Fast ionic conductors, i.e. materials in which charge transport mainly occurs through the motion of ions, are an important class of materials with immense scope for industrial applications. There are different classes of fast ionic conductors e.g. polymer electrolytes, glasses, oxide ion conductors etc. and they find applications such as solid electrolytes in batteries, in fuel cells and in electro active sensors. There are mixed conducting materials as well which have both ions and electrons as conducting species that are used as electrode materials. Specifically, polymer electrolytes 1−3 have been in use in lithium polymer batteries, which have much more advantages compared to other secondary batteries. Polymer electrolyte membranes have been in use in direct methanol fuel cells (DMFC). The membranes act as proton conductors and allow the protons produced from the fuel (methanol) to pass through. Oxide ion conductors are used in high temperature solid oxide fuel cells (SOFC) and they conduct via oxygen ion vacancies. Fuel cells are rapidly replacing the internal combustion engines, because they are more energy efficient and environment friendly. The present thesis is concerned with the preparation, characterization and conductivity studies on the following fast ionic conductors: (MPEG)xLiClO4, (MPEG)xLiCF3SO3 where (MPEG) is methoxy poly(ethylene glycol), the hydrotalcite [Mg0.66Al0.33(OH)2][(CO3)0.17.mH2O] and the nanocomposite SPE, (PEG)46 LiClO4 with dispersed nanoparticles of hydrotalcite. We also present our investigations of spin probe electron spin resonance (SPESR) as a possible technique to determine the glass transition temperature (Tg) of polymer electrolytes where the conventional technique of Tg determination, namely, differential scanning calorimetry, (DSC), is not useful due to the high crystallinity of the polymers. In the following we summarize the main contents of the thesis. In Chapter 1 we provide a brief introduction to the phenomenon of fast ionic conduction. A description of the different experimental techniques used as well as the relevant theories is also given in this chapter. In most solid polymer electrolytes (SPE), the usability is limited by the low value of the ionic conductivity. A number of different routes to enhance the electrical, thermal and mechanical properties of these materials is presently under investigation. One such route to enhance the ionic conductivity in polymer electrolytes is by irradiating the polymer electrolyte with gamma rays, electron beam, ion beams etc. In Chapter 2, we describe our work on the effect of electron beam (e-beam) irradiation on the solid polymer electrolytes (MPEG)xLiClO4 and (MPEG)xLiCF3SO3. The polymer used is methoxy poly(ethylene glycol) or poly(ethylene glycol) methyl ether with a molecular weight 2000. Salts used are LiClO4 and LiCF3SO3. ’x’ in the subscript is a measure of the salt concentration; it is the ratio of the number of ether oxygens in the polymer chain to that of the Li+ ion. ’x’ values chosen are 100, 46, 30 and 16. Nearly one order of magnitude increase in the conductivity is observed for samples (MPEG)100LiClO4 and (MPEG)16LiCF3SO3 on irradiation. It was found that the increase in the net ionic conductivity is a function of both the irradiation dose and the salt concentration. The enhanced ionic conductivity remains constant for ∼ 100 hrs, which signifies a possible near permanent change in the polymer electrolyte system due to irradiation. The samples were also characterized using DSC and Fourier transform infrared spectroscopy (FTIR). DSC results could be correlated with conductivity findings, giving low Tg values for samples having high conductivity. It was also found that there is a small increase in the crystalline fraction of the samples on irradiation, which agrees with earlier reports on samples irradiated with low dosage. FTIR results are suggestive of decreased cross linking as the reason for increased ionic conductivity. However, this aspect needs a further confirmatory look before the findings can be termed conclusive. In Chapter 3, we describe the studies we have carried out on Li -doped hydrotalcite. We report the details of preparation and characterization of hydrotalcite as well as NMR and ionic conductivity measurements on both doped (with Li+ ions) and undoped hydrotalcite. Hydrotalcite was prepared by co-precipitation method and the composition of hydrotalcite was chosen as [Mg0.66Al0.33(OH)2][(CO3)0.17.mH2O]. Samples were prepared with salt (LiClO4) concentration 5 %, 10 %, 15 %, 20 % and 25 %. It was found that the highest ionic conductivity occurs for the sample with 20 % doping. 