Characterization of Acetonitrile Isotopologues as Vibrational Probes of Electrolytes
Dereka, B (Dereka, Bogdan); Lewis, NHC (Lewis, Nicholas H. C.); Keim, JH (Keim, Jonathan H.); Snyder, SA (Snyder, Scott A.); Tokmakoff, A (Tokmakoff, Andrei)
Journal of Physical Chemistry B, 2022, Volume 126, pp. 278-291.
Acetonitrile has emerged as a solvent candidate for novel electrolyte formulations in metal-ion batteries and supercapacitors. It features a bright local C N stretch vibrational mode whose infrared (IR) signature is sensitive to battery-relevant cations (Li+, Mg2+, Zn2+, Ca2+) both in pure form and in the presence of water admixture across a full possible range of concentrations from the dilute to the superconcentrated regime. Stationary and time-resolved IR spectroscopy thus emerges as a natural tool to study site-specific intermolecular interactions from the solvent perspective without introducing an extrinsic probe that perturbs solution morphology and may not represent the intrinsic dynamics in these electrolytes. The metal-coordinated acetonitrile, water-separated metal-acetonitrile pair, and free solvent each have a distinct vibrational signature that allows their unambiguous differentiation. The IR band frequency of the metal-coordinated acetonitrile depends on the ion charge density. To study the ion transport dynamics, it is necessary to differentiate energy-transfer processes from structural interconversions in these electrolytes. Isotope labeling the solvent is a necessary prerequisite to separate these processes. We discuss the design principles and choice of the (CD3CN)-C-13 label and characterize its vibrational spectroscopy in these electrolytes. The Fermi resonance between C-13 = N and C-D stretches complicates the spectral response but does not prevent its effective utilization. Time-resolved two-dimensional (2D) IR spectroscopy can be performed on a mixture of acetonitrile isotopologues and much can be learned about the structural dynamics of various species in these formulations. |
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