Unravelling asynchronous oxidation of carbon and lithium carbonate during charging in lithium-carbon dioxide battery

Lithium carbonate (Li₂CO₃) and carbon (C) play crucial roles as primary discharge products in lithium-carbon dioxide (Li-CO₂) batteries. Understanding the reversible formation and oxidation of Li₂CO₃ and C during charge-discharge cycles is essential for the cyclic performance of Li-CO₂ batteries. However, the role of the decomposition mechanisms of Li₂CO₃ and the C substrate remains debated, especially under real operating conditions. Here, we find that the discharge product C undergoes oxidation during charging, displaying non-synchronous oxidation compared to Li₂CO₃. Oxidation primarily involves C and the electrolyte in the early charging stages, producing CO₂ and CO. In the later stages, the decomposition of Li₂CO₃ predominates, producing highly reactive CO₃^·- intermediates. Interestingly, after prolonged ball milling of lithium carbonate and carbon, the C elements can be exchanged through Li₂CO₃•C composite materials. By forming Li₂CO₃•C composites, C can be oxidized synchronously during the charging. Therefore, designing a catalyst to promote the reversible formation/decomposition of Li₂CO₃•C could be vital to achieving reversible cycling in Li-CO₂ batteries.