Extending the low-temperature operation of sodium metal batteries combining linear and cyclic ether-based electrolyte solutions

Abstract Despite the promise of sodium electrochemistry at ambient temperature, the performance of sodium metal batteries in frigid environments is hindered by high internal resistance and unstable solid electrolyte interphase (SEI), which strongly depends on the electrolyte composition. Here we present an electrolyte formulation consisting of acyclic/cyclic ethers as binary solvents and a compatible sodium salt, which is thermally stable down to − 150°C. This electrolyte not only provides low resistance but enables a protective SEI in cold environments. Long-term sodium-metal cycling is demonstrated at the gelid temperature of − 80°C, showing small overpotentials of ~ 150 mV for over 750 hours. Both X-ray photoelectron spectroscopy and cryogenic transmission electron microscopy are performed to elucidate the temperature-dependent chemistry of the interphase. Full cells are further demonstrated, exhibiting a low decay rate (< 0.089% per cycle) and a high average Coulombic efficiency (> 99.5%) at temperatures as low as − 60°C.