Anion Chemistry: Structure, Electrochemistry and Stability of NASICON Cathodes

The development of high-energy-density sodium-ion batteries places stringent requirements on cathode materials to simultaneously achieve high operating voltages, rapid Na⁺ ions transport, and long-term structural and interfacial stability. Polyanionic NASICON-type frameworks have emerged as a compelling cathode platform due to their robust three-dimensional Na⁺ ions diffusion networks, strong inductive effects, and excellent thermal stability. Within this materials family, fluorophosphate NASICON cathodes offer elevated redox potentials, while targeted anion chemistry modulation provides an effective strategy to tune electronic structure, ion migration, and interfacial reactivity. Representative vanadium-based fluorophosphate cathodes, Na₃V₂(PO₄)₂F₃ (NVPF) and Na₃V₂O₂(PO₄)₂F (NVOPF), exhibit closely related crystal frameworks yet display distinct sodium storage behavior and degradation characteristics under high-voltage operation. In NVOPF, partial substitution of F^- by O^2- enhances electronic conductivity and Na⁺ ions transport kinetics, while NVPF maintains a higher redox potential associated with the V³⁺/V⁴⁺ couple. This review critically compares NVPF- and NVOPF-based cathodes in terms of crystal structure, sodium storage mechanism, synthesis and modification strategies, and high-voltage cathode/electrolyte interfacial stability. By correlating structural chemistry with electrochemical and interfacial evolution, this work provides general insights and design guidelines for high-voltage NASICON-type cathodes in sodium-ion batteries.