Letter pubs.acs.org/NanoLett
High Ion Conducting Polymer Nanocomposite Electrolytes Using Hybrid Nanofillers Changyu Tang,†,‡ Ken Hackenberg,‡ Qiang Fu,† Pulickel M. Ajayan,‡ and Haleh Ardebili*,§ †
Department of Polymer Science and Materials, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, China ‡ Mechanical Engineering and Materials Science Department, Rice University, Houston, Texas 77251-1892, United States § Mechanical Engineering Department, University of Houston, Houston, Texas 77004, United States S Supporting Information *
ABSTRACT: There is a growing shift from liquid electrolytes toward solid polymer electrolytes, in energy storage devices, due to the many advantages of the latter such as enhanced safety, flexibility, and manufacturability. The main issue with polymer electrolytes is their lower ionic conductivity compared to that of liquid electrolytes. Nanoscale fillers such as silica and alumina nanoparticles are known to enhance the ionic conductivity of polymer electrolytes. Although carbon nanotubes have been used as fillers for polymers in various applications, they have not yet been used in polymer electrolytes as they are conductive and can pose the risk of electrical shorting. In this study, we show that nanotubes can be packaged within insulating clay layers to form effective 3D nanofillers. We show that such hybrid nanofillers increase the lithium ion conductivity of PEO electrolyte by almost 2 orders of magnitude. Furthermore, significant improvement in mechanical properties were observed where only 5 wt % addition of the filler led to 160% increase in the tensile strength of the polymer. This new approach of embedding conducting−insulating hybrid nanofillers could lead to the development of a new generation of polymer nanocomposite electrolytes with high ion conductivity and improved mechanical properties. KEYWORDS: Polymer electrolyte, ion conductivity, hybrid nanofillers, lithium ion batteries
S
can potentially find low-energy paths along the filler/matrix interface.26 The strong affinity between the CNT’s rich electron cloud and positive lithium ions can further facilitate lithium salt dissociation and ion transport through the polymer electrolyte. However, the electrical conductivity of CNTs and the risk of battery shorting have restricted the application of CNTs in the field of polymer electrolytes. In this study, we have successfully grown one-dimensional CNTs within two-dimensional montmorillonite clay platelets to create hybrid 3D nanofillers27−29 and have shown that the ion conductivity and mechanical properties of the hybrid nanofilled polymer electrolyte are significantly enhanced. Previous studies on the untreated clays embedded in polymer electrolyte22,23 and studies involving polymer electrolyte intercalated in synthetic lithium hectorite (SLH) clay layers24 have shown improvement in ion conductivity of the polymer electrolyte. The growth of CNTs within clay interlayers leads to partial exfoliation of clay platelets and the formation of a hybrid filler with high aspect ratio. Furthermore, electron conduction in CNTs is blocked by clay platelets that are attached to the CNTs, eliminating the risk of electrical shorting. Therefore,
olid polymer electrolytes (SPEs) such as poly(ethylene oxide) (PEO) are promising candidates for replacing liquid electrolytes in lithium ion batteries. SPEs are flexible and can conform to any battery shape; they eliminate the need for a separator and can significantly improve the safety and stability of batteries due to their nonleakage and nonreactive characteristics.1−3 However, the ion conductivities of polymers can be significantly lower than that of liquid electrolytes.4−6 To overcome this problem, several techniques have been attempted such as increasing the amount of lithium salt and adding low-molecular-weight liquid plasticizers such as ethylene carbonate and propylene carbonate into the polymer matrix.7−9 These efforts have been successful in further increasing the ion conductivity but have also led to compromises in mechanical properties since the plasticized polymer chains become more flexible and amorphous, leading to reduction in mechanical stability and strength of the polymer electrolyte. Nanocomposite technology has offered a breakthrough to the above dilemma.10 Researchers have reported that the addition of nanoscale ceramic fillers (i.e., SiO2, Al2O3, and TiO2)10−21 and clays22−24 into polymer electrolytes not only improved the ionic conductivity but also enhanced the mechanical strength and stability of the polymer electrolyte. Carbon nanotubes (CNTs) are considered as ideal mechanical reinforcement for polymer matrix due to their high aspect ratio and excellent strength and modulus.25 Also, the lithium ions © 2012 American Chemical Society
Received: August 3, 2011 Revised: February 7, 2012 Published: February 27, 2012 1152
dx.doi.org/10.1021/nl202692y | Nano Lett. 2012, 12, 1152−1156
Nano Letters
Letter
Figure 1. (a) Schematics of montmorillonite clay−CNT hybrid fillers. (b−d) SEM images of clay−CNT structures at various magnifications.
