Ag+-ion conducting Nano-Composite Polymer Electrolytes (NCPEs): Synthesis, characterization and all-solid-battery studies

Journal of Non-Crystalline Solids 391 (2014) 91–95

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Ag+-ion conducting Nano-Composite Polymer Electrolytes (NCPEs): Synthesis, characterization and all-solid-battery studies Rehana Ashrafi a, Dinesh K. Sahu a,b, Priyanka Kesharwani a, Manju Ganjir a, R.C. Agrawal a,⁎ a b

Solid State Ionics Research Laboratory, School of Studies in Physics & Astrophysics, Pt. Ravishankar Shukla University, Raipur 492010, C.G., India Rungta Engineering College, Raipur, India

a r t i c l e

i n f o

Article history: Received 13 December 2013 Received in revised form 11 March 2014 Available online 6 April 2014 Keywords: Nano Composite Polymer Electrolytes (NCPEs); Hot-press film casting; Ionic conductivity; Ionic/cationic transference number; All-Solid-State battery

a b s t r a c t Ag+-ion conducting Nano Composite Polymer Electrolyte (NCPE) films: [90PEO: 10AgCF3SO3] + xAl2O3, where x = 0.5, 1, 1.5, 2, 3, 4, 5 wt.(%), have been prepared by a completely dry hot-press technique in place of the traditional solution-cast method. NCPE film, basically a two-phase organic composite polymer electrolyte, has been synthesized using Solid Polymer Electrolyte (SPE) composition: [90PEO: 10AgCF3SO3], identified as one of the high conducting films with room temperature (300 K) ionic conductivity (σrt) ~ 7.12 × 10−7 S/cm and having superior mechanical flexibility, as 1st-phase host and nano-particles (size b 50 nm) of an insulating/inert filler material Al2O3 as 2nd-phase dispersoid. Filler particle concentration dependent conductivity measurements revealed NCPE film: [(90PEO: 10AgCF3SO3) + 3Al2O3] as Optimum Conducting Composition (OCC) with σrt ~ 2.57 × 10−6 S/cm. A conductivity enhancement of more than 3-fold could be achieved in SPE as a consequence of dispersal of Al2O3 nanoparticles. Also, NCPE OCC film physically appeared relatively more stable/flexible as compared to SPE host film. The characterization of ion transport properties in SPE host and NCPE OCC films has been done in terms of ionic conductivity (σ) and total ionic (tion)/cation (t+) transference numbers. These ionic parameters have been evaluated experimentally using different ac/dc techniques. The temperature dependent conductivity has also been studied and the activation energy (Ea) was computed by liner least square fitting of Arrhenius plot. The material characterization was done by analyzing X-ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) responses on the film samples. All-Solid-State batteries in the cell configuration Ag (anode)//NCPE OCC film//(I2 + C + NCPE) (cathode) have been fabricated and the cell-potential discharge performances have been studied under varying load conditions. An Open Circuit Voltage (OCV) ~ 0.56 V was obtained. Some important cell parameters have been evaluated from the cell potential discharge profiles. The battery performed quite satisfactorily especially under low current drain states. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Pure polymers often exhibit poor electrical conductivity. However, they can be made ion conducting by complexing/dissolving suitable ionic salts into them. Ion conducting polymers in thin/flexible film form show great technological promises to fabricate All-Solid-State mini/micro electrochemical power sources viz. batteries, fuel cells, or supercapacitors. Ion conduction in polymers was reported for the first time in 1973 in a dry Solid Polymer Electrolyte (SPE): poly (ethylene oxide) PEO complexed with alkali salt [1]. Eventually, the first practical battery based on SPE film: PEO polymer complexed with Li+-ion salt was demonstrated in 1979 [2]. Since then, a large number of Solid Polymer Electrolytes (SPEs) involving different mobile ions viz. H+, Ag+, Cu+, Li+, Na+, K+, and Mg2+ have been reported [3–11]. Majority of SPE films, reported in the past used PEO as a common polymeric host ⁎ Corresponding author. Tel.: +91 771 226286, +91 9425203030 (mobile); fax: +91 771 2262583. E-mail address: [email protected] (R.C. Agrawal).

