Ag

Electrochimica Acta 53 (2008) 8134–8137

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Lithium insertion in ultra-thin nanobelts of Ag2 V4 O11 /Ag Zhanjun Chen a,b , Shaokang Gao a,b , Ronghua Li b , Mingdeng Wei a,c,∗ , Kemei Wei c , Haoshen Zhou d,∗∗ a

Institute of New Energy Technology and Nano-Materials, Fuzhou University, Fuzhou, Fujian 350002, China College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou, Fujian 350002, China c National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, China d National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan b

a r t i c l e

i n f o

Article history: Received 8 May 2008 Accepted 6 June 2008 Available online 20 June 2008 Keywords: Ag2 V4 O11 /Ag Ultra-thin nanobelts Lithium intercalation Electrochemical behavior

a b s t r a c t In this study, ultra-thin nanobelts of Ag2 V4 O11 /Ag were successfully synthesized. The synthesized ultrathin nanobelts of Ag2 V4 O11 /Ag are highly crystalline and the thickness is found to be about 5 nm. A lithium battery using ultra-thin nanobelts of Ag2 V4 O11 /Ag as the active materials of the positive electrode exhibits a high initial discharge capacity of 276 mAh g−1 , corresponding to the formation of Lix Ag2 V4 O11 (x = 6). With increased cycling, the electrode made of ultra-thin nanobelts of Ag2 V4 O11 /Ag tends to loose electrochemical activity due to Ag+ ions in the ultra-thin nanobelts of Ag2 V4 O11 were reduced and new phase was formed. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction One-dimensional (1D) nanomaterials, including nanotubes, nanowires, nanobelts, nanoribbions, nanofibers and nanorods, have exhibited specific chemical and physical properties due to their low dimensions of nanometer-size magnitude, which differs greatly from that of their bulk counterparts [1]. So far, a large number of 1D structural nanomaterials, such as TiO2 [2], V2 O5 [3], V2 O4 ·0.25H2 O [4], VO2 [5], Agx V2 O5 [6], AgVO3 [7], and Ag2 V4 O11 [8], have been widely synthesized. Among them, silver vanadates are important materials owing to their ionic properties and the potential applications as cathode material in rechargeable lithium batteries. The detailed structural and phase analysis of silver vanadates have been widely reported [9]. Recently, much attention has been attracted to insert other cations into silver vanadates to obtain novel materials with a high conductivity [10]. Over the past few years, the synthesis of 1D nanostructural silver vanadium oxides has been widely reported. Qian and coworkers [11] prepared single-crystal nanowires of ␤-Ag0.33 V2 O5 by the hydrothermal reaction of AgNO3 and NH4 VO3 at 180 ◦ C. Using a template method, ultra-long nanoribbon bundles of ␤-AgVO3 were synthesized [12]. Mao et al. [13] obtained the single-crystal

∗ Corresponding author at: Institute of New Energy Technology and NanoMaterials, Fuzhou University, Industrial Road 523, Fuzhou, Fujian 350002, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (M. Wei), [email protected] (H. Zhou). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.06.014

Ag2 V4 O11 nanobelts using vanadium pentoxide powder and silver nitrate as reagents in the presence of 1,6-hexanediamine. Recently, Shen and Chen [8] and Zhang et al. [14] also reported the synthesis of Ag2 V4 O11 nanowires. In this work, ultra-thin nanobelts of Ag2 V4 O11 /Ag were successfully synthesized using a simple synthetic route. The electrochemical properties of ultrathin nanobelts of Ag2 V4 O11 /Ag were also investigated and reveal a large initial capacity of 276 mAh g−1 at a current density of 0.02 Ag−1 . 2. Experimental 2.1. Synthesis The ultra-thin nanobelts of Ag2 V4 O11 /Ag were prepared on the basis of a published procedure with modification [8]. At first, 0.068 g of commercial V2 O5 powder was dispersed into 70 ml of deionized water, and was then transferred to a 95 ml Teflon-lined autoclave in an oven at 170 ◦ C for 24 h. After the reaction, a yellow solution was obtained. Secondly, 0.117 g of Ag2 SO4 powder was added into the above yellow solution, and was then transferred to autoclave again, and kept at 170 ◦ C for 48 h. The product was washed and filtered, and then dried at 60 ◦ C for 4 h in vacuum. 2.2. Characterizations X-ray powder diffraction (XRD) patterns were obtained using a diffractometer (Cu K␣, PANalytical, X’Pert). Scanning electron microscope (SEM) and transmission electron micrograph (TEM)

