In Situ Multilength-Scale Tracking of Dimensional and Viscoelastic

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In Situ Multilength-Scale Tracking of Dimensional and Viscoelastic Changes in Composite Battery Electrodes Vadim Dargel,† Nicolas Jac̈ kel,‡,§ Netanel Shpigel,† Sergey Sigalov,† Mikhael D. Levi,*,† Leonid Daikhin,∥ Volker Presser,‡,§ and Doron Aurbach*,† †

Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany § Department of Materials Science and Engineering, Saarland University, 66123 Saarbrücken, Germany ∥ School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat Aviv 69978, Israel ‡

S Supporting Information *

ABSTRACT: 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 (EQCMD). 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 intercalationinduced 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. KEYWORDS: QCM-D, EQCM, lithium ion battery, composite electrodes, polymer binders, PVdF, NaCMC

1. INTRODUCTION Polymeric binders (PBs), representing only a small fraction of composite battery electrode total mass, affect their cycling performance keeping mechanical integrity of the electrode during its entire cycling life. Often overlooked aspects like binder properties, concentration, and distribution within the composite electrodes bulk may also have a tremendous impact on the cycling performance.1 In short, the PBs affect the electrode cycling performance in a number of ways such as (i) via the adhesion strength to the neighboring intercalation particles and current collector,2−5 (ii) mechanical resistance to appearance of cracks during its excessive intercalation-induced expansion of the electrode (e.g., inside the intercalation particles and/or within the PBs),6−9 (iii) continuous swelling © 2017 American Chemical Society

of the electrodes when in contact with battery electrolyte solutions,4 and (iv) via the intrinsic viscoelastic properties of the composite electrodes and their intercalation-induced changes during long-term cycling under different cycling conditions.10 One of the most attractive anodes of high specific capacity for Li-ion battery is a composite silicon electrode which experiences large intercalation-induced volume changes and, as a consequence, undergoes rapid deterioration of its cycling performance.5,9,11−16 An extremely high intercalation-induced Received: May 4, 2017 Accepted: August 4, 2017 Published: August 4, 2017 27664

DOI: 10.1021/acsami.7b06243 ACS Appl. Mater. Interfaces 2017, 9, 27664−27675

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ACS Applied Materials & Interfaces volume change of Si intercalation particles (ca. by 350% of the initial value) greatly affects the mechanical properties of the PBs via the related stress−strain relationship imposing constraints on the desirable mechanical properties of the PBs used. In fact, a stiff (in aprotic solutions) binder consisting of Na salt of carboxymethyl cellulose (NaCMC)3,5,14,17−19 and especially alginate (a high-modulus natural polysaccharide obtained from brown algae)13 is believed to resist intercalation-induced fragmentation of the enormously expanded Si electrode host more effectively than the much softer binder comprised of polyvinyl difluoride (PVdF). So-called self-curing PBs maintain even more efficiently the mechanical integrity of the composite Si electrode occurring after its fragmentation.20,21 For intercalation electrodes with a smaller volume change (compared to that of Si electrode), stiff binders are not necessarily more advantageous compared to the soft ones, with the latter better accommodating volume changes of the electrode. Among the most common techniques for PBs characterization in composite battery electrodes, microscratching22,23 is certainly invasive and is usually applied under ex situ conditions. The nanoindentation by atomic force microscopy (AFM), although being suitable for in situ application during electrochemical operation,24 provides a local rather than the area-averaged response linked to such characteristics defining electrode cycling performance as capacity retention, Faradaic efficiency, etc. Hence, the development of noninvasive in situ techniques for the continuous monitoring of stiffness/softness of composite electrodes under different cycling conditions is a challenge for a better understanding of the factors limiting cycling life of energy-storage electrodes. Based on the use of a commercial analytical instrument, multiharmonic electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), we developed herein a general platform for an advanced viscoelastic characterization of thin composite battery electrodes in different environments (gaseous or liquid), in contact with battery electrolyte solutions under open-circuit conditions during storage and applied potential during electrode charging/cycling. The method ensures extremely high sensitivity in detection of intercalation-induced gravimetric, dimensional, and viscoelastic changes. Furthermore, probing of the adhesion strength of PBs to the current collector surface as well as between the intercalation particles is also possible (Figure 1a). We selected a composite LiFePO4 (LFP) electrode with differently sized intercalation

particles because the EQCM-D response is very sensitive to particles size and, more broadly, to the electrode’s geometry and porosity (Figure 1a,b). Further, we studied two PBs with contrastingly different behavior: in aqueous solutions of Li2SO4 polyvinylidene fluoride (PVdF) and sodium carboxymethyl cellulose (NaCMC) they remain completely rigid and soft, respectively. The high sensitivity and selectivity in probing intrinsic gravimetric and mechanical properties of electrode materials having a complex geometric structure is achieved by utilizing (i) a small-load approximation of EQCM-D method (i.e., the measured complex frequency shifts are always much smaller than the resonance frequency itself)25 and (ii) the development of a variety of hydrodynamic26−31 and viscoelastic25,32−38 models which are fitted to the experimental EQCM-D data collected for the electrodes with rigid or soft binders in order to retrieve the geometric and mechanical structural characteristics of the probed electrodes. Our study suggests that an ideal binder for a composite LFP electrode cycled in aqueous solutions should have a moderately large storage (elastic) modulus and small loss modulus to allow full reversible intercalation-induced volume changes of the composite electrode but preventing a viscous-like behavior of the binder deteriorating electronic conductance of the composite electrodes. The developed methodology allows assessing of intrinsic mechanical properties of binders in ionsinsertion battery electrodes and porous electrodes for supercapacitors in contact with various kinds of electrolyte solutions.

