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Fabrication of LiCoO2 films for lithium rechargeable microbattery in an aqueous solution by electrochemical reflux method
Electrochimica Acta 50 (2004) 467–471
Fabrication of LiCoO2 films for lithium rechargeable microbattery in an aqueous solution by electrochemical reflux method Jin-Ho Leea , Kyoo-Seung Hana,∗ , Bum-Jae Leea,∗∗ , Seong-Il Seob , Masahiro Yoshimurac a
c
Department of Fine Chemicals Engineering and Chemistry, Center for Ultramicrochemical Process Systems (CUPS), Chungnam National University, 220 Kung-dong, Daeduck Science Town, Yuseong-gu, Taejeon 305-764, South Korea b College of Medicine, Catholic University of Korea, Seoul 137-701, South Korea Materials and Structures Laboratory, Center for Materials Design, Tokyo Institute of Technology, Yokohama 226, Japan Received 2 June 2003; received in revised form 2 April 2004; accepted 2 April 2004 Available online 6 October 2004
Abstract LiCoO2 films were directly deposited on electron-conducting substrates using electrochemical reflux method in an aqueous solution under ambient atmosphere at a fixed temperature between 100 and 200 ◦ C with a fixed current density between 0.1 and 1.0 mA cm−2 . The structural and compositional purities of the deposited LiCoO2 film were confirmed by elemental analyses, X-ray diffraction pattern analyses, and Raman spectroscopy. According to the Raman spectroscopy and the voltage versus capacity profiles for the deposited LiCoO2 film, it appears that the ¯ deposited film consists of layered LiCoO2 phase (space group R3m). Although the deposited LiCoO2 film was fabricated in a very economical and simple way, it exhibits an initial discharge capacity of 54.1 Ah/cm2 m and the discharge capacity retention of 85.6% over 15 cycles. © 2004 Elsevier Ltd. All rights reserved. Keywords: Electrochemical reflux method; LiCoO2 ; Lithium rechargeable battery; Thin film
1. Introduction Improvements in the miniaturization of electrical equipments give rise to a requirement to develop very small power sources. All-solid-state lithium rechargeable microbatteries seem to meet the needs due to their high energy density, fine power rates, longevity, low self-discharge, lightweight, etc. [1–5]. An all-solid-state lithium rechargeable microbattery basically comprises a cathode film, an electrolyte film, and an anode film. First, a cathode film is deposited on a current collector. In previous works, thin film of cathode materials such as LiCoO2 , LiNiO2 , LiMn2 O4 or V2 O5 has been prepared ∗
Corresponding author. Tel.: +82 42 822 6637; fax: +82 42 821 6297. E-mail addresses: [email protected] (K.-S. Han), [email protected] (B.-J. Lee). ∗∗ Co-corresponding author. 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.04.049
primarily using gas phase and/or vacuum systems such as sputtering technique and laser ablation method [3–12]. Then, an electrolyte film is sputter-deposited on the cathode film. Recently, LIPON film is being extensively used as an electrolyte film [3–5,12,13]. Finally, Li metal film is thermally evaporated [1–5,12,13]. Here, we should note that the major difficulty in fabricating microbatteries is in the preparation of a cathode film. It is due to difficulties to make gas phase from solid-state lithiated transition metal oxide. Accordingly, such synthetic procedures unfortunately require huge energy and material consumption as well as sophisticated instrumentation. Therefore, the development of an alternative synthetic route in an economical and environmentally friendly way has been extensively investigated using various techniques, such as spin coating, deep coating, electrostatic spray deposition, soft solution processing, etc. [14–22]. Recently, we found that an aqueous solution reaction combined with electrochemical and hydrothermal reactions is
468
J.-H. Lee et al. / Electrochimica Acta 50 (2004) 467–471
capable of fabricating LiCoO2 film. In addition, we have proposed the reaction pathway to obtain LiCoO2 film as [18,20–22] Co + 3OH− → HCoO2 – + H2 O + 2e– (oxidative dissolution)
(1)
HCoO2 – + OH– → CoO2 – + H2 O + e– (presence of excess OH– at T
High )
(2)
CoO2 – + Li+ → LiCoO2 (precipitation)
(3)
According to the given reaction pathway, hydroxyl groups play a key role in obtaining LiCoO2 film. Because excess hydroxyl groups can be supplied by the electrolysis of water (cathode: 2H2 O + 2e− → 2OH− + H2 ), the electrochemical reaction should be combined with the aqueous solution reaction [22]. However, we wonder whether the hydrothermal reaction should be combined with the aqueous solution reaction. The hydrothermal procedure requires the usage of a closed batch system such as autoclave. As shown in Eq. (2), although the reaction temperature is an obvious reaction factor, the reaction pressure seems not to do. Therefore, if the desired reaction temperature can be achieved without the usage of autoclave, LiCoO2 film can be obtained in a more economical way. In this paper, we present the results of an attempt to implement the electrochemical reflux method to fabricate LiCoO2 film in an aqueous solution and in an open system.
