Structural Elucidation of Na2/3NiO2, A Dynamically Stabilised Cathode Phase with Ni Charge- and Na Vacancy- Ordering

NaNiO2 (NNO) has been investigated as a promising sodium-ion battery (NIB) cathode material, but it is limited by degradation-induced capacity fade. On desodiation, NNO forms multiple phases with large superstructures due in part to Na+-ion vacancy ordering, however, their structures are unknown. Here, we report a structural solution to the Na2/3NiO2 (P/3) de-sodiated phase using combined Rietveld refinement of high-resolution synchrotron X-ray (SXRD) and neutron diffraction (NPD) data, magnetic susceptibility, and 23Na solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Our experimental results are compared to Ab Initio Molecular Dynamics (AIMD) simulations, which indicate multiple low-energy structures that are dynamically populated. We observe a combination of competing effects which contribute to the resultant dynamic nature of the structure, including honeycomb ordering of mixed-valence Ni, orbital-ordering of Jahn-Teller (JT) distorted Ni3+, and zigzag Na+/vacancy ordering. Our work provides evidence of the multiple contributions to the structures of de-sodiated Na2/3NiO2, along with a framework for investigating the other unsolved de-sodiated structures. This work may also inform understanding of the Jahn-Teller evolution in other nickel-rich lithium and sodium ion cathodes, such as LiNiO2.