Structure and electrochemistry of Li1±yNiO2 and a new Li2NiO2 phase with the Ni (OH)2 structure

We discuss the preparation and structure of 3R-LixNi2-xO2 for x near 1 and report the variation of the voltage of Li/Li1± yNiO2 cells versus y. Using electrochemical techniques and in situ X-ray diffraction we show that Li can be reversibly intercalated in Li1-yNiO2 for 0≤y≤0.6 and that over this range of y the lattice constants of the 3R phase change reversibly. Additional lithium can also be inserted electrochemically into LiNiO2 to form a new Li2 phase with the Ni(OH)2 structure. When the additional Li is removed from Li2NiO2, the host lattice reverts to the LiNiO2 structure, but there is substantial hysteresis in the voltage of Li/Li1±yNiO2 cells during this phase transition. The analogous phase, Li2CoO2, does not form when additional Li is added to LiCoO2 by electrochemical methods.