Demystifying Activity Origin of M-N-C Single-Atomic Mediators Toward Expedited Rate-Determining Step in Li-S...

Sluggish sulfur reduction reaction (SRR) kinetics remains a formidable challenge in Li-S electrochemistry. In this sense, the rational design of single-atom species has become a burgeoning practice to expedite sulfur redox, where the underlying catalytic mechanism otherwise remains elusive. Herein, a class of metal single-atom modified porous carbon nanofiber films (MSA PCNFs, M = Fe, Co, or Ni), fabricated via a generic synthetic strategy, as mediators to boost SRR kinetics is reported. Throughout electrokinetic measurement and operando instrumental probing, NiSA PCNF is evidenced to harness the catalytic superiority toward the rate-determining step (i.e., liquid-solid conversion) of the SRR process. Density functional theory (DFT) simulations further reveal that the catalytic features of M-N-C moieties in catalyzing the Li₂S precipitation rely heavily upon the coordination environments of adjacent carbon atoms and d-orbital configurations of metal centers. In response, the thus-derived S/NiSA PCNF cathode realizes an encouraging areal capacity of 14.12 mAh cm^-2 under elevated sulfur loading (10.2 mg cm^-2) and lean electrolyte usage (E/S ratio ≈ 5.5 μL mg^-1). This work offers insight into the identification of exact catalytic moieties for different transition metal M-N-C single-atom SRR mediators, showcasing a meaningful guidance and potential impact on Li-S catalysis.