Location- and Orientation-Dependent Progressive Crack Propagation in Cylindrical Graphite Electrode Particles

Abstract

Crack propagation in graphite electrodes has been discovered to facilitate the growth of solid electrolyte interphases (SEI) that greatly affect the long-term capacity of lithium-ion batteries. In order to maintain the charge capacity of these batteries over a number of years, crack propagation must be understood and minimized. Using cohesive zone models in finite element calculations, we have studied crack propagation in cylindrical graphite anodes by considering the progressive growth of preexisting defects during cyclic charging. We have found that for a defect to grow, it must be situated far from the center of the particle in order to be placed under high tensile stress, and it must also be closely aligned with the radial direction so the components of stress normal to the defect are high enough to cause crack propagation. Such defects begin to grow during the delithiation - not the lithiation - phase of the charging cycle due to the state of high tensile hoop stress that occurs in the outer region of the particle during lithium de-intercalation. Upon subsequent cycles, the cracks progressively grow further, until complete failure of the particle is observed. Our simulations show that for typical charging conditions, defects situated within $88\%$88% of the particle’s radius from the center, and those mis-aligned from radial lines by more that ${26}^{\circ }$26∘ , will not propagate. A failure diagram that demarcates safe and unsafe crack growth regimes is presented as a function of the location and orientation of defects. We also discuss the influence of particle size, crack microstructure, and charging rate on the failure map. The main results dealing with location-dependent failure are qualitatively the same when fully anisotropic material behavior is considered as opposed to an isotropic assumption.