Nanostructured Polymer Particles as Additives for High Conductivity

Jun 15, 2017 - Emmanouil GlynosParaskevi PetropoulouEmmanouil MygiakisAlkmini D. NegaWenyang PanLampros PapoutsakisEmmanuel P...
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Nanostructured Polymer Particles as Additives for High Conductivity, High Modulus Solid Polymer Electrolytes Emmanouil Glynos,*,† Lampros Papoutsakis,†,‡ Wenyang Pan,§ Emmanuel P. Giannelis,§ Alkmini D. Nega,∥ Emmanouil Mygiakis,∥ Georgios Sakellariou,*,∥ and Spiros H. Anastasiadis†,‡ †

Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, P.O. Box 1385, 711 10 Heraklion, Crete, Greece ‡ Department of Chemistry, University of Crete, P.O. Box 2208, 710 03 Heraklion, Crete, Greece § Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States ∥ Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografrou, 15 771 Athens, Greece S Supporting Information *

ABSTRACT: For the next generation of safe and high energy rechargeable lithium metal batteries, we introduce nanostructured polymer particles of asymmetric miktoarm star copolymers as additives to liquid electrolytes for use as solid polymer electrolytes (SPE). The mechanical properties of the resulting SPEs are dramatically improved compared to the pure liquid electrolyte (the elastic modulus increased by up to 8 orders of magnitude), while the ionic conductivity was maintained close to that of the pure liquid electrolyte. In particular, the addition of 44 wt % miktoarm stars, composed of ion conducting poly(ethylene oxide), PEO, arms that complement stiff insulating polystyrene arms, PS ((PS)n(PEO)n, where n = 30 the number of arms), in a low molecular weight PEO doped with lithium bis(trifluoromethane)sulfonamide (LiTFSI), resulted in SPEs with a shear modulus of G′ ∼ 0.1 GPa and ion conductivity σ ∼ 10−4 S/cm. The SPEs show a strong decoupling between the mechanical behavior and the ionic conductivity as G′ remains fairly constant for temperatures up to the glass transition temperature of the PS blocks, while the conductivity monotonically increases reaching σ ∼ 10−2 S/cm. Our strategy offers tremendous potential for the design of all-polymer nanostructured materials with optimized mechanical properties and ionic conductivity over a wide temperature window for advanced lithium battery technology.

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INTRODUCTION Solid polymer electrolytes (SPEs) hold promise as viable candidates to alleviate the safety hazards associated with the flammable nature of the conventional liquid electrolytes, and they possess great potential for use in lithium metal batteries.1,2 The use of lithium metal as an electrode could increase considerably the energy density of lithium-ion batteries, making them theoretically comparable in energy density to gasoline, which would enable their use in emerging technologies such as electric vehicles and as efficient electrical energy storage systems to be used in conjunction with wind and solar energy sources. After the theoretical prediction by Monroe and Newman3 that a mechanically robust electrolyte with a shear modulus of a few GPa would eliminate macroscopic dendrite formation (a problem associated with lithium metal batteries), significant research efforts have focused on the development of solid polymer electrolytes capable of suppressing the dendrite formation while exhibiting high ionic conductivity that is necessary for practical applications. To this end, an effective SPE should simultaneously possess a high ionic conductivity (σ > 10−4 S/cm) and a high shear modulus (G′ > 0.1 GPa) both at room temperature. Unfortunately, in homogeneous polymer © XXXX American Chemical Society

materials, ion motion/transport is coupled to segmental dynamics, and any attempt to improve conductivity via faster polymer motions results in a decrease in stiffness.4,5 The “stateof-the-art” approach to decouple the two antagonistic parameters is the generation of multiphase nanostructured materials, where one phase conducts ions (soft/mobile phase needed for fast ion motion) while the insulating glassy phase imparts the desired mechanical properties.1,6−8 The two most promising lines of investigation involve the use of block copolymers (BCP’s)7−9 or the introduction of nanosized inorganic fillers to a polymer electrolyte for the fabrication of nanoscale functional hybrid materials.10−16 In the first case, linear BCP’s, in which one block is soft and ionconducting while the other is insulating and stiff/glassy, open a wide range of morphologies due to their ability to self-assemble in a variety of structures.17 For instance, a prominent example is the use polystyrene-block-poly(ethylene oxide), PS-b-PEO, in the presence of a lithium salt.18−26 PEO provides solvation of Li Received: April 16, 2017 Revised: June 7, 2017

