Investigation of Mg2+ ion on Structural, Morphological, FTIR, Dielectric and AC Conductivity of PVDF-HFP Based Solid Polymer Electrolytes and Application to Electrochemical Cell

In this paper, solid polymer electrolytes comprising of Poly (vinylidene-fluoride-hexafluoropropylene) (PVDF-HFP) polymer and Mg (ClO 4 ) 2 salt were prepared by employing the solution casting technique. The fabricated polymer-salt electrolyte membranes are exposed to XRD, FTIR and SEM studies. The real and imaginary part of dielectric permittivity is illustrated with the Cole-Cole plot. Static dielectric constant (𝜀 𝑠 ) , dynamic dielectric constant (𝜀 ∞ ) , dielectric strength (Δ𝜀 ), dielectric loss (t anδ) and relaxation time (τ) are determined using the Cole-Cole plot. The electrochemical properties; cell stability, cell discharge characteristics, conductivity are analyzed. Structural studies of XRD peaks are broadened to confirm the amorphous phase of polymer matrix. Morphological studies shows the presence of interlinked micro-pores promote for ease of mobility of Mg 2+ ions which attribute to enhance ionic conductivity. The static dielectric constant (𝜀 𝑠 ) , dynamic dielectric constant (𝜀 ∞ ) , dielectric strength (Δ𝜀 ), dielectric loss (t anδ) reach m aximum but relaxation time (τ) decreases for an optimal concentration ratio of (100:40) PVDF-HFP: Mg (ClO 4 ) 2 that reveals fast hopping of ions from one site of the polymer chain to another. The highest ionic conductivity of 7.73333x 10 -4 Scm -1 is obtained at room temperature for [PVDF-HFP: Mg(ClO 4 ) 2 ] polymer-salt electrolyte. The cell discharge characteristics of OCV and SCC of Mg/ [PVDF-HFP: Mg(ClO 4 ) 2 ] /I+C cell are found to be 1.8 V and 120 mA respectively The electrochemical stability was observed with a constant voltage of 0.43volt in a positive cycle and 0.4 volts of negative potential which favors an electrochemical membrane for battery applications Interface potentiostat. And the Electrical impedance spectroscopic studies (EIS) are characterized with E4980A Precision LCR Meter.


Introduction
Solid-state electrochemical cells have high demand at present in modern technology. It has been developed by modifying the polymer electrolytes to enhance conductivity and stability at ambient temperature for potential applications and in energy storage devices. In the process of developing good ionic conductive polymer electrolytes, plasticizers are used which are made of complex polymer electrolyte like PEO/PC/EC/lithium, PAN-PC/EC-LiClO4, PAN-PC/EC-LiCF3SO3, PAN-PC/EC-LiAsF6, and PMMA-PC/EC-LiAsF6 [1 -4]. These complex composite solid polymer electrolytes have potential advantages over conventional solid polymer electrolytes as they exhibit better mechanical strength, higher ionic conductivity, and better temperature stability. Major research work is carried out with various lithium salts LiX (X = I, Cl, Br, 4 , 3 3 , 4 , 6, 6 etc) of low lattice energies dissolved in high molecular weight solid polymer electrolytes. Despite Liion salt complex composite structured polymer electrolytes, the magnesium-based salts are used, and prepared to attain good mechanical strength, temperature stability, and high ionic conductivity. Mg 2+ ion-based polymer electrolytes are of low cost, easy to handle, and have a divalent cationic conductivity mechanism. Therefore Mg(ClO4)2 salt is incorporated in PVDF-HFP polymer to fabricate Mg-based solid polymer electrolyte membranes [5].
Besides, magnesium-based batteries are safe and reliable for electric vehicles and domestic applications. The battery performance of magnesium ion electrolytes is very close to lithium ion but avoids explosive hazards that occur in lithium ion batteries [5,6]. The Li-ion is monovalent but the Mg-ion is divalent in nature. Even Mg is a relatively earth-abundant material, cheaper, lightweight and environmentally friendly [7]. Magnesium perchlorate Mg(ClO4)2 is a fast ion conducting salt and its incorporation in a polymeric system is expected to get fair complexation with the polymer due to its large anions. Mg 2+ ion decoupling from the segmental motion of polymer chain is occurring due to large ionic aggregates [8 -10]. The improvised maximum ionic conductivity is found to be of magnitude 10 -5 S cm -1 at room temperature. The increased conductivity depends on ion migration through interlinked network of pores which is an important factor for ionic mobility. It has been observed that the critical moisture effect of Mg(ClO4)2 is very less when compared with various lithium salts. Magnesium batteries may turn up as an alternate for the next rank batteries due to the intrinsic advantage of the Mg metal.
Owing to the divalent property of Mg 2+ , this battery can provide a higher theoretical volumetric capacity (3832 mAh•cm -3 ) than Li (2062 mAh•cm -3 ). So, Mg batteries are spirited for energy storage devices [10,11]. The Mg 2+ ionic radius is near to Li ionic radius [12]. Hence, Mg(ClO4)2 is incorporated in PVDF-HFP polymer which has enough amorphous nature with a large number of interlinked micro-pores and, results are reported in this paper. Based on this polymer electrolyte an electrochemical cell has been fabricated and their battery discharge characteristic is studied.

