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Spectroscopic Investigation of Interactions among Components and Ion Transport

Mechanism in Polyacrylonitrile Based Electrolytes

Zhaoxiang Wang, Biying Huang, Rongjian Xue, Xuejie Huang, and Liquan Chen*

Laboratory for Solid state Ionics, Institute of Physics, Chinese Academy of Sciences

P.O.Box 603-29, Beijing 100080, China

*Correspondence author

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Abstract

Raman, infrared (IR), fluorescence, X-ray photoelectron (XPS), and nuclear magnetic resonance (NMR)

spectra of various combinations of the components of the polyacrylonitrile-based electrolytes have been

studied. It is found that the so-called composite- or mixed-phase polymer electrolytes are not simple mixtures

of electrolytic salt, plasticizer and polymer; instead there are very strong interactions between each two of the

components of the electrolytes though different systems may show different interaction characteristics. By

analyzing the important functions of the plasticizer, a possible mechanism of the ion transport in the polymer

electrolyte is proposed.

Introduction

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Since the discovery that a number of polymer-salt systems exhibit considerable conductivity1-3

, much

research effort has been made to find the optimal combination of host polymer and dopant salt for fast ionic

transport. In 1990, Abraham et al.4

synthesized polyacrylonitrile (PAN)-based lithium-salt electrolytes with

ionic conductivities higher than 10-3

S/cm at room temperature, breaking through the prediction of 10-4

S/cm.

These electrolytes are ionically conducting, plasticizer-dissolved salt in a polymer matrix. The polymer

contributes the mechanical strength and the plasticizer-dissolved salt allows the relatively high conductivity at

ambient temperature. These systems combine the desirable mechanical properties of polymers (e.g. low

density, high flexibility, ease of fabrication, etc.) and the potential of high conductivity of the liquid

electrolytes.

This laboratory has been studying PAN-based electrolytes by way of co-complex or plasticizing. A

good deal of knowledge of the synthesis of these new polymer electrolytes has thus been obtained, especially

concerning the factors that promote ionic mobility. Huang5

has synthesized a dozen of PAN-based solid

electrolytes containing plasticizers, including propylene carbonate (PC), ethylene carbonate (EC),

dimethylformamide (DMF), dimethylsulfoxide (DMSO), sulfolane (SL), -butyrolactone (BL), etc., and their

mixtures. It was found that these electrolytes had conductivities as high as 2.5x10-3

S/cm at room temperature

and were compatible with metallic lithium quite well. Moreover, these polymer electrolytes still show good

elasticity after kept in water-proof environment for one year. Such polymer electrolytes are promising

candidates of electrolytes in lithium batteries used with many applications6.

As to the functions of the plasticizer in the polymer electrolyte system, there have been various

suppositions. Some authors suppose that the only functions of the plasticizer are to decrease the glass transition

temperature, dissolve the salt, make the polymer become amorphous and therefore allow the ions or ionic

carriers to move freely so as to raise the ionic conductivity of the electrolyte. Other authors believe, however,

that besides the plasticizing effect, one more important function of the plasticizer is to associate with the ionic

carriers and allow them to move faster. To date, most researchers treat the polymer electrolytes containing

plasticizer as mixed phases or composite electrolytes. This means that the interactions between the components

of the polymer electrolytes have been neglected.

Strong Coulombic attractions between the dissolved salt ions result in formation of ion pairs, triplets,

and even multiples with higher degrees of association in concentrated (polymer) electrolytes. Since neutral ion

pairs or neutral multiples do not contribute to the conductivity, the number of charge carriers will not be

proportional to the stoichiometric concentration of the salt. As a result, the conductivity is reduced. In

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addition, the solution becomes more viscous with the addition of the salt. In turn, the higher viscosity

reduces the mobility of charge carriers, leading to a further drop of the ionic conductivity.

In this paper, the recent experimental results of this laboratory are summarized on the interactions

between each two components of the PAN-based electrolytes by Raman, infrared (IR), fluorescence, X-ray

photoelectron (XPS) and nuclear magnetic resonance (NMR) spectroscopy. By analyzing the functions of the

plasticizers, a possible transport mechanism of the ions in the PAN-based electrolytes is proposed.

Experimental

Preparation of the samples.

The reagents used in the experiments, EC, PC, DMF, DMSO, and LiClO4 (all from Aldrich), and

PAN (Fushun, China) were analytically pure. The solvents were distilled whereas the salt and the polymer

reagents were dried in a vacuum ovenprior to use. Then they were mixed thoroughly at the required molar

ratios, sealed in a bottle and heated at 110C for 2 hours for the liquid electrolytes but for 12 hours for the

polymer eletrolytes, and subsequently cooled down to room temperature (20C).

Raman and IR spectrometers and the experimental geometry

The IR and Raman spectra were all recorded with an IFS66+FRA106 type Fourier transform IR-

Raman spectrometer (Bruker Instruments Corporation, Germany). The laser power on the sample was 200

mW with a radiation spot of 0.1 mm in diameter. The excitation line was 1.064 m. The scattered light was

collected in a direction of 180 to the incident light (back-scattering geometry). Each of the Raman spectra was

the average of 200 scans while the IR spectra were average of 16 scans, at a speed of two seconds per scan. The

resolution was 4 cm-1

for both the Raman and IR spectra. A germanium detector used to measure the scattered

light was cooled in liquid nitrogen. An OS/2 computer operating system was used to collect and process the

data. For the liquid samples, optical glass test tubes were used for the Raman spectra while for the gel

samples the Raman spectra were recorded directly. The gel samples were pressed into very thin films (less

than 10 m) and supported with a KBr pellet for the IR measurement.

