Temperature-Dependent Reaction Pathways in FeS2: Reversibility and the Electrochemical Formation of Fe3S4

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 degrees 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 (similar to 60 degrees 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.

Automated summary

This study investigates the rechargeable Li-FeS2 system using various characterization techniques and an ionic liquid electrolyte. The results reveal new reaction pathways and products during the initial lithiation and subsequent cycling of FeS2. It is found that Li2S and hexagonal FeS are formed as intermediate phases during the initial lithiation, and greigite (Fe3S4) is produced as a charge product during the rechargeable reaction, which differs from previous reports. The formation of Fe3S4 is dependent on temperature and the availability of sulfur. Upon further cycling, Fe3S4 transforms to a lower sulfur content iron sulfide phase. The findings highlight the importance of retaining sulfur in the active material for capacity retention.