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solid polymer electrolyte interface properties
Electrochimica Acta 52 (2006) 1082–1086
Effects of the surface treatment of the Al2O3 filler on the lithium electrode/solid polymer electrolyte interface properties Minato Egashira, Yumi Utsunomiya, Nobuko Yoshimoto, Masayuki Morita ∗ Department of Applied Chemistry, Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan Received 8 May 2006; received in revised form 22 July 2006; accepted 29 July 2006 Available online 1 September 2006
Abstract The lithium deposition–dissolution process in solid polymer electrolytes containing Al2 O3 filler treated under different conditions has been investigated comparing with the ionic conduction behavior of the electrolyte. The composite electrolytes were prepared from poly(ethylene oxide) (PEO), LiBF4 and ␣-Al2 O3 filler by using a dry process, where the surface of ␣-Al2 O3 was beforehand modified by a wet process. The exchange current densities, i0 , of the lithium electrode process in P(EO)20 LiBF4 with and without Al2 O3 filler were determined by a micro-polarization method. The temperature dependence of i0 provided similar values for activation energy, ca. 25 and 70 kJ mol−1 in both temperature regions above and below 60 ◦ C, respectively. The effect of the surface treatment of the filler on the lithium electrode process gave a different tendency from that on the ionic conductivity. The Al2 O3 surface treated by alkali solution enhanced the electrode process to the largest extent among the fillers used here, while it led to rather poor cycling stability in voltammetry. The enhanced reaction rate at the lithium electrode/solid polymer electrolyte interface has probably resulted in the improved ion dissociation by the surface groups of the Al2 O3 filler. © 2006 Elsevier Ltd. All rights reserved. Keywords: Solid polymer electrolyte; Surface-modified alumina; Lithium electrode process
1. Introduction Since the proposal by Armand et al. [1], solid polymer electrolyte has been expected as a potential component of an advanced lithium (Li) battery system with improved reliability and safety. For the practical Li battery application, properties such as high conductivity, high Li+ -ion mobility, compatibility with electrodes, and mechanical strength are recommended for solid polymer electrolyte system. The reversibility and reaction rate of the electrode processes are also considered to be important for the battery systems using solid electrolytes. To our knowledge, however, much less attention has been paid to the interface property between lithium electrode and polymer electrolyte [2,3] than those to the ionic conductivity of the polymer electrolyte. The addition of ceramic fillers, such as SiO2 , Al2 O3 , and TiO2 , in polymer electrolyte systems has been proposed to improve their conductivity and mechanical strength [4–8]. These
∗
Corresponding author. E-mail address: [email protected] (M. Morita).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.07.058
fillers are effective to enhance the ionic conductivity of polymer electrolytes. Several mechanisms have so far been proposed about the conductivity enhancement by the addition of the fillers [9–13]. For such ceramic fillers as SiO2 and Al2 O3 , it is acceptable that the surface of the filler influences the mobility of Li+ . For example, Chung et al. [9] claimed a model based on Lewis acid–base interaction for the enhancement of the Li+ mobility under coexistence of the ceramic filler. That is, the surface of the filler possibly weakens the interaction between polymer chain and Li+ , or accelerates the dissociation of the Li salt, and then creates a new Li+ -conducting pathway in the polymer complex. In contrast, the effects of the ceramic fillers on the electrochemical processes at practical positive and negative electrodes are still unclear, despite of the reaction rates being one of the important factors that determine the cell performances in a practical system. In particular, precise work on a Li metal electrode is important not only from a practical viewpoint due to its potential use as a high-energy electrode, but from fundamental one, for example, utilizing it as a reference electrode for electrochemical measurements. In the present study, the electrode process of Li metal has been examined in poly(ethylene oxide) (PEO)-based solid polymer electrolyte containing ␣-Al2 O3 powder as a filler.
