Polymer Electrolyte Nanocomposites

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Encyclopedia of Nanoscience and Nanotechnology

Polymer Electrolyte Nanocomposites

Mikrajuddin Abdullah 1 , Wuled Lenggoro, Kikuo OkuyamaHiroshima University, Hiroshima, Japan

CONTENTS

1. Introduction2. Conductivity Enhancement

in Polymer Electrolytes3. Development of Polymer

Electrolyte Nanocomposites4. Preparation Methods5. Important Parameters6. Charge Transport Characterizations7. Spectroscopic Characterizations8. Microscopic Analysis9. Thermal Characterizations

10. Density Method11. Electrical Properties12. Mechanical Properties13. Thermal Properties14. Luminescent Composites15. Conclusion

GlossaryReferences

1. INTRODUCTIONRechargeable cells are key components in mobile technolo-gies, such as portable consumer electronics and electric vehi-cles [1]. A search for batteries that provide high energydensity and multiple rechargeability has been a subject of considerable attentions. Even though battery technology

developed one hundred years ago, progress and improve-ments in technology have been slow, particularly when com-pared to the growth of computer technology [2].

A Li-based battery provides a high density and exibilityof design. Todays lithium battery has a high specic energy(> 130 W h kg1), a high energy density (> 300 W h L 1),

1 Permanent address: Department of Physics, Bandung Institute of Technology, Jalan Ganeca 10 Bandung 40132, Indonesia.

high cell voltage (3.5 V), as well as a long cycl(5001000) charge/discharge. Worldwide production ofdevices exceeded 200 million in 1997 and it will be aimately three times that number during 2001 [1]. Slithium produces an explosion reaction with water-belectrolytes, a search for nonaqueous electrolytes is crit

important to the production of the next-generation lithbattery, using electrolytes in an solid phase in an effodevelop more environmentally friendly materials. Polelectrolytes are potential candidates for replacing the ventional aqueous electrolytes in lithium batteries. Polycontaining esters, ethers, or mixtures thereof which havability to dissolve salts are the base materials for polelectrolytes. Polymer electrolytes are generally preparemixing high molecular weight polymers (HMWPs) wsalt solution. The polymer serves as solid solvent, thusmitting the salt to dissociate into anions and cations. Sthe mass of a cation is much smaller than that of an anthe electrical conductivity is dominated by cation tranLithium salts are usually used for this purpose since

are the most electropositive of materials (3 04 V relativeto the standard hydrogen electrodes) as well as the ligmetal (atomic mass 6.94 g/mol, and density 0.53 g/cm3) andthus facilitate the design of storage systems with high edensity (Watt hour/kg) [1]. Table 1 shows a comparisothe electrochemical properties of several metals.

Until presently, however, no polymer electrolyte-blithium batteries are commercially available in the maTherefore, worldwide research is being focused on the dopment of high power and high energy density polymertrolytes with a major attention to safety, performance,reliability. A battery contains two electrodes: positivenegative (both sources of chemical reactions), separatean electrolyte that contains dissociated salts through wion carriers ow. Once these electrodes are connecteexternal circuits, chemical reaction appears at both etrodes to result in a deliverance of electrons to the extecircuits. The properties of a battery thus strongly depenthe electrolyte, anode, and cathode. With the use of pmer electrolytes in lithium batteries, high specic energspecic power, safe operation, exibility in packaginglow cost in fabrication as well as low internal voltageat relatively large current withdraw is expected [3].

ISBN: 1-58883-064-0/$35.00Copyright 2004 by American Scientic Publishers All rights of reproduction in any form reserved.

Encyclopedia of Nanoscience and NanotechnologyEdited by H. S. Nalwa

Volume 8: Pages (731762


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732 Polymer Electrolyte Nanocomposite

Table 1. Electrochemical properties of several metals that have poten-tial applications for use in batteries.

Atomic weight Valence Specic charge ElectrodeMetal [g/mol] charge [A h kg1] potential [V]

Li 694 1 3862 305Na 2299 1 1166 271

Mg 2431 2 2205 238Zn 6538 2 820 076Cd 11241 2 477 040Pd 20720 2 250 013

Realization of commercial polymer electrolyte batter-ies is actively investigated in many companies worldwide. A major effort to develop advanced polymer batteries forelectric vehicles began in the early 1990s by 3M and Hydro-Quebec [4]. The battery contains a lithium metal anode, apolyethylene oxide (PEO)-based polymer electrolyte, and a vanadium oxide (VOx) cathode. The reversibility of lithiumintercalation and deintercalation in the VOx is quite goodbut the average discharge of the cell is low. PolyPlus Bat-tery company in the United States is developing poly-mer electrolyte-based lithium battery which would operateat room temperature with specic energy as high as 500W h kg1 [3]. In a prototype cell, using cathode made of lithium intercalated disulde polymer, a specic energy ashigh as 100 W h kg1 and charge and discharge cycles almostreproducible for over 350 cycles were observed at 90 C [5].Moltec company reported a specic density of 180 W h kg1for an AA-sized battery based on organosulfur cathode [6].Ultrane Battery company reported a room-temperaturesolid polymer battery based on intercalation type electrode with a specic energy 125 W h kg1 and charge/dischargecycling time of 500 [3]. This performance is still below theconsumer expectation threshold. In 1995, Turrentine andKurani in the United States did a survey on demand foralternative fuel cell for vehicles and found that consumersagreed to buy electric vehicles which would run for at least200 km per battery [7].

2. CONDUCTIVITY ENHANCEMENTIN POLYMER ELECTROLYTES

It is believed that in the polymer electrolytes, the cationsare coiled by polymer segment leaving the anions to occupyseparate positions [8]. Battery performance is limited by thespeed of cation diffusion. The transport of cations takesplace if there is a relaxation of the polymer segments sothat cations are released from a segment and then occupyanother segment. Segmental relaxation requires the pres-ence of free volume in the polymer matrix, a condition thatcan be attained if the polymer is in an amorphous state.Unfortunately, most HMWPs crystallize at ambient temper-atures. Ions are transported with difculty in a crystallinematrix since no chain relaxation occurs and, as a result, theconductivity of polymer electrolytes in this phase (at ambi-ent temperature) is depressed. The transport of ions in thisstate is dominated by the jumping of cations to the nearestlocation, which depends on the blocking potential (activa-tion energy). This is similar to the jump of charge carriers in

crystalline solids. The characteristic time for jumping is portional to the exponential of the blocking potential. Tresults in a conductivity of the order of 108 S/cm, a valuethat is far below the desired value of about 104 S/cm [9].When it enters the amorphous state, that is, at temperaturabove the melting point, a high conductivity appears. a commonly used polymer, that is, polyethylene oxide,

melting temperature is 65 C. This is, of course, impracticasince the operating temperature for most electronic deviis room temperature. In addition, at temperatures above tmelting point, the polymer becomes soft, causing the sostate properties to degrade. Initiated by the work of Wriand Armand [1012], several kinds of polymer electrolhave been intensively investigated around the world. Tabdisplays examples of polymer electrolytes and their msured conductivities at 20 C [13].

Improvements in the electrical conductivity of pomer electrolytes at ambient temperature is therefore critical importance for technological applications. Sevapproaches have been explored to realize this aim. Sofrequently used methods will be explained briey here.

2.1. Preparing Low Degreeof Crystallinity Polymers

By considering that the presence of amorphous statestrictly important for improving the conductivity, the mstrategy is to enhance the amorphous state at low teperatures. The rst approach is to prepare low degree crystallinity polymer from initial. It includes cross-linof two polymers [13, 14], synthesis of new polymer, clinking high molecular weight polymer through -irradiation[15, 16], addition of plasticizers in polymer electrolytes, tion of llers, and bending of two polymers [17, 18]. Anostrategy is to prepare an amorphous polymer so as to obta polymer that is composed of four to ve monomeric unFor this system, the chains must be sufciently long to etively complex cations but too short to crystallize at low peratures. Thus the matrix would still be in the amorphstate even at low temperatures. The polymer host servesa solvent and does not include any organic liquids.

2.2. Addition of Side Chains An alternative way to decrease the crystallinity of polymatrix is to introduce side chain to the polymer main chTheoretically, chain ends and branch can be thought oimpurities, which depress the melting point of the polymSimple mathematical formulation can be used to explainmelting point lowering by the presence of chain ends branch. If X i is the mole fraction of impurities (chain endside chains, and branch), then the melting point of polymT m , decreases according to [19]

1T m

1T om =

RH u

ln 1X i (1)

where T om =melting point of polymer containing only pomer chain with innite chain length, R = gas constant,and H u = enthalpy of fusion per mole of repeat uniChung and Sohn showed that the XRD intensity of poly


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Polymer Electrolyte Nanocomposites 733

Table 2. Examples of polymer electrolytes with their corresponding electrical conductivities at 20 C.

Example polymer ConductivityPolymer host Repeat unit electrolyte (S/cm) at 20 CPoly(ethylene oxide), PEO (PEO)8:LiClO4 108CH 2 CH 2 O n

Poly(oxymethylene), POM POM:LiClO4 108CH 2 O n

Poly(propylene oxide), PPO (PPO)8LiClO4 108(CH 3)CH 2CH 2O nPoly(oxymethylene- (POO)25:LiCF3SO3 3105oligo-ethylene), POO (CH 2O)(CH 2CH 2O) nPoly(dimethyl siloxane), DMS DMS:LiClO4 104(CH 3)2SiO nUnsaturated ethylene UP:LiClO4 105

Oxide segmented, UP (EO:Li+ =32 1)HC=CH(CH 2)4O(CH 2CH 2O) n(CH 2)4 x

Poly[(2-methoxy)ethyl (PMEGE)8:LiClO4 105glycidyl ether], PMEGE CH 2CHO

CH 2(OCH 2CH 2)2OCH 3

n

Poly[(methoxy) poly(ethylene PMG22:LiCF3SO3 3105glycol)] methacrylate, PMGn(EO:Ll+ =181)

CH 2C

(CH 2CH 2O) xCH 3

CO

3

n

(PEO-PPO-PEO)-SC (PEO-PPO-PEO)-SC: 13105SC=siloxane crosslinked LiClO4 (4:1 molar)CH 3 CH 3

CH 3 CH 3

O O

PEO(CH 2)3SiOSi(CH 2)3PEO

PEO(CH 2)3SiOSi(CH 2)3PEO

PEO grafted polysiloxane, PGPS PGPS:LiClO4 104

SiO

CH 2 CH 2 PEO

CH 3

n

Poly[bis-2-(2-methoxyethoxy) (MEEP)4:LiBF4 2105ethoxy]phosphazene,MEEP (MEEP)4:LiN(CF3SO2)4 5105(MEEP)4:LiC(CF3SO2)4 104P=NOCH 2 CH 2 OCH 2 CH 2 OCH 3

H 2 H 2 H 2 H 2 H 3

n

decreases with the increase in the length of chain of comb-shaped polymer [20].

