POLYMER BASED NANOCOMPOSITE ELECTROLYTES FOR Li-ION BATTERIES

Abstract

Novel studies of Wright and Armand in the late 1970s on ionically conducting polymers known as “polymer electrolytes” have opened an area of materials research with potential application in the power industry. The electrolytes have been known by their technological applications in the areas of energy conversion and storage. As compared to liquid electrolytes, the polymer electrolytes have several merits such as these materials have no leakage problem, ability to operate with highly reactive electrodes over a wide range of temperature and the possibility of miniaturization of device assembly. These advantages have drawn many researchers attention towards the development of lithium polymer batteries and other electrochemical devices such as electrochromic windows, supercapacitors, sensors etc. The polymer electrolytes create a new market of the secondary battery and „Sony‟ is the number one worldwide in the production of lithium-ion gel polymer batteries. The utilization of the lithium-ion gel polymer battery has been supported by the technical innovation of polymer electrolyte materials. Batteries based on polymer electrolytes are the subject of active research and development. In order to keep abreast with the rapid development of portable electronic equipment, improving the performance of polymer electrolytes has therefore become a goal of research. The polymers to act as electrolyte are desirable to possess the following characteristics: (i) high ionic conductivity, (ii) good dimensional stability, (iii) high cationic transport number, (iv) improved mechanical stability, (v) high chemical, electrochemical and thermal stability and (vi) compatibility with the materials of both anode and cathode. The Ph.D. thesis deals with the synthesis and characterization of gel polymer electrolytes (GPEs) based on (i) PMMA-(PC+DEC)-LiClO4 and (ii) P(VdF-HFP)-(PC+DEC)-LiClO4 systems. Dispersoids used are multi-walled carbon nanotube (MWCNT), carbon nanofiber (CNF), silicon dioxide (SiO2) nanofiber and titanium dioxide (TiO2) nanofiber for the preparation of nanocomposite gel polymer electrolytes (NCGPEs). The various categories of polymer electrolytes are (a) Conventional dry/solid polymer electrolytes, (b) Plasticized solid polymer electrolytes, (c) Rubbery electrolytes, (d) Polyelectrolytes, (e) Gel polymer electrolytes and (f) Composite polymer ii electrolytes. Each type is differentiated from another by a characteristic conductivity range. Gel polymer electrolytes have a relatively higher ionic conductivity and therefore are contemplated to be a potential candidate for application in high performance lithium ion battery. Though various researchers have conducted investigations on gel cast electrolytes, however, the primary focus of their studies are on achieving higher ionic conductivity of these materials. In the present study, the GPEs and NCGPEs have been synthesized in the form of film by solution casting technique. An attempt has been made to integrate the investigation on mechanical characteristics such as strength of the films with desired ionic conductivity and thermal stability. It is needless to mention that mechanical strength is an essential requirement for the films to be inserted in the cell assembly. Different types of nanofiller (carbon and oxide based) were selected and for each type of nanofiller, several compositions were used to study the effect of filler quantity on the various properties of the NCGPEs. Dispersion of nanofiller in precursor polymer solution was made using ultrasonic probe. While the stress-strain characteristics were measured for the individual films, the ionic conductivity of MWCNT and CNF dispersed films were measured by adopting three-layer-film fabrication. The central layer of MWCNT/CNF dispersed layer was sandwiched both sides with filler free gel cast polymer layers. Such arrangement ensures the avoidance of electrical shorting for the electrical conductivity measurements. The presentation of the entire thesis has been divided in six chapters. Chapter I presents the introduction of the work stating the objective of the Ph.D. dissertation. Chapter II presents the literature survey describing the state of the art of different types of polymer electrolytes in general and nanocomposite gel polymer electrolytes in particular. A brief description of ion transport models has also been made. The justification of taking up the problem has been highlighted. Chapter III presents the properties of constituent materials viz. polymers (PMMA and P(VdFHFP)), plasticizers (PC+DEC), salt (LiClO4) and inorganic fillers (MWCNT, CNF, SiO2 nanofiber and TiO2 nanofiber) used in preparing GPEs and NCGPEs. Synthesis techniques for the preparation of GPEs and NCGPEs by solution casting method have been explained. Principles of various characterization tools and instruments employed to characterize the GPEs and NCGPEs in iii the present work have been briefly explained. The ionic, mechanical, thermal characteristics and electrochemical analysis of the GPEs and NCGPEs have also been discussed. Chapter IV presents results and discussion on the synthesis of the GPEs based on (i) PMMA- (PC+DEC)-LiClO4 and (ii) P(VdF-HFP)-(PC+DEC)-LiClO4 systems along with their physical, chemical and electrochemical characteristics. Chapter V presents results and discussion on the synthesis of various types of NCGPEs dispersed with different types of nanofillers viz. MWCNTs, CNFs, SiO2 nanofiber and TiO2 nanofiber in both the systems i.e PMMA-(PC+DEC)-LiClO4 and P(VdF-HFP)-(PC+DEC)-LiClO4. The enhancement in the mechanical, thermal and electrochemical properties that has been observed for all the NCGPE films with respect to their GPE counterpart has been discussed. Chapter VI outlines the major conclusions drawn from the work done. The future scope of the related field is also briefly stated.