Results and Discussion

1. Interactions between LiClO4 and plasticizers

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A. Interactions between Li+

ion and Plasticizers

Fig.1 shows the Raman spectra of ethylene carbonate (EC) with different concentrations of LiClO4. It

can be seen that the spectral changes of the ring bending mode of EC at 715 cm-1

are obvious. With the

increase of the LiClO4 concentration, a shoulder appears at 727 cm-1

at firs t and its relative intensity

increases with increasing solution concentration. Finally the shoulder becomes so strong that it dominates

the ring bending band. This agrees with the IR spectra of the mode. Similar band splitting phenomena

have also been observed in the IR and Raman spectra of the ring breathing mode (the mother peak at 893

cm-1

is split into 893 and 903 cm-1

components in concentrated solutions).

It is well known that the solvation effect of the ClO4 anion is very weak in nonaqueous solutions

7

due to its large size and will not influence the Raman or IR spectra of EC significantly. Therefore, the band

splitting phenomena observed here should be ascribed to the interaction between the Li+ ion from LiClO4

and the oxygen atoms of the ring group of EC (including both the C=O and C-O bonds). Considering

other spectral changes, such as the C=O stretching band (moves from 1771 cm-1

in pure EC to 1776 cm-1

in

concentrated solutions) in the IR spectra, we assume that the interactions mainly occur between the C=O

group and Li+ ion. This viewpoint is also true of the following discussion of interaction between LiClO 4

and other solvents.

There are lone pairs of electrons on both the oxygen and the nitrogen atoms of the O=C-N group of

dimethylformamide (DMF). When different contents of LiClO4 are added into DMF, it is found that a

shoulder appears on the high-wavenumber side ( at 670 cm-1

) in both the Raman and IR spectra of the

O=C-N deformation band (at 658 cm-1

in pure DMF). This shoulder has a changing feature similar to the

ring bending and ring breathing bands of EC containing LiClO4 but its change is more obvious (Fig.2). In

addition, the CH3-N stretching band at 1095 cm-1

in pure DMF also moves to 1100 cm-1

in the Raman

spectra of concentrated solutions. Changes in relative intensities of other bands related to the N or O atoms

were also observed in the experiments. These indicate that Li+ ions can interact with both the oxygen and

the nitrogen atoms of DMF in the system of LiClO4/DMF.

Due to the severe self-association in dimethylsulfoxide (DMSO) by way of SLLO of neighboring

DMSO molecules , most of the DMSO molecules exist in the form of dimers or polymers at room

temperature. When LiClO4 is introduced into DMSO, however, the interaction between LiClO4 and DMSO

molecules destructs the cyclic dimers at first, making them become monomers. Then the Li+ ions interact

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with the monomers, forming an ion-molecule associate. In the meantime, due to the interaction between Li+

ion and the S=O bond, the CH3 groups linked to the S atoms are also affected by way of an induction effect

of Li+ ions transferred along the molecular chain. As a result, its Raman shift moves towards the high-

wavenumber side of the Raman spectrum with increasing content of LiClO4 (Fig.3).

The above discussion has demonstrated that no matter the Li+ ions associate with the O or the N

atoms of the solvents, the vibrational frequencies of the related structures increase. This is explainable. When

Li+ ions associate with the O or N atoms of the O=C, S=O or CN (in PAN, see the following section)

structures, their binding energies are expected to be lowered to some degree, which will bring a decrease in the

vibrational frequencies of the groups. On the other hand, the Li + ion association results in an even greater

degree of reduction in the effective mass of the associates (=mLi0/(mLi+0), where 0=C=O , S=O or NC,

mLi

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increase of the concentration of LiClO4 has resulted in perturbation to the ClO4 anion itself and formation of

ion associates, such as solvent-separated ion pairs, Li+-solvent-ClO4 (corresponding to the appearance of a

new components at ca. 940 cm-1

for the1 mode), contact ion pairs, [Li+ClO4

] (corresponding to the ca. 948

cm-1

component), and multiple ion aggregates, {Li+ClO4}n (n2) (corresponding to the component at 955

cm-1

) in the Raman spectrum. Increase of these aggregates will lead to decrease of the concentration of the

charge carriers and increase of solution viscosity, which reduces the mobility of the charge carriers. Both of

these effects will decrease the ionic conductivity of the electrolytes.

C. Coordination number of Li+ ion in EC and the concentration of free ions in EC/LiClO4 solution.

In order to determine concentrations of the free and associated solvent molecules from Raman spectra of

different salt concentrations, the splitting and the intensity change of the ring bending band of EC (at 715 cm-

1) was detected quantitatively. By checking the spectra of EC with different LiClO4 contents, the ring

stretching band of EC (at 1230 cm-1

) was taken as the internal standard.

By band-fitting, it is easy to deconvolute the ring-bending band and calculate the integrated intensity

of each component relative to that of the standard band. Table 1 shows the concentration-dependent relative

intensities of I715/I1230, I725/I1230, and (I715 + I725)/I1230. The intensity of the band at 715 cm-1

decreases while the

intensity of the band at 725 cm-1

increases as the concentration of the salt increases. This fact supports the

assignment of the two bands to two different species, i.e., bulk (or free) EC and associated EC. The total

intensity, (I715 + I725)/I1230, decreases slowly in a large range of salt content.

If Cfand Cb are defined as the concentrations of the free and associated EC molecules in the electrolyte,

respectively, and Ifand Ib are the relative intensities of the bands related to the free (I715/I1230) and the associated

EC (I725/I1230) species, the following relationships can be obtained:

If = CfJf (1a)

Ib = CbJb (1b)

where Jf and Jb are coefficients related to the molecular species and their scattering conditions. Thus, after

Deng and Irish7, 8

, the total relative intensity of the band is

It = I f + Ib

= CfJf + CbJb

= (1 - Jb/Jf)If + CJb (2)

where C = Cf + Cb is a constant ( for EC, C = 11.36 mol/Kg ).

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Equation (2) indicates that the relation between It and If is linear, with (1-Jb/Jf) as the slope and (CJb)

as the intercept. Fig.5 is a plot of Itvs If. It can be seen that the relation between I t and If is roughly linear.

