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Co-continuous structural electrolytes based on ionic liquid, epoxy resin and organoclay: Effects of organoclay content
Materials and Design 104 (2016) 126–133
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Co-continuous structural electrolytes based on ionic liquid, epoxy resin and organoclay: Effects of organoclay content Yalin Yu a,⁎, Boming Zhang a, Yang Wang b, Guocheng Qi a, Fangyu Tian a, Jiping Yang a, Shubin Wang a a b
School of Materials Science and Engineering, Beihang University, Beijing 100191, China School of Transportation Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 6 March 2016 Received in revised form 1 May 2016 Accepted 4 May 2016 Available online 06 May 2016 Keywords: Structural electrolyte Organoclay Mechanical properties Ionic conductivity
a b s t r a c t As a vital part of structural power composites, structural electrolyte should achieve ionic conducting and load bearing functions simultaneously. In the present work, a novel method was established for optimizing the morphology and multifunctionality of a liquid/epoxy based structural electrolyte by adding different contents of organoclay (organically modified layered silicates, OLS). The OLS content had a substantial effect on the liquid/ epoxy co-continuous morphology. The introduction of OLS changed the epoxy microstructure from the interconnected flakes to the parallel ridges. The increase of OLS content led to a more compact epoxy network and a more interpenetrated conductive phase. However, an excessive amount of OLS could cause the agglomeration and make the morphology parallel interconnected bars. The variation in morphology accounted for the multifunctionality increase in the structural electrolytes. The optimal formulation was determined with the best properties of 211 MPa in tensile modulus and 0.09 mS/cm in ionic conductivity, approaching the inferior limit of the multifunctional performance for a structural electrolyte. As a promising candidate for structural power composites, this co-continuous structural electrolyte containing ionic liquid, epoxy resin and OLS shows great potential to be used in the multifunctional energy storage devices. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Broad efforts have been devoted to lightweight and multifunctional materials for the reduction of the energy consumption in many engineering applications [1]. The structural power composite (SPC) can simultaneously realize the goals of load bearing and energy storage and is considered as one of the most promising alternative materials for manufacturing energy storage devices [1–3]. The structural electrolyte, typically modified polymer materials, is a vital component in SPC to provide high ionic conductivity as well as the ability to transfer load [4,5,6]. Apart from the conventional research approaches of polymer matrix, the investigations on the electrochemical performances such as the ionic conductivity and dielectric properties are also utilized for the development of the structural electrolytes [7–10]. Studies have been mainly focused on two major strategies for the applications of the polymer electrolyte systems: One is the development of polyethylene oxide (PEO) based solid polymer electrolytes where the ions move via the local relaxation and segmental motion of the PEO chains, the other is the preparation of epoxy resin blended with gel or liquid electrolyte to achieve the mechanical and electrochemical functions separately in one polymer system. Since Wright ⁎ Corresponding author at: Room D542, New Main Building, No. 37 Xueyuan Road, Haidian District, Beijing, China. E-mail address: [email protected] (Y. Yu).
http://dx.doi.org/10.1016/j.matdes.2016.05.004 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
[11] firstly discovered that the complex of PEO and alkaline salts has the ability of ionic conductivity in 1973, numerous researchers have deeply investigated the ionic conduction mechanisms and multifunctional performance of PEO based electrolytes. Given the potential use of the vinyl ester derivatives of poly(ethylene glycol) (PEG), Snyder and co-workers formulated random copolymers containing ethylene oxide (EO) groups [12–14]. In this way a structural scaffold is generated through crosslinking while the PEG sidechains can enhance the ion transport. Willgert et al. [15] also conducted a systematic study on photoinduced curing of PEO-dimethacrylate/PEO-monomethacrylate lithium salt mixtures to form thermoset electrolytes and addressed the importance of seeking for an acceptable balance between mechanical and conductive properties. However, despite the advantage that the manufacture process for the vinyl ester derivatives of PEG is simple, the trade-off between mechanical and conductive properties makes it very difficult for the PEO based electrolytes to satisfy the inferior limit of the multifunctional performance for the SPC [13–16]. Epoxy resin is one class of the most used structural polymer matrices with high mechanical modulus and strength. Adding the immiscible electrolyte into epoxy/amine system can generate the co-continuous phase structure, with the epoxy networks providing the load-bearing function while the electrolyte phase supplying the ion mobility [17,18]. Recently, poly(ethylene-glycol) (PEG) or propylene carbonate (PC) as the additive was explored to form a bicomponent structural electrolyte [16]. Low multifunctional performance was obtained due to a combination
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of plasticization of the structural network and limited percolation of the liquid network. Besides, several authors chose ionic liquids (ILs) as the ionic conducting agents for the polymer electrolytes and highlighted the influence of the ILs on the dual-phase formation and structuration [19–23]. The addition of ILs led to a good multifunctionality with a room temperature ionic conductivity reaching 0.8 mS/cm and a Young's modulus of 180 MPa [21]. Gienger et al. compared the properties of the in-situ cured electrolyte and the porous epoxy resin backfilled with liquid electrolyte. The latter showed a better multifunctional performance with a room temperature ionic conductivity reaching 1.5 mS/cm and a Young's modulus of 120 MPa [18]. In summary, only when the formation and the control of the interconnected microstructure are well realized, can the multifunctional performance of the epoxy modified electrolyte be improved greatly. Thus the epoxy modified electrolyte has shown great potential to be the component of energy storage devices. It is well acknowledged that the addition of particle fillers into epoxy based hybrids are beneficial to the miscibility, phase separation and morphology with the fracture toughness being improved simultaneously [24–28]. As a result, incorporating nanoparticles into epoxy-based multi-phase blends is considered as the most effective method to phase separation and morphology controlling. Tanaka and the coworkers employed glass spheres into the polymer blends to affect the dynamic coupling between phase separation and wetting, and indicated the possibilities to control the final domain size in phase separation and the final morphology [24]. Zhang et al. devoted their efforts to the modification of the epoxy/PEI resin using nano-scale silica particles. The dispersion of the silica particles led to the occurrence of phase inversion structure and the size change of the secondary epoxy rich phase [25]. In recent years, there has been a growing interest in the utilization of organoclays (organically