Despite depressing the melting point of polymer, the pres-ence of side chain also promotes the solvating of a saltas reported by Ikeda and co-workers [21, 22]. The sidechain has shorter relaxation time compared to the mainchain. The coupling of the side chain with the ion carrier,therefore, results in an increase in the conductivity. Watan-abe et al. designed comb-shaped polyether host with shortpolyether side chain [23]. However, the mechanical prop-erties decreased even as the conductivity increased. Highconductivity with good mechanical properties was obtainedby designing a polymer of high molecular weight with tri-oxyethylene side chain as reported also by Ikeda et al. [24].With 18 mol.% of side chain, the conductivity was measuredto be 1.5104 S/cm at 40 C and raised to 1.4103 S/cmat 80 C.

Composite of polymer with room-temperature molten slatis also an interesting approach to improve the conductivity of polymer electrolytes. Watanabe et al. reported the composite

consisting of chloroaluminate molten salt that possesconductivity of 2 103 S/cm at 303 K [25, 26]. Howevthe disadvantage of chloroaluminate is its hygroscopic erties such that it is impractical in application. The unon-chloroaluminate molten salt, therefore, is requireavoid the hygroscopic problem. Tsuda et al. reported a ductivity of 2.3 102 S/cm in composite of polymer anroom-temperature molten uorohydrogenates [27].

2.3. Addition of Plasticizers Another approach to improve the conductivity is by tion of additional material into the host polymer. Tapproach appears to be the simplest since a pre-produpolymer can be used to make the polymer electrolytes. viously, low molecular weight polymers were usually ureduce the operation temperature of polymer electrolyThe low molecular weight polymers which were addthe matrix of HMWP to reduce the crystallinity attemperatures are frequently known as liquid plastici


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Feullade and Perche demonstrated the idea of plasticizingthe polymer with an aprotic solution containing alkali metalsalt in which the organic solution of the alkali metal saltremained trapped within the matrix of solid polymer matrix [28]. Such mixing results in formation of gels with ionic con-ductivity close to the liquid electrolytes. Less evaporatingsolvents such as ethylene carbonate (EC), propylene car-

bonate (PC), dimethyl formamide (DMF), diethyl phthalate(DEP), diethyl carbonate (DEC), methyl ethyl carbonate(MEC), dimethyl carbonate (DMC), -butyrolactone (BL),glycol sulde (GS), and alkyl phthalates have been com-monly investigated as plasticizers for the gel electrolytes.

Figure 1 shows the effect of plasticizer content tetraglyme(tetraethylene glycol dimethyl ether) on the glass temper-ature of a system of PEO-co-PPO (3:1):LiClO4 [29]. Thedecrease in the glass temperature can be simply explainedusing a Fox equation:

1T g =

W 1T g1 +

W 2T g2

(2)

where W 1 and W 2 denote the weight fractions of compo-nent 1 and component 2, respectively, and T g1 and T g2 aretheir corresponding glass transitions. This equation tells thatthe glass temperature of the composite locates between theglass temperature of the components. This relation is alsoapplicable for copolymer where T g1 and T g2 denote the glasstemperature of polymers forming the copolymer. Reduc-tion in the glass temperature means the enhancement in theamorphous state at low temperature, and therefore improvesthe conductivity at low temperatures. Figure 2 shows theenhancement of conductivity by the addition of plasticizertetraglyme on the system of PEO-co-PPO (3:1):LiClO4,measured at 25 C [29]. The decrease in the glass transi-tion results in the improvement in the fraction of amor-

phous state at room temperature, therefore improving theconductivity.However, an improvement in conductivity is adversely

accompanied by a degradation in solid-state congurationand a loss of compatibility with the lithium electrode [9],

0 25 50 75 100

180

200

220

wt.% tetraglyme

T g

[ K ]

Figure 1. Effect of plasticizer weight fraction on the glass temperatureof a PEO-co-PPO (3:1):LiClO4 using plasticizer tetraglyme (tetraethy-lene glycol dimethyl ether). Data points were derived from [29], D. R.MacFarlene et al., Electrochim. Acta 40, 2131 (1995).

0105

104

103

102

25 50 10075wt.% tetraglyme

[ S / c m

]

Figure 2. Effect of plasticizer weight fraction on the conductivity25 C of a PEO-co-PPO (3:1):LiClO4 using plasticizer tetraglyme(tetraethylene glycol dimethyl ether). Data points were extracted f[29], D. R. MacFarlene et al., Electrochim. Acta 40, 2131 (1995).

particularly when the fraction of plasticizers is too hFor example, the modulus of elasticity and elastic strensignicantly decreases by addition of plasticizers. Thbecause the plasticizers are usually low molecular wepolymer having low mechanical strength. Therefore, ation of plasticizers decreases the mechanical strength ofhost polymer. Figure 3 shows the effect of plasticizer glyme content on the elastic modulus and tensile strenof PEO-co-PPO (3:1):LiClO4 [29]. The use of moderate orlarge quantities of plasticizer results in the production

Elasticmodulus

Tensile

Strength

0 20 40 600.0

1.0

2.0

wt.% tetraglyme

M o d u l u s [ M p a ]

Figure 3. Effect of plasticizer weight fraction on the modulus of a Pco-PPO (3:1):LiClO4 using plasticizer tetraglyme (tetraethylene glycdimethyl ether). Data points were extracted from [29], D. R. Maclene et al., Electrochim. Acta 40, 2131 (1995).


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gel electrolyte. The presence of some plasticizer may alsogive rise to problems caused by its reaction with the lithiumanode. The poor mechanical stability was accounted to bemainly due to the solubility of the polymer matrix in theplasticizer [30]. Cross-linking of the polymer with ultravioletradiation [31], thermally [32], by photopolymerization [33],or electron beam radiation polymerization [34] was found

to reduce the solubility of polymer in the solvent and alsohelped to trap liquid electrolytes within the polymer matrix.

3. DEVELOPMENT OF POLYMERELECTROLYTE NANOCOMPOSITES

Currently, one popular approach to improve the conduc-tivity involves dispersing ceramic llers (solid plasticizers)in the polymer matrix, producing what is currently knownas composite polymer electrolytes. This approach was rstintroduced by Weston and Steele [35]. Ceramic ller wasused to reduce the glass transition temperature and crys-tallinity of the polymer and thus allow the amorphous

polymer to maintain the liquid-like characteristic at themicroscopic level. Ceramic llers that are frequently usedhave particle sizes in the range of about several ten nano-meters up to several micrometers. Fortunately, such llermaterials are commercially available in various sizes at lowprices. Figure 4 shows the effect of ller content on the con-ductivity of polymer electrolytes PEO:LiClO4 [9]. Table 3displays examples of polymer electrolyte nanocompositesand their conductivities at around room temperature.

The inorganic ller also acts as a support matrix for thepolymer, so that even at high temperature, the compositeremains solid. However, at the microscopic level, it main-tains a liquid-like structure, which is important for suf-cient conductivity. The ller particles, due to high surface

area, prevent the recrystallization of polymer when annealedabove the melting point. The acid-base interaction betweenthe ller surface group and the oxygen of the PEO leads toa Lewis acid characteristic of the inorganic ller and favors

2.4 2.6 2.8 3.0 3.2 3.4 3.6

8

6

4

2

L o g

[ S / c m ]

1000/T [1/K]

VTF

Arrhenius

Figure 4. Arrhenius plot of electrical conductivities of: (solid) ceramic-free PEO:LiClO4, (triangle) PEO:LiClO4 containing 10 wt.% Al2O3(5.8 nm), and (square) PEO:LiClO4 containing 10 wt.% TiO2 (13 nm).Data points were extracted from [9], F. Croce et al., Nature 394, 456(1998).

Table 3. Examples of polymer electrolyte nanocomposites with corresponding electrical conductivities at around room temperatu

Conductivityat around

Polymer electrolytes Fillers rt (S/cm) R

PEO:LiBF4 nano-sized-Al2O3 104micro-sized-Al2O3 10

5

[36]PEO:LiCIO4 SiC 107 [37]EO-co-PO:LiCF3SO3 Li1 3 Al0 3Ti17(PO4)3 2104 (40 C) [38]Brached-poly(ethylene silica (12 nm size) 107 [39]imine):H3PO4PEO:LiClO4 -Al2O3 105 [40]PEO:LiClO4 AlCl3 105 [40]PEO:LiClO4 NNPAAM > 105 [40]PEO-PEG:LiI Al2O3 106 [41]PEO-PMMA:EC:LiI Al2O3 108 [41]PEG:LiCF3O4 SiO2 105 [42]PEG:LiCF3O4 C12H25OSO3Li 5105 [43]coated-SiO2EC:PC:PAN:LiAsF6 porous zeolite 103 [44]PEO:LiClO4 AlN, BaTiO3, Bi2O3 107106B4C, BN, CaSiO3

CeO2, Fe2O3, MoS2,PbTiO3, Si3N4,

PEO:LiClO4 carbon black [46]PEO:Li[(SO2CF3)2N] -LiAlO2 4106 [47]PEO:PMMA:EC:LiI MgO 107 [48]PEO:AgSCN Al2O3 8 8104 [49]PEO:AgSCN Fe2O3 1 1105 [50]PEO:AgSCN SO2 3106 [51]PEO:NaClO4 Na2SiO3 2106 [52]PEO:LiCF3SO3 mineral clay 103 [53]PEO:NH4I PbS 0 99106 [54]PEO:NH4I CdS 0 96106 [54]PEO:NH4I PbxCd1xS 0 63084106 [54]

the formation of complexes with PEO. The ller thenas cross-linking center for the PEO, reducing the tensiothe polymer for self-organization and promoting stiffOn the other hand, the acid-base interaction betweenpolar surface group of the ller and electrolyte ions profavors the dissolution of the salt.

