NMR Methodology for Measuring Dissolved O2 and Transport in Lithium-Air Batteries.

Abstract

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 O2, 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 1H, 13C, 7Li, and 19F NMR spectroscopy, with the results showing that both the 1H, 13C, 7Li, and 19F bulk magnetic susceptibility shifts and the change in 19F relaxation times were accurate measures of dissolved O2 concentration. O2 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 O2 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 O2 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 O2 evolution was successfully quantified. Our work demonstrates the first usage of this NMR methodology to quantify O2 in LAB electrolytes, experimentally demonstrate solvation environments of O2, and detect O2 evolution in situ in a LAB flow cell.