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Preparation of RF-sputtered lithium cobalt oxide nanorods by using porous anodic alumina (PAA) template
Journal of Alloys and Compounds 414 (2006) 302–309
Preparation of RF-sputtered lithium cobalt oxide nanorods by using porous anodic alumina (PAA) template C.L. Liao a , M.T. Wu a , J.H. Yen a , I.C. Leu b , K.Z. Fung a,∗ a
Department of Materials Science and Engineering, National Cheng Kung University, No. 1 Ta-Hsueh Road, Tainan 70101, Taiwan b Department of Electronic Engineering, Kun Shan University, No. 949 Da Wan Road, Yung-Kang City, Tainan Hsien 710, Taiwan
Received 11 April 2005; received in revised form 22 July 2005; accepted 25 July 2005 Available online 8 September 2005
Abstract Recent years have shown a growing interest in the investigation of lithium microbatteries. In this study, LiCoO2 thin-films and nanorods were obtained by radio frequency magnetron sputtering. In order to obtain well-crystallized LiCoO2 nanorods exhibiting excellent electrochemical properties, LiCoO2 films prepared at various sputtering conditions were first investigated by XRD, Raman, SEM, and charge–discharge test to find out a suitable sputtering condition for the nanorod preparation. The 600 ◦ C-annealed LiCoO2 films deposited at 250 ◦ C exhibited the best cycleability and capability, and consequently, these conditions were used to deposit the LiCoO2 nanorods. According to the SEM observations, the LiCoO2 nanorods were successfully deposited into the porous anodic alumina template (thickness and diameter were 300 and 60 nm) under the sputtering conditions. The cyclic voltammogram of 600 ◦ C-annealed LiCoO2 nanorods showed that the sputtered LiCoO2 nanorods exhibited excellent electrochemical reversibility to be used as the cathode material. © 2005 Elsevier B.V. All rights reserved. Keywords: Lithium cobalt oxide; RF-sputtering; Nanodot; Porous anodic alumina
1. Introduction Within the past 10 years, lithium batteries have been an essential energy storage component for the portable electronics. The performance of lithium batteries is highly contingent upon the properties of cathode materials. Therefore, cathode materials such as LiCoO2 , LiNiO2 , and LiMn2 O4 have been widely investigated. Today, LiCoO2 is the primary commercial cathode material in lithium batteries due to its advantages of high specific capacity, high operating voltage, and long cycle life [1–5]. For electrode materials in electrochemical devices, a reduction in dimensions becomes a significant demand in order to increase the effective reaction area of the electrodes. Moreover, it is well known that the electrochemical ∗
Corresponding author. Tel.: +886 6 2380208; fax: +886 6 2380208. E-mail address: [email protected] (K.Z. Fung).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.07.042
properties of the thin-film electrode could be enhanced by increasing its surface area due to the minimization of the diffusion paths of lithium ions. Thus, there is a great demand for the investigation of materials with special structures such as nanorods, nanowires, nanotubes, nanodots, and nano-arrayed membranes. A way to obtain these structures is to use the template-assisted method combined with several deposition methods such as sol–gel, chemical vapor deposition (CVD), physical vapor deposition (PVD), electroplating, and electrophoretic deposition [6–10]. Although it is now common to find LiCoO2 nanowires prepared by the sol–gel method using the template-assisted method [6], but there are currently no reports on LiCoO2 nanorods prepared by RF-sputtering using the template-assisted method. In addition, in order to investigate the effect of the nano-structure on electrochemical behaviors of RF-sputtered LiCoO2 cathode, the methods to prepare the thin-films and nanorods should be the same. Consequently, in this work, LiCoO2 nanorods were obtained
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by radio frequency (RF) magnetron sputtering and templateassisted method. In general, the RF-sputtered material should then undergo a post-annealing to enhance its crystallinity and electrochemical properties (consequently, during the annealing the crystallinity of porous anodic alumina (PAA) would be enhanced at the same time). Yet, in our experiments it was difficult for the NaOH solution to remove the crystallized PAA. Therefore, it was necessary to find out a suitable sputtering condition in order to obtain well-crystallized LiCoO2 and remove the PAA easily. In conducting this investigation, the structure and electrochemical properties of LiCoO2 films prepared under several conditions were investigated first. Finally, LiCoO2 nanorods prepared by RF-sputtering using the template-assisted method were obtained using the best sputtering deposition conditions. 2. Experimental 2.1. LiCoO2 thin-film deposition In order to find out a suitable sputtering condition for the deposition of LiCoO2 nanorods, LiCoO2 films obtained at various sputtering conditions were first investigated. Ptcoated silicon wafers were used as substrates to deposit LiCoO2 films. All of the LiCoO2 films and nanorods were deposited by RF magnetron sputtering from the LiCoO2 target. The 2 in. diameter LiCoO2 target was sintered at 700 ◦ C for 2 h by hot-pressing LiCoO2 powders that were calcined in an oxygen atmosphere from Li2 CO3 and CoCO3 at 700 ◦ C for 12 h. After presputtering the target for 15 min, the sputtering deposition was carried out under a pressure of 20 m Torr and a gas flow rate of 12 sccm. The Ar and O2 gas flow rate was 3:1. The RF power and the distance between substrate and target were 100 W and 40 mm, respectively. During the deposition process, the substrate temperature was controlled by a heater and held at ambient temperature, 250 and 600 ◦ C. In order to enhance the crystallinity of the LiCoO2 films, the films were then annealed at 600 ◦ C for 2 h in a controlled oxygen atmosphere. An X-ray diffractometer using Cu K␣ radiation ˚ and Raman spectroscopy (Coherent Innova (λ = 1.5418 A) 90) were used to characterize the structure and the crystallinity of LiCoO2 films. The morphologies and crosssections of LiCoO2 films were observed by scanning electron microscopy (Philips, XL-40FEG). The charge–discharge characteristics (measured by Arbin, BT2043) of LiCoO2 were investigated on the Li|LiCoO2 cell using polycrystalline LiCoO2 films as the cathode (the active area is 1 cm2 ), lithium metal as the anode, and 1 M LiPF6 in propylene carbonate as the electrolyte. 2.2. LiCoO2 nanowires deposition LiCoO2 nanorods were finally prepared under the same conditions as the 250 ◦ C-deposited LiCoO2 film. PAA/Si
