Understanding edge effect for Li atom insertion: Nano layered transition metal dichalcogenides

Although exposing more edges by nanostructure engineering has been one of main ways to increase Li diffusion channels and shorten Li diffusion paths for layered transition metal dichalcogenides (TMDs) as anode material of lithium-ion batteries (LIBs), few studies have focused on the influence of layered TMDs edges on Li insertion. By density functional theory (DFT) calculations, we investigated the edge effect of layered TMDs on Li insertion. The Li insertion diffusion barriers of 2H(c)-MoS2, 1T'-MoS2, 2H(a)-NbS2 and 1T-TiS2 flush edge indicated only the edge reconstruction of 2H(c)-MoS2 flush edge hinders Li insertion and others almost have no hindrance. Through analyzing the Li binding energies and electronic structure along the Li insertion pathways, we found the edge reconstruction increases the electronegativity of edge S atom for 2H(c)-MoS2 resulting in the insertion obstruction. One criterion was established to efficiently evaluate Li insertion obstruction for 25 kinds of layered TMDs with flush edge including 2H(c), 2H(a) and 1T phase structures. It was found that almost layered TMDs with Group-VIB transition metal exhibit Li insertion obstruction. Considering the actual edge structure and the factor causing the hindrance, we studied the edge effect of the step and S-defect contained edge on Li insertion to explore the methods for decreasing or eliminating Li insertion obstruction in view of atomic edge structure. Our results provide new insights toward the influence of layered TMDs edge on Li insertion.