Scalable Integration of Li 5 FeO 4 towards Robust, High‐Performance Lithium‐Ion Hybrid Capacitors

Lithium‐ion hybrid capacitors have attracted great interest due to their high specific energy relative to conventional electrical double‐layer capacitors. Nevertheless, the safety issue still remains a drawback for lithium‐ion capacitors in practical operational environments because of the use of metallic lithium. Herein, single‐phase Li 5 FeO 4 with an antifluorite structure that acts as an alternative lithium source (instead of metallic lithium) is employed and its potential use for lithium‐ion capacitors is verified. Abundant Li + amounts can be extracted from Li 5 FeO 4 incorporated in the positive electrode and efficiently doped into the negative electrode during the first electrochemical charging. After the first Li + extraction, Li + does not return to the Li 5 FeO 4 host structure and is steadily involved in the electrochemical reactions of the negative electrode during subsequent cycling. Various electrochemical and structural analyses support its superior characteristics for use as a promising lithium source. This versatile approach can yield a sufficient Li + ‐doping efficiency of >90 % and improved safety as a result of the removal of metallic lithium from the cell.