Preparation and characterization of hot-pressed solid polymer electrolytes:

Indian Journal of Pure & Applied Physics Vol. 54, September 2016, pp. 583-588 Preparation and characterization of hot-pressed solid polymer electrolytes: (1-x)PEO: xnabr Angesh Chandra* Department of Applied Physics, Shri Shankaracharya Institute of Professional Management & Technology, Raipur 492 015, India Received 26 November 2014; revised 8 July 2016; accepted 10 August 2016 Preparation and ionic transport characterizations of a new Na + ion conducting hot-pressed solid polymer electrolytes (SPEs) (1-x)PEO: xnabr, where 0 < x < 50 in wt%, have been reported. SPE films have been prepared using a hot-press technique in place of the traditional solution cast method. Two orders of magnitude of the conductivity enhancement achieved in SPE film composition (70PEO: at room temperature. Polymer-salt complexations and thermal behavior of present SPEs have been explained with the help of X-ray diffraction (XRD), differential scanning calorimetric (DSC), thermogrevimetric analysis (TGA) and scanning electron microscopy (SEM) technique. The Na + ion conduction in the present SPEs has been discussed on the basis of experimental measurements on its ionic conductivity σ, ionic mobility µ and ionic transference number t ion at room temperature. Temperature dependent ionic conductivity measurement has also been carried out to calculate the activation energy E a. Keywords: Solid polymer electrolytes, Ionic conductivity, Hot-press technique, XRD, FTIR, DSC, TGA 1 Introduction The development of new poly(ethylene oxide) (PEO) based solid polymer electrolytes (SPEs), complexed with variety of ionizable alkali metal salts show tremendous promises specially in thin film battery technology in last three decades 1-3. The main materials advantages of PEO-based solid polymer electrolytes are their good mechanical processibility, ease of fabrication in the thin flexible film forms which make them as appropriate choices in fabricating all-solid-state thin film electrochemical devices viz mini/ micro batteries of desirable shapes/sizes. Moreover, SPEs provide very good electrode/ electrolyte contacts. Ion conduction in polymers was reported for the first time by Fenton et al 4 in 1973. Since then, a large number of solid polymer electrolytes (SPEs) involving different kinds of ion conductors viz Ag +, Na +, K +, Mg ++, NH + 4 etc has been reported 5-7. Among the known ion conductors, Na + ion salt shows various advantage such as its availability in abundance at a cheaper cost than lithium and more softness of the material makes it easier to achieve good contact with electrode and electrolyte in the solid state devices viz. polymeric batteries fabrication 8-10. The interaction between other alkali ions and the polar groups of polymer is stronger *Corresponding author (E-mail: chandrassi@gmail.com) than that of Na + ion. Thus in lithium or other alkali ions, more activation energy required for ionic conduction thane Na + ion in polymer electrolyte. Hence the Na + ion salt has been selected as a dopant to prepare the present SPE. SPE films are prepared, in general, by a traditional solution-cast/ sol-gel methods. But these traditional techniques have some drawbacks such as relatively very longer procedure, more expensive etc. To overcome these difficulties recently, a novel hot-press technique has been developed for casting solid polymer electrolyte (SPE)/nano composite polymer electrolyte (NCPE) films. Hot-press technique has several procedural conveniences viz solution free/dry, least expensive, quicker procedure etc than those of traditional methods 11-15. In the present investigation, preparation and characterization of a new Na + ion conducting PEObased solid polymer electrolytes (SPEs) (1-x)PEO: xnabr, where 0 < x < 50 wt% is reported. Materials characterization and ion transport property studies of the present SPE films are reported with the help of various experimental methods and previously proposed models. 2 Experimental Solid Polymer Electrolytes (SPEs) (1-x)PEO: xnabr, where 0 < x < 50 wt.%, were prepared by using the precursor chemicals PEO (10 5 Mw, Aldrich,

