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containing 75wt % sulfur exhibited a high initial capacity of 1368 mAh g -1 at 0.5 C and a small capacity decay rate of 0.034% per cycle at 2.0 C. The existence of CoS2 not only anchored the polar polysulfides, but also accelerated the charge transfer at the polysulfide/CoS2 interface, as illustrated in Figure 4b, enhancing the electrochemical performance of Li-S batteries. Co9S8 is another kind of metal sulfide with high conductivity of 290 S cm-1 at room temperature. Unique Co9S8-inlaid carbon hollow nanopolyhedra were synthesized and then used as an efficient sulfur host for Li-S batteries by Chen et al., as shown in Figure 4c.[71] These hollow nanopolyhedra with large void space not only accommodate high sulfur mass loading, but also buffer the volume expansion. The highly polar embedded Co9S8 crystals can strongly bind polysulfides and therefore restrict their outward diffusion. In order to probe the chemisorption capability of Co9S8 nanocrystals towards polysulfides, DFT calculations were conducted, as shown in Figure 4d. The (202) planes of Co9S8 were chosen as the representative crystalline planes for the simulations, because they are close to the stoichiometric Co/S ratio of 5:4. The absorption energies between Co9S8 (202) and the different Li2Sn species (Li2S8, Li2S6, Li2S4, Li2S2, Li2S) are -6.08, -4.03, -2.97, -4.52, and - 5.51 eV, respectively. This strong chemical interaction of Li2Sn with Co9S8 resulted from their highly polar nature. A visual adsorption experiment was further carried out, as shown in Figure 4e. The color of a Li2S4 solution mixed with Co9S8/C hollow nanocrystals turned almost colorless after 120 min, confirming that the Co9S8/C hollow nanocrystals had stronger adsorption capability towards Li2S4 than common porous carbon. The Co9S8/C-S composite cathode still exhibited high discharge capacity of 560 mAh g-1 after 1000 cycles at 2.0 C, corresponding to a low capacity decay of 0.041% per cycle. In addition, the improved rate performance indicates that Co9S8 can also significantly improve the reaction kinetics of the polysulfide redox reactions. 10PDF Image | Advances in Polar Materials for Lithium-Sulfur Batteries
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