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 center dot- 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 CO3 center dot- 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 center dot C composite after ball milling. By forming the Na2CO3 center dot 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 center dot C might be the key to realizing the reversible cycling of Na-CO2 batteries.