Multifunctional Epoxy-Based Solid Polymer Electrolytes for Solid-State

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Multifunctional Epoxy-Based Solid Polymer Electrolytes for Solid-State Supercapacitors Suk Jin Kwon, Taehoon Kim, Byung Mun Jung, Sang Bok Lee, and U Hyeok Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11016 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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ACS Applied Materials & Interfaces

Multifunctional Epoxy-Based Solid Polymer Electrolytes for Solid-State Supercapacitors Suk Jin Kwon1, Taehoon Kim1, Byung Mun Jung1, Sang Bok Lee*1 and U Hyeok Choi*2 1

Functional Composite Department, Korea Institute of Materials Science (KIMS), Changwon

51508, Korea 2

Department of Polymer Engineering, Pukyong National University, Busan 48547 Korea

*Corresponding authors E-mail: [email protected] & [email protected]

ABSTRACT Solid polymer electrolytes (SPEs) have drawn attention for promising multifunctional electrolytes requiring high mechanical properties and ionic conductivity. To develop a safe SPE for energy storage applications, mechanically robust cross-linked epoxy matrix is combined with fast ion diffusing ionic liquid/lithium salt electrolyte (ILE) via a simple onepot curing process. The epoxy-rich SPEs show higher Young’s modulus (E) with higher glass transition temperature (Tg), but lower ionic conductivity (σDC) with a higher activation energy, compared to the ILE-rich SPEs. The incorporation of inorganic robust Al2O3 nanowire simultaneously provides excellent mechanical robustness (E ~ 1 GPa at 25 °C) and good conductivity (σDC ~ 2.9×10−4 S/cm at 25 °C) to the SPE. This suggests that the SPE has a bicontinuous microphase separation into ILE-rich and epoxy-rich microdomain, where ILE continuous conducting phases are intertwined with a sturdy cross-linked amorphous epoxy framework, supported by the observation of the two Tgs and low tortuosity as well as the microstructural investigation. After assembling the SPE with activated carbon electrodes, we 1

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successfully demonstrate the supercapacitor performance, exhibiting high energy and power density (75 Wh/kg at 382 W/kg and 9.3 kW/kg at 44 Wh/kg). This facile strategy holds tremendous potential to advance multifunctional energy storage technology for next generation electric vehicles.

KEYWORDS: solid polymer electrolytes, supercapacitors, ionic conductivity, bicontinuous composite electrolyte, energy density, power density

1. Introduction Future automotive industries are required for the weight reduction of vehicles, allowing to reduce the carbon emissions and to increase the fuel efficiency.1-3 Integrating a structural energy storage system (multifunctional safe electrolytes with the capability to ensure structural integrity and store and release electrical energy) into electric vehicles (EVs) makes it possible to diminish the net weight and volume, compared to the use of independent monofunctional materials.4 Solid polymer electrolytes (SPEs) are of great interest as promising solutions to offer such a multifunctional characteristic, owing to their mechanically rigid, nonvolatile, nonflammable, and electrochemical stable characteristics.5-11 However, the SPEs have been faced with the difficulty to accomplish high modulus without having a loss of ionic conductivity, since ion transport in SPEs is necessarily lower than that of conventional liquid electrolytes.12,13 Therefore, the reliable SPEs must show a combination of considerable mechanical integrity and high room temperature ionic conductivities, capable of withstanding a weight or strain while efficiently transporting energy at the same time.14 2


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Among the various SPEs, poly(ethylene oxide) (PEO)12 or poly(acrylonitrile) (PAN)15 based SPEs are the most extensively researched SPEs,16,17 due to their strong cation solvating ability. Both the ether oxygen (EO) on a PEO and the cyano (CN) group on a PAN coordinate with cations, thereby allowing the electrolytic cation/anion pair to be separated as charge carrier that is diffused by polymer chain motion. Although the semicrystalline polymers can provide the needed mechanical strength for energy storage systems, the crystallization of PEO and PAN has been proved as the main factor for the detriment of the ion transport, owing to the slowing down of polymer chain dynamics upon crystallization.18 Therefore, it is necessary to maintain an amorphous nature for better conductivity in PEO and PAN, and that preventing crystallization is one approach to that end. The effective strategy to suppress the crystallization is the addition of liquid plasticizers into the SPE (so-called gel polymer electrolytes),18 allowing the polymer chain to be more flexible and lowering glass transition temperature, but inevitably compromising mechanical strength of the SPE.19 Gadjourova et al.20 reported another method to reduce the polymer crystallinity by the addition of an inorganic filler such as TiO2, SiO2, or Al2O3 into a PEO-based polymer matrix. This revealed an increase in the ionic conductivity without significantly reducing the mechanical properties. Both the ion conductivity and mechanical modulus strongly depended on the inorganic filler shape and size as well as the degree of the filler dispersion. Liu et al.21 used an active ceramic nanowire filler (Li0.33La0.557TiO3), which participates in lithium ion transport. These composite polymer electrolytes showed three orders of magnitude enhancement of ionic conductivity in PAN/LiClO4-based SPEs (σDC ~ 2.4×10−4 S/cm at 25 °C). Although improvement of ionic conductivity by liquid or inorganic nanoparticle plasticization has been well established in the PEO or PAN-based nanocomposite 3



