Effects of a Block Copolymer as Multifunctional Fillers on Ionic

Oct 11, 2010 - R. Arunkumar , Ravi Shanker Babu , M. Usha Rani , S. Kalainathan. Journal of Applied Polymer Science 2017 134 (27), 44939 ...
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J. Phys. Chem. B 2010, 114, 13637–13643

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Effects of a Block Copolymer as Multifunctional Fillers on Ionic Conductivity, Mechanical Properties, and Dimensional Stability of Solid Polymer Electrolytes Jianying Ji, Bin Li, and Wei-Hong Zhong* School of Mechanical and Materials Engineering, Washington State UniVersity, Pullman, Washington 99164, United States ReceiVed: June 23, 2010; ReVised Manuscript ReceiVed: September 14, 2010

Solid polymer electrolytes with high ionic conductivities, good mechanical properties, dimensional stability, and easy processability were obtained from poly(ethylene oxide)-block-polyethylene (PEO-b-PE)-loaded poly(ethylene oxide) (PEO)/lithium perchlorate (LiClO4). In this article, we reported that the ionic conductivity and mechanical properties were remarkably increased due to the addition of the PEO-b-PE compared to that of PEO/LiClO4 electrolyte. Scanning electron microscope (SEM), optical micrograph, and X-ray diffraction (XRD) results indicate that the addition of the copolymer, PEO-b-PE, decreased the defects of the PEO electrolyte films. Good dimensional stability was observed by dynamic rheological techniques up to 100 °C (higher than the melting point of PEO, 65 °C). A bicontinuous phase structure, that is crystalline PE domains within a matrix of PEO/salt, was proposed as the mechanism for such comprehensive enhancements in the ionic conductivity, mechanical properties, and dimensional stability obtained simultaneously in this study through a facile approach based on incorporation of a copolymer. Introduction Solid polymer electrolytes (SPEs) have attracted worldwide attention due to their unique characteristic properties wherein ions can be transported inside a polymer matrix under an external electrical field. Thus, SPEs will make it possible to fabricate thin film lithium batteries with high specific energy and specific power, safe operation, flexibility in packaging, and low cost.1–3 Compared with the other three kinds of electrolytes used in lithium ion battery, organic-carbonate-based electrolytes, ionic liquid-based electrolytes, and polymer gel electrolytes, the intrinsic problem of SPEs is their very low ionic conductivity.4–7 Many approaches have been developed to improve their ionic conductivity such as incorporation of suitable amounts of plasticizer to form a kind of plasticized polymer electrolyte.8 However, a high-performance SPE material not only should possess high ionic conductivity but also meet other property requirements, such as mechanical properties, dimensional stability, processability, and so forth. Sufficient mechanical strength, for example, is particularly important for SPE applications in a lithium ion battery to withstand internal temperature and pressure buildup during the battery operation and to eliminate the possibility of internal short circuits.9 Furthermore, high dimensional stability should be maintained during the application temperature range for longer cycling life.10 Good processability can meet the requirements of size and shape, thus offering a wide range of designs.11 One of challenging issues for SPEs is to incorporate high ionic conductivity while maintaining good mechanical properties. Unfortunately, the ion conductivity of SPEs is commonly decreased with increasing mechanical properties due to the practice of doping of ions. Both types of properties in an SPE are related to the movement of the polymer chains. The high chain mobility can lead to high ion conductivity but at the same * To whom correspondence should be addressed. Tel.: +1 509 335 7658. Fax: +1 509 335 4662. E-mail: [email protected].

time can lead to lower mechanical properties, that is enhancements in both properties are usually in conflict.12,13 Inert nanoparticulate fillers have been incorporated into polymer matrices as a means to increase mechanical stability while the ionic conductivity does not decrease. However, the dispersion of the fillers and their interaction with the polymer matrices are still critical issues for achieving quality stable SPE materials.14–19 Another approach is to synthesize new structures with low crystallinity at room temperatures, and hence with low glass transition temperatures, forming structures with graft polymer,20 block copolymer,21 and cross-linked polymer networks.22 Numerous research related to this has been reported but the outcomes are not yet satisfactory. For example, in a cross-linking polymer, increasing the cross-linking density results in an increase in mechanical strength but reduces the conductivity at the same time because of the reduction in the polymer chains’ mobility.23 Block copolymers, are usually used as matrices for SPEs. In these systems, the conducting pathways are provided by polar groups contained blocks, called conductivity blocks; whereas a nonconducting block imparting the desired mechanical properties is called a reinforcement block.24 A typical one is the PEO block contained in polymer electrolytes, in which PEO block used as a conductivity block is responsible for transporting ions, whereas the other block (such as PS block in block copolymer PEO-b-PS) is used as the reinforcement block to offer mechanical properties.25 A high percentage of PEO structure is requested for achieving high ionic conductivity. However, this can lower the mechanical properties of the electrolytes due to the lower percentage of reinforcement block. In other words, if the conducting block, PEO block, is the major component/continuous phase, the reinforcement block as a discontinuous phase will be dispersed in the continuous PEO block, and thus the mechanical properties will be lower. Similarly, if the reinforcement block is major component/ continuous phase, the conducting block PEO as discontinuous phase will be dispersed in a continuous reinforcement block, which will result in lower ionic conductivity. The final properties

