Structural Elucidation of Na_2/3NiO₂, a Dynamically Stabilized Cathode Phase with Nickel Charge and Sodium Vacancy Ordering

NaNiO₂ (NNO) has been investigated as a promising sodium-ion battery 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 Na_2/3NiO₂ (P^'3) desodiated phase using combined Rietveld refinement of high-resolution synchrotron X-ray (SXRD) and neutron powder diffraction (NPD) data, magnetic susceptibility, and ²³Na solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Our experimental results are compared to <i>ab initio</i> molecular dynamics (AIMD) simulations, which indicate multiple low-energy structures that are dynamically populated. We observe a combination of competing effects that 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 multiple contributions to the structures of desodiated Na_2/3NiO₂, along with a framework for investigating the other unsolved desodiated structures. This work may also inform our understanding of the Jahn-Teller evolution in other nickel-rich lithium- and sodium-ion cathodes, such as LiNiO₂.