modified layered silicates, OLS) to control the morphology and phase separation kinetics for its distinctive intercalated and/or exfoliated structures [26–29]. The onset of phase separation and the gelation or vitrification time were greatly brought forward and the periodic distance of phase-separated structure was reduced when OLS was incorporated [28]. The phase diagram of epoxy blend was indicated to shift to a higher temperature with the increase in OLS content due to the easy penetration of the blends into the clay galleries [29]. Hence OLS has a dramatic impact to the phase separation process and the final phase morphology of binary polymer mixtures. However, to the best of our knowledge, these studies are mainly focused on the application of OLS into the thermoplastic/thermosetting mixtures but no literature is available regarding the effects of OLS on the morphology and multifunctional performance of the liquid/polymer co-continuous system. In the present paper, the effects of OLS content on the multifunctional performance of the structural electrolytes were discussed. Epoxy was blended with liquid electrolyte and OLS to form the co-continuous microstructures. Bisphenol A epoxy/polyether amine resin system was
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utilized to form the crosslinked network for load-bearing, and IL dissolved with PC and lithium salt as the liquid phase provided the ionic conductivity. We optimized the phase separation and final microstructure by the addition of different mass contents of nano-scaled OLS, and evaluated the morphology, mechanical and conductive properties of the structural electrolytes. The influence of OLS content on the multifunctional performance was summarized and the optimal formulation of the structural electrolyte based on ionic liquid, epoxy resin and OLS was determined. 2. Experimental 2.1. Materials The epoxy resin system used in this study was composed of liquid diglycidyl ether of bisphenol A (DGEBA) type epoxy resin E51 with an epoxy value of 0.51 from Lanxing Wuxi Resin Co. Ltd. and tetrafunctional epoxy resin AG-80 with an epoxy value of 0.85 from Shanghai Huayi Resin Co. Ltd. The curing agent was JEFFAMINE D400 polyetheramine bought from Huntsman International LLC, with an amine hydrogen equivalent weight of 115. The organoclay (Nanomer® I.·30P from Nanocor Inc.) employed in this work was a montmorillonite with octadecylamine modified as the intercalation reagent. The mixture of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (EMIM-Tf2N, 99%, Shanghai Cheng Jie Chemical Co. Ltd.) and propylene carbonate (PC, 99%, Kuer Chemistry Co. Ltd.) was utilized as the liquid electrolyte with the corresponding lithium salt of lithium bis(trifluoromethanesulphonyl)imide (LiTf2N, 99.9%, Chinese Special Gases Department of Purification Equipment Research Institute). All these materials were kept in a vacuum drier at ambient temperature in order to avoid absorbing moisture. 2.2. Polymer preparation The liquid electrolyte component was prepared with LiTf2N dissolved in the mixture of 99 wt.% EMIM-Tf2N and 1 wt.% PC at the concentration of 2.3 mol/L. E51, AG80 and the liquid electrolyte were roughly blended following the component ratio listed in Table 1 and placed in an oven at 55 °C for 5 min in order to decrease the viscosity. The OLS was dispersed finely in the preheated blends by high-shear mixer at 8000 rpm for 1 h in an external ice bath to keep the system from local overheating before the formulated addition of D400. Finally all the components were stirred using a power mixer until a uniform solution was obtained. The resin plaques with the thickness of 2 mm were manufactured using two paralleled glass plates separated by a silicone spacer. The two glass plates were treated with the mold release agent and clamped by bulldog clips. The cylindrical spacer with a diameter of 2 mm was placed in a U-shape between the glass plates, as shown in Fig. 2. The
Table 1 Compositions and multifunctional performances of the structural electrolytes based on ionic liquid, epoxy resin and OLS. Sample label
S A B C Epoxy-S Epoxy-A Epoxy-B Epoxy-C Ionic liquid Conductive phase
Structural phase
Liquid phase
Multifunctional performance
E51 (g)
AG-80 (g)
D400 (g)
OLS (g)
EMIM-Tf2N (g)
PC (g)
LiTf2N (g)
Tensile modulus (MPa)
Tensile strength (MPa)
Ionic conductivity (mS/cm)
70 70 70 70 70 70 70 70 – –
30 30 30 30 30 30 30 30 – –
73.49 73.49 73.49 73.49 73.49 73.49 73.49 73.49 – –
0 (0 wt.%) 8.99 (2.5 wt.%) 17.99 (5.0 wt.%) 26.98 (7.5 wt.%) 0 (0 wt.%) 8.99 (2.5 wt.%) 17.99 (5.0 wt.%) 26.98 (7.5 wt.%) – –
128.81 128.81 128.81 128.81 – – – – 128.81 128.81
1.30 1.30 1.30 1.30 – – – – – 1.30
56.14 56.14 56.14 56.14 – – – – – 56.14
373.25 ± 36.71 136.89 ± 7.88 211.57 ± 11.10 198.94 ± 9.67 2800.69 ± 24.28 2753.75 ± 52.23 3024.64 ± 57.56 3115.89 ± 57.72 – –
9.27 ± 1.34 9.75 ± 0.80 10.92 ± 0.35 9.07 ± 0.09 52.93 ± 0.43 44.20 ± 0.21 47.32 ± 0.25 44.84 ± 0.33 – –
(5.67 ± 0.42) × 10−2 (4.74 ± 0.30) × 10−2 (8.97 ± 0.54) × 10−2 (8.42 ± 0.41) × 10−2 – – – – 7.01 ± 0.03 1.66 ± 0.02
The concentration of LiTf2N in the liquid phase was kept constant at 30.14 wt.% and weight ratio (E51 + AG-80 + D400): (PC + EMIM-Tf2N) at 100:75.
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Fig. 1. Procedure for the preparation of the specimens.
homogenous solution was poured into the preheated mold and degassed in the vacuum oven at 45 °C for 20 min. The curing cycle was 75 °C for 2 h, 110 °C for 2 h and 150 °C for 2 h. All the heating and cooling ramps were carried out at the same rate of 2 °C/min. Macroscopically separated phases cannot be observed in any of the cured resin plaques as shown in Fig. 1. All the specimens can be considered as isotropic. This is because the rate of phase separation is slower compared with the polymer formation at the curing temperature [20,21]. After demolding, the plaques were cut into three specimen types: discs (20 mm diameter) for electrochemical impedance spectroscopy and morphology observation, bars (45 ∗ 8 ∗ 2 mm) for dynamic mechanical analysis, and dumbbell specimens for tensile testing. All specimens were kept in the vacuum drier at ambient temperature before the characterizations in order to avoid absorbing moisture.
amplitude of 5 μm. The DMA samples were heated from − 50 °C to 100 °C at a heating rate of 3 °C/min. The glass transition temperature (Tg) of each specimen was determined from the tan δ peak.
2.3. Characterization 2.3.1. Scanning electron microscope (SEM) The SEM specimens with different OLS content were submerged in ethanol which was changed every other 12 h to extract the liquid phase. After seven days, the specimens were placed in the oven at 75 °C under vacuum until each sample mass was constant. The morphology was observed using the scanning electron microscope (CamScan JEOL 6010). 2.3.2. Fourier-transform infrared spectroscopy (FT-IR) The Fourier-Transform Infrared Spectroscopy (FT-IR) was conducted on a Nicolet Avatar 360 in order to confirm whether the chemical compositions of the specimens with different formulations were the same. 32 scans at a resolution of ±4 cm−1 were averaged for each specimen. All the IR measurements were performed in reflection mode with a frequency range of 4000–400 cm−1 at ambient temperature. 2.3.3. Dynamical mechanical analysis (DMA) Dynamic mechanical thermal analysis (Mettler Toledo) was performed in a three-point bending mode at a frequency of 1 Hz with an
Fig. 2. FT-IR spectra of structural electrolytes with different OLS contents (S, A, B and C), OLS/epoxy nanocomposite (Epoxy-B) and conductive phase.
