Structure characterization and electrochemical properties of RF sputtered lithium nickel cobalt oxide thin films

In this study, lithium nickel cobalt oxide thin-film cathode was grown on Pt- coated silicon substrate by radio frequency (RF) sputtering. From XRD, TEM, and Raman spectra analyses, the structure of the 250°C as-deposited LiNiCoO₂ film with thickness of 0.6 μm was analyzed to exhibit layered (R3̄m symmetry) crystalline structure with (104) out-of-plane texture. From XRD and Raman spectra analyses, the degree of crystallization of the as-deposited films was enhanced by postannealing due to the rearrangement of atoms during the annealing process. The SEM top-view observations showed that the as-deposited film possessed smooth surface morphology and nanocrystalline grains that were smaller than 50 nm. After the annealing treatment, the morphology of the films became roughly and the grains enlarged. The grains even seemed to be slightly sintered and the films became slightly porous after 700°C annealing. The SEM cross-sections showed that the films had columnar structure that was commonly observed from RF sputtered films. The discharge capacity of the LiNiCoCO₂ thin-film cathode was direct proportion to the annealing temperature. In other words, the discharge capacity was direct proportion to the degree of crystallization of the films. The 1st discharge capacities of 700, 600, and 500°C-annealed films were 60, 52, and 43 μAh cm^-2 μm^-1, respectively. The 50th discharge capacity remained 70% of the initial discharge capacity.