Gel Polymer Electrolytes with High Thermal Stability for Safe Lithium Metal Batteries

Abstract

The poor thermal stability of polypropylene (PP) separators poses risks of electrolyte leakage and battery short-circuiting, limiting their application in lithium metal batteries (LMBs). To address these challenges, a gel polymer membrane was designed using polymer blending technology. This membrane effectively retains the electrolyte, provides a stable environment, enhances thermal stability, and significantly decreases the risk of battery explosions and side reactions between the lithium metal and the electrolyte. Compared to commercial PP separators, the developed blend-type gel polymer electrolyte (b-GPE) demonstrates a superior performance, including structural stability at temperatures up to 150 °C and a high lithium-ion transference number ($t_{{Li}^{+}}$) of 0.513. Furthermore, a cell with a LiCoO₂ cathode operated at a 1 C rate retains 97.4% of its capacity after 300 cycles. After exposure to 120 °C, the b-GPE-120 demonstrates that its performance is comparable to that of the b-GPE, such as a $t_{{Li}^{+}}$ of 0.506, a high electrolyte absorption rate, and a wide electrochemical window of 5.2 V.

1. Introduction

Lithium (Li) metal is regarded as an optimal anode material for high-energy density batteries because of its impressive specific capacity of 3860 mAh g⁻¹ and its low redox potential of −3.04 V relative to the standard hydrogen electrode [1,2,3]. However, the use of traditional liquid electrolytes raises significant safety hazards, including liquid leakage, a low flash point, and battery explosions [4,5]. These issues have impeded the widespread adoption of LMBs in applications like electric vehicles and renewable energy storage systems [6,7]. To address these challenges, researchers have explored alternative electrolyte systems, such us solid-state electrolytes and gel polymer electrolytes (GPEs) [8,9,10]. Solid electrolytes provide superior mechanical strength and electrochemical stability, preventing dendrite penetration thus enhancing the safety of the battery [11,12,13]. However, their relatively low ionic conductivity limits the charge/discharge rates and cycling performance [14,15]. Additionally, solid electrolytes face challenges related to interface stability and charge transfer efficiency [16,17,18].

In comparison, GPEs integrate the benefits of both solid-state and liquid electrolytes, exhibiting high ionic conductivity, good elasticity and flexibility, and specific mechanical properties [19]. GPEs can form stable and uniform interfaces within the battery, reducing performance degradation over long-term use or under extreme conditions [20,21]. Several kinds of gel electrolytes have been developed and studied, including those based on polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), and polyvinylidene fluoride (PVDF) [22,23,24,25].

Table 1. The performance of different base gel electrolytes.

Matrix	Ionic Conductivity	Security	Mechanical Strength	Temperature Range	Disadvantages
PEO	~10−4 S cm−1	high	high	40–100 °C	limited electrochemical stability
PAN	~10−4 S cm−1	high	high	-	passivation on contact with electrodes
PMMA	~10−3 S cm−1	high	low	−20–125 °C	poor mechanical strength
PVDF	~10−3 S cm−1	high	high	<150 °C	higher cost

summarizes the performance advantages and disadvantages of different base gel electrolytes. PEO is one of the earliest materials researched for GPEs, known for its high ionic conductivity, good electrochemical stability, wide operating temperature range, and moderate mechanical strength. However, it is prone to degradation during long-term use, which limits its long-term stability and service life [26]. PAN-based gel electrolytes offer excellent electrochemical stability, heat resistance, and flame retardancy, with a high electrochemical window and significant ion migration [27]. Nevertheless, the highly polar groups (like -CN) on its chains can lead to severe passivation at the interface with the Li anode, resulting in a decreased GPE performance [28]. PMMA, with carbonyl side chains in its structure, can interact strongly with oxygen atoms in carbonate-based solvents, allowing it to adsorb a substantial quantity of electrolyte and demonstrate a high ionic conductivity; however, GPEs prepared from PMMA have relatively poor mechanical strength and cannot be used alone [29].