7Li NMR plots for all the samples clearly show an overlap of a Gaussian and a Lorentzian lineshape. The Gaussian line is because of the presence of a less mobile fraction of the 7Li+ ions and the Lorentzian line is because of the presence of a more mobile fraction of 7Li+ ions. The highest ionic conductivity was found for the salt concentration 20 % and from the room temperature 7Li NMR studies we found that for this particular concentration, the mobile fraction of the 7Li ion is also maximum. Without the salt doping, the conductivity of the sample was too small to be measured. Temperature variation of both 1H and 7Li NMR was also done, to compare the ionic conductivities from NMR. Another method to obtain enhanced properties in polymer electrolytes is by forming ’nanocomposite’ polymer electrolytes. Nanocomposites are formed by dispersing nanoparticles of certain materials in the polymer electrolyte matrix. Till now, nanoparticles used are mostly oxides of metals, e.g. Al2O3, TiO2, MgO, SiO2 etc and clays like montmorillonite, liponite, hydrotalcite etc. Chapter 4 describes the preparation and characterization of the nanocomposite polymer electrolyte (PEG)46LiClO4 formed with hydrotalcite nanoparticles. The polymer used is PEG, poly(ethylene glycol) of molecular weight 2000, and salt used is LiClO4. The salt concentration is selected so as to give the highest ionic conductivity for the solid polymer electrolyte. Hydrotalcite belongs to a class of materials called LDH, layered double hydroxides. The composition selected is [Mg0.66Al0.33(OH)2][(CO3)0.17 .mH2O], since this is the most stable composition. These materials are easy to prepare in the nano size and are being used in a number of applications. These are characterized by the presence of layers of positively charged double hydroxides separated by layers of anions and water molecules. The water molecules give stability to the structure. Nanoparticles of hydrotalcite were prepared in the laboratory itself. XRD data of hydrotalcite confirm the crystal structure. TEM data show the particle size to be ∼ 50 nm. The polymer electrolyte (PEG)46LiClO4 was doped with these nanoparticles and the doping levels are 1.8 %, 2.1 %, 2.7 %, 3.6 % and 4.5 % by weight. Impedance spectroscopy was used to find the ionic conductivity. We have found that the sample with a doping of 3.6 % by weight gives the highest ionic conductivity and the increase in ionic conductivity is nearly one order of magnitude. DSC was used for thermal characterization of these nanocomposites. The glass transition temperatures, Tg , found from DSC measurements corroborates the ionic conductivity data, giving the lowest Tg for the sample with highest conductivity. Temperature variation of the ionic conductivity shows Arrhenius behavior. 7Li NMR was done on the pristine SPE (PEG)46LiClO4 and the nanocomposite of (PEG)46LiClO4 with 3.6 % filler. The ionic conductivity was also estimated from the temperature variation of 7Li NMR line widths. Studies on the DSC endotherms of the nanocomposites give the fractional crystallinity of the samples. From these studies it can be concluded that the variation in ionic conductivity can be attributed to the change in fractional crystallinity; the nanocomposite polymer electrolyte having highest ionic conductivity, i.e. the NCPE with filler concentration of 3.6 % also has the lowest fractional crystallinity. Additionally, a possible increase in the segmental motion inferred from a reduction in the glass transition temperature coupled with a lowering of the activation energy may also contribute to the increased ionic conductivity in the nanocomposite polymer electrolyte. Glass transition temperature Tg has a very important role in studying the dynamics of polymer electrolytes. In Chapter 5, we explore the possibility of using spin probe electron spin resonance (SPESR) as a tool to study the glass transition temperature of polymer electrolytes. When the temperature of the polymer is increased across the glass transition, the viscosity of the sample decreases. This corresponds to a transition from a slow tumbling regime with τc = 10−6 s to a fast tumbling regime with τc = 10−9 s where τc is the correlation time for the probe dynamics. Spin probe ESR can be used to probe this transition in polymers. We have used 4-hydroxy tempo (TEMPOL) as the spin probe which is dispersed in the nanocomposite polymer electrolyte based on (PEG)46LiClO4 and hydrotalcite. Below and across the glass transition, this nitroxide probe exhibits a powder pattern showing both Zeeman (g) and hyperfine (hf) interaction anisotropy. When the frequency of the dynamics increases such that the jump frequency f is of the same order of magnitude as the anisotropy of the hf interaction, i.e., ∼ 108 Hz, the anisotropy of the interactions averages out and a spectrum of reduced splitting and increased symmetry in the line shape is observed. This splitting corresponds to the nonvanishing isotropic value of the hyperfine tensor and is observed at a temperature higher than but correlated with Tg. The crossover from the anisotropic to isotropic spectrum is reflected in a sharp reduction in the separation between the two outermost components of the ESR spectrum, which corresponds to twice the value of the z-principal component of the nitrogen hyperfine tensor, 2Azz, from ∼75 G to ∼ 35 G. In our study, we have varied the concentration of the nano-fillers. The Tg for all the samples were estimated from the measurement of T50G and the known correlation between 4 T50G and Tg, where T50G is the temperature at which the extrema separation (2Azz) of the ESR spectra becomes 50 Gauss. The values obtained from this method are compared with the values found from DSC done on the same samples. Within experimental error, these two techniques give reasonably close values. Tg’s were also estimated by a cross over in the correlation time (τc) vs temperature plot. The τc values were calculated using a spectral simulation program. We conclude that spin probe ESR can be an alternative to the DSC technique for polymers with high fraction of crystallinity, for which DSC often does not give any glass transition signature. In Appendix I, ionic conductivity studies on quenched and gamma irradiated polymer electrolytes (PEG)46LiClO4 and (MPEG)16LiClO4 is done. It is observed that, (i) the samples quenched to 77 K after melting show enhancement of ionic conductivity by a factor of 3 & 4; (ii) on irradiation, the ionic conductivity decreases for a dose of 5 kGy and subsequently, keeps on increasing for higher doses of 10 kGy and 15 kGy. In Appendix II, the BASIC language program (eq-res.bas) used for impedance data analysis is given.