Figure 2. (a) Fraction of dissociated salt ions (ClO4− anions) based on FTIR analysis of pure and filled PEO electrolyte. (b) Schematics of the interactions between clay, carbon nanotubes, polymer chains, and lithium salt ions.
XRD peaks indicate ordered stacked-layer structures,26 and for clay−CNT hybrid, the characteristic peaks of clay disappear, indicating that the growth of CNTs among clays can lead to the disruption of the ordered structure of clay. The strong characteristic XRD peaks of PEO indicate the presence of crystalline regions in the polymer, and the broadened characteristic peaks of clay observed in both types of PEO composites can be attributed to the formation of intercalated structure of clay.30,31 With increasing nanofiller content (clay and clay−CNT) in the polymer electrolyte, the crystalline peaks become broader and weaker, suggesting nanofillers suppress the crystallization of PEO. In the case of clay−CNTfilled PEO, the CNTs attached to clay surface can entangle with PEO chains from different directions, further facilitating the suppression of the PEO crystallization.
CNTs hybridized within insulating ceramic clays can open the door to the utilization of CNTs in polymer electrolytes for enhancement of various critical properties. The preparation techniques for the montmorillonite clay− CNT hybrid fillers and PEO nanocomposites are described in the Methods section. Figure 1a shows the schematics of the 3D hybrid clay−CNT structure. Figure 1b−d shows the scanning electron microscopy (SEM) images of clay−CNT hybrid. The CNTs are randomly attached to the clay surfaces and platelets and thus are electrically insulated. The SEM images of PEO nanocomposites are shown in Supporting Information Figures 1a−c and 2a−c (UV etched). The X-ray diffractions of the clay and clay−CNT hybrids, PEO electrolyte, and its composites are included in the Supporting Information Figures 3 and 4. For pristine clay, the 1153
dx.doi.org/10.1021/nl202692y | Nano Lett. 2012, 12, 1152−1156
Nano Letters
Letter
The Fourier transform infrared (FTIR) spectra were used to study the ion−ion interaction in the polymer electrolyte with lithium perchlorate salt (LiClO4). The ClO4− in PEO generally shows two bands in the spectra:32−34 one band located in the range of 620−624 cm−1 is assigned to ClO4− “free” ion vibration, and the other one located in the range of 630−635 cm−1 is associated with the vibration of Li+ClO4− contact-ion pairs. The fraction of “free” ions and ion pairs can be calculated by integrating the area under the two peaks. The spectroscopically “free” ions are believed to be responsible for ionic charge transport in polymer electrolyte. They can be temporarily complexed or attached to the polymer chain (Li+ can attach to oxygen atoms on PEO) during segmental motion of the polymer and then hop to the next site. The fraction of “free” ions will indicate the effectiveness of various electrolyte components/fillers in increasing charge concentration and subsequent ion conduction. Figure 2a shows the fraction of free ClO4− ions calculated from FTIR spectra for pure PEO and its composites. The FTIR spectra are included in Supporting Information Figures 5−10. The dissociated lithium salt ions in pure PEO count for 82.2%. With incorporation of nanofillers (clay and clay−CNT) the free ions increase. The addition of clay show limited effect on the dissociation of lithium salt. However, adding clay−CNT hybrids into PEO electrolyte can result in increase of free ions, and a maximum of 95.7% is reached in 10% clay−CNT PEO composite. This indicates that clay−CNT 3D hybrids can more effectively facilitate the dissociation of lithium salt by weakening the interaction between contact-ion pairs. Figure 2b depicts the schematics of the interaction between clay, carbon nanotubes, polymer chains, and lithium salt. Positive lithium ions can exhibit strong affinity to the high negative charge of the electron cloud in the outer surface of CNTs as well as the negative oxygen atoms on the clay, which can result in the separation of the contact-ion pairs.34 Furthermore, the polymer chains can be unfolded, leading to increase in free volume due to the high aspect ratio of the 3D hybrid nanoscale fillers. Increase in free volume has been associated with higher mobility of ions due to least resistance paths in the polymer. Generally, the ionic conductivity of electrolyte σ relates to the number of the charge carriers (ni), ionic charge (zi), and their mobility (μi) in the electrolyte defined as follows:1 σ=
Figure 3. (a) Ionic conductivity of pure PEO and composites at varying temperatures. (b) Ion conductivity of PEO composites at 25 °C with varying hybrid filler contents.
