Ionic Conductivity, Self-Assembly, and Viscoelasticity in Poly(styrene-b

Institute of Physics and Max Planck Institute for Polymer Research, Johannes Gutenberg University Mainz, 55128 Mainz, Germany. Macromolecules , 2015, ...
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Ionic Conductivity, Self-Assembly, and Viscoelasticity in Poly(styrene‑b‑ethylene oxide) Electrolytes Doped with LiTf George Zardalidis,*,† Katerina Gatsouli,‡ Stergios Pispas,‡ Markus Mezger,§ and George Floudas*,† †

Department of Physics, University of Ioannina, P.O. Box 1186, 451 10 Ioannina, Greece Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 116 35 Athens, Greece § Institute of Physics and Max Planck Institute for Polymer Research, Johannes Gutenberg University Mainz, 55128 Mainz, Germany ‡

S Supporting Information *

ABSTRACT: Diblock copolymers of poly(styrene-b-ethylene oxide), PS-b-PEO, are employed together with lithium triflate (CF3SO3Li, LiTf) at several [EO]:[Li] ratios as solid polymer electrolytes. Their thermodynamic state, self-assembly, and viscoelastic properties are discussed in conjunction with the ionic conductivity. PS-b-PEO/LiTf differs from the wellinvestigated PS-b-PEO/LiTFSI system in that the electrolyte in the former binds intramolecularly to PEO chains. Microscopic and macroscopic parameters affecting ion transport are discussed. From a microscopic point of view different parameters were considered as potential regulators of ion transport: the characteristic domain spacing, d, the interfacial thickness, Δ, and the ratio of Δ/d. By comparing two block copolymer electrolytes (PS-b-PEO and PI-b-PEO) bearing the same conducting block (PEO) and the same electrolyte (LiTf) but in the presence of different interactions, among the microscopic parameters it is the domain spacing that appears to have the most decisive role in ionic conductivity. Ion conductivity in PS-b-PEO/LiTf exhibits a molecular weight dependence similar to that reported for the PS-b-PEO/LiTFSI system, however, with somewhat lower values reflecting anion size effects. Among the macroscopic factors that limit ionic conductivity, the possible preferential wetting of the electrodes by either of the constituent phases can lead to an orientation that effectively blocks ion transport. The viscoelastic properties of the block copolymer electrolytes differ substantially from the neat block copolymers. Li-ion coordination affects not only the PEO segments but also, surprisingly, the PS segments. An increase in PS glass temperature by ∼10 K is reported. In addition, the viscoelastic properties suggest the formation of transient structures in the molten complex.

1. INTRODUCTION There is extensive research toward the aim of achieving truly solid polymer electrolytes (SPEs) for applications in batteries.1−4 Potential candidates should have (i) high ionic conductivity, (ii) high lithium ion transference number,5 and, at the same time, (iii) high elastic modulus (∼0.1 GPa). The first requirement is obvious. The second relies on the fact that in typical lithium batteries only the cations are electroactive toward the electrodes. Hence, high cationic transference numbers are of importance. The third requirement is a necessity to prevent dendritic growth at the electrodes.6 Moreover, materials must comply with the requirements of safety and have environmentally friendly constituents. Polymeric materials are good candidates for meeting these requirements. Among them, poly(ethylene oxide) (PEO) is a possible polymeric matrix. Its low glass temperature Tg, the concomitant high segmental mobility,7 the relatively high dielectric constant (dielectric permittivity, ε′ ∼ 5, at ambient temperature) that facilitates charge separation, and the solvating capability for a number of lithium salts make PEObased matrices promising candidates for battery applications. © XXXX American Chemical Society

However, at temperatures above its melting point PEO has a low modulus that does not comply with the requirement for mechanical stability. In this respect, block copolymer electrolytes composed of soft/hard nanophases offer particular advantages.8 The soft nanophase serves as ion-conducting phase, whereas the hard nanophase imparts the required mechanical strength. Among the different SPEs, the phase behavior of lithium salt-doped poly(styrene-b-ethylene oxide) (PS-b-PEO) copolymers was extensively investigated as a function of LiX salt concentration and for different counterions (X = ClO4−, CF3SO3−, AsF6−).9−23 However, the most popular lithium salt is composed of the large bis(trifluoromethane)sulfonamide (TFSI−) anion. It was shown that the formation of PEO−salt complexes can lead to changes in block copolymer nanostructure and domain spacing.22,10,7 Other studies on PSb-PEO electrolytes as a function of molecular weight revealed that ion conductivity is governed by two competing factors: the Received: July 17, 2015 Revised: September 6, 2015

