Electrochemistry of Sputtered and Ball Milled Si-Fe-O Alloys in Li Cells

Abstract

Si-Fe-O samples were produced using combinatorial sputtering and ball milling methods. Their structures were characterized by X-ray diffraction and Mössbauer spectroscopy, and electrochemical characterization was performed in Li cells. All samples were amorphous or nanostructured, however alloy phase composition had a strong dependence on synthesis method. Sputtered samples comprised amorphous Si, Fe and Si-O phases, while those made by ball milling were found to contain Si, α-FeSi2 and Si-O phases. This resulted in different reversible capacities, depending on the alloy synthesis method. In all alloys, the presence of oxygen results in low initial coulombic efficiency (ICE) below 0.9 V, due to the formation Li-O species that irreversibly trap Li. However, the ICEs increase as the upper cutoff voltage is increased, and at an upper cutoff of 2.5 V the ICE has little dependence on oxygen content. Electrodes prepared from Si-Fe-O powders mixed with graphite have good cycling performance and high volumetric capacities (e.g. ∼1938 Ah/L for a Si0.80Fe0.08O0.12/graphite electrode).