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Table 2 Mass and volume contents of different phases in the structural electrolytes. Sample no. Mass fraction
Volume fraction
Structural phase
S A B C
Conductive phase
Structural phase
Conductive phase
Nominal (wt.%)
Practical (wt.%) Nominal (wt.%) Practical (wt.%) Nominal (vol.%) Practical (vol.%) Nominal (vol.%) Practical (vol.%) Difference (vol.%)
48.23 49.48 50.69 51.84
51.77 50.52 49.31 48.16
56.33 56.16 55.43 59.12
43.67 43.84 44.57 40.88
62.32 63.49 64.61 65.65
68.21 68.32 68.01 70.84
37.68 36.51 35.39 34.35
31.79 31.68 31.99 29.16
5.89 4.83 3.40 5.20
Fig. 3. SEM micrographs of structural electrolytes with different OLS contents after extraction of the liquid phase: (a) Sample S; (b) Sample A; (c) Sample B; (d) Sample C.
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2.3.4. Tensile testing Tensile tests were conducted for mechanical evaluation according to ASTM D790. Tensile strength and tensile modulus were measured using an electrical universal testing machine (Instron 5565) at a crosshead speed of 5 mm/min. The tensile force was measured by a load cell with a capacity of 500 N. For each formulation, eight specimens were tested in order to guarantee at least 5 effective data of tensile properties. 2.3.5. Electrochemical impedance spectroscopy (EIS) The EIS was measured using electrochemical workstation (CHI660D, Shanghai Chenhua Device Company, China) in a frequency range from 1 to 106 Hz. All specimens were dried in a vacuum drier at ambient temperature before testing. Each disc was coated with silver paste on both surfaces before placed into a fixer for clamping the discs with two stainless steel electrodes. Five specimens were tested for each formulation. The ionic conductivity of each specimen can be calculated by the following equation: σ¼
d Rs A
ð1Þ
where σ, d, Rs and A represent the ionic conductivity, the distance between the electrodes, the bulk resistance, and the cross-section area of the sample, respectively. 3. Results and discussion 3.1. Effects of OLS content on the composition and morphology The structural electrolytes with different OLS contents, as well as the distinct structural and conductive phases, were successfully prepared
according to the process in Section 2.2. FT-IR was utilized in order to determine the composition of the samples. From the spectrum of conductive phase, the band at 1635 cm−1 could be assigned to the in-plane C\\C and C\\N stretching vibration of the imidazole ring. Besides, the observed bands from 1578 cm−1 to 1057 cm−1 were assigned to the stretching modes of S\\O, S\\N and C\\F. As there was no difference in the chemical compositions between the Epoxy-S, A, B and C, only Epoxy-B was characterized by FT-IR on behalf of all the structural phases. For the spectrum of Epoxy-B, the absence of epoxy band at 915 cm−1 indicated the E51 and AG-80 resin was entirely cured. The bands from 1611 cm−1 to 1457 cm−1 corresponded to the C_C stretching vibration of the benzene ring. The C\\N stretching vibration was assigned to the band at 1351 cm−1. The characteristic bands of C\\C stretching mode, C\\O stretching mode and C\\H bending mode at 1196 cm−1, 1142 cm−1, and 1060 cm−1 respectively were also observed in the spectrum of Epoxy-B. The spectra of A, B, C and S in Fig. 2 show almost the same characteristic peaks, which suggests there is no difference in the chemical composition between the four samples. Furthermore, it can be inferred that the discrepancies in multifunctional properties, which will be discussed in the following context, are resulted from the differences in component microstructure instead of chemical composition. The specific composition of these samples can be analyzed by spectra comparison with pure structural and conductive phase. Like the spectrum of Epoxy-B, the absence of epoxy band at 915 cm−1 indicated the entire cure of epoxy resin. In addition, the S_O band at 1578 cm−1 in the conductive phase was overlapped by the bands corresponding to C_C stretching vibration of the benzene ring in Epoxy-B. The S\\O, S\\N and C\\F bands in the conductive phase were also covered by the C\\C, C\\O and C\\H bands, respectively. However, the band at 1635 cm−1 corresponding to the C\\C and C\\N stretching vibration of the imidazole ring could be observed in the
Fig. 4. Effects of OLS content for four different structural electrolyte samples on (a) DMA results and (b) the glass transfer temperature.
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spectra of all the structural electrolyte samples. This certificated the existence of conductive phase, which means all the specimens were bicomponent systems containing both epoxy resin and liquid electrolyte. In order to evaluate the phase contents, the liquid phase (conductive phase) of all these samples was extracted using ethanol and all the samples were dried in the oven. The drying process were not be halted until the specimen weight reached a constant value. Since the amount of D400 in the liquid phase could not be assessed precisely, the liquid electrolyte was assumed to be ideal without D400. According to the measured mass and the theoretical density of each material, the volume fractions of both structural and liquid phase can be calculated. The nominal and practical results were summarized in Table 2. The difference between nominal and practical volume fractions of liquid phase verified that a small part of liquid electrolyte was locked in the structural phase during the curing process. The practical data of the conductive phase corresponded to the results of interpenetrated liquid phase and the difference between nominal and practical values corresponded to the results of locked liquid phase. With the increase of OLS content, the amount of interpenetrated liquid electrolyte was increased at first and decreased afterwards, while the amount of locked liquid electrolyte showed an opposite tendency. All the samples exhibited a co-continuous microstructure characteristic with a porous continuous epoxy network in Fig. 3. However, the epoxy morphologies were highly dependent on the OLS content. In the cross-section of the structural electrolyte without OLS (Fig. 3(a)), the continuous phase was composed of interconnected epoxy flakes with a dimension of approximately 10 μm and discrete epoxy spheres with a diameter of 1–2 μm. The irregular-shaped holes penetrated with each other had a non-uniform size varying from 2 μm to 4 μm. After the addition of 2.5 wt.% OLS (Fig. 3(b)), the epoxy microstructure was transformed from flakes into ridges. The OLS/epoxy pieces dispersed alongside every epoxy ridge with nearly equal space like two rows of hackles. The distance between the ridges was close to that between the OLS/epoxy pieces, with a dimension of roughly 3–5 μm. The interpenetrated holes with a size of about 5 μm were relatively uniformly distributed in the epoxy matrix. For the sample with an OLS content of 5 wt.% (Fig. 3(c)), the space between ridges decreased to 1–2 μm and the space between hackles was narrowed to 3–5 μm. The holes distributed non-uniformly with the size reduced to less than 3 μm. Besides, many small epoxy spheres with a diameter of 1–2 μm were spread on the ridges and hackles as a new morphology. When the OLS content was increased to 7.5 wt.% (Fig. 3(d)), the OLS had a spatially nonuniform dispersion in the epoxy resin. The morphology of the epoxyrich region circled in red was similar with that in Sample S (Fig. 3(a)). The OLS-rich region circled in black was mainly composed of ridges with a larger dimension. The OLS agglomeration made the epoxy network look more like parallel interconnected bars with a space of about 3 μm. It is interesting to find such linear features with the addition of OLS, which may be caused by the gravity effect of the OLS or the rough surface of the glass plates. Efforts will be devoted to the phase separation process to further analyze the formation of the linear features.