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were taken on a Philip-XL30 instrument and a JEOL 2010 instrument, respectively. 2.3. Electrochemical measurements For electrochemical measurement, the samples were mixed and ground with 5 wt.% Teflon (poly(tetrafluoroethane)) powder as a binder and 45 wt.% acetylene black carbon (AB) powder as the conductive assistant materials. The mixture was spread and pressed on a 0.25 cm2 nickel mesh (100 mesh) as the working electrode (WE). The reference (RE) and counter electrode (CE) were prepared by spreading and pressing lithium metal onto a similar nickel mesh. The electrolyte was 1 M LiClO4 in ethyl carbonate (EC) and dimethyl carbonate (DMC) (EC/DMC = 1:1, v/v). Cell assembly was carried out in a glove box under an argon atmosphere. CV studies were performed over a potential range of 1.5–3.75 V (Li+ /Li) at scan rates of 0.1 mV s−1 . Galvanostatic discharge–charge was performed in a potential range of 1.5–3.75 V (Li+ /Li) under constant current densities of 0.02, 0.05 and 0.1 Ag−1 . The weight was based on the active materials (Ag2 V4 O11 , excluding AB and Teflon, the amount of Ag nanoparticles is too small to be calculated). 3. Results and discussion Fig. 1 shows the XRD patterns of the product synthesized at 170 ◦ C for 48 h. All the diffraction peaks can be indexed to pure phase of Ag2 V4 O11 with a monoclinic structure (JCPDS 20-1385). The morphologies of the product can be confirmed by using SEM and TEM measurements. As depicted in Fig. 2a, a large number of nanobelts were found in the product. These nanobelts lie close to each other to form bundle morphology and their length was up to several tens of micrometers. The morphology of the products can be further confirmed from TEM measurements. As shown in Fig. 2b,

Fig. 1. XRD pattern of sample synthesized at 170 ◦ C for 48 h.

it clearly reveals the presence of a large number of nanobelts in the product. A TEM image in Fig. 2c clearly indicates that the nanobelts are very thin and their thickness is found to be about 5 nm. Apart from the nanobelts, numerous small particles on the surface of nanobelts were also detected. The Ag particles are uniform and the size ranges from 2 to 5 nm. As depicted in Fig. 2d, the lattice fringe of particles corresponds to a d spacing of 0.23 nm, which is in good agreement with the d1 1 1 spacing in the XRD pattern of Ag (JCPDS 04-0783). However, Ag was not detected by XRD measurement, indicating amount of Ag is too small to be detected. Fig. 2d also exhibits highly crystalline nanobelts, and the lattice fringe corresponds to a d spacing of 0.34 nm. This agrees with the d−2 0 2 spacing

Fig. 2. SEM and TEM images of sample synthesized at 170 ◦ C for 48 h. (a) SEM image, (b) low magnification TEM image, and (c and d) high magnification images.

