Silicon-based composites as anodes for lithium ion batteries

Abstract

Owing to the rapid development of portable electronic devices, there has been an increasing demand for lithium-ion batteries (LIBs) with high energy densities and long cycle life. The energy densities and performances of LIBs largely depend on the physical and chemical properties of the electrode materials. Silicon-based anode has emerged as one of the most promising next generation anode material due to its ultrahigh theoretical capacity (4200 mAh·g-1) and low working potential. Despite these distinct benefits, applications of silicon-based anode are affected by significant defects, such as capacity degradation and electrode pulverization due to the huge volume change of ~400 % during electrochemical cycling. This thesis is dedicated to synthesizing silicon-based anode materials with various morphologies, such as yolk-shell structure, 2D nanosheet and 3D hierarchical structure through different methods, to solve the application challenges related to silicon. Chapter 1 introduces the background and principles of lithium-ion battery and provides an outline of my research direction. Chapter 2 reviews previous literature addressing problems caused by the volume change of the silicon anode, including morphology, composite materials, electrolyte, binders, etc. Chapter 3 introduces the methodologies implemented within this thesis which include that of synthesis, characterization, and testing. In Chapter 4, a variety of yolk-shell carbon-coated silicon nanoparticles are successfully synthesized based on an optimal design guideline. In situ lithiation and delithiation of the yolk-shell nanoparticles under transmission electron microscopy clearly reveals the working mechanism of the silicon-based yolk-shell structured composite material, and also clarifies how carbon shell protect silicon nanoparticles from pulverization during cycling. Optimised yolk-shell silicon-based nanocomposite yields a capacity of 2219.6 mAh·g-1 and achieves a reversible capacity of 1666.9 mAh·g-1 after 80 cycles with 75.1% capacity retention. These results are significant for the design of yolk-shell structured anode materials for high-capacity LIBs and superior cycling performance. However, the application of the design of yolk-shell structured anode materials is limited due to the complexity of synthesizing yolk-shell structure and the uncontrollable morphology of commercial silicon nanoparticles. To tackle this problem, the two-dimensional (2D) porous silicon nanosheets (Si-NSs) are synthesized using graphene oxide nanosheets template as introduced in Chapter 5 of this thesis. The obtained Si-NSs, which are aggregations of silicon nanocrystals of ~10nm, deliver a reversible capacity of 800 mAh·g-1 after 900 cycles at a rate as high as 8.4 A·g-1. The superior electrochemical performance of the Si-NSs anode can be attributed to their unique 2D mesoporous structure as demonstrated by the ex-situ measurements and in-situ characterizations. This study provides a new methodology for the design and fabrication of 2D Si-NSs with enhanced electrochemical performance. In Chapter 5, the magnesiothermic reduction reaction with 2D SiO2/r-GO is performed, aiming to obtain a silicon-carbon composite to solve the challenge of unstable cycling performance. However, due to the high temperature of magnesiothermic reduction, the forming of SiC results in capacity attenuation. Therefore, in Chapter 6, SiO2/g-C3N4 is used as a precursor, and 3D N-doped Si/C is successfully synthesized by magnesiothermic reduction without any SiC formation. The obtained N-doped Si/C exhibits a porous structure composed of nitrogen doping wrinkled silicon-carbon nanocomposite which displays a superior reversible specific capacity of 1005.3 mAh·g-1 at 4.2 A·g-1 after 250 cycles with a more stable cycling performance and rate capability. This outstanding performance can be attributed to the novel 3D hierarchical porous nanostructure, which reduces lithiation-introduced volume expansion, accelerates lithium-ion transmission and improves conductivity. Chapter 7 of this thesis discusses the conclusions of this research and sets forward proposals for future research directions.