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Topochemical fluorination and defluorination in the context of fluoride-ion batteries and tuning of magnetic properties

Wissel, Kerstin (2020):
Topochemical fluorination and defluorination in the context of fluoride-ion batteries and tuning of magnetic properties.
Darmstadt, Technische Universität,
DOI: 10.25534/tuprints-00013383,
[Ph.D. Thesis]

Abstract

Within this work, it was demonstrated that topochemical modifications of the anion sublattices of Ruddlesden-Popper-type oxides An+1BnO3n+1 and derived metastable oxyfluorides An+1BnO3n+1-xF2x with 0 < x ≤ 2 have a significant influence on the crystal and electronic structures of the newly synthesised phases. This could be used to effectively tailor and reversibly tune magnetic properties. Different non-oxidative, reductive or oxidative modification routes, leading to fluoride intercalation, exchange and/or deintercalation processes, were investigated. Such topochemical reactions have been also found to take place upon the electrochemical fluorination of Ruddlesden-Popper-type oxides in fluoride-ion batteries and have led to the development of intercalation-based cathode materials. For the development of novel intercalation-based electrodes, oxyfluorides, obtained via a prior non-oxidative topochemical fluorination of the respective oxides, were examined concerning their potential use as active anode or cathode materials. During charging, the use of the oxyfluoride as active anode material results in defluorination, whereas additional fluorination occurs when the oxyfluoride contains additional vacancies and is used as active cathode material. For both cases, the oxyfluoride represents the discharged state of the electrode material. These additional topochemical modifications of the parent oxyfluorides could be also achieved via chemical preparation approaches. For the chemical preparation of the anode material in the charged state, a reductive defluorination method based on sodium hydride was developed. The additional fluorination was performed using highly oxidising F2 gas. The non-oxidatively fluorinated oxyfluorides Sr2TiO3F2, Sr3Ti2O5F4 and La2NiO3F2 were modified accordingly. A focus was set on the defluorination of these phases, since this is related to the development of intercalation-based anode materials, a field, which has been conceptionally unexplored prior to this work. However, the structural stability of the oxyfluorides within the electrode composites was found only for Sr3Ti2O5F2 and La2NiO3F2, of which primarily La2NiO3F2 showed redox activity. This Ni-based phase could be successfully electrochemically defluorinated as well as additionally fluorinated, showing its potential to serve as both, active anode and cathode material. The resulting composition-induced alterations of the crystal structure and magnetic properties of the chemically and electrochemically obtained phases were analysed by a variety of characterisation techniques, including different diffraction and spectroscopy methods, DFT-based calculations and magnetic measurements. The chemically and electrochemically formed phases showed to be structurally related. Therefore, the structural and magnetic characteristics of the chemically prepared phases, which were analysed in-depth, could be transferred to the electrochemically synthesised phases. Magnetic properties, related to the presence or absence of unpaired electrons and the strength of exchange interactions, were found to be highly dependent on the structural modifications and transition metal cation oxidations states. Even though a generally detrimental effect of irreversible side reactions, resulting in the progressive decrease of the electronic conductivity of the carbon additive, was found to exist, the reversibility of the structural changes over extended cycling was observed. This was found to offer the possibility to switch reversibly between different magnetic states of the charged and discharged phases. A detailed investigation of magnetoelectric switching due to reversible fluoride intercalation was performed on La1.3Sr1.7Mn2O7. A switching between a strongly and weakly ferromagnetic state could be achieved, resulting in high relative changes of the magnetisation with one of the highest reported magnetoelectric voltage couplings reported for tuneable magnetic systems.

Item Type: Ph.D. Thesis
Erschienen: 2020
Creators: Wissel, Kerstin
Title: Topochemical fluorination and defluorination in the context of fluoride-ion batteries and tuning of magnetic properties
Language: English
Abstract:

Within this work, it was demonstrated that topochemical modifications of the anion sublattices of Ruddlesden-Popper-type oxides An+1BnO3n+1 and derived metastable oxyfluorides An+1BnO3n+1-xF2x with 0 < x ≤ 2 have a significant influence on the crystal and electronic structures of the newly synthesised phases. This could be used to effectively tailor and reversibly tune magnetic properties. Different non-oxidative, reductive or oxidative modification routes, leading to fluoride intercalation, exchange and/or deintercalation processes, were investigated. Such topochemical reactions have been also found to take place upon the electrochemical fluorination of Ruddlesden-Popper-type oxides in fluoride-ion batteries and have led to the development of intercalation-based cathode materials. For the development of novel intercalation-based electrodes, oxyfluorides, obtained via a prior non-oxidative topochemical fluorination of the respective oxides, were examined concerning their potential use as active anode or cathode materials. During charging, the use of the oxyfluoride as active anode material results in defluorination, whereas additional fluorination occurs when the oxyfluoride contains additional vacancies and is used as active cathode material. For both cases, the oxyfluoride represents the discharged state of the electrode material. These additional topochemical modifications of the parent oxyfluorides could be also achieved via chemical preparation approaches. For the chemical preparation of the anode material in the charged state, a reductive defluorination method based on sodium hydride was developed. The additional fluorination was performed using highly oxidising F2 gas. The non-oxidatively fluorinated oxyfluorides Sr2TiO3F2, Sr3Ti2O5F4 and La2NiO3F2 were modified accordingly. A focus was set on the defluorination of these phases, since this is related to the development of intercalation-based anode materials, a field, which has been conceptionally unexplored prior to this work. However, the structural stability of the oxyfluorides within the electrode composites was found only for Sr3Ti2O5F2 and La2NiO3F2, of which primarily La2NiO3F2 showed redox activity. This Ni-based phase could be successfully electrochemically defluorinated as well as additionally fluorinated, showing its potential to serve as both, active anode and cathode material. The resulting composition-induced alterations of the crystal structure and magnetic properties of the chemically and electrochemically obtained phases were analysed by a variety of characterisation techniques, including different diffraction and spectroscopy methods, DFT-based calculations and magnetic measurements. The chemically and electrochemically formed phases showed to be structurally related. Therefore, the structural and magnetic characteristics of the chemically prepared phases, which were analysed in-depth, could be transferred to the electrochemically synthesised phases. Magnetic properties, related to the presence or absence of unpaired electrons and the strength of exchange interactions, were found to be highly dependent on the structural modifications and transition metal cation oxidations states. Even though a generally detrimental effect of irreversible side reactions, resulting in the progressive decrease of the electronic conductivity of the carbon additive, was found to exist, the reversibility of the structural changes over extended cycling was observed. This was found to offer the possibility to switch reversibly between different magnetic states of the charged and discharged phases. A detailed investigation of magnetoelectric switching due to reversible fluoride intercalation was performed on La1.3Sr1.7Mn2O7. A switching between a strongly and weakly ferromagnetic state could be achieved, resulting in high relative changes of the magnetisation with one of the highest reported magnetoelectric voltage couplings reported for tuneable magnetic systems.

