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Giant electric field-induced strain in PVDF-based battery separator membranes probed by Electrochemical Strain Microscopy Konstantin Romanyuk, Carlos M. Costa, Sergey Yu. Luchkin, Andrei Leonidovitch Kholkin, and Senentxu Lanceros-Mendez Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01018 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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Giant electric field-induced strain in PVDF-based battery separator membranes probed by Electrochemical Strain Microscopy Konstantin Romanyuk1,2,#, Carlos M. Costa3,#, Sergey Yu. Luchkin1,$ Andrei L. Kholkin1,2*, Senentxu Lanceros-Méndez3,4,5 #

1

equal contribution

Physics Department & CICECO – Aveiro Institute of Materials, University of Aveiro,

3810-193 Aveiro, Portugal 2

Institute of Natural Sciences, Ural Federal University, 620000 Ekaterinburg, Russia

3

Centro/Departamento de Física, Universidade do Minho, Campus de Gualtar, 4710-057

Braga, Portugal 4

BCMaterials, Parque Científico y Tecnológico de Bizkaia, 48160-Derio, Spain

5

IKERBASQUE, Basque Foundation for Science, Bilbao, Spain

KEYWORDS: PVDF, battery separator, electrolyte, ion diffusion, Electrochemical Strain Microscopy.

*Corresponding author: [email protected] $ Current affiliation: Skolkovo Institute of Science and Technology, Skolkovo Innovation Center, 3 Nobel St., Moscow 143026, Russia ACS Paragon Plus Environment

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Abstract Efficiency of lithium-ion batteries largely relies on the performance of battery separator membrane as it controls the mobility and concentration of Li-ions between the anode and cathode electrodes. Recent advances in Electrochemical Strain Microscopy (ESM) prompted to study Li diffusion and transport at the nanoscale via electromechanical strain developed under an application of inhomogeneous electric field applied via the sharp ESM tip. In this work, we observed unexpectedly high electromechanical strain developed in polymer membranes based on

porous

poly(vinylidene

fluoride)

(PVDF)

and

poly(vinylidene

fluoride-co-

chlorotrifluoroethylene) (PVDF-CTFE) and, using it, could study a dynamics of electroosmotic flow of electrolyte inside the pores. We show that, independently of the separator membrane, electric field-induced deformation observed by ESM on wetted membrane surfaces can reach up to 10 nm under a moderate bias of 1 V (i.e., more than an order of magnitude higher than that in best piezoceramics). Such a high strain is explained by the electroosmotic flow in a porous media composed of PVDF. It is shown that the strain-based ESM method can be used to extract valuable information such as average pore size, porosity, elasticity of membrane in electrolyte solvent, and membrane-electrolyte affinity expressed in terms of zeta potential. Besides, such systems can, in principle, serve as actuators even in the absence of apparent piezoelectricity in amorphous PVDF.

ACS Paragon Plus Environment

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Introduction Efficient storage of energy is one of the most critical issues of the modern society still to be solved for the next technological revolution. Widely used systems to store energy are batteries, which are ubiquitous components in numerous devices ranging from mobile phones to electric vehicles.1,2 A critical part affecting the battery performance is the membrane separator that is placed between the anode and cathode serving as a medium for charge transfer.3,4 Typical membrane separator consists of porous material with salts (composite membrane) or with an embedded electrolyte solution. The electrolyte solution is composed of salts that are dissolved in a solvent, such as water or organic molecules.5 Typically, functionality of the separator membrane is largely determined by several parameters such as degree of porosity, average pore size, and tortuosity.6 All these affect the uptake process of the electrolyte solution and, therefore, the ionic conductivity.7,8 Bruggeman exponent that quantifies electronic conductivity of the membrane depends on both tortuosity and degree of porosity. Commonly, the Bruggeman exponent is assumed to be 1.59 but different authors refer to values ranging from 2.4 to 4.5.10 Furthermore, the use of different salts (LiBF4, LiTFSI, etc.) in electrolyte solution for a given degree of porosity of polymer membrane leads to the variation of tortuosity in the range between 3.3 and 4.1.11 Generally, ionic conductivity depends not only on the porosity and tortuosity but also on the apparent affinity, i.e, on the interaction between the salt and the polymer chains within the membrane. Polymers such as poly(vinylidene fluoride) (PVDF) and its copolymers poly(vinylidene fluoride-co-trifluoroethylene)

