Hidden phases and colossal insulator-metal transition in single-crystalline T-Nb2O5 thin films accessed by lithium intercalation

Fast migration of lithium (Li)-ions in oxide materials is fundamental to the operation of Li-ion batteries. The intercalation of Li-ions into oxides can further lead to emergent electronic property changes. Some of the fastest Li-ion conductors are 4d oxides, and of these, the niobium oxide polymorph T-Nb2O5 is especially interesting with its two-dimensional fast ion migration channels. However, the growth of single-crystalline T-Nb2O5 films is challenging due to its stability over only a limited synthesis temperature window, the existence of many other polymorphs, and its large orthorhombic unit cell. Here, we first demonstrate the epitaxial growth of single domain T-Nb2O5 thin films, critically with the ion channels oriented perpendicular to the film's surface. We show that the insertion of just a small amount of Li using ionic liquids results in conversion of the initially insulating film to a metallic state with a colossal change in resistivity of almost eleven orders of magnitude. In situ experiments, in conjunction with theoretical calculations, reveal a series of transitions between distinct crystal and electronic structures as the lithium content is systematically increased. These include hidden phases that have not previously been identified. Furthermore, replacing the Au electrode with a Li-oxide electrode allows for a significant reduction of the gate voltage at which metallization takes place. Our study opens a new path towards the exploration of hidden phases and the development of novel electrochemically controlled electronic devices.