PVDF is well known for its high dielectric constant, excellent film-forming ability, thermal stability, and chemical resistance [30,31]. The strong electronegativity of its -CF side chain groups provides an outstanding resistance to electrochemical oxidation, making it a promising material for the enhancement of battery performance and the extension of the cycle life [32]. However, the regular structure of PVDF tends to form crystalline phases that impede lithium-ion conduction. To address this issue, researchers have introduced hexafluoropropylene (HFP) into PVDF to produce the copolymer PVDF-HFP. This modification reduces crystallinity, boosts the polymer’s absorption of the electrolyte, and improves the GPEs’ conductivity [33]. Additionally, blending PVDF with other polymers or inorganic fillers can further decrease crystallinity, enhance the continuity of the amorphous phase, and improve the lithium-ion transport [34]. Table S1 summarizes the performance characteristics of PVDF-based GPEs, highlighting the enhanced thermal stability reported in various studies. For instance, Chen et al. fabricated composite nanofibers utilizing coaxial electrospinning, featuring a core–shell structure of boehmite-doped PVDF-HFP/PAN. These nanofibers successfully served as separators in lithium-ion batteries, where PAN provided robust heat resistance. Notably, the separator exhibited minimal shrinkage even after heating at 160 °C for 0.5 h, while the outer layer of this separator facilitated electrolyte permeation and Li⁺ migration through its -CF and -OH groups [35]. Tan et al. constructed an electrolyte via electrospinning, incorporating ZIF-8 as an active filler within PVDF-HFP/PEO. This electrolyte demonstrated an impressive thermal decomposition stability up to 400 °C. The Lewis acid–base interaction between ZIF-8 and the polymer effectively reduced the crystallinity, restricted the mobility of TFSI⁻ ions, and enhanced the Li⁺ transport [36]. Hu et al. developed an asymmetric PVDF-HFP/PVA-SiO₂ gel electrolyte, utilizing nano-silica as a filler additive to enhance the flame retardancy of the polymer system. This GPE maintained its structural integrity even after heating at 160 °C for 0.5 h [37]. Hsu et al. synthesized a PVDF-graft-PAN copolymer using ozone-induced polymerization. When blended with a PEO polymer, this copolymer formed a modified PVDF. This modified electrolyte had a significantly improved electrolyte compatibility, thermal performance, and membrane wettability, ultimately achieving a high-performance and high-safety electrolyte. Importantly, the improved m-PVDF membrane exhibited a minimal size change after heating at 150 °C for 0.5 h, demonstrating its outstanding thermal stability [38]. However, these studies primarily focused on the high temperature resistance and structural stability of the gel membranes, without thoroughly investigating their electrochemical performance after their exposure to elevated temperatures. As a result, the ability of these membranes to maintain stable electrochemical properties following thermal stress remains an open question.

To enhance the performance of gel electrolytes, researchers have explored multi-polymer blending strategies. Hsu et al. reported that an ozone polymerization treatment significantly improved the properties of a b-PVDF electrolyte membrane composed of PVDF, PAN, and PEO. Specifically, the electrochemical stability window was expanded from 4.8 V to 5.5 V, and the capacity retention remained at 91% after 300 cycles at 0.5 C. Additionally, the material exhibited excellent thermal stability, maintaining its structural integrity up to 150 °C, both before and after modification. Building upon this approach, we incorporated PVA into the existing PVDF/PAN/PEO polymer blend and employed a water-phase inversion method to develop a novel gel polymer electrolyte (GPE) with a three-dimensional network structure. In this design, PVDF-HFP serves as the primary framework, while the interactions among PAN, PEO, and PVA induce irregular molecular arrangements, promoting the formation of a porous membrane. This structure increases the porosity, enhances the pore distribution uniformity, and improves the electrolyte absorption (electrolyte contact angle of 21.15°). Electrochemical performance tests revealed that the b-GPE exhibits a wide electrochemical stability window of 5.2 V and a favorable lithium-ion transference number. After 300 cycles at 1 C, it retained 97.4% of its initial capacity, demonstrating a superior cycling stability. Furthermore, the b-GPE exhibited excellent safety in pouch cell damage tests. Even after heating at 150 °C for 2 h, it maintained its structural integrity with no significant changes. Additionally, we optimized the thermal treatment conditions and found that the b-GPE-120, processed at 120 °C, exhibited a performance nearly equivalent to the b-GPE. Its lithium-ion transference number, long-cycle stability, and rate performance in lithium metal batteries (LMBs) were comparable, making it a promising candidate for high-performance energy storage applications.

