Improvement of Cycling Capability of Li₂S-FeS Composite Positive Electrode Materials by Surface Coating With Titanium Oxide

High-energy-density batteries are actively sought after for next-generation electric vehicle (EV) applications, triggering the search for sulfur-based positive electrode materials with high capacity for extended driving range. One of the primary issues of sulfur cathodes is the rapid capacity decay caused by the dissolution of sulfur in the electrolyte and the resulting parasitic reactions. This can be prevented by incorporating a suitable amount of carbon material into the sulfur electrodes, because of its inherent properties; however, introducing a large amount of low-density carbon materials will lead to a decrease in the energy density of the battery. Therefore, an alternative method to suppress sulfur dissolution is required for commercial applications. Recently, we developed a Fe-containing Li₂S-based material (Li₈FeS₅) as the sulfur cathode; it has a relatively high initial discharge capacity (>700 mAh·g⁻¹) and electric conductivity. However, rapid capacity degradation was observed during the initial several cycles, due to some unknown reactions between the electrolyte and Li₈FeS₅. In this study we demonstrate an effective method to improve the cycle performance of Li₈FeS₅ by coating its surface with a stabilizing material. We selected titanium oxide as the coating material, based on its high stability toward liquid electrolytes and its strong interaction with sulfur. Hence, we obtained TiO_x-coated Li₈FeS₅ particles (Li₈FeS₅-TiO_x) by a liquid-phase reaction. The transmission electron microscopy observation revealed that the Li₈FeS₅ particles were coated with several tens of nanometers of TiO_x layers. The Li₈FeS₅-TiO_x cells exhibited improved cycling performance with a carbonate electrolyte. We also observed that the capacity originating from an iron redox was not affected by coating during the cycling test; in contrast, the degradation of the cell capacity corresponding to the sulfur redox was suppressed significantly after coating with the TiO_x layer. Thus, the surface reaction of Li₈FeS₅ with the electrolyte, which could be the main reason for the cycling degradation, was effectively suppressed by coating with the TiO_x layer.