Highly-dispersed nickel on 2D graphitic carbon nitrides (g-C3N4) for facilitating reaction kinetics of lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are promising next-generation energy storage devices due to high theoretical energy density and low-cost. Nevertheless, the practical applications are hindered by polysulfide shuttling effect, low electrical conductivity of sulfur, and slower conversion kinetics. Here, the graphited g-C3N4 assembled with highly-dispersed nickel (HDNi@g-C3N4) is designed as a catalyst to accelerate the reaction kinetics of lithium polysulfide. The oxidized Ni sites of HDNi@g-C3N4 molecules significantly accommodate the orbital for the electron clouds of polysulfide by forming S-n(2-)center dot center dot center dot Ni-N active site, thus efficiently improving redox kinetics and mitigating shuttle effects. Based on density functional theory (DFT) calculations, HDNi@g-C3N4 exhibits a superior metallicity with increased density of states (DOS) at the Fermi energy level. Then, the narrowed energy gap between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) level contributes to the enhanced conductivity of catalyst molecular and fast combination between electrons and Li+ ions. Moreover, the positive Gibbs free energy change is significantly decreased for the HDNi@g-C3N4 cathode. The Li-S battery exhibits a high reversible capacity of 1, 271.6 mAh g(-1) at 0.1 C and a high rate capacity of 571.96 mAh g(-1) at 2.0 C, a preferable cycling stability with a capacity retention of 53 % even after 500 cycles at a 1.0 C, and an average decay rate of 0.733 % per cycle.