The Influences of 1-Butyl-3-Methylimidazolium Tetrafluoroborate on Electrochemical, Thermal and Structural Studies as Ionic Liquid Gel Polymer Electrolyte

After decades of development, ionic liquid gel polymer electrolytes (ILGPEs) are currently experiencing a renaissance as a promising electrolyte to be used in electrochemical devices. Their inherent tendency towards poor electrochemical properties have limited their applications and commercialization activities. Henceforth, gel polymer electrolyte (GPE) is being introduced to alleviate the abovementioned issues. In this work, the assessment of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] in poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) to form ILGPE was done. The relationship of [BMIM][BF4] towards the dielectric properties at different wt. % ratios and temperature was ascertained. The results indicated that [BMIM]BF4 is able to facilitate fast conduction. Moreover, it was found that [BMIM][BF4] could serve as an effective agent in reducing crystallinity and glass transition temperature of the polymer and thus enhanced the ionic conductivity of the samples. Notwithstanding, the ILGPE sample possessed a high thermal stability up to 300 °C and good electrochemical stability of 4.2 V which are beneficial for operation in electrochemical devices. All in all, the correlation between the ionic liquid chemistry and electrochemical performances could provide a valuable insight to rational selection and design for ILGPE electrolytes.


Introduction
The discovery of electrolytes was in the work of Alessandro Volta in the year 1800 when he invented the first electrochemical cell, which consisted of two dissimilar electrodes dipped in an electrolyte solution of saltwater brine, producing a steady electric current [1]. Ever since, various types of liquid electrolytes have been developed and were commercialized for use in electrochemical devices such as rechargeable batteries, solar cells, and fuel cell applications. Liquid electrolytes often achieve high ionic conductivity values due to high ion mobility in electrolytes, especially in less viscous electrolytes. Nonetheless, the use of liquid electrolyte still encounters several safety problems. It is highly volatile and flammable due to the use of organic solvents. In addition, liquid electrolyte represents a greater risk of leakage. These factors have resulted in severe combustion in some cases. For this reason, a tremendous research effort in search of a safer electrolyte has led to a solid-state electrolyte (SSE). Solid polymer electrolyte (SPE) is one of the most promising SSEs and comprises polymer and salt. The SPEs offer some distinct advantages over electrolytes dissolved in organic solvents. The use of SPE reduces the risk of leakage room temperature conductivity of~3 mS cm −1 and electrochemical stability of~4.0 V. Furthermore, it does not have a melting point but low glass transition temperature at ca. −85 • C and viscosity value of 219 cP (25 • C), whereas BMIMPF 6 has higher T g (−76 • C) and viscosity (450 cP at 25 • C) [23,24]. These are good requirements when it comes to LIB application which required low viscosity.
Herein, this paper demonstrates the improvement of ILGPE based PVDF-HFP using [BMIM]BF 4 ionic liquid. Studies on effects of electrical, electrochemical, thermal, and structural properties of the polymer electrolytes when adding high amounts of ionic liquid in the system will be shown. In addition, we present a facile ILGPE preparation of stable ILGPE films with a high loading amount of ionic liquid (>50%) through solution casting. It exhibited good electrochemical and thermal properties, which we believe could act as a potential electrolyte to be applied in electrochemical devices.

Preparation of Ionic Liquid Gel Polymer Electrolyte
ILGPE was prepared by dissolving an appropriate amount of PVDF-HFP in 10 mL of acetone solvent and stirred for 24 h at room temperature. Afterward, [BMIM][BF 4 ] as a plasticizing agent was added and the mixture were further stirred for 24 h to achieve a homogenous solution. The electrolyte solution was then cast onto a Teflon dish and the solvent was let to evaporate slowly for 48 h. The dried gel was peeled off and a freestanding and flexible film was attained. The obtained ILGPE films were further dried in vacuum oven to ensure that it is free from solvent and moisture, then stored in a desiccator until further analysis. The above samples were prepared at different ratios to PVDF-HFP:[BMIM][BF 4 ] using the same procedures.

