Experimental and Theoretical Study of Ionic Pair Dissociation in a

Article pubs.acs.org/JPCB

Experimental and Theoretical Study of Ionic Pair Dissociation in a Lithium Ion−Linear Polyethylenimine−Polyacrylonitrile Blend for Solid Polymer Electrolytes Fernando Pignanelli, Mariano Romero,* Ricardo Faccio,* and Á lvaro W. Mombrú Centro NanoMat/CryssMat/Física, DETEMA, Facultad de Química, Universidad de la República, C.P. 11800 Montevideo, Uruguay ABSTRACT: Herein, we report the preparation and characterization of a novel polymeric blend between linear polyethylene imine (PEI) and polyacrylonitrile (PAN), with the purpose of facilitating the dissociation of lithium perchlorate salt (LiClO4) and thus to enhance Li ion transport. It is a joint theoretical and experimental procedure for evaluating and thus demonstrating the lithium salt dissociation. The procedure implies the correlation between the theoretical pair distribution function (PDF) and conventional X-ray diffraction (XRD) by means of a molecular dynamics (MD) approach. Additionally, we correlated the experimental and theoretical Raman and infrared spectroscopy for vibrational characterization of the lithium salt after dissociation in the polymeric blend. We also performed confocal Raman microscopy analysis to evidence the homogeneity on the distribution of all components and the LiClO4 dissociation in the polymer blend. The electrochemical impedance analysis confirmed that the Li−PAN−PEI blend presents a slightly better lithium conductivity of ∼8 × 10−7 S cm−1. These results suggest that this polymer blend material is promising for the development of novel fluorine-free solid polymer lithium ion electrolytes, and the methodology is suitable for characterizing similar polymeric systems.

1. INTRODUCTION Solid polymer electrolytes (SPEs) are being studied due to their safer design, hermetic sealing, flame resistance, and shape suitability, compared to liquid electrolytes, in order to fulfill the safety issues of lithium ion batteries.1−3 The improvement in the performance of these SPEs is basically related to the reduced crystallinity and capability of coordination to lithium ions of the polymer host. The most common polymers used as the polymer host in SPE are poly(methyl methacrylate) (PMMA),4,5 poly(ethylene oxide) (PEO),6,7 and polyacrylonitrile (PAN)8,9 because of their capability of coordination to lithium ions through carbonyl (CO), ether (C−O−C), or nitrile groups (CN), respectively. On the other hand, the lithium electrolyte sources are usually lithium perchlorate (LiClO4),8 lithium tetrafluoroborate (LiBF4),10 lithium trifluoromethanesulfonate (LiCF3SO3),11 and also imidazolederived lithium salts such as lithium 4,5-dicyano-2-(trifluoromethyl) imidazolide (LiTDI),12 but the latter are more commonly used in liquid solvents. The use of these lithium salts is to weaken the chemical bonds between the anion and lithium ion in order to prevent ionic pair formation, thus enhancing the lithium transference number and conductivity.8,11,12 There are a lot of experimental reports dedicated to understanding the lithium environment in SPEs by using Xray diffraction (XRD),13 nuclear magnetic resonance,14 and microRaman spectroscopy.15 However, there are only very few recent reports focused on theoretical simulation of lithium dissociation in both liquids and SPEs.16,17 Theoretical insight © XXXX American Chemical Society

could be of great significance to get a better understanding and correlation with more realistic systems formed by the lithium salt in the presence of more than one component. In a previous report, we showed that the addition of fluorinefree imidazole compounds have shown a strong interaction through the amine (N−H) groups, mediated by hydrogen bonds with a nitrate anion.18 On the basis of a similar principle, in the present report, we study the preparation of a linear polyethylenimine (PEI) and PAN polymer blend in order to study, both theoretically and experimentally, the dissociation of LiClO4 and the ionic transport. The hypothesis is based on the affinity of amine (N−H) groups of PEI and nitrile (CN) groups of PAN for perchlorate and lithium ions, respectively. A one-step preparation is reported for the Li−PEI−PAN polymer blend for SPE. It is important to mention that the polymeric systems separately present Li ion conductivities of ∼1 × 10−7 and ∼1 × 10−8 S cm−1 for Li−PAN8 and Li−PEI, respectively, at room temperature.19,20 In this work, we present how the Li ion conductivity could be improved to 8 × 10−7 S cm−1. Additionally, we report theoretical simulations by means of molecular dynamics (MD) and density functional theory (DFT) to understand the lithium dissociation process and correlate with the pair distribution function (PDF), X-ray Received: May 14, 2017 Revised: June 14, 2017 Published: June 21, 2017 A

