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Article Fabrication of PEO-PMMA-LiClO4-Based Solid Containing Silica Aerogel for All-Solid-State Batteries

Ye Sol Lim, Hyun-Ah Jung and Haejin Hwang * Department of Science and Engineering, Inha University, 100 Inha-ro, Michuhol-gu, Incheon 22212, Korea; [email protected] (Y.S.L.); [email protected] (H.-A.J.) * Correspondence: [email protected]; Tel.: +82-32-860-7521

 Received: 3 September 2018; Accepted: 19 September 2018; Published: 26 September 2018 

Abstract: To improve the ionic conductivity and thermal stability of a (PEO)-ethylene carbonate (EC)-LiClO4-based solid polymer for lithium- 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-LiClO4 solid polymer electrolyte were studied by X-ray diffraction, Fourier-transform 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 LiClO4 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.

Keywords: solid polymer electrolyte; all-solid-state lithium battery; polyethylene oxide; conductivity; silica aerogel

1. 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 electrolyte, whose leakage may cause fire and explosions, thus raising safety issues [2,3]. As a possible to this problem, all-solid-state lithium-ion batteries are being developed using a polymer or -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 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 transition . 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 of PEO increases at room temperature, whereas the mobility of lithium and thus the ionic conductivity are reduced [10]. In addition, the stability of PEO decreases at high , so that easily occurs [11]. In order to reduce and improve thermal stability, a PEO electrolyte was blended with TiO2, SiO2, and Al2O3 as inert inorganic fillers. organic frameworks (MOF) with organic functional

Energies 2018, 11, 2559; doi:10.3390/en11102559 www.mdpi.com/journal/energies Energies 2018, 11, 2559 2 of 10 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 [14–16]. Recently, polymers including PEO have been prepared by a photo-induced polymerization technique which is a fast, environmentally benign and energy-efficient process [17–19]. 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 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.

2. Materials and Methods The polymer electrolyte was prepared from commercially available PEO (molecular weight, Mw = 5,000,000, Sigma-Aldrich, Gyeonggi, Korea) and PMMA (Mw = 120,000, Sigma-Aldrich, Gyeonggi, Korea). Acetonitrile (ACN, 99.8%, Samchun Pure Chemical, Pyeongtaek, Korea), LiClO4 (99.99%, Sigma-Aldrich, Gyeonggi, Korea), and ethylene carbonate (EC, 98%, Sigma-Aldrich, Gyeonggi, Korea) were used as the solvents for lithium salt and plasticizer, respectively. A silica aerogel (Enova Aerogel IC3100) was purchased from Cabot Co. 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 LiClO4 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 . The molar ratios of polymer to LiClO4 and EC were 6:1 and 9:1, respectively. A total of 35.3 wt% of LiClO4 was added to the polymers (PEO and PMMA). Finally, the polymer/lithium salt/silica aerogel 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 Table1.

Table 1. Composition of the solid polymer electrolytes.

Sample Name PEO:PMMA (Molar Ratio) Silica Aerogel (wt%) SP1 1:0 - SP2 12:1 - SP3 10:1 - SP4 8:1 - SP5 4:1 - SP6 2:1 - SP4-1 8:1 1 SP4-2 8:1 2 SP4-3 8:1 4 SP4-4 8:1 8 SP4-5 8:1 16

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 (Tg) and (Tm) 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 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 Energiesstructures2018, 11of, 2559the polymer, lithium salt, and silica aerogel were analyzed by Fourier transform infrared3 of 10 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 in the 600–3000 cm−1 range, at a resolution of 4 cm−1. To measure the thermal stability of the polymer the polymer electrolyte samples, the film was placed and maintained for 6 h in a hot oven at 80 °C. 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) 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 impedance spectroscopy (IM6e, Zahner, Kronach, Germany) measurements in a frequency range from from 1 Hz to 1 MHz, at 30 °C. Pt blocking electrodes with a diameter of 10 mm were coated on both 1 Hz to 1 MHz, at 30 ◦C. Pt blocking electrodes with a diameter of 10 mm were coated on both surfaces surfaces of the solid electrolyte and placed in contact with two aluminum disks. of the solid electrolyte and placed in contact with two aluminum disks.

