Silicon clathrates as anode materials for lithium ion batteries?

Abstract

Very recently, Si clathrates have been proposed to be useful as anodes in lithium ion batteries (LIBs). To explore this possibility, a systematic study was performed on the type-II Si clathrate (Si₁₃₆) using first-principles electronic structure calculations. The investigation focuses on five key properties of a good anode material, viz. structural stability, electrical conductivity, intercalation voltage, Li ion mobility, as well as storage capacity. Since pristine Si₁₃₆ is thermodynamically less stable with respect to crystalline Si, we focus on clathrates which are stabilized by doping Ba atoms in the large cages of empty Si₁₃₆. The lithiation of Ba_xSi₁₃₆ (6 ≤ x ≤ 8) clathrates was then studied. It is found that the volume change associated with lithiation is negligibly small. The intercalation voltages are very low (∼0.4 V) and almost constant with Li concentration. Two distinct diffusion mechanisms for Li ion transport in the clathrates were identified. The lowest energy diffusion pathway (0.33 eV) is through the hexagonal face shared by two adjacent large 5¹²6⁴ cages. The Li diffusion between 5¹² and 5¹² or 5¹² and 5¹²6⁴ cages through a pentagonal face has a higher energy barrier of 0.73–0.85 eV. The Li diffusion through a pentagonal face is found to be a complicated process involving sequential breaking and reformation of a particular Si–Si bond from the clathrate structure. Moreover, the activation barrier is reduced substantially from 2.08 eV by the simultaneous presence of Ba atoms in the two neighboring 5¹²6⁴ cages connected by the Si–Si bond. Although the maximum theoretical capacity (123 mA h g⁻¹) is lower than that of the graphite anode, it is still comparable with the current cathode materials. The information obtained in this study will be helpful to assess whether Si clathrates can be used as anode materials.