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nanosheet as host,[55] polypyrrole-MnO2 coaxial nanotube host,[56] and MnO2@carbon hollow nanoboxes as host[57] Realizing the drawback from the highly insulating nature of these metal oxides mentioned above, the nanostructured Magnéli phase oxide Ti4O7 was further developed as a polar host for Li-S batteries, because it combines metallic conductivity with strong chemical binding ability towards polysulfides. Ti4O7 contains polar O-Ti-O units that have a high affinity for polysulfide, and the theoretical bulk conductivity is as high as 2 × 103 S cm-1 at 298 K. Pang et al. visually tested the interaction between Ti4O7 and Li2S4.[58] The Li2S4 solvent with the addition of Ti4O7 became light yellow immediately and almost completely colorless after 1 h, while the control samples (graphite and carbon in equivalent amounts based on surface area) remained intensely yellow-gold (Figure 3a), indicating a strong interaction between Li2S4 and polar Ti4O7. The potential mechanism of the chemical interaction was explored by analyzing S 2p X-ray photoelectron spectroscopy (XPS) spectra (Figure 3b). The ST-1 and SB0 peaks, corresponding to the terminal and bridging sulfur atoms, are shifted to higher energies by 2.7 and 1.7 eV upon contact between Li2S4 and Ti4O7. This indicates interaction of both the terminal and bridging sulfur in the lithium polysulfides with the Ti4O7 surface, resulting in the polarization of electrons away from the sulfur atoms by the electropositive titanium and/or oxygen vacancies at the interface. In addition, the electron transfer to chemically bonded lithium polysulfides is enhanced on the metallic surface of Ti4O7, leading to surface-mediated deposition of the discharge product Li2S. This was evidenced by an operando X-ray absorption near-edge spectroscopy (XANES) study on the Ti4O7/S electrode (Figure 3c). At almost the same time, Tao et al. also investigated the Ti4O7 materials as host materials in Li-S batteries.[59] The DFT calculation results for the bonding properties of Sx (x = 1, 2, and 4) and Li2Sx (x = 1, 2, and 4) on Ti4O7 (1-20) and TiO2 (110) is shown in Figure 3d, which indicates that the low coordinated Ti in titanium oxide can stabilize the sulfur clusters, while oxygen- 8PDF Image | Advances in Polar Materials for Lithium-Sulfur Batteries
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