Influence of the anode graphite particle size on the SEI film formation in lithium-ion cells

Electrochemical impedance spectroscopy (EIS) is used as a tool to investigate the formation process of a lithium-ion cell. The usability of EIS was demonstrated for two anode active materials with different particle sizes. The initial charging of the cell was interrupted when a defined anode-half cell potential (vs. Li/Li+) was reached, in order to measure an impedance spectrum. This was fitted with a common equivalent circuit model and the SEI film resistance (RSEI) was extracted. Results show that for both active materials RSEI initially increases until an anode half-cell potential of approximately 0.2 V is reached. Subsequently the RSEI shows a sharp decline for both active materials. The SEI film resistance is significantly higher for smaller particles, indicating that a less conductive SEI is built on smaller particles during formation.