Ionic Liquid-Based Polymer Electrolytes via Surfactant-Assisted

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Ionic Liquid-based Polymer Electrolytes via Surfactantassisted Polymerization at the Plasma-Liquid Interface Quoc Chinh Tran, Van-Tien Bui, Van-Duong Dao, Joong Kee Lee, and Ho-Suk Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04947 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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ACS Applied Materials & Interfaces

Ionic Liquid-based Polymer Electrolytes via Surfactant-assisted Polymerization at the Plasma-Liquid Interface Quoc Chinh Trana†, Van-Tien Buia,b†, Van-Duong Daoa, Joong-Kee Lee c, Ho-Suk Choia* a

Department of Chemical Engineering and Applied Chemistry, Chungnam National University,

220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, Republic of Korea b c

Institute of Research and Development, Duy Tan University, Da Nang, Viet Nam

Energy Storage Research Center, Korea Institute of Science and Technology, P.O. Box 131,

Cheongryang, Seoul 130-650, Republic of Korea *

E-mail: [email protected]

ABSTRACT We first report an innovative method, which we refer to as interfacial liquid plasma polymerization, to chemically cross-link ionic liquids (ILs). By this method, a series of all-solid state, free-standing polymer electrolytes is successfully fabricated where ILs are used as building blocks and ethylene oxide-based surfactants are employed as an assisted-crosslinking agent. The thickness of the films is controlled by the plasma exposure time or the ratio of surfactant to ILs. The chemical structure and properties of the polymer electrolyte are characterized by scanning electron microscopy (SEM), Fourier transformation infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and electrochemical impedance spectroscopy (EIS). Importantly, the underlying polymerization mechanism of the crosslinked IL-based polymer electrolyte is studied to show that fluoroborate or halide anions of ILs together with the aid of a small amount of surfactants having ethylene oxide groups is necessary to form crosslinked network structures of the polymer electrolyte. The ionic conductivity of the obtained polymer electrolyte is 2.28×103

S.cm-1, which is a relatively high value for solid polymer electrolytes synthesized at room

temperature. This study can serve as a cornerstone for developing all-solid state polymer electrolytes with promising properties for next-generation electrochemical devices.

Keyword: liquid plasma, polymerization, ionic liquid, ion conductivity, surfactant, triton X100

ACS Paragon Plus Environment


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1. INTRODUCTION Electrochemical devices such as lithium ion batteries, supercapacitors, fuel cells, and solar cells have attracted enormous attention from the scientific community due to their wide range of 1-3

applications in the industries of transportation, portable electronics, and aerospace.

Among the

many parts of electronic devices, the electrolyte is one of the most important components, 4

playing a key role in determining device performance, durability, and safety. Unfortunately, most commercially available electrochemical devices using liquid electrolytes accompany potential safety problems and other obstacles including short-term stability due to leakage, 5

thermal decomposition, electrode corrosion, and narrow electrochemical windows. To address these issues, solid state electrolytes that can feasibly overcome the current problems of liquid electrolytes have been developed. These solid electrolytes improved the safety, durability, and ease of processing while allowing flexible device design owing to their good mechanical strength, high thermal stability, and large electrochemical window. Normally, however, they possess low ion conductivity when compared to non-solid electrolytes.

6-8

For the development of new

generation solid electrolytes, ionic liquids (ILs), which offer efficient ion conduction, negligible vapour pressure, and a large thermal and electrochemical stability window, have been incorporated into a mechanical polymer matrix to form solid-state IL-incorporated gel polymer 4,9-12

electrolytes (ILGPEs).

Depending on the method of preparation, IL-based polymer

electrolytes can be classified into three categories including doping of polymer with ILs, in situ polymerization or crosslinking of the monomer in ILs, and synthesis of polymeric ionic liquids (PILs).

13,14

In the two former approaches, ILs are incorporated into a conventional polymer

matrix to form an IL-based polymer gel electrolyte (ILPGE). Although ILPGEs offer a simple way for preparing polymer electrolytes with ease of processing and relatively high ion conductivity, they still are impeded by the limit of IL content since ILs are employed as plasticizers. The ion conductivity of ILPGEs can be enhanced by introducing a greater quantity of ILs to the polymer matrix. However, this can cause some serious problems such as phase separation, leakage of ILs, reduced mechanical strength, and lower dimensional stability during 15

its applications.