10.1021/jp105816s  2010 American Chemical Society Published on Web 10/11/2010

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are primarily up to the continuous phase. Accordingly, to emphasize any one of the two phases could inevitably sacrifice the other one; it will be much harder to improve the processability simultaneously. Therefore, if one can improve ionic conductivity while improving or maintaining mechanical properties for SPEs and afford good processability by designing a bicontinuous phase structure, it will be a significant breakthrough for SPE materials development. Through our series of studies, we found a polymer electrolyte with bicontinuous phase structure that can effectively be prepared using self-assembling block copolymers, consisting of two kinds of blocks. One block is ionophilic, which is able to facilitate the transport of ions, and the other one is ionophobic, which is used to offer mechanical properties. In this study, we used poly(ethylene oxide) (PEO) as the electrolyte matrix because PEO has been found to be the most successful host material for solid polymer electrolytes. Typical double-crystalline diblock copolymer, poly(ethylene oxide)block-polyethylene (PEO-b-PE), is used as a multifunctional filler, for enhancing performances of the PEO-based electrolytes. PEO-based electrolyte with lithium perchlorate (LiClO4), PEO/ LiClO4, and two PEO-b-PE loaded electrolytes with a low and a high concentration of PEO-b-PE, PEO/PEO-b-PE/LiClO4 (6: 1:1 and 6:6:1), were prepared. Results showed that high ionic conductivities, good mechanical properties, dimensional stability and easy processability were obtained from the PEO-b-PE loaded PEO-based electrolyte. Experimental Section PEO with a molecular weight of 4 000 000 g mol-1 was purchased from Aldrich. Commercially available block copolymer, PEO-b-PE, with an average molecular weight of 1400 g/mol and ether oxide 50 wt % purchased from Aldrich) was dried at 50 °C for 4 days prior to use. Lithium perchlorate (LiClO4, Aldrich) salt was dried at 70 °C under vacuum for 24 h before use. The concentration of LiClO4 was fixed at 15 wt % for all samples. The amount of PEO-b-PE is same as the weight of LiClO4 and PEO respectively, denoted as PEO/PEOb-PE/LiClO4 (6:1:1) and PEO/PEO-b-PE/LiClO4 (6:6:1). Calculated amounts of PEO, LiClO4, and PEO-b-PE were dissolved in acetonitrile and then stirred by magnetic stirring. A homogeneous solution was casted onto a glass plate after 3 h ultrasonic treatment for deaerating. The solvent was allowed to slowly evaporate at room temperature in a hood. All of the films were put in vacuum oven at 40 °C before any tests to wipe off residue solvent traces. The ionic conductivity was measured using a Universal Dielectric Spectrometer BDS 20 in the frequency range from 10-2 to 106 Hz at room temperature. The film was sandwiched between two gold electrodes of 2 cm diameter. The input voltage (Vrms) was 1 V. The tensile strength of polymer electrolyte films was measured by an Instron Model 4466 materials tester. The electrolyte films were cut into rectangular strips, and their widths and thicknesses were measured by a micrometer before taking measurements. Ten specimens were prepared for each tensile test. All measurements were carried out at a crosshead speed of 24.5 mm min-1. The rheology experiments were performed in a stress-controlled rheometer, RDA0 at 100 °C. Frequency sweep tests were performed between 1 and 500 rad s-1. A 20 mm diameter steel parallel-plate geometry is used on all samples. Scanning electron microscope (FEI 200F) at the magnification of 1000× and optical light microscope (Olympus BX51) at the magnification of 50× were used to analyze the morphology and microstructure of films. X-ray diffraction (XRD) measurements

