Anion/Cation Solvation Engineering for a Ternary Low‐Concentration Electrolyte toward High‐Voltage and Long‐Life Sodium‐Ion Batteries

Abstract High‐voltage sodium‐ion batteries (SIBs) are one of the most promising energy storage technologies for abundant resources and cost‐efficiency. However, their low energy density compared with lithium‐ion batteries (LIBs) hinders their practical applications. The high reactivity of high‐voltage cathodes, the primary factor, leads to deterioration of electrode/electrolyte interphase. Herein, a novel anion/cation solvation strategy is innovatively proposed for a ternary low‐concentration electrolyte that tackles the critical bottleneck of unstable electrode/electrolyte interphase. Especially, the intermolecular interaction within as‐designed electrolyte is remodeled by weakly polar fluorinated co‐solvent (ethoxy(pentafluoro)cyclotriphosphazene, PFPN) compared with traditional carbonate‐based electrolytes. PFPN can not only stabilize propylene carbonate (PC), reduce the interaction of Na⁺‐PC, ClO₄⁻, accelerating Na⁺ desolvation, but also weaken the interaction of anions to form stable organic/inorganic composite cathode electrolyte interphase (CEI). In this work, the ionic conductivity of low‐concentration electrolyte increases to 6.12 mS cm⁻¹ and the oxidation stability is successfully extended to 4.84 V. And Na₃V₂(PO₄)₂F₃ (NVPF)||Na half‐cells present excellent cycling performance with average coulombic efficiency of 99.5% after 2000 cycles at 4.5 V and NVPF||hard carbon (HC) full cells with relatively high energy density (≈450 Wh kg⁻¹) competitive to commercial LIBs, which are expected to be coupled with higher voltage cathodes to achieve higher energy density in future.