Fabrication of PEO-PMMA-LiClO 4 -Based Solid Polymer Electrolytes Containing Silica Aerogel Particles for All-Solid-State Lithium Batteries

: To improve the ionic conductivity and thermal stability of a polyethylene oxide (PEO)-ethylene carbonate (EC)-LiClO 4 -based solid polymer electrolyte for lithium-ion batteries, polymethyl methacrylate (PMMA) and silica aerogel were incorporated into the PEO matrix. The effects of the PEO:PMMA molar ratio and the amount of silica aerogel on the structure of the PEO-PMMA-LiClO 4 solid polymer electrolyte were studied by X-ray diffraction, Fourier-transform infrared spectroscopy and alternating current (AC) impedance measurements. The solid polymer electrolyte with PEO:PMMA = 8:1 and 8 wt% silica aerogel exhibited the highest lithium-ion conductivity (1.35 × 10 − 4 S · cm − 1 at 30 ◦ C) and good mechanical stability. The enhanced amorphous character and high degree of dissociation of the LiClO 4 salt were responsible for the high lithium-ion conductivity observed. Silica aerogels with a high speciﬁc surface area and mesoporosity could thus play an important role in the development of solid polymer electrolytes with improved structure and stability.


Introduction
The constant development and widespread use of mobile devices are leading to increasing demand for secondary batteries for electrical energy storage. In particular, with the rapid growth in the smartphone market and the development of electric vehicles, the need for secondary batteries will continue to increase [1]. Current lithium-ion secondary batteries use a liquid electrolyte, whose leakage may cause fire and explosions, thus raising safety issues [2,3].
As a possible solution to this problem, all-solid-state lithium-ion batteries are being developed using a polymer or ceramic-based solid electrolyte [4,5]. The use of a solid electrolyte avoids risks of fire or explosions due to leakage and may also provide high capacity by reducing the cathodic limit of the potential window [6]. Among various solid electrolyte materials, solid polymers have been actively studied, owing to their good flexibility and interfacial stability [7].
Polyethylene oxide (PEO) presents various advantages, such as excellent interfacial stability with lithium, the formation of complexes with lithium salts, and a low glass transition temperature. However, the commercialization of PEO is difficult because its room-temperature ionic conductivity is as low as 10 −7 -10 −6 S·cm −1 [8,9]. This is because the degree of crystallization of PEO increases at room temperature, whereas the mobility of lithium ions and thus the ionic conductivity are reduced [10]. In addition, the stability of PEO decreases at high temperatures, so that deformation easily occurs [11].
In order to reduce crystallinity and improve thermal stability, a PEO electrolyte was blended with TiO 2 , SiO 2 , and Al 2 O 3 as inert inorganic fillers. Metal organic frameworks (MOF) with organic functional groups are known to be an effective additive to improve lithium-ion conductivity and to control interfacial resistance between PEO-based solid electrolytes and electrodes [12,13]. Another method to improve the properties of the polymer electrolyte involves copolymerization with two or more polymers [14][15][16]. Recently, polymers including PEO have been prepared by a photo-induced polymerization technique which is a fast, environmentally benign and energy-efficient process [17][18][19].
In this study, PEO was copolymerized with polymethyl methacrylate (PMMA) to obtain a solid polymer electrolyte with improved ionic conductivity and thermal stability. In addition, silica aerogel particles were incorporated into the copolymerized PEO-PMMA electrolyte as inert inorganic fillers. Silica aerogel is a mesoporous material with extremely high porosity and a specific surface area. Further experiments were carried out to optimize the PEO:PMMA ratio and the silica aerogel content.