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Fig. 5. Mechanical properties of the structural electrolytes with different OLS contents.
system, the latter played the dominant role. It suggested that the average length of polymer chain segment was increased with an increasing OLS content, which can result in the decrease in structural modulus. However, the storage modulus showed a different tendency. The storage modulus at ambient temperature ranged from 200 MPa to nearly 500 MPa. The addition of OLS resulted in a decrease of storage modulus to 50% value. With the increase of the OLS content, the storage modulus was slightly increased and decreased afterwards. In order to further analyze the relation between the mechanical performance and OLS content, the static tensile test was conducted on all these structural electrolytes (Fig. 5). The results showed that the tensile strength was increased firstly and decreased afterwards with very little difference between these values of each sample. The tendency of the tensile modulus was in accordance with the storage modulus. The addition of OLS decreased the modulus substantially from 373 MPa to 137 MPa, which can be explained by the morphology in Fig. 3. The ridges of Sample A were only connected by the OLS/epoxy pieces, while the epoxy network of the Sample S was composed of epoxy flakes connected with each other in three dimensions, which contributed to a more stable structure and a higher tensile modulus. Besides, more pores in larger size were found in Sample A than Sample S. However, For Sample B with an OLS content of 5 wt.%, the ridge network became more compact, with a lower ridge space and a higher epoxy volume fraction. Consequently, the tensile modulus was increased to 211 MPa. When 7.5 wt.% OLS was mixed with the electrolyte system, OLS agglomeration and the increase of volume fraction of the structural phase
3.2. Effects of OLS content on the properties The effect of the OLS content on the dynamical mechanical properties of the structural electrolytes can be seen in Fig. 4. The Tg linearly decreased with the increase of OLS content. With the OLS content increasing to 7.5 wt.%, Tg dropped about 10 °C. On one hand, the OLS with a large specific surface area had a strong bonding at the OLS/ epoxy interface, which restricted the movement of the epoxy polymer chain segments as well as lowered the free volume. As a result, the thermo-stability was improved and the Tg should be increased. On the other hand, the intercalation-agent, namely octadecylamine in this study, acted as the plasticizer to increase the free volume of the polymer chain segment, which led to the decrease of the Tg. In this co-continuous
Fig. 6. Ionic conductivity of the structural electrolytes with different OLS contents.
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Fig. 7. Multifunctionality of the structural electrolytes with different OLS contents: (a) Ionic conductivity plotted against tensile modulus, and (b) Comparison in multifunctional properties between our experimental results and existing literature data. “♦”—PEO:PEO-b-PE 50:50 [30]; “◊”—PMMA-TEOS:IL 50:50 [31]; “ ”—epoxy-PEO/IL 60:40 [32]; “●”—PEGDGE/IL 60:40 [33]; “▼”—BADGE/TetradX/TEPA:IL 50:50 [17]; “★”—formulations based on VTM266 [21]; “☆”—epoxy porous monolith backfilled with PC [18]; “□”—formulations based on MVR444 [21]; “○”—formulations based on MTM57 [20]; “■”—liquid/epoxy system mixed with different contents of OLS.
interacted with each other, which slightly reduced the tensile modulus to 200 MPa. Moreover, the CV of the tensile mechanical performance decreased with an increasing OLS content, which implied that the addition of OLS can lower the data scatter of mechanical performance. Generally the OLS makes a contribution to the ionic conducting in the co-continuous electrolyte systems. The relation between OLS content and ionic conductivity has the same tendency with the Young's modulus (Fig. 6). The ionic conductivity is greatly influenced by the volume fraction of the liquid phase [3,4,12] and the interpenetration of the microstructure [2,17]. The epoxy network is insulating and the liquid phase plays a vital role in conducting ions. As a result, the increase in the amount of the liquid phase can promote the ionic conductivity [13,14]. Additionally, the structural electrolyte with a percolated phase-separated structure in scale and a very fine epoxy network provides more elaborate passageways for the ion transport [18,19]. Compared with the ridge network in Sample A, the epoxy flake network in Sample S had a better interconnectivity and more interconnected micro-sized holes. As a consequence, Sample A with 2.5 wt.% OLS had a lower ionic conductivity. But the ionic conductivity reached the highest value of 0.09 mS/cm with an OLS content of 5 wt.%, which may attribute to the highest volume fraction of interpenetrated liquid electrolyte phase (Table 2). Besides, the increase of OLS content can lead to a better percolated microstructure consistent with the micrograph in Fig. 3(c). When OLS content increased to 7.5 wt.%, the two factors interacted with each other and resulted in the decline in the ionic conductivity finally. With an excessive amount of OLS, the irregular distribution of the ridges and flakes as well as the agglomeration of OLS caused the distortion of the epoxy network. This morphological change brought about the decreasing percolation on the conductive phase and blocked a part of the channels for ion transport. Moreover, the practical volume fraction of the liquid phase reaches the lowest value at 7.5 wt.% OLS content in Table 2. This parameter represented volume fraction of the interpenetrated liquid phase. With the fewest mobile lithium ions in amount, the ionic conductivity ought to be decreased. 3.3. Multifunctional performance It is well known that the mechanical performance and ionic conductivity has a trade-off relationship. The former needs the stiffness of polymer chain segments and the undamaged microstructure, while the latter needs a soft chain segments and interpenetrated passageways throughout the epoxy [15,19]. Obviously, it is very difficult to improve the mechanical and electrochemical performances simultaneously. The effective way to overcoming such a conflict is controlling of the SPE microstructure at different length-scales [2], but it needs the studies
of different curing chemistries and novel matrix formulations. Nevertheless, a proper addition of OLS can be seen as a simple method for the increase of multifunctionality. Fig. 7(a) illustrated that the introduction of OLS firstly led to the reduction of both the properties, but with the increase of OLS content the structural property was linearly correlated with ionic conductivity. According to the literature, a reasonable goal for a structural electrolyte is to achieve both mechanical stiffness and ion conductivity within one order of magnitude of conventional materials [20,21]. In our study, the structural electrolyte should achieve at least 200 MPa in Young's modulus and 0.1 mS/cm in ionic conductivity. The literature data of structural electrolytes were collected in Fig. 7(b). Compared with PEO based solid polymer electrolytes, the epoxy/liquid systems with co-continuous structure are closer to the multifunctional goal. Furthermore, the structural electrolyte in our study with the primary epoxy/amine system approaches the threshold, showing an exciting combination between the ionic conductivity and Young's modulus. The multifunctional performance may have a further improvement by the modification of the material system, such as increasing the concentrate of lithium salt, and employing another epoxy system with a higher stiffness. In general, the co-continuous structural electrolyte based on ionic liquid, epoxy resin and OLS is a promising candidate for structural power composites and able to reach the goals of load bearing and energy storage simultaneously.
