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Title: Mechanisms of phase transformations in aluminum anodes for lithium-based batteries
Authors: Zheng, Tianye
Degree: Ph.D.
Issue Date: 2021
Abstract: Lithium storage in aluminum stems from a phase transformation from lithium-poor α phase (Al, face-centered cubic) to lithium-rich β phase (LiAl, cubic) at room temperature. The intrinsic properties, such as high capacity, light weight, low cost, and the potential of simplifying the manufacturing processes make Al a competitive anode material in lithium-based batteries. However, utilization of Al-based anodes is still not fully viable at this moment due to the drastic capacity fading during charge and discharge, thus drawing less attention compared to other anode candidates. In this Ph.D. project, the initial step is to gain fundamental understandings of the α to β phase transformation through operando light microscopy and kinetic analysis. It is visually revealed that nuclei appear at random positions and expand to form quasi-circular patches that grow and merge until the phase transformation is complete. Interestingly, the growth of the quasi-circular patches exhibits anisotropy at the granular level. Together with the electron backscatter diffraction technique, the lithiation of Al is suggested to be a whole-grain process that is influenced by grain textures, and the grain with a preferred out-of-plane <111> orientation may inhibit the phase transformation. As for the reversed β to α transition, the extraction of Li from the β phase is accompanied by fracture and crack formation leading to the detachment of the α phase from the rest of the electrode. The mechanical stress in Al thin film electrodes shows a strong stress asymmetry during (de-)lithiation.
Then the investigations have been extended towards bulky Al foil electrodes, of which the typical features remain. However, the considerable thickness of foils facilitates a quasi-1D in-depth phase propagation once the surface is fully covered with the β-LiAl. The cross-section of a partly lithiated Al foil exhibits unique features under an electron microscope. Combining with operando x-ray diffraction, relevant scientific insights are yielded: 1) plastic deformation zone is characterized next to the β-LiAl, causing an extremely small size of Al grains before being transformed; 2) significant compression may prevent certain Al regions from being nucleated; 3) delithiation makes Al matrix nanoporous with a neglected volume contraction, thereby accumulating the electrode thickness over cycling. Lastly, the β-LiAl is found to be the only crystalline phase at room temperature. Li solubility within the β phase is suggested to take over the suspected formation of Li-rich phases beyond the β-LiAl. The solubility range of the β phase is determined to be ~5 at% by potentiostatic charge counting experiments. Moreover, the cyclic voltammetry of partially lithiated Al foils shows that the β phase can be (de-)saturated without propagating the phase front towards the α phase. Through delicate manipulation by solely engaging the solubility range, i.e., preventing the problematic α/β/α phase transformations from occuring, the cycling life of β-LiAl anode can be significantly improved and compete with the state-of-the-art LIB anodes. Not only does this thesis provide fundamental understandings for the β-LiAl phase at room temperature that complements the existing phase diagrams, but also implies that Al foils hold great potential as an anode material for lithium-based energy storage.
Subjects: Lithium-ion batteries
Storage batteries
Phase transformations (Statistical physics)
Hong Kong Polytechnic University -- Dissertations
Pages: 163 pages : color illustrations
Appears in Collections:Thesis

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