Fully Reversible Homogeneous and Heterogeneous Li Storage in RuO2 with High Capacity

Abstract In this paper, we report that Li can be stored in RuO 2 with an unusually high coulombic efficiency. The process involves three electrochemical steps: i) formation of a Ru/Li 2 O nanocomposite, ii) formation of a Li‐containing surface film, and iii) interfacial deposition of Li within the Ru/Li 2 O matrix. Corresponding to the storage of 5.6 mole of Li ions per mole of RuO 2 , a high capacity of 1130 mAh g –1 is achieved. Furthermore, virtually all inserted Li ions can be extracted, corresponding to a nearly 100 % coulombic efficiency at the first cycle. Achieving a complete reversibility for such a Li storage system through complex heterogeneous solid‐state electrochemical reactions is possible because of the formation of nanoscale Ru/Li 2 O during Li insertion and nano‐RuO 2 during Li extraction, in addition to the favorable transport properties of RuO 2 itself.