SYNTHESIS AND CHARACTERIZATION OF LiFePO4/CPOLYMER CATHODE MATERIAL FOR Li-ION BATTERY

Abstract

Due to the economic liability of fossil fuels and increasing global warming concerns, it is required to develop alternative renewable and clean energy sources (such as solar, wind, and hydroelectric power) intensively at wide scale. These developments have raised a strong demand of high capacity energy storage systems with lower production costs and safety requirements. Li-ion batteries (LIBs) have been considered as one of the most promising energy storage system to meet these requirements. The first commercial Li-ion battery was developed by Sony Company in 1991. The ever increasing demand of producing a high energy storage system at lower cost drives the continuous research on various components of LIB. There are three key components in a LIB system: cathode, anode and electrolyte. The modern cathode materials have been prepared in their lithiated (discharged) state so that they can be paired with delithiated anode with a minimum possibility of accidental short-circuiting during the assembling of a battery. Hence, cathode materials always have a keen interest in research to enhance the storage capacity and fast charging of a LIB. The commonly used cathode materials are based on transition metals oxides or phosphates active material like LiCoO2, LiMn2O4, and LiFePO4 etc. LiFePO4 is a potential material for the cathode of Li-ion battery because it possesses superior performance as it has high discharge capacity of 170 mAh/g, flat discharge voltage (3.4 V) profile, excellent thermal stability both at the charged and discharged states, high cyclability, low cost and is environmental friendly. However, a few limitations such as nominal discharge voltage of 3.4 V vs Li/Li-ion, low electronic conductivity (10-9 S/cm), lower Li-ion diffusion coefficient and poor performance at low temperature restrict LiFePO4 material from being used as cathode in commercial applications. The limitation of low redox voltage of 3.4 V for Fe2+/Fe3+ couple can be solved by substituting Fe with the transition metals such as Ni, Co and Mn in the structure of LiFePO4. The issues related to conductivity and diffusivity can be solved by 1) powder particles size reduction, 2) doping and 3) conductive surface coating such as with carbon and/or conducting polymers. The reduced particle size of powder offers several advantages such as short diffusion length, improved electron propagation, large electrolyte/electrode interface area and modified chemical ii potential. In view of the above advantages the particles size reduction can improve the highrate capability and cycling stability of LiFePO4 material. The coating with conducting materials of carbon and polymers improves the electrical conductivity and Li-ion diffusivity of LiFePO4 resulting in its improved capacity and rate capabiliy. The carbon coating is a potential approach to meet the theoretical capacity of LiFePO4 at a nominal current rate. The effect of carbon coating is to impart high electronic conductivity, enhance the Li-ion diffusion in LiFePO4/C composite. Despite of many advantages carbon coating decreases tap density and volumetric density of cathode, and form inactive Fe2P layer on LiFePO4 active particles. Substituting the inactive carbon and binder with electrochemically active polymer can enhance the electrochemical performance of LiFePO4. The conducting polymers viz. polypyrrole, polythiophene and polyaniline are electrochemically active in the range of 2.2 - 3.8 V which overlaps with the redox range (3.4 V for Fe2+/Fe3+) for LiFePO4. Therefore, these conducting polymers can act not only as conducting agent but also as host material for Li-ion insertion/extraction. This PhD thesis aims at exploring and investigating different polymers coated LiFePO4/C cathode materials for lithium ion battery. Systematic studies on the synthesis, characterization, cell fabrication and electrochemical testing have been performed. The thesis has been presented in six chapters. Chapter 1 presents a brief overview on non-conventional and conventional energy sources, history of batteries development, and working principle of the lithium ion battery. This chapter also introduces several advantages of lithium ion battery over the other type of secondary batteries. Chapter 2 presents a brief description of the various components of Lithium ion battery. The present status of various anode materials has been summarized. The detailed survey on cathode