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.
(© 2022. The Author(s).)