Characterizations
Dielectric studies were conducted by extracting the impedance data measured using a potentiostat model VersaSTAT 4 by Princeton Applied Research (Oak Ridge, Tennessee, USA) at room and elevated temperature. Prior to analysis, ILGPE films were punched into small disc shape with an area of 2.01 cm 2 . The measurement was performed in a stainless steel (SS) holder with the configuration of SS/ILGPE film/SS. The whole assembly was kept in a temperature-controlled oven and the measurement was taken in the temperature range of 303 K to 373 K. Dielectric properties of the ILGPE films; dielectric constant (ε ) and dielectric loss (ε ) were calculated using the following equations.
where Z i and Z r are the imaginary and real impedance, respectively. Angular frequency, ω = 2π f , where f is the frequency, and C 0 = ε o A/l in which ε o = permittivity of free space, A = effective contact area of electrolyte and the electrode, and l = thickness of the sample. Further analysis from the impedance data such as dielectric modulus and loss tangent (tan δ) were carried out. Dielectric modulus, which contains two parts, real (M ) and imaginary (M ), were calculated using the equations that follow: Meanwhile, the ratio between ε and ε quantified as loss tangent (tan δ) was expressed below: Electrochemical stability window of the highest conducting film was determined using Wonatech ZIVE MP2 Multichannel electrochemical workstation (Seoul, Korea). ILGPE film was placed in between two stainless steel blocking electrodes and the analysis was carried out from 0 to 5 V with scan rate of 1 mVs −1 . Thermal characteristics of ILGPE films were investigated using DSC 8822 e Mettler Toledo (Greifensee, Switzerland). Two cycles of heating were performed on each film. The first cycle began from −50 to 100 • C whilst the second cycle was heated up to 200 • C with a heating rate of 10 • C min −1 . Thermogravimetric analyzer, which is Mettler-Toledo TGA/SDTA 851 was used to determine the thermal stabilities of ILGPE films in the temperature range of 25 • C to 800 • C under inert atmosphere at a heating rate of 10 • C min −1 . The films were carefully weighed in the range of about 7.0 mg. The crystallinity of the ILGPE films were investigated using XRD D5000 Siemens (Munich, Bavaria, Germany). Diffraction angle, 2θ, was taken from 3 • to 60 • at a rate of 0.05 • s −1 . Figure 1 shows the physical observation of ILGPE films at various percentage ratios of PVDF-HFP to [BMIM] [BF 4 ]. Generally, all the films display good homogeneity despite mixing two different types of entities. This is reflected through good transparency, softness, and flexibility as shown in Figure 1. However, at higher rations of [BMIM] [BF 4 ], the samples were mechanically unstable and became gel-like. In accordance with the intended purpose, samples above the ratio of 60:40 will not be considered for further examination and study.

Dielectric-Temperature-Dependent Studies
Ionic conductivity is one of the key indicators for high-performance electrolyte in any electrochemical devices. For example, PEMFCs and LIBs applications demand a minimum ionic conductivity of 10 −3 S cm −1 to be functioned. The influences of [BMIM][BF4] on

