Diffusion-Induced Transient Stresses in Li-Battery Electrodes Imaged

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Diffusion-Induced Transient Stresses in Li-Battery Electrodes Imaged by EQCM-D and Environmental SEM Netanel Shpigel, Mikhael Levi, Xiaopeng Cheng, Tianci Cao, Rui Wu, Tyler S. Mathis, Yuefei Zhang, Doron Aurbach, and Yury Gogotsi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00884 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Energy Letters

Diffusion-Induced Transient Stresses in LiBattery Electrodes Imaged by EQCM-D and Environmental SEM Netanel Shpigel,1 Mikhael D. Levi,1 Xiaopeng Cheng,2 Tianci Cao,2 Rui Wu,2 Tyler S. Mathis,3 Yuefei Zhang,*,2 Doron Aurbach,*,1 Yury Gogotsi 3

1

Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel

2

Beijing University of Technology, Beijing, 100124 (P. R. China)

3

Department of Materials Science and Engineering and A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, PA 19104, USA

Corresponding Authors* D. Aurbach, e-mail: [email protected] (D.A.)

Yuefei Zhang, e-mail: [email protected] (Y.Z.)

Y. Gogotsi, e-mail: [email protected] (Y.G.)

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ABSTRACT

Quick charging of Li-ion batteries is often accompanied by rapid expansion of composite battery electrodes resulting in appearance of transient stresses inside the electrodes bulk. Although predicted theoretically, they have never been tracked by direct in situ measurements.

Herein,

using

multi-harmonic

electrochemical

quartz

crystal

microbalance with dissipation monitoring (EQCM-D), acoustic images of strong transient deformations in LiFePO4 electrodes were obtained in the form of giant resonance frequency and resonance width shifts. The formation of cracks was verified by scanning electron microscopy. The effects of charging rate, stiffness of the polymeric binder and solution concentration have been identified. The attractive feature of EQCM-D is its high sensitivity for selective probing of average mechanical characteristics of the operated electrodes, especially of the particle-binder interactions, directly linked to the electrode cycling performance. Using EQCM-D, an inexpensive, simple, and fast method of structural health monitoring for battery electrodes can be intelligently designed.


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Next-generation electrochemical energy storage devices are designed to provide high energy and power densities during safe and long-term operation under specified conditions.1,2 Unfortunately, high rates of charging/discharging are accompanied by strong concentration-induced stresses causing microcracking of the electrode particles, and eventually deteriorating the entire electrode cycling performance3,4. A variety of coupled electrochemical-micromechanical models (analytical5 and numerical ones6,7) have been proposed to quantify the concentration-induced mechanical stresses in battery electrodes under different cycling conditions. During the last two decades8-12 direct in situ stress measurements of battery electrodes have become available (see recent comprehensive reviews) 13,14. The main problem of these measurements is that all of them, without exception, are applied under slow charging rates or stationary conditions,

10,11,14,15

hence, lacking the

short-time resolution required to monitor the stresses appearing at high rates of charging which are transient in nature. When these stresses exceed a critical level, 3 ACS Paragon Plus Environment


giving rise to the formation of microcracks, the latter can be imaged by a variety of microscopy techniques14. However, of crucial importance is early identification and continuous assessment of the transient stresses in real time, and their long-term mechanical consequences under operando conditions, which is an essential element for intelligently designed structural health monitoring of battery electrodes. In order to meeting this growing demand in the energy storage field, we propose herein the use of in situ surface acoustic spectroscopy based on EQCM-D (Electrochemical Quartz Crystal Microbalance with Dissipation monitoring)16,17 as a fully dynamic and highly sensitive technique to uniquely monitor fast transient deformations during the cycling life of the entire composite electrode via its easily recorded effective viscoelastic characteristics. Fast Li-ion intercalation into electrode particles embedded in a stiff network of polymer binder results in non-uniform expansion (due to the large misfit strain between the FePO4 and LiFePO4 phases) producing a temporary (transient) breakdown of the intermolecular bonds of the polymer macromolecules in close vicinity to the expanded parts of the electrode particles (see Fig. 1). As follows from acoustic multi-layer viscoelastic modeling, these momentary electrode particle-binder mechanical interactions cause a drastic decrease of the effective shear viscosity of the electrode, giving rise to giant transients in resonance frequency and resonance width shifts as a function of the overtone order. The proposed acoustic model is fully consistent and does not necessarily require transformation of the recorded viscoelastic parameters into the values of stresses or deformations when the continuous monitoring of fast mechanical changes, which affect the electrode cycling performance, is the primary goal. Taking into account that the technique of in situ acoustic hydrodynamic spectroscopy developed by us has been already successfully applied in tracking the intercalation-induced porous structure changes in stiff electrodes,18 and viscoelastic modeling was implemented for determining in situ mechanical characteristics of soft

battery and supercapacitor electrodes under

stationary conditions/slow charging rates,19-21 (reviewed in ref. 22), we believe that this newly proposed application of EQCM-D in combination with ESEM technique will open a 4 ACS Paragon Plus Environment

