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 (S 8 to Li 2 S 8 ), liquid–liquid (Li 2 S 8 to Li 2 S x, 8 > x > 4), liquid–solid (Li 2 S x to Li 2 S 2 ), and solid–solid (Li 2 S 2 to Li 2 S) 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 CoS 2 composite embedded in an ultrathin nitrogen-doped carbon layer (Cu-CoS 2 @NC) is designed to accelerate polysulfide conversion kinetics in lithium–sulfur batteries (LSBs). Batteries assembled with a Cu-CoS 2 @NC-coated commercial polypropylene separator (Cu-CoS 2 @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-CoS 2 @NC//PP is attributed to Cu-induced lattice distortion in the CoS 2 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.