In situ ionogel electrolyte with anion-anchoring sites and covalent bonded phosphorus enables high-performance sodium metal batteries

Abstract

Sodium metal batteries (SMBs) containing gel polymer electrolytes (GPEs) are considered a promising candidate for energy storage systems owing to their high theoretical capacity and low cost. Nevertheless, the intrinsic combustibility and uncontrollable dendrite growth caused by organic components and the low cation transference number of GPEs hinder their practical application. Herein, a fireproof and improved conductive ionic-liquid-based GPE was fabricated through in situ thermal polymerization using diethyl vinylphosphonate, poly(ethylene glycol) diacrylate and nonflammable ionic liquids as the flame retardant, cross-linker and plasticizer, respectively, which polymerized within a designed anion-anchored fiber framework. The thermostable ionic-liquid-incorporated GPE with polyethyleneimine (termed PPGPE) exhibits self-extinguishing behavior due to a dual-phase flame-retardant mechanism. The fiber skeleton with abundant Lewis acid sites functions with anions and facilitates Na⁺ migration, contributing to the improved ionic conductivity (1.2 × 10⁻⁴ S cm⁻¹) and Na⁺ transference number (0.42) of the PPGPE at 25 °C. Moreover, the PPGPE could power symmetrical cells for over 1000 h at 0.05 mA cm⁻² with a 0.3 V overpotential and support 150 cycles of Na₃V₂(PO₄)₃ (NVP)-SMBs at 0.5 C with a capacity retention of 70.4%. This work provides a simple but effective approach to simultaneously improve the safety and electrochemical performance of SMBs for practical application.