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Journal of Non-Crystalline Solids 353 (2007) 2104–2108

Sol–gel derived organic–inorganic hybrid electrolytes for thinfilm electrochromic devices

Elzbieta Zelazowska a,*, Maria Borczuch-Laczka b, Ewa Rysiakiewicz-Pasek c

a Institute of Glass and Ceramics, Cracow Branch, 30-702 Krakow, ul. Lipowa 3, Polandb Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krakow, Poland

c Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland

Available online 2 April 2007

Abstract

Sol–gel derived, lithium ion conducting organic–inorganic hybrid electrolytes for ambient temperatures applications, have been syn-thesized from tetraethyl orthosilicate (TEOS), poly(ethylene oxide) (PEO), propylene carbonate (PC), propylene oxide, butyl acrylate,butyl methacrylate, ethyl acetoacetate and LiClO4 precursors. Mass fractions of the organic additions in the gels were of ca 30 mass%for gels 0/B, F–H and 40 mass% for gel J. The colorless transparent or translucent hybrid materials obtained in this work were aged atroom temperature for at least three weeks and then dried at 80 �C for 3 h. The morphology and structure of all compositions were inves-tigated by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDX), Fourier transform infraredspectroscopy and 29Si MAS nuclear magnetic resonance. Amorphous nature of the hybrids was confirmed by X-ray diffraction. SEM,FTIR and NMR analysis showed structural properties and [SiO4] tetrahedrons poly-condensation process to be strongly influenced byorganic additives have been employed. Room temperature ionic conductivities of the hybrid electrolytes were in a range of 9.84 · 10�4–1.56 · 10�3 X�1 cm�1.� 2007 Elsevier B.V. All rights reserved.

PACS: 81.07.Pr; 81.20.Fw; 61.20.Qg; 66.30.Dn

Keywords: Fast ion conduction; Sol–gel, aerogel and solution chemistry; Organic–inorganic hybrids

1. Introduction

Solid materials with sufficiently high ionic conductivitiesat ambient temperatures have attracted much attention inthe recent years, because of their potentially wide rangeof applications as electrolytes in advanced optoelectronicand electrochemical devices, such as rechargeable lithiumbatteries, electrochromic windows and displays [1–3]. Theelectrolytes for application in electrochromic windows areadditionally required to have both an ionic conductivityof minimum 10�4–10�3 X�1 cm�1 as well as possibility to

0022-3093/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2007.01.077

* Corresponding author. Tel.: +48 012 423 67 77; fax: +48 012 423 5836.

E-mail address: ezelazowska@isic.krakow.pl (E. Zelazowska).

be used in a transparent and colorless thin layer form witha sufficient durability [3,4].

The complexes of alkali metal salts with long chainpolyethers, and especially poly (ethylene oxide) PEO areregarded to comply majority of these requirements apartfrom too low ionic conductivity in the room temperaturesrange [5,6]. At room temperatures, the simple electrolytesof these systems are known to consist of crystalline phasesof PEO and alkali metal salt–PEO complexes with a rela-tively small addition of the non crystalline regions, whilebelow the normal melting temperature of PEO,Tm = 65 �C, ionic conductivity is almost exclusively con-nected with ions mobility through the amorphous phase[7–9]. On the other hand, due to starting from solutionsof precursors prepared at room temperature, sol–gelmethod has been successfully employed for producing

E. Zelazowska et al. / Journal of Non-Crystalline Solids 353 (2007) 2104–2108 2105

organically modified amorphous transparent materials[10,11]. In the last years, sol–gel derived organic–inorganichybrids have attracted much interest for the solid electro-lyte applications [12–14].