M. Egashira et al. / Electrochimica Acta 52 (2006) 1082–1086
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A dry process was applied for preparing the polymer–ceramics composite electrolyte films [14] to ensure the homogeneous dispersion of the ceramics filler in the polymer. It can also exclude any possible influences of the residual solvent that is used in a conventional solution-casting method for the composite preparation. Effects of the surface modification of Al2 O3 on the reaction kinetics of Li were investigated using a micro-polarization technique. The temperature dependence of the reaction rate is discussed in detail, compared with that of the ionic conductivity of the composites containing differently treated Al2 O3 . 2. Experimental 2.1. Preparation of polymer electrolyte Poly(ethylene oxide) (Mw = 1,000,000, Aldrich) was used as received. LiBF4 (Tomiyama Pure Chemical Co., Japan) was dried for 24 h at 120 ◦ C. The surface modification of ␣-Al2 O3 having 2–3 m of average particle size (Institute for Highly Pure Chemistry) was carried out by the following procedure: Al2 O3 was immersed in 5 mol dm−3 H2 SO4 or 15 mol dm−3 NaOH aqueous solution for 4 days at 95 ◦ C, and then thoroughly rinsed by de-ionized water until the filtrates were neutralized. The residue Al2 O3 powder was dried under a vacuum. The amounts of the acidic site at the surface of Al2 O3 after acid-treatment and the basic site after alkali-treatment were determined per mass by acid–base titration [15]. They were found to be 1.2 mmol g−1 of acid for acid-treatment and 0.6 mmol g−1 of base for alkalitreatment. The sample without treatment (as-dried) gave too low titration values to determine the acidic and basic sites quantitatively. Following three materials; Al2 O3 without treatment, with acid and alkali treatments, were used as the filler and referred as neutral, acidic, and basic Al2 O3 , respectively. Components, LiBF4 , PEO, and Al2 O3 in a prescribed ratio, were mixed and pressed into a homogenous pellet form with ca. 0.6 mm thickness in an Ar-filled glove box. 2.2. Electrochemical measurements A three-electrode cell consisting of Li metal foils as working, counter and reference electrodes was assembled for cyclic voltammetry. The measurement was carried out using a potentiostat (HA-301; Hokuto Denko Co., Japan) equipped with a function generator (HB-104; Hokuto Denko Co.), under a scan rate of 10 mV s−1 and a potential range from −3 to +3 V. During the measurement, the cell temperature was kept at 70 ◦ C. A micro-polarization technique was also employed using the
Fig. 1. Temperature dependence of ionic conductivity of P(EO)20 LiBF4 with and without 10 wt.% of various Al2 O3 filler.
same setup. The current flowing through the cell was controlled, and the electrode potential (overvoltage) was monitored at the steady-state. The current was stepwise increased until the steadystate overvoltage reached to ±5 mV. The ionic conductivity of the polymer electrolytes was measured by an ac method using a two-electrode cell with stainlesssteel electrodes and a frequency response analyzer (NF, S5020C). The frequency was scanned from 100 kHz to 10 Hz, and the ac amplitude was 500 mV. The apparent ionic mobility of Li+ (uLi + ) was determined by a combination method of ac impedance with direct current polarization [16], where the same two-electrode cell, as for the conductivity measurement, was used. 3. Results Fig. 1 shows the temperature dependence of the ionic conductivity of P(EO)20 LiBF4 with and without Al2 O3 filler in an Arrhenius-type format. As reported frequently [9,17,18], PEO-based electrolytes show two separated lines divided at 60 ◦ C regardless of the coexistence or the types of Al2 O3 filler. This profile is associated with the state of the PEO-based electrolyte where a part of PEO phase is fused above its melting point around 60 ◦ C. However, the P(EO)20 LiBF4 composite itself remains apparently solid even at 70 ◦ C. While the coexistence of neutral Al2 O3 improved the conductivity in measured temperature range, acidic Al2 O3 rather decreased the electrolyte
Table 1 Activation energy for ionic conduction and Li electrode process for P(EO)20 LiBF4 electrolyte
Without Al2 O3 With neutral Al2 O3 With acidic Al2 O3 With basic Al2 O3
Ea for ionic conduction (kJ mol−1 )
Ea for Li electrode process (kJ mol−1 )
High T region
Low T region
High T region
Low T region
11 8 8 10
47 37 44 37
25 26 24 28
69 70 66 66
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conductivity. Such a difference suggests that the surface state of Al2 O3 filler greatly influences the ionic conductivity of the PEO-based electrolyte system. The activation energy, Ea , for ionic conduction at high and low temperature regions is summarized in Table 1. The activation energies appeared to be similar regardless of the existence of Al2 O3 filler. The experimental results on the cation transport number (tLi + ) combined with the ionic conductivity showed certain effects of the Al2 O3 filler on the Li+ ionic mobility (uLi + ) in the polymeric solid electrolyte, as similar to that reported previously [18,19]. That is, the addition of neutral Al2 O3 provided rather high value of uLi + . This result is also consistent with those reported by Chung et al., where the Li+ diffusion coefficient was influenced by the addition of ceramic fillers in the liquid electrolyte systems consisting of low molecular weight poly(ethylene glycol) [18]. Despite of the difference in the preparation method, the filler in the electrolyte prepared here appeared to provide the effect similar to that reported in the polymer electrolyte systems. However, effects of the surface treatment of Al2 O3 on the Li+ mobility itself were not obvious in the present solid system. The total ionic conductivity of the system containing the ceramic filler was somewhat influenced by the difference in the surface treatment. These observations may lead to a conclusion that the surface treatment of Al2 O3 influences the anion mobility in the present system, which will be discussed in the latter part of this paper. Fig. 2 shows the cyclic voltammograms recorded at 70 ◦ C for lithium electrode with P(EO)20 LiBF4 polymer electrolytes containing Al2 O3 fillers with and without surface modification. In each case, a redox couple, attributed to the deposition−dissolution of Li, Eq. (1), was clearly observed around 0 V versus Li/Li+ . Li+ + e− = Li
Fig. 2. Cyclic voltammograms of Li in P(EO)20 LiBF4 with and without various Al2 O3 filler. Sweep rate: 10 mV s−1 , temperature: 70 ◦ C.
