The Influence of Composition and Microstructure of the Cathode on the Impedance Response in Lithium-Sulfur Batteries

Abstract

Lithium-sulfur batteries represent one of the most promising candidates for next-generation batteries due to their high specific energy density. Nonetheless, this battery chemistry is also facing obstacles such as the polysulfide-shuttle effect and the insulative nature of sulfur. In attempt to counteract these undesirable effects, several cathode-focused approaches were examined in the scientific community, including the infiltration and confinement of the active material in a conductive carbon host material or utilizing covalently bound sulfur within a polymeric backbone. However, these concepts also alter the known redox-mechanisms of lithium-sulfur cell chemistry as polysulfide dissolution is suppressed. In this work, various cathode systems including sulfur-carbon composite and sulfurized polyacrylonitrile (SPAN) are studied comprehensively by means of electrochemical impedance spectroscopy. Symmetrical two- or three-electrode cell arrangement is employed in order to analyze cathodic mechanisms without anode-induced or asymmetry artifacts. Transmission line models characteristic of porous electrodes have been implemented to understand the contribution of the different processes to the impedance of these cathodes. The influence of parameters such as microstructure of these cathodes, in particular porosity, on the impedance response is discussed in detail. Moreover, the investigation of impedance evolution upon various states of charge allows the identification of the highly diverse underlying processes related to the cathode, specifically a range of charge transfer mechanisms including quasi-solid-state and polysulfide-based redox reactions.