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

Solid State Ionics 164 (2003) 81 86 www.elsevier.com/locate/ssi 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 LiClO 4 as doping salts and silane-modified SiO 2 as filler. Electrochemical, thermal and mechanical properties of PEO-based polymer electrolytes mixed with modified SiO 2 were 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 SiO 2 as inert filler, the addition of silane-modified SiO 2 increases the ionic conductivity of the (PEO) 16 LiClO 4 more noticeably, and leads to remarkable enhancement in the mechanical properties of polymer electrolytes. D 2003 Elsevier B.V. All rights reserved. PACS: 82.35.Rs; 73.61.Ph; 72.80.Tm Keywords: Polymer electrolytes; Ionic conductivity; PEO; SiO 2 1. Introduction Solid-state, high-density, rechargeable batteries are important to the development of several applications, from portable electronics to electric vehicles to backup power sources in aircraft. Polymer electrolytes have been quite attractive because they can lead * Corresponding author. Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. Tel./fax: +86-10-6277-3587. E-mail address: cwnan@tsinghua.edu.cn (C.-W. Nan). 0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.08.004 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 polymer 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 maintaining their flexibility and mechanical stability, 转 载

82 L. Fan et al. / Solid State Ionics 164 (2003) 81 86 which extends over a wide temperature range [2]. Among them, the addition of inert phases into polymer electrolytes has attracted considerable attention due to its improved mechanical stabilities and enhanced ionic conductivities and electrolyte/electrode interface stability [3 8]. The increase in conductivity 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 decrease in T g and an increase in the crystalline fraction when adding 22 kinds of fillers into (PEO) 16 LiClO 4. 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 electrolytes. We use silane-coupling agent KH550 to modify SiO 2 particles in order to improve the dispersion of inert SiO 2 particles in the polymer matrix. The modification by the silane molecules that were attached to the SiO 2 surface could make the polar SiO 2 particle surface become a much less polarsililated surface [15,16]. Thermal, electrical and mechanical properties of (PEO) 16 LiClO 4 electrolyte complexed with modified and unmodified SiO 2 were compared. 2. Experimental 2.1. Sample preparation PEO (Alfa Aesar) with an average molecular weight of 300000 was used. LiClO 4 (Alfa Aesar) was dried in a vacuum oven at 100 jc 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. SiO 2 powder with a specific surface area of 249 m 2 /g (BET absorption measurement) and particle size of about 10 nm was used as inert filler. The SiO 2 particles were dried in vacuum oven at 80 jc for 2 days. KH550 silane-coupling agent (Nanjing Chemical Engineering Factory, China) was used to modify the SiO 2 particle surface. The chemical structure of the silane-coupling agent molecule is 2.2. Modification method of SiO 2 particles A certain amount of KH550 silane-coupling agent was dissolved in absolute alcohol and then SiO 2 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 SiO 2 was centrifuged and washed with absolute alcohol to remove the residual silane-coupling agent. Finally, the SiO 2 particles were dried in vacuum oven at 80 jc for 2 days. After treated, the reactions between active silane and SiO 2 would lead to the hydrolysis of three Si O C 2 H 3 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 SiO 2 to form SiUOUSi bonds and release H 2 O as the condensation product [15]. The silane molecules attached to the SiO 2 surface might further link together to form a cross-linking structure of short polyether units surrounding the silica particles. 2.3. Preparation of polymer electrolytes The PEO LiClO 4 concentration ratio was fixed to 16 and the SiO 2 weight percent (both unmodified and modified SiO 2 ) is the amount of SiO 2 added to the total (PEO) 16 LiClO 4 weight. The composites were prepared first by dispersion of dried SiO 2 in acetonitrile with the aid of ultrasonic dispersion, followed by the addition of predetermined amounts of PEO and LiClO 4. 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 jc for 48 h to form the films of about 150 Am in thickness.