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parameter significantly, one exerts control over the microscopic parameters (domain size and interfacial width) that could possibly affect ion transport. Lastly, we are comparing the viscoelastic properties of the SEO/LiTf block copolymer electrolytes with the bulk polymer electrolyte PEO/LiTf. Here, the aim is to explore the viscoelastic characteristics of the “phase” bearing the highest ionic conductivity. As expected, this phase forms at temperatures above the dissolution of the complex crystal. However, unexpectedly, it has a viscoelastic signature distinctly different from the bulk copolymer. We show that there exist some transient structures in SEO/LiTf even at elevated temperatures.

glass temperature of the PS phase and the channel width of the conducting PEO phase.15,16,22,23 It was suggested that longrange order impedes ion transport.24 Subsequently, selfconsistent field theory addressed the inhomogeneous distribution of Li+ ions within the conducting phase (they tend to occupy the central region of the PEO phase15), the preferential solvation energy of anions, the translational entropy of anions, the ion-pair equilibrium between polymer-bound Li+ and anion, and the changes in the interaction parameter, χ, due to the bound ions.25−27 There is consensus that both macroscopic and microscopic factors affect ion transport in SPEs. On the macroscopic scales, nanophase separated block copolymers consist of a large number of grains with a varying size and orientation. Ions must find their path from one electrode to the other by moving always within the PEO phase by traversing several grain boundaries. In general, ion diffusion takes place in three dimensions. However, regarding intragrain transport ions are restricted essentially in two dimensions within the lamellar geometry of PS-b-PEO. Combining this fact with the random orientation of the lamellae plains, it is obvious that ion movement is subject to significant constrains within a single grain and over several grain boundaries at longer distances.20,24 These factors have been summarized in

σ = wbmϕσ0

2. EXPERIMENTAL SECTION 2.1. Materials. Synthesis of Block Copolymers. The synthesis of the PS-b-PEO block copolymers was made by anionic polymerization high-vacuum techniques35 employing sec-butyllithium as initiator and benzene as the solvent. Styrene was the first monomer to be polymerized at 298 K, for 24 h, followed by the polymerization of ethylene oxide, in the presence of phosphazene base (t-BuP4), at a temperature of 313 K for 48 h, as described before.36 The block copolymers, S93EO237 and S467EO940 (denoted as SxEOy, with x and y being the degrees of polymerization of PS and PEO, respectively), were characterized by size exclusion chromatography and 1H NMR spectroscopy in order to obtain the molecular weight and the composition of the copolymers as well as to confirm the uniformity of the materials. Preparation of Block Copolymer/LiTF Composites. For preparing the PS−PEO/LiTF composites the block copolymers were dissolved in dry THF (stock solutions of 5% w/v concentration were prepared), followed by a 2 h heating at 333 K in order to ensure complete dissolution. A stock solution of LiTF (Aldrich) in dry THF (5% w/v) was also prepared. Mixing of the two solutions took place in the appropriate volume ratios, followed by evaporation of the solvent at ca. 303 K, in order to obtain composites of different Li:EO ratios. 2.2. Characterization Methods. Differential Scanning Calorimetry. The thermal properties of the block copolymer electrolytes were studied with a Q2000 (TA Instruments) differential scanning calorimeter (DSC). Cooling and heating cycles were performed at a rate of 10 K/min and in a temperature range between 173 and 473 K. The instrument was calibrated for best performance on the specific temperature range and heating/cooling rate. The calibration sequence included a baseline calibration for the determination of the time constants and capacitances of the sample and reference sensor using a sapphire standard, an enthalpy, temperature calibration for the correction of thermal resistance using indium as standard (ΔH = 28.71 J/g, Tm = 428.8 K), and a heat capacity calibration with sapphire standard. X-ray Scattering. Small-angle X-ray scattering (SAXS) measurements were made using Cu Kα radiation (Rigaku MicroMax 007 X-ray generator, Osmic Confocal Max-Flux curved multilayer optics). Oriented fibers of 1.0 mm diameter were prepared by a mini-extruder. 2D scattering patterns were recorded on a Mar345 image plate at a sample-to-detector distance of 1.8 m. Radial intensity distributions are presented as a function of the modulus of the scattering vector q = (4π/λ) sin(2θ/2), where 2θ is the scattering angle. Temperaturedependent SAXS measurements in the range from 303 to 473 K in steps of 10 K were made on heating and on subsequent cooling. Dielectric Spectroscopy (DS). Sample capacitors were prepared in a glovebox under a controlled nitrogen atmosphere. Samples were heated up to the melting temperature of the complex and hot pressed between two mirror polished brass electrodes to a thickness of 100 μm maintained with Teflon spacers. Dielectric spectroscopy measurements were made with a Novocontrol BDS system composed of a frequency response analyzer (Solartron Schlumberger FRA 1260) and a broadband dielectric converter. The experiments were performed at atmospheric pressure in the temperature range from 193 to 473 K and for frequencies in the range from 0.01 to 106 Hz. The conductivity has

