Entering a new era of green energy, several criteria such as cost, cycle life, safety, efficiency, energy, and power need to be considered in developing electrical energy storage systems for transportation and grid storage. Lithium-sulfur (Li-S) batteries are one of the prospective candidates in this regard as sulfur offers a high theoretical capacity of 1675 mAh g⁻¹ at a safer operating voltage range of ~ 2.1 V and low-cost benefit. This dissertation explores various original designs of novel electrodes, new cell configurations, and recharge strategies that can boost the cycle performance of Li-S cells. An in situ sulfur deposition route has been developed for synthesizing sulfur-carbon composites as cathode materials. This facile synthesis method involves the precipitation of elemental sulfur at the interspaces between carbon nanoparticles in aqueous solution at room temperature. Thus, a sulfur/multi-wall carbon nanotube (MWCNT) composite cathode with high-rate cyclability has been synthesized by the same process. Due to the self-weaving behavior of MWCNTs, extra cell components such as binders and current collectors are rendered unnecessary, thereby streamlining the electrode manufacturing process and decreasing the cell weight. A novel Li-S cell configuration with a carbon interlayer inserted between the separator and cathode has been designed to enhance the battery cyclability as well. A conductive MWCNT interlayer acting as a pseudo-upper current collector not only reduces the charge transfer resistance of sulfur cathodes significantly, but also localizes and retains the dissolved active material during cycling. Moreover, with a bi-functional microporous carbon paper intrerlayer, we observe a significant improvement not only in the active material utilization but also in capacity retention, without involving complex synthesis or surface modification. The kinetics of the sulfur/long-chain polysulfide redox couple (S₈ [double-sided arrow] Li₂S₄, theoretical capacity = 419 mAh g⁻¹) is experimentally proven to be very fast in the Li-S system. The Li-S cell with a blended carbon interlayer retains excellent cycle stability and possesses a high percentage of active material utilization over 250 cycles at high C rates (up to 15C). The meso-/micro- pores in the interlayer are in charge of accommodating the shuttling polysulfides and offering sufficient electrolyte accessibility. An appropriate and applicable way to recharge Li-S cells within the lower plateau region has been designed to offer tremendous improvement with various Li-S battery systems. Adjusting the charging condition led to long cycle life (over 500 cycles) with excellent capacity retention (> 99%) by inhibiting the electrochemical reactions along with polysulfide dissolution. In addition, the redox products determined by ex situ x-ray photoelectron spectroscopy (XPS) further clarify the mechanism of polysulfide formation upon cycling, which is partially different from the general consensus. These approaches of novel electrode designs, new cell configurations, charging strategy, and understanding of the reactions in different discharge steps could progress the development and advancement of Li-S batteries. / text

This dissertation is composed of seven chapters, detailing advances within the area of sulfur polymer chemistry and processing, and highlights the relevance of the work to the fields of polymer science, energy storage, and optics that are enabled through the development of novel high sulfur-content copolymers as discussed in the following chapters. The first chapter is a review summarizing both the historical forays into utilization of elemental sulfur in high sulfur-content materials and the current research on the incorporation of sulfur into novel copolymers and composites for high value added applications such as energy production/storage, polymeric optical components, and dynamic/self-healing materials. Although recent efforts by the materials and polymer chemistry communities have afforded innovative sulfur containing materials, many studies fail to take advantage of the low cost and incredible abundance of sulfur by incorporating only minimal quantities into the end products. A fundamental challenge in the preparation of sulfur-containing polymers is simultaneous incorporation of high sulfur-content through facile chemical methods, to truly use the element as a novel feedstock in copolymerizations. Contributing to the challenge are the intrinsic limitations of sulfur (i.e., low miscibility with organic solvents, high crystallinity, and poor processability). The emphasis in chapter 1 is the critical development of utilizing sulfur as both a reagent and solvent in a bulk reaction, termed inverse vulcanization. Through this methodology we can directly prepare materials which retain the advantageous properties of elemental sulfur (i.e., high electrochemical capacity, high refractive index, and liable bond character), obviate the processing challenges, and enable precise control over composition and properties in a facile manner. The second chapter focuses on advancement in colloid synthesis, specifically an example mediated by in-situ reduction of organometallic precursors (ClAu^IPPh₃) by elemental sulfur at high temperatures. In chapter 2, elemental sulfur is employed both as a reactant and novel solvent, generating