Polymer Electrolyte Nanocomposites

Encyclopedia of Nanoscience and Nanotechnology

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Polymer Electrolyte Nanocomposites Mikrajuddin Abdullah1 , Wuled Lenggoro, Kikuo Okuyama Hiroshima University, Hiroshima, Japan

CONTENTS 1. Introduction 2. Conductivity Enhancement in Polymer Electrolytes 3. Development of Polymer Electrolyte Nanocomposites 4. Preparation Methods 5. Important Parameters 6. Charge Transport Characterizations 7. Spectroscopic Characterizations 8. Microscopic Analysis 9. Thermal Characterizations 10. Density Method 11. Electrical Properties 12. Mechanical Properties 13. Thermal Properties 14. Luminescent Composites 15. Conclusion Glossary References

1. INTRODUCTION Rechargeable cells are key components in mobile technologies, such as portable consumer electronics and electric vehicles [1]. A search for batteries that provide high energy density and multiple rechargeability has been a subject of considerable attentions. Even though battery technology developed one hundred years ago, progress and improvements in technology have been slow, particularly when compared to the growth of computer technology [2]. A Li-based battery provides a high density and flexibility of design. Today’s lithium battery has a high specific energy (>130 W h kg−1 ), 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.

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

high cell voltage (3.5 V), as well as a long cycle life (500–1000) charge/discharge. Worldwide production of such devices exceeded 200 million in 1997 and it will be approximately three times that number during 2001 [1]. Since lithium produces an explosion reaction with water-based electrolytes, a search for nonaqueous electrolytes is critically important to the production of the next-generation lithium battery, using electrolytes in an solid phase in an effort to develop more environmentally friendly materials. Polymer electrolytes are potential candidates for replacing the conventional aqueous electrolytes in lithium batteries. Polymers containing esters, ethers, or mixtures thereof which have the ability to dissolve salts are the base materials for polymer electrolytes. Polymer electrolytes are generally prepared by mixing high molecular weight polymers (HMWPs) with a salt solution. The polymer serves as solid solvent, thus permitting the salt to dissociate into anions and cations. Since the mass of a cation is much smaller than that of an anion, the electrical conductivity is dominated by cation transfer. Lithium salts are usually used for this purpose since they are the most electropositive of materials (−304 V relative to the standard hydrogen electrodes) as well as the lightest metal (atomic mass 6.94 g/mol, and density 0.53 g/cm3 ) and thus facilitate the design of storage systems with high energy density (Watt hour/kg) [1]. Table 1 shows a comparison of the electrochemical properties of several metals. Until presently, however, no polymer electrolyte-based lithium batteries are commercially available in the market. Therefore, worldwide research is being focused on the development of high power and high energy density polymer electrolytes with a major attention to safety, performance, and reliability. A battery contains two electrodes: positive and negative (both sources of chemical reactions), separated by an electrolyte that contains dissociated salts through which ion carriers flow. Once these electrodes are connected to external circuits, chemical reaction appears at both electrodes to result in a deliverance of electrons to the external circuits. The properties of a battery thus strongly depend on the electrolyte, anode, and cathode. With the use of polymer electrolytes in lithium batteries, high specific energy and specific power, safe operation, flexibility in packaging, and low cost in fabrication as well as low internal voltage drop at relatively large current withdraw is expected [3]. Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (731–762)

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Polymer Electrolyte Nanocomposites

Table 1. Electrochemical properties of several metals that have potential applications for use in batteries. Metal Li Na Mg Zn Cd Pd

Atomic weight [g/mol]

Valence charge

Specific charge [A h kg−1 ]

Electrode potential [V]

694 2299 2431 6538 11241 20720

1 1 2 2 2 2

3862 1166 2205 820 477 250

305 271 238 076 040 013

Realization of commercial polymer electrolyte batteries is actively investigated in many companies worldwide. A major effort to develop advanced polymer batteries for electric vehicles began in the early 1990s by 3M and HydroQuebec [4]. The battery contains a lithium metal anode, a polyethylene oxide (PEO)-based polymer electrolyte, and a vanadium oxide (VOx ) cathode. The reversibility of lithium intercalation and deintercalation in the VOx is quite good but the average discharge of the cell is low. PolyPlus Battery company in the United States is developing polymer electrolyte-based lithium battery which would operate at room temperature with specific energy as high as 500 W h kg−1 [3]. In a prototype cell, using cathode made of lithium intercalated disulfide polymer, a specific energy as high as 100 W h kg−1 and charge and discharge cycles almost reproducible for over 350 cycles were observed at 90  C [5]. Moltec company reported a specific density of 180 W h kg−1 for an AA-sized battery based on organosulfur cathode [6]. Ultrafine Battery company reported a room-temperature solid polymer battery based on intercalation type electrode with a specific energy 125 W h kg−1 and charge/discharge cycling time of 500 [3]. This performance is still below the consumer expectation threshold. In 1995, Turrentine and Kurani in the United States did a survey on demand for alternative fuel cell for vehicles and found that consumers agreed to buy electric vehicles which would run for at least 200 km per battery [7].

2. CONDUCTIVITY ENHANCEMENT IN POLYMER ELECTROLYTES It is believed that in the polymer electrolytes, the cations are coiled by polymer segment leaving the anions to occupy separate positions [8]. Battery performance is limited by the speed of cation diffusion. The transport of cations takes place if there is a relaxation of the polymer segments so that cations are released from a segment and then occupy another segment. Segmental relaxation requires the presence of free volume in the polymer matrix, a condition that can be attained if the polymer is in an amorphous state. Unfortunately, most HMWPs crystallize at ambient temperatures. Ions are transported with difficulty in a crystalline matrix since no chain relaxation occurs and, as a result, the conductivity of polymer electrolytes in this phase (at ambient temperature) is depressed. The transport of ions in this state is dominated by the jumping of cations to the nearest location, which depends on the blocking potential (activation energy). This is similar to the jump of charge carriers in

crystalline solids. The characteristic time for jumping is proportional to the exponential of the blocking potential. This results in a conductivity of the order of 10−8 S/cm, a value that is far below the desired value of about 10−4 S/cm [9]. When it enters the amorphous state, that is, at temperatures above the melting point, a high conductivity appears. For a commonly used polymer, that is, polyethylene oxide, the melting temperature is 65  C. This is, of course, impractical since the operating temperature for most electronic devices is room temperature. In addition, at temperatures above the melting point, the polymer becomes soft, causing the solidstate properties to degrade. Initiated by the work of Wright and Armand [10–12], several kinds of polymer electrolytes have been intensively investigated around the world. Table 2 displays examples of polymer electrolytes and their measured conductivities at 20  C [13]. Improvements in the electrical conductivity of polymer electrolytes at ambient temperature is therefore of critical importance for technological applications. Several approaches have been explored to realize this aim. Some frequently used methods will be explained briefly here.

2.1. Preparing Low Degree of Crystallinity Polymers By considering that the presence of amorphous state is strictly important for improving the conductivity, the main strategy is to enhance the amorphous state at low temperatures. The first approach is to prepare low degree of crystallinity polymer from initial. It includes cross-linking of two polymers [13, 14], synthesis of new polymer, crosslinking high molecular weight polymer through -irradiation [15, 16], addition of plasticizers in polymer electrolytes, addition of fillers, and bending of two polymers [17, 18]. Another strategy is to prepare an amorphous polymer so as to obtain a polymer that is composed of four to five monomeric units. For this system, the chains must be sufficiently long to effectively complex cations but too short to crystallize at low temperatures. Thus the matrix would still be in the amorphous state even at low temperatures. The polymer host serves as a solvent and does not include any organic liquids.

2.2. Addition of Side Chains An alternative way to decrease the crystallinity of polymer matrix is to introduce side chain to the polymer main chain. Theoretically, chain ends and branch can be thought of as impurities, which depress the melting point of the polymer. Simple mathematical formulation can be used to explain the melting point lowering by the presence of chain ends and branch. If Xi is the mole fraction of impurities (chain ends, side chains, and branch), then the melting point of polymer, Tm , decreases according to [19] 1 1 R − o = ln1 − Xi  Tm Tm Hu

(1)

where Tmo = melting point of polymer containing only polymer chain with infinite chain length, R = gas constant, and Hu = enthalpy of fusion per mole of repeat unit. Chung and Sohn showed that the XRD intensity of polymer

733

Polymer Electrolyte Nanocomposites Table 2. Examples of polymer electrolytes with their corresponding electrical conductivities at 20  C. Polymer host

10−8

— CH2O — n

POM:LiClO4

10−8

— (CH3)CH2CH2O — n

(PPO)8 LiClO4

10−8

— CH2CH2O — n

Poly(oxymethylene), POM Poly(propylene oxide), PPO Poly(oxymethyleneoligo-ethylene), POO Poly(dimethyl siloxane), DMS

Conductivity (S/cm) at 20  C

(PEO)8 :LiClO4

Poly(ethylene oxide), PEO

— (CH2O)(CH2CH2O) — n — (CH3)2SiO — n — HC=CH(CH2)4O(CH2CH2O)n(CH2)4 — x — CH2CHO — n

(POO)25 :LiCF3 SO3

3×10−5

DMS:LiClO4

10−4

UP:LiClO4 (EO:Li+ = 32  1) (PMEGE)8 :LiClO4

10−5 10−5

—

Unsaturated ethylene Oxide segmented, UP Poly[(2-methoxy)ethyl glycidyl ether], PMEGE

Example polymer electrolyte

Repeat unit

CH2(OCH2CH2)2OCH3 PMG22 :LiCF3 SO3

CH3

—

3×10−5

— CH2C — n — — — —

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

C

O

O — (CH2CH2O)xCH3 —

—

—

—

—

—

—

CH3 CH3

—

(PEO-PPO-PEO)-SC SC = siloxane crosslinked

PEO–(CH2)3–Si–O–Si–(CH2)3–PEO O

(PEO-PPO-PEO)-SC: LiClO4 (4:1 molar)

1–3×10−5

O

PEO–(CH2)3–Si–O–Si–(CH2)3–PEO CH3 CH3

PEO grafted polysiloxane, PGPS

PGPS:LiClO4

—

CH3

10−4

—

—SiO— n CH2CH2PEO

OCH2CH2OCH2CH2OCH3

—

Poly[bis-2-(2-methoxyethoxy) ethoxy]phosphazene,MEEP

—

— P=N — n

(MEEP)4 :LiBF4 (MEEP)4 :LiN(CF3 SO2 )4 (MEEP)4 :LiC(CF3 SO2 )4

2×10−5 5×10−5 10−4

OCH2CH2OCH2CH2OCH3

decreases with the increase in the length of chain of combshaped polymer [20]. Despite depressing the melting point of polymer, the presence of side chain also promotes the solvating of a salt as reported by Ikeda and co-workers [21, 22]. The side chain has shorter relaxation time compared to the main chain. The coupling of the side chain with the ion carrier, therefore, results in an increase in the conductivity. Watanabe et al. designed comb-shaped polyether host with short polyether side chain [23]. However, the mechanical properties decreased even as the conductivity increased. High conductivity with good mechanical properties was obtained by designing a polymer of high molecular weight with trioxyethylene side chain as reported also by Ikeda et al. [24]. With 18 mol.% of side chain, the conductivity was measured to be 1.5 × 10−4 S/cm at 40  C and raised to 1.4 × 10−3 S/cm at 80  C. Composite of polymer with room-temperature molten slat is also an interesting approach to improve the conductivity of polymer electrolytes. Watanabe et al. reported the composite

consisting of chloroaluminate molten salt that possesses a conductivity of 2 × 10−3 S/cm at 303 K [25, 26]. However, the disadvantage of chloroaluminate is its hygroscopic properties such that it is impractical in application. The use of non-chloroaluminate molten salt, therefore, is required to avoid the hygroscopic problem. Tsuda et al. reported a conductivity of 2.3 × 10−2 S/cm in composite of polymer and room-temperature molten fluorohydrogenates [27].

2.3. Addition of Plasticizers Another approach to improve the conductivity is by addition of additional material into the host polymer. This approach appears to be the simplest since a pre-produced polymer can be used to make the polymer electrolytes. Previously, low molecular weight polymers were usually used to reduce the operation temperature of polymer electrolytes. The low molecular weight polymers which were added to the matrix of HMWP to reduce the crystallinity at low temperatures are frequently known as liquid plasticizers.

734

Polymer Electrolyte Nanocomposites

1 W W = 1 + 2 Tg Tg1 Tg2

(2)

10–3

10–4

10

–5

0

25

50

220

75

100

wt.% tetraglyme

Figure 2. Effect of plasticizer weight fraction on the conductivity at 25  C of a PEO-co-PPO (3:1):LiClO4 using plasticizer tetraglyme (tetraethylene glycol dimethyl ether). Data points were extracted from [29], D. R. MacFarlene et al., Electrochim. Acta 40, 2131 (1995).

particularly when the fraction of plasticizers is too high. For example, the modulus of elasticity and elastic strength significantly decreases by addition of plasticizers. This is because the plasticizers are usually low molecular weight polymer having low mechanical strength. Therefore, addition of plasticizers decreases the mechanical strength of the host polymer. Figure 3 shows the effect of plasticizer troglyme content on the elastic modulus and tensile strength of PEO-co-PPO (3:1):LiClO4 [29]. The use of moderate or large quantities of plasticizer results in the production of

2.0

Elastic modulus

Modulus [Mpa]

where W1 and W2 denote the weight fractions of component 1 and component 2, respectively, and Tg1 and Tg2 are their corresponding glass transitions. This equation tells that the glass temperature of the composite locates between the glass temperature of the components. This relation is also applicable for copolymer where Tg1 and Tg2 denote the glass temperature of polymers forming the copolymer. Reduction in the glass temperature means the enhancement in the amorphous state at low temperature, and therefore improves the conductivity at low temperatures. Figure 2 shows the enhancement of conductivity by the addition of plasticizer tetraglyme on the system of PEO-co-PPO (3:1):LiClO4 , measured at 25  C [29]. The decrease in the glass transition results in the improvement in the fraction of amorphous state at room temperature, therefore improving the conductivity. However, an improvement in conductivity is adversely accompanied by a degradation in solid-state configuration and a loss of compatibility with the lithium electrode [9],

10–2

σ [S/cm]

Feullade and Perche demonstrated the idea of plasticizing the polymer with an aprotic solution containing alkali metal salt in which the organic solution of the alkali metal salt remained trapped within the matrix of solid polymer matrix [28]. Such mixing results in formation of gels with ionic conductivity close to the liquid electrolytes. Less evaporating solvents such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl formamide (DMF), diethyl phthalate (DEP), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), dimethyl carbonate (DMC), -butyrolactone (BL), glycol sulfide (GS), and alkyl phthalates have been commonly investigated as plasticizers for the gel electrolytes. Figure 1 shows the effect of plasticizer content tetraglyme (tetraethylene glycol dimethyl ether) on the glass temperature of a system of PEO-co-PPO (3:1):LiClO4 [29]. The decrease in the glass temperature can be simply explained using a Fox equation:

Tg[K]

1.0

200

Tensile Strength

180

0

25

50

75

100

wt.% tetraglyme

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

0.0

0

20

40

60

wt.% tetraglyme

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

735

Polymer Electrolyte Nanocomposites

gel electrolyte. The presence of some plasticizer may also give rise to problems caused by its reaction with the lithium anode. The poor mechanical stability was accounted to be mainly due to the solubility of the polymer matrix in the plasticizer [30]. Cross-linking of the polymer with ultraviolet radiation [31], thermally [32], by photopolymerization [33], or electron beam radiation polymerization [34] was found to reduce the solubility of polymer in the solvent and also helped to trap liquid electrolytes within the polymer matrix.

