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Fabrications and electrochemical properties of fluorine-modified spinel LiMn2O4 for lithium ion batteries
Solid State Ionics 152 – 153 (2002) 327 – 334 www.elsevier.com/locate/ssi
Fabrications and electrochemical properties of fluorine-modified spinel LiMn2O4 for lithium ion batteries Chuan Wu a,b,c, Feng Wu a,c, Liquan Chen b, Xuejie Huang b,* a
School of Chemical Engineering and Materials Science, Beijing Institute of Technology, Beijing 100081, China b Laboratory for Solid State Ionics, Institute of Physics, Chinese Academy of Science, Beijing 100080, China c National Development Center for High Technology Green Materials, Beijing 100081, China Accepted 14 February 2002
Abstract Spinel LiMn2O4 was modified by an aqueous fluoride modification method (AFMM). It is found that tiny crystals of potassium manganese trifluoride (KMnF3) grew on the surface of the spinel particles during the electrochemical process. The quantity and the integrality of the KMnF3 crystals were controlled by varying current density and processing time. The mechanisms of the formation of KMnF3 were discussed. The harmful two-phase reaction of LiMn2O4 upon cycling was suppressed after the cathode was processed by AFMM. The chemical diffusion coefficients (Ds) of Li+ were measured by a potential relaxation technique (PRT). The values of Ds obtained in this way are of the order of 1010 – 109 cm2/s. D 2002 Elsevier Science B.V. All rights reserved. Keywords: LiMn2O4; Surface modification; KMnF3; Chemical diffusion coefficient; Potential relaxation technique
1. Introduction As a promising candidate of the cathode materials for lithium ion batteries, spinel LiMn2O4 have been studied extensively. To improve the cyclic performance of this material, much attention has been focused on the doping of the host structure [1,2]. Recently, it was proved by Cho et al. [3] that the sol – gel coating on the surface could improve the structural stability of spinel LiMn2O4 and layered LiCoO2 [4 –6]. According to our previous work [7], surface modification of spinel LiMn2O4 is also an attractive way to improve the properties of this material. The spinel structure of *
Corresponding author. Tel.: +86-10-82649046; fax: +86-1082649050. E-mail address: [email protected] (X. Huang).
LiMn2O4 can be retained and the electrochemical performance can be enhanced in this way. The chemical diffusion coefficient of Li+ is considered as an important kinetic parameter for characterizing the extraction and insertion properties of spinel LiMn2O4 [8 –13]. In the present paper, the Li+ chemical diffusion coefficient of the fluorine-modified LiMn2O4 was studied by adopting a potential relaxation technique (PRT) [14].
2. Experimental Spinel LiMn2O4 had been modified by using an electrochemical method as previously described [7]. Fig. 1 illustrates the experimental set up in this work. An electrolytic cell, which contains potassium fluoride
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Fig. 1. Diagram of the electrochemical system for fluorinemodification; (1) aluminum foil, (2) LiMn2O4 film, (3) nickel foil, (4) electrolytic cell.
aqueous solution, was used as the electrochemical modification system. Hence, we here name this method as aqueous fluoride modification method (AFMM). LiMn2O4 films and nickel foils served as the working electrode and the counter electrode, respectively. The electrolytic cell was charged at different current densities. Subsequently, the modified LiMn2O4 films were washed with distilled water, and vacuum dried at 55 jC for 2 days to remove the residual water. The XRD patterns for the fluorine-modified samples were obtained with a Rigaku B/max-2400 X-ray diffractometer using CuKa radiation. SEM studies were performed by an Oxford S-4200 scanning electron microscope. To calculate the chemical diffusion coefficients of Li+, PRT measurements were carried out on a CHI660A electrochemical workstation. For the electrochemical tests, the prepared samples were assembled in Swagelok two-electrode cells [15,16]. Metal lithium foil served as the counter electrode. The electrolyte was 1 M LiPF6/EC-DMC (1:1 in volume). The cells was cycled in the voltage rang of 3.0– 4.5 V with a current density of 0.2 mA/cm2.
