Study on plasticized Poly (vinylidene chloride- co- acrylonitrile) polymer electrolytes for battery applications

Plasticized Poly (vinylidene chloride- co- acrylonitrile) [P(VdC-co-AN)] polymer electrolytes comprising of Lithium Perchloride (LiClO4) as complexing salt and plasticizers such as Propylene carbonate (PC) and β-butyrolactone (βbl) is prepared by solution casting technique. Polymer electrolytes were prepared in the ratio [(x)PVdC-co-PAN+(100-x-z) Plasticizer+(z) LiClO4] and were subjected to various characterizations. X-ray Diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) were carried out to study the structural and functional groups present in the material. Impedance spectroscopy to find the ionic conductivity of the material. The maximum ionic conductivity at room temperature was exhibited by the samples containing 68% PC (9.237 × 10−4 S cm−1) and 66% of βbl (2.284 × 10−4 S cm−1). Samples exhibiting higher ionic conductivity (68% PC and 66% of βbl) are subjected to Linear sweep voltammetry and transference number measurements. The electrochemical stability is 4.5 v for the both films, whereas transference number is 0.955 and 0.94, respectively. Thermogravimetry/Differential Thermal analysis (TG/DTA) shows the prepared films doesn’t not undergo any weight loss till 220 °C (thermally stable). The surface morphology of the polymer membrane was explored through Atomic force microscopy (AFM).


Introduction
Polymer plays a vital role in everyday life due to its less weight, non-combustible nature, and conformability to our required size and shape [1][2][3]. A broad theory of creating solid electrical conductors similar to polymer electrolytes is dissolving lithium salts in host polymers. Polymer electrolytes are classified based on the incorporation of dopants used in these lithium-ion batteries: solid polymer electrolyte, gel polymer electrolyte, and composite polymer electrolyte. Polymer electrolytes (PE) are used in lithium-ion batteries due to their excellent chemical and thermal stability also it has good mechanical strength so it is used to fabricate a flexible solid-state generator, sensors, and fuel cells with a high energy density [4][5][6][7][8]. Each type of electrolyte has its own advantages and disadvantages [9][10][11]. The major setback in solid polymer electrolytes is low ionic conductivity at ambient temperature, so plasticizers were used to overcome this drawback [12][13][14]. Numerous techniques, such as copolymerization, grafting, physical cross-linking, blending, plasticization, and the addition of inert ceramic oxides into the matrix, have been reported for increasing the ionic conductivity of polymer electrolytes. Apart from that, there are few other techniques for alignment of monomers of polymers in polymer films like film stretching, flow-induced crystallization, electrospinning etc [15,16].
Ardent research is carried out on polymer hosts, such as poly (ethylene oxide) (PEO), poly (methyl methacrylate) (PMMA), poly (vinyl chloride) (PVC), poly(acrylonitrile) (PAN), poly (vinylidene fluoride) (PVDF), and poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) etc [5,[17][18][19][20][21]. Enhancement in ionic conductivity is also done in polymer electrolytes by adding lithium salts with a polymer host, some lithium salts such as (LiBF 4 [24]. Recently researchers started to focus on Poly (vinylidene chloride-co-acrylonitrile) [ P(VdC-co-AN)] polymer due to its versatile properties [25,26]. Copolymers are preferred over polymers due to their unique features, such as high conductivity and good mechanical stability, which are essential for efficient electrochemical cells [27]. P(VdC-co-AN) has good thermal and mechanical stability, excellent air barrier properties, and broader electrochemical stability [28]. This material is said to be stable and safe to handle and is not environmentally harmful under normal conditions [25]. The work focuses on finding the appropriate plasticizer and its concentration which yields appreciable ionic conductivity with good stability. Poly (vinylidene chloride-co-acrylonitrile) as the host polymer, plasticizers -Propylene carbonate (PC) and β-Butyrolactone (βbl), and the complexing salt as lithium perchloride (LiClO 4 ) is being chosen. These polymer electrolytes are prepared by a solution casting technique and subjected to various characterizations.

P(VdC-co-AN)-LiClO 4 film preparation
Polymer electrolyte was prepared by solution casting technique [29]. The weighed Poly (vinylidene chloride-coacrylonitrile) and Lithium perchlorate were kept in a vacuum oven for 12 h at 60°C and 70°C to improve the purity. The samples were treated with DMF and left undisturbed for 24 h. The dissolved sample and the dissolved salt are stirred in a conical flask continuously for 24 h. The appropriate amount of plasticizers are added to the solution and stirred for 5 h to obtain a homogeneous solution. The obtained solution is stirred at an elevated temperature of 60°C-70°C until a homogeneous slurry state was obtained. The slurry obtained is cast onto Teflon-coated glass plates and kept in the vacuum oven for drying at room temperature for 24 h and at 65°C for 10 h to evaporate any residual solvents [30]. The resulting film was visually examined for its dryness and freestanding nature. The thickness of the prepared films was measured using digital screw gauge and were found to be of thickness 0.1 mm. The schematic representation of preparation of polymer membrane with chemical structure is mentioned in figure 1.

