LAUSR

Iron pyrite (FeS2) is a promising lithium ion battery cathode material because of its low cost and ultra-high energy density (1671 Wh kg-1). However, its reaction mechanisms are still controversial. In this work, we find that different from the conventional beliefs that a formation of an intermediate phase Li2FeS2 and then 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 current rate and temperature. In the charge process, it undergoes a two-step reaction: a phase transition Fe + Li2S → FeS at about 1.74 V; a generation of elemental sulfur (Li2S → S, 2.30 V). FeS is a mackinawite phase which is formed on the interface of Li2S via a hetero-epitaxial growth. In the subsequent cycles, it is a combination reaction behavior of FeS and S. The reaction mechanism suggests that FeS2 suffers from the challenges of conversion reaction materials both FeS and S, such as big volume change, voltage hysteresis, and polysulfide dissolution. These findings would help understand the intrinsic capacity fading of FeS2 and provide guidelines to improve its electrochemical performances.