electrolytes increases with the clay−CNT content, and the maximum conductivity (2.07 × 10−5 S/cm) is reached at 10% clay−CNT. The combination of salt dissociation and ion mobility enhancements appears to be highly effective in overall increase of almost 2 orders of magnitude in ion conductivity. Upon further increase in clay−CNT content above 10%, the ionic conductivity shows a slight decrease, indicating potential aggregation of hybrid fillers impeding the lithium ion transport in PEO.12 Figure 4a,b shows the mechanical properties of PEO electrolyte composites with clay and clay−CNT hybrid fillers. In Figure 4a, the tensile strength of pure PEO electrolyte is 0.37 MPa. By comparing the composites with clay and clay−CNT, it is found that the clay−CNT hybrid has much better reinforcing
∑ niziμi
The enhanced dissociation of salt induced by clay−CNT hybrids and the subsequent increase of the number of charge carriers can directly improve ionic conductivity in the PEO electrolyte. Furthermore, the embedded hybrid nanofillers can decrease the crystallinity of PEO and increase free volume, leading to higher ion mobility and ion conductivity of the electrolyte. Figure 3a shows the ion conductivities of pure PEO electrolyte and various clay and clay−CNT composites with Li salt as a function of temperature; 10% clay−CNT composite exhibits the highest conductivity value. We also performed impedance spectroscopy of the PEO filled with clay−CNT without Li salt and found resistance values above 108 ohm, indicating good electrical insulation (Supporting Information Figures 11 and 12). Figure 3b shows the ionic conductivity of PEO electrolytes with various clay−CNT contents at room temperature (25 °C). The ionic conductivity of PEO composite
Figure 4. (a) Tensile strength with varying hybrid filler content. (b, inset) Stress−strain curves of pure PEO and composites with varying hybrid filler content. 1154
dx.doi.org/10.1021/nl202692y | Nano Lett. 2012, 12, 1152−1156
Nano Letters
Letter
room temperature, and the mixture was subjected to 30 min of sonication. Gel permeation chromatography (GPC) was performed to examine the effect of sonication on PEO chains. GPC results (Supporting Information Figure 13 and Table 1) indicate slight reduction of Mw; however, complex impedance tests before and after sonication show that the plasticizing effect on PEO ion conductivity is negligible. Upon formation of homogeneous solution, the temperature of above mixture was raised to 80 °C with continuous stirring to obtain viscous solution by evaporating excessive solvent. Subsequently, the homogeneous mixture solution was vacuumed for 1 h at room temperature and dried at 50 °C under vacuum for overnight to obtain a film with a thickness of 230−260 μm. The nanocomposite films with clay only were also prepared by similar procedure as stated above. All samples were stored in a desiccator under vacuum for further characterization. Wide-angle X-ray diffraction (XRD) pattern of the samples were obtained by Rigaku D/Max diffractometer with Cu Kα radiation (λ = 0.154 18 nm) under a voltage of 40 kV and a current of 40 mA. Samples were scanned over the range of diffraction angle 2θ = 2°−40°, with a scan speed of 3°/min at room temperature. The morphologies of powder and polymer films were observed by scanning electron microscopy (SEM) using FEI Quanta 400 under an acceleration voltage of 20 kV. All samples were coated with gold to increase the electrical conduction. Ionic conductivity was determined using the complex impedance method in the temperature range 23−90 °C. The film samples were sandwiched between stainless steel electrode disks and placed in a temperature-controlled oven. The impedance measurements were carried out with Autolab in the frequency range of 1 Hz−1 MHz. The mechanical properties of the samples were measured on an Instron Universal Testing Machine E3000 at room temperature with gauge length of 20 mm and a tensile rate of 5 mm/min according to ASTM D882. The specimen dimension was 60.00 mm in length and 8.00 mm in width. Four parallel measurements were carried out for each sample.
