Activating three electron reaction in sodium vanadium ferric phosphate toward high energy density for Na-ion batteries

Na3V2(PO4)3 (NVP) with a robust sodium superionic conductor (NASICON) structure and high voltage platform has been regarded as one of the most prospective cathode materials for sodium-ion batteries (SIBs). Nevertheless, the increasing demand for energy density encourages us to activate multielectron reactions in NVP-based materials to achieve higher voltage/capacity to meet the challenge of commercialization of SIBs. Herein, NASICON-structured Na3V2−xFex(PO4)3 [NVP-Fe(x)] has been developed through a solid state reaction. The introduction of Fe3+ successfully activates high-voltage-platform redox couple of V4+/V5+ in Na3V2−xFex(PO4)3 and thus achieves three-electron redox reactions, resulting in significantly enhanced energy density (417.3 Wh kg−1) and superhigh initial capacity (166.4 mA h g−1 at 1 C). This work excites the generation of three-electron redox reactions in Fe3+-doped NVP for the first time, providing an important guidance for designing high-energy-density polyanionic materials for sodium storage by inducing multielectron redox reactions.