Realizing Scalable Nano-SiO₂-Aerogel-Reinforced Composite Polymer Electrolytes with High Ionic Conductivity via Rheology-Tuning UV Polymerization

Abstract

Polymer electrolytes for lithium metal batteries have aroused widespread interest because of their flexibility and excellent processability. However, the low ambient ionic conductivity and conventional fabrication process hinder their large-scale application. Herein, a novel polyethylene-oxide-based composite polymer electrolyte is designed and fabricated by introducing nano-SiO₂ aerogel as an inorganic filler. The Lewis acid–base interaction between SiO₂ and anions from Li salts facilitates the dissociation of Li⁺. Moreover, the SiO₂ interacts with ether oxygen (EO) groups, which weakens the interaction between Li⁺ and EO groups. This synergistic effect produces more free Li⁺ in the electrolyte. Additionally, the facile rheology-tuning UV polymerization method achieves continuous coating and has potential for scalable fabrication. The composite polymer electrolyte exhibits high ambient ionic conductivity (0.68 mS cm⁻¹) and mechanical properties (e.g., the elastic modulus of 150 MPa). Stable lithium plating/stripping for 1400 h in Li//Li symmetrical cells at 0.1 mA cm⁻² is achieved. Furthermore, LiFePO₄//Li full cells deliver superior discharge capacity (153 mAh g⁻¹ at 0.5 C) and cycling stability (with a retention rate of 92.3% at 0.5 C after 250 cycles) at ambient temperature. This work provides a promising strategy for polymer-based lithium metal batteries.

1. Introduction

With the ever-increasing demand for efficient energy storage devices, lithium ion batteries (LIBs) have been widely used in modern society [1,2,3]. However, LIBs based on graphite anodes encounter a bottleneck because of their energy density limit. Due to the high theoretical capacity and low electrochemical potential of lithium metal, lithium metal batteries (LMBs) have been considered as promising alternatives [4]. Nevertheless, the organic liquid electrolytes used in LMBs are flammable and volatile, which presents serious safety issues [5]. Compared with organic liquid electrolytes, solid state electrolytes (SSEs) hold great promise in reducing safety risks due to their low flammability and lack of leakage. Moreover, SSEs with satisfactory mechanical properties are capable of suppressing the growth of Li dendrites [6].

As one of the important branches in SSEs, solid polymer electrolytes (SPEs) exhibit excellent structure flexibility and processability [7]. Poly (ethylene oxide) (PEO) is the most studied polymer type among numerous SPEs due to the high solvation capacity of the ether groups and chain mobility. However, the low ionic conductivity at room temperature and poor mechanical properties are persistent obstacles to the further applications of PEO-based electrolytes [8,9,10]. To tackle these limitations, various approaches have been conducted with PEO-based electrolytes. Plasticizers are introduced into polymer electrolytes to enhance the ionic conductivity, and the electrolytes obtained are called quasi-solid polymer electrolytes (QPEs) [11,12,13,14]. For example, the introduction of 60% succinonitrile (SN) endows poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/ LiTFSI with a high ionic conductivity of 1 mS cm⁻¹ at 0 °C [15]. The PEG/LiSTFSI/EC system shows excellent ionic conductivity of 0.19 mS cm⁻¹ at room temperature [16]. The plasticizers promote the dissociation of Li salts due to their plasticity and high polarity. Meanwhile, they increase the amorphous content contributing to chain motion. Hence, QPEs with plasticizers typically exhibit high conductivity. However, this solution is realized at the expense of diminished mechanical strength, which leads to an increased risk of lithium dendrite penetration. Dispersing inorganic filler nanoparticles into PEO-based electrolytes to obtain composite polymer electrolytes is an effective way to enhance ionic conductivity as well as mechanical properties [17,18,19,20]. For example, Cui et al. have developed a composite electrolyte with an interconnected SiO₂ aerogel backbone [9]. The electrolyte shows an enhanced ionic conductivity of 0.6 mS cm⁻¹ at 30 °C and an elastic modulus of up to 430 MPa.

Poly (ethylene glycol) acrylates (PEGAs) belonging to PEO-based electrolytes are widely investigated due to their high solvation ability of Li salts, enhanced electrochemical stability, and low crystallization. The solventless UV polymerization is an efficient and environmentally friendly method to fabricate PEGA electrolytes. Molded casting and the in situ method are dominant in the current reported solventless UV polymerization [16,21,22]. Nevertheless, due to the low viscosity of the electrolyte slurry, it is challenging to obtain the PEGA electrolytes with the desired membrane thickness and shape after casting using the above methods. Moreover, the mentioned methods are noncontinuous and not suitable for mass production. Hence, it is necessary to develop a fabrication method promising continuous casting production at a large scale.