Another potential application of polymer electronanocomposites is for making solar cells [55]. Dye-senssolar cells have attracted great scientic and technocal interest as potential alternatives to classical photovdevices. The cell operation mechanism involves abtion of visible light by the chemisorbed dye, followthe electron injection from the excited synthesizer int

semiconductor conduction band. The selection of lielectrolytes, usually containing organic solvent such atonitrile and propylene carbonate, assures the perfect reeration of the dye by direct interaction of the dye oxidstate and I /I3 redox couple and leads to impressively hsolar-to-electrical conversion efciencies (711%) [56However, the stability and long-term operation of theare affected by solvent evaporation or leakage. Thus mercial exploitation of these devices needs the repment of the liquid electrolyte by a solid charge transmedium, which not only offers hermetic sealing and sity but also reduces design restriction and endows the


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with shape choices and exibility. Katsaros et al. investi-gated solid-state dye-sensitized solar cells using compositepolymer electrolytes using PEO and TiO2 in the presenceof I /I3 redox couple [55]. Initially, dye:Ru(dcbpy)2(NCS)2(dcbpy is 4,4-dicarboxylic acid-2-2-bipyridine) was attachedon the surface of TiO2 nanoparticles by immersion of theTiO2 thin-lm electrode overnight in ethanolic solution of

the complex, followed by drying. The functionalized TiO2nanoparticles, I /I3 , and PEO were put in acetonitrile, fol-lowed by heating and drying to evaporate the solvent. Maxi-mum incident photon to current efciencies as high as 40% were obtained at 520 nm, only two times lower that thanobtained using liquid electrolytes [58]. The overall conver-sion efciency was 0.96%. For all-solid-state devices, suchefciency can be considered to be sufciently high [59].

4. PREPARATION METHODSNow we will briey explain several methods of preparationof polymer electrolyte nanocomposites that are commonlyused. Which method should be used, of course, depends onthe materials and the form of sample to be formed. Onemethod can only produce sample in the form of thick lm,and another one can produce a sample in the form of lmof submicrometer thickness.

4.1. Casting MethodThis method is frequently used due to its simplicity. It canproduce polymer lm from several micrometers up to sev-eral millimeters thickness. Generally, this method includesthe following steps:

(a) dispersion of ceramic llers in a salt solution,(b) addition of a specied amount of polymer to the

mixture,(c) mixing by means of stirrer or ultrasonic equipment to

disperse the particles homogeneously in the polymermatrix,

(d) casting the mixture on a substrate,(e) nally drying in vacuum or in an atmosphere of argon. All these steps are usually performed in a glove box lled

with argon gas and excluding oxygen and water to levelsbelow 20 parts per million (ppm), to avoid the possibleoccurrence of a dangerous reaction between water andlithium. The solvent must be water-free and should be com-mon solvent for both the salt and the polymer. Since themelting point of several polymers is as high as 65 C, thesolvent must also easily evaporate so that drying can be per-formed at temperatures of around 65 C. Organic solventssuch as acetonitrile, cyclopentanone, and propylene carbon-ate, plus inorganic solvents such as thionyl chloride (SOCl2),are typically used.

Sometimes, the insertion of salt is performed after cast-ing the lm. For example, Ardel et al. prepared PVDF2801 (Kynar)-based polymer electrolyte composites accord-ing to the following steps [60]. First, Kynar was dissolvedinto cyclopentanone. Nanoparticles of silica were added andthe mixture was mixed for 24 h at room temperature to gethomogeneous slurry. After complete dissolution, the slurry was cast on the Teon support and spread with the use of

doctor blade technique. To prevent surface irregularities, lm was then covered with a box pierced with holes allowed a slow evaporation of the cyclopentanone. Acomplete evaporation of the cyclopentanone, the polymmembrane was soaked in a lithium ion solution for 4Several fresh lithium solutions for each soaking can be uto ensure a complete impregnation of lithium ion into

membrane.

4.2. Spin CoatingThe spin-coating method is very similar to the castmethod. Instead of casting the lm on a substrate, in tmethod, the mixture is dropped on a substrate and placin a spin coater that can be rotated at adjustable rottion speed. The lm thickness can be controlled easilyadjusting the viscosity (concentration) of the mixture the speed of rotation. However, this method is only avable if the viscosity of the mixture is not too high. For amixture, the spin coater rotation is not enough to spread mixture droplet to form thin lm.

4.3. Hot PressHot press technique equipment is illustrated in FigureThe equipment consists of: (A) weighing cylinder, (B) hing chamber, (C) basement, and (T) temperature controllProper amounts of polymer, salt, and ller are mixed imortar for about several minutes. The powder mixturethen sandwiched between two sheets of Mylar or other mrials, and positioned inside the heating chamber that is ctrolled at temperatures lightly above the melting point ofpolymer. If PEO is used as polymer matrix, temperature80 C is suitable [61]. The sample is then pressed overn with a pressure that can be controlled by weighing cylin After heating and pressing, the sample is then slowly coto room temperature. The sample is then separated frothe Mylar sheet and placed in a glove box.

T

A

B

C Sample

Figure 5. Illustration of hot press equipment: (A) weighing cylind(B) heater, (C) base, and (T) temperature controller.


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4.4. In-situ Preparation In-situ preparation explained here is the preparation of nanoparticles in the polymer matrix. Mikrajuddin et al.produced polymer electrolytes of polyethylene glycol withlithium ion by in-situ production of ZnO nanoparticlesin the polymer matrix [62, 63]. The preparation methods will be briey described as follows. Zinc acetate dihydrate,(CH3COO)2Zn2H2O 0.1 M in 100 mL ethanol 99.5, washeated with stirring in distillation equipment at tempera-ture of 80 C to produce about 60 mL condensate and40 mL of hygroscopic solution. Lithium hydroxide monohy-drate, LiOHH2O, of various concentrations was suspendedin 40 mL ethanol and stirred until all the granular materialdissolved. Polyethylene glycol (PEG) (Mn=2,000,000) wassuspended into each LiOH solution and then stirred withheating at around 60 C until homogeneous gel-type mix-tures were obtained. The mixture temperatures were thenleft to go down several minutes, after which 10 mL of hygro-scopic CH3COO2Zn2H2O solution was added into eachmixture. The new mixtures were then homogeneously mixedand then dried in an oven that was kept at temperature of 40C during three days. The schematic of sample preparations

is displayed in Figure 6.There are many differences between the present method

and the commonly used ones. In the present approach:(a) Nanoparticles are grown in-situ in polymer matrix.(b) The size of dispersed particles is controllable. (c) Ioncarriers are inserted in-situ in the polymer matrix duringthe growing process. (d) Finally, since the grown nano-particles are luminescent, we obtain a new class of polymerelectrolytes, namely luminescent polymer electrolytes withnanoparticles as luminescence centers. Based on the TEMpicture, we found that the size of ZnO nanoparticles was5 nm.

ZnAc 2

2H2O

EthanolEthanol

LiOH

H2O

PEG

Dried at 40C,3 days

Mixed at 60C

Distilled at 80C

Cooled at 0C

Mixed

severalminutes

Mixed around 10 min

Left cooling

unused

ZnO nanoparticlesare formed in the polymer matrix

Characterizations

Condensate(60%)

Hygroscopicsolution (40%)

Figure 6. Diagram of in-situ preparation of PEG:Li containing nano-particles of ZnO. Adapted with permission from [62], Mikrajuddinet al., J. Electrochem. Soc. 149, H107 (2002). 2002, Elsevier.

Chandra et al. produced polymer electrolytes PEO:N4Icontaining nanometer-sized semiconductor particles CdS, PbxCd1xS [54]. Methanolic solution of PEO aNH4I was rst stirred roughly at 40 C for 810 h, whichresulted in viscous solution of the ion conducting compof PEO/NH4I. To this solution, a solution Pb(CH3COO)2,Cd(CH3COO)2, or Pd(CH3COO)2 + Cd(CH3COO)2 in adesired fraction was added. The stirring was continuedthe viscosity was back to the value it was before addinacetate compounds. Subsequently, H2S was bubbled throughit giving PbS, CdS, or PbxCd1xS. The nal viscous solutio was poured in a petri dish for obtaining solution-castThen the lm was dried in vacuum.

5. IMPORTANT PARAMETERSTo bring polymer electrolytes as well as polymer electcomposites, these materials should provide enough valuseveral properties as follows.

5.1. Electrical ConductivityConductivity denes the density of current that can be tported in the material by applying a certain electric eelectric eld E is applied in the material, the current dens will be proportional to the applied electric eld, wherproportional constant is the conductivity, or,

J = E (3) with J the current density (A/m2) and (S/m or S/cm) theelectrical conductivity. It is clear that high conductivityrial will produce high current density upon applying a celectric eld. The value of conductivity is determined bdensity of mobile ions (ion carriers) in the material (n), the

scattering time of the ion ( ), the ion charge (q ), as well asthe mass of ion carrier (m), according to a relation

= nq 2

m (4)

This equation gives the reason why most polymer trolytes use lithium ions as ion carriers. The mass of litis the smallest among all metals, so it produces the hiconductivity.

For industrial application, the conductivity of polyelectrolytes must be as high as 102 S/cm. However, untilpresently, this conductivity can only be achieved at highperatures in which the polymer is present in the soft por even liquid phase. The conductivity at room temperof most reported polymers is still below 104 S/cm.

