Flexible poly(vinylidene fluoride-co-hexafluoropropylene)-based gel polymer electrolyte for high-performance lithium-ion batteries

Abstract

Gel polymer electrolytes (GPEs) have attracted ever-increasing attention in Li-ion batteries, due to their great thermal stability and excellent electrochemical performance. Here, a flexible poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-based GPE doped with an appropriate proportion of the PEO and SiO₂ is developed through a universal immersion precipitation method. This porous PVDF-HFP-PEO-SiO₂ GPE with high ionic conductivity and lithium-ion transference number (t_Li⁺) can enhance the electrochemical performance of LiFePO₄ cells, leading to superior rate capability and excellent cycling stability. Moreover, the PVDF-HFP-PEO-SiO₂ GPE effectively inhibits the lithium dendrite growth, thereby improving the safety of Li-ion batteries. In view of the simplicity in using the gel polymer electrolyte, it is believed that this novel GPE can be used as a potential candidate for high-performance Li-ion batteries.

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1. Introduction

With the rapid development of portable electronic devices and electric vehicles, lithium-ion batteries (LIBs) have been widely used due to their high specific capacity, low self-discharge, high battery voltage, long cycling life and environmental friendliness.^1–3 However, the organic solvent in the traditional liquid electrolyte is flammable, thus it can cause spontaneous combustion and even explosion accidents, which severely hinders the development of LIBs.⁴ Therefore, to improve the safety of LIBs, solid electrolytes have received extensive attention from researchers due to their non-flammability and wide working temperature range, and lithium metal can be chosen as the anode.^5,6 However, the low ionic conductivity and poor interfacial contact between electrodes and solid-state electrolytes restrict the practical application.^7,8

Gel polymer electrolytes (GPEs) have an intermediate state between solid and liquid, formed by immersing the polymer membrane in the liquid electrolyte, which can simultaneously provide the high ionic conductivity of liquid electrolytes at room temperature and the excellent safety of solid-state electrolytes.^9,10 GPEs are mainly composed of three parts: polymer matrix, lithium salt and low-molecular organic solvent (plasticizer). The polymer matrix is the core of the GPEs structure, and plays a role in adsorbing plasticizers.^11,12 The ideal GPEs must exhibit high room temperature ion conductivity, lithium-ion transference number, electrochemical stability window and mechanical strength, etc.^13–16 Recently, the polymers have been widely studied, such as poly(ethylene oxide) (PEO),¹⁷ poly(methyl methacrylate) (PMMA),^18–20 poly(vinyl chloride) (PVC),²¹ poly(acrylonitrile) (PAN),^22–24 poly(vinylidene fluoride) (PVDF),²⁵ and poly(vinylidene fluoride-co-hexafluorpropylene) (PVDF-HFP).^1,26,27 Among them, the high electron-absorption C–F bonds of PVDF-HFP polymer has attracted great interest from scientific researchers, due to its excellent thermal endurance, superior dielectric constant, good mechanical performance, and high electrochemical stability.²⁷ The copolymer PVDF-HFP obtained by introducing amorphous HFP into PVDF can disrupt the symmetric and regular chain structure of pure PVDF to reduce its crystallinity, thereby enhancing the adsorption of the polymer membrane to the liquid electrolyte and improving the conductivity of GPEs.

The addition of ceramic nanoparticles such as SiO₂,^28,29 TiO₂,³⁰ Al₂O₃,³¹ and ZrO₂ (ref. 32) can further improve the performance of GPEs. The inorganic filler in the polymer matrix can increase the proportion of the amorphous region of the polymer chain, reduce the crystallinity of the polymer membrane, as well as increase the Li⁺ conductivity. Because of the high heat resistance of inorganic fillers, they are beneficial to improve the thermal stability of GPE. In addition, inorganic nanoparticles can also enhance the mechanical strength and interface stability of GPEs.