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ACS Energy Letters

broad new spectrum of analytical applications, especially for structural health monitoring of Li-ion battery electrodes. For a first demonstration, it was important to choose a well-known electrode material that apparently undergoes a simple first order phase transition upon intercalation/deintercalation of Li ions, in this case LiFePO4. Nevertheless, even these ‘simple’ electrodes undergo pronounced morphological changes that may endanger their stability during fast operation. This emphasizes the importance of developing new in situ methodologies for studying the mechanical and morphological stability of intercalation electrodes in advanced batteries, which is vital for fast charging. All the experimental and theoretical background required, even for people outside the field, is provided in depth, in a very educational manner, in the Supporting Information (SI). The perspective message conveyed by this paper relates to a demonstration how powerful may be EQCM-D based methodologies for the study of composite electrodes, highly important for energy storage and conversion devices.

Figure 1. Scheme of the mechanical electrode particles-binder interactions indicating the origin of giant EQCM-D transients during Li-ions insertion into a composite LFP electrode. The deintercalated state, partially lithiated state (transient at high charging rate), and fully intercalated state are shown in panels a, b and c, respectively. The (transient) breakdown of intermolecular bonds between the polymeric macromolecules of the binder caused by rapidly expanding intercalation particles results in a drastic decrease of shear viscosity generating giant EQCM-D transients on high overtone orders.



Acoustic probing of stationary and transient deformations by EQCM-D. This relates to the background of the EQCM-D technique given detail in the SI section 1 (stationary and transient deformations probed by EQCM-D and illustrated in Fig. S1). In the following study we will deal with two coupled experimental EQCM-D quantities, measured at any odd overtone order, n: the resonance frequency change caused by the intercalation of Li-ions into a battery electrode, f/n, and the related resonance width change, W/n, or equivalently, the change of the dissipation factor, D = (W/n)/(f/n) = W0/f0 where f0 is the fundamental frequency (n=1), and W0 is the resonance width shift measured at n=1. The non-stationary (transient) stresses and the related deformations developed in battery electrodes during their charging originate from the non-uniform character of the intercalation process which depends on the electrode thickness, C-rate of the electrode charge/discharge, and the concentration of the electrolyte solution. We start with the effect of the electrode thickness on the measured EQCM-D responses of thin and thick composite LiFePO4 electrodes (chosen as the most suitable demonstrator in this pioneering work). Thin-layer electrodes. These electrodes’ coating consists of a single-layer of intercalation particles which are in direct contact with Au current collector of the crystal on one side, and the electrolyte solution on the other side. The polyvinylidene fluoride (PVdF) binder network rigidly links the particles between themselves in the layer and with the current collector (i.e. Au-coated quartz crystal surface). The thickness of the coating was around 150 nm as estimated by AFM. SEM image of this electrode is shown in Fig. S8a). Fig. 2 from panel a to panel g shows the electrochemical behavior of a thinlayer composite LFP electrode at two different cyclic voltammetry (CV) scan rates (a) together with the related f/n and D changes during the entire lithiation-delithiation cycle (b and e). It is seen that a perfect gravimetric response described by Sauerbrey equation (see SI Eq. S1) is observed at the smallest scan rate whereas at higher scan rates the characteristic dispersion of the f/n and D changes with the overtone order, n, is observed (for the criterion of the gravimetric behavior see SI section 1). Panels c and f show long-term cycling behavior of the same electrode and the related f/n and 6 ACS Paragon Plus Environment

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D changes. The electrode reveals excellent capacity retention after 200 cycles at 100 mV/s scan rate, and the related very stable f/n and D changes as functions of the cycle number. The SEM image (Fig. S2) made after cycling does not reveal any microcracking of the intercalation particles. Thus, we conclude that only slight transient deformations tracked by EQCM-D are typical for fast cycling of the thin electrode, which have no detrimental effect on its cycling performance.



Figure 2. Frequency and dissipation changes measured with thin LFP electrode. 0.1M Li2SO4 was used as the electrolyte solution. The CVs in the form of differential capacity obtained at different scan rates are presented in panel a; f/n and D changes as a function of time during the entire charging-discharging cycle performed at 5 and 150 mV/s, (b, and e respectively). The black, blue and red lines show the


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changes in both resonance frequency and dissipation at 3rd, 7th, and 11th overtone orders as indicated in panels b and f. The intercalation and deintercalation domains are indicated by the hatched and empty areas, respectively. FFaraday denotes the frequency change calculated from the intercalation charge using Faraday law and Sauerbrey equation (S1). The intercalation charge related to the electrode cycled at 5 mV/s during its entire cycling life is shown in panel c. f/n and D changes as functions of cycle number are shown in panel f at a scan rate 150 mV/s.

