Zinc-Based and Flexible Energy Storage Devices

鋅基與柔性儲能器件

Student thesis: Doctoral Thesis

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Award date4 Dec 2019

Abstract

The ever-growing demands for efficient harvesting, storage and utilization of renewable energy have stimulated the pursuit of alternative advanced batteries. To date, Li-ion batteries have been the protagonist since their first report in 1991 due to its stable output and versatility. Unfortunately, their limitations of high cost, insufficient energy density, and blemished safety are never too negligible to be ignored. As one of the most abundant elements in the earth's crust, zinc has a relative high capacity density of 820 mAh g−1 as well as the feature of safety and nontoxicity. Those features indeed make Zinc-based batteries such as Zn-air batteries and Zn-ion batteries are receiving increased attentions due to the low cost, high safety, and high eco-efficiency. In addition, zinc-based batteries are also suitable power source candidates for safe and flexible devices because of their intrinsic advantage in safety which negates the requirement for a rigid protective casing.

Zn-air battery require active and durable electrocatalysts on the cathode side to catalyse oxygen reduction reaction (ORR) during discharge process. Although traditional noble-metal materials like Pt-based compounds perform excellent electrocatalytic activities toward the reduction of oxygen, their scarcity, poor durability and reaction selectivity are inherent obstacles for wide commercial applications. Herein, inspired by inherent anisotropic structure for high-efficient ions transport in natural wood, a N, S co-doped carbon skeleton with a unique 3D architecture derived from bass wood was fabricated served as an ORR catalyst. With initial heteroatom embedment, followed by ammonia activation, surface functionalities, porous structures and specific surface areas of the wood carbons are simultaneously optimized. Accordingly, the resultant wood carbons exhibit superior ORR performances with a half-wave potential of 0.86 V (vs. reversible hydrogen electrode) in alkaline media, in conjunction with superb methanol cross-over tolerance and long-term stability, which then served as suitable and durable air-cathode catalysts for Zn-air battery. Simultaneously, the as-prepared materials also deliver much enhanced electrochemical performances as self-supported supercapacitor electrodes.

As aforementioned, rechargeable aqueous Zn-ion (Zn/MnO2) battery is particularly advantageous by virtue of inherent safety, negligible toxicity, and low flammability compared with traditional lithium ion batteries. Zn/MnO2 microbattery (ZIMB) was well designed to serve as a miniaturized power sources with high performance, low-cost, environmental friendliness and integratable characteristics for Internet of Things (IoT) applications. By using a simple and scalable fabrication strategy, the ZIMB revealed a superior capacity and excellent rate performances, substantially higher than those of the reported lithium-ion MBs. More importantly, the most attractive advantage of ZIMBs is the strong ability of monolithically integration with other devices and its provision of aesthetic versatility. To demonstrate the versatility in real world applications, several flexible MB arrays were fabricated to power LEDs, smart watch and electroluminescent panel. With the proposed strategy, the fabricated ZIMBs can be seamlessly integrated into any printed circuit, sensors, or artwork; the properties of these ZIMBs can be easily tuned by simple pattern design, fulfilling the increasing demand of highly customized power systems in the IoT and flexible/wearable electronics industries.

As the mechanical reliability of flexible energy storage devices requires more attention than traditional power source, we tried to build fair methodology for flexibility and softness evaluation, where flexible Zn-ion (Zn/MnO2) battery was thus selected to construct flexible energy storage devices. Various parameters adopted in bending and stretching tests were discussed in detail to evaluate the flexibility and reliability. Then, referring to the international standards to evaluate the softness of leather, a softness parameter was proposed for assessing the as-fabricated flexible batteries. Softness was more associated with wearability, while wearability influenced how people feel when they used flexible electronics. A systematic and commonly acceptable standard test methodology was designed to accurately evaluate the flexibility, stretchability, and wearability of flexible energy storage devices.

In summary, this thesis explored suitable and effective electrode/electrolyte materials as well as more preferable cell configuration and structural designs to develop zinc-based batteries with better electrochemical performances together with enhanced flexibility and integrability for integration into flexible electronics and IoT industries. It is believed the strategies and results would inspire the ingenious designs and developments of energy storage and conversion devices for the future soft electronic applications.

    Research areas

  • zinc-based battery, flexible energy storage devices