In this work, a PVDF-HFP-based porous GPE with a proper proportion of the PEO and SiO₂ was prepared by immersion precipitation method. Since the oxygen-containing functional groups of PEO segment have excess electrons, lithium ions become easy to coordinate with the ether oxygen functional groups.¹⁷ The hydrophilicity of PEO can increase the exchange rate of solvent and non-solvent, thereby promoting the formation of finger-like pore structure.^33–35 Herein, with the addition of the PEO and SiO₂, the PVDF-HFP-PEO-SiO₂ GPE showed low crystallinity, excellent thermal stability, wide electrochemical window, high lithium-ion transference number and ionic conductivity. The 3D porous structure of PVDF-HFP-PEO-SiO₂ GPE provided higher porosity and liquid electrolyte uptake than pure PVDF-HFP GPE. Moreover, the obtained CPE exhibited a wide electrochemical window up to 4.7 V, enhanced ionic conductivity of 1.12 × 10⁻³ S cm⁻¹, and higher lithium-ion transference number (t_Li⁺) of 0.48. Notably, the capacity of the LiFePO₄ cell with PVDF-HFP-PEO-SiO₂ GPE remained at 146.3 mA h g⁻¹ even after 200 cycles at 0.1C. The lithium symmetric battery assembled with the PVDF-HFP-PEO-SiO₂ GPE also displayed ultralong cycling life. It is believed that this novel GPE has a great potential to be used for high safety lithium metal batteries in practical application.

2. Experimental

2.1 Materials

Poly(ethylene oxide) (PEO, M_w ≈ 600 000), bis(trifluoromethanesulfoneimide) lithium salt (LiTFSI, 99.9%), and nano SiO₂ powder was purchased from Macklin. Poly(vinylidene fluoride-co-hexafluorpropylene) (PVDF-HFP, M_w ≈ 455 000) was purchased from Sigma-Aldrich. N-Methyl-2-pyrrolidone (NMP) and N,N-dimethylformamide (DMF) was purchased from Sigma-Aldrich. Poly(vinylidene fluoride) (PVDF), lithium iron phosphate (LiFePO₄), conductive carbon black (Super P), ethylene carbonate (EC) and dimethyl carbonate (DMC) were purchased from Guangdong Canrd New Energy Technology Co., Ltd.

2.2 Preparation of the PVDF-HFP-PEO-SiO₂ gel polymer electrolyte

The composite polymer membrane with porous structure was prepared by immersion precipitation method. PVDF-HFP was dissolved in DMF with a weight percent of 10 wt% and magnetically stirred at 60 °C for 2 h to form a homogeneous and transparent solution. After that, PEO and SiO₂ (both of them were 10 wt% of PVDF-HFP) were added to the above solution and stirred at 60 °C for 2 h. The obtained viscous solutions were casted on a clean glass plate, and then put into deionized water. After the phase inversion was completed, the PVDF-HFP-PEO-SiO₂ membrane fell off the glass plate. Finally, freeze-dry the PVDF-HFP-PEO-SiO₂ membrane for 24 h to completely remove the water and residual DMF. For comparison, the pure PVDF-HFP membrane was also prepared by the same method. All the as-prepared polymer membranes were cut into 18 mm in diameter and transferred into an Ar-filled glovebox (H₂O < 0.1 ppm, O₂ < 0.1 ppm) for further testing and characterization. The prepared PVDF-HFP-PEO-SiO₂ membrane was soaked in the liquid electrolyte (1 M LiTFSI in EC/DMC, 1/1 by volume) for 2 h, and the excess liquid electrolyte on the membrane was removed by wiping with a filter paper.

2.3 Characterization of the composite separators

The surface morphology of all the samples was characterized by scanning electron microscope (SEM, Hitachi S-4800). Thermal gravimetric analysis (TGA) curves were performed under nitrogen (N₂) atmosphere with a heating rate of 10 °C min⁻¹ using a Netzsch STA 2500. The crystallinity was measured by X-ray diffraction (XRD, Bruker D8 Advance) using Cu Ka radiation. The diffraction angle (2θ) was between 5° and 80° with a scan rate of 5° min⁻¹. The tensile mechanical performance tests were performed on an electronic universal testing mechine (UTM, 6530, Shenzhen Kay Strength Test Instrument Co., Ltd.) at room temperature with a stretching speed of 20 mm min⁻¹.

