Novel Composite Gel Electrolytes with Enhanced Electrical

Jun 8, 2017 - 2.3.5Conductivity Measurement and Electrochemical Stability Window. The conductivity of the electrolytes was measured by electrochemical...
1 downloads 9 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Novel Composite Gel Electrolytes with Enhanced Electrical Conductivity and Thermal Stability Prepared Using Self-Assembled Nanofibrillar Networks Wei-Chi Lai,*,†,‡ Li-Jie Liu,† and Po-Hsun Huang† †

Department of Chemical and Materials Engineering and ‡Energy and Optoelectronic Materials Research Center, Tamkang University, No. 151, Yingzhuan Rd., Tamsui Dist., New Taipei City 25137, Taiwan S Supporting Information *

ABSTRACT: Novel composite gel electrolytes were prepared using self-assembled organogels as scaffolds. Mixing silica with the low-molecular-weight poly(ethylene glycol) (PEG)-based electrolytes resulted in precipitation due to significant aggregation of silica. However, clear and transparent PEG− silica composite gel electrolytes were obtained with 1,3:2,4dibenzylidene-D-sorbitol (DBS) organogels. The organogels resulted from the formation of DBS nanofibrillar networks in which the diameter sizes of the nanofibrils ranged from 10 to 100 nm, as observed by transmission electron microscopy. These three-dimensional nanofibrillar networks entrapped the silica and prevented its aggregation. The thermal properties, such as gel dissolution and thermal degradation temperatures, of the composite gels significantly increased with increasing silica content, as determined by polarizing optical microscopy and thermogravimetric analysis. The conductivity of the prepared composite gel electrolytes was clearly enhanced by increasing the silica content. The silica was well dispersed along the DBS nanofibrillar networks, establishing homogeneous microstructures and effective contact with other components of the electrolytes, leading to an increase in the conductivity. polymer gel electrolytes.12−17 The most frequently investigated polymers are PEG and PEO because they are chemically stable, polar, and exhibit appreciable conductivities.15 Because of the high degree of crystallinity of PEG and PEO, the ionic conductivity is often low. In our previous study,18 we proposed a novel method for preparing polymer gel electrolytes using gelators in low-molecular-weight (liquid) PEG-based electrolytes (not using organic solvents) for dye-sensitized solar cells. The conductivity of the prepared PEG gel electrolytes was similar to that of the pure PEG liquid electrolytes. An inexpensive gelator, 1,3:2,4-dibenzylidene-D-sorbitol (DBS), was selected for use in the PEG-based electrolyte in this study. DBS is an amphiphilic molecule derived from the sugar alcohol D-glucitol that contains hydrophobic phenyl rings and hydroxyl groups. The chemical structure of DBS is depicted in Figure 1. DBS has been shown to self-assemble into 3-D nanofibrillar networks through hydrogen bonding at low concentrations in a variety of organic solvents and lowmolecular-weight polymers to produce organogels.19−21 The average diameter of the resulting fibrils ranged from 10 nm to 1 μm depending on the solvent and polymer matrix. Applications for DBS gels include templated synthesis, antiperspirants, drugdelivery systems, and separations.22,23 DBS gels have also been

1. INTRODUCTION Lithium-ion batteries are the most common type of rechargeable batteries in electrochemical devices for portable electronics, electric vehicles, and aerospace applications.1 The electrolyte, which allows for ionic movement, is a major component of a lithium-ion battery. Liquid electrolytes in lithium-ion batteries consist of lithium salts in an organic solvent, such as ethylene carbonate, propylene carbonate, dimethyl carbonate, or diethyl carbonate.1 Liquid electrolytes are most common and are typically prepared in volatile organic solvents, which often results in solvent volatilization, flammability, and leakage during long-term operation.2,3 Solid electrolytes possess advantageous characteristics that include high reliability and nonleakage of electrolyte solutions; however, the ionic conductivity of solid electrolytes is significantly lower than liquid electrolytes.4 Gel electrolytes, which can solve the solvent leakage problem and have conductivities only slightly lower than liquid electrolytes, have become widely attractive in both academic and industrial areas.5,6 However, several problems still exist, including volatility of the solvent and limited current due to high viscosity of the electrolytes.4,7 Gel electrolytes are typically prepared by adding low-molecular-weight gelators or polymers to organic solvent-based electrolyte solutions.8−11 For example, polymers such as poly(ethylene glycol) (PEG), poly(ethylene oxide) (PEO), poly(acrylonitrile), poly(methyl methacrylate), and poly(vinylidene fluoride) have been used to prepare © 2017 American Chemical Society