http://dx.doi.org/10.1016/j.jnoncrysol.2014.03.013 0022-3093/© 2014 Elsevier B.V. All rights reserved.

and exhibited room temperature ionic conductivity (σrt) much lower than 10− 4 S cm−1. However, for practical battery operating at room temperature, SPE should have σrt ≥ 10−4S/cm. The ionic conductivity can be improved substantially by dispersing low dimension (μm/nm) particles of an insulating/inert filler material such as Al2O3, SiO2, and TiO2 as 2nd-phase dispersoid into SPE which acts as 1st-phase host [10,11]. Akin to 2-phase inorganic composite solid electrolytes [12], these systems are referred to as 2-phase organic composite electrolytes viz. Composite Polymer Electrolytes (CPEs), in general and in particular, Nano-Composite Polymer Electrolytes (NCPEs), when dispersed with nano-filler particles. Particle size of the filler material plays a vital role in improving the room temperature conductivity in 2-phase solid electrolyte systems. Hence, the dispersal of nano-particles may be more effective as far as the enhancement in the room temperature conductivity and improvements in other physical properties are concerned [10–12]. SPE/NCPE films are usually prepared by the traditional ‘solution cast’ method. However, in the recent past a completely- dry/solution-free procedure, referred to as ‘hot-press’ technique, has been developed for casting dry polymer electrolyte films viz. SPE/CPE/NCPE [9–11]. This

R. Ashrafi et al. / Journal of Non-Crystalline Solids 391 (2014) 91–95

technique has several merits over the traditional method, hence, it is currently getting wide acceptability. This paper reports hot-press casting of NCPE membranes: [90PEO: 10AgCF3SO3] + xAl2O3 in which SPE film composition: [90PEO: 10AgCF3SO3], identified as one of the high conducting film and also physically appeared having superior mechanical flexibility, has been selected as 1st-phase host and used nano-sized (b 50 nm) particles of filler material Al2O3 as 2nd-phase dispersoid. Further, from the filler particledependent conductivity studies, NCPE film: [90PEO: 10AgCF3SO3] + 3Al2O3 was identified as Optimum Conducting Composition (OCC) at room temperature. Suthanthiraraj and coworkers [13–18] have done extensive studies on Silver Triflet based dry polymer electrolytes systems viz. SPE/NCPE and gel polymer electrolytes, including the present NCPE film. However, they prepared NCPE films by ‘solution cast’ method and identified NCPE film: (PEO50–AgCF3SO3) + 5Al2O3, as optimum conducting film at room temperature [13]. Their result and that reported in the present paper match well qualitatively except for the composition of the optimum conducting film. The characterization of ion transport behavior in the present SPE host and NCPE OCC films has been done in terms of some basic ionic parameters viz. ionic conductivity (σ) and total ionic (tion)/cationic (t+) transport numbers. The activation energy (Ea) has been evaluated by linear least square fitting of the data obtained from ‘logσ-1/T’ Arrhenius plot. All-Solid-State batteries have been fabricated by sandwiching NCPE OCC film between appropriate anode/cathode couple and the cell performance has been studied under varying load conditions.