L. Fan et al. / Solid State Ionics 164 (2003) 81 86 83 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 electrodes (1 cm in diameter). The impedance response was gauged in air over the range 25 80 jc. Differential scanning calorimetry (DSC) measurements 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 10 jc per min. TGA measurements were carried out under nitrogen atmosphere with DuPont TGA 2050. The mechanical strength of the polymer electrolytes 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 electrolytes was determined at 80 jc by running a linear sweep voltammetry, using stainless steel as a working electrode and lithium as a reference electrode. Shanghai 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 LiClO 4 with various contents of SiO 2 and KH550- modified SiO 2. The glass transition temperature (T g ) 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 T g for (PEO) 16 LiClO 4 with various weight contents of SiO 2 and KH550-modified SiO 2. It shows that T g is lowered when SiO 2 and KH550-modified SiO 2 are added. As T g lowers, the amorphous phase becomes more flexible and the ionic conductivity should be enhanced at low temperature. Fig. 1b shows that T m mostly decreases when SiO 2 and KH550-modified SiO 2 are added. The decrease in T m upon SiO 2 addition is attributed to the appearance of smaller crystallites due to the presence of ceramic Fig. 1. Thermal properties of (PEO) 16 LiClO 4 with various contents of SiO 2 and KH550-modified SiO 2 : (a) glass transition temperature T g, (b) melting temperature T m and (c) heat of melting H m. particles [14]. The area under the curve for the melting endotherm (H m ) is related to crystallinity in the species. Fig. 1c shows that H m increases when the

84 L. Fan et al. / Solid State Ionics 164 (2003) 81 86 SiO 2 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 conduction 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) 16 LiClO 4 /SiO 2, (PEO) 16 LiClO 4 /KH550-modified SiO 2 and the (PEO) 16 LiClO 4, implying that the plasiticizing effect resulting from the residual solvent should be similar for the electrolytes of both unmodified and KH550-modified SiO 2 filler. 3.2. Ionic conductivity Fig. 3 shows temperature dependence of the ionic conductivity for the (PEO) 16 LiClO 4 /SiO 2 composite electrolytes. The increase in ionic conductivity of the composite polymer electrolytes at low temperature is attributed to the two competing effects. The decrease in T g implies an increase in segmental motion of the polymer, and thus a conductivity enhancement, whereas increase in H m implies an increase in crystallinity and thus a conductivity reduction. The conductivity values of the composite polymer electrolytes Fig. 2. TGA of (PEO) 16 LiClO 4 /10 wt.% of SiO 2 and KH550- modified SiO 2 composite electrolytes. Fig. 3. Temperature dependence of ionic conductivity for (PEO) 16 LiClO 4 /SiO 2 composite electrolytes: (a) SiO 2 and (b) KH550-modified SiO 2. depend on the relative content of these two competing effects [14]. At high temperature, where the polymer electrolytes are mostly amorphous, the ionic conductivity increases in comparison with pure (PEO) 16 Li- ClO 4. It indicates that the interface between SiO 2 and polymer/salt electrolyte has crucial importance for ionic conduction. It has been suggested that the weakening of the polyether cation association induced by the ceramic particles might be important for ionic conduction [17]. As seen from Fig. 3, the conductivity increases with the increase in SiO 2 content and then attains a maximum value when SiO 2 concentration is at about 10 wt.% for both SiO 2 and KH550-modified SiO 2. Subsequently, the conductivity decreases with further increasing SiO 2 content. The enhancement in ionic conductivity due to the addition of ceramic fillers has

L. Fan et al. / Solid State Ionics 164 (2003) 81 86 85 Fig. 4. Ionic conductivity for (PEO) 16 LiClO 4 with various contents of SiO 2 and KH550-modified SiO 2 at 30 jc. been explained by an improved effective-medium theory (EMT) [10 13]. From a microscopic perspective, the composite electrolyte can be treated as a quasi two-phase system, which consists of a polymeric ion-conducting matrix with dispersed composite units. The ionic conductivity could arise from the existence of a highly conducting layer at the electrolyte/filler interface [10 13]. This interface layer could be an amorphous polymer layer surrounding SiO 2 [12,13] and/or a space-charge layer [18,19]. At low ceramic filler loadings, the conductivity increases with SiO 2 content basically due to the increase in amount of the conductive layers. The conductivity does not continue to rise indefinitely, with increasing concentration of SiO 2 particles. It falls once an optimum concentration of SiO 2 is crossed. This behavior is a direct consequence of the high concentration of SiO 2, which tends to impede ionic movement by acting as mere insulators [9 11]. For further illustration, Fig. 4 shows the ionic conductivity of (PEO) 16 LiClO 4 with various contents of SiO 2 and KH550-modified SiO 2 at 30 jc. The comparison of Fig. 4 shows that the KH550-modified SiO 2 can produce larger enhancement than unmodified SiO 2, especially at low loading levels. Thus the surface modification of SiO 2 is an effective approach to prepare composite electrolytes with enhanced performance [15]. The modified SiO 2 particles are more compatible with the PEO LiClO 4 solution because the silane molecule carries a short PEO block. As discussed above, the interaction between active silane and SiO 2 leads to a monolayer coverage of the short polyether units on the SiO 2 surface [15,16], and the oxygen atoms from the short polyether units on the SiO 2 surface can compete with oxygen atoms in the PEO backbone for complexation with Li + ions, resulting in a more relaxed coordination between oxygen atoms and Li + ions and thereby facilitating the transport of Li + ions through the polymer [15]. On the other hand, the silane moieties attached on the SiO 2 particle surface can effectively improve the dispersion of the SiO 2 particles in the PEO matrix during blending because of steric repulsive actions. In comparison with the case of unmodified SiO 2 powder, a higher interfacial area between the polymer and fillers can be reached. The increase in the polymer SiO 2 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 composite electrolyte as a function of SiO 2 content is given in Fig. 5. The addition of inert filler, both SiO 2 and KH550-modified SiO 2, apparently increases the tensile strength of polymer films. The reinforcement mechanism is attributed to the adhesion of inorganic Fig. 5. Tensile strength of various PEO SiO 2 composite electrolytes.