Thick-layer electrode. To prove that transient deformations are caused by a nonuniform character of the lithiation process at high charging rates, especially when performed in a dilute electrolyte solution, thick electrodes (thickness around 700-800 nm as estimated by AFM) were prepared. Fig. 3 shows sequentially the data obtained in 0.1 M and 2 M Li2SO4 solutions, respectively: CVs of this electrode (a, e), the corresponding intercalation charge, Q (b, f), the related f/n and D changes during 75 cycles (c, g), and finally, a detailed profile of the overtone order-dependent change of f/n and D along the entire selected cycle (d, h). Poor cycling performance for the thick electrode relative to the thin one when measured in diluted solution becomes immediately clear not only from the deteriorated cycling performance, i.e. from the consecutive CVs (a), revealing the decrease of the corresponding intercalation charge (b), but also from the most dramatic changes of the f/n and D during 75 cycles (c), and from the very significant characteristic changes of the shape of the f/n and D plots during the entirety of the selected cycle (d). It is seen (d) that the quantities f/n and D measured in the dilute solution reveal strong downward and upward shifts, respectively, with the overtone order in the form of the peak evolution during a lithiation half-cycle; characteristically, the peaks disappear completely at the end of the lithiation stage. The acoustic response is thus transient in its nature implying that high rates of lithiation of the electrode results in non-uniform Li-ion intercalation across the electrode which causes the diffusion-induced transient stress and the related deformation sensitively captured in the f/n and D responses. At the end of lithiation the Li-ions tend to spread uniformly throughout the sample, and the transient deformation completely vanishes. The higher concentration of 9 ACS Paragon Plus Environment


the electrolyte solution in contact with the porous composite electrode is known to form a more uniform distribution of Li-ions across the electrode, hence favoring the suppression of the transient deformation. As presented in the right-side panels of Fig. 3 the cycling performance of the thick electrode in 2 M Li2SO4 is much better compared to that in 0.1 M Li2SO4, and most importantly, the characteristic sharp f/n and D transients completely disappear from the EQCM-D response (compare d with h). Panel c shows very clearly that after cycle #40 the spikes of the f/n and D tend to rise sharply for the higher overtone orders, the baseline of the f/n change crosses the zero line and takes positive values. After cycle #60, the f/n and D changes for the highest overtone order become discontinuous (no such behavior was observed with the electrode cycled in 2 M Li2SO4, see panel g). This characteristic behavior of the acoustic response implies that the limit of mechanical integrity of the electrode has been crossed due to the fast lithiation rate: the very strong transient deformations across the electrode thickness give rise to the formation and growth of microcracks. Indeed, as shown in Fig. 4 many microcracks are seen on the surface of the intercalation particles of the thick electrodes (c and d); in contrast, microcracks are absent in the pristine electrode and the electrode cycled in 2 M Li2SO4 (a, and b). These images establish a direct correlation between the EQCM-D signatures of the non-uniform lithiation of the electrodes (i.e. the characteristic coupling between the f/n and D responses at different n), and their cycling performance, thus producing a kind of acoustic image of strong transient stresses/deformations.


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Figure 3. Frequency and dissipation changes measured with thick LFP electrode. 0.1 M and 2M solutions of Li2SO4 were used (left, a-d, and right, e-h, panels, respectively). The CVs were measured at 100 mV/s for 75 cycles (a and e), the corresponding intercalation-deintercalation charges are shown in panels b and f, and the related changes of f/n and D as a function of the cycle number are presented in panels c and g. th

Detailed plots of f/n and D changes as a function of time during the entire 35 cycle for the different overtone orders are shown in panels d and h, separately. The intercalation and deintercalation domains are indicated by the hatched and empty areas, respectively.

(a)

(b)

(c)

(d)

Figure 4. SEM images of pristine and long-term cycled thick LFP electrodes. The electrodes contain 2-3 layers of intercalation particles on Au-covered quartz crystals. Images of pristine electrode and the electrodes after 75 cycles in 2M Li2SO4 and 0.1M Li2SO4 are shown in panels a, b and c. Larger magnification of cracked particles presented in panel d.