(1)

where σ0 is the conductivity of the respective homopolymer electrolyte (in this case PEO/LiTf) and φ is the volume fraction of the conducting phase; m describes the effect of morphology and accounts for the effect of the dimensionality of the conducting phase. For randomly oriented lamellae (cylinders) two-thirds (one-third) of the domain orientations contribute to ion transport direction; hence m = 2/3 (1/3).28 Furthermore, the lamellar plains get twisted and bended on grain boundaries. These deformations result in a local distortion in the dimensions and connectivity of the conducting paths between adjacent PEO regions. TEM studies as well as molecular dynamics simulations have shown a decrease at the cross section of the conducting paths, at the points where neighboring grains meet.20,30,31 The parameter b accounts for these grain boundary and grain size effects. Lastly, w includes the influence of electrode wettability by either phase. Recently, it was shown that a propensity by one of the blocks to wet the electrodes can lead to blocking orientation at the electrode surface, at least for some layers.29 Therefore, it is not surprising that the ion transport in SPEs shows a strong influence on annealing.24 The present study employs the same PS-b-PEO copolymer but uses a different electrolyte, namely LiCF3SO3 (LiTf), that is considered as the archetypal polymer electrolyte.7 In PEO/LiTf, each Li ion coordinates with three ether oxygens and with an oxygen from each of two adjacent CF3SO3− groups but in the absence of interchain links. 32 Hence, in contrast to intermolecular bound LiTFSI electrolytes, LiTf binds intramolecularly to the PEO chains. Therefore, the aim is to explore the influence of the anion type on the ion transport (dc conductivity) as a function of the block copolymer molecular weight and as a function of increasing interaction parameter. The latter is facilitated by comparing PS-b-PEO with poly(isoprene)-b-poly(ethylene oxide) (PI-b-PEO), enriched with the same salt, but in the presence of significantly different interaction parameters for the neat block copolymers (χSEO = 0.043 and χIEO = 0.31 at 423 K33,34). By tuning the interaction B

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Macromolecules been studied using the analysis of the complex conductivity function through σ* = σ′ + iσ″, which is related to the complex dielectric permittivity with σ* = iωε0ε*.37,38 The dc conductivity has been analyzed with respect to the random free energy barrier model by J. Dyre according to which the charge carriers are hopping in a spatially randomly varying energy barriers.39 According to the model, the onset of dc conductivity is determined by crossing the highest energy barrier. The model provides an analytical expression for the complex dielectric function as ε*(ω) = ε∞ + σ0τe/[ε0 ln(1 + iωτe)], where ε∞ is the value of ε′ in the limit of high frequencies and σ0 and τe are the dc conductivity and characteristic time of ion motion. It predicts a universal shape for the conductivity contribution based on these two parameters. The model predictions are tested earlier against the experimental data for the PEO/LiTf electrolyte. The predictions can only partially fit the experimental data and within a limited frequency range. We attribute these deviations to the nature of polymer electrolyte. The present system cannot be considered as disordered, and in addition, formation of the complex is strongly temperature dependent. Alternatively, the dc conductivity can be obtained by the plateau in the real part, σ′, without invoking any model. Rheology. A TA Instruments AR-G2 with a magnetic bearing that allows for nanotorque control was used for recording the viscoelastic properties of the polymer electrolytes. Measurements were made with the environmental test chamber as a function of temperature. Samples were prepared on the lower rheometer plate (8 mm), the upper plate was brought into contact, and the gap thickness was adjusted. The linear and nonlinear viscoelastic regions were determined by the strain amplitude dependence of the complex shear modulus |G*| at ω = 10 rad/s. Measurements involved isothermal frequency scans within the range 10−1 < ω < 102 rad/s at selected temperatures and isochronal temperature ramps with ω = 10 rad/s between 333 and 403 K.

