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

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.