CoN//Co9S8 co-embedded in reduced graphene oxide as catalyst for catalytic conversion of polysulfides

Lithium-sulfur (Li-S) batteries with high theoretical specific capacity of 1675 mAh g-1 and energy density of 2600 Wh kg-1 are considered as a promising candidate for energy conversion and storage devices in future. However, some critical dilemmas such as low real energy density, high solubility of ether-based electrolytes and slow redox kinetics of long-chain polysulfides remain unsettled hindering their development. Here we design and propose a high-performance sulfur host material constructed by growing CoN/Co9S8 nanoparticles enclosed in reduced graphene oxide (RGO) and Co9S8 modified separator to enhance the electrochemical performance of Li-S batteries. The results of first principal calculations confirm that the calculated adsorption energies of Li2S8, Li2S6, Li2S4 and Li2S2 on the Co9S8 (111) surface are lower than that of the CoN (111) surface, indicating that the Co9S8 has stronger adsorbing effect with polysulfides. Furthermore, the catalytic effect in Li2S6 symmetric batteries and the nucleation of the Li2S tests display that the CoN/Co9S8@RGO has a fast conversion reaction kinetics and catalytic effect of polysulfides, thereby inhibiting the leakage of the polysulfides. Consequently, the S@CoN/Co9S8@RGO cathode with Co9S8 layer represents a high discharge capacity of 1367.7 mAh g-1 at 0.1C and a prolonged lifespan over 700 cycles at 1 C with an extremely low decay rate of 0.06 % per cycle and high cycling stability.