Dielectric-Temperature-Dependent Studies
Ionic conductivity is one of the key indicators for high-performance electrolyte in any electrochemical devices. For example, PEMFCs and LIBs applications demand a minimum ionic conductivity of 10 −3 S cm −1 to be functioned. The influences of [BMIM][BF 4 ] on conductivity and conduction behavior of the ILGPEs have been discussed in our previous work [25]. At optimum ratio of 40:60 (PVDF-HFP to [BMIM][BF 4 ]) sample, the highest conductivity of 2.84 × 10 −3 S cm −1 with transference number (TN) value of 0.998 are obtained. The near unity value of 1 in TN indicates ion conduction is primary predominately by ions and not electron conduction [26,27]. Thereby, the studies on dielectric properties will provide an important insight on the polarization effect at the electrode-electrolyte interfaces, ion conduction behavior and relaxation mechanism of the electrolyte system. In the field of polymer electrolytes, dielectric constant (ε') is a measure of the polymer electrolyte's capacity to store charge carriers and it represents the amount of dipole alignment in a given volume. On the other hand, dielectric loss (ε") is a measure of energy loss due to the motion of free charge carriers and dipoles alignment when the polarity of electric field reverses rapidly [28]. All the dielectric and modulus data are calculated using the data collected from impedance spectroscopy (IS) which can be found in our previous work [26]. Figure 2 shows the graph of frequency dependence of ε and ε at various PVDF-HFP to [BMIM][BF 4 ] contents. It is observed from both Figure 2a and b that ε and ε display similar pattern in which there is an increasing peak at the lower frequency region and a gradual decrease at the higher frequency region for all ILGPEs. The high value of ε observed at low frequency region is due to the accumulations of charge at electrodeelectrolyte interface resulted from slow periodic reversal effect of the applied field [29]. Whereas, at the higher frequency region, the periodic reversal of the applied field occurred so fast that the charge carriers and dipoles in the polymer chain are incapable to cope up with the speed and orient themselves according to the direction of the applied field. Hence, the value of ε at high frequency region is small as there is no excess of ion diffusion [30,31]. Apart from that, the ε value increased with increasing percentage ratio of [BMIM] [BF 4 ]. This insinuates that the charge carrier density increases with higher IL loading in the ILGPE system. A similar trend has also been reported for other polymer electrolyte systems [32,33]. loading in the ILGPE system. A similar trend has also been reported for other polymer electrolyte systems [32,33].   Figure 3a and b represents the plots of and for the highest conducting film with a 60:40 ratio at various temperatures (303 K to 373 K). Based on both figures, it can be seen that the values of and gradually increase with temperature. Presumably, the increase of temperature may increase the free volume and mobility of polymer segments, which facilitates the dipole orientation due to the greater movement of the dipole molecular chain of ILGPE [28,33].  Figure 3a,b represents the plots of ε and ε for the highest conducting film with a 60:40 ratio at various temperatures (303 K to 373 K). Based on both figures, it can be seen that the values of ε and ε gradually increase with temperature. Presumably, the increase of temperature may increase the free volume and mobility of polymer segments, which facilitates the dipole orientation due to the greater movement of the dipole molecular chain of ILGPE [28,33].
Further interpretation of the impedance data was carried out through dielectric modulus which study the behavior of electrode-electrolyte polarization effect at different frequencies. Figure 4a (3) and (4), respectively. As the frequency increases, M and M also increased due to the distribution of the relaxation process and attain maximum value in the higher frequency range. From both figures, M and M were observed to have peak dispersion in the high frequency region and decreases towards the low frequency region. The peaks at high frequencies highlighted on the bulk effect revealed that the ILGPE films are predominantly ionic conductors [34]. As the loading of IL increases, the spectra are shifted to the right beyond the frequency range used in this experiment indicating that the ILGPE film are purely an ionic conducting system. On the contrary, M and M values approaching zero in the low frequency region are associated to the electrode-electrolyte polarization effect. Generally, the polarization effect at electrode-electrolyte interfaces is often suppressed at lower frequency, giving small values (tending zero). This indicates the removal of electrode polarization [35]. The plots that display a long tail close to the x-axis are due to large capacitance value that correlates with the large polarization effect at the electrode-electrolyte interface [27]. Therefore, the loading of 60% [BMIM][BF 4 ] incorporated into the system proves that the films are purely due to ionic conductors. Further interpretation of the impedance data was carried out through dielectric modulus which study the behavior of electrode-electrolyte polarization effect at different frequencies. Figure 4a,b display the dielectric modulus of real ( ) and imaginary modulus part ( ) as a function of frequency for the samples with different percentage ratios of PVDF-HFP to [BMIM][BF4]. The and ′ are calculated based on equation (3) and (4), respectively. As the frequency increases, and ′ also increased due to the distribution of the relaxation process and attain maximum value in the higher frequency range. From both figures, and were observed to have peak dispersion in the high frequency region and decreases towards the low frequency region. The peaks at high frequencies highlighted on the bulk effect revealed that the ILGPE films are predominantly Analysis of loss tangent (tan δ) was carried out to observe the effect of IL loading towards the relaxation behavior. Variation of tan δ as a function with frequency for different percentage ratios of PVDF-HFP to [BMIM] [BF 4 ] is presented in Figure 5a. Tanδ or also known as dissipation factor, is a measurement of the ratio of energy loss to energy stored determined by the expression in equation (6). The occurrence of peak maxima of tan δ seen in the figure imply the presence of relaxation dipoles in the system [36]. It can be seen that the peak has shifted towards the high frequency region as the loading of [BMIM][BF 4 ] is increased. At the same time, it also indicates the reduction of relaxation time. The relation between the relaxation time (τ) and frequency ( f max ) corresponding to the maximum frequency peak expressed as below:   The relaxation time is associated with the change diffusion process in the sample and is correlated with the plateau of the real part of the conductivity [31]. The reduction in relaxation time implies that the polymer chain can easily be oriented through the increment of free volume due to the incorporation of more amounts of ions. Presented in Figure 5b is the graph of relaxation time and ionic conductivity at various loading of PVDF-HFP to [BMIM] [BF 4 ]. It can be seen that the relation between relaxation time and ionic conductivity at various IL loadings is the opposite. As the ionic conductivity increases, the relaxation time decreases. This shows that increasing the loading of [BMIM][BF 4 ] into the ILGPE system resulted in increased ionic conductivity which then helped in reducing the relaxation time during ionic movement in the sample [37]. Along with that, increase in height of tan δ for each ILGPE film also suggested the increment in charge carrier density owed to the increase of [BMIM][BF 4 ] [38].

