Surface-modified ZrO₂ fillers strongly facilitate Li⁺ transport in PEO-based electrolytes, but even simple addition of the modifier serves to greatly improve ionic conductivity, proving the crucial role of the additive–matrix interaction.

Solid‐state lithium–sulfur batteries (SSLSBs) promise high theoretical capacity and enhanced safety. However, the use of lithium metal anodes comes with side reactions and limited interfacial stability, creating a demand for advanced engineering of the interface between the anode and the solid‐state electrolyte (SSE). In this work, a protective Li _x AlS _y layer is added to the lithium surface via chemical reaction steps to improve the interfacial stability. We demonstrate that this protective layer can inhibit further side reactions between the SSE and the lithium anode. SSLSBs with modified lithium anodes exhibit elevated cycling performance starting from the first cycle, delivering a specific discharge capacity of 340 mAh g ⁻¹ (S) at 0.1C. After 50 cycles, they still maintain nearly twice the discharge capacity compared to the reference cells. Insights from impedance analysis and the distribution of relaxation time (DRT) analysis indicate that the protective layer keeps the charge transfer resistance constant and facilitates the migration of lithium ions. This work thus provides a sound strategy to effectively solve the interfacial issues between the lithium metal anode and SSE, thereby addressing key obstacles in applying Li metal anodes in SSLSBs. Our findings offer a new perspective for understanding and optimizing solid‐state battery interfaces.