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and surface hydroxyl groups, which play a decisive role in strongly binding with polysulfides. Many kinds of nanostructured TiO2, such as hollow spheres,[37] mesoporous spheres,[38,39] nanowires,[40] nanotubes,[41-43] nanofibers,[44] nanoparticles,[45] and feather duster,[46] were then explored to host sulfur in the cathode. Nb2O5 is an electronic semi-conductor with orthorhombic phase structure, and its bulk electrical conductivity reaches to 3.4 × 10-6 S cm-1.[47] The Li+ intercalation pseudocapacitance behavior of Nb2O5 was discovered by Dunn et al.,[48] as the unique Nb-O crystalline structure could provide fast two-dimensional transport paths for Li+ between atomic layers, resulting in the formation of highly conductive lithiated compounds. Tao et al. designed a new effective sulfur host, in which Nb2O5 was dispersed into mesoporous carbon microspheres (MCM/Nb2O5) through a wet impregnation method (Figure 2a).[49] With the combination of the high conductivity of the MCM and the high polarity of the Nb2O5 nanocrystals, MCM/ Nb2O5 cathodes delivered an initial discharge capacity of 1289 mAh g-1 at 0.5 C and excellent capacity retention with a fade rate as low as 0.14% per cycle over 200 cycles, as well as good rate capacity of 640 mAh g-1 at 5 C. According to the density functional theory (DFT) calculations, the binding energy between Li2S6 and Nb2O5 is 65.16 kcal mol-1, while it is only 9.84 kcal between Li2S6 and sp2 carbon (Figure 2b). Figure 2c displays the conformation of the binding system. The shortest distance between Li and O is 0.19 nm, indicating a strong ionic Li-O bond, which mainly accounts for the binding energy between Li2S6 and Nb2O5. The distance between Nb and S is 0.24-0.27 nm, which also partially improves the interaction between Li2S6 and Nb2O5 due to the van der Waals attraction and weak ionic bonds. The distance between Li and C is relatively long (0.24 nm), however, resulting in weaker attraction than for the Li-O bond. Additionally, there are four Li-O and four Nb-S pair in the Li2S6-Nb2O5 system, while there is only one Li-C pair in the Li2S6-C system. Furthermore, an electrochemical kinetic study further revealed that the Nb2O5 nanocrystals serve as an electrocatalyst, accelerating the multiple polysulfide transformation. 6PDF Image | Advances in Polar Materials for Lithium-Sulfur Batteries
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