2. Experimental Section

2.1. Materials

Polyvinylidene–hexafluoropropylene (PVDF-HFP, M_w~400,000), polyvinyl oxide (PEO, M_w~5,000,000), polyacrylonitrile (PAN, M_w~150,000), polyvinyl alcohol (PVA, M_w~145,000), >-methylpyrrolidone (NMP, >99.5%), and butyl alcohol (99.5%) were all provided by Macklin (Shanghai, China). Commercial electrolyte (1M LiPF₆ in EC:DEC = 1:1 Vol % with 5% FEC), LiCoO₂, PVDF and Super P were all purchased from Guangdong Canrd New Energy Technology Co., LTD (Guangdong, China).

2.2. Preparation of b-PVDF and b-GPE

Using PVDF-HFP as the matrix, a mixture of PVDF-HFP, PAN, PEO, and PVA in a ratio of 6:2:1:1 (or others) was added to NMP solvent at a total polymer mass concentration of 8% wt. The solution was subsequently heated and stirred at 60 °C until fully dissolved, resulting in a thick translucent liquid. This liquid was subsequently dispensed onto aluminum foil using a dropper and scraped with a blade before being immersed in dispersant water. After standing for some time, the reverse phase transitioned into a gel state; the gel was then peeled off from the aluminum foil, dried at 50 °C for 8 h, cut into 19 mm diameter disks for all subsequent tests, placed in a glove box with argon, and soaked with commercial electrolytes to obtain b-GPE. The synthesis method for b-PVDF remains consistent, but with the mass ratio of PVDF-HFP, PAN, and PEO adjusted to 6:2:2.

2.3. Preparation of LiCoO₂ Cathode

First, prepare a 2.5% wt. PVDF-NMP solvent. Weigh a specific amount of LiCoO₂ and SP powder, then mix and grind them uniformly in an 8:1 ratio before drying in an oven at 50 °C. Next, add a predetermined quantity of PVDF-NMP solvent (maintaining a mass ratio of PVDF to LiCoO₂ of 1:8) and achieve a homogeneous slurry through magnetic stirring for 8 h. The resulting mixture is evenly scraped and coated onto clean aluminum foil using a tetrahedron preparation device, followed by overnight drying at 110 °C. After cutting, the LiCoO₂ positive electrode sheet is obtained, and the active material loading for each pole plate ranges from 2 to 2.4 mg cm⁻².

2.4. Material Characterizations

Scanning electron microscopy (Nova Nano SEM450, FEI company, Brno, Czech Republic) was used to characterize the b-GPE separators, and the morphology and elemental composition of the fiber film were observed. The functional group composition of the sample was tested by Fourier Transform Infrared Spectrometer (FT-IR, Nicolet 6700, Thermal power Company, Waltham, MA, USA) and Raman Spectrometer (DXR3, Thermo Fisher Scientific, Waltham, MA, USA). The surface morphology of Li metal was characterized by optical microscopy (Industrial Digital Camera, Shanghai, China), and the infiltrability of the diaphragm was characterized by contact angle meter (DSA100, KRUSS company, Hamburg, Germany).

2.5. Assembly and Electrochemical Measurements of LMBs

With LiCoO₂ as the positive electrode and pre-treated b-GPE as the separator and electrolyte, first place them into the electrode shell, then add Li metal as the negative electrode. Finally, place the gasket, spring, and negative electrode shell, and seal the battery using a battery sealing machine. After resting, the battery is ready for electrochemical testing. The performance was evaluated using the CHI760e electrochemical workstation, using electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), current–voltage curve (CA), and cyclic performance of the land system. EIS and CA are measured by passing the assembled symmetric cell (Li|b-GPE|Li), and LSV is tested by assembled button cell of Li|b-GPE|steel.

3. Results and Discussion

We designed and synthesized a gel polymer electrolyte, b-GPE, using polymer blending and water phase inversion, as illustrated in Figure 1. The four polymers were blended in the appropriate proportions and dissolved in NMP, which was followed by heating and stirring until the solution became clear and transparent. The resulting solution was subsequently spread onto aluminum foil, then immersed in water, where the phase inversion occurred, forming a three-dimensional network structure. In this structure, PVDF-HFP serves as the primary matrix, interwoven with other polymers to create a supportive network for the gel system. Upon the evaporation of NMP and water, the material was subsequently immersed in the electrolyte to yield the GPE, and the anions and cations in the Li salt are evenly distributed within the three-dimensional pores. To evaluate the optimal mixing ratio of the raw materials, the PVDF-HFP-based polymer membranes synthesized with four components at different proportions were compared regarding their lithium metal surface morphology after 20 cycles at a 0.5 mAh cm⁻² current density. As shown in Figure S1, lithium metal surfaces with a 50% PVDF-HFP content exhibited several byproduct formations and an inferior protective performance of the gel polymer films. A PVDF-HFP ratio of 60% demonstrated enhanced effectiveness compared to 70%, while among the three ratios (6:2:1:1, 6:1:2:1, and 6:1:1:2), the 6:2:1:1 formulation achieved a superior metallic luster on the lithium surface and optimal protection. This ratio was therefore selected as the focus of this study. As shown in Figure S2, the gel polymer membrane was cut to a suitable size by a tablet press, resulting in a measured thickness of approximately 75 μm. When soaking in the electrolyte, the membrane changed from white in its dry state to translucent.

