Influence of Fe2O3 Nanofiller Shape on the Conductivity and Thermal

Sep 28, 2012 - Department of Electrical Engineering Technology, Purdue University, South Bend, Indiana 46634, United States. J. Phys. Chem. C , 2012, ...
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Influence of Fe2O3 Nanofiller Shape on the Conductivity and Thermal Properties of Solid Polymer Electrolytes: Nanorods versus Nanospheres Nhu Suong T. Do,†,‡ Dean M. Schaetzl,§ Barnali Dey,† Alan C. Seabaugh,† and Susan K. Fullerton-Shirey*,† †

Department of Electrical Engineering, Notre Dame Center for Nano Science and Technology, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Mathematics, Saint Mary’s College, Notre Dame, Indiana 46556, United States § Department of Electrical Engineering Technology, Purdue University, South Bend, Indiana 46634, United States S Supporting Information *

ABSTRACT: The influence of nanofiller shape on the ionic conductivity and thermal properties of solid polymer electrolytes is investigated. Electrolytes of polyethylene oxide (PEO) and LiClO4 filled with 1−20 wt % spherical Fe2O3 nanoparticles and 0.5−10 wt % Fe2O3 nanorods are measured at an ether oxygen to Li ratio of 10:1. Nanorods improve the ionic conductivity to a similar extent as spherical nanoparticles, except at concentrations 10−20 times lower. The maximum conductivity improvement occurs at a spherical metal oxide nanoparticle loading of 10 wt %; however, an equivalent nanorod loading decreases the conductivity below that of the unfilled electrolyte. This result suggests that the long-range morphology of the two nanocomposites differs widely, where high concentrations of nanorods will inhibit instead of enhance Li transport. The shape of the nanofiller also affects the crystallization rate and resulting crystal structure. Differential scanning calorimetry measurements show that samples containing nanorods crystallize faster than those containing spherical nanoparticles, and nanorods favor formation of the (PEO)6:LiClO4 crystal phase. Previous studies have shown that this channel-like structure is more conductive than the amorphous phase. If nanorods could be used to induce the formation and alignment of this conductive structure normal to the electrode surface, perhaps ionic conductivity could be further enhanced in nanofilled solid polymer electrolytes where the nanoscale structure is precisely controlled.

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reported for both PEO/LiTFSI6,7 and PEO/LiClO4.8 Slow recrystallization kinetics are the consequence of coordination between Li cations and multiple ether oxygen atoms in PEO. When provided sufficient time to crystallize, the ionic conductivity typically decreases precipitously at temperatures below the melting point of the polymer. However, there exists evidence that a certain crystal phase can enhance Li-ion transport beyond that of the amorphous phase.9 Specifically, a channel-like structure is formed by PEO, with Li ions located inside the channel and anions between the channels. This structure, identified as (PEO)6:LiX, has been detected in SPEs, where X = AsF6,10 PF6,11 and SbF611 via powder diffraction and X = N(CF3SO2)212 by molecular dynamics simulation. Moreover, remnants of this structure have been detected at temperatures above the melting point by neutron diffraction for X = N(CF3SO2)213 and ClO4.14

INTRODUCTION Solid polymer electrolytes (SPEs) consist of a salt dissolved in a polymer. They have a variety of potential applications, including rechargeable batteries, where they could replace liquid- or gel-phase electrolytes. Without the need to encapsulate a liquid-phase electrolyte with a rigid casing, SPEs could potentially decrease device weight and perhaps lead to mechanically flexible batteries with new applications. Moreover, they offer improved chemical stability and help prevent the growth of dendrites. However, after decades of research, ion mobility through a truly solid-state polymer electrolyte remains low at room temperature1,2 (∼10−6 S/cm). Polyethylene oxide (PEO) is the most commonly studied polymer for SPEs, and popular Li salts include LiClO4, LiN(CF3SO2)2 (LiTFSI), and LiCF3SO3 (LiTf). In the amorphous phase, it is well established that ion mobility increases with the segmental mobility of the polymer. However, at room temperature, most SPEs have both amorphous and crystalline domains according to their phase diagrams.3−5 Depending on the identity and concentration of Li salt, crystallization can proceed slowly (i.e., hours or days), as © 2012 American Chemical Society

Received: June 16, 2012 Revised: September 13, 2012 Published: September 28, 2012 21216

