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.