Exchange-Mediated Transport in Battery Electrolytes: Ultrafast or Ultraslow?

:understanding the mechanisms of charge transport in batteries is important forthe rational design of new electrolyte formulations. Persistent questions about ion transportmechanisms in battery electrolytes are often framed in terms of vehicular diffusion bypersistent ion-solvent complexes versus structural diffusion through the breaking andreformation of ion-solvent contacts, i.e., solvent exchange events. Ultrafast two-dimensional(2D) IR spectroscopy can probe exchange processes directly via the evolution of the cross-peaks on picosecond time scales. However, vibrational energy transfer in the absence ofsolvent exchange gives rise to the same spectral signatures, hiding the desired processes. Weemploy 2D IR on solvent resonances of a mixture of acetonitrile isotopologues to differentiatechemical exchange and energy-transfer dynamics in a comprehensive series of Li+,Mg2+,Zn2+,Ca2+,andBa2+bis(trifluoromethylsulfonyl)imide electrolytes from the dilute to thesuperconcentrated regime. No exchange phenomena occur within at least 100 ps, regardlessof the ion identity, salt concentration, and presence of water. All of the observed spectraldynamics originate from the intermolecular energy transfer. These results place the lower experimental boundary on the ion-solventresidence times to several hundred picoseconds, much slower than previously suggested. With the help of MD simulations andconductivity measurements on the Li+and Zn2+systems, we discuss these results as a continuum of vehicular and structuralmodalities that vary with concentration and emphasize the importance of collective electrolyte motions to ion transport. Theseresults hold broadly applicable to many battery-relevant ions and solvents.

Automated summary

Understanding charge transport mechanisms in batteries is crucial for designing new electrolyte formulations. This study used ultrafast two-dimensional infrared (2D IR) spectroscopy to investigate exchange processes in Li+, Mg2+, Zn2+, Ca2+, and Ba2+ electrolytes. The results showed that the observed spectral dynamics mainly originated from intermolecular energy transfer, with no solvent exchange phenomena observed.