Electrochemical Stability Study-Linear Sweep Voltammetry
Electrochemical stability window (ESW) of the electrolyte plays a vital role especially when being applied in electrochemical device applications. ESW of the ILGPE film was measured by examining the breakdown voltage through LSV analysis [39]. The measurement was conducted using inert symmetrical electrode SS/ILGPE/SS cell configuration. Figure 6 shows the voltammogram of the optimum ILGPE sample at a scan rate of 1 mVs −1 . As presented in the Figure 6, it is found that the breakdown voltage of the ILGPE occurs at 4.2 V. Similar result was obtained in the previous work by TianKhoon et. al, whereby the sudden change of anodic current occurred at around 3.8 V which indicates the start of the oxidation of [BF 4 ] − anions [40]. Studies conducted by Kazemiabnavi et al. revealed that anions that are highly fluorinated such as BF 4 − tend to be electrochemically more stable against reduction and oxidation as compared to other anions [41]. As a result, fluorinated anions have wider ESW when coupled with an ionic liquid cation. In addition, the high stability value of 4.2 V is in good agreement with the previously reported value by Kazemiabnavi et al. and it also meets the requirement as a potential electrolyte for use in LIBs and PEMFC applications [41].

Thermal Analysis (DSC and TGA)
3.4.1. Differential Scanning Calorimetry DSC analysis presents the essential information on the thermal characteristics of the electrolyte such as glass transition temperature ( ), melting temperature ( ) as well as the crystallization temperature ( ). Figure 7 depicts the heat flow curves in the tempera-

