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

Abstract

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.