1. Production of non-porous and porous Cu-Sn/C Multilayered system via Electron Beam Evaporation Techniques B.D. Polat 1, N. Sezgin 1, K.Kazmanlı 1, Ö. Keleş 1*, A. Abouimrane 2, K.Amine 2 1 ITU Faculty of Chemical and Metallurgical and Material Engineering, Maslak – İstanbul, 34469, Turkey 2 Argonne National Laboratories, Chicaho Illinois, USA In this paper, non porous and porous Cu-Sn/C multi-layered thin films were deposited on a copper substrate by electron beam evaporation technique. During the deposition, first Cu-Sn thin film was formed then very thin C layer was deposited subsequently. Oblique angle deposition (OAD) technique was utilized to generate porous Cu-Sn/C multi-layered thin film. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the structure, the morphology and composition of the electrodes respectively. Cyclic voltammetry (CV) and galvanostatic charge–discharge measurements were carried to characterize the electrochemical performance of the electrodes. XRD data showed that for both sample additional amount of carbon decreased the grain size of Cu 6 Sn 5 thin film, which increase the cycle life by improving its tolerance to high volumetric changes occurred during discharge-charge reactions. Added to this, the results of electrochemical analyses prove that inclined nano columns resulted from OAD method, facilitate lithium ions movement within the structure compared to flat Cu-Sn/C thin film, which also improves the battery life of the anode material. Keywords: multilayer thin film, Cu-Sn composite thin film, C effect in the thin film structure, morphology effect in electrochemical performance, lithium ion batteries. Introduction Recently, tin has been widely studied because it has a higher theoretical capacity (991mAh/g) than graphite (372mAhg/1) [1]. However, the use of pure tin as electrode was found impractical due to large volume change (up to 360 %) occurs when tin reacts with lithium ions, which results in a decrease in the cycling stability of the electrode [2]. Up to now, different researchers proved that in Cu–Sn alloys, in particular, Cu 6 Sn 5 intermetallic compound is considered as the most promising alternative anode material [3]. Moreover, previous researches proved that [4-5] the nanostructured electrodes are the best suited for high volume expansion. Thus, different methodologies have been used to decrease the particle size of Cu 6 Sn 5. Adding carbon into thin film is an alternative method used to decrease the grain size [6-10]. Up to now, there has been no published work where the electrochemical performances of C contained nonporous and nanostructured Cu-Sn thin films were compared. In this paper, two thin films with non porous and nano porous structure were deposited by electron beam deposition process. The deposited nanostructured thin film made of inclined (slanted) columns with porosities, was formed by using oblique angle deposition (OAD) method. The sizes of the columns were in nano sized range owing to the use of carbon. The thin films were characterized by using X- ray diffraction, scanning electron microscopy (SEM). The electrochemical properties of the thin films were compared and the potential viability of the Cu–Sn/C multilayered thin film as negative electrodes for a Li-ion battery was discussed at the end of the paper. Introduction Recently, tin has been widely studied because it has a higher theoretical capacity (991mAh/g) than graphite (372mAhg/1) [1]. However, the use of pure tin as electrode was found impractical due to large volume change (up to 360 %) occurs when tin reacts with lithium ions, which results in a decrease in the cycling stability of the electrode [2]. Up to now, different researchers proved that in Cu–Sn alloys, in particular, Cu 6 Sn 5 intermetallic compound is considered as the most promising alternative anode material [3]. Moreover, previous researches proved that [4-5] the nanostructured electrodes are the best suited for high volume expansion. Thus, different methodologies have been used to decrease the particle size of Cu 6 Sn 5. Adding carbon into thin film is an alternative method used to decrease the grain size [6-10]. Up to now, there has been no published work where the electrochemical performances of C contained nonporous and nanostructured Cu-Sn thin films were compared. In this paper, two thin films with