corresponding to the melting of the complex crystal. On increasing salt concentration to [EO]:[Li+] = 8:1, PEO crystals melt at ∼313 K whereas the complex crystal melts at 398 K. A similar behavior is seen at the [EO]:[Li+] = 6:1 composition with respective melting temperatures at 318 and 419 K for PEO crystals and crystalline complex. At the higher salt concentrations studied, [EO]:[Li+] = 4:1 and 3:1, PEO crystallization is fully suppressed, suggesting that the entire PEO chain is coordinated with lithium ions. The corresponding melting temperatures for the complex are now at 420 and 417 K for the 4:1 and 3:1 concentrations, respectively. The PEO crystal and complex crystal degrees of crystallinity are calculated using XC = ΔH/(wXΔH0X), where ΔH is the change in enthalpy during melting as determined by DSC, wX is the weight fraction of PEO, and ΔH0X is the heat of fusion of the ideal crystal (ΔH0PEO = 196 J/g, ΔH0compl = 127 J/g). The latter (ΔH0compl) was estimated from the (PEO)4LiCF3SO3 change in enthalpy during melting of the complex crystal.7 Figure 1b depicts the DSC traces of the high molecular weight electrolyte S467EO940/LiTf. The thermal behavior is qualitatively similar to the respective homopolymer case. On heating, cold crystallization of PEO crystals and the formation of complex crystal are also observed at the lower salt concentration [EO]:[Li+] = 12:1. Here melting of PEO crystals takes place at 330 K and of the complex crystal at 394 K. At [EO]:[Li+] = 8:1, PEO crystals melt at 330 K while melting of the complex crystal is a broad process with a maximum at 405 K. At the higher salt concentration studied, [EO]:[Li+] = 4:1, crystalline complex is the only ordered structure that melts at 443 K. The calculated degrees of crystallinity for PEO crystals and crystalline complex are summarized in Figure 2 for the studied

3. RESULTS AND DISCUSSION 3.1. Thermal Properties. The thermal characteristics of SEO electrolytes are discussed with respect to the DSC traces of Figure 1. The homopolymer electrolyte PEO318/LiTf traces

Figure 2. Degree of crystallinity for PEO crystals (top) and PEO/LiTf complex crystals (bottom) for the homopolymer PEO318/LiTf and the two block copolymer electrolytes SEO/LiTf. Figure 1. DSC traces of (a) S93EO237/LiTf and (b) S467EO940/LiTf block copolymer electrolytes obtained on heating with a rate of 10 K/ min. The dashed lines give the traces of the corresponding PEO316/ LiTf homopolymer electrolytes. Arrows indicate the melting temperature of the complex crystals.

SEO/LiTf copolymers in comparison to the homopolymer electrolyte PEO/LiTf. There are two main findings with respect to Figure 2. First, the PEO degrees of crystallinity in the homopolymer electrolyte are substantially higher than in the block copolymer electrolytes. Interfacial mixing and/or chain stretching in the block copolymer electrolytes significantly reduce PEO crystal order and complex crystal order. Between the two SEO copolymers, the copolymer having the lower molecular weight (S93/EO237) has a reduced PEO degree of crystallinity. This reflects the higher interfacial mixing for the lower molecular weight copolymer.8 Second, complex crystal formation grows at the expense of the PEO crystal for both the homopolymer and block copolymer electrolytes. This is

under the same conditions7 are also depicted for comparison. Figure 1a shows the traces for the lower molecular weight block copolymer electrolyte, S93EO237/LiTf. At low salt concentration, i.e., [EO]:[Li+] = 12:1, the behavior is qualitatively similar to the homopolymer case. It shows the characteristic cold crystallization of PEO and complex crystal formation that are followed by the melting of crystalline PEO at 320 K and by a broad endothermic process with a peak around 372 K C

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Macromolecules understandable as at the stoichiometric concentration PEO crystallization is completely suppressed whereas decreasing electrolyte content allows for the formation of PEO crystals as well. 3.2. Structural Characteristics. Figure 3 shows the SAXS patterns of S93EO237/LiTf electrolytes as a function of salt

Figure 4. (left) SAXS pattern of the neat S467EO940 block copolymer and its electrolytes at 423 K. Vertical lines indicate the position of the calculated first-order reflections. Arrows indicate higher-order reflections. (right) POM images from the same S467EO940/LiTf electrolytes (left column) in comparison to the PEO/LiTf homopolymer electrolytes (right column) at 353 K. The scale bar in all POM images is 250 μm.

Figure 3. (left) SAXS pattern of the S93EO237/LiTf block copolymer electrolytes for different salt concentrations at 473 K. Vertical lines indicate the position of the first-order reflections. Arrows indicate higher-order reflections. (right) POM images from the same S93EO237/ LiTf electrolytes (left column) in comparison to the PEO/LiTf homopolymer electrolytes (right column) at 353 K. The scale bar in all POM images is 250 μm.

