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What Triggers Oxygen Loss in Oxygen Redox Cathode Materials?
Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England.
Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England;Justus Liebig Univ Giessen, Inst Phys Chem, Heinrich Buff Ring 17,Room B48, D-35392 Giessen, Germany.
Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England.
Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England.
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2019 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 31, no 9, p. 3293-3300Article in journal (Refereed) Published
Abstract [en]

It is possible to increase the charge capacity of transition metal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, we have compared two analogous Na-ion cathodes, P2-Na0.67Mg0.28Mn0.72O2 and P2-Na0.78Li0.25Mn0.75O2. On charging to 4.5 V, >0.4e(-) are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na0.78Li0.25Mn0.25O2 but displaced from the TM to the alkali metal layers, showing that vacancies in the TM layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li[Li0.2Ni0.2Mn0.6]O-2, are not a trigger for oxygen loss. On charging at 5 V, P2-Na0.78Li0.25Mn0.75O2 exhibits oxygen loss, whereas P2-Na0.67Mg0.28Mn0.72O2 does not. Under these conditions, both Na+ and Li+ are removed from P2-Na0.78Li0.25Mn0.75O2, resulting in underbonded oxygen (fewer than 3 cations coordinating oxygen) and surface-localized O loss. In contrast, for P2-Na0.67Mg0.28Mn0.72O2, oxygen remains coordinated by at least 2 Mn4+ and 1 Mg2+ ions, stabilizing the oxygen and avoiding oxygen loss.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC , 2019. Vol. 31, no 9, p. 3293-3300
National Category
Materials Chemistry Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-386161DOI: 10.1021/acs.chemmater.9b00227ISI: 000468242300025OAI: oai:DiVA.org:uu-386161DiVA, id: diva2:1328120
Available from: 2019-06-20 Created: 2019-06-20 Last updated: 2019-08-13Bibliographically approved
In thesis
1. Anion redox processes in novel battery cathode materials investigated by soft X-ray spectroscopy
Open this publication in new window or tab >>Anion redox processes in novel battery cathode materials investigated by soft X-ray spectroscopy
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents experimental investigations of the electronic structure of emerging and novel cathode materials used in lithium- and sodium-ion batteries. The investigated materials include a range of oxide materials containing the elements nickel and manganese. Central goals are to find fundamental explanations for favorable, respectively, unfavorable electrochemical cycling behavior and to arrive at a better understanding of the roles that the different elemental constituents of the compounds play. The experiments are based on the application of X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) in the soft X-ray region and have been performed at synchrotron radiation facilities such as The Advanced Light Source (USA), The Swiss Light Source (Switzerland) and SPring-8 (Japan).

 XAS and RIXS of spinel LiNi0.44Mn1.56O4 at the O K-edge as well as the Ni and Mn L-edges were measured for two different crystal structures, namely, transition-metal-ordered and -disordered, respectively. The results show that both Ni and O contribute strongly as redox centers for the charge compensation during electrochemical cycling. The Ni L-RIXS spectra show evidence of a more stable Ni--O bond in the disordered material.

 In the layered manganese oxide materials Li[Li0.2Ni0.2Mn0.6]O2, Na0.67[Mg0.28Mn0.72]O2, and Na0.78[Li0.25Mn0.75]O2, as well as the disordered Li1.9Mn0.95O2.05F0.95 one observes that reversible O redox leads to two distinct features in O K-RIXS. Both features resonate in a narrow incident energy range suggesting that localized O hole states are formed, one close to the elastic peak and the other as a strong emission peak at an energy loss of about 8 eV. These features appear reversibly on the voltage plateau of the charge-discharge curve and can be used to identify a certain type of O redox reactions.

The work also includes investigations that compare two different compositions of the structurally related material Li2MnO3 grown epitaxially as thin films. Evidence is found for anionic activity during the initial cycle that is of a different kind than the above as no evidence for localized O holes is found. Instead, excess Li in the transition metal layer is shown to lead to a more rapid loss of covalency in the Mn--O bonds.

In short, this work presents some of the first explorations into the role of different types of anionic redox centers in cathodes, by means of XAS and RIXS thereby also demonstrating the utility and power of synchrotron based techniques for gaining atomic-level understanding of battery electrode materials.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 73
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1835
Keywords
soft X-ray spectroscopy, X-ray absorption spectroscopy (XAS), resonant inelastic X-ray scattering (RIXS), lithium-ion battery (LIB), sodium-ion battery (SIB), anionic redox, cathode materials, layered manganese oxide, spinel LNMO
National Category
Condensed Matter Physics Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-390623 (URN)978-91-513-0714-5 (ISBN)
Public defence
2019-09-27, Room 80101, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
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Available from: 2019-09-06 Created: 2019-08-13 Last updated: 2019-09-17

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Naylor, Andrew J.Duda, LaurentMassel, Felix

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