polymer electrolytes, their Young’s modulus values (E ~ 10 – 140 MPa for PEO SPE22,23 and E ~ 0.01 – 0.4 MPa for PAN SPE24) are not enough to withstand mechanical force from an accident with EVs, leading to high safety risks such as fire or explosion, limiting the use on a large scale. For safe and useful SPEs, an alternative novel polymer matrix is therefore necessary to substitute for the conventional polymer matrices. In order to overcome these challenges, microphase-separated morphologies have been explored, where a conducting soft phase is the channel for fast ion transport and another insulating glassy phase provides robust mechanical properties. This solution allows high modulus with reasonable ionic conductivity. The nanostructured ion-conducting block copolymers (BCPs), composed of a glassy polystyrene (PS) block and a low-Tg PEO block, have been synthesized via radical polymerization and studied to independently adjust the ionic conductivity and mechanical strength.25 Although the PS-PEO BCP reached a shear modulus on the order of 100 MPa at 90 °C, the BCP exhibited the ionic conductivity (σDC > 10-4 S/cm) only above 90 °C, owing to the crystallization of the PEO block.26 Schulze et al.27 and Chopade et al.28 developed robust SPEs with high ambient-temperature ion conductivity via sequential reversible-addition fragmentation chain transfer polymerization, inducing the SPE to polymerization-induced microphase separation (PIMS). The PIMS allowed the SPE to be separated into conducting PEO nanochannels having lithium salts and plastic crystal from a mechanically cross-linked polystyrene framework, resulting in high ionic conductivity (σDC ~ 3.5 x 10-4 S/cm at 30 °C) and modest modulus (E ~ 300 MPa at 30 °C). However, such complex radical initiation polymerization processes have intrinsic drawbacks in that free radical initiators or residual monomers can easily react with electrodes, thereby increasing the electrolyte-electrode resistance and rapidly deteriorating energy storage capacity. Thus, it is 4


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vital to develop an effective method to prepare an SPE having a bicontinuous, conducting, and strong network by a simple cross-linking reaction without producing any by-products. In this study, we report facile one-pot synthesis strategy based on an epoxy ring-opening polymerization to prepare a robust and conductive network SPE, leading to an integration of high ionic conductivity, mechanical strength, and specific capacitance, for the development of multifunctional structural supercapacitors. This epoxy-based SPE contains an ionic liquid, a lithium salt, and an inorganic Al2O3 nanowire, with the structures shown in Figure 1. The highly cross-linkable, easily processible, and commercial available diglycidyl ether of bisphenol-A (DGEBA) epoxy resin is used as the supporting SPE matrix to supply the mechanical strength of the completely amorphous polymer network, because of its high Young’s modulus (~3 GPa) and tensile strength (~70 MPa) after the crosslinking reaction. The ionic liquid electrolyte (ILE) containing high conducting 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide (BMIM-TFSI) and low-lattice-energy bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) is co-cross-linked with the DGEBA to maximize the SPE ionic conductivity. Moreover, the thermal curing process of ILE confinement by epoxy resins has revealed a 3D cross-linked network microstructure, where the conducting ILE electrolyte is micro-separated from the cross-linked epoxy framework via PIMS,29-32 enabling bicontinuous morphology. The selected Al2O3 nanowire, consisting of a chemical compound of aluminum and oxygen with strong ionic internal bonding, is one of the strongest and stiffest oxide ceramics, giving rise to a further enhancement of the thermal, mechanical, and electrochemical stability of SPEs.33 The role of the concentration of epoxy, ILE and Al2O3 on ion transport and mechanical strength of the epoxy-based SPEs, such as Tg, σDC, and E, is systematically explored using DSC, dielectric spectroscopy, and mechanical testing. As there is an intimate connection among phase5



separated microstructure, ion transport, and mechanical strength, these investigations are complemented by a microstructural study from FE-SEM. In addition, after assembling the SPE with two symmetric electrodes, its specific capacitance and energy/power density are also investigated by electrochemical measurements of the solid-state supercapacitor. It is possible that this proposed strategy is ideal for designing an SPE satisfying the required characteristics, such as simple process (applicability on a large scale), high modulus (electrochemical and dimensional stability), and high ionic conductivity (faster recharging), of low-cost and highperformance EVs

2. Experimental 2.1 Materials An epoxy resin DGEBA (KUKDO Chemical, Korea), a hardening agent, methyl tetrahydrophthalic anhydride (MeTHPA, SHIN-A T&C, Korea), and a catalyst, Nbenzyldimethylamine (BDMA, Sigma Aldrich, Korea) were used to construct an epoxy matrix of our bifunctional SPEs. Their conducting phase consists of a LiTFSI (Sigma Aldrich, Korea), a BMIM-TFSI (Sigma Aldrich, Korea), and an aluminum oxide (Al2O3) nanowire (diameter 2 – 6 nm and length 200 – 400 nm, Sigma Aldrich, Korea). To fabricate a carbon electrode, an activated carbon powder with a surface area of 2000 m2/g (MTI, Korea), a conductive carbon with a surface area of 62 m2/g (MTI, Korea), a poly(vinylidene fluoride-cohexafluoropropylene) (PVDF, Sigma Aldrich, Korea), and l-methyl-2- pyrrolidinone (NMP, Sigma Aldrich, Korea) were used. Among these chemicals, LiTFSI was particularly dried at 120 °C for 24 h in vacuo prior to use, and the others were used as received. 6


