- Email: [email protected]
Microstructure and electrochemical properties of electron-beam deposited Sn–Cu thin film anodes for thin film lithium ion batteries
Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 3377–3385
Microstructure and electrochemical properties of electron-beam deposited Sn–Cu thin film anodes for thin film lithium ion batteries R.Z. Hu, Y. Zhang, M. Zhu ∗ School of Mechanical Engineering, South China University of Technology, Guangzhou 510640, PR China Received 13 August 2007; received in revised form 22 November 2007; accepted 26 November 2007 Available online 3 December 2007
Abstract Thin film Sn–Cu anodes with high Cu content were prepared by electron-beam evaporation deposition using Cu substrate as current collector. Annealing, with the condition being determined by DSC, was used to improve the performance of these electrodes. X-ray diffraction (XRD), scanning probe microscopy (SPM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the structure and composition of the Sn–Cu thin film electrodes. Cyclic voltammetry and galvanostatical charge–discharge measurement were carried out to characterize the electrochemical properties of the as-deposited and annealed electrodes. -Cu3 Sn intermetallic phase was formed and interface strength between deposited active materials layer and current collector was enhanced by annealing the as-deposited film under suitable condition. The annealed thin film electrode showed good cycleability and had no phase change during cycling. Although large initial capacity loss was found associated with SEI formation due to increase of surface roughness of annealed electrode, a stable discharge capacity near 300 mAh/g with Coulomb efficiency of about 96% was obtained at voltage window of 0.1–2.0 V and a discharge capacity of about 200 mAh/g and Coulomb efficiency of 97% were kept stable up to 30th cycle at a narrower voltage window of 0.2–1.5 V versus Li/Li+ . © 2007 Elsevier Ltd. All rights reserved. Keywords: Sn–Cu thin film anode; Electron-beam evaporation deposition; Electrochemical cycleability; Thin film lithium ion batteries
1. Introduction Thin-film lithium ion batteries (TFB) have been intensively studied as power sources for varieties of microelectronic devices [1–3], in which a lithium metal film has been used as the anode. However, high activity of lithium metal with air and moisture makes the fabrication process of TFB complicated and prevents it from operating under harsh environment [4,5]. Therefore, many kinds of materials, especially Sn-based compounds and composites such as Sn-based oxides and oxynitrides [6–10] have been considered as the alternative anodes. Although Sn-based oxide and oxynitride thin films show promising cycleability with high capacity, the formation of Li2 O during the first cycle causes a large initial irreversible capacity loss. Another way of improving the capacity and cycleability of Sn-based materials is to form metallic phases and/or intermetallic compounds with the metal elements which are inactive against Li insertion/extraction. This metal element can act as the matrix to buffer ∗
Corresponding author. Tel.: +86 20 87113924; fax: +86 20 87111317. E-mail address: [email protected] (M. Zhu).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.11.064
the volume expansion related to insertion/extraction of lithium, and thereby, overcome the mechanical failure of the electrode. Kim et al. [11] reported that the structure of Ni3 Sn2 film electrode prepared by electron-beam evaporation was unchanged upon lithium insertion/extraction even after 500 cycles. Sn–Cu alloy, especially Cu6 Sn5 intermetallic phase, has been considered as a promising alternative anode material for conventional lithium ion battery. Various preparation methods, including mechanical alloying and reductive precipitation and so on, have been used to prepare Sn–Cu alloys containing mainly Cu6 Sn5 phase [12–23]. By in situ XRD study, Dahn and co-workers [12] found that Cu6 Sn5 experienced the following two stages of reversible transitions upon lithium insertion/ extraction: Cu6 Sn5 ↔ Li2 CuSn
and
Li2 CuSn ↔ Li4.4 Sn + Cu,
It was also reported that capacity retention could be improved by increasing the ratio of Cu/Sn to be higher than that of Cu6 Sn5 in Sn–Cu alloys [15,20], because the excess Cu element can act as the buffer and “matrix glue” for the active material particles in the electrodes. According to the phase diagram of Cu–Sn, Cu3 Sn
3378
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
phase is the phase with Cu/Sn ratio higher than Cu6 Sn5 . However, there is little information on the electrochemical properties of Cu3 Sn phase as negative electrode material yet. It should be pointed out that the preparation method was found to have significant effect on their electrochemical performances besides the phase compositions of Sn–Cu alloys. E-beam evaporation deposition (EBD) has been demonstrated to be a suitable method to fabricate thin film anode for TFB. The thin films prepared by EBD can have unique structure and show interesting electrochemical properties [11,24–26]. In this work, bi-layer Sn–Cu thin films (Sn–Cu/Cu) were deposited by EBD and heat-treated to promote the formation of Cu3 Sn phase and improve their electrochemical characteristics. The effects of the heat-treatment and cut-off potentials on the cycling behavior, and failure mechanism of these thin film electrodes were studied.
trodes were made after the samples were washed in acetone and anhydrous alcohol to remove the electrolyte. Electrochemical measurements were carried out using threeelectrode cells. The counter and reference electrodes were lithium metal foils, and the working electrodes were Sn–Cu film of 10 mm × 10 mm in area. The electrolyte was 1 M LiPF6 in EC + DEC + EMC (1:1:1, v/v/v) (Guangzhou Tinci High-Tech Material Co. Ltd). The cells were assembled and measured in an argon-filled glove box. The amount of the active material was determined by weighting the substrate before and after evaporation deposition using analytical balance. Cyclic voltammetry was performed at 0.5 mV/s between 2.0 and 0.1 V. Charge–discharge cyclic performances of the cells were measured at cut-off potentials of 0.1–2.0 and 0.2–1.5 V, respectively, with a current density of 100 A/cm2 at room temperature. All the electrochemical measurements were carried out on an Auto Lab Electrochemical System (ECO Chemie).
2. Experimental The Sn–Cu thin films were deposited on the Cu foil of about 10 m in thickness by an electron-beam evaporator with two source units. The Cu and Sn with purity of 99.9% were kept in graphite crucibles and evaporated by electron beam, respectively. The base vacuum of the evaporation chamber was below 2.0 × 10−3 Pa. The Cu foil substrate was cleaned successively by dilute hydrochloric solution, distilled water and anhydrous alcohol. Because the evaporation rates of Cu and Sn are different, a Cu layer film was first deposited on the current collector and then a Sn–Cu thin film was co-deposited to gain sufficient Cu content in the deposited thin film. In addition, the pre-deposited Cu layer would be favor for the interdiffusion between Cu and Sn during heating in comparing with the Cu substrate because the deposited Cu has high density of defects such as crystal defects, microvoids and microcracks. For film deposition, the relative amount of Cu and Sn in thin films was controlled by adjusting the evaporation rate from each single source. The film thickness was controlled by the input power and evaporation time and measured using the quartz crystal oscillation that installed in the deposition chamber. The thickness was about 1 m of the as-deposited Sn–Cu thin film used for electrode properties measurement. A thicker film, with thickness of 4–6 m, was also prepared for the purpose of investigating the microstructure and composition variation along the cross-section of the film during heating. DSC heating scan was carried out to reveal phase transformation process of the as-deposited Sn–Cu thin film in heating process using a Pyris Diamond DSC (PerkinElmer, USA). The scanning was carried out from 100 to 400 ◦ C at heating rate of 10 ◦ C/min under N2 protection. Phase structure, morphology and composition of the thin films experienced different preparing conditions and electrochemical cycling were characterized by XRD, SEM and EDX analyses using a Philips x’pert MPD with Cu K␣ radiation, a LEO 1530 VP FE-SEM and EDX accessory (INCA300) attached to SEM. Surface roughness of thin films were analyzed by SPM using a SPA-300HV. Cross-sectional samples were prepared by burying the films in epoxy resin. SEM and XRD analysis of electrochemical measured thin film elec-
3. Results and discussion 3.1. The effects of heat treatment on the structure of Sn–Cu thin film electrodes Fig. 1 shows the DSC heating scan curve of the as-deposited thin film. It can be observed that an endothermic peak appears at 218.4 ◦ C, which should be induced by both the melting of Sn and eutectic reaction of Sn and Sn–Cu phases. However, the eutectic temperature was depressed owing to the small Sn grains and microcrystals in the E-beam deposited film. As the heat treatment conditions for the Sn–Cu electrodes in references were quite different [13,18,19,21,22], the as-deposited Sn–Cu thin films were annealed at 185, 200 and 220 ◦ C for 10 h, respectively, and then analyzed by XRD to obtain optimal heat treatment condition. Fig. 2 gives the XRD patterns of the as-deposited thin film and thin films annealed at different temperature. Diffractogram (a) in Fig. 2 shows that the as-deposited thin film electrode is
Fig. 1. DSC heating scan curve of the as-deposited Sn–Cu thin film electrode, the heating rate is 10 ◦ C/min.
