Chemical Physics Letters 393 (2004) 271–276

www.elsevier.com/locate/cplett

Nanocomposite polymer electrolyte based on Poly(ethylene oxide)and solid super acid for lithium polymer battery

Jingyu Xi *, Xiaozhen Tang *

School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

Received 15 March 2004; in final form 7 June 2004

Available online 2 July 2004

Abstract

This Letter reports a novel PEO-based nanocomposite polymer electrolyte by using solid super acid SO2�4 /ZrO2 as filler. XRD,

DSC, and FT-IR results prove the strong Lewis acid–base interactions between SO2�4 /ZrO2 and PEO chains. The addition of SO2�

4 /

ZrO2 can enhance the ionic conductivity and the lithium ion transference number of the electrolyte. The highest room temperature

ionic conductivity of 2.1� 10�5 S cm�1 is obtained for the sample PEO12–LiClO4–7%SO2�4 /ZrO2. The excellent performances such

as good compatibility with lithium electrode, and broad electrochemical stability window suggest that PEO–LiClO4–SO2�4 /ZrO2

nanocomposite electrolyte can be used as electrolyte materials for lithium polymer batteries.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

All solid-state lithium polymer batteries may be one

of the best choices for electrochemical power source of

the future characterized by its high energy densities,

good cyclability, reliability and safety [1,2]. PEO–LiX

based polymer electrolytes have received extensive at-

tentions [3–6], for its potential capability to replace

traditional liquid electrolytes, since Wright and co-workers [7] found that the complex of PEO and alkaline

salts had the ability of ionic conductivity in 1973.

The general concept of the transport of Liþ in the

polymer electrolyte is coupled with the local relaxation

and segmental motion of the PEO chains [6], which can

only be obtained when PEO is in its amorphous state.

However, due to its particular structure, PEO often

shows much higher crystalline ratios at sub-ambienttemperature regions, corresponding to ionic conductiv-

ity lower than 10�5 S cm�1 [8–11]. This limitation is, of

course, a drawback for applications in the consumer

electronic market, such as cell phone and notebook PC.

* Corresponding authors. Fax: +86-21-5474-3264.

E-mail addresses: jyxi@sjtu.edu.cn (J. Xi), xtang@sjtu.edu.c (X. Tang).

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.06.054

On the other hand, the ionic conductivity of PEO basedpolymer electrolytes can reach to the appreciable level

(10�4–10�3 S cm�1), fit able for numerous other appli-

cations such as for electric vehicles (EV), hybrid electric

vehicles (HEV), energy storage, and load levering, when

the operation temperature is higher than the melting

point of the PEO [2,12].

Development of PEO-based electrolytes capable of

combining high ionic conductivity with superior inter-facial stability towards the lithium metal anode, and

good mechanical properties is the key problem for the

R&D of all solid-state lithium polymer batteries [6,11].

When the third component, such as inorganic fillers, was

introduced into the PEO-based electrolytes to form the

composite polymer electrolytes (CPEs), all of above

performances could be improved [13]. The fillers, which

have been used in CPEs, can be generally classified intothree families: nanooxides, e.g., SiO2 [14], Al2O3 [8,15],

TiO2 [16], ZnO [17], ZrO2 [18], etc., layered clays, e.g.,

montmorillonite [19,20], and porous materials [21–23].

Solid super acid, e.g., SO2�4 /ZrO2, SO

2�4 /Fe2O3, and

SO2�4 /TiO2, are useful acid catalysts in the field such as

selective hydrocarbon isomerisatios, acylations and es-

terification reactions due to their strong Lewis acidity

[24–26]. Ionic conductivity of PEO-based composite

272 J. Xi, X. Tang / Chemical Physics Letters 393 (2004) 271–276

electrolytes can be enhanced through the well-known

Lewis acid–base interactions between the filler and PEO

chains [6]. The Lewis acidity values of solid super acid

are stronger than that of the traditional ceramic fillers

like SiO2 and Al2O3. This inspired us to develop a novelnanocomposite electrolyte PEO–LiClO4–SO

2�4 /ZrO2, in

which solid super acid SO2�4 /ZrO2 filler can obviously

improve the ionic conductivity and other electrochemi-

cal properties of PEO–LiClO4 electrolyte. The en-

hancement mechanisms of SO2�4 /ZrO2 are also studied

by XRD, FT-IR, DSC, and SEM techniques.