2. Experimental
Fig. 1. Schematization of the experimental conditions and the apparatus for electrochemical reflux method. (a) Condenser; (b) a fixed galvanostatic current between 0.1 and 1.0 mA cm−2 ; (c) thermocouple; (d) oil-bath for iso-temperature; (e) immersion heater; (f) immersion circulator; (g) cobalt metal powder; (h) electron-conducting substrate as a cathode; (i) electronconducting substrate as an anode; (j) a mixture of LiOH and KOH solutions; and (k) polytetrafluoroethylene vessel.
synthetic schematization of the experimental conditions and the apparatus is shown in Fig. 1. The freshly obtained films were washed several times with doubly distilled water to eliminate residual LiOH and KOH solution, and then dried at 80 ◦ C for 2 h.
2.1. Materials 2.2. Film characterization LiCoO2 films were directly deposited on electronconducting substrates such as platinum or stainless steel plates in a mixture of 1 M LiOH and 12 M KOH solutions using electrochemical reflux method. Cobalt metal powders (99.99% Kanto Chemical Co., Tokyo, Japan) as a Co source were immersed in the solution, and located on the bottom of the reaction vessel and at a distance of 20 cm from the electron-conducting substrates. Due to the high pH of the solution, a laboratory made polytetrafluoroethylene (PTFE) vessel was used as a reaction vessel. For the electrochemical reflux reaction, a cooling condenser was installed on the top of the PTFE vessel. The temperature in the reaction vessel was regulated using an external heating system, a Chromel–Alumel thermocouple and an automatic controller. The electrochemical reflux reaction was carried out at a fixed temperature between 100 and 200 ◦ C. The heating processed to the fixed temperature with an approximate heating rate of 3.0 ◦ C/min and the subsequent isothermal process followed by the cooling process. During the electrochemical reflux reaction, the working electrode was galvanostatically charged at a fixed current density between 0.1 and 1.0 mA cm−2 . The
The scanning electron microscope (SEM) image of the film inclined at 30◦ was obtained using an Hitachi SEM S-4500. The X-ray diffraction (XRD) pattern analysis of the obtained films was achieved by using a Mac Science M03XHF22 diffractometer and Cu K␣ radiation (λ ˚ operated with 30 mA and 40 kV. The diffrac= 1.5405 A) tograms were recorded in the 2θ range of 5–90◦ with 0.02◦ intervals and scanning rate of 2◦ /min. To determine the contents of lithium and cobalt in the obtained films, elemental analysis was carried out by ICP-AES (Jobin Yvon, JY38S). Room temperature Raman measurements were performed using a Jobin Yvon/Atago Bussan T64000 triple spectrometer with a liquid nitrogen cooled CCD detector for 300–600 s. The laser beam (λ = 514.5 nm) was focused to a ∼3 m diameter spot by a 90× microscope objective. The spectral resolution was 2–3 cm−1 . To make an all-solid-state electrochemical cell, the solid electrolyte, LIPON, was deposited on the prepared film by RF magnetron sputtering of an Li3 PO4 target in a nitrogen atmosphere. Then, the Li metal film was thermally vapor-deposited rather then sputtered over the electrolyte in
J.-H. Lee et al. / Electrochimica Acta 50 (2004) 467–471
469
Fig. 2. SEM image of the LiCoO2 film inclined at 30◦ and deposited on Pt substrate at 150 ◦ C with the current density of 0.3 mA cm−2 for 8 h in 1 M LiOH and 12 M KOH solution.
an argon-filled glove box. The fabricated microbattery cell was cycled at a current density of ±10 A cm−2 between 4.3 and 2.7 V in an argon-filled glove box at room temperature.
