Alkali Metals Extraction Reactions with the Silicides Li15Si4 and Li3NaSi6 : Amorphous Si versus allo-Si

The silicides Li15Si4 and Li3NaSi6 were subjected to chemical extraction of the alkali metal component by liquid ammonia and ethanol, respectively, which after washing yielded black powders of amorphous silicon. The investigated reactions are interesting with respect to both the formation of novel Si modifications and the delithiation process in Si anode materials. The products termed a-Si (from Li15Si4) and a-allo-Si (from Li3NaSi6) were characterized by a combination of methodologies (X-ray diffraction, transmission electron microscopy, differential thermal analysis, Raman, atomic absorption, and energy-dispersive X-ray spectroscopy) which revealed (i) a porous microstructure for a-Si built from spherically shaped particles with sizes around 10 nm, (ii) partial surface oxidation of both materials and (iii) the presence of nanocrystalline Si in both materials. The result of the protic oxidation of Li3NaSi6 is at variance with earlier findings reporting the formation of a crystalline bulk allotrope of silicon (allo-Si) from the topotactic combination of silicon layers present as polyanions in Li3NaSi6. Additionally, quantum chemical calculations show that silicon layers in Li3NaSi6 cannot combine to energetically favorable allotropic forms of Si. This is different from Li7Ge12, where polyanionic germanium layers topotactically convert to the germanium allotrope m-allo-Ge upon oxidation.