Conductivity Enhancement Studies on Poly -Poly Composite Polymer Electrolytes

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

Conductivity Enhancement Studies on Poly (Acrylonitrile)-Poly (Vinylidene Fluoride) Composite Polymer Electrolytes 1

M. Usha Rani1, Ravi Shanker Babu1, S. Rajendran2, R. Arunkumar1 1 – Department of Physics, School of Advanced Sciences, VIT University, Vellore 2 – Department of Physics, Alagappa University, Karaikudi, India DOI 10.2412/mmse.8.72.942 provided by Seo4U.link

Keywords: polymer electrolyte, composite, inert filler, plasticizer, impedance studies.

ABSTRACT. Composite electrolyte films consisting of poly (acrylonitrile), poly (vinylidene Fluoride), ethylene carbonate, propylene carbonate, lithium tetra fluoroborate (LiBF4) and also titanium dioxide (TiO2) particles have been prepared by solution casting technique. The effect of inorganic filler on the conductivity of the blended polymer electrolyte has been studied. A conductivity of 3.1 x 10 -5 S cm-1 is achieved at room temperature for the composition PAN-PVdF–LiBF4-EC-PC (21-10-8-33.3-27.7), whereas it improves two orders of magnitude (i.e. 5.624 ×10 −3 S cm−1) upon dispersing fine particles of TiO2 as inert filler into the matrix. The role of ceramic phase is to increase the ionic conductivity and to reduce the melting temperature which is ascertained from conductivity and thermo gravimetric/differential thermal analysis respectively.

Introduction. Decades ago, Wright and co-workers [1] pioneered the research on solid polymer electrolytes and later Armand et al. [2] realized the potential applications of these materials in batteries with high specific energy and other ionic devices. Even though, polymer electrolytes are advantageous in terms of shape, geometry, mechanical strength and the potential for strong electrode electrolyte contact, they have some disadvantages like, poor interfacial properties, low ionic conductivity at ambient temperature [3]. Generally polymer electrolytes show practical ionic conductivity only at higher temperatures, and their melting points, and at such high temperatures, they exist in a ‘quasi-liquid’ state and become very flexible, and therefore show very poor dimensional stability. A dimensionally polymer electrolyte film easily cause a short circuit between a cathode and an anode when it is applied to all solid-state lithium battery. Increasing ionic conductivity by increasing the salt concentration is ruled out because, higher salt content may favour reduction in crystalline fraction of polymer but causes high ion-pairing interaction, which lead to salt aggregation [4].

Hitherto several studies have been made primarily on the enhancement of ionic conductivity at ambient temperature via various approaches such as blends, copolymers, comb-shaped polymers, cross-linked networks, addition of plasticizers and incorporation of ceramic fillers onto the polymer matrix [5]. Studies have revealed that plasticized polymer electrolytes lose their mechanical strength upon addition of plasticizer and lead poor interfacial properties. The mechanical properties of the polymer electrolytes can be increased either by chemical or physical curing which incurs high processing cost. Recently, phase-inversion technique has drawn the attention of many researchers, despite its advantages it suffers from poor rate capability [6]. Very recently, studies reveal that the composite polymer electrolytes could offer lithium batteries with reliability and improved safety [6].

Many reports are available on the effect of ceramic oxides on polymer electrolytes such as, physical and electrochemical properties, increase in cation transference number and improvement of interfacial stability between the composite polymer electrolyte and lithium metal. In this work, novel composite polymer electrolyte composed of PAN-PVdF-EC-PC-LiBF4-TiO2 as promising electrolyte 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

for all solid-state lithium-ion batteries was prepared, and optimization for high ionic conductivity was carried out by investigating the amount of ceramic filler incorporated.

Experimental. Poly (acrylonitrile) (PAN) (average molecular weight: 94000) and poly (vinylidene fluoride) (PVdF) (average molecular weight: 534000) bought from Aldrich, USA were dried at 353K under vacuum for 10 h; lithium tetra fluoroborate (LiBF4) (Aldrich) was dried at 343K under vacuum for 24h. Plasticizer ethylene carbonate (EC) propylene carbonate (PC) (Aldrich) was used without further purification. Titanium dioxide (TiO2) procured from Aldrich, USA of particle size <5 µm was used after annealing at 373K for 10 h. All the electrolytes were prepared by solution casting technique.

