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

Fe 7 S 8 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 Fe 7 S 8 @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 Fe 7 S 8 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 Fe 7 S 8 @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 cm 2 /s) than its pure Fe 7 S 8 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.