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 (C2O62-) 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.