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improve the conductivity, whereas those without hydroxyl termination (i.e., basic surface chemistry) have no effect.19 The authors suggest that because OH groups on the nanoparticle surface interact with PEO ether oxygens and salt anions, this will (1) increase salt dissociation, liberating Li cations for conduction, and (2) increase the amorphous fraction of PEO by preventing crystallization. With this mechanism, Li-ion mobility is enhanced at the interface between the nanoparticle and the polymer electrolyte. Molecular dynamics simulations of PEO/LiClO4 confirm that interactions between acidic sites and anions alter the local environment of the Li cations.34 The second mechanism pertains to the way nanoparticles organize over large length scales. If the interface between the particle and the electrolyte favors ion mobility, then the formation of percolating pathways via nanoparticle surfaces would provide long, conductive pathways over which ion mobility could proceed at a faster rate than through the bulk. Adebahr and co-workers reported 7Li NMR and 1H NMR measurements of PEO/LiClO4 and 3PEG/LiClO4, where 3PEG is trihydroxypoly(ethylene oxide-co-propylene oxide), suggesting that Li ions interact directly with the nanoparticles, creating conductive pathways at the particle/polymer interface.35 They propose that the maximum conductivity improvement with nanoparticles likely corresponds to the maximum number of percolating pathways.35 A small angle neutron scattering (SANS) study of 3PEG/LiClO4 with Al2O3 and TiO2 nanoparticles supports the existence of percolating pathways.36 The data were fit to a model describing polymer-like structures with a fractal dimension of 2 that were one or two primary particles wide and 100 to at least 400 nm long. However, SANS data for PEO/LiClO4 with Al2O3 particles indicate no such polymer-like aggregates.32 Instead, the majority of nanoparticles cluster into dense aggregates larger than 200 nm. Together, these results suggest that the identity of the polymer and the sample preparation procedure will largely determine whether or not percolating pathways will form. The third and more recently proposed mechanism suggests that nanoparticles may help stabilize the conductive (PEO)6:LiX channels described above,32 which can persist at temperatures above the melting point.13,14 Quasi-elastic neutron scattering (QENS) measurements on PEO/LiClO4 reveal the presence of a second dynamic process over long time scales (i.e., nanoseconds).37 The second process has been observed in other SPEs where the (PEO)6:LiX structure can form, and it is interpreted as the restricted rotation of the conductive, channel-like structure described above.37 QENS data reveal that this rotation becomes more restricted in the presence of Al2O3 nanoparticles.32 It is therefore possible that the nanoparticle surface helps to stabilize the conductive structures. The conductivity improvement observed with the addition of metal oxide nanofillers is likely a combination of the three mechanisms describe above. With this in mind, we propose that nanofillers with a high aspect ratio (i.e., nanorods) may be better suited to improve ion mobility. Similar to spherical nanoparticles, the surface chemistry can be controlled to maximize the number of acidic surface groups. Because the percolation threshold scales inversely with aspect ratio,38 we expect nanorods to achieve percolation more easily than spherical nanoparticles, potentially creating longer conductive pathways. Additionally, if nanofillers do help stabilize conductive polymer channels, which can persist above the melting point, then we might expect long, thin nanofillers to

Although conductivity in a fully crystalline sample of (PEO)6:LiX made with low molecular weight PEO (100 000 g/mol) to create a free-standing film, and increase the conductivity by increasing the fraction and mobility of the amorphous phase. This is commonly accomplished by adding small-molecule plasticizers to the SPE.15−17 This modification improves conductivity but at the expense of the electrolyte’s mechanical properties. In contrast, adding metal oxide nanoparticles is a modification that both increases the ionic conductivity and improves the mechanical properties, but only modest conductivity values have been reported at room temperature (e.g., within the range of 1 × 10−6 to 1 × 10−4 S/cm depending on the extent of crystallization1,18−21). Over the past 15 years, at least a dozen different types of metal oxide nanoparticles have been studied over a wide concentration range.2,21−31 Regardless of the chemical identity of the salt or the nanoparticle, the maximum conductivity improvement typically occurs in the particle concentration range of 10−15 wt %.2,18,21−24,32,33 This is illustrated in Figure 1, where the

Figure 1. Conductivity boost versus nanoparticle filler concentration from data in refs 2, 18, 21−24, 32, and 33 for PEO-based SPEs at temperatures within the range 70−80 °C.

