CARBON NANOSTRUCTURES-DEVELOPMENT AND APPLICATION IN THE ANODE OF LI-ION BATTERY

Abstract

Nanotechnology, although the term was coined by Norio Taniguchi in the 1970's, has witnessed its first breakthrough with the discovery of Buckeyballs (fullerene) in eighties and nanotubes in the nineties. The properties and applicability of carbon nanostructures in general have produced tremendous excitement and interest among scientists, engineers and technologists. The synthesis of carbon nanostructures has progressed significantly and now further advancement depends critically on better understanding of the steps involved in growth for each type of carbon nanostructures. The first segment of the present study aims to contribute to the understanding of the growth of carbon nanostructures in catalytic chemical It vapour deposition method with particular emphasis on the conditions favouring the development of different morphologies viz, single walled carbon nanotube, multi walled carbon nanotube, carbon nanofiber and carbon nanotape around catalyst nanoparticles, so that the method may be adapted to large scale production of carbon nanostructures of a given morphology. Since the commercial introduction of lithium-ion batteries by Sony Corporation in the early 1990, there is continuing global effort to develop lithium ion batteries with higher capacity and better stability in order to extend their application, particularly in mobile tools and automobiles. The battery performance solely depends upon the capacity of electrode materials to hold active species of lithium and discharge/charge reversibly. The currently used commercial anode of graphite has excellent stability and low cost but could be intercalated by the active species of lithium up to a maximum limit given by the chemical formula LiC6, which leads to a theoretical limit of its specific charge capacity, 372 mAhg* In order to overcome the limitation of charge capacity, attempts have been made to replace the existing graphite material with different nanostructures of carbon. The objective of the second segment of the present study is to understand the relationship between the morphology and the electrochemical properties of carbon nanostructures and further to examine their electrochemical performance in terms of capacity for practical application in rechargeable lithium ion batteries. The present work compares the electrochemical performance of different morphologies of carbon nanostructures viz., single walled carbon nanotube, double walled carbon nanotube, multi walled carbon nanotube and carbon nano fiber. H In the present study, catalytic chemical vapour deposition method has been used to grow carbon nanostructures at 640 °C by decomposition of acetylene gas using nanoparticles of cobalt and nickel oxides with and without doping by copper oxide. The oxide nanoparticles with various combinations of doping level and the particle size synthesised by sol-gel technique were dispersed on the substrate of porous anodic aluminium oxide obtained by two step anodization of pure aluminium. Teflon made electrochemical cells were assembled using the carbon nanostructure based electrode as the working electrode and lithium metal as the reference electrode for charge/discharge cycling using electrochemical analyzer. It has been observed that there is melting or surface melting of oxide nanoparticles at the temperature prevailing during growth of carbon nanostructures. The melting/surface melting characteristics of the oxide nanoparticles having different sizes with and without doping have been extensively investigated and in a pioneering approach, the temperature at the start of melting has been taken as the characteristic parameter of melting to rationalise the observations on the growth morphology of carbon nanostructure. It has also been observed that there is reduction of oxide in the chamber of catalytic chemical vapour deposition set-up, which contains potent reducing agents like carbon and hydrogen. The surface melting of oxide perhaps helps in dissolution of carbon and relatively faster transport of the reducing agents inside the particles. I)issolution of carbon following deposition also helps to prevent the formation of thick deposit of carbon on the oxide nanoparticles, which results in the formation of nanobeads. The rate of flow of carbon bearing gas is therefore important as it controls the rate of decomposition and deposition of carbon over the nanoparticles. The dissolution and diffusion of the reducing agents inside oxide nanoparticles may take place radially inside the oxide particles. Assuming a given temperature profile within the oxide particle, it may safely be presumed in general that the molten part of oxide at the surface remains molten even after reduction as metal has in general a lower melting point than its oxide. The thickness of the molten surface layer of metal may even be more than that of the molten layer of oxide before reduction. However, cobalt oxide, C0304, has a lower melting point than metallic cobalt and the thickness of the molten layer could be less than the thickness of molten layer in oxide. One may perceive that graphene may nucleate on the template of hexagonal close packed plane in the metal clusters created by reduction on the surface of solid oxide core. Once graphene has nucleated and grown around the particle there will be two direction of growth radially along the thickness and axially along the length. The liux of carbon entering the particle will have to be used for two purposes - (i) to cause II reduction of oxide (along with hydrogen), and (ii) to provide carbon for axial growth as well as graphitic growth in thickness direction. Since growth in the thickness direction takes place by addition of more graphene layers and hence it is termed as graphitic growth, which may take place both outside and inside of the initially developed graphene tube. On the outside, this growth will take place provided there is molten layer or else there will be no graphitic growth and only loose carbon deposit may form as it has been commonly observed. The possibility of growing inside will depend on: (i) flux of carbon inside the graphene tube possibly through a dynamic process of dissolution and reformation of the graphene layer as the dissolved part of the layer will be reformed by flux of carbon arriving from outside, and (ii) progress of reduction and melting. Melting inside is required not only for part dissolution of graphene and high flux of carbon inside but also for squeezing out the melt along the axial hole of the tube to make room for the graphitic growth inside. This could be the explanation for the shape change of catalyst particles as it has been observed inside carbon nanostructure. If the entire core melts due to reduction it will pave a way for the growth of carbon nanofiber and partial melting will lead to multi walled carbon nanotube. When the oxide particle is very small and there is not enough thickness in the liquid layer for radial graphitic growth either inside or outside, single walled carbon nanotube is expected to grow only by axial growth. The core melting in the reaction chamber of catalytic chemical vapour deposition depends both on the melting characteristics of the oxide nanoparticle, indicated by the temperature at the start of surface melting, T, which in turn depends on the chemical constitution as modified by doping and the size of the oxide particle. When the temperature at the start of melting is normalized with respect to the growth temperature, Tg, of carbon nanostructure as A = (Ts Tg)/Ts and A is plotted with the minimum size of the oxide nanoparticle used in an experiment for the growth of carbon nanostructure, it was observed that there is clustering of the experimental points representing the carbon nanostructure with the same morphology irrespective of the type of catalyst used. The anode based on carbon nanostructures shows superior electrochemical properties and better coulombic efficiency as compared to graphite. Depending on the type of nanostructure, reversible capacity obtained lies in the range from 450 to 600 mAhg and the coulombic efficiency beyond the third cycle is within the range from 85 to 98%. The content of the thesis has been presented in six chapters which are outlined below: Chapter 1 presents a brief introduction describing the importance of present study. in Chapter 2 presents an overview of different types of carbon nanostructures, their structural difference, properties and method of production. The current state of growth mechanism and selective synthesis processes, have been reviewed. Application of carbon based materials for energy storage application with a special emphasis on their use in the anode of lithium-ion battery system has been highlighted. The present state of knowledge available in the area has been discussed in order to identify the gap towards which the present investigation has been directed. Chapter 3 describes the details of the experimental work carried out in the present study. Chapter 4 presents results and discussion on oxide nanoparticle synthesis, nanoporous substrate preparation and synthesis of carbon nanostructure using nanoparticles of cobalt and nickel oxides with and without doping by copper oxide, having different size distribution and melting characteristics. Chapter 5 presents electrochemical characterization of the different types of carbon nanostructure based anode materials to determine their suitability as anode materials for lithium ion batteries. Chapter 6 summarizes the major conclusions.