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

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 Fe[Fe(CN)₆]/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.