Stark Tuning Rates of Organic Carbonates Used in Electrochemical Energy Storage Devices

Lithium ion batteries frequently employ carbonate-based electrolyte solvents to support reversible lithium ion storage in response to electric fields applied to the electrode/electrolyte junction. Although these fields are critical for controlling beneficial and deleterious electrochemical reactions alike, quantifying their magnitude is a persistent challenge that inhibits our fundamental understanding of high-voltage electrochemical energy storage devices. In this study, we utilize complementary experimental techniques of vibrational Stark spectroscopy and vibrational solvatochromism in conjunction with molecular dynamics simulations to determine the vibrational sensitivity (Stark tuning rate, Δμ ⇀) of the carbonyl group (C=O) in response to an electric field for diethyl carbonate (DEC), ethylene carbonate (EC), and fluoroethylene carbonate (FEC). We first determine that the response of the C=O group in each solvent to an externally applied electric field exhibits a second derivative line shape characteristic of the linear Stark effect. We find the magnitude of this response to be unique for each carbonate solvent based on a field-frequency calibration; Δμ ⇀DEC = 0.37 cm-1/(MV/cm), Δμ ⇀EC = 0.31 cm-1/(MV/cm), and Δμ ⇀FEC = 0.57 cm-1/(MV/cm). We then leverage two electrostatic expressions to converge upon an angle-dependent equilibrium (open circuit) interfacial field for archetypal Li-ion battery electrode/electrolyte junctions. Based upon this convergence model, which depends explicitly on the dielectric function of the electrode interface and the projection of the field onto the dipole axis of the C=O group, we estimate local fields spanning approximately 30-50 MV/cm at LiCoO2 and 84-132 MV/cm at graphite interfaces. This quantitative benchmark of Δμ ⇀ for some of the most commonly used electrolyte solvents lays the groundwork for proofing future electrostatic materials design strategies, for example, by controlling electrochemical reaction dynamics using extrinsic interface modifiers.