First-shell sulfur doping induced asymmetric FeN₃S₁ coordination for accelerating sulfur redox kinetics in Li–S batteries

The asymmetric coordination engineering of single-atom catalysts emerges as a promising strategy to accelerate the sluggish redox kinetics of sulfur cathodes in Li–S batteries. Herein, an asymmetric coordination of Fe single-atom catalyst was exploited by simultaneously introducing sulfur atom and nitrogen atoms into the first coordination shell, where each Fe atom was connected with one sulfur and three nitrogen atoms (FeN₃S₁). This asymmetric coordination redistributed the electron density around the Fe center, which upshifted the d-band center and shortened the Fe–S(ads) bond length between the Fe site and lithium polysulfides (LiPSs), collectively strengthening LiPSs adsorption (where (ads) denotes the adsorbed atom). Moreover, these electronic modulations endowed the asymmetric FeN₃S₁ site, lowering the free energy barriers of the rate-determining steps (Li₂S₄ to Li₂S₂/Li₂S), accelerating the sulfur redox kinetics. Consequently, the S@Fe,S-NC^15-1 (NC stands for nitrogen-doped carbon) cathode maintains a low capacity decay rate of only 0.05% per cycle over 500 cycles at 4.0 C. This work provides a rational asymmetric coordination engineering strategy toward high performance Li–S batteries.