In this work, organic–inorganic hybrid electrolytes havebeen obtained using tetraethyl orthosilicate TEOS, butylacrylate, butyl methacrylate, poly(ethylene oxide) PEO,propylene oxide, ethyl acetoacetate, propylene carbonate,dichloromethane, ethanol and acetonitrile precursors andsolvents. Lithium perchlorate (LiClO4), dissolved in PCor ethanol, has been employed as a salt for doping atweight ratio of about 0.01 with respect to the mass of freshgels, The hybrids obtained were investigated for morphol-ogy, structural properties and ionic conductivity with aimto determine their potential to be useful as transparentsolid electrolytes for the room temperature applications.

2. Experimental procedures

Silica component of the organic–inorganic hybrids andpure silica gel prepared for comparison, were producedby mixing TEOS Si(OC2H5)4 and distilled water with themolar ratio of TEOS:H2O = 1:4. As a catalyst, 36.6%HCl was added drop by drop up to pH 2. Organic partswere produced from butyl acrylate (BA, C7H12O2), butylmethacrylate (BMA, C8H14O2), PEO, (–(CH2CH2O)n–,M � 600.000), propylene oxide (PO, C3H6O), ethyl aceto-acetate (EAA, C6H10O3), propylene carbonate (PC,C4H6O3), dichloromethane (CH2Cl2), ethanol (C2H5OH)and acetonitrile (CH3CN) precursors and solvents. As asalt for doping at the weight ratio of about 0.01 withrespect to the mass of fresh gels, LiClO4 dissolved in PCor ethanol was applied. Components (at least of reagentgrade, Merck and Aldrich) employed for preparing gels,are listed in Table 1. Mass fractions of the organic precur-sors were calculated on ca 30 or 40 mass%, at the weightconcentration of PEO (or PO):(BA, BMA and/or EAA)equal to 1. Solutions of TEOS, after stirring for 1 h, weremixed with solutions of organic compounds in appropriatesolvents (dichloromethane, acetonitrile and/or ethanol).

Table 1Components of the starting solutions

Sample Components Appearance,remarks

E TEOS (sol) Colorless,transparent

F TEOS (sol), BMA, LiClO4/PC Colorless,transparent

G TEOS (sol), BA, PO, EAA, LiClO4/PC Colorless,transparent

H TEOS (sol), BA, BMA, EAA, LiClO4/PC Colorless,transparent

J TEOS (sol), BA, BMA, EAA, PEO/CH3CN + ethanol, CH2Cl2, LiClO4/ethanol

Colorless,slightlyopalescent

0/B TEOS (sol), PEO/CH2Cl2, PC, BMA,LiClO4/ethanol

Colorless,transparent

The resulting mixtures were stirred for 3 h. Gelation pro-cess has occurred within 1–1.5 days. Fresh gels were driedfor three weeks at ambient temperature and for 3 h in anelectric drier at 80 �C.

Scanning electron microscopy equipped with energy dis-persive X-ray spectroscopy SEM/EDX, (JEOL JSM 5400with LINK An 10/5, NOVA NANOSEM-FEI), Fouriertransform infrared spectroscopy (Bio-Rad FTS-60VMFTIR spectrometer, KBr technique), nuclear magnetic res-onance 29Si MAS NMR (NMR spectrometer at the mag-netic field 7.05 T) and X-ray diffraction (XRD 7, Seifertdiffractometer) were used for examination of morphologyand structure of the hybrids and silica gel obtained in thiswork. Ac conductivity was measured with RCL-meter(HIOKI 3532-50 RCL HiTESTER). The current–voltagecharacteristics at ±polarized dc potential applied througha laboratory-made potentiostat/galvanostat were observedfor thin film electrochromic cell based on WO3 (ca 120 nmthick) and NiO (ca 160 nm thick) active electrode andcounter-electrode, respectively. The electrochromic filmswere coated onto the low-emissivity glass (K-Glass, Pil-kington) using pyrolysis method.

3. Results

The appearance of the gels after heat treatment isdescribed in Table 1. All the obtained gels have revealedan amorphous structure under XRD examination. TheXRD pattern, typical of hybrids under investigation, isshown in Fig. 1.

