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Study of the electrochemical performance of spinel LiMn2O4 at high temperature based on the polymer modified electrode
Electrochemistry Communications 7 (2005) 383–388 www.elsevier.com/locate/elecom
Study of the electrochemical performance of spinel LiMn2O4 at high temperature based on the polymer modified electrode Guohe Hu a,b, Xiaobing Wang a,b, Fang Chen a,b, Jianyin Zhou a,b, Rengui Li a, Zhenghua Deng a,* a
Chengdu Institute of Organic Chemistry, the Chinese Academy of Sciences, 9, RenMin South Road 4, Chengdu 610041, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 4 January 2005; received in revised form 8 February 2005; accepted 8 February 2005 Available online 26 February 2005
Abstract A novel method improving the performance of spinel LiMn2O4 at high temperature is studied. The copolymer was put into the interspaces of the LiMn2O4 electrode layer prepared by maleic anhydride and acrylamide for decorating LiMn2O4, involving groups to compound with the manganese-ion. It is found that with the act of modification of the functional copolymer, the 45th discharge capacity was improved at 55 C from 56.8 to 81.4 mAh g1 on the LiMn2O4 electrode. Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron micrographs (SEM), inductively coupled plasma atomic emission spectrometry (ICP-AES) and a series of electrochemical measurements have been conducted in this work, and the effects of modification of the polymer layer are discussed. 2005 Elsevier B.V. All rights reserved. Keywords: Cathode; Spinel LiMn2O4; Modified electrode; Copolymer; Capacity fading
1. Introduction The spinel-structure lithium manganese oxide has been realized as the most promising positive-electrode material for lithium-ion batteries because of such properties as low cost, abundant precursors, non-toxicity and environmental advantages [1–3]. The rechargeability has been greatly improved. However, spinel LiMn2O4 electrode has an obstacle that its capacity fades faster on cycling at elevated temperature than at room temperature, namely, its electrochemical performance is poor at elevated temperature. The reasons for capacity fading and mechanism of spinel LiMn2O4 at elevated temperature have been reported over the last years, as widely be*
Corresponding author. Tel.: +86 28 85229252; fax: +86 28 85233426/85250719. E-mail address: [email protected] (Z. Deng). 1388-2481/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.02.010
lieved: an unstable two-phase structure co-existing changed into a stable one-phase structure in the highvoltage (4.15 V) region and the manganese oxide directly dissolved in the electrolyte and the electrolyte solution decomposed on the electrode [4,6]. It was also comprehended that for the spinel structure the disproportionate reaction 4þ 2þ 2Mn3þ ðsolÞ ! MnðsolÞ þ MnðliqÞ
was accelerated along with the increasing of temperature, which caused to increase the entropy of the positive-ion, forming the out-of-order spinel structure [5,7], at the same time this process accelerated the dissolution of manganese cations and in evidence it also catalyzed the electrolyte solution decomposition [8]. The decomposition of the electrolyte solution and existing of a small amount of water molecules in electrolyte could supply hydrogen ions [9], the reaction between H+ and
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LiMn2O4 again accelerated the dissolution of manganese cations. This bad cycling expedites the capacity of spinel LiMn2O4 fading at elevated temperature, ultimately, the protonic c-MnO2 without the electrochemical activity was produced [10–12]. In order to improve the stability of LiMn2O4 spinel cathodes at high temperature, several methods have been investigated. Among them, doping partial substitution of manganese ions by other metal ions [13–17], coating of the surface of LiMn2O4 particles by organic oxides [18–21] and modification of the active conducting polymer were mainly reported [22–25], some works on modification of electrolyte were reported [26–28], too. But the practicability of the three methods above was still not perfect. In this manuscript, we studied the high-temperature electrochemical performance of spinel LiMn2O4 through treating the surface of the spinel electrode by a functional copolymer. The polymer should possess the anti-oxidative capability and be tiny expanding instead of dissolving while dipping in the electrolyte for a long time. Thus, the polymer contained groups to compound with those manganese cations for decorating LiMn2O4 was designed. Based on above requirements, we used the copolymer of maleic anhydride (MA) and acrylamide (AM) to modify the LiMn2O4 spinel.
cm2 between 3.00 and 4.30 V. Cyclic voltammetry was performed on the LiMn2O4 electrode with a scan rate of 0.1mV s1 in the potential range of 3.0–4.6 V (vs. Li/Li+) by an Arbin BT2000 system. Electrochemical impedance measurements were performed with a Lock in amplifier (PE model 5210) electrochemical interface under remote control by personal computer. The frequency range between 100 kHz and 100 mHz.
