Structural and Electrochemical Properties of Nanocomposite Polymer

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Structural and electrochemical properties of nanocomposite polymer electrolyte for electrochemical devices Mohan L Verma, Manickam Minakshi, and Nirbhay K Singh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie502615w • Publication Date (Web): 05 Sep 2014 Downloaded from http://pubs.acs.org on September 8, 2014

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Industrial & Engineering Chemistry Research

Structural and electrochemical properties of nanocomposite polymer electrolyte for electrochemical devices Mohan L Verma1, Manickam Minakshi2,* and Nirbhay K Singh3 1

Condensed Matter Physics Laboratory, Department of Applied Physics, Shri Shankaracharya Group of Institutions, Junwani, Bhilai (Chhattisgarh) 490020, India 2

School of Engineering and Information Technology, Murdoch University, WA 6150, Australia 3

Department of Applied Physics, Shri Shankrachayra Institute of Engineering and Technology, Khapri Durg (Chhattisgarh) 490020, India

AUTHOR INFORMATION:

Email: [email protected] TEL: +61 8 9360 2017

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Abstract This report describes the structural and electrochemical properties of the nanocomposite polymer electrolyte (70PEO:30AgI) incorporating SiO2 filler of different weight percentage (wt. %) ranging between 1 and 10 wt. %. Studies on inorganic filler in polymer matrix could be quite familiar but the chosen polymer and the effect of concentration exhibited the highest conductivity and its use for capacitor studies could be novel and studied with the aid of impedance spectroscopy. The optimum composite composition (OCC) of 5 wt. % loading, 95(70PEO:30AgI):5SiO2, exhibited the best performance of conductivity σ ~2.50 10−3 S/cm and with a low activation energy of 0.1 eV. The impedance, capacitive and other structural behaviour of the polymer electrolyte validated the proposed conceptual idea. The conductivity enhancement has been well correlated with the change in structure and morphology of the polymer electrolyte suggesting the amorphous domain is found to be suitable for solid state capacitor applications.

Keywords: nanocomposite polymer electrolyte; impedance, filler; SiO2; Capacitor


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Introduction Ionically conducting nanocomposite polymer electrolytes are important materials in solid state electrochemical devices, including rechargeable batteries and capacitors1. In all solid state capacitor both the electrodes and electrolyte consist of conducting polymers and the cell operates by the capacitive processes of the polymer backbone2. During the electrochemical processes (discharging / charging) the ions in the electrolyte adsorb and desorb on the porous polymer electrode and the system allows for very rapid charging and discharging of the polymer so that capacitor like performance is obtained. The solid polymer electrolyte are defined as water free system with an ion conducting phase formed by dissolved salts in a host polymer matrix3-5. The advantages of solid electrolyte in devices are that there is no need of separator as a component and there won’t be any leakage6. In the current work, polyethylene oxide (PEO) based on high molecular weight is used as polymer backbone with AgI salt to provide sufficiently high ionic conductivity7-8. However, the ionic conductivities of PEO based polymer electrolytes are still only in the range of 10−7 S cm-1 due to its largely crystalline in nature9-10. Also, the main disadvantage of solid electrolyte is that their conductivity is lower than that of the liquid counterparts. In this regard, composite polymer electrolyte consists of sources which benefit from both the solid and liquid electrolytes. Numerous research works were carried out over a decade towards the incorporation of ceramic fillers into electrolytes, an effective strategy to improve not only the conductivity but also the mechanical properties of solid polymer electrolytes. Various fillers such as Al2O3, CeO2, TiO2 and SiO2 were reported11-16 and it is widely known that these additives acts as charge carriers and have interaction between surface groups on the polymer chain that reduces the degree of PEO crystallinity9. Thus, the conductivity of the PEO based electrolyte having fillers is gradually enhanced17. It’s also been reported that increase in the content of fillers after certain level lead to a decrease in conductivity18-19. Among the fillers studied, 3 ACS Paragon Plus Environment


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SiO2 plays a crucial role in terms of enhancing ionic conductivity. To our best knowledge, an effect of concentration added to the polymer electrolyte [70PEO:30AgI] and its electrochemical properties in particular to its potential use for electrochemical double layer capacitor has not been explored until now. The impedance spectroscopy and other physical techniques employed in the current work will bring new insights in the field and helps in understanding the molecular level interactions to determine the polymer-salt complexes and role of SiO2 filler. The determined relaxation processes may provide valuable information of mobile ionic species on the surface of the fillers. In the current work, a fine powder of SiO2 filler (~ 5 µm; see Fig. 8f) has been dispersed on the host matrix having a composition [70PEO:30AgI] termed as “solid polymer electrolyte having optimal composition of AgI” (SPE (OCC)) 8, 20. Solid polymer electrolytes (SPE OCC) incorporated with SiO2 filler particles are referred as “nanocomposite polymer electrolytes” (NCPE). The NCPE electrolytes are synthesized via hot press technique. The SiO2 filler at the optimum amount was found to be very effective in reducing the crystallinity of PEO based polymer matrix. The effect of SiO2 filler and its influence on structural (physical), electrochemical and capacitive properties have been investigated. The presence of the optimum amount of filler (5 wt. %) showed the maximum ionic conductivity of σ ~2.50 10−3 S/cm which has not ever been reported for this polymer electrolyte with SiO2 filler earlier21-23. The electrochemical double layer capacitor (EDLC) involving electrostatic phenomenon (non-faradaic) is reported. A new class of solid state capacitor consisting 70(70PEO: 30AgI):30 activated carbon as the electrodes sandwiched with synthesized NCPE (OCC) electrolyte is presented and their results are discussed.

Experimental Poly ethylene oxide (PEO; Mw of 105, Aldrich, USA) based conducting electrode 1-x (70PEO:30AgI): xSiO2 (where x = 0 ≤ x ≤ 10 wt. %) were prepared by using a hot press 4 ACS Paragon Plus Environment

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method in place of the conventional solution-cast technique. The precursors AgI (98%), SiO2 (99%) and activated carbon (AC) (99%) were purchased from Redial laboratory reagent, Sigma – Aldrich and Loba Chemie Pvt. Ltd, respectively. Initially, solid polymer electrolyte (SPE) with the composition (70PEO:30AgI) is prepared from the pure PEO mixed with a stoichiometric ratio of fine powder of AgI. The mixture was thoroughly ground for 15 min and then heated for 20 min at 100 °C. The obtained slurry is then pressed between stainless steel blocks by applying about 350 psi. The detailed and advantages related to hot pressing technique have been given elsewhere20. The synthesized standard electrolyte (70PEO:30AgI) was used as the host polymer-salt matrix (as the base material). Required amount of insoluble SiO2 precursor in appropriate wt. % is dispersed to the host polymer-salt complex and the ingredients were thoroughly ground at room temperature for about 10 min using an agate mortar and pestle. The physically mixed salt mixtures of different compositions were then heated close to the melting point in a muffle furnace for about 20 min. This resulted in soft slurry of polymer encapsulated with AgI salt and SiO2 as filler. The obtained slurry was then pressed at 350 psi. This was resulted in thin polymer layer of area ~1.85 cm2 having uniform thicknesses of 0.04, 0.037, 0.031, 0.029 and 0.017 cm corresponding to 1, 3, 5, 7 and 10 wt. % of SiO2 contents in nanocomposite polymer electrodes (NCPE). The masses of PEO and AgI in the composites were kept identical. It shows that addition of filler decrease the thicknesses of sample could be due to decrease in degree of crystallinity. All solid-state symmetric capacitor has been constructed using the synthesized 70(70PEO:30AgI):30AC (activated carbon) termed as “PAC electrodes” on both sides as “symmetric device” and the nanocomposite polymer electrolyte with the composition 95(70PEO:30AgI):5SiO2 is sandwiched in between these electrodes. The electrochemical impedance measurements were carried out in a two electrode system and the schematic representation of the cell has been reported by us earlier20. A Princeton Applied Research (Versa Stat III model) at a frequency range of 1 MHz to 10 MHz has been used for impedance analysis and the applied potential 5 ACS Paragon Plus Environment


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amplitude was varied from 5 mV. Cyclic voltammetry has been performed between the voltage limits (0 – 1.2 V) at a scan rate of 1 mV/s to evaluate the suitability for supercapacitor applications. The structural determination of the synthesized electrode was investigated through X-ray diffraction technique using Siemens X-ray diffractometer with Philips Co Kα radiation. The microscopic analysis of the material was conducted by using a scanning electron microscope (Philips Analytical XL series 20) coupled with energy dispersive analysis (EDS).

Results and Discussion (a) Electrochemical characterization of 1-x (70PEO:30AgI) : xSiO2 nanocomposite polymer electrolyte: Impedance spectra and conductivity measurements The complex impedance plot of 1-x (70PEO:30AgI):xSiO2 nanocomposite polymer electrolyte (NCPE) at various concentration of SiO2 as filler at different temperatures is shown in Fig. 1. The prepared electrolyte samples were placed between two stainless steel plates that acted as the active electrodes for the conductivity measurements. Typical plot consists of the high frequency distorted semicircle representing the parallel combination of bulk resistance and capacitance which could be due to the bulk conductivity of the solid polymer electrolytes24 and the low frequency inclined spike (non-vertical) like region attributed to the ion diffusion in polymer electrolyte25. The low frequency tail indicates the capacitive nature of the interface and the absence of electronic conductivity while the distorted semicircle is representative of grain boundary effects in the sample as reported by Scrosati et al25. From the Fig. 1, it has been noted that the diameter of the semicircle decreases with the increase in temperature and on the other hand for a given specific temperature (say 80 °C) the diameter and intensity of the semicircle decreases with the increase in the filler concentration of SiO2 until 5 wt. % and thereafter increases (x and y axes in the plots are different). Based on the previous works 24-25 this could be interpreted in a way


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that number of charge carriers and the mobility of ions are higher and hence the obtained lower bulk resistance (Rb ~ 10 Ω) for 3 and 5 wt.% SiO2. The bulk resistance for all concentrations shown in Fig. 1 has been calculated from the intercept of the semicircle with real axis in the low frequency region. The value of the bulk resistance (Rb) decreases from 25 to 7 Ω (5 wt% SiO2) with increasing temperature for a given electrolyte composition. Gondaliya et al26 reported a similar observation for the polymer electrolyte that with the increase in the temperature, the intersection of the impedance plot on the real axis (Z') shifts towards the origin reflecting a decrease in the bulk resistance of the sample. The bulk resistance initially decreases with the concentration of SiO2 filler and reaches minimum for 3 and 5 wt. % (~ 7 Ω) and then any further addition of SiO2 increased the bulk resistance (~ 70 Ω). For the 7 wt. % of filler content, only the resistive component of polymer electrolyte exists without any form of capacitor27 implying an immobile polymer chain could be disintegrated. This is discussed in more detail in the physical characterization section having data drawn from XRD and SEM analyses. The ion conductivity of the solid polymer electrolyte with various concentrations of SiO2 can be calculated using the equation σ = L /Rb x A; where L is the thickness of the electrolyte film and A the surface area. A very similar

trend has been found from the conductivity measurements that until a particular composition of the electrolyte the conductivity increases and thereafter the conductivity decreases which is detailed in the subsequent section. The conductivity measurements were carried out on all the synthesized NCPEs to understand the behaviour of its mechanism in the presence of SiO2 filler. The conductivity of the NCPE against the reciprocal absolute temperature is displayed in Fig. 2 and the system does not show any quantum jump in conductivity with respect to temperature, reflecting there is no change in phase and the synthesised electrolyte exhibits less crystalline28. Michael et al28 reported a linear variation of conductivity represents the fact that polymer electrolyte (PEO) blends exhibit a completely amorphous structure. Moreover, the solid polymer 7 ACS Paragon Plus Environment


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electrolyte (Fig. 2) with 1 wt. % SiO2 exhibits a low ionic conductivity (σ ~1.12 10−3 S/cm) however, the conductivity increases with increase in SiO2 concentration up to 5 wt. % (σ ~ 2.5 10−3 S/cm) and comparable to that of the reported value for AgI nanoplates by Maier et al29. This could be due to the presence of filler facilitating the path for ionic transport and polymer segmental motion7, 30. At a lower temperature this activity is hindered but at higher temperature the polymer host matrix can expand that enhances the overall mobility of the ions led to higher ionic conductivity. The addition of ceramic filler also prohibits recrystallization of the electrolytes and promotes the amorphous regions11. Dispersion of more SiO2 (5 wt. %) in the host polymer electrolyte makes the amorphous phase of the electrolyte more flexible and hence the ionic conductivity increases31. However, a decrease in conductivity is observed in Fig. 2 for > 5 wt. %, (σ ~2.5 10−4 S/cm; difference in an order of magnitude) this could be explained in a way that excessive fillers in the solid polymer electrolyte may lead to ion pairs and ion aggregation inhibiting the ionic conduction and slowing its mobility23, 32-33 in an amorphous phase. A further increase of filler concentration (> 7 wt. %) causes a significant decrease in conductivity (σ ~5 10−4 S/cm) due to an increase in glass transition temperature21. The obtained value of conductivity for 5 wt. % SiO2 appears to be maximum of (σ ~2.50 10−3 S/cm) and comparable to that of the values reported by Tan et al24 (σ ~2.05 10−4 S/cm) and Senaviratne et al34 (σ ~1.24 10−4 S/cm) but exceeded with those values reported earlier21-23. Liu and Dissanayake et al22-23 also reported similar trend of behavior for the composite polymer electrolyte added with SiO2 as filler. The ionic conductivity decreases after surpassing the optimum level of the filler concentrations, however, their optimum filler concentration was reported to be 15 wt.% while we have found 5 wt.% exhibited the maximum enhancement of conductivity. The temperature dependence of the samples shows the conductivity increases with temperature and it essentially follows a near Arrhenius-type equation. Through the conductivity versus 1/T plot, the activation energy can be calculated and the obtained values are plotted in Fig. 3. The migration of ion depends 8 ACS Paragon Plus Environment

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mainly on the segmental movement of the polymer chain (PEO) in the amorphous region of the electrolyte. The activation energy decreases from 0.6 to 0.1 eV with the increase of SiO2 filler between 1 and 5 wt. % SiO2. Further increasing the SiO2 content causes an increase in the activation energy to 1.2 eV for 10 wt. % SiO2. The lowest activation energy of NCPE is found to be 0.1 eV corresponding to 5 wt. % of SiO2 and in excellent agreement with previously reported results of 0.16 eV for silver ion polymer electrolytes35-36. In the case of Al2O3 and ZrO2 as fillers in the PEO electrolyte system, the reported activation energies are found to be 0.3 and 0.58 eV

19-20

, and 0.025 eV respectively37. It is also generally believed

that the content of the amorphosity in the polymer host will directly correlate the lowest amount of activation energy (Ea) that required for the defect formation and migration of the Ag ion. Thus, when the concentration of filler increases, the Ea decreased and conductivity increased with the increase of SiO2 filler and the optimum value is found to be 5 wt. % implying that the Ag ions to migrate from one to another site require lower energy of just 0.1 eV suggesting high mobility of ions present in the NCPE sample. The overall effect reiterate the well-known mechanism that the addition of filler increases the conductivity through inhibiting the crystallization of PEO chains in the host polymer while providing the conducive pathway for Ag ions to travel at the surface of the SiO2 through Lewis Acid base interactions between the filler and the polymer38-39. Based on the impedance and conductivity studies, it is obviously seen that 5 wt. % of SiO2 filler shows the best performance and chosen for further impedance measurements and this sample is denoted as NCPE (OCC). Figure 4a shows the variation of the real part of impedance (Z') versus logarithmic frequency at different temperatures. The magnitude of Z' is observed to be high in intensity at a lower frequency and the typical peak like curve decreases in intensity as the temperature (343 K – 373 K) increases but for 393 K the intensity again increased. The broadening of the peaks suggests that there is a spread of relaxation times. The significant widening of peaks (in the x-axis of Fig. 4a) on increasing temperature suggests the presence of a temperature9 ACS Paragon Plus Environment


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dependent relaxation process in the material. The relaxation species may be possibly due to ions at low temperature and presence of defects at higher temperature. The shift of the peak (maximum Z') to lower frequency is observed for higher temperatures. In comparison to that of the spectrum obtained for 0 wt. % added SiO2 (in Fig. 5a), the magnitude of Z' is observed to be maximum at higher frequencies but the trend is quite similar to that observed for 5 wt. % material (Fig. 4a). The observed difference in peak positions explains the increase in conductivity with the increase in temperature and frequency for SiO2 added NCPE. Fig. 4b shows the variation of the imaginary part of impedance (Z") with frequency at the different temperature. The curves show that the Z" values reach a maxima peak (Z"max) and the value of this peak shifts to higher frequencies with increasing temperature from 353 K to 393 K indicating the relaxation time of the mobile ions to adjust themselves in the host polymer network40. Figure 5b shows the spectra for 0 wt. % added SiO2, where no such maxima peak have been identified but curve displayed a plateau in the low frequency. The plots of impedance parameters (Z'/Z"max) of NCPE (OCC) as a function of logarithmic frequency are plotted in Fig. 6. It is clearly seen from the plot that the presence of relaxation peak could be attributed to the localised (defect) relaxation or non-localised conduction (ionic or electronic conductivity)41. At the peak, the relaxation is defined by the relation ωmτm = 1, where τm is relaxation time. A broadened spectrum for the temperatures (343 – 373 K) on the high frequency side is the indication of the wide distribution of relaxation times and large deviation from Debye’s theory42. The overlapping of the relaxation peaks for the temperatures (343 – 373 K) indicates that the dynamical processes are nearly temperature independent. The dielectric relaxation process of the host polymer NCPE may be explained by loss tangent (tan δ). Figure 7 presents the plot of frequency dependence of tan δ at different temperatures. It can be seen from the Figure 7 that the maximum of tan δ shifts towards higher frequency and the height of the peak decreases with increasing temperature. The appearance of peak suggests the presence of relaxing dipoles in all the samples invariable 10 ACS Paragon Plus Environment

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to the temperature. A quite similar behaviour is described for electrolyte films having parallel resistance and capacitance elements43. The observed little changes predicts that the polymer chain appears to be very rigid to allow any changes and hence the loss tangent occurs at a low frequency associated with a higher relaxation peak as observed in Fig. 6. (b) Physical Characterization of 1-x (70PEO:30AgI ): xSiO2 nanocomposite polymer electrolyte: SEM, EDS, DSC and XRD analyses The

Scanning

electron

micrographs

(SEM)

of

1-x

(70PEO:30AgI):xSiO2

nanocomposite polymer electrolyte for different concentrations of SiO2 filler is shown in Fig. 8a-e. The micrograph of the precursor SiO2 used in this study is given in Fig. 8f for a comparison. The important features to examine between the micrographs are its morphology and change in crystallinity. The image in Fig. 8a having no SiO2 filler termed as “SPE (OCC)” represents crystalline in nature with well-defined individual grains. This morphology gradually diminishes with the addition of SiO2 in the parent polymer (Fig. 8b-d) that decreases the crystallinity and the extent being more for higher filler content. In the case of 7 wt. % SiO2 (Fig. 8e), the polymer appears to lose its integrity with a severe particle aggregation. Although, materials comprising 5 and 7 wt. % SiO2 both show an amorphous character (Fig. 8d-e) but particle ordering is seen only for the optimal addition of 5 wt. % SiO2 (Fig. 8d). The 5 wt. % SiO2 termed as “NCPE (OCC)” image (in Fig. 8d) showed that the SiO2 fillers are widely dispersed in the host polymer matrix. The brighter portions in the image (compare Fig. 8f) represent the presence of SiO2 particles. This is evidenced through the energy dispersive analysis (EDS) technique shown in Fig. 9 and its composition is tabulated in Table 1. The presence of filler and its ordering aids the Ag ion transport in electrolyte to enhance the ionic conductivity. The morphological features described by Yap et al7 for the PEO based film with SiO2 filler showed a homogenous film but with lower degree of amorphosity and hence they have 11 ACS Paragon Plus Environment


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observed an improvement in the ionic conductivity of ~3.57 10−5 S/cm. In our studies, we observed a similar trend that the presence of spherulites in Fig. 8a represents the crystallinity of PEO, whereas less defined particles are observed for the filler added polymer (Fig. 8b-e). This means that amorphosity of PEO increases by the addition of SiO2 filler. The filler particles occupied the pores and break the grain boundaries between PEO-AgI complexes and allowed the transport of Ag+ ion in faster way. A smooth and less defined morphology enhanced the conductivity to ~2.50 10−3 S/cm (as reported in the earlier section). The obtained results matches well with the conductivity study reported in the literature44, where the

ethylene

carbonate/propylene

carbonate

mixture

is

added

into

the

poly(methlmethacrylate) PMMA electrolyte which exhibits an increase in conductivity as compared to the pure PMMA. Based on these facts, we can conclude that NCPE (OCC) electrolyte has not only possessed a high conductivity but also distinct effect on the surface morphology and semi-crystalline behaviour. To aid this conceptual, differential scanning calorimetry (DSC) analysis (not included here) has been performed. Among the concentrations studied, NCPE (OCC) electrolyte showed the lowest glass transition temperature (Tg = − 67 oC) which enable the ion transport to occur in the host by the higher segmental motion of the polymer backbone. The observed low glass transition and melting temperatures evidently comes from the increased segmental flexibility and amorphous nature of the host polymer45. To determine if there are any changes occurred in the polymer structure, apart from the morphological variations, X-ray diffraction (XRD) was carried out on the 1-x (70PEO:30AgI):xSiO2 nanocomposite polymer electrolyte for different concentrations of SiO2 and it is given in Fig. 10. The X-ray diffraction patterns of synthesized materials show characteristic peaks of the polymer electrolyte at 2θ values 24ο, 39.3ο, 46.3ο and 62.2ο

46-48

.

The peaks at 24ο and 62.2ο correspond to PEO while the peaks at 39.3ο and 46.3ο correspond to AgI. Comparing the XRD pattern of SPE (OCC) having no SiO2 filler (Fig. 10), the peaks 12 ACS Paragon Plus Environment

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for filler added samples are lower in intensity with broader peak widths which is an indication of reduction in degree of crystallinity. The full width at half maximum of the peak for SPE (OCC) and NCPE (OCC) samples are 1.01ο and 1.35ο and the extent being more marked for 7 % SiO2 with 1.65ο. The calculated particle size using the Scherer’s method for the SPE (OCC) and NCPE (OCC) samples was found to be 334 nm and 34 nm respectively. The decrease in intensities and the sharpness of the prominent peaks are referred to the disruption of the PEO crystalline structure by SiO2 filler indicating that complexation has taken place in the amorphous phase46-48. The obtained NCPE (OCC) amorphous polymer has flexible backbone that resulted in greater ionic diffusivity with high ionic conductivity. All the above characterizations concluded that 5 wt. % SiO2 showed the best performance in terms of conductivity, morphology and crystallinity that are essential factors for making an electrochemical device. (c) Solid State capacitor studies of 95 (70PEO:30AgI) : 5SiO2 polymer electrolyte The best performed 5 wt. % SiO2 is chosen as an electrolyte for constructing a solid state capacitor. The cell is constructed using 5 wt. % SiO2 termed NCPE (OCC) as electrolyte with solid polymer electrode (PAC) (70(70PEO:30AgI):30 activated carbon) on either side. The potentiostatic, galvanostatic and cycling stability of this cell is given in Figs. 11-13. A typical cyclic voltammetry profile of the cell (solid state capacitor) is shown in Fig. 11. The scan was swept at a constant rate of 1 mV/s between the voltage range 0 and 1.2 V and the corresponding current was recorded. The electrode was started with an anodic scan at 0.2 V and finished at 1.2 V then reversed back to the original potential via cathodic scan. The obtained data shows the evidence of capacitance properties with a near rectangular curve, equivalent to 20 F/g. The specific capacitance obtained from the cyclic voltammogram data was calculated using the equation C = I / (V x m), where I is the redox current, V being the scan rate, and the m is the area of the electrode. The distortion of curve from the ideal 13 ACS Paragon Plus Environment


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rectangular shape could be due to high capacitive reactance49. The mechanism involved in the solid state capacitor is based on adsorption/desorption of Ag ions in the electrodes49 rather an intercalation mechanism of Ag or I occurring in the polymer (PAC) electrode50. The redox additives in the electrolytes exhibiting a pair of reduction and oxidation peaks are reported in the literature50-51. In our current studies, a typical near rectangular curve without any welldefined redox peaks, indicates that Ag/I is found to adsorb and desorb (non-faradaic) on the surface of polymer (PAC) electrode rather an electron transfer to occur in the bulk electrode material. The typical charge-discharge curves for the solid state capacitor at a range of current densities are shown in Fig. 12. It can be seen that at lower current densities the obtained specific capacitance for the capacitor is calculated to be 20 F/g but when the current density is increased from 0.4 A/g to 1.25 A/g the available capacitance reduced to 50% delivering only 10 F/g. The observed low capacitance is due to the smaller contact area of the active polymer material in the electrode and the solid state electrolyte in the device. Nonetheless, the rate performance of the solid state capacitor is competitive and the observed capacitance (20 F/g) is comparable to the values reported for this system52-54. The symmetric charge-discharge behavior again represents the non-faradaic behaviour of the electrode. The electrochemical stability of the PAC electrodes with NCPE (OCC) electrolyte is presented in Fig. 13. The available capacitance is found to be stable over numerous cycles and interestingly, the coulombic efficiency is close to 92% suggesting the all solid state device is suitable for long term electrochemical cycling.

Conclusions PEO and AgI based nanocomposites polymer electrolytes with SiO2 in different weight percentage (wt. %) have been synthesised by hot press technique and reported. The XRD and SEM analyses reveal the presence of filler reduces the degree of crystallinity of the polymer electrolyte. The highest conductivity of σ ~2.50 10−3 S/cm was obtained for the composition 14 ACS Paragon Plus Environment

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95(70PEO:30AgI):5 SiO2 and this value exceeds those of the reported values for SiO2 filler in polymer electrolyte studies. The activation energy of the high conductivity sample is calculated using the Arrhenius plot and it has been found to be 0.1 eV. With the aid of impedance measurements the mobility of charge carriers and their influence in filler concentration have been determined. The imaginary part of the impedance spectra suggests that the distribution of relaxation times is temperature independent. The relaxation frequency complexes have been calculated by loss tangent spectra. The NCPE (OCC) sample of the composition 95(70PEO:30AgI)5 SiO2 that possesses highest conductivity, low activation energy and high amorphous nature has been chosen for all solid state capacitor application and it showed the capacitive behaviour involving adsorption/desorption of Ag ions in the polymer electrolyte. The solid state capacitor exhibited 20 F/g at a current density of 0.4 A/g and the available capacitance is stable for multiple cycles. Acknowledgements The author (M. M) wishes to acknowledge the funding bodies, Australia-India Strategic Research Fund (AISRF) and Australian Research Council (ARC). A part of this work was supported under Australian Research Council (ARC) Discovery Project funding scheme DP1092543 and Australia-India Early Career Research Fellowship. Authors (M. M.) wishes to acknowledge the receipt of SEIT seed grant through which part of the electrochemical work has been generated.



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alternating

copolymers

of

vinyl

ethers

with

methoxy

oligo(ethleneoxy)ethyl groups and vinylene carbonate. Electrochim. Acta 2013, 112, 221. (6) Scrosati, B. Recent advances in lithium-ion battery materials. Electrochim. Acta 2000, 45, 2461. (7) Yap, Y. L.; You, A. H.; Teo, L. L.; Hanapei, H. Inorganic filler sizes effect on ionic conductivity in polyethylene oxide (PEO) composite polymer electrolyte. Int J. Electrochem. Soc. 2013, 8, 2154. (8) Agrawal, R. C.; Pandey, G. P. Solid polymer electrolytes: materials designing and allsolid-state battery applications: an overview. J. Phys. D: Appl. Phys. 2008, 41, 223001. (9) Zhang, X –W.; Wang, C.; Appleby, A. J.; Little, F. E. Characteristics of lithium-ionconducting composite polymer-glass secondary cell electrolytes. J. Power Sources 2002, 112, 209. (10) Persi, L.; Croce, F.; Scrosati, B.; Plichta, E.; Hendrickson, M. A. Poly(ethylene oxide)based, nanocomposite electrolytes as improved separators for rechargeable lithium polymer batteries: The Li/LiMn3O6 case. J. Electrochem. Soc. 2002, 149, A212. (11) Croce, F. Appetecchi, G. B.; Persi, L.; Scrosati, B. Nanocomposite polymer electrolytes for lithium batteries. Nature 1998, 394, 456. 16 ACS Paragon Plus Environment

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(12) Sun, J. K.; Moon, H. S.; Kim, J. W.; Park, J. W. Role of functional nano-sized inorganic fillers in poly(ethylene) oxide-based polymer electrolytes. J. Power Sources 2003, 117, 124. (13) Nan, C. W.; Fan, L. Yin, Y.; Cai, Q. Enhanced ionic conductivity of polymer electrolytes containing nanocomposite SiO2 particles. Phys. Rev. Lett. 2003, 91, 266104. (14) Yuan, A.; Zhao, J. Composite alkaline polymer electrolytes and its application to nickelmetal hydride batteries. Electrochim. Acta 2006, 51, 2454. (15) Morita, M.; Noborio, H.; Yoshimoto, N.; Ishikawa, M. Ionic conductance of composite electrolytes based on network polymer with ceramic powder. Solid State Ionics 2006, 177, 715. (16) Dey, A.; Karan, S.; De, S. K. Thermal and electric properties of CeO2 nanoparticles dispersed in polyethylene oxide: NH4ClO4 complex. Solid State Ionics 2008, 178, 1963. (17) Shen, C.; Wang, J.; Tang, Z.; Wang, H.; Lian, H.; Zhang, J.; Cao, C. Physicochemical properties of poly(ethylene oxide)-based composite polymer electrolytes with a silanemodified mesoporous silica SBA-15. Electrochim. Acta 2009, 54, 3490. (18) Sownthari, K.; Suthanthiraraj, S. A. Structural and AC impedance studies on nanocomposite polymer electrolytes based on poly(ε-caprolactone). J. Appl. Polymer Sci. 2014, 131, 40524. (19) Polu, A. R.; Kumar, R. Effect of Al2O3 ceramic filler on PEG-based composite polymer electrolytes for magnesium batteries. Adv. Mat. Lett. 2013, 4, 543. (20) Verma, M. L.; Minakshi, M.; Singh, N. K. Synthesis and characterization of solid polymer electrolyte based on activated carbon for solid state capacitor. Electrochim. Acta 2014, 137, 497. (21) Fan, L.; Nan, C.-W.; Zhao, S. Effect of modified SiO2 on the properties of PEO-based polymer electrolytes. Solid State Ionics 2003, 164, 81. (22) Liu, Y.; Lee, J. Y.; Hong, L. Functionalised SiO2 in poly(ethylene oxide)-based polymer electrolytes. J. Power Sources 2002, 109, 507. (23) Dissanayake, M. A. K. L.; Jayathilaka, P. A. R. D.; Bokalawala, R. S. P.; Albinsson, I.; Mellander, B.-E. Effect of concentration and grain size of alumina filler on the ionic



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conductivity enhancement of the (PEO)9LiCF3SO3:Al2O3 composite polymer electrolyte. J. Power Sources 2003, 119-121, 409. (24) Tan, C. G.; Siew, W.O.; Pang, W. L.; Osman, Z.; Chew, K. W. The effects of ceramic fillers on the PMMA-based polymer electrolyte systems. Ionics 2007, 13, 361. (25) Scrosati, B.; Croce, F.; Persi, L. Impedance spectroscopy study of PEO-based nanocomposite based electrolytes. J. Electrochem. Soc. 2000, 147, 1718. (26) Gondaliya, N.; Kanchan, D. K.; Sharma, P.; Jayswal, M. S. Dielectric and electric properties of plasticized PEO-AgCF3SO3-SiO2 nanocomposite polymer electrolyte system. Polymer Composites 2012, 33, 2195 (27) Ramesh, S.; Arof, A. K. Ionic conductivity studies of plasticized poly(vinyl chloride) polymer electrolytes. Mater. Sci. Eng. B 2001, 85, 11. (28) Michael, M. S.; Jacob, M. M. E.; Prabhaharan, S. R. S.; Radhakrishna, S. Enhanced lithium ion transport in PEO-based solid polymer electrolytes employing a novel class of plasticizers. Solid State Ionics 1997, 98, 167. (29) Guo, Y. G.; Lee, J. S.; Maier, J. AgI nanoplates with mesoscopic superionic conductivity at room temperature. Adv. Mater. 2005, 17, 2815. (30) Maier, J. Ionic transport in nano-sized systems. Solid State Ionics 2004, 175, 7. (31) Capiglia, C.; Mustarelli, P.; Quartarone, E.; Tomasi, C.; Magistris, A. Effects of nanoscale SiO2 on the thermal and transport properties of solvent-free, poly(ethylene oxide) (PEO)-based polymer electrolytes. Solid State Ionics 1999, 118, 73. (32) Sharma, J. P.; Sekhon, S. S. Nanodispersed polymer gel electrolytes: Conductivity modification with the addition of PMMA and fumed silica. Solid State Ionics 2007, 178, 439. (33) Saikia, D.; Chen-Yang, Y. W.; Chen, Y. T.; Li, Y. K.; Lin, S. I. 7Li NMR spectroscopy and ion conduction mechanism of composite gel polymer electrolyte: A comparative study with variation of salt and plasticizer with filler. Electrochim. Acta 2009, 54, 1218. (34) Dissanayake, M. A. K. L.; Rupasinghe, W. N. S.; Jayasundara, J. M. N. I.; Ekanayake, P.; Bandara, T. M. W. J.; Thalawala, S. N.; Seneviratne, V. A. Ionic conductivity enhancement in the solid polymer electrolyte PEO9LiTf by nanosilica filler from rice husk ash. J. Solid State Electrochem. 2013, 17, 1775. 18 ACS Paragon Plus Environment

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(35) Stevens, J. R.; Mellander, B. –E. Poly(ethylene oxide)-alkali metal-silver halide salt systems with high ionic conductivity at room temperature. Solid State Ionics 1986, 21, 203. (36) Pappenfus, T. M.; Henderson, W. A.; Owens, B. B.; Mann, K. R.; Smyrl, W. A. Ionic conductivity of poly(vinylpyridinium)/silver iodide solid polymer electrolyte system. Solid State Ionics 2004, 171, 41. (37) Dey, A.; Karan, S.; De, S. K. Effect of nanoadditives on ionic conductivity of solid polymer electrolyte. Indian J. Pure and Appl. Phys. 2013, 51, 281. (38) Croce, F.; Perse, L.; Scrosati, B.; Serraino-Fiory, F.; Plichta, E.; Hendrickson, M A. Role of the ceramic fillers in enhancing the transport properties of composite polymer electrolytes. Electrochim. Acta 2001, 46, 2457. (39) Chung, S. H.; Wang, Y.; Persi, L.; Croce, F.; Greenbaum, S. G.; Scrosati, B.; Plichta, E. Enhancement of ion transport in polymer electrolytes by addition of nanoscale inorganic oxides. J. Power Sources 2001, 97, 644. (40) Funke, K.; Roling, B.; Lange, M. Dynamics of mobile ions in crystals, glasses and melts. Solid State Ionics 1998, 105, 195. (41) Gerhardt, R. Impedance and dielectric spectroscopy revisited: Distinguishing localised relaxation from long-range conductivity. J. Phys. Chem. Solids 1994, 55, 1491. (42) Gondaliya, N. Kanchan, D. K.; Sharma, P.; Jayswal, M. S.; Pant, M. Conductivity and dielectric behavior of AgCF3SO3 doped PEO polymer films. Integr. Ferroelectr. 2010, 119, 1. (43) Nadkarni, G.; Simmons, J. G. Electrical properties of evaporated molybdenum oxide films. J. App. Phys. 1970, 41, 545. (44) Low, S. P.; Ahmad, A.; Rahman, M. Y. A. Effect of ethylene carbonate plasticizer and TiO2 nanoparticles on 49% poly(methyl methacrylate) grafted natural rubber-based polymer electrolyte. Ionics 2010, 16, 821. (45) Wang, M.; Qi, L.; Zhao, F.; Dong, S. A novel comb-like copolymer based polymer electrolyte for Li batteries. J. Power Sources 2005, 139, 223.



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Page 20 of 30

Page 21 of 30

20

1 SiO2

60 oC

0

- Z'' / Ohm

- Z'' / Ohm

80 oC o 90 C 100 oC

5

0

5

70 oC

6 4

100 oC

80 oC 90 oC

2 0

10 15 20 25 30 35 40 Z' / Ohm

0

5

10

15 20 Z' / Ohm

25

70

10

5 SiO2

30

35

7 SiO2

60

8

60 oC

- Z'' / Ohm

50 70 oC

6 4

80 oC 90 oC

2 0

60 oC

8

70 oC 10

3 SiO2

10

15

- Z'' / Ohm

100 oC 0

5

40 80 oC

30 20

100 oC

10 10 15 Z' / Ohm

15

20

0

25

90 oC 0

20

40 60 Z' / Ohm

80

100

70 oC

9

60 oC

80 oC

6

90 oC 100 oC

3 0

70 oC

10 SiO2

12 - Z'' / Ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

5

10

15 20 Z' Ohm

25

30

35

Figure 1 Cole-Cole (complex impedance) spectra of the nanocomposite solid polymer electrolyte ((1-x(70PEO:30AgI):xSiO2)) for different SiO2 concentrations at different temperatures indicated in the figure.



