Composite solid electrolytes (CSEs) combine the flexibility of polymers with the stability of inorganic electrolytes, making them promising candidates for next-generation solid-state lithium-metal batteries (LMBs). However, their practical application is limited by low room-temperature ionic conductivity, primarily due to poor polymer-inorganic interfacial compatibility that hinders Li+ transport. In this work, we introduce a polymer-compatible ionic liquid (IL) to mediate the interphase between the polymer and ceramic components, simultaneously preventing ceramic particle aggregation for uniform dispersion and activating ceramic-polymer interfaces to construct continuous Li+ transport pathways across ceramic domains and interfacial boundaries. The interfacial engineered CSEs exhibit a substantial enhancement in room-temperature ionic conductivity to 1.64 × 10−3 S cm−1. At the ambient temperature, the Li||Li symmetric cells demonstrate stable and reversible lithium plating/stripping for 4000 h, and the Li||LiFePO4 cell delivers an initial specific capacity of 172.1 mAh g−1 at 0.5C with 90.4% capacity retention after 300 cycles. Furthermore, the Li||LiNi0.8Co0.1Mn0.1O2 cells demonstrate stable performance even under high-voltage operation (4.5 V). This work provides a practical interfacial design strategy for developing high-performance CSEs in the next-generation solid-state LMBs.