The degree of crystallinity (X_c) of the different membranes was obtained by the following eqn (1):³⁶

The electrolytes uptake of the different membranes was calculated by the following eqn (2):¹

where M₁ and M₂ are the weights of the different membranes before and after immersion in the liquid electrolytes.

The porosity was obtained from the following method. The different membranes were first soaked in n-butanol for 2 h, and then wiped off the excess n-butanol on the membrane surface with a filter paper. The wet membranes were weighed immediately. The porosity of the membranes was obtained by the following eqn (3):^1,36

ΔM is the mass of the membranes after and before the adsorption of n-butanol, ρ is the density of n-butanol, and V₀ is the membrane volume before the adsorption of n-butanol.

2.4 Measurements of electrochemical performance

The ionic conductivity was tested by an electrochemical workstation. The separators were sandwiched between two stainless steel (SS) electrodes, and measured by AC impedance spectroscopy in the frequency range from 1.0 MHz to 0.01 Hz with an AC amplitude of 10 mV. The ionic conductivity was calculated by the following eqn (4):^13,36where σ is the ionic conductivity, d is the thickness of the separators, R is the bulk resistance, and S is the area of the separators.

The lithium-ion transference number (t_Li+) of the symmetric Li|GPE|Li battery was measured by the chronoamperometry and the AC impedance spectroscopy with a voltage of 10 mV. The lithium-ion transference number (t_Li+) was obtained by the following eqn (5):³⁶

where I₀ and I_ss are the initial and steady-state current, respectively. R₀ and R_ss are the initial and steady-state interfacial resistance, respectively. ΔV is the polarization potential (10 mV).

The electrochemical stability window was tested by sandwiching the separators between stainless steel (SS) and lithium metal as the working electrode and the counter electrode, respectively. The voltage range was from 0 to 6.0 V with a scan rate of 10 mV s⁻¹. The electrochemical properties were carried out using CR2032 coin-type of Li/GPE/LiFePO₄ on the battery test system (Wuhan Land Electronic Co. Ltd., China). The LiFePO₄ cathode was prepared by completely mixing LiFePO₄ active powder, Super P, and PVDF with a mass ratio of 80 : 10 : 10 in N-methyl-2-pyrrolidone (NMP) solvent. Subsequently, the slurry was coated onto an aluminum foil, and dried at 100 °C for 24 h in a vacuum oven, and then punched to 10 mm diameter disk for testing. The mass loading of the as-prepared LiFePO₄ cathode was about 1.25 mg cm⁻². Cells were assembled in a glove-box filled with argon. The galvanostatic charge–discharge tests were carried out at a constant current density in a voltage range of 2.6–4.0 V. The Li/Li symmetric cells were assembled by sandwiching the GPE between two lithium foils.

3. Results and discussion

SEM was used to illustrate the influence of PEO introduction on the structure of the polymer membrane. Fig. 1a–f shows the SEM images of the bottom surface, top surface and cross-section of the pure PVDF-HFP and PVDF-HFP-PEO-SiO₂ membrane, respectively. As shown in Fig. 1a, almost no penetrating pores were found on the bottom surface of the pure PVDF-HFP membrane, but only slight concave was observed on the bottom surface. The pure PVDF-HFP membrane showed a relatively dense top surface (Fig. 1b), and the thickness of the pure PVDF-HFP membrane was about 100 μm (Fig. 1c). It was further confirmed that the length of the finger-like pore structure was about one-third of the entire cross-section, and the remaining parts formed a sponge-like pore structure. After the addition of the PEO and SiO₂, the microstructure of the polymer membrane became more porous with improved pore interconnectivity, which can be observed from Fig. 1d and e. Meanwhile, it can be observed that the cross-section of the PVDF-HFP-PEO-SiO₂ membrane with penetrating finger-like pore structure (Fig. 1f). The PEO was a hydrophilic polymer, which could increase the exchange rate of solvent and non-solvent during the preparation process, thereby promoting the formation of finger-like pore structure.³⁴ Since the porous structure was beneficial for absorbing more liquid electrolyte, the ionic conductivity of the GPE can be enhanced significantly.^1,36

Fig. 1 SEM images of (a) bottom surface, (b) top surface and (c) cross section of PVDF-HFP membrane. SEM images of (d) bottom surface, (e) top surface and (f) cross section of PVDF-HFP-PEO-SiO₂ membrane.

