PVDF-HFP

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Charge Carrier Relaxation in Different Plasticized PEO/PVDF-HFP Blend Solid Polymer Electrolytes Sayan Das, and Aswini Ghosh J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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The Journal of Physical Chemistry

Charge Carrier Relaxation in Different Plasticized PEO/PVDF-HFP Blend Solid Polymer Electrolytes S. Das and A. Ghosh* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India * Corresponding author, E-mail: [email protected]

ABSTRACT In this paper, we report firstly the effect of concentration of ethylene carbonate plasticizer on conduction and relaxation of charge carriers in PEO / PVDF-HFP - LiClO4 blend electrolytes. Secondly, the results for different plasticizers such as ethylene carbonate, propylene carbonate and dimethyle carbonate on the conductivity and relaxation in these blend electrolytes are compared. We have followed a new approach for the analysis of the conductivity data. The frequency dependent conductivity is analyzed using random freeenergy barrier model, taking into consideration the low frequency polarization effect. The temperature dependences of the ionic conductivity and the relaxation time obtained from the model exhibit Vogel-Tammann-Fulcher behaviour. Using the scaling of the ac conductivity spectra it is observed that the relaxation dynamics of charge carriers in blend electrolytes are independent of temperature but depends on the nature of plasticizers. The electric modulus is studied using Havriliak–Negami function for the understanding of ionic relaxation. The modulus data are also analyzed using non-exponential Kohlrausch–Williams–Watts function. The temperature dependence of the relaxation time obtained from modulus analysis follows Vogel–Tammann–Fulcher relation for all plasticized electrolytes. It is observed that the 1 ACS Paragon Plus Environment


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stretched exponent is much lower than unity, which suggests that charge carrier relaxation is highly non-exponential in these plasticized electrolytes. INTRODUCTION Polymer electrolytes are of great interest because of their potential application in electrochemical devices such as battery, fuel cell, super-capacitors etc.

1-5

. Solid polymer

electrolytes are promising candidates compared to liquid and gel electrolytes, as they are flexible, easily shaped and safe due to no leakage6. These polymer electrolytes are commonly doped with alkali salts in the host polymer to achieve high ionic conductivity7-10. Amongst the polymer hosts of practical interest, polyethylene oxide (PEO) is the most promising for applications in rechargeable lithium ion batteries. The conductivity of polymer electrolyte depends on ionic and segmental motion of the host polymer. Though, PEO has better flexibility, good complexation properties and good mechanical stability

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, the conductivity

below melting point of PEO is low due to high crystalline phase 2. Now-a-days poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) is immensely considered as a polymer matrix for its good mechanical stability and low crystalline nature12-16. This copolymer contains PVDF as crystalline phase ensuring good mechanical support of polymer matrix and HFP as amorphous phase which immobilizes large amount of liquid electrolyte. Some techniques such as addition of plasticizer, insertion of nano-particles, addition of ionic liquid, blending of two polymers, etc. have been adopted to enhance the ionic conductivity of these polymer electrolytes17-22. The study of ion transport mechanism in polymer electrolytes is a great challenge due to inherent complexity of polymer structure 23. Impedance spectroscopy has been widely used to study conduction and relaxation processes in polymer materials. Ion conduction in polymer electrolytes depends on ion hopping process and also on the dynamics of segmental motion of 2 ACS Paragon Plus Environment

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the polymer complex. The frequency dependence of the ac conductivity in these materials shows power law behaviour such as ω n, where ω is angular frequency and n is frequency exponent. The value of n provides information about conduction mechanism24. A simple random free-energy barrier model (RBM) proposed by Dyre is considered to describe such typical behaviour of the ac conductivity

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. The relaxation dynamics can be expressed in

different representations such as conductivity, dielectric permittivity and modulus formalism 26-27

. In the low frequency region, dielectric permittivity data are concealed by interfacial and

electrode polarization and the relaxation process is hardly observed. But in modulus representation the relaxation phenomenon becomes distinct as electrode and interfacial polarizations are suppressed. In this paper, we have studied first the effect of concentration of ethylene carbonate (EC) plasticizer on ion conduction and relaxation dynamics of blend polymer electrolyte consisting of polyethylene oxide (PEO) and poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) with lithium perchlorate (LiClO4) as dopant salt. Secondly, the results for ethylene carbonate, propylene carbonate and dimethyle carbonate (DMC) plasticizers on the conductivity and ionic relaxation in blend electrolytes have been compared. We have analyzed the results using a new approach by taking into account the contribution of electrode polarization, calculated considering the fractal nature of electrode-sample interface28. EXPERIMENTAL Sample Preparation PVDF-HFP (M.W 460000 g/mol), PEO (M.W 400000 g/mol), and LiClO4 (SigmaAldrich) were dried in vacuum prior to use. Acetonitrile and acetone (Sigma-Aldrich) were used as solvents. The molar ratio of ethylene oxide segments to lithium ions was kept fixed at EO: Li = 9:1. Appropriate amounts of PEO and LiClO4 were dissolved in acetonitrile and 3 ACS Paragon Plus Environment