Figure 1. Schematics of a combined EQCM-D study of gravimetric, dimensional, viscoelastic, and adhesion properties of thin composite battery electrodes containing large and small intercalation particles (a). Scanning electron micrograph of the LFP electrode containing large agglomerates (bumps) and small particles (b).

3. RESULTS AND DISCUSSION 3.1. Background and Basic Definitions. The EQCM technique has been used successfully in its gravimetric mode for characterization of Li-battery42 and supercapacitor39,41,43−46

2. EXPERIMENTAL SECTION 2.1. Samples Preparation and EQCM-D Characterization. Electrodes for EQCM-D study were prepared by coating of LFP/ PVdF in NMP or LFP/NaCMC in water on 5 MHz Au polished quartz crystals (14 mm diameter, Biolin Scientific) by airbrush method.30,39−41 Multiharmonic EQCM-D measurements were performed with Q-Sense E1 module (QCM-D from Biolin Scientific) using overtone orders from 3 to 13. LiFePO4 (LFP) electrodes containing “small particles” were prepared in two steps. First, LFP powder (battery grade powder purchased from Süd-Chemie) was dispersed and settled as sediment in ethanol. After sedimentation, a supernatant with small particles (average particle size ∼300 nm) was separated and dried on a hot plate. Second, PVdF (HSV 900, Kynar) or NaCMC binder (SigmaAldrich) was added to the LFP powder containing only small particles, and all of the components were dispersed in NMP (Sigma-Aldrich) or double-distilled H2O by sonication. A slurry containing LFP powder (“large particles”) had no sedimentation stage. Together with the binder they were dispersed in a suitable solvent by ultrasonication method and immediately sprayed on the surface of a quartz crystal. A components’ ratio in LFP electrodes containing small and large particles with respect to the binder was 90:10 (by mass). Different LFP-to-PVDF ratios were obtained by the subsequent impregnation of PVdF solution (LFP-to-PVdF ratio 99:1 by mass). 2.2. Electrochemical Characterization. Electrochemical measurements were performed using a potentiostat BioLogic VSP-300. The coated Au-quartz crystal was set as the working electrode in a homemade electrochemical flooded cell with Pt counter electrode and Ag/AgCl/KCl (sat.) reference electrode. The electrolyte solution was aqueous 0.1 M Li2SO4 (Sigma-Aldrich) in double-distilled H2O. A cyclic voltammetry experiment using a 5 mV/s scan rate was applied between +0.45 and −0.15 V vs Ag/AgCl.

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potential,30 the characteristic dependences of Δf/n and ΔW/n as functions of δ reflect the heterogeneous geometric electrode structure or its potential-dependent changes. They are further treated by the use of suitable hydrodynamic impedance models (based on the initially suggested electrode’s geometric structure) to retrieve the structural characteristics of the rigid porous electrode host using the fitting routine. This is the main principle of in situ hydrodynamic spectroscopy of battery electrodes as reported in our recent paper.30 The fitting procedure somewhat resembles determination of equivalent electric circuit parameters from the electrochemical impedance spectra of battery electrodes (for details see our recent review article in ref 29). Figure 1a shows a sketch of the composite LFP electrodes consisting of either small intercalation particles or a mixture of small and large particles, with the use of either rigid or soft binders, namely, PVdF and NaCMC, respectively. From Figure 1a we see that the extent of hydrodynamic interactions reflected by Δf/n and ΔW/n should be different for the two electrodes, depending on the geometry of the electrode. We note that a rough external surface and porous interior part of the electrode create additional viscous contributions to Δf/n and ΔW/n.29 Besides probing dimensional and porous structure changes in composite electrodes containing a rigid binder, the adhesion strength of binders forming a continuous network (in which the intercalation particles are embedded) linked to the quartz crystal surface can be also probed with high sensitivity. As follows from scanning electron micrographs (Figure 1b), larger particles are less covered by binders, and their connection to the quartz crystal surface is consequently weaker. This is due to a nonhomogeneous distribution of the binder in thin electrode layers prepared by spraying of the electrode slurry using an airbrush. A weaker adherence of large intercalation particles to quartz crystal surface results in their sliding with friction across the crystal surface during oscillations (Figure 1a). We propose herein a model describing the experimental changes of Δf/n and ΔW/n accounting for the sliding friction effect. In contrast, small intercalation particles are well embedded into the continuous polymeric network. These two contrastingly different composite electrode layers can be considered as a rigid uniform porous layer and a continuous soft (viscoelastic) layer, respectively. The predominantly rigid or soft-type behavior of small-particles electrodes are easily identified by the characteristic dependences of Δf/n and ΔW/n on δ in the former case, and on the overtone order, n, in the latter case (as presented in detail in this paper). The choice of the independent variable requires some clarification. The parameter δ is the sole natural independent variable of the hydrodynamic problem for rigid porous solids because the velocity profile of the oscillation wave within the rigid electrode host is the same as that in the quartz plate; hence, dissipation of the oscillation energy occurs exclusively across the porous rigid host/contacting liquid interphase. In contrast, in a viscoelastic coating attached to quartz crystal surface on one side and contacting with liquid on the other side, the velocity of the oscillation differs significantly from that in the quartz crystal plate depending in a complicated manner on the viscoelastic parameters of the layer and viscosity/density ratio of the contacting liquid. Hence δ is not the single parameter affecting the values of Δf/n and ΔW/n. A more convenient parameter to solve the viscoelastic problem is the overtone order, n, since the contributions to Δf/ n and ΔW/n from the viscoelastic and liquid layers are both

