Sergey Sigalov

Electrochemical quartz crystal microbalance (EQCM) studies of ions and solvents insertion into highly porous activated carbons.

Electrochemical quartz crystal microbalance (EQCM) technique provides a direct assessment to the behavior of electroadsorbed ions and solvent molecules confined in micropores of activated carbon electrodes in contact with practically important aprotic electrolyte solutions. The estimated value of the solvation number equal to 3 is evident for a partial desolvation of Li(+) cations when adsorbed in carbon micropores.

Assessing the Solvation Numbers of Electrolytic Ions Confined in Carbon Nanopores under Dynamic Charging Conditions.

We propose herein a new reliable approach to assess solvation numbers of ions confined in carbon nanopores based on dynamic quartz crystal measurements. This was proved for the entire families of alkaline, alkaline-earth cations, and halogen anions. As-assessed hydration numbers appear in the sequence characteristic of a transition from the cosmotropic to a chaotropic-type behavior with the decrease of the ions charge-to-size ratio. The information on the behavior of ions confined in nanometric space of different (especially charged) carbon materials is in high demand for the development of powerful supercapacitors, nanofiltration membranes, and chemical/biochemical sensors.

The Effect of Specific Adsorption of Cations and Their Size on the Charge‐Compensation Mechanism in Carbon Micropores: The Role of Anion Desorption

Combined application of cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) technique reveals a complicated interplay between the adsorption of ammonium and lower molecular weight tetraalkyl ammonium cations and desorption of Cl(-) anions inside carbon micropores at low surface charge densities, which results in failure of their permselectivity. Higher negative surface charge densities induce complete exclusion (desorption) of the Cl(-) co-ions, which imparts purely permselective behavior on the carbon micropores. The second fundamental effect discovered herein relates to the dominant role of anion desorption (as compared to cation adsorption), that is, overwhelming failure of permselectivity extends to high negative charge densities of the electrode in the presence of bulky tetraalkyl ammonium cations, which tend to be confined in the micropores of the carbon. The results obtained are important for advancement of high power density carbon-based supercapacitors, nanofiltration technologies with porous carbon membranes, and studies of ionic transport across biological membranes.

In situ hydrodynamic spectroscopy for structure characterization of porous energy storage electrodes

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.

Non‐Invasive In Situ Dynamic Monitoring of Elastic Properties of Composite Battery Electrodes by EQCM‐D

Reversible Li-ion intercalation into composite Li-ion battery (LIB) electrodes is often accompanied by significant dimensional electrode changes (deformation) resulting in significant deterioration of the cycling performance. Viscoelastic properties of polymeric binders affected by intercalation-induced deformation of composite LIB electrodes have never been probed in situ on operating electrochemical cells. Here, we introduce a newly developed noninvasive method, namely electrochemical quartz-crystal microbalance with dissipation monitoring (EQCM-D), for in situ monitoring of elastic properties of polymeric binders during charging of LIB electrodes. As such, we find EQCM-D as a uniquely suitable tool to track the binders structural rigidity/softness in composite Li insertion electrodes in real-time by the characteristic increase/decrease of the dissipation factor during the charging-discharging process. The binders partially swollen in aprotic solutions demonstrate intermediate viscoelastic charge-rate-dependent behavior, revealing rigid/soft behavior at high/low charging rates, respectively. The method can be adjusted for continuous monitoring of elastic properties of the polymeric binders over the entire LIB electrodes cycling life.

Novel in situ multiharmonic EQCM-D approach to characterize complex carbon pore architectures for capacitive deionization of brackish water.

Multiharmonic analysis by electrochemical quartz-crystal microbalance with dissipation monitoring (EQCM-D) is introduced as an excellent tool for quantitative studying electrosorption of ions from aqueous solution in mesoporous (BP-880) or mixed micro-mesoporous (BP-2000) carbon electrodes. Finding the optimal conditions for gravimetric analysis of the ionic content in the charged carbon electrodes, we propose a novel approach to modeling the charge-dependent gravimetric characteristics by incorporation of Gouy-Chapman-Stern electric double layer model for ions electrosorption into meso- and micro-mesoporous carbon electrodes. All three parameters of the gravimetric equation evaluated by fitting it to the experimental mass changes curves were validated using supplementary nitrogen gas sorption analysis and complementing atomic force microscopy. Important overlap between gravimetric EQCM-D analysis of the ionic content of porous carbon electrodes and the classical capacitive deionization models has been established. The necessity and usefulness of non-gravimetric EQCM-D characterizations of complex carbon architectures, providing insight into their unique viscoelastic behavior and porous structure changes, have been discussed in detail.