2. Experimental

The zirconia support was prepared by hydrolysis of

zirconium oxychloride with ammonia, as already de-

scribed [24,25]. The precipitate zirconium hydroxide was

washed with deionized water until no ammonia and

chloride ions were detected in the washings, the absence

of which was confirmed through the phenolphthaleinand silver nitrate tests, respectively. The white precipi-

tate zirconium hydroxide was then dried in an oven at

383 K for 24 h. Sulphation of this hydrous zirconia was

done by percolating 1 N H2SO4 solution through it. The

sulphated zirconium hydroxide was then calcined in air

at 923 K for 3 h to obtain the final SO2�4 /ZrO2. The

Hammet acidity function H0 of as prepared SO2�4 /ZrO2

is )16.0, confirmed the successful preparing of this solidsuper acid.

The preparation of nanocomposite electrolyte films

involved first the dispersion of the SO2�4 /ZrO2 powder

and LiClO4 in anhydrous acetonitrile, followed by the

addition of the PEO component and a thorough mixing

of the resulting slurry. The slurry was cast on to a Teflon

plate and then the plate was placed into a self-designed

equipment, under the sweep of dry air with a flow rate of10 Lmin�1, in order to let the solvent slowly evaporate.

Finally, the resulting films were dried under vacuum at

50 �C for 24 h to get rid of the residue solvent. These

procedures yielded translucent homogenous films of

thickness range from 100 to 200 lm (see the inset of

Fig. 4). The samples used in this study were denoted as

PEO12–LiClO4–X%SO2�4 /ZrO2, in which the EO/Li ra-

tio was fixed to 12 for all samples and the content ofSO2�

4 /ZrO2, X, was ranged from 2 to 20 wt% of the PEO

weight.

X-ray diffraction (XRD) patterns were recorded by

using a Bruker D8 Advance instrument equipped with

Cu Ka radiation and were performed at 40 kV and 40

mA with a scanning rate of 4� min�1. Infrared spectra

were recorded on a PE PARAGN1000 instrument with

a wavenumber resolution of 2 cm�1. Differential scan-ning calorimeter (DSC) measurements were carried out

on a PE Pyris-1 analyzer at a heating rate of 10

�Cmin�1. Surface morphology of the sample was stud-

ied by the scanning electron microscopy (SEM) using

Hitachi (model S-2150) instrument with gold sputtered-

coated films.

Ionic conductivity of the electrolyte films was deter-

mined by AC impedance spectroscopy in the 1 MHz–1 Hz frequency range using a Solartron 1260 Impedance/

Gain-Phase Analyzer coupled with a Solartron 1287

Electrochemical Interface. The electrolyte film was

sandwiched between two stainless steel (SS) blocking

electrodes to form a symmetrical SS/polymer electrolyte/

SS cell. Lithium ion transference number, TLiþ , was

evaluated using the method of AC impedance combined

with steady-state current technique, proposed by Vin-cent and Bruce [27,28]. The electrolyte film was sand-

wiched between two lithium-unblocking electrodes to

form a symmetrical Li/polymer electrolyte/Li cell. The

cell was assembled and sealed in an argon-filled UNI-

LAB glove box (O2 < 0.1 ppm; H2O< 0.1 ppm). Elec-

trochemical stability window of the electrolyte was

determined by running a linear sweep voltammetry in

three-electrode cells using stainless steel as the blockingworking electrode, lithium as both the counter and the

reference electrode. A Solartron 1287 Electrochemical

Interface was used to run the voltammetry at a scan rate

of 1 mVS�1.