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
3379
Fig. 2. XRD patterns of Sn–Cu thin film electrodes annealed under different conditions: (a) as-deposited; (b) 185 ◦ C for 10 h; (c) 200 ◦ C for 10 h; (d) 220 ◦ C for 10 h.
mainly composed of Sn and Cu phase in addition to the existence of a small amount of Cu6 Sn5 phase. The formation of Cu6 Sn5 phase should be due to partially alloying between Cu, including Cu co-deposited with Sn and pre-deposited Cu layer, and Sn during evaporation because the vaporized particles has high energy. After annealing at 185 ◦ C for 10 h, as shown in Fig. 2b, no peak of Sn could be detected and -Cu3 Sn phase (orthorhombic structure, space group: Cmcm) was formed, but the peak intensity of Cu3 Sn was quite weak. When the film was annealed at 200 ◦ C, as shown in Fig. 2c, both Cu6 Sn5 and Cu3 Sn phases could be distinctly observed. When the annealing temperature was 220 ◦ C, the peaks of Cu6 Sn5 disappeared and peaks of Cu3 Sn phase weakened while strong peaks of Cu appeared. Considering that the Cu3 Sn phase was mainly interested in this investigation, and multi-phase materials show better electrochemical performance than the single-phase materials [27], annealing at 200 ◦ C for 10 h was selected as the optimal heat treatment parameter for Sn–Cu thin film electrodes. Fig. 3a and b shows the plane-view surface morphology of the thin films. The surface of the as-deposited film, shown in Fig. 3a was composed of evaporated particles, which were mainly the uniform crystallite of Sn. The cross-sectional image, given in the inset of the figure, shows that deposited layer has columnar structure along growth direction and its thickness is about 1 m. After annealing at 200 ◦ C, as shown in Fig. 3b, the crystallite became smaller comparing with the as-deposited film. The surface roughness of thin film electrode was also dramatically increased after heat treatment, which was revealed by the AFM images shown in Fig. 4. The roughness (Img. Rms) was about 62 nm and 105 nm for the as-deposited film and annealed film, respectively. Compositions of the thin films were determined by EDX analysis on the plane-view film and the results are listed in Table 1. The Cu/Sn ratio of the as deposited film was 2.7/1, and it changed to 4.5/1 after annealing at 200 ◦ C for 10 h. The Cu/Sn ratio of 2.7/1 is less than the stoichiometry of Cu3 Sn while
Fig. 3. SEM images of the surface morphology of the thin film electrodes: (a) as-deposited and (b) after heated at 200 ◦ C for 10 h; the insets in the figures show the unpolishing cross-section of electrodes.
the ratio of 4.5/1 is higher than the stoichiometry of the Cu3 Sn. The increased Cu content in the film should be provided by the pre-deposited Cu layer. Based on the structure, morphology and composition analysis described above, it can be concluded that the initial mixture of Cu and Sn phases in the as-deposited thin film were transformed to Cu3 Sn and Cu6 Sn5 phase and the crystalline grains of them are smaller than those of original Sn phase. In order to investigate the difference in the interface region between the deposited layer and the current collector before and after annealing, a thick film with thickness of 4–6 m was prepared for cross-sectional analysis. As it is shown in Fig. 5a, two layers, which are Sn and Cu as identified from BSE image and EDX analysis, can be clearly observed in the cross-section. It should be pointed out, however, that the deposited Cu layer cannot be distinguished from the Cu substrate in the BSE image Table 1 The compositions of Sn–Cu thin film anodes determined by EDX analysis on the plane view surface Atomic%
As-deposited film Annealed film
Weight%
Cu
Sn
Cu/Sn
Cu
Sn
73.39 81.72
26.61 18.28
2.7/1 4.5/1
59.62 70.53
40.38 29.47
3380
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
Fig. 4. AFM images (20 m × 20 m) and roughness analysis of Cu–Sn thin film electrode: (a) as-deposited and (b) after heated at 200 ◦ C for 10 h.
Fig. 5. BSE images of cross-section of Sn–Cu films and EDX line-scanning profile along the cross-section: (a) as-deposited and (b) annealed at 200 ◦ C for 10 h.
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
3381
Fig. 6. Cycle performance of the as-deposited thin film electrode. Electrode potential range, 0.1–2.0 V; current density, 100 A/cm2 .
because their contrasts are same. It has been found that the deposited layer contains some pores, which indicates that the as-deposited layer was not fully dense. For the annealed sample, as shown in Fig. 5b, there are two intermediate layers formed mainly on the deposited layer in addition to the Sn and Cu layer. This is clearly shown by the contrast change stepwise from dark to light gray from the substrate to the surface. These results proved that Sn and Cu interdiffused and formed Cu3 Sn or Cu6 Sn5 layer from Cu side to Sn side. Thus, the interface binding strength between active materials layer and current collector would be enhanced by annealing. This would be beneficial for the cycleability of this film anode. 3.2. Electrochemical properties of the thin film electrodes Fig. 6 shows the cycle performance of the charge–discharge capacity and Coulomb efficiency of the as-deposited Sn–Cu thin film anode. The initial discharge capacity is 756 mAh/g, which is 76% of the theoretical limit of Sn metal (991 mAh/g). It decreases sharply to 210.7 mAh/g in the following ten cycles, and could only keep 84.8 mAh/g in the 25th cycle. These results show that the cycle performance of the as-deposited thin film anode is very poor. The Coulomb efficiency of the anode is about 84% in the first cycle and increases to 91.6% in the 10th cycle. It rises to 96.5% in 24th cycle. These results show that the stability of the electrochemical properties in the interface between anode and electrolyte increases with the proceeding of cycling, which improves the reversibility of the charge and discharge of the anode. The cycle performances of annealed Sn–Cu thin film anodes cycled under different potential regions, which were 0.1–2.0 and 0.2–1.5 V, respectively, are shown in Fig. 7. When the anode annealed after 200 ◦ C was cycled between 0.1 and 2.0 V, as shown in Fig. 7a, the initial discharge capacity was 731 mAh/g, just 25 mAh/g less than the unannealed one. However, it decreased sharply to 462.6 mAh/g in the 2nd cycle and the Coulomb efficiency of 1st cycle was just 61%, which indicated large initial irreversible capacity. The discharge capacity decreased gradually to 253 mAh/g after 25 cycles. The annealed
Fig. 7. Cycle performance of the annealed thin film electrodes. Anodes annealed at 200 ◦ C were cycled among potential ranges of 0.1–2.0 V (a) and 0.2–1.5 V (c) and anodes annealed at 185 and 220 ◦ C were cycled between 0.1 and 2.0 V (b). Current density: 100 A/cm2 .
electrode lost about 1.8% discharge capacity in each cycle. The Coulomb efficiency increased to 91.3% in 2nd cycle, and remained over 96% till 25th cycle. In order to investigate the influences of annealing conditions for the cycle performance of anodes, the films that annealed after 185 and 220 ◦ C were also electrochemically characterized, respectively, and the obtained charge/discharge performances of them were shown in Fig. 7b. It was found that the capacity retention of film anodes annealed at 185 and 220 ◦ C could be also improved dramatically compared with that of the as-deposited film anode, which shown in Fig. 6. However, all these annealed thin film anodes existed large initial irreversible capacity and showed low Coulomb efficiencies in the first cycle. These results indicated that the cycle performance of the evaporation deposited Sn–Cu thin film anode had been improved by heat treatment. It is assumed that the improvement is owing to the facts that the binding strength between active materials layer and the current collector was enhanced by heat treatment. In addition, the intermetallic compounds of Cu3 Sn and Cu6 Sn5 formed in the thin films should reduce volume change during lithium insertion and extraction.