Giant f/n and D transient responses within the extended potential windows. A spectacular sharpening of the f/n and D transients occurs when a high rate of 12 ACS Paragon Plus Environment

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ACS Energy Letters

discharge is coupled with expansion of the electrochemical window (Fig. 5) showing a drastic dependence of the transient deformations on the amount of intercalation charge. Whereas the dispersion of f/n and D with n at slow scan rates (insets in b and c) is overwhelmingly smaller when compared to that seen at high scan rates, the direction of their changes with n is reverse with respect to that observed at high scan rates. This type of quasi-stationary response was repeatedly observed in thick electrodes previously, which was ascribed to a quasi-stationary viscoelastic behavior of the thick electrodes probed by EQCM-D. 22 The transient responses for the higher n are expressed by very sharp peaks of f/n and D, requiring adequate acoustic modeling which should throw light on their origin. Modeling of f/n and D transients. For this purpose, we selected the highest transient denoted (iii) in Fig. 5b and c (the enlarged view of this transient is shown in Fig. S3). For consistency, we also modeled the drastically smaller transients of the thin electrode shown in Fig. 2d. The two-layer viscoelastic model is based on the non-uniform character of the intercalation process (schematically shown in Fig. S1g) further described in SI sections 2 and 3. Viscoelastic modeling is widely used in energy storage field to quantify mechanical properties of different battery electrodes16,23-29.




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Figure 5. Giant transients of f/n and D obtained with thick LFP electrode within the extended electrochemical windows. CVs (in the form of the differential capacity) were measured at different scan rates and within the extended potential windows (a). f/n and D changes as a function of scan rate and width of the electrochemical windows (b and c, respectively). The enlarged view of f/n and D changes at a scan rate 100 mV/s is shown in d (from peaks (i) to (ii)). The hatched and non-hatched areas in d denote the intercalation and deintercalation domains, respectively. Responses on the different overtone orders are indicated. Insets in b and c present enlarged views of the small n-dispersion of the f/n and D responses measured at slow scan rates.

Each solid layer is characterized by its own set of viscoelastic parameters such as the shear storage (G’) and loss (G”) moduli, and the layer thickness, h. The first layer is the external part of the electrode (facing solution) which experiences the transient deformation during fast Li-ion insertion, and the other part (i.e. the internal layer) relates to the deintercalated state. The two-layer model was fitted to the experimental transient peaks of f/n and D as a function of n, denoted by symbols, and the model (fitted) curves are given by the dashed lines in Fig. 6 for the thick electrode, and in Fig. S4 for the thin electrode, respectively. The related fitting viscoelastic parameters are listed in Table S2 and S1, respectively. The main conclusion from Table S1 is that the n-dependence of the f/n and D changes of the thin electrode during the intercalation reaction occurring at a high rate is mainly due to a decrease of the shear viscosity (hence of the loss modulus, G’’) by about a factor of two. The data from Table S2 shows that the giant transients of the ndependent f/n and D changes during the fast Li-ions insertion are caused by the same reason, namely, by the decrease of the shear viscosity (and hence of the loss modulus) but in this case, the viscosity decreases dramatically, by a factor of 200. The decrease of the shear viscosity is assigned to the development of a strong transient stress within the multilayered electrode owing to the enhanced non-homogeneous insertion of Li-ions into the LFP electrode. The stress is further transferred to the stiff polymeric binder network disturbing its initial structure (mainly the Van-der-Waals bonds are affected), see Fig. 1. This effect is essentially similar to shear thinning 15 ACS Paragon Plus Environment


occurring in hydrogels.

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Once the stress (and the deformation) decreases due to

reaching a more uniform distribution of Li-ions in the electrode at a higher lithiation level, the shear viscosity returns to its initial (large) value due to the relaxation of the stressed polymer binder network to its initial state. The parameters fitted to transients (i), (ii) and (iii) (Table S2) indicate that the model used here is fully self-consistent since all the parameters for the three peaks were virtually constant except for the increasing thickness of the layer. This increase is ascribed to the transient deformation (intercalation), at the expense of the decreased thickness of the non-deformed layer which corresponds to the deintercalated state of the electrode.

Figure 6. Fitting of two-layer viscoelastic model to the experimental f/n and D transients of thick electrode during the course of the intercalation process. The experimental values are designated by symbols for three transients marked as (i), (ii), and (iii) in Fig. 5. The model curves are shown by the dashed lines. The fitted parameters are listed in Tables S2.


