Microstructures and electrochemical properties of Si-RE and Si-Fe anode materials for rechargeable Li-ion batteries

The effects of iron and rare-earth (RE) elements on the microstructures and the electrochemical properties of rapidly solidified Si alloys were investigated. Rapidly solidified Si-Fe and Si-RE ribbons were prepared by using a melt-spinning process. The thicknesses of the obtained ribbons were 9 - 13 mu m. The ribbons were fragmented by using a ball-milling process to produce powders. Each of the alloy powders was mixed with a conductive material and a binder and then dissolved in deionized water and SFG(6) to form electrodes. The electrolyte used was 1.5-M LiPF6 dissolved in ethyl carbonate/dimethyl carbonate/fluoroethylene carbonate. The results showed that the microstructures of the ribbons consisted of Si particles, which were present in inactive phases composed of the intermetallic compound FeSi2 or (Ce,La)Si-2. The diameters of the Si particles in the melt-spun Si-RE ribbons were 60 - 90 nm whereas those of the particles in the Si-Fe alloy were approximately 0.8 - 12 mu m. The Si-RE alloy exhibited an initial irreversible capacity that was half that of the Si-Fe alloy. The higher Coulombic efficiency and capacity retention rate of the Si-RE alloy were attributable to its nanosized particles and the effects of the enthalpy of solution.