Abstract
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 CD313CN label and characterize its vibrational spectroscopy in these electrolytes. The Fermi resonance between 13C≡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.
Highlights
Sustainable energy is one of the most urgent needs in modern society
The acetonitrile molecule features a prominent local C N stretching vibrational mode that presents an opportunity to use IR spectroscopy to study its interactions with electrolyte salts from the solvent perspective without introducing any extrinsic reporters
Using the acetonitrile C N spectroscopy in electrolytes is complicated by the Fermi resonance (FR) between this mode and nearby dark transitions
Summary
Sustainable energy is one of the most urgent needs in modern society. Especially Li-ion batteries (LIB), have been at the forefront of the efforts to meet these challenges. The current state of LIB has almost reached its limit in energy density, and future innovations are needed to ramp up the capacity, safety, rate performance, electrochemical stability, lifespan, and reduce the cost of future batteries.[3] Combined with the limited and costly supply of the raw materials for LIBs,[4,5] as well as ethical concerns related to their mining,[6] trailblazing metal-ion battery technologies emerge based on abundant and cheap metal ions serving as charge carriers, such as Zn2+,7 Mg2+,8 and Ca2+.9. Combined with the limited and costly supply of the raw materials for LIBs,[4,5] as well as ethical concerns related to their mining,[6] trailblazing metal-ion battery technologies emerge based on abundant and cheap metal ions serving as charge carriers, such as Zn2+,7 Mg2+,8 and Ca2+.9 The path beyond Li ions dwells on the development of new chemistries for electrolyte formulations: new salts,[10] additives,[3,10] revisited or exotic solvents,[3,11] and exploration of unusual concentration regimes, such as superconcentration.[12−14] Understanding the microscopic structure and dynamics occurring on a wide range of time scales in these novel electrolytes is a prerequisite for revealing the functional correlations between the formulation content and its performance and will aid in the data-driven design of future electrolytes
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