Accelerating Rate Calorimetry Tests of 18650 Li-Ion Cells before and after Storage Degradation at High Temperature

Abstract

The use of Li-ion cells has been growing globally with a large number of cells powering a wide range of applications in variety of environments, and there have been several reported incidents raising safety concerns. Some of the cases have been related to overheating (thermal runaway) of Li-ion cells. Thermal runaway occurs when the exothermic reactions go out of control, thus the self-heating rate of the cell increase to the point that it begins to generate more heat than what can be dissipated. Understanding the behavior of Li-ion cells during thermal runaway is critical to evaluate the safety of these energy storage devices under outstanding conditions. In this work we analyze the thermal runaway behavior of 18650 Li-ion cells before and after storage degradation at high temperatures. The thermal behavior of the cells is analyzed using accelerating rate calorimetry. Non-degraded and degraded commercial available 18650 Li-ion secondary cells were tested. The nominal capacity of the cells is 2550 mAh anode and cathode materials are graphite and LiCoO2 + additives, respectively. In order to degrade the cells, these were stored at 80°C and at different state of charges. The cells were tested inside an accelerating rate calorimeter (ARC, 2000TM Columbia Scientific Industries) to record exothermic and thermal runaway reactions under adiabatic conditions. The cells were charged/discharged using a battery tester (KIKUSUI, PFX2011). Figure 1 shows the exothermic degree of non-degraded and degraded Li-ion cells as a function of state of charge. The data were generated from the self-heating rates of the cells obtained during the ARC tests at 180°C, where the self-heating rates of the cells were normalized based on the highest self-heating rate. This plot is useful to compare the exothermic behavior of the Li-ion cells at different state of charges. The non-degraded cell exhibited the largest exothermic degree at 100% SOC and the lowest degree at 0% SOC. The degraded cells presented lower exothermic degrees than that of the non-degraded cell at 100% SOC but higher exothermic degrees at 0% SOC. Figure 1