Abstract
With rising energy concerns, efficient energy conversion and storage devices are urgently required to provide a sustainable, green energy supply. Electrochemical energy storage devices, such as supercapacitors and batteries, have been proven to be the most effective energy conversion and storage technologies for practical application. Supercapacitors and lithium-based batteries are particularly promising because of their excellent power density and energy density. However, further development of these energy storage devices is hindered by their poor electrode performance. The carbon materials used in supercapacitors and batteries, such as graphite, activated carbons and various nanostructured carbon materials (ordered porous carbon, CNT, graphene etc.), are often derived from non-renewable resources under relatively harsh environments. Natural abundant biomass is a green, alternative carbon source with many desired properties to derive renewable carbon materials for supercapacitors and lithium-based batteries. Therefore, it has been of great social and economic significance to develop renewable carbon materials from natural abundant biomass materials in order to realize sustainable battery materials sourcing.
It has also been predicted that next-generation electronics will be flexible and wearable. Many efforts have been devoted to developing safe, lightweight and flexible power sources with the goal of meeting the increasing need for future wearable/flexible electronics. This dissertation aims to study the mechanism and technologies for converting cotton textile into renewable, flexible and conductive carbon substrate at a low cost, high throughput way for flexible energy storage applications. In this dissertation, cotton textile, as the commonly overlooked everyday households, has been chosen as the starting precursor materials to prepare renewable flexible conductive substrates for different energy storage systems.
Specifically, in Chapter 2 & 3, natural abundant cotton textiles were first converted to flexible, conductive activated cotton textile (ACTs). The obtained ACT was further chosen as a flexible substrate to design flexible supercapacitor. In order to push up the energy storage capability, high energy density metal oxides, such as core/shell NiCo2O4@NiCo2O4, 3D porous CoO@NiO, were controllably deposited on the ACT fiber with desired microstructure. Both symmetric and asymmetric supercapacitors were assembled and tested. Inspired by using flexible ACTs for supercapacitors application, in Chapter 4, NiS2 nanobowls wrapped with conductive graphene sheets (ACT/NiS2-graphene) were deposited on ACT fibers by a simple two-step heat treatment method to fabricate binder-free electrode for flexible lithium-ion battery. When used as a binder-free electrode, the ACT/NiS2-graphene electrode exhibited an exceptional electrochemical performance. Encouraged by the success of the design of flexible lithium-ion batteries with flexible ACTs, in Chapter 5, we extended the application of flexible ACTs for lithium-sulfur (Li-S) battery applications. The assembled lithium-sulfur cell also exhibited exceptional rate capability and durable cyclic performance (with a well-retained capacity of ~1016 mAh g−1 even after 200 cycles). A flexible Li-S cell with ACT/S-rGO as a cathode was also assembled to demonstrate its superior potential as flexible power sources. In Chapter 6, a new built-in magnetic field enhanced polysulfide trapping mechanism was discovered by introducing ferromagnetic iron/iron carbide (Fe/Fe3C) nanoparticles with a graphene shell (Fe/Fe3C/graphene) onto a flexible activated cotton textile (ACT) fiber to prepare the ACT@Fe/Fe3C/graphene sulfur host. The novel trapping mechanism is demonstrated by significant differences in the diffusion behaviour of polysulfides in a custom-designed liquid cell compared to a pure ACT/S cathode. In Chapter 7, we proposed a scalable roll-to-roll manufacturing approach to integrating a flexible solar cell with a flexible home-made flexible supercapacitor to fabricate self-sustainable power pack, which demonstrates huge potential for future off-grid and micro-grid application.
These research activities not only brought new insights on the deriving renewable carbon materials from natural abundant biomass resources but also boosted the design and fabrication of next-generation flexible energy-storage devices, which hold great promise for future wearable/flexible electronics.
