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A. A. Azli, N. S. A. Manan, M. F. Z. Kadir, "Conductivity and Dielectric Studies of Lithium Trifluoromethanesulfonate Doped Polyethylene Oxide-Graphene Oxide Blend Based Electrolytes", Advances in Materials Science and Engineering, vol. 2015, Article ID 145735, 10 pages, 2015. https://doi.org/10.1155/2015/145735
Conductivity and Dielectric Studies of Lithium Trifluoromethanesulfonate Doped Polyethylene Oxide-Graphene Oxide Blend Based Electrolytes
Series of polymer blend consisting of polyethylene oxide (PEO) and graphene oxide (GO) as co-host polymer were prepared using solution cast method. The most amorphous PEO-GO blend was obtained using 90 wt.% of PEO and 10 wt.% of GO as recorded by X-ray diffraction (XRD). Fourier transform infrared spectroscopy (FTIR) analysis proved the interaction between PEO, GO, lithium trifluoromethanesulfonate (LiCF3SO3), and ethylene sulfite (ES). Incorporation of 25 wt.% LiCF3SO3 into the PEO-GO blend increases the conductivity to S cm−1. The conductivity starts to decrease when more than 25 wt.% salt is doped into the polymer blend. The addition of 1 wt.% ES into the polymer electrolyte has increased the conductivity to S cm−1. Dielectric studies show that all the electrolytes obey non-Debye behavior.
Polymer electrolytes are an important component of many electrochemical devices such as lithium batteries, capacitors, fuel cells, and solar cells. Armand et al.  first proposed the use of polyethylene oxide (PEO) and alkali salts system as solid polymer electrolytes (SPE) for lithium batteries, which immediately spurred an enormous amount of research worldwide. Application of SPE especially in electrochemical devices offers advantages in terms of ease of fabrication as thin-film materials, high reliability without leakage problem of liquid electrolytes, and higher temperatures of operation and miniaturization to ensure high energy density . Despite its advantages, the low ionic conductivity of SPE at room temperature often limits their applications .
PEO is a widely used film-forming polymer with high tensile strength and flexibility [4, 5]. PEO also exhibits many other useful properties such as chemical stability, ease of electrolyte fabrication, and excellent complexation with many ionic salts [6, 7]. However, most PEO based polymer electrolytes have low conductivity due to its semicrystalline polymer nature in which, below the melting temperature of PEO at 330 K, the polymer may consist of both lamellar crystals and amorphous regions, as well as intermediate regions at the crystal/amorphous interphase . Crystalline polymer electrolytes have long been considered as insulators while ionic conductivity is confined exclusively to the amorphous state . The ion migration in SPE is believed to occur predominantly in the amorphous rich phase; thus reducing crystallinity of PEO by blending PEO with other materials as a co-host polymer increases the conductivity of PEO based SPE.
Blend polymer of PEO with carbon materials can be considered as new perspective for electrochemical applications, especially for the storage of energy because the existence of different carbon allotropes (graphite, diamond, and fullerenes/nanotubes), various microtextures (more or less ordered) owing to the degree of graphitization, and a rich variety of dimensionality from 0 to 3D and ability for existence under different forms (from powders to fibres, foams, fabrics, and composites) indicate that carbon is a very attractive material to explore . Graphene oxide (GO) is a highly oxygenated, hydrophilic layered material derived from graphite oxide resulting from oxidation of graphite by strong oxidising agents. Under the application of external energy, such as from ultrasonic vibration or thermal shock, three-dimensional graphite oxide can be exfoliated into two-dimensional graphite oxide nanosheets of single layer graphite oxide [11, 12]. Graphite oxide nanosheets can be as thin as a one carbon layer and can then be described as GO nanosheets . Reinforcement of nanometer sized materials may enhance most properties of polymer blend if they are well dispersed in the matrix polymer . Other interesting features of GO are that it can be chemically functionalized and modified for the preparation of polymer-based composite materials and it is also precursor materials for preparing graphene nanosheets in large quantity for many other functional applications such as secondary batteries, ionic conductors, and supercapacitors [15, 16].
