1 Introduction

Many types of ionic conducting polymers, generally classified as polymer electrolytes, have been developed and characterized in recent years. Special interest today is focused on composite gel polymer electrolyte systems having high ionic conductivity at ambient temperatures, since they may find unique applications, for example in rechargeable lithium batteries, separators, and fuel cells. Polyacrylonitrile (PAN) [1], poly(vinylidene fluoride) (PVDF) [2–4], poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) [5, 6] and poly(methyl methacrylate) (PMMA) [7–9] are the most common host polymers used for preparing gel electrolytes. Ceramic fillers such as SiO2 [10], Al2O3 [11], TiO2 [12] and BaTiO3 [13] have been incorporated along with the host polymer in order to obtain composite polymer electrolytes with improved electrical and mechanical properties. The ceramic fillers can promote electrochemical properties, but only by physical action without directly contributing to the lithium ion transport process. By suitable surface modification of the ceramic particles, they can also act as the source of charge [14, 15].

These considerations motivated us to modify the polymer matrix by introducing inorganic materials containing dissociative lithium ions. In this study, CMGPE based on PVDF which was doped with SiO2(Li+) were prepared and characterized.

2 Experimental

2.1 Materials

PVDF homopolymer (Mn, ca. 1.66 × 105; Mw, ca. 3.31 × 105; Mw/Mn = 1.99, Shanghai Ofluorine Chemical) was used as polymer matrix and Tetraethylorthosilicate(TEOS,Sinopharm Chemical Reagent Co., Ltd) γ-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560, Guotai-Huarong Chemical) were employed to provide silica source. LiOH·H2O(Sinopharm Chemical) was used as received without further purification. Organic liquid electrolyte (EC/EMC/DMC 1:1:1(W/W/W) LiPF61 mol/l, Guotai-Huarong Chemical).

2.2 Synthesis of SiO2(Li+)

TEOS and KH560 were dissolved in ethanol at mol rate of 2:1, a little of concentrated hydrochloric acid solution (36 wt%) was added into the obtained solution to adjust its pH to about 2.0. The mixture were heated to 75 °C with magnetic force stirring for 6 h to get the silica SiO2, then calculated amounts of standard solutions of the lithium hydroxide in methanol were added. The reaction last for about 2 h until silica SiO2(Li+) were prepared. And the white powder SiO2(Li+) can be obtained after being dried under vacuum at 100 °C for 24 h.

2.3 Preparation of Composite Microporous Gel Polymer Electrolyte

The CMGPEs doped with SiO2(Li+) were fabricated by standard solution-casting technique coupled with traditional phase inversion method. A proper amount of PVDF was first dissolved in the solvent (N,N-dimethylformamide, DMF), it was stirred for about 18 h at room temperature. Then, a proper amount of SiO2(Li+) were added into the polymer solution and the solution were stirred for other 6 h to make them dissolved completely. The relative weight ratio of SiO2(Li+) to PVDF was varied from 1 to 10 wt%. The polymer solution was cast on a glass with a stainless steel blade in a thickness of ~150 μm, then it was put into methanol for 45 s, and then dried in the vacuum oven at 100 °C for 24 h. To get the CMGPEs, the prepared membranes were transferred to the glove box and immersed in the organic liquid electrolyte. After activation, the membranes were removed from the liquid electrolyte and excess electrolyte solution on the surface was wiped with a filter paper. For convenience, different samples obtained from different relative concentration of SiO2(Li+) to PVDF were denoted as PVDF–X %SiO2(Li+).

2.4 Measurements

FT-IR measurements were carried out on BRUKER VECTOR-22 spectrometer at room temperature. The spectra were collected over the range 400–4,000 cm−1 by averaging 128 scans at a maximum resolution of 2 cm−1.

DSC measurements were carried out using a DSC Q100 (TA Instruments) (TA Instruments, USA) over a temperature of 20–200 °C at a scan rate of 10 °C/min. All the thermograms were base line corrected and calibrated using Indium metal. The experimental specimens (8–10 mg) were dried at 60 °C under vacuum for 24 h before being measured. All the samples were firstly annealed at 120 °C for 3 min, cooled to 20 °C using liquid nitrogen and then scanned for the measurement.

X-ray diffractions analysis of the polymer films were performed using a D/MAX-RA X-ray diffractometer (Rigaku) with 2θ values between 3°and 60°.

Morphology of the composite polymer membranes were observed by scanning electron microscopy (SEM) (XL30-ESEM, PHILIPS) under vacuum.

Tensile properties of the composite polymer membranes were measured through a 10KN electromechanical tensile testing machine (CMT5104, China) at room temperature. The tensile speed was 40 mm/min.