5.2. Transference NumberSince the electrochemical process in lithium battinvolves the intercalation and de-intercalation of lithcations throughout host compound lattice, solid polyelectrolytes with cation transference number (t+) approach-ing unity are desirable for avoiding a concentration gent during repeated charge-discharge cycles. The repot+ value for dried polymer electrolytes range from 0.00.2 [64]. For a gel polymer system, t+ value of 0.40.5 ha


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been found for poly(bis-methoxy ethoxy)phosphazene [65],and 0.56 in a system of UV-cured gel polymer electrolytesbased on polyethylene glycol diacrylate/polyvinylidene uo-ride [66].

Transference number of a particle is dened as the ratioof the conductivity due to it and the total conductivity. Assume the total conductivity is due to ionic, ion, and to

electronic, e, then = ion + e (5)

The ionic and electronic transference numbers are then

ti = ion

(6)

and

te = e

(7)

For pure ionic, ti = 1, and for pure electronic, te = 1. Forpolymer electrolyte composites, a general condition satisedis 0 < t i t e < 1.

5.3. CrystallinityCrystallinity plays an important role in determining theconductivity of polymer electrolytes. At crystalline phase,the transport of ion carriers is very difcult so that theconductivity is very low. At amorphous phase, there is asegmental motion of polymer chain that also assists thedisplacement of ions. As a result, the transport of ions is rel-atively easy. Thus, high conductivity will result. One majorroute to improve the conductivity of polymer electrolytesis by increasing the fraction of amorphous states. Additionof ceramic llers, addition of plasticizer, and production of branch polymer are efforts to improve the amorphous statein the polymer.

5.4. Mechanical StrengthOne objective of the use of polymer electrolyte is to make abattery or fuel cell with a strength comparable to that of liq-uid electrolytes. Therefore, it is expected that the improve-ment of conductivity is not accompanied by a decrease inthe mechanical strength. It is why the addition of ceramicller has received more attention, since the conductivity andthe mechanical strength can be improved simultaneously.In contrast, the use of liquid plasticizer, although it canenhance the conductivity much higher than the addition of ceramic ller, involves such a degradation in the mechanicalstrength as to make this approach less interesting.

5.5. Storage TimeBattery or fuel cell made from polymer electrolytes shouldhave to operate several weeks or several months. Thus, theproperties of polymer electrolytes should not change toomuch during this time. For example, the conductivity shouldnot depend so much on the storage time. Ideally, the prop-erties should be time independent. However, in reality, theproperties tend to degrade with storage time.

6. CHARGE TRANSPORTCHARACTERIZATIONS

Electrical conductivity is the critical parameter for polyelectrolyte composites. One target of the present researchthis eld is to produce polymer electrolyte nanocomposthat exhibit a high electrical conductivity, especially at r

temperature. Conductivity at around 102

103

S/cm isrequired to bring this material into industry. The electriconductivity relates to the value of current that can be pduced by the battery. The potential produced by the battdepends on the reaction of the battery with the electroEven though the electrode reaction can produce high eltrical potential, the use of low conductive electrolytes produce only small amount of electric current. And sithe power can be calculated simply by the relation Power =Voltage Current , the use of low conductive materials wproduce a low specic energy battery.

6.1. d.c. ConductivityIdeally, the d.c. conductivity should be measured in orto be sure that the values pertain to long-range ion moment instead of dielectric losses such as would be assated with limited or localized rattling of ions within caHowever, the difculty in making a d.c. measuremenin nding an electrode material that is compatible wthe electrolyte composites. For example, if stainless selectrodes are attached to an electrolyte composite, as dplayed in Figure 7a, and small voltage is applied acrthe electrodes, Li+ ions migrate preferentially toward thcathode, but pile up without being discharged at the staless/electrolyte interface. A Li+ ion decient layer forms atthe electrolyte/stainless steel interface.

The cell therefore behaves like a capacitor. There is accumulation of ions at interface region of electrode acomposite. A large instantaneous current I o presents whenthe cell is switched on, whose magnitude is related toapplied voltage and the resistance of the electrolytes then falls exponentially with time, as illustrated in FigureThe characteristic time of current decreasing is relativelyso that it is difcult to make an accurate measurement.

Therefore, the a.c. method is commonly used in tpresent to make measurement over a wide range of fquencies. The d.c. value can be extracted from the adata. Many a.c. measurements are performed with block

I

Io

time

+

+

CPE

(a) (b)

Figure 7. (a) Polymer electrolyte composites sandwiched between blocking electrodes. (b) The decay of current when a constant d.c. age is applied between two electrodes.


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electrode such that no discharge or reaction occurs at theelectrode/electrolyte interface. Because the current will owback and forth, no ions pile up on electrode surface, espe-cially when using a high a.c. frequency. This is why the a.c.resistance (impedance) tends to decrease with increase inthe frequency. The electrodes that are commonly used areplatinum, stainless steel, gold, and indium tin oxide (ITO)

glass. The complex impedance method is widely used todetermine the resistance of the sample. The principle of themethod is based on measurements of cell impedance, whichare taken over a wide range of frequency and then analyzedin the complex impedance plane which is useful for deter-mining the appropriate equivalent circuits for a system andfor estimating the values of the circuit parameters.

Impedance is nothing but the a.c. resistance of the cell.The value in general contains the real and the imaginarypart. An electrochemical cell, in general, exhibits resistive,capacitive, as well as inductive properties. The resistiveproperty contributes to the real part of the impedance, whilethe capacitive and the inductive properties contribute tothe imaginary part of the impedance. Therefore, an elec-trochemical cell can be considered as a network of resis-tor, capacitor, as well as conductor. Which arrangement for which cell is usually determined after performing a measure-ment, by analyzing the form of impedance curve. A capac-itor that presents as an open circuit in a d.c. network andan inductor that appears as a straight conductor wire in ad.c. circuit, both appear as imaginary resistors in the a.c.circuit. Until presently, the inductive properties of the elec-trochemical cell are ignored so that the polymer electrolytecomposite is considered only as a network of resistor andcapacitor.

The complex impedance can be written in a general formas

Z =Z iZ (8) where is the frequency, Z is the real part of impedance, contributed by resistive part, Z is the imag-inary part of impedance, contributed by capacitive part, andi = 1, the imaginary number. As an illustration, Figure 8 shows examples of simple RCcircuits and the corresponding plot of impedance (Nyquistplot). For a serial arrangement of a resistor and a capacitor,as displayed in Figure 8b left, the impedance can be writtenas

Z =R iC

(9)

orZ =R (10a)Z =

1C

(10b)

It is clear that the real part of impedance is constant, inde-pendent of the frequency, while the imaginary part dependson the frequency. For very small frequency, the imaginarypart is very large and this value decreases inversely withfrequency. For frequency approaches to innity, the imag-inary part of impedance closes to zero and the impedance

R

R

R

Z [ ]

Z [ ]

Z [ ]

Z [ ]

Z [ ]

Z [ ]

Z [ ]

Z [ ]

R

mC 1R = 1

mCR = 1

R

C1

C 2

C

R

R C

R

(a)

(b)

(c)

(d)

Figure 8. Examples of simple RC circuits and the corresponimpedance (Nyquist) plots.

value at this very high frequency equals to resistance.the Nyquist plot for this arrangement appears as a vcal straight line, starting from a lower frequency valthe upper part downwards when the frequency increaseshown in Figure 8b right. The intersection of this linehorizontal axis (the real value of impedance) corresponthe resistance.

For a parallel arrangement of resistor R and capacitanceC , as appears in Figure 8c left, the real and imaginary pof the impedance are given by

Z = R

1+ RC 2 (11a)

and

Z

=R

RC

1+ RC2 (11b)

and the corresponding Nyquist plot appears in Figurright. The Nyquist plot appears as an arc. The intersecof this arc with the vertical axis at a low frequency (righcorresponds to the resistance. The frequency at the peathe arc, m , satises the relation

mRC =1 (12)From the intersection point at the low frequency regionthe position of the arc peak, the resistance and the captance of the system can be determined.


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PEO3:LiClO4 + -Al2O3, that both dc and p satisfy the Arrhenius expression

C =C o exp E kT

(18)

where C is either dc or p , and C o is the corresponding

prefactor.Furthermore, the temperature-dependent dielectric con-stant can also be obtained from the ac data. The real partof the dielectric constant, , can be expressed as

= ac

o(19)

where ac is the imaginary part of the a.c. conducttivity,and o is the permitivity of vacuum. The complex dielectricconstant can be written as = +i , and theimaginary part can be obtained from the real part using aKramerKronig relation

= 2 P

0

ss2 2 ds (20)

where P denotes the principal part of the integral [70]. Onthe other hand, if the imaginary part has been known, thereal part can be determined using a relation

= 2

P 0 s ss2 2 ds (21)6.3. Diffusion CoefcientElectrical conductivity can also be determined by measuringthe diffusion coefcient. From the temperature-dependentdiffusion coefcient, the temperature dependence of elec-

trical conductivity can be determined using NerstEinsteinequation

= ne2D

kT (22)

where n =charge carrier concentration e =electron charge,and D =diffusion coefcient.Salt diffusion coefcient can be obtained by galvanostati-cally polarizing a symmetric cell containing no-blocking elec-trode for a short period of time. For example, assume a cellcontaining Li-based polymer electrolytes and lithium elec-trodes at both sides. When the current is turned off, theinduced concentration prole is allowed to relax. At longtime after the current interrupt, the following equation isapplicable [71]:

ln = 2D s

L 2 t +A1 (23)

where = measured cell potential, Ds = salt diffusioncoefcient, L =electrolyte thickness, t =time, and A1 =aconstant.It appears that D s is proportional to the slope of curve ln with respect to t . The dependence of salt diffusion con-

stant on the salt concentration is displayed in Figure 10 [71]for system of PEO:NaCF3SO3 at 83 C. The Ds decreases

0.08.0

7.8

7.6

7.4

7.2

7.0

0.5 1.0 1.5 2.0 2.5

L o g

D s

[ c m

2 / s ]

Salt concentration [mol/L]

Figure 10. Effect of salt concentration on the diffusion coefcienPEO:NaCF3SO3 system at 83 C. Data points were extracted from [7Y. Ma et al., J. Electrochem. Soc. 142, 1859 (1995).

as the salt concentration increases from about 8

108 cms

for dilute solution.Diffusion coefcient can also be determined fromNyquist plot as discussed by Strauss et al. [72]. The mfrequency arc is attributed to the solid/electrolyte inter At lower frequencies, the impedance is affected by contration gradient (diffusion) and ionic aggregates. Thefusion impedance of symmetric electrolyte with no-bloelectrode, such as Li/CPE/Li, can be written as

D = RTL

n2F 2C bZ DC (24)

where R = the gas constant, n = ratio of EO/cations, F =Faraday number, C b = bulk concentration of cation, T =temperature, and L =electrolyte thickness.Lorimer also introduced another formula for calculathe diffusion constant, that is [73],

D = mL 2

2 54 (25)

where m =the frequency at the maxima of low frequearc, and L =electrolyte thickness. The values predictedEq. (25), however, are around 610 times as large aspredicted by Eq. (14). The error can be contributed byshift of m due to the formation of ion pairs [72].