Fig. 2 shows SEM images of hybrids G and 0/B. TheSEM/EDX examination of the hybrids obtained in thiswork has revealed fine and smooth surface morphologies,while there in the fractured surface images, the micron-sized particles linked into the chains with the voids betweenthem can be observed at the high magnifications.

FTIR and 29Si MAS NMR spectra of the hybrid gelsafter heat treatment at 80 �C are shown in Figs. 3 and 4,respectively. Assignments of the characteristic bands in

0

100

200

300

400

500

600

700

800

900

1000

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00

2 Theta (degree)

Co

un

ts (

a.u

.)

Fig. 1. XRD pattern of the hybrid J.

Fig. 2. SEM images of hybrids at magnification of 50000·: (a) G (surface view) and (b) 0/B (fractured surface).

Fig. 3. FTIR spectra of the organic–inorganic hybrid electrolytes underinvestigation 0/B, E–J.

-180-160-140-120-100-80-60-40-20

Q2 Q

3 Q

4

ppm from TMS

29Si NMR4 kHz MAS

F

G

H

J

0/B

Fig. 4. 29Si MAS NMR spectra for hybrid gels 0/B and F–J.

2106 E. Zelazowska et al. / Journal of Non-Crystalline Solids 353 (2007) 2104–2108

the FTIR spectra are given in the Table 2. Results of 29SiNMR spectroscopy are shown in Table 3.

29Si MAS NMR spectra and calculated results (Fig. 4,Table 3, respectively) of the hybrid electrolytes and puresilica gel (E) given for comparison, exhibit peak profileswith different amounts of the Q4, Q3 and Q2 structuralunits, corresponding to the silicon Si in coordination 4, 3or 2 in respect to the bridging oxygen atoms. The analysisof these spectra was based on the numerical values of theparameter A1 equal to the ratio of Q4/Q3 and parameterA2 equal to the ratio of Q4/Q2 calculated from the relativefractions of the peak area, corresponding to the appropri-ate Q species, where Q4 value at approximately �109 ppmcorresponds to [SiO4] tetrahedrons. The observed chemical

shifts were referenced to the signal of tetramethyl silane(TMS).

Ac measurements of the ionic conductivities have beencarried out for all the hybrids doped with lithium perchlo-rate. The measured values of the ionic conductivity r25 in[X�1 cm�1] at lithium salt doping of 0.01 mass% in respectto mass of the fresh gels, were 1.32 · 10�3, 1.24 · 10�3,1.56 · 10�3, 1.45 · 10�3 for samples 0/B, F, G and H,respectively and 9.84 · 10�4 for hybrid J, at the ±0.08%basic accuracy of the measurement. The best value(1.56 · 10�3X�1 cm�1) was indicated for hybrid electrolyteH with additive both, BA and BMA.

All the organic–inorganic hybrid materials obtained inthis work were examined as electrolytes for cells based on

Table 2Assignments of the characteristic bands in the FTIR spectra

F G H J O/B E Structural unit

452 453 451 448 448 460 q Si–O–Si, bending582 575 585 583 573 571 Li in LiClO4, Li connected with organics, Si–O ring (4)637 637 637 637 628 ClO�4715 714 715 714 714 Organics, vibrations in propylene carbonate (PC) molecule779–815 779–815 779–815 779–815 779–815 799 Pas Si–CH2, –O–Si–CnHm, ms Si–O–Si stretching947 948 942 948 942 948 mas Si–OH, stretching1078–1121 1079–1120 1078–1120 1078–1121 1056–1121 1083–1121 mas Si–O, stretching, ClO�4 , m Si–O–R,1187 1186 1186 1186 1186 C–O, organics1358 1297–1359 1359 1357 1357 Organic parts, C–H in CnHm, m C–O–C1392 1392 1392 13921485 1485 1485 1457–1485 1485 Organic groups1559 1558 1559 1558 1559 mas CO(OH) (acetate)group1636 1636 1635 1636 1637 1637 H–O–H in H2O1743–1791 1747–1794 1747–1793 1747–1793 1747–1793 Si–CO(OH) (acetate) group, organic parts, C@O stretching2939–2990 2939–2990 2937–2990 2938–2987 2938–2993 CH3, mas CH2, organic parts, C–H3390 3387 3388 3418 3418 3443 OH in H2O

