The present study has used a variety of characterization techniques to determine the products and reaction pathways involved in the rechargeable Li-FeS2 system. We revisit both the initial lithiation and subsequent cycling of FeS2 employing an ionic liquid electrolyte to investigate the intermediate and final charge products formed under varying thermal conditions (room temperature to 100 °C). The detection of Li2S and hexagonal FeS as the intermediate phases in the initial lithiation and the electrochemical formation of greigite, Fe3S4, as a charge product in the rechargeable reaction differ significantly from previous reports. The conditions for Fe3S4 formation are shown to be dependent on both the temperature (∼60 °C) and the availability of sulfur to drive a FeS to Fe3S4 transformation. Upon further cycling, Fe3S4 transforms to a lower sulfur content iron sulfide phase, a process which coincides with the loss of sulfur based on the new reaction pathways established in this work. The connection between sulfur loss, capacity fade, and charge product composition highlights the critical need to retain sulfur in the active material upon cycling.
Conversion cathodes represent a viable route to improve rechargeable Li+ battery energy densities, but their poor electrochemical stability and power density have impeded their practical implementation. Here, we explore the impact cell fabrication, electrolyte interaction, and current density have on the electrochemical performance of FeS2/Li cells by deconvoluting the contributions of the various conversion and intercalation reactions to the overall capacity. By varying the slurry composition and applied pressure, we determine that the capacity loss is primarily due to the large volume changes during (de)lithiation, leading to a degradation of the conductive matrix. Through the application of an external pressure, the loss is minimized by maintaining the conductive matrix. Further, we determine that polysulfide loss can be minimized by increasing the current density (>C/10), thus reducing the sulfur formation period. Analysis of the kinetics determines that the conversion reactions are rate-limiting, specifically the formation of metallic iron at rates above C/8. While focused on FeS2, our findings on the influence of pressure, electrolyte interaction, and kinetics are broadly applicable to other conversion cathode systems.