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We present a novel 'shifting-reference concentration-cell' method, altering the traditional protocol for measuring liquid-junction potentials by using a sequence of reference concentrations in regularly spaced intervals, rather than a fixed reference. The method, applied to solutions of lithium hexafluorophosphate (LiPF6) in propylene carbonate (PC) and ethyl methyl carbonate (EMC) at 25 °C, helps to determine thermodynamic factors more accurately, and is useful across a wider concentration range. For LiPF6:PC, good agreement with prior fixed-reference measurements is shown, and new data at low concentrations is consistent with Debye-Hückel theory. Original composition-dependent property correlations are produced for LiPF6:EMC up to 2 M, including the density and thermodynamic factor, as well as isothermal-transport properties such as transference number, conductivity, diffusivity, and viscosity. Polarization-relaxation simulations validate these correlations. For LiPF6:EMC, the low thermodynamic factor and cation/anion Stefan-Maxwell diffusivity, as well as Walden analysis, suggest that ion association dominates, even at high dilution.

We present a method to produce a novel magnesium/aluminum chloride complex (MaCC) electrolyte, involving a new promoter, CuCl 2 . The chemically formed electrolytic solution is observed to perform Mg deposition/dissolution with high coulombic efficiency. We further propose a new mechanism for MaCC synthesis, which suggests an alternative way to synthesize Mg-conductive electrolytes, by reacting metal chloride salts with nonaqueous suspensions of metallic magnesium.A set of preliminary experiments verifies that efficient Mg deposition/dissolution can be achieved from an electrolytic solution formed by direct reaction of FeCl 2 with a nonaqueous suspension of Mg powder.

Accurate models of ion transport in isothermal binary solutions require measurements of various properties, including ionic conductivity, salt diffusivity, cation transference number (t+ 0), and the thermodynamic Darken factor (χ). In this work a novel method is proposed to quantify composition-dependent values of χ(1-t+ 0 ). Recent studies of highly concentrated “solvent-in-salt” electrolytes suggest a strong variation of transference number with respect to composition. In fact t+ 0 for electrolyte systems may double in magnitude in the highly concentrated regimes.1 Previous studies have quantified the Darken factor by assuming that t+ 0 is relatively constant across the concentration range of interest. Furthermore, concentration-cell experiments typically fix arbitrary reference concentrations that may be far from the test concentration across the liquid junction. Such methods employ integral values of t+ 0; the accuracy of thermodynamic properties, and thereby numerical models, may be compromised if there are substantial concentration differences and concomitant local changes in t+ 0.2 An alternative approach to concentration-cell experiments would apply differential forms of the equations relating potential difference, composition, thermodynamic factor, and cation transference number.3 We propose a novel approach to concentration-cell experiments in which a matrix of liquid-junction potentials banded about a variable reference concentration is employed. When coupled with independent Hittorf-cell measurements of t+ 0, this 3-dimensional surface for potential difference can be numerically fitted to reliably and accurately determine differential values of the thermodynamic factor χ across the solubility range of an electrolyte. We demonstrate this method with the electrolyte lithium hexafluorophosphate in propylene carbonate (LiPF6:PC) to determine concentration-cell liquid-junction potential (U) across a wide the solubility range. Transference-number measurements from Hittorf experiments are then used to isolate χ from differential measurements of χ(1-t+ 0 ) as a function of electrolyte particle fraction (y). Suo, L., Hu, Y. S., Li, H., Armand, M. & Chen, L. A new class of Solvent-in-Salt electrolyte for high-energy rechargeable metallic lithium batteries. Nat. Commun. 4, 1–9 (2013). Lundgren, H., Scheers, J., Behm, M. & Lindbergh, G. Characterization of the Mass-Transport Phenomena in a Superconcentrated LiTFSI:Acetonitrile Electrolyte. J. Electrochem. Soc. 162, A1334–A1340 (2015). Newman, J. & Thomas-Alyea, K. E. Electrochemical Systems. (John Wiley & Sons, 2004). Figure 1

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