The Structure Modification of Substrate for Alloying Materials in Li-Ion Secondary Battery

Abstract

Li-ion secondary batteries possessing high energy and power density have been extensively researched to satisfy the needs for large-scale vehicles and storage system [1]. Graphite, most commercially used these days, has a stable cycle behavior during the charge/discharge process, however, it shows a relatively low specific capacity (372 mAh/g). Much as the intercalation materials including graphite can react with Li ion reversibly, they have a low specific capacity because of the host network structure. In general, the materials that experience a conversion and alloying reaction with Li ion during the charge/discharge process show high energy density [2]. The alloying materials such as Si (4200 mAh/g for Li4.4Si), Sn (994 mAh/g for Li4.4Sn), and Sb (660 mAh/g for Li3Sb) have a much higher energy density compared to the conventional graphite electrode, and thus the development of negative electrode with the aforementioned elements should be the most reliable approach to achieve high energy density [3,4].Recently, Si has been extensively studied owing to its highest theoretical capacity among the negative electrode materials. Furthermore, Si is very inexpensive, easy to handle, and low electrochemical potential, with respect to Li metal. Besides, Sn has been also investigated as a high energy material. However, many researches have been addressing the critical issue of Si and Sn, leading to severe volume change during the charge/discharge process. The volume change is up to ~400% for Li4.4Si and Li4.4Sn, and it finally pulverizes and exfoliates Si and Sn from the current collector. In the last decade, many approaches have been made to overcome the critical issues of Si and Sn material [5]. The structural modification of substrate, which enlarge the surface area of active materials, could be an effective way to reduce the stress induced during the charge/discharge cycle. The substrate is able to maintain its initial structure and consistently affects active materials.In this study, we attempted to make micrometer-size modification of the substrate structure in sawtooth and pyramidal shape and nanometer-size modification in nanowire structure. The microelectromechanical systems (MEMS) allowed us to fabricate the sawtooth-shape (width of 85 and 170 μm, S-85 and S-170, respectively) and pyramidal-shape (50 and 100 μm, P-50 and P-100, respectively) Si wafer substrateas shown in Figure 1(a). The deposited Si film was verified by the cross-sectional FE-SEM analysis as shown in Figure 1(b). We also prepared nanowire structure on Cu foil. Figure 2 shows the surface images of nanowire structure and Sn-electrodeposited nanowire.In this presentation, we studied the electrochemical properties and the changes in the electrode surface according with the existence of structure modification. We sought to correlate the capacity retentions and the surface coverage of Si on the electrode. By confirming the regions in which the majority of the Si remained, the relaxation effect was verified. Moreover, the capacity retention and rate capability of the Sn electrodeposited nanowire electrode were also investigated by electrochemical analysis.