Role of Oxygen Holes in ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ Revealed 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.

Figure 1

O $1s$ x-ray absorption spectroscopy. (a) Schematic crystal structure of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$. Co ions are surrounded by six oxygen ions and form a triangular lattice layer. Li ions are sandwiched by the Co-O layers. The oxygen sites with higher O $2p$ hole concentration are indicated by the yellow (shaded) circles. (b) Electronic configurations and O $1s$ XAS processes for the ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ sites. (c) O $2p$ orbitals located in between the ${\mathrm{Co}}^{3+}$ site and the ${\mathrm{Co}}^{4+}$ site. The blue (red) O $2p$ orbitals can hybridize with the ${\mathrm{Co}}^{3+}$ (${\mathrm{Co}}^{4+}$) site. (d) O $1s$ XAS spectra of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ ($x=0.99$, 0.66, 0.46, and 0.25) taken at incidence angle of 60 degrees.

Figure 2

(a) Schematic picture of the charge ordered states for $x=1.00$, 0.89, 0.66, 0.44, and 0.22. The ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ sites are arranged with the ratio of $x$:$1-x$. (b) The Co $3d$ and O $2p$ PDOS calculated by the model Hartree-Fock methods for the charge ordered states.

Figure 3

O $1s$ XAS spectra of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ at 20 K as a function of incidence angle for (a) $x=0.99$, (b) $x=0.66$, (c) $x=0.46$.

Figure 4

(a) Co $2p$ XAS spectra of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ ($x=0.99$, 0.66, 0.46, and 0.25) at 20 K for the incidence angle $\theta$ of 60°. (b) Co $2p$ XAS spectra of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ ($x=0.99$) at 20 K as a function of incidence angle. (c) Co $2p$ XAS spectra of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ ($x=0.66$) at 20 K as a function of incidence angle.