Therefore, Fig.8 implies that

1-Jb/Jf= 0.238 (3a)

CJ b = 1.386 (3b)

As a result,Jf = 0.160 (4a)

Jb = 0.122 (4b)

The coordination number of the lithium cation, equivalent to the average ligand number of lithium, ns,

is defined as

ns = Cb/CLi+ = Ib/(CLi+Jb) (5)

Based on Table 1 and eq.5 a coordination number of six is suggested for the lithium ions in EC. This value

is larger than the coordination number of LiAsF6 in acetone and in methyl acetate or methyl formamide by

Deng and Irish7, 8

, and LiClO4 in EC by Hyodo and Okabayashi9, but is still within the range of values

commonly found in lithium cation ionic crystals. The most favorable n s value for lithium ions are expected to

be four, five and six10, 11

. Moreover, it is worth noticing that the calculated ns value decreases with the increase

of the solution concentration. This is due to the assumption that all the LiClO4 has been solvated in EC

when CLi+ is replaced with CLiClO4. That is, CLi+ in eq.5 should be the concentration of free Li+ cations

instead of the concentration of LiClO4 in EC, CLiClO4. Due to the ion-pairing effect, CLi+ is smaller than CLiClO4

and the difference between them increases with the solution concentration when ion pairing becomes more

significant. Only in infinitely dilute solutions where the ion-pairing effect is negligible, can CLiClO4 and CLi+ be

equal. That is why a solvation number of six is suggested for Li+ in EC though smaller coordination numbers

are also measured at concentrated LiClO4/EC solutions.

Many authors have reported ion pairing or ion association of LiClO4 in both aqueous and non-aqueous

solutions. However, it seems that few researchers have reported a quantitative dependence between the free

anion content in the solution and the vibrational spectra of the ClO4 anions

12, 13.

Analogous to the above calculation, the concentrations of the free and perturbed ClO4 anions were

evaluated. The solution concentration (mol/Kg) dependences of free and perturbed ClO4 ions are shown in

Fig.6. It can be seen that the concentration of the free anions has a maximum at [LiClO 4] = 3 mol/Kg. The

solution concentration dependence of the free ion concentration is comparable to the plot of solution

concentration versus conductivity. For example, the conductivity maximum appears at ca. 1 M in

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PC/LiClO414

solution and at weight ratio of 5:20:75 in LiClO4:PAN:plasticizer (solvent)5. It is reasonable that

the conductivity increases with the concentration of free ions. Another factor that influences the conductivity is

the viscosity of the medium which monotonously increases with the increase of the salt concentration.

Therefore the conductivity maximum is reached at a lower salt concentration than the maximum of free ion

concentration in the same solution electrolyte.

2. Interactions between Polyacrylonitrile and Plasticizers

Analyzing the Raman spectra of pure EC, pure PAN and their mixtures with different molar ratios it is

found that there is a striking influence of PAN on the spectra of EC. The spectral band that changes most

significantly is the C=O stretching mode of EC at 1762 cm-1

. It changes from a medium peak in pure EC to a

weak peak in a mixture of EC/PAN and its Raman shift moves 10 cm-1

towards the high frequency side of the

spectra (Fig.7). In addition, its ring stretching mode moves from 1059 to 1072 cm-1

, accompanied with

decreasing intensity and increasing linewidth. These indicate that there is a rather strong interaction between

PAN and EC. On the other hand, the influence of EC on the spectral features of PAN is not obvious. A

plausible explanation is that the CN bond is much stronger than the C=O bond; an interaction of the same

strength can bring different effects on different bonds. It is possible that this interaction occurs through the

dipolar repulsive interaction of the C=O group of EC and the CN group of PAN, resulting in an increase of

their polarities.

When PAN is added into DMSO, the spectral changes of DMSO become obvious at a very low molar

ratio of PAN:DMSO. A new component appears at 1070 cm-1

(corresponding to the monomers of DMSO)

while the intensities of the components at 1013 and 1030 cm-1

(corresponding to the linear dimer and the

polymers of DMSO, respectively) decrease. It is also observed that the linewidth of the S=O bending mode at

303 cm-1

increases with the increasing content of PAN in the mixture. Obvious spectral changes are also

observed in other vibrational modes, such as the CSC stretching mode. These indicate that the influence of

PAN on DMSO is by way of the S=O group and dissociates the linear dimers of the DMSO molecules. On the

other hand, DMSO also exerts influences on the PAN molecules. This influence is not obvious at low content

of PAN. When the molar ratio reaches 0.8, however, the Raman scattering cross section of the CN stretching

mode (at 2240 cm-1

) becomes small (Fig.8).

3. Interactions between LiClO4 and Polyacrylonitrile

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For the convenience of comparison, the Raman spectra of EC/PAN at the mass ratio of 3:2 and of

EC/LiClO4 at the mass ratio of 7:3, together with the Raman spectrum of EC/PAN/LiClO4 sample at the

mass ratio of 50:24:26 are shown in Fig.9. Although the CN stretching band shows no obvious change in

the Raman shift, a weak but obvious shoulder appears on the high frequency side of the CN stretching band.

This shoulder cannot be assigned to EC or LiClO4 or to the interaction between EC and LiClO4 or between

EC and PAN because no such a peak or shoulder is observed in this region in these systems. Therefore, it can

be reasonably believed that the appearance of the weak shoulder at 2270 cm-1

in Fig.9 results from strong

interaction between LiClO4 and PAN molecules.

The supposition that the new shoulder at about 2270 cm-1

reflects the interaction between Li+ ions and

the CN group also agrees with the analysis of the chemical structure of PAN. There are a pair of unbounded

electrons on the nitrogen atom in the CN group and the Li+ ion has an empty orbital. Therefore it is

possible for Li+ ions to bond with the nitrogen atom of CN group and an aggregate is formed. Because the

intensity of the CN group is very strong, the split shoulder at 2270 cm-1

is too easy to be overlapped by the

strong 2244 cm-1

band when the number of Li+ ions is not large enough. Therefore in the normally used

concentrations of LiClO4 and PAN, no trace of strong interaction between PAN and LiClO4 could be observed

by spectroscopy.