3. Results and discussion
The SEM images of the modified spinel samples MF9, MF13, MF14, MF31 are shown in Fig. 2. It is evident that the quantity of the tiny crystals can be controlled by the current density. While the processing time is fixed to 12 h, there are more and more tiny crystals grown on the spinel particles as the current density increases from 1.25 to 6.25 AA/cm2 (see Fig. 2a –c). It indicates that the processing current density is a crucial factor affecting the growth of KMnF3. As we expected, sample MF31 might have more integral KMnF3 crystals than sample MF14 because the processing current density of the former is 10-folds as large as that of the latter. However, the SEM image of sample MF31, which was treated for 2 h at the current density of 12.5 AA/cm2 is different from the others. The product seems like an amorphous phase rather than a crystal phase. It may be illustrated by the X-ray powder diffraction studies as follows. 3.2. XRD patterns In Fig. 3, the XRD patterns of the modified samples show the effect of the processing time on the growth of KMnF3. When the processing time is fixed to 12 h, there are notable diffraction peaks of KMnF3 in all the XRD patterns. The sharp peaks indicate the integrity of KMnF3 crystals. However, while the processing time is merely 2 h (sample MF31), the diffraction peaks of KMnF3 cannot be detected, though the processing current density is as high as 12.5 AA/cm2. This indicates that the processing time is another crucial factor that affects the growth of KMnF3. Diffraction peaks of lithium fluoride (LiF) are also found in the XRD patterns. Even though these peaks are very weak, LiF can be detected in all the modified samples, including sample MF31, which has no inteTable 1 Electrochemical processing conditions of fluorine-modified spinel LiMn2O4 Sample
Current density (AA/cm2)
Processing time (h)
Composition
MF9 MF13 MF14 MF31
6.25 2.50 1.25 12.5
12 12 12 2
LiMn2O4, KMnF3, LiF LiMn2O4, KMnF3, LiF LiMn2O4, KMnF3, LiF LiMn2O4, amorphous phase, LiF
3.1. SEM images To understand the factors influencing the growth of KMnF3 on the surface of the spinel particles, a series of experiments were carried out by varying the current density and the processing time, as shown in Table 1.
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Fig. 2. SEM images of the fluorine-modified samples processed with various current densities and processing times: (a) sample MF14 (1.25 AA/ cm2, 12 h), the white bar is 10 Am; (b) sample MF13 (2.50 AA/cm2, 12 h), the white bar is 10 Am; (c) sample MF9 (6.25 AA/cm2, 12 h), the white bar is 10 Am; (d) sample MF31 (12.5 AA/cm2, 2 h), the white bar is 1 Am.
gral crystals on the spinel surface. It indicates that there must be a common reaction, which produces small amount of LiF electrochemically. In the previous work [7], Eqs. (1) and (2) were adopted to interpret the formation of KMnF3. Here an additional reaction equation (3) is adopted to describe the formation of LiF. 2Mn3þ ! Mn2þ þ Mn4þ þ
2þ
K þ Mn
þ 3F ! KMnF3
Liþ þ F ! LiF
ð1Þ ð2Þ ð3Þ
It is well known that lithium ions are extracted from the spinel framework during the charge process of lithium-ion batteries. In the present study, when the electrochemical process begins, a disproportion reac-
tion of Mn3+ takes place in the electrolyte system combined with the extraction of Li+. Namely, reactions (1) and (3) take place at the same time, and a small amount of LiF is produced in this procedure. After the Mn2+ ions are formed and migrate to the surface of the spinel particles, reaction (2) takes place and becomes dominant. In the meanwhile, the formation of LiF is suppressed. Therefore, the amount of KMnF3 increases continuously, but the amount of LiF is kept at a constant level. According to the SEM and XRD studies described above, the growth of the tiny crystals of KMnF3 is affected by both the processing time and the current density. However, it is still our concern whether the two factors function separately or not. Further steps to study this issue are in progress.
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Fig. 3. XRD patterns of the fluorine-modified samples; (a) sample MF14, (b) sample MF13, (c) sample MF9, (d) sample MF31.
3.3. Galvanostatic cycling properties According to our previous study [7], tiny crystals of potassium manganese trifluoride (KMnF3) could be electrochemically produced on the surface of the spinel particles. However, when the sample was cycled at a heavy current, it showed a dramatic low capacity because the integral KMnF3 crystals blocked the diffusion of Li+ and caused a remarkable polarization of the cell. Therefore, an amorphous surface layer may be superior to a crystal one for Li+ diffusion. Fig. 4 shows the galvanostatic charge– discharge curve of sample MF31 with an amorphous surface layer. The modified sample was assembled in a Swagelok cell [15,16] in an argon-filled glove box. Metal lithium foil served as the counter electrode. The electrolyte was 1 M LiPF6/EC-DMC (1:1 in volume). The prepared cell was cycled at a constant current density of 0.2 mA/cm2 in the voltage rang 3.0 –4.5 V.
