ARTICLE pubs.acs.org/JPCC
Ionic Conductivity Enhancement of Polyethylene Oxide-LiClO4 Electrolyte by Adding Functionalized Multi-Walled Carbon Nanotubes Dan Zhou,† Xiaoguang Mei,† and Jianyong Ouyang* Department of Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 ABSTRACT: Polymer electrolytes are needed in many solid-state electronic and energy devices. But polymer electrolytes usually have a low ionic conductivity. This work reports the enhancement in the ionic conductivity of polyethylene oxide (PEO)-LiClO4 electrolyte by adding functionalized multiwalled carbon nanotubes (MWCNTs). MWCNTs are functionalized with carboxylic groups through oxidation with acids. They can be dispersed in acetonitrile solutions of PEO and LiClO4. The presence of functionalized MWCNTs can effectively enhance the ionic conductivity of the PEO-LiClO4 electrolyte, and the ionic conductivity enhancement depends on the loading of the functionalized MWCNTs. Enhancement by a factor of 3.3 was observed. The enhancement in the ionic conductivity is attributed to the functionalized MWCNT-induced decrease in the crystallinity of PEO and increase in the salt dissociation due to the Lewis acidbase interaction of the functionalized MWCNTs with PEO and LiClO4. The addition of functionalized MWCNTs can also effectively improve the mechanical properties of PEO films.
’ INTRODUCTION Many electronic and energy devices, including light-emitting electrochemical cells, lithium batteries, dye-sensitized solar cells, supercapacitors, and electrochromic windows, involve the ionic transport through electrolyte.17 Though water and organic solvents are traditionally used as the solvents in electrolytes, they have problems of volatility and leakage. These problems may be overcome by using polymer electrolytes in which polymers are exploited to replace the liquid solvents. Solid polymer electrolytes can give rise to solid-state energy devices. The ionic conductivity of polymer electrolytes is affected by the salt dissociation and the ionic transport in polymers. Polyethylene oxide (PEO) is the most popular polymer in polymer electrolytes due to its high capability in forming complexes with salts and its chemical stability.8 The complex formation of PEO with ions, particularly Li+, facilitates the salt dissociation and the ion transport. However, PEO tends to crystallize due to its highly ordered chain structure, which impedes the ion transport, so that the ionic conductivity of PEO electrolytes is far below that of liquid electrolytes. This causes a high internal resistance in the electronic and energy devices and lowers the device performance. It is important to significantly improve the ionic conductivity of polymer electrolytes for high-performance solid energy devices. One strategy is to reduce the crystallinity of PEO. The early effort is to reduce the crystallinity by introducing plasticizers, such as ethylene carbonate, propylene carbonate and tetraethylene glycol, into polymer electrolytes.9,10 However, these plasticizers can bring problems of not only poor mechanical properties but also interfacial instability.11 Another strategy reported recently is to blend ceramic nanoparticles, such as TiO2, SiO2, Al2O3, ZnO, and LiAlO2, into polymer electrolytes.1225 These nanoparticles r 2011 American Chemical Society
have a large surface area and a lot of functional groups such as COOH and OH on the surface. These functional groups of nanoparticles can interact with the polymers and salts through the Lewis acidbase interactions, which can reduce the crystallinity of the polymers and promote the disassociation of the salts into free ions, and thus contribute to the ionic conductivity enhancement.26 In addition, the ceramic nanoparticles can also increase the mechanical strength. Other nanoparticles, such as carbon powders,27 clays,28 or modified montmorillonite29 were also used as additives to improve the ionic conductivity of polymer electrolytes. The mechanism for the ionic conductivity enhancement may be similar to that by ceramic nanoparticles. In this work, we report the improvement of the ionic conductivity of the PEO-LiClO4 electrolyte by blending oxidized multiwalled carbon nanotubes (oMWCNTs) that are functionalized with COOH groups on the outside. MWCNTs are chosen because they are one-dimensional nanometer materials and have excellent mechanical strength while low density.3034 When oMWCNTs are blended with polymers, the polymer chains wrap the MWCNTs. This can decrease the crystallinity of the polymers. Although composites of MWCNTs with polymers have been extensively investigated and MWCNTs have even been used with poly(vinylidene fluorideco-hexafluoropropylene) (PVdF-HFP)35 and a blend of poly(vinyl acetate) and PVdFHFP36 to form gel electrolytes, they have not been used as an additive to improve the ionic conductivity of solid polymer electrolytes. We found that blending oMWCNTs into a Received: April 7, 2011 Revised: July 11, 2011 Published: July 17, 2011 16688
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PEO/LiClO4 solid electrolyte can increase the ionic conductivity by a factor of 3.3.
