Deciphering and Enhancing Rate-Determining Step of Sodium Deposition towards Ultralow-Temperature Sodium Metal Batteries

Achieving high ionic conductivity and stable performance at low temperatures remains a significant challenge in sodium-metal batteries (SMBs). In this study, we propose a novel electrolyte design strategy that elucidates the solvation structure-function relationship within mixed solvent systems. A mixture of diglyme and 1,3-dioxolane was developed to optimize the solvation structure towards superior low-temperature electrolyte. Molecular dynamics simulations and Raman spectra results reveal the solvent-separated ion pairs and contact ion pairs dominated solvation structure in the designed electrolyte, displaying a superior ionic conductivity of 1.78×10-3 S cm-1 at -40 °C. Besides, comprehensive kinetic analysis shows Na+ transportation in the electrolyte shows a greater impact on sodium plating than Na+ transport through the solid electrolyte interphase or charge transfer. As a result, the electrolyte enables stable operation for over 12,000 hours in Na||Na cells at -40 °C. In Na||Na2/3Ni1/4Cu1/12Mn2/3O2 full cells, it maintains a high capacity retention of 92.4% over 600 cycles with an initial specific capacity of 89.4 mAh g-1 at -40 °C, and achieves 81.7% capacity retention after 50 cycles with an initial specific capacity of 75.3 mAh g-1 at -78 °C. These results pave the way for the development of high-performance SMBs capable of operating under ultralow temperatures.