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Excess lithium in transition metal layers of epitaxially grown thin film cathodes of Li2MnO3 leads to rapid loss of covalency during first battery cycle
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.ORCID iD: 0000-0001-5949-0997
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(English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455Article in journal (Other academic) Submitted
Abstract [en]

We have investigated the initial-cycle battery behavior of epitaxial thin films of Li2MnO3-cathodes by employing resonant inelastic X-ray scattering (RIXS) at the O K- and Mn L3-edges. Thin films (25 nm thickness) with Li/Mn-ratios of 2.06 (stoichiometric) and 2.27 (over-stoichiometric), respectively, were epitaxially grown by pulsed laser deposition and electrochemically cycled as battery cathodes in half-cell setup, stopped at potentials for full charge (delithiation) and complete discharge (relithiation), respectively, for X-ray analysis. Using RIXS, we find that significant anionic reactions take place in both materials upon initial delithiation. However, no signatures of localized oxygen holes are found in O K-RIXS of the Li2MnO3 regardless of Li/Mn-ratio. Instead, the top of the oxygen valence band is depleted of electrons forming delocalized empty states upon delithiation. Mn L-RIXS of the over-stoichiometric cathode material shows a progressive loss of charge transfer state intensity during the first battery cycle, revealing a more rapid loss of Mn--O covalency in the over-stoichiometric material.

Keywords [en]
Li-ion battery, Li-rich lithium manganese oxide cathode, pulsed laser deposition (PLD), thin film, resonant inelastic X-ray scattering (RIXS), soft X-ray absorption spectroscopy (XAS)
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-390620OAI: oai:DiVA.org:uu-390620DiVA, id: diva2:1342153
Available from: 2019-08-12 Created: 2019-08-12 Last updated: 2019-08-13
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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Massel, FelixRensmo, HåkanXu, ChaoYounesi, RezaHahlin, MariaDuda, Laurent

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