Thermal Properties of Composite Polymer Electrolytes Poly(Ethylene Oxide)/Sodium Triﬂuoroacetate/Aluminum Oxide (PEO)10Cf3coona + X Wt.% Al2o3

materials

Article Thermal Properties of Composite Polymer Electrolytes Poly(Ethylene Oxide)/Sodium Triﬂuoroacetate/Aluminum Oxide (PEO)10CF3COONa + x wt.% Al2O3

Miguel I. Delgado Rosero *, Nori M. Jurado Meneses and Ramiro Uribe Kaﬀure

Physics Department, University of Tolima, Ibagué, CP 730006, Colombia; [email protected] (N.M.J.M.); [email protected] (R.U.K.) * Correspondence: [email protected]

Received: 27 March 2019; Accepted: 18 April 2019; Published: 7 May 2019

Abstract: Polymeric membranes of poly(ethylene oxide) (PEO) and sodium trifluoroacetate (PEO:CF3COONa) combined with different concentrations of aluminum oxide (Al2O3) particles were analyzed by impedance spectroscopy, differential scanning calorimetry (DSC) and thermogravimetry (TGA). DSC results show changes in the crystalline fraction of PEO when the concentration of Al2O3 is increased. TGA analysis showed thermal stability up to 430 K showing small changes with the addition of alumina particles. The decrease in crystalline fraction for membranes with low Al2O3 concentration is associated with the increase in conductivity of (PEO)10CF3COONa + x wt.% Al2O3 composites.

Keywords: polymer electrolytes; DSC; TGA; phase transitions

1. Introduction Solid polymer electrolytes (SPEs) are materials that have been widely investigated for their potential use in a variety of electrochemical devices such as fuel cells, batteries, electrochromic windows, supercapacitors, among others [1]. To be used in these devices, SPEs must show high ionic conductivity, good electrochemical stability, and a wide thermal stability range [2]. Improvements in these physicochemical and structural characteristics have been reported when SPEs are added with a variety of ﬁllers [3]. For instance, SPEs formed by poly(ethylene oxide) (PEO) and alkaline metallic salts such as Na+, K+ and Li+, show a reduction of the crystalline phase and an increase in dissolved ions mobility. These changes lead to systems with relatively high ionic conductivity values (σ ~ 1 10 5 S cm 1)[4,5]. × − − However, the need for even higher conductivity values, such as those required in various technological applications, has led to strategies to further increase the amorphous phase by also adding plasticizers and/or ceramic materials [6,7]. Recent reports have shown a meaningful improvement in the electrical, thermal and mechanical properties of polymeric electrolyte membranes synthesized with the addition of inert particles of aluminum oxide (Al2O3), titanium dioxide (TiO2) or silicon dioxide (SiO2), among others [8–10]. As a result of their large speciﬁc surface, these inorganic oxides show strong Lewis acid interactions upon the PEO matrix that create additional hopping sites and adequate pathways for ionic motion. Particularly, in a previous study, the electrical properties of the (PEO)10CF3COONa + x wt.% Al2O3 composite system were analyzed and it was found that, for low alumina concentrations, ionic conductivity increased by two orders of magnitude relative to the pure polymer [11].

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The improvement in thermal, electrical and mechanical properties of electrolytes based on PEO added with ceramic particles, can be explained by the decline of the kinetics of polymer crystallization, which increases the amorphous phase in localized regions and contributes to the formation of highly conductive pathways [12,13]. Relevant information related to the fraction of crystalline phase in these systems, can be obtained from the measurement of changes in melting points and the enthalpy of phase transitions [14]. In this work, we conducted a thermal analysis of (PEO)10CF3COONa + x wt.% Al2O3 systems, through the diﬀerential scanning calorimetry (DSC) and thermogravimetry (TGA), to determine present phases and thermal stability and to correlate these results with those of conductivity obtained by impedance spectroscopy.

