High-capacity, nanocrystalline Li2RuO3-LiCoO2 cathodes for flexible solid-state thin film batteries

Abstract

Safe, energy-dense batteries are needed to power the next generation of flexible, wearable microelectronic devices. Solid-state Li metal thin-film batteries (TFBs) offer compelling performance but their thin film cathodes require high temperature annealing for optimal electrochemical performance, making integration with low-cost, flexible, thermoplastic substrates difficult. In this study, thin film cathodes based on 0.8Li2RuO3-0.2LiCoO2-xLi2O (LRCO) are reported for the first time. LRCO thin-film cathodes prepared by radio-frequency (RF) sputtering provide excellent energy storage performance without requiring a thermal annealing step. X-ray diffraction shows that the LRCO is nanocrystalline, and X-ray photoelectron spectroscopy confirms the 4+ and 3+ oxidation states of Ru and Co, respectively. Energy storage performance as a function of the charge potential was investigated via galvanostatic cycling to identify the maximum safe potential to avoid molecular oxygen evolution from the cathode lattice. Nanocrystalline LRCO delivers a discharge capacity of >110 μAh cm−2 μm−1 at 0.3C which compares favorably to the state-of-art LiCoO2 thin-film cathode at 69 μAh cm−2 μm−1. The feasibility of fabricating and performance on flexible thermoplastic susbstrates is also demonstrated. On polyethylene terephthalate and polyimide susbtrates, the discharge capacity remains >100 μAh cm−2 μm−1 and the capacity retention is 97.5% over 120 cycles in a bent state.