View article

Layered sodium transition metal oxides with the P2 structure, e.g. Na_2/3[Ni_1/3Mn_2/3]O₂, are regarded as candidates for Na-ion battery cathodes. On charging, extraction of Na destabilizes the P2 phase (ABBA oxide ion stacking) in which Na⁺ is in trigonal prismatic coordination, resulting in layer gliding and formation of an O2 phase (ABAC stacking) with octahedral coordination. However, many related compounds do not exhibit such a simple P2 to O2 transition but rather form a so called “Z”-phase. Substituting Ni by Fe in Na_2/3[Ni_1/3Mn_2/3]O₂ is attractive as it reduces cost. The Fe containing compounds, such as Na_2/3[Ni_1/6Mn_1/2Fe_1/3]O₂, form a “Z”-phase when charged above 4.1 V vs. Na⁺/Na. By combining ex situ and operando X-ray diffraction with scanning transmission electron microscopy and simulated diffraction patterns, we demonstrate that the “Z”-phase is most accurately described as a continuously …