ELECTRICAL PROPERTIES OF PVDF-HFP BASED POLYMER ELECTROLYTES CONTAINING IONIC LIQUID

ELECTRICAL PROPERTIES OF PVDF-HFP BASED POLYMER ELECTROLYTES CONTAINING IONIC

LIQUID

M.L.Kaushal

Assistant Professor, Department of Physics, Goswami Ganesh Dutt Sanatan Dharam College, Hariana, Hoshiarpur, Punjab (India)

ABSTRACT

Ionic liquids have unique properties like high ionic conductivity, immeasurable vapour pressure, non- flammability, good thermal and electrochemical stability etc. These properties make them suitable materials to be used for the synthesis of polymer electrolytes. Polymer electrolytes have been prepared by incorporating 2, 3-dimethyl-1-octylimidazolium hexafluorophosphate (DMOImPF₆) in polyvinylidene fluoride-co- hexafluoropropylene (PVdF-HFP) using solution casting technique. The variation of conductivity with the concentration of ionic liquid, ammonium hexafluorophosphate (NH₄PF₆), plasticizers (dimethylacetamide (DMA), propylene carbonate (PC)) and temperature has been investigated. The highest conductivity of 2.2x10^-4 S/cm at room temperature has been obtained for PVdF-HFP + 60wt%DMOImPF₆ + 0.5M NH₄PF₆ + 0.1ml PC.

Keywords: Conductivity, Ionic Liquid, Plasticizer, PVdF-HFP

I. INTRODUCTION

The complexation of metal salt (MX) with polyethylene oxide (PEO) was firstly reported by Wright et. al, however the technological importance of these polymer electrolytes was highlighted in 1978 by Armand et. al [1-2]. Then different metal salts viz sodium iodide (NaI), sodium thiocyanide (NaSCN), sodium perchlorate (NaClO₄), potassium iodide (KI), ammonium triflate (NH₄CF₃SO₃), calcium perchlorate (CaClO₄), lithium triflate (LiCF₃SO₃), lithium perchlorate (LiClO₄) etc were used for complexation with PEO. However, the conductivity of polymer electrolytes containing different lithium salts in the range of 10^-6 – 10^-8 S/cm at room temperature and 10^-3 – 10^-4 S/cm at 100^oC has been observed [3-5]. The high value of ionic conductivity is obtained above melting temperature (65^oC) of crystalline phase of PEO. Above 65^oC the polymer electrolyte exist in amorphous phase, which is high conducting phase for polymer electrolytes. So PEO is mostly studied polymer as a host for ions because of its flexible macromolecular structure and good ability of the ion dissociation. Then various polymer matrix systems have been studied for this purpose includes polyethylene oxide (PEO), polymethylmethacrylate (PMMA) [6-14]. Polymer electrolytes have been prepared by incorporating 2, 3-dimethyl-1-octylimidazolium hexafluorophosphate (DMOImPF₆) in polyvinylidene fluoride- co-hexafluoropropylene (PVdF-HFP). The variation of conductivity with the concentration of ionic liquid, salt

(NH₄PF₆), plasticizers (dimethylacetamide (DMA), propylene carbonate (PC)) and temperature has been investigated. The effect of dielectric constant of plasticizer has also been studied.

II. MATERIALS AND CHARACTERIZATION

DMOImPF₆ was prepared by the anion exchange reaction of equimolar amounts of NH₄PF₆ in acetonitrile with DMOImBr. First, DMOImBr was dissolved in acetonitrile in a round bottom flask and then NH₄PF₆ was added alongwith continuous stirring. After 48 hours of stirring, the precipitates of NH4Br settled down which were removed by filtration and DMOImPF₆ in acetonitrile was obtained. To remove the organic impurities, 2-3gms of neutral alumina was added in the solution containing ionic liquid and acetonitrile and stirred for 2 hours. After that neutral alumina was removed by filtration. The acetonitrile was removed by vacuum evaporation under reduced pressure and DMOImPF₆ was obtained in a quantitative yield. The conductivity of electrolytes has been measured by HP 4284A precision LCR meter (frequency range 20 Hz – 1 MHz) and computer interfaced Hioki 3532 – 50 LCR HiTester (frequency range 42 Hz – 5 MHz) [15-17].