4. Competition between plasticizer and polymer on associating with Li+ ions

A. Comparison of Li+

-association in liquid and gel electrolytes with different plasticizers (solvents).

Fig.10 shows the Raman spectra of the O=C-N deformation mode of DMF with PAN dissolved in the

solution of DMF/LiClO4. Comparing Figs.2A and 10, it can be seen that the Raman spectrum of this mode

in DMF/LiClO4/PAN system is very similar to that in DMF/LiClO4. At each comparable molar ratio, the

spectra show no obvious changes before and after PAN is added to the solution. This indicates that the

addition of PAN does not influence significantly the interaction between DMF and Li+ ions. Similar

phenomena have also been observed in the system of DMSO/LiClO4/PAN systems.

However, the spectral difference of the ring deformation mode of propylene carbonate (PC) is very

obvious before and after PAN is dissolved in the solution of PC/LiClO4 (Fig.11, for the detail of the evolution

process of the ring deformation mode of PC containing different contents of LiClO4, see Ref.14). Firstly, the

addition of PAN into the solution makes the Li+-solvent (or plasticizer) interaction less observable. In the

system of PC/LiClO414

, the 722-cm-1

(split from ring deformation mode of PC at 712 cm-1

) component

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becomes apparent at PC/LiClO4 = 10:1. However, this component is still undetectable at PC/LiClO 4 = 7:1

when the content of PAN is very high (i.e., from PC:LiClO4:PAN=7:1:3 to 7:1:5) in PC/LiClO4 /PAN.

Secondly, Fig.11 indicates that the splitting of the ring deformation mode is strongly dependent on the

content of PAN in PC/LiClO4 solution. If the PAN content is very high, the Li+-solvent interaction becomes

apparent only at very high PC:LiClO 4 ratios. In addition, the Li+-solvent interaction becomes apparent at a

lower PC:LiClO4 ratio if the PAN content is very low.

Similar to the system of EC/LiClO4/PAN, the interaction between Li+ ion and PAN can be seen

clearly in the system PC/LiClO4/PAN (Fig.12). When the molar fraction of LiClO4 is much smaller than that

of PAN, there is almost no difference in the shape of the nitrile stretching band before and after PAN is

plasticized with PC. When the molar fraction of PAN in the electrolyte is further decreased, the shape of the

2244 cm-1

band begins to deviate from its originally symmetric shape.

The Raman spectra of DMF/LiClO4/PAN electrolytes are also measured at molar ratios from 10:1:3 to

10:3:3 and 10:2:4. However, changes in the molar ratio of DMF:LiClO4:PAN over this range do not influence

the shape or the vibrational frequency of the nitrile stretching band (at 2244 cm-1). This means that no strong

interaction is detectable between the Li+ ions and the nitrile group. These results agree with those observed in

the DMSO/LiClO4/PAN system.

Summarizing the above results and discussion, it seems that the polymer host and the plasticizer

interfere with each other in their association with the Li+ ions. On one hand, the addition of PAN into

PC/LiClO4 solution suppresses the Li+-PC interaction, and the observation of the Li+-PAN interaction is

related to the PAN content in the polymer electrolyte as well as to the plasticizer. On the other hand, PAN

only weakly interferes with the Li+-DMF interaction and no Li+-PAN interaction is observed over a wide range

of molar ratio of DMF (or DMSO):LiClO4:PAN. A competition exists between the association of plasticizer

and PAN with the Li+ ions. This assumption is supported by the following experimental facts.

B. Interaction between Li+ ion and PAN in plasticizer-removed electrolytes.

The three-component polymer electrolyte (plasticizer, polymer, and salt) system is much more

complicated than a two-component one (such as polyethylene oxide (PEO)-salt solid electrolyte). If the

plasticizer is removed from the solid (gel) electrolyte, the plasticizer-free PAN-based electrolyte (PAN-salt)

will be similar in some respects to the PEO-salt solid electrolyte. By studying the properties of such simple

PAN-salt systems, some information can be obtained to understand the ion(s) transport mechanism in

practical PAN-based gel electrolytes.

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Gels composed of LiClO4, PAN and plasticizer (PC, for example) were cast on to a glass plate and

heated at 120 to 140C for 6 hours to remove most of the plasticizer. Then the sample was transferred to a

vacuum chamber (0.1 Pa) and stored there for a week to eliminate the residue of the plasticizer. Finally the

plasticizer-removed PAN-LiClO4 film was peeled off from the glass plate and ready for the measurements. The

Raman and IR spectroscopic measurements showed that there was no detectable trace of plasticizer in the

samples. For brevity, the plasticizer-removed gel electrolytes and plasticizer/PAN are denoted as LiClO4/PAN

and PAN, respectively) in the following discussion.

Fig.13 shows the Raman (A) and IR (B) spectra of the CN stretching mode of PAN and LiClO4/PAN

at various molar ratios. It can be seen from Fig.13A that the Li +-PAN interaction is evident at a LiClO4/PAN

molar ratio as low as 1:7, whereas in Fig.13B the splitting of the CN stretching mode of PAN is still

observable at a molar ratio of LiClO4:PAN = 1:10. The common spectral features of Fig.13 can be

summarized as follows. Firstly, the relative intensity related to the Li+-perturbed CN stretching band is

proportional to the LiClO4:PAN molar ratio over a large range. This is similar to the Li+-solvent interaction

previously discussed. Secondly, the interaction between Li+ ion and PAN occurs and can be detected at very

low LiClO4:PAN molar ratios. These characteristics have never been observed or reported in

plasticizer/LiClO4/PAN systems before. Since different plasticizers were employed in synthesizing the gel

electrolytes and similar results were obtained in LiClO4/PAN, it can be reasonably assumed that the Li+-PAN

interaction observed in the LiClO4/PAN is independent of the plasticizer originally employed. Therefore, it

can be concluded that the lack of Li+-PAN interaction in the plasticizer/LiClO4/PAN system at low

LiClO4:PAN molar ratios is due to competition for the Li+ ions between the plasticizer and the PAN host in

the gel electrolytes.

