Engineering atomic Fe sites with asymmetrical nitrogen/sulfur coordination for stable and boosted sulfur conversion

Abstract

Introducing nonmetallic heteroatoms into the conventional metal-N-C configuration is thought to be an effective strategy to regulate the coordinated chemistry environment of atomic metal centers and further improve the sulfur electrochemistry behavior in lithium-sulfur batteries. Nevertheless, the modulation of heteroatoms junction states (doped or coordinated) in carbocycle is rarely explored. Herein, based on the stable pre-coordination of Fe3+ and terminal sulfhydryl/amino groups in cysteine, an asymmetrical Fe-N3S1 coordination structure in cysteine-derived graphene (Fe/CG) is rationally designed and prepared. The intervention of S atom at the first-coordination shell of Fe centers could induce the distorted spatial coordination structure, which greatly reduces the Li2S decomposition barrier. Moreover, the bonded S atom increases the d-band center (Ɛda) of Fe and pronouncedly activates the surrounding atoms, enhancing the electronic conductivity and reaction activity/affinity of Fe/CG, which was evidenced through theoretical calculations. The serial electro-kinetic analysis further confirms the bidirectional catalytic effect of Li2S redox conversions on Fe/CG. Consequently, the S@Fe/CG cathodes exhibit outstanding cycling stability (718.8 mAh g−1 after 500 cycles at 1 C with a decay of 0.037 % per cycle) and rate performance (639 mAh g−1 at 4 C). This work provides a new perspective to regulate the local chemistry environment of metal centers via coordination engineering.