Valence-Modulated Na4Fe3(PO4)2(P2O7) Cathode Tuned by Orbital-Delocalization for Extreme-Temperature Sodium Storage

Abstract

Iron-based polyanionic Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is recognized as a promising cathode for sodium-ion batteries (SIBs) with its cost-effectiveness and stable framework. However, its commercialization is seriously hindered by sluggish Na⁺ kinetics, and insufficient capacity utilization. Herein, an orbital-delocalization assisted valence modulated strategy is proposed to address these challenges. The lattice is stabilized by high-valence Mo⁶⁺ through robust Mo─O bonds, simultaneously reducing Na⁺ diffusion barriers and activating the inert Na2 sites, while electron delocalization is effectively promoted by its partially filled 3d orbitals to enhance electronic conductivity. Concurrently, additional charge compensation is also provided by Mo⁴⁺ via a reversible Mo⁴⁺/Mo⁶⁺ redox couple, enabling complete Na⁺ extraction/insertion and suppression of structure distortion. A record-high discharge capacity of 130.74 mAh g⁻¹ at 0.1 C is delivered by the optimized Na₄Fe_2.91Mo_0.09(PO₄)₂(P₂O₇) cathode, with 87.23% capacity retained after 10 000 cycles at 50 C, along with stable operation from −40 to 60 °C. A universal paradigm for high-performance polyanionic cathodes is established by this synergistic reinforcement approach, advancing durable and high-power SIBs.