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2.2 SPE Preparation For these SPEs, 1 M LiTFSI was completely dissolved in BMIM-TFSI to make an ionic liquidbased electrolyte (referred to as ILE). The matrix epoxy components (referred to as EPOXY, composed of DGEBA, MeTHPA, and BDMA in the ratio of 100: 90: 1.5 by weight) were then added into the ILE solution, and the mixture was stirred until its homogeneous mixture was formed. Different ratios of EPOXY and ILE [X vol%_ILE, where X is the ILE content (X = 0, 30, 50, 70, and 100)] were weighted into 10 mL vials. For the SPEs containing inorganic Al2O3 nanowires, Al2O3 was added into 50 vol%_ILE with various contents (ILE50 + Y vol%_Al2O3, where Y = 2, 5, 6, or 8 is the Al2O3 volume fraction). The ILE/EPOXY/Al2O3 mixtures were cured at 80 °C for 2 h, 120 °C for 1 h, and then 150 °C for 2 h in an oven, after being under vacuum at 60 °C for 30 min. 2.3 Preparation of Activated Carbon Electrodes and Supercapacitors For a carbon slurry of supercapacitor carbon electrodes, PVDF as a binder was dissolved in NMP solvent, and the solution was then mixed with an activated carbon (AC) and a conductive carbon (CC), where the mass ratio of AC, CC and PVDF was 8:1:1. The mixtures were stirred until forming a uniformed slurry. The slurry was coated onto an aluminum foil substrate using a film applicator. The AC electrode coated with the carbon slurry was dried at 60 °C in a vacuum oven and then at 120 °C for 3 hr to remove the remaining solvents in the carbon materials. Then, a supercapacitor was fabricated by sandwiching an epoxy-based SPE (ILE50) between a pair of the AC carbon electrodes. The SPE was also served as the separator for the supercapacitor. 2.4 Characterization 7



Fourier Transform Infrared Spectroscopy (FTIR) spectra and mechanical properties of samples were measured by using a Nicolet iS10 FTIR spectrometer (Thermo Scientific), with a diamond attenuated total reflectance (ATR) cell and using a Q800 Dynamic Mechanical Analyzer from TA Instruments, respectively, at room temperature. The detailed experimental description has already been provided in the earlier works.14,34 Thermal measurements were conducted using a TA Instruments Q2000 differential scanning calorimeter (DSC) using 10 K/min heating and cooling rates under inert gas atmosphere to measure possible phase transition in the SPEs. Samples weighing 5-10 mg were hermetically sealed in an aluminum pan. The first heating scan was conducted at a ramp rate of 10 K/min from 25 to 150 °C to remove adsorbed water. For the SPE morphological investigation, prior to Field Emission-Scanning Electron Microscopy (FE-SEM) analysis, each sample was placed in ethanol during 3 days at 60 °C and then subsequently dried in a vacuum oven without the heat on (i.e. room temperature) under reduced pressure to evaporate the ethanol. The FE-SEM images of the sample were then recorded on a JEOL, JSM-5800 SEM with a voltage of 10 kV, after platinum sputter coating for 120 s. The ionic conductivities of the SPEs were measured by broadband dielectric relaxation spectroscopy using a Novocontrol GmbH Concept 40.14 Samples were covered with two different size polished brass electrodes, such as top (10 mm diameter) and bottom (20 mm diameter) electrodes. Silica fibers were placed on top of the sample to control thickness of the sample at 100 μm. The SPE sandwiched between the two electrodes was placed on a parallel plate capacitor cell and loaded into a cryo-system with nitrogen atmosphere. Each sample was 8


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annealed in the instrument at 120 °C to remove any moisture. Data were collected in isothermal frequency sweeps from 120 °C to -100 °C. Electrochemical performances on the SPE/AC symmetric cell were evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) experiments. CV and GCD were performed using theVSP300 from Bio-Logic Science Instruments. Potentiostats were measured in the potential range of 0 to 2.5 V to examine the influence of the potential sweep rate, and the CV was measured at a sweep rate of 10, 20, 50 and 100 mV/s. To test the GCD performance, the electrochemical cell was charged and discharged with the potential range 0 to 2.5 V at the constant current 0.13, 0.20, 0.33, 0.50, 0.66, and 1.00 A/g.

3. Results and Discussion 3.1 ATR-FTIR In order to confirm the cross-linking between DGEBA and MeTHPA in these epoxy-based electrolytes, the FTIR measurements were performed (Supporting Information, Figure S1), where the FTIR spectra of the uncured DGEBA, MeTHPA, and SPE were compared with those of the cured SPEs without and with Al2O3 (ILE50 after curing and 8 vol%_Al2O3). The peak at 913 cm-1 is associated with the epoxide ring stretching band of DGEBA, and that at 921 cm-1 is related to the anhydride C-O stretching band of MeTHPA.14 After the thermal curing process, the two peaks disappear in the spectrum of the SPEs (ILE50 after curing and 8 vol %_Al2O3), indicating the formation of ether linkages by the complete reaction of epoxide rings. In addition, the two characteristic peaks at 1850 and 1770 cm-1 are associated with the anhydride carbonyl C=O group asymmetric and symmetric stretching vibration of the MeTHPA, respectively. After 9



curing, however, it is observed that for the SPEs, not only the absence or weakening of the anhydride C=O bands but also the appearance of the aliphatic ester stretching band at 1730 cm1

are observed, owing to the esterification reaction between MeTHPA and DGEBA.14 This is

another evidence to identify the ring-opening polymerization reaction in these SPEs.

