Fe-N-C Single Atom Catalysts Facilitate the Fast Electron Transfer with MoS2/SnS2 Cathode in Lithium Sulfur Batteries

Lithiumsulfur batteries (LSBs) emerge as promising next-generation energy storage systems offering cost-effectiveness, environmental friendliness, and high theoretical energy density. The practical implementation of LSBs faces significant hindrances due to the shuttle effect and sluggish redox reactions. To address these challenges, single-atom catalysts (SACs) from d-block elements can offer increased active catalytic sites, rapid charge transfer, accelerated electron migration, and fast sulfur redox conversion kinetics of lithium polysulfides (LiPSs). In this study, we fabricated three different LSB cathodes: pure S, S@MoS2/SnS2, and S@Fe-MoS2/SnS2. These cathodes were then used to explore the cycle life, capacity, rate capability, and redox kinetic reactions of LiPSs, while assessing the influence of Fe-SACs on their performance. As a result, LSBs with S@Fe-MoS2/SnS2 cathodes demonstrate an extended cycle life of 1000 cycles at a C-rate of 0.2 C, maintaining a capacity close to 500 mA h g–1, a highest initial discharge capacity of 1622 mA h g–1 and 1066 mA h g–1 at 0.05 C and 0.2 C, and excellent rate capabilities of 708 mA h g–1 and 558 mA h g–1 at 1 C and 2 C, respectively. The synergistic effect of the Fe-SAC-based cathode (S@Fe-MoS2/SnS2) creates plentiful adsorptive and highly active catalytic sites resulting in substantially enhanced capacity for adsorbing soluble long-chain LiPSs. This facilitates ultra-fast redox kinetics, surpassing the performance of the S@MoS2/SnS2 and pure S cathodes. In the ex situ analysis, results from powder X-ray diffraction (XRD) to observe the new phase, soft X-ray absorption spectroscopy (XAS) to investigate the electronic structure, and X-ray photoelectron spectroscopy (XPS) with different energies (900 eV, 2000 eV, and 6000 eV) to track the chemical-state evolution of Fe-SACs cathodes displayed a notable electrochemical reversibility involving S8 ⇄ LiPSs ⇄ Li2S conversion even after 1000 cycles. Additionally, in situ operando Raman analysis can unveil a novel catalytic mechanism of Fe-SACs “facilitating rapid electron transfer” during the discharge and charge processes of LSBs involving the conversion of S8 ⇄ long-chain LiPSs ⇄ Li2S2/Li2S. This study elucidates the working mechanism of Fe-SACs cathodes, offering insights into overcoming the shuttle effect and facilitating sulfur redox kinetics for the advancement of commercial LSBs.