Studies on 1-Butyl 3-Methylimidazolium Hexafluorophosphate Incorporated

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Studies on 1-Butyl 3-Methylimidazolium Hexafluorophosphate Incorporated PVC-PBMA Polymer Electrolytes 1

R. Arunkumar1, Ravi Shanker Babu1, M. Usha Rani1 1 – Department of Physics, School of Advanced Sciences, VIT University, Vellore, Tamilnadu, India DOI 10.2412/mmse.59.9.873 provided by Seo4U.link

Keywords: ionic liquids, LiPF6, solution casting techniques, impedance analysis, SEM analysis.

ABSTRACT. Polymer electrolytes consisting of polyvinyl chloride (PVC) and poly (butyl methacrylate) (PBMA) as polymers, lithium hexafluorophosphate (LiPF6) as complex salt and 1-butyl 3-methylimidazolium hexafluorophosphate (BmImPF6) as plasticizer were prepared by solution casting technique. The effect of ionic liquid (BmImPF6) on PVCPBMA blend polymer electrolytes are investigated by AC impedance, dielectric and SEM analysis to elucidate their electrical, dielectric and surface morphological assessment. Ionic conductivity of the prepared polymer electrolytes is found to be in the order of 10-3 S cm-1. Polymer electrolyte with PVC-PBMA-LiPF6-BmImPF6 (17-17-06-60) is found to be a potential candidate in battery applications.

Introduction. Tremendous research on solid polymer electrolytes with salt/plasticizer/ceramics replaces liquid electrolytes in lithium batteries because of limitations in liquid electrolytes like corrosion, leakage, flammability, degradation, etc. Solid polymer electrolytes (SPE) fulfil those drawbacks and further SPE are highly compatible with electrodes when compare to its counterparts [1], [2], [3]. Solid polymer electrolytes based lithium batteries are used as power sources (electronic devices) in laptops, digital cameras, mobile phones, hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and as batteries for electric vehicles (EVs) [4], [5]. The main drawback with solid polymer electrolyte battery is its poor ionic conductivity at room temperature. To increase the room temperature ionic conductivity, several methods like addition of plasticizer, blending of polymers, crosslinking of polymer, incorporation of inorganic fillers and ionic liquids (ILS) are reported. Among this, the present work focuses on ionic liquid based polymer electrolytes. Ionic liquid doped polymer electrolytes have good ionic conductivity at low temperature because it can dissociate the anion and cation easily at lower temperature due to the molten state of ILS is below 373K. Further ionic liquid doped polymer electrolytes have good thermal stability, non-flammability, non-volatility, non-toxicity and wide electrochemical window [6]. The ionic liquid incorporated polymer electrolyte batteries are operated at low temperature (313K) [7]. Ionic liquid have the advantages of the previously mentioned properties when compared to those with organic solvents [8].

The present work reports the investigation on ILS doped polymer electrolytes. Extensively studied on the variation in ionic conductivity, dielectric behaviour and morphology of polymer electrolytes with BmImPF6carried out using ac impedance, dielectric and SEM analysis respectively.

Experimental. Polyvinyl chloride with average molecular weight 48000 g/mol, poly (butyl methacrylate) with average molecular weight 337000 g/mol, lithium hexafluorophosphateand 1-butyl 3-methylimidazolium hexafluorophosphatewere procured from sigma Aldrich, USA. PVC-PBMA polymer electrolytes with addition of BmImPF6at different ratios were prepared by solution casting technique. The required amounts of substances were dried at 373K under vacuum at 10-3 millibar for 10h. The dried polymers and salt were treated with pre-distilled tetrahydrofuran and left undisturbed. The solution of PVC and PBMA were mixed together for 24h using a magnetic 1

© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

stirrer followed by the addition of LiPF6 and BmImPF6 after 5h. The homogenous mixture was stirred at elevated temperature until slurry was formed. The slurry transformed into Teflon coated glass plate/petridishes and was left in vacuum atmosphere to evaporate the remaining solvent. The resultant film was subjected to heat treatment toevaporatetheresidual solvent if any.

Conductivity and dielectric measurement of ILS incorporated PVC-PBMA polymer electrolytes were carried out by using HIOKI 3532-50 LCR Hi TESTER meter in frequency range of 50Hz to 5MHz with temperature difference 303 to 373K. Surface morphology of PVC-PBMApolymer electrolytes were analysed by SEM analysis using Carl Zeiss EVO/185H,UK instrument and accelerating voltage at 10kV. Results and discussion Conductivity studies The ionic conductivity of the polymer electrolyte were calculated using the following relation:

ď ł=đ?‘…

đ??ż đ?‘?đ??´

(1)

where L –thickness of the sample measured with using peacock meter; A – area of the film (A= π r2); Rb – bulk resistance obtained from intercept on X-axis in Cole-Cole plot.

Temperature dependent ionic conductivity of ionic liquid doped PVC-PBMA polymer electrolytes are depicted in Fig. 1a. Ionic conductivity of the polymer electrolytes increase with increase in temperature. As the temperature increases polymer electrolyte can expand and produce more free volume. In free volume, ionic transportation can occur between electrodes, which lead to enhancement in ionic conductivity. Dependence of ionic conductivity on temperature for polymer electrolytes doped with ionic liquid exhibited an increase of two orders of magnitude at 373K (Table.1). Temperature dependent ionic conductivity of polymer electrolytes are found to obeys the Vogel TammannFulcher (VTF) relation and it confirms the ionic conductivity occurs due to migration of ions in a viscous matrix [9].

