  Angle-resolved photoemission spectroscopy is used to study the band-structure
of superconducting electrochemically intercalated Cu$_x$Bi$_2$Se$_3$. We find
that in these samples the band-gap at the $\Gamma$ point is much larger than in
pristine Bi$_2$Se$_3$. Comparison to the results of band-structure calculations
indicates that the origin of this large gap is internal stress caused by
disorder created by the Cu intercalation. We suggest that the internal pressure
may be necessary for superconductivity in Cu$_x$Bi$_2$Se$_3$.
