First-Principles Study of Sodium Intercalation in Crystalline Nax Si24 (0 ≤ x ≤ 4) as Anode Material for Na-ion Batteries

The search for Si-based anodes capable of undergoing low volume changes during electrochemical operation in rechargeable batteries is ample and active. Here we focus on crystalline Si-24, a recently discovered open-cage allotrope of silicon, to thoroughly investigate its electrochemical performance using density functional theory calculations. In particular, we examine the phase stability of NaxSi24 along the whole composition range (0 <= x <= 4), volume and voltage changes during the (de)sodiation process, and sodium ion mobility. We show that NaxSi24 forms a solid solution with minimal volume changes. Yet sodium diffusion is predicted to be insufficiently fast for facile kinetics of Na-ion intake. Considering these advantages and limitations, we discuss the potential usefulness of Si-24 as anode material for Na-ion batteries.