Flexible/Wearable Zinc Based Energy Storage Devices

Abstract

Flexible and wearable technologies are leading the trend of next-generation electronic products. Wearable devices such as Apple Watch and Google Glass are emerging in the mainstream as young people’s new favorites. It is believed that these electronics will bring significant change to human lifestyle in the near future due to the infinite possibilities they could offer. These electronics usually need suitable energy storage devices such as batteries to power them. However, the current available power sources including alkaline batteries and lithium-ion batteries are usually heavy, bulky and rigid, leading to the poor compatibility with flexible and wearable electronics. Hence, it is crucial to develop energy storage devices with competitive electrochemical performance and miniaturized size, as well as the flexibility that allows their function in wearable situations.

Polymer electrolyte holds the key for the development of high-performance solid-state energy storage devices. Herein, we first developed a novel hierarchical gelatin and polyacrylamide (PAM) based electrolyte (HPE) and constructed an extremely safe solid-state rechargeable and wearable Zn/MnO₂ battery (ZIB) based on such polymer electrolyte. The grafting of PAM on gelatin hydrogel significantly enhances the mechanical strength and ionic conductivity of the HPE film, while the polyacrylonitrile (PAN) fiber membrane efficiently reduces the possibility of a battery short circuit in addition to further improving the strength of HPE. The ZIB with HPE delivers a high areal energy density and power density (6.18 mWh cm⁻² and 148.2 mW cm⁻², respectively), high specific capacity (306 mAh g^-1) and excellent cycling stability (97% capacity retention after 1000 cycles at 2772 mA g^-1). More importantly, the solid-state ZIB exhibits excellent safety and wearability, demonstrating the ability to work under a number of severe conditions including being cut, bent, hammered, punctured, set on fire, sewed by a commercial sewing machine and even washed in water without any packaging. For demonstration, ZIBs were connected in series to power a commercial smart watch, a wearable pulse senor and a smart insole.

Next, a high-performance waterproof, tailorable and stretchable yarn-shaped ZIB based on a novel cross-linked PAM electrolyte and double helix yarn electrodes was developed. Benefiting from the high ionic conductivity of the PAM electrolyte and high loading of MnO₂ nanorods in helix structured electrodes, the yarn ZIB delivers a high specific capacity and volumetric energy density (302.1 mAh g^-1 and 53.8 mWh cm^-3, respectively) as well as excellent cycling stability (98.5% capacity retention after 500 cycles). More importantly, the quasi-solid-state yarn ZIB also demonstrates superior knittability, good stretchability (up to 300% strain) and excellent waterproof ability (high capacity retention of 96.5% after 12 h underwater operation). Besides, the long yarn ZIB can be tailored into short ones and each part still functions well. For demonstration, a 1.1-m-long yarn ZIB was cut into 8 parts and woven into a textile which was used to power a long flexible belt with 100 LEDs embedded and a 100 cm² flexible electroluminescent panel.

Finally, we explored MoS₂ with expanded inter-layer spacing (E-MoS₂) as a promising cathode candidate for rechargeable and flexible ZIBs. By X-ray diffraction (XRD) and Raman studies, a reversible Zn²⁺ ion intercalation/deintercalation mechanism was revealed. The E-MoS₂ electrode delivers a specific capacity of 202.6 mA h g^-1 at 0.1 A g^-1, a desirable energy density of 148.2 Wh kg^-1 and good cycle stability (98.6 % capacity retained over 600 cycles). By using the newly-developed starch/ polyacrylamide (PAM) based polymer electrolyte with high zinc ion conductivity, a quasi-solid Zn/E-MoS₂ battery was developed, which exhibits decent electrochemical performance even under various heavy deformations.

In summary, different polymer electrolytes and cathode materials as well as flexible/wearable zinc based energy storage devices have been studied in depth in this thesis, targeting at obtaining high electrochemical performance together with enhanced safety property as well as superior flexibility. It is believed that strategies and results in the thesis can shed new light on the design and development of other multi-functional flexible and wearable devices in the near future.

Date of Award	2 Jan 2019
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunyi ZHI (Supervisor)