Neodymium doped, sol-gel processed polymer electrolytes

170

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Neodymium Doped, Sol-Gel Processed Polymer Electrolytes M.M.

S i l v a ~, V. de Z e a B e r m u d e z 2, L.D. C a r l o s 3 a n d M. J. S m i t h I

llnstituto de Materiais, Universidade do Minho, Gualtar, 4719 Braga Codex, Portugal 2SecVao de Quimica, Universidade de Tr~s-os-Montes e Alto Douro Quinta de Prados, Apartado 202, 5001 Vila Real Codex. Portugal 3Departamento de Fisica, Universidade de Aveiro, 3810 Aveiro, Portugal

Polymer films prepared by the sol-gel method and containing neodymium trifluoromethanesulphonate (triflate) have been characterized by thermal, X-ray diffraction and conductivity measurements. The films were produced in a laboratory atmosphere and subsequently subjected to an appropriate drying pre-treatment before storage and study within an argon-filled drybox. The results of these experiments confirm that the amorphous hybrid host matrix has characteristics which are significantly better than poly(ethylene oxide)-based materials. Abstract.

1.

Introduction

Since the introduction of the solvent-free solid polymer electrolyte concept in 1973, research in this domain [1, 2] has been dominated by applications of electrolytes in advanced batteries. Recent studies [3-7], carried out on materials containing lanthanide salts, have demonstrated that novel applications in the fields of flexible phosphors and solid state lasers may also be possible. Studies of macromolecular solutions of photoluminescent species based on poly(ethylene oxide) [8-10], (PEt), have shown that within certain electrolyte composition ranges high molecular weight homopolymers show a marked tendency to form crystalline spherulites which compromise the optical quality of the sample. The use of sol-gel procedures offers a route by which suitable inorganic/organic hybrid matrices may be prepared. These hybrid matrices, commonly designated as ORganically MOdified SILicateS (or ORMOSILS), conveniently combine a thermally and mechanically stable silica network with organic segments which plastify the material and may contribute to solubilization of ionic guest species. As hydrolytic stability is an essential feature of the hybrid structure, the inorganic/organic graft must be achieved with a suitably stable crosslink. We have recently synthesized and characterized a novel series of Eu(m)-doped ormosils [6, 7, 11-14]. These

ormolytes (ORganically MOdified silicate electroLYTES) were prepared by bonding short (OCH2CH2) chains onto the silica network through urea bridges, -NH(C=O)NH-, and therefore the members of the resulting ormosil subclass have been designated as ureasilicates or ureasils [11]. The hybrid structure readily dissolves large quantities of the neodymium salt and the free-standing polymer films produced show moderate levels of ionic conductivity (approximately 10.5 ~ l c m l at room temperature) over a wide range of salt concentration. Sample transparency, an aspect of fundamental importance for optical applications, is significantly better than that observed in films based on commercial polymers with the same guest species. 2.

Experimental

Details

Sol-gel electrolytes were prepared in two stages. In the preliminary step 2.5 g of tx,o~-diaminepoly(oxyethyleneco-polypropylene) (commercially available as Jeffamine ED 2001 | from Fluka Chemie AG) was dried under vacuum at 90 ~ for several days. The diamine was then dissolved in 10 ml of tetrahydrofuran (Riedel de Haen, p. a.) which was dried over molecular sieves and used without further purification. A volume of 0.625 ml of alkoxysilane precursor, 3-isocyanatepropyltriethoxysilane, (ICPTES, Fluka) was then added to the diamine in a fume cupboard and the reaction flask was sealed and stirred

Ionics 4 (1998)

