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In Situ Monitoring of Gravimetric and Viscoelastic Changes in 2D Intercalation Electrodes Netanel Shpigel,† Maria R. Lukatskaya,‡,# Sergey Sigalov,† Chang E. Ren,‡ Prasant Nayak,† Mikhael D. Levi,*,† Leonid Daikhin,§ Doron Aurbach,*,† and Yury Gogotsi*,‡ †

Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Department of Materials Science and Engineering, and A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, United States § School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat Aviv 69978, Israel ‡

S Supporting Information *

ABSTRACT: Viscoelastic properties of battery electrodes in contact with electrolyte solutions may affect the electrodes’ cycling performance. However, they are not easily assessed by in situ measurements. Herein, we show that an electrochemical quartz-crystal microbalance with dissipation (EQCM-D) enables extraordinary sensitive probing of intrinsic electrodes materials’ properties such as intercalationinduced gravimetric and viscoelastic changes, using Ti3C2(OH)2 (MXene) as a classical 2D intercalation model material. The insertion of each Li-ion into thin electrodes comprising this MXene is accompanied by insertion of one water molecule. Solvent-dependent viscoelastic changes and periodic stiffening/softening upon fully reversible Li-ion intercalation/deintercalation into an MXene electrode correlates well with its excellent long-term cycling performance. The experimental platform based on a commercial instrument, EQCM-D monitoring, and advanced viscoelastic modeling (extended Voight-type model) can be used for in situ real time characterization of intrinsic materials’ properties of practical composite battery electrodes important for a deeper understanding of the factors controlling their cycling performance.

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the MAX phase; for example, the pristine Ti3AlC2 MAX phase (M = Ti, A = Al, X = C) loses all Al layers, acquiring surface functionalities (often ended with O2−, OH−, or F−). The intercalated ions (possibly with accompanying solvent molecules) easily enter the layer-to-layer spacing of Ti3C2(OH)2,16 resulting in intercalation-induced deformation17 and also elastic changes in this electrode.18 When produced by etching in LiF−HCl solutions, Ti3C2(OH)2 behaves like a clay thanks to the intercalated solvated Li+-ions occupying interlayer space.11 Many MXenes, including Ti3C2Tx, where T stands for surface termination, possess metallic electrical conductivity19 and demonstrate promise for energy storage applications,20 in particular, in supercapacitors and Li-ion and Na-ion hybrid capacitors,21 due to their high volumetric capacitance, excellent cyclability (no degradation after >10 000 cycles in aqueous electrolytes), and high-rate capabilities.22−24 Ti3C2(OH)2

dvanced electrochemical energy storage devices are expected to offer high energy and power densities during long-term safe operation.1−4 However, repeated Li-ion insertion/extraction can result in significant dimensional and porous structure changes in cycled battery electrodes.5,6 The developed structural changes may not entirely relax at the end of each cycle, tending to accumulate in the electrode’s bulk, eventually resulting in mechanical failure.6 One of the problems of great fundamental and practical importance, but almost untouched in present Li-battery research, is the role of electrode materials’ viscoelasticity in their long-term cycling performance. Among the functional materials widely used in energy storage devices (batteries and supercapacitors) whose elastic properties have been characterized are anisotropic two-dimensional (2D) materials like graphene,7 graphene oxide,8 TiS2,9 MoS210 (and other transition metal dichalcogenides). Recently, new 2D transition metal carbides, under the general name MXenes, where M stands for a transition metal, such as Ti, V, Nb, Mo, and so forth;11−15 X stands for carbon or nitrogen. The latter are easily fabricated by selective etching of © 2017 American Chemical Society

