Sodium Vacancy, Nickel Charge and Orbital Order: Structural Evolution in Electrochemically Desodiated NaxNiO2

NaNiO2 is a promising cathode material for sodium-ion batteries due to its high theoretical capacity of 235.8 mAh.g-1. However, as with many Na-ion cathode materials, a series of poorly understood phase transitions occur on electrochemical cycling, inducing volume mismatch-based stress/strain and resulting in particle cracking, electrochemically disconnected particles, and therefore, irreversible capacity loss. This behaviour is one key obstacle in the development of long-lasting, high-performance Na-ion batteries. Although the series of phases that form as NaxNiO2 is electrochemically cycled have been previously identified, their structures remained unsolved, limiting our ability to understand and control the phase transition behaviour. Following our recent structure solution of the first desodiated phase Pʹ3-Na2/3NiO2, we now report structural solutions based on Rietveld refinement of high-resolution synchrotron X-ray diffraction (SXRD) and neutron powder diffraction (NPD) for the remaining phases obtained on desodiation: Pʹʹ3-Na1/2NiO2, Oʹʹ3-Na2/5NiO2, and Oʹʹʹ3-Na1/3NiO2. Each phase contains a unique Na+/vacancy ordering, which minimises intralayer electrostatic repulsions between Na+ ions, and Nix+-charge ordering which reduces interlayer repulsions through location of lower valence Nix+ nearer to vacancies. Using these structures, we conduct sequential Rietveld refinement of operando SXRD data, which supports prior identification of a transient Pʹʹʹ3-Na1/2<x<2/3NiO2 phase, which is not isolable ex situ. Our operando data also identify the presence of a solid-solution phase Oʹʹ3δ-Na1/3<x<2/5NiO2 and second-order behaviour of the Oʹʹ3-Na2/5NiO2 -> Oʹʹʹ3-Na1/3NiO2 phase transition at the top of charge. This work provides unprecedented insight into structural evolution during electrochemical cycling in Ni-rich Na cathodes (and likely, Li analogues), paving the way towards rational doping regimes designed to disrupt degradation-inducing phase transitions to increase capacity and cycle lifetime, and improve performance of Co-free Na and Li cathodes.