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Validation of the acoustic effective medium model is provided in SI Section 4. Experimental details are presented in SI Section 5. Transient deformations and mechanical properties of binders. High rate of lithiation, sufficient electrode thickness and the presence of a dilute electrolyte solution are the dominant factors for the development of high transient stresses and deformations. One more important condition is the sufficiently high stiffness of the binder, since viscoelastic binders tend to accommodate not only quasi-stationary volume changes of lithiated composite electrodes but also transient stresses. Indeed, the transient deformations were not observed with thick LFP electrodes containing PVdF binder cycled in aprotic solutions (see Fig. S5a,b) in which PVdF possesses viscoelastic (soft) properties. We also prepared the LFP electrode without any binder, containing small electrode particles dispersed in ethanol. The EQCM-D response measured at comparable cycling rates and the related SEM images did not reveal the presence of any transients or cracks (Fig. S5c,d, and S6 respectively): transient stresses do not appear in this case in view of the absence of constricting pressure from the binder. Environmental SEM (ESEM) imaging. We used this technique to prove the link between the strong transient deformations and the formation of cracks. The ESEM study was conducted with thick LFP electrodes cycled in 1.2M LiFSI in PYR13FSI ionic liquid (aqueous solutions could not be used due to high volatility of water). PVdF binder was proved to be rigid in this solution. When the thick electrode (5.7 m, in this case) was cycled in a narrow potential window from 2.5 to 4.2 V (vs. Li) at 3C rate no cracks were visible in the FIB-made cross-section of the electrode (Fig. 7d). In contrast when the same electrode was cycled between 2.2 and 4.7 V (vs. Li) at 6C, i.e. under conditions favorable for development of non-uniform Li-ion intercalation, numerous cracks are seen on the surface of the intercalation particles (Fig. 7c). No cracks were formed during cycling of the thinner electrode (2.2 m, see cross-sections of thin and thick electrodes in Fig. S8c and d, respectively).



Figure 7. The preparation process of LFP electrode via FIB. (a,b) LFP electrode after FIB cross sectioning. Cross-sectional SEM images of the electrode after long term cycling under fast charging (cycling from 2.2 to 4.7 V at 6C), and slow charging (cycling from 2.5 to 4.2 V at 3C) using 1.2M LiFSI in PYR13FSI as an electrolyte. The yellow arrows denote the cracks locations (c and d). STEM–HAADF (High-angle annular dark-field) and the related EDS mapping images for the fast charged and the slow charged electrode (e and f respectively).

Discussion. The data presented here clearly shows the great advantages of acoustic probing of Li-intercalated electrodes cycled at different C-rates. In fact, we have obtained a high-frequency (MHz range) acoustic reflection (image) of the transient deformations in both single- and multi-layered LFP electrodes confirming theoretical predictions of the development of C-rate dependent diffusion-induced stresses in 18 ACS Paragon Plus Environment

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intercalation electrodes.5,6 In addition, we have experimentally confirmed that multilayered electrodes of similar thicknesses cycled at slow and fast rates result in either stable cycling with good capacity retention, or in a complete deterioration of the electrode capacity, respectively. Cycling the composite electrode containing a stiff binder at high rates of charging clearly demonstrates strong transient stresses, generated initially along the [100] direction with the largest misfit strain between FePO4 and LiFePO4

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revealed by the mechanical interaction of the rapidly strained LiFePO4

layer with the stiff binder. This stress is transferred to the surrounding stiff polymer binder network, and at this point, is reflected acoustically in the form of the giant ndependent transients of f/n and D. With a stiff binder, the accumulated stresses between the FePO4 and LiFePO4 layers result eventually in intense crack formation as seen in conventional ex situ SEM and in situ ESEM images for the fast-cycled electrodes. Consistently, EQCM-D transients as well as cracks are not observed for all those systems in which the effective mechanisms of stress relaxation are realized. The systems ensuring effective stress relaxation mechanisms imply the use of binders possessing viscoelastic properties in the electrolyte solutions under study, and combination of experimental conditions allowing to avoid non-uniform lithiation processes in the electrode (i.e. using thin electrodes’ coatings, low rate of the electrode cycling, more conductive solutions, etc). It should be absolutely clear that the mechanism we are proposing herein for mechanical failure of composite electrodes relates to the C-rate dependent transient stresses, and are completely different from the earlier reported data on microcracks following dislocation-based fracture mechanism 32,33. Dislocations initially present in the intercalation particles, move and grow in number during fast electrodes charging i.e. mainly during the course of electrochemical delithiation (especially, fast chemical delithiation) rather than originating from the diffusion-induced deformation mechanism. It should be also noted that the proposed mechanism of transient stresses in LFP electrodes is drastically different from the so-called C-rate independent



