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 ∼4.5 mg/cm2 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.