Introduction

Lithium-ion batteries are widely used in electronic devices, hybrid vehicles, and vehicles [1,2,3]. However, lithium-ion batteries have reached their bottleneck, in which energy density and power density are limited by the electrode materials [4, 5]. Solid-state lithium metal batteries are one of the candidate’s electrolytes for achieving high energy density [6, 7]. Solid-state lithium metal batteries use lithium metal as the electrode, and its theoretical capacity is ten times than that of the graphite. Furthermore, solid-state lithium metal batteries use solid electrolytes to replace commercial organic liquid electrolytes, which can solve the safety issues.

Generally, solid-state electrolytes are classified into inorganic solid-state electrolytes, polymer electrolytes, and gel polymer electrolytes [8, 9]. Inorganic solid electrolytes are rigid, which causes interfacial contact issues and extremely high interfacial resistance [10,11,12]. Polymer electrolytes have low ionic conductivity at room temperature (< 10−4 S cm−1) [13, 14]. Gel polymer electrolytes are prepared by adding a plasticizer to polymer electrolytes to improve the ionic conductivity, where ionic conductivity is about 10−3 S cm−1 [15,16,17,18,19]. However, the mechanical properties of gel polymer electrolytes are poor, and filling with inorganic particles can improve that. It is a common method to improve the mechanical properties of polymer electrolytes by adding inorganic particles. Therefore, many inorganic particles have been composited into polymer electrolytes to improve mechanical properties and electrochemical performance [20,21,22,23,24]. Tambelli et al. [25] reported that the highest room temperature ionic conductivity of PEO-Al2O3 composite polymer electrolytes was less than 10−4 S cm−1. Xiao et al. [25] fabricated a high-performance composite polymer electrolyte doped with Li0.1Ca0.9TiO3 particles with a high room temperature ionic conductivity of 3.947 mS cm−1, where the as-prepared CPE membranes needed to be immersed into an organic electrolyte for 0.5 h before the test. Liu group [26] used Li0.33La0.557TiO3 nanowires as fillers to enhance the ionic conductivity of polymer electrolyte, and the ionic conductivity was 0.24 mS cm−1 at room temperature. Others [27,28,29,30,31,32] reported inorganic particle composite polymer electrolytes, where the room temperature ionic conductivity was around < 10−4 S cm−1.

In our previous work [33], we prepared a high-performance ionic liquid gel polymer electrolyte (ILGPE), which exhibited good battery performance. We utilized an ionic liquid as polymer electrolytes plasticizer to enhance their ionic conductivity, and the optimized ILGPE had a high ionic conductivity of 1.3 mS cm−1 at 23 °C. To improve the ionic conductivity and mechanical properties of the optimized ILGPE, we combined ILGPE and TiO2, MgO, SiO2, ZrO2, CeO2, NiO, Bi2O3, and Al2O3 inorganic electrolytes to fabricate composite gel polymer electrolytes, and the properties of these electrolytes were tested. The interaction of TiO2 in ILGPE was investigated, and the safety of TiO2-ILGPE was discussed.

Methods and materials

Fabrication of composite gel polymer electrolytes

The preparation of ionic liquid gel polymer electrolytes (ILGPE) and composite ionic liquid gel polymer electrolytes (CILGPE) filled with inorganic particles (TiO2, MgO, SiO2, ZrO2, CeO2, NiO, Bi2O3, and Al2O3, 99.8%, Aladdin) followed a previous report [33]. N-Methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI, Lanzhou Greenchem ILs) ionic liquid, LiTFSI (98+%, Alfa Aesar), and poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP, MW ~ 455,000, Sigma) were dissolved in N-methyl-2-pyrrolidone (NMP) with the weight ratio of 3:1:1, and then vigorously stirred to obtain a homogenous electrolyte slurry at room temperature. For xwt% CILGPE (x = 0~20), x g inorganic particles were dispersed in the NMP solvent by ultrasonic treatment for 15 min and then mixed with the 100 g ILGPE slurry. Finally, the resulting slurry was cast onto a glass plate and dried in a vacuum oven for 48 h at 80 °C. The dried polymer electrolytes were transferred to an Ar-filled glovebox and then punched into round disks with a diameter of 19 mm.

