Ionics (2008) 14:515–523 DOI 10.1007/s11581-008-0210-7
ORIGINAL PAPER
Structural, thermal and ion transport studies on nanocomposite polymer electrolyte-{(PEO + SiO2):NH4SCN} system Kamlesh Pandey & Mrigank Mauli Dwivedi & Mridula Tripathi & Markandey Singh & S. L. Agrawal
Received: 17 August 2007 / Revised: 21 November 2007 / Accepted: 18 January 2008 / Published online: 4 March 2008 # Springer-Verlag 2008
Abstract Development and characterisation of polyethylene oxide (PEO)-based nanocomposite polymer electrolytes comprising of (PEO-SiO2): NH4SCN is reported. For synthesis of the said electrolyte, polyethylene oxide has been taken as polymer host and NH4SCN as an ionic charge supplier. Sol–gel-derived silica powder of nano dimension has been used as ceramic filler for development of nanocomposite electrolytes. The maximum conductivity of electrolyte ∼2.0 × 10−6 S/cm is observed for samples containing 30 wt.% silica. The temperature dependence of conductivity seems to follow an Arrhenius-type, thermally activated process over a limited temperature range. Keywords Polymer nanocomposite . PEO: SiO2 . Ionic transference number . Electrical conductivity
K. Pandey (*) : M. M. Dwivedi National Centre of Experimental Mineralogy and Petrology, University of Allahabad, Allahabad 211 002, India e-mail:
[email protected] M. M. Dwivedi e-mail:
[email protected] M. Tripathi Department of Chemistry, CMP Degree College, Allahabad, India e-mail:
[email protected] M. Singh : S. L. Agrawal Department of Physics, Awadhesh Pratap Singh University, Rewa, Madhya Pradesh, India S. L. Agrawal e-mail:
[email protected]
Introduction With the rapid growth of portable electronic devices, the demand of compact lightweight, high-capacity, solid-state rechargeable batteries have also increased tremendously over the years [1]. The polymer electrolytes have recently become a hot contender due to their significant theoretical interest as well as practical importance for the development of solid-state electrochemical devices beside other distinctive properties [2–3]. A large number of solid polymeric electrolytes with appreciably high ionic conductivity have been investigated in the past three decades by complexing polar polymers [e.g. polyethylene oxide (PEO) and polypropylene oxide (PPO)], having strong solvating ability with a number of alkali, alkaline and transition metal salts (e.g. LiClO4, Mg(ClO4)4, LiI, NaI, AgNO3 etc) [4–5]. Amongst these polymers, the polyethylene oxide is a semicrystalline polymer at room temperature and has an exceptional property to dissolve high concentration of a wide variety of dopants [6]. Recently, particular attention has been devoted to introduce some structural modification in polymer electrolyte in order to increase their electrical conductivity and improve their thermal, mechanical and electrochemical properties to provide commercial acceptability in electrochemical devices. Various techniques (like plasticization, co-polymerization, etc.) have been adopted to achieve the desired objective in these polymer electrolytes [7–8]. In this process, another class of polymer electrolyte referred to as “composite polymer electrolyte (CPE)” has been developed. These polymer electrolytes are dispersed with ceramic or inorganic or high molecular weight organic fillers to enhance electrical conductivity and to improve thermal, mechanical and electrochemical stability of the polymer film. Such a dispersion of fillers
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was first suggested by Weston and Steele [9]. Since then, a number of inorganic or ceramic and organic additives have been reported [10–13]. Recently, few works have been reported on the synthesis and characterization of nanocomposite polymer system [14–16] where due to small particle size, the electrochemical, magnetic and optical behaviour have been shown to improve drastically [17]. These materials have become more attractive and useful for the medical, technical and industrial use [18]. Prompted by these considerations, attempts have been made here to develop a proton or ammonium ion conducting nanocomposite electrolyte based on host polymer PEO. The present paper reports a nanocomposite polymer electrolyte PEO: SiO2: NH4SCN system. The effect of ceramic additive (silica) and a salt ammonium thiocyanate on PEO, with respect to morphology and electrical conductivity have been investigated via Differential scanning calorimetry (DSC), X-ray diffraction (XRD), Optical microscopy, scanning electron microscopy (SEM), infrared (IR) spectroscopy and ionic transport measurement. The change in the morphology and structure of CPE was studied by XRD, optical microscopy and SEM. The electrical conductivity has been evaluated from complex–impedance plot and interpreted as a function of composition and temperature of material.
