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

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.


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 SiO 2 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-SiO 2 -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 LiFePO 4 //Li fullcells exhibit superior electrochemical stability with the SiO 2 -modified QPEs. This work provides new perspectives on how to design and fabricate QPEs for practical lithium metal batteries.

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 shearthinning 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-SiO 2 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. 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.

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. To optimize the SiO 2 content, electrolyte membranes containing 0 wt%, 3 wt%, and 5 wt% SiO 2 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 SiO 2 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 SiO 2 in experimental electrolytes. Figure S2 illustrates that the Li//Li cells employing electrolytes with 0 wt%, 3 wt%, and 5 wt% SiO 2 exhibit a similar polarization voltage of approximately 70 mV. Nevertheless, QPEs with 0 wt% and 3 wt% SiO 2 exhibit short circuits within only 600 h and 750 h, respectively. By contrast, RTS-5% SiO 2 QPE holds stable cycling for 750 h, which indicates that RTS-5% SiO 2 QPE provides the highest mechanical strength to resist Li dendritic penetration in our exploration. Therefore, 5 wt% is selected as the optimized content of SiO 2 in the following research.

Physicochemical Characterization
The digital photo of the RTS-5% SiO 2 QPE membrane is presented in Figure 2a. The RTS-5% SiO 2 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 SiO 2 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% SiO 2 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% SiO 2 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 Tg 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). The X-ray diffraction (XRD) patterns of RTS QPE and RTS-5% SiO 2 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% SiO 2 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 SiO 2 exist in RTS-5% SiO 2 QPE, indicating that the original structure of SiO 2 remains after being mixed with the polymer slurry. There are no chemical changes that can be observed (Figure 3c).   (Table 1) indicate that the addition of SiO 2 enhances the tensile strength, maximum load, and elastic modulus, which contributes to suppressing the growth of Li dendrites and increasing the lifetime of cells.

Electrochemical Behaviors of RTS-5% SiO 2 QPE
To evaluate the Li + transference ability in QPE, the electrochemical impedance spectra are conducted at room temperature (Figure 4a). The RTS-5% SiO 2 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-SiO 2 aerogel. Firstly, the Lewisacidic sites on SiO 2 interact with EO groups, which decreases the polymer crystallinity and weakens the interaction between Li + and EO groups [24][25][26]. Second, the Lewis acidbase interaction between the SiO 2 and TFSIfrom 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% SiO 2 QPE is electrochemically stable up to 4.55 V, which is higher than RTS QPE (4.22 V) at room temperature. It demonstrates that SiO 2 is beneficial for improving the electrochemical stability of QPE. The Li + transference number of RTS-5% SiO 2 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 TFSIanions in RTS-5% SiO 2 QPE.

Electrochemical Behaviors of RTS-5% SiO2 QPE
To evaluate the Li + transference ability in QPE, the electrochemical impedance tra are conducted at room temperature (Figure 4a). The RTS-5% SiO2 QPE exhibits a h conductivity (0.68 mS cm −1 ) than the RTS QPE (0.18 mS cm −1 ) does. The increased conductivity can be attributed to the role of the nano-SiO2 aerogel. Firstly, the Lewis-a sites on SiO2 interact with EO groups, which decreases the polymer crystallinity weakens the interaction between Li + and EO groups [24][25][26]. Second, the Lewis acid interaction between the SiO2 and TFSIfrom Li salts promotes the dissociation of Li [27,28]. This synergistic effect produces more free Li + in QPEs. As illustrated in Figu RTS-5% SiO2 QPE is electrochemically stable up to 4.55 V, which is higher than RTS (4.22 V) at room temperature. It demonstrates that SiO2 is beneficial for improvin electrochemical stability of QPE. The Li + transference number of RTS-5% SiO2 is ar 0.51, higher than that of RTS QPE (t (Li + ) = 0.27) ( Figure S3). This is due to an increa free Li + and a decrease in free TFSIanions 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% SiO 2 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% SiO 2 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% SiO 2 QPE/Li cells is lower than that of RTS-SiO 2 QPE (Figure 4e), which can be attributed to the increased ionic conductivity of the electrolyte.

Electrochemical Performance of RTS-5%-SiO 2 -QPE-Based FullCells
To verify the practical application of the QPEs, LiFePO 4 /QPE/Li fullcells are assembled and tested at ambient temperature. As illustrated in Figure 5a, the RTS-5% SiO 2 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% SiO 2 QPE. Meanwhile, it proves the higher ionic conductivity of RTS-5% SiO 2 QPE than that of RTS QPE. Cyclic voltammetry measurements are conducted to evaluate the electrochemical redox kinetics. The redox peak intensities of RTS-5% SiO 2 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 LiFePO 4 /RTS-5% SiO 2 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 LiFePO 4 /RTS-5% SiO 2 /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% SiO 2 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%-SiO 2 -QPE-based cells exhibit higher specific capacities and lower polarization voltages. 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.

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 Molecules 2023, 28, 756 7 of 10 cells employing RTS-5% SiO 2 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% SiO 2 QPE. The galvanostatic charge/discharge curves of the RTS-5%-SiO 2 -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% SiO 2 QPE. In general, LiFePO 4 /RTS-5% SiO 2 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.

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 rheologytuning 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.

RTS-5% SiO 2 Recipe Slurry
A total of 0.1 g of nano-SiO 2 aerogel was added into 2 g of RTS recipe slurry and stirred continuously until the SiO 2 was completely dispersed. After that, 0.01 g of photoinitiator was added and stirred.

RTS-5% SiO 2 QPE
RTS-5% SiO 2 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% SiO 2 QPEs.
The obtained electrolytes are named RTS-x% SiO 2 QPEs (x for the mass percentage of SiO 2 in the electrolytes, detailed in Table S2), while those without SiO 2 are labeled as RTS QPEs.

Preparation of LiFePO 4 Cathode
The LiFePO 4 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 .

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.

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: 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: where ∆V 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.

Conclusions
In summary, a nano-SiO 2 -aerogel-modified QPE is prepared via a rheology-tuning UV-initiated polymerization. The RTS-5% SiO 2 QPE possesses a high ionic conductivity of 0.68 mS cm −1 and mechanical strength at room temperature. The Lewis acid sites on SiO 2 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 SiO 2 -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. LiFePO 4 /RTS-5% SiO 2 /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.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest:
The authors declare no conflict of interest.