Further enhancements regarding the energy density and specific power of lithium ion batteries are absolutely necessary in order to satisfy the increasing requirements for automotive applications e.g. extended driving ranges. One way to realize such improvements, depicts the replacement of commonly used carbon-based anode materials with high capacity anodes.¹ In this regard silicon (Si) containing composites are considered promising candidates for the replacement of carbonaceous anode materials due to the significantly higher specific capacity of Si. However, the use of Si is still hindered by several challenges that have to be overcome for a successful application. One major issue of Si-based anode materials is the strong capacity decay due to the huge volume changes (~ 300%) of Si during the lithiation/de-lithiation process, leading to an recurring solid electrolyte interphase (SEI) reformation, which results in the ongoing consumption of active lithium from the cathode.² One concept to alleviate the detrimental effects previously mentioned and to boost the performance of such anode materials, comprises the combination of Si with a matrix material. The general idea behind this approach is to combine Si with a second phase that enhances the mechanical stability and can buffer the volumetric changes of Si and thus, enables the formation of a stable SEI.^3,4

In this study, we present a silicon iron (Fe) composite that contains two phases, a crystalline Si phase and a second, intermetallic Fe_xSi_y-phase. The applied synthesis route yields materials that combine a porous structure with the aforementioned matrix approach. This composite design is believed to have a beneficial effect on the capacity retention during cycling since it may buffers the volumetric changes of the Si and simultaneously provides extra space for the volume changes. The applied synthesis route contains a ball-milling and washing step, and subsequently the addition of a thin carbon coating. The composites, synthesized this way, are investigated via scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction and thermogravimetric analysis in order to characterize their structure, morphology and composition. Moreover, electrochemical studies on the long-term cycling and rate performance with regard to the application as anodes in lithium ion batteries are conducted. Thereby, this work focuses on the influence of a high temperature treatment, as well as the influence of the Fe to Si ratio on the electrochemical performance. Furthermore, the role of the stabilizing Fe_xSi_y matrix phase in the composite is investigated regarding the question whether this phase is inactive towards lithiation or if it contributes to the lithiation/de-lithiation capacity of the composite.

References

1 Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M. Lithium ion, lithium metal, and alternative rechargeable battery technologies: The odyssey for high energy density. Journal of Solid State Electrochemistry 2017, 21, 1939-1964.

2 Wu, H.; Cui, Y. Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today 2012, 7, 414–429.

3 Besenhard, J. O.; Yang, J.; Winter, M. Will advanced lithium-alloy anodes have a chance in lithium-ion batteries? Journal of Power Sources 1997, 68, 87–90.

4 Park, C.-M.; Kim, J.-H.; Kim, H.; Sohn, H.-J. Li-alloy based anode materials for Li secondary batteries. Chemical Society reviews 2010, 39, 3115–3141.