  The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its
bulk form especially for applications in fast-charging batteries and
electrochemical (pseudo)capacitors. Its crystal structure that has
two-dimensional (2D) layers with very low steric hindrance allows for fast
Li-ion migration. However, since its discovery in 1941, the growth of
single-crystalline thin films and its electronic applications have not yet been
realized, likely due to its large orthorhombic unit cell along with the
existence of many polymorphs. Here we demonstrate the epitaxial growth of
single-crystalline T-Nb2O5 thin films, critically with the ionic transport
channels oriented perpendicular to the film's surface. These vertical 2D
channels enable fast Li-ion migration which we show gives rise to a colossal
insulator-metal transition where the resistivity drops by eleven orders of
magnitude due to the population of the initially empty Nb 4d0 states by
electrons. Moreover, we reveal multiple unexplored phase transitions with
distinct crystal and electronic structures over a wide range of Li-ion
concentrations by comprehensive in situ experiments and theoretical
calculations, that allow for the reversible and repeatable manipulation of
these phases and their distinct electronic properties. This work paves the way
to the exploration of novel thin films with ionic channels and their potential
applications.
