A 4 V Na⁺ Intercalation Material in a New Na‐Ion Cathode Family

Large‐scale electrochemical energy storage is a critical factor in the development of renewable energy sources to enable their intermittent power to become dispatchable. In this context, Na‐ion batteries are seen as promising alternatives to Li‐ion batteries, but their advancement requires the discovery of new materials, their electrochemical properties, and a better understanding of structure–property relationships that underpin the electrochemistry. This study presents a new class of Na + insertion materials for Na‐ion batteries. By virtue of its moderately inductive polyanionic framework, the air and moisture stable selenite Na 2 Co 2 (SeO 3 ) 3 displays a highly suitable redox potential of ≈ 4 V versus Na/Na + based on the Co 2+ /Co 3+ couple, rendering it compatible with conventional liquid organic electrolytes. A microwave hydrothermal synthesis route is developed for the rapid synthesis of nanostructured Na 2 Co 2 (SeO 3 ) 3 and its conductive graphene oxide composite. The electrochemistry and structural evolution of Na 2 Co 2 (SeO 3 ) 3 determined on cycling the cathode in a Na battery was investigated by operando X‐ray diffraction, X‐ray photoelectron spectroscopy, and temperature dependent magnetic susceptibility measurements. These studies reveal good structural and electrochemical reversibility.