Zhao, Fangjia; (2024) Multi-dimensional Materials Design for Advanced Zinc-Ion Batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Zinc-ion batteries have drawn intensive research interest due to their ease of fabrication and less-flammable electrolyte. However, the discovery of the unique properties of the aqueous environment of aqueous zinc-ion batteries remains a challenge. For example, the most significant obstacle for zinc-ion batteries is water-related side reactions. An advantage of zinc-ion batteries is that the aqueous environment provides an additional charge carrier: hydronium ions. To fully exploit the differences in zinc-ion batteries, it is essential to address these interconnected challenges from the fundamental aspect of aqueous zinc-ion batteries: water. The unregulated zinc deposition process can lead to zinc dendrite formation due to the ‘tip effect’. These dendrites can penetrate the separator and cause micro short-circuits. To protect the zinc anode from dendrite issues and water-related side reactions, a carbon layer doped with nitrogen and oxygen was investigated using a rapid sputtering method. This method increased the zinc nuclei density and achieved a uniform zinc deposition process.Water-related side reactions were further mitigated using perfluorooctanoic acid as an electrolyte additive. This fluorinated surfactant has low surface energy, forming an adsorption protection layer with fluorinated tails to limit the amount of water molecules near the interface, thus reducing side reactions. It also reduces the surface tension of the electrolyte, leading to enhanced wettability on both the anode and cathode, which improves rate performance due to sufficient mass transfer. To further understand the positive aspects of aqueous zinc-ion batteries, a series of manganese oxides were synthesised, and the synergistic collaboration between vacancies, lattice water, and nickel ions, which enhances hydrated proton hopping via the Grotthuss mechanism, was studied. Protons preferentially bond with O2- ions in the Mn-O layer, and proton transfer is favoured in the presence of vacancies. The Grotthuss mechanism enables efficient proton charge transfer without the long-distance movement of the molecule, leading to high ionic conductivity and high specific capacity. This thesis emphasises the significant role of water molecules and their impact on the overall performance of aqueous zinc-ion batteries. It demonstrates that water-related side reactions in aqueous zinc-ion batteries can be mitigated using practical strategies, such as a minute amount of PFOA additive and the rapid sputtering process, both of which require minimal raw materials. More importantly, it shows that aqueous zinc-ion batteries can be further enhanced via the Grotthuss-like mechanism by utilising hydronium as a charge carrier in the aqueous electrolyte. These strategies and the Grotthuss mechanism can be further applied to other aqueous systems to enhance the energy density and lifespan.

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