PMMA was dissolved using ACN and the solution was stirred at 60 • C for 1 h. Then, PEO was added to the PMMA solution and mixed under the same conditions until a clear solution was obtained. EC and LiClO 4 were then added sequentially to the polymer solution and stirred at 60 • C for 12 h. Then, the silica aerogel powder was added to the polymer solution and stirred for another 24 h. Since EC and lithium salts are highly reactive with atmospheric moisture, the reaction was carried out in a glove box under a nitrogen atmosphere. The molar ratios of polymer to LiClO 4 and EC were 6:1 and 9:1, respectively. A total of 35.3 wt% of LiClO 4 was added to the polymers (PEO and PMMA). Finally, the polymer/lithium salt/silica aerogel mixture solution was poured into a mold and dried at 40 • C for 24 h under vacuum conditions, to obtain a thick film of solid electrolyte. Eleven (SP1 to SP6 and SP4-1 to SP4-5) solid polymer samples were prepared in this study and the PEO:PMMA ratios and silica aerogel content are shown in Table 1. X-ray diffraction (XRD, DMAX 2200, Rigaku, Tokyo, Japan) measurements were performed to determine the degree of crystallinity of the solid polymer electrolyte. Thermal properties such as glass transition (T g ) and melting (T m ) temperatures were measured by differential scanning calorimetry (DSC, Perkin Elmer, Santa Clara, CA, USA). The DSC analysis was carried out at a heating rate of 10 • C·min −1 under a nitrogen atmosphere, in the temperature range of −40 to 200 • C. The chemical structures of the polymer, lithium salt, and silica aerogel were analyzed by Fourier transform infrared spectroscopy (FT-IR, Vertex 80, Bruker, Ettlingen, Germany). The FT-IR measurements were performed in the 600-3000 cm −1 range, at a resolution of 4 cm −1 . To measure the thermal stability of the polymer electrolyte samples, the film was placed and maintained for 6 h in a hot oven at 80 • C.
The lithium-ion conductivity of the solid electrolyte was analyzed by alternating current (AC) impedance spectroscopy (IM6e, Zahner, Kronach, Germany) measurements in a frequency range from 1 Hz to 1 MHz, at 30 • C. Pt blocking electrodes with a diameter of 10 mm were coated on both surfaces of the solid electrolyte and placed in contact with two aluminum disks.

Results and Discussion
The XRD patterns of various solid polymer electrolytes are shown in Figure 1. No peaks corresponding to crystalline PEO or PMMA are visible in the patterns of the SP1 to SP4 samples, consisting of PEO, LiClO 4 and EC, or PEO, PMMA, LiClO 4 , and EC. This indicates that the SP1, SP2, SP3, and SP4 samples are amorphous. In addition, no peaks corresponding to LiClO 4 could be identified in these four samples. This result suggests the complete dissolution of LiClO 4 in PEO or the blended polymer electrolytes with a PEO:PMMA ratio greater than eight [20].
Energies 2018, 11, x FOR PEER REVIEW 3 of 10 (DSC, Perkin Elmer, Santa Clara, CA, USA). The DSC analysis was carried out at a heating rate of 10 °C•min −1 under a nitrogen atmosphere, in the temperature range of −40 to 200 °C. The chemical structures of the polymer, lithium salt, and silica aerogel were analyzed by Fourier transform infrared spectroscopy (FT-IR, Vertex 80, Bruker, Ettlingen, Germany). The FT-IR measurements were performed in the 600-3000 cm −1 range, at a resolution of 4 cm −1 . To measure the thermal stability of the polymer electrolyte samples, the film was placed and maintained for 6 h in a hot oven at 80 °C. The lithium-ion conductivity of the solid electrolyte was analyzed by alternating current (AC) impedance spectroscopy (IM6e, Zahner, Kronach, Germany) measurements in a frequency range from 1 Hz to 1 MHz, at 30 °C. Pt blocking electrodes with a diameter of 10 mm were coated on both surfaces of the solid electrolyte and placed in contact with two aluminum disks.

