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 Na 3 V 2 (PO 4 ) 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 Mn 3+ for V 3+ in Na 3 V 2 (PO 4 ) 3 is proposed to activate V 4+ /V 5+ redox couple for boosting energy density of the cathodes (Na 3 V 2‒ x Mn x (PO 4 ) 3 ). With the introduction of Mn 3+ into Na 3 V 2 (PO 4 ) 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 (V 2+ /V 3+ , V 3+ /V 4+ , and V 4+ /V 5+ ) 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 Na 3 V 2 (PO 4 ) 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.