Received: March 27, 2017 Revised: May 31, 2017 Published: June 8, 2017 6390

DOI: 10.1021/acs.langmuir.7b01053 Langmuir 2017, 33, 6390−6397

Article

Langmuir

Lithium perchlorate (LiClO4) (anhydrous and purity: 95.0%) was purchased from Alfa Aesar. 2.2. Sample Preparation. 2.2.1. DBS/PEG/Silica Composite Gels. Varying amounts of colloidal silica were mixed with fixed amounts of PEG at room temperature until the solutions became clear. Next, DBS/PEG/silica samples were prepared by dissolving 3 wt % DBS into the PEG/silica solutions at 160 °C for 2 h on a hot plate under constant agitation. After the DBS dissolved completely, the clear solutions were removed from the plate and cooled to room temperature to induce gelation. Finally, the samples were stored at 25 °C for a week prior to conducting experiments. 2.2.2. DBS/PEG/Silica Composite Gel Electrolytes. DBS (3 wt %) and varying amounts of silica were mixed with PEG-based electrolytes (weight fraction of PEG/LiClO4 = 15 with a fixed LiClO4 amount (0.2 g)) and heated to 160 °C on a hot plate for 2 h under constant agitation. The mixtures were cooled to room temperature, producing the composite gel electrolytes. To prepare the DBS/PC/silica gel electrolytes, DBS (1 wt %) and varying amounts of silica (0−2 wt %) were mixed with PC-based electrolytes (weight fraction of PC/LiClO4 = 15 with a fixed LiClO4 amount (0.2 g)) and heated to 160 °C on a hot plate under constant agitation. The samples were cooled to room temperature, producing the gel electrolytes. For the pure PEG and PC electrolytes containing varying amounts of silica, the preparation method was the same as those for the gel systems. Both pure systems did not contain DBS and existed in a liquid state. All the samples were stored at 25 °C for a week prior to conducting experiments. 2.3. Measurements. 2.3.1. Structures: Polarized Optical Microscopy (POM). The morphologies of the composite gels were observed using a polarized optical microscope (Leica DM2700 M RL/ TL) equipped with a camera under air. After heat treatment at 160 °C to erase the thermal history and removed the water, the solutions were dropped onto a glass plate and quickly covered with a coverslip. The samples were cooled to room temperature and stored at 25 °C for a week before the POM images were taken. Repeated experiments were conducted, and similar observations were obtained. 2.3.2. Structures: Transmission Electron Microscopy (TEM). The microstructures of the composite gels were observed by field emission transmission electron microscopy (JEOL JEM-2100F). The samples were prepared by completely dissolving them at 160 °C on a hot plate, and the solution was dripped onto carbon-supported TEM grids to form gels, which were then placed in a vacuum for a week at 25 °C. Finally, the samples were washed with water to remove the PEG components and placed in a vacuum oven at 60 °C for 12 h. 2.3.3. Thermal Properties: Gel Dissolution Temperature. The gel dissolution temperatures of the composite gels were determined via POM by measuring at which temperature birefringence completely disappeared during heating. To measure the gel dissolution temperatures, samples of the composite gels on glass plates covered with a coverslip were reused after the POM observations at room temperature. The samples were heated from 25 to 160 °C at a heating rate of 3 °C/min using a hot stage (Linkam THMS600), and POM images were taken at different temperatures. 2.3.4. Thermal Properties: Thermal Degradation Temperature. Thermal degradation studies of the composite gels were performed with a thermogravimetric analyzer (TA Instruments Hi-Res TGA 2950). The samples were heated at 10 °C/min under a flow of nitrogen. The thermal decomposition temperature corresponded to the temperature at which the maximum rate of weight loss occurred. 2.3.5. Conductivity Measurement and Electrochemical Stability Window. The conductivity of the electrolytes was measured by electrochemical impedance spectroscopy (EIS). EIS measurements of the assembled cells were performed with an advanced electrochemical system (Ametek Parstat 2263) in a 1−200 000 Hz frequency range and a 10 mV amplitude at room temperature. The electrolytes were sandwiched between two parallel ITO glasses. The conductivity (σ) was calculated from the electrolyte resistance (R) from the equation

Figure 1. Chemical structure of 1,3:2,4-dibenzylidene-D-sorbitol (DBS).

