Characterization of thin-film rechargeable lithium batteries with lithium cobalt oxide cathodes

Thin-film rechargeable lithium batteries with amorphous and crystalline LiCoO2 cathodes were investigated. The lithium cobalt oxide films were deposited by radio-frequency (RF) magnetron sputtering of an LiCoO2 target in a 3:1 Ar/O-2 mixture gas. From proton-induced gamma-ray emission analysis (PIGE) and Rutherford backscattering spectrometry (RES), the average composition of these films was determined to be Li1.15CoO2.16 or, within experimental uncertainty, LiCoO2 + 0.08 Li2O. The x-ray powder diffraction patterns of films annealed in air at 500 to 700 degrees C were consistent with the regular hexagonal structure observed for crystalline LiCoO2. The discharge curves of the cells with amorphous LiCoO2 cathodes showed no obvious structural transition between 4.2 and 2.0 V, while the discharge curves of the cells with polycrystalline cathodes were consistent with a two-phase potential plateau at similar to 3.9 V with a relatively large capacity. Two lower capacity plateaus were observed at similar to 4.2 and 4.1 V with the 600 and 700 degrees C annealed cathodes; the -dq/dV peaks were broader and weaker for the 600 degrees C annealed cathodes and were not present at all with the 500 degrees C annealed films. The chemical diffusion coefficients of Li+ in the cathodes obtained from ac impedance measurements at cell potentials of similar to 4 V ranged from similar to 10-(12) cm(2)/s for the as-deposited amorphous cathodes to similar to 10(-9) cm(2)/s for the films annealed at 700 degrees C. The capacity loss on extended cycling of the thin-film cells varied with the crystallinity and thickness of the cathodes and with temperature. With the highly crystalline, 700 degrees C annealed material, losses on cycling between 4.2 and 3.8 V at 25 degrees C ranged from 0.0001%/cycle (>10(4) cycles) to 0.002%/cycle for cells with cathodes from 0.05 to 0.5 mu m thick.