non porous and nano porous structure were deposited by electron beam deposition process. The deposited nanostructured thin film made of inclined (slanted) columns with porosities, was formed by using oblique angle deposition (OAD) method. The sizes of the columns were in nano sized range owing to the use of carbon. The thin films were characterized by using X- ray diffraction, scanning electron microscopy (SEM). The electrochemical properties of the thin films were compared and the potential viability of the Cu–Sn/C multilayered thin film as negative electrodes for a Li-ion battery was discussed at the end of the paper. Experimental Non porous and porous, multilayered Cu-Sn/C composite thin films were deposited on a copper substrate (with 15.5 mm diameter and 1.5mm thickness) by an electron beam evaporator (in 10 -4 Torr vacuum level). In the deposition of thin films, since carbon has a comparatively low sputtering rate two graphite crucibles were used: one contained Cu-Sn mixture (2/3 (weight%)) and the other had carbon merely. The non porous Cu-Sn/C thin film was generated by depositing Cu-Sn thin film on a copper substrate with a glancing angle of 0 o, then C was deposited on it subsequently. In order to produce nano-structured Cu-Sn/C multi-layered thin film, first Cu-Sn flat non porous thin film was deposited on a copper substrate with a glancing angle of 0 o. Then, a subsequent deposition of Cu- Sn thin film on this flat non-porous composite thin film and carbon on to this nanostructured composite thin film were done when there was 80 o angle between the substrate’s and the target’s surface normals. And, on these layers, Cu-Sn, carbon and Cu-Sn thin films layers were coated respectively via oblique angled deposition method when the deposition flux was hit to the substrate surface with glancing angle of 80°. The compositions of the non porous and porous films were determined by energy dispersive X-ray spectroscopy (EDS) analyses. The surface microstructure as well as the thin film thickness was measured by utilizing field- emission scanning electron microscopy (JEOL JSM 7000F). The phases present in the coating were determined by using Philips PW3710 System (with CuKα at 40 kV and 30 mA). The X-ray data were collected in the 2θ range of 20–80° in steps of 0.05 . CR2032 coin cells were fabricated to test the electrochemical properties of thin-film electrodes. All cells were tested at room temperature at a constant current in the voltage range of 0.0–2.5 V versus Li/Li+ with a rate of 50mAh/g. Experimental Non porous and porous, multilayered Cu-Sn/C composite thin films were deposited on a copper substrate (with 15.5 mm diameter and 1.5mm thickness) by an electron beam evaporator (in 10 -4 Torr vacuum level). In the deposition of thin films, since carbon has a comparatively low sputtering rate two graphite crucibles were used: one contained Cu-Sn mixture (2/3 (weight%)) and the other had carbon merely. The non porous Cu-Sn/C thin film was generated by depositing Cu-Sn thin film on a copper substrate with a glancing angle of 0 o, then C was deposited on it subsequently. In order to produce nano-structured Cu-Sn/C multi-layered thin film, first Cu-Sn flat non porous thin film was deposited on a copper substrate with a glancing angle of 0 o. Then, a subsequent deposition of Cu- Sn thin film on this flat non-porous composite thin film and carbon on to this nanostructured composite thin film were done when there was 80 o angle between the substrate’s and the target’s surface normals. And, on these layers, Cu-Sn, carbon and Cu-Sn thin films layers were coated respectively via oblique angled deposition method when the deposition flux was hit to the substrate surface with glancing angle of 80°. The compositions of the non porous and porous films were determined by energy dispersive X-ray spectroscopy (EDS) analyses. The surface microstructure as well as the thin film thickness was measured by utilizing field- emission scanning electron microscopy (JEOL JSM 7000F). The phases present in the coating were determined by using Philips PW3710 System (with CuKα at 40 kV and 30 mA). The X-ray data were collected in the 2θ range of 20–80° in steps of 0.05 . CR2032 coin cells were fabricated to test the electrochemical properties of thin-film electrodes. All cells were tested at room temperature at a constant current in the voltage range of 0.0–2.5 V versus Li/Li+ with a rate of 50mAh/g. References 1. C.K. Chen, X.F. Zhang and Y. Cui, Nano Letters 8 (2008) 307. 