Ionic conductivity of electrolytes

Ionic conductivity of the polymer electrolytes was tested by potentiostat electrochemical impedance spectroscopy (EIS) on the Princeton Applied Research VestaSTAT4 by using a stainless steel/stainless steel (SS/SS) structure in a coin cell as previously reported [15, 33]. The frequency ranged from 100 kHz to 1 Hz with an AC amplitude of 5 mV. The bulk resistance (Rb) of the polymer electrolytes was obtained by the equivalent circuit of EIS Nyquist plots; the area of the polymer electrolyte was S (cm2); the thickness (L) of the polymer electrolyte was measured by a digital thickness gauge (Mitutoyo 547-526S 0–12 mm), and the ionic conductivity (σ) was calculated using the following equation:

$$ \sigma =\frac{L}{R_{\mathrm{b}}S} $$
(1)

Mechanical test

Tensile specimens of the polymer electrolytes were cut off in the form of “dogbone shaped” strips from the molded plate (the size of the specimens was a length of 9 mm, the width of 3.5 mm, and thickness of around 200~300 μm). The measurements were performed on a TA Instruments DMA Q800 V21.2 with a test speed of 1.5 N min−1.

Cell performance testing

Five wt% TiO2-ILGPE as the working electrode, Li-metal as the counter electrode, and the reference electrode, glass fiber as the separator, and 1 M LiPF6 EC/DMC (v/v = 1/1) as the electrolyte at C/10 and 23 °C. The mass loading of the active material (TiO2) of the working electrode is about 0.09 mg cm−2. The 1C is defined as 335 mAh g−1.

Results and discussions

Ionic conductivity of electrolytes

Table S1 shows the ionic conductivity of ILGPE with various inorganic particles with weight percentages of 2.5~20% at 30 °C. The highest ionic conductivity of CILGPE shows various weight percentage of inorganic particles. The highest conductivity with optimized composition is shown in Fig. 1.

Fig. 1
figure 1

The optimized composite gel polymer electrolytes and their ionic conductivity at 30 °C

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Figure 1 displays the highest ionic conductivity of inorganic composite ionic liquid gel polymer electrolytes (CILGPE) with their optimized compositions at 30 °C. As can be observed in Fig. 1, the 2.5 wt% SiO2-ILGPE has the highest ionic conductivity of 2.5 mS cm−1 at 30 °C, while that of 15% Bi2O3-ILGPE and 5 wt% TiO2-ILGPE is 1.5 mS cm−1 at 30 °C. However, the 2.5 wt% NiO-ILGPE show the lowest ionic conductivity amount of these polymer electrolytes. α-Al2O3, selected as a filer in the work, is a lithium-ion nonconductor, and the ionic conductivity of 2.5 wt% α-Al2O3-ILGPE is relatively low. The ionic conductivity of 20 wt% ZrO2-ILGPE and 5 wt% MgO-ILGPE is acceptable. The ionic conductivity of these polymer electrolytes with inorganic particles is ~ 10−3 S cm−1.

Furthermore, there is a sudden increase in ionic conductivity with x% addition of inorganic fillers (Table S1). While higher or lower than that concentration of x, the ionic conductivity is obviously low. For instance, the ionic conductivity of 2.5 wt% TiO2-ILGPE and 10 wt% TiO2-ILGPE is 1.1 mS cm−1 and 0.98 mS cm−1, respectively. The ionic conductivity of polymer electrolytes could exhibit an optimized value with the addition of 5 wt% TiO2. The results are in agreement with those described in the literature, where a maximum in the ionic conductivity occurs as the increase of the filler concentration [25, 34], followed by a decrease in ionic conductivity values for higher filler weight percentage. Additionally, the decrease observed in high filler concentration could be associated with the blocking effect of the inorganic fillers in the conductivity pathway. The increase, for low filler concentration, could be, on the other hand, related to the reduction in the fusion enthalpy, which can be obtained by DSC measurement. Therefore, there is a maximum value of ionic conductivity with the optimized composition of composite ionic liquid gel polymer electrolytes.

