Reversible Activation of V 4+ /V 5+ Redox Couples in NASICON Phosphate Cathodes

Abstract Na superionic conductor structured Na 3 V 2 (PO 4 ) 3 cathodes have attracted great interest due to their long cycling lifespan and high thermal stability rendered by the robust 3D framework. However, their practical application is still hindered by the high cost of raw materials and limited energy density. Herein, a doping strategy with low‐cost Fe 2+ is developed to activate V 4+ /V 5+ redox, in an attempt to increase the energy density of phosphate cathodes. It is also revealed that reversible activation of V 4+ /V 5+ redox is related to the Na positions (Na1, 6b; Na2, 18e). Only the V‐based compounds with enough Na2 content can activate the V 4+ /V 5+ reversibly. More importantly, without presodiation treatment and addition of any sodiation agent, Na 3.4 V 1.6 Fe 0.4 (PO 4 ) 3 is delicately designed as both cathode and the Na self‐compensation agent in full cells, allowing a promising energy density of ≈260 Wh kg −1 . This work sheds light on enhancing the energy density, and designing Na self‐compensation for practical Na‐ions batteries.