For the [EO]:[Li+] = 8:1 composition, the first-order reflection is observed at q = 0.083 nm−1. Higher-order reflections appear at relative positions 1:3:5:7. In the [EO]:[Li+] = 4:1 composition the first-order reflection appears at 0.067 nm−1 with higher order reflections at relative positions 1:2:3:4:5:6. In all cases, an increase in d-spacing with increasing electrolyte content was observed in agreement with earlier studies.10,19,22,29 The increase in d-spacing is mainly associated with an increase in the effective interaction parameter χeff that characterizes the unfavorable interactions between unlike blocks and to a lesser extent the increasing volume fraction of the PEO/LiTf domains. Increasing d-spacing can have different origins: In one picture, Li ions coordinated to ether oxygens stiffen the PEO backbone. This induces a change in nanodomain structure toward more segregated domains. In this view, the effective interaction parameter between PS and the PEO/LiTf complex, χeff, can be obtained from the nanodomain spacing. An alternative is provided by a recent self-consistent field theory.25−27 In this picture the key parameter is the preferential solvation energy of the anions. Different solvation energies create an effective repulsion between the blocks that increase the domain spacing. In most cases, the effective interaction parameter χeff can be obtained from disordered phase structure factor, S(q). However, the SEO/LiTf system is nanophase separated up to the highest temperature investigated (SSL) (Figure S1, Supporting Information). Presently, there are two alternative ways of extracting χeff. The first is based on the SSL prediction for the lattice spacing d ∼ χeff1/6N2/3 (Figure S2).8,40 Specifically, for the PS-b-PEO systems the dependence of d on χ1/6N2/3 is defined by using the present bulk SEO as well as literature data from other SEO copolymers.41 Subsequently, the same dependence is assumed for the SEO/LiTf electrolytes, and the corresponding χeff value is obtained from the measured

concentration at 473 K. At a [EO]:[Li+] = 12:1 composition, the scattering pattern exhibits a peak at a modulus of scattering vector of q1 = 0.28 nm−1. The peak at q3 = 0.82 nm−1 ≈ 3q1 is attributed to a higher-order reflection. This suggests a lamellar structure of equal domain thicknesses. In the [EO]:[Li+] = 8:1 composition three peaks can be seen with relative positions 1:2:3 with respect to the first reflection at 0.265 nm−1. The existence of an even-order reflection suggests a lamellar structure of unequal PS vs PEO domain thicknesses opposed to the 12:1 composition. For the [EO]:[Li+] = 6:1 and 4:1 compositions we find four peaks with relative positions 1:2:3:4 and a first-order reflection at 0.243 and 0.253 nm−1, respectively. Increasing salt concentration leads to a more ordered lamellar structure as it is evident by the peaks. At the stoichiometric composition [EO]:[Li+] = 3:1 the first-order reflection appears at q1 = 0.23 nm−1 with a second-order one at q1 = 0.46 nm−1. Figure 4 shows the SAXS patterns for the neat high molecular weight diblock copolymer S467 EO940 and for the corresponding S467EO940/LiTf electrolytes at 423 K. In the neat copolymer the scattering pattern displays a broad peak at q = 0.105 nm−1. Its width and the absence of higher-order reflections indicate the absence of long-range order. Even with a small amount of salt, i.e., [EO]:[Li+] = 12:1, pronounced higher-order reflections appear at relative positions 1:3:5:7:9 with respect to q1 = 0.101 nm−1. The absence of even-order reflections suggests the formation of long-range ordered lamellae composed of PS and PEO domains of equal thickness. D

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Macromolecules d-spacing. The second approach assumes the same dependence for χeff(N,r), where r = [Li+]:[EO], as in the SEO/LiTFSI system studied earlier. This approach is obtained from the variation of the disordered state structure factor, following predictions of the random phase approximation for PS-b-PEO/ LiTFS systems22 χeff =

⎛ 22.4Tr ⎞⎤ 10.2 1850 0.0101T ⎡ ⎟ + + ⎢⎣1 − exp⎜⎝ − ⎥ T TN N N ⎠⎦ (2)

where T is the temperature and N is the degree of polymerization. Figure 5 depicts the result of the calculated effective interaction parameter with the two methods. Clearly, the two

Figure 6. (top) Nanodomain d-spacing, (middle) interfacial thickness Δ, and (bottom) ratio Δ/d as a function of salt concentration. The blue and green symbols correspond to the high and low molecular weight SEO electrolytes. Red symbols correspond to IEO/LiTf. The filled and open symbols of Δ and Δ/d are obtained based on the two assumptions used in extracting the effective interaction parameter (from d ∼ χ1/6N2/3 and from using eq 2, respectively). Lines are guides for the eye.

