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

Abstract

In this work, unique Co3 O4 /N-doped reduced graphene oxide (Co3 O4 /N-rGO)composites as favorable sulfur immobilizers and promoters for lithium-sulfur (Li-S) batteries are developed. The prepared Co3 O4 nanopolyhedrons (Co3 O4 -NP) and Co3 O4 nanocubes mainly expose (112) and (001) surfaces, respectively, with different atomic configurations of Co2+ /Co3+ sites. Experiments and theoretical calculations confirm that the octahedral coordination Co3+ (Co3+ Oh ) siteswith different oxidation states from tetrahedralcoordination Co2+ sites optimize the adsorption and catalytic conversion of lithium polysulfides. Specially, the Co3 O4 -NP crystals loaded on N-rGO expose (112) planes with ample Co3+ Oh active sites, exhibiting stronger adsorbability and superior catalytic activity for polysulfides, thus inhibiting the shuttle effect. Therefore, the S@Co3 O4 -NP/N-rGO cathodes deliver excellent electrochemical properties, for example, stable cyclability at 1C with a low capacity decay rate of 0.058% over 500cycles, superb rate capability up to 3C, and high areal capacity of 4.1mAhcm-2 . 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.