3.2 Thermal Analysis Figure 2 correlates glass transition temperature that is the only phase transition observed in these SPEs by DSC, with either ILE or Al2O3 content, emphasizing the connection between composition variation and Tg. The host cross-linked epoxy matrix without ILE (ILE0) exhibits the highest Tg = 407 K among all the materials studied (Figure 2 and Supporting Information, Table S1), whereas the Tg = 204 K of the neat ionic liquid electrolyte (ILE100), consisting of BMIM-TFSI and LiTFSI, is ~200 K lower than that of ILE0 and ~17 K higher than that of the pure BMIM-TFSI (Tg = 187 K).35 The increase in Tg has been also observed in another binary mixture of imidazolium-based ionic liquid with lithium salt because more ion pairs associate to form physical cross-links, leading to consistently higher Tg.36 After mixing and curing, the two components have significantly different Tg values. The SPE with less than 50 vol% ILE (ILE30) in Figure 2a exhibits a Tg = 369 K, which is 38 K lower than the ILE0 Tg, indicating that the incorporation of ILE into epoxy matrix enhances the epoxy chain mobility (i.e., the plasticization effect of ILE). A single Tg for lower ILE content SPE (ILE30) suggests continuous epoxy-rich domains in which ions such as BMIM+, Li+, and TFSI- are randomly distributed. As ILE content increases to 50 and 70 vol% ILE, however, two Tg values are observed for the SPEs (ILE50 and ILE70, Figure 2a and Figure S2), suggesting microphase 10


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separation into ILE-rich and epoxy-rich microdomains. The ILE-rich microphase Tg (referred to as TgILE , open symbols in Figure 2) barely changes, while the Tg of the epoxy-rich microphase (referred to as Tgepoxy , filled symbols in Figure 2) steadily decreases with ILE content. The characteristic decrease in Tgepoxy of the epoxy-rich microdomains with , ILEX increasing ILE content was compared to predictions from the Fox equation37 (eq 1, dashed line in Figure 2a) and Gordon-Taylor equation38 (eq 2, solid line in Figure 2a),

1 epoxy g , ILEX

T

epoxy g , ILEX

T





1   ILE  ILE  ILE100 TgILE0 Tg

(1   ILE )TgILE0  K  ILETgILE100 (1   ILE )  K  ILE

(1)

(2)

wherein  ILE is the volume fraction of the ILE, TgILE0  407 K , TgILE100  204 K , and K is an adjustable parameter practically related to the degree of curvature of the Tg vs  ILE curve. The fact that the positive Tg deviations from the Fox equation and the agreement between the experimental Tg and Gordon-Taylor equation with the best fit K = 0.59 reflects that there are favorable intermolecular interactions between the components.39 This presumably indicates that at higher ILE contents, most of the ions form the ILE-rich microphase, separated from the epoxy-rich microphase via PIMS, and some of the ions can be also incorporated in the epoxyrich domains and act as plasticizers that lower Tgepoxy . For the Al2O3 containing SPEs, where Al2O3 nanowires with various contents are added to the ILE50, they also exhibit two Tg values: one for Tgepoxy and the other for TgILE , as shown in Figure 2b. Increasing the Al2O3 content from 2 to 5 and 6 vol% leads to a ~20 K increase in Tgepoxy (filled symbols in Figure 2b) and 11



adding further Al2O3 lowers Tgepoxy slightly. Note that all the Al2O3-containing SPEs have lower Tgepoxy than their host Al2O3-free SPE (ILE50), presumably due to Al2O3 plasticization. However, an effect from Al2O3 content on TgILE (open columns in Figure 2b) is more subtle; that is, the Al2O3 containing SPEs have a quite similar TgILE ~ 200 K within DSC experimental uncertainty (as indicated in Figure 2b and Table S1). This similar TgILE regardless of Al2O3 content is consistent with the results from ionic conductivity described in a later section.

3.3 Ionic Conductivity of Epoxy-based SPEs with and without Al2O3 The ionic DC conductivity  DC , shown in Figures 3a-b, was evaluated from the plateau region in the frequency

f

dependence of the in-phase part of conductivity  '( f )

(Supporting Information, Figure S3). The ILE content significantly affects the  DC for these SPEs as shown in Figure 3a. Compare to the room temperature conductivity of the neat ILE100 (  DC ~ 1.3 103 S/cm ), the ILE-rich ILE70 and ILE50 with the two Tgs (Figure 2a) show slightly lower  DC ~ 6.9 104 S/cm and  DC ~ 2.6 104 S/cm , respectively, while the epoxy-rich ILE30 with a single Tg (Figure 2a) shows three orders of magnitude lower

