Electrochemical Cycling of Sodium-Filled Silicon Clathrate

Due to their high charge-storage capacity (i.e. >3000 mAhg ), there has been much research activity in the development of silicon-based anodes for high-energy-density lithium-ion batteries. In recent years, there has been immense interest in understanding the electrochemical, 2] structural, and mechanical properties of both diamond cubic (c-Si) and amorphous silicon (a-Si) under reaction with lithium. Silicon with a clathrate or cage-like structure has been recently investigated as a potential anode, with both theoretical and experimental verification by Langer et al. that lithiation is feasible. Since prior studies on silicon clathrates have focused predominately on their superconducting and thermoelectric properties, this is a potentially new and exciting application for this class of materials. Type-I clathrates of the form M8Si46, where M is a guest atom intercalated into the structure, are made of two pentagonal dodecahedra (Si20) cages and six tetrakaidecahedra (Si24 cages) per unit cell and crystallize in the Pm 3n space group (Figure 1a). Type-II clathrates of the form MxSi136 (0<x<24), are made of sixteen pentagonal dodecahedra plus eight hexakaidecahedra (Si28 cages) per unit cell and crystallize in the Fd 3m space group (Figure 1b). Detailed X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) studies of the lithiation process into practically guest free type-II clathrate Na1.3Si136 [10] showed that the clathrate structure could be maintained until insertion of 24 Li per formula unit. Upon insertion of more Li, the clathrate structure became amorphous and eventually transformed into crystalline Li15Si4 (c-Li15Si4), much like is observed during lithiation of c-Si. However, no delithiation behavior was shown in this previous work, and no extended cycling data were shown. To this end, we have conducted an extended electrochemical, structural, and theoretical study of sodium-filled silicon clathrate to better understand its properties as an anode for lithium-ion batteries. Sodium-filled silicon clathrate was synthesized from the decomposition of Zintl phase NaSi. XRD patterns of the as-