Unique Material Characterizations of Energy Storage and Conversion Electrodes Using Acoustic Spectroscopy by EQCM-D (Electrochemical Quartz Crystal Microbalance with Dissipation Monitoring)

Depending on a variety of factors (electrode thickness, morphology, mechanical properties such as stiffness/compliance), the quartz crystal resonator coated with thin electrode coatings oscillates in a typically MHz frequency range on multiple harmonics operating in either gravimetric or beyond-the-gravimetric modes. The latter mode provides important real time information about the mechanical properties of the electrodes during their charging/discharging. Starting from a simple gravimetric monitoring of dynamics of ions adsorption into nanoporous carbon supercapacitors, 1 we have further proved that the use of multiharmonic Electrochemical Quartz Crystal Microbalance with Dissipation Monitoring (EQCM-D) provides the basis for the new powerful in situ method of highly sensitive monitoring of intercalation-induced dimensional and porous structure changes in operating battery 2 and supercapacitor 3 electrodes. The resonance frequency and dissipation factor changes caused by contact of a porous solid electrode with electrolyte solutions are recorded on multiple harmonics and fitted to a suitable hydrodynamic impedance model returning structural parameters of the electrode. In the second step, this new methodology was extended to a continuous monitoring of gravimetric, dimensional and viscoelastic changes of binder-free 2D electrodes such as Mxene (Ti3C2(OH)x) caused by insertion of water molecules modulated by intercalation-deintercalation of Li-ions in aqueous solutions. 4-7 Monitoring viscoelastic properties of solid-electrolyte interface (SEI) formed on the surface of a high-voltage anode such as LTO was shown to be an extremely effective means for fast screening of electrolyte solutions to optimize the cycling behavior of this electrode. 8 Our recent paper made focus on in situ acoustic diagnostics of particle-binder interactions in battery electrodes: the accommodation of intercalation-induced volume changes significantly depends on the stiffness/softness of the binder used, 9 on one hand, and on the extent to which the size of the guest cation matches the size of the host accommodation sites. 10 The different worked examples of the successful use of EQCM-D-based surface-acoustic-wave spectroscopy for material characterization of energy storage electrodes have been summarized in recent reviews. 11,12 More examples of innovative methods of material characterizations by QCM-D will be presented in the talk.