Iron pyrite (FeS 2 ) 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 Li 2 FeS 2 is formed followed by Fe/Li 2 S composites at the initial discharge, it undergoes a one-step reaction (FeS 2 → Fe + Li 2 S) or a two-step reaction (FeS 2 → FeS + Li 2 S → Fe + Li 2 S), which depends on the current rate and temperature. In the charge process, it undergoes a two-step reaction: phase transition Fe + Li 2 S → FeS at about 1.74 V and generation of elemental sulfur (Li 2 S → S, 2.30 V). FeS is a mackinawite phase that is formed on the interface of Li 2 S via heteroepitaxial growth. Subsequent cycles involves a combination reaction of FeS and S. The reaction mechanism suggests that FeS 2 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 2 and provide guidelines to improve its electrochemical performances.

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