Figure 5. Effective interaction parameter, χeff, of SEO/LiTf (green and blue symbols) and IEO/LiTf (red symbols) block copolymer electrolytes calculated from the d ∼ χ1/6N2/3 dependency (filled symbols) or via eq 2 (empty symbols).

electrolyte are included (obtained from the first method). Notice the same domain spacing for the S92EO237/LiTf and I111EO201/LiTf electrolytes. This is fortuitous and reflects the higher bare interaction parameter and smaller N, for the latter system. Nevertheless, it provides the possibility of comparing ion conductivities under conditions of identical domain spacing but with different interfacial area and hence different ratio of Δ/d. In the next section we discuss the ion conductivity in relation to these structural features. 3.3. Ionic Conductivity. The dc conductivities of the SEO/ LiTf block copolymer electrolytes at a range of temperatures and salt concentrations are depicted in Figure 7. The figure depicts the logarithm of normalized conductivities, log(σSEO−salt/(φEO−saltσPEO−salt)), where σSEO−salt refers to the conductivity of the block copolymer electrolyte, σPEO−salt is the conductivity of the homopolymer electrolyte, and φEO−salt is the volume fraction of the conducting phase, for the various salt concentrations of the block copolymer electrolytes. Because different salt concentrations have different melting temperatures for the complex (Figure 1), the temperature axis is also normalized with respect to the corresponding complex melting temperature. Comparing the two SEO molecular weights, it is the higher molecular weight that shows systematically higher conductivities independent of composition. Such a difference could be a result of the microscopic parameters d and Δ, suggesting that the smaller nanodomain spacing and wider interface of S92EO237/LiTf hinders ion movement through the existence of the glassy PS phase, as pointed out earlier.16 In addition, independent of molecular weight, it is the [EO]:[Li+] = 12:1 composition that shows the highest conductivity. This suggests the importance of an amorphous part of PEO chain

methods give quite different results. The values obtained from eq 2 show only a weak dependence on the salt concentration. The first approach shows a strong dependence, which is very evident for the higher molecular weight copolymer. In the same figure the effective interaction parameter from another block copolymer electrolyte, namely PI-b-PEO(IEO)/LiTf, bearing the same salt but a different (PI) block is also depicted for comparison.29 The higher interaction parameter of the neat copolymer is evident, while the incorporation of salt has little effect on χeff. Subsequently, the effective interaction parameter, χeff, as extracted by the two methods is employed in calculating the interfacial thickness, Δ, according to40,42 Δ=

⎞ ⎛ 2a ⎜ 1.34 ⎟ 1 + ⎟ 6χeff ⎜⎝ (χeff N )1/3 ⎠

(3)

where α is the characteristic segment length. Figure 6 depicts the main microscopic structural factors that could influence ionic conductivity as a function of salt concentration. They are the nanodomain spacing d (top), interfacial thickness Δ (middle), and the ratio Δ/d (bottom). As expected, the two approaches in determining χeff give different results for Δ and Δ/d. Nevertheless, the trend for Δ/d is similar. Comparing the two SEO/LiTf copolymers, we find a significantly higher ratio Δ/d for the lower molecular weight copolymer. A higher Δ/d means that a larger ion fraction resides in the vicinity or even inside the interfacial area. Therefore, they experience a different friction factor. In the same figure the data for the IEO/LiTf E

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different anions follow the same trend but with somewhat lower values for the SEO/LiTf system. It is likely that the larger TFSI− anion with the more delocalized charge has a lower binding to Li+, giving rise to more uncomplexed Li+ cations available for charge transport. Recent works emphasized the effects of charge delocalization and of ion size in ion dissociation and transport.43,44 In addition to the microscopic effects discussed above, macroscopic effects can also influence the measured conductivity (eq 1). The highest normalized conductivity observed here is for the S467EO940/LiTf system with [EO]:[Li+] = 12:1. However, even in this case the normalized conductivity is about 0.3, which is lower than the 2/3 factor expected for a random lamellar orientation.24,28 This can be discussed in terms of morphology and grain boundary effects (b in eq 1). In fact, we have observed that annealing reduces the measured conductivity (Figure S3). In addition, a recent investigation29 suggested that a propensity of one of the blocks for preferential wetting of the electrodes used in DS could result in an additional reduction in dc conductivity. Hence, we explored the wetting properties of the two “blocks” (PS and PEO/salt) on the electrodes used in DS studies. Contact angle measurements were performed at 458 K, just above the melting temperature of the complex and well above the PS glass temperature. Figure 9