3. Results and discussion 3.1. Characteristics of the copolymer Infrared spectroscopy has been used to characterize the chain structure of polymers. Fourier transformed infrared spectroscopy of the copolymer p(MA-AM) has been shown in Fig. 1. It exhibits that, two characteristic peaks at 1775.9 and 1843.0 cm1 represent the typical anhydride group. The peak at 3440.4 cm1 is the characteristic peak of the –NH2 group, while that frequency 1667.2 cm1 indicates the AC@O double bond structure. Based on our experimental conditions, we consider that this copolymer is p(MA-AM) production of the above-mentioned polymerization reactions. 3.2. Scanning electron micrographs
2. Experimental The copolymer was prepared by maleic anhydride (MA) and acrylamide (AM). These polymerization reactions were in the butanone solution at 50 C about 8 h. The product of the polymerization was filtrated, and washed by the butanone solution then dried at 100 C. The composition of these polymers was determined by Fourier transformed infrared spectroscopy. The modified LiMn2O4 electrode was prepared through two steps, as described below. First, the electrode with composition of 90:7:3(wt.) LiMn2O4 (purchased from Huilong company, YunNan, China): acetylene: LA132 (an aqueous binder, Indigo company, ChengDu, China) was coated onto an aluminum foil as the current collector followed by drying at 70 C. Secondly, it was soaked into the N,N-dimethylformamide solution of p(MA-AM) several minutes and dried at about 100 C. The charge-discharge characteristics of the modified electrode were examined in a laboratory cell consisting of a positive electrode and a negative electrode that separated by a porous polypropylene membrane. The cell negative electrode was lithium metal, which was in excess. 1 M LiPF6–ethylene carbonate/dimethyl carbonate (1:1 in weight) was used as the electrolyte. Cycling performance of the cell was measured with a BS-9300 Secondary Battery Tester (QingTian, China) at 0.65 mA/
We consider the LiMn2O4 electrode film as an inorganic coat with the binder and conducting carbon black. Inside the coat there always exist a large lot of interspaces. Hereby, if a functional polymer put into these interspaces, forming the spinel LiMn2O4 modified electrode decorate LiMn2O4, the modification must be effective. Fig. 2 shows the differences scanning electron micrographs (SEM) images of the spinel LiMn2O4 film before and after dipping into the copolymer butanone solution.
Fig. 1. FTIR spectra of p(MA-AM).
G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388
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Fig. 2. SEM images of the spinel LiMn2O4 electrode (a) unmodified electrode; (b) modified electrode.
From Fig. 2(a) it can be seen that in the spinel LiMn2O4 there exists a great quantity of different-scaled interspaces. From Fig. 2(b) we consider the copolymer p(MA-AM) could completely soak into interspaces, forming a reticulate structure. 3.3. Inductively coupled plasma atomic emission spectrometry In these experiments, the dissolvable manganese solution in LiPF4/EC + DMC electrolyte was prepared. The porous polypropylene membrane and the electrolyte and the two electrodes washed by dilute hydrochloric acid, and the quality of the manganese element was directly determined by using inductively coupled plasma atomic emission spectrometry (ICP-AES). It can be seen in Table 1, after the 45th cycle at 55 C the dissolved quality of Mn of the modified and unmodified LiMn2O4 electrode was 6.4 and 12.4 lg, respectively. Correspondingly, the dissolved quality percentage of manganese element of modified electrode in electrolyte (0.097%) was merely one half of that of the unmodified LiMn2O4 electrode (0.190%). Therefore, with the polymer solution treatment, the p(MA-AM) penetrate these interspaces of the LiMn2O4 film was successful in reducing the dissolution of the manganese oxide. 3.4. X-ray photoelectron spectroscopy Fig. 3 illustrates the XPS emission of Mn(2p) of LiMn2O4 electrode and modified electrode. The binding energy values of Mn(2p) of the spinels are list in Table 2. It is very interesting that the peak of Mn(2p3/2) emission
XPS Intensity (a.u)
a
b
660
Unmodified Modified
The quality of Mn on the LiMn2O4 electrode (mg) 6.5 6.6
The dissolved quality of Mn (lg) 12.4a 6.4b
The dissolved quality percentage of Mn (%) 0.190% 0.097%
a,b shown is the 45th cycle of the spinel LiMn2O4 unmodified and modified electrode at 55 C, respectively.
648
642
636
Binding Energy (eV) Fig. 3. XPS emission of Mn(2p) of spinels LiMn2O4 (a) unmodified electrode; (b) modified electrode.
Table 2 XPS emission of Mn(2p) of LiMn2O4 spinels Electrode
Binding energy (eV) Mn(2p3/2)
Mn(2p1/2)
Mn(2p)
Unmodified electrode Modified electrode
641.420 642.080
652.926 653.370
11.506 11.290
shifts from 641.420 to 642.080 eV. This phenomenon is possibly due to the interaction of electron and polarization at the spinel LiMn2O4 interface, which are groups of the copolymer such as anhydride groups and CO– NH2 groups to compound with the manganese-ion. On the other hand, the disproportionate reaction 4þ 2þ 2Mn3þ ðsolÞ ! MnðsolÞ þ MnðliqÞ becomes difficult because it requires extra energy to destroy the spinel structure. Thus, the copolymer p(MA-AM) put into these interspaces could be likely to play a rule in protecting and stabilizing the spinel structure. ? O Mn
Mn
H O Mn
O
H O
O Mn
? Mn
Mn
O
O
the LiMn2O4 electrode
Table 1 Amount of LiMn2O4 dissolved in LiPF4/EC + DMC electrolyte Electrode
654
CH
n O O Mn
Mn O
CH
HC
CH2
n
NH2
O H O
H
O
O
O Mn
Mn
Mn
O Mn
O
O
the LiMn2O4 electrode Fig. 4. Model of anhydride and amid groupsÕ complexing with the surface manganese cations.
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G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388 140
unmodified electrode modified electrode
130
specific discharge (mAh/g)
120 110 100 90 80 70 60 50 40 30 0
5
10
15
20
25
30
35
40
45
50
cyclenumber Fig. 6. Typical cycle life curves (specific discharge capacity vs. cycle number) modified and unmodified spinel LiMn2O4. Lithium batteries, 55 C, 1 M LiPF6/EC–DMC (1:1), C/6 rates.