’ EXPERIMENTAL SECTION Materials. Pristine MWCNTs without functionalization were purchased from Chengdu Organic Chemicals Co. Ltd. in China. They had an outer diameter of 815 nm and a length of 0.52 μm. PEO (Mw = 100 000 g mol1), lithium perchlorate (LiClO4), and anhydrous acetonitrile were obtained from Sigma-Aldrich. HNO3, H2SO4, and other reagents with analytical grade were purchased from Fisher Scientific Inc. Pristine MWCNTs were oxidized by refluxing in a mixture of HNO3 and H2SO4 (volume ratio = 1:3) for 6 h. After the oxidation, oMWCNTs were collected by filtration and then rinsed with deionized water. They were dried at 110 °C in vacuum overnight. Preparation of Polymer Electrolytes. oMWCNTs, PEO, and LiClO4 were dried in vacuum and kept in a glovebox filled with nitrogen prior to use. Various amounts of oMWCNTs were dispersed in acetonitrile under sonication for 60 min. PEO and LiClO4 were then added into these solutions in succession, and the obtained mixtures were vigorously stirred at 50 °C for 24 h. The molar ratio of the repeating unit of PEO, ethyleneoxide, to lithium ion was kept as 8, while the weight fraction of oMWCNTs with respect to PEO was varied. The resulting homogeneous mixtures were cast in Petri dishes and dried in vacuum for 72 h to completely remove the solvent. The dried electrolyte samples were sandwiched between two fluorinedoped tin oxide (FTO) conducting glasses in the glovebox for the ionic conductivity investigation. PEO-LiClO4 electrolytes without oMWCNTs were also prepared for comparison. Characterization of Materials. The surface and crosssectional scanning electron microscopic (SEM) images were taken with a ZEISS SUPRA 40 scanning electron microscope. Differential scanning calorimetry (DSC) of the samples was carried out using a DSC-2920 by TA Instruments. The samples were heated from 30 to 100 °C at a heating rate of 10 °C min1 under a constant flow (70 mL min1) of nitrogen gas. The Fourier transform infrared (FTIR) spectra were obtained with a VARIAN 3100 FT-IR excalibur series spectrometer. Pristine MWCNT and oMWCNT powder samples were pressed into KBr pellets for FTIR spectroscopy. PEO, PEO-LiClO4, and PEO-LiClO4oMWCNTs films for FTIR study were prepared by casting their solutions on KBr pellets and dried in vacuum. The viscosity values of acetonitrile solutions of PEO and PEO-oMWCNTs at 20 °C were measured with a Brookfield DV-III Ultra Programmable Rheometer. The X-ray diffraction (XRD) patterns were measured with a Bruker BraggBrentano theta X-ray diffractometer. The ac impedances of the cells were measured using an AUTOLAB PGSTAT 30 potentiostat/galvanostat analyzer equipped with a frequency response analyzer module. The ac amplitude was 10 mV, and the frequency range was from 100 000 to 0.1 Hz. The ionic conductivity of the polymer electrolytes were measured in an oven in the temperature range of 2080 °C. The system was thermally equilibrated at each temperature for at least 30 min before measurement. The ionic conductivity (σ) of each sample was calculated according to the equation σ = L/RbA, where Rb is the bulk resistance of the electrolyte obtained from the ac impedance, L is the thickness, and A is the area of the sample.
Figure 1. FTIR spectra of pristine MWCNTs and oMWCNTs.
