Surface modification of Lithium metal anode for advanced Lithium batteries

Abstract

Moving toward full electrifying the automotive applications and utility grids aims to reflect environmental issues, such as global warming and concerns for fossil fuel depletion and the needs for intelligent energy utilization systems. Even when current lithium ion battery (LIB) was fully developed, the highest energy storage that LIBs can deliver is too low to meet the demands of key markets, such as transport, in the long term. In this regard, exploration of new chemistry, especially electrochemistry, and new materials is urgently needed to build up ad-vanced battery systems. As candidates for advanced lithium batteries, two systems will be addressed in this thesis

lithium-air (oxygen) ($Li-O_2$) and lithium (Li) metal-based batteries (LMBs). Especially with respect to cell configurations, both systems share the Li metal as an anode material owing to its ultimately high theoretical capacity ($3860 mAh g^{-1}$) and lowest redox potential (-3.04 V vs. SHE). Nevertheless, the use of metallic Li has called into question due to its critical challeng-es

uncontrolled Li deposition characteristics with Li dendrite and poor Li cycling efficiency. Such challenges hinder the development and practical applications of $Li-O_2$ batteries and LMBs. This thesis begins with some brief on general backgrounds of LIBs and basic principles of $Li-O_2$ battery systems. After that, the superiority of $Li-O_2$ and LMB systems compared to with LIBs will be featured in terms of energy density. At the end of introduction part, funda-mentals for understanding the characteristics of Li metal electrode and recent progresses on Li metal stabilizations will be carefully discussed. The first part of this thesis presents an importance on Li metal stabilization toward long-term operation of $Li-O_2$ system (Chap. 2, 3 and 4). The point of Chap. 2 is to problematize the severity of oxygen-induced Li metal surface deterioration and, its detrimental effect on $Li-O_2$ cell performance will be discussed. Next, a simple protection technology for Li metal stabili-zation will be presented using inorganic/organic hybrid composite protective layer (CPL). It was confirmed that a cell with the CPL-coated Li metal anode exhibited two-fold improved cycleability by reducing the electrolyte decomposition at Li metal surface (Chap. 3). Having extended the concept, further examination with the electrolyte containing redox mediator was carried out to confirm the effectiveness of CPL strategy on its problematic effect, i.e. redox shuttle which reduces the efficacy of redox mediation and impairs cycling stability. The find-ings on deceleration of redox shuttle by CPL-coating on Li metal will provide on impetus for sustainable redox mediation in $Li-O_2$ batteries (Chap. 4). The second part takes it as focal point of Li metal stabilization and its efficacy on perfor-mance enhancements for LMB system under much higher current condition. Especially, planar-type protection (CPL) and three-dimensional (3D) conductive interlayer on Li metal were comparatively discussed in Chap. 5 and 6, respectively. Based on, required shear modulus of protective layer for efficient suppression of Li dendrite formation was examined by quantifying mechanical strength of CPL with nanoindentation. Moreover, through comprehensive study on Li metal failure mechanism, it was found that the formation of deactivated porous layer and its explosive growth is main origin of sudden capacity decay of LMBs. Hence, control and re-utilization of deactivated porous layer is crucial to enhance the reversibility of Li metal electrode. Introduction of fibril stainless steel (FSS) felt as 3D conductive interlayer enables to mitigate the escalation of cell impedance by facile electron transfer into porous layer and extends the Li utilization by shifting a growth direction of porous layer. Li metal batteries using the FSS/Li anode exhibited a two-fold increase in cycling stability compared with the bare Li metal electrode.