2. Experimental For hot-press casting of dry polymer electrolyte films, AR grade (purity N 99%) chemicals: poly (ethylene oxide) PEO (Mw ~ 6 × 105), silver triflet (AgCF3SO3), Al2O3 (particle size b 50 nm) procured from Sigma-Aldrich, USA, have been used after pre-drying at ~50 °C for ~3 h. Firstly, SPE films in varying salt concentrations were prepared by hotpress casting method. Dry powders of polymeric host PEO and complexing salt: AgCF3SO3 in appropriate wt.(%) ratio were mixed physically for ~ 30–60 min, then heated close to the melting point of PEO (~70 °C) with mixing continued for another ~30–60 min. This gave rise to a soft slurry/lump which was then pressed between two cold Stainless Steel (SS) blocks to form SPE film of uniform thickness (~100–150 μm). The details on film casting procedure have been discussed earlier [10]. Based on salt-concentration-dependent-conductivity studies at room temperature, SPE film composition: [90PEO: 10AgCF3SO3], exhibiting

-5.4

300K -5.6

log σ (S/cm)

92

-6

-6.2

-6.4 0

2

4

6

x Al2O3 wt.(%) Fig. 2. Conductivity as function of concentration of filler material particles for different hot-press NCPE films: [90PEO: 10AgCF3SO3] + xAl2O3, dashed line is drawn as guide to the eyes.

relatively higher conductivity and physically appeared having superior flexibility, was chosen as 1st-phase host to prepare NCPE films: [90PEO: 10AgCF3SO3] + xAl2O3, where the filler material Al2O3 (particle size b 50 nm) in different wt.(%) ratio: x = 0.5, 1, 1.5, 2, 3, 4, and 5 was dispersed as 2nd-phase dispersoid. To hot-press cast NCPE films, dry powders of 1st-phase SPE composition and 2nd-phase filler material in different wt.(%) ratios were mixed separately physically, then heated close to melting point of PEO and pressed between two cold SS blocks giving rise to NCPE films of uniform thickness, as before. Further from the filler-particle-concentration-dependent-conductivity measurements NCPE OCC film has been identified. The conductivity measurements were carried out at fixed ac frequency (i.e. at 5 kHz). Total ionic transference number (tion) was evaluated by dc polarization Transient Ionic Current (TIC) technique [19,20] and cationic (Ag+) transference number (t+) by a combined ac/dc technique [21]. The material characterization and confirmation of complexation of salt/dispersal of filler particles have been done by XRD (Model: D2-PHASER, Bruker, CuKα radiation, 2θ from 5° to 70°) and FTIR (IR Affinity-1, Shimadzu) studies in the open ambience. Finally, All-Solid-State batteries in the cell configuration: Ag (anode)//NCPE OCC film//(I2 + C + Electrolyte) (cathode) have been fabricated and the performance was studied by discharging the cells at room temperature through load resistances: 60 kΩ, 100 kΩ. Some important cell parameters were evaluated from the plateau region of the cell potential discharge profiles.

-3

-5.5

300K

-6

-3.5 -4

log σ (S/cm)

-6.5

log σ (S/cm)

-5.8

-7 -7.5

-4.5 -5

-8

-5.5

-8.5

-6 -6.5

-9 0

2

4

6

8

10

12

14

AgCF3SO3 wt(%) Fig. 1. Conductivity as function of salt concentration for different hot-press SPE films: [PEO: AgCF3SO3], dashed line is drawn as guide to the eyes.

2.6

2.8

3

3.2

3.4

1000/T (K-1) Fig. 3. ‘Logσ−1/T’ plot for hot-press cast SPE host ( ) and NCPE OCC ( ) films, dashed line is drawn as guide to the eyes.

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Table 1 Room temperature (300 K) conductivity (σrt), activation energy (Ea) and total ionic (tion)/cationic (t+) transport numbers for SPE host/NCPE OCC films. σrt of pure PEO film is also given. Film Pure PEO SPE:[90PEO: 10AgCF3SO3] NCPE:[90PEO:10AgCF3SO3] + 3Al2O3