Figure 7. Normalized conductivities for the PS-b-PEO/LiTf and PI-bPEO/LiTf block copolymer electrolytes. The temperature is normalized with respect to the complex melting temperature Tcomp m .

where ions are continuously incorporated following progressive melting of the stoichiometric complex. Because in the two SEO/LiTf systems all microscopic parameters (d, Δ, and Δ/d) vary according to Figure 6, it is not possible to draw a decisive conclusion on the single microscopic parameter that affects most ion transport. To this end, we include dc conductivities from the respective PI-bPEO/LiTf electrolyte.29 Interestingly, the dc conductivity of the latter system at [EO]:[Li+] = 8:1 is comparable to that of SEO/ LiTf. The two systems possess identical d but different Δ and Δ/d, revealing that among the different microscopic parameters it is the domain spacing that controls ion transport. Next we explore the effect of anion type on ion transport. Figure 8 compiles literature data16,17 for the normalized dc

Figure 9. Wetting of electrodes by a drop of PS (left) and PEO/LiTf (right) for three different electrodes: gold plated (a, b), stainless steel (c, d), and brass (e, f). The temperature is 458 K.

presents the different contact angles on three types of electrodes. A preferential wetting of one of the blocks is evident by the higher contact angles of PEO/salt, which could result in several layers of the conducting phase in a blocking orientation, i.e., with the lamellar normal perpendicular to the electrode surface. This explains the origin of the w factor appearing in eq 1. 3.4. Viscoelastic Behavior. Apart from high ionic conductivity SPEs should have an elastic modulus of the order of 0.1 GPa in the temperature range from 303 to 353 K. Such moduli prohibit dendritic growth on the electrodes during charge/discharge cycles.6 The viscoelastic properties of the homopolymer electrolytes, PEO/LiTf, with the same LiTf compositions have been examined earlier.7 It was found at the stoichiometric composition the crystalline complex shows a predominantly elastic response whereas the structure formed at more dilute concentrations exhibits a viscoelastic response. Another feature of the crystalline complex in the polymer electrolytes (PEO/LiTf electrolyte and SEO/LiTf electrolytes) is that they are very susceptible to strain. Thus, low strain amplitudes were employed that correspond to the linear viscoelastic range. Figure 10 (left) shows the storage and loss moduli of S92EO237/LiTf under isochronal conditions (ω = 10 rad/s). The neat block copolymer shows a PS glass temperature at around 353 K. The block copolymer electrolyte has a significantly higher elastic modulus up to 363 K. In addition,

Figure 8. Normalized conductivity, σSEO−salt/(φEO−saltσPEO−salt), as a function of SEO block copolymer molecular weight. Red triangles are from ref 17, and black circles are from ref 16. The salt in this case was LiTFSI. The green squares refer to SEO copolymer but with a different electrolyte (LiTf, this work).

conductivities of SEO diblock copolymers bearing either LiTFSI or LiTf as a function of copolymer molecular weight. The overall molecular weight dependence was discussed in terms of two competing factors: interfacial mixing for the lower molecular weights and width of conducting channel for the higher molecular weights. Normalized conductivities for the F

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the first time that changes in the PS glass temperature as well as different viscoelastic properties of block copolymer electrolytes with respect to the neat block copolymers have been reported for temperatures higher than the melting of the complex. This shows that the state with the highest ionic conductivity is characterized by some association of the electrolyte with the PEO chains.

