NMR Methodology for Measuring Dissolved O and Transport in Lithium-Air Batteries.

Similar to fuel cells, poor mass transport of redox active species, such as dissolved oxygen gas, is one of the challenges faced by lithium-air batteries (LABs). Capitalizing on the paramagnetic properties of O, we used nuclear magnetic resonance (NMR) spectroscopy to measure oxygen concentration and transport in LAB electrolytes. Lithium bis(trifluoromethane) sulfonylimide (LiTFSI) in glymes or dimethyl sulfoxide (DMSO) solvents were investigated with H, C, Li, and F NMR spectroscopy, with the results showing that both the H, C, Li, and F bulk magnetic susceptibility shifts and the change in F relaxation times were accurate measures of dissolved O concentration. O saturation concentrations and diffusion coefficients were extracted that are comparable to values measured by electrochemical or pressure methods reported in the literature, highlighting the validity of this new methodology. This method also provides experimental evidence of the local O solvation environment, with results again comparable to previous literature and supported by our molecular dynamics simulations. A preliminary in situ application of our NMR methodology is demonstrated by measuring O evolution during LAB charging using LiTFSI in the glyme electrolyte. While the in situ LAB cell showed poor coulombic efficiency, since no additives were used, the O evolution was successfully quantified. Our work demonstrates the first usage of this NMR methodology to quantify O in LAB electrolytes, experimentally demonstrate solvation environments of O, and detect O evolution in situ in a LAB flow cell.