To obtain thermal dimensional stability of the polymer membrane, the Celgard 2325 separator, pure PVDF-HFP and PVDF-HFP-PEO-SiO₂ were heat-treated for 1 h at 150 °C and 175 °C, respectively. Fig. 2a and b display photographs of the separators and the corresponding shrinkage ratios before and after heat treatment, respectively. The Celgard 2325 separator exhibited serious shrinkage after thermal treatment at elevated temperature. When the temperature increased from 150 °C to 175 °C, the shrinkage ratio of the Celgard 2325 separator changed from 22% to 36%. In contrast, the dimensional changes of the pure PVDF-HFP and PVDF-HFP-PEO-SiO₂ were only 4% at 150 °C. Furthermore, both the pure PVDF-HFP and PVDF-HFP-PEO-SiO₂ did not exhibit significant dimensional changes at higher temperature of 175 °C. These results suggested that the PVDF-HFP-based membranes had much better thermal stability compared to the Celgard 2325 separator, which implied that the PVDF-HFP-based membrane as a separator can improve the battery safety by reducing the risk of short circuit.

Fig. 2 (a) Photographs of the Celgard 2325 separator and PVDF-HFP-based membranes before and after being exposed to 150 °C and 175 °C for 1 h. (b) Thermal shrinkage of membranes after heat-treatment. (c) TG curves for various membranes.

Fig. 2c shows TG curves of the Celgard 2325 separator and PVDF-HFP-based membranes. A strong mass loss occurred at about 260 °C for the Celgard 2325 separator, while the PVDF-HFP membrane started to decompose at around 415 °C, indicating the good thermal stability of polymer PVDF-HFP. However, the PVDF-HFP-PEO-SiO₂ has a relatively lower initial decomposition temperature of ∼350 °C than pure PVDF-HFP with the addition of PEO and SiO₂. The lower initial decomposition temperature should be related to the addition of PEO.⁴ In addition, PVDF-HFP-based membranes display a higher residual amount than Celgard 2325 separator. Hence, PVDF-HFP-PEO-SiO₂ membrane with great thermal stability can be utilized as the separator for applications in LIBs with high safety.

The crystalline structures of the Celgard 2325 separator, pure PVDF-HFP and PVDF-HFP-PEO-SiO₂ membranes were performed by XRD, as shown in Fig. 3a. Obviously, two intense characteristic peaks at 18.3° and 19.9° were observed for the pure PVDF-HFP membrane. After addition of the PEO and SiO₂, the characteristic peak of the PVDF-HFP membrane slightly decreased. In addition, the degree of crystallinity (X_c) was calculated by the eqn (1). The X_c of the Celgard 2325 separator, pure PVDF-HFP and PVDF-HFP-PEO-SiO₂ membrane were 36%, 39% and 27%, respectively. The introduction of the PEO and SiO₂ can significantly reduce the crystallinity of the PVDF-HFP-based membrane, which was agreed well with the XRD results. The decrease of crystallinity can be considered as the addition of nano-filler SiO₂ disrupted the chain structure of pure PVDF-HFP, resulting in the reduce of crystalline zone, and thus enhanced the transport of lithium ions.^17,37

Fig. 3 (a) XRD patterns, (b) porosity and electrolyte uptake of the Celgard 2325 separator, pure PVDF-HFP and PVDF-HFP-PEO-SiO₂, (c) stress–strain curve of the PVDF-HFP-PEO-SiO₂ GPE.