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clusters, etc., as the competing core−core attractions and brush interactions are modified by changes in the grafting density and/or in the length of the grafts.40−43 Theory and computer simulations have shown that this self-assembly behavior is driven by the immiscibility between the inorganic particle cores and the organic grafted chains, a process analogous to the selfassembly of block copolymers.41,43 Star-shaped PEO polymers have been used for polymer electrolytes as their structure inhibits the crystallization of PEO increasing the ionic conductivity.44,45 Despite the means for controlling the crystallization behavior that branched polymer offer, due to the coupling of ion and polymer motion in any homogeneous polymer electrolytes, any attempt to improve conductivity moved the mechanical properties in the wrong direction. In particular, star PEO/lithium imide salt complexes formed viscous liquids, which did not provide the necessary mechanical properties.44 Herein, we move toward a different direction where we use high functionality miktoarm star-shaped copolymers as additives to polymer liquid electrolytes for the synthesis of all-polymer nanostructured solid electrolytes that exhibit an unprecedented combination of high modulus and ionic conductivity at room temperature. In particular, we demonstrate the use of asymmetric miktoarm star-shaped copolymers composed of ion conducting PEO arms that complement shorter stiff/glassy insulating polystyrene (PS) arms as nanoparticle additives to low-molecular-weight polymer electrolytes for the development of nanostructured solid polymer electrolytes, where both the ion conducting phase and the mechanical reinforcing phase are both interconnected. The novelty in our approach lies on the fact that because of the high functionality (n = 30, i.e., 60 arms in total) and the chemical composition of the arms, these miktoarm (PS)n(PEO)n stars can be considered as ion-conducting stiff polymeric nanoparticles for temperatures below the glass transition temperature of the PS arms. Furthermore, having the PEO arms slightly longer the PS arms resulted in the selfassembly of (PS)n(PEO)n stars in interconnected structures leading to the optimization of their mechanical properties. The formation of SPEs that have simultaneously both the ionconducting and the mechanical reinforcing phase highly interconnected resulted in materials with room temperature modulus of the order of 0.1 GPa (i.e., 8 orders of magnitude higher compared to the pure PEO liquid) while maintaining the ionic conductivity near that of pure liquid electrolytes with values acceptable for applications of ∼10−4 S/cm.

ions and constitutes the conducting phase while PS constitutes the rigid phase.9 In linear BCP’s, the ionic conductivity, σ, has been described by the Sax−Ottino model,27 σ = αϕcσc, where ϕc is the volume fraction of the conductive phase, σc is the intrinsic conductivity of the conductive phase, and α is the morphology factor that accounts for the geometry, shape, tortuosity, and connectivity of the conductive phase. When the macroscopic structure of the material is isotropic, α = 1/3 for cylinders and α = 2/3 for lamellae, since, on average, only onethird and two-thirds of the grains, respectively, will contribute to the ion transport in a specific direction, while biocontinuous gyroid morphologies exhibit value of α = 1. Nevertheless, the Sax−Ottino model does not account for grain boundary defects and bending/torsion of the conductive channels, and the conductivity of linear BCP systems is lower than the theoretical values. For the most well-studied BCP system of PS-b-PEO blended with an ionic liquid or a lithium salt, a PS/PEO molar ratio around unity provides a good balance between mechanical strength and ionic conductivity. Despite the good mechanical properties of linear BCP’s over a wide range of temperatures, the conductivity is still ∼10−6 S/cm at room temperature (2 orders of magnitude lower than what is required for any practical application), and only at temperatures higher than ∼80 °C, σ(T) ∼ 10−4 S/cm in the best performing linear BCP/ Li salt electrolytes.7 On the contrary, hybrid polymer electrolyte nanocomposites, i.e., blends of nanosized fillers with polymer electrolytes, offer the possibility of mechanically robust polymeric electrolytes that under certain conditions are grain-free and consist of highly interconnected ionic pathways/channels.28−32 Controlling the state of particle dispersion within the ion-conducting polymeric host is a key parameter for obtaining the ideal morphology in polymer electrolyte nanocomposites. Achieving uniform particle dispersion has been the focus of considerable research as it has been long believed that is necessary in order to optimize a specific property of the nanohybrid. A route toward this is the use of PEO-grafted nanoparticles of high grafting densities as additives to mechanically reinforce lowmolecular-weight PEO electrolytes that are liquid at room temperature with a G′ ∼ 10 Pa but exhibit high ionic conductivity.33−35 The presence of PEO-grafted chains that are chemically compatible with the host drives the dispersion of the particles. When PEO-grafted nanoparticles were added to lowmolecular-weight liquid PEO electrolytes at particle loadings at or near the onset of particle percolation, ϕp ∼ 0.3, the mechanical modulus increased by more than 6 orders of magnitude, reaching a storage modulus of G′ ∼ 0.1 MPa while maintaining σ ∼ 10−4 S/cm.35,36 For ϕp of 0.4 and higher, G′ increased to ∼1 MPa, but due to the high volume fraction of the insulating particles or equivalently the low volume fraction of the conductive phase, the ionic conductivity deteriorated to σ < 10−5 S/cm. It is important to point out that at ϕp ∼ 0.4 G′ remained about 3 orders of magnitude lower than the target of G′ > 1 GPa. While a uniform dispersion of the nanofillers in the polymer host is believed to be the most effective state for the optimal properties of hybrid materials, Torquato and co-workers37−39 showed that in a two-phase system, where phase A is the mechanical reinforcing and phase B is the conducting phase, for the two properties to be simultaneously optimized, both phases should be simultaneously interconnected. To this end, Kumar and co-workers have shown that polymer-grafted particles may self-assemble in structures, like sheets, vesicles, percolating