(σrt) (S/cm) −9

3.20 × 10 7.12 × 10−7 2.57 × 10−6

Ea (eV)

tion

t+

– 0.60 0.52

– 0.99 0.98

– 0.20 0.27

3. Results and discussion 3.1. Ion transport characterization studies Fig. 1 shows the room temperature conductivity (σrt) variation as a function of concentration of complexing salt in wt.(%) for different hotpress cast SPE films: [PEO: AgCF3SO3]. One can note that σrt increased rapidly on initial addition of 1–2 wt.(%) salt, then gradually on further addition of salt up to ~12 wt.(%). The increase in the conductivity, as usual, can be attributed to an increase in the mobile ion carriers as a result of dissociation of salt by polymeric host [10,11]. SPE films beyond 12 wt.(%) salt, physically appeared brittle/less flexible mechanically. Hence, for this reason, SPE film composition: [90PEO: 10AgCF3SO3], exhibiting room temperature (300 K) conductivity (σrt) ~ 7.12 × 10−7 S/cm and having relatively superior flexibility, has been chosen as 1st-phase host for hot-press casting NCPE films: [90PEO: 10AgCF3SO3] + xAl2O3, where the filler material Al2O3 nano-particles in different wt.(%) ratios x = 0.5, 1, 1.5, 2, 3, 4, and 5 were used as 2nd-phase dispersoid. Fig. 2 shows the ‘logσ − x’ plot for NCPE films. It can be noted that dispersal of filler particles, in general, resulted into an enhancement in the room temperature conductivity in all NCPE films. However, two σpeaks appeared at x = 1 and 3 wt.(%). The existence of two σ-maxima in ‘logσ − x’ plot has been observed in number of NCPE films reported in the past [9–11] and has been attributed to two kinds of ion conduction mechanisms operative in the system [22]. First σ-maxima is due to the dissociation of ion aggregates and/or undissociated salt which resulted into further generation of free ion carriers, while second σ-maxima as

Fig. 4. XRD pattern for: (a) pure PEO, (b) SPE host: (90PEO: 10AgCF3SO3) and (c) NCPE OCC: (90PEO: 10AgCF3SO3) + 3Al2O3.

Fig. 5. FTIR spectral response for: (a) pure PEO, (b) SPE host: (90PEO: 10AgCF3SO3) and (c) NCPE OCC: (90PEO: 10 AgCF3SO3) + 3 Al2O3.

well as the conductivity variation around this maxima could be related to the well-known 2-phase composite effect and can be explained on the basis of space-charge and/or percolation model [12,23,24]. One can also note that NCPE film: [90PEO: 10AgCF3SO3] + 3Al2O3 exhibited relatively higher σrt ~ 2.57 × 10−6 S/cm, hence, has been referred to as NCPE OCC film. As a consequence of dispersal of nano-particles of filler material in 1st-phase SPE host, σrt-value in NCPE OCC film increased by more than 3-fold. Further, on physical inspection/handling, this NCPE OCC film appeared more stable/flexible as compared SPE host film. Fig. 3 shows ‘logσ − 1/T’ plots for NCPE OCC and SPE host films. It can be observed that the conductivity of both the films increased almost linearly as the temperature increases up to ~ 60 °C. However, a slight change in the slope could be noticed around 60–70 °C. In fact, this temperature region (60–70 °C) corresponds to the well-known semicrystalline to amorphous phase transition of polymer PEO which occurs ~ 65 °C [10]. Temperature dependent conductivity variation of Fig. 3 below this temperature can very well be expressed by Arrhenius equation: logσ = logσo − Ea/kT, which is indicative of ion transport

Fig. 6. Cell potential profile as a function of time when discharged under load resistances: 100 kΩ ( ), 60 kΩ ( ).