6.4. Transference Number

Transference number can be calculated by analyzing thimpedance spectrum of symmetrical cell with no bloelectrode. The transference number can be calculatedcomparing the width of the skew low frequency semicZ d , with the value of the bulk resistance, that is [74],

t+ = 1

1+Z d /R b(26)

Transference number can also be determined by measment of the electrochemical potential of the cell as itrated in Figure 11 [75]. Suppose the polymer compos


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CPE

Electrode 1 Electrode 2

1 2

Figure 11. A simple experiment for determining the transference num-ber to electrodes with differential chemical potentials.

sandwiched between two electrodes with different chemicalpotential 1 and 2. The electrochemical potential acrossthis cell is given by

E = 1z F

2

1t i d =

1z F

t i 2 1 (27) where z =absolute value of the valence of the mobile ionin the electrolyte; and F =Faraday number.

For pure ionic composite, ti =1 so thatE pure = 2 1 z 1F 1 (28)

ThusE =t i E pure (29)

By measuring E and calculating E pure, we can obtain t i. Another method based on a combination of d.c. polariza-

tion and a.c. impedance has been introduced by Evans et al.This method involves measuring the resistance and currentacross a symmetrical Li/electrolyte/Li cell polarized by a d.c. voltage [76]. The t+ is given by

t+ = I S V

I oRo

I o V I S RS (30) where V =d.c. voltage applied to the cell, Ro =initial resis-tance of the passivating layer, RS = steady-state resistanceof the passivating layer, I o =initial current, and I S =steady-state current.

The d.c. polarization potential usually used is several tensof millivolts. This equation is applicable for ideal, dilutesolutions. However, Doyle and Newman state that althoughthis equation is not strictly applicable in concentrated elec-trolytes, the ratio of steady-state to initial current providesuseful information on the contribution by organic additivesto the ionic conductivity of polymer electrolytes [77]. Thesimplication of Eq. (30) was also used, that is, t+ =I SS / I O .However, signicant errors resulted from neglect of kineticresistances at the electrode/electrolyte interface [78]. Trans-ference numbers of some composites appear in Table 4.

7. SPECTROSCOPICCHARACTERIZATIONS

7.1. NMR Spectroscopy A moving ion would substantially modify the interaction of electromagnetic waves with matter. Investigating this inter-action gives a better understanding of ion dynamics on

Table 4. Transference number of several composites.

TransferenceComposites number Temperature Ref.

PEO:LiCF3SO3 + -LiAlO2 (4 m) 0.29 90 C [79]PEO:LiBF4 + -LiAlO2 (4 m) 0.26 90 C [79]PEO:LiClO4+TiO2 (13 nm) 0.50.6 90 C [79]PEO:LiClO4+ Al2O3 (6 nm) 0.310.33 90 C [80](PEO)30LiClO4 0.180.19 100 C [80](PEO)8LiClO4 0.190.20 90 C [80](PEO)8LiClO4+SiO2 0.220.23 100 C [80]

a microscopic scale. An example of method for studythe ion dynamics is nuclear magnetic resonance (NMspectroscopy. This method probes the spin of ion usan electromagnetic wave in radio frequency. In amorphsingle-phase polymer electrolytes, there is usually founstraight relationship between polymer segmental motion ionic mobility by observing a strong correlation betweenonset of NMR line-narrowing and the glass transition [NMR has contributed signicantly to the understanding

the physical properties of the composite polymer electrolmainly because it offers the possibility to selectively sthe ionic and polymer chain dynamics. For example, msurement of the temperature dependence of 7Li lineshapesand spin-lattice relaxation allows the determination of activation energy and the correlation time of the catimotion. Gang et al. described the 7Li line-narrowing in thecomposite of PEO:LiBF4 + -LiAlO2 (1030 wt%) in thetemperature range of 270270 K [82]. Dai et al. repor wide line and high resolution solid-state 7Li NMR [83].

In material, each spin interacts with other spins, givrise to spin-spin interaction or relaxation time T 2. Further-more, a new thermal equilibrium distribution of the sp which has to be mediated through lattice, is forced by

magnetic eld. The characterization time required for excess energy to be given to the lattice or for attainmof new thermal equilibrium is expressed in terms of splattice or thermal relaxation time T 1. Under simultaneousapplication of static and radio frequency magnetic elperpendicular direction,

H z =H o (31a)H x =2H 1 cos t (31b)

The interaction of this magnetic eld with the nuclear sresults in the Bloch susceptibility [84]

= 1

2 o oT 2

T 2 o 1+T 32 o 2 + 2H 21T 1T 2

(32a)

= 12 o oT 2

11+T 22 o 2 + 2H 21T 1T 2

(32b)

where o = H o , and is the gyromagnetic ratio of thespin. For low RF eld H 1, 2H 21T 1T 2 1, then Eqs. (32a)and (32b) give the familiar absorption curve with half-w

= o 1/T 2 (33)However, the exact lineshape and linewidth can be de

mined using Van Vleck method of moment. This meth


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allows the connection of the absorption line with themotional behavior of the nuclei. The second moment, M 2 isgiven by

M 2 = o2f d

= 3

4 4 2I I

+1

13 cos ij 2r 6ij

(34)

where I is the nuclear spin [85].Nuclear spin and NMR frequencies of some nuclei at

H o =10 000 Gauss are given in Table 5 [86]. Based on thisdata, we can select the magnetic eld H o at around 10,000Gauss (1 Tesla) and frequency o of around 16.574 MHz, sothat the absorption of the target spin can be observed. Forexample, in order to detect the absorption of lithium ion inthe lithium-based polymer electrolyte, the magnetic eld H ocan be set at around 16,574 Gauss.

If the spins are in motion, such as in uid or intramolec-ular motion of liquid-like lattice, the value of M 2 is smallbecause of the small time-average local eld component (the

average of cos2

ij =1/ 3 . The linewidth decreases as o M

1/ 22 (35)

If c is the uctuation time, M 1/ 22 c 1 corresponds to rigidlattice and motional narrowing corresponds to M 1/ 22 c 1.The region with 1/ c approximate to M 1/ 22 represents, theregion of the onset of motional narrowing. Motional nar-rowing is used to study the ionic motion. Considerableline-narrowing takes place above a certain temperature,indicating diffusional motion.

A rough estimation of the activation energy for motioncan be obtained by using a modied Bloembergen, Purcell,and Pound expression [87]:

c = 1 c =

tan 22B2A2B2

(36)

where c = jump frequency, = measured linewidth attemperature T , A =unnarrowed linewidth of the rigid lat-tice, B =fully narrowed linewidth, and is a constant.

Table 5. Nuclear spin and resonance frequency at 10,000Gauss of several nuclei.

Magnetic resonancefrequency (MHz)

Nuclei Nuclear spin at 10,000 Gauss1H 1/2 425776Li 1 62657Li 3/2 1657417O 5/2 57219F 1/2 4005523Na 3/2 1126239K 3/2 198741K 3/2 109263Cu 3/2 1128565Cu 3/2 12090107 Ag 1/2 1220109 Ag 1/2 1981

Fitting c/ with Arrhenius expression,

c

= o exp

E kT

(37)

one obtains the value of E based on the Arrhenius plot oc/ . Another expression for determining the activation en

is given by Hendrickson and Bray [88]:

ln 1

D 1A =

E kT +ln

1B

1A

(38)

where D is a temperature-independent constant of a linbroadening term.

The relation between spin-lattice relaxation time T 1 andthe uctuation time c can be written as [89]

1T 1 =S

c1+ 2o 2c +

4 c1+4 2o 2c

(39)

where S is a constant. For an approximate case, in wh

relaxation from a pair of nuclei of xed internuclear spr only is considered, one obtains [90]

S = 25r 6

4 2I I +1 (40)The spin-spin relaxation time T 2 is given by [87]

1T 2 =S

52

c1+ 2o 2c +

c1+4 2o 2c +

32 c (41)

To study dynamics, the relaxation time is measured function of temperature so that c can be calculated. Usingan Arrhenius relation

c = o exp E/kT (42)the activation energy can be calculated.

