Redox Mediators for Faster Lithium Peroxide Oxidation in a Lithium–Oxygen Cell: A Scanning Electrochemical Microscopy Study

The reoxidation of Li2O2 as a discharge product of a Li–O2 cell occurs with high overpotential at the three-phase boundary between the electron conductor of the positive gas-diffusion electrode, the organic electrolyte, and solid Li2O2. Indirect oxidation of Li2O2 by means of redox mediators is a way to overcome the high overpotential of this reaction, decrease the formation of reactive oxygen species, and convert the entire amount of Li2O2 formed during the discharge step. The rate constants of the redox reaction between oxidized mediators and solid Li2O2 were determined for five organic redox mediators in two glyme electrolytes and two electrolytes based on ionic liquids. Specifically, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), 1,2,4,5-tetramethoxybenzene (TMB), 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB), 2,2′-bis(1,3-dithiolylidene) (TTF), and N-methylphenothiazine (MPT) were used in electrolytes based on 1-methoxy-2-(2-methoxyethoxy)ethane (diglyme) and 1-methoxy-2-[2-[2-(2-methoxyethoxy)ethoxy]ethoxy]ethane (tetraglyme) as well as in the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Py1,4]TFSI). The suitability of the method was carefully evaluated. The highest rate constant was found for DBDMB in diglyme. TTF showed the lowest rate constant in all tested electrolytes. The rate constants of all mediators in ionic liquids are at least 1 order of magnitude lower than in the ether-based electrolytes although the viscosity of [EMIm]TFSI is comparable to that of tetraglyme. This must be considered when devising Li–O2 cells with such nonvolatile electrolytes.