Formation of LiAlyNi1-yO2 solid solutions under high and atmospheric pressure

Three methods were used for the synthesis of LiAlyNi1-yO2 solid solutions with layered crystal structure: citrate and hydroxide precursor methods at atmospheric pressure and high-pressure synthesis in oxygen-rich atmosphere (3GPa). Structural characterization of the oxides was performed by powder XRD analysis and electron paramagnetic resonance (EPR) spectroscopy. Irrespective of the different preparation techniques used, it was found that LiAlyNi1-yO2 solid solutions can be formed in the limited concentration range of 0=<y=<0.5 and 0.75=<y=<1.0. The unit cell parameter a decreases linearly with the Al content whereas the unit cell parameter c increases sharper as compared to the linear interpolation of the c parameter calculated for the two end compositions LiNiO2 and LiAlO2. In these compositions, aluminum substitutes for Ni in the NiO2-layer, the mean AlyNi1-y-O bond length decreasing. The extent of the trigonal distortion of AlyNi1-yO6 and LiO6-octahedra varies with the aluminum content and depends on the synthesis procedure used. The LiO6-octahedra are more flexible to tolerate the increased trigonal distortion as compared to the AlyNi1-yO6-octahedra. High-pressure synthesis favors the formation of oxides with a higher extent of trigonal distortion of both AlyNi1-yO6 and LiO6-octahedra. From EPR measurements, it was shown that local cationic distribution in LiAlyNi1-yO2 depends on the synthesis temperature. At atmospheric pressure, higher synthesis temperatures promote the reaction of cation mixing between the layers