3. DEVELOPMENT OF POLYMER ELECTROLYTE NANOCOMPOSITES Currently, one popular approach to improve the conductivity involves dispersing ceramic fillers (solid plasticizers) in the polymer matrix, producing what is currently known as composite polymer electrolytes. This approach was first introduced by Weston and Steele [35]. Ceramic filler was used to reduce the glass transition temperature and crystallinity of the polymer and thus allow the amorphous polymer to maintain the liquid-like characteristic at the microscopic level. Ceramic fillers that are frequently used have particle sizes in the range of about several ten nanometers up to several micrometers. Fortunately, such filler materials are commercially available in various sizes at low prices. Figure 4 shows the effect of filler content on the conductivity of polymer electrolytes PEO:LiClO4 [9]. Table 3 displays examples of polymer electrolyte nanocomposites and their conductivities at around room temperature. The inorganic filler also acts as a support matrix for the polymer, so that even at high temperature, the composite remains solid. However, at the microscopic level, it maintains a liquid-like structure, which is important for sufficient conductivity. The filler particles, due to high surface area, prevent the recrystallization of polymer when annealed above the melting point. The acid-base interaction between the filler surface group and the oxygen of the PEO leads to a Lewis acid characteristic of the inorganic filler and favors –2

Log σ [S/cm]

–4 VTF

–6

Arrhenius

–8

2.4

2.6

2.8

3.0

3.2

3.4

3.6

1000/T [1/K]

Figure 4. Arrhenius plot of electrical conductivities of: (solid) ceramicfree PEO:LiClO4 , (triangle) PEO:LiClO4 containing 10 wt.% Al2 O3 (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 their corresponding electrical conductivities at around room temperature.

Polymer electrolytes PEO:LiBF4

Fillers

nano-sized-Al2 O3 micro-sized-Al2 O3 PEO:LiCIO4 SiC EO-co-PO:LiCF3 SO3 Li13 Al03 Ti17 (PO4)3 Brached-poly(ethylene silica (12 nm size) imine):H3 PO4 PEO:LiClO4 -Al2 O3 PEO:LiClO4 AlCl3 NNPAAM PEO:LiClO4 PEO-PEG:LiI Al2 O3 PEO-PMMA:EC:LiI Al2 O3 PEG:LiCF3 O4 SiO2 C12 H25 OSO3 Li PEG:LiCF3 O4 coated-SiO2 EC:PC:PAN:LiAsF6 porous zeolite PEO:LiClO4 AlN, BaTiO3 , Bi2 O3 B4 C, BN, CaSiO3 CeO2 , Fe2 O3 , MoS2 , PbTiO3 , Si3 N4 , carbon black PEO:LiClO4 PEO:Li[(SO2 CF3 )2 N] -LiAlO2 PEO:PMMA:EC:LiI MgO PEO:AgSCN Al2 O3 PEO:AgSCN Fe2 O3 PEO:AgSCN SO2 Na2 SiO3 PEO:NaClO4 PEO:LiCF3 SO3 mineral clay PbS PEO:NH4 I CdS PEO:NH4 I Pbx Cd1−x S PEO:NH4 I

Conductivity at around rt (S/cm)

Ref.

∼10−4 ∼10−5 ∼10−7 ∼2×10−4 (40  C) ∼10−7

[36] [37] [38] [39]

∼10−5 ∼10−5 >∼10−5 ∼10−6 ∼10−8 ∼10−5 ∼5 × 10−5

[40] [40] [40] [41] [41] [42] [43]

∼10−3 ∼10−7 –10−6

[44]

∼4 × 10−6 ∼10−7 ∼88 × 10−4 ∼11 × 10−5 ∼3 × 10−6 ∼2 × 10−6 ∼10−3 ∼099 × 10−6 ∼096 × 10−6 ∼063–084 × 10−6

[46] [47] [48] [49] [50] [51] [52] [53] [54] [54] [54]

the formation of complexes with PEO. The filler then acts as cross-linking center for the PEO, reducing the tension of the polymer for self-organization and promoting stiffness. On the other hand, the acid-base interaction between the polar surface group of the filler and electrolyte ions probably favors the dissolution of the salt. Another potential application of polymer electrolyte nanocomposites is for making solar cells [55]. Dye-sensitized solar cells have attracted great scientific and technological interest as potential alternatives to classical photovoltaic devices. The cell operation mechanism involves absorption of visible light by the chemisorbed dye, followed by the electron injection from the excited synthesizer into the semiconductor conduction band. The selection of liquid electrolytes, usually containing organic solvent such as acetonitrile and propylene carbonate, assures the perfect regeneration of the dye by direct interaction of the dye oxidized state and I− /I− 3 redox couple and leads to impressively high solar-to-electrical conversion efficiencies (7–11%) [56, 57]. However, the stability and long-term operation of the cell are affected by solvent evaporation or leakage. Thus commercial exploitation of these devices needs the replacement of the liquid electrolyte by a solid charge transport medium, which not only offers hermetic sealing and stability but also reduces design restriction and endows the cell

736 with shape choices and flexibility. Katsaros et al. investigated solid-state dye-sensitized solar cells using composite polymer electrolytes using PEO and TiO2 in the presence of I− /I− 3 redox couple [55]. Initially, dye:Ru(dcbpy)2 (NCS)2 (dcbpy is 4,4’-dicarboxylic acid-2-2 -bipyridine) was attached on the surface of TiO2 nanoparticles by immersion of the TiO2 thin-film electrode overnight in ethanolic solution of the complex, followed by drying. The functionalized TiO2 nanoparticles, I− /I− 3 , and PEO were put in acetonitrile, followed by heating and drying to evaporate the solvent. Maximum incident photon to current efficiencies as high as 40% were obtained at 520 nm, only two times lower that than obtained using liquid electrolytes [58]. The overall conversion efficiency was 0.96%. For all-solid-state devices, such efficiency can be considered to be sufficiently high [59].

4. PREPARATION METHODS Now we will briefly explain several methods of preparation of polymer electrolyte nanocomposites that are commonly used. Which method should be used, of course, depends on the materials and the form of sample to be formed. One method can only produce sample in the form of thick film, and another one can produce a sample in the form of film of submicrometer thickness.

4.1. Casting Method This method is frequently used due to its simplicity. It can produce polymer film from several micrometers up to several millimeters thickness. Generally, this method includes the following steps: (a) dispersion of ceramic fillers in a salt solution, (b) addition of a specified amount of polymer to the mixture, (c) mixing by means of stirrer or ultrasonic equipment to disperse the particles homogeneously in the polymer matrix, (d) casting the mixture on a substrate, (e) finally drying in vacuum or in an atmosphere of argon. All these steps are usually performed in a glove box filled with argon gas and excluding oxygen and water to levels below 20 parts per million (ppm), to avoid the possible occurrence of a “dangerous reaction” between water and lithium. The solvent must be water-free and should be common solvent for both the salt and the polymer. Since the melting point of several polymers is as high as 65  C, the solvent must also easily evaporate so that drying can be performed at temperatures of around 65  C. Organic solvents such as acetonitrile, cyclopentanone, and propylene carbonate, plus inorganic solvents such as thionyl chloride (SOCl2 ), are typically used. Sometimes, the insertion of salt is performed after casting the film. For example, Ardel et al. prepared PVDF 2801 (Kynar)-based polymer electrolyte composites according to the following steps [60]. First, Kynar was dissolved into cyclopentanone. Nanoparticles of silica were added and the mixture was mixed for 24 h at room temperature to get homogeneous slurry. After complete dissolution, the slurry was cast on the Teflon support and spread with the use of

Polymer Electrolyte Nanocomposites

doctor blade technique. To prevent surface irregularities, the film was then covered with a box pierced with holes that allowed a slow evaporation of the cyclopentanone. After complete evaporation of the cyclopentanone, the polymer membrane was soaked in a lithium ion solution for 48 h. Several fresh lithium solutions for each soaking can be used to ensure a complete impregnation of lithium ion into the membrane.

4.2. Spin Coating The spin-coating method is very similar to the casting method. Instead of casting the film on a substrate, in this method, the mixture is dropped on a substrate and placed in a spin coater that can be rotated at adjustable rotation speed. The film thickness can be controlled easily by adjusting the viscosity (concentration) of the mixture and the speed of rotation. However, this method is only available if the viscosity of the mixture is not too high. For a gel mixture, the spin coater rotation is not enough to spread the mixture droplet to form thin film.

4.3. Hot Press Hot press technique equipment is illustrated in Figure 5. The equipment consists of: (A) weighing cylinder, (B) heating chamber, (C) basement, and (T) temperature controller. Proper amounts of polymer, salt, and filler are mixed in a mortar for about several minutes. The powder mixture is then sandwiched between two sheets of Mylar or other materials, and positioned inside the heating chamber that is controlled at temperatures lightly above the melting point of the polymer. If PEO is used as polymer matrix, temperature of 80  C is suitable [61]. The sample is then pressed overnight with a pressure that can be controlled by weighing cylinder. After heating and pressing, the sample is then slowly cooled to room temperature. The sample is then separated from the Mylar sheet and placed in a glove box.

A

B

C

T

Sample

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

737

Polymer Electrolyte Nanocomposites

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 with lithium ion by in-situ production of ZnO nanoparticles in the polymer matrix [62, 63]. The preparation methods will be briefly described as follows. Zinc acetate dihydrate, (CH3 COO)2 Zn · 2H2 O 0.1 M in 100 mL ethanol 99.5, was heated with stirring in distillation equipment at temperature of 80  C to produce about 60 mL condensate and 40 mL of hygroscopic solution. Lithium hydroxide monohydrate, LiOH · H2 O, of various concentrations was suspended in 40 mL ethanol and stirred until all the granular material dissolved. Polyethylene glycol (PEG) (Mn = 2,000,000) was suspended into each LiOH solution and then stirred with heating at around 60  C until homogeneous gel-type mixtures were obtained. The mixture temperatures were then left to go down several minutes, after which 10 mL of hygroscopic CH3 COO2 Zn · 2H2 O solution was added into each mixture. The new mixtures were then homogeneously mixed and then dried in an oven that was kept at temperature of 40  C 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) Ion carriers are inserted in-situ in the polymer matrix during the growing process. (d) Finally, since the grown nanoparticles are luminescent, we obtain a new class of polymer electrolytes, namely luminescent polymer electrolytes with nanoparticles as luminescence centers. Based on the TEM picture, we found that the size of ZnO nanoparticles was 5 nm.

Ethanol

ZnAc2•2H2O

LiOH•H2O

Distilled at 80°C

Mixed around 10 min

Ethanol

Condensate (60%) Mixed at 60°C unused

PEG

Cooled at 0°C

Left cooling

Hygroscopic solution (40%) Mixed several minutes

Dried at 40°C, 3 days

ZnO nanoparticles are formed in the polymer matrix

Characterizations

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

Chandra et al. produced polymer electrolytes PEO:NH4 I containing nanometer-sized semiconductor particles PbS, CdS, Pbx Cd1−x S [54]. Methanolic solution of PEO and NH4 I was first stirred roughly at 40  C for 8–10 h, which resulted in viscous solution of the ion conducting complexes of PEO/NH4 I. To this solution, a solution Pb(CH3 COO)2 , Cd(CH3 COO)2 , or Pd(CH3 COO)2 + Cd(CH3 COO)2 in a desired fraction was added. The stirring was continued until the viscosity was back to the value it was before adding the acetate compounds. Subsequently, H2 S was bubbled through it giving PbS, CdS, or Pbx Cd1−x S. The final viscous solution was poured in a petri dish for obtaining solution-cast film. Then the film was dried in vacuum.

5. IMPORTANT PARAMETERS To bring polymer electrolytes as well as polymer electrolyte composites, these materials should provide enough values of several properties as follows.

5.1. Electrical Conductivity Conductivity defines the density of current that can be transported in the material by applying a certain electric field. If electric field E is applied in the material, the current density will be proportional to the applied electric field, where the proportional constant is the conductivity, or, J = E

(3)

with J the current density (A/m2 ) and  (S/m or S/cm) the electrical conductivity. It is clear that high conductivity material will produce high current density upon applying a certain electric field. The value of conductivity is determined by the density of mobile ions (ion carriers) in the material (n), the scattering time of the ion (), the ion charge (q), as well as the mass of ion carrier (m), according to a relation =

nq 2  m

(4)

This equation gives the reason why most polymer electrolytes use lithium ions as ion carriers. The mass of lithium is the smallest among all metals, so it produces the highest conductivity. For industrial application, the conductivity of polymer electrolytes must be as high as 10−2 S/cm. However, until presently, this conductivity can only be achieved at high temperatures in which the polymer is present in the soft phase, or even liquid phase. The conductivity at room temperature of most reported polymers is still below 10−4 S/cm.

5.2. Transference Number Since the electrochemical process in lithium batteries involves the intercalation and de-intercalation of lithium cations throughout host compound lattice, solid polymer electrolytes with cation transference number (t + ) approaching unity are desirable for avoiding a concentration gradient during repeated charge-discharge cycles. The reported t + value for dried polymer electrolytes range from 0.06 to 0.2 [64]. For a gel polymer system, t + value of 0.4–0.5 has

738

Polymer Electrolyte Nanocomposites

been found for poly(bis-methoxy ethoxy)phosphazene [65], and 0.56 in a system of UV-cured gel polymer electrolytes based on polyethylene glycol diacrylate/polyvinylidene fluoride [66]. Transference number of a particle is defined as the ratio of 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)

te =

e 

(7)

and

For pure ionic, ti = 1, and for pure electronic, te = 1. For polymer electrolyte composites, a general condition satisfied is 0 < ti  te < 1.

