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TiS2 also has been studied in Li-S batteries due to its high electronic conductivity and the polar nature of its surface. Cui and co-workers proposed a Li2S@TiS2 core-shell nanostructure synthesized by an in-situ reaction method as cathode for Li-S batteries (Figure 5a).[72] The TiCl4 precursor reacts directly with some of the Li2S on the surface to form a TiS2 coating (TiCl4 + 2Li2S = TiS2 + 4LiCl), resulting in the formation of the Li2S@TiS2 core-shell nanostructure. On the one hand, the electronic conductivity of the Li2S@TiS2 structures was measured to be 5.1 × 10-3 S cm-1, which is 10 orders of magnitude higher than that of bare Li2S (10-3 S cm-1), leading to fast electron transport. On the other hand, polar Ti-S groups in the TiS2 coating layer can potentially interact strongly with Li2Sn species, as evidenced by the DFT calculations (Figure 5b). The binding energy between Li2S and a single layer of TiS2 was 2.99 eV, while only 0.29 eV was calculated between Li2S and a single layer of carbon- based graphene.[73] The much stronger interaction between Li2S and TiS2 is attributed to their similar polar nature, unlike the nonpolar nature of graphene. In addition, this similar reaction mechanism can be extended to the synthesis of Li2S@ZrS2 and Li2S@VS2 core-shell nanostructures. The calculated binding energies of Li2S to ZrS2 and VS2 are 2.70 and 2.94 eV, respectively (Figure 5b). Compared to the bare Li2S cathodes, the Li2S@TiS2 cathodes exhibited a high initial specific capacity of 806 mAh g-1, which was calculated based on Li2S, stable cycling performance at 0.2 C, and good rate capability from 0.2 to 4 C (Figure 5c). Another study of TiS2 foam supported sulfur cathode with high sulfur loading was conducted by Archer and co-workers.[74] The improved performance of the materials was also attributed to the high conductivity and strong affinity towards soluble polysulfides of polar TiS2. Various other metal sulfides, such as MoS2,[75,76] SnS2,[77,78] NiS2,[79] WS2,[80] MnS,[81] CuS,[82] and FeS2,[83] have also been investigated as polar hosts in Li-S batteries. Although the conductivity of the metal sulfides is much higher than for metal oxides, carbon-based materials are still introduced, in order to further decrease the internal resistance and enhance the utilization of the active material. In addition, most of the adsorption mechanisms between 11PDF Image | Advances in Polar Materials for Lithium-Sulfur Batteries
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