3. Results and discussion

Fig. 1 shows the XRD patterns of ZrO2 and SO2�4 /

ZrO2. It is obvious that the characteristic peak of ZrO2

(Fig. 1a) becomes weaker after sulphated (Fig. 1b),

suggesting that most of the ZrO2 in SO2�4 /ZrO2 is in the

amorphous state. The SEM image (inset of Fig. 1)

clearly shows that the particles of ZrO2 and SO2�4 /ZrO2

have average diameters of about 40–50 nm and 60–70

nm, respectively.Fig. 2 displays the X-ray diffraction patterns of pure

PEO, PEO12–LiClO4, and PEO12–LiClO4–X%SO2�4 /

ZrO2. The characteristic diffraction peaks of crystalline

PEO are 2h ¼ 19� and 23.5�. These diffraction peaks

become weaker and broader when the Li salt is intro-

duced in the PEO, suggesting that the coordination in-

teractions between the ether O atoms of PEO and Liþ

can reduce the crystallinity of PEO effectively. With theaddition of SO2�

4 /ZrO2 into PEO12–LiClO4 complex, the

peak intensities of the crystalline PEO decrease further

and only trace of crystalline PEO can be detected when

the content of SO2�4 /ZrO2, X, is higher than 10. SO2�

4 /

ZrO2 may decrease the crystallinity of PEO through the

Lewis acid–base interactions between the ether O of

PEO and Lewis acid sites of SO2�4 /ZrO2, like the case of

SiO2 and Al2O3 [6,10].Thermodynamic properties of PEO12–LiClO4–

X%SO2�4 /ZrO2 obtained from DSC test are summarized

in Table 1. The percentage of crystalline PEO, Xc, can be

Fig. 1. X-ray diffraction patterns of ZrO2 (a) and SO2�4 /ZrO2 (b). The inset shows a relative SEM image. The scale bar is 1 lm in each panel.

J. Xi, X. Tang / Chemical Physics Letters 393 (2004) 271–276 273

calculated with the equation Xc ¼ DHm=DH �m, where

DH �m is the melting enthalpy of a completely crystalline

PEO sample [29]. It can be seen from Table 1 that both

10 20 30 40 50

Inte

nsity

PEO12

-LiClO4-20%SO

4

2-/ZrO2

PEO12

-LiClO4-10%SO

4

2-/ZrO2

PEO12

-LiClO4-7%SO

4

2-/ZrO2

PEO12

-LiClO4-5%SO

4

2-/ZrO2

PEO12

-LiClO4-2%SO

4

2-/ZrO

2

PEO12

-LiClO4

PEO

2q / (˚)

Fig. 2. X-ray diffraction patterns of pure PEO, PEO12–LiClO4 and

PEO12–LiClO4–X%SO2�4 /ZrO2.

Table 1

Thermodynamic and electrochemical properties of PEO12–LiClO4–X%SO2�4

X Tg (�C) Tm (�C) DHm (J g�1) X

0 )32.8 54.4 83.4 3

2 )41.9 49.5 58.8 2

5 )46.2 48.8 55.6 2

7 )45.7 48.5 53.4 2

10 )47.1 48.0 52.1 2

13 )47.1 48.1 47.4 2

16 )50.5 47.5 47.3 2

20 )51.7 44.2 35.3 1

aXc ¼ ðDH samplem =DH �

mÞ � 100, DH�m ¼ 213:7 (J g�1).

b Calculated form the impedance response of Li/electrolyte/Li cells at 70 �c Tested by AC impedance combined with steady-state current method atdObtained from linear voltage sweep test at 90 �C.

the melting temperature ðTmÞ and Xc decrease with the

increase of SO2�4 /ZrO2 content nearly in the whole X

range, in agreement with the XRD results. The glass

transition temperature, Tg, of PEO also decreases with

the increase of SO2�4 /ZrO2. The decrease of Tg and Xc

will increase the flexibility of the PEO chains and the

ratio of amorphous state PEO, respectively. And as a

result, the ionic conductivity should be enhanced at low

temperature regions.