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substrate was used to deposit LiCoO2 nanorods. It was fabricated by anodizing the Al/Au/Ti/SiO2 /Si substrates via a two-step anodization process. During this process, a thin SiO2 layer of 20 nm thickness was prepared by thermal oxidation using a high-temperature tube furnace, and a Ti layer of 20 nm thickness was deposited by E-beam evaporation. The SiO2 layer was introduced as a barrier layer against the oxidation of Si when the Al film was consumed completely during anodization. In addition, the Ti layer served as an adhesive layer to increase the adhesion of Al and Si substrate. After the deposition of SiO2 and Ti layers, the Al layer was finally deposited on the substrate by E-beam evaporation. The first-step anodization was conducted in 0.3 M oxalic acid solution in a cooling circulation bath at 13 ◦ C and with an anodizing voltage of 40 V. The anodized alumina formed in the first anodization step was removed by the mixture of 12 wt.% H3 PO4 and 6 wt.% H2 Cr2 O4 . The remaining aluminum was anodized again until it was completely converted into alumina. After the two-step anodization, the thickness of PAA stood at about 300 nm. Finally, PAA was immersed into 5 wt.% H3 PO4 for 30 min to remove the barrier layer between alumina and Au/Ti/SiO2 /Si substrate. After depositing LiCoO2 onto the PAA templates, the annealing process was carried out with the PAA template because the as-deposited LiCoO2 would otherwise have been damaged by the NaOH solution due to its poor crystallinity. Therefore, the removal of PAA proceeded after the conclusion of the annealing process. After removing the PAA template by 2 wt.% NaOH solution, the LiCoO2 nanorods were obtained. SEM (Philips, XL-40FEG) was used to observe the morphologies of LiCoO2 nanorods. Afterwards, the structure of the LiCoO2 nanorods was analyzed by TEM (JEOL, JEM-3010) analysis. The cyclic voltammetric characteristic (measured by EG&G, Potentiostat 273A) of LiCoO2 nanorod was also investigated on a Li|LiCoO2 cell using LiCoO2 nanorods as the cathode, lithium metal as the anode, and 1 M LiPF6 in propylene carbonate as the electrolyte under a sweep rate of 100 V/s with the voltage ranging from 3.0 to 4.2 V.
3. Results and discussion 3.1. Structural analysis of RF-sputtered LiCoO2 films It is known that LiCoO2 exhibits three different symmetries: hexagonal with R3m symmetry, cubic with Fm3m symmetry, and spinel with Fd3m symmetry [11]. Previous researches have shown Raman spectroscopy to be a powerful instrument in distinguishing these three different symmetries due to the different Raman spectra [12–15]. Fig. 1(a) shows the Raman spectrum of as-deposited LiCoO2 films obtained at ambient temperature. In this figure, there are two RS peaks located at 580.3 and 467.2 cm−1 , which respectively correspond to A1g and Eg of the hexagonal LiCoO2 with R3m symmetry. The XRD result (not shown here) also indicates that the as-deposited LiCoO2 films obtained at ambient tem-
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Fig. 1. Raman spectrum of (a) as-deposited and (b) 600 ◦ C-annealed LiCoO2 films deposited at ambient temperature.
perature exhibit crystalline structure with (1 0 4) out-of-plane texture. They also indicate that the crystallized LiCoO2 films exhibit only hexagonal LiCoO2 without any second phase. Thus, it is suggested that the sputtered atoms might stack on the substrate in short-range order, and, consequently, the films shows nano-crystallized structure. This result is different from some articles reported earlier [16–19], which indicated that as-deposited films without substrate heating or post-annealing exhibited amorphous structure. Nevertheless, a recent publication by Whitacre et al. also reported that as-deposited LiCoO2 films obtained at ambient temperature by RF-sputtering exhibited nano-crystallized structure and (1 0 4) out-of-plane texture [20]. Therefore, in this work, the as-deposited films exhibit nano-crystallized structure is expected. According to our previous study [21], the as-deposited LiCoO2 films with nanocrystalline structure did not show excellent electrochemical properties due to insufficient crystallinity. However, after the LiCoO2 films underwent high temperature annealing, the electrochemical properties were enhanced significantly. Therefore, we concluded that postannealing was a necessary step to enhance the crystallinity and the electrochemical properties of the films. Unfortunately, along with enhancing the electrochemical properties of LiCoO2 by annealing, the annealing process enhanced the crystallinity of the PAA template as well. Our recent tests showed that the 700 ◦ C-crystallized PAA was difficult to remove in nanorod preparation. However, the 600 ◦ C-crystallized PAA was easier to remove, and, consequently, the annealing temperature was chosen at 600 ◦ C for the nanorod preparation. The Raman spectrum of 600 ◦ Cannealed LiCoO2 film is shown in Fig. 1(b). One can see from the figure that the 600 ◦ C-annealed LiCoO2 film deposited at ambient temperature still consists of LiCoO2 phase because there are only two well-defined RS peaks of hexagonal LiCoO2 located at 591.5 and 479.3 cm−1 . The
Fig. 2. XRD patterns of 600 ◦ C-annealed LiCoO2 films deposited at substrate temperature of (a) ambient temperature, (b) 250 ◦ C, and (c) the pattern of LiCoO2 film deposited at substrate temperature of 600 ◦ C without any post-annealing.
