Bivalent Cobalt as Efficient Catalyst Intercalation Layer Improves Polysulfide Conversion in Lithium-Sulfur Batteries

Herein, we investigated in detail the effect of metal valences in different cobalt-based organic framework compounds on the kinetics of sulfur reaction in lithium-sulfur batteries (LSBs). On this basis, two organic framework compounds of zeolite-imidazole-based cobalt organic framework compound (Co-ZIF) and tetrakis(4-benzoic acid) porphyrinato-Co^III chloride [Co-TBP(III)] with different valences were constructed as the functional intercalation separators of LSBs, and explored the effects of different valences on improving the reaction kinetics of polysulfides and inhibiting the shuttle effect. Experiments and theoretical calculations prove that Co^II exhibits the best catalytic activity. This is mainly due to the fact that +2 valence shows a strong adsorption energy for polysulfides and a higher Fermi level compared with +3 valence, thus improving the efficiency of the rapid catalytic conversion of sulfur species. As expected, the discharge specific capacity of Co-ZIF as the catalytic layer of the LSBs reached 772.7 mAh g^-1 at a high current density of 5 C. More importantly, the initial specific capacity is 839.6 mAh g^-1 at high current 3 C, and after 720 cycles, the attenuation rate of per cycle is only 0.092 %, and the coulombic efficiency remains above 92 %.