Advanced Nanostructure Materials for Hybrid Supercapacitors

Abstract

Hybrid supercapacitors (HSCs) are electrochemical devices that combine the characteristics of batteries and supercapacitors in one asymmetric cell. Lithium ion batteries (LIBs) and supercapacitors (SCs) represent two ends of the power and energy density spectrum. On one end of the Ragone plot spectrum, LIBs utilize faradaic reactions to provide high energy densities (150–250) Whkg-1, however, this relatively slow reaction process limits the power density of LIBs (<1000 W kg-1). The faradic mechanism intercalates/de-intercalates lithium into the active material, which causes changes in the chemical phase and increases the likelihood of material degradation, resulting in a limited cycle life (500-300 cycles). On the other end of the spectrum, SCs are based on electrostatic charge collection, which involve fast, reversible adsorption and desorption of ions on the surface of the active material without phase change or chemical reactions. For this reason, SCs are well known for their high power densities (~10 000 W kg-1) and long cyclability (>100 000 cycles), however SCs suffer from limited energy densities (<10 Whkg-1). Combining half-battery and half supercapacitor in one device to bridge the energy/power density gap, while improving cycle life is a promising solution to meet the evolving energy requirements. However, the rapid fading of the power density, reduced capacitance retention and reduced cyclability at high power rates are the main challenges hindering the development of HSC devices. The power density decays significantly because the faradic material at the battery part can’t adjust its rate of charge-discharge reactions to match the adsorption-desorption rate of SCs. The capacitance retention and cycle life correspond to the stability of the material on the battery component. To overcome these challenges, it is important to develop a new type of nanostructure materials with improved electrochemical capabilities. In this work, we investigate a new class of nanostructure materials with high stability and improved reaction kinetics for the faradic component of the HSC. The development strategy introduces alterations to the intrinsic characteristics of the materials, without changing their chemical phase. The investigated materials included: 1D nickel doped lithium titanate oxide nanofibers, 2D vanadium-modified chalcogens nanosheets anchored graphene nanosheets, and reconciled 2D vanadium disulfide nanosheets with prominent 3D ultra-small nanoparticles attached to graphene nanosheets. The developed materials exhibit outstanding electrochemical performance. In chapter 5, we report how a simple and scalable electrospinning technique was utilized to synthesis 1D nickel doped lithium titanate oxide nanofibers (Ni-LTONF10). The physiochemical characterization confirmed: 1) the successful insertion of nickel into the lattice of the lithium titanate oxide nanofibers (LTONF) without changing its chemical structure and, 2) that the nickel was homogenously distributed throughout the nanofibers to the atomic level, resulting in significantly enhanced ion diffusion and electrical conductivity. This unique coupling of 1D morphology and nickel doping of LTO was investigated as the anode material for lithium ion batteries capable of demonstrating outstanding rate capabilities (up to 50 times higher than theoretical capacity (50 C)). The investigated nanofibers also performs 3 times better than nickel-doped nanoparticles demonstrated in other recent reports and shows outstanding ability to maintain high capacity even at 50 C. Ni-LTONF10 shows 20 times higher capacity compared to un-doped lithium titanate nanofibers at 50 C. Specifically, Ni-LTONF10 displays an initial capacity of 190 mAhg-1 at 0.2 C which is 9% higher than the theoretical capacity of LTO, 150 mAhg-1 at 5 C, 116 mAhg-1 at 20 C and 63 mAhg-1 at 50 C. Additionaly, a hybrid supercapacitor was fabricated using Ni-LTONF10, showing superior energy density at high power density. The device was capable of delivering an energy density of 60 Wh kg -1 at a power density of 1.5 kW kg-1 and also retained a high energy density of 35 Wh kg -1 at 5 kW kg-1. In chapter 6, we discuss the development of vanadium-modified binary chalcogens (NiCo2S4) wrapped with graphene (VNCS), forming tuned 2D sheet-on-sheet nanostructure. This unique material has been synthesized using a facile solvothermal method and is used as an electrode material for supercapacitors capable of demonstrating outstanding improvement in cyclability and capacitance retention at high power rates. The VNCS material shows a superior performance