One alternative way to achieve high IL content is the synthesis of polymer

ionic liquids (PILs) in which IL based-polymerizable monomers (oligomers) are polymerized to form PILs. It should be noted that this strategy includes a number of steps involving organic


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synthesis or modification of macromolecules and complex purification, and also requires control of the polymerization conditions.16,17 Thus, the development of a simple, economical, and environmentally friendly method for crosslinking diversely commercialized ILs is highly desirable. Recently, plasma polymerization, including plasma-state polymerization and plasma-induced polymerization, has been used as an alternative method to the conventional polymerization 18

techniques.

Plasma-state polymerization is a plasma-assisted chemical vapour deposition

process used to fabricate an ultra-thin film on a surface that is placed in a plasma flame. In plasma-induced polymerization, the energy of plasma is transferred from excited species to the organic monomers in a liquid or solid phase. This induces the formation of radicals or ions, causing polymerization. As a result, the conventional plasma polymerization methods require low operating pressure or the use of polymerizable monomers. Thus, they are not applicable to polymerize non-volatile ILs that do not have any functionally reactive groups. In order to overcome the limitations of the above mentioned approaches and meet the requirements of commercialization, a novel and single-step strategy for the direct polymerization of diversely commercialized ILs to prepare polymer electrolytes was proposed. The developed strategy is based on plasma interfacial polymerization with the aid of ethylene oxide-based surfactants. First, we show the successful synthesis of PILs using radio frequency plasma under atmospheric pressure with the assistance of surfactants. A series of polymer electrolytes were prepared with different ILs and surfactants. Second, we suggest the formation mechanism of the cross-linked polymer electrolyte. Finally, we investigate the effect of the initial monomer ratios on the thickness and ion conductivity of the obtained polymer films. 2. EXPERIMENTAL SECTION 2.1. Materials Triton-X100, terpineol (95%), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), 1butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM] Tf2N), acetone, and ethanol were purchased from Sigma-Aldrich, USA. Argon gas was obtained from Yonhap L.P.G, Korea. Aluminum-laminate was obtained from MTI, Korea. A solution of 1M lithium



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hexafluorophosphate (LiPF6, 99.99%, Sigma-Aldrich) in dimethyl carbonate (DMC, 99%, Sigma-Aldrich, USA) was used as an electrolyte. 2.2. Synthesis of Polymer A homogenous liquid mixture containing 1.5% molar percent of Triton X-100 in [BMIM]BF4 was initially prepared by using a Vortex Mixer-KMC-1300V for 15 min. The mixture was then coated on a 20x20 mm glass plate by dropping of 0.5ml of the solution. The polymerization of a thin liquid film on the glass plate was carried out in an atmospheric pressure plasma system with power of 150 W, an Ar gas flow rate of 5 lpm, and plasma treatment time of 10 min. Noted that, the atmospheric pressure plasma system was described in our previous reports,

19-21

and the gap

between the power electrode and the liquid film was 2 mm. After plasma polymerization, the synthesized polymer film was immersed into ethanol to separate the film from the glass plate. Next, the polymer film was washed with acetone followed by distilled water several times and then dried in oven at 60°C for 1 h. The film thickness was controlled by both plasma treatment time and mixture composition. 2.3. Characterization of Polymer The thickness and the surface structure of the synthesized polymer film were measured using a scanning electron microscope (SEM) (JSM-7000F, JEOL, Japan). The molecular chemical features were analyzed by Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) (Thermo Scientific, USA). The elemental and chemical compositions of the polymer were characterized by X-ray photoelectron spectroscopy (XPS) using a Sigma Probe Thermo Fisher VG Scientific spectrometer (MULTILAB 2000, Thermo Scientific, USA) equipped with a monochromatic Al Kα X-ray source. The structure and the cross-links of the polymer were investigated by

13

C MAS NMR and 1H-NMR using a Solid 400MHz NB NMR

and a HFXY 1.6mm probe spectrometer (Agilent Technologies, USA) at 15 kHz. The thermal stability of the polymer was determined using a thermal gravimetric analysis (TGA) (MettlerToledo Inc., USA). 2.4. Test Cell Assembly Conductivity measurements were performed by sandwiching the polymer matrix between two stainless steel plates (SS) (MTI, KOREA). These plates (15.6 mm of diameter x 1 mm of


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thickness) were first carefully washed several times with acetone in an ultrasonicator for 10 min and then dried at 50 °C for 2 h in a vacuum oven. Next, the synthesized polymer film was transferred onto one SS plate. After that, 0.5 ml of 1M LiPF6/DMC solution was dropped on the prepared electrode. Finally, the samples were sandwiched between two SS electrodes in an aluminum-laminated case. An aluminum-laminated wire had previously been fixed onto the SS electrode. The cells were then transferred into a vacuum sealer to seal the aluminum-laminated case. 2.5. Measurement of Ion Conductivity The ionic conductivity of the PILs was measured by electrochemical impedance spectroscopy (EIS) using an IviumStat device. The EIS data were collected with a frequency range of 100 mHz to 100 kHz and a perturbation amplitude of 10 mV for an open-circuit condition. A 10 mV AC amplitude was applied with a frequency sweep from 1 Hz to 1 MHz. All measurements were carried out at room temperature. The obtained spectra were fitted using the Z-View software (v2.8d, Scribner Associates, Inc.) with reference to the proposed equivalent circuit.



