CHANGES TO THE ELECTROLYTE IN AGED LI-ION CELLS
Electrolyte composition is crucial to the lifetime, cycling capability and safety of Li-ion cells. Many studies have measured electrolyte changes after formation or during the first few charge and discharge cycles, while very few consider long-term changes that occur to Li-ion electrolytes. This work highlights some previous research and describes rigorous electrolyte analysis methods. The development of the electrolyte extraction method, which allows for the reconstruction of electrolyte composition, is discussed and applied to cells which have been cycled for many months. The developed centrifuge and GC-MS/ICP-MS methods are presented in this work along with results from cells cycled to various upper cutoff potentials and from cells containing a low-viscosity co-solvent. Results from 𝜇-XRF transition metal analysis of the negative electrode are also presented. The findings indicate some salt loss from electrolyte in cells cycled to the higher upper cutoff potentials and minimal Mn deposition found on the negative electrode. Potential reaction mechanisms to describe the observed changes are discussed and compared to previous studies. Now that the methods for rigorous analysis of the electrolyte have been established, many additional projects have been proposed as future work.