AbstractThermal treatment of tavorite‐type FePO4·H2O leads to the formation of H0.8FePO4.4 (space group C2/c) and H0.4FePO4.2 (space group I41/amd).

Thermal treatment of the Tavorite-type material FePO(4)·H(2)O leads to the formation of two crystallized iron phosphates, very similar in structure. Their structural description is proposed taking into account results obtained from complementary characterization tools (thermal analyses, diffraction, and spectroscopy). These structures are similar to that of the pristine material FePO(4)·H(2)O: iron atoms are distributed between the chains of corner-sharing FeO(6) octahedra observed in FePO(4)·H(2)O and the octahedra from the tunnels previously empty, in good agreement with the formation of a Fe(4/3)PO(4)(OH)-type phase. The formation of an extra disordered phase was also proposed. These samples obtained by thermal-treatment of FePO(4)·H(2)O also intercalate lithium ions through the reduction of Fe(3+) to Fe(2+) at an average voltage of ~2.6 V (vs Li(+)/Li), with a good cyclability and a reversible capacity around 120 mA h g(-1) (>160 mA h g(-1) during the first discharge).

Following our previous work on the tavorite-like LiFePO4 center dot OH and FePO4 center dot H2O phases, we report here the magnetic and NMR characterizations of analogous LiMnPO4 center dot OH, MnPO4 center dot H2O and VPO4 center dot H2O phases together with the DFT calculations of the NMR shifts. The first two compounds exhibit Curie-Weiss type magnetic behavior with Curie constants close to the theoretical ones for HS Mn3+, while the vanadium compound is very close to a pure Curie-type behavior. Li-7, P-31 and H-1 MAS NMR spectra are reported for the three compounds, and show strong Fermi-contact shifts for the first two nuclei, while the sign and magnitude of the H-1 shifts are very different for the three phases.DFT calculations (FLAPW in GCA+U approximation) using the WIEN2k code and the experimental susceptibilities are shown to reproduce closely the experimental data. This situation is compared to the case of the homologous and isostructural Fe compounds, which exhibit much more complex magnetic behaviors. (C) 2011 Elsevier Inc. All rights reserved.

7Li, 31P, and 1H MAS NMR spectra and magnetic properties are reported for LiFePO4·OH and FePO4·H2O. The former shows no Curie–Weiss-type behavior up to room temperature, while the latter tends to such a behavior in a restricted temperature range. Calculation strategies are discussed for the NMR shifts that result from Fermi contact interaction with the high spin Fe3+ ions. Zero Kelvin electron spin densities obtained by averaging over the ion size using VASP (with PAW potentials) range with those obtained at the nucleus from WIEN2k, with the GGA and GGA+U methods. The latter values have been scaled with the temperature of the NMR measurement by using the experimental magnetic susceptibility, yielding calculated NMR shifts. The agreement is quite satisfactory, but very much dependent on the exchange correlation potential used for the calculation. Possible reasons for this are discussed, also considering the difference in magnetic behaviors.

A new iron(III) phosphate FePO4·H2O, isostructural to already reported VPO4·H2O and MnPO4·H2O phases, was obtained from tavorite LiFePO4(OH) through a Li+/H+ exchange. The composition of this new phase was confirmed by different chemical analyses. The ion-exchange reaction was shown to be topotactic; indeed the structures of LiFePO4OH and FePO4·H2O are very similar and both are characterized by chains of FeO6 octahedra, interconnected through PO4 tetrahedra, such as the resulting frameworks enclose different types of tunnels. A neutron diffraction study has allowed the localization of hydrogens in the FePO4·H2O structure, revealing that the two hydrogen atoms are linked to the oxygen atoms shared by two adjacent FeO6 octahedra. The presence of these “H2O-like” groups inserted along the FeO6 chains leads to a considerable distortion of the FeO6 octahedra. The nature of the −OH and −OH2 groups in LiFePO4OH and FePO4·H2O, respectively, was confirmed by vibrational spectroscopies. Lithium intercalation was sh...