Received: February 19, 2017 Accepted: May 5, 2017 Published: May 5, 2017 1407

DOI: 10.1021/acsenergylett.7b00133 ACS Energy Lett. 2017, 2, 1407−1415

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to a wide variety of electrode materials and electrolyte solutions. Background of Methodology and Experimental Details. Essential features of the EQCM-D method and measurement protocols are included in sections 1−5 of the Supporting Information (SI). Synthesis and preparation of Ti3C2Tx electrodes and instrumentation are described in SI sections 6 and 7, respectively. Instrumentation. Multiharmonic quartz-crystal measurements using EQCM-D were performed using a Q-Sense E1 module (QCM-D from Bioline Scientific) at various overtone orders from 3rd to 13th (for further details, see SI section 7). Viscoelastic Modeling of a Thin MXene Electrode in Contact with Water and Li2SO4 Aqueous Solutions. The theory of acoustic response of viscoelastic coatings on quartz-crystal surfaces in contact with a liquid phase (e.g., electrolyte solutions) is presented in ref25. Biolin Scientific (Sweden), the company that commercialized QCM-D, supplied this instrument with original software for data presentation and treatment called Qtools (we used version 3.1.25.604). The principle of viscoelastic modeling and the related viscoelastic parameters are considered in the SI section 5.

(MXene) was selected for the present study as a classical electrochemically active 2D intercalation material, with which new methodologies for measuring in situ gravimetric and viscoelastic properties of ion insertion electrodes can be demonstrated, as probes for assessing their reversibility, stability, and possible failure mechanisms.

Development of a highly sensitive QCM-D platform for in situ real time monitoring of gravimetric and viscoelastic changes in energy-storage electrodes during their long-term cycling is demonstrated. MXene electrodes undergo obvious elastic changes upon interaction with ions and solvent molecules. A recent paper describing nanoscale intercalation-induced elastic changes in 2D Ti3C2Tx (i.e., potential-dependent changes in the material’s elastic modulus)18 demonstrated for the first time the power of contact resonance (CR) frequency imaging by atomic force microscopy (CR-AFM). However, in this study, only the elastic (storage) modulus was probed rather than the imaginary component of the complex bulk modulus (i.e., loss modulus). Herein, on the basis of a commercial analytical instrument, an electrochemical quartz-crystal microbalance with dissipation monitoring (EQCM-D from Biolin Scientific, Sweden), we introduce an extraordinary sensitive experimental and theoretical platform for combined in situ monitoring of minute gravimetric and viscoelastic intercalation-induced changes on the order of a few ng/cm2 and a few tens of Pa, respectively. In addition, the viscoelastic state of the electrodes is easily assessed for nonelectrochemical processes, such as for the dry states of the electrodes, and immediately after their immersion into neat solvents and electrolyte solutions under open-circuit conditions (OCVs), so that we can easily distinguish between mere swelling of the electrodes without changing their shear modulus and swelling-induced material’s softening. The developed experimental and theoretical platform according to the authors’ estimation is suitable for highly sensitive gravimetric and viscoelastic characterization of composite electrodes comprising polymeric binders that can soften after contact with electrolyte solutions. However, for a comprehensive presentation of the power of this new methodology, we selected binder-free MXene electrodes, which soften after contact with water and/or aqueous solutions and periodically stiffen/soften during Li-ion insertion/extraction (see Figure 1). Good agreement between periodic potential-dependent viscoelastic changes and long-term cycling performance of the electrode has been found. Although this first demonstration relates to MXene charging in aqueous solutions, the methodology and the related modeling routine can be extended

Highly sensitive intercalation-induced gravimetric and viscoelastic changes in battery electrode materials are documented for thin and thick electrode coatings, respectively. Key Def initions. The absolute resonant frequencies on different overtone orders, n, are designated as f; they are mostly used as normalized quantities, f/n (the fundamental frequency is denoted as f 0). During EQCM-D measurements in air and in solutions, the loading of the crystal due to the adhered viscoelastic layer changes not only f but also the dissipation factor, D, defined as the ratio of the full resonance peak width at half-height, W, and the resonant frequency, f: D = (W/n)/( f/n). Once a neat quartz crystal is loaded by rigidly attached mass in the gas phase or in solution (e.g., by fabricating an electrode coating in air, or by viscous loading when immersed into a solution, or by mass loading due to intercalation of Li-ions), we are interested in changes of f/n and W/n. These changes designated as Δf/n and ΔW/n are referenced to either neat or coated quartz crystals in air for viscoelastic and rigid hydrodynamic modeling, respectively (for a detailed description of the reference states, see our recent review).26 When a porous/rough electrode is considered as rigid (i.e., its elastic shear modulus, G′, is on the order of that of the quartz plate whereas the dissipation factor D is zero or negligibly small), then after its immersing from air (or any other gas phase) into liquid, the experimental changes in Δf/n and ΔW/n originate solely from the hydrodynamic interactions