Appropriate quantities of PAN, PVdF, LiBF4(Table 1) were dissolved by adding in sequence to predistilled DMF (dimethylformamide. E. Merck, Germany). After incorporating the required amount of plasticizer EC and PC, inorganic filler TiO2was suspended in the solution, stirred for about 48 hours at room temperature, and then at 333K for 4 h before the electrolytes were cast on finely polished Teflon supports or Teflon covered glass plates. The films were dried in vacuum oven at 333K at a pressure of 10−3Torr for 24 h. The thus obtained film was visually examined for its dryness and free-standing nature. The obtained films were characterized by XRD, FTIR, conductivity, TG/DTA and SEM analysis.

Results and discussion Structural Analysis. The X-ray diffraction method has been used only in a limited perspective to identify or confirm the amorphicity, complexation of the polymer electrolyte films. The X-ray diffraction pattern of pure PAN, PVdF, LiBF4, TiO2 and complexes are shown in Fig 1.(I) respectively. Fig 1 [I. (b), (c) and (d)] reveals the crystalline nature of PVdF (with sharp peaks at 14º, 19º & 22 º), LiBF4 and TiO2 respectively. From the diffraction patterns it is obvious that there is a decrease in relative intensity and broadening of the peak in the complexes. It may be due to the addition of salt and blending of amorphous PAN, which induces a change in the crystallographic organization in the crystalline PVdF. This result can be interpreted by considering the Hodge et al.

[7] criterion, which establishes a correlation between the height of the peak and the degree of crystallinity. The effect of adding TiO2 to the polymer complex is to improve ionic conductivity and thermal stability. The sharp peaks in the spectrum of polymer complex [Fig.1. I. (g), (h) and (i)] reveal the presence of undissolved TiO2 in the polymer matrix, Fig 1. I. (e & f) show the effect of Tio2 upon PVdF which shows the reduction of crystallinity of PVdF. The diffraction peaks are found with lower intensity till 10 wt% and found to increase on further addition indicating the increase in crystallinity of the polymer electrolyte which may be responsible for the lowering of ionic conductivity. The maximum ionic conductivity is found for PAN-PVdF-EC-PC-TiO2 (10 wt %) polymer electrolyte system which may be due to the higher amorphicity of the polymer electrolyte.

FT-IR spectroscopy is used to establish interaction between the constituents used in the complex. In the present case, FT-IR is used to establish the interaction between the polymers, salt and plasticizers.

Such interaction can induce changes in vibrational modes of the atoms or molecules in the material.

The FTIR spectra obtained for pure PAN, PVdF, LiBF4, EC, PC, TiO2 and the complexes in the range of 4000 to 400 cm-1 is shown in Fig. 1 [II (a-k)] respectively. The vibrational bands at 2942, 2245, 1250, 1074 cm-1 in pure PAN is assigned to C-H stretching, CN stretching, C-N stretching, C-C stretching respectively. The characteristic frequency of PVdF occurring at 1277, 1185, 854 cm - 1 are assigned to C-F stretching, C-F2 stretching and characteristic frequency of vinylidine compound respectively. These characteristic frequencies of PAN and PVdF mentioned above are found to be shifted to 2952, 2243, 1076 and 1174, 881cm-1 for PAN and PVdF respectively. The characteristic frequency corresponding to C-N and C-F stretching are found to be absent in the complex. Some of the absorption peaks corresponding to the primary constituents of the polymer complexes were found to be shifted in the polymer complex. The absorption bands at (1455, 781, 640 cm-1) of PAN, (2996, 1554, 1165 cm-1) of EC, (2933, 1484 cm-1) of PC and 1320 cm-1 of LiBF4 are shifted to (1452, 777, 645 cm-1), (2291, 1556, 1170 cm-1), (2937, 1483 cm-1) and 1313 cm-1 respectively. Apart from the

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shift in peaks, there are some new peaks obtained at (2980, 2519, 1969, 1570, 974, 717 and 482 cm- 1) in the complexes. The above analysis establishes the confirmation of complex formation.