Table 3Isotropic chemical shifts (d, ppm), line widths (half width at half maximum: hwhm, ppm) and relative fraction (%) of Qn units in hybrid electrolytes andsilica gel (E)

Sample Q2 �d, hwhm (ppm);relative share (%)

Q3 �d, hwhm (ppm);relative share (%)

Q4 �d, hwhm (ppm);relative share (%)

A1 = Q4/Q3 A2 = Q4/Q2

E �92.2 (5.5) 7 �101.0 (7.2) 51 �109.9 (7.2) 42 0.82 6.00F �91.0 (3.3) 4 �100.8 (6.1) 44 �109.6 (8.6) 52 1.18 13.00G �92.0 (4.3) 4 �100.8 (5.6) 44 �109.8 (8.0) 52 1.18 13.00H �92.0 (7.1) 10 �100.8 (5.9) 48 �109.6 (7.5) 42 0.88 4.20J �91.2 (6.9) 9 �100.8 (5.4) 46 �109.9 (8.3) 45 0.98 5.000/B �90.6 (3.6) 5 �100.8 (5.5) 45 �109.8 (8.1) 50 1.11 10.00

WO3/NiO, +-1.5 V

-20

-10

0

10

20

30

0 10 20

t [s]

I [m

A ]

WO3/NiO +-1.5 V

-20

-10

0

10

20

-2 0 2

U (V)

I (m

A)

a b

Fig. 5. Current response (a) and cyclic voltammogram (b) for thin filmtungsten oxide/organic–inorganic hybrid electrolyte F/nickel oxide elec-trochromic cell, cycled with voltage of ±1.5 V (cycled area 3 cm2, scan rate50 mV/s).

E. Zelazowska et al. / Journal of Non-Crystalline Solids 353 (2007) 2104–2108 2107

WO3 thin film with an electrochromic window arrange-ment. A typical response to a rectangular shaped currentsignal and a cyclic voltammogram (CV) taken for the elec-trochromic cell under (±)polarized low dc voltage areshown in Fig. 5(a) and (b), respectively.

4. Discussion

The SEM/EDX examination of the hybrids obtained inthis work at their fractured surfaces has revealed micron-

sized particles connected together into some of chains withthe voids between them, which can be observed at the highmagnifications. This phenomenon, observed especially inthe hybrids J and 0/B, containing butyl methacrylate andPEO, seems to be connected with crosslinking polymeriza-tion, most likely due to bonding inorganic and inorganicparts, such as polyether chains fragments by means of car-boxylic and hydroxyl groups present in acrylates. The car-boxylic groups of the acrylic acid derivatives have beenreported as crosslinkers of the polymer particles and hybridorganic–inorganic systems, e.g. by Lee et al. [15] and Zhouet al. [16], respectively. Grow of particles in such systemshave been ascribed to crosslinking of the polymer particlesby means of carboxylic groups, connected with capturingof monomers, nuclei and/or oligomeric radicals from themedium during poly-condensation. In this case, the disper-sion polymerization observed in hybrids under investiga-tion can be due to condensation of silica particles fromthe hydrolysis and polymerization reactions of TEOS,overlapped with crosslinking activity of the carboxylicand OH� groups.

