Safe solid-state lithium metal batteries (SSLMBs) with high energy density are in great demand for electrical vehicles and large-scale energy storage systems. The interfacial challenges including large interface resistance and untoward reactions have an enormous impact on the rate performance and cycle stability of SSLMBs. Hence, in this work, plastic crystal-based materials are proposed as the interface layers to reduce the interfacial impedance and prevent side reactions between solid electrolytes and Li anode. The plastic crystal-based materials can enable in-situ solidification between solid electrolytes and Li anode and show high ion conductivity of up to [Formula: see text]10[Formula: see text] S ⋅ cm[Formula: see text], high Li ion transference number, and good chemical and electrochemical stability against Li metal, indicating they are suitable to be used as the interface layers for SSLMBs.