Strong localization of oxidized Co3+ state in cobalt-hexacyanoferrate

Secondary batteries are important energy storage devices for a mobile equipment, an electric car, and a large-scale energy storage. Nevertheless, variation of the local electronic state of the battery materials in the charge (or oxidization) process are still unclear. Here, we investigated the local electronic state of cobalt-hexacyanoferrate (Na$_x$Co[Fe(CN)$_6$]$_{0.9}$), by means of resonant inelastic X-ray scattering (RIXS) with high energy resolution (~100 meV). The L-edge RIXS is one of the most powerful spectroscopic technique with element- and valence-selectivity. We found that the local electronic state around Co$^{2+}$ in the partially-charged Na$_{1.1}$Co$^{2+}$$_{0.5}$Co$^{3+}$$_{0.5}$[Fe$^{2+}$(CN)$_6$]$_{0.9}$ film (x = 1.1) is the same as that of the discharged Na$_{1.6}$Co$^{2+}$[Fe$^{2+}$(CN)$_6$]$_{0.9}$ film (x = 1.6) within the energy resolution, indicating that the local electronic state around Co$^{2+}$ is invariant against the partial oxidization. In addition, the local electronic state around the oxidized Co$^{3+}$ is essentially the same as that of the fully-charged film Co$^{3+}$[Fe$^{2+}$(CN)$_6$]$_{0.3}$[Fe$^{3+}$(CN)$_6$]$_{0.6}$ (x = 0.0) film. Such a strong localization of the oxidized Co$^{3+}$ state is advantageous for the reversibility of the redox process, since the localization reduces extra reaction within the materials and resultant deterioration.