(P(VDF-TrFE)),

poly(vinylidene

fluoride-co-

hexafluoropropylene), P(VDF-HFP), and poly(vinylidene fluoride-co-chlorotrifluoroethylene), P(VDF-CTFE) are being intensively investigated for their use as battery separator membranes due to their high polarity, excellent thermal and mechanical properties,12 controllable porosity and wettability offered by organic solvents, and chemical inertness and stability in cathodic


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environment.6 PVDF-based electrolytes have higher conductivities (about 10 times) than the corresponding PVS-based electrolytes but may have lower chemical stability.13 Higher ionic conductivity of PVDF is due to several reasons, in particular (i) high dielectric constant (ε=6-12) which helps to enhance ionic dissociation in the electrolyte resulting in higher concentration of ionic charge carriers, (ii) weaker coupling of ions with the polymer chains. Different modifications of PVDF - α, β, γ – phases (P(VDF-TrFE), P(VDF-CTFE), P(VDF-HFP) show different electrical properties and chemical stabilities, thus allowing flexibility in the separator designs for specific applications.12 In order to improve the performance of battery separator membranes based on PVDF, it is prerequisite to study their microstructure and ionic conductivity at both nano- and microscales. Ionic conductivity of the battery separator membrane affects the entire battery performance and is determined by the interaction between the salts and the polymer chains.14 One of the few experimental techniques capable to probe these properties locally (i.e. at the scale of several tens of nm) is the method of Scanning Probe Microscopy (SPM) and, in particular, Electrochemical Strain Microscopy (ESM), the novel technique that emerged in recent years.15 Electrochemical mechanisms and nature of ionic transport determined at the nanoscale by ESM are fundamental for the understanding and thus improving the performance of the materials for battery applications.16 The principle of this technique is similar to Piezoresponse Force Microscopy (PFM), in which the cantilever in the contact regime measures the variation in the local strain of a surface caused by piezoelectric effect.17,18 In the case of ESM, the variation of local strain under applied bias is related to the ionic motion within the materials, either due to electrode polarization or surface electrochemical processes.19 The main advantage of this technique is the ability to monitor the electrochemical phenomena in solids at the nanometer scale by measuring dynamic strain, rather than current.15,19 This technique has been already validated on cathode


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materials such as LiCoO217 and LiFePO420, on Li-ion conductive glass-ceramics,21 and on ionconductive oxides such as YSZ22 and NiO.23 As for LiCoO2 cathode material, it has been shown that lithium ion diffusivity is higher for certain grains and grain boundaries and thus it is necessary to optimize the ionic conductivity of the grain boundary network, rather than the material itself.17 Using ESM method it has been revealed that the nanocrystalline LiFePO4 (LFP) exhibits higher Li-ion diffusivity and lower energy dissipation as compared to microcrystalline LFP. This explained its higher capacity observed at the macroscopic (battery) level.20 This work focuses on studying both ESM response (i.e. local electrochemical activity of electrolyte at high frequency) and dc mediated strain in two polymer battery separators based on PVDF and PVDF-CTFE. The choice of these materials is due to their essentially different structure. PVDF is a semi-crystalline polymer in which the amorphous part is embedded between the lamellar crystalline structures with a crystallinity degree ranging from 40 to 60%. PVDF-CTFE is also a semi-crystalline polymer; however, its degree of crystallinity is significantly lower due to the addition of chlorotrifluoroethylene (CTFE). This should result in different ionic transport at the nanoscale. In this work, we show that both the dc strain and ESM response observed in separator membranes are a result of electroosmotic flow (EOF) of the electrolyte in the porous media rather than due to Vegard’s expansion induced by ionic motion within the membrane. The results demonstrate that the ESM can be successfully used to study a new class of battery materials and present a novel mechanism of electric field induced strain in non-piezoelectric media. Results PVDF and PVDF-CTFE-based membranes were prepared by dissolving the polymer material in N, N-dimethylformamide at a 15/85 polymer/solvent weight ratio (see Materials and Methods section). This experimental procedure leads to a polymer membrane with a well-


