Effects of ZnO coating on electrochemical performance and thermal stability of LiCoO2 as cathode material for lithium-ion batteries

ZnO-coated LiCoO2 particles are prepared by plasma-enhanced chemical vapour deposition (PE-CVD) in a coating range from 0.08 to 0.49 wt.%, and examined using field emission-scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and atomic absorption spectroscopy (AAS), with particular focus on surface characteristics. From charge-discharge cycling tests in the range of 3.0–4.5 V, the ZnO coating has little effect on the discharge capacity in the first few cycles, but the coating effectively improves the capacity retention after prolonged cycling. In the experimental range studied, the optimum amount of ZnO coating which maximizes the capacity retention is found to be 0.21 wt.%. An excessive amount of ZnO coating causes a decrease in both cyclic performance and thermal stability. The possible reasons for enhanced cycleability and thermal stability afforded by the ZnO coating are discussed from the viewpoint of the surface morphology of the bare and coated LiCoO2 particles and their impedance spectra.