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3. RESULTS AND DISCUSSION 3.1 Synthesis and Characterization of Polymer Electrolyte

Figure 1. (a) Scheme illustrating the synthesis route of the polymer electrolyte from IL and surfactant via interfacial liquid plasma polymerization. (b) Photographs of polymer electrolyte films on glass substrates with different plasma exposure times. (c) Surface and cross-sectional morphology of polymer electrolyte film. (d) Thermal properties of polymer electrolyte. The polymer electrolytes used in (c, d) were prepared with Triton X100 molar content of 1.5% and plasma exposure time of 600s. The whole preparation process for the production of a free-standing IL based-polymer electrolyte film by using interfacial liquid plasma polymerization is illustrated in Figure 1(a). As shown in Figure 1(a), the polymer electrolyte was simply synthesized by placing a mixture of ILs and surfactants in the plasma region. Notably, neither ILs nor surfactants, when solely used, are able to generate a polymer film via interfacial liquid plasma polymerization. However, a polymer film


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was formed when a mixture of ILs and surfactants was exposed to plasma for a certain time, as shown in Figure 1(b). Note that the ratio of surfactants to ILs is small (normally around 1.5 mol. %) and thus the surfactant could be denoted as an additive for the polymerization of IL. As seen in Figure 1(b), a thin polymer film was observed on the liquid surface at only 30s exposure. In addition, the color of the obtained polymer films also changes with respect to the exposure time. With exposure time smaller than 120s, the obtained film was quite transparent. However, the film color turned opaque white yellow with a further increase of exposure time. This is reasonable because the thickness of the polymer film became thicker with increasing plasma exposure time. Additionally, the influence of the plasma exposure time on the film thickness was comprehensively investigated (SI, Figure S1). As shown in Figure S1, the thickness of the polymer electrolyte film progressively increases with increasing exposure time and reaches a saturated value at about 600s. As a result, plasma exposure time of 600s is appropriate for synthesis of the polymer electrolyte film, and thus it was chosen for all later experiments to investigate the influence of other conditions on polymer electrolyte properties. The surface and the cross-sectional morphology of the polymer electrolyte film prepared with plasma exposure time of 600s are presented in Figure 1(c). The figure shows that a dense continuous polymer electrolyte film with uniform thickness of around 3 µm was formed. The thermal properties of the obtained polymer film were characterized and are shown in Figure 1(d). The DSC curve exhibits a low transition temperature (Tg around 2.87 °C) of the polymer electrolyte. The melting temperature of IL-based polymer electrolyte is assigned to strong and wide peak at 79.8 °C. The wide range of melting temperature suggests a complexity of polymer structure. The coexistence of the low Tg and Tm is because IL incorporated in the polymer network may reduce the crystalline portion of the polymer, resulting in a low transition temperature. The two successive endothermic peaks at about 236 °C and 306 °C may be caused by rearrangement (or 22-25

deconstruction) and decomposition of polymer network structure, respectively.

The results

suggest that the IL-based polymer electrolyte has high segment mobility, which is favorable for high ionic conductivity. While having a low transition temperature, however, the polymer electrolyte still possesses relatively good thermal stability, as can be seen in the TGA thermal diagram. The polymer starts to decompose at around 145 °C. There was about 12% of residues after thermal gravimetric analysis because of the existence of fluorine-based component in crosslinked polymer chain which can cause ash after decomposing cross-linked network structure of



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26,27

polymer electrolyte.

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We suggest that the thermal stability of the polymer electrolyte can be

improved by using high molecular weight materials such as polyethylene glycol instead of the 28,29

surfactant.

