New Insights on Reaction Pathways for FeS₂ Cathodes

Rechargeable FeS₂ has recently re-emerged as a 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 over the intermediate and charge products formed still remain. Improved understanding of these reactions is key for combatting the pervasive capacity fade problems experienced by Li/FeS₂ secondary batteries.<br/><br/>In our work, we combine a suite of 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. In particular, 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, in contrast to the often cited Li₂FeS₂. 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. To emphasize the similarity between hexagonal FeS and FeS₂, we demonstrate that a composite of Li₂S: hexagonal FeS (1:1 mol) presents an electrochemically equivalent system to Li/FeS₂.<br/><br/>Second, upon charging to 3.0 V (vs Li/Li⁺) at various temperatures, we report, for the first time, 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 (appearing at ~40 ^oC) and the availability of sulfur to drive FeS to Fe₃S₄. Interestingly, this reaction has been reported in the geology literature as a pathway to pyrite formation. While Fe₃S₄ forms reversibly for the first few cycles at 60 ^oC, its long-term formation is inhibited by the availability of sulfur due to the solubility of sulfur and polysulfides in the electrolyte as indicated by UV-VIS. After 5 cycles, this loss of sulfur inhibits Fe₃S₄ formation and the room temperature charge product, nanocrystalline tetragonal FeS, is formed instead, accounting for the capacity degradation observed within the first few cycles. Upon further cycling, we find that the Li/FeS₂ capacity stabilizes to 500 mAh g^-1after 50 cycles. The insights presented here underscore the importance of understanding the reversibility of each of these charge products to improve capacity retention. These features are also likely to impact the emerging interest in solid-state FeS₂ systems.