Solute and Solvent Structural Effects in Concentrated-Electrolyte Transport

Abstract

Super-concentration of electrolytic solutions has been hypothesized as a route to improve battery performance [1]. To predict electrolyte behavior accurately in superconcentrated systems, one must account for their fundamentally different molecular structures. Interactions between species such as ion pairs in concentrated electrolytes may impact the transport laws governing diffusion, migration, and convection, and solute-volume effects may be large because the solution volume fraction occupied by salt is extremely high. Transport phenomena relating to solute and solvent structure, such as Faradaic convection and the excluded-volume effect, can become dominant in the superconcentrated regime [2]. In very concentrated electrolytes convection can be important because when the salt occupies a large volume fraction of the solution, salt flux across the boundary of a solution drives a bulk solution velocity through overall mass continuity. Experiments demonstrate that at high concentration, viscosity no longer determines ion mobility, i.e., the Stokes–Einstein relation fails. The dynamics of ion-pair formation and salt association/dissociation kinetics both can impact the availability of charge-carrying species in an electrolyte. Experimentally, Faradaic convection can be quantified by combining densitometry with electrochemical measurements. Ion speciation and extent of dissociation can be quantified with spectroscopic techniques. Newman’s application of Onsager–Stefan–Maxwell theory to electrochemical transport considers solute/solute interactions neglected in Nernst–Planck dilute solution theory [3]. We will add to this perspective by discussing how solute-volume effects and ion-association equilibria additionally influence apparent flux-explicit transport properties such as conductivity, diffusivity and transference number [4]. [1] Yamada, Y., Wang, J., Ko, S., Watanabe, E. & Yamada, A. Advances and issues in developing salt-concentrated battery electrolytes. Nat. Energy 4, 269–280 (2019). [2] Liu, J. & Monroe, C. W. Solute-volume effects in electrolyte transport. Electrochim. Acta 135, 447–460 (2014). [3] Monroe, C. W. & Delacourt, C. Continuum transport laws for locally non-neutral concentrated electrolytes. Electrochim. Acta 114, 649–657 (2013). [4] Hou, T. & Monroe, C. W. Composition-dependent thermodynamic and mass-transport characterization of lithium hexafluorophosphate in propylene carbonate. Electrochim. Acta (2019). Figure 1