3.3. Results Results and and Discussion Discussion TheThe XRD XRD patterns patterns of of various various solid solid polymer polymer electrolytes electrolytes are are shown shown in in Figure Figure1. 1. No No peaks peaks correspondingcorresponding toto crystalline crystalline PEOPEO oror PMMAPMMA areare visiblevisible inin thethe patternspatterns ofof thethe SP1SP1 toto SP4SP4 samples,samples, consisting of PEO, LiClO4 and EC, or PEO, PMMA, LiClO4, and EC. This indicates that the SP1, SP2, consisting of PEO, LiClO4 and EC, or PEO, PMMA, LiClO4, and EC. This indicates that the SP1, SP3, and SP4 samples are amorphous. In addition, no peaks corresponding to LiClO4 could be 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 identified in these four samples. This result suggests the complete dissolution of LiClO4 in PEO or the blendedthe blended polymer polymer electrolytes electrolytes with with a PEO:PMMA a PEO:PMMA ratio ratio greater greater than eightthan eight [20]. [20].

FigureFigure 1. 1.XRD XRD patterns patterns of of solid solid polymer polymer electrolyte electrolyte samples. samples.

Conversely,Conversely, several several sharp sharp peaks peaks associated associated with with crystalline crystalline PEO, PEO, PMMA, PMMA, PEO/LiClO PEO/LiClO4,4,and and PMMA/LiClOPMMA/LiClO44 phasesphases werewere observedobserved inin thethe SP5SP5 andand SP6SP6 samples,samples, confirmingconfirming thatthat thethe blend blend with with a a PEO:PMMAPEO:PMMA ratio ratio of of 4:1 4:1 lies lies at at the the boundary boundary of of the the /immiscibility miscibility/immiscibility regions regions [ 21[21].]. The The relative relative intensitiesintensities of of all all peaks peaks corresponding corresponding to the to crystalline the crystalline phases phases increased increased with increasing with increasing PMMA PMMA molar amounts.molar amount The degrees. The of crystallinity, degree of crystallinity, thus, showed thus, a strong showed dependence a strong on the dependence molar PEO:PMMA on the molarratio. ◦ ◦ InPEO:PMMA the case of the ratio. SP5 In and the SP6 case samples, of the SP5 the and peaks SP observed6 samples, at the 2θ =peaks 22.5 observedand 23.5 atwere 2θ = assigned 22.5° and to 23.5° the crystallinewere assigned LiClO to4 the crystalline [9,22]. In addition, LiClO4 phase their intensities[9,22]. In addition, increased their with intensities increasing increasedPMMA molar with amounts.increasing Therefore, PMMA molar the incorporation amounts. Therefore, of large the amounts incorporation of PMMA of large appears amounts to have of hadPMMA a negative appears impactto have on had the a LiClO negative4 complexation impact on the with LiClO the4 polymer complexation electrolyte. with the polymer electrolyte. Contrastingly,Contrastingly, the the addition addition of of silica silica aerogel aerogel powder powder to to the the PEO-PMMA PEO-PMMA polymer polymer did did not not have have anan effect effect on on the the miscibility miscibility or or the the degree degree of of crystallinity. crystallinity. All All solid solid polymer polymer samples samples containing containing silica silica aerogelaerogel powder powder were were found found to be to amorphous, be amorphous, with no with visible no XRD visible peaks XRD associated peaks associated with the crystalline with the phases or LiClO4. 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, x FOR PEER REVIEW 4 of 10 crystalline phases or LiClO4. 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 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. 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 LiClO4 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 LiClO4 formed several crystalline complexes, such as (PEO)6:LiClO4 and (PEO)3:LiClO4, depending on the PEO:LiClO4 ratio, temperature and thermal history [26,27]. The melting temperature of the (PEO)3:LiClO4 complex was 150 °C, while (PEO)6:LiClO4 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: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. EnergiesThe2018 glass, 11, 2559transition temperatures of the samples can be estimated from the derivatives of4 ofthe 10 DSC curves (Figure 2b). The Tg was found to be −8.4 °C for the PEO-LiClO4 complex solid polymer, andoccurred gradually in the decreased amorphous with region, theso increasing that the segmental PEO:PMMA motion ratio of the for PEO-PMMA the PEO-PMMA matrix-LiClO was4 further solid polymerimproved electrolytes. in the composite It is known polymer that electrolyte the Tg of samplesPEO not [complexed23]. by LiClO4 is −55 °C, and the Tg valueFigure increases2a,b with show increasing the DSC and LiClO derivative4 molar amount curves,s respectively, [29]. The Tg of measured the solid polymerfor SP1 wa electrolytes.s in good agreementNo endothermic with the peak results was obtained observed by in Fullerton the SP1 toet SP4al. [2 and4]. However SP4-1 to, SP4-4 the Tg samples, of the solid whereas polymer two electrolytesendothermic containing peaks were silica found aerogel for thepowder SP5 andwas SP6higher samples. than that However, of the silica all solid aerogel polymer-free samples. samples Inertexhibited ceramic an exothermicfillers such as peak silica at aroundaerogel −can15 alter to − 5the◦C, crystallinity which can beor attributedthe stiffness to of the polymer glass transition chains, resultingof PEO- orin PEO-PMMA-basedan increase in the T solidg of solid polymer polymer electrolytes. electrolytes [30,31].

