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Nature of the "Z"-phase in layered Na-ion battery cathodes
Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England.
Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England;Etteplan Sweden AB, SE-17154 Solna, Sweden.
Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England;Univ Lancaster, Dept Chem, Lancaster, England.
Paul Scherrer Inst, Electrochem Lab, CH-5232 Villigen, Switzerland.
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2019 (English)In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 12, no 7, p. 2223-2232Article in journal (Refereed) Published
Abstract [en]

Layered sodium transition metal oxides with the P2 structure, e.g. Na-2/3[Ni1/3Mn2/3]O-2, 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[Ni1/3Mn2/3]O-2 is attractive as it reduces cost. The Fe containing compounds, such as Na-2/3[Ni1/6Mn1/2Fe1/3]O-2, 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 Fe-57-Mossbauer spectroscopy. It was, however, found that the Fe-substitution serves to disrupt cation ordering in the material.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2019. Vol. 12, no 7, p. 2223-2232
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Inorganic Chemistry
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URN: urn:nbn:se:uu:diva-392868DOI: 10.1039/c8ee02991aISI: 000477950000008OAI: oai:DiVA.org:uu-392868DiVA, id: diva2:1354751
Available from: 2019-09-26 Created: 2019-09-26 Last updated: 2019-09-26Bibliographically approved

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Ericsson, ToreHäggström, Lennart
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