spectra and SEM or optical microscopy. The XRD pattern was recorded between 2θ values 15–60° at room temperature using Phillips X-Pert diffractometer. The Infrared spectrums were recorded on Perkin Elmer IR Spectrophotometer in a range 400–4,000 cm−1. The optical micrographs of the film were recorded using computer controlled Leica DMLP polarizing microscope and the Scanning Electron Micrograph of the films were taken by Jeol JXA 8100 EPMA at 15 kV. DSC data were collected with a Perkin Elmer DSC unit in the temperature range RT to 150 °C at a heating rate of 5 °C/min under N2 atmosphere to access thermal behaviour of composite polymer electrolyte films. The transport number (tion) was determined by the Wagner’s method of polarisation [21]. In this method, a fixed d.c. potential (×0.5 V) was applied across the sample and the current was monitored with time for a sufficiently large time to allow the sample to get fully polarised. The electrical conductivity was evaluated from complex–impedance plot obtained using computer controlled Hioki (Japan) LCZ HI Tester (model 3520-01). During all measurements, humidity level was maintained to eliminate its effect. The humidity level was maintained by a constant humidity chamber, in which an oversaturated salt solution (i.e. Ca(NH)3. 4H2O for humidity 51%) was used for maintaining the humidity level.
Experimental
Result and discussion
The polymer PEO (M.W. ∼6×105, ACROS Organics) and the salt Ammonium thiocyanate (NH4SCN, Rankem India) of AR grade were used in synthesis of electrolyte. For the synthesis of ceramic filler (SiO2), tetraethyl orthosilicate (TEOS, Aldrich) was used as precursor material, ethanol as solvent and Ammonia solution as a catalyst. To obtain nano fillers, TEOS was hydrolyzed through the two-step hydrolysis process [19]. The pH of the solution was kept at ∼10 (i.e. basic medium) to obtain the filler particles. A part of this solution was jellified and dried in the form of powder. The refractive index (RI) of this powder was measured (RI=1.426) by index-matching technique [20] to confirm formation of silica powder. Rest part of the solution was subsequently admixed stoichiometrically in PEO solution (PEO dissolved in de-ionised (DI) water at 40 °C) and stirred for 12 h continuously. This gelatinous polymer solution was finally cast in a polypropylene petri dish. To synthesize the polymer electrolyte films, vacuum-dried NH4SCN was also added stoichiometrically in solution of PEO and SiO2 in DI water. The solution cast film was finally dried at room temperature for obtaining free standing films of CPE. Structural behavior of PEO–SiO2 and PEO–SiO2– NH4SCN system was studied with the aid of XRD, IR
Structural and thermal studies X-ray diffraction Figure 1 depicts the XRD pattern of water-casted PEO film doped with different SiO2 contents with pristine PEO film. All the curves show the presence of background modulation—a feature prevalent in polymeric systems. Though the crystal structure of PEO is well known, the XRD pattern of PEO film was recorded to ascertain its structure and morphology. The XRD pattern of pure PEO film shows sharp and intense peak at 2θ = 19° and 23°, which correspond to values reported elsewhere [22]. The unit cell of polyethylene oxide is a repeat unit of (O–CH2–CH2) having bond length of the order of 19.2 Å [23]. Existence of broad peaks, in addition to few sharp reflections, further confirms its partial crystalline and partial amorphous nature. Owing to this type of morphology, pristine-solvent free polymer electrolytes fail to deliver high conductivity values. To overcome the problem of partial crystalline and partial amorphous structure, ceramic filler SiO2 was doped in pristine electrolyte. The intercalation of polymer chain with ceramic filler (silica) usually increases the interlayer spacing of ceramic material. This effect leads to shift of
