Probing Lithium Carbonate Formation in Trace-O2-Assisted Aprotic Li-CO2 Batteries Using in Situ Surface-Enhanced Raman Spectroscopy

A trace-O₂-assisted aprotic Li-CO₂ battery represents a promising approach for CO₂ recycling. However, cathode passivation and large overpotential are frequently observed for current Li-CO₂ batteries because of the insolubility and nonconductivity of the discharge product of lithium carbonate (Li₂CO₃). Toward maximizing the energy capabilities of the Li-CO₂ electrochemistry, it is crucially important to have a fundamental understanding of the Li₂CO₃ formation mechanism in Li-CO₂ batteries. In this report, the discharge reaction of a trace-O₂-assisted Li-CO₂ battery has been interrogated with in situ surface-enhanced Raman spectroscopy. It was found that in high-donor-number (DN) solvents Li₂CO₃ formation proceeds primarily via an "electrochemical solution route", with peroxodicarbonate (C₂O₆^2-) as the key intermediate, whereas in low-DN solvents Li₂CO₃ forms via a chemical reaction of Li₂O₂ and CO₂ on the cathode surface, namely, a "chemical surface route". It is notable that during discharge the trace-O₂ acts as a "pseudo-catalyst" to activate CO₂ in high-DN solvents but not in low-DN solvents. The mechanistic study presented here will assist us in tailor-designing better electrolyte systems and enable more energetic electrochemistry operation far away from the poison of Li₂CO₃.