Mechanics-based investigation into the structural integrity and optimization of core-shell nanostructured electrode materials for lithium ion batteries

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Mechanics-based investigation into the structural integrity and optimization of core-shell nanostructured electrode materials for lithium ion batteries


Author: Li, Weiqun
Title: Mechanics-based investigation into the structural integrity and optimization of core-shell nanostructured electrode materials for lithium ion batteries
Degree: Ph.D.
Year: 2017
Subject: Lithium ion batteries.
Nanostructured materials.
Energy storage.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xxiv, 176 pages : color illustrations
Language: English
InnoPac Record:
Abstract: Lithium ion battery is an efficient energy storage and conversion device in our daily life and industry production. Several problems still limit its wider application, such as the low reversible capacity, short cycle life, slow charging rate, safety issues, etc. In order to solve these problems, the high-capacity anode materials, such as tin dioxide (SnO₂, 781 mA h·g⁻¹) and silicon (Si, 4200 mA h·g⁻¹), have been studied. However, SnO₂ and Si anode materials suffer from large volume change during electrochemical cycling, leading to fracture and pulverization of the anode materials and degradation and failure of the lithium ion batteries. Therefore, carbon coating technique has been used to constrain the volume change and alleviate the fracture. The structural integrity is guaranteed at the expense of overall capacity due to the lower theoretical capacity of carbon (372 mA h·g-1). This thesis is focused on the structural integrity and the capacity maximization of the core-shell nanostructured anode materials with either carbon or Si as the coating materials. Chapters 1-3 discussed the research background, literature reviews and research methods related to materials and mechanics for lithium ion batteries. From chapter 4 to 7, four kinds of core-shell nanostructured electrode materials were studied by experimental and/or theoretical approaches. Finally, conclusions, discussions and future works were presented in Chapter 8. In Chapter 4, the carbon-coated SnO₂ nanowire anode material was studied by using theoretical methods. The fracture of the carbon shell was found to be thickness-dependent during lithiation. If the thickness is lower than a critical value of ~9 nm, the carbon shell is prone to being cracked. A theoretical model, based on the theory of diffusion, was established to calculate the concentration of lithium ions in the nanowire during lithiation. By analogously treating the lithiation-induced expansion as thermal expansion, the evolution of stress and deformation upon lithiation was analyzed by using finite element simulation. An optimal thickness (~9 nm) of carbon shell was obtained so that the structural integrity and maximum capacity can be achieved for the carbon-coated SnO₂ nanowire electrode materials. In order to further improve the capacity of the anode material, the carbon-coated Si nanoparticle electrode was studied in Chapter 5. The carbon shell was found to expedite the fracture of the synthesized carbon-coated Si nanoparticles during lithiation. The fracture occurrence was summarized to be dependent on the geometrical dimensions, including the thickness of carbon shell and the diameter of Si core. The finite element simulation results showed that the carbon shell is firstly cracked due to the lithiation-induced expansion of Si core. Such crack propagates into the lithiated Si core upon further lithiation due to the elevated energy release rate near the tip of crack caused by the material's inhomogeneity along the direction of crack extension. An optimal design guideline was thereby proposed to prevent the fracture and maximize the capacity for the carbon-coated Si nanoparticle electrode materials.
To further improve the stability and capacity of the Si-based anode materials, the yolk-shell carbon-coated Si nanoparticles, which contain a void space between the yolk and shell, were studied through in situ lithiation and theoretical modeling, as discussed in Chapter 6. The geometrical dimension-dependent fracture of the nanoparticles was revealed from the experimental studies. A mechanics-based theoretical model was proposed to calculate the stress states in the carbon shell upon full lithiation. A design guideline was provided to maintain the structural integrity and maximize the capacity by optimizing the geometrical dimensions of the yolk-shell carbon-coated Si nanoparticles. Apart from voiding the fracture, interfacial stability between electrodes and cooper (Cu) current collector is also important for improving the performance of the Si-based electrode materials. In Chapter 7, the Si-coated Cu nanowires were synthesized though hydrothermal method and magnetron sputtering technique. The lithium nanostructures formed on the surface of Si shell during delithiation. The bulk lithium nanostructures reacted with the delithiated Si shell to form LixSi, inducing the fracture of the Si shell. However, the Si shell adhered well with the Cu core, indicating a stable cycling performance. These results showed the potential application of the Si-coated Cu nanowire structured anode materials for lithium ion batteries. Through the comprehensive studies of the core-shell nanostructured electrode materials, the lithiation/delithiation and fracture mechanisms of the high-capacity core-shell nanostructured electrode materials were analyzed. The experimental and theoretical approaches should be beneficial for the study of other electrode materials. The optimal design guidelines proposed in this thesis should be of great value for the design of the core-shell structured electrode materials with supreme structural integrity and high capacity for lithium ion batteries.

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