171 using a type K thermocouple

EtO\ HOII HOH ..OEt I II I I It I EIO-SifCH,)3 N C N CH CH2(OCltCHz).,(OCHzCH2)b (OC1t2CIH)c N C N (Ctt2) 3S i - O E t "O Et EtO / CH 3 CH 3 CH 3

with the sensor placed close to the electtrolyte disk. Conductivity measurements were effected using the ac

where OEt represents -OCH2CH3, a + c = 2.5 and b = 40.5 Scheme 1 overnight. The formation of the intermediate compound, designated as ureapropyltriethoxysilane (UPTES) with the structure illustrated in scheme 1, was followed by infrared spectroscopy. The reaction was taken to be complete when the strong band at 2277 cm 1, assigned to the stretching vibration of the NCO group, disappeared from the recorded spectra. In the following step an accurately known mass of neodymium triflate, Nd(CF3SO3) 3, was prepared by the method of Massaux and Duyckaerts [15] and dissolved in known volumes of ethanol (Merck, p. a.) and distilled water. The mass of the salt used was chosen to yield the desired electrolyte composition and the volumes of ethanol and water were calculated to maintain the molar ratio of 1 UPTES : 4 ethanol : 1.5 water. This mixture was added to the solution of THF and UPTES and stirred in a sealed reaction flask for approximately 30 minutes. After this period the contents of the flask were transferred to a Teflon | mould, covered with a perforated membrane of Parafilm | and left to evaporate for 24 hours. The film was subsequently placed in an oven at 40 ~ for a week and finally subjected to a thermal treatment of 80 ~ for

impedance technique (Solartron 1250 FRA and 1286 ECI) during a heating cycle. Thermal analysis of electrolyte samples was undertaken by heat-flux DSC and thermogravimetric methods. Disks of 5 m m diameter were cut from films and located in 40 ktl cans which were sealed hermetically within the drybox. These samples were characterized between -40 and 350 ~

using a Mettler TC 11 controller coupled to a

DSC 20 oven operating under a constant flux of high purity argon and at a 5 ~ min ~ heating rate. A Perkin Elmer TAC 7/DX controller and thermobalance was used to apply a 10 ~ min ~ heating rate to 5 m m diameter disk samples presented in open platinum cans at temperatures between 30 and 500 ~ Powder X-ray diffraction patterns were recorded using a Rigaku Geigerflex D/max-c diffractometer system. The samples were exposed to the CuKt~ radiation at room temperature in a range of 20 between 4 and 80 ~ 3. Results and D i s c u s s i o n The most surprising features of the behaviour of the neodymium triflate-based ormolytes were the relatively high ionic conductivities observed and the apparent weak dependence of the conductivity on the electrolyte com-4.0

three weeks to form a transparent, elastomeric, fir.estanding monolith with a light yellow coloration. A range of ormolytes containing different concentra-

m

-5.0

9

tions of neodymium triflate were prepared using the pre-

9

&A

9

viously described. The composition of electrolyte films was identified by the number n of (OCH2CH2) monomer units in the host polymer per mole of neodymium triflate. Ormolyte samples were produced with values of n between 1 and 200. Disks of slightly greater than 10 mm diameter were removed from films and located between two 10 mm gold disk electrodes. The electrode/electrolyte/electrode assembly was then installed in a cell support which has been previouslym described [16]. Measurements of the total ionic conductivity of the sample were carried out within a drybox, under an argon atmosphere, at temperatures between 20 and 100 ~ and at approximately 5 ~ intervals. The temperature of the electrolyte was evaluated

r~

-6.0

o -7.0

-8.0

IBm I

2.6

I

218

I

310

i

3.2

i

3

1000 / K Fig. 1. Conductivity behaviour of U(2000)nNd(CF3SO3)3 ormolytes with compositions of 9 n = 100, 9 50, 9 20 and I1.

