High-Performance Li-S Batteries with a Minimum Shuttle Effect: Disproportionation of Dissolved Polysulfide to Elemental Sulfur Catalyzed by a Bifunctional Carbon Host.

A long cycle-life Li-S battery (both the coin cell and pouch cell) is reported with minimum shuttle effect. The performance was achieved with a bifunctional carbon material with three unique features. The carbon can catalyze the disproportionation of dissolved long-chain polysulfide ions to elemental sulfur; the carbon can ensure homogeneous precipitation of Li sulfide on the host carbon, and the carbon has a honeycomb porous structure, which can store sulfur better. All the features are demonstrated experimentally and reported in this paper. Few dissolved polysulfides are found by high-performance liquid chromatography in the electrolyte of the Li-S batteries during cycling, and only dissolved elemental sulfur is detected. The unique porous structure of the carbon made from raw silk is revealed by scanning electron microscopy. The N-containing functionalities that were introduced to carbon from the amino acids of raw silk can catalyze the disproportionation of the dissolved S to solid S at the cathode side, thereby mitigating the shuttle effect. In addition, the hierarchical honeycomb porous structures generated by a carbonization process can physically trap high-order lithium polysulfides and sustain the volume change of sulfur. With the synergistic effects of the unique structures and characteristics of the carbon prepared at 800 °C, the sulfur/carbon composite delivers a high reversible capacity of over 1000 mAh g after 100 cycles with a sulfur content of 1.2 mg cm in a pouch cell.