 DC ~ 2.5 106 S/cm . On the other hand, the Al2O3 containing SPEs exhibit similar conductivities (see Figure 3b). The temperature dependence  DC for the neat ILE100 and the epoxy-based SPEs with and without Al2O3 is well described by a Vogel-Tammann-Fulcher (VTF) equation40-43 (the solid curves in Figures 3a-b and Supporting Information, Figure S4)

12


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

Ea    R(T  T0 ) 

 DC (T )  AT 1/ 2 exp 

(3)

wherein A is the pre-exponential constant at infinite temperature, proportional to the number of charge carriers, Ea is equivalent to the activation energy for ion conduction, R is the gas constant, and T0 is the Vogel temperature, typically ~50 K below Tg ; those fitting parameters are listed in Table 1. As expected, Table 1 shows not only TgILE  T0  48 K for all the SPEs, but also an increase in the A value with increasing ILE content, indicating that the addition of ILE into the epoxy matrix clearly enhances the number of charge carriers and conductivity for these SPEs. Furthermore, for the ILE-rich SPEs (ILE70, ILE50, ILE50+2, 5, 6, and 8 vol% Al2O3), their activation energies Ea ~ 8.4 kJ/mol are similar to that of the neat ILE100 ( Ea ~ 8.2 kJ/mol ), whereas for the epoxy-rich ILE30, decreasing ILE content significantly increases the activation energy by 2X ( Ea ~ 16.3 kJ/mol ) (Figure 3c and Table 1), resulting in the lowest ionic conductivity among all the SPEs studied. To explore the relation between conducting network topology and ionic conductivity for these SPEs, the conductivity data  ILEX shown in Figures 3a-b were normalized by the conductivity  ILE100 of the neat ILE100 and its volume fraction  ILE in the SPEs:

 nor 

 ILEX  ILE ILE100

(4)

The normalized conductivity  nor  1 assumes the SPE conducting phase to behave identically to that of the neat BMIM-TFSI + LiTFSI electrolyte mixture (ILE100).44 Figure 3d

13



displays the  nor at 298 K for the SPEs and shows that the epoxy-rich ILE30 has nearly two orders of magnitude lower  nor than the ILE-rich SPEs (  nor  0.7 for ILE70 and

 nor  0.4 for ILE50). The  nor values observed in these SPEs are higher than those in other epoxy-based SPEs14,31 containing either ionic liquid (star symbols in Figure 3d) or a mixture of succinonitrile and Li salt (hexagon symbols in Figure 3d). The large  nor is intimately related to the morphological advantage of the SPEs containing more than 50 wt% ILE, where the ionic channels are effectively arranged along the continuous ILE-rich microdomains separated from the epoxy-rich microdomains. This conducting network topology has been quantified by the tortuosity   1/  nor .45 In numbers of literature28,46,47, the 1.5    3 are expected for small molecule transport in media with bicontinuous network morphologies. In the present study, the  values obtained from the normalized conductivity are 1.3    3.4 , which is similar to the prediction of the network structure, as shown in Figure 3e.

3.4 Relationship among Young’s Modulus, Morphology, and Ionic Conductivity The effect of ILE and Al2O3 on mechanical properties of these epoxy-based SPEs was investigated from stress-strain curves (Supporting Information, Figure S5). The chemically cross-linked epoxy acting as internal framework provides SPEs with superior mechanical properties since Young’s modulus of the pure epoxy (ILE0) is about 3 GPa at 298 K. Upon the incorporation of ILE into the epoxy matrix, it is observed that with increasing the ILE content, Young’s modulus decreases to 2 GPa for ILE30 and 0.5 GPa for ILE50, and further increasing ILE content significantly drops Young’s modulus by 100-fold (40 MPa for ILE70) as shown in Figure 4a. The modulus decrease to 2 GPa is expected due to the reduction in the epoxy volume 14


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fraction, with further reductions to 0.5 GPa and 40 MPa, owing to the development of the porous microstructure, as shown in the FESEM micrographs of these SPEs (Figures 4b-d). The porous morphology, however, gives rise to an advantage to generate ion path channels inside the epoxy matrix, resulting in a considerable increase in the conductivity from ~10-6 (ILE30) to 10-4 S/cm (ILE50 and ILE70) by 100-fold (Figure 4a). This result indicates an inverse relationship between Young’s modulus (filled symbols in Figure 4a) and the ionic conductivity (open symbols in Figure 4a). In Figure 4b, the FE-SEM image of ILE30 reveals the small size-pores randomly dispersed throughout the robust but nonconducting epoxy matrix. This is consistent with the observation of the single Tg (Figure 2a), the lowest ionic conductivity (Figure 3a), and the large tortuosity

