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 Na3V2(PO4)2F3-yO2y (0 ≤ y ≤ 1), in particular, has proven the ability to deliver ≈500 Wh kg-1, operating on the V3+/V4+ (y = 0) or V4+/V5+ redox couples (y = 1). This paper reports here on a significant increase in specific energy by enabling sodium insertion into Na3V2(PO4)2FO2 to reach Na4V2(PO4)2FO2 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 Na4V2(PO4)2FO2–NaV2(PO4)2FO2 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.