V─O─Ti Bridge-Bond Polarity Boosts Na⁺ Migration in V-Based Fluorophosphates for Fast Sodium Storage

Vanadium-based fluorophosphates are promising high-energy-density cathodes for next-generation sodium-ion batteries (SIBs). However, the pronounced degradation of Na⁺ diffusion at high voltage hinders their fast-charging capability. Herein, a Ti⁴⁺-doping-induced bridge-bond-polarity-driven strategy is proposed to reshape the V electronic structure for facilitating V─O electron exchange. Correspondingly, voltage hysteresis, polarization, and structural distortion at high voltage are suppressed, accompanied by enhanced electronic conductivity. Meanwhile, the polar V─O─Ti bridge bonds expand the dominant one- and two-dimensional Na⁺ transport channels by inducing bridging-O displacement, further boosting Na⁺ migration capability. Therefore, Na₃V_1.92Ti_0.08O(PO₄)₂F₂ with V─O─Ti delivers high capacities of 121.8 and 62.1 mA h g^-1 at 0.1C and 50C, respectively, while maintaining excellent cycling stability. Additionally, the 0.92-Ah Na₃V_1.92Ti_0.08O(PO₄)₂F₂//HC pouch cell exhibits 80% capacity retention after 1000 cycles at 1C. At the 9 Ah scale, the pouch cell achieves a high energy density of 154 Wh kg^-1. This synergistic reinforcement strategy driven by bridge-bond polarity paves the way for sustainable high-power polyanionic SIBs.