Results and Discussion
The XRD patterns of various solid polymer electrolytes are shown in Figure 1. No peaks corresponding to crystalline PEO or PMMA are visible in the patterns of the SP1 to SP4 samples, consisting of PEO, LiClO4 and EC, or PEO, PMMA, LiClO4, and EC. This indicates that the SP1, SP2, SP3, and SP4 samples are amorphous. In addition, no peaks corresponding to LiClO4 could be identified in these four samples. This result suggests the complete dissolution of LiClO4 in PEO or the blended polymer electrolytes with a PEO:PMMA ratio greater than eight [20]. Conversely, several sharp peaks associated with crystalline PEO, PMMA, PEO/LiClO4, and PMMA/LiClO4 phases were observed in the SP5 and SP6 samples, confirming that the blend with a PEO:PMMA ratio of 4:1 lies at the boundary of the miscibility/immiscibility regions [21]. The relative intensities of all peaks corresponding to the crystalline phases increased with increasing PMMA molar amounts. The degree of crystallinity, thus, showed a strong dependence on the molar PEO:PMMA ratio. In the case of the SP5 and SP6 samples, the peaks observed at 2θ = 22.5° and 23.5° were assigned to the crystalline LiClO4 phase [9,22]. In addition, their intensities increased with increasing PMMA molar amounts. Therefore, the incorporation of large amounts of PMMA appears to have had a negative impact on the LiClO4 complexation with the polymer electrolyte.
Contrastingly, the addition of silica aerogel powder to the PEO-PMMA polymer did not have an effect on the miscibility or the degree of crystallinity. All solid polymer samples containing silica aerogel powder were found to be amorphous, with no visible XRD peaks associated with the Conversely, several sharp peaks associated with crystalline PEO, PMMA, PEO/LiClO 4, and PMMA/LiClO 4 phases were observed in the SP5 and SP6 samples, confirming that the blend with a PEO:PMMA ratio of 4:1 lies at the boundary of the miscibility/immiscibility regions [21]. The relative intensities of all peaks corresponding to the crystalline phases increased with increasing PMMA molar amounts. The degree of crystallinity, thus, showed a strong dependence on the molar PEO:PMMA ratio. In the case of the SP5 and SP6 samples, the peaks observed at 2θ = 22.5 • and 23.5 • were assigned to the crystalline LiClO 4 phase [9,22]. In addition, their intensities increased with increasing PMMA molar amounts. Therefore, the incorporation of large amounts of PMMA appears to have had a negative impact on the LiClO 4 complexation with the polymer electrolyte.
Contrastingly, the addition of silica aerogel powder to the PEO-PMMA polymer did not have an effect on the miscibility or the degree of crystallinity. All solid polymer samples containing silica aerogel powder were found to be amorphous, with no visible XRD peaks associated with the crystalline phases or LiClO 4 . The complete dissolution of the Li salt in the polymer matrix was confirmed in the case of the SP1 to SP4 and SP4-1 to SP4-4 samples. This suggests that lithium-ion conduction Energies 2018, 11, 2559 4 of 10 occurred in the amorphous region, so that the segmental motion of the PEO-PMMA matrix was further improved in the composite polymer electrolyte samples [23]. Figure 2a,b show the DSC and derivative curves, respectively, of the solid polymer electrolytes. No endothermic peak was observed in the SP1 to SP4 and SP4-1 to SP4-4 samples, whereas two endothermic peaks were found for the SP5 and SP6 samples. However, all solid polymer samples exhibited an exothermic peak at around −15 to −5 • C, which can be attributed to the glass transition of PEO-or PEO-PMMA-based solid polymer electrolytes.
50 and 65 °C [21]. The endothermic peak at ~120 °C observed for SP5 and SP6 can be assigned to the crystalline (PEO)3:LiClO4 phase, because the melting temperature of PEO was 44.04 °C and PMMA dis not show endothermic peaks associated with melting in the temperature range from −40 to 200 °C [28]. The slightly lower melting temperature observed for SP5 and SP6 partially resulted from their high LiClO4:PEO ratio, because these samples were blended with PMMA.