Many NMR experiments have been performed on pmer electrolyte nanocomposites [9195]. Figure 12 sthe Arrhenius plot of spin-lattice relaxation time PEO8:LiClO4 polymer electrolytes and the composite ppared with 5.3 wt.% -Al2O3 detected at a Larmor fre-quency of o =1554 MHz [96]. The attempt frequency 1/ ocan be interpreted as a vibrational frequency, of ordeoptical phonon frequency (10121012 Hz). It can be calcu-lated (e.g., using Mathematica software) that the func1/T 1 nds a maximum value at o c =0 613. The maximum value of 1/T 1 appears at temperature of T 330 K for bothsamples. Using a Larmor frequency of o = 1554 MHz,one obtains c = 0 613/(1.554 108 = 394 109 s. Forcomposite of PEO:LiClO4+carbon black, Franco et al. estmated the relation time of about 4.4 109 s based on the1H resonance measurement [97]. Using Eq. (42) and assing 1/ o 1012, we have E 0 24 eV for both samples.The electrical conductivity can be determined fEq. (42) and different form of NernstEinstein equatthat is,

= Nd 2q 2

6 ckT (43)


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2.5

10 1

10 0

3.5 4.5 5.5

1000/T [K 1]

S p

i n - l a t t i c e r e

l a x a t i o n r a t e ( 1 / T

1 ) [ s

1 ]

Figure 12. Arrhenius plot of spin-lattice relaxation time (1/T1) of 7Li NMR spectra for (open circle) PEO:LiClO4 and (solid circle)PEO:LiClO4 containing carbon black. Data points were extracted from[96], A. C. Bloise et al., Electrochim. Acta 46, 1571 (2001).

where N = lithium concentration per unit volume, d =average ionic jump distance, and q =ionic charge.For example, the value of N in PEO8:LiClO4 was deter-mined from the molecular weights of PEO and LiClO4 andthe density of the electrolytes (1 3 g/ cm3) [96], to resultin N 1 71021 cm3. Considering the average Li-Li dis-tance of 4 [96], the conductivity at 300 K is about 71104 S/cm.

Temperature-dependent linewidth of composite of PO1 5LiI+6 v% Al2O3 is displayed in Figure 13 [83]. Itcan be seen that there is a drastic change in the line-width when temperature is changed from 30 C to 50 C. It isassumed that the onset of the motional narrowing is at

temperature of 40 C. At 20 C the sample exhibits arigid limit lineshape with baseline due to a distributionof 7Li nuclear quadrupole satellite transition [98]. As thetemperature is raised, partial line-narrowing results.

Forsyth et al. reported the effect of ller content onthe 7Li linewidth for a composite of copolymer trihydroxypoly(ethylene oxide-co-propylene oxide) with an EO/PO

20 0 20 40 60 80 100 120

1

2

3

4

5

6

7

7 L i N M R l i n e w

i d t h [ k H z ]

Temperature [C]

Figure 13. Temperature dependence of 7Li NMR linewidth of PEO15LiI containing 6 vol.% Al2O3. Data points were extracted from [83],Y. Dai et al., Electrochim. Acta 43, 1557 (1998).

ratio of 3:1 containg LiClO4 and TiO2 nanoparticles [99].The linewidth increases with increase in the ller contand reaches a plateau after about 10 wt.%, as observedFigure 14. Although the linewidth relates to the mobilitlithium ion, in which the line-narrowing represents moion, an interpretation that the increase of linewidth relato decrease in the mobility of lithium ions will be contra

tory with the reported result. The reported result conrmthat addition of ller at lower content increases the lithiion mobility and therefore increases the conductivity. A sible interpretation is the lithium ion environment chan with the addition of ller and the linewidth changes more likely to reect the changing environment rather ththe changing mobility. A possible interpretation is chemshift dispersion, that is, lithium ion occupying many dient state environments (but magnetically degenerate) suthat a distribution of chemical shift is obtained.

The broad signal of resonance is attributed to crystallstate, while the narrow component is attributed to amphous phase. This difference contribution can be usedpredict the crystallinity of polymer in composite. For ex

ple, Singh et al. observed the 1

H resonance of a systemof PEG46:LiClO4 +nanoparticles of Mn0 03Zn0 97 Al2O4 [95].They tted the broad part of the signal with a Gaussian fution and the narrow component with the Lorentzian fution. The crystallinity of the polymer equals to the fracof the area of narrow and broad components of the signFor pure PEG they found a crystalline of about 83% usthis method.

7.2. Raman ScatteringRaman spectroscopy is important to probe molecules wanisotropic polarizability. The vibrating atoms are not to follow the incident radiation frequency if it is much hi

than the phonon frequency. However, the electron cloof the vibrating atom can interact with the frequencythe incident radiation. These oscillating dipoles can abenergy from the radiation eld and re-emit radiation of same frequency. This radiation is detected as scattered li

0 4 8 12 16200

300

400

500

600

wt.% TiO 2

7 L i L i n e w i d t h [ H z ]

Figure 14. Effect of ller weight fraction on the 7Li NMR linewidth of copolymer of trihydroxypoly(EO-co-PO) with (EO/PO = 3/1) contain-ing LiClO4 and TiO2 nanoparticles. Data points were extracted from[99], M. Forsyth et al., Solid State Ionics 147, 203 (2002).


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and is known as Rayleigh scattering. Moreover, the oscillat-ing dipoles see the force eld of the vibrating atomic nucleialso as the nuclei oscillate around the equilibrium position;the deformability of the electron cloud varies with the oscil-lation frequency of the nuclei.

The Raman scattering is described in a simple term here.If a time-dependent electric eld, E t , is applied to a

molecule, it produces an induced dipole moment, t ,t = t E t (44)

where t is the polarizability of molecule. If the inci-dent frequency and the polarizability of the molecule changebetween min and max of frequency int as a result of itsrotation and vibration, we can write [100]

t = + 12 cos intt E o cost (45)

with

= max min (46)Therefore, we obtain,

t = E o cos t + 12 E o

cos + int t +cos int t (47)The induced dipole moment has then components as fol-lows: unshifted frequency, , known as Rayleigh line; lowerfrequency, int, known as Stokes line; and higher fre-quency, + int, known as anti-Stokes line.Raman spectra can be used to determine the concen-tration of free ions in the electrolytes. For example, theRaman spectra of LiCF3SO3 have been tabulated as appearin Table 6 [101]. A composite containing SO

3, the Raman

spectrum of the S (SO3) spectral region of the triate anionof poly(ethylene oxide) dimetyl ether (400) complexed withLiCF3SO3, along with the three-component curves t areshown in Figure 15. It was explained that the compo-nent observed at 1032 cm1 corresponds to free anions notinteracting directly with lithium cations. The component of 1042 cm1 has been attributed to contact pair and the com-ponent of 1052 cm1 has been attributed to Li2CF3SO3 tripleions [102].

Because of the multicomponent nature of the spectrum,it can be concluded that the ion-ion interaction is present in

Table 6. LiCF3SO3 vibrational assignments.

Band Wavenumber (cm1) Assignment

s(SO3) 1033 free ion1043 monodenate ion pairs, LiX,

also LiX2 and LiX231053 Li2X+ aggregate1062 LiX23

as(SO3) 1272 free ion1257, 1302 monodenate ion pairs, LiX,

also LiX2 , LiX231270, 1308 Li2X+ aggregate

1288 Li3X2+ aggregate

I n t e n s i t y [ a

. u . ]

1042 cm 1

1052 cm 1

1060 1050 1040

Raman shift [cm 1 ]

1030 1020 1010

1032 cm 1

O:M = 110:1

Figure 15. Raman spectra of LiCF3SO3. Reprinted with permissionfrom [102], A. Ferry et al., Electrochim. Acta. 43, 1471 (1998). 1998Elsevier Science.

the system even down to the concentration of O:M=563:1.The relative amount of anions not interacting directlylithium ions, that is, spectroscopically free, increasesincreasing concentration from approximately 22% at 563:1 to 40% at O:M = 110 and then falls off slightly higher concentration. The 1042 cm1 band initially decreasesin relative intensity with increasing concentration andlevels off at 54% in the upper concentration range.

The fraction of free ion obtained from Raman spectra system of PPO:NaCF3SO3 is displayed in Figure 16 [10for a system of hydroxyl end capped PPO with NaCF3SO3.

7.3. FTIR Spectroscopy Atoms in solid vibrate at frequency approximately 10121013Hz. Vibration mode involving pairs of groups of boatoms can be excited to higher energy by absorption ofation at appropriate frequency. In the infrared (IR) techogy, the frequency at the incident radiation is varied an

200 250 300 3500.0

0.2

0.4

0.6

0.8

1.0

F r a c t i o n

o f a n

i o n s

Temperature [K]

(OH-PPO400) 16:NaCF 3SO3

(OH-PPO4000) 16:NaCF 3SO 3

Figure 16. Effect of temperature on the fraction of anions obtaifrom the Raman spectrometry. Data points were extracted from H. Ericson et al., Electrochim. Acta 43, 1401 (1998).


16/32


17/32


For the infrared spectra of PEG:LiClO4:AlBr3 1 mass%,the 4(ClO4 ) spectra can be separated into two contribu-tions with maxima at 623 and 633 cm1 [106]. The 623 cm1band is attributed to free anions and the 633 cm1 peak isrelated to bound or contact (ClO4 ) [107]. The fraction of free anions can be calculated as the fraction of area under623 cm1 mode to the total area of 4(ClO4 ) envelope.

Compared to composite in the absence of AlBr3, there is adramatic increase in the fraction of free anions when AlBr3is dispersed in the electrolytes.

7.4. X-ray Photoelectron SpectroscopyX-ray photoelectron spectroscopy (XPS) is a powerful tech-nique for studying the surface of solids. The data obtainedusing this technique are mainly used to extract the infor-mation regarding the bonding energies of various core-levelelectrons from different elements of solid materials. These values are then interpreted as the bonding between the ele-ment under consideration with their neighbor. Informationabout the local structure and interaction of an element withits neighbor can be extracted from the XPS data [108].In the development of battery, the interaction between theelectrolyte and the electrode determines the performanceof the battery. Understanding this interaction is importantto optimize the preparation parameters for realizing high-performance batteries. Therefore, surface studies, for exam-ple using XPS, may be of great signicance to understandthe interaction of polymer electrolytes with electrode.

In principle, this technique measures the kinetic energy of electrons that are emitted from matter as a consequence of bombardment with ionizing radiation or high-energy parti-cles. If the process results in an ionization of electrons fromthe bombarded material, the kinetic energy of the electron will satisfy

E k =h E b (50) where h =energy of incident radiation, and E b =bindingenergy of electron.

For a given atom, a range of E b values is possible corre-sponding to ionization of electrons from different inner andouter valence shells. Measurement of the value of E k , andtherefore E b, provides a means of identication of atoms.In the XPS method, the ionizing radiation is usually MgK (1254 eV) or AlK (1487 eV) monochromatic radiation.