Place of Publication: Darmstadt
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Fachgebiet Materialdesign durch Synthese
Date Deposited: 22 Sep 2020 14:01
DOI: 10.25534/tuprints-00013383
Official URL: https://tuprints.ulb.tu-darmstadt.de/13383
URN: urn:nbn:de:tuda-tuprints-133834
Referees: Clemens, Prof. Dr. Oliver and Slater, Prof. Dr. Peter R.
Refereed / Verteidigung / mdl. Prüfung: 20 August 2020
Alternative Abstract:
Alternative abstract Language
In der vorliegenden Arbeit wurde aufgezeigt, dass topochemische Modifikationen des Anionenteilgitters von Ruddlesden-Popper-artigen Oxiden An+1BnO3n+1 und davon abgeleiteten metastabilen Oxyfluoriden An+1BnO3n+1-xF2x mit 0 < x ≤ 2 einen erheblichen Einfluss auf die Kristall- sowie elektronische Struktur der neu hergestellten Phasen hat. Dies kann genutzt werden, um magnetische Eigenschaften effektiv anzupassen und reversible einzustellen. Verschiedene nicht-oxidative, reduktive oder oxidative Modifikationsrouten, die zu Fluorideinlagerung, -austausch und/oder –auslagerung führen, wurden untersucht. Es wurde bereits in früheren Studien aufgezeigt, dass derartige topochemische Reaktionen auch durch die elektrochemische Fluorierung von Ruddlesden-Popper-artigen Oxiden in Fluoridionenbatterien hervorgerufen werden kann. Dies hat zur Entwicklung von interkalationsbasierten Kathodenmaterialien geführt. Für die Entwicklung neuartiger interkalationsbasierter Elektroden wurden Oxyfluoride, die über eine vorherige nicht-oxidative topochemische Fluorierung der entsprechenden Oxide gewonnen wurden, in Hinblick auf einen potentiellen Einsatz als aktives Anoden- oder Kathodenmaterial untersucht. Während des Ladens führt die Nutzung des Oxyfluorids als aktives Anodenmaterial zu einer Defluorierung, während eine zusätzliche Fluorierung beobachtet wird, wenn das Oxyfluorid weitere Leerstellen besitzt und als Kathodenmaterial eingesetzt wird. In beiden Fällen stellt das Oxyfluorid den entladenen Zustand des Elektrodenmaterials dar. Diese zusätzlichen topochemischen Modifikationen der Ausgangsoxyfluoride konnten zudem über chemische Synthesewege erzielt werden. Für die chemische Präparation des Anodenmaterials im geladenen Zustand wurde eine reduktive Defluorierungsmethode basierend auf Natriumhydrid entwickelt. Die zusätzliche Fluorierung wurde durchgeführt mithilfe von stark oxidierendem F2-Gas. Die nicht-oxidativ fluorierten Oxyfluoride Sr2TiO3F2, Sr3Ti2O5F4 und La2NiO3F2 wurden entsprechend modifiziert. Ein Fokus wurde gesetzt auf die Defluorierung dieser Phasen, da dies mit der Entwicklung von interkalationsbasierten Anodenmaterialien verbunden ist, was zu Beginn dieser Arbeit ein konzeptionell unerforschtes Feld darstellte. Strukturelle Stabilität der Oxyfluoride in den Elektrodenkompositen konnte jedoch nur für Sr3Ti2O5F2 und La2NiO3F2 bestätigt werden. Von diesen Phasen zeigte vor allem La2NiO3F2 Redox-Aktivität. Diese Ni-basierte Phase konnte erfolgreich elektrochemisch defluoriert und zusätzlich fluoriert werden. Dies zeigt das Potential von La2NiO3F2 auf, sowohl als aktives Anoden-, als auch Kathodenmaterial eingesetzt werden zu können. Die resultierenden zusammensetzungsinduzierten Änderungen der Kristallstruktur und der magnetischen Eigenschaften der chemisch und elektrochemisch erzeugten Phasen wurde mittels einer Vielzahl an Charakterisierungsmethoden analysiert, die verschiedene Diffraktions- und Spektroskopiemethoden, DFT-basierte Berechnungen und magnetische Messungen umfassten. Die chemisch und elektrochemisch hergestellten Phasen zeigten eine strukturelle Ähnlichkeit. Daher konnten strukturelle und magnetische Charakteristika der chemisch hergestellten Phasen, die experimentell eingehender untersucht werden konnten, auf die elektrochemisch synthetisierten Phasen übertragen werden. Magnetische Eigenschaften, die auf die Präsenz oder das Fehlen von ungepaarten Elektronen und die Stärke der Austauschwechselwirkungen zurückgeführt werden konnten, zeigten eine starke Abhängigkeit von strukturellen Modifikationen und von den Oxidationszuständen der Übergangsmetallkationen. Obwohl ein prinzipiell nachteiliger Effekt von irreversiblen Nebenreaktionen, die zu einer zunehmenden Abnahme der elektronischen Leitfähigkeit des Kohlenstoffadditivs führten, nachgewiesen werden konnte, konnte die Reversibilität der strukturellen Änderungen während andauerndem Zyklierens bestätigt werden. Dies eröffnete Möglichkeiten, zwischen verschiedenen magnetischen Zuständen der geladenen und entladenen Phasen reversible zu schalten. Eine detaillierte Untersuchung dieses magnetoelektrischen Schaltens aufgrund einer reversiblen Einlagerung von Fluoridionen wurde an La1.3Sr1.7Mn2O7 durchgeführt. Schalten zwischen einem stark und schwach ferromagnetischen Zustand konnte erzielt werden, was zu hohen relativen Änderungen der Magnetisierung mit einem der höchsten berichteten magnetoelektrischen Spannungskopplungen für schaltbare magnetische Systeme einherging.German
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