Additional Sodium Insertion into Polyanionic Cathodes for Higher‐Energy Na‐Ion Batteries

Na‐ion technology is increasingly studied as a low‐cost solution for grid storage applications. Many positive electrode materials have been reported, mainly among layered oxides and polyanionic compounds. The vanadium oxy/flurophosphate solid solution Na 3 V 2 (PO 4 ) 2 F 3‐ y O 2 y (0 ≤ y ≤ 1), in particular, has proven the ability to deliver ≈500 Wh kg ‐1 , operating on the V 3+ /V 4+ ( y = 0) or V 4+ /V 5+ redox couples ( y = 1). This paper reports here on a significant increase in specific energy by enabling sodium insertion into Na 3 V 2 (PO 4 ) 2 FO 2 to reach Na 4 V 2 (PO 4 ) 2 FO 2 upon discharge. This occurs at ≈1.6 V and increases the theoretical specific energy to 600 Wh kg −1 , rivaling that of several Li‐ion battery cathodes. This improvement is achieved by the judicious modification of the composition either as O for F substitution, or Al for V substitution, both of which disrupt Na‐ion ordering and thereby enable insertion of the 4th Na. This paper furthermore shows from operando X‐Ray Diffraction (XRD) that this energy is obtained in the cycling range Na 4 V 2 (PO 4 ) 2 FO 2 –NaV 2 (PO 4 ) 2 FO 2 with a very small overall volume change of 1.7%, which is one of the smallest volume changes for Na‐ion cathodes and which is a crucial requisite for stable long‐term cycling.