Bruce Dunn

University of California, Los Angeles,

New Insights on Reaction Pathways for FeS2 Cathodes

Rechargeable FeS2 has recently re-emerged as cathode candidate for high energy density batteries. With a theoretical capacity of 894 mAh g-1 combined with being the most abundant metal sulfide on earth, FeS2 holds great promise to provide a means toward higher energy density batteries in a sustainable manner. However, the complex reaction pathways of FeS2 after the initial lithiation are not well understood and discrepancies regarding the intermediate and charge products formed still remain.
In the research reported here, we combine a suite of ex-situ techniques (XRD, XANES, XPS) to investigate the lithiation and delithiation reaction pathways under a wide temperature range (RT to 100 ºC), enabled by the use of an ionic liquid electrolyte. We report two features which have been largely overlooked in prior studies. First, from the initial lithiation reaction, we identify hexagonal FeS and Li2S as the intermediates formed in the two-step reaction. The detection of hexagonal FeS as an intermediate phase suggests that the electrochemical pathways for FeS and FeS2 are more similar than previously thought, with both iron sulfides exhibiting an irreversible lithiation. The second feature involves charging reactions. Upon charging to 3.0 V (vs Li/Li+) at various temperatures, we report the electrochemical formation of Greigite Fe3S4 as a charge product. The formation of this sulfur-rich iron sulfide compound is found to be highly dependent on both temperature (~40 ºC) and availability of sulfur to drive FeS to Fe3S4. While Fe3S4 forms reversibly for the first few cycles, its long-term formation is inhibited by the availability of sulfur due to the solubility of sulfur and polysulfides (PS) in the electrolyte. The connection between sulfur loss, capacity fade, and charge product composition highlights the critical need to retain sulfur in the active material upon cycling.

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