The role of electrolyte solvent stability and electrolyte impurities in the electrooxidation of Li₂O₂ in Li-O₂ batteries

The charging process of micrometer-sized, electrically insulating Li ₂O₂ 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 Li ₂O₂. 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.