Synergetic effect of TiO2 nano filler additives on conductivity and dielectric properties of PEO/PVP nanocomposite electrolytes for electrochemical cell applications

Sodium-ion conducting PEO/PVP blend based solid polymer electrolyte films complexed with NaIO4 salt and nano-sized TiO2 fillers are fabricated by employing a solution casting technique for Na-ion battery applications. Measurements of X-ray diffraction (XRD) and thermogravimetric analysis (TGA) are carried out to investigate the crystallinity and thermal stability of the solid polymer electrolytes. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) studies are performed to understand the modifications in surface morphological features and to evaluate the size and distribution of dispersed nano-sized TiO2 fillers. The room temperature ionic conductivities of polymer electrolyte films are investigated by impedance analysis in the frequency range 1 MHz - 1 Hz. The nano-sized TiO2 (3 wt%) filled composite electrolyte of ‘PEO/PVP/NaIO4 (10 wt%)’ demonstrates a maximum room temperature conductivity of 9.82 X 10-6 S/cm. The influence of TiO2 filler on conductivity and dielectric properties are presented in this report.


Introduction
Although lithium ion batteries (LIBs) have been recognized as a promising energy storage devices [1][2][3] their high cost, less abundance, environmental impact and safety limitations impede the widespread implementation of lithium insertion materials in future battery technologies [4][5][6][7][8]. Therefore, there is urgent necessity to search for alternative energy storage system technology capable of complementing the current Li-ion battery technology. Amongst the accessible battery chemistries, sodium (Na) based rechargeable batteries (SIBs) have recently captured much attention because they are environmentally friendly, non-toxic, low cost and abundant materials [9,10]. Substantial efforts of research, development, and demonstration are currently in progress to replicate the performance of wellestablished Li-ion batteries using sodium ion batteries [11][12][13]. Solid polymer electrolytes (SPEs) have attracted intensive studies during the past decades due to their potential applications in various advanced devices including metal ion batteries, supercapacitors, and stretchable actuators. Compared to the traditional organic liquid electrolytes, SPEs offer enhanced safety, stability and thin film manufacturability but their low ionic conductivity especially at room temperature has suppressed their development [14][15][16]. Polyethylene oxide (PEO) based polymer electrolytes have been employed extensively for battery applications since Armand et al. have demonstrated the feasibility of using PEO as a potential ion conductive electrolyte [17]. PEO is one of the widely investigated host polymers used for synthesizing SPEs to employ in alkali metal ion-conducting batteries due to its high electrochemical stability, good solvation, complexation and ion dissociation abilities [18][19][20][21][22][23][24][25]. Several researchers have reported about various sodium salt complexes of PEO based polymer electrolytes for sodium ion battery applications [26,27]. However based on previous reports, the semicrystalline nature of PEO at room temperature, subsequently limits its ionic conductivity [28]. The most straight forward approach to overcome this problem is modifying the PEO matrix in order to decrease its degree of crystallinity. One of the most promising alternate choices of enhancing the amorphous phase in PEO based electrolyte systems is blending of PEO with suitable higher amorphous polymer [29][30][31][32]. Polyvinyl pyrrolidone (PVP) has been identified as a compatible partner to PEO which exhibits higher order of amorphosity. PVP has high glass transition temperature, mechanical and thermal stabilities, provided its carbonyl group (C = O) enables to form different complexes of alkali metal ion salts at wide range of concentrations. In particular, reinforcement of nano-sized materials with polymer electrolyte systems can modify their microstructural properties and enhance their conductivity properties [33]. Several researchers have reported about micro-or nano-sized ceramic fillers doped sodium-ion polymer electrolytes, such as ZrO 2 , BaTiO 3 ,Sb 2 O 3 ,Al 2 O 3 or silica nanoparticles [34,35]. In the present report, NaIO 4 salt complexed and nano-sized TiO 2 fillers doped PEO/PVP blend based electrolyte systems are prepared by the solution cast technique and investigated the influence of nano-sized TiO 2 fillers on conductivity and dielectric properties of salt complexed PEO/PVP blends solid polymer electrolytes.