The porosity (P) of CPE membranes can be calculated by Eq. (1) after the membranes were immersed into n-butanol for 30 min, where ρa and ρb are the density of n-butanol and the dry CPE membrane, respectively; ma and mb are the mass of the membrane with n-butanol and the dry membrane, respectively.

$$ P = \frac{{{\text{m}}_{\text{a}} /{{\uprho}}_{\text{a}} }}{{({\text{m}}_{\text{a}} /{{\uprho}}_{\text{b}} ) + {\text{m}}_{\text{b}} /{{\uprho}}_{\text{b}} }} $$

Swelling behavior of the CMGPEs were processed in a dry glove box. The film samples were first cut into a specimen of 2 cm in diameter. After the mass was weighted, they were immersed in the liquid electrolyte. The CPEs were taken out at different times until no weight changed. The swelling capability of electrolyte solution contents was determined as follows: Sw = 100(W−W0)/W0, where W0 and W were weights of the initial and wet films.

The ionic conductivity (σ) of the CMGPEs were determined by AC impedance spectroscopy (EG&G Model 273A potentiostat). It was sandwiched between two parallel stainless steel (SUS) discs (Ф: 1 cm) and mounted in a sealed coin cell to prevent contamination of the sample. The frequency ranged from 100 kHz to 10 Hz at a perturbation voltage of 5 mV. The ionic conductivity (σ) was then calculated from the electrolyte resistance (Rb) obtained from the intercept of the Nyquist plot with the real axis, the membrane thickness(l), and the electrode area(A) according to the equation σ = l/ARb.

The electrochemical stability window of the semi-IPN gel polymer electrolyte was measured by linear sweep voltammetry (LSV) at a scanning rate of 0.05 V/s. Three electrode-laminated cell was assembled inside a glove box. Stainless steel (SS) was used as working electrode and lithium metal was used both as a counter and as a reference electrode. LSV measurement was carried out using CHI660A (CH Instruments. Inc).

3 Results and Discussion

3.1 FT-IR Spectrum Measurements

Figure 1 shows the FT-IR spectrum for SiO2(Li+). The characteristic absorption peaks 3,440 cm−1(νSi–OH), 2,940 cm−1(νC–H), 1,060 cm−1(δas,Si–O–Si), and 1,785 cm−1(νC=O) are observed in IR spectrum, and the disappearance absorption peaks of 950–815 cm−1 for epoxy bond means that LiOH have reacted with the modified SiO2 and SiO2(Li+) were prepared according to above reaction.

Fig. 1
figure 1

FT-IR spectrum for SiO2(Li+)

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Figure 2 shows the characteristic FT-IR spectra for PVDF and PVDF-5 %SiO2(Li+) respectively. α- and γ-phases of PVDF characterizing bands are observed in the range of 400–1,600 cm−1. The assignment of FT-IR spectra of the samples have reported as follows: α-phase (TGTG−); 472 and 509 cm−1 (CF2 bending), 1,070 cm−1 (CH2 wagging) and γ-phase (TTTGTTTG−); 841 cm−1 (CH out-of-plane deformation and CH2 rocking), 881 cm−1 (CF2 symmetric stretching), 1,280 cm−1 (CF out-of-plane deformation), 1,400 cm−1 (CH2 wagging) [16]. The enhancement of the intensity of absorption peak in the range of 1,280–1,070 cm−1 means that the SiO2(Li+) is added into the composite polymer membrane. However, with the addition of SiO2(Li+), no visible shifting for PVDF characterizing bonds are observed.

Fig. 2
figure 2

FT-IR spectra of a pure PVDF, b PVDF-5 %SiO2(Li+)

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3.2 DSC Measurements

Figure 3 displays the typical DSC thermograms obtained for composite polymer membranes varied with the content of SiO2(Li+), and their dynamic data are listed in the Table 1.

Fig. 3
figure 3

DSC curves for composite membranes a PVDF-1 %SiO2(Li+), b PVDF-2 %SiO2(Li+), c PVDF-5 %SiO2(Li+)

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Table 1 Properties of PVDF and the cross-linked copolymer coated PVDF
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It is found that there is a crystal melting peak from 150 to 170 °C which is attributed to partly crystallinity of the PVDF. With the content of SiO2(Li+) increases, the crystalline melting temperature(Tm) and crystallinity for the composite polymer membranes are both decreased. The result suggests that the polymer chains may be hindered by the cross-linking centers formed by the interaction of the Lewis acid groups ceramics (e.g. the –OLi groups and –OH on the SiO2(Li+)) with the polar groups (e.g. the –F groups of the polymer chains). So, the degree of crystallization of polymer matrix decreases with addition of SiO2(Li+). The added Silicas are far more receptive to the segmental chain motion of the polymer [17]. Such an interaction stabilizes the amorphous structure and enhances the ionic conductivity of CMGPEs [18].