conductivity boost, defined as the conductivity of the filled sample normalized by the unfilled sample, is plotted versus filler concentration for 10 PEO-based SPEs filled with various metal oxide nanoparticles in the temperature range 70−80 °C. Because the thermal history, which strongly affects conductivity, varies between studies, we only include conductivity data at temperatures above the melting point of pure PEO (Tm > 60 °C) where the samples are amorphous. The conductivity boost averaged over all the studies at each filler concentration is also included. Understanding the mechanism by which nanofillers improve conductivity could enable rational design of nanostructured SPEs that further improve conductivity. Several mechanisms have been suggested, and we will review three here. The first pertains to the surface chemistry of the nanoparticle. Croce and co-workers showed that Al2O3 nanoparticles with surfaces terminated by hydroxyl groups (i.e., acidic surface chemistry) 21217

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Nanofiller Characterization. We imaged the NPs and NRs with an FEI Magellan 400 field emission scanning electron microscope (FE SEM) at 5 kV. Nanofiller size was determined by averaging 100 individual nanofillers identified in the SEM images. The elemental composition was verified with a QUANTAX 200 energy dispersive X-ray spectrometer (EDS), coupled to the SEM. SPE Sample Preparation. Polyethylene oxide (PEO, Mw = 600 000 g/mol) and LiClO4 were dissolved in acetonitrile at ether oxygen to Li molar ratios of 8:1 and 10:1 by stirring for 3 h at room temperature. We prepared SPE samples with NP concentrations in the range of 1 − 20 wt % and NR concentrations in the range of 0.5−10 wt %. For samples filled with nanofillers, we first sonicated the nanofillers in a vial filled with acetonitrile for several minutes using a Branson 2510 sonicator. Because the nanofillers are magnetic, we removed the stir bar from the PEO/LiClO4/acetonitrile solution prior to adding the solution containing Fe2O3 nanofillers. The mixture was sonicated until the acetonitrile evaporated. The solid films were dried in a vacuum oven at 100 °C for 1 week. Because the SPEs are hygroscopic, samples prepared for DSC and conductivity measurements were stored under a vacuum and held at 100 °C for at least 24 h prior to the measurement. Conductivity Measurements. The SPEs were hot pressed between Teflon-coated plates to a thickness of ∼100 μm. The resulting films were free-standing, including those without nanofillers. The films were placed between two stainless steel electrodes of diameter 12.7 mm and hot pressed until the diameter of the film equaled that of the electrodes. The final SPE thickness ranged from 95 to 105 μm. Samples were equilibrated at 100 °C, and then cooled to room temperature at a rate of 16 °C/min. Conductivity data were collected upon heating from 23 to 100 °C and equilibrated for 10 min at each temperature. The sample thickness remained unchanged throughout the experiment. This was determined by measuring the thickness at room temperature and after equilibration at the highest temperature (100 °C). The temperature was controlled by a Temptronic TP03000A-X300 ThermoChuck systems to within ±0.1 °C. Impedance measurements were made under nitrogen on a Cascade Microtech Summit 11861 probe Station using an Agilent 4294A Precision Impedance Analyzer in the frequency range 40 Hz − 110 MHz with a 500 mV AC field and 0 V DC bias. We calculate the AC conductivity using eq 1

