Role of Oxygen Holes in LixCoO2 Revealed by Soft X-Ray Spectroscopy

The fundamental electronic structure of the widely used battery material LixCoO2 still remains a mystery. Soft x-ray absorption spectroscopy of LixCoO2 reveals that holes with strong O 2p character play an essential role in the electronic conductivity of the Co3+/Co4+ mixed valence CoO2 layer. The oxygen holes are bound to the Co4+ 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 LixCoO2 is unique among the various oxide-based battery materials and is one of the key ingredients to improving their electronic and Li-ion conductivities.