Mechanical property of electrolytes

Figure 2 shows the mechanical properties of these CILGPE by tensile tests. As shown in Fig. 2, the mechanical properties of these composite ILGPE were affected by the addition of inorganic particles. The addition of TiO2 could increase their mechanical properties; the addition of Al2O3 and NiO would cause a poor mechanical property, while the concentration of CeO2, SiO2, Bi2O3, ZrO3, and MgO would show the various effects on their mechanical properties, such as 15 wt% Bi2O3-ILGPE and 2.5 wt% display a poor mechanical property. By the mechanical properties and ionic conductivity measurements, we selected TiO2 as a filler for the gel polymer electrolytes in this study.

Fig. 2
figure 2

Tensile test for composite gel polymer electrolytes. Tensile strength was tested five times for each sample to ensure that these results were reproducible

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Characterization of electrolytes

According to the results of ionic conductivity measurements, we observe that the concentration of TiO2 nanoparticles in the polymer matrix could affect ionic conductivity. As known to all, TiO2 nanoparticles cannot conduct lithium-ions. Thereby, the addition of TiO2 would sit in the lithium-ion pathway. The addition of high amounts of TiO2 filler can reduce the ionic conductivity. The mechanism scheme is shown in Figure S1, indicating that the concentration of non-conductive fillers cannot be over the optimizing ratio. When the 5 wt% TiO2 is added into ILGPE, the highest ionic conductivity is obtained and the value is 1.51 mS cm−1.

To investigate the effects of TiO2, the interaction between TiO2, PP13TFSI, LiTFSI, and PVDF-HFP in composite gel polymer electrolytes was investigated by FT-IR and Raman spectroscopy (Fig. 3). By the FT-IR spectroscopy, the TiO2 does not react with the PP13TFIS, LiTFSI, and PVDF-HFP during ILGPE preparation (Fig. 3a). In the ILGPE matrix, PP13TFSI, LiTFSI, and PVDF-HFP have “coordination,” such as Raman shift of 315 cm−1 and 740 cm−1 (Fig. 3b). After the addition of TiO2, however, these “coordination” disappeared or weakened, while the new “coordination” appeared around 1600–1800 cm−1. The results indicate that TiO2 could break the “original coordination” between PP13TFSI, LiTFSI, and PVDF-HFP, and then form a “new coordination” with PP13TFSI, LiTFSI, and PVDF-HFP. Thus, TiO2 particles sit in the lithium-ion pathway and decrease lithium-ion transport [35].

Fig. 3
figure 3

FT-IR and Raman spectra of ILGPE and 5 wt% TiO2-ILGPE in a range of 400–4000 cm−1 and 150–2000 cm−1, respectively

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Besides, the XRD pattern of 5 wt% TiO2-ILGPE shows a new diffraction peak at 25° (2θ), indicating that the amorphous phase of PVDF-HFP increases by the 5 wt% TiO2 (Figure S2). There is literature reporting that the amorphous phase of the polymer can increase the ionic conductivity of polymer electrolytes. That is the reason that the ionic conductivity increases and reaches the highest value and then decreases after that (5 wt%) [36,37,38,39].

Figure 4 shows the SEM images of the CILGPE with various weight percentages of TiO2. ILGPE is a porous film and the pore size is less than 5 μm (as shown in Fig. 4a and a-magnified). With the addition of 5 wt% TiO2, the pores of ILGPE are filled and the surface of CILGPE is getting smooth and flat (Fig. 4b), while with the weight percentage of TiO2 increase, the surface of the CILGPE turns ununiform due to the TiO2 particle aggregation (Fig. 4e). The compact surface of 5 wt% TiO2 probably could inhibit lithium dendrites’ growth and solve the safety issue of lithium metal batteries.

