Somerville, James W.Sobkowiak, AdamTapia-Ruiz, NuriaBillaud, JulietteLozano Suárez, Juan GabrielHouse, Robert A.Gallington, Leighanne C.Ericsson, ToreHäggström, LennartRoberts, Matthew R.Maitra, UrmimalaBruce, Peter G.2019-09-182019-09-182019-05-17Somerville, J.W., Sobkowiak, A., Tapia-Ruiz, N., Billaud, J., Lozano Suárez, J.G., House, R.A.,...,Bruce, P.G. (2019). Nature of the Z-phase in layered Na-ion battery cathodes. Energy and Environmental Science, 12 (7), 2223-2232.1754-56921754-5706https://hdl.handle.net/11441/89185Layered sodium transition metal oxides with the P2 structure, e.g. Na2/3[Ni1/3Mn2/3]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 Na2/3[Ni1/3Mn2/3]O2 is attractive as it reduces cost. The Fe containing compounds, such as Na2/3[Ni1/6Mn1/2Fe1/3]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 changing intergrowth structure which evolves from P2 to O2 through the OP4 structure as an intermediate. On charging, Na+ removal results in O-type stacking faults within the P2 structure which increase in proportion. At 50% O-type stacking faults, the ordered OP4 phase forms and on further charging more O-type stacking faults are formed progressing towards a pure O2 structure. This gives the superficial appearance of a solid solution. Furthermore, in contrast to some previous studies, we did not detect Fe migration at any state-of-charge using 57Fe-Mössbauer spectroscopy. It was, however, found that the Fe-substitution serves to disrupt cation ordering in the material.application/pdfengAttribution-NonCommercial-NoDerivatives 4.0 InternacionalAttribution-NonCommercial-NoDerivatives 4.0 Internacionalhttp://creativecommons.org/licenses/by-nc-nd/4.0/info:eu-repo/semantics/articleinfo:eu-repo/semantics/openAccesshttps://doi.org/10.1039/c8ee02991a