3. Results and discussion The formation of a dark-gray film on the electrochemical reflux reaction was visually detected only on the cathode substrate. Although the anode and cathode substrates were located at the same intervals from the cobalt metal powders, no film was deposited on the anode substrate. This demonstrates the key role of excess hydroxyl groups in obtaining LiCoO2 film and the possibility of a continuous fabrication of LiCoO2 film in an open system. The deposited films were subsequently analyzed with respect to their crystallinity, chemical composition, structure, and electrochemical activity. The SEM image in Fig. 2 simultaneously shows the morphologies of the surface and cross section of the deposited film. This image directly demonstrates the formation of the homogeneous film with film thickness of about 12 m without discontinuity and peeling of the film. A very important evidence for the chemical composition of the deposited film can be obtained by the ICP-AES results. The Li/Co molar ratio in the film, calculated from the ICPAES results, was close to 1.0, which is in accordance with expected stoichiometry. The XRD patterns of the LiCoO2 reference bulk phase, the deposited LiCoO2 film, and the platinum substrate are shown in Fig. 3. Almost all XRD peaks characteristic of the space
Fig. 3. X-ray diffraction patterns for (a) the layered LiCoO2 reference powder, (b) the LiCoO2 film deposited on Pt substrate at 150 ◦ C with the current density of 0.3 mA cm−2 for 8 h in 1 M LiOH and 12 M KOH solution, and (c) Pt substrate.
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Fig. 5. Voltage profile at ±10 A cm−2 for the LiCoO2 film deposited on Pt substrate at 150 ◦ C with the current density of 0.3 mA cm−2 for 8 h in 1 M LiOH and 12 M KOH solution.
Fig. 4. Raman spectra excited at 514.5 nm for (a) Co(OH)2 , (b) the spinel Li1−x CoO2 reference powder, (c) the layered LiCoO2 reference powder, (d) the prepared LiCoO2 film, (e) the layered LiCoO2 phase prepared at 150 ◦ C for 24 h in 4 M LiOH solution by hydrothermal reaction [20,22], and (f) the spinel Li1−x CoO2 phase prepared at 50 ◦ C for 24 h in 4 M LiOH solution by hydrothermal reaction [20,22].
¯ as shown in the LiCoO2 reference bulk phase can group R3m be found in the X-ray diffractogram of the deposited LiCoO2 film. In addition, no XRD peaks of possible impurities, such as Co(OH)2 , CoOOH, and Co3 O4 phases, are detected. The Raman spectra for the Co(OH)2 reference bulk phase, the spinel Li1−x CoO2 reference bulk phase, the layered LiCoO2 reference bulk phase, the deposited LiCoO2 film, the layered LiCoO2 phase prepared at 150 ◦ C for 24 h in 4 M LiOH solution by hydrothermal reaction, and the spinel Li1−x CoO2 phase prepared at 50 ◦ C for 24 h in 4 M LiOH solution by hydrothermal reaction are compared in Fig. 4 [20,22]. As shown in Fig. 4, the different crystal structures results in different vibration modes, i.e., two vibration modes (A1g , Eg ) in the case of layered LiCoO2 phase (space group ¯ and four vibration modes (A1g , E2g , 2F1g ) in the case R3m) of spinel Li1−x CoO2 phase (space group Fd3m) [23,24]. Because the XRD patterns of layered LiCoO2 and spinel Li1−x CoO2 phases are unfortunately quite similar, the Raman spectroscopy can be a conclusive evidence for the structural characterization of lithiated cobalt oxide phases. As shown in Fig. 4(c)–(e), it appears that the deposited film consists of the layered LiCoO2 phase. Because the layered LiCoO2 phase exhibits much better electrochemical activity for a lithium rechargeable battery than the spinel Li1−x CoO2 phase, the deposited film is expected to show a good battery performance. Fig. 5 shows the voltage versus capacity profiles for the deposited LiCoO2 film. Open-circuit voltage of freshly pre-
pared cell is 3.2 V, which is similar to that of layered LiCoO2 phase. In addition, both charge and discharge curves show a potential plateau at 3.8–3.9 V, that is one of the typical properties for the layered LiCoO2 phase [8,25–27]. Thus, these two points are consistent with the results of Raman spectroscopy. The evolution of the discharge capacity versus cycle number evaluated from the voltage versus capacity profiles is shown in Fig. 6. The deposited LiCoO2 film exhibits an initial discharge capacity of 54.1 Ah/cm2 m and the discharge capacity retention of 85.6% over 15 cycles. Considering the theoretically feasible discharge capacity of 68.9 Ah/cm2 m for Lix CoO2 (0.5 ≤ x ≤ 1.0) phase, the discharge capacity of 54.1 Ah/cm2 m for the deposited LiCoO2 film should be ascribed to smaller density of the
Fig. 6. Volumetric discharge capacity vs. cycle number for the LiCoO2 film deposited on Pt substrate at 150 ◦ C with the current density of 0.3 mA cm−2 for 8 h in 1 M LiOH and 12 M KOH solution.
J.-H. Lee et al. / Electrochimica Acta 50 (2004) 467–471
deposited film than the theoretical density of 5.06 g/cm3 for LiCoO2 phase. Although the present electrochemical activity of the LiCoO2 film deposited by electrochemical reflux method is inferior than that of the sputter-deposited LiCoO2 film, we expect that advances in the optimization of reaction conditions may make a better battery performance.
4. Conclusion Well crystallized and electrochemically active LiCoO2 films with constant film thickness without discontinuity and peeling of the film are deposited using electrochemical reflux method. Despite of the simple and economical fabrication, the measured film properties show the deposited films to be prospective as a cathode film for lithium rechargeable microbatteries. Considering the facility in fabricating, upsizing and mass-producing of LiCoO2 films using electrochemical reflux method, this method should be expected to serve as an economical alternative synthetic route for the fabrication of lithiated cathode films.
Acknowledgement This research was funded by Korea Research Foundation Grant (KRF-2001-005-E00033).
References [1] S.D. Jones, J.R. Akridge, F.K. Shokoohi, Solid State Ionics 69 (1994) 357. [2] S.D. Jones, J.R. Akridge, Solid State Ionics 53–56 (1992) 628. [3] N.J. Dudney, J.B. Bates, R.A. Zuhr, S. Young, J.D. Robertson, H.P. Jun, S.A. Hackney, J. Electrochem. Soc. 146 (1999) 2455. [4] J.B. Bates, G.R. Gruzalski, N.J. Dudney, C.F. Luck, X. Yu, Solid State Ionics 70/71 (1994) 619.
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[5] Y.S. Park, S.H. Lee, B.I. Lee, S.K. Joo, Electrochem. Solid State Lett. 2 (1999) 58. [6] P. Liu, J.G. Zhang, J.A. Turner, C.E. Tracy, D.K. Benson, J. Electrochem. Soc. 146 (1999) 2001. [7] K.A. Striebel, A. Rougier, C.R. Horne, R.P. Reade, E.J. Cairns, J. Electrochem. Soc. 146 (1999) 4339. [8] K.A. Striebel, C.Z. Deng, S.J. Wen, E.J. Cairns, J. Electrochem. Soc. 143 (1996) 1821. [9] F.K. Shokoohi, J.M. Tarascon, B.J. Wilkens, D. Guyomard, C.C. Chang, J. Electrochem. Soc. 139 (1992) 1845. [10] M. Antaya, K. Cearns, J.S. Preston, J.N. Reimers, J.R. Dahn, J. Appl. Phys. 76 (1994) 2799. [11] Y. Iriyama, M. Inaba, T. Abe, Z. Ogumi, J. Power Sources 94 (2001) 175. [12] J.B. Bates, N.J. Dudney, B.J. Neudecker, F.X. Hart, H.P. Jun, S.A. Hackney, J. Electrochem. Soc. 147 (2000) 59. [13] B.J. Neudecker, R.A. Zuhr, J.D. Robertson, J.B. Bates, J. Electrochem. Soc. 145 (1998) 4160. [14] C.H. Chen, E.M. Kelder, M.J.G. Jak, J. Schoonman, Solid State Ionics 86–88 (1996) 1301. [15] Y.J. Park, J.G. Kim, M.K. Kim, H.T. Chung, H.G. Kim, Solid State Ionics 130 (2000) 203. [16] Y.J. Park, K.S. Park, J.G. Kim, M.K. Kim, H.G. Kim, H.T. Chung, J. Power Sources 88 (2000) 250. [17] P. Fragnaud, T. Brousse, D.M. Schleich, J. Power Sources 63 (1996) 187. [18] K.S. Han, S.W. Song, M. Yoshimura, Chem. Mater. 10 (1998) 2183. [19] K.S. Han, S.W. Song, T. Watanabe, M. Yoshimura, Electrochem, Solid State Lett. 2 (1999) 63. [20] S.W. Song, K.S. Han, M. Yoshimura, J. Am. Ceram. Soc. 83 (2000) 2839. [21] K.S. Han, S.W. Song, H. Fujita, M. Yoshimura, Solid State Ionics 135 (2000) 273. [22] K.S. Han, S.W. Song, H. Fujita, M. Yoshimura, E.J. Cairns, S.H. Chang, J. Am. Ceram. Soc. 85 (2002) 2444. [23] W.G. Fately, Infrared and Raman Selection Rules for Molecular and Lattice Vibrations: The Correlation Method, Wiley-Interscience, New York, 1972. [24] M. Inaba, Y. Iriyama, Z. Ogumi, Y. Todzuka, A. Tasaka, J. Raman Spectrosc. 28 (1997) 613. [25] J.M. Paulsen, J.R. Mueller-Neuhaus, J.R. Dahn, J. Electrochem. Soc. 147 (2000) 508. [26] T. Ohzuku, A. Ueda, J. Electrochem. Soc. 141 (1994) 2972. [27] G.G. Amatucci, J.M. Tarascon, L.C. Klein, J. Electrochem. Soc. 143 (1996) 1114.
Fabrication of LiCoO2 films for lithium rechargeable microbattery in an aqueous solution by electrochemical reflux method Jin-Ho Leea , Kyoo-Seung Hana,∗ , Bum-Jae Leea,∗∗ , Seong-Il Seob , Masahiro Yoshimurac a
c
Department of Fine Chemicals Engineering and Chemistry, Center for Ultramicrochemical Process Systems (CUPS), Chungnam National University, 220 Kung-dong, Daeduck Science Town, Yuseong-gu, Taejeon 305-764, South Korea b College of Medicine, Catholic University of Korea, Seoul 137-701, South Korea Materials and Structures Laboratory, Center for Materials Design, Tokyo Institute of Technology, Yokohama 226, Japan Received 2 June 2003; received in revised form 2 April 2004; accepted 2 April 2004 Available online 6 October 2004
Abstract LiCoO2 films were directly deposited on electron-conducting substrates using electrochemical reflux method in an aqueous solution under ambient atmosphere at a fixed temperature between 100 and 200 ◦ C with a fixed current density between 0.1 and 1.0 mA cm−2 . The structural and compositional purities of the deposited LiCoO2 film were confirmed by elemental analyses, X-ray diffraction pattern analyses, and Raman spectroscopy. According to the Raman spectroscopy and the voltage versus capacity profiles for the deposited LiCoO2 film, it appears that the ¯ deposited film consists of layered LiCoO2 phase (space group R3m). Although the deposited LiCoO2 film was fabricated in a very economical and simple way, it exhibits an initial discharge capacity of 54.1 Ah/cm2 m and the discharge capacity retention of 85.6% over 15 cycles. © 2004 Elsevier Ltd. All rights reserved. Keywords: Electrochemical reflux method; LiCoO2 ; Lithium rechargeable battery; Thin film
1. Introduction Improvements in the miniaturization of electrical equipments give rise to a requirement to develop very small power sources. All-solid-state lithium rechargeable microbatteries seem to meet the needs due to their high energy density, fine power rates, longevity, low self-discharge, lightweight, etc. [1–5]. An all-solid-state lithium rechargeable microbattery basically comprises a cathode film, an electrolyte film, and an anode film. First, a cathode film is deposited on a current collector. In previous works, thin film of cathode materials such as LiCoO2 , LiNiO2 , LiMn2 O4 or V2 O5 has been prepared ∗
Corresponding author. Tel.: +82 42 822 6637; fax: +82 42 821 6297. E-mail addresses: [email protected] (K.-S. Han), [email protected] (B.-J. Lee). ∗∗ Co-corresponding author. 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.04.049
primarily using gas phase and/or vacuum systems such as sputtering technique and laser ablation method [3–12]. Then, an electrolyte film is sputter-deposited on the cathode film. Recently, LIPON film is being extensively used as an electrolyte film [3–5,12,13]. Finally, Li metal film is thermally evaporated [1–5,12,13]. Here, we should note that the major difficulty in fabricating microbatteries is in the preparation of a cathode film. It is due to difficulties to make gas phase from solid-state lithiated transition metal oxide. Accordingly, such synthetic procedures unfortunately require huge energy and material consumption as well as sophisticated instrumentation. Therefore, the development of an alternative synthetic route in an economical and environmentally friendly way has been extensively investigated using various techniques, such as spin coating, deep coating, electrostatic spray deposition, soft solution processing, etc. [14–22]. Recently, we found that an aqueous solution reaction combined with electrochemical and hydrothermal reactions is
468
J.-H. Lee et al. / Electrochimica Acta 50 (2004) 467–471
capable of fabricating LiCoO2 film. In addition, we have proposed the reaction pathway to obtain LiCoO2 film as [18,20–22] Co + 3OH− → HCoO2 – + H2 O + 2e– (oxidative dissolution)
(1)
HCoO2 – + OH– → CoO2 – + H2 O + e– (presence of excess OH– at T
High )
(2)
CoO2 – + Li+ → LiCoO2 (precipitation)
(3)
According to the given reaction pathway, hydroxyl groups play a key role in obtaining LiCoO2 film. Because excess hydroxyl groups can be supplied by the electrolysis of water (cathode: 2H2 O + 2e− → 2OH− + H2 ), the electrochemical reaction should be combined with the aqueous solution reaction [22]. However, we wonder whether the hydrothermal reaction should be combined with the aqueous solution reaction. The hydrothermal procedure requires the usage of a closed batch system such as autoclave. As shown in Eq. (2), although the reaction temperature is an obvious reaction factor, the reaction pressure seems not to do. Therefore, if the desired reaction temperature can be achieved without the usage of autoclave, LiCoO2 film can be obtained in a more economical way. In this paper, we present the results of an attempt to implement the electrochemical reflux method to fabricate LiCoO2 film in an aqueous solution and in an open system.
2. Experimental
Fig. 1. Schematization of the experimental conditions and the apparatus for electrochemical reflux method. (a) Condenser; (b) a fixed galvanostatic current between 0.1 and 1.0 mA cm−2 ; (c) thermocouple; (d) oil-bath for iso-temperature; (e) immersion heater; (f) immersion circulator; (g) cobalt metal powder; (h) electron-conducting substrate as a cathode; (i) electronconducting substrate as an anode; (j) a mixture of LiOH and KOH solutions; and (k) polytetrafluoroethylene vessel.
synthetic schematization of the experimental conditions and the apparatus is shown in Fig. 1. The freshly obtained films were washed several times with doubly distilled water to eliminate residual LiOH and KOH solution, and then dried at 80 ◦ C for 2 h.
2.1. Materials 2.2. Film characterization LiCoO2 films were directly deposited on electronconducting substrates such as platinum or stainless steel plates in a mixture of 1 M LiOH and 12 M KOH solutions using electrochemical reflux method. Cobalt metal powders (99.99% Kanto Chemical Co., Tokyo, Japan) as a Co source were immersed in the solution, and located on the bottom of the reaction vessel and at a distance of 20 cm from the electron-conducting substrates. Due to the high pH of the solution, a laboratory made polytetrafluoroethylene (PTFE) vessel was used as a reaction vessel. For the electrochemical reflux reaction, a cooling condenser was installed on the top of the PTFE vessel. The temperature in the reaction vessel was regulated using an external heating system, a Chromel–Alumel thermocouple and an automatic controller. The electrochemical reflux reaction was carried out at a fixed temperature between 100 and 200 ◦ C. The heating processed to the fixed temperature with an approximate heating rate of 3.0 ◦ C/min and the subsequent isothermal process followed by the cooling process. During the electrochemical reflux reaction, the working electrode was galvanostatically charged at a fixed current density between 0.1 and 1.0 mA cm−2 . The
The scanning electron microscope (SEM) image of the film inclined at 30◦ was obtained using an Hitachi SEM S-4500. The X-ray diffraction (XRD) pattern analysis of the obtained films was achieved by using a Mac Science M03XHF22 diffractometer and Cu K␣ radiation (λ ˚ operated with 30 mA and 40 kV. The diffrac= 1.5405 A) tograms were recorded in the 2θ range of 5–90◦ with 0.02◦ intervals and scanning rate of 2◦ /min. To determine the contents of lithium and cobalt in the obtained films, elemental analysis was carried out by ICP-AES (Jobin Yvon, JY38S). Room temperature Raman measurements were performed using a Jobin Yvon/Atago Bussan T64000 triple spectrometer with a liquid nitrogen cooled CCD detector for 300–600 s. The laser beam (λ = 514.5 nm) was focused to a ∼3 m diameter spot by a 90× microscope objective. The spectral resolution was 2–3 cm−1 . To make an all-solid-state electrochemical cell, the solid electrolyte, LIPON, was deposited on the prepared film by RF magnetron sputtering of an Li3 PO4 target in a nitrogen atmosphere. Then, the Li metal film was thermally vapor-deposited rather then sputtered over the electrolyte in
J.-H. Lee et al. / Electrochimica Acta 50 (2004) 467–471
469
Fig. 2. SEM image of the LiCoO2 film inclined at 30◦ and deposited on Pt substrate at 150 ◦ C with the current density of 0.3 mA cm−2 for 8 h in 1 M LiOH and 12 M KOH solution.
an argon-filled glove box. The fabricated microbattery cell was cycled at a current density of ±10 A cm−2 between 4.3 and 2.7 V in an argon-filled glove box at room temperature.
3. Results and discussion The formation of a dark-gray film on the electrochemical reflux reaction was visually detected only on the cathode substrate. Although the anode and cathode substrates were located at the same intervals from the cobalt metal powders, no film was deposited on the anode substrate. This demonstrates the key role of excess hydroxyl groups in obtaining LiCoO2 film and the possibility of a continuous fabrication of LiCoO2 film in an open system. The deposited films were subsequently analyzed with respect to their crystallinity, chemical composition, structure, and electrochemical activity. The SEM image in Fig. 2 simultaneously shows the morphologies of the surface and cross section of the deposited film. This image directly demonstrates the formation of the homogeneous film with film thickness of about 12 m without discontinuity and peeling of the film. A very important evidence for the chemical composition of the deposited film can be obtained by the ICP-AES results. The Li/Co molar ratio in the film, calculated from the ICPAES results, was close to 1.0, which is in accordance with expected stoichiometry. The XRD patterns of the LiCoO2 reference bulk phase, the deposited LiCoO2 film, and the platinum substrate are shown in Fig. 3. Almost all XRD peaks characteristic of the space
Fig. 3. X-ray diffraction patterns for (a) the layered LiCoO2 reference powder, (b) the LiCoO2 film deposited on Pt substrate at 150 ◦ C with the current density of 0.3 mA cm−2 for 8 h in 1 M LiOH and 12 M KOH solution, and (c) Pt substrate.
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Fig. 5. Voltage profile at ±10 A cm−2 for the LiCoO2 film deposited on Pt substrate at 150 ◦ C with the current density of 0.3 mA cm−2 for 8 h in 1 M LiOH and 12 M KOH solution.
Fig. 4. Raman spectra excited at 514.5 nm for (a) Co(OH)2 , (b) the spinel Li1−x CoO2 reference powder, (c) the layered LiCoO2 reference powder, (d) the prepared LiCoO2 film, (e) the layered LiCoO2 phase prepared at 150 ◦ C for 24 h in 4 M LiOH solution by hydrothermal reaction [20,22], and (f) the spinel Li1−x CoO2 phase prepared at 50 ◦ C for 24 h in 4 M LiOH solution by hydrothermal reaction [20,22].
¯ as shown in the LiCoO2 reference bulk phase can group R3m be found in the X-ray diffractogram of the deposited LiCoO2 film. In addition, no XRD peaks of possible impurities, such as Co(OH)2 , CoOOH, and Co3 O4 phases, are detected. The Raman spectra for the Co(OH)2 reference bulk phase, the spinel Li1−x CoO2 reference bulk phase, the layered LiCoO2 reference bulk phase, the deposited LiCoO2 film, the layered LiCoO2 phase prepared at 150 ◦ C for 24 h in 4 M LiOH solution by hydrothermal reaction, and the spinel Li1−x CoO2 phase prepared at 50 ◦ C for 24 h in 4 M LiOH solution by hydrothermal reaction are compared in Fig. 4 [20,22]. As shown in Fig. 4, the different crystal structures results in different vibration modes, i.e., two vibration modes (A1g , Eg ) in the case of layered LiCoO2 phase (space group ¯ and four vibration modes (A1g , E2g , 2F1g ) in the case R3m) of spinel Li1−x CoO2 phase (space group Fd3m) [23,24]. Because the XRD patterns of layered LiCoO2 and spinel Li1−x CoO2 phases are unfortunately quite similar, the Raman spectroscopy can be a conclusive evidence for the structural characterization of lithiated cobalt oxide phases. As shown in Fig. 4(c)–(e), it appears that the deposited film consists of the layered LiCoO2 phase. Because the layered LiCoO2 phase exhibits much better electrochemical activity for a lithium rechargeable battery than the spinel Li1−x CoO2 phase, the deposited film is expected to show a good battery performance. Fig. 5 shows the voltage versus capacity profiles for the deposited LiCoO2 film. Open-circuit voltage of freshly pre-
pared cell is 3.2 V, which is similar to that of layered LiCoO2 phase. In addition, both charge and discharge curves show a potential plateau at 3.8–3.9 V, that is one of the typical properties for the layered LiCoO2 phase [8,25–27]. Thus, these two points are consistent with the results of Raman spectroscopy. The evolution of the discharge capacity versus cycle number evaluated from the voltage versus capacity profiles is shown in Fig. 6. The deposited LiCoO2 film exhibits an initial discharge capacity of 54.1 Ah/cm2 m and the discharge capacity retention of 85.6% over 15 cycles. Considering the theoretically feasible discharge capacity of 68.9 Ah/cm2 m for Lix CoO2 (0.5 ≤ x ≤ 1.0) phase, the discharge capacity of 54.1 Ah/cm2 m for the deposited LiCoO2 film should be ascribed to smaller density of the
Fig. 6. Volumetric discharge capacity vs. cycle number for the LiCoO2 film deposited on Pt substrate at 150 ◦ C with the current density of 0.3 mA cm−2 for 8 h in 1 M LiOH and 12 M KOH solution.
J.-H. Lee et al. / Electrochimica Acta 50 (2004) 467–471
deposited film than the theoretical density of 5.06 g/cm3 for LiCoO2 phase. Although the present electrochemical activity of the LiCoO2 film deposited by electrochemical reflux method is inferior than that of the sputter-deposited LiCoO2 film, we expect that advances in the optimization of reaction conditions may make a better battery performance.
4. Conclusion Well crystallized and electrochemically active LiCoO2 films with constant film thickness without discontinuity and peeling of the film are deposited using electrochemical reflux method. Despite of the simple and economical fabrication, the measured film properties show the deposited films to be prospective as a cathode film for lithium rechargeable microbatteries. Considering the facility in fabricating, upsizing and mass-producing of LiCoO2 films using electrochemical reflux method, this method should be expected to serve as an economical alternative synthetic route for the fabrication of lithiated cathode films.
Acknowledgement This research was funded by Korea Research Foundation Grant (KRF-2001-005-E00033).
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