High performance sputtered LiCoO2 thin films obtained at a moderate annealing treatment combined to a bias effect

LiCoO2 thin films have been successfully deposited by radio frequency magnetron sputtering onto aluminum substrate and post-annealed at a lower temperature of 500 °C compared to usual annealing treatments. We demonstrate that an appropriate combination of key deposition parameters including a high working pressure and a substrate bias enables for the first time the reproducible synthesis at this moderate temperature of high performance layered HT-LiCoO2 thin films. The morphology, the structure and the electrochemical behaviour of the deposits have been examined as a function of the working gas pressure ranging from 0.55 Pa to 3 Pa. Raman microspectrometry has been thoroughly used to characterize the structure of the thin films: a mixture of the layered HT-LiCoO2 and the cubic LT-LiCoO2 phases is systematically revealed in the absence of substrate bias and for the first time, the access to the relative amount of both compounds is allowed. A promoting effect of increasing gas pressure on the formation of the layered HT-LiCoO2 phase is demonstrated, the HT/LT relative amount increasing from 1:3 at 0.55 Pa to 3:4 at 3 Pa. In a second step, we show that the effect of substrate bias combined with high values of working gas pressure and a moderate heat treatment (500 °C) leads to the formation of pure HT-LiCoO2 thin films at 3 Pa, with very attractive electrochemical properties: a high specific capacity of 67 μA h cm−2 μm−1 at C/5 rate, a good adherence to the substrate and the best capacity retention known for a LiCoO2 film in liquid electrolyte (4.2–3 V).