Influence of binary lithium salts on 49% poly(methyl methacrylate) grafted natural rubber based solid polymer electrolytes

2.1 Materials

MG49 (M_w = 33000 g mol⁻¹) was obtained under commercial name “MEGAPOLY” from Green HPSP (Malaysia) Sdn. Bhd., Petaling Jaya, Malaysia. LiBF₄ (M_w = 93.75 g mol⁻¹ & 98% (w/w)), LiCF₃SO₃ (M_w = 156.01 g mol⁻¹ & 96% (w/w)) and LiI (M_w = 133.85 g mol⁻¹ & 99% (w/w)) salts were supplied by Sigma-Aldrich and tetrahydrofuran (THF) (M_w = 72.11 g mol⁻¹ & 99.9% (w/v)) was supplied by J.T Baker. All the materials were used without further purification.

2.2 Sample preparation

All the polymer electrolyte samples were prepared by solution casting technique and kept in a glove box, under controlled O₂ (>1.0 ppm) and H₂O (>0.1 ppm). An appropriate amount of MG49 (1.5 g) was dissolved in stoppered flasks containing 30 ml of THF and left to swell for 24 h. After 24 h, the solution was stirred using a magnetic stirrer for another 24 h until complete dissolution of MG49 was achieved. Binary lithium salt LiBF₄:LiCF₃SO₃ (25 wt%) with different percentages ratios (100:0), (90:10), (70:30), (50:50), (30:70), (10:90) and (0:100) respectively, were dissolved in 10 ml THF in different small bottles for 30 min and were stirred using a magnetic stirrer for another 30 min until the binary salt dissolved completely. The binary salt solutions were then added to the M49 and continuously stirred for 24 h. The homogenous solutions were poured into different Teflon dishes and left to dry slowly under fume hood to form films. Further drying was carried out in a vacuum oven at 50 °C for 24 h under the vacuum condition to remove any trapped solvent. Eventually, the free-standing films were stored in the dry cabinet to avoid any moisture or contaminations before further analysis. The same experimental procedure was repeated for binary lithium salt LiBF4: LiI.

2.3 Characterization

FTIR analysis was carried out using a computer interfaced with Perkin Elmer spectrum 2000 imaging system equipped with attenuated total reflection (ATR) accessory. The electrolytes were analyzed in the frequency range of 4000 cm⁻¹ to 650 cm⁻¹ with a scanning resolution of 2 cm⁻¹ (Aravindan et al., 2008; Su’ait et al., 2011). XRD model D-5000 Siemen was employed to analyze the crystalline behavior in the electrolyte film. The data were recorded in the range of diffraction angle 2θ from 5° to 80° at a scanning rate of 0.025° s⁻¹ (Ue et al., 2002; Ulaganathan and Rajendran, 2010). DSC analysis was carried out, under nitrogen flow rate of 50 ml min⁻¹ and the temperature range between −40 and 300 °C, using a METTLER TOLEDO Thermal Analyzer model DSC 822e as the main unit and STARe software. The samples were heated/cooled two runs from −40 °C to 300 °C at a heating rate of 10 °C min⁻¹ to study thermal transitions of the polymer matrix. For EIS analysis the samples were punched into disc shape by tabletop press tool of 16 & 20 mm diameter and taken three samples for each of film. The samples were then sandwiched between two stainless steel blocking electrodes after measuring the average of thickness that was between (0.016–0.026 cm) and connected to a frequency resonance analyzer (FRA) model VersaSTAT4 Princeton Applied Research with applied frequency from 1 MHz to 0.1 Hz at 100 mV amplitude. The conductivity (σ) was measured 5–9 times for each of sample. The conductivity calculated from the bulk resistance (R_b), which was obtained from the intercept on the real impedance axis (Z' axis) and the film thickness (l) and the contact area of the thin film (A = πr²) in accordance with the equation σ =  $\frac{l}{(Rb\times A)}$ . Ion transference number (t_ion), the samples were sandwiched between two stainless steel blocking electrodes and connected to VersaSTAT4 with applied potential 10 mV and a period time 12,600 S. The t_ion measurements were carried out two times. t_ion was calculated using Wagner's dc polarization equation, t_ion = 1−(I_ss/I_t) = I_t −I_ss/I_t, where I_ss and I_t represent steady-state current and total current (ionic and electronic) at starting time, respectively. The t_ion measurements were carried out two times as shown in Supplementary Information B.