5.3. Crystallinity Crystallinity plays an important role in determining the conductivity of polymer electrolytes. At crystalline phase, the transport of ion carriers is very difficult so that the conductivity is very low. At amorphous phase, there is a segmental motion of polymer chain that also assists the displacement of ions. As a result, the transport of ions is relatively easy. Thus, high conductivity will result. One major route to improve the conductivity of polymer electrolytes is by increasing the fraction of amorphous states. Addition of ceramic fillers, addition of plasticizer, and production of branch polymer are efforts to improve the amorphous state in the polymer.

5.4. Mechanical Strength One objective of the use of polymer electrolyte is to make a battery or fuel cell with a strength comparable to that of liquid electrolytes. Therefore, it is expected that the improvement of conductivity is not accompanied by a decrease in the mechanical strength. It is why the addition of ceramic filler has received more attention, since the conductivity and the mechanical strength can be improved simultaneously. In contrast, the use of liquid plasticizer, although it can enhance the conductivity much higher than the addition of ceramic filler, involves such a degradation in the mechanical strength as to make this approach less interesting.

6. CHARGE TRANSPORT CHARACTERIZATIONS Electrical conductivity is the critical parameter for polymer electrolyte composites. One target of the present research in this field is to produce polymer electrolyte nanocomposites that exhibit a high electrical conductivity, especially at room temperature. Conductivity at around 10−2 –10−3 S/cm is required to bring this material into industry. The electrical conductivity relates to the value of current that can be produced by the battery. The potential produced by the battery depends on the reaction of the battery with the electrode. Even though the electrode reaction can produce high electrical potential, the use of low conductive electrolytes can produce only small amount of electric current. And since the power can be calculated simply by the relation Power = Voltage × Current, the use of low conductive materials will produce a low specific energy battery.

6.1. d.c. Conductivity Ideally, the d.c. conductivity should be measured in order to be sure that the values pertain to long-range ion movement instead of dielectric losses such as would be associated with limited or localized rattling of ions within cages. However, the difficulty in making a d.c. measurement is in finding an electrode material that is compatible with the electrolyte composites. For example, if stainless steel electrodes are attached to an electrolyte composite, as displayed in Figure 7a, and small voltage is applied across the electrodes, Li+ ions migrate preferentially toward the cathode, but pile up without being discharged at the stainless/electrolyte interface. A Li+ ion deficient layer forms at the electrolyte/stainless steel interface. The cell therefore behaves like a capacitor. There is an accumulation of ions at interface region of electrode and composite. A large instantaneous current Io presents when the cell is switched on, whose magnitude is related to the applied voltage and the resistance of the electrolytes but then falls exponentially with time, as illustrated in Figure 7b. The characteristic time of current decreasing is relatively fast so that it is difficult to make an accurate measurement. Therefore, the a.c. method is commonly used in the present to make measurement over a wide range of frequencies. The d.c. value can be extracted from the a.c. data. Many a.c. measurements are performed with blocking I

(a) +

(b)

Io

CPE

5.5. Storage Time Battery or fuel cell made from polymer electrolytes should have to operate several weeks or several months. Thus, the properties of polymer electrolytes should not change too much during this time. For example, the conductivity should not depend so much on the storage time. Ideally, the properties should be time independent. However, in reality, the properties tend to degrade with storage time.

+

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

739

Polymer Electrolyte Nanocomposites

Z  = Z    − iZ   

(8)

where is the frequency, Z    is the real part of impedance, contributed by resistive part, Z    is the imaginary√part of impedance, contributed by capacitive part, and i = −1, the imaginary number. As an illustration, Figure 8 shows examples of simple RC circuits and the corresponding plot of impedance (Nyquist plot). For a serial arrangement of a resistor and a capacitor, as displayed in Figure 8b left, the impedance can be written as i Z =R− C

(9)

or Z = R Z  =

1 C

(10a) (10b)

It is clear that the real part of impedance is constant, independent of the frequency, while the imaginary part depends on the frequency. For very small frequency, the imaginary part is very large and this value decreases inversely with frequency. For frequency approaches to infinity, the imaginary part of impedance closes to zero and the impedance

(a)

Z′′ [Ω]

R

Z′ [Ω]

R

C

R •

Z′′ [Ω]

(b)

R Z′ [Ω] R Z′′ [Ω]

(c)

C

(d)

ωmCR = 1 •

•

Z′ [Ω]

R

R C2

Z′′ [Ω]

electrode such that no discharge or reaction occurs at the electrode/electrolyte interface. Because the current will flow back and forth, no ions pile up on electrode surface, especially when using a high a.c. frequency. This is why the a.c. resistance (impedance) tends to decrease with increase in the frequency. The electrodes that are commonly used are platinum, stainless steel, gold, and indium tin oxide (ITO) glass. The complex impedance method is widely used to determine the resistance of the sample. The principle of the method is based on measurements of cell impedance, which are taken over a wide range of frequency and then analyzed in the complex impedance plane which is useful for determining the appropriate equivalent circuits for a system and for 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 imaginary part. An electrochemical cell, in general, exhibits resistive, capacitive, as well as inductive properties. The resistive property contributes to the real part of the impedance, while the capacitive and the inductive properties contribute to the imaginary part of the impedance. Therefore, an electrochemical cell can be considered as a network of resistor, capacitor, as well as conductor. Which arrangement for which cell is usually determined after performing a measurement, by analyzing the form of impedance curve. A capacitor that presents as an open circuit in a d.c. network and an inductor that appears as a straight conductor wire in a d.c. circuit, both appear as imaginary resistors in the a.c. circuit. Until presently, the inductive properties of the electrochemical cell are ignored so that the polymer electrolyte composite is considered only as a network of resistor and capacitor. The complex impedance can be written in a general form as

ωmC1R = 1 • R

C1

•

Z′ [Ω]

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

value at this very high frequency equals to resistance. Thus the Nyquist plot for this arrangement appears as a vertical straight line, starting from a lower frequency value at the upper part downwards when the frequency increases, as shown in Figure 8b right. The intersection of this line with horizontal axis (the real value of impedance) corresponds to the resistance. For a parallel arrangement of resistor R and capacitance C, as appears in Figure 8c left, the real and imaginary parts of the impedance are given by Z =

R 1 +  RC2

(11a)

and Z  = R

RC 1 +  RC2

(11b)

and the corresponding Nyquist plot appears in Figure 8c right. The Nyquist plot appears as an arc. The intersection of this arc with the vertical axis at a low frequency (right arc) corresponds to the resistance. The frequency at the peak of the arc, m , satisfies the relation m RC

=1

(12)

From the intersection point at the low frequency region and the position of the arc peak, the resistance and the capacitance of the system can be determined.

740

Polymer Electrolyte Nanocomposites

More complex arrangements, have a more complex expression for the impedance. For example, a combination of serial and parallel circuit as appears in Figure 8d left has the impedance as −1
1 1 + (13) + i C1 Z= R i C2 with the corresponding Nyquist plot appearing in Figure 8d right. It contains a vertical line that intersects the horizontal axis at Z  = R, and an arc with the peak satisfies m RC1 = 1. Again, from these two values, one can determine R and C1 . The value of C2 is determined by measuring the vertical component of impedance at a certain frequency, say ∗ . If  the vertical component of impedance at this point is Z ∗ , the value of C2 satisfies Z

 ∗

=

1 ∗C 2

(14)

Sometimes, the form of curve is not as simple as that described here. However, in principle, we can find some circuit arrangement such that the theoretical Nyquist plot is in agreement with the measured data. Some computer software is commercially available for extracting the equivalent circuit for measured data. As an example, the impedance measurement of a system of PEO:LiCF3 SO3 containing Li14 Al04 Ge16 (PO4 3 fillers is displayed in Figure 9a [67]. The corresponding a.c. circuit that can produce this impedance data appears in Figure 9b, with Rb = bulk resistance, Rpb = phase boundary resistance, Rint = interfacial resistance, Cpb = phase boundary capacity, Cint = interfacial capacity, Zd = diffusive impedance. The corresponding parameter values that can properly fit the measured data are

Z′′ [kΩ]

=

1 $ Re A

(15)

where Re = resistance of polymer electrolyte, $ = material thickness (electrode spacing), and A = material cross section. The common procedure for measuring the temperature dependence of conductivity is as follows. (a) Heat the sample at a required temperature. Sometimes, it needs a half hour or more to equilibrate the sample temperature. (b) Measure the impedance at all range of frequency. Sometimes, it can take from tens of mHz up to several MHz. The computerized measurement is usually performed since a great number of data should be collected for each setting temperature. (c) Change the setting temperature, and again collect the impedance data in all frequency regions. (d) Analyze the collected data and find the equivalent circuit. (e) Determine the resistance of the electrolytes based on the impedance plot and the equivalent circuit at each setting temperature. (f) Calculate the conductivity at each setting temperature.

6.2. a.c. Conductivity

0.6

Despite the d.c. conductivity, the a.c. conductivity sometimes gives important information such as the dielectric properties of the composites. The frequency dependence of a.c. conductivity in polymer electrolytes can be written as [38]

0.4

ac   = dc + A

0.2

0

Rb = 593 ", Rpb = 1637 ", Cpb = 31 nF, and Cint = 19 #F [67]. From the measured resistance of polymer electrolytes, the electrical conductivity can be calculated using a simple equation

0

0.5

1.0

1.5

2.0

2.5

3.0

Z′ [kΩ] Rpb

Rint

Rb

Zd

Cpb

(16)

where dc = d.c. conductivity, A and n are the material parameters, 0 < n < 1, and is an angular frequency. The curve might consist of three regions, a spike at low frequency, followed by a plateau at medium frequency, and another spike at high frequency. The high frequency part corresponds to bulk relaxation phenomena, while the plateau region is connected to d.c. part of conductivity. The lower spike is connected to electrode/electrolyte phenomena. Fitting the curve with Eq. (16), one can determine the parameters A and n, and from those parameters, the hopping frequency [68],

Cint

Figure 9. (Top) Impedance plot PEO:LiCF3 SO3 containing Li14 Al04 Ge16 (PO4 3 obtained from experiment. Data points were extracted from [67], C. J. Leo et al., Solid State Ionics 148, 159 (2002). (Bottom) The suggested RC circuit for data in (a). See text for the explanation of symbols.

n

p

=

  1/n dc

A

(17)

By fitting the experimental data of ac − , one can determine the dc and p at each temperature. Using this approach, Siekierski et al. [69] found in a system of

741

Polymer Electrolyte Nanocomposites p

–7.2

(18)

where C is either dc or p , and Co is the corresponding prefactor. Furthermore, the temperature-dependent dielectric constant can also be obtained from the ac data. The real part of the dielectric constant, , , can be expressed as   , = ac ,o

  , s 2 ds P 2 2 0 s −

2   s, s ds P 2 2 0 s −

(20)

(21)

6.3. Diffusion Coefficient Electrical conductivity can also be determined by measuring the diffusion coefficient. From the temperature-dependent diffusion coefficient, the temperature dependence of electrical conductivity can be determined using Nerst–Einstein equation =

ne2 D kT

- 2 Ds t + A1 L2

–7.8

0.0

0.5

1.0

(23)

where 1 = measured cell potential, Ds = salt diffusion coefficient, L = electrolyte thickness, t = time, and A1 = a constant. It appears that Ds is proportional to the slope of curve ln 1 with respect to t. The dependence of salt diffusion constant on the salt concentration is displayed in Figure 10 [71] for system of PEO:NaCF3 SO3 at 83  C. The Ds decreases

1.5

2.0

2.5

Salt concentration [mol/L]

Figure 10. Effect of salt concentration on the diffusion coefficient for PEO:NaCF3 SO3 system at 83  C. Data points were extracted from [71], Y. Ma et al., J. Electrochem. Soc. 142, 1859 (1995).

as the salt concentration increases from about 8 ×10−8 cm s for dilute solution. Diffusion coefficient can also be determined from the Nyquist plot as discussed by Strauss et al. [72]. The medium frequency arc is attributed to the solid/electrolyte interface. At lower frequencies, the impedance is affected by concentration gradient (diffusion) and ionic aggregates. The diffusion impedance of symmetric electrolyte with no-blocking electrode, such as Li/CPE/Li, can be written as D=

RT L n2 F 2 Cb ZDC

(24)

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

(22)

where n = charge carrier concentration e = electron charge, and D = diffusion coefficient. Salt diffusion coefficient can be obtained by galvanostatically polarizing a symmetric cell containing no-blocking electrode for a short period of time. For example, assume a cell containing Li-based polymer electrolytes and lithium electrodes at both sides. When the current is turned off, the induced concentration profile is allowed to relax. At long time after the current interrupt, the following equation is applicable [71]: ln 1 =

–7.6

–8.0

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

–7.4

(19)

 is the imaginary part of the a.c. conducttivity, where ac and ,o is the permitivity of vacuum. The complex dielectric constant can be written as ,  = ,   + i,  , and the imaginary part can be obtained from the real part using a Kramer–Kronig relation

,   = −

–7.0

satisfy the

Log Ds [cm2/s]

PEO3 :LiClO4 + -Al2 O3 , that both dc and Arrhenius expression   E C = Co exp − kT

D=

mL

2

254

(25)

where m = the frequency at the maxima of low frequency arc, and L = electrolyte thickness. The values predicted by Eq. (25), however, are around 6–10 times as large as that predicted by Eq. (14). The error can be contributed by the shift of m due to the formation of ion pairs [72].

6.4. Transference Number Transference number can be calculated by analyzing the arc impedance spectrum of symmetrical cell with no blocking electrode. The transference number can be calculated by comparing the width of the skew low frequency semicircle, Zd , with the value of the bulk resistance, that is [74], t+ =

1 1 + Zd /Rb

(26)

Transference number can also be determined by measurement of the electrochemical potential of the cell as illustrated in Figure 11 [75]. Suppose the polymer composite is

742

Polymer Electrolyte Nanocomposites Electrode 1

Table 4. Transference number of several composites.

Electrode 2

Transference number Temperature Ref.

