R. Brec

Electrochemical Cyclability of Orthorhombic LiMnO2 Characterization of Cycled Materials

The electrochemical removal of lithium from orthorhombic LiMnO 2 (o-LiMnO 2 ) leads to a phase transition with a first plateau at about 3.7 V. This corresponded to the formation of a spinel-like material; a possible transition to a rhombohedral Li 2 MnO 2 phase was ruled out through structural and crystal-site energy considerations. Several electrochemical cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals the fewer the number of cycles needed. The capacity difference between large and small crystallite/crystal compounds was ascribed to kinetic reasons as shown by ex situ x-ray diffraction analyses and quasi-equilibrium electrochemical studies. Capacities as high as 200 Ah/kg were found for 0.3 μm crystal size materials. Contrary to the spinel prepared at high temperature, the electrochemically obtained spinel-like phase cycled very well in the 2.5 to 4.3 V range, suggesting structural differences between the two materials. An extended x-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the manganese second neighbors for two compounds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO 2 .

Electrochemical cyclability of orthorhombic LiMnO{sub 2}: Characterization of cycled materials

The electrochemical removal of lithium from orthorhombic LiMnO 2 (o-LiMnO 2 ) leads to a phase transition with a first plateau at about 3.7 V. This corresponded to the formation of a spinel-like material; a possible transition to a rhombohedral Li 2 MnO 2 phase was ruled out through structural and crystal-site energy considerations. Several electrochemical cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals the fewer the number of cycles needed. The capacity difference between large and small crystallite/crystal compounds was ascribed to kinetic reasons as shown by ex situ x-ray diffraction analyses and quasi-equilibrium electrochemical studies. Capacities as high as 200 Ah/kg were found for 0.3 μm crystal size materials. Contrary to the spinel prepared at high temperature, the electrochemically obtained spinel-like phase cycled very well in the 2.5 to 4.3 V range, suggesting structural differences between the two materials. An extended x-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the manganese second neighbors for two compounds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO 2 .

Importance of short interlayer Te···Te contacts for the structural distortions and physical properties of CdI2-type layered transition-metal ditellurides

Abstract We examined how the short intra- and interlayer Te···Te contacts of layered transition-metal tellurides affect their structures and physical properties by carrying out tight-binding band electronic structure calculations for the CdI 2 -type layered transition-metal dichalcogenides Ti X 2 (X =S, Se, Te) and M Te 2 (M =V, Nb, Ta) on the basis of the extended Huockel method. In the CdI 2 -type tellurides, the top portion of the Te p -block bands overlaps significantly with the bottom portion of the metal d -block bands, thereby causing a substantial electron transfer from the p - to the d -block bands. For this p → d electron transfer, the interlayer Te···Te contacts are found to be essential because the overlap between the Te p z -orbitals (perpendicular to the layer) associated with the interlayer Te···Te contacts is most effective in raising the top portion of the Te p -block bands. As a consequence, layered transition-metal tellurides are likely to possess a three-dimensional metallic character, and a slight change in their interlayer Te···Te contacts significantly affects their electrical and other physical properties.

Comparative study of some rare earth sulfides: doped γ-[A]M2S3 (M = La, Ce and Nd, A = Na, K and Ca) and undoped γ-M2S3 (M = La, Ce and Nd)

Abstract The crystal structure determination of γ-M2S3 compounds (M = La, Ce, and Nd) has been carried out for the first time from single crystals obtained through high-temperature melting under sulfur pressure. The three phase structures do not depart from the cubic Th3P4 structural type, with a statistical filling of the dodecahedral sites by the cations. The γ-Na0.5Ce2.5S4-doped phase structure has also been determined from a powder neutron diffraction study. Na+ was found to be located at the dodecahedral site, in agreement with the composition limit of Na/Ce = 0.20 as determined by cell parameter variation versus composition. A powder X-ray diffraction study of the potassium- and calcium-doped derivatives (γ-K0.46C2.54S4 and γ-Ca0.89Ce2.07S4) confirmed the results obtained for the sodium-doped phase. In no case, at least in the phases studied, does the alkali or alkaline earth metal occupy the inter-dodecahedral tetrahedral sites. The electronic band structures of Ce2S3 and of Ce3−xS4 ( 0 1 3 ) indicate an insulating behavior for the former compound and a metallic behavior for the latter, assuming in this case a rigid band model. In Ce3S4, the electronic conductivity takes place along the CeCe bonds. No SS bonding was found in the two binaries. It seems possible to assign the color of some γ-M2S3 materials to electronic transitions to the conduction band from (i) the valence band (La2S3), (ii) the 4f level (Ce2S3) and (iii) the 4f and valence band (Nd2S3).

Electrochemical behavior of orthorhombic LiMnO2: influence of the grain size and cationic disorder

Abstract Stoichiometric orthorhombic O-LiMnO 2 , prepared at high temperature from a mixture of Mn 2 O 3 and LiOH.H 2 O, is electrochemically active in a lithium deintercalation/intercalation process. The samples with the smallest grains ( O μ m), and corresponding to the phases prepared with some lithium hydroxide deficiency developed the higher weight capacities. These could reach up to 160 mAh/g under a C/15 regime following a forming that spanned over about thirty cycles. The cells capacity remained constant after forming, showing the very good cyclability of the system. It has not been clearly demonstrated whether the Li/Mn substitution in the O-LiMnO 2 samples played an important role in the electrochemical characteristics of the Li/Li x MnO 2 cells, although the group with small grains also showed, on average, a higher cationic disorder. A phase transition took place upon the first oxidation of O-LiMnO 2 . The new phase presented an electrochemical behavior resembling that of spinel LiMn 2 O 4 . A modeling of the discharge curves showed a progressive forming of the materials during a few cycles.