Since part of the Li+ and Mn3+ are consumed during the electrochemical modification process, as described in Eqs. (1) –(3), the specific capacity of the modified sample is lower than that of non-modified LiMn2O4. For stoichiometric LiMn2O4, the extraction and insertion of Li+ is illustrated by a two-step process [17 –19]. Firstly, the Li+ with strong Li –Li interaction are removed from half of the tetrahedral positions at lower potential, and some of the Mn3+ are converted to Mn4+. As the content of Mn4+ increases, the second half of Li+ is extracted with more energy at higher potential. Namely, there are two binary equilibrium systems during Li extraction or insertion, which are considered as LiMn2O4 – Li0.5Mn2O4 and Li0.5Mn2O4 – k-MnO2 [17]. As for the fluorine-modified spinel, the characteristic two 4-V plateaus of LiMn2O4 in the galvanostatic charge – discharge curves cannot be clearly distinguished (see Fig. 4). This indicates that the phase changes of the modified spinel are not so drastic when compared with the non-modified LiMn2O4. According to Xia and Yoshio [20], the shape of the charge– discharge curve will gradually transform to a quasione plateau as the vacancy rate increases in spinel LiMn2O4. In our study, the consumption of Li+ and Mn3+ during the electrochemical modification process may lead to a vacancy-rich structure, causing the quasi-one plateau of the modified sample. In the charge– discharge curve, the high voltage region changes to an S-shape, implying that the two-phase structure is transformed to a one-phase structure [21]. Namely, the harmful two-phase reaction of LiMn2O4 upon cycling was suppressed after the cathode was processed by AFMM. Although the Li insertion/extraction reaction regions cannot be distinguished definitely in Fig. 4, they are distinct in the dQ/dV vs. V curve, as shown in Fig. 5. Since the host structure of LiMn2O4 is retained after fluorine-modification processing, the characteristic redox peaks of LiMn2O4 are kept. There are two pairs of oxidation and reduction peaks located at 4.06 and 4.17 V, and 4.07 and 3.97 V. This indicates the reversibility of the electrochemical reactions in the charge– discharge process. The intensities of the oxidation peaks are approximately the same. However, the intensity of the first reduction peak is much weaker than that of the second one. The delay of the first Li+ insertion process is responsible for this phenomenon.
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Fig. 4. Typical galvanostatic charge – discharge curve of fluorine-modified sample MF31.
3.4. Chemical diffusion coefficients A PRT measurement was adopted to measure the chemical diffusion coefficients of Li+ in the fluorine-
modified sample. This technique has the capability to gain the chemical diffusion coefficients during both charge and discharge processes. The modified sample MF31 was assembled in a Swagelok cell in an argon-
Fig. 5. dQ/dV vs. V plot derived from the charge – discharge curve of Fig. 4.
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Fig. 6. Typical potential relaxation plot of sample MF31 during the charging process. The starting potential of the relaxation is 3.90 V.
filled glove box, as described in the former section. The thickness of the complex cathode is 6.1103 cm. To avoid the effects of side reactions, the cell was cycled till the coulombic efficiency was above 97%.
The potential relaxation curves were recorded by using a chronopotentiometry technique of the CHI660A electrochemical workstation. After the cell was charged or discharged to a certain potential at the
Fig. 7. ln[exp(ulut)F/RT1] vs. t plot at the potential of 3.90 V.
C. Wu et al. / Solid State Ionics 152 – 153 (2002) 327–334 Table 2 Chemical diffusion coefficients of Li+ for fluorine-modified sample MF31 Cyclic process
Starting potential (V)
Chemical diffusion coefficient (cm2/s)
Charge
3.90 4.09 4.18 4.34 4.11 4.03 3.84 3.25
1.95109 2.741010 2.171010 7.211010 3.18109 2.19109 7.321010 6.301010
Discharge
current density of 0.2 mA/cm2, the cathodic or anodic current was dropped to zero, then the open circuit voltage (OCV) was recorded by adopting an interval of one record per second. Eq. (4) describes the diffusion properties of the lithium ions during the extraction and insertion procedure in the bulk electrode. The detailed derivation of this equation was described elsewhere [14]. h u u i p2 t ln exp l F 1 ¼ lnN Ds t ð4Þ RT L2 ut represents the electrode potential as the relaxation time equal to t, and ul is the potential when the relaxation time tends to be infinite. Another parameter L is the thickness of the working electrode. The value of the chemical diffusion coefficient Ds can be calculated from the linear fit slope of ln[exp(ulut)F/ RT1] vs. t. Here it is unnecessary to consider the intercept ln N of the fitted plot. Fig. 6 shows the typical potential relaxation plot of sample MF31 during charge process. Although the cell has been charged to 3.95 V, the start point of the relaxation is 3.90 V because of the polarization of the electrode. It can be seen that the OCV vs. Li/Li+ decreases rapidly in the initial stages. When the relaxation time is long enough, the OCV tends to be constant. Considering the self-discharge of the lithium ion batteries, it is insignificant to record the OCVafter a very long time. Therefore, we chose the OCV corresponding with t=86,400 s (24 h) as the value of ul. The typical plot of ln[exp(ulut)F/RT1] vs. t is shown in Fig. 7. Ds evaluated from the slope of the solid line is 1.95109 cm2/s. Chemical diffusion coefficients of charge and discharge process at various potentials are listed in Table 2.