used in batteries as gel electrolytes.18,24 These previous reports determined that electrolytes prepared by the addition of DBS or DBS derivatives showed improved electrochemical performances. Several reports investigated the addition of inorganic fillers, such as silica (SiO2), copper oxide (CuO), and titania (TiO2) particles, into liquid or polymer electrolytes to enhance the conductivity and thermal stability.25−31 Bhattacharyya et al.25 found that the addition of specific amounts of oxide particles (SiO2 and TiO2) to organic solvent-based (liquid) electrolytes enhanced the conductivities of the composites relative to the pure liquid electrolytes. This phenomenon was explained using heterogeneous percolation theories at the interfaces.26 The acidic or basic surfaces of oxide fillers could adsorb salt ions in liquid electrolytes, leading to the formation of high concentrations at the solid−liquid interfaces. Wang et al.27 investigated synthetic silica network structures to increase the effective contact and interactions between the silica and other components of the system, leading to an increase in the conductivity. Zhao et al.28 synthesized a series of composite electrolytes that were composed of polyelectrolyte-grafted fumed silica nanoparticles and poly(ethylene glycol) dimethyl ether. Composite nanoparticles grafted with poly(lithium 4styrenesulfonate) exhibited enhanced conductivity. Recently, Bhattacharyya et al.29−31 found that the oxide additive acted as a heterogeneous doping creating free charge carriers and increasing the ion transport. Furthermore, a stable particle network (the gel formation) can facilitate the long-range transport because the network structure resulted in overlaps of space charge and percolation in conductivity. In the present study, the facile preparation and thorough characterization of novel PEG−silica composite gel electrolytes using an inexpensive gelator, DBS, is described. Colloidal silica (a suspension of silica particles in an aqueous solution) was chosen to mix into the DBS/PEG systems. The silica that was mixed with the PEG solution aggregated and eventually precipitated. However, clear and transparent composite gels were obtained using the DBS organogels. Our results indicated that these three-dimensional DBS nanofibrillar networks entrapped the silica and prevented its aggregation. The conductivity of the prepared composite gel electrolytes increased significantly with the amount of silica. This is the first report of using DBS and silica in PEG-based gel electrolytes.

2. EXPERIMENTAL SECTION 2.1. Materials. 1,3:2,4-Dibenzylidene-D-sorbitol (DBS) was obtained from Milliken Chemical. Poly(ethylene glycol) (PEG) with a weight-average molecular weight of 400 g/mol was purchased from Acros. Colloidal silica (50 wt % suspension in H2O) was obtained from Aldrich. Propylene carbonate (PC) was obtained from Acros.

σ= 6391

L R×A DOI: 10.1021/acs.langmuir.7b01053 Langmuir 2017, 33, 6390−6397

Article

Langmuir where L is the thickness of the electrolytes and A is the contact area. The electrolyte resistance (R) was obtained from the impedance diagrams. In this study, L was fixed as 0.005 cm, and A was fixed as 1 cm2. The conductivities were obtained by averaging at least five sample cells measurements. The electrochemical stability window of the electrolytes was determined by linear sweep voltammetry at the scanning rate of 1 mV/s.

3.1.2. Structures: TEM. Figure 3 depicts the TEM micrographs of the DBS gel and silica. As seen in Figure 3a, DBS

3. RESULTS AND DISCUSSION 3.1. Composite Gels. 3.1.1. Structures: POM. We first investigated the phase behaviors of the samples prepared from dissolution of varying amounts of DBS in PEG by visual observation. All samples were stored at 25 °C under vacuum for 1 week before observation. As the DBS amounts reached 3 wt %, transparent gels formed. When silica was added, DBS/PEG/ silica composite gels formed for silica amounts below 7 wt %. When the silica content was above 7 wt %, the sample became opaque, indicating that the macroscopic phase separation occurred (see Figure S1a,b of the Supporting Information). Therefore, composite gels containing 3 wt % DBS and 0−7 wt % silica were chosen for study. The gels were characterized by the dynamic rheological measurement (see Figure S2). We next examined the microstructures of these composite gels. A prior study determined that neat DBS samples revealed spherulitelike morphologies during cooling from the melt, as observed by POM.32 Moreover, these spherulite-like textures were also observed for DBS in various organic solvents and liquid polymers.18 Electron microscopy verified that these morphologies were formed due to the presence of DBS nanofibrillar networks.18 Figure 2 displays the POM photographs of our

Figure 3. TEM micrographs of the (a) DBS gel and (b) silica with different magnifications.

networks consisting of fibrils approximately 20 nm in diameter were observed. The diameter of these DBS nanofibrils was quite similar to those reported for DBS in organic solvents and liquid polymers.6−8 In the silica sample, the silica was sphereshaped and the average diameter was approximately 25 nm (see Figure 3b). Figure 4 portrays the TEM micrographs of the composite gels containing varying amounts of silica. Table 1 lists the average diameters of the nanofibrils in the composite gels containing varying amounts of silica. As seen in Table 1, the average diameters of the nanofibrils increased with increasing silica content. The self-assembly of DBS is predominantly controlled by intermolecular hydrogen bonding between DBS and the solvent matrix (PEG).33 Silica, which contains hydroxyl groups, reduces the intermolecular interactions between DBS and PEG, leading to an increase in the self-assembly of DBS. Therefore, larger diameter sizes of the nanofibrils were observed as more silica was added. In addition, the silica was well dispersed along these DBS nanofibrillar networks. 3.1.3. Thermal Properties: Gel Dissolution Temperature. Figure 5 is a series of POM micrographs of the composite gels with varying silica amounts at different temperatures. As seen in Figure 5, when the temperature increased, the spherulite-like textures became less distinct. For example, in the neat DBS gel (without silica), birefringence began to fade at approximately 81.3 °C and eventually disappeared completely at 92.4 °C. The disappearance of the birefringent morphology was presumed to correspond to the dissolution of the DBS nanofibrillar network in the PEG matrix. The gel dissolution temperatures of the composite gels with varying silica contents are listed in Table 2. The gel dissolution temperatures were determined from POM by observing the temperature at which birefringence completely disappeared during heating. From Table 2, the gel dissolution temperatures increased with increasing silica content. As observed in the structures of these composite gels, a large diameter size of the nanofibrils was found for higher silica contents. Therefore, more thermal energy was required to disrupt the gel networks containing more extensive aggregates of DBS nanofibrils with higher silica concentrations. In