Table 1. Polymerization capability of several ILs and surfactants using plasma Initial

Triton X100

Terpineol

[BMIM]BF4

polymerized

Not

[BMIM]Cl

polymerized

Not

[BMIM]Br

polymerized

Not

[BMIM]TFSI

Not

Not

[BMIM]I

polymerized

Not

To verify the polymerization capability, a number of ILs and surfactants were employed and the results are listed in Table 1. From Table 1, it can be concluded that ILs having fluoroborate or halide anions together with surfactants containing an ethylene oxide group are necessary for the formation of a polymer electrolyte. In the absence of one of the aforementioned components it is not feasible to generate the polymer electrolyte film. Overall, the strategy provides a unique way to produce an IL-based polymer electrolyte with high IL content, high dimensional stability, and especially high ionic conductivity. Moreover, the strategy offers the following advantages: i)

The preparation procedure is extremely simple and cost-effective with an open air process. Moreover, the strategy may be applicable for continuous synthesis systems.

ii)

The strategy can employ commercially available ILs, which do not have any functional reactive groups and thus cannot be polymerized by conventional polymerization methods, as starting materials for building the polymer network. As a result, the strategy avoids the initial steps of designing and synthesizing special monomers and complicated purification processes, and it also does not require control of polymerization conditions. Additionally, it should be noted that residual initiators may adversely affect polymer properties during usage. These residual initiators normally remain in as-synthesized polymers. In the developed strategy, however, they are completely eliminated.


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iii)

The strategy is feasible to prepare IL-based polymer electrolytes with unique network architectures and compositions that are inaccessible by other methods. Thus, a variety of polymer electrolytes with diverse and promising properties can be created.

Figure 2. (a) FTIR spectra of [BMIM]BF4, Triton X100, and polymer electrolyte. (b) Deconvoluted peak of polymer electrolyte in a range of 900 to 1200 cm-1. (c, d) 1H and 13C solid state MAS NMR spectra of polymer electrolyte, respectively. (e, f) Survey and deconvolved high-resolution C1s XPS spectra of polymer electrolyte. All polymer films were prepared at Triton X100 molar content of 1.5% and plasma exposure time of 600s.



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In order to investigate the chemical features of the synthesized polymer, an ATR-FTIR analysis was first considered. For reference, IR spectra of pure IL and Triton X100 were also characterized. The results are presented in Figure 2(a). It is seen that the IR spectrum of [BMIM]BF4 is characterized by three group peaks. The group peaks in the range from 2800 to 3000 cm-1 are ascribed to the stretch mode of butyl constituent (C(5-8)H) attached to the imidazolium ring, the peaks at 3162 and 3122 cm-1 are attributed to the C−H vibrational mode of the imidazolium ring (C(2-4)H), and the peak at 1053 cm-1 is assigned to B−F.

30,31

On the other

hand, the IR spectrum of Triton X100 showed three distinct absorption peaks. The peaks around 3000 cm-1 are assigned to C−H alkyl groups. The strong peak at 1109 cm-1 is assigned to ether groups. A broad peak at 3300-3600 cm-1 is attributed to hydroxyl groups. However, in the IR spectrum of the polymer synthesized from IL and Trition X100, some characteristic peaks of individual components were changed. In detail, the peaks at 3122-3162 cm-1 and 3300-3600 cm-1 disappeared. The results indicate that the imidazolium ring conjugated system and hydrogen groups decomposed through the polymerization reaction. Additionally, they also imply complete washing of residual non-reacted IL and Triton X100 from the as-synthesized polymer film. Two new bands at 1725 and 1660 cm-1 in the IR spectrum of the synthesized polymer correspond to C=O and C=C stretching, respectively. In the fingerprint region of 1540-1204 cm-1of polymer, the small peaks, even intense band at 1511 cm-1, can be characterized as the stretching vibration of the benzenoid group in Triton X100, as well as CH3, CH2 asymmetric bending and CH3 30,32

deformation of alkyl substituents in ILs.

The major peak of the polymer film was observed

in a range of 900 to 1200 cm-1 (Figure 2b). After deconvolution, these two peaks at 1109 cm-1 for C−O−C and 1053 cm-1 for B−F are clearly visible. The C−O−C bonding is associated with the presence of the ethylene oxide units in the Triton X100 structure,

33

while the B−F stretch is

ascribed to the BFସି anions in ILs, as discussed above. The results confirm the contribution of both IL and Triton X100 to the polymer network structure, and thus the characterized peaks of ILs and Triton X100 can be partially overlapped. For more insight into the structure, solid state 1H and

13

C MAS NMR study of the polymer

electrolyte was conducted. The results are depicted in Figures 2(c,d). In general solid state NMR spectra, the characteristic peaks are usually wide and overlapped with each other. Thus, deconvolution of the original spectra is necessary for the analysis of solid state NMR. Figure 2(c)


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presents the deconvoluted 1H MAS NMR. As seen in Figure 2(c), the 1H NMR spectra showed lower shifted peaks at 1.22, 3.31, and 3.87 ppm, which are attributed to the alkyl chain in the IL and the hydrophobic tail of Triton X100 with a downshift in relation to the pure IL and Triton X100.

