Multiscale mechanical modeling of high capacity anode in lithium ion batteries

Abstract

Challenges to battery technologies have been traditionally understood in terms of the electrochemistry of materials. A number of disadvantages are associated with lithium ion battery systems, which include stress-induced material damage and large volume deformation due to lithium ion swelling. Therefore, there is a need for better design, operation, and control of lithium ion batteries to meet the growing demands of energy storage. Physics-based modeling and simulation methods provide the best and most accurate approach for addressing such issues in lithium ion battery systems. This dissertation aims at investigating the mechanical behavior of high capacity negative electrodes in Li-ion batteries within the multiscale framework. To achieve this goal, a coupled equation based on continuum formulas for diffusion and stresses are developed. However, this model does not predict the concentration effects on the material properties of electrode materials. Therefore, the variation of mechanical stiffness and Li diffusivity as a function of Li concentration will be calculated by the density functional theory method. The modified diffusion-induced stress model is reformulated so that I can apply it to the changing of material and discretized for finite element analysis. The model is used to investigate several important issues of stress distribution in Si and prelithiated Si nanowires. This thesis outlines the investigation of material properties, electrode particle size, and charging rate effect on stress evolution of electrodes in Li-ion batteries. To explore the failure mechanism of the Si anode, the elasto-plastic stress analysis is performed. The microscopic mechanism of large plastic deformation is explained, which is to continuous Li-assisted breaking and reforming of Si-Si and Li-Si bonds. The stress evolution in finite element analysis is calculated to compare the elastic and the plastic effect. For rational design of Si-based anodes, a silicon dioxide electrode is suggested for next-generation high-capacity anode materials for lithium-ion batteries. With a purpose of clarifying Li+ insertion behavior into silicon dioxide (SiO2) at an atomic level, lithiation mechanisms, structural evolution, and voltage profiles of silicon dioxide are calculated by density functional theory calculations. In the theoretical calculations, we elucidate that amorphization of SiO2 is an effective way to improve its reactivity and reversibility toward Li+ insertion and extraction. The possible reaction mechanism of amorphous SiO2 is proposed based on the theoretical calculation and experimental validation in this thesis. Furthermore, to overcome the shortcomings of silicon dioxide, a silicon sub-oxide anode is suggested, which is fabricated by using an oxygen reduction method in the presence of a transition metal. Li-ion batteries are an emerging field that couple electrochemistry and mechanics. This thesis aims to understand the deformation mechanism, stresses, and fracture associated with the lithiation reaction in Li-ion batteries, and hopes to provide new insight into the rational design of robust, high-capacity Si-based anode materials.