(a) (b)

FigureFigure 2. 2. DSCDSC (a (a) )and and derivative derivative ( (bb)) curves curves of of solid solid polymer polymer electrolyte electrolyte samples. samples.

FigureBased on3 shows the data the inFT Figure-IR spectra1, the of absence the solid of anpolymer endothermic electrolyte peak samples. (corresponding The peak to at the 1100 melting cm−1 canof crystallinebe assigned PEO to the and C- PEO-PMMA)O-C stretching in of theether SP1 links to SP4in PEO. and In SP4-1 addition, to SP4-4 a peak samples attributed was expected,to the C- −1 Hbecause stretching the PEOof PEO and was PEO-PMMA observed near samples 2900 complexed cm , and its by intensity LiClO4 were decreased amorphous with increasing [24,25]. In contrast,PMMA molarthe endothermic amounts. The peaks peaks observed observed in SP5at 625 and and SP6 939 corresponded cm−1 were assigned to the melting to ClO of4− [ crystalline32], while a phases peak such as PEO, PMMA and other complex phases.

In general, PEO and LiClO4 formed several crystalline complexes, such as (PEO)6:LiClO4 and (PEO)3:LiClO4, depending on the PEO:LiClO4 ratio, temperature and thermal history [26,27]. ◦ The melting temperature of the (PEO)3:LiClO4 complex was 150 C, while (PEO)6:LiClO4 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: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 (Figure2b). 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]. Figure3 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 −1 − molar amounts. The peaks observed at 625 and 939 cm were assigned to ClO4 [32], while a peak EnergiesEnergies 20182018,, 1111,, x 2559 FOR PEER REVIEW 5 5of of 10 10 corresponding to uncomplexed LiClO4 was observed at 1640 cm−1 in all solid polymer electrolyte −1 samplescorresponding [33]. to uncomplexed LiClO4 was observed at 1640 cm in all solid polymer electrolyte samples [33].

FFigureigure 3. 3. FTFT-IR-IR spectra spectra of of solid solid polymer polymer electrolyte electrolyte samples. samples.

