Solid‐State‐Enabled Three‐Stage Delithiation Pathway of Li 5 FeO 4 for Silicon‐Based All‐Solid‐State Batteries

ABSTRACT Li 5 FeO 4 is a prelithiation additive for compensating irreversible lithium loss in lithium‐ion batteries, yet its function and mechanism in all‐solid‐state batteries (ASSBs) remain largely unexplored. Here, we demonstrate that nanosized Li 5 FeO 4 serves as an effective cathode prelithiation additive in halide‐based ASSBs. Through mechanochemical milling, pristine micrometer‐sized Li 5 FeO 4 is reduced to approximately 500 nm, leading to a significant enhancement in delithiation capacity from 43.1 to 752.5 mAh g ‒1 . This enhancement arises from nanosizing‐induced local structural disorder and improved electronic conductivity. Mechanistic analyses reveal a unique three‐stage delithiation pathway in the all‐solid‐state reaction environment, involving low‐potential lattice oxygen oxidation, coupled oxygen/iron oxidation, and lattice oxygen oxidation accompanied by iron reduction. Guided by this mechanism, nanosized Li 5 FeO 4 replenishes lithium consumed by silicon anodes, enabling room‐temperature silicon‐based ASSBs with 70% capacity retention over 1000 cycles and stable operation from −10°C to 55°C. Moreover, a silicon‐based all‐solid‐state pouch cell delivers an energy density of 473.3 Wh kg ‒1 (calculated on electrode mass) and retains 86.1% capacity after 500 cycles under 24.5 MPa. This work reveals the distinct delithiation pathway of Li 5 FeO 4 in an all‐solid‐state reaction environment, thus providing mechanistic guidance for developing efficient prelithiation strategies for high‐energy‐density ASSBs.