The porosity and liquid electrolyte uptake were used to further explore the porous structure of the polymer membrane. Fig. 3b shows the porosity of various membranes, together with the corresponding liquid electrolyte uptake. The porosity of PVDF-HFP-PEO-SiO₂ reached 74%, which was much higher than the Celgard 2325 separator (25%) and the pure PVDF-HFP (41%). The corresponding liquid electrolyte uptake of the Celgard 2325 separator, pure PVDF-HFP and PVDF-HFP-PEO-SiO₂ were 126%, 181% and 539%, respectively. Obviously, the addition of PEO and SiO₂ effectively increased the porosity of PVDF-HFP, which can absorb more liquid electrolyte and get higher lithium ion conductivity.¹ Fig. 3c shows the mechanical property of the PVDF-HFP-PEO-SiO₂ GPE. The tensile strength of the PVDF-HFP-PEO-SiO₂ GPE was 6.72 MPa with an elongation-at-break at 80%. The higher mechanical property of PVDF-HFP-PEO-SiO₂ GPE decreased the risk of short-circuit of lithium batteries, improving their safety and stability.

The electrochemical window of the Celgard 2325 separator with liquid electrolyte and PVDF-HFP-based GPEs were recorded with linear sweeping voltammetry (LSV) curves in the voltage range of 0–5.5 V at a scan rate of 10 mV s⁻¹. As shown in Fig. 4a, the anodic limiting potential of PVDF-HFP-based GPEs was higher than that of the Celgard 2325 separator absorbed liquid electrolyte (∼4.5 V). Moreover, it can be observed that the PVDF-HFP-PEO-SiO₂ GPE showed the highest anodic limiting potential of 4.7 V. Hence, the addition of the PEO and SiO₂ was supposed to improve the electrochemical stability of the PVDF-HFP-PEO-SiO₂ GPE for LIBs.^17,37

Fig. 4 (a) LSV curves and (b) impedance spectra of the Celgard 2325 separator absorbed liquid electrolyte and PVDF-HFP-based GPEs.

The ionic conductivity was measured by an AC impedance analyzer technique, and the values were calculated by the eqn (4). As shown in Fig. 4b, the ionic conductivity of the PVDF-HFP-based GPEs was much higher than that of the Celgard 2325 separator absorbed liquid electrolyte. The ionic conductivity of the Celgard 2325 separator that absorbed liquid electrolyte, pure PVDF-HFP and PVDF-HFP-PEO-SiO₂ GPEs can be calculated to be 4.33 × 10⁻⁵ S cm⁻¹, 2.46 × 10⁻⁴ S cm⁻¹, and 1.12 × 10⁻³ S cm⁻¹, respectively. Moreover, the ionic conductivity of PVDF-HFP-PEO-SiO₂ GPE was slightly lower than that of the liquid electrolyte (7.86 × 10⁻³ S cm⁻¹). The improvement of ion conductivity was attributed to the following reasons. First, fluorine atom of PVDF-HFP has strong electronegativity, which can facilitate the lithium ions to transfer easily in the amorphous region. Second, the oxygen-containing functional groups of PEO segment have excess electrons, while the electron orbitals of lithium ions contain empty orbitals, thus lithium ion becomes easy to coordinate with the ether oxygen functional groups.^17,19 Third, the PVDF-HFP-PEO-SiO₂ GPE with high porosity and good pore interconnectivity increased the electrolyte uptake ratio.¹ The high ionic conductivity of the PVDF-HFP-PEO-SiO₂ GPE should be beneficial for the effective enhancement of battery performance.

The lithium-ion transference number (t_Li⁺) is an important parameter of the GPEs, which influences the electrochemical properties of LIBs. The value of t_Li⁺ was calculated by the eqn (5), where the initial and steady-state current were obtained by the polarization curve, and the initial and steady-state interface impedances were obtained by AC impedance, as shown in Fig. 5. The PVDF-HFP-PEO-SiO₂ GPE reached a high t_Li⁺ of 0.49, which was higher than those of the pure PVDF-HFP GPE (t_Li⁺ = 0.39) and the Celgard 2325 separator absorbed liquid electrolyte (t_Li⁺ = 0.27). The higher t_Li⁺ of the PVDF-HFP-PEO-SiO₂ GPE was ascribed to the high porosity and typical structure with good pore interconnectivity, resulting in an increase amount of the electrolyte uptake.