3.1 FTIR analysis

FTIR spectroscopy was used to observe the vibrational energies of covalent bonds in the polymer host and the chemical interactions that occur in complexes of polymer-salts. Due to the fact that any type of bond has a different natural frequency of vibration, the identification of the absorption peaks in the infrared region of the vibration portion represents a specific type of bond (Su’ait et al., 2011). The major regions of interest were the oxygen atoms of the carbonyl group (—C⚌O) (1750–1730 cm⁻¹) and the ether group (C—O—C) (1300–1000 cm⁻¹) from the MMA structure in MG49 (Su’ait et al., 2011). Fig. 2 shows a structure of the MG49 and its functional group’s vibrational mode.

Close

Fig. 2 Structure of the MG49 and its functional group’s vibrational mode.

Export to PPT

Fig. 3(a) and (b) illustrates the symmetrical stretching of the carbonyl group v(—C⚌O), from the MMA structure in the MG49 that mixed with LiBF₄:LiCF₃SO₃ and LiBF₄:LiI, respectively. It was found the symmetrical stretch bands belong to v(—C⚌O) frequency of MMA give rise to an intense, very strong and sharp peak at 1733 cm⁻¹ has shifted the most prominent carbonyl band, (C⚌O) peak of MG49 to a lower wavenumber from 1733 cm⁻¹ to between 1725 cm⁻¹ to 1727 cm⁻¹ after addition of various lithium salts, LiBF₄:LiCF₃SO₃ and LiBF₄:LiI, respectively. The C⚌O functional group experiences a weak stretching due to strong intermolecular interaction. It can be suggested that the weak stretching mode and thus, longer their bond length of v(—C⚌O) in MMA due to the high electronegative value of oxygen atoms (Pauling scale 3.5 of oxygen) which pulled Li⁺ ions (Pauling scale 1.0 of lithium) towards the central atoms. Thus, stretched the bond between carbon (Pauling scale 2.5 of carbon) and oxygen in carbonyl group. The interaction also affects the vibration band for —O—CH₃ group at 1455 cm⁻¹ (shifted to 1446 cm⁻¹) for all salts ratios after the addition of the binary lithium salts in the polymer matrix, indicating weaker asymmetry stretch between oxygen and methylene group. This proving the occurrence of polymer–salt complex is formed (complete list of the peak’s shift is provided in Supplementary Information C). The Li⁺ cations can form a dative bond with the oxygen atom in the carbonyl group resulting in the downshift of the carbonyl band. This finding is consistent with the observations reported by Kumutha and Alias (2006).

Close

Fig. 3 FTIR spectra of MG49 and MG49 with various ratios of (a) LiBF₄:LiCF₃SO₃ and (b) LiBF₄:LiI in the spectral region of 1800–1300 cm⁻¹.

Export to PPT

Furthermore, Fig. 4(a) and (b) indicated the peak corresponding to asymmetrical stretching of v_as(C—O—C) at 1150 cm⁻¹ except for (70:30) ratio of LiBF₄:LiI and all samples of LiBF₄:LiCF₃SO₃ whom show the opposite behavior and v_s(—C(O)O—) at 1275 cm⁻¹ has shifted to the higher wavenumber, after the addition of both binary lithium salts. This strong stretching is due to the changes in electronegativity of double bond’s carbon next oxygen which coordinated with Li⁺. Obtained results indicate a strong intermolecular interaction between oxygen from the carbonyl group of MMA in MG49 with lithium ions in both doping salts. In addition to the work of the reported literature, the oxygen atoms on the polymer chain act as an electron donor and form a coordinate/dative bond with the lithium ions from the doping salts to form a polymer-salt complex (Su’ait et al., 2011). As indicates from Fig. 3(a) and (b) and Fig. 4(a) and (b), the changes in the specific wavenumber gives a conclusive proof for specific intermolecular interaction and confirmed the formation of polymer-salt complexes as discussed above.

Close

Fig. 4 FTIR spectra of MG49 and MG49 with various ratios of (a) LiBF₄:LiCF₃SO₃ and (b) LiBF₄:LiI in the spectral region of 1300–1000 cm⁻¹.

Export to PPT

Although, the peak corresponding to (—C⚌C—) symmetric stretching of isoprene in natural rubber at 1604 cm⁻¹ showed no changes in terms of both the peak shift and intensity of the vibration for each sample after adding both binary lithium salts in the polymer matrix. This indicates that there was no interaction between binary Li salts with the non-polar group in the polymer chain and lithium salt concentration, implied the impossibility of lithium-olefin complex formation during preparation.

