Secondary lithium electrochemical cells, and particularly lithium batteries, using an intercalation compound as the positive electrode have been studied intensively during the past decade. Heretofore, the cathode material used in these batteries was typically a lithiated cobalt oxide, nickel oxide, or manganese oxide. The earliest reports of rechargeable lithium batteries occurred more than a decade ago, and are shown in, for example, U.S. Pat. Nos. 4,302,518 and 4,357,215 to Goodenough, et al.
Secondary lithium batteries using polymer electrolytes offer substantial advantages over lithium ion batteries with liquid electrolytes as are currently known in the field. Among these advantages are enhanced safety, long-cycle life, high energy density, and flexibility. Most of all, secondary lithium batteries using polymer electrolyte holds great promise to be manufactured with ease, since thin film processes in the polymer industry can be used or adapted to the production of secondary lithium batteries. One of the key issues in making secondary lithium polymer batteries is the preparation of composite electrodes which possess good mechanical strength and high conductivity, both in terms of ionic conductivity and electronic conductivity. High conductivity, both ionic and electronic, is essential for high rate operation of the lithium battery. Good mechanical strength is also necessary for large scale processing.
Composite electrodes used in secondary lithium polymer batteries typically contain an electrode material providing active mass and polymer electrolyte providing mechanical integrity and ionic conductivity. The polymer electrolyte used in the composite electrodes of the prior art are identical to the polymer used in the electrolyte layer of the device, and have, heretofore been fabricated of, for example, poly(ethylene oxide) or poly(vinylidene fluoride).
Examples of this electrochemical device configuration can be found in, for example, U.S. Pat. No. 5,296,318 to Gozdz, et al., in which poly(vinylidene fluoride) copolymer was used in the composite electrode and as the electrolyte layer. Other polymers which were used in the same fashion include polyethylene oxide and poly(acrylonitrile).
These devices, while acceptable, did not demonstrate the high levels of ionic conductivity and the high performance rate characteristics required to make lithium polymer batteries successful in the marketplace. Moreover, due to inherent limitations with the polymer itself, mechanical integrity, and hence cycle life of the material were compromised.
Accordingly, there exists a need for a method of preparing composite electrodes, having high ionic and electronic conductivity, as well as mechanical strength which enables easy and inexpensive production of secondary lithium polymer batteries.