Fast ion adsorption processes in supercapacitors enable quick storage/ delivery of signifi cant amounts of energy, while ion intercalation in battery materials leads to even larger amounts of energy stored, but at substantially lower rates due to diffusional limitations. Intercalation of ions into the recently discovered 2D Ti 3 C 2 T x (MXene) occurs with a very high rate and leads to high capacitance, posing a paradox. Herein, by characterizing the mechanical deformations of MXene electrode materials at various states-ofcharge with a variety of cations (Li, Na, K, Cs, Mg, Ca, Ba, and three tetraalkylammonium cations) during cycling by electrochemical quartz-crystal admittance (EQCA, quartz-crystal microbalance with dissipation monitoring) combined with in situ electronic conductance and electrochemical impedance, light is shone on this paradox. Based on this work, it appears that the capacitive paradox stems from cationic insertion, accompanied by signifi cant deformation of the MXene particles, that occurs so rapidly so as to resemble 2D ion adsorption at solid-liquid interfaces. The latter is greatly facilitated by the presence of water molecules between the MXene sheets.

The effect of surface roughness on the response of the QCM has been considered, both theoretically and experimentally. A new theoretical approach to the description of the effect of roughness on the response of the QCM is proposed that accounts for the multiscale nature of roughness. Performing experiments in liquids having a wide range of viscosity and density made it possible to understand, for the first time, what characteristics of roughness influence the QCM experiments. The most important conclusion of the current study is that, to understand the experimental data, one has to take into account at least two types of roughness: slight and strong. We found that measurements of the frequency shift observed are not sufficient for the interpretations of the experimental data observed, and a full analysis of the impedance spectrum is called for.

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The state-of-the-art in supercapacitors and capacitive deionization technology utilizes high surface area carbons which store energy in electric double layers (EDLs) at the carbon surface/electrolyte solution interface. To reach high energy density, a fundamental understanding of the nature of ion and solvent adsorption into their porous volume is required. We review herein a novel approach to in situ gravimetric monitoring of the compositional changes in porous carbon electrodes during their charging using the electrochemical quartz crystal admittance (EQCA) method. In addition to gravimetric sensing, mechanical changes in the electrode coating under different charge/discharge/storage regimes can be monitored with high precision and sensitivity. Adsorption of ions in charged meso- and microporous carbons studied by EQCA was interpreted by linking them to the classical EDL models and capacitive deionization theory. Focus is provided for application of EQCA in characterizing adsorption of different series of ions, including determination of the number of solvent molecules accompanying ions during their adsorption and monitoring the molecular interactions between the adsorbed ions and bulk water.

Quartz crystal microbalance with dissipation monitoring (QCM-D) generates surface-acoustic waves in quartz crystal plates that can effectively probe the structure of films, particulate composite electrodes of complex geometry rigidly attached to quartz crystal surface on one side and contacting a gas or liquid phase on the other side. The output QCM-D characteristics consist of the resonance frequency (MHz frequency range) and resonance bandwidth measured with extra-ordinary precision of a few tenths of Hz. Depending on the electrodes stiffness/softness, QCM-D operates either as a gravimetric or complex mechanical probe of their intrinsic structure. For at least 20 years, QCM-D has been successfully used in biochemical and environmental science and technology for its ability to probe the structure of soft solvated interfaces. Practical battery and supercapacitor electrodes appear frequently as porous solids with their stiffness changing due to interactions with electrolyte solutions or as a result of ion intercalation/adsorption and long-term electrode cycling. Unfortunately, most QCM measurements with electrochemical systems are carried out based on a single (fundamental) frequency and, as such, provided that the resonance bandwidth remains constant, are suitable for only gravimetric sensing. The multiharmonic measurements have been carried out mainly on conducting/redox polymer films rather than on typical composite battery/supercapacitor electrodes. Here, we summarize the most recent publications devoted to the development of electrochemical QCM-D (EQCM-D)-based methodology for systematic characterization of mechanical properties of operating battery/supercapacitor electrodes. By varying the electrodes' composition and structure (thin/thick layers, small/large particles, binders with different mechanical properties, etc.), nature of the electrolyte solutions and charging/cycling conditions, the method is shown to be operated in different application modes. A variety of useful electrode-material properties are assessed noninvasively, in situ, and in real time frames of ion intercalation into the electrodes of interest. A detailed algorithm for the mechanical characterization of battery electrodes kept in the gas phase and immersed into the electrolyte solutions has been developed for fast recognition of stiff and viscoelastic materials in terms of EQCM-D signatures treated by the hydrodynamic and viscoelastic models. Working examples of the use of in situ hydrodynamic spectroscopy to characterize stiff rough/porous solids of complex geometry and viscoelastic characterization of soft electrodes are presented. The most demonstrative example relates to the formation of solid electrolyte interphase on LiTiO electrodes in the presence of different electrolyte solutions and additives: only a few cycles (an experiment during ∼30 min) were required for screening the electrolyte systems for their ability to form high-quality surface films in experimental EQCM-D cells as compared to 100 cycles (200 h cycling) in conventi...

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A primary atomic-scale effect accompanying Li-ion insertion into rechargeable battery electrodes is a significant intercalation-induced change of the unit cell volume of the crystalline material. This generates a variety of secondary multiscale dimensional changes and causes a deterioration in the energy storage performance stability. Although traditional in situ height-sensing techniques (atomic force microscopy or electrochemical dilatometry) are able to sense electrode thickness changes at a nanometre scale, they are much less informative concerning intercalation-induced changes of the porous electrode structure at a mesoscopic scale. Based on a electrochemical quartz-crystal microbalance with dissipation monitoring on multiple overtone orders, herein we introduce an in situ hydrodynamic spectroscopic method for porous electrode structure characterization. This new method will enable future developments and applications in the fields of battery and supercapacitor research, especially for diagnostics of viscoelastic properties of binders for composite electrodes and probing the micromechanical stability of their internal electrode porous structure and interfaces.