Josette Olivier-Fourcade

Changes in oxidation state and magnetic order of iron atoms during the electrochemical reaction of lithium with NiFe2O4

Abstract The mixed transition-metal spinel oxide NiFe2O4 is used for the first time as active electrode materials vs. lithium metal in test cells. Reversible capacities close to 900 mAh/g are found. Due to the poorly crystalline nature of the products of electrochemical reaction, diffraction procedures give little information about the iron forms in the reaction products. The use of 57 Fe Mossbauer spectroscopy allows to monitor the mechanism of the electrochemical reaction with lithium of the spinel. During the first discharge, the amorphization process is accompanied by metal reduction. The reversible redox reaction FeIII↔Fe0 accounts for the cycling capacity. However, magnetic ordering is lost during the first discharge and is not recovered in subsequent cycling or in the iron metal products.

SnO reduction in lithium cells: study by X-ray absorption, 119Sn Mössbauer spectroscopy and X-ray diffraction

Abstract We studied the reduction mechanism of SnO in lithium cells by X-ray absorption near OK and SnL I edge spectroscopy, 119 Sn Mossbauer spectroscopy and X-ray diffraction. The reduction mechanism is complex, involving mixed valence intermediate compounds. In the interval 0≤Li/Sn≤2 the main reaction corresponds to a partial reduction of Sn II , which is partially reversible, regenerating the tin oxide during the charge. For Li/Sn greater than two, LiSn bonds are formed, but SnO interactions are still present. The charge within this interval also partially regenerates tin oxide, but the reversibility does not extend to Li/Sn lower than two. At low voltage and very large depth of discharge, the formation of LiSn alloys and Li 2 O clearly takes place, with negligible SnO interactions.

Reactivity of Antimony Oxides and MSb2O6 ( M = Cu , Ni , Co ) , Trirutile-type Phases with Metallic Lithium

Isostrutural MSb 2 O 6 (M = Cu,Ni,Co) trirutile-type phases were prepared by solid-state synthesis and their electrochemical activity towards lithium was investigated. NiSb 2 O 6 and CoSb 2 O 6 showed similar electrochemical behavior, with an uptake of about 18-19 Li per formula unit along the first reduction and only 6-7 Li reversibly removed upon subsequent cycling. This totally differs from the behavior of CuSb 2 O 6 that was found to react with only about 7 Li during the first reduction, without any capacity recovered on subsequent charge. From X-ray diffraction data, both Cu and Co phases lead to an amorphous composite down to 0 V. From high-resolution transmission electron microscopy observations, reduced CoSb 2 O 6 consists of 10-50 nm Co particles dispersed in a Li 3 Sb + Li 2 O matrix. For CuSb 2 O 6 , our results and observations enlightened a two-step reduction: First, the formation of Cu clusters through an electrochemically-driven exchange reaction leading to Li 2 Sb v 2 O 6 , and then reduction of this matrix into 5 nm Sb domains dispersed in an insulating amorphous Li-rich Li-Sb v -O matrix, preventing any further alloying reaction and any charge reaction. The complete charge irreversibility observed on Li/Sb 2 O 5 half-cells confirmed this point, while the Li reduction of Sb 2 O 3 emphasized a close similarity with the reactivity of the (Ni/Co)Sb 2 O 6 phases, suggesting a first reduction step of Sb 5+ into Sb 3+ , assuring conduction and subsequent cycling capacity. Despite the composite nature of the as-formed electrode and the very fine Sb particles, the reversibility of the alloying reaction was not found to be satisfactory for Li-ion cells.

119Sn Mössbauer study of LixSn alloys prepared electrochemically

Abstract A study by 119 Sn Mossbauer spectroscopy of lithiated tin β-Sn powder is presented for between 0 and 3.4 lithiums inserted per formula and for a thin foil of β-tin with between 0 and 2.8 lithiums inserted. This identifies the different Li–Sn alloys obtained electrochemically. We have identified two different sites. The first site corresponds to a tin with Sn–Sn bonds in a slightly distorted site. The second site is representative of Li–Sn bonds for tin in a more distorted site. In other respects, this study has allowed a better understanding of the complex tin reduction, which occurs during lithium insertion in tin oxide SnO.

Lithium insertion mechanism in Sb-based electrode materials from 121Sb Mössbauer spectrometry

Abstract Lithium insertion mechanism in some antimony-based compounds: SnSb, CoSb3, CrSb2, TiSb2 has been studied by means of 121 Sb Mossbauer spectrometry which gives valuable information about the local electronic structure of the probed element (Sb). The structural and electronic modifications induced by insertion of lithium have been characterized. For these Sb-based materials the lithium insertion mechanisms involve Li3Sb formation and composite multi-phase separations with one component displaced from the pristine compound.

ROLE OF IRON OXIDES IN THE PHOSPHATE ADSORPTION PROPERTIES OF KAOLINITES FROM THE IVORY COAST

The phosphate adsorption properties of three clay samples, with kaolinite as the dominant mineral, from different deposits in the Ivory Coast have been investigated. The clays contain varying amounts of crystalline Fe oxides and kaolinite with structural Fe. All measurements were made in dilute suspension under controlled conditions of temperature, pH, ionic strength and saturating cation. Data have been fitted to Langmuir adsorption isotherms. Both P adsorption and surface area measurements have been made on samples before and after chemical removal of Fe oxides. The samples have large P adsorption capacities, which are not entirely explained by their large specific surface areas. The presence of Fe oxides makes a strong contribution to the surface area and enhances the adsorption capacities. There is little evidence that structural Fe makes a strong contribution to the enhanced P adsorption capacity.