composite composed of well-defined gold nanoparticles (Au NPs) fully dispersed in a sulfur matrix. While the synthesis of Au NPs in molten sulfur was a novel development the challenge of analyzing the particles directly within the sulfur composite matrix by microscopy techniques required improvement of the composites mechanical properties. To overcome this issue, a one-pot reaction in which the Au NPs were initially synthesized, was vulcanized through an ambient atmosphere-tolerant bulk copolymerization by the addition of a difunctional comonomer (divinylbenzene). The improved composite integrity enabled microtoming and transmission electron microscopy analysis of the particles within the crosslinked reaction matrix. Due to the facile capabilities of directly dissolving the comonomers within the molten sulfur the inverse vulcanization methodology provides a simple route to prepare stable, high sulfur-content copolymers in a single one-pot reaction. The third chapter expands upon the methodology for direct dissolution of difunctional comonomers into molten elemental sulfur to afford chemically stable copolymer. A major challenge associated with the high temperature (i.e., 185 °C) bulk copolymerization reactions between sulfur and vinyl comonomers (i.e., divinylbenzene, DVB) is the high volatility of the organic monomers at elevated temperatures (BP of DVB = 195 °C). To obviate this problem required a novel monomer with an increased boiling point for successful scaling of the inverse vulcanization methodology. The work presented in chapter 3 details the employment of 1,3-diisopropenylbenzene (DIB, BP = 231 °C) to enable larger scale bulk inverse vulcanization reactions, allowing facile control over thermomechanical properties by simple variation in copolymer composition (50–90-wt% S₈, 10–50-wt% DIB). Poly(Sulfur-random-1,3-diisopropenylbenzene) ((poly(S-r-DIB)) copolymers prepared via the inverse vulcanization methodology possess substantially improved processing capabilities compared with elemental sulfur. A facile demonstration of improved processability is the generation of free-standing micropatterned structures using a high sulfur content liquid pre-polymer resin that can be poured into a mold and cured into the desired final form. The highest weight percentage copolymer (i.e., 90-wt% S₈) was also demonstrated to improve cycle lifetimes and capacity retention (823 mAh•g⁻¹ at 100 cycles) of a Lithium-Sulfur (Li-S) cell when the copolymer was utilized as the active material instead of elemental sulfur. Chapter four focuses on the optimization of Li-S cell performance as a function of copolymer composition and provides a more thorough understanding of the means by which copolymer active material improves battery performance. A substantial challenge associated with Li-S cells is the fast capacity fade and short cycle lifetimes that result from loss of the active material (i.e., sulfur) during normal cycling processes. The field has generally addressed these issues by encapsulation of the sulfur in a protective shell (e.g., polymeric, carbonaceous, or metal oxide in nature) in an attempt to sequester the active material. However, encapsulation of sulfur is non-trivial and leads to low loadings of sulfur, resulting in a low energy density within the final cell. To address the challenges associated with maintaining high capacity and long cycle lifetimes while employing an active material which is low cost, generated in a facile manner, and has a high sulfur content required a novel approach. In the work presented in chapter 4 we prepared high sulfur content copolymers via the inverse vulcanization methodology, which meet all the requirements necessary of an active material, and investigated the performance of Li-S batteries as a function of the copolymer composition. A survey of several poly(S-r-DIB) copolymer compositions were prepared with DIB compositions ranging from 1-50-wt% DIB (i.e., 50-99 wt% sulfur) and screened to determine optimal compositions for optimal Li-S battery performance. From this analysis it was determined that copolymers with 10-wt% DIB (90-wt% S₈) were optimal for producing Li-S batteries with high capacity and long cycle lifetimes. 10-wt% DIB copolymers batteries ultimately achieved long cyclic lifetimes and maintained high capacity (>600 mAh/g at 500 cycles). Chapter five details the optimization of conditions necessary to generate large scale (>100 g) inversely vulcanized sulfur copolymers and their application towards Li-S batteries. As previously stated a significant challenge in the Li-S battery field is the production of a Li-S active material with improved performance that is low cost, synthesized in a facile manner, and possesses high sulfur content. To date poly(S-r-DIB) copolymers prepared via the inverse vulcanization methodology afford some of the longest cycle lifetimes and highest capacity retention for polymeric active materials. However, initial inverse vulcanization reactions investigated for preparing active materials were performed on 10 gram scales. The goal of the work presented in chapter 5 was to prepare materials on a scale applicable to fabrication of several prismatic Li-S cells, each of which requires several grams of active material. However, scaling up of the reaction to a kilogram and utilizing the traditional inverse vulcanization conditions (i.e., 185 °C) results in catastrophic degradation as a consequence of the Trommsdorf effect. To address this challenge required decreasing the radical concentration within the bulk copolymerization, which necessitated performing the kilogram scale inverse vulcanization reactions at lower temperatures (i.e., 130 °C) over a longer reaction period. Decreasing the temperature generates materials that are nearly identical in thermomechanical properties to smaller scale samples and the battery performance is likewise comparable (>600 mAh/g at 500 cycles). The key advantage of performing the inverse vulcanization reaction at lower temperatures is that additional monomers, with lower boiling points or degradation issues, can be utilized and the increased gelation time, enables facile incorporation of additives (e.g., carbon black or nanoparticles) into the reaction. Chapter six focuses on the development of poly(S-r-DIB) copolymers as novel mid-infrared (mid-IR) transmitting materials and the analysis of the optical properties as a function of copolymer composition. A challenge in the optical science community is the limited number of materials applicable to the development of innovative optical components capable of functioning in the mid and far-IR regions. Semi-conductor and chalcogenide glasses have been widely applied as device components in infrared optics due to their high refractive indices (n ~2.0–4.0) and high transparency in the infrared region (1–10 μm). However, such materials are also expensive, difficult to fabricate, and toxic in comparison to organic polymers. On the other hand organic polymers are easily processed, low cost, and generated from easily accessible raw materials. Unfortunately, polymeric materials generally have low refractive indices (n<1.65) and are prepared from monomers with functional groups that are highly absorbing at mid-IR and longer wavelengths. Chapter 6 details the realization through the inverse vulcanization methodology of the first example of a material that is high refractive index and low mid-IR absorption, but also low cost and easily processable. Critical to achieving a polymeric material which was appropriate for mid-IR applications was the high sulfur content and the absence of functional groups, both of which are afforded by the facile copolymerization process. By simply controlling copolymer composition the optical properties of the material were tailorable; allowing adjustment of the refractive index from ~1.75 (50-wt% DIB) to ~1.875 (20-wt% DIB). Finally, through facile techniques, high quality copolymers lenses were prepared and we demonstrated the high optical transparency over several regions of the optical spectrum, from the visible (400–700 nm) all the way to the mid-IR (3–5μm). Poly(S-r-DIB) copolymers demonstrated high transparency to mid-IR light, but still maintain the processing capabilities of an organic polymer, the first example of such a material to possess both qualities. Ultimately the inverse vulcanization methodology offers a novel route to low cost, high refractive index, IR transparent materials, opening up unique opportunities for polymeric optical components within the optical sciences field. The seventh chapter discusses utilization of the inverse vulcanization methodology as a means to prepare and control the dynamic behavior of sulfur copolymers for potential applications towards self-healing materials. The incorporation of dynamic covalent bonds into conventional polymer architectures, either directly within the backbone or as side-chain groups, offers the stability of covalent bonds but with the ability of stimuli-responsive behavior to afford a change in chemical makeup or morphology. Traditionally the installation of such functionality requires the use of disparate, orthogonally polymerizable functional groups (i.e., vinyl) and discrete design of the comonomers utilized to generate a responsive copolymer. Therefore, a challenge in developing novel dynamic copolymers is the ability to install stimuli-responsive functionality directly as a result of the copolymerization without the need for rigorous synthetic monomer design and complex copolymerization techniques. In chapter 7 we discuss the analysis of poly(S-r-DIB) copolymers with rheological techniques to assess the composition dependent dynamic behavior. Aided by the bulk nature of copolymerization, the feed ratio of S₈ and DIB directly dictates copolymer microstructure; thus the sulfur rank between the organic groups (i.e., DIB) was tailorable from a single sulfur (thioether) to multiple sulfurs (pentasulfide). Control over sulfur content and number of S–S enables control over the dynamic behavior, as monitored via in-situ rheological techniques. The highest sulfur-content copolymers (80-wt% S₈, 20-wt% DIB) showed the fastest response when under shear stress due to the large number of S–S bonds. On the other hand when no dynamic bonds were present in the copolymer (i.e.; 35-wt% S₈, 65-wt% DIB) there is no dynamic behavior and full recovery of the pristine mechanical properties was not observed. The facile synthesis and simple control over copolymer microstructure affords the inverse vulcanization methodology an advantage over other dynamic materials, and provides potential secondary qualities (i.e., high refractive index) built directly into the structure.

