Effect of modified SiO 2 on the properties of PEO-based polymer electrolytes

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Effect of modified SiO

₂

on the properties of PEO-based

polymer electrolytes

Lizhen Fan

a,b

, Ce-Wen Nan

a,b,

*, Shujin Zhao

c a

State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China b

Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c

Department of Materials Engineering, Jiamusi University, Jiamusi 154007, China

Received 9 May 2003; received in revised form 20 July 2003; accepted 15 August 2003

Abstract

Composite polymer electrolytes based on poly(ethylene oxide) (PEO) were prepared by using LiClO4as doping salts and

silane-modified SiO2as filler. Electrochemical, thermal and mechanical properties of PEO-based polymer electrolytes mixed

with modified SiO2were studied. Differential scanning calorimetry (DSC) results showed that in the presence of the filler,

there is a decrease in the glass transition temperature of the electrolyte, whereas, on the other hand, enhances the crystallinity of the sample. Compared with unmodified SiO2 as inert filler, the addition of silane-modified SiO2 increases the ionic

conductivity of the (PEO)16LiClO4more noticeably, and leads to remarkable enhancement in the mechanical properties of

polymer electrolytes.

Keywords:Polymer electrolytes; Ionic conductivity; PEO; SiO2

1. Introduction

Solid-state, high-density, rechargeable batteries are important to the development of several applica-tions, from portable electronics to electric vehicles to backup power sources in aircraft. Polymer electro-lytes have been quite attractive because they can lead

to flexible, compact, laminated solid-state structures free from leaks and available in different geometries

[1]. A solid polymer electrolyte serves as both a separator to prevent electrodes from coming into physical contact, and more importantly, as an ionic conductor. Poly(ethylene oxide) (PEO)-based poly-mer electrolytes have been the most extensively studied polymer ionic conductors because of the beneficial structure in supporting fast ion transport. Unfortunately, a high crystalline phase concentration limits the conductivity of PEO-based electrolytes. Various methods have been applied to reduce the crystallinity of PEO-based electrolytes while main-taining their flexibility and mechanical stability,

* Corresponding author. Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. Tel./fax: +86-10-6277-3587.

E-mail address:[email protected] (C.-W. Nan).

www.elsevier.com/locate/ssi Solid State Ionics 164 (2003) 81 – 86

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which extends over a wide temperature range [2]. Among them, the addition of inert phases into polymer electrolytes has attracted considerable atten-tion due to its improved mechanical stabilities and enhanced ionic conductivities and electrolyte/elec-trode interface stability [3 – 8]. The increase in con-ductivity has been found to depend upon the concentration and particle size of the inert phases

[9 – 13]. The improvement in the ionic conduction observed has been generally assigned to the decrease of polymeric crystallinity after the addition of the fillers [3]. However, Choi et al. [14] found a

de-crease in Tg and an increase in the crystalline

fraction when adding 22 kinds of fillers into (PEO)16LiClO4. The exact role played by such oxide

fillers in PEO-based composite electrolytes still remains to be quantified.

The principal motivation of this work is to investigate the effect of interaction between the polymer and the oxide fillers by modification of filler surfaces on the properties of composite electro-lytes. We use silane-coupling agent KH550 to mod-ify SiO2particles in order to improve the dispersion

of inert SiO2 particles in the polymer matrix. The

modification by the silane molecules that were attached to the SiO2 surface could make the polar

SiO2 particle surface become a much less

polar-sililated surface [15,16]. Thermal, electrical and mechanical properties of (PEO)16LiClO4 electrolyte

complexed with modified and unmodified SiO2were

compared.

2. Experimental

2.1. Sample preparation

PEO (Alfa Aesar) with an average molecular

weight of 300 000 was used. LiClO4 (Alfa Aesar)

was dried in a vacuum oven at 100j_{C for 24 h and}

then stored in a desiccator, prior to use. Acetonitrile was refluxed at a suitable temperature under nitrogen atmosphere, prior to use. SiO2powder with a specific

surface area of 249 m2/g (BET absorption measure-ment) and particle size of about 10 nm was used as inert filler. The SiO2particles were dried in vacuum

oven at 80 j_{C for 2 days. KH550 silane-coupling}

agent (Nanjing Chemical Engineering Factory, China)

was used to modify the SiO2 particle surface. The

chemical structure of the silane-coupling agent mole-cule is

2.2. Modification method of SiO2particles

A certain amount of KH550 silane-coupling agent was dissolved in absolute alcohol and then SiO2 was added to form a suspension. The mixture

was stirred at room temperature for 24 h and stayed still for 24 h. Then the resultant SiO2 was

centri-fuged and washed with absolute alcohol to remove the residual silane-coupling agent. Finally, the SiO2

particles were dried in vacuum oven at 80 j_{C for 2}

days. After treated, the reactions between active

silane and SiO2 would lead to the hydrolysis of

three Si – O – C2H3 groups in the silane molecule

[15,16], which produces three Si – OH groups. These OH groups then reacted with the OH groups on the

surface of SiO2 to form SiUOUSi bonds and

release H2O as the condensation product [15]. The

silane molecules attached to the SiO2 surface might

further link together to form a cross-linking struc-ture of short polyether units surrounding the silica particles.