(1)
The voltammograms measured at lower temperatures (<70 ◦ C) showed similar profiles but higher overvoltage mainly due to higher ohmic resistance of the bulk electrolyte. All polymer electrolytes, except for the one containing basic Al2 O3 , exhibited a reversible cycle behavior. The redox peak current was highest when the polymer electrolyte contained neutral Al2 O3 . The result shown in Fig. 2 suggests that the surface nature of the filler did affect the Li electrode process in the present system. In order to discuss the effect of the filler on the Li electrode process quantitatively, apparent exchange current density was determined in each electrolyte by a micro-polarization technique. When a small extent of overpotential η (|η| ≤ 5 mV) is applied to a cell consisting of Li working, counter, and reference electrodes, the output steady-state current i will have a linear relationship with η as the following equation: RT η= i (2) nFi0
Fig. 3 shows typical i−η relationships, for P(EO)20 LiBF4 with and without the neutral filler, obtained from the polarization measurement at 70 ◦ C. The i0 values were determined from the slope of the polarization curves measured for different electrolyte composition and at various temperatures. Fig. 4
where i0 is exchange current density, and other symbols have their usual meanings. From Eq. (2), a polarization resistance Rp can be written as follows: Rp =
RT nFi0
(3)
Fig. 3. Typical micro-polarization plots for Li in the polymer electrolyte with and without neutral Al2 O3 . Electrolyte: P(EO)20 LiBF4 , temperature: 70 ◦ C.
M. Egashira et al. / Electrochimica Acta 52 (2006) 1082–1086
Fig. 4. Exchange current density as a function of Al2 O3 content in P(EO)20 LiBF4 . Temperature: 70 ◦ C.
shows the i0 values at 70 ◦ C as a function of the Al2 O3 content in P(EO)20 LiBF4 . The i0 value increased with an increase in the Al2 O3 content up to 10 wt.%, but further addition of Al2 O3 tended to decrease the i0 value. Such variation in i0 with the Al2 O3 content suggests that, with as much as 10 wt.% of Al2 O3 , the interface between Li and P(EO)20 LiBF4 accelerates the electrode process, while excess Al2 O3 may decrease the effective Li+ concentration at the interface. The effects of the surface treatment of the filler on the i0 value are also included in Fig. 4 for the case of 10 wt.% addition. Fig. 5 shows Arrhenius-type temperature dependences of the exchange current density i0 for P(EO)20 LiBF4 with and without 10 wt.% of the Al2 O3 filler. The i0 values are the same order of magnitude as those reported previously for poly(ethylene glycol)-based polymer electrolytes [2,20]. The composite electrolyte containing the basic Al2 O3 had a significant effect for increasing the i0 value. All electrolyte systems in the figure showed essentially similar temperature dependence: the slope
Fig. 5. Temperature dependence of exchange current density for Li electrode process in P(EO)20 LiBF4 with and without 10 wt.% of various Al2 O3 filler.