more effectively help induce the formation and propagation of long, thin channel-like structures. Recently, Zhang and co-workers added SiO2 nanowires to a gel-phase SPE comprised of poly(vinylidene-fluoride-co-hexafluoropropylene) (P(VDF-HFP)) soaked in an electrolyte solution of LiPF6 and three plasticizers.39 In addition to improving the mechanical properties, ionic conductivity increased with nanowire loading up to the maximum measured concentration of 10 wt %. However, this electrolyte is a gel, and it remains unclear how nanowires would affect a polymer electrolyte without plasticizers. Moreover, conductivity data were not reported for the same system loaded with spherical nanoparticles, so it is unknown if and how nanoparticle anisotropy affects the conductive properties of the gel. In the current study, we add iron oxide (α-Fe2O3) nanoparticles with an aspect ratio of ∼7 (i.e., nanorods) to a SPE of PEO and LiClO4, and directly compare the performance to the same SPE filled with spherical α-Fe2O3 nanoparticles. Throughout the remainder of the manuscript, we will refer to nanorods as “NRs”, and spherical nanoparticles as “NPs”. The NRs and NPs have the same composition and have comparable diameters (NRs, 10−20 nm; NPs, 20−30 nm), allowing us to isolate the influence of nanofiller shape. We choose iron oxide (α-Fe2O3) nanofillers because NRs of α-Fe2O3 can be readily synthesized, and the α-phase provides optimal surface chemistry (i.e., hydroxyl termination)27,28 for better ion mobility.40,41 We choose PEO/LiClO4 because this is the most well studied SPE, providing a model system. We measure SPEs filled with Fe2O3 NPs and NRs in the concentration range 0−20 wt %. The majority of our analysis focuses on SPEs with an ether oxygen to Li ratio of 10:1. This is the eutectic concentration where the formation of two crystalline phases is equally energetically favored (pure PEO and (PEO)6:LiClO4),3 resulting in one melting point. A previous study shows that γ-Al2O3 NPs have a maximum effect on the conductivity at this concentration.32 To determine whether or not nanofiller shape affects the relative formation of one crystalline phase over the other, we also measure the thermal properties of several samples at an ether oxygen to Li ratio of 8:1, where the melting features of the two crystalline phases are well separated. We measure the ionic conductivity using impedance spectroscopy and the glass transition temperature, melting point, and heat of fusion using differential scanning calorimetry (DSC).

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σ (f ) =

EXPERIMENTAL METHODS α-Fe2O3 (Hematite) Nanofillers. Spherical α-Fe2O3 nanoparticles (NPs) (>99.5%, 30 nm diameter) were purchased from US Research Nanomaterials, Inc., Houston, TX. To synthesize the α-Fe2O3 nanorods (NRs), we followed a procedure described by Tang and co-workers.42 All chemicals were purchased from Sigma Aldrich and used without further purification. We added 0.556 g of FeSO4·7H2O and 0.328 g of anhydrous CH3COONa to 80 mL of deionized (DI) water and stirred vigorously for 20 min. The orange suspension was held at 100 °C for 8 h in a 125 mL Teflon-lined autoclave. The autoclave cooled to room temperature after several hours, and we centrifuged the solid α-FeOOH at 7500 rpm for 5 min and washed it three times with DI water and twice with ethanol. To drive off the excess ethanol, we dried the centrifuged solution in air at 50 °C for 4 h, followed by 3 h at 260 °C to obtain the final α-Fe2O3 NR product. The electrical conductivity of polycrystalline hematite is ∼10−14 S/cm at room temperature.43

d cos θ(f ) A | Z(f )|

(1)

where f is the frequency, d is the SPE thickness, A is the electrode area, |Z| is the magnitude of the measured impedance, and θ is the phase angle. The AC conductivity is obtained as the best fit to the conductivity versus frequency data in the frequency range where conductivity has a slope of zero.44 Examples of the raw data and calculated conductivity values for the unfilled sample are provided in the Supporting Information. Two different SPE samples of the same composition were prepared and measured at each nanofiller concentration and temperature to assign error bars to the data. Differential Scanning Calorimetry (DSC). We measured the glass transition temperature (Tg) and heat of fusion (Hf) using a TA Instruments 2920 Modulated DSC with aluminum sample pans. Sample weights were 9−11 mg, and measurements were performed under nitrogen. A heat/cool/heat cycle in the range of −70 to 100 °C was used to evaluate the Tg, with 21218

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heating and cooling rates of 2 °C/min in the range of −70 to 10 °C and 5 °C/min in the range of 10 to 100 °C. Because the SPEs do not fully crystallize immediately upon cooling to room temperature, we carefully monitored the thermal treatment when evaluating the extent of crystallization. We heated the sample pan to 100 °C, and equilibrated at room temperature under vacuum for varying time periods: 10 min (corresponding to the thermal treatment applied during the conductivity measurements), 30 min, and 24 h for the 10:1 concentration, and 7 days for the 8:1 concentration because this sample requires a longer time to recrystallize. After equilibrating for each specified time period, the pan was transferred to the DSC and heated from 22 to 70 °C at 5 °C/min to measure the melting features. Two different SPE samples of the same composition were prepared and measured at each nanofiller concentration.

