Introduction

Solid polymer electrolytes (SPE) currently receive a great deal of attention because of their proposed large-scale use in secondary lithium ion batteries [1]. The development of SPE has drawn much interest among researchers [2–4]. The high molecular weight polyethylene oxide (PEO)-based composite polymer electrolytes are gaining popularity as the best candidates for polymer matrix due to their salvation power, complexation ability and ion transport mechanism directly connected to the alkaline cell [5]. The desirable characteristics of SPE are: good compatibility with lithium metal, no leakage, low self-discharge in batteries, elastic relaxation under stress, ease in processing and continuous production [6, 7]. However, the low ionic conductivity of SPE limits their applications. It is known that the ionic transport of SPE occurs only in the amorphous polymer regions and is often governed by the segmental motion of polymer chain [8, 9]. It is clear that the presence of a flexible, amorphous phase in SPE is essential for higher ionic conductivity. Although many kinds of polymeric electrolytes such as poly(acrylonitrile) [10], poly(propylene oxide) [11], poly(methyl methacrylate) [12] and poly(maleic anhydridestyrene) [11] have been studied, high molecular weight PEO-based SPE is actively researched for technological applications and fundamental studies [6, 13].

The high ionic conductivities observed in polymer/alkali metal salt complexes, particularly those involving PEO and lithium salts, have stimulated great interest for all solid-state, high-energy-density batteries [14, 15]. PEO, the most extensively studied polymer, can form complexes with a wide range of metal salts including alkali, alkaline earth and transition metal salts. PEO/salt electrolytes suffer one major drawback, whereby PEO is semi-crystalline at ambient temperature. Much effort has been devoted to solid polymer electrolytes in order to understand the mechanisms and governing structure of polymer electrolytes and their ability to solvate ions and yield high ionic conductivity. Several salts have been used to increase the conductivity of polymer electrolytes such as Mg(NO3)2 [16], LiCF3SO3 [17], KI [18], NaClO3 [19] and LiPF6 [20]. From the various salts mentioned, lithium hexafluorophosphate (LiPF6) remains the only salt used in practical lithium ion batteries [21]. The conductivity increases with increasing concentrations of LiPF6, up to 15 wt.% concentration. The conductivity for 20 wt.% LiPF6 at room temperature is about 4.10 × 10−5 S cm−1, which is the highest conductivity value reported in the literature [16–20]. In addition, LiPF6 is the most stable salt at room temperature, provided that the amount of H2O is very low (less than 1 mg L−1) [20]. Owing to the benefits of LiPF6, this paper is focused on the effects of various LiPF6 salt concentrations on the properties of solid polymer electrolytes.

Experimental

The electrolytes were prepared using PEO (Aldrich, with an average molecular weight of 6 × 106) which was dried at 50 °C under vacuum for 48 h, and LiPF6 was used without further purification. PEO and various ratios of LiPF6 were composed and dissolved, in sequence, with acetonnitrile and are listed in Table 1. The mixture was stirred for 24 h, and the resulting solution was poured into a petri dish. The solvent was left to evaporate at room temperature. The prepared electrolytes were subjected to several characterization studies. Conductivity studies were carried out using HIOKI 3532–50 LCR HiTESTER, and differential scanning calorimetry (DSC) thermographs were obtained using DSC 820 fitted with 56 thermocouples. Fourier transform infrared spectroscopy (FTIR) spectra were obtained using Perkin Elmer Spectrum 400, and X-ray diffraction (XRD) analysis was conducted using Bruker model D8 Advance.

Table 1 Thermal properties of salted polymer electrolyte
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Results and discussion

Conductivity studies

Figure 1 shows the variation of room temperature conductivity as a function of salt concentration. The dependence of conductivity on salt concentration provides information on the specific interaction between salt and the polymer matrix. It is observed that the conductivity increases upon the addition of salt. The highest conductivity is obtained at 4.1 × 10−5 S cm−1 for 20 wt.% of LiPF6. The conductivity is expected to decrease beyond 20 wt.% of LiPF6 due to a decrease in the number of charge carriers which lower the segmental motion of the polymer chain [18].

