Destabilization of NaBH4 by Transition Metal Fluorides.

In order to improve the suitability of NaBH4 as a clean fuel, its decomposition temperature needs to be decreased to below 535 °C, while its hydrogen release must be as high as possible. In this work, the influence of a collection of first and second period transition metal fluorides on the destabilization of NaBH4 is studied on samples produced by ball milling NaBH4 with 2 mol% of a metal fluoride additive. The effects obtained by increasing additive amount and changing oxidation state are also evaluated for NbF5, CeF3, and CeF4. The as-milled products are characterized by in-house power X-ray diffraction, while the hydrogen release and decomposition are monitored by temperature programmed desorption with residual gas analysis, differential scanning calorimetry, and thermogravimetry. The screening of samples containing 2 mol% of additive shows that distinctive groups of transition metal fluorides affect the ball milling process differently depending on their enthalpy of formation, melting point, or their ability to react at the temperatures achieved during ball milling. This leads to the formation of NaBF4 in the case of TiF4, MnF3, VF4, CdF2, NbF5, AgF, and CeF3 and the presence of the metal in CrF3, CuF2, and AgF. There is no linear correlation between the position of the transition metal in the periodic table and the observed behavior. The thermal behavior of the products after milling is given by the remaining NaBH4, fluoride, and the formation of intermediate metastable compounds. A noticeable decrease of the decomposition temperature is seen for the majority of the products, with the exceptions of the samples containing YF3, AgF, and CeF3. The largest decrease of the decomposition temperature is observed for NbF5. When comparing increasing amounts of the same additive, the largest decrease of the decomposition temperature is observed for 10 mol% of NbF5. Higher amounts of additive result in the loss of the NaBH4 thermal signal and ultimately the loss of the crystalline borohydride. When comparing additives with the same transition metal and different oxidation states, the most efficient additive is found to be the one with a higher oxidation state. Furthermore, among all the samples studied, higher oxidation state metal fluorides are found to be the most destabilizing agents for NaBH4. Overall, the present study shows that there is no single parameter affecting the destabilization of NaBH4 by transition metal fluorides. Instead, parameters such as the transition metal electronegativity and oxidation state or the enthalpy of formation of the fluoride and its melting point are competing to influence the destabilization. In particular, it is found that the combination of a high metal oxidation state and a low fluoride melting point will enhance destabilization. This is observed for MnF3, NbF5, NiF2, and CuF2, which lead to high gas releases from the decomposition of NaBH4 at the lowest decomposition temperatures.

Anchoring Nanostructured Manganese Fluoride on Few-Layer Graphene Nanosheets as Anode for Enhanced Lithium Storage.

Manganese fluoride (MnF2)/few-layer graphene nanosheets (GNS) composites are successfully prepared via a facile solvothermal method. It is found that in situ formed tetragonal MnF2 submicron crystals (50-200 nm) with good crystallinity anchoring homogeneously onto conducting GNS, allows the electrically insulating MnF2 particles to be wired up to the current collector with enhanced electron transport pathway. The MnF2/GNS composites act as anode in LIBs and display prominently improved electrochemical performance in comparison to that of pure MnF2, on account of the close interactions between the underlying graphene nanosheets and MnF2 particles grown atop. Distinctly enhanced capacity as high as 489 mAh g(-1) after 100 cycles can be obtained at 600 mA g(-1), while the self-activation process can be greatly accelerated at 6000 mA g(-1) with a maximum specific capacity of 530 mAh g(-1). With long cycling stability for 4000 cycles at 6000 mA g(-1), the MnF2/GNS composite can be deemed as an attractive candidate anode for high-capacity, long cycle life, and environmentally friendly LIBs.

Perspectives on Li and transition metal fluoride phosphates as cathode materials for a new generation of Li-ion batteries.

To satisfy the needs of rapidly growing applications, Li-ion batteries require further significant improvements of their key properties: specific energy and power, cyclability, safety and costs. The first generation of cathode materials for Li-ion batteries based on mixed oxides with either spinel or rock-salt derivatives has already been widely commercialized, but the potential to improve the performance of these materials further is almost exhausted. Li and transition metal inorganic compounds containing different polyanions are now considered as the most promising cathode materials for the next generation of Li-ion batteries. Further advances in cathode materials are considered to lie in combining different anions [such as (XO4) (n-) and F(-)] in the anion sublattice, which is expected to enhance the specific energy and power of these materials. This review focuses on recent advances related to the new class of cathode materials for Li-ion batteries containing phosphate and fluoride anions. Special attention is given to their crystal structures and the relationships between structure and properties, which are important for their possible practical applications.