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electrochemical shock mechanism which is valid for stress accommodation in binderfree electrodes. 32 The difference in the mechanical and, in particular, fracture behavior of the slow- and fast- cycled single-layer and multi-layer LFP electrodes as follows from our surface acoustic study and ESEM imaging are in good agreement with recently published data on the operando full-field X-ray microscopic study of similar LFP electrodes.33 Indeed, the major result found by here is that a slow rate of charging for thick electrodes induces

homogeneous

phase

distribution

occurring

via

concurrent

phase

transformations across the entire electrode in agreement with the results obtained by full-field X-ray microscopy (no transient stress appears as we believe). In contrast, at high rates of charging, two-phase zones coexist during the phase transformation33 bringing about a strong C-rate dependent stress as confirmed by our surface acoustic spectroscopic studies. Since the EQCM-D approach to characterization of composite battery electrodes is based on the effective medium consideration, the method cannot provide a detailed microscopic view of the transient stresses originating from the abrupt compositional gradients (this is beyond its intrinsic resolution, which is related to the acoustic penetration depth). At the same time the method possesses a tremendous advantage with respect to other local high-resolution techniques because it represents a fully dynamic and highly sensitive in situ diagnostic tool capable of monitoring the appearance and growth of strong transient stresses in cycled electrodes. The characteristic size of inhomogeneities in most practical battery electrodes rarely exceed the penetration depth of the acoustic waves generated in the quartz crystals, and for this reason the EQCM-D signatures (images) are obtained at the very same scale as the macroscopic electrochemical characteristics of practical battery electrodes such as current, reversible and irreversible capacities, capacity retention, Faradaic efficiency, rate capability, etc. In particular, the EQCM-D signature of the surface cracks in the electrode will become visible when the cracks reach a sufficiently high concentration. 20 ACS Paragon Plus Environment

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Obviously, the high-resolution techniques are sensitive to a much lower concentration of cracks, they can track their geometric location, direction of growth, etc., but all these microscopic (or nanoscopic) characteristics do not directly correlate with the macroscopic characteristics of battery performance, unless the microscopic models contain a priori information how these microscopic characteristics are averaged at least on a mesoscopic scale. Hence a combination of high-resolution techniques with a mesoscopic-scale resolution EQCM-D technique seems to be the most promising in the energy storage field. The next challenge is the possibility of tracking grain boundary microfracture in binder-free electrodes experiencing C-rate independent (stationary) stresses 34: recording of in situ acoustic emission events revealed microfractures in these systems which may change the effective stiffness of the electrode coatings. The latter can be sensitively tracked by EQCM-D. Present and future perspectives of the use of EQCM-D for improved electrode designs. A versatile in situ multi-harmonic EQCM-D method has been developed for acoustic imaging and quantitative analysis of charging rate-dependent transient deformations in composite battery electrodes. These electrodes contain intercalation particles arranged in layers embedded into a stiff or viscoelastic polymeric binder network. As a typical example, fast Li-ion intercalation into a composite olivine electrode, FePO4, accompanied by a moderate volume expansion (6 %) was considered for this study, since for such a first demonstration the use of well-known electrode’s material is very important. Having a highly dynamic nature, EQCM-D sensitively captures both transient and quasi-stationary micromechanical electrode/binder interactions in terms of effective macroscopic viscoelastic characteristics that can be quantified by acoustic viscoelastic modeling. The transient deformations that develop during rapid Li-ion intercalation into composite battery electrodes with stiff binder networks have characteristic sharp peaks in their resonance frequency responses and dissipation factors that are caused by drastic mechanically-induced decreases in the local viscosity of the polymeric binder network. The characteristic coupling of the resonance frequency and dissipation factor changes (i.e. the EQCM-D signature) depends not only on the 21 ACS Paragon Plus Environment


electrode/binder nature but also on the electrode volume expansion, depth of discharge, electrolyte solution composition (which affects the viscoelastic properties of the binders), electrode particle size and thickness, rate of discharge, cycle number, microstructural design of the composite electrode, the electrochemical cell geometry, etc. The factors listed above not only define the processing-induced microstructure of a composite electrode (e.g. spatial arrangement of the intercalation particles and binder network, multi-scaled porosity, etc.)35 but, most importantly, strongly modify these interactions during electrodes discharge under different cycling conditions. The fast temporal evolution of the micromechanical electrode/binder interactions, which are usually coupled with deterioration of the electrodes cycling performance, is very difficult to document in real time even when taking into account the latest improvements in visualization techniques. This, however, is the very niche in which EQCM-D has an important advantage over the alternative techniques, where it reveals a direct relationship between the electrode cycling performance and the corresponding dynamically resolved changes in the EQCM-D response. For example, transient deformations that develop in composite LiFePO4 electrodes with a rigid polymeric binder as was shown in this paper were characterized by overtone-order dependent sharp peaks in the resonance frequency and the related dissipation changes (i.e. EQCM-D signatures). These changes were shown to be strictly reproducible when measured on different overtone orders. However, when the transient deformations are accumulated during long-term cycling of this electrode, they result in micro-cracks forming in the intercalation particles, which have a noticeably different EQCM-D signature: the frequency and dissipation changes measured for high overtone orders enormously increase, even approaching the instrument limits. From these results, the former EQCM-D signature can be used for early diagnosis of the appearance of transient stress/deformation, whereas the latter EQCM-D signature is indicative of micro-cracking of the intercalation particles. A second example relates to electrochemical insertion of relatively large Na-ions into a FePO4 host (initially tailored for accommodation of much smaller Li-ions), which even at 22 ACS Paragon Plus Environment

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ACS Energy Letters

a slow rate of discharge causes large quasi-stationary volume changes of the electrode’s active mass around 17%, i.e. 3 times larger than that observed in case of LiFePO4

21.

