Research Of Pouch Lithium-ion Battery Thermal Stress And Thickness Variation

The lithium-ion battery has become the key to the development of electric vehicle. The pouch lithium-ion battery thickness changes and thermal stress produced at charge and discharge processes, while the lithium-ion de/intercalate into the electrode material and temperature rise. The thickness and stress changes may affect battery performance, life and pack design. Therefore this paper focuses on lithium-ion battery temperature field, thermal stress and thickness variation.This paper studies the distribution of lithium-ion battery temperature field and thermal stress field combined with theoretical calculation and simulation method. Firstly, it is simplified into a laminated material with energy storage function according to the pouch lithium-ion battery structure characteristics. We got the mechanical parameters and coefficient of thermal expansion by laminated material model and the effective thermal expansion theory. Then the steady-state temperature field of the lithium-ion battery was calculated, which was based on the heat balance equation and heat transfer boundary conditions. Finally, the battery thermal stress approximate solution is calculated by the temperature field equations and prevent strain method, and ANSYS is also used to simulate the lithium-ion battery temperature and thermal stress field under normal condition. At the last of the paper, we measured the thickness variation of a pouch lithium-ion battery at different rate by a device designed with a linear displacement transducer (LVDT). And the relationship between cell thickness change and SOC was briefly deduced. Some conclusions were obtained.The steady-state temperature field of pouch lithium ion battery is ellipsoid equation, and the isothermal surface is ellipsoid. The highest temperature in the geometric center increases linearly with the heat generation rate and inverse correlation with the convective heat transfer coefficient. While the maximum temperature difference increases linearly with the heat generation rate and the square of the sizes. When the heat rate is8500W/m3and ambient temperature is20℃, the battery maximum temperature is23.4℃and the maximum temperature difference is0.6℃.The lithium-ion battery thermal stress distribution is not uniform. Battery center area, where is the high-temperature region, is the compressive stress. While battery side central is tensile stress. The thermal stress concentration occurs at the center of the sides. Along the x direction of the maximum compressive stress is15.0KPa and the maximum tensile stress is31.4KPa. The maximum tensile stress is about twice the maximum compressive stress. The maximum von miss stress is29.7KPa and the minimum is0.56KPa. The shear stress in the symmetry plane is approximately zero and minimum values is-5.4KPa at2/3length and width.The thickness change of the pouch lithium-ion battery is related to the SOC, the original thickness of the battery, electrode material, cycle life, C rate, temperature and so on. The thickness change of the battery is similar at1/3C,1/2C and1.0C. The battery thickness changes reversible, increased at charge and decreased at discharge. The experimental data shows that the change ofthickness is not linear to the change of the SOC. It increases rapidly in the low range of SOC(0-40%), the amount of change is about0.040-0.050mm,70-80%of the total variation. Then it becomes flat for the mid-range of SOC(40-80%) and finally increases again when SOC is larger than80%. At large discharge current the battery temperature rise is large, and the change of battery thickness caused by temperature is large too. The thermal expansion coefficient along the battery thickness direction is about1.9×10-4/℃.