Thermally Adaptive PVDF-HFP/Ionic Liquid Electrolyte Enabling Stable SEI/CEI Formation in Sodium Metal Batteries

Sodium-metal batteries (SMBs) offer exceptional energy density but face persistent interfacial instability and electrolyte degradation challenges. Herein, to address these limitations, an adaptive polymer electrolyte membrane (EM) is developed from poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) integrated with 1-ethyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [EMPYR][TFSI] ionic liquid (IL). The membrane exhibits a smart thermal response, functioning as a solid electrolyte at 55 °C while forming a gel electrolyte at 25 °C upon incorporation of a conventional liquid electrolyte. This reversible transition effectively mitigates cathode-electrolyte side reactions at elevated temperature by reducing solvent mobility and facilitating the formation of a stable cathode-electrolyte interphase (CEI). Combined experimental and theoretical analyses reveal that optimized PVDF-HFP/IL coordination enables dual interfacial stabilization, forming a robust solid-electrolyte interphase (SEI) on sodium metal and a protective CEI on Na₃V₂(PO₄)₃ cathodes. The optimized PVDF-HFP/IL-EM enables dendrite-free Na plating/stripping for over 1,000 h at 0.1 mA cm⁻² with 98.2% Coulombic efficiency. The Na | PVDF-HFP-3(GPE) | Na₃V₂(PO₄)₃full cell demonstrates high-rate capability and excellent cycling stability, retaining 86% capacity after 1000 cycles at 1C. This work establishes a new paradigm of adaptive polymer electrolytes, achieving simultaneous interfacial stabilization, thermal resilience, and high-rate performance for next-generation sodium-metal batteries.