Fe-single atom catalysts facilitate fast electron transfer with MoS2/SnS2 cathodes in lithium–sulfur batteries

Abstract

Lithium–sulfur 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 catalyst (SAC) based combination materials 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/SnS_2. 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 0.2C, maintaining a capacity close to 500 mA h g⁻¹, the highest initial discharge capacity of 1622 mA h g⁻¹ and 1066 mA h g⁻¹ at 0.05C and 0.2C, and excellent rate capabilities of 708 mA h g⁻¹ and 558 mA h g⁻¹ at 1C and 2C, respectively. The synergistic effect of the Fe-SAC-based combination 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 hard X-ray photoelectron microscopy (HAXPES) with different energies (900 eV, 2000 eV, and 6000 eV) to track the chemical-state evolution of Fe-SACs in MoS₂/SnS₂ cathodes displayed 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 in MoS₂/SnS₂ “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-SAC cathodes, offering insights into overcoming the shuttle effect and facilitating sulfur redox kinetics to advance commercial LSBs.