Introduction

Magnesium-based solid polymer electrolyte (SPE) membranes have been studied for their potential application in magnesium solid state batteries [1]. A wide variety of magnesium salts were used, for example, MgCl2 [2], Mg(NO3)2 [3], Mg(CH3COO)2 [4], MgSO4 [5], Mg(ClO4)2 [6], Mg(CF3SO3)2 or Mg(Tf)2 [7], and Mg(N(CF3SO2)2)2 or Mg(TFSI)2 [6], dissolved in various polymeric systems, such as polyethylene oxide (PEO) [2], polyvinyl acetate (PVA) [3], polyvinyl pyrrolidone (PVP) [5], polyethylene carbonate (PEC) [6], polyethylene glycol (PEG) [4], and polyallyl glycidyl ether (PAGE) [8]. Likewise, there are several reports on SPE membranes prepared with copolymers and polymer blends as poly(ε-caprolactone-co-trimethylene carbonate) PCL-PTMC [9], biopolymers [10], the mixtures PEO-PVDF [11], and PEO-PVP [12].

The SPE membranes have low conductivities, therefore, is necessary to carry out various strategies to increase the ion mobility. Among the most used is gelation with solvents such as glymes (mono-, di-, tri-, and tetra-) [13] and carbonate esters (ethylene-, propylene-, or diethylene-) [14]. On the other hand, the plasticizers succinonitrile ((NCCH2)2) [15] and recently urea ((NH2)2CO) [16] were studied as additives to polymer magnesium salt system. A different approach for nanoparticles (MgO, TiO2, Al2O3, SiO2, and B2O3) as ionic conductivity changing agents in SPE membranes showed promising results [17].

The PEO is the most studied polymer [2], but its electrochemical and mechanical properties are poor. However, if a second polymer as polyvinylidene fluoride (PVDF) [11] or PVP [12] is added, these properties are enhanced. Dhatarwal and Sengwa [18] reported a decrease in the intensity of the X-ray peaks as the PVDF content in the polymer mixture increases. Considering the effect of incorporation PVDF into PEO, it is desirable to have a larger amount of PEO in the mixture since the oxygens in the PEO chains are responsible for cation solvation. Otherwise, a problem may arise because PVDF is immiscible in small amounts in PEO [19]. Broadly, the mechanical and electrochemical performance of SPE membranes prepared using PVDF as an additive can be improved [20]. It was observed in the literature [21] that adding 10 wt. % of PVDF to PEO and combined it with an electrolytic salt of either lithium, sodium, or magnesium provides a solid polymeric structure suitable for preparing SPE membranes with acceptable ionic conductivity.

In the last years, ionic liquids (ILs) have been used as plasticizing or gelling agents to increase conductivity in SPE membranes. Maheshwaran et al. [22] published a study on the effect of 1-ethyl-3-methyl imidazolium tetrafluoroborate [EMIM][BF4] incorporation within PEO/Mg(CF3SO3)2 system, reporting a marked increase of ionic conductivity and Mg2+ ion transport number ∼ 0.22. Tang et al. [23] elaborated an electrolytic system formed by the copolymer polyvinylidene fluoride-co-hexafluoropropylene PVDF-HFP and the salt Mg(CF3SO3)2 modified by the ionic liquid 1-ethyl-3-methyl imidazolium trifluoromethane sulfonate [EMIM] [CF3SO3], which decreased the crystallinity in PVDF-HFP and increased the conductivity of SPE membranes. Rathika et al. [11] prepared solid polymer blend electrolytes by optimized blend of PVA-Mg(CF3SO3)2-[EMIM][CF3SO3] system and the maximum ionic conductivity obtained at room temperature was 1.2 × 10−5 S cm−1 with the blend containing 15 wt. % salt.