Fig. 5 Chronoamperometry curves of (a) Celgard 2325 separator, (b) PVDF-HFP, (c) PVDF-HFP-PEO-SiO₂ in the symmetrical Li/GPE/Li cells. AC impedances before and after polarization are shown in the inset.

To evaluate the electrochemical properties of the PVDF-HFP-PEO-SiO₂ GPE, the cycling stability and rate capability were examined by CR2032 coin-type half-cells. The schematic of LiFePO₄/PVDF-HFP-PEO-SiO₂ GPE/Li battery is shown in Fig. 6a. The electrochemical properties of the LiFePO₄/GPEs/Li cells were examined at a constant current density, as shown in Fig. 6b–e. For comparison, the battery assembled with the Celgard 2325 separator that absorbed liquid electrolyte was denoted by LiFePO₄/Celgard 2325 separator/Li. Fig. 6b depicts the initial charge/discharge curves of the cells with various GPEs. The same charge/discharge voltage plateaus were found in the curves, while the battery with PVDF-HFP-PEO-SiO₂ GPE exhibited the highest discharge capacity of 152.5 mA h g⁻¹ and initial coulombic efficiency of 94.5%. Fig. 6c displays the cycling stability of the LiFePO₄/GPEs/Li cells in a voltage range between 2.6 and 4.0 V at a current density of 0.1C. It was found that the discharge capacity of the cell with PVDF-HFP-PEO-SiO₂ GPE reached a high capacity of 146.3 mA h g⁻¹ even after 200 cycles, with the capacity retention of ∼100%. For comparison, the LiFePO₄/Celgard 2325 separator/Li cell showed a significant capacity fading after 50 cycles, and remained the discharge capacity of 85.8 mA h g⁻¹ after 200 cycles, with the capacity retention of only 58%. The LiFePO₄/PVDF-HFP GPE/Li cell exhibited a serious capacity fading after 30 cycles, and only retained the capacity of ∼50 mA h g⁻¹ after 100 cycles. Fig. 6d shows the discharge capacities of cells at different rates from 0.1C to 2C. It can be seen that PVDF-HFP-PEO-SiO₂ GPE showed outstanding rate performance. Its discharge capacity was 115.5 mA h g⁻¹ even at a high rate of 2C, which was higher than LiFePO₄/Celgard 2325 separator/Li cell (109 mA h g⁻¹). When the current density was returned to 0.1C, the discharge capacity of LiFePO₄/PVDF-HFP-PEO-SiO₂ GPE/Li cell recovered to 147.7 mA h g⁻¹, while LiFePO₄/Celgard 2325 separator/Li cell exhibited a low capacity of 117 mA h g⁻¹. Fig. 6e shows the charge/discharge profiles of LiFePO₄/PVDF-HFP-PEO-SiO₂ GPE/Li cell at various current densities from 0.1 to 2C. The overpotential slightly increased with the increase of current density, showing a relatively low polarization. This enhanced rate capability was attributed to the high ionic conductivity and lithium-ion transference number of PVDF-HFP-PEO-SiO₂ GPE.^3,11 Therefore, the PVDF-HFP-PEO-SiO₂ GPE with the great rate capability and excellent cycling stability can be considered as a candidate for the high-performance electrolyte in LIBs.

Fig. 6 (a) Schematic illustration of LiFePO₄/PVDF-HFP-PEO-SiO₂ GPE/Li battery and PVDF-HFP-PEO-SiO₂ membrane. (b) Charge and discharge curves of LiFePO₄/GPEs/Li cells during the first cycle. (c) Cycling performance of LiFePO₄/GPEs/Li cells for 200 cycles at 0.1C rate. (d) Rate capability of LiFePO₄/GPE/Li cells based on Celgard 2325 separator, PVDF-HFP, and PVDF-HFP-PEO-SiO₂ GPEs. (e) Charge and discharge curves of LiFePO₄/PVDF-HFP-PEO-SiO₂ GPE/Li cell at various current densities from 0.1C to 2C.