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Fig. 1 a XRD Pattern of xPEO+(1−x) SiO2 film for different x values (wt.%). b XRD Pattern of 95[xPEO+(1−x) SiO2]+5 NH4SCN film for different x values (wt.%)
diffraction peaks towards the lower 2θ value which are related through the Bragg’s relation
1 ¼ 2d sin θ
ð1Þ
The XRD pattern of different compositional ratios of PEO and Silica with addition of salt is shown in Fig. 1a and b, respectively. From Fig. 1a, it is clear that addition of ceramic
filler (SiO2) in polymer host (PEO) reduces the intensity of the main peaks (in PEO, 2θ=19° and 23°) followed by broadening of the peak area, which is an indication of reduction in degree of crystallinity. When SiO2 concentration exceeds 50 wt.% in PEO: SiO2, peaks get submerged in broadening and few new peaks of SiO2 appear. From this diffractogram, it is also inferred that at lower weight percentage of SiO2, the crystallite size of silica is large but
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as SiO2 concentration increases, the dispersal becomes homogeneous followed by reduction in size. At higher ratio of SiO2 due to the cluster formation, the peak of SiO2 reappears and crystallinity decreases. Moreover, addition of silica shifts diffraction maxima to lower 2θ value. The magnitude of shift varies with doping ratio. When salt is doped to form CPE, no new peak appears; instead, the existing peaks of PEO:SiO2 reappear at the same position but with reduced intensities (Fig. 1b). Samples with the higher weight content of NH4SCN give a completely amorphous film. As a result, polymer–salt interaction cannot be ruled out. The average crystallite size of crystals in different compositions were calculated (given in Table 1) by the Scherrer formula [24], and it was found to be ∼30–50 nm. Thus, transformation of crystalline phase into amorphous phase is concluded from XRD measurements. SEM and optical study Optical micrographs of pure PEO and with the addition of ceramic and salt are shown in Fig. 2. In this figure, distinct spherulites are observed in the pure polymer, which is indicative of the lamellar microstructure of the pure polymer. The dark boundaries observed between the spherulites show presence of partial amorphous phase (Fig. 2a). Morphology of the film changes substantially upon addition of SiO2 in different weight percentage (Fig. 2b). Micrographs also show the heterogeneous dispersion for low percentage of ceramic additive which transforms into homogeneous distribution at higher SiO2 contents (Fig. 2c). After the addition of salt, no significant change is observed in the optical micrograph (Fig. 2d). The SEM image of the different compositions is shown in Fig. 3. Pure PEO film revealed partial crystalline structure of PEO (Fig. 3a). The crystalline structure formed due to longer evaporation time. Addition of ceramic filler makes the original entity lose this character (Fig. 3b).
