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.