Sulfurized Polyacrylonitrile (SPAN) Cathodes in Lithium-Sulfur Batteries: Investigating the Influence of Cathode Microstructure and Electrolyte System on Electrochemical Performance

Abstract

With their unmatched specific energy density, lithium-sulfur batteries are an outstanding example for the potential of next-generation energy storage. However, major challenges such as the well-known polysulfide shuttle effect and the inherent insulating nature of sulfur stand in the way of widespread application. To combat these problems, numerous approaches have been explored, largely focusing on the cathode. [1] One of the most promising approaches is the application of covalently bound sulfur within an organic backbone, which leads to better cyclability by eliminating or suppressing sulfur dissolution. [2] In this work, a promising organosulfur namely sulfurized polyacrylonitrile (SPAN) serving as the cathode in lithium-sulfur batteries is deeply studied. In particular, the effect of the cathode microstructure including porosity caused from varying degrees of calendering on electrochemical performance is investigated. To elucidate the effect of such microstructure variation the electrochemical impedance spectroscopy in a symmetrical two- or three-electrode system is employed. The notable strength of this particular setup lies in the prevention of undesired anode and asymmetry artifacts in the recorded spectra while enabling in-situ measurements at various states of charge. [3] Suitable impedance modelling and fitting are deployed to differentiate between various processes on the cathode side. The combination of morphological examination and electrochemical analysis of the heavily calendered SPAN cathodes indicates damage to the electrically conductive network while the active material does not appear to be affected during the calendering process. Moreover, increased overpotentials are detected in the calendered cathodes, leading to incomplete charging and fast capacity decay. Additionally, emphasis is placed on the effect of the electrolyte system, in particular ether and carbonate electrolyte, recognizing their capability to influence redox pathways and capacity retention. It is shown that SPAN cathodes are exhibiting higher compatibility with the carbonate-based electrolyte. The differences in electrochemical stability and impedance response for these two electrolyte systems are being discussed in detail.