Effects Of External Mechanical Field On Electrochemical Properties Of Lithium Ion Battery Anode Materials

Lithium-ion batteries show excellent performance and have been applied in many fields.During the charging and discharging process,the repeated insertion and extraction of lithium ions between the electrode and the electrolyte lead to changes in the internal mechanical properties of electrode materials.At the same time,when the electrode materials are subjected an external mechanical field,the internal mechanical field and concentration field will be changed,which affects the performance of lithiumion batteries.Therefore,it is of great significance to study the influence of force on the electrochemical properties of electrode materials.The electrode material is inevitably subjected external mechanical field in practical applications,however the current research mainly focuses on internal stress.Thus,based on thermodynamic theory,and combined with Fick's first law and Arrhenius dynamics method,the theoretical framework of lithium-ion battery electrode materials during the charging process is constructed.Also,the finite element model is established to analyze the influence of external mechanical field on the field of stress,concentration and electrode capacity of lithium ion battery electrode materials.In this thesis,graphite electrodes are considered.The main research contents and main conclusions are as follows:(1)Based on the thermodynamic theory and Fick's first law,the theoretical framework of the insertion for lithium ion in electrode materials is established,and two types of electrode models for the infinite electrode and finite electrode are constructed.It is found that in the absence of external force,the hydrostatic stress of the electrode material is compressive stress,and the lower the concentration with the higher the compressive stress.The hydrostatic stress and concentration on the surface of the infinite electrode is uniform.The internal stress difference leads to the electrode capacity of finite electrode higher than that of infinite electrode.(2)The effects of boundary forces on the mechanical and electrochemical properties of finite electrodes and infinite electrodes are investigated.The results show that the boundary pressure makes the hydrostatic stress of the electrode increase and the concentration decreases.The boundary tension force leads to decreasing of the hydrostatic stress and increasing of the concentration.The boundary tension force can increase the capacity of the electrode and promote ion diffusion.Conversely,the boundary pressure force reduces the electrode capacity and inhibits ion diffusion.(3)We study the effects of uniform stress and gradient stress on the mechanical and electrochemical properties of electrode materials.It is found that the uniform radial tensile stress enhances the ion diffusion rate and increases the capacity of the electrode.The uniform axial tensile stress decreases the ion transport rate and electrode capacity.The result of uniform compressive stress is opposite to tensile stress.Radial gradient tensile stress reduces hydrostatic stress,increases ion diffusion rate and electrode capacity.Axial gradient tensile stress increases hydrostatic stress,while ion diffusion rate decreases,and electrode capacity decreases.Gradient compressive stress shows an opposite tend.At the same time,it is found that the uniform stress is more affected than the gradient stress,and the radial stress is more affected than the axial stress.(4)The effects of uniform strain and gradient strain on the mechanical and electrochemical properties of electrode materials are investigated.It is found that uniform compressive strain and gradient compressive strain can reduce hydrostatic stress,accelerate ion diffusion rate and increase electrode capacity.The effect of tensile strain has an opposite tend.Comparing the effects of uniform strain and gradient strain,it is shown that the uniform strain has a greater influence than that of gradient strain.