Probing Local Ion Insertion via Electro-Chemo-Mechanical Coupling Responses

Abstract

Electrochemical energy storage devices (e.g., Li-ion batteries and supercapacitors) store energy through electrolytic ion insertion in the electrode. During the ion insertion process, the electrode needs to open up or reorganize its crystallographic structure in order to host the ions, which results in macroscopic and microscopic deformation. The amount of energy barrier required for electrode structural change during the ion insertion process dictates the energy and power density and cycle life of the device. Therefore, understanding the electrode deformation and changes in mechanical responses during electrochemical events and their impact on devices’ performance 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 operationIn this work, we demonstrate the electro-chemo-mechanical coupling behaviors of proton insertion into WO3 materials via in-situ AFM. The concept of mechanical cyclic voltammetry (mCV) curves was developed, and the relationship between electrochemical current and strain were investigated with simplified models. The results revealed multiple ion-intercalation process 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 corelated to local morphology, crystal orientations or chemical compositions. Besides providing information of redox heterogeneity, the mechanical CV methodology allows us to circumvent the common issues that are often encountered in the global electrochemical characterizations (ex: cell resistance and unwanted parasitic reaction). We further demonstrate that the mCV approach is applicable to a variety of energy storage materials with 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.