Aintzane Goñi

High voltage cathode materials for Na-ion batteries of general formula Na3V2O2x(PO4)2F3−2x

Different samples of the sodium–vanadium fluorophosphate cathodic materials have been synthesized via the hydrothermal method, varying the type and content of carbon used in the synthesis. Structural characterization of the composites was performed by powder X-ray diffraction. Magnetic susceptibility measurements and EPR (Electron Paramagnetic Resonance) polycrystalline spectra indicate that some of the samples exhibit V3+/V4+ mixed valence, with the general formula Na3V2O2x(PO4)2F3−2x where 0 ≤ x < 1. The morphology of the materials was analyzed by Transmission Electron Microscopy (TEM). A correlation between the type and content of carbon with the electrochemical behavior of the different samples was established. Electrochemical measurements conducted using Swagelok-type cells showed two voltage plateaux at 3.6 and 4.1 V vs. Na/Na+. The best performing sample, which comprised a moderate percentage of electrochemical grade carbon as additive, exhibited specific capacity values of about 100 mA h g−1 at 1C (≈80% of theoretical specific capacity). Cyclability tests at 1C proved good reversibility of the material that maintained 98% of initial specific capacity for 30 cycles.

Synthesis, crystal structure and spectroscopic properties of the NH4NiPO4·nH2O (n= 1,6) compounds; magnetic behaviour of the monohydrated phase

NH4NiPO4·6H2O and NH4NiPO4·H2O have been obtained by adding different concentrations of H3PO4 to dilute solutions of NiCl2·6H2O with special attention to the control of the pH in the solvent medium, which was regulated by addition of NH4OH. The NH4NiPO4·6H2O compound crystallizes in the orthorhombic Pmn21space group with cell parameters a= 6.9032(8), b= 6.0907(5) and c= 11.1402(8)A, V= 468.39(7)A3, Z= 2, R= 23 and Rw= 2.3%. The structure is three-dimensional and consists of Ni[O(w)]6 octahedra [O(w)= oxygen from a water molecule] linked to PO4 and NH4 tetrahedra by hydrogen bonds. All polyhedra are quite regular in this compound. The NH4NiPO4·H2O phase crystallizes in the Pmn2l space group with cell parameters a= 5.5698(2), b= 8.7668(2) and c= 4.7460(2)A. The structure of this compound has been refined with the Rietveld method using the coordinates of the KMnPO4·H2O phase as a starting model. The final residual factors were Rwp= 7.37, RB= 2.68%. The structure is formed from sheets of distorted NiO6 corner-sharing octahedra bridged through the oxygen atoms of the phosphate tetrahedra. These layers are pillared along the b direction and are interconnected by hydrogen bonds with the NH4+ cations, which are inserted between the sheets. The spectroscopic properties of both compounds are in good agreement with the symmetry observed in each phase. The values of the nephelauxetic ratio, β, are 0.89 and 0.94 for the hexahydrated and monohydrated compounds respectively. Magnetic susceptibility and specific heat results obtained for NH4NiPO4·H2O show an essentially two-dimensional antiferromagnetic exchange coupling, which becomes of a more three-dimensional behaviour with decreasing temperature.

Influence of Carbon Content on LiFePO4 / C Samples Synthesized by Freeze-Drying Process

The influence of the carbon content in LiFeP0 4 /C composites synthesized by the freeze-drying method was studied by varying the citric acid (chelating agent): Fe ratio. Diminishing this ratio from 1:1 to 0.33:1 led to a gradual reduction of the carbon content from 16.1 to 7.2% wt and different morphologies. Transmission electron microscopy micrographs of the composite with the greatest carbon percentage (16%) show mainly 30 nm LiFeP0 4 particles homogeneously embedded in a carbon network. Samples containing less carbon exhibit only one type of morphology, 200-700 nm aggregates made up of an intimate mixture of LiFeP0 4 particles and carbon. Galvanostatic cycling from 2 to 4 V vs Li/Li + evidences the typical LiFePO 4 redox behavior at 3.4 V, and a second contribution at 2.65 V probably related to the carbon content. At a high rate, a good specific capacity value is observed for the nanoparticulate sample (16% wt C), whereas poorer performance is observed for low carbon content samples (11 and 7.2 wt % C). Heterogeneous and insufficient carbon covering together with phosphate particle aggregation in these latter samples can account for this behavior. Two carbon distribution models are proposed to explain different electrochemical responses. In all cases, a good capacity retention is observed after prolonged cycling.

