Electrochemical Characterization and Temperature Dependency of Mass-Transport Properties of LiPF₆in EC:DEC

Mass transport in the electrolyte is one of the limiting processes when it comes to the power density and energy efficiency of lithium-ion batteries. Electrolyte characterizations are therefore of utmost importance. This study reports the ionic conductivity, diffusion coefficient, lithium-ion transport number, and thermodynamic enhancement factor, as well as density and viscosity, for the electrolyte LiPF 6 in EC:DEC (1:1, by weight) at 10°C, 25°C, and 40°C and for concentrations between 0.5 M and 1.5 M. By combining mathematical modeling and three experiments: conductivity measurements, concentration cells, and galvanostatic polarizations, the mass transport phenomena were fully characterized. All parameters were found to vary strongly with both concentration and temperature proving that temperature dependent parameters are essential when studying thermal behavior of lithium-ion batteries. Moreover, conductivity increased with temperature and showed a local maximum at around 1 M within the concentration range at all temperatures. The other parameters either showed a continuous decrease (diffusion coefficient and lithium-ion transport number) or increase (thermodynamic enhancement factor) with concentration at all temperatures. Limited liquid range leading to solvent crystallization at 10°C leads to very poor performance, possibly due to the strong coordination between the lithium ion and the crystallizing species, EC. Overall, the studied electrolyte is found to perform poorly compared to previously studied systems.