The Interplay between Salt Association and the Dielectric Properties of Low Permittivity Electrolytes: The Case of LiPF₆ and LiAsF₆ in Dimethyl Carbonate

Abstract

In this article, we present evidence that the dielectric constant of an electrolyte solution can be effectively used to infer the association regime of the salt species from computational methods. As case studies, we consider the low dielectric constant solvent dimethyl carbonate with LiAsF6 and LiPF6 salts at low concentrations. Using both quantum “ab initio” methods as well classical molecular dynamics simulations, we elucidate the salt’s contribution to the dielectric constant as well as the dipolar relaxation times, which act as quantitative signatures. By comparing to previously published measurements, we provide strong evidence for the presence of contact-ion pairs at these low concentrations. Interestingly, these ion pairs increase the dielectric constant of the solution, allowing for significantly improved ionic conductivity as a function of salt concentrations. We also discuss the role of multimeric equilibrium species as contributors to the functional properties of designer electrolytes, such as dielectric properties of the solution and ionic conductivity.

The critical role of configurational flexibility in facilitating reversible reactive metal deposition from borohydride solutions

“…Molecular dynamics (MD) simulations were carried out with the GROMACS software, 26 and initial configurations were prepared using Packmol. 27 Forcefield parameters for THF were taken from Spoel et al, 28 Ca 2+ parameters from the standard OPLS library, and BH4parameters from Rajput et al [29][30][31] The methodology used to calculate permittivity change from individual salt species follows Self et al 32 Further details of the MD simulations can be found in the supplementary information document. Quantum chemistry calculations were undertaken with Gaussian 16 software.…”

“…long-range electrostatics), may affect the various cluster equilibria differently, which we plan to investigate in upcoming work. To test whether the presence of dipolar clusters could facilitate formation of ionic species at high concentration through permittivity increases, as previously observed in other strongly associating electrolytes, 32,[40][41][44][45][46] we conducted simulations to determine the influence of several proposed clusters on the solution dielectric constant (supplementary information, Figure S6). The computed lowest free energy electrolyte structure, THF3Ca(BH4)2, possesses a bent geometry and a calculated dipole moment of 5 Debye.…”

An Atomic Insight into the Chemical Origin and Variation of the Dielectric Constant in Liquid Electrolytes

“…In experiments,t he dielectric constant at magnitude of million or billion Hz is used to represent the static dielectric constant of electrolytes and the dissociated ion pairs are no longer an unreasonable contribution to e. [17] Therefore,only contact ion pairs (CIPs) and solvents are considered when calculating the dielectric constant of electrolytes through formula S1. [18] On the other hand, cations will be solvated by solvents.P ositive charges carried by spherical cations can induce the centripetal orientation of solvent dipoles,f urther influencing the dielectric constant.…”

Ion Association Constants for Lithium Ion Battery Electrolytes from First-Principles Quantum Chemistry

“…66,67 Another example of binary salt systems for LIBs is Li + electrolytes with added CsPF 6 or CsTFSI, which have helped in passivation phenomena. [68][69][70] In low permittivity electrolytes, where polar associated salt species can help increase solution permittivity via polar contact-ion pairs, 3,5,9 the addition of a preferentially associated salt may be useful, as long as formation of larger but less polar aggregates (e.g. quadrupolar dimers) is minimized.…”

“…Conventional wisdom holds that ion association is deleterious as it reduces the conductivity of LIB electrolytes. 1,2 However, newly emerging LIB electrolyte formulations, such as those described as low permittivity [3][4][5][6] or superconcentrated, 7,8 are promising alternative candidates. In these active research areas, associated salt plays unconventional roles.…”