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

Abstract 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.