Mechanochemical Synthesis of Fe–Si-Based Anode Materials for High-Energy Lithium Ion Full-Cells

The application of silicon (Si)-based negative electrode materials depicts one possibility to increase the energy density of lithium ion batteries (LIBs) due to the high gravimetric and volumetric capacity of Si. Huge volumetric changes during the lithiation/delithiation and the consequences resulting thereof, for example, mechanical instability and loss of active lithium, lead to a very poor cycling stability of Si though. In this work, we combine Si with iron (Fe) by a mechanochemical ball mill synthesis in order to design FexSiy materials and to improve the cycling performance by forming a stabilizing inactive buffer phase. Thereby, we comprehensively study how the electrochemical performance of the FexSiy materials is influenced by different factors, namely the milling time, the variation of the Fe/Si ratio, the application of heat-treatment, and the combination of the FexSiy material with carbon. Those materials showing the most promising electrochemical performance in FexSiy || Li metal cells, that is, a stable cycling performance and high initial Coulombic efficiency (CEff), are further evaluated in a NMC-111 || FexSiy full-cell setup, allowing a more practical assessment of their practical performance, as the active lithium content is limited in these cells. Because of their high tap density (>1 g cm–3) and high initial CEff (86–92%), the FexSiy materials can reach a high areal capacity and can be applied without an additional prelithiation step in a LIB full-cell. In comparison to a commercial Si–C composite material, the FexSiy materials show a superior gravimetric energy and improved cycling stability in full-cells.