Abstract
Wearable electronics require the matching batteries with excellent flexibility, high safety, and superior electrochemical performance. Lithium-ion batteries (LIBs) and Zn-based batteries currently can meet the requirements of superior electrochemical performance and high safety, respectively, while other properties of these two batteries need to be improved. Achieving high flexibility is crucial for promoting the application of LIBs in wearable electronics, given their high energy density and long cycling performance. For intrinsically safe aqueous Zn-based batteries, it is imperative to improve their electrochemical performance, particularly the operating voltage which typically hovers around 1 V, which severely limits their practical applications. Therefore, to advance the process of the flexible energy storage devices, it is necessary to conduct studies from two perspectives: developing flexible LIBs with excellent flexibility and Zn-based batteries with high operating voltage.Simultaneously achieving a small bending angle, multiple deformation modes, superior mechanical durability and high energy density of flexible LIBs remains a challenge. Here, inspired by a human joint, I fabricate a novel and rational structural design for flexible LIBs. In the battery, thick and rigid stacks for storing the main energy are equipped with cumbered surfaces and interconnected thin parts, imitating the articular surface-ligament structure of a human joint providing flexibility for the whole battery. The configuration of thick stacks can be changed by different winding technologies, which endows the battery with abundant deformability, including bending, twisting, stretching and even winding. Finite element simulation confirms that our designed battery will not lead to the irreversible plastic deformation of metal current collectors under various harsh and complex deformations. The flexible battery with cubic energy storage units exhibits a high energy density of 371.9 W h L−1, which is 92.9% of a conventional pouch cell. Furthermore, it can maintain stable cycling performances, even undergoing over 200 000 times dynamic bending and 25 000 times dynamic twisting deformations. The battery with cylindrical energy storage units can withstand more harsh and complex deformations. After undergoing over 100 000 times dynamic stretching, 20 000 times twisting and 100 000 times bending deformations, a high-capacity retention of over 88% can be attained. Accordingly, the novel and unique flexible LIBs provide great promise for their practical applications in wearable electronics.
Next, considering the operating voltages of Zn-based batteries are normally below 2 V with conventional electrolytes. We develop a unique phase-separation electrolyte (PSE) consisting of a completely immiscible aqueous phase and an oil phase. The alkaline aqueous electrolyte can take advantage of the low electrode potential of the zinc anode. The interface ion transfer electrochemistry in the PSE can further boost the operating voltage by ~0.35 V. Accordingly, our developed Zn-based batteries deliver an unprecedented average operating voltage of 3.41 V, approaching the voltage of LIBs. More interestingly, the liquid-liquid interface in the PSE can entirely intercept the propagation of Zn dendrites, benefiting from the completely blocked diffusion of Zn2+ into the oil phase. The Zn-based batteries with LiMn2O4 cathodes deliver an excellent cycle performance over 600 cycles with a 99.6% coulombic efficiency. To further demonstrate practicality, I fabricated a full cell commercial-level cathode mass loading of 18.3 mg cm–2, achieving a zinc-based battery with a high voltage of 2.56 V and a high areal capacity of 2.14 mAh cm–2.
Finally, since graphite has the advantages of low cost, abundant resources, environmentally friendless, and high redox potential, I developed a Zn||Graphite dual-ion battery with PSE. The energy state of Zn2+ ions in electrolytes can be significantly reduced by forming stable Zn(OH)42−, which enables the Zn||Graphite dual-ion batteries to exhibit high operating voltage. In addition, benefiting from the stable CEI formed on the surface of graphite in oil-phase electrolyte, the co-intercalation of solvent molecules into graphite is illuminated, resulting in excellent cycle performance of the dual-ion battery. As a result, the Zn||Graphite dual-ion batteries deliver a record-high operating voltage of over 3 V, which is comparable to alkali metal batteries. After 1000 cycles, a high-capacity retention of 75% is achieved with an average coulombic efficiency of over 98.2%. To further demonstrate practicality, I also fabricate batteries with high mass loading cathode of 22 mg cm−2 and commercial graphite paper cathode, delivering operating voltages of over 2.2 V and 2.6 V, respectively.
In summary, the rational structural designs for flexible LIBs are investigated to achieve high flexibility, excellent mechanical durability, and stable electrochemical performance. The electrochemical performances of Zn-based batteries with PSE also have been studied in depth in this thesis. These strategies and results are believed to have significant implications for promoting the practical application of flexible energy storage devices.
| Date of Award | 25 Jul 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chunyi ZHI (Supervisor) & Ji-jung KAI (Co-supervisor) |