Synthesis of (Li₂Fe_1–yMn_y)SO Antiperovskites with Comprehensive Investigations of (Li₂Fe_0.5Mn_0.5)SO as Cathode in Li-ion Batteries

Abstract

A series of solid solutions (Li 2 Fe 1−y Mn y )SO with a cubic antiperovskite structure was successfully synthesized. The composition (Li 2 Fe 0.5 Mn 0.5 )SO was intensively studied as a cathode in Li-ion batteries showing a reversible specific capacity of 120 mA h g −1 and almost a 100% Coulombic efficiency after 50 cycles at 0.1C meaning extraction/insertion of 1 Li per formula unit during 10 h. Operando X-ray absorption spectroscopy confirmed the redox activity of both Fe 2+ and Mn 2+ cations during battery charge and discharge, while operando synchrotron X-ray diffraction studies revealed a reversible formation of a second isostructural phase upon Li-removal and insertion at least for the first several cycles. In comparison to (Li 2 Fe)SO, the presence of Mn stabilizes the crystal structure of (Li 2 Fe 0.5 Mn 0.5 )SO during battery operation, although post mortem TEM studies confirmed a gradual amorphization after 50 cycles. A lower specific capacity of (Li 2 Fe 0.5 Mn 0.5 )SO in comparison to (Li 2 Fe)SO is probably caused by slower kinetics, especially in the two-phase region, as confirmed by Li-diffusion coefficient measurements.

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 presence of some amount of sulfur in the gas phase during the reaction time was often confirmed after the synthesis by an off-white scurflike layer on the wall of the ampule and therefore, a slight amount of impurities was likely to form. When the initial argon pressure inside the tube was increased in comparison to the earlier reported synthetic conditions for Li 2 Fe 1 − x M x SO series (Gorbunov et al, 2020), the scurf disappeared, in accordance with the well-known Le Chatelier's principle (Le Chatelier and Boudouard, 1898). A value of 0.5 bar Ar in the tube before sealing provided a reasonable balancing between the material purity and the stability of the ampule against the internal gas pressure, increasing with the temperature.…”

“…Afterward, the ampule with the pellet was put in an oven, heated up to 750 • C with a rate of 50 • C/h, kept there for 3 h, and immediately cooled down to room temperature by quenching it in melting ice. In general, the synthesis route was taken from the previous reports on antiperovskites (Lai et al, 2018;Gorbunov et al, 2020); however, some parameters were varied to optimize the quality of the obtained materials.…”

“…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).…”

“…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). The structure of antiperovskites differs significantly from known tunnel-like (olivine) (Padhi et al, 1997), two-dimensional (α-NaFeO 2 ) (Demourgues Guerlou et al, 1993), three-dimensional (spinel LiMn 2 O 4 ) (Thackeray et al, 1983), or polyanionic NASICON-or LISICON-type structures (Bukun et al, 1998), commonly used as cathodes for Li-ion or Na-ion batteries.…”