Liu, Mingqiang; (2024) Modulation on Ion Transport Behavior and Interfacial Reaction for Highly Sustainable Aqueous Zinc Metal Batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Aqueous metal batteries (AMBs) with non-organic electrolytes are a promising energy storage technology, compared to the batteries with organic electrolytes such as lithium-ion batteries, primarily due to their low costs, environmental friendliness and intrinsic safety as they can be operated in non-flammable and non-toxic aqueous solutions. Aqueous zinc-ion batteries (AZIBs), based on Zn metal anode (ZMA), have many particular advantages such as abundant natural reserves, inexpensive price, high hydrogen evolution overpotential, high specific energy, which makes them suitable for mass production. However, the plating-stripping electrochemistry of zinc anodes in aqueous electrolytes has to date suffered from a low Coulombic efficiency (CE), caused by hydrogen evolution reaction (HER) corrosion side reactions associated with irreversible byproducts. Large and disruptive hexagonal dendrites are easily formed in many AZIBs, due to inhomogeneous deposition on the surface of ZMA caused by the disordered diffusion and uneven distribution of cations in the electrolyte, leading to capacity fading, consumption of electrolyte, and short circuits. Hence, it is imperative to modulate ion transport behaviors (e.g., diffusion and migration) and interfacial reactions to improve the electrochemical performance of metallic zinc anode. Here I tried to modulate ion transport behavior and interfacial reaction from three aspects (electrolyte additive, interfacial modification and electrolyte nanostructure), thus achieving high performance AZIBs. The details of these three main works in the PhD project are as follows: 1) Electrolyte plays a central role in AZIBs, the solvation structure of zinc ions dictates the charge transfer resistance, reaction kinetics and the amount of side reactions. This is because the H2O molecules in Zn(H2O)62+ typical solvation sheath can easily decompose and form H2 and OH-, resulting in the formation of ZnO22- or Zn(OH)42-, which can easily generate ZnO and Zn4(OH)6SO4 by-products on zinc metal surface. In addition, the adsorption of the H2 on the surface of zinc metal will hinder the nucleation of zinc, resulting in uneven distribution of the interfacial electric field and elevated overpotential, which further exacerbates the inhomogeneous deposition of zinc ions, making it easier to formation of large dendrites. Thus, regulation of zinc ions solvation structure and interfacial reaction is an easily scalable way to achieve outstanding effects on stabilizing the Zn anode. Hence, I propose a unique route to fabricate ultrafine grainy dendrite-free zinc metal anodes by decreasing the electrolyte surface tension and passivating the metallic Zn. Specifically, an organic molecule, ethylene glycol monomethyl ether (EGME), was introduced into a traditional ZnSO4 electrolyte, which acts to both coordinate with Zn2+ ions to modulate the solvation structure and to chemisorb to the metallic Zn surface, thereby controllably increasing the driving force for zinc nucleation and growth, promoting uniform deposition and preventing side reactions. It is demonstrated that the refined zinc anodes can realize a high average CE of 99.5% and promote ultralong-term cycling stability over 8800 cycles (366 days). AC (activated carbon)|| Zn full cells are also shown to display excellent cyclability, with negligible capacity fading over 10,000 cycles, and larger pouch cells maintain near 100% retention over 1000 cycles, while full ZnVO||Zn cells demonstrate that EGME is compatible with state-of the-art ZIB cathodes. This simple method for controlling zinc anode interfacial structure and increasing AZB stability offers both a promising direction for understanding the mechanisms of zinc nucleation and growth in applied battery systems and a route for practically managing the anode instability issues that hold back the commercialization of AZBs and other energy storage systems. 2) The fundamental solution to metallic zinc protection lies in solving the boundary issues between the electrode and the electrolyte. While other commercially relevant battery systems benefit from a spontaneously forming solid electrolyte interphase (SEI) layer that stabilizes their anode, no such layer forms in AZIBs. Herein, I have designed and engineered an artificial composite metallic alloy interphase for AZIBs. This interfacial layer is initially deposited in the form of a thin film of Ag and In, but develops to become an intimate mix of an AgxZny alloy and metallic indium. Importantly, this layer acts to synergistically regulate the migration of zinc ions through the SEI layer and enable the dense and dendrite-free deposition Zn, simultaneously overcoming all of the primary drivers of Zn anode degradation. As a result, the novel ZMA delivers high CE (99.8%) and excellent long-term cycle life for both zinc symmetric cells (over 8000 cycles) and Zn@(Ag-In)||ZnVO full cells with 90.2% capacity retention over 10,000 cycles. Hence, this scalable approach represents a viable route towards the commercial utilization of this next-generation energy storage system. 3) The electrolyte plays a central role in renewable energy storage and conversion devices. The ion transport dictates voltage, current, and concentration distributions in electrolytes. While the microstructure of electrolyte affects the ion transport behavior, which will further influence the interfacial reactions (e.g., SEI formation and cation stripping/plating) on electrodes and the whole cell performance. Despite their complex chemistry and atomically solvated structures, electrolytes in batteries are usually regarded as macroscopically homogeneous ion-transporting media, and their microstructural characterization remains a knowledge gap. Here, I synthesized a novel F-containing material zinc(II)Bis(nonafluoro-1-butanesulfonate) (Zn(NFBS)2), which consists of two hydrophilic groups and hydrophobic groups, and comprehensively studied the structure-effectiveness relationships between the electrolyte micro-configuration containing Zn(NFBS)2 salt and the battery performance. Experiments showed that the configuration of amphiphilic anion NFBS- self-assemblies is concentration- and temperature- dependent. Specifically, Synchrotron based SAXS/WAXS analysis revealed that there is an evolution from disorder to order, as well as from spherical to cylindrical at dilute to high concentration. Consequently, the novel zinc salt solution with ordered micro-configuration exhibited excellent wide-temperature electrochemical performance from -30 °C to 80 °C.

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