Mixed ion‐electron conducting Li3P for efficient cathode prelithiation of all‐solid‐state Li‐ion batteries

All‐solid‐state batteries (ASSBs) using sulfide electrolytes hold promise for next‐generation battery technology. Although using a pure Li metal anode is believed to maximize battery energy density, numerous recent studies have implicated that Li‐ion anodes (e.g., graphite and Si) are more realistic candidates due to their interfacial compatibility with sulfide electrolytes. However, those Li‐ion ASSBs suffer from an issue similar to liquid Li‐ion batteries, which is a loss of active Li inventory owing to interfacial side reactions between electrode components, resulting in reduced available capacities and shortened cycle life. Herein, for the first time, we explore the potential of Li3P for cathode prelithiation of Li‐ion ASSBs. We identify that the crystallized Li3P (c‐Li3P) has room‐temperature ionic and electronic conductivities of both over 10−4 S/cm. Such a mixed ion‐electron conducting feature ensures that the neat c‐Li3P affords a high Li+‐releasing capacity of 983 mAh/g in ASSBs during the first charging. Moreover, the electrochemical delithiation of c‐Li3P takes place below 2 V versus Li+/Li, while its lithiation dominates below 1 V versus Li+/Li. Once used as a cathode prelithiation regent for ASSBs, c‐Li3P only functions as a Li+ donor without lithiation activity and can adequately compensate for the Li loss with minimal dosage added. Besides mitigating first‐cycle Li loss, c‐Li3P prelithiation can also improve the battery cyclability by sustained release of low‐dosage Li+ ions in subsequent cycles, which have been embodied in several full ASSBs by coupling a LiCoO2 cathode with various types of anodes (including graphite, in foil, Sb, and Si anode). Our work provides a universal cathode prelithiation strategy for high‐efficiency Li‐ion ASSBs.