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

Abstract

LiGa_yCo_1−yO₂ solid solutions with a layered crystal structure (0 ≤ y < 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 Ga_yCo_1−yO₂ layer, thus indicating a random Co/Ga distribution. The electrochemical performance of LiGa_yCo_1−yO₂ as a cathode material in lithium ion cells has been evaluated in potentiostatic and galvanostatic experiments. The de-intercalation voltage of the LiGa_yCo_1−yO₂ solid solutions increases and the reversible capacity decreases with increasing gallium content.