While ILs are relatively expensive, Gupta et al. [24] presented a relatively cheap alternative variant using the ethyl sulfate anion (SO4CH2CH3- or ES-), which displayed an electrochemical window of nearly 4 V in glassy carbon electrodes. Also, the magnesium ion is electrochemically active with polyaniline/platinum cathodes when dissolved in the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][ESO4] [25]. Therefore, ionic liquid [EMIM][ESO4] is a viable candidate as an additive in SPE membranes that uses magnesium salt due to their excellent properties, such as high thermal stability, high ionic conductivity, low viscosity, wide electrochemical stability window, and protic nature [26,27,28,29].

The type and percentage of magnesium salt are important for possible technological application in the future. Salts with anions [CF3SO3]− and [N(CF3SO2)2]− exhibit remarkable properties but at an unaffordable cost. Other anions such as NO3− and SO42− show a smaller electrochemical window, lower conductivities, and are therefore not entirely desirable. Contrarily, the ClO4− anion has a wide electrochemical window and consequently the Mg(ClO4)2 salt has been tested in polyelectrolyte systems [6].

To the best of our knowledge, there exists no previous experimental work describing the development of SPE membranes based on PEO/PVDF/Mg(ClO4)2-[EMIM][ESO4] system. Hence, the purpose of this work is to prepare SPE membranes  in which the Mg(ClO4)2 salt was selected as Mg2+ ions supplier, to understand the effect of adding ionic liquid [EMIM][ESO4] within the SPE-based electrolyte, evaluating their electrochemical performance in magnesium battery application.

Experimental section

Polyethylene oxide (PEO, Sigma Aldrich Mv ~ 900,000), polyvinylidene fluoride (PVDF, Sigma Aldrich Mw ~ 534,000), 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM] [ESO4], Sigma Aldrich, purity ≥ 95 %), and magnesium perchlorate (Mg(ClO4)2, Sigma Aldrich ACS reagent) were used to prepare SPE membranes. Acetone (ACE, Sigma Aldrich ACS reagent, purity ≥ 99.5 %) and dimethylacetamide (DMAc, Sigma Aldrich ReagentPlus, purity ≥ 99 %) were used as solvents in a 70:30 vol. % ratio, respectively. Stoichiometric amounts of PEO:PVDF (90:10 wt. %), Mg(ClO4)2, and [EMIM][ESO4] were added to a glass vial in 5 ml solvent mixture; this mixture was sonicated for 12 h at 40 °C. Subsequently, the viscous mixture was cast over Teflon dishes and solvents were evaporated to obtain free-standing SPE membranes with a thickness from 200 to 300 µm. The SPE membranes were dried at 50 °C on a heating plate and then under vacuum to be stored in a dry box with an argon atmosphere. Table 1 lists all the mixtures prepared with the materials. The concentration of IL, magnesium salt, and polymers in SPE membranes was determined by the molar ratio of CH2–O–CH2 (EO)/Mg2+/IL.

Table 1 Raw material properties and SPE membranes showing the percentage of crystallization
Full size table

Structural characterization

The structural characterization of the materials was analyzed by X-ray diffraction (XRD, D2 Phaser, Bruker) and Fourier transform infrared spectroscopy (FTIR, Interspec 200-X). The thermal stability was evaluated by differential thermal analysis (DTA-50, Shimadzu), and the semicrystalline nature of the SPE membranes was evidence with optical microscopy studies (MV-439, National Instruments).

Coin cell assembly

CR2032 coin cells, made of SS316 for its excellent properties [30], were assembled with two configurations: SS||SPE||SS and Mg||SPE||Mg, to determine the total ion transport number and the Mg2+ ion transport number, respectively. The impedance of the Mg||SPE||Mg cell was measured immediately before and after polarization. Electrochemical impedance spectroscopy (EIS) was evaluated in the frequency range of 1 Hz to 1 MHz by applying 10 mV amplitude signal. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) studies were performed using a VMP3 potentiostat/galvanostat (Biologic Science Instruments) at scan rate of 5 mV s−1. All cells were assembled in a dry glove box (Omni-Lab 0210, VAC) filled with argon and H2O < 1 ppm.

