Thin-film rechargeable lithium batteries with amorphous and crystalline LiCoO{sub 2} cathodes were investigated. The lithium cobalt oxide films were deposited by radio-frequency (RF) magnetron sputtering of an LiCoO{sub 2} target in a 3:1 Ar/O{sub 2} mixture gas. From proton-induced {gamma}-ray emission analysis (PIGE) and Rutherford backscattering spectrometry (RBS), the average composition of these films was determined to be Li{sub 1.15}CoO{sub 2.16} or, within experimental uncertainty, LiCoO{sub 2} + 0.08 Li{sub 2}O. The X-ray powder diffraction patterns of films annealed in air at 500 to 700 C were consistent with the regular hexagonal structure observed for crystalline LiCoO{sub 2}. The discharge curves of the cells with amorphous LiCoO{sub 2} 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 {approximately}3.9 V with a relatively large capacity. Two lower capacity plateaus were observed at {approximately} 4.2 and 4.1 V with the 600 and 700 C annealed cathodes; the {minus}dq/dV peaks were broader and weaker for the 600 C annealed cathodes and were not present at all with the 500 C annealed films. The chemical diffusion coefficients of Li{sup +} in the cathodes obtainedmore » from ac impedance measurements at cell potentials of {approximately} 4 V ranged from {approximately} 10{sup {minus}12} cm{sup 2}/s for the as-deposited amorphous cathodes to {approximately} 10{sup {minus}9} cm{sup 2}/s for the films annealed at 700 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 C annealed material, losses on cycling between 4.2 and 3.8 V at 25 C ranged from 0.0001%/cycle (> 10{sup 4} cycles) to 0.002%/cycle for cells with cathodes form 0.05 to 0.5 {micro}m thick.« less