The top-view SEM images reveal a surface with uniformly distributed pores ranging from 2 to 5 μm (Figure 2a and Figure S3a), while the cross-sectional views display a fibrous network structure (Figure 2b and Figure S3b), where fiber chains are interwoven randomly or in an ordered fashion, forming a highly porous three-dimensional network. Figure 2c,d show top and cross-sectional images of the b-GPE after the thermal treatment at 120 °C for 2 h. Compared with the untreated samples, some surface degradation is evident; however, the internal structure remains stable, demonstrating the excellent thermal and structural stability of the b-GPE based on PVDF-HFP. Even after 2 h of exposure to a temperature of 150 °C, there were no significant changes in the internal structure were observed (Figure S4). Energy dispersive X-ray spectroscopy (EDS) confirmed the primary composition of the b-GPE. As shown in Figure S5, the uniform distribution of the C, N, O and F elements confirmed the homogeneous blending of the four polymers, with C and F being the most abundant elements. In the FT-IR spectrum of the b-GPE (Figure 2e), the transmission peaks closely resemble those of PVDF-HFP. The peaks at 840 and 884 cm⁻¹ are associated with crystalline phase vibrations. The peak at 1073 cm⁻¹ corresponds to the C-O-C stretching vibration of PEO, while the peak at 1189 cm⁻¹ arises from the stretching vibration of -CF₂ groups. The peaks at 1402 and 1737 cm⁻¹ are attributed to the bending vibration and stretching vibration of -CH₂ groups, respectively. Notably, the peak at 2245 cm⁻¹ in the b-GPE is assigned to the -CN stretching vibration of PAN. Additionally, the broad absorption band around 3400 cm⁻¹ exhibits an enhanced intensity compared to PVDF-HFP, originating from the -OH groups of PVA [39]. Raman spectra of the b-GPE and the four polymers are shown in Figure 2f. Both PVA and PEO display weak Raman signals, with characteristic peaks in the 2850–3000 cm⁻¹ region corresponding to -OH and -CH₂ groups [40]. The peak profile of the b-GPE in this region closely matches that of PVDF-HFP, reflecting the C-H bond stretching vibrations, while peak enhancements are observed due to contributions from PVA, PEO, and PAN. The peak at 1430 cm⁻¹ is ascribed to the bending or twisting vibrations of -CH₂ groups in PAN, and the prominent peak at 2243 cm⁻¹ unequivocally originates from the -CN functional group of PAN. This further illustrates the blending of the four polymers while retaining some of the characteristics of each polymer.

In the thermal stability tests shown in Figure 3a, both the b-GPE and commercial PP were subjected to a temperature of 60 °C for 2 h, revealing no significant changes. However, at 90 °C, PP began to significantly curl due to its inferior thermal stability, while the b-GPE remained stable. As the temperature increased to 120 °C, the b-GPE exhibited minimal shrinkage, whereas PP was completely curled. At 150 °C, the b-GPE showed more noticeable shrinkage, and at 200 °C after 2 h, both materials underwent carbonization. These results confirm that the b-GPE has a superior thermal stability compared to PP up to 120 °C. For further validation, the b-GPE treated at 120 °C for 12 h (b-GPE-120) was selected for subsequent electrochemical testing. High electrolyte uptake and wettability mean that the gel polymer membrane can effectively absorb and retain the electrolyte. After swelling in the organic electrolyte, the resulting b-GPE exhibits both flexibility and extensibility, ensuring good contact with the electrode interface. The excellent wettability of the electrolyte helps maintain thermal stability under overheating conditions, reducing the risk of leakage or fire and thereby improving the safety performance of the battery [41].