increasing intensity and the shift of RS peaks indicate that the 600 ◦ C-annealing process enhance the crystallinity of sputtered LiCoO2 films. Fig. 2(a) and (b) shows the XRD patterns of 600 ◦ Cannealed LiCoO2 films obtained at ambient temperature and 250 ◦ C, respectively. Fig. 2(c) shows the film directly deposited at a substrate temperature of 600 ◦ C. From Fig. 2(a) and (b), it is evident that both the 600 ◦ C-annealed films consist of LiCoO2 single phase and exhibit (1 0 4)-preferred orientation. Also, the film directly deposited at 600 ◦ C exhibits (1 0 1)-preferred orientation. These results are similar to the article reported by Bates et al., which concluded that the (1 0 1)-, (1 1 0)-, and (1 0 4)-preferred orientation were favored to minimize the volume strain energy of the LiCoO2 film [3]. Therefore, it is expected that even though the LiCoO2 films are obtained from various deposition conditions, they would all consist of these specific orientations in order to minimize the volume strain energy. 3.2. SEM observations of RF-sputtered LiCoO2 films Fig. 3(a) and (b) shows the SEM top-view observations of the 600 ◦ C-annealed LiCoO2 films deposited at ambient temperature and 250 ◦ C, respectively. Fig. 3(c) illustrates the film directly obtained at a substrate temperature of 600 ◦ C. It is clear that the films obtained at various conditions all show continuous and uniform morphologies. It is also obvious that the 600 ◦ C-annealed films exhibit nano-sized grains, however, the films directly deposited at substrate temperature of
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Fig. 3. SEM images of 600 ◦ C-annealed LiCoO2 films deposited at (a) ambient temperature and (b) 250 ◦ C, and (c) the SEM image of LiCoO2 film deposited at a substrate temperature of 600 ◦ C without any post-annealing.
600 ◦ C show submicron and needle-like grains as well. The different morphologies of these films are due to the various substrate temperatures during the deposition process. It is suggested that the higher substrate temperature is similar to anneal the films during the sputtering deposition process, thus, the grain growth is conspicuous, and consequently, the films show submicron grains.
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Fig. 4. (a) Discharge curves of the Li//LiCoO2 cells with LiCoO2 films obtained from various conditions (deposited at ambient temperature and annealed at 600 ◦ C (- - -), 250 ◦ C-deposited and annealed at 600 ◦ C (—), and 600 ◦ C-deposited (– – –)) and (b) discharge capacity vs. cycle number plot of LiCoO2 films obtained from various conditions (deposited at ambient temperature and annealed at 600 ◦ C (), 250 ◦ C-deposited and annealed at 600 ◦ C (), and 600 ◦ C-deposited (䊉)).
3.3. Electrochemical properties of RF-sputtered LiCoO2 films Fig. 4(a) shows the discharge curves of LiCoO2 films deposited at various sputtering conditions. The electrochemical measurements were carried out under a current density of 10 A/cm2 in the potential range of 4.2–3.0 V. In Fig. 4(a), the 600 ◦ C-annealed films obtained at ambient temperature and 250 ◦ C show a well-defined discharge plateau
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at about 3.9 V. This discharge plateau corresponds to the main de-intercalation reaction of Li ions from the LiCoO2 layered structure. After the voltage discharging to lower than 3.9 V, the voltage drops rapidly to the cut-off voltage of 3.0 V. The first discharge capacity of 600 ◦ C-annealed films obtained at ambient temperature and 250 ◦ C are 48.18 and 50.39 Ah/cm2 m, respectively. Nevertheless, for lack of post-annealing at 600 ◦ C, the discharge curve of LiCoO2 film directly deposited at 600 ◦ C shows an unobvious discharge plateau that is due to the lower crystallinity of LiCoO2 film. Furthermore, the 1st discharge capacity of the film is only 42.49 Ah/cm2 m. These results indicate that the crystallinity and the capability of 600 ◦ C-annealed films are superior to the film directly deposited at 600 ◦ C. Fig. 4(b) gives the discharge capacity versus cycle number plot of LiCoO2 films. Obviously, the 10th discharge capacities of 600 ◦ C-annealed films deposited at ambient temperature and 250 ◦ C are 42.58 and 49.86 Ah/cm2 m, respectively, while the 10th discharge capacity of 600 ◦ C-deposited
film is only 30.65 Ah/cm2 m. The charge–discharge tests show that the 600 ◦ C-annealed LiCoO2 film obtained at 250 ◦ C exhibits the largest discharge capacity and the best cycle retention. Consequently, the following LiCoO2 nanorod preparation was carried out under this sputtering condition (substrate temperature of 250 ◦ C). 3.4. SEM observations of RF-sputtered LiCoO2 nanorods Based on the charge–discharge tests of LiCoO2 films obtained under various sputtering conditions, the 250 ◦ Cdeposited film that underwent a 600 ◦ C post-annealing exhibited excellent capacity and cycleability. Therefore, the sputtering parameters for depositing LiCoO2 nanorods using the template-assisted method were chosen at RF power of 100 W, working pressure of 20 m Torr, gas flow rate of 9 sccm Ar and 3 sccm O2 , substrate temperature of 250 ◦ C, and postannealing at 600 ◦ C.
Fig. 5. SEM top-view images of PAA (a) before and (b) after deposition, and SEM cross-section observations of LiCoO2 nanorods deposited into PAA template for which thicknesses were (c) 300 nm and (d) 500 nm and (e) without PAA template (removed by 2 wt.% NaOH).