Figure 1. Scheme for Li-ion insertion/extraction-induced viscoelastic changes in MXene electrodes. 1408

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of the rigid/rough solid host with the liquid phase during the crystal shear oscillation. The natural independent variable of the hydrodynamic problem is the penetration depth, δ, showing decay of the oscillation wave in a viscous liquid in close proximity to the coated quartz crystal surface (see below). Thus, we are interested in recording and modeling of Δf/n and ΔW/n changes as functions of δ, which retrieves the structural characteristics of the rigid electrode host or their potentialdependent changes. In the opposite case of viscoelastic electrode coatings, the reference state relates to the neat crystal in air; the independent variable of the viscoelastic problem is the overtone order, n, so that Δf/n and ΔW/n are presented as functions of n and are further modeled with extraction of both components of the complex shear modulus, G*, called storage and loss moduli G′ and G″, respectively, and the density and thickness of the solid electrode coating, ρS and h, respectively. Rigid and Viscoelastic Behavior of MXene Electrodes in Air. As is seen from Figure S1, MXene electrodes of different submicrometric thicknesses were used either as thin or thick coatings with respective mass loadings of 21 and 140 μg cm−2; the related effective Sauerbrey’s thicknesses were found to be 70 and 470 nm, respectively (independently checked by AFM; see Figure S2). Rigid behavior of both electrode coatings in air is evidenced by the overtone-order-independent frequency change caused by the rigidly attached mass of the electrode and by relatively small absolute values of the dissipation factor, D, comparable to the intrinsically low value of D for the uncoated Au crystal. In this case, the Sauerbrey’s equation transforms the frequency change (Δf/n) on any overtone order, n, into the related surface mass density change, Δm = −C · Δf/n,27 where C = (Zq/(2f 02) is the mass sensitivity constant (f 0 is fundamental frequency, and Zq is the acoustic wave impedance of AT-cut quartz equal to 8.8 × 105 g/(cm2s−1); for a 5 MHz crystal, C = 17.7 ng/(cm2Hz). Characterization of MXene Electrodes in Neat Solvents. From the change in Δf/n and ΔW/n as functions of n (when the coated crystal is immersed from the gas phase into liquid), one can easily distinguish whether the electrode is rigid or soft in liquid. According to the in situ hydrodynamic spectroscopy principle, all rigid coatings (independent of their porous structure) demonstrate a monotonic increase of Δf/n as the overtone order, n, increases or, equivalently, as the penetration depth, δ, decreases.28−31 The penetration depth reflects the exponential decay of the velocity amplitude of the movable liquid due to dissipation of oscillation energy in the viscous liquid’s interior. It depends on the dynamic viscosity and density of the liquid, ηL and ρL, respectively, and on the overtone order, n: δ = (ηL /πnfoρL)1/2.28,29,32 In the case of the ideally flat surface of a crystal or a nonporous electrode coating, the changes in Δf/n and ΔW/n (after immersion of the crystal from air into liquid) are linear functions of δ with the slopes dependent on only the mechanical properties of the quartzcrystal plate (see solid lines in Figure 2 designated as “plane”). Rigid porous coatings result in larger characteristic changes of Δf/n and ΔW/n (thus deviating from the straight lines), as is the case for the MXene electrode in contact with inert solvent hexane (see blue squares) in Figure 2 (the parameters of the porous structure calculated using the equations derived previously in refs 28, 29, 32, and 33 and reproduced for convenience in SI eqs S3 and S4 are listed in the figure’s caption).