  138 . On the contrary, for the ILE-rich SPEs in Figures 4c-d, their FE-SEM images depict a disordered porous structure with an array of interconnected epoxy microdomains at a submicron scale, validating a microphase-separated bicontinuous network morphology of interpenetrating domains of conducting phase and cross-linked epoxy. This is why ILE50 and ILE70 exhibit the two Tgs from DSC (Figure 2a) and the relatively high normalized conductivity and low tortuosity from conductivity measurement (Figures 3d & 3e). Consequently, ILE50 with E ~ 0.5 GPa,   2.3 , and  DC ~ 104 S/cm at 298 K appears to be the best choice of SPE among the five samples studied here (Figure 3). The combination of epoxy and ILE can tune Tg (from single Tg to two Tgs) and microstructure (from island to bicontinuous morphology) to optimize Young’s modulus and ionic conductivity. Figure 5a displays the effect of incorporating Al2O3 nanowires as inorganic fillers to further increase Young’s modulus without sacrificing ionic conductivity. After addition of up to 5 vol% of Al2O3 into ILE50, its Young’s modulus approaches ~1 GPa, which is two times higher than 15



the host Al2O3-free ILE50 (~0.5 GPa). This is accompanied by an increase in the ionic conductivity (  DC ~ 2.9 104 S/cm at 298 K) as shown in Figure 5a. Beyond 6 vol% Al2O3, however, both the Young’s modulus and the ionic conductivity begin to decrease to 0.67 GPa and  DC ~ 1.8 104 S/cm , respectively. The nonmonotonic behavior on the Al2O3 containing SPEs is also observed in their glass transition temperatures (Figure 2b); i.e., the Tgepoxy of epoxy-rich microdomains and the TgILE of ILE-rich microdomains are consistent with the results of Young’s modulus and conductivity, respectively. Furthermore, from the representative FE-SEM images of the SPEs with 5 and 8 vol% Al2O3 in Figures 5b-c, both the SPEs also exhibit a bicontinuous morphology with the interconnected epoxy microdomains, but for the 8 vol% Al2O3 SPE, the dimensions of the epoxy microdomains are much smaller than for the 5 vol% Al2O3, indicating that the microphase separation took place on a much shorter length scale.31 When the Al2O3 nanoparticles are uniformly dispersed in the epoxy matrix (Supporting Information, Figure S6 showing elemental mapping of oxygen and aluminium of the SPE with 5 vol% Al2O3), this can significantly enhance the mechanical properties, due to the Al2O3 having superior mechanical characteristics, and increase the ionic conductivity. However, as the Al2O3 nanowires with high surface area48 are further incorporated, it is possible to easily produce the Al2O3 aggregation, eventually leading to a decrease in Young’s modulus and loss of ionic conductivity. Compared to the host ILE50, the improvement in ionic conductivity on the addition of Al2O3 nanowires is presumably due to the favor interaction between the nanowire surface group and ionic species by Lewis acid-base interaction.49 This allows ion pairs to be easily dissociated, hence boosting the concentration of conducting ions that can transport rapidly throughout the inorganic surface as the conductive pathways.50 Eventually, optimization of the compatibility between high modulus and high ionic 16


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conductivity is achieved in the ILE50 with 5 vol% Al2O3 that exhibits E ~ 1 GPa and  DC ~ 2.9 x 10-4 S/cm at room temperature. The development of the room temperature ionic conductivity  DC and corresponding Young’s modulus E of various SPEs14,27,28,31,51 is shown in Figure S7, where there is little such an SPE exhibiting E  1 GPa and  DC > 10-4 S/cm concurrently. It is worth proposing that our work reaches the optimal combination of modulus and conductivity, providing competitive energy storage characteristics as well as maintaining enough structural integrity as a robust solid polymer electrolyte for supercapacitors.

3.5 Electrochemical Performance of Solid-State Supercapacitor The electrochemical measurements of a solid-state supercapacitor with centimeter dimensions (Figure 6), consisting of the epoxy-based SPE (ILE50) and activated carbon electrodes, are conducted in the symmetric cell at room temperature. Figure 6a displays scan rate-dependent CV curves of the full supercapacitor in the voltage window from 0 to 2.5 V, indicating a stable voltage window near 2.5 V. Although at the low scan rate, the CV curves exhibit a rectangular shape, the deviation of CV curves from a rectangular shape of ideal electrical double-layer capacitors is observed at the high scan rate. This is presumably due to the relatively large resistance and overpotential.51 The capacitance of the supercapacitor is further evaluated by GCD measurements at various current densities (Figure 6b). The linear voltage-time relation (i.e., isosceles triangle shape of GCD curve) reveals a double-layer supercapacitor, consistent with the observation from the CV curves. To further demonstrate the advantage of epoxy-based 17



Page 18 of 35

SPE for the performance of the fabricated supercapacitor, the specific capacitance as a function of current density for the solid-state supercapacitor is shown in Figure 6c. The specific capacitance ( CsGCD , filled symbols in Figure 6c) was calculated from the GCD curves (Figure 6b) using the following equation52

CsGCD  2

I m  dV / dt

(5)

wherein I is the applied current, m is the mass of active material in an electrode, and

dV / dt is the slope of the discharge curve. As expected, the specific capacitance decrease gradually with increasing discharge current density due to the internal resistance of electrode (or large IR drop at a large discharge current density).52 In Figure 6c, the high specific capacitance of ~90 F/g is obtained from the epoxy SPE-based supercapacitor at a current density of 0.13 A/g. The capacitance obtained here is much higher than those obtained from liquid electrolytes combined with activated carbon (AC)-based electrodes in previous reports;53-55 for example, 60 F/g at 100 mA/g for MnO2/LiOH/AC and 80 F/g at 10 mA/cm2 for AC/ionic liquid/AC.56-58 The Ragone plot of the supercapacitor displaying power density as a function of energy density based on the GCD tests is shown in Figure 6c, where a maximum energy density of 75 Wh/kg at 382 W/kg is obtained for our symmetric supercapacitor, and even at a high power density of 9.3 kW/kg, the device still displays an energy density of 44 Wh/kg. This performance is superior to that of other symmetric supercapacitors, such as Si/epoxy-based SPE/Si (5 – 8 Wh/kg and 4 kW/kg)51 and AC/V2O5/AC (24 Wh/kg at 2.5 kW/kg).59 The supercapacitor also exhibits reasonably cycling stability with a high capacitance retention of 90% (after 100 charge-discharge cycles) and 61% (after 1000 cycles) as shown in Figure S8. We further demonstrate the practical application of our supercapacitor by powering 18