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Fig. 5(a) shows the SEM top-view image of the PAA template before the LiCoO2 deposition. This SEM image shows that the PAA exhibits fine ordering arrangement pores over the micron area, and the average pore diameter is about 60 nm. In addition, Fig. 5(b) shows the SEM top-view image of the PAA template after the deposition of LiCoO2 . It indicated that after the deposition of LiCoO2 , the average pore size of the PAA template reduces from 60 to 30 nm. It is suggested that the LiCoO2 could be deposited into the channels of the PAA template. Fig. 5(c) and (d) shows the SEM cross-sectional images of RF-sputtered LiCoO2 deposited onto the PAA/Si substrates whose thicknesses are 300 and 500 nm, respectively. Interestingly, in Fig. 5(c), the LiCoO2 is successfully deposited into the channels of 300 nm-thick PAA template; but as the PAA thickness is thicker than 500 nm, the PAA template with empty channels can be observed in Fig. 5(d). According to the step coverage limitation of sputtering technique, the PAA channels with appropriate aspect ratio could be an important parameter in obtaining sputtered nanorods on PAA/Si substrate. Instead, it could only be deposited into the top of the channels when the aspect ratio of PAA template is too large and, subsequently, just formed a film covering the PAA surface. Fig. 5(e) shows the LiCoO2 nanorods without the PAA template that was removed by 2 wt.% NaOH solution. The diameter of the LiCoO2 nanorods illustrated in Fig. 5(e) is about 60 nm and is similar to that of the PAA channels. Furthermore, the LiCoO2 nanorods tends to aggregate to each other is due to the Van der Waal’s force. In addition, it is well known without any applied bias that the sputtered atoms would not have the trend to move in a direction that parallel to PAA channels. However, the sputtered atoms might collide with the wall of the PAA channel and deposit into the deeper pits of the PAA channels. A sketch of this description for LiCoO2 nanorods deposition is shown in Fig. 6. Consequently, after the deposition, annealing, and removal of the PAA, the LiCoO2 nanorods were obtained. Based on the SEM results, it was suggested that 300 nm is the thickness limitation of the PAA template (its pore diameter is 60 nm) to fabricate LiCoO2 nanorods by sputtering technique. 3.5. Structural characterization of RF-sputtered LiCoO2 nanorods TEM with electron diffraction is used to verify the crystal structure of LiCoO2 nanorods obtained from RF-sputtering deposition. The TEM bright field image and diffraction pattern of LiCoO2 nanorods are given in Fig. 7(a) and (b), respectively. The LiCoO2 nanorods were deposited at the sputtering parameters described in previous section. As seen in Fig. 7(a), which illustrates the TEM bright field image of LiCoO2 nanorods, the dimension of LiCoO2 nanorods is in agreement with that of the PAA channel. Moreover, the SAED pattern of LiCoO2 nanorods consisting of (0 0 3), (1 0 1), (1 0 4), and (1 1 0) reflections is a typical diffraction of LiCoO2 with R3m symmetry. All the reflections corre-
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Fig. 6. A sketch of the LiCoO2 nanorods deposited into the PAA template.
spond to LiCoO2, and no second phase to be observed. Thus, using template-assisted method and RF-sputtering technique, well-crystallized LiCoO2 nanorods could be successfully fabricated. 3.6. Cyclic voltammetric characteristic of LiCoO2 nanorods According to the most important aspect of electrode materials is the electrochemical activity; the cyclic voltammetric (CV) test is used to investigate the redox of the materials. Fig. 8 shows the CV measurement of sputtered LiCoO2 nanorods obtained from a 300 nm-thick PAA template. The CV plots exhibit one conjugate pair of redox peaks corresponding to an anodic peak (a) and a cathodic peak (b). These anodic and cathodic peaks are attributed to the reactions of Li ions de-intercalated from and intercalated into the LiCoO2 layer structure [22,23]. The anodic peak in the 1st
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cycle that shifts to higher voltage (4.01 V) is attributed to the over-potential to activate the material. After the LiCoO2 is activated, the subsequent anodic and cathodic peaks relocate to about 3.95 and 3.85 V, respectively. After the first cycle testing, the subsequent CV plots (second to fifth cycle) are approximately similar. These results indicate that the LiCoO2 nanorods obtained by RF-sputtering exhibit well enough reversibility (or cycleability) to be used as a cathode in lithium batteries. Nonetheless, the well reversibility might be relative to the specific nanorod structure due to the consideration of the nanorod structure could reduce the path of Li diffusion and enhance its electrochemical properties of the electrodes. Consequently, the detailed charge–discharge measurements with various current densities will be reported and discussed in the near future.
4. Conclusion
Fig. 7. (a) TEM bright field image and (b) SAED pattern of a 600 ◦ Cannealed LiCoO2 nanorod that deposited at 250 ◦ C by RF-sputtering.
LiCoO2 films deposited under various conditions all showed a single phase of LiCoO2 without any second phase. When comparing the electrochemical properties of the films obtained at various conditions, the 600 ◦ C-annealed LiCoO2 films deposited at 250 ◦ C showed the highest discharging capacity (capability) and best cycle retention (cycleability). According to the SEM observation and TEM analysis, we confirmed that the well-crystallized LiCoO2 nanorods could be obtained under this condition using the 300 nm-thick PAA template (its pore diameter was about 60 nm). As the PAA thickness increased, the LiCoO2 nanorods could not be obtained due to the step coverage limitation of sputtering technique. Also, the CV measurement showed that the RFsputtered LiCoO2 nanorods exhibited well electrochemical reversibility to be used as cathode material.
Acknowledgement This work was supported under the grant No. NSC922120-M-006-003 by National Science Council (NSC), Taiwan.
References
Fig. 8. Cyclic voltammogram of 600 ◦ C-annealed LiCoO2 nanorods at a sweep rate of 100 V/s.
