Epitaxial Film Growth of LixCoO2(0.6 ≤x≤ 0.9) via Topotactic Ion Exchange of Na0.8CoO2†

High-quality epitaxial films of Li0.93CoO2 were successfully grown on the (0001)-face of α-Al2O3 substrates via the topotactic ion exchange of Na0.8CoO2 epitaxial films. The average x-value of the LixCoO2 films was controlled from 0.60 to 0.93 by immersing in a K2S2O8 aqueous solution. The metal (xmet. ≤ 0.72)/insulator (xins. ≥ 0.83) phase separation was clearly observed in the range of 0.72 0.72, indicating that the metal phase dominates the S-value of the metal/insulator mixture.