3.3 TGA Measurement

Figure 4 shows the TGA results for pure PVDF membrane and composite polymer membrane (PVDF-5 %SiO2(Li+)) respectively. No significant weight loss are observed until 430 °C for the composite polymer membrane. It can improve the heat resistant of PVDF based composite polymer membrane by addition of SiO2(Li+). It is very interesting that amount of residue of PVDF membrane is higher than the composite membrane when the temperature exceed 500 °C. SiO2(Li+) is prepared by sol–gel reaction of TEOS and KH560, which has a few organic groups in the Silica. The decomposed organic groups after thermal decomposition of platforms may cause this phenomenon.

Fig. 4
figure 4

Thermograms of polymer membranes a pure PVDF, b PVDF-5 %SiO2(Li+)

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3.4 XRD Measurements

The X-ray diffraction patterns of composite polymer membrane varied with the content of SiO2(Li+) are presented in Fig. 2. It shows that the relative intensity of the broad diffraction peak at Brag angle of 2θ = 20.50° decreases with increasing of the concentration of SiO2(Li+), which suggests that the added SiO2(Li+) can lower the crystallinity of the polymer membrane. Moreover, the XRD pattern shows broader and fewer peaks after adding SiO2(Li+) into polymer matrix, which suggests that the CPE membrane may possess more amorphous areas for lithium-ion transfer. The interaction of PVDF chains between SiO2(Li+) may lead to lower crystallinity and rich amorphous phase for the composite polymer membrane. This result can be interpreted by considering the Hodge et al. [19] criterion which establishes a correlation between the height of the peak and the degree of crystallinity. In addition, no characteristic peak of SiO2(Li+) is found confirms that the particles is successfully encapsulated by polymer membranes. Thus, the X-ray study reveals the complex formation in the composite polymer membrane and the result is consistent with DSC result.(Fig. 5)

Fig. 5
figure 5

XRD patterns for PVDF based composite polymer membrane varied with different content of SiO2(Li+)

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3.5 SEM Measurements

Figure 6a–f show the microphotographs of composite polymer membranes varied with the content of SiO2(Li+) to PVDF. It can be found that the pore size of the membrane increased with the content of the SiO2(Li+). It might be caused by a portion of SiO2(Li+) dispersed in the methanol when the membrane was immersed in the solution. Presence of microspores in the membrane leaded to efficient uptake of the liquid electrolyte when it was soaked in an electrolyte solution, and finally they benefit to the gelation of the microporous membrane [20]. However, excess pores in the membrane could deteriorate the mechanical properties of the gel polymer electrolyte. When the content of the SiO2(Li+) reached 10 %, the prepared gel polymer electrolyte was very fragile which could not be handled by hand.

Fig. 6
figure 6

Scanning electron micrograph images of surface of the composite polymer membranes: a pure PVDF, b PVDF-1 %SiO2(Li+), c PVDF-2 %SiO2(Li+), d PVDF-5 %SiO2(Li+), e PVDF-10 %SiO2(Li+)

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3.6 Porosity and Swelling Measurements

It can be seen from Table 2 that the porosity of pure microporous PVDF membrane is 31.65 %, and equilibrium swelling ratio Sw is 287.32. From the result of SEM and porosity measurement, the pore size and porosity of the composite polymer membrane are both increased with the content of the SiO2(Li+), which can make the liquid electrolyte be absorbed and gelification easily, and lead to the higher ionic conductivity in the end. When the content of SiO2(Li+) is 5 %, the porosity of the composite polymer membrane and the Sw are 51.08 and 362.45 respectively, and the ionic conductivity at room temperature reaches 3.83 × 10−2S/cm.

Table 2 Physical properties and ionic conductivity of CMGPEs
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3.7 Stress–Strain Response

Figure 7 show typical engineering stress–strain behavior for the composite polymer membranes along the machine direction, and the results of the stress and strain for them are list in the Table 1. The elongation at break increases with increasing the content of SiO2(Li+). With the character of small size and large surface, the SiO2(Li+) can contact with the polymer chains adequately, which can absorb the impact forces and prevent further expansion of the crack of the materials when the composite polymer membranes are be attacked. The results show that the composite gel polymer electrolyte can be applied in the polymer lithium-ion battery with the addition a proper amount of SiO2(Li+).