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Preparation of RF-sputtered lithium cobalt oxide nanorods by using porous anodic alumina (PAA) template C.L. Liao a , M.T. Wu a , J.H. Yen a , I.C. Leu b , K.Z. Fung a,∗ a
Department of Materials Science and Engineering, National Cheng Kung University, No. 1 Ta-Hsueh Road, Tainan 70101, Taiwan b Department of Electronic Engineering, Kun Shan University, No. 949 Da Wan Road, Yung-Kang City, Tainan Hsien 710, Taiwan
Received 11 April 2005; received in revised form 22 July 2005; accepted 25 July 2005 Available online 8 September 2005
Abstract Recent years have shown a growing interest in the investigation of lithium microbatteries. In this study, LiCoO2 thin-films and nanorods were obtained by radio frequency magnetron sputtering. In order to obtain well-crystallized LiCoO2 nanorods exhibiting excellent electrochemical properties, LiCoO2 films prepared at various sputtering conditions were first investigated by XRD, Raman, SEM, and charge–discharge test to find out a suitable sputtering condition for the nanorod preparation. The 600 ◦ C-annealed LiCoO2 films deposited at 250 ◦ C exhibited the best cycleability and capability, and consequently, these conditions were used to deposit the LiCoO2 nanorods. According to the SEM observations, the LiCoO2 nanorods were successfully deposited into the porous anodic alumina template (thickness and diameter were 300 and 60 nm) under the sputtering conditions. The cyclic voltammogram of 600 ◦ C-annealed LiCoO2 nanorods showed that the sputtered LiCoO2 nanorods exhibited excellent electrochemical reversibility to be used as the cathode material. © 2005 Elsevier B.V. All rights reserved. Keywords: Lithium cobalt oxide; RF-sputtering; Nanodot; Porous anodic alumina
1. Introduction Within the past 10 years, lithium batteries have been an essential energy storage component for the portable electronics. The performance of lithium batteries is highly contingent upon the properties of cathode materials. Therefore, cathode materials such as LiCoO2 , LiNiO2 , and LiMn2 O4 have been widely investigated. Today, LiCoO2 is the primary commercial cathode material in lithium batteries due to its advantages of high specific capacity, high operating voltage, and long cycle life [1–5]. For electrode materials in electrochemical devices, a reduction in dimensions becomes a significant demand in order to increase the effective reaction area of the electrodes. Moreover, it is well known that the electrochemical ∗
Corresponding author. Tel.: +886 6 2380208; fax: +886 6 2380208. E-mail address: [email protected] (K.Z. Fung).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.07.042
properties of the thin-film electrode could be enhanced by increasing its surface area due to the minimization of the diffusion paths of lithium ions. Thus, there is a great demand for the investigation of materials with special structures such as nanorods, nanowires, nanotubes, nanodots, and nano-arrayed membranes. A way to obtain these structures is to use the template-assisted method combined with several deposition methods such as sol–gel, chemical vapor deposition (CVD), physical vapor deposition (PVD), electroplating, and electrophoretic deposition [6–10]. Although it is now common to find LiCoO2 nanowires prepared by the sol–gel method using the template-assisted method [6], but there are currently no reports on LiCoO2 nanorods prepared by RF-sputtering using the template-assisted method. In addition, in order to investigate the effect of the nano-structure on electrochemical behaviors of RF-sputtered LiCoO2 cathode, the methods to prepare the thin-films and nanorods should be the same. Consequently, in this work, LiCoO2 nanorods were obtained
C.L. Liao et al. / Journal of Alloys and Compounds 414 (2006) 302–309
by radio frequency (RF) magnetron sputtering and templateassisted method. In general, the RF-sputtered material should then undergo a post-annealing to enhance its crystallinity and electrochemical properties (consequently, during the annealing the crystallinity of porous anodic alumina (PAA) would be enhanced at the same time). Yet, in our experiments it was difficult for the NaOH solution to remove the crystallized PAA. Therefore, it was necessary to find out a suitable sputtering condition in order to obtain well-crystallized LiCoO2 and remove the PAA easily. In conducting this investigation, the structure and electrochemical properties of LiCoO2 films prepared under several conditions were investigated first. Finally, LiCoO2 nanorods prepared by RF-sputtering using the template-assisted method were obtained using the best sputtering deposition conditions. 2. Experimental 2.1. LiCoO2 thin-film deposition In order to find out a suitable sputtering condition for the deposition of LiCoO2 nanorods, LiCoO2 films obtained at various sputtering conditions were first investigated. Ptcoated silicon wafers were used as substrates to deposit LiCoO2 films. All of the LiCoO2 films and nanorods were deposited by RF magnetron sputtering from the LiCoO2 target. The 2 in. diameter LiCoO2 target was sintered at 700 ◦ C for 2 h by hot-pressing LiCoO2 powders that were calcined in an oxygen atmosphere from Li2 CO3 and CoCO3 at 700 ◦ C for 12 h. After presputtering the target for 15 min, the sputtering deposition was carried out under a pressure of 20 m Torr and a gas flow rate of 12 sccm. The Ar and O2 gas flow rate was 3:1. The RF power and the distance between substrate and target were 100 W and 40 mm, respectively. During the deposition process, the substrate temperature was controlled by a heater and held at ambient temperature, 250 and 600 ◦ C. In order to enhance the crystallinity of the LiCoO2 films, the films were then annealed at 600 ◦ C for 2 h in a controlled oxygen atmosphere. An X-ray diffractometer using Cu K␣ radiation ˚ and Raman spectroscopy (Coherent Innova (λ = 1.5418 A) 90) were used to characterize the structure and the crystallinity of LiCoO2 films. The morphologies and crosssections of LiCoO2 films were observed by scanning electron microscopy (Philips, XL-40FEG). The charge–discharge characteristics (measured by Arbin, BT2043) of LiCoO2 were investigated on the Li|LiCoO2 cell using polycrystalline LiCoO2 films as the cathode (the active area is 1 cm2 ), lithium metal as the anode, and 1 M LiPF6 in propylene carbonate as the electrolyte. 2.2. LiCoO2 nanowires deposition LiCoO2 nanorods were finally prepared under the same conditions as the 250 ◦ C-deposited LiCoO2 film. PAA/Si