The long-term electrochemical stability of GPEs against the Li dendrites growth was evaluated by the Li/GPEs/Li cells. Fig. 7a presents the lithium plating/striping voltage profile of the symmetric cells under a current density of 0.1 mA cm⁻². Obviously, the Li/PVDF-HFP-PEO-SiO₂ GPE/Li symmetric cell exhibited low polarization voltage after 1000 h of cycles, and also no short-circuit phenomenon occurred. For comparison, the overpotential of the Celgard 2325 separator saturated with the liquid electrolyte gradually increased in the first 400 h of cycles and then dropped significantly, while the overpotential of PVDF-HFP increased sharply after only 150 h of cycles, implied that a severe lithium dendrite growth happened to further cause short-circuit. Furthermore, the Li/PVDF-HFP-PEO-SiO₂ GPE/Li symmetric cell also can be stably cycled for 600 h at a high current density of 0.2 mA cm⁻² (Fig. 7b). SEM images of the lithium metals before and after cycling for various symmetric cells are shown in Fig. 7c–f. It can be seen that the surface of the lithium metal was smooth before cycling (Fig. 7c). After 500 h cycling, cracks and uneven lithium deposition of lithium were found for the cell assembled with the Celgard 2325 separator absorbed liquid electrolyte (Fig. 7d). Meanwhile, the lithium metal of Li/PVDF-HFP GPE/Li cell after being cycled for 300 h exhibited massive and particulate lithium crystal, as shown in Fig. 7e. In surprise, the surface of the lithium metal of Li/PVDF-HFP-PEO-SiO₂ GPE/Li cell was still relatively smooth even after 1000 h cycles, in which the growth of lithium dendrite was not observed (Fig. 7f). Based on these results, the typical structure of PVDF-HFP-PEO-SiO₂ GPE effectively inhibited the lithium dendrite growth, thereby improving the safety of the lithium metal batteries.³

Fig. 7 (a) Voltage profiles of the lithium plating/striping cycling of symmetrical Li/Li cells with Celgard 2325 separator, PVDF-HFP and PVDF-HFP-PEO-SiO₂ GPEs at a current density of 0.1 mA cm⁻². (b) Voltage profiles of the lithium plating/striping cycling of PVDF-HFP-PEO-SiO₂ GPE at a current density of 0.2 mA cm⁻². SEM images of (c) Li metal before cycling, (d) Li metal of Li/Celgard 2325 separator/Li cell after 500 h cycles, (e) Li metal of Li/PVDF-HFP GPE/Li cell after 310 h cycles, and (f) Li metal of Li/PVDF-HFP-PEO-SiO₂ GPE/Li cell after 1000 h cycles at a current density of 0.1 mA cm⁻².

4. Conclusions

We have developed a PVDF-HFP-PEO-SiO₂ composite membrane with porous structure by immersion precipitation method. By having the advantage of a finger-like porous structure with high porosity, this designed membrane can effectively improve liquid electrolyte uptake, resulting in the high ionic conductivity of 1.12 × 10⁻³ S cm⁻¹ and lithium-ion transference number (t_Li⁺) of 0.48. Hence, the outstanding cycling performance (147.7 mA h g⁻¹ after 200 cycles) was demonstrated using the LiFePO₄ cathode based on the PVDF-HFP-PEO-SiO₂ GPE. Moreover, it can assist in inhibition of the growth of lithium dendrites, resulting in a superior compatibility with lithium metal. Taking the facile fabrication process and the eco-friendly and low-cost qualities in account, this novel developed GPE has a great potential to be utilized for high-performance and safety LIBs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21606135), the Science Technology Innovation Research Program of Ningbo (2019B10113, 2020Z024). This work was also sponsored by K. C. Wong Magna Fund in Ningbo University.

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