Rapid Na+ Transport Pathway and Stable Interface Design Enabling Ultralong Life Solid‐State Sodium Metal Batteries

Sodium‐metal batteries (SMBs) using solid‐state polymer electrolytes (SPEs) show impressive superiority in energy density and safety. As promising candidates for SPEs, solid‐state plastic crystal electrolytes (SPCE) based on succinonitrile (SN) plastic crystal could achieve high ion conductivity and wide voltage window. Nonetheless, the notorious SN decomposition reaction on the electrode/electrolyte interface seriously challenges the stable operation of the battery. To address this drawback, we commence with the structural engineering of the polymer chain segments in SPCE and employ intermolecular interactions to optimize the composition of solid electrolyte interface (SEI). Moreover, this study elucidates the migration mechanism of sodium ions in SPCE in detail. The assembled sodium symmetric cells display a high critical current density of up to 2.7 mA cm‐2 and stable cycling performance for 700 hours at 0.5 mA cm‐2. Furthermore, the Na/SPCE‐9/Na3V2(PO4)3 maintains a discharge specific capacity of up to 76.8 mAh g‐1 at 10 C and shows impressive long‐cycle stability, retaining 86.2% of initial capacity over 5000 cycles with an average coulombic efficiency of 99.9%. The stability of the SEI construction is meaningful to the cycle life. Our work presents a high‐performance SPCE with intrinsic safety, providing valuable insights for the future design of solid‐state SMBs.