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EXPERIMENTAL SECTION

2.1. Materials. Synthesis of Miktoarm Asymmetric Star-Shaped Copolymers. The asymmetric miktoarm star poly(ethylene oxide)− polystyrene copolymers, (PS)n(PEO)n with n = 30 number of arms, were synthesized by the “arm-first” method. Sequential anionic polymerization of styrene and divinylbenzene (DVB) leads to the formation of relatively well-defined star-shaped polymers with anionically active cores.46,47 We utilized these living carbanionic sites present in the core to form a second set of PEO arms growing out from the star core. Schematically, the structure of the asymmetric miktoarm star-shaped copolymer synthesized is seen in Figure 1a. Size exclusion chromatography (SEC), nuclear magnetic resonance spectroscopy (NMR), and light scattering have been employed for the characterization of the synthesized macromolecules. The functionality was estimated to be n = 30, while the molecular weights per arm Mwarm were 3.7 and 8.5 kg/mol for the PEO and PS, respectively. More details on the synthesis and characterization of the star-shaped copolymer can be found in the Supporting Information. The SPEs were prepared by blending different amounts of (PS)30(PEO)30 and B

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The sample thickness was tuned to approximately 100−110 nm thick. The samples were subsequently annealed under vacuum at a temperature around 120 °C for about 24 h. Before the TEM measurements, the samples were stained with RuO4 for 5 min, where the PEO phase was preferentially stained. Electrochemical Characterization. The ionic conductivity of the SPEs was measured as a function of temperature using a Novocontrol N40 broadband dielectric spectrometer (BDS). The samples were prepared in a glovebox under a controlled nitrogen environment. Samples were heated up until melted and pressed between the electrodes; short-circuiting was prevented with a donut-shaped Teflon ring. The measurements were performed over a temperature range of 20−100 °C; prior to the measurements the samples were equilibrated at 130 °C in the BDS under a nitrogen atmosphere. The frequency range of the measurements was varied from 0.1 to 107 Hz. The dc conductivity was estimated from the plateau value of the highfrequency plateau region of the real part of conductivity versus frequency data without invoking any model and as described by Jonscher.5 The samples were examined on subsequent cooling and heating to ensure the reproducibility of the measurements and equilibrium properties. Mechanical Properties. The mechanical response of the SPEs was evaluated in an ARES 100 FRTN1 strain-controlled rheometer (TA Instruments) equipped with an ARES convection oven, ensuring excellent temperature control. Parallel plate geometry of 8 mm diameter was utilized for the linear tests. The samples for the rheological measurements were prepared by shaping them to 8 mm discoid specimens by compression molding in a vacuum. The molding procedure was carried out for about 5 min, and then the samples were left to cool down to room temperature. The discoid specimens were placed in a Teflon mold with an 8 mm diameter and annealed in an oven under vacuum at temperatures ∼100 °C for about 2 days. Prior to rheological measurements, the discoids were placed at the lower rheometer plate, the temperature was subsequently increased to approximately 90−120 °C (depending on the sample), the samples were left at this temperature for 20 min, and then the upper plate was brought in contact and the gap thickness was adjusted to about 1 mm. The linear and nonlinear viscoelastic regimes were determined by the strain amplitude dependence of the complex shear modulus at ω = 100 and 10 rad/s at selected temperatures. Measurements involved also isothermal frequency scans in the range of 10−1 < ω < 102 rad/s.

