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Electrochimica Acta xxx (2011) xxx– xxx

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Electrochimica Acta

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ffect of lithium salt concentrations on blended 49% poly(methyl methacrylate)rafted natural rubber and poly(methyl methacrylate)ased solid polymer electrolyte

.S. Su’ait a,b,∗, A. Ahmada,b, H. Hamzaha,b, M.Y.A. Rahmanc

Polymer Research Center, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, MalaysiaSchool of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, MalaysiaCollege of Engineering, Universiti Tenaga Nasional, 43009 Kajang, Selangor, Malaysia

r t i c l e i n f o

rticle history:eceived 30 November 2010eceived in revised form 5 June 2011ccepted 6 June 2011vailable online xxx

eywords:9% poly(methyl methacrylate)-graftedatural rubber (MG49)

onic conductivityithium perchlorate (LiClO4)ithium tetrafluoroborate (LiBF4)oly(methyl methacrylate) (PMMA)olid polymer electrolyte

a b s t r a c t

The effect of lithium salts (lithium tetrafluoroborate, LiBF4 and lithium perchlorate, LiClO4) as dop-ing salts in rubber-polymer blends, 49% poly(methyl methacrylate) grafted natural rubber (MG49) andpoly(methyl methacrylate) (PMMA) in solid polymer electrolyte (SPE) film for electrochemical devicesapplication was investigated. The electrolyte films were prepared via the solution casting technique using0–25 wt.% lithium salt. The effect of the lithium salts on chemical interaction, ionic conductivity and struc-tural and morphological studies of (70:30) MG49-PMMA films was analyzed using Fourier TransformInfrared (FT-IR) Spectroscopy, Electrochemical Impedance Spectroscopy (EIS), X-ray Diffraction (XRD)and Scanning Electron Microscopy (SEM). Infrared analysis showed that the interactions between lithiumions and oxygen atoms occur at the ether group (C–O–C) (1500–1100 cm−1) on the MMA structure in bothMG49 and PMMA. The oxygen atoms in the structure of the polymer host act as electron donor atomsand form a coordinate bond with the lithium ions from the doping salt to form polymer–salt complexes.The ionic conductivity was investigated at room temperature as well as at a temperature range from303 K to 373 K. The ionic conductivity without the addition of salt was 1.1 × 10−12 S cm−1. The highestconductivity at room temperature for (70:30) MG49-PMMA–LiBF4 was 8.6 × 10−6 S cm−1 at 25 wt.% ofLiBF4. The ionic conductivity of (70:30) MG49-PMMA–LiClO4 was 1.5 × 10−8 S cm−1 at 25 wt.% of LiClO4.However, both electrolyte systems do not exhibit Arrhenius-like behavior. Systems with LiBF4 salt havehigher ionic conductivity than those with LiClO4 salt because of the differences in anionic size and latticeenergy of the appropriate salt. The observations from structural and morphology studies showed that
complexation and re-crystallization occur in the system. The XRD studies showed a reduction of the MMApeak intensity at 29.5◦ after the addition of 5–25 wt.% LiBF4 salt due to ion dissociation in the electrolytesystem. Thus, this contributed to the increase of ionic conductivity in (70:30) MG49-PMMA–LiBF4. Mor-phological studies showed that (70:30) MG49-PMMA is homogenously blended, and no phase separationoccurred. The addition of lithium salts changed the topological texture from a smooth and dark surfaceto a rough and bright surface.
. Introduction

The observation of ionic conductivity in polymer materials com-lexed with salts by Fenton et al. [1] has led to the development

Please cite this article in press as: M.S. Su’ait, et al., Effect of lithium saltnatural rubber and poly(methyl methacrylate) based solid polymer electro

f electrochemical device applications [2]. Thus far, research onoly(methyl methacrylate) (PMMA) based polymer electrolytes hasnly been conducted in 1985 by Iijima and co-workers [3]. PMMA

∗ Corresponding author at: Polymer Research Center, Faculty of Science and Tech-ology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia.el.: +60 389215439; fax: +60 389215410.