4. Conclusions We established a novel method to optimize the morphology and multifunctional performance of the structural electrolyte with distinct conductive and structural phases by adding different contents of OLS. The introduction of OLS into the epoxy/ionic liquid system changed the continuous phase from the interconnected epoxy flakes with attached micro-spheres to the parallel epoxy ridges connected by OLS/ epoxy hackles, which resulted in the decrease of mechanical and ionic conductive properties. However, with the increase of OLS content, a more compact epoxy network was developed with a more cramped ridge space and a more interpenetrated conductive phase, which increased the mechanical properties and ionic conductivity simultaneously. When OLS content reached 7.5%, OLS agglomeration and the decreasing of the conductive phase content led to the reduction of multifunctional performance. The highest multifunctional properties were obtained at 5% OLS content, with 211 MPa in tensile modulus and 0.09 mS/cm in ionic conductivity. This reaches the performance inferior limit and shows great potential for the application in the energy storage devices.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Co-continuous structural electrolytes based on ionic liquid, epoxy resin and organoclay: Effects of organoclay content Yalin Yu a,⁎, Boming Zhang a, Yang Wang b, Guocheng Qi a, Fangyu Tian a, Jiping Yang a, Shubin Wang a a b
School of Materials Science and Engineering, Beihang University, Beijing 100191, China School of Transportation Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 6 March 2016 Received in revised form 1 May 2016 Accepted 4 May 2016 Available online 06 May 2016 Keywords: Structural electrolyte Organoclay Mechanical properties Ionic conductivity
a b s t r a c t As a vital part of structural power composites, structural electrolyte should achieve ionic conducting and load bearing functions simultaneously. In the present work, a novel method was established for optimizing the morphology and multifunctionality of a liquid/epoxy based structural electrolyte by adding different contents of organoclay (organically modified layered silicates, OLS). The OLS content had a substantial effect on the liquid/ epoxy co-continuous morphology. The introduction of OLS changed the epoxy microstructure from the interconnected flakes to the parallel ridges. The increase of OLS content led to a more compact epoxy network and a more interpenetrated conductive phase. However, an excessive amount of OLS could cause the agglomeration and make the morphology parallel interconnected bars. The variation in morphology accounted for the multifunctionality increase in the structural electrolytes. The optimal formulation was determined with the best properties of 211 MPa in tensile modulus and 0.09 mS/cm in ionic conductivity, approaching the inferior limit of the multifunctional performance for a structural electrolyte. As a promising candidate for structural power composites, this co-continuous structural electrolyte containing ionic liquid, epoxy resin and OLS shows great potential to be used in the multifunctional energy storage devices. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Broad efforts have been devoted to lightweight and multifunctional materials for the reduction of the energy consumption in many engineering applications [1]. The structural power composite (SPC) can simultaneously realize the goals of load bearing and energy storage and is considered as one of the most promising alternative materials for manufacturing energy storage devices [1–3]. The structural electrolyte, typically modified polymer materials, is a vital component in SPC to provide high ionic conductivity as well as the ability to transfer load [4,5,6]. Apart from the conventional research approaches of polymer matrix, the investigations on the electrochemical performances such as the ionic conductivity and dielectric properties are also utilized for the development of the structural electrolytes [7–10]. Studies have been mainly focused on two major strategies for the applications of the polymer electrolyte systems: One is the development of polyethylene oxide (PEO) based solid polymer electrolytes where the ions move via the local relaxation and segmental motion of the PEO chains, the other is the preparation of epoxy resin blended with gel or liquid electrolyte to achieve the mechanical and electrochemical functions separately in one polymer system. Since Wright ⁎ Corresponding author at: Room D542, New Main Building, No. 37 Xueyuan Road, Haidian District, Beijing, China. E-mail address: [email protected] (Y. Yu).
http://dx.doi.org/10.1016/j.matdes.2016.05.004 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
[11] firstly discovered that the complex of PEO and alkaline salts has the ability of ionic conductivity in 1973, numerous researchers have deeply investigated the ionic conduction mechanisms and multifunctional performance of PEO based electrolytes. Given the potential use of the vinyl ester derivatives of poly(ethylene glycol) (PEG), Snyder and co-workers formulated random copolymers containing ethylene oxide (EO) groups [12–14]. In this way a structural scaffold is generated through crosslinking while the PEG sidechains can enhance the ion transport. Willgert et al. [15] also conducted a systematic study on photoinduced curing of PEO-dimethacrylate/PEO-monomethacrylate lithium salt mixtures to form thermoset electrolytes and addressed the importance of seeking for an acceptable balance between mechanical and conductive properties. However, despite the advantage that the manufacture process for the vinyl ester derivatives of PEG is simple, the trade-off between mechanical and conductive properties makes it very difficult for the PEO based electrolytes to satisfy the inferior limit of the multifunctional performance for the SPC [13–16]. Epoxy resin is one class of the most used structural polymer matrices with high mechanical modulus and strength. Adding the immiscible electrolyte into epoxy/amine system can generate the co-continuous phase structure, with the epoxy networks providing the load-bearing function while the electrolyte phase supplying the ion mobility [17,18]. Recently, poly(ethylene-glycol) (PEG) or propylene carbonate (PC) as the additive was explored to form a bicomponent structural electrolyte [16]. Low multifunctional performance was obtained due to a combination
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of plasticization of the structural network and limited percolation of the liquid network. Besides, several authors chose ionic liquids (ILs) as the ionic conducting agents for the polymer electrolytes and highlighted the influence of the ILs on the dual-phase formation and structuration [19–23]. The addition of ILs led to a good multifunctionality with a room temperature ionic conductivity reaching 0.8 mS/cm and a Young's modulus of 180 MPa [21]. Gienger et al. compared the properties of the in-situ cured electrolyte and the porous epoxy resin backfilled with liquid electrolyte. The latter showed a better multifunctional performance with a room temperature ionic conductivity reaching 1.5 mS/cm and a Young's modulus of 120 MPa [18]. In summary, only when the formation and the control of the interconnected microstructure are well realized, can the multifunctional performance of the epoxy modified electrolyte be improved greatly. Thus the epoxy modified electrolyte has shown great potential to be the component of energy storage devices. It is well acknowledged that the addition of particle fillers into epoxy based hybrids are beneficial to the miscibility, phase separation and morphology with the fracture toughness being improved simultaneously [24–28]. As a result, incorporating nanoparticles into epoxy-based multi-phase blends is considered as the most effective method to phase separation and morphology controlling. Tanaka and the coworkers employed glass spheres into the polymer blends to affect the dynamic coupling between phase separation and wetting, and