Fig. 1
figure 1

Variation of conductivity as a function of LiPF6 content at room temperature

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As the salt content increases, the conductivity increases because the number density of mobile ions increases. Therefore, the polymer segment's motion is promoted [22]. The results agree well with the Rice and Roth model [17, 23]:

$$ \sigma = \frac{2}{3}\left[ {\frac{{{{\left( {Ze} \right)}^2}}}{{{k_B}Tm}}} \right]\eta {E_A}\tau \exp \left( { - \frac{{{E_A}}}{{{k_B}T}}} \right) $$
(1)

where σ, η, Z, e, E A and m represent the conductivity, density, charge carrier, elementary charge, activation energy and mass of the conducting ions, respectively. Furthermore, the parameters T, k B, e and τ represent the absolute temperature, Boltzman constant, electronic charge and time travel of ions between sites, respectively. From Eq. 1, it can be seen that the conductivity increases when the density of mobile ions increases.

Figure 2 shows the complex impedance plot for the PEO–LiPF6 complex at room temperature. The absence of a high-frequency, semi-circular region at a higher salt content verifies that the current carriers are ions. Therefore, the total conductivity of the complexes is mainly due to the ion conduction.

Fig. 2
figure 2

Impedance plots of samples containing a 0 wt.%, b 5 wt.%, 10 wt.%, c 15 wt.%, 20 wt.%

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Figure 2a–d shows the complex impedance plots for samples with 0, 5, 10, 15, 20 wt.% LiPF6 at room temperature, respectively. A part of a depressed semicircle can be observed for the sample with 0 wt.% LiPF6. The semicircle can be represented by a parallel combination of a capacitor, which is due to the immobile polymer chain [24]. As the salt concentration begins to increase, the semicircle in the plots is observed to lessen and finally results in a low-frequency spike. This observation suggests that only the resistive component of the polymer electrolytes exists [24, 25], which is due to the mobile ions inside the polymer matrix.

The R b for all samples was calculated from the low-frequency intercept of the semicircle or high-frequency intercept of the spike on the real axis. It is found that the R b decreases with an increase in salt concentration. This may be due to an increase in mobile charge carriers upon the addition of LiPF6. The R b is obtained to be 5.36 × 106 S cm−1, 4.64 × 103 S cm−1, 5.19 × 102 S cm−1, 2.52 × 102 S cm−1 and 1.61 × 102 S cm−1 for 0, 5, 10, 15 and 20 wt.% concentration of LiPF6, respectively.

The conductivities of PEO complex films are found to increase with temperature from 303 K to 373 K, and the films are found to be mechanically stable [26]. At higher temperatures, the thermal movement of polymer chain segments and the dissociation of salts improve, which increase conductivity. In the complex impedance plots shown in Fig. 3, the disappearance of a depressed semicircle at high frequencies reveals the absence of capacitive nature, and therefore, only diffusion processes take place [27]. The intercept at the real axis gives the electrolytes' resistance, and the values decrease with increasing sample temperature and ionic mobility. However, the charge concentrations do not necessarily increase with temperature for polymeric electrolytes [23].

Fig. 3
figure 3

Impedance plot of samples at temperatures of a 303–313 K and b 343–373 K

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Figure 4 shows the Arrhenius plot of ionic conductivity at various weight percents of LiPF6. The temperature dependence of the ionic conductivity is not linear and obeys the Vogel–Tamman–Fulcher (VTF) relationship.

Fig. 4
figure 4

Conductivity dependence temperature of complexes at various weight percents of LiPF6. a 5, b 10, c 15, d 20

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In PEO–LiPF6 films, 20 wt.% LiPF6 with PEO is the highest composition of solid films achievable. At higher compositions of LiPF6, films fail to form and remain in a gel-like state. This may be caused by the presence of water molecules. It is known that the LiPF6 salt possesses a high capability to absorb water molecules at high temperatures of approximately 40 °C [20]. Thus, at higher compositions of LiPF6 salt, more moisture is absorbed, which increases the mobility of the PEO–LiPF6 system. Consequently, water molecules form hydrogen bonds with PEO and disrupt the linkage between PEO–LiPF6 systems [17]. To analyse the mechanisms of ionic conduction, PEO–LiPF6 systems were investigated at a temperature range of 298 to 373 K, as shown in Fig. 4.

