Understanding Full-Cell Evolution and Non-chemical Electrode Crosstalk of Li-Ion Batteries

The confined, sealed design of a commercial lithium-ion battery makes it difficult to probe and understand how the system evolves during cycling. In this work, we investigate the full-cell evolution of lithium-ion batteries using two complementary techniques, electrochemical impedance spectroscopy (EIS) and ultrasonic time-of-flight analysis, which make it possible to couple electrochemical and material property changes during cycling. It is found that there is a post-formation break-in'' period before full-cell behavior stabilizes. This period is signified by an increased swelling of the graphite anode, likely caused by side reactions, which increases the pressure within the cell. The increased pressure forces electrolyte to wet previously inactive portions of the lithium cobalt oxide cathode, lowering the cell impedance. The results demonstrate how the full-cell performance can be greatly affected by non-chemical crosstalk between the two electrodes and indicates the importance of using multiple complementary experimental techniques.