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RESULTS AND DISCUSSION Prior to preparing and characterizing the nanofilled SPEs, we imaged the α-Fe2O3 NRs and NPs using an SEM (Figure 2A

Figure 3. Ionic conductivity versus temperature for PEO/LiClO4 SPEs filled with (A) α-Fe2O3 NPs and (B) α-Fe2O3 NRs. The symbols represent the average of two measurements, and error represents the largest and smallest measured values.

samples loaded with (A) NPs and (B) NRs. The conductivity data can be divided into two regimes: one above the melting point (48 °C) where the samples are completely amorphous and one below where the samples are semicrystalline. A melting point at this temperature is consistent with the phase diagram for PEO/LiClO4 at 10:1, where both crystalline PEO and (PEO)6:LiClO4 share the same melting point at the eutectic concentration.3 As discussed in the Introduction, pure crystalline PEO will block ion transport, while (PEO)6:LiClO4 can be highly conductive if the structure is properly aligned.45 For those samples filled with NPs (Figure 3A), the maximum conductivity improvement occurs at a loading of 10 wt %, regardless of temperature. This result is consistent with previous studies of SPEs filled with spherical metal oxide nanofillers, where the maximum improvement typically occurs in the range 10−15 wt % (Figure 1). However, unlike SPEs filled with NPs, the maximum conductivity improvement with NRs occurs at significantly lower concentrations: 0.5−1 wt % (Figure 3B). As described above, one possible mechanism to explain conductivity improvement with the addition of nanofillers is the creation of percolating pathways for ion conduction. Because the percolation threshold is inversely proportional to aspect ratio, NRs will achieve percolation at concentrations lower than that of spherical NPs. We did not study the nanoscale morphology of the samples investigated in this study, but it is possible that the small amount of NRs required to increase the

Figure 2. Scanning electron micrographs of (A) α-Fe2O3 nanorods (NR) with average length 105 ± 32 nm and average diameter 16 ± 5.4 nm and (B) α-Fe2O3 nanoparticles (NP) with average diameter 29 ± 11 nm. The error represents one standard deviation from the mean.

and B). The dimensions vary widely for both types of nanofillers. To estimate the average size and capture the extent of polydispersity, we used several SEM images for which the dimensions could be clearly distinguished. We averaged the lengths of 50 NRs and the diameters of 100 NRs and NPs. The NRs have an average length and diameter of 105 ± 32 and 16 ± 5.4 nm, respectively, giving a nominal aspect ratio of 6.6. The NPs have an average diameter of 29 ± 11 nm. The EDS data, provided in the Supporting Information, reveal only those peaks associated with Fe and O. We added the α-Fe2O3 NPs and NRs to the PEO/LiClO4 electrolyte at concentrations varying from 0.5 to 20 wt %. The ionic conductivities for SPEs at an ether oxygen to Li ratio of 10:1 are illustrated in Figure 3 as a function of temperature for 21219

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samples at temperatures above the melting point (52, 75, and 100 °C, Figure 4A) and at room temperature (Figure 4B). First, we consider the data at temperatures above the melting point where polymer crystallization is not a factor. The conductivity is maximized at a loading of 10 wt %, consistent with the literature for a variety of spherical metal oxide nanofillers (Figure 1). In the case of NRs, the maximum conductivity boost is shifted to lower values. At 10 wt % and T > Tm, it even decreases below that of the unfilled sample. At this NR concentration, we would expect the largest number of percolating pathways. Because our samples undergo no special treatment to align the NRs, these pathways would extend in all directions, and those oriented parallel to the electrode surface could serve as ion traps, inhibiting ion motion normal to the electrodes and decreasing the conductivity. The conductivity improvement above the melting point is often attributed to improved polymer mobility induced by the nanofiller; however, previous QENS measurements have shown this is not the case for PEO/LiClO4 filled with α-Al2O3.32 We used DSC to measure the glass transition temperature of all the samples with a concentration of 10:1, and we detect no Tg change upon nanofiller addition. This result means that the polymer mobility is unaffected by nanofiller addition to an extent detectable by DSC. The onset, average, and final temperatures of the transition are reported in Table 1. Three

