Gaining Control over the Mechanism of Oxygen Reduction in Organic Electrolytes: The Effect of Solvent Properties

In the present study, we employ differential electrochemical mass spectroscopy (DEMS) to investigate how additives to lithium containing organic electrolytes influence the mechanism of electrochemical oxygen reduction reaction. We find that some additives favor the formation of superoxide over the formation of lithium peroxide. We have tried to relate the influence of the additive on superoxide formation to properties of the pure solvent: Additives with a large donor number do not exert any influence. Although all additives with a large acceptor number facilitate superoxide formation, we have ascribed the effect to the polarity of the additive. We discuss in the present paper how the polarity of the additive increases solvent rearrangement energies. Following the Marcus–Hush theory this introduces an activation barrier for the second electron transfer. Therefore, electrochemical oxygen reduction stops at the superoxide stage, and larger overpotentials are required to facilitate the second electron transfer. From a broader perspective this finding is of significance as solvent effects in electrocatalysis have so far been neither well investigated nor understood.