indicated the possibilities to control the final domain size in phase separation and the final morphology [24]. Zhang et al. devoted their efforts to the modification of the epoxy/PEI resin using nano-scale silica particles. The dispersion of the silica particles led to the occurrence of phase inversion structure and the size change of the secondary epoxy rich phase [25]. In recent years, there has been a growing interest in the utilization of organoclays (organically modified layered silicates, OLS) to control the morphology and phase separation kinetics for its distinctive intercalated and/or exfoliated structures [26–29]. The onset of phase separation and the gelation or vitrification time were greatly brought forward and the periodic distance of phase-separated structure was reduced when OLS was incorporated [28]. The phase diagram of epoxy blend was indicated to shift to a higher temperature with the increase in OLS content due to the easy penetration of the blends into the clay galleries [29]. Hence OLS has a dramatic impact to the phase separation process and the final phase morphology of binary polymer mixtures. However, to the best of our knowledge, these studies are mainly focused on the application of OLS into the thermoplastic/thermosetting mixtures but no literature is available regarding the effects of OLS on the morphology and multifunctional performance of the liquid/polymer co-continuous system. In the present paper, the effects of OLS content on the multifunctional performance of the structural electrolytes were discussed. Epoxy was blended with liquid electrolyte and OLS to form the co-continuous microstructures. Bisphenol A epoxy/polyether amine resin system was
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utilized to form the crosslinked network for load-bearing, and IL dissolved with PC and lithium salt as the liquid phase provided the ionic conductivity. We optimized the phase separation and final microstructure by the addition of different mass contents of nano-scaled OLS, and evaluated the morphology, mechanical and conductive properties of the structural electrolytes. The influence of OLS content on the multifunctional performance was summarized and the optimal formulation of the structural electrolyte based on ionic liquid, epoxy resin and OLS was determined. 2. Experimental 2.1. Materials The epoxy resin system used in this study was composed of liquid diglycidyl ether of bisphenol A (DGEBA) type epoxy resin E51 with an epoxy value of 0.51 from Lanxing Wuxi Resin Co. Ltd. and tetrafunctional epoxy resin AG-80 with an epoxy value of 0.85 from Shanghai Huayi Resin Co. Ltd. The curing agent was JEFFAMINE D400 polyetheramine bought from Huntsman International LLC, with an amine hydrogen equivalent weight of 115. The organoclay (Nanomer® I.·30P from Nanocor Inc.) employed in this work was a montmorillonite with octadecylamine modified as the intercalation reagent. The mixture of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (EMIM-Tf2N, 99%, Shanghai Cheng Jie Chemical Co. Ltd.) and propylene carbonate (PC, 99%, Kuer Chemistry Co. Ltd.) was utilized as the liquid electrolyte with the corresponding lithium salt of lithium bis(trifluoromethanesulphonyl)imide (LiTf2N, 99.9%, Chinese Special Gases Department of Purification Equipment Research Institute). All these materials were kept in a vacuum drier at ambient temperature in order to avoid absorbing moisture. 2.2. Polymer preparation The liquid electrolyte component was prepared with LiTf2N dissolved in the mixture of 99 wt.% EMIM-Tf2N and 1 wt.% PC at the concentration of 2.3 mol/L. E51, AG80 and the liquid electrolyte were roughly blended following the component ratio listed in Table 1 and placed in an oven at 55 °C for 5 min in order to decrease the viscosity. The OLS was dispersed finely in the preheated blends by high-shear mixer at 8000 rpm for 1 h in an external ice bath to keep the system from local overheating before the formulated addition of D400. Finally all the components were stirred using a power mixer until a uniform solution was obtained. The resin plaques with the thickness of 2 mm were manufactured using two paralleled glass plates separated by a silicone spacer. The two glass plates were treated with the mold release agent and clamped by bulldog clips. The cylindrical spacer with a diameter of 2 mm was placed in a U-shape between the glass plates, as shown in Fig. 2. The
Table 1 Compositions and multifunctional performances of the structural electrolytes based on ionic liquid, epoxy resin and OLS. Sample label
S A B C Epoxy-S Epoxy-A Epoxy-B Epoxy-C Ionic liquid Conductive phase
Structural phase
Liquid phase
Multifunctional performance
E51 (g)
AG-80 (g)
D400 (g)
OLS (g)
EMIM-Tf2N (g)
PC (g)
LiTf2N (g)
Tensile modulus (MPa)
Tensile strength (MPa)
Ionic conductivity (mS/cm)
70 70 70 70 70 70 70 70 – –
30 30 30 30 30 30 30 30 – –
73.49 73.49 73.49 73.49 73.49 73.49 73.49 73.49 – –
0 (0 wt.%) 8.99 (2.5 wt.%) 17.99 (5.0 wt.%) 26.98 (7.5 wt.%) 0 (0 wt.%) 8.99 (2.5 wt.%) 17.99 (5.0 wt.%) 26.98 (7.5 wt.%) – –
128.81 128.81 128.81 128.81 – – – – 128.81 128.81
1.30 1.30 1.30 1.30 – – – – – 1.30
56.14 56.14 56.14 56.14 – – – – – 56.14
373.25 ± 36.71 136.89 ± 7.88 211.57 ± 11.10 198.94 ± 9.67 2800.69 ± 24.28 2753.75 ± 52.23 3024.64 ± 57.56 3115.89 ± 57.72 – –
9.27 ± 1.34 9.75 ± 0.80 10.92 ± 0.35 9.07 ± 0.09 52.93 ± 0.43 44.20 ± 0.21 47.32 ± 0.25 44.84 ± 0.33 – –
(5.67 ± 0.42) × 10−2 (4.74 ± 0.30) × 10−2 (8.97 ± 0.54) × 10−2 (8.42 ± 0.41) × 10−2 – – – – 7.01 ± 0.03 1.66 ± 0.02
The concentration of LiTf2N in the liquid phase was kept constant at 30.14 wt.% and weight ratio (E51 + AG-80 + D400): (PC + EMIM-Tf2N) at 100:75.
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Fig. 1. Procedure for the preparation of the specimens.
homogenous solution was poured into the preheated mold and degassed in the vacuum oven at 45 °C for 20 min. The curing cycle was 75 °C for 2 h, 110 °C for 2 h and 150 °C for 2 h. All the heating and cooling ramps were carried out at the same rate of 2 °C/min. Macroscopically separated phases cannot be observed in any of the cured resin plaques as shown in Fig. 1. All the specimens can be considered as isotropic. This is because the rate of phase separation is slower compared with the polymer formation at the curing temperature [20,21]. After demolding, the plaques were cut into three specimen types: discs (20 mm diameter) for electrochemical impedance spectroscopy and morphology observation, bars (45 ∗ 8 ∗ 2 mm) for dynamic mechanical analysis, and dumbbell specimens for tensile testing. All specimens were kept in the vacuum drier at ambient temperature before the characterizations in order to avoid absorbing moisture.
amplitude of 5 μm. The DMA samples were heated from − 50 °C to 100 °C at a heating rate of 3 °C/min. The glass transition temperature (Tg) of each specimen was determined from the tan δ peak.
2.3. Characterization 2.3.1. Scanning electron microscope (SEM) The SEM specimens with different OLS content were submerged in ethanol which was changed every other 12 h to extract the liquid phase. After seven days, the specimens were placed in the oven at 75 °C under vacuum until each sample mass was constant. The morphology was observed using the scanning electron microscope (CamScan JEOL 6010). 2.3.2. Fourier-transform infrared spectroscopy (FT-IR) The Fourier-Transform Infrared Spectroscopy (FT-IR) was conducted on a Nicolet Avatar 360 in order to confirm whether the chemical compositions of the specimens with different formulations were the same. 32 scans at a resolution of ±4 cm−1 were averaged for each specimen. All the IR measurements were performed in reflection mode with a frequency range of 4000–400 cm−1 at ambient temperature. 2.3.3. Dynamical mechanical analysis (DMA) Dynamic mechanical thermal analysis (Mettler Toledo) was performed in a three-point bending mode at a frequency of 1 Hz with an
Fig. 2. FT-IR spectra of structural electrolytes with different OLS contents (S, A, B and C), OLS/epoxy nanocomposite (Epoxy-B) and conductive phase.