The glass transition temperatures of the samples can be estimated from the derivatives of the DSC curves (Figure 2b). The Tg was found to be −8.4 °C for the PEO-LiClO4 complex solid polymer, and gradually decreased with the increasing PEO:PMMA ratio for the PEO-PMMA-LiClO4 solid polymer electrolytes. It is known that the Tg of PEO not complexed by LiClO4 is −55 °C, and the Tg value increases with increasing LiClO4 molar amounts [29]. The Tg measured for SP1 was in good agreement with the results obtained by Fullerton et al. [24]. However, the Tg of the solid polymer electrolytes containing silica aerogel powder was higher than that of the silica aerogel-free samples. Inert ceramic fillers such as silica aerogel can alter the crystallinity or the stiffness of polymer chains, resulting in an increase in the Tg of solid polymer electrolytes [30,31].   Based on the data in Figure 1, the absence of an endothermic peak (corresponding to the melting of crystalline PEO and PEO-PMMA) in the SP1 to SP4 and SP4-1 to SP4-4 samples was expected, because the PEO and PEO-PMMA samples complexed by LiClO 4 were amorphous [24,25]. In contrast, the endothermic peaks observed in SP5 and SP6 corresponded to the melting of crystalline phases such as PEO, PMMA and other complex phases.
In general, PEO and LiClO 4 formed several crystalline complexes, such as (PEO) 6 :LiClO 4 and (PEO) 3 :LiClO 4 , depending on the PEO:LiClO 4 ratio, temperature and thermal history [26,27]. The melting temperature of the (PEO) 3 :LiClO 4 complex was 150 • C, while (PEO) 6 :LiClO 4 melts between 50 and 65 • C [21]. The endothermic peak at~120 • C observed for SP5 and SP6 can be assigned to the crystalline (PEO) 3 :LiClO 4 phase, because the melting temperature of PEO was 44.04 • C and PMMA dis not show endothermic peaks associated with melting in the temperature range from −40 to 200 • C [28]. The slightly lower melting temperature observed for SP5 and SP6 partially resulted from their high LiClO 4 :PEO ratio, because these samples were blended with PMMA.
The glass transition temperatures of the samples can be estimated from the derivatives of the DSC curves (Figure 2b). The T g was found to be −8.4 • C for the PEO-LiClO 4 complex solid polymer, and gradually decreased with the increasing PEO:PMMA ratio for the PEO-PMMA-LiClO 4 solid polymer electrolytes. It is known that the T g of PEO not complexed by LiClO 4 is −55 • C, and the T g value increases with increasing LiClO 4 molar amounts [29]. The T g measured for SP1 was in good agreement with the results obtained by Fullerton et al. [24]. However, the T g of the solid polymer electrolytes containing silica aerogel powder was higher than that of the silica aerogel-free samples. Inert ceramic fillers such as silica aerogel can alter the crystallinity or the stiffness of polymer chains, resulting in an increase in the T g of solid polymer electrolytes [30,31]. Figure 3 shows the FT-IR spectra of the solid polymer electrolyte samples. The peak at 1100 cm −1 can be assigned to the C-O-C stretching of ether links in PEO. In addition, a peak attributed to the C-H stretching of PEO was observed near 2900 cm −1 , and its intensity decreased with increasing PMMA molar amounts. The peaks observed at 625 and 939 cm −1 were assigned to ClO 4 − [32], while a peak corresponding to uncomplexed LiClO 4 was observed at 1640 cm −1 in all solid polymer electrolyte samples [33].
Energies 2018, 11, x FOR PEER REVIEW 5 of 10 corresponding to uncomplexed LiClO4 was observed at 1640 cm −1 in all solid polymer electrolyte samples [33]. The intensity of the peak corresponding to LiClO4 was slightly higher in the SP5 and SP6 than the SP1 to SP4 samples. This is in good agreement with the XRD results in Figure 1. In contrast, the intensity of the free ClO4 − peak at 625 cm −1 increased as the PEO:PMMA molar ratio decreased to eight; however, the ClO4 − peak intensity decreased with increasing PMMA molar content. These results suggest that PMMA blending with the PEO matrix favors the dissociation of LiClO4 and the formation of PEO-LiClO4 complexes. However, when the PEO to PMMA molar ratio decreased to four and three, phase separation between PEO and PMMA occurred, which resulted in a decrease in the degree of dissociation of the lithium salt.
