Structure and electrochemistry of Li xFe yNi 1-yO 2

Abstract

The solid solution series LiFe y Ni 1- y O 2 (0.1≤ y≤0.5) has been prepared from various starting materials. Using powder X-ray diffraction and Rietveld profile refinement, we show that this solid solution is only iso-structural with hexagonal LiNiO 2 in the conposition range 0≤ y≲0.23. For 0.23≲ y≲0.48 phase coexistence occurs between hexagonal LiFe 0.23Ni 0.77O 2 and cubic (NiO structure) LiFe 0.48Ni 0.52O 2. For all samples in the hexagonal phase, cation mixing between transition metal and lithium layers, was observed. Using anomalous X-ray techniques we were able to determine that the cation mixing was primarily between Fe and Li. The cation mixing and the presence of the cubic phase for y>0.2, profoundly affects the electrochemical de-intercalation of Li from these materials. Electrochemical measurements using Li/Li 1− x Fe y Ni 1− y O 2 cells show that as y increases the amount of Li which can be reversibly cycled, Δ x, decreases, thus limiting the practical usefulness of these materials.