Fig. 5. Cyclic voltammogram of the unmodified (a) and modified (b) LiMn2O4 /Li cell in 1 M LiPF6 ethylene carbonate/dimethyl carbonate (1:1 in weight) solution at room temperature. The applied potential was scanned from 3.0 to 4.6 V with a scan rate of 0.1 mV s.
In addition, it has been reported [18–20] that surface treatments could improve the elevated temperature storage characteristic of spinel LiMn2O4. As described in Ref. [11], the method of surface treatment was performed with the agent acetylaceton, manganese cations existing on the surface of the Æ1 1 1æ planes are not fully coordinated by oxygen anions. It utilized the dangling bonds of the manganese cations in the Æ1 1 1æ planes on the surface of the LiMn2O4 particles. It is conceivable that, these anion groups also utilize a complexing reaction to remove the active manganese cations of the surface. So, we imitate the model of G.G. Amatucci as shown in Fig. 4, the anhydride groups and the amide groups complex directly with these cations on the surface of the LiMn2O4 particles, and it could remove the active sites of manganese cations, and that those the dangling bonds of these surface cations may contribute to a large extent of the catalytic properties of the spinel toward electrolyte oxidation. As results in Fig. 7, we found it could improve the elevated temperature performance of LiMn2O4 spinel.
typical oxidation processes of LiMn2O4 in 4.05 and 4.15 V (vs. Li/Li+) and de-oxidation processes in the 3.9 V domain, which involves phase transitions [4,6]. As is seen in Fig. 5(b), the CV peaks related to the p(MA-AM) modified are much more steady, compared to the peaks of the unmodified LiMn2O4 electrode. These differences mean that the cyclestability of the modified of p(MA-AM) is better than that of the standard, unmodified material. Correspondingly, it also demonstrates the above-mentioned results. The relationship between specific discharge capacity and cycle number of a lithium battery cycled at a constant current rate of C/6 between 3.0 and 4.3 V at 55 C is depicted in Fig. 6. As shown, the lithium battery with modified electrode as the cathode active material has better rechargeability than LiMn2O4 electrode. In general, both electrodes exhibited high initial discharge capacity. However, modified electrode shows the better
3.5. Electrochemical behavior To investigate the performance of the cell, we used the LiMn2O4 electrode as positive electrode and Li metal as negative electrode. Fig. 5 shows eight consecutive cyclic voltammogram curves of unmodified (a) and modified (b) LiMn2O4 electrode, measured at room temperature. These CV curves are very typical for LiMn2O4 electrode [4]. They show two sets of peaks reflecting the
Fig. 7. Nyquist plots of different LiMn2O4/graphite cells in 1 M LiPF6 ethylene carbonate and dimethyl carbonate solution (a) unmodified LiMn2O4/graphite cell; (b) modified LiMn2O4/graphite cell.
G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388
cycleability. The 45th discharge capacity of the modified and unmodified LiMn2O4 electrode reaches 81.4 and 56.8 mAh g1 at 55 C, respectively. The capacity fading rate is 0.79 mAh g1 per cycle for modified electrode, 1.34 mAh g1 for the LiMn2O4 electrode, which indicates that the above-mentioned copolymer could obviously improve the electrochemical performance of the spinel-structure LiMn2O4 at high temperature. This surface treatment is successful in reducing electrolyte oxidization by placing a physical barrier between the oxidizing spinel and electrolyte as described in Ref. [11]. This technology offers increased resistance towards irreversible capacity loss during elevated temperature storage, namely, the functional copolymer p(MA-AM) plays a rule in protecting and stabilizing the spinel structure. According to the above point of views, we wellfounded consider that this method has a possibly major reason on reducing the dissolution of the manganese cations of surface of the LiMn2O4 film based on the complexing reaction between the manganese cations and the anion groups. Electrochemical impedance spectroscopy is a powerful method for studying electrochemical mechanisms especially in the field of lithium ion batteries [29]. Impedance analysis was used to investigate the interface reaction about the conductivity and the process of lithium intercalation and deintercalation [30]. It is widely believed that the kinetics of coloration of metal oxides and the cathode process in secondary lithium ion batteries is controlled by the diffusion of the inserted species through the electrode [31–34]. Fig. 7(a) and (b) present Nyquist plots of two LiMn2O4/graphite (unmodified and modified) cells that have been run for four cycles. These Nyquist plots are very typical of these electrodes. Both curves are composed of two depressed semicircle in the high and medium frequencies range. They reflect the serial of the process of lithium intercalation and deintercalation, as already demonstrated and reported [31–33]. It is obvious that the block resistance produced by the modified LiMn2O4 electrode exceeds the unmodified ones approximate 50 X. According to depictions of Ref. [31,33], increase of the block resistance could be attributed to the conducting substrate resistances after modifying LiMn2O4 electrode, which is eliminated the resistance of the same electrolyte effect. However, in the medium frequency range, notice that the diameter of two arcs is approximate 300 X, i.e., the SEI layer is formed after two LiMn2O4/graphite cells have been run four cycles, the resistance of ionic charge transfer between the surface of the electrode and the electrolyte solution without transformation. These two LiMn2O4/graphite (modified and un-modified) cells have the same slope line which is inclined at 45 to Z1 (real impedance) approximate. This straight line in the spectra indicates that the solid-state diffusion of lithium ions remove through the homoge-
387
neous LiMn2O4 phase between the absence range and the content range, forming the movement of the phase boundary and separating two phases coexisting in equilibrium. In contrast, the copolymer in the surface of LiMn2O4 electrode does not participate in and affect lithium ion diffusion in the oxide electrode. Furthermore, they have the impedance about 170 X. From the result of electrochemical impedance spectroscopy, it is clear that the resistance of ionic charge transfer between the surface of the electrode and the electrolyte solution always is the determinative process of the kinetic mechanisms whether LiMn2O4 electrode decorating with p(MA-AM) or not.
