Carboxylate-Based Electrolyte with Bilateral Functions Enable Working Sodium-Metal Batteries at -60°C

Sluggish interfacial reaction kinetics and unstable electrode/electrolyte interphases seriously hinder the practical applications of sodium-metal batteries (SMBs) with extremely low-temperature performance. Here, we propose an ester-based electrolyte featuring solvent-anion synergistic and self-purification functions that empowers SMBs to operate stably at record-low temperatures down to -60 °C. The weakly solvating fluorinated solvent (ethyl difluoroacetate, [EDFA]) constructs an anion-dominated solvated structure, where the high-donor-number trifluoroacetate anion (TFA^-) synergistically reduces the Na⁺ desolvation energy barrier while triggering the defluorination of EDFA. This dynamic process in situ generates inorganic components (e.g., NaF) in the solid electrolyte interphase (SEI). Further, tris(trimethylsilyl) phosphite (TMSPi) additive effectively scavenges corrosive HF species through a self-purification mechanism, ensuring durable cathode-electrolyte interphase formation. As a result, the ester-based SMBs render a capacity retention of 96% at -40 °C (relative to the room-temperature capacity), exceptional rate capacity (81.64 mAh g^-1 at 400 mA g^-1 with 70.26% retention at -40 °C, which is the best rate capability at extremely low temperature compared to other ester-based SMBs), and extended cycle ability (>300 cycles at -40 °C and 100 cycles at -60 °C). This work establishes a new paradigm for developing ester-based electrolytes to address the extreme-temperature operation challenges of SMBs.