The initial increase in conductivity is presumably due to an increase in the number of charge carriers in the matrix. For higher concentrations of LiPF6, the charge build-up is offset by the retarding effect of ion aggregates such as ion pairs and ion triplet formation, which causes constraints in ionic and polymer segmental mobility [1]. Hence, the conductivity reaches the maximum value at 20 wt.% LiPF6 complex. This shows a balance between the two opposing forces, which is an increase in the number of charge-carrier ions and a decrease in ionic mobility [1].

Even though pure PEO is an insulator, pure PEO shows a conductivity of 10−10 at room temperature. It may be noted that the small value of conductivity in pure polymers is commonly due to the presence of impurities trapped during the preparation of the complex. The polar and flexible PEO main chain dissociates LiPF6 to generate carrier ions, and the migration of these ions through interchain and intrachain polymer segments in the amorphous region of the complex [28] is responsible for the increase in conductivity. The increase of salt concentration above 20 wt.% leads to a decrease in conductivity, which may be due to ion association in the polymer complexes by reducing the availability of vacant coordinating sites [28].

A number of studies have been conducted to investigate PEO used in polymer electrolytes. Ramesh et al. [17] proved that in a PEO–LiCF3SO3 system, a PEO having 12 wt.% LiCF3SO3 exhibits the highest conductivity with a value of 1.10 × 10−6 S cm−1, whereas a PEO with 5 wt.% Li2SO4 possesses the lowest conductivity with a value of 7.27 × 10−10 S cm−1. Paruthimal et al. [18] investigated PEO complexed with KI salt and obtained a conductivity of about 6.23 × 10−5 S cm−1 at 303 K for a molar ratio of 12:1 (PEO/KI) [18]. Siva Kumar et al. [19] studied PEO complexed with NaClO3 and obtained a conductivity value of 3.36 × 10−7 S cm−1 at 303 K. In this research, the conductivity for pure PEO is found to be 5.28 × 10−10 S cm−1 at 303 K, and the value increases gradually to 6.76 × 10−5 S cm−1 when 20% LiPF6 was added into the PEO.

DSC studies

Figure 5 shows the DSC curves of complexes at various weight percents of LiPF6. A sharp endothermic peak was observed at a temperature near 65 °C for pure PEO during the heating process, as shown in Fig. 5a. The addition of salt causes peak broadening as well as movement of its position to lower temperatures. Table 1 summarizes the DSC results. It shows that the melting temperature of PEO complexes with LiPF6 decreases with increasing LiPF6 concentration. The decrease in PEO melting temperature by LiPF6 addition also indicates the complexation of LiPF6 and PEO. The destruction of PEO crystallinity is clearly observed. This result is also confirmed by the XRD analysis, as shown in Fig. 6.

Fig. 5
figure 5

DSC curves of salted polymer electrolyte at various weight percents of LiPF6. a 0, b 5, c 10, d 15, e 20

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Fig. 6
figure 6

XRD pattern of salted polymer electrolyte at various weight percents of LiPF6. a 5, b 10, c 15, d 20