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RESULTS AND DISCUSSION The asymmetric polystyrene−poly(ethylene oxide) miktoarm star copolymers, (PS)n(PEO)n with n = 30 number of arms, were synthesized by the “arm-first” method. Sequential anionic polymerization of styrene and divinylbenzene (DVB) leads to the formation of relatively well-defined star-shaped polymers with anionically active cores.46,47 We utilized these living carbanionic sites present in the core to synthesize the PEO arms growing out from the star core. The structure of the asymmetric miktoarm star-shaped copolymer synthesized is schematically pictured in Figure 1a. The functionality was estimated to be n = 30, while the molecular weights per arm, Mwarm, were 3.7 and 8.5 kg/mol for the PEO and PS, respectively. More details on the synthesis and characterization of the star-shaped copolymer can be found in the Supporting Information. The SPEs were prepared by blending different amounts of (PS)30(PEO)30 and low-Mw linear PEO (Mw = 0.5 kg/mol) together with lithium bis(trifluoromethane)sulfonamide, LiTFSI, as the lithium salt. The ethylene oxide/ lithium molar ratio was kept constant at [EO]/[Li+] = 8. Figure 1b shows the transmission electron microscopy (TEM) images for specimens containing 30 wt % (PS)30(PEO). A bicontinuous nanostructured material is seen with highly interpenetrating and percolating domains of (PS)30(PEO)30 star-shaped copolymers within the oligomeric

Figure 1. (a) Schematic of the DVB synthetic route for the synthesis of PS (red arms) and PEO (blue arms) asymmetric miktoarm starshaped copolymers. (b, c) TEM micrographs of a 30 and 44 wt % (PS)30(PEO)30/linear PEO-0.55k blend, respectively; the inset in (b) is a schematic that represents the organization of (PS)30(PEO)30 particles within the PEO host. The PEO domains appear dark after staining with RuO4 for 5 min. low Mw linear PEO (LPEO, with Mw = 0.5 kg/mol) together with lithium bis(trifluoromethane)sulfonamide, LiTFSI, as the lithium salt. The fraction of ethylene oxide/lithium molar ratio was kept constant at [EO]/[Li+] = 8. 2.2. Characterization. Differential Scanning Calorimetry. The thermal properties of the polymer electrolytes were examined with a PerkinElmer differential scanning calorimeter (DSC). The temperature range from −100 to 140 °C was covered with a heating/cooling rate of 10 °C/min. In all cases, the crystallization, melting, and glass transitions were obtained from the second heating and cooling, respectively, in order to eliminate thermal history and any remaining humidity. During the measurements, the samples were maintained under constant nitrogen flow. Morphology. The morphology of the SPEs was studied using a high resolution transmission electron microscope (HR TEM) equipped with a field emission gun. The TEM samples were prepared by spincoating a solution of the sample under investigation on a TEM grid; this procedure ensured the formation of relatively uniform film whose thickness could be tuned by the concentration of the polymer solution. C

