Composite Solid Polymer Electrolytes Based on PEO and Surface-Modified Al2O3 Fillers for Solid-State Sodium Ion Batteries

Abstract Poly(ethylene oxide) (PEO)-based solid polymer electrolytes suffer from low ionic conductivity and limited Na+ transport capability, which restrict their application in solid-state sodium metal batteries. Herein, a Na+-coordinating filler (Al2O3@KHMF) is prepared by grafting organic chains onto the Al2O3 surface through silane coupling followed by a Schiff-base reaction. The grafted chains, bearing imine nitrogen and carbonyl oxygen groups, provide abundant coordination sites for Na+ ions and simultaneously enhance the interfacial suitability for the polymer matrix. These functional groups facilitate NaTFSI dissociation and weaken the interaction between Na+ ions and PEO chains, thereby promoting Na+ transport. Consequently, the optimized PEO-NaTFSI-Al2O3@KHMF composite electrolyte achieves an ionic conductivity of 5.10 × 10–3 S cm–1 and a Na+ transference number of 0.44 at 60 °C. The electrolyte enables stable Na plating/stripping for over 800 h at 0.1 mA cm–2. Furthermore, the assembled Na3V2(PO4)3||Na cell delivers a reversible capacity of 74.7 mAh g–1 after 200 cycles at 2 C, demonstrating enhanced cycling stability when operating at a high rate. This work offers an effective molecular interface engineering strategy for developing high-performance solid-state sodium metal batteries.