4. Conclusion In this paper, we managed to improve the electrochemical storage properties of the spinel at 55 C based on the LiMn2O4 film surface decorating with the functional polymer. It is obvious that with the act of modification of the functional copolymer p(MA-AM), the 45th discharge capacity was improved from 56.8 to 81.4 mAh g1 at 55 C on the LiMn2O4 electrode. Furthermore, electrochemical impedance spectra shows the resistance of ionic charge transfer between the surface of the electrode and the electrolyte solution is the determinative process of the kinetic mechanisms. Although this treatment does not completely solve elevated temperature storage issues of the spinel-structure LiMn2O4, we consider that this method is promising in the developing of improvements the spinel materials because this technology can be easily put into practice and taken for preferable industrialization value, comparing with other methods as coating of the surface of LiMn2O4 particles by inorganic oxide or modification of the active conducting polymers.
References [1] A. Momchilov, V. Manev, A. Nassalevska, A. Kozawa, J. Power Sources 41 (1993) 305. [2] R.M. Dell, Solid State Ionics 134 (2000) 139. [3] J.M. Tarascon, E. wang, F.K. Shokoohi, W.R. Mckinnon, S. Colson, J. Electrochem. Soc. 138 (1991) 2859. [4] X. Yongyao, Masaki Yoshio, J. Power Sources 66 (1997) 129. [5] G.G. Amatucci, C.N. Schmutz, A. Blyr, C. Sigala, A.S. Gozdz, D. Larcher, J.M. Tarascon, J. Power Sources 69 (1997) 11. [6] J.M. Tarascon, F. Coowar, G. Amatuci, F.K. Shokoohi, D.G. Guyomard, J. Power Sources 54 (1995) 103. [7] N. Sato, Surf. Coat. Technol. 169–170 (2003) 758. [8] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc. 143 (1996) 2204. [9] Dong H. Jang, Seung M. Oh, J. Electrochem. Soc. 144 (1997) 3342. [10] J. Cho, M.M. Thackeray, J. Electrochem. Soc. 146 (1999) 3577. [11] G.G. Amatucci, A. Blyr, C. Sigala, P. Alfonse, J.M. Tarascon, Solid State Ionics 104 (1997) 13.
388
G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388
[12] H. Yamaguchi, A. Yangda, H. Uwe, Phys. Rev. B 58 (1998) 8. [13] Gui-Ming Song, Wen-Jiang Li, Yu Zhou, Mater. Chem. Phys. 87 (2004) 162. [14] A.J.M. Schmets, G.J. Kearley, H. Mutka, I.M. de Schepper, Phys. B. 350 (2004) 991. [15] B.h. Deng, H. Nakamura, Q. Zhang, M. Yoshio, Y. Xia, Electrochim. Acta 49 (2004) 1823. [16] K.W. Kim, Sun-Woo Lee, Kyoo-Seung Han, Hyun Jin Chung, Seong Ihl Woo, Electrochim. Acta 48 (2003) 4223. [17] Hsien-Cheng Wang, Chung-Hsin Lu, J. Power Sources 119 (2003) 738. [18] J.T. Son, K.S. Park, H.G. Kim, H.T. Chung, J. Power Sources 126 (2004) 18. [19] J.S. Gnanaraj, V.G. Pol, A. Gedanken, D. Aurbach, Electrochem. Commun. 5 (2003) 940. [20] J. Cho, Solid State Ionics 160 (2003) 241. [21] Sung-Chul Park, You-Min Kim, Yong-Mook Kang, Ki-Tae Kim, Paul S. Lee, Jai-Young Lee, J. Power Sources 10 (2001) 86. [22] C. Polo Fonseca, S. Neves, J. Power Sources 135 (2004) 249. [23] C. Arbizzani, A. Balducci, M. Mastragostino, M. Rossi, F. Soavi, J. Electroanal. Chem. 553 (2003) 125.
[24] Jong-Uk Kim, In-Seong Jeong, Seong-In Moon, Hal-Bon Gu, J. Power Sources 97–98 (2001) 450. [25] S. Kuwabata, S. Masui, H. Yoneyama, Electrochim. Acta 44 (1999) 4593. [26] S. Komaba, B. Kaplan, T. Ohtsuka, Y. Kataoka, N. Kumagai, H. Groult, J. Power Sources 119–121 (2003) 378. [27] C. Arbizzani, A. Balducci, M. Mastragostino, M. Rossi, F. Soavi, J. Power Sources 119–121 (2003) 695. [28] D. Aurbach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, U. Heider, Electrochim. Acta 47 (2002) 1423. [29] D.A. Harrington, P. van den Driessche, J. Electroanal. Chem. 567 (2004) 153. [30] J. Bisquert, G. Garcia-Belmonte, P. Bueno, E. Longo, L.O.S. Bulhoes, J. Electroanal. Chem. 452 (1998) 229. [31] Young-Min Choi, Su-Il Pyun, Solid State Ionics 99 (1997) 173. [32] M. Itagaki, N. Kobari, S. Yotsuda, K. Watanabe, S. Kinoshita, M. Ue, J. Power Sources 135 (2004) 255. [33] J.L. Shui, G.S. Jiang, S. Xie, C.H. Chen, Electrochim. Acta 49 (2004) 2209. [34] Young-Min, Su-Il Pyun, Joon-Sung, Seong-In Moon, J. Power Sources 56 (1995) 25.