Composites

µ1

CPE

µ2

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

sandwiched between two electrodes with different chemical potential #1 and #2 . The electrochemical potential across this cell is given by 1  #2 1 E= t d# = (27) t # − #1  z F #1 i z F i 2 where z = absolute value of the valence of the mobile ion in the electrolyte; and F = Faraday number. For pure ionic composite, ti = 1 so that Epure = #2 − #1  z −1 F −1

(28)

E = ti Epure

(29)

Thus By measuring E and calculating Epure , we can obtain ti . Another method based on a combination of d.c. polarization and a.c. impedance has been introduced by Evans et al. This method involves measuring the resistance and current across a symmetrical Li/electrolyte/Li cell polarized by a d.c. voltage [76]. The t + is given by t+ =

IS V − Io Ro  Io V − IS RS 

(30)

where V = d.c. voltage applied to the cell, Ro = initial resistance of the passivating layer, RS = steady-state resistance of the passivating layer, Io = initial current, and IS = steadystate current. The d.c. polarization potential usually used is several tens of millivolts. This equation is applicable for ideal, dilute solutions. However, Doyle and Newman state that although this equation is not strictly applicable in concentrated electrolytes, the ratio of steady-state to initial current provides useful information on the contribution by organic additives to the ionic conductivity of polymer electrolytes [77]. The simplification of Eq. (30) was also used, that is, t + = ISS /IO . However, significant errors resulted from neglect of kinetic resistances at the electrode/electrolyte interface [78]. Transference numbers of some composites appear in Table 4.

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

PEO:LiCF3 SO3 + -LiAlO2 (4 #m) PEO:LiBF4 + -LiAlO2 (4 #m) PEO:LiClO4 + TiO2 (13 nm) PEO:LiClO4 + Al2 O3 (6 nm) (PEO)30 LiClO4 (PEO)8 LiClO4 (PEO)8 LiClO4 + SiO2

0.29 0.26 0.5–0.6 0.31–0.33 0.18–0.19 0.19–0.20 0.22–0.23

90 90 90 90 100 90 100



C C  C  C  C  C  C 

[79] [79] [79] [80] [80] [80] [80]

a microscopic scale. An example of method for studying the ion dynamics is nuclear magnetic resonance (NMR) spectroscopy. This method probes the spin of ion using an electromagnetic wave in radio frequency. In amorphous single-phase polymer electrolytes, there is usually found a straight relationship between polymer segmental motion and ionic mobility by observing a strong correlation between the onset of NMR line-narrowing and the glass transition [81]. NMR has contributed significantly to the understanding of the physical properties of the composite polymer electrolytes mainly because it offers the possibility to selectively study the ionic and polymer chain dynamics. For example, measurement of the temperature dependence of 7 Li lineshapes and spin-lattice relaxation allows the determination of the activation energy and the correlation time of the cation motion. Gang et al. described the 7 Li line-narrowing in the composite of PEO:LiBF4 + -LiAlO2 (10–30 wt%) in the temperature range of 270–270 K [82]. Dai et al. reported wide line and high resolution solid-state 7 Li NMR [83]. In material, each spin interacts with other spins, giving rise to spin-spin interaction or relaxation time T2 . Furthermore, a new thermal equilibrium distribution of the spin, which has to be mediated through lattice, is forced by the magnetic field. The characterization time required for the excess energy to be given to the lattice or for attainment of new thermal equilibrium is expressed in terms of spinlattice or thermal relaxation time T1 . Under simultaneous application of static and radio frequency magnetic field in perpendicular direction, Hz = Ho

(31a)

Hx = 2H1 cos t

(31b)

The interaction of this magnetic field with the nuclear spin results in the Bloch susceptibility [84] 9 =

1 9 2 o

o T2

9  =

1 9 2 o

o T2

1 + T23 

T2  o −  2 2 2 o −  +  H 1 T 1 T2

(32a)

1 + T22 

1 − 2 +  2 H12 T1 T2 o

(32b)

where o = Ho , and  is the gyromagnetic ratio of the spin. For low RF field H1 ,  2 H12 T1 T2 1, then Eqs. (32a) and (32b) give the familiar absorption curve with half-width  =

o

−

 1/T2

(33)

However, the exact lineshape and linewidth can be determined using Van Vleck method of moment. This method

743

Polymer Electrolyte Nanocomposites

allows the connection of the absorption line with the motional behavior of the nuclei. The second moment, M2 is given by   − o 2 f   d M2 = −

=

 1 − 3 cos
2

(34)

where I is the nuclear spin [85]. Nuclear spin and NMR frequencies of some nuclei at Ho = 10 000 Gauss are given in Table 5 [86]. Based on this data, we can select the magnetic field Ho at around 10,000 Gauss (1 Tesla) and frequency o of around 16.574 MHz, so that the absorption of the target spin can be observed. For example, in order to detect the absorption of lithium ion in the lithium-based polymer electrolyte, the magnetic field Ho can be set at around 16,574 Gauss. If the spins are in motion, such as in fluid or intramolecular motion of liquid-like lattice, the value of M2 is small because of the small time-average local field component (the average of cos2
o

∝ M21/2

(35)

If c is the fluctuation time, M21/2 c 1 corresponds to rigid lattice and motional narrowing corresponds to M21/2 c  1. The region with 1/c approximate to M21/2 represents, the region of the onset of motional narrowing. Motional narrowing is used to study the ionic motion. Considerable line-narrowing takes place above a certain temperature, indicating diffusional motion. A rough estimation of the activation energy for motion can be obtained by using a modified Bloembergen, Purcell, and Pound expression [87]: ?c =

1  

 = -  2 −B 2 c tan 2 A2 −B2

(36)

where ?c = jump frequency,  = measured linewidth at temperature T , A = unnarrowed linewidth of the rigid lattice, B = fully narrowed linewidth, and  is a constant. Table 5. Nuclear spin and resonance frequency at 10,000 Gauss of several nuclei.

Nuclei 1

H Li 7 Li 17 O 19 F 23 Na 39 K 41 K 63 Cu 65 Cu 107 Ag 109 Ag 6

Nuclear spin

Magnetic resonance frequency (MHz) at 10,000 Gauss

1/2 1 3/2 5/2 1/2 3/2 3/2 3/2 3/2 3/2 1/2 1/2

42577 6265 16574 572 40055 11262 1987 1092 11285 12090 1220 1981

Fitting ?c / with Arrhenius expression,   ?c ? E = o exp −   kT

(37)

one obtains the value of E based on the Arrhenius plot of ?c /. Another expression for determining the activation energy is given by Hendrickson and Bray [88]:

 1 1 E 1 1 ln − =− + ln − (38)  −D A kT B A where D is a temperature-independent constant of a linebroadening term. The relation between spin-lattice relaxation time T1 and the fluctuation time c can be written as [89]
 1 c 4c =S (39) + T1 1 + 2o c2 1 + 4 2o c2 where S is a constant. For an approximate case, in which relaxation from a pair of nuclei of fixed internuclear spacing r only is considered, one obtains [90]
 2 S=  4
2 II + 1 (40) 5r 6 The spin-spin relaxation time T2 is given by [87] 
1 c 5 c 3 =S + + c T2 2 1 + 2o c2 1 + 4 2o c2 2

(41)

To study dynamics, the relaxation time is measured as a function of temperature so that c can be calculated. Using an Arrhenius relation c = o expE/kT 

(42)

the activation energy can be calculated. Many NMR experiments have been performed on polymer electrolyte nanocomposites [91–95]. Figure 12 shows the Arrhenius plot of spin-lattice relaxation time for PEO8 :LiClO4 polymer electrolytes and the composite prepared with 5.3 wt.% -Al2 O3 detected at a Larmor frequency of o = 1554 MHz [96]. The attempt frequency 1/o can be interpreted as a vibrational frequency, of order of optical phonon frequency (1012 –1012 Hz). It can be calculated (e.g., using Mathematica software) that the function 1/T1 finds a maximum value at o c = 0613. The maximum value of 1/T1 appears at temperature of T  330 K for both samples. Using a Larmor frequency of o = 1554 MHz, one obtains c = 0613/(1.554 × 108  = 394 × 10−9 s. For composite of PEO:LiClO4 + carbon black, Franco et al. estimated the relation time of about 4.4 × 10−9 s based on the 1 H resonance measurement [97]. Using Eq. (42) and assuming 1/o ≈ 1012 , we have E  024 eV for both samples. The electrical conductivity can be determined from Eq. (42) and different form of Nernst–Einstein equation, that is, =

Nd 2 q 2 6c kT

(43)

Spin-lattice relaxation rate (1/T1)[s–1]

744

Polymer Electrolyte Nanocomposites

100

10–1

2.5

3.5

1000/T [K–1]

4.5

5.5

Figure 12. Arrhenius plot of spin-lattice relaxation time (1/T1 ) of 7 Li 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 determined from the molecular weights of PEO and LiClO4 and the density of the electrolytes (B ≈ 13 g/cm3 ) [96], to result in N ≈ 17 × 1021 cm−3 . Considering the average Li-Li distance of ∼4 Å [96], the conductivity at 300 K is about 71 × 10−4 S/cm. Temperature-dependent linewidth of composite of PO15 LiI + 6 v% Al2 O3 is displayed in Figure 13 [83]. It can be seen that there is a drastic change in the line-width when temperature is changed from 30  C to 50  C. It is assumed that the onset of the motional narrowing is at temperature of 40  C. At −20  C the sample exhibits a rigid limit lineshape with baseline due to a distribution of 7 Li nuclear quadrupole satellite transition [98]. As the temperature is raised, partial line-narrowing results. Forsyth et al. reported the effect of filler content on the 7 Li linewidth for a composite of copolymer trihydroxypoly(ethylene oxide-co-propylene oxide) with an EO/PO

ratio of 3:1 containg LiClO4 and TiO2 nanoparticles [99]. The linewidth increases with increase in the filler content and reaches a plateau after about 10 wt.%, as observed in Figure 14. Although the linewidth relates to the mobility of lithium ion, in which the line-narrowing represents mobile ion, an interpretation that the increase of linewidth relates to decrease in the mobility of lithium ions will be contradictory with the reported result. The reported result confirmed that addition of filler at lower content increases the lithium ion mobility and therefore increases the conductivity. A possible interpretation is the lithium ion environment changes with the addition of filler and the linewidth changes are more likely to reflect the changing environment rather than the changing mobility. A possible interpretation is chemical shift dispersion, that is, lithium ion occupying many different state environments (but magnetically degenerate) such that a distribution of chemical shift is obtained. The broad signal of resonance is attributed to crystalline state, while the narrow component is attributed to amorphous phase. This difference contribution can be used to predict the crystallinity of polymer in composite. For example, Singh et al. observed the 1 H resonance of a system of PEG46 :LiClO4 + nanoparticles of Mn003 Zn097 Al2 O4 [95]. They fitted the broad part of the signal with a Gaussian function and the narrow component with the Lorentzian function. The crystallinity of the polymer equals to the fraction of the area of narrow and broad components of the signal. For pure PEG they found a crystalline of about 83% using this method.

7.2. Raman Scattering Raman spectroscopy is important to probe molecules with anisotropic polarizability. The vibrating atoms are not able to follow the incident radiation frequency if it is much higher than the phonon frequency. However, the electron cloud of the vibrating atom can interact with the frequency of the incident radiation. These oscillating dipoles can absorb energy from the radiation field and re-emit radiation of the same frequency. This radiation is detected as scattered light 600

7

7

Li Line width [Hz]

5 4

500

400

7

Li NMR linewidth [kHz]

6

3 300 2 200

1 –20

0

20

40

60

80

100

120

0

4

8

12

16

wt.% TiO2

Temperature [°C]

Figure 13. Temperature dependence of 7 Li NMR linewidth of PEO15 LiI containing 6 vol.% Al2 O3 . Data points were extracted from [83], Y. Dai et al., Electrochim. Acta 43, 1557 (1998).

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

745

Polymer Electrolyte Nanocomposites

(44)

where t is the polarizability of molecule. If the incident frequency and the polarizability of the molecule change between min and max of frequency int as a result of its rotation and vibration, we can write [100]
 1 #t =  +  cos int t Eo cos t (45) 2 with , = max − min

(46)

Therefore, we obtain, 1 #t = Eo cos t + Eo 2 × Ccos + int t + cos −

int tD

(47)

The induced dipole moment has then components as follows: unshifted frequency, , known as Rayleigh line; lower frequency, − int , known as Stokes line; and higher frequency, + int , known as anti-Stokes line. Raman spectra can be used to determine the concentration of free ions in the electrolytes. For example, the Raman spectra of LiCF3 SO3 have been tabulated as appear in Table 6 [101]. A composite containing SO3 , the Raman spectrum of the ?S (SO3 ) spectral region of the triflate anion of poly(ethylene oxide) dimetyl ether (400) complexed with LiCF3 SO3 , along with the three-component curves fit are shown in Figure 15. It was explained that the component observed at 1032 cm−1 corresponds to free anions not interacting directly with lithium cations. The component of 1042 cm−1 has been attributed to contact pair and the component of 1052 cm−1 has been attributed to Li2 CF3 SO3 triple ions [102]. Because of the multicomponent nature of the spectrum, it can be concluded that the ion-ion interaction is present in Table 6. LiCF3 SO3 vibrational assignments. Band ?s (SO3 )

?as (SO3 )

Wavenumber (cm−1 )

Assignment

1033 1043

free ion monodenate ion pairs, LiX, also LiX−2 and LiX2− 3 Li2 X+ aggregate 2− LiX3 free ion monodenate ion pairs, LiX, also LiX−2 , LiX2− 3 Li2 X+ aggregate Li3 X2+ aggregate

1053 1062 1272 1257, 1302 1270, 1308 1288

1042 cm

Intensity [a.u.]

#t = tEt

O:M = 110:1 –1

1032 cm–1

1052 cm–1

1060

1050

1040

1030

1020

1010

–1

Raman shift [cm ]

Figure 15. Raman spectra of LiCF3 SO3 . Reprinted with permission from [102], A. Ferry et al., Electrochim. Acta. 43, 1471 (1998). © 1998, Elsevier Science.

the system even down to the concentration of O:M = 563:1. The relative amount of anions not interacting directly with lithium ions, that is, spectroscopically free, increases with increasing concentration from approximately 22% at O:M 563:1 to 40% at O:M = 110 and then falls off slightly at higher concentration. The 1042 cm−1 band initially decreases in relative intensity with increasing concentration and then levels off at 54% in the upper concentration range. The fraction of free ion obtained from Raman spectra in a system of PPO:NaCF3 SO3 is displayed in Figure 16 [103], for a system of hydroxyl end capped PPO with NaCF3 SO3 .