34

These results indicate that ethylene oxide radical units of Triton X100 bind with IL; this 35

leads to a decrease in the electron cloud density on oxygen atoms of the ethylene oxide units.

Thus, a lower chemical shift was observed. In addition, higher chemical shift peaks at 7.10 and 8.31 ppm contribute to the imadazolium ring of the IL with a downshift in relation to the pure IL. It is concluded that the IL is combined with Triton X100 through the imidazolium radical of the 36,37

IL and Triton X100 radicals.

In the meantime, the

13

C MAS NMR results of the polymer

showed a combination of the corresponding signals of the IL and Triton X-100, as shown in Figure 2(d).

36,37

This can be attributed to a binding of the IL and Triton X100 for the formation

of the polymer chain. In order to investigate the chemical composition of the synthesized polymer, XPS characterization of the polymer film synthesized with Triton X100 molar content of 1.5% at plasma exposure time of 600s was carried out. The results are shown in Figures 2(e, f). As can be seen in Figure 2 (e), the wide-scan survey spectrum of the polymer mainly contains five peaks, for C1s, O1s, B1s, F1s, and N1s, where the nitrogen, boron, and fluorine elements come from the ILs. The results again confirm the contribution of the IL to the polymer structure. Furthermore, in order to visualize the polymer network structure, the contribution of the carbon-containing functional groups to the formation of the polymer was clarified through deconvolution of the XPS C1s spectra. The fitting was done after Shirley+Liner background subtraction on the fitting interval. The results are shown in Figure 2(f). The C1s spectra consist of five peaks at 283.8, 284.4, 285.8, 286.1, and 287.7 eV, corresponding to the carbon atoms of C=C, C−C, C−O−C, 38-40

C=N, and C=O groups, respectively.

It has been reported that the formation of C=C and

C=O bonds is a result of either disproportionation of the ethylene oxide unit macro-radicals in 41

the region with a lower concentration of O2 or decomposition of these radicals (SI, Figure S2).

The presence of functional group peaks after the deconvolution is consistent with the FTIR results.



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3.2 Polymerization Mechanism of Cross-Linked Polymer Electrolyte

Figure 3. Scheme representing a possible polymerization mechanism of the cross-linked polymer network induced by interfacial liquid plasma polymerization. For interfacial liquid plasma polymerization, high energy species in plasma can transfer their energy to molecules in a liquid when they contact the liquid surface. Thus, plasma plays an important role as an effective way to cause the generation of radicals, and consequently results in polymerization. A polymerization mechanism initiated by interfacial liquid plasma is thus suggested based on three stages: initiation, propagation, and termination. Based on the analysis of the structure and chemical composition as discussed above, a possible underlying mechanism for forming the cross-linked polymer from [BMIM]BF4 and Triton X100 was suggested, as illustrated in Figure 3.


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In the initiation stage, the radical generation is mainly attributed to hydrogen abstraction, chain scission of ethylene oxide groups, and fluorine dissociation of fluoroborate groups under a high energy dose of plasma irradiation. It has been reported that oxygen-containing functionalities such as ethers, carboxylic acid, esters, and hydroxyl groups show high plasma susceptibility and 42-44

the radicals formed are determined by the resonance stability of the radicals.

Based on this,

hydrogen radicals would be abstracted from the oligomer chain at α-C of ether or hydroxyl end groups induced by inelastic collisions of plasma species and the oligomer chain, as shown in Eq. 1. Indeed, the decomposition of the hydroxyl groups was confirmed by the disappearance of their characteristic peaks in the IR spectrum of the polymer film. At the same time, the surfactant could be degraded by chain scission at the ether bond near the aromatic group to form two macro-radicals, as shown in Eq.2. It is also reported that the bond between fluorine and boron is quite weak and it can be dissociated under irradiation.

