Study of lithium storage properties of the Sn-Ni alloys prepared by magnetic sputtering technology

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1 Available online at Acta Metall. Sin.(Engl. Lett.)Vol.23 No.5 pp October 2010 Study of lithium storage properties of the Sn-Ni alloys prepared by magnetic sputtering technology Xianhua HOU, Shejun HU, Wei PENG, Zhiwen ZHANG and Qiang RU School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou , China Manuscript received 21 May 2010; in revised form 26 July 2010 Nano-level Sn-Ni alloy thin-film electrode materials prepared by magnetic sputtering technology are characterized with X-ray diffraction (XRD), atom force microscopy (AFM) and scanning electron microscopy (SEM). The charge/discharge and cyclic voltammograms (CV) of the films electrodes are tested by the battery testing system of high precision. The results indicate that the materials prepared by direct current (DC) and radio frequency (RF) methods differ greatly in their performance. Ni 3 Sn 2 alloy phase constitutes the main components prepared by DC method, the particles on the surface are tiny and show steady cycling performance, the deficiency is that they have low initial efficiency and small discharge capacity of 72% and 108 ma h/g, respectively. Contrary to the former, Ni 3Sn 4 alloy phase constitutes the main components prepared by RF method, the particles on the surface appear comparatively larger, their discharge capacity did not decline in the first 15 times, keeping above 500 ma h/g, but began to decline after 15 times. KEY WORDS Lithium ion battery; Sn-Ni alloy; Electrochemical property 1 Introduction Li rechargeable batteries have become one of the most developing power sources in the 21st century [1 3]. Recently, lithium-ion batteries are widely used as power sources for portable electronic devices. In the future, they may be adopted as power supply for the hybrid electric vehicle (HEV) and fully electric vehicle (EV). For applications such as HEV and EV, high capacity and long cycle life of the batteries have become the urge target of current research [4,5]. Tin-based anode materials have attracted much interests as alternatives to commercial carbon for lithium-ion batteries due to higher specific capacity and stable plateau potential [6 8]. It is well known that Sn-Ni alloy electrode provide longer cyclability than that of pure tin to a great extent, but the whole electrochemical properties have enormous differences with various synthesized methods. Mukaibo et al. [9] reported that electrodeposited Sn-Ni alloy from different compositions show considerably different performances as anode materials, and the performance was remarkably well (ca. 650 ma h/g at 70 th cycle) when the Corresponding author. Professor, PhD; Tel: ; Fax: address: husj@scnu.edu.cn; houxh5697@163.com (Shejun HU)

