In-situ electrochemical surface engineering of electrodes for the advanced Li metal batteries

Abstract

The growing concern of climate change have led to the worldwide investigation of renewable energy sources and energy storage system which play important role for carbon neutrality. Driven by such paradigm shift from fossil fuel to renewable energy, the emerging market growth of the electric vehicles (EVs) and energy storage systems (ESS) occurred. This rapid market growth led to the accelerated exploration of energy storage system with high power and energy density. Among various energy storage systems, Li-ion batteries (LIBs) are widely used battery system which has been already commercialized since 1991. However, state-of-art LiBs utilizing graphite as anode have almost reached its theoretical energy density limit due to material limitation, still insufficient to meet the growing energy demands. To overcome this challenge, replacing the graphite anode with a lithium metal anode (LMA) offers a promising solution, as LMA provides about ten times higher theoretical capacity (3,860 mAh g-1) and operates at a lower potential (-3.04 V vs. S.H.E.) compared to graphite (372 mAh g-1, -2.84 V vs. S.H.E.). Consequently, lithium metal batteries (LMBs) incorporating LMAs are regarded as the "holy grail" of battery systems, with the potential to deliver twice the energy density of conventional LiBs. However, practical application of LMBs are hindered by uncontrollable Li dendritic growth and formation of fragile native solid electrolyte interphase (SEI) layer. Li dendrite is provoked by inhomogeneous initial nucleation of Li and further deposition due to the lithiophobic nature of current collector. Moreover, fragile native SEI layer could be broken during dendrite formation which can proliferate dendrite growth onto the crack, by acting as ‘hot spot’. As grown Li dendrite could penetrate the separator to lead ‘short circuit’ leading to cell explosion. Moreover, Li dendrite can break the fragile nature solid electrolyte interphase (SEI) layer and results in rapid consumption of electrolyte. Even more, such dendritic Li can be electrically isolated to become ‘dead Li’ during the Li deposition/stripping process. Those side reactions lead to safety issues and rapid failure of LMBs, hence suppression of Li dendrite and reinforcement of native SEI layer is urgent issue in this society. To suppress Li dendrite growth and reinforcement of native SEI layer, in this thesis, various surface engineering strategies of electrode (current collector) will be suggested. Specifically, surface modification approaches based on the in-situ electrochemical methods to form favorable interface for Li deposition/stripping are intensively studied. In-situ electrochemical surface engineering has several advantages as follows; 1) it is cost-effective over other methods as it only requires electrochemical signals (current, potential) for surface treatment, 2) we can design the electrode surface by controlling the redox reaction of electrolyte component, 3) it is universal method for various electrolyte systems and electrodes. The following contents will be presented throughout this thesis. 1. In the introduction section (section 1), the brief review of Li metal battery systems, problems of those systems and literature survey of current solutions will be presented. 2. In the section 2, a topic related to the formation of artificial SEI layer (ASEI) layer by electrochemical method will be presented. In particular, this topic will present the in-situ method to form inorganic-rich ASEI layer on current collector (CC) via catalyzing reduction of certain electrolyte additive, and mechanism of catalyzed reduction will be briefly presented. 3. In the section 3, surface modification method introducing lithiophilic materials on CC by in- situ method will be presented. Specifically, the one-pot fabrication method of the lithiophilic LMA which unifying surface treatment and lithiation via in-situ oxidation reaction of functional additive will be presented. 4. In the last section, in-situ surface modification method in the anode-free Li metal batteries (AFLMBs) will be presented. This topic will present the electrochemical pretreatment method to form lithiophilic materials and robust SEI on the CC in the working AFLMBs.