Study of the electrochemical performance of spinel LiMn2O4 at high temperature based on the polymer modified electrode Guohe Hu a,b, Xiaobing Wang a,b, Fang Chen a,b, Jianyin Zhou a,b, Rengui Li a, Zhenghua Deng a,* a
Chengdu Institute of Organic Chemistry, the Chinese Academy of Sciences, 9, RenMin South Road 4, Chengdu 610041, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 4 January 2005; received in revised form 8 February 2005; accepted 8 February 2005 Available online 26 February 2005
Abstract A novel method improving the performance of spinel LiMn2O4 at high temperature is studied. The copolymer was put into the interspaces of the LiMn2O4 electrode layer prepared by maleic anhydride and acrylamide for decorating LiMn2O4, involving groups to compound with the manganese-ion. It is found that with the act of modification of the functional copolymer, the 45th discharge capacity was improved at 55 C from 56.8 to 81.4 mAh g1 on the LiMn2O4 electrode. Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron micrographs (SEM), inductively coupled plasma atomic emission spectrometry (ICP-AES) and a series of electrochemical measurements have been conducted in this work, and the effects of modification of the polymer layer are discussed. 2005 Elsevier B.V. All rights reserved. Keywords: Cathode; Spinel LiMn2O4; Modified electrode; Copolymer; Capacity fading
1. Introduction The spinel-structure lithium manganese oxide has been realized as the most promising positive-electrode material for lithium-ion batteries because of such properties as low cost, abundant precursors, non-toxicity and environmental advantages [1–3]. The rechargeability has been greatly improved. However, spinel LiMn2O4 electrode has an obstacle that its capacity fades faster on cycling at elevated temperature than at room temperature, namely, its electrochemical performance is poor at elevated temperature. The reasons for capacity fading and mechanism of spinel LiMn2O4 at elevated temperature have been reported over the last years, as widely be*
Corresponding author. Tel.: +86 28 85229252; fax: +86 28 85233426/85250719. E-mail address: [email protected] (Z. Deng). 1388-2481/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.02.010
lieved: an unstable two-phase structure co-existing changed into a stable one-phase structure in the highvoltage (4.15 V) region and the manganese oxide directly dissolved in the electrolyte and the electrolyte solution decomposed on the electrode [4,6]. It was also comprehended that for the spinel structure the disproportionate reaction 4þ 2þ 2Mn3þ ðsolÞ ! MnðsolÞ þ MnðliqÞ
was accelerated along with the increasing of temperature, which caused to increase the entropy of the positive-ion, forming the out-of-order spinel structure [5,7], at the same time this process accelerated the dissolution of manganese cations and in evidence it also catalyzed the electrolyte solution decomposition [8]. The decomposition of the electrolyte solution and existing of a small amount of water molecules in electrolyte could supply hydrogen ions [9], the reaction between H+ and
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G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388
LiMn2O4 again accelerated the dissolution of manganese cations. This bad cycling expedites the capacity of spinel LiMn2O4 fading at elevated temperature, ultimately, the protonic c-MnO2 without the electrochemical activity was produced [10–12]. In order to improve the stability of LiMn2O4 spinel cathodes at high temperature, several methods have been investigated. Among them, doping partial substitution of manganese ions by other metal ions [13–17], coating of the surface of LiMn2O4 particles by organic oxides [18–21] and modification of the active conducting polymer were mainly reported [22–25], some works on modification of electrolyte were reported [26–28], too. But the practicability of the three methods above was still not perfect. In this manuscript, we studied the high-temperature electrochemical performance of spinel LiMn2O4 through treating the surface of the spinel electrode by a functional copolymer. The polymer should possess the anti-oxidative capability and be tiny expanding instead of dissolving while dipping in the electrolyte for a long time. Thus, the polymer contained groups to compound with those manganese cations for decorating LiMn2O4 was designed. Based on above requirements, we used the copolymer of maleic anhydride (MA) and acrylamide (AM) to modify the LiMn2O4 spinel.
cm2 between 3.00 and 4.30 V. Cyclic voltammetry was performed on the LiMn2O4 electrode with a scan rate of 0.1mV s1 in the potential range of 3.0–4.6 V (vs. Li/Li+) by an Arbin BT2000 system. Electrochemical impedance measurements were performed with a Lock in amplifier (PE model 5210) electrochemical interface under remote control by personal computer. The frequency range between 100 kHz and 100 mHz.