In situ real-time gravimetric and viscoelastic probing of surface films formation on lithium batteries electrodes

It is generally accepted that solid–electrolyte interphase formed on the surface of lithium-battery electrodes play a key role in controlling their cycling performance. Although a large variety of surface-sensitive spectroscopies and microscopies were used for their characterization, the focus was on surface species nature rather than on the mechanical properties of the surface films. Here we report a highly sensitive method of gravimetric and viscoelastic probing of the formation of surface films on composite Li4Ti5O12 electrode coupled with lithium ions intercalation into this electrode. Electrochemical quartz-crystal microbalance with dissipation monitoring measurements were performed with LiTFSI, LiPF6, and LiPF6 + 2% vinylene carbonate solutions from which structural parameters of the surface films were returned by fitting to a multilayer viscoelastic model. Only a few fast cycles are required to qualify surface films on Li4Ti5O12 anode improving in the sequence LiPF6 < LiPF6 + 2% vinylene carbonate << LiTFSI.The solid-electrolyte interphase formed on Li-battery electrodes strongly affects their cycling performance, however the mechanical properties of the surface films are not well-known. Here the authors report a sensitive gravimetric/viscoelastic method to probe surface film formation on composite electrodes, coupled with Li-ion intercalation.

Electrochemical quartz crystal admittance studies of ion adsorption on nanoporous composite carbon electrodes in aprotic solutions

This paper describes the application of Electrochemical Quartz Crystal Admittance (EQCA) methodology to the tracking of ion adsorption on composite electrode coatings consisting of highly porous activated carbon particles and polyvinylidene difluoride (PVdF) binder rigidly attached to quartz crystal surfaces. Solutions of LiBF4 and (C2H5)4NBF4 in propylene carbonate (PC) were used in this study. At small charge densities, the effect of frequency change is nearly of gravimetric nature. We propose a new method to determine the mass contribution to the resonance frequency shift due to adsorption of ions and accompanying solvent molecules, revealing different ion/solvent population ratios for Li+, (C2H5)4 N+ and BF4− ions correlated to the ion solvation ability. The EQCA model applied describes the change in the frequency and in resonance peak width in terms of dimensional changes of large carbon particles (bumps) and of pseudo-uniform layers of smaller particles mixed with PVdF. The type of oscillation energy dissipation in composite carbon electrodes with PVdF binder strongly depends on non-uniform potential-induced deformations of electrode particles, and this suggests a strong effect of solvent nature on the mechanical properties of polymeric binders. EQCA may provide important information on the role of polymeric binders during cycling of composite electrodes both for supercapacitors and for Li-ion batteries electrodes.

In Situ Multilength-Scale Tracking of Dimensional and Viscoelastic Changes in Composite Battery Electrodes

Intercalation-induced dimensional changes in a composite battery electrode (comprising a polymeric binder) are one of the major factors limiting electrode cycling performance. Since electrode performance is expressed by the quantities averaged over its entire surface area (e.g., capacity retention, Faradaic efficiency, rate capability), significant efforts have been made to develop a methodology allowing its facile mechanical diagnostics at the same areal scale. Herein we introduce such a generic methodology for a highly sensitive in situ monitoring of intrinsic mechanical properties of composite battery electrodes. The gravimetric, dimensional, viscoelastic, and adhesive changes in the composite electrodes caused by Li-ions intercalation are assessed noninvasively and in real time by electrochemical quartz-crystal microbalance with dissipation monitoring (EQCM-D). Multiharmonic acoustic waves generated by EQCM-D penetrate into thin porous electrodes comprising either rigid or a soft binder resulting in frequency and dissipation changes quantified by analytical acoustic load impedance models. As a first demonstration, we used a composite LiFePO4 (LFP) electrode containing either polyvinylidene dichloride (PVdF) or Na carboximethyl cellulose (NaCMC) as rigid and viscoelastic binders, respectively, in aqueous electrolytes. The intercalation-induced volume changes of LFP electrode were evaluated from a hydrodynamic correction to the mass effect of the intercalated ions for PVdF, and both components of the effective complex shear modulus (i.e., storage and loss moduli) in case of NaCMC binder have been extracted. The sliding friction coefficients for large particles bound at their bottom to the quartz crystal surface (a measure of the adhesion strength of binders) has also been evaluated. Tracking the mechanical properties of the composite electrodes in different environments and charging/cycling conditions in a self-consistent manner provides all necessary conditions for an optimal selection of the polymeric binders resistant to intercalation-induced volume changes of intercalation particles.

Direct Assessment of Nanoconfined Water in 2D Ti3C2 Electrode Interspaces by a Surface Acoustic Technique

Although significant progress has been achieved in understanding of ion-exchange mechanisms in the new family of 2D transition metal carbides and nitrides known as MXenes, direct gravimetric assessment of water insertion into the MXene interlayer spaces and mesopores has not been reported so far. Concurrently, the latest research on MXene and Birnessite electrodes shows that nanoconfined water dramatically improves their gravimetric capacity and rate capability. Hence, quantification of the amount of confined water in solvated electrodes is becoming an important goal of energy-related research. Using the recently developed and highly sensitive method of in situ hydrodynamic spectroscopy (based on surface-acoustic probing of solvated interfaces), we provide clear evidence that typical cosmotropic cations (Li+, Mg2+, and Al3+) are inserted into the MXene interspaces in their partially hydrated form, in contrast to the insertion of chaotropic cations (Cs+ and TEA+), which effectively dehydrate the MXene. These new findings provide important information about the charge-storage mechanisms in layered materials by direct quantification and efficient control (management) over the amount of confined fluid in a variety of solvated battery/supercapacitor electrodes. We believe that the proposed monitoring of water content as a function of the nature of ions can be equally applied to solvated biointerfaces, such as the ion channels of membrane proteins.