Table 1 Calculated average particle size of undoped and doped PEO: SiO2 film for different compositions Sample
Average particle size (nm)
Pure PEO film 90 wt.% PEO+10 wt.% SiO2 film 80 wt.% PEO+20 wt.% SiO2 film 70 wt.% PEO+30 wt.% SiO2 film 60 wt.% PEO+40 wt.% SiO2 film 20 wt.% PEO+80 wt.% SiO2 powder [90 wt.% PEO+10 wt.% SiO2]95: (NH4SCN)5 film [80 wt.% PEO+20 wt.% SiO2]95: (NH4SCN)5 film SiO2 powder
45.2 23.7 22.8 20.7 21.2 31.8 32.5 25.7 50–60
Higher SiO2 ratio distribution in PEO becomes homogeneous followed by cluster formation. Scanning electron images show that the addition of SiO2 disturbs the crystalline nature of pure matrix (Fig. 3c). Similarly, doping of salt shows the same type of structure with separate entity (Fig. 3d). The higher ratio of SiO2 content shows the more homogeneous with a particle of the limit of nanometer size (∼100 nm) which is suggestive of nanosize format. Infrared spectroscopic study Infrared spectra of PEO:SiO2 film and x (PEO:SiO2):(1−x) NH4SCN salt (in the range of 4,000–400 cm−1) are shown in Fig. 4 and peak assignments are given in Table 2. The main feature of the PEO: SiO2 spectrum is presence of the broad peaks at 3,700 and 1,620 cm−1 due to –OH stretching and –OH bending. The existence of peaks at 3,000–3,100, 1,700, 1,410 and 1,390 cm−1 are related to C–H stretching, C–H in plane bending, C–O stretching and ν CH2–O, respectively. Other silica related peaks are at 941 and 802 cm−1 [25]. Crystalline PEO has a triplet peak at 1,149, 1,109, 1,061 cm−1 and another peak at 1,280 cm−1 (related to –C–H twisting) [26]. All these peaks completely vanish after introduction of ceramic filler (SiO2) in present studies, which also indicates reduction in crystallinity of the system. It is evident that SiO2 acts as inert filler in PEO matrix, which only modifies the morphology of the system. Addition of salt NH4SCN gives few new peaks at 2,000– 2,100 and 860 cm−1, which indicates formation of crystalline complex of x (PEO:SiO2):(1−x) NH4SCN. The new band at 2,100 cm−1 is ascribable to the contact ion pair and solvent-separated diamers. The salt addition reduces the intensities of different original peaks. Other important feature in IR spectrum is the relative intensity of the band at 2,060 cm−1, which increased at the expense of 2,025 cm−1. This indicates the disintegration of crystalline PEO/ PEO: SiO2 phase and results in decrease of crystallinity. At a local level, after the addition of NH4SCN, the intensity of band 860 cm−1 slightly enhanced, which particularly were sensitive to the local conformation of O–C–C–O tortional angle. This type of complexation suggests gauche coordination. For CH2 wagging modes at 1,343, 1,360 cm−1, the intensity decreases drastically and is replaced by a sharp band around 1,350 cm−1 upon the addition of NH4SCN. The reduced intensities of various characteristic peaks (triplet band of crystalline PEO at 1,280, 1,360 and 3,100 cm−1) confirms the fall in degree of crystallinity after addition of ceramic filler and salt. Differential scanning calorimetry The DSC curve of Pure PEO, PEO: SiO2 with different filler concentration and x (PEO:SiO2):(1−x) NH4SCN are
Ionics (2008) 14:515–523 Fig. 2 Optical micrographs of pure PEO film (a), 90 PEO+10 SiO2 film (b), 70 PEO+30 SiO2 film (c) and 95[90 PEO+10 SiO2]+5 NH4SCN film (d)
Fig. 3 SEM image of pure PEO film (a), 90 PEO+10 SiO2 film (b), 70 PEO+30 SiO2 film (c) and 95[90 PEO+10 SiO2]+5 NH4SCN film (d)
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the calculated crystallinity of PEO:SiO2 and with NH4SCN are lower than that of PEO film casted in water and is in agreement of XRD results. This also suggests that silica is compatible with PEO and changes the morphology of system through reduction in crystallinity of the complex. The enhancement of amorphosity is favourable for the ionic mobility and, hence, its ion transport behaviour in the PEObased solid polymer electrolyte. Ion transport studies Polarisation studies
Fig. 4 Infrared spectra of a 90 PEO+10 SiO2 and b 95 [90 PEO+ 10 SiO2]+5 NH4SCN film (pure PEO spectrum in inset [24])
shown in Fig. 5a and b, respectively. Diffractogram of pure PEO film shows two peaks, one endothermic around 69 °C and another exothermic around 105 °C. The former peak is related to the melting of pure PEO while the latter peak is related to evaporation of adsorbed water. After 125 °C, the sample starts dissociating. Addition of ceramic filler (SiO2) in PEO matrix continuously reduces the melting temperature of the resulting system. The melting temperature of polymer changes from 69 °C to 64 °C after the addition of 30 wt.% SiO2. Further crystallinity of the host polymer is seen to reduce as indicated by enthalpy change after addition of filler. This effect is possibly due to the formation of more amorphous domains with partial miscibility of filler with polymer host. Similar observation has been recorded even after the addition of salt. A further reduction in melting temperature (Tm) was noticed in case of composite polymer electrolytes. Such a reduction is possibly due to the formation of amorphous PEO: NH4SCN complexes witnessed in IR and XRD studies. The ΔHm and
In the present study, the likely mobile species are protonic and, thus, thick silver electrodes were used in Wagner Polarisation method to assess the nature of ion transport and evaluate total transference number from current time plot. The variation of current with time for two different composition i.e. x[90 PEO:10 SiO2]:(1−x)NH4SCN and x[80 PEO:20 SiO2]:(1−x)NH4SCN are given in Fig. 6a and b, respectively. From these plots, the initial current iinitial and final current ifinal is evaluated, and total ionic transference number (tion) was calculated using the relation tion ¼
iinitial ifinal iinitial
ð2Þ
The calculated values of tion for different composite films are listed in Table 3. From the table, it is apparent that polyethylene oxide with the filler SiO2 only is essentially a nonionic material, but after doping samples with ammonium thiocyanate, nature of material changes and become completely ionic. These values are at best qualitative due to (a) nonavailability of ideal blocking electrode for protonic or gaseous species and (b) uncertainty in the measurement of initial current due to quick onset of polarisation.
Table 2 Assignment of different IR peaks of 90PEO:10SiO2 film and 95[90PEO: 10SiO2]: 5NH4SCN film Peak position in 90 PEO:10 SiO2 film
Peak position in 95(90 PEO:10 SiO2):5 NH4SCN film
Assignments
Broad peak at 3,650–3,800 cm−1 Broad peak at 3,100 cm−1 2,902 cm−1 —— 1,780 cm−1 1,600 cm−1 1,410,1390 cm−1 941 cm−1 —— 802 cm−1 Broad peak at 780 cm−1
Broad peak 3,650–3,800 cm−1 — — 2,100 cm−1 — 1,620 cm−1 — — 860 cm−1 — Broad peak at 780 cm−1 with reduced intensity
ν OH, adsorbed H–O–H, =C–H stretching νas CH2, ν C–Hx organic groups SCN organics, C–O H–O–H, molecular C–H in plane bending, ν CH2–O νas Si–OH SCN peak νs Si–O–Si, νas Si–C, Si–O–Si–CnHm ν SiO–C2H5
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Fig. 6 Current vs time curve for a x[90 PEO:10 SiO2]:(1−x) NH4SCN and b x[80 PEO:20 SiO2]:(1−x) NH4SCN
Fig. 5 a DSC curve for xPEO+(1−x) SiO2 with different x values (x in wt.%). b DSC curve for [xPEO+(1−x) SiO2] undoped and doped with 5 wt.% NH4SCN
Electrical conductivity measurement The variation of electrical conductivity (σRT) of the composite polymer electrolyte film as a function of filler composition (i.e. SiO2 content) is shown in Fig. 7. This indicates that the conductivity increases with the increase of SiO2 content up to 30 wt.% (σmax =2.37×10−6 S/cm) and,