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of the Arrhenius plot was changed at 60 ◦ C. This boundary temperature is known as related to the melting point of PEO. This result suggests that the Li electrode process on the face of P(EO)20 LiBF4 electrolyte was influenced by the fused state of PEO. The activation energy, Ea , of the Li electrode process calculated from the slope of the plots for the high temperature (60–70 ◦ C) and for the low temperature (35–60 ◦ C) regions is also summarized in Table 1. The Ea values were around 25 and 70 kJ mol−1 at high temperature region and low temperature region, respectively, regardless of the existence and the type of the Al2 O3 filler. The activation energy at high temperature region is very close to that observed in 0.5 mol dm−3 LiCF3 SO3 /poly(ethylene glycol) dimethyl ether (PEGDME) viscous liquid electrolyte system reported by Kato et al. [21]. On the other hand, the activation energy at lower temperature end is close to that for lithium ion transfer between PEO-based polymer electrolyte and non-graphitizable carbon electrode, which suggests that the dominant process would be the Li+ -ion migration into passivating layer on the electrode surface [22]. 4. Discussion The activation energy for the Li electrode process in P(EO)20 LiBF4 polymer electrolyte varies with temperature, either higher or lower than 60 ◦ C. That is, the rate-controlling step in the electrode process should be changed around 60 ◦ C. In general, a mass transfer process can be rate-determining for Li electrode process in polymer electrolytes where the Li+ mobility is limited at a low level. The slope change in the Arrhenius plot of i0 at around 60 ◦ C may also remind the process being masscontrolling because of similar temperature profiles between the ionic conductivity and the exchange current density. However, the mass transfer process appears not to be a rate-determining step here because the Ea values for the electrode process are clearly different from those for the ionic conductivity, at both high and low temperature regions. The effect of Al2 O3 filler addition is also different in these cases. The phase transition of polymer (at around 60 ◦ C) may influence the charge transfer process of Li itself through some changes in the coordination circumstance of Li+ in the electrolyte. The conductance behavior of the polymer electrolytes containing surface-modified Al2 O3 indicates that the Al2 O3 surface contributes to the conduction of Li+ in the polymer electrolytes. Basic sites may have an alternative function from acidic sites toward the conduction of Li+ . If the neutral Al2 O3 contains almost equivalent amounts of acidic and basic sites, function of the acidic site will be compensated by that of the basic site. The functions of both acidic and basic sites have been proposed by Park et al. [12]. According to their discussion, the acidic site acts as an anion trapper while the basic site accelerates the dissociation of LiBF4 . This mechanism can explain the difference in the results on effects of the surface treatment on the ionic conductivity and the Li electrode process. The Li electrode process appears to be enhanced by the presence of Al2 O3 filler, while the effect varied with the surface treatment of the filler. It is difficult to visualize such an effect
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because Al2 O3 does not fully cover the electrode surface. Therefore, it is natural to assume that Al2 O3 influences the Li electrode process by changing the property of the bulk polymer electrolyte. In view of the reaction kinetics for the Li electrode process, the Al2 O3 filler does not change Ea at both high temperature and low temperature regions. This suggests that the reaction mechanism is essentially unchangeable regardless the presence of the filler. In such systems Al2 O3 may only increase the activity of Li+ through accelerating the dissociation of LiBF4 salt. Apart from the ionic conductivity, basic Al2 O3 was the most effective for the enhancement of i0 of the Li electrode process. One possible explanation for such behavior is that only a basic site of the Al2 O3 surface is effective to enhance the activity of Li+ at the vicinity of the Li electrode surface while an acidic site provides a small contribution. An acidic site may be reactive toward Li when it has a direct contact, otherwise the effect of an acidic site may be cancelled by the electric field at the vicinity of the lithium electrode. The beneficial effect of the alkali-treatment of Al2 O3 on the enhancement of i0 value may be balanced by rather poor cycle behavior of Li electrode, as shown in Fig. 2. This is probably due to instability at the interface between metal Li and the alkali-treated Al2 O3 . 5. Conclusion The effect of the surface modification of Al2 O3 filler in poly(ethylene oxide)-based electrolyte on the Li electrode process has been investigated by using Al2 O3 filler treated with acid and alkali solutions. The surface-treatment of Al2 O3 influences both ionic conductivity of the polymer electrolyte and the exchange current density of the Li electrode process. The addition of Al2 O3 filler, regardless of its surface conditions, did not affect the activation energy of the Li electrode process as well as the ionic conduction. That is, the Al2 O3 addition enhances the activity of Li+ in the polymer electrolyte. The Al2 O3 filler treated with alkali solution is preferred to enhance the Li electrode process, while it led to rather poor cycling stability in voltammetry. The neutral Al2 O3 is the most effective to increase the