DOI: 10.1021/acs.jpcb.7b04634 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. Li−PEI−PAN model after 2 ns of MD simulation and classical geometry optimization. References for atoms are lithium (brown), chloride (green), oxygen (red), nitrogen (blue), and carbon (black).

and 0.2 cm thickness with sputtered gold blocking electrodes. EIS data was obtained using a 10 mV ac voltage amplitude in the 0.1 Hz−1 MHz frequency regime at 20 °C using a Gamry Reference 3000 impedance analyzer. Data processing was performed using Echem Analyst software. 2.3. Theoretical Simulation of the Li−PEI−PAN Polymer Blend. The methodology used to simulate the Li− PEI−PAN polymer blend at large scale was MD as implemented in the software LAMMPS.21 The force field applied was OPLS-AA (Optimized Potential for Liquid Simulations − All Atom), which can better describe the possible conformations of the polymer chain by adding more torsion parameters than other force fields.22,23 The OPLS-AA can also more accurately describe the interactions between single atoms due to its all-atom force field characteristic. A 776 atoms cubic box with periodic boundary conditions in all directions was adjusted to obtain a density of 1.2 g/mL. In order to verify that the size of the system was adequate for the map of the configuration space, we performed MD simulations using a 2 × 2 × 2 supercell, with a total of 6128 atoms. The PDF confirmed that both simulations led to the same results. The initial geometry was optimized and then used as an input for a 2 ns MD simulation under a NVT ensemble. In addition, we also applied final geometrical optimization using DFT24,25 through the SIESTA code26 using the exchange−correlation functional GGA-PBE.27,28 XRD pattern simulation was calculated using the Debye method as implemented in Debyer29 for a 2 × 2 × 2 supercell of the original, totaling a 6128 atoms cluster. Raman spectra simulation was performed for ∼60 atoms clusters in the surrounding of lithium and perchlorate using DFT with the hybrid exchange−correlation potential B3LYP 30−33 for a 6-31G(d,p) basis set, as implemented in Gaussian 09,34 preceded by geometric optimization.

diffraction (XRD), infrared and Raman spectroscopy, confocal Raman microscopy, and EIS.

2. METHODS 2.1. Preparation of the Li−PEI−PAN Polymer Blend. LiClO4 (99%), PEI (99%), and PAN (99%) were provided from Sigma-Aldrich and used without prior purification. The preparation of the Li−PEI−PAN blend started by dissolving separately LiClO4 in ethanol, PEI in ethanol, and PAN in dimethyl sulfoxide (DMSO) and then mixing the three solutions under vigorous stirring at T = 100 °C until a transparent light yellow homogeneous solution was obtained. In order to obtain a Li−PEI−PAN blends with a 1:10:10 molar ratio, the respective amounts of the reagents mentioned above were used. The mixed Li−PEI−PAN solution was kept under vigorous stirring until dryness, and the obtained solid was additionally dried under vacuum (P ≈ 10 mPa) at T = 70 °C for 10 h until complete removal of residual solvent was achieved. The dried powder samples were finally pressed into pellets at 50 kN/cm2 pressure to obtain the Li−PEI−PAN polymer blend in its final form. 2.2. Characterization of the Li−PEI−PAN Polymer Blend. XRD measurements were performed using a Rigaku Ultima IV diffractometer with CuKα radiation in the 2θ = 5− 60° range using 2θ steps of 0.02° with a 5 s integration time per step. FT-IR spectroscopy was performed using Shimadzu Prestige 21 after previous dilution in KBr and pelletizing. FT-IR spectra were recorded in the range of 400−4000 cm−1 by averaging 20 scans with a resolution of 4 cm−1. Confocal Raman spectroscopy was performed using a WITec Alpha 300RA confocal Raman spectrometer. The laser wavelength used in this experiment was 785 nm to avoid sample fluorescence, and the laser power was set to ∼10 mW in order to avoid sample decomposition. Raman spectra were obtained by averaging a set of 400 spectra with a 0.2 s integration time each. Electrochemical impedance spectroscopy (EIS) measurements were performed for the Li−PAN−PEI pellets with a 1.2 cm diameter B