3382
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
With respective to the large initial capacity loss, it was possible induced by the large changes, especially those of morphology and/or composition, on the surfaces of thin film during annealing. The cycle performance of annealed anode cycled in the range from 0.2 to 1.5 V is shown in Fig. 7c. It shows that the charge/discharge capacity is lower than that of the electrode cycled at cut-off potential of 0.1–2.0 V, and the initial discharge capacity is 497 mAh/g, which is 240 mAh/g less than that shown in Fig. 7a. Although there is a 260 mAh/g irreversible loss of capacity in the first cycle, however, discharge capacity of about 200 mAh/g and Coulomb efficiency of 97% are kept stable up to 30th cycle. The capacity loss in each cycle is less than 1 mAh/g. The above result demonstrated that the cycle performance of thin film anode at lower cut-off potential was better than that at higher cut-off potential. This is in agreement with the results reported in previous work [15,20,23]. However, the capacities and Coulomb efficiencies of the film electrode prepared by EBD in the present work were higher than those Sn–Cu alloy electrodes with similar compositions prepared by mechanical alloying [13] and electrodeposition methods [20]. Fig. 8 shows the charge/discharge curves of the first four cycles of the as-deposited Sn–Cu thin film electrode in cut off potential window of 0.1–2.0 V. It is clear from the curve in Fig. 8 that the as-deposited film exhibits three plateaus on discharge (lithiation), which is quite different from the charge/discharge curves of the electrodeposited Sn–Cu film electrodes [13,19,28]. The potentials of the plateaus are about 0.67, 0.55 and 0.45 V, respectively, which are close to the plateau potential for formation of LiSn, Li7 Sn3 , Li5Sn2 and Li13 Sn5 phase at room temperature [29]. There are also three plateaus in the charge (delithiation) curves, which are 0.56, 0.70 and 0.77 V, respectively, and should correspond to the delithiation potentials of those Li–Sn phases mentioned above. The initial four charge/discharge curves of the annealed Sn–Cu thin film electrode, as shown in Fig. 9, are quite different from those of the as-deposited film electrode. There are two plateaus on the discharge curve of the first cycle, which dis-
Fig. 8. Initial charge–discharge curves of the as-deposited Sn–Cu thin film electrode cycled between 0.1 and 2.0 V vs. Li/Li+ .
Fig. 9. Initial charge–discharge curves of the annealed Sn–Cu thin film electrode cycled between 0.1 and 2.0 V vs. Li/Li+ .
Fig. 10. Cyclic voltammograms of Sn–Cu thin film electrodes: (a and b) are unannealed and annealed film in LiPF6 (1 M)/EC + DEC + EMC (1:1:1, v/v/v), respectively. Scan rate: 0.5 mV/s. The inset in (a) shows magnified voltammograms in the range of 1.35–1.60 V.
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
appeared in the following cycles. The potential ranges of the two slope plateaus are 1.3–1.2 and 0.9–0.8 V, respectively. They are higher than the potentials of lithium ion insert into Sn [28,29] or Cu6 Sn5 [12,30], but in the potential region where electrolyte reduction and subsequent SEI formation take place [27]. Thus, these plateau regions are related to the formation of SEI by catalytic decomposition of the electrolyte on the annealed thin film electrode, which can consume a lot of electrochemical active material and, consequently, results in large capacity loss capacity in the first cycle [11,28]. They are also possibly associated with the oxidation of the electrode during annealing, but no oxygen or oxide was detected by EDX and XRD analyses. The reason for the large irreversible capacity of the annealed electrode will be discussed later in detail. Fig. 10a and b shows the cyclic voltammograms at 0.5 mV/s of the as-deposited and annealed Sn–Cu thin film anode, respectively. With respect to the as-deposited film, two cathodic peaks at 0.33 and 0.64 V can be observed, while three anodic peaks can be observed at the potentials of 0.64, 0.73 and 0.82 V, respectively. All the cathodic peaks and anodic peaks correspond to lithium alloying and de-alloying with the active materials, respectively. Besides those reversible peaks, as shown in the inset of Fig. 10a, an irreversible peak, which is most probably related to SEI formation, appears in the first two cycles in the range of 1.4–1.55 V. With respect to annealed thin film electrode, two cathodic peaks at the potential range of 0.1–0.5 V and two anodic peaks
3383
at 0.4–0.7 V were observed. They were reversible. The cathodic peaks correspond to inserting of Li into Cu3 Sn and Cu6 Sn5 forming Lix Cu3 Sn and Lix Cu6 Sn5 phases [12,17]. To the contrary, the anodic peaks are attributed to the delithiation of the Li–Sn–Cu phases. It should be noted that there are two irreversible peaks, as indicated by the arrows in Fig. 10b, during cathodic sweep at the first cycle, and the potential ranges are 0.8–0.9 and 1.2–1.3 V, respectively, which are in good agreement with the potential plateaus observed in Fig. 9. It can also be seen that the irreversible peaks only appear in the first cycle and disappear in following cycles. This indicates that the surface of the electrode became stable and solvent decomposition decreased significantly after formation of SEI layer in the first cycle, which resulted in a decrease in capacity loss and improvement in the cycleability of the electrode as shown in Figs. 9 and 7. However, it was worth to note that there was an initial irreversible capacity of about 260 mAh/g for the annealed film electrode while only about 80 mAh/g for the as-deposited electrode, which indicated that annealing aggravate initial capacity lose of the thin film electrode. This was probably due to the refining of crystalline grain and increase of surface roughness of the film after it had been annealed at 200 ◦ C for 10 h (see in Figs. 3 and 4). As well known, the SEI film was apt to be formed as the surface roughness of electrode increasing. Therefore, the high surface roughness and fine crystal grain of thin film would bring high interface area for reaction with the electrolyte and would aggravate the formation of SEI film.
Fig. 11. SEM images of electrodes after cycling: (a) as-deposited electrode after 20 cycles and (b) annealed electrode after 25 cycles. (a2 and b2 ) are enlarged images of the electrode (a) and (b), respectively.
3384
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
Table 2 The EDX analysis results in different position of Sn–Cu thin film anodes after cycling Atomic% Cu
Weight% Sn
Cu/Sn
Cu
Sn
As-deposited electrode after 20 cycles Spectrum 1 90.35 9.65 Spectrum 2 52.57 47.28 Spectrum 3 97.71 1.29
9.4/1 1.12/1 76.5/1
83.36 37.38 97.61
16.64 62.62 2.39
Annealed electrode after 25 cycles Spectrum 1 80.50 19.50 Spectrum 2 78.18 21.82
4.13/1 3.58/1
68.84 65.76
31.16 34.24
The selected areas of spectrums are shown in Fig. 10a and b.