thereafter, it starts decreasing with the increasing concentration of filler before saturating at large SiO2 content. Similar effect has also been observed with increasing concentration of salt. This is possibly due to modification or increase of transition temperature of the polymer composite electrolyte with increase in filler or dopant concentration. In addition, the mechanical stability of the composite materials has also been found to improve with increasing concentration (up to 30% only) of filler and salt. Because morphology of films is directly linked to mobility of charge carrier to migrate upon the application of electric field, ionic conductivity of polymer electrolytes is bound to be influenced by morphology of the system. In view of this explanation, conductivity of polymer electrolyte modified with nanofillers enhances until 40 wt.% SiO2 and then diminishes. Here, it is remarkable that as SiO2 content increases, the brittleness of film increases. Beyond 40 wt.%
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Table 3 Ionic transference number for the nanocomposite polymeric film Sample
Ionic transference number
90 PEO:10 SiO2 95[90 PEO:10 SiO2]:5 NH4SCN 90[90 PEO:10 SiO2]:10 NH4SCN 80 PEO:20 SiO2 95[80 PEO:20 SiO2]:5 NH4SCN 90[80 PEO:20 SiO2]:10 NH4SCN
0.00 0.90 0.92 0.00 0.91 0.94
addition of SiO2, it is impossible to synthesise the film; rather, the system is like fine powder. The temperature dependence of the conductivity i.e. σ vs 1/T plot of x[90 PEO:10 SiO2]:(1−x)NH4SCN and x[80 PEO:20 SiO2]: (1 − x)NH4SCN are shown in Fig. 8. Siekierski [27] has studied ion transport properties of PEO-NH4SCN system. He has shown a variation of ionic conductivity from 3×10−9 S/cm for (PEO)40NH4SCN to 7× 10−7 S/cm for (PEO)20NH4SCN. After addition of nanofiller, a further increase of twofold is noticed in conductivity. Further, these plots show insignificant variation in conductivity PEO:SiO2 (90:10 and 80:20 wt.% ratio) over a wide range below melting temperature of PEO. After the melting temperature (∼66 °C) of PEO, the conductivity starts increasing with some fluctuations. This is indicative of unstable nature of the PEO:SiO2 film after the Tm of PEO. The addition of salt NH4SCN shows a sudden jump in the conductivity after Tm, in each of the four composition shown in Fig. 8. This has been explained on the basis of semicrystalline to amorphous phase transition. The almost linear variation in conductivity as a function of temperature follows apparently an Arrhenius type thermally activated process below Tm (in all samples); above Tm (only with salt composition films), it exhibits Vogel–Tamman–Fulcher type character. The calculated values of activation energy of different composition are given in Table 4, which suggests continuous fall in activation energy on account
Fig. 8 Variation of conductivity with temperature in x[90 PEO: 10 SiO2]:(1−x) NH4SCN and x[80 PEO:20 SiO2]:(1−x) NH4SCN polymer films
of morphological improvement in films upon addition of nanosized fillers.
Conclusions The experimental studies through XRD, IR, SEM and DSC showed that ceramic filler SiO2 was able to decrease the crystalline content and enhanced salt dissociation of x (PEO:SiO2):(1−x)NH4SCN. The intercalated silica in PEO polymer host also produces a huge interfacial area with better mechanical and thermal property of the solid composite electrolyte. The XRD data and SEM show the average particle size in the film to be in nanosize format. The composite polymer electrolyte film of composition of x[90 PEO:10 SiO2]:(1−x)NH4SCN and x[80 PEO:20 SiO2]: (1−x)NH4SCN show reasonably good ionic conductivity with the complete ionic nature. The temperature dependence of conductivity of SPEs show the Arrhenius type thermally activated process with two different activation energies, before and after the melting temperature.
Table 4 Activation energy of different nano-polymer composite electrolyte films Sample
Fig. 7 Composition dependence of conductivity of PEO: SiO2 polymer film
[90 [90 [80 [80
PEO+10 PEO+10 PEO+20 PEO+20
Activation energy (eV) SiO2]95:(NH4SCN)5 SiO2]90:(NH4SCN)10 SiO2]95:(NH4SCN)5 SiO2]90:(NH4SCN)10
33.5 3.5 7.5 1.8
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