M. Anne

Structure and conductivity of Cu and Ni-substituted Bi4V2O11 compounds

The partially Cu- or Ni-substituted compounds (Bi4V2(1−x)M2xO11−3x;M=Cu, Ni) are highly oxygen-conducting. Three phases (α, β, γ) are observed in the unsubstituted compound; α is the low-conducting room temperature phase and γ the high-conducting phase at high temperature. Structure and conductivity are studied as a function of the substitution on the vanadium sites. Between 0 and 6% at room temperature, the Cu compound remains in the orthorhombic α phase and its ionic conductivity increases. A strong anisotropic conductivity is observed. For 0.07≤x≤0.12, the average structure is tetragonal (γ-type) at room temperature. The conductivity is very high and does not vary very much over this substitution range. Impedance spectroscopy measurements have also been carried out on the x=0.07 Ni-substituted compound. Commensurate or incommensurate superstructures are observed for all of these compounds.

Cation ordering in Li2Mn3MO8 spinels: structural and vibration spectroscopy studies

Abstract Lithium-manganese oxide spinels with 1/4 manganese replaced by Mg, Ti, Co, Ni, Cu, Zn and Ga, yielding formula LiMn1.5M0.5O4 (or Li2Mn3MO8) have been prepared. Cationic ordering was known previously for M=Mg and Zn, resulting in a superstructure with primitive cubic symmetry. Given the poor chemical contrast of X-ray diffraction between Mn and Ti, Co, Ni, Cu or Ga, neutron diffraction studies were carried out. Evidence of cation ordering is found for M=Ni and Cu, but not for Ti, Co or Ga. These results are confirmed by FTIR and Raman spectroscopies. Doubly-substituted samples (Li0.5M0.5)[Mn1.5M0.5]O4 (overall formula LiMn3M2O8) were also prepared for M=Mg and Zn. These do not form the primitive superstructure, a result ascribed to the lower manganese valence with respect to LiMn1.5M0.5O4. Zn-containing spinels give rise to an extensive Li/Zn cation inversion, which also shows up as additional high-frequency bands in IR and Raman spectroscopies. This investigation shows that the cell volume is determined by the average octahedral-site cation radius, and that the main driving force for octahedral cation ordering is the charge difference between Mn and M atoms.

Lithium intercalation in LiMgMnO and LiAlMnO spinels

LiAlMnO4 and LiMg0.5Mn1.5O4 have been investigated as replacements of LiMn2O4 for lithium intercalation below 3 V. The decomposition of acetate and carbonate precursors was studied by thermogravimetry. Solid state reactions yielded ‘LiAlMnO4’ with manganese valence < 4 and always containing impurity phases such as LiAl5O8 or LiMn2O4. The intercalation behaviour was studied potentiostatically in lithium cells using both liquid and solid electrolytes, and by X-ray diffraction on intercalated cathodes. The structural and electrochemical behaviour of ‘LiAlMnO4’ is very similar to that of LiMn2O4, and gives lower capacities on cycling. Lithium intercalation in LiMg0.5Mn1.5O4 includes three reduction steps between 2.8 and 1.6 V, corresponding to the intercalation of ca. 0.58, 0.22 and 0.25 Li atoms, respectively. The total initial capacity is 180 mAh/g, but also drops on cycling, mainly due to the collapse of the second step corresponding to the critical threshold around Mn+3.5, where the tetragonal distortion due to the Jahn—Teller effect takes place.

Crystal structure determination of α, β and γ-Bi4V2O11 polymorphs. Part I: γ and β-Bi4V2O11

Abstract Using combined X-ray single crystal and neutron powder thermodiffraction data, the crystal structure of the high temperature γ -form of Bi 4 V 2 O 11 was confirmed and accurately refined in the I4/mmm space group and that of the β -form was entirely determined in the centrosymmetric Amam space group. The two-fold superlattice characterising the β structure is the result of an ordering process involving corner-sharing V–O tetrahedra and disordered trigonal bipyramids. A possible scheme for the γ ↔ β phase transition is proposed.

Bi4V2O11 polymorph crystal structures related to their electrical properties

Abstract Combining X-ray single crystal and neutron powder diffraction data, the crystal structures of the three polymorphs of Bi4V2O11 were refined. These polymorphs can be described from a common orthorhombic mean cell with approximate parameters: am≈5.5, bm≈5.6 and cm≈15.3 A. They are all built upon well-defined Bi2O22+ layers spaced by perovskite-like VO3.5 slabs. A large disorder of the oxygen located in these slabs was observed for the γ phase in which the diffusion is very fast. The decrease in temperature leads to an ordering of the oxygen vacancies in the β and α-polymorphs. The oxygen packing in the β-polymorph remains close to that of a distorted perovskite with a vacancy located on one site. This perovskite lattice is not maintained in the α-polymorph. The β to α transition is reconstructive with the formation of typical trigonal bipyramids leading to a 6am superstructure. Because of the high parameter to data ratio, the structure of this α-polymorph was modelled in the smaller 3am superstructure. This superstructure can be explained by an ordering of some oxygen vacancies along [010]. However, this ordering could be more complex and some disorder may remain in this area, which would explain the low activation energy observed for this α-polymorph. The specific structural characteristics of the three polymorphs of Bi4V2O11 are correlated with their corresponding conductivity and activation energy.