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Fig. 3. CV curve of an electrode using ultra-thin nanobelts of Ag2 V4 O11 /Ag as the cathode and lithium metal as the anode over the voltage range of 1.5–3.75 V.

in the XRD pattern of Ag2 V4 O11 . Thus, the synthesized products are composed of Ag2 V4 O11 nanobelts and Ag nanoparticles, denominated as Ag2 V4 O11 /Ag. To obtain single-phase Ag2 V4 O11 nanobelts, a series of synthetic experiments were performed. Unfortunately, the single-phase Ag2 V4 O11 nanobelts were not formed. High-valent vanadium oxide (V2 O5 ) is well known to act as electrode material for lithium intercalation (xLi+ + xe− + V2 O5 → Lix V2 O5 ). Here, the electrochemical properties of a lithium ion’s intercalation/deintercalation into/from the Ag2 V4 O11 nanobelts are characterized. The cyclic voltammetry (CV) curves of the ultra-thin nanobelts of Ag2 V4 O11 /Ag at a scan speed of 0.1 mV s−1 in a potential window of 3.75–1.5 V (vs. Li+ /Li), which is the potential range of V5+ ↔ V4+ and V4+ ↔ V3.5+ , give the sharp reversible intercalation and deintercalation peaks shown in Fig. 3. The lithium ion’s insertion and extraction process can be described by following equation. +

−

xLi + xe + Ag2 V4 O11 /Ag ↔ Lix Ag2 V4 O11 /Ag

(1)

The reduction peaks located at 2.76 and 2.39 V (vs. Li+ /Li) resulted from the V5+ → V4+ and V4+ → V3.5+ , respectively, while the oxidation peaks located at 3.52 and 2.70 V (vs. Li+ /Li) resulted from the V4+ → V5+ and V3.5+ → V4+ , respectively.

Fig. 4. The discharge–charge profiles of ultra-thin nanobelts of Ag2 V4 O11 /Ag at different current densities. (a) 0.02, (b) 0.05, and (c) 0.1 Ag−1 .

Fig. 5. The discharge–charge profiles (a) and cycle performance (b) of electrodes composed of Ag2 V4 O11 /Ag nanobelts at a current density of 0.02 Ag−1 .

Fig. 4 depicts the first discharge–charge curves of the ultra-thin nanobelts of Ag2 V4 O11 /Ag for different current densities. The initial discharge capacity was found to be 276 mAh g−1 and is close to the theoretic capacity of 270 mAh g−1 , corresponding to the formation of Lix Ag2 V4 O11 /Ag (x = 6). Based on the discharge curves, the process of Li insertion into the ultra-thin nanobelts of Ag2 V4 O11 /Ag can be divided into three stages. First, a large amount of Li ions are associated with a long sloped region before the voltage plateau about 2.5 V (Li+ /Li), corresponding to the formation of Li2.67 Ag2 V4 O11 /Ag. Secondly, the discharge curve has a shorter voltage plateau around 2.5–2.4 V (Li+ /Li), indicating that the Li ions inserted into the crystal lattice of Ag2.67 V4 O11 with another 1.33 mol Li to form Li4.0 Ag2 V4 O11 /Ag. Finally, a sloped region (from 2.4 to 1.5 V (Li+ /Li)) also exhibits another 2 mol Li’s insertion with another long slope region to form Li6 Ag2 V4 O11 /Ag. Here, the compositions are only calculated based on the corresponding capacitances in the discharge curve. It is also found from Fig. 4 that the initial capacity decreased significantly with increasing current densities. For example, the capacity decreased from 276 to 150 mAh g−1 when the current density increased from 0.02 to 0.1 Ag−1 which means the electronic conductivity of the ultra-thin nanobelts of Ag2 V4 O11 /Ag is poor. Fig. 5 shows the discharge–charge curves and cycle performance of the electrode made of the ultra-thin nanobelts of Ag2 V4 O11 /Ag at 1st, 2nd, 5th, 10th, and 20th cycle at a current density of 0.02 Ag−1 . It clearly shows in Fig. 5a that the initial capacity decreased significantly with increasing cycling. The length of plateau (at ca.

