Author: Yin, Qifang
Title: Optimization of mechanical and electrochemical performances of silicon electrode in lithium-ion batteries : molecular dynamics simulations
Advisors: Yao, Haimin (ME)
Zhou, Li-min (ME)
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Lithium cells
Storage batteries
Department: Department of Mechanical Engineering
Pages: xix, 148 pages : color illustrations
Language: English
Abstract: Lithium-ion batteries (LIBs) are becoming more and more indispensable for the contemporary lives and industrials. Prevailing LIBs tend to be unable to fulfill the increased requirements for better performances. Silicon (Si), with the highest theoretical capacity (~4200 mAh/g) and abundant reserves in the Earth's Crust, stands out as one of the most promising candidates for electrode material. However, Si suffers from great volume expansion during the interaction with lithium (Li) ions, leading to a rapid decay of the Si-based electrode and low cycling life of the LIBs. In order to alleviate the problem and to develop Si-based LIBs, considerable efforts have been devoted. Si electrodes with extraordinary performances have been created in laboratory. To date, it is advocated that more attention should be paid to the optimization of the Si electrode for higher volumetric/areal capacity and lower fabrication cost. This thesis is concentrated on the optimizations of the nanostructures of Si electrode. Chapter 1 is about the fundamental knowledge concerning the Si-based LIBs, and then a literature review on the progress achieved in recent years to improve the performances of Si-based LIBs will be presented. Chapter 2 serves as an introduction to the methodology implemented in this thesis, including molecular dynamics simulations and finite element analysis. In Chapter 3, the methods developed to realize the simulations of dynamic processes during the reaction between Li and Si will be elaborated. To simulate the dynamic intercalation of Li in Si at room temperature, a slight adaptation of the potential is made, and justification of the results derived from the adapted potential is given. It is found that the adapted potential is capable of qualitative predictions. For the simulation of the dynamic extraction of Li out of Li-Si alloy, a home-made algorithm is developed to approximate the natural diffusion process. The results are in qualitative agreement with the ones by theoretical derivation based on diffusion theory.
In Chapter 4, a method is developed to visualize the lithiation-induced stress field of Si nanowire anode. It is shown that the stress field obtained by molecular dynamics simulations is qualitatively consistent with the ones derived from theoretical modeling. In Chapter 5 molecular dynamics simulations and theoretical deductions are conducted to determine the optimal porosity that can endow the porous Si nanosheet anode with a long cycle life and high volumetric capacity. Additionally, the evolution and survivability of the porous structure in the Si anode during the charging-discharging cycles is revealed by simulations. It is found that the anode with higher original porosity can survive more cycles before the collapse of the nanopores. This work can provide guidelines for the optimal design of the porous structure in Si anode. In Chapter 6, we studied the effects of the mechanical constraint from the accessory materials in the electrode on the performance of Si nanosheet anode material. Intercalation of Li into mechanical constrained Si anode is simulated to scrutinize the influence on the capacity and lithiation rate. Results reveal that strong mechanical constraint would greatly reduce the capacity and lithiation rate of Si active material. Several suggestions are made for improving the performances of Si nanosheet anode. And finally, in Chapter 7 the conclusions of the thesis will be presented, and two pieces of future work will be proposed. Our work can not only shed light on the atomistic details in the Si electrode during the interaction with Li, but also serve as a guideline for the optimal design of the Si-based anode material. We believe that the conclusions of this thesis would accelerate the advent of the next generation LIBs.
Rights: All rights reserved
Access: open access

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