Experimental
Polymer PVDF-HFP from Sigma Aldrich with average molecular weight 4x10 5

XRD analysis
XRD spectral data of (PVDF-HFP: Mg (ClO4)2) electrolyte membranes are used to study the crystalline/ amorphous nature of the polymer-salt electrolyte. Fig. 2 shows the XRD patterns with Bragg's angle 2θ versus relative intensity at room  It is obvious that Mg(ClO4)2 interaction with host polymer PVDF-HFP matrix leads to a decrease of intermolecular interaction among the polymer chain which in turn reduces the crystalline phase and hence improves the amorphous region [13]. Thus XRD graph suggests an increase in the amorphous phase at an optimum concentration of polymer-salt electrolyte ratio. Further increase in the concentration of Mg(ClO4)2, the intensity of XRD peak is slightly increased representing again acquiring the semi-crystalline nature of the PVDF-HFP polymer as clearly observed in Fig. 2(f). Similar results are obtained for PVDF-HFP with different salts due to interaction as discussed by the researchers [13,14].  [15 -17]. Both α and β phases are two types of crystalline phases of PVDF units [13, 18 -20] while the γ phase represents an amorphous phase of HFP units of PVDF-HFP [18,21]. The low transmittance peaks at 844 cm -1 and 1286 cm -1 , assigned to the long trans-sequence of ferroelectric β-phase of PVDF-HFP are absolved due to the effect of Mg 2+ ion interaction with PVDF-HFP polymer [22 -24]. Moreover the intensity peak of PVDF-HFP at 1286 cm -1 decreases and shifts to 1352 cm -1 due to the effect of Mg 2+ ion on the bond length of host polymer chains. The absorption peak of PVDF-HFP at 804 cm -1 predicts -C-F-wagging and the disappearance of this peak provides evidence of interaction of Mg 2+ with the polymer host. The peak assigned at 798 cm -1 corresponds to skeleton deformation which is found to be reduced due to the complexation of polymer salt indicating a phase transition from crystalline to amorphous phase. The vibration peak of PVDF-HFP polymer assigned at 871cm -1 corresponds to CH2 rocking has disappeared and a new vibrational band appeared at 1041 cm -1 represents the amorphous phase [25].

FTIR analysis
The bending of C-C-band is observed at 1070 cm -1 and the peak at 1400 cm -1 corresponds to the vibrational frequency of CH2 which deforms and the intensity of  smooth texture on the surface with a large number of micro-pores which confirms a phase transition from semi-crystalline to amorphous nature [31]. The pores result in microstructure due to evaporation of solvent [35].

DC Conductivity
The DC ionic conductivity of the polymer electrolyte can be calculated using the equation  This shows well compatibility and proper blending of polymer-salt electrolytes. It is clear that at an optimum concentration Mg 2+ and ClO4ions dissociate freely and generate free charge carriers. Hence, the crystalline phase of the polymer is suppressed, and increased in the amorphous phase is responsible to enhance conductivity which is established in the XRD and SEM results.

Structural Model
The    is dropped to 1.5 V when it is connected to a load of 1K. It is stable for 70 hrs. and a sharp decrease is noticed in the voltage, it may be due to the polarization and formation of a thin layer of salt at the electrode-electrolyte interface [38,39]. Further work is in progress to improve the cell parameters and capacity.   (3)

