Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2

Thermoelectric properties of the layered cobalt oxide system LixCoO2 were\ninvestigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase\nbulk samples of LixCoO2 were successfully obtained through electrochemical\ndeintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >=\n0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35)\nexhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K\n(S293K) significantly depends on the Li content (x). The S293K value is as\nlarge as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K\nto +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50.\nThis behavior is in sharp contrast to the results of x <= 0.40 for which the\nS293K value is small and independent of x (+10 uV/K), indicating that a\ndiscontinuous change in the thermoelectric characteristics takes place at x =\n0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity\nare found to coexist in a narrow range of Li composition at about x = 0.75. The\ncoexistence, which leads to an enhanced thermoelectric power factor, may be\nattributed to unusual electronic structure of the two-dimensional CoO2 block.\n