TheThe intensity of thethe peakpeak correspondingcorresponding to to LiClO LiClO4 was4 was slightly slightly higher higher in in the the SP5 SP5 and and SP6 SP6 than than the theSP1 SP1 to SP4 to SP4 samples. samples. This This is in goodis in good agreement agreement with the with XRD the results XRD results in Figure in1 .Figure In contrast, 1. In contrast, the intensity the − −1 intensityof the free of ClOthe 4freepeak ClO4 at− peak 625 cmat 625increased cm−1 increased as the as PEO:PMMA the PEO:PMMA molar molar ratio decreasedratio decreased to eight; to − eighthowever,; however, the ClO the4 ClOpeak4− intensitypeak intensity decreased decreased with increasing with increasing PMMA PMMA molar content. molar content. These results These resultssuggest suggest that PMMA that PMMA blending blending with the with PEO the matrix PEO favors matrix the favors dissociation the dissociation of LiClO4 ofand LiClO the formation4 and the formationof PEO-LiClO of PEO4 complexes.-LiClO4 complexes. However, when However, the PEO when to PMMAthe PEO molar to PMMA ratio decreased molar ratio to four decreased and three, to fphaseour and separation three, phase between separation PEO andbetween PMMA PEO occurred, and PMMA which occurred, resulted which in a decreaseresulted in a the decrease degree in of thedissociation degree ofof dissociation the lithium of salt. the lithium salt. AnAn interesting feature feature observed observed in in Figure Figure3 is 3 that is that the addition the addition of silica of silica aerogel aerogel to the PEO-PMMA to the PEO- PMMApolymer polymer electrolyte electr favoredolyte favor the dissociationed the dissociation of LiClO of4 LiClOand its4 and complexation its complexation with the with polymer the polymer matrix. matrix.As a result, As a theresult, intensity the intensity of the peak of the corresponding peak corresponding to LiClO to4 decreased LiClO4 decreased in the SP4-1 in the to SP4 SP4-4-1 to samples, SP4-4 − −1 samples,whereas thewhereas intensity the intensity of the free of ClO the4 freepeak ClO at4− 600peak cm at 600increased. cm−1 increased. FigureFigure 44 showsshows thethe ACAC impedanceimpedance spectraspectra ofof symmetricalsymmetrical cellscells consistingconsisting ofof aa solidsolid polymerpolymer ◦ electrolyteelectrolyte andand twotwo Pt Pt blocking blocking electrodes, electrodes, measured measured at 30at 30C, along°C, along with with their equivalenttheir equivalent circuit circuit model. model.The spectra The spectra show a semicircleshow a semicircle at high frequency at high frequency followed byfollowed a spike by at lowa spike frequency, at low oftenfrequency, observed often in observedsymmetrical in symmetrical cells with blocking cells with electrodes. blocking The electrodes. equivalent The circuit equivalent is composed circuit ofis Rcomposedb and Cb inof parallel,Rb and Cconnectedb in parallel, in seriesconnected with Cine ,series where withRb and Ce, whereCb are R theb and bulk Cb resistance are the bulk and resistance and of capacitance the electrolyte, of therespectively, electrolyte and, respectively,Ce is the interfacial and Ce is capacitance. the interfacial Among capacitance. these circuit Among elements, these thecircuit bulk elements, resistance the of bulkthe solid resistance polymer of the electrolyte solid polymer corresponds electrolyte to the corresponds diameter of theto the semicircle diameter in theof the Nyquist semicircle plot shownin the Nyquistin Figure plot4. The shown lithium-ion in Figure conductivity 4. The lithium can be- calculatedion conductivity from the can bulk be resistance, calculated the from thickness the bulk of resistance,the electrolyte, the thickness and the area of the of theelectrolyte, electrode. and Table the 2area shows of the the electrode. lithium-ion Table conductivity 2 shows the of thelithium solid- ionpolymer conductivit electrolytey of the samples. solid polymer electrolyte samples.

Energies 2018, 11, 2559 6 of 10 Energies 2018, 11, x FOR PEER REVIEW 6 of 10

FigureFigure 4.4. ACAC impedanceimpedance spectraspectra ofof solidsolid polymerpolymer electrolyte electrolyte samples.samples.

