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Electrochemical reactivity mechanism of CuInSe2 with lithium
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Thin Solid Films 516 (2008) 8386 – 8392 www.elsevier.com/locate/tsf
Electrochemical reactivity mechanism of CuInSe2 with lithium Ming-Zhe Xue, Zheng-Wen Fu ⁎ Department of Chemistry & Laser Chemistry Institute, Shanghai Key laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai, 200433, PR China Received 24 April 2007; received in revised form 25 February 2008; accepted 7 April 2008 Available online 12 April 2008
Abstract CuInSe2 thin films were fabricated by pulsed laser deposition and its electrochemistry with lithium was investigated. The reversible discharge capacity of the CuInSe2/Li cell cycled between 0.01 V and 2.5 V was found to be in the range of 438 to 647 mAh g− 1 during the first 100 cycles. By using ex situ X-ray diffraction, transmission electron microscopy, selected-area electron diffraction measurements and X-ray photoelectron spectroscopy, both classical alloying process and separated selenization/reduction of nanosized metallic In and Cu were proposed for the lithium electrochemical reaction of CuInSe2, indicating one of the features of the lithium electrochemistry of CuInSe2. CuInSe2 has high reversible capacity and good cycle performance, which makes it potential anode material for future lithium-ion batteries. © 2008 Elsevier B.V. All rights reserved. Keywords: CuInSe2; Pulsed laser deposition; Thin film; Anode; Lithium-ion batteries
1. Introduction Since tin-based amorphous oxides and 3d transition metal oxides were reported as anode materials for lithium ion batteries [1,2], the electrochemical properties of various metal compounds, for example fluorides, nitrides, phosphides and sulfides, have been widely investigated. Li et al. studied a series of metal fluorides [3]. Their results showed that metal fluorides could also store lithium heterogeneously via phase formation as found for various oxides. Pereira et al. examined the electrochemistry of Cu3N with lithium and showed a similar reversible lithium/ copper nitride conversion process driven by Cu nanoparticles as transition metal oxides [4]. In addition, they found that oxidation of Cu metal into Cu2+ formed copper oxide, and they believed it to be associated with electrolyte degradation. Souza et al. reported on the electrochemical properties of MnP4 and supposed a topotactic first-order transition between the crystal structures of MnP4 and Li7MnP4 [5]. This mechanism was confirmed and further extended by Gillot et al. [6]. Their results showed that after the first conversion process that transforms binary MnP4 electrode into ternary Li7MnP4, a partial and irreversible decomposition of Li7MnP4 into Li3P+Mn0 was achieved and should be responsible for the poor electrodes ⁎ Corresponding author. E-mail address: [email protected] (Z.-W. Fu). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.04.050
capacity retention. Débart et al. investigated the electrochemical reactivity of some transition metal (Co, Ni, Cu) sulfides versus lithium [7]. They showed that cobalt and nickel sulfides react versus lithium through conversion reactions similarly to their homologous oxides while in contrast, the electrochemical reactivity of CuS towards Li was shown to follow a displacement reaction leading to the growth and disappearance of large copper dendrites with a concomitant reversible decomposition/recrystallization of the initial electrode material. Apparently, the lithium electrochemical reactions with metal compounds are complex. More work should be done to search new compounds and understand whether there are other mechanisms. Recently we fabricated a series of metal selenides thin films by pulsed laser deposition and found that most of them could react with lithium reversibly. Our results showed versatile lithium electrochemistry of these materials. For some lithium alloying metals, the corresponding metal selenides, for instance, SnSe, Ag2Se and ZnSe, all react with lithium according to a twosteps mechanism including both classical alloying/de-alloying processes and selenization/reduction of nanosized metal [8–10]. On the contrary, the 3d transition metal selenides showed more abundant electrochemical properties. The electrochemical reaction mechanism of α-MnSe thin films with lithium upon cycling involving the irreversible decomposition of α-MnSe and reversible formation of β-MnSe [11]. The reversible electrochemical conversion between NiSe2 and Ni+Li2Se is via two
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intermediates, β-NiSe and Ni3Se2 [12]. Both CuSe2 and CuSe reversibly react with lithium heterogeneously to form Cu3Se2 and Cu2Se while Cu2Se may undergo a “displacement” reaction mechanism with lithium [13]. However, all of these selenides are binary compounds. The legitimate question remains whether ternary metal selenides, for example, CuInSe2 could store Li reversibly. CuInSe2 is an interesting metal chalcogenide semiconductor material. Lattice parameters of bulk polycrystalline CuInSe2 material with chalcopyrite structure were reported to be a = 5.78 Å and c = 11.62 Å [14]. Due to its band gap of 1.04 eV, which is within the maximum solar absorption region, it is one of the most promising materials for creation of highefficiency polycrystalline thin film solar cells. Besides these, it is also extensively used in the area of visible, infrared lightemitting diodes, infrared detectors and optical parametric oscillators [15]. There is no available report on the possibility of CuInSe2 as anode material for lithium-ion batteries. There are many techniques currently in use for the fabrication of CuInSe2 thin films such as flash-evaporation [16], electrodeposition [17], metal organic chemical vapor deposition [18], selenization of Cu–In alloy precursors [19] and pulsed laser deposition (PLD) [20–22]. Among them, PLD technique is very simple and effective method for the deposition of thin film electrodes. In previous reports on pulsed deposition of CuInSe2 thin film, sintered CuInSe2 powders were pressed into the target. In this work, we used a mixed target consisting of Cu, In and Se powders for pulsed laser ablation and got polycrystalline CuInSe2 thin film on stainless steel substrates. The electrochemical behavior, structure, composition and morphology of CuInSe2 thin film were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS). The motivation of this work is to explore the possibility of using CuInSe2 as anode material for lithium-ion batteries and to elucidate its electrochemical reaction mechanism with lithium. 1.1. Experimental The apparatus used for pulsed laser deposition has been described elsewhere [11]. Experimental conditions for depositing thin films are described briefly as follows. A 355 nm laser beam, provided by the third harmonic frequency of a Q-switched Nd: yttrium aluminum garnet (YAG) laser (Quanta-Ray GCR150) was focused onto the surface of the target. The incident angle between the laser beam and the target surface normal was 45°. The laser energy intensity was about 2 J cm− 2. The repetition rate and pulse width of the laser was 10 Hz and 10 ns, respectively. The targets were made from Cu, In and Se powders (both pure 99.9%), they were mixed and ground in certain element molar ratios of Cu:In:Se = 1:1:2.5, then were pressed to form a 1.3 cm diameter pellet as the ablated target. An excess of Se in the mixture target can compensate Se loss derived from its vacuum sublimation during laser ablation. The base pressure of the chamber was 10− 2 Pa, and the ambient Ar gas pressure during deposition was kept at 8 Pa by a needle valve. The thin
