Highly stable rare earth YS₂ and ScS₂ monolayers for potassium-ion batteries: first-principles calculations

Most currently reported anode materials for potassium-ion batteries (KIBs) face a significant trade-off between high potassium capacity and stability, limiting their practical applications. It is widely recognized that reversible potassium intercalation during potassiation and depotassiation processes offers a promising approach to achieving long-term cycle stability. In this study, we conducted a comprehensive investigation of the MS₂ monolayer family as anodes for K-ion batteries, utilizing density functional theory (DFT) computations. We demonstrated that the lowest unoccupied states (E_LUS) of MS₂ monolayers can serve as a simple yet effective descriptor for evaluating potassium adsorption ability. It was revealed that a lower E_LUS of the material can lead to more energetically favourable electron occupation, resulting in stronger K adsorption. The proposed potassiation mechanisms were largely dependent on the delicate competition between the K-MS₂ interaction (E_ads) and the M-S bonding interaction (ΔH_f) within the MS₂ structure. Our computations indicated that most of the MS₂ monolayers (except for CoS₂, NiS₂, and PdS₂) could suppress the conversion reaction after K-ion insertion owing to the less electrovalent K-S bond. By evaluating the theoretical capacities, diffusion barriers, and electronic characteristics, the rare earth Sc- and Y-mediated MS₂ monolayers were identified as the most promising intercalation candidates for KIB anodes with maximum theoretical capacities of 509.28 and 461.09 mA h g^-1 and exceptionally low ion diffusion barriers of 0.11 eV and 0.12 eV, respectively. This study provides an effective strategy for designing stable and high-performance electrodes for potassium-ion batteries, thereby advancing the development of next-generation energy storage systems.