effect for PEO electrolyte than clay. The incorporation of 5% clay−CNT hybrids leads to 160% increase in tensile strength of composite compared to pure PEO electrolyte. This can be due to the clay−CNT hybrid filler with larger aspect ratio and rough surface, wrapped well with the polymer chains, leading to strong interface between nanofillers and polymer. The tensile strength of PEO nanocomposite increases with the clay−CNT content. At 10% clay−CNT content, a combination of relatively high tensile strength and elongation is achieved. This percentage corresponds to the maximum ion conductivity in Figure 3b, suggesting that 10% clay−CNT content may offer optimum combination of mechanical properties and ion conductivity enhancements. In summary, we have grown and insulated CNTs within clay platelets and created 3D hybrid CNT/clay nanofillers. The lithium ion conductivity of PEO nanocomposite electrolyte with hybrid fillers is enhanced by almost 2 orders of magnitude. We have also observed significant enhancement in mechanical properties of the hybrid-filled PEO electrolyte up to 160% increase in tensile strength. The future trend of energy storage devices (e.g., lithium ion batteries and supercapacitors) is toward polymer-based technologies due to manufacturability, multifunctionality, and safety and flexibility of polymers and their nanocomposites. This new approach of embedding conducting−insulating hybrid nanofillers could lead to the development of a new generation of polymer nanocomposite electrolytes with high ion conductivity and improved mechanical properties. Methods. Fe(NO3)3·9H2O (>99%) was purchased from Fisher Scientific. Clay (sodium montmorillonite) with a cationic exchange capacity (CEC) of 96 mequiv/100 g was purchased from Southern Clay Products, Inc. (Gonzales, TX). Sulfuric acid (98%) and nitric acid (65%) were purchased from Fisher Scientific. Twelve grams of Fe(NO3)3 was first dissolved in 100 mL of distilled water. Eight grams of montmorillonite was suspended in 200 mL of distilled water and then mixed with the above Fe(NO3)3 solution in a 500 mL round-bottom flask, followed by vigorous stirring and refluxing in a oil bath overnight (12 h). The resulting mixed suspension was filtered with cellulose paper and washed with DI water many times until the filtrate became fully transparent. The residue water in wet clay was exchanged with ethanol. Next, the clay was exposed to air to naturally dry and was then ground into fine powder. Finally, the treated clay powder was calcined at 450 °C for 4 h to become the clay-supported iron oxide. The above powder was then ground in mortar and transferred into a ceramic boat. The boat with treated powder was placed into the center of quartz tube. The temperature of quartz tube was raised to 775 °C with an atmosphere of argon and hydrogen (80:20) at a flow rate of 1300 mL/min. The ethylene (100 mL/min) and water vapor were introduced into the tube to allow nanotube growth for 20 min. The black products were put into the mixture of nitric acid and sulfuric acid and stirred for 6 h to remove the catalysts. The purified clay−CNT hybrids were obtained after many times of washing with water followed by vacuumed drying. PEO (average Mw = 100 000), lithium perchlorate (battery grade), and acetonitrile were purchased from Sigma-Aldrich. The molar ratios of O/Li in all samples were kept as 16:1. Desired amounts of clay−CNT hybrid and lithium salt (LiClO4) were added into 30 mL of acetonitrile and sonicated for 30 min to form a uniform suspension. Then, PEO powder was added into the suspension following stirring for 5 h at
■
ASSOCIATED CONTENT
S Supporting Information *
Table 1 and Figures 1−13. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS C.T. acknowledges financial support from National Natural Science Foundation of China (51103141) and CAEP fund (2011B0302053). H.A. acknowledges partial support from TcSUH seed funding.
