In Situ Surface-Enhanced Infrared Spectroscopy to Identify Oxygen Reduction Products in Nonaqueous Metal-Oxygen Batteries

We report on the detection of metastable, solvated, and surface adsorbed alkali metal-oxygen (M-O2) discharge species using in situ attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). Oxygen-oxygen stretching bands (vO-O) of superoxide species formed during M-O2 battery discharge have been challenging to observe by conventional infrared (IR) techniques, and because of this, there has been limited use of IR techniques for in situ monitoring of the discharge products at the cathode in metal-O2 batteries. We explore SEIRAS technique to investigate lithium-oxygen and sodium-oxygen electrochemistry in acetonitrile (MeCN; a low Gutmann donor number solvent) as well as dimethyl sulfoxide (DMSO; a high Gutmann donor number solvent) in order to demonstrate the feasibility of our approach in the ongoing efforts toward the realization of M-O2 battery technology. In situ IR spectroscopy studies, together with a coupled-cluster method including perturbative triple excitations [CCSD(T)] calculations, establishes that certain M-O and O-O stretching bands (vM-O and vO-O) of metal superoxide and peroxide molecular species are IR active, although these vibrational modes are silent or suppressed in their crystalline forms. An in situ IR spectroscopy based approach to distinguish between "solution mediated" and "surface confined" discharge pathways in nonaqueous M-O2 batteries is demonstrated