2. Materials and Methods

6 The PEO powder (molecular weight Mw = 1 10 ) and CF COONa from Aldrich (Darmstadt, × 3 Germany) were vacuum dried at room temperature for 24 h and then stored in a silica gel dryer. The polymer and the salt were weighted in a 10:1 (EO:Na) ratio and then separately dissolved in acetonitrile under magnetic stirring for 4 h. The two obtained solutions were combined and stirred for 4 additional hours. Then, Al2O3, from Aldrich (~150 mesh, or <104 µm, and 50 Å of size pore, Darmstadt, Germany), were added to x = 0.0, 3.0, 6.0, 10.0, 20.0 and 30.0% concentrations (x = wt. Al2O3 100%/(wt. Al2O3 + wt. (PEO)10CF3COONa)). The mixture was kept on a low frequency magnetic agitation to avoid decantation of Al2O3 particles and to ensure a uniform dispersion. When the mixture reached the viscous liquid properties, it was cast on a Petri dish and then stored in a dry atmosphere to let the solvent slowly evaporate. The resulting membranes show a mechanical consistency and their thickness varies from 150 to 200 µm. Samples were analyzed by DSC (MDSC 2920 TA Instruments, New Castle, DE, USA) from 220 to 450 K, at 10 K/min heating rate; nitrogen was used as a carrier gas. Thermogravimetric analysis were performed by a 2050 TA instruments, with 10 K/min heating rate, from 303 to 660 K using nitrogen as carrier gas. The conductivity values were obtained by the impedance spectroscopy in a frequency ranging from 50 Hz to 5 MHz, using blocking platinum electrodes. The impedance measurements were carried out by using a HIOKI 3532-50 LCR impedance analyzer (Nagano, Japan), and the dc conductivity (σ) was calculated using the relation: l σ = , AR where l is the thickness, A is the area and R is the resistance of the sample.

3. Results and Discussion The DSC thermogram for the pure PEO membrane is shown in Figure1a. There, two anomalies can be observed: One endothermic about 330 K, which corresponds to the PEO crystalline phase melting, and one exothermic about 443 K corresponding to the polymer decomposition. The DSC thermogram corresponding to the CF3COONa salt is shown in Figure1b; on it, an endothermic anomaly about 480 K can be observed, due to salt melting. Figure1c shows DSC results for the solid polymer electrolyte (PEO)10CF3COONa; this thermogram shows two endothermic anomalies: One about 334 K, usual in this type of membrane and that corresponds to the PEO crystalline phase melting [15–17]. The other endothermic anomaly is observed about 387 K and corresponds to the melting point of a new crystalline phase of a complex formed by the combination of polymer and salt [18]. In Figure2, DSC thermograms of (PEO) 10CF3COONa + x wt.% Al2O3 composite are shown for the diﬀerent concentrations of Al2O3 studied. From these thermograms the values of melting temperature Materials 2019, 12, x FOR PEER REVIEW 3 of 6

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(Tm) and enthalpy (∆Hm) are obtained; these data are given in Table1. The relative percentage of crystallinity (χc(%)) was calculated by:

∆H χ (%) = m 100, c 0 ∆Hm ×

Materialswhere ∆2019H0, 12=, x203 FOR J PEERg 1 was REVIEW used as standard enthalpy of fusion for 100% crystalline PEO [19]. 3 of 6 m · −

Figure 1. Differential scanning calorimetry (DSC) thermograph for: (a) pure poly(ethylene oxide) (PEO); (b) pure CF3COONa salt and (c) (PEO)10CF3COONa solid electrolyte.

In Figure 2, DSC thermograms of (PEO)10CF3COONa + x wt.% Al2O3 composite are shown for the different concentrations of Al2O3 studied. From these thermograms the values of melting temperature (Tm) and enthalpy (ΔHm) are obtained; these data are given in Table 1. The relative percentage of crystallinity (χc(%)) was calculated by: ΔH χ (%) =×m 100 , c ΔH 0 m Δ ⋅ −1 whereFigure 1. Di203ﬀerential J g was scanning used as calorimetry standard enthalpy (DSC) thermograph of fusion for for: 100% (a) pure crystalline poly(ethylene PEO [19]. oxide) Figure 1. Differential scanning calorimetry (DSC) thermograph for: (a) pure poly(ethylene oxide) (PEO); (b) pure CF3COONa salt and (c) (PEO)10CF3COONa solid electrolyte. (PEO); (b) pure CF3COONa salt and (c) (PEO)10CF3COONa solid electrolyte.