7.3. FTIR Spectroscopy Atoms in solid vibrate at frequency approximately 1012 –1013 Hz. Vibration mode involving pairs of groups of bonded atoms can be excited to higher energy by absorption of radiation at appropriate frequency. In the infrared (IR) technology, the frequency at the incident radiation is varied and the 1.0

0.8

Fraction of anions

and is known as Rayleigh scattering. Moreover, the oscillating dipoles see the force field of the vibrating atomic nuclei also as the nuclei oscillate around the equilibrium position; the deformability of the electron cloud varies with the oscillation frequency of the nuclei. The Raman scattering is described in a simple term here. If a time-dependent electric field, Et, is applied to a molecule, it produces an induced dipole moment, #t,

0.6

0.4

0.2

(OH-PPO400)16:NaCF3SO3 (OH-PPO4000)16:NaCF3SO3

0.0 200

250

300

350

Temperature [K]

Figure 16. Effect of temperature on the fraction of anions obtained from the Raman spectrometry. Data points were extracted from [103], H. Ericson et al., Electrochim. Acta 43, 1401 (1998).

746

Polymer Electrolyte Nanocomposites

quantity of radiation absorption or transmission by sample is observed. In the Raman technique, on the other hand, the sample is illuminated with monochromatic light, usually generated by a laser. In order for a particular mode to be IR active, the associated dipole moment must vary during the vibrational cycle. Therefore, centrosymmetric vibrational modes are IR inactive. The principal selection rule for a vibrational mode to be Raman active is that the nuclear motions involved must produce a change in polarizability. In-situ FTIR spectroscopy can also be used to determine the conductivity of ion in composites. For example, a system of poly(methyl methacrylate-co-alkali metal methacrylate)ethylene carbonate:LiClO4 is displayed in Figure 17a. A typical spectrum of carbonyl stretches region to appear in four peaks at 1730 cm−1 , 1751 cm−1 , 1775 cm−1 , and 1806 cm−1 [104]. The evolution of carbonyl peak was observed with time under the application of d.c. field such that the ion transport in the composite is shown in Figure 17b [104]. The change in the intensity of carbonyl group in the ethylene carbonate represents the different chemical environments within the composites. The intensities of the peak at 1730 cm−1 , 1775 cm−1 , and 1806 cm−1 were found to be fairly

(a)

Intensity

EC/LiClO4 = 3/1

2100

EC/LiClO4 = 4/1 EC/LiClO4 = 5/1

2000

1900

1800

1700

1600

Wavenumbers (cm–1) 50

EC E2 C EC = D 2 − #E Et Ex Ex

(48)

where C = concentration of charge carriers, represented by the intensity of the 1750 cm−1 peak, D = diffusion coefficient, # = mobility of the charge carriers, and E = electric field. The solution of Eq. (48) at a fixed position, x = 0, can be expressed as   #E2 t (49) Cx = 0 t = A + B exp − 4D where A and B are constants. From the slope of curve intensity of 1750 cm−1 peak, one can determine the mobility of the ion carriers. Measuring the mobility at various ion concentrations, we can determine the mobility at various ion concentrations as appears in Figure 18 [104]. The ion movement can also be analyzed based on the change in the intensity of 624 cm−1 , arising from ClO− 4 stretching band [105]. Kim et al. showed the dependence of 624 cm−1 intensity on time at 25  C and found that the mobility calculated from that data is similar to that obtained from the 1750 cm−1 absorption of ethylene carbonate [104]. The change of anionic mobility with the ion content of ionomers was found to be nearly the same as that of cation mobility.

1729 cm–1

(b)

1750 cm–1

40

1775 cm–1

–4.5

1805 cm–1 30

µ [cm2/s V]

Intensity [au]

constant with time under the electric field. It indicated that these three peaks might not be correlated with movement of charge carriers. However, the intensity of the 1750 cm−1 peak decreased with the time, eventually reaching a limit value. The decrease in the intensity of the 1750 cm−1 peak related to the change in the concentration of the plasticizer solvating the salt. The change in the peak intensity corresponding to various chemical environments is due to the migration of the charge carrier. The concentration of the plasticizer to solvate the salt was found to decrease, which implies that the movement of cations in the electrolytes is strongly correlated with the motion of the plasticizers. The change in the peak intensity of the plasticizer solvating salt can be analyzed solving the following transport equation [104]:

20

–5.0

10

0

0

4

8

12

Time (min)

Figure 17. (a) IR spectra of poly(methyl methacrylate-co-alkali metal methacrylate)-ethylene carbonate:LiClO4 and (b) the decay of the observed peaks when the applied d.c. potential was switched off. Data points we extracted from [104], C. H. Kim et al., Electrochim. Acta 43, 1421 (1998).

–5.5

2

4

6

8

10

12

Ion content [mol%]

Figure 18. The effect of ion content on the mobility of poly(methyl methacrylate-co-alkali metal methacrylate)-ethylene carbonate:LiClO4 determined from the decay curve of 1729 cm−1 peak. Data points we extracted from [104], C. H. Kim et al., Electrochim. Acta 43, 1421 (1998).

747

Polymer Electrolyte Nanocomposites

For the infrared spectra of PEG:LiClO4 :AlBr3 1 mass%, the ?4 (ClO− 4 ) spectra can be separated into two contributions with maxima at 623 and 633 cm−1 [106]. The 623 cm−1 band is attributed to free anions and the 633 cm−1 peak is related to bound or contact (ClO− 4 ) [107]. The fraction of free anions can be calculated as the fraction of area under 623 cm−1 mode to the total area of ?4 (ClO− 4 ) envelope. Compared to composite in the absence of AlBr3 , there is a dramatic increase in the fraction of free anions when AlBr3 is dispersed in the electrolytes.

7.4. X-ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) is a powerful technique for studying the surface of solids. The data obtained using this technique are mainly used to extract the information regarding the bonding energies of various core-level electrons from different elements of solid materials. These values are then interpreted as the bonding between the element under consideration with their neighbor. Information about the local structure and interaction of an element with its neighbor can be extracted from the XPS data [108]. In the development of battery, the interaction between the electrolyte and the electrode determines the performance of the battery. Understanding this interaction is important to optimize the preparation parameters for realizing highperformance batteries. Therefore, surface studies, for example using XPS, may be of great significance to understand the 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 particles. If the process results in an ionization of electrons from the bombarded material, the kinetic energy of the electron will satisfy Ek = h? − Eb

(50)

where h? = energy of incident radiation, and Eb = binding energy of electron. For a given atom, a range of Eb values is possible corresponding to ionization of electrons from different inner and outer valence shells. Measurement of the value of Ek , and therefore Eb , provides a means of identification 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 investigated the interaction of salt with the polymer chain in the systems of PEOn LiCF3 SO3 and PEOn Cu(CF3 SO3 2 [108]. There is evidence of the interaction of the carbon from –CH2 -CH2 O- polymer chains with the cation of the salt through the ether oxygen of PEO and anion of the salt. A complexation between the action of the salt and the oxygen of the PEO is identified using this method. It is also observed that for the PEOn Cu(CF3 SO3 2 system, the change in the nature of the complex occurs at low salt concentrations. However, for PEOn LiCF3 SO3 system, such a change is identified at higher salt concentration. Liu et al. prepared polymer electrolyte nanocomposites using SiO2 filler and 2-[methoxy(polyethylenoxy)-propyl] trimethoxy silane coated SiO2 [109]. The XPS method has

been utilized to confirm the success of coating. During treatment with 2-[methoxy(polyethylenoxy)-propyl] trimethoxy silane, the OH- groups of the surface of the SiO2 react with partially hydrolized silane. For the untreated SiO2 , the XPS data can be fitted by band centering at 284.6 eV, which is often characteristic of adventitious CH2 species in the measurement chamber. On the other hand, the coated SiO2 can be fitted with two bands, centered at 284.6 eV and 286.2 eV. The former corresponds to the adventitious CH2 species while the latter corresponds to C-O-C group that appears as a result of the functionalization reaction.

7.5. X-ray Diffraction X-rays are electromagnetic radiation with a wavelength around 1 Å (10–10 m). The X-rays which are used in almost all diffraction experiments are produced by accelerating an electron beam through 30000 V and permitting it to strike a metal target, such as copper. The incident electron has sufficient energy to ionize some of the copper 1s (K shell) electrons. An electron in an outer orbital (2p or 3p) immediately drops down to occupy the vacant 1s level and the energy released in the transition appears as X-radiation. The transition energies have fixed values and so create the spectra of characteristic X-radiation. For copper, the 2p → 1s transition, called K , has a wavelength of 1.5418 Å and the 3p → 1s transition, KG , has a wavelength of 1.3922 Å. If the average crystalline size is below a critical limit (∼200 nm diameter), a broadening of diffraction X-ray beam occurs [110]. The commonly accepted formula for particle size broadening is the Scheerer formula d=

09H B cos
(51)

where d = crystalline radius, H = X-ray wavelength,
(52)

where BM = measured peak width in radiation at half peak height, BS = width of a peak of a standard material mixed with the sample, whose particle size is considerably greater than 200 nm and which has a diffraction peak near the relevant peak of the sample. For relatively large particle, the width of peak (unbroadened peak) is very small, so that BM  BS . Therefore, it is often possible to approximate B with BM . Figure 19 shows the XRD spectra of polyethylene glycolbased polymer electrolyte nanocomposites containing ZnO nanoparticles [62]. Using 110 with a position at around 2
748

Polymer Electrolyte Nanocomposites

Intensity [au]

120

(d)

80

(c)

0

50

55

60

65

70

2θ [°]

Intensity (A.U.)

40

(b)

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

simple equation

(a)

xc =

Ac Ac + A a

(53)

Shin et al. observed the effect of filler content on the XRD pattern on PEO in a system of PEO10 LiCF3 SO3 containing TinO2n−1 . As observed in Figure 21, the PEO peak intensities decrease by the increase in the volume fraction of filler content [111]. It indicates that the crystallinity of the sample decreases by increasing the volume fraction of filler content. Similar results have also been reported by Leo et al. in a system of PEO:LiCF3 SO3 containing filler of Li14 (Al04 Ge16 )(PO4 3 [112]. With addition of filler, the intensity of the crystalline peaks has decreased and a noticeable 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 ANALYSIS

10

20

30

40

50

2θ (degree)

Figure 21. XRD patterns of PEO10 LiCF3 SO3 polymer electrolytes with (a) 0, (b) 5, (c) 10, and (d) 15 wt.% Tin O2n−1 . (•, 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 advanced field emission SEM. Wen et al. compared the SEM picture of PEO:LiClO4 and PEO:LiClO4 containing alumina whisker. For PEO:LiClO4 , great amounts of microcracks were observed on the surface, as observed in Figure 22a [113]. The size of islands are several micrometers, compared to the PEO spherulite [114]. Addition of 10 wt.%

8.1. Scanning Electron Microscopy

Intensity [au]

The morphology of the sample surface can be observed using scanning electron microscopy (SEM). The surface smoothness and the presence of holes in the scale down to

(a)

(b)

(c) Ac Aa

2θ

Figure 20. Typical form of XRD pattern of high molecular weight polymer.

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

749

Polymer Electrolyte Nanocomposites

8.2. Transmission Electron Microscopy In the case of polymer electrolyte nanocomposites, transmission electron microscopy (TEM) is usually used for determining the dispersion of nanoparticles in the polymer matrix and the size of nanoparticles. Capiglia et al. showed that larger filler (submicron size) is quite well dispersed in the polymer matrix while the smaller filler (tens of nanometers size) is not well distributed in the polymer matrix [61]. Instead, the small filler is condensed in large blocks of size up to 1 #m. Based on the TEM photograph, Mikrajuddin et al. showed that PEG based polymer electrolyte nanocomposites made by in-situ growth of ZnO nanoparticles in the polymer matrix and in-situ insertion of lithium ion during nanoparticles growth have particle size of about 6 nm [63]. This result is consistent with the calculation of the particle size using size-dependent bandgap equation [117–119]. Chandra et al. also observed the size of nanoparticles synthesized in-situ in polymer electrolyte nanocomposites using TEM and found that small content nanoparticles (about 1 wt.%) have smaller size [54].

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

8.4. Optical Microscopy Optical microscopy characterization of polymer electrolyte nanocomposites was not reported too much. To date, Kim et al. reported the optical micrograph of a system of PEO16 : LiClO4 containing various filler content, taken under crossed polarizers [121]. They observed well-defined spherulitic morphologies. The spherulites were observed in thin films deposited on the glass substrate whose typical thickness was about 20 #m. They proposed that the size and the morphology of the spherulites can be related to the melting point or glass temperature of the composites. This was based on the

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

9. THERMAL CHARACTERIZATIONS 9.1. Thermogravimetry Thermogravimetry (TG) is a technique for measuring the change in the weight of a substance as a function of temperature or time. The result usually appears as a continuous chart record, as displayed in Figure 23. The sample, usually a few milligrams in weight, is heated at a constant rate, typically 1–20  C/min. It has constant weight until it begins to decompose at temperature Ti . Decomposition usually takes place over a range of temperature Ti to Tf and second constant plateau is then observed above Tf , which corresponds to the weight of residue. A TGA curve of composite made by PEO:LiBF4 containing 2-[methoxy(polyethylenoxy)-propyl]trimethoxy silanecoated SiO2 nanoparticles is displayed in Figure 24 [109]. At temperatures below 180–200  C, the sample experiences only a small weight loss due to removal of residual water on the surface of SiO2 . At increasing temperatures, the OH groups on the SiO2 surfaces begin to decompose to give rise to slight increase in the weight loss. In addition to the decomposition of the OH groups, another weight loss take place at about 350  C that can be attributed to the decomposition of silane molecules that are bonded on the surface of SiO2 . Liu et al. suggested that the adsorption of containing 2-[methoxy(polyethylenoxy)-propyl]trimethoxy silane on the surface of SiO2 is likely to give a sub-monolayer coverage [109].

9.2. Differential Thermal Analysis Differential thermal analysis (DTA) is a technique in which the temperature of a sample is compared with that of inert reference material during a programmed change of temperature. The temperature of sample and reference should be the same until some thermal event such as melting, decomposition, or change in crystal structure occurs in the sample.

Wi

Weight

whisker in the polymer electrolytes inhibited the formation of 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 formation of microcracks (Fig. 22c). Golodnitsky et al. observed the SEM picture of filler-free PEOn :LiI and PEOn :LiI containing alumina [115]. PEOn :LiI containing high concentration salt is made up of units whose area is hundreds of square microns. Addition of alumina causes a minor reduction in the grain size. In a PVdF gel polymer electrolyte containing CuO filler, Wang and Gu found that the surface presented a multitude of PVdF grains and pores of average size of about 2–4 #m in diameter [116]. They assumed the CuO nanoparticles distributed uniformly in the polymer matrix.