E-mail addresses: [email protected], ady [email protected] (M.S. Su’ait).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.06.015

© 2011 Elsevier Ltd. All rights reserved.

has been used as a polymer host due to its high stability at thelithium-electrolyte surface and because it is less reactive towardsthe lithium electrode [4]. It induces a more favorable passivationfilm on the electrode surface. Therefore, a higher cyclability oflithium electrodes in PMMA-based electrolytes is expected. In addi-tion, the MMA monomer in PMMA has a polar functional group inthe main polymer chain that has a high affinity for lithium ions,which are transported. Oxygen atoms from the MMA structure willform a coordinate bond with the lithium ion from doping salts

concentrations on blended 49% poly(methyl methacrylate) graftedlyte. Electrochim. Acta (2011), doi:10.1016/j.electacta.2011.06.015

[5,6]. This polar group in the MMA acts as a stiffener that increasesion transport, which occurs through a continuous conduction paththat does not affect the electrochemical stability of the electrolyte[7]. However, a plasticized gel-based PMMA electrolyte system

dx.doi.org/10.1016/j.electacta.2011.06.015


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Fig. 1. Structure of the MG monomer.

hows poor mechanical properties towards the electrode. This isecause of the insufficient mechanical strength to hold the pressureetween the anode and cathode. In this study, PMMA was blendedith modified natural rubber to improve the mechanical properties

nd to overcome the brittle properties.Recently, modified natural rubber (NR), such as epoxidized NR

ENR) and PMMA-grafted NR (MG) based polymer electrolytes, hasrawn the attention of many researchers [6,8–14]. Modified NR hasttractive attributes, such as low glass transition temperature (Tg);g ENR = −25 ◦C, Tg MG49 = −60 ◦C [8], soft elastomer characteriza-ion at room temperature and good elasticity. A suitable elasticityan result in flat and flexible film. Therefore, an excellent contact isxpected between an electrolytic layer and an electrode in a batteryystem. However, ENR has some drawbacks due to its mechan-cal properties, such as a slight stickiness and difficulty peelingff of the substrate [9,12,15], compared to MG film that is moreree-standing, elastic and flexible. However, modified NR, such as

G30 and MG49, has an oxygen atom in the MMA monomer, whichan act as an electron donor atom in the structure of the poly-er host. The oxygen atom with a lone pair of electrons forms a

oordinate bond with the cation from metal salts, resulting in theormation of polymer-complexes [11,13]. MG49 was selected forhis study because 49% is the highest percentage of PMMA that cane grafted on natural rubber, whereas that percentage is just 30%MMA for MG30. Fig. 1 shows the structure of the MG monomer14]. Furthermore, modified NR with a polar group can also act as aolymeric solvent, and the ionic conductivity is higher in compar-

son to the glassy or crystalline states of the polymer [2]. Previoustudies on various MG rubbers have been conducted elsewhere6,10,11,14].

Metal salts play an important role in ion-conducting elec-rolytes. Therefore, the selection of suitable metal salts, with regardo the cation and anion size, needs to be performed prior to theesign of an electrolyte system. In this research, lithium salt washosen because it contains the lightest properties of all metals,roviding high gravimetric coulombic density despite having the

ow transition number of one electron per lithium atom [16]. Themaller cation size of the lithium ion could contribute to ion dissoci-tion resulting from coulombic interaction forces between the twoppositely charged ions [17]. Other factors, such as cation polaritynd a large anion size, are required for the delocalization of ionicharge, which could minimize the lattice energy [2]. Gray [2] andhanshan Wang [18] explained that the anion size affects the solu-ility of the salts. High ionic conductivity can be achieved with the


ncrease of anionic size, as shown below:

Cl− < I− ∼ SCN− < ClO4− ∼ CF3SO3

− < BF4− ∼ AsF6

−

PRESS Acta xxx (2011) xxx– xxx

Lithium salts, such as LiClO4 and LiBF4, are used as dopants inpolymer electrolyte systems because they can behave as a Lewisacid. Therefore, they can interact with electron donor centers. Inaddition, these types of salt have low lattice energy, which maxi-mizes the ionic conductivity [19]. Other studies have reported thatLiClO4 salt is very stable at ambient moisture and less hygroscopicin comparison to LiCF3SO3 salt. LiClO4 salt gives a high solubilitydue to its low lattice energy value in plasticized solvent, with ionicconductivity around 10−4 to 10−3 S cm−1 [20]. However, the highoxidation state of chlorine (VII) in perchlorate makes it a strongoxidant, which readily reacts with most organic species in violentways under certain conditions, such as high temperature and highcurrent charge [21]. LiBF4 is a salt based on an inorganic super-acidanion and has less toxicity in comparison to LiAsF6 and LiClO4. Inaddition, BF4

− anions possess high ionic mobility compared withother lithium salts, even though it has a lower dissociation con-stant than LiPF6 and LiAsF6 [22]. Takami et al. [23] found thatthin lithium-ions batteries using LiBF4 salts give excellent perfor-mance and are very promising thin rechargeable batteries with highenergy density, high discharge performance, very low swelling forhigh-temperature storage and excellent safety.

