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

Abstract

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.

Electrochemical Modeling of GITT Measurements for Improved Solid-State Diffusion Coefficient Evaluation

“…FeS 2 is a conversion cathode material that undergoes a four-electron redox process to produce Li 2 S and Fe. Prior to undergoing the conversion reaction, a significant portion of the total capacity results from lithium intercalation into the active material. − To study the intercalation region, FeS 2 coin cells were first lithiated down to 1 V at a slow rate of C/50 to form Li 2 S and Fe, and then subjected to a partial delithiation up to 2.4 V to avoid the upper conversion reaction . At 2.4 V, the system is expected to be composed of Li 1.2 FeS 2 , the delithiated state of the intercalation reaction. , From this point on, the cycling window was limited to 1.6–2.4 V and GITT was performed at C /20 (based on a theoretical capacity of 894 mA h/g) for 20 min, followed by a 4 h rest period (Figure e).…”

“…23−27 To study the intercalation region, FeS 2 coin cells were first lithiated down to 1 V at a slow rate of C/50 to form Li 2 S and Fe, and then subjected to a partial delithiation up to 2.4 V to avoid the upper conversion reaction. 25 At 2.4 V, the system is expected to be composed of Li 1.2 FeS 2 , the delithiated state of the intercalation reaction. 23,28 From this point on, the cycling window was limited to 1.6−2.4 V and GITT was performed at C/20 (based on a theoretical capacity of 894 mA h/ g) 23 for 20 min, followed by a 4 h rest period (Figure 1e).…”

Recent Developments of Cathode Materials for Thermal Batteries

“…Such method can be easily scaled up, but increases the internal resistance of the battery due to the presence of different polymer binders ( Masset et al, 2005 ; Cha et al, 2018 ). Various studies shows the phase transformations of FeS 2 discharging process as follows ( Tomczuk et al, 1982 ; Choi et al, 2014 ; Chen et al, 2017 ; Cha et al, 2018 ; Zhang and Tran, 2018 ; Kim et al, 2020 ; Zou et al, 2020 ; Thu et al, 2021 ): FeS 2 →Li 3 Fe 2 S 4 →Li 2+x Fe 1-x S 2 →Fe.…”

Electrochemical Modeling of GITT Measurements for Improved Solid-State Diffusion Coefficient Evaluation

“…FeS 2 slurries were punched out into 3/8 inch diameter disks FeS 2 is a conversion cathode material that undergoes a four-electron redox process to produce Li 2 S and Fe. Prior to undergoing the conversion reaction, a significant portion of the total capacity results from lithium intercalation into the active material 18,[26][27][28][29] . To study the intercalation region, FeS 2 coin cells were first lithiated down to 1 V at a slow rate of C/50 to form Li 2 S and Fe, and then subjected to a partial delithiation up to 2.4 V to avoid the upper conversion reaction 27 .…”