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

TL;DR

This study investigates how varying lithium content in LixCoO2 affects its thermoelectric properties, revealing a sharp transition in behavior around x=0.40-0.50 and coexistence of high Seebeck coefficient with metallic conductivity near x=0.75.

Contribution

It provides detailed analysis of thermoelectric properties across a wide Li composition range, highlighting a discontinuous change and coexistence phenomena not previously reported.

Findings

Seebeck coefficient varies significantly with Li content.

Metallic conductivity observed in highly Li-deficient phases.

Large Seebeck coefficient coexists with metallic behavior near x=0.75.

Abstract

Thermoelectric properties of the layered cobalt oxide system LixCoO2 were investigated in a wide range of Li composition, 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 Seebeck coefficient at 293 K (S293K) significantly depends on the Li content (x). The S293K value is as large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K to +10 uV/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 (+10 uV/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.