conductivity results from the fact that percolation can be achieved at a lower concentration. The concept of high-aspect-ratio nanofillers improving the properties of polymer nanocomposites at low concentration is not new. For example, an electrically conductive network can be achieved in a polymer host using a concentration of multiwall carbon nanotubes that is 2 orders of magnitude lower than spherical carbon black particles.46 The percolation threshold decreases further when the nanotubes are aligned.46 Nanofillers with a large aspect ratio can also improve the mechanical properties of nanocomposites beyond that of spherical nanofillers. When spherical carbon particles are replaced with one-half the amount of single or multiwalled carbon nanotubes in a poly(methyl methacrylate) (PMMA) matrix, the rheological behavior changes from liquid-like to gellike.47 Moreover, the flammability of the nanocomposite is lowered due to the formation of a network structure via the carbon nanotubes.47 These examples demonstrate how smaller amounts of anisotropic nanofillers are required to improve electrical and mechanical properties of a polymer nanocomposite, and the data in Figure 3 show that this extends to ion mobility as well. It is difficult to compare the magnitude of the conductivity improvement for NP versus NR based on Figure 3 alone; therefore, we normalize the conductivity of the filled samples by the unfilled sample and plot this as the “conductivity boost” versus filler concentration in Figure 4. In order to distinguish between samples that are completely amorphous and those that are semicrystalline, the conductivity boost is reported for

Table 1. Glass Transition Temperatures (Tg) of All Samples with an Ether Oxygen to Li Ratio of 10:1a concentration (wt %)

Tgonset (°C)

Tgavg (°C)

Tgfinal (°C)

0 Nanorods (NR) 0.5 1 2.5 5 10 Nanoparticles (NP) 1 2.5 5 10

−36.6 ± 1.5

−33.6 ± 1.5

−31.9 ± 1.9

−33.9 −35.7 −35.8 −34.6 −34.2

± ± ± ± ±

0.4 1.7 1.3 1.8 0.4

−31.3 −32.9 −33.0 −32.7 −32.4

± ± ± ± ±

0.4 1.0 1.0 2.3 0.5

−29.5 −31.7 −31.2 −30.5 −29.9

± ± ± ± ±

0.1 1.0 1.9 2.2 1.3

−32.6 −34.8 −33.1 −33.9

± ± ± ±

0.3 1.2 0.9 0.6

−30.1 −32.5 −29.9 −31.4

± ± ± ±

0.2 0.7 0.3 0.1

−29.2 −30.4 −28.5 −29.3

± ± ± ±

1.0 0.7 0.2 0.6

a

Tgonset and Tgfinal represent the beginning and the end of the transition. The reported value is the average of two measurements, and the error represents the largest and smallest measured values.

possible mechanisms, outlined in the Introduction, could explain why the conductivity improvement persists at temperatures greater than the melting point: nanofiller surface chemistry, percolating pathways of nanofillers, or the stabilization of conductive (PEO)6 remnants by the nanofiller. It is possible that more than one of these mechanisms could be involved. Next, we consider data at temperatures below the melting point where pure crystalline PEO and (PEO)6:LiClO4 can form (Figure 4B). In this region, the conductivity is exponentially dependent on temperature with an activation energy of approximately 1.4 eV for the unfilled sample. NRs lower the activation energy at small loadings (0.5−1 wt %), whereas 10 wt % of the NPs are required to lower the activation energy to a similar extent. Improved conductivity at temperatures below the melting point is typically attributed to the ability of nanofillers to suppress the formation of crystal structures that

Figure 4. Conductivity boost versus nanofiller concentration (A) above the melting point and (B) at room temperature. The data in part A represent the average of six data points (two at each temperature: 52, 78, and 100 °C), and the error bars represent one standard deviation from the average. The data in part B represent two measurements made at each concentration at 22 °C. The dotted lines are included to guide the eye. 21220

