Medium‐Entropy Doping Stimulated Multielectron Redox for Ultrastable High‐Capacity Na ₃ V ₂ (PO ₄

Polyanion‐based sodium‐ion cathodes are attractive candidates for next‐generation energy storage owing to the excellent structural stability, yet the practical deployment is hindered by limited specific capacity. Here, a medium‐entropy Na superionic conductor (NASICON)‐type Na ₃ Fe _0.3 Cr _0.3 V _1.1 Ti _0.3 (PO ₄ ) ₃ (ME‐NVP) cathode is proposed to enhance redox utilization while reducing vanadium content. Medium‐entropy doping effectively activates the V ⁴⁺ /V ⁵⁺ couple, enabling high reversible electrochemical behavior over a wide voltage window of 1.5 to 4.6 V. ME‐NVP achieves a high specific capacity of 160.1 mAh g ⁻¹ at 0.1 C through the stepwise multielectron redox reactions including Ti ³⁺ /Ti ⁴⁺ , Fe ²⁺ /Fe ³⁺ , V ²⁺ /V ³⁺ , V ³⁺ /V ⁴⁺ , and V ⁴⁺ /V ⁵⁺ , delivering an energy density of 475 Wh kg ⁻¹ . More impressively, the synergistic effect among multiple transition‐metal species endows ME‐NVP with outstanding rate capability and cycling stability, maintaining 100% capacity retention after 4000 cycles at 20 C. Combined density functional theory (DFT) calculations and in situ X‐ray diffraction (XRD) analyses reveal reduced Na ⁺ migration barriers, enhanced conductivity, and solid‐solution‐type Na ⁺ storage behavior, which together accounts for the improved performance. This work establishes medium‐entropy engineering as a viable strategy for designing high‐performance multianion electrode materials.