Figure 2. POM micrographs of the composite gels with (a) 0, (b) 3, (c) 5, and (d) 7 wt % silica at room temperature.

prepared composite gels containing varying amounts of silica. As seen in Figure 2, all samples exhibited spherulite-like morphologies, and the spherulite-like sizes ranged from 200 to 1000 μm. The addition of silica did not significantly influence the spherulite-like morphologies of the samples. In addition, the samples containing silica exhibited explicit spherulite boundaries (indicated by the arrows). It was assumed that the silica was excluded after the formation of the DBS nanofibrillar networks. TEM was then used to observe these structures. 6392

DOI: 10.1021/acs.langmuir.7b01053 Langmuir 2017, 33, 6390−6397

Article

Langmuir

degradation temperatures of organic polymers.34 Therefore, in our system, the addition of the silica did improve the thermal stability of the composite gel samples. 3.2. Composite Gel Electrolytes: Conductivity. The composite gel electrolytes were prepared by adding DBS, LiClO4 salts, and varying amounts of the silica to low molecular weight liquid PEG. The gel formation time of these composite gel electrolytes was influenced by the addition of LiClO4 salts; however, these composite gel electrolytes (with 0−7 wt % silica) still retained the behavior of a gel. Figure 7 presents the effect of the LiClO4 amount on the conductivity of pure PEG (liquid) electrolytes at room temperature. The conductivity of the samples was initially enhanced by increasing the LiClO4 amount. The maximum conductivity value was achieved for a PEG/LiClO4 weight ratio of 15. At lower salt concentrations, the number of charge carriers increased with increasing salt concentration, leading to an enhancement in conductivity.35 However, the conductivity decreased after reaching the maximum conductivity upon further addition of salts. At higher salt concentrations, the viscosity increased and the mobility of entire polymer backbone decreased.35 As a result, the conductivity decreased. In this study, the PEG/LiClO4 weight fraction was fixed at 15. Figure 8 displays the conductivities of the composite gel electrolytes containing varying amounts of silica and a fixed LiClO4 content (weight fraction of PEG/LiClO4 = 15). Note that PEG/silica systems (without DBS) were found with macroscopic phase separation and precipitation (see Figure S1c). When the DBS molecules were allowed to self-assemble, clear and transparent composite gel electrolytes were obtained. No obvious and significant aggregation of silica was observed in the samples containing silica. These systems (0−7 wt % silica) still exhibited gel behavior,and no precipitation occurred, even over a period as long as 6 months. As seen in Figure 8, the conductivity of the composite gel electrolytes with silica lower than 7 wt % significantly increased with increasing silica content. This can be explained as follows. Silica, an oxide additive, acted as a heterogeneous doping creating free charge carriers and increasing the ion transport. A stable network further facilitated the long-range transport.29−31 As observed in Figure 4, the silica networks formed, which were along the DBS nanofibrillar networks. Therefore, with the increase in silica amounts, the number of spanning pathways increased, and the ionic conductivity had a maximum. In addition, Wang et al.27 investigated the formation of gel electrolytes from sol−gel silica networks that possessed homogeneous structures within a polymer matrix, which increased the contact between the silica and other components of the electrolyte systems, leading to an increase in the conductivity. In our systems, the silica network was well dispersed along the DBS nanofibrillar networks, which could increase the effective contact with other components of the electrolytes and enhance the conductivity. However, for the samples with 9 wt % silica, the samples were opaque, probably due to the large amounts of silica particles aggregated (see Figure S1a,b). The significant decrease in the conductivity may result from the aggregated silica blocking the pathways for the migration of ions.29−31 Table 3 lists the conductivities of selected PEO-based gel electrolytes for lithium-ion batteries.36−42 Note that PEO and PEG are the same polymeric material with different molecular weights. The conductivities of the solid PEO-based electrolytes are usually around 10−8 S/cm.7 Most researchers studied the

Figure 4. TEM micrographs of the composite gels with (a) 3, (b) 5, and (c) 7 wt % silica.

