Molecular Engineering of Weakly Solvating Dinitrile Electrolytes for Long‐Lasting Room‐Temperature Lithium Metal Batteries

The development of lithium metal batteries (LMBs) relies on advanced solid‐state electrolytes. Solid‐state polymer electrolytes (SPEs) offer advantages such as cost‐effectiveness, mechanical flexibility, and light weight; however, their practical use is hindered by limited ionic conductivity, narrow electrochemical stability window, and suboptimal cycling performance. Herein, we present a class of weakly‐solvating dinitrile‐based SPEs (DBSPEs) composed of a poly(2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl acrylate) network coupled with structurally modulated dinitrile solvents. Through rationale molecular engineering, the synergistic combination of short ethylene oxide side chains and weakly coordinating dinitrile solvents enables weak hybrid solvation of lithium ions, thereby facilitating rapid ion migration and uniform lithium deposition. The optimized DBSPE‐2 exhibits a high ionic conductivity (~2.2×10‐3 S cm‐1) and an extremely wide electrochemical window (~5.7 V). Notably, the Li||DBSPE‐2||Li symmetric cell yields stable lithium plating/stripping over 6900 hours at 0.1 mA cm−2 under ambient conditions (26 °C). When paired with LiFePO4 in a full cell, the LMBs deliver a superior fast‐charging capability, realizing 80.78% capacity retention after 1500 cycles at 2.0 C rate. This work provides fundamental insights into the solvation structure and accordingly realizes long‐lasting LMBs with weak solvents.