3.3. Structure change of thin film anodes during charge–discharge cycling Fig. 11a and b are SEM images of the as-deposited and annealed thin film electrodes after cycling measurement, respectively. As it is shown in Fig. 11a, the as-deposited thin film appeared to be pulverized after 20 cycles. Comparing with Fig. 3a, it can be found that the deposited Sn–Cu layer had been damaged and the active materials particles agglomerated, which resulted in nonuniform surface of the electrode. The EDX results listed in Table 2 show that area 2 (i.e. spectrum 2 in Fig. 11a) mainly contains Sn, while area 3 (i.e. spectrum 3 in Fig. 11a) is almost Cu. The Cu/Sn ratio of the as-deposited electrode after 20 cycles reached to 9.4/1, which is about 3 times of that before cycling, being 2.7/1 as given in Table 1. These results indicate that part of the Sn had been peeled off from the Cu foil, which was resulted from large volume change and
phase transition in the Sn–Cu thin film anode. Thus, the discharge capacity of the as-deposited electrode decayed rapidly, and exhibited poor cycleability as shown in Fig. 6. After annealing, however, the formation Cu3 Sn and Cu6 Sn5 intermetallic phases in the electrode could relax the volume expansion and inhibit the cracking generation caused by delithiation and lithiation. Therefore, structure stability and cycle performance of the electrode was improved. As shown in Fig. 11b, no obvious pulverization and cracks can be observed on the annealed thin film electrode after 25 cycles. Comparing with the SEM image in Fig. 3b, uniform crystalline grains of Sn–Cu alloys and smaller grains can be seen after cycling. These might be related to the protection of the SEI layer formed in the first discharge cycle and recrystallization of Sn–Cu alloy phases during cycling. The EDX results listed in Table 2 indicate that the Cu/Sn ratio of the annealed electrode did not increase after 25 cycles. They are 4.13/1 and 3.58/1 in spectrum 1 and spectrum 2 labeled in Fig. 11b, respectively, which is close to Cu/Sn ratio, being 4.5/1, of the film before cycling. However, it was possible that the Sn of inner layer was gradually diffused to the surface layer of the film, and resulted in a bit decreasing of Cu/Sn ratio on the surface of annealed film electrode after cycling. To prove some of the above considerations further, the film electrodes were characterized by XRD after charge–discharge cycling. Fig. 12a and b shows the XRD patterns obtained for unannealed electrode after 20 cycles (charge) and annealed electrode after 25 cycles (charge), respectively. In comparing with the diffractograms of both electrodes before cycling (also shown in Fig. 12), no new peaks have been observed on the diffractograms in both electrodes undergone cycling. The unannealed electrode mainly contains Sn, while the annealed electrode is composed of Cu3 Sn and Cu6 Sn5 except peak from Cu foil. It
Fig. 12. XRD patterns of Sn–Cu thin film electrodes after cycling: (a) unannealed electrode after 20 cycles and (b) annealed electrode after 25 cycles.
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
means that the annealed thin film anode remained stable and did not undergo obvious phase change during cycling. 4. Conclusion Sn–Cu anode for thin film lithium ion batteries was prepared by depositing Sn–Cu thin film of high Cu content on Cu foil with method of electron-beam evaporation. An endothermic peak at 218.4 ◦ C was detected in DSC heating scan curve for the as-deposited thin film. After the film was heated at 200 ◦ C for 10 h, it was composed of -Cu3 Sn and a small mount of Cu6 Sn5 intermetallic phases, and the interface strength between active materials layer and current collector was remarkably improved. A stable discharge capacity of more than 250 mAh/g was obtained at voltage window of 0.1–2.0 V, and a discharge capacity of about 200 mAh/g was kept stable up to 30th cycle at a narrower potential window of 0.2–1.5 V with a current density of 100 m/cm2 . The reversible capacities, Coulomb efficiencies and cycleabiltiy of the film prepared by the present method were higher than those of the high Cu content Sn–Cu alloy electrodes prepared by mechanical alloying and electrodeposition methods. It was found that large irreversible discharge capacity was associated with SEI formation due to increase of surface roughness of annealed film electrode. The annealed thin film anode remained stable without obvious phase changing and crack forming up to 25 cycles. Acknowledgements This research was supported by Ministry of Education under project No. IRT0551 and Guangdong Provincial Natural Science Foundation under the Team Project. References [1] J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans, Solid State Ionics 135 (2000) 33. [2] J.L. Souquet, M. Duclot, Solid State Ionics 148 (2002) 375. [3] N.J. Dudney, Mater. Sci. Eng. B 116 (2005) 245.
3385
[4] B.J. Neudecker, R.A. Zuhr, J.B. Bates, J. Power Sources 81 (1999) 27. [5] B.J. Neudecker, N.J. Dudney, J.B. Bates, J. Electrochem. Soc. 147 (2000) 517. [6] T. Brousse, R. Retoux, U. Herterich, D.M. Schleich, J. Electrochem. Soc. 145 (1998) 1. [7] F. Ding, Z. Fu, M. Zhou, Q. Qin, J. Electrochem. Soc. 146 (1999) 3554. [8] C. Branci, N. Benjelloun, J. Sarradin, M. Ribes, Solid State Ionics 135 (2000) 169. [9] K.S. Parka, Y.J. Parkb, M.K. Kima, J.T. Sona, H.G. Kima, S.J. Kim, J. Power Sources 103 (2001) 67. [10] Y.I. Kim, C.S. Yoon, J.W. Park, J. Solid State Chem. 160 (2001) 388. [11] Y.L. Kim, H.Y. Lee, S.W. Jang, S.J. Lee, H.K. Baik, Y.S. Yoon, Y.S. Park, S.M. Lee, Solid State Ionics 160 (2003) 235. [12] D. Larcher, L.Y. Beaulieu, D.D. Macneil, J.R. Dahn, J. Electrochem. Soc. 147 (2000) 1658. [13] N. Tamura, R. Ohshita, M. Fujimota, S. Fujitani, M. Kamino, I. Yonezu, J. Power Sources 107 (2002) 48. [14] G.X. Wang, L. Sun, D.H. Bradhurst, S.X. Dou, H.K. Liu, J. Alloys Compd. 299 (2000) L12. [15] Y.Y. Xia, T. Sakai, T. Fujiedu, M. Wada, H. Yoshinaga, J. Electrochem. Soc. 148 (2001) A471. [16] J. Wolfenstine, S. Campos, D. Foster, J. Read, W.K. Behl, J. Power Sources 109 (2002) 230. [17] D.G. Kim, H. Kim, H.J. Sohn, T. Kang, J. Power Sources 104 (2002) 221. [18] N. Tamura, R. Ohshita, M. Fujimoto, M. Kamino, S. Fujitani, J. Electrochem. Soc. 150 (2003) A679. [19] H. Morimoto, S. Tobishima, H. Negishi, J. Power Sources 146 (2005) 469. [20] S.D. Beattie, J.R. Dahn, J. Electrochem. Soc. 150 (2003) A894. [21] W.H. Pu, X.M. He, J.G. Ren, C.R. Wan, C.Y. Jiang, Electrochim. Acta 50 (2005) 4140. [22] F.S. Ke, L. Huang, J.S. Cai, S.G. Sun, Electrochim. Acta 52 (2007) 6741. [23] L. Wamg, S. Kitamura, K. Obata, S. Tanase, T. Saki, J. Power Sources 141 (2005) 286. [24] Y.L. Kim, H.Y. Lee, S.W. Jang, S.H. Lim, S.J. Lee, H.K. Baik, Y.S. Yoon, S.M. Lee, Electrochim. Acta 48 (2003) 2593. [25] J.B. Kim, B.S. Jun, S.M. Lee, Electrochim. Acta 50 (2005) 3390. [26] J.B. Kim, H.Y. Lee, K.S. Lee, S.H. Lim, S.M. Lee, Electrochem. Commun. 5 (2003) 544. [27] M. Winter, W.K. Appel, B. Evers, T. Hodal, K.C. M¨oller, I. Schneider, M. Wachtler, M.R. Wagner, G.H. Wrodning, J.O. Besenhard, Monatshefite f¨ur Chemie 132 (2001) 473. [28] M. Inaba, T. Uno, A. Tasaka, J. Power Sources 146 (2005) 473. [29] M. Winter, J.O. Besenhard, Electrochim. Acta 45 (1999) 31. [30] J.J. Zhang, Y.M. Zhang, X. Zhang, Y.Y. Xia, J. Power Sources 167 (2007) 171.
