Publications

The phase stability of liquid electrolytes for lithium-ion batteries is often a limiting factor for their operation, particularly under low temperature conditions and near the solubility limits. Despite the central role of the thermodynamic activities of nonaqueous electrolyte components in governing phase behavior, they generally remain poorly investigated in concentrated regimes. In the current work, we investigate concentrated electrolytes via the study of the thermodynamic activities of species in solution up to the first solid solvate composition for two binary lithium-ion battery carbonate electrolytes: LiPF6 in ethylene carbonate and LiPF6 in dimethyl carbonate. The enthalpies of fusion of the relevant solid solvates (EC)4LiPF6 and (DMC)3LiPF6 were measured to determine the activities of the species from reported solid−liquid equilibrium data up to the first solvate composition. For LiPF6 in ethylene carbonate, we find that deviations from ideality continue to increase with concentration beyond the dilute limit up to 3.54 m. For LiPF6 in dimethyl carbonate, we find that the salt activity coefficient continues to decrease beyond the dilute limit to moderate concentrations before increasing monotonically until the solvate composition. Our approach taken herein to use binary activity data in the study of liquidus lines and solution energetics will aid in the study of practical ternary Li-ion battery electrolytes, for which thermal stability is important but generally unresolved.

The activities of individual solvent species in multi-solvent ternary liquid electrolytes dictate liquid-solid equilibria and are pertinent to transport phenomena. Nonetheless, considerable scholarship has assumed that such electrolytes can be modeled as single-solvent electrolytes. Relaxing this assumption, we show in the present work that knowledge of the activity of ionic species, described by the salt thermodynamic factor and the transfer activity coefficients at infinite dilution, is sufficient to calculate activity changes of individual solvent species for ternary multi-solvent electrolytes. We also propose measurements of an individual solvent species activity as a method to study (ionic) solvent transfer energies. We apply the derived relationships to the well-characterized liquid electrolyte system LiCl in H 2 O-EtOH. We also study a non-aqueous electrolyte solution, LiPF 6 in EC-EMC, which is important for lithium-ion battery technology. For the latter, we show that in the studied composition space there are non-negligible transfer activity coefficients, highlighting the importance of a multisolvent description of the ternary electrolytes.

One of the current efforts of Li-ion battery technology research is to enable the operation of lithium-ion batteries in low temperature environments (<0 °C). However, the ternary phase diagrams of liquid electrolytes commonly used in lithium-ion cells are generally unknown. We herein study the liquidus surface of a ternary liquid electrolyte up to 1 m LiPF6 in ethylene carbonate and ethyl methyl carbonate. We find that literature electrochemical and thermodynamic measurements of the composing binary electrolytes, in addition to limited ternary liquidus data, can be used to recover the liquidus surface via appropriate mixing terms.

Accessing the energy density and sustainability of calcium metal batteries requires mastering reversible calcium electrodeposition through electrolyte design. Several electrolytes support reversible, ambient temperature deposition but at utilization and rate too low for practical applications. These challenges stem from solvation structures characterized by either high barriers for cation desolvation or thermodynamic instability, leading to parasitic decomposition of the salt and solvent. The optimal solvation structure for the effective delivery of calcium to the electrode interface is not known. In this work, we show that adding a relatively small amount of a weakly associating calcium salt (calcium carba-closo-dodecaborate) to an otherwise strongly associated solution (calcium borohydride in tetrahydrofuran) produces a surprising population of fully solvent-coordinated Ca2+ cations in the form of solvent-separated ion pairs (SSIPs). We further demonstrate that the formation of these SSIPs beneficially impacts the kinetics and thermodynamics of calcium electrodeposition, revealing the unexpected finding that direct coordination of Ca2+ by the BH4− anion limits the electrodeposition process. These findings reveal how the competition between solvent and anion coordination to Ca2+ affects calcium deposition kinetics and cycling stability, setting the stage for a new calcium electrolyte design based on mixed anion electrolytes.