Incorporating appropriate salts into the PEO-GO host polymer matrix provides charge carriers to the polymer matrix in order to increase the ionic conductivity of the system. High molecular weight PEO based polymer electrolytes containing lithium salts are of particular interest because of their easy formation of complexes with lithium salts and high mobility of charge carriers particularly lithium trifluoromethanesulfonate (LiCF3SO3) because the ionic conduction of LiCF3SO3 salt is ambipolar, which has lower Columbic potential and possesses higher segmental mobility of polymer chains [6, 17, 18]. Many studies on PEO-lithium salt system have reported that doping salt into the system has increased conductivity significantly [6, 18, 19]. The major drawback of these PEO-Li electrolyte systems for SPE is that they tend to crystallize at room temperatures resulting in low ionic conductivity [20, 21]. Plasticization is one of the common ways of reducing crystallinity and also increasing the amorphous phase content of polymer electrolytes. Previous work has reported that the use of ethylene sulfite (ES) as plasticizer increased the room temperature conductivity to the order of ~10−4 s cm−1 on PEO-chitosan blend polymer system . ES has dielectric constant, of 39.6, which is higher than other common plasticizers, that is, dimethylacetamide and diethyl carbonate . Increase in electrolytes conductivity has been reported with incorporation of plasticizer with higher dielectric constant . Besides, ES also has low melting point (−17°C) which can ensure the plasticizer molecules will not solidify at room temperature which may limit the ion mobility . The feasibility of ES in improving conductivity of the PEO based SPE can be investigated further and small loadings of ES contents minimize deterioration of the mechanical properties and the potential stability of the polymer films with the addition of plasticizers .
In this work, we prepare salted and plasticized SPE systems based on PEO-GO polymer blend doped with LiCF3SO3 as doping salt; meanwhile ES is reinforced as plasticizer in order to enhance the conductivity. The conductivity and dielectric properties of the electrolytes will be discussed.
2. Materials and Methods
2.1. Preparation of the Graphene Oxide (GO)
GO nanosheets were prepared by a modified Hummers method . Any extra acids and salts were removed through washing process using distilled water (DI). Mild sonication was applied to the suspension for about 10 min to exfoliate graphite oxide that still remains in the GO suspension. Finally, GO suspension was put into the oven for 48 hours at 60°C to discard water present, thus obtaining dark brown GO nanosheets.
2.2. Preparation and Characterization of Various PEO-GO Blend Films
Different weight percentages (x wt.%) of GO nanosheets were mixed in 50 mL DI and homogenized using ultrasonicator XO-650D (Nanjing Xianou Instruments Co. Ltd.) for about 15 min at 50% crusher capacity. After the solutions cooled at room temperature, () wt.% of PEO (Mv~300,000 containing 200–500 ppm butylated hydroxytoluene as inhibitor, Aldrich Chemistry) was then added to the GO solutions. The mixtures were stirred until homogeneous solutions were obtained. All solutions were cast onto different plastic Petri dishes and left to dry at room temperature and then kept in a desiccator filled with silica gel desiccants for further drying.
X-ray diffraction (XRD) measurements of the polymer blend films were carried out using a Siemens D5000 X-ray diffractometer where X-rays of 1.54 Å wavelengths were generated by a Cu Kα source. The 2θ angle was varied from 5° to 80°.
2.3. Preparation of Electrolytes
The polymer blend electrolyte system was prepared by homogenizing 10 wt.% of GO nanosheets in 50 mL DI from total of 0.5 g polymer blend. After the solution was cooled to room temperature, 90 wt.% of PEO was added and stirred until homogenous solution was obtained. Prior to the preparation of the salted electrolytes, LiCF3SO3 (Aldrich Chemistry) was dried at 100°C for 1 hour to eliminate trace amounts of water in the material. Different amounts of LiCF3SO3 were added to the PEO-GO solutions and stirred until complete dissolution. For the preparation of plasticized system, different amounts of ES (Aldrich Chemistry) were added to the highest conducting salted electrolyte solutions and stirred until complete dissolution. All homogenous solutions were cast onto plastic Petri dishes and left to dry at room temperature for 2-3 days to form film. The dry films were then kept in desiccators filled with silica gel desiccants for several hours before being characterized to avoid any trace of moisture.
2.4. Characterization of Electrolytes
Impedance measurements were conducted using HIOKI 3532-50 LCR HiTESTER from room temperature to 373 K in the frequency range of 50 Hz to 5 MHz. The electrolyte films were sandwiched between two stainless steel electrodes of a conductivity holder with diameter of 1.9 cm. The value of bulk resistance was determined from the Cole-Cole plots. Conductivity was calculated using the following equation:where is the thickness of the electrolyte samples and is the electrode-electrolyte contact area. A digital thickness gauge (Mitutoyo Corp.) was employed to measure the thickness of the electrolytes.
