Synergistics of Fe₃C and Fe on Mesoporous Fe-N-C Sulfur Host for Nearly Complete and Fast Lithium Polysulfide Conversion

The practical deployment of advanced Li-S batteries is severely constrained by the uncontrollable lithium polysulfide conversion under realistic conditions. Although a plethora of advanced sulfur hosts and electrocatalysts have been examined, the fundamental mechanisms are still elusive and predictive design approaches have not yet been established. Here, we examined a series of well-defined Fe-N-C sulfur hosts with systematically varied and strongly coupled Fe3C and Fe electrocatalysts, prepared by one-step pyrolysis of a novel Fex[Fe(CN)6]y/polypyrrole composite at different temperatures. We revealed the key roles of Fe3C and metallic Fe on modulating polysulfide conversion, in that the polar Fe3C strongly adsorbs polysulfide whereas the Fe particles catalyze fast polysulfide conversion. We then highlight the superior performance of the rational host with strongly coupled Fe3C and Fe on mesoporous Fe-N-C host on promoting nearly complete polysulfide conversion, especially for the challenging short-chain Li2S4 conversion to Li2S. The electrodeposited Li2S on this host was extremely reactive and can be readily charged back to S with minimal activation overpotential. Overall, Li-S batteries equipped with the novel sulfur host delivered a high specific capacity of 1350 mAh g-1 at 0.1C with a capacity retention of 96% after 200 cycles. This work provides new insights on the functional mechanism of advanced sulfur hosts, which could eventually translate into new design principles for practical Li-S batteries.