Comparison of the phase relationships of chemically delithiated layered Li1−xCo1−yMyO2 (M=Al and Mg) oxides

Lithium has been chemically extracted with NO2BF4 in acetonitrile medium from LiCo1−yAlyO2 (y=0.1 and 0.25) and LiCo1−yMgyO2 (y=0.06 and 0.1). The products have been characterized by a Rietveld refinement of the X-ray diffraction data, wet chemical analyses for lithium and oxygen contents (where possible), and infrared spectroscopy for metallic conduction. While all the lithium could be extracted from LiCo0.94Mg0.06O2, only 80%, 70%, and 80% of the lithium could be extracted, respectively, from LiCo0.9Al0.1O2, LiCo0.75Al0.25O2, and LiCo0.9Mg0.1O2. The Li1−xCo0.9Al0.1O2 system maintains the initial O3-type structure for 0.45≤(1−x)≤1.0 and a two-phase mixture of O3+O3′ phases for 0.2≤(1−x)<0.45, while the Li1−xCo0.75Al0.25O2 system maintains the initial O3-type structure for the entire range of 0.3≤(1−x)≤1.0. Similarly, Li1−xCo0.94Mg0.06O2 maintains the initial O3-type structure for 0.6≤(1−x)≤1.0 and a mixture of the O3- and P3-type phases for 0<(1−x)≤0.6, while Li1−xCo0.9Mg0.1O2 maintains the initial O3-type structure for the entire range of 0.2≤(1−x)≤1.0. The observed phase relationships are compared with those of the previously investigated Li1−xCo1−yMyO2 and Li1−xNi1−yMnyO2 systems, and the results are rationalized on the basis of electrostatic considerations.