Table 1. Average Diameters of the Nanofibrils of Composite Gels Containing Varying Amounts of Silica sample 0 3 5 7

wt wt wt wt

% % % %

silica silica silica silica

average diameters of nanofibrils (nm) 20.6 35.7 55.7 78.6

± ± ± ±

8.6 9.6 8.7 9.7

addition, the spherulite-like morphologies formed again upon subsequent cooling, and thus we confirmed that these composite gels were thermally reversible. 3.1.4. Thermal Properties: Thermal Degradation Temperature. Figure 6 displays the TGA curves of the composite gels containing varying amounts of silica. The thermal decomposition temperature is the temperature at which the maximum rate of weight loss occurs. The original TGA curves (weight (%) vs temperature) are shown in Figure S3. As seen in Figure 6, the thermal degradation temperatures were 309, 326, 328, and 331 °C for neat DBS gel and the composite gels with 3, 5, and 7 wt % silica, respectively. When silica was added, the thermal degradation temperatures significantly increased with increasing silica content. In general, incorporating an inorganic additive or filler, such as TiO2 or SiO2, increases the thermal 6393

DOI: 10.1021/acs.langmuir.7b01053 Langmuir 2017, 33, 6390−6397

Article

Langmuir

Figure 5. POM micrographs of the composite gels with (a) 0, (b) 3, (c) 5, and (d) 7 wt % silica at different temperatures.

Table 2. Gel Dissolution Temperatures of Composite Gels with Varying Silica Contents sample 0 3 5 7

wt wt wt wt

% % % %

silica silica silica silica

gel dissolution temperature (°C) 92.4 97.6 102.5 110.3

enhancement of the conductivities of the PEO-based electrolytes by adding plasticizers (organic solvents), such as propylene carbonate (PC) and ethylene carbonate (EC), to decrease the degree of the crystallinities.36−38 As seen in Table 3, Bandara et al.36 examined the PEO−LiCF3SO3 systems plasticized with 50% PC and 50% EC. The conductivity was around 9 × 10 −4 S/cm; however, the mechanical stability was poor. The systems with cross-linked PEO and PC exhibited higher mechanical stability and good conductivity.37 Other plasticizers, including dibutyl, dimethyl, and dioctyl phthalate (DOP), were added into PEO−LiClO4 systems, where the DOP systems showed the best conductivity (9.7 × 10 −5 S/ cm).38 On the other hand, several studies investigated the composite PEO-based electrolytes with inorganic fillers such as silica and Al2O3.39−41 These composite gel electrolytes have shown increases in conductivity and thermal properties. The preparation of solid PEO−LiCF3SO3 electrolytes with nano-

Figure 6. TGA curves of the composite gels containing varying amounts of silica.

sized silica filler by using the solution casting method was proposed by Abdullah et al.39 The systems with 10 wt % silica exhibited the maximum conductivity (7.1 × 10−5 S/cm).39 The addition of PC and EC to the PEO-based composite gel electrolytes with inorganic fillers showed significantly enhanced conductivity up to 10−4 S/cm.40,41 Recently, the gel electrolytes 6394

DOI: 10.1021/acs.langmuir.7b01053 Langmuir 2017, 33, 6390−6397

Article

Langmuir

temperature of PEG is greater than 300 °C (see Figure 6), which means no solvent evaporation happened. In addition, we also found that the addition of the inorganic filler, silica, significantly increased the conductivity and thermal stability. 3.3. Other Organic Solvent-Based Electrolyte Systems with DBS and Silica: Conductivity. In addition to the PEGbased electrolytes, conductivity studies of the most common organic alkyl carbonate solvent, PC, were also conducted. Figure 9 shows the conductivity of DBS/PC gel electrolytes

Figure 7. Effect of the LiClO4 amount on the conductivity of pure PEG electrolytes at room temperature.

Figure 9. Conductivity of the DBS/PC gel electrolytes containing varying amounts of silica and a fixed LiClO4 content (weight fraction of PC/LiClO4 = 15).

containing varying amounts of silica and a fixed LiClO4 content (weight fraction of PC/LiClO4 = 15). According to our previous study,30 the solvent (matrix) polarity and DBS concentration influence the gel formation of the DBS organogels; therefore, the extent of gel formation in the DBS/PC gels was different from that in the DBS/PEG gel systems. Gels formed when the DBS content reached 1 wt % in the DBS/PC systems. As the silica content reached 3 wt %, precipitation and macroscopic phase separation occurred. In this study, silica concentrations below 3 wt % with a fixed DBS content of 1 wt % were chosen for the preparation of PC-based gel electrolytes. As seen in Figure 9, similar results were observed as those shown in Figure 8, in which the conductivity of the gel electrolytes containing DBS increased with increasing silica content. Therefore, we propose that the formation of selfassembled DBS organogels, which result from the presence of nanofibrillar networks, prevents the aggregation of silica, and leads to an increase in conductivity. 3.4. Electrochemical Stability. Figure 10 displays the electrochemical window for the prepared composite gel electrolytes with 7 wt % silica. The operating voltage was performed under the scanning range between 0 and 5 V by linear sweep voltammetry. The voltage at which the current rapidly increases defines the electrochemical stability window. It was found that our prepared composite gel electrolyte was stable electrochemically up to a potential of 5 V and has good oxidation stability in the 0−5 V operating voltage environment.