4. CONCLUSION Application of block copolymers bearing a PEO block as solid polymer electrolytes requires a high modulus and suppressed PEO crystallinity as well as low melting temperatures for the complex. The PS-b-PEO/LiTf system fulfills these requirements. The analysis of the thermodynamic state of the PS-bPEO/LiTf block copolymer electrolytes in comparison to the homopolymer electrolyte (PEO/LiTf) revealed a strong effect of interfacial mixing and chain stretching on reducing the crystallinity of both PEO crystal and the complex crystal. Despite this, ionic conductivity was lower than in the corresponding homopolymer electrolyte. The thermodynamic state of the PS-b-PEO/LiTf block copolymer electrolytes poses some constrains concerning ion transport. From a microscopic point of view, different parameters were considered as potential regulators of ion transport: the characteristic d-spacing, the interfacial thickness Δ, and the ratio Δ/d. Interfacial area, where ions experience a different environment, play a crucial role in controlling ion transport. We compared two block copolymer electrolytes (PS-b-PEO and PI-b-PEO) bearing the same conducting block (PEO) and the same electrolyte (LiTf). Among the different microscopic parameters, the d-spacing affected by different interaction parameters appears to have the most decisive role on ionic conductivity. The ion conductivity in PS-b-PEO/LiTf electrolytes exhibits a molecular weight dependence similar to that reported for the PS-b-PEO/LiTFSI system. However, anion size effects lead to a slightly lower conductivity. A macroscopic factor that further limits ionic conductivity is the organization of the block copolymers near the electrode surface. Preferential wetting of the electrodes by either of the constituent phases can lead to an orientation that effectively blocks ion transport. The viscoelastic properties of the block copolymer electrolytes differ substantially from the neat block copolymers. Li-ion coordination affects not only the PEO segments but also the PS segments. An increase in PS glass temperature by ∼10 K is reported. In addition, the viscoelastic properties suggest the formation of transient structures at temperatures higher than the melting of complex. At such temperatures the moduli of the block copolymer electrolyte exceed by 1 decade the moduli of the neat block copolymer. Further studies are needed to clarify the local structure responsible for the higher moduli and ion conductivity. In conclusion, both microscopic and macroscopic features should be considered in the design of efficient SPEs for battery applications.

Figure 10. Storage (filled symbols) and loss (open synbols) moduli: (left) during cooling with 1 K/min at a frequency of 10 rad/s. Vertical arrows indicate the PS glass temperature and complex melting temperature; (right) versus frequency at 353 and 393 K for the neat S92EO237 (red) and its electrolyte (blue) with [EO]:[Li+] = 12:1 salt concentration. A line with a slope of 1/2 is shown.

the PS glass temperature has increased by roughly 10 K. This effect likely results from the stiffening of the PEO and PS chains at the interfacial region. Such an effect for the PS phase has not been reported before since the DSC trace is dominated by the complex melting process (Figure 1). In addition, at temperatures above the melting of the complex (at T ∼ 383 K), the storage modulus remains higher than the loss modulus (i.e., G′ ≥ G″), revealing the combined effects of nanodomain structure elasticity and transient “cross-linking” of PEO chains by Li+. More information on the exact viscoelastic state can be obtained by frequency-dependent measurements. In the temperature range investigated several factors determine the shear moduli. These include the glassy state of one of the blocks (PS), the PEO semicrystalline structure, the complex crystal, and the nanodomain structure as well as a possibly modified amorphous phase at lower and higher temperatures, respectively. At 353 K, neat SEO shows a strong dependence on frequency, originating from the transition zone starting at the PS glass temperature and the nanodomain structure. At the same temperature the SEO/LiTf electrolyte exhibits a significantly higher modulus with only a weak dependence on frequency with G′(ω) ∼ ω0.16 and G″(ω) ∼ ω0, indicating an elastic behavior. This drastic change in the viscoelastic behavior of SEO/LiTf with respect to PEO/LiTf, both with the same salt concentration ([EO]:[Li+] = 12:1), reflects the higher PS glass temperature in the former. More information on the viscoelastic properties can be obtained by examining the response at higher temperatures, preferably at temperatures above the melting point for the complex. At 393 K the SEO copolymer has G′(ω) ∼ G″(ω) ∼ ω1/2 which characterizes nanophase-separated block copolymers with a lamellar morphology.8 At the same temperature, located above the melting of the crystalline complex, the SEO/LiTf electrolyte exhibits moduli that are frequency dependent but with values about 1 decade higher than in bulk SEO. Even a shift by 10 K due to the increased polystyrene Tg cannot compensate for this difference (Figure 10, left). Increasing moduli suggest the formation of transient structures in SEO/LiTf electrolytes at temperatures even above the melting of complex. Further structural investigations are needed to explore the local structure at these high temperatures. To our knowledge, it is

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01596. Small-angle X-ray scattering patterns as a function of temperature, nanodomain spacings, and annealing effects G

DOI: 10.1021/acs.macromol.5b01596 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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on the ionic conductivity of block copolymer electrolytes (DS) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: gfl[email protected] (G.F.). *E-mail: [email protected] (G.Z.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was cofinanced by the E.U.-European Social Fund and the Greek Ministry of Development-GSRT in the framework of the THALIS program and the “Excellence in the Research Institutes” program. The current work was also supported by the Research unit on Dynamics and Thermodynamics of the UoI cofinanced by the European Union and the Greek state under NSRF 2007-2013 (Region of Epirus, call 18).

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DOI: 10.1021/acs.macromol.5b01596 Macromolecules XXXX, XXX, XXX−XXX