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Electrochemical behavior of thin-film LiMn2O4 electrode in aqueous media
Electrochimica Acta 47 (2001) 495– 499 www.elsevier.com/locate/electacta
Electrochemical behavior of thin-film LiMn2O4 electrode in aqueous media Ali Eftekhari * Department of Chemistry, K N Toosi Uni6ersity of Technology, P.O. Box 15875 -4416, Tehran, Iran Received 14 May 2001; received in revised form 25 July 2001
Abstract The electrochemical properties of a thin-film LiMn2O4 electrode were studied in an aqueous medium. The studies were done in a saturated aqueous medium to increase similarity of the electrolyte solution with a non-aqueous medium. The apparent diffusion coefficient was calculated and discussed for different potentials at the aqueous electrolyte. In addition, stability of the thin solid film was investigated during potential cycling, which indicates that the electrode is highly stable in aqueous medium as well as in non-aqueous media. The obtained results from electrochemical measurements indicate that the properties of the thin-film LiMn2O4 electrode in aqueous medium are similar to those that have been found in non-aqueous media. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: LiMn2O4; Thin film; Aqueous media; Diffusion coefficient; Electrochemical stability; Lithium batteries
1. Introduction Studies on the LiMn2O4 spinel are one of the most rapidly growing and advancing areas in lithium secondary battery research because of cost and toxicity reasons [1–10]. Moreover, LiMn2O4 is less expensive in comparison with similar compounds such as LiCoO2 and LiNiO2. The charge/discharge reaction of LiMn2O4 is as shown in the following equation, which occurs at a high potential of about 4 V (vs. Li/Li+): LiMn2O4 = Li1 − x Mn2O4 +xLi+ +xe−
(0 5 x 5 1) (1)
where the forward reaction is related to the charge process and the reverse one is discharge. Due to the existence of a wide range of spinel LiMnO compounds with different Li/Mn ratios [11 – 13], the electrochemical properties of LiMn2O4 are highly sensitive to the synthesis route, processing conditions and thermal history of the samples. Several methods have been proposed for the preparation of LiMn2O4 for lithium-ion battery applications [2,14 –17]. However, most electrochemical * Tel.: +98-21-204-2549; fax: +98-21-205-7621. E-mail address: [email protected] (A. Eftekhari).
studies of LiMn2O4 have been done in non-aqueous media. There are a few number of papers dealing with studies of electrochemical properties of lithium transition metal oxides in aqueous media [18 –25]. Dahn et al. [19 –21] have studied the charge and discharge properties of LiMn2O4 in aqueous solutions of LiOH and LiNO3. The rate capabilities of nanostructured electrodes of LiMn2O4 with different thickness have been investigated in aqueous electrolyte [22]. The effect of different aqueous electrolytes has also been studied for voltammetric behavior of LiNiO2 prepared by selfpropagating high-temperature combustion (SPHTC) [24]. One of the main problems in lithium batteries is plating of the metallic lithium. Thus, no trickle charge can be applied because the Li-ion is unable to absorb any overcharge. Trickle charge could cause plating of the metallic lithium, a condition that would make the cell unstable. Instead, a brief topping charge should be applied from time to time to compensate for the small amount of self-discharge the battery and its protective circuit consume. On the other hand, the applied medium is not highly effective on such behavior, as reported for suitable media with ignorable Li plating for aqueous solution [18 –25].
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In the present paper, we wish to study the electrochemical behavior of LiMn2O4 as a lithium transition metal oxide in an aqueous medium. To obtain an aqueous medium similar to non-aqueous media used in the literature, a saturated solution of LiNO3 was used as the electrolyte. The study is of interest for both researchers studying the properties of materials in different media and those who are studying lithium-ion batteries, as aqueous media can be used for their preliminary studies.
Experiments were done at a conventional three-electrode cell containing a platinum plate and a saturated calomel electrode (SCE) as counter and reference electrodes, respectively. The thin-film LiMn2O4 electrode was the working electrode of the electrochemical cell. A Luggin capillary was placed between the reference electrode and the working electrode to minimize the iRdrop of the system. The electrolyte solution was prepared from saturated LiNO3 ( 9 M).
