(Invited) Type I and Type II Silicon Clathrates As Novel Anodes for Lithium Batteries

Abstract

Due to their high charge storage capacities upon reaction with Li, diamond cubic (c-Si) and amorphous silicon (a-Si) are attractive anode materials for the development of high energy density lithium-ion batteries. However, the 300% change in volume between the unlithiated and lithiated phases results in stress formation that can lead to pulverization of the silicon. A great deal of research has been devoted to better understanding these phase transformations. While nanostructured electrodes, such as those involving silicon nanowire, nanotube, nanocrystal, or nanoporous morphologies have successfully demonstrated long-term cycling without pulverization, this strategy relies on the use of engineered space within or between the silicon to allow free expansion and contraction. Agglomeration and degradation of the engineered structure after many lithiation/delithiation cycles may lower the effectiveness of this strategy. The current approach of nanostructuring also does not address the key fundamental problem with silicon, namely its large structural and volume changes upon lithiation. To this end, investigation of new anode materials with less drastic changes during reaction with Li is still required. Silicon clathrates consist of silicon covalently bonded in face-sharing Si20, Si24, and/or Si28 clusters with guest atoms occupying interstices inside the polyhedra. Their properties arise largely from their unique cage-like structures and interactions between guest atoms with the clathrate framework. They are considered promising materials for superconducting and thermoelectric applications. To date, their potential as energy storage materials has not been fully explored and their electrochemical properties are largely unknown. In this talk, recent first principles and experimental results from our lab and collaborators regarding the electrochemical properties of type I and type II silicon clathrates will be discussed. Type I clathrates based on MxSi46 are composed of two Si20 cages plus six tetrakaidecahedra (Si24 cages) per unit cell. The guest atoms, M, are located in centers of the Si24 and Si20 cages. Type II clathrates are composed of sixteen pentagonal dodecahedra (Si20 cages) plus eight hexakaidecahedra (Si28 cages) per unit cell. While type II clathrate based on Na24Si136 was found to become amorphous upon lithiation and transform to crystalline Li15Si4, similar to c-Si, electrochemical and structural analysis using X-ray diffraction and nuclear magnetic resonance showed that reversible lithium intercalation can occur in type I clathrate without discernable changes to the silicon framework. Density functional theory was used to understand how different possible sites for Li affect the framework structure and bond lengths, formation energy, and electronic band structure. Our results show that silicon clathrates may be attractive as durable anodes with high volumetric capacity.