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 (RLi/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 RLi/Al has a significant impact on the types of products. Only at the RLi/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 RLi/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.