The present thesis demonstrates and discusses polymeric ion and mixed ion-electron conductors for rechargeable batteries based on lithium viz. lithium-ion and lithium-sulphur batteries. The proposed polymer ion conductors in the thesis are discussed primarily as potential alternatives to conventional liquid and solid-crystalline electrolytes in lithium-ion batteries. These discussions are part of Chapters 2-4. On the other hand, the polymer based mixed ion-electron conductor is demonstrated as a novel electrode for lithium-Sulphur battery in Chapter 5. Possibility of application of polymer ion conductors is discussed in the context of Li-S battery in Chapter 6. A distinct correlation between the physical properties and electrochemical performance of the proposed conductors is highlighted in detail in this thesis. Systematic investigation of the ion transport mechanism in the polymeric ion conductors has been carried out using various spectroscopic techniques at different time and length scales. Such detailed investigations demonstrate the key structural and physical parameters for design of alternative polymer conductors for rechargeable batteries. Though the thesis discusses the various polymeric conductors in the context of lithium-based batteries, it is strongly felt that the design strategies are equally likely to be beneficial for different battery chemistries as well as for other electrochemical generation and storage devices. A brief discussion of the contents and highlights of the individual chapters are described below: The thesis comprises of six Chapters. Chapter 1 briefly reviews the important developments and materials of lithium-based batteries, with specific focus on Li-ion and Li-S batteries. It starts with discussions on different types of liquid, solid crystalline and solid-like electrolytes. Their materials characteristics, advantages and disadvantages are discussed in the context of secondary batteries such as lithium-ion and lithium-sulphur batteries. As prospective alternative electrolytes polymer based soft matter electrolytes are discussed in detail. Special emphasis is given to the recent developments in polymer electrolytes and their ion conduction mechanism, which are central themes to this thesis. The importance of investigation of charge transport, typically ion, on electrochemical processes is also briefly discussed in Chapter 1. A brief discussion about the characteristics, materials and non-trivialities of the electrochemical storage process in Li-S battery is also reviewed. Chapter 2A demonstrates a binary polymer physical network based gel (PN-x) electrolyte, comprising of an ionic liquid confined inside a binary polymer system for electrochemical devices such as secondary batteries. The synthesis, physical property and electrochemical performances are studied as a function of content of one of the polymers in this Chapter. A physical network of two polymers with different functional groups leads to multiple interesting consequences. The polymer physical network characteristics determine all physical properties including electrochemical property of the ionic liquid integrated PN based GPE. The conductivities of the proposed gel are nearly an order in magnitude higher than the unconfined ionic liquid electrolyte and displays good dimensional stability and electrochemical performance in a separator-free battery configuration. The ac-impedance spectroscopy, steady shear viscosity measurement, dynamic rheology are employed to study physical properties of the proposed gel polymer electrolyte. Chapter 2B discusses the detailed investigations of the ion transport mechanism of the gel polymer electrolyte, as discussed in Chapter 2A. Ion conduction mechanism is investigated in the light of ion diffusion and solvent dynamics of the entrapped ionic liquid inside the polymer. The studies reveal a heavy influence of network characteristics on the ion conduction mechanism. The influence of solvent dynamics on the ion transport is drastically altered by polymer physical network. Consequently, a drastic change in the ion mobility and nature of predominant charge carrier is observed in the polymer physical network based gel electrolyte. A clear transformation from dual ion conductivity to a predominantly anion conductivity is observed on going from single polymer to a dual polymer network. The spectroscopic tools such as pulsed field gradient nuclear magnetic resonance (PFG–NMR), Brillouin light scattering spectroscopy, ac-impedance spectroscopy, FT-Raman and FTIR spectroscopy were used to elucidate the ion transport mechanism in the Chapter. Chapter 3 demonstrates a simple design strategy of gel polymer electrolyte comprising of a lithium salt (lithium bis(trifluoromethanesulfonyl) imide, LiTFSI) solvated by two plastic crystalline solvents, one a solid (succinonitrile, abbreviated as SN) and another a (room temperature) ionic liquid (1-butyl-1-methyl-pyrrolidinium bis(trifluoromethane sulfonyl) imide, (abbreviated as IL) confined inside a linear network of poly(methyl methacrylate) (PMMA). The concentration of the IL component determines the physical properties of the unconfined electrolyte and when confined inside the polymer network in gel polymer electrolyte. Intrinsic dynamics of one plastic crystal influences the conduction mechanism of gel polymer electrolytes. The enhanced disordering in the plastic phase of succinonitrile by IL doping alters both the local ion environment and viscosity. The proposed plastic crystal electrolytes show predominantly anion conduction (tTFSI ≈ 0.5) however, lithium transference number (tLi ≈ 0.2) is nearly an order higher than the ionic liquid electrolyte (IL-LiTFSI) (tLi ≈ 0.02-0.06), discussed in Chapter 2. The gel polymer electrolyte displayed high mechanical compliability, stable Li-electrode | electrolyte interface, low rate of Al corrosion and stable cyclability. The promising electrochemical performance further justifies simple strategy of employing mixed physical state plasticizers to tune the physical properties of polymer electrolytes requisite for application in rechargeable batteries. Chapter 4A proposes a novel liquid dendrimer–based single ion conducting liquid electrolyte as potential alternative to conventional molecular liquid solvent–salt solutions and conventional solid polymer electrolytes for rechargeable batteries, sensors and actuators. The physical properties are investigated as a function of peripheral functionalities in the first generation poly(propyl ether imine) (G1-PETIM)–lithium salt complexes. The change in peripheral group simultaneously affects the effective physical properties viz. viscosity, ionic conductivity, ion diffusion coefficients, transference numbers and also the electrochemical response. The specific change from ester (–COOR) to cyano (–CN) terminated peripheral group resulted in a remarkable switch over from a high cation (tLi+ = 0.9 for –COOR) to a high anion (tPF6- = 0.8 for –CN) transference number. Chapter 4B presents an analysis of the frequency dependent ionic conductivity of single ion dendrimer conductors by using time temperature scaling principles (TTSPs) and dielectric modeling of the electrode polarization. The TTSP provides information on the salt dissociation and number density of mobile charges and hence provides direct insights into the ion conduction mechanism. Summerfield and Baranovskii–Cordes scaling laws, which are well known TTSPs, have been applied to analyze the ion conductivity. The electrode polarization, which quantifies the number density of mobile charges and ionic mobility, is studied using Macdonald-Coelho model of electrode polarization. The combination of these two theoretical investigations of the experimental data emanating from one technique i.e. ac– impedance spectroscopy, predicts independently the contributions of the effect of mobile ion charges and ionic mobility to ion conduction mechanism. In Chapter 5 focus shifts from polymer ion conductors to polymer mixed ion-electron conductor. The polymer mixed ion-electron conductor is demonstrated as a novel electrode material for Li-S battery. A simple strategy to overcome the challenges towards practical realization of a stable high performance Li–S battery is discussed. A soft mixed conducting polymeric network is utilized to configure sulphur nanoparticle. The soft matter network provides efficient and distinct pathways for lithium and electron conduction simultaneously. A lithiated polyethylene glycol (PEG) based surfactant tethered on ultra-small sulphur nanoparticles and wrapped up with polyaniline (PAni) (abbreviated as S-MIEC) is demonstrated here as an exceptional cathode for Li–S batteries. The S-MIEC is characterized by several methods: powder-X-ray diffraction (PXRD), thermo gravimetric analysis (TGA), fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), ac-impedance spectroscopy and dc current-voltage measurements are performed to evaluate conductivity of S-MIEC cathode. Electrochemical studies such as cyclic voltammetry, galvanostatic charge-discharge cycling, galvanostatic intermittent titration (GITT) are performed to demonstrate feasibility of S-MIEC in the Li–S battery performance. Chapter 6 provides a brief summary of the work carried out as part of this thesis and also demonstrates the future perspective of the present work. Potential of the polymer physical network based gel polymer electrolytes, which are discussed in Chapter 2A-B for lithium-ion batteries, are demonstrated in Li-S battery. The proposed polymer physical network confines higher order lithium polysulfides (typically Li2S8) dissolved in tetraethylene glycol dimethyl ether (TEGDME) based electrolyte (TEGDME-1M LiTFSI). The three dimensional polymer network is proposed to be formed by physical blending of the poly(acrylonitrile) (PAN) with the copolymer of AN and poly(ethylene glycol) methyl ether methacrylate (PEGMA), [ P(AN–co–PEGMA)]. We extend here the similar synthetic approaches as described in Chapter 2A. The approach proposed and demonstrated in this concluding Chapter is expected to mitigate some of the major issues of Li-S chemistry. The proposed Li2S8 confined gel electrolyte exhibits moderately high values of ionic conductivity, 2 × 10-3 Ω-1cm-1 and shows a stable capacity of 350 mAhg-1 over 30 days in a separator free Li-S battery.