STRUCTURE AND ELECTROCHEMISTRY OF LIXFEYNI1-YO2

The solid solution series LiFeyNi1-yO2 (0.1 less-than-or-equal-to Y less-than-or-equal-to 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 LiNiO2 in the composition range 0 less-than-or-equal-to y less than or similar 0.23. For 0.23 less than or similar y less than or similar 0.48 phase coexistence occurs between hexagonal LiFe0.23Ni0.77O2 and cubic (NiO structure) LiFe0.48Ni0.5202. 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 gamma > 0.2, profoundly affects the electrochemical de-intercalation of Li from these materials. Electrochemical measurements using Li/Li1-xFeyNi1-yO2 cells show that as y increases the amount of Li which can be reversibly cycled, DELTAx, decreases, thus limiting the practical usefulness of these materials.