III. RESULTS AND DISCUSSION

The conductivity of polymer electrolytes containing ionic liquids has been measured by impedance spectroscopy using a cell with pressure contact stainless steel electrodes [17]. Due to unique porperties of ionic liquids such as high ionic conductivity, immeasurable vapour pressure, good thermal stability, wide electrochemical window etc., they have been used to prepare polmer electrolytes [18-20]. Polymer electrolytes containing different concentrations of DMOImPF₆ in PVdF-HFP were prepared. The effect of concentration of ionic liquid on the conductivity of electrolytes was studied. The variation of conductivity of polymer electrolytes (PVdF-HFP +x wt%DMOImPF₆) (at 30^oC) as a function of the concentration of ionic liquid is given in Fig.1. The conductivity of PVdF-HFP (~10^-9 S/cm) increases by four orders of magnitude with the addition of ionic liquid, and reaches a value of 4.1x10^-5 S/cm for electrolytes containing 60wt% ionic liquid in PVdF-HFP. The conductivity shows a small decrease at higher concentrations of ionic liquid. The decrease in conductivity at higher concentrations of ionic liquid may be possibly due to the formation of ion aggregates, as a large number of ions are present in these electrolytes, and these ion aggregates do not take part in the conduction process. However the decrease in conductivity at higher concentration of ionic liquid is small and is by a factor only. Polymer electrolytes having composition PVdF-HFP + 60wt%DMOImPF6 possess maximum conductivity at room temperature and were selected for further studies on these materials. The conductivity of the same order has also been reported for polymer electrolytes containing ionic liquids [21-25].

The effect of addition of NH₄PF₆ – salt containing the same anion (PF₆^-) as the ionic liquid (DMOImPF₆) – on the conductivity of polymer electrolytes having composition PVdF-HFP+60wt%DMOImPF₆ was studied and the variation of conductivity (at 30^oC) with the concentration of salt is given in Fig.2. The addition of NH₄PF₆ has been observed to result in a small decrease in conductivity. This may be due to the presence of ion aggregates in polymer electrolytes containing ionic liquid and salt (NH₄PF₆). Both ionic liquid and salt contributes to the formation of ion aggregates, which do not take part in the conduction process and hence a small decrease in conductivity has been observed.

The conductivity of polymer electrolytes follows the general equation of conductivity,  = nq (where n is number of free ions, q is charge and  is mobility of charge carriers). The conductivity can be increased by either increasing the number of free ions or mobility of the free ions in the polymer electrolytes, or by increasing both number and mobility of free ions. Many reports have been available in which improvement in conductivity is observed with the addition of high dielectric constant plasticizers [26-28]. This was investigated in the present case by adding plasticizers with relatively higher value of dielectric constant. The addition of high dielectric constant plasticizer provides the liquid phase (which is a high mobility phase) in polymer electrolyte and hence increases the mobility of free ions. Due to high dielectric constant of plasticizer which helps to dissociate the ion aggregates present in the polymer electrolytes, which results in an increase in free ion concentration.

Figure 1 Variation of Conductivity (at 30

C) for Polymer Electrolytes (PVdF-HFP + x wt%DMOImPF

) with the Concentration of Ionic Liquid.

Figure 2 Variation of Conductivity (at 30

C) for PVdF-HFP + 60wt% DMOImPF

+x (M)

NH

PF

with the Concentration (x) of Salt.

The plasticizers used in the present work are dimethylacetamide (DMA) (=37.8) and propylene carbonate (PC) (=64.4), which have high values of dielectric constant. Plasticized polymer electrolytes were prepared by adding different concentrations of DMA in polymer electrolytes having composition PVdF-HFP + 60wt%DMOImPF₆ + 0.5M NH₄PF₆. The variation of conductivity with the concentration of DMA is given in Fig. 3. The conductivity of electrolytes increases from 3.0x10^-5 S/cm to 8.7x10^-5 S/cm with the addition of 22wt% DMA. More DMA could not be added, as it deteriorates the mechanical properties of plasticized polymer electrolytes. The conductivity (at 30^oC) of plasticized polymer electrolytes containing 0.1ml (16wt%) DMA is 7.2x10^-5 S/cm [5]. Polymer electrolytes containing PC as the plasticizer instead of DMA were also prepared and those having composition PVdF-HFP + 60wt% DMOImPF₆ + 0.5M NH₄PF₆ + 0.1ml PC shows conductivity of 2.2x 10^-4 S/cm at room temperature and this value has also been shown in Fig. 3. The conductivity of plasticized polymer shows the conductivity of the electrolyte containing equal volume of PC.

