  Infinite-layer nickelate thin films materialize an intriguing new platform
for high-temperature unconventional superconductivity, with LaNiO$_2$/SrTiO$_3$
as reference setup. We discuss the relative stability of the elementary
interfaces of this system and determine the corresponding electronic band
structure. We find substantial changes compared to the bulk, in particular in
relation to the 5$d$ orbital contributions to the low-energy physics which can
be totally replaced by purely Ni-3$d$ flat bands. The $d^9$ configuration
characteristic of cuprates can thus be supplemented by an extra interfacial
ingredient destabilizing the normal non-superconducting state in these
heterostructures.