Crystal structure determination of α-, β- and γ-Bi4V2O11 polymorphs. Part II: crystal structure of α-Bi4V2O11

The crystal structure of α-Bi 4 V 2 O 11 was solved in the A2 space group and refined using combined X-ray single crystal and neutron powder diffraction data in the following unit cell a = 16.5949(3) A (3 x a m = 5.5316 A), b = 5.6106(1) A, c = 15.2707(3) A, γ = 90.260(2)°. It is built upon [Bi 2 O 2 ] 2+ layers spaced with vanadium-oxygen slabs where the vanadium atoms exhibit three different oxygen environments. The main characteristic of these V-O slabs is a well defined dimeric unit with two trigonal bipyramids sharing one edge and connected to two VO tetrahedra. These rigid blocks extend along [100] and are spaced with a disordered area where different V-O trigonal bipyramids are interconnected. A possible scheme to explain the actual 6a m superlattice is proposed. It also accounts for the diffusion lines systematically observed along [100] by SAED. A relationship between the crystal structures of the three Bi 4 V 2 O 11 polymorphs and their corresponding conductivity is tentatively suggested.

Thermal behaviour of Bi4V2O11 : X-ray diffraction and impedance spectroscopy studies

Abstract Bi 4 V 2 O 11 powdered samples and single crystals were studied by high temperature X-ray diffraction and impedance spectroscopy to characterize the phase transitions. From high temperature X-ray diffraction on powders and single crystals, the α ⇆ β and β ⇆ γ reversible phase transitions were observed. The β ⇆ γ one is ferroelastic ⇆ paraelastic but surprisingly the α ⇆ β transition also exhibits a ferroelastic character, with a 90 ° switching of the a and b axis on cooling and/or, more scarcely, on heating. Impedance spectroscopy measurements were carried out using platelet shaped single crystals with well developed (001) faces. The corresponding σ ∥ (001 plane) and σ ⊥ ( c direction) bulk conductivities were obtained and compared with values from ceramic pellets, σ ∥ values are close to those characterizing the pellets, and the anisotropy of the conductivity is evidenced by σ ∥ values about 2 orders of magnitude larger than σ ⊥ ones. Slope changes observed in Arrhenius plots are in agreement with the phase transitions.

BICOVOX family of oxide anion conductors: chemical, electrical and structural studies

The BICOVOX family of compounds (cobalt-doped Bi4V2O11) has been characterised combining chemical, structural and electrical approaches. The compositional extent of the single-phase BICOVOX solid solution, stable at ambient temperature, was determined. Single-crystal and powder X-ray diffraction data, combined with density measurements, enabled us to formulate the solid solution as Bi2(V1–x–yCoxBiy)Oz. The maximum x value (0.25) is reached when y= 0.04, and the maximum y value (0.06), when x= 0.10. The evolution of the Arrhenius conductivity plots vs. x and y has been studied within the tetragonal domain. The maximum σ value is roughly independent of composition for 0.10 ⩽x⩽ 0.15 and 0 ⩽y⩽ 0.04.

Electrochemical lithium intercalation in disordered manganese oxides

Abstract Four highly disordered manganese oxides were prepared by reduction of sodium permanganate by chloride, iodide, hydrogen peroxide or oxalate in aqueous medium containing a large excess of Li + ions, yielding hydrated oxides with Mn valence in the range 3.80–3.92. Thermogravimetric studies showed that the iodide-reduced oxide can be dehydrated to 92% at 240°C, while the other three ones retain water at temperatures up to ca. 400°C, where crystallization is significant. The electrochemical behaviour was studied potentiostatically and galvanostatically in lithium cells on samples dried at 240°C. All samples give a unique, broad, reversible oxidation–reduction peak in the range of 2.0–3.6 V. Cycling capacities vary in decreasing order iodide>hydrogen peroxide>chloride>oxalate. The best compound, Li 0.60 Na 0.16 MnO 2.33 I ≈0.05 , has an initial capacity of >170 Ah/kg, slowly decreasing on cycling to reach 140 Ah/kg after 70 cycles. These performances, which are far superior to those of manganese spinels, are compared to those of recently reported amorphous manganese oxi-iodides prepared in anhydrous conditions.

Reversibility of lithium intercalation in lithium and sodium phyllomanganates

The cycling of sodium and lithium phyllomanganates in liquid lithium batteries was investigated both by galvanostatical and potentiostatical methods. Slow-scanning voltammograms show the occurrence of a single-phase reaction extending from 3.1 to 2.6 V on discharge for both compounds. On cycling, the voltammogram of Li phyllomanganate smears out, while the current peak narrows in the case of Na. In both cases, the initial capacity of ∼240 Ah/kg drops continuously on cycling between 2 and 4 V. X-ray diffraction shows an important disordering with cycling, with the possible emergence of a cubic-packed, spinel-like structure.