context: In 2015, unique graphene-like Co9S8 with a high surface area and hierarchical porosity was first proposed by Nazar's group to afford superior LiPS adsorptivity for high-performance Li–S batteries. The Co9S8/S composite cathode not only manifests up to a factor of 10 increase of cycling stability (0.045% fading per cycle over 1500 cycles at C/2 at a sulfur loading of 1.5 mg cm−2) compared to standard porous carbons but also, more importantly, enables a high-loading sulfur electrode with up to 75 wt% sulfur and up to 4.5 mg cm−2 areal sulfur loading. First-principles calculations and spectroscopic studies consistently demonstrate that a synergistic dual interaction based on Sn2− → Coδ+ and Li+ → Sδ− exists between the metal sulfide-based host material and LiPSs. In particular, it is revealed that the nature of the crystallographic surface plays a very crucial role. The binding energy of 2.22 eV for Li2S2 on the (002) surface (Co/S = 1:4) increases to 3.24 eV for the (202) surface (Co/S = 5:4) and further to 6.06 eV for the purely Co-terminated (008) plane. This finding enables a better understanding of the interaction mechanism for metal sulfides and provides guidance for fabricating host materials with preferred crystal faces, but unfortunately, the mechanism by which the polar surface accelerates LiPS redox has not been clearly revealed yet. Soon after, Yuan et al. demonstrated that simply introducing half-metallic CoS2 microparticles into graphene electrodes largely amplified the electrochemical current response and reduced charge transfer resistance in LiPS symmetrical cells. More importantly, it is found that CoS2 serves as a mediator to provide strong CoS2–polysulfide interaction and to facilitate the liquid–liquid redox of polysulfides, further affecting the correlated liquid–solid transformation, viz., the prior formation of Li2S nuclei. With a sulfur loading of 0.4 mg cm−2, a high initial capacity of 1368 mA h g−1 at 0.5C, an increase of 10% in energy efficiency, and a low decay rate of 0.034% per cycle for 2000 cycles at 2.0C are realized through the incorporation of mechanically milled CoS2 with a low specific surface area. These observations suggest that manipulating the redox reactivity of polysulfides upon a conducting surface is a more effective strategy for improving the Li–S battery performance, in comparison to tuning the dissolvability and transport behavior of polysulfides in aprotic electrolytes. Given that the metal sulfide herein employed is in the bulk form, therefore, the performance enhancement can be further achieved by optimizing its structure and properties, including engineering the crystal facets to mediate the adsorbate–adsorbent interaction, reducing the dimension to maximize the exposed polar surface and introducing a strong coupling effect between substrates and supported metal sulfides for intriguing modulation of material properties.
answer: Co9S8/S composite