3. Results and discussion 2. Experimental The LiMn2O4 sample was synthesized by solid-state reaction according to the procedure described in the literature [17]. Briefly, it was done by calcination of a mixture of LiOH and MnO2 (CMD) in a 1:2 molar ratio at 400 °C for 10 h and sintering at a constant temperature for 48 h in air with intermediate grinding. To achieve the aim of this research, LiMn2O4 was only prepared by sintering at 850 °C. The sample was cooled with a slow cooling rate of 1°C/min. The complete morphology and structure of the sample prepared using the above-mentioned method has been reported previously by Ahn and Song [17]. A Pt substrate electrode with geometrical area of 0.9 cm2 was used for the deposition of a thin film of LiMn2O4. During the film deposition, the surface of the Pt electrode was covered to limit the exposed area to a circle with certain diameter. The amount of deposited film was estimated by weighing the substrate electrode before and after the deposition. The amount of LiMn2O4 used for the electrode was 2593 mg by weighing the electrode before and after the deposition. Electrochemical measurements were performed using a low-noise home-made potentiostat connected to a microcomputer by running the CORRVIEW software.
Fig. 1. Typical cyclic voltammogram of the thin-film LiMn2O4 electrode in aqueous solution (dE/dt= w =0.5 mV/s).
Fig. 1 shows a typical cyclic voltammogram of the thin-film LiMn2O4 electrode in a saturated solution of LiNO3 under quasi-steady state condition with slow potential scan rate (w= 0.5 mV/s). Two couples of oxidation and reduction are found near 0.5 and 0.6 V (vs. SCE), accompanied by extraction/insertion of lithium ions from/into the manganese spinel phase [15,26,27]. It indicates that the oxidation and reduction of LiMn2O4, which is according to Eq. (1), occurs in two steps. The first (the lower-potential peaks) is related to lithium ion insertion/extraction in the range 05 x5 0.5 in Lix Mn2O4 and the other (the high-potential couple) for the insertion/extraction of lithium ions in the range 0.55 x51. As the first peak’s couple (occurs at less positive potentials) corresponds to the first step of electrochemical reaction of the LiMn2O4 redox system, the first couple was chosen for further voltammetric studies of the system under investigation. It also should be noted that the obtained results from electrochemical studies of the second couple show similar voltammetric characteristics. However, the data reported in the present paper are related to the first couple, but similar data can be obtained for the second couple. The effect of potential scan rate on the peak current of cyclic voltammograms is presented in Fig. 2. For scan rates lower than 6 mV/s, there is a relationship between the peak current and scan rate, due to the existence of very small particles, which cause the complete electrolysis in the long period of the experiment [24]. On the other hand, this behavior comes from the fact that the film is thin enough that at the scan rate employed, the diffusion layer reaches the film edge, which is called ‘finite diffusion’. For higher values of scan rate, the peak current is in proportion to the square root of the scan rate. However, the obtained result shows that electrochemical reaction of the lithium insertion/extraction is kinetically limited by diffusion of lithium ions within the solid phase. Nevertheless, as will be shown later, the diffusion coefficient of lithium ions is dependent on the electrode potential. As expected, the peak separation increases by increasing the potential scan rate, as the anodic peaks
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Luggin capillary used between the working and reference electrodes and the mentioned conductivity of the electrolyte solution, the iR-drop of the system in cyclic voltammetric measurements was less than 1 mV. These behaviors could be ascribed to the effect of concentration polarization and maybe slow electron transfer [28]. To study electrochemical stability of the thin-film LiMn2O4 electrode, voltammetric experiments were done during potential cycling. It has been reported [29] that the lithium extraction around 4.1 V (vs. Li/Li+) results in an irreversible structural change to some extent. Fig. 4(a) shows only a 10% decrease in the peak current after 100 potential cycles with potential scan rate of 1 mV/s. Moreover, it should be noted that the thin-film electrode is very stable during potential cycling and this decay in the current is related to slow scan rate. By applying a scan rate of 100 mV/s (Fig. 4(b)), decrease in the electrode current after 1000 potential cycles is about half of that which occurs after only 100 cycles with a scan rate of 1 mV/s. As expected, time is effective on the stability of the electrode as well as applied potential. It is clear that the timescale of the experiments related to the slow scan rate is 100 times larger than the experiment done at a fast scan rate.
