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Access Type

WSU Access

Date of Award

January 2020

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics and Astronomy

First Advisor

Ratna . Naik

Abstract

Solid electrolytes including ceramics and polymers are considered to be the ultimate substitute for organic liquid electrolytes currently used in commercialized lithium ion batteries to address the safety concerns due to Li dendrite growth and internal short circuiting. However, low ionic conductivity due to high grain boundary resistance in ceramics and semi-crystalline nature of polymers has held back the solid electrolytes from being used in Li-ion batteries. Polyethylene oxide (PEO), complexed with a Li-salt, is a well-studied polymer electrolyte showing ionic conductivity properties at room temperature. However, the coexistence of amorphous and crystalline regions at room temperature (< Tm, the melting temperature) has

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limited its ionic conductivity to ~10 - 10 S cm . Li7La3Zr2O12 (LLZO), a fast Li-ion conductor in

its cubic crystalline form, has shown high ionic conductivity up to 10-4 S cm-1 at room 90

temperature. In the present thesis, we have investigated hybrid composite solid polymer electrolyte (CSPE) films of PEO-LLZO-LiClO4 with different [EO]:[Li] ratio and LLZO concentrations.

The aim of this thesis was to investigate the dependence of conductivity of CSPE films on Li-ion and LLZO content, and optimize their near room temperature conductivity while maintaining the robust mechanical properties of CSPE films. We have synthesized free-standing CSPEs films of PEO-LLZO-LiClO4 with a thickness of ~100-130 m, comprised of sub-micron sized aluminum substituted cubic LLZO particles (purchased from MSE) dispersed in PEO (Mw=106 g/mol)-LiClO4 matrix using a solution cast method. The structural, morphological and electrochemical properties were studied using x-ray diffraction, scanning electron microscopy, electrical impedance spectroscopy (EIS) and linear sweep voltammetry. The AC dielectric permittivity and ionic conductivity of the films, determined using EIS measurements, were analyzed using an expression that generalizes the power-law dependence of complex dielectric permittivity and conductivity including electrode polarization. The temperature dependence of ionic conductivity were investigated, and the data were analyzed using Vogel-Tammann- Fulcher- (VTF) empirical equation to estimate the changes in the activation energy of PEO- LiClO4 upon the addition of LLZO particles.

The PEO-50 wt% LLZO-LiClO4 films with [EO]:[Li] = 15:1 and 12:1 have shown high ionic conductivity of ~ 10-4 S cm-1 at room temperature, that are mechanically robust, stable against Li metal up to 5 V, and thermally stable up to 120 C.

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