  Theoretical studies of penetration of various alkali atoms (Li, Na, Rb, Cs)
throughout graphene membrane grown on silicon carbide substrate are reported
and compared with recent experimental results. Results of first principles
modeling demonstrate rather low (about 0.8 eV) energy barrier for the formation
of temporary defects in carbon layer required for the penetration of Li at high
concentration of adatoms, higher (about 2 eV) barrier for Na, and barriers
above 4 eV for Rb and Cs. Experiments prove migration of lithium adatoms from
graphene surface to the buffer layer and SiC substrate at room temperature,
sodium at 100{\deg}C and impenetrability of graphene membrane for Rb and Cs.
Differences between epitaxial and free standing graphene for the penetration of
alkali ions are also discussed.