■
REFERENCES
(1) Meyer, W. H. Polymer electrolytes for lithium-ion batteries. Adv. Mater. 1998, 10, 439−448. (2) Armand, M.; Tarascon, J. M. Building better batteries. Nature 2008, 451, 652−657. 1155
dx.doi.org/10.1021/nl202692y | Nano Lett. 2012, 12, 1152−1156
Nano Letters
Letter
(3) Dias, F. B.; Plomp, L.; Veldhuis, J. B. J. Trends in polymer electrolytes for secondary lithium batteries. J. Power Sources 2000, 88, 169−191. (4) Armand, M. B. Polymer electrolytes. Annu. Rev. Mater. Sci. 1986, 16, 245−61. (5) Tarascon, J. M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359−367. (6) Angell, A. A.; Liu, C.; Sanchez, E. Rubbery solid electrolytes with dominant cationic transport and high ambient conductivity. Nature 1993, 362, 137−139. (7) Sheldon, M. H.; Glasse, M. D.; Latham, R. J.; Linford, R. G. The effect of plasticiser on zinc polymer electrolytes. Solid State Ionics 1989, 34, 135−138. (8) Yang, X. Q.; Lee, H. S.; Hanson, L.; McBreen, J.; Okamoto, Y. Development of a new plasticizer for poly(ethylene oxide)-based polymer electrolyte and the investigation of their ion-pair dissociation effect. J. Power Sources 1995, 54, 198−204. (9) Lee, H. S.; Yang, X. Q.; McBreen, J.; Xu, Z. S.; Skotheim, T. A.; Okamoto, Y. Ionic conductivity of a polymer electrolyte with modified carbonate as a plasticizer for poly(ethylene oxide). J. Electrochem. Soc. 1994, 141, 886−889. (10) Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nanocomposite polymer electrolytes for lithium batteries. Nature 1998, 394, 456−458. (11) Stephan, A. M.; Nahm, K. S. Review on composite polymer electrolytes for lithium batteries. Polymer 2006, 47, 5952−5964. (12) Baxter, J.; et al. Nanoscale design to enable the revolution in renewable energy. Energy Environ. Sci. 2009, 2, 559−588. (13) Ji, K. S.; Moon, H. S.; Kim, J. W.; Park, J. W. Role of functional nano-sized inorganic fillers in poly(ethylene) oxide-based polymer electrolytes. J. Power Sources 2003, 117, 124−130. (14) Ahn, J. H.; Wang, G. X.; Liu, H. K.; Dou, S. X. Nanoparticledispersed PEO polymer electrolytes for Li batteries. J. Power Sources 2003, 119−121, 422−426. (15) Weston, J. E.; Steele, B. C. H. Effects of inert fillers on the mechanical and electrochemical properties of lithium salt-poly(ethylene oxide) polymer electrolytes. Solid State Ionics 1982, 7, 75− 79. (16) Sun, H. Y.; Sohn, H. J.; Yamamoto, O.; Takeda, Y.; Imanishi, N. Enhanced lithium-ion transport in PEO-based composite polymer electrolytes with ferroelectric BaTiO3. J. Electrochem. Soc. 1999, 146, 1672−1676. (17) Byrne, M. T.; Gun’ko, Y. K. Recent advances in research on carbon nanotube-polymer composites. Adv. Mater. 