Impact of Different Conductive Polymers on the Performance of the Sulfur Positive Electrode in Li-S Batteries

Abstract

Sulfur particles were coated with conductive polymer layers by dielectric barrier discharge (DBD) plasma technology under atmospheric conditions (ambient pressure and low temperature). The DBD plasma process is a dry and sustainable (solvent-free, limited energy consumption technique compatible with upscaling. Different conductive coated sulfur materials were produced and labeled as "PEDOT-S" [poly(3,4-ethylene dioxythiophene-sulfur)], "PANI-S" (polyaniline-sulfur), "PTs-S" (polythiophene-sulfur), and "PPy-S" (polypyrrole-sulfur). The corresponding electrical conductivities were measured at 10(-5), 10(-6), 10(-7), and 10(-8) S/cm, respectively. The role of the conductive coating is to enhance the electrochemical performance of Li-S cells by improving the electronic conductivity of the sulfur particles and preventing the well-known polysulfide shuttle phenomenon. A vast range of characterization methods including conductivity analysis, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and carbon-13 NMR (nuclear magnetic resonance spectroscopy) were used to assess the chemical characteristics using the different conductive polymer-coated sulfur materials. In the coated sulfur samples, fragmentation of aromatic rings was observed, 88% for the PTs-S and 42% for the PEDOT-S, while it is very limited for the PANI-S. Such a phenomenon has never been reported in the literature. The uncoated and coated sulfur powders were used (as active material) in positive electrodes of Li-S cells with a relatively high sulfur loading of similar to 4.5 mg/cm(2) using LiPAA (lithium polyacrylate) as an (aqueous) binder. Long-term galvanostatic cycling at C/10 and multi-C-rate tests showed the capacity fade and rate capability losses to be highly mitigated for cells containing conductive polymer-coated sulfur in comparison to cells using the uncoated sulfur. Kinetic investigations by cyclic voltammetry and electrochemical impedance spectroscopy analyses undoubtedly confirm improved electron and Li-ion transport within the electrodes containing conductive polymer-coated sulfur. The electrochemical performance can be ranked as PEDOT-S > PANI-S > PTs-S > PPy-S > raw sulfur.