The mechanical properties of PEO and PEO-oMWCNTs (1 wt %) films were measured using a LLOYD tensile testing machine at a crosshead speed of 10 mm min1 according to ASTM D882. The PEO-oMWCNTs samples were prepared by the following process. At first, oMWCNTs were dispersed in acetonitrile under sonication for 60 min. PEO was then added into the solution, and the obtained mixture was vigorously stirred at 50 °C for 24 h. Finally, the samples were prepared by drop casting the mixture and subsequently drying in vacuum for 72 h. The samples had a size of 100 10 0.2 mm. PEO samples without oMWCNTs were prepared by dropping acetonitrile solution of PEO and subsequently drying in vacuum for 72 h.
’ RESULTS AND DISCUSSION Characterization of PEO-LiClO4-oMWCNT Electrolytes. Pristine MWCNTs bundle together and cannot be directly dispersed in solution. MWCNTs functionalized with carboxylic groups can be prepared through oxidation with strong acids. Figure 1 shows the FTIR spectra of pristine MWCNTs and oMWCNTs. Two bands at 1716 and 1220 cm1 appear on the FTIR spectrum of oMWCNTs while they are absent on that of pristine MWCNTs. These two bands can be assigned to the CdO and CO stretching mode, respectively. The FTIR spectra confirm the existence of COOH groups on oMWCNTs. The band at 1583 cm1, which corresponds to the CdC stretching of MWCNTs,37 becomes prominent after the oxidation. The oxidized MWCNTs (oMWCNTs) can be dispersed in various solvents, including water and polar organic solvents. Figure 2 presents the SEM images of pristine MWCNTs and oMWCNTs. Pristine MWCNTs bind together, so that they cannot be dispersed in water or normal organic solvents. The intertube interactions are reduced after the oxidation. Thus, oMWCNTs do not bind together, and they can be dispersed in water and polar organic solvents. oMWCNTs were well dispersed in acetonitrile after sonication for one hour. Then, PEO was added while stirring. Finally, LiClO4 was added. The addition sequence of PEO and LiClO4 is important for obtaining a stable dispersion of the three components. If LiClO4 is directly added into the solution of oMWCNTs prior to PEO, then oMWCNTs will precipitate. oMWCNTs do not precipitate, when LiClO4 is added after the PEO addition. Probably, PEO can interact with the carboxylic groups of oMWCNTs, which shields the interaction between oMWCNTs and LiClO4 in solution and avoids the oMWCNT precipitation. 16689
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Figure 2. SEM images of (a) pristine MWCNTs and (b) oMWCNTs.
Figure 3. Surface SEM images of (a) PEO (100 ), (b) PEO-LiClO4 (100 ), (c) PEO-LiClO4-oMWCNTs (1 wt %) (100 ), (d) PEO-LiClO4oMWCNTs (1 wt %) (10000 ), and (e) PEO-LiClO4-oMWCNTs (3 wt %) (10000 ), and cross-sectional image SEM image of (f) PEO-LiClO4oMWCNTs (1 wt %) (10000 ).
The interaction between PEO and oMWCNTs is evidenced by the oMWCNT effect on the viscosity of PEO solution. An acetonitrile solution of 5 wt % PEO had a viscosity of 12.80 mPa 3 s. The viscosity increased to 14.21 mPa 3 s after the
addition of 1 wt % oMWCNTs into the solution. This implies that oMWCNTs can cross-link PEO chains. The interaction between oMWCNTs and PEO can be attributed to the hydrogen bond between the OH groups of oMWCNTs and the oxygen atoms of PEO. 16690
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Table 1. Tm, ΔHm, and Xc of PEO, PEO-LiClO4, and PEOLiClO4-oMWCNTs with oMWCNT loadings of 0.5, 1, 3, 6, and 10 wt % Tm (°C)
ΔHm (J/g)
PEO
67.8
178.4
83.5
PEO-LiClO4
66.6
61.2
28.6
PEO-LiClO4-oMWCNTs (0.5 wt %)
62.3
24.0
11.2
PEO-LiClO4-oMWCNTs (1 wt %)
61.5
21.7
10.1
PEO-LiClO4-oMWCNTs (3 wt %) PEO-LiClO4-oMWCNTs (6 wt %)
61.2 61.9
21.6 33.1
10.1 15.4
PEO-LiClO4-oMWCNTs (10 wt %)
61.3
38.9
18.2
sample
Xc (%)
Figure 4. XRD patterns of PEO with various oMWCNT loading.