94

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Table 2 Some important cell parameters obtained in the plateau region of the cell potential profiles of Fig. 6. Load

OCV (Volt)

Current density (μA/cm2)

Discharge capacity (mA·h)

Specific energy (mWh/g)

Specific power (mW/kg)

100 kΩ 60 kΩ

0.56 0.56

2.9 4.3

0.256 0.380

1.36 1.481

91.12 99.22

via jump/hop mechanism. The activation energy (Ea) has been computed by least square linear fitting of the slope in the temperature region below ~ 60 °C. ‘Logσ − 1/T’ plots for other NCPE films dispersed with Al2O3 filler particle concentrations x = 1, 2, 4, 5 wt.(%), exhibited almost parallel slopes in this temperature region and hence, had nearly same Ea-value. Table 1 lists σrt of pure PEO, SPE host and NCPE OCC films along with ‘Ea’ and tion/t+ values (discussed below) for SPE/NCPE OCC films. One can note, Ea-value decreased slightly for NCPE OCC film. This is indicative of a relatively easy ion migration supported by the enhanced degree of amorphous region in polymer host as a consequence of dispersal of filler material particles. Total ionic (tion) and cation (t+) transference numbers in NCPE OCC/SPE host materials were evaluated using dc polarization TIC and combined ac/dc techniques respectively, as mentioned in Section 2. tion ~0.98–0.99 which is very close to unity, hence, indicating the fact that both the film materials are predominantly ion conducting medium. However, t+ ~ 0.20 (SPE host) and ~ 0.27 (NCPE OCC) which are significantly low and for practical device applications t+ should be at least ≥0.5 [10]. Nevertheless, t+ in SPE host film improved moderately on the dispersal of filler material particles into it i.e. in NCPE OCC film. 3.2. Materials characterization studies The complexation of salt in PEO as well as the dispersal of nanoparticle of filler material Al2O3 in SPE host have been confirmed by XRD and FTIR analysis. Fig. 4 shows the XRD pattern for pure PEO, SPE host and NCPE OCC films. It is normally difficult to assign crystalline planes and phases in XRD pattern of polymeric materials. However, one can clearly note that the intensity of two main peaks (at 2θ = 19.5 & 24°) of pure PEO (pattern a) decreased substantially after salt complexation in PEO (i.e. pattern b for SPE host) and/or dispersal of filler material in SPE host (i.e. pattern c for NCPE OCC) [10]. These peaks also shifted slightly towards lower 2θ value. Such changes in XRD patterns usually indicated the complexation of salt in PEO and/or dispersal of filler material particles in SPE host. This has been further confirmed by FTIR studies. Fig. 5 shows the FTIR spectra for pure PEO, SPE host and NCPE OCC films. The main characteristic peaks at ~2238, 2163 and 1963 cm−1 for pure PEO are all present in the FTIR spectra of SPE host/NCPE OCC films but with significant changes in the peak intensities. Intensity of peaks at ~525–530, ~1200 cm−1 which corresponds to the C\O\C bending, stretching respectively in polymer PEO, decreased substantially after salt complexation (in SPE) and/or dispersal of filler particles (in NCPE). Also, the peak intensities corresponding to CH2 bending and CH2 rocking modes at ~1475 and ~845 cm−1 respectively also decreased as well as shifted slightly towards lower wavenumbers. These spectral changes once again confirmed the complexation of salt in PEO and/or dispersal of inert filler material into SEP host. 3.3. All-Solid-State battery: Cell performance studies All-Solid-State batteries, in the cell configuration mentioned in Section 2, have been fabricated and their performances have been tested. An Open Circuit Voltage (OCV) ~ 0.56 V was obtained. The room temperature conductivity of present NCPE film is low (~2.57 × 10−6 S/cm) and hence, it would be suitable only for low current drain applications. For this reason, these batteries were discharged through high load resistances i.e. 100 kΩ and 60 kΩ. In fact, with low load (viz. 5, 10, 20 kΩ)

