Rational Design of Yolk–Shell Fe7S8@C-N for High Rate and Long Cycle Li-Ion Batteries

Fe7S8 with large capacity shows high potential for Li-ion batteries, while it still suffers large volume expansion, resulting in fast capacity fading. Herein, a novel yolk-shell structural Fe7S8@C-N is rationally designed, in which the N-doped carbon layer with superior mechanical flexibility enables one to accommodate the volume expansion of the Fe7S8 core and promote its electronic transportation. Besides, the surface porous morphology is believed to facilitate electrolyte infiltration and Li-ion diffusion as well. Therefore, this modified Fe7S8@C-N electrode exhibits lower expansivity (∼28.0% vs ∼87.4%), smaller voltage hysteresis, higher conductivity (1.6 × 10-2 S/m) and better Li-diffusivity (1.09 × 10-12 cm2/s) than its pure Fe7S8 powder; thus better cyclability (458 mAh/g vs 121 mAh/g after 150 cycles) and rate-capability improvement (546 mAh/g vs 125 mAh/g at 2000 mA/g) can be achieved. Such a yolk-shell structural design strategy can be easily extended to other conversion or alloying type materials for advanced energy storage.