Results and discussion

X-ray diffraction studies

Figure 1a displays the XRD pattern of raw materials, while Fig. 1b shows the XRD pattern of the SPE membranes compared to the base membrane 16 Mg(ClO4)2–84(PEO:PVDF (90:10 wt. %)) or SPE:IL (100:0 wt. %).

Fig. 1
figure 1

XRD pattern of a PEO, Mg(ClO4)2, PVDF, and b SPE membranes

Full size image

Based on Fig. 1a, PEO presents a maximum peak intensity at 23.6°, along with a second intense peak at 19.5°. Two lower intensity peaks can be observed at 26.6° and 27.2°, corresponding to diffractions from (112), (120), (131), and (041) planes, respectively, according to ICDD crystallographic file 00–057-1528. PVDF exhibits two main peaks at 18.5° and 20.0° and another minor peak at 26.7°, corresponding to crystallographic planes (110), (020), and (021), respectively, according to crystallographic file JCPDS No. 44–0141 [31]. The Mg(ClO4)2 shows three main peaks at 21.5°, 23.0°, and 31.6° corresponding to (021), (121), and (221) planes, respectively, according to JCPDS crystallographic file No. 85–0609 [32].

From Fig. 1b, the diffractogram of the SPE membrane with a composition PEO:PVDF (90:10 wt. %) exhibits a clear decrease in the intensity counts of the main peaks (5 times relative to PEO) located at 23.0° and 18.8°. Moreover, the disappearance of the main peaks associated with PVDF (18.5° and 20.0°) was observed, indicating that the polymeric chain of PVDF can plasticize PEO and significantly reduce its degree of crystallinity, which is helpful for the transport of Mg2+ ions [12]. In addition, other minor peaks were found in the diffractogram of the SPE membrane with composition PEO:PVDF (90:10 wt. %) located at 26.2°, 27.0°, 33.3°, 36.1°, and 39.6°, associated with PEO.

In the SPE membrane SPE:IL (100:0 wt. %), the (021) and (221) planes of Mg(ClO4)2 were not observed, possibly due to its efficient incorporation into the PEO-PVDF (90:10 wt. %) system. As a result of this incorporation, there is a decrease in crystallinity as well as a reduction in peak intensity counts from 3000 to 1500, which contributes to the ionic conductivity of the SPE membranes [33].

The Mg(ClO4)2 and [EMIM][ESO4] incorporation into SPE membranes produce an increase in the 10° to 40° “hump,” showing a decrease in crystalline nature of PEO, previously mentioned by number of counts, with a 10 wt. % PVDF addition. To quantify these observations, a formal analysis [34] using the percentage of crystallinity (\({X}_{c}\)) was carried out using the area of crystalline peaks (\({A}_{C}\)) and the combined area of crystalline and amorphous peaks (without “hump”) and diffractogram total area (\({A}_{T}\)), from 10 to 40°.

$${X}_{c}=\left(\frac{{A}_{C}}{{A}_{T}}\right)\cdot 100$$
(1)

Table 1 shows the crystallization percentage results (calculated by Origin Pro) for diffractograms shown in Fig. 1. These observations confirm a considerable decrease from 53 to 23 % in degree of crystallization when only 10 wt. % of PVDF was added to PEO network.

The percentage of crystallinity gradually decreased with the addition of Mg(ClO4)2 in the SPE membranes. However, the crystallinity did not decrease with the incorporation of the ionic liquid, except in the case of the SPE membrane SPE:IL (80:20 wt. %), indicating a stronger plasticizing effect with a small proportion of PVDF in the mixture.

A shift toward lower Bragg angles was likewise observed with the addition of IL at concentrations above 20 wt. %, indicating an effective interaction between IL and the PEO semicrystalline interlaminar layers, which increases the interlaminar spacing value. Besides, an increase in crystallite size was observed, as evidenced by interlaminar spacing measurements, SEM studies, Fig. S1, and Table S1 in the supplementary information.

