Development of vanadium-based polyanion positive electrode active materials for high-voltage sodium-based batteries

Polyanion compounds offer a playground for designing prospective electrode active materials for sodium-ion storage due to their structural diversity and chemical variety. Here, by combining a NaVPO(4)F composition and KTiOPO(4)-type framework via a low-temperature (e.g., 190 °C) ion-exchange synthesis approach, we develop a high-capacity and high-voltage positive electrode active material. When tested in a coin cell configuration in combination with a Na metal negative electrode and a NaPF(6)-based non-aqueous electrolyte solution, this cathode active material enables a discharge capacity of 136 mAh g(−1) at 14.3 mA g(−1) with an average cell discharge voltage of about 4.0 V. Furthermore, a specific discharge capacity of 123 mAh g(−1) at 5.7 A g(−1) is also reported for the same cell configuration. Through ex situ and operando structural characterizations, we also demonstrate that the reversible Na-ion storage at the positive electrode occurs mostly via a solid-solution de/insertion mechanism.