Probing Lithium Carbonate Formation in Trace-O-Assisted Aprotic Li-CO Batteries Using in Situ Surface-Enhanced Raman Spectroscopy.

A trace-O-assisted aprotic Li-CO battery represents a promising approach for CO recycling. However, cathode passivation and large overpotential are frequently observed for current Li-CO batteries because of the insolubility and nonconductivity of the discharge product of lithium carbonate (LiCO). Toward maximizing the energy capabilities of the Li-CO electrochemistry, it is crucially important to have a fundamental understanding of the LiCO formation mechanism in Li-CO batteries. In this report, the discharge reaction of a trace-O-assisted Li-CO battery has been interrogated with in situ surface-enhanced Raman spectroscopy. It was found that in high-donor-number (DN) solvents LiCO formation proceeds primarily via an "electrochemical solution route", with peroxodicarbonate (CO) as the key intermediate, whereas in low-DN solvents LiCO forms via a chemical reaction of LiO and CO on the cathode surface, namely, a "chemical surface route". It is notable that during discharge the trace-O acts as a "pseudo-catalyst" to activate CO 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 LiCO.