Jump to main content
Jump to site search
Access to RSC content Close the message box

Continue to access RSC content when you are not at your institution. Follow our step-by-step guide.

Issue 7, 2019
Previous Article Next Article

Nature of the “Z”-phase in layered Na-ion battery cathodes

Author affiliations


Layered 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.

Graphical abstract: Nature of the “Z”-phase in layered Na-ion battery cathodes

Back to tab navigation

Supplementary files

Article information

10 Oct 2018
17 Apr 2019
First published
17 May 2019

This article is Open Access

Energy Environ. Sci., 2019,12, 2223-2232
Article type

Nature of the “Z”-phase in layered Na-ion battery cathodes

J. W. Somerville, A. Sobkowiak, N. Tapia-Ruiz, J. Billaud, J. G. Lozano, R. A. House, L. C. Gallington, T. Ericsson, L. Häggström, M. R. Roberts, U. Maitra and P. G. Bruce, Energy Environ. Sci., 2019, 12, 2223
DOI: 10.1039/C8EE02991A

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Material from this article can be used in other publications provided that the correct acknowledgement is given with the reproduced material.

Reproduced material should be attributed as follows:

  • For reproduction of material from NJC:
    [Original citation] - Published by The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC.
  • For reproduction of material from PCCP:
    [Original citation] - Published by the PCCP Owner Societies.
  • For reproduction of material from PPS:
    [Original citation] - Published by The Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology, the European Photochemistry Association, and RSC.
  • For reproduction of material from all other RSC journals:
    [Original citation] - Published by The Royal Society of Chemistry.

Information about reproducing material from RSC articles with different licences is available on our Permission Requests page.

Social activity

Search articles by author