conductivity of the polymer electrolyte among the three kinds of fillers. Acknowledgement This work was financially supported by a Grant-in-Aid for Scientific Research (No. 17550168) from JSPS. References [1] M.B. Armand, J.M. Chabagno, M.J. Duclot, Fast Ion Transport in Solids, Elsevier, Amsterdam, 1979. [2] N. Munichandraiah, L.G. Scanlon, R.A. Marsh, J. Power Sources 72 (1998) 203. [3] B. Bouchet, S. Lascaud, M. Rosso, J. Electrochem. Soc. 150 (2003) A1385. [4] W. Wieczonek, K. Such, H. Wycislik, J. Plochorski, Solid State Ionics 36 (1989) 25. [5] F. Capuano, F. Croce, B. Scrosati, J. Electrochem. Soc. 138 (1991) 1918. [6] M.C. Borghini, M. Mastragostino, S. Passerini, B. Scrosati, J. Electrochem. Soc. 142 (1995) 2118. [7] F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nature 394 (1998) 496. [8] G.B. Appetecchi, F. Croce, L. Persi, F. Ronci, B. Scrosati, Electrochim. Acta 45 (2000) 1481. [9] S.H. Chung, Y. Wang, L. Persi, F. Croce, S.G. Greeenbaum, B. Scrosati, E. Plichta, J. Power Sources 97–98 (2001) 644. [10] B. Kumar, L.G. Scanlon, R.J. Spry, J. Power Sources 96 (2001) 337. [11] H.Y. Sun, Y. Takeda, N. Imanishi, O. Yamamoto, H.-J. Sohn, J. Electrochem. Soc. 147 (2000) 2462. [12] C.H. Park, D.W. Kim, J. Prakash, Y.-K. Sun, Solid State Ionics 159 (2003) 111. [13] P. Johansson, P. Jacobsson, Solid State Ionics 170 (2004) 73. [14] G.B. Appetecchi, F. Croce, J. Hassoun, B. Scrosati, M. Salomon, F. Cassel, J. Power Sources 114 (2003) 105. [15] H. Uchida, M. Tenma, Shokubai (Catalysis in Japanese) 4 (1962) 353. [16] J. Evans, C.A. Vincent, P.G. Bruce, Polymer 28 (1987) 2324. [17] P. Bruce, C.A. Vincent, J. Electroanal. Chem. 225 (1989) 1. [18] S.H. Chung, Y. Wang, S.G. Greenbaum, M. Marcinek, L. Persi, F. Croce, W. Wieczorek, B. Scrosati, J. Phys.: Condens. Matter 13 (2001) 11763. [19] J.H. Shin, Y.T. Lim, K.W. Kim, H.J. Ahn, J.H. Ahn, J. Power Sources 107 (2002) 103. [20] D. Rahner, S. Machill, G. Ludwig, J. Power Sources 54 (1995) 378. [21] Y. Kato, T. Ishihara, Y. Uchimoto, M. Wakihara, J. Phys. Chem. B 108 (2004) 4794. [22] T. Doi, Y. Iriyama, T. Abe, Z. Ogumi, J. Power Sources 142 (2005) 329.
Effects of the surface treatment of the Al2O3 filler on the lithium electrode/solid polymer electrolyte interface properties Minato Egashira, Yumi Utsunomiya, Nobuko Yoshimoto, Masayuki Morita ∗ Department of Applied Chemistry, Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan Received 8 May 2006; received in revised form 22 July 2006; accepted 29 July 2006 Available online 1 September 2006
Abstract The lithium deposition–dissolution process in solid polymer electrolytes containing Al2 O3 filler treated under different conditions has been investigated comparing with the ionic conduction behavior of the electrolyte. The composite electrolytes were prepared from poly(ethylene oxide) (PEO), LiBF4 and ␣-Al2 O3 filler by using a dry process, where the surface of ␣-Al2 O3 was beforehand modified by a wet process. The exchange current densities, i0 , of the lithium electrode process in P(EO)20 LiBF4 with and without Al2 O3 filler were determined by a micro-polarization method. The temperature dependence of i0 provided similar values for activation energy, ca. 25 and 70 kJ mol−1 in both temperature regions above and below 60 ◦ C, respectively. The effect of the surface treatment of the filler on the lithium electrode process gave a different tendency from that on the ionic conductivity. The Al2 O3 surface treated by alkali solution enhanced the electrode process to the largest extent among the fillers used here, while it led to rather poor cycling stability in voltammetry. The enhanced reaction rate at the lithium electrode/solid polymer electrolyte interface has probably resulted in the improved ion dissociation by the surface groups of the Al2 O3 filler. © 2006 Elsevier Ltd. All rights reserved. Keywords: Solid polymer electrolyte; Surface-modified alumina; Lithium electrode process
1. Introduction Since the proposal by Armand et al. [1], solid polymer electrolyte has been expected as a potential component of an advanced lithium (Li) battery system with improved reliability and safety. For the practical Li battery application, properties such as high conductivity, high Li+ -ion mobility, compatibility with electrodes, and mechanical strength are recommended for solid polymer electrolyte system. The reversibility and reaction rate of the electrode processes are also considered to be important for the battery systems using solid electrolytes. To our knowledge, however, much less attention has been paid to the interface property between lithium electrode and polymer electrolyte [2,3] than those to the ionic conductivity of the polymer electrolyte. The addition of ceramic fillers, such as SiO2 , Al2 O3 , and TiO2 , in polymer electrolyte systems has been proposed to improve their conductivity and mechanical strength [4–8]. These
∗
Corresponding author. E-mail address: [email protected] (M. Morita).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.07.058
fillers are effective to enhance the ionic conductivity of polymer electrolytes. Several mechanisms have so far been proposed about the conductivity enhancement by the addition of the fillers [9–13]. For such ceramic fillers as SiO2 and Al2 O3 , it is acceptable that the surface of the filler influences the mobility of Li+ . For example, Chung et al. [9] claimed a model based on Lewis acid–base interaction for the enhancement of the Li+ mobility under coexistence of the ceramic filler. That is, the surface of the filler possibly weakens the interaction between polymer chain and Li+ , or accelerates the dissociation of the Li salt, and then creates a new Li+ -conducting pathway in the polymer complex. In contrast, the effects of the ceramic fillers on the electrochemical processes at practical positive and negative electrodes are still unclear, despite of the reaction rates being one of the important factors that determine the cell performances in a practical system. In particular, precise work on a Li metal electrode is important not only from a practical viewpoint due to its potential use as a high-energy electrode, but from fundamental one, for example, utilizing it as a reference electrode for electrochemical measurements. In the present study, the electrode process of Li metal has been examined in poly(ethylene oxide) (PEO)-based solid polymer electrolyte containing ␣-Al2 O3 powder as a filler.
