Factors influencing the crystal chemistry of chemically delithiated layered HxNi1−y−zMnyCozO2

Abstract

Layered H_xNi_1−y−zMn_yCo_zO₂ oxides have been synthesized by chemically extracting lithium from the corresponding LiNi_1−y−zMn_yCo_zO₂ with the oxidizer NO₂BF₄ in acetonitrile. Rietveld analysis of the X-ray diffraction data reveals that the structure of the delithiated H_xNi_0.5−yMn_0.5−yCo_2yO₂ phases obtained from LiNi_0.5−yMn_0.5−yCo_2yO₂ synthesized at 900 °C changes from P3 type (in H_xCoO₂) to O1 type (in H_xNi_0.33Mn_0.33Co_0.33O₂) to O3 type (in H_xNi_0.5Mn_0.5O₂) with decreasing Co content. In addition, the tendency to form the P3 type phase instead of the O1 type phase in the case of H_xNi_0.33Mn_0.33Co_0.33O₂ increases as the synthesis temperature of the parent LiNi_0.33Mn_0.33Co_0.33O₂ decreases from 900 to 600 °C. A similar tendency to form the P3 type phase instead of the O3 type phase is also seen with the H_xNi_0.5Co_0.5O₂ system with decreasing synthesis temperature of LiNi_0.5Co_0.5O₂. These observed variations in the type of phases formed are explained on the basis of (i) the differences in the chemical lithium extraction rate caused by the differences in the degree of cation disorder and surface area, and (ii) the proton content inserted during chemical lithium extraction.