86 L. Fan et al. / Solid State Ionics 164 (2003) 81 86 (PEO) 16 LiClO 4 -based composite polymer electrolytes containing silane-modified SiO 2 have been fabricated. 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 SiO 2, modified SiO 2 in PEO-based polymer electrolytes effectively leads to higher enhancement in ionic conductivity and mechanical stability. The enhancement can be attributed to the good dispersion of silane-modified SiO 2 in polymer electrolyte. Acknowledgements Fig. 6. Current voltage response of (PEO) 16 LiClO 4 /SiO 2 electrolytes at 80 jc on stainless steel electrode as a working electrode (scanning rate: 10 mv s 1 ). filler to the macromolecular chain and thus immobilizes the polymer chains [20]. The enhancement in tensile strength of polymer electrolytes caused by KH550-modified SiO 2 is larger than that by unmodified SiO 2, 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 LiClO 4 SiO 2. Both cases exhibit good electrochemical stability up to 5.0 V. For composite complexes with KH550-modified SiO 2, the electrochemical stability window is a little higher than that with unmodified SiO 2. The high window might be related to the high dispersion of SiO 2 particles when they are modified with KH550 silane-coupling agent. The measurement of variation in the ionic conductivity 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 LiClO 4 /10 wt.% TiO 2 polymer electrolyte. 4. Conclusions This work was supported by the MOE of China under grant 20020003079 and Heilongjiang Province Natural Science Foundation (grant No. E0213). References [1] M.M.E. Jacob, E. Hackett, E.P. Giannelis, J. Mater. Chem. 13 (2003) 1. [2] E. Quartarone, P. Mustarelli, A. Magistris, Solid State Ionics 110 (1998) 1. [3] F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nature 394 (1998) 456. [4] L.Z. Fan, C.-W. Nan, Z.M. Dang, Electrochim. Acta 47 (2002) 3541. [5] L.Z. Fan, C.-W. Nan, M. Li, Chem. Phys. Lett. 369 (2003) 698. [6] B. Scrosati, F. Croce, L. Persi, J. Electrochem. Soc. 5 (2000) 1718. [7] G.B. Appetecchi, F. Croce, L. Persi, F. Ronci, B. Scrosati, Electrochim. Acta 45 (2000) 1481. [8] W. Wieczorek, J.R. Stevens, Z. Florjanczyk, Solid State Ionics 85 (1996) 76. [9] C.-W. Nan, Acta Phys. Sin. 36 (1987) 191. [10] C.-W. Nan, D.M. Smith, Mater. Sci. Eng., B 10 (1991) 99. [11] C.-W. Nan, Prog. Mater. Sci. 37 (1993) 1. [12] W. Wieczorek, D. Raducha, A. Zalewska, J. Phys. Chem., B 102 (1998) 8725. [13] W. Wieczorek, A. Zalewska, D. Raducha, Z. Florjanczyk, J.R. Stevens, Macromolecules 29 (1996) 143. [14] B.K. Choi, Y.W. Kim, K.H. Shin, J. Power Sources 68 (1997) 357. [15] Y. Liu, J.Y. Lee, L. Hong, J. Power Sources 109 (2002) 507. [16] Silanes, Silicones and Metal Organics, Gelest Catalog, 2000, p. 75. [17] S.H. Chung, Y. Wang, L. Persi, F. Croce, S.G. Greenbaum, J. Power Sources 97 98 (2001) 644. [18] J. Maier, Prog. Solid State Chem. 23 (1995) 171. [19] N. Sata, K. Eberman, K. Eberl, J. Maier, Nature 408 (2000) 946. [20] R.B. Seymour, Reinforced Plastics, Properties and Application, ASM International, Philadelphia, USA, 1991, p. 52.