Electrochimica Acta 53 (2008) 3377–3385
Microstructure and electrochemical properties of electron-beam deposited Sn–Cu thin film anodes for thin film lithium ion batteries R.Z. Hu, Y. Zhang, M. Zhu ∗ School of Mechanical Engineering, South China University of Technology, Guangzhou 510640, PR China Received 13 August 2007; received in revised form 22 November 2007; accepted 26 November 2007 Available online 3 December 2007
Abstract Thin film Sn–Cu anodes with high Cu content were prepared by electron-beam evaporation deposition using Cu substrate as current collector. Annealing, with the condition being determined by DSC, was used to improve the performance of these electrodes. X-ray diffraction (XRD), scanning probe microscopy (SPM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the structure and composition of the Sn–Cu thin film electrodes. Cyclic voltammetry and galvanostatical charge–discharge measurement were carried out to characterize the electrochemical properties of the as-deposited and annealed electrodes. -Cu3 Sn intermetallic phase was formed and interface strength between deposited active materials layer and current collector was enhanced by annealing the as-deposited film under suitable condition. The annealed thin film electrode showed good cycleability and had no phase change during cycling. Although large initial capacity loss was found associated with SEI formation due to increase of surface roughness of annealed electrode, a stable discharge capacity near 300 mAh/g with Coulomb efficiency of about 96% was obtained at voltage window of 0.1–2.0 V and a discharge capacity of about 200 mAh/g and Coulomb efficiency of 97% were kept stable up to 30th cycle at a narrower voltage window of 0.2–1.5 V versus Li/Li+ . © 2007 Elsevier Ltd. All rights reserved. Keywords: Sn–Cu thin film anode; Electron-beam evaporation deposition; Electrochemical cycleability; Thin film lithium ion batteries
1. Introduction Thin-film lithium ion batteries (TFB) have been intensively studied as power sources for varieties of microelectronic devices [1–3], in which a lithium metal film has been used as the anode. However, high activity of lithium metal with air and moisture makes the fabrication process of TFB complicated and prevents it from operating under harsh environment [4,5]. Therefore, many kinds of materials, especially Sn-based compounds and composites such as Sn-based oxides and oxynitrides [6–10] have been considered as the alternative anodes. Although Sn-based oxide and oxynitride thin films show promising cycleability with high capacity, the formation of Li2 O during the first cycle causes a large initial irreversible capacity loss. Another way of improving the capacity and cycleability of Sn-based materials is to form metallic phases and/or intermetallic compounds with the metal elements which are inactive against Li insertion/extraction. This metal element can act as the matrix to buffer ∗
Corresponding author. Tel.: +86 20 87113924; fax: +86 20 87111317. E-mail address: [email protected] (M. Zhu).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.11.064
the volume expansion related to insertion/extraction of lithium, and thereby, overcome the mechanical failure of the electrode. Kim et al. [11] reported that the structure of Ni3 Sn2 film electrode prepared by electron-beam evaporation was unchanged upon lithium insertion/extraction even after 500 cycles. Sn–Cu alloy, especially Cu6 Sn5 intermetallic phase, has been considered as a promising alternative anode material for conventional lithium ion battery. Various preparation methods, including mechanical alloying and reductive precipitation and so on, have been used to prepare Sn–Cu alloys containing mainly Cu6 Sn5 phase [12–23]. By in situ XRD study, Dahn and co-workers [12] found that Cu6 Sn5 experienced the following two stages of reversible transitions upon lithium insertion/ extraction: Cu6 Sn5 ↔ Li2 CuSn
and
Li2 CuSn ↔ Li4.4 Sn + Cu,
It was also reported that capacity retention could be improved by increasing the ratio of Cu/Sn to be higher than that of Cu6 Sn5 in Sn–Cu alloys [15,20], because the excess Cu element can act as the buffer and “matrix glue” for the active material particles in the electrodes. According to the phase diagram of Cu–Sn, Cu3 Sn
3378
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
phase is the phase with Cu/Sn ratio higher than Cu6 Sn5 . However, there is little information on the electrochemical properties of Cu3 Sn phase as negative electrode material yet. It should be pointed out that the preparation method was found to have significant effect on their electrochemical performances besides the phase compositions of Sn–Cu alloys. E-beam evaporation deposition (EBD) has been demonstrated to be a suitable method to fabricate thin film anode for TFB. The thin films prepared by EBD can have unique structure and show interesting electrochemical properties [11,24–26]. In this work, bi-layer Sn–Cu thin films (Sn–Cu/Cu) were deposited by EBD and heat-treated to promote the formation of Cu3 Sn phase and improve their electrochemical characteristics. The effects of the heat-treatment and cut-off potentials on the cycling behavior, and failure mechanism of these thin film electrodes were studied.
trodes were made after the samples were washed in acetone and anhydrous alcohol to remove the electrolyte. Electrochemical measurements were carried out using threeelectrode cells. The counter and reference electrodes were lithium metal foils, and the working electrodes were Sn–Cu film of 10 mm × 10 mm in area. The electrolyte was 1 M LiPF6 in EC + DEC + EMC (1:1:1, v/v/v) (Guangzhou Tinci High-Tech Material Co. Ltd). The cells were assembled and measured in an argon-filled glove box. The amount of the active material was determined by weighting the substrate before and after evaporation deposition using analytical balance. Cyclic voltammetry was performed at 0.5 mV/s between 2.0 and 0.1 V. Charge–discharge cyclic performances of the cells were measured at cut-off potentials of 0.1–2.0 and 0.2–1.5 V, respectively, with a current density of 100 A/cm2 at room temperature. All the electrochemical measurements were carried out on an Auto Lab Electrochemical System (ECO Chemie).
2. Experimental The Sn–Cu thin films were deposited on the Cu foil of about 10 m in thickness by an electron-beam evaporator with two source units. The Cu and Sn with purity of 99.9% were kept in graphite crucibles and evaporated by electron beam, respectively. The base vacuum of the evaporation chamber was below 2.0 × 10−3 Pa. The Cu foil substrate was cleaned successively by dilute hydrochloric solution, distilled water and anhydrous alcohol. Because the evaporation rates of Cu and Sn are different, a Cu layer film was first deposited on the current collector and then a Sn–Cu thin film was co-deposited to gain sufficient Cu content in the deposited thin film. In addition, the pre-deposited Cu layer would be favor for the interdiffusion between Cu and Sn during heating in comparing with the Cu substrate because the deposited Cu has high density of defects such as crystal defects, microvoids and microcracks. For film deposition, the relative amount of Cu and Sn in thin films was controlled by adjusting the evaporation rate from each single source. The film thickness was controlled by the input power and evaporation time and measured using the quartz crystal oscillation that installed in the deposition chamber. The thickness was about 1 m of the as-deposited Sn–Cu thin film used for electrode properties measurement. A thicker film, with thickness of 4–6 m, was also prepared for the purpose of investigating the microstructure and composition variation along the cross-section of the film during heating. DSC heating scan was carried out to reveal phase transformation process of the as-deposited Sn–Cu thin film in heating process using a Pyris Diamond DSC (PerkinElmer, USA). The scanning was carried out from 100 to 400 ◦ C at heating rate of 10 ◦ C/min under N2 protection. Phase structure, morphology and composition of the thin films experienced different preparing conditions and electrochemical cycling were characterized by XRD, SEM and EDX analyses using a Philips x’pert MPD with Cu K␣ radiation, a LEO 1530 VP FE-SEM and EDX accessory (INCA300) attached to SEM. Surface roughness of thin films were analyzed by SPM using a SPA-300HV. Cross-sectional samples were prepared by burying the films in epoxy resin. SEM and XRD analysis of electrochemical measured thin film elec-
3. Results and discussion 3.1. The effects of heat treatment on the structure of Sn–Cu thin film electrodes Fig. 1 shows the DSC heating scan curve of the as-deposited thin film. It can be observed that an endothermic peak appears at 218.4 ◦ C, which should be induced by both the melting of Sn and eutectic reaction of Sn and Sn–Cu phases. However, the eutectic temperature was depressed owing to the small Sn grains and microcrystals in the E-beam deposited film. As the heat treatment conditions for the Sn–Cu electrodes in references were quite different [13,18,19,21,22], the as-deposited Sn–Cu thin films were annealed at 185, 200 and 220 ◦ C for 10 h, respectively, and then analyzed by XRD to obtain optimal heat treatment condition. Fig. 2 gives the XRD patterns of the as-deposited thin film and thin films annealed at different temperature. Diffractogram (a) in Fig. 2 shows that the as-deposited thin film electrode is
Fig. 1. DSC heating scan curve of the as-deposited Sn–Cu thin film electrode, the heating rate is 10 ◦ C/min.
