Electrochemical-mechanical-Thermal Coupling Modeling And Safety Application Of Lithium Ion Battery

With the goal of "peak carbon dioxide emissions and carbon neutrality" proposed by President Xi Jinping,lithium-ion batteries are favored as new energy storage carrier due to high-efficiency.The charge and discharge of lithium-ion batteries are a complex process of coupling multiphysics such as electrochemical,mechanical and thermal.Traditional experimental methods are difficult to obtain the coupling characteristics and mechanism from the microscopic to the macroscopic within the battery.In this thesis,supplemented by experimental characterization and validation,numerical simulation is mainly presented from model development,mechanism elucidation and model application towards the intricate relationship and unclear mechanism of electrochemical-mechanical-thermal coupling.The coupling models between two physics are first developed during discharge process from the microscopic to the macroscopic level in order to unveil the relationship between the electrochemicalmechanical-thermal characteristics.Subsequently,three-physics coupling model is proposed with emphasis put on the comparison of electrochemical-mechanical-thermal behavior between charge and discharge process.Finally,the application scenario of lithium-ion battery model is constructed with a design strategy proposed to improve lithium-ion battery safety.The main contents are as follows:(1)The electrochemical-mechanical model is presented at particle level to unveil relationship between diffusion-induced stress and lithium(Li)concentration inside the battery.The Li intercalation and de-intercalation behavior is found to follow the principle of "proximity"-the intercalation and de-intercalation of Li occurred preferentially in the positive and negative electrode particles closer to the separator.The correlation between electrochemical and mechanical properties,as well as the diffusion-induced stress formation mechanism during delithiation of graphite particles are revealed.During the delithiation process,the center of the particle exhibited radial compressive stress,while the surface exhibited tangential tensile stress,the largest stress is occurred at the conjunction between particles.(2)A three-dimensional layered electrochemical-thermal coupled model is developed at the electrode level to elucidate the interconnection among current density,potential gradient,mass transfer process and temperature gradient.It is found that the current density and current density gradient are the highest at the interface between tab and electrode,which also resulted in large potential gradient,solid-phase Li concentration gradient and liquid-phase Li+concentration gradient as well as uneven temperature distribution at this position.(3)A three-dimensional mechanical-thermal model is established at cell level to examine temperature difference induced thermal stress and expansion behavior along different geometric directions.The relationship among thermal stress,expansion behavior as well as the depth of discharge and rate is quantified.The von Mises thermal stress,strain and displacement deformation of the battery along different geometric directions are also calculated-.It is detected that the battery is more likely to expand along the thickness direction mechanically,and the maximum displacement occurred in the thickness direction at the end of discharge.The maximum displacement for a prismatic battery with rated capacity of 38 Ah during 1.0 C discharge is 1.09 μm,which is only 0.008%of half the thickness of the battery.(4)A multi-scale and multi-dimensional electrochemical-mechanical-thermal coupling model is developed to systematically compare the internal electrochemical processes,temperature variations,diffusion-induced stress and thermal stress formation during the charging and discharging process.It is found that driven by diffusioninduced stress,particles are more partial to rupture at the end of constant current charging;driven by thermal stress,the moment when the cell is most likely to be damaged is related to the charge and discharge rate.Furthermore,temperature evolutions during charge and discharge are compared,heat generation discrepancy and mechanism during charging and discharging are also revealed,and the heat contribution of each heat source is quantified.It is found that the anode polarization heat dominated during charging while the cathode polarization heat dominated during discharge.Finally,the diffusion-induced stress and thermal stress are systematically compared to discover that the thermal stress is several orders of magnitude smaller than the diffusion-induced stress.The maximum thermal stress is 131.7 kPa while the maximum diffusion-induced stress is 30 MPa at 1.5 C rate for a cylindrical battery with rated capacity of 5 Ah.(5)The model finally settled down to guide the safety design of lithium-ion batteries to realize safety application.On the basis of the aforementioned modeling,the application of the model is realized from the perspective of battery safety detection and design towards the predominant phenomenon that threatens lithium-ion batteries safety—Li plating.An electrochemical Li plating model is presented to accomplish indepth Li plating detection.Two concepts with "complete Li plating" and "incomplete Li plating" are proposed according to whether the Li plating proceeds to the current collector interface.The "depth of Li plating" is also defined to estimate Li plating risk voltage range,a method for alleviating and inhibiting Li plating from electrode design perspective is explored.This model assisted Li plating detection method is simple and easy to embed in battery management system(BMS)of electric vehicles and energy storage system for safety detection.This thesis aims to improve the multiphysics coupling modeling capabilities of lithium-ion batteries and develop cutting-edge theories of electrochemical-mechanicalthermal.At the same time,the data can provide numerical ranges and theoretical guidance for lithium-ion battery safety management system models.