Fluorine-free acetonitrile-based electrolytes with anion-reinforced solvation chemistry for sustainable sodium-ion full batteries

Acetonitrile-based electrolytes have garnered significant attention for sodium-ion batteries due to their high ionic conductivity and favorable oxidation resistance. However, their practical implementation is hindered by an unsatisfactory anti-reduction ability, leading to poor compatibility with hard carbon anode and, consequently, rapid capacity degradation. Herein, we employ a widely used commercial carbonate ester solvent (ethyl methyl carbonate) to modulate the ion-dipole interactions in fluorine-free acetonitrile-based electrolytes. The relatively weak solvation ability of ethyl methyl carbonate facilitates more anions to participate the inner solvation sheath, thereby promoting the formation of robust electrode-electrolyte interface at a wide temperature range. This stabilized interface effectively suppresses continuous electrolyte decomposition, significantly improving the compatibility between fluorine-free acetonitrile-based electrolytes and hard carbon anodes. As a result, the optimized electrolyte enables stable operation of hard carbon ||Prussian blue full cells over a wide temperature range (25-100 °C), demonstrating a capacity retention of 62.5% after 600 cycles at 55 °C and 1.0 C. It is worth noting that Ampere-hour-level hard carbon ||Prussian blue pouch cells deliver consistent electrochemical performance at both 25 °C and 55 °C. This study elucidates the mechanistic role of cosolvent engineering in enhancing the reductive stability of fluorine-free acetonitrile-based electrolytes and provides a viable electrolyte design strategy for their practical implementation in sodium-ion batteries. Acetonitrile-based electrolytes have poor stability in sodium-ion batteries. Here, the authors introduce ethyl methyl carbonate to modulate anion-enhanced solvation, forming inorganic-rich interfaces that improve compatibility with hard-carbon anodes and enable long-term stable battery performance.