X-ray diffraction, 57Fe Mössbauer and step potential electrochemical spectroscopy study of LiFeyCo1−yO2 compounds

Abstract With the aim of finding new cathode materials for lithium-ion batteries, the synthesis of layered solid solutions with LiFe y Co 1− y O 2 stoichiometries has been studied in this work. X-ray single phase products were obtained by a ceramic procedure for 0≤ y ≤0.2. The unit cell dimensions of the powdered solids increase with the iron content. The Rietveld analysis of a sample with y =0.1 using anisotropic thermal parameters led to R BRAGG =3.37. The hexagonal unit cell parameters of this solid were a =2.8271(1) A and c =14.1266(7) A. The site occupancy used in the Rietveld procedure was: (Fe T 0.0086 ) 6c [Li 0.9868 Fe O 0.0046 ] 3b [Li 0.0132 Co 0.9000 Fe O 0.0868 ] 3a O 2 according to the intensity of the signals observed in the Mossbauer spectrum. This consists of one intense (87%) quadrupole split signal with isomer shift of ca. 0.316(3) mm/s is ascribable to Fe(III) replacing cobalt in the CoO 2 layers. Two weaker quadrupole signals result from small amounts of iron in octahedral 3b and pseudotetrahedral 6c sites of the LiO 2 layers. The presence of pseudotetrahedral iron puts obstacles to the lithium ion diffusivity. In consequence, the electrochemical spectra evidence an increased cell polarization as increases. The lithium extraction at the end of the first charge decreases with iron content, with a maximum of 0.6 Li per formula for y =0.1. The introduction of nickel in the composition of these solids may be useful to improve the electrochemical performance of the solid solutions. Ternary systems show an improved electrochemical behaviour.

Copper in In2S3 : a study by X-ray diffraction, diffuse reflectance and X-ray absorption

Abstract A study of the In2S3Cu2SCuS system has shown the existence of a solid solution range with the In2S3 vacant spinel crystal structure. An X-ray diffraction study has allowed the identification of two redox processes in the formation of solid solutions with CuS, which is a mixed valency copper sulphide, Cu21Cu11(S2)2−S−. At low CuS concentrations all copper(I) is oxidized to copper(II) and the substitution of indium(III) by copper(II) proceeds without sulphur loss. At higher CuS concentrations the redox reaction is restricted to the sulphur atoms and the substitution of indium(III) by copper(I) and copper(II) is accompanied by sulphur losses. The presence of copper in two oxidation states has been confirmed by diffuse reflectance and X-ray absorption at the Cu K edge.

Interpretation of the 119Sn Mössbauer parameters

The variations of the 119Sn Mössbauer isomer shift δ are interpreted for tin compounds from a semi-empirical tight-binding calculation of the electronic density at the nucleus ρ(0). A molecular model is proposed in order to relate the variations of ρ(0) for the Sn(IV) chalcogenides to the changes in the Sn environment. The variations of the experimental values of the quadrupole splitting δ are linearly correlated to the values of the electric field gradients (EFG) calculated by the full-potential linearized-augmented-plane-wave (FP-LAPW) method. The value of the 119Sn nuclear quadrupole moment is found to be |Q| = 10.5 ± 0.2 fm2. Finally, the relation between the EFG and the Sn environment is discussed for SnO.

Sur les chalcogeno-iodures d'antimoine SbXI(X =S, Se, Te): Structures et spectroscopie Mo¨ssbauer de121Sb

The crystal structure of SbXI(X =Se, Te) compounds has been determined by means of three-dimensional intensity data. The crystal structure of SbSeI, orthorhombic, space groupPnma witha = 8.698(2), b = 4.127(1), c = 10.412(2) A, was refined at several temperatures (180 K,R = 0.021; 293 K,R = 0.020; 320 K,R = 0.023) in correlation with the paraelectric structure or SbSI stable above 293 K. The crystal structure of SbTeI, triclinic, space groupP1¯, witha = 7.570(3), b = 7.159(3), c = 4.228(3) A, α = 107.22(5), β = 106.18(4), γ = 77.19(3)° has been determined by symbolic addition method and refined to a finalR value of 0.035. These structures are built up from infinite weakly linked ribbons (SbX2)n of trigonal SbX3 with SbX bonds of 2.605(1), 2.795(1)A(X =Se), and 2.829(1), 2.953(1), 2.955(1)A(X =Te). The nature of SbX and SbI bonds is discussed in terms of the S, Se, Te substitution. Antimony-121 Mo¨ssbauer spectra have been recorded at liquid helium temperature. The data are discussed with regard to the stereochimical activity of the antimony (III) lone pair of electrons. For SbTeI the Mo¨ssbauer parameters are interpreted in terms of direct population of conductance bands by nonbonding electron pairs.