Fig. 2. Relationship of the peak current to potential scan rate for (a) slow and (b) fast scans.
shift towards a more positive potential and the cathodic peaks shift to the negative ones. Fig. 3 shows scan rate dependency of the peak separation. However, these behaviors are not due to the increased iR-drop because of the system’s resistance, as the ionic conductivity of the LiNO3 solution is very high. For example, it has been reported that the ionic conductivity of a 5.7 M LiNO3 solution is 0.11 V/cm [21]. According to the
Fig. 3. Effect of potential scan rate on the peak separation.
Fig. 4. Stability of the electrode during potential cycling for (a) slow scan rate (1 mV/s) and (b) fast scan rate (100 mV/s). The experiments were done by repetitive cyclic voltammetry in a potential range located between 0.0 and 0.8 V.
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diffusion limited. The value of the diffusion coefficient can be obtained from chronoamperometric measurements. It has been reported [30,31] that for the time domain of th 2/p2D, the current is dependent on the time as shown in the following equation: ln(i )= ln(2nFADDC/h)− (p2Dapp/4h 2)t
Fig. 5. Changes in discharge capacity of thin-film LiMn2O4 electrode during charge/discharge cycles.
In addition, the discharge capacity of the thin-film LiMn2O4 electrode was investigated at moderate potential scan rate of 5 mV/s. Fig. 5 shows the cyclic behavior of the electrode determined by the integration of currents of repetitive cyclic voltammograms. The potential cycling was done as potential was scanned between 0.0 and 0.8 V versus SCE. As seen, less than 10% decrease is observed after 100 cycles. It indicates that the cycle stability of the thin-film LiMn2O4 electrode is good in aqueous medium of LiNO3, indicating that the material is stable against lithium-ion extraction/insertion. As the kinetics of a solid-state diffusion process limit the total reaction rate, diffusion coefficient is one of the most important kinetic parameters for ion-insertion compounds. It also is very important in battery application as the total reaction rate is related to the maximum current of the battery. However, as noted above, the electrochemical lithium insertion/extraction reaction is
(2)
where h and A are the thickness and the surface area of the electroactive film, and DC is the variation of lithium concentration in the film during potential step. For the case of electrodes with spherical shapes, h must be replaced by a, the radius of the spherical particle. Although the above-mentioned equation often has been used for spherical electrodes [30–32], this is also true for planar electrodes [9]. However, for using this equation, applying h or a must be taken into account to be applicable for different electrodes. According to Eq. (2), there are two ways to calculate the diffusion coefficient: from the intercept of the log i versus t curve using the first part of the equation (D), and from the slope of the log i versus t curve using the last part of the equation (Dapp). As the precise measurement of DC is very difficult, the latter approach is more suitable for this purpose. Therefore, the second approach was used for the determination of the apparent diffusion coefficient (Dapp). The dependence of the apparent diffusion coefficient on the electrode potential is presented in Fig. 6 by applying the 10 mV potential steps. As seen, the apparent diffusion coefficient varies over two decades from 10 − 9 to 10 − 11 cm2/s. By comparing the obtained curve with the cyclic voltammogram (Fig. 1), it can be observed that they are similar and the apparent diffusion coefficient curve shows minima corresponding to the intercalation peaks in cyclic voltammograms. The electrochemical properties of LiMn2O4 have been extensively studied in the past few years due to its importance for lithium battery and related applications. The obtained results from electrochemical studies of LiMn2O4 in aqueous solution shows that there is no significant difference with those reported for nonaqueous solutions. It is very interesting to use aqueous electrolytes instead of non-aqueous electrolytes even for studies of lithium battery cathodes.