45,46

The dissociation of fluoroborate

anions generates fluorine free radicals and fluoroborate radical anions, as presented in Eq.3. It should be noted that fluorine and hydrogen free radicals are the most reactive species. They can attack otherwise inert materials to induce secondary radicals. In the propagation stage, hydrogen and fluorine radicals produced by hydrogen abstraction and fluorine dissociation will react with the surfactant and IL to generate the macro-radicals, as presented in Eqs.4,5,6. As shown in Eq.4, hydrogen radicals should attack positively charged nitrogen due to their high electronegativity to form intermediate radical cations. It should be noted that cationic radical species are much less stable, they should be transformed into more stable radicals through transposition. In this case, the imidazolium radical cation will withdraw one electron from hydrogen substituent to form an imidazolium radical together with fluoroborate acid. The decomposition of the imidazolium conjugated system was also confirmed in the IR spectra of the polymer. Moreover, hydrogen radicals may cause the generation of additional fluorine radicals (Eq.5). Then fluorine radicals will probably react with the surfactant to induce more macro-radicals (Eq.6). Although macroradicals should have higher stability and lower reactivity than fluorine and hydrogen radicals, they are still able to react with each other or even neutral molecules, including ILs. Note that the nitrogen atoms in ILs are screened by the ring and methyl group. Therefore, macro-radicals may have difficulty approaching a nitrogen positive charge because of steric hindrance. Consequently, the macro-radicals would react with ILs at the C=C double bond of the imidazolium ring, as presented in Eq.7. As a conclusion, hydrogen and fluorine radicals generated by plasma will



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cause the formation of many different kinds of macro-radicals. The molecular weight of the polymer progressively increases by recombining radicals in a chain reaction. Finally, in the terminal stage, the cross-linked network structure of the polymer will be completely formed when all radicals are consumed or the plasma is switched off. 3.3 Effect of Initial Mixture Composition on the Thickness and Ion Conductivity of Polymer Films

Figure 4. Cross-sectional FESEM images of polymer electrolyte films prepared with different Triton X100 molar content: (a) 1.5, (b) 3, (c) 6, (d) 12, and (e) 24%. All polymer films were prepared at plasma exposure time of 600s. Figure 4 shows cross-sectional FESEM images of polymer electrolyte films prepared with different Triton X100 molar content. As can be seen in Figure 4, uniform polymer electrolyte films were successfully synthesized on glass substrates through interfacial liquid plasma polymerization under atmospheric pressure. The difference in the thickness of the polymer film was obtained with different ratios of Triton X100 molar content. Namely, the thickness is around 3.04 µm for polymer film prepared with Triton X100 molar content of 1.5% (Figure 4a). It became thinner with increasing Triton X100 molar content in the liquid mixture, as shown in Figures 4(a-e). The thickness was found to be 0.94 µm at Triton X100 molar content of 24% in


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the liquid mixture (Figure 4e). It should be noted that the polymer film was not formed at Triton X100 molar content significantly higher than 24%. The changes in the polymer film thickness could be explained by the change in the degree of crosslinking together with the reaction rate between radicals extracted from Triton X100 and IL building materials.

Figure 5. (a) C1s XPS survey spectrum of polymers with different Triton X100 molar content and (b-f) the deconvoluted C1s core level for polymers with different Triton X100 molar content. All polymer films were prepared at plasma exposure time of 600s. The fitting was done after Shirley + Liner background subtraction on the fitting interval. The spectra are fit by LorentianGaussian functions.



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In order to understand the effect of cross-linking on the thickness of the polymer films, we conducted XPS measurements. The results are given in Figure 5. Figure 5(a) shows C1s core level XPS spectra of polymer films synthesized with different Triton X100 molar content. As can be seen, the C peak was a single peak at Triton X100 molar content of 1.5%, but it gradually separated into two regions with increasing Triton X100 content from 1.5% to 24%. This result is related with the change in composition of the polymer with different Triton X100 content. To investigate the change in composition of the polymer films, we deconvoluted the C1s into five different functional groups, as described in Figure 2(f). The results are presented in Figures 5(bf). As observed, C=N bonding, which carbon and nitrogen atoms in the imidazolium ring in the IL contribute to, decreases with an increase in the Triton X100 molar content, suggesting that the concentration of imidazolium ring units in the polymer branch network is decreased. Furthermore, the ionic liquid radicals such as F* and [BMIM]* decrease with increasing Triton X100 molar content. It has been reported that the ionic liquid radicals play an important function 47

in controlling the ion conductivity of the polymer.

