A STUDY OF THE TRANSPORT PROPERTIES OF ELECTROLYTES FOR LI-ION BATTERIES

Abstract

Measuring the transport properties of electrolyte solutions can be an effective way to screen for candidate electrolytes for use in a given Li-ion battery application. In this thesis, ionic conductivity and viscosity are measured for a wide range of electrolytes falling under two main classes: carbonate-based electrolytes with added low-viscosity co-solvents to increase ionic conductivity for high charge rate applications, and carbonate electrolytes that do not contain ethylene carbonate (EC), which has been found to negatively affect cell lifetime at high voltage. Of the different co-solvents studied, the ester, methyl acetate (MA) gave one of the largest increases in conductivity in these electrolytes. The impact of MA on electrolyte conductivity and viscosity was studied as a function of salt concentration, solvent composition, and temperature. Removing EC from the electrolyte caused a decrease in the solution viscosity, but also a decrease in the maximum conductivity. This phenomenon was investigated further using Walden-type analyses, concluding that significant ion association occurs in solutions with low EC content. Charge-discharge data for full Li-ion cells confirm that electrolytes containing MA improve the charge rate capability of cells, and removing EC improves long-term cycling at elevated voltage. The data collected in this thesis was compared to a theoretical model of electrolyte properties, the Advanced Electrolyte Model (AEM), which was able to correctly predict the viscosity and conductivity of the different electrolytes studied in this thesis in its current form.