M. Egashira et al. / Electrochimica Acta 52 (2006) 1082–1086
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A dry process was applied for preparing the polymer–ceramics composite electrolyte films [14] to ensure the homogeneous dispersion of the ceramics filler in the polymer. It can also exclude any possible influences of the residual solvent that is used in a conventional solution-casting method for the composite preparation. Effects of the surface modification of Al2 O3 on the reaction kinetics of Li were investigated using a micro-polarization technique. The temperature dependence of the reaction rate is discussed in detail, compared with that of the ionic conductivity of the composites containing differently treated Al2 O3 . 2. Experimental 2.1. Preparation of polymer electrolyte Poly(ethylene oxide) (Mw = 1,000,000, Aldrich) was used as received. LiBF4 (Tomiyama Pure Chemical Co., Japan) was dried for 24 h at 120 ◦ C. The surface modification of ␣-Al2 O3 having 2–3 m of average particle size (Institute for Highly Pure Chemistry) was carried out by the following procedure: Al2 O3 was immersed in 5 mol dm−3 H2 SO4 or 15 mol dm−3 NaOH aqueous solution for 4 days at 95 ◦ C, and then thoroughly rinsed by de-ionized water until the filtrates were neutralized. The residue Al2 O3 powder was dried under a vacuum. The amounts of the acidic site at the surface of Al2 O3 after acid-treatment and the basic site after alkali-treatment were determined per mass by acid–base titration [15]. They were found to be 1.2 mmol g−1 of acid for acid-treatment and 0.6 mmol g−1 of base for alkalitreatment. The sample without treatment (as-dried) gave too low titration values to determine the acidic and basic sites quantitatively. Following three materials; Al2 O3 without treatment, with acid and alkali treatments, were used as the filler and referred as neutral, acidic, and basic Al2 O3 , respectively. Components, LiBF4 , PEO, and Al2 O3 in a prescribed ratio, were mixed and pressed into a homogenous pellet form with ca. 0.6 mm thickness in an Ar-filled glove box. 2.2. Electrochemical measurements A three-electrode cell consisting of Li metal foils as working, counter and reference electrodes was assembled for cyclic voltammetry. The measurement was carried out using a potentiostat (HA-301; Hokuto Denko Co., Japan) equipped with a function generator (HB-104; Hokuto Denko Co.), under a scan rate of 10 mV s−1 and a potential range from −3 to +3 V. During the measurement, the cell temperature was kept at 70 ◦ C. A micro-polarization technique was also employed using the
Fig. 1. Temperature dependence of ionic conductivity of P(EO)20 LiBF4 with and without 10 wt.% of various Al2 O3 filler.
same setup. The current flowing through the cell was controlled, and the electrode potential (overvoltage) was monitored at the steady-state. The current was stepwise increased until the steadystate overvoltage reached to ±5 mV. The ionic conductivity of the polymer electrolytes was measured by an ac method using a two-electrode cell with stainlesssteel electrodes and a frequency response analyzer (NF, S5020C). The frequency was scanned from 100 kHz to 10 Hz, and the ac amplitude was 500 mV. The apparent ionic mobility of Li+ (uLi + ) was determined by a combination method of ac impedance with direct current polarization [16], where the same two-electrode cell, as for the conductivity measurement, was used. 3. Results Fig. 1 shows the temperature dependence of the ionic conductivity of P(EO)20 LiBF4 with and without Al2 O3 filler in an Arrhenius-type format. As reported frequently [9,17,18], PEO-based electrolytes show two separated lines divided at 60 ◦ C regardless of the coexistence or the types of Al2 O3 filler. This profile is associated with the state of the PEO-based electrolyte where a part of PEO phase is fused above its melting point around 60 ◦ C. However, the P(EO)20 LiBF4 composite itself remains apparently solid even at 70 ◦ C. While the coexistence of neutral Al2 O3 improved the conductivity in measured temperature range, acidic Al2 O3 rather decreased the electrolyte
Table 1 Activation energy for ionic conduction and Li electrode process for P(EO)20 LiBF4 electrolyte
Without Al2 O3 With neutral Al2 O3 With acidic Al2 O3 With basic Al2 O3
Ea for ionic conduction (kJ mol−1 )
Ea for Li electrode process (kJ mol−1 )
High T region
Low T region
High T region
Low T region
11 8 8 10
47 37 44 37
25 26 24 28
69 70 66 66