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films were deposited on stainless steel (SS) substrates, which were kept at 200 °C. The distance between the target and substrate was 40 mm. The deposition time is fixed at 30 min. The weight of the thin film was directly obtained by subtracting the original substrate weight from total weight of the substrate and the deposited thin film onto its surface, which were examined by electrobalance (BP 211D, Sartorius), and was about 0.10 mg for an area of 1.0 cm2. The precision of the weight is ± 0.01 mg. XRD patterns and the morphology of the thin film electrodes were investigated by Bruker D8 advance diffractormeter equipped with Cu-Kα radiation (λ = 1.5406 Å) and SEM (Philips XL30 microscope) with the operating voltage of 25 kV, respectively. TEM and SAED measurements were carried out by a JEOL 2010 TEM at 160 kVaccelerating voltage. XPS measurements were performed on a Perkin-Elmer PHI 5000C ECSA system with monochromatic AlKα (1486.6 eV) irradiation. The spectra were taken with the X-ray source operating at 400 W (15 kV–27 mA). The specimens were analyzed at an electron take-off angle of 70o, measured with respect to the surface plane. The pressure of vacuum is 10− 7 Pa. No sputter-etching was performed on the sample prior to the XPS examination. To correct possible charging of the films by X-ray irradiation, the binding energy was calibrated using the C 1s (284.6 eV) spectrum of hydrocarbon that remained in the XPS analysis chamber as a contaminant. For the electrochemical measurements, the cells were constructed using the as-deposited thin films as a working electrode and a lithium sheet as a counter electrode. The electrolyte is 1 M LiPF6 in a nonaqueous solution of ethylene carbonate and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). The cells were assembled in an Ar filled glove box. Galvanostatic cycling measurements were carried out at room temperature with a Land CT 2001A battery test system. The cells were cycled between 0.01 and 2.5 V vs. Li+/Li at a current density of 5 μA cm− 2. Cyclic voltammetry (CV) tests were performed with a scanning rate of 0.1 mV s− 1 between 0.01 V and 2.5 V on CHI660A electrochemical working station (CHI Instruments, TN). In order to gain insight into the reaction mechanism of copper indium selenide with lithium, ex situ XRD, TEM, SAED and XPS measurements were carried out to reveal the structure and morphology change of CuInSe2 thin film electrodes at selected voltage points during the initial discharge and charge process. The model cells were dismantled in an Ar filled glove box and the electrodes were rinsed in anhydrous DMC to eliminate residual salts. For TEM and SAED measurements, the active materials were scratched from the SS substrate. The loose powders were then mixed with ethanol to prepare slurry, out of which one drop was taken, and deposited on a copper grid. To avoid exposure to oxygen or water, the thin films or copper grids were rapidly transferred into the chambers for cleanliness. 2. Results and discussions Fig. 1(a) shows the XRD pattern of the target used in pulsed laser deposition. Several peaks at 2θ = 23.5o, 29.7o, 33.0o, 36.3o, 39.2o, 41.2o, 43.3o, 45.2o, 50.5o, 51.6o, 54.5o, 55.7o and 56.0o
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Fig. 1. XRD patterns of (a) the target and (b) the as-deposited thin film. The peaks marked with asterisk corresponding to stainless steel substrate.
are in good accord with the diffraction peaks of copper [23], indium [24] and selenium [25]. After laser ablation of the mixed target, black-colored thin films were deposited on the SS substrate in our experimental condition. The XRD pattern of the as-deposited thin film was shown in Fig. 1(b). Apart from two diffraction peaks appearing at 2θ = 43.6o and 50.8o corresponding to the SS substrate (marked by asterisks), two peaks at 2θ = 26.5o and 44.1o could be assigned to the (112) and (204) (or (220)) reflection of the CuInSe2 [26]. The average crystallite size calculated by the Scherrer formula is estimated to be 15 nm (using the strongest peak at 26.5o). XRD data suggest that crystalline CuInSe2 thin film could be prepared by pulsed laser ablation of the mixed target of copper, indium and selenium. Fig. 2 shows the typical SEM image of the as-deposited CuInSe2 thin film. It exhibits a well-defined surface texture and is composed of small particles with an average size of about 100 nm. In addition, some particles agglomerates can be viewed in the SEM image. This phenomenon is also observed in some other metal selenide thin films and it seems to be a feature of thin film prepared by pulsed laser deposition [10–12]. The galvanostatic cycling profiles of the CuInSe2/Li cell cycled between 0.01 Vand 2.5 V under a current density of 5 μA
Fig. 2. SEM image of the as-deposited CuInSe2 thin film.
cm− 2 is shown in Fig. 3(a). The open circuit voltage (OCV) lies at 2.41 V. The first lithium insertion into CuInSe2 process is characterized by a large flat voltage plateau at 1.20 V, a large sloping plateau under 0.70 V and three small plateaus at 1.99, 1.47 and 1.31 V. These discharge plateaus can be attributed to the electrochemical reaction of the CuInSe2 thin film electrode with lithium. In the second cycle, the discharge curves differ from the first in a certain extent. The sloping plateau under 0.70 V is nearly unchanged while four higher plateaus are replaced by three sloping ones at 2.04, 1.51 and 1.18 V. The specific capacity of CuInSe2 thin film obtained as a function of the cycle number is shown in Fig. 3 (b). For comparison, the cycle performance of In thin film prepared by pulsed laser deposition is also showed. The initial discharge capacity of the as-deposited CuInSe2 thin film electrode is 647 mAh g− 1. This corresponds to 8.1 Li per CuInSe2 as shown in Fig. 3 (a). The second discharge process yields a reversible capacity of about 555 mAh g− 1, corresponding to 7.0 Li per CuInSe2. The irreversibility between the first two cycles is 14%. Afterward the cycling becomes more stable. The discharge capacity of the 100th cycle is 438 mAh g− 1, keeping 68% of the initial one. Although In thin film electrode has higher initial discharge capacity than CuInSe2, its capacity retention is poorer. After 100th cycles, the discharge capacity is only 58 mAh g− 1,
Fig. 3. (a) Galvanostatic curves of CuInSe2/Li cell and (b) the discharge capacities of CuInSe2 and In thin films as a function of cycle number.
