A. Verbaere

LiMBO3 (M=Mn, Fe, Co):: synthesis, crystal structure and lithium deinsertion/insertion properties

Abstract The LiMBO3 (M=Mn, Fe, Co) compounds were synthesized by a solid state reaction. LiFeBO3 and h-LiMnBO3 crystal structures were determined from single crystal data. LiFeBO3 exhibits the same structure as that of other already described LiMBO3 compounds (M=Mg, Mn, Co, Zn). The structure of h-LiMnBO3 is isotypic with the hexagonal form of LiCdBO3. The electrochemical study shows that a very small amount of lithium (less than 0.04 Li per formula unit) can be deinserted reversibly from the three compounds. According to the thermodynamic study performed in the case of LiFeBO3, the Fe3+/Fe2+ redox couple lies between 3.1 and 2.9 V/Li, demonstrating an important inductive effect of the BO3 group.

Electrochemically synthesized vanadium oxides as lithium insertion hosts

The electrochemical oxidation of vanadyl cations in aqueous solution leads to a solid deposit on the working electrode, called electrolytic vanadium oxide (e-V2O5). The electrodeposition reaction occurs in two steps including an oxidation into soluble species followed by a precipitation. Electrodeposited compounds are mixed valence, hydrated vanadic acids. Their chemical formula can be written H0.4V2O5.2−δ·nH2O with 0.04<δ<0.2 and 0<n<1.8. These two latter parameters depend on the current density applied during electrodeposition, the duration and the temperature of a subsequent mild thermal treatment in air. e-V2O5 materials are porous, poorly crystallized layered compounds. At 260°C, they become completely anhydrous and undergo a phase transformation into α-V2O5. The electrochemical intercalation of lithium into these compounds shows two main single phase phenomena near 2.6 and 3.1 V/Li. This reduction induces a lengthening of the average vanadium oxygen bond, and a decrease of the lithium diffusion coefficient. e-V2O5 compounds reversibly intercalate 1.4≅Li per formula unit at an average voltage of 2.8≅V/Li, at a rate of C/50 in the 4–2 V range, and this capacity is maintained during several tens of discharge/charge cycles. The electrochemical behavior is slightly dependent on the VIV content and the crystallization state of the compounds.

The Origin of Capacity Fading upon Lithium Cycling in Li1.1V3O8

Two Li 1 . 1 V 3 O 8 samples prepared by two different routes were investigated: SG350, obtained by firing a sol-gel precursor at 350°C, and SS580, obtained by solid-state reaction at 580°C. The electrochemical lithium insertion behaviors were studied and revealed important differences, especially on capacity fading upon lithium cycling. This article is focused on the lithium insertion mechanism in Li 1 . 1 V 3 O 8 and on its consequences on cyclability of the materials studied. This study shows that the capacity fading due to the two-phased phenomenon at 2.6 V could be related to an increase of the drastic change in cell lattice constants leading to local damage of the crystal structure. It seems also to depend on the morphology of the compound studied. The dissolution of small quantity of V I I I in the electrolyte may occur during the last electrochemical phenomenon at 2.35 V. It is also evidenced that this dissolution depends on the morphology of the compound studied. Discussions are provided on possible mechanisms for the dissolution process and on the relationships between the dissolution reaction and the capacity fading upon cycling.

Influence of the morphology on the Li insertion properties of Li1.1V3O8

Two Li1.1V3O8 samples have been prepared by heating a sol–gel precursor at 350 and 650 °C, i.e. below and above the melting point, respectively. Their electrochemical lithium insertion behavior was investigated after different grinding treatments. Important differences were observed, both in initial capacity and in cyclability. Attempts were made to correlate these differences to the material characteristics (composition, morphology and structure). The importance of grain morphology (size, size distribution and shape) and texture (agglomeration of smaller particles or not) has been evidenced. The size and agglomeration of the grains play a major role on the initial capacity, while their crystal shape (well formed crystals or no crystal shape) seems to be the main factor influencing the cyclability. This latter morphology feature was also shown to affect differently the partial capacity fading occurring on each electrochemical Li insertion process.

