Accelerating polysulfide redox conversion on bifunctional electrocatalytic electrode for stable Li-S batteries

Lithium-sulfur (Li-S) batteries are very appealing power source with high energy density. However, their performance is limited by polysulfide shuttling problem and sluggish kinetics of Li-S reactions. Notorious polysulfide shuttling can be effectively overcome by physicochemical adsorption of polysulfides in porous electrodes. Thereafter, slow conversion rate of polysulfide adsorbed on the electrode emerged as a new bottle-neck issue to Li-S battery performance. Herein, we report an bifunctional metal-nitrogen-carbon electrocatalytic electrode made of Co-embedded N-doped graphitic carbon decorated on free-standing N-doped carbon fabric for electrocatalysis of polysulfide conversion and the understanding on its electrocatalytic effect. The Co-embedded N-doped graphitic carbon is identified as the bifunctional active center to electrocatalytically accelerate the oxidation of Li2S and the polysulfide conversion. First-principles calculations reveals that the graphitic N sites neighbored to Co in carbon matrix exhibit the modest reactivity for strong yet favorable adsorption/dissociation of polysulfide species. It leads to greatly reduced energy and kinetics barrier for polysulfide conversion without weakening the polysulfide adsorption on the electrode. The integrated and hierarchically porous electrode design further maximizes the electrocatalytic effect in thick electrode with high sulfur loading. The electrocatalytic electrodes with sulfur loading up to 6.5 mg cm-2 can be directly applied in Li-S batteries without tedious electrode fabrication, exhibiting very low capacity decay rate of 0.034% for 500 cycles and excellent high-rate capability up to 5 C. This work may provide the new insight into the understanding on vital role and chemical nature of electrocatalysis in promoting Li-S battery performance.