Operando Studies of Antiperovskite Lithium Battery Cathode Material (Li₂Fe)SO

Abstract

Spectroscopic and X-ray diffraction operando techniques were used to investigate polycrystalline antiperovskite (Li 2 Fe)SO as cathode materials in a Li-battery setup. During Li removal, several intermediate, relatively stable phases exist. At low charging, Fe is oxidized from +2 to +3, but at higher charging, S 2− is also partly oxidized to elemental sulfur, suggesting a cathode bifunctionality, and both redox processes seem reversible. On cycling (Li 2 Fe)SO in a battery, spectroscopy data suggest that a part of the Fe atoms irreversibly vacate the high-symmetry positions in the crystal lattice, in line with the broadening of X-ray diffraction peaks. Instead, new, relatively broad reflections appear in the X-ray patterns that might be explained by a crystallographic superstructure, corresponding to a doubling of the cubic unit cell axis, but the peak broadness indicates a lowering in crystallographic symmetry. Using a standard electrolyte and a moderate charging rate of C/10 results in typical capacity loss per cycle, but by using an electrolyte with low sulfur solubility, the (Li 2 Fe)SO cathode is stabilized, and charge densities of more than 200 mAh g −1 at a 1C charging rate are obtained. Additionally, a Li-deficient precursor (Li 0.8 Fe)SO served as a cathode material in a Na battery, providing presumably reversible Na intercalation and removal.

Studies of Li2Fe0.9M0.1SO Antiperovskite Materials for Lithium–Ion Batteries: The Role of Partial Fe2+ to M2+ Substitution

“…As one would expect from the values of ionic radii for transition metal cations (Shannon, 1976), partial substitution of high-spin HS-Fe 2+ (0.78 Å) by HS-Co 2+ (0.745 Å) must result in a slight shrinkage of the crystallographic unit cell, whereas HS-Mn 2+ (0.83 Å) enhances the cell parameter. The values of cell metrics for Li 2 Fe 0.9 Mn 0.1 SO and pure Li 2 FeSO obtained in this work agree well with the results of the previous research related to these materials (Mikhailova et al, 2018;Gorbunov et al, 2020). For the Co-containing phase as expected, the determined cell parameter of 3.9082(5) Å is slightly smaller than that of unsubstituted Li 2 FeSO material (3.914 Å).…”

“…The structural behavior of the Li 2 Fe 0.9 Co 0.1 SO material (Figure 5) strongly resembles that of Li 2 FeSO. No formation of a second isostructural phase was observed, and the change of the lattice parameter is highly non-linear during the charging process, similar to the reported behavior for Li 2 FeSO (Mikhailova et al, 2018). The amount of extracted lithium before this nonlinearity appears, is different, though.…”

“…Moreover, the rapid deterioration of the diffusion process at the end of the charge, corresponding to a significant decrease of the diffusion coefficient, occurs earlier for the substituted materials (2.7 V vs. 2.88 V for Li 2 FeSO). However, the effect of partial substitution is opposite for the discharge process: the average Li-diffusion coefficient for substituted phases is one order of magnitude higher than for Li 2 FeSO (Mikhailova et al, 2018). No direct correlation between the lattice parameter a and the lithium diffusion coefficient was observed.…”

“…(D) Average oxidation states of metal cations in Li 2 Fe 0.9 Co 0.1 SO estimated from the electrochemical measurements (EC) and XAS studies. The position of the Fe K-edge was defined as the energy at the X-ray absorption coefficient µ(E) = 0.8 of normalized post-edge intensities, similar to Mikhailova et al (2018) and Gorbunov et al (2020).…”

“…Recently reported compounds with the antiperovskite structure represent an interesting class of materials for possible application as cathodes in Li-ion batteries (Lai et al, 2017(Lai et al, , 2018Mikhailova et al, 2018;Gorbunov et al, 2020). Among the studied materials, Li 2 CoSO and Li 2 MnSO (Lai et al, 2018), Li 2 FeSO (Lai et al, 2017;Mikhailova et al, 2018) and Li 2 Fe 0.5 Mn 0.5 SO (Gorbunov et al, 2020), the Li 2 FeSO compound demonstrates the most promising characteristics in terms of specific electrochemical capacities during galvanostatic cycling and rate capability. Other materials mentioned above show much lower specific capacity values not exceeding 100 mAh/g after few cycles (Lai et al, 2018) and quite a poor cycling behavior in lithium cells (Gorbunov et al, 2020).…”