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conductivity. Such a difference suggests that the surface state of Al2 O3 filler greatly influences the ionic conductivity of the PEO-based electrolyte system. The activation energy, Ea , for ionic conduction at high and low temperature regions is summarized in Table 1. The activation energies appeared to be similar regardless of the existence of Al2 O3 filler. The experimental results on the cation transport number (tLi + ) combined with the ionic conductivity showed certain effects of the Al2 O3 filler on the Li+ ionic mobility (uLi + ) in the polymeric solid electrolyte, as similar to that reported previously [18,19]. That is, the addition of neutral Al2 O3 provided rather high value of uLi + . This result is also consistent with those reported by Chung et al., where the Li+ diffusion coefficient was influenced by the addition of ceramic fillers in the liquid electrolyte systems consisting of low molecular weight poly(ethylene glycol) [18]. Despite of the difference in the preparation method, the filler in the electrolyte prepared here appeared to provide the effect similar to that reported in the polymer electrolyte systems. However, effects of the surface treatment of Al2 O3 on the Li+ mobility itself were not obvious in the present solid system. The total ionic conductivity of the system containing the ceramic filler was somewhat influenced by the difference in the surface treatment. These observations may lead to a conclusion that the surface treatment of Al2 O3 influences the anion mobility in the present system, which will be discussed in the latter part of this paper. Fig. 2 shows the cyclic voltammograms recorded at 70 ◦ C for lithium electrode with P(EO)20 LiBF4 polymer electrolytes containing Al2 O3 fillers with and without surface modification. In each case, a redox couple, attributed to the deposition−dissolution of Li, Eq. (1), was clearly observed around 0 V versus Li/Li+ . Li+ + e− = Li
Fig. 2. Cyclic voltammograms of Li in P(EO)20 LiBF4 with and without various Al2 O3 filler. Sweep rate: 10 mV s−1 , temperature: 70 ◦ C.
(1)
The voltammograms measured at lower temperatures (<70 ◦ C) showed similar profiles but higher overvoltage mainly due to higher ohmic resistance of the bulk electrolyte. All polymer electrolytes, except for the one containing basic Al2 O3 , exhibited a reversible cycle behavior. The redox peak current was highest when the polymer electrolyte contained neutral Al2 O3 . The result shown in Fig. 2 suggests that the surface nature of the filler did affect the Li electrode process in the present system. In order to discuss the effect of the filler on the Li electrode process quantitatively, apparent exchange current density was determined in each electrolyte by a micro-polarization technique. When a small extent of overpotential η (|η| ≤ 5 mV) is applied to a cell consisting of Li working, counter, and reference electrodes, the output steady-state current i will have a linear relationship with η as the following equation: RT η= i (2) nFi0
Fig. 3 shows typical i−η relationships, for P(EO)20 LiBF4 with and without the neutral filler, obtained from the polarization measurement at 70 ◦ C. The i0 values were determined from the slope of the polarization curves measured for different electrolyte composition and at various temperatures. Fig. 4
where i0 is exchange current density, and other symbols have their usual meanings. From Eq. (2), a polarization resistance Rp can be written as follows: Rp =
RT nFi0
(3)
Fig. 3. Typical micro-polarization plots for Li in the polymer electrolyte with and without neutral Al2 O3 . Electrolyte: P(EO)20 LiBF4 , temperature: 70 ◦ C.
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Fig. 4. Exchange current density as a function of Al2 O3 content in P(EO)20 LiBF4 . Temperature: 70 ◦ C.
shows the i0 values at 70 ◦ C as a function of the Al2 O3 content in P(EO)20 LiBF4 . The i0 value increased with an increase in the Al2 O3 content up to 10 wt.%, but further addition of Al2 O3 tended to decrease the i0 value. Such variation in i0 with the Al2 O3 content suggests that, with as much as 10 wt.% of Al2 O3 , the interface between Li and P(EO)20 LiBF4 accelerates the electrode process, while excess Al2 O3 may decrease the effective Li+ concentration at the interface. The effects of the surface treatment of the filler on the i0 value are also included in Fig. 4 for the case of 10 wt.% addition. Fig. 5 shows Arrhenius-type temperature dependences of the exchange current density i0 for P(EO)20 LiBF4 with and without 10 wt.% of the Al2 O3 filler. The i0 values are the same order of magnitude as those reported previously for poly(ethylene glycol)-based polymer electrolytes [2,20]. The composite electrolyte containing the basic Al2 O3 had a significant effect for increasing the i0 value. All electrolyte systems in the figure showed essentially similar temperature dependence: the slope
Fig. 5. Temperature dependence of exchange current density for Li electrode process in P(EO)20 LiBF4 with and without 10 wt.% of various Al2 O3 filler.