4. Conclusions
Fig. 6. Dependence of the apparent diffusion coefficient on the electrode potential. The data (values for the apparent diffusion coefficient, Dapp) were obtained from chronoamperometric measurements at each potential.
The electrochemical behavior of the thin-film LiMn2O4 was studied in an aqueous solution. Voltammetric characteristics of the electrode were obtained by studying the effects of potential scan rate on the peak current and peak separation. In addition, it was shown that the electrode is electrochemically stable in aqueous medium during potential cycling. It is of interest and important because the most important applications of
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LiMn2O4 and similar compounds are as cathode materials for lithium batteries. Thus, excellent electrochemical stability is required for using them in an electrochemical cell (condition). The aqueous electrolyte was a saturated solution, which is the most similar condition to a non-aqueous medium. The obtained results from experimental measurements reveal that the electrode shows similar behavior to that observed for non-aqueous media. It can be used to replace non-aqueous media with aqueous media for a preliminary study of cathode materials for lithium battery applications. Due to the many advantages of aqueous media, it is very useful for the purpose of preliminary investigations. Of course, this is suitable only for preliminary studies and further studies for battery application must be carried out under the same condition as for lithium batteries (non-aqueous media). Nevertheless, this statement is only related to the study of lithium batteries, whereas the research is useful for deep and whole studies for different applications. Indeed, without attention to the application of LiMn2O4 for lithium battery technology, the present paper studies the electrochemical behavior of a thin-film LiMn2O4 electrode, which is of interest from electrochemical (e.g. basic and fundamental aspects for electrochemical reactions) point of view and scientific application of this science. Further investigation of the thin-film LiMn2O4 electrode and the other layered oxides is now in progress, which may lead us to a classification of the electrochemical behavior of this type of materials in aqueous media.
Acknowledgements The author would like to thank Professor Isamu Uchida (Tohoku University, Japan) for providing the background of this research.
References [1] R.J. Gummow, A. de Kock, M.M. Thackeray, Solid State Ionics 69 (1994) 59. [2] J.M. Tarascon, W.R. McKinnon, F. Coowar, T.N. Bowmer, G. Amatucci, D. Guyomard, J. Electrochem. Soc. 141 (1994) 1421.
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[3] A. Yamada, K. Miura, K. Hinokuma, M. Tanaka, J. Electrochem. Soc. 142 (1995) 2149. [4] G. Pistoia, D. Zane, Y. Zhang, J. Electrochem. Soc. 142 (1995) 2551. [5] Y. Gao, J.R. Dahn, J. Electrochem. Soc. 143 (1996) 1783 (see also p. 100). [6] J.M. Tarascon, F. Coowar, G. Amatucci, F.K. Shokoohi, D.G. Gyomard, J. Power Sources 54 (1995) 103. [7] M.M. Thackeray, M.F. Mansuetto, J.B. Bates, J. Power Sources 68 (1997) 153. [8] A. Blur, C. Sigala, G. Amatucci, D. Guyomard, Y. Chabre, J.M. Tarascon, J. Electrochem. Soc. 145 (1998) 194. [9] M. Nishizawa, T. Uchiyama, K. Dokko, K. Yamada, T. Matsue, I. Uchida, Bull. Chem. Soc. Jpn. 71 (1998) 2011. [10] M. Nishizawa, T. Uchiyama, T. Itoch, T. Abe, I. Uchida, Langmuir 15 (1999) 4949. [11] F. Le Cras, P. Strobel, M. Anne, D. Bloch, J.B. Soupart, J.C. Rousche, Eur. J. Solid State Inorg. Chem. 33 (1996) 67. [12] P. Strobel, F. Le Cras, M. Anne, J. Solid State Chem. 124 (1996) 83. [13] C. Masquelier, M. Tabuchi, K. Ado, R. Kanno, Y. Kobayashi, Y. Maki, O. Nakamura, J.B. Goodenough, J. Solid