Structural and Chemical Characterization of Layered Li₁_-<i>_x</i>Ni_1-<i>_y</i>Mn<i>_y</i>O₂_-_δ (<i>y</i> = 0.25 and 0.5, and 0 ≤ (1 − <i>x</i>) ≤ 1) Oxides

Abstract

Lithium has been chemically extracted from the O3-type layered LiNi 1 - y Mn y O 2 ( y = 0.25 and 0.5) oxides with the oxidizer NO 2 BF 4 in acetonitrile medium, and the products Li 1 - x Ni 1 - y Mn y O 2 (0 ≤ (1 − x ) ≤ 1) have been characterized by X-ray diffraction, redox titrations, and infrared spectroscopy. Rietveld refinement of the X-ray diffraction data indicates that the Li 1 - x Ni 0.5 Mn 0.5 O 2 - δ system maintains the parent O3-type structure without the formation of any new phases for the entire lithium content 0 ≤ (1 − x ) ≤ 1. In contrast, the Li 1 - x Ni 0.75 Mn 0.25 O 2 - δ system begins to form a new O3‘ phase with a smaller c parameter for (1 − x ) < 0.29, a two-phase region consisting of O3 and O3‘ phases for 0 < (1 − x ) < 0.29, and a single phase O3‘ phase for x = 0. Wet-chemical redox titration analysis indicates that the y = 0.5 system has a greater tendency to lose oxygen from the lattice than the x = 0.25 sample due to a greater overlap of the metal:3d band with top of the O 2 -:2p band, which is consistent with the observed voltage profiles. Nevertheless, both the systems remain as semiconductors for the entire 0 ≤ (1 − x ) ≤ 1. The results are compared with those found before for the Li 1 - x CoO 2 - δ and Li 1 - x Ni 0.85 Co 0.15 O 2 - δ systems.