In our previous study, we designed and fabricated a QPE with PEGAs as the polymer matrix, tetramethyl urea (TMU) as the plasticizer, poly(ethylene glycol) diacrylate (PEGDA) as the cross-linker, and polyethylene glycol terephthalate (PET) nonwoven as the supported framework [23]. However, the ionic conductivity (0.18 mS cm⁻¹) and mechanical properties (e.g., the elastic modulus of 60 MPa) still need further improvement. Herein, we report a nano-SiO₂-aerogel-reinforced composite QPE prepared via a facile rheology-tuning UV-initiated polymerization. The rheology-tuning slurry (RTS) possesses a suitable viscosity for continuous coating processes, promising scalable production. Furthermore, the modified QPE shows a high ionic conductivity of 0.68 mS cm⁻¹ and an elastic modulus (150 MPa) at room temperature. The Li//Li symmetric cells and LiFePO₄//Li fullcells exhibit superior electrochemical stability with the SiO₂-modified QPEs. This work provides new perspectives on how to design and fabricate QPEs for practical lithium metal batteries.

2. Results and Discussion

2.1. Rheology-Tuning UV Polymerization

In this work, rheology tuning is achieved by adjusting the viscosity and thixotropy via UV polymerization. Figure 1 illustrates the mechanism of rheology-tuning polymerization. First, poly(ethyleneglycol) methyl ether acrylate (PEGMEA) monomers are mixed with initiators, and the mixture is exposed to a UV lamp for several minutes until it turns into a viscous slurry. In this step, part of the monomers undergoes C=C bond polymerization. The RTS with polymers as solute and unreacted PEGMEA monomers as solvent belongs to a typical non-Newtonian fluid, possessing the shear-thinning characteristic. The viscosity of the RTS decreases along with the increasing shear rate, proving its shear-thinning behavior (Figure S1). In contrast, the PEGMEA monomers lacking rheology tuning show no shear-thinning behavior and quite low viscosities. Therefore, the RTS is suitable for a continuous coating process. After rheology tuning, the RTS is mixed with PEGDA, Li salts, plasticizer, nano-SiO₂ aerogel, and photoinitiator, coated on both sides of PET nonwoven, and cured under the UV lamp. The thickness of the electrolyte can be changed by adjusting the gap of the scraper.

To optimize the SiO₂ content, electrolyte membranes containing 0 wt%, 3 wt%, and 5 wt% SiO₂ are prepared and tested for their cycling stabilities in Li//Li symmetric cells at 0.1 mA cm⁻² and 0.1 mAh cm⁻². When the content of SiO₂ increases to more than 5 wt%, the electrolyte slurry becomes too viscous to be mixed up evenly. Hence, 5 wt% is selected as the highest content of SiO₂ in experimental electrolytes. Figure S2 illustrates that the Li//Li cells employing electrolytes with 0 wt%, 3 wt%, and 5 wt% SiO₂ exhibit a similar polarization voltage of approximately 70 mV. Nevertheless, QPEs with 0 wt% and 3 wt% SiO₂ exhibit short circuits within only 600 h and 750 h, respectively. By contrast, RTS-5% SiO₂ QPE holds stable cycling for 750 h, which indicates that RTS-5% SiO₂ QPE provides the highest mechanical strength to resist Li dendritic penetration in our exploration. Therefore, 5 wt% is selected as the optimized content of SiO₂ in the following research.

2.2. Physicochemical Characterization

The digital photo of the RTS-5% SiO₂ QPE membrane is presented in Figure 2a. The RTS-5% SiO₂ QPE membrane exhibits a flat and smooth surface. As illustrated in Figure 2b, a compact surface without any cracks is obtained on the QPE film. The SiO₂ is distributed uniformly, and no obvious agglomeration can be observed. Figure 2c shows the cross section of QPE with a thickness of 39 μm. The RTS-5% SiO₂ QPE slurry is supported by the PET nonwoven and cures successfully. Furthermore, the EDS mapping images are shown in Figure 2d–i. The S, Si, C, O, F, and N elements of RTS-5% SiO₂ QPE are distributed evenly, which further suggests that the QPE slurry has been coated on PET nonwoven uniformly and compactly.