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4. Conclusions In summary, ultra-thin nanobelts of Ag2 V4 O11 /Ag were successfully synthesized. The synthesized ultra-thin nanobelts of Ag2 V4 O11 /Ag are highly crystalline and the thickness is found to be about 5 nm. The results of electrochemical measurements indicate that an electrode composed of ultra-thin nanobelts of Ag2 V4 O11 /Ag exhibits a high initial discharge capacity of 276 mAh g−1 , corresponding to the formation of Lix Ag2 V4 O11 /Ag (x = 6). However, it tended to loose electrochemical activity with cycling. This is due to Ag+ ions in the Ag2 V4 O11 nanobelts were reduced into metallic Ag and new phase was formed, indicating that the electrode composed of the ultra-thin nanobelts of Ag2 V4 O11 /Ag is unstable. Acknowledgements

Fig. 6. XRD pattern of electrode made of Ag2 V4 O11 /Ag nanobelts after lithium ions intercalation/deintercalation scycling.

2.5 V) becomes shorter and tends to disappear, indicating that the insertion of Li ion into the matrix of Ag2 V4 O11 becomes more and more difficult and tends to loose activity. As depicted in Fig. 5b, the capacity decreased slightly in few initial cycles and dropped significantly to 30 mAh g−1 after 20 cycles, indicating that the electrode made of the ultra-thin nanobelts of Ag2 V4 O11 /Ag is unstable. Fig. 6 shows the XRD pattern of electrode made of the ultra-thin nanobelts of Ag2 V4 O11 /Ag after cycling. As shown in Fig. 6, apart from the peaks of metallic Ag, new phase of Ag1−x V2 O5 was formed, indicating that Ag+ ions in the Ag2 V4 O11 nanobelts were reduced into metal Ag and Ag1−x V2 O5 phase was formed with cycling. Thus, the reaction can be formulated as follows [15]. xe− + xLi+ + Ag2 V4 O11 → Ag2−x Lix V4 O11 + xAg

(2)

This might be a reason that the electrode made of the ultra-thin nanobelts of Ag2 V4 O11 /Ag tends to loose electrochemical activity.

This study was financially supported by the National High Technology Research and Development Program (“863”) under Grant No. 2007AA05Z438, Science and Technology Program from Fujian Province (No. 2005HZ01-2-4; 2005HZ-01-2-5; 2007HZ0005-1) and the startup fund from Fuzhou University. References [1] A.P. Alivisatos, Science 271 (1999) 933. [2] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999) 1307. [3] N. Pinna, M. Willinger, K. Weiss, J. Urban, R. Schlögl, Nano Lett. 3 (2003) 1131. [4] M.D. Wei, H. Sugihara, I. Honma, M. Ichihara, H.S. Zhou, Adv. Mater. 17 (2005) 2964. [5] B.S. Guiton, Q. Gu, A.L. Prieto, M.S. Gudiksen, H.K. Park, J. Am. Chem. Soc. 127 (2005) 298. [6] C.R. Xiong, A.E. Aliev, B. Gnade, K.J. Balkus Jr., ACS Nano 2 (2007) 293. [7] Y. Liu, Y.G. Zhang, Y.H. Hu, Y.T. Qian, Chem. Lett. 34 (2005) 146. [8] G.Z. Shen, D. Chen, J. Am. Chem. Soc. 128 (2006) 11762. [9] S. Sharma, M. Panthofer, M. Jansen, A. Ramanan, Mater. Chem. Phys. 91 (2005) 257. [10] T. Kato, Y. Katayama, T. Miura, T. Kishi, Solid State Ionics 134 (2000) 209. [11] Y. Liu, Y.G. Zhang, M. Zhang, Y.T. Qian, J. Cryst. Growth 289 (2006) 197. [12] M. Li, M.W. Shao, H.Z. Ban, H. Wang, H.Z. Gao, Solid States Ionics 178 (2007) 775. [13] C.J. Mao, X.C. Wu, H.C. Pan, J.J. Zhu, H.Y. Chen, Nanotechnology 16 (2005) 2892. [14] S.Y. Zhang, W.Y. Li, C.S. Li, J. Chen, J. Phys. Chem. B 110 (2006) 24855. [15] F. García-Alvarado, J.M. Tarascon, Solid State Ionics 73 (1994) 247.