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As previously reported [8 –13], chemical diffusion coefficients of Li+ for LiMn2O4 are in the range from 1010 to 108 cm2/s. In the present study, the values of Ds in both charge and discharge processes are conveniently determined. Those of the fluorine-modified LiMn2O4 are of the order of 1010 –109 cm2/s.
4. Conclusion Spinel LiMn2O4 was modified by using an aqueous fluoride modification method (AFMM). We showed the possibility of modifying LiMn2O4 by simple electrochemical processing. Tiny crystals of potassium manganese trifluoride (KMnF3) were formed on the spinel particles. The formation of the KMnF3 crystals was affected by current density and processing time. The harmful two-phase reaction of LiMn2O4 upon cycling was suppressed after the cathode was processed by AFMM. As compared with previous study [7], amorphous surface layers of the modified samples may be superior to more crystalline layers for Li+ diffusion. A potential relaxation technique (PRT) was adopted to measure the chemical diffusion coefficient of Li+ (Ds) for the fluorine-modifed samples. The values of Ds are of the order of 1010 – 109 cm2/s, similar to those of non-modified LiMn2O4.
Acknowledgements The authors would like to show gratitude to NSFC (No. 59972041) and National 863 key program (No. 2001AA320301) for financial support.
References [1] C. Wu, H. Li, F. Wu, X. Huang, in: B.V.R. Chowdari, W. Wang (Eds.), Proceeding of the 7th Asian Conference on Solid State Ionics, World Scientific Press, Fuzhou, China, Oct. 29, 2000, pp. 261 – 265. [2] J.H. Lee, J.K. Hong, D.H. Jang, Y.-K. Sun, S.M. Oh, J. Power Sources 89 (2000) 7. [3] J. Cho, G.B. Kim, H.S. Lim, C.-S. Kim, S.I. Yoo, Electrochem. Solid-State Lett. 2 (1999) 607. [4] J. Cho, G. Kim, Electrochem. Solid-State Lett. 2 (1999) 253. [5] J. Cho, Y.J. Kim, B. Park, Chem. Mater. 12 (2000) 3788. [6] J. Cho, C.S. Kim, S.I. Yoo, Electrochem. Solid-State Lett. 3 (2000) 362.
334
C. Wu et al. / Solid State Ionics 152 – 153 (2002) 327–334
[7] C. Wu, F. Wu, X. Huang, in: B.V.R. Chowdari, W. Wang (Eds.), Proceeding of the 7th Asian Conference on Solid State Ionics, World Scientific Press, Fuzhou, China, Oct. 29, 2000, pp. 267 – 270. [8] L. Chen, J. Schoonman, Solid State Ionics 67 (1994) 17. [9] L. Chen, X. Huang, E. Kelder, J. Schoonman, Solid State Ionics 76 (1995) 91. [10] M.Y. Saidi, J. Barker, R. Koksbang, J. Solid State Chem. 122 (1996) 195. [11] M. Wakihara, L. Guohua, H. Ikuta, T. Uchida, Solid State Ionics 86 – 88 (1996) 907. [12] W. Yang, G. Zhang, S. Lu, J. Xie, Q. Liu, Solid State Ionics 121 (1999) 85. [13] S. Chitra, P. Kalyani, T. Mohan, R. Gangadharan, B. Yebka, S. Castro-Garcia, M. Massot, C. Julien, J. Electroceram. 3 (1999) 433.
[14] Q. Wang, H. Li, X. Huang, L. Chen, J. Electrochem. Soc. 148 (2001) A737. [15] J.M. Tarascon, F.J. Disalvo, D.W. Murphy, G.W. Hull, J.V. Waszczak, J. Solid State Chem. 54 (1984) 204. [16] P. Arora, B.N. Popov, R.E. White, J. Electrochem. Soc. 145 (1998) 807. [17] M.M. Thackeray, P.J. Johnson, L.A. de Picciotto, Mater. Res. Bull. 19 (1984) 179. [18] W. Liu, G.C. Farrington, F. Chaput, B. Dunn, J. Electrochem. Soc. 143 (1996) 879. [19] A. Ott, P. Endres, V. Klein, B. Fuchs, A. Jager, H.A. Mayer, S. Kemmler-Sack, H.-W. Praas, K. Brandt, G. Filoti, V. Kunczer, M. Rosenberg, J. Power Sources 72 (1998) 1. [20] Y. Xia, M. Yoshio, J. Electrochem. Soc. 144 (1997) 4186. [21] Y. Xia, M. Yoshio, J. Electrochem. Soc. 143 (1996) 825.