Fig. 7
figure 7

Stress–strain curves of the composite membranes a PVDF-1 %SiO2(Li+), b PVDF-2 %SiO2(Li+) and c PVDF-5 %SiO2(Li+)

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3.8 Ionic Conductivity Measurement

Figure 8a–c are the typical AC impedance spectra for composite gel polymer electrolyte varied with the content of SiO2(Li+). No semicircle is observed at high frequency. The result suggests that only the resistive component of composite gel polymer electrolyte could be considered at the high amount of plasticizing electrolyte. It is possible to construct a local effective pathway in liquid phase and in gel phase for ionic conduction [21]. With the assistance of the lithium-ion in the SiO2(Li+), the lithium salt in the composite gel polymer electrolyte could disassociate easily, and the ion mobility is decoupled with the segmental motion of the polymer chain, and it is transferred through gel electrolyte [22]. Lithium-ion can transport quickly in these phases as the electric potential alternates between positive electrode and negative electrode in an AC field.

Fig. 8
figure 8

AC impedance spectra of composite gel electrolyte membrane in different temperature: a PVDF-1 %SiO2(Li+), b PVDF-2 %SiO2(Li+) and c PVDF-5 %SiO2(Li+)

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The σ vs. 1/T plots for the composite gel polymer electrolyte is shown in Fig. 9. It obviously shows that the ionic conductivity increases with the temperature. This behavior can be rationalized by recognizing the free-volume model [15, 23]. As the temperature increases, it results in an increase in the overall mobility of the ions and polymer chains. The linear relationship suggests that the conductivity is thermally activated. The conductivity relationship can be expressed as.

Fig. 9
figure 9

Dependence of conductivity on the reciprocal of temperature for composite gel polymer electrolyte (wt%)

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$$ \sigma = A\,\exp ( - E_{a} /RT) $$

where E a is the activation energy, A is a constant and T is the absolute temperature. The corresponding of the activation energy of the electrolyte material means that the charge transport in the composite gel polymer electrolyte is independent of the segmental movements of the PVDF, and thus a ‘fast’ charge transport might occur in the composite gel polymer electrolyte in the temperature range 20–70 °C.

It is well known that there are three phases in the composite gel polymer electrolyte: solid state PVDF polymer matrix, the gelled polymeric matrix coming from the swelling of organic components in non-aqueous electrolyte, and absorbed liquid electrolyte. The solid state polymer matrix ensures the mechanical strength of the gel polymer electrolyte, and the others provide a high ionic conductivity. In our case, there is a new phase, SiO2(Li+), in the composite gel polymer electrolytes. The nano-particles influence, on one hand, the porous structure of the polymer membrane. On the other hand, Lewis acid–base effect, which comes from the interaction between –OLi groups on the surface of SiO2(Li+) and polar CF2 groups of polymer chains, can weaken the interaction the interaction between Li+ ions and anions and facilitating their migration [24, 25]. However excess silica may destroy the mechanical properties of the composite gel polymer electrolyte. At 5 wt%, the composite polymer membrane reaches to the highest porosity, so its apparent activation energy for ions transport is down to the minimum value and the ionic conductivity reaches to the highest value.(Table 3)

Table 3 Mechanical properties of composite polymer membranes with different content of SiO2(Li+)
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3.9 Electrochemistry Stability Measurements

For a lithium-ion polymer battery, the cell potential can approach as high as 4.5 V versus Li/Li+, implying that the composite polymer electrolyte should be electrochemically stable up to at least 4.5 V [25]. To ascertain the electrochemical stability of the CMGPE, linear sweep voltammogram of the laminated three electrode cells was performed at ambient temperature. The electrochemical stability of the samples are presented in Fig. 10. The working electrode potential of the cell was varied from 3.0 to 6.0 V (vs. Li) at sweep rate of 0.05 V/s.

Fig. 10
figure 10

Electrochemical stability window of CMGPEs varied with the addition of SiO2(Li+)

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It is evident from the figure that there is no electrochemical reaction in the potential range 3.0–5.1 V. The onset of current flow at 5.1 V is associated with the decomposition of the electrolyte. So the anodic stability limit of the electrolyte is 5.1 V versus that of Li, which is sufficiently high for the lithium oxide cathodes LiCoO2 and Li2Mn2O4 [26]. Hence it may be concluded that the added SiO2(Li+) can improve the electrochemical stability of the CMGPEs due to its property of electrochemical inert.

4 Conclusion

A new type of composite gel polymer electrolyte varied with the content of SiO2(Li+) was prepared by immersion deposition technique. The crystallinity of the composite microporous polymer membrane decreases with the increase of the incorporated amount of SiO2(Li+). While the Porosity and liquid electrolyte swelling ability increase with it, this is beneficial to absorb more liquid electrolytes. However, excessive SiO2(Li+) could destroy the mechanical properties of the microporous membrane. At the content of 5 %, the Sw of CMGPEs is 362.45, and the ionic conductivity at room temperature could reach 3.87 × 10−2S/cm, and its electrochemical stability window is about 5.1 V. The addition of the SiO2(Li+) can enhance the properties of the CMGPEs and make it potential candidate for application as polymer electrolyte in devices.