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substrate was used to deposit LiCoO2 nanorods. It was fabricated by anodizing the Al/Au/Ti/SiO2 /Si substrates via a two-step anodization process. During this process, a thin SiO2 layer of 20 nm thickness was prepared by thermal oxidation using a high-temperature tube furnace, and a Ti layer of 20 nm thickness was deposited by E-beam evaporation. The SiO2 layer was introduced as a barrier layer against the oxidation of Si when the Al film was consumed completely during anodization. In addition, the Ti layer served as an adhesive layer to increase the adhesion of Al and Si substrate. After the deposition of SiO2 and Ti layers, the Al layer was finally deposited on the substrate by E-beam evaporation. The first-step anodization was conducted in 0.3 M oxalic acid solution in a cooling circulation bath at 13 ◦ C and with an anodizing voltage of 40 V. The anodized alumina formed in the first anodization step was removed by the mixture of 12 wt.% H3 PO4 and 6 wt.% H2 Cr2 O4 . The remaining aluminum was anodized again until it was completely converted into alumina. After the two-step anodization, the thickness of PAA stood at about 300 nm. Finally, PAA was immersed into 5 wt.% H3 PO4 for 30 min to remove the barrier layer between alumina and Au/Ti/SiO2 /Si substrate. After depositing LiCoO2 onto the PAA templates, the annealing process was carried out with the PAA template because the as-deposited LiCoO2 would otherwise have been damaged by the NaOH solution due to its poor crystallinity. Therefore, the removal of PAA proceeded after the conclusion of the annealing process. After removing the PAA template by 2 wt.% NaOH solution, the LiCoO2 nanorods were obtained. SEM (Philips, XL-40FEG) was used to observe the morphologies of LiCoO2 nanorods. Afterwards, the structure of the LiCoO2 nanorods was analyzed by TEM (JEOL, JEM-3010) analysis. The cyclic voltammetric characteristic (measured by EG&G, Potentiostat 273A) of LiCoO2 nanorod was also investigated on a Li|LiCoO2 cell using LiCoO2 nanorods as the cathode, lithium metal as the anode, and 1 M LiPF6 in propylene carbonate as the electrolyte under a sweep rate of 100 V/s with the voltage ranging from 3.0 to 4.2 V.
3. Results and discussion 3.1. Structural analysis of RF-sputtered LiCoO2 films It is known that LiCoO2 exhibits three different symmetries: hexagonal with R3m symmetry, cubic with Fm3m symmetry, and spinel with Fd3m symmetry [11]. Previous researches have shown Raman spectroscopy to be a powerful instrument in distinguishing these three different symmetries due to the different Raman spectra [12–15]. Fig. 1(a) shows the Raman spectrum of as-deposited LiCoO2 films obtained at ambient temperature. In this figure, there are two RS peaks located at 580.3 and 467.2 cm−1 , which respectively correspond to A1g and Eg of the hexagonal LiCoO2 with R3m symmetry. The XRD result (not shown here) also indicates that the as-deposited LiCoO2 films obtained at ambient tem-
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Fig. 1. Raman spectrum of (a) as-deposited and (b) 600 ◦ C-annealed LiCoO2 films deposited at ambient temperature.
perature exhibit crystalline structure with (1 0 4) out-of-plane texture. They also indicate that the crystallized LiCoO2 films exhibit only hexagonal LiCoO2 without any second phase. Thus, it is suggested that the sputtered atoms might stack on the substrate in short-range order, and, consequently, the films shows nano-crystallized structure. This result is different from some articles reported earlier [16–19], which indicated that as-deposited films without substrate heating or post-annealing exhibited amorphous structure. Nevertheless, a recent publication by Whitacre et al. also reported that as-deposited LiCoO2 films obtained at ambient temperature by RF-sputtering exhibited nano-crystallized structure and (1 0 4) out-of-plane texture [20]. Therefore, in this work, the as-deposited films exhibit nano-crystallized structure is expected. According to our previous study [21], the as-deposited LiCoO2 films with nanocrystalline structure did not show excellent electrochemical properties due to insufficient crystallinity. However, after the LiCoO2 films underwent high temperature annealing, the electrochemical properties were enhanced significantly. Therefore, we concluded that postannealing was a necessary step to enhance the crystallinity and the electrochemical properties of the films. Unfortunately, along with enhancing the electrochemical properties of LiCoO2 by annealing, the annealing process enhanced the crystallinity of the PAA template as well. Our recent tests showed that the 700 ◦ C-crystallized PAA was difficult to remove in nanorod preparation. However, the 600 ◦ C-crystallized PAA was easier to remove, and, consequently, the annealing temperature was chosen at 600 ◦ C for the nanorod preparation. The Raman spectrum of 600 ◦ Cannealed LiCoO2 film is shown in Fig. 1(b). One can see from the figure that the 600 ◦ C-annealed LiCoO2 film deposited at ambient temperature still consists of LiCoO2 phase because there are only two well-defined RS peaks of hexagonal LiCoO2 located at 591.5 and 479.3 cm−1 . The
Fig. 2. XRD patterns of 600 ◦ C-annealed LiCoO2 films deposited at substrate temperature of (a) ambient temperature, (b) 250 ◦ C, and (c) the pattern of LiCoO2 film deposited at substrate temperature of 600 ◦ C without any post-annealing.
