The influence of cobalt substitution on the local structural changes around Co atoms in the layered lithium nitridocobaltates Li(3-2x)Co(x)N for 0.05 ≤ x ≤ 0.44 is investigated using Co K-edge X-ray absorption spectroscopy (EXAFS and XANES). The Co-N bond length in Li(3-2x)Co(x)N compounds is obtained vs x by performing EXAFS fitting and found to be shorter (1.80 Å) than for x = 0 (Li3N), and its value does not change with x. A comparison of EXAFS data with XRD results is discussed. We show that the continuous decrease of interlayer distance versus Co content (x), described from XRD data, accounts for an average of the Co-N and Li-N distances, weighted by the number of these bond lengths. In addition, the present work supports the proposal that the Li1b-N bonds contract with x due to a significant increase of Coulombic attractive forces locally induced by the progressive Li(+)/Co(2+) substitution. XRD studies suggested that divalent Co ions bond to two nitrogen in Li(3-2x)Co(x)N. Although additional works are still needed to prove its valence, the present XAFS findings complements the local structure found by XRD, in good accord with the electrochemical properties previously reported.

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, Li 2 O, LiOH, Li 2 CO 3 , LiOCH 3 and LiOC 2 H 5 , on the cycling ability of a graphite or LiNiMnCoO 2 electrodes [3] in Li half cells containing the standard (EC/PC/3DMC + 1 M LiPF 6 ) electrolyte. These lithium salts are soluble (LiF), slightly soluble (LiOCH 3 and LiOC 2 H 5 ) or almost insoluble (Li 2 O, LiOH, Li 2 CO 3 ) 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, Li 2 O and LiF (at high concentration) degrade battery performances by forming a resistive film at the electrode surface. On the contrary, LiOCH 3 and Li 2 CO 3 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.

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.

The Solid Electrolyte Interphase (SEI), formed during the first cycles of life in lithium-ion batteries, contains a variety of lithium salts, with direct effect on the aging performance of the battery. In this work, we investigate the impact of addition of SEI lithium salts (LiF, Li2CO3, LiOH, Li2O, LiOCH3 and LiOC2H5) in the electrolyte on the cycling ability of graphite and LiNi1/3Mn1/3Co1/3O2 (NMC) electrodes. Results show that NMC is more sensitive to salt addition than graphite material. Furthermore, results demonstrate that both LiOH and Li2O have a negative effect on the SEI formation. Conversely, Li2CO3, LiOCH3 and LiOC2H5 are beneficial and promote the formation of a polymeric coating on the SEI. Finally, the impact of the presence of LiF on the SEI depends mainly on its concentration. The effect of the presence of additives capable of complexing lithium salts such as the glyme series, CH3O[CH2CH2O]nCH3 (Gn, with n = 2, 3 or 4), is investigated by cyclic voltammetry, galvanostatic charge–discharge tests and electrochemical impedance spectroscopy (EIS). Results show that the glymes chain length is a determining factor in their complexation mechanism, which depends on both the nature and the concentration of the lithium salt.

Li-ion batteries are undoubtedly the most popular and mature electrochemical storage devices. Numerous studies deal with the development of new materials and electrolytes for such systems [1-3]. However, there are several factors that limit the development of this technology, and one of the key questions is the limitation the life time. Efficient cycling of lithium-ion batteries is associated with the formation of a passivation layer at the electrodes surface, referred to as the SEI (Solid Electrolyte Interphase) [4]. The quality of this film is decisive for battery performance: it must prevent solvent intercalation in the electrode, limit electrolyte degradation upon cycling, but still allow an optimal transport of Li ions between the electrode and the electrolyte.

The chemical reactivity in air of the promising metallic nitride Li7MnN4 as negative electrode for Li-ion batteries has been studied. The evolution of the ageing process with exposure time has been examined by XRD and the degradation kinetics has been determined. This cubic metallic nitride is found to be much less sensitive to the air atmosphere than that exhibited by the lamellar nitridocobaltates Li3−2xCoxN (0.1⩽x⩽0.44). A fast oxidation step in air leads to the delithiated Li6.2MnN4 phase after a few hours and is thereafter progressively destroyed by reaction with moisture leading to the formation of LiOH⋅H2O and Li2CO3. Electrochemical experiments on various aged samples support these results. The demonstration of Li7MnN4 storage in dry air is showed to constitute a relevant solution to preserve the integrity of the nitride host lattice and then its excellent electrochemical properties.

Iron trifluoride has been known for a long time and here we provide new insights into its detailed synthesis. Anhydrous iron trifluorides with different structures have been synthesized via one-shot and stepwise gas-solid fluorination under a gaseous flow of molecular fluorine from 150 degrees C. The degree of hydration of the precursors has a direct influence on the crystalline phase obtained: one-shot fluorination of FeCl2.4H(2)O leads to the hexagonal tungsten bronze-type FeF3 while fluorination of dehydrated iron chlorides gives the rhombohedral FeF3 with traces of oxy(hydroxy) fluorides. A stepwise procedure at 150, 250 and 350 degrees C tends to lead to pure phases and avoid the crystallization of oxy(hydroxy) fluorides in the final powder.

