Topochemical fluorination reactions in the context of fluoride ion batteries, tailored properties and for the modification of thin films

• Applicant: Professor Dr. Oliver Clemens
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Synthesis and Properties of Functional Materials
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 285249272

The project aims to investigate topochemical fluorination reactions of oxides within the perovskite type and similar structures (AxByOz, A = alkali or alkali earth, B = transition metal). This reaction type is of special interest in the fields of fluoride ion batteries, tailored material properties as well as thin film technologies. According to this, the project is structured in three subprojects. New electrochemical fluorination reactions are under concern in part (a). We aim to find and characterize new compounds which are capable for this type of reaction (e. g. SrFeO2--> SrFeO2F or LaSrMnO4 --> LaSrMnO4F2-d). To prove fluoride incorporation we aim to use Mössbauer as well as X-ray absorption spectroscopy in addition to XRD experiments. In contrast to chemical fluorination methods (XeF2, F2), electrochemical fluorination reactions can be expected to have the potential to adjust fluorine contents and average transition metal oxidation states (which play a crucial role on properties such as magnetism, conductivity, superconductivity) precisely. A special interest is located in the identification of compounds which are capable for reversible fluorine intercalation and deintercalation, which would facilitate building completely intercalation based fluoride ion batteries. In relation to part (a), chemical defluorination methods will be investigated in part (b) using reductive chemical agents (e. g. metal organic compounds, alkali metals) with oxyfluoride compounds. We target to invest the fluorination behavior of further materials, among them Bi1-xAxFeO3-y and La1 xSrxMnO3, where a strong change of material properties (magnetism, conductivity, ferroelectric properties) is to be expected and investigated. If fluorination of the latter systems will show to be reversible, the project will be additionally expanded towards electrochemically adjustable material properties. Investigation of ferroelectric properties requires the preparation of dense thin film oxyfluoride films. This is explained due to the fact that oxyfluorides are thermodynamically unstable and therefore cannot be sintered. We aim to investigate the fluorination and subsequent characterisation of thin perovskite films prepared by pulsed laser deposition in subproject (c). We will study the interplay between substrate strain and chemical strain due to fluorination on structure and properties of epitaxial grown films. In addition, thin porous fluorinated films will be investigated regarding their properties which are relevant for solid oxide fuel cell technologies (stability, conductivity, polarization resistances). Finally, we strive for theoretical validation of experimental results by means of DFT based calculations for all three subprojects.

• DFG Programme: Independent Junior Research Groups