Abstract
Electrochemical strain microscopy (ESM) has been developed with the aim of measuring Vegard strains in mixed ionic-electronic conductors (MIECs), such as electrode materials for Li-ion batteries, caused by local changes in the chemical composition. In this technique, a voltage-biased AFM tip is used in contact resonance mode. However, extracting quantitative strain information from ESM experiments is highly challenging due to the complexity of the signal generation process. In particular, electrostatic interactions between tip and sample contribute significantly to the measured ESM signals, and the separation of Vegard strain-induced signal contributions from electrostatically induced signal contributions is by no means a trivial task. Recently, we have published a compensation method for eliminating frequency-independent electrostatic contributions in ESM measurements. Here, we demonstrate the potential of this method for detecting Vegard strain in MIECs by choosing Cu_2Mo_6S_8 as a model-type MIEC with an exceptionally high Cu chemical diffusion coefficient. Even for this material, Vegard strains are only measurable around and above room-temperature and with proper elimination of electrostatics. The analyis of the measured Vegards strains gives strong indication that due to a high charge transfer resistance at the tip/interface, the local Cu concentration variations are much smaller than predicted by the local Nernst equation. This suggests that charge transfer resistances have to be analyzed in more detail in future ESM studies.
Highlights
The growing efforts towards ecological sustainability put an ever increasing focus on the problem of energy storage, which is e.g. crucial to replace fossil fuels by renewable energies in the automotive sector
In order to eliminate the influence of electrostatics and contact mechanical signal contributions without loosing resonance enhancement, we recently proposed a new compensation method based on band excitation, which exploits the distinct frequency dependences of Vegard strain and electrostatic contributions[47]
All electrochemical strain microscopy (ESM) measurements were performed on Cu2Mo6S8 using an atomic force microscope under ultrahigh vacuum conditions, where the sample temperature T was systematically varied between 200 K and 400 K
Summary
The growing efforts towards ecological sustainability put an ever increasing focus on the problem of energy storage, which is e.g. crucial to replace fossil fuels by renewable energies in the automotive sector In this context, the optimization of electrochemical systems for energy storage does involve the application of materials with optimized chemical compositions[1], but recent analysis has highlighted the potential impact of the materials nanostructures, with e.g. grain boundaries or interface regimes showing improved transport p roperties[2,3,4,5,6,7,8,9]. For the local detection of such Vegard strains, the so-called electrochemical strain microscopy (ESM) technique has been d eveloped[28,29] In this technique, an ac voltage bias applied to the tip at the contact resonance frequency leads to local chemical composition changes underneath the tip and to a local Vegard strain (see Fig. 1). The electrostatically induced ESM signals were considered to be even the dominant s ignals[38,39], and were, for instance, recently exploited for the analysis of local chemical distributions in solid state e lectrolytes[40]
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