High rate bias sputtered LiCoO2 thinfilms as positive electrode for all-solid-state lithium microbatteries

A LiCoO2 thin film with a crystalline structure and free of the cubic phase is obtained by adopting a two-step treatment with a combination of bias substrate during RF sputtering and a post-annealing treatment at moderate temperature of 500 °C or 400 °C. All-solid-state Li/LiPON/LiCoO2 lithium batteries are fabricated using such crystalline bias LiCoO2 thin films. The crack-free surface of the cathode thin film and the nanosized LiCoO2 particles ensure an efficient electrochemical behavior and high performance of the battery. The rate capability and the influence of the voltage range on the rechargeable capacity delivered by the battery are investigated as well as cycling properties. A high capacity close to 50 μAh cm−2 μm−1 is demonstrated at 10 μA. cm−2 with an excellent rate capability since 86% of this value is recovered at 30 C without any further damage of the cathode thin film. An excellent cycling behavior of the biased film is demonstrated since after a capacity decay of 12% over the first 100 cycles, the specific capacity practically does not decline over 800 additional cycles at 0.4 C rate. These results compare very well with the best all-solid-state Li microbatteries built with LiCoO2 heat-treated at 700 °C or more. It exemplifies the relevance of using biased LiCoO2 thin films heat-treated at moderate temperature (400 °C-500 °C).