Compressibility of Lithium Hexafluorophosphate Solutions in Two Carbonate Solvents

Speed-of-sound measurements are performed to establish how the isentropic bulk modulus Ks 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 Ks (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 Ks 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.

Automated summary

Speed-of-sound measurements were conducted to investigate the changes of isentropic bulk modulus Ks in an electrolyte system composed of lithium hexafluorophospate (LiPF6) in blends of propylene carbonate (PC) and ethyl methyl carbonate (EMC). The Ks values were accurately determined for nine different compositions, including salt molality m, mass fraction of PC in the PC:EMC cosolvent f, and temperature T. The results showed that the compressibility of the electrolyte varied with the solvent ratio, salt content, and temperature, with Ks ranging from 1 to 3 GPa. The acoustical properties related to the composition provided insights into the speciation and solvation states in the bulk electrolytes.