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
3379
Fig. 2. XRD patterns of Sn–Cu thin film electrodes annealed under different conditions: (a) as-deposited; (b) 185 ◦ C for 10 h; (c) 200 ◦ C for 10 h; (d) 220 ◦ C for 10 h.
mainly composed of Sn and Cu phase in addition to the existence of a small amount of Cu6 Sn5 phase. The formation of Cu6 Sn5 phase should be due to partially alloying between Cu, including Cu co-deposited with Sn and pre-deposited Cu layer, and Sn during evaporation because the vaporized particles has high energy. After annealing at 185 ◦ C for 10 h, as shown in Fig. 2b, no peak of Sn could be detected and -Cu3 Sn phase (orthorhombic structure, space group: Cmcm) was formed, but the peak intensity of Cu3 Sn was quite weak. When the film was annealed at 200 ◦ C, as shown in Fig. 2c, both Cu6 Sn5 and Cu3 Sn phases could be distinctly observed. When the annealing temperature was 220 ◦ C, the peaks of Cu6 Sn5 disappeared and peaks of Cu3 Sn phase weakened while strong peaks of Cu appeared. Considering that the Cu3 Sn phase was mainly interested in this investigation, and multi-phase materials show better electrochemical performance than the single-phase materials [27], annealing at 200 ◦ C for 10 h was selected as the optimal heat treatment parameter for Sn–Cu thin film electrodes. Fig. 3a and b shows the plane-view surface morphology of the thin films. The surface of the as-deposited film, shown in Fig. 3a was composed of evaporated particles, which were mainly the uniform crystallite of Sn. The cross-sectional image, given in the inset of the figure, shows that deposited layer has columnar structure along growth direction and its thickness is about 1 m. After annealing at 200 ◦ C, as shown in Fig. 3b, the crystallite became smaller comparing with the as-deposited film. The surface roughness of thin film electrode was also dramatically increased after heat treatment, which was revealed by the AFM images shown in Fig. 4. The roughness (Img. Rms) was about 62 nm and 105 nm for the as-deposited film and annealed film, respectively. Compositions of the thin films were determined by EDX analysis on the plane-view film and the results are listed in Table 1. The Cu/Sn ratio of the as deposited film was 2.7/1, and it changed to 4.5/1 after annealing at 200 ◦ C for 10 h. The Cu/Sn ratio of 2.7/1 is less than the stoichiometry of Cu3 Sn while
Fig. 3. SEM images of the surface morphology of the thin film electrodes: (a) as-deposited and (b) after heated at 200 ◦ C for 10 h; the insets in the figures show the unpolishing cross-section of electrodes.
the ratio of 4.5/1 is higher than the stoichiometry of the Cu3 Sn. The increased Cu content in the film should be provided by the pre-deposited Cu layer. Based on the structure, morphology and composition analysis described above, it can be concluded that the initial mixture of Cu and Sn phases in the as-deposited thin film were transformed to Cu3 Sn and Cu6 Sn5 phase and the crystalline grains of them are smaller than those of original Sn phase. In order to investigate the difference in the interface region between the deposited layer and the current collector before and after annealing, a thick film with thickness of 4–6 m was prepared for cross-sectional analysis. As it is shown in Fig. 5a, two layers, which are Sn and Cu as identified from BSE image and EDX analysis, can be clearly observed in the cross-section. It should be pointed out, however, that the deposited Cu layer cannot be distinguished from the Cu substrate in the BSE image Table 1 The compositions of Sn–Cu thin film anodes determined by EDX analysis on the plane view surface Atomic%
As-deposited film Annealed film
Weight%
Cu
Sn
Cu/Sn
Cu
Sn
73.39 81.72
26.61 18.28
2.7/1 4.5/1
59.62 70.53
40.38 29.47
3380
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
Fig. 4. AFM images (20 m × 20 m) and roughness analysis of Cu–Sn thin film electrode: (a) as-deposited and (b) after heated at 200 ◦ C for 10 h.
Fig. 5. BSE images of cross-section of Sn–Cu films and EDX line-scanning profile along the cross-section: (a) as-deposited and (b) annealed at 200 ◦ C for 10 h.
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
3381
Fig. 6. Cycle performance of the as-deposited thin film electrode. Electrode potential range, 0.1–2.0 V; current density, 100 A/cm2 .
because their contrasts are same. It has been found that the deposited layer contains some pores, which indicates that the as-deposited layer was not fully dense. For the annealed sample, as shown in Fig. 5b, there are two intermediate layers formed mainly on the deposited layer in addition to the Sn and Cu layer. This is clearly shown by the contrast change stepwise from dark to light gray from the substrate to the surface. These results proved that Sn and Cu interdiffused and formed Cu3 Sn or Cu6 Sn5 layer from Cu side to Sn side. Thus, the interface binding strength between active materials layer and current collector would be enhanced by annealing. This would be beneficial for the cycleability of this film anode. 3.2. Electrochemical properties of the thin film electrodes Fig. 6 shows the cycle performance of the charge–discharge capacity and Coulomb efficiency of the as-deposited Sn–Cu thin film anode. The initial discharge capacity is 756 mAh/g, which is 76% of the theoretical limit of Sn metal (991 mAh/g). It decreases sharply to 210.7 mAh/g in the following ten cycles, and could only keep 84.8 mAh/g in the 25th cycle. These results show that the cycle performance of the as-deposited thin film anode is very poor. The Coulomb efficiency of the anode is about 84% in the first cycle and increases to 91.6% in the 10th cycle. It rises to 96.5% in 24th cycle. These results show that the stability of the electrochemical properties in the interface between anode and electrolyte increases with the proceeding of cycling, which improves the reversibility of the charge and discharge of the anode. The cycle performances of annealed Sn–Cu thin film anodes cycled under different potential regions, which were 0.1–2.0 and 0.2–1.5 V, respectively, are shown in Fig. 7. When the anode annealed after 200 ◦ C was cycled between 0.1 and 2.0 V, as shown in Fig. 7a, the initial discharge capacity was 731 mAh/g, just 25 mAh/g less than the unannealed one. However, it decreased sharply to 462.6 mAh/g in the 2nd cycle and the Coulomb efficiency of 1st cycle was just 61%, which indicated large initial irreversible capacity. The discharge capacity decreased gradually to 253 mAh/g after 25 cycles. The annealed
Fig. 7. Cycle performance of the annealed thin film electrodes. Anodes annealed at 200 ◦ C were cycled among potential ranges of 0.1–2.0 V (a) and 0.2–1.5 V (c) and anodes annealed at 185 and 220 ◦ C were cycled between 0.1 and 2.0 V (b). Current density: 100 A/cm2 .
electrode lost about 1.8% discharge capacity in each cycle. The Coulomb efficiency increased to 91.3% in 2nd cycle, and remained over 96% till 25th cycle. In order to investigate the influences of annealing conditions for the cycle performance of anodes, the films that annealed after 185 and 220 ◦ C were also electrochemically characterized, respectively, and the obtained charge/discharge performances of them were shown in Fig. 7b. It was found that the capacity retention of film anodes annealed at 185 and 220 ◦ C could be also improved dramatically compared with that of the as-deposited film anode, which shown in Fig. 6. However, all these annealed thin film anodes existed large initial irreversible capacity and showed low Coulomb efficiencies in the first cycle. These results indicated that the cycle performance of the evaporation deposited Sn–Cu thin film anode had been improved by heat treatment. It is assumed that the improvement is owing to the facts that the binding strength between active materials layer and the current collector was enhanced by heat treatment. In addition, the intermetallic compounds of Cu3 Sn and Cu6 Sn5 formed in the thin films should reduce volume change during lithium insertion and extraction.
