Mechanical and electrochemical degradation of Silicon anode for Li-ion batteries

Abstract

Silicon is one of a promising candidate for a conventional anode material of graphite to increase energy density as well as power density in LIB system. Silicon is abundant on earth and it has the ability to absorb large quantities of lithium. The theoretical electric capacity of Si is greater ten times than that of graphite. But a large amount of absorbed Li induces a large volume expansion which leads the fracture of silicon significantly. Consequently, the capacity degradation occurs within a few charge-discharge cycling. In order to overcome this limitation and to enhance Si performance, experiment and simulation studies have been performed to understand the mechanics and kinetics of Si anode. For example, a variety of nanostructures of Si, such as nanowire, nanoparticle, and hollow nanoparticle have been developed and tested to enhance the mechanical stability. Furthermore, the electrochemical response of Si-based materials has been also studied extensively by assembling a half cell. In computer-aided engineering (CAE) research, a chemo-mechanical model has been developed to analyze the internal stress evolution in host material during lithiation/delithiation process. And an electrochemical model was constructed to predict the electrochemical performance of Si anodes. However, new phenomenon have been reported, and new technology has developed, advanced models are required to provide deeper insight into the mechanical and the electrochemical behavior of Si anode materials. In this study, we suggest advanced models overcome the limitation of previous research and we investigate the mechanical and electrochemical degradation mechanisms of silicon anode material for lithium-ion batteries. We first investigate the fracture behavior of crystalline silicon nanowires by combining of Li diffusion, larger deformation, and fracture mechanics. Based on the experimental observations, two phase Li diffusion model is employed to create a sequence of core-shell structures for the stress analysis. To account for large lithiation swelling, a chemo-mechanical model is constructed based on the nonlinear large deformation theory. We then analyze the fracture behavior of crystalline silicon nanowires by combining of a bilinear cohesive zone model. The crack is initiated from the surface of Si nanowires and it propagates to the center after the amount of Li inserted. Moreover, simulation results demonstrate that there is a critical size of the particle, below which fracture can be averted and there is a safe state of charge (SOC) depending on particle size. Secondly, we investigate the electrochemical behavior of silicon anode material in Li-ion batteries within the multiscale framework because the electrochemical performance of Si strongly depends on the interaction between atomic scale and microscale phenomena. In the atomic-scale, the stress-dependent energy barrier for the migration of lithium and the molar excess Gibbs free energy were calculated using density functional theory (DFT). In the micro-scale, we considered the coupled diffusion and large deformation model to determine the non-equilibrium cell potential. These simulation results demonstrate that the multi-scale model is consistent with experimental observations at different C-rates with constant diffusion coefficient. Finally, we suggest an electrochemical degradation model with crack growth. This model is combined previous two modes including both the mechanical failure and the electrochemical response of the patterned Si. A crack in silicon is considered as the main reason for capacity reduction for lithium ion batteries due to the inhomogeneity of Li within the Si and the isolation of Si resulting from complete debonding. When Si size is large, a crack is formed in the silicon and it propagates during lithiation/delithiation, which hinders Li diffusion, resulting in the capacity loss. The mixed-mode failure is considered for the fracture behavior of the patterned Si and Butler-Volmer equation for electrode kinetics is used to obtain the cells voltage and capacity. This study mainly investigates the relation between the electrochemical performance of LIB and the crack growth with taking into account the impacts of charge/discharge rate condition.