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stirred in a magnetic stirrer. As PVDF-HFP is not dissolved in acetonitrile, we have used acetone and the solution was stirred for 4-5 hours in a magnetic stirrer and heated at 50 oC. Then both the solutions were mixed. The resulting solution was stirred for 20 hours. The solution became viscous due to evaporation of the solvent and was cast in a polytetrafluoroethylene (PTFE) container and kept for 24 hours for normal evaporation. Finally, it was dried for 24 hours at 50 oC in vacuum to form free standing homogeneous film. For the preparation of the PEO / PVDF-HFP - LiClO4 - X wt. % EC polymer electrolytes, appropriate amounts of EC were added to mixed solutions of PEO and PVDFHFP under stirring condition. The same procedure as stated above was followed to get films of PEO / PVDF-HFP - LiClO4 - X wt. % EC polymer electrolytes. The ratio of PEO:PVDFHFP was maintained at 2:3 as the conductivity for this ratio exhibits maximum18. For the preparation of polymer electrolytes with PC and DMC plasticizers, fixed amounts (30 wt. %) of PC and DMC were added separately to the solution of PEO and LiClO4 in acetonitrile under stirring condition. Thereafter, the same technique as stated above was followed to get films of PEO / PVDF-HFP - LiClO4 - 30 wt. % PC and PEO / PVDFHFP - LiClO4 - 30 wt. % DMC polymer electrolytes. The thickness of the polymer films varied between 0.2 mm and 0.4 mm. We have fixed 30 wt. % for PC and DMC, since we observed maximum conductivity for 30 wt. % in case of EC plasticized electrolyte as discussed later in the text. Characterization X-ray diffraction (XRD) patterns of the prepared films were recorded in an X-ray diffractometer (D8 ADVANCE, BRUKER AXS) using Cu Kα radiation (0.154 nm wavelength) at a scan rate of 0.3 degree min-1. Differential scanning calorimetry (DSC) experiments were performed in a DSC instrument (TA, model Q2000 and model Q600) in N2 4 ACS Paragon Plus Environment

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atmosphere with a heating rate of 10 oC min-1. The surface morphology of platinum sputtered polymer films was studied using a field emission scanning electron microscope (FE-SEM) (JEOL JSM-6700F). Fourier transform infrared (FTIR) spectra of these samples were recorded in the wave number range 400–4000 cm–1 in a FTIR spectrometer with (model SHIMADZU 8400S). The resolution of spectrometer is 4 cm-1. For electrical measurements the samples were sandwiched between two stainless steel blocking electrodes of a conductivity cell. The measurements of capacitance and conductance of the films were carried out in a RLC meter (Quad Tech, model 7600) in the frequency range of 10 Hz – 2 MHz in vacuum in the temperature range of 210 – 340 K with a stability of ± 0.10 K. The ionic conductivity of the films at different temperatures was obtained from the complex impedance plots. RESULTS AND DISCUSSION X-Ray Diffraction XRD patterns of the PEO, PVDF-HFP, PEO / PVDF-HFP, PEO / PVDF-HFP LiClO4 - X wt. % EC are shown in Fig. 1, while the inset shows the same for PEO / PVDFHFP - LiClO4-30 wt. % PC and PEO / PVDF-HFP - LiClO4 - 30 wt. % DMC. It is noted that the diffraction peaks, characteristic of PEO at 2θ = 19° and 23.5° and characteristic of PVDFHFP at 2θ = 18.26°, 20.01° and 26.66°, are clearly observed in the PEO / PVDF-HFP blend 29

