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Materials 2022, 15, 3037 6 of 11 Such greater discharge capacity at the initial cycle was previously reported for both Li2TiS3 and other compositions (Li3NbS4 and Li3SnS4) [16,24,25]. We have to keep in mind that this phenomenon will require the addition of “lithium sacrificial salt” inside Li-ion cells if this material is used in front of a graphite anode instead of metal lithium. Another way to overcome this phenomenon could be to use the material in all solid-state batteries, with metal lithium as anode. In all cases, the origin of the extra capacity remains unknown, and further structural analyses are needed to explain it. We explain the possible redox mechanism in Li2TiS3 with a hypothesis whereby all the atoms are stabilized at their valence states (Ti4+ and S2−) in the pristine electrode, and there is no loss of sulfur atoms during the charge–discharge process. Based on this hypothesis, only sulfur redox should be active during charging, since Ti is already at its maximum valence of Ti4+. Therefore, charge capacity may have been produced by anionic (sulfur) redox, with some part of the S2− atoms oxidized into S22 (Equation (1)). Moreover, the discharge capacity could have been provided by either anionic redox (S2−/S22−) or both anionic and cationic (Ti4+/Ti3+) redox processes. A reversible sulfur redox process was previously mentioned in the literature [32]. Reversible formation and dissociation of covalent S-S bonds in an Li2TiS3 electrode were detected in both pair distribution function analyses and ab initio molecular dynamics calculations, and this was later attributed to reversible sulfur redox processes. Li2TiSeS2 and Li2TiSe2S cells were also tested with the same cycling schedule applied to Li2TiS3 cells, and 179 mAh·g−1 charge and 310 mAh·g−1 discharge capacities were delivered between 3 and 1.5 V vs. Li+/Li. The average charge and discharge potentials were measured to be 2.34 V and 2.06 V, respectively, indicating that the average potentials are reduced by selenium substitution. At the end of the first cycle, we again detected a large discharge capacity. More than 2.0 Li+ ions were inserted into the structure, and the composition changed into Li3.08TiSeS2, which can be regarded as the average (theoretical) composition. Such capacity was even greater than the theoretical capacity of Li2TiSeS2 cells (261 mAh·g−1 based on two electron exchange processes). At the subsequent cycles, a reversible cycling curve was observed. If we keep the same hypothesis that we previously used to explain the redox process of Li2TiS3 cells, the charge capacity should be produced by either sulfur (S2−/S22−) or selenium (Se2−/Se22−) redox processes, which is active between 3 and 1.5 V [29,30]. Here, again, the discharge capacity should be provided by either anionic (S2−/S22− or Se2−/Se22−) or both anionic and cationic (Ti4+/Ti3+) redox processes. Li2TiSe2S cells delivered charge and discharge capacities of 149 mAh·g−1 and 379 mAh·g−1, respectively. Much lower average charge and discharge potentials (2.24 V and 1.98 V) were detected. At the end of the initial discharge, 3.44 Li+ ions were inserted into the cubic rocksalt structure of Li2TiSe2S, and the theoretical composition became equiv- alent to Li4.17TiSe2S. We again detected extra discharge capacity, and this was even greater than the theoretical capacity of Li2TiSe2S (213 mAh·g−1 based on two electron exchange processes). Now, we cannot explain more than a three Li+ uptake with the same hypothesis; a combination of anionic and Ti3+/Ti4+ redox processes. In the cycling curve of Li2TiSe2S cells, we observed that the second and third cycles were reversible; however, smaller charge and discharge capacities, as well as a rapid capacity fading, were detected. To describe the possible redox processes taking place in Li2TiSexS3−x cells, we con- ducted cyclic voltammetry measurements (Figure 5). Li2TiS3 cells showed one oxidative and one reductive peak that resulted from sulfur redox reaction (S2−/S22− and S22−/S2−) at 2.69 V and 2.28 V. Moreover, 2.51 V charge and 2.20 V discharge potentials were detected in Li2TiSeS2 cells, and 2.43 V charge and 2.12 V discharge potentials were detected in Li2TiSe2S cells, in accordance with previous results showing a reduced working potential for Se-substituted materials. During discharge, the apparition of a second reduction peak could be observed as a shoulder of the main peak, only for the Se-substituted materials. This peak is reversible and can be observed in the second cycle of Li2TiSexS3−x cells. Therefore, we can assume that the substitution of S byPDF Image | Lithium-Rich Rock Salt Type Sulfides-Selenides
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