Flexible Energy Storage: From Yarn Ni-Co//Zn Batteries to Functional Supercapacitors

Abstract

With the development of smart electronic devices, research in the related fields such as LED screens, energy storage devices and packaging materials has begun to develop. The energy storage part, as a key device to determine the endurance of electronic equipment, has naturally attracted widespread attention. Considering the development trend of intelligent, flexible and multi-functional electronic devices, higher requirements are placed on the energy density and flexibility of electrical storage devices.

Flexible electrical storage devices are a type of electrical storage devices developed based on the needs of flexible electronic devices. Generally, flexible substrates and current collectors (such as metal foil, carbon cloth, etc.) are used to realize the flexibility of the device, while conventional electrode materials are used to obtain energy storage. After achieving flexibility, due to the requirements of wearable electronic devices, the overall softness, comfort, breathability and safety of the device have further attracted the attention of researchers, and the realization of these mechanical properties depends on the innovation and development in both material selection and preparation technology.

After comprehensively analyzing the preparation methods of various flexible electrical storage devices, this thesis proposes methods for preparing flexible electrical storage devices based on different functional levels, including yarn-like Ni-Co//Zn flexible batteries based on stainless steel yarns, two-dimensional wearable supercapacitor based on gold leaf gliding polyester cloth and stretchable all-polymer supercapacitor based on double network agar/polyacrylic hydrogel. The comfort, gas permeability and stretchability of the electrical storage device are achieved by changing the active material and the fabrication method.

Firstly, a yarn-shaped battery based on Ni-Co//Zn aqueous system was designed. Through the design of one-dimensional yarn, the softness and safety compatibility of flexible wearable battery were realized as well as the high conductivity of metal current collector. Metals are widely used as substrate materials and current collectors for electrical storage devices due to their good electrical conductivity, low cost and compatibility with active materials. However, due to the hardness of the metal, it is difficult to be applied directly to a flexible electrical storage device. By making the conventional stainless steel into a one-dimensional stainless steel yarn, not only the flexibility of the metal can be achieved, but also the structural design can be further carried out. Ni-Co and Zn electrodes can be obtained by electrochemical deposition, and the yarn structure has a large specific surface area, thereby increasing the adhesion amount of the active material and therefore improving its electrical storage performance. At the same time, the one-dimensional yarn structure also gives the device greater flexibility, not only as a yarn like battery, but also as a raw material for the further textile preparation. By controlling the composition of the electrolyte, the composition ratio of the Ni-Co composite electrode can be controlled. At the same time, the addition of Co and the use of a mild solid electrolyte significantly improve the stability and cycle performance of the electrode. In the aqueous solution, the battery capacity can be maintained above 120 mAh/g, and a high capacity of 82.4 mAh/g can also be achieved in the solid electrolyte. More than 70% of the capacity can be maintained even after 1000 cycles. The yarn-like structure provides greater flexibility and designability for this battery, effectively extending its range of applications.

Although metal can be used as a substrate to obtain a higher energy density, it is difficult to meet the requirements of comfort and wearability because of its high density and hardness. By paving the gold leaf on the polyester cloth with electrostatic attraction, a flexible substrate achieving both gas permeability and electrical conductivity is obtained. The softness and the tailorability of this substrate are similar to the commercial fabric, and the mechanical properties are also comparable to those of the common fabrics. At the same time, the gold leaf provides excellent electrical conductivity. The polypyrrole nanowires are deposited on the substrate electrochemically as an active material. The network structure provides a larger contact area for the active sites and the electrolyte, and the fluffy network layer can also provide a looser, softer texture layer for a flexible electrode material that withstands bending, cutting, sewing while maintaining the original performance. This composite electrode can be further prepared as a soft, breathable and wearable supercapacitor with an area ratio capacitance of 17.14 mF cm^-2 at 0.2 mA cm^-2, a volumetric energy density of 161.6 kW m^-3 at the current density of 1 mA cm^-2, and a power density of 13.5 kWm^-3. At the same time, the fabrication technology of the PET/gold leaf substrate also guarantees the designability of the whole device. It can design the series and parallel circuit of the whole device by designing the area and the shape of the gold leaf overlay, which provides an idea for the design and preparation of the wearable supercapacitor component.

The stretchable device is a branch of a flexible electrical storage device which performs the stretching function through a pleated/corrugated structure or due to its pristine elasticity. However, due to the limitation of electrolyte materials, most stretchable devices can only be stretched for a limited number of times, and cannot completely recover to the original state. By preparing an Agar/hydrophobically associated polyacrylamide double-network hydrogel and combining a physical network with a chemical network, a hydrogel electrolyte that can be stretched-recovered for multiple times is obtained. The double network hydrogel had a residual strain of less than 5% after 30 stretching cycles of 500% deformation, and a residual strain of 6.2% after 1000 stretching cycles of 100% deformation, as well as still maintained good ion conductivity. The annealed PPy film worked as the electrode material, and the annealing process can make the structure of PPy regular, which effectively improve the conductivity of PPy. The PPy film is thin and soft, which can adhere to the surface of the hydrogel and form a pleated structure without mechanical cracking, which can effectively ensure the stretching-recovery performance of the overall device. Based on the above materials, an all-polymer stretchable supercapacitor can be prepared, which performs a residual strain of only 10.2% after 1000 stretching-recovery cycles of 100% deformation, and a capacitance of up to 79.7 mF cm^-2 which can be maintained even after 1000 stretching cycles.

Date of Award	30 Jan 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunyi ZHI (Supervisor) & Shanyi Du (External Supervisor)