Letter pubs.acs.org/NanoLett
Tuning Ion Conducting Pathways Using Holographic Polymerization Derrick M. Smith,† Bin Dong,† Russell W. Marron,† Michael J. Birnkrant,† Yossef A. Elabd,‡ Lalgudi V. Natarajan,§ Vincent P. Tondiglia,§ Timothy J. Bunning,∥ and Christopher Y. Li*,† †
A. J. Drexel Nanotechnology Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States ‡ Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States § Science Applications International Corporation, Colonel Glenn Highway, Dayton, Ohio 45431, United States ∥ Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States S Supporting Information *
ABSTRACT: Polymer electrolyte membranes (PEMs) with high and controlled ionic conductivity are important for energyrelated applications, such as solid-state batteries and fuel cells. Herein we disclose a new strategy to fabricate long-range ordered PEMs with tunable ion conducting pathways using a holographic polymerization (HP) method. By incorporating polymer electrolyte into the carefully selected HP system, electrolyte layers/channels with length scales of a few tens of nanometers to micrometers can be formed with controlled orientation and anisotropy; ionic conductivity anisotropy as high as 37 has been achieved. KEYWORDS: Polymer electrolyte membrane, lithium ion battery, holographic polymerization, ion conductivity
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Herein we disclose a new strategy to fabricate long-range ordered PEMs with tunable ion conducting pathways using a holographic polymerization (HP) method. We introduce the name holographic PEM (hPEM) for these unique membranes. Similar to interference lithography, HP has recently been used as a quick and efficient top-down technique to fabricate 1D, 2D, and 3D defect-free nanoscale photonic structures with excellent structural control.22−24 During the HP process, a photopolymerizable mixture is exposed to two or more coherent laser beams whose interference leads to a standing wave pattern. Higher intensity regions within the interference pattern result in a locally faster polymerization process and reaction rate anisotropy, which in turn leads to a spatial distribution of high molecular weight polymers. Liquid crystals,22 nanoparticles,25,26 PS latex spheres (260 nm diameter), and silicate nanoplates have been patterned using HP. Using a similar technique,27−29 3D bicontinuous polymer microframes with length−scale dependent mechanical properties can be fabricated; furthermore, these structures have been implemented as skeletons for further functionalization.30 Most recently, we demonstrated that optical gratings can be fabricated by incorporating and spatially segregating inert macromolecules using this approach. PEO homopolymer and PEO-b-polycaprolactone (PEO-b-
olymer electrolyte membranes (PEMs) with high ionic conductivity are important for energy-related applications, such as solid-state batteries and fuel cells.1,2 In order to achieve improved performance and longevity of the device, mechanical properties of the membrane must be promoted to the highest critical factor, particularly in solid-state lithium ion batteries.1−6 This is because the formation of lithium dendrites, which is detrimental to the device, can be stopped if the shear modulus of PEMs can be dramatically increased without sacrificing its ionic conductivity. Studies have shown that cation transport is coupled with segmental motion of the polymer chain, and high ionic conductivity can be obtained in soft polymers, such as rubbery polyethylene oxide (PEO).7 One of the most promising approaches to increasing the mechanical properties of the PEM without sacrificing its ion conductivity is using a block copolymer (BCP) strategy.1,5,8−21 By synthesizing BCPs (or similar structures) consisting of PEO and a high modulus segment (such as polystyrene, PS), it has been demonstrated that exploiting BCP phase separation at ∼10−100 nm scales, ionic conducting channels can be formed in the PEO domains, while PS provides a high modulus of the film. Although BCP contributes an elegant solution to the currently technological hurdles, it also has numerous intrinsic shortcomings, including, e.g., poor long-range order, high-defect content, and complex phase structures, particularly in ion/BCP blends. Correlation between BCP phase morphology and ionic conductivity is still a subject of intense study.1,5,8,18−21 © 2011 American Chemical Society
Received: October 12, 2011 Revised: December 7, 2011 Published: December 12, 2011 310
dx.doi.org/10.1021/nl203599y | Nano Lett. 2012, 12, 310−314
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PCL) have been patterned into a 1D grating, which subsequently exhibited a narrow optical diffraction notch.31−34 The optical properties of the spatially anisotropic structure could be modulated with temperature due to local changes in the refractive index induced by polymer crystallization/melting processes. In this paper, we show that HP can be used to fabricate hPEMs with controlled ion conducting pathways. By incorporating polymer electrolytes into the carefully selected HP system, electrolyte layers/ channels with length scales of a few tens of nanometers to micrometers can be formed. Anisotropic conducting behavior was observed for these hPEMs due to the long-range ordered nanostructures. The hPEM approach mimicks the BCP, and the cross-linked polymer network can provide a necessary scaffold to mechanically stabilize the PEM for device applications. Electrolytes used in this study consist of 400 g/mol PEO with bis trifluoromethane sulfonimide lithium salt (TFSI-Li) at a salt weight percent of 25%, corresponding to a [Li]:[EO] molar ratio of 1:19.7 These ingredients were selected because our previous work showed that PEO can be uniformly patterned into various nanostructures using HP, and PEO/Li+ is the standard electrolyte material for polymer lithium ion battery studies. Norland 65 was used as the cross-linkable monomer similar to our previous work.31−34 hPEMs were fabricated with various electrolyte volume fractions using two different HP optical setups shown in Figure 1. The targeted
Figure 2. UV−vis spectra of TFSI-Li hPEM reflection grating films at electrolyte volume fraction loadings of 34.8−55.5%. The spectra are offset for clarity with a scale bar denoting 40% transmission. Inset shows DE change with respect to electrolytes loading.
