Integrating Prelithiation and Interface Protection to Achieve High‐Energy All‐Solid‐State Batteries

All‐solid‐state batteries (ASSBs), particularly those with Li‐free anodes or even anode‐free configurations, have attracted extensive attention due to high safety and energy density. However, chemical‐mechanical degradation typically deteriorates the cycle life and energy of Li‐free anode ASSBs with the absence of Li inventory. Here, the prelithiation agent Li5FeO4 (LFO) coated Ni‐rich layered oxide is developed as the cathode for Li‐free anode ASSBs. The coated LFO acts as an interfacial protective layer to prevent the highly oxidizing Ni‐rich cathode from reacting with sulfide solid‐state electrolytes (SSEs), mitigating the structural degradation of Ni‐rich cathodes and the decomposition of SSE, resulting in excellent cycle life. Beneficial from the coated LFO in the cathode of the Li‐free anode ASSBs, the reversible capacity improves from 174.7 mAh g−1 to 199.7 mAh g−1, and the capacity retention is enhanced from 33.8 % to 84.8 % after 100 cycles. Additionally, an ultrahigh energy density of 440 Wh kg−1, based on the mass of the composite cathode, Li‐free anode, and SSE, is obtained in a Li‐free anode all‐solid‐state pouch cell equipped with the LFO‐coated cathode. The coated Li5FeO4 (LFO) on the surface of LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode can serve both as a prelithiation agent to compensate for the rapid active Li depletion in the Li‐free anode ASSB, and as a protective layer between NCM811 and Li6PS5Cl (LPSC) to suppress the structural degradation of NCM811 and the decomposition of LPSC.