materials with main emphasis on LiFePO4 has been conducted. The factors affecting the carbon coating on LiFePO4 have been described. The promising methods of synthesis of electrochemically active polymers have been discussed. Finally the recent development on iii electrochemically active polymer coatings on the potential cathode materials has been summarized. Chapter 3 deals with the synthesis and characterization of LiFePO4/C and polymers coated LiFePO4/C. The LiFePO4/C nano-particles were synthesized by chemical precipitation method and the in-situ polymer coatings on LiFePO4/C were developed by oxidation polymerization method. Three types of electrochemically active polymer viz. polyaniline, polypyrrole and polythiophene have been used to coat LiFePO4/C material. Chapter 4 presents the process of LiFePO4 synthesis. The effects of 1.0 ml and 0.5 ml aniline monomers on structure and thickness of carbon coating as well as on the particle size of LiFePO4 have been investigated. The decomposition of the polymer coatings has resulted in good graphitized carbon coating on the LiFePO4 particles. The electrochemical study shows that discharge capacity of LiFePO4/C powder synthesized using 1.0ml aniline is higher than the powder synthesized using 0.5ml aniline. The higher rate capability and cyclability of LiFePO4 synthesized using 1.0ml aniline were attributed to the smaller particle size, higher Li-ion diffusion coefficient and higher electronic conductivity. Chapter 5 is broadly divided into three sections and presents the detailed investigation on mechanism of formation of polymer coatings viz. of polypyrrole, polythiophene and polyaniline on LiFePO4/C particles. In section-I, the Polypyrrole coating on LiFePO4/C (LiFePO4/C-PPy) particles has been discussed. The XRD pattern of the polypyrrole coated LiFePO4/C composite confirms the formation of two electrochemically active phases of LiFePO4 and Li0.05FePO4. The less porosity in the polypyrrole coating grown in ACN solvent produces good connectivity of the core particles resulting in higher electrical conductivity and Li-ion diffusivity. The composite LiFePO4/C-PPy polymerized in ACN shows appreciable rate capability of 82 mAh g-1 compared to 40 mAh g-1 for sample LiFePO4/C at higher current rate of 20C. The better rate capability of LiFePO4/C-PPy polymerized in ACN was due to the higher electrical conductivity and higher Li-ion diffusivity as confirmed by electrochemical impedance spectroscopy (EIS) measurements. iv The section-II presents the investigation on polythiophene coated LiFePO4/C material. To suppress the growth of Li3PO4 impurity phases, polythiophene coating was developed on Lideficient and Li-excess samples of LiFePO4. It has been observed that only the polythiophene coated Li-deficient Li0.95FePO4 material was capable of suppressing the Li3PO4 impurity phase. Polythiophene with different quantities of 4, 6, 8 and 12 wt% were deposited on Li0.95FePO4 material and the resultant materials are designated as Li0.95FePO4/C-PTh(4), Li0.95FePO4/CPTh( 6), Li0.95FePO4/C-PTh(8) and Li0.95FePO4/C-PTh(12) respectively. The EIS measurements show that all the polythiophene coated Li0.95FePO4/C materials exhibit higher Li-ion diffusivity and low charge transfer resistance (Rct). As the quantity of polythiophene increases in composites the conductivity also increase. The improved rate capability of Li0.95FePO4/CPTh( 8) composite was due to optimum quantity of polythiophene covering and interconnecting the LiFePO4/C particles. In section-III detailed investigations on the polyaniline coated LiFePO4/C composite has been presented. The in-situ polyaniline coating on LiFePO4 particles was done using three different oxidizing agents viz. (NH4)2S2O8, KMnO4 and K2Cr2O7. Polyaniline (PANI) samples have been grown separately by self oxidation process using the above oxidizing agents. XRD analysis reveals the formation of single phase pure active material LiFePO4/C and mixed phase containing LiFePO4 and FePO4 for polymer coated LiFePO4/C composite. The amounts of polymer in the polyaniline coated LiFePO4/C composites synthesized using (NH4)2S2O8, KMnO4 and K2Cr2O7 are 14, 15 and 17 wt% respectively. Electrical conductivities of the composite materials were determined by Impedance spectroscopy method. The composite material synthesized with (NH4)2S2O8 has higher conductivity compared to those synthesized with KMnO4 and K2Cr2O7. Chapter 6 presents the major conclusions of the present study and scope of the future work.