Figure 5. DSC curves of (a) PEO, (b) PEO-LiClO4 and PEO-LiClO4oMWCNTs electrolytes with (c) 0.5 wt %, (d) 1 wt %, (e) 3 wt %, (f) 6 wt %, and (g) 10 wt % of oMWCNTs.
Figure 3 presents the surface SEM images of pure PEO, PEOLiClO4, PEO-LiClO4-oMWCNTs (1 wt %), and PEO-LiClO4oMWCNTs (3 wt %) and the cross-sectional SEM image of PEO-LiClO4-oMWCNTs (1 wt %). The pure PEO has a rough surface. The domains with a diameter of tens micrometers are the PEO crystalline structure.38 The surface becomes smooth after blending of LiClO4 and oMWCNTs, and the large crystalline domains disappear. This change indicates that the presence of LiClO4 and oMWCNTs can reduce the crystallinity of PEO due to the interactions of PEO with LiClO4 and oMWCNTs. The surface and cross-sectional SEM images of PEO-LiClO4-oMWCNTs (1 wt %) suggest the homogeneous dispersion of oMWCNTs in the electrolyte. The morphology of PEO-LiClO4-oMWCNTs is affected by the oMWCNT loading. No remarkable oMWCNT bundles were observed when the oMWCNT loading was 1 wt %, while some oMWCNTs aggregated when oMWCNTs was increased to 3 wt %. Figure 4 presents the XRD patterns of PEO samples with various oMWCNT loadings. There was no LiClO4 in these samples, because LiClO4 is highly hygroscopic in ambient condition and the absorbed water affects the XRD results. The XRD
Figure 6. FTIR spectra of LiClO4, PEO, PEO-LiClO4, and PEOLiClO4-oMWCNTs (1 wt %).
patterns imply that pure PEO has a high crystallinity and the crystallinity decreases with the increasing oMWCNT loading. The effect of oMWCNTs on the crystallinity of PEO-LiClO4 was further investigated by DSC in the temperature range of 30 to 100 °C (Figure 5).39 The crystal melting temperature (Tm) and melting enthalpy (ΔHm) values of the electrolytes were obtained from the DSC curves, and degrees of crystallinity (Xc) were estimated in terms of the ratios of the experimentally obtained ΔHm values with respect to the value of 213.7 J/g that is the melting enthalpy of 100% crystalline PEO.40,41 The values of these parameters are summarized in Table 1. LiClO4 lowers Tm, ΔHm and Xc of PEO, and the addition of oMWCNT further 16691
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The Journal of Physical Chemistry C decreases the values of these three parameters. Thus, blending oMWCNTs can reduce the crystallinity of PEO-LiClO4 electrolytes, which is consistent with the XRD patterns. This can be attributed to the wrapping of oMWCNTs by PEO. oMWCNTs with a large surface area and a lot of COOH groups can disrupt the chain folding and restrict the organization of PEO chains, leading to the decrease in the crystallinity and the enhancement in the ionic conductivity. As shown in Table 1, the optimal oMWCNT loading is 1 wt % for Tm, ΔHm, and Xc. When the oMWCNT loading is 3 wt % or above, Tm, ΔHm, and Xc are higher than that with a low oMWCNT loading. This can be attributed to the aggregation of oMWCNTs in PEO-LiClO4 electrolytes at a high loading as observed in the SEM image (Figure 3e). The oMWCNT aggregation lowers their ability in reducing the crystallinity of PEO. The interactions of PEO with LiClO4 and oMWCNTs in these polymer electrolytes were further studied by the FTIR spectroscopy. Figure 6a shows the FTIR spectra of PEO, LiClO4, PEO-LiClO4, and PEO-LiClO4-oMWCNTs (1 wt %). Two frequency regions are of particular interest. One is near 1100 cm1. The triple bands at 1147, 1114, and 1060 cm1 observed on the FTIR spectrum of PEO originate from the COC symmetric and asymmetric stretchings. They confirm the presence of crystalline PEO.21,42 The triple bands become broad and merge into a single peak with the absorption maximum at 1093 cm1, when LiClO4 is added into the PEO matrix. The absorption maximum shifts to 1112 cm1 when oMWCNTs are blended into the electrolyte. Another interesting IR