Figure 2. QCM-D normalized frequency (top) and resonance width changes (bottom) as a function of penetration depth, δ, for the MXene electrode transferred from air into four neat solvents. Application of a rigid homogeneous porous layer model for the data for hexane (see SI eqs S3 and S4) returns the permeability length, ξ = 10 nm, and the effective thickness of the boundary hydrodynamic layer, h = 45 nm. Ethanol, methanol, and water demonstrate an increase in the electrode viscoelasticity (see the text).

The situation drastically changes when the electrode is brought into contact with methanol, ethanol, and water; the viscoelastic feature as defined as the downward deviation of Δf/n and upward deviation of ΔW/n for higher harmonics (lower values of δ) increases in the order ethanol < methanol < water. The viscoelastic properties of the MXene electrode after its contact with protic solvents presumably originate from hydrogen bonding of the molecules of alcohols and water to O- and OH-terminations of the MXene interspaces.16 In contrast to the situation of the rigid electrode, the viscoelastic electrode coating in contact with liquid is fully characterized by Δf/n and ΔW/n as functions of n such that the characteristic probing length of the oscillation becomes the shear wavelength of sound, λ, in the electrode coating (ranging from several tens to several hundreds of microns depending on the shear modulus and density of the coating and also on the overtone order). This relatively large value of λ in the electrode coating is very beneficial for characterization of practical laterally heterogeneous electrodes because all kinds of nonhomogeneities at a smaller than λ scale (such as micro- and mesopores filled with solution, pieces of binder or carbon black particles in composite electrodes) are accounted for in the form of an effectivemedium theory. The smaller-scale inhomogeneities affect the related effective viscoelastic parameters (such an approach was proved to be useful in colloidal science).34 Note a perfect correspondence between the characteristic scales of effective viscoelastic changes in nonhomogeneous electrodes and areaaveraged characteristics of electrode charging and cycling performance (the latter depends on smaller-scale inhomogeneities). Examples of viscoelastic modeling of MXene electrodes in contact with neat water or an aqueous Li2SO4 solution under OCV and during Li-ion intercalation will be further considered in full detail. Behavior of a 70 nm Thick MXene Electrode (Thin-Layer Regime). Figure 3 shows differential capacitance curves for these thin electrodes. The rectangular shape of the curves reflects 1409

