Unravelling the Complex Na₂CO₃ Electrochemical Process in Rechargeable Na-CO₂ Batteries

Rechargeable sodium-carbon dioxide batteries utilize CO₂ directly and use abundant and low-cost sodium instead of lithium. Sodium carbonate is an important discharge product in Na-CO₂ 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 Na₂CO₃ decomposition during the charging process follows a different pathway to lithium carbonate decomposition. It proceeds via a reactive CO₃^•− intermediate instead of a singlet oxygen intermediate, and thus CO and O₂ evolution are not identified during charging. Calculation results show that the O-O distance between two adjacent CO₃^•− in solid Na₂CO₃ is longer than that in lithium carbonate and thus forming the C₂O₆²⁻ dimer and singlet oxygen is kinetically disfavored in Na₂CO₃. Surprisingly, the carbon element in Na₂CO₃ and carbon substrate can exchange via a Na₂CO₃•C composite after ball milling. By forming the Na₂CO₃•C composite, carbon can participate in the charging process and be fully oxidized. Therefore, designing a catalyst to encourage the reversible formation/decomposition of Na₂CO₃•C might be the key to realizing the reversible cycling of Na-CO₂ batteries.