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Macromolecules PEO. This behavior is driven by the immiscibility between the PS cores and the low-molecular-weight PEO host, where neither the functionality of the PEO arms (n = 30) nor the length is high and long enough, respectively, to avoid their aggregation. It should be noted that in the TEM images presented here the samples were labeled with RuO4 that preferentially stains the PEO phase for the staining time used, i.e., 5 min; thus, the PEO phase appears black and the PS phase light dark.48,49 With increasing the (PS)30(PEO)30 loading (44 wt %, Figure 1c), a similar interconnected structure is observed with a decreasing domain size. This attests to the effectiveness of synthesizing asymmetric miktoarm stars with the PEO arms slightly longer than those of PS for the preparation of SPEs having both the ion-conducting and the mechanical reinforcing phases highly interconnected. Differential scanning calorimetry (DSC) measurements for the pure (PS)30(PEO)30 miktoarm star-shaped molecules indicate that branching and associated geometrical constrains significantly affect the crystallization behavior of (PS)30(PEO)30, as already reported.44 In particular, the crystallization temperature of the pure (PS)30(PEO)30 stars was Tc ∼ −30 °C. Figure S6 of the Supporting Information shows the DSC heat flow of the (PS)30(PEO)30 stars during cooling together with the corresponding curves of various linear PEO’s with a range of Mw’s between 1.9 and 10 kg/mol; it is clear that for the linear PEO’s the Tc’s are above room temperature. The PEO arms in the (PS)30(PEO)30 star-shaped copolymers stay amorphous when cooled from high temperatures to room temperature, which is of significant importance for ion conductivity. Finally, the Tg of the PS arms was ∼85 °C. The effectiveness of using (PS)30(PEO)30 star-shaped copolymers to mechanically reinforce the liquid PEO electrolytes is illustrated by the storage, G′, and loss shear moduli, G″, data shown in Figure 2. It is immediately apparent that at room temperature (i.e., T ∼ 24 °C) the addition of 30 wt % (PS)30(PEO)30 nanoparticles provides an elastic solid-like behavior to the composite polymer electrolyte, since the storage modulus G′ dominates the loss modulus G″, i.e., G′ ≫ G″, and they are both fairly independent of the shear frequency ω (Figure 2a). It should be noted that the pure low-molecularweight PEO is liquid at room temperature (i.e., G′ ∼ ω2 and G″ ∼ ω) with G′ ∼ 10 Pa at ω = 10 rad/s and G″ ≫ G′. It should be noticed that the mechanical response of the electrolyte with 30 wt % (PS)30(PEO)30 remains fairly unaffected when the temperature increases up to T ∼ 85 °C. For T > 85 °C, that is, around the onset of the glass transition temperature of the PS arms, a weak power-law dependence on frequency, G′ ∼ ωm, begins to appear at low frequencies with m increasing with increasing temperature. Eventually a transition to a liquid-like (G′ < G″) behavior is observed. Figure 2b shows G′ and G″ for various (PS)30(PEO)30 weight fractions at ω = 10 rad/s. At room temperature, G′ ≈ 25 kPa for the 15 wt % (PS)30(PEO)30 hybrid, increases to G′ ≈ 25 MPa for the 30 wt % one and reaches G′ ∼ 0.1 GPa for the 44 wt % (PS)30(PEO)30 sample (for the pure (PS) 30 (PEO) 30, G′ ≈ 6 GPa at room temperature). In other words, at room temperature there is a 4, 7, and 8 orders of magnitude increase of the elastic modulus compared to the pure liquid PEO (∼10 Pa at 10 rad/s) for the 15, 30, and 44 wt % (PS)30(PEO)30 blends, respectively. Significantly, the mechanical response of the electrolytes remains fairly unaffected by increasing temperature up to ∼85 °C.

Figure 2. (a) Storage modulus, G′ (solid symbols), and loss modulus, G″ (open symbols), for the 30 wt % (PS)30(PEO)30 blend with [EO]/ [Li] = 8 at 24, 80, 100, and 110 °C (black, red, blue, and green symbols, respectively). (b) G′ (solid symbols) and G″ (open symbols) as a function of temperature obtained from the frequency sweep in the linear regime at ω = 10 rad/s for the 15, 30, and 44 wt % (PS)30(PEO)30 (red triangles, gray squares, and black circles, respectively). (c) G′ (solid symbols) and G″ (open symbols) of the 30 wt % (PS)30(PEO)30 blend electrolyte as a function of shear strain at ω = 10 rad/s and for 24, 80, and 110 °C (black squares, red circles, and green squares, respectively); the inset is the corresponding data for 85 °C, i.e., at a temperature close to the TgPS.

Figure 2c shows the shear strain response of the 30 wt % (PS)30(PEO)30 sample at different temperatures. At T = 24 °C (black squares in Figure 2c), the material rapidly softens at a shear strain higher than a critical value while a pronounced maximum in G″ is observed; in the small strain regime, G′ is fairly independent of strain (linear regime), while for higher strains the system behaves as a simple fluid, i.e., G″ > G′. These results strongly suggest that our system is processable for strains higher than the critical shear strain, which is particularly important for processing and eventually manufacturing. Note that this behavior remains fairly constant for temperatures up to the onset of the glass transition of the PS arms, i.e., up to T ∼ D

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Figure 3. (a) The dc ionic conductivity as a function of temperature for pure linear PEO (black squares), pure (PS)30(PEO)30 (dark yellow stars), and blends of 15, 30, and 44 wt % (PS)30(PEO)30 (red circles, blue triangles, and green rhombus, respectively); the gray region highlights the σ ≥ 10−4 S/cm area. (b−d) Temperature dependence of ionic conductivity of the various polymer blend electrolytes with 15 (b), 30 (c), and 44 wt % (d) (PS)30(PEO)30 nanoparticles together with the expected reduction of conductivity for tortuous ion-conducting channels in a heterogeneous electrolyte with 1.5 ≤ τ ≤ 3 (solid line: τ = 1.5; dashed line: τ = 3).