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block Li transport, suggesting that the SPEs filled with 1 wt % NRs and 10 wt % NPs have the lowest crystal fractions because they yield the highest conductivity (Figure 4B). To quantify the extent of crystallization, we measure the melting point and heat of fusion using DSC. As discussed in the Introduction, recrystallization kinetics can be slow in SPEs; therefore, we measure the Tm and Hf 10 and 30 min and 24 h after cooling to 22 °C. We measure samples at a concentration of 10:1, filled with 1 and 10 wt % NRs and NPs because these are the best and worst performing samples for each nanofiller shape, and we measure the unfilled sample as our control. Because these samples are at the eutectic concentration, we expect only one melting point near 48 °C according to the phase diagram.3 Therefore, Hf will represent contributions from both crystalline PEO and (PEO)6:LiClO4, making it impossible to decouple the crystal fraction of each phase. The DSC scans indicating the Tm and Hf values are provided in Figure 5. For the SPE filled with 10 wt % NPs (Figure 5B), the data support the concept that conductivity will be maximized in a sample where crystallization is suppressed. After 10 and 30 min of equilibration, the 10 wt % NP sample remains completely amorphous and the conductivity is improved by a factor of ∼8. However, this relationship does not hold for the SPE filled with 1 wt % NRs: the conductivity improves by a factor of ∼7, yet the sample is semicrystalline after 10 min (Figure 5C). Generally, the data indicate that recrystallization occurs at a faster rate for SPEs filled with NRs compared to NPs. One explanation is that the NRs are more highly aggregated than the NPs, and therefore are less effective at preventing crystallization. While we expect both types of nanofillers to be heavily aggregated, we would expect the NRs to aggregate more strongly than NPs because there is more interparticle interaction (i.e., NRs interact along a line, whereas NPs interact at a point). On the basis of the average nanofiller dimensions from the SEM images, the 10 wt % NP sample has 8 times more Fe2O3 surface area available to interact with the polymer electrolyte than the 1 wt % NR sample. Together with the concept that NRs will aggregate more strongly than NPs, this calculation suggests that the conductivity improvement at 1 wt % NR concentration may depend more on long-range ordering than the total surface area in contact with the electrolyte. A second possibility is that long-range ordering of the NRs induces polymer ordering, and perhaps the formation of the conductive (PEO)6:LiClO4 phase is specifically favored. This could account for why the 1 wt % NR sample improves conductivity by a factor of ∼7 while remaining semicrystalline. One piece of evidence from the 10:1 DSC data supports this hypothesis: after the longest equilibration time (i.e., 24 h), the melting point of the sample filled with 10 wt % NRs has shifted to higher temperature. Of the two crystalline phases, (PEO)6:LiClO4 has the higher melting point (see phase diagram in ref 3); thus, the shift in Tm suggests the presence of regions rich in (PEO)6:LiClO4. However, the presence of only one melting point at 10:1 makes it difficult to show clearly that NRs induce the formation of the higher melting point (PEO)6:LiClO4 structure. Thus, we also prepared SPEs at ether oxygen to Li ratios of 8:1 filled with 1 and 10 wt % NPs and NRs, and measured the thermal properties with DSC. At this ratio, both phases do not melt at the same temperature, and two distinct melting features will be present: Tm ∼ 60 °C for (PEO)6:LiClO4 and Tm ∼ 50 °C for pure PEO.3 The presence of two peaks make it easier to identify any effect the nanofiller

Figure 5. DSC scans for SPEs with an ether oxygen to Li ratio of 10:1. Data are provided for the (A) unfilled samples and those loaded with (B) 1 and 10 wt % NPs and (C) NRs after 10 min, 30 min, and 24 h of equilibration. The Hf values indicated on the plot are the average of two measurements, and the error represents the largest and smallest measured values. The approximate locations of Tm are indicated by vertical lines. The data are shifted on the y axis for clarity.

shape might have on specific crystal structures. Because SPEs at this Li concentration require days to recrystallize,8 we equilibrate the samples at room temperature for 7 days in a vacuum prior to the DSC measurement. The DSC data for samples at the 8:1 concentration are provided in Figure 6. SPEs containing 1 and 10 wt % spherical NPs appear to contain more pure crystalline PEO, because the low melting point feature associated with pure PEO is larger and more well-defined. The 1 wt % NR sample is very similar to the unfilled sample; 21221

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Figure 7. Ionic conductivity as a function of time for unfilled, 1 wt % NR, and 10 wt % NP SPEs. The sample cools to 22 °C after 5 min, as indicated on the figure. Figure 6. DSC scans for SPEs with an ether oxygen to Li ratio of 8:1 after 24 h of equilibration at room temperature. The Hf values indicated on the plot are the average of two measurements, and the error represents the largest and smallest measured values. The approximate locations of Tm are indicated by vertical lines. The data are shifted on the y axis for clarity.

higher value, where the (PEO)6 phase is more favored than the pure crystalline PEO phase.

