(Digital Presentation) Lithiation/De-Lithiation of a Unique C-S Species in Amorphous FeS₄/C Cathodes for Li Batteries

When paired with Li metal anodes, Fe-based conversion cathodes like iron sulfide present a promising material for creating energy dense rechargeable batteries for energy storage. While iron sulfide cathodes (e.g. FeS2) typically suffer from deleterious polysulfide formation, recent work with a class of amorphous iron sulfide-carbon composites (e.g. FeS4/C) has shown minimal evidence of polysulfide formation during lithiation and de-lithiation, and thus it may represent a pathway to stable, polysulfide-free iron sulfide batteries. Prior reports indicate that the active component of the composite is the FeS4 and that the amorphous carbon does not exhibit any electrochemical response. While studying this material, however, we have found evidence to suggest that the cycling of FeS4 alone does not fully explain the electrochemical and chemical behavior observed for the FeS4/C cathode, and that the carbon plays a significant role in the overall electrochemistry of the composite. In this presentation, we will present a study of this amorphous FeS4/C composite as a cathode for use in Li batteries and provide evidence of the electrochemical lithiation and de-lithiation of a previously unstudied and unreported C-S compound found in the composite material. Our results indicate that this C-S species is amorphous and is (electro)chemically similar to sulfurized poly(acrylonitrile) (SPAN) and carbyne polysulfide (C-PS). By coupling ex situ Raman and X-ray photoelectron spectroscopy (XPS) to galvanostatic battery cycling, we show how chemical changes in the FeS4/C cathode correspond to electrochemical features associated with the lithiation and de-lithiation of both iron sulfide and the C-S species. Our results indicate that the C-S species is electrochemically active at all stages of cycling, while the FeS4 loses activity within approximately 15-20 cycles. This differs significantly from earlier reports and suggests that the lack of polysulfide formation when testing the FeS4/C composite may in fact be due to the cycling of this C-S species rather than iron sulfide. These results serve as an initial foray into the electrochemistry of this new material and should provide a basis for additional study of this new and promising C-S based cathode material for Li batteries. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.