3382
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
With respective to the large initial capacity loss, it was possible induced by the large changes, especially those of morphology and/or composition, on the surfaces of thin film during annealing. The cycle performance of annealed anode cycled in the range from 0.2 to 1.5 V is shown in Fig. 7c. It shows that the charge/discharge capacity is lower than that of the electrode cycled at cut-off potential of 0.1–2.0 V, and the initial discharge capacity is 497 mAh/g, which is 240 mAh/g less than that shown in Fig. 7a. Although there is a 260 mAh/g irreversible loss of capacity in the first cycle, however, discharge capacity of about 200 mAh/g and Coulomb efficiency of 97% are kept stable up to 30th cycle. The capacity loss in each cycle is less than 1 mAh/g. The above result demonstrated that the cycle performance of thin film anode at lower cut-off potential was better than that at higher cut-off potential. This is in agreement with the results reported in previous work [15,20,23]. However, the capacities and Coulomb efficiencies of the film electrode prepared by EBD in the present work were higher than those Sn–Cu alloy electrodes with similar compositions prepared by mechanical alloying [13] and electrodeposition methods [20]. Fig. 8 shows the charge/discharge curves of the first four cycles of the as-deposited Sn–Cu thin film electrode in cut off potential window of 0.1–2.0 V. It is clear from the curve in Fig. 8 that the as-deposited film exhibits three plateaus on discharge (lithiation), which is quite different from the charge/discharge curves of the electrodeposited Sn–Cu film electrodes [13,19,28]. The potentials of the plateaus are about 0.67, 0.55 and 0.45 V, respectively, which are close to the plateau potential for formation of LiSn, Li7 Sn3 , Li5Sn2 and Li13 Sn5 phase at room temperature [29]. There are also three plateaus in the charge (delithiation) curves, which are 0.56, 0.70 and 0.77 V, respectively, and should correspond to the delithiation potentials of those Li–Sn phases mentioned above. The initial four charge/discharge curves of the annealed Sn–Cu thin film electrode, as shown in Fig. 9, are quite different from those of the as-deposited film electrode. There are two plateaus on the discharge curve of the first cycle, which dis-
Fig. 8. Initial charge–discharge curves of the as-deposited Sn–Cu thin film electrode cycled between 0.1 and 2.0 V vs. Li/Li+ .
Fig. 9. Initial charge–discharge curves of the annealed Sn–Cu thin film electrode cycled between 0.1 and 2.0 V vs. Li/Li+ .
Fig. 10. Cyclic voltammograms of Sn–Cu thin film electrodes: (a and b) are unannealed and annealed film in LiPF6 (1 M)/EC + DEC + EMC (1:1:1, v/v/v), respectively. Scan rate: 0.5 mV/s. The inset in (a) shows magnified voltammograms in the range of 1.35–1.60 V.
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
appeared in the following cycles. The potential ranges of the two slope plateaus are 1.3–1.2 and 0.9–0.8 V, respectively. They are higher than the potentials of lithium ion insert into Sn [28,29] or Cu6 Sn5 [12,30], but in the potential region where electrolyte reduction and subsequent SEI formation take place [27]. Thus, these plateau regions are related to the formation of SEI by catalytic decomposition of the electrolyte on the annealed thin film electrode, which can consume a lot of electrochemical active material and, consequently, results in large capacity loss capacity in the first cycle [11,28]. They are also possibly associated with the oxidation of the electrode during annealing, but no oxygen or oxide was detected by EDX and XRD analyses. The reason for the large irreversible capacity of the annealed electrode will be discussed later in detail. Fig. 10a and b shows the cyclic voltammograms at 0.5 mV/s of the as-deposited and annealed Sn–Cu thin film anode, respectively. With respect to the as-deposited film, two cathodic peaks at 0.33 and 0.64 V can be observed, while three anodic peaks can be observed at the potentials of 0.64, 0.73 and 0.82 V, respectively. All the cathodic peaks and anodic peaks correspond to lithium alloying and de-alloying with the active materials, respectively. Besides those reversible peaks, as shown in the inset of Fig. 10a, an irreversible peak, which is most probably related to SEI formation, appears in the first two cycles in the range of 1.4–1.55 V. With respect to annealed thin film electrode, two cathodic peaks at the potential range of 0.1–0.5 V and two anodic peaks
3383
at 0.4–0.7 V were observed. They were reversible. The cathodic peaks correspond to inserting of Li into Cu3 Sn and Cu6 Sn5 forming Lix Cu3 Sn and Lix Cu6 Sn5 phases [12,17]. To the contrary, the anodic peaks are attributed to the delithiation of the Li–Sn–Cu phases. It should be noted that there are two irreversible peaks, as indicated by the arrows in Fig. 10b, during cathodic sweep at the first cycle, and the potential ranges are 0.8–0.9 and 1.2–1.3 V, respectively, which are in good agreement with the potential plateaus observed in Fig. 9. It can also be seen that the irreversible peaks only appear in the first cycle and disappear in following cycles. This indicates that the surface of the electrode became stable and solvent decomposition decreased significantly after formation of SEI layer in the first cycle, which resulted in a decrease in capacity loss and improvement in the cycleability of the electrode as shown in Figs. 9 and 7. However, it was worth to note that there was an initial irreversible capacity of about 260 mAh/g for the annealed film electrode while only about 80 mAh/g for the as-deposited electrode, which indicated that annealing aggravate initial capacity lose of the thin film electrode. This was probably due to the refining of crystalline grain and increase of surface roughness of the film after it had been annealed at 200 ◦ C for 10 h (see in Figs. 3 and 4). As well known, the SEI film was apt to be formed as the surface roughness of electrode increasing. Therefore, the high surface roughness and fine crystal grain of thin film would bring high interface area for reaction with the electrolyte and would aggravate the formation of SEI film.
Fig. 11. SEM images of electrodes after cycling: (a) as-deposited electrode after 20 cycles and (b) annealed electrode after 25 cycles. (a2 and b2 ) are enlarged images of the electrode (a) and (b), respectively.
3384
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
Table 2 The EDX analysis results in different position of Sn–Cu thin film anodes after cycling Atomic% Cu
Weight% Sn
Cu/Sn
Cu
Sn
As-deposited electrode after 20 cycles Spectrum 1 90.35 9.65 Spectrum 2 52.57 47.28 Spectrum 3 97.71 1.29
9.4/1 1.12/1 76.5/1
83.36 37.38 97.61
16.64 62.62 2.39
Annealed electrode after 25 cycles Spectrum 1 80.50 19.50 Spectrum 2 78.18 21.82
4.13/1 3.58/1
68.84 65.76
31.16 34.24
The selected areas of spectrums are shown in Fig. 10a and b.