FTIR studies

Figure 2 displays FTIR spectra of starting materials (Fig. 2a) and the prepared SPE membranes (Fig. 2b). The spectra of PEO and PVDF were consistent with the FTIR spectra, published elsewhere [18]; the spectrum of Mg(ClO4)2 is coincident with FTIR spectra reported by Reddy and Chu [35]; and the spectrum of IL [EMIM][ESO4] was similar to FTIR spectra reported by Nkuna et al. [36]. The SPE membrane formed by the incorporation of PVDF to the PEO matrix (10:90 wt. %, respectively) generally displays typical PEO bands, such as C–O stretching at 833 cm−1 and CH2 asymmetric bending at 952 cm−1. Triplet splittings formed by symmetric and asymmetric stretching of C–O–C band were observed at 1055, 1100, and 1150 cm−1 due to the effect of crystallinity decrease in PEO by PVDF addition. The vibrational bands observed in the wavenumbers 1240 and 1280 cm−1, corresponding to CH2 symmetric and asymmetric torsions, the doublet at 1345 and 1355 cm−1, as well as 1471 cm−1, were assigned to CH2 bending. For relatively low percentage of PVDF in the base membrane (10 wt. %), fewer characteristic signals were observed for this compound, which were located at 875 cm−1, corresponding to CH2 stretching of PVDF, and possibly band at 750 cm−1 with assignment to CF2 bending (both signals assigned to the PVDF α phase). The addition of Mg(ClO4)2 to PEO:PVDF (90:10 wt. %) system with a 16 wt. % to form a SPE membrane shows some characteristic weak bands, such as ClO4− asymmetric bending at 624 cm−1 and 1405 cm−1, and a small band corresponding to ClO4− symmetric stretching [37]. On the other hand, the strong vibration of Mg(ClO4)2 at 1642 cm−1 disappears completely in all SPE membranes. These observations suggest the possible interaction of the Mg2+ ions of salt with ether oxygen of PEO [22]. The absorption bands at 755, 905, 1015, 1175, 1210, and 1570 cm−1 were assigned to C–O–S–O bending, C–O–SO3 system, C–O–SO3 symmetric stretching, an imidazolium ring asymmetric stretching in-plane, C–O–SO3 asymmetric stretching, and C = N stretching, respectively [38]. In particular, it was observed an increase in the band at 1015 cm−1, as well as decrease in the band at 1150 cm−1, as the ionic liquid in the base membrane increases, which suggests possible interactions of [EMIM]+ cation with the oxygens in the PEO polymeric chain [39].

Fig. 2
figure 2

FTIR spectrum of a PEO, Mg(ClO4)2, PVDF, and b SPE membranes

Full size image

Thermal studies

Figure 3 shows a DTA analysis performed in SPE membranes. PVDF showed a melting temperature (Tm) at 157 °C and PEO at 65 °C; similarly, the SPE membrane PEO:PVDF (90:10 wt. %) showed both Tm (Fig. 3a). The PEO thermogram evidenced a typical semi-crystalline to crystalline transformation process at 67.4 °C (see Table 1). The addition of PVDF indicates a slight reduction in the transition temperature to 65 °C as a function of the highest percentage (90 wt. %) was constituted by PEO. The addition of Mg(ClO4)2 to PEO:PVDF (90:10 wt. %) system produces a slight increase in the transition at 58.6 °C and the addition of 10 wt. % ionic liquid produces a decrease with a transition temperature at 58.0 °C, which increases with the increment of up to 20 % of [EMIM][ESO4] with a value of 59.2 °C. Therefore, a higher ionic liquid addition produces reductions at the melting points, as seen in Fig. 3b. On the other hand, both the transition enthalpy and the relative percentage of crystallization (% XC,el = 100(∆Ht,membrana)⁄(∆Ht,PEO)) show a decrease due to the PVDF addition (50 %, see Table 1). Additionally, both are reduced with the addition of 12 wt. % Mg(ClO4)2. These results, added to XRD analysis, where a decrease in crystallinity with the addition of ionic liquid from 10 wt. % to 20 wt. %, and an increase in crystallinity percentage with the addition of [EMIM][ESO4]. This suggests that the addition of ionic liquid has a significant impact on the crystallinity of the mixture [40]. The largest reduction in crystallinity was produced by the combined addition of 10 wt. % PVDF to PEO and 16 wt. % Mg(ClO4)2, and moderate increases in crystallinity were achieved with the addition of ionic species such as Mg2+, [ClO4]−, [EMIM]+, and [ESO4]− (see Figs. S2, S3, and S4 in the supplementary information for optical microscopy studies and density functional theory (DFT) studies, respectively).

