List of interlayer expansions and average crystallite size of the nanocomposites.
We report, for the first time, the intercalation of poly[oligo(ethylene glycol) oxalate] (POEGO) and POEGO lithium salt (LiCF3SO3) complex (POEGO-LiCF3SO3) into vanadium pentoxide xerogel (V2O5nH2O). The effect of changing the polymer concentration on the interlayer expansion of the layered host was studied, and the optimal intercalation ratio was determined to be 1:2. The intercalates were characterized by powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, and AC impedance spectroscopy.
- Layered structures
- Electrical properties
- Infrared (IR) spectroscopy
- X-ray diffraction
Intercalation chemistry is a versatile technique for developing new materials at moderate conditions. Unlike other techniques such as organic syntheses where rigorous conditions are used and purification of products is mandatory, intercalation chemistry provides an excellent route to combine the properties of two materials, which often does not require product purification. Therefore, intercalation chemistry is a “green shift” for developing new materials. Vanadium pentoxide xerogel (V2O5
Intercalation of polymers into layered structures is a growing field of research with a wide range of potential applications . For example, organic–inorganic nanocomposites offer promise for new engineering composites in the automotive, packaging, and aerospace industry because of their improved mechanical properties . An organic–inorganic nanocomposite is a two-phase material in which the organic and inorganic phases are distributed in each other at the nanolevel. Therefore, with careful selection of the inorganic host and the guest polymer, it is possible to design materials that can be used in electrochemical energy storage devices such as in Li-ion batteries. Normally, the nanocomposite composition is controlled in order to increase its ionic conductivity at ambient temperatures so that it can be used as a solid electrolyte and/or as a cathode.
Vanadium pentoxide xerogel is a good host material for guest molecules and ions. The intercalation of guest species into vanadium pentoxide xerogel may occur via dipole–dipole interaction, ion exchange, acid–base, coordination, and redox reactions, enabling the system to accept both neutral and charged guest species [7, 8]. Vanadium pentoxide xerogel has also shown promising redox reactions that can be utilized in Li-ion batteries. For example, Passerini et al. demonstrated that V2O5
Current conventional Li-ion batteries utilize liquid organic electrolytes [10, 11] that come with several shortcomings, which limit their widespread usage in large load applications, such as electric vehicles and stationary power. Most of these liabilities are safety-related, which include electrolyte leakage, decomposition, flammability, and a propensity to develop catastrophic short circuits. To circumvent these problems, polymer electrolytes have been extensively studied as an alternative. The search for polymer electrolytes was initiated by the discovery of ionic conduction in complexes of poly(ethylene oxide) (PEO) containing alkali metal salts [12, 13] and the suggestion that such ionic conductors could be used as electrolytes in electrochemical devices . Since then many PEO derivatives have been developed with efforts to improve ionic conductivity [15, 16]. There are two main strategies used for developing PEO derivatives with increased ionic conductivity: one approach focuses on increasing ionic mobility, that is, developing polymers that are flexible and amorphous with low glass transition temperatures. The second strategy focuses on increasing ionic dissociation by placing polar subunits such as acrylamide, acrylonitrile, maleic anhydride, and carbonate along the chains to increase the polymer’s dielectric constant [17, 18]. Based on the second strategy, poly[oligo(ethylene glycol)-oxalate] (POEGO) was developed by Xu et al. . They reported a maximum conductivity of 5.9 × 10−5 S cm−1 at 25°C with the complex of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), with etheric oxygen to Li-ion ratio of [EO]/[Li+] = 16. They also reported that POEGO–LiTFSI complex showed good electrochemical stability up to 4.4 V versus Li+/Li. These properties make POEGO a viable candidate for developing solid electrolytes for Li-ion batteries.