An interesting feature observed in Figure 3 is that the addition of silica aerogel to the PEO-PMMA polymer electrolyte favored the dissociation of LiClO4 and its complexation with the polymer matrix. As a result, the intensity of the peak corresponding to LiClO4 decreased in the SP4-1 to SP4-4 samples, whereas the intensity of the free ClO4 − peak at 600 cm −1 increased. Figure 4 shows the AC impedance spectra of symmetrical cells consisting of a solid polymer electrolyte and two Pt blocking electrodes, measured at 30 °C, along with their equivalent circuit model. The spectra show a semicircle at high frequency followed by a spike at low frequency, often observed in symmetrical cells with blocking electrodes. The equivalent circuit is composed of Rb and Cb in parallel, connected in series with Ce, where Rb and Cb are the bulk resistance and capacitance of the electrolyte, respectively, and Ce is the interfacial capacitance. Among these circuit elements, the bulk resistance of the solid polymer electrolyte corresponds to the diameter of the semicircle in the Nyquist plot shown in Figure 4. The lithium-ion conductivity can be calculated from the bulk resistance, the thickness of the electrolyte, and the area of the electrode. Table 2 shows the lithiumion conductivity of the solid polymer electrolyte samples. The intensity of the peak corresponding to LiClO 4 was slightly higher in the SP5 and SP6 than the SP1 to SP4 samples. This is in good agreement with the XRD results in Figure 1. In contrast, the intensity of the free ClO 4 − peak at 625 cm −1 increased as the PEO:PMMA molar ratio decreased to eight; however, the ClO 4 − peak intensity decreased with increasing PMMA molar content. These results suggest that PMMA blending with the PEO matrix favors the dissociation of LiClO 4 and the formation of PEO-LiClO 4 complexes. However, when the PEO to PMMA molar ratio decreased to four and three, phase separation between PEO and PMMA occurred, which resulted in a decrease in the degree of dissociation of the lithium salt. An interesting feature observed in Figure 3 is that the addition of silica aerogel to the PEO-PMMA polymer electrolyte favored the dissociation of LiClO 4 and its complexation with the polymer matrix. As a result, the intensity of the peak corresponding to LiClO 4 decreased in the SP4-1 to SP4-4 samples, whereas the intensity of the free ClO 4 − peak at 600 cm −1 increased. Figure 4 shows the AC impedance spectra of symmetrical cells consisting of a solid polymer electrolyte and two Pt blocking electrodes, measured at 30 • C, along with their equivalent circuit model. The spectra show a semicircle at high frequency followed by a spike at low frequency, often observed in symmetrical cells with blocking electrodes. The equivalent circuit is composed of R b and C b in parallel, connected in series with C e , where R b and C b are the bulk resistance and capacitance of the electrolyte, respectively, and C e is the interfacial capacitance. Among these circuit elements, the bulk resistance of the solid polymer electrolyte corresponds to the diameter of the semicircle in the Nyquist plot shown in Figure 4. The lithium-ion conductivity can be calculated from the bulk resistance, the thickness of the electrolyte, and the area of the electrode. Table 2 shows the lithium-ion conductivity of the solid polymer electrolyte samples.    Figure 5a shows the lithium-ion conductivities of the solid polymer electrolyte samples as a function of the PMMA molar ratio. The Li-ion conductivity increases with increasing PMMA molar amounts. The highest conductivity was obtained for the solid polymer electrolyte sample with PEO:PMMA = 8:1. The ion conductivity of PEO depends on the degree of crystallinity and the solubility of the lithium salt. Blending PMMA with PEO can result in a PEO-PMMA polymer with low degree of crystallinity, along with a low glass transition temperature. As was shown in Figure 2, the Tg of the PEO-PMMA blended polymer decreased with the addition of PMMA [34,35]. However, the further addition of PMMA led to a gradual decrease in conductivity. This phenomenon can be explained by the immiscibility between PEO and PMMA in solid polymer samples with high PMMA molar amounts, as shown in Figure 1. The formation of crystalline PEO-and PMMA-based polymers resulted in a severe decrease in carrier (i.e., lithium-ion) mobility.    