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light-emitting diodes (LEDs). Owing to the high capacitance, the device could power two red LEDs (1.8 ~ 2.1 V, 20 mA, 3 mm diameter) as shown in Figure 6d.

4. Conclusion We have successfully developed a cross-linked epoxy-based SPEs including a plasticizing ionic liquid, a weak-binding lithium salt, and an inorganic robust Al2O3, by single-pot ring opening polymerization enabling easy processing control, mild reaction conditions, and high reactivity. The ionic conductivity is enhanced with an increase of the ILE concentration, and 70 vol% ILE results in the highest room temperature conductivity (  DC ~ 6.8 x 10-4 S/cm) among the SPEs under investigation. Increasing ILE content, on the other hands, leads to a decrease in Young’s modulus. The incorporation of 5 vol% Al2O3 allows the SPEs to overcome such a trade-off and promotes Young’s modulus as E ~ 1 GPa without compromising ionic conductivity (  DC ~ 2.9 x 10-4 S/cm). The superior performance was due to the long-range continuity of the conducting channels that facilitates ion transportation over long distances and chemically cross-linked mechanical phase, consistent with the thermal, morphological, and ion transporting observation. Crucially, we fabricate the solid-state supercapacitor and observe its electrochemical performance with high energy/power density. This work opens the door for development of multifunctional safe solid electrolytes maintaining structural integrity and efficiently transporting electrical energy in next generation energy storage applications. Furthermore, future work will connect the epoxy resin-based membrane with an SPE in a lithium metal battery, due to its dense cross-linked structure and good adhesion with the electrode,60 enabling effective suppression Li dendrite growth.61,62 19



Supporting Information. ATR-FTIR spectra, DSC thermograms, in-phase part of conductivity, residual sum of squares from the VFT fitting to ionic conductivity, stress-strain curves, EDS mapping, multifunctional plot of Young’s modulus vs ionic conductivity, cycle stability, and EIS spectra.

Acknowledgements. This study was financially supported by the Fundamental Research Program (PNK5830) of the Korean Institute of Materials Science (KIMS) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (Grant No. 20174010201460).

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27



Page 28 of 35

Table 1. Fitting Parameters of the VTF (Eq 3) Temperature Dependence of the Ionic Conductivity

Sample

A [S/cm K1/2]

Ea [kJ/mol]

T0 [K]

TgILE  T0

ILE30

4.6

16.3

128

-

ILE50

5.2

8.5

155

49

ILE70

15.3

8.5

158

47

ILE100

27.0

8.2

160

44

ILE50+2 vol%_Al2O3

4.8

8.2

156

49

ILE50+5 vol%_Al2O3

5.9

8.6

154

46

ILE50+6 vol%_Al2O3

5.8

8.7

153

51

ILE50+8 vol%_Al2O3

3.0

8.3

155

50

28


Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Chemical structures of DGEBA epoxy resin, MeTHPA curing agent, BDMA catalyst, BMIM-TFSI ionic liquid, and LiTFSI salt.

29



epoxy-rich domain Tgepoxy (filled symbols)

400

(b)

TgILE

ILE-rich domain (open symbols) Gordon-Taylor Eq (K=0.59) Fox Eq

350 300

ILE0 ILE30 ILE50 ILE70 ILE100

250 200 0.0

0.2

400 350

Tg [K]

(a)

Tg [K]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

epoxy-rich domain Tgepoxy (filled columns) ILE-rich domain TgILE (open columns)

ILE50+2 vol%_Al2O3 ILE50+5 vol%_Al2O3 ILE50+6 vol%_Al2O3 ILE50+8 vol%_Al2O3

300 250 200

0.4

0.6

0.8

150 0

1.0

0.02

ILE

0.05

0.06

0.08

Al2O3

Figure 2. Compositional variation in the glass transition temperature Tg for (a) the neat epoxy (ILE0), net ILE (ILE100), and the Al2O3 free SPEs (ILE30, ILE50, and ILE70) and for (b) the Al2O3 containing SPEs (ILE50+X vol%_Al2O3, where X is 2, 5, 6, or 8), where  ILE and  Al2O3 are the volume fraction of ILE and Al2O3, respectively. The dashed line is the Fox eq 1 and the solid line is the Gordon-Taylor eq 2 fit to the Tg data with adjustable parameter K = 0.59.