Whereas this large volume change can be accommodated by a viscoelastic binder network, the use of a stiff binder results in the appearance of significant stresses inside the binder network. Similar to the transient deformations in LiFePO4, these quasistationary deformations can accumulate during consecutive electrode cycling. Eventually, complete mechanical degradation of a stiff binder in composite NaFePO4 electrodes occurs, whereas no cracks were found in the intercalation particles. The large volume changes of the intercalation particles occurring at a slow rate are transferred to the polymeric binder network causing the appearance of strong, unrelaxing stresses. The related EQCM-D signature (a gradual increase of the effective electrode softness mainly via a decrease of electrode’s effective elastic modulus) is completely different from that found for transient deformations, reflecting the different nature of the micromechanical electrode-binder interactions. A third example of a variety of electrode-binder interactions relates to the use of intercalation particles of large and small size (micron- and submicron-size, respectively). The stiffness of the contact area between the current collector and the first layer of the intercalation particles depends critically on the electrode’s particles size and the amount of binder used 19. It was observed that when the amount of binder is not sufficient to ensure the required stiffness in the contact region, larger particles will start to experience sliding friction. The acoustic energy dissipates during sliding friction to a much larger extent than that accompanying the transient deformations. Of course, this sliding friction requires adequate acoustic modeling which can quantify the adhesion strength of the polymeric binder in the vicinity of the quartz crystal current collector 19. These three examples clearly show that micromechanical electrode-binder interactions are quantifiable by the use of analytical acoustic models in terms of their effective viscoelastic or sliding friction parameters. This is possible only when using simplified electrode geometries. If the goal of the researcher is early identification of appearance of transient deformations or the formation of microcracks inside the intercalation 23 ACS Paragon Plus Environment


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particles, the use of simple electrodes as demonstrated herein is sufficient for the EQCM-D analysis (thus, using more precise or more complex electrode geometry is not required). Alternatively, if the goal of the researcher is a rigorous description of the transient deformations that evolve during micro-cracking of intercalation particles, numerical computations can be performed for the area-averaged stress by using the finite element method and the (actual) precise electrode geometry

16.

However, the

effort required is much larger when compared with that related to the use of the analytical models with simpler electrode geometry. Finally, we believe that tracking intercalation-induced mechanical changes in battery electrodes by EQCM-D can be advantageously integrated with newly developing methods of characterizing and modeling of the microstructures of composite battery electrodes36. The combination of these techniques including real time EQCM-D operated in gravimetric, viscoelastic and hydrodynamic modes

22

will soon be very suitable for

dynamic structural health monitoring of a variety of different types of battery electrodes.

ORCID Mikhael D. Levi: 0000-0002-4984-6678 Doron Aurbach: 0000-0001-8047-9020 Yury Gogotsi: 0000-0001-9423-4032

AUTHORS BIOGRAPHIES


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Netanel Shpigel is a PhD 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.

Mikhael D. Levi is a Professor at Bar-Ilan University. He received his PhD in 1976 under the supervision of Academician A.N. Frumkin in Moscow State University. Since 2009 has been developing non-gravimetric EQCM method adjusted for simultaneous tracking of gravimetric, viscoelastic and porous structure changes in battery materials. Xiaopeng Cheng is now a Ph.D. candidate supervised by Prof. Yuefei Zhang at institute of microstructure and property of advanced materials, Beijing University of Technology (BJUT). His research currently focus on Cs-TEM/STEM characterization of high-performance batteries materials. Tianci Cao is a Postgraduate student at Beijing University of Technology guided by Profs. Yuefei Zhang and Xianqiang Liu. He is working on advanced applications of in-situ SEM for characterization of energy storage materials. Rui Wu is a PH.D. Student at institute of microstructure and property of advanced materials, Beijing University Of Technology which supervised by Profs. Yuefei Zhang. He is specialized in advanced applications of in-situ environment SEM and Cs-TEM/STEM characterization.



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Tyler S. Mathis received his B.S. in Biochemistry and Molecular Biology from the University of California at Santa Cruz in 2015. His research, under the advisement of Dr. Yury Gogotsi, is focused on the mechanisms of charge storage and the transportation ions in MXenes in electrochemical systems, with an emphasis on MXenes used in supercapacitor systems. Yuefei Zhang is a professor in institute of microstructure and property of advanced materials, Beijing University of Technology. His research group interests are focus on the in situ TEM/SEM experiments to reveal the microscopic mechanisms that govern the performance of energy storage materials. Doron Aurbach is a full professor, head of the electrochemistry group at BarIlan

University.

