Study on energy efficiency and cyclability of anode material in Li ion battery

Abstract

In this dissertation, a study on the energy dissipation and performance degradation of silicon anode of lithium ion batteries have been investigated by chemo-mechanical coupling finite element method. Galvanostatic charge – discharge cycle is simulated, and the energy dissipation is calculated from entropy production due to diffusion and plastic deformation. Both silicon nanowire and silicon nanofilm anode have been simulated to observe the effect of anode geometry on dissipation. Yield strength, charge range and charge speed are varied in order to study the effect of material properties and charging condition. It is observed that whereas diffusion dissipation dominates in nanowire anode, plastic dissipation dominates in nanofilm anode. Energy dissipation as a function of charge range and charge speed have been presented for both nanowire and nanofilm andes. On the other hand, the initiation and propagation of crack in galvanostatic charge – discharge cycle have been simulated using element failure method. Tensile strength is used as the crack initiation criteria, and the fracture energy and J-integral is used as the crack propagation criteria. Crack propagation direction is determined by the direction of first pricipal stress. When a crack exists in a nanowire, the dissipation by diffusion and plastic has been increased, apart from the dissipation by crack. The increase of plastic and dissipation energy dissipation owing to the existence of crack is ploted, and the value increases as the nanowire radius increases. Calculation has been performed for the initial center crack and initial surface crack case, and it is observed whereas the initial center crack tends to grow straightly, the initial surface crack tends to be vent owing to higher compressive stress in the inner part at discharging state.