Electrode Modification and Fluoroethy Lene Carbonate Additive Optimization for Ultra-Low Temperature Sodium Storage

Sodium-ion batteries (SIBs) are promising for large-scale energy storage owing to the low cost and abundance of sodium, but their ultra-low-temperature application is hampered by sluggish Na+ kinetics, unstable electrolyte interfaces, and severe capacity decay. In this work, Al-doped Na 3 V 2 (PO 4 ) 3 with carbon coating was prepared, and FEC additive was introduced to optimize the electrolyte interface. The modified electrodes were tested from −60 to −80°C. The average capacity decay reaches 0.3%, 0.4%, and 0.6% per cycle at −60, −70, and −80°C, respectively, indicating that the target of ≤0.3% decay per cycle below −40°C is not achieved. Nyquist analysis shows that rising charge-transfer resistance and restricted Na+ diffusion dominate the failure. This work reveals the ultra-low-temperature failure mechanism and offers guidance for designing durable low-temperature SIBs.