High‐Energy‐Density Cathode Achieved via the Activation of a Three‐Electron Reaction in Sodium Manganese Vanadium Phosphate for Sodium‐Ion Batteries
Yuxiang Chen, Qingping Li, Peng Wang, Xiangyue Liao, Ji Chen, Xiaoqin Zhang, Qiaoji Zheng, Dunmin Lin, Kwok Ho Lam
Abstract
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.