Oxidation States Regulation of Cobalt Active Sites through Crystal Surface Engineering for Enhanced Polysulfide Conversion in Lithium-Sulfur Batteries.

In this work, unique Co O /N-doped reduced graphene oxide (Co O /N-rGO) composites as favorable sulfur immobilizers and promoters for lithium-sulfur (Li-S) batteries are developed. The prepared Co O  nanopolyhedrons (Co O -NP) and Co O  nanocubes mainly expose (112) and (001) surfaces, respectively, with different atomic configurations of Co /Co sites. Experiments and theoretical calculations confirm that the octahedral coordination Co (Co ) sites with different oxidation states from tetrahedral coordination Co sites optimize the adsorption and catalytic conversion of lithium polysulfides. Specially, the Co O -NP crystals loaded on N-rGO expose (112) planes with ample Co active sites, exhibiting stronger adsorbability and superior catalytic activity for polysulfides, thus inhibiting the shuttle effect. Therefore, the S@Co O -NP/N-rGO cathodes deliver excellent electrochemical properties, for example, stable cyclability at 1 C with a low capacity decay rate of 0.058% over 500 cycles, superb rate capability up to 3 C, and high areal capacity of 4.1 mAh cm . This catalyst's design incorporating crystal surface engineering and oxidation state regulation strategies also provides new approaches for addressing the complicated issues of Li-S batteries.