Langmuir 1999, 15, 4949-4951
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Electrochemical Quartz Crystal Microbalance for Insertion/Extraction of Lithium in Spinel LiMn2O4 Thin Films Matsuhiko Nishizawa, Takayuki Uchiyama, Takashi Itoh, Takayuki Abe, and Isamu Uchida* Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 07 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan Received March 8, 1999. In Final Form: May 28, 1999 The electrochemical quartz crystal microbalance (EQCM) technique was successfully used to investigate the lithium insertion/extraction reaction in LiMn2O4 spinel. A uniform and dense film of LiMn2O4 was prepared by electrostatic spray deposition (ESD) onto an Au-coated quartz plate, which was used as an electrode for the EQCM experiments. The ESD method enabled us to perform the low-temperature spinel synthesis at 400 °C to avoid the thermal damage of the quartz. The electrochemical properties of the LiMn2O4 film in a 1 M LiClO4 carbonate solution were studied at 50 °C . The data obtained for the lithium insertion/extraction were discussed for the first time in connection with the mass change of the LiMn2O4 specimen.
Introduction The electrochemical quartz crystal microbalance (EQCM) technique has been established as a powerful tool for in situ monitoring of mass variation during electrochemical reactions.1,2 This technique is particularly attractive for lithium ion battery research because an insertion/extraction reaction of lithium ions in active materials is accompanied by mass changes.3-6 Park et al.3 studied lithium insertion into V2O5 xerogel film by means of the EQCM in addition to electrochemical methods; however, little or no data has been reported on the EQCM-based evaluation of spinel LiMn2O4, which is considered a promising cathode-active material.7-11 The EQCM technique allows direct measurement of the manganese dissolution in a LiPF6 electrolyte solution, which is believed to be responsible for the capacity fading at elevated temperatures.12-15 The preparation of LiMn2O4 films usually requires a high operating temperature around 800 °C,7-15 which is beyond the temperature of phase transition of quartz (573 °C).16 The thermal damage * To whom correspondence should be addressed. Fax: +81-22214-8646. E-mail:
[email protected]. (1) Buttry, D. A. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel-Dekker: New York, 1990; Vol. 17. (2) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (3) Park, H.-K.; Smyrl, W. H.; Ward, M. D. J. Electrochem. Soc. 1995, 142, 1068. (4) Chi, Q.; Tatsuma, T.; Ozaki, M.; Sotomura, T.; Oyama, N. J. Electrochem. Soc. 1998, 145, 2369. (5) Aurbach, D.; Moshkovich, M. J. Electrochem. Soc. 1998, 145, 2629. (6) Mori, M.; Naruoka, Y.; Naoi, K.; Fauteux, D. J. Electrochem. Soc. 1998, 145, 2340. (7) Bruce, P. G. Chem. Commun. 1997, 1817. (8) Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461. (9) Tarascon, J. M.; Guyomard, D. Electrochim. Acta 1993, 38, 1221. (10) Ohzuku, T.; Kitagawa, M.; Hirai, T. J. Electrochem. Soc. 1990, 137, 769. (11) Lithium Batteries, New Materials, Developments and Perspectives; Pistoia, G. Ed.; Elsevier: Amsterdam, 1994. (12) Tarascon, J. M.; Coowar, F.; Amatuci, G.; Shokoohi, F. K.; Guyomard, D. G. J. Power Sources 1995, 54, 103. (13) Gummow, R. J.; de Kock, A.; Thackerary, M. M. Solid State Ionics 1994, 69, 59. (14) Jang, D. H.; Oh, S. M. J. Electrochem. Soc. 1997, 144, 2593. (15) Jang, D. H.; Oh, S. M. Electrochim. Acta 1998, 43, 1023. (16) West, A. R. Solid State Chemistry and Its Applications; John Wiley & Sons: New York, 1984; p 431.
to the quartz during the sample preparation may be the reason for avoiding the use of the EQCM technique for the investigation of LiMn2O4. In the present work, we achieved the synthesis of a uniform film of spinel LiMn2O4 at 400 °C by adopting the electrostatic spray deposition (ESD)17,18 method to avoid the thermal damage of the quartz electrode. Consequently, we were able for the first time to perform the quantitative analysis of EQCM data for the lithium insertion/extraction reaction in LiMn2O4. Although more attention is focused on LiMn2O4 capacity fading in a LiPF6 electrolyte solution,12-15 we report here the results obtained in a stable electrolyte system with LiClO4 to demonstrate the applicability of the EQCM technique to study LiMn2O4 film. Experimental Section The ESD setup used in this work and its working principles are described elsewhere.17,18 A high d.c. voltage (typically, 12 kV) was applied between an electrically conductive substrate and a metal capillary nozzle, which is connected to a precursor solution. Under a preset flow rate, the precursor solution is atomized at the orifice of the nozzle, generating a spray. The spray moves toward the heated substrate because of the electrostatic force and, owing to the pyrolysis of precursors, a thin layer is formed on the substrate surface. As illustrated in Figure 1, an ethanol precursor solution of 25 mM LiNO3 + 50 mM Mn(NO3)2‚6H2O was pumped at 2 mL h-1 through a stainless steel nozzle (inner diameter, 0.8 mm), which was placed 2.5 cm above the electrode substrate, e.g., the shear mode 9 MHz ATcut quartz crystal Au electrode (SEIKO EG & G). The temperature of the electrode substrate was kept at 400 °C during the deposition. The amount of deposited material was calculated from the change in resonant frequency of the EQCM electrode before and after the deposition by using the Sauerbrey equation.1,2 The film morphology was studied with a scanning electron microscope (SEM) (JSM-5310LV). Electrochemical measurements were carried out by a commercially available EQCM system (SEIKO EG&G, QCA917) in a dry bag filled with Argon gas. The assembly was placed inside an incubator (SANYO, MIR162), and the temperature of the cell was controlled between 25 and 50 °C with an accuracy of (2 °C. (17) Chen, C.; Kelder, E. M.; Jak, M. J. G.; Schoonman, J. Solid State Ionics 1996, 86, 1301. (18) Nishizawa, M.; Uchiyama, T.; Dokko, K.; Yamada, K.; Matsue, T.; Uchida, I. Bull. Chem. Soc. Jpn. 1998, 71, 2011.
10.1021/la990270z CCC: $18.00 © 1999 American Chemical Society Published on Web 06/26/1999
4950 Langmuir, Vol. 15, No. 15, 1999
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Figure 1. Schematic drawing of the apparatus for the electrostatic spray deposition (ESD). The electrode substrates were partially masked to define the exposed area.
Figure 3. (a) Cyclic voltammogram of a LiMn2O4 ESD film at 0.1 mV s-1 (second cycle). (b) Variation of mass during the voltammetry (solid curve) and the theoretical variation (dashed curve) assuming one electron transfer per equivalent of lithium. (c) Mass accumulated per mole of electron transported (mpe) for every 20-mV anodic potential scan. Experiments were conducted in 1 M LiClO4/PC + EC at 50 °C.
Figure 2. SEM images showing cross sections of LiMn2O4 films prepared onto Pt/SiO2/Si substrates by (a) ESD at 400 °C and (b) ESD at 100 °C followed by annealing at 400 °C in air. The electrolyte solution was 1 M LiClO4/propylene carbonate (PC) + ethylene carbonate (EC) (1:1 in volume) (Li ion battery grade, Mitsubishi Kagaku Co.), with less than 20 ppm water content. The cyclic voltammetry and the constant-current chronopotentiometry (charge-discharge test) were conducted using Li foils as the reference and counter electrodes, respectively.
Results and Discussion Figure 2a shows SEM images of a LiMn2O4 film prepared in 30 min of deposition time onto a Pt-coated Si substrate maintained at 400 °C. The film seems to be uniform and dense, with a thickness of ca. 0.5 µm. In additional experiments, we prepared an ESD film at 100 °C, followed by annealing at 400 °C, in accordance with the general sol-gel method. In this case, the obtained film was significantly porous as shown in Figure 2b, probably due to gas evolution during the decomposition and crystallization of the precursor. These results show that the in situ decomposition of the precursor, which is characteristic of the ESD method, is crucial for preparing a uniform LiMn2O4 film. The formation of a compact film is required
in electrochemical characterizations of insertion materials to eliminate complications resulting from the porous structure. The XRD spectrum of the ESD film was well-assigned to spinel LiMn2O4,18 justifying the formation of the spinel phase at 400 °C. It is worthwhile to emphasize that the LiMn2O4 spinel is routinely synthesized at temperatures over 700 °C.7-15 The low-temperature synthesis is critical for the EQCM experiments because the R-β phase transition of quartz occurs at 573 °C.16 Although the phase transition is reversible, the heating at 700 °C affects the resonant properties of the EQCM electrodes in fact. Shokoohi et al.19 attempted to prepare a spinel LiMn2O4 film at low temperature by the reactive electron-beam evaporation. They reported that the single-phase polycrystalline LiMn2O4 spinel films can be fabricated at crystallization temperatures as low as 400 °C through the postdeposition annealing in an oxygen environment. However, the annealing after exposure to air requires temperatures over 800 °C, probably due to carbonates formed at the surface.19 In contrast, the ESD method employed in this work is an in situ crystallization process that allows the formation of the oxide films at lower temperatures. Figure 3a shows a cyclic voltammogram (CV) recorded at a 0.1 mV s-1 scan rate for the LiMn2O4 ESD film at 50 °C. The amount and thickness of the deposited LiMn2O4 were 15.7 µg and ca. 0.2 µm, respectively. Two couples of oxidation and reduction peaks appeared at ca. 4.0 and 4.15 V vs Li/Li+, which is in agreement with results reported by several researchers.7-11 The CV presented in Figure 3a was obtained during the second potential cycle. Figure 3b presents the mass variation (solid curve) measured simultaneously with the CV. The increase in the resonant frequency was converted to the mass decrease by the Sauerbrey equation.1,2 The profile of the variation of mass has two steps corresponding to the current peaks at the CV. The mass of the film decreased with the (19) Shokoohi, F. K.; Tarascon, J. M.; Wilkens, B. J.; Guyomard, D.; Chang, C. C. J. Electrochem. Soc. 1992, 139, 1845.
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oxidative lithium extraction from the LiMn2O4 and reverted to the initial value upon the reinsertion of the lithium ion, which demonstrates the reversibility of the insertion reaction. The dashed curve is for the calculated values from the CV, assuming one electron transfer per equivalent of Li. For the anodic potential scan in the 4.0-V region, calculated results agreed with the observed mass change in both the shape and amplitude. However, the calculated decrease in mass in the 4.15-V region was larger than the measured one, indicating that another electrochemical reaction took place in this potential range. Figure 3c represents the mass accumulated per mole of electron transported (mpe) for every 20-mV anodic potential scan. It is noted that the mpe in the 4-V region were around -7, the value expected for lithium extraction, while the side reactions in the higher potential region decreased the |mpe|. The oxidative decomposition of the solvent at the electrode surface could be the principal side reaction at the high potential. It is evident that the EQCM measurements can distinguish between the insertion/extraction reactions and the other electrochemical side reactions. Figure 4a shows the constant-current (100 µA cm-2) charge-discharge curve at 50 °C with 3.6- and 4.3-V cutoff voltages vs Li/Li+. The amount and thickness of the deposited LiMn2O4 were 15.6 µg and ca. 0.2 µm, respectively, and the potential plateaus at 4.0 and 4.15 V correspond to the CV peaks. As shown in Figure 4b, the mass response (solid curve) fits well to the theoretical straight line (dashed one), assuming one electron transfer per equivalent of Li. The maximum deviation was only 5% of the total mass change, indicating that the amount of electrochemical side reaction, such as the solvent oxidation, is negligible during the constant-current charge-discharge experiments, even at 50 °C. Galvanostatic experiments prevent unnecessary polarization; for example, the plot in Figure 4b shows that the electrode potential was higher than 4.2 V for less than 2 min during this polarization. We also investigated the chargedischarge cycle performance of the LiMn2O4 film electrode in 1 M LiClO4/PC + EC at 50 °C. The electrode maintained more than 90% of its capacity even after 100 cycles, and thus the loss of mass was too small to discuss quantitatively. This result demonstrates the reliability of the EQCM system as well as the stability of LiMn2O4 in a LiClO4 electrolyte solution.
Langmuir, Vol. 15, No. 15, 1999 4951
Figure 4. (a) Charge-discharge curve of a LiMn2O4 ESD film at 0.1 mA cm-2 (second cycle). (b) Variation of mass during the charge-discharge cycle (solid line) and the theoretical straight line (dashed one) assuming one electron transfer per equivalent of lithium. Experiments were conducted in 1 M LiClO4/PC + EC at 50 °C.
In conclusion, we studied in this work the applicability of the EQCM technique to the reaction of LiMn2O4 film. The low-temperature film preparation, which is necessary to achieve the EQCM measurement, was realized by adopting the ESD method. Vast research interest regarding the LiMn2O4 has recently been focused on its serious capacity fading in the presence of LiPF6 as an electrolyte. Therefore, the present focus of our work is to study the mechanism of capacity fading in the LiMn2O4/PF6- system by using the EQCM technique of which its applicability is reported herein. Acknowledgment. This work was partly supported by Grant-in-Aids for Scientific Research on Priority Area (No. 10131211 for “Electrochemistry of Ordered Interfaces”) from the Ministry of Education, Science, Sports and Culture, Japan, and by Lithium Battery Energy Storage Technology Research Association (LIBES), Japan. LA990270Z
