Species association – including effects such as ion pairing and solvent complexation – can complicate electrochemical transport dynamics. As a liquid electrolyte becomes more concentrated, driving forces for association become stronger and may profoundly impact system performance. Ion pairing has been proposed to explain the extended diffuse double layers observed at ionic-liquid/electrode interfaces [1]. Researchers also have justified the low or negative cation transference numbers observed for many lithium electrolytes by suggesting that ions associate [2]. These discussions motivated our systematic study of species association effects in concentrated electrolytes.

The Nernst–Planck theory has been used to study reactive multi-species liquid electrolytes [3], but it ignores solute/solute interactions, limiting applicability to dilute solutions [4]. In concentrated solutions, such interactions can alter transport dynamics substantially, even when species concentrations are relatively low. Monroe has illustrated that interactions between lithium salts and dissolved oxygen can manifest diffusion potentials that change open-circuit voltages of metal-air batteries considerably [5]. Accordingly, even the minor species produced by weak association should not be ignored in concentrated electrolytes. A multi-species concentrated-solution model considering species association will provide a more general and comprehensive interpretation of experimental transport-property data.

Monroe and Delacourt derived flux-explicit transport laws from Onsager–Stefan–Maxwell equations, and emphasized the need for thermodynamic rigor in multi-species transport characterization [6]. The present research advances their framework further, by accounting for species association. In systems were species associate, the Onsager–Stefan–Maxwell transport equations are further constrained by local association equilibria and kinetics. Thus some transport properties may be intrinsically correlated. We will present reformed flux-explicit transport equations and clarify how locally equilibrated association affects apparent transport dynamics.

Three systems, namely, doped ion-conductive ceramics, ionic liquids, and binary solutions, will be analyzed: we will investigate carrier/carrier interaction in doped solid electrolytes, and illustrate how defects or doped elements induce diffusion polarization; we will model the ionic-liquid double layer in cases where ions can combine to form a neutral pair; and we will discuss how the association of solutes affects transference-number measurements in liquid binary solutions.

[1] M.A. Gebbie, et al., PNAS. 110, 9674-9679 (2013)

[2] G. Richardson, et al., J. Electrochem. Soc., 165(9), H561-H567 (2018)

[3] S. Clark, et al., ChemSusChem,10, 4735 – 4747 (2017)

[4] A.M. Bizeray, et al., J. Electrochem. Soc., 163(8), E223-E229

[5] C. W. Monroe, J. Electrochem. Soc., 164(11), E3547-E3551 (2017)

[6] C. W. Monroe and C. Delacourt, Electrochimica Acta, 114, 649–657 (2013)