Mechanism for Singlet Oxygen Production in Li-Ion and Metal–Air Batteries

Singlet oxygen has emerged as a real mystery puzzling battery science, having been observed in Li–O2 and Na–O2 batteries, in conventional Li-ion batteries with NMC cathodes, and during the oxidation of Li2CO3. The formation of singlet oxygen has been directly linked to the degradation and catastrophic fade seen in these battery chemistries. While there are several proposed hypothesis for its origin, the exact mechanism for the formation of singlet oxygen remains unclear. In this Letter, we attempt to unify these findings by proposing a mechanism of singlet oxygen production in metal–air and Li-ion batteries. We show that a potential dependence of surface termination explains the onset potentials of singlet oxygen release, and in all considered cases the mechanism of singlet oxygen generation is through the chemical disproportionation of the uncoordinated superoxide anion in solution; therefore, the singlet oxygen yield is determined by the concentration of free superoxide versus alkali superoxide ion pairs in solution.