Figure 5a shows the lithium-ion conductivities of the solid polymer electrolyte samples as a function of the PMMA molar ratio. The Li-ion conductivity increases with increasing PMMA molar amounts. The highest conductivity was obtained for the solid polymer electrolyte sample with PEO:PMMA = 8:1. The ion conductivity of PEO depends on the degree of crystallinity and the solubility of the lithium salt. Blending PMMA with PEO can result in a PEO-PMMA polymer with low degree of crystallinity, along with a low glass transition temperature. As was shown in Figure 2, the T g of the PEO-PMMA blended polymer decreased with the addition of PMMA [34,35]. However, the further addition of PMMA led to a gradual decrease in conductivity. This phenomenon can be explained by the immiscibility between PEO and PMMA in solid polymer samples with high PMMA molar amounts, as shown in Figure 1. The formation of crystalline PEO-and PMMA-based polymers resulted in a severe decrease in carrier (i.e., lithium-ion) mobility. The addition of silica aerogel powder to the PEO-PMMA blended polymer matrix effectively enhanced its conductivity. As the silica aerogel content was increased, the conductivity first increased, reaching a maximum at 8 wt%, and then decreased upon the further addition of silica aerogel. The highest conductivity of 1.35 × 10 −4 S•cm −1 was achieved for the SP4-3 sample. The incorporation of nanosized ceramic fillers such as TiO2, SiO2, ZrO2, and Al2O3 has been extensively used to enhance the lithium-ion conductivity of PEO and its related polymer electrolytes, as first proposed by Wieczorek et al. [36].
Ceramic fillers, and especially TiO2 nanoparticles, provide Lewis acid centers that can induce Lewis acid-based interactions between the polar surface of ceramic particles and lithium ions, which in turn create highly conducting transient pathways for lithium ions in the polymer electrolyte [37][38][39]. Although the strength of the Lewis acid center of SiO2 is much lower than that of TiO2, the Lewis acid-based interactions may be enhanced in the solid polymer electrolyte samples containing silica aerogel due to the extremely large specific surface area of the silica aerogel powder used in this study. In addition, the silica aerogel promotes Li salt dissociation, because the Lewis acid-base interactions between silica aerogel and lithium ions weaken the Li + -ClO4 − ionic couple [40]. As is shown in Figure  3, the FT-IR intensity of the ClO4 − peak increased with the increasing silica aerogel content. The increase in conductivity of the polymer electrolyte samples containing silica aerogel powder could be ascribed to the enhanced dissociation of the Li salt promoted by the silica aerogel.
The SP4-1, SP4-2, and SP4-3 samples exhibited lower conductivity than SP4. This was attributed to the fact that the high degree of dissociation induced by the silica aerogel was counteracted by the volumetric effect. When the silica aerogel content exceeded 8 wt%, e.g., in the SP4-5 sample, it was difficult to obtain a homogeneous film without cracks or defects.
Solid polymers used as electrolytes for lithium-ion secondary batteries are required to withstand unexpected temperature increases. Therefore, it is extremely important to develop solid polymer electrolytes with high stability at elevated temperatures. The thermal and mechanical stabilities of the solid polymer electrolyte samples are displayed in Figure 6. The polymer samples were heattreated at 80 °C for 6 h and subsequently folded or stretched. The SP1 sample, consisting of PEO and LiClO4, exhibited poor thermal and mechanical stability compared to samples blended with PMMA or silica aerogel powder. In particular, Figure 6b,c show that the SP1 sample did not recover its original film shape after being folded, and tore upon stretching.
In contrast, the SP4 and SP4-3 samples exhibited superior thermal and mechanical properties to SP1, which suggests that PMMA blending and incorporation of silica aerogel powder effectively allows the acquisition of thermally and mechanically stable solid electrolytes [41]. In particular, the SP4-3 sample, containing 8 wt% silica aerogel and possessing the highest lithium-ion conductivity, showed no shrinkage or shape change after the application of thermal and mechanical stresses. The addition of silica aerogel powder to the PEO-PMMA blended polymer matrix effectively enhanced its conductivity. As the silica aerogel content was increased, the conductivity first increased, reaching a maximum at 8 wt%, and then decreased upon the further addition of silica aerogel. The highest conductivity of 1.35 × 10 −4 S·cm −1 was achieved for the SP4-3 sample. The incorporation of nanosized ceramic fillers such as TiO 2 , SiO 2 , ZrO 2 , and Al 2 O 3 has been extensively used to enhance the lithium-ion conductivity of PEO and its related polymer electrolytes, as first proposed by Wieczorek et al. [36].