2 364 composition was controlled to Sn 62 Ni 38. Lee et al. [10] reported that nanocrystalline Ni 3 Sn 4 alloy powders prepared by high energy ball milling were examined as an anode for lithium ion batteries, and the specific capacity was less than 100 ma h/g after 100 cycles, and so on. There are three main stable phases Ni 3 Sn, Ni 3 Sn 2 and Ni 3 Sn 4 in practical electrode materials, their effects on the whole system are not very clear. The proper concentration of each phase, which plays a crucial role in preparing the effective electrode materials, is still unknown. Therefore, the lithiation mechanism investigation of various mesophases plays an important role in understanding the performance of Sn-Ni alloy electrode in rechargeable lithium ion batteries. In previous work, the mesophase Ni 3 Sn was studied by the first principle calculation. It showed that this phase was a devastating phase for whole alloy electrode because of the exhibiting relatively larger expansion ratio and specific low capacity [11]. Meanwhile, the physical characteristic and electrochemical parameters of the mesophase Ni 3 Sn 4 were investigated by the same theoretical method [12]. The result indicated that the Ni 3 Sn 4 alloy phase has relatively smaller expansion ratio and fluctuated electrochemical potentiality. This paper tries to focus on the lithium storage properties of the Ni 3 Sn 2 and Ni 3 Sn 4 alloy electrode prepared by direct current magnetic sputtering (DCMS) and radio frequency magnetic sputtering (RFMS) technology respectively. The corresponding electrochemical cycling performance was investigated via coin cells. 2 Experimental The Sn-Ni thin films electrode was deposited on copper-foil substrates in a deposition chamber by means of the direct current and radio frequency magnetic sputtering at room temperature respectively. The Sn-Ni alloy target with Sn and Ni can reach the ratio of 97 to 3. The copper-foil substrates were cleaned by a conventional procedure with alcohol and acetone ultrasonic cleaner for 15 min. The stable pressure in the sputtering chamber was Pa and the working pressure was limited to 3.0 Pa in the argon atmosphere. Firstly, radio frequency power of 100 W was used to bombard the substrates for 5 min to eliminate the impurity of the surface before depositing the thin film. Subsequently, the Sn-Ni thin films were obtained by sputtering Sn-Ni alloy target with D.C. power of 100 W and R.F. power of 250 W, respectively. Morphologies of the samples using the atom force microscope and the scanning electron microscope were observed. X-ray diffraction measurement was carried out over the 2θ ranging from 15 to 95 with a Philips 3100E diffractometer using CuK α radiation (λ=0.154 nm). The contents of Sn and Ni on the thin films were analyzed by using inductively coupled plasma-atomic emission spectrometry (ICP-AES) with IRIS Advantage The anode disks (10 mm in diameter) were punched and weighed before drying the vacuum at 100 C for 10 h, and the anode were incorporated into button cells with a lithium foil counter electrode. The cells were assembled in an argon-filled glove box (Mikrouna, Sukei1220/750) with less than 1 ppm each of oxygen and moisture. LiPF 6 solution (1 mol/l) dissolved with the mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) as the electrolyte at the proportion of 1:1:1, and Celgard 2400 as the separator. The cycling performance of the Sn-Ni thin film electrodes

3 365 were measured within a range from 0.0 to 2.0 V (vs. Li/Li + ), and the cycling was tested using LAND-2001A battery tester. Cyclic voltammetry was performed on a two-electrode hermetically sealed button cell, using a Solartron1480 electrochemical interface controlled by Corr-ware. The scan rate was 0.2 mv/s. 3 Results and Discussion The X-ray diffraction patterns for the Sn-Ni alloy thin films at different deposited methods are given in Fig.1. It shows from Fig.1a that the alloy thin film prepared by the direct current magnetic sputtering is composed of main Ni 3 Sn 2 phase and a small amount of pure Sn and Ni 3 Sn 4 phase. However, the alloy thin film prepared by radio frequency magnetic sputtering is composed of main Ni 3 Sn 4 phase and a small amount of pure Sn and Ni 3 Sn 2 phase as shown in Fig.1b. From the above discussion, we can know that the differences of the crystalline structure have a great influence on the properties of electrochemical performance. Following this, we will analyze the tested results and electrochemical properties. Fig.1 X-ray diffraction patterns of Sn-Ni alloy thin films for direct current magnetic sputtering (a) and radio frequency magnetic sputtering (b). The AFM images of the Sn-Ni alloy thin films are shown in Fig.2. As can be seen in Fig.2, the two films both are with rough surface. Obviously, the surface harshness of Sn-Ni thin films prepared by DCMS is much lower than that by RFMS. The film prepared by DCMS is composed of spherical grains with size ranges from 500 nm to 1 µm. The compact structure of the film deposited in DCMC process may due to the low depositing rate, this special structure is beneficial to improve the stability of the electrode circulation. However, the film prepared by RFMS consists of bigger grains of about 1 µm. Especially, a large amount of pores or voids have been formed in this films, which is beneficial to lithium insertion/extraction in the host material. Compared with the DCMS, RFMS has a relatively faster depositing rate, which results in the incomplete crystalline structure and harsh surface. When it is charging and discharging, the film electrode is apt to dissolve with electrolyte, resulting in much energy released from the active lithium ion. From this, we know that the properties weakening of recycling is quickened just because of the high irreversible capacity of the electrode. In addition, the shaggy structure can contribute to the large surface area of electrical contact. The large surface area can consume a lot of

4 366 electrochemical capacity for the formation of the solid electrolyte interface (SEI), which is attributed to the decomposition of the electrolyte on the surface of the active particles. Thus, the capacity will be rapidly decreased with increasing cycles. The SEM morphologies of the Ni 3 Sn 2 and Ni 3 Sn 4 alloy phases are presented in Figs.3a and 3b. The Ni 3 Sn 2 alloy presents as the squama and compact structure. Such a configuration seems to be able to reduce the lithium ion diffusion caused by narrow walls and Fig.2 AFM images of Sn-Ni films prepared by direct current magnetic sputtering (a) and radio frequency magnetic sputtering (b), respectively. Fig.3 SEM images of the Ni 3Sn 2 (a, c) and Ni 3Sn 4 (b, d) alloys before (a, b) and after (c, d) cycling 30 times.

