Porous Molybdenum Carbide Nanorods as Novel "Bifunctional" Cathode Material for Li-S Batteries.

The shuttle effect of polysulfide is the primary obstacle in Li-S batteries. Although some effort has been made to suppress the shuttle process, more highly conductive materials with stronger adsorption ability to polysulfides are still in urgent demand. Another important problem is the deteriorating performance of Li-S batteries with prolonged cycling owing to irreversible deposition of lithium sulfide (Li2 S). However, a host material for the effective conversion of insoluble Li2 S has been elusive up to now. Herein, porous molybdenum carbide nanorods (Mo2 C NRs), with high catalytic activity for Li2 S and ultrastrong adsorption for polysulfides, are used as a "bifunctional" host material and incorporated into sulfur cathodes for the first time. The "bifunctional" Mo2 C NRs have the advantage of immobilizing polysulfides over the conventional host, with adsorption energies from -4.89 to -8.20 eV for Li2 Sx (x=1, 2, 4, 6, and 8). The electrochemical analysis shows that Mo2 C NRs, more than an anchor, can activate Li2 S by greatly reducing the charging overpotential. Therefore, the irreversible deposition of polysulfides is effectively restrained and the utilization of Li2 S is clearly enhanced. The Mo2 C NRs-sulfur composites (Mo2 C NRs-S) cathodes provide a capacity of 1298 mA h g-1 at 0.1 C in the Li-S batteries. More importantly, the battery with Mo2 C NRs-S cathodes exhibits a much lower capacity decay of 0.062 % per cycle at 1 C over 500 cycles.