Enhanced Electrochemical Stability of Sulfide Electrolytes with Surface Modification for High‐Performance LiNiO₂ Based All‐Solid‐State Lithium Batteries

All-solid-state lithium batteries (ASSLBs) equipped with layered Ni-rich cathodes hold great promise for achieving high-specific energy and enhanced safety. Although LiNiO₂ (LNO) cathode theoretically offers superior specific capacity (≈275 mAh g^-1) and cost efficiency, its practical application in ASSLBs is hindered by significant interfacial incompatibility with solid electrolytes, resulting in parasitic side reactions and sluggish charge transport, particularly under high-voltage operation. Here, a facile ball-milling strategy is presented to engineer an in situ protective layer on Li_5.5PS_4.5Cl_1.5 (LPSC) through a spontaneous chemical reaction with Li₂SO₄, which effectively mitigates interfacial electrochemical instability. This modified electrolyte enables ASSLBs with LNO to achieve a record-high discharge capacity of 231.3 mAh g^-1 at 0.2 C and 45 °C, alongside remarkable cycling stability (95% capacity retention after 200 cycles at 4.4 V). Additionally, even under an ultra-high voltage of 4.6 V, the battery can still retain 95% capacity over 140 cycles. Multimodal spectroscopic analyses confirm that the designed coating suppresses interfacial decomposition while maintaining rapid Li⁺ transport. This work establishes a scalable, cost-effective approach to interfacial engineering, unlocking the potential of LNO-based ASSLBs for high-specific energy applications.