Changes to the Electrolyte in NMC640/Graphite Li-Ion Pouch Cells Tested for One Year at 85 °C

Single crystal Li[Ni 0.6 Mn 0.4 Co 0.0 ]O 2 /graphite pouch cells with electrolytes of varying lithium bis(fluorosulfonyl)imide (LiFSI) concentrations from 0.2 to 1.0 M underwent about one year of cycling between 3.0 and 3.9 V at 85 °C before reaching 80% remaining capacity. Cycling performance was shown to depend on the permeation of electrolyte solvent through pouch bag seals and on the initial concentration of salt in the electrolyte, with the 0.4 M LiFSI cells demonstrating the best capacity retention. The cells were then used both to investigate the effects of varying LiFSI concentration on electrolyte degradation after one year of testing, and to validate an updated method of electrolyte analysis using a benchtop nuclear magnetic resonance (NMR) spectrometer. Electrolyte analysis using NMR provided information about the changes in electrolyte composition post-cycling, and this method was validated by Li-ion differential thermal analysis. Minimal consumption of electrolyte salt was observed, even after one year cycling at 85 °C, which explains why cells initially made with only 0.2 M salt concentration performed well.