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Figure 1. Frequency dependence of conductivity of pure PEO and PEO-based polymer electrolyte films.

were conducted on a diffractometer Siemens D-500 with Cu-KR radiation. Results and Discussion Ionic Conductivity. Ionic conductivity is one of the most important properties for a high-performance polymer electrolyte. Figure 1 shows the plots of the conductivity variations as a function of frequency for pure PEO, PEO/LiClO4 and PEO/ PEO-b-PE/LiClO4. The conductivity of all the samples increased with increasing frequency. Generally, the ionic conductivity of an electrolyte, σ, related to the number of the charge carriers and their mobility in the electrolyte, is often defined as follows:

σ ) neµ where n, e, and µ refer to the number of charge carriers, the ionic charge, and the ionic mobility, respectively.26 The results in Figure 1 show that the ionic conductivity increased with the concentration of the copolymer. It is attributed to the increase of the n and µ values. The presence of an additional ethylene oxide (EO) group from PEO-b-PE weakens the interaction between LiClO4 and EO in PEO. The competition between the EO groups to coordinate Li+ leads to lowering in crystallinity and also likely increases the free Li+ concentration. Similar competitive interaction between composite components having the same functional groups have already been observed in PANbased polymer electrolyte composites, which is due to the increase of the dissociation of the lithium salt.27 Furthermore, the addition of the copolymer decreased the crystallinity, which enhanced the mobility of polymer segment further because of the lithium ion movement. Finally, the ionic conductivity in the PEO/PEO-b-PE/LiClO4 improved compared with PEO/ LiClO4. The maximum value of conductivity obtained from PEO/PEO-b-PE/LiClO4(6:6:1), which was more than 10 times higher than that obtained from PEO/LiClO4. Mechanical Properties. Tensile tests were conducted for the thin films first. The stress-strain curves for the polymer electrolytes are presented in Figure 2. The Young’s modulus values for all samples that are calculated from the initial slope of stress-strain curve are listed in Table 1. All the samples, pure PEO, PEO/LiClO4, PEO/PEO-b-PE/ LiClO4 (6:1:1) and PEO/PEO-b-PE/LiClO4 (6:6:1), exhibit two

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Figure 2. Stress-strain curves of pure PEO, PEO/LiClO4, PEO/PEOb-PE/LiClO4 (6:1:1), and PEO/PEO-b-PE/LiClO4 (6:6:1) electrolyte films: (a) complete curves and (b) part of the curves with the strain lower than 50%.

TABLE 1: Stress-Strain Results of Pure PEO and PEO-Based Polymer Electrolyte Films

samples PEO PEO/LiClO4 PEO/PEO-PE/ LiClO4(6:1:1) PEO/PEO-PE/ LiClO4(6:6:1)

ultimate stress (MPa)

modulus (MPa)

strain at breaking point (%)

8.35 ( 1.65 0.31 ( 0.05 1.96 ( 0.53

84.5 ( 10.3 14.5 ( 6.4 56.4 ( 4.4

360 ( 100 300 ( 200 860 ( 150

7.29 ( 1.06

114.2 ( 9.3

1140 ( 200

characteristic regions of deformation in their stress-strain curves, that is first an elastic stage: at low strains (less than 10%) the stress increases rapidly with increasing strain, and the steep initial slopes can be observed in this elastic region, which quantitatively gives an indicator for elastic modulus (part b of Figure 2); second is nonlinear stage: at high strains the stress only displays a slow increase with strain until failure occurs, reflecting the ductile property of a polymer electrolyte. For the bicomponent SPE film, PEO/LiClO4, the mechanical properties dramatically reduced when the Li ions were added into the PEO matrix, in particular, the ultimate stress and modulus are extremely lower than that of pure PEO. This sharp decrease is considered due to the reduced cross-linking density in PEO matrix caused by the interaction between lithium ions and the polymer matrix.28 These low mechanical properties are also related to their microstructures and morphologies (analyzed in the following section). The much lower mechanical properties inevitably make preparation of the films much difficult. With the addition of the copolymer, the mechanical properties of the electrolytes increased obviously versus the PEO-based electro-