inset) with 100% background transmission, suggesting a Bragg grating of excellent DE was formed. Upon further increasing electrolyte loading, the DE decreases significantly to approximately 20%. The formation of sharp notches in the UV−vis spectra suggests that an appreciable refractive index modulation was achieved from the formation of electrolytes and Norland 65 layers. To confirm the structure, ∼80 nm thick sections of the reflection grating hPEM along the film normal (left column in Figure 1) were obtained using ultramicrotomy; Figure 3a−c shows TEM images of xz plane of the film. The samples were stained with RuO4; the dark domains are PEO-rich, while the gray domains are cross-linked Norland. Phase separation between the electrolyte and cross-linked Norland resin can be clearly seen in all three images. Morphological evolution is also evident as we increase the electrolyte loading. At lower electrolyte loadings (Figure 3a, 34.8 v/v %), electrolytes formed nanosized droplets (∼ 50−200 nm in diameter) which were aligned into uniform layers separated by cross-linked Norland resin. As the electrolyte volume fraction is increased to 50.5 v/v %, long anisotropic electrolyte “blocks” were observed with the long axis of the “blocks” parallel to the HP layers (a “brick and mortar-like” structure). Because the image is an xz cross section of the hPEM and the hPEM structure should be isotropic within the xy plane in the present experimental condition, Figure 3b suggests that electrolytes formed nearly 2D, micrometer scale, confined layers that are parallel to the hPEM surface. “Cross talk” also occurs between the adjacent layers, which should play a significant role in conductivity performance of the hPEM. Further increasing in electrolyte loading led to holographic films with poor defined structure, where the layered architecture disappeared and only large-scale phase separation was observed. This is because the polymerization and diffusion kinetics were not enough in harmony to create uniformly separated layers of Norland and electrolyte during the HP process.22 While phase separation did take place in this case, the separated domains do not show long-range order. Note that the dark regions correspond to the electrolyte regions; most of the samples in Figure 3a−c appeared to have more dark region than the loaded electrolyte, even for the hPEM with 34.8 v/v % electrolytes loading. The 50.5% reflection sample has dark regions covering nearly 65−70% of the area. This may suggest that there is noticeable miscibility
Figure 1. Schematic of optical setups for reflection (left) and transmission (right) gratings using holographic polymerization (top), and the resulting electrode configuration for electrochemical impedance spectroscopy (EIS) measurement (bottom). The blue and red layers represent Norland resin-rich and electrolyte-rich areas, respectively.
nanostructure is layer-by-layer lamellae with the lamellar normal parallel (left, via reflection grating) or perpendicular (right, via transmission grating) to the hPEM surface.33 Figure 2 shows UV−vis spectra for the reflection grating hPEM. The spectra are offset for clarity purpose. All the spectra from the varying electrolyte loadings show 100% background transmission within the optical range of the spectra (400−800 nm) with sharp notches at ∼560−600 nm. The depth of the notch is defined as the diffraction efficiency (DE) of the grating. At lower electrolyte loading (∼35−45 v/v %), the DE is about 50−60%. As the electrolyte loading increased to 50.5 v/v %, a DE notch of nearly 100% was observed (see Figure 2, 311
dx.doi.org/10.1021/nl203599y | Nano Lett. 2012, 12, 310−314
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Figure 3. TEM micrographs of reflection gratings hPEMs (a−c) and transmission gratings hPEMs (d−f) at denoted percent electrolyte loadings. Dark and light regions correspond to electrolyte-rich and Norland-rich areas, respectively. Scale bars are 1 μm for the large micrographs and 200 nm in white for the inset micrographs. Schematics of the hPEM cross sections (g−i) with blue regions representing electrolyte domains.
