Author: Shi, Fangyi
Title: Studying liquid sulfur in lithium-sulfur batteries
Advisors: Lau, Shu Ping (AP)
Xu, Zheng-long (ISE)
Degree: Ph.D.
Year: 2024
Subject: Lithium sulfur batteries
Storage batteries -- Materials
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Physics
Pages: xix, 136 pages : color illustrations
Language: English
Abstract: The lithium-sulfur batteries have entered a stage of flourishing development for their attractive gravimetric energy density of 2567 Wh/kg and low cost of sulfur stock. Through decades of efforts, electrochemical performance, such as specific capacity, cycling stability, and rate performance, has been greatly improved. However, some problems still need to be overcome to achieve practical Li-S batteries, including the insulating nature of S/Li2S, the shuttle effect of soluble lithium polysulfides, and the slow reaction kinetics of the Li2S deposition. In addition, the real reaction process of Li-S batteries is still unclear due to the sensitivity of sulfur species to the environment and the complex reaction mechanisms. Liquid sulfur has been recently discovered in the charging process of Li-S batteries using optical microscopy and in situ Raman spectroscopy on an optical cell. Liquid sulfur has specific advantages over solid sulfur: i. The reshaping ability brings liquid sulfur a higher charging capacity even at a large current density. ii. The lower reaction barrier of liquid-liquid conversion leads to fast reaction kinetics. This thesis explores the role and application of liquid sulfur in Li-S batteries.
Firstly, we observed the sulfur generation and growth on the thick MoS2 nanoflakes. Liquid sulfur can be generated on the basal plane of MoS2 but would be solidified quickly when it comes into contact with crystalline sulfur growing from the edges. Annealing MoS2 nanoflakes in the H2 atmosphere can introduce oxide around the edges and sulfur vacancies on the basal plane. These two factors enable the generation of liquid sulfur throughout the whole charging process, even at large overpotentials and low temperatures. The same annealing method can also be applied to other transition metal dichalcogenide (TMD) materials (WS2 and MoSe2). This research suggests that functionalized TMD materials have the potential to achieve a pure liquid sulfur-lithium battery system.
Compared to TMD materials, carbon is more common in Li-S batteries. To better understand the liquid sulfur, we observe the nucleation and growth of sulfur on single-layer graphene by in situ Raman and optical microscopy. Due to its metastable characterization, we found the supercooled liquid sulfur hard to keep as the final charging product. The important role of liquid sulfur has been verified through analysis of the growth dynamic of liquid sulfur in the charging process. Furthermore, we found that the current density strongly influenced the morphology and density of liquid sulfur, while the charge capacity was limited. Based on these findings, a cathode host supporting liquid sulfur formation has been designed to achieve high electrochemical performance at a high charging rate.
The third work has inspected the effects of charge transfer and mass transport of reaction species on liquid sulfur deposition. The electrochemical reaction kinetics of liquid sulfur was systematically studied by gradually increasing the conductivity of the substrate, introducing catalysts, and changing the temperature. This work reveals that excellent conductivity, efficient catalyst, and suitable temperature favor liquid sulfur deposition kinetics.
Rights: All rights reserved
Access: open access

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