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

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.