2.3. Preparation of polymer electrolytes

The PEO – LiClO4 concentration ratio was fixed

to 16 and the SiO2weight percent (both unmodified

and modified SiO2) is the amount of SiO2added to

the total (PEO)16LiClO4 weight. The composites

were prepared first by dispersion of dried SiO2 in

acetonitrile with the aid of ultrasonic dispersion, followed by the addition of predetermined amounts of PEO and LiClO4. This solution was stirred at

room temperature for approximately 24 h until the mixture appeared to be homogeneous. The mixture was cast on a Teflon plate followed by evaporating solvent in an argon-filled glove box for 24 h. Finally, the samples were dried under vacuum at 80 j_{C for 48 h to form the films of about 150} Am in thickness.

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2.4. Characterization of polymer electrolytes

The complex impedance was measured by using an HP 4192A LF impedance analyzer in the frequency range from 5 Hz to 2 MHz. The composite film was sandwiched between stainless steel blocking electro-des (1 cm in diameter). The impedance response was gauged in air over the range 25 – 80j_C.

Differential scanning calorimetry (DSC) measure-ments were carried out under nitrogen atmosphere with DuPont TA 2910-modulated DSC. Samples were loaded in hermetically sealed aluminum pans and measurements were taken at a heating rate of 10j_C

per min. TGA measurements were carried out under nitrogen atmosphere with DuPont TGA 2050.

The mechanical strength of the polymer electro-lytes was measured from stress – strain tests using a Shimadzu AGS-10KNG instrument. The samples had been kept in a desiccator until the experiment, which lasted for at most 5 min for each sample.

The electrochemical stability of the polymer elec-trolytes was determined at 80j_{C by running a linear}

sweep voltammetry, using stainless steel as a working electrode and lithium as a reference electrode. Shang-hai CHI 660A electrochemical workstation was used for voltammetry measurement.

3. Results and discussion

3.1. Thermal analysis of polymer electrolyte

Fig. 1 shows the thermal properties of (PEO)16

LiClO4 with various contents of SiO2 and

KH550-modified SiO2. The glass transition temperature (Tg)

involves the freezing of large-scale molecular motion without a change in structure at which a glassy phase of the sample becomes a rubbery amorphous phase on heating. Fig. 1a shows Tg for (PEO)16LiClO4 with

various weight contents of SiO2and KH550-modified

SiO2. It shows that Tg is lowered when SiO2 and

KH550-modified SiO2 are added. As Tg lowers, the

amorphous phase becomes more flexible and the ionic conductivity should be enhanced at low temperature.

Fig. 1b shows that Tm mostly decreases when SiO2

and KH550-modified SiO2are added. The decrease in Tmupon SiO2addition is attributed to the appearance

of smaller crystallites due to the presence of ceramic

particles[14]. The area under the curve for the melting endotherm (Hm) is related to crystallinity in the

species. Fig. 1c shows that Hm increases when the

Fig. 1. Thermal properties of (PEO)16LiClO4with various contents

of SiO2and KH550-modified SiO2: (a) glass transition temperature Tg, (b) melting temperatureTmand (c) heat of meltingHm.

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SiO2 particles are added. It means that the volume

fraction of crystalline phase increases. This is opposite to suggestions that inert fillers enhance the formation of an amorphous phase[3,6,8]. But it is in accordance with the results recently by Choi et al. [14], who reported that the formation of crystalline phase is possible if the filler particle acts as a nucleation center of the crystalline polymer phase. Because ionic con-duction takes place primarily through the amorphous phase of the polymer, high crystallinity would lead to low conductivity.

The TGA measurement (Fig. 2) shows that the

amount of the residual solvent is similar in the (PEO)16LiClO4/SiO2, (PEO)16LiClO4

/KH550-modi-fied SiO2and the (PEO)16LiClO4, implying that the

plasiticizing effect resulting from the residual solvent should be similar for the electrolytes of both unmod-ified and KH550-modunmod-ified SiO2filler.

3.2. Ionic conductivity

Fig. 3shows temperature dependence of the ionic conductivity for the (PEO)16LiClO4/SiO2 composite

electrolytes. The increase in ionic conductivity of the composite polymer electrolytes at low temperature is attributed to the two competing effects. The decrease inTgimplies an increase in segmental motion of the

polymer, and thus a conductivity enhancement, whereas increase in Hm implies an increase in

crys-tallinity and thus a conductivity reduction. The con-ductivity values of the composite polymer electrolytes

depend on the relative content of these two competing effects[14]. At high temperature, where the polymer electrolytes are mostly amorphous, the ionic conduc-tivity increases in comparison with pure (PEO)16

Li-ClO4. It indicates that the interface between SiO2and

polymer/salt electrolyte has crucial importance for ionic conduction. It has been suggested that the weakening of the polyether – cation association in-duced by the ceramic particles might be important for ionic conduction[17].