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Table 2 Mass and volume contents of different phases in the structural electrolytes. Sample no. Mass fraction
Volume fraction
Structural phase
S A B C
Conductive phase
Structural phase
Conductive phase
Nominal (wt.%)
Practical (wt.%) Nominal (wt.%) Practical (wt.%) Nominal (vol.%) Practical (vol.%) Nominal (vol.%) Practical (vol.%) Difference (vol.%)
48.23 49.48 50.69 51.84
51.77 50.52 49.31 48.16
56.33 56.16 55.43 59.12
43.67 43.84 44.57 40.88
62.32 63.49 64.61 65.65
68.21 68.32 68.01 70.84
37.68 36.51 35.39 34.35
31.79 31.68 31.99 29.16
5.89 4.83 3.40 5.20
Fig. 3. SEM micrographs of structural electrolytes with different OLS contents after extraction of the liquid phase: (a) Sample S; (b) Sample A; (c) Sample B; (d) Sample C.
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2.3.4. Tensile testing Tensile tests were conducted for mechanical evaluation according to ASTM D790. Tensile strength and tensile modulus were measured using an electrical universal testing machine (Instron 5565) at a crosshead speed of 5 mm/min. The tensile force was measured by a load cell with a capacity of 500 N. For each formulation, eight specimens were tested in order to guarantee at least 5 effective data of tensile properties. 2.3.5. Electrochemical impedance spectroscopy (EIS) The EIS was measured using electrochemical workstation (CHI660D, Shanghai Chenhua Device Company, China) in a frequency range from 1 to 106 Hz. All specimens were dried in a vacuum drier at ambient temperature before testing. Each disc was coated with silver paste on both surfaces before placed into a fixer for clamping the discs with two stainless steel electrodes. Five specimens were tested for each formulation. The ionic conductivity of each specimen can be calculated by the following equation: σ¼
d Rs A
ð1Þ
where σ, d, Rs and A represent the ionic conductivity, the distance between the electrodes, the bulk resistance, and the cross-section area of the sample, respectively. 3. Results and discussion 3.1. Effects of OLS content on the composition and morphology The structural electrolytes with different OLS contents, as well as the distinct structural and conductive phases, were successfully prepared
according to the process in Section 2.2. FT-IR was utilized in order to determine the composition of the samples. From the spectrum of conductive phase, the band at 1635 cm−1 could be assigned to the in-plane C\\C and C\\N stretching vibration of the imidazole ring. Besides, the observed bands from 1578 cm−1 to 1057 cm−1 were assigned to the stretching modes of S\\O, S\\N and C\\F. As there was no difference in the chemical compositions between the Epoxy-S, A, B and C, only Epoxy-B was characterized by FT-IR on behalf of all the structural phases. For the spectrum of Epoxy-B, the absence of epoxy band at 915 cm−1 indicated the E51 and AG-80 resin was entirely cured. The bands from 1611 cm−1 to 1457 cm−1 corresponded to the C_C stretching vibration of the benzene ring. The C\\N stretching vibration was assigned to the band at 1351 cm−1. The characteristic bands of C\\C stretching mode, C\\O stretching mode and C\\H bending mode at 1196 cm−1, 1142 cm−1, and 1060 cm−1 respectively were also observed in the spectrum of Epoxy-B. The spectra of A, B, C and S in Fig. 2 show almost the same characteristic peaks, which suggests there is no difference in the chemical composition between the four samples. Furthermore, it can be inferred that the discrepancies in multifunctional properties, which will be discussed in the following context, are resulted from the differences in component microstructure instead of chemical composition. The specific composition of these samples can be analyzed by spectra comparison with pure structural and conductive phase. Like the spectrum of Epoxy-B, the absence of epoxy band at 915 cm−1 indicated the entire cure of epoxy resin. In addition, the S_O band at 1578 cm−1 in the conductive phase was overlapped by the bands corresponding to C_C stretching vibration of the benzene ring in Epoxy-B. The S\\O, S\\N and C\\F bands in the conductive phase were also covered by the C\\C, C\\O and C\\H bands, respectively. However, the band at 1635 cm−1 corresponding to the C\\C and C\\N stretching vibration of the imidazole ring could be observed in the
Fig. 4. Effects of OLS content for four different structural electrolyte samples on (a) DMA results and (b) the glass transfer temperature.
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spectra of all the structural electrolyte samples. This certificated the existence of conductive phase, which means all the specimens were bicomponent systems containing both epoxy resin and liquid electrolyte. In order to evaluate the phase contents, the liquid phase (conductive phase) of all these samples was extracted using ethanol and all the samples were dried in the oven. The drying process were not be halted until the specimen weight reached a constant value. Since the amount of D400 in the liquid phase could not be assessed precisely, the liquid electrolyte was assumed to be ideal without D400. According to the measured mass and the theoretical density of each material, the volume fractions of both structural and liquid phase can be calculated. The nominal and practical results were summarized in Table 2. The difference between nominal and practical volume fractions of liquid phase verified that a small part of liquid electrolyte was locked in the structural phase during the curing process. The practical data of the conductive phase corresponded to the results of interpenetrated liquid phase and the difference between nominal and practical values corresponded to the results of locked liquid phase. With the increase of OLS content, the amount of interpenetrated liquid electrolyte was increased at first and decreased afterwards, while the amount of locked liquid electrolyte showed an opposite tendency. All the samples exhibited a co-continuous microstructure characteristic with a porous continuous epoxy network in Fig. 3. However, the epoxy morphologies were highly dependent on the OLS content. In the cross-section of the structural electrolyte without OLS (Fig. 3(a)), the continuous phase was composed of interconnected epoxy flakes with a dimension of approximately 10 μm and discrete epoxy spheres with a diameter of 1–2 μm. The irregular-shaped holes penetrated with each other had a non-uniform size varying from 2 μm to 4 μm. After the addition of 2.5 wt.% OLS (Fig. 3(b)), the epoxy microstructure was transformed from flakes into ridges. The OLS/epoxy pieces dispersed alongside every epoxy ridge with nearly equal space like two rows of hackles. The distance between the ridges was close to that between the OLS/epoxy pieces, with a dimension of roughly 3–5 μm. The interpenetrated holes with a size of about 5 μm were relatively uniformly distributed in the epoxy matrix. For the sample with an OLS content of 5 wt.% (Fig. 3(c)), the space between ridges decreased to 1–2 μm and the space between hackles was narrowed to 3–5 μm. The holes distributed non-uniformly with the size reduced to less than 3 μm. Besides, many small epoxy spheres with a diameter of 1–2 μm were spread on the ridges and hackles as a new morphology. When the OLS content was increased to 7.5 wt.% (Fig. 3(d)), the OLS had a spatially nonuniform dispersion in the epoxy resin. The morphology of the epoxyrich region circled in red was similar with that in Sample S (Fig. 3(a)). The OLS-rich region circled in black was mainly composed of ridges with a larger dimension. The OLS agglomeration made the epoxy network look more like parallel interconnected bars with a space of about 3 μm. It is interesting to find such linear features with the addition of OLS, which may be caused by the gravity effect of the OLS or the rough surface of the glass plates. Efforts will be devoted to the phase separation process to further analyze the formation of the linear features.