State Chem. 123 (1996) 255. [14] A. Momchilov, V. Manev, A. Nassalevska, J. Power Sources 41 (1993) 305. [15] D. Guyomard, J.M. Tarascon, J. Electrochem. Soc. 139 (1992) 937. [16] Y. Xia, M. Yoshia, J. Electrochem. Soc. 143 (1996) 825. [17] D.S. Ahn, M.Y. Song, J. Electrochem. Soc. 147 (2000) 874. [18] Y. Kanzaki, A. Taniguchi, M. Abe, J. Electrochem. Soc. 138 (1991) 334. [19] W. Li, W.R. McKinnon, J.R. Dahn, J. Electrochem. Soc. 141 (1994) 2310. [20] W. Li, J.R. Dahn, D.S. Wainwright, Science 264 (1994) 1115. [21] W. Li, J.R. Dahn, J. Electrochem. Soc. 142 (1995) 1742. [22] N. Li, C.J. Patrissi, G. Che, C.R. Martin, J. Electrochem. Soc. 147 (2000) 2044. [23] H. Schlorb, M. Bungs, W. Plieth, Electrochim. Acta 42 (1997) 2619. [24] M.M. Rao, M. Jayalakshmi, O. Schaf, U. Guth, H. Wulff, F. Scholz, J. Solid State Electrochem. 4 (1999) 17. [25] M.M. Rao, M. Jayalakshmi, O. Schaf, U. Wulff, U. Guth, F. Scholz, J. Solid State Electrochem. 5 (2001) 50. [26] T. Ohzuku, M. Kitagawa, T. Hirai, J. Electrochem. Soc. 137 (1990) 769. [27] A.H. Gemeay, H. Nishiyama, S. Kuwabata, H. Yoneyama, J. Electrochem. Soc. 142 (1995) 4190. [28] G. Che, K.B. Jirage, E.R. Fischer, C.R. Martin, H. Yoneyama, J. Electrochem. Soc. 144 (1997) 4296. [29] K. Kanamura, H. Naito, T. Yao, Z. Takehara, J. Mater. Chem. 6 (1996) 33. [30] H. Ura, T. Nishina, I. Uchida, J. Electroanal. Chem. 369 (1995) 169. [31] T. Nishina, H. Ura, I. Uchida, J. Electrochem. Soc. 144 (1997) 1273. [32] M. Nishizawa, R. Hashitani, T. Itoh, T. Matsue, I. Uchida, Electrochem. Solid State Lett. 1 (1998) 10.
Electrochemical behavior of thin-film LiMn2O4 electrode in aqueous media Ali Eftekhari * Department of Chemistry, K N Toosi Uni6ersity of Technology, P.O. Box 15875 -4416, Tehran, Iran Received 14 May 2001; received in revised form 25 July 2001
Abstract The electrochemical properties of a thin-film LiMn2O4 electrode were studied in an aqueous medium. The studies were done in a saturated aqueous medium to increase similarity of the electrolyte solution with a non-aqueous medium. The apparent diffusion coefficient was calculated and discussed for different potentials at the aqueous electrolyte. In addition, stability of the thin solid film was investigated during potential cycling, which indicates that the electrode is highly stable in aqueous medium as well as in non-aqueous media. The obtained results from electrochemical measurements indicate that the properties of the thin-film LiMn2O4 electrode in aqueous medium are similar to those that have been found in non-aqueous media. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: LiMn2O4; Thin film; Aqueous media; Diffusion coefficient; Electrochemical stability; Lithium batteries
1. Introduction Studies on the LiMn2O4 spinel are one of the most rapidly growing and advancing areas in lithium secondary battery research because of cost and toxicity reasons [1–10]. Moreover, LiMn2O4 is less expensive in comparison with similar compounds such as LiCoO2 and LiNiO2. The charge/discharge reaction of LiMn2O4 is as shown in the following equation, which occurs at a high potential of about 4 V (vs. Li/Li+): LiMn2O4 = Li1 − x Mn2O4 +xLi+ +xe−
(0 5 x 5 1) (1)
where the forward reaction is related to the charge process and the reverse one is discharge. Due to the existence of a wide range of spinel LiMnO compounds with different Li/Mn ratios [11 – 13], the electrochemical properties of LiMn2O4 are highly sensitive to the synthesis route, processing conditions and thermal history of the samples. Several methods have been proposed for the preparation of LiMn2O4 for lithium-ion battery applications [2,14 –17]. However, most electrochemical * Tel.: +98-21-204-2549; fax: +98-21-205-7621. E-mail address: [email protected] (A. Eftekhari).