FTIR spectroscopy (Fig. 3, Table 2) has revealed thebands characteristic of water (H–O–H at around 1635–1638 cm�1, OH� in H2O in a range of 3375–3443 cm�1)and those of the Si–O–Si bonds: Si–O–Si asymmetric stretch-ing vibrations in a range of 1056–1121 cm�1, Si–OH

2108 E. Zelazowska et al. / Journal of Non-Crystalline Solids 353 (2007) 2104–2108

asymmetric stretching at around 942–948 cm�1, Si–O–Sisymmetric stretching at around 815 cm�1 and bendingvibration of the Si–O–Si groups in a range of around 447–460 cm�1. These bands were found in the FTIR spectra ofall the gels under investigation. Besides of gel E, there inthe spectra of hybrid gels, the sharp bands characteristic oforganic parts as well as ClO4 groups are present [17,18]. Dif-ferences observed in the amount, position and intensity ofbands from organics: (at around 715; 779; 1121–1187;1297–1359; 1559; 1743–1798; 2937–2993 cm�1) are evidentlyconnected with organic additives have been employed [17–19]. FTIR results indicate formation of the direct bondingbetween inorganic part and organic parts by Si–CH2, –O–Si–CnHm, Si–O–R and Si–CO(OH).

Besides of lithium in LiClO4 and that bonded to organ-ics at around 571–585 cm�1, there in the spectra of thehybrid electrolytes the peaks from ClO�4 at 628–637 cm�1

can be observed, indicating presence of the free lithiumions. Such a conclusion seems to be in a good agreementwith results of the ionic conductivity measurements, rela-tively high for the hybrids under investigation.

The 29Si NMR measurements under magic angle spin-ning conditions (Fig. 4, Table 3) with a good agreementwith results of FTIR and SEM/EDX investigation, haverevealed a relatively high intensity of the peaks of the Q4

species originating, in the spectra of the hybrids F, G and0/B. All these hybrids contain only one butyl compound,i.e. BMA (F, 0/B) or BA (G). It can be concluded there-fore, that higher poly-condensation level of an inorganicnetwork occurs in these hybrids, while mixing of two,organic and inorganic networks occurs at presence of thehigher amount of the organic precursors, and especiallytwo another butyl species and/or PEO. In that case forma-tion of the direct bonds between organic and inorganicparts during the poly-condensation process is likely over-lapped with crosslinking polymerization. Such effect canbe ascribed to crosslinking effect of the carboxyl andhydroxyl groups of the acrylic acid butyl derivatives takingpart in the polymerization process.

The hybrid materials obtained in this work have provedto be able to take part in electrochromic reactions. Suchreactions depend on the insertion/extraction of the lithiumions and injection/extraction of electrons under cyclingwith ±polarized low dc voltage and are resulted in revers-ible dark blue or colorless states of the tungsten oxidelayer. The symmetric situation and shape of cathodic andanodic peaks (Fig. 5(a) and (b)) for active and counter-elec-trode due to ion insertion and extraction, respectively, indi-cate materials under investigation to be a sufficient host forreversible insertion/extraction of the lithium ions. On theother hand, relatively sharp peaks at the highest values ofthe voltage applied makes possible both lithium and protonconductance, connected with presence of water, which hasbeen revealed by infrared spectroscopy in the gels underinvestigation.

5. Conclusion

Sol–gel derived, amorphous Li-ion conductive organic–inorganic hybrid electrolytes with the ionic conductivitiesof 10�4–10�3 X�1 cm�1 were obtained using tetraethylorthosilicate TEOS, poly(ethylene oxide), propylene oxide,propylene carbonate, butyl acrylate, butyl methacrylate,ethyl acetoacetate and lithium perchlorate. FTIR analysisresults have revealed presence of the free Li ions besidesof those in the LiClO4 form and bonded to organics. Directchemical bonding between the inorganic and organic partshave been revealed from FTIR and 29Si MAS NMR spec-tra. The poly-condensation process overlapped with cross-linking polymerization has been observed, especially in thehybrids containing both, butyl methacrylate and poly (eth-ylene oxide). All the hybrid materials obtained in this workhave proved to be able to take part in the electrochromicreactions depending on reversible insertion/extraction ofthe lithium ions.

Acknowledgement

Financial support of this work provided by the Ministryof Science and Higher Education, Department of ScientificResearch, Poland, Grant No. 4 T08D 001 25 is gratefullyacknowledged.

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