The Fourier transform infrared spectroscopy (FTIR) studies were performed using attenuated total reflection infrared spectroscopy (ATR-IR) Perkin Elmer Spectrum 400 GladiATR in the wavenumber range of 400–4000 cm−1. The objective of FTIR was to confirm complexation between polymer and salt.
3. Results and Discussion
3.1. XRD of PEO-GO Blends
The choice of a proper ratio of polymer blend is crucial to its ability to serve as a polymer host in electrolytes because suitable ratio of polymer blend controls the reduction in crystallinity of the polymer blend. Qualitative identification of crystallinity structure can be made by comparing the position of XRD peaks and intensity of the peaks for polymer blends with standard patterns of pure material. Figure 1 shows XRD pattern for (a) pure PEO and (b) pure GO films. The maximum intensity peaks of pure PEO film were observed at (i) 2θ = 19.1° and at (ii) 2θ = 23°. These two peaks of PEO also have been reported by Kumar et al.  at 2θ = 19.36° and at 2θ = 23.72° and other authors [27, 28]. Several less intense peaks are exhibited around 2θ = 26.3°, 27.0°, 35.6°, 36.4°, 39.9°, and 43.1°. The sharp peaks indicate the crystalline phase of PEO, which originates from the ordering of polyether side chains because there is strong intermolecular interaction between PEO chains through the hydrogen bonding . Meanwhile, a very intense and sharp peak of pure GO film was found to be at 2θ = 10.7° on Figure 1. According to Mural et al. , XRD peak of graphite, that is, starting material of GO occurring at 2θ = 26.46°, has shifted to lower angle at 2θ = 9.5° due to oxidative functionalization of graphitic structures.
Figure 2 shows the XRD pattern for different ratio (wt.%) of PEO-GO films in comparison to the pure PEO film (100 wt.% PEO) and pure GO film (100 wt.% GO). Changes in peak position or peak shape can be seen consequently in Figure 2. The very sharp peak of 100 wt.% GO shifts in the lower angle 2θ = 9.8° with the presence of 10 wt.% of PEO in the blend and the trend is similar for 20 wt.% of PEO (2θ = 8.7°), 30 wt.% PEO (2θ = 7.8°), and 40 wt.% of PEO (2θ = 6.8°). At 50 wt.% PEO-50 wt.% GO blend, GO peak has already shifted to slightly higher angle of 2θ = 7.4° and the trend of GO peak follows for up to 80 wt.% PEO-20 wt.% GO blend. The shift in high or lower angle for every different ratio proved that PEO and GO have been blended together. It was observed that, at 90 wt.% PEO-10 wt.% GO blend, the peak shape of GO has become broad compared to others indicating that high intensity of GO peak has been lowered at this particular ratio. Throughout Figure 2, the peaks of PEO are not visible because those peaks have been superimposed by the maximum intensity of the GO peak. Since the relative intensities of 90 wt.% PEO-10 wt.% GO crystalline peaks are decreased, it is chosen in this work as polymer host because less crystallinity or amorphous phase observed signifies disruption of the rigid bonding of the material which enables ion migration for ionic conductivity .
3.2. FTIR Analysis
The complexation between the materials was studied using FTIR technique. Figure 3 shows two regions of FTIR spectra in which Figure 3(a) represents O-H band for pure GO while Figure 3(b) represents C-H stretching for pure PEO. These two main characteristics for each material were compared to the 90 wt.% PEO-10 wt.% GO blend which is selected based on XRD results. The FTIR spectra of the graphene oxide nanosheets show a broad absorption band at 3190 cm−1 due to the typical carbonyl and carboxyl groups present in the GO structure. Wang et al.  and Cao et al.  also have reported the presence of O-H band in GO nanosheets near 3300 cm−1 and 3390 cm−1. Upon addition of 90 wt.% PEO through polymer blending, the O-H band has shifted to 3223 cm−1; meanwhile Figure 3(b) depicts the C-H stretching mode for pure PEO at 2871 cm−1 also has shifted to 2877 cm−1 when 10 wt.% GO was added to the polymer host. Alteration in wavenumber of O-H band and C-H stretching indicates that interaction in PEO-GO blend has occurred as reported by Mural et al. in .
