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

Abstract

Thermoelectric properties of the layered cobalt oxide system Li${}_x$CoO${}_2$ were investigated in a wide range of Li compositions, 0.98 ⩾ $x$ ⩾ 0.35. Single-phase bulk samples of Li${}_x$CoO${}_2$ were successfully obtained through electrochemical deintercalation of Li from the pristine LiCoO${}_2$ phase. While Li${}_x$CoO${}_2$ 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 ($S_{293\mathrm{K}}$) significantly depends on the Li content ($x$). The $S_{293\mathrm{K}}$ value is as large as $+$70 up to $+$100 μV/K for $x$ ⩾ 0.94, and it rapidly decreases from $+$90 μV/K to $+$10 μV/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 $S_{293\mathrm{K}}$ value is small and independent of $x$ (+10 μV/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 CoO${}_2$ block.