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Electrochemical reaction of nanocrystalline Co3O4 thin film with Lithium
Solid State Ionics 170 (2004) 105 – 109 www.elsevier.com/locate/ssi
Electrochemical reaction of nanocrystalline Co3O4 thin film with Lithium Zheng-Wen Fu, Ying Wang, Ye Zhang, Qi-Zong Qin * Department of Chemistry, Laser Chemistry Institute, Fudan University, 220 Handan Road, Shanghai 200433, China Received 4 October 2003; received in revised form 16 February 2004; accepted 24 February 2004
Abstract The electrochemical reaction of lithium and nanocrystalline Co3O4 thin films fabricated by reactive pulsed laser deposition was investigated using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and galvanostatic measurements. Our new finding is that during the first discharge process of nanocrystalline Co3O4 film-based lithium cell Co3O4 thin film reacts with lithium and forms nano-sized metallic Co and Li2O, but after charging to 3.0 V CoO instead of Co3O4 is generated and the subsequent discharge/charge processes are reversible. A reaction mechanism of nanocrystalline Co3O4 thin film with lithium is proposed. D 2004 Elsevier B.V. All rights reserved. PACS: 80.45; 81.15.F Keywords: Nanocrystalline thin film; Co3O4 thin film; Pulsed laser deposition; Electrochemical reaction of Co3O4 thin film with lithium
1. Introduction Nano-sized transition metal oxides have been widely studied in recent years in search of new anode materials for Li-ion batteries. Tarascon et al. [1,2] investigated a variety of transition metal oxides MxOy such as FeO, CoO, NiO, CuO, Cu2O, or Co3O4 as anode materials and proposed a new mechanism of Li reactivity in such materials different from the classical Li insertion/deinsertion or Li-alloying process, involving the reversible formation and decomposition of Li2O, and accompanying in situ nanoparticles formation and their reduction and oxidation. Obrovac et al. [3] has investigated CoO-based electrode and the electrochemical reaction of a-CoO with lithium ions using in situ X-ray diffraction and in situ Mo¨ssbauer measurement. They suggested a similar reaction mechanism including the immediate decomposition of metal oxide to form nano-sized metal particles, but the reduced metal was oxidized to h-CoO by a displacement reaction where the metal ions displaced the Li ion in Li2O via an ion exchange process. The reactivity of Co3O4 was recently reinvestigated [4 –7]. Larcher et al. [4] found that two competing mechanisms to reach the full decomposition of Co3O4 electrode into a composite * Corresponding author. Tel./fax: +86-21-651-0277. E-mail address: [email protected] (Q.-Z. Qin). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.020
consisting of Co nanograins embedded in a Li2O existed and the intermediates were LixCo3O4 and a-CoO, depending on the texture of the starting material, and both of the intermediates decomposed into Co nanograins and Li2O on further reduction. It is well known that thin films deposited on electronically conductive substrates are more suitable for fundamental electrochemical studies of transition metal oxides due to avoiding the interference from additives in polymer-bonded electrodes. Thin films of cobalt oxides prepared by various methods such as electron-beam evaporation [8], electrochemical-deposition [9], chemical vapor deposition [10], the sol –gel route [11] have been reported. Recently we have reported the investigation on the electrochemical performance of nanocrystalline Co3O4 thin film fabricated by pulsed laser deposition (PLD) [12]. PLD is a simple method to deposit thin film electrodes and has demonstrated its potential in preparing nanostructured thin films [13,14]. In this paper, we report the electrochemical reaction of lithium with nanocrystalline Co3O4 thin films prepared by pulsed laser deposition. The structure, composition and morphology of as-deposited, lithiated and delithiated thin films of cobalt oxides are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photo-electron spectroscopy (XPS). An electrochemical reaction mechanism of nanocrystalline Co3O4 thin film with lithium is proposed.
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2. Experimental A pulsed laser deposition (PLD) system for preparing thin films of cobalt oxides has been described elsewhere [15]. A 355 nm laser beam with a repetition rate of 10 Hz and a pulse width of 6 ns was provided by the third harmonic frequency of a Q-switched Nd:YAG laser (Spectra Physics GCR-150) and directed through a quartz window onto a rotating target. The films were deposited on a stainless steel substrate heated to a temperature of 600 jC at a laser fluence of 2 J/cm2, and postannealed at 300 jC for 2 h in an oxygen environment. Metal Co target was prepared by pressing Co powder (99.99%) to form a 1.3 cm diameter pellet. The O2 ambient gas pressure was maintained at 40 Pa during the reactive pulsed laser deposition. The deposition rate of the films under above conditions was estimated to be 4 nm/min. X-ray diffraction (XRD) patterns of the composite film were determined by using a Rigata/max-C diffractometer ˚ ). The film with Cu-Ka radiation source (k = 1.5406 A thickness was measured by scanning electron microscopy (SEM) (Philips XL 30) and a profilometry (Tencor AlphaStep 200). XPS measurements were performed on a Perkin Elmer PHI 6000C ECSA system with monochromatic Al Ka (1486.6 eV) irradiation. To correct possible charge-up of the films by X-ray irradiation, the binding energy was calibrated using C1s (284.6 eV) spectrum of hydrocarbon that remained in the XPS analysis chamber as a contaminant. The electrochemical performance of the thin film electrodes of cobalt oxides was measured using the same method as described in Ref. [12]. The lithiated films were handled in an Ar-filled glove box in order to avoid exposure to oxygen and water, and were rapidly transferred to the XPS and SEM measurement chambers. XPS spectra of the samples with and without the bombardment of the surface by Ar ion sputtering for 15 min were identical, indicating that the sample surfaces were kept clean under our experimental conditions.
3. Results In order to identify the chemical composition, crystallinity and valence state of as-deposited films prepared by reactive PLD in O2 ambient using Co as a target, we measured the XRD and XPS spectra of as-deposited thin films of cobalt oxide. Fig. 1 shows the typical XRD patterns of the as-deposited film and the stainless steel substrate. The diffraction peaks shown in Fig. 1(b) at 2h = 19.0j, 31.3j, 36.6j, 59.2j and 65.1j can be attributed to the spinel type cubic structure of Co3O4 with Fd3m space group [16,17]. The other two diffraction peaks appearing at 2h = 43.3j and 74.7j come from the stainless steel substrate (see Fig. 1(a)). The crystallite size of Co3O4 calculated by the Scherrer’s formula is estimated to be about 50nm, indicating the formation of nanostructured polycrystalline Co3O4 thin film.
Fig. 1. X-ray diffraction patterns: (a) stainless steel substrate, (b) the asdeposited film of cobalt oxides, and (c) the lithiated film electrode.
A high-resolution XPS of the as-deposited thin film of cobalt oxide in the binding energy from 770 to 815 eV is shown in Fig. 2(a). Two peaks at binding energies of 779.5 and 794.8 eV with the separation of 15.3 eV correspond to the spin-orbit doublet of the Co 2p. Satellite features at 788.6 and 803.7 eV are also observed. The Co 2p spectrum is consistent with the XPS spectrum of Co3O4 [18 – 21]. Both XRD and XPS measurements clearly indicate that the as-deposited film is composed of Co3O4. The electrochemical performance of Co3O4 films were investigated by galvanostatic cycling measurements. Fig. 3(a) shows the voltage-capacity profiles of Li/cobalt-oxide cells cycled between 0.01 and 3.0 V at a constant current density of 10 AA/cm2. The initial open-circuit voltage of the cell is 3.2 V. During the first discharge, two different sloping voltages ranges between 0.93 –0.7 and 0.7– 0.02 V, but in the following discharge the sloping voltages shift to 1.36– 1 and 0.98– 0.02 V, respectively. The plateaus in the second and the subsequent discharge curves are similar, but different from the first one. The big difference between the first and the subsequent voltage-capacity profiles implies the occurrence of electrochemical Li-driven irreversible structural and textural changes of the Co3O4 electrode. From the voltage-capacity profiles shown in Fig. 3, the irreversible specific capacity obtained is 913 mAh/g (equivalent to 8 Li per Co3O4) and 6 lithium ions can be removed upon the first charge leading to a reversible capacity of 640 mAh/g. It should also be noted that the existence of discharge plateaus in the discharge/charge curves exhibits the characteristic behavior of film electrodes with nanocrystalline particles. The first and second cyclic voltammograms (CV) for the as-deposited film electrode of cobalt oxides measured at a scan rate of 0.1 mV/s between 0.1 and 3.6 V are shown in Fig. 4. In the first cycle, a large cathodic current peak with a maximum at about 0.87 V and a small one peaked at 0.48 V are observed. But in the second cycle, these two peaks shift to 0.98 and 1.36 V, respectively, which do not change in the subsequent cycles. The anodic peaks located at 1.68 and 2.0
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Fig. 3. Voltage-capacity profiles for the Co3O4 film/Li cell. The cell was cycled between 0.01 and 3.0 V at a current density of 10 AA/cm2.
were rinsed by dimethyl carbonate and dried. Fig. 2(b) shows the Co 2p core level spectra of the lithiated cobalt oxide film, and their binding energy peaks of the lithiated film are clearly different from those of the as-deposited film. The electron binding energy of Co 2p3/2 and 2p1/2 of the lithiated film shift to 778.8 and 792.1 eV, respectively, and no satellite features are observed, indicating that the electrochemical reaction of Li with nanocrystalline cobalt oxides took place. From the binding energy data reported in the literature [20] the Co 2p3/2 and 2p1/2 peaked at 778.0 and 792.7 eV can be attributed to metallic cobalt. Fig. 2(c) shows the XPS spectrum of the thin film charged to 3.0 V. It is surprising to note that the Co 2p3/2 and 2p1/2 peaked at 781.0 and 797.3 eV with satellite peaks located at 787.1 and 803.7 [18,19] corresponding to that of CoO, implying that the recharged thin film is composed of CoO not Co3O4. Fig. 1(c) shows the XRD patterns of the lithiated Co3O4 films. It is interesting to see that there is no clear diffraction peaks, revealing that the lithiated film formed by the electrochemical reaction of Co3O4 and lithium either becomes amorphous or consists of nano-particles with the size smaller than the X-ray coherence wavelength. As we illustrated before the voltage-capacity profiles already exhibit the crystalline behavior of the film electrode, and it Fig. 2. XPS spectrum of Co 2p: (a) the as-deposited film of cobalt oxides, (b) the lithiated film of cobalt oxides, (c) the delithiated film of cobalt oxides.
V keep unchanged. The voltage-capacity profiles and cyclic voltammogram measurements are in good agreement and both indicate that the electrochemical processes in the second and subsequent cycles are different from the first one. To further examine the electrochemical processes of cobalt oxides upon the first cycling the lithiated and delithiated films were characterized by XRD and XPS measurements. The Co3O4 film electrode was firstly lithiated by discharging a Co3O4film/Li cell to 0.01 V and then delithiated by charging the cell to 3.0 V at a constant current density of 5 AA cm2. Right before these measurements the lithiated and delithiated Co3O4 film taken out from the cell
Fig. 4. Cyclic voltammograms of the as-deposited film electrode of cobalt oxides cycled between 0.1 and 3.6 V in 1M LiPF6-1: 1 EC: DMC electrolyte.
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seems most likely that the lithiated film is composed of nano-sized metallic cobalt embedded in Li2O matrix. This result is similar to the Li ion insertion reaction of 3dtransition metal oxide, such as NiO and FeO used as anodes in Li-ion batteries, which was confirmed by in situ XRD and TEM observation [1].
4. Discussion From the first discharge (Fig. 3) and voltammogram curves (Fig. 4) of Co3O4 film electrode, it can be seen that the discharge plateaus at 0.91 and 0.6 V in the first discharge are in good agreement with the cathodic current peaks in the CV curve, but the subsequent discharge plateaus are at 0.9 and 1.4 V. Connar and Irvine [24] investigated the lithium insertion behaviors of Co3O4 and showed that both the initial and subsequent discharge plateaus appeared at 1.2 V. Poizot et al. [22] reported that the plateau of the first discharge and that of the subsequent discharge of Co3O4 appeared at 1.18 and 1.2 V, respectively, and those of CoO appeared at 0.8 and 1.5 V, respectively. We note that the second discharge behavior of the Co3O4 film electrode fabricated by PLD in this work is different from that of Co3O4 bulk powder, but similar to that of CoO bulk powder. In addition, we have found that the irreversible specific capacity of the film electrode is 913 mAh/g, close to the theoretical specific capacity of Co3O4 and the reversible specific capacity was 640 mAh/g, which is close to the theoretical specific capacity of CoO. Our results clearly give the implication that the first discharge behavior demonstrates the electrochemical characteristics of Co3O4 film electrode, but the subsequent discharge and charge behavior presents that of CoO film electrode. The binding energy, the line shapes and the intensity of the satellite peaks of XPS have usually been used for the identification of the cobalt species. Our XPS results indicate Co3O4 to be present on the surface of the asdeposited film. After lithiation, metallic Co is found in Co 2p spectrum of the lithiated film electrode of cobaltoxides. In addition, Li 1s spectrum of lithiated electrodes also indicates the formation of Li2O. It is obvious that the reaction products of metallic Co and Li2O are produced from the electrochemical reaction of nanocrystalline Co3O4 with Li. However, after the film electrode is delithiated, the metal Co converts to CoO, not Co3O4, These evidences demonstrate that the electrochemical reaction of Li with nanocrystalline Co3O4 is irreversible in the first charged/discharge process. Recently, Tarascon’s group also pointed out that the re-oxidation process of CuO occurred nonhomogeneously leading to the coexistence of Cu, Cu2O, and CuO phases, which was supported by their TEM and selected area electron diffraction (SAED) results obtained at the end of the first charge [5]. Thackeray et al. [23] reported that the structure of lithiated Co3O4 was predominantly rock salt-like, due to
the fact that cationic species (Co2 +) initially present in the tetrahedral sites were displaced during lithiated process into empty neighbor octahedral sites leading to rock salt type compounds. Recently, Obrovac et al. [3] studied the electrochemical reaction of a-CoO and Li and found that nano-sized metal cobalt and Li2O were formed afterdischarge and h-CoO was generated after charging to 3 V, since Li2O had antifluorite structure while h-CoO had a zinc blende type lattice, and the oxygen lattices in both structures were nearly the same. Based on our results mentioned above, we propose that the reduction and oxidation reactions in the discharge/ charge processes of nanocrystalline Co3O4 thin film-based lithium cell involve the following steps. First discharge Co3 O4 þ 8Li ! 3 Co þ 4Li2 O Subsequent cycling Co þ Li2 O X CoO þ 2Li The first step is an irreversible reaction of lithium and Co3O4 film, which produces nano-sized metallic cobalt, embedded in Li2O. While charging, the metallic cobalt embedded in Li2O matrix generates CoO instead of Co3O4 due to the similarity of oxygen lattices in Li2O and h-CoO. In the subsequent steps, the original oxygen lattice is preserved and the reaction of CoO with Li becomes reversible during discharge/charge cycles.
5. Conclusions Pulsed laser deposition method has been successfully used to fabricate the film electrode composed of nano-sized Co3O4. Voltage profile and cyclic voltammetry measurements of the film electrode show an irreversible reduction process in the first cycle. Consequently, the film electrode exhibits good cycle performance with a reversible capacity as high as 640 mAh/g at 10 AA/cm2. The detailed XPS analysis demonstrates that the reaction mechanism for Li reaction with cubic spinel Co3O4 involves the formation of Li2O and cobalt metal nano-particles during the first discharge process. After charging to 3.0 V, CoO instead of Co3O4 is formed in the thin film, which has been confirmed by XPS measurements. We propose a reaction mechanism that the discharge/charge processes of the nanocrystalline Co3O4-based lithium cell include two steps. The first step is an irreversible reaction, producing nano-sized metallic cobalt and Li2O, and the subsequent steps are reversible processes involving the oxidation and reduction reactions between Co and CoO.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Project No. 200083001).
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References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [2] S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P. Pizot, J.-M. Tarascon, J. Electrochem. Soc. 148 (2001) A285. [3] M.N. Obrovac, R.A. Dunlap, R.J. Sanderson, J.R. Dahn, J. Electrochem. Soc. 148 (2001) A576. [4] D. Larcher, G. Sudant, J.-B. Leriche, Y. Chabre, J.-M. Tarascon, J. Electrochem. Soc. 149 (2002) A234 – A241. [5] G.X. Wang, Y. Chen, K. Konstantinov, J. Yao, J.-H. Ahn, H.K. Liu, S.X. Dou, J. Alloys Compd. 340 (2002) L5 – L10. [6] P.A. Connor, J.T.S. Irvine, Electrochim. Acta 47 (2002) 2885 – 2892. [7] G.X. Wang, C. Chen, K. Konstantinov, M. Lindsay, H.K. Liu, S.X. Dou, J. Power Source 109 (2002) 142 – 147. [8] T. Seike, J. Nagai, Sol. Energy Mater. 22 (1991) 107. [9] C.N.P. da Fonseca, M.-A. de Paoli, A. Gorenstein, Adv. Mater. 3 (1991) 553. [10] T. Maruyama, S. Arai, J. Electrochem. Soc. 143 (1996) 1383. [11] F. Svegl, B. Orel, I.G. Svegl, V. Kaucic, Electrochim. Acta 45 (2000) 4359.
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[12] Y. Wang, Z.-W. Fu, Q.-Z. Qin, Thin Solid Films 441 (2003) 19. [13] Y. Wang, Q.-Z. Qin, J. Electrochem. Soc. 149 (2002) A849. [14] K.M. Beck, T. Sasaki, N. Koshizaki, Chem. Phys. Lett. 301 (1999) 336. [15] Z.W. Fu, Q.Z. Qin, J. Electrochem. Soc. 147 (2000) 4610. [16] M.M. Thackeray, W.I.F. David, J.B. Goodenough, Mater. Res. Bull., 17 (1982) 785. [17] J.D. Hanawalt, Goodenough, Anal. Chem. 10 (1938) 475. [18] N.S. Mclntyre, M.G. Cook, Anal. Chem. 47 (1975) 2208. [19] T.L. Barr, J. Phys. Chem. 82 (1978) 1801. [20] I.G. Casella, M.R. Guascito, J. Electroanal. Chem. 54 (1999) 746. [21] B. Raquet, R. Mamy, J.C. Ousset, N. Negre, M. Goiran, C.G. Piecourt, J. Magn. Magn. Mater. 184 (1998) 41. [22] P. Poizot, S. Larulle, S. Grugeon, L. Dupont, B. Beaudoin, J.M. Tarascon, C. R. Acad. Sci., Ser. Chim. (Paris) 3 (2000) 681. [23] M.M. Thackeray, S.D. Baker, K.T. Adendorff, Solid State Ionics 17 (1985) 175. [24] P.A. Connar, J.T.S. Irvine, Electrochim. Acta 47 (2002) 2885.
Electrochemical reaction of nanocrystalline Co3O4 thin film with Lithium Zheng-Wen Fu, Ying Wang, Ye Zhang, Qi-Zong Qin * Department of Chemistry, Laser Chemistry Institute, Fudan University, 220 Handan Road, Shanghai 200433, China Received 4 October 2003; received in revised form 16 February 2004; accepted 24 February 2004
Abstract The electrochemical reaction of lithium and nanocrystalline Co3O4 thin films fabricated by reactive pulsed laser deposition was investigated using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and galvanostatic measurements. Our new finding is that during the first discharge process of nanocrystalline Co3O4 film-based lithium cell Co3O4 thin film reacts with lithium and forms nano-sized metallic Co and Li2O, but after charging to 3.0 V CoO instead of Co3O4 is generated and the subsequent discharge/charge processes are reversible. A reaction mechanism of nanocrystalline Co3O4 thin film with lithium is proposed. D 2004 Elsevier B.V. All rights reserved. PACS: 80.45; 81.15.F Keywords: Nanocrystalline thin film; Co3O4 thin film; Pulsed laser deposition; Electrochemical reaction of Co3O4 thin film with lithium
1. Introduction Nano-sized transition metal oxides have been widely studied in recent years in search of new anode materials for Li-ion batteries. Tarascon et al. [1,2] investigated a variety of transition metal oxides MxOy such as FeO, CoO, NiO, CuO, Cu2O, or Co3O4 as anode materials and proposed a new mechanism of Li reactivity in such materials different from the classical Li insertion/deinsertion or Li-alloying process, involving the reversible formation and decomposition of Li2O, and accompanying in situ nanoparticles formation and their reduction and oxidation. Obrovac et al. [3] has investigated CoO-based electrode and the electrochemical reaction of a-CoO with lithium ions using in situ X-ray diffraction and in situ Mo¨ssbauer measurement. They suggested a similar reaction mechanism including the immediate decomposition of metal oxide to form nano-sized metal particles, but the reduced metal was oxidized to h-CoO by a displacement reaction where the metal ions displaced the Li ion in Li2O via an ion exchange process. The reactivity of Co3O4 was recently reinvestigated [4 –7]. Larcher et al. [4] found that two competing mechanisms to reach the full decomposition of Co3O4 electrode into a composite * Corresponding author. Tel./fax: +86-21-651-0277. E-mail address: [email protected] (Q.-Z. Qin). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.020
consisting of Co nanograins embedded in a Li2O existed and the intermediates were LixCo3O4 and a-CoO, depending on the texture of the starting material, and both of the intermediates decomposed into Co nanograins and Li2O on further reduction. It is well known that thin films deposited on electronically conductive substrates are more suitable for fundamental electrochemical studies of transition metal oxides due to avoiding the interference from additives in polymer-bonded electrodes. Thin films of cobalt oxides prepared by various methods such as electron-beam evaporation [8], electrochemical-deposition [9], chemical vapor deposition [10], the sol –gel route [11] have been reported. Recently we have reported the investigation on the electrochemical performance of nanocrystalline Co3O4 thin film fabricated by pulsed laser deposition (PLD) [12]. PLD is a simple method to deposit thin film electrodes and has demonstrated its potential in preparing nanostructured thin films [13,14]. In this paper, we report the electrochemical reaction of lithium with nanocrystalline Co3O4 thin films prepared by pulsed laser deposition. The structure, composition and morphology of as-deposited, lithiated and delithiated thin films of cobalt oxides are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photo-electron spectroscopy (XPS). An electrochemical reaction mechanism of nanocrystalline Co3O4 thin film with lithium is proposed.
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2. Experimental A pulsed laser deposition (PLD) system for preparing thin films of cobalt oxides has been described elsewhere [15]. A 355 nm laser beam with a repetition rate of 10 Hz and a pulse width of 6 ns was provided by the third harmonic frequency of a Q-switched Nd:YAG laser (Spectra Physics GCR-150) and directed through a quartz window onto a rotating target. The films were deposited on a stainless steel substrate heated to a temperature of 600 jC at a laser fluence of 2 J/cm2, and postannealed at 300 jC for 2 h in an oxygen environment. Metal Co target was prepared by pressing Co powder (99.99%) to form a 1.3 cm diameter pellet. The O2 ambient gas pressure was maintained at 40 Pa during the reactive pulsed laser deposition. The deposition rate of the films under above conditions was estimated to be 4 nm/min. X-ray diffraction (XRD) patterns of the composite film were determined by using a Rigata/max-C diffractometer ˚ ). The film with Cu-Ka radiation source (k = 1.5406 A thickness was measured by scanning electron microscopy (SEM) (Philips XL 30) and a profilometry (Tencor AlphaStep 200). XPS measurements were performed on a Perkin Elmer PHI 6000C ECSA system with monochromatic Al Ka (1486.6 eV) irradiation. To correct possible charge-up of the films by X-ray irradiation, the binding energy was calibrated using C1s (284.6 eV) spectrum of hydrocarbon that remained in the XPS analysis chamber as a contaminant. The electrochemical performance of the thin film electrodes of cobalt oxides was measured using the same method as described in Ref. [12]. The lithiated films were handled in an Ar-filled glove box in order to avoid exposure to oxygen and water, and were rapidly transferred to the XPS and SEM measurement chambers. XPS spectra of the samples with and without the bombardment of the surface by Ar ion sputtering for 15 min were identical, indicating that the sample surfaces were kept clean under our experimental conditions.
3. Results In order to identify the chemical composition, crystallinity and valence state of as-deposited films prepared by reactive PLD in O2 ambient using Co as a target, we measured the XRD and XPS spectra of as-deposited thin films of cobalt oxide. Fig. 1 shows the typical XRD patterns of the as-deposited film and the stainless steel substrate. The diffraction peaks shown in Fig. 1(b) at 2h = 19.0j, 31.3j, 36.6j, 59.2j and 65.1j can be attributed to the spinel type cubic structure of Co3O4 with Fd3m space group [16,17]. The other two diffraction peaks appearing at 2h = 43.3j and 74.7j come from the stainless steel substrate (see Fig. 1(a)). The crystallite size of Co3O4 calculated by the Scherrer’s formula is estimated to be about 50nm, indicating the formation of nanostructured polycrystalline Co3O4 thin film.
Fig. 1. X-ray diffraction patterns: (a) stainless steel substrate, (b) the asdeposited film of cobalt oxides, and (c) the lithiated film electrode.
A high-resolution XPS of the as-deposited thin film of cobalt oxide in the binding energy from 770 to 815 eV is shown in Fig. 2(a). Two peaks at binding energies of 779.5 and 794.8 eV with the separation of 15.3 eV correspond to the spin-orbit doublet of the Co 2p. Satellite features at 788.6 and 803.7 eV are also observed. The Co 2p spectrum is consistent with the XPS spectrum of Co3O4 [18 – 21]. Both XRD and XPS measurements clearly indicate that the as-deposited film is composed of Co3O4. The electrochemical performance of Co3O4 films were investigated by galvanostatic cycling measurements. Fig. 3(a) shows the voltage-capacity profiles of Li/cobalt-oxide cells cycled between 0.01 and 3.0 V at a constant current density of 10 AA/cm2. The initial open-circuit voltage of the cell is 3.2 V. During the first discharge, two different sloping voltages ranges between 0.93 –0.7 and 0.7– 0.02 V, but in the following discharge the sloping voltages shift to 1.36– 1 and 0.98– 0.02 V, respectively. The plateaus in the second and the subsequent discharge curves are similar, but different from the first one. The big difference between the first and the subsequent voltage-capacity profiles implies the occurrence of electrochemical Li-driven irreversible structural and textural changes of the Co3O4 electrode. From the voltage-capacity profiles shown in Fig. 3, the irreversible specific capacity obtained is 913 mAh/g (equivalent to 8 Li per Co3O4) and 6 lithium ions can be removed upon the first charge leading to a reversible capacity of 640 mAh/g. It should also be noted that the existence of discharge plateaus in the discharge/charge curves exhibits the characteristic behavior of film electrodes with nanocrystalline particles. The first and second cyclic voltammograms (CV) for the as-deposited film electrode of cobalt oxides measured at a scan rate of 0.1 mV/s between 0.1 and 3.6 V are shown in Fig. 4. In the first cycle, a large cathodic current peak with a maximum at about 0.87 V and a small one peaked at 0.48 V are observed. But in the second cycle, these two peaks shift to 0.98 and 1.36 V, respectively, which do not change in the subsequent cycles. The anodic peaks located at 1.68 and 2.0
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Fig. 3. Voltage-capacity profiles for the Co3O4 film/Li cell. The cell was cycled between 0.01 and 3.0 V at a current density of 10 AA/cm2.
were rinsed by dimethyl carbonate and dried. Fig. 2(b) shows the Co 2p core level spectra of the lithiated cobalt oxide film, and their binding energy peaks of the lithiated film are clearly different from those of the as-deposited film. The electron binding energy of Co 2p3/2 and 2p1/2 of the lithiated film shift to 778.8 and 792.1 eV, respectively, and no satellite features are observed, indicating that the electrochemical reaction of Li with nanocrystalline cobalt oxides took place. From the binding energy data reported in the literature [20] the Co 2p3/2 and 2p1/2 peaked at 778.0 and 792.7 eV can be attributed to metallic cobalt. Fig. 2(c) shows the XPS spectrum of the thin film charged to 3.0 V. It is surprising to note that the Co 2p3/2 and 2p1/2 peaked at 781.0 and 797.3 eV with satellite peaks located at 787.1 and 803.7 [18,19] corresponding to that of CoO, implying that the recharged thin film is composed of CoO not Co3O4. Fig. 1(c) shows the XRD patterns of the lithiated Co3O4 films. It is interesting to see that there is no clear diffraction peaks, revealing that the lithiated film formed by the electrochemical reaction of Co3O4 and lithium either becomes amorphous or consists of nano-particles with the size smaller than the X-ray coherence wavelength. As we illustrated before the voltage-capacity profiles already exhibit the crystalline behavior of the film electrode, and it Fig. 2. XPS spectrum of Co 2p: (a) the as-deposited film of cobalt oxides, (b) the lithiated film of cobalt oxides, (c) the delithiated film of cobalt oxides.
V keep unchanged. The voltage-capacity profiles and cyclic voltammogram measurements are in good agreement and both indicate that the electrochemical processes in the second and subsequent cycles are different from the first one. To further examine the electrochemical processes of cobalt oxides upon the first cycling the lithiated and delithiated films were characterized by XRD and XPS measurements. The Co3O4 film electrode was firstly lithiated by discharging a Co3O4film/Li cell to 0.01 V and then delithiated by charging the cell to 3.0 V at a constant current density of 5 AA cm2. Right before these measurements the lithiated and delithiated Co3O4 film taken out from the cell
Fig. 4. Cyclic voltammograms of the as-deposited film electrode of cobalt oxides cycled between 0.1 and 3.6 V in 1M LiPF6-1: 1 EC: DMC electrolyte.
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seems most likely that the lithiated film is composed of nano-sized metallic cobalt embedded in Li2O matrix. This result is similar to the Li ion insertion reaction of 3dtransition metal oxide, such as NiO and FeO used as anodes in Li-ion batteries, which was confirmed by in situ XRD and TEM observation [1].
4. Discussion From the first discharge (Fig. 3) and voltammogram curves (Fig. 4) of Co3O4 film electrode, it can be seen that the discharge plateaus at 0.91 and 0.6 V in the first discharge are in good agreement with the cathodic current peaks in the CV curve, but the subsequent discharge plateaus are at 0.9 and 1.4 V. Connar and Irvine [24] investigated the lithium insertion behaviors of Co3O4 and showed that both the initial and subsequent discharge plateaus appeared at 1.2 V. Poizot et al. [22] reported that the plateau of the first discharge and that of the subsequent discharge of Co3O4 appeared at 1.18 and 1.2 V, respectively, and those of CoO appeared at 0.8 and 1.5 V, respectively. We note that the second discharge behavior of the Co3O4 film electrode fabricated by PLD in this work is different from that of Co3O4 bulk powder, but similar to that of CoO bulk powder. In addition, we have found that the irreversible specific capacity of the film electrode is 913 mAh/g, close to the theoretical specific capacity of Co3O4 and the reversible specific capacity was 640 mAh/g, which is close to the theoretical specific capacity of CoO. Our results clearly give the implication that the first discharge behavior demonstrates the electrochemical characteristics of Co3O4 film electrode, but the subsequent discharge and charge behavior presents that of CoO film electrode. The binding energy, the line shapes and the intensity of the satellite peaks of XPS have usually been used for the identification of the cobalt species. Our XPS results indicate Co3O4 to be present on the surface of the asdeposited film. After lithiation, metallic Co is found in Co 2p spectrum of the lithiated film electrode of cobaltoxides. In addition, Li 1s spectrum of lithiated electrodes also indicates the formation of Li2O. It is obvious that the reaction products of metallic Co and Li2O are produced from the electrochemical reaction of nanocrystalline Co3O4 with Li. However, after the film electrode is delithiated, the metal Co converts to CoO, not Co3O4, These evidences demonstrate that the electrochemical reaction of Li with nanocrystalline Co3O4 is irreversible in the first charged/discharge process. Recently, Tarascon’s group also pointed out that the re-oxidation process of CuO occurred nonhomogeneously leading to the coexistence of Cu, Cu2O, and CuO phases, which was supported by their TEM and selected area electron diffraction (SAED) results obtained at the end of the first charge [5]. Thackeray et al. [23] reported that the structure of lithiated Co3O4 was predominantly rock salt-like, due to
the fact that cationic species (Co2 +) initially present in the tetrahedral sites were displaced during lithiated process into empty neighbor octahedral sites leading to rock salt type compounds. Recently, Obrovac et al. [3] studied the electrochemical reaction of a-CoO and Li and found that nano-sized metal cobalt and Li2O were formed afterdischarge and h-CoO was generated after charging to 3 V, since Li2O had antifluorite structure while h-CoO had a zinc blende type lattice, and the oxygen lattices in both structures were nearly the same. Based on our results mentioned above, we propose that the reduction and oxidation reactions in the discharge/ charge processes of nanocrystalline Co3O4 thin film-based lithium cell involve the following steps. First discharge Co3 O4 þ 8Li ! 3 Co þ 4Li2 O Subsequent cycling Co þ Li2 O X CoO þ 2Li The first step is an irreversible reaction of lithium and Co3O4 film, which produces nano-sized metallic cobalt, embedded in Li2O. While charging, the metallic cobalt embedded in Li2O matrix generates CoO instead of Co3O4 due to the similarity of oxygen lattices in Li2O and h-CoO. In the subsequent steps, the original oxygen lattice is preserved and the reaction of CoO with Li becomes reversible during discharge/charge cycles.
5. Conclusions Pulsed laser deposition method has been successfully used to fabricate the film electrode composed of nano-sized Co3O4. Voltage profile and cyclic voltammetry measurements of the film electrode show an irreversible reduction process in the first cycle. Consequently, the film electrode exhibits good cycle performance with a reversible capacity as high as 640 mAh/g at 10 AA/cm2. The detailed XPS analysis demonstrates that the reaction mechanism for Li reaction with cubic spinel Co3O4 involves the formation of Li2O and cobalt metal nano-particles during the first discharge process. After charging to 3.0 V, CoO instead of Co3O4 is formed in the thin film, which has been confirmed by XPS measurements. We propose a reaction mechanism that the discharge/charge processes of the nanocrystalline Co3O4-based lithium cell include two steps. The first step is an irreversible reaction, producing nano-sized metallic cobalt and Li2O, and the subsequent steps are reversible processes involving the oxidation and reduction reactions between Co and CoO.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Project No. 200083001).
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