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defined porous microstructure.24 Though the porous structure is homogeneous all along the sample thickness, the bottom of the sample in contact with the Petri dish shows a thin glossy surface (solvent is evaporated to and from the top of the sample). This is relevant for the ESM measurements, as will be delineated below. Separator membranes will be called hereafter by their polymer name: PVDF and PVDF-CTFE. Table 1 presents the relevant parameters of studied membranes: the porosity, the average pore size, and both the β-phase content and the amorphous phase content. The values were calculated by the pycnometer method, by using Scanning Electron Microscopy images, by Fourier Transformed Infrared Spectroscopy and by Differential Scanning Calorimetry, respectively, following the procedures described in detail in Refs. 25 and 26. Table 1. Porosity, average pore size, β-phase content and amorphous content of the separator membranes. Separator

Porosity

Average

membrane

/± 3%

size / ± 0.5µm

PVDF

65

1

70

40

60

7

32

73

PVDF-CTFE

pore β-phase content Amorphous / ± 2%

content / ± 2%

The experimental setup of Electrochemical Strain Microscopy (ESM) measurements is schematically shown in Fig. 1. Before measurements the separator membrane was placed on top of electrolyte droplet and maintained for 10-15 minutes in order to let the electrolyte to soak in (Fig. 2(b)). DC strain measurements were performed by applying positive and negative voltage pulses of 1 V in amplitude. Strain was measured as a function of time by tracking the displacement of the piezoscanner in the constant deflection regime. ESM measurements could be performed on membranes with both porous and glossy surfaces. Apparently, ESM measurements on porous surface were not stable and reproducible due to a very high roughness and wide open pores (Fig. 2(a)). Due to inherent drift of the scanning system and sample deformation during measurements, the mechanical and electrical


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contact of the tip with the porous surface resulted in hysteresis loops of variable shapes and magnitudes upon repetitive acquisition. If the shape and magnitude of the loops did not change under consecutive voltage ramping, then the ESM measurements were considered as reproducible. Typical surface topography of the porous separator membrane is shown in Fig. 2(a). On the glossy surface (that was in contact with the Petri dish during preparation) the ESM measurements were much more reproducible. Typical topography of the glossy surface is shown in Fig. 2(b). Due to better stability the results presented below are obtained from the glossy surface.

BE

Figure 1. Schematic illustration of the ESM experimental setup. Probing ac and bias dc voltages are applied between the steel back electrode (BE) and the conducting tip. First measurements were performed on samples placed on the steel substrate without electrolyte (Fig. 1). Due to the absence of mobile electrolyte ions in dry membranes no ESM response was detected. Further measurements were carried out by placing the sample on the droplet of the electrolyte as described in the experimental section. During 10-15 minutes prior to measurements the electrolyte molecules penetrate into the pores from the bottom side, while the surface of the sample remains dry (as schematically illustrated in Fig. 2(c)). After the tip approaches the surface and ac/dc voltage is applied, a small meniscus of electrolyte can be formed around the tip (Fig. 2(d)), indicating that electrolyte appears at the surface. After scanning the tip was moved to the predetermined locations and ESM voltage spectroscopy measurements were performed by applying a sequence of ac and dc pulses. During this process


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the wetting meniscus can increase in size as it is shown in Fig. 2(e). As a consequence, the apparent ESM response also increased and equilibrated at a certain level.