Figure 4(a) shows the FTIR spectra of salted system of selected sample in the two main regions of O-H band 3050–3650 cm−1 and also in the lower region around 630–650 cm−1 as presented in Figure 4(b). According to Starkey and Frech , band at 764 cm−1 is assigned to the (CF3) mode of the triflate ion . The appearance of a peak at 638 cm−1 for 5 wt.% LiCF3SO3 could be attributed to triflate ion in the pure LiCF3SO3 salt as shown in Figure 4(b)(ii) because it is understood that the vibrational spectrum of triflate ion is very sensitive to its state of coordination, producing a series of different FTIR active bands as its environment changes . It can been seen that, with increasing of the LiCF3SO3 content, intensities of the region around 635 cm−1 have become higher compared to the intensity of the peak for 5 wt.% LiCF3SO3. For O-H band, the wavenumber has shifted to the higher wavenumber values as depicted in Figure 4(a). Addition of 10 wt.% LiCF3SO3 to the 90 wt.% PEO-10 wt.% GO blend has shifted the value number from 3223 cm−1 to 3434 cm−1. Upon addition of 40 wt.% salt, the O-H band shifts to lower wavenumber of 3468 cm−1 indicating a decrease in conductivity of the electrolyte. This result indicates that more salt aggregates have been formed which limit the conduction of ions. The increased wavenumber suggested that interaction in polymer blend with doping salt has occurred. Previous work by Karan et al.  proposed the coordination of the Li ions to the oxygen atoms of polymer backbone of PEO due to an increase in the glass transition temperature of the PEO-LiCF3SO3 electrolytes with increasing Li salt content. Another work by Yadav et al.  on graphene oxide/carboxymethylcellulose/alginate (GO/CMC/Alg) composite blends stated that there are hydrogen bonding-type interactions due to the occurrence of such shifting in the FTIR spectra. The hydrophilic oxygenated functional groups on the surface or at the edge of the GO sheets played a critical role in improving the compatibility between GO and the polymer matrix.
Figure 5 represents the FTIR spectra of selected sample from PEO-GO-LiCF3SO3-ES system in the region of O-H band at 3400–3550 cm−1. From salted system, the O-H band appearing at 3456 cm−1 has shifted to 3461 cm−1 with incorporation of 0.2 wt.% ES. The increase in wavenumber for each ES loading suggested that complexation of ES into the PEO-GO-LiCF3SO3 electrolyte system occurred. From Figure 5(b), the C=O band appearing at 1645 cm−1 shifts to lower wavenumber of 1641 cm−1 with incorporation of 0.2 wt.% ES and the same peak can be observed for addition of 0.4 wt.% ES–1.4 wt.% ES in the polymer blend. The peak shape also has become broad and less intense compared to the peak without addition of ES. Bhatt and O’Dwyer  used computational studies to predict theoretical FTIR spectra on ES as additives in propylene carbonate-based Li-ion battery electrolyte in which wavenumber of C=O of propylene carbonate increases as a result of a significant change in the bond length due to the interaction of ES on C=O frequency. This demonstrates that changes in wavenumber could be possibly due to interaction between ES and PEO-GO-LiCF3SO3 system that alter the bond length of C=O of the polymer host.
3.3. Conductivity Analysis
The effect of LiCF3SO3 concentration on room temperature conductivity is represented in Figure 6(a). The conductivity of pure PEO-GO film at room temperature is () × 10−11 S cm−1, which is relatively low since no mobile ions are provided within the film. On addition of 5 wt.% LiCF3SO3, the conductivity increases to () × 10−9 S cm−1. The conductivity further increases to () × 10−6 S cm−1 on addition of 25 wt.% LiCF3SO3. The conductivity of PEO-LiCF3SO3 electrolyte is reported to be in the order of ~10−6 S cm−1 [6, 27, 36]. The conductivity decreases to () × 10−7 S cm−1 with the addition of 30 wt.% LiCF3SO3. The conductivity further decreases as salt concentration increases to 40 wt.%. This phenomenon can be attributed to the influence of the ion pairs and higher ion aggregations, reducing the overall mobility and degree of freedom, hence decreasing the conductivity .
Apart from polymer blending, plasticization can also assist the conductivity enhancement by weakening the Columbic force between anions and cations of salt to promote salt dissociation. Since the 25 wt.% LiCF3SO3 added electrolyte obtained the highest conductivity in salted system, the electrolyte was incorporated with different amounts of ES to enhance the conductivity. From Figure 6(b), the conductivity increases to () × 10−5 S cm−1 upon addition of 1 wt.% ES. The incorporation of ES to the electrolyte assists the production of free mobile ions and lowered the viscosity of the electrolyte . More free mobile ions are produced with addition of ES because the presence of plasticizer further weakens the Columbic force between cations and anions of the LiCF3SO3 salt, hence promoting more salts dissociation [22, 38, 39]. The use of ES in PEO-KOH films has been reported to increase the number density of mobile ions indicating that ES has dissociated more salts into ions and thus elevated the conductivity of SPE system . The presence of plasticizer also creates alternative pathways for ions conduction which lead to conductivity enhancement . This is because ES creates network formation through oxygen atoms in its molecules and thus increases the conductivity of the electrolyte system at room temperature.
