Orchestrating Dynamic Ion Pairs via Co-Solvent Engineering for High-Voltage Sodium-Ion Batteries

Elevating cathode operating voltages represents a definitive and efficient strategy to boost the energy density of sodium-ion batteries (SIBs), yet high-potential interfacial instability and electrolyte oxidation severely hinder its practical application. Inorganic salt additives have proven effective in constructing a protective cathode-electrolyte interphase (CEI) layer, but their applicability is fundamentally constrained by an intrinsic solvation-structure trade-off: solvent-separated ion pairs (SIPs) guarantee fast ionic transportation but fail to support robust interfacial CEI formation, whereas contact ion pairs (CIPs) favor CEI construction but suffer from sluggish ion migration. Herein, we transcend this impasse by proposing a co-solvent engineering strategy to construct dynamic ion pairs (DIPs). By integrating NaPO₂F₂ into a tailored triethylene glycol dimethyl ether/tetrahydrofuran binary solvent, we establish a reversible ligand-exchange equilibrium that dynamically shuttles between dissociated and associated states. This DIP configuration harnesses strong Na⁺-PO₂F₂ ^- association to drive the growth of an inorganic-rich CEI, while simultaneously ensuring rapid anion migration. The molecular-level modulation significantly reduces Na⁺ desolvation energy and achieves a high anodic stability. Consequently, the Na||NFM cell retains 91.77% capacity after 1000 high-voltage cycles. This work highlights the critical role of dynamic ion-pair regulation and provides a universal electrolyte design principle for high-energy, long-lifespan SIBs.