3.2 Structural analysis

XRD analysis is one of the common ways to investigate the nature of crystallinity and amorphousity of a material (Ramesh and Arof, 2001). Fig. 5(a) and (b) shows XRD patterns of MG49, LiBF₄, LiCF₃SO₃, LiI and MG49 with various LiBF₄:LiCF₃SO₃ and LiBF₄:LiI. The XRD pattern of MG49 shows a hump in the region between 10° and 23° and a peak with high intensity which is attributed to MMA at 2θ = 29.5°. These observations were demonstrated the semicrystalline nature of MG49 (Gao et al., 2011; Su’ait et al., 2011). When the lithium salts were incorporated into the polymer host, the intensity of the peak at 2θ = 29.5° was not found. This is due to the reduction in the semi-crystalline phase in the polymer salt complexes at various weight percentages of all lithium salts into MG49 which is mostly attributable to the reduced configurational entropy due to the presence of crystalline segments (Huang and Paul, 2007). Besides, it is due to the plasticizing effect which prefers the formation of an amorphous phase rather than the crystalline phase (Yang and Farrington, 1993).

Close

Fig. 5 XRD patterns of MG49, LiBF₄, LiCF₃SO₃, LiI, and MG49 with various ratios of (a) LiBF₄:LiCF₃SO₃ and (b) LiBF₄:LiI.

Export to PPT

Besides, there are noticeable changes in the peaks of LiBF₄, LiCF₃SO₃ and LiI salts after the addition of the polymeric host. The presence peaks of LiCF₃SO₃ at the angles between 10° to 50°, LiBF₄ at the angles of 13.5°, 18.8°, 21.5°, 23.6°, 26.8°, 28.2°, 32.0°, 36 °, 42 ° and 55° and LiI at the angles of 20.3°, 25.4°, 29.3°, 36.3°, 47°, 49°, 52.4°and 54° were not seen at any of the polymer-salt complexes which confirms that both binary lithium salts were solvated well in the MG49 matrix (Su’ait et al., 2011; Yap et al., 2012). Moreover, there is no discernible change of peaks in XRD patterns observed upon the incorporation of LiBF₄, LiCF₃SO₃ and LiI, the intensity of the peaks showed a decrease together with broadening of XRD patterns with the incorporation of lithium salts. This indicates the decrease in crystallinity of the samples and increased amorphous phase. Dispersing lithium salt in the SPE system prevents the polymer chains from reorganizing, resulting in the reduction of crystallinity and subsequently improves ionic conduction as observed in FTIR analysis (Ramesh and Lu, 2011).

The salt’s solvation is assumed to increase the degree of salt dissociation which also affects the ionic conductivity and ion transference number. This factors correlated to crystallinity will be discussed in the ionic conductivity’s section. A slight change was observed at a high concentration of salt. In addition, new peaks at 28° for LiBF₄ and 21° to 22° for LiCF₃SO₃ in the polymer-salt complexes which indicates re-crystallization occurred in the polymer host. Table 1 illustrates the increase of the crystallinity with the increase of a ratio secondary salt in all MG49-LiBF₄:LiCF₃SO₃ polymer electrolytes due to the lower degree of the amorphous polymer matrix that contributes to less disorder arrangement of polymeric chain and thus decreasing the flexibility of the polymer matrix. However, the exception can be seen in the composition between (50:50), (30:70) and (10:90) LiBF₄: LiI. The results demonstrates LiBF₄:LiI at (30:70) ratio gives the lowest crystallinity due to disruption of physical crosslink by strong intermolecular interaction between v(—C⚌O) of MMA with LiBF₄:LiI (30:70) as shown by the significant shift in wavenumber compared the other ratios and further increase in amorphous nature at this polymer electrolyte. Moreover, the reduction of the crystallinity in this ratio and enhancement the degree of amorphicity. This is due to the intermolecular cross-linking of the polymer chains has been distrupted and the degree of salt dissociation increases (Rajendran and Ramesh Prabhu, 2010) which further affects the ionic conductivity. Ra et al. (2011) suggested the amorphous nature results in a greater ionic diffusivity and high ionic conductivity, which can be obtained in amorphous polymers which possess a flexible backbone as discussed in the EIS section. Besides, this may be a strong interaction between v(—C⚌O) of MMA with LiBF₄:LiI (30:70) as shown by the significant shifting in wavenumber out of all samples. This phenomenon may be related to the disrupter of the existing physically cross-linked between the polymeric chains, which contributes to semi-crystallinity properties of the polymeric materials. This behavior will be explained by thermal analysis in the next section.