30


Page 31 of 35

-1 (a) 10

10

10-5 10-7 ILE100 ILE70 ILE50 ILE30

10-9 10-11 10-13

2.5

3.0

VTF fits (solid lines)

10-3 10-5

DC [S/cm]

DC [S/cm]

-1 (b) 10

VTF fits (solid lines)

-3

10-7 ILE50 ILE50+2 vol%_Al2O3 ILE50+5 vol%_Al2O3 ILE50+6 vol%_Al2O3 ILE50+8 vol%_Al2O3

10-9 10-11

3.5

4.0

4.5

10-13

5.0

2.5

3.0

3.5

1000/T [K-1]

4.0

4.5

5.0

1000/T [K-1]

Al2O3 0.00

0.02

0.04

0.06

0.08

(c) 14 12 10

(d) 100

ILE100 ILE70 ILE50 ILE30

10-3

ILE50 ILE50+2 vol%_Al2O3 ILE50+5 vol%_Al2O3 ILE50+6 vol%_Al2O3 ILE50+8 vol%_Al2O3

nor

16

Ea [kJ/mol]

T = 298 K ILE100 ILE70 ILE50 ILE30

10-6

Shirshova et al. @ 303 K (ref. 31) Jang et al. @ 298 K (ref. 14)

8 10-9 0.2

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

ILE

0.4

0.02

(e)

ILE70 ILE50

4

0.04

0.6

0.8

1.0

ILE

Al2O3 0.06

0.08

ILE50+2 vol%_Al2O3 ILE50+5 vol%_Al2O3 ILE50+6 vol%_Al2O3 ILE50+8 vol%_Al2O3

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 T = 298 K

1 0.4

0.5

0.6

0.7

ILE

Figure 3. Temperature dependence of ionic conductivity for SPEs containing different concentration of (a) ILE (filled symbols) and (b) Al2O3 (open symbols): solid curves indicate fits to eq 3. ILE and Al2O3 compositional variation in (c) the activation energy Ea , (d) the normalized conductivity  nor at 298 K, compared with literature values14,31 (hexagon symbols at 298 K and star symbols at 303 K), and (e) tortuosity  at 298 K for epoxy-based SPEs, where  ILE and  Al2O3 are the volume fraction of ILE and Al2O3, respectively.

31



Figure. 4 (a) ILE compositional variation in Young’s modulus ( E , filled symbols, left axis) and ionic conductivity (  DC , open symbols, right axis) at 298 K: solid and dashed lines are only guides for the eyes. FE-SEM micrographs of (b) ILE30, (c) ILE50, and (d) ILE70 after extraction of the conducting electrolyte phase: the bright regions in the micrographs are the epoxy phase.

32


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Page 33 of 35

3.5 1.0

3.0

0.9

0.8

2.5

0.7 ILE50 ILE50+2 vol%_Al2O3 ILE50+5 vol%_Al2O3 ILE50+6 vol%_Al2O3 ILE50+8 vol%_Al2O3

0.6

(b) 5 vol%_Al2O3

(a)

T = 298 K E (filled symbols) DC (open symbols)

2.0

0.5

DC [x10-4 S/cm]

E (Young's Modulus) [GPa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c) 8 vol%_Al2O3

1.5 0.00

0.02

0.04

Al2O3

0.06

0.08

Figure 5. (a) Al2O3 compositional variation in Young’s modulus ( E , filled symbols, left axis) and ionic conductivity (  DC , open symbols, right axis) at 298 K: solid and dashed lines are only guides for the eyes. Representative FESEM micrographs of (b) ILE50+5 vol%_Al2O3 and (c) ILE50+8 vol%_Al2O3 after extraction of the conducting electrolyte phase: the bright regions in the micrographs are the epoxy phase.

33



Figure. 6 Electrochemical measurements for the supercapacitor, composed of the epoxy-based SPE (ILE50) and activated carbon electrodes. (a) Cyclic voltammetry (CV) profiles at different scan rates from 10 to 100 mV/s. (b) Galvanostatic charge/discharge (GCD) profiles at various currents density from 0.13 to 1.00 A/g. (c) Specific capacitance as a function of the current density (filled symbols, left and bottom axes) from the GCD measurements and Ragone plot of the power density vs energy density of the supercapacitor (open symbols, right and top axes). (d) Photographs show the supercapacitor with dimensions [120 (length) x 110 (width) x 0.65 (thickness) mm] powering LEDs.

34


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3.5 T = 298 K E (filled symbols) DC (open symbols)

1.0 0.9

3.0

0.8

2.5

0.7 ILE50 ILE50+2 vol%_Al2O3 ILE50+5 vol%_Al2O3 ILE50+6 vol%_Al2O3 ILE50+8 vol%_Al2O3

0.6

2.0

0.5

DC [X10-4 S/cm]

E (Young's Modulus) [GPa]

Table of Contents (TOC)

1.5 0.00

0.02

0.04

Al2O3

0.06

0.08

Energy density [Wh/kg]

C H

H 3C

3

O

O O H C H

3

N F 3C C H

3

O S O

N

3

O

O

N

C H

O S O

C F

O O

n

3

F 3C

O S O

N

Li

O S O

C F

3

40 120

50

60

70

80 104

110 100

103

90 80 70

102

60 50 0

200

400

600

800

1000

Power density [W/kg]

H 3C

Specific capacitance [F/g]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


101

Current density [mA/g]

35


Multifunctional Epoxy-Based Solid Polymer Electrolytes for Solid-State

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