frequency region is near 625 cm1 (Figure 6b). The IR envelope corresponds to the ν4 vibration of ClO4. The band has an asymmetrical shape and can be devoluted into two peaks at 623 and 635 cm1.43,44 The former arises from free anions, while the latter originates from the bound or contact anions. The intensity of the peak at 635 cm1 increases after the addition of oMWCNTs. The oMWCNT effect on the FTIR spectrum is similar to that of adding acidic Al2O3 into PEO-salt electrolyte.23,25,4345 According to Wieczorek et al., the COC symmetric and asymmetric stretchings is sensitive to the PEO-LiClO4 interactions, while the ν4 vibration of ClO4 is related to the ionion interactions. They interpreted the change in the FTIR spectrum after the Al2O3 addition as the result of Lewis acidbase interaction. Acidic Al2O3 can interact with the two Lewis bases, the oxygen atoms of PEO and ClO4. The Lewis acidbase interaction model is applicable for our system, because COOH is the surface group of oMWCNT just like acidic Al2O3. The red shift of the COC stretchings of PEO caused by LiClO4 is due to the formation of the transient cross-linking complex of the Li+ with the ether oxygen of PEO, which weakens the COC stretchings and decreases the crystallinity of PEO. After the addition of oMWCNTs, the OH groups of oMWCNTs form hydrogen bond with the oxygen atoms of PEO. This hydrogen band can be regarded as a Lewis acidbase interaction as well. Thus, the polymer chains become less flexible due to the crosslinking of PEO chains through oMWCNTs. The frequencies COC stretchings shift to high frequency. The increase in the high frequency component of the ClO4 ν4 mode can be attributed to the Lewis acidbase interaction between OH groups of oMWCNTs and ClO4. This leads to the bound of the anions and the increase in the frequency. The bound of the ClO4 anions to oMWCNTs promote the dissociation of LiClO4.
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Figure 7. Ionic conductivities of PEO-LiClO4 and PEO-LiClO4oMWCNTs electrolytes with different oMWCNT loadings at 20 °C.
Figure 8. Arrhenius plot of temperature dependences of ionic conductivities of PEO-LiClO4-oMWCNTs electrolytes with different oMWCNT loadings at 2080 °C.
Other characteristic peaks of PEO also change after the addition of oMWCNTs, such as the CH2 bending at 1467 cm1, doublet CH2 swinging vibrations at 1359 and 1342 cm1, CH2 rocking and COC asymmetric stretching at 962 cm1, and CH2 asymmetric rocking at 843 cm1. The band at 1643 cm1 is due to the vibration of LiClO4.46 Ionic Conductivity of PEO-LiClO4-oMWCNTs Electrolytes. The decrease in the crystallinity of PEO and the increase in LiClO4 dissociation caused by oMWCNTs can give rise to the enhancement in the ionic conductivity of the polymer electrolytes. Figure 7 illustrates the ionic conductivities of PEO-LiClO4oMWCNTs with different oMWCNT loadings. The ionic conductivities of PEO-LiClO4-oMWCNTs electrolytes are higher than that of PEO-LiClO4. The optimal oMWCNT loading is 1 wt %, which is consistent with the DSC study. PEO-LiClO4oMWCNTs (1 wt %) has an ionic conductivity as 3.3 times as that of PEO-LiClO4. The decrease in the ionic conductivity after the further increase of oMWCNT is probably due to the oMWCNT aggregation in the electrolytes, which leads to increase of the crystallinity.47 Figure 8 presents the Arrhenius plots for the temperature dependences of the ionic conductivities of PEO-LiClO4 and PEO-LiClO4-oMWCNTs with different oMWCNT loadings in the temperature range of 2080 °C. The ionic conductivities of all the electrolytes increased with the temperature increase, and 16692
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Figure 9. VTF plot of temperature dependences of the ionic conductivities of PEO-LiClO4-oMWCNTs electrolytes with different oMWCNT loadings at 2080 °C.