Rechargeable lithium sulfur (Li-S) batteries are potentially safe, environmentally friendly and economical alternative energy storage systems that can potentially be combined with renewable sources including wind solar and wave energy. Sulfur has a high theoretical specific capacity of ~1680 mAh/g, attainable through the reversible redox reaction denoted as S8+16Li ↔8Li¬2S, which yields an average cell voltage of ~2.2 V. However, two detrimental factors prevent the achievement of the full potential of the Li-S batteries. First, the poor electrical/ionic conductivity of elemental sulfur and Li2S severely hampers the utilization of active material. Second, dissolution of intermediate long-chain polysulfides (Li2Sn, 2<n<7) into the electrolyte and their shuttle between cathode and anode lead to fast capacity degradation and low Coulombic efficiency. As a result of this shuttle process, insoluble and insulating Li2S/Li2S2 precipitate on the surface of electrodes causing loss of active material and rendering the electrode surface electrochemically inactive. Extensive research efforts have been devoted to overcome the aforementioned problems, such as combination of sulfur with conductive polymers, and encapsulation or coating of elemental sulfur in different nanostructured carbonaceous materials. Noteworthy, sulfur-polyacrylonitrile (SPAN) composites, wherein sulfur is chemically bond to the polymer backbone and PAN acts as a conducting matrix, have shown some success in suppressing the shuttle effect. However, due to the limited electrical conductivity of polyacrylonitrile, the capacity retention and rate performance of the SPAN systems are still very modest, which shows only 67 % retention of the initial capacity after 50 cycles for the binary system. Recently, graphene has been intensively investigated for enhancing the rate and cycling performance of lithium sulfur batteries. Graphene, which has a two-dimensional, one-atom-thick nanosheet structure, offers extraordinary electronic, thermal and mechanical properties. Herein, a sulfur-polyacrylonitrile/reduced graphene oxide (SPAN/RGO) composite with unique electrochemical properties was prepared. PAN is deposited on the surface of RGO sheets followed by ball milling with sulfur and heat treatment. Infrared spectroscopy and microscopy studies indicate that the composite consists of RGO decorated with SPAN particles of 100 nm average size. The PAN/RGO composite shows good overall electrochemical performance when used in Li/S batteries. It exhibits ~85% retention of the initial reversible capacity of 1467 mAh/g over 100 cycles at a constant current rate of 0.1 C and retains 1100 mAh/g after 200 cycles. In addition, the composite displays excellent Coulombic efficiency and rate capability, delivering up to 828 mAh/g reversible capacity at 2 C. The improved performance stems from composition and structure of the composite, wherein RGO renders a robust electron transport framework and PAN acts as sulfur/polysulfide absorber.

Rechargeable lithium sulfur (Li-S) batteries are potentially safe, environmentally friendly and economical alternative energy storage systems that can potentially be combined with renewable sources including wind solar and wave energy. Sulfur has a high theoretical specific capacity of ~1680 mAh/g, attainable through the reversible redox reaction denoted as S8+16Li ↔8Li¬2S, which yields an average cell voltage of ~2.2 V. However, two detrimental factors prevent the achievement of the full potential of the Li-S batteries. First, the poor electrical/ionic conductivity of elemental sulfur and Li2S severely hampers the utilization of active material. Second, dissolution of intermediate long-chain polysulfides (Li2Sn, 2<n<7) into the electrolyte and their shuttle between cathode and anode lead to fast capacity degradation and low Coulombic efficiency. As a result of this shuttle process, insoluble and insulating Li2S/Li2S2 precipitate on the surface of electrodes causing loss of active material and rendering the electrode surface electrochemically inactive. Extensive research efforts have been devoted to overcome the aforementioned problems, such as combination of sulfur with conductive polymers, and encapsulation or coating of elemental sulfur in different nanostructured carbonaceous materials. Noteworthy, sulfur-polyacrylonitrile (SPAN) composites, wherein sulfur is chemically bond to the polymer backbone and PAN acts as a conducting matrix, have shown some success in suppressing the shuttle effect. However, due to the limited electrical conductivity of polyacrylonitrile, the capacity retention and rate performance of the SPAN systems are still very modest, which shows only 67 % retention of the initial capacity after 50 cycles for the binary system. Recently, graphene has been intensively investigated for enhancing the rate and cycling performance of lithium sulfur batteries. Graphene, which has a two-dimensional, one-atom-thick nanosheet structure, offers extraordinary electronic, thermal and mechanical properties. Herein, a sulfur-polyacrylonitrile/reduced graphene oxide (SPAN/RGO) composite with unique electrochemical properties was prepared. PAN is deposited on the surface of RGO sheets followed by ball milling with sulfur and heat treatment. Infrared spectroscopy and microscopy studies indicate that the composite consists of RGO decorated with SPAN particles of 100 nm average size. The PAN/RGO composite shows good overall electrochemical performance when used in Li/S batteries. It exhibits ~85% retention of the initial reversible capacity of 1467 mAh/g over 100 cycles at a constant current rate of 0.1 C and retains 1100 mAh/g after 200 cycles. In addition, the composite displays excellent Coulombic efficiency and rate capability, delivering up to 828 mAh/g reversible capacity at 2 C. The improved performance stems from composition and structure of the composite, wherein RGO renders a robust electron transport framework and PAN acts as sulfur/polysulfide absorber.