As seen from Fig. 3, the conductivity increases with the increase in SiO2content and then attains a

maximum value when SiO2concentration is at about

10 wt.% for both SiO2 and KH550-modified SiO2.

Subsequently, the conductivity decreases with further increasing SiO2 content. The enhancement in ionic

conductivity due to the addition of ceramic fillers has

Fig. 2. TGA of (PEO)16LiClO4/10 wt.% of SiO2 and

KH550-modified SiO2composite electrolytes.

Fig. 3. Temperature dependence of ionic conductivity for (PEO)16LiClO4/SiO2 composite electrolytes: (a) SiO2 and (b)

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been explained by an improved effective-medium theory (EMT)[10 – 13]. From a microscopic perspec-tive, the composite electrolyte can be treated as a quasi two-phase system, which consists of a polymer-ic ion-conducting matrix with dispersed composite units. The ionic conductivity could arise from the existence of a highly conducting layer at the electro-lyte/filler interface[10 – 13]. This interface layer could

be an amorphous polymer layer surrounding SiO2

[12,13] and/or a space-charge layer [18,19]. At low ceramic filler loadings, the conductivity increases with SiO2 content basically due to the increase in

amount of the conductive layers. The conductivity does not continue to rise indefinitely, with increasing concentration of SiO2particles. It falls once an

opti-mum concentration of SiO2is crossed. This behavior

is a direct consequence of the high concentration of

SiO2, which tends to impede ionic movement by

acting as mere insulators[9 – 11].

For further illustration, Fig. 4 shows the ionic conductivity of (PEO)16LiClO4with various contents

of SiO2 and KH550-modified SiO2 at 30 jC. The

comparison ofFig. 4shows that the KH550-modified SiO2 can produce larger enhancement than

unmodi-fied SiO2, especially at low loading levels. Thus the

surface modification of SiO2is an effective approach

to prepare composite electrolytes with enhanced per-formance[15]. The modified SiO2particles are more

compatible with the PEO – LiClO4 solution because

the silane molecule carries a short PEO block. As

discussed above, the interaction between active silane and SiO2leads to a monolayer coverage of the short

polyether units on the SiO2surface [15,16], and the

oxygen atoms from the short polyether units on the SiO2surface can compete with oxygen atoms in the

PEO backbone for complexation with Li+ions, result-ing in a more relaxed coordination between oxygen atoms and Li+ ions and thereby facilitating the trans-port of Li+ ions through the polymer [15]. On the other hand, the silane moieties attached on the SiO2

particle surface can effectively improve the dispersion

of the SiO2 particles in the PEO matrix during

blending because of steric repulsive actions. In com-parison with the case of unmodified SiO2powder, a

higher interfacial area between the polymer and fillers can be reached. The increase in the polymer – SiO2

interfaces raises the proportion of effective media for ion conduction in the electrolyte, which leads to enhancement in ionic conductivity.

3.3. Mechanical properties

The mechanical property of a polymer electrolyte during charge/discharge cycles is vital for a safe and endurable battery. The tensile strength of the compos-ite electrolyte as a function of SiO2content is given in

Fig. 5. The addition of inert filler, both SiO2 and

KH550-modified SiO2, apparently increases the

ten-sile strength of polymer films. The reinforcement mechanism is attributed to the adhesion of inorganic

Fig. 4. Ionic conductivity for (PEO)16LiClO4with various contents

of SiO2and KH550-modified SiO2at 30jC.

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filler to the macromolecular chain and thus immobil-izes the polymer chains [20]. The enhancement in tensile strength of polymer electrolytes caused by KH550-modified SiO2is larger than that by

unmod-ified SiO2, which could be attributed to the structure

of silane[15,16].

3.4. Electrochemistry stability window

Fig. 6 shows the current – voltage response obtained from polymer electrolytes based on PEO – LiClO4– SiO2. Both cases exhibit good

electrochem-ical stability up to 5.0 V. For composite complexes with KH550-modified SiO2, the electrochemical

sta-bility window is a little higher than that with unmod-ified SiO2. The high window might be related to the

high dispersion of SiO2 particles when they are

modified with KH550 silane-coupling agent.

The measurement of variation in the ionic conduc-tivity of these composite electrolytes with time (not presented here) showed that there was no significant change in their conductivity with time, as observed previously by Croce et al. [3] for PEO – LiClO4/10

wt.% TiO2polymer electrolyte.

4. Conclusions

(PEO)16LiClO4-based composite polymer

electro-lytes containing silane-modified SiO2have been

fab-ricated. DSC results indicate that the volume fraction of crystalline phase in composite polymer electrolytes increases but the amorphous phase becomes more flexible. Compared with unmodified SiO2, modified

SiO2 in PEO-based polymer electrolytes effectively

leads to higher enhancement in ionic conductivity and mechanical stability. The enhancement can be attrib-uted to the good dispersion of silane-modified SiO2in

polymer electrolyte.

Acknowledgements

This work was supported by the MOE of China under grant 20020003079 and Heilongjiang Province Natural Science Foundation (grant No. E0213).

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