Sensitivity of electrodes in a lithium ion cell to temperature and SOC

In the design and development of electric vehicles (EVs), the variation in lithium ion cells (LICs) is one of the most important safety issues as it can cause a decrease in the life of the battery systems and shorten the mileage range. This problem is rooted in the design accuracy and rationality of the process values for the battery electrodes, and defining the effects of temperature and state of charge (SOC) on the electrodes is a critical step toward improving the variation in LICs. In this paper, the electrochemical impedance spectroscopy (EIS) method was adopted to study the 2.8 A·h 18650 cell. Firstly, the cells was dissembled and then attached the positive/negative electrodes separately to coin cells with an Li plate as the count electrode. Secondly, the impedance changes at different temperatures (25, 10 and-5℃) and the SOCs for these coin cells were studied using EIS. The results show that for 20% -100% state of charge at different temperatures, the negative electrode is the control electrode; electrochemical impedance is several times that of the positive electrode, especially, at-5℃, it reaches 4 times. Therefore, the negative electrode is the control factor in the kinetic variation process. For 0-20% SOC at 25 and 10℃, the electrochemical impedance of the positive electrode is larger than that of the negative electrode and it becomes the control electrode. Regarding EVs:1) the normal SOC usage range is 20% -95%; and 2) the working temperature range is above 0℃ to satisfy cell variations in the pack and benefit the life of the cells. Combined with the above results, it can be concluded that improving the variation in the negative electrodes is most useful to the variation in the 2.8 A·h 18650 cell. Above all, in the design process for LICs, the variation of electrodes should be improved as a target for improving costs and yields.