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, which 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, probably 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 11 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, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications.

氧化铌多晶型物T -Nb ₂ O ₅以其散装形式已被广泛研究，特别是在快速充电电池和电化学（赝）电容器中的应用。其晶体结构具有空间位阻极低的二维 (2D) 层，可实现锂离子的快速迁移。然而，自1941年被发现以来，单晶薄膜的生长及其电子应用尚未实现，可能是由于其较大的斜方晶胞以及许多多晶型物的存在。在这里，我们演示了单晶T -Nb ₂ O ₅薄膜的外延生长，关键是离子传输通道垂直于薄膜表面。这些垂直的 2D 通道能够实现快速的锂离子迁移，我们证明这会产生巨大的绝缘体-金属转变，其中由于电子填充最初为空的 Nb 4 d 0 态，电阻率下降了 11 个数量级^。此外，我们通过全面的原位实验和理论计算，揭示了在广泛的锂离子浓度范围内具有不同晶体和电子结构的多个未探索的相变，这允许对这些相及其独特的电子特性进行可逆和可重复的操纵。这项工作为探索具有离子通道的新型薄膜及其潜在应用铺平了道路。