5 367 improve the cycling stability. In addition, the Ni 3 Sn 4 alloy presents as a loose and porous morphological structure. The distance between two particles is sufficient to form the transportation channel of lithium ion and electrons. This structure has contribution to specific capacity of alloy electrode, but is against the cycle performance. SEM photographs of Ni 3 Sn 2 and Ni 3 Sn 4 alloy phases after cycling 30 times are shown in Figs.3c and 3d. It needs to be noted that good structure retention of the Ni 3 Sn 2 presents excellent cycling stability. Obviously, Ni 3 Sn 4 alloy film appears dramatical mechanical disintegration due to larger volume expansion during the charge and discharge process. Therefore, the practical and effective Sn-Ni alloy electrodes need to be controlled within the concentration of various mesophases and synthesis multiphases of alloy materials. Fig.4 compares the cycling performance of the Sn-Ni films prepared by DCMS with that prepared by RFMS. It is obvious in Fig.4a that the capacity recession of the Ni 3 Sn 2 alloy electrode is very obvious in the first 5 cycles. Subsequently, the charge/discharge capacity is stable and coulombic efficiency remains at 99%. Ni 3 Sn 2 alloy electrode has a stable reversible capacity of 108 ma h/g. Meanwhile, Ehrlich et al. [13] indicated that Ni 3 Sn 2 alloy electrode prepared by mechanical ball milling method had stable reversible capacity of 81 ma h/g after 100 cycles. Kim et al. [14] also proposed that Ni 3 Sn 2.1 films prepared by an e-beam evaporation process could present as an excellent cycle performance with no capacity recession after 500 cycles (800 ma h/cm 3 ). Therefore, Ni 3 Sn 2 alloy phase plays an important role in improving the cycle stability for thin film rechargeable lithium batteries. As shown in Fig.4b, the reversible capacity of Ni 3 Sn 4 maintains more than 540 ma h/g during the first 15 cycles without obvious capacity recession, perhaps the element Ni is an inactive matrix and can buffer the volume change of Sn during alloying/de-alloying with Li, while the discharge capacity decreases rapidly after 15 cycles. Fig.5 shows the first- and second-cycle curves of the cell in its charge and discharge processes, employing the Sn-Ni alloy as a working electrode incorporated with a lithium metal as a counter electrode. As shown in Fig.5a, the 1st discharge capacity of Ni 3 Sn 2 can be reached to 393 ma h/g and the 1st charge capacity is estimated to reach to ma h/g, and the coulombic efficiency in the 1st cycle is 83.1%. Thus, the initial irreversible capacity loss is attributed to the formation of the solid electrolyte interphase (SEI) film [15] due to the decomposition of the electrolyte on the surface of the active particles and the alloy phases of high formation energy [11,16] due to the reaction between active particles and lithium ions. Evidently, when the cell is discharged at 0.03 ma in the first lithiation, there Fig.4 Capacity and coulombic efficiency vs. cycle number for alloy electrodes: (a) Ni 3 Sn 2 and (b) Ni 3 Sn 4.