2010, 22, 1672− 1688. (18) Walls, H. J.; Zhou, J.; Yerian, J. A.; Fedkiw, P. S.; Khan, S. A.; Stowe, M. K.; Baker, G. L. Fumed silica-based composite polymer electrolytes: synthesis, rheology, and electrochemistry. Power Sources 2000, 89, 156−162. (19) Forsyth, M.; MacFarlane, D. R.; Best, A.; Adebahr, J.; Jacobsson, P.; Hill, A. J. The effect of nano-particle TiO2 fillers on structure and transport in polymer electrolytes. Solid State Ionics 2002, 147, 203− 211. (20) Zhou, J.; Fedkiw, P. S. Ionic conductivity of composite electrolytes based on oligo(ethylene oxide) and fumed oxides. Solid State Ionics 2004, 166, 275−293. (21) Fan, J.; Raghavan, S. R.; Yu, X. Y.; Khan, S. A.; Fedkiw, P. S.; Hou, J.; Baker, G. L. Composite polymer electrolytes using surfacemodified fumed silicas: conductivity and rheology. Solid State Ionics 1998, 111, 117−123. (22) Chen, H. W.; Chiu, C. Y.; Wu, H. D.; Shen, I. W.; Chang, F. C. Solid-state electrolyte nanocomposites based on poly(ethylene oxide), poly(oxypropylene) diamine, mineral clay and lithium perchlorate. Polymer 2002, 43, 5011−5016. (23) Kim, S.; Hwang, E. J.; Jung, Y.; Han, M.; Park, S. J. Ionic conductivity of polymeric nanocomposite electrolytes based on poly(ethylene oxide) and organo-clay materials. Colloids Surf., A 2008, 313−314, 216−219.
(24) Sandi, G.; Carrado, K. A.; Joachin, H.; Lu, W.; Prakash, J. Polymer nanocomposites for lithium battery applications. J. Power Sources 2003, 119−121, 492−496. (25) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Y. K. Small but strong: A review of the mechanical properties of carbon nanotubepolymer composites. Carbon 2006, 44, 1624−1652. (26) Udomvech, A.; Kerdcharoen, T.; Osotchan, T. First principles study of Li and Li+ adsorbed on carbon nanotube: Variation of tubule diameter and length. Chem. Phys. Lett. 2005, 406, 161−166. (27) Zhang, W. D.; Phang, I. Y.; Liu, T. X. Growth of carbon nanotubes on clay: Unique nanostructured filler for high-performance polymer nanocomposites. Adv. Mater. 2006, 18, 73−77. (28) Wang, Z.; Xu, C.; Zhao, Y.; Zhao, D.; Wang, Z.; Li, H.; Lau, K. T. Fabrication and mechanical properties of exfoliated clay−CNTs/ epoxy nanocomposites. Mater. Sci. Eng., A 2008, 490, 481−487. (29) Zhao, Y. Q.; Lau, K. T.; Wang, Z.; Wang, Z. C.; Cheung, H. Y.; Yang, Z.; Li, H. L. Fabrication and properties of clay-supported carbon nanotube/poly (vinyl alcohol) nanocomposites. Polym. Compos. 2009, 30, 702−707. (30) Ray, S. S.; Okamoto, M. Polymer/layered silicate nanocomposites: A review from preparation to processing. Prog. Polym. Sci. 2003, 28, 1539−1641. (31) Yue, R.; Niu, Y.; Wang, Z.; Douglas, J. F.; Zhu, X.; Chen, E. Suppression of cystallization in a plastic crystal electrolyte (SN/ LiClO4) by a polymeric additive (polyethyleneoxide) for battery applications. Polymer 2009, 50, 1288−1296. (32) Wieczorek, W.; Raducha, D.; Zalewska, A.; Stevens, J. Effect of salt concentration on the conductivity of PEO-based composite polymeric electrolytes. J. Phys. Chem. B 1998, 102, 8725−8731. (33) Shriver, D. F.; Papke, B. L.; Ratner, M. A.; Dupon, R.; Wong, T.; Brodwin, M. Structure and ion transport in polymer-salt complexes. Solid State Ionics. 1981, 5, 83−88. (34) Sim, L. H.; Gan, S. N.; Chan, C. H.; Yahya, R. ATR-FTIR studies on ion interaction of lithium perchlorate in polyacrylate/ poly(ethylene oxide) blends. Spectrochim. Acta, Part A 2010, 76, 287− 292.
1156
dx.doi.org/10.1021/nl202692y | Nano Lett. 2012, 12, 1152−1156
