Development of Electrode Materials and Architectures for Safe and Efficient Energy Storage in Li-Ion Batteries and Supercapacitors

Abstract

Occasional reports on accidental explosions that appear in the news, increases the safety concerns of using Li ion batteries (LIBs). The scope of LIB is expanding toward large-scale energy storage systems such as electrified transportation and smart grids. Considering these facts, manufacturers and consumers now have concerns regarding the safe use of such devices. The high voltage anode materials have greatly relieved the unstable state of LIB during operation because they can exclude the uneven plating of lithium and reductive electrolyte decomposition. Although various titanium based oxides have been developed as high voltage anode, low capacity or poor potential flatness still severely hinder their industrial usage. In this dissertation, two types of titanium-based crystals are proposed, and they exhibited enhanced Li storage performance as high voltage anode materials as follows. A c-channel that is formed inside stacked (001) planes in rutile TiO2 exhibits the lowest energy barrier for Li migration. In this regard, the rational design of a TiO2 architecture for stacked (001) planes is needed in order to maximize Li storage. Here, a three-dimensional and dendritic TiO2 sphere comprised of c-channel specialized nanorods is proposed, which can be prepared via the specific adsorption of chlorine ion on the (110) plane. Along with a confined Li pathway, such radially assembled TiO2 nanorods show a low intercrystalline resistance. When fabricated into an electrode, the three-dimensional and dendritic TiO2 sphere is capable of delivering an almost 100% Coulombic efficiency in conjunction with long cycle charge/discharge stability (300 cycles). This approach, which is based on theoretical studies and experimental validation, provides guidance for tailoring electrode materials for use in Li storage systems. H2Ti12O25 (HTO), a recently discovered anode material for LIB, has outstanding electrochemical performance compared to other high voltage materials. However, its thermodynamic/kinetic properties as a Li host have not been thoroughly investigated yet. In this study, the Li storing behavior of HTO was intensively characterized by a combined theoretical experimental study. In addition, the strong dependence of electrochemical performance on Li diffusion kinetics stimulated us to develop a nanostructured HTO which provides incorporated Li with a short diffusion length inside a nano-crystal. As a result, the nanostructured HTO showed upgraded Li storage performance. This work suggests that the HTO is one of the most competitive material found to date for the construction of advanced LIB with excellent safety and stability. Electrochemical capacitors (so-called supercapacitors), with high power density and superior cycling ability, are considered to be one of the most stable and safe energy storage systems. High power as well as reasonably high energy density are provided, supercapacitors are likely to be regarded as a representative energy storage system along with batteries. The common challenge in supercapacitor design is the resistive behavior originated from sluggish ion transport, poor electrical conductivity, and high charge transfer resistance, which increases equivalent series resistance of overall cell. In addition, a variety of activation process for the increase of surface area not only are involved in harmful chemicals or toxic metals but also lower its cost-effectiveness. In this dissertation, two strategies for resolving above-mentioned challenging points are proposed

3D bicontinuous metal-carbon hybrid and organic gel-wrapped MnO2 thin fim electrode. The three dimensionally aligned bicontinuous carbon and metal hybrids are synthesized. The resulting material shows a significantly high rate capability up to 1,000 V s-1. The proposed strategy exploits an agarose gel as a template for the simultaneous construction of three dimensional (3D) carbon structures filled with a metal-lined architecture in supercapacitors. The carbon framework with three dimensional and interconnected metal lines inside minimizes both empty inner pores and electron transfer barriers, which results in a substantial reduction of the overall electrode resistance. In addition, it can be used for the filteration of voltage ripples due to the ultrafas7t response with high efficiency. A robust hybrid film containing MnO2 was prepared for achieving large areal capacitances. An agarose gel, as an ion-permeable and elastic layer coated on a current collector, plays a key role in stabilizing the deposited pseudocapacitive MnO2. Cyclic voltammetry and electrochemical impedance spectroscopy data indicate that the hybrid electrode is capable of exhibiting a high areal capacitance up to 52.55 mF cm-2, with its superior structural integrity and adhesiveness to the current collector being maintained, even at a high MnO2 loading. The emergence of body-centric power generators such as piezoelectric/solar cell and the functional/morphological evolution of smart phones have created a need for advanced devices for energy storage. In order to meet the power demand of such devices, the construction of a series of unconventional energy storage systems have been developed in the past few years, which enable one to achieve flexible, foldable, and stretchable characteristics. In order to secure their stability and safety of energy storage platform under mechanically stressed conditions, the primary requirements of electrode materials include robust electrical connectivity and mechanical endurance. In the last part of this dissertation, a lint taping method is described for the fabrication of thin layer of conducting network using graphite felt. An all-solid-state, completely foldable and washable energy storage platform was fabricated. By adopting it as a supercapacitor electrode, the physical characteristics and electrochemical properties of such a GFCN are identified. A constructed graphitic fiber network derived from conventional graphite felt was readily assembled into a full-cell by its self-adhering architecture. The as-prepared system exhibits high mechanical properties under various folding motifs and washable characteristics without capacitance fading by virtue of the robust electrical connectivity of the fibrous graphite network and intimate contact between the polymeric gel electrolyte and the electrodes. The collected results suggest that this supercapacitor system is a promising candidate for practically available and wearable energy storage systems with high cost-effectiveness and scalability.