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sound in the electrode coating, λ, scaled by 2π (the argument of the tangent of the viscoelastic equation should be much smaller than 1: (h × 2π)/λ ≪ 1).15 The experimental approach to the thin-layer approximation allows one to understand an important gravimetric aspect of the electrodes used for energy storage (especially battery and supercapacitor electrodes containing solvent molecules):35,36 whether Li-ions are inserted into the electrode effectively in their fully or partially desolvated form. The dashed red line in Figure 3b is calculated from the electrode charge with Mi = 25 (sum of molecular weights of the Li-ion and H2O molecule), implying that insertion of each Li-ion is accompanied by insertion of one water molecule. Viscoelastic Behavior of Thicker MXene Electrodes. In contrast to thin MXene electrodes, the thick electrodes behave strongly nongravimetrically, as is seen from the large n-dependent shifts of both Δf/n and ΔW/n (Figure 4c,d) during prolonged charge/discharge cycling of 310 nm thick MXene electrodes; the CVs are shown as an inset in panel (a), whereas major panels (b)−(d) display charge and Δf/n and ΔW/n changes during 100 consecutive cycles. Panels (e)−(g) show the corresponding changes at around cycle #50 at a higher resolution. The characteristic pattern of their n dependence during Li-ion intercalation (a drastic increase in Δf/n and the corresponding decrease in ΔW/n) implies significant changes in the electrode’s viscoelasticity during ion insertion/extraction. Unexpectedly, the continuous cycling of this thick MXene electrode only improves its electrochemical response, as expressed by the rectangular shape of their CV curves (Figure 4a) along with excellent Coulombic efficiency. The improvement could be due to the retarded impregnation of thick electrodes with the electrolyte solution. The baseline for the total intercalation charge (Figure 4b) and the baselines for Δf/n (Figure 4c) and ΔD (Figure 4d) show negligibly small changes toward the end of cycling, signifying excellent cycling performance of this electrode, which is fully correlated with the accompanying changes of the electrode’s viscoelasticity. In addition, periodic changes of Δf/n and ΔD with potential do not depend on the scan rate even for the thickest MXene electrodes studied (470 nm thick; see SI Figure S3), confirming the absence of dependence of the viscoelastic behavior on the rate of deformation caused by charging/discharging of this MXene electrode. HR SEM images of the fresh and cycled electrodes look almost identical (Figure 5), demonstrating their excellent electrochemical stability. The last important note is that one should not confuse the rates of intercalation-induced deformation (the perturbing stimulus) occurring at frequencies many orders of magnitude smaller than the MHz frequency range of crystal oscillation (at which the electrode’s viscoelasticity is probed); the former are seen by QCM-D as virtually stationary states. This is a great advantage of QCM-D in characterizing intercalation-induced viscoelastic changes in electrode materials in a wide range of charging rates. It is important to note that the data presented above is a typical example of the information that can be obtained by EQCM-D on the intrinsic behavior of most types of electrode materials used in energy storage devices. Such information when treated by appropriate models can provide unique insight about the stability of electrode materials and their possible failure mechanisms. Viscoelastic Modeling and Parameter Validation. Taking into account the effective homogeneity of the MXene electrodes’ coatings (compared to the wavelength of sound in the electrode, λ; see the discussion above), we used an extended

Rigid in the gas phase, the MXene electrode softens after contact with protic solvents (especially with water); an extended viscoelastic model provides a full explanation of the softening effect.

Figure 3. Electrochemical (a) and EQCM-D (b) behavior of thin electrodes in 2 M Li2SO4 solution in the gravimetric (Sauerbrey) limit; scan rates are indicated. The time dependence of the associated CV currents (top panel) with simultaneously measured Δf/n (middle panel) and ΔW/n (bottom panel) for thin electrodes at a scan rate of 20 mV/s is shown in (b). The state of charge and discharge as well as the responses on different harmonics is indicated.

purely capacitive-type cyclic voltammetry responses. The perfect symmetry of the curves with respect to the potential axis and practical independence of the capacitance on the scan rate used prove a completely quasi-equilibrium character of charging−discharging of these thin MXene electrodes. The potential-induced shifts of Δf/n and ΔW/n for the same electrode are depicted in Figure 3b, revealing its entirely gravimetric behavior (Δf/n is independent of n, and ΔW/n = 0; in addition, the frequency shift, ΔFtheo due to the mass of intercalated Li-ions was calculated from the combination of the Faraday law, Δm= (Mi × σ)/(zi × Fc) with the Sauerbrey’s equation (here Mi and zi are the molecular mass and ion’s charge, respectively, σ is the electrode charge density, and Fc is the Faraday constant). One should correctly understand the origin of the thin-layer gravimetric limit; actually, water molecules soften this thin MXene electrode; however, the related viscoelastic correction to the mass effect is negligibly small in this limit because the characteristic thickness of the coating, h, is much smaller than the probing wavelength of 1410

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Figure 4. Long-term cycling of a 310 nm thick MXene electrode. (a) CV curves for the 1st and 100th cycles. Panels (b), (c), and (e) show the cumulative intercalation charge, the related Δf/n, and ΔD changes, respectively, for different overtone orders (the first 15 and last 15 cycles are shown). Detailed profiles at the midpoint of the electrode cycling life are shown in panels (e)−(g), respectively. Different overtone orders are indicated.

Figure 5. SEM images of a freshly prepared (a) and cycled (b) MXene electrode. The scale bar is 500 nm.