Several studies have been conducted where POEGO is intercalated into various layered structures. The intercalation is meant to improve the mechanical properties of POEGO in order to eliminate its tendency to flow under pressure, while retaining its ionic conductivity, and hence make it more suitable for use as a solid electrolyte in Li-ion batteries. For example, POEGO has been intercalated into hectorite , tin disulfide , graphite oxide , and molybdenum disulfide . To our best knowledge, no work has been reported on the intercalation of POEGO into V2O5
The focus of this chapter is to report on the intercalation of POEGO and LiCF3SO3–POEGO complex into vanadium pentoxide xerogel and the potentials of these nanocomposites in the Li-ion battery applications. In this chapter, the lithium salt (LiCF3SO3,) and a complex of POEGO with LiCF3SO3 will be abbreviated as Li
Sodium metavanadate was purchased from Fluka. Dowex 50W-X 8, 20–50 mesh resin, and oxalic acid dihydrate were purchased from Baker. HPLC-grade methanol was purchased from Caledon. Benzene 99.9% was purchased from Aldrich. All were used as received. Poly(ethylene glycol) (PEG 400), purchased from Aldrich was dried over 3 Å molecular sieves under nitrogen purge from a Schlenk line.
2.2. Vanadium pentoxide xerogel (V2O5
The synthesis of V2O5
2.3. Synthesis of poly[oligo(ethylene glycol) oxalate] (POEGO)
POEGO was synthesized as reported in the literature . In a typical experiment, PEG 400 (15.0 g, 0.0375 mol) was added to oxalic acid dihydrate (5.00 g, 0.0397 mol) in benzene (100 mL), and the mixture was refluxed for 4 days in a 250 mL round-bottom flask while stirring. Benzene was then removed under reduced pressure. The reaction mixture was then heated in a vacuum oven at 120°C until a clear viscous product was obtained. The use of PEG with a molecular weight of 400 means that there are, on average, nine ethyleneoxy repeating units in each oligo(ethylene glycol) oxalate group in our POEGO .
A Li-POEGO complex was prepared using the optimum ionic conductivity ratio of [EO]/Li+ = 16 as reported in the literature .
2.4. Synthesis of nanocomposites
The nanocomposites were prepared by adding POEGO dissolved in methanol to the aqueous solution of V2O5
To confirm that the polymer chains were actually intercalated into V2O5
2.5. Materials characterization
Powder X-ray diffraction (XRD) data were collected on a Bruker AXS D8 Advance instrument equipped with a graphite monochromator, variable divergence slit, variable antiscatter slit, and a scintillation detector. Cu (K
Thermal properties of the samples were investigated using TA instruments. Thermogravimetric analysis (TGA) data were collected on a Q500 in dry air or nitrogen purge using a heating rate of 10°C/min up to 680°C. Differential scanning calorimetry (DSC) was performed on a Q100 under dry nitrogen purge using heating and cooling rates of 10°C/min and 5°C/min, respectively. The TGA and DSC data were processed using the TA Universal Analysis 2000 software. The samples were freeze-dried using a Virtis Benchtop 3.3/Vacu-Freeze dryer and left in a desiccator overnight, before analysis.
Reflectance Fourier transform infrared (FTIR-ATR) spectra were obtained in the range 4000–400 cm−1 on a Bruker ALPHA FT-IR spectrometer equipped with attenuated total reflectance (ATR) sampling unit. The resolution of the instrument was 0.9 cm−1, and 128 scans were used.
Conductivity data were collected by using AC impedance spectroscopy. A Solartron 1250 frequency response analyzer, along with a home-built current-preamplifier circuit was utilized. The amplitude of the sine wave perturbation was 50 mV, and a frequency range from 10 kHz to 0.01 Hz was used. The samples were run as cast films on rectangular glass substrates. Silver paste was placed on the two ends of the films as electrodes, so that current flow was along the film, parallel to the substrate. Prior to the conductivity measurements, samples were held in vacuum for at least 20 h at room temperature to remove any moisture and adsorbed volatile materials. During conductivity measurements, the samples remained in vacuum and were in thermal contact with an electrically heated copper sample holder cooled by a Cryodyne 350 refrigerator. A Lakeshore Cryotronics Model 321 temperature controller was used for temperature regulation. After the conductivity measurements, the glass substrates were cleaved so that the film thickness could be measured with an optical microscope.