electrodes, but its use in a much more powerful nongravimetric mode is not so common. The gravimetric EQCM relies on a change of the single output parameter, namely, the resonant frequency, Δf, related to the quartz crystal oscillation on the fundamental frequency, f 0. In the framework of gravimetric EQCM the resonance width peak, W, is constant by definition; that means, it is assumed that the electrochemical processes do not cause any change of dissipation of oscillation energy in the electrode coating in contact with the electrolyte solution. Hence, a large variety of processes occurring in battery and supercapacitor electrodes during their charging such as the intercalation-induced dimensional and viscoelastic changes are obviously beyond the gravimetric EQCM approach. Consequently, understanding of complex mechanical phenomena accompanying intercalation processes in porous composite electrodes result in significant changes in Δf and ΔW obtained on different overtone orders, n, treated by nongravimetric EQCM in terms of acoustic load impedance models.29,30 It is important to note that, when the electrode coating is deposited onto quartz crystal surface, then immersed into electrolyte solution, and finally subjected to external electrode polarization, instead of treating absolute values of Δf and ΔW on different harmonics, it is much more convenient to deal with their normalized changes, Δf/n and ΔW/n, correctly selecting the reference state (Δ stands for the frequency and resonance width changes for a given state of the electrode with respect to that for the selected reference state). For example, characterizing electrode coating by QCM-D in air, the reference state is neat crystal in air. If a thin electrode coating is rigidly attached to quartz crystal surface then the resonance width peak remains constant, and the frequency change due to dry electrode coating, Δf/n, is transformed into the related change of the surface mass density, Δm, using Sauerbrey’s equation:47 Δm = −C Δf /n

(1)

where C = (Zq/(2f 02) is the mass sensitivity constant (f 0 is the fundamental frequency and Zq is the acoustic wave impedance of AT-cut quartz equal to 8.8 × 105 g/(cm2 × s)); for 5 MHz crystal C = 17.7 ng/(cm2Hz). Equation 1 shows that gravimetric determinations do not necessarily require the use of multiple harmonics because Δf/n must be independent of n in this case. The hydrodynamic interaction of the oscillating porous solid with contacting liquid implies that the independent variable of the solution to the hydrodynamic problem is the penetration depth of the transverse oscillation wave, δ, in liquid. This wave is generated by the oscillating quartz crystal and spreads into the contacting liquid experiencing exponential decay of its amplitude with the distance from the crystal surface. The penetration depth depends on the liquid’s dynamic viscosity to density ratio, ηL/ρL, and, most importantly for the electrochemical measurements, on the overtone order, n:26,27,29,30,32,48

δ = (ηL /πnfo ρL )1/2

(2)

For electrochemical characterization of battery electrodes, the liquid’s viscosity and density are fixed parameters due to a selected electrolyte solution; however, δ can be varied by using multiple harmonics during EQCM-D measurements. Depending on the relation between δ and geometric elements of the electrode structure (e.g., pore widths for a uniform porous layer affecting the layer’s permeability length, ξ, when liquid moves across the porous body; ref 48) and their changes with 27666


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Figure 2. Nonprocessed EQCM-D data (plots of Δf/n and ΔD for the overtone orders from 3 to 13 as functions of time after immersing of the coated crystal in aqueous 0.1 M Li2SO4): neat PVdF film and composite LFP electrodes with PVdF containing small and large particles (a, c, and e, respectively); neat NaCMC film and composite LFP electrodes with NaCMC containing small and large particles (b, d, and f, respectively). Dashed arrows show the direction of increasing n. Offsets for f/n and D in liquid are referenced to the values of these quantities for the dry coatings in air.

functions of n. For this reason, the reference state for a viscoelastic film in contact with a liquid is neat uncovered crystal in the air whereas viscoelastic and liquid layers are the simplest particular case of acoustic multilayer formalism.25,38 We use herein an extended viscoelastic model with fixed parameters of liquid layer (dynamic viscosity, ηL, and density, ρL) and five parameters of the viscoelastic layer: the shear storage (elastic) modulus, G′, shear viscosity, ηS linked to shear loss modulus, G″, by the equation G″ = 2πnfoηS′, electrode thickness, h, and power law exponents for G′ and ηS, designated as β′ and β″.49 Our viscoelastic model is then fitted to the experimental values of Δf/n and ΔW/n, by which the five viscoelastic parameters mentioned before are returned. Some works on viscoelastic modeling replace the resonance width change, ΔW/n, with the change of the dissipation factor on any overtone order, ΔD = (ΔW/n)/( f/n); the dissipation factor along with frequency, f/n, present two output characteristics of the EQCM-D instrument. 3.2. Viscoelastic Behavior of Neat Polymeric Films and Composite Electrodes in Contact with Electrolyte Solutions. The information on stiffness/viscoelasticity of neat polymer films and thin composite electrode coatings containing these polymers is already contained in the raw EQCM-D response, that is, by the values of Δf/n and ΔD, see Figure 2. From this figure it is seen that Δf/n and ΔD for all six samples

depend on the overtone order n. The physical origin of this dependence is different for the rigid and soft coatings, reflecting hydrodynamic solid/liquid interactions and viscoelastic dissipation of oscillation energy for the former and latter case, respectively. Figure 2 shows that for neat PVdF film and PVdFcontaining composite electrodes the values of Δf/n and ΔD are independent of the time of immersion of these samples from the air in the electrolyte solution; this matches the expected behavior for rigid coatings. In contrast, neat NaCMC films and NaCMC-based electrodes reveal a significant increase in the resonant frequency with time signifying the so-called loss of mass, which is typical for a progressing material softening.25,38,49 Prior to a quantitative treatment of the raw EQCM-D responses in terms of hydrodynamic models for rigid binder and viscoelastic models for the soft binder, we will introduce more raw data related to the electrochemical cycling of four different LFP electrodes (comprising large and small particles with two binders) and the behavior of some of these electrodes in air. This is an important feature of our approach: a selfconsistent modeling must relate to the entire set of data collected in air, in electrolyte solution under open-circuit voltage (OCV), and during electrochemical cycling. Such unprocessed EQCM-D data are shown in Figure 3. 27667


