We have employed the rotating ring disk electrode (RRDE) technique to study the oxygen reduction reaction (ORR) on gold and glassy carbon cathodes in dimethyl sulfoxide (DMSO) electrolytes containing lithium salts. At the gold ring electrode at 3.0 V vs. Li/Li + (0.1 M LiPF 6 ) soluble superoxide radical anion undergoes oxidation to O 2 under convective-diffusion conditions. For both glassy carbon and gold cathodes, typical oxygen reduction current-potential curves are sensitive to rotation speed and undergo a maximum and further electrode passivation by formation of Li 2 O 2 while the Au ring electrode currents follow the same peak shape with detection of soluble superoxide at the ring downstream in the electrolyte solution. Unlike the behavior in acetonitrile-lithium solutions, LiO 2 is more stable in DMSO and can diffuse out in solution and be detected at the ring electrode. While in cyclic voltammetry both time and potential effects are convoluted, we have carried out RRDE chrono-amperometry experiments at the disk electrode with detection of superoxide at the Au ring so that thus potential and time effects were clearly separated. The superoxide oxidation ring currents exhibit a maximum at 2.2 V due to the interplay of O 2 − formation by one-electron O 2 reduction, Li 2 O 2 disproportionation and two-electron O 2 reduction.

The O2/Li2O2 electrode reaction has been studied on low surface area Au electrodes in three solvent-electrolyte pairs (0.1 M LiPF6/DMSO, LiPF6/ACN, and LiBF4/ACN) using an electrochemical cell coupled to UHV XPS spectrometer, EQCM, AFM, and DEMS. The XPS spectra of the surfaces after treatment at selected electrode potentials for the O2 reduction and reoxidation of the surface show the presence of C and S from solvent decomposition and of F and P from electrolyte decomposition. Furthermore, Li 1s and O 1s peaks due to Li2O2 and decomposition products such as carbonate, organics, LiF, high oxidation sulfur, and phosphorus compounds were also observed. Using ACN instead of DMSO results in less solvent decomposition, whereas using LiBF4 results in less electrolyte decomposition. XPS, AFM, and EQCM show that O2 reduction products removal only takes place at very high overpotentials. In agreement with XPS which shows removal of carbonate surface species, DEMS confirms evolution of CO2 and consumption of O2 at 4.5 V, but LiF cannot be removed completely in a round trip of the Li-O2 battery cathode.

In situ infrared subtractive normalized Fourier transform infrared spectroscopy (SNIFTIRS) experiments were performed simultaneously with the electrochemical experiments relevant to Li-air battery operation on gold cathodes in ionic liquid PYR14TFSI based electrolyte. Ionic liquid anion was found to be stable, while the cation PYR14+ was found to decompose in studied conditions. In oxygen saturated LiTFSI containing PYR14TFSI electrolyte carbon dioxide and water were formed at potential 4.3 V either with or without previous oxygen electro-reduction reaction. However in deoxygenated LiTFSI contacting ionic liquid no formation of CO2 or water was observed, suggesting oxygen presence to be crucial in carbon dioxide production.

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We examine for the first time the ability of inert carbon free-radicals as soluble redox mediators to catalyze and enhance the oxygen reduction reaction in a (TEGDME)-based electrolyte. We demonstrate that the tris(2,4,6-trichlorophenyl)methyl (TTM) radical is capable of chemically favoring the oxygen reduction reaction improving significantly the Li-O2 battery performance.

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Ex situ atomic force microscopy (AFM) has been used to study the morphology of oxygen reduction products in the LiPF6-dimethyl sulfoxide (DMSO) electrolyte, i.e. Li2O2 on a highly oriented pyrolytic graphite (HOPG) surface. Both cyclic voltammetry and chronoamperometry have shown that at low cathodic polarization the initial deposits decorate the edge steps of HOPG. At higher overpotentials a massive deposit covers the terraces. Upon charging the battery cathode Li2O2 oxidation and dissolution do not take place until high overpotentials are reached at which solvent decomposition has been demonstrated by in situ FTIR studies.