Multiple Knee Points in the Degradation of a Commercial Lithium-ion Battery: A Case Study of the NCM/Graphite System

Lithium-ion batteries are used on an increasingly large scale, making their lifetime prediction a critical issue. Especially, a rapid decrease in capacity after the mild degradation period, referred to as the knee point, is often observed, and thus understanding the knee point and its mechanism is a critical issue. Although numerous studies have been dedicated to the analysis of this phenomenon, studies on a commercial large-format lithium-ion battery are lacking. Further studies are required to continuously track changes over time and elucidate the source of knee points during operation. Herein, we conduct degradation cycle tests using a large-format commercial lithium-ion battery (> 50 Wh, LiNi0.5Co0.2Mn0.3O2/graphite) and analyze the knee points during cycling by employing electrochemical impedance spectroscopy (EIS) at different states of charge (SOCs) to track the degradation state over time, in combination with differential analysis and post-mortem methods. As a result, two knee points appear during degradation, caused by increases in resistance that are mainly derived from electrolyte depletion and Li plating at the anode. These observations are described based on the SOC dependency of the EIS results, which can be leveraged to identify the cause of knee points.