Fundamental Study of the Mechanical Failure of Silicon Based Electrodes for Li-Ion Batteries Using a Novel Multi-Physics Computational Modeling Framework

摘要

Li-ion batteries are the currently accepted flagship energy storage system with several cathode systems identified over the years. However, graphite has always remained the commercial anode material of choice. Silicon has been identified as the next-generation anode for Li-ion systems with a high theoretical capacity (4200 mAhg-1) compared to graphite (372 mAhg-1) and has been the focus of much research over the past decade. Silicon unfortunately, undergoes large volumetric expansion (312%) upon Li diffusion generating considerable diffusion-induced stresses. Presence of high stress leads to mechanical failure of Si resulting in capacity fade due to loss of electrical contact with the current collector impeding commercialization. The mechanical response of the electrode depends on the electrode properties comprising the active (Si) and passive components (current collector, mechanical supports). The objective of this thesis is to gain a mechanistic understanding of the interactions between the electrode components and their effect on the overall mechanical integrity of the Si based anode assembly, which can aid in the design of failure resistant, next-generation, high capacity anodes.udTo achieve this objective, a custom nonlinear finite element modeling software that can model coupled diffusion induced large elasto-plastic deformation of Si, surface electrochemical reaction kinetics and eventual mechanical failure response of the electrode system was utilized. This modeling framework is first used to understand the effect of passive components (current collector and Si-Cu interface properties) on the mechanical stability of an a-Si thin film anode system. To unlock the mechanisms behind the gradual interfacial delamination of the Si film from the underlying Cu current collector in an a-Si thin film anode system, a detailed parametric study is performed to analyze effect of the mechanical properties of the current collector and the Si-Cu interface on the delamination at Si-Cu interface. The knowledge gained from these studies is further bolstered by examining the mechanical stability of a-Si patterned thin film anodes upon insertion of a thin elastic buffer layer between a square Si thin film pattern and the current collector. Finally, the modeling framework is utilized to understand the effect of active material geometry in Si-carbon nanotube (CNT) heterostructured anodes. ud