Investigation Of Ion Transport Mechanism In Succinonitrile Based Plastic Crystalline Electrolytes
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The present thesis deals in detail the influence of solvent dynamics and solvation on ion transport in succinonitrile based plastic crystalline electrolytes. The main objective of correlating plastic solvent characteristics with ion transport was achieved by probing the electrolyte using characterization techniques at various length and time scales. Although majority of the results presented in this thesis focus on a prototype succinonitrile electrolyte (succinonitrile-lithium perchlorate, SN-LiClO4), the conclusions drawn from the results on SN-LiClO4 are quite general and can be extended to various types of salts as well as plastic crystalline matrices. Chapters 2-5 demonstrate in a systematic and detailed manner the beneficial influence of solvent dynamics on ion transport in the solid state. The thesis comprises of six chapters. A brief discussion of the contents and highlights of the individual chapters are described below: Chapter 1 briefly reviews the importance of various types of electrolytes for electrochemical applications. The chapter starts with a discussion on different types of liquid and solid crystalline electrolytes and their drawbacks in electrochemical devices such as lithium-ion batteries. Following the discussion on the two extremes of electrolytes viz. liquid and solid electrolytes, various soft matter electrolytes including polymer and plastic crystalline materials are discussed. Aims of the thesis are specified in chapter 1. Chapter 2 discusses plastic crystalline electrolytes as prospective electrolytes for electrochemical applications. In this chapter, we present a detailed study of correlation of ion transport with solvent structure and dynamics in lithium perchlorate (LiClO4)-succinonitrile (SN), a prototype succinonitrile based plastic crystalline electrolyte. Significant influences of the salt on the crystallographic structure, trans-gauche isomerism and solvation properties of succinonitrile (SN) are observed. Ionic conductivity (ac-impedance spectroscopy) and single crystal X-ray studies (in-situ cryo crystallography) reveal the influence of configurational isomerism and ion solvation on ion transport in LiClO4-SN. We quantify the ion association using theoretical analysis of Fuoss-Onsager formalism for various LiX-SN (typically X = ClO4-, CF3SO3-, TFSI-) electrolytes. Thermal (differential scanning caloriemetry) and spectroscopic (Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR)) studies have also been discussed in the chapter to support our proposition. Chapter 3 describes our investigation on issues other than salt that are likely to affect ion transport in Li-salt-SN based plastic crystalline electrolytes such as water in sample and sample thermal history. We investigate here in a detailed manner the influence of water and thermal history on SN configurational isomerism and solvation in LiClO4-SN. LiClO4 in SN electrolyte samples were prepared in various ways for the fulfillment of the objectives of the study of the present chapter. Correlation of water and thermal history on ion transport were studied via ac-impedance spectroscopy and room temperature Fourier transform infrared (FTIR) spectroscopy. The ionic conductivity and infra-red findings were supplemented via differential scanning calorimetry (DSC). Chapter 4 presents dielectric relaxation spectroscopy (DRS) to study the various relaxation processes of the plastic crystalline solvent and ionic species responsible for ion-transport in succinonitrile-based electrolytes. For the DRS study, we select the same system i.e. SN-LiClO4 for which the role of solvent dynamics and ion-association on ion transport was discussed in detail in chapter 2. We supplement the ionic conductivity and various spectroscopic investigations highlighted in chapter 2 via study of the frequency dependence of dielectric function. The permittivity data are further analyzed using Havriliak-Negami (HN) and Kohlrausch-Williams-Watta (KWW) functions for identification of various processes and also for detailed insight on the ion transport mechanism. Chapter 5 comprises of the temperature dependence the bulk acoustic phonons in SN and SN-LiX (X = ClO4-, CF3SO3-, TFSI-, Cl-) electrolytes from (200-300) K. Room temperature Brillouin spectra of SN based plastic crystalline electrolytes with different cationic salts (MClO4-, M = Li+, Na+, Rb+) were also measured. The influences of salt concentration and temperature on solvent dynamics and ion-association effect have been investigated in detail for the SN-LiClO4 electrolyte. The Brillouin data were further analyzed using Lorentzian and Fano resonance function for identification of behavior of various Brillouin modes. An attempt was made to understand ion transport mechanism in SN-LiX plastic crystalline electrolytes based on the concept of molecular liquids as opposed to conventional solid state defect chemistry. The chapter also discusses preliminary results on the relaxational dynamics of SN and SN-LiClO4 in the plastic phase examined using quasi elastic neutron scattering (QENS) facility at ILL-Grenoble IN16 beamline. Chapter 6 provides a brief summary of the work presented in the thesis and discusses how knowledge from the present work (chapters 2-5) can be utilized to generate new electrolytes. The system proposed is a liquid electrolyte based on bis-nitrile (G0-CN) which does not possess majority of the detrimental issues associate with conventional liquids and various improvisations of polymer electrolytes. We also show that the various dendrimer generations obtained from the monomer bis-nitrile (G0-CN) can also be utilized as an alternative solvent for generation of liquid electrolytes for electrochemical devices such as (primary/secondary) batteries. In a way, we discuss a novel liquid electrolyte system whose physical (viscosity, dielectric constant) and solvation properties can be tuned easily to fulfill task of specific objectives. The preliminary ionic conductivity, viscosity and electrochemical studies of the Gn-CN-Li-salt (n=0-2) liquid electrolytes show considerable promise. Though the prospective dendrimer solvent is a liquid, we envisage that in future compounds with similar chemical properties can also be synthesized in the soft matter state.
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