Scalable integration of Li₅FeO₄ 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 Li₅FeO₄ 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 Li₅FeO₄ 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 Li₅FeO₄ 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.