Solid-State-Enabled Three-Stage Delithiation Pathway of Li₅FeO₄ for Silicon-Based All-Solid-State Batteries

Li₅FeO₄ 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 Li₅FeO₄ serves as an effective cathode prelithiation additive in halide-based ASSBs. Through mechanochemical milling, pristine micrometer-sized Li₅FeO₄ 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 Li₅FeO₄ 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 Li₅FeO₄ in an all-solid-state reaction environment, thus providing mechanistic guidance for developing efficient prelithiation strategies for high-energy-density ASSBs.