Properties of Ion Complexes and Their Impact on Charge Transport in Organic Solvent-Based Electrolyte Solutions for Lithium Batteries: Insights from a Theoretical Perspective

Abstract

Electrolyte formulations in standard lithium ion and lithium metal batteries are complex mixtures of various components. In this article, we review molecular key principles of ion complexes in multicomponent electrolyte solutions in regards of their influence on charge transport mechanisms. We outline basic concepts for the description of ion–solvent and ion–ion interactions, which can be used to rationalize recent experimental and numerical findings concerning modern electrolyte formulations. Furthermore, we discuss benefits and drawbacks of empirical concepts in comparison to molecular theories of solution for a more refined understanding of ion behavior in organic solvents. The outcomes of our discussion provide a rational for beneficial properties of ions, solvent, co-solvent and additive molecules, and highlight possible routes for further improvement of novel electrolyte solutions.

Theoretical and Computational Insight into Solvent and Specific Ion Effects for Polyelectrolytes: The Importance of Local Molecular Interactions

“…All studies reveal significant ordering effects when compared to bulk phase due to the polarity of water molecules. Hence, and in agreement with experimental findings, a reasonable assumption for highly polar solvents like water due to charge-induced ordering effects reads M 2 trans (r) < M 2 trans which reveals a decreased local dielectric constant in presence of charged species when compared to pure solvents [34,50]. In consequence, the local dielectric constant is lower when compared to the bulk dielectric constant in accordance with r (r) < , which means that electrostatic effects become more pronounced for polar solvents in close distance to charged interfaces.…”

“…In contrast to previous mean field considerations as discussed in the last section, it was shown that specifically the molecular interactions between the polyelectrolyte, the ions as well as the solvent molecules play a crucial role [34]. In this section, we will concentrate on the influence of solvent-related contributions in more detail.…”

“…The value of the Bjerrum length corresponds to the distance between the charges, where electrostatic interactions are of comparable magnitude as the thermal energy k B T. In accordance with the theory [44], strong counterion condensation sets in for values ξ ≥ 1, a condition which is met for polyelectrolytes with small b and solvents with large λ B (Equation (6)). Thus, even at very dilute or vanishing salt concentrations, electrostatic interactions between the ions can be ignored for distances r ≥ λ B , as induced by dielectric screening mechanisms of the surrounding solvent molecules [34,50]. An explicit expression for the number of condensed counterions can be derived as follows [44].…”

“…In terms of a phenomenological explanation for dielectric decrement effects, the electrostatic field around the ions E(r) perturbs the relaxation of the surrounding solvent molecules in terms of multipole interactions according to [34,50] r (r) = r + βE(r) 2 (18) where β < 0 denotes a solvent-and concentration-specific coupling constant. Beyond a critical salt concentration, it can be shown that all solvent molecules are part of the first or second solvation shell around the ions which rationalizes global changes in the bulk dielectric constant as discussed in Refs.…”

“…The molecular arrangement of the solvent molecules is influenced by the presence of the polyelectrolyte as manifested by local variations of the dielectric constant, charge hydration asymmetry effects or modified charge transport mechanisms [26][27][28]. Comparable molecular interactions also dominate the occurrence of various polyelectrolyte structures in bulk phase, as can be seen by the formation of polyelectrolyte micelles, pearl-necklace structures, the onset of microphase separation processes between polar and apolar regions as well as the formation of polyelectrolyte complexes [29][30][31][32][33][34] and coacervates [35]. With regard to the broad plethora of observations and applications, it becomes clear that a more fundamental understanding of local interactions and short-range effects on polyelectrolytes is of urgent need.…”