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3. RESULTS AND DISCUSSION 3.1. Pair Distribution Function (PDF). The Li−PEI− PAN cluster model after MD simulation is shown in Figure 1. The comparison of the PDFs obtained from the classical force relaxation using MD and the geometry optimization using DFT are shown in Figure 2. The different position of some peaks is

in the polymer blend. In order to have an idea of the degree of dissociation, it is important to note that the r(Li−ClO−4) mean distance in the crystal system is about ∼3.3 Å.37 It is also important to remark that both MD and DFT simulations showed similar results, differing in a constant shift down to ∼0.1 Å, originating from utilization of different methodologies. Nevertheless, these results allow us to validate and thus to estimate the lithium dissociation in these systems. 3.2. X-ray Powder Diffraction (XRD). As mentioned before, the simulation of the diffraction pattern was obtained utilizing the Debye method for a 2 × 2 × 2 supercell of the box shown in Figure 1. We observed that this number of atoms in the box was large enough to have good statistics for mean coherence distances in this polymer blend model. The simulation of the diffraction pattern using the Debye method showed excellent agreement with the experimental data, as shown in Figure 3. Experimentally, the isolated polymer

Figure 2. Theoretical PDF [G(r)] for the Li−PEI−PAN model using MD and DFT. The energy convergence for MD simulation is shown in the inset.

probably based on the fact that classical methods usually find the local energy minima near the equilibrium values of bond stretching and bending modes. However, in both cases, the appearance of a peak at r ≈ 1.25 Å can be attributed to the carbon−hydrogen (C−H) bond lengths, and the strong peak at r ≈ 1.36 Å is probably associated with the carbon−carbon (C− C) bond length in both PAN and PEI polymers. It is important to remark that a shortened peak, typically r ≈ 1.5 Å,35 can be associated with the presence of the strong polar nitrile (CN) group. The peak at r ≈ 1.45 Å is probably ascribed to the carbon−nitrogen (C−N) bond lengths of PEI polymer, as already observed using theoretical simulations.36 In addition, other peaks at r ≈ 1.81−1.92 Å could be associated with hydrogen bonds (NH···O) between amine and perchlorate groups. Finally, the presence of diffuse peaks at r ≈ 2.3−2.8 Å can be associated with polymer backbone interchain correlations. The lithium-to-perchlorate distances r(Li− ClO−4) for the Li−PEI−PAN model are summarized in Table 1. Briefly, there are at least three well-defined populations of lithium-to-perchlorate distances with r(Li−ClO−4) ≈ 3.4, 5.3, and 8.3 Å, suggesting different levels of lithium dissociation

Figure 3. Experimental XRD patterns for LiClO4, PEI, PAN, and Li− PEI−PAN and theoretical simulation of XRD patterns for Li−PEI− PAN.

references diffraction peaks were also analyzed in order to evaluate the blending process. Pure PAN showed a sharp diffraction peak at 2θ ≈ 17.0° associated with the (100) plane of a hexagonal structure and a broad peak at 2θ ≈ 28.0° ascribed to the amorphous region.38 On the other hand, pure PEI showed sharp peaks at 2θ ≈ 13.5, 20.3, and 27.3°, associated with its dihydrated crystalline form.39 The Li−PEI− PAN polymer blend showed the presence of well-defined peaks at 2θ ≈ 17.0, 18.0, 23.5, and 26° associated with mean correlation lengths d ≈ 5.2, 4.9, 3.8, and 3.4 Å, respectively. The latter three peaks are probably a consequence of the shifting of the PEI peaks, suggesting that effectively the blending process has taken place. However, it is important to remark that the shifting of these PEI peaks has also been observed in the recrystallization process using other solvents such as DMSO.39 In addition, two lower relative intensity peaks were observed at 2θ ≈ 29.0 and 32.0° associated with mean correlation lengths of d ≈ 3.1 and 2.8 Å and a broad peak at 2θ ≈ 23° ascribed to the amorphous zones of the polymer blend. 3.3. Raman and Infrared Spectroscopy. Experimental Raman and FT-IR spectra for LiClO4, PEI, PAN, and Li−PEI− PAN are shown in Figure 4. FT-IR spectra for the LiClO4 reference showed typical broad peaks at ∼1100 and 1650 cm−1, attributed to stretching and bending modes of the perchlorate

