Unraveling the Reaction Mechanism of FeS2 as a Li-Ion Battery Cathode.

Iron pyrite (FeS2) is a promising lithium-ion battery cathode material because of its low cost and ultrahigh energy density (1671 Wh kg-1). However, its reaction mechanisms are still controversial. In this work, we find that different from the conventional belief that an intermediate phase Li2FeS2 is formed followed by Fe/Li2S composites at the initial discharge, it undergoes a one-step reaction (FeS2 → Fe + Li2S) or a two-step reaction (FeS2 → FeS + Li2S → Fe + Li2S), which depends on the current rate and temperature. In the charge process, it undergoes a two-step reaction: phase transition Fe + Li2S → FeS at about 1.74 V and generation of elemental sulfur (Li2S → S, 2.30 V). FeS is a mackinawite phase that is formed on the interface of Li2S via heteroepitaxial growth. Subsequent cycles involves a combination reaction of FeS and S. The reaction mechanism suggests that FeS2 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 FeS2 and provide guidelines to improve its electrochemical performances.