Search

Reactivity of pure molecular fluorine F2 allows the creation of new materials with unique electrochemical properties. We demonstrate that titanium oxyfluoride TiOF2 can be obtained under molecular fluorine from anatase titanium oxide TiO2, while the fluorination of rutile TiO2 leads only to pure fluoride form TiF4. Contrary to most fluorides, TiOF2 is an air-stable oxyfluoride that is a potential electrode material for Li-ion secondary batteries systems. It shows capacities as high as 220 mAh g-1 and good cyclability at high current rates in a potential window of 4-1.2V. In such window, only Li+ insertion occurs, as proven by in operando XRD/electrochemistry experiments.

Metal fluorides are low cost inorganic compounds that have been studied for their industrial applications such as smelting in metallurgy, uranium enrichment processor optical material production. This applied development of the synthesis of inorganic fluorides allowed intense fundamental researches in the 70’s and 80’s as regards their electronic, and more particularly magnetic, properties. In the last decades, research subjects have been shifted toward the nanostructuration of inorganic fluorides, due to potential applications such as in secondary lithium battery. Indeed, nanostructured metal fluoride materials are potential candidate materials as cathodes for lithium-ion batteries through both insertion and conversion reactions and they could improve the theoretical specific capacity of the positive electrode. Among the MF3 fluorides (M = metal), iron trifluoride, FeF3, is one of the most promising candidate. However, its application as a cathode material for lithium ion battery has been largely hindered by both its low conductivity and its high sensibility to air humidity leading to cyclability rather limited. This work will present some new strategies to develop nanostructured iron fluoride of high chemical stability. They have been prepared through the reactivity of some unusual metal precursors with molecular fluorine gas. In order to favor the nanostructuration of the metal fluoride, the kinetic of fluorination has been followed by in-situ Infra-Red measurements. All the metal fluorides obtained have been then characterized by Scanning Electronic Microscopy, XRD, IR, MOSSBAUER, and Raman spectroscopies. Finally, the electrochemical performances will be presented and correlated with both the fluorination conditions and the physicochemical characteristics of the samples prepared.

Lithium-ion batteries (LIB) are undoubtedly the most popular and mature electrochemical storage devices and, for future use, a higher degree of safety, a longer cycle life, and a higher specific energy are desirable. In order to achieve this goal, numerous publications have been devoted to new active materials and new electrolytes [1] but far less to interfaces. As a matter of fact the solid-electrolyte interphase (SEI) formed through the reductive decomposition of solvent molecules plays a crucial role in the performances of LIB. The robustness and efficiency of the SEI [2] therefore significantly affect the power capability, stability, safety and cycle life of a LIB. In this study, we investigate the impact of several lithium salts, already present in the SEI like LiF, Li2O, LiOH, Li2CO3, LiOCH3 and LiOC2H5, on the cycling ability of a graphite or LiNiMnCoO2 electrodes [3] in Li half cells containing the standard (EC/PC/3DMC + 1 M LiPF6) electrolyte. These lithium salts are soluble (LiF), slightly soluble (LiOCH3 and LiOC2H5) or almost insoluble (Li2O, LiOH, Li2CO3) but present in solution as a colloidal suspension at saturation. Results show that these lithium salts, dissolved or in suspension in the electrolyte have a non-negligible impact on the electrode capacity during cycling, depending on the type of electrode and nature of the salt. Mineral salts like LiOH, Li2O and LiF (at high concentration) degrade battery performances by forming a resistive film at the electrode surface. On the contrary, LiOCH3 and Li2CO3 are beneficial to cycling performances owing to the formation of a Li conductive coating. In order to avoid the negative impact of dissolved LiF, glymes like diglyme (G2), triglyme (G3) and tetraglyme (G4) have been added as these compounds are known to complex strongly Li+ ions. A complementary approach has been the use of fluorinated surfactant additives able to modify the interfacial material/electrolyte free energy. Electrochemical characterization by galvanostatic cycling and cyclic voltammetry, combined with XPS and SEM surface characterization have been performed to evaluate the impact of each additive and to clarify its mechanism of action. Results show that the addition of glymes limits the negative effect of SEI lithium salts, while the surfactant improves the wettability of the electrodes leading to better cycling performances. [1] J.B. Goodenough, Y. Kim, Chemistry of Materials, 22 (2010) 587-603. [2] P. Verma, P. Maire, P. Novák, Electrochimica Acta, 55 (2010) 6332-6341. [3] Fabien Chrétien, Jennifer Jones, Christine Damas, Daniel Lemordant, Patrick Willmann, M. Anouti, Journal of Power Sources, 248 (2014) 969-977.

not Available.

not Available.

not Available.

not Available.

not Available.

not Available.