Storage Studies on Li/Graphite Cells and the Impact of So-Called SEI-Forming Electrolyte Additives

The growth of the solid electrolyte interphase on graphite electrodes during storage at different temperatures was studied using the High Precision Automated Storage System at Dalhousie University. The Li/graphite cells were precycled at 30°C before storage. The open circuit potential was automatically measured every six hours during storage and the capacity before and after storage was measured with high accuracy. The mechanism responsible for the capacity loss during storage is the growth and reformation of solid electrolyte interphase (SEI) on the graphite surface. The experimental results show that the addition of the electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) increase the capacity loss during storage compared to the control electrolyte at 30, 40, 50 and 60°C. A beneficial effect of VC was not even observed at 60°C and cells with the FEC electrolyte additive showed substantially higher capacity loss at 60°C than control cells. Storage experiments on negative electrodes can lead to a better understanding of the solid electrolyte interphase and also can be used to screen for better SEI-forming electrolyte additives.