Synthesis of Cathode and Anode Materials via Top-Down Approach for Li-ion Batteries
Graduate school of UNIST
Battery Science and Technology
xii, 162 p.
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Creating green energy solutions has become crucial to society. However, to achieve a clean and renewable energy system, significant developments must be made not only in energy conversion technologies (such as solar panels and wind turbines), but also in the feasibility and capabilities of stationary, electric-energy storage (EES). Many types of EES systems have been considered such as pumped hydroelectric storage, compressed air energy storage (CAES), flywheels, and electrochemical storage. Among them, electrochemical storage such as batteries has the advantage of being more efficient compared to other candidates because it is more suitable in scalability, efficiency, lifetime,discharge time, weight, and/or mobility of the system. Currently, lithium (Li)-ion rechargeable batteries have become very important in recent years due to their great promise as power source, but the batteries are limited by their materials' performance. Accordingly, the development of high performance materials has been main focus in materials science research. Here, the achievements of cathode, anode,and current collector are described that they are synthesized via top-down approach to enhance their performance in Li-ion batteries. In cathode research, in spite of that there have been many reports dealing with nanostructured cathode materials, none of previous works have been reported the morphology transition of cathode materials via chemical etching. In this study, we found that a selective chemical etching method using PVP and AgNO3 is very promising for obtaining significantly improved electrochemical performance of the cathode materials even at high voltage range. This etching method spontaneously turns to layered morphology with a layer thickness of 10 nm. Furthermore, we found that the concurrent modification of layered LiCoO2 with a nanoscale Co3O4 coating layer by chemical etching to minimize the capacity loss and to maximize the rate capability of the cathode without the loss of the electrode density. In anode research, a novel architecture consisting of Si nanowires internally grown from pores in the etched graphite with high electrode density of 1.5 g/cm3 is introduced. In previous works, various nano-engineering concepts were introduced to overcome a volume related problem of Si during cycling.However, although these strategies exhibited a superior performance such as high capacity and good cycling stability, they cannot be satisfied with electrode density which is highly required to determine high energy density in practical approach. In this point of view, this work provides new strategy to design electrode material with practically required electrode density and high volumetric capacity. Simply, porous graphite as template for Si nanowires growth is designed via hydrogenation and Si nanowires are internally grown from pores in etched graphite via Vapor-Liquid-Solid process.Especially, porous graphite, which first is reported as top-down approach, plays a key role of good electrochemical performance in this work. In this system, not only porous graphite can offer free space to accommodate the volume change of Si nanowires, but also efficiently improve the electron transport between active materials. In current collector research, more advanced nanostructure anodes of uniform 3D Cu-Si core shell structured arrays with 250 and 500 nm diameter are produced using top-down processes. This nanostructured anodes improved in cycle stability and rate performance, even at 20 C rate. As a current collector, each Cu nanopillar substrate provides a high surface area for better mass accommodation of Si deposition while the space between them enhances the electrochemical reaction between the electrode and electrolyte and accommodates the volume change during cycling. In addition, because the fabrication of the Cu nanopillar substrate only involves conventional top-down processes, the nanopillars can be generated through a facile and fast process with control of the surface area and simple modulation of the nanopillar density or diameter. Remarkably, the well-patterned nanopillar substrate imparts a significantly enhanced connection between the current collector and active materials without a binder, and also provides free space to accommodate Si expansion without pulverization during cycling. As an additional part, owing to an introduction of devices that required a flexible energy storage,Li-ion batteries (LIBs) as a leading candidate have been widely considered due to high electrochemical performance. To approach a flexible property in LIBs, the system has highly required an electrode with flexible characteristic, therefore, graphene based composites has been strongly attractive due to the large surface area and electron transport with high mechanical strength. Also, as another field in anode material, vanadium sulfides (VS4) have been paid much attention due to high specific capacity and rate capability of lithium storage in these days. Accordingly, the composite consisting of grapheme and VS4 is synthesized and characterized to describe the mechanism for lithium storage and high electrochemical performance in LIBs.