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indicating the collapse of the crystal structure of CuInSe2 and the formation of much more crystalline In. On the other hand, we could not observe any diffraction of copper-based compounds. This may be due to the size of formed copperbased compound less than the X-ray coherence length (6 nm), which could not be identified by XRD techniques. However, combined with the charge-discharge curve of the first cycle, which shows that the intercalated number of Li is about of 4 when discharging to 0.9 V, it is reasonable to deduce that CuInSe2 has fully decomposed to form In, Cu and Li2Se as the following reaction: CuInSe2 þ 4Li→Cu þ In þ 2Li2 Se
Fig. 4. First three cyclic voltammograms for the as-deposited CuInSe2 thin film electrode.
much lower than the corresponding value of CuInSe2 thin film. Apparently, CuInSe2 can improve the reversible reaction of In with Li. Fig. 4 shows the first three cyclic voltammograms of the asdeposited CuInSe2 thin film electrode between 0.01 and 2.5 V measured at a scan rate of 0.1 mV s− 1. Six cathodic current peaks at 2.01, 1.42, 1.28, 1.16, 0.58 and nearly 0.00 V are observed in the first reduction process. Compared with the CV curves of In, In2O3 and InSb [27,28], CV curves at a low voltage range (vs. Li from 0.9 to 0 V) should mainly result from the alloying/dealloying reaction of In with Li during cycling. In the subsequent cycles, there are no obvious differences in this range. On the other hand, the profiles of the higher voltage range of 0.9 ~ –2.5 V are changed. In the second cycle, three peaks at 2.03, 1.47 and 1.14 V emerge and remain in the following cycles. These results are in good accord with the charge–discharge curves and imply the change of structure and composition of the thin film electrode after the first cycle. To determine the structural modification of CuInSe2 induced by Li uptake/removal, ex situ XRD measurements were performed upon CuInSe2 thin film electrode at various states of the cell cycled between 0.01 and 2.5 V at a constant current of 5 μA cm− 2. Fig. 5 shows XRD patterns obtained at different states of the first discharge and charge cycle. For comparison, XRD pattern of the as-deposited CuInSe2 thin film is also included (Fig. 5 (a)). After discharging to 1.35 V, the diffraction peaks of CuInSe2 only slightly shift to higher positions (from 26.5o to 26.6o and from 44.1o to 44.3o) and become broader, indicating that the reservation of the tetragonal structure (Fig. 5 (b)). Simultaneously, a new weak peak is observed at 32.9o and could be assigned to metal In [24], indicating the formation of metal In. These results imply that the discharge behavior from OCV to 1.35 V may be related to an intercalation process, in which lithium is gradually inserting into CuInSe2 structure and pulling out indium. When further discharging to 0.9 V, the diffraction peaks of CuInSe2 vanish and the peak assigned to indium becomes stronger and more keen-edged (Fig. 5 (c)),
ð1Þ
When finally discharging to 0.01 V, the peak of metal In disappears meanwhile a new peak at 21.4o is observed and could be attributed to the (311) diffraction of cubic In3Li13 [29] (Fig. 5 (d)), indicating the Li-alloying reaction of metal In. The observed slight 2θ shift toward small angle in peak position between the standard value and the experiment value is most likely due to the formation of nonstoichiometric InLix particles. Electrochemical reactions often form nonstoichiometric products [13,30]. Upon charging process, lithium is gradually extracted from the lithiated electrode. When charging to 1.0 V, diffraction peak from metal In reappears as shown in Fig. 5 (e). This means that Li+ is extruded from In–Li alloy and In framework is renewed. Finally, when charging to 2.5 V, the diffraction peak of In disappears, indicating that it takes part in the electrochemical reaction. Instead, two peaks centered at 26.8o and 44.5o emerge (Fig. 5 (f)). It is surprising to find that these two peaks could be assigned to the (111) and (220) diffraction of Cu2Se [31]. This interesting result suggests that Cu2Se is formed as the charging product. Nevertheless, although it is unambiguous that metal indium take part in the electrochemical reaction, we could not detect
Fig. 5. Ex situ XRD pattern of CuInSe2 thin film at various states during the first cycle of CuInSe2/Li cell: (a) the as-deposited; (b) discharging to 1.35 V; (c) discharging to 1.0 V; (d) discharging to 0.01 V; (e) charging to 1.0 V; (f) charging to 2.5 V. The peaks marked with asterisk corresponding to stainless steel substrate.
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techniques were utilized. The high resolution transmission electron microscopy image of the delithiated CuInSe2 thin film is shown in Fig. 6(a). The crystallites with clear and coherent stripes are observed (see the white arrow). SAED pattern in this region shows several clear rings made up of discrete spots (Fig. 6(b)). All d-spacings derived from the SAED pattern are shown in Table 1 and could be indexed to Cu2Se [31], which has been detected by XRD techniques before. TEM and SAED further confirm the existence of Cu2Se, but they also could not give any information about In-based products. This means that after charging to 2.5 V, In-based products may form an extremely amorphous phase which could not be detected by SAED. XPS measurements were performed to examine the valence change of indium in the thin film electrodes of charging of 1.0 V and charging of 2.5 V. Fig. 7(a) shows the In 3d spectra of the thin films. For calibration, the C 1 s spectra was also measured and shown in Fig. 7(b). The In 3d5/2 binding energy of the thin films of charging to 1.0 V (Fig. 7(b)-(1)) and charging to 2.5 V (Fig. 7(a)-(2)) is 443.5 and 443.8 eV, respectively. According to the previous studies, they could be attributed to In 3d5/2 binding energy for In and In2Se3 [32,33]. XPS results show that metal In is reoxidized to form In2Se3 after charging to 2.5 V.
