Non-flammable polymeric ionic liquid hybrid electrolytes for high-stability solid-state sodium metal batteries

Solid-state sodium metal batteries (SMBs) show promise for next-generation energy storage, but challenges remain in developing electrolytes that balance safety, performance, and longevity. This study introduces a hybrid electrolyte system for SMBs, combining a poly(diallyldimethylammonium) bis(fluorosulfonyl)imide polymer matrix with a sodium bis(fluorosulfonyl)imide in N-butyl- N -methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide ionic liquid electrolyte. The optimized hybrid electrolyte exhibits a sodium ion conductivity of 2.1 mS cm −1 at 70 °C and an oxidative stability of 4.8 V vs Na + /Na. Electrochemical characterization reveals efficient sodium plating/stripping with a critical current density of 8.5 mA cm −2 . X-ray photoelectron spectroscopy identifies a NaF-rich interphase at the sodium metal surface, contributing to interfacial stability. An all-solid-state SMB utilizing this electrolyte and a sodium vanadium phosphate cathode demonstrates stable cycling at 60 °C, retaining 75 % capacity after 2000 cycles. These results highlight the potential of hybrid electrolytes in advancing safe, high-performance solid-state SMBs for future energy storage applications. • Non-flammable PDADMAFSI-ionic liquid hybrid electrolyte for sodium metal batteries • Achieves 2.1 mS cm −1 Na + conductivity at 70 °C and 4.8 V vs Na + /Na oxidative stability • Enables stable Na plating/stripping with 8.5 mA/cm 2 critical current density • XPS reveals NaF-rich interphase that enhances interfacial and cycling stability. • Full cells retain 75 % capacity after 2000 cycles at 60 °C with NVP cathode.