  LaNiO$_{3}$-BaTiO$_{3}$ superlattices with different types of interfaces are
studied from first-principles density-functional theory. It is revealed that
the ferroelectricity in the superlattice with (NiO$_2$)$^-$/(BaO)$^0$
interfaces is enhanced from that of the superlattice with
(LaO)$^+$/(TiO$_2$)$^0$ interfaces. The origin lies at the polar discontinuity
at the interface, which makes the holes localized within the
(NiO$_2$)$^-$/(BaO)$^0$ interface, but drives a penetration of electrons into
BaTiO$_3$ component near (LaO)$^+$/(TiO$_2$)$^0$ interface. Our calculations
demonstrate an effective avenue to the robust ferroelectricity in BaTiO$_3$
ultrathin films.
