Anode Properties of Silicide/Si Composite Alloy Electrodes Prepared By Rapid Quenching for Lithium-Ion Battery

Abstract

1. Introduction Si is attractive as an anode active material for lithium ion battery (LIB) due to its high theoretical capacity. However, large volume expansion and contraction of Si occur during charge and discharge reactions, respectively, which results in pulverization after cycling. Therefore, Si electrode shows poor cyclability. On the other hand, composite electrodes consisting of transition metal silicide and Si exhibit advanced cycling performance compared to Si-alone electrode. We have reported that the following four properties are required for silicide phase: (1) mechanical properties suitable for relaxation of the stress from Si, (2) high electronic conductivity, (3) moderate reactivity with Li+, and (4) high thermodynamic stability[1-5].Rapid quenching is one of the preparing methods of silicide/Si composite alloy. In this study, we focused that this method can control positional relation between Si and silicide phase by changing additives or its amounts. We prepared various Si-alloys and investigated relationship between their anode properties and the arrangement.1. Experiment Si-Fe and Si-Zr alloy were prepared by rapid quenching. First, ingot of Si-alloy was melted at 1500 oC by induction furnace. After melting, molten alloy was dropped on Cu roll at rotating speed of 40 m/s. Thus, molten alloy was quenched and Si-alloy (silicide/Si composite) ribbons were prepared. The ratio of Si phase to silicide phase was set to be 33 : 67 mass% (ideal discharge capacity of Si-alloy : 1200 mA h g-1). LIB anode properties of Si-alloy electrodes were investigated using slurry electrode consisting of 80 wt.% active material, 5 wt.% ketjen black, and 15 wt.% polyamic-acid. The slurry was spread out onto SUS316L foil with a thickness of 20 mm and a loading amount of about 30 mg cm-2, and then dried at 70oC to obtain a working electrode. A 2032 type coin cell was comprised the working electrode, a separator of glass fiber filter, and a counter electrode of Li metal sheet (thickness: 1 mm, diameter: 12 mm). 1M LiPF6 dissolved in ethylene carbonate (EC) : diethyl carbonate (DEC) (1 : 1 vol.%) was employed as an electrolyte.The charge-discharge testing was performed at 25oC and the current density of 0.07 A g-1 (0.05C), during the first cycle, and 0.3 A g-1 (0.2C) during subsequent cycles in the potential range of 0.002–1.0 V vs. Li/Li+.1. Results and discussions Fig. 1 shows the field emission-electron probe micro analysis (FE-EPMA) results of cross-section of Si-Fe alloy and Si-Zr alloy ribbons prepared by rapid quenching method. In Fig. 1(a), white areas were located around black areas. Corresponding element mapping revealed that black and white areas represent Si and FeSi2 phases, respectively. Therefore, the results indicate that silicide phase surrounds Si phase in Si-Fe alloy. On the other hand, ZrSi2 phase surrounded Si phase in Fig. 1(b). Thus, we could prepare various Si-alloys having different positional relation between Si and silicide phases by changing additive elements.Fig. 2 shows cycling performance of Si-alloy electrodes in 1 M LiPF6/EC:DEC(1:1 vol.%). Si-Zr alloy electrode exhibited higher capacity than Si-Fe alloy. In addition, cycle stability of Si-Zr alloy was almost the same as that of Si-Fe alloy in spite of high capacity. Fig. 3 shows the cross-sectional scanning electron microscope (SEM) images of Si-alloy electrodes before and after the 50th cycle. In Si-Fe alloy, particles of Si-Fe alloy were collapsed after the cycling and the electrode thickness significantly increased after the 50th cycle. On the other hand, particle shape of Si-Zr alloy was maintained after the 50th cycle and change in the thickness was much smaller than Si-Fe alloy.These results indicate that positional relation between Si and silicide phases influences on collapse behavior of Si-alloy during charge-discharge and the collapse of Si-alloy can be suppressed by Si phase surrounding silicide phase like Si-Zr alloy.