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Figure 3. CVs measured at 2 mV/s for the composite electrodes with PVdF and NaCMC binders (a and b, respectively) and the related EQCM-D data for the LFP/PVdF (b and c, for small and large intercalation particles, respectively) and LFP/NaCMC (e and f, for small and large intercalation particles, respectively). At t = 0 and 600 s the LFP electrode is in the completely intercalated state (hence offset relates to the intercalated state); at t = 300 s the electrode is in the completely deintercalated state. The solid dark-yellow lines in panels b, c, e, and f relate to the frequency change Ftheo calculated using Sauerbrey’s equation based on the intercalation charge determined from the related CV. A scratch on the electrode coating composed of predominantly single-layer small intercalation particles was imaged by SEM at low (g) and higher resolutions (h).

deformation at some point exceeding the deformation at yield. Surely, the imaged piece of the free-standing coating is the one acoustically sensed by EQCM-D when it is rigidly attached to the quartz crystal surface. The coating presents a single-layer of platelet LFP intercalation particles embedded into the PVdF network (the front side of the coating is shown in Figure 3h). The bright and dark spots in Figure 3g correspond to the pieces of the closely arranged intercalation particles and the separations between them, respectively. The dark spots in the image are the noncovered crystal surface or covered with a neat PVdF binder. Using AFM, we have measured the step between the crystal surface and the outer surface of the coating: in average, a singlelayer electrode is 150 nm thick. The relatively rough and porous electrode structure is the reason for a considerable

When cycled at a slow scan rate of 2 mV/s, these electrodes reveal virtually the same shape of their cyclic voltammograms (CVs) for PVdF and NaCMC binders (Figure 3a,b). The EQCM-D responses of LFP/PVdF and LFP/NaCMC electrodes containing small and large particles are shown in Figure 3b,c and e,f, respectively. From panels b and e it is seen that the nature of binder significantly affects the functional dependence of Δf/n and ΔW/n on the overtone order, n. To better understand the reason behind this dependence, we first visualized the composite electrode coatings that are characterized by EQCM-D. For this purpose, the electrode containing small LFP particles and PVdF binder (the response is shown in Figure 3b) was gently scratched and imaged by SEM at different resolutions (Figure 3g,h). Scratches imply the application of a large shearing stress so that the PVdF binder of the composite electrode most probably experiences plastic 27668


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Figure 4. QCM-D characterization of neat polymeric films and composite LFP electrodes containing PVdF and NaCMC binders in air. Dissipation vs frequency shift in air (ΔD vs Δf/n) measured at different overtone orders for neat polymeric films and composite electrodes comprising either large or small particles as indicated (a). ΔD vs Δf/n in air for the LFP electrodes containing the same amount of large active particles and increasing content of PVdF binder as indicated (b). Best fit is shown by solid squares (fitted with eq S7, Supporting Information); the full set of parameters is listed in Tables S1 and S2). Panel c shows a SEM image of large particle poorly adhered to crystal surface at the PVdF content of 0.1 mass % in contrast to a much better adhesion of the particles at 10 mass % content of PVdF (panel d).

deviation of the EQCM-D response from that of the plane surface (as is shown in Figure 5c and discussed later). Despite the above deviation, Figure 3b showing the EQCMD response of this electrode during Li-ions deintercalation is characterized by the following specific features: (i) the appearance of a negligibly small dispersion of Δf/n on different overtone orders, (ii) the closeness of Δf/n to the frequency shift calculated from the intercalation charge using Sauerbrey’s equation (designated as Ftheo), and (iii) a negligibly small change of ΔW/n. Hence the use of a thin porous rigid coating results in although small but still measurable hydrodynamic correction29 to the mass change effect due to Li-ion insertion. The quantification of this small hydrodynamic correction caused by the intercalation-induced volume changes of electrode particles is further discussed in relation to Figure 5e. In contrast, the response of the LFP electrode containing small particles and viscoelastic NaCMC binder (Figure 3e) implies quite different coupling of Δf/n and ΔW/n as a function of n which can be described only by the classical viscoelastic model extracting the potential-dependent viscoelastic parameters (further discussed in relation to Figure 5d). Finally, the EQCM-D behavior of LFP electrodes containing large particles with both PVdF and NaCMC binders (panels c and f) is different from the behavior of the electrodes containing small particles by their characteristic n-dependences of Δf/n and ΔW/n. This difference is explained below in relation to Figure 5f in terms of the model of sliding friction of large particles. Summarizing the raw EQCM-D data related to LFP particles of different size in combination with either rigid or viscoelastic