85 °C. For T < 85 °C, the mechanical response of the 30 wt % (PS)30(PEO)30 specimen originates from the percolated network of the (PS)30(PEO)30 stars formed within the PEO host (Figure 1a) where the interconnected PS glassy phase offers direct pathways for the propagation of the stress. This is consistent with observations by Kumar and co-workers for polymer-grafted silica particles for short grafting chains and low grafting densities even at 5 wt % loadings.40 The maximum in G″ indicates the critical strain for particles to escape the formed structures. For T > 85 °C, a simple strain softening behavior is observed with no maximum in G″ (inset in Figure 2c) as the PS arms undergo their glass transition, the interconnected PS phase loses its mechanical integrity, and the material flows. A qualitatively similar mechanical response was observed for the 44 wt % (PS)30(PEO)30 nanostructured polymer electrolytes as well (as is clear in Figure S7) where G′ increased by an order of magnitude to G′ ∼ 0.1 GPa for T < 85 °C. Despite the significant enhancement in the mechanical strength of the polymer electrolytes, the addition of the (PS)30(PEO)30 nanoparticles results in relatively small changes in the ionic conductivity compared to that of pure liquid PEO. In particular, for the 15 and 30 wt % (PS)30(PEO)30 samples the ionic conductivity, σ, is constantly above 10−4 S/cm; i.e., it remains within the practical-for-application conductivity range (gray area in Figure 3a), which is between a half and a quarter of that of the pure liquid electrolyte at the same [EO]/[Li+] (black squares in Figure 3a). For the 44 wt % (PS)30(PEO)30 specimen, σ ≅ 6 × 10−5 S/cm at T = 20 °C and σ ≅ 2 × 10−4 S/cm at T = 30 °C. For the 15 and 30 wt % samples, σ > 10−4

S/cm even for T = 20 °C. For all samples, the ionic conductivity increases with increasing temperature, exceeding the critical benchmark value of 1 mS/cm at ∼40, 55, and 70 °C for the 15, 30, and 44 wt % (PS)30(PEO)30, respectively. It is important to point out that SPEs composed entirely by (PS)30(PEO)30 macromolecules, where the weight fraction of the conductive phase is ∼18%, exhibit a finite, even low, ionic conductivity (dark yellow stars in Figure 3a). This highlights that in our system these nanoparticles are by design intramolecular nanostructured entities that may allow ion conduction throughout their volume; i.e., they allow intraparticle ion conduction. The conductivity in nanostructured materials, σ(T), may be expressed as a fraction of the conductivity of the conductive phase, σcond(T), as measured in our case for the low molecular weight PEO electrolyte (black squares in Figure 3a) as σ (T ) =

ϕσ (T ) c cond τ

where ϕc is the volume fraction of the conductive phase and τ is the tortuosity factor, which describes the distance that an ion must travel relative to a straight path. In other words, τ accounts for the morphology and interconnectivity of the conductive phase given that it is isotropic. We used this equation to generate the solid and dashed curves in Figures 3b−d assuming the range of τ values between 1.5 and 3, which have been shown to describe the diffusion of small molecules in one phase disordered and continuous network of morphologies.50−52 It is clear that the data for the 15 wt % E

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Macromolecules (PS)30(PEO)30 can be described by a value τ = 1.5, indicating that the conducting channels are continuous over macroscopic distances. The ionic conductivity of 30 wt % (PS)30(PEO)30 remains fairly well in the range τ = 1.5−3 while that of the 44 wt % (PS)30(PEO)30 is quite lower than the anticipated range; only at higher temperatures do ionic conductivities reside within the range. These small deviations, especially for the 44 wt % (PS)30(PEO)30, may be understood in terms of narrower conductive channels with increased amount of conducting “dead ends” (Figure 1c) due to the relatively high loading of the (PS)30(PEO)30. Nevertheless, the presence of the large fraction of the (PS)30(PEO)30 stars ensured high mechanical modulus without dropping the ion conductivity below ∼10−4 S/cm. Much of the research efforts in the design of polymer electrolytes suitable for applications have focused on the decoupling between the mechanical properties and the ionic conductivity. Our system reveals an astonishing decoupling of this kind, as revealed in Figure 4, where G′ is plotted as a

