Classifying Electrolyte Solutions by Comparing Charge and Mass Transport

Abstract

The conventional classification of electrolyte solutions as “strong” or “weak” accounts for their charge transport properties, but neglects their mass transport properties, and is not readily applicable to highly concentrated solutions. Here, we use the Onsager transport formalism in combination with linear response theory to attain a more general classification taking into account both charge and mass transport properties. To this end, we define a molar mass transport coefficient , which is related to equilibrium center‐of‐mass fluctuations of the mobile ions and which is the mass‐transport analogue of the molar ionic conductivity . Three classes of electrolyte solution are then distinguished: (i) “Strong electrolytes” with ; (ii) “weak charge transport electrolytes” with ; and (iii) “weak mass transport electrolytes” with . While classes (i) and (ii) encompass the classical “strong” and “weak” electrolytes, respectively, many highly concentrated electrolytes fall into class (iii) and thus exhibit transport properties clearly distinct from classical strong and weak electrolytes.

Mechanistic understanding of the correlation between structure and dynamics of liquid carbonate electrolytes: impact of polarization

“…39,40 On the other hand, these can readily be calculated from MD simulations, 30,[41][42][43] given sufficient statistics. Knowledge of the Onsager coefficients in turn allows one to identify whether a given electrolyte is limited by charge or mass transport, 44,45 as well as the calculation of the transference number for different boundary conditions 43 or reference frames. 46 Due to these reasons, combined experimental and numerical approaches 30,43 seem especially fruitful.…”

“…In this way, we obtain a detailed mechanistic understanding, thereby quantifying the wellknown property of EC molecules to dissociate lithium-anion pairs. Finally, we apply a concept, recently introduced by the Roling group, 44,45 to characterize the transport mechanisms and to elucidate for different compositions whether the transport is charge-limited or mass-limited.…”

Ion Dynamics in Concentrated Electrolyte Solutions: Relating Equilibrium Fluctuations of the Ions to Transport Properties in Battery Cells

“…The classification approach is substantiated by showing that both fast charge transport and fast mass transport are indispensable for fast Li + transport under anion-blocking conditions in lithium-ion batteries. Furthermore, we consider the influence of cross correlations between center-of-mass and collective translational dipole fluctuations on the transport properties, which were neglected in Roling et al [33]…”

“…Although a number of transport models for concentrated electrolyte solutions has been developed, [ 22–26 ] their classification in terms of the classical scheme “strong” or “weak” has been controversially discussed in the literature. [ 27–32 ] In a recent Frontier Outlook, [ 33 ] we have suggested a more general classification, which is based on a comparison of charge and mass transport in the electrolyte. To this end, we defined a molar mass transport coefficient , which is the mass transport analogue of the molar ionic conductivity and which, according to linear response theory, is related to center‐of‐mass fluctuations of the ions in thermal equilibrium.…”

“…To this end, we defined a molar mass transport coefficient , which is the mass transport analogue of the molar ionic conductivity and which, according to linear response theory, is related to center‐of‐mass fluctuations of the ions in thermal equilibrium. [ 33 ] Based on this, we distinguished “weak charge transport electrolytes” with from “weak mass transport electrolytes” with . In “weak charge transport electrolytes,” the strong interactions between cations and anion as compared to ion/solvent interactions leads to charge transport limitations, see Figure a.…”