binder in thin composite electrode coatings, we come to the important conclusion: the characteristic patterns of Δf/n and ΔW/n dependences on n are due to the physically different mechanisms of the EQCM-D responses which must be captured by the related rigid hydrodynamic and viscoelastic models for small intercalation particles and sliding friction for the large particles. Below is a brief list of the location of energy dissipation dependent on the electrode geometry and viscoelastic properties of binders. The dissipation in rigid LFP/PVdF electrode (small particles) occurs in relatively wide pores and at the contact of external electrode surface with the electrolyte solution (dissipation does not occur in rigid porous matrix). The dissipation in the viscoelastic LFP/NaCMC electrode (small particles) occurs inside of the effectively (pseudo)homogeneous electrode layer and at the contact with the electrolyte solution. Finally, the dissipation in the electrodes containing large particles weakly attached to the quartz crystal surface occurs at the contact area of the sliding friction between the crystal surface and large intercalation particle and of course owing to a considerable hydrodynamic interaction with the contacting liquid. Note an important difference between the electrochemical behaviors of practical thick LFP electrodes containing either small or large intercalation particles: The most significant difference is the lower rate capability typical for the electrode containing larger particles. However, when arranging differently sized intercalation particles in thin layer electrodes in contact with the quartz crystal surface, the intrinsic mechanical properties of the polymeric binders and their dependence on the electrode volume changes can be easily assessed. This is not 27669


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Figure 5. Hydrodynamic and viscoelastic modeling of EQCM-D responses for PVdF and NaCMC neat films (a and b, respectively) and for the composite electrodes containing small LFP particles in both intercalated state (under OCV) and deintercalated state (c,e and d,f, respectively). The experimental data are shown by the solid spheres. For rigid modeling, the values of Δf/n and ΔW/n normalized by ρL × f 02 (dimension (cm3 s)/g) are presented as functions of the penetration depth, δ, calculated for the different overtone orders (a and c). Equations A2 and A3 for the shallow rough surface and rigid porous uniform layer, respectively, from ref 29 were used for fitting the results in panels a and c, respectively. The full set of fitting parameters of the results in panel c is listed in Table S4. The response due to ideally plane surface in panels a and c was calculated using Kanazawa equation (eq A1 from ref 29). For the electrochemical deintercalation, the raw data for Δf/n and ΔW/n were used (e). For the viscoelastic modeling of the neat NaCMC film Δf/n and ΔW/n are presented as functions of the overtone order, n (b, d, and f). The dashed lines represent the best fitted curves to the experimental data. Fitting was performed using eq 10.2.3 from ref 25. The full sets of viscoelastic model parameters for the neat NaCMC film and composite electrode with this binder are listed in Tables S3 and S5.

for the independence of Δf/n on n. This means that the electrodes containing a larger amount of PVdF are effectively more rigid than those containing smaller amounts of PVdF although the dissipation factor does not become zero: some dissipation mechanisms still exist in the structures with a large amount of PVdF. SEM images of the electrodes containing 1 mass % and 10 mass % PVdF (Figure 4c,d) make it obvious that the large intercalation particles adhere to the quartz crystal surface not strongly enough if the amount of PVdF is small. Large particles experience sliding friction with respect to the oscillating surface (i.e., they move with some delay) which increases the dissipation factor requiring theoretical modeling. An original theory of the effect of sliding friction of large intercalation particles probed acoustically in the air and in the electrolyte has been elaborated in this paper. The adhesion strength of the composite electrode comprising large intercalation particles is quantified by the sliding friction coefficient.

surprising as a few layers of intercalation particles close to the quartz crystal surface is the acoustic sensing depth of EQCM-D beyond the gravimetric limit. 3.3. Modeling of Response of LFP/PVdF Electrodes Containing Large Particles in Contact with Air. The EQCM-D study of these electrodes in air reveals their contrastingly different behavior from that for the electrodes containing small particles and neat polymeric films (Figure 4). Small values of D for the latter electrodes and for neat polymeric films imply rigid behavior in the air in contrast to the large values of D (especially for the higher overtone orders) for the electrodes with large sliding particles. For a better understanding of the mechanism of dissipation of oscillation energy, we characterized a series of LFP composite electrodes with large particles as a function of the PVdF content. As seen from Figure 4b, as the content of PVdF increases, the characteristic shape of the ΔD vs Δf/n curve changes from a quarter-of-circle to the essentially vertical line, revealing a trend 27670


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Figure 6. Modeling of LFP electrodes comprising large particles with sliding friction along the crystal surface. Additional contribution of either rigid or viscoelastic layers of smaller particles to Δf/n and ΔW/n changes were considered for PVdF and NaCMC binders, respectively (see Supporting Information, section 2). The experimental values of Δf/n and ΔW/n are shown in all panels by solid black and blue spheres, respectively. The reference state for the responses at OCV (intercalated state) relates to that of the neat crystal in air. Panels a and b show responses for the LFP electrode with PVdF binder under OCV and after complete Li-ions extraction, respectively. Panels c and d show similar data for NaCMC binder. Dashed lines in all panels are the best fitted curves with the full set of the related parameters listed in Tables S6 and S7 for PVdF and NaCMC, respectively.