Figure 2. AFM surface topography of (a) porous separator surface and (b) glossy surface (measured in tapping mode). Schematic illustration of ESM measurements on glossy surface (without big open pores): (c) no meniscus – dry surface; (d) small meniscus; (e) large meniscus – wet surface. Figure 3 illustrates the ESM response measured on a dry glossy surface of PVDF-CTFE (see schematic in Fig. 2(c)). ESM response is notably higher on topographical depressions (Figs. 3(a) and 4(b)). Electrolyte molecules penetrate into the pores under capillary action and, therefore, higher electrolyte content in PVDF and higher ESM response are usually observed on the depressions rather than on protrusions. In a number of measurements, the tip stuck on the surface if the electrolyte remained attached. Consequently, some images were smoothed and distorted due to tip buckling, but correlation between topography and ESM response is still recognizable (Fig. 3 (c,d)).


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Figure 3. (a) Topography and (b) corresponding ESM magnitude of the PVDF-CTFE membrane. (c) Topography and (d) corresponding ESM magnitude of the PVDF membrane. Voltage spectroscopy measurements were performed by applying square dc pulses while increasing and decreasing their heights. Probing ac voltage (1 V, 10 – 20 kHz) was applied both on top of the dc pulses (on-field) and between the dc pulses (off-field) to measure the ESM response (Fig. 4(a)). The shape of the ESM hysteresis loop represents kinetics of nonequilibrium electromigration processes induced in the material by the applied electric field. In solid materials it usually opens up when the dc bias changes the total number of mobile ions below the tip. Figure 5 illustrates hysteresis loops obtained on dry surfaces. The ESM response from PVDF-CTFE sample is higher than that from the PVDF membrane, the loops having also different shapes. The evolution of the response over acquisition time is indicated by color contrast.


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Figure 4. Voltage spectroscopy: (a) schematic of dc and ac pulses; (b) schematic of the loop formation: dc on – on field, dc off – off field. Color scale indicates the time of migration/relaxation of the ESM amplitude during/after a single dc pulse. Figure 6 shows hysteresis loops obtained on wet surfaces. The response from PVDFCTFE sample is also higher than from PVDF. ESM hysteresis loops show similar relaxation trends. Loops from PVDF-CTFE membranes are largely open, while the ones from PVDF sample are more closed without significant hysteresis. The response from the wet surface (see schematic in Fig. 3 (c-e)) was found to be about one order of magnitude higher than that from dry surfaces and was proportionally scaled for both PVDF and PVDF-CTFE samples. One can see that the loops shown in Figs. 6(a,b) and 6(e,f) are similar, while the loops in Figs. 6(c,d) and 6(g, h) are significantly different. Loops shown in Fig. 9 were measured for shorter pulse period – 0.6 sec. In this case, the loops are more open. Such behavior can be


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associated with the ion relaxation processes. At different polarities (left and right branches of the loops) of DC pulses the directions of relaxation are opposite. Loops from the PVDF-CTFE sample clearly indicate polar behavior of the relaxations.

Figure 5. ESM loops measured on dry surfaces of PVDF-CTFE sample: (a) on field, (b) off field. PVDF sample: (c) on field, (d) off field. Pulse period 2 sec.

According to Tselev et al27 vertical surface displacement (ESM amplitude), proportional to the change of total number of excess ions (average concentration variation) in the diffuse layer per unit area for the ac voltage oscillation half-period, can be estimated as:

u ∝ δC (t ) ∝

eCi DiVac fk BTR0 ,

(1)

where e is the elementary charge, Ci is the concentration of mobile ions, Di is the diffusion coefficient, Vac is the applied periodic electric field with frequency f , and R0 is the radius of the tip-sample contact area. The behavior shown in Figs. 5, 6 cannot be described by only one type of mobile ions contributing to local strain in the framework of the standard ESM theory. It


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can only illustrate multiple processes and their different contributions when studied surface is either dry or wet.