3. Results and discussion
3.1. X-ray diffraction (XRD)
Powder XRD patterns were used to confirm the successful intercalation of POEGO and Li-POEGO into V2O5
crystallite size (Å)
The XRD patterns of the nanocomposites showed complete intercalation of POEGO in the gallery space of the host, based on the absence of the pristine V2O5
Table 1 shows that there is no significant increase in
From the XRD patterns, the average crystallite size of the nanomaterials was calculated by using the Scherrer equation. The Scherrer equation is , where
The Scherrer equation is , where
All the nanocomposite mole ratios prepared formed homogeneous solutions that were cast into thin films for XRD and conductivity studies. However, upon freeze-drying, the solutions from mole ratios 1:3 and 1:4 separated into two solid phases. One phase was a fine powder and the other phase was a sticky mass. The two phases were characterized separately with the other techniques (TGA, DSC, and FTIR).
3.2. Thermogravimetric analysis (TGA)
The thermostability and stoichiometric composition of the nanocomposites were determined using TGA data. Figure 2 shows the decomposition profile for POEGO, V2O5MeOH, V2O5POEGO 1:1, and V2O5POEGO 1:4 (the fine powder and the sticky mass). The thermograms were obtained in air using a heating rate of 10°C min−1. POEGO decomposed completely in air, as shown in Figure 2, curve a. The control sample (V2O5MeOH) has the highest residue percentage of 89% as shown in Figure 2, curve e. Curves b and c of Figure 2 show the decomposition profiles for V2O5POEGO 1:4 (sticky mass) and V2O5POEGO 1:4 (fine powder), respectively, while that of V2O5POEGO 1:1 is depicted by curve d. A complete analysis of the TGA data is summarized in Table 2. The nanocomposites have a weight residue percentage that corresponds to the amount of V2O5
|Material||Decomposition in air||Residue (wt.%)|
|Mole ratios used||Composition ratios||Weight (%)||Temperature (°C)2|
||V2O5(H2O)1.9||10.3, 2.5||49; (321, 364)||84.3|
|V2O5POEGO (1:3) f||V2O5(POEGO)0.6(H2O)0.7||59.0||(205, 254)||38.1|
|V2O5POEGO (1:3) s||V2O5(POEGO)1.1(H2O)3.2||68.2||209||24.1|
|V2O5POEGO (1:4) f||V2O5(POEGO)0.4(H2O)0.3||49.6||(195, 245)||48.6|
|V2O5POEGO (1:4) s||V2O5(POEGO)1.2(H2O)0.6||72.9||260||25.5|
The thermogram of the control sample (Figure 2, curve e) shows a constant weight residue above 100°C, that is, after the evaporation of methanol. Therefore, the weight loss process above 100°C for the nanocomposites is associated with the polymer decomposition (see Figure 2). Increasing the amount of POEGO in the nanocomposites decreases the residue percentage, due to the decreased concentration of V2O5
For the nanocomposites which separated into two phases, the fine powder phases (Figure 2, curve c) yielded higher residue percentages compared to the corresponding sticky mass phases (Figure 2, curve b). These data mean that the sticky phases have higher polymer component than the fine powder. Interestingly, the residue percentages of the sticky phases have similar weight percentages of 24.1% and 25.5% for V2O5POEGO 1:3 and V2O5POEGO 1:4, respectively. On the other hand, the corresponding residue percentages of the fine powders are significantly different, having a difference of more than 10%. This observation means that during the phase separation, the composition of the sticky phase is independent of the amount of V2O5
The decomposition profiles can be divided into three stages as shown in Figure 2. In stage I (<120°C), the weight loss corresponds to the evaporation of water/solvent. The weight loss in stage I varies randomly with no correlation to the amount of polymer used. In stage II (120–400°C), the weight loss corresponds to the decomposition of the polymer. At this stage, the percentage weight loss is directly proportional to the amount of the polymer present in the nanocomposite. Finally, stage III (>400°C) corresponds to the residue which has a constant weight percentage. The residue, yellow in color, was identified with XRD to be orthorhombic V2O5 crystals. 2 The temperature values were taken from the peak maximum of the derivative plot. Where the derivative peaks were not well resolved (due to overlapping weight loss steps), the temperature values of the peaks are enclosed in brackets.
