Design and Development of Lithium-Sulphur Batteries: Operando Studies using X-ray Absorption Spectroscopy

Advanced power sources are urgently needed to meet the ever-increasing demand of modern society, especially battery requirements for extended-range electric vehicles and grid energy storage applications. [1] However, current rechargeable lithium-ion batteries are not able to fulfil the demand for future energy demands.

The Li-S batteries represent a very promising electrochemical system for these applications that offers high theoretical capacity of 1673 mAh g-1, low material cost, and relative safety. [2] Despite these attractive advantages, Li-S batteries are still hindered from large scale commercialisation, due to a number of challenges. [3] The major concern is thought to be caused by the shuttle effect, originating from the dissolution of cathode species into the electrolyte and migration towards the anode, where they react with the lithium metal, resulting in loss of active sulphur material. [4] This leads to poor cycling stability, low Coulombic efficiency, and Li corrosion. The sulphur cathode undergoes very complicated reaction step during cell cycling but the complete reaction mechanisms are still not established.

Herein, an operando XAS technique is employed to gain deep insights into the mechanism of Li-S cell reaction. This measurement technique offers the advantage to be element specific that is sensitive to oxidation states and chemical bonding of the atoms of interest. XAS in the S K-edge region is a powerful tool for the detection of the reaction product of Li-S batteries. The fingerprint analysis consisting of a comparison of XAS spectra between different sulphur species and measured data is helpful for distinguishing different species exist in the cell during the battery operation.

Operando measurements were performed utilising a modified operando cell specifically designed for X-ray experiments and provide accurate information of electrochemical process. The design and specification of the operando cell are described in chapter 2.

The objective of chapter 3 was to investigate the differences in the sulphur reaction mechanisms in variation of the electrolyte compositions including Li salts, solvent systems, and LiNO3 additive. The S K-edge XANES measurements demonstrated that the electrolyte compositions influence the self-discharge rate and the conversion reaction mechanisms of sulphur chemistry.

Chapter 4 systematically investigated the function of inorganic metallic additives on sulphur electrodes with the aim of redox-shuttle suppression, catalytic activities, selfdischarge mitigations, and adsorption of polysulphides. This chapter highlights the unique operando XAS measurements of the S and Ti K-edges using a single test cell. This revealed details about the response of the TiO2 and S chemistry during lithiation and de-lithiation. Reversibility of the oxidation states of TiO2 was demonstrated. Additionally, the importance of the operando analysis is demonstrated by comparison with the post-mortem analysis.

Chapter 5 demonstrates the environmentally friendly and cost-effective fabrication method of sulphur electrodes. Potential hazardous material is eliminated from the sulphur electrode system and replaced with conductive polymers, PANI and PEDOT:PSS, and their cell performances were investigated using operando XAS technique.