Table 1. Theoretical Distance between the Lithium Cation and Perchlorate Anion r(Li−ClO−4) for the Li−PEI−PAN Model Using MD and DFT Calculations r(Li−ClO−4) (Å)a

[OPLS-AA]

[GGA-PBE]

Li-1 Li-2 Li-3 Li-4 Li-5

3.49 3.36 5.34 5.33 8.35

3.50 3.33 5.26 5.45 8.20

a

The theoretical distances between the lithium cation and perchlorate anion r(Li−ClO−4) were extracted as Li−Cl distances. C


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Table 2. Experimental Infrared (I) and Raman (R) Frequencies for Most Relevant Vibrational Modes for LiClO4, PEI, PAN, and Li−PEI−PAN LiClO4

PEI

PAN

Li−PEI−PAN

assignmentsa

270m

265m

oop(C2C−N) [R] ν2(ClO4) [R] δ(C−CN) [R] ν4(ClO4) [R] mixed [I] ν1(ClO4) [R] ν(C−C) [I] mixed [I] twist,wag(CH2) [R/I] δ(C−C−C) [R] ν(C−N) [R/I] ν(C−N) [I] ν(C−N) [R] mixed [I] twist(CH2) [R] δ(CH2) [R] δ(CH2) [I] δ(CH2) [R/I] mixed [I] δ(CH2) [R] ν(C−N) [R/I] ν(C−H) [I] ν(N−H) [I]

467w 530m 623w 893m 967s

933s 1040m 1076m 1090s

1090m 1103m

1121s 1144s 1163m

1121m 1144m 1250m

1297m 1360m 1385m 1454s

1385s 1458m

1460m 1491m 2243s 2851m 2920m

2243m 2849m 2915m

a

Experimental FT-IR and Raman vibrational mode assignments were based on literature and DFT theoretical simulations.

Figure 4. Experimental (a) FT-IR and (b) Raman spectra for LiClO4, PAN, PEI, and Li−PEI−PAN.

∼266, 531, 1108, and 1456 cm−1, revealing no drastic modifications from PAN vibrational modes. However, a sharp peak at ∼934 cm−1 was observed in relation with a shift of the perchlorate symmetric stretching mode, originally located at ∼965 cm−1. The shift to lower-wavenumber values could probably be ascribed to dissociation of the lithium ion from the perchlorate anion, as already reported.41 In order to explain Raman spectra of the Li−PEI−PAN polymer blend with more accuracy, we performed theoretical Raman spectra simulation using DFT. We evaluate the effect of two lithium-to-perchlorate distances based on the PDF analysis discussed previously. The two models evaluated for this purpose are shown in Figure 5. Li−PEI−PAN-A and Li−PEI−PAN-B models represent clusters with lithium-to-perchlorate distances of r(Li−ClO−4) ≈ 3.5 and 5.4 Å, respectively. Theoretical infrared and Raman spectra for Li−PEI−PAN-A and Li−PEI−PAN-B models are shown in Figure 6. The most important features that can be extracted from this analysis are the suppression of the relative intensity of the CN stretching mode at ∼2340 cm−1 for both infrared and Raman spectra when the lithium-to-perchlorate distance increases, as shown in Figure 6. This is in agreement with the experimental data, suggesting that when lithium and perchlorate are separated enough, there is strong coordination of the lithium ion with the nitrile (CN) group of PAN. Moreover, the peaks at ∼1160−1220 cm−1 ascribed to PEI C− N stretching modes showed a global shift to lower wavenumber at ∼1080−1160 cm−1 when the lithium-to-perchlorate distance increased, as shown in Figure 6. This is in agreement with experimental data, suggesting that the PEI polymer is probably interacting with the perchlorate anion via hydrogen bonding. In addition, the Raman peak associated with perchlorate symmetric stretching was located at ∼846 and 834 cm−1 for