Fig. 1. Ionic conductivity of PVC-PBMA polymer electrolytes depends on (a) temperature and (b) various concentrations of BmImPF6.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 1 (b) shows the variation of conductivity with the variation of ionic liquid concentration. The best room temperature ionic conductivity of the polymer electrolytes is found to be 1.284 x 10-3S cm1 at 303K for film A5 which is four orders higher than the polymer electrolytes (0.017 x 10-5S cm-1at 303K) without ionic liquid. The increase in ionic conductivity with BmImPF6 concentration is due to (i) large number of ionic charge carriers provide by ILS since it has cautions (BmIm+) as well as anions (PF6−), further (ii) the low viscosity of BmImPF6 also assist in increasing amorphicity or reducing crystalinity of the polymer electrolytes which would ensure conformations in polymer chain leading to segmental motion resulting in higher conductivity [10].

Table 1. Ionic conductivity of PVC-PBMA polymer electrolytes.

Sample code

PVC:PBMA:LiPF6: BmImPF6

A1

Ionic conductivity 10-5 (S cm-1) 303K

318K

333K

353K

373K

47:47:06:00

0.017

0.022

0.035

0.108

0.254

A2

37:37:06:20

0.052

0.093

0.162

0.461

1.320

A3

27:27:06:40

0.122

0.230

0.514

2.952

7.371

A4

17:17:06:60

3.912

6.631

8.256

55.06

153.1

A5

07:07:06:80

128.4

228.5

342.0

467.5

961.4

Dielectric studies The real and imaginary part of dielectric constant (Ć?Ęš&Ć?Ęş) of PVC-PBMA polymer electrolytes are evaluated using the following relation, đ??śđ?‘‘

Ć?Ęš =Ć?đ?‘œ đ??´ ď ł

Ć?Ęş = đ?œ” Ć?đ?‘œ

(2) (3)

where C – capacitance; d – thickness; A – area of the polymer electrolyte membrane;

ď ł – conductivity; ω – angular frequency; Ć?o – is permittivity of free space (8.854 x 10-12 F/m).

The real (Ć?Ęš) and imaginary part (Ć?Ęş)of dielectric constant as function of frequency for 60 wt% of ionic liquid incorporated PVC-PBMA polymer electrolytes at different temperature are depicated in Fig. 2(a, b). The real and imaginary part of dielectric constant increase with increase in temperature, which is due to increase in free ions and charge carrier density. The dielectric constants (Ć?Ęš and Ć?Ęş) of PVC-PMA polymer electrolytes are high at low frequency and it decrease gradually with increase in frequency and tends almost to zero at higher frequency denoting the presence of electrode polarization effect.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig.2. Temperature dependent dielectric constant of PVC-PBMA polymer electrolytes (a) real part and (b) imaginary part.

Dielectric modulus The dielectric modulus was introduced by Macedo et al and is inversely proportional to the dielectric constant. The real (Mʹ) and imaginary part (Mʺ) of dielectric modulus of PVC-PBMA polymer electrolytes have been calculated by using the following relation: Ɛʹ

Mʹ =(Ɛʹ)2 +(Ɛʺ)2 Ɛʺ

Mʺ = (Ɛʹ)2 +(Ɛʺ)2

(4) (5)

where Ɛʹ, Ɛʺ– are real and imaginary part of dielectric constant respectively.

Frequency dependent real and imaginary dielectric modulus for 60 wt% of BmImPF6 incorporated PVC-PBMA polymer electrolytes are depicted in Fig.3a&b respectively. Both real and imaginary part of dielectric modulus found to decreases at low frequencies, which implies negligible contribution due to electrode polarization. The peak intensity for both real and imaginary modulus is high for lower temperature at higher frequency region. The peak intensity of the dielectric modulus decrease with increase in temperature may be due to the presence of plurality of relaxation mechanism. The presence of long tail at low frequency is due to large capacitance associated with the electrodes.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 3. Temperature dependent dielectric modulus of PVC-PBMA polymer electrolytes (a) real part and (b) imaginary part.

SEM analysis

Fig. 4. SEM image for 60 wt % BmImPF6 incorporated PVC-PBMA polymer electrolytes at different magnification (a) 3000x and (b) 7000x.

The surface morphology of the PVC-PBMA polymer electrolytes doped with 60 wt%BmImPF6at different magnifications (3000&7000x) is shown in Fig. 4 (a, b).The presences of smooth and ununiformed sized pores and further its helps in enhancing the ionic conductivity of the polymer electrolytes.

Summary. PVC-PBMA blend polymer electrolytes with BmImPF6at different concentration were prepared by solution casting technique. The temperature dependent ionic conductivity of PVC-PBMA polymer electrolytes obeys Vogel Tammann Fulcher relation. The detailed frequency dependent dielectric behaviour (Ɛʹ, Ɛʺ, Mʹ and Mʺ) of PVC-PBMA polymer electrolytes are discussed and reported. These supporting the defence from conductivity studies which proves the high ionic conductivity of PVC-PBMA polymer electrolyte with 60 wt% of BmImPF6 exhibiting good stability suitable for battery applications.

References

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

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