Using XPS method, Vosshage and Chowdary investigatedthe interaction of salt with the polymer chain in the systemsof PEOnLiCF3SO3 and PEOnCu(CF3SO3 2 [108]. There isevidence of the interaction of the carbon from CH2-CH2-O- polymer chains with the cation of the salt through theether oxygen of PEO and anion of the salt. A complexationbetween the action of the salt and the oxygen of the PEOis identied using this method. It is also observed that forthe PEOnCu(CF3SO3 2 system, the change in the nature of the complex occurs at low salt concentrations. However, forPEOnLiCF3SO3 system, such a change is identied at highersalt concentration.

Liu et al. prepared polymer electrolyte nanocompositesusing SiO2 ller and 2-[methoxy(polyethylenoxy)-propyl]trimethoxy silane coated SiO2 [109]. The XPS method has

been utilized to conrm the success of coating. During ment with 2-[methoxy(polyethylenoxy)-propyl] trimsilane, the OH- groups of the surface of the SiO2 react withpartially hydrolized silane. For the untreated SiO2, the XPSdata can be tted by band centering at 284.6 eV, whioften characteristic of adventitious CH2 species in the mea-surement chamber. On the other hand, the coated SiO2 can

be tted with two bands, centered at 284.6 eV and 286.The former corresponds to the adventitious CH2 species while the latter corresponds to C-O-C group that appeaa result of the functionalization reaction.

7.5. X-ray DiffractionX-rays are electromagnetic radiation with a wavelearound 1 (1010 m). The X-rays which are used in aall diffraction experiments are produced by acceleratinelectron beam through 30000 V and permitting it to sa metal target, such as copper. The incident electron sufcient energy to ionize some of the copper 1s (K shell)electrons. An electron in an outer orbital (2p or 3p ) imme-diately drops down to occupy the vacant 1s level and theenergy released in the transition appears as X-radiation.transition energies have xed values and so create the tra of characteristic X-radiation. For copper, the 2p 1stransition, called K , has a wavelength of 1.5418 and 3p 1s transition, K , has a wavelength of 1.3922 .If the average crystalline size is below a critical ( 200 nm diameter), a broadening of diffraction Xbeam occurs [110]. The commonly accepted formula foticle size broadening is the Scheerer formula

d = 09B cos B

(51)

where d = crystalline radius, = X-ray wavelength, B =Bragg angle, B =the line-broadening, given by Warren fmula

B2 =B2M B2S (52) where BM =measured peak width in radiation at half peheight, BS = width of a peak of a standard material mix with the sample, whose particle size is considerably gthan 200 nm and which has a diffraction peak near theevant peak of the sample. For relatively large particle width of peak (unbroadened peak) is very small, soBM BS . Therefore, it is often possible to approximat B with BM .

Figure 19 shows the XRD spectra of polyethylene gbased polymer electrolyte nanocomposites containing nanoparticles [62]. Using 110 with a position at around2 B 567 , and the width of the peak of around 2 B1 25 , we obtain B 0 5 rad and B 0 022 rad. Thus thepredicted nanoparticle size is around d 7 2 nm.

The degree of crystallinity of polymer can be determalso by wide-angle X-ray scattering (WAXS). Figure 20trates the XRD spectra of polymer, where the relativsharp peaks are due to scattering from the crystalline regand the broad underlying hump is due to scattering fnoncrystalline region. The degree of crystallinity can bdicted based on the measurement of the area under sharp peak (A c) and the broad hump (A a) and using a


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50 60 700

40

80

120

2 []55 65

I n

t e n s

i t y

[ a u

]

Figure 19. XRD pattern of PEG containing Li ions and ZnO nano-particles. Data were extracted from [62], Mikrajuddin et al., J. Elec-trochem. Soc. 149, H107 (2002).

simple equation

xc

= Ac

A c +A a(53)

Shin et al. observed the effect of ller content on the XRDpattern on PEO in a system of PEO10LiCF3SO3 contain-ing TinO2n1. As observed in Figure 21, the PEO peakintensities decrease by the increase in the volume fractionof ller content [111]. It indicates that the crystallinity of the sample decreases by increasing the volume fraction of ller content. Similar results have also been reported byLeo et al. in a system of PEO:LiCF3SO3 containing llerof Li1 4(Al0 4Ge1 6)(PO4 3 [112]. With addition of ller, theintensity of the crystalline peaks has decreased and a notice-able broadening of the area under the peak was observed.It is a clear indication of the reduction of the crystallinity of

the polymer.

8. MICROSCOPIC ANALYSIS8.1. Scanning Electron MicroscopyThe morphology of the sample surface can be observedusing scanning electron microscopy (SEM). The surfacesmoothness and the presence of holes in the scale down to

2

I n

t e n s

i t y

[ a u

]

A a

A c

Figure 20. Typical form of XRD pattern of high molecular weight poly-mer.

10

(a)

(b)

(c)

(d)

20 302 (degree)

I n t e n s i t y ( A

. U . )

40 50

Figure 21. XRD patterns of PEO10LiCF3SO3 polymer electrolytes with(a) 0, (b) 5, (c) 10, and (d) 15 wt.% TinO2n1. (, crystalline of PEO).Reprinted with permission from [111], J. H. Shin et al., Mater. Sci. Eng. B 95, 148 (2002). 2002, Elsevier Ltd.

several tens of nanometers can be viewed using the advaneld emission SEM. Wen et al. compared the SEM pture of PEO:LiClO4 and PEO:LiClO4 containing alumina whisker. For PEO:LiClO

4, great amounts of microcracks

were observed on the surface, as observed in Figure [113]. The size of islands are several micrometers, copared to the PEO spherulite [114]. Addition of 10 wt

(a) (b)

(c)

Figure 22. SEM photograph of (a) PEO:LiClO4, (b) PEO:LiClO4 con-taining 10 wt.% whisker, and (c) PEO:LiClO4 containing 20 wt.% whisker. Reprinted with permission from [113], Z. Wen et al., SolidState Ionics 148, 185 (2002). 2002, Elsevier Ltd.


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whisker in the polymer electrolytes inhibited the formationof microcracks inside the composite polymer electrolytes(Fig. 22b). Further addition of the amount of the whisker,for example 20 wt.%, is more effective in avoiding the for-mation of microcracks (Fig. 22c).

Golodnitsky et al. observed the SEM picture of ller-freePEOn:LiI and PEOn:LiI containing alumina [115]. PEOn:LiI

containing high concentration salt is made up of units whosearea is hundreds of square microns. Addition of aluminacauses a minor reduction in the grain size. In a PVdF gelpolymer electrolyte containing CuO ller, Wang and Gufound that the surface presented a multitude of PVdF grainsand pores of average size of about 24 m in diameter [116].They assumed the CuO nanoparticles distributed uniformlyin the polymer matrix.

8.2. Transmission Electron MicroscopyIn the case of polymer electrolyte nanocomposites, transmis-sion electron microscopy (TEM) is usually used for deter-mining the dispersion of nanoparticles in the polymer matrix

and the size of nanoparticles. Capiglia et al. showed thatlarger ller (submicron size) is quite well dispersed in thepolymer matrix while the smaller ller (tens of nanometerssize) is not well distributed in the polymer matrix [61].Instead, the small ller is condensed in large blocks of sizeup to 1 m. Based on the TEM photograph, Mikrajuddinet al. showed that PEG based polymer electrolyte nano-composites made by in-situ growth of ZnO nanoparticles inthe polymer matrix and in-situ insertion of lithium ion dur-ing nanoparticles growth have particle size of about 6 nm[63]. This result is consistent with the calculation of the par-ticle size using size-dependent bandgap equation [117119].Chandra et al. also observed the size of nanoparticles syn-thesized in-situ in polymer electrolyte nanocomposites usingTEM and found that small content nanoparticles (about1 wt.%) have smaller size [54].

8.3. Atomic Force MicroscopyThere are not many reports on the atomic force microscopy(AFM) investigation of polymer electrolytes. Instead of investigating the polymer electrolytes themselves, Granvalet-Manchini et al. investigated the change in the surface of lithium electrode when making contact with polymer elec-trolytes [120]. After about three days contact with polymerelectrolytes, self-assembled polymer layer is developed onthe surface of lithium electrode.

8.4. Optical MicroscopyOptical microscopy characterization of polymer electrolytenanocomposites was not reported too much. To date, Kimet al. reported the optical micrograph of a system of PEO16:LiClO4 containing various ller content, taken under crossedpolarizers [121]. They observed well-dened spherulitic mor-phologies. The spherulites were observed in thin lmsdeposited on the glass substrate whose typical thickness wasabout 20 m. They proposed that the size and the morphol-ogy of the spherulites can be related to the melting point orglass temperature of the composites. This was based on the

fact that the spherulites have a lamellar structure for almall polymers and the increase in the lamella thickness rein the increase in the melting temperature [122].

9. THERMAL CHARACTERIZATIONS

9.1. ThermogravimetryThermogravimetry (TG) is a technique for measuringchange in the weight of a substance as a function of perature or time. The result usually appears as a continuchart record, as displayed in Figure 23. The sample, usa few milligrams in weight, is heated at a constant rateically 120 C/min. It has constant weight until it begindecompose at temperature T i. Decomposition usually takeplace over a range of temperature T i to T f and second con-stant plateau is then observed above T f , which correspondsto the weight of residue.

A TGA curve of composite made by PEO:LiBF4 contain-ing 2-[methoxy(polyethylenoxy)-propyl]trimethoxy coated SiO2 nanoparticles is displayed in Figure 24 [1 At temperatures below 180200 C, the sample experienceonly a small weight loss due to removal of residual on the surface of SiO2. At increasing temperatures, the OHgroups on the SiO2 surfaces begin to decompose to girise to slight increase in the weight loss. In addition tdecomposition of the OH groups, another weight loss place at about 350 C that can be attributed to the decompsition of silane molecules that are bonded on the surfacSiO2. Liu et al. suggested that the adsorption of contain2-[methoxy(polyethylenoxy)-propyl]trimethoxy silane surface of SiO2 is likely to give a sub-monolayer cover[109].