The X-ray diffraction (XRD) patterns of RTS QPE and RTS-5% SiO₂ QPE membranes are exhibited in Figure 3a. No crystal state peaks can be observed in the XRD results. The amorphous state leads to improved ionic conductivity of the electrolyte. The electrolytes are also characterized by Differential Scanning Calorimetry (DSC). The T_g values of RTS-5% SiO₂ QPE and RTS QPE are −72.26 °C and −71.18 °C, respectively. It indicates that the electrolytes are amorphous at room temperature, which contributes to the mobility of polymer chains. Fourier transform infrared (FTIR) spectroscopy demonstrates that the characteristic peaks of SiO₂ exist in RTS-5% SiO₂ QPE, indicating that the original structure of SiO₂ remains after being mixed with the polymer slurry. There are no chemical changes that can be observed (Figure 3c).

For polymer electrolytes, satisfactory mechanical properties are essential indices for practical application in batteries. The stress–strain curve results in Figure 3d demonstrate that the RTS-5% SiO₂ QPE exhibits higher tensile strength (17.38 MPa) than the RTS QPE (7.29 MPa). Moreover, the detailed mechanical properties (

Table 1. Mechanical properties of polymer electrolyte membranes.

	RTS QPE	RTS-5% SiO2 QPE
Tensile strength (MPa)	7.29	17.38
Breaking elongation (%)	40.93	37.03
Maximum load (MPa)	9.01	23.47
Elastic modulus (MPa)	60	150

) indicate that the addition of SiO₂ enhances the tensile strength, maximum load, and elastic modulus, which contributes to suppressing the growth of Li dendrites and increasing the lifetime of cells.

2.3. Electrochemical Behaviors of RTS-5% SiO₂ QPE

To evaluate the Li⁺ transference ability in QPE, the electrochemical impedance spectra are conducted at room temperature (Figure 4a). The RTS-5% SiO₂ QPE exhibits a higher conductivity (0.68 mS cm⁻¹) than the RTS QPE (0.18 mS cm⁻¹) does. The increased ionic conductivity can be attributed to the role of the nano-SiO₂ aerogel. Firstly, the Lewis-acidic sites on SiO₂ interact with EO groups, which decreases the polymer crystallinity and weakens the interaction between Li⁺ and EO groups [24,25,26]. Second, the Lewis acid–base interaction between the SiO₂ and TFSI^- from Li salts promotes the dissociation of Li salts [27,28]. This synergistic effect produces more free Li⁺ in QPEs. As illustrated in Figure 4b, RTS-5% SiO₂ QPE is electrochemically stable up to 4.55 V, which is higher than RTS QPE (4.22 V) at room temperature. It demonstrates that SiO₂ is beneficial for improving the electrochemical stability of QPE. The Li⁺ transference number of RTS-5% SiO₂ is around 0.51, higher than that of RTS QPE (t (Li⁺) = 0.27) (Figure S3). This is due to an increase in free Li⁺ and a decrease in free TFSI^- anions in RTS-5% SiO₂ QPE.

To confirm the compatibility of the electrolyte with lithium metal, Li//Li symmetric cells sandwiched with RTS QPE and RTS-5% SiO₂ QPE are tested at 0.1 mA cm⁻², with a fixed capacity of 0.1 mAh cm⁻² (Figure 4d). The overpotential of Li/RTS QPE/Li cells suddenly increases after 600 h of cycling, indicating that the conduction of lithium ions in the electrolyte is blocked. The Li/RTS-5% SiO₂ QPE/Li, on the other hand, can cycle stably for 1400 h without short circuiting, indicating that the interface between the Li metal and electrolyte is stable during Li deposition and stripping. There are no lithium-ion-conductive obstructions or lithium-dendrite-piercing phenomena. The enlarged curves at 400−406 h demonstrate that the overpotential of Li/RTS-5% SiO₂ QPE/Li cells is lower than that of RTS-SiO₂ QPE (Figure 4e), which can be attributed to the increased ionic conductivity of the electrolyte.