with 430% improvement in capacitance retention at high power rates (50 A g-1) and 140% improvement in capacitance retention after 10,000 cycles at 10 A g-1, when compared to the un-modified material. Specifically, the VNCS showed an initial capacitance of 1340 F g-1 at 2 A g-1 and outstanding capacitance retention at 50 A g-1 (1024 F g-1). Impressively, the capacitance retention after 10,000 cycles at 10 A g-1 exceeds 90%. These exceptional results are considered at the top of reported work in literature as discussed in chapter 6. Moreover, a hybrid supercapacitor (HSC) was fabricated using the VNCS showing superior performance. The HSC delivers an energy density of 45.9 Wh kg -1 at 0.87 kW kg-1 and maintains a superior energy density of 33.6 Wh kg -1 at 9 kW kg-1 indicating the excellent potential of this material in hybrid supercapacitor applications The sheet-on-sheet structure reduced particle aggregation, provided larger surface areas with more electroactive sites for ion diffusion, enhanced the charge-discharge kinetics, which allows for faster electron transport. The morphology and structure characterization techniques confirmed that the vanadium is homogenously distributed throughout the binary chalcogens, resulting in significantly enhanced material stability at high power rates. HRTEM analysis confirmed the role of vanadium in fine-tuning the nano-architecture of the material and showed the dislocation in material structure. In chapter 7, we introduce the use of a safe and simple solvothermal method to synthesize a distinctive, flower bouquet-like, 2D vanadium disulfide (VS2) nanosheet structure with ultra-small prominent 3D VS2 nanoparticles (10-25 nm) on its surface and anchored on the surface of graphene nanosheets (VS2/G). This inimitable material has been tested in supercapacitors and showed superior capacitance, cyclability and capacitance retention at high power rates. The VS2/G showed 130 % higher capacitance at 1 A g-1 compared to other recent reports and remarkably improved capacitance at higher current densities. The material also showed a distinctive ability to maintain capacitance after long cycles at high current densities. Specifically, VS2/G showed 211 F g-1 at 1 A g-1, 135 F g-1 at 20 A g-1 and 97 % capacitance retention after 8000 cycles at 5 A g-1. The VS2/G was tested in a full cell HSC and showed superior energy density of 46.93 Wh kg-1 at a power density of 0.91 kW kg-1 and retained high energy density of 23.11 Wh kg-1 even when the power density was increased ten-fold (9.40 kW k g-1) highlighting the excellent potential of this material to bridge the gap between battery and supercapacitor technologies. The unique morphology of VS2 nanosheets embedded on graphene nanosheets with ultra-small VS2 nanoparticles distributed uniformly at the surface of the nanosheets was confirmed by different characterization techniques including SEM and TEM. The presence of graphene and the harmonized synergy between the 2D sheet-on-sheet morphology with the 3D ultra-small VS2 nanoparticles has a number of advantages. As such, it 1) hinders the agglomeration of the material and provides a large contact area with the electrolyte; and 2) generates strong covalent interactions between the VS2 with the graphene surface. These characteristics lead to an increase in capacitance due to the increase in the number of electroactive sites and improve the charge transfer kinetics, while paving shorter ion diffusion pathways, all resulting in stable and reversible charge transfer processes. Chapter 8 summarizes and concludes the thesis and suggests potential future works for capitalizing on the reported scientific achievements. The introduction of effective changes to the intrinsic characteristics of materials and the development of tuned and novel nanostructure materials, using simple and inexpensive methods, pave the way toward the development of commercial and industrial scale hybrid supercapacitor devices. Future work can investigate the design and development of thick electrodes in an attempt to exploit the high stability of the developed materials and increase the material loading on the electrodes leading to higher energy densities without scarifying the power densities. The increased working voltage and the long cycling life along with the high capacitance retention of the developed materials at high power rates can be used as a base to investigate the design of multi-stack device for industrial scale applications such as electrical vehicles, backup systems, transportation, etc. It is also recommended to further investigate incorporating the developed material in other energy storage and conversion devices.