Study of Analysis of Internal Current Distribution of Lithium Ion Pouch Cells By Magnetic Field Measurement

Electric vehicles are expected to become more widespread toward the realization of a low-carbon society. As electric vehicles become more widespread, the market for lithium-ion batteries (LiBs) used in batteries will expand. However, degradation of the LiBs causes deterioration in performance such as battery capacity and cycle life. Moreover, it reduced safety by the internal short-circuits and heat generation during charging and discharging. Therefore, there is a need to establish a method for determining the degradation of LiBs. Electrochemical reactions in LiBs take place in the electrode planes. When LiBs are locally degraded, the current distribution differs from normal conditions by non-uniform reactions. Therefore, it is necessary to measure the current distribution to evaluate this reaction. However, measuring current distribution has not been established. In our previous study, the current distribution in the cell surface using a magnetic sensor in a fuel cell were calculated and controlled the fuel cell. In this study, we proposed a method to analyze the current distribution of LiBs by measuring the magnetic field. As a preliminary step for the diagnosis of LiB degradation, we evaluated the current distribution of single-layer pouch cells in different states. Compared to other types of LiBs, the single-layer pouch cell has a simple internal structure and a flat shape, making it possible to analyze the current distribution in two dimensions. In addition, the cells have the characteristic of the distance between the electrodes can be easily changed. In this study, the distance between the electrodes of a part of the cell plane was changed by applying local pressure to the single-layer pouch cell from both sides using clips. The purpose of this study was to evaluate the change in current distribution caused by this pressure using magnetic sensors. In this study, the current intensity was calculated using an inverse problem based on Biot-Savart's law. The current distribution was calculated by applying inverse matrix operations to the initial conditions, boundary conditions, and measured magnetic field. Using the correction term, the current intensity was calculated by the difference between the measured value and the initial measured value. In this study, a total of four cases were set up using two types of single-layer pouch cells. “no pressure” and “locally pressurized with a clip”. Generally, a single-layer pouch cell is pressurized evenly from both sides to make the distance between electrodes uniform and stable across the entire cell plane. Therefore, if the cell is not pressurized, the inter-electrode distance will be non-uniform. In addition, if pressure is applied locally only to the top of the cell, the inter-electrode distance in that area will narrow. In this study, we compare the difference in current distribution between these two cases by analyzing the internal currents based on magnetic field measurements from four sensors. In the results of the experiment, the current intensity of “no pressure” case was measured to change in the upper part of the cell over the course of the discharge. This result indicates that the distance between the electrodes was non-uniform over the entire surface of the cell. On the other hand, current intensity of “locally pressurized with a clip” case was not changed the discharge. It is considered that the distance between the electrodes in the area where the pressure is applied locally shortens, and the current is concentrated due to the decrease in internal resistance. In the pouch cells of different types, the points where changes in current intensity appeared over the course of discharge without applying pressure were different. This result suggests that the unevenness in the distance between the electrodes during the manufacturing process of each sample of the two types of cells was responsible for the differences in the current intensity distribution. This study found that the use of four magnetic sensors can be used to observe changes in the current distribution caused by the distance between the electrodes.