XRD studies
XRD diffraction is a technique to analyze the nature and structural characteristics of the films prepared [31]. The structural characteristics are carried out for all pristine polymer, salt and prepared electrolytes by X-Ray diffractometer, Bruker D8 Advance with CuKα radiation (λ = 1.5403 Ǻ) in the range from 10°-90°.

FTIR analysis
The Fourier transform infrared spectrum (FTIR) is used to evaluate the molecular interaction in a miscible and immiscible polymer [32]. There will be a change in the inter and intramolecular interactions of the compounds, indicating a change in vibrational spectra from pristine components. The FTIR spectra of the prepared sample were carried out using a SHIMADZU IR AFFINITY spectrometer in the range of 4000 to 400 cm −1 at room temperature.

AC impedance analysis
An effective technique for examining the conduction mechanism in polymer electrolytes is ac impedance analysis. Ac impedance spectroscopy is an excellent tool to investigate the conduction in polymer electrolytes. The ionic conductivity is the crucial parameter which tunes the electrical properties of the material. Normally ionic conductivity of the polymer electrolyte depends on two main parameters: the actual concentration of the conducting species and the mobility of conducting species. The ionic conductivity of polymer electrolyte can be calculated by the relation where L-thickness of the film, A -area of the film (πr 2 ), R b -bulk resistance [33]. The variation in ionic conductivity of the polymer electrolyte is recorded in the range of 303-373 K.

Linear sweep voltammetry
The linear sweep voltammetry was carried out by sandwiching polymer electrolyte between stainless steel electrodes with a constant scanning rate of 0.1 mV s −1 . LSV was carried out for free standing films with highest ionic conductivity.

Transference number measurement
The transference number measurement was introduced by Wagner [34]. This study was carried out by sandwiching polymer electrolyte between the stainless-steel electrodes. The electrodes are connected to the DC power supply with a constant voltage of 1.5 V. The change in current with respect to time is measured. This characterization was carried out for the free-standing films with highest ionic conductivity.

Thermogravimetry analysis
The thermal stability of the as prepared polymer electrolyte is ascertained from Thermogravimetric/Differential thermal analysis (TG/DTA). The thermal behaviour of the polymer material is considered to be an essential property considering the safety measures for battery application. The thermal analysis was carried out for two samples [A4 and B3], which possess high ionic conductivity using NETZSCH STA 2500 STA2500A-0061-N in the range of 20°C to 500°C at the heating range of 20°C per min.

Atomic force spectroscopy
Atomic force microscopy is a versatile and powerful tool to study the 2-dimensional and 3-dimensional surfaces of synthesized material [35]. AFM shows the changes in the surface topography and phase of the material. Atomic force microscopy is performed on the polymer films exhibiting maximum ionic conductivity.  were added [36]. The plasticizers ratio will increase the amorphous nature of PE, which in turn increases the ionic diffusivity of conductivity. Figure 3 shows the pristine peaks of Poly (vinylidene chloride-co-acrylonitrile), Lithium perchlorate, propylene carbonate, Beta butyrolactone. Figure 3 Also shows the vibration peaks of prepared polymer electrolytes with various plasticizers. The strong vibrational peaks at 2237 cm −1 are attributed to the nitrile group C≡N Stretching of pristine P(Vdc-co-AN). The peaks observed at 1038-1073 cm −1 , and 1242 cm −1 are assigned to CH 2 asymmetric stretching and wagging of pristine P(Vdc-co-AN). The characteristic peak at 1658 cm −1 indicates the C=O stretching of pure LiClO 4 . The C-H stretching, C=O stretching, CH 2 wagging, and O-CH 2 stretching are the vibrations that are present in pristine PC and βbl [37][38][39].C=O stretching, CH 2 wagging is all common in the P(VdC-co-AN) and in plasticizers.  Figure 4 depicts the temperature-dependent ionic conductivity of Poly (vinylidene chloride-co-acrylonitrile) -Lithium perchlorate with different concentrations of plasticizers [propylene carbonate and Beta butyrolactone. Films A4 and B3 exhibit a conductivity of 9.237 × 10 −4 S cm −1 and 2.284 × 10 −4 S cm −1 respectively at room temperature, which is higher when compared to the other films. It is observed that despite the higher conductivity of films A5, B4, and B5 the films were found to be jelly in nature. The ionic conductivity of plasticized polymer electrolytes is found to increase with the increase in temperature ( Table 1), which could be attributed to the increase in the volume of the matrix. These polymer electrolytes can expand on the increase in temperature, which enhances the segmental motion of the polymer chain and thus produces free volume in polymer electrolytes. The higher ionic conductivity of polymer electrolyte consisting of PC compared to that of βbl may be due to significant loss of later during drying the film. This may be due to the reduction in boiling point of βbl due to the influence of the pressure (400 mmHg) during the vacuum drying process. Thus the loss of plasticizer increases the glass transition temperature, thus reducing the segmental motion of polymer chains which in turn decreases the ionic conductivity of the polymer membrane [42]. This is evident from the conductivity table [ Table 1]. The temperature-dependent ionic conductivity of plasticized P(VdC-co-AN) polymer electrolyte obeys the VTF relation, elucidating that ionic conductivity is due to the migration of ions. Figure 5 shows the dependence of ionic conductivity with plasticizer concentrations. The conductivity is found to increase with the increase in plasticizer concentration. The incorporation of plasticizers into the polymer host will increase the amorphous nature of the polymer, ensuing polymer flexibility. This increases ion density due to high dissociation of salt, and ion mobility due to flexible polymer chains enhancing ionic conductivity. Plasticizers also decrease the viscosity of polymer electrolytes, which help in ionic migration between the electrodes [17].

