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.