2.4. Electrochemical Performance of RTS-5%-SiO₂-QPE-Based FullCells

To verify the practical application of the QPEs, LiFePO₄/QPE/Li fullcells are assembled and tested at ambient temperature. As illustrated in Figure 5a, the RTS-5% SiO₂ QPE exhibits a lower electrochemical impedance (≈163 Ω) than the RTS QPE (≈270 Ω), implying that the fast ion transference is conducted in the RTS-5% SiO₂ QPE. Meanwhile, it proves the higher ionic conductivity of RTS-5% SiO₂ QPE than that of RTS QPE. Cyclic voltammetry measurements are conducted to evaluate the electrochemical redox kinetics. The redox peak intensities of RTS-5% SiO₂ QPE are higher than those of RTS QPE, indicating the former QPE delivers a higher charge/discharge capacity (Figure 5b). It corresponds to the higher initial discharge capacity of LiFePO₄/RTS-5% SiO₂ QPE /Li cells at 1 C (147.5 mAh g⁻¹, Figure S4). The rate capability of fullcells is evaluated at various C-rates from 0.2 C to 1.5 C (Figure 5c). The LiFePO₄/RTS-5% SiO₂/Li cells deliver specific discharge capacities of 168.3, 157.3, 147.5, and 136.6 mAh g⁻¹ at 0.2, 0.5, 1, and 1.5 C. Moreover, the discharge capacity returns to the original values along with the return of current densities. Compared with the RTS-QPE-based batteries, the RTS-5% SiO₂ QPE shows an excellent rate performance and reversibility. The charge/discharge curves at various C-rates are presented in Figure 5d. In contrast to RTS-QPE-based cells, the RTS-5%-SiO₂-QPE-based cells exhibit higher specific capacities and lower polarization voltages.

To further test the electrochemical cyclic stability at high current density, galvanostatic charge/discharge cycling measurements are conducted at 0.5 C. After 250 cycles, the full cells employing RTS-5% SiO₂ QPE exhibit a higher specific capacity (140.7 mAh g⁻¹) and capacity retention (92.3%) than RTS QPE (Figure 5e), indicating excellent cycling stability and good interfacial contact between the electrode and RTS-5% SiO₂ QPE. The galvanostatic charge/discharge curves of the RTS-5%-SiO₂-QPE-based cells at the first, twentieth, fiftieth, and one hundred fiftieth cycles are presented in Figure 5f. During the first cycle to the one hundredth cycle, the charge/discharge curves almost overlap, and there is no obvious capacity decay, which further proves the superior cycle stability of RTS-5% SiO₂ QPE. In general, LiFePO₄/RTS-5% SiO₂ QPE/Li cells exhibit excellent capacity and electrochemical stability. The comparison for the electrochemical performance of polymer solid state electrolytes is shown in Table S1.

3. Materials and Methods

3.1. Preparation of RTS Recipe Slurry

First, 50 g of PEGMEA (M_w = 518; Sartomer CD551) and 0.02 g of 2,2-dimethoxy-2-phenylacetophenone photoinitiator were added into a flask filled with nitrogen. The mixture was stirred for 10 min to obtain a uniform solution. The solution was then placed under a 365 nm UV lamp for several minutes until a viscous slurry formed. In the meantime, the UV lamp and nitrogen were removed to stop the reaction. Second, 2 g of rheology-tuning slurry and 1 g of PEGDA (M_W = 608; Sartomer SR610) were added into a brown glass bottle for preliminary mixing, and then 3 g of TMU, 2 g of LiTFSI, and 0.01 g of photoinitiator were added in turn and stirred to obtain the RTS recipe slurry.

3.2. Preparation of SiO₂-Modified Quasi-Solid Polymer Electrolyte Membrane

3.2.1. RTS-5% SiO₂ Recipe Slurry

A total of 0.1 g of nano-SiO₂ aerogel was added into 2 g of RTS recipe slurry and stirred continuously until the SiO₂ was completely dispersed. After that, 0.01 g of photoinitiator was added and stirred.

3.2.2. RTS-5% SiO₂ QPE

RTS-5% SiO₂ recipe slurry was cast coated on both sides of PET nonwoven and then cured under a 365 nm UV lamp for several minutes to obtain the uniform RTS-5% SiO₂ QPEs.

The obtained electrolytes are named RTS-x% SiO₂ QPEs (x for the mass percentage of SiO₂ in the electrolytes, detailed in Table S2), while those without SiO₂ are labeled as RTS QPEs.

3.3. Preparation of LiFePO₄ Cathode

The LiFePO₄ cathode was fabricated by coating the slurry consisting of LFP (active material, 80 wt%), PVDF (binder, 6 wt%), KS-6 (conductive agent, 2 wt%), Super P (2 wt%), and RTS QPE (10 wt%) on the carbon-coated Al foil (C-Al) and then drying at 60 °C overnight. The C-Al was punched into pellets with a diameter of 8 mm. The mass loading of the cathode obtained was 2.16 mg cm⁻².

