A Long Cycle Li-S Battery with Minimum Shuttle Effect - Catalytic Disproportionation of Dissolved Polysulfide to Elemental Sulfur

A Long cycle-life Li-S battery pouch cell with high sulfur loading (5 mg per cm²) is reported with the mitigation of the 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 homogenous precipitation of Li sulfide on the host carbon and the carbon has honeycomb porous structure which can store sulfur batter. All the features will be demonstrated experimentally and reported.<br/>Through an ex-situ, postmortem analysis in which a Li-S cell was disassembled after many cycles, few dissolved polysulfides were found in the electrolyte of the Li-S cell made with the bifunctional carbon in comparison with a control Li-S cell. A HPLC was used to determine the distribution of dissolved elemental sulfur and polysulfide ions in the electrolyte of the Li-S batteries during cycling. Only dissolved elemental sulfur was detected in the cell with the bifunctional carbon, while a distribution of polysulfide ions of various -S-S- chain length was observed in the control Li-S cell. A catalytic polysulfide disproportionation reaction mechanism was proposed, in which polysulfide ions can be catalytically disproportionate to elemental sulfur and Li₂S₂ and/or Li₂S precipitates. Since the dissolved polysulfides are the engine driving the shuttle effect, the detrimental shuttling in a Li-S cell can be mitigated through the removal of dissolved polysulfide ions in the electrolyte during the discharge and recharge.<br/>The unique porous structure of the bifunctional carbon which was made from a raw silk was revealed by a SEM and a N₂-absorption isotherm. The pore structure was believed to store sulfur uniformly and ensured the homogeneous deposition of Li₂S₂ and/or Li₂S. The N-containing functionalities that were introduced to carbon from the amino acids of raw silk can catalyze the disproportionation of the dissolved S_n^2-to solid S₈at 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, the sulfur/carbon composite using delivers a high reversible capacity of over 1000 mAh g^–1 and over 600 mAh g^–1 with a sulfur content of 1.2 mg cm^–2 5 mg cm^–2 in a pouch cell, respectively.