*ORMOCERs as inorganicorganic electrolytesfor new solid state lithium batteries and

supercapacitors

M. Popall,a{ M. Andrei,b J. Kappel,a J. Kron,a K. Olmaa and B. Olsowskia

aFraunhofer-Institut fur Silicatforschung, Neunerplatz 2, D-97082 Wurzburg, GermanybEniricerche, 1-20097 S. Donato Milanese, Milan, Italy

(Received 16 September 1996)

AbstractORMOCERs (ORganically MOdified CERamics) are inorganicorganic copolymers which aresynthesized as matrix for Li-ion conduction. The inorganic oxidic backbone of these materials results frompolycondensation of alkoxy compounds whereas the organic network is formed from reactive functionalgroups R of alkoxysilanes of the type RSi(OR)3, or by co-polymerising reactive organic monomers withreactive functionalized alkoxysilanes. Depending on the reactive organic functionalities and their thermaland uv- initiated organic crosslinking reactions the materials were adapted to the needs of battery andsupercapacitor manufacturing. For ionic conductivity polyethers with dierent chainlengths and functiona-lized (eg epoxy) termination sites were synthesized and attached to organically functionalized oxidic oligo-mers. Conductivities of up to 104 O1 cm1 at room temperature (RT) were achieved without plasticizer.The electrolytes form an amorphous network with configuration temperatures T0 (according to VogelTammannFulcher) close to 808C, several degrees below the transformation temperature Tg (measured byDSC) in agreement with conventional configuration theory. The activation energies correlate favourablywith results for good polymer electrolytes. # 1998 Elsevier Science Ltd. All rights reserved

Key words: inorganicorganic copolymers, ORMOCERs, amorphous matrix, attachment of ethylene oxidemonomers, modular-design principle.

INTRODUCTION

Since the first publications and fundamental investi-

gations of Wright [1] and Armand [2] in the early

eighties polymer ionic conductors became interest-

ing for applied research. Driving forces were on the

one hand the growing interest in low and medium

temperature fuel cells, on the other hand there is a

large market for environmentally friendly light-

weight rechargeable batteries and supercapacitors.

Polyethylene oxide (PEO), polyethylene imine,

polysulfides, etc. [3], were investigated, but PEO

still is the most commonly used material for hosting

lithium salts. As the conductivity at room tempera-

ture in pure PEO is limited by partial crystallization

various attempts were undertaken to achieve a

more amorphous and plasticized material. A large

number of investigations describe the use of comb-

branch polymers or copolymers with ionic conduc-

tivities up to 105 O1 cm1 [4]. Others in additionmodify the polymer electrolyte by using plasticizers

resulting in so-called gel-electrolytes [5]. Finally, as

an alternative, Armand et al. established plasticized

salts [6] eg lithium sulfonylimides like LiTFSI, to

achieve the plasticizing eect (disordering of the

crystalline structure of PEO) as well as better dis-

sociation of the salts.

Our attempts towards better conductivity are

based on amorphous networks formed from reac-

tive nano-sized functionalized oxidic oligomers.

Organic crosslinking reactions will result in inorga-

nicorganic hybrid materials.

These inorganicorganic copolymers

(ORMOCERs) in general are hybrid polymeric ma-

terials consisting of interconnected inorganic oxidic

Electrochimica Acta, Vol. 43, Nos 1011, pp. 11551161, 1998# 1998 Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain00134686/98 $19.00+0.00PII: S0013-4686(97)10014-7

*Registered trademark of Fraunhofer-Gesellschaft zurForderung der angewandten Forschung e. V., Munchen(FRG){Author to whom correspondence should be addressed.

1155

(Si, Al, Zr, etc.) and organic (polyethylene, poly-(meth-)acrylate, polyethylene oxide, etc.) com-

ponents.Synthesis is carried out via polycondensation

reactions (solgel processing) in combination with

organic crosslinking reactions of reactive functiona-lized organosilanes. Depending on the chemicalcomposition and processing parameters, special

properties are available to meet the requirementsfor the application technology [710].Previous work on ORMOCERs as polymer

lithium ionic conductors is described [11, 12].This short communication provides selected

aspects of the synthesis from a more applied pointof view. Some material properties and application

demands as well as the resulting conductivity willbe discussed. The use of these electrolytes in sec-ondary lithium batteries, eg their stability versus

lithium and their use as binder for cathode ma-terials, documented in long time cycling exper-iments, are published by Skaarup et al. [13]. For

first information on application in supercapacitorssee Ingram et al. [14].

EXPERIMENTAL

Synthesis of the functionalized oxidic backbone

2 mol 3-glycidyloxypropyltrimethoxysilane(GLYMO) are hydrolyzed with 7.5 mol of water,

including dissolved NH4F (0.02 mol) as catalyst.After 3 days at 508C the resulting emulsion isextracted with CH2Cl2 and dried.