In this work, MG49-PMMA polymer blends with a ratio of 70:30were doped with lithium salts for the preparation of solid polymerelectrolytes (SPE) by solution blending via the casting technique. Allsamples were characterized using Fourier Transform Infrared (FT-IR) Spectroscopy, AC electrochemical impedances spectroscopy(EIS), X-ray Diffraction (XRD) and Scanning Electron Microscopy(SEM). It was expected that the lithium salts would raise theconductivity in (70:30) MG49-PMMA polymer blends and the con-ductivity between LiBF4 and LiClO4 salts would be significantlydifferent.

2. Materials and methods

2.1. Materials

MG49 was obtained under commercial name “MEGAPOLY” fromGreen HPSP (Malaysia) Sdn. Bhd., Petaling Jaya, Malaysia. PMMAwith low molecular weight, as well as the LiClO4 and LiBF4 salts,were supplied by Fluka Chemicals, Germany. All the materials wereused without further purification.

2.2. Sample preparation

All samples were prepared by solution blending via the cast-ing technique. MG49 was dissolved in stoppered flasks containingtoluene. After 24 h, the solution was stirred using efficient mag-netic stirring for the following 24 h until complete dissolutionof MG49 was achieved. PMMA solution was prepared in anotherstoppered flask containing toluene and stirred for 24 h. These twosolutions were then mixed for 24 h to obtain a homogenous solu-tion. LiClO4 salt was dissolved in THF solution for 12 h and addedto the solution for the next 24 h with continuous stirring. Theelectrolyte solution was then cast onto a glass petri dish, andthe solvent was allowed to slowly evaporate in a fume hood atroom temperature. Residual solvent was then removed in a vac-uum oven for 48 h at 50 ◦C. The samples were then stored in adesiccator until further use. The same experimental procedurewas repeated for different weight percents of LiClO4 and LiBF4salt.

2.3. Characterization


The FT-IR spectrum was recorded by a computer interfaced witha Perkin Elmer GX Spectrometer. The electrolyte was cast onto NaClwindows and analyzed in the frequency range of 4000 cm−1 to


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00 cm−1, with a scanning resolution of 4 cm−1. The ionic conduc-ivity measurements were carried out by EIS using a high frequencyesonance analyzer (HFRA) model 1255 with applied frequencyrom 1 MHz to 0.1 Hz at 1000 mV amplitude. The conductivity mea-urements were conducted at room temperature at a temperatureange of 303–373 K. The 16 mm in diameter disc-shaped sampleas sandwiched between two stainless steel block electrodes. The

onic conductivity (�) was calculated from the bulk resistance (Rb),hich was obtained from the intercept on the real impedance axis

Z′ axis) and the film thickness (l) and contact area of the thin filmA = �r2 = �(1.60 cm/2)2 = 2.01 cm2), in accordance with the equa-ion � = [l/(A·Rb)]. The conductivity, � dependence on temperature, is given by the Arrhenius equation, � = �oe(−Ea/kT), where �o, Ea

nd k represent the pre-exponential factor, activation energy andoltzmann constant, respectively.

The [O/Li+] ratio for the polymer blend was calculated by theollowing equation [24]:

O/Li+]

= MT × MW salt

Msalt × ((MW m/nm) + (MW MG49/nMG49) + (MW PMMA/nPMMA))

here MT = total mass of the polymer (MG49 + PMMA) (g),w salt = the molecular weight of salt (g mol−1), Msalt = the mass

f salt (g), Mw m = the molecular weight of monomer (g mol−1),w MG49 = the molecular weight of monomer MG49 (g mol−1), Mw

MMA = the molecular weight of monomer PMMA (g mol−1), nm = theotal number of oxygen atoms per repeated unit of monomerMG49 + PMMA), nMG49 = the number of oxygen atoms per repeatednit of monomer MG49 and nPMMA = the number of oxygen atomser repeated unit of monomer PMMA.

XRD model D-5000 Siemen was used to observe the appearancend disappearance of the crystalline or amorphous phase with theresence of lithium salts. The data were collected in the range ofiffraction angle 2� from 5◦ to 35◦ at a scanning rate 0.04◦ s−1. Theurface morphology of the sample was observed by using the SEM


odel Philips XL30 with 1000× magnification at 20 kV electroneam. The sample was fractured in liquid nitrogen and coated with

gold sputtered-coated machine before the analysis. The analysisas carried out at room temperature.