3. Results and discussion 3.1. Characteristics of the copolymer Infrared spectroscopy has been used to characterize the chain structure of polymers. Fourier transformed infrared spectroscopy of the copolymer p(MA-AM) has been shown in Fig. 1. It exhibits that, two characteristic peaks at 1775.9 and 1843.0 cm1 represent the typical anhydride group. The peak at 3440.4 cm1 is the characteristic peak of the –NH2 group, while that frequency 1667.2 cm1 indicates the AC@O double bond structure. Based on our experimental conditions, we consider that this copolymer is p(MA-AM) production of the above-mentioned polymerization reactions. 3.2. Scanning electron micrographs
2. Experimental The copolymer was prepared by maleic anhydride (MA) and acrylamide (AM). These polymerization reactions were in the butanone solution at 50 C about 8 h. The product of the polymerization was filtrated, and washed by the butanone solution then dried at 100 C. The composition of these polymers was determined by Fourier transformed infrared spectroscopy. The modified LiMn2O4 electrode was prepared through two steps, as described below. First, the electrode with composition of 90:7:3(wt.) LiMn2O4 (purchased from Huilong company, YunNan, China): acetylene: LA132 (an aqueous binder, Indigo company, ChengDu, China) was coated onto an aluminum foil as the current collector followed by drying at 70 C. Secondly, it was soaked into the N,N-dimethylformamide solution of p(MA-AM) several minutes and dried at about 100 C. The charge-discharge characteristics of the modified electrode were examined in a laboratory cell consisting of a positive electrode and a negative electrode that separated by a porous polypropylene membrane. The cell negative electrode was lithium metal, which was in excess. 1 M LiPF6–ethylene carbonate/dimethyl carbonate (1:1 in weight) was used as the electrolyte. Cycling performance of the cell was measured with a BS-9300 Secondary Battery Tester (QingTian, China) at 0.65 mA/
We consider the LiMn2O4 electrode film as an inorganic coat with the binder and conducting carbon black. Inside the coat there always exist a large lot of interspaces. Hereby, if a functional polymer put into these interspaces, forming the spinel LiMn2O4 modified electrode decorate LiMn2O4, the modification must be effective. Fig. 2 shows the differences scanning electron micrographs (SEM) images of the spinel LiMn2O4 film before and after dipping into the copolymer butanone solution.
Fig. 1. FTIR spectra of p(MA-AM).
G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388
385
Fig. 2. SEM images of the spinel LiMn2O4 electrode (a) unmodified electrode; (b) modified electrode.
From Fig. 2(a) it can be seen that in the spinel LiMn2O4 there exists a great quantity of different-scaled interspaces. From Fig. 2(b) we consider the copolymer p(MA-AM) could completely soak into interspaces, forming a reticulate structure. 3.3. Inductively coupled plasma atomic emission spectrometry In these experiments, the dissolvable manganese solution in LiPF4/EC + DMC electrolyte was prepared. The porous polypropylene membrane and the electrolyte and the two electrodes washed by dilute hydrochloric acid, and the quality of the manganese element was directly determined by using inductively coupled plasma atomic emission spectrometry (ICP-AES). It can be seen in Table 1, after the 45th cycle at 55 C the dissolved quality of Mn of the modified and unmodified LiMn2O4 electrode was 6.4 and 12.4 lg, respectively. Correspondingly, the dissolved quality percentage of manganese element of modified electrode in electrolyte (0.097%) was merely one half of that of the unmodified LiMn2O4 electrode (0.190%). Therefore, with the polymer solution treatment, the p(MA-AM) penetrate these interspaces of the LiMn2O4 film was successful in reducing the dissolution of the manganese oxide. 3.4. X-ray photoelectron spectroscopy Fig. 3 illustrates the XPS emission of Mn(2p) of LiMn2O4 electrode and modified electrode. The binding energy values of Mn(2p) of the spinels are list in Table 2. It is very interesting that the peak of Mn(2p3/2) emission
XPS Intensity (a.u)
a
b
660
Unmodified Modified
The quality of Mn on the LiMn2O4 electrode (mg) 6.5 6.6
The dissolved quality of Mn (lg) 12.4a 6.4b
The dissolved quality percentage of Mn (%) 0.190% 0.097%
a,b shown is the 45th cycle of the spinel LiMn2O4 unmodified and modified electrode at 55 C, respectively.
648
642
636
Binding Energy (eV) Fig. 3. XPS emission of Mn(2p) of spinels LiMn2O4 (a) unmodified electrode; (b) modified electrode.
Table 2 XPS emission of Mn(2p) of LiMn2O4 spinels Electrode
Binding energy (eV) Mn(2p3/2)
Mn(2p1/2)
Mn(2p)
Unmodified electrode Modified electrode
641.420 642.080
652.926 653.370
11.506 11.290
shifts from 641.420 to 642.080 eV. This phenomenon is possibly due to the interaction of electron and polarization at the spinel LiMn2O4 interface, which are groups of the copolymer such as anhydride groups and CO– NH2 groups to compound with the manganese-ion. On the other hand, the disproportionate reaction 4þ 2þ 2Mn3þ ðsolÞ ! MnðsolÞ þ MnðliqÞ becomes difficult because it requires extra energy to destroy the spinel structure. Thus, the copolymer p(MA-AM) put into these interspaces could be likely to play a rule in protecting and stabilizing the spinel structure. ? O Mn
Mn
H O Mn
O
H O
O Mn
? Mn
Mn
O
O
the LiMn2O4 electrode
Table 1 Amount of LiMn2O4 dissolved in LiPF4/EC + DMC electrolyte Electrode
654
CH
n O O Mn
Mn O
CH
HC
CH2
n
NH2
O H O
H
O
O
O Mn
Mn
Mn
O Mn
O
O
the LiMn2O4 electrode Fig. 4. Model of anhydride and amid groupsÕ complexing with the surface manganese cations.
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G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388 140
unmodified electrode modified electrode
130
specific discharge (mAh/g)
120 110 100 90 80 70 60 50 40 30 0
5
10
15
20
25
30
35
40
45
50
cyclenumber Fig. 6. Typical cycle life curves (specific discharge capacity vs. cycle number) modified and unmodified spinel LiMn2O4. Lithium batteries, 55 C, 1 M LiPF6/EC–DMC (1:1), C/6 rates.
Fig. 5. Cyclic voltammogram of the unmodified (a) and modified (b) LiMn2O4 /Li cell in 1 M LiPF6 ethylene carbonate/dimethyl carbonate (1:1 in weight) solution at room temperature. The applied potential was scanned from 3.0 to 4.6 V with a scan rate of 0.1 mV s.
