Copper-Doping-Induced Lattice Distortion in CoS ₂ Nanocages for Enhanced Polysulfide Conversion in Advanced Li–S Batteries

In lithium–sulfur batteries (LSBs), sulfur conversion involves a multistep process encompassing solid–liquid (S8 to Li2S8), liquid–liquid (Li2S8 to Li2Sx, 8 > x > 4), liquid–solid (Li2Sx to Li2S2), and solid–solid (Li2S2 to Li2S) transformations. Among these, the solid–solid phase transition represents the slowest step in the complex sulfur reduction reaction (SRR). In this study, a copper-doped CoS2 composite embedded in an ultrathin nitrogen-doped carbon layer (Cu-CoS2@NC) is designed to accelerate polysulfide conversion kinetics in lithium–sulfur batteries (LSBs). Batteries assembled with a Cu-CoS2@NC-coated commercial polypropylene separator (Cu-CoS2@NC//PP) as a functional interlayer demonstrate excellent rate performance (754 mAh g–1 at 5.0C) and long-term cycling stability with an ultralow capacity fading rate of only 0.079% per cycle over 500 cycles at 1.0C. Even under high sulfur loading (4.99 mg cm–2) and low current density (0.1C), a high initial capacity of 886.25 mAh g–1 is achieved. The excellent performance of Cu-CoS2@NC//PP is attributed to Cu-induced lattice distortion in the CoS2 framework, which exposes more active sites and enhances SRR kinetics, while the ultrathin N-doped carbon layer facilitates efficient electron/ion transport and promotes rapid Li+ migration. This work provides atomic-level insight into transition metal doping–regulated SRR kinetics and offers a promising design strategy for the development of high-performance LSBs.