According to the above results, the competitive capability of these three polar materials to form ion

associates with the Li+ ions is: DMF>PC>PAN. Because the competition of PAN is much weaker than that

of DMF, it is difficult to observe the Li+-PAN associates when PAN and DMF coexist. Most of the Li + ions

have been associated with DMF. However, in the PC/LiClO 4/PAN electrolyte, such Li+-PAN associates are

observed much more easily. When the plasticizer is removed, no plasticizer would compete with the polymer

host for Li+ ions. It is reasonable that the Li+-PAN interaction can be easily observed in this case even when

the molar fraction of LiClO4 is very low (

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The Raman spectra of the1 mode of ClO4 anion at different LiClO4/PAN molar ratios are shown in

Fig.14. It indicates that two Lorentzian components, at 933 and 954 cm-1

, respectively, are sufficient to

deconvolute the Raman envelope of the 1 mode quite well when the molar fraction of LiClO4 is high. The

933 cm-1

band is from the unperturbed ClO4 anions. Based on the assignment in Section 1B, it seems

plausible to attribute the 954-component to the multiple ion aggregate in LiClO4/PAN (i.e.,{Li+ClO4

}n).

However, such an attribution contradicts the observation of the ion aggregates in LiClO4-solutions. Usually,

the multiple ion aggregates follow the appearance of the solvent-separated ion pairs and the contact ion pairs in

a liquid perchlorate salt solution. An explanation is that the perturbation of PAN on the ClO 4 anion is even

weaker than that of the ordinary solvents of LiClO4 and the 954 cm-1

component comes from recrystallized

LiClO4. This may also be the reason why PAN is not as competitive as the plasticizer (for example, PC and

DMF) on associating with the Li+ ions. The activated IR spectra of the 1 mode agree well with the Raman

spectra (not shown).

6. Coordination number of Li+ ion in plasticizer-removed PAN based electrolytes

Fig.15 shows the XPS spectra of N 1s of the samples. In pure PAN, N 1s spectrum has a single peak

at ca. 397.6 eV, corresponding to the single nitrogen atom of the nitrile group in each repeat unit of PAN.

This band moves to ca. 398.4 eV when LiClO4 is added into the polymer. It has been verified repeatedly that

in the PAN-based electrolytes, PAN-plasticizer-LiClO4, only the polar groups of the plasticizer or the Li+ ions

can interact directly with the nitrile groups11,13,14

. Therefore, in the present plasticizer-removed PAN-LiClO4

system, it is reasonable to attribute the chemical shift of this band to the association between the Li+ ion and

the N atom on the frame of PAN chain. When the Li+ ion associates with the N atoms, the density of the

electronic nebula around the N atom becomes lower and the strength of the nitrile bonding becomes weaker.

Hence, the electronegativity of the N atom becomes larger. As a result, the binding energy of the nitrile

becomes higher. Moreover, it is also worth noticing that the binding energies of N 1s in PAN-3

(LiClO4:PAN=1:3) and PAN-4 (LiClO4:PAN=1:4) are quite similar, though the XPS spectra of N 1s are

significantly different before and after adding LiClO4 into PAN. It seems that the spectral changes of the N 1s

has reached its saturation at the molar ratio of PAN:LiClO4=4:1 or larger than 4:1.

XPS spectra of Li 1s, Cl 2p and O 1s agree with the results from N 1s spectra. These results combine

to suggest that the coordination number of Li+ ion in PAN should not be larger than four. Therefore, a

coordination number of four is suggested for LiClO4 in plasticizer-removed polymer electrolytes.

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7. 7Li NMR Spectra of the practically used PAN-based electrolyte

A spectrum of PAN-EC-PC-DMSO-LiClO4 at room temperature is shown in Fig.16. It can be seen that

the spectrum is composed of three components: a narrow and strong Gaussian peak, a broad Lorentzian peak

and a small peak with a chemical shift. Chemical shift represents the change of chemical environment around

the subject ion. Usually the linewidth of an NMR peak reflects the ion mobility in the system. The faster the

ions move, the narrower its related peak will be. Fig.16 indicates that there are three kinds of Li+ ions

corresponding to the three components in the NMR spectrum. The narrowest one is caused by the fast-moving

ions in the gel state; the broad peak corresponds to the slow-moving ions in solid PAN and the chemical shifts

comes from the ions associated with the plasticizer.

Because gel is the major phase in PAN-based electrolytes, fast ion transport in the gel state is the major

contribution to the high conductivity of the PAN-based electrolytes. The temperature-dependent linewidth of

the7Li spectra shows a motion narrowing temperature at ca.200 K (Fig.17). Below 200 K the spectra are very

broad, indicating a slow mobility of most of the Li+ ions. As the temperature increases, a narrow peak appears

and becomes the dominant component around room temperature. Using HB equation,

log(1/W) = Ea/(2.303RT) + log(1/B).

an activation energy of 0.28 eV is obtained at ca. 25C, slightly smaller than the value from the VTF

equation. A plausible explanation is that triplet ion aggregates may also contribute to the ionic conductivities

as well as the free ions. In a HB equation, the B value represents the extent to which the dipole-induced line

broadening can be eliminated at infinitely high temperatures. If the process is a long-range one, the dipolar

interactions are completely averaged out and the linewidth would be narrowed to its lifetime values (B10-4

KHz) if the motion

is short-ranged. The values measured from our sample at 25C are about 10-7

KHz. It can be concluded that the

Li+ ion motion in the electrolyte is mainly a long-range process.