Wavenumber/cm-1 17001720174017601780

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(a)LiClO 4

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0 %

10%

Fig. 2. FT-IR spectrum of the symmetrical stretching of th

PRESS Acta xxx (2011) xxx– xxx 3

3. Results and discussion

3.1. FT-IR analysis

FTIR spectroscopy was used to observe the vibration energies ofcovalent bonds in the polymer host and the interactions that occurin the polymer–salt complexes. Due to the fact that each type ofbond has a different natural frequency of vibration, the identifica-tion of the absorption peak in the vibration portion of the infraredregion gives a specific type of bonding [25,26]. The main regionsof interest were the oxygen atoms of the carbonyl group (C O)(1750–1730 cm−1) and the ether group (C–O–C) (1300–1000 cm−1)from the MMA structure in MG49 and PMMA [25]. According tothe literature, the oxygen atoms in the structure of the polymerhost act as electron donor atoms and form a coordinate/dative bondwith the lithium ions from the doping salts to form a polymer–salt[4,5,11,13,20,27–31]. The vibration frequency of the polymer–saltcomplexes is subsequently shifted to lower a wavenumber by about15–25 cm−1 compared to the polymer hosts [25].

Fig. 2(a) and (b) shows the FT-IR spectrum of the symmetricalstretching of the carbonyl group, v(C O), from the MMA structurein the MG49-PMMA blend for LiClO4 and LiBF4, respectively. Thev(C O) frequency of MMA gave rise to an intense, very strong andsharp peak at 1733 cm−1. With the addition of 10 wt.% and 20 wt.%lithium salt in both systems, the intensity of the v(C O) of the MMApeak was reduced and shifted to 1735 cm−1. However, because theresolution was only within 4 cm−1, this result was not significantenough to prove the interaction that occurs in the carbonyl group.

The specific vibration mode of the ether group (C–O–C) fromthe MMA structure in the MG49-PMMA blend for LiClO4 and LiBF4could be observed at a stretching mode of –COO– and v(C–O), a sym-metrical stretching mode of vs(C–O–C), an asymmetrical stretchingmode of vas(C–O–C) and an –CH3 asymmetric deformation of MMA,ı(O–CH3). However, the vibration at the stretching mode of –COO–and v(C–O) at 1272 cm−1 and at the symmetrical stretching modeof vs(C–O–C) at 985 cm−1 was not significant enough to indicate theinteraction that occurred. Fig. 3(a) and (b) shows the asymmetricalstretching mode of vas(C–O–C) at 1150 cm−1 for the MG49-PMMA


blend. With the presence of 20 wt.% LiBF4 salts, the intensity ofvas(C–O–C) in MG49 systems reduced until it became lower thanits neighboring peak at 1178 cm−1. The peak shifted approximately28 cm−1 from its original position. Furthermore, the peak shifts

Wavenumber/cm-1

17001720174017601780

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20

40

60

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(b )

0 %2 0 %

1 0 %

LiBF4

e carbonyl group, v(C O) in (a) LiClO4 and (b) LiBF4.


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4 M.S. Su’ait et al. / Electrochimica Acta xxx (2011) xxx– xxx

-1

110011201140116011801200

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LiClO4

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70

80

(b)

0%

10%

20%

LiBF4

g of th

ooPrttTfpsClfg

Wavenumber/c m

Fig. 3. FT-IR spectrum of the asymmetrical stretchin

bserved in Fig. 4(a) and (b) suggests the asymmetric deformationf the MMA group, from ı(O–CH3) of 1455 cm−1 in the MG49-MMA blend to 1460 cm−1 and 1461 cm−1 for LiClO4 and LiBF4,espectively. The interactions between the atoms were weaker dueo the distraction that occurs on the molecule chain, indicated byhe frequency of the infrared spectrum that was shifted to the left.he peak shift confirms the interaction between the lithium ionsrom the doping salt and the oxygen atoms in the structure of theolymer host. The reduction, with broadening of intensity and peakhift, indicates that the remaining vibration frequency between


–O–CH3 bonding was disturbed by the interaction between theithium ions and oxygen atoms. This is because a new bond wasormed between the lithium ions from the doping salt and the oxy-en atoms in the structure of the polymer host, also known as a

Wavenumber/cm-1

140014201440146 014801500

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smitt

ance

(%)

0

20

40

60

80

(a)

0%

10%

20%

LiCl O4

Fig. 4. FT-IR spectrum of the asymmetric deformation

Wavenumber/cm

e ether group, vas(C–O–C) in (a) LiClO4 and (b) LiBF4.

coordinate/dative bond, which led to the formation of polymer–saltcomplexes [5,11].

The symmetric stretching of polyisoprene in the rubber chainof vs(C C) at 1604 cm−1 showed no changes in terms of both thepeak shift and intensity of the vibration for each sample [6]. Thissignifies that there was no interaction with the non-polar group inthe polymer chain.

