Preparation of the layered double hydroxide (LDH) LiAl₂(OH)₇·2H₂O, by gel to crystallite conversion and a hydrothermal method, and its conversion to lithium aluminates

Abstract

A layered double hydroxide (LDH) with chemical composition LiAl₂(OH)₇ ·2H₂O was prepared via a wet chemical route of gel to crystallite (G–C) conversion at 80 °C involving the reaction of hydrated alumina gel, Al₂O₃ ·yH₂O (80<y<120) with LiOH (Li₂O/Al₂O₃ ≥0.5) in presence of hydrophilic solvents such as ethanol under refluxing conditions. The hydrothermal synthesis was carried out using the same reactants by heating to ≤140 °C in a Teflon-lined autoclave under autogenerated pressure (≤20 MPa). Transmission electron microscopy showed needle-shaped aggregates of size 0.04–0.1 µm for the gel to crystallite conversion product, whereas the hydrothermal products consisted of individual lamellar crystallites of size 0.2–0.5 µm with hexagonal morphology. The LDH prepared through the gel to crystallite conversion could be converted into LiAl(OH)₄ ·H₂O or LiAl(OH)₃NO₃ ·H₂O by imbibition of LiOH or LiNO₃ , respectively, under hydrothermal conditions. Thermal decomposition of LDH above 1400 °C gave rise to LiAl₅O₈ accompanied by the evaporation of Li₂O. LiAl(OH)₄ ·H₂O and LiAl(OH)₃NO₃ ·H₂O decomposed in the temperature range 400–1000 °C to α- or β-LiAlO₂ . The compositional dependence of the product, the intermediate phases formed during the heat treatment and the possible reactions involved are described in detail.

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