Alkali metal has predicted to be a promising candidate for decorating silicene surface to obtain the high hydrogen storage capacity, owing to their physical properties of lightweight, lower cohesive energy, and appropriate strength of the interaction with H_2 molecules. However, though the high potential in hydrogen storage of alkali metal adatoms-decorated silicene under the fixed adatom adsorption component is well known, the evidence for the hydrogen storage capacity of alkali metal adatoms-decorated sil...