Single Transition Metal Atom Catalyst for a High-Performance Li–S Battery with a Graphdiyne–Graphene Heterostructure Host: A DFT Investigation + ML Predictions

Shuttling of lithium polysulfides (LiPSs) and slow kinetics of the sulfur reduction reaction (SRR) are considered as the major roadblocks for achieving high-performance lithium–sulfur batteries (LSBs). The solution lies in optimizing the binding strength of LiPSs and catalyzing the SRR. In this work, with the aid of density functional theory calculations, ab initio molecular dynamics simulation, and machine learning (ML), we show that a heterostructure made out of graphene (Gra) and transition metal (TM) atom-anchored graphdiyne (GDY) effectively addresses both these issues. Our results show that the large triangular pores of GDY allow easy penetration of Li+ ions into the sulfur-intercalated TM-GDY/Gra heterostructures and result in LiPSs. The sparsely distributed TM atoms on the GDY surface tune the binding strength of LiPSs and act as catalysts for SRR. Based on the binding strength of LiPSs, TM atom catalysts are categorized into strong, moderate, and weak. Gibbs's free-energy calculations reveal that heterostructures with moderate binding strength are best suited for SRR catalytic activity with barriers smaller than ∼0.4 eV. Furthermore, a Li2S decomposition barrier for the charging process is 3 times lower in the moderate class compared to pristine Gra. Feature importance analysis based on a gradient boosting regression ML model shows that the binding strength of LiPSs in the heterostructures is closely related to intrinsic electronic properties of TM and sulfur atoms, i.e., valence electronic configuration of the TM atom, electronegativity ratio of S to TM atom, and ionic radii of TM and S atoms. Furthermore, it also reveals that the energy barriers for the elementary steps of the SRR are related to the difference in the binding strength of LiPSs corresponding to the conversion step. This study elucidates the suitability of moderate binding heterostructures for LSBs; Fe, Co, Mn, and Rh are preferred single-atom catalysts to serve the purpose.