High‐Energy‐Density Cathode Achieved via the Activation of a Three‐Electron Reaction in Sodium Manganese Vanadium Phosphate for Sodium‐Ion Batteries

Abstract

Sodium superionic conductor (NASICON)‐type Na₃V₂(PO₄)₃ has attracted considerable interest owing to its stable three‐dimensional framework and high operating voltage; however, it suffers from a low‐energy density due to the poor intrinsic electronic conductivity and limited redox couples. Herein, the partial substitution of Mn³⁺ for V³⁺ in Na₃V₂(PO₄)₃ is proposed to activate V⁴⁺/V⁵⁺ redox couple for boosting energy density of the cathodes (Na₃V_2‒xMn_x(PO₄)₃). With the introduction of Mn³⁺ into Na₃V₂(PO₄)₃, the band gap is significantly reduced by 1.406 eV and thus the electronic conductivity is greatly enhanced. The successive conversions of four stable oxidation states (V²⁺/V³⁺, V³⁺/V⁴⁺, and V⁴⁺/V⁵⁺) are also successfully achieved in the voltage window of 1.4–4.0 V, corresponding to three electrons involved in the reversible reaction. Consequently, the cathode with x = 0.5 exhibits a high reversible discharge capacity of 170.9 mAh g⁻¹ at 0.5 C with an ultrahigh energy density of 577 Wh kg⁻¹. Ex‐situ x‐ray diffraction (XRD) analysis reveals that the sodium‐storage mechanism for Mn‐doped Na₃V₂(PO₄)₃ consists of single‐phase and bi‐phase reactions. This work deepens the understanding of the activation of reversible three‐electron reaction in NASICON‐structured polyanionic phosphates and provides a feasible strategy to develop high‐energy‐density cathodes for sodium‐ion batteries.