Leading

INREP–Israel

National

Research

center

for

Electrochemical Propulsion (22 research groups), MRS, ECS, 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, super-capacitors. Website: https://ch.biu.ac.il/aurbach


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ACS Energy Letters

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 Highly Cited Researcher by Thomson-Reuters in 2014-2016, and elected a Fellow of AAAS, MRS, ECS, RSC, ACerS and the World Academy of Ceramics. Website: https://nano.materials.drexel.edu/

NOTES

The authors declare no competing financial interest.

SUPPORTING INFORMATION.

Contains 6 sections: 1. Stationary and transient deformations probed by EQCMD; 2.Conventional single-layer viscoelastic modeling; 3. Two-layer viscoelastic model describing transient deformations; 4. Validation of the effective medium viscoelastic model; 5. Experimental details, 6. Supplementary Figures (Figures S1 – S8), and Tables S1 and S2.



ACKNOWLEDGEMENT The authors acknowledge funding from the Binational Science Foundation (BSF) USAIsrael via Research Grant Agreement 2014083/2016and from the Israeli Ministry of Science and Technology and Space Grant 66032. N.S. thanks the Israel Ministry of Science Technology and Space for their financial support. The authors declare no competing financial interest. Y.Z. thanks the National Natural Science Foundation of China (grant no.21676005) and Great Wall Scholarship Project (grant no. CIT&TCD20170306) for financial support. We are grateful to Dr. M. Verbrugge (General Motors) for useful discussion of the manuscript of this paper. 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 2012, 11, 19-29. 2) Goodenough, J. B.; Park, K.-S.: The Li-ion rechargeable battery: a perspective. J. Am. Chem. Soc. 2013, 135, 1167-1176. 3) Hu, Y.; Zhao, X.; Suo, Z.: Averting cracks caused by insertion reaction in lithium–ion batteries. J. Mater. Res. 2010, 25, 1007-1010. 4) Woodford, W. H.; Chiang, Y.-M.; Carter, W. C.: “Electrochemical Shock” of Intercalation Electrodes: A Fracture Mechanics Analysis. J. Electrochem. Soc. 2010, 157, A1052-A1059. 5) Cheng, Y.-T.; Verbrugge, M. W.: Diffusion-induced stress, interfacial charge transfer, and criteria for avoiding crack initiation of electrode particles. Journal of the Electrochemical Society 2010, 157, A508-A516. 6) ChiuHuang, C.-K.; Huang, H.-Y. S.: Critical lithiation for C-rate dependent mechanical stresses in LiFePO4. Journal of Solid State Electrochemistry 2015, 19, 2245-2253. 7) Stamps, M.; Eischen, J. W.; Huang, H.-Y. S.: Particle-and crack-size dependency of lithium-ion battery materials LiFePO4. AIMS Mater. Sci 2016, 3, 190. 8) Chou, S.-L.; Pan, Y.; Wang, J.-Z.; Liu, H.-K.; Dou, S.-X.: Small things make a big difference: binder effects on the performance of Li and Na batteries. PCCP 2014, 16, 20347-20359. 9) Jung, H.; Gerasopoulos, K.; Talin, A. A.; Ghodssi, R.: In situ characterization of charge rate dependent stress and structure changes in V 2 O 5 cathode prepared by atomic layer deposition. Journal of Power Sources 2017, 340, 89-97.


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20) Shpigel, N.; Lukatskaya, M.; Sigalov, S.; Ren, C. E.; Nayak, P. K.; Levi, M. D.; Daikhin, L.; Aurbach, D.; Gogotsi, Y.: In situ Monitoring of Gravimetric and Viscoelastic Changes in 2D Intercalation Electrodes. ACS Energy Letters 2017, 2, 1407-1415. 21) Shpigel, N.; Sigalov, S.; Levi, M. D.; Mathis, T.; Daikhin, L.; Janes, A.; Lust, E.; Gogotsi, Y.; Aurbach, D.: In Situ Acoustic Diagnostics of Particle-Binder Interactions in Battery Electrodes. Joule 2018, 2, 988-1003. 22) Shpigel, N.; Levi, M. D.; Sigalov, S.; Daikhin, L.; Aurbach, D.: In situ real-time mechanical and morphological characterization of electrodes for electrochemical energy storage 29 ACS Paragon Plus Environment


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36) Liu, Z.; Mukherjee, P. P.: Microstructure evolution in lithium-ion battery electrode processing. J. Electrochem. Soc. 2014, 161, E3248-E3258.

Two were used


Diffusion-Induced Transient Stresses in Li-Battery Electrodes Imaged

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