solution (contributing to Δf/n rather than to ΔW/n). The neat NaCMC film with a thickness of 39.8 nm was modeled by the viscoelastic model25,32−38 yielding an elastic shear modulus of 50.4 kPa and loss modulus of 40.8 kPa; other parameters are listed in Table S3. The parameters appeared to be close to the literature data.50 The behavior of LFP electrodes containing small intercalation particles with either PVdF or NaCMC binder (Figure 5b,e) were similarly modeled by the rigid hydrodynamic model of the uniform porous layer and by the extended viscoelastic model in both intercalated (OCV) and deintercalated states. The use of the viscoelastic model for the case of the porous composite electrode containing NaCMC is substantiated by the fact that the characteristic size of the electrode heterogeneity (e.g., the pore width and the size of NaCMC partitions between the particles) is smaller than the wavelength of sound in the electrode material, λ. This limit is often called the effective medium theory: on the length scale of λ, the composite material is effectively pseudohomogeneous. The results of fitting are shown in Figure 5c,d for the rigid and viscoelastic models; the fitting parameters are listed in Tables S4 and S5. Li-ions extraction was accompanied by the decrease of the electrode layer thickness by 0.4% and 0.6%: The softer binder allows somewhat larger deformation. This implies that the rigid binder restricts the extent of the intercalation-induced deformation compared to that for the softer polymeric binder.

3.4. Modeling of EQCM-D Response for Neat Binders and Composite Electrodes Containing Small Particles. For the rigid PVdF binder, the results of the hydrodynamic modeling of Δf/n and ΔW/n as functions of the penetration depth, δ, are presented in Figure 5a,c,d. We used eq A3 for the rigid porous uniform layer from ref 29. The response due to the ideally plane surface was calculated using Kanazawa equation (eq A1 from ref 29). In contrast, for the extended viscoelastic modeling, the experimental data are presented as functions of the overtone order, n. In all cases, symbols denote the experimental data and the dotted lines show the best fit using either hydrodynamic or viscoelastic models. A detailed description of how the hydrodynamic models should be used to fit the experimental EQCM-D data for the battery electrodes has been reported elsewhere.27,29−31 The use of the extended viscoelastic model, mainly for the characterization of polymeric and biologically active coatings has been described in previous literature.25 For fitting of the experimental data by classical viscoelastic model we used Qtools software 3.125.604 from Biolin Scientific. For analysis of the behavior of viscoelastic electrode coatings (small particles) in air and in contact with liquid, this software relies on eqs 10.1.8 and 10.2.3, respectively, from ref 25. The hydrodynamic model used for fitting the PVdF film behavior shows that the film 70 nm thick (loading mass density 8.83 μg/cm2) had a slightly rough external surface (roughness coefficient: 1.035) and contained 4.2% of trapped electrolyte 27671


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Figure 7. Continuous monitoring of Δf/n and ΔD changes during cycling of the composite electrodes containing large LFP particles and either PVdF or NaCMC binders (arrows show the direction of increasing n). The experimental EQCM-D data along with the related CVs changes (at 2 mV/s) are shown in panels a,c and b,d for PVdF and NaCMC, respectively. The different periodicity of Δf/n and ΔD changes in panels a and c is due to the different rates of charging varied from 2 to 20 mV/s.

higher frequency (i.e., at the rate faster than the relaxation rate). On the contrary, as the frequency is lower, the higher rate of dissipation makes the loss modulus the major contribution to the complex shear modulus. 3.5. Modeling of EQCM-D Response of Composite Electrodes Containing Large Particles. This subsection presents our original theoretical acoustic approach to the description of the behavior of large LFP intercalation particles probing their adhesion strength to the current collector and between the particles as a function of the binder’s content. The binder-free electrodes would imply the existence of sliding friction of the particles as the sole contribution to the frequency and dissipation changes. An insufficient amount of binder results in a partial contribution of the sliding friction to the total EQCM-D response, and a larger amount of binder facilitates rigid attachment (adherence) of the particles to the crystal surface (i.e., sliding friction disappears). The theory of sliding friction of semispherical bumps (approximating real particles) on the quartz crystal surface in the air and in the liquid is presented in detail in the Supporting Information, section 1.

The comparison of the viscoelastic parameters of the neat NaCMC film with that of the composite electrode containing 10% NaCMC is of extreme importance: as seen from Tables S3 and S5 for the neat NaCMC film, values for G′ and G″ are very similar, whereas for the composite electrode, the loss modulus, G″, is by a factor of 2 larger than the storage modulus, G′. Stiff intercalation particles occupying a large part of the composite electrode volume interact with polar functionalities of NaCMC molecules, thus increasing the effective storage modulus as compared to that of the neat film. However, the remaining NaCMC beyond the layer of the closest approach to the surface of intercalation particles loses the intermolecular bonds existing in the neat NaCMC film. Thereby, the viscous contribution to the complex shear modulus of the electrode is increased. As expected for G″> G′, the mechanical relaxation in the partitions of the NaCMC binder occurs with the rates overlapping with the characteristic frequency range of EQCM-D experiment (oscillation frequency between 15 and 70 MHz). The exponent of the elastic modulus, b′ = 0.94 (Table S5) shows that the elastic energy is stored more efficiently when excited at the 27672


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EQCM-D cell comprises a very small mass of the composite electrode coatings (typically between 20 and 150 μg/cm2). In addition, these cells contain a very large amount of electrolyte solution in direct contact with the electrode compared to that in practical coin cells. Thus, EQCM-D is very sensitive to any kind of viscoelastic changes because of the excessive swelling of small-mass electrode by a large amount electrolyte solution. As seen from Figure 7c,d, the soft binder results in a significant decrease of the electrode capacity retention (compare CVs in Figure 7b,d): The intercalation-induced volume changes of the electrode particles facilitates soft binder flow (correlated with the relative increase of the loss modulus with respect to the storage modulus) which gradually decreases the electronic conductance of the composite electrode in contrast to the unchanged electronic conductance of the more rigid binder. However, the rigid binder does not allow developing full and reversible intercalation-induced volume changes which result in the accumulation of stresses in the contact area and may in some severe cases result eventually in the formation of cracks deteriorating the composite electrode cycling performance.

