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 with1H,13C,7Li, and19F NMR spectroscopy, with
the results showing that both the1H,13C,7Li, and19F bulk magnetic susceptibility shifts
and the change in19F relaxation times were accurate measures
of dissolved O2concentration. O2saturation
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 O2solvation 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 O2evolution 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 O2evolution
was successfully quantified. Our work demonstrates the first usage
of this NMR methodology to quantify O2in LAB electrolytes,
experimentally demonstrate solvation environments of O2, and detect O2evolution in situ in a LAB flow cell.