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Anion redox processes in novel battery cathode materials investigated by soft X-ray spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
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 [en]
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: urn:nbn:se:uu:diva-390623ISBN: 978-91-513-0714-5 (print)OAI: oai:DiVA.org:uu-390623DiVA, id: diva2:1342156
Public defence
2019-09-27, Room 80101, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-09-06 Created: 2019-08-13 Last updated: 2019-09-17
List of papers
1. The role of anionic processes in Li1xNi0.44Mn1.56O4 studied by resonant inelastic X-ray scattering
Open this publication in new window or tab >>The role of anionic processes in Li1xNi0.44Mn1.56O4 studied by resonant inelastic X-ray scattering
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-389847 (URN)
Available from: 2019-07-29 Created: 2019-07-29 Last updated: 2019-08-13
2. How Mn/Ni ordering controls electrochemical performance in high-voltage spinel LiNi0.44Mn1.56O4 (LNMO) with fixed oxygen content
Open this publication in new window or tab >>How Mn/Ni ordering controls electrochemical performance in high-voltage spinel LiNi0.44Mn1.56O4 (LNMO) with fixed oxygen content
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(English)Manuscript (preprint) (Other academic)
Keywords
High voltage spinel, LNMO, cation ordering, oxygen deficiency, rock-salt, anionic redox activity
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-389799 (URN)
Available from: 2019-07-28 Created: 2019-07-28 Last updated: 2019-08-13
3. Excess lithium in transition metal layers of epitaxially grown thin film cathodes of Li2MnO3 leads to rapid loss of covalency during first battery cycle
Open this publication in new window or tab >>Excess lithium in transition metal layers of epitaxially grown thin film cathodes of Li2MnO3 leads to rapid loss of covalency during first battery cycle
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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
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:nbn:se:uu:diva-390620 (URN)
Available from: 2019-08-12 Created: 2019-08-12 Last updated: 2019-08-13
4. Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li0.2Ni0.2Mn0.6]O-2
Open this publication in new window or tab >>Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li0.2Ni0.2Mn0.6]O-2
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2016 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 138, no 35, p. 11211-11218Article in journal (Refereed) Published
Abstract [en]

Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g., Mn3+/4+ in LiMn2O4, and this limits the energy storage of Li-ion batteries. Compounds such as Li[Li0.2Ni0.2Mn0.6]O-2 exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V versus Li+/Li. The capacity at 4.5 V is dominated by oxidation of the O-2(-) anions accounting for similar to 0.43 e(-)/formula unit, with an additional 0.06 e(-)/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V is mainly O loss, similar to 0.08 e(-)/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn4+/Li+). The results have been obtained by combining operando electrochemical mass spec on 180 labeled Li[Li0.2Ni0.2Mn0.6]O-2 with XANES, soft X-ray spectroscopy, resonant inelastic X-ray spectroscopy, and Raman spectroscopy. Finally the general features of O redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal oxygen interaction on anion redox in lithium rich cathode materials.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-305471 (URN)10.1021/jacs.6b05111 (DOI)000382901800031 ()27498756 (PubMedID)
Funder
Swedish Research Council
Available from: 2016-10-19 Created: 2016-10-18 Last updated: 2019-08-13Bibliographically approved
5. Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox
Open this publication in new window or tab >>Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox
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2018 (English)In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 11, no 4, p. 926-932Article in journal (Refereed) Published
Abstract [en]

The quantity of charge stored in transition metal oxide intercalation cathodes for Li or Na batteries is not limited by transition metal redox reactions but can also access redox reactions on O; examples include Li1.2Ni0.13Mn0.54Co0.13O2, Li2Ru0.75Sn0.25O3, Li1.2Nb0.3Mn0.4O2, Na2RuO3 and Na2/3Mg0.28Mn0.72O2. Here we show that oxyfluorides can also exhibit charge storage by O-redox. We report the discovery of lithium manganese oxyfluoride, specifically the composition, Li1.9Mn0.95O2.05F0.95, with a high capacity to store charge of 280 mA h g(-1) (corresponding to 960 W h kg(-1)) of which almost half, 130 mA h g(-1), arises from O-redox. This material has a disordered cubic rocksalt structure and the voltage-composition curve is significantly more reversible compared with ordered Li-rich layered cathodes. Unlike lithium manganese oxides such as the ordered layered rocksalt Li2MnO3, Li1.9Mn0.95O2.05F0.95 does not exhibit O loss from the lattice. The material is synthesised using a simple, one-pot mechanochemical procedure.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-354239 (URN)10.1039/c7ee03195e (DOI)000430537000017 ()
Available from: 2018-06-29 Created: 2018-06-29 Last updated: 2019-08-13Bibliographically approved
6. Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2
Open this publication in new window or tab >>Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2
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2018 (English)In: Nature Chemistry, ISSN 1755-4330, E-ISSN 1755-4349, Vol. 10, p. 288-295Article in journal (Refereed) Published
Abstract [en]

The search for improved energy-storage materials has revealed Li-and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li+-O(2p)-Li+ interactions). Na2/3[Mg0.28Mn0.72]O2 exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg2+ resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na2/3[Mg0.28Mn0.72]O2 does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkalirich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg2+ remains in Na2/3[Mg0.28Mn0.72]O2, which stabilizes oxygen.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-344548 (URN)10.1038/nchem.2923 (DOI)000425589000009 ()29461536 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 290605
Available from: 2018-03-06 Created: 2018-03-06 Last updated: 2019-08-13
7. What Triggers Oxygen Loss in Oxygen Redox Cathode Materials?
Open this publication in new window or tab >>What Triggers Oxygen Loss in Oxygen Redox Cathode Materials?
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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
National Category
Materials Chemistry Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-386161 (URN)10.1021/acs.chemmater.9b00227 (DOI)000468242300025 ()
Available from: 2019-06-20 Created: 2019-06-20 Last updated: 2019-08-13Bibliographically approved
8. Understanding charge compensation mechanisms in Na0.56Mg0.04Ni0.19Mn0.70O2
Open this publication in new window or tab >>Understanding charge compensation mechanisms in Na0.56Mg0.04Ni0.19Mn0.70O2
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(English)In: Article in journal (Other academic) Submitted
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-390622 (URN)
Available from: 2019-08-12 Created: 2019-08-12 Last updated: 2019-08-13

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