(Invited) Electrochemical and Postmortem Analyses of 18650-Type Li-Ion Secondary Battery Degraded at High and Low Temperature Environments

Abstract

Lithium-ion secondary batteries have been widely used as small-size and lightweight energy sources for applications of laptops, robots and electric vehicles. For realizing a high performance, the energy sources are required to have high energy density with high reliability. The batteries are charged and discharged repeatedly based on anode and cathode reactions, which both accompany side reactions to a greater or less extent. The battery degradation phenomena are well explained by kinetics of chemical reactions evaluated by non-destructive examination.1 To understand the battery degradation mechanism for developing a durable one, it is indispensable to clarify the side reactions occurred to cause the degradation. For that purpose, differential capacity curves (dQ/dV vs. V) and electrochemical impedance spectroscopy (EIS) were employed as electrochemical measurements.2 To support the mechanism estimated by the measurements, postmortem analyses were conducted using SEM, XPS and XRD. These investigations were performed to the commercially available 18650 cell consisting of Li(Ni, M)O2 (M: metal elements to partially replace Ni) as the cathode material and graphite as the anode material. The employed loads were calendar deterioration and charge/discharge cycle deterioration.3,4 Firstly, the calendar deterioration behavior of the 18650-type battery was evaluated by storing them at 80 °C for different durations. The results indicate that the battery capacity decreased with the increasing number of storage days in a high-temperature environment. From the differential capacity curves, we found that the change in the overvoltage, which was caused by the electrode reaction, that appeared at approximately 4.2 V was most significant. To analyze this phenomenon, EIS of the deteriorated battery was measured at 4.2 V, and it was confirmed that the cathode-based resistance of the battery increased significantly at this potential. Furthermore, postmortem analysis revealed that the cathode active material of the tested batteries clearly deteriorated; however, no significant change in the active material in the anode was observed. Therefore, it is considered that the cathode materials of nickel-based lithium-ion secondary batteries deteriorate by storing at 80 °C, thereby resulting in reduced battery capacity and increased cathode component resistance after storing at high temperatures.Secondly, cycle deterioration behaviors of the lithium-ion batteries in different operating temperature were studied. Compared with the deterioration in the high-temperature range, the degree of decrease in battery capacity at the low temperature was greater, and the more severe the deterioration was shown at temperatures far from the safe temperature range. From the differential capacity analysis, the peaks corresponding to the anode was significantly reduced at the voltage of the initial charging stage after cycle deterioration at low temperature. This means that the lithium ions intercalated into/deintercalated from the graphite layer decreased. In addition, EIS of the batteries before and after cycle deterioration was measured at each deteriorated temperature. Based on these, postmortem analysis was conducted with a major focus on the anode after 20-cycle deterioration at low temperature. From the SEM observation, the anode can be known to grow thicker with generating cracks and gas pockets. According to the XPS measurements, it is found that Li2CO3 and LiF are generated by decomposition of electrolytic solution at the anode surface.Overall, it was clarified that the deterioration at high temperature affected the cathode and that at low temperature affected the anode.