采用阻抗谱技术,对2.8 A·h 18650电芯进行拆解解析,单独分析正负极电极在不同温度下(25、10和-5℃),不同荷电状态下的阻抗变化.结果表明:在不同温度下,在20%~100%荷电状态下,负极作为控制电极,其反应电化学阻抗是正极的数倍,尤其是在-5℃,达到了4倍,负极是电芯一致性问题中动力学因素的控制主因;在0~20%荷电状态下,在10和25℃下,正极的反应电化学阻抗要远远大于负极,正极成为控制端.结合目前电动车上动力电池的实用荷电状态一般在20%~95%,针对该2.8 A·h 18650电芯,提高负极电极的一致性是核心所在.同理,对其他类型电芯而言,在电芯设计过程中,在综合考虑成本的前提下,需要更有针对性地提高正负极的一致性标准,从而更为有效地改善整个电芯产品的一致性.
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.