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/LiCoO₂ half cells as well as both graphite/LiCoO₂ and graphite/LiMn₂O₄ Li-ion cells are used to illustrate how high precision coulometry results can be used to help elucidate cell degradation mechanisms.