The TGA curves of the polymer membranes before and after PVA incorporation are shown in Figure 3b. The thermal decomposition temperature range of b-PVDF (without PVA) was observed between 318.6 and 458.7 °C, while that of the b-GPE shifted to 342.7–497.3 °C, demonstrating a 24.1 °C enhancement in thermal stability through the PVA addition. This improvement can be attributed to the unique molecular chain characteristics of PVA, which combines an appropriate rigidity and flexibility. These structural features enable the b-GPE to maintain its structural integrity at elevated temperatures. Furthermore, the incorporation of PVA enhances intermolecular interactions between polymer chains, thereby reinforcing the overall thermal stability of the composite system. For polymer membranes as porous materials, the porosity can be determined via the butyl alcohol immersion method. Specifically, after the membrane is thoroughly dried, a disk-shaped sample is prepared using a cutting device, and its mass is recorded as the dry weight ($M_1$). Subsequently, the sample is immersed in n-butanol for 4 h. Excess n-butanol on the surface is carefully removed, and the wet weight ($M_2$) is immediately measured. The porosity ($P$) of the membrane is calculated using Equation (1):

$$P={\displaystyle \frac{M_2-M_1}{\rho \times V}}$$

(1)

where $V$ represents the geometric volume of the membrane and $\rho$ denotes the density of butyl alcohol. The final porosities of the PP, b-PVDF, and b-GPE were determined to be 41.80%, 67.52%, and 77.79%, respectively, suggesting that polymeric films with three-dimensional network structures exhibit an enhanced porosity (Figure 3c). The incorporation of PVA further increased the porosity of b-PVDF, which is likely attributed to the molecular-level interpenetration and entanglement between PVA and the other three polymers. This interaction facilitates the formation of an intricate three-dimensional network structure, thereby improving both the quantity and uniformity of the pores. As illustrated in Figure 3d, compared to PP with only a 76.8% electrolyte uptake rate, the b-GPE achieves 1228.2%, while b-GPE-120 shows a slight reduction to 1160.3% due to some dimensional shrinkage. In contact angle testing experiments, the initial contact angle of PP after the electrolyte application is 32.25°, which remains unchanged over time. In contrast, the b-GPE and b-GPE-120 show initial contact angles of 21.15° and 19.84°, respectively, with the electrolyte fully absorbed within 1 s, resulting in a contact angle of 0°, indicating their excellent wettability (Figure 3e).

In LSV tests, we observed that the electrochemical window of the battery assemblies using the three types of fiber membranes reached 5.2 V, indicating a wide voltage range (Figure 4a). Compared with PP, the gel membrane before and after the heat treatment did not reduce the stability of the electrochemical window, indicating that the synthesized gel membrane would not have side reactions with the electrolyte in the process of gelation, which would lead to changes in the electrochemical window. Figure 4b presents the Tafel plots for the three types of gel fiber membranes, indicating that the exchange current density of the b-GPE curve is the highest (−3.605), indicating that it has a lower electrochemical activation energy and better reaction kinetics, which enhances the migration efficiency of lithium-ions. Next is b-GPE-120 (−4.410), with PP (−4.654) having the lowest exchange current density. Subsequent transference number tests further confirm this point. Figure 4c shows the ionic conductivity of the b-GPE and b-GPE-120, which can be obtained by the following formula:

$$\sigma ={\displaystyle \frac{d}{R\times S}}$$

(2)

where $\sigma$ is the ionic conductivity, $d$ is the diaphragm thickness, $R$ is the body resistance, and $S$ is the effective electrode test area. The calculated ionic conductivity of the b-GPE and b-GPE-120 is 5.80 $\times$ 10⁻³ and 2.82 $\times$ 10⁻³ S cm⁻¹, respectively.

While assembling the symmetric lithium batteries, a small and constant potential difference $∆V$ (10 mV) was applied using the potentiostatic polarization method, while monitoring the change in the current over time. Figure 4d–f show the $t_{{Li}^{+}}$ for the PP, b-GPE, and b-GPE-120, calculated using the following formula:

$$t_{{Li}^{+}}={\displaystyle \frac{I_S\left(∆{V-I}_0{R}_0\right)}{I_0\left(∆{V-I}_S{R}_S\right)}}$$

(3)

where $I_0$ represents the initial current. As polarization progresses, a stable ion concentration gradient gradually forms within the battery, suppressing the migration of anions, and the system’s current is predominantly contributed to by cations, resulting in the steady-state current $I_S$; $R_0$ and $R_S$ are the interfacial resistances between the electrode and the electrolyte before and after polarization, respectively. The lithium-ion transference numbers for batteries assembled with the three materials are 0.308, 0.513, and 0.506, respectively. This indicates that both the b-GPE and b-GPE-120 have a high lithium-ion migration efficiency, which is beneficial for enhancing the energy efficiency during charging and discharging processes, reducing polarization phenomena, and maintaining a more stable voltage output.