Table 2. Lithium-ion conductivities of solid polymer electrolyte samples. Table 2. Lithium-ion conductivities of solid polymer electrolyte samples. PEO:PMMA Silica Aerogel Ion Conductivity Sample PEO:PMMA Silica Aerogel −1 Ion ◦Conductivity Sample (Molar Ratio) (wt%) (S·cm , at 30 C) (Molar Ratio) (wt%) (S·cm−1, at 30 °C ) SP1 1:0 - 2.36 × 10−5 −5 SP1 SP21:0 12:1 - - 3.64 × 10−5 2.36 × 10 SP2 SP312:1 10:1 - - 6.18 × 10−5 3.64 × 10−5 −5 SP3 SP410:1 8:1 - - 6.70 × 10 6.18 × 10−5 SP5 4:1 - 4.63 × 10−5 SP4 8:1 - 6.70 × 10−5 SP6 2:1 - 4.40 × 10−5 −5 SP5 SP4-14:1 8:1 1 - 2.52 × 10−5 4.63 × 10 SP6 SP4-22:1 8:1 2 - 2.83 × 10−5 4.40 × 10−5 −5 SP4-1 SP4-38:1 8:1 4 1 5.37 × 10 2.52 × 10−5 SP4-4 8:1 8 1.35 × 10−4 SP4-2 8:1 2 2.83 × 10−5 SP4-5 8:1 16 1.02 × 10−4 SP4-3 8:1 4 5.37 × 10−5 SP4-4 8:1 8 1.35 × 10−4 Figure5a shows the lithium-ion conductivities of the solid polymer electrolyte samples as a SP4-5 8:1 16 1.02 × 10−4 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:PMMAFigure 5a = shows8:1. The the ion lithium conductivity-ion conductivities of PEO depends of the solidon the polymer degree electrolyte of crystallinity samples and as the a solubilityfunction of of the the PMMA lithium molar salt.Blending ratio. The PMMA Li-ion conductivity with PEO can increases result in with a PEO-PMMA increasing polymerPMMA molar with lowamount degrees. The of crystallinity, highest conductivity along with was a low obtained glass transition for the temperature. solid polymer As elect wasrolyte shown sample in Figure with2, thePEO:PMMATg of the PEO-PMMA = 8:1. The ion blended conductivity polymer of decreased PEO depends with the on addition the degree of PMMA of crystallinity [34,35]. However, and the thesolubility further of addition the lithium of PMMA salt. Blending led to a PMMA gradual with decrease PEO incan conductivity. result in a PEO This-PMMA phenomenon polymer can with be explainedlow degree by of the crystallinity, immiscibility along between with a PEO low andglass PMMA transition in solid temperature. polymer samplesAs was shown with high in Figure PMMA 2, molarthe Tg amounts,of the PEO as-PMMA shown inblended Figure polymer1. The formation decreased of with crystalline the addition PEO- and of PMMA PMMA-based [34,35]. polymersHowever, resultedthe further in aaddition severe decrease of PMMA in carrierled to (i.e.,a gradual lithium-ion) decrease mobility. 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. Energies 2018, 11, 2559 7 of 10 Energies 2018, 11, x FOR PEER REVIEW 7 of 10

(a) (b)

Figure 5. LithiumLithium-ion-ion conductivit conductivityy of of solid solid polymer polymer electrolyte electrolyte samples samples as as a a function of of PMMA PMMA ( (aa)) and silica aerogel (b) content.

