Salt and Solvent Activities up to the Solvate Composition for LiPF ₆ in Ethylene Carbonate and LiPF ₆ in Dimethyl Carbonate

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.