Voigt-type model25,30,37 to extract viscoelastic parameters of the electrode in its three different states: (i) in contact with pure water, (ii) in contact with 2 M Li2SO4 solution at OCV, and (iii) for the maximal degree of Li-ion insertion into the MXene electrode (for details, see the SI). All of the fitting parameters are listed in a table in the SI, and among them are five viscoelastic parameters: the shear storage and loss moduli, G′ and G″ (the latter was calculated from shear viscosity, ηS, G″ = 2πnfoηS), the electrode thickness, h, and power law exponents for G′ and ηS, that is, β′ and β″. From this table, it is seen that in all three cases G′ ≫ G″, showing an almost frequencyindependent storage modulus, whereas the related loss modulus linearly depends on frequency in double-logarithmic coordinates. Small values of exponents testify that the effect of the viscoelastic dispersion in the MXene electrode is relatively small, which is typical for the predominantly elastic solids. For this reason, the most demonstrative is the trend in the change of the shear storage modulus, G′. A satisfactory fit to the

Li-ion intercalation/deintercalation processes are reflected by periodic stiffening/softening of MXene electrodes, demonstrating their high mechanical stability during long-term cycling in aqueous Li2SO4 solutions along with excellent cycling performance of this electrode. experimental frequency and dissipation factor changes with n for the MXene active mass in contact with water was achieved with G′ = 0.26 GPa (see Figure 6a), implying the electrode’s softening as compared to the rigid state of the electrode in air and in contact with hexane. During subsequent replacement of pure water by a 2 M Li2SO4 solution, G′ increased to 0.32 GPa, 1411

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van der Waals slits, causing their expansion.40 Weakening of the initial hydrogen bonds (ensuring rigid structure in air) causes effective electrode softening. Using the gravimetric thin-layer limit of EQCM-D, we have found that intercalation of Li-ions into MXene is accompanied by insertion of ∼1 molecules of H2O, on average, per each Li-ion. Such a small number of accompanying water molecules means that neighboring Li-ions tend to share all of the available water of the confined space of the van der Waals slit, forming common solvation shells. This makes the electrodes effectively stiffer. An additional source of stiffening could be electrostatic attraction between the negatively charged surfaces of the walls of the van der Waals slits and Li+ cations. State-of-the-Art and Future Opportunities. We have shown herein that the multiharmonic EQCM-D method can be used as a versatile tool that collects important generic information about intercalation-induced mechanical changes in battery electrodes, no matter whether they are stiff or soft. The use of EQCM working on the single fundamental frequency only (so common in energy storage research) is valuable exclusively for truly gravimetric conditions (zero shift of the dissipation factor is the necessary condition). Recently, we used EQCM-D for hydrodynamic spectroscopy (working with 6 odd overtone orders from 3 to 13) to characterize dimensional and porous structure changes in the rigid LiMn2O4 electrode in situ and in real time.32 In the present Perspective, the multiharmonic analysis has been adjusted to probing, with extraordinary high sensitivity of the gravimetric and viscoelastic changes of titanium carbide MXene (Ti3C2Tx) electrodes as a typical example for ion insertion electrode materials used in batteries and supercapacitors. We used simple binder-free MXene electrodes in this proof-of-concept work to demonstrate all necessary steps of material characterization and the model description: in air (or inert gas), in liquids including electrolyte solutions, and during intercalation of ions. This initial demonstration of the use of EQCM-D for characterization of charging of MXene electrodes in aqueous solutions allowed us, in a first demonstrative work, to avoid complicated effects related to SEI formation, viscoelastic behavior of binders (and hence the entire composite electrodes), the possibility of changing the local viscosity of the electrolyte solution due to polymerization of organic additives, gas evolution reactions, and so forth. In this respect, thinking about the complex processes occurring in practical battery electrodes during their charging, one should note the pioneering work carried out at Argonne National Laboratory in which EQCM-D was used to quantitatively characterize SEI formation on Sn and Si electrodes in aprotic solutions of different compositions.41−44 EQCM-D study of the formation of SEI on different battery electrodes will surely become one of the forefronts of advanced Li-battery research. However, the readers of this Perspective article should not be left with the opinion that only EQCM-D provides the assessment of dimensional, porous structure and viscoelastic changes in battery electrodes during their charging; utilization of network analyzers working on multiple harmonics results in essentially similar information.30 In recent work, it was nicely demonstrated that coupling of viscoelastic effects of different electrode hosts with the shallow roughness of their external surface can be rigorously quantified in terms of quartz crystal impedance analysis.45−50 We should emphasize that in order to reach an extraordinary high sensitivity of in situ gravimetric and viscoelastic probing of electrodes for energy storage, only small-mass

