Bipolar CoSe₂ nanocrystals embedded in porous carbon nanocages as an efficient electrocatalyst for Li-S batteries

The confinement and catalysis properties of orthorhombic CoSe₂ as a host material for Li-S battery are studied by theoretical and experimental methods. First-principles calculations show that the (111) surface of CoSe₂ possesses adjacent nucleophilic and electrophilic centers. This bi-polar surface can easily bond the polysulfide molecules and catalyze their conversion reactions. As a result, the decomposition energy barriers of polysulfides are greatly decreased from ∼3.0 eV to 0.2–0.4 eV. Following theoretical prediction, CoSe₂ nanocrystals with specific (111) surface are prepared and embedded in porous carbon nanocages using ZIF-67 metal organic framework as a template. Benefited from the advantages of porous C and CoSe₂ (111) surface, the shuttling of polysulfides is suppressed and their conversion kinetics is facilitated. The Li-S cell using this host material exhibits high capacity and remarkable cycle stability, showing a discharge capacity of 1199 mA·h·g⁻¹ at 0.2C and 400 stable cycles at 1.0C.