EQCM-D technique for complex mechanical characterization of energy storage electrodes: Background and practical guide

We summarize herein our four years’ experience in application of Electrochemical Quartz Crystal Microbalance with Dissipation Monitoring (EQCM-D) method used to characterize the electrode materials for energy storage and conversion. A special focus of this review is on the fundamental aspects of acoustic probing of electrode films rigidly attached to the surface of a quartz crystal sensor oscillating in thickness shear mode on multiple overtone orders. It is shown that the concept of acoustic load impedance and the related complex frequency change is of key importance to collect and quantitatively analyze diverse information on in situ acoustic properties of real energy storage electrodes. We provide a comprehensive description of the principles of hydrodynamic modeling of acoustic load impedance related to stiff electrodes with complex geometry/morphology (e.g. rough and porous structures), and viscoelastic modeling of the load impedance of the softer electrodes. These models are fitted to the experimental complex frequency changes (i.e. resonance frequency and resonance width changes) such that the fitted parameters present a complete set of gravimetric, electrochemical and mechanical characteristics of the operated electrodes. A practical application of the concept of acoustic load impedance enables to provide the viable solutions to the various problems of electrodes used in energy storage devices. This is demonstrated herein taking as a typical example a new 2D layered material Ti₃C₂ (MXene). A short section of the review is devoted to details of electrode coatings fabrication which allows the operation of the EQCM-D as a universal gravimetric, hydrodynamic and viscoelastic probe of the electrodes state of health under different conditions of charging, cycling and aging.