3.3 Thermal analysis

Fig. 6(a) and (b) shows DSC thermograms of MG49 with different composition of LiBF₄:LiCF₃SO₃ and LiBF4:LiI ratios, whereas the extracted information of glass transition temperature (T_g) and melting temperature (T_m) are listed in Table 2. A small drop in heat flow from exothermic reaction to endothermic reaction was initially observed and it was commonly known as glass transition temperature (T_g). Thus, heat flow of polymer matrix decreases monotonically, up to above 50 °C, and followed by a weak endothermic peak which is known as the melting point of the polymer electrolytes, T_m. As reported by Idris and co-worker (Glasse et al., 2002), MG49 has a low glass transition temperature (T_g = −60 °C) due to its soft elastomer at ambient temperature. Due to our instrumentation limitation, the T_g for MG49 is not been able to observe.

Close

Fig. 6 DSC thermograms of MG49 with various ratios of (a) LiBF₄:LiCF₃SO₃ and (b) LiBF₄:LiI.

Export to PPT

From DSC analysis, there are only one T_g profile was started to display at the ratio of (50:50) to (0:100) for LiBF₄:LiCF₃SO₃ that indicates there is no phase separation, as well as the electrolyte samples, were homogeneously prepared (Ahmad et al., 2006). The T_g peaks at the highest percentage of LiBF₄ salt ratios were unable to observe, most probably due to the instrumentation limitation. It is expected that the T_g for the system is approaching T_g for MG49 (T_g = −60 °C). In addition, (30:70) ratio of LiBF₄:LiI exhibited the lowest T_g and T_m value, thus indicating the lowest crystallinity as proven in XRD studies and suggests that lithium salts contribute to the reduction of crystal phase.

Whereas, (70:30) ratio of LiBF₄:LiI demonstrated an opposite phenomenon; which leads to higher crystallinity. Based on the observed trends, the T_g value is found to decrease at the binary salts composition between (70:30), (50:50) and (30:70) LiBF₄:LiI; which indicates the increase in the amorphous nature of polymer electrolytes. The higher degree of amorphous polymer matrix contributes to more disorder arrangement of the polymeric chain and causes a reduction in the energy barrier for the segmental motion of the polymer electrolyte. Thus, increases the flexibility of polymer matrix and charge transport (Ulaganathan and Rajendran, 2010). The T_m also exhibits the same feature as T_g as shown in Fig. 6(a) and Table 2. Indeed, more asymmetric endothermic (the crystalline structure will mostly transform into amorphous form) melting peaks are being depicted in the DSC thermograms. This divulges a decrease in crystalline degree and/or an increase in the amorphous region of both polymer electrolytes (Pandey and Hashmi, 2009). It was clear from the obtained values of T_m where (50:50) ratio gave the lower T_m value than (0:100) for both salts. The decrease in T_g and T_m imply the indicates the increase in amorphous proportion in polymer electrolytes which leads to enhancement of flexibility and softening of the polymer backbone and improves the segmental movement of polymer chains (Nicholson and Snelling, 1955; Ramesh and Arof, 2001). Hence the ions flow more easily throughout the polymer chain. It can be said that the decrease of T_g at (30:70) LiBF₄:LiI leads to the segmental mobility increase. In addition, the reduction of its T_m compared to the other ratios in both binary lithium salts systems indicated that lithium salt and the polymer matrix affect the main chain dynamics of the polymer. The MG49 polymer is disturbed by binary lithium salts due to the coordination interactions between the Li⁺ ions and O atoms which causes the increase in amorphous nature of the polymer electrolytes as a result of a better ionic diffusivity and high ionic conductivity (Ra et al., 2011). This result is in a good agreement with XRD result as well.

3.4 Impedance analysis

The typical Nyquist plots for all samples of LiBF₄:LiCF₃SO₃ (100:0) and LiBF₄:LiI (30:70) are shown in (Supplementary Information D: Figure D). A semicircle (combination between resistance and capacitance) was observed for these impedance plots. This indicates that the resistance is due to ions migration. The bulk resistance (R_b) for all samples had been obtained from the low-frequency intercept of the semicircle or high- frequency intercept of the spike on the real axis (Osaka et al., 1998).