Fabrications and electrochemical properties of fluorine-modified spinel LiMn2O4 for lithium ion batteries Chuan Wu a,b,c, Feng Wu a,c, Liquan Chen b, Xuejie Huang b,* a
School of Chemical Engineering and Materials Science, Beijing Institute of Technology, Beijing 100081, China b Laboratory for Solid State Ionics, Institute of Physics, Chinese Academy of Science, Beijing 100080, China c National Development Center for High Technology Green Materials, Beijing 100081, China Accepted 14 February 2002
Abstract Spinel LiMn2O4 was modified by an aqueous fluoride modification method (AFMM). It is found that tiny crystals of potassium manganese trifluoride (KMnF3) grew on the surface of the spinel particles during the electrochemical process. The quantity and the integrality of the KMnF3 crystals were controlled by varying current density and processing time. The mechanisms of the formation of KMnF3 were discussed. The harmful two-phase reaction of LiMn2O4 upon cycling was suppressed after the cathode was processed by AFMM. The chemical diffusion coefficients (Ds) of Li+ were measured by a potential relaxation technique (PRT). The values of Ds obtained in this way are of the order of 1010 – 109 cm2/s. D 2002 Elsevier Science B.V. All rights reserved. Keywords: LiMn2O4; Surface modification; KMnF3; Chemical diffusion coefficient; Potential relaxation technique
1. Introduction As a promising candidate of the cathode materials for lithium ion batteries, spinel LiMn2O4 have been studied extensively. To improve the cyclic performance of this material, much attention has been focused on the doping of the host structure [1,2]. Recently, it was proved by Cho et al. [3] that the sol – gel coating on the surface could improve the structural stability of spinel LiMn2O4 and layered LiCoO2 [4 –6]. According to our previous work [7], surface modification of spinel LiMn2O4 is also an attractive way to improve the properties of this material. The spinel structure of *
Corresponding author. Tel.: +86-10-82649046; fax: +86-1082649050. E-mail address: [email protected] (X. Huang).
LiMn2O4 can be retained and the electrochemical performance can be enhanced in this way. The chemical diffusion coefficient of Li+ is considered as an important kinetic parameter for characterizing the extraction and insertion properties of spinel LiMn2O4 [8 –13]. In the present paper, the Li+ chemical diffusion coefficient of the fluorine-modified LiMn2O4 was studied by adopting a potential relaxation technique (PRT) [14].
2. Experimental Spinel LiMn2O4 had been modified by using an electrochemical method as previously described [7]. Fig. 1 illustrates the experimental set up in this work. An electrolytic cell, which contains potassium fluoride
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 3 2 6 - 0
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Fig. 1. Diagram of the electrochemical system for fluorinemodification; (1) aluminum foil, (2) LiMn2O4 film, (3) nickel foil, (4) electrolytic cell.
aqueous solution, was used as the electrochemical modification system. Hence, we here name this method as aqueous fluoride modification method (AFMM). LiMn2O4 films and nickel foils served as the working electrode and the counter electrode, respectively. The electrolytic cell was charged at different current densities. Subsequently, the modified LiMn2O4 films were washed with distilled water, and vacuum dried at 55 jC for 2 days to remove the residual water. The XRD patterns for the fluorine-modified samples were obtained with a Rigaku B/max-2400 X-ray diffractometer using CuKa radiation. SEM studies were performed by an Oxford S-4200 scanning electron microscope. To calculate the chemical diffusion coefficients of Li+, PRT measurements were carried out on a CHI660A electrochemical workstation. For the electrochemical tests, the prepared samples were assembled in Swagelok two-electrode cells [15,16]. Metal lithium foil served as the counter electrode. The electrolyte was 1 M LiPF6/EC-DMC (1:1 in volume). The cells was cycled in the voltage rang of 3.0– 4.5 V with a current density of 0.2 mA/cm2.
