Among different electrochemical energy storage devices, the ones that store a higher amount of energy (e.g., batteries) often undergo larger ion insertion-induced structural transformation and volume change, which results in lower rate performance. The ones that store a lower amount of energy (e.g., supercapacitors) usually show gradual structural change and smaller volume change. There is a strong correlation between electrochemistry and mechanical properties. Understanding the electro-chemo-mechanical coupling and controlling the ion insertion-induced strain is crucial for achieving high power and high energy storage devices. In-situ atomic force microscopy (AFM) is well suited to tackle this task as it allows tracking local volume changes and other mechanical responses sub-nanometer spatial resolution under the conditions close to the device operation

In this work, we monitored electrode volume change via in-situ AFM and demonstrated the electro-chemo-mechanical coupling behaviors during proton insertion into WO₃ materials. The concept of mechanical cyclic voltammetry (mCV) curves was developed, and the relationship between electrochemical current and strain was investigated with simplified models. The results revealed multiple ion-intercalation processes with different mechanical responses are involved during electrode cycling. Local heterogeneity was investigated via mCV mapping, confirming that the charging mechanisms varied across the electrode. These local variations could be further correlated to local morphology, crystal orientations, or chemical compositions. We further demonstrate that the mCV approach is applicable to a variety of energy storage materials (e.g., birnessite and MXene) with the increasing complexity of current-deformation relationships.

The work was supported by the Fluid Interface Reactions, Structures, and Transport (FIRST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.