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of the Arrhenius plot was changed at 60 ◦ C. This boundary temperature is known as related to the melting point of PEO. This result suggests that the Li electrode process on the face of P(EO)20 LiBF4 electrolyte was influenced by the fused state of PEO. The activation energy, Ea , of the Li electrode process calculated from the slope of the plots for the high temperature (60–70 ◦ C) and for the low temperature (35–60 ◦ C) regions is also summarized in Table 1. The Ea values were around 25 and 70 kJ mol−1 at high temperature region and low temperature region, respectively, regardless of the existence and the type of the Al2 O3 filler. The activation energy at high temperature region is very close to that observed in 0.5 mol dm−3 LiCF3 SO3 /poly(ethylene glycol) dimethyl ether (PEGDME) viscous liquid electrolyte system reported by Kato et al. [21]. On the other hand, the activation energy at lower temperature end is close to that for lithium ion transfer between PEO-based polymer electrolyte and non-graphitizable carbon electrode, which suggests that the dominant process would be the Li+ -ion migration into passivating layer on the electrode surface [22]. 4. Discussion The activation energy for the Li electrode process in P(EO)20 LiBF4 polymer electrolyte varies with temperature, either higher or lower than 60 ◦ C. That is, the rate-controlling step in the electrode process should be changed around 60 ◦ C. In general, a mass transfer process can be rate-determining for Li electrode process in polymer electrolytes where the Li+ mobility is limited at a low level. The slope change in the Arrhenius plot of i0 at around 60 ◦ C may also remind the process being masscontrolling because of similar temperature profiles between the ionic conductivity and the exchange current density. However, the mass transfer process appears not to be a rate-determining step here because the Ea values for the electrode process are clearly different from those for the ionic conductivity, at both high and low temperature regions. The effect of Al2 O3 filler addition is also different in these cases. The phase transition of polymer (at around 60 ◦ C) may influence the charge transfer process of Li itself through some changes in the coordination circumstance of Li+ in the electrolyte. The conductance behavior of the polymer electrolytes containing surface-modified Al2 O3 indicates that the Al2 O3 surface contributes to the conduction of Li+ in the polymer electrolytes. Basic sites may have an alternative function from acidic sites toward the conduction of Li+ . If the neutral Al2 O3 contains almost equivalent amounts of acidic and basic sites, function of the acidic site will be compensated by that of the basic site. The functions of both acidic and basic sites have been proposed by Park et al. [12]. According to their discussion, the acidic site acts as an anion trapper while the basic site accelerates the dissociation of LiBF4 . This mechanism can explain the difference in the results on effects of the surface treatment on the ionic conductivity and the Li electrode process. The Li electrode process appears to be enhanced by the presence of Al2 O3 filler, while the effect varied with the surface treatment of the filler. It is difficult to visualize such an effect
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because Al2 O3 does not fully cover the electrode surface. Therefore, it is natural to assume that Al2 O3 influences the Li electrode process by changing the property of the bulk polymer electrolyte. In view of the reaction kinetics for the Li electrode process, the Al2 O3 filler does not change Ea at both high temperature and low temperature regions. This suggests that the reaction mechanism is essentially unchangeable regardless the presence of the filler. In such systems Al2 O3 may only increase the activity of Li+ through accelerating the dissociation of LiBF4 salt. Apart from the ionic conductivity, basic Al2 O3 was the most effective for the enhancement of i0 of the Li electrode process. One possible explanation for such behavior is that only a basic site of the Al2 O3 surface is effective to enhance the activity of Li+ at the vicinity of the Li electrode surface while an acidic site provides a small contribution. An acidic site may be reactive toward Li when it has a direct contact, otherwise the effect of an acidic site may be cancelled by the electric field at the vicinity of the lithium electrode. The beneficial effect of the alkali-treatment of Al2 O3 on the enhancement of i0 value may be balanced by rather poor cycle behavior of Li electrode, as shown in Fig. 2. This is probably due to instability at the interface between metal Li and the alkali-treated Al2 O3 . 5. Conclusion The effect of the surface modification of Al2 O3 filler in poly(ethylene oxide)-based electrolyte on the Li electrode process has been investigated by using Al2 O3 filler treated with acid and alkali solutions. The surface-treatment of Al2 O3 influences both ionic conductivity of the polymer electrolyte and the exchange current density of the Li electrode process. The addition of Al2 O3 filler, regardless of its surface conditions, did not affect the activation energy of the Li electrode process as well as the ionic conduction. That is, the Al2 O3 addition enhances the activity of Li+ in the polymer electrolyte. The Al2 O3 filler treated with alkali solution is preferred to enhance the Li electrode process, while it led to rather poor cycling stability in voltammetry. The neutral Al2 O3 is the most effective to increase the
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