Figure 6. Fitting of the viscoelastic model (dashed lines) to the experimental Δf/n and ΔW/n changes (solid circles and squares as indicated) for the MXene electrode in contact with pure water (a) and in contact with 2 M Li2SO4 under OCV and during Li-ion intercalation, respectively (b). All of the fitting parameters are listed in a table in the SI.

indicating that the presence of electrolyte in the aqueous solutions results in MXene’s stiffening (see Figure 6b). Exposure of the MXene electrodes to lithium sulfate solution results in intercalation of Li-ions in between the Ti3C2Tx layers and causes further intercalation-induced stiffening; G′ increases to 0.53 GPa, whereas the electrode thickness, h, decreases from 298 to 291 nm, implying that Li-ion insertion causes MXene contraction (Figure 6b). This conclusion is in good agreement with tracking the MXene electrodes’ deformation caused by Li-ion insertion using in situ AFM17 and XRD measurements.13 In what follows, we are trying to provide a qualitative molecular explanation of the change of the elastic modulus of MXene electrodes in three different states based on the concept of hydrogen bonds in this material. MXenes are 2D solids composed of closely packed single sheets containing surfaceterminal groups like −OH, −O, and −F12−14,38,39 that lead to hydrogen bonding between neighboring MXene layers. When the electrodes are immersed in water, the intercalated layers of water molecules disturb the initial hydrogen bonds in the 1412

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Although this first demonstration relates to new (binder-free) 2D MXene electrodes, there is no practical limitation to extend the developed acoustic methodology described herein to noninvasive in situ tracking of gravimetric and viscoelastic changes in 3D hosts composing battery electrodes together with rigid or viscoelastic binders.

2015 from Drexel University under the supervision of Prof. Gogotsi; her Ph.D. research focused on capacitive performance and understanding the mechanism of charge storage in 2D carbides (MXenes). Sergey Sigalov is a postdoctoral student at Bar-Ilan University guided by Profs. M. D. Levi and D. Aurbach. He received his Ph.D. degree at Bar-Ilan University and continues to work on the use of EQCM-D for characterization of battery and supercapacitor materials. Chang E. Ren received her B.E. in Materials Science and Engineering from Northwestern Polytechnical University in 2012. Chang is currently a fifth year Ph.D. candidate at A.J. Drexel Nanomaterials Institute and the Department of Material Science and Engineering at Drexel University. Her research, under the advisement of Dr. Yury Gogotsi, focuses on developing MXene films for ionically selective applications, specifically water purification and supercapacitors.

(up to 100−120 μg/cm2) samples should be used (QCM smallload approximation30) because heavier electrodes’ coatings suppress the crystal oscillations. Hence, an inevitable price for reaching a high sensitivity in assessing intrinsic (true) active electrode materials’ properties is the necessity to operate with small-mass/thin electrodes. The work is in progress in our laboratory with moderately flooded experimental EQCM-D cells allowing optimization of electrolyte solution compositions for the best cycling performance of various battery electrode materials via amazingly fast assessment of their intrinsic in situ mechanical properties.

