Boron-Fluoride Anion-Derived Layers Facilitate Long-Cycle and High-Capacity Sodium-Ion Batteries

Ether-based electrolytes perform exceptionally well in sodium-ion batteries, but their compatibility with electrodes with a long life span remains poor. This study innovatively proposes a synergized ether electrolyte with a high-content NaBF4 salt strategy, enabling a uniform, fluoride- and boride-rich electrolyte interphase film on electrode surfaces. This interphase facilitates Na+ transport and inhibits electrolyte decomposition. Our results show that the boron-fluoride anion-derived interfacial film significantly helps the hard carbon anode to achieve a life span of over 10 months (1480 cycles) with an average Coulombic efficiency of up to 99.93%. This work provides valuable insights for designing advanced sodium-ion electrolytes and anion-derived interfacial films.