  As a storage material for Li-ion batteries, graphene/molybdenum disulfide
(Gr/MoS2) composites have been intensively studied in experiments. But the
relevant theoretical works from first-principles are lacking. In the current
work, van-der-Waals-corrected density functional theory calculations are
performed to investigate the interaction of Li in Gr/MoS2 composites. Three
interesting features are revealed for the intercalated Gr/Li(n)/MoS2 composites
(n = 1 to 9). One is the reason for large Li storage capacity of Gr/MoS2: due
to the binding energies per Li atom increase with the increasing number of
intercalated Li atoms. Secondly, the band gap opening of Gr is found, and the
band gap is enlarged with the increasing number of intercalated Li atoms, up to
160 meV with nine Li; hence these results suggest an efficient way to tune the
band gap of graphene. Thirdly, the Dirac cone of Gr always preserve for
different number of ionic bonded Li atoms.
