Issue 7, 2019

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


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

Supplementary files

Article information

Article type
10 Oct 2018
17 Apr 2019
First published
17 May 2019
This article is Open Access
Creative Commons BY license

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

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. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

Read more about how to correctly acknowledge RSC content.

Social activity