lyte, PEO/LiClO4.With the lower concentration of the copolymer, the electrolyte PEO/PEO-b-PE/LiClO4 (6:1:1) shows a 532% increase in strength, 289% increase in modulus, and 1.84 times increase in breaking elongation versus PEO/LiClO4. This implies that copolymer dramatically impacts the mechanical performance of the electrolytes, though the ultimate stress and modulus of this ternary electrolyte with lower amount of copolymer are lower that of pure PEO but much higher than that of the PEO/LiClO4. More remarkably, with an addition of higher amounts of the copolymer into the PEO electrolyte system, the electrolyte film of PEO/PEO-b-PE/LiClO4(6:6:1) had the ultimate stress of 7.3 MPa (close to pure PEO level), modulus 114.2 MPa, and breaking strain 1143%, which are 2252%, 688%, and 274% higher than that of PEO/LiClO4, respectively. Attention should also be paid to the strain deviation levels. For PEO/LiClO4, the strain deviation ratio is 68.2%, which is very high but is reasonable because during the preparation of the film samples we observed that the films of PEO/LiClO4 are extremely soft and sticky. This phenomenon implies that the PEO/LiClO4 samples were too weak to be fabricated stably, which results in difficulty in SPE film quality control. However, quality stability and processability are greatly important for the application of the SPE materials in the battery industry. It is believed that high ionic conductivity, good mechanical properties, and good processability are simultaneously critical for a high-performance SPE material. In our studies, it was observed that, after addition of the copolymer, nonsticky and quality stable films were produced. The tensile testing results indicated that the strain deviation ratios decreased to 17.5% and 17.6% in PEO/PEO-b-PE/LiClO4 (6:1:1) and PEO/PEO-b-PE/LiClO4 (6: 6:1), respectively. These mechanical property results confirm the processability of the electrolytes improved significantly with the addition of the copolymer. Therefore, incorporation of the copolymer into the PEO electrolyte to form a ternary electrolyte system, PEO/PEO-bPE/LiClO4, dramatically enhanced the mechanical properties for the PEO electrolyte. More significantly, such high enhancements in mechanical properties and improved processability while possessing 10 times higher in ionic conductivity have never been reported. Morphology and Crystal Structure Studies. It is wellknown that PEO-based polymer electrolytes exhibit three phases: the crystalline PEO phase, the crystalline PEO-lithium salt complex phase, and an amorphous phase. From the SEM image, parts a and c of Figure 3, the morphology of PEO and PEO/ LiClO4 are porous showing separated granules, whereas with addition of the copolymer to the PEO/LiClO4, the morphology of PEO/PEO-b-PE/LiClO4 (6:6:1) becomes smoother and there is no obvious phase separation, indicating that the PEO-b-PE and PEO have a good compatibility (part e of Figure 3). In the optical micrographs (parts b and d of Figure 3), spherulitic morphology is evident for all the samples in which sizes of the spherulites decreased with the addition of LiClO4 into the system. From part f of Figure 3, we know the surface morphology was severely changed due to the addition of copolymer. From the marked information in part c of Figure 3, we observed the microcracks existing in the film. The microcracks not only decreased the mechanical properties but also are very harmful to their applications in solid-state lithium batteries because they could easily lead to a short circuit of anode and cathode and therefore decreasing the cycling life of the batteries. In part e of Figure 3, which shows the surface morphology of

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Figure 3. SEM (a) PEO, (c) PEO/LiClO4, (e) PEO/PEO-b-PE/LiClO4 (6:6:1) and optical micrograph (b) PEO, (d) PEO/LiClO4 and (f) PEO/PEOb-PE/LiClO4 (6:6:1) images of the films.

Figure 4. XRD patterns of pure PEO, PEO-b-PE, PEO-based electrolyte films.

the polymer electrolytes containing copolymer, it is well demonstrated that the addition of the copolymer could effectively inhibit the formation of microcracks. A similar crack phenomenon was also reported indicating that the addition of alumina whisker decreased the percentage of microcracks in the films due to the much higher tensile strength of the alumina ceramic whisker.29

X-ray diffraction (Figure 4) measurements were made for pure PEO, PEO-b-PE, PEO-based electrolyte films to examine the crystal structures. Two kinds of characteristic diffraction peaks, located at 2θ ) 19.5° and 23.5°, are apparent in pure PEO.30 The incorporation of LiClO4 into the PEO matrix caused a decrease in the intensity of PEO characteristic peaks, which indicates that the LiClO4 addition decreased the crystallinity of

Effects of a Block Copolymer

Figure 5. Rheological curves vs frequency for pure PEO, PEO-b-PE, PEO/LiClO4, and PEO/PEO-b-PE/LiClO4 (6:6:1): (a) elastic modulus (G′, shown with solid symbols) and the viscous modulus (G′′ shown with hollow symbols); (b) complex viscosity (η*) and (c) G′′/G′ ) tan δ at 100 °C.

