The fundamental electronic structure of the widely used battery material Li(x)CoO(2) still remains a mystery. Soft x-ray absorption spectroscopy of Li(x)CoO(2) reveals that holes with strong O 2p character play an essential role in the electronic conductivity of the Co(3+)/Co(4+) mixed valence CoO(2) layer. The oxygen holes are bound to the Co(4+) sites and the Li-ion vacancy, suggesting that the Li-ion flow can be stabilized by oxygen hole back flow. Such an oxygen hole state of Li(x)CoO(2) is unique among the various oxide-based battery materials and is one of the key ingredients to improving their electronic and Li-ion conductivities.

Measurements of electrical resistivity ͑͒, dc magnetization ͑M͒, and specific heat ͑C͒ have been performed on layered oxide Li x CoO 2 ͑0.25Յ x Յ 0.99͒ using single-crystal specimens. The versus temperature ͑T͒ curve for x = 0.90 and 0.99 is found to be insulating but a metallic behavior is observed for 0.25Յ x Յ 0.71. At T S ϳ 155 K, a sharp anomaly is observed in the -T, M-T, and C / T-T curves for x = 0.66 with thermal hysteresis, indicating the first-order character of the transition. The transition at T S ϳ 155 K is observed for the wide range of x = 0.46-0.71. It is found that the M-T curve measured after rapid cool becomes different from that after slow cool below T F , which is ϳ130 K for x = 0.46-0.71. T F is found to agree with the temperature at which the motional narrowing in the 7 Li NMR linewidth is observed, indicating that the Li ions stop diffusing and order at the regular site below T F . The ordering of Li ions below T F ϳ 130 K is likely to be triggered and stabilized by the charge ordering in CoO 2 layers below T S .

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We report an angle-resolved photoemission spectroscopy (ARPES) study of Li x CoO 2 single crystals which have a hole-doped CoO 2 triangular lattice. Similar to Na x CoO 2 , the Co 3d a 1g band crosses the Fermi level with strongly renormalized band dispersion while the Co 3d e ′ g bands are fully occupied in Li x CoO 2 (x=0.46 and 0.71). At x=0.46, the Fermi surface area is consistent with the bulk hole concentration indicating that the ARPES result represents the bulk electronic structure. On the other hand, at x=0.71, the Fermi surface area is larger than the expectation which can be associated to the inhomogeneous distribution of Li reported in the previous scanning tunneling microscopy study by Iwaya et al. [Phys. Rev. Lett. 111, 126104 (2013)]. However, the Co 3d peak is systematically shifted towards the Fermi level with hole doping excluding phase separation between hole rich and hole poor regions in the bulk. Therefore, the deviation of the Fermi surface area at x=0.71 can be attributed to hole redistribution at the surface avoiding polar catastrophe. The bulk Fermi surface of Co 3d a 1g is very robust around x=0.5 even in the topmost CoO 2 layer due to the absence of the polar catastrophe.