NMR Methodology for Measuring Dissolved O2 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 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.


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: Photograph of the sample prepared for parallel alignment with the applied magnetic field. The 7 mm PEEK tube was inserted into the 10 mm glass NMR tube and stuck to the glass walls.  LiTFSI in tetraglyme and triglyme using   Equation 11 from the main text, , where is the concentration

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of O 2 as a function of time, is the saturated oxygen concentration, is directly proportional to diffusivity, and is the surface area to volume ratio of the liquid.

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Details of molecular dynamics simulations: The molecular dynamics (MD) simulation took the experimental concentrations of O 2 and LiTFSI in diglyme and used them as the basis for creating a simulation box, using Packmol. 1 The number of molecules were 1 O 2 , 36 Li + , 36 TFSI and 1000 diglyme. This box was then minimised with Gromacs version 2022.4, 2 which was used for the rest of the simulations.
The OPSL-AA force field was used for the diglyme (C-H bonds used constraints), while the TFSI used the parameters from Canongia Lopes and Pádua. 3 The molecular oxygen used the parameters from Arora and Sandler. 4 This particular combination of force fields has been tested previously and found to work well by Haas et al. 5 Following the minimisation, an initial 100 ps NVT run was run with a Nosé-Hoover thermostat set to 298K with a 1 ps time constant. This was followed with a 5 ns NPT equilibration run, where the Parrinello-Rahman barostat was set to 1.01325 bar (1 atm) and a time constant of 5 ps and a compressibility of 4.5e-5. For the final NPT production run, the simulation was run for 10 ns, with data being stored every 2500 step. The simulations used a time step of 1 fs, throughout. Van der Waals and Coulombic interactions were cutoff at 1.2 nm. Following the simulation the built-in Gromacs toolchain, e.g. gmx rdf, was used to explore the structure of the electrolyte. Figure S4: Radial distribution functions for F atoms, H atoms, and Li + relative to the O 2 molecule (centre of mass). No coordination of the O 2 to the Li + is observed. Figure S5 Discharge/charge voltage profile and operando pressure measurement for a LAB (Swagelok cell). The cell was cycled at 0.1 mA/cm 2 using a 0.25M LiTFSI in diglyme electrolyte, Li metal anode, and mesoporous carbon coated carbon paper. More electrons are consumed on charge than gas is evolved compared to the ideal ratio of 2 mol eto 1 mol O 2 gas (Li 2 O 2  2Li + + 2e -+ O 2 (gas) ): 2.0 mole of electrons were consumed per mole of O 2 consumed on discharge and 2.8 mole of electrons were consumed per mole O 2 evolved on charge.
Time / h Figure S6 Subsequent discharge/charge voltage profile for the LAB cell shown in Figure 5, main text.
Calculation of the ratio of moles eto moles O 2 evolved from the operando NMR relaxometry experiment ( Figure 5 and Table 4, main text) Moles of electrons consumed measured via total charged passed: