Conductivity and structural studies of PVA based mixed-ion composite polymer electrolytes

The solid membranes having different ratios of poly-vinyl alcohol (PVA), sodium perchlorate (NaClO4) and lithium perchlorate (LiClO4) were prepared using solution casting technique. The mixed-ion composite polymer electrolytes were characterized by X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR) and conductivity measurement investigations. The XRD study confirms the amorphous nature of the mixed-ion composite polymer electrolytes. FTIR analysis has been used to characterize the structure of polymer which confirms the polymer and salt complex formation. The temperature dependent nature of ionic conductivity of the mixed-ion composite polymer electrolytes was determined by using conductivity meter (EC-035WP ERMA Inc, made in Japan). The ionic conductivity of the electrolyte was found in the range of 10 10 S/cm at room temperature.


Introduction
The application of solid polymer electrolytes in electrochemical storage devices, super capacitors, fuel cells and sensors are increasing day by day since last few decades [1][2][3][4]. Extensive studies and research have been committed towards improvement in conductivity of solid polymer electrolyte materials at ambient temperature for their potential application in electrochemical devices. Several modifications such as incorporation of various salts [5][6][7][8][9][10][11], plasticizers [12][13][14] and fillers [15][16] in solid polymer electrolytes have been explored to improve their performance in electrochemical device applications. The exploration of new materials has been done during the past few decades that can replace the conventional materials with improved characteristics. The mixed ion composite polymers are fulfilling the requirements of such materials in modern era. The mixed-ion composite polymers are produced by combination of two or more components and are heterogeneous on microscopic scale [9][10]. The present work aims at the introduction of an economical mixed-ion composite polymer material which can be used in electrochemical devices with good performance comparable with present electrolytes of mixed-ion nature. This paper gives an account of crystallization character, structural analysis and ionic conduction of the new mixed-ion composites of PVA with inorganic salts.

Experimental
Poly (vinyl alcohol) PVA (molecular weight = 1,25,000) was purchased from CDH, sodium perchlorate (NaClO4) and lithium perchlorate (LiClO 4 ) were purchased from Sigma Aldrich. Weighing of polymers and salts was done on Toledo Metller electronic balance. The stock solutions of sodium perchlorate (NaClO4) and lithium perchlorate (LiClO 4 ) were prepared and later used to prepare different mixed ion composite polymer electrolytes. The salts were added to the polymer in various concentrations (5%, 10%, 15%, 20%, 25% and 30% w/v) to make composites of different compositions. The solutions of electrolytes were prepared by mixing of PVA (20%) with NaClO4 and LiClO 4 at different ratios in distilled water to get PVA+NaClO4, PVA+LiClO4 and PVA+NaClO4+LiClO4 complex electrolytes. The solutions were then stirred continuously using stirrer for 8 hours on a hot plate at 60º C to obtain a homogeneous mixture. Finally, prepared solutions of mixed ion composites were placed in 80 × 15 mm diameter Borosil petri dishes for further annealing at room temperature. The petri dishes were covered with aluminium foil to avoid any contamination from environment. Five different samples of polymer electrolytes were prepared and subjected to characterization.

Characterization techniques
Temperature dependent conductivity of the samples was measured by pen type conductivity meter (EC-035WP ERMA Inc, made in Japan) at varying temperatures. Mixed-ion composite polymer electrolyte samples of 1 mm thickness and 10 mm × 10 mm dimension were placed between the electrodes of conductivity meter. The thicknesses of samples were measured by screw gauge.
Fourier Transform Infrared Spectrum is a fingerprint which provides us ample information about the structure of compound. In order to elucidate the temperament and complexation between polymer and salt, FTIR spectra have been recorded on Bruker Tensor-37 spectrophotometer in the wavenumber range between 4000-500 cm -1 . The XRD pattern analysis of the synthesized samples was carried out on RIGAKU JAPAN ULTIMA-IV X-ray diffractometer to observe the nature of the developed films. The sample of 10 mm size was mounted on sample holder and placed in diffractometer. The samples were scanned in 2θ range of 10º-80º with X-rays using Cu target source for crystalline phase identification of these mixed ion polymeric composites.

FTIR spectroscopy
In order to determine the possible nature of bonding between various functional groups in the samples, FTIR spectroscopy was used and the results are presented in figure-1. The complex formation in the polymer matrices was evidenced through FTIR studies. FTIR spectra of composite prepared by mixing PVA with sodium perchlorate, lithium perchlorate and both lithium and sodium perchlorate, is shown in Fig. 1(b to d) and compared with that of pure PVA (Fig. 1a). Predominant absorption bands were observed at around 3365-3297 cm -1 in IR spectrum of pure PVA which were displaced to higher wavenumbers indicating specific interactions in the polymer matrices [17]. These bands can be attributed to stretching vibrations of OHgroups originated from residual water and those from the organic counterpart (PVA) [18]. Additionally, in the IR spectrum of pure PVA, absorption signal at about 2951 cm -1 was also observed which was resulting from C-H asymmetric stretching vibrations of CH and CH2 groups. This band was shifted to 2954-2957 cm -1 in PVA based complexes. The peaks appearing at 1257 cm -1 and 1128 cm -1 were assigned as originating from C-O bond deformation [19] and C-C stretching vibrations respectively. However, the corresponding peaks are not clearly visible in the spectra of PVA based complexes due the appearance of a strong band at 1089-1070 cm -1 . These intense peaks were due to the ClO4asymmetric stretching vibrations [20]. The peaks appearing in the range of 802-808 cm -1 in IR spectra of PVA and its composite were assigned as arising from C-H rocking mode [17][18]. The peaks at 616-621 cm −1 in PVA based complexes are assigned to ClO4asymmetric bending type of vibrations [20].

X-ray analysis
To scrutinize the nature of crystallinity of pure PVA film and PVA/salt composite polymer electrolyte films, X-ray diffraction patterns were obtained from X-ray diffractometer. X-ray diffraction patterns o f p o l y m e r / s a l t composite systems of PVA, PVA/NaClO4, PVA/LiClO 4 and PVA/NaClO4+LiClO 4 in different proportions are shown in Fig.-2. It is observed that the maximum intensity of diffraction spectrum for pure PVA film is at 2θ = 20º which shows a typical semi-crystalline structure of PVA [21]. The nature of the peak and its intensity was also altered due to the addition of salts in various proportions [5][6], [22][23][24][25]. The reduction in peak intensity shows the amorphous nature of

Temperature dependence of conductivity
The conductivity values of various samples are shown in Table-1. The conductivity followed the typical trend of polymer salt complexes [27]. An increase in conductivity of mixed-ion composite liquid polymer electrolyte was noticed on increasing the temperature which may be attributed to the complete dissociation of salt in liquid resulting in higher mobile ion concentration and flexibility of polymer electrolyte [28][29]. The variations of conductivity (ln σ) with reciprocal of temperature (K -1 ) for various mixed-ion polymer composites are shown in Fig.-3. The temperature dependent conductivity of mixed-ion composite polymer follows a linear pattern which indicates Arrhenius behavior. Hence the activation energy was calculated from the following relation: where σo is the pre-exponential factor, EA is the activation energy, k is Boltzmann constant, and T is absolute temperature. The calculated values of activation energy for mixed-ion composites range from 0.09 to 0.10 eV. These low values of activation energy suggest an amorphous nature of the mixed-ion composite polymer electrolyte.

Conclusions
In the present report, we have synthesized a mixed-ion composite polymer electrolyte by addition of sodium perchlorate (NaClO4) and lithium perchlorate (LiClO 4 ) to PVA solution in water. The observed conductivity of PVA/sodium perchlorate and PVA/lithium perchlorate composites were 1.14×10 -3 S/cm and 1.22×10 -3 S/cm respectively at 298K. The conductivity of mixedion composite polymer electrolyte having high concentrations of PVA was found to be 1.57×10 -3 S/cm at 298K. On increasing the temperature, a significant decrease in the conductivity values was evident for PVA/sodium perchlorate and PVA/lithium perchlorate composites reaching upto 8.20×10 -4 S/cm & 8.91×10 -4 S/cm respectively at 335K. On the other hand, negligible reduction in the conductivity value could be observed for mixed-ion composite polymer electrolyte PVA+NaClO4+LiClO4. The temperature dependence conductivity obeyed Arrhenius behavior. The complexation and bonding between host polymer and salts to form PVA−NaClO4−LiClO4 complex ions was evident from FTIR spectroscopy. X-ray diffraction analysis of mixed ion composites revealed the transition from crystalline to amorphous structure by addition of the salts. These studies suggest that new mixed-ion composite polymer electrolyte can be used in electrochemical devices.