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

High-quality epitaxial films of Li 0.93 CoO 2 were successfully grown on the (0001)-face of α-Al 2 O 3 substrates via the topotactic ion exchange of Na 0.8 CoO 2 epitaxial films. The average x -value of the Li x CoO 2 films was controlled from 0.60 to 0.93 by immersing in a K 2 S 2 O 8 aqueous solution. The metal ( x met. ≤ 0.72)/insulator ( x ins. ≥ 0.83) phase separation was clearly observed in the range of 0.72 < x < 0.87. Seebeck coefficient ( S ) for the Li 0.6 CoO 2 film was +38 µV·K −1, and it increased gradually with the x -value ( x ≤ 0.72), while no significant x -value dependence of S was observed at x > 0.72, indicating that the metal phase dominates the S -value of the metal/insulator mixture.