Ceramic fillers, and especially TiO 2 nanoparticles, provide Lewis acid centers that can induce Lewis acid-based interactions between the polar surface of ceramic particles and lithium ions, which in turn create highly conducting transient pathways for lithium ions in the polymer electrolyte [37][38][39]. Although the strength of the Lewis acid center of SiO 2 is much lower than that of TiO 2 , the Lewis acid-based interactions may be enhanced in the solid polymer electrolyte samples containing silica aerogel due to the extremely large specific surface area of the silica aerogel powder used in this study. In addition, the silica aerogel promotes Li salt dissociation, because the Lewis acid-base interactions between silica aerogel and lithium ions weaken the Li + -ClO 4 − ionic couple [40]. As is shown in Figure 3, the FT-IR intensity of the ClO 4 − peak increased with the increasing silica aerogel content.
The increase in conductivity of the polymer electrolyte samples containing silica aerogel powder could be ascribed to the enhanced dissociation of the Li salt promoted by the silica aerogel. The SP4-1, SP4-2, and SP4-3 samples exhibited lower conductivity than SP4. This was attributed to the fact that the high degree of dissociation induced by the silica aerogel was counteracted by the volumetric effect. When the silica aerogel content exceeded 8 wt%, e.g., in the SP4-5 sample, it was difficult to obtain a homogeneous film without cracks or defects.
Solid polymers used as electrolytes for lithium-ion secondary batteries are required to withstand unexpected temperature increases. Therefore, it is extremely important to develop solid polymer electrolytes with high stability at elevated temperatures. The thermal and mechanical stabilities of the solid polymer electrolyte samples are displayed in Figure 6. The polymer samples were heat-treated at 80 • C for 6 h and subsequently folded or stretched. The SP1 sample, consisting of PEO and LiClO 4 , exhibited poor thermal and mechanical stability compared to samples blended with PMMA or silica aerogel powder. In particular, Figure 6b,c show that the SP1 sample did not recover its original film shape after being folded, and tore upon stretching.
In contrast, the SP4 and SP4-3 samples exhibited superior thermal and mechanical properties to SP1, which suggests that PMMA blending and incorporation of silica aerogel powder effectively allows the acquisition of thermally and mechanically stable solid electrolytes [41]. In particular, the SP4-3 sample, containing 8 wt% silica aerogel and possessing the highest lithium-ion conductivity, showed no shrinkage or shape change after the application of thermal and mechanical stresses.

Conclusions
We fabricated PEO-PMMA-EC-LiClO 4 -based solid polymer electrolytes containing silica aerogel particles and investigated the dependence of their structure, thermal behavior, and lithium-ion conductivity on the PEO:PMMA ratio and silica aerogel content. The XRD and FT-IR analyses show that blending PMMA with PEO effectively retards the crystallization of the polymers and reduces their glass transition temperature, which in turn leads to enhanced lithium-ion conductivity. The solid polymer sample with PEO:PMMA = 8:1 (SP4) exhibited good conductivity (6.90 × 10 −5 S·cm −1 ). The ionic conductivity of the blended PEO-PMMA-EC-LiClO 4 solid polymer electrolyte was further increased by incorporating silica aerogel powder. The conductivity increased with the silica aerogel content, which might be due to the higher degree of lithium salt dissociation. The sample incorporating 8 wt% silica aerogel showed the highest conductivity, with a value of 1.35 × 10 −4 S·cm −1 . However, the incorporation of 16 wt% silica aerogel did not result in a further increase in conductivity. This effect is due to the aggregation of silica aerogel, which reduces the mobility of lithium ions. Finally, the addition of PMMA and silica aerogel significantly improved the thermal and mechanical stabilities of the PEO-based solid polymer electrolyte.