Surface and Interface Engineering of Carbon-based Materials for Advanced Lithium Storage Systems

Abstract

Carbon-based materials have been considered as versatile candidates in active anode and supportive host in lithium-based batteries owing to their unique intrinsic characteristics such as abundant resource, lightweight, excellent chemical resistance, and high electrical conductivity. However, low specific capacity and poor rate-capability of commercial graphite anode still impede next-generation batteries for high-energy density. Carbon-based materials are able to complement their lacking of inherent capacity and electrochemical kinetic limitations because their properties can be easily manipulated by surface and interface engineering such as diverse dimensionalities, chemical composition, internal electronic structure, and morphological change. Therefore, it is required to strategically modify structures, chemical components, and morphologies of carbon-based materials in accordance with ultimate targets. In this dissertation, we focus on surface and interface engineering of carbon-based materials for advanced lithium storage systems. Chapter 1 describes the backgrounds and principle of lithium-based batteries to emphasize the necessity of advanced lithium-based batteries and to understand the effect factors of electrochemical performance in battery systems. In addition, we deal with the content of state-of-the-art strategies and their limitations for carbon-based materials in lithium-based batteries. In Chapter 2, we introduce strategies to improve the specific capacity in lithium-ion batteries, employing graphite-based composites incorporated with molybdenum sulfides. To prepare for a well-constructed composite, the structures and chemical components of graphite are modified by physical and chemical processes. By ball-milling process, the number of edges in graphite is exposed and the oxygen functional groups are formed at graphite edges by mild oxidation. The number of exposed edges provides large sites for molybdenum sulfide loading. Furthermore, the oxygen species contributes to anchoring between modified graphite and molybdenum sulfide. Molybdenum sulfide embedded within the edges of modified graphite facilitates the improvement of Li storage capacity and Li+ ion diffusion, resulting in a high specific capacity of 1239 mAh g−1 at 0.13 A−1 g and excellent rate-capability at various current densities. Furthermore, the graphite-based composite shows good cycling stability for 100 cycles owing to the enhanced interaction between modified graphite and molybdenum sulfide. Although graphite-based composite leads to high electrochemical performance compared to commercial graphite, its composite still remains poor rate-capability and low capacity at a high current density. Based on these results, we further explore the routes which are improving the capacity and reducing the polarization. The systematic analyses, including electrochemical measurements and ex-situ XPS depth profile, reveal that the main capacity in molybdenum sulfide is depended on the number of sulfur contents. Moreover, graphene, which can provide a large surface area, is used to reduce Li diffusion length between electrolyte and electrode. Based on the understanding on the role of oxygen species, we synthesize the nano-sized compound of molybdenum linked rich-sulfur chemically and uniformly grown on 2D graphene. Through the nano-structural and dimensional engineering, graphene-based composite shows higher capacity (465 mAh g−1) at a high current density of 1 A g−1 than that of graphite-based composite (232 mAh g−1). The research in Chapter 3 describes the change in the chemical composition for improving Li+ ion kinetics during the electrochemical reaction. Among heteroatoms, fluorine (F) atom with the highest electronegativity and low polarizability is adopted into the graphene framework, and their influence on electrochemical performance is investigated in terms of F content. To thoroughly understand the effect of C−F bonds in graphene, moreover, C−F configurations are regulated by facile thermal treatment. Above a certain content, the F atom provides high Li-ion adsorption, fast electron transfer, and accelerated Li-ion diffusion because of tuning the electrical and chemical structure. The resulting fluorinated graphene shows a high capacity of 1365 mAh g−1 at 0.1 A g−1, remarkable rate capability, and good stability with 64% retention after 1,000 cycles. From the control of C−F bond by annealing, it is confirmed that the C−F configurations play a significant role in the formation of solid electrolyte interface (SEI) layer. The covalent C−F bond is electrochemically stronger than another bonds (ionic and semi-ionic C−F bonds), leading that the stable and thin SEI layer is formed on its surface. Hence, the fluorinated graphene with covalent-rich C−F bond exhibits higher long-term stability with 80% retention after 3,000 cycles than that of fluorinated graphene before thermal annealing. In Chapter 4, we focus on morphological engineering to provide stable Li plaiting during operating at low potential. To prevent the growth of Li dendrite, a hierarchical surface is constructed on carbon fiber (CF) using binders in fabricated CF paper (CFP). Through low-temperature carbonization, the binders (polyacrylic acid and sodium carboxymethyl cellulose) at the CF surface are transformed into oxygen-containing amorphous carbon and sodium carbonate (Na2CO3). This carbonization process results in the hierarchical morphology on the CF surface. In particular, the entirely covered oxygen-containing amorphous carbon serves as the lithiophilic sites on the CF surface, guiding uniform Li nucleation. Furthermore, the Na2CO3 can preferentially decomposes the bis(trifluoromethanesulfonyl)imide (TFSI−) anions to form inorganic-rich SEI layer. In the electrochemical test, prepared CFP shows a low Li nucleation overpotential and smooth dendrite-free Li plating. In full-cell consisting of a LiFePO4 cathode with high loading (~13 mg cm−2), we achieve a high-energy density of 428 Wh kg−1 and excellent capacity retention of 85% after 300 cycles. This dissertation offers effective surface and interface engineering of carbon-based materials for high electrochemical performance in lithium-based batteries through structural, chemical, and morphological changes. Our approaches using modified carbon-based materials can be utilized as a potential anode for high electrochemical performance in practical energy fields.