FigureFigure 2. 2. DSCDSC thermograms for (PEO)10CFCF33COONaCOONa ++ xx wt.%wt.% Al Al22OO3 3compositecomposite (x (x == 0.0,0.0, 3.0, 3.0, 6.0, 6.0, 10.0, 10.0, 20.0,20.0, and and 30.0). 30.0).

Table 1. Endothermic anomaly enthalpies for diﬀerent composites (PEO)10CF3COONa + x wt.% Al2O3. For the sample corresponding to x = 3.0%, a significant decrease in enthalpy, and therefore in the percentage of crystallinity,x wt.% is observedMelting Temperaturein relation to the sampleEnthalpy that does Crystallinitynot contain alumina (x = 0.0%). This decreaseAl2O in3 the percentageTm of(K) crystallinity in∆ theHm system(J/g) could beχc associated(%) with the interaction between0.0 the electrolyte and 319.54 Al2O3: The dispersed 67.70 Al2O3 particles 33.3 are coated by an amorphous material,3.0 which interrupts 320.30 the alignment of the 26.61 polymer chains 13.1 and decreases the crystallinity of the system6.0 [20,21]. 332.19 53.68 26.4 10.0 332.06 56.42 27.8 20.0 332.10 58.71 28.9 30.0 335.28 62.43 30.8 Figure 2. DSC thermograms for (PEO)10CF3COONa + x wt.% Al2O3 composite (x = 0.0, 3.0, 6.0, 10.0, 20.0, and 30.0).

For the sample corresponding to x = 3.0%, a significant decrease in enthalpy, and therefore in the percentage of crystallinity, is observed in relation to the sample that does not contain alumina (x = 0.0%). This decrease in the percentage of crystallinity in the system could be associated with the interaction between the electrolyte and Al2O3: The dispersed Al2O3 particles are coated by an amorphous material, which interrupts the alignment of the polymer chains and decreases the crystallinity of the system [20,21].

Table 1. Endothermic anomaly enthalpies for different composites (PEO)10CF3COONa + x wt.% Al2O3.

x wt.% Melting Temperature Enthalpy Crystallinity Materials 2019, 12, 1464 Δ 4 of 6 Al2O3 Tm (K) Hm (J/g) % 0.0 319.54 67.70 33.3 For the sample corresponding3.0 to320.30x = 3.0%, a signiﬁcant26.61 decrease in13.1 enthalpy, and therefore in the percentage of crystallinity,6.0 is observed332.19 in relation to the sample53.68 that does26.4 not contain alumina (x = 0.0%). This decrease in the10.0 percentage of crystallinity332.06 in the system 56.42 could be27.8 associated with the interaction between the electrolyte20.0 and Al2O3332.10: The dispersed Al 58.712O3 particles28.9 are coated by an amorphous material, which interrupts30.0 the alignment335.28 of the polymer 62.43 chains and decreases30.8 the crystallinity of the system [20,21]. 2 3 However,However, when when the the percentage percentage of ofAl AlO2O added3 added is isincreased, increased, i.e., i.e., for for samples samples with with alumina alumina concentrationconcentration values values of ofx ≥x 6.0%,6.0%, the the percentage percentage of ofcrystallinity crystallinity again again increases increases (see (see Table Table 1).1 ).This This isis 2 3 ≥ possiblypossibly due due to toAl AlO2O particle3 particle aggregation aggregation that that causes causes increases increases in incrystallinity crystallinity [22–24]. [22–24 ]. 10 3 2 3 TheThe TGA TGA thermograms thermograms for for the the (PEO) (PEO)CF10CFCOONa3COONa + x+ wt.%x wt.% Al AlO2 Osystem3 system are are shown shown in Figure in Figure 3. 3. AlthoughAlthough not not visible visible to the to thenaked naked eye, eye,in all in presented all presented thermograms thermograms (x = 3.0, (x 10.0,= 3.0, 20.0 10.0, and 20.0 30.0), and there 30.0), wasthere a loss was of a lossmass of less mass than less 4% than around 4% around 325 K. 325 This K. Thisloss losscorresponds corresponds to the to theevaporation evaporation of ofsolvent solvent residuesresidues (acetonitrile). (acetonitrile). Figure Figure 33 showsshows the decompositiondecomposition ofof thethe sample sample in in two two stages stages around around 496 496 K andK and663 663 K. Also,K. Also, the maximum the maximum mass lossesmass arelosses shown, are obtained shown, fromobtained the derivative from the of derivative mass percentage of mass with percentagerespect to with temperature respect (dottedto temperature line on the (dotted right-hand line scale).on the Theseright-hand thermograms scale). These show thatthermograms membranes show that membranes are thermally stable up to 430 K, with slight variations due to the effect of the are thermally stable up to 430 K, with slight variations due to the eﬀect of the Al2O3 concentration. Al2O3 concentration.

