Asymmetric sulfonamide design enabling high-voltage sodium-ion pouch cells in wide temperature

To advance toward commercial viability, sodium-ion batteries are required to operate under high cut-off voltages and low temperatures. This necessitates electrolyte designs that provide sufficient oxidative stability for high-voltage cycling while maintaining high ion mobility at low temperatures. Herein we design a non-flammable sulfonamide solvent molecule, N-ethyl-N-methyl-trifluoromethanesulfonamide, by introducing asymmetric alkyl substituents that create a geometric kink to hinder efficient crystal packing during cooling, leading to a low melting point (-86 °C). The resulting sulfonamide-based electrolyte exhibits weak ion-dipole interaction and favorable solvation structures enriched in contact ion pairs and aggregates, which promote the formation of highly stable and conductive interphases with both the positive and negative electrodes. Benefiting from these features, the sulfonamide-based electrolyte enables 1-Ah-level hard carbon||NaNi_1/3Fe_1/3Mn_1/3O₂ pouch cells to retain 69.8% and 42.3% of room-temperature capacity even at -60 °C and -70 °C, while achieving capacity retentions of 90.0% and 81.6% after 1500 and 1000 cycles at high upper cut-off voltages of 4.15 V and 4.2 V, respectively. The sulfonamide-based electrolyte also improves the high-temperature cycling stability and delays the onset and trigger of thermal runaway at the pouch-cell level. This work offers fundamental insights into solvent molecule and electrolyte design for advancing high-energy and wide-temperature sodium-ion batteries.