Understanding the Electron State Effect of Iron Single-Atom for Enhancing Solid–Solid Conversion Kinetics of Sulfur Cathodes

Abstract

Optimizing the solid–solid conversion kinetics has been challenging in lithium–sulfur batteries (LSBs). In this study, a nitrogen and boron dual-coordinated Fe single-atom catalyst (Fe-N₂B₂/C) was exploited by inducing boron atoms into the coordination shell to disrupt the nitrogen-only coordinated configuration (Fe-N₄/C). The intervention of boron reduced the oxidation state of Fe atoms, which increased electron density of the Fe 3d orbital and narrowed band gap between the conduction and valence bands. Furthermore, the elevated d-band center of Fe in Fe-N₂B₂/C raised the antibonding orbital energy, providing sites for charge transfer and polysulfide adsorption. These electronic modulations endowed Fe-N₂B₂/C with prominent anchoring capacity and catalytic activity. Consequently, in the ether-based electrolyte, the S@Fe-N₂B₂/C sulfur cathode delivered an initial capacity of 786 mAh g⁻¹ at 4.0 C, maintaining an impressive capacity retention of 82.7% after 200 cycles and exhibiting a sluggish capacity decay of 0.08% after 500 cycles. Simultaneously, in the all-solid-state system based on halide electrolytes (HEs), the S@Fe-N₂B₂/C cathode achieved a remarkable discharge capacity (1066 mAh g⁻¹, 0.1 C), high average Coulombic efficiency (>99%) and excellent cyclic stability (0.068%, 0.2 C). This study uncovers the origin of outstanding activity of Fe single-atom catalyst and provides a promising strategy for HEs-based all-solid-state system.