studies of LiMn2O4 have been done in non-aqueous media. There are a few number of papers dealing with studies of electrochemical properties of lithium transition metal oxides in aqueous media [18 –25]. Dahn et al. [19 –21] have studied the charge and discharge properties of LiMn2O4 in aqueous solutions of LiOH and LiNO3. The rate capabilities of nanostructured electrodes of LiMn2O4 with different thickness have been investigated in aqueous electrolyte [22]. The effect of different aqueous electrolytes has also been studied for voltammetric behavior of LiNiO2 prepared by selfpropagating high-temperature combustion (SPHTC) [24]. One of the main problems in lithium batteries is plating of the metallic lithium. Thus, no trickle charge can be applied because the Li-ion is unable to absorb any overcharge. Trickle charge could cause plating of the metallic lithium, a condition that would make the cell unstable. Instead, a brief topping charge should be applied from time to time to compensate for the small amount of self-discharge the battery and its protective circuit consume. On the other hand, the applied medium is not highly effective on such behavior, as reported for suitable media with ignorable Li plating for aqueous solution [18 –25].
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In the present paper, we wish to study the electrochemical behavior of LiMn2O4 as a lithium transition metal oxide in an aqueous medium. To obtain an aqueous medium similar to non-aqueous media used in the literature, a saturated solution of LiNO3 was used as the electrolyte. The study is of interest for both researchers studying the properties of materials in different media and those who are studying lithium-ion batteries, as aqueous media can be used for their preliminary studies.
Experiments were done at a conventional three-electrode cell containing a platinum plate and a saturated calomel electrode (SCE) as counter and reference electrodes, respectively. The thin-film LiMn2O4 electrode was the working electrode of the electrochemical cell. A Luggin capillary was placed between the reference electrode and the working electrode to minimize the iRdrop of the system. The electrolyte solution was prepared from saturated LiNO3 ( 9 M).
3. Results and discussion 2. Experimental The LiMn2O4 sample was synthesized by solid-state reaction according to the procedure described in the literature [17]. Briefly, it was done by calcination of a mixture of LiOH and MnO2 (CMD) in a 1:2 molar ratio at 400 °C for 10 h and sintering at a constant temperature for 48 h in air with intermediate grinding. To achieve the aim of this research, LiMn2O4 was only prepared by sintering at 850 °C. The sample was cooled with a slow cooling rate of 1°C/min. The complete morphology and structure of the sample prepared using the above-mentioned method has been reported previously by Ahn and Song [17]. A Pt substrate electrode with geometrical area of 0.9 cm2 was used for the deposition of a thin film of LiMn2O4. During the film deposition, the surface of the Pt electrode was covered to limit the exposed area to a circle with certain diameter. The amount of deposited film was estimated by weighing the substrate electrode before and after the deposition. The amount of LiMn2O4 used for the electrode was 2593 mg by weighing the electrode before and after the deposition. Electrochemical measurements were performed using a low-noise home-made potentiostat connected to a microcomputer by running the CORRVIEW software.
Fig. 1. Typical cyclic voltammogram of the thin-film LiMn2O4 electrode in aqueous solution (dE/dt= w =0.5 mV/s).