In addition, it has been reported [18–20] that surface treatments could improve the elevated temperature storage characteristic of spinel LiMn2O4. As described in Ref. [11], the method of surface treatment was performed with the agent acetylaceton, manganese cations existing on the surface of the Æ1 1 1æ planes are not fully coordinated by oxygen anions. It utilized the dangling bonds of the manganese cations in the Æ1 1 1æ planes on the surface of the LiMn2O4 particles. It is conceivable that, these anion groups also utilize a complexing reaction to remove the active manganese cations of the surface. So, we imitate the model of G.G. Amatucci as shown in Fig. 4, the anhydride groups and the amide groups complex directly with these cations on the surface of the LiMn2O4 particles, and it could remove the active sites of manganese cations, and that those the dangling bonds of these surface cations may contribute to a large extent of the catalytic properties of the spinel toward electrolyte oxidation. As results in Fig. 7, we found it could improve the elevated temperature performance of LiMn2O4 spinel.
typical oxidation processes of LiMn2O4 in 4.05 and 4.15 V (vs. Li/Li+) and de-oxidation processes in the 3.9 V domain, which involves phase transitions [4,6]. As is seen in Fig. 5(b), the CV peaks related to the p(MA-AM) modified are much more steady, compared to the peaks of the unmodified LiMn2O4 electrode. These differences mean that the cyclestability of the modified of p(MA-AM) is better than that of the standard, unmodified material. Correspondingly, it also demonstrates the above-mentioned results. The relationship between specific discharge capacity and cycle number of a lithium battery cycled at a constant current rate of C/6 between 3.0 and 4.3 V at 55 C is depicted in Fig. 6. As shown, the lithium battery with modified electrode as the cathode active material has better rechargeability than LiMn2O4 electrode. In general, both electrodes exhibited high initial discharge capacity. However, modified electrode shows the better
3.5. Electrochemical behavior To investigate the performance of the cell, we used the LiMn2O4 electrode as positive electrode and Li metal as negative electrode. Fig. 5 shows eight consecutive cyclic voltammogram curves of unmodified (a) and modified (b) LiMn2O4 electrode, measured at room temperature. These CV curves are very typical for LiMn2O4 electrode [4]. They show two sets of peaks reflecting the
Fig. 7. Nyquist plots of different LiMn2O4/graphite cells in 1 M LiPF6 ethylene carbonate and dimethyl carbonate solution (a) unmodified LiMn2O4/graphite cell; (b) modified LiMn2O4/graphite cell.
G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388
cycleability. The 45th discharge capacity of the modified and unmodified LiMn2O4 electrode reaches 81.4 and 56.8 mAh g1 at 55 C, respectively. The capacity fading rate is 0.79 mAh g1 per cycle for modified electrode, 1.34 mAh g1 for the LiMn2O4 electrode, which indicates that the above-mentioned copolymer could obviously improve the electrochemical performance of the spinel-structure LiMn2O4 at high temperature. This surface treatment is successful in reducing electrolyte oxidization by placing a physical barrier between the oxidizing spinel and electrolyte as described in Ref. [11]. This technology offers increased resistance towards irreversible capacity loss during elevated temperature storage, namely, the functional copolymer p(MA-AM) plays a rule in protecting and stabilizing the spinel structure. According to the above point of views, we wellfounded consider that this method has a possibly major reason on reducing the dissolution of the manganese cations of surface of the LiMn2O4 film based on the complexing reaction between the manganese cations and the anion groups. Electrochemical impedance spectroscopy is a powerful method for studying electrochemical mechanisms especially in the field of lithium ion batteries [29]. Impedance analysis was used to investigate the interface reaction about the conductivity and the process of lithium intercalation and deintercalation [30]. It is widely believed that the kinetics of coloration of metal oxides and the cathode process in secondary lithium ion batteries is controlled by the diffusion of the inserted species through the electrode [31–34]. Fig. 7(a) and (b) present Nyquist plots of two LiMn2O4/graphite (unmodified and modified) cells that have been run for four cycles. These Nyquist plots are very typical of these electrodes. Both curves are composed of two depressed semicircle in the high and medium frequencies range. They reflect the serial of the process of lithium intercalation and deintercalation, as already demonstrated and reported [31–33]. It is obvious that the block resistance produced by the modified LiMn2O4 electrode exceeds the unmodified ones approximate 50 X. According to depictions of Ref. [31,33], increase of the block resistance could be attributed to the conducting substrate resistances after modifying LiMn2O4 electrode, which is eliminated the resistance of the same electrolyte effect. However, in the medium frequency range, notice that the diameter of two arcs is approximate 300 X, i.e., the SEI layer is formed after two LiMn2O4/graphite cells have been run four cycles, the resistance of ionic charge transfer between the surface of the electrode and the electrolyte solution without transformation. These two LiMn2O4/graphite (modified and un-modified) cells have the same slope line which is inclined at 45 to Z1 (real impedance) approximate. This straight line in the spectra indicates that the solid-state diffusion of lithium ions remove through the homoge-
387
neous LiMn2O4 phase between the absence range and the content range, forming the movement of the phase boundary and separating two phases coexisting in equilibrium. In contrast, the copolymer in the surface of LiMn2O4 electrode does not participate in and affect lithium ion diffusion in the oxide electrode. Furthermore, they have the impedance about 170 X. From the result of electrochemical impedance spectroscopy, it is clear that the resistance of ionic charge transfer between the surface of the electrode and the electrolyte solution always is the determinative process of the kinetic mechanisms whether LiMn2O4 electrode decorating with p(MA-AM) or not.
