Rational Electrolyte Formulation for Sodium Metal Batteries Operating in Extremely Cold Environments

Sodium metal batteries have shown considerable potential when operated at ambient temperatures. However, their performance in cold environments is constrained by increased electrolyte resistance with decreasing temperature and dendritic sodium plating associated with unstable solid electrolyte interphase (SEI), which are primarily influenced by the electrolyte composition. In this study, we present an electrolyte formulation that remains thermally stable down to −150 °C, which not only facilitates low internal resistance but also contributes to the formation of a protective SEI under cryogenic conditions. When cycled at −40 °C at 1 mA cm−2, the sodium metal electrode exhibits a low overpotential of only 16 mV over 750 h; even at an ultra-low temperature of −80 °C, the electrode demonstrates remarkable long-term stability with a low overpotential of 54 mV sustained over 1500 h at 0.5 mA cm−2. Furthermore, full cell evaluations when paring with Na3V2(PO4)3 cathode reveal a high average Coulombic efficiency exceeding 99.1% and a capacity retention over 83% after 100 cycles at both −40 °C and −80 °C.