GRAPHENE-BASED MATERIALS FOR LI-ION CAPACITORS (LICS)

Abstract

Li-ion capacitors (LICs) are a relatively new type of energy storage devices that merge the features of Li-ion batteries (LIBs) and supercapacitors (SCs), providing the potential to combine the best characteristics of both technologies. However, currently, most LICs tend to achieve high energy density at the expense of high power and long cyclability, and vice versa. This is because of the slow reaction kinetics of the LIB-type anodes, which fail to match the rapid adsorption at the SC-type cathodes. Moreover, SC-type cathodes often have a lower storage capacity than LIB-type anodes, creating a storage capacity mismatch between the two electrodes. The holy grail of the present research is to create a system with high energy and power outputs, in addition to long cyclability, to fulfil the dramatically growing demand for 21st century green technology, from electric cars to mobile phones, laptops, portable healthcare monitors, etc. Therefore, this thesis specifically aims to develop and synthesise novel graphene-based materials using easy, cost-effective, and scalable synthesis methods for both anodes and cathodes of LICs. For the anode, a three-dimensional (3D) nitrogen-doped rGO-siloxene nanocomposite (NGSil-4) has been synthesised using an easy and low-cost hydrothermal treatment, and its electrochemical performance as an anode material for LICs has been demonstrated. The obtained nanocomposite has 3D interconnected macroporous channels with defects, resulting in improved kinetics and structural stability with high reversible capacity (472.07 mAh g−1 at 0.1 A g-1), good rate performance (176.63 mAh g−1 at 6 A g-1), and good cyclability. For the cathode, a 3D rGO with porous structure and nitrogen and phosphorus heteroatom doping (NPHG) has been synthesised using an easy and cost-effective hydrothermal treatment. Its potential as a SC-type cathode for LICs has also been explored. In the prepared NPHG, the micro-/meso-/and macroporous structure combined with the N and P heteroatom doping. Consequently, the NPHG cathode achieved a good reversible specific capacity of ~120 mAh g−1 at 0.1 A g−1 and excellent long-term cycling performance, in which 92% of its capacity was retained after 1000 cycles at 1 Ag-1. Finally, the energy densities, power densities and cycle lifetime performances of the fabricated LIC with the NGSil-4 anode and the NPHG cathode have been evaluated. The LIC delivered an energy density of 145.86 Wh Kg-1 at a high-power density of 200.2 W Kg-1, with a considerably high cycling performance (84% after 1000 cycles). These good results are due to the excellent electrochemical properties of the NGSil-4 anode and the hierarchical porous NPHG cathode, showing that these graphene-based materials are a way forward to obtain high energy densities and high-power densities.