anion, respectively.40 All of the relevant signals are presented in Table 2. In addition, Raman spectra for solid LiClO4 showed a sharp peak at ∼967 cm−1 ascribed to the symmetric stretching mode (ν1) of the perchlorate anion and two other peaks with lower intensity at ∼623 and 467 cm−1, ascribed to the ν4 and ν2 modes, respectively.36 FT-IR spectra for PAN reference showed two sharp peaks at ∼1452 and 2243 cm−1, ascribed to CH2 bending and CN stretching modes, respectively, and other broad and low-intensity peaks. Raman spectra showed broad peaks at ∼270, 530, 1103, 1330, and 1454 cm−1 associated with the C2CN out-of-plane and bending modes of C−CN, C−C−C, CH2, and CH3 groups, respectively. FT-IR spectra for PEI reference showed sharp peaks at ∼1088, 1119, and 1144 cm−1 related to CH2 twisting and two C−N stretching modes, respectively. Raman spectra for the PEI polymer showed peaks at ∼1120, 1163, 1297, and 1491 cm−1, associated with two C− N stretching and CH 2 twisting and bending modes, respectively. Li−PEI−PAN polymer blend FT-IR spectra showed peaks at 1087, 1118, and 1138 cm−1, suggesting no drastic modification from PEI vibrational modes, except for a slight shift of the C−N stretching mode from 1144 to 1138 cm−1, which could be possibly associated with the hydrogen bond interactions (NH···O) with perchlorate anions. The presence of a sharp peak at 1385 cm−1 is probably ascribed to the shift of the CH2 bending mode, originally at ∼1452 cm−1 for pure PAN polymer, also suggesting some kind of interaction with PEI or a perchlorate anion. In addition, no drastic modifications from other broad peaks were observed, but a decrease in the CN stretching mode sharp peak at ∼2243 cm −1 was found, suggesting the possibility of partial coordination to the lithium ion. On the other hand, Raman spectra of the Li−PEI−PAN polymer blend showed peaks at D


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Figure 5. (a) Li−PEI−PAN-A and (b) Li−PEI−PAN-B cluster models after DFT optimization. References for atoms are lithium (brown), chloride (green), oxygen (red), nitrogen (blue), and carbon (black).

Figure 7. Confocal Raman imaging (a) filtered by components and (b) by I[ν(ClO−4]/I[δ(CH2)] for the Li−PEI−PAN polymer blend. Figure 6. Theoretical (a) FT-IR and (b) Raman spectra for Li−PEI− PAN-A and Li−PEI−PAN-B models.

respectively. The three components showed good homogeneity in the 40 × 40 μm2 analyzed image shown in Figure 7. This means that all of the components, that is, PAN, PEI, and LiClO4, are very well dispersed in the polymer blend, showing no microscopic segregation of any of their single components. The Raman spectra for selected 266 × 266 nm2 pixels associated with three rich-component selected areas are shown in Figure 8. The homogeneity of the polymer blending process is evidenced by the very similar Raman spectra for the three component-rich zones, for which only slight variations in the relative intensity were observed, revealing that even for the smallest pixel of the image, that is, ∼300 nm, still no segregation of single components is observed but a mixture of all of them. These slight increments in the relative intensity were associated, as expected, with the oop(C2CN) PAN mode for the PAN-rich zone, the ν(ClO−4) mode for the