The peak of m(ClO�4 ) band at the region of 650–

600 cm�1 in the FT-IR spectra is frequently used to

analyze ion–ion interactions in PEO–LiClO4 based

electrolytes [30,31]. In PEO12–LiClO4 (Fig. 3b) case, the

peak of m(ClO�4 ) band split into two peaks at �624 and

�635 cm�1, demonstrates that two different kind of

ClO�4 anions exist in this complex. Salomon et al. [30]

suggest that the m(ClO�4 ) band centered between 630 and

635 cm�1 is associated with the presence of contact-ionpairs, whereas the band centered at about 623 cm�1 can

be attributed to free ClO�4 anions. As can be seen from

Fig. 3b, the peak characteristic for ‘free’ ClO�4 is much

larger than that of contact-ion pairs. This is because that

the relative low content of LiClO4 (O/Li¼ 12) is helpful

for its dissolving in the PEO matrix. For nanocomposite

/ZrO2 nanocomposite electrolytes

ca (%) Rinter

b (X) TLiþc Decomposition

voltage d (V)

9.0 72.3 0.198 4.73

7.5 65.9 0.213 4.80

6.0 54.8 0.230 4.82

4.9 31.6 0.287 4.95

4.4 47.5 0.292 4.91

2.2 65.8 0.289 4.93

2.1 75.8 0.297 4.89

6.5 115.6 0.299 4.90

C.

70 �C.

660 650 640 630 620 610 600

free ClO4

-

Li+-ClO4

- contact ion pairs

(g)

(f)

(e)

(d)

(c)

(b)

(a)

Tra

nsm

ittan

ce (

a.u.

)

Wavenumbers / cm-1

Fig. 3. FT-IR spectra of pure PEO (a), PEO12–LiClO4 (b) and PEO12–

LiClO4–X%SO2�4 /ZrO2: (c) X ¼ 5; (d) X ¼ 7; (e) X ¼ 10; (f) X ¼ 16;

(g) X ¼ 20 in the wavenumber range 660–600 cm�1.

274 J. Xi, X. Tang / Chemical Physics Letters 393 (2004) 271–276

electrolytes containing SO2�4 /ZrO2, the peak character-

istic to contact-ion pairs at �635 cm�1 becomes more

smaller and only a trace shoulder peak can be found

when the content of SO2�4 /ZrO2 reaches 20 wt% of PEO

(Fig. 3c–g). The strong Lewis acid–base interactions

between the SO2�4 /ZrO2 (Lewis acid) and O atoms

(Lewis base) both in PEO segments and ClO�4 anions

can weaken its interactions with Liþ and thereby releasemore ‘free’ Liþ.

SEM is often used to study the compatibility between

the various components of the composite electrolytes

through the detection of phase separations and inter-

faces [23,32]. The compatibility between the polymer

Fig. 4. SEM images and digital photos of pure PEO membrane (a), PEO12–

(e) X ¼ 10; (f) X ¼ 20.

matrix and the inorganic fillers has great influence on

the properties (mechanical, thermal, ionic conductivity,

and interface with the lithium anode) of the PEO-based

composite electrolytes. Both Li salt and SO2�4 /ZrO2

modified the surface morphology of the PEO electro-lytes. Fig. 4a is the surface image of pure PEO film. The

image shows a rough morphology with a great deal of

micro-pores, a common occurrence for PEO-based

electrolytes prepared by the solvent casting method.

These small pores are caused by the fast evaporation of

the acetonitrile solvent during the preparation process.

A dramatic improvement of surface morphology from

rough to smooth is achieved (Fig. 4b) after the additionof Li salt. The smooth surface morphology is closely

related to the reduction of PEO crystallinity via the in-

teraction between PEO segments and lithium cations.

The incorporation and high dispersion of SO2�4 /ZrO2 in

PEO12–LiClO4 complex further improves the surface

morphology (Fig. 4c–e). However, the morphology of

the electrolyte becomes rough when the content of

SO2�4 /ZrO2 further increased (Fig. 4f), this can be ex-

plained by the aggregation of SO2�4 /ZrO2 at high load-

ing content.