Fig. 1. (I). XRD patterns of (a) PAN, (b) PVdF, (c) LiBF4, (d) TiO2 and complexes PAN(21)PVdF(10)-LiBF4(8)-EC(33.3)-PC(27.7)-TiO2(X), (e) 0, (f) 5, (g) 10, (h) 15, (i) 20. (II). FTIR Spectra of (a) PAN (b) PVdF, (c) LiBF4, (d) EC, (e) PC, (f) TiO2 and complexes PAN(21)-PVdF (10)LiBF4(8)-EC(33.3)-PC(27.7)-TiO2(X), (g) 0, (h) 5, (i) 10, (j) 15, (k) 20.

Conductivity measurements. Inorganic filler (TiO2) dependent ionic conductivity of PAN-PVdF composite polymer electrolytes are depicted in Fig.2. Evident from the isotherm (Fig. 2a), that, as the concentration of ceramic increases, the conductivity is found to increase up to a certain concentration (10 wt %) and then decrease Table 1. This increase in conductivity due to ceramic addition can be attributed to (a) the ceramic particles acting as nucleation centres for the formation of minute crystallites. (b) The ceramic particles aiding in the formation of amorphous phase in the polymer electrolyte. (c) The formation of a new kinetic path via polymer ceramic boundaries (i.e. mobility of ions through ceramic rich phase which entraps the residual solvents ensuing ion mobility). The conductivity is not a linear function of filler concentration, at low concentration levels of TiO2, the dilution effect which tends to depress the conductivity is efficiently contrasted by the specific interactions of ceramic surface, which promote fast transport thus the net result is a progressive enhancement of the conductivity. At higher filler content the dilution effect predominates and the conductivity decays.

Temperature dependence of ionic conductivity is found to increase with increase in temperature (Fig. 2b) is due to polymer segmental motion. At higher temperature, the segmental motion either permits the ions to hop from one site to another or provides necessary voids for ions to move in the polymer matrix. As the temperature increases, polymer chains acquire faster internal modes in which the bond rotations produce faster segmental motion. This in turn, favours the hoping inter and intra-chain ion movements and the conductivity of the polymer electrolyte increases accordingly. The temperature MMSE Journal. Open Access www.mmse.xyz

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

dependence of electrical conductivity (log  vs. 1/T) indicates that the ionic conductivity obeys VTF relation, which describes the transport properties in a viscous matrix. However, at lower temperature, the presence of Li salt lead to salt-polymer or cation-dipole interaction, which increase the cohesive energy of polymer networks. As the free volume decreases, polymer segmental motion and ionic mobility are hindered, hence ionic conductivity decreases. It is found that PAN-PVdF-LiBF4-EC-PC complex with 10 wt. % of TiO2 has got the maximum room temperature conductivity of 5.624 X 103 S/cm which is higher compared to the system bereft of ceramic oxide.

Fig. 2. (a). Conductivity of PAN-PVdF-LiBF4-EC-PC System as a function of TiO2 concentration, (b). Arrhenius plot of log σ Vs reciprocal temperature of PAN( 21)-PVdF(10)-LiBF4(8)- PC(27.7)EC(33.3)- TiO2(X wt. %) 0 (1), 5 (2), 10 (3), 15 (4), 20 wt. %.

Table 1. Conductivity values of PAN(21)-PVdF(10)-LiBF4(8)-EC(33.3)-PC(27.7) with 5 different composition of TiO2 at different temperatures.

Films

Composition of TiO2

Conductivity values of PAN: PVdF : LiBF4: X TiO2 in x 10-3 S cm-1 303 K

318 K

333 K

353 K

373 K

0

0

0.031

0.076

0.166

0.240

0.372

1

5

0.127

0.240

0.378

0.566

0.831

2

10

5.624

1.413

2.741

4.258

5.505

3

15

0.355

0.933

1.860

3.155

4.168

4

20

0.217

0.644

1.410

2.195

2.792

TG / DTA analysis Thermal analysis of PAN-PVdF-EC-PC-LiBF4-TiO2 (10 wt. %) system which shows maximum ionic conductivity was carried out using PERKIN ELMER (Pyris Diamond) USA in the range 32 to 825°C at a heating rate of 10°C/min. The TG/DTA spectrum of the sample mentioned above is shown in Fig. 3.

