COMPOSITIONAL AND FUNDAMENTAL CHARACTERIZATION OF LI-ION CELL ELECTROLYTES

Abstract

The electrolyte is crucial to successful lithium ion cell operation. The composition, including the solvents and additives used will dictate how long the cell lasts. There are only a few studies that study the electrolytes from cells that have gone through a prolonged cycling sequence, even fewer that do this while focusing on specific additive combinations. This work studies a matrix of Li-ion cells with additives vinylene carbonate and ethylene sulfate to determine which additive combination is optimal for long cell lifetimes. The results are determined through a novel electrolyte extraction technique that uses NMR and GC-MS to quantify known electrolyte components and identify unknowns. The extraction method and NMR/GC-MS method are presented in this work along with the results from the tested cells. Cells with 1 or 2% VC and 1% DTD had the most stable cycling performance but a surprising amount of electrolyte salt consumption. μ-XRF transition metal analysis of the negative electrodes are also presented. The possible degradation mechanisms of electrolytes were described but the cells tested were absent of any such products. Many additional projects using the NMR/GC-MS method described are proposed as future work. Lastly, NMR was used to additionally determine some transport properties of Li-ion cell electrolytes. Results found salts in methyl acetate to have faster diffusion rates compared to a DMC solution.