The potential changes during continuous charge–discharge cycles of symmetric Li batteries reflect the stability of Li metal deposition and dissolution processes. Figure 5a presents the time–voltage curves for different materials under a current density of 0.5 mA cm⁻² for an hour of charging and discharging (with an inset showing the polarization curves over various cycle durations). From these curves, it can be observed that batteries assembled with PP separators begin to polarize after reaching a steady state (overpotential of 32 mV) at 200 h, likely due to the growth of Li dendrites. The polarization voltage gradually increases, and at 860 h, the separator is penetrated by dendrites, leading to a short circuit. In contrast, batteries assembled with the b-GPE and b-GPE-120 exhibit stable polarization voltages with overpotentials of 24 mV and 22 mV, respectively, and the cycle life of the b-GPE and b-GPE-120 are more than 1000 h. The lower polarization voltage and longer cycle life indicate that the b-GPE can effectively inhibit the growth of Li dendrites. Figure S6 shows the cycling curves under high current densities of 1 mA cm⁻² and 5 mA cm⁻². Here again, the b-GPE and b-GPE-120 exhibit the lower overpotential at 28 mV and 48 mV (33 mV and 80 mV), which are both significantly more stable than PP’s 105 mV and 213 mV. Moreover, testing at various current densities (0.5, 1, 3, 5 mA cm⁻²) and the same capacity density (0.5 mAh cm⁻²) revealed that the b-GPE and b-GPE-120 consistently showed a lower overpotential than PP, indicating good rate capability (Figure 5b). After 20 cycles at 0.5 mA cm⁻², SEM tests on the Li anodes from symmetric batteries assembled with the three types of separators (Figure 5c–e and Figure S7) show a stark contrast: the b-GPE and b-GPE-120 surfaces are smooth and free of Li dendrites or byproducts, whereas the PP surface clearly exhibits various deposits, reaffirming the b-GPE’s protective efficacy against Li metal. Even if the b-GPE is exposed to a high temperature for a period of time, it still protects the Li before the treatment and has good thermal stability.

To elucidate the composition of the solid electrolyte interphase (SEI) layer in Li metal batteries, the cycled Li symmetric cells (20 cycles at 0.2 mA cm⁻²) were disassembled, and the post-cycled Li metal anode was characterized by XPS. The peak deconvolution results of the four elemental spectra revealed that the SEI primarily consists of Li₂CO₃ and LiF (Figure 6). In the Li 1s spectrum, two dominant peaks at 55.0 eV and 55.8 eV were identified, corresponding to Li₂CO₃ and LiF, respectively [42]. Compared to the PP electrolyte, where Li₂CO₃ accounted for 60% of the SEI composition, the b-GPE exhibited a higher proportion of LiF (Figure 6(a1,a2)). The LiF-rich SEI formed in the b-GPE facilitates a uniform Li-ion deposition and suppresses Li dendrite growth [43]. The C 1s spectra showed minimal differences in the peak area between PP and the b-GPE, indicating no adverse effects from the b-GPE on carbonaceous components (Figure 6(b1,b2)). For the O 1s spectra, PP exhibited a higher Li₂CO₃ content, which impeded ion transport (Figure 6(c1,c2)). In the F 1s spectra (Figure 6(d1,d2)), the peak at 686.5 eV corresponded to Li_xPO_yF_z compounds derived from LiPF₆ decomposition [44]. Notably, the PP-based SEI contained more Li_xPO_yF_z, whereas the b-GPE exhibited a higher LiF content, further confirming the enhanced stability of the SEI formed by the b-GPE.

Furthermore, Figure 7 displays the optical morphology of the Li anode surfaces after 50 cycles at a current density of 0.5 mA cm⁻². The Li anode surface remains relatively smooth with no significant byproducts in b-GPE based batteries. In contrast, the surface of the Li anode with PP separators shows many black byproducts, likely Li carbonate formed due to side reactions with the electrolyte. The corrosion level of the Li anode surface protected by b-GPE-120 is similar to that of the b-GPE, indicating its structure is stable and exhibits a good electrochemical performance after a high-temperature pretreatment.