Fig. 4
figure 4

SEM images of various weight percentage of TiO2 composite ionic liquid gel polymer electrolytes. a, a-magnified ILGPE, b 5 wt% TiO2-ILGPE, c 10 wt% TiO2-ILGPE, d 15 wt% TiO2-ILGPE, and e 20 wt% TiO2-ILGPE

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Safety of TiO2-ILGPE

We assembled a Cu/ILGPE/Li cell and a Cu/5 wt% TiO2-ILGPE/Li cell, and then the lithium stripping/plating was carried out with a current density of 0.5 mA cm−2 for long-term cycles. After the Cu/Li cells shorted, we disassemble them and the digital photos of ILGPE and 5 wt% TiO2-ILGPE are shown in Figure S3. Figure S4 displays the pristine ILGPE and 5 wt% TiO2-ILGPE before cycling, and these electrolytes are free-standing membranes. The color of the polymer electrolytes surface becomes “black” in some regions after cycling, because the deposited lithium metal adheres to the surface of the polymer electrolytes. The ILGPE membrane is still transparent, and the 5 wt% TiO2-ILGPE membrane remains light yellow and nontransparent. The results indicate that TiO2 is not reduced by lithium metal during the charging/discharging. Meanwhile, we performed cycle voltammetry (CV) and battery measurements for the Li/5 wt% TiO2-ILGPE battery with 1 M LiPF6 EC/DMC (v/v = 1/1) as the electrolyte and glass fiber as the separator (Fig. 5). The CV curves do not show any redox peak, indicating that the Li/5 wt% TiO2-ILGPE battery does not have any redox reaction in the measured voltage range (Fig. 5a). The cells cannot deliver a noticeable capacity during the constant current charging/discharging, indicating that TiO2 particles in the ILGPE are not redox-active (Fig. 5b). In a word, TiO2 fillers in the CILGPE do not reduce against lithium metal anode and its safety to add to polymer electrolytes.

Fig. 5
figure 5

a CV curves for the cell with 5 wt% TiO2-ILGPE as the working electrode and Li-metal as the counter electrode (glass fiber as the separator, and 1 M LiPF6 EC/DMC (v/v = 1/1) as the electrolyte) in a voltage range of 0–3 V (vs. Li+/Li) at a scan rate of 1 mV s−1. b The voltage profile of Li/5 wt% TiO2-ILGPE battery (5 wt% TiO2-ILGPE as the working electrode, Li-metal as the counter electrode and the reference electrode, glass fiber as the separator, and 1 M LiPF6 EC/DMC (v/v = 1/1) as the electrolyte) at C/10 and 23 °C. The 1C is 335 mAh g−1. The mass loading of the active material (TiO2) of the working electrode is about 0.09 mg cm−2

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The combustion test of the polymer electrolytes is performed, and the digital images are shown in Fig. 6. As shown in Fig. 6a, the PVDF-HFP membrane is flammable. When putting the PVDF-HFP membrane on fire, it shrinks immediately (~ 3 s) and gets on fire in a short time (~ 5 s). Finally, it turns “black” in color. The ILGPE is a non-flammable membrane, but it shrinks at ~ 5 s without blackening. While 5 wt% TiO2-ILGPE is a non-flammable membrane, it displays a perfect flame retarding ability. Furthermore, both ILGPE and TiO2-ILGPE are a promising electrolyte for safe lithium metal batteries.

Fig. 6
figure 6

Combustion test of (a) PVDF-HFP membrane, (b) ILGPE, and (c) 5 wt% TiO2-ILGPE

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Conclusion

In this study, the non-ionic conductive inorganic particles were used to improve the mechanical properties and ionic conductivity of gel polymer electrolytes. By the properties of these composite gel polymer electrolytes, TiO2 particles were selected as the filler of gel polymer electrolytes. The addition of TiO2 particles not only can fill the pores of gel polymer electrolytes to obtain dense electrolytes membrane but also can coordinate with the PP13TFSI or/and LiTFSI inside the composite gel polymer electrolytes. The safety of TiO2-ILGPE was evaluated by cyclic voltammetry, battery performance, and combustion test. With TiO2-ILGPE as an anode and Li-metal as a cathode, we assembled TiO2-ILGPE/Li cells to investigate the stability of TiO2-ILGPE against lithium metal. The results indicated that the TiO2 particles would not undergo a redox reaction during TiO2-ILGPE/Li cells cycling, and the cells delivered a charging specific capacity of 0.02 μAh g−1 at C/10, which could be neglected. Besides, the TiO2-ILGPE is a non-flammable membrane and a promising polymer electrolyte for the next generation of safe lithium metal batteries.