Ionics 4 (1998)

172

-3

clear similarities can be detected in the variation of the total ionic conductivities with concentration over the range of temperature and composition studied. The most notable difference between the two systems is that the

-4 -5

/I--i__l/I

i--I--I/i~i

or~ -6 "$

conductivity of the sol-gel derived material is much less dependant on the sample temperature (Fig. 2). This be-

(~C

havior is expected as a result of the marked crystallinity of the linear homopolymer based system. The xerogel host, with a prodominantly amorphous morphology, ex0

20

20

40

40

60

6'0

80

100

8'0

120

100

n (O/Nd) Fig. 2. Conductivity isotherms of U(2000)nNd(CF3SO3)3 ormolytes at 9 90 ~ 9 60 ~ 9 50 ~ and 9 30 ~ Inset illustrates results obtained with the same guest salt in a poly(ethylene oxide) host polymer (reprinted with permission of the editor of Portugaliae Electrochemica Acta).

hibits a lower activation energy for ion transport. At higher temperatures, between 80 and 100 ~ the results obtained over the entire composition range of n from 5 to approximately 100, are very similar in these two systems. The amorphous morphology of the ureasils U(2000)nNd(CF3SO3) 3, with compositions of n between 20 and 30, is confirmed by the results of thermal analysis using heat-flux DSC, which is included in Fig. 3. At values of n between 40 and 100 the endothermic peak

position (Fig. 1). This behaviour is unlike that observed

which corresponds to the fusion of polymer hedrites in-

in most monovalent cation based systems but has been found in other multivalent systems, in particular in re-

creases in intensity, with a decreasing amount of guest salt species, as would also be expected in a PEO-like structure. In semi-crystalline samples a poorly formed

cently studied Eu(KI)-doped ureasils [7]. The neodymium salt-based electrolytes can be most directly compared to the system prepared by solvent casting the same salt but with PEO as host polymer [17]. The behaviour of this latter system is illustrated in the inset graph of Fig. 2 and n=

~

n= ~

~

20

of salt-polymer complexes and may be associated with the initiation of decomposition. Further studies of these

n =40 n =50 n=60

n = 100

'

'

'

I 0

. . . .

[ 100

....

] ' ' ' ' [ .... 200

300

Temperature (~ Fig. 3. Results of thermat analysis Nd(CF3SO3)3 ormolytes by heat-flux DSC.

certain compositions of electrolyte, peaks of low intensity are observed at temperatures between 200 and 300

l

n=30

'~

exothermic peak can be discerned at temperatures of between -20 and 0 ~ This thermal event may be assigned to cold crystallization of the host polymer. In

of U(2000) n

These peaks do not seem to be caused by the fusion

samples are necessary to clarify the origin of this thermal event. Figure 4 shows the X-ray powder diffraction (XRD) patterns of U(2000)nNd(CF3SO3)3, at n = 20, 40, 60, 80 and 100. For n between 40 and 100 the diffractograms are dominated by the presence of two intense and well-defined Bragg peaks at 19.1-19.2 ~ and 23.3-23.4 ~ These peaks are associated with the diffraction of crystalline poly(ethylene glycol) [13], PEG (molecular weight 2000, with 40.5 monomer unit, similar to the number present in the U(2000) hybrid). In the salt-rich region of electrolyte composition, at close to n = 20, XRD results confirm that the pure crystalline polymer phase disappears, in agreement with the results obtained by thermal analysis (Fig. 3), and the diffraction pattern shows only a broad hump centred at 20.5-21.0 ~, which is caused by the presence of amorphous silica [13]. This feature is also clearly identified in ureasils with n = 80 and 100.

Ionics 4 (1998)

173

I , I , I ' I ' I ' I ' I ' I ' I ' I ~ I ' I ' I ' I ' I ' I ' I ' ~

105-I

'

n=20

n=40

85

=

75 n=60

~=~ r~

=

=

85

40

754 n=80 95n = I00 ,I,

4

I , l , l l l J l , l J l l l , | , l , l , l , I

lll,I

85-

.1"~

75

8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80

65

20 (degrees)

55

Fig. 4. X-ray diffraction patterns of with n = 20, 40, 60, 80 and 100.