PEO-LiClO4-oMWCNTs (1 wt %) always has the highest ionic conductivity at every temperature. The Arrhenius plots are not linear. The temperature (T) dependence of the ionic conductivities (σ) is replotted in terms of the VogelTammanFulcher (VTF) equation (Figure 9), σ ¼ σ0 T 1=2 exp½ Ea =RðT T0 Þ where σ0 , E a, R, and T 0 are the pre-exponential factor, the activation energy, the gas constant, and ideal glass transition temperature of the host polymer (216 K for PEO), respectively. 48 The Ea value by the VTF analysis is 8.90 kJ mol 1 for PEO-LiClO 4 , and it decreases to 7.74 kJ mol1 for PEO-LiClO 4 -oMWCNTs (1 wt %). The change in the activation energy is consistent with the oMWCNT effect on the ionic conductivity. We also tried to blend pristine MWCNTs in PEO-LiClO4 electrolyte. But MWCNTs can be dispersed neither in acetonitrile nor acetonitrile solution of PEO because the MWCNTs bind together. Pristine MWCNTs aggregate in the PEO-LiClO4 electrolyte and do not make any remarkable effect on the ionic conductivity. Mechanical Properties of PEO-oMWCNTs. Beside ionic conductivity, the mechanical properties are also very important for the application of polymer electrolytes. The superior mechanical properties of MWCNTs imply that the oMWCNTs can increase not only the ionic conductivity but also the mechanical properties of polymer electrolytes. We studied the mechanical properties of PEO and PEO-oMWCNTs (1 wt %). LiClO4 was not added in these materials, because the water absorption by the highly hygroscopic LiClO4 strongly affected the accuracy of the results. Figure 10 shows the stressstrain curves of PEO and PEO-oMWCNTs (1 wt %). The Young’s moduli of the two samples are calculated in term of their stressstrain curves at a low strain. The Young’s modulus and the tensile strength of PEO are 74 and 0.5 MPa, respectively. They increase to 474 and 3.7 MPa, respectively, after the addition of 1 wt % oMWCNTs. The increase in the mechanical properties by oMWCNTs is significantly different from that by adding plasticizers. Although plasticizers can increase the ionic conductivity, they also reduce the mechanical properties.
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Figure 10. Stressstrain curves of PEO and PEO-oMWCNTs (1 wt %) films.
’ CONCLUSIONS The ionic conductivity of PEO-LiClO4 electrolyte can be enhanced by blending oMWCNTs that are prepared by oxidation of pristine MWCNTs with acids. Enhancement by a factor of 3.3 was observed. The mechanism for the ionic conductivity enhancement was studied by FTIR, SEM, DSC, and temperature dependence of the ionic conductivities. The oMWCNT effect on the ionic conductivity is attributed to the reduction in the crystallinity of PEO and the Lewis acidbase interactions of oMWCNT with PEO and LiClO4. The Lewis acidbase interactions can reduce the crystallinity of the polymer electrolyte, lower the activation energy, and increase the salt dissociation. Consequently, the ionic conductivity is enhanced by oMWCNTs. In addition, the addition of oMWCNTs can effectively improve the mechanical property of the PEO film. ’ AUTHOR INFORMATION Corresponding Author
*Phone: 65-6516-1472; Fax: 65-6776-3604; E-mail: mseoj@ nus.edu.sg. Author Contributions †
These authors contributed equally to this work.
’ ACKNOWLEDGMENT This research work was financially supported by a research grant from the Ministry of Education, Singapore (Grant No.: R-284-000086-112). ’ REFERENCES (1) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086–1088. (2) Gadjourova, Z.; Andreev, Y. G.; Tunstall, D. P.; Bruce, P. G. Nature 2001, 412, 520–523. (3) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359–367. (4) Li, Z. H.; Xiao, Q. Z.; Zhang, P.; Zhang, H. P.; Wu, Y. P.; Van Ree, T. Funct. Mater. Lett. 2008, 1, 139–143. (5) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; van Schalkwijk, W. Nat. Mater. 2005, 4, 366–377. (6) Fan, B. H.; Mei, X. G.; Sun, K.; Ouyang, J. Y. Appl. Phys. Lett. 2008, 93, 143103. 16693
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