Analysis of thermal propagation behaviour of Li-ion cells under various boundary conditions considering breathing and swelling mechanisms

In literature the importance of the thermal behavior of lithium-ion batteries is amply illustrated. In fact, various methods have been shown for analyzing fundamental thermal parameters, such as heat flux, and "in-plane" and "through-plane" thermal conductivities. However, the disadvantage of the methods present in the literature is that they do not consider the mechanical behaviors and specifically the phenomena of breathing and swelling, during the charging and discharging cycles, as they can influence the thermal behavior. Therefore, in general a legitimate question to ask is how the change in volume can affect the variation of the thermal parameters of a lithium-ion battery. To this end, the purpose of the thesis is divided into two parts. A first part which deals with a thermal characterization, through a software simulation (COMSOL Multiphysics), of the lithium-ion cells with an existing device to study the different values of the thermal conductivity, of the heat flux, and the temperature distribution on the cell surface with different values of thickness. To model the thermal behavior of the Li-ion battery cell a 3D single-layer approach is adopted in order to simplify the simulation. On the other hand, the idea is to measure the heat flux and heat transfer inside the battery by applying different pretension force values to different battery temperature values, in order to recreate the different cell volume changes, understand how thickness and area of the battery are connected to the thermal parameters and how the latter change according to an external mechanical force applied and / or even try to understand if it is possible to counteract the swelling phenomenon and therefore the degradation of the cell itself thanks to an external pressure . Since the existing device considered in this study is composed of several sensors, including the one considered in this case the heat flux sensor, it is advisable to understand what the maximum load it can withstand is. Hence, it is possible to introduce the second part on which the thesis is focused, which also represents the crucial point of the whole work and on which the research question of this study is based. In fact, the research question of this thesis is to study by applying an external pretension force with a range of [0-20]kN, how a heat-flux sensor can react, what is the maximum applicable force value that the sensor can withstand before breaking, and if changing different values of force and temperature the sensor output is always the same or can vary. Therefore, the purpose of the second part is to analyze and simulate via software the behavior of the heat flux sensor FHF02, creating a suitable structure in which there is an upper part that acts as a heat source and a lower part that acts as a cooling system plus an external device that gradually applies, at different values of temperature difference ΔT, increasing values of force up to the limit of the sensor itself.