2 The temperature values were taken from the peak maximum of the derivative plot. Where the derivative peaks were not well resolved (due to overlapping weight loss steps), the temperature values of the peaks are enclosed in brackets.
The stoichiometric compositions were calculated based on the mass loss at each of the first two stages, the mass of the residue, and the corresponding molecular weight of the compound. The mass loss at stage I, stage II, and the mass of the residue (stage III) were assumed to correspond to water, POEGO, and V2O5, respectively. The moles of water, POEGO, and V2O5 were calculated for each sample. The mole ratios were expressed with respect to the moles of V2O5 (e.g., the composition for V2O5POEGO 1:1 was determined to be V2O5(POEGO)0.3(H2O)1.8).
The mass loss at stage I, stage II, and the mass of the residue (stage III) were assumed to correspond to water, POEGO, and V2O5, respectively. The moles of water, POEGO, and V2O5 were calculated for each sample. The mole ratios were expressed with respect to the moles of V2O5 (e.g., the composition for V2O5POEGO 1:1 was determined to be V2O5(POEGO)0.3(H2O)1.8).
3.3. Differential scanning calorimetry (DSC)
DSC provided important information on
|Materials (mole ratio)||Phase transitions||Δ
|V2O5POEGO (1:3) f||−30.4||115||145.4|
|V2O5POEGO (1:3) s||−35.3||113||148.3|
|V2O5POEGO (1:4) f||−30.0||115||145.0|
|V2O5POEGO (1:4) s||−31.6||116||147.6|
All samples show negative sloping baselines after the glass transition temperature, which indicates the slow heat flow to the sample, which may correspond to the energy used to vaporize volatiles from the samples during heating. The horizontal baseline displayed by the sticky phase (Figure 3, curve d) means that the sample and the reference cell are in thermodynamic equilibrium. This observation is characteristic for amorphous and flexible materials .
The blending temperature being below the decomposition temperature (<200°C) means that thin films can be obtained by hot pressing the fine powder or the sticky mass up to 100°C, without any decomposition or damage.
The DSC comparison between POEGO (Figure 3, curve a) and V2O5POEGO 1:4 sticky mass phase (Figure 3, curve d) showed that the sticky mass phase did not have any crystallization or melting
The temperature gap (Δ
3.4. Fourier transform infrared (FTIR)
FTIR spectroscopy was used to investigate the type of chemical bonds present in the nanocomposites in comparison to POEGO and pristine V2O5
It is important to note that comparison of the IR spectra for V2O5
The absorption bands for C=O and V=O were significantly perturbed in the nanocomposites, an indication for a chemical reaction between POEGO and V2O5
3.5. AC impedance spectroscopy
The impedance experiment involved applying an AC voltage to the sample and measuring the real and imaginary parts of the resulting current. Curve (a) in Figure 5 is a complex plane plot of the impedance of a Li-POEGO cast-film sample at a temperature of 310 K. High-frequency data is near the origin. A semicircle like this, with a low-frequency diagonal spur on the right, is characteristic of an ionic conductor between blocking electrodes . Also shown is a fit to the equivalent-circuit model shown in the inset in Figure 5. In this circuit,
Impedance data for a typical nanocomposite, V2O5POEGO 1:4 (batch EM1-77), at 300 K, is also shown in Figure 5 (curve b). The plots for all our nanocomposites are semicircles, similar to the plots for a simple parallel RC circuit, indicating that the nanocomposites are electronic conductors. Here,
Electronic conductivity data for V2O5
The results for the nanocomposite samples from sample group EM1-33, which were all made at the same time, using the same V2O5
Conductivities of a few V2O5LiPOEGO nanocomposites were also measured, and one typical sample is included in Figure 6. In general, the electrical conductivity of V2O5LiPOEGO was less than that of V2O5POEGO. We were not able to detect any signs of ionic conductivity in the V2O5LiPOEGO nanocomposites. However, with the impedance technique used in this work, if the ionic conductivity is much smaller than the electronic conductivity, it will not be detected. A material like V2O5LiPOEGO 1:4 is expected to conduct lithium ions, and these experiments do not rule that out, since as long as the ionic conductivity is below about 10−5 S/cm at 300 K, we would not be able to observe it.