PEO. The characteristic diffraction peaks of PEO-b-PE are apparent at 21.5° and 23.8°, which are orthorhombic (110) and (200) reflections, respectively.31 When addition of the copolymer into PEO/LiClO4 system to form PEO/PEO-b-PE/LiClO4 (6:6: 1) is done, all of the diffraction peaks become broader and less prominent, consistent with a large decrease in the crystallinity of the PEO/PEO-b-PE/LiClO4 (6:6:1). The more interesting result is that the characteristic peak of PEO-b-PE (2θ)21.5°) is still narrow and sharp in PEO/PEO-b-PE/LiClO4 (6:6:1), which indicates that in the PEO/PEO-b-PE/LiClO4(6:6:1) system, the residue crystallinity is attributed to the PEO-b-PE. Rheology Studies. An SPE in lithium battery is not only used as an ion transport medium but also as an electrode separator. Hence, it is very important for the electrolyte to remain mechanically and dimensionally stable during the application temperature range. However, above the melting point of PEO (65 °C), the material can easily lose its dimensional stability and behave as an extremely viscous low-modulus liquid, which will reduce the cycling life of the battery. To evaluate the characteristics of a viscoelastic material, it is customary to examine the elastic (G′) and viscous modulus (G′′) as a function of frequency. In Figure 5, we show the

J. Phys. Chem. B, Vol. 114, No. 43, 2010 13641 frequency (ω) dependence of the elastic modulus (G′) and viscous modulus (G′′), respectively, of the pure PEO, PEO-bPE, PEO/LiClO4, and PEO/PEO-b-PE/LiClO4 (6:6:1) at 100 °C, higher than the melting point of PEO (65 °C). By comparing the order of magnitude of these two types of modulus for each sample, it can be found that G′ is always higher than G′′ over the entire frequency range except the PEO/LiClO4, which indicates that solid polymer electrolyte with the copolymer, PEO/PEO-b-PE/LiClO4, are solid elastic, which corresponds to the results of the mechanical tests, that showed the strain increased sharply when the copolymer incorporated into the PEO/LiClO4. In general, if the degree of crystallinity is low enough, the G′ can fall off more steeply reflecting changes in the molecular mobility in the amorphous domains.32 In our samples, according to the XRD results we know the crystallinity of PEO/PEO-bPE/LiClO4 (6:6:1) is lower than that in the PEO/LiClO4. From part a of Figure 5, the modulus G′ of ternary electrolyte, PEO/ PEO-b-PE/LiClO4 (6:6:1), increased nearly 2 orders of magnitude compared with the bicomponent electrolyte, PEO/LiClO4. The enhancement in modulus may be due to the confinement of PEO polymer chains within the continuous PE crystalline phase. This hypothesis can be further confirmed by complex viscosity. From part b of Figure 5, the viscosity of the ternary electrolyte system, PEO/PEO-b-PE/LiClO4, is higher than that of the bicomponent system PEO/LiClO4. It is known that, a higher viscosity corresponds to lower free volume. The addition of lithium salt and copolymer leads to decrease in crystallinity that generates a larger free volume, whereas the existence of PE structure limited the mobility of polymer chains, which increased the viscosity. Another point we should pay attention to is that the similar trend can be seen in G′′. In other words, the modulus ratio (G′′/G′ ) tan δ) (shown in part c of Figure 5) is kept constant over the whole investigated frequency range, evidencing no structural changes in the sample PEO/PEO-bPE/LiClO4 (6:6:1) compared to PEO-b-PE, in the frequency sweep range.33 As discussed above, at 100 °C, higher than the PEO melting point (65 °C), the elastic modulus increased nearly two orders of the magnitude of the ternary electrolyte system, PEO/PEOb-PE/LiClO4, compared with the bicomponent system, PEO/ LiClO4, which indicated that the dimensional stability of the electrolytes were dramatically improved by the addition of the copolymer. Mechanism Analysis. For the mechanism of the enhancements in ionic and mechanical properties as well as dimensional stability, we propose a micro bicontinuous phase structure for the ternary electrolyte, PEO/PEO-b-PE/LiClO4, with a continuous crystal PE phase-layered-continuous amorphous PEO phase, as shown in Figure 6. The PEO matrix is compatible with the copolymer by the miscible PEO block, and the lithium ions are transported in the amorphous PEO phase. According to literature,34,35 because of the slight solubility of PE blocks in solvent acetonitrile, the PEO blocks in the copolymer PEO-bPE are always confined by the pre-existing PE crystal phase. Finally, the crystalline lamellae of PE and PEO are alternately stacked. For the 1400 g mol-1 copolymer used in our study, when the sample crystallized at 30 °C, only PE blocks crystallized and formed a layered stack-structure.36 As is wellknown, blending the copolymer with the homopolymer of either component is feasible without phase separation, as the homopolymer aggregates in domains with its block copolymer counterpart. In the PEO/PEO-b-PE/LiClO4 system, PE blocks are not compatible with ions, thus, ion transport is restricted to

