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. Batteries designed for electric vehicles must be able to provide constant, reliable performance for 10–15 years. Variations in performance at any point in a battery's life make it difficult for vehicle manufacturers to provide accurate diagnostics (e.g., remaining range and required charge time). In this work, we show that as-received batteries, which have already undergone a cycling protocol from the manufacturer (known as the formation process), can still undergo a post-formation break-in period. The break-in period is signified by an initial, rapid evolution of the physical and electrochemical properties before reaching stable values. This work demonstrates that, even after an initial cycling protocol, periods of rapid property change can still exist in a battery. Such periods could be important for understanding how electric vehicles operate after transition points in their use (i.e., after long periods of disuse or changes in the use profile). This study investigates the evolution of material and electrochemical properties in commercial lithium-ion batteries during cycling. Results indicate that as-received batteries undergo a post-formation break-in period, which is signified by an initial, rapid evolution of the battery's properties before stabilizing. Break-in corresponds to non-chemical crosstalk, whereby physical changes in the negative electrode affect the electrochemical performance of the positive electrode. These findings demonstrate how interplay between components during early cycles can affect the future battery performance.