8. Ion Transport Mechanisms in PAN Based Electrolytes

The conductivity of an electrolyte is determined by the concentration of the charge carriers and their

mobilities. The extent to which ion-pairs are dissociated to form charge carrying ions is determined by the

dielectric constant of the plasticized polymer with a plasticizer.

Based on the above knowledge about the microstructure of a PAN-based electrolyte, it is possible to

depict the total structural geometry in PAN-based electrolytes. When the concentration of LiClO4 is very low

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in a LiClO4-solvent (plasticizer) system, most of the LiClO4 is dissociated. The ionic conductivity of the

solution increases with increasing LiClO4 content (Scheme 1a). However, with increasing LiClO4

concentration, ion pairs appear and the association becomes dominant, leading to a decrease of concentration of

free ionic carriers and therefore, a decrease of the ionic conductivity (Scheme 1b through d). The number of the

EC molecules around one Li+ ion can be as large as six in the EC/LiClO4 solution.

In a (plasticizer-removed) PAN/LiClO4 system, the Li+ ions are associated with the PAN chain by way of the

comb-like CN group. One Li+ ion can associate with as many as four CN groups. Such association is

shown in Scheme 2a. ClO4 anion, due to its very weak interaction with PAN, is separated from the Li+ ions

and exists as a lone ionic carrier. When the content of LiClO4 is further increased, the surplus Li+ ions (in

respect to the coordination number of four) recrystallized with ClO4 anions (Scheme 2b). As the Li+ ions are

held in the polymer matrix by means of the coordination bond with the nitrogen and the chain of the polymer

is rigid at room temperature (without plasticizer, the glass transition temperature of PAN is as high as

169C), the ionic conductivity of PAN/LiClO4 system would be very low and the contribution of the anion to

the total ionic conductivity is dominant, i.e. the transference number t>>t+).

When LiClO4 and PAN are dissolved in a plasticizer the coordination bonds of Li+ ion to PAN can be

decoupled due to the strong competitive capability of the plasticizer for the Li + ions. In a PC- or EC-

plasticized polymer electrolyte, the decoupling behavior becomes obvious with increasing molar ratio of the

plasticizer as shown in Scheme 3. The decoupling behavior becomes more obvious due to the stronger

competitive capability of the plasticizer in DMF- or DMSO-plasticized polymer electrolyte than that of PC or

EC (Scheme 3d and e). In such an electrolyte, no Li+-coordinated PAN molecules can be detected by the

Raman or IR spectroscopy as long as the LiClO4/PAN mixture can be dissolved by the plasticizer. Therefore,

the Li+-coordinated PAN molecules must be very scarce, if exist, in such a system. Besides the decoupling

effect of the plasticizer to the Li+-PAN coordinate species, repulsions also exist between the polymer and the

plasticizer through their dipoles, which has been shown in Scheme 3 as well.

It has been verified repeatedly that the amorphous region in the PEO-salt electrolytes is responsible for

the high mobility of the Li+

ions. XRD patterns by Huang et al5

demonstrated that this is also the case in the

PAN-based electrolytes.7Li NMR studies on the PAN-based electrolytes indicates that there are at least three

states of Li+ ions in the plasticized PAN-based electrolytes: i. Li+

ions in the gel state, which constitutes the

most important part of the polymer electrolytes; ii. Li+ ions moving along the PAN chain; and iii. Li+ ions

associated with the plasticizer molecules. Moreover, analysis of the temperature-dependent linewidth of the7Li

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NMR spectra demonstrates that the movement of most of the Li+ ions is between a long-range and a short-

range one. As to the long-range movement, it is quite possible that the Li+ ions will move along the side

chain of PAN, jumping from one site to the next. According to the Raman, IR and XPS spectra this site is

most probably provided by the CN group of PAN. Based on polymer physics, the length of a segmental

chain of PAN is a dozen of PAN basic units (-CH2-CH-CN-), a few nanometers long. As the polymer chain

moves in the form of segmental chain movement, it is easy to understand that the Li+ ion movement with the

polymer is a long-range one in the solid state. If the Li+ ions move in the liquid medium, its movement will

be strongly affected by the Brown movement of the solvent molecules and will be a short-range one. NMR

spectrum (Fig.16) has shown that the dominant part of the PAN-based electrolytes is the one with a gel

structure (corresponding to the strongest Lorentzian peak). Gel is a state between solid and liquid, the

movement of the Li+ ions in the gel is between a short-range one and a long-range one. As a result, the Li +

ions moving in the gel contribute the most to the ionic conductivity of the polymer electrolyte.

There are also Li+ ions moving along the segmental chain of PAN. The transport mechanism of these

ions are quite similar to those in the PEO-salt electrolyte. By coordinating with the side chain of the polymer,

the movement of the Li+ ions is quite similar to that of the segmental chain in a PEO-salt electrolyte.

Therefore, their movement is a long-range one. However the Li +ion movement along the segmental chain

cannot be the major contribution to the ionic conductivity in a PAN-based electrolyte at room temperature

because the movement of the segmental chain of PAN is slow at room temperature in respect to the ions in

the gel. This may be quite different from the PEO-salt electrolyte, where the segmental chain movement is the

most important to the conductivity. Of course, when the temperature is elevated, the movement of the

segmental chain becomes faster. So the total ionic conductivity of the polymer electrolyte tends to be similar

to that of the liquid electrolyte. This hypothesis agrees with the experimental results of Olsen and

Koksbang15

.

We also notice that the transference number of the anion16

( 0.64) is usually larger than that of the

cation ( 0.36) in the PAN-based electrolyte at room temperature. The movement of ClO4 anion in the gel is

similar to that in a liquid medium; instead of moving along the segmental chain by jumping from one site to

the next, these anions move outside the tunnel provided by the PAN-chain due to the weak interaction

between the anion and the polymer. On one hand, the weak solvation effect with the plasticizer allows the

anions to move faster than the cations. On the other hand, the large ionic size of the anion blocks its fast

movement more severely in the gel than in the liquid due to the much larger viscosity of the gel than of the

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liquid. These two effects coexist and combine to make the movement of the anions move slower in a

plasticized polymer electrolyte than in a liquid electrolyte of the same solvent.