3.4. Physical Characterization

A Carl Zeiss SEM was conducted to observe the morphologies of the QPE. The Rigaku MiniFlex (Rigaku) X-ray diffractometer (XRD) was used for the crystal phase test. The test was performed under the following conditions: Cu Kα ray, voltage 35 kV, current 30 mA, scanning step diameter 0.01 s⁻¹, and scanning rate 5° min⁻¹. The scanning range of the sample was 3−80°. Fourier transform infrared (FTIR) spectra using Nicolet-IS50 (Thermo Fisher Scientific) were utilized to investigate the structure. Differential Scanning Calorimetry (DSC) was conducted on the Polyma 214 instrument. The mechanical properties were evaluated using the Instron 5967 tensile test machine.

3.5. Electrochemical Characterization

Electrochemical impedance spectroscopy (EIS) was used to evaluate the ionic conductivity of the QPE. The test was performed on Autolab Electrochemical Instrumentation (Metrohm) in the frequency range of 1 Hz to 100 kHz. The C-Al/QPE/C-Al batteries were assembled for the test. The ionic conductivity (σ) was calculated by the following equation:

$$\mathsf{\sigma }=\frac{\mathrm{L}}{\mathrm{RA}}$$

where L represents the thickness of the QPE membrane, R is the bulk resistance obtained from alternating current impedance analysis, and A is the contact area between C-Al and the electrolyte.

Li⁺ transference number (t_Li⁺) was obtained in Li/electrolyte/Li cells at room temperature by combining DC polarization and AC impedance via the following equation:

$${\mathrm{t}}_{{\mathrm{Li}}^{+}}=\frac{{\mathrm{I}}_{\mathrm{ap}}\left(\Delta \mathrm{V}-{\mathrm{I}}_{\mathrm{bp}}{\mathrm{R}}_{\mathrm{bp}}\right)}{{\mathrm{I}}_{\mathrm{bp}}\left(\Delta \mathrm{V}-{\mathrm{I}}_{\mathrm{ap}}{\mathrm{R}}_{\mathrm{ap}}\right)}$$

where ΔV is the voltage pulse at DC polarization applied to the symmetric cells, and its value is 10 mV. I_bp and I_ap represent the initial and steady currents, respectively. R_bp and R_ap represent the resistance before and after polarization, respectively. The linear sweep voltammetry was implemented in Li/QPE/stainless steel (SS) cells at a sweep rate of 10 mV s⁻¹ and a voltage range of 2.0 V to 6.0 V. The cyclic voltammetry (CV) was conducted at 0.1 mV s⁻¹ in a voltage range of 2.3–4.2 V. To obtain Li symmetric cells, QPE was sandwiched with two Li foils (diameter = 10 mm) in CR2016-type coin cells. To do the full cell assembly, a CR2016-type coin cell was assembled by contacting, in sequence, the LFP electrode (d = 8 mm), the QPE membrane (d = 19 mm), and a lithium foil (d = 10 mm). All batteries were assembled in an argon-filled glove box (H₂O < 0.1 ppm, O₂ < 0.1 ppm). On a LAND-CT3001A battery tester (Wuhan LAND Electronic Co., Ltd.), galvanostatic charge/discharge measurements were conducted to test the cyclic performance of Li symmetrical cells and fullcells LiFePO₄/QPE/Li. The LiFePO₄/QPE/Li fullcells were tested at a voltage range of 2.5 V−4.0 V. The electrochemical measurement tests described above were all conducted at 27 °C.

4. Conclusions

In summary, a nano-SiO₂-aerogel-modified QPE is prepared via a rheology-tuning UV-initiated polymerization. The RTS-5% SiO₂ QPE possesses a high ionic conductivity of 0.68 mS cm⁻¹ and mechanical strength at room temperature. The Lewis acid sites on SiO₂ aerogel interact with EO groups and anions from Li salts, which decreases the polymer crystallinity, boosts the number of free Li⁺, and thus promotes the ionic conductivity. The SiO₂-aerogel-reinforced electrolyte can not only suppress Li dendrites, but also contribute to the stability of electrochemical cycling. The Li//Li symmetric cells cycle stably over 1400 h. LiFePO₄/RTS-5% SiO₂/Li cells deliver impressive cycling stability, with a discharge capacity of 140.7 mAh g⁻¹ and 92.3% retention after 250 cycles at 0.5 C. UV-initiated solventless polymerization holds promise for large-scale continuous coating. This work gives new insights into the design and fabrication of composite polymer electrolytes for lithium metal batteries.