|dc.description.abstract||With the continued increase in energy demand for portable electronics, grid storage, and electric vehicles, more attention is being placed on the development of advanced energy conversion and storage systems such as metal ion batteries and fuel cells. Recently, lithium-ion batteries (LIBs) have dominated the electronic applications market such as consumer electronics, power tools, and medical devices. Moreover, LIBs have been used in the transportation sector in electric vehicles (EV) and electric bicycles. High capacity retention and long cycle life are essential, especially for the EV market. However, due to the limited energy density and high cost of large LIBs packs, the current battery technology is not satisfactory for the widespread application in EVs. Therefore, development of battery technology with high-energy density and low-cost materials can lead to significant improvements in the performance and lifetimes of products that use LIBs.
To improve the energy density of LIBs, conventional anode materials (graphite) need to be replaced by novel electrode materials and improved electrode designs with a higher capacity and more reliable performance. Silicon (Si) is an exciting and promising candidate for use as active material in the negative electrode to develop the next generation LIBs due to its natural abundance, high safety, low-cost, environmentally friendliness, and high theoretical specific capacity reaching 4200 mAh g-1 compared with 372 mAh g-1 of graphite. However, the critical challenge with Si is the huge volume changes during the lithiation and delithiation processes, which causes mechanical fractures and delamination of the electrode. In addition, solid electrolyte interphase (SEI) formation disrupts the electrical contact between Si particles during cycling, which lead to degradation of the electrode and rapid capacity fading. These issues limit the wide commercialization of Si as anode material for LIBs.
In this thesis work, different categories of advanced nanostructure materials have been designed and developed to serve as a conductive network for nanostructured Si morphologies with high capacity and better mechanical stability to enable the evolution of the next generation of LIBs. This thesis starts with a brief introduction to LIBs, followed by the objectives and approaches taken in this PhD project. A literature review on the main battery components and the operation principles of rechargeable LIBs with a focus on the development of the electrode materials will be discussed. A survey of the experimental procedures, characterization techniques, and performance testing procedures are provided. Specific research projects are proposed, and specifically demonstrated in the projects presented in this thesis. This will provide readers with a comprehensive overview of the field of study, and detailed project plans in order to successfully develop novel advanced electrode materials for high energy density and reliable rechargeable LIBs.
The first approach of my PhD thesis is focused on developing flexible and conductive carbon networks to improve the stability of Si-based anodes. At this stage, we have designed a polymer blend of polyvinylpyrrolidone (PVP) and polyacrylonitrile (PAN) which was self-assembled onto the surface of Si nanoparticles (SiNPs) allowing for the generation of a very intimate coating of Si dioxide and nitrogen-rich carbon shell upon slow heat treatment. This methodology capitalizes on the surface interaction of PVP with SiNPs to provide a sturdy nanoarchitecture. The addition of PVP improves the stability and adhesion of PAN to the carbon-based matrix which surrounds the Si particles, leading to enhance the stability of the Si anode. In addition to being a very scalable fabrication process, our novel blend of PVP and PAN allowed for an electrode with high reversibility. When compared with a standard electrode Si/PVDF framework, this material of PVP/PAN demonstrated a significantly superior first discharge capacity of 2736 mAh g-1, high Coulombic efficiency, and excellent rate capability, as well as excellent cycle stability for 600 cycles at a high rate of 3000 mA g-1.
Even though we achieved considerable improvements to the Si-based anode, we still need to improve the electrode capacity with long cycle stability and high areal capacity. In the second part of this thesis, a multifunctional composite binder was developed by cross-linking a poly(acrylic acid) (PAA) and carboxymethyl cellulose (CMC) spine with PAN through a slow heat treatment process. The composite binder strongly interacts with Si, providing a sturdy structure with efficient pathways for both Li-ion and electron transport. The cross-linked carboxyl groups from PAA and CMC offered a robust 3D cross-linked network, anchoring SiO2 coated Si nanoparticles onto a highly-porous carbon scaffold, forming stable a solid electrolyte interphase. This composite anode not only exhibits a high initial capacity of 3472.6 mAh g−1 with an initial Coulombic efficiency of 89.1%, but also provides excellent cycling stability for 650 cycles at a high current density of 3000 mA g−1.
While excellent rate performance and dramatic enhancement of Si-based anode were obtained using cross-linking of CMC-PAA with g-PAN, we looked to further improve the cycle life with high capacity using reinforcement additives. In the last part of this thesis, a novel multi-leveled design of webs-like morphology is reported as a robust and highly stable 3D interconnected network to mass-produce nanostructured Si composite anode. This sturdy composite consisting of nano-size Si particles (NSi), nitrogen-doped carbon nanotubes (N-CNTs), and graphenized polyacrylonitrile (g-PAN) is prepared via a simple and low-cost method as a negative electrode for LIBs. The NSi@N-CNT/g-PAN composite integrates the benefits from its components, where NSi-interactive materials deliver high capacity, N-CNTs with nitrogen functionalizations act as electron highways and flexible network to connect NSi particles, and g-PAN with nitrogen-rich provides nitrogen-doped graphene sheets, which wrapped the whole structure network of NSi@N-CNTs. The stable interaction between the Si particles and N-CNTs enhances electron transport, while g-PAN effectively improves the capacity and conductivity of the whole electrode and provides a porous skeleton allowing convenient ion diffusion leading to longevity in battery operations. We found that only when all three components are introduced will significant enhancement in performance be observed. This nanocomposite anode exhibits superior cycling stability with a reversible capacity of ~1361 mAh g-1 for a remarkably long-life of 1100 cycles when cycled at a high current density of 3000 mA g-1. Moreover, high loading cycling of up to 3 mAh cm-2 at ~1 mgSi cm-2 was achieved at a current density of 500 mA g-1. This effective strategy could potentially be applied to prepare large-scale production of a high-performance electrode for LIBs.||en