(Digital Presentation) The Effect of Dominant Operational Conditions on Capacity Deterioration of Commercial Li-Ion Cells Under Accelerated Testing

Abstract

Li-ion batteries (LIBs) have been used in a variety of applications including consumer electronics, electric vehicles, space exploration, the defense sector, etc. As a result, the performance and reliability of the commercial LIBs have become extremely crucial which can be monitored by the deterioration in the charge storage over continuous charge-discharge cycling [1]. However, cycling LIBs at the nominal current rate involves excessive time and resource consumption resulting in a slow battery qualification process. Accelerated testing has been proposed to examine the reliability test and for the efficient qualification process of batteries [2]. It is conducted by increasing the operational (stress factors) conditions of the LIBs that have direct implications on the performance and cycle life of a battery. During accelerated testing, capacity deterioration and increase in impedance are recorded periodically, which serve as indicators of degradation of the batteries [3]. Batteries under consideration for the degradation process are closely monitored and the above degradation indicators are recorded at elevated levels of the operational conditions than normal. This process is known as the accelerated degradation test. In literature, there are several operational conditions that have been reported to affect the capacity fade and cycle life of a battery [1-4]. However, the C-rate and ambient temperature are the widely believed dominant factors that have a significant effect on the performance of a battery [2, 4].In this work, we will present a systematic study on the performance degradation of commercial LIBs under accelerated testing using dominant operational conditions viz. C-rate and testing ambient temperature. Capacity fading of commercial cylindrical 18650-type LIBs is monitored at 1 and 2 C-rate while maintaining the ambient temperature during testing at 25 and 35 ℃ in a thermal chamber. Capacity degradation is highly sensitive to C-rate and causes faster fading at higher current rates due to mechanical degradation of electrodes, formation of SEI layer and side reactions. Discharging at high C-rates also causes ohmic heating in the cells increasing their internal temperature which further influences the chemical reaction kinetics [4]. The ambient temperature also has a strong influence on the capacity loss due to the degeneration of the positive electrode and SEI layer formation at the negative electrode [1, 4]. Accelerated testing including the synergistic effect of the ambient temperature and C-rate on the capacity fading and performance evaluation of the commercial LIBs till the end of life or 20% capacity reduction would be presented. Post cycling analysis of the Li-ion cells, in particular, electrode materials and periodic impedance measurements would enable a better understanding of battery failure under such accelerated testing. We believe that the results of this study would pave the way to shorten the duration in the detection of a battery failure and enable an efficient battery qualification process.