these batteries discharged within few hours. It is well known that AllSolid-State batteries based on Ag+-ion conducting inorganic solid electrolytes exhibit the highest OCV ~ 0.687 V when sandwiched between Ag/elemental I2 (anode/cathode) couple [25]. The lower value of OCV in the present batteries based Ag+-ion conducting organic NCPE film and coupled between similar Ag/I2 electrodes, may be attributed to the low value of cation transport number (t+). Fig. 6 shows the cell potential discharge profile as a function of time. Except for an initial drop in the cell potential, which is due to usual polarization build-up effect, the cell potential remained almost stable for more than 50 h when discharged through both the load resistances. Some important cell parameters have been calculated in the plateau regions of the discharge profiles and listed in Table 2. These results indicated that these batteries can be used satisfactorily only during low current drain states. 4. Conclusions Ag+-ion conducting NCPE film: [90PEO: 10AgCF3SO3] + 3 Al2O3 has been synthesized by hot-press technique. This is basically a 2-phase organic composite electrolyte system in thin/flexible film form in which SPE composition: [90PEO: 10AgCF3SO3] has been used as 1st-phase host and nano-particles of filler material Al2O3 as 2nd-phase dispersoid. The ion transport properties have been characterized in terms of some basic ionic parameters viz. σ, tion, and t+. tion was found almost close to unity which is indicative of the fact that the newly synthesized NCPE film is a pure ion conducting medium. The complexation of salt in polymer and/or dispersal of filler material particles in SPE host has been confirmed by XRD and FTIR analysis. All-Solid-State batteries have been fabricated by sandwiching NCPE film between Ag (anode) and (C + I2 + Electrolyte) (cathode). The cell performances have been studied under varying load conditions. The cell performance studies indicated that these batteries are useful only during low current drain applications. References [1] D.E. Fenton, J.M. Parker, P.V. Wrigth, Polymer 14 (1973) 589. [2] M.P. Armand, J.M. Chabagno, M. Diadat, Fast Ion Transport in Solids, in: P. Vashistha, J.M. Mundy, G.K. Sheny (Eds.), 1979, p. 135, (North Holland). [3] M.B. Armand, Annu. Rev. Mater. Sci. 16 (1986) 245. [4] M.A. Ratner, D.F. Shriver, Chem. Rev. 88 (1988) 109. [5] J.R. Mac Callum, C.A. Vincent (Eds.), Polymer Electrolyte Reviews, vol. 1, 2, Elsevier Applied Science Publisher, London, 1987 & 1989. [6] K. Murata, Electrochim. Acta 40 (1995) 2177. [7] P.G. Bruce (Ed.), Solid State Chemistry, Cambridge University Press, Cambridge, 1995. [8] F.M. Gray, Polymer Electrolytes, Royal Society of Chemistry, Cambridge, 1997. (Monographs). [9] F. Capuano, F. Croce, B. Scrosati, J. Electrochem. Soc. 138 (1991) 1918; F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nature 394 (1998) 456; G.B. Appetecchi, F. Croce, L. Persi, F. Ronci, B. Scrosati, Electrochim. Acta 45 (2000) 1481; G.B. Appetecchi, J. Hassoun, B. Scrosati, F. Croce, F. Cassel, M. Salomon, J. Power Sources 124 (2003) 246. [10] R.C. Agrawal, G.P. Pandey, J. Phys. D. Appl. Phys. 41 (2008) 055409 (Topical Review); R.C. Agrawal, D.K. Sahu, Y.K. Mahipal, R. Ashrafi, Matl. Chem. Phys. 139 (2013) 410. [11] E. Quartarone, P. Mustarelli, Chem. Soc. Rev. 40 (2011) 2025 (Critical Review). [12] R.C. Agrawal, R.K. Gupta, Superionic solids: composite electrolyte phase — an overview, J. Mater. Sci. 34 (1999) 1131. [13] S.A. Suthanthiraraj, D.J. Sheeba, Indian J. Phys. 79 (2005) 807; S.A. Suthanthiraraj, D.J. Sheeba, Ionics 13 (2007) 447. [14] S.A. Suthanthiraraj, R. Kumar, B. Joseph Paul, Spectrochim. Acta A 71 (2009) 2012–2015.

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