∆W Wf

Ti

Tf

Figure 23. Typical form of TGA curve.

Temperature

750

Polymer Electrolyte Nanocomposites 100

Weight percentage [%]

(PEO)8LiCF3SO3/clay (wt %) 99

(f) 50/50 OH-decomposition

(e) 70/30

98 (d) 91/9 silane decomposition 97 (c) 97/3

0

100

200

300

400

500

Temperature [°C]

Figure 24. TGA curve of PEO:LiBF4 containing 2-[methoxy (polyethylenoxy)-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 temperature, 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 usually a few milligrams and heating and cooling rate is usually 1–50  C/min. The difference in the sample and reference temperature will appear as Figure 25. If calorimetric data is required, it is better to use differential scanning calorimetry (DSC). DSC is very similar to DTA. A sample and an inert reference are also used in DSC system but the cell is designed differently. The sample and the reference are maintained at the same temperature during the heating program and extra heat input to the sample (or to the reference if sample undergoes an exothermic change) is required to maintain this balance. Enthalpy changes are therefore measured directly. Examples of DSC curves for various PEO:LiCF3 SO3 + clay composites are displayed in Figure 26 [123]. From this figure, we can extract several parameters like the glass temperature, the melting point, and enthalpy. These values were found to depend on the filler content, as summarized in Table 7. A pure PEO has one first-order endothermic transition at around 70  C, corresponding to the melting of the

Heating

Polymorphic change

(b) 1/0

Endothermal

96

(a) undoped PEO

–90

–60

∆T

EXO

Temperature

Figure 25. Typical form of DTA curve.

Solidification

30

60

90

120

150

180

Figure 26. DSC curve of PEO:LiCF3 SO3 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 minor endothermic transition was observed at around 140–150  C, due to the melting of crystalline complex phase formed by PEO and LiCF3 SO3 [124–126]. The melting temperature of the crystalline PEO phase depends on the filler content. The Tm initially shifted to higher temperature when the filler content increased, and reached the highest value at 9 wt.% clay, and then decreased with further increase in the clay content. Table 7. The parameters of polymer electrolyte nanocomposites extracted from the DSC curve.

Melting

ENDO

Polymorphic change

0

Temperature (°C)

Samples

Cooling

–30

PEO PEO:LiCF3 SO3 PEO:LiCF3 SO3 + clay 3 wt% PEO:LiCF3 SO3 + clay 9 wt% PEO:LiCF3 SO3 + clay 30 wt% PEO:LiCF3 SO3 + clay 50 wt%

Tg (K)

Tm (K)

H (J/g)

Xc

Tmc (K)

−46 −50

7050 6984 7158

1655 599 689

362 416

1520 1497

−43

774

778

470

1569

−46

5493

401

242

1426

−48

4599

291

176

1324

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

751

Polymer Electrolyte Nanocomposites

The PEO crystallinity (expressed by the area covered by transition curve) is also dependent on the clay content. The PEO crystallinity initially increases with increasing clay content up to 9 wt.% and then decreases with further increase in the clay content. Melt-crystallized polymers are never completely crystallized. This is because there are an enormous number of chain entanglements in the melt and it is impossible for the organization to form 100% crystalline polymer during crystallization. The degree of crystallinity is therefore of great technological importance. The degree of crystallinity can be deduced from the DTA data, in which the melting enthalpy can be obtained. The crystallinity can be calculated using a simple equation [127]: Hm × fPEO Hm c

(54)

where Hm = melting enthalpy measured, Hm c = melting enthalpy of 100% crystalline (for PEO, Hm c = 1964 J/g), and fPEO = weight fraction of PEO in polymer electrolytes.

10. DENSITY METHOD Another method for determining the crystallinity is based on the knowledge of density of crystalline and amorphous phases as well as the density of the polymer specimen. The crystallization of polymer from melt is accompanied by the reduction in the volume due to an increase in density. The crystals have a higher density than the molten or noncrystalline polymer since the last two contain also free volume. Based on this difference, the density method can be utilized to determine the degree of crystallinity. If V is volume of polymer specimen, and Va and Vc are volumes of amorphous and crystalline regions, respectively, we have V = Va + Vc . The relation of polymer specimen mass and the amorphous and the crystalline region masses is given by BV = Ba Va + Bc Vc

Bc B




B − Ba Bc − B a

The role of the ceramic filler is to influence the recrystallization kinetics of the polymer matrix chains, thereby ultimately promoting localized amorphous regions and thus enhancing the transport of cations [9]. To produce a high fraction of amorphous state in the composite, the as-prepared composite is first heated above the melting point so that all parts of the polymer are converted to the amorphous state. During the cooling process, the matrix part around particles remains in the amorphous state, even when the temperature drops below the melting point. Therefore, a high electrical conductivity would be expected to appear at ambient temperatures. Indeed, enhancement in conductivity of up to about three orders of magnitude at low temperatures and about one order of magnitude at high temperatures has been reported for the system of poly(ethylene oxide)-LiClO4 containing ceramic fillers [9]. In addition, composites containing ceramic fillers in the nanoscale particle size exhibit both excellent mechanical stability (promoted by the network of the fillers into the polymer bulk) and high ionic conductivity (promoted by the high surface area of the dispersed filler). The volume fraction of filler particles affects the conductivity of a composite. The symbols in Figure 27 display the effect of filler loading on the electrical conductivity of a poly(ethylene oxide)/NaI containing filler of I-Al2 O3 [128]. The conductivity increases with an increase in the volume fraction of fillers, reaches a maximum at a certain value of filler particles, and then decreases toward zero for further increases in the volume fraction of fillers. This observation 10–4

10–5

(55)

where B = density of polymer specimen, Ba = density of amorphous region, and Bc = density of crystalline region. By defining the crystallinity as xc = Bc Vc /BV , we obtain xc =

11.1. Effect of Filler Content on Conductivity

10–6

σ [S/cm]

Xc =

11. ELECTRICAL PROPERTIES

10–7

 (56)

The density of polymer specimen can be determined by simply measuring the volume and weighing the mass. The density of crystalline region can be calculated from the knowledge of the crystal structure. The density of amorphous phase can be determined by measuring the density of almost completely amorphous polymer, such as polymer obtained by rapid cooling from polymer melt. Equation (56), however, is valid if polymer specimen contains no holes which are often present in molded samples. In addition, since packing of the molecules in amorphous region is random, it is likely that the density of amorphous phase depends on the thermal treatment of the specimen.

10–8

10–9 0.0

0.1

0.2

0.3

0.4

0.5

Volume fraction of fillers

Figure 27. Effect of filler volume fraction on the electrical conductivity at 25  C for composites of (open circle) PEO:LiClO4 + PAAM and (solid circle) PEO:NaI +I-Al2 O3 . Date points were extracted from Y. Liu et al., J. Power Sources 109, 507 (2002). Curves were obtained from theoretical calculation to fit the data using (t/R = 06) for fitting the open circle data and (t/R = 116) for fitting the solid circle data. Curves were replotted from [128], J. Przyluski et al., Electrochim. Acta 40, 2102 (1995).

752

Polymer Electrolyte Nanocomposites

can be explained as follows. By increasing the fraction of fillers, the total amount of amorphous state around the fillers increases since the surface area increases, thus increasing the conductivity. If the filler content is so high, some of the filler agglomerates (making contact) so that the surface area is reduced, thus reducing the fraction of amorphous state around the filler, thereby reducing the conductivity. At a specified amount of filler fraction, the continuous network of amorphous state disappears so that the transport of cations is blocked. The conductivity of composites can be approximated with the conductivity of filler particles. The effective medium approximation was used to explain the conductivity enhancement in polymer electrolyte composites [128]. Dispersed ceramics create an amorphous layer around the particles, which have a high conductivity. The composite is considered to be a two-phase system: particles and amorphous layer as one phase and the rest of the polymer as the other phase. The conductivity of the second phase (polymer matrix) is equal to the conductivity of the polymer electrolytes free of dispersed particles. The conductivity of the particle–amorphous layer can be calculated using Maxwell–Garnett equation [129],  c = 1

21 + 2Y 2 − 1  21 + 2 − Y 2 − 1 

(57)

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

1 1 + t/R3

(58)

where t = thickness of the conducting layer, and R = radius of filler. The system is analogous to a system containing conducting particles dispersed in a polymer matrix in which the conducting particles correspond to the particle–amorphous layer. The improved conductivity of grain and polymer medium is calculated by the Nakamura [130] or Nan and Smith [131, 132] equation, ca = 2c

Vc 3 − Vc

(59a)

ae = 2e

1 − Vc 2 + Vc

(59b)

where Vc = V2 /Y , and V2 = volume fraction of filler in a bulk electrolyte. The dependence of composite conductivity on the load fraction of dispersed particles and particle size can be calculated using the effective medium theory [133]: 

 ca −m ea −m V2 V2 + 1− =0 Y m +pc ca −m  Y m +pc ea −m  (60) where m is the conductivity of the composite and pc is the continuous percolation threshold for the composite grain. Pc can be taken to be 0.28 on the basis of general percolation theory [131, 132]. Lines in Figure 27 are the theoretical prediction for a system of PEO:LiClO4 + PAAM and

PEO:NaI + I-Al2 O3 [128]. The data were collected at 25  C. The theoretical curves were calculated using t/R = 06 for solid line and t/R = 116 for dashed line. However, this theory cannot explain the enhancement of conductivity at high temperatures compared to polymer electrolytes which are free of filler as shown in Figure 4. An alternative approach has been used to explain conductivity enhancement due to filler dispersion [134]. The presence of a filler induces the formation of an amorphous layer around the filler particles as well as the presence of an ionic layer around the particles. Typically, an amorphous layer around the filler particles is effectively created if the polymer containing a filler is heated above the melting point and then cooled down. The presence of a ceramic filler inhibits the recrystallization of a polymer layer around the particles so that layer remains in the amorphous state after the polymer reaches room temperature. That means that, without heating above the melting point, the conductivity of polymer electrolytes containing a filler is close to that of filler-free polymer electrolytes. However, by assuming the presence of an ionic layer around the filler which has a conductivity higher than that when the ionic layer is absent, the conductivity is then enhanced even though all parts of the polymer are in the crystalline phase or in the amorphous phase. This assumption explains the enhancement in conductivity at high temperatures even when all parts of the polymer are in the amorphous state. This approach is similar to the observation of conductivity enhancement in solid-state ionic composites, into which ceramic fillers are dispersed [135]. The ionic transport in solid-state ionics is very similar to polymer electrolytes. Polymer electrolytes in the crystalline phase can be thought of as analogous to solid-state ionics having a low conductivity, and polymer electrolytes in the amorphous phase can be analogous to solid-state ionics having a high conductivity. Therefore, since the ionic layer is assumed to be present in solid-state ionic materials when the fillers are dispersed, a similar layer would be expected to appear in polymer electrolytes containing a ceramic filler. The conductivity of the ionic layer can be roughly approximated with l =  + , in which  is the conductivity of the medium around the particles in the absence of an ionic layer, and  is the excess conductivity due to the presence of a charged layer. Thus, if the polymer around the filler is in the crystalline state, the conductivity of the layer around the particles would be higher than the conductivity of this crystalline state. If the polymer medium around the particles is in the amorphous state, the conductivity of the ionic layer would be higher than the conductivity of the amorphous polymer. Mikrajuddin et al. calculated the loading effect on the electrical conductivity of composites by assuming a simple cubic packing of filler particles and obtained a bell-shaped conductivity variation [134], similar to that obtained experimentally. Most of fillers used for making polymer electrolyte composites are restricted to nonconductive materials. Recent reports of the addition of small amount of conducting materials such as carbon (1.5 wt.%) with surface area of 60 m2 /g showed an enhancement of conductivity and interfacial stability compared to carbon-free composites [136].

753

Polymer Electrolyte Nanocomposites

11.2. Effect of Salt Concentration on Conductivity Another potential approach is to increase the concentration of salt ions in the matrix. However, this approach is unpredictable. Figure 28 shows the effect of ion content on the electrical conductivity of a system of PEG:LiClO4 + -Al2 O3 at 25  C [137]. By increasing the concentration of salt ions, the electrical conductivity initially increases, followed by a decrease after passing a specified salt concentration. Explanations have been proposed to explain this observation. Doeff et al. explained this behavior in terms of a trade-off between an increasing number of charge carriers and ion aggregation and increased viscosity due to ionic cross-linking, which lowers the conductivity as the salt concentration passes a critical value [138]. Dupon et al. explained this dependence as being due to strong ion pairing, which effectively traps the mobile cations, and therefore significantly reduces the ionic conductivity relative to nonion-paired complexes on a similar structure [139]. Marcinek [137] found that up to LiClO4 concentration equal to 0.25 mol/kg, the conductivity of both systems PEG:LiClO4 and PEG:LiClO4 + -Al2 O3 system of PEG:LiClO4 + -Al2 O3 do not differ from each other by more than 30–40%. However, for high salt concentrations, the conductivity of electrolytes containing -Al2 O3 are much higher. The difference rises up to 30–40 times for a system with the highest salt concentration.

11.3. Effect of Temperature on Conductivity Since the transport of ions in the amorphous state is assisted by chain segment relaxation, the temperature dependence for ion transport in this state follows the temperature dependence of chain relaxation. A Vogell–Tamman–Fulcher (VTF) dependence of electrical conductivity then results, that is,   A B T  = √ exp − (61) T − To T

in which A, B, and To are constants and T is the temperature. The value of the constant parameter depends on the type of composites as well as the conditions used in preparing the polymer. The value of To depends on the glass transition of the polymer. A generally accepted relation is To = Tg + 50, in which Tg denotes the glass transition of the polymer [140]. The glass transition of most polymers is related to the melting point that satisfies Tg /Tm = 05–0.8 [19]. Parameter B can be expressed as a linear function of To [134]. The value of VTF parameters was found to depend on the concentration of salt. For example, in a system of PPG4000:AgCF3 SO3 , the value of VTF parameters at various salt concentrations was reported by Eliasson et al. [141]. The VTF parameters also depend on the salt type, as reported by Florzanczyk et al. [142]. On the other hand, since the transport of ions in the crystalline state is dominated by ion carriers jumping to the nearby locations, the temperature dependence of conductivity then follows the Arrhenius law,   Ea T  = A exp − kT

(62)

in which Ea is the activation energy. The value of Ea /q, in which q is the charge of the ion carrier, represents the blocking potential that must be overcome for an ion carrier to jump to the nearest locations. For polymer electrolytes only (free of ceramic fillers), a VTF behavior of conductivity is observed at temperatures above the melting point; Arrhenius behavior prevails at low temperatures. A drastic depression of electrical conductivity occurs at temperatures where the VTF behavior changes to Arrhenius behavior [9]. Since the dispersion of filler particles will generate amorphous states even at low temperatures, VTF behavior is observed even at low temperatures for conductivity, as can also be seen in Figure 2. Continuing the VTF behavior up to ambient temperature overcomes a drastic depression of conductivity so that high conductivity prevails at low temperatures.

