Epitaxial NaxCoO2 Thin Films via Molecular-Beam Epitaxy and Topotactic Transformation: a Model System for Sodium Intercalation

Renewable energy sources such as solar and wind are critical to combatting global warming. Nevertheless, their intermittent energy generation requires the development of large-scale grid energy storage, in contrast to the on-demand generation of coal-based power plants. Sodium-ion batteries offer a promising potential technology, yet because sodium ions are larger than lithium ions, sodium-ion intercalation results in more drastic structural rearrangements. An improved understanding of structural dynamics and ionic diffusion pathways is crucial to developing more durable sodium-ion batteries. Here we synthesize epitaxial NaxCoO2 by using molecular-beam epitaxy and topotactic transformation. In the synthesized epitaxial films, the CoO2 layers are canted with respect to the film surface, allowing electrochemical extraction of sodium ions, which we confirm via ex-situ x-ray diffraction. We anticipate the epitaxial thin films reported here to enable future operando studies of interfaces, subtle lattice distortions, and microstructure during electrochemical cycling.