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 (Na3V2‒xMnx(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.