Figure 6. ESM loops measured on the wet surface of PVDF-CTFE sample: (a),(e) on field, (b), (f) off field. PVDF sample: (c), (g) on field, (d), (h) off field. (a-d) pulse period 2 sec. (e-h) pulse period 0.6 sec.


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Discussion ESM response shown in Fig. 3(b,d) represents the combined contribution of several factors and cannot be directly used to distinguish among different mechanisms involved in signal formation. It only illustrates how different elements of topography respond to the ac voltage excitation, while understanding and interpretation of the results require the knowledge of actual mechanisms of the response. ESM response is typically attributed to local strain caused by Vegard’s expansion.28,29 However, the surface displacement of any nature can be detected by the sensitive ESM setup and contribute to the total response. In materials with more than one possible contribution this fact complicates the interpretation of obtained results. In PVDF and its copolymers filled with electrolyte the applied voltage produces electric field that can cause a number of effects: (i) piezoelectric/ferroelectric response30, (ii) ac diffusioosmosis and electroosmosis,31,32 (iii) electrostriction – indraught and orientation of the polar LiTFSI+PC molecules (dipoles), (iv) surface effects – change of surface tension of PVDF due to change of chemical composition at the PVDF/electrolyte interface, and (v) Li abstraction from the electrolyte molecule and migration of Li+ (cation) and negatively charged bis-trifluoromethanesulfonimide molecule (anion). Below we consider some of these mechanisms and their possible contribution to the experimentally observed strain. PVDF and PVDF-CTFE are semi-crystalline polymers with a spherulite structure. Typical dimension of the spherulites is 1-4 µm and depends on the crystallization temperature. This structure is composed of small crystallites with typical dimensions of ≈ 10 nm organized in groups of lamellae, with partially oriented amorphous intralamellar phase. Further, due to the monomer structure, PVDF and co-polymer show high dielectric constant (up to ~12) and highly polar polymer chains, in particular in the electroactive β-phase.33,34


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Taking into account the energy and structural differences between the amorphous and crystalline regions, the electrolyte uptake can occur in the amorphous region which undergo swelling to accommodate the electrolyte.35 In this way, ion transport in the amorphous region should be taken into account for the ESM measurements.7,36 It is well known that, when poled in an electric field, PVDF and its copolymers exhibit ferroelectric and, therefore, also piezoelectric properties.30 This response is attributed to PVDF and – if exists – should be present with or without the presence of an electrolyte. The ESM measurements without electrolyte did not reveal any ac response (without dc field) so the samples do not polar even under an applied electric field. Therefore, piezoelectric/ferroelectric nature of the response should be ruled out. Volumetric forces acting on the dielectric fluid in nonuniform electric field are determined by the Helmholtz equation.37,38,39

r ε ε  ∂ε  ∇P = F = eδnE − 0 E 2∇ε + 0 ∇ E 2 r ρ  2 2  ∂ρ  , where for polar dielectrics

(2)

∂ε r ρ = αε r . Here α ≤ 1.5 is the empirical factor for most of ∂ρ

the studied polar dielectric liquids40,41, E is the electric field, ρ is the mass density, and ε r is the relative permittivity of the material ( ε 0 is the vacuum permittivity). If we disregard forces acting due to liquid inhomogeneity, we obtain:

r ε ∇P = F = eδnE + α diel ∇E 2 2 .

(3)

Here and further, ε diel = ε 0ε r . The second term in Eq. 3 is the electrostriction equation, and the first term is associated with the electroosmotic flow (EOF) of electric double-layer at charged interface in a solution. Electroosmotic flow can be expected in the porous polymers filled by electrolyte.42 Together with the electrostriction force (Eq. 3) it can induce differential pressure in the porous structure resulting in surface deformations which can be detected by the