45-

U(2000)nNd(CF3SO3) 3

35 25

The results of thermogravimetric analysis are illustrated in Fig. 5 and confirm that the onset of thermal decomposition occurs at temperatures between 300 and 350 ~ significantly higher than that of similar Eu(m)doped materials. One of the advantages of the sol-gel preparative technique is that the final product is presented without the anti-oxidants and thermal stabilizers, which are normally added to commercial PEO and which improve the resistance of the host polymer to thermal degradation. Even in the absence of these additives, the xerogels show adequate thermal stability for most applications.

perties are expected to improve the optical characteristics of the materials and enhance their performance in potentially interesting emerging technological domains [18].

4.

5.

Conclusions

Solid, solvent-free, transparent ormolytes prepared using the sol-gel method and based on neodymium triflate have been shown to sustain levels of conductivity similar to those of doped commercial poly(ethylene oxide)-based material. The amorphous domain of the Nd(III)-doped ureasils extends over a larger composition range and the thermal stability of these latter materials is not markedly different from that of films based on PEO. These pro-

15

25

75

189 175 225 27S 325 375 425 47S Temperature (~

Fig. 5. Thermogravimetric analysis curves of selected U(2000)nNd(CF3SO3)3 ormolytes.

Acknowledgements

The authors wish to thank the Funda~o de Ci~ncias e Tecnologia for financial support through contract PBIC/ CTM/1965/95. .

References

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174 [2] F.M. Gray, Polymer Electrolytes, RSC Materials Monographs, The Royal Society of Chemistry, London (1997). [3] L.D. Carlos, M. Assunqao, T.M. Abrantes and L. Alc~icer, Mat. Res. Soc. Symp. Proc. 293, 117 (1993). [4] L.D. Carlos and A.L.L. Videira, Physical Review B 49, 11721 (1994). [5] L.D. Carlos and M. Assunq~o, J. Mater. Res. 11, 2104 (1996). [6] L.D. Carlos, V. de Zea Bermudez, M.C. Duarte, M.M. Silva, C.R. Silva and M.J. Smith, Luminescent Materials, The Electrochemical Society Proceedings Series, PV 97-29, 352 (1998). [7] V. de Zea Bermudez, L.D. Carlos, M.C. Duarte, M.M. Silva, C.R. Silva, M.J. Smith, M. Assunqao and L. Alc~icer, J. Alloys and Compounds, in press (1998). [8] M.M. Silva, N. Gon~alves, M.J. Smith and P. Lightfoot, Electrochimica Acta 43, 1511 (1998). [9] M.M. Silva and M.J. Smith, lonics 3, 134 (1997) [10] A. Bernson and J. Lindgren, Solid State Ionics 60, 31 (1993). [11] V. de Zea Bermudez, D. Baril, J.-Y. Sanchez, M. Armand and C. Poinsignon, Optical Materials for Energy Efficiency and Solar Energy Conversion XI. Chromogenics for Smart Windows (A. Hugot-Le

Ionics 4 (1998)

[12] [13]

[14] [15] [16] [17] [18]

Goff, C. G. Granqvist and C. M. Lampert, Eds.) Proceedings SPIE 1728, 180 (1992). V. de Zea Bermudez, L. D. Carlos and L. Alc~icer, Chem. Mater. (1998) accepted. L.D. Carlos, V. de Zea Bermudez, R.A. Szl Ferreira, L. Marques and M. Assunq~o, Chem. Mater. (1998) accepted. L.D. Carlos, V. de Zea Bermudez and R.A. S~ Ferreira, J. Non-Cryst. Solids (1998) submitted. J. Massaux and G. Duyckaerts, Anal. Chim. Acta 73, 416 (1974). M.J. Smith and C.J.R. Silva, Electrochimica Acta 40, 2389 (1995). M.J. Smith and C.J.R. Silva, Portugaliae Electrochimica Acta 10, 153 (1992). L.D. Carlos, M. Assunq~o and L. Alc~icer, Electrochimica Acta 43, 1365 (1998).

Paper presented at the 5th Euroconference on Solid State Ionics, Benalmddena, Spain, Sept. 13-20, 1998 Manuscript rec. Sept. 9, 1998; rev. Sept. 14, 1998; acc. Sept. 15, 1998.