Experimental
PEO and PVP of molecular weights of 5 X 10 6 and 3.6 X 10 5 respectively, were procured from Aldrich and employed without any further purification to prepare PEO/PVP solid state blend electrolytes. Sodium periodate salt (NaIO 4 , Sigma Aldrich) and Titanium dioxide nano powder (TiO 2 , Sigma Aldrich) were used as additives and methanol (Aldrich) was used as a solvent. Appropriate amounts of PEO and PVP polymers were dissolved in methanol solution and followed by mechanical stirring at room temperature for 15 hours to obtain uniform mixture of blend. In the meanwhile, 10 wt% of NaIO 4 salt was dissolved separately in methanol solution and added to the prepared viscous PEO/PVP polymer solutions to obtain 'PEO/PVP/NaIO 4 ' polymer complex. Nano-sized TiO 2 powder (3 wt%) was dispersed in methanol solution separately, which was sonicated for 30 minutes, and added to 'PEO/PVP/NaIO 4 (10 wt%)' electrolyte solution to prepare 'PEO/PVP/NaIO 4 (10 wt%)/TiO 2 (3 wt%)' nano composite electrolyte. The as-prepared viscous solutions were poured into polypropylene dishes and the solvent (methanol) was allowed to evaporate slowly at room temperature to harvest free-standing polymer electrolyte films. All electrolyte films were, vacuum dried at 45 o C to remove traces of methanol solvent and kept in desiccators filled with silica gel desiccants for several hours before being characterized to avoid any traces of moisture.
The structural properties of the electrolytes were analysed by recording X-ray diffraction patterns (XRD) in the 2θ range of Thermal stabilities of solid polymer electrolytes were studied by TGA carried out under nitrogen gas atmosphere by using TGA Q50 TA operating in the temperature range 303 K to 1073 K and at a heating rate of 10 0 C/min. Solid polymer electrolyte films were sandwiched between two copper electrodes and ionic conductivity studies were carried out at room temperature by a.c. impedance measurements using Biologic potentiostat/galvanostat (SP -200) in the frequency range of 1 MHz -1 Hz.  It evidences for further increase of amorphous portion in the matrix of salt complexed nano composites, which favours for the enhancement of ionic conductivity [38][39][40]. From the TEM image (figure 2(c)) the average size of nano fillers is found to be 10 nm and dispersed nano particles are observed to be uniformly distributed in the matrix of salt complexed blend electrolytes. The nanocomposite polymer electrolytes should have good thermal stability at high temperatures to meet the criteria for employing them as solid electrolytes in sodium ion batteries. Figure 2d represents TGA curves for pure PEO/PVP blend (black line), NaIO 4 (10 wt%) salt complexed PEO/PVP blend (red line) and nano composite 'PEO/PVP/NaIO 4 (10 wt%)' (blue line) electrolytes. The first weight loss region at lower temperatures (< 100 0 C) originates from the loss of water absorbed during the loading of samples and a major weight loss region at higher temperatures (> at above 300 0 C) is associated with the decomposition of pure PEO and nano composite matrices [41]. Evidently, nano-sized TiO 2 filler (3 wt%) doped PEO/PVP/NaIO 4 (10 wt%) polymer electrolyte is demonstrated appreciable thermal stability.   The ionic conductivity of the samples is calculated by the equation, σ=t/(R b *A), where t and A represent thickness and area of the electroldes, respectively. The bulk resistance is obtained from the intercept of the semicircle at the high frequency side (1 MHz to 1 Hz) of the plot, with the real axis (as shown in figure 3).The estimated room temperature conductivity of pure PEO/PVP blend electrolyte is 2.24 X 10 -9 S/cm and associated ionic conductivity for 'PEO/PVP/NaIO 4 (10 wt%)' electrolyte increased to 1.57 X 10 -7 S/cm. As a result of inclusion of nano-sized TiO 2 (3 wt%) of filler, the intercept of higher end of semicircle corresponding to 'PEO/PVP/NaIO 4 (10 wt%) electrolyte' on the | -axis tends to move towards lower value as shown in inset of figure 3. The corresponding room temperature conductivity of nan-composite electrolyte is increased to 9.82 X 10 -6 S/cm. The enhancement of conductivity as a result of inclusion of TiO 2 nano fillers could be due to reduction in crystallinity of the polymer chains in blend polymer electrolyte and improve the capacity of sodium ion transport in the electrolyte film and hence improve the ionic conductivity of the polymer electrolyte film [42,43]. Dielectric materials are recognized as a media that have an ability to store electrical energy. This property of dielectric materials can be estimate by measuring is the permittivity or di-electric constant of the material. In alternating electric fields the relative permittivity exhibits complex behavior and is defined by * = ′ − ′′ . The real (Z r ) and imaginary (Z i ) parts of complex impedance ( * ) are also used for the evaluation of real and imaginary parts of dielectric permittivity using the following equations [44]

Conductivity and Dielectric properties
Here C o is the vacuum capacitance and given by ε o A/d, where ε o is a permittivity of free space and is equal to 8.85 × 10 −12 Fm −1 . The angular frequency is given as ω = 2πf , where f is the frequency of applied field. The real part of complex dielectric permittivity ′ has the same significance as that of the ordinary dielectric constant of the material. It measures the energy stored in the material during each cycle, to be returned to the electric field at the end of the cycle. Figure 4 describes the variation dielectric constant values of nano-composite 'PEO/PVP/NaIO 4 (10 wt%)/TiO 2 (3 wt%)' (blue line) electrolyte. Significantly, nano-composite electrolyte films demonstrate relatively higher dielectric constant values in comparison to pure and salt complexed PEO/PVP electrolytes films ( as shown in inset of figure 4). This may be due to capability of TiO 2 nano particles leads to dissociation of undissociated salt/ion aggregates into free ions (anions) in the matrix of PEO/PVP/NaIO 4 (10 wt%) electrolyte and supports for the enhancement of dielectric constant values [45].

Conclusions
In the present report, NaIO 4 salt complexed and nano-sized TiO 2 fillers doped free standing PEO/PVP blend electrolyte films were prepared by using conventional solution cast technique. XRD studies revealed the increase of amorphous portion in the matrix of TiO 2 nano composite blend electrolytes. Increase of surface smoothness of blend electrolytes upon doping TiO 2 nano fillers confirmed the enhancement of degree of amorphicity in the blend electrolytes. Relatively, TiO 2 nano composite blend electrolytes showed good thermal stability. The nano-composite 'PEO/PVP/NaIO 4 (10 wt%)/TiO 2 (3 wt%)' electrolyte demonstrated higher room temperature ionic conductivity of 9.82 X 10 -6 S/cm. The increase in the dielectric constants as a result of addition of 10 wt% of salt and 3 wt% TiO 2 nano fillers evidenced for increase in number of mobile ions and their mobility in the matrix of PEO/PVP blend electrolytes.