Differential Scanning Calorimetry
DSC analysis presents the essential information on the thermal characteristics of the electrolyte such as glass transition temperature (T g ), melting temperature (T m ) as well as the crystallization temperature (T c ). Figure 7 depicts the heat flow curves in the temperature range of −100 • C to 180 • C for different percentage ratios of ILGPEs and Table 1 provides the thermal properties of reference for PVDF-HFP and [BMIM] [BF 4 ] [22,[42][43][44][45]. In ensuring the ILGPE films are completely free from any remnant trace of solvent and trapped moisture, all DSC traces were taken during the second heating. Based on the graph shown in Figure 7, it can be seen that upon higher incorporation of [BMIM][BF 4 ], T g and T m values for all films shifted to lower temperature region, from −33.16 • C to −73.78 • C and 126.13 • C to 100.67 • C, respectively [46]. Along with that, there is an appearance of exothermic crystallization peak at higher percentage ratio, notably at 50: 50 4 ] in the polymer matrix and that the plasticizing effect of the IL have weaken the intermolecular interactions between the polymer chains [47]. The weakening of the forces increases the flexibility of the polymer backbone and therefore, give rise to the ionic conductivity of the ILGPE [48,49]. Meanwhile, the shifting of T m to a lower temperature region suggests that there are more amorphous phases present in the ILGPE system. Pandey et al. [50] reported that a broader T m peak is attributed to the presence of liquid components that cause an increase in the amorphous proportion in the overall material. the ionic conductivity of the ILGPE [48,49]. Meanwhile, the shifting of to a lower temperature region suggests that there are more amorphous phases present in the ILGPE system. Pandey et al. [50] reported that a broader peak is attributed to the presence of liquid components that cause an increase in the amorphous proportion in the overall material.

Thermogravimetric Analysis
Thermal stability of an electrolyte is extremely vital to guarantee acceptable performance in any electrochemical devices and seemed to be a critical issue over the past decades. It is of great importance to develop an electrolyte with high thermal stability which

Thermogravimetric Analysis
Thermal stability of an electrolyte is extremely vital to guarantee acceptable performance in any electrochemical devices and seemed to be a critical issue over the past decades. It is of great importance to develop an electrolyte with high thermal stability which can enhance the safety property in energy storage and conversion devices. For example, lowtemperature PEMFC and high-temperature PEMFC operate in a range of 60-80 • C and 110-180 • C, respectively. Meanwhile, LIBs have typical temperature working range of −20-60 • C. Figure 8 depicts thermogram of TGA and differential gravimetric analysis (DTG) of the ILGPE films with different percentage ratio of PVDF-HFP to [BMIM] [BF 4 ]. According to the previous report, thermal degradation temperature (T d ) for pure PVDF-HFP is 470 • C whilst T d for [BMIM] [BF 4 ] is in the range of 360 • C to 420 • C [51,52]. TGA curves for all films showed high thermal stabilities up to 300 • C in which it is a major desirable property for the application of any electrochemical devices. No weight loss observed below 200 • C for all ILGPE films revealed that the sample is free from solvent and moisture. This is in good agreement with the hydrophobic nature of the fluoropolymer and moisture stable of imidazolium-based IL [10]. The first degradation step, which is in the range of 350 • C to 420 • C is believed due to the degradation of IL, since the T d of [BMIM][BF 4 ] is lower compared to PVDF-HFP. Meanwhile, the following step from 420 • C to 465 • C may be due to the degradation of its polymer host [51]. The maximum weight loss percentage of the ILGPE is also in accordance with the loading of [BMIM][BF 4 ] in the system. desirable property for the application of any electrochemical devices. No weight loss observed below 200 °C for all ILGPE films revealed that the sample is free from solvent and moisture. This is in good agreement with the hydrophobic nature of the fluoropolymer and moisture stable of imidazolium-based IL [10]. The first degradation step, which is in the range of 350 °C to 420 °C is believed due to the degradation of IL, since the of [BMIM][BF4] is lower compared to PVDF-HFP. Meanwhile, the following step from 420 °C to 465 °C may be due to the degradation of its polymer host [51]. The maximum weight loss percentage of the ILGPE is also in accordance with the loading of [BMIM][BF4] in the system.
The DTG peak in Figure 8b shows a sharp peak of degradation temperature ( ) for each film.  [51]. From the results, the decrease in thermal stability proves that polymer matrix and IL incorporated well with each other and was still in the right temperature working range that is suitable for most applications in electrochemical devices, in particular at high temperature.