Fig. 6. TEM image (a) and corresponding SAED pattern (b) of the delithiated CuInSe2 thin film electrode at the first charging to 2.5 V.
any crystalline In-based products by XRD techniques. This may be due to the amorphous or nanocrystalline feature of them. In order to further reveal the structure and composition of the thin film electrode after charging to 2.5 V, ex situ TEM and SAED
Table 1 d-spacings (Å) derived from SAED analysis of the delithiated CuInSe2 thin film electrode after charging to 2.5 V. JCPDS standard for Cu2Se (65-2982) is shown for reference Charging to 2.5 V Cu2Se — Fm30304 m
3.33 2.04 1.74 1.33 1.18 a = 5.77. ± 0.01
d (hkl)
int-f
3.33 (111) 2.04 (220) 1.74 (311) 1.32 (331) 1.18 (422)
447 999 161 52 213 a = 5.76
Fig. 7. (a) In 3d and (b) C 1 s XPS spectra and of the delithiated CuInSe2 thin film electrode (1) at the first charging to 1.0 V and (2) at the first charging to 2.5 V.
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Based on the above results, the electrochemical reaction mechanism of CuInSe2 with lithium involving the following steps is proposed CuInSe2 ðcrystallineÞ þ 4Li→Cu þ In þ 2Li2 Se
ð2Þ
3In þ 13Li↔In3 Li13
ð3Þ
2In þ 2Cu þ 4Li2 SeLi2 Se↔Cu2 SeðcrystallineÞ þ In2 Se3 ðamorphousÞ þ 8Li
ð4Þ
The first step occurs in the initial discharge process. It is an irreversible reaction with the collapse of pristine CuInSe2 structure and the formation of Cu, Li2Se and In. The subsequent cycling processes involve the second step. It includes the alloying/dealloying reaction of In and two selenization/reduction reaction between crystalline Cu2Se and Cu, and amorphous In2Se3 with In. The difference between electrochemically formed In2Se3 and Cu2Se and the original tetragonal CuInSe2 could cause the first discharge process different from the sequent ones. Nowadays the most common anode material of commercial lithium-ion batteries is graphite, whose theoretical capacity is only 372 mAh g− 1 [34]. Although some improvements were used to modify the carbon-based materials and the reversible capacities of around 450 mAh g− 1 are now being reached, it is not satisfactory. So many research efforts now are focused on searching new materials. In this work, we proposed a new anode material, CuInSe2, for lithium-ion batteries. Compared with graphite and other carbon-based anode materials, CuInSe2 has higher reversible capacity (of about 555 mAh g− 1), which makes it potential anode material for lithium-ion batteries. Recently we have fabricated several kinds of binary metal selenides and investigated their lithium electrochemical reaction mechanism [8–13]. Our results show that although all these compounds decomposed in the first discharge process, most of them would reform a metal selenide phase whose structure or chemical composition is similar to the original one in the charge process. Nevertheless, in this study we surprised to find that after charging to 2.5 V, In2Se3 and Cu2Se, instead of CuInSe2, are the final products. In order to explain this interesting result, we attempt to calculate the Gibbs free energies and theoretical electromotive force (EMF) values of three following reactions by Nernst equation. Free energy data of these materials at 25 °C are taken from the literature [35,36]. In þ Cu þ 2Li2 SeLi2 Se→CuInSe2 þ 4Li
ð5Þ
Cu þ 1=2Li2 SeLi2 Se→1=2Cu2 Se þ Li
ð6Þ
In þ 3=2Li2 SeLi2 Se→1=2In2 Se3 þ 3Li
ð7Þ
1=2In2 Se3 þ Cu þ 1=2Li2 SeLi2 Se→CuInSe2 þ Li
ð8Þ
1=2Cu2 Se þ In þ 3=2Li2 SeLi2 Se→CuInSe2 þ 3Li
ð9Þ
The calculated values are shown in Table 2. It is seen that reaction (7) has lower EMF value than reactions (5) and (6).
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Table 2 Gibbs free energy and EMF values of the corresponding reactions Reaction
ΔG (kJ mol− 1)
EMF (V)
(5) (6) (7) (8) (9)
630.64 169.45 458.66 171.95 461.18
1.63 1.76 1.58 1.78 1.59
These results suggest that if only considering the thermodynamic factor, Li2Se is more facile to only react with metal In first and form In2Se3. After this process, Cu and residual Li2Se will also take part in the electrochemical reaction. The possible reaction is Eqs. (6) or (8). Because the EMF values of these two reactions are very close, it is hard for us to explain the present results that only reaction (6) occurs. Then we suppose that dynamic factors may play an important role in this process, as the galvanostatic cycling is performed under room temperature, which could not give sufficient activation energy to form CuInSe2. As a matter of fact, the asdeposited CuInSe2 thin film is fabricated under 200 °C. However, the intrinsic factors are still unknown. More work should be done to explore the essence of the lithium electrochemistry of CuInSe2. 3. Conclusions Thin films of copper indium selenide have been fabricated by reactive pulsed laser deposition. Galvanostatic cycling measurements and CV were used to examine the electrochemical behavior of CuInSe2/Li cell. A large reversible discharge capacity of 555 mAh g− 1 of CuInSe2/Li cell was achieved. By using ex situ XRD, TEM, SAED and XPS, both classical alloying process and separated selenization/reduction of nanosized metallic In and Cu were proposed in the lithium electrochemical reaction of CuInSe2. The formation of In2Se3 and Cu2Se as final products in the charge process could be attributed to both thermodynamic and dynamic factors. CuInSe2 has high reversible capacity and stable cycle performance, which make it potential anode material for lithium ion batteries. References [1] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395. [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [3] H. Li, G. Richter, J. Maier, Adv. Mater. 15 (2003) 736. [4] N. Pereira, L. Dupont, J.M. Tarascon, L.C. Klein, G.G. Amatucci, J. Electrochem. Soc. 150 (2003) A1273. [5] D.C.S. Souza, V. Pralong, A.J. Jacobson, L.F. Nazar, Science 296 (2002) 2012. [6] F. Gillot, L. Monconduit, M.-L. Doublet, Chem. Mater. 17 (2005) 5817. [7] A. Débart, L. Dupont, R. Patrice, J.-M. Tarascon, Solid State Sci. 8 (2006) 640. [8] M.Z. Xue, J. Yao, S.C. Cheng, Z.W. Fu, J. Electrochem. Soc. 153 (2006) A270. [9] M.Z. Xue, S.C. Cheng, J. Yao, Z.W. Fu, Electrochim. Acta 51 (2006) 3287. [10] M.Z. Xue, Z.W. Fu, Electrochim. Acta 52 (2006) 988.