Electrochemical stability of polymer electrolyte (Cyclic
∞ is also known as high-frequency dielectric constant. The rise in the value of the high-frequency dielectric constant represents the response of ions for the highfrequency electric field as shown in Fig. 11 b). And the right-side intersecting point of a semicircle indicates the static dielectric constant or low-frequency dielectric constant; which is directly linked with the number of charge carriers at lowfrequency. As the concentration of Mg (ClO4)2 salt is increased in the PVDF-HFP polymer, there is complete disassociation of salt which is evidence of accumulation of charge at the electrodes. This can be understood by the highest value of lowfrequency dielectric constant as noticed in Fig. 11 c). In Fig. 11 Fig. 11 c). It is clear from Fig. 11 b) and Fig. 11 c) that the dynamic and static dielectric constant attain the highest for the optimal concentration the weight ratio of (10:04) (PVDF -HFP: Mg (ClO4)2) polymer-salt electrolyte. It is noticed that the diameter of the circle becomes large i.e. the dielectric strength (Δ = ∞ − ) also increases as it is proportionate to the diameter of a circle (as illustrated in Fig. 11 a)   It is obvious from Fig. 12 a) that the tangent loss (or a measure of dielectric loss) increases with an increase in the frequency of electric field and attains a maximum peak at an appropriate optimal concentration ratio of (100:400) (PVDF-HFP: Mg(ClO4)2) polymer electrolyte and it decreases for the higher-frequency. To understand the better effect of the ac electric field on the polymer electrolyte, Fig 12   a) can be divided into three regions. The initial or first region is low-frequency, the second region is the moderate frequency and the third region called the high frequency. The increase in the domination of the ohmic element is greater than the loss of the capacitive element which can be represented as the low-frequency region.
It is observed that there is an increase in the loss of electric energy because more relaxation time ( ) is required for the alignment of ions in the direction of the ac electric field. It is observed that a maximum peak is obtained when the frequency of the electric field perfectly matches with the rotational frequency of the polymer molecule and this maximum peak is represented as a moderate-frequency region and energy loss is also high in this region. The maximum energy loss or tangent occurs at a certain frequency known as the resonance frequency. This may lead to maximum power transfer to the dipoles in the polymer matrix system and hence maximum power loss is releases in the form of heat. The corresponding frequency of high dielectric loss satisfies the equation = 1 [40,43]. The resonance frequency occurs if the frequency of Ohmic and Capacitance components matches with the frequency of the ac electric field that can be noted from Fig. 12 a) and hence the relaxation time is calculated from the maximum peak which is also called as average relaxation time of the polymer electrolyte. But at higher frequencies the capacitance component dominates, hence the domination of the Ohmic element becomes independent of frequency. It is noticed that the relaxation time decreases with the ion orientation in the polymer electrolyte. Moreover, even though the dielectric loss at a higher frequency is less ( ), the response of the ions is good to orient and align along the field direction. Hence the polymer electrolyte attains high ionic conductivity and possesses a good dielectric constant. It is perceived that as the Mg(ClO4)2 salt concentration increases, the relaxation peak shifts from lowfrequency to high-frequency owing that the relaxation time decreases which reveals the rapid hopping of Mg ions from one polymer site to another site of the polymer matrix [43,44]. And a very less relaxation time is obtained for an optimal concentration ratio of (100:40) PVDF-HFP: Mg (ClO4)2 polymer electrolyte that reveals fast hopping of ions in the polymer matrix from one site of the polymer chain to another as shown in Fig. 12 b). Further, a high tangent loss occurs due to resonance between the applied electric field and more ions in the polymer electrolyte. Also, a substantial segmental motion of ions attains good ionic conductivity at the higher frequency of the polymer electrolyte.

The real part of complex AC conductivity
The ac electrical conductivity of the polymer electrolytes is calculated with the equation given below where is the angular frequency, is the permittivity of free space, 0 = 8.854 10 −12 / and tan = dielectric loss The complex ac conductivity of the polymer electrolyte can be expressed as Where ′ and ′′ are the real and imaginary part of the dielectric permittivity respectively, is the angular frequency, 0 is the capacitance of vacuum, ′ and ′′ are real and imaginary parts of the electrical impedance [43], which can be measured by an LCR meter. Further, complex conductivity is the combination of real ′ and imaginary ′′ part of conductivity. The variation of the real part of AC conductivity with the change of frequency is shown in Fig. 13. Three distinct regions are observed in Fig. 13 that depend upon the frequency of an AC electric field. The low-frequency region is the frequency region-I, it is noticed that there is a sharp rise in conductivity occurred due to electrode polarization. This is illustrated as the EP window in Fig. 13. The next followed region is the intermediate region which is also known as the frequencyindependent region; here the conductivity does not depend on the frequency, which reveals the long-range conduction of the charge carriers, and dc ionic transport (hopping) associated with the AC conductivity can be easily understood. In region II there is a gradual increase of ion conductivity for an optimum concentration ratio of power law [45,46].
where ( ) denotes the total conductivity , ′ ′ is the dc conductivity of the sample, is the dispersive component of the ionic conductivity of AC current.
′ ′ is the angular frequency, ′ ′ is the degree of freedom between the mobile ions and ′ ′ is a constant which indicates the strength of polarizability. It is observed in same initial site after forward-backward motion with respect to frequency is called unsuccessful hoping mechanism which is not noticed in the polymer sample [45].
Hence a successful hoping ion dispersion is seen in Fig. 13 for the composition ratio

Figure 13
Variation of real part of conductivity against frequency for different concentration of Mg(ClO4)2 salt in PVDF-HFP solid polymer electrolyte