Characterization of Thin‐Film Rechargeable Lithium Batteries with Lithium Cobalt Oxide Cathodes

Thin‐film rechargeable lithium batteries with amorphous and crystalline cathodes were investigated. The lithium cobalt oxide films were deposited by radio‐frequency (RF) magnetron sputtering of an target in a 3:1 Ar/ mixture gas. From proton‐induced γ‐ray emission analysis (PIGE) and Rutherford backscattering spectrometry (RBS), the average composition of these films was determined to be or, within experimental uncertainty, . 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 . The discharge curves of the cells with amorphous 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 ∼3.9 V with a relatively large capacity. Two lower capacity plateaus were observed at ∼4.2 and 4.1 V with the 600 and 700°C annealed cathodes; the −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 in the cathodes obtained from ac impedance measurements at cell potentials of ∼4V ranged from ∼10⁻¹² cm²/s for the as‐deposited amorphous cathodes to ∼10⁻⁹ cm²/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 (>10⁴ cycles) to 0.002%/cycle for cells with cathodes from 0.05 to 0.5 μm thick.