Figure 8. Conductivity of the composite gel electrolytes containing varying amounts of silica and a fixed LiClO4 content (weight fraction of PEG/LiClO4 = 15).

Table 3. Conductivities of Selected PEO-Based Gel Electrolytes for Lithium-Ion Batteries PEO-based gel electrolyte PEO−LiCF3SO3 (PC + EC) cross-linked PEO−LiClO4 (PC) PEO−LiClO4 (DOP) PEO−LiClO4 (PC + EC + 15 wt % SiO2) PEO−LiTf (PC + EC + 15 wt % Al2O3) PEO block copolymer−LiClO4 PEG−LiClO4 (3 wt % DBS + 7 wt % SiO2)

conductivity (S/cm) 9.0 8.0 9.7 2.0 1.2 1.0 5.0

× × × × × × ×

10−4 10−4 10−5 10−4 10−4 10−5 10−4

consisting of block copolymer and liquid PEO were synthesized by Rolland et al.42 The maximum conductivities of around 1 × 10−5 S/cm were attained. In summary, most studies investigated the preparation of PEO-based gel electrolytes by adding plasticizers (organic solvents) to increase the conductivity approaching around 10−4 S/cm. As mentioned in the Introduction, the similar potential problems associated with liquid electrolytes may occur, such as the volatilization and flammability, during long-term operation. In our systems, no organic solvents were used, and a method was proposed to prepare the gel electrolytes using DBS in the low-molecular-weight PEG-based electrolytes. The conductivities were all higher than 10−4 S/cm. Moreover, the degradation

4. CONCLUSIONS The facile preparation and thorough characterization of novel PEG−silica composite gel electrolytes with self-assembled DBS was performed in this study. All of the gel systems with 0−7 wt % silica exhibited spherulite-like morphologies, and TEM 6395

DOI: 10.1021/acs.langmuir.7b01053 Langmuir 2017, 33, 6390−6397

Article

Langmuir

(5) Kubo, W.; Kambe, S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Photocurrent-Determining Processes in QuasiSolid-State Dye-Sensitized Solar Cells Using Ionic Gel Electrolytes. J. Phys. Chem. B 2003, 107, 4374−4381. (6) Zhang, S.; Lee, K. H.; Sun, J.; Frisbie, C. D.; Lodge, T. P. Viscoelastic Properties, Ionic Conductivity, and Materials Design Considerations for Poly(styrene-b-ethylene oxide-b-styrene)-Based Ion Gel Electrolytes. Macromolecules 2011, 44, 8981−8989. (7) Stephan, A. M. Review on Gel Polymer Electrolytes for Lithium Batteries. Eur. Polym. J. 2006, 42, 21−42. (8) Huo, Z.; Dai, S.; Zhang, C.; Kong, F.; Fang, X.; Guo, L.; Liu, W.; Hu, L.; Pan, X.; Wang, K. Low Molecular Mass Organogelator Based Gel Electrolyte with Effective Charge Transport Property for LongTerm Stable Quasi-Solid-State Dye-Sensitized Solar Cells. J. Phys. Chem. B 2008, 112, 12927−12933. (9) Tao, L.; Huo, Z.; Dai, S.; Ding, Y.; Zhu, J.; Zhang, C.; Zhang, B.; Yao, J.; Nazeeruddin, M. K.; Gratzel, M. Stable Quasi-Solid-State DyeSensitized Solar Cells Using Novel Low Molecular Mass Organogelators and Room-Temperature Molten Salts. J. Phys. Chem. C 2014, 118, 16718−16726. (10) Suait, M. S.; Rahman, M. Y. A.; Ahmad, A. Review on Polymer Electrolyte in Dye-Sensitized Solar Cells (DSSCs). Sol. Energy 2015, 115, 452−470. (11) Bidikoudi, M.; Perganti, D.; Karagianni, C.-S. Polycarpos Falaras Solidification of Ionic Liquid Redox Electrolytes Using Agarose Biopolymer for Highly Performing Dye-Sensitized Solar Cells. Electrochim. Acta 2015, 179, 228−236. (12) Lan, Z.; Wu, J.; Lin, J.; Huang, M.; Yin, S.; Sato, T. Influence of Molecular Weight of PEG on the Property of Polymer Gel Electrolyte and Performance of Quasi-Solid-State Dye-Sensitized Solar Cells. Electrochim. Acta 2007, 52, 6673−6678. (13) Song, J. Y.; Wang, Y. Y.; Wan, C. C. Review of Gel-Type Polymer Electrolytes for Lithium-Ion Batteries. J. Power Sources 1999, 77, 183−197. (14) Santhosh, P.; Vasudevan, T.; Gopalan, A.; Lee, K. P. Preparation and Properties of New Cross-Linked Polyurethane Acrylate Electrolytes for Lithium Batteries. J. Power Sources 2006, 160, 609−620. (15) Kim, J. H.; Kang, M.-S.; Kim, Y. J.; Won, J.; Park, N.-G.; Kang, Y. S. Dye-Sensitized Nanocrystalline Solar Cells Based on Composite Polymer Electrolytes Containing Fumed Silica Nanoparticles. Chem. Commun. 2004, 1662−1663. (16) Reddy, M. J.; Kumar, J. S.; Rao, U. V. S.; Chu, P. P. Structural and Ionic Conductivity of PEO Blend PEG Solid Polymer Electrolyte. Solid State Ionics 2006, 177, 253−256. (17) Kang, M. S.; Kim, Y. J.; Won, J.; Kang, Y. S. Roles of Terminal Groups of Oligomer Electrolytes in Determining Photovoltaic Performances of Dye-Sensitized Solar Cells. Chem. Commun. 2005, 2686−2688. (18) Lai, W.-C.; Chen, C.-C. Novel Poly (ethylene glycol) Gel Electrolytes Prepared Using Self-Assembled 1, 3:2, 4-Dibenzylidene-dSorbitol. Soft Matter 2014, 10, 312−319. (19) Cornwell, D. J.; Daubney, O. J.; Smith, D. K. Photopatterned Multidomain Gels: Multi-Component Self-Assembled Hydrogels Based on Partially Self-Sorting 1, 3:2, 4-Dibenzylidene-d-sorbitol Derivatives. J. Am. Chem. Soc. 2015, 137, 15486−15492. (20) Diehn, K. K.; Oh, H.; Hashemipour, R.; Weiss, R. G.; Raghavan, S. R. Insights into Organogelation and its Kinetics from Hansen Solubility Parameters. Toward a Priori Predictions of Molecular Gelation. Soft Matter 2014, 10, 2632−2640. (21) Takeno, H.; Kuribayashi, Y. A Synchrotron Small-Angle X-ray Scattering Study on Structures of 1,3:2,4-Dibenzylidene Sorbitol Gels. Colloids Surf., A 2015, 467, 173−179. (22) Lai, W.-C.; Tseng, S.-J.; Chao, Y.-S. Effect of Hydrophobicity of Monomers on the Structures and Properties of 1,3:2,4-Dibenzylidened-sorbitol Organogels and Polymers Prepared by Templating the Gels. Langmuir 2011, 27, 12630−12635. (23) Howe, E.; Babatunde, J.; Okesola, O.; Smith, D. K. SelfAssembled Sorbitol-Derived Supramolecular Hydrogels for the

