Li iontronics in single-crystalline T -Nb 2 O 5 thin films with vertical ionic transport channels

Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 737109. Funding has been provided by the Alexander von Humboldt Foundation in the framework of the Alexander von Humboldt Professorship to S.S.P.P. endowed by the Federal Ministry of Education and Research. The electrochemical theory of Z.J. and A.K. was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under award no. DE-SC0019281. F.N.S. acknowledges funding from the Faraday Institution CATMAT project (FIRG016). The oxide structure and phase transition theory of A.M.S. and A.M.R. was supported by the Office of Naval Research, under grant N00014-20-1-2701. The authors acknowledge computational support from the National Energy Research Scientific Computing Center (NERSC) of the DOE and the High-Performance Computing Modernization Office (HPCMO) of the US Department of Defense (DOD). Use of the APS at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357. We thank C. Guillemard at ALBA synchrotron and J. H. Jin at GERI for their assistance with gating of XANES and TEM samples, respectively.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