A High Precision Coulometry Study of the SEI Growth in Li/Graphite Cells

The charge and discharge endpoint capacities as well as the coulombic efficiency of Li/graphite coin cells have been examined using the high precision charger at Dalhousie University. Cells were charged and discharged at different C-rates and temperatures to observe trends in the formation of the solid electrolyte interphase (SEI) on the graphite electrode. The experiments show that time and temperature, not cycle count, are the dominant contributors to the growth of the SEI. The charge consumed by the SEI and hence the SEI thickness, increase approximately with time1/2 consistent with a process where the temperature-dependent SEI growth rate is inversely proportional to the SEI thickness. The charge consumed by the SEI is proportional to the electrode surface area and this increased consumption on high surface area electrodes continues during cycling, at least with the 1 M LiPF6 ethylene carbonate:diethyl carbonate electrolyte used.