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Figure 6. Schematic representation of possible structure of PEO/PEO-b-PE/LiClO4 electrolyte.

the microphase of PEO block.37 The LiClO4 interacted with the PEO (matrix chains and PEO blocks). From the XRD curve, we know the PE blocks still crystallized in the PEO/PEO-bPE/LiClO4 (6:6:1) system. Hence, bicontinuous phase structure, that is crystalline PE domains within a matrix of PEO/salt structure can be formed. In this special structured film, the highly continuous amorphous phase of PEO blocks/PEO matrix is responsible for the high ionic conductivity, whereas the crystal PE blocks are imparting mechanical strength and maintaining dimensional stability due to the higher melting point of PE blocks. Conclusions High ionic conductivity, dramatically enhanced mechanical properties, excellent dimensional stability, and easy processability were achieved simultaneously by a simple copolymer addition method. In this article, we reported the results of ionic conductivity and mechanical property enhancements with the addition of the copolymer, PE-b-PEO. From the microstructure studies, it was observed that morphological defects in the copolymer loaded PEO electrolyte films were visibly decreased. Rheological studies revealed that the ternary system, PEO/PEOb-PE/LiClO4 (6:6:1) shows addition of the copolymer improved dimensional stability over the pure PEO-based SPE, PEO/LiClO4 up to 100 °C. A bicontinuous phase structure os proposed, that is PE crystal phase-layered-continuous PEO amorphous structure, contributes to all of the property enhancements including mechanical properties reported in this article. Such comprehensive enhancements in the ionic conductivity, mechanical properties, and dimensional stability, achieved at the same time in

this study, have never been previously reported. The method reported in this article is very simple and efficient for creating multiple high performances for SPEs. References and Notes (1) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587–603. (2) Srivastava, N.; Tiwari, T. e-Polym. 2009, 146, 1–17. (3) Meyer, W. H. AdV. Mater. 1998, 10, 439–448. (4) Abraham, K. M.; Jiang, Z.; Carroll, B. Chem. Mater. 1997, 9, 1978– 1988. (5) Patil, A.; Patil, V.; Shin, D. W.; Choi, J. W.; Paik, D. S.; Yoon, S. J. Mater. Res. Bull. 2008, 43, 1913–1942. (6) Agrawal, R. C.; Gupta, R. K. J. Mater. Sci. 1999, 34, 1131–1162. (7) Aurbach, D.; Levi, M. D.; Levi, E. Solid State Ionics 2008, 179, 742–751. (8) Song, J. Y.; Wang, Y. Y.; Wan, C. C. J. Power Sources 1999, 77, 183–197. (9) Dias, F. B.; Plomp, L.; Veldhuis, J. B. J. J. Power Sources 2000, 88, 169–191. (10) Raghavan, S. R.; Riley, M. W.; Fedkiw, P. S.; Khan, S. A. Chem. Mater. 1998, 10, 244–251. (11) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11–27. (12) Fan, L.; Dang, Z.; Nan, C.; Li, M. Electrochim. Acta 2002, 48, 205–209. (13) Singh, M.; Odusanya, O.; Wilmes, G. M.; Eitouni, H. B.; Gomez, E. D.; Patel, A. J.; Chen, V. L.; Park, M. J.; Fragouli, P.; Iatrou, H.; Hadjichristidis, N.; Cookson, D.; Balsara, N. P. Macromolecules 2007, 40, 4578–4585. (14) Song, J. Y.; Wang, Y. Y.; Wan, C. C. J. Power Sources 1999, 77, 183–197. (15) Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nature 1998, 394, 456–458. (16) Xi, J.; Tang, X. Electrochim. Acta 2006, 51, 4765–4770. (17) Bronstein, L. M.; Joo, C.; Karlinsey, R.; Ryder, A.; Zwanziger, J. W. Chem. Mater. 2001, 13, 3678–3684.

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