CHARACTERIZATION OF THIN-FILM RECHARGEABLE LITHIUM BATTERIES WITH LITHIUM COBALT OXIDE CATHODES

Thin-film rechargeable lithium batteries with amorphous and crystallin e LiCoO2 cathodes were investigated. The lithium cobalt oxide films we re 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 emi ssion analysis (PIGE) and Rutherford backscattering spectrometry (RES) , the average composition of these films was determined to be Li1.15Co O2.16 or, within experimental uncertainty, LiCoO2 + 0.08 Li2O. The x-r ay powder diffraction patterns of films annealed in air at 500 to 700 degrees C were consistent with the regular hexagonal structure observe d for crystalline LiCoO2. The discharge curves of the cells with amorp hous LiCoO2 cathodes showed no obvious structural transition between 4 .2 and 2.0 V, while the discharge curves of the cells with polycrystal line cathodes were consistent with a two-phase potential plateau at si milar to 3.9 V with a relatively large capacity. Two lower capacity pl ateaus 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 wi th the 500 degrees C annealed films. The chemical diffusion coefficien ts of Li+ in the cathodes obtained from ac impedance measurements at c ell 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 extende d cycling of the thin-film cells varied with the crystallinity and thi ckness of the cathodes and with temperature. With the highly crystalli ne, 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.