  Conversion reaction is one of the most important chemical processes in energy
storage such as lithium ion batteries. While it is generally assumed that the
conversion reaction is initiated by ion intercalation into the electrode
material, solid evidence of intercalation and the subsequent transition
mechanism to conversion remain elusive. Here, using well-defined WO3 single
crystalline thin films grown on Nb doped SrTiO3(001) as a model electrode, we
elucidate the conversion reaction mechanisms during Li+, Na+ and Ca2+ insertion
into WO3 by in situ transmission electron microscopy studies. Intercalation
reactions are explicitly revealed for all ion insertions. With corroboration
from first principle molecular simulations, it is found that, beyond
intercalation, ion-oxygen bonding destabilize the W framework, which gradually
collapses to pseudo-amorphous structure. In addition, we show the interfacial
tensile strain imposed by the SrTiO3 substrate can preserve the structure of an
ultra-thin layer of WO3, offering a possible engineering solution to improve
the cyclability of electrode materials. This study provides a detailed
atomistic picture on the conversion-type electrodes in secondary ion batteries.
