Thermodynamic rebalancing of dissociation-coordination effect in weakly solvating electrolytes for high-performance sodium-ion batteries

The weakly solvating electrolytes (WSEs) offer a promising solution to lower desolvation barriers and construct inorganic-rich solid electrolyte interphase (SEI) for sodium-ion batteries (SIBs). However, it remains challenging for WSEs to simultaneously obtain competitive ionic conductivity and anion-rich solvation chemistry. Herein, we propose a thermodynamic rebalancing strategy by introducing dimethyl sulfite (DMS), a medium-polarity co-solvent, into a classic WSE system (1 M sodium hexafluorophosphate (NaPF₆) in fluoroethylene carbonate/methyl propionate). Such electrolyte achieves a self-optimized solvation structure that simultaneously enhances salt dissociation and maintains an anion-dominated primary solvation sheath. This unique coordination environment yields markedly improved bulk-phase ion transport kinetics. Notably, DMS selectively dissolves unstable organic components within the SEI during cycling, preserving a robust inorganic-rich interface that enables uniform sodium deposition and effectively suppresses dendrite growth at low temperatures. Consequently, Na||hard carbon (HC) half-cell delivers a high reversible capacity of 215.7 mAh g⁻¹ with 71.21% capacity retention over 1000 cycles. Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) ||HC full cell exhibits exceptional cycling stability and a high specific capacity, even at −20 °C. This work presents a promising electrolyte design that decouples the trade-off of dissociation-coordination, paving the way for high-performance SIBs.