3. Results and discussion
The SEM images of the modified spinel samples MF9, MF13, MF14, MF31 are shown in Fig. 2. It is evident that the quantity of the tiny crystals can be controlled by the current density. While the processing time is fixed to 12 h, there are more and more tiny crystals grown on the spinel particles as the current density increases from 1.25 to 6.25 AA/cm2 (see Fig. 2a –c). It indicates that the processing current density is a crucial factor affecting the growth of KMnF3. As we expected, sample MF31 might have more integral KMnF3 crystals than sample MF14 because the processing current density of the former is 10-folds as large as that of the latter. However, the SEM image of sample MF31, which was treated for 2 h at the current density of 12.5 AA/cm2 is different from the others. The product seems like an amorphous phase rather than a crystal phase. It may be illustrated by the X-ray powder diffraction studies as follows. 3.2. XRD patterns In Fig. 3, the XRD patterns of the modified samples show the effect of the processing time on the growth of KMnF3. When the processing time is fixed to 12 h, there are notable diffraction peaks of KMnF3 in all the XRD patterns. The sharp peaks indicate the integrity of KMnF3 crystals. However, while the processing time is merely 2 h (sample MF31), the diffraction peaks of KMnF3 cannot be detected, though the processing current density is as high as 12.5 AA/cm2. This indicates that the processing time is another crucial factor that affects the growth of KMnF3. Diffraction peaks of lithium fluoride (LiF) are also found in the XRD patterns. Even though these peaks are very weak, LiF can be detected in all the modified samples, including sample MF31, which has no inteTable 1 Electrochemical processing conditions of fluorine-modified spinel LiMn2O4 Sample
Current density (AA/cm2)
Processing time (h)
Composition
MF9 MF13 MF14 MF31
6.25 2.50 1.25 12.5
12 12 12 2
LiMn2O4, KMnF3, LiF LiMn2O4, KMnF3, LiF LiMn2O4, KMnF3, LiF LiMn2O4, amorphous phase, LiF
3.1. SEM images To understand the factors influencing the growth of KMnF3 on the surface of the spinel particles, a series of experiments were carried out by varying the current density and the processing time, as shown in Table 1.
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Fig. 2. SEM images of the fluorine-modified samples processed with various current densities and processing times: (a) sample MF14 (1.25 AA/ cm2, 12 h), the white bar is 10 Am; (b) sample MF13 (2.50 AA/cm2, 12 h), the white bar is 10 Am; (c) sample MF9 (6.25 AA/cm2, 12 h), the white bar is 10 Am; (d) sample MF31 (12.5 AA/cm2, 2 h), the white bar is 1 Am.
gral crystals on the spinel surface. It indicates that there must be a common reaction, which produces small amount of LiF electrochemically. In the previous work [7], Eqs. (1) and (2) were adopted to interpret the formation of KMnF3. Here an additional reaction equation (3) is adopted to describe the formation of LiF. 2Mn3þ ! Mn2þ þ Mn4þ þ
2þ
K þ Mn
þ 3F ! KMnF3
Liþ þ F ! LiF
ð1Þ ð2Þ ð3Þ
It is well known that lithium ions are extracted from the spinel framework during the charge process of lithium-ion batteries. In the present study, when the electrochemical process begins, a disproportion reac-
tion of Mn3+ takes place in the electrolyte system combined with the extraction of Li+. Namely, reactions (1) and (3) take place at the same time, and a small amount of LiF is produced in this procedure. After the Mn2+ ions are formed and migrate to the surface of the spinel particles, reaction (2) takes place and becomes dominant. In the meanwhile, the formation of LiF is suppressed. Therefore, the amount of KMnF3 increases continuously, but the amount of LiF is kept at a constant level. According to the SEM and XRD studies described above, the growth of the tiny crystals of KMnF3 is affected by both the processing time and the current density. However, it is still our concern whether the two factors function separately or not. Further steps to study this issue are in progress.
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Fig. 3. XRD patterns of the fluorine-modified samples; (a) sample MF14, (b) sample MF13, (c) sample MF9, (d) sample MF31.
3.3. Galvanostatic cycling properties According to our previous study [7], tiny crystals of potassium manganese trifluoride (KMnF3) could be electrochemically produced on the surface of the spinel particles. However, when the sample was cycled at a heavy current, it showed a dramatic low capacity because the integral KMnF3 crystals blocked the diffusion of Li+ and caused a remarkable polarization of the cell. Therefore, an amorphous surface layer may be superior to a crystal one for Li+ diffusion. Fig. 4 shows the galvanostatic charge– discharge curve of sample MF31 with an amorphous surface layer. The modified sample was assembled in a Swagelok cell [15,16] in an argon-filled glove box. Metal lithium foil served as the counter electrode. The electrolyte was 1 M LiPF6/EC-DMC (1:1 in volume). The prepared cell was cycled at a constant current density of 0.2 mA/cm2 in the voltage rang 3.0 –4.5 V.
