Abstract: With the continuous advancement of industrialization, sodium-ion batteries (SIBs) need to operate in various challenging circumstances, particularly in extremely cold conditions. However, at ultra-low temperatures, the reduced ionic conductivity and sluggish Na⁺ migration of commonly carbonate-based electrolytes will inevitably lead to a sharp decrease in the capacity of SIBs. Herein, we design a carboxylate ester-based electrolyte with excellent ultra-low temperature performance by straightforward cosolvent strategy. Due to the low viscosity, melting point, and sufficient ionic conductivity of the designed electrolyte, the resulting Na||Na₃V₂(PO₄)₂O₂F can achieve the capacity retention of 96% (100 cycles at 0.1 C) at -40 °C and can also operate stably even at -50 °C. Besides, galvanostatic intermittent titration technique (GITT), ex-situ X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (TEM) tests are employed to analyze and confirm that the carboxylate ester-based electrolyte promotes robust and uniform cathode/electrolyte interface layer formation and accelerates ion diffusion kinetics, which collectively facilitates the better low-temperature performance. In addition, the assembled hard carbon||NVPOF full cells further prove the practicability of the carboxylate ester-based electrolyte at low-temperature, which delivers high discharge capacity of 108.4 and 73.0 mAh g^-1 at -25 and -40 °C. This work affords a new avenue for designing advanced low-temperature electrolytes for SIBs.

Key words: Sodium-ion batteries, Ester-based electrolyte, Ultra-low temperature, Cathode electrolyte interface, Ionic conductivity