Figure 10. Electrochemical window for the composite gel electrolytes with 7 wt % silica.

verified that these morphologies were formed due to the presence of DBS nanofibrillar networks. The formation of nanofibrillar networks entrapped the silica and prevented its aggregation. The conductivity was found to increase with the addition of silica. In addition, the thermal stability of our prepared composite gels was significantly improved by the addition of silica. Furthermore, the gel electrolyte was stable electrochemically up to a potential of 5 V.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01053. Phase behaviors of PEG/DBS/silica and PEG/silica samples (Figure S1); rheological measurement of the PEG/DBS/silica samples with 0 and 7 wt % silica (Figure S2); TGA curves of the PEG/DBS/silica samples (Figure S3) (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Phone +886-2-2621-5656 ext 3516; Fax +886-2-2620-9887; email [email protected] (W.-C.L.). ORCID

Wei-Chi Lai: 0000-0003-3179-7756 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministry of Science and Technology of Taiwan. REFERENCES

(1) Ritchie, A. G. Recent Developments and Future Prospects for Lithium Rechargeable Batteries. J. Power Sources 2001, 96, 1−4. (2) Wakihara, M. Recent Developments in Lithium Ion Batteries. Mater. Sci. Eng., R 2001, 33, 109−134. (3) Papageorgiou, N.; Maier, W. F.; Gratzel, M. An Iodine/Triiodide Reduction Electrocatalyst for Aqueous and Organic Media. J. Electrochem. Soc. 1997, 144, 876−884. (4) Wang, Y. Recent Research Progress on Polymer Electrolytes for Dye-Sensitized Solar Cells. Sol. Energy Mater. Sol. Cells 2009, 93, 1167−1175. 6396

