Interaction of FeS₂ and sulfur in Li-S battery system

Many transition metal sulfides are electronically conductive, electrochemically active and reversible in reactions with lithium. However, the application of transition metal sulfides as sulfur cathode additives in lithium-sulfur (Li-S) batteries has not been fully explored. In this study, Pyrite (FeS₂) is studied as a capacity contributing conductive additive in sulfur cathode for Li-S batteries. Electrochemically discharging the S-FeS₂ composite electrodes to 1.0 V activates the FeS₂ component, contributing to the improved Li-S cell discharge energy density. However, direct activation of the FeS₂component in a fresh S-FeS₂ cell results in a significant shuttling effect in the subsequent charging process, preventing further cell cycling. The slight FeS₂ solubility in electrolyte and its activation alone in S-FeS₂ cells are not the root causes of the severe shuttling effect. The observed severe shuttling effect is strongly correlated to the 1^st charging of the activated S-FeS₂ electrode that promotes iron dissolution in electrolyte and the deposition of electronically conductive FeS on the anode SEI. Pre-cycling of the S-FeS₂ cell prior to the FeS₂activation or the use of LiNO₃ electrolyte additive help to prevent the severe shuttling effect and allow the cell to cycle between 2.6 V to 1.0 V with an extra capacity contribution from the FeS₂ components. However, a more effective method of anode pre-passivation is still needed to fully protect the lithium surface from FeS deposition and allow the S-FeS₂ electrode to maintain high energy density over extended cycles. A mechanism explaining the observed phenomena based on the experimental data is proposed and discussed.