The electrochemical reaction of bronze-type FeF3 center dot 0.33H(2)O, synthesized via a simple room-temperature solution route, with lithium was investigated by operando Mossbauer spectroscopy and X-ray diffraction. The two techniques revealed a complex electrochemical mechanism where the pristine crystalline compound is gradually transformed into an amorphous material containing nanodomains of a FeF2-like rutile structure mixed to iron nanoparticles. Upon charge, two steps are dominating the electrochemical behavior: partial reformation of the initial bronze structure and oxidation to Fe(III). This reaction mechanism, however, is not constant, and noticeable variation can be observed during galvanostatic cycling (up to 55 cycles) until eventually an amorphous material containing rutile nanodomains composes the final active electrode. The material performance, under the form of a fluoride/graphene oxide composite, is also assessed with respect to the long-term effect of the depth of first discharge.

Vertically aligned carbon nanotubes grown directly onto a metallic substrate without external catalyst can be used as the active electrode material and the current collector, respectively.

A simple microwave-assisted fluorolytic sol-gel synthesis of 30 nm sized FeF3 nanocrystals supported on partially reduced graphene oxide is reported. The as-fabricated nanostructures used as positive electrodes in Li-ion batteries show capacity retention as high as 150 mA h g(-1) even after having sustained repeated charge-discharge cycles.

Among the many industrial and research level applications of fluorides like microelectronics, polymers, agronomy, imaging, dental composites, uranium enrichment processes or catalysis, there is a recent and growing interest for these ionic compounds in the field of energy storage and conversion. Li-ion technology in particular, has been attracted to fluorides due to the possibility of more-than-one electrode reaction. In fact, these reactions can potentially enhance the energy stored in commercial cells (conversion reactions). By reaction with lithium, transition metal fluorides such as MnF3, FeF3, CoF3 or CuF2, can be reduced to the respective zero valent metal/LiF mixture with resulting high theoretical capacities ranging from 3 to 4.2 times the amount currently delivered by commercial LiFePO4 (170 mAh g−1). Since, in practice, the most of the research efforts are being put into iron trifluoride due to chemical (relatively mild synthesis conditions), electrochemical (lower polarization than MnF3 for example) and environmental (Fe is more environmentally friendly than Co) reasons, in this review we will focus on the electrochemistry, synthesis strategies and amelioration techniques for the high oxidation state ferric fluoride and its derivatives.

The benzyl alcohol route is used to decorate reduced graphene oxide (RGO) nanosheets with iron oxide (FeOx) nano-particles. The resulting FeOx/RGO composites combine both electrical double-layer capacitive and pseudocapacitive behaviors of RGO and FeOx, respectively, owing to the hybrid nano-structure. In 1m Na2SO4 aqueous electrolyte, the capacitance of the FeOx/RGO electrode reaches 126 Fg(-1) in a negative potential range from -0.8 to 0.0 V (vs. Ag/AgCl) and 97 Fg(-1) in the extended potential window from -0.8 to + 0.7 V (vs. Ag/AgCl). By using the benzyl alcohol route, a higher concentration of iron precursor leads to full surface coverage of the RGO nanosheets by smaller FeOx nanoparticles, forming a close-packed layer that limits the restacking of RGO nonosheets in the dry state. The resulting FeOx/RGO composite gains the same power capability as RGO and a higher energy density than raw FeOx, owing to this well-designed morphology.

Crystalline SnO2/reduced graphene oxide (RGO) nanocomposites were synthesized by a one-pot microwave-assisted non-aqueous sol–gel method, in which partially reduction of the graphene oxide and nanoparticle formation occurs simultaneously. Composite samples with different SnO2 loadings on the RGO were prepared and characterized by TEM, XRD, TGA-DSC and FT-IR. Chemoresistive devices, consisting of a thick layer of the samples synthesized on alumina substrates provided with Pt interdigitated electrodes, were fabricated and their electrical and NO2 sensing characteristics investigated. The results obtained have shown the possibility of a fine tuning of the sensing characteristics of the devices fabricated by simply controlling the amount of metal oxide nanoparticles loaded onto the reduced graphene oxide sheets. This was explained on the basis of the critical role played by the n-SnO2/p-RGO heterojunction formed on the composite materials.

Non-aqueous sol-gel routes involving the reaction of metal oxide precursors in organic solvents (e.g., benzyl alcohol) at moderate temperature and pressure, offer advantages such as high purity, high reproducibility and the ability to control the crystal growth without the need of using additional ligands. In this paper, a study carried out on a series of iron oxide/reduced graphene oxide composites is presented to elucidate a structure-properties relationship leading to an improved electrochemical performance of such composites. Moreover, it is demonstrated that the easy production of the composites in a variety of temperature and composition ranges, allows a fine control over the final particles size, density and distribution. The materials obtained are remarkable in terms of the particle's size homogeneity and dispersion onto the reduced graphene oxide surface. Moreover, the synthesis method used to obtain the graphene oxide clearly affects the performances of the final composites through the control of the restacking of the reduced graphene oxide sheets. It is shown that a homogeneous and less defective reduced graphene oxide enables good electrochemical performances even at high current densities (over 500 mAh/g delivered at current densities as high as 1600 mA/g). The electrochemical properties of improved samples reach the best compromise between specific capacity, rate capability and cycle stability reported so far.