Synthesis, crystal structure, and magnetic properties of NH4CuPO4·H2O

In this paper is described the synthesis and characterization of a new layered phosphate, NH4CuPO4 ·H2O, and its magnetic properties. Its crystal structure has been solved at room temperature. It crystallizes in the P21 /a monoclinic space group, with a=7.3907(8), b=7.5191(6), c=8.651(1) A, β=94.54(1)° and Z=4. The compound presents a layered structure, with copper phosphate sheets linked by NH4+ cations. These layers are parallel to the (001) plane and are interconnected by hydrogen bonds with the NH4+ cations. The layers are formed by centrosymmetric dimers of CuO5 edge-sharing distorted square pyramids, crosslinked by corner-sharing phosphate tetrahedra. Each copper(ii) cation is bonded to three phosphate oxygens and one water molecule forming the base of the square pyramid, and a symmetry related phosphate oxygen in the axial position. The crystal structure of NH4CuPO4 ·H2O is related to the layered dittmarite (NH4MgPO4 ·H2O) type structure. However, in the dittmarite family there are MO6 octahedra separated by the phosphate groups. The title compound is the first one related to the dittmarite family which exhibits layers formed by centrosymmetric Cu2O8 dimers crosslinked by phosphate tetrahedra. The IR data of this layered phosphate are in good agreement with the symmetry observed in the phase. EPR data and magnetic studies show the existence of antiferromagnetic interactions and the presence of predominant short range interactions. In spite of the layered structure exhibited by the compound, the magnetic study indicates that the magnetic behaviour is consistent with a dimeric structure with a J/k value of 4.93 K. A 2D ordering, as exhibited by other related compounds, may be reached at temperatures lower than 1.8 K.

Clustering of Fe3+ in the Li1−3xFexMgPO4 (0<x<0.1) solid solution

Abstract The Li 1−3 x Fe x MgPO 4 (0 x 4 . Fe 3+ substitutes part of the Li + ions in the channels of the LiMgPO 4 structure along the [010] direction, creating cation vacancies. The IR bands corresponding to the vibrational modes of the phosphate groups undergo a gradual widening with the amount of inserted iron as a consequence of the increase of disorder in the structure. The EPR spectra show signals with an effective g ′=4.0. This fact can be attributed to the presence of high spin Fe 3+ ions in orthorhombic symmetry. The increase of Fe 3+ in the compounds leads to a broadening of the Lorentzian EPR signals indicating the existence of magnetic interactions between the Fe 3+ ions. Magnetic susceptibility measurements on the Li 1−3 x Fe x MgPO 4 (0 x 3+ ions exhibit a short range magnetic order, forming clusters associated with the vacancies in the structure.

Unexpected substitution in the Li1 − 3xFexNiPO4 (0 < x < 0.15) solid solution. Weak ferromagnetic behaviour

Li1 − 3xFexNiPO4 (0 < x < 0.15) solid solution phases have been synthesized by solid state reaction. X-Ray studies on polycrystalline samples show that all phases are isostructural with the parent LiNiPO4 compound, crystallizing in the orthorhombic system, space group Pnma. The evolution of the lattice parameters with the iron content follows Vegards law. The IR spectra show split bands for the phosphate groups, in good agreement with the distortion observed in the crystal structure. The EPR spectra for all phases show a broad band centred at zero field. The absence of a signal corresponding to Fe3+ ions suggests the presence of FeIII–NiII interactions strongly affected by the zero field splitting of the nickel(II) ions. The magnetic behaviour of the Li1 − 3xFexNiPO4 (0 < x < 0.15) phases can be described as antiferromagnetic with the presence of weak ferromagnetism below the ordering temperature. The magnitude of the remanent ferromagnetic moment is indicative of the existence of iron clustering in the samples. The FeO6 octahedra form finite chains in which the magnetic moments of the Fe3+ ions are antiferromagnetically aligned with a small canting angle.