Figure 3 Variation of Room Temperature Conductivity for PVdF-HFP + 60wt%DMOImPF

+ 0.5M NH

PF

+ DMA with the Concentration of Plasticizer (DMA).

electrolytes containing PC is one order of magnitude higher than for electrolytes containing 16wt% DMA, and is due to the higher dielectric constant of PC (ε=64.4) as compared with DMA (ε=37.8). The addition of plasticizers (DMA, PC) to polymer electrolytes containing ionic liquid contributes to an increase in the dielectric constant of electrolytes, which enhances the dissociation of ion aggregates present in these electrolytes and the liquid nature of plasticizer increases the mobility of free ions. Hence both these factors leads to an increase in free ions concentration (n) and mobility () of ions, and hence an increase in conductivity has been observed (Fig. 3).

The conductivity of polymer electrolytes having composition PVdF-HFP + xwt%DMOImPF6 (x=50, 60, 65 and 70) was measured at different temperatures in the 30–120^oC temperature range. The variation of log conductivity with reciprocal temperature is given in Fig. 4. For all the concentrations of ionic liquid in polymer electrolyte, conductivity increases with an increase in temperature. The highest conductivity (4.2 x 10^-5 S/cm at

30^oC and 1.4 x 10^-3 S/cm at 120^oC) at all temperatures has been observed for polymer electrolyte containing 60wt%DMOImPF₆ in PVdF-HFP. With an increase in temperature from 30 to 120^oC, an increase by two orders of magnitude in conductivity has been observed. At low temperature, the difference between conductivity values for polymer electrolytes containing different concentrations of ionic liquid is small, however the difference between the conductivity values decreases above melting point [5].

Figure 4 Variation of Conductivity with Temperature for PVdF-HFP + xwt% DMOImPF

(x=50(●), 60(■), 65 (▲) and 70(○))

The conductivity of polymer electrolytes having composition PVdF-HFP + 60wt%DMOImPF6 + x(M) NH₄PF₆ (with x = 0.5, 1.0 and 2.0) was measured at different temperatures and the change in log conductivity with reciprocal temperature is given in Fig.5. At room temperature, the conductivity of electrolyte containing 0.5(M) NH₄PF₆ is higher as compared to other concentrations of salt (NH₄PF₆) in the electrolytes, but with an increase in temperature, the difference in conductivity for different concentrations of salt decreases and at temperatures above 70^oC, all the electrolytes containing different concentrations of salt show nearly the same value of conductivity. At the highest temperature, the conductivity of electrolytes containing 0.5M NH₄PF₆ is 1.57 x 10^-3 S/cm which is higher than for electrolytes containing 2M NH₄PF₆ i.e.1.2 x 10^-3 S/cm . This may be due to the reason that, at high concentration of salt, large number of ions hinders the motion of charge carries and hence decreases the conductivity of the electrolyte [5, 28].

Figure 5 Variation of Conductivity with Temperature for PVdF-HFP + 60wt% DMOImPF

+x (M) NH

PF

(x=0.5(●), 1.0(▲),and 2.0 (○))

IV. CONCLUSIONS

- Incorporation of ionic liquid in PVdF-HFP increases the conductivity of polymer electrolyte upto 60wt%

DMOImPF₆ with highest conductivity of 4.1x10^-5 S/cm at room temperature.

- The addition of NH₄PF₆ in PVdF-HFP+60wt%DMOImPF₆ decreases the conductivity of the polymer electrolytes due to formation of ion aggregates.

- The addition of plasticizers, propylene carbonate (PC) and dimethylacetamide (DMA), enhances the conductivity of polymer electrolytes. The highest value of conductivity 2.2x 10^-4 S/cm at room temperature has been obtained for PVdF-HFP + 60wt%DMOImPF₆ + 0.5M NH₄PF₆ + 0.1ml PC.

- For all polymer electrolytes, conductivity increases with increase in temperature. The conductivity of electrolytes containing 0.5M NH₄PF₆ is 1.6 x 10^-3 S/cm at 120^oC.

- The dielectric constant of plasticized polymer electrolyte containing PC have high value of dielectric constant and hence possesed the maximum conductvity.

V. ACKNOWLEDGEMENTS

The author is thankful to Dr. Rajiv Kumar for fruitful discussion about ionic liquids and various characterization techniques for preparation of ionic liquids.

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