Trace NaBF4 Modulated Ultralow-Concentration Ether Electrolyte for Durable High-Voltage Sodium-Ion Batteries

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 Na₃(VOPO₄)₂F (NVOPF) cathode, but their specific capacity rapidly depletes to exhaustion during long-term cycling. To address this issue, trace NaBF₄ (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 PF₆⁻ 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 NaBF₄ 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.