3.2. Ionic conductivity


The typical impedance spectra are shown in Fig. 5. The compleximpedance spectra show two well-defined regions; a semicircle inthe high frequency range that is related to the conduction processin the bulk of complex, and the linear region in the low frequency

Wavenumber/cm -1

140 01420144 014601480150 0

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(%)

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(b)

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of MMA at ı(O–CH3) in (a) LiClO4 and (b) LiBF4.




M.S. Su’ait et al. / Electrochimica Acta xxx (2011) xxx– xxx 5

MA w

rasv[tbmtcbtT2TratrtTc

lcresbadUi

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Fig. 5. Impedance plot of SPE (70:30) MG49-PM

ange that is attributed to the bulk effect of blocking electrodes. Inn ideal case at low frequency, the complex impedance plot wouldhow a straight line parallel to the imaginary axis; however, the cur-ature was caused by the double layer at the blocking electrodes5,32]. No charge crossed the electrodes from the dielectric materialo the metal electrode for the blocking contacts, and vice versa. Theehavior of dielectrics under the application of steady voltage isainly dependent on the type of contacts between the metal elec-

rodes and the dielectric material. Therefore, the observed transienturrent was due to the polarization of the material, which may haveeen be caused by the hopping positive and/or negative charges inhe polymer electrolytes system, as reported elsewhere [33–35].he impedance spectra for (70:30) MG49-PMMA with (a) 0 wt.% to5 wt.% LiClO4 and (b) 0 wt.% to 25 wt.% LiBF4 are shown in Fig. 5.he impedance spectrum shows a semicircle in the high frequencyange, which resulting a smaller value of bulk resistant with theddition of lithium salts. This indicates that the resistance withinhe material decreased with the increasing amount of charge car-iers in the lithium salts. According to the equation � = [l/(A·Rb)],he bulk resistance (Rb) is the inverse of the ionic conductivity (�).herefore, the decrease of bulk resistance will increase the ioniconductivity of the polymer electrolytes.

The ionic conductivity and [O/Li+] ratio of (70:30) MG49-PMMA-ithium salts is shown in Table 1. The ionic conductivity without saltontent was 1.1 × 10−12 S cm−1. MG49-PMMA–LiClO4 salts gaveise to 1.5 × 10−8 S cm−1 at 25 wt.% of LiClO4. Meanwhile, the high-st conductivity was observed at 8.6 × 10−6 S cm−1 with LiBF4alts at room temperature, with the same salt concentration andlend composition. The increase in ionic conductivity of LiBF4 was


pproximately 600 times higher than that of LiClO4 because of theifference in the anion size. The atomic radius (r) was calculated bye [22] using the van der Waals volume of each ion in the follow-

ng equation: r = (3 V/4�)1/3. In that study the atomic radius for BF4−

able 1he ionic conductivity and [O/Li+] ratio (70:30) of MG49-PMMA-lithium salts at room tem

Sample LiClO4

Ionic conductivity, � (S cm−1) [O/Li+] ratio

0 wt.% 1.1 × 10−12 –

5 wt.% 7.0 × 10−11 30/1

10 wt.% 1.2 × 10−09 15/1

15 wt.% 7.8 × 10−09 10/1

20 wt.% 1.4 × 10−08 8/1

25 wt.% 1.5 × 10−08 6/1

ith 0 wt.% to 25 wt.% (a) LiClO4 and (b) LiBF4 salt.

was found to be 0.218 r/nm in comparison to ClO4− at 0.215 r/nm

[22]. Gray [2] and Shanshan Wang [8] explained that the size ofthe anion affects the solubility of the salts. The large anion size isrequired for the delocalization of the ionic charge, which could min-imize the lattice energy between the cation and anion. The smallersize of the lithium cation could contribute to the increase of iondissociation resulting from coulombic interaction forces betweenthe lithium cation and the large anion [17].

The ionic conductivity increased as the addition of salt increasedto its maximum level in the polymer host. The conductivity wasapproximately 7.8 × 106 times higher after the addition of 25 wt.%LiBF4 salt to the polymer host. This was due to the increase ofthe number of conducting species in the electrolyte. The optimumvalue indicates the maximum and effective interactions betweenthe oxygen atoms and Li+ ions in the electrolyte. This interac-tion was explained by FT-IR investigated by other researchers[5,6,10–13,28]. It was discovered that a coordinate bond wasformed in the complexes between the lithium ions and oxygenatoms from the polymer host. The maximum and effective inter-actions between oxygen atoms and lithium ions in the electrolytesystem can be presented by the ratio of oxygen atoms to lithiumions and can be simply written as the [O/Li+] ratio. Different val-ues of the [O/Li+] ratio could be due to the difference in molecularweight and weight percent of the lithium salt added. The ionicconductivity increased with increasing salt addition up to themaximum level in the polymer host due to the increasing LiBF4salt content that contributed to the ion dissociation of LiBF4 saltsinto Li+ and BF4