3.4. Dielectric Studies
Dielectric study is useful in revealing the conductivity behavior of polymer electrolyte. This study gives an important insight into the polarization effect at the electrode/electrolyte interface and further understanding in conductivity trend . The conductivity trend of the salted system and plasticized system of PEO-GO polymer host can be further verified by dielectric studies. Dielectric constant is representative of stored charge in a material while dielectric loss is a measure of energy losses to move ions when the polarity of electric field reverses rapidly [43, 44]. The frequency dependence of and at room temperature for salted system (PEO-GO-LiCF3SO3) and plasticized system (PEO-GO-LiCF3SO3-ES) is shown in Figures 7(a) and 7(b) and Figures 8(a) and 8(b), respectively. The equations for the dielectric constant and dielectric loss are as follows:where and are the real and imaginary parts of the impedance, is angular frequency, and is the vacuum capacitance. Based on the frequency dependence of real and imaginary parts of the for both salted and plasticized system, there are no appreciable relaxation peaks observed in the frequency range use in this study. No relaxation peaks are observed signifying that the increase in conductivity is mainly due to the increase in number density of mobile ions . The rises sharply towards low frequencies for salted system and selected sample of plasticized system (Figure 7). This is due probably to the electrode polarization effects . At high frequencies, the periodic reversal of the electric field occurs so fast that there is no excess ion diffusion in the direction of the field. The polarization due to the charge accumulation decreased, leading to the observed decrease in the value of at high frequencies as reported by Ramesh et al. on PEO-LiCF3SO3 system . Based on Figure 8(a) of salted system, the higher value of dielectric loss at low frequency is due to the free charge motion within material and also addition on the plasticizer increases in the lower frequency region which reflects the enhancement of mobility charge carrier . From Figure 8(b), first decreases with rise in frequency in low frequency region followed by a peak in the loss spectra. The appearance of peak is attributed to the relaxation phenomena of polymer (motion of salt free chain segment) . Similar types of observations have been reported in the literature .
The plot of loss tangent (tan ) as a function of frequency can obtain the information of the relaxation phenomena. The value of tan was calculated using
The frequency dependence of tan for selected electrolyte samples in salted and plasticized systems at room temperature is shown in Figures 9(a) and 9(b), respectively. The maximum of tan (), which represents the relaxation peak, is located at higher frequency for higher conducting electrolyte. The relaxation time for each electrolyte was obtained from the relationwhere is the angular frequency of the relaxation peak. The occurrence of relaxation time is the result of the efforts carried out by ionic charge carriers to obey the change in the direction of the applied field . The values of for the salted system and selected samples of plasticized systems are shown in Tables 1 and 2, respectively. From Table 1, the highest conducting electrolyte with incorporation of 25 wt.% LiCF3SO3 possesses the lowest value of 7.23 × 10−6 s. In plasticized system, value is observed to decrease to 2.41 × 10−6 s as ES content increases to 1 wt.% as shown in Table 2. Other reports also show that the higher conducting electrolytes have the lower values of [48, 49].
Electrolyte systems based on PEO-GO blend doped with LiCF3SO3 have been successfully prepared via solution cast technique. Based on the XRD results, the less crystallinity was obtained using 90 wt.% of PEO and 10 wt.% of GO as polymer host. In salted system, highest conductivity of the PEO-GO blend polymer was achieved utilizing 25 wt.% LiCF3SO3 with the value of () × 10−6 S cm−1 at room temperature. In plasticized system, the addition of 1.0 wt.% ES has further enhanced the conductivity to () × 10−5 S cm−1. The relaxation time of the electrolytes is found to decrease with increasing conductivity at room temperature for both salted and plasticized system. From FTIR analysis, the complexation between the electrolyte components is proven by the shifting of O-H band at 3000–3700 cm−1, C-H stretch at 2800–2900 cm−1, CF3 vibration at 600–700 cm−1, and C=O band at 1500–1700 cm−1.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank the University of Malaya for the financial support (UMRG: RP010B-13AFR).
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