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Thin Solid Films 516 (2008) 8386 – 8392 www.elsevier.com/locate/tsf
Electrochemical reactivity mechanism of CuInSe2 with lithium Ming-Zhe Xue, Zheng-Wen Fu ⁎ Department of Chemistry & Laser Chemistry Institute, Shanghai Key laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai, 200433, PR China Received 24 April 2007; received in revised form 25 February 2008; accepted 7 April 2008 Available online 12 April 2008
Abstract CuInSe2 thin films were fabricated by pulsed laser deposition and its electrochemistry with lithium was investigated. The reversible discharge capacity of the CuInSe2/Li cell cycled between 0.01 V and 2.5 V was found to be in the range of 438 to 647 mAh g− 1 during the first 100 cycles. By using ex situ X-ray diffraction, transmission electron microscopy, selected-area electron diffraction measurements and X-ray photoelectron spectroscopy, both classical alloying process and separated selenization/reduction of nanosized metallic In and Cu were proposed for the lithium electrochemical reaction of CuInSe2, indicating one of the features of the lithium electrochemistry of CuInSe2. CuInSe2 has high reversible capacity and good cycle performance, which makes it potential anode material for future lithium-ion batteries. © 2008 Elsevier B.V. All rights reserved. Keywords: CuInSe2; Pulsed laser deposition; Thin film; Anode; Lithium-ion batteries
1. Introduction Since tin-based amorphous oxides and 3d transition metal oxides were reported as anode materials for lithium ion batteries [1,2], the electrochemical properties of various metal compounds, for example fluorides, nitrides, phosphides and sulfides, have been widely investigated. Li et al. studied a series of metal fluorides [3]. Their results showed that metal fluorides could also store lithium heterogeneously via phase formation as found for various oxides. Pereira et al. examined the electrochemistry of Cu3N with lithium and showed a similar reversible lithium/ copper nitride conversion process driven by Cu nanoparticles as transition metal oxides [4]. In addition, they found that oxidation of Cu metal into Cu2+ formed copper oxide, and they believed it to be associated with electrolyte degradation. Souza et al. reported on the electrochemical properties of MnP4 and supposed a topotactic first-order transition between the crystal structures of MnP4 and Li7MnP4 [5]. This mechanism was confirmed and further extended by Gillot et al. [6]. Their results showed that after the first conversion process that transforms binary MnP4 electrode into ternary Li7MnP4, a partial and irreversible decomposition of Li7MnP4 into Li3P+Mn0 was achieved and should be responsible for the poor electrodes ⁎ Corresponding author. E-mail address: [email protected] (Z.-W. Fu). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.04.050
capacity retention. Débart et al. investigated the electrochemical reactivity of some transition metal (Co, Ni, Cu) sulfides versus lithium [7]. They showed that cobalt and nickel sulfides react versus lithium through conversion reactions similarly to their homologous oxides while in contrast, the electrochemical reactivity of CuS towards Li was shown to follow a displacement reaction leading to the growth and disappearance of large copper dendrites with a concomitant reversible decomposition/recrystallization of the initial electrode material. Apparently, the lithium electrochemical reactions with metal compounds are complex. More work should be done to search new compounds and understand whether there are other mechanisms. Recently we fabricated a series of metal selenides thin films by pulsed laser deposition and found that most of them could react with lithium reversibly. Our results showed versatile lithium electrochemistry of these materials. For some lithium alloying metals, the corresponding metal selenides, for instance, SnSe, Ag2Se and ZnSe, all react with lithium according to a twosteps mechanism including both classical alloying/de-alloying processes and selenization/reduction of nanosized metal [8–10]. On the contrary, the 3d transition metal selenides showed more abundant electrochemical properties. The electrochemical reaction mechanism of α-MnSe thin films with lithium upon cycling involving the irreversible decomposition of α-MnSe and reversible formation of β-MnSe [11]. The reversible electrochemical conversion between NiSe2 and Ni+Li2Se is via two
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intermediates, β-NiSe and Ni3Se2 [12]. Both CuSe2 and CuSe reversibly react with lithium heterogeneously to form Cu3Se2 and Cu2Se while Cu2Se may undergo a “displacement” reaction mechanism with lithium [13]. However, all of these selenides are binary compounds. The legitimate question remains whether ternary metal selenides, for example, CuInSe2 could store Li reversibly. CuInSe2 is an interesting metal chalcogenide semiconductor material. Lattice parameters of bulk polycrystalline CuInSe2 material with chalcopyrite structure were reported to be a = 5.78 Å and c = 11.62 Å [14]. Due to its band gap of 1.04 eV, which is within the maximum solar absorption region, it is one of the most promising materials for creation of highefficiency polycrystalline thin film solar cells. Besides these, it is also extensively used in the area of visible, infrared lightemitting diodes, infrared detectors and optical parametric oscillators [15]. There is no available report on the possibility of CuInSe2 as anode material for lithium-ion batteries. There are many techniques currently in use for the fabrication of CuInSe2 thin films such as flash-evaporation [16], electrodeposition [17], metal organic chemical vapor deposition [18], selenization of Cu–In alloy precursors [19] and pulsed laser deposition (PLD) [20–22]. Among them, PLD technique is very simple and effective method for the deposition of thin film electrodes. In previous reports on pulsed deposition of CuInSe2 thin film, sintered CuInSe2 powders were pressed into the target. In this work, we used a mixed target consisting of Cu, In and Se powders for pulsed laser ablation and got polycrystalline CuInSe2 thin film on stainless steel substrates. The electrochemical behavior, structure, composition and morphology of CuInSe2 thin film were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS). The motivation of this work is to explore the possibility of using CuInSe2 as anode material for lithium-ion batteries and to elucidate its electrochemical reaction mechanism with lithium. 1.1. Experimental The apparatus used for pulsed laser deposition has been described elsewhere [11]. Experimental conditions for depositing thin films are described briefly as follows. A 355 nm laser beam, provided by the third harmonic frequency of a Q-switched Nd: yttrium aluminum garnet (YAG) laser (Quanta-Ray GCR150) was focused onto the surface of the target. The incident angle between the laser beam and the target surface normal was 45°. The laser energy intensity was about 2 J cm− 2. The repetition rate and pulse width of the laser was 10 Hz and 10 ns, respectively. The targets were made from Cu, In and Se powders (both pure 99.9%), they were mixed and ground in certain element molar ratios of Cu:In:Se = 1:1:2.5, then were pressed to form a 1.3 cm diameter pellet as the ablated target. An excess of Se in the mixture target can compensate Se loss derived from its vacuum sublimation during laser ablation. The base pressure of the chamber was 10− 2 Pa, and the ambient Ar gas pressure during deposition was kept at 8 Pa by a needle valve. The thin