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XRD studies

The X-ray diffractograms for various concentrations of PEO-LiPF6 complexes are shown in Fig. 6. The crystalline nature for each sample is compared with that of pure PEO. Sharp and intense diffraction lines occur at 2u-20° and 24°, indicating the crystalline nature of pure PEO [23]. The intensities of PEO peaks at 2u-20° and 24° are observed to decrease upon the addition of salt. However, it can be seen that the diffraction pattern resembles that of pure PEO. The sharpness of the diffraction lines remains unchanged, suggesting that a portion of the PEO remains salt-free, and the other portion of PEO containing salt transforms to an amorphous phase. This indicates that the crystalline phase of uncomplexed PEO exists in the solid electrolyte films. As the salt concentration increases, it can be observed that there are changes in 2u from 2u-24° to 2u-23.3° and 2u-20° to 2u-19.2°. In this salt concentration regime, it can be noticed that the intensities of the diffraction peaks at 2u-23.3° and 2u-19.2° decrease slightly. Therefore, the angles 2u-23.3° and 2u-19.2° appear to be the preferred crystallographic direction, whereby an interaction of PEO and salt takes place. The variation in intensity of the diffraction peaks at different doping salt concentrations from 5 to 20 wt.% of LiPF6 indicates the increase in the number of preferred oriental ions for each concentration, which leads to an increase in electrical conductivity in the salt concentration regime. New peaks at 2u-29.4° begin to appear in the diffractogram for films with 20 wt.% of LiPF6, showing the formation of a new crystalline phase. The new peaks suggest the formation of crystalline PEO–LiPF6 complexes.

FTIR studies

Figure 7 depicts the IR transmittance spectra of the samples recorded at room temperature in the region of 4,000–500 cm−1. FTIR transmittance bands' positions and assignments of the individual materials are listed in Table 2. It is observed that peaks occur between 4,000–3,100 cm−1, corresponding to the OH group. The peaks broaden with an increase in LiPF6 salt concentration. This is due to the decomposition of LiPF6 to solid LiF and gases of OPF3 and HF, which is represented by the following equation [29]:

Fig. 7
figure 7

FTIR spectra of pure PEO and various weight percents of LiPF6 in complexes a 0, b 5, c 10, d 15, e 20

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Table 2 FTIR transmittance bands' positions and their assignments
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$$ {\hbox{LiP}}{{\hbox{F}}_{{6}}}{ + }{{\hbox{H}}_{{2}}} \to {\hbox{LiF}}\left( {\hbox{s}} \right) + {\hbox{OP}}{{\hbox{F}}_{{3}}}\left( {\hbox{g}} \right) + {\hbox{HF}}\left( {\hbox{g}} \right) $$
(2)

As LiPF6 salt concentration increases, the intensity of symmetric stretching vibration band of pure PEO increases and shifts to 2,869 cm−1 (Fig. 6a), 2,873 cm−1 (Fig. 6b–c) and 2,890 cm−1 (Fig. 6d). This is due to the additional amount of Li+ in the complexes. The anti-symmetric stretching vibration band at 1,100 cm−1 of pure PEO is shifted to 1,094 cm−1 (Fig. 6a), 1,090 cm−1 (Fig. 6b–c) and 1,110 cm−1 (Fig. 6d) as LiPF6 concentration increases. The peaks broaden with the exception of 20 wt.% of LiPF6. This may be due to the ion aggregates such as ion pairs and ion triplet formation, which cause constraints to ions and polymer segmental mobility. This explains the nature of conductivity result for 20 wt.% of LiPF6 shown in Fig. 3. A similar reaction occurs to the twisting vibration bands at 842 and 963 cm−1 for pure PEO. When the weight percent of LiPF6 is increased, the peaks are shifted to 839 and 944 cm−1 (Fig. 6a), 945 and 837 cm−1 (Fig. 6b), 945 and 835 cm−1 (Fig. 6c) and 963 and 844 cm−1 (Fig. 6d). The peaks' intensities are increased with the exception of 20 wt.% of LiPF6.

Conclusions

PEO-based polymer electrolyte systems with various weight percents of LiPF6 have been synthesized using solution casting technique. The system with 20 wt.% LiPF6 exhibits a maximum conductivity of 10−5 S cm−1 at room temperature. The temperature dependence of ionic conductivity of the electrolytes obeys the VTF relationship. At the molecular level, FTIR studies provide strong evidence that there is a specific interaction between PEO and LiPF6. The change in peak intensity, shape and position confirms the complexation process of PEO–LiPF6 systems. The DSC curves indicate that the addition of various weight percents of LiPF6 on polymer electrolytes decreases the melting transition temperature (T m) and degree of crystallinity of the system.