Atomic Conversion Reaction Mechanism of WO3 in Secondary Ion Batteries

TL;DR

This study uses in situ electron microscopy and simulations to reveal the detailed atomistic mechanisms of ion insertion and conversion in WO3 electrodes, providing insights into energy storage reactions.

Contribution

It provides the first direct visualization and understanding of the conversion reaction mechanisms in WO3 during ion insertion, combining experimental and computational approaches.

Findings

Intercalation reactions occur for Li+, Na+, and Ca2+ in WO3.

Ion-oxygen bonding destabilizes the W framework, leading to collapse.

Interfacial strain can preserve WO3 structure, improving cyclability.

Abstract

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.