increasing intensity and the shift of RS peaks indicate that the 600 ◦ C-annealing process enhance the crystallinity of sputtered LiCoO2 films. Fig. 2(a) and (b) shows the XRD patterns of 600 ◦ Cannealed LiCoO2 films obtained at ambient temperature and 250 ◦ C, respectively. Fig. 2(c) shows the film directly deposited at a substrate temperature of 600 ◦ C. From Fig. 2(a) and (b), it is evident that both the 600 ◦ C-annealed films consist of LiCoO2 single phase and exhibit (1 0 4)-preferred orientation. Also, the film directly deposited at 600 ◦ C exhibits (1 0 1)-preferred orientation. These results are similar to the article reported by Bates et al., which concluded that the (1 0 1)-, (1 1 0)-, and (1 0 4)-preferred orientation were favored to minimize the volume strain energy of the LiCoO2 film [3]. Therefore, it is expected that even though the LiCoO2 films are obtained from various deposition conditions, they would all consist of these specific orientations in order to minimize the volume strain energy. 3.2. SEM observations of RF-sputtered LiCoO2 films Fig. 3(a) and (b) shows the SEM top-view observations of the 600 ◦ C-annealed LiCoO2 films deposited at ambient temperature and 250 ◦ C, respectively. Fig. 3(c) illustrates the film directly obtained at a substrate temperature of 600 ◦ C. It is clear that the films obtained at various conditions all show continuous and uniform morphologies. It is also obvious that the 600 ◦ C-annealed films exhibit nano-sized grains, however, the films directly deposited at substrate temperature of
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Fig. 3. SEM images of 600 ◦ C-annealed LiCoO2 films deposited at (a) ambient temperature and (b) 250 ◦ C, and (c) the SEM image of LiCoO2 film deposited at a substrate temperature of 600 ◦ C without any post-annealing.
600 ◦ C show submicron and needle-like grains as well. The different morphologies of these films are due to the various substrate temperatures during the deposition process. It is suggested that the higher substrate temperature is similar to anneal the films during the sputtering deposition process, thus, the grain growth is conspicuous, and consequently, the films show submicron grains.
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Fig. 4. (a) Discharge curves of the Li//LiCoO2 cells with LiCoO2 films obtained from various conditions (deposited at ambient temperature and annealed at 600 ◦ C (- - -), 250 ◦ C-deposited and annealed at 600 ◦ C (—), and 600 ◦ C-deposited (– – –)) and (b) discharge capacity vs. cycle number plot of LiCoO2 films obtained from various conditions (deposited at ambient temperature and annealed at 600 ◦ C (), 250 ◦ C-deposited and annealed at 600 ◦ C (), and 600 ◦ C-deposited (䊉)).
3.3. Electrochemical properties of RF-sputtered LiCoO2 films Fig. 4(a) shows the discharge curves of LiCoO2 films deposited at various sputtering conditions. The electrochemical measurements were carried out under a current density of 10 A/cm2 in the potential range of 4.2–3.0 V. In Fig. 4(a), the 600 ◦ C-annealed films obtained at ambient temperature and 250 ◦ C show a well-defined discharge plateau
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at about 3.9 V. This discharge plateau corresponds to the main de-intercalation reaction of Li ions from the LiCoO2 layered structure. After the voltage discharging to lower than 3.9 V, the voltage drops rapidly to the cut-off voltage of 3.0 V. The first discharge capacity of 600 ◦ C-annealed films obtained at ambient temperature and 250 ◦ C are 48.18 and 50.39 Ah/cm2 m, respectively. Nevertheless, for lack of post-annealing at 600 ◦ C, the discharge curve of LiCoO2 film directly deposited at 600 ◦ C shows an unobvious discharge plateau that is due to the lower crystallinity of LiCoO2 film. Furthermore, the 1st discharge capacity of the film is only 42.49 Ah/cm2 m. These results indicate that the crystallinity and the capability of 600 ◦ C-annealed films are superior to the film directly deposited at 600 ◦ C. Fig. 4(b) gives the discharge capacity versus cycle number plot of LiCoO2 films. Obviously, the 10th discharge capacities of 600 ◦ C-annealed films deposited at ambient temperature and 250 ◦ C are 42.58 and 49.86 Ah/cm2 m, respectively, while the 10th discharge capacity of 600 ◦ C-deposited
film is only 30.65 Ah/cm2 m. The charge–discharge tests show that the 600 ◦ C-annealed LiCoO2 film obtained at 250 ◦ C exhibits the largest discharge capacity and the best cycle retention. Consequently, the following LiCoO2 nanorod preparation was carried out under this sputtering condition (substrate temperature of 250 ◦ C). 3.4. SEM observations of RF-sputtered LiCoO2 nanorods Based on the charge–discharge tests of LiCoO2 films obtained under various sputtering conditions, the 250 ◦ Cdeposited film that underwent a 600 ◦ C post-annealing exhibited excellent capacity and cycleability. Therefore, the sputtering parameters for depositing LiCoO2 nanorods using the template-assisted method were chosen at RF power of 100 W, working pressure of 20 m Torr, gas flow rate of 9 sccm Ar and 3 sccm O2 , substrate temperature of 250 ◦ C, and postannealing at 600 ◦ C.
Fig. 5. SEM top-view images of PAA (a) before and (b) after deposition, and SEM cross-section observations of LiCoO2 nanorods deposited into PAA template for which thicknesses were (c) 300 nm and (d) 500 nm and (e) without PAA template (removed by 2 wt.% NaOH).
