Unravelling the Complex Na ₂ CO ₃ Electrochemical Process in Rechargeable Na‐CO ₂ Batteries

Abstract Rechargeable sodium‐carbon dioxide batteries utilize CO 2 directly and use abundant and low‐cost sodium instead of lithium. Sodium carbonate is an important discharge product in Na‐CO 2 batteries and its oxidative decomposition during charging determines cell performance (i.e., overpotentials and cyclability) but the decomposition mechanism has not been addressed yet. Herein, it is found that Na 2 CO 3 decomposition during the charging process follows a different pathway to lithium carbonate decomposition. It proceeds via a reactive CO 3 •− intermediate instead of a singlet oxygen intermediate, and thus CO and O 2 evolution are not identified during charging. Calculation results show that the OO distance between two adjacent CO 3 •− in solid Na 2 CO 3 is longer than that in lithium carbonate and thus forming the C 2 O 6 2− dimer and singlet oxygen is kinetically disfavored in Na 2 CO 3 . Surprisingly, the carbon element in Na 2 CO 3 and carbon substrate can exchange via a Na 2 CO 3 •C composite after ball milling. By forming the Na 2 CO 3 •C composite, carbon can participate in the charging process and be fully oxidized. Therefore, designing a catalyst to encourage the reversible formation/decomposition of Na 2 CO 3 •C might be the key to realizing the reversible cycling of Na‐CO 2 batteries.