Dependence of Al_2O_3 Coating Thickness and Annealing Conditions on Microstructural and Electrochemical Properties of LiCoO_2 Film

摘要

We studied the dependence of Al_2O_3 coating thickness and annealing conditions on the surface morphology and electrochemical properties of Al_2O_3 coated LiCoO_2 films. The optimum coating thickness allowing for the highest capacity retention was about 24 nm. A sample consisting of Al_2O_3 coated on annealed LiCoO_2 film with additional annealing at 400 °C had a uniform coating layer between the coating materials and cathode films. This sample showed the best capacity retention of approx 91 % with a charge-cut off of 4.5 V after 30 cycles, while the bare cathode film showed a capacity retention of approx 32 % under the same conditions. The formation of second phases such as Co-Al-O was observed in the coating films by X-ray photoelectron spectroscopy (XPS). The Co-Al-O containing samples showed a higher initial capacity because of their smaller grain size, but less capacity retention than the Al_2O_3 containing samples.