A carboxylate ester cosolvent strategy for wide-temperature sodium-ion batteries

Sodium-ion batteries (SIBs) have emerged as a promising alternative for next-generation energy storage, yet their low-temperature performance remains limited, primarily due to the high viscosity, sluggish desolvation kinetics, and unstable interfacial reactions of conventional carbonate-based electrolytes. Here, carboxylate esters are introduced as cosolvents into the carbonate electrolyte to address these challenges. Unlike short-chain ethyl acetate with poor reduction stability or long-chain propyl propionate with high viscosity and poor interfacial compatibility, ethyl propionate (EP) strikes an optimal balance between low-temperature fluidity and interfacial compatibility. The resulting EC/PC/EP electrolyte exhibits high conductivity of 0.482 mS cm −1 at −60 °C and high oxidation potential > 4.5 V, while its anion participated solvation structure enhances interfacial stability. Benefitting from these attributes, the EC/PC/EP electrolyte enables exceptional performance: the Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) electrode retains 96.2% capacity after 500 cycles at −20 °C and 76.8% after 300 cycles at −40 °C, and remains functional even at −70 °C. Moreover, 1 Ah hard carbon//NFPP pouch cell delivers outstanding wide-temperature-range performance, retaining 86.4% capacity after 3000 cycles at 25 °C and 90.2% after 1000 cycles at 60 °C, while maintaining operability at −70 °C. This work provides insights into tailoring Na + solvation through carboxylate ester modulation and offers a rational strategy for developing advanced electrolytes to enable high-performance SIBs over an exceptionally wide temperature range.