. As the plasticizer content is increased in polymer blend, the intensity of the diffraction

peaks gradually decreases due to the increase of amorphous phase in the polymer blends. Differential scanning calorimetry The DSC traces of PEO / PVDF-HFP - LiClO4 - X wt. % EC, PEO / (PVDF-HFP) – LiClO4-30 wt. % PC and PEO / (PVDF-HFP) – LiClO4 - 30 wt. % DMC blend polymer electrolytes are shown in Fig. 2. It is observed that all samples exhibit glass transition followed by melting. The glass transition temperature (Tg), melting temperatures (Tm) and 5 ACS Paragon Plus Environment


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melting enthalpy (∆Hm) have been calculated from the DSC traces and are listed in Table 1. It is noted that incorporation of plasticizers in polymer blend decreases melting enthalpy which is minimum for 30 wt. % EC. The glass transition temperature Tg which governs segmental motion, becomes minimum for 30 wt. % EC concentration. The lowest melting temperature and enthalpy have been obtained for 30 wt. % EC content in polymer electrolyte, when compared to blend polymer electrolytes with 30 wt% PC and DMC plasticizers. The relative percentage of crystallinity (Xc) has been calculated using the relation Xc = ∆Hm/ ∆H0, where ∆Hm is melting enthalpy obtained from DSC traces and ∆H0 is melting enthalpy of 100% crystalline PVDF-HFP (104.7 g/J-1) 30 and is listed in Table 1 for all compositions. It is noted in the table that the minimum crystallinity is obtained for 30 wt. % EC content. It is observed in Table 1 that glass transition temperature and crystallinity for composition PEO / PVDFHFP - LiClO4 - 30 wt. % EC are the lowest of PEO / PVDF-HFP - LiClO4 - 30 wt. % PC and PEO / PVDF-HFP - LiClO4 - 30 wt. % DMC blend electrolytes. FE-SEM micrographs Fig. 3 (a) shows the FE-SEM image of the PEO/PVDF-HFP-LiClO4 electrolyte, while the FE-SEM images of the plasticized polymer electrolyte films are shown in Figs. 3(b)–3(d). The sponge-like homogeneous structure with micropores is observed in the films due to porous structure of the PVDF-HFP polymer31. Small spherulites with micropores are observed on the surface texture with the increase of the plasticizer content. Reduction of the crystalline phase in the electrolyte is related to the smooth surface morphology of the polymer electrolytes. It is clear that the incorporation of EC plasticizer reduces crystalline phase of host polymers, thus enhancing overall amorphous phases. FTIR Spectra

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FTIR spectral analysis is a powerful technique to characterise molecular interaction and chemical bonding in polymer electrolytes. Fig. 4 (a) shows the FTIR spectra in the wave range 400 – 4000 cm-1 of all polymer electrolytes. The wagging and bending vibrations of – CF2 of PVDF-HFP at 416 and 502 cm-1 get shifted to 483 and 511 cm-1 due to plasticization. The peaks at 608 and 763 cm-1 observed for the crystalline phase of PVDF-HFP is shifted to 624 and 775 cm-1 respectively in plasticized samples, while the peak at 881 cm-1 due to amorphous phase of PVDF-HFP remains unaltered. The peaks at 1173 and 1390 cm-1 observed for PVDF-HFP due to symmetrical stretching of –CF2 and –CH2 groups shift slightly to 1178 and 1407 cm-1 respectively32. The two bands observed at 840 and 950 cm-1 belong to CH2 rocking vibrations of methylene groups and are related to helical structural group of PEO33. The vibrational band at 1100 cm-1 is assigned to C-O-C (symmetric and asymmetric) stretching of PEO. The peak observed at 1236 cm-1 due to CH2 symmetric twisting of PEO shifts to 1233 cm-1 in the plasticized samples. Two bands at 1359 and 1343 cm-1 belonging to CH2 wagging and CH2 bending get shifted to 1330 and 1350 cm-1 respectively. To study ion-ion interaction the (ClO4-) mode appearing at ~ 620 - 640 cm-1 has been analyzed. The (ClO4-) envelope consists of two peaks at 624 cm-1 and 632 cm-1 which correspond to ClO4- ‘free’ ion vibration and Li+ ClO4- contact-ion pairs respectively34. The de-convolution of the peak, using Gaussian-Lorentzian product function, is shown in Fig. 4 (b) for a composition. It is observed that the relative intensity of the free ions, which is proportional to the relative concentration of free ions are 95.53, 91.90 and 82.96 for EC, PC and DMC based electrolytes. So the addition of DMC decreases concentration of free ions, which is responsible for the decrease of their ionic conductivity as shown later. Dc Conductivity