The addition of silica aerogel powder to the PEO-PMMAPEO-PMMA blended polymer matrix effectively enhanced its its conductivity. conductivity. As As the the silica sil aerogelica aerogel content content was increased, was increased, the conductivity the conductivity first increased, first increased,reaching a reaching maximum a maximum at 8 wt%, at and 8 wt%, then and decreased then decreased upon the upon further theaddition further addition of silica of aerogel. silica Theaerogel. highest The conductivity highest conductivity of 1.35 × 10 of− 4 1.35S·cm ×− 1 10was−4 S∙cm achieved−1 was for achieved the SP4-3 for sample. the SP4 The-3 incorporation sample. The incorporof nanosizedation ceramicof nanosized fillers ceramic such as fillers TiO 2such, SiO as2, TiO ZrO22, ,SiO and2, ZrO Al2O2, 3andhas Al been2O3 has extensively been extensively used to usedenhance to enhance the lithium-ion the lithium conductivity-ion conductivity of PEO and of itsPEO related and polymerits related electrolytes, polymer electrolytes, as first proposed as first by Wieczorekproposed by et Wieczorek al. [36]. et al. [36]. Ceramic fillers, fillers, and especially TiO2 , provide Lewis acid centers that can induce Lewis acid acid-based-based interactions between the polar surface of ceramic particles and lithium ions, which in turn createcreate highlyhighly conductingconducting transient transient pathways pathways for for lithium lithium ions ions in in the the polymer polym electrolyteer electrolyte [37 –[3739–]. Although39]. Although the strengththe strength ofthe of the Lewis Lewis acid acid center center of of SiO SiO2 is2 is much much lower lower than than that that of of TiO TiO22,, thethe LewisLewis acidacid-based-based interactions may be enhanced in the solid polymer electrolyte samples containing silica aerogel due to the extremely extremely large large specific specific surface area area of of the the silica silica aerogel aerogel powder powder used used in in this this study. study. In addition, the silica aerogel promotes Li salt dissociation,dissociation, because the Lewis acid-baseacid-base interactions + + − − between silica silica aerogel aerogel and and lithium lithium ions ions weaken weaken the the Li Li-ClO-ClO4 ionic4 ionic couple couple [40]. A [40s is]. shown As is shownin Figure in − − 3,Figure the 3 FT, the-IR FT-IR intensity intensity of the of ClO the4 ClO peak4 increasedpeak increased with the with increasing the increasing silica silica aerogel aerogel content. content. The Theincrease increase in conductivity in conductivity of the of thepolymer polymer electrolyte electrolyte samples samples containing containing silica silica aerogel aerogel powder powder could could be ascribed to the enhanced dissociationdissociation of the Li salt promoted byby the silica aerogel.aerogel. The SP4 SP4-1,-1, SP4 SP4-2,-2, and SP4 SP4-3-3 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 the volumetricvolumetric effect. When the the silica aerogel content exceeded 8 8 wt%, e.g., in the SP4 SP4-5-5 sample, it was difficultdifficult to obtain a homogeneous filmfilm without crackscracks oror defects.defects. Solid polymers used as electrolytes electrolytes for lithium lithium-ion-ion secondary batteries are required to withstan withstandd unexpected temperature increases. Therefore, Therefore, it it is is extremely important to develop solid polymer electrolytes withwith highhigh stabilitystability at at elevated elevated temperatures. temperatures. The The thermal thermal and and mechanical mechanical stabilities stabilities of the of thesolid solid polymer polymer electrolyte electrolyte samples samples are displayed are displayed in Figure in 6Fi. Thegure polymer 6. The polymer samples weresamples heat-treated were heat at- ◦ treated80 C for at 680 h °C and for subsequently 6 h and subsequently folded or folded stretched. or stretched. The SP1 The sample, SP1 consistingsample, consisting of PEO andof PEO LiClO and4, LiClOexhibited4, exhibited poor thermal poor thermal and mechanical and mechanical stability stability compared compared to samples to samples blended blended with PMMA with orPMMA silica oraerogel silica powder. aerogel Inpowde particular,r. In particular, Figure6b,c Fig showure that6b,c the show SP1 that sample the didSP1 not sample recover didits not original recover film its originalshape after film being shape folded, after being and tore folded upon, and stretching. 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.