Fig. 1 shows a typical cyclic voltammogram of the thin-film LiMn2O4 electrode in a saturated solution of LiNO3 under quasi-steady state condition with slow potential scan rate (w= 0.5 mV/s). Two couples of oxidation and reduction are found near 0.5 and 0.6 V (vs. SCE), accompanied by extraction/insertion of lithium ions from/into the manganese spinel phase [15,26,27]. It indicates that the oxidation and reduction of LiMn2O4, which is according to Eq. (1), occurs in two steps. The first (the lower-potential peaks) is related to lithium ion insertion/extraction in the range 05 x5 0.5 in Lix Mn2O4 and the other (the high-potential couple) for the insertion/extraction of lithium ions in the range 0.55 x51. As the first peak’s couple (occurs at less positive potentials) corresponds to the first step of electrochemical reaction of the LiMn2O4 redox system, the first couple was chosen for further voltammetric studies of the system under investigation. It also should be noted that the obtained results from electrochemical studies of the second couple show similar voltammetric characteristics. However, the data reported in the present paper are related to the first couple, but similar data can be obtained for the second couple. The effect of potential scan rate on the peak current of cyclic voltammograms is presented in Fig. 2. For scan rates lower than 6 mV/s, there is a relationship between the peak current and scan rate, due to the existence of very small particles, which cause the complete electrolysis in the long period of the experiment [24]. On the other hand, this behavior comes from the fact that the film is thin enough that at the scan rate employed, the diffusion layer reaches the film edge, which is called ‘finite diffusion’. For higher values of scan rate, the peak current is in proportion to the square root of the scan rate. However, the obtained result shows that electrochemical reaction of the lithium insertion/extraction is kinetically limited by diffusion of lithium ions within the solid phase. Nevertheless, as will be shown later, the diffusion coefficient of lithium ions is dependent on the electrode potential. As expected, the peak separation increases by increasing the potential scan rate, as the anodic peaks
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Luggin capillary used between the working and reference electrodes and the mentioned conductivity of the electrolyte solution, the iR-drop of the system in cyclic voltammetric measurements was less than 1 mV. These behaviors could be ascribed to the effect of concentration polarization and maybe slow electron transfer [28]. To study electrochemical stability of the thin-film LiMn2O4 electrode, voltammetric experiments were done during potential cycling. It has been reported [29] that the lithium extraction around 4.1 V (vs. Li/Li+) results in an irreversible structural change to some extent. Fig. 4(a) shows only a 10% decrease in the peak current after 100 potential cycles with potential scan rate of 1 mV/s. Moreover, it should be noted that the thin-film electrode is very stable during potential cycling and this decay in the current is related to slow scan rate. By applying a scan rate of 100 mV/s (Fig. 4(b)), decrease in the electrode current after 1000 potential cycles is about half of that which occurs after only 100 cycles with a scan rate of 1 mV/s. As expected, time is effective on the stability of the electrode as well as applied potential. It is clear that the timescale of the experiments related to the slow scan rate is 100 times larger than the experiment done at a fast scan rate.
Fig. 2. Relationship of the peak current to potential scan rate for (a) slow and (b) fast scans.
shift towards a more positive potential and the cathodic peaks shift to the negative ones. Fig. 3 shows scan rate dependency of the peak separation. However, these behaviors are not due to the increased iR-drop because of the system’s resistance, as the ionic conductivity of the LiNO3 solution is very high. For example, it has been reported that the ionic conductivity of a 5.7 M LiNO3 solution is 0.11 V/cm [21]. According to the
Fig. 3. Effect of potential scan rate on the peak separation.
Fig. 4. Stability of the electrode during potential cycling for (a) slow scan rate (1 mV/s) and (b) fast scan rate (100 mV/s). The experiments were done by repetitive cyclic voltammetry in a potential range located between 0.0 and 0.8 V.
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diffusion limited. The value of the diffusion coefficient can be obtained from chronoamperometric measurements. It has been reported [30,31] that for the time domain of th 2/p2D, the current is dependent on the time as shown in the following equation: ln(i )= ln(2nFADDC/h)− (p2Dapp/4h 2)t
Fig. 5. Changes in discharge capacity of thin-film LiMn2O4 electrode during charge/discharge cycles.
