Interaction of FeS2 and Sulfur in Li-S Battery System

Abstract

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 FeS2 component 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 1st 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 FeS2 activation 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.