Fig. 5 exhibits the temperature dependence of ionic

conductivity of PEO12–LiClO4 and PEO12–LiClO4–

X%SO2�4 /ZrO2 nanocomposite electrolytes. All samples

show a break around the temperature range from 45 to

60 �C, near the Tm of PEO. However, the degree of this

break in the case of PEO12–LiClO4–X%SO2�4 /ZrO2 is

obviously lower than that in the case of PEO12–LiClO4,

especially at high SO2�4 /ZrO2 loading content, suggest-

ing that the addition of SO2�4 /ZrO2 can, in part, restrain

the recrystallization of PEO in the nanocomposite

LiClO4 (b) and PEO12–LiClO4–X%SO2�4 /ZrO2: (c) X ¼ 5; (d) X ¼ 7;

2.8 3.0 3.2 3.41E-7

1E-6

1E-5

1E-4

1E-3

84.1 60.3 39.5 21.1

T / oCC

ondu

ctiv

ity /

S cm

-1

1000T /K-1 -1

X=0X=2X=5X=7X=10X=13X=16X=20

Fig. 5. Temperature dependence of ionic conductivity of PEO12–

LiClO4–X%SO2�4 /ZrO2 nanocomposite electrolytes.

J. Xi, X. Tang / Chemical Physics Letters 393 (2004) 271–276 275

electrolytes, in accord with the results obtained from

XRD and DSC studies. The maximum value of the ionic

conductivity of PEO12–LiClO4–X%SO2�4 /ZrO2 is ob-

tained at about 7 wt% of SO2�4 /ZrO2 loading content at

all temperatures. In other cases, the filler content has to

be higher (generally, 10%) [6,33]. The super Lewisacidity of SO2�

4 /ZrO2 ensures the much stronger Lewis

acid–base interactions between it and PEO chains, and

thus a small amount of SO2�4 /ZrO2 can effectively im-

prove the ionic conductivity of the electrolyte.

Other electrochemical properties such as interfacial

resistance ðRinterÞ between the electrolyte and the lithium

metal electrode, lithium ion transference number ðTLiþÞ,and electrochemical stability window of the PEO12–Li-ClO4–X%SO2�

4 /ZrO2 nanocomposite electrolyte are also

summarized in Table 1. The existence of the lowest Rinter

can be well explained by the above discussions in SEM

section. The increase of TLiþ of PEO12–LiClO4–SO2�4 /

ZrO2 can be attributed to the increase of ‘free’ Liþ,which further proves the strong Lewis acid–base inter-

actions between SO2�4 /ZrO2 and PEO chains, as men-

tioned in the IR studies. The decomposition voltageof the PEO12–LiClO4–SO

2�4 /ZrO2 increases of about

0.1–0.2 V comparing with the PEO12–LiClO4 complex

and is higher than 4.8 V vs Liþ/Li.

4. Conclusions

A novel PEO-based all solid-state nanocompositepolymer electrolyte is obtained by using solid super acid

SO2�4 /ZrO2 as the filler for the first time. A combination

of X-ray diffraction, thermal analysis, and FT-IR tech-

niques show that SO2�4 /ZrO2 can reduce the crystallinity

of PEO effectively through the strong Lewis acid–base

interactions with PEO chains, leading to obvious en-

hancement of ionic conductivity of the PEO12–LiClO4–

SO2�4 /ZrO2. In addition, SO2�

4 /ZrO2 can also enhancethe lithium ion transference number of the nanocom-

posite electrolyte. The high ionic conductivity, good

interfacial compatibility with lithium metal electrode

and high decomposition voltage indicate that PEO12–

LiClO4–SO2�4 /ZrO2 electrolyte is promising for all solid-

state rechargeable lithium ion batteries.

Acknowledgements

The authors would like to thank Prof. Jun Yang for

helpful discussions. This work was financially supported

by the Key Science and Technology Project of Shanghai

under Grant 02dz11002.

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