Linear sweep voltammetry
The study was aimed to determine the electrochemical stability window of the polymer electrolyte between 0 and 6 V. Figure 6, the current is stable up to 4.5 V for the polymeric film A4, whereas in B3 stability window reaches the maximum value up to 5 V. From the literature is evident that lithium rechargeable battery will operate between 3 to 5 V [43]. Therefore, these two films find their potentiality to be used in lithium polymer batteries.

Transference number measurement
The purpose of this investigation is to ascertain which charge species is enhancing the conductivity of the produced polymer electrolytes [44]. The high ionic conductivity samples (A4 and B3) are subjected to the transference number measurement. A4 and B3 samples are tested for the transference number measurement with applied constant voltage (1.5 V). The plot between time and current is shown in figure 7. As time increases, it is observed that the polarization current decreases and it reaches a saturation state. Transference number is measured using formula    Where, I + is the initial current and Iis at constant current.
from the above equation, the transference number (t + ) of the high ionic conductivity samples A4 and B3 is found to be 0.95 and 0.94. This demonstrates that conduction happens on the samples for the electrolyte's transit method. Figure 8 shows the TG/DTA of the polymer membranes prepared. From the TG graph, it is observed that the meagre initial weight loss of (2.1%) occurring before 111°C may be due to the removal of residual solvent, if any. A remarkable weight loss of (10%) associated with an exothermic peak at 207°C may be due to the phase change occurring in the pristine electrolyte membrane. A major weight loss of (60%) is observed around 371°C is attributed to the polymer degradation [45]. Hence it is concluded that the polymer film is stable up to 220°C. It is observed that beyond 260°C the polymer decomposed completely. As the films show stability till 220°C with appreciable conductivity. Hence it can be concluded that these films can be used as an electrolyte in lithium-ion polymer batteries.  film of A4 is 0.2887, and B3 is 0.2879 μm. The presence of porous in the polymer films is due to plasticizer retention, which is responsible for the high ionic conduction of the polymer electrolyte.

Conclusion
Polymer electrolyte systems consisting of P(VdC-co-AN)-LiClO 4 -X [X = PC, βbl] were prepared and subjected to studies to identify their potentiality in applications. Complex formation and structural elucidation of P(VdC-co-AN)-LiClO 4 -Plasticizers (PC and βbl) polymer complex is confirmed from FTIR and XRD studies. Films with different concentrations were subjected to AC impedance spectroscopy to understand their mechanism of conductivity. The conductivity was observed to increase with the increase in plasticizer concentration, as well as with the increase in temperature. Film A4 consisting of 68% of PC and B3 with 66% of βbl, were found to exhibit high ionic conductivity of 9.237 × 10 −4 Scm −1 and 2.284 × 10 −4 Scm −1, respectively, with appreciable mechanical stability compared with other films. The linear sweep voltammetry was taken for the polymer electrolyte with high ionic conductivity and is predicted to lie in the range of 4 and 4.5 V. The transference number for the samples lie at 0.95 and 0.94. The thermal analysis performed by TG/DTA shows the stability of the polymer electrolyte up to 220°C. All the characterization results show that P(VdC-co-AN): LiClO 4 : PC can be used in energy storage applications.