Compressibility of Lithium Hexafluorophosphate Solutions in Two Carbonate Solvents

Speed-of-sound measurements are performed to establish how the isentropic bulk modulus K _s of the electrolyte system comprising lithium hexafluorophospate (LiPF₆) in blends of propylene carbonate (PC) and ethyl methyl carbonate (EMC) varies with salt molality m, mass fraction of PC in the PC:EMC cosolvent f, and temperature T. Bulk moduli are calculated by combining acoustic time-of-flight data between parallel walls of a liquid-filled cuvette with densitometric data for a sequence of binary and ternary salt solutions. Correlations are presented to yield K _s (m, f, T) accurately for nine compositions spanning the range m = 0-2 mol kg^-1 and f = 0-1, at temperatures T ranging from 283.15 to 313.15 K. Electrolyte compressibility varies most with solvent ratio, followed by salt content and temperature, with K _s ranging from 1 to 3 GPa. Composition-dependent acoustical properties elucidate the nature of speciation and solvation states in bulk electrolytes, and could be useful to identify the features of individual phases within solution-permeated porous electrodes.