Fig. 3
figure 3

DTA curves of a PEO:PVDF (90:10 wt. %), PEO, PVDF, and b SPE membranes

Full size image

Ionic conductivity

The temperature effect on SPE membranes with different ionic liquid contents is shown in Fig. 4. Conductivity measurements were performed by intercalating SPE membranes between two stainless steel (SS) separators. The dependence of ionic conductivity on temperature was recorded over a range from 30 to 100 °C. An expected behavior of conductivity in SPE:IL (100:0 wt. %), with a certain degree of amorphization, was observed. This behavior can be easily modeled using the VTF equation (Vogel-Tammann-Fulcher) [41]. The addition of ionic liquid [EMIM][ESO4] at 10–20 wt. % in the base membrane produced a discontinuity around the thermal transition zone or from the semi-crystalline to amorphous state in SPE membranes. The discontinuity disappeared when higher amounts of [EMIM][ESO4] (30–40 wt. %) were added. The results confirmed the plasticizing role of [EMIM][ESO4] in SPE membranes, reducing the degree of crystallization, as noted in XRD and DTA studies. Furthermore, this effect has been widely reported in the literature [42]. On the other hand, a quasi-linear behavior of the logarithm of conductivity (log σ) versus reciprocal temperature (1/T), denoting a typical Arrhenius plot, was observed at temperatures below or above the transition zone. This behavior was modeled using the following equation:

Fig. 4
figure 4

Arrhenius plot of temperature dependence ionic conductivity in SPE membranes. Lines are just guides to the eye. ACE boiling point is 329 K

Full size image
$$log\left(\frac{\sigma }{{\sigma }_{o}}\right)=-\frac{{E}_{a}}{RT}$$
(2)

The conduction process activation energy (\({E}_{a}\)), pre-exponential factor (collision frequency) (\({\sigma }_{o}\)), universal gas constant (R) (8.6 × 10−5 eV K−1), and absolute temperature (T) [22] are shown in Table 2. A constant decrease was observed as the ionic liquid concentration increased at room temperature. At 80 °C (a temperature above the semicrystalline-amorphous transition), a large decrease in activation energy was noted, indicating that temperature plays a major role, while the effects of [EMIM][ESO4] addition play a minor role.

Table 2 Electrical properties of SPE membranes
Full size table

Electrochemical properties and transport number

The total ion transport number (\({t}_{ion}\)) was estimated by chronoamperometry (CA) measurements. In this method the polarization current was controlled as a function of time for the SPE membrane SPE:IL (60:40 wt. %) intercalated between two stainless steel blocking electrodes, employing a symmetrical SS||SPE:IL (60:40 wt. %)||SS cell configuration. A 0.75 V DC was applied across the sample, and the \({t}_{\mathrm{ion}}\) of the SPE membrane was estimated using Wagner’s method [43], with the following equation:

$${t}_{\mathrm{ion}} = \frac{{I}_{i}-{I}_{f}}{{I}_{i}}$$
(3)

where \({I}_{i}\) (~ 2 6.78 µA) is the initial current and \({I}_{f}\) (~ 0.43 µA) is the final steady-state current (Fig. 5a). The \({t}_{\mathrm{ion}}\) value was obtained as 0.98, suggesting the purely ionic nature of the solid polymer electrolyte system.

