Effect of Noncovalent Dispersion of Poly(Ethylene Oxide) in Columnar Polyether-Based Discotic Liquid Crystal on the Ionic Conductivity and Dynamics of Lithium Ions

Jih-Dar Hwang,Chi Wi Ong,Shang-Wu Ding,Po-Ying Chen

doi:10.3390/cryst9120627

Abstract

The ionic conductivity of an electrolyte is represented by a product of carrier density, charge (electric), and ionic mobility. The overall goal of this study was to provide an insight into the influence of lithium ion conductivity and dynamic when a continuous discotic liquid crystal (DLC) matrix of hexaazatrinapthylene-polyether, HATN-TEG-1, is doped with a small amount of polyethylene oxide (PEO, 5% of MW 8000). The favorable non-covalent interactions between PEO and the DLC triethylene glycol side-chains is supported by the maintenance of the mesophase. The lithium ionic conductivity of HATN-TEG-1 was found to be 1.1 × 10−6 S cm−1, which is better than the corresponding HATN-TEG-1-5%PEO-8000 with a value of 6.06 × 10−7 S cm−1. These results are further supported by the dynamics of the lithium ions in HATN-TEG-1 and HATN-TEG-1-5%PEO-8000 as characterized by 7Li, and 1H NMR spin-lattice relaxation time and self-diffusion coefficient measurements. Though the additional PEO was found to increase the ion carriers, the significant lowering of the ionic conductivity may be attributed to the more pronounced decrease of the mobility of the ionic part when the HATN-TEG-1 matrix is dispersed with PEO. This finding indicates that the doping of 5% PEO onto the matrix of HATN-TEG-1 DLC has an adverse effect on both its diffusion rate and ion conductivity.

Highlights

Solid electrolytes with high ionic conductivity are crucial for Li-ion battery performance and have received considerable attention due to the safety considerations arising from the demand for the replacement of liquid electrolytes that are prone to the formation of lithium dendrites that can cause short circuiting and overheating [1,2]
5% polyethylene oxide (PEO) onto the matrix of HATN-TEG-1 discotic liquid crystal (DLC) has an adverse effect on both its diffusion rate and ion conductivity
A lithium-polyethylene oxide (PEO) system with an ease of fabrication and low cost has attracted the most attention to date, but it suffers from its low ion conductivity

Summary

Introduction

Solid electrolytes with high ionic conductivity are crucial for Li-ion battery performance and have received considerable attention due to the safety considerations arising from the demand for the replacement of liquid electrolytes that are prone to the formation of lithium dendrites that can cause short circuiting and overheating [1,2]. Though the use of solid electrolytes, especially polymers, may circumvent the restriction of liquid electrolytes, they exhibit low conductivity through the loss of segmental chain motions required for ion-transport upon cooling below the glass transition temperature. For improving Li-ion conductivity, blended polyethylene oxide-based polymer electrolytes with the addition of liquid electrolytes, plasticizers or inorganic fillers have been designed out to lower the Tg and increase the void volume [4].

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