584 INDIAN J PURE & APPL PHYS, VOL 54, SEPTEMBER 2016 USA) and NaBr (purity > 98%, Merck, India). The dry powders of NaBr, as complexing salts and PEO, as polymeric host, in different wt% ratios were ground thoroughly at room temperature for about ~1 h in an agate pestle and mortar. The physically mixed PEO salt mixtures of different compositions were heated separately close to the melting point of PEO (~70 C) for ~30 min with mixing continuing. This resulted in hot/soft slurry of polymer complexed with the salt. The slurry so obtained was then pressed between two SS-cold blocks at ~1.25 ton cm 2 which gave rise to a uniform thin film of thickness ~0.015 cm. Polymer-salt complexation and materials characterization were done with the help of X-ray diffraction (XRD) patterns using Shimadzu X-ray diffractometer at Cu-Kα radiation, fourier transform infra-red (FTIR) (model: Shimadzu-8400), scanning electron microscopy (SEM) (model: JEOL, JXA- 8100, Japan), differential scanning calorimetry (DSC) (model: Perkin Elmer) and thermogrevimetric analysis (TGA) (model: SDT Universal). The ionic conductivity σ measurements were carried out by the equation [σ = l/(r b.a)], where R b is the bulk resistance, l is the thickness and A is the cross sectional area of the polymeric film. The bulk resistance R b was determined on different polymeric films using an LCR- bridge (model: HIOKI 3520-01, Japan). The ionic mobility was determined with the help of well-known equation: [µ = d 2 / (V.τ) ], where d is the thickness of SPE films, V is the applied external dc potential and τ is the time of flight. The time of flight τ was determined directly employing dc polarization transient ionic current (TIC) technique using an x-y-t recorder (model: Graphtec WX 2300-1L, Japan) 16. Ionic transference number t ion has also been evaluated at different temperatures using dc polarization technique with the help of well known equation t ion = [1 (I e /I T )], where I e is the electronic current and I T is the total current of the cell [SS // SPE OCC // SS]. 3 Results and Discussion Figure 1 shows the room temperature conductivity σ variations for the hot-pressed SPE films (1-x) PEO: x NaBr, where 0 < x < 50 wt%. To determine the bulk resistance R b, the impedance plot for highest conducting composition of SPE at room temperature is shown in bottom inset of Fig. 1. The conductivity increased when the ionic salt NaBr concentration increased initially from 0 to 30 wt% and then decreased on further addition of salts. The conductivity maxima appeared at x = 30, i.e., the composition (70PEO: with σ ~ 7.58 10-7 Scm -1. This highest conducting composition has been referred to as optimum conducting composition (OCC). SPE films beyond 50 wt% salt concentration appeared, unstable and brittle. A conductivity increase of more than two orders of magnitude was obtained in SPE OCC from that of the pure PEO (σ ~ 3.2 10-9 Scm -1 ). The increase in conductivity is due to the increase in degree of amorphousity and increase in ionic mobility µ and mobile ion concentration n, which can be also be explained by the various models viz mobility enhancement model, percolation model etc. 3,17. To explain the conductivity enhancement in the present SPE OCC again, the ionic mobility µ and mobile ion concentration n studies have been done with the help of dc polarization transient ionic current (TIC) technique, as mentioned in experimental section. Figure 2 shows log µ-x and log n-x plots for SPEs (1-x)PEO: xnabr. One can clearly notice that the existence of maxima for both µ and n at x = 30 wt% observed. This explains the fact that the overall increase in ionic conductivity of SPE OCC has been due to increase in both µ and n. Table 1 shows the some important ion transport parameters of hotpressed SPE OCC and ionic conductivity of pure PEO at room temperature. Figure 3 shows the XRD patterns of pure PEO, pure NaBr and SPE OCC: (70PEO:. It can be clearly seen from the figure that characteristics Fig. 1 Logσ x plot of hot-pressed SPEs (1-x)PEO: xnabr. Bottom inset shows the impedance plot for SPE OCC (70PEO:

CHANDRA: HOT-PRESSED SOLID POLYMER ELECTROLYTES 585 peaks of pure PEO appeared between 2θ = 15-30 o drastically decreases after the addition of salt NaBr. Some of the peaks of pure PEO became relatively lessprominent/feeble after salt complexation. However, no additional diffraction peaks corresponding to the formation of a stable ion-peo complex can be identified. This is usually attributed to the increase in the degree of amorphicity and confirmation of formation of SPE. Figure 4 shows the FTIR spectra for the pure PEO, NaBr and hot-pressed SPE OCC (70PEO:. The possible configuration of newly synthesized hotpressed SPE OCC is: (ii) C-C stretching at 1721 cm -1, as indicated by the star (iii) C-O-C asymmetric stretching at 962 cm -1 (iv) It is clearly indicated from the spectra that the width of the C-O-C and C-H stretching modes decreases after addition of salts at ~1100 cm -1 and ~ 2900 cm -1, respectively and broadening of peaks at ~ 820 cm -1, as indicated by the arrow The followings points have been observed in the present FTIR spectra: (i) The absorption features in PEO are CH 2 bending at 1466 cm -1 Table 1 Some important ion transport parameters of hotpressed SPE OCC and ionic conductivity of pure PEO at room temperature Systems σ (Scm -1 ) µ (cm 2 V -1 s -1 ) n (cm -3 ) Pure PEO 3.20 10-9 SPE OCC (70PEO: 7.58 10-7 7.2 10-3 6.57 10 14 ~ 0.95 t ion Fig. 3 XRD patterns (a) pure NaBr, (b) pure PEO and (c) SPE OCC (70PEO: Fig. 2 Log µ x and log n-x plots of hot-pressed SPEs (1-x) PEO: NaBr Fig. 4 FTIR spectra (a) pure PEO, (b) Pure NaBr and (c) SPE OCC (70PEO:

586 INDIAN J PURE & APPL PHYS, VOL 54, SEPTEMBER 2016 All these above results and symmetrical and asymmetrical changes in vibrational bands with ether oxygen, indicate that polymer-salt complexations takes place. To explain the surface morphology of the present hot-pressed SPE OCC: (70PEO:, SEM images have been determined. Figure 5 shows surface morphology of pure PEO and SPE OCC (70PEO:. The smooth surface morphology of SPE OCC is clear indication of increase in degree of amorphicity/reduction in the crystallinity of pure PEO after salt complexations. It is also closely related to the enhancement of degree of amorphicity with polymer-salt complexations, while pure PEO usually shows a rough morphology 17,18. Figure 6 shows the DSC thermogrames for pure PEO and hot-pressed SPE OCC (70PEO:. It can be noticed from both the thermogrames that both the broad endothermic peaks were observed in polymeric films at ~ 60-70 C and it is corresponding to the melting point temperature of pure PEO. It is clearly shown from the figure that both the melting temperature T m and glass transition temperature T g have decreased from 68 to 66 C and 65 to 71 C, respectively. The decrease in temperature is due to the increase in degree of amorphicity and also structural modifications in SPE OCC and hence it is also directly related to increase in ionic conductivity. Figure 7 shows the thermogrevimetric analysis (TGA) curves for pure PEO and SPE OCC (70PEO:. It can be clearly seen from the figure that the total weight loss for pure PEO is larger as compared to the SPE OCC. The total weight loss for pure PEO is ~ 95% and SPE OCC is ~ 68%. The minimum weight loss in SPE OCC as compared to pure PEO is due to the addition of salt in polymer/ formation of SPE, similar types of behavior with alkali metal ions have also been observed by the various workers 19-21. The degree of crystallinity χ of SPE OCC and pure PEO has also been calculated with the help of well-known relation, as reported Fig. 6 DSC thermogrames (a) pure PEO and (b) SPE OCC (70PEO: Fig. 5 SEM images (a) pure PEO 17,18 (70PEO: and (b) SPE OCC Fig. 7 TGA curves (a) pure PEO and (b) SPE OCC (70PEO:

CHANDRA: HOT-PRESSED SOLID POLYMER ELECTROLYTES 587 elsewhere 22. Table 2 lists of melting temperatures T m, glass transition temperature T g, degree of crystalline χ and weight loss for pure PEO and SPE OCC films for direct comparison of different parameters. The ionic transference number t ion, a direct quantitative measure of extent of ionic contribution to the total conductivity, was also determined for SPE OCC film using TIC current time plot and obtained t ion ~ 0.95, as shown in Fig. 8. This clearly indicated Table 2 Melting temperatures T m, glass transition temperature T g, degree of crystalline χ and weight loss for pure PEO and SPE OCC films Systems T m ( C) T g ( C) χ (%) Weight loss (%) Pure PEO 68-65 100 95 SPE OCC (70PEO: 66-71 78 68 Fig. 8 Current vs time plot for SPE OCC (70PEO: Fig. 9 Log σ 1/T plot for hot-pressed SPE OCC (70PEO: that SPE OCC film material is purely an ion conducting system and hence, can be appropriately employed as electrolyte to fabricate thin film electrochemical devices viz. batteries. The major ion conducting species in this SPE OCC are yet to be identified, as both cations and anions may be mobile. However, cations Na +, being relatively smaller in size than anion Br -, are expected to be the main transporting ionic species in this SPE material. Figure 9 shows log σ 1/T plot for the