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films were deposited on stainless steel (SS) substrates, which were kept at 200 °C. The distance between the target and substrate was 40 mm. The deposition time is fixed at 30 min. The weight of the thin film was directly obtained by subtracting the original substrate weight from total weight of the substrate and the deposited thin film onto its surface, which were examined by electrobalance (BP 211D, Sartorius), and was about 0.10 mg for an area of 1.0 cm2. The precision of the weight is ± 0.01 mg. XRD patterns and the morphology of the thin film electrodes were investigated by Bruker D8 advance diffractormeter equipped with Cu-Kα radiation (λ = 1.5406 Å) and SEM (Philips XL30 microscope) with the operating voltage of 25 kV, respectively. TEM and SAED measurements were carried out by a JEOL 2010 TEM at 160 kVaccelerating voltage. XPS measurements were performed on a Perkin-Elmer PHI 5000C ECSA system with monochromatic AlKα (1486.6 eV) irradiation. The spectra were taken with the X-ray source operating at 400 W (15 kV–27 mA). The specimens were analyzed at an electron take-off angle of 70o, measured with respect to the surface plane. The pressure of vacuum is 10− 7 Pa. No sputter-etching was performed on the sample prior to the XPS examination. To correct possible charging of the films by X-ray irradiation, the binding energy was calibrated using the C 1s (284.6 eV) spectrum of hydrocarbon that remained in the XPS analysis chamber as a contaminant. For the electrochemical measurements, the cells were constructed using the as-deposited thin films as a working electrode and a lithium sheet as a counter electrode. The electrolyte is 1 M LiPF6 in a nonaqueous solution of ethylene carbonate and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). The cells were assembled in an Ar filled glove box. Galvanostatic cycling measurements were carried out at room temperature with a Land CT 2001A battery test system. The cells were cycled between 0.01 and 2.5 V vs. Li+/Li at a current density of 5 μA cm− 2. Cyclic voltammetry (CV) tests were performed with a scanning rate of 0.1 mV s− 1 between 0.01 V and 2.5 V on CHI660A electrochemical working station (CHI Instruments, TN). In order to gain insight into the reaction mechanism of copper indium selenide with lithium, ex situ XRD, TEM, SAED and XPS measurements were carried out to reveal the structure and morphology change of CuInSe2 thin film electrodes at selected voltage points during the initial discharge and charge process. The model cells were dismantled in an Ar filled glove box and the electrodes were rinsed in anhydrous DMC to eliminate residual salts. For TEM and SAED measurements, the active materials were scratched from the SS substrate. The loose powders were then mixed with ethanol to prepare slurry, out of which one drop was taken, and deposited on a copper grid. To avoid exposure to oxygen or water, the thin films or copper grids were rapidly transferred into the chambers for cleanliness. 2. Results and discussions Fig. 1(a) shows the XRD pattern of the target used in pulsed laser deposition. Several peaks at 2θ = 23.5o, 29.7o, 33.0o, 36.3o, 39.2o, 41.2o, 43.3o, 45.2o, 50.5o, 51.6o, 54.5o, 55.7o and 56.0o
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Fig. 1. XRD patterns of (a) the target and (b) the as-deposited thin film. The peaks marked with asterisk corresponding to stainless steel substrate.
are in good accord with the diffraction peaks of copper [23], indium [24] and selenium [25]. After laser ablation of the mixed target, black-colored thin films were deposited on the SS substrate in our experimental condition. The XRD pattern of the as-deposited thin film was shown in Fig. 1(b). Apart from two diffraction peaks appearing at 2θ = 43.6o and 50.8o corresponding to the SS substrate (marked by asterisks), two peaks at 2θ = 26.5o and 44.1o could be assigned to the (112) and (204) (or (220)) reflection of the CuInSe2 [26]. The average crystallite size calculated by the Scherrer formula is estimated to be 15 nm (using the strongest peak at 26.5o). XRD data suggest that crystalline CuInSe2 thin film could be prepared by pulsed laser ablation of the mixed target of copper, indium and selenium. Fig. 2 shows the typical SEM image of the as-deposited CuInSe2 thin film. It exhibits a well-defined surface texture and is composed of small particles with an average size of about 100 nm. In addition, some particles agglomerates can be viewed in the SEM image. This phenomenon is also observed in some other metal selenide thin films and it seems to be a feature of thin film prepared by pulsed laser deposition [10–12]. The galvanostatic cycling profiles of the CuInSe2/Li cell cycled between 0.01 Vand 2.5 V under a current density of 5 μA
Fig. 2. SEM image of the as-deposited CuInSe2 thin film.
cm− 2 is shown in Fig. 3(a). The open circuit voltage (OCV) lies at 2.41 V. The first lithium insertion into CuInSe2 process is characterized by a large flat voltage plateau at 1.20 V, a large sloping plateau under 0.70 V and three small plateaus at 1.99, 1.47 and 1.31 V. These discharge plateaus can be attributed to the electrochemical reaction of the CuInSe2 thin film electrode with lithium. In the second cycle, the discharge curves differ from the first in a certain extent. The sloping plateau under 0.70 V is nearly unchanged while four higher plateaus are replaced by three sloping ones at 2.04, 1.51 and 1.18 V. The specific capacity of CuInSe2 thin film obtained as a function of the cycle number is shown in Fig. 3 (b). For comparison, the cycle performance of In thin film prepared by pulsed laser deposition is also showed. The initial discharge capacity of the as-deposited CuInSe2 thin film electrode is 647 mAh g− 1. This corresponds to 8.1 Li per CuInSe2 as shown in Fig. 3 (a). The second discharge process yields a reversible capacity of about 555 mAh g− 1, corresponding to 7.0 Li per CuInSe2. The irreversibility between the first two cycles is 14%. Afterward the cycling becomes more stable. The discharge capacity of the 100th cycle is 438 mAh g− 1, keeping 68% of the initial one. Although In thin film electrode has higher initial discharge capacity than CuInSe2, its capacity retention is poorer. After 100th cycles, the discharge capacity is only 58 mAh g− 1,
Fig. 3. (a) Galvanostatic curves of CuInSe2/Li cell and (b) the discharge capacities of CuInSe2 and In thin films as a function of cycle number.
