In Situ Study of Oxygen Reduction in Dimethyl Sulfoxide (DMSO) Solution: A Fundamental Study for Development of the Lithium-Oxygen Battery

To develop a lithium-oxygen (Li-O-2) battery with an extremely high specific energy, mechanisms for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) on a flat gold electrode in the aprotic polar solvent of dimethyl sulfoxide (DMSO) has been systematically investigated by electrochemistry in combination with in situ UV-vis absorption spectroscopy, surface-enhanced Raman vibrational spectroscopy (SERS), and ex situ infrared spectroscopy. In the Li-free DMSO solution, O-2 is efficiently reduced to the superoxide, which forms an ion-pair with the tetrabutylammonium (TBA) cation and shows an excellent stability in DMSO. The adsorption of the superoxide on the gold electrode surface has been observed by an in situ SERS measurement. When the Li-ion is included in the DMSO, O-2 can be further electrochemically reduced to lithium peroxide (Li2O2) and deposited on the electrode surface, although a large amount of superoxide is still produced in the solution. The latter oxide shows a UV-vis absorption spectrum similar to that polarized in the Li-free DMSO solution, implying that Li-ions solvated by DMSO make ion-pairs with superoxide (denoted as LiO2) in solution. No evidence for the disproportionation reaction of LiO2 to Li2O2, one of the known reaction mechanisms proposed before, has been obtained in the bulk solution and on the electrode surface in the present study. The formation of Li2O2 is controlled by reaction kinetics and Li-ion diffusion. The Li2O2 thin-film is terminated at a certain film thickness on the electrode surface. On the basis of the quantitative analyses of the in situ spectroscopic observations, the partial yields for LiO2 and Li2O2 have been estimated to elucidate the mechanism for the ORR/OER processes. The present results are discussed in comparison to previous observations on porous carbon cathodes regarding the surface area, morphology, and three-phase interface on the electrode and solution interface.