A tandem catalysis to accelerate solid–state sulfur conversion in lithium–sulfur batteries

Single-atom catalysts (SACs) have emerged as an effective approach to improve the utilization efficiency of sulfur cathodes in lithium–sulfur (Li–S) batteries, owing to their unique atomic-scale catalytic interfaces. However, achieving fast reaction kinetics for both the liquid–solid and solid–solid conversion of lithium polysulfides (LiPSs) demands a tandem catalysis mechanism with multiple active sites, which remains a major challenge. Herein, we report the first example of an asymmetric Ta–N₅ single-atom catalyst (denoted Ta SAs/NC) with dual active sites engineered to trigger tandem catalysis and thereby promote the stepwise conversion of LiPSs. The asymmetric Ta–N₅ coordination environment induces electron redistribution, thereby intensifying d–p orbital hybridization between the Ta 5d and S 3p orbitals. In this tandem catalytic process, the Ta–S bond predominantly accelerates the liquid–solid conversion of LiPSs, whereas the N5–Li (N5 represents the fifth N atom in Ta SAs/NC) bond serves as the principal active site for the subsequent solid–solid conversion. Li–S batteries with the Ta SAs/NC interlayer exhibit a high capacity retention of 90.42 % after 500 cycles at 7 C. In addition, the cells with a high sulfur loading of 5.03 mg cm⁻² deliver an exceptional initial areal capacity of 5.72 mAh cm⁻² and maintain 5.01 mAh cm⁻² after 30 cycles. Moreover, the pouch cell based on this interlayer exhibits excellent capacity decay of 0.21 % per cycle after 200 cycles.