Understanding Li-Al-CO₃ layered double hydroxides. (I) Urea-supported hydrothermal synthesis

In the current work, Li-Al-CO₃ layered double hydroxides (LDHs) were synthesized by a hydrothermal method using LiCl, AlCl₃, and urea as raw materials. The effect of LiCl/AlCl₃ molar ratio (R_Li/Al) on the composition of products was investigated. Special emphasis was placed on the formation process of Li-Al-CO₃ LDHs. It was found that the R_Li/Al has a significant impact on the types of products. Only at the R_Li/Al being higher than unity, the pure LDHs phase with a chemical formula of [LiAl₂(OH)₆](CO₃)_0.5·2H₂O can be formed. Low R_Li/Al values (≤0.25) lead to the formation of an ammonium aluminum carbonate hydroxide phase. The formation of the LDHs is essentially via a Li⁺-intercalation mechanism. Amorphous Al(OH)₃ is first formed, and Li⁺ ions are then intercalated into the vacancies in the Al(OH)₃-layers to form crystalline Li-Al-CO₃ LDHs. To the best of our knowledge, this is the first report on the hydrothermal formation process of Li-Al LDHs. This work provides a better understanding for the features of Li-Al LDHs.