Role of silicon as structural stabilizer and redox-active element in multicomponent (Sn, Sb, Fe, Si) alloy electrodes for Na-ion batteries

Metal alloys are attractive electrode materials for Na-ion batteries (SIBs) thanks to their high theoretical capacities. Because of its good availability and promising use in Li-ion batteries, silicon is also interesting for SIBs. However, so far its use has not been very successful because formation of the NaSi phase (954 mAh g-1) is kinetically unfavourable. Here we investigate the role of Si in multicomponent alloys composed of redox active metals tin (Sn), antimony (Sb), and redox inactive iron (Fe), which acts as a structural stabilizer. The composites are prepared by high-energy ball milling. Obtained particles and agglomerates are typically in the lower μm-range (0.5 – 10 μm) and contain the different metals and/or intermetallic phases as determined by SEM and XRD. It is found that Si remains electrochemically inactive irrespective of its chemical state but still provides some benefits. Similar to Fe, Si acts as a structural stabilizer for composite electrodes. Excellent rate capability is demonstrated by the Sn:Si:Fe alloy, while highest capacity and long-term stability are found for the Sn:Sb:Si (2:2:1) alloy. The stabilizing effect of Si (and Fe) is observed through operando electrochemical dilatometry, which shows a much smaller degree in electrode breathing compared to the Si/Fe-free electrode.