− species. The maximum and effective interactionfor LiClO4 salt was [6/1]. Meanwhile, the maximum and effective


interaction for LiBF4 salt was [5/1]. The increase of the conduct-ing species in the electrolyte helped to increase the conductivity[27]. Moreover, the ionic conductivity only increased by about1.3 × 104 after the addition of LiClO4 to the polymer host, which

perature.

LiBF4

Ionic conductivity, � (S cm−1) [O/Li+] ratio

1.1 × 10−12 –4.0 × 10−11 29/11.7 × 10−08 14/15.2 × 10−08 10/11.8 × 10−06 7/18.6 × 10−06 5/1




6 M.S. Su’ait et al. / Electrochimica Acta xxx (2011) xxx– xxx

Ft

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after they plotted a loss tangent against temperature for the high-est conductivity of PMMA-EC-LiBF4 samples. The ˇ-relaxation peakat 333 K was attributed to the main chain mobility below the glasstransition temperature, Tg. This indicates that the increase in seg-

ig. 6. Arrhenius plot for the (70:30) MG49-PMMA electrolyte at 25 wt.% salt con-ent.

s approximately 600 times lower than LiBF4. As previously men-ioned, LiBF4 (0.218 r/nm) has a larger anion size in comparisono LiClO4 (0.215 r/nm) [22]. The large anion size led to delocal-zation of the ionic charge that could have minimized the latticenergy between the Li+ and BF4

−. Therefore, the ion dissociationf the smaller cation, Li+ could have increased, resulting fromhe weakening of coulombic interaction forces in the media. Thiseakening of coulombic interactions between opposite charges isescribed by the dielectric constant of the medium. The polariz-bility of a medium can arise from the polarizabilities of the atomsr molecules that it is composed of, even if they have no permanentipole moment. Atoms or molecules that lack permanent dipolesave electronic polarizability, a tendency of electronic charge dis-ributions to shift slightly within the atom, in response to an electriceld.

The conductivity, � dependence on temperature T, is given byrrhenius equation: � = �oe(−Ea/kT), where �o, Ea and k represent there-exponential factor, activation energy and Boltzmann constant,espectively. The value for �o and Ea can be calculated from the-axis and plot intercept between log � and 1000/T [36].

Fig. 6 shows the dependence of conductivity on temperature byhe Arrhenius plot for the MG49-PMMA blend at 25 wt.% LiClO4 andiBF4 salts. It was observed that the conductivity increased with thencrease of temperature from 303 K to 373 K. However, the relation-hip between conductivity and temperature was non-linear. Thisndicates that neither electrolyte system exhibited Arrhenius-likeehavior. Therefore, the pre-exponential factor �o and activationnergy Ea of the electrolyte cannot be estimated from the plot.on-Arrhenius-like behavior has been explained by Kincs and Mar-

in [37], and the non-Arrhenius-like behavior was associated withynamic temperature dependence restructuring of the anion “sub-

attice”. Meanwhile, Noor et al. [24] suggested that the value of Ea

s due to the energy required to provide a conductive conditionor the migration of ions. This is because the activation energy is

combination of the energy of the charge carrier creation (defectormation) and the energy of ion migration that can be evaluated by

linear fit of the plot. However, Rajendran and Uma (2000) indicatehat the non-Arrhenius-like behavior corresponding to ion trans-ort in polymer electrolytes is dependent on the segmental motionf the polymer. Thus, the results may be effectively represented byhe empirical Vogel–Tamman–Fulcher (VTF) equation based on the


ree volume concept:

= AT − 1/2 exp[ −B

T − To

]

Fig. 7. XRD pattern of (70:30) MG49-PMMA–LiClO4.

where A and B are constants, T is the temperature in Kelvin (K) andTo = (Tg − 50), the temperature taken 50 K below the glass transitiontemperature (Tg). Constant A in the VTF equation is related to thenumber of charge carriers in the electrolyte system, and constant Bis related to the activation energy of ion transport associated withthe configurational entropy of the polymer chains.