the particle loading, facilitate the formation of an astonishingly mechanically robust electrolyte with G′ ∼ 0.1 GPa while maintaining a high ionic conductivity of σ ∼ 10−4 S/cm were achieved. Key to their performance is their morphology that stems from the ability of the (PS)30(PEO)30 nanoparticles to self-assembly in highly interconnected structures within the liquid electrolytes host. Since the mechanical properties of the PS-based nanoparticles are influenced primarily by the high glass transition temperature of the PS arms, the mechanical response of the composite electrolytes remains relatively unaffected (G′ ∼ 0.1 GPa) for T < Tg of PS. Over the same temperature window, ionic conductivity, which is coupled to the dynamics of the PEO phase, monotonically increases and reaches values of σ ∼ 10−3 S/cm. This feature reveals that due to their macromolecular design, the proposed systems offer the means for decoupling the antagonistic properties, which limit the realization of solid polymer electrolytes in secondary lithium metal batteries both at ambient conditions and at elevated temperatures. The electrochemical performance of the solid polymer electrolytes and the possibility of their integration in Li batteries are beyond the scope of the present work. Herein, the utilization of novel nanostructured polymer “particles”, based on high functionality miktoarm copolymers, as additives to a liquid electrolyte is reported for the formation of nanostructured materials where the ion conducting phase and the mechanical reinforcing phase are both interconnected. This behavior is driven by the immiscibility between the PS cores and the low-molecular-weight PEO host, where neither the functionality of the PEO arms (n = 30) nor their length is high and long enough, respectively, to lead to a homogeneous dispersion and avoid their aggregation. The reported nanostructured materials showed a remarkable decoupling between ionic conductivity and mechanical properties, which highlight a new approach for the synthesis of nanostructured materials that may be important for application beyond Li metal batteries. The electrochemical performance of the resulting electrolytes is part of our ongoing work that will focus on the possibility of their utilization in Li batteries.

Figure 4. Storage modulus versus dc ionic conductivity for the 15, 30, and 44 wt % (PS)30(PEO)30 blends (red circles, blue triangles, and green rhombus, respectively).

function of ion conductivity for all the different samples examined here. In particular, the use of (PS)30(PEO)30 copolymer nanoparticles allows the increase of ionic conductivity by more than an order of magnitude without deterioration of its mechanical properties. This unique characteristic is attributed to the presence of the high-Tg PS arms, which ensures the mechanical stiffness of the nanoparticles for temperature up to the glass transition temperature of PS as well as to the presence of the slightly longer PEO arms that resulted in the formation of nanostructured materials with simultaneously highly interconnected ion-conducting channels and glassy insulating phases. As discussed in the Introduction, such morphology is of vital importance for the two properties to be simultaneously optimized.37−39

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00789. Synthesis details, SEC, 1H NMR, FTIR characterization, and molecular characteristics of the miktoarm star (PS)n(PEO)n copolymers; DSC and strain amplitude dependence data of the SPEs (PDF)

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CONCLUSIONS In summary, mechanically robust and processable all-polymer solid electrolytes with high ionic conductivities at room temperature were synthesized using miktoarm (PS)30(PEO)30 star-shaped copolymers dispersed in a liquid PEO host. Because of the colloid-like behavior of the (PS)30(PEO)30 molecules resulting from the high number of arms and the presence of the glassy PS arms, these macromolecules are stiff nanoparticles at temperatures below the Tg of PS. Isotropic nanostructured materials with highly interconnecting phases that, depending on

AUTHOR INFORMATION

Corresponding Authors

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

Emmanouil Glynos: 0000-0002-0623-8402 Spiros H. Anastasiadis: 0000-0003-0936-1614 Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This work was partially performed in the framework of the PROENYL research project (Action KRIPIS, project MIS448305, 2013SE01380034), funded by the General Secretariat for Research and Technology, Ministry of Education, Greece and the European Regional Development Fund (Sectoral Operational Programme: Competitiveness and Entrepreneurship, NSRF 2007-2013)/European Commission.

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DOI: 10.1021/acs.macromol.7b00789 Macromolecules XXXX, XXX, XXX−XXX