Structural Analysis (X-ray Diffractometry)
XRD was performed to determine crystalline state of each ILGPE film. Figure 9 rep resents the XRD patterns of pure PVDF-HFP and ILGPE films with different percentag ratios of PVDG-HFP to [BMIM][BF4]. The semicrystalline structure of pure PVDF-HF film is confirmed by the coexistence of crystalline peaks (18° to 20°) in the amorphou region (15° to 25°) [53]. Upon addition of [BMIM][BF4], the prominent crystalline peak a 20° diminished and ceased to be visible, indicating that IL is an effective plasticizer for th PVDF-HFP polymer [50]. Besides, the degree of crystallinity (DC) for pure PVDF-HFP found to be 59%. With as much as 10 wt.% of [BMIM][BF4] incorporated into the system the DC decreased to 46.4% and subsequently reduced to 35.6% at 60 wt.% of IL loadin This implies that [BMIM][BF4] is not just as a charge carrier, but it also plays role as plasticizer and thus, lowers the crystallinity as well as results in increased ionic condu tivity of the ILGPE system [53].  4 ] of 50% and 60%, there is another peak with T d2 at 419 • C. This peak corresponds to the degradation of PVDF-HFP [51]. From the results, the decrease in thermal stability proves that polymer matrix and IL incorporated well with each other and was still in the right temperature working range that is suitable for most applications in electrochemical devices, in particular at high temperature.

Structural Analysis (X-Ray Diffractometry)
XRD was performed to determine crystalline state of each ILGPE film. Figure 9 represents the XRD patterns of pure PVDF-HFP and ILGPE films with different percentage ratios of PVDG-HFP to [BMIM] [BF 4 ]. The semicrystalline structure of pure PVDF-HFP film is confirmed by the coexistence of crystalline peaks (18 • to 20 • ) in the amorphous region (15 • to 25 • ) [53]. Upon addition of [BMIM][BF 4 ], the prominent crystalline peak at 20 • diminished and ceased to be visible, indicating that IL is an effective plasticizer for the PVDF-HFP polymer [50]. Besides, the degree of crystallinity (DC) for pure PVDF-HFP is found to be 59%. With as much as 10 4 ] is not just as a charge carrier, but it also plays role as a plasticizer and thus, lowers the crystallinity as well as results in increased ionic conductivity of the ILGPE system [53].

Conclusions
Overall

Conclusions
Overall, PVDF-HFP incorporated with ionic liquid [BMIM][BF 4 ] gel polymer electrolytes have substantially improved. These improvements have been reflected in the dielectric constant, dielectric loss, and tangent loss values of the ILGPE films that were found to increase upon addition of [BMIM]BF 4 . The values are higher when the percentage ratios of [BMIM] [BF 4 ] to PVDF-HFP increase, due to the increase in the number of charge carriers supplied by [BMIM] [BF 4 ]. The appearance of peaks in the modulus formalism spectra at high frequencies implies that the ILGPE films are predominantly ionic conductors. The DSC and XRD results showed that the crystallinity of ILGPE film significantly decreased with [BMIM] [BF 4 ]. The reduction of crystallinity helps by increasing of ionic conductivity. Meanwhile, all ILGPE films have high thermal stability up to 300 • C and lower weight loss which signify the improvement in thermal stability. Further investigation of the electrochemical stability carried out through LSV analysis revealed high ESW value of 4.2 V. In general, our findings showed that [BMIM][BF 4 ] could serve as a reinforced agent to replace the volatile and flammable organic solvent for better GPE performance. Nevertheless, future works need to further study the compatibility of ILGPE-based PVDF-HFP/[BMIM]BF 4 with electrodes and the solubility of salts such as LiTFSI for application in LIB.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions.