Study On The Mechanisms Of Diffusion Induced Dislocation And Lithiation Behavior In Lithium Ion Battery Electrodes

In essence, the charge or discharge cycling process in a lithium ion battery is a multi-system crossed problem including lithium ion transport,electrochemical reaction and solid mechanics. The extremely complex electrochemical reaction process is affected by many factors. Meanwhile, many new reaction mechanisms are found by researchers and some new challenges need to be solved. In order to prolong the span for repetitive use and obtain an optimal design for electrode particles, this thesis aims at systematically investigating diffusion induced dislocation and lithiation behavior mechanisms in lithium ion battery electrodes.The main study tasks and contributions of this thesis as follows:(1) Under the condition of different lithium ion diffusion capacities, the generation and action mechanisms of diffusion induced dislocation are studied. Diffusion induced stress field, elastic strain energy and the effects of dislocation mechanisms on them are analyzed in detail. Results show that diffusion induced stress field can create dislocations, and in turn dislocations relieve diffusion induced radial stress partially.Furthermore, the tensile radial stress is converted to a compressive one near the electrode surface. Under small electrochemical Biot number, the degree of dislocation induced stress relieving the radial stress is obvious and meanwhile the influence of dislocation strain energy upon total strain energy is more significant. The smaller the electrode radius is, the greater the relieved degree of the radial stress is. Both electrochemical Biot number and dislocation density can be reasonably controlled and coordinated based on nanotechnology so as to prevent the fracture failure of the electrode and prolong the cycle life.(2) Based on diffusion kinetics and heteroepitaxial strained layer theory, a theoretical model considering the generation of misfit dislocations in a nanofilm anode is proposed. By utilizing the calculation method of energy minimization, the generation and distribution of diffusion induced misfit dislocation under galvanostatic and potentiostatic conditions are analyzed respectively in detail. In addition, the homogeneous strain energy and minimum total strain energy are discussed,respectively. Results show that, when the insertion time and diffused layer thickness are less than their respective critical values, the monotone decreasing trend for the total strain energy with the increasing misfit dislocation space indicates that no misfitdislocation exists in the diffused layer. Under potentiostatic condition, the nucleation time and speed of misfit dislocations are shorter and faster than that under galvanostatic condition. A certain region immediately possesses much lower dislocation density under the surface of the thin film electrode as compared to the region below it. In order to relax diffusion induced strain and relieve diffusion induced stress, charge parameters and misfit dislocation density can be tuned reasonably with the purpose of improving the durability of lithium ion batteries.(3) According to elastic-plastic deformation theory, a lithiation phase transformation model considering the concurrent propagation of two lithiation fronts and lithiation induced volume expansion is developed. Based on theoretical calculation, the contribution of lithiation induced stress field to lithiation driving force is analyzed. Stress-dependent lithiation driving forces under different amorphous silicon shell sizes are discussed. The possible fracture and debonding failures for the composite structure of amorphous silicon and carbon nanofiber are predicted. Results show that the magnitudes of the tangential and axial stresses in the remaining unlithiated silicon shell are much lower than those in the lithiated silicon shells. Lithiation reaction resistance at the lithiation front which propagates towards the center of the hollow silicon nanowire is lower, and thus resulting in the thicker lithiation layer at the external surface. The smaller the outer radius of the hollow silicon nanowire is, the lower lithiation reaction resistances the two lithiation fronts undergo. These quantitative results can provide some theoretical support for preventing the fracture and debonding failures and prolonging the service life and a few basic guidelines concerning optimal design.