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Fig. 3. TG/DTA curve for PAN(21)-PVdF(10)-LiBF4(8)-EC(33.3)-PC(27.7)-TiO2(10).

This shows an endothermic peak in DTA around 49-50°C associated with a weight loss of 9% which may be due to the evaporation of moisture absorbed by the sample during loading. The polymer electrolyte film is found to be stable till 279°C associated with a weight loss of 15% which could be confirmed by the exothermic peak obtained around 253°C-321°C with a peak maximum at 290°C. The weight loss of the polymer beyond 290°C is heavy which may be due to the decomposition of the electrolyte constituents indicating the temperature range for efficient usage. Hence it is concluded that the polymer electrolyte PAN(21)-PVdF(10) –EC(33.3)-PC(27.7)-LiBF4(8)-TiO2(10) can be effectively used in lithium polymer battery applications.

Scanning electron microscopic studies.

The microstructure of polymer blend films plays a vital role for effective use in practical applications.

Fig.4 exhibits the photographs of PAN (21) – PVdF (10) – LiBF4 (8)- EC (33.3)-PC (27.7) - TiO2 (10) at two different magnification i.e. 200 and 1000. Under these magnifications, it is seen that the ceramic particles are so closely packed which would offer low unstable interfacial resistance by reducing the growth of lithium passivation. Moreover the presence of ceramic fillers could accommodate more amount of plasticizer and polymer matrix in between them (evident from Fig. 4a and b respectively), which would help in withstanding the stress produced during fabrication and functioning of this electrolyte in battery applications.

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

Fig. 4. SEM photographs of PAN(21)-PVdF(10)-LiBF4(8)-EC(33.3)-PC(27.7)-TiO2(10) at (a 200 (b) 1000 magnifications.

Summary. Five different polymer electrolyte systems consisting of PAN–PVdF–LiBF4–EC- PCTiO2 [TiO2=0, 5, 10, 15, 20 wt. %] have been studied. Of the five films, the film 2 is found to be the best on the basis of conductivity and mechanical stability. The conductivity of the polymer electrolyte PAN (21)-PVdF (10) –EC (33.3)-PC (27.7)-LiBF4 (8)-TiO2 (10) is found to be maximum (5.624 X 10-3 S/cm). The thermal stability of the film is estimated as 280°C. Hence, the properties (based on the studies reported) of PAN (21)-PVdF (10) –EC (33.3)-PC (27.7)-LiBF4 (8)-TiO2 (10) polymer electrolyte look very promising for Li battery applications and could be used effectively.

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[2] Armand, M. B., Chabagno, J. M., Duclot, N. J., &Vashishta, P. (1979). Mundy, Shenoy (Eds.), Fast Ion Transport in Solids.

[3] Appetecchi, G. B., Scaccia, S., Passerini, S. (2000). Investigation on the Stability of the Lithium‐ Polymer Electrolyte Interface. Journal of the Electrochemical Society, Vol. 147(12), 4448-4452, DOI 10.1149/1.1394084 [4] Reddy, M. J., Chu, P. P. (2002). Ion pair formation and its effect in PEO: Mg solid polymer electrolyte system. Journal of power sources, Vol. 109(2), 340-346.

[5] Marcinek, M., Syzdek, J., Marczewski, M., Piszcz, M., Niedzicki, L., Kalita, M., Kasprzyk, M. (2015). Electrolytes for Li-ion transport–Review. Solid State Ionics, Vol. 276, 107-126.M. [6] Appetecchi, G. B., Croce, F., Persi, L., Ronci, F., Scrosati, B. (2000). Transport and interfacial properties of composite polymer electrolytes. Electrochimica Acta, Vol. 45(8), 1481-1490.

[7] Hodge, R. M., Edward, G. H., & Simon, G. P. (1996). Water absorption and states of water in semi crystalline poly (vinyl alcohol) films. Polymer, Vol. 37(8), 1371-1376.

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