High-pressure synthesis of Ga-substituted LiCoO2 with layered crystal structure

LiGayCo1-yO2 solid solutions with a layered crystal structure (0less than or equal toy<0.75) were prepared under high-pressure (3 GPa) at 850°C using a piston-cylinder type apparatus. This is in contrast to the solubility of Ga in the layered LiCoO2 structure at atmospheric pressure, which is strongly limited, and reaches a maximum value of y=0.1 at 700°C. The structure of Ga substituted LiCoO2 is characterized by XRD analysis and IR spectroscopy. It has been found that Ga substitutes for Co in the CoO2 layer (3a site), while Li and O are in their normal positions (3b and 6c, respectively). The progressive replacement of Co by Ga leads to a linear increase in the mean M-O bond distance and to a smooth decrease in frequency of the main vibration of the GayCo1-yO2 layer, thus indicating a random Co/Ga distribution. The electrochemical performance of LiGayCo1-yO2 as a cathode material in lithium ion cells has been evaluated in potentiostatic and galvanostatic experiments. The de-intercalation voltage of the LiGayCo1-yO2 solid solutions increases and the reversible capacity decreases with increasing gallium content.