Thermal decomposition of large-size prismatic lithium battery

Taking into account the space utilization efficiency and the characteristics of the battery pack thermal control system, the current passenger electric vehicle battery packs generally use square lithium batteries. Compared with cylindrical lithium batteries, square lithium batteries have their own characteristics and characteristics in terms of thermal characteristics. advantage. Thomas Grandjean et al. [1] of the University of Warwick in the United Kingdom conducted research on large-capacity square lithium batteries. Thomas Grandjean found that as the size of lithium batteries increases, the number of pole pieces in the battery increases, and the heat dissipation of the battery is hindered to a certain extent. There is a certain temperature gradient in the direction perpendicular to the pole piece and the horizontal direction inside the battery, which will also bring no small challenge to the thermal management system of the battery pack. In order to know the thermal characteristics of large-size prismatic lithium batteries, Thomas Grandjean conducted a specific study on the temperature gradient distribution of commercial 20Ah prismatic LFP batteries at different discharge rates. The study found that both the ambient temperature and the discharge current of the battery have an impact on the battery’s The temperature is closely related. The following table specifically analyzes the temperature rise of the battery under different discharge currents and ambient temperatures. It can be seen from the following data that the lower the ambient temperature, the greater the discharge rate, the greater the temperature rise. The current distribution and heat dissipation in the lithium battery are affected by factors such as the battery structure, the shape and position of the tabs, and so on. Therefore, during the discharge process, the temperature changes of different parts of the battery are also very uneven. The temperature rise diagrams of different parts of the battery are shown in the figure below. From the point of view of the temperature rise distribution, the temperature distribution in the battery is extremely uneven at a larger discharge rate. The temperature of the middle part of the battery is much higher than that of the other parts. Part of the temperature is the lowest under most conditions (it is important to take away some heat from the connection with the wire), but at a rate of 10C, the temperature of the Al tab is higher due to its own heat generation. Due to the small thermal conductivity of the pole piece, Thomas Grandjean found that the battery also has a large temperature gradient in the vertical battery direction, and this temperature gradient is greatly affected by the battery discharge rate, as shown in the figure below, at 10C The temperature difference between the two sides of the battery can reach about 20 ℃ under the high magnification. As the current drops, the temperature gradient gradually decreases. The existence of temperature gradient will lead to uneven current distribution and SoC during use of the battery, leading to accelerated local aging, which in turn affects the service life of the battery. In the process of using electric vehicles, they will face special conditions such as start-up and rapid acceleration. Lithium batteries are required to discharge high currents. Due to the polarization and ohmic impedance of lithium batteries, the batteries will generate heat rapidly under high current conditions. Therefore, the research on the thermal characteristics of large-size batteries under high current is also the focus of lithium batteries in practical use. C.Veth and others of Dt.ACCUmotiveGmbHu0026CoKG, a subsidiary of Daimler Motors that process lithium batteries (powered lithium-ion batteries for Mercedes-Benz), etc. [2] evaluated the thermal characteristics of 50Ah prismatic batteries at high currents. The specific research provided high-quality data for the establishment of the electric-thermal model of the battery, and provided data support for predicting the distribution of temperature, current, voltage, SoC and SoH in large-size batteries. The surface temperature change of the 50Ah NMC/C prismatic battery during discharge at 300A is shown in the following figure (Figure a, 10s at the beginning of discharge; Figure b, 250s in the middle of discharge; Figure c, 585s at the end of discharge), which can be seen from the figure At the beginning of the discharge, the high temperature area should be on the side close to the negative electrode tab. This is mainly because the copper foil is relatively thin, resulting in a higher resistance value than the thicker Al foil, but in the later stage of the discharge, it is affected by the battery boundary conditions and the battery. As a result of the shape, the high temperature area of u200bu200bthe battery is transferred to the middle part of the battery. C.Veth also found that the temperature gradient of the single cell in the battery pack increases with the increase of the discharge current, and decreases with the increase of the temperature, as shown in the figure below. Different aging modes of lithium batteries will have different effects on the battery, which will also affect the thermal characteristics of lithium batteries under high current. C.Veth research found that different battery aging modes will have completely different thermal characteristics of lithium batteries. The following figure shows the temperature distribution image of a calendar-aging battery (Figure a), a large-current cycle aging battery (Figure b), and a low-current cycle aging battery (Figure c) after discharge at 250A. It can be seen that, The hottest area of u200bu200bthe battery is also different under different aging modes. Due to the existence of air chambers in the calendar aging battery, there is an inactive area at the corners of the battery. The high-rate cyclic aging battery has internal resistance near the position of the negative electrode tab. The newly-added inactive area is more prominent in low-rate cycling batteries, and the newly-added inactive area for internal resistance only expands in one step. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: About the common failure modes of lithium battery cathode materials and the corresponding preventive measures