Carine Rongeat

Impact of the nature of carbon on the electrochemical performance of four C–FeF₂ nanocomposites derived from CFx and Fe(CO)₅.

Two heat-resistant ferritic cast steels, 1.4476 and 1.4777 with a similar high Cr concentration, have been analyzed regarding their microstructures, mechanical properties, corrosion resistance and aging behavior. The differences observed between the grades are related to their different C contents. A higher C concentration ensures a higher mechanical resistance, in particular at high temperature (850°C) and also a better high-temperature (850°C) corrosion resistance. This is related to the formation of numerous carbides. However, this study also reveals that the microstructure of these steels is greatly affected by aging. In particular, the precipitation of the sigma phase and the formation of nitride precipitates could lead to failure during service at high temperature.

Trustworthy preparation and contacting of micron‐sized batteries is an essential task to enable reliable in situ TEM studies during electrochemical biasing. Some of the challenges and solutions for the preparation of all‐solid‐state batteries for in situ TEM electrochemical studies are discussed using an optimized focused ion beam (FIB) approach. In particular redeposition, resistivity, porosity of the electrodes/electrolyte and leakage current are addressed. Overcoming these challenges, an all‐solid‐state fluoride ion battery has been prepared as a model system for in situ TEM electrochemical biasing studies and first results on a Bi/La_0.9Ba_0.1F_2.9 half‐cell are presented. Microsc. Res. Tech. 79:615–624, 2016.

Fluoride ion batteries prepared in the discharged state using an anode composite of a mixture of Mg and MgF₂ show promising cycling performances.

Here we report on the first electrochemical fluorination exemplarily performed on perovskite type BaFeO_2.5. A cell setup of the type BaFeO_2.5 II La_0.9Ba_0.1F_2.9 II MF_x (with MF_x being MgF₂ and CeF₃) was used to perform the reaction, charging the cell up to voltages of about 4 V. Formation of a compound of approximate composition BaFeO_2.5F_∼0.5 was observed, in agreement with diffraction studies of the independently performed chemically fluorinated compound using F₂ gas, and also possessing a capacity which is close to the theoretical capacity of the material. This new method gives an alternative towards the use of highly reactive and toxic F₂ gas, and provides potential in adjusting the chemical potential for oxidative chemical fluorinations.

Batteries based on a fluoride shuttle (fluoride ion battery, FIB) can theoretically provide high energy densities and can thus be considered as an interesting alternative to Li-ion batteries. Large improvements are still needed regarding their actual performance, in particular for the ionic conductivity of the solid electrolyte. At the current state of the art, two types of fluoride families can be considered for electrolyte applications: alkaline-earth fluorides having a fluorite-type structure and rare-earth fluorides having a tysonite-type structure. As regard to the latter, high ionic conductivities have been reported for doped LaF₃ single crystals. However, polycrystalline materials would be easier to implement in a FIB due to practical reasons in the cell manufacturing. Hence, we have analyzed in detail the ionic conductivity of La_1–yBa_yF_3–y (0 ≤ y ≤ 0.15) solid solutions prepared by ball milling. The combination of DC and AC conductivity analyses provides a better understanding of the conduction mechanism in tysonite-type fluorides with a blocking effect of the grain boundaries. Heat treatment of the electrolyte material was performed and leads to an improvement of the ionic conductivity. This confirms the detrimental effect of grain boundaries and opens new route for the development of solid electrolytes for FIB with high ionic conductivities.

The hydrogen sorption properties of Li₃N under reactive milling conditions have been investigated in- and ex-situ as a function of polytype structure (α vs. β), focusing on the influence of the micro-structure and/or the crystal structure upon hydrogen uptake. LiNH₂ and LiH were synthesized by reactive milling of Li₃N at 20 bar hydrogen pressure for 4 h. Reactive milling represents a quick and effective technique to produce LiNH₂ by hydrogenation of Li₃N at low hydrogen pressure and without any need for heating. As to our knowledge, we present a full hydrogenation of Li₃N under the aforementioned conditions for the first time. The (de)hydrogenation and rehydrogenation behaviour of milled amides was evaluated using a combination of powder X-ray diffraction, differential scanning calorimetry, thermogravimetry and in situRaman spectroscopy. In situRaman spectroscopy showed a shift in the lithium amide stretching modes upon hydrogenation supporting a non-stoichiometric storage mechanism consistent with the literature. The microstructure and polytype composition of the Li₃N dehydrogenated materials had no effect on the hydrogenation products and only minor effects on the hydrogen uptake profile during milling.