Figure 8 and Figure 9 provide optical and microscopic images of the Li anode after 50 cycles at 1.0 mA cm⁻² and 5.0 mA cm⁻². With an increasing current density, a pronounced intensification of electrochemical corrosion on the lithium metal anode was observed, which manifested as surface darkening and the proliferation of dark-colored deposits under microscopic examination. Notably, Li electrodes protected by the b-GPE and b-GPE-120 maintained a partial metallic luster, demonstrating a superior resistance to electrochemical corrosion compared to those utilizing polypropylene (PP) separators.

The performance of different separators was evaluated using the cells with LiCoO₂ and Li metal as cathodes and anodes. Figure 10a displays the discharge capacity, coulombic efficiency, and cycle number relationship under 1 C conditions for three types of separators. The graph shows that the Li metal batteries using b-GPE separators exhibited the highest discharge capacity, maintaining over 115 mAh g⁻¹ even after 300 cycles with a capacity retention of 97.4%. Although the discharge capacity was lower than that of the batteries with PP separators after 500 cycles, the b-GPE batteries had a longer cycle life, retaining 50% of their capacity even after 1500 cycles. The batteries with b-GPE-120 separators showed a slightly reduced performance compared to the b-GPE but still performed better than those with PP separators. Under 2 C conditions, the performance ranking of the LMB assemblies with the three separators remained the same (Figure 10b), and the b-GPE has a longer cycle life and a higher capacity retention rate.

Figure 10c compares the capacity performance of the Li metal batteries assembled with the three types of separators at different current densities (0.2 C, 0.5 C, 1 C, 2 C, 4 C, and 0.2 C). At any given current density, the batteries with b-GPE separators had the highest discharge capacities, reaching 133.4 mAh g⁻¹ at 0.2 C, with capacities of 126.5, 122.5, 104.2, and 74.7 mAh g⁻¹ at 0.5 C, 1 C, 2 C, and 4 C, respectively. When returned to 0.2 C conditions, the capacity increased back to 134.4 mAh g⁻¹, indicating an excellent rate performance. Similarly, the charge–discharge curves for the button cells assembled with the three types of separators under varying rate conditions in Figure 10c clearly demonstrate the highest discharge capacity for the b-GPE. Scaling up from button cells, the same structure was used to assemble flexible pouch batteries to meet different space requirements of devices. A freshly assembled pouch could illuminate a green light bulb, which remained bright even after folding and cutting experiments (Figure 10d,e). When cut with ceramic scissors, the light bulb briefly dimmed, likely due to the damage to the internal structure of the pouch during cutting, which increased the internal resistance and affected the uniformity and efficiency of the internal chemical reactions, thus causing a drop in the output voltage and a decrease in the current flowing through the light bulb. Once the internal reactions in the pouch stabilized, the light bulb illuminated again. Despite exposing the Li sheet, gel electrolyte, and cathode to the air, the light bulb remained bright (Figure 10f), demonstrating the excellent safety of the b-GPE and its adaptability to various working environments in the pouch format assembled with it.

4. Conclusions

This paper describes the fabrication of a novel gel polymer electrolyte (b-GPE) based on a PVDF-HFP matrix using the phase inversion method, intended for use as a separator in LMBs. The PVDF-HFP forms the primary framework, with the incorporation of PAN, PEO, and PVA enhancing the polymer’s porous structure and increasing both the porosity and uniformity of the pore distribution, thereby facilitating more lithium-ion channels. The PVDF-HFP-based gel electrolyte maintains its performance at elevated temperatures, with the addition of PVA improving the thermal stability of the polymer electrolyte. Even at 150 °C, it retains its complete microstructure, whereas PP separators almost entirely shrink. Electrochemical tests reveal that the b-GPE has a wide electrochemical window, higher lithium-ion transference number, greater capacity retention, and superior cyclic performance. Moreover, pouch batteries assembled with the b-GPE maintain their integrity even after folding and cutting and are still capable of lighting a small light bulb, demonstrating excellent safety. Therefore, selecting a polymer matrix with a high thermal stability (PVDF and its copolymer PVDF-HFP) and employing polymer blending effectively enhances the overall performance of gel polymer electrolytes in LMBs. Moving forward, we anticipate that further innovations in GPE performance will arise from optimizing interactions between various polymers and other components, such as inorganic fillers or ionic liquids.