When conductivity data plot as straight lines in an Arrhenius-type plot like Figure 6, this indicates a thermally activated conduction process described by an equation of the form
Note that the band gap does not depend on the V2O5:POEGO ratio. Our XRD data shows that interlayer expansion is also almost independent of V2O5:POEGO ratio, indicating that the interlayer spaces are fully occupied with a bilayer arrangement of POEGO chains even before the ratio reaches 1:1. Since increasing the amount of POEGO further does not result in more POEGO between the V2O5 layers, some of the POEGO must be outside the nanocomposite crystallites. This interpretation of the XRD data explains two features of the conductivity. First, assuming the increase in activation energy is due to separation of the V2O5 layers and/or interaction between the V2O5 layers and the inserted polymers, there should be no further change in activation energy once the interlayer spaces are full. Second, as the amount of POEGO is increased beyond what is needed to fill the interlayer spaces, regions of electrically insulating polymer will form outside the nanocomposite crystallites, blocking some charge-transport channels and reducing the conductivity through the sample.
Since the conductivity measurements were made with cast films in which phase separation does not occur, we have not yet acquired the conductivity data on the separate fine powder and sticky mass phases obtained from freeze-dried V2O5POEGO 1:3 and V2O5POEGO 1:4. Characterization of charge transport in these phases would be an interesting topic for future work. Because of the high polymer content in the sticky mass phase, complexing it with a lithium salt may yield a material with higher ionic conductivity.
In summary, successful intercalation of POEGO and Li-POEGO into V2O5
The authors are grateful for the financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada, Canada Foundation for Innovation (CFI), Atlantic Innovation Fund of Canada (AIF), Innovation Prince Edward Island (PEI), and University of Prince Edward Island (UPEI).
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- The Scherrer equation is Dhkl=Kλ57.3βcosθ, where Dhkl = average crystallite size (Å), λ is the wavelength of the Cu (Kα) radiation used (λ = 1.5406 Å), β = peak width at half height (2θ), and θ = position of the peak in degrees. The constant 57.3 is the conversion factor from radians to degrees. K is a constant that depends on the shape of the crystallites. The shape of our crystallites is not known. However, we use K = 0.9 (for spheres) for all samples, since we are primarily concerned with the trends, rather than the actual values.
- The temperature values were taken from the peak maximum of the derivative plot. Where the derivative peaks were not well resolved (due to overlapping weight loss steps), the temperature values of the peaks are enclosed in brackets.
- The mass loss at stage I, stage II, and the mass of the residue (stage III) were assumed to correspond to water, POEGO, and V2O5, respectively. The moles of water, POEGO, and V2O5 were calculated for each sample. The mole ratios were expressed with respect to the moles of V2O5 (e.g., the composition for V2O5POEGO 1:1 was determined to be V2O5(POEGO)0.3(H2O)1.8).