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.