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.

There's something in the air … A nanocomposite consisting of well-dispersed SnO(2) and Pt nanoparticles on reduced graphene oxide (see the high-resolution TEM image) exhibited very high responses to hydrogen at concentrations between 0.5 and 3% in air, with response times of 3-7 s and recovery times of 2-6 s. The sensor was prepared by a straightforward microwave-assisted non-aqueous sol-gel approach.

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.

In this paper, we report an investigation of three tin-based composite materials as negative electrodes for lithium-ion batteries. Theses composites were synthesized by solid state reaction from dispersion of micrometric tin into BPO4, Li-doped BPO4 (LiBPO) and Na-substituted BPO4 (NaBPO) matrix, respectively. We have investigated more particularly the influence of the two alkaline ions (Li+, Na+) introduced into the matrix on electrochemical performances. The morphology of powders was observed by SEM and the composition studied by EDX analysis. The conductivity measurements showed that the modified BPO4 matrixes (Li or Na) exhibit improved conductivity (σRT=2×10−11Scm−1 for NaBPO). A focus of our interest was to relate the nature and structural composition of the composite interface between active tin and inactive matrix to the irreversible capacity in this type of composite materials. The electrochemical analysis shows a decrease of the irreversible capacity for the composite based on modified matrixes (around 150 and 190mAhg−1 for SnNaBPO and SnLiBPO, respectively) with respect to the reference composite SnBPO (245mAhg−1).

A comparative study of two Sn-based composite materials as negative electrode for Li-ion accumulators is presented. The former SnB0.6P0.4O2.9 obtained by in-situ dispersion of SnO in an oxide matrix is shown to be an amorphous tin composite oxide (ATCO). The latter Sn-0.72[BPO4](0.28) obtained by ex-situ dispersion of Sri in a borophosphate matrix consists of Sri particles embedded in a crystalline BPO4 matrix. The electrochemical responses of ATCO and Sn-0.72.[BPO4](0.28) composite in galvanostatic mode show reversible capacities of about 450 and 530 mAhg(-1), respectively, with different irreversible capacities (60% and 29%). Analysis of these composite materials by Sn-119 Mossbauer spectroscopy in transmission (TMS) and emission (CEMS) modes confirms that ATCO is an amorphous Sn-II composite oxide and shows that in the case of Sn-0.72[BPO4](0.28), the Surface of the tin clusters is mainly formed by Sn-II in an amorphous interface whereas the bulk of the clusters is mainly formed by Sn-0. The determination of the recoilless free fractions f (Lamb-Mossbauer factors) leads to the effective fraction of both Sn-0 and Sn-II species in such composites. The influence of chemical composition and especially of the surface-to-bulk tin species ratio oil the electrochemical behaviour has been analysed for several Sn-x[BPO4](1-x) composite materials (0.17 < x < 0.91). The cell using the compound Sn-0.72[BPO4](0.28) as active material exhibits interesting electrochemical performances (reversible capacity of 500 mAh g(-1) at C/5 rate). (C) 2009 Elsevier Inc. All rights reserved.

Three different synthesis techniques (hydrothermal synthesis, modified Pechini synthesis and Pechini synthesis) were successfully used for preparation of Li2FeSiO4 samples. The obtained samples possess some differences in the morphology and in the particle size, as well as in the presence of in situ formed carbon. The best electrochemical performance has been obtained with the smallest particles embedded into carbon matrix. Such a Li2FeSiO4/C composite contains the highest amounts of impurities (Fe2O3, SiO2 and Li2SiO3) and only 68.8at.% of iron is in the form of FeII as detected by Mössbauer spectroscopy, respectively. Despite the highest amount of impurities, the sample shows the highest reversible capacity (approximately 100mAhg−1 based on whole silicate-derived material). With the proper structuring of Li2FeSiO4/C composites, utilisation of large part of capacity is also possible at current densities corresponding to C/5 and C/2 cycling rate. A lower amount of impurities was found in the samples that do not contain any in situ carbon after synthesis. Among them, the highest purity is possessed by the sample prepared at 900°C, as determined using Mössbauer spectroscopy. The results obtained by Mössbauer spectroscopy and XRD analysis indicate on the differences in the crystal structure between the thermally treated samples and the sample prepared by hydrothermal synthesis.