Thus, we could project that the ionic

conductivity would be decreased with increasing Triton X100 molar content. Furthermore, C-C bond corresponds to carbon atoms in the alkyl chains of ILs and the hydrophobic tail of Triton X100, while C−O−C bond is assigned to carbon atoms in the ethylene oxide units of Triton X100.48-50 As shown in Figure 5, the C−O−C bonding percentage was higher than that of C−C bonding. The results indicate that the ethylene oxide units of Triton X100 are the main and most important units in the polymer branch networks. It should be emphasized that a polymer film was not formed with the pure liquid mixture containing only IL or pure Triton X100 under the same plasma conditions. We expected that a change of the C−O−C bonding percentage related to the change in the polymer film thickness. As analyzed by the FTIR spectrum, C=C and C=O bonds were formed after cross-linking of the IL and the Triton X100 mixture. As discussed in in previous section, the formation of the C=C group is a result of degradation and disproportionation of −CH2−CH2−O−CH2−CH•−O− to form −CH2−CH2−O−CH=CH−O−; meanwhile C=O group is formed as a result of chain scission from −CH2−CH2−O−CH2−CH•−O− to −CH2−CH2−O−C•H2 and −O−CH=O.

41

This implies that the

formation of C=C bonds causes a reduction of the concentration of active radicals that bind to create crosslinking. As a consequence, the crosslinking density is somewhat lower with respect


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to an increase in intensity of C=C bonds. Indeed, the ratio of the intensity peaks of C=C stretch (1660 cm-1) and C=O stretch (1725 cm-1) extracted from the FTIR spectrum is increased linearly with increasing Triton X100 content, as shown in Table S1. Thus, the changes of the C=C and C=O bonding percentage and the ratio of C=C/C=O could be exploited as a tool to measure the 51

degree of cross-linking, which revealed the thickness of the synthesized polymer films.

We next considered the relationship between the ratio of C−O−C/C−C bonding and C=C/C=O bonding obtained from the C1s spectrum and the thickness of polymer films. The results are illustrated in Figure S3 (Supporting Information). As can be seen in Figure S3, both ratios of C−O−C/C−C and C=C/C=O increase with decreasing polymer film thickness. This is consistent with the FTIR results, as shown in Table S1. This once again revealed that the relative C=C and C=O bonds correspond to the degree of crosslinking, which affects the thickness of the polymer 51

film.

As a result, we can conclude that at a small amount of Triton X100, the number of

radicals induced by plasma is directly proportional to the Triton X100 content. Large linkages are consequently formed, resulting in the formation of a thicker polymer film. However, with excess Triton X100, a large amount of radical formation with a high reaction rate may cause a reaction between the radicals to form inactive species (−CH2−CH2−O−CH=CH−O−), resulting in decreased polymer film thickness.



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Figure 6. (a) Impedance spectra for polymer electrolyte with cell structures: SS/polymer/SS at condition: AC=10 mV, T= 25oC. The inset Figure 6(a) shows the equivalent circuit diagram used to fit the observed impedance spectra and Nyquist plot of cells equipped with different polymer film thickness. Rh: ohmic serial resistance (Ω); Rb: the bulk resistance (Ω); CPE: constant phase element; Cdl: double layer capacitor of polymer/blocking electrode interface (F); Rdl: charge transfer resistance at polymer/blocking electrode interface (Ω). (b) Changes of atomic percent, ion conductivity, and film thickness corresponding to Triton X100 molar content. All polymer films were prepared at plasma exposure time of 600s.


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We investigated the effect of polymer thickness on ionic conductivity. The structure of the fabricated SS/polymer film/SS cell, as described in Figure S4 (see supporting information). The impedance spectrum for the SS/polymer/SS system was obtained and presented in Figure 6 (a). The fitting parameters are summarized in Table 2. We found that there was only one semicircle in the Nyquist plots. The results indicate that there was no electrochemical reaction at the electrode/electrolyte interfaces. Furthermore, the linear diffusion process of oxidized/reduced species, and the migration of ions did not occur, as indicated by the absence of a straight line. 52-56

We found that the Rb became smaller with decreasing polymer film thickness, as presented

in Figure 6(b). The smallest value of Rb was 0.07 (Ω) for the cell fabricated with the polymer synthesized with Triton X100 molar content of 1.5% in the liquid mixture. The ionic conductivity (σ) of PILs can be calculated through the expression σ = d/RbA, where Rb, A, and d 57

are the bulk electrolyte ohmic resistance, the area, and the thickness, respectively.