Considering the above transport mechanisms of the ions (anion and cation), the functions of the

plasticizer in the PAN-based electrolytes may be summarized as the following:

i. decreasing the glass transition temperature of the polymer and dissolving the crystallites of the

polymer. This will lead to the increase of the mobility of the segmental chain of the polymer and help the

transport of the charge carriers depending on the movement of the segmental chain (ions moving in the solid

state polymer);

ii. dissolving the electrolytic salt so as to provide charge carriers for the polymer electrolyte;

iii. interacting with the polymer molecules by way of dipolar interactions and therefore, increasing the

polarities of the polymer and the plasticizer itself which, in turn, will help the dissociation of the electrolyte in

the polymer electrolyte;

iv. destructing the coordination bonds of the Li+ ions with PAN and making more Li+ ions move in

the gel instead in the solid state. This will help to increase the mobility of the Li + ions in the polymer

electrolytes.

Based on the above functions of the plasticizer in a polymer electrolyte, some suggestions may be

proposed for selecting the plasticizer of the polymer electrolyte:

i. compatible with the polymer and the electrodes;

ii. thermodynamically stable;

iii. low viscosity;

iv. high dielectric constant;

v. low melting point and high boiling point;

vi. inexotic and easily obtainable.

The above six points are conventional requirements to the plasticizer for polymer electrolytes.

However, according to our research results, one more requirement must be added to the plasticizer:

vii. the plasticizer should have a strong competitive capability to decouple the coordination bond of

PAN-Li+, but this capability should not be too strong in order to avoid any strong Li+-plasticizer-ClO4

coordination. This requirement is different from the above six requirements. Dielectric constant of a plasticizer

has no direct relationship with its competitive capability. For example, Li+-PAN associates are easier to be

formed in EC (or PC)/PAN/LiClO4 system than in DMF or DMSO/PAN/LiClO4 system though PC and EC

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have very high dielectric constant compared with DMF or DMSO. On the other hand, although DMF has a

stronger competitive ability to decouple the Li+-PAN coordination bond, it has a lower dissolving capability

for LiClO4 and Li+-DMF associates are much easier to be formed in DMF/PAN/LiClO 4 than in EC (or

PC)/PAN/LiClO4 system.

Usually a single-component plasticizer cannot satisfactorily meet all the above requirements. Therefore,

a multi-component plasticizer is necessary to prepare polymer electrolytes with high ionic conductivity,

electrochemical capability with the electrode, thermodynamic stability and enough mechanical strength, etc..

Conclusions

By analyzing the Raman, IR, fluorescence, X-ray photoelectron and the7Li

NMR spectra of the

components of PAN based electrolytes, we can conclude:

I. Strong interactions between plasticizer and the Li+ ions mainly occur on the O atoms of its C=O or

S=O bonds. The increase of the LiClO4 concentration not only affects the structure of the plasticizer but also

perturbing the symmetry of the ClO4 anion itself, forming ion associates, such as solvent separated ion pairs,

contact ion pairs and even multiple ion aggregates. This will result in a decrease of the concentration of the

free charge carriers. A coordination number of 6 is proposed for the Li + ions in EC/LiClO4 solutions. The

quantitative relationship between the solution concentration and the concentration of free ions is roughly

discussed.

II. Evidence of strong interactions between plasticizers and PAN is experimentally found. Interactions

of PAN with different plasticizers show different characteristics. The interaction between EC and PAN mainly

occurs by way of dipolar repulsions, while the self-associated DMSO linear dimers must be first decoupled

before they can interact with PAN as monomers in a DMSO/PAN system.

III. For the first time, interactions between the Li+ ions and the PAN were evidently observed in

polymer electrolytes with EC and PC as plasticizers. These interactions occur through the Li+ ion and the

nitrogen atom of the CN bonds in PAN molecules. However, similar interactions failed to be observed in

polymer electrolytes plasticized with DMF or DMSO. This is ascribed to the strong competition between the

plasticizer and the polymer on associating with the Li+ ions, evidenced by comparing the Raman and IR

spectra of the polymer electrolytes before and after the plasticizer is removed. XPS studies indicate that the

coordination number of the Li+ ion in plasticizer-removed PAN-based electrolyte is four.

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IV. Combining the results from the Raman and IR spectra with the7Li NMR results, three existence

states are identified for the Li+ ions in plasticized PAN-based electrolytes: the narrow Gaussian peak is

ascribed to the fast moving Li+ ions in the gel state, the most important phase for the ionic conductivity of the

electrolytes. The broad peak is attributed to the slow moving Li+ ions in the solid phase of PAN, which

contributes little to the ionic conductivity of the polymer electrolyte due to their slow mobility. Finally, the

chemical shift observed is assigned to the association between the Li+ ion and the plasticizer, a popular

phenomenon observed in the plasticized polymer electrolytes.

V. Considering the strong competition effects between the plasticizer and the polymer and the

differences of the charge carriers moving in the gel and the solid phases, the function of the plasticizer for the

mobility of the ions in the electrolytes is emphasized. It is suggested that on choosing a plasticizer for the

PAN based electrolytes, an important factor that should be considered is the competitive capability of the

plasticizer with the polymer on associating with the Li+ ions. In order that, more Li+ ions can move in the gel

state instead of moving in the solid phase of PAN or as plasticizer-separated ion pairs, which will drop the

ionic conductivity.

Acknowledgments

This work was supported by the National Science Foundation of China (Contract No.59672027).