FigureFigure 3. Thermogravimetry 3. Thermogravimetry (TGA) (TGA) thermographs thermographs with with thei theirr derivatives derivatives (pointed (pointed lines lines referred referred to tothe the

right-handright-hand scale) scale) for for different diﬀerent (PEO) (PEO)10CF10CF3COONa3COONa + x+ wt.%x wt.% Al2 AlO32 composites.O3 composites.

FigureFigure 4 4shows shows conductivity conductivity results results as a asfunction a function of the of inverse the inverse of temperature, of temperature, obtained obtained by the impedanceby the impedance spectroscopy spectroscopy for: Pure for: PEO Pure PEOpolymer polymer membranes, membranes, solid solid polymer polymer electrolyte electrolyte (PEO)(PEO)10CF10CF3COONa,3COONa, and and (PEO) (PEO)10CF103CFCOONa3COONa + x +wt.%x wt.% Al2O Al3 2compositesO3 composites analyzed. analyzed. From From the graphs the graphs it canit canbe seen be seen that, that, for almost for almost the thewhole whole temperatur temperaturee range range analyzed, analyzed, the the highest highest conductivity conductivity values values correspondcorrespond to tothe the x =x 3.0%= 3.0% composite. composite. These These results results are are consis consistenttent with with the the increase increase of ofthe the amorphous amorphous phasephase around around the the dispersed dispersed Al Al2O23O particles,3 particles, which which would would create create adequate adequate pathways pathways to toincrease increase ionic ionic mobilitymobility and and thus thus improve improve conductivity conductivity values. values. FigureFigure 44 also also shows shows the ArrheniusArrhenius andand Vogel Vogel Tammann-Fulcher Tammann-Fulcher (VTF) (VTF) fitting fitting in two in two regions: regions: From From298 to298 333 to K,333 and K, fromand from 393 to 393 433 to K. 433 For K. temperatures For temperatures ranging ranging from 333 from to 393 333 K, to it 393 is not K, possibleit is not to possiblemake ato fitting make with a fitting either with model, either because model, the beca phaseuse the transitions phase transitions observed inobserved DSC occur. in DSC Parameters occur. Parametersobtained byobtained these fittings by these are fittings in agreement are in ag withreement those with previously those previously reported [ 11reported]. [11].

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Figure 4. Conductivity as a function of the inverse of temperature for PEO, (PEO)10CF3COONa Figure 4. Conductivity as a function of the inverse of temperature for PEO, (PEO)10CF3COONa polymer electrolyte and (PEO)10CF3COONa + x wt.% Al2O3 composite. The solid line represents the polymer electrolyte and (PEO)10CF3COONa + x wt.% Al2O3 composite. The solid line represents the ArrheniusArrhenius ﬁtting fitting and and the the dotted dotted line line represents represents the the Vogel Vogel Tammann-Fulcher Tammann-Fulcher (VTF) (VTF) ﬁtting. fitting.

