Analysis of the charge/discharge mechanism of an Fe-containing Li₂S positive electrode material and its visualization...

The structure and charge/discharge mechanism of an Fe-containing Li₂S-based positive electrode material (Li₈FeS₅) were investigated using X-ray scattering and absorption spectroscopy analyses. Conventional XRD measurements indicated that Li₈FeS₅ adopts a low-crystalline Li₂S structure (antifluorite; space group Fm3̄m), and pair distribution function (PDF) analyses suggested that Fe ions occupy the vacant cation sites. Structural rearrangement occurred during the first charge, resulting in the formation of an inhomogeneous local structure around S atoms. The structural change was irreversible when charged to 3.0 V but not at 2.6 V vs. Li⁺/Li, which may be the reason for the low discharge capacity in a normal electrochemical test. Significant disproportionation of S-S distances (ca. 4.0 Å, corresponding to the nearest-neighbor S-S distance in the antifluorite structure) was observed when charged to 3.0 V, and such irreversible disproportionation was observed mainly around the S-S pairs away from the Fe atoms. The atomic rearrangements during charge/discharge processes were also modeled using neural network potential calculations, which were reasonably consistent with the PDF results.