Trace NaBF ₄ Modulated Ultralow‐Concentration Ether Electrolyte for Durable High‐Voltage Sodium‐Ion Batteries

Abstract Ultralow‐concentration ether electrolytes hold great promise for cost‐effective sodium‐ion batteries (SIBs), while their inferior cycle stability under high voltages remains an awkward challenge. Herein, ultralow‐concentration diglyme (G2)‐based electrolytes with single sodium salt are found to manifest high‐rate capability when employed for high‐voltage Na ₃ (VOPO ₄ ) ₂ F (NVOPF) cathode, but their specific capacity rapidly depletes to exhaustion during long‐term cycling. To address this issue, trace NaBF ₄ (0.03 m ) as electrolyte additive is introduced, which minimally affects ion conductivity of the pristine electrolyte, yet weakens the coordination between Na ⁺ ions and G2 molecules. This allows more PF ₆ ⁻ to enter the solvation sheath of Na ⁺ ions, forming a more stable cathode electrolyte interphase and enhancing the cycle performance without sacrificing high‐rate performance (up to 20 C). As a result, the trace NaBF ₄ modulated G2‐based electrolyte enables the NVOPF cathode to cycle steadily, with a capacity retention of 94.2% over 1000 cycles at a low rate of 1 C. This work provides valuable insights into the modulation of ultralow‐concentration ether electrolytes for use in durable high‐voltage SIBs.