A NEW PERSPECTIVE ON VANADYL TARTRATE DIMERS. PART II. STRUCTURE AND SPECTROSCOPIC PROPERTIES OF CALCIUM VANADYL TARTRATE TETRAHYDRATE

Abstract The calcium vanadyl tartrate complex [Ca(VO)(d,l-C4H2O6)(H2O)4] has been synthesized and characterized by spectroscopic methods. Its crystal structure was solved by X-ray methods. The compound is monoclinic, space group P21/c, with a = 8.0282(5), b = 17.1568(8), c = 7.6113(3)A, β = 94.269(4)° and Z = 4. The structure consists of centrosymmetric vanadyl tartrate dimers, [(VO)(d,l-C4H2O6)]2 4-, and calcium cations placed between them. As a result, dimers form chains in the [101] direction. Neighbouring chains are linked by the coordination of the calcium ion to the oxygen atom of a vanadyl group of a different chain, thus forming a two-dimensional structure. Different layers are linked by hydrogen bonds. Spectroscopic studies show the existence of intra-dimeric interactions between vanadium atoms.

Effect of Carbon Coating on the Physicochemical and Electrochemical Properties of Fe2O3 Nanoparticles for Anode Application in High Performance Lithium Ion Batteries

Nanoparticulate Fe2O3 and Fe2O3/C composites with different carbon proportions have been prepared for anode application in lithium ion batteries (LIBs). Morphological studies revealed that particles of Fe2O3 in the composites were well-dispersed in the matrix of amorphous carbon. The properties of the γ-Fe2O3 nanoparticles and the correlation with the particle size and connectivity were studied by electron paramagnetic resonance, magnetic, and Mössbauer measurements. The electrochemical study revealed that composites with carbon have promising electrochemical performances. These samples yielded specific discharge capacities of 1200 mAh/g after operating for 100 cycles at 1C. These excellent results could be explained by the homogeneity of particle size and structure as well as the uniform distribution of γ-Fe2O3 nanoparticles in the in situ generated amorphous carbon matrix.

High-voltage cathode materials for lithium-ion batteries: freeze-dried LiMn0.8Fe0.1M0.1PO4/C (M = Fe, Co, Ni, Cu) nanocomposites.

Four LiMn0.8Fe0.1M0.1PO4/C (M = Fe, Co, Ni, Cu) cathode materials have been synthesized via a freeze-drying method. The samples have been characterized by powder X-ray diffraction, transmission electron microscopy, magnetic susceptibility, and electrochemical measurements. The composition and effective insertion of the transition-metal substituents in LiMnPO4 have been corroborated by elemental analysis, the evolution of the crystallographic parameters, and the magnetic properties. The morphological characterization of the composites has demonstrated that the phosphate nanoparticles are enclosed in a matrix of amorphous carbon. Among them, LiMn0.8Fe0.1Ni0.1PO4/C is the most promising cathode material, providing a good electrochemical performance in all aspects: high voltage and specific capacity values, excellent cyclability, and good rate capability. This result has been attributed to several factors, such as the suitable morphology of the sample, the good connection afforded by the in situ generated carbon, and the amelioration of the structural stress provided by the presence of Ni(2+) and Fe(2+) in the olivine structure.

Structural, magnetic and electrochemical study of a new active phase obtained by oxidation of a LiFePO4/C composite

In this work, a new phase produced by controlled oxidation of a LiFePO4/C composite has been isolated and characterized. This new compound preserves mainly an olivine structure, but the complete oxidation of Fe is implied. A significant iron mis-site disorder and vacancy formation is proposed. The new phase demonstrated possession of different spectroscopic, magnetic and electrochemical properties from triphilite–heterosite. AC and DC magnetic susceptibility and specific heat measurements showed that the new phase presented spin-glass behavior. Electrochemical cycling showed that the new phase reacted at 2.5 V, which is a different potential to the heterositeversus a Li anode. Moreover, it provoked reversion to the triphilite–heterosite system, although a fraction of the material remained as the oxidized phase.