In addition, the discharge capacity of the thin-film LiMn2O4 electrode was investigated at moderate potential scan rate of 5 mV/s. Fig. 5 shows the cyclic behavior of the electrode determined by the integration of currents of repetitive cyclic voltammograms. The potential cycling was done as potential was scanned between 0.0 and 0.8 V versus SCE. As seen, less than 10% decrease is observed after 100 cycles. It indicates that the cycle stability of the thin-film LiMn2O4 electrode is good in aqueous medium of LiNO3, indicating that the material is stable against lithium-ion extraction/insertion. As the kinetics of a solid-state diffusion process limit the total reaction rate, diffusion coefficient is one of the most important kinetic parameters for ion-insertion compounds. It also is very important in battery application as the total reaction rate is related to the maximum current of the battery. However, as noted above, the electrochemical lithium insertion/extraction reaction is
(2)
where h and A are the thickness and the surface area of the electroactive film, and DC is the variation of lithium concentration in the film during potential step. For the case of electrodes with spherical shapes, h must be replaced by a, the radius of the spherical particle. Although the above-mentioned equation often has been used for spherical electrodes [30–32], this is also true for planar electrodes [9]. However, for using this equation, applying h or a must be taken into account to be applicable for different electrodes. According to Eq. (2), there are two ways to calculate the diffusion coefficient: from the intercept of the log i versus t curve using the first part of the equation (D), and from the slope of the log i versus t curve using the last part of the equation (Dapp). As the precise measurement of DC is very difficult, the latter approach is more suitable for this purpose. Therefore, the second approach was used for the determination of the apparent diffusion coefficient (Dapp). The dependence of the apparent diffusion coefficient on the electrode potential is presented in Fig. 6 by applying the 10 mV potential steps. As seen, the apparent diffusion coefficient varies over two decades from 10 − 9 to 10 − 11 cm2/s. By comparing the obtained curve with the cyclic voltammogram (Fig. 1), it can be observed that they are similar and the apparent diffusion coefficient curve shows minima corresponding to the intercalation peaks in cyclic voltammograms. The electrochemical properties of LiMn2O4 have been extensively studied in the past few years due to its importance for lithium battery and related applications. The obtained results from electrochemical studies of LiMn2O4 in aqueous solution shows that there is no significant difference with those reported for nonaqueous solutions. It is very interesting to use aqueous electrolytes instead of non-aqueous electrolytes even for studies of lithium battery cathodes.
4. Conclusions
Fig. 6. Dependence of the apparent diffusion coefficient on the electrode potential. The data (values for the apparent diffusion coefficient, Dapp) were obtained from chronoamperometric measurements at each potential.
The electrochemical behavior of the thin-film LiMn2O4 was studied in an aqueous solution. Voltammetric characteristics of the electrode were obtained by studying the effects of potential scan rate on the peak current and peak separation. In addition, it was shown that the electrode is electrochemically stable in aqueous medium during potential cycling. It is of interest and important because the most important applications of
A. Eftekhari / Electrochimica Acta 47 (2001) 495–499
LiMn2O4 and similar compounds are as cathode materials for lithium batteries. Thus, excellent electrochemical stability is required for using them in an electrochemical cell (condition). The aqueous electrolyte was a saturated solution, which is the most similar condition to a non-aqueous medium. The obtained results from experimental measurements reveal that the electrode shows similar behavior to that observed for non-aqueous media. It can be used to replace non-aqueous media with aqueous media for a preliminary study of cathode materials for lithium battery applications. Due to the many advantages of aqueous media, it is very useful for the purpose of preliminary investigations. Of course, this is suitable only for preliminary studies and further studies for battery application must be carried out under the same condition as for lithium batteries (non-aqueous media). Nevertheless, this statement is only related to the study of lithium batteries, whereas the research is useful for deep and whole studies for different applications. Indeed, without attention to the application of LiMn2O4 for lithium battery technology, the present paper studies the electrochemical behavior of a thin-film LiMn2O4 electrode, which is of interest from electrochemical (e.g. basic and fundamental aspects for electrochemical reactions) point of view and scientific application of this science. Further investigation of the thin-film LiMn2O4 electrode and the other layered oxides is now in progress, which may lead us to a classification of the electrochemical behavior of this type of materials in aqueous media.
Acknowledgements The author would like to thank Professor Isamu Uchida (Tohoku University, Japan) for providing the background of this research.
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