9.2. Differential Thermal AnalysisDifferential thermal analysis (DTA) is a technique in wthe temperature of a sample is compared with that of ireference material during a programmed change of temature. The temperature of sample and reference shouldthe same until some thermal event such as melting, decposition, or change in crystal structure occurs in the sam

W

Ti

W i

W f

Tf Temperature

W e i g h t

Figure 23. Typical form of TGA curve.


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0 100 200 300 400 500

Temperature [C]

96

97

98

99

100

W e i g h t p e r c e n t a g e [ % ]

OH-decomposition

silane decomposition

Figure 24. TGA curve of PEO:LiBF4 containing 2-[methoxy (polyethyl-enoxy)-propyl]trimethoxy silane coated SiO2 nanoparticles. The curve was replotted from [109], Y. Liu et al., J. Power Sources 109, 507 (2002).

If the sample temperature lags behind the reference temper-ature, the process is called endothermic. On the contrary,if the sample temperature leads the reference temperature,the process is known as exothermic. The sample size is usu-ally a few milligrams and heating and cooling rate is usually150 C/min. The difference in the sample and referencetemperature will appear as Figure 25.

If calorimetric data is required, it is better to use differen-tial scanning calorimetry (DSC). DSC is very similar to DTA. A sample and an inert reference are also used in DSC sys-tem but the cell is designed differently. The sample and thereference are maintained at the same temperature duringthe heating program and extra heat input to the sample (orto the reference if sample undergoes an exothermic change)is required to maintain this balance. Enthalpy changes aretherefore measured directly.

Examples of DSC curves for various PEO:LiCF3SO3 +clay composites are displayed in Figure 26 [123]. From thisgure, we can extract several parameters like the glass tem-perature, the melting point, and enthalpy. These values werefound to depend on the ller content, as summarized inTable 7. A pure PEO has one rst-order endothermic tran-sition at around 70 C, corresponding to the melting of the

Temperature

Heating

Cooling

ENDO

EXO

Polymorphicchange Melting

SolidificationPolymorphicchange

T

Figure 25. Typical form of DTA curve.

90 90 120 150 18060 60

Temperature (C)

(PEO) 8LiCF 3SO 3 /clay (wt %)

E n d o t h e r m a l

30

(a) undoped PEO

(b) 1/0

(c) 97/3

(d) 91/9

(e) 70/30

(f) 50/50

300

Figure 26. DSC curve of PEO:LiCF3SO3 containing clay. Reprinted with permission from [123], H.-W. Chen and F.-C. Chang, Polymer 42,9763 (2001). 2001, Elsevier Ltd.

PEO crystalline phase. When salt is added, a second miendothermic transition was observed at around 140150 C,due to the melting of crystalline complex phase formedPEO and LiCF3SO3 [124126]. The melting temperature othe crystalline PEO phase depends on the ller content. TT m initially shifted to higher temperature when the ller ctent increased, and reached the highest value at 9 wt.% cand then decreased with further increase in the clay cont

Table 7. The parameters of polymer electrolyte nanocomposiextracted from the DSC curve.

Samples T g (K) T m (K) H (J/g) X c T mc (K)

PEO 7050 1655PEO:LiCF3SO3 46 6984 599 362 1520PEO:LiCF3SO3+ 50 7158 689 416 1497clay 3 wt%PEO:LiCF3SO3+ 43 774 778 470 1569clay 9 wt%PEO:LiCF3SO3+ 46 5493 401 242 1426clay 30 wt%PEO:LiCF3SO3+ 48 4599 291 176 1324clay 50 wt%

Adapted with permission from [123], H.-W. Chen and F.-C. Chang, Polymer 42,9763 (2001). 2001, Elsevier.


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The PEO crystallinity (expressed by the area covered bytransition curve) is also dependent on the clay content. ThePEO crystallinity initially increases with increasing clay con-tent up to 9 wt.% and then decreases with further increasein the clay content.

Melt-crystallized polymers are never completely crystal-lized. This is because there are an enormous number of

chain entanglements in the melt and it is impossible for theorganization to form 100% crystalline polymer during crys-tallization. The degree of crystallinity is therefore of greattechnological importance. The degree of crystallinity can bededuced from the DTA data, in which the melting enthalpycan be obtained. The crystallinity can be calculated using asimple equation [127]:

X c = H mH m c f PEO (54)

where H m =melting enthalpy measured, H m c =meltingenthalpy of 100% crystalline (for PEO, H m c =1964 J/g),and f PEO =

weight fraction of PEO in polymer electrolytes.

10. DENSITY METHOD Another method for determining the crystallinity is basedon the knowledge of density of crystalline and amorphousphases as well as the density of the polymer specimen.The crystallization of polymer from melt is accompanied bythe reduction in the volume due to an increase in density.The crystals have a higher density than the molten or non-crystalline polymer since the last two contain also free vol-ume. Based on this difference, the density method can beutilized to determine the degree of crystallinity.

If V is volume of polymer specimen, and V a and V c are

volumes of amorphous and crystalline regions, respectively, we have V = V a +V c. The relation of polymer specimenmass and the amorphous and the crystalline region massesis given by

V = a V a + cV c (55) where = density of polymer specimen, a = density of amorphous region, and c =density of crystalline region.By dening the crystallinity as xc = cV c/ V , we obtain

xc = c a

c a (56)

The density of polymer specimen can be determined bysimply measuring the volume and weighing the mass. Thedensity of crystalline region can be calculated from theknowledge of the crystal structure. The density of amor-phous phase can be determined by measuring the densityof almost completely amorphous polymer, such as polymerobtained by rapid cooling from polymer melt.

Equation (56), however, is valid if polymer specimen con-tains no holes which are often present in molded samples.In addition, since packing of the molecules in amorphousregion is random, it is likely that the density of amorphousphase depends on the thermal treatment of the specimen.

11. ELECTRICAL PROPERTIES11.1. Effect of Filler Content on ConductivityThe role of the ceramic ller is to inuence the recrystation kinetics of the polymer matrix chains, thereby ultimpromoting localized amorphous regions and thus enhanthe transport of cations [9]. To produce a high fractioamorphous state in the composite, the as-prepared comite is rst heated above the melting point so that all pof the polymer are converted to the amorphous state. Ding the cooling process, the matrix part around partremains in the amorphous state, even when the tempture drops below the melting point. Therefore, a high trical conductivity would be expected to appear at amtemperatures. Indeed, enhancement in conductivity of uabout three orders of magnitude at low temperatures about one order of magnitude at high temperatures has breported for the system of poly(ethylene oxide)-LiClO4 con-taining ceramic llers [9]. In addition, composites coning ceramic llers in the nanoscale particle size exhibitexcellent mechanical stability (promoted by the netwothe llers into the polymer bulk) and high ionic conduc(promoted by the high surface area of the dispersed l

The volume fraction of ller particles affects the contivity of a composite. The symbols in Figure 27 displeffect of ller loading on the electrical conductivitypoly(ethylene oxide)/NaI containing ller of -Al2O3 [128].The conductivity increases with an increase in the vofraction of llers, reaches a maximum at a certain valuller particles, and then decreases toward zero for furincreases in the volume fraction of llers. This observ

0.010

9

108

107

106

105

104

0.1 0.2 0.3 0.4 0.5

Volume fraction of fillers

[

S / c m

]

Figure 27. Effect of ller volume fraction on the electrical contivity at 25 C for composites of (open circle) PEO:LiClO4+PAAMand (solid circle) PEO:NaI+ -Al2O3. Date points were extracted fromY. Liu et al., J. Power Sources 109, 507 (2002). Curves were obtainfrom theoretical calculation to t the data using (t / R =0 6) for ttingthe open circle data and (t / R = 1 16) for tting the solid circle datCurves were replotted from [128], J. Przyluski et al., Electrochim. Acta40, 2102 (1995).


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can be explained as follows. By increasing the fraction of llers, the total amount of amorphous state around the llersincreases since the surface area increases, thus increasingthe conductivity. If the ller content is so high, some of theller agglomerates (making contact) so that the surface areais reduced, thus reducing the fraction of amorphous statearound the ller, thereby reducing the conductivity. At a

specied amount of ller fraction, the continuous network of amorphous state disappears so that the transport of cationsis blocked. The conductivity of composites can be approxi-mated with the conductivity of ller particles.

The effective medium approximation was used to explainthe conductivity enhancement in polymer electrolyte com-posites [128]. Dispersed ceramics create an amorphous layeraround the particles, which have a high conductivity. Thecomposite is considered to be a two-phase system: parti-cles and amorphous layer as one phase and the rest of thepolymer as the other phase. The conductivity of the secondphase (polymer matrix) is equal to the conductivity of thepolymer electrolytes free of dispersed particles. The con-ductivity of the particleamorphous layer can be calculated

using MaxwellGarnett equation [129], c = 1

2 1 +2Y 2 12 1 + 2 Y 2 1

(57)

where 1 = conductivity of the interface layer, 2 =conductivity of dispersed grain, and Y = volume fraction of ller in the composite, according to equation [128]Y =

11+t/R 3

(58)

where t =thickness of the conducting layer, and R =radiusof ller.The system is analogous to a system containing conduct-

ing particles dispersed in a polymer matrix in which theconducting particles correspond to the particleamorphouslayer. The improved conductivity of grain and polymermedium is calculated by the Nakamura [130] or Nan andSmith [131, 132] equation,

ac =2 cV c

3V c(59a)

ea =2 e1V c2+V c

(59b)

where V c = V 2/Y , and V 2 = volume fraction of ller in abulk electrolyte.The dependence of composite conductivity on the load

fraction of dispersed particles and particle size can be cal-culated using the effective medium theory [133]:

V 2Y

ac m m +p c ac m +

1V 2Y

ae m m +p c ae m =

0(60)

where m is the conductivity of the composite and p c is thecontinuous percolation

Polymer Electrolyte Nanocomposites

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