DOI: 10.1021/acs.langmuir.7b01053 Langmuir 2017, 33, 6390−6397

Article

Langmuir Controlled Encapsulation and Release of Active Pharmaceutical Ingredients. Chem. Commun. 2015, 51, 7451−7454. (24) Mohmeyer, N.; Wang, P.; Schmidt, H.-W.; Zakeeruddin, S. M.; Gratzel, M. Quasi-Solid-State Dye Sensitized Solar Cells with 1, 3:2, 4di-O-Benzylidene-D-Sorbitol Derivatives as Low Molecular Weight Organic Gelators. J. Mater. Chem. 2004, 14, 1905−1909. (25) Bhattacharyya, A. J.; Maier, J. Second Phase Effects on the Conductivity of Non-Aqueous Salt Solutions:“Soggy Sand Electrolytes. Adv. Mater. 2004, 16, 811−814. (26) Maier, J. Ionic Conduction in Space Charge Regions. Prog. Solid State Chem. 1995, 23, 171−263. (27) Wang, X.-L.; Cai, Q.; Fan, L.-Z.; Hua, T.; Lin, Y.-H.; Nan, C.-W. Gel-Based Composite Polymer Electrolytes with Novel Hierarchical Mesoporous Silica Network for Lithium Batteries. Electrochim. Acta 2008, 53, 8001−8007. (28) Zhao, H.; Jia, Z.; Yuan, W.; Hu, H.; Fu, Y.; Baker, G. L.; Liu, G. Fumed Silica-Based Single-Ion Nanocomposite Electrolyte for Lithium Batteries. ACS Appl. Mater. Interfaces 2015, 7, 19335−19341. (29) Santhosha, A. L.; Bhattacharyya, A. J. A Few Case Studies on the Correlation of Particle Network and Its Stability on the Ionic Conductivity of Solid-Liquid Composite Electrolytes. J. Phys. Chem. B 2015, 119, 11317−11325. (30) Das, S. K.; Bhattacharyya, A. J. Oxide Particle Surface Chemistry and Ion Transport in “Soggy Sand” Electrolytes. J. Phys. Chem. C 2009, 113, 6699−6705. (31) Bhattacharyya, A. J. Ion Transport in Liquid Salt Solutions with Oxide Dispersions:“Soggy Sand” Electrolytes. J. Phys. Chem. Lett. 2012, 3, 744−750. (32) Lai, W.-C.; Wu, C.-H. Studies on the Self-Assembly of Neat DBS and DBS/PPG Oganogels. J. Appl. Polym. Sci. 2010, 115, 1113− 1119. (33) Lai, W.-C.; Lee, Y.-C. Self-Assembly Behavior of Gels Composed of Dibenzylidene Sorbitol Derivatives and Poly(ethylene glycol). RSC Adv. 2016, 6, 98042−98051. (34) Kuan, H.-C.; Chiu, S.-L.; Chen, C.-H.; Kuan, C. F.; Chiang, C.L. Synthesis, Characterization, and Tthermal Stability of PMMA/ SiO2/TiO2 Tertiary Nanocomposites via Non-Hydrolytic Sol-Gel Method. J. Appl. Polym. Sci. 2009, 113, 1959−1965. (35) Li, P. J.; Wu, J. H.; Huang, M. L.; Hao, S. C.; Lan, Z.; Li, Q.; Kang, S. The Application of P(MMA-co-MAA)/PEG Polyblend Gel Electrolyte in Quasi-Solid State Dye-Sensitized Solar Cell at Higher Temperature. Electrochim. Acta 2007, 53, 903−908. (36) Bandara, L. R. A. K.; Dissanayake, M. A. K. L.; Mellander, B.-E. Ionic Conductivity of Plasticized (PEO)-LiCF3SO3 Electrolytes. Electrochim. Acta 1998, 43, 1447−1551. (37) Dias, F. B.; Plomp, L.; Veldhuis, J. B. J. Trends in Polymer Electrolytes for Secondary Lithium Batteries. J. Power Sources 2000, 88, 169−6191. (38) Michael, M. S.; Jacob, M. M. E.; Prabaharan, S. R. S.; Radhakrishna, S. Enhanced Lithium Ion Transport in PEO-based Solid Polymer Electrolytes Employing a Novel Class of Plasticizers. Solid State Ionics 1997, 98, 167−174. (39) Abdullah, A.; Abdullah, S. Z.; Ali, A. M. M.; Winie, T.; Subban, R. H. Y. Electrical Properties of PEO-LiCF3SO3-SiO2 NanoComposite Polymer Electrolytes. Solid State Sci. Technol. 2009, 17, 110−118. (40) Pitawala, H. M. J. C.; Dissanayake, M. A. K. L.; Seneviratne, V. A.; Mellander, B.-E.; Albinson, I. Effect of Plasticizers (EC or PC) on the Ionic Conductivity and Thermal Properties of the (PEO)9LiTf: Al2O3 nanocomposite polymer electrolyte system. J. Solid State Electrochem. 2008, 12, 783−789. (41) Nan, C.-W.; Fan, L.; Lin, Y.; Cai, Q. Enhanced Ionic Conductivity of Polymer Electrolytes Containing Nanocomposite SiO2 Particles. Phys. Rev. Lett. 2003, 91, 266104. (42) Rolland, J.; Brassinne, J.; Bourgeois, J.-P.; Poggi, E.; Vlad, A.; Gohy, J.-F. Chemically Anchored Liquid-PEO Based Block Copolymer Electrolytes for Solid-State Lithium-Ion Batteries. J. Mater. Chem. A 2014, 2, 11839−11846.

6397

DOI: 10.1021/acs.langmuir.7b01053 Langmuir 2017, 33, 6390−6397