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CONCLUSIONS We measured the ionic conductivity and thermal properties of SPEs filled with spherical Fe2O3 nanoparticles and Fe2O3 nanorods in the range 0.5−20 wt % using impedance spectroscopy and DSC. We demonstrated that NRs improve the conductivity over all measured temperatures (22−100 °C), and do so at concentrations 10−20 times lower than spherical Fe2O3 NPs. One possible explanation is that high-aspect-ratio Fe2O3 NRs create more percolating pathways for ion conduction at lower concentrations than spherical Fe2O3 NPs. The glass transition temperature is independent of nanofiller concentration as measured by DSC, suggesting that nanofillers to do not affect polymer mobility. This is consistent with previous QENS and DSC measurements for a similar system.32 With respect to electrolyte structure, DSC data at ether oxygen to Li ratios of 8:1 and 10:1 suggest that Fe2O3 NRs enhance the formation of the (PEO)6:LiClO4 crystalline phase. This crystal structure is highly conductive and can persist to some extent in the amorphous phase. Thus, a nanofiller that induces the formation of this phase would be beneficialespecially if the conductive structures were aligned normal to the electrode surfaces to allow for direct Li transport. The NRs in our system are randomly dispersed, and we observe a decrease in the conductivity over longer time scales due to crystallization of both PEO and (PEO)6:LiClO4. Regardless of the nanofiller concentration or chemical identity, nanofillers can only increase the conductivity to a modest extent, and therefore, the use of metal oxide nanofillers has essentially been abandoned by the polymer electrolyte community as a tool to improve the conductivity in SPEs. The current study demonstrates that changing the nanofiller shape does not further improve the conductivity but shows that anisotropy affects the concentration at which Fe2O3 nanofillers have a maximum effect. If we can understand why NRs are more effective than spherical NPs per unit volume, and exploit the fact that NRs induce the formation of the highly conductive (PEO)6:LiClO4 phase, perhaps we could use this information to engineer fillers and architectures that would boost the conductivity further while retaining the solid state property of the material.

however, at 10 wt % NR loading, the melting feature associated with pure PEO disappears entirely, leaving only the melting feature associated with (PEO)6:LiClO4. In combination, the DSC data for SPEs at 8:1 and 10:1 loaded with 10 wt % NR provide evidence that NRs can induce the formation of the (PEO)6:LiClO4 structure if present in sufficient concentrations. Although the formation of the PEO6 structure is desirable, the conductivity of the 10:1 sample loaded with 10 wt % NR is lower than the unfilled sample (Figures 3 and 4). Staunton and co-workers showed that it is insufficient simply for the (PEO)6 structure to be present in the SPEthe channels must also be aligned properly.45 A large fraction of misaligned channels may account for the decreased conductivity at 10 wt % NR loading. These results suggest that one key to further improving the conductivity is to align the NRs perpendicular to the electrodean approach we are currently pursuing. We point out that this shift to a higher melting temperature was not observed in the 1 wt % NR sample at either Li concentration (8:1 or 10:1). However, this does not rule out the possibility that (PEO)6 formation at the interface between the electrolyte and the NR could account for the conductivity improvement at 1 wt % NR loading. At this small NR concentration, the additional formation of (PEO)6 may not contribute significantly to the DSC measurement. The DSC data at an ether oxygen to Li ratio of 10:1 suggest that conductivity will vary as a function of time due to recyrstallization. For the unfilled and best performing samples (1 wt % NR and 10 wt % NP), we monitored the roomtemperature conductivity for 30 min after cooling from above the melting point. The data, illustrated in Figure 7, show that the conductivity improvement observed in the 1 wt % NR sample does not persist over longer times. Keeping in mind that the unfilled sample and the 1 wt % NR samples crystallize to a similar extent after 10 min, one possible explanation is that (PEO)6:LiClO4 forms immediately in the 1 wt % NR sample, accounting for the high conductivity at 10 min, and nonconductive pure crystalline PEO forms over longer time scales, leading to the decreased conductivity. These results suggest that the Li concentration should be increased to a 21222

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ASSOCIATED CONTENT

S Supporting Information *

Examples of the impedance, phase angle, and conductivity data are provided as a function of frequency and temperature. The EDS data for the Fe2O3 NPs and NRs are also provided. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 (574) 631-1367. Notes

The authors declare no competing financial interest. References herein to any specific commercial company, product, process, or service by trade name, trademark, maufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the Department of the Army (DoA). The opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or the DoA, and shall not be used for advertising or product endorsement purposes.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the Notre Dame Center for Nano Science and Technology (NDnano) and the U.S. Army TARDEC under Contract No. W56HZV08-C-0236, through a subcontract with Mississippi State University. Any opinions, findings and conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. Army TARDEC. We thank Prof. Paul McGinn at the University of Notre Dame for use of his DSC.

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