1. USE OF NANOSTRUCTURED Sn THIN FILM ANODES FOR LITHIUM ION BATTERIES B.D. Polat, N. Sezgin, O.Keleş, K.Kazmanlı, A. Abouimbrane Istanbul Technical University Department of Metallurgical and Materials Engineering İstanbul 34469, Turkey Argonne National Laboratory Chemical Sciences and Engineering Division 9700 South, IL 60439-4837 Abstract Electron beam evaporation deposition method was used to produce nano porous Sn thin film. XRD analyses proved that the thin film was made of nano crystalline Sn particles. The galvanostatic charge discharge results showed that the nano porous Sn thin film had a high first discharge capacity although some capacity loss was observed the capacities became steady at high values afterwards Keywords:Tin anode, lithium ion batteries, electron beam evaporation deposition. Introduction The lithium ion battery industry is continuously looking for materials with higher charge-discharge capacities. In this context, Sn has attracted much attention as an anode material because of its high lithium packing density at a proper operating voltage window and high theoretical capacity (up to about 994mAhg -1 ). However, tin anodes, similar to other metallic materials, suffer from excessive volume changes, which occur during cycling. In order to overcome this problem some researchers have suggested to keep a narrow cycling potential range to get lower lithium containing compounds like LiSn so that less mechanical stress would be expected in the electrode structure [1-10]. On the other hand, there are some other research groups working on alloying Sn with different materials such as antimony [11-12], cobalt [13], copper [14-15], lanthanum [16], manganese [17], nickel [18], silver [19], titanium [20] or iron [21]. Another point of view on the subject was focused on the improvement of Sn architecture in order to be able to control the volumetric change of the anode structure. Production of nano- structured Sn thin films have been evaluated to represent an alternative solution as a new generation anode material in lithium ion batteries. Since the performance of a lithium ion battery electrode is limited by mass transport of anions and cations throughout the bulk electrode; an electrode with open porous architecture having a high accessible surface area is considered to be the optimum nanostructured electrode [22-27]. In this paper, electron beam evaporation method was used to produce nano porous Sn thin film anodes. The electrodes were characterized by using X-ray diffraction, field emission scanning electron microscopy (FE-SEM) and via electrochemical measurements and their electrochemical performance was discussed based on their morphology. Experimental Nano porous structured Sn thin films were deposited on a 15.5 mm diameter copper substrate (with 1.5mm thickness) by an electron beam evaporator from a pure metal (Sn) source. The film thickness was controlled by evaporation time and measured by using field-emission scanning electron microscopy (FE-SEM, JEOL JSM 7000F). The composition of the films was determined by energy dispersive X-ray spectroscopy (EDS) analysis. The surface microstructure was imaged with FE-SEM. X-ray diffraction (XRD) experiments were carried out with using Cu Kα radiation (Philips PW 3710). CR2032 coin cells were fabricated in order to test the electrochemical properties of thin-film electrodes. The cells were assembled in a glove-box (MBRAUN) under argon atmosphere. The testing cell is based on the following sequence: i) a working electrode, ii) a 1M LiPF 6 in ethylene carbonate-dimethyl carbonate, EC:DMC 1:1 (Merck Battery Grade) electrolyte solution iii) separator (Celgrad2400) and iv) a lithium metal foil counter electrode. All cells were tested at room temperature at a constant current in the voltage range of 0.05V–2.5 V versus Li/Li+ with a rate of 50mAh/g. Cyclic voltammetry (CV) was performed in the potential range of 0.05V-2.5V versus Li/Li + at a scan rate of with a rate of 0.03 mV/s. Results and Discussion Conclusion In this work, nano porous structured tin films were produced via electron beam deposition method. The study showed that the porous structured electrode had a high capacity value close to the theoretical capacity of Sn metal. Although some capacity loss was observed after the 25 th cycle, high charge-discharge capacities were still determined by the porous structured electrode. References M.H. Chen, Z.C. Huang, G.T. Wu, G.M. Zhu, J.K. You and Z.G. Lin, Materials Research Bulletin 5 831 (2003). Q. Wang, H. Li, L. Chen and X. Huang, Solid State Ionics 152-153 43 (2002). J.C. Arrebola, A. Caballero, J.L.G. Camer, L. Hernen, J. Morales and L. Sanchez, Electrochem. Commun. 11, 1061 (2009). M.J. Lindsay, G.X. Wang and H.K. Li, J. Power Sources 119 84 (2003). M. Wachtler, J.O. Besenhard and M. Winter, J. 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Power Sources, 81-82, 13 (1999). Figure 1. XRD patterns of Sn thin film anodes having a.nonporous, b.nano porous structures Figure 2. FE-SEM a) surface and b) cross sectional views of Sn thin film anodes having ab Figure 3 shows the capacity delivered during charge-discharge when nano porous thin films were cycled versus lithium foil counter electrode. The charge capacity in the first cycle of sample was around 1000 mAh/g, almost equal to the theoretical capacity, which may indicate Li 22 Sn 5 formation. This high performance proved that the mass transport of anions and cations through the bulk electrodes limited the performance of lithium ion batteries. [24]. The pillar like structure of the sample facilitated the lithium movement within the electrode and shortened the Li + diffusion distances hence, less time was needed to achieve full charge or discharge at the same current density compared to the first sample. In addition, the larger surface area of the electrode reduces the local current density and decreases the polarization. Besides, the electrode with a continuous assembly of pillar like structures can enhance the contact with substrate material and suppresses free particle movement, to delay aggregation of tin particles on electrode materials [24]. Figure 4 shows cyclic voltammograms (CV) of the tin anodes having nanoporous structures. An irreversible reduction peak was shown approximately at 0.7V versus Li/Li+, in addition, at 0.5V and 0.3 V some other reduction peaks were observed related to the formation of Li-Sn alloys, with Li deficient and Li-rich phases respectively (Figure 3) [28, 29]. Results and Discussion XRD results showed the presence of Sn crystallines deposited on copper substrate (Figure 1). The structure and the morphology of the thin films seemed to be related to the kinetics of the deposition process, which in turn were affected by the position of the substrate to the crucible (metal vapor source), solidification time and vacuum level. Surface views of samples (Figures 2a) reveal that porous structured Sn film has a remarkable pillar like structure and the thin film is fairly porous. Moreover, the cross sectional views (Figures 2b) demonstrated that films were very thin and had a thickness around 150  50nm. Figure 3. Charge-Discharge capacity delivered upon cycling Figure 4. Cyclic voltammograms of electron beam deposited Sn thin