hPEMs is clearly more favorable for PEM applications as opposed to defect-rich BCPs. Electrochemical impedance spectroscopy (EIS) was used to measure the conductivity at room temperature. Two-electrode geometry was applied to both reflection and transmission hPEMs as shown in Figure 1. Herein we also define the measured conductivity of reflection and transmission grating hPEMs as σr and σt, respectively. In hPEMs, the NOA 65 layers can be considered as a nonionically conducting barrier to influence the ionic diffusion behavior during EIS measurements. If perfect NOA65 layers were formed with no cross-talking, σt should be much greater than σr. Therefore σr and σt correlate the morphology of the hPEMs and their conductivity performance.
between NOA65 and the electrolyte layers, and the relatively broad phase boundaries between electrolyte-rich and Nolandrich domains also appeared as dark in the TEM image. Nevertheless, from the UV−vis and TEM results, it is safe to conclude that uniform, long-range ordered, controlled phase separation between electrolyte and Norland has been created in these hPEMs. Transmission optical setup (Figure 1 right column) was also utilized in order to create hPEMs with the electrolyte layers perpendicular to the film surface. TEM micrographs of the yz cross sections of these films are shown in Figure 3d−f. Similar morphologies were observed: nanosized electrolyte droplets, anisotropic electrolyte layers, and randomly separated structures were formed at different electrolyte loadings. Note that the layer direction in Figure 3d−f is parallel to z, while it is perpendicular to z in Figure 3a−c. Figure 3g−i depicts a schematic of the structures formed at the different electrolyte loadings. At lower electrolyte loadings, droplets form in confined layers; as the electrolyte volume is increased, the layers fill out with electrolytes to create a brick and mortar-like structure. There is a low density of crosstalk between the layers with this brick and mortar formation. Once a certain electrolyte volume is reached, the layers’ integrity is compromised, and a randomly phase separated structure is formed. Formation of the long-range ordered nanostructures between electrolyte and polymer resin is encouraging. It has been hypothesized, and extensively pursued, that nanoscale phase separations of electrolyte and polymeric materials with controlled nanostructure could eventually lead to stable, more durable PEMs for devices.1,2 The leading candidate to create such phase separation is BCPs due to its well-known phase structures.35,36 Figure 3 shows that hPEMs mimick BCP perforated lamellar structure; Norland and electrolyte form adjacent layers, and the Norland layer selectively perforated the electrolyte layers. This particular structure could be excellent for PEM applications because the resin could provide a robust scaffold for electrolyte layers and prevent possible dendrite formation during battery operation. The long-range order in
Figure 4. Ionic conductivity versus electrolyte content of hPEMs for transmission (blue circles), reflection gratings (red diamonds), and isotropic samples (iPEM) written with no holographic patterning (green triangles). Inset shows anisotropic ionic conductivity plot of reflection versus transmission gratings.
Figure 4 shows the room temperature conductivity data of hPEMs with respect to the electrolyte loading (f in volume percentage). A detailed calculation method to obtain the 312
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mechanical stretching. It is, however, lower than those obtained in LC systems. Figure 3 suggests that complete phase separation was not formed in the present system. The best sample (Figure 3b,e) shows electrolyte channel/layers on the length scale of a few micrometers. Crosstalk between adjacent layers also dramatically decreased the ionic anisotropy. We anticipate that by careful material tuning of the thermodynamics of the cross-linkable resin and the electrolytes, higher σt and larger conductivity anisotropy can be achieved. Nevertheless, our preliminary work demonstrated that a model system to study ionic conductivity had been fabricated in a onestep HP process. Compared with LC systems, hPEMs are mechanically stable and are therefore preferable for lithium ion battery or fuel cell applications. Compared with BCPs, hPEMs also show the following advantages. First, ion conducting pathways with length scales ranging from a few tens to hundreds of nanometers can be fabricated, while most of the BCP systems can only offer ion channel size of
