Molecular Tailoring of Succinonitrile Reshapes Na+ Solvation and Interfacial Chemistry in High-Voltage Solid-State Polymer Electrolytes for Sodium Metal Batteries

Solid-state polymer electrolytes based on poly(ethylene oxide) are pivotal for high-safety, high-energy-density sodium metal batteries, yet suffer from inadequate room-temperature ionic conductivity and unstable electrode interfaces. Here, molecular tailoring of succinonitrile yields a novel plasticizer (SGa) featuring retained amide groups and a stable gauche conformation. This design suppresses the plasticizer self-aggregation within the poly(ethylene oxide) matrix, enhances polymer chain interactions, and optimizes Na⁺ solvation structures. Consequently, the resultant SPE achieves a high ionic conductivity of 0.352 mS cm^–1 and a Na⁺ transference number of 0.71. Molecular dynamics simulations reveal that SGa weakens strong Na⁺ coordination while anchoring TFSI^– anions, establishing a Na⁺-dominated fast conduction network. Crucially, the modified SPE induces a uniform, robust solid electrolyte interphase enriched in inorganic Na₃N, significantly stabilizing the Na anode. Na₃V₂(PO₄)₃|SPE|Na cell delivers 90.6% capacity retention after 1500 cycles at 1 C (versus 485 cycles for the pristine system) and exhibits superior rate capability (61.42 mAh g^–1 at 4 C) alongside high-voltage tolerance up to 4.5 V. Beyond addressing critical bottlenecks in sodium metal batteries, this work demonstrates the effectiveness of molecular customization in designing high-performance polymer electrolytes.