2. A.R. Kamali and D.J. Fray, Re. Adv. Mater. Sci. 27 (2011) 14. 3. K.D.Kepler, J.T.Vaughey, M.M.Thackeray, Electrochem. Solid-State Lett. 2 (1999) 307. 4. J. R. Dahn, R. E. Mar, and A. Abouzeid, J. Electrochem. Soc., 153, A361, 2006. 5. P. P. Ferguson, M. L. Martine, A. E. George, and J. R. Dahn, J. Power Sources, 194, 794 2009. 6. C.K. Chen, X.F. Zhang and Y. Cui, Nano Letters 8 (2008) 307. 7. R.Z.Hu, M.Q.Zeng, M.Zhu, Electrochimica Acta, 54 (2009) 2843. 8. A. D. W. Todd, R. E. Mar, and J. R. Dahn, J. Electrochem. Soc., 154, A597, 2007. 9. A. D. W. Todd, P. P. Ferguson, J. G. Barker, M. D. Fleischauer, and J. R. Dahn, J. Electrochem. Soc., 156, A1034 2009. 10. W. Cui, F. Li, H. Liu, C. Wang, and Y. Xia, J. Mater. Chem., 19, 7202 2009. 11. J. Yao, G.X. Wang, J.H. Ahn, H.K. Liu, S.X. Dou, J. Power Sources 114 (2003) 292. 12. P. Guo, Z.W. Zhao, H.K. Liu, S.X. Dou, Carbon 43 (2005) 1392. References 1. C.K. Chen, X.F. Zhang and Y. Cui, Nano Letters 8 (2008) 307. 2. A.R. Kamali and D.J. Fray, Re. Adv. Mater. Sci. 27 (2011) 14. 3. K.D.Kepler, J.T.Vaughey, M.M.Thackeray, Electrochem. Solid-State Lett. 2 (1999) 307. 4. J. R. Dahn, R. E. Mar, and A. Abouzeid, J. Electrochem. Soc., 153, A361, 2006. 5. P. P. Ferguson, M. L. Martine, A. E. George, and J. R. Dahn, J. Power Sources, 194, 794 2009. 6. C.K. Chen, X.F. Zhang and Y. Cui, Nano Letters 8 (2008) 307. 7. R.Z.Hu, M.Q.Zeng, M.Zhu, Electrochimica Acta, 54 (2009) 2843. 8. A. D. W. Todd, R. E. Mar, and J. R. Dahn, J. Electrochem. Soc., 154, A597, 2007. 9. A. D. W. Todd, P. P. Ferguson, J. G. Barker, M. D. Fleischauer, and J. R. Dahn, J. Electrochem. Soc., 156, A1034 2009. 10. W. Cui, F. Li, H. Liu, C. Wang, and Y. Xia, J. Mater. Chem., 19, 7202 2009. 11. J. Yao, G.X. Wang, J.H. Ahn, H.K. Liu, S.X. Dou, J. Power Sources 114 (2003) 292. 12. P. Guo, Z.W. Zhao, H.K. Liu, S.X. Dou, Carbon 43 (2005) 1392. Results and Discussion Figure 2. XRD data of a) non porous, b) porous Figure 3. CV data of a) non porous, b) porous thin films thin films. In the initial scanning cycle of the non porous thin film two sharp and one very small anodic peaks were detected. One broad reduction peaks was also noticed in Figure 3a. On the other hand, for nano porous structured Cu-Sn/C thin film, three reduction and three oxidation peaks were detected. The reduction peaks in the potential range 0.5–1.0 V were ascribed to the formation of solid electrolyte interface (SEI) film on the surface of electrode [11], and the third reduction peak close to 0.0 V versus Li/Li+ was attributed to the formation of Li 4.4 Sn [12]. Figure 4 shows the capacity delivered during charge-discharge when Cu-Sn/C non-porous and porous nanostructured multilayered thin films were cycled versus lithium foil counter electrode. For non-porous thin film, the initial capacity was noted as (650 mAh g -1 ), then decreased to 500 after the first cycle. A progressive decrease in capacity was detected with some fluctuation in Figure 4a. This fluctuation might be explained considering the carbon content of the thin film, which might decrease the homogeneity of the thin film. After the 20 th cycle the capacity felt down to 90 mAh g -1, then at the 35 th cycle the sample failed. The short cycle life and low capacity performance of the thin film could be elucidating considering its non-porous structure and relatively bigger particle size. Even the initial discharge value is lower compared to that of the non porous thin film, an improvement in the cycleability of the porous nanostructured Cu-Sn/C thin film was noticed in Figure 4b. EDS analyses justifies the reason of this low initial capacity value. Non porous Cu-Sn/C thin film having (at%) 49.20% Cu, 47.94% Sn and 2.86% C, and the porous thin film has 29.47% Cu, 29.03 %Sn, 41.5%. The samples having rough surface owing to the presence of the slanted columns, displayed large initial capacity loss after the first cycle associated with SEI formation. This decline in capacity continues up to 100 th cycles, where the capacity fell down to 90 mAh g -1. When the trend in capacity performance of the thin films was analysed, it is seen that for both sample, the first discharge capacity value exceeded the theoretical