designs, Hitachi Maxell has released a cell that features foil-like thinness and can-like ruggedness. The vendor has exploited an aluminum alloy that contains greater than 4.5% magnesium to produce a rigid material with just 0.15-mm thickness. That's almost as thin as a foil pouch, but unlike the pouch, Hitachi Maxell's metal can resists bending, cracking, and piercing.
Using this metal can, the company has produced a 2.8- by 34- by 65-mm Li-ion prismatic that provides just over 600 mAh of capacity (Table 1, again). That's deemed just enough for cell phones with fewer features. Nevertheless, the cell's capacity can be modified by changing length and width dimensions. (For a description of how this cell is fabricated, read the online sidebar, "Canning Thin Li-Ion Cells.")
The availability of Li-ion cells with thicknesses below 3 mm creates competition with Li-polymer cells whose main claim to fame is their thinness. Moreover, others are developing Li-ion cells almost that thin. Panasonic plans to offer Li-ion cells housed in aluminum cans with dimensions as low as 4.3 mm this year.
Meanwhile, NEC Electronics continues to exploit manganese-based cathodes (less common than the cobalt-based cells) to build Li-ion cells in foil packages. Among the vendor's recent introductions are two Bluetooth oriented cells—a 3.9-mm thick Li-polymer cell with 150-mAh capacity, and a 5.6-mm thick version with 300 mAh. The company plans to develop this Li-ion technology further and introduce cells with 4- to 8-mm thicknesses.
Another packaging development comes from GS-Melcotec. Originally offered in the foil pouch, its LY series of Li-polymer cells is now being produced in a Buttercup-shaped aluminum-laminated film case. The squared-off shape fits better than the pouch in some designs.
The availability of coin-type cells is another packaging twist seen in recent product introductions. Panasonic, for one, has developed a Li-ion coin cell that offers 130 mAh in a 30-mm (diameter) by 3.2-mm (thick) cell, the CGL3032. Another vendor, Korea Power Cell (www.powercellkorea.com), has announced plans to offer a Li-polymer of the same dimensions that will deliver 180 mAh (PD3032 or PowerDisc).