The highest ionic conductivity for LiClO4 was 2.0 × 10−7 S cm−1

at 373 K. However, the highest ionic conductivity for LiBF4 was5.4 × 10−4 S cm−1, which is three magnitudes higher than that ofLiClO4 at the same temperature. The bulk resistance of the elec-trolyte could not be observed above the impedance spectrum atthis temperature because the sample was unstable in temperatureshigher than 373 K. Rahman et al. [38] believed that the increasedconductivity with temperature can be linked to the increasing chainflexibility of the polymer chain in the electrolyte. Thus, increasesthe segmental motion of the polymer chain. This will lead to theincrease in the dissociation rate of Li+, thus improving the mobil-ity of the charge carrier. Hence, the maximum conductivity of theelectrolyte occurs at a temperature below the glass transition tem-perature (Tg). However, the gap observed at temperatures above343 K for LiBF4 and below 333 K for LiClO4 in Fig. 6 was due to the ˇ-relaxation of the PMMA segment as reported by Othman et al. [36],


Fig. 8. XRD pattern of MG49-PMMA–LiBF4.


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ental motion of the polymer chain will also enhance the transportf ions in the polymer blend, resulting in the conductivity enhance-ent for the LiBF4 system above the relaxation temperature, 333 K.owever, the conductivity for LiClO4 was almost constant at 333 K,

ndicating that the segmental motion of the polymer chain doesot affect the conductivity of LiClO4 salts. As can be observed by


EM micrograph in Fig. 9(c) and (e), the number of salt particles inhe LiClO4 system was less than that observed in the LiBF4 system.f the remaining LiClO4 salt particles dissolve at high temperature,

Fig. 9. SEM micrograph of (70:30) MG49-PMMA at (a) 0 wt.%, (b) 15 wt.%


it will lead to the complete dissociation of its ionic species. Thissignifies that the number of conducting species has exceeded themaximum number of ionic species at this temperature. In addi-tion, this observation may be due to the thermal decomposition ofLiClO4. Lu et al. [39] reported that the first mass loss of LiClO4 saltswas observed at 335 K, which is related to the removal of HCl and


thermal decomposition reaction. The discharge of the oxygen ele-ment from LiClO4 during the decomposition process (oxygenation)could sometimes occur even under a nitrogen atmosphere, result-

and (c) 25 wt.% LiClO4 salt and (d) 15 wt.% and (e) 25 wt.% LiBF4 salt.


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ng in an overall exothermic effect. Meanwhile, the decompositionf LiBF4 requires energy to break some of its chemical bands. There-ore, the process showed endothermic behavior; in fact, LiBF4 salts an ate-complex of LiF and BF3. Consequently, they separate intoheir original composition at a high temperature, and the processs endothermic.

From our preliminary studies on the MG49 solid polymerlectrolyte [14], LiBF4 salt gives a higher ionic conductivity in com-arison to LiClO4 salt due to the difference in anion size and latticenergy of the appropriate salt. LiBF4 salt has low lattice energy dueo the delocalization of the charge of the large anion in compari-on to that of the LiClO4 salt [2]. The size of the large anion as wells the low lattice energy of the lithium salt are generally expectedo promote greater dissociation of the salts, and thus provides aigher concentration of ions to mobile [40]. In contrast, these find-

ngs are slightly lower than the finding of Idris et al. [8] and Alit al. [30] because in this work, no plasticizers such as polypropy-ene carbonate (PC) and ethylene carbonate (EC) were added intohe electrolyte system, and a different type of lithium salt was used.he presence of PC and EC in polymer electrolyte can easily corrodehe lithium metal electrode in the electrochemical cell [15].

.3. Structural studies

The XRD analysis was used to determine the structure andrystallization of the polymer–salt complexes by observing theppearance and disappearance of the crystalline or the amorphousegion. The appearance of the amorphous region or the reduction ofhe crystalline region would give a high ionic conductivity in com-arison to the crystalline or semi-crystalline region, as reportedlsewhere [2,9,28–31,40]. Fig. 7 shows the XRD pattern of (70:30)G49-PMMA–LiClO4. Meanwhile, Fig. 8 shows the XRD pattern

f (70:30) MG49-PMMA–LiBF4. From Fig. 7, pure LiClO4 salt gaveighly intense peaks at 13.5◦, 21.1◦, 23.3◦, 31.7◦ and 33.1◦, and theresence of pure LiBF4 peaks can be observed at the angles of 13.5◦,8.8◦, 21.5◦, 23.6◦, 26.8◦, 28.2◦, 32.0◦ and 32.8◦.

