An optimized electrically conductive Si-Fe matrix to boost the performance of Si electrodes in Li-ion batteries

The development of Si-based anodes has opened the venue to increase the energy density in lithium-ion batteries (LIBs). Nonetheless, the use of Si-based electrodes usually leads to a gradual loss in the cell's electrochemical performance due to the significant volume expansion of silicon in electrode reactions. Combining silicon, a poor electronic conductor, with an electronically conductive Li-inactive phase, is a promising strategy to alleviate the volume expansion of silicon during lithiation and delithiation while providing a robust electronic network. Si-Fe alloys are prospective candidates which could be used to maintain the electronic network in the silicon electrodes. In this study, different Si-Fe alloys are synthesized using ball-milling (BM) and arc melting (AM) techniques, leading to highly different chemical compositions and powder morphologies to better understand the role of iron silicide inactive phases in electrochemical reactions and optimize their performance. The use of AM results in the formation of Si and alpha-Fe2Si5 conducting matrix in a desired ratio, as expected from the Si-Fe binary phase diagram, while BM generates a mixture of phases, including undesirable products. Thanks to the presence of the inactive iron silicide phase (alpha-Fe2Si5), the electrical conductivity of the Si/alpha-Fe2Si5 composite can be increased up to 103 S m-1, five orders of magnitude compared to pristine Si. The electrochemical testing results show that the performance of such a composite is strongly influenced by the balance between Si and inactive iron silicide phase, as well as their interparticle contact. Dilatometry tests in full cell configuration also demonstrate the advantage of using alpha-Fe2Si5 as a matrix to buffer Si volume change, prevent the loss of active material, and maintain a reversible swelling of 24 % throughout cycling up to the 45th cycle. After optimization of electrode and electrolyte formulations, such composites could significantly outperform current Si/C electrodes in terms of volumetric capacity, rate capability and long-term cycling.