References

1. P. V. Wright, Brit. Polym. J. 7(1975) 319.

2. P. V. Wright, J. Polym. Sci. Polym. Phys. Ed. 14(1976) 955.

3. M. Armand, J. M. Chabagno, and M. J. Duclot, 2nd International Conference on Solid Electrolytes, St

Andrews paper 6.5, 1978.

4. K. M. Abraham and M. Alamgir, J. Electrochem. Soc., 137(1990)1657

5. Biying Huang, Thesis for Ph.D, Institute of Physics, Chinese Academy of Sciences (1995).

6. G.Pistoria, Bull. Electrochem., 7(1991)524.

7. Z. Deng and D. E. Irish, J. Chem. Soc. Faraday Trans. 88 (1992) 2891.

8. Z. Deng and D. E. Irish, Can. J. Chem. 69 (1991) 1766.

9. S. Hyodo and K. Okabayashi, Electrochim. Acta, 34(1989) 1551

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Fig.4 The IR spectral evolution of the1 mode (ca. 930 cm-1) of ClO4

with different LiClO4:EC molar ratios

in LiClO4/EC solution: a. 0.008; b. 0.04; c. 0.09; d. 0.21 and e. 0.35.

Fig.5 The relative intensity of the ring-bending mode.of free EC (If) vs that of the total (free and bound) EC

(It) in LiClO4/EC solutions.

Fig.6 The relationship between the concentrations of free and bound ClO4-anions and the LiClO4/EC solution

concentration.

Fig.7 The Raman spectra of the C=O stretching mode of EC at 1762 cm-1

with different EC:PAN molar

ratios: a. pure EC; b. 4:1; and c. 3:2.

Fig.8 The molar ratio dependence of the relative intensity of the nitrile stretching mode to different modes of

DMSO.

Fig.9 The Raman spectra of EC/PAN/LiClO4 at different mass ratio of EC:PAN:LiClO4: a. 7:0:3; b. pure

PAN heat-treated at 110C in air for 10 h; c. 3:2:0; and d. 49:25:26.

Fig.10 The Raman spectra of the O=C-N deformation mode of DMF (at 658 cm-1

in pure DMF) and the 4

mode of ClO4 anion (at 624 cm

-1) in DMF/LiClO4/PAN electrolytes with different molar ratios of

DMF:LiClO4:PAN (the band with an arrow overhead is from the ClO4 anion)

Fig.11 Lorentzian band fitting to the Raman spectra of the ring deformation mode (at 712cm-1

) of the LiClO4-

perturbed PC in PC/LiClO4/PAN electrolyte at various molar ratios of PC:LiClO4:PAN (note: the

band with an arrow overhead is from the4 mode of ClO4).

Fig.12 The Raman spectra in the CN stretching region of PAN in PC/LiClO4/PAN at various molar ratios

of PC:LiClO4:PAN.

Fig.13 The Raman (A) and the IR (B) spectra of the CN stretching mode of PAN in plasticizer-free

LiClO4/PAN with various LiClO4:PAN molar ratios.

Fig.14 The Raman spectra of the1 mode of the ClO4-at different molar ratios of LiClO4:PAN in plasticizer-

removed solid electrolyte.

Fig.15 The XPS spectra of N 1s in pure PAN and PAN containing different contents of LiClO4 in plasticizer-

free solid electrolyte.

Fig.16 A typical7Li NMR spectrum of PAN-EC-PC-DMSO-LiClO4 at room temperature (Frequency:

34.964 MHz).

Fig.17 The evolution process of the line width with the change of temperature of PAN-EC-PC-DMSO-

LiClO4 electrolyte.

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Scheme 1 An evolution process of the ionic species in a LiClO4-solvent solution.

a. at very low concentration, the salt is completely dissolved and there are only solvated Li+ ions;

b. when the concentration is raised solvent-separated ion pairs appear in the solution;

c. further increase of slt concentration leads to ion pairs and the ionic atmosphere is damaged;

d. in concentrated solution, ionic aggregates appear and dominate the solution while the concentration

of the free Li+ ions drops dramatically.

Scheme 2 Association of the ionic species in a plasticizer-removed LiClO4/PAN system.

a. the Li+ ions are associated with the CN groups of the PAN molecules. The coordination number

could be as large as 4. The ClO4 ions move outside the PAN matrix;

b. when the salt content is further increased, the extra ions will turn to form ion pairs, [Li+ClO4].

Scheme 3 Association of ionic species in a plasticized LiClO4/PAN system.

a. when a plasticizer is added to a LiClO4/PAN system, the Li-PAN associates and the ion pairs are

decoupled due to the solvation and strong competition effects of the plasticizer for the Li +ions (ref.

Scheme 2a); a repulsion exists between the polymer and the plasticizer;

b and c. When more plasticizer is added, more and more ion associates are decoupled and the content of

the solvent (plasticizer)-separated ion pairs is low; a gel polymer electrolyte with high ionic

conductivity is obtained.

d. if a plsticizer with very strong competitive capability is added to the LiClO 4/PAN system, the Li+-

PAN associates are deccoupled and solvent-separated ion pairs tends to be formed. The appearance of

the ion pairs will reduce the concentration of free charge carriers.

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Table 1. Relative integrated intensity of the ring-bending mode of EC at 715 cm-1

(If) and 725 cm-1

(Ib) and

their sum (It) at different concentrations of LiClO4 with the corresponding I1230 as the standard.

Cont. (%) peak (cm-1

) If peak (cm-1

) Ib It

1 716.5 1.6873 726.0 0.0623 1.7496

5 716.4 1.3996 727.5 0.3063 1.7059

10 716.4 1.1542 727.2 0.5215 1.6757

20 716.1 0.5839 727.5 0.9445 1.5284

30 715.7 0.4764 727.4 1.0164 1.4927

37.6 717.9 0.3277 728.5 1.1384 1.4662

45.4 718.9 0.2325 728.5 1.2040 1.4365

51.7 -- 0 729.8 1.3824 1.3824