The introduction of lithium salts to the polymer host was foundo decrease the semi-crystalline phase of MMA by reducing the

MA hump to a broad shape in the region between 10◦ and 20◦.eanwhile, an identical peak for MMA at the angle of 29.5◦ was

ound to decrease with the addition of LiBF4 salt. The presence ofiClO4 and LiBF4 peaks at a high salt concentration at the anglesf 13.5◦, 23.0◦, 31.4◦ and 28.0◦ indicates that re-crystallizationccurred in the polymer host. The re-crystallization of lithium saltsccurred because of the ion association between the Li+ cationnd the anion in the electrolyte at a high salt concentration [6].herefore, it provided low conductivity despite the addition ofithium salt to its maximum level [41]. Nevertheless, this findings similar to those reported elsewhere, in which the ionic con-uctivity still occurred by either the reduction of the crystallinehase or the enhancement of the amorphous phase in the polymerost [2,6,28–31,40]. The salt affects the overall ionic conductivityhrough the formation of crystalline complexes, the intramolec-lar cross-linking of the polymer chains and the degree of saltissociation-number of charge carriers [2].

.4. Morphology studies

Morphology studies were carried out by SEM to investigatehe effect of lithium salt content on the fractured surface of thelended MG49-PMMA. Fig. 9 shows the SEM micrograph of MG49-MMA with (a) 0 wt.%, (b) 15 wt.% and (c) 25 wt.% LiClO4 salt and


d) 15 wt.% and (e) 25 wt.% LiBF4 salt. The SEM micrograph inig. 9(a) shows a homogenous surface of rubber-polymer blendssing the solution blending technique. No phase separation coulde observed by either physical observation or SEM in the blended

PRESS Acta xxx (2011) xxx– xxx

MG49-PMMA blank and blended MG49-PMMA with the additionof salt. This observation proves the existence of an intermolecularinteraction between the oxygen atoms and the methylene groupin the MMA structure [24]. The properties of flexibility and elas-ticity of the blended films were attributed to the polyisoprenesegment in MG49, while the hard segment was contributed by theMMA monomer that has been grafted onto the rubber chain [27].Monikowska et al. [42] suggested that the dark region in the SEMmicrograph represents an amorphous phase. Meanwhile, the crys-talline phase is contributed by the bright region in the polymerelectrolyte system. In this study, the dark region was contributedby a rubber chain that has an amorphous characteristic. The brightregion, which contributes to the crystalline phase, was given by thepresence of PMMA and lithium salts. As can be observed in Fig. 9,the topological texture of blended MG49-PMMA changed from asmooth and dark fractured surface to a rough and brighter surfaceafter the addition of lithium salt.

The cross-sectional view of the sample in Fig. 9(c)–(e) shows theformation of micro-pores due to the interaction between the sol-vent and the polymer host, as reported by Ahmad and co-workers[43]. The size of the micro-pores in Fig. 9(e) appears to be larger thanthe micro-pores in Fig. 9(d). According to [42], the presence of poreswill give a compensating effect on the transporting properties of Li+

ions by increasing the surface area. Hence, it improves the conduc-tivity of the electrolytes. The SEM micrograph in Fig. 9(e) also showsan increase of bright surface with the increase of pores size on thepolymer host. The increment of the bright surface gives the crys-talline properties in the polymer host. Further additions of lithiumsalt in this system will lead to the re-crystallization of lithium saltitself due to a high salt concentration in the electrolyte system. Thehigh salt concentration gives a high tendency of the ionic species toassociate or aggregate with each other [34,36,41,44]. This ionic ten-dency will decrease the number of the conducting species and theionic mobility. Thus, it congests the ionic migration in the segmen-tal polymer chain. This process will disturb the conducting processin the electrolyte systems and provide a low conductivity in thesystems.

4. Conclusions

The solid polymer electrolyte MG49-PMMA at the ratio(70:30) doped with two different lithium salts, LiClO4 and LiBF4,was successfully prepared by a solution casting technique. Thehighest conductivity at room temperature was approximately∼10−6 S cm−1 at 25 wt.% of LiBF4, which is two magnitudes higherthan the ionic conductivity found in the LiClO4 system with thesame concentration. This was due to the difference in the size ofthe anion and lattice energy of appropriate salt. Infrared analy-sis showed that the interaction between lithium ions and oxygenatoms occurred at the ether group (C–O–C) (1500–1100 cm−1)on the MMA structure in both MG49 and PMMA. The structuralanalysis recorded by XRD showed the reduction of the MMA crys-tallinity phase at the highest conductivity. The observations fromthe morphology studies by SEM showed that complexation andcrystallization have occurred in the system.

Acknowledgements

The authors would like to extend their gratitude towards theUniversiti Kebangsaan Malaysia for allowing this research to becarried out. This work is supported by the MOSTI grant 03-01-02-SF0423.


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M.S. Suait Et Al 2011 Electrochimica Acta