4.5. ConclusionsConclusions

DSCDSC analyses analyses on (PEO)on (PEO)10CF103CFCOONa3COONa+ x wt.%+ x Alwt.%2O3 compositesAl2O3 composites showed changesshowed inchanges the crystalline in the phasecrystalline fraction phase of thefraction system, of the for system, all concentration for all concentration values of values Al2O 3ofadded. Al2O3 added. These These changes changes in the in crystallinitythe crystallinity of the of new thesystem new system resulted resulted in changes in ch inanges the conductivity in the conductivity of the electrolyte of the electrolyte by creating by pathwayscreating pathways to increase to ionic increase mobility. ionic The mobility. sample Th withe sample 3.0% alumina with 3.0% concentration alumina concentration showed the highestshowed conductivitythe highest conductivity at the same time at the as same the highest time as percentage the highest of percentage the amorphous of the phase. amorphous phase. ThermogravimetricThermogravimetric analysesanalyses indicateindicate thermalthermal stabilitystability inin thethe membranesmembranes upup toto 430430 K,K, which,which, togethertogether with with the the increase increase in conductivity in conductivity as a function as a function of temperature, of temperature, suggests that suggests these composites that these cancomposites be used ascan electrolyte be used separatorsas electrolyte in electrochemicalseparators in electrochemical cells such as batteries cells such and as gas batteries sensors. and gas sensors. Author Contributions: Conceptualization, M.I.D.R, N.M.J.M. and R.U.K.; methodology, M.I.D.R. and N.M.J.M.; validation,Author Contributions: M.I.D.R, N.M.J.M. conceptualization, and R.U.K.; M.I.D.R, formal analysis,N.M.J.M. N.M.J.M.;and R.U.K.; investigation, methodology, M.I.D.R. M.I.D.R. and and N.M.J.M.; N.M.J.M.; resources,validation, M.I.D.R, M.I.D.R, N.M.J.M. N.M.J.M. and and R.U.K.; R.U.K.; data formal curation, analysis, N.M.J.M.; N.M.J.M.; writing—original investigation, draft M.I.D.R. preparation, and N.M.J.M.; M.I.D.R, N.M.J.M.resources, and M.I.D.R, R.U.K.; N.M.J.M. writing—review and R.U.K.; and data editing, curation, M.I.D.R, N.M.J.M.; N.M.J.M. writing—original and R.U.K.; supervision, draft preparation, M.I.D.R.; M.I.D.R, project administration, M.I.D.R.; funding acquisition, M.I.D.R, N.M.J.M. and R.U.K. N.M.J.M. and R.U.K.; writing—review and editing, M.I.D.R, N.M.J.M. and R.U.K.; supervision, M.I.D.R.; project Funding:administration,This work M.I.D.R.; was funded funding by acquis COLCIENCIASition, M.I.D.R, and UniversityN.M.J.M. and of Tolima R.U.K. through the Central Research Oﬃce.