SPE OCC (70PEO:. The conductivity increased almost linearly with temperature up to ~ 65-70 C at which an upward jump in conductivity was observed. The increase in ionic conductivity with temperature can be explained in terms of polymer segmental motion, which results in an increase of free volume and available of free conducting paths 23-25. The jump in conductivity at this temperature corresponds to semicrystalline to amorphous phase transition temperature T m of PEO. The ionic conductivity of SPE OCC at different temperatures is also listed for direct comparison in Table 3. The linear portion of log σ 1/T plots below T m (indicated by the arrow) can be expressed by following Arrhenius type equation: log σ(t) = 3.19 10-1 exp (- 0.33/ kt) where 0.33 ev is the activation energy E a computed by least square linear fitting of the data. The low E a in the present SPE OCC is indicative of relatively easier ion migration and this can be potentially used as electrolyte for electrochemical device applications. However, the ionic conductivity in the present hotpressed SPE is lower but this is the primary Table 3 Ionic conductivity values of SPE OCC (70PEO: at different temperatures Temperature ( C) Conductivity (Scm -1 ) 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 7.58 10-7 9.33 10-7 1.09 10-6 1.41 10-6 1.65 10-6 1.86 10-6 2.57 10-6 8.70 10-6 1.51 10-5 2.90 10-5 2.69 10-5 2.95 10-5 3.31 10-5 3.80 10-5 4.46 10-5

588 INDIAN J PURE & APPL PHYS, VOL 54, SEPTEMBER 2016 investigation and further more experiment is on progress. The present result is also helpful for the development of ion conducting nanocomposite polymer electrolytes by the dispersion of insulating nanomaterials in the present SPE. 4 Conclusions A new Na + ion conducting hot-pressed solid polymer electrolyte (70PEO: with ionic conductivity 7.58 10-7 Scm -1 has been prepared by hotpress method. Materials characterizations and polymersalt complexations have been done by using XRD, FTIR, SEM, DSC and TGA techniques. Ionic mobility and mobile ion concentration both are responsible for conductivity enhancements in the present SPE. The ionic transference number measurement (t ion ~ 0.95) indicated that SPE film material is a pure ion conducting system with cation Na + -ions as the principal charge carriers. The low activation energy (E a = 0.33 ev) for the present SPE indicative of relatively easier ion transport in the newly synthesized system and hence it can be potentially used for electrochemical device applications. Acknowledgements The author gratefully acknowledge to SERB-DST, New Delhi for providing financial assistance through the SERB DST Research Project (No. SR/ FTP/ PS- 23/ 2009). References 1 Armand M B, Adv Mater, 2 (1990) 278. 2 Abraham K M, Applications of electroactive polymers, (Chapman and Hall, Springer, London), 1993. 3 Agrawal R C & Pandey G P, J Phys D Appl Phys, 41 (2008) 223001. 4 Fenton D E, Parker J M & Wright P V, Polymer, 14 (1973) 589. 5 Kumar J S, Reddy M J & Rao U V S, Mater Sci Lett, 41 (2006) 6171. 6 Agrawal R C, Sahu D K, Mahipal Y K & Ashrafi, Mater Chem Phys, 139 (2013) 410. 7 Chandra A, Agrawal R C & Mahipal Y K, J Phys D Appl Phys, 42 (2009) 135107. 8 Bhide A & Kariharan K, J Power Sources, 159 (2006) 1450. 9 Ellis B L & Nazar L F, Curr Opin Solid State Mater Sci, 16 (2012) 168. 10 Chandra A, Chandra A & Thakur K, Eur Phys J Appl Phys, 69 (2015) 20901. 11 Appetecchi G B, Croce F, Hasson J, Scrosati B, Salomon M & Cassel F, J Power Sources, 114 (2003) 105. 12 Agrawal R C & Chandra A, J Phys D Appl Phys, 40 (2007) 7024. 13 Sengwa R J, Sankhla S & Choudhary S, Ionics, 16 (2010) 697. 14 Chandra A, Eur Phys J Appl Phys, 50 (2010) 21103. 15 Chandra A, Indian J Phys, 90 (2016) 759. 16 Chandra S, Tolpadi S K & Hashmi S A, Solid State Ionics, 28-30 (1988) 651. 17 Chandra A & Chandra A, Hot-pressed polymer electrolytes: synthesis and characterization, (Lambert Academic Pub, Germany), 2010. 18 Reddy M J & Chu P P, Electrochimica Acta, 47 (2002) 1189. 19 Dey A, Karan S & De S K, Solid State Commun, 149 (2009) 1282. 20 Poul A R, Kumar R & Kumar K V, Adv Mater Lett, 3 (2012) 406. 21 Chandra A, Chandra A & Thakur K, Chin J Polymer Sci, 31 (2013) 302. 22 Chandra A, Chandra A & Thakur K, Polym Bull, 71 (2014) 181. 23 Druger S D, Nitzan A & Ratner M A, J Chem Phys, 79 (1983) 3133. 24 Druger S D, Nitzan A & Ratner M A, Phys Rev B, 31 (1985) 3939. 25 Reddy M J, Sreekanth T & Rao U V S, Solid State Ionics, 126 (1999) 55.