Unraveling the Reaction Mechanism of FeS as a Li-Ion Battery Cathode.

Iron pyrite (FeS) is a promising lithium-ion battery cathode material because of its low cost and ultrahigh energy density (1671 Wh kg). However, its reaction mechanisms are still controversial. In this work, we find that different from the conventional belief that an intermediate phase LiFeS is formed followed by Fe/LiS composites at the initial discharge, it undergoes a one-step reaction (FeS → Fe + LiS) or a two-step reaction (FeS → FeS + LiS → Fe + LiS), which depends on the current rate and temperature. In the charge process, it undergoes a two-step reaction: phase transition Fe + LiS → FeS at about 1.74 V and generation of elemental sulfur (LiS → S, 2.30 V). FeS is a mackinawite phase that is formed on the interface of LiS via heteroepitaxial growth. Subsequent cycles involves a combination reaction of FeS and S. The reaction mechanism suggests that FeS suffers from the demerits of both FeS and S, such as a large volume change, voltage hysteresis, and polysulfide dissolution. These findings would help us to understand the intrinsic capacity fading of FeS and provide guidelines to improve its electrochemical performances.