11.4. Effect of Particle Surface on Conductivity

σ [S/cm]

10–6

10–6

–6

10

0.001

0.01

0.1

1

Concentration [mol/kg]

Figure 28. Effect of salt concentration on the conductivity at 25  C of PEG:LiClO4 + -Al2 O3 . Data points were extracted from [137], M. Marcinek et al., Solid State Ionics 136–137, 1175 (2000).

As observed in Figure 4, at high temperatures where all parts of the polymer are in an amorphous state, the conductivity of a composite is higher than that of polymer electrolytes which are free of fillers. This indicates that the amorphous layer around filler particles should not be the only contributor to conductivity enhancement. It has been proposed that the surface properties of the dispersed filler also play a significant role in the electrical conductivity of polymer electrolyte nanocomposites. Prior to use, the fillers are usually cleaned by drying at certain temperatures in vacuum or washing with a solution followed by drying in vacuum to remove contaminants on the surface. Matsuo and Kuwano observed an enhancement in conductivity by a factor of five to ten when a lithium dodecylsulfate surfactant was added to the surface of an SiO2 in a system of PEG-LiCF3 SO3 -SiO2 [43]. Prior to the synthesis of composites, the silica nanoparticles were dispersed in a

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Polymer Electrolyte Nanocomposites

solution of the surfactant to produce a self-assembly of surfactant molecules on the particle surface. Figure 29 shows the effect of surfactant content (lithium dodecylsulfate, LDS (C12 H25 OSO3 Li)) on the electrical conductivity of a system of poly(ethylene oxide)-LiCF3 SO3 -SiO2 . The ionic conductivity rapidly increases with increasing LDS content and then decreases after reaching a critical value at a certain weight fraction of LDS. The weight fraction of LDS to produce the conductivity peaks is the amount necessary for producing a monolayer on the surface of SiO2 . Further increases in LDS content led to a reduction in conductivity. Functionalization of SiO2 in a poly(ethylene glycol)-based polymer electrolyte using 2-[methoxy(polyethylenoxy)propyl] trimethoxy silane was reported to enhance electrical conductivity as well as the tensile strength of composites (the tensile strength was increased by about 1 Mpa compared to that using pristine SiO2  [109]. A small improvement in conductivity compared to pristine SiO2 was observed for small loading levels of surfactant. It was assumed that silane moieties attached on the SiO2 surface effectively prevent coagulation of the SiO2 particles during blending because of steric repulsive forces. It then increases the dispersibility of SiO2 powder in the PEO matrix [109]. The relaxation of chain segments is also improved by adding treated SiO2 . In poly(ethylene oxide)-based electrolytes, Li+ ions are coordinated to oxygen atoms in the polymer chains. Movement of the dissociated Li+ ions can be constrained by multiple oxygen atoms coordinated to the same Li+ central ion. Upon the addition of functionalized SiO2 , the oxygen atoms from the short polyether units on the surface of SiO2 compete with the oxygen atoms from the polymer chains for complexion with Li+ . The result is a more relaxed coordination between oxygen atoms and Li+ ions, which in turn, facilitates the transport of Li+ ions through the polymer. Croce et al. observed a difference in conductivity when acidic, neutral, and basic Al2 O3 were used as fillers [143]. A specific interaction between the surface group of ceramic particles and both the PEO segment and lithium salt ions

Log σ [S/cm]

–4.5

T = 40 °C

–5.0

was assumed to exist, which can be attributed to Lewis acidbase interactions. This interaction may act as cross-linking centers for the PEO segments and for the X− anions to lower the PEO reorganization tendency and thus promoting the structure modification of the polymer chains. The effect promotes Li+ conducting pathways at the ceramic surface. Second, Lewis acid-base interactions between the electrolyte ionic species lower ionic coupling. The expected result is the promotion of salt dissociation via a type of “ion-ceramic complex” formation. The surface of the filler also affects the transference number of ion in the composites. Table 8 shows the effect of acid-base properties of the filler on the transference number in a system of P(EO)20 :LiCF3 SO3 containing Al2 O3 [143]. Capiglia et al. investigated the properties of composite electrolytes consisting of SiO2 powder with different surface features in PEO:LiClO4 system. They removed the hydroxyl groups on SiO2 by calcinations at 900  C in order to reduce the hygroscopic properties of the particles. The reduction of hydroxyl group on the particle surface was found to increase the ionic conductivity [144]. Walls et al. prepared series of SiO2 material with hydrophilic and hydrophobic surface groups and used them as filler in PEGDM system. They showed clearly a cause and effect relationship between the observed properties and the surface chemistry [145]. Fan et al. attached different surface group to the silica and used the functionalized particles in PEGDM-based composite electrolyte. They found that the silica surface chemistry affected mostly the rheological properties, but not the ionic conductivity [146].

11.5. Effect of Particle Size on Conductivity The use of nanometer- and micrometer-sized Al2 O3 in a system of PEO:LiBF4 improved the electrolyte mechanical and electrochemical performance with decreasing size of ceramic filler. Nanometer-sized particles provide an order increase in conductivity compared to micrometer-sized filler [36]. Chandra et al. observed a decrease in the conductivity at 333 K of two orders of magnitude when particle size was increased from 10 to 100 #m. However, only a small decrease was observed at 373 K when the particle size was increased in the same range [147]. Wieczorek et al. reported a decrease in conductivity when the particle size is increased from 2 to 7 #m [148]. Figure 30 shows the Arrhenius plot of electrical conductivity of a system of PEO-based polymer electrolyte nanocomposites at different filler sizes [148]. The dependence of conductivity on the filler size can be described qualitatively using a space-charge layer concept Table 8. Lithium transference number at 95  C.

–5.5 0

10

20

30

40

50

LDS/(SiO2+LDS) wt%

Figure 29. Effect of surfactant weight fraction on the electrical conductivity of PEG:LiCF3 SO3 containing LDS coated SiO2 nanoparticles. Data points were extracted from [43], Y. Matsuo and J. Kuwano, Solid State Ionics 79, 295 (1995).

Samples

t+

P(EO)20 LiCF3 SO3 P(EO)20 LiCF3 SO3 + 10%Al2 O3 (basic) P(EO)20 LiCF3 SO3 + 10%Al2 O3 (neutral) P(EO)20 LiCF3 SO3 + 10%Al2 O3 (acidic)

046 048 054 063

Adapted with permission from [143], F. Croce et al., Electrochim. Acta 46, 2457 (2001). © 2001, Elsevier.

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Polymer Electrolyte Nanocomposites –3

Table 9. Effect of EC and PEG plasticizers on the electrical conductivity at 120  C.

–4

Log σ [S/cm]

Electrolytes

Plasticizer

PEO9 PMMA05 :Lil PEO9 PMMA05 :Lil PEO6 PMMA06 :Lil PEO6 PMMA06 :Lil

–5

–6

Conductivity (S/cm)

Filler Al2 O3 Al2 O3 Al2 O3 Al2 O3

ECl PEG05

6 6 6 6

0.6×10−3 1.2×10−3 0.4×10−3 1.0×10−3

wt.% wt.% wt.% wt.%

Adapted with permission from [151], D. Golodnitsky et al., Solid State Ionics 85, 231 (1996). © 1996, Elsevier. –7

–8

2.83

3

3.19

3.41

1000/T [K–1]

Figure 30. Arrhenius plot of (square) PEO:LiBF4 , (diamond) PEO: LiBF4 containing 10 wt.% Al2 O3 (micrometer size), and (circle) PEO:LiBF4 containing 10 wt.% Al2 O3 (nanometer size). Data points were extracted from [148], W. Wieczorek et al., Electrcochim. Acta 40, 2251 (1995).

that is commonly used to explain similar effect in a composite of solid-state ionics. The origin of space-charge layer is ascribed to the difference in the free energy formation of the individual vacancies in case of Schottky disorder and of the vacancies and the interstices in the case of Frenkel disorder. The space-charge layer for a two-phase composite of solid-state ionics has been introduced by Maier, who found the enhanced conductivity of the composite material as [149]
 √ L ,,o RT #v Nvo (63)  = 1 − Lo + 3 2GL r Vm where o = conductivity of filler-free polymer electrolytes, L = volume fraction of filler, GL = a constant to account for the topology of filler, r = filler particle radius, , = dielectric constant, ,o = permittivity of vacuum, Vm = bulk molar volume of the matrix phase, #v = the mobility of dominant defect, and Nvo = the mole fraction of dominant defect at the interface. This equation can fit the data of conductivity of NaCl containing various sizes of Al2 O3 [150].

type for all plasticizer content. However, above 50  C, the temperature dependence of conductivity seems to change with the change in the content of plasticizer. For 5 wt.% plasticizer (ethylene carbonate), there is a temperature Tm above which the Arrhenius plot of conductivity was observed and below which a qualitative change in the curve was observed. The discontinuity in the slope of the curve of the unplasticized composite disappears. At higher plasticizer content, a distinct change in the temperature-dependent conductivity was observed.

11.7. Effect of Storage Time on Conductivity Shin et al. measured the effect of storage time on the electrical conductivity of polymer electrolyte composites of PEO10 :LiCF3 SO3 containing 5–15 wt.% of Tin O2n−1 stored at 90  C. The conductivity increased with increase in the storage time until approximately 5 h, when the conductivity reached a steady-state value. The conductivity of polymer electrolytes containing ceramics reaches the steady-state value faster than ceramic-free polymer electrolytes [153]. The conductivity of PEO8 :LiClO4 containing 10 wt.% SiO2 is relatively stable up to two weeks storage as displayed in Figure 31 [154]. Croce et al. suggested that the recrystallization may occur with kinetics which appear to be critically dependent on the annealing condition [9]. 2.50

2.25

11.6. Effect of Plasticizers on Conductivity

2.00

σ [×10–5 S/cm]

The plasticizers have a great influence on the electrical, chemical, and electrochemical properties of the polymer electrolyte composites. The addition of ethylene carbonate results in a more homogeneous and transparent film. Golodnitsky et al. observed a conductivity enhancement by a factor of two in system of LiI:PEO9 PMMA05 + 6 vol.% Al2 O3 when ECl is added [151]. Enhancement about seven times was observed in a system of LiI:PEO6 PMMA05 + 6 vol.% Al2 O3 if PEG05 is added as appears in Table 9 [151]. Leo et al. [152] observed the different temperature dependence of conductivity when plasticizer is added into the polymer composites. Different weight percent of plasticizer alters the shape of log  against 1/T . The temperature dependence of conductivity below 50  C shows an Arrhenius

1.75

1.50

1.25

1.00

0

2

4

6

8

10

12

14

Time [days]

Figure 31. Effect of storage time on the conductivity of PEO8 :LiClO4 containing 10 wt.% SiO2 . Data points were extracted from [154], G. Appetecchi et al., Electrochim. Acta 45, 1481 (2000).

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Polymer Electrolyte Nanocomposites

Appetecchi et al. compared the conductivity of ECDMC-PAN:LiPF6 containing 6 wt.% Al2 O3 and EC-DMCPAN:LiPF6 free of Al2 O3 . They found that, after 30 days aging at 75  C, there was no significant change in the conductivity of ceramics containing polymer electrolytes, while the ceramic-free polymer suffered a depression in conductivity to become about 10% of the initial value [155].

12. MECHANICAL PROPERTIES 12.1. Effect of Filler Content Weston and Steele used ceramic filler to improve the mechanical properties of PEO films [156]. Addition of ceramic filler to improve the mechanical properties of polymer is not new. The addition of carbon black filler can extend the lifetime of tires from 5,000 miles if no carbon black is used to a potential 80,000 miles in some current tires. Figure 32 shows the effect of filler content on the elastic modulus of a system of poly(ethylene glycol) dimethyl ether:LiN(CF3 SO2 2 containing fumed silica with a surface group modified to become Si-C8 H17 instead of Si-OH. Increasing the concentration of the filler generates a stronger network structure with the modulus increasing by two orders of magnitude [157]. Although addition of filler is effective in hardening the polymer electrolytes at lower temperature, the effectiveness becomes lower at temperatures higher than the melting point of the matrix. Wen et al. used a whisker to improve the mechanical strength of polymer electrolyte composites. The work is based on the morphology of whisker which has a special shape similar to rigid network formed inside the polymer electrolytes. The mechanical strength of the polymers was evaluated with their thermal creep behavior, indicated by the relative resistance change of the polymer electrolyte films after being kept for some time under a certain pressure [113]. Addition of ceramic filler in a composite of PEO8 :LiClO4 remarkably affected their

thermal creep properties, in particular at high temperatures. The electrolytes containing whisker filler are much more mechanically stable than those containing nanosized -Al2 O3 . However, at low temperatures (below the melting point), the thermal creep properties of both composites are similar. Leo et al. observed improvement of mechanical strength of PEO-based polymer electrolytes by a factor of three when 7.5 wt.% Nasicon glass-ceramic fillers is added [152]. This filler content also increased the glass temperature of about 14  C. It is suggested that the addition of filler increases the stiffness of polymer segment, thereby suppressing the polymer chain motion [158].

