Chain Length‐Mediated Crosslinked Polymer Design for Succinonitrile Electrolytes with Optimized Interfacial Chemistry Toward Long‐Cycle Sodium Metal Batteries

Sodium metal solid‐state batteries (SMSSBs) are attractive because of the elemental abundance of sodium, reduced cost, and improved safety. Succinonitrile (SN)‐based plastic crystal–polymer composite electrolytes (PCCEs) are promising for SMSSBs due to their high ionic conductivities, but their applications have been plagued with unwanted SN‐induced side reactions at sodium metal anodes. Here, we considerably stabilize sodium metal interfaces by regulating the chain length between diacrylate crosslinking points in PCCEs. Mechanistic analysis suggests that longer segmental chains with increased ether groups impose steric hindrance and stronger interactions on SN that restrict its mobility. The populated ether groups also tend to strongly coordinate with Na + , effectively repelling SN from its solvation shell. Both the steric confinement effect and the competitive coordination with SN help mitigate SN‐induced parasitic processes at the interphase, promoting uniform, reversible sodium stripping/deposition reactions. Using the optimized PCCE, 1000P, Na||Cu cells exhibit excellent average Coulombic efficiencies of 97.4% with highly reversible Na stripping and deposition cycles over 1000 h. The resulting Na|1000P|Na 3 V 2 (PO 4 ) 3 full cells retain 89.3% capacity after ∼8000 cycles at 2C, while Na|1000P|Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 cells retain 97.9% after ∼400 cycles at 0.5C. This study demonstrates an effective strategy to address the sodium‐metal interfacial issue for SMSSBs.