Materials Design and Engineering for High Energy-Density Rechargeable Zinc-air Batteries
by Hua Zhang
Date of Examination:2022-04-12
Date of issue:2022-06-09
Advisor:Prof. Dr. Kai Zhang
Referee:Prof. Dr. Kai Zhang
Referee:Prof. Dr. Franc Meyer
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Name:PhD thesis-Hua Zhang--without CV.pdf
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EnglishThe growing demands for energy and the associated environmental pollution have sparked a great deal of interest in the development of clean energy technologies including fuel cells, metal-air batteries, supercapacitors, and hydrogen production. Among them, Zinc-air batteries (ZABs) are potential possibilities because of their low cost, high theoretical energy density, high-level safety, and environmental friendliness. In addition, zinc (Zn) is abundant in the environment and far less expensive than lithium metals. Thus, ZABs have been proposed as promising energy sources for grid-scale energy storage systems, as well as to replace or supplement lithium-ion batteries for next-generation electric vehicles. However, the development of rechargeable ZABs with high energy output and long lifespan remains a big challenge. For instance, a typical ZAB has a nominal cell voltage of 1.65 V, while most of the reported ZAB devices are capable of only 1.3 V or 1.4 V, which is much far below lithium-ion batteries. As a result, the energy density of ZABs is often lower than 1000 Wh kg-1, far away from the predicted value of 1353 Wh kg-1 (excluding oxygen). In this regard, it is vital to investigate the synthesis of advanced materials and unique structural designs to further increase the operating voltage and energy density of ZABs. Besides, due to the rapid development of lightweight and portable smart electronic devices, the miniaturization of ZABs with high energy density is highly desirable but challenging. To this end, this thesis mainly focuses on the development of high-performance catalyst and separator materials accompanied by engineering and configuration design to fabricate high energy-output ZAB systems including miniaturization ZABs at the microscale and asymmetric-electrolyte ZABs with high voltage. This work starts with an overview of the current status and scientific challenges of rechargeable ZABs. We first provide the operating principle and current configurations of ZABs, as well as their advantages and disadvantages. Then, the chemistry and major developments of key factors determining the performance of ZABs are conscientiously discussed, such as the Zn electrode, electrolytes, separators, and bifunctional air electrode. Briefly, an understanding of the issues hindering the electrochemical performance (e.g., efficiency, durability, and cycle-life) of ZABs are presented, which are mainly caused by serious corrosion of the Zn electrode, unsatisfied bifunctional electrocatalysts, and the carbonate formation at the air cathode. Particularly, bifunctional electrocatalysts with high activity and durability on the air cathode are highly desirable, but they are challenging to create. To address this issue, a Co2+-coordinated porphyrin-based organic covalent framework (POF) was fabricated as an effective bifunctional electrocatalyst for ZABs by blending Co catalytic units into the POF and then hybridizing with conductive scaffold CNTs. Controlling the C-N coordination and the valence of Co units can regulate the bifunctional catalytic activity of the CoPOF@CNT catalyst. The CoPOF@CNT catalyst was then combined with a porous gas diffusion layer to create a high-performance carvable air cathode. In a poly(vinyl alcohol-co-poly(acrylic acid) hydrogel (PVA-co-PAA) electrolyte, the resultant air cathode demonstrated a high peak power density of 89 mW cm-2 and cycling durability for 110 cycles. Based on the high-performance carvable air cathode and highly adhesive PVA-co-PAA gel, a microimprint manufacture route was proposed for assembling an on-chip Zn-air rechargeable microbattery (μZAB), which avoids the difficulty of the catalyst integration on the chip at a target location. Impressively, the on-chip μZAB demonstrated a record-high volumetric power density of 570 mW cm-3 and a volumetric energy density of 413 Wh L-1, about 3 times that of a commercial compact primary ZAB. The on-chip μZAB also reaches a lifecycle capacity of 4.5 mAh, which is roughly double that of the current commercial on-chip tiny lithium-ion battery. Our method closes the gap between advanced material production and on-chip integration, opening the door for high-performance on-chip ZABs. After that, a quasi-solid-state ZAB with asymmetry-electrolyte (sAZAB) having both improved discharge voltage and substantially high energy density is presented. To achieve such a sAZAB, we proposed a combination strategy including the fabrication of a low-cost Zn2+-conductive polyimide (ZnPI) separator and an efficient COF-based bifunctional electrocatalyst (CoPOF@MXene). The coordination of carbonyl groups and Zn2+ ions renders ZnPI with highly selective transportation of Zn2+ ions. In terms of the bifunctional catalyst, the synergistic interaction between Co-Nx active sites and highly conductive MXene substrate contributes to high catalytic activity in an acidic medium. The sAZAB was then assembled by using a Zn anode, a CoPOF@MXene based air cathode, and a ZnPI separator sandwiched in between the alkaline anolyte (PAA-co-PVA + 6 M KOH) and the acidic catholyte (PAM-co-PVA gel + 3 M H3PO4). As a result, the as-fabricated sAZAB demonstrated excellent rate performance and cycling stability (~100 h), as well as a high open-circuit voltage of up to 2.1 V and an average round-trip efficiency of 70%. Moreover, to improve the battery performance at a high discharge of depth, an optimized sAZAB with the enhanced power density and specific capacity was further constructed by combining the (002)-textured Zn electrode and Zn3(PO4)2-based catholyte. At a current density of 5 mA cm-2, the optimized sAZAB coin cell achieves an impressive specific capacity of 742.4 mAh gZn-1 and an energy density of 1425.4 Wh kgZn-1, outperforming reported conventional quasi-solid-state ZABs. At the cell level, an ultrahigh project energy density of 55.5 Wh kgcell-1 is attainable, which has surpassed most of the reported works. This work closes the gap between fundamental research and the practical application of rechargeable ZABs, paving the way toward advances in materials science and structural engineering for electrochemical energy and storage systems. The presented study is a monography work containing two publications. One of them has been already published and the other one is under submission. The general background and main topic blocks are presented in chapters 2-4.
Keywords: Zn-air batteries; High energy output; Materials design; porphyrin-based organic covalent framework