A new iron(III) phosphate FePO4 center dot H2O, isostructural to already reported VPO4 center dot H2O and MnPO4 center dot H2O phases, was obtained from tavorite LiFePO4(OH) through a Li+/H+ exchange. The composition of this new phase was confirmed by different chemical analyses. The ion-exchange reaction was shown to be topotactic; indeed the structures of LiFePO4OH and FePO4 center dot H2O are very similar and both are characterized by chains of FeO6 octahedia. Interconnected through PO4 tetrahedra, such as the resulting frameworks enclose different types of tunnels. A neutron diffraction study has allowed the localization of hydrogens in the FePO4 center dot H2O structure, revealing that the two hydrogen atoms are linked to the oxygen atoms shared by two adjacent FeO6 octahedra. The presence of these "H2O-like" groups inserted along the FeO6 chains leads to a considerable distortion of the FeO6 octahedra. The nature of the -OH and -OH2 groups in the LiFePO4OH and FePO4 center dot H2O, respectively, was confirmed by vibrational spectroscopies. Lithium interacalation was shown to occur in FePO4 center dot H2O through the reduction of Fe3+ to Fe2+ at an average voltage of similar to 2.8 V (vs Li+/Li) with a good cyclability.

Ce travail porte sur la synthese et la caracterisation de nouveaux materiaux d’electrodes positives pour batteries au lithium. Nos recherches se sont principalement orientees vers les materiaux de type phosphates de metaux de transition, et notamment vers la famille des tavorites de composition (Li,H)FePO4(OH), qui presente une structure tridimensionnelle comportant plusieurs types de tunnels propices a l’insertion d’ions lithium. La structure du materiau LiFePO4(OH) a ainsi ete parfaitement resolue, de meme que celle du materiau FePO4.H2O, qui est un nouveau phosphate de fer (III) decouvert au cours de ces travaux. Ces deux materiaux, ainsi que ceux obtenus par traitement thermique de la phase FePO4.H2O, ont ete caracterises a l’aide de differentes techniques d’analyse physico-chimiques. Leur comportement electrochimique vis-a-vis de l’intercalation / desintercalation du lithium a ete etudie, ainsi que les mecanismes redox et structuraux associes mis en jeu.

Pure tavorite LiFePO4(OH) was synthesized through a hydrothermal route. A fine structural analysis was done by X-ray and neutron diffraction techniques. The structure consists of a three-dimensional network with iron(III) octahedra (FeO6) sharing corners, forming chains that run along the b direction. These chains are interconnected by PO4 tetrahedra, such as the resulting framework encloses tunnels of two different sizes running along the a and c axis. The lithium and hydrogen atoms were precisely localized in these tunnels. Theoretical (GGA + U) calculations performed for LiFePO4X materials (X = OH, F) confirmed our results and revealed that a unique lithium position is expected in LiFePO4(OH), as experimentally observed. For the first time, lithium intercalation was shown to occur in LiFePO4(OH) through the reduction of Fe3+ to Fe2+ at an average voltage of ~2.3 V (vs. Li(+)/Li) with a good cyclability.

AbstractFePO4·H2O is prepared by Li+/H+ exchange from an aqueous HNO3 suspension of LiFePO4(OH) at about 60 °C.

The hydrophilic Molecule phenylbenzimidazolesulfonic acid (PBSA) has been coencapsulated with cetyltrimethylammonium chloride (CTAC), an amphiphilic surfactant, via all in Situ method in a mesoporous Silica matrix. The effect of coencapsulation on texture and structure of the mesoporous silica host has been Studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen sorption experiments, and Si-29 magic-angle spinning (MAS) NMR spectroscopy. It was clearly observed that the presence of PBSA led to different characteristics compared to the reference MCM-41 type organized mesoporous Silica (OMS), which suggests that the active molecules perturb the silica network formation. The encapsulation of the hydrophilic PBSA molecule was found to be stable in aqueous media, indicating that strong interactions exist between PBSA and its environment (surfactant and silica framework). H-1-Si-29 and H-1-C-13 MAS NMR experiments evidenced their spatial proximity and confirmed the presence of interaction between PBSA and both CTA(+) molecules and silanol of the silica framework. This NMR Study gave an overall Picture of the organic/inorganic interface.