In situ TEM investigation of hexagonal WO3 irreversible transformation to Li2WO4

Lithium-ion migration at the WO3/electrolyte interface is governed by the phase transformation mechanism by which metastable states are replaced from one phase to another. Herein, an in situ single nanowire-based cell is constructed to investigate the dynamic phase transformation and morphology evolution of h-WO3 nanowire in real-time during its deep lithiation. One of the most significant features of h-WO3 lithiation is the irreversible transformation from h-WO3 to Li2WO4. There are a large number of deep ion-trapping sites composed of 4 O atoms in Li2WO4 crystal structure, that is, inserted lithium ions are irreversibly bound in these traps and nearly cannot be deintercalated. A mechanism on irreversible WO3-to-Li2WO4 transformation in deep lithiation reaction is demonstrated. With the aid of in situ transmission electron microscopy, we controllably triggered lithium-ion migration at the WO3/electrolyte interface and directly visualized the nanoscale lithium-ion migration as a stop motion movie.