Acknowledgments:Funding: This workThe was authors funded thank by COLCIENCIAS the Non-Metallic and Systems University Phase of TransitionTolima through Group the at UniversidadCentral Research del Valle, Colombia, for the technical support. Office. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments: The authors thank the Non-Metallic Systems Phase Transition Group at Universidad del Valle, Colombia, for the technical support. References Conflicts of Interest: The authors declare no conflict of interest. 1. Mindemark, J.; Lacey, M.J.; Bowden, T.; Brandell, D. Beyond PEO—Alternative host materials for Li+-conducting solid polymer electrolytes. Prog. Polym. Sci. 2018, 81, 114–143. [CrossRef] References 2. Judez, X.; Zhang, H.; Li, C.; Eshetu, G.G.; González-Marcos, J.A.; Armand, M.; Rodriguez-Martinez, L.M. 1. SolidMindemark, Electrolytes J.; Lacey, for Safe M.J.; and Bowden, High Energy T.; Brandell, Density D. Lithium-Sulfur Beyond PEO—Alternative Batteries: Promises host materials and Challenges. for Li+- J.conducting Electrochem. solid Soc. polymer2018, 165 electrolytes., A6008–A6016. Prog. [CrossRef Polym. Sci] . 2018, 81, 114–143. 3.2. Jinisha,Judez, X.; B.; Zhang, Anilkumar, H.; Li, K.M.; C.; Eshetu, Manoj, G.G.; M.; González Abhilash,-Marcos, A.; Pradeep, J.A.; Armand, V.S.; Jayalekshmi, M.; Rodriguez-Martinez, S. Poly (ethylene L.M. oxide)Solid Electrolytes (PEO)-based, for sodium Safe and ion-conducting‚ High Energy Density solid polymer Lithium-Sulfur electrolyte Batteries: films, dispersed Promises withand Challenges. Al2O3 filler, J. forElectrochem. applications Soc. in 2018 sodium, 165, ion A6008–A6016. cells. Ionics 2018, 24, 1675–1683. [CrossRef] 4.3. Delgado,Jinisha, B.; I.; Anilkumar, Castillo, J.;K.M.; Chac Manoj,ón, M.; M.; Abhilash, Vargas, R.A. A.; Pradeep, Ionic conductivity V.S.; Jayalekshmi, in the S. polymer Poly (ethylene electrolytes oxide)

PEO(PEO)-based,/CF3COONa. sodiumPhys. ion-conducting‚ Status Solidi B 2000 solid, 220 polymer, 625–629. electrolyte [CrossRef films,] dispersed with Al2O3 filler, for applications in sodium ion cells. Ionics 2018, 24, 1675–1683.

Materials 2019, 12, 1464 6 of 6

5. ElBellihi, A.A.; Bayoumy, W.A.; Masoud, E.M.; Mousa, M.A. Preparation, Characterizations and Conductivity of Composite Polymer Electrolytes. Polym. Nano Compos. Electrolytes 2012, 33, 1–6. 6. Liang, B.; Tang, S.; Jiang, Q.; Chen, C.; Chen, X.; Li, S.; Yan, X. Preparation and characterization of PEO-PMMA

polymer composite electrolytes doped with nano-Al2O3. Electrochim. Acta 2015, 169, 334–341. [CrossRef] 7. Klongkan, S.; Pumchusak, J. Eﬀects of Nano Alumina and Plasticizers on Morphology, Ionic Conductivity,

Thermal and Mechanical Properties of PEO-LiCF3SO3 Solid Polymer Electrolyte. Electrochim. Acta 2015, 161, 171–176. [CrossRef] 8. Agrawal, A.; Satapathy, A. Thermal and dielectric behaviour of polypropylene composites reinforced with ceramic ﬁllers. J. Mater. Sci. Mater. Electron. 2015, 26, 103–112. [CrossRef] 9. Rybalko, V.P.; Nikityuk, A.I.; Pisarenko, E.I.; Kuznetsova, T.I.; D’yachenko, P.B.; Guseinov, S.L.; Malashin, A.S.; Korchmarek, A.S.; Kireev, V.V. Eﬀect of inorganic nanopowders on properties of acrylic composites. Russ. J. Appl. Chem. 2015, 88, 826–832. [CrossRef] 10. Jurado-Meneses, N.M.; Delgado-Rosero, M.I.; Meléndez-Lira, M.A. Structural and vibrational studies on composites

polymer electrolytes (PEO)10CF3COONa + x wt.% Al2O3. Rev. Fac. Ing. Univ. Antioquia 2017, 83, 43–49. [CrossRef] 11. Jurado Meneses, N.M.; Delgado Rosero, M.I.; Vargas Zapata, R.A. Conductividad iónica en nuevos compositos

