Hydrothermal Syntheses of Layered Lithium Nickel Manganese Oxides from Mixed Layered Ni(OH)₂−Manganese Oxides

A series of mixed layered Ni(OH) 2 −manganese oxides with Ni/Mn molar ratios up to near 1 were prepared by using a novel exfoliation−restacking hydrothermal method and a hydrothermal intercalation method. A series of lithium nickel manganese oxides with hexagonal layered structure (space group R 3̄ m ) can be obtained by hydrothermal treatment of the mixed layered Ni(OH) 2 −manganese oxides in a concentrated LiOH solution at 200 °C. Another type of lithium nickel manganese oxide with an α-LiFeO 2 -like cubic structure can be obtained by using NiMnO 3 as precursor under similar hydrothermal conditions. The synthesis reactions and the products were investigated by XRD, TEM, TG−DTA, and XPS studies. The mixed layered structure of Ni(OH) 2 −manganese oxides can be described by two sublattices having different hexagonal unit-cell parameters in the ab plane but a common periodicity perpendicular to them along c . The layered lithium nickel manganese oxides can be expressed by a general formula of {Li x }[Li y Ni n Mn m ]O 2 ( x ≤ 1, y ≤ 1 / 3, ( y + n + m ) ≤ 1). The cubic phase of lithium nickel manganese oxide is metastable, which transforms to the hexagonal layered phase after heating at 500 °C in air.