Salts having low lattice energy are generally expected to promote greater dissociation of the salt, thereby providing more ions (Ramesh and Arof, 2000). In this work, LiBF₄, LiCF₃SO₃ and LiI with large anions and low lattice energies have been used. Fig. 7 illustrates the variation of the logarithm of ionic conductivities of MG49 based polymer electrolytes with respect to various ratios of binary lithium salt compositions; LiBF₄:LiCF₃SO₃ and LiBF₄:LiI (at 25 wt% of total salts) at room temperature. The results clearly showed that ionic conductivity increases as the ratio of LiBF₄ increases as a comparison to the LiCF₃SO₃ for the system MG49-LiBF₄:LiCF₃SO₃. The highest room temperature conductivity was obtained at 1.42 × 10⁻⁸ S cm⁻¹ for LiBF₄:LiCF₃SO₃ (100:0). The electrolyte containing a higher content ratio of LiBF₄ was shown the highest ionic conductivity which mainly due to the lowest lattice energy (−699 KJ mol⁻¹) compared to LiCF₃SO₃, hence easier dissociation of Li ions by the polymer host (Ahmad, 2009; Ulaganathan and Rajendran, 2010). Besides, the rise in ionic conductivity with the increase of the ratio of LiBF₄ for LiBF₄:LiCF₃SO₃ system was due to the decrease in crystallinity of the polymer electrolyte. This was clearly observed in XRD results that showed the ratio LiBF₄:LiCF₃SO₃ (100:0) has the lowest crystallinity and it was compatible with EIS results. However, the results indicated that at the higher ratio of LiCF₃SO₃, the conductivity drops which is due to that LiBF₄ has a higher degree of dissociation compared to LiCF₃SO₃. In addition, the ratio LiBF₄:LiCF₃SO₃ (0:100) was given the highest of T_g from DSC analysis, which indicated the polymer have more rigid structure at the high content of LiCF₃SO₃ salt. Furthermore, among the lithium salts used here, LiBF₄ had higher ionic mobility due to its lowest lattice energy.

Close

Fig. 7 Variation of a logarithm of ionic conductivities of MG49 with respect to various ratios of binary lithium salts LiBF₄:LiCF₃SO₃ and LiBF₄:LiI (at 25 wt% of total salts).

Export to PPT

Whereas, the ionic conductivity was variable when the ratio of LiBF₄ increased as a comparison to the LiI as tabulated in Table 3. LiBF_4:LiI polymer electrolyte system shows an increase in ionic conductivity and 1.89 × 10⁻⁶ S cm⁻¹ is achieved at LiBF₄:LiI (30:70) as shown in Fig. 7. This is due to that the ratio LiBF₄:LiI (30:70) had the lowest crystallinity and the lowest of T_g. Moreover, the segmental motion of the polymer chain and the number of charge carriers are controlled by LiBF₄:LiI (30:70) more than the other ratios. According to Chowdari et al. (1992), it is possible that the two competing factors, segmental mobility which affects the mobility of charge carriers and the mobile charge carrier concentration which depends on the nature of inorganic salt and its interaction with the polymer chain, are optimized for the composition, resulting in higher conductivity. However, the ionic conductivity decreases at (70:30) reflecting a strong stretching of v_as(C—O—C) due to the changes in electronegativity of double bond’s carbon next oxygen which coordinated with Li⁺. The effect of large anion size required for ion delocalization charge was not seen in this work. Even, LiBF₄ and LiI with (30:70) have smaller anion size and LiI have higher lattice energy (LiBF₄: 699 KJ mol⁻¹, LiCF₃SO₃: 725 KJ mol⁻¹ & LiI: 730 KJ mol⁻¹) (Ponnamma et al., 2016; Scrosati, 1993), but they provided higher ionic conductivity in comparison to LiBF₄:LiCF₃SO₃ (30:70). This is mainly due to LiBF₄:LiCF₃SO₃ (30:70) controls the conduction process in the samples. Besides, conductivity requires both a high dissociation and a high mobility.

Table 4 indicates t_ion values of MG49 with various ratios of both binary lithium salts (25 wt% of total polymer) based polymer electrolytes using Wagner's dc polarization technique that was mentioned earlier. It was found the values of ion transport (t_ion) for MG49-LiBF₄:LiCF₃SO₃ (30:70) was only 0.71 in comparison to 1.00 for MG49-LiBF₄:LiI at the same binary lithium salts ratios. The results showed an increase in t_ion values with increasing ionic conductivity as shown in Table 4 and Fig. 8. t_ion was measured for the optimum and control samples for comparison. It was found the values of t_ion increases with the increase in ionic conductivity. This is due to the increasing number of conduction species in the electrolyte which contributed to total ions of lithium (Li⁺) and anions. For the lowest values of (t_ion) were also compatible with the lowest ionic conductivity. An explained in initial part, the high transference number for LiBF₄:LiI (30:70) can be related to the effect of weaker bonding, thus leads for high dissociation and a high mobility of ions.

Close

Fig. 8 Chronoamperometery plot for various ratios of MG49:LiBF₄:LiCF₃SO₃ & MG49: LiBF₄:LiI.

Export to PPT