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indicating the collapse of the crystal structure of CuInSe2 and the formation of much more crystalline In. On the other hand, we could not observe any diffraction of copper-based compounds. This may be due to the size of formed copperbased compound less than the X-ray coherence length (6 nm), which could not be identified by XRD techniques. However, combined with the charge-discharge curve of the first cycle, which shows that the intercalated number of Li is about of 4 when discharging to 0.9 V, it is reasonable to deduce that CuInSe2 has fully decomposed to form In, Cu and Li2Se as the following reaction: CuInSe2 þ 4Li→Cu þ In þ 2Li2 Se
Fig. 4. First three cyclic voltammograms for the as-deposited CuInSe2 thin film electrode.
much lower than the corresponding value of CuInSe2 thin film. Apparently, CuInSe2 can improve the reversible reaction of In with Li. Fig. 4 shows the first three cyclic voltammograms of the asdeposited CuInSe2 thin film electrode between 0.01 and 2.5 V measured at a scan rate of 0.1 mV s− 1. Six cathodic current peaks at 2.01, 1.42, 1.28, 1.16, 0.58 and nearly 0.00 V are observed in the first reduction process. Compared with the CV curves of In, In2O3 and InSb [27,28], CV curves at a low voltage range (vs. Li from 0.9 to 0 V) should mainly result from the alloying/dealloying reaction of In with Li during cycling. In the subsequent cycles, there are no obvious differences in this range. On the other hand, the profiles of the higher voltage range of 0.9 ~ –2.5 V are changed. In the second cycle, three peaks at 2.03, 1.47 and 1.14 V emerge and remain in the following cycles. These results are in good accord with the charge–discharge curves and imply the change of structure and composition of the thin film electrode after the first cycle. To determine the structural modification of CuInSe2 induced by Li uptake/removal, ex situ XRD measurements were performed upon CuInSe2 thin film electrode at various states of the cell cycled between 0.01 and 2.5 V at a constant current of 5 μA cm− 2. Fig. 5 shows XRD patterns obtained at different states of the first discharge and charge cycle. For comparison, XRD pattern of the as-deposited CuInSe2 thin film is also included (Fig. 5 (a)). After discharging to 1.35 V, the diffraction peaks of CuInSe2 only slightly shift to higher positions (from 26.5o to 26.6o and from 44.1o to 44.3o) and become broader, indicating that the reservation of the tetragonal structure (Fig. 5 (b)). Simultaneously, a new weak peak is observed at 32.9o and could be assigned to metal In [24], indicating the formation of metal In. These results imply that the discharge behavior from OCV to 1.35 V may be related to an intercalation process, in which lithium is gradually inserting into CuInSe2 structure and pulling out indium. When further discharging to 0.9 V, the diffraction peaks of CuInSe2 vanish and the peak assigned to indium becomes stronger and more keen-edged (Fig. 5 (c)),
ð1Þ
When finally discharging to 0.01 V, the peak of metal In disappears meanwhile a new peak at 21.4o is observed and could be attributed to the (311) diffraction of cubic In3Li13 [29] (Fig. 5 (d)), indicating the Li-alloying reaction of metal In. The observed slight 2θ shift toward small angle in peak position between the standard value and the experiment value is most likely due to the formation of nonstoichiometric InLix particles. Electrochemical reactions often form nonstoichiometric products [13,30]. Upon charging process, lithium is gradually extracted from the lithiated electrode. When charging to 1.0 V, diffraction peak from metal In reappears as shown in Fig. 5 (e). This means that Li+ is extruded from In–Li alloy and In framework is renewed. Finally, when charging to 2.5 V, the diffraction peak of In disappears, indicating that it takes part in the electrochemical reaction. Instead, two peaks centered at 26.8o and 44.5o emerge (Fig. 5 (f)). It is surprising to find that these two peaks could be assigned to the (111) and (220) diffraction of Cu2Se [31]. This interesting result suggests that Cu2Se is formed as the charging product. Nevertheless, although it is unambiguous that metal indium take part in the electrochemical reaction, we could not detect
Fig. 5. Ex situ XRD pattern of CuInSe2 thin film at various states during the first cycle of CuInSe2/Li cell: (a) the as-deposited; (b) discharging to 1.35 V; (c) discharging to 1.0 V; (d) discharging to 0.01 V; (e) charging to 1.0 V; (f) charging to 2.5 V. The peaks marked with asterisk corresponding to stainless steel substrate.
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techniques were utilized. The high resolution transmission electron microscopy image of the delithiated CuInSe2 thin film is shown in Fig. 6(a). The crystallites with clear and coherent stripes are observed (see the white arrow). SAED pattern in this region shows several clear rings made up of discrete spots (Fig. 6(b)). All d-spacings derived from the SAED pattern are shown in Table 1 and could be indexed to Cu2Se [31], which has been detected by XRD techniques before. TEM and SAED further confirm the existence of Cu2Se, but they also could not give any information about In-based products. This means that after charging to 2.5 V, In-based products may form an extremely amorphous phase which could not be detected by SAED. XPS measurements were performed to examine the valence change of indium in the thin film electrodes of charging of 1.0 V and charging of 2.5 V. Fig. 7(a) shows the In 3d spectra of the thin films. For calibration, the C 1 s spectra was also measured and shown in Fig. 7(b). The In 3d5/2 binding energy of the thin films of charging to 1.0 V (Fig. 7(b)-(1)) and charging to 2.5 V (Fig. 7(a)-(2)) is 443.5 and 443.8 eV, respectively. According to the previous studies, they could be attributed to In 3d5/2 binding energy for In and In2Se3 [32,33]. XPS results show that metal In is reoxidized to form In2Se3 after charging to 2.5 V.
Fig. 6. TEM image (a) and corresponding SAED pattern (b) of the delithiated CuInSe2 thin film electrode at the first charging to 2.5 V.
any crystalline In-based products by XRD techniques. This may be due to the amorphous or nanocrystalline feature of them. In order to further reveal the structure and composition of the thin film electrode after charging to 2.5 V, ex situ TEM and SAED
Table 1 d-spacings (Å) derived from SAED analysis of the delithiated CuInSe2 thin film electrode after charging to 2.5 V. JCPDS standard for Cu2Se (65-2982) is shown for reference Charging to 2.5 V Cu2Se — Fm30304 m
3.33 2.04 1.74 1.33 1.18 a = 5.77. ± 0.01
d (hkl)
int-f
3.33 (111) 2.04 (220) 1.74 (311) 1.32 (331) 1.18 (422)
447 999 161 52 213 a = 5.76
Fig. 7. (a) In 3d and (b) C 1 s XPS spectra and of the delithiated CuInSe2 thin film electrode (1) at the first charging to 1.0 V and (2) at the first charging to 2.5 V.
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Based on the above results, the electrochemical reaction mechanism of CuInSe2 with lithium involving the following steps is proposed CuInSe2 ðcrystallineÞ þ 4Li→Cu þ In þ 2Li2 Se
ð2Þ
3In þ 13Li↔In3 Li13
ð3Þ
2In þ 2Cu þ 4Li2 SeLi2 Se↔Cu2 SeðcrystallineÞ þ In2 Se3 ðamorphousÞ þ 8Li
ð4Þ
The first step occurs in the initial discharge process. It is an irreversible reaction with the collapse of pristine CuInSe2 structure and the formation of Cu, Li2Se and In. The subsequent cycling processes involve the second step. It includes the alloying/dealloying reaction of In and two selenization/reduction reaction between crystalline Cu2Se and Cu, and amorphous In2Se3 with In. The difference between electrochemically formed In2Se3 and Cu2Se and the original tetragonal CuInSe2 could cause the first discharge process different from the sequent ones. Nowadays the most common anode material of commercial lithium-ion batteries is graphite, whose theoretical capacity is only 372 mAh g− 1 [34]. Although some improvements were used to modify the carbon-based materials and the reversible capacities of around 450 mAh g− 1 are now being reached, it is not satisfactory. So many research efforts now are focused on searching new materials. In this work, we proposed a new anode material, CuInSe2, for lithium-ion batteries. Compared with graphite and other carbon-based anode materials, CuInSe2 has higher reversible capacity (of about 555 mAh g− 1), which makes it potential anode material for lithium-ion batteries. Recently we have fabricated several kinds of binary metal selenides and investigated their lithium electrochemical reaction mechanism [8–13]. Our results show that although all these compounds decomposed in the first discharge process, most of them would reform a metal selenide phase whose structure or chemical composition is similar to the original one in the charge process. Nevertheless, in this study we surprised to find that after charging to 2.5 V, In2Se3 and Cu2Se, instead of CuInSe2, are the final products. In order to explain this interesting result, we attempt to calculate the Gibbs free energies and theoretical electromotive force (EMF) values of three following reactions by Nernst equation. Free energy data of these materials at 25 °C are taken from the literature [35,36]. In þ Cu þ 2Li2 SeLi2 Se→CuInSe2 þ 4Li
ð5Þ
Cu þ 1=2Li2 SeLi2 Se→1=2Cu2 Se þ Li
ð6Þ
In þ 3=2Li2 SeLi2 Se→1=2In2 Se3 þ 3Li
ð7Þ
1=2In2 Se3 þ Cu þ 1=2Li2 SeLi2 Se→CuInSe2 þ Li
ð8Þ
1=2Cu2 Se þ In þ 3=2Li2 SeLi2 Se→CuInSe2 þ 3Li
ð9Þ
The calculated values are shown in Table 2. It is seen that reaction (7) has lower EMF value than reactions (5) and (6).
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Table 2 Gibbs free energy and EMF values of the corresponding reactions Reaction
ΔG (kJ mol− 1)
EMF (V)
(5) (6) (7) (8) (9)
630.64 169.45 458.66 171.95 461.18
1.63 1.76 1.58 1.78 1.59
These results suggest that if only considering the thermodynamic factor, Li2Se is more facile to only react with metal In first and form In2Se3. After this process, Cu and residual Li2Se will also take part in the electrochemical reaction. The possible reaction is Eqs. (6) or (8). Because the EMF values of these two reactions are very close, it is hard for us to explain the present results that only reaction (6) occurs. Then we suppose that dynamic factors may play an important role in this process, as the galvanostatic cycling is performed under room temperature, which could not give sufficient activation energy to form CuInSe2. As a matter of fact, the asdeposited CuInSe2 thin film is fabricated under 200 °C. However, the intrinsic factors are still unknown. More work should be done to explore the essence of the lithium electrochemistry of CuInSe2. 3. Conclusions Thin films of copper indium selenide have been fabricated by reactive pulsed laser deposition. Galvanostatic cycling measurements and CV were used to examine the electrochemical behavior of CuInSe2/Li cell. A large reversible discharge capacity of 555 mAh g− 1 of CuInSe2/Li cell was achieved. By using ex situ XRD, TEM, SAED and XPS, both classical alloying process and separated selenization/reduction of nanosized metallic In and Cu were proposed in the lithium electrochemical reaction of CuInSe2. The formation of In2Se3 and Cu2Se as final products in the charge process could be attributed to both thermodynamic and dynamic factors. CuInSe2 has high reversible capacity and stable cycle performance, which make it potential anode material for lithium ion batteries. References [1] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395. [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [3] H. Li, G. Richter, J. Maier, Adv. Mater. 15 (2003) 736. [4] N. Pereira, L. Dupont, J.M. Tarascon, L.C. Klein, G.G. Amatucci, J. Electrochem. Soc. 150 (2003) A1273. [5] D.C.S. Souza, V. Pralong, A.J. Jacobson, L.F. Nazar, Science 296 (2002) 2012. [6] F. Gillot, L. Monconduit, M.-L. Doublet, Chem. Mater. 17 (2005) 5817. [7] A. Débart, L. Dupont, R. Patrice, J.-M. Tarascon, Solid State Sci. 8 (2006) 640. [8] M.Z. Xue, J. Yao, S.C. Cheng, Z.W. Fu, J. Electrochem. Soc. 153 (2006) A270. [9] M.Z. Xue, S.C. Cheng, J. Yao, Z.W. Fu, Electrochim. Acta 51 (2006) 3287. [10] M.Z. Xue, Z.W. Fu, Electrochim. Acta 52 (2006) 988.
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