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Fig. 5. Mechanical properties of the structural electrolytes with different OLS contents.
system, the latter played the dominant role. It suggested that the average length of polymer chain segment was increased with an increasing OLS content, which can result in the decrease in structural modulus. However, the storage modulus showed a different tendency. The storage modulus at ambient temperature ranged from 200 MPa to nearly 500 MPa. The addition of OLS resulted in a decrease of storage modulus to 50% value. With the increase of the OLS content, the storage modulus was slightly increased and decreased afterwards. In order to further analyze the relation between the mechanical performance and OLS content, the static tensile test was conducted on all these structural electrolytes (Fig. 5). The results showed that the tensile strength was increased firstly and decreased afterwards with very little difference between these values of each sample. The tendency of the tensile modulus was in accordance with the storage modulus. The addition of OLS decreased the modulus substantially from 373 MPa to 137 MPa, which can be explained by the morphology in Fig. 3. The ridges of Sample A were only connected by the OLS/epoxy pieces, while the epoxy network of the Sample S was composed of epoxy flakes connected with each other in three dimensions, which contributed to a more stable structure and a higher tensile modulus. Besides, more pores in larger size were found in Sample A than Sample S. However, For Sample B with an OLS content of 5 wt.%, the ridge network became more compact, with a lower ridge space and a higher epoxy volume fraction. Consequently, the tensile modulus was increased to 211 MPa. When 7.5 wt.% OLS was mixed with the electrolyte system, OLS agglomeration and the increase of volume fraction of the structural phase
3.2. Effects of OLS content on the properties The effect of the OLS content on the dynamical mechanical properties of the structural electrolytes can be seen in Fig. 4. The Tg linearly decreased with the increase of OLS content. With the OLS content increasing to 7.5 wt.%, Tg dropped about 10 °C. On one hand, the OLS with a large specific surface area had a strong bonding at the OLS/ epoxy interface, which restricted the movement of the epoxy polymer chain segments as well as lowered the free volume. As a result, the thermo-stability was improved and the Tg should be increased. On the other hand, the intercalation-agent, namely octadecylamine in this study, acted as the plasticizer to increase the free volume of the polymer chain segment, which led to the decrease of the Tg. In this co-continuous
Fig. 6. Ionic conductivity of the structural electrolytes with different OLS contents.
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Fig. 7. Multifunctionality of the structural electrolytes with different OLS contents: (a) Ionic conductivity plotted against tensile modulus, and (b) Comparison in multifunctional properties between our experimental results and existing literature data. “♦”—PEO:PEO-b-PE 50:50 [30]; “◊”—PMMA-TEOS:IL 50:50 [31]; “ ”—epoxy-PEO/IL 60:40 [32]; “●”—PEGDGE/IL 60:40 [33]; “▼”—BADGE/TetradX/TEPA:IL 50:50 [17]; “★”—formulations based on VTM266 [21]; “☆”—epoxy porous monolith backfilled with PC [18]; “□”—formulations based on MVR444 [21]; “○”—formulations based on MTM57 [20]; “■”—liquid/epoxy system mixed with different contents of OLS.
interacted with each other, which slightly reduced the tensile modulus to 200 MPa. Moreover, the CV of the tensile mechanical performance decreased with an increasing OLS content, which implied that the addition of OLS can lower the data scatter of mechanical performance. Generally the OLS makes a contribution to the ionic conducting in the co-continuous electrolyte systems. The relation between OLS content and ionic conductivity has the same tendency with the Young's modulus (Fig. 6). The ionic conductivity is greatly influenced by the volume fraction of the liquid phase [3,4,12] and the interpenetration of the microstructure [2,17]. The epoxy network is insulating and the liquid phase plays a vital role in conducting ions. As a result, the increase in the amount of the liquid phase can promote the ionic conductivity [13,14]. Additionally, the structural electrolyte with a percolated phase-separated structure in scale and a very fine epoxy network provides more elaborate passageways for the ion transport [18,19]. Compared with the ridge network in Sample A, the epoxy flake network in Sample S had a better interconnectivity and more interconnected micro-sized holes. As a consequence, Sample A with 2.5 wt.% OLS had a lower ionic conductivity. But the ionic conductivity reached the highest value of 0.09 mS/cm with an OLS content of 5 wt.%, which may attribute to the highest volume fraction of interpenetrated liquid electrolyte phase (Table 2). Besides, the increase of OLS content can lead to a better percolated microstructure consistent with the micrograph in Fig. 3(c). When OLS content increased to 7.5 wt.%, the two factors interacted with each other and resulted in the decline in the ionic conductivity finally. With an excessive amount of OLS, the irregular distribution of the ridges and flakes as well as the agglomeration of OLS caused the distortion of the epoxy network. This morphological change brought about the decreasing percolation on the conductive phase and blocked a part of the channels for ion transport. Moreover, the practical volume fraction of the liquid phase reaches the lowest value at 7.5 wt.% OLS content in Table 2. This parameter represented volume fraction of the interpenetrated liquid phase. With the fewest mobile lithium ions in amount, the ionic conductivity ought to be decreased. 3.3. Multifunctional performance It is well known that the mechanical performance and ionic conductivity has a trade-off relationship. The former needs the stiffness of polymer chain segments and the undamaged microstructure, while the latter needs a soft chain segments and interpenetrated passageways throughout the epoxy [15,19]. Obviously, it is very difficult to improve the mechanical and electrochemical performances simultaneously. The effective way to overcoming such a conflict is controlling of the SPE microstructure at different length-scales [2], but it needs the studies
of different curing chemistries and novel matrix formulations. Nevertheless, a proper addition of OLS can be seen as a simple method for the increase of multifunctionality. Fig. 7(a) illustrated that the introduction of OLS firstly led to the reduction of both the properties, but with the increase of OLS content the structural property was linearly correlated with ionic conductivity. According to the literature, a reasonable goal for a structural electrolyte is to achieve both mechanical stiffness and ion conductivity within one order of magnitude of conventional materials [20,21]. In our study, the structural electrolyte should achieve at least 200 MPa in Young's modulus and 0.1 mS/cm in ionic conductivity. The literature data of structural electrolytes were collected in Fig. 7(b). Compared with PEO based solid polymer electrolytes, the epoxy/liquid systems with co-continuous structure are closer to the multifunctional goal. Furthermore, the structural electrolyte in our study with the primary epoxy/amine system approaches the threshold, showing an exciting combination between the ionic conductivity and Young's modulus. The multifunctional performance may have a further improvement by the modification of the material system, such as increasing the concentrate of lithium salt, and employing another epoxy system with a higher stiffness. In general, the co-continuous structural electrolyte based on ionic liquid, epoxy resin and OLS is a promising candidate for structural power composites and able to reach the goals of load bearing and energy storage simultaneously.
4. Conclusions We established a novel method to optimize the morphology and multifunctional performance of the structural electrolyte with distinct conductive and structural phases by adding different contents of OLS. The introduction of OLS into the epoxy/ionic liquid system changed the continuous phase from the interconnected epoxy flakes with attached micro-spheres to the parallel epoxy ridges connected by OLS/ epoxy hackles, which resulted in the decrease of mechanical and ionic conductive properties. However, with the increase of OLS content, a more compact epoxy network was developed with a more cramped ridge space and a more interpenetrated conductive phase, which increased the mechanical properties and ionic conductivity simultaneously. When OLS content reached 7.5%, OLS agglomeration and the decreasing of the conductive phase content led to the reduction of multifunctional performance. The highest multifunctional properties were obtained at 5% OLS content, with 211 MPa in tensile modulus and 0.09 mS/cm in ionic conductivity. This reaches the performance inferior limit and shows great potential for the application in the energy storage devices.
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