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ESM tip. Dipolar response was reported by Chen et al in 43. In our case, indraught into stronger electric field near the tip and orientation of polar (LiTFSI+PC) molecules (dipoles) can lead to a significant dipolar moment and corresponding strain response. The difference between the electroosmotic and electrostriction responses in Eq. 3 can be separated due to their polar (for electroosmotic) and nonpolar (for electrostriction) contributions. Deformation of porous membranes with developed surface induced by the change of surface tension in variable media can give a notable contribution. When PVDF membrane is filled with electrolyte, it apparently increases its volume due to the reduction of PVDF surface tension in the electrolyte media. Electric field applied to ESM tip induces ionic transport which can change electrolyte composition and the surface state in the PVDF pores thus influencing the PVDF surface tension and giving rise to volume deformation of the membrane. Typical surface tension of PVDF calculated by different authors ranges from ~ 30 to ~ 40 mJ/m2.44,45 Change (reduction) of the PVDF surface tension in electrolyte due to interaction with surfactant molecules or due to variable electrolyte composition can thus induce deformations of the highly porous membranes up to several %. Another possible mechanism assumes that free Li+ cations and TFSI- anions can move in the electric field, penetrating mainly within the amorphous phase of the polymers (ionic absorption into PVDF polymer was reported by Faria et al.46 This effect is followed by the local volume change (expansion) and corresponding surface displacement that can be detected by ESM. Amplitude of the detected ESM response depends on the local change of concentration of the anions and cations and their impact on the volume expansion. The volume expansion can be described similarly to the Vegard’s expansion in alloys by Vegard’s like coefficients βTFSI for anions and βLi for cations. The measured response can be thus attributed to the combination of both. Contribution of anions and cations can be determined from the analysis of the relaxation kinetics after dc pulses. PVDF is known as a good insulator, therefore electric charge transport


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is mainly provided by electrolyte through the pores of PVDF membrane. Therefore, we will consider this contribution as an additional mechanism. Further we consider electroosmotic contribution as main contribution in a more detail. Solids in polar solvents usually develop surface charges, due to either dissociation of chemical groups on the surface, or to chemical binding or physical adsorption of ions from the electrolyte. This surface charge is balanced by an equal and opposite net charge of ions in the electrolyte.47 Electroosmotic flow (EOF) is the motion of a liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or any other fluid conduit.48 Electroosmotic flow is caused by the Coulomb force induced by an electric field on net mobile electric charge in a solution. Because the chemical equilibrium between the solid surface and electrolyte solution typically leads to the interface acquiring a net fixed electrical charge, a layer of mobile ions, known as an electrical double layer or Debye layer, forms in the region near the interface. When the high enough electric field is applied to the fluid (usually via electrodes), the net charge in the electrical double layer is induced to move by the resulting Coulomb force. The resulting flow is termed electroosmotic flow. Typical PVDF and PVDF-CTFE contain polar functional group -C-F. We assume that one of the possible mechanisms can be as follows: fluorine ions can dissociate into electrolyte solution, induce negatively charged diffuse layer and positively charged capillary wall (Fig. 7). The applied voltage induces electrophoresis flow of electrolyte through the porous media resulting in the deformation of porous PVDF-CTFE under the tip. For simplicity, a microporous membrane is assumed to be a matrix composed of many micropores in the shape of uniform long cylinders of radius ρ joining spherical cavities (see Supporting Information for details). Equation describing dynamics of local deformation of the elastic porous membrane filled with incompressible isotropic fluid at large scale > ρ can be written (see Supporting Information) as:

∂ε = D ⋅ ∆ε , ∂t ACS Paragon Plus Environment

(4)

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Figure 7. Capillary electrophoresis in PVDF and PVDF-CTFE. Typical PVDF and PVDFCTFE contain polar functional group -C-F. Schematic presentation of the possible mechanism of formation negatively charged diffuse layer and positively charged capillary wall – dissociation fluorine ions into electrolyte solution. where D =

σ b ρ~02 Y (1 − ν ) , ε is the strain of isotropic elastic media, ρ~ = ρ~0 (1 + ε ) is 2σ 0 η (1 + ν )(1 − 2ν )

the effective radius of the capillaries at ε

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