The importance of bond covalency for the activation of multielectron reactions in phosphate cathodes for sodium-ion batteries

Due to the uniform distribution of sodium in the crust, the increasing demand for high specific energy and long life in terms of energy storage is placing higher requests on sodium-ion battery technology. The activation of multi-electron reactions in NASICON sodium-ion battery cathode materials can not only reduce the use of vanadium, but also effectively increase the specific energy, therefore, it has received enormous attention from researchers. In this report, the importance of the covalent proportion of metal-oxygen bond in activating the V⁴⁺/V⁵⁺ redox with higher discharge voltage in Na₃V₂(PO₄)₃ has been extensively studied. Advanced characterization methods and theoretical calculations comprehensively explain the effects of Zn²⁺, Mg²⁺, and Cu²⁺ on V³⁺ substitution and the corresponding electrochemical behavior and structural change behavior. Moreover, a complete two-phase reaction in the high voltage reaction region (3.88 V) was observed in Na₃V_1.5Cu_0.5(PO₄)₃. Our results highlight the importance of the covalent proportion of metal-oxygen bonds in activating multi-electron reactions in NASICON structures.