Nanocrystalline LixMn2-yO4 cathodes for solid-state thin-film rechargeable lithium batteries

Thin-film cathodes of lithium manganese oxide, 0.3-3 mu m thick, were deposited by rf magnetron Sputtering of a LiMn2O4 ceramic target onto unheated substrates. The resulting films were dense, similar to 4.2 g/cm(3), with a similar to 50 Angstrom nanocrystalline spinel structure. The film composition was typically LixMn2-yO4 With y similar to 0.3 and 1.2 < x < 2.2. When cycled in a thin-film rechargeable lithium battery, specific cathode capacities of 145 +/- 23 and similar to 270 mAh/g were realized for discharge from 4.5 V to either 2.5 or 1.5 V,respectively. The discharge and charge current densities were limited by the resistivity of lithium transport into and through the cathode. After thousands of cycles at 25 degrees C, there was a small increase in cell resistance. After several hundred cycles at 100 degrees C, the discharge curves developed a stable knee at similar to 4 V characteristic of crystalline LiMn2O4 cathodes. The polarization of the discharge/charge cycles were interpreted in terms of free energy of mixing curves. (C) 1999 The Electrochemical Society. S0013-4651(98)11-060-1. All rights reserved.