Understanding electrolyte salt chemistry for advanced potassium storage performances of transition-metal sulfides

Molybdenum disulfide/carbon nanotubes assembled by ultrathin nanosheets are synthesized to illustrate the electrolyte salt chemistry via potassium bis-(fluorosulfonyl)imide (KFSI) versus potassium hexafluorophosphate (KPF6). Compared to the case of KPF6, the electrochemical performances using KFSI as the electrolyte salt are greatly improved: ~275 mAh g−1 after 15,000 cycles at 1 A g−1, or ~172 mAh g−1 even at 40 A g−1. These results represent one of the best performances for the reported anode materials. The enhanced performances could be attributed to the FSI-induced changes in the solvate structures, that is, a large solvation energy, a high lowest unoccupied molecular orbital, and a small bonding dissociation energy of S–F. In this case, a uniform and robust solid–electrolyte interphase (SEI) is produced, improving the mechanical properties and the interface integrity. Then, the uncontrollable fracture and repeated growth of SEI, which always lead to the dissolution of sulfur species and the blockage of charge transfer in the case of KPF6, are well inhibited. This similar enhancement works for other sulfides by KFSI, demonstrating the general importance of this electrolyte salt chemistry.