Abstract
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 Si24, 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 Si24 as anode material for Na-ion batteries.
Highlights
Li-ion rechargeable batteries are the energy storage system for current commercial portable electronics
The main problem associated to the electrochemical ion insertion of silicon is thatintercalation is accompanied with large volume changes and mechanical stresses that weaken the host lattice
Recent ab initio molecular dynamics simulations showed that the volume expansion of amorphous Li-Si alloys increases linearly with lithium content up to 160%, whereas Na-Si alloys expand to about 230% for the same alkali content[30]
Summary
Li-ion rechargeable batteries are the energy storage system for current commercial portable electronics. The main problem associated to the electrochemical ion insertion of silicon is that (de)intercalation is accompanied with large volume changes and mechanical stresses that weaken the host lattice This degrades the morphology of the anode during cycling, with large irreversible capacity losses[27,28,29]. Marzouk et al.[43] have recently explored the insertion of sodium into different allotropic forms of crystalline NaxSi24 (with x = 1, 2, 3, 4, 5, and 6): (i) the experimentally observed open-cage Si2433–38 and (ii) additional metastable allotropes found computationally using evolutionary metadynamics They showed that sodiation in all these systems is energetically favourable up to four sodium atoms per Si24, with small volume expansions and low intercalation potentials. Follow-up studies to fully understand the electrochemical behaviour of such compounds are timely and highly desirable
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