Role of Oxygen Holes inLixCoO2Revealed by Soft X-Ray Spectroscopy

The fundamental electronic structure of the widely used battery material ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ still remains a mystery. Soft x-ray absorption spectroscopy of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ reveals that holes with strong O $2p$ character play an essential role in the electronic conductivity of the ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ mixed valence ${\mathrm{CoO}}_{2}$ layer. The oxygen holes are bound to the ${\mathrm{Co}}^{4+}$ 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 ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ is unique among the various oxide-based battery materials and is one of the key ingredients to improving their electronic and Li-ion conductivities.