4. Conclusion In this paper, we managed to improve the electrochemical storage properties of the spinel at 55 C based on the LiMn2O4 film surface decorating with the functional polymer. It is obvious that with the act of modification of the functional copolymer p(MA-AM), the 45th discharge capacity was improved from 56.8 to 81.4 mAh g1 at 55 C on the LiMn2O4 electrode. Furthermore, electrochemical impedance spectra shows the resistance of ionic charge transfer between the surface of the electrode and the electrolyte solution is the determinative process of the kinetic mechanisms. Although this treatment does not completely solve elevated temperature storage issues of the spinel-structure LiMn2O4, we consider that this method is promising in the developing of improvements the spinel materials because this technology can be easily put into practice and taken for preferable industrialization value, comparing with other methods as coating of the surface of LiMn2O4 particles by inorganic oxide or modification of the active conducting polymers.
References [1] A. Momchilov, V. Manev, A. Nassalevska, A. Kozawa, J. Power Sources 41 (1993) 305. [2] R.M. Dell, Solid State Ionics 134 (2000) 139. [3] J.M. Tarascon, E. wang, F.K. Shokoohi, W.R. Mckinnon, S. Colson, J. Electrochem. Soc. 138 (1991) 2859. [4] X. Yongyao, Masaki Yoshio, J. Power Sources 66 (1997) 129. [5] G.G. Amatucci, C.N. Schmutz, A. Blyr, C. Sigala, A.S. Gozdz, D. Larcher, J.M. Tarascon, J. Power Sources 69 (1997) 11. [6] J.M. Tarascon, F. Coowar, G. Amatuci, F.K. Shokoohi, D.G. Guyomard, J. Power Sources 54 (1995) 103. [7] N. Sato, Surf. Coat. Technol. 169–170 (2003) 758. [8] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc. 143 (1996) 2204. [9] Dong H. Jang, Seung M. Oh, J. Electrochem. Soc. 144 (1997) 3342. [10] J. Cho, M.M. Thackeray, J. Electrochem. Soc. 146 (1999) 3577. [11] G.G. Amatucci, A. Blyr, C. Sigala, P. Alfonse, J.M. Tarascon, Solid State Ionics 104 (1997) 13.
388
G. Hu et al. / Electrochemistry Communications 7 (2005) 383–388
[12] H. Yamaguchi, A. Yangda, H. Uwe, Phys. Rev. B 58 (1998) 8. [13] Gui-Ming Song, Wen-Jiang Li, Yu Zhou, Mater. Chem. Phys. 87 (2004) 162. [14] A.J.M. Schmets, G.J. Kearley, H. Mutka, I.M. de Schepper, Phys. B. 350 (2004) 991. [15] B.h. Deng, H. Nakamura, Q. Zhang, M. Yoshio, Y. Xia, Electrochim. Acta 49 (2004) 1823. [16] K.W. Kim, Sun-Woo Lee, Kyoo-Seung Han, Hyun Jin Chung, Seong Ihl Woo, Electrochim. Acta 48 (2003) 4223. [17] Hsien-Cheng Wang, Chung-Hsin Lu, J. Power Sources 119 (2003) 738. [18] J.T. Son, K.S. Park, H.G. Kim, H.T. Chung, J. Power Sources 126 (2004) 18. [19] J.S. Gnanaraj, V.G. Pol, A. Gedanken, D. Aurbach, Electrochem. Commun. 5 (2003) 940. [20] J. Cho, Solid State Ionics 160 (2003) 241. [21] Sung-Chul Park, You-Min Kim, Yong-Mook Kang, Ki-Tae Kim, Paul S. Lee, Jai-Young Lee, J. Power Sources 10 (2001) 86. [22] C. Polo Fonseca, S. Neves, J. Power Sources 135 (2004) 249. [23] C. Arbizzani, A. Balducci, M. Mastragostino, M. Rossi, F. Soavi, J. Electroanal. Chem. 553 (2003) 125.
[24] Jong-Uk Kim, In-Seong Jeong, Seong-In Moon, Hal-Bon Gu, J. Power Sources 97–98 (2001) 450. [25] S. Kuwabata, S. Masui, H. Yoneyama, Electrochim. Acta 44 (1999) 4593. [26] S. Komaba, B. Kaplan, T. Ohtsuka, Y. Kataoka, N. Kumagai, H. Groult, J. Power Sources 119–121 (2003) 378. [27] C. Arbizzani, A. Balducci, M. Mastragostino, M. Rossi, F. Soavi, J. Power Sources 119–121 (2003) 695. [28] D. Aurbach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, U. Heider, Electrochim. Acta 47 (2002) 1423. [29] D.A. Harrington, P. van den Driessche, J. Electroanal. Chem. 567 (2004) 153. [30] J. Bisquert, G. Garcia-Belmonte, P. Bueno, E. Longo, L.O.S. Bulhoes, J. Electroanal. Chem. 452 (1998) 229. [31] Young-Min Choi, Su-Il Pyun, Solid State Ionics 99 (1997) 173. [32] M. Itagaki, N. Kobari, S. Yotsuda, K. Watanabe, S. Kinoshita, M. Ue, J. Power Sources 135 (2004) 255. [33] J.L. Shui, G.S. Jiang, S. Xie, C.H. Chen, Electrochim. Acta 49 (2004) 2209. [34] Young-Min, Su-Il Pyun, Joon-Sung, Seong-In Moon, J. Power Sources 56 (1995) 25.