The

calculated results of ionic conductivity are given in Table 2. It is seen that a decrease in the Rb value denotes an increase in ionic conductivity. Indeed, as seen in Table 2, the ion conductivity decreases with decreasing polymer film thickness. The highest ionic conductivity was 2.28 x 10-3 S.cm-1 at 25oC for the cell fabricated with the polymer synthesized with Triton X100 molar content of 1.5 % in the liquid mixture. This value is comparable to that of a commercial polymer electrolyte at ambient temperature. The trend of the change in ionic conductivity with respect to Triton X100 content is similar to the trend of change in the polymer film thickness. The results are clearly depicted in Figure 6 (b). Table 2. The extracted values of the equivalent circuit PIL 1.5% 3% 6% 12% 24%

Rb (Ω) 0.07 0.42 0.55 4.92 4.58

d (x10-4) (cm) 3.04 2.75 2.37 1.68 0.94

A (cm2) 1.91 1.91 1.91 1.91 1.91

σ (x103) (S.cm-1) 2.28 0.34 0.23 1.79x10-2 1.07x10-2

In order to further understand the effect of polymer film thickness or Triton X100 content on the ionic conductivity, we determined the relationship between the polymer film thickness, ionic conductivity and N, F atomic percentages in polymer films with respect to the Trion X100 content. The results are shown in Figure 6(b). Note that the N, F atomic percentages in the



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polymer films were obtained from the XPS results in Figure 5. Obviously, a high atomic content of N, F in the polymer network reflects a high ionic liquid content in the polymer network. As can be seen in Figure 6(b), the changes in atomic percentages of N and F are in good agreement with the changes in the polymer film thickness and ionic conductivity. Indeed, the thickness of the polymer electrolyte film decreases with an increase in the Triton X100 molar content in the prepared mixture. The decrease of ion conductivity could be explained by the decrease of ionic liquid molar content, resulting in a decrease of the ion transport in the polymer network. The decrease in the ionic liquid content linked to the polymer is also demonstrated by the decrease of concentration of N and F atoms in the polymer. This results in a lack of carrier ions in the polymer electrolyte. Therefore, the interaction between the ionic liquid and Triton X-100 plays an important function to form a cross-linking polymer through plasma induced polymerization. Furthermore, it should be noted that the permeability of the plasma polymer also greatly differs 58

from that of the conventional polymer.

It can promote ion diffusion, which enhances the ion

conductivity of the plasma polymer. Increasing the Triton X100 content in the liquid mixture results in a decrease of the crosslinking density and thereby reduces the stability of the branched network, leading to a collapse of many voids which occurred as a direct consequence of the 59

hyper-cross linking to form the branch network structure of the as-synthesized polymer.

This

results in the formation of a smaller number of voids through which ions feasibly can permeate through the polymer film, leading to a decrease of ion conductivity. Finally, the ionic liquid content suggests a significant contribution of the polymer matrix toward ion permeability of the polymer, and promotes the formation of an ionic double layer at the electrode, which is related to 60

ion mobility.

CONCLUSION We reported a simple and innovative method to produce a crosslinked polymer electrolyte with high ion conductivity and chemical stability. This is the first time that ILs have been directly polymerized by atmospheric plasma with the assistance of surfactants. The thickness of the polymer electrolyte film can be controlled by varying the ratio of ILs to surfactant or plasma exposure time. Importantly, we suggested a possible polymerization mechanism of the crosslinked polymer electrolyte where plasma discharge is an efficient way to generate free radicals and the reaction of the radicals results in the formation of a crosslinked network


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structure of the polymer electrolyte. The suggested mechanism was also proved and validated by characterization using FTIR, solid state NMR, and XPS. The obtained PILs are promising because they demonstrate a high ion conductivity of 2.28×10-3 S.cm-1 at room temperature. This study provides groundwork for the development of all-solid state polymer electrolytes with promising properties for next-generation of electrochemical devices. ASSOCIATED CONTENT Supporting Information Change of film thickness of polymer electrolyte corresponding to plasma exposure time (crosssectional SEM images and plot of film thickness as a function of exposure time). FTIR spectra of polymer electrolytes. Table of IC=C/IC=O ratios. Plot of correlation between C−O−C/C−C bonding and C=C/C=O bonding percentage ratio and the film thickness. The polymer film/SS blocking electrode system with the electrical double layer at the blocking electrode. This material is available free of charge online at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

(H.S.C) E-mail: [email protected]

Author Contributions † These authors contributed equally to the work.

ACKNOWLEDGEMENTS This research was supported through a National Research Foundation (NRF) grant (2014R1A2A2A01006994) and the Korea Research Fellowship Program (2015H1D3A1061830) funded by the Ministry of Science, ICT and Future Planning through the National Research Foundation of Korea, and also through the Technology Innovation Program (G01201406010606) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).



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Ionic Liquid-Based Polymer Electrolytes via Surfactant-Assisted

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