6 368 Fig.5 First- and second-cycle charge-discharge curves of alloy electrodes: (a) Ni 3 Sn 2 and (b) Ni 3Sn 4. is a long plateaus voltage at about 0.3 V, and there is a long plateaus voltage with an average voltage of 0.75 V during the 1st charge process. As shown in the cyclic voltammogram of Ni 3 Sn 2 in Fig.6, the main peak is near 0.8 V corresponding to the long plateaus voltage. On the other hand, for Ni 3 Sn 4 alloy, the cell is discharged at the same condition mentioned above, the 1st discharge capacity reached up to ma h/g and the 1st charge capacity is estimated to ma h/g, and the coulombic efficiency in the 1st cycle is 98.8%. There are four plateaus voltages at about 0.75, 0.6, 0.5 and 0.3 V, respectively, in which all fours are with the 2nd charge-discharge process. Meanwhile, according to CV curve of Ni 3 Sn 4 in Fig.6b, we found that the small peaks during the lithium insertion process depend on the four plateaus voltage of chargedischarge curves. This may be ascribed to the formation of high lithiation formation energy alloy phases. Fig.6 Cyclic voltammograms of the Ni 3Sn 2 (a) and Ni 3Sn 4 (b) electrodes. 4 Conclusions The Nano-level Sn-Ni alloy thin-film electrode materials were prepared by DCMS and RFMS. The electrode materials prepared by DCMS are mainly composed of Ni 3 Sn 2 alloy phase, while Ni 3 Sn 4 alloy phase chiefly constitutes the electrode materials by RFMS. Both

7 369 of the two ways have their own advantages. By DCMS, there would appear some tiny compact particles on the film surface, the corresponding charge/discharge process has good cyclical stability due to the relatively slow deposition rate. By RFMS, because of the quick deposition rate, the particles on the film surface would be comparatively larger and the connection between particles is loose, the corresponding charge/discharge capacity is higher but declines quickly when it cycles. Acknowledgements This work was supported by the National Natural Science Foundation of China (No ) and the authors would like to thank Prof. Weishan LI for beneficial suggestions. REFERENCES [1] K. Kang, Y.S. Meng, J. Breger, C.P. Grey and G. Ceder, Science 311 (2006) 977. [2] X.H. Hou, S.J. Hu, W.S Li, L.Z. Zhao, Q. Ru, H.W. Yu and Z.W. Huang, Chin Sci 53 (2008) [3] X.F. Ouyang, S.Q. Shi, C.Y. Ouyang, D.Y. Jiang, D.S Liu, Z.Q. Ye and M.S Lei, Chin Phys B 16 (2007) [4] T. Noriyuki, Q. Ryuji, F. Masahisa, F. Shin, K. Maruo and Y. Ikuo, J Power Sources 107 (2002) 48. [5] M.S. Park, Y.M. Kang, S. Rajendran, H.S. Kwon and J.Y. Lee, Mater Chem Phys 100 (2006) 496. [6] H. Li, X.J. Huang, L.Q. Chen, Z.G. Wu and Y. Liang, Electrochem Solid-State Lett 2 (1999) 47. [7] M.S. Park, S. Rajendran, Y.M. Kang, K.S. Han, Y.S. Han and J.Y. Lee, J Power Source 158 (2006) 650. [8] X.H. Hou, S.J. Hu and L. Shi, Acta Phys Sin 59 (2010) [9] H. Mukaibo, T. Momma and T. Osaka, J Power Sources 146 (2005) 457. [10] H.Y. Lee, S.W. Jang, S.M. Lee, S.J. Lee and H.K. Baik, J Power Sources 112 (2002) 8. [11] X.H. Hou, S.J. Hu, W.S. Li, Z.W. Huang, Q. Ru and H.W. Yu, Chin Phys B 17 (2008) [12] P. Lavela, F. Nacimiento, G.F. Ortiz and J.L. Tirado, J Solid State Electrochem 14 (2010) 139. [13] G.M. Ehrlich, C. Durand, X. Chen and T.A. Hugener, F. Spiess and S.L. Suib, J Electrochem Soc 147 (2000) 886. [14] Y.L. Kim, H.Y. Lee, S.W. Jang, S.J. Lee, H.K. Baik, Y.S. Yoon, Y.S. Park and S.M. Lee, Solid State Ionics 160 (2003) 235. [15] G.A. Nazri and G. Pistoia, Lithium Ion Batteries (Kluwer Academi Pub, Massachusettes, 2004) p.167. [16] X.H. Hou, S.J. Hu, W.S. Li, L.Z. Zhao, H.W. Yu and C.L. Tan, Acta Phys Sin 57 (2008) 2374.