A study on the electrochemical evaluation of degradation, and surface treatments of electrodes in Li–Sulfur/Chalcogens batteries

Abstract

The demands for batteries with higher energy densities than commercial lithium-ion batteries (LiBs) are increasing in lots of applications such as electric vehicles (EVs) and energy storage systems (ESSs). Among various candidates for battery systems, Lithium-Sulfur/Chalcogens (Li-S/Chalcogens) batteries, which employs sulfur (S), selenium (Se), and tellurium (Te) as cathode materials and Li as an anode material, have been suggested as next-generation batteries due to their higher energy densities and a lower cost (S) compared to LiBs. However, critical issues such as the dendrite formation on Li anode, the volume expansion of S and chalcogens, and the dissolution of the reaction intermediates have made their practical use difficult. Above all, the dissolution of intermediates from the cathode, which causes the (i) loss of active materials, (ii) the shuttle effect by transport of intermediates between the cathode and Li anode, (iii) and the subsequent passivation of Li anode by chemical reduction of intermediates on Li anode, has seriously degraded the electrochemical performances. Therefore, the strategic approaches to alleviate the dissolution from S/Chalcogens electrode have been suggested to be most effective to improve electrochemical performance. However, ex situ preparation of electrode materials in these studies, which requires complex multistep processes, has rendered these strategies non-cost-competitive for practical use. Moreover, the fundamental understanding and evaluation of dissolution have hardly recognized the chemical between reaction intermediates, which affect the electrochemical performance. In this thesis, both of fundamental and strategic approaches to evaluating and mitigating the dissolution (degradation) in Li-S/chalcogens batteries will be proposed. Particularly, the dissolution will be studied and evaluated using the electrochemical method. Based on this fundamental study, the strategy to design the electrode to mitigate the dissolution will be further suggested focusing on the effect of strategy on the dissolution behavior.

The first topic of this thesis (Chapter 1) includes an overview of the background knowledge of LiBs such as the general configuration of the device and its working mechanism.

The second topic of this thesis (Chapter 2) is the development of surface treatment of electrodes for highly stable Li-S/Chalcogens (Se and Te) batteries. The electrode to alleviate the degradation of the electrode is designed using chemical/electrochemical surface treatments.

The final topic of this thesis (Chapter 3) is a study on the electrochemical evaluation method on the degradation of the electrode in Li-S batteries. This topic contains the development of electrochemical methods and the strategic approach to mitigating the degradation is suggested.