3.3. Structure change of thin film anodes during charge–discharge cycling Fig. 11a and b are SEM images of the as-deposited and annealed thin film electrodes after cycling measurement, respectively. As it is shown in Fig. 11a, the as-deposited thin film appeared to be pulverized after 20 cycles. Comparing with Fig. 3a, it can be found that the deposited Sn–Cu layer had been damaged and the active materials particles agglomerated, which resulted in nonuniform surface of the electrode. The EDX results listed in Table 2 show that area 2 (i.e. spectrum 2 in Fig. 11a) mainly contains Sn, while area 3 (i.e. spectrum 3 in Fig. 11a) is almost Cu. The Cu/Sn ratio of the as-deposited electrode after 20 cycles reached to 9.4/1, which is about 3 times of that before cycling, being 2.7/1 as given in Table 1. These results indicate that part of the Sn had been peeled off from the Cu foil, which was resulted from large volume change and
phase transition in the Sn–Cu thin film anode. Thus, the discharge capacity of the as-deposited electrode decayed rapidly, and exhibited poor cycleability as shown in Fig. 6. After annealing, however, the formation Cu3 Sn and Cu6 Sn5 intermetallic phases in the electrode could relax the volume expansion and inhibit the cracking generation caused by delithiation and lithiation. Therefore, structure stability and cycle performance of the electrode was improved. As shown in Fig. 11b, no obvious pulverization and cracks can be observed on the annealed thin film electrode after 25 cycles. Comparing with the SEM image in Fig. 3b, uniform crystalline grains of Sn–Cu alloys and smaller grains can be seen after cycling. These might be related to the protection of the SEI layer formed in the first discharge cycle and recrystallization of Sn–Cu alloy phases during cycling. The EDX results listed in Table 2 indicate that the Cu/Sn ratio of the annealed electrode did not increase after 25 cycles. They are 4.13/1 and 3.58/1 in spectrum 1 and spectrum 2 labeled in Fig. 11b, respectively, which is close to Cu/Sn ratio, being 4.5/1, of the film before cycling. However, it was possible that the Sn of inner layer was gradually diffused to the surface layer of the film, and resulted in a bit decreasing of Cu/Sn ratio on the surface of annealed film electrode after cycling. To prove some of the above considerations further, the film electrodes were characterized by XRD after charge–discharge cycling. Fig. 12a and b shows the XRD patterns obtained for unannealed electrode after 20 cycles (charge) and annealed electrode after 25 cycles (charge), respectively. In comparing with the diffractograms of both electrodes before cycling (also shown in Fig. 12), no new peaks have been observed on the diffractograms in both electrodes undergone cycling. The unannealed electrode mainly contains Sn, while the annealed electrode is composed of Cu3 Sn and Cu6 Sn5 except peak from Cu foil. It
Fig. 12. XRD patterns of Sn–Cu thin film electrodes after cycling: (a) unannealed electrode after 20 cycles and (b) annealed electrode after 25 cycles.
R.Z. Hu et al. / Electrochimica Acta 53 (2008) 3377–3385
means that the annealed thin film anode remained stable and did not undergo obvious phase change during cycling. 4. Conclusion Sn–Cu anode for thin film lithium ion batteries was prepared by depositing Sn–Cu thin film of high Cu content on Cu foil with method of electron-beam evaporation. An endothermic peak at 218.4 ◦ C was detected in DSC heating scan curve for the as-deposited thin film. After the film was heated at 200 ◦ C for 10 h, it was composed of -Cu3 Sn and a small mount of Cu6 Sn5 intermetallic phases, and the interface strength between active materials layer and current collector was remarkably improved. A stable discharge capacity of more than 250 mAh/g was obtained at voltage window of 0.1–2.0 V, and a discharge capacity of about 200 mAh/g was kept stable up to 30th cycle at a narrower potential window of 0.2–1.5 V with a current density of 100 m/cm2 . The reversible capacities, Coulomb efficiencies and cycleabiltiy of the film prepared by the present method were higher than those of the high Cu content Sn–Cu alloy electrodes prepared by mechanical alloying and electrodeposition methods. It was found that large irreversible discharge capacity was associated with SEI formation due to increase of surface roughness of annealed film electrode. The annealed thin film anode remained stable without obvious phase changing and crack forming up to 25 cycles. Acknowledgements This research was supported by Ministry of Education under project No. IRT0551 and Guangdong Provincial Natural Science Foundation under the Team Project. References [1] J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans, Solid State Ionics 135 (2000) 33. [2] J.L. Souquet, M. Duclot, Solid State Ionics 148 (2002) 375. [3] N.J. Dudney, Mater. Sci. Eng. B 116 (2005) 245.
3385
[4] B.J. Neudecker, R.A. Zuhr, J.B. Bates, J. Power Sources 81 (1999) 27. [5] B.J. Neudecker, N.J. Dudney, J.B. Bates, J. Electrochem. Soc. 147 (2000) 517. [6] T. Brousse, R. Retoux, U. Herterich, D.M. Schleich, J. Electrochem. Soc. 145 (1998) 1. [7] F. Ding, Z. Fu, M. Zhou, Q. Qin, J. Electrochem. Soc. 146 (1999) 3554. [8] C. Branci, N. Benjelloun, J. Sarradin, M. Ribes, Solid State Ionics 135 (2000) 169. [9] K.S. Parka, Y.J. Parkb, M.K. Kima, J.T. Sona, H.G. Kima, S.J. Kim, J. Power Sources 103 (2001) 67. [10] Y.I. Kim, C.S. Yoon, J.W. Park, J. Solid State Chem. 160 (2001) 388. [11] Y.L. Kim, H.Y. Lee, S.W. Jang, S.J. Lee, H.K. Baik, Y.S. Yoon, Y.S. Park, S.M. Lee, Solid State Ionics 160 (2003) 235. [12] D. Larcher, L.Y. Beaulieu, D.D. Macneil, J.R. Dahn, J. Electrochem. Soc. 147 (2000) 1658. [13] N. Tamura, R. Ohshita, M. Fujimota, S. Fujitani, M. Kamino, I. Yonezu, J. Power Sources 107 (2002) 48. [14] G.X. Wang, L. Sun, D.H. Bradhurst, S.X. Dou, H.K. Liu, J. Alloys Compd. 299 (2000) L12. [15] Y.Y. Xia, T. Sakai, T. Fujiedu, M. Wada, H. Yoshinaga, J. Electrochem. Soc. 148 (2001) A471. [16] J. Wolfenstine, S. Campos, D. Foster, J. Read, W.K. Behl, J. Power Sources 109 (2002) 230. [17] D.G. Kim, H. Kim, H.J. Sohn, T. Kang, J. Power Sources 104 (2002) 221. [18] N. Tamura, R. Ohshita, M. Fujimoto, M. Kamino, S. Fujitani, J. Electrochem. Soc. 150 (2003) A679. [19] H. Morimoto, S. Tobishima, H. Negishi, J. Power Sources 146 (2005) 469. [20] S.D. Beattie, J.R. Dahn, J. Electrochem. Soc. 150 (2003) A894. [21] W.H. Pu, X.M. He, J.G. Ren, C.R. Wan, C.Y. Jiang, Electrochim. Acta 50 (2005) 4140. [22] F.S. Ke, L. Huang, J.S. Cai, S.G. Sun, Electrochim. Acta 52 (2007) 6741. [23] L. Wamg, S. Kitamura, K. Obata, S. Tanase, T. Saki, J. Power Sources 141 (2005) 286. [24] Y.L. Kim, H.Y. Lee, S.W. Jang, S.H. Lim, S.J. Lee, H.K. Baik, Y.S. Yoon, S.M. Lee, Electrochim. Acta 48 (2003) 2593. [25] J.B. Kim, B.S. Jun, S.M. Lee, Electrochim. Acta 50 (2005) 3390. [26] J.B. Kim, H.Y. Lee, K.S. Lee, S.H. Lim, S.M. Lee, Electrochem. Commun. 5 (2003) 544. [27] M. Winter, W.K. Appel, B. Evers, T. Hodal, K.C. M¨oller, I. Schneider, M. Wachtler, M.R. Wagner, G.H. Wrodning, J.O. Besenhard, Monatshefite f¨ur Chemie 132 (2001) 473. [28] M. Inaba, T. Uno, A. Tasaka, J. Power Sources 146 (2005) 473. [29] M. Winter, J.O. Besenhard, Electrochim. Acta 45 (1999) 31. [30] J.J. Zhang, Y.M. Zhang, X. Zhang, Y.Y. Xia, J. Power Sources 167 (2007) 171.