Optimization Of Thermal Performance Of Kirigami Lithium Ion Battery Based On Composite Phase Change Material

With the development of science and technology,individuals’ requirements for electronic devices are increasing,and flexible batteries have been gradually applied to wearable electronic devices,such as smart bracelets.Kirigami based lithium ion battery can achieve flexibility and stretchability through its unique structure.However,during the process of charging or discharging,lithium-ion batteries will inevitably generate large amounts of heat due to detachment,insertion of lithium ions,polarization effects etc.If the generated heat cannot be dissipated in time,it will cause the battery to deteriorate,accelerate aging,run out of heat and other dangerous situations.Therefore,in order to improve heat dissipation of the batteries,battery thermal management system is widely used.At present,the research on the thermal management systems of common cylindrical batteries and prismatic batteries is relatively mature,but the research on the thermal performance of flexible and stretchable batteries is relatively scarce.And there is no standard guide for the design of thermal management systems for stretchable batteries.Therefore,this paper aims to study the thermal behavior of Kirigami lithium ion batteries,propose and optimize a thermal management system which is suitable for this battery.The specific method is as follows:First of all,in order to simulate the heat generation of the lithium-ion battery,an electrochemical-thermal coupling model of this battery is established based on finite element method based on commercial software COMSOL MULTIPHYSICS.Specifically,a three-dimensional thermal model is constructed based on the geometry of the Kirigami battery;a pseudo two-dimensional electrochemical model is established based on the internal structure and working principle of the lithium-ion battery;and these two models are coupled through the transient heat transfer equation.Then,the established model is compared with the experimental data to verify the accuracy of the model.Secondly,based on this finite element model,the heat generation on the surface of lithium-ion batteries at different discharge rates（1C,3C and 5C）,ambient temperature（288.15K,298.15K and 308.15K）and convection heat transfer coefficient（0,5 W/（m²·K）and 25 W/（m²·K））are studied and compared.Moreover,the influence of the positions of the tabs（tabs distributed on single side and double sides）on the heat generation of the battery is explored.Thirdly,because the paraffin/expanded graphite composite phase change material has the advantages of low cost,stable shape,and no leakage,it is applied to the thermal management system of the Kirigami lithium ion batteries.The specific heat capacity and thermal conductivity of this composite phase change material based thermal management system are determined by defining the minimum heat transfer unit.Based on the thermal contour of the lithium ion battery at a discharge rate of 5C,a novel distribution of the thermal management system is proposed.The heat dissipation under different designs of thermal management units（such as square and circle）is studied.At last,the thermal management system is optimized online based on software MATLAB.In order to analyze the influence of different design parameters on the objective functions(maximum temperature on the battery surface T_max and maximum temperature difference △T_max),a global sensitivity analysis is used to quantify the influence of the design parameters and filter the factors that have little effect on the thermal performance.The relationship between the design variables and the objective function is established through surrogate models,and the accuracy is analyzed.In order to avoid the contingency of evolutionary algorithms,different multi-objective evolutionary algorithms are used for online optimization,and the optimal results are compared.To sum up,this research establishes an electrochemical-thermal coupling model of Kirigami lithium ion batteries based on the finite element method.A thermal management system based on paraffin/expanded graphite composite phase change material is proposed and optimized to reduce the maximum temperature of the battery surface and improve the uniformity of the temperature on the battery surface.This article provides a promising solution and theoretical guidance for the design of thermal management system for stretchable lithium-ion batteries.