Note that we consider the contribution to the total values of Δf/n and ΔW/n from either rigid porous or viscoelastic layers formed by the small particles in parallel (i.e., additive) to the contribution due to the sliding friction of large particles. This is because the part of the quartz crystal surface is occupied by the small particles, whereas the rest is occupied by the large particles, and the lateral sizes of these pieces of the surface are larger than the wavelength of sound, λ. We applied this model for the description of the electrodes behavior both in the air and in the electrolyte solution. As shown in Figure 4b, the model fits the ΔD vs Δf/n dependencies for the composite LFP electrode containing 1 and 10 mass % of PVdF binder measured in air. The most important parameter of the model of the sliding friction of large particles in air is Bair, which is inversely proportional to the sliding friction coefficient, χ, and the latter appears to be two times smaller for the electrode containing 1 mass % PVdF (facilitating sliding friction) than that for the electrode containing 10 mass % PVdF (leading to a stronger adherence of the particles to the crystal surface). The fitting results for the LFP electrodes containing PVdF and NaCMC binders, respectively, in contact with the electrolyte solution (these data are presented in Figure 3c,f) are shown in Figure 6a−d. The related parameters are listed in Tables S6 and S7. For the LFP electrode containing large particles and PVdF binder, 75% of the electrode mass relates to the intercalation into the sliding particles, whereas the remaining 25% correspond to the mass contribution due to the intercalation into the small particles (Δf1,layer = −310 Hz in Figure 6a compared to Δf1,bump = −915 Hz in Table S6). As seen from Figure 6a the model values of Δf/n corresponding to the related model values of ΔW/n differ from the experimental values of Δf/n by a constant, n-independent value assigned to the mass contribution of the rigid layer due to the intercalation into the small particles, Δf1,layer = −310 Hz. The most important parameter of the sliding particles model in solution is the parameter Bliquid inversely proportional to the sliding friction coefficient, χ (see SI, section 1.2). As expected, χ is 1.7 times larger for LFP/PVdF than that for LFP/NaCMC (i.e., PVdF holds large intercalation particles stronger than NaCMC), see Table S6 and S7. Finally, deintercalation of Liions results in shrinkage of the large particles by about 1%. 3.6. Continuous Consecutive Cycling of Composite LFP Electrode Containing PVdF and NaCMC Binders. Figure 7 shows unprocessed EQCM-D data and the related treated CV responses for the electrodes containing large particles and either PVdF or NaCMC binder. For the PVdFcontaining electrode, continuous cycling was performed by an uninterrupted series of charge−discharge cycles with different scan rates as indicated. This allows us to observe simultaneously the amplitude of the periodic change of the frequency and dissipation as a function of the scan rate which slightly decrease as the scan rate increases resembling a similar decrease of the intercalation charge (Figure S1) and the background line of EQCM-D response with time. Disregarding a small effect of the background (long-time) stabilization of Δf/n and ΔD with time after the beginning of the cycling (Figure 7a), the general shape of this dependence is like that of the neat PVdF film (Figure 2a) considered being rigid in the tested solution. Contrarily, the behavior of the composite electrode containing NaCMC binder (Figure 7c) resembles the behavior of the neat NaCMC film (Figure 2b) identified as the viscoelastic one in the same solution. Our

4. CONCLUSIONS EQCM-D senses with high precision the mechanical state of the composite electrodes both in air and after their impregnation in the electrolyte solutions and under applied potential during electrodes charging/cycling. When penetrating a single/few layers of the differently sized intercalation particles separated by either rigid or viscoelastic binder, the acoustic waves of different overtone orders subject the layers to the acoustic deformation thus probing their mechanical state. We focused the analysis of experimental data for the porous electrodes with different binders on the electrode geometry and the specific effect of the binder nature on the EQCM-D response. It appears that small intercalation particles especially those in direct contact with quartz crystal surface when embedded into the rigid binder such as PVdF in aqueous solutions experience intercalation-dependent volume changes reflected by solid−liquid hydrodynamic interactions and can be quantified by the related load impedance model (dissipation in solid matrix of the composite electrode is absent). However, when the polymeric matrix is soft, the EQCM-D response reflects the viscoelastic properties of the composite electrode. The application of the viscoelastic model returns the values of the effective shear storage and loss moduli and also the electrode thickness. Large intercalation particles in contact with crystal surface with a small concentration of PVdF in the composite electrode experience sliding friction. The theory of sliding friction of large intercalation particles has been developed to probe the adhesion ability of the polymeric binder. A detailed EQCM-D study of neat polymer binder films and composite LFP electrodes provides a clear clue which binder should be selected to accommodate moderate intercalation-induced volume changes of the composite LFP electrode. Such a binder must have a moderately high elastic modulus to allow reversible intercalation-induced volume changes of the composite electrode, but the binder should not be too soft (i.e., should have a moderate loss modulus) to prevent the deterioration of the electronic conductance of the composite electrode due to undesirable flow of the binder. The binders with the required viscoelastic properties can be fast screened among the many possible candidates using the 27673