Since part of the Li+ and Mn3+ are consumed during the electrochemical modification process, as described in Eqs. (1) –(3), the specific capacity of the modified sample is lower than that of non-modified LiMn2O4. For stoichiometric LiMn2O4, the extraction and insertion of Li+ is illustrated by a two-step process [17 –19]. Firstly, the Li+ with strong Li –Li interaction are removed from half of the tetrahedral positions at lower potential, and some of the Mn3+ are converted to Mn4+. As the content of Mn4+ increases, the second half of Li+ is extracted with more energy at higher potential. Namely, there are two binary equilibrium systems during Li extraction or insertion, which are considered as LiMn2O4 – Li0.5Mn2O4 and Li0.5Mn2O4 – k-MnO2 [17]. As for the fluorine-modified spinel, the characteristic two 4-V plateaus of LiMn2O4 in the galvanostatic charge – discharge curves cannot be clearly distinguished (see Fig. 4). This indicates that the phase changes of the modified spinel are not so drastic when compared with the non-modified LiMn2O4. According to Xia and Yoshio [20], the shape of the charge– discharge curve will gradually transform to a quasione plateau as the vacancy rate increases in spinel LiMn2O4. In our study, the consumption of Li+ and Mn3+ during the electrochemical modification process may lead to a vacancy-rich structure, causing the quasi-one plateau of the modified sample. In the charge– discharge curve, the high voltage region changes to an S-shape, implying that the two-phase structure is transformed to a one-phase structure [21]. Namely, the harmful two-phase reaction of LiMn2O4 upon cycling was suppressed after the cathode was processed by AFMM. Although the Li insertion/extraction reaction regions cannot be distinguished definitely in Fig. 4, they are distinct in the dQ/dV vs. V curve, as shown in Fig. 5. Since the host structure of LiMn2O4 is retained after fluorine-modification processing, the characteristic redox peaks of LiMn2O4 are kept. There are two pairs of oxidation and reduction peaks located at 4.06 and 4.17 V, and 4.07 and 3.97 V. This indicates the reversibility of the electrochemical reactions in the charge– discharge process. The intensities of the oxidation peaks are approximately the same. However, the intensity of the first reduction peak is much weaker than that of the second one. The delay of the first Li+ insertion process is responsible for this phenomenon.
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Fig. 4. Typical galvanostatic charge – discharge curve of fluorine-modified sample MF31.
3.4. Chemical diffusion coefficients A PRT measurement was adopted to measure the chemical diffusion coefficients of Li+ in the fluorine-
modified sample. This technique has the capability to gain the chemical diffusion coefficients during both charge and discharge processes. The modified sample MF31 was assembled in a Swagelok cell in an argon-
Fig. 5. dQ/dV vs. V plot derived from the charge – discharge curve of Fig. 4.
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Fig. 6. Typical potential relaxation plot of sample MF31 during the charging process. The starting potential of the relaxation is 3.90 V.
filled glove box, as described in the former section. The thickness of the complex cathode is 6.1103 cm. To avoid the effects of side reactions, the cell was cycled till the coulombic efficiency was above 97%.
The potential relaxation curves were recorded by using a chronopotentiometry technique of the CHI660A electrochemical workstation. After the cell was charged or discharged to a certain potential at the
Fig. 7. ln[exp(ulut)F/RT1] vs. t plot at the potential of 3.90 V.
C. Wu et al. / Solid State Ionics 152 – 153 (2002) 327–334 Table 2 Chemical diffusion coefficients of Li+ for fluorine-modified sample MF31 Cyclic process
Starting potential (V)
Chemical diffusion coefficient (cm2/s)
Charge
3.90 4.09 4.18 4.34 4.11 4.03 3.84 3.25
1.95109 2.741010 2.171010 7.211010 3.18109 2.19109 7.321010 6.301010
Discharge
current density of 0.2 mA/cm2, the cathodic or anodic current was dropped to zero, then the open circuit voltage (OCV) was recorded by adopting an interval of one record per second. Eq. (4) describes the diffusion properties of the lithium ions during the extraction and insertion procedure in the bulk electrode. The detailed derivation of this equation was described elsewhere [14]. h u u i p2 t ln exp l F 1 ¼ lnN Ds t ð4Þ RT L2 ut represents the electrode potential as the relaxation time equal to t, and ul is the potential when the relaxation time tends to be infinite. Another parameter L is the thickness of the working electrode. The value of the chemical diffusion coefficient Ds can be calculated from the linear fit slope of ln[exp(ulut)F/ RT1] vs. t. Here it is unnecessary to consider the intercept ln N of the fitted plot. Fig. 6 shows the typical potential relaxation plot of sample MF31 during charge process. Although the cell has been charged to 3.95 V, the start point of the relaxation is 3.90 V because of the polarization of the electrode. It can be seen that the OCV vs. Li/Li+ decreases rapidly in the initial stages. When the relaxation time is long enough, the OCV tends to be constant. Considering the self-discharge of the lithium ion batteries, it is insignificant to record the OCVafter a very long time. Therefore, we chose the OCV corresponding with t=86,400 s (24 h) as the value of ul. The typical plot of ln[exp(ulut)F/RT1] vs. t is shown in Fig. 7. Ds evaluated from the slope of the solid line is 1.95109 cm2/s. Chemical diffusion coefficients of charge and discharge process at various potentials are listed in Table 2.
