A study on the deformation and fracture of anodes in Li ion batteries

Abstract

Silicon and germanium are considered as one of the best candidates of anode material for the Li-ion batteries due to its enormous capacity. However, it has been known that the Li ion insertion and extraction during electrochemical cycling results in a huge volume expansion and shrinkage, which can lead to fracture of the anode. This mechanical degradation can lead to the fading of the capacity of the battery by isolating active materials, and by creating new surface area for solid electrolyte interface (SEI) growth. Thus, understanding of how electrodes are able to sustain electrochemical reaction without mechanical degradation is essential for the development of high-capacity Li-ion batteries. This thesis explores the deformation and fracture of anode materials during electrochemical cycling to provide a guideline for the practical design of high-capacity lithium ion batteries to avoid fracture. First, we report observations of microstructural changes in {100} and {110} oriented silicon wafers during initial lithiation under relatively high current densities. Evolution of the microstructure during lithiation was found to depend on the crystallographic orientation of the silicon wafers. In {110} silicon wafers, the phase boundary between silicon and LixSi remained flat and parallel to the surface. In contrast, lithiation of the {100} oriented substrate resulted in a complex vein-like microstructure of LixSi in a crystalline silicon matrix. A simple calculation demonstrates that the formation of such structures is energetically unfavorable in the absence of defects due to the large hydrostatic stresses that develop. However, TEM observations revealed micro-cracks in the {100} silicon wafer, which can create fast diffusion paths for lithium and contribute to the formation of a complex vein-like LixSi network. This defect-induced microstructure can significantly affect the subsequent delithiation and following cycles, resulting in degradation of the electrode. Second, we have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration using a simple bending test. Silicon thin-films on copper substrates were lithiated to various states of charge. Then, bending tests were performed by deforming the substrate to a pre-defined shape, allowing for a variation in the curvature along the length of the electrode. From the bending test, the critical strains at which cracks initiate in the lithiated silicon were determined. Using the substrate curvature technique, we also measured the elastic modulus and stresses that develop in the electrodes during electrochemical lithiation. From these measurements, the fracture energy was calculated as a function of lithium concentration using a finite element simulation of fracture of an elastic film on an elastic-plastic substrate. The fracture energy was determined to be 12.0 ± 3.0 J m-2 for amorphous silicon and 10.0 ± 3.6 J m-2 for Li3.28Si, with little variation in the fracture energy for intermediate Li concentrations. Third, we measure stresses that develop in sputter-deposited amorphous Ge thin films during electrochemical lithiation and delithiation. Amorphous LixGe electrodes are found to deform plastically at stresses that are significantly smaller than those of their amorphous LixSi counterparts. The stress measurements allow for quantification of the elastic modulus of amorphous LixGe as a function of lithium concentration, indicating a much-reduced stiffness compared to pure Ge. Additionally, we observe that thinner films of Ge survive a cycle of lithiation and delithiation, whereas thicker films fracture. By monitoring the critical conditions for crack formation, the fracture energy is calculated using an analysis from fracture mechanics. The fracture energies are determined to be 8.0 J m-2 for a-Li0.3Ge and 5.6 J m-2 for a-Li1.6Ge. These values are similar to the fracture energy of pure Ge and are typical for brittle fracture. Despite being brittle, the ability of amorphous LixGe to deform at relatively small stresses during lithiation results in an enhanced ability of Ge electrodes to endure electrochemical cycling without fracture.