Energies 2018, 11, 2559 8 of 10 Energies 2018, 11, x FOR PEER REVIEW 8 of 10

FigureFigure 6. 6.Photographs Photographs of ( ofa) As-prepared(a) As-prepared solid solidpolymer polymer electrolyte electrolyte samples, samples (b) Solid, ( polymerb) Solid samples polymer whichsample weres which heat-treated were heat at- 80treated◦C for at 6 80 h and °C for then 6 folded,h and then and (foldedc) Solid, and polymer (c) Solid samples polymer whichsamples were heat-treatedwhich were at heat 80 -◦treatedC for 6 ath and80 °C then for stretched.6 h and then stretched.

4. ConclusionsIn 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 We fabricated PEO-PMMA-EC-LiClO4-based solid polymer electrolytes containing silica aerogel the acquisition of thermally and mechanically stable solid electrolytes [41]. In particular, the SP4-3 particles and investigated the dependence of their structure, thermal behavior, and lithium-ion sample, containing 8 wt% silica aerogel and possessing the highest lithium-ion conductivity, showed no conductivity on the PEO:PMMA ratio and silica aerogel content. The XRD and FT-IR analyses show shrinkage or shape change after the application of thermal and mechanical stresses. that blending PMMA with PEO effectively retards the crystallization of the polymers and reduces 4.their Conclusions glass transition temperature, which in turn 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). We fabricated PEO-PMMA-EC-LiClO -based solid polymer electrolytes containing silica aerogel The ionic conductivity of the blended PE4O-PMMA-EC-LiClO4 solid polymer electrolyte was further particlesincreased and by investigatedincorporating the silica dependence aerogel powder. of their The structure, conductivity thermal increased behavior, with andthe silica lithium-ion aerogel conductivitycontent, which on the might PEO:PMMA be due ratio to the and higher silica aerogel degree content. of lithium The XRD salt dissociation.and FT-IR analyses The sample show thatincorporat blendinging PMMA 8 wt% silica with aerogel PEO effectively showed the retards highest the conductivity, crystallization with of thea value polymers of 1.35 and × 10 reduces−4 S∙cm−1. theirHowever glass, transition the incorporation temperature, of which 16 wt% in turn silica leads aerogel to enhanced did not lithium-ion result in conductivity. a further increase The solid in × −5 · −1 polymerconductivity. sample This with effect PEO:PMMA is due to the = 8:1 aggregation (SP4) exhibited of silica good aerogel, conductivity which reduces (6.90 the10 mobilityS cm ).of Thelithium ionic ions. conductivity Finally, ofthe the addition blended ofPEO-PMMA-EC-LiClO PMMA and silica aerogel4 solid significantly polymer electrolyte improved was the furtherthermal increasedand mechanical by incorporating stabilities of silica the aerogelPEO-based powder. solid Thepolymer conductivity electrolyte. increased with the silica aerogel content, which might be due to the higher degree of lithium salt dissociation. The sample incorporating −4 −1 8Author wt% silica Contributions: aerogel showed Conceptualization, the highest H conductivity,.H.; investigation, with H a.-A value.J.; supervision, of 1.35 × H10.H.; andS·cm writing. However,—original thedraft, incorporation Y.S.L. 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 Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the additionKorea government of PMMA (MSIT) and silica (No. aerogel2016R1A2B2011517 significantly). This improved work was the supported thermal and by the mechanical Korea Institute stabilities of Energy of theTechnology PEO-based Evaluation solid polymer and Planning electrolyte. (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20174030201500). Author Contributions: Conceptualization, H.H.; investigation, H.-A.J.; supervision, H.H.; and writing—original draft,Conflicts Y.S.L. of Interest: The authors declare no conflicts of interest. Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2016R1A2B2011517). This work was supported by the Korea Institute of Energy Energies 2018, 11, 2559 9 of 10

Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20174030201500). Conflicts of Interest: The authors declare no conflicts of interest.

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