The theoretical volumetric capacity of silicon (2194 Ah/L) is much larger than that of graphite (756 Ah/L). However, the large volume expansion of silicon is still a main concern for practical utilization. One method of reducing the volume expansion of Si is to alloy it with an inactive element. Both iron and manganese are cheap and abundant transition metals and are therefore interesting candidate inactive alloying metals for silicon based anodes. The binary systems, including Si-Fe and Si-Mn, have been previously studied in our group and by others. It has been found that adding Mn or Fe to Si reduces volume expansion during lithiation by forming inactive phases with Si and by inhibiting Li₁₅Si₄ formation. The silicides of Fe and Mn are different in structure and stoichiometry. Ball milled alloys in the Si-Fe-Mn ternary system have not been previously reported. Therefore it is meaningful to investigate the Si-Fe-Mn ternary system.

Here, Si_100-x-yMn_xFe_y ((x,y)=(0,15), (5,12), (10,9), (15,6), (20,3), (25,0)) alloys were prepared by ball milling and their phase composition, microstructure and electrochemistry in Li half cells were studied. The XRD results show that α-Si₂Fe and β-Si₂Fe are formed in low Mn content samples, i.e. Si₈₅Fe₁₅, Si₈₃Mn₅Fe₁₂. As the Mn content is increased, only Si₁₉Mn₁₁ phase is detected. However, the continuous peak shifts in XRD profiles and Mossbauer spectroscopy, shown in Figure 1, demonstrate that Fe atoms are present in the Si₁₉Mn₁₁ structure, to form ternary solid solutions.

The potential and differential capacity curves of Si_100-x-yMn_xFe_y alloys are typical of alloys in which Si is the only active phase. The formation of Li₁₅Si₄ is successfully suppressed in all the samples in the initial cycles. However, Li₁₅Si₄ forms after about 10 cycles, as shown in Figure 2. The volumetric capacity and volume expansion of the materials increase with increasing silicon ratio, as shown in Figure 3. In this presentation the composition, microstructure and electrochemistry of alloys in the Si-Mn-Fe system will be thoroughly discussed.