The reaction mechanisms of Li with Sn/BPO4 composites to be used as negative electrode materials for Li-ion batteries were studied during electrochemical cycling by operando Mössbauer spectroscopy and X-ray diffraction using a specifically conceived in situ electrochemical cell. The starting composites consist of three main components: β-Sn particles as the electrochemically active species, an inactive matrix of BPO4 and an amorphous SnII-borophosphate interfacial phase linking the two former components and improving the cohesion of the composite. During the first discharge, the latter Sn(II) species are first reduced to zerovalent tin forming Li-poor Li–Sn alloys. After its complete reduction, the reaction of Li continues with β-Sn leading to Li–Sn alloys increasingly rich in Li, with a final composition between those of Li7Sn2 and Li13Sn5. X-ray diffraction shows a progressive loss of long range order of the composites with the suppression of the diffraction peaks of the initial β-Sn and the formation of an ill-defined mixture of Li–Sn alloys. The evolution of this mechanism is investigated on going from a reference Sn/BPO4 composite prepared by conventional ceramic methods with common micrometric BPO4 to a new improved material prepared by carbothermal synthesis starting from nanometric BPO4. With the new composite prepared by carbothermal synthesis, a significant improvement of the reversible capacity at the first cycle is obtained together with a slight improvement of the cycling behaviour. An additional improvement can be obtained by increasing the rate of the first discharge, and thus hampering the formation of the thermodynamically stable LiSn intermetallic.

Novel composites consisting of tin particles associated to graphite were prepared by chemical reduction of tin(+2) chloride byt-BuONa-activated sodium hydride in the presence of graphite. The samples obtained using various C/Sn ratios were investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and elemental analyses. The largest tin particles associated to graphite layers were observed for the material with a C/Sn ratio of 16. For the materials with C/Sn ratios of 42 and 24, SEM and TEM experiments demonstrated that Sn aggregates of ca. 250 nm length and composed of Sn particles with an average diameter of ca. 50 nm were homogeneously distributed at the surface of graphite. Electrodes prepared from theC/Sn=42material exhibit a high reversible capacity of over 470 mAhg−1up to twenty cycles with stable cyclic performances.

The SEI (Solid Electrolyte Interphase) at the surface of electrodes in lithium-ion batteries is composed of various lithium compounds, organic or mineral, which have a direct impact on cycling performance. The main lithium species constituting the SEI and selected in this study are lithium fluoride LiF, lithium carbonate Li2CO3, lithium hydroxide LiOH, lithium oxide Li2O, lithium methoxide LiOCH3 and lithium ethoxide LiOC2H5. Their solubilities were determined in ethylene, propylene, dimethyl, diethyl and vinylene carbonates (EC, PC, DMC, DEC and VC) and in one of their mixtures commonly used in lithium-ion batteries (EC/PC/3DMC) by mean of atomic absorption spectroscopy (AAS). These solutions were also investigated by EIS (Electrochemical Impedance Spectroscopy) and conductimetry measurements. Results show that while solubilization properties differ between LiF and other lithium compounds considered, their association pattern in solution is identical and solutions are mainly constituted of quadrupolar aggregates. (C) 2011 Elsevier B.V. All rights reserved.

Monodisperse and small tin nanoparticles were prepared from a 2,2′-bipyridine–tin(+2) chloride complex using sodium borohydride as reducing agent. When the synthesis was conducted in the presence of graphite, Sn particles with an average diameter of ca. 29nm well-dispersed at the surface of graphite were obtained. Electrochemical lithium insertion was carried out in these materials. A stable reversible capacity of ca. 480mAhg−1, value 37% higher than that of pure graphite, was found.

The Sn[BPO4]0.4 composite material is composed of three main constituents: the electrochemically active Sn0 species, the BPO4 buffer matrix, and an interfacial amorphous SnII borophosphate phase which acts as a link between the other two, improving the cohesion of the whole composite. In this paper, we report an investigation of the effect of the reaction time on structure and size of this interfacial layer formed between the Sn0 and the BPO4 particles. 119Sn Mössbauer spectroscopy shows an increase in the amount of the oxidized SnII species with the increase of the reaction time following a quasi-kinetic profile. Operando Mössbauer spectroscopy allowed the study of the transformation of the interface at the beginning of the first electrochemical cycle, indicating that the big irreversible loss during the first discharge is closely related to the increase of the amount of SnII in the amorphous interface.

The structural and textural properties of a Sn-0.4BPO(4) composite material synthesized by ex situ dispersion of beta-Sn in a BPO4 matrix were investigated by using several complementary techniques to study the global order (XRD, TGA-DSC, SEM-XEDS) and the local order (FT-IR, Sn-119 Mossbauer spectroscopy and X-ray absorption spectroscopy). The results reveal that the composite material consists of three main components: an electrochemically active species "Sn", an inactive matrix "BPO4", and an amorphous Sn(II) borophosphate which acts as a link between the two former and which improves the cohesion of the composite. The electrochemical performances of the composite material were tested in Swagelok-type cells with metallic Li as counter-electrode. It shows a high reversible capacity of about 500 mAh g(-1) at a C/20 rate, and a very good stability under cycling even at very fast rates of C or C/13. Crown Copyright (C) 2009 Published by Elsevier Masson SAS. All rights reserved.