(PEO)10 (CF3COONa)-X % Al2O3. Univ. Sci. 2013, 18, 173–180. [CrossRef] 12. Ahn, J.H.; Wang, G.X.; Liu, H.K.; Dou, S.X. Nanoparticle-dispersed PEO polymer electrolytes for Li batteries. J. Power Sources 2003, 119, 422–426. [CrossRef]

13. Köster, T.K.J.; van Wüllen, L. Cation-anion coordination, ion mobility and the eﬀect of Al2O3 addition in PEO based polymer electrolytes. Solid State Ionics 2010, 181, 489–495. [CrossRef] 14. Vijayalekshmi, V.; Khastgir, D. Chitosan/partially sulfonated poly(vinylidene ﬂuoride) blends as polymer electrolyte membranes for direct methanol fuel cell applications. Cellulose 2018, 25, 661–681. [CrossRef]

15. Yang, R.; Zhang, S.; Zhang, L.; Bi, X. Eﬀects of LiClO4 on the Characteristics and Ionic Conductivity of the Solid Polymer Electrolytes Composed of PEO, LiClO4 and PLiAA. Mater. Sci. Forum 2013, 743, 53–58. [CrossRef] 16. Gurusiddappa, J.; Madhuri, W.; Suvarna, R.P.; Dasan, K.P. Studies on the morphology and conductivity of

PEO/LiClO4. Mater. Today Proc. 2016, 3, 1451–1459. [CrossRef] 17. Martinez-cisneros, C.S.; Levenfeld, B.; Varez, A.; Sanchez, J.Y. Development of sodium-conducting polymer electrolytes: comparison between ﬁlm-casting and ﬁ lms obtained via green processes. Electrochim. Acta 2016, 192, 456–466. [CrossRef] 18. Delgado, M.I.; Jurado, N.M.; Vargas, R.A. Phase diagram of polymer electrolyte: (x)(PEO)–(1 x)CF COOLi. − 3 Rev. Fac. Ing. Univ. Antioquia 2012, 62, 77–82. 19. Wu, X.L.; Xin, S.; Seo, H.H.; Kim, J.; Guo, Y.G.; Lee, J.S. Enhanced Li+ conductivity in PEO–LiBOB polymer electrolytes by using succinonitrile as a plasticizer. Solid State Ionics 2011, 186, 1–6. [CrossRef] 20. Polu, A.R.; Rhee, H.W. Nanocomposite solid polymer electrolytes based on poly(ethylene oxide)/POSS-PEG (n = 13.3) hybrid nanoparticles for lithium ion batteries. J. Ind. Eng. Chem. 2015, 31, 323–329. [CrossRef] 21. Langer, F.; Bardenhagen, I.; Glenneberg, J.; Kun, R. Microstructure and temperature dependent lithium ion transport of ceramic–polymer composite electrolyte for solid-state lithium ion batteries based on garnet-type Li7La3Zr2O. Solid State Ionics 2016, 291, 8–13. [CrossRef]

22. Polu, A.R.; Kumar, R. Mg2+-ion conducting poly(ethylene glycol)-TiO2 composite polymer electrolytes for solid-state batteries. Mater. Express 2014, 4, 79–84. [CrossRef] 23. Rajesh Kumar, S.; Juan, C.H.; Liao, G.M.; Lin, J.S.; Yang, C.C.; Ma, W.T.; You, J.H.; Jessie Lue, S. Fumed Silica Nanoparticles Incorporated in Quaternized Poly(Vinyl Alcohol) Nanocomposite Membrane for Enhanced Power Densities in Direct Alcohol Alkaline Fuel Cells. Energies 2016, 9, 15. [CrossRef] 24. Saikia, D.; Chen-Yang, Y.W.; Chen, Y.T.; Li, Y.K.; Lin, S.I. Investigation of ionic conductivity of composite gel polymer electrolyte membranes based on P(VDF-HFP), LiClO4 and silica aerogel for lithium ion battery. Desalination 2008, 234, 24–32. [CrossRef]

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