1-dimensional lepidocrocite titania nanostructures applications in lithium sulfur batteries and in-operando X-ray diffraction of gamma sulfur in lithium sulfur batteries

Neal Amadeus Cardoza

doi:10.17918/00010746

The last few decades have seen a massive development in areas of portable electronics and electric vehicles, made in part possible by the rise of lithium-ion (Li-ion) batteries. These rechargeable batteries have transformed the world; however they are reaching their limits of specific capacity. Additionally, Li-ion is dependent on elements like nickel and cobalt, which come with their own supply chain issues. New chemistries are needed that are lower in cost, higher in energy density, and utilize abundant elements. Lithium-sulfur (Li-S) has garnered a lot of attention in the last decade as the most promising next generation chemistry, owing this title largely to its higher specific capacity and the abundance of sulfur. However, Li-S has many challenges preventing its commercialization. The primary issue being the polysulfide shuttle, with polysulfides (PSs) being the intermediary compounds of lithium (Li) and sulfur (S) generated during the discharge/reduction of S. The polysulfide shuttle results in poor cyclability, active material loss, and self-discharge. Furthermore, Li-S relies on the use of Li metal anodes, which are prone to dendrites and poor cyclability. Given that Li metal anodes are the primary anodes for Li-S, it is crucial to address issues on both sides of the cell. In this thesis, I aim to do address issues with S and Li, describing my work using 1-dimensional titania nanostructures as S hosts and Li metal hosts, and providing insight into the evolution of gamma sulfur ([gamma]-S) within Li-S during cycling. In investigating 1DLs as possible S hosts for Li-S batteries, we first developed an understanding of PSs binding to 1DL. By first using ultraviolet-visible spectroscopy to show that 1DL binds a higher concentration of PSs compared to carbon - a typical S host. Furthermore, we show improved PS binding of 1DL over its precursor using a PS shuttle current measurement. Post- mortem x-ray photo spectroscopy was leveraged to gain further insights into the chemical environment of the 1DL surface after interactions with PS. This spectroscopic technique revealed two binding mechanisms: Lewis acid-base interactions and polythionates/thionates binding. These mechanisms are also observed with the widely used nanomaterial, MXenes, yet 1DLs offer lower synthesis complexity and greater scalability. Chapter 2 illustrates how the novel 1DL can serve as an effective S host. To further improve the PS and 1DL interactions, we demonstrate a novel 1-step aqueous surface functionalization of lepidocrocite titania using dopamine. This surface functionalization shows a 63% reduction in the optical bandgap of 1DL and results in 2.6x increase in the electronic conductivity. Electron microscopy and energy dispersive spectroscopy show a more uniform homogenous coating of S on the surface of the porous meso-particles of 1DL. This yields an increase in S utilization based on galvanostatic measurements, and improved PS shuttle current response. Furthermore, this functionalization improves the polythionate/thionate binding of PSs at the expense of Lewis acid-base interactions. We further investigate the binding of dopamine to the surface of 1DL using Fourier transform infrared spectroscopy, showing the disappearance of the hydroxyl group from dopamine and the formation of a Ti-O-C bond on the 1DL. Suggesting that dopamine binds to 1DL via its hydroxyl group, resulting in bi-dentate or chelating binding on the titanium 1DL sites. Lastly, as dopamine contains dual hydroxyl groups at the 1,2 positions on its phenol ring, it belongs to a large class of molecules known as catechol. As dopamine binds to 1DL through hydroxyl groups, other catechol can be used to further tailor 1DL for other applications. This thesis explores the novel gamma-S ([gamma]-S), attempting through in-operando x-ray diffraction to unravel the phase change of [gamma]-S. Despite earlier work with host particles binding to PSs, eventually these PSs still participate in the shuttle leading to the eventual decline in performance of the cell. Despite PSs being bound, these S host materials are unable to function in commercial Li-ion electrolyte - a significant hurdle towards commercialization. As such it is unique that a non- confined S existing as a different allotrope is capable of cycling in commercial Li-ion carbonate electrolyte, which typically degrades in the presence of PSs. In this thesis, chapter 6 shows how the atmosphere of the reactor is critical to the stabilization of [gamma]-S, along with the substrate which was shown in prior work. The in-operando work in this chapter shows the conversion of [gamma]-S 0k0 to Li₂S h0l at the lab scale. However, due to the nanocrystalline size of S, x-ray energies at the synchrotron level were needed to delve further into the S phase change. In the synchrotron data, it is evident that the aluminum packaging commonly used in in-operando setups interferes with the Li₂S h0l signal. Nevertheless, other smaller peaks of Li₂S can be seen arising and disappearing in a cyclic fashion corresponding to the discharge and charge cycle respectively. Additionally, the initial S deposited consists of a mixed phase of the common [alpha]-S, along with the [beta]-S and [gamma]-S. It is conceivable that there is some contamination of [alpha]-S with the [gamma]-S. However, as [beta]-S has been seen with other Li-S systems as a final discharge product, it is possible that merely during the resting of the cell that some [gamma]-S converts to [beta]-S. As the cell is cycled further, we see the conversion towards more [beta]-S. This work, despite its experimental issues, leads us to improved understanding of [gamma]-S and improved experimental design for future synchrotron experiments with [gamma]-S. These future experiments would ultimately improve our understanding of how non-confined [gamma]-S converts directly to Li₂S without the formation of PSs. Lastly, the Li-S system faces significant challenges related to both Li and S, making it insufficient to address only the S issues for the commercial viability of Li-S batteries. As a result, we explore the potential of 1DL as a Li metal host for improved Li performance. 1DL can function as a Li metal host, owing to its hydroxyl groups, lithiophilicity through ion-exchanged Li, layered structure, and its ability to intercalate Li ions. Here 1DL nanoflakes are orientated vertically to guide Li-metal deposition and serve as a Li metal scaffold. This is achieved through controlled directional freezing of blade casted aqueous slurries. These vertical-1DL (V-1DL) enable better Li plating and stripping than Cu. The long range order of V-1DL and its vertical alignment contribute to improved Li-ion flux. Through dead Li measurements, V-1DL demonstrates reduced Li loss to dead Li. Post-mortem depth profiling XPS measurements show increased fluorine, nitrogen, and lithium hydroxide in the solid electrolyte interface layer on Li, compared to Cu. These measurements show 1DL through its lithiophilicity and surface hydroxyl groups leads to improved Li stability.

1-dimensional lepidocrocite titania nanostructures applications in lithium sulfur batteries and in-operando X-ray diffraction of gamma sulfur in lithium sulfur batteries

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1-dimensional lepidocrocite titania nanostructures applications in lithium sulfur batteries and in-operando X-ray diffraction of gamma sulfur in lithium sulfur batteries

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