Dual Ionic-Electronic Conduction Engineering Unlocks High-Rate Kinetics in Long-Cycling Na₄Fe₃(PO₄)₂P₂O₇ Cathodes

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) holds great potential as a promising cathode material for sodium-ion batteries. However, its practical viability remains severely constrained by the formation of electrochemically inactive maricite-NaFePO₄ during synthesis, low inherent conductivity, and intrinsically sluggish Na⁺ transport. Herein, we present a cathode featuring a cheese-like porous framework, which effectively overcomes existing limitations, enabling highly efficient dual ionic-electronic conduction. The interconnected porous architecture dramatically shortens Na⁺ diffusion pathways and facilitates rapid electrolyte infiltration, thereby accelerating ion-transport kinetics, while the mechanically resilient block morphology efficiently buffers the substantial volume fluctuations during charge-discharge cycling. Concurrently, the incorporation of Cr³⁺ markedly accelerates charge-transfer kinetics while effectively suppressing electrostatic repulsion during (de)sodiation, resulting in a reduced Na⁺ migration barrier and the activation of Na3 sites. Consequently, this design enables a highly reversible capacity of 117.9 mAh g^-1 at 0.1C together with exceptional long-term stability, retaining 90.7% capacity after 3000 cycles at 20C. This study establishes a powerful structural-engineering paradigm for the rational design and modification of high-performance sodium-ion cathode materials.