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The reciprocal temperature dependence of the ionic conductivity (σdc), obtained from the complex impedance plots, are shown in Fig. 5 (a) for PEO / PVDF-HFP - LiClO4 - X wt. % EC electrolytes. The same for PEO / PVDF-HFP - LiClO4 - 30 wt. % PC and PEO / PVDF-HFP - LiClO4 - 30 wt. % DMC is shown in Fig. 5 (b) along with PEO / PVDF-HFP LiClO4 - 30 wt. % EC for comparison. It is noted in Figs. 5 (a) and (b) that temperature dependence of the ionic conductivity follows VTF relation given by 35-37 σ dc = σ 0T −1/ 2exp  − E σ / k B ( T − T0 ) 

(1)

where σ0 is the pre-exponential factor, kB is the Boltzman constant, Eσ is the pseudoactivation energy, T0 is the Vogel scaling temperature and T is the absolute temperature. The experimental data in Figs. 5 (a) and 3 (b) were fitted to Eq. (1). The values of the Eσ and T0 obtained from best fits for different compositions are listed in Table 2. It is noted that the values of Eσ and T0 are the lowest and highest respectively for electrolyte with 30 wt. % EC among other electrolytes with 30 wt. % of PC and DMC. The composition dependence of the conductivity for PEO / PVDF-HFP - LiClO4 - x wt. % EC electrolytes is shown in Fig. 5 (c). It is observed in Fig. 5 (c) that the conductivity shows a maximum for 30 wt. % EC. It is also in Table 2 that the maximum ionic conductivity is obtained for PEO / PVDF-HFP - LiClO4 30 wt. % EC electrolytes, while minimum conductivity is observed for electrolyte with 30 wt. % DMC. It may be noted that the addition of DMC, which has lower dielectric constant (ε = 3.1) than those of PEO (ε ≈ 5) and PVDF-HFP (ε ≈ 11.38) decreases the conductivity of PEO / PVDF-HFP - LiClO4 electrolyte. But EC (ε = 89.78) and PC (ε = 66.14) plasticizers with higher dielectric constants than those of PEO and PVDF-HFP increase ionic conductivity of PEO / PVDF-HFP - LiClO4 electrolyte. Thus, the value of dielectric constant of the plasticizer should be higher than that of the polymers for the enhancement of the conductivity. Otherwise, it shows anti-plasticization effect as in the case of DMC. Similar


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effects of plasticizers on the conductivity of other polymer electrolytes have been also reported in the literature38-41.

Ac conductivity and scaling The frequency dependence of the real (σ′) and imaginary part (σ″) of complex conductivity (σ*) for PEO / PVDF-HFP - LiClO4 - 30 wt. % EC electrolyte at several temperatures are shown in Figs. 6 (a) and 6 (b), respectively. It is noted in Fig. 6 (a) that at very low frequency σ′ decreases rapidly due to interfacial and electrode polarization effects. It is noted also in Fig. 6 (a) that at comparatively high temperature σ′ is independent of frequency, corresponding to the dc conductivity. However, at higher frequencies, σ′ increases with the increase in frequency showing a dispersive behaviour. The imaginary part (σ″) shows a gradual increase with increasing frequency at higher frequencies. As the frequency is decreased, σ″ decreases up to a certain frequency where polarization effect starts and below that frequency σ″ shows an increasing trend. On further decreasing the frequency σ″ spectra show a maximum, indicating full development of polarization and σ″ again decreases at very low frequencies. The same nature is observed for other electrolyte films at different temperatures. The frequency dependence of the ac conductivity spectra can be analyzed using the random free-energy barrier model (RBM) as proposed by Dyre

25

. In this model, the

conduction is assumed to take place via hoping of the charge carriers in conducting materials, which are subjected to spatially random energy landscapes. According to this model, the complex conductivity function is given by

 iωτe  σ*(ω) = σdc    ln(1 + iωτe ) 

(2)

where τe = 1/ ωe, ωe is the attempt frequency to overcome the largest barrier determining the dc conductivity σdc. As electrode polarization is dominant in the low frequency region, we 9 ACS Paragon Plus Environment


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have taken into account the electrode polarization effect which was calculated considering fractal nature of the electrode-polymer interface by several authors28, 42 and is given by

σ*(ω) = σ0 +

(σdc − σ0 ) (1 + iωτ J )−α

(3)

Where σdc is the dc conductivity and σ0 does not coincide with the value σdc in the presence of the electrode polarization effect. α (0

PVDF-HFP

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