Temperature Dependent Local Structure of NaxCoO2 Cathode Material for Rechargeable Sodium-Ion Batteries

We have investigated the local structure of differently charged NaxCoO2, cathode material as a function of temperature by Co K-edge X-ray absorption fine structure (EXAFS) measurements. We have found that the Charge/discharge process has a direct effect on the bond characteristics of the cathode in the Na-ion batteries. The results reveal that the local Co-O bonds get softer, while the Co Co bonds hardly show any change during discharge (sodiation). The present study underlines the key role of local atomic displacements in diffusion and the reversibility of ions in cathodes for batteries and points toward the feasibility of NaxCoO2 to be used as a cathode material. The results are discussed in comparison with the lithiation/delithiation of LixCoO2 battery materials.