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

概要

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Thermoelectric properties of the layered cobalt oxide system LixCoO2 were investigated in a wide range of Li compositions, 0.98 ≥ x ≥ 0.35. Single-phase bulk samples of LixCoO2 were successfully obtained through electrochemical deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x ≥ 0.94 is semiconductive, the highly Li-deficient phase (0.75 ≥ x ≥ 0.35) exhibits metallic conductivity. The magnitude of the Seebeck coefficient at 293 K (S293K) significantly depends on the Li content (x). The S293K value is as large as +70 up to +100μV/K for x ≥ 0.94, and it rapidly decreases from +90μV/K to +10μV/K as x is lowered within a Li composition range of 0.75 ≥ x ≥ 0.50. This behavior is in sharp contrast to the results of x ≤ 0.40 for which the S293K value is small and independent of x (+10μV/K), indicating that a discontinuous change in the thermoelectric characteristics takes place at x = 0.40-0.50. The unusually large Seebeck coefficient and metallic conductivity are found to coexist in a narrow range of Li composition at about x = 0.75. The coexistence, which leads to an enhanced thermoelectric power factor, may be attributed to unusual electronic structure of the two-dimensional CoO2 block.
2011-05-15