capacity of Cu 6 Sn 5. Chiu et. al. have explained this fact by the formation of solid electrolyte interface layer (SEI), which was resulted from the active elements reaction with electrolyte [5]. Figure 4. Charge-discharge capacity delivered by the sample which was made of subsequent deposition of Cu-Sn/C multilayer thin films. a)b)c) Results and Discussion Figure 1a. Surface view, b. Cross sectional view of of non porous film, c. Surface view, b. Cross sectional view of of porous film Figures 1 a,d showed SEM images of the as deposited non porous and porous Cu-Sn/C multilayered thin films produced by electron beam deposition process. The sample having porous structure had remarkable slanted nano-columns compared to non porous structure thin film which had no remarkable 3D structure. Figures 1.a-d show that since the deposition rate of the thin film was constant (3Å/s), the thin film thickness and the morphology were varied depending on the deposition angle. Figures 1.a-d indicated that even the process duration was the same for both sample, the deposited film had  490 nm and  264 nm for non porous and porous thin films. XRD data of the multi-layered thin films revealed that both films were made Cu 6 Sn 5 compounds. The graph (Figures 2.a-b) proved the effect of carbon on the porous thin film morphology. The presence of carbon reduced the particles’ size to nanometer scale, which made the thin film structure more amorphous than crystalline. Using the Retvield refinement method, calculated lattice parameters for Cu 6.26 Sn 5 in reference to Cu 6 Sn 5 are found as a: 4.234 Å, c: 5.144 Å and a: 4.206 Å, c: 5.097 Å respectively. Results and Discussion Figure 1a. Surface view, b. Cross sectional view of of non porous film, c. Surface view, b. Cross sectional view of of porous film Figures 1 a,d showed SEM images of the as deposited non porous and porous Cu-Sn/C multilayered thin films produced by electron beam deposition process. The sample having porous structure had remarkable slanted nano-columns compared to non porous structure thin film which had no remarkable 3D structure. Figures 1.a-d show that since the deposition rate of the thin film was constant (3Å/s), the thin film thickness and the morphology were varied depending on the deposition angle. Figures 1.a-d indicated that even the process duration was the same for both sample, the deposited film had  490 nm and  264 nm for non porous and porous thin films. XRD data of the multi-layered thin films revealed that both films were made Cu 6 Sn 5 compounds. The graph (Figures 2.a-b) proved the effect of carbon on the porous thin film morphology. The presence of carbon reduced the particles’ size to nanometer scale, which made the thin film structure more amorphous than crystalline. Using the Retvield refinement method, calculated lattice parameters for Cu 6.26 Sn 5 in reference to Cu 6 Sn 5 are found as a: 4.234 Å, c: 5.144 Å and a: 4.206 Å, c: 5.097 Å respectively. a) b) Conclusion In this paper, non-porous and porous nanostructured Cu-Sn/C multi-layered thin film was deposited on copper substrates by electron beam evaporation method. The oblique (glancing) angle electron beam evaporation technique was used to produce porous nanostructured Cu-Sn/C thin film. The results showed that additional amount of carbon decreased the grain size of Cu 6 Sn 5 to nanosized structure and the slanted nano columnar structure produced by oblique(glancing) angle depsition technique increased the cycle life of the battery. However, because of the increase in surface roughness, the samples displayed large initial capacity loss after first cycle associated with SEI formation. Conclusion In this paper, non-porous and porous nanostructured Cu-Sn/C multi-layered thin film was deposited on copper substrates by electron beam evaporation method. The oblique (glancing) angle electron beam evaporation technique was used to produce porous nanostructured Cu-Sn/C thin film. The results showed that additional amount of carbon decreased the grain size of Cu 6 Sn 5 to nanosized structure and the slanted nano columnar structure produced by oblique(glancing) angle depsition technique increased the cycle life of the battery. However, because of the increase in surface roughness, the samples displayed large initial capacity loss after first cycle associated with SEI formation.