Fig. 5
figure 5

a Chronoamperometric curve at an applied voltage of 0.75 V and 20 mV on symmetrical cells; SS||SPE:IL (60:40 wt. %)||SS and b Mg||SPE:IL (60:40 wt. %)||Mg, respectively. Inset: Impedance curves for symmetrical cell; Mg||SPE:IL (60:40 wt. %)||Mg. c Cyclic voltammograms of symmetrical cells; SS||SPE:IL (60:40 wt. %)||SS and Mg||SPE:IL (60:40 wt. %)||Mg. All the electrochemical measurements were carried out at room temperature (30 °C)

Full size image

To determine the transport number (\({t}_{{\mathrm{Mg}}^{2+}}\)), a symmetric cell (Mg||SPE:IL (60:40 wt. %)||Mg) was evaluated using a combination of impedance spectroscopy and polarization studies, as suggested in the literature [44,45,46,47,48,49,50,51], by applying small constant voltage of 20 mV for 1 h. The \({t}_{{\mathrm{Mg}}^{2+}}\) value was calculated from Eq. (4):

$${t}_{{\mathrm{Mg}}^{2+}} = \frac{{I}_{s}(\Delta V-{R}_{o}{I}_{o})}{{I}_{o}(\Delta V-{R}_{s}{I}_{s})}$$
(4)

where \({I}_{o}\) (~ 8.29 µA) and \({I}_{s}\) (~ 2.90 µA) are the initial and final steady state currents, and \({R}_{o}\) (~ 1123 Ω) and \({R}_{s}\) (~ 3116 Ω) are the cell resistance before and after applying the polarization voltage, respectively, as seen in Fig. 5b. Using Eq. (4), the magnesium-ion-transport number is determined to be 0.34. This indicates significant Mg2+ ion transport number for the SPE system of this report.

Furthermore, Fig. 5c displays CV tests in symmetrical cells recorded at a scan rate of 5 mV s−1. No redox peaks were observed in the SS||SPE:IL (60:40 wt. %)||SS cell, in contrast to the Mg||SPE:IL (60:40 wt. %)||Mg cell, which exhibited reversible redox peaks. This confirms Mg2+ ions transfer at the electrolyte/electrode interface [52].

Finally, LSV measurement was employed using SS as working electrode and magnesium disc as counter electrode (Fig. 6), obtaining a high value of ~ 4.2 V, whose electrochemical stability window allowed its application in magnesium ion batteries (see Fig. S5 in the supplementary information for electrochemical performance studies).

Fig. 6
figure 6

LSV test for the cell configuration SS||SPE:IL (60:40 wt. %)||Mg measured at a scan rate of 5 mV s.−1

Full size image

Conclusions

Solid polymer electrolyte (SPE) membranes based on a PEO/PVDF/Mg(ClO4)2-[EMIM][ESO4] ternary system were characterized by structural and thermal studies. XRD analysis showed that the SPE membranes had a semicrystalline structure, which promoted good Mg2+ ions diffusion. FTIR analysis showed coordination between the EMIM+ cations and Mg2+ ions with the oxygens in the PEO polymer, as well as complex formation inside the SPE membrane with a concentration of PEO/PVDF/Mg(ClO4)2–40 wt. % of [EMIM][ESO4]. The DTA study confirmed that Tm decreased as the IL concentration increased, suggesting an increase in SPE membrane chain flexibility. An ionic conductivity of ~ 5.4 × 10−5 S cm−1 was observed for the SPE membrane SPE:IL (60:40 wt. %) from the Arrhenius plot, which also showed a total ion transport number of ~ 0.98. These results support a higher contribution of Mg2+ ions and a negligible contribution of electrons (~ 0.02) within the SPE system, as well as a high Mg2+ ion transport number (~ 0.34). Preliminary studies on rechargeable batteries with a Mg||SPE:IL (60:40 wt. %)||MoS2 configuration showed a discharge capacity of 40 mAh g−1.