Li−PEI−PAN-A and Li−PEI−PAN-B, respectively. This is also in agreement with experimental data, evidencing that the shift to lower wavenumber of the perchlorate symmetric stretching is directly related to the increase of the lithium dissociation. 3.4. Confocal Raman Microscopy. In order to obtain information about lithium dissociation in the Li−PEI−PAN polymer blend, we also performed confocal Raman microscopy, as shown in Figure 7. The confocal Raman image filtered by components shown in Figure 7 was obtained using the out-ofplane oop(C2CN) PAN mode at 265 cm−1, the symmetric stretching ν(ClO−4) perchlorate mode at 935 cm−1, and the bending δ(CH2) PEI mode at 1465 cm−1 to obtain PAN-rich (red), perchlorate-rich (green), and PEI-rich (blue) zones, E


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Figure 8. Raman spectra for selected 266 × 266 nm2 pixels associated with perchlorate-rich (green), PEI-rich (blue), and PAN-rich (red) zones in Figure 7a for the Li−PEI−PAN polymer blend.

perchlorate-rich zone, and the PEI δ(CH2) for the PEI-rich zone. The good homogeneity of the confocal Raman image is evidence of the intimate mixing at the nanoscale level, thus revealing the success of the blending process for the Li−PEI− PAN polymer blend. We use the I[ν(ClO−4)]/I[δ(CH2)] relative intensities ratio between the characteristic perchlorate symmetric stretching mode at ∼934 cm−1 and the polymer CH2 bending mode at ∼1452 cm−1 to obtain the contrast image shown in Figure 7. Using this correction to the confocal Raman image, we can determine more precisely the zones were the prechlorate anion is present in a higher concentration for a selected area of the Li−PEI−PAN polymer blend. The zone where the I[ν(ClO−4)]/I[δ(CH2)] ratio increased is represented in red using the scale reference shown in Figure 7. On the basis of this correction, we can have a quantitative approach to the LiClO4 concentration if further calibration curves are performed. 3.5. Electrochemical impedance Spectroscopy (EIS). EIS for the Li−PEI−PAN polymer blend is shown in Figure 9. The Nyquist plot represented as imaginary (Z″) versus real (Z′) impedance is shown in Figure 9, in which a typical arc circle is observed in relation with the parallel combination of the bulk resistance (R) and constant phase element (CPE). The irregularities at the electrode/electrolyte contact are observed at low frequencies and can be modeled using a CPE connected in series,42 as shown in Figure 9. The phase (ϕ) versus frequency plot is shown in Figure 9, which showed resistive (ϕ ≈ 0°) to capacitive (ϕ ≈ −80°) transition behavior with increasing frequency. The model fitting showed good agreement with both Nyquist and phase plots, and the total resistance (R) showed R = 3.8 × 108 ohm, which is associated with a lithium conductivity of σLi = RA/t = 8.0 × 10−7 S cm−1, with A being the area of the electrodes and t the thickness of the pellet. This lithium conductivity is on the order of those observed for other SPEs,42 proving its good characteristics as a promising polymer host for future lithium ion electrolytes.

Figure 9. EIS analysis for the Li−PEI−PAN polymer blend.

blending process. Raman and infrared spectroscopy experimental and theoretical analyses revealed the lithium dissociation process acting as a predicting technique to determine the degree of dissociation of the lithium−perchlorate ionic pair in these polymer blend electrolytes. Confocal Raman microscopy showed evidence of the homogeneity of all components and the lithium−perchlorate distribution in the polymer blend electrolyte. Finally, EIS analysis showed lithium conductivity of ∼8 × 10−7 S cm−1, as typically observed for SPEs, emerging as a promising material for the development of novel fluorine-free solid polymer lithium ion electrolytes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.F.). *E-mail: [email protected] (M.R.). ORCID

Ricardo Faccio: 0000-0003-1650-7677 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the Uruguayan funding institutions CSIC, ANII, and PEDECIBA. We would like to acknowledge financial support from the EQC-X-2012-1-14 ANII research project.

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4. CONCLUSIONS We report the successful preparation and characterization of a novel PEI and PAN polymer blend in order to study the dissociation of LiClO4 and the ionic transport. Experimental and theoretical structural studies by means of PDF and XRD revealed a semicrystalline structure, evidencing the polymer

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Experimental and Theoretical Study of Ionic Pair Dissociation in a

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