-2.6 5% -2.8 -3.0 log(σ ) / S cm

-1

3% -3.2

1%

-3.4

7%

-3.6 10 %

-3.8 -4.0 2.5

2.6

2.7

2.8

2.9

3.0

-1

(1000 / T) / K

Figure 2 Variation of ionic conductivity with inverse temperature of the 1-x (70PEO:30AgI):xSiO2 (x = wt. % denoted in the graph) samples with different compositions of SiO2 concentration.

1.2 1.0 0.8

Ea / eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

0.6 0.4 0.2 0.0 0

2

4

6

8

10

Wt. % (SiO2)

Figure 3 Variation of activation energy (Ea) of NCPE (OCC) as a function of SiO2 concentration. The composition with 95(70PEO:30AgI):5SiO2 is referred as NCPE (OCC). 22 ACS Paragon Plus Environment

Page 23 of 30

40

(a) 393 K

Z' x 103 / Ohm

35

343 K

30 353 K 25

373 K 20 4.50

4.75

5.00

5.25

5.50

log (ω) / Hz

(b) 60

343 K 393 K

50

Z''x 103 / Ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


373 K 353 K 40

30

20 4.5

4.6

4.7

4.8

4.9

5.0

5.1

5.2

5.3

log (ω) / Hz

Figure 4 Variation of (a) real and (b) imaginary parts of impedance spectra with frequency dependence of NCPE (OCC) at various temperatures quoted in the figure. The composition with 95(70PEO:30AgI):5SiO2 is referred as NCPE (OCC)



3

(a)

3 Z' x 10 / Ohm

353 K 2 373 K

1

393 K

0 3

4

5 log (ω) / Hz

6

7 353 K

5 Z' x 10 / Ohm

7

(b)

6

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

373 K

4 3

393 K

2 1 0 3

4

5

6

7

log (ω) / Hz

Figure 5 Variation of real (a) and imaginary (b) parts of impedance spectra with frequency dependence of SPE (OCC) at various temperatures quoted in the figure (to compare the behaviour with NCPE (OCC) in figure 4). The composition (70PEO:30AgI) is termed as SPE (OCC). 24 ACS Paragon Plus Environment

Page 25 of 30

40 343 K 353 K 373 K 393 K

35

Z' / Z''max

30 25 20 15 10 5 0 -4

-3

-2

-1

0

log ω /ωmax

Figure 6 Frequency dependence of Z"/Z"max for NCPE (OCC)

8 343 K 353 K 373 K 393 K

7 6 5 tan δ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 3 2 1 0 2

3

4 5 log(ω) / Hz

6

7

Figure 7 Variation of loss tangent as a function of temperature for NCPE (OCC)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

(a)

(b)

(c)

(d)

(e)

(f)

Figure 8 Scanning electron micrographs of the nanocomposite solid polymer electrolyte ((1-x(70PEO:30AgI):xSiO2)) for different SiO2 concentrations. Micrographs represent (a) 0, (b) 1, (c) 2, (d) 5, and (e) 7 SiO2 concentrations. The micrograph for SiO2 precursor used in this study is given in (f) for a comparison. The (a) 0 % SiO2 termed as SPE (OCC) indicating crystalline features and (d) 5 % SiO2 termed as NCPE (OCC) implying less defined particles.


Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9 Energy dispersive analysis (EDS) spectra of NCPE (OCC) obtained from the micrograph of Fig. 8 (d).

Table 1 Elemental composition of NCPE (OCC) electrolyte Element

Weight %

Atomic %

Si (K)

5.65

9.83

Ag (L)

34.96

15.85

I (L)

40.13

15.46

O (K)

19.26

58.86

Total

100.00


o

(311)

o

(220)

•

Page 28 of 30

• PEO o AgI 7% SiO2

•

5% SiO2

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(120)


2% SiO2 1% SiO2 0% SiO2

20

30

40 50 2θ / degrees

60

70

Figure 10 X-ray diffraction pattern of the nanocomposite solid polymer electrolyte ((1x(70PEO:30AgI):xSiO2)) for different SiO2 concentrations as labelled in the figure. The 5 % SiO2 termed as NCPE (OCC) and 0 % SiO2 termed as SPE (OCC).


Voltage / V Figure 11 A cyclic voltammetric profile for a solid state capacitor built with NCPE (OCC) electrolyte using a solid polymer electrode. The electrolyte is sandwiched between two identical polymer electrodes 70(70PEO:30AgI):30 activated carbon.

1.2 1.0 Cell Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current / A

Page 29 of 30

0.8

0.4 A.g-1

0.6

0.6 A.g-1

0.4

1.0 A.g-1

0.2

1.25 A.g-1

0.0 0

20

40

60

80

100

120

Time / S Figure 12 Charge-discharge profiles of a solid state capacitor built with NCPE (OCC) electrolyte using a solid polymer electrode. The electrolyte is sandwiched between two identical polymer electrodes 70(70PEO:30AgI):30 activated carbon. 29 ACS Paragon Plus Environment


50

100

40

80

30

60

20

40

10

20

0

Coulombic efficiency (%)

-1 Cell capacitance (F g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

0 0

40

80 120 160 Cycle number /

200

240

Figure 13 Cycling stability of a solid state capacitor built with NCPE (OCC) electrolyte using a solid polymer electrode at 1 A/g. The electrolyte is sandwiched between two identical polymer electrodes 70(70PEO:30AgI):30 activated carbon. The cell shows a stable capacitance for over 250 cycles.


Structural and Electrochemical Properties of Nanocomposite Polymer

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