Na ion batteries attract significant research interest since they provide potentially high energy density while using low cost and abundant sodium as the active ion. Si has been extensively investigated for its operation as anode in lithium based batteries since it has a high theoretical lithiation capacity up to Li_4.4Si. However, the sodium analogue with successful reversible electrochemical sodiation of Si has not been reported yet. Si sodiation is anticipated to be different with respect to phase behaviour, insertion voltages and kinetic barriers when compared to Li ion, for instance because of the difference in ionic radius of Na⁺ (0.97 Å) and Li⁺(0.68 Å). From thermal synthesis of Na-Si materials it is known that NaSi is the most Na rich phase for Na-Si binary compounds, which would enable a sizeable capacity of 954 mAh/g.

Anticipating that nanoscaling is of advantage for the kinetics of ion insertion and extraction, we studied Si particles with much reduced size (~ 20 nm) containing a large fraction of amorphous Si. The nano particles were obtained from Expanding Thermal Plasma Chemical Vapour Deposition (ETPCVD) of silane. The materials consist of aggregated individual nano particles with a very thin silicon oxide layer that forms after contact with air. The work presented here reports, to our knowledge for the first time, reversible electrochemical Na ion uptake in these Si based materials for a significant capacity. The desodiation capacity is stable around 260 mAh/g, while the initial sodiation shows a large irreversible Na loss attributed to SEI formation. The experiments show that during Na insertion a two phase equilibrium may be present between NaSi and Si:        x Na + Si  ->  x NaSi + (1-x) Si, where the NaSi becomes amorphous. For desodiation a sloping voltage profile is observed indicative of a solid solution behaviour. Therefore we propose that desodiation occurs as NaSi ->  Na_1-xSi + x Na, where the Na_1-xSi  phase remains amorphous. After 100 cycles all Si has been transformed to amorphous phases, and no crystalline nano Si particles remain.

In conclusion, nanoparticles containing both amorphous and crystalline Si, produced by ETPCVD, demonstrate an excellent reversible capacity of 279 mAh/g for Si at 10 mA/g and a capacity retention of 248 mA/g after 100 cycles at 20 mA/g. Significant reversible capacities can be achieved at high dis-/charge rates as well. Nanoscaling benefits the Na insertion and extraction kinetics in Si although reversible Na uptake and release for the full theoretical capacity has not been reached. Initial amorphous and crystalline Si are performing an equally active role in the electrochemical sodiation. Coexistence of Si and NaSi may occur during Na insertion into amorphous Si and on the surface of the Si crystallites; while a solid solution desodiation reaction is evidenced when Na is being extracted.

Y. Xu, E. Swaans, S. Basak, H. W. Zandbergen, D. M. Borsa, F. M. Mulder, Adv. Energy Mater. 2015, doi 10.1002/aenm.201501436 early view