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Prasant Nayak is a postdoc guided by Profs. Aurbach and M. D. Levi. He received his Ph.D. degree from Bangalore (India). He specializes in materials synthesis and characterizations. Mikhael D. Levi is a Professor at Bar-Ilan University working in the lab headed by Prof. D. Aurbach. He received his Ph.D. in 1976 under the supervision of Academician Professor A. N. Frumkin at Moscow State University. He specializes in the development of fine electroanalytical methods for characterizations of battery materials. Since 2009, he has been developing a nongravimetric EQCM method adjusted for simultaneous tracking of gravimetric, viscoelastic, and porous structure changes in battery materials.

ASSOCIATED CONTENT

S Supporting Information *

Leonid Daikhin is a Professor at Tel-Aviv University. He is involved in theoretical work related to hydrodynamic and viscoelastic modeling.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00133. Essentials of the EQCM-D method, types of dimensional and porous electrode structure changes in battery electrodes probed by EQCM-D, hydrodynamic modeling of thin rigid MXene electrodes in contact with hexane, EQCM-D measurements in air, EQCM-D measurements in liquids: rigid and viscoelastic behaviors, synthesis and preparation of Ti3C2Tx electrodes, and instrumentation, figures showing frequency and dissipation factors, AFM images, and viscoelastic changes, and a table of the fitting parameters for the curves in Figure 7 (PDF)

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Doron Aurbach is a full professor, senate member, and head of the electrochemistry group at Bar-Ilan University. He leads INREP  Israel National Research center for Electrochemical Propulsion (22 research groups) and is an MRS, ECS, and ISE fellow. He is involved in all aspects of Li, Na, Mg, Li−S, and Li−O2 battery research, development of new electrolyte solutions, surface and materials science, new analytical methodologies, and supercapacitors. Yury Gogotsi is Distinguished University Professor and Trustee Chair of Materials Science and Engineering at Drexel University. He works on nanostructured carbons and two-dimensional carbides and nitrides for energy storage and related applications. He was recognized as a Highly Cited Researcher by Thomson-Reuters in 2014−2016 and was elected a Fellow of AAAS, MRS, ECS, RSC, ACerS, and the World Academy of Ceramics.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.D.L.). *E-mail: [email protected] (D.A.). *E-mail: [email protected] (Y.G.).

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ACKNOWLEDGMENTS The authors acknowledge funding from the Binational Science Foundation (BSF) USA−Israel via Research Grant Agreement 2014083/2016. N.S. thanks the Israel Ministry of Science Technology and Space.

ORCID

Doron Aurbach: 0000-0002-1151-546X Yury Gogotsi: 0000-0001-9423-4032

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Present Address #

M.R.L.: Department of Chemical Engineering, Stanford University, Stanford, CA 94305, U.S.A.

REFERENCES

(1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li-O2 and Li-S batteries with high energy storage. Nat. Mater. 2011, 11, 19−29. (2) Erickson, E. M.; Ghanty, C.; Aurbach, D. New Horizons for Conventional Lithium Ion Battery Technology. J. Phys. Chem. Lett. 2014, 5, 3313−3324. (3) Goodenough, J. B.; Park, K.-S. The Li-ion rechargeable battery: a perspective. J. Am. Chem. Soc. 2013, 135, 1167−1176. (4) Scrosati, B.; Garche, J.; Tillmetz, W. Advances in Battery Technologies for Electric Vehicles; Elsevier Science: London, U.K., 2015. (5) Levi, M. D.; Sigalov, S.; Salitra, G.; Elazari, R.; Aurbach, D.; Daikhin, L.; Presser, V. In situ tracking of ion insertion in iron

Notes

The authors declare no competing financial interest. Biographies Netanel Shpigel is a Ph.D. student at Bar-Ilan University guided by Profs. M. D. Levi and D. Aurbach. He is specialized in advanced applications of EQCM-D and AFM for characterization of energy storage materials. Maria R. Lukatskaya, currently a postdoctoral researcher at Stanford University, is working on electrolyte development as well as capacitance of coordination polymers. She received her Ph.D. in 1413

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