Polar and conductive iron carbide@N-doped porous carbon nanosheets as a sulfur host for high performance lithium sulfur batteries

Abstract

The practical application of Li-S batteries is inherently limited by the low conductivity of sulfur and the dissolution of polysulfides. The enhancement of conductivity and introduction of polarity to the sulfur host is an effective strategy to overcome these long-standing challenges. In this work, a facile one-step carbonizing process is used to synthesize a unique porous and conductive nanocomposite where Fe3C nanoparticles are embedded in nitrogen-doped porous carbon sheets (Fe3C@NPCS). As demonstrated in Li2S4 adsorption experiments and confirmed by density functional theory calculations, Fe3C nanoparticles are able to strongly adsorb polysulfides via strong Fe-S chemical bonds. Due to the remarkable conductivity, Fe3C nanoparticles can also accelerate the electrochemical reactions of the polysulfides through fast electron transportation. The carbon sheets prevent the aggregation of Fe3C nanoparticles and accommodate sulfur and its volume expansion during charge/discharge processes. With these features, the resultant sulfur cathodes (Fe3C@NPCS-S) deliver an excellent cycling performance with a capacity decay of 0.036% per cycle, and high rate performance of ca. 1127, 1020, 907, 802, 731 and 647 mAh g−1 under current densities of 0.3, 0.5, 1, 2, 3 and 5 C, respectively.