The condensed GLYMO, dissolved in CH2Cl2 iscooled to 158C, 1 mol trivinylchlorosilane (VINYL)is added dropwise and the mixture is stirred forcompletion of the reaction. HCl, CH2Cl2 and the

surplus of VINYL are then distilled o. After sev-eral treatments with dry ethanol followed by distil-lation (to remove HCl) the resulting resin is dried in

high vacuum. FTIR spectroscopy and Karl-Fischer titration was used for detection of traces ofwater and0Si-OH.

Attachment of the epoxide monomers

The epoxy monomers RO(CH2CH2O)5(6)CH2CH(O)-CH2 (EPOX5 and EPOX6) are attached

to the oligomers in 1:1 molar ratio correspondingto GLYMO using catalysts for epoxy-polymeriz-ation and temperatures up to 1008C.In a similar way the diepoxide monomers

CH2-(O)CHCH2O(CH2CH2O)1(3)CH2CH (O)-CH2(EGDE and DIEPOX3) were reacted. The reactions

were monitored by FTIR spectroscopy and epox-ide titration.

Establishing of the inorganicorganic Li-electrolyte

Lithium salts were dissolved in the dried functio-nalized resins by stirring and initiators for thermalor uv-curing were added. After casting in moulds

(diameter: 10 resp. 50 mm) the resin was applied to

uv-curing (high pressure mercury lamp) or heated in

an oven.

Dierential scanning calorimetry (DSC) investi-

gations were done in N2. Ac complex impedance

spectroscopy with stainless steel blocking electrodes

was used to measure the ionic conductivity of flat

samples (diameter: 10 mm, height: ca. 0.5 mm, cov-

ered on both sides with graphite) at dierent tem-

peratures. In general after measurement of

conductivity at RT the samples were heated to

1008C in vacuum, held at those conditions over-

Fig. 1. Scheme for solgel synthesis of oligomers establish-

ing the inorganic oxidic backbone.

M. Popall et al.1156

Fig.2.Schem

eforprincipleattachmentofmonoepoxideEO-m

onomersbyepoxypolymerisation(x=

2forEPOX5andx=

3forEPOX6).

ORMOCERs as inorganicorganic electrolytes 1157

Fig.3.Schem

eforestablishingoftheprinciplenetwork

oftheinorganicorganicLi-electrolytebyaddinglithium

saltsfollowed

byuv-orthermalinitiatedcuringreactions.

M. Popall et al.1158

night, then the temperature was decreased to RTwhile measuring conductivity.

RESULTS AND DISCUSSION

Discussion of synthesis

Synthesis of the epoxide monomers. Mono-epoxidemonomers were synthesized by the functionalization

of an ethyleneglycol monoethyl or monomethy-lether, having the required number of EO units,with epichlorohydrine using the phase transfer cata-

lyst tetrabutylammonium hydrogensulfate, accord-ing to [15, 16] in very high yields. As commercialglycols are only available having up to 3 EO units,

longer EO chains were synthesized following theroute described in [16]. The synthesized monomerswere characterized by 1H-, 13C-NMR and FT-IR-spectroscopy.

Synthesis of the functionalized oxidic backbone.GLYMO was hydrolyzed and condensed (solgelprocessing) at 508C by adding water using ammo-niumfluoride as catalyst. The resulting mixture (twophases) was extracted or separated respectively. Asol of nanosized oxidic particles functionalized with

epoxy groups results. According to FT-IR-spec-troscopy there is still a small quantity of remaining0Si-OH units. To remove them, they were reactedwith alkenyl chlorosilanes like VINYL (see Fig. 1).

Attachment of the epoxide monomers. Afterextraction of HCl and evaporation of all solventsand traces of HCl, the epoxide monomers EPOX5

and EPOX6 in 1:1 molar ratio corresponding toGLYMO were attached as ion hosts by epoxy-poly-merisation (see Fig. 2). As catalyst various lithium

compounds could be used. The use of lithium buty-late will in addition lithiate all remaining alkyl-OH,resulting from epoxypolymerisation. According to

FT-IR measurements and epoxide titration there isnearly no remaining epoxy-functionality.Establishing of the inorganicorganic Li-electro-

lyte. After careful drying of the resins of EO-func-

tionalized oligomers (monitored by FT-IR-

spectroscopy), lithium salts were dissolved (LiClO4,other salts like LiBF4 or LiSO3CF3 were also

tested). After application as a coating or casting inmoulds the resins were thermally cured at 1008C(see Fig. 3). Depending on the application as binder

for the cathode material or as separator in batteriesand supercapacitors, ethyleneoxide oligomers canbe added as plasticizer.

