Electrochemical Lithiation of Silicon Clathrate-II

The clathrate-II compound NaxSi136 (x → 0) was synthesized as a precursor for electrochemical lithiation via sodium extraction from the Zintl-Phase NaSi. The final composition was determined to be Na1.3Si136 via a Rietveld refinement. Electrochemical lithiation of this clathrate-II compound yielded a potential plateau at 300 mV whose capacity is in good agreement with 24 lithium atoms being inserted in the clathrate-II structure. In order to prove this assumption, several clathrate-containing electrodes were prepared and electrochemically lithiated. Afterwards the electrode material was investigated using 7Li magic-angle spinning (MAS)-NMR spectroscopy. Two signals in a 2:1 ratio at extremely high chemical shifts of 412 and 433 ppm, respectively, relative to 1M aqueous lithium chloride solution, were detected and give the indisputable evidence that lithiation of the clathrate-II framework was successful. Hence, this work presents a new method for filling clathrate cages and moreover, the first lithium containing clathrate compound.