Deijun Xiong

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 time 1/2 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 LiPF 6 ethylene carbonate:diethyl carbonate electrolyte used.

Interpreting High Precision Coulometry Results on Li-ion Cells

The High Precision Charger at Dalhousie University can be used to accurately measure the Coulombic Efficiency (CE) and the charge and discharge capacity endpoint slippages per cycle (Δ d and Δ c , respectively) of Li-ion full cells and Li/electrode half cells. If the CE is not exactly 1.0000 and the endpoints slip then this must be due to parasitic reactions between the electrode materials and the electrolyte in the cell. The various parasitic currents and charges associated with: solid electrolyte interface growth; electrolyte oxidation; transition metal dissolution and positive electrode damage are considered using a Li inventory model. The mathematical relations between the parasitic currents and the measured CE, Δ d and Δ c are derived. Example data collected on both Li/graphite, Li/LiCoO 2 half cells as well as both graphite/LiCoO 2 and graphite/LiMn 2 O 4 Li-ion cells are used to illustrate how high precision coulometry results can be used to help elucidate cell degradation mechanisms.