Les batteries lithium-air et Li-S sont des techniques prometteuses pour un stockage efficace d’énergie électrochimique. Les principaux défis sont de développer un électrolyte solide à haute conductivité ionique et des cathodes efficaces. Dans ce travail, des aérogels de carbone conducteurs avec une double porosité ont été synthétisés en utilisant la méthode de sol-gel. Ils ont été utilisés comme cathode dans des batteries lithium-air. Ces cathodes peuvent fournir deux types de canaux pour le stockage de produits de décharge, facilitant la diffusion gaz-liquide et réduisant ainsi le risque de colmatage. Presque 100 cycles été obtenus avec une capacité de 0,4 mAh et une densité de courant de 0,1 mA/cm². Pour le développement d'électrolyte solide stable et conducteur, les sulfures, en particulier Li4SnS4 et son dérivé Li10SnP2S12 ont été particulièrement étudiés. Ces composés ont été synthétisés en utilisant une technique en deux étapes comprenant la mécanosynthèse et un traitement thermique à température relativement basse qui a été optimisé afin d'améliorer la conductivité ionique. La meilleure conductivité obtenue est de 8,27×10-4 S / cm à 25°C et ces électrolytes présentent une grande stabilité électrochimique sur une large gamme de voltage de 0,5 à 7V. Les couches minces ont également été déposées en utilisant la technique de pulvérisation cathodique, avec en général une conductivité ionique améliorée. La performance des batteries Li-S assemblées avec ces électrolytes massifs doit être améliorée, en particulier en améliorant la conductivité ionique de l'électrolyte. / Lithium-air and Li-S batteries are promising techniques for high power density storage. The main challenges are to develop solid electrolyte with high ionic conductivity and highly efficient catalyzed cathode. In this work, highly conductive carbon aerogels with dual-pore structure have been synthesized by using sol-gel method, and have been used as air cathode in Lithium-air batteries. This dual- pore structure can provide two types of channels for storing discharge products and for gas-liquid diffusion, thus reducing the risk of clogging. Nearly 100 cycles with a capacity of 0.4mAh at a current density of 0.1 mA cm-2 have been obtained. For developing stable and highly conductive solid electrolyte, sulfides, especially Li4SnS4 and its phosphorous derivative Li10SnP2S12 have been particularly investigated. These compounds have been synthesized by using a two-step technique including ball milling and a relatively low temperature heat treatment. The heat treatment has been carefully optimized in order to enhance the ionic conductivity. The best-obtained conductivity is 8.27×10-4 S/cm at 25°C and the electrolytes show high electrochemical stability over a wide working range of 0.5 – 7V. Thin films have also been deposited by using the sputtering technique, with generally improved ionic conductivity. The performance of the Li-S batteries assembled with these bulk electrolytes is still to be improved, particularly by improving the ionic conductivity of the electrolyte.

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.

Graphene-directed two-dimensional (2D) nitrogen-doped porous carbon frameworks (GPF) as the hosts for sulfur were constructed via the ionothermal polymerization of 1,4-dicyanobenzene directed by the polyacrylonitrile functionalized graphene nanosheets. As cathodes for lithium–sulfur (Li–S) batteries, the prepared GPF/sulfur nanocomposites exhibited a high capacity up to 962 mA h g⁻¹ after 120 cycles at 2 A g⁻¹. A high reversible capacity of 591 mA h g⁻¹ was still retained even at an extremely large current density of 20 A g⁻¹. Such impressive electrochemical performance of GPF should benefit from the 2D hierarchical porous architecture with an extremely high specific surface area, which could facilitate the efficient entrapment of sulfur and polysulfides and afford rapid charge transfer, fast electronic conduction as well as intimate contact between active materials and the electrolyte during cycling.