12.2. Effect of Particle Surface Inorganic fillers are actually bonded to the molecular chain and thereby immobilize the polymer chain. The degree of adhesion between the polymer matrix and the fillers, the surface area of the filler, and the packing characteristic of the filler are important factors that determine the mechanical characteristics of the composites [159]. Liu et al. compared the effect of tensile strength of composites of PEO:LiBF4 +SiO2 containing pristine SiO2 and organic coated SiO2 [109]. The SiO2 was coated using 2-[methoxy(polyethylenoxy)-propyl] trimethoxy silane. Figure 33 shows the effect of filler loading on the tensile strength. It can be seen that the trend in the tensile strength variation is similar for both composites. However, the tensile strength of composite made using coated SiO2 filler is always larger than that of material made by pristine SiO2 . A double enhancement was observed in all filler loading region. Functionalized SiO2 might be considered as a filler which carries a pendant coupling agent. The coupling agent has two types of functional groups, one capable of forming chemical bond with the surface of the filler and the other capable of entangling with PEO chains [160]. 3

6

PEO + pristine SiO2

10

Tensile strength [Mpa]

Elastic modulus [Pa]

PEO + funcionalized SiO2

105

104

2

1

PEO 3

10

0 0

5

10

15

20

25

Fumed silica weight %

Figure 32. Effect of weight fraction of filler on the elastic modulus of PEG dimethyl ether:LiN(CF3 SO2 2 containing C8 H17 coated fumed silica. Data points were extracted from [152], C. J. Leo et al., Solid State Ionics 148, 159 (2002).

4

8

12

16

20

wt.% SiO2

Figure 33. Effect of filler content on the tensile strength of PEO:LiBF4 containing pristine SiO2 (open circle) and functionalized SiO2 (solid circle). Data points were extracted from [109], Y. Liu et al., J. Power Sources 109, 507 (2002), and lines are for eye guide.

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Polymer Electrolyte Nanocomposites

13. THERMAL PROPERTIES

80

13.1. Effect of Filler Content on the Glass Temperature

70

60

∆Hm [J/g]

Franco et al. observed the reduction in the glass transition temperature in composite of PEO:LiClO4 containing carbon black (conducting filler) [97]. Tg = 220 K was observed for composite containing 10 wt.% carbon black, 210 K for that containing 20 wt.% carbon black, and 202 K for that containing 30 wt.% carbon black. Choi and Shin measured the effect of the filler loading on the glass temperature of a system of PEO16 LiClO4 + SiC. Tg first decreases rapidly with the increase in the filler content and then reaches a plateau beyond which it further drops as can be seen in Figure 34 [161]. On the contrary, Chung and Sohn observed also an enhancement of glass temperature as the concentration of salt increases in a system of polymer comb-shaped polymer matrix by attaching triethoxyethylene side chain on the main chain of polyethylene oxide [162]. Wieczorek et al. observed that the SiO2 filler which enhanced the conductivity raised the Tg [163], while the -Al2 O3 filler which reduced the conductivity above room temperature lowered the Tg [164]. The data indicated that the variation of Tg neither exhibits systematic results nor meets the expected variation of conductivity. Filler content also affects the melting enthalpy of composites. Figure 35 shows the effect of filler loading on the melting enthalply extracted from DCS curve for system of PEO16 :LiClO4 +SiC, and the normalization relative to the mass fraction of pure (PEO)16 :LiClO4 [161]. Using Eq. (54) and remembering Hm c is proportional to mass fraction of pure (PEO)16 :LiClO4 , curve (b) reflects the crystallinity of the composite. It is clear that the crystallinity increases with the loading of the filler at low loading and then decreases with further increase in the filler content. Choi and Shin proposed the enhancement of crystallinity with the loading content of filler is due to the role of the filler which may act as nucleation centers of crystalline polymer phase. The

50

40

30

20

0

10

20

30

40

50

60

70

SiC wt.%

Figure 35. Effect of filler content on the melting enthalpy (solid circle) and crystallinity (open circle) of PEO16 :LiClO4 containing SiC. Data points were extracted from [161], B. Choi and K. Shin, Solid State Ionics 86–88, 303 (1996).

nucleation effect was considered to be sufficient to overcome the hindrance of crystallization due to the enhancement of the segmental motion of the PEO [161].

13.2. Effect of Salt Concentration on the Glass Temperature Marcinek et al. observed the effect of salt concentration of the system of PEG:LiClO4 + -Al2 O3 and found that the glass temperature also increases with the salt concentration as appears in Figure 36 [137]. For concentrations up to 0.25 mol/kg salt concentration, the Tg is almost constant. At higher salt concentrations, the plot of Tg with respect to the logarithm of salt concentration likely increases with an exponential trend. They also compared the variation of glass temperature of system free of filler and found that at up to 0.26 mol/kg salt concentration, the Tg of both samples is almost similar.

–25 230 –30

Tg [K]

Tg [°C]

220 –35

–40

–45

–50

210

200

0

20

40

60

SiC wt.%

190

10–6

10–4

10–2

100

Salt cocentration [mol/kg]

Figure 34. Effect of filler content on the glass temperature of PEO16 : LiClO4 containing SiC. Data points were extracted from [161], B. Choi and K. Shin, Solid State Ionics 86–88, 303 (1996), and line is for eye guide.

Figure 36. Effect of salt concentration on the glass temperature of PEG:LiClO4 . containing -Al2 O3 . Data points were extracted from [137], M. Marcinek et al., Solid State Ionics 136–137, 1175 (2000).

758

14. LUMINESCENT COMPOSITES Most fillers used in polymer electrolyte nanocomposites are “optically inactive,” that is, they do not have any luminescence properties. It would be interesting to investigate the properties of composite if the fillers loaded in polymer emit luminescence. This approach would have interesting applications such as the possibility of obtaining information regarding the degradation of battery or fuel cells using such composites, for example based on the luminescence intensity emitted by the filler. Changes in properties of the polymer when the battery or fuel cell degrades from the initial performance would be expected. This indicates that the properties of the medium around the filler would change when the battery or fuel cell degrades. It would affect the detected luminescence intensity emitted by the filler change. Based on this detected luminescence, the current performance of the battery or fuel cell could be determined. Zinc oxide (ZnO) is one of the promising materials for preparing composite polymer electrolytes that are luminescent. ZnO nanoparticles play a role as an agent for reducing the tendency of the polymer matrix to crystallize and simultaneously as luminescence centers. Since the luminescence spectra of ZnO are dependent on crystalline size (quantum size effect), the emitted wavelength can be controlled by adjusting the size of the ZnO nanocrystallites. ZnO particles of different sizes can be produced easily using, for example, a colloidal process, so that the “color” of the composite can be easily tuned. A “blue composite” can be prepared using smaller ZnO nanoparticles, while a “yellow composite” can be produced using large ZnO particle size. This composite has also considerable potential for fabricating devices that simultaneously produce both electrical current and light, such as luminescent electrochemical cells, self-powered displays, etc. Figure 37 (right) shows an example of a photoluminescence (PL) spectrum of composite polymer electrolytes containing ZnO nanoparticles [62]. The sample produced a highly intense green luminescence. The color of those samples can clearly be observed by the naked eye even using a hand-held UV lamp as background illumination in the laboratory. The PL peaks can be shifted to lower or higher wavelengths by altering the concentration of the precursors. The luminescence spectrum was produced by an electron transition from the bottom of conduction band to a state located near the center of the band. The corresponding excitation spectra of the samples are shown in Figure 37 (left). A peak is related to the bandedge transition. The wavelength peak is smaller than that observed for bulk ZnO, which shows a peak at 365 nm (energy about 3.4 eV). The shift of the band-edge peak to lower wavelengths indicates the presence of a quantum size

Intensity [-]

At higher salt concentration, the Tg of filler-free samples is higher that that of sample containing filler. Sun et al. found that the glass temperature of a system of poly(N ,N dimethylaminoethyl methacrylate)-tetraglyme:LiClO4 polymer electrolytes increases strongly with the increasing salt concentration over the first 0.5 mol/kg addition of LiClO4 . Further addition of salt results in only a minor increase in the glass temperature [165].

Polymer Electrolyte Nanocomposites

Excitation

300

Luminescence

400

500

600

Wavelength [nm]

Figure 37. Left: Excitation spectra of PEG:Li containing ZnO nanoparticles. Right: The corresponding luminescence spectra. Data were extracted from [62], Mikrajuddin et al., J. Electrochem. Soc. 149, H107 (2002).

effect of the optical bandgap. Using the size-dependent optical bandgap equation, the size of ZnO nanoparticles in the matrix is estimated to be around 6 nm. The in-situ growth of nanoparticles in the polymer matrix ensures the dispersion of nanoparticles in polymer bulk easily without the need for an intensive mixing process. The selection of a precursor containing lithium ions permits the insertion of cations during the growth process. This approach has been used to produce composite polymer electrolytes containing ZnO nanoparticles using lithium hydroxide as one precursor and a hygroscopic solution of zinc acetate as another precursor. A high molecular weight polyethylene glycol was first mixed with a solution of lithium hydroxide and then further mixed with a hygroscopic solution of zinc acetate. Since lithium did not participate in the formation of zinc oxide, lithium ions were left in the polymer matrix and were then able to take part as ion carriers.

15. CONCLUSION Stable and high electrical conductivity polymer electrolytes are required for fabricating flexible and environmentally friendly fuel cells and batteries. Some important parameters critical to polymer electrolytes in order to bring this material into industries have been discussed in this review. Although high electrical conductivity of polymer electrolytes usually appears at high temperatures, many efforts to improve the electrical conductivity at room temperature have been introduced by researchers all around the world. They include the synthesis of amorphous polymer matrix, addition of second phase in the host polymer matrix (plasticizers) like low molecular weight polymers, and addition of size chains. The most popular approach now is the addition of ceramic filler in nanometer scale. This approach becomes interesting since the addition of nanometer-sized ceramic filler improves not only the electrical conductivity of polymer electrolytes but also the mechanical properties. This material, usually named

Polymer Electrolyte Nanocomposites

as polymer electrolyte nanocomposite, attracts a lot of attention because of the simplicity in the processing. Various kinds of polymer in a lot of molecular weights as well as various kinds of ceramic filler in a lot of sizes are available in the market. The process of production of composites is relatively simple, that is, by just mixing the polymer, salt, and filler in a common solution, ended with drying. Methods of preparation of polymer electrolyte nanocomposites were briefly discussed in this review. Characterizations of the prepared films are important to understand the properties as well as to find the optimum preparation conditions. Various methods of characterization, including electrical, spectroscopic, microscopic, and thermal characterizations, were discussed briefly. Detailed explanations of the methods might be found in some analytical chemistry books. In the final part, we discussed some parameters which affect the properties of polymer electrolyte composites. It was found that the properties of the composites are sensitive to the preparation condition as well as material used for making the composites. For example, the electrical conductivity is greatly dependent on the volume fraction of fillers, surface property, filler size, type of salt, side chain, etc. Also the mechanical properties are affected by filler content, surface property, and salt concentration.

GLOSSARY Anion An ionic species having a negative charge. Battery A device that stores energy and makes it available in an electrical form. Blending Physically mixing several types of materials so thoroughly that they appear to be indistinguishable from each other in the product. Casting method A method of forming sheets of composite materials by pouring fluid materials onto a flat surface. Cation An ionic species having a positive charge. Charging Supply electric current to a battery. Copolymerization Polymerization with two or more different monomers. Cycle life The number of charge/discharge cycles that are possible before failure occurs. Differential scanning calorimetry A materials characterization laboratory technique by which the temperature of a sample of the substance in question is raised in increments while a reference is heated in the same rate. The amount of heat that is required to heat the sample and the reference to each temperature is recorded and can be plotted. From these plots, melting points, phase change temperatures, chemical reaction temperatures, and glass transition temperature of polymers can be determined. Discharging Withdrawing charge from a battery. Doctor blade technique A technique using a flat bar used for regulating the amount of liquid material on the rollers of a coating machine, or to control the thickness of a coating after it has been applied to a substrate. Electrical conductivity A measure of how well a material accommodates the transport of electric charge.

759 Energy density The energy obtainable per unit volume. Energy efficiency (energy released on discharge)/(energy required for charge). Filler Inorganic particles added to polymer matrix. Fourier transform spectroscopy 1A measurement technique whereby spectra are collected based on the response from a pulse of electromagnetic radiation. Fourier transform spectroscopy is more sensitive and has a much shorter sampling time than conventional spectroscopic techniques. Free volume The extrapolated differences in volume between a glass and the extrapolation of the melt curve is called the free volume. The free volume is associated with the space between molecules in a sample. Glass transition temperature (Tg ) The temperature below which molecules have very little mobility. On a larger scale, polymers are rigid and brittle below their glass transition temperature and elastic above it. Tg is usually applicable to amorphous phases and is commonly applicable to glasses and plastics. Hot press The forming of a compact material at temperatures sufficiently high to cause concurrent sintering. Melting point The temperature at which it changes state from solid to liquid. When considered as the temperature of the reverse change, the temperature is referred to as the freezing point. Nuclear magnetic resonance (NMR) A physical phenomenon described independently by Felix Bloch and Edward Mills Purcell in 1946. It involves the interaction of atomic nuclei placed in an external magnetic field with an applied electromagnetic field oscillating at a particular frequency. Magnetic conditions within the material are measured by monitoring the radiation absorbed and emitted by the atomic nuclei. NMR is used as a spectroscopy technique to obtain physical, chemical, and electronic properties of molecules. It is also the underlying principle of Magnetic Resonance Imaging. NMR is one of the techniques used to build quantum computers. Number average molecular weight A way of determining the molecular weight of a polymer. Polymer molecules, even ones of the same type, come in different sizes (chain lengths, for linear polymers), so we have to take an average of some kind. The number average molecular weight is the common average of the molecular weights of the individual polymers. It is determined by measuring the molecular weight of n polymer molecules, summing the weights, and dividing by n. The number average molecular weight of a polymer can be determined by osmometry, end-group titration, and colligative properties. Plasticizers Materials added to polymer matrix to improve the fraction of amorphous state. Polymer electrolytes Electrolytes which using polymer as a media to dissociate ions. Rechargeable batteries Batteries that can be restored to full charge by the application of electricity. They come in many different designs using different chemistry. Small angle x-ray scattering A laboratory technique in which photons are elastically scattered from a sample. The sample can not be too thick. Liquids can be examined. Features on the nanometer length scale can be examined.

760 Specific energy The energy obtainable per unit weight. Weight average molecular weight A way of determining the molecular weight of a polymer. Polymer molecules, even if of the same type, come in different sizes (chain lengths, for linear polymers), so we have to take an average of some kind. For the weight average molecular weight, this is done as follows: weigh a number of polymer molecules, add the squares of these weights, and then divide by the total weight of the molecules. Intuitively, if the weight average molecular weight is w, and you pick a random monomer, then the polymer it belongs to will have a weight of w on average. The weight average molecular weight can be determined by light scattering, small angle neutron scattering (SANS), and by sedimentation velocity. Wide angle x-ray scattering An X-ray diffraction technique that is often used to determine the crystalline structure of polymers.

ACKNOWLEDGMENT Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship for Mikrajuddin Abdullah is gratefully acknowledged.

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