Occurrence and characterization of anionic bondings in transition metal dichalcogenides

Abstract Transition metal dichalcogenides MX2 crystallize in either two-dimensional or three-dimensional (3D) structures. This originates from the competition between cationic d levels and anionic sp levels. The occurrence of a chalcogen pairing may be obtained through oxidation of a ternary phase: Li2FeS2 leads to a metastable new binary compound Fe3+S2−(S2)2−. Such an electronic situation may also be found within the 3D family, the IrX2 (XS, Se) and RhSe2 compounds with quite elongated XX bonds attributed to strong strains. IrTe2 should confirm the structural type presented by sulphur and selenium derivatives. Its previously reported CdI2-like structure is in fact based on a polymeric network with multiple TeTe bonds (Irn3+(Te−1.5)2n) as confirmed by integrated overlap population calculations. This polymeric modification is presented by several other MTe2 phases and explains the very low c a value (1.38) of the hexagonal cell observed in this family. The polymerization phenomena must be generalized to most pyrite-like MTe2 with the noticeable exception of MnTe2. The layered binary Cr 2 3 □ 1 3 Te 2 is another example of tellurium polymeric bondings. Finally a classification of structures taking into account not only the dimensionality but also the polymerization degree of such materials is suggested. From many examples, it is shown that the polymerizing behaviour of the heavy chalcogen anion seems to be much more general than expected and should lead to many charge transfer studies.

Infrared studies of lithium intercalation in the FePS3 and NiPS3 layer-type compounds

Infrared spectra (700-30 cm-1) of several lithium intercalates (chemically prepared) LixMPS3, with M=Fe, Ni and 0<x<1.5, have been recorded and compared with those known for the corresponding host lattices. These lithium intercalates are mainly characterized by new absorption bands at 336 cm-1 and 310 cm-1 for the iron and nickel compounds, respectively. These bands assigned to lithium vibrations increase progressively with lithium content : it is concluded that Li+ ions are more likely to occupy the 2d and 4h “octahedral” sites in the gaps. In addition, the spectra of the nickel derivatives reveal some geometric distortion within the layers and a progressive strengthening of the Ni-S interactions. These results are correlated to the best energy yields obtained in NiPS3/lithium batteries.

Synthesis and crystal structure of a new layered phase: The chromium hexatellurosilicate Cr2Si2Te6

Abstract Cr2Si2Te6, a new layered material belonging to the hexatellurosilicate family, was synthesized from the pure elements heated in an evacuated Pyrex tube for 10 days at 500°C. The crystal symmetry is rhombohedral, space group R3, with the cell parameters a = b = 6.7578 (6) A, c = 20.665 (3) A, V = 817.3 (2) A3, and Z = 3. The X-ray crystal structure was determined from 456 independent reflections and 31 variables. The final R value is 0.033. The structure, built from a hexagonal close packing of tellurium atoms in the AB sequence, is isostructural with Fe2P2Se6. Between the anionic layers, chromium atoms and (Si2) pairs fill the octahedral sites in a 2 : 1 ratio, leaving alternate octahedral site planes empty. Distorted octahedral (Si2Te6) and (CrTe6) groups (mean dSiTe = 2.509 (7) A, dSiSi = 2.265 (7) A, and mean dCrTe = 2.781 (14) A) are found in 2D Cr2Si2Te6. Comparisons are made with other tellurosilicates containing (Si2Te6) units with silicon pairs.

Redox processes in the LixFeS2/Li electrochemical system studied through crystal, Mössbauer, and EXAFS analyses

Li2FeS2 is a compound that can be de-intercalated, leading to a new FeS2 cathodic material. The structure of the lithiated phase is developed from hexagonal-close-packing of S2− anions with tetrahedral iron ions, constituting (2D) infinite sheets. Lithium fills some of the tetrahedral and octahedral voids of the host in equivalent proportion. The first lithium removal corresponds to the oxidation of iron and to an important shift from its tetrahedral sites to others in the structure, as evidenced by Mossbauer and EXAFS measurements. De-intercalation of the second lithium results in the oxidation of sulfur, the final structure of the completely oxidized phase being Fe3+(Td)S2−(S2)2−12. As shown earlier, this charged material can be re-intercalated up to its original composition.

Infrared study of lithium intercalated phases in the LixFeS2 system (0 ⩽ x ⩽ 2). Characterization of a new iron disulfide

Abstract Infrared spectra (550−200cm−1) of the Li2FeS2 phase, of several chemically and electrochemically oxydized LixFeS2 samples with x=1.64, 0.98, 0.72, 0.28, 0.14 and ∼0.0, and of some reintercalated phases with x=0.69, 0.93 and 1.50 have been recorded and analyzed in terms of stretching vibrations of LiS6 or LiS4 entities, of S2−2 anionic pairs and of Fe3+ −S bonds. From a comparison of the spectra, it is concluded that lithium ions occupy “octahedral” and “tetrahedral” sites in the lithium rich phases, and that they are mainly localized in “octahedral” positions for all the other phases (0 ⩽ × 1 2 S−2 is found.