Localisation du doublet solitaire dans les composés oxygènés cristallisés du thallium I

Resume Nos etudes sur les composes oxygenes du thallium I nous ont conduit a proposer un modele permettant le calcul, sur une base electrostatique, de la position du doublet solitaire (appele paire non liee par divers auteurs). Le principe de ce calcul consiste a evaluer le champ au niveau du thallium I et, connaissant sa polarisabilite, a determiner la position du doublet solitaire dont le deplacement traduit la polarisation du thallium I. La methode exposee est applicable a tout ion comportant un doublet solitaire et dont la polarisabilite est connue (ex. Pb II ). Les approximations liees a la nature du modele propose et les limites de validite sont discutees.

Lead magnoniobate crystal structure determination

The single crystal structure of lead magnoniobate PbMg13Nb23O3 was determined at 20°C using data collected with an X-ray automatic four circle diffractometer. The average structure is cubic with space group Pm3m and exhibits important atomic shifts from the special positions of the cubic perovskite structure. The disordered local positions as well as the thermal parameters have been refined to final R index and weighted RW index of 0.0368 and 0.0378, respectively. Results are discussed in comparison with previous ones obtained by other techniques.

Nanofibrous α ­ , β ­ , γ­ and α⋅γ -Manganese Dioxides Prepared by the Hydrothermal-Electrochemical Technique I. Synthesis and Characterization

The hydrothermal-electrochemical method has been used for the synthesis of manganese dioxides from acidic MnSO 4 or A 2 SO 4 /MnSO 4 (A = Li, Na, K, NH 4 ) solutions. This method produced manganese dioxides of either the β-, γ- or α- structure types, mixtures of these structure types, α/γ or γ/β, or interconnected α- and γ-phases (α.γ). The structure of the material obtained can be controlled by adjusting the parameters of the synthesis: temperature, acidity of the solution, composition of the solution, or applied current density. The γ-MnO 2 materials synthesized are characterized by exceptionally low amounts of microtwinning defects. These materials can be synthesized over a wide range of P, values, including in the range usually reserved for chemically prepared (chemical manganese dioxide, CMD) materials. Materials exhibiting lower P, values are favored when the pH or the temperature of the synthesis is decreased. The lowest P r values were found in materials synthesized at 92°C and pH 0. A study of the morphology of the deposits has shown that it can be controlled by changing the experimental conditions.

γ-MnO2 for Li batteries: Part I. γ-MnO2: Relationships between synthesis conditions, material characteristics and performances in lithium batteries

Abstract With the use of various electrodeposition conditions (concentration and pH of the bulk MnSO 4 solution, temperature, synthetic mode) and thermal treatments, γ-MnO 2 materials exhibiting rather different physico-chemical and structural characteristics were prepared. Relationships were established between synthesis conditions and such characteristics, showing that to some extent, these physico-chemical and structural parameters can be tuned. The electrochemical behavior of lithiated phases obtained upon the first Li insertion into these γ-MnO 2 materials has been investigated. It is shown that the maximum Li reversible intercalation capacity strongly depends on the structural parameters (concentration of De Wolff defects and degree of microtwinning) of the starting γ-MnO 2 samples.

β-Zr2(PO4)2SO4: A zirconium phosphato-sulfate with a Sc2(WO4)3 structure. A comparison between garnet, nasicon, and Sc2(WO4)3 structure types

Abstract The single-phase compound β-Zr2(PO4)2SO4 has been prepared by refluxing zirconium phosphate gel in sulfuric acid at 200°C. It crystallizes in the orthorhombic system, space group Pbcn with a = 12.3742(9), b = 8.867(2), c = 8.951(1), A, Z = 4. The structure was determined from 560 reflections collected on a Nonius CAD4 automatic diffractometer with Mo K α radiation. The final R index and weighted Rw index are 0.049 and 0.062, respectively. The structure, built up from ZrO6 octahedra and PO4 and SO4 tetrahedra linked by corners, is isotypic with that of Sc2(WO4)3 and is closely related to that of garnet and nasicon. In these three structure types the arrangements of octahedral cations are very similar.

The potassium niobyl cyclotetrasilicate K2(NbO)2Si4O12

Abstract The single-phase compound K2(NbO)2Si4O12 was prepared by solid-state reaction. It crystallizes in the tetragonal system, space group P4bm with a = 8.7404(8)A and c = 8.136(1)A, Z = 2. The structure was determined from 484 reflections collected with a Nonius CAD4 diffractometer with MoKα radiation. The final R index and weighted Rw index are 0.021 and 0.022, respectively. This framework structure is built up from chains of corner-shared NbO6 octahedra running parallel to the four-fold axes and linked together by four-membered Si4O12 single rings. This structure is very similar to that of K4(ScOH)2 Si4O12.