Mechanochemical Synthesis of Iron Silicon Alloys and Their Electrochemical Characterization As High Energy Anode Materials
Authors: Ruttert Mirco , Siozios Vassilios , Winter Martin , Placke Tobias 
Published: February 2020

Lithium ion batteries (LIBs) are the dominating energy storage devices in the field of portable consumer electronics. They are also considered to be the most promising technology for the application in electric vehicles due to their high energy density. Nonetheless, their power and energy density need to be further improved to meet the challenging requirements for automotive applications, such as extended driving ranges and fast charging capabilities. [1,2] The partial substitution of the commonly used anode material graphite with silicon (Si) depicts one possible approach to increase the energy density significantly, since Si offers a higher specific capacity compared to graphite. However, an insufficient capacity retention of Si-based anodes during cycling represents a major challenge, regarding the commercial application. The poor cycling performance is caused by enormous volume changes, accompanying the lithiation/de-lithiation process of Si, resulting in the deterioration of the electrode due to pulverization of Si particles and contact loss between the active material and the current collector. Additionally, the consumption of active lithium and electrolyte during the formation of a new passivating solid electrolyte interphase (SEI) in every cycle contributes to a strong capacity decay. [3] The performance of Si-based anodes can be improved by embedding Si in different matrices, e.g. graphite or amorphous carbon. Furthermore, the alloying of Si with an inactive metal, such as iron (Fe) can significantly increase the performance by forming electrochemical inactive metal silicides that can stabilize the whole structure and suppress volume changes. [4, 5]

In this contribution, we present the synthesis of Fe/Si-alloys via a simple high energy ball milling process, using elemental Fe and Si as the precursor materials. The influence of different Fe:Si ratios, the addition of carbon, as well as the influence of different heat-treatment conditions on the structure and electrochemical performance are investigated. Therefore, the synthesized active/inactive Si/FexSiy-composites are analyzed by X-ray diffraction and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy in order to identify the formed intermetallic phases, their morphology and elemental distribution within the material. Nitrogen adsorption experiments are carried out to determine the specific surface area of Si/FexSiy-composites. To evaluate the suitability of the composites as high-energy anode in LIBs, their electrochemical performance is characterized regarding their long-term cycling stability and rate capability.

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