Evaluation of HEV Batteries for Recycle~Investigation for Practical Use of the Low Capacity Retention Sorting Technology By Electrochemical Impedance Spectroscopy~

Abstract

Introduction Recently, hybrid electric vehicle (HEV) is getting popular in the car market. After the end of life of HEV, Almost all the Nickel-metal hydride (Ni-MH) battery packs used for HEV go to the recycle process. There is a possibility that some of the batteries remain capacity and we could reuse these batteries without any troubles. Regarding Ni-MH battery as well as other secondary batteries, capacity decreased during the actual usage, and batteries should be thrown away if the capacity drops below threshold value. In other words, the capacity estimation method is necessary to reuse used batteries. Generally, the battery capacity is measured with the full charge and discharge cycle. But it takes much time to do the full charge and discharge cycle. This results in low productivity of the reuse used battery. Therefore we have investigated the quick and easy way to measure the battery capacity. We reported that the low capacity retention batteries caused by decrease of the hydrogen amount in the negative electrode could be sorted out with the electrochemical impedance spectroscopy (EIS) in SOC 0%. And this technology has been put to practical use in our factory [1, 2]. In the other session of this meeting, we plan to make a presentation that the low capacity retention batteries due to the deterioration of the positive electrodes, such as the memory effect and material degradation, could be sorted out by EIS in a charge state. In this study, we would like to report the test results which we have carried out the evaluation of the deteriorated batteries aiming for practical use of screening technology of a low capacity retention batteries caused by positive electrodes. Experimental We evaluated the two types of deteriorated batteries. One was the single cell whose capacity decreased by charge and discharge cycles. Another was 6-cell-battery which was collected from the market. In order to confirm the capacity decrease reason, batteries were measured the imaginary value of Nyquist plots at 0.1 Hz. Based on the test results, these batteries turned out to be deteriorated by positive electrodes. After the impedance measurement of these batteries at 3.9Ah charging state, the battery capacity was measured by 0.4C charge and discharge cycle from SOC 0% to 100% at 25deg.C. The EIS was done with 0.15C of AC signal of the frequency from 100kHz to 10mHz. Results and discussion Figure 1 shows Nyquist plots of the batteries whose capacity decreased by charge and discharge cycles up to 6.5Ah, 6.0Ah, and 4.1Ah. Nyquist plots of Ni-MH battery shows semicircle which indicates the charge transfer reaction and linear part which indicates the diffusion resistance. The semicircle in Nyquist plot of Ni-MH impedance is mainly attributed to the negative electrode. On the other hand, the linear part is mainly attributed to the positive electrode. Nyquist plots were affected by difference of the battery capacity which was caused by positive electrode. So we found that the imaginary value get higher with the decrease of battery capacity. (ΔZ "/ Δ (ω-1))-1[A · s · V-1] was calculated as the frequency response of the linear part image value in a diffusion resistance. The relationship between the calculated results and battery capacity is shown in figure 2. Figure 2 indicated that (ΔZ "/ Δ (ω-1))–1 and capacity have positive correlation. This results suggested that the battery capacity, which was decreased by charge and discharge cycles, and the collected batteries from the market could be estimated by using (ΔZ "/ Δ (ω-1))–1as the frequency response of linear part of Nyquist plots. Detailed results and discussion will be given in the presentation.