Diffusion Stress And Residual Stress Analysis Of Li-ion Battery Based On Electrochemical-Stress Coupling Model

At present,the new energy automobile industry is developing rapidly,and the demand for high-performance lithium-ion batteries is quite urgent.However,due to possible design problems in the internal structure of the battery and improper external using conditions in the later period,lithium-ion batteries endure severe diffusion stress during operation,resulting in attenuation of battery capacity and deconstruction in electrode structure,and ultimately lead to battery electrode failure and battery life attenuation.Based on the electrochemical principle and diffusion mechanics principle of the battery,this paper adopts the research method of combining experiment and simulation to conduct in-depth research on various factors that affect the diffusion stress and residual stress of the battery.The choice of battery and the ideal operating environment when the battery is working provide theoretical support.The main research content of this paper is in following aspects:(1)In order to obtain the electrochemical performance parameters and the charging and discharging curves of different working conditions,we conduct charging and discharging tests of commercial ternary lithium-ion energy batteries under different working conditions,which is for the construction of the later model and the accuracy of the model.The test content mainly includes battery capacity calibration,different ambient temperature and different discharge rate tests,hybrid pulse power characteristic(HPPC)test and electrode reference potential test.(2)Based on the electrochemical principles and diffusion mechanics principles of lithium-ion batteries,we use the large-scale simulation software COMSOL to build the electrochemical-mechanical coupling model of the battery,and modify the key parameters of the model to improve its accuracy.Since it is impossible to measure the real-time stress of the battery during the discharge process,this paper mainly adopts the indirect verification method to verify the coupling model under different ambient temperatures,different discharge rates and dynamic current conditions.The results show that this electrochemical-mechanical coupling model has high accuracy and good applicability.The error of charging and discharging under different magnifications and different temperatures is less than 2%,and the error of working under dynamic current conditions is also less than 3.5%.It can be used as the suitable model for the follow-up study of diffusion stress.(3)Based on the constructed electrochemical-mechanical coupling model,single-factor analysis is applied for various factors affecting diffusion stress.The external factors that affect the stress include the discharge current,the ambient temperature of the battery,the thickness of the active material of the negative electrode sheet,and the internal factors include the porosity,particle size,elastic modulus and Poisson’s ratio of the material.The results show that lithium ions are released from the particles when the battery is discharged,while the particles are in a shrinked state.The tangential stress and radial stress at the center of the particle are equal.The Von Mises stress increases from the center of the particle to the surface,and the surface is most prone to tensile fracture.Among the internal and external factors,the porosity,particle size,discharge current and ambient temperature of the active material have a greater impact on the stress.Therefore,porosity and particle size should be given priority when designing the battery.(4)After disassembling the tested lithium-ion battery cell,we perform XRD stress test and SEM scanning electron microscope analysis.The results show that the lattice spacing of the active material particles and the fracture degree of the material surface increase with the increase of the discharge current and the increase of the number of cycles.The increasing thickness of the active material has a good resistance to the diffusion stress of the battery,but too thick active material layer will cause lithium ions to accumulate in the current fluid,causing the increase of residual stress of the electrode and the peeling off of active material from the current collector.