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Fig. 5(a) shows the SEM top-view image of the PAA template before the LiCoO2 deposition. This SEM image shows that the PAA exhibits fine ordering arrangement pores over the micron area, and the average pore diameter is about 60 nm. In addition, Fig. 5(b) shows the SEM top-view image of the PAA template after the deposition of LiCoO2 . It indicated that after the deposition of LiCoO2 , the average pore size of the PAA template reduces from 60 to 30 nm. It is suggested that the LiCoO2 could be deposited into the channels of the PAA template. Fig. 5(c) and (d) shows the SEM cross-sectional images of RF-sputtered LiCoO2 deposited onto the PAA/Si substrates whose thicknesses are 300 and 500 nm, respectively. Interestingly, in Fig. 5(c), the LiCoO2 is successfully deposited into the channels of 300 nm-thick PAA template; but as the PAA thickness is thicker than 500 nm, the PAA template with empty channels can be observed in Fig. 5(d). According to the step coverage limitation of sputtering technique, the PAA channels with appropriate aspect ratio could be an important parameter in obtaining sputtered nanorods on PAA/Si substrate. Instead, it could only be deposited into the top of the channels when the aspect ratio of PAA template is too large and, subsequently, just formed a film covering the PAA surface. Fig. 5(e) shows the LiCoO2 nanorods without the PAA template that was removed by 2 wt.% NaOH solution. The diameter of the LiCoO2 nanorods illustrated in Fig. 5(e) is about 60 nm and is similar to that of the PAA channels. Furthermore, the LiCoO2 nanorods tends to aggregate to each other is due to the Van der Waal’s force. In addition, it is well known without any applied bias that the sputtered atoms would not have the trend to move in a direction that parallel to PAA channels. However, the sputtered atoms might collide with the wall of the PAA channel and deposit into the deeper pits of the PAA channels. A sketch of this description for LiCoO2 nanorods deposition is shown in Fig. 6. Consequently, after the deposition, annealing, and removal of the PAA, the LiCoO2 nanorods were obtained. Based on the SEM results, it was suggested that 300 nm is the thickness limitation of the PAA template (its pore diameter is 60 nm) to fabricate LiCoO2 nanorods by sputtering technique. 3.5. Structural characterization of RF-sputtered LiCoO2 nanorods TEM with electron diffraction is used to verify the crystal structure of LiCoO2 nanorods obtained from RF-sputtering deposition. The TEM bright field image and diffraction pattern of LiCoO2 nanorods are given in Fig. 7(a) and (b), respectively. The LiCoO2 nanorods were deposited at the sputtering parameters described in previous section. As seen in Fig. 7(a), which illustrates the TEM bright field image of LiCoO2 nanorods, the dimension of LiCoO2 nanorods is in agreement with that of the PAA channel. Moreover, the SAED pattern of LiCoO2 nanorods consisting of (0 0 3), (1 0 1), (1 0 4), and (1 1 0) reflections is a typical diffraction of LiCoO2 with R3m symmetry. All the reflections corre-
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Fig. 6. A sketch of the LiCoO2 nanorods deposited into the PAA template.
spond to LiCoO2, and no second phase to be observed. Thus, using template-assisted method and RF-sputtering technique, well-crystallized LiCoO2 nanorods could be successfully fabricated. 3.6. Cyclic voltammetric characteristic of LiCoO2 nanorods According to the most important aspect of electrode materials is the electrochemical activity; the cyclic voltammetric (CV) test is used to investigate the redox of the materials. Fig. 8 shows the CV measurement of sputtered LiCoO2 nanorods obtained from a 300 nm-thick PAA template. The CV plots exhibit one conjugate pair of redox peaks corresponding to an anodic peak (a) and a cathodic peak (b). These anodic and cathodic peaks are attributed to the reactions of Li ions de-intercalated from and intercalated into the LiCoO2 layer structure [22,23]. The anodic peak in the 1st
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cycle that shifts to higher voltage (4.01 V) is attributed to the over-potential to activate the material. After the LiCoO2 is activated, the subsequent anodic and cathodic peaks relocate to about 3.95 and 3.85 V, respectively. After the first cycle testing, the subsequent CV plots (second to fifth cycle) are approximately similar. These results indicate that the LiCoO2 nanorods obtained by RF-sputtering exhibit well enough reversibility (or cycleability) to be used as a cathode in lithium batteries. Nonetheless, the well reversibility might be relative to the specific nanorod structure due to the consideration of the nanorod structure could reduce the path of Li diffusion and enhance its electrochemical properties of the electrodes. Consequently, the detailed charge–discharge measurements with various current densities will be reported and discussed in the near future.
4. Conclusion
Fig. 7. (a) TEM bright field image and (b) SAED pattern of a 600 ◦ Cannealed LiCoO2 nanorod that deposited at 250 ◦ C by RF-sputtering.
LiCoO2 films deposited under various conditions all showed a single phase of LiCoO2 without any second phase. When comparing the electrochemical properties of the films obtained at various conditions, the 600 ◦ C-annealed LiCoO2 films deposited at 250 ◦ C showed the highest discharging capacity (capability) and best cycle retention (cycleability). According to the SEM observation and TEM analysis, we confirmed that the well-crystallized LiCoO2 nanorods could be obtained under this condition using the 300 nm-thick PAA template (its pore diameter was about 60 nm). As the PAA thickness increased, the LiCoO2 nanorods could not be obtained due to the step coverage limitation of sputtering technique. Also, the CV measurement showed that the RFsputtered LiCoO2 nanorods exhibited well electrochemical reversibility to be used as cathode material.
Acknowledgement This work was supported under the grant No. NSC922120-M-006-003 by National Science Council (NSC), Taiwan.
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Fig. 8. Cyclic voltammogram of 600 ◦ C-annealed LiCoO2 nanorods at a sweep rate of 100 V/s.
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