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As previously reported [8 –13], chemical diffusion coefficients of Li+ for LiMn2O4 are in the range from 1010 to 108 cm2/s. In the present study, the values of Ds in both charge and discharge processes are conveniently determined. Those of the fluorine-modified LiMn2O4 are of the order of 1010 –109 cm2/s.
4. Conclusion Spinel LiMn2O4 was modified by using an aqueous fluoride modification method (AFMM). We showed the possibility of modifying LiMn2O4 by simple electrochemical processing. Tiny crystals of potassium manganese trifluoride (KMnF3) were formed on the spinel particles. The formation of the KMnF3 crystals was affected by current density and processing time. The harmful two-phase reaction of LiMn2O4 upon cycling was suppressed after the cathode was processed by AFMM. As compared with previous study [7], amorphous surface layers of the modified samples may be superior to more crystalline layers for Li+ diffusion. A potential relaxation technique (PRT) was adopted to measure the chemical diffusion coefficient of Li+ (Ds) for the fluorine-modifed samples. The values of Ds are of the order of 1010 – 109 cm2/s, similar to those of non-modified LiMn2O4.
Acknowledgements The authors would like to show gratitude to NSFC (No. 59972041) and National 863 key program (No. 2001AA320301) for financial support.
References [1] C. Wu, H. Li, F. Wu, X. Huang, in: B.V.R. Chowdari, W. Wang (Eds.), Proceeding of the 7th Asian Conference on Solid State Ionics, World Scientific Press, Fuzhou, China, Oct. 29, 2000, pp. 261 – 265. [2] J.H. Lee, J.K. Hong, D.H. Jang, Y.-K. Sun, S.M. Oh, J. Power Sources 89 (2000) 7. [3] J. Cho, G.B. Kim, H.S. Lim, C.-S. Kim, S.I. Yoo, Electrochem. Solid-State Lett. 2 (1999) 607. [4] J. Cho, G. Kim, Electrochem. Solid-State Lett. 2 (1999) 253. [5] J. Cho, Y.J. Kim, B. Park, Chem. Mater. 12 (2000) 3788. [6] J. Cho, C.S. Kim, S.I. Yoo, Electrochem. Solid-State Lett. 3 (2000) 362.
334
C. Wu et al. / Solid State Ionics 152 – 153 (2002) 327–334
[7] C. Wu, F. Wu, X. Huang, in: B.V.R. Chowdari, W. Wang (Eds.), Proceeding of the 7th Asian Conference on Solid State Ionics, World Scientific Press, Fuzhou, China, Oct. 29, 2000, pp. 267 – 270. [8] L. Chen, J. Schoonman, Solid State Ionics 67 (1994) 17. [9] L. Chen, X. Huang, E. Kelder, J. Schoonman, Solid State Ionics 76 (1995) 91. [10] M.Y. Saidi, J. Barker, R. Koksbang, J. Solid State Chem. 122 (1996) 195. [11] M. Wakihara, L. Guohua, H. Ikuta, T. Uchida, Solid State Ionics 86 – 88 (1996) 907. [12] W. Yang, G. Zhang, S. Lu, J. Xie, Q. Liu, Solid State Ionics 121 (1999) 85. [13] S. Chitra, P. Kalyani, T. Mohan, R. Gangadharan, B. Yebka, S. Castro-Garcia, M. Massot, C. Julien, J. Electroceram. 3 (1999) 433.
[14] Q. Wang, H. Li, X. Huang, L. Chen, J. Electrochem. Soc. 148 (2001) A737. [15] J.M. Tarascon, F.J. Disalvo, D.W. Murphy, G.W. Hull, J.V. Waszczak, J. Solid State Chem. 54 (1984) 204. [16] P. Arora, B.N. Popov, R.E. White, J. Electrochem. Soc. 145 (1998) 807. [17] M.M. Thackeray, P.J. Johnson, L.A. de Picciotto, Mater. Res. Bull. 19 (1984) 179. [18] W. Liu, G.C. Farrington, F. Chaput, B. Dunn, J. Electrochem. Soc. 143 (1996) 879. [19] A. Ott, P. Endres, V. Klein, B. Fuchs, A. Jager, H.A. Mayer, S. Kemmler-Sack, H.-W. Praas, K. Brandt, G. Filoti, V. Kunczer, M. Rosenberg, J. Power Sources 72 (1998) 1. [20] Y. Xia, M. Yoshio, J. Electrochem. Soc. 144 (1997) 4186. [21] Y. Xia, M. Yoshio, J. Electrochem. Soc. 143 (1996) 825.