The Role of Electrolyte Solvent Stability and Electrolyte Impurities in the Electrooxidation of Li2O2in Li-O2Batteries

The charging process of micrometer-sized, electrically insulating Li2O2 particles in discharged Li-air battery cathodes is still poorly understood. In this work, we focused on the charging mechanism of model Li2O2/carbon pre-filled electrodes in electrolytes based on several solvents (a glyme, alkyl carbonates, and the ionic liquid (Pyr14TFSI) with and without redox mediator (LiI) or water as additives. For electrolytes that are highly reactive with lithium metal and/or lead to shuttling currents within the cell, a 2-compartment cell using a solid Li+-conducting electrolyte was employed. On the basis of oxygen evolution rates determined by online electrochemical mass spectrometry (OEMS), we formulated a mechanistic hypothesis wherein electrolyte degradation products produced at high potentials upon Li2O2 electrooxidation are either acting as redox mediators or enhance solubility of Li2O2. The presence of water during the charging process enables the solution-transfer of Li2O2 via the formation of soluble H2O2, which is electrooxidized at the electrode surface. Water is formed during the initially high charging potentials, particularly in glyme based electrolytes. Water and/or redox mediators enable the enhanced mass transport and/or charge transfer between electrode surface and electrically insulating Li2O2 particles, leading to their complete electrooxidation even without direct contact with the electronically conductive electrode matrix.