Effect of Ti-Doped Al₂O₃ coating thickness and annealed condition on microstructure and electrochemical properties of LiCoO₂ thin-film cathode

We investigated the dependence of the various annealing conditions and thickness (6-45 nm) of the Ti-doped Al₂O₃ coating on the electrochemical properties and the capacity fading of Ti-doped Al₂O₃ coated LiCoO₂ films. The Ti-doped-Al₂O₃-coating layer and the cathode films were deposited on Al₂O₃ plate substrates by RF-magnetron sputter. Microstructural and electrochemical properties of Ti-doped-Al₂O₃-coated LiCoO₂ films were investigated by transmission electron microscopy (TEM) and a dc four-point probe method, respectively. The cycling performance of Ti-doped Al₂O₃ coated LiCoO₂ film was improved at higher cut-off voltage. But it has different electrochemical properties with various annealing conditions. They were related on the microstructure, surface morphology and the interface condition. Suppression of Li-ion migration is dominant at the coating thickness >24 nm during charge/discharge processes. It is due to the electrochemically passive nature of the Ti-doped Al₂O₃ films. The sample be made up of Ti-doped Al₂O₃ coated on annealed LiCoO₂ film with additional annealing at 400°C had good adhesion between coating layer and cathode films. This sample showed the best capacity retention of ∼92% with a charge cut off of 4.5 V after 50 cycles. The Ti-doped Al₂O₃ film was an amorphous phase and it has a higher electrical conductivity than that of the Al₂O₃ film. Therefore, the Ti-doped Al₂O₃ coated improved the cycle performance and the capacity retention at high voltage (4.5 V) of LiCoO₂ films.