Material properties

The materials as resins are transparent and

slightly yellowish, caused by traces of impurities ofthe epoxide monomer (not detectable with standardmethods). The resulting conducting membranes and

coatings are transparent, optically homogeneous,slightly brownish and strongly hygroscopic.According to X-ray diraction analysis the elctro-lytes are amorphous.

Table 1 summarizes the values from the VTFequation: This equation, which describes the depen-dence of conductivity s on absolute temperature T:s(T) = AT1/2 exp[EA/R/(TT0)], (A is a constantproportional to the number of ion carriers, EA isthe activation energy, R the gas constant and T0 a

temperature close to Tg), was fitted to measuredvalues of temperature dependent conductivity.Additional DSC measurements gave the values ofthe transformation temperatures of the dierent

electrolytes. The T0 and Tg values are

rheology of the resins and therefore, the more

or less sterically hindered polymerisation of the

epoxy- and vinyl-functionalities. Random border

mechanisms [17] should of course influence the ion

conductivity, based on the dierent oligomer sizes.

The less distinct dierence in conductivity despite

the drastic dierence in plasticity (oil = system 1 vs

hard gumlike membrane = system 2) could be a

hint but not more. The Tg of system 1 should beattributed more to some kind of relaxation of the

network than to a transformation.

To get more information about the best way of

attaching EO-units (as bridge between the oxidic

oligomers or as attached unit, free in rotation)

diepoxy endcapped EO-monomers like ethylenegly-

coldiglycidylether (EGDE) were chosen for copoly-

merisation with the monoepoxide EO-monomers

and the epoxy groups of condensed GLYMO in

system 3 (oligomer size similar to system 2). For

comparison the O-concentration referring to the

organic network was kept constant compared to the

other electrolytes. As expected softer membranes

than in system 2 series result, caused by the plasti-

cizing polymerisation of EGDE and copolymerisa-

tion of EGDE with the monoepoxy EO-monomers.The fact that despite this plasticizing eect the con-

ductivity is even lower could possibly be explained

by a lack of sterically unhindered EO-units close to

the oxidic backbone (in comparison to system 2).

This again would indicate that random border con-ductivities close to the more inorganic oxidicareas will play an important role for the total con-

ductivity. The decrease in conductivity at high saltconcentrations conforms to the measured increaseof Tg.

For more information about the influence ofmore or less mobile ion hosts the monoepoxide EO-monomers were replaced by DIEPOX3, an EO-

monomer with 3 EO-units, both sides endcappedwith epoxide units. The plots in Fig. 4 show VTFbehaviour. The calculated curves fit very well withthe experimental values. System 4 (composition, see

Fig. 4) having the same O concentration as theothers (referring to the organic network), showspoor conductivity, confirming that EO-units brid-

ging inorganic areas are less promoting conduc-tivity by stiening the whole network and enablingonly a sterically hindered handshaking mechanism

along the borders to the oxidic areas. AddingPEO 400 (P400) or PEO 500 (P500) (concentration,see Fig. 4) result in flexible membranes with ionic

conductivities of up to 2.4 104 O1 cm1 at RT (seeFig. 4).

CONCLUSION

This short communication demonstrates the syn-thesis of inorganicorganic polymer electrolytes fol-

Fig. 4. VTF plots of systems 4 (O:Li = 20:1), ORMOCERs and PEO interpenetrated ORMOCERs: the dots represent

the measured values whereas the curves were calculated.

ORMOCER

GLYMO VINYL DIEPOX3 P500 P400 LiClO4 s25

Sample no. [wt.%] [wt.%] [wt.%] [wt.%] [wt.%] [wt.%] [O1cm1](a) 21.7 7.6 70.7 10.7 3.30109(b) 17.4 6.1 56.5 20 10.6 2.28106(c) 17.4 6.1 56.5 20 10.6 5.15106(d) 47.5 16.7 35.8 8.4 9.00105(e) 17.4 6.1 76.5 9.8 2.40104

M. Popall et al.1160

lowing a modular-design principle. The inorganicpart not only guarantees the amorphous behaviour,

but the size of the oxidic oligomers and its kind ofEO-functionality have direct influence on the con-ductivity and the mechanical properties. All the sys-

tems discussed show ideal VTF behaviour anddierent routes towards high conductivity aredescribed. Finally, the mechanical stability coupled

with easy processing as a coating indicates promis-ing material properties.

ACKNOWLEDGEMENTS

This fundamental and applied research is part of aBrite/EuRam project (BE 7251) funded by the

European Union. We wish further to thank thetechnical sta at FhG-ISC and Eniricerche as wellas our project partners at Sonnenschein Lithium,

Thomson LCC, Universities of Denmark (S.Skaarup et al.) and Scotland (M. Ingram et al.).

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ORMOCERs as inorganicorganic electrolytes 1161