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(10) Calvo, E.; Etchenique, R.; Bartlett, P.; Singhal, K.; Santamaria, C. Quartz Crystal Impedance Studies at 10 MHz of Viscoelastic Liquids and Films. Faraday Discuss. 1997, 107, 141−157. (11) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243−3262. (12) Goriparti, S.; Miele, E.; De Angelis, F.; Di Fabrizio, E.; Proietti Zaccaria, R.; Capiglia, C. Review on Recent Progress of Nanostructured Anode Materials for Li-Ion Batteries. J. Power Sources 2014, 257, 421−443. (13) Kovalenko, I.; Zdyrko, B.; Magasinski, A.; Hertzberg, B.; Milicev, Z.; Burtovyy, R.; Luzinov, I.; Yushin, G. A Major Constituent of Brown Algae for Use in High-Capacity Li-Ion Batteries. Science 2011, 334, 75−79. (14) Li, J.; Lewis, R.; Dahn, J. Sodium Carboxymethyl Cellulose a Potential Binder for Si Negative Electrodes for Li-Ion Batteries. Electrochem. Solid-State Lett. 2007, 10, A17−A20. (15) Ma, D.; Cao, Z.; Hu, A. Si-Based Anode Materials for Li-Ion Batteries: A Mini Review. Nano-Micro Lett. 2014, 6, 347−358. (16) Zhang, W.-J. A Review of the Electrochemical Performance of Alloy Anodes for Lithium-Ion Batteries. J. Power Sources 2011, 196, 13−24. (17) Komaba, S.; Shimomura, K.; Yabuuchi, N.; Ozeki, T.; Yui, H.; Konno, K. Study on Polymer Binders for High-Capacity SiO Negative Electrode of Li-Ion Batteries. J. Phys. Chem. C 2011, 115, 13487− 13495. (18) Lux, S. F.; Balducci, A.; Schappacher, F. M.; Passerini, S.; Winter, M. (Invited) Na-CMC as Possible Binder for LiFePO4/C Composite Electrodes: The Role of the Drying Procedure. ECS Trans. 2009, 25, 265−270. (19) Moretti, A.; Kim, G.-T.; Bresser, D.; Renger, K.; Paillard, E.; Marassi, R.; Winter, M.; Passerini, S. Investigation of Different Binding Agents for Nanocrystalline Anatase TiO2 Anodes and Its Application in a Novel, Green Lithium-Ion Battery. J. Power Sources 2013, 221, 419−426. (20) Herbst, F.; Döhler, D.; Michael, P.; Binder, W. H. Self-Healing Polymers via Supramolecular Forces. Macromol. Rapid Commun. 2013, 34, 203−220. (21) Wang, C.; Wu, H.; Chen, Z.; McDowell, M. T.; Cui, Y.; Bao, Z. Self-Healing Chemistry Enables the Stable Operation of Silicon Microparticle Anodes for High-Energy Lithium-Ion Batteries. Nat. Chem. 2013, 5, 1042−1048. (22) Aslan, M.; Weingarth, D.; Herbeck-Engel, P.; Grobelsek, I.; Presser, V. Polyvinylpyrrolidone/Polyvinyl Butyral Composite as a Stable Binder for Castable Supercapacitor Electrodes in Aqueous Electrolytes. J. Power Sources 2015, 279, 323−333. (23) Aslan, M.; Weingarth, D.; Jäckel, N.; Atchison, J.; Grobelsek, I.; Presser, V. Polyvinylpyrrolidone as Binder for Castable Supercapacitor Electrodes with High Electrochemical Performance in Organic Electrolytes. J. Power Sources 2014, 266, 374−383. (24) McAllister, Q. P.; Strawhecker, K. E.; Becker, C. R.; Lundgren, C. A. In Situ Atomic Force Microscopy Nanoindentation of Lithiated Silicon Nanopillars for Lithium Ion Batteries. J. Power Sources 2014, 257, 380−387. (25) Johannsmann, D. The Quartz Crystal Microbalance in Soft Matter Research; Springer: Berlin, 2014. (26) Daikhin, L.; Sigalov, S.; Levi, M. D.; Salitra, G.; Aurbach, D. Quartz Crystal Impedance Response of Nonhomogenous Composite Electrodes in Contact With Liquids. Anal. Chem. 2011, 83, 9614− 9621. (27) Levi, M. D.; Daikhin, L.; Aurbach, D.; Presser, V. Quartz Crystal Microbalance with Dissipation Monitoring (EQCM-D) for In-Situ Studies of Electrodes for Supercapacitors and Batteries: A MiniReview. Electrochem. Commun. 2016, 67, 16−21. (28) Levi, M. D.; Lukatskaya, M. R.; Sigalov, S.; Beidaghi, M.; Shpigel, N.; Daikhin, L.; Aurbach, D.; Barsoum, M. W.; Gogotsi, Y. Solving the Capacitive Paradox of 2D MXene using Electrochemical Quartz-Crystal Admittance and In Situ Electronic Conductance Measurements. Adv. Energy. Mater. 2015, 5, 1400815.

noninvasive in situ EQCM-D method in conjunction with the relevant rigid hydrodynamic or viscoelastic modeling.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06243. Section 1 “Theory of QCM-D response for sliding friction of bumps” with 14 SI equations, section 2 containing a supplementary figure, and section 3 “Supplementary Tables” containing 7 tables listing the best fitted values of the parameters of hydrodynamic, viscoelastic, and sliding friction models. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Vadim Dargel: 0000-0002-6363-273X Volker Presser: 0000-0003-2181-0590 Doron Aurbach: 0000-0002-1151-546X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding from the German-Israeli Foundation for Scientific Research and Development (GIF) via Research Grant Agreement No. 1-1237-302.5/2014. N.J. and V.P. thank Prof. Eduard Arzt (INM) for his continuing support.

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REFERENCES

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