Delphine Flahaut

One dimensional compounds with large thermoelectric power: Ca3Co2O6 and Ca3CoMO6 with M = Ir4+ and Rh4+

Abstract In the Ca–Co–O system, there exists at least two identified phases, Ca 3 Co 4 O 9 and Ca 3 Co 2 O 6 exhibiting 2D CoO 2 planes and 1D Co–Co chains, respectively. Both oxides are characterized by large thermopower values at room temperature, the largest value being obtained for Ca 3 Co 2 O 6 (Seebeck coefficient, S 300 K =450 μV K −1 ). However, the electrical resistivity of the latter is too large (typically 50 Ω cm) for applications as thermopower materials. Interestingly, the substitutions of Ir 4+ or Rh 4+ for one cobalt out of two in the chain do not greatly affect the electronic properties in contrast to the substitution by Sc 3+ (3d 0 ). This emphasizes that the Ir or Rh cations can play a similar role to that of the Co cations. This opens a new route for the search of new thermoelectric materials alternative to the cobaltites.

XPS investigation of surface reactivity of electrode materials: effect of the transition metal.

The role of the transition metal nature and Al2O3 coating on the surface reactivity of LiCoO2 and LiNi(1/3)Mn(1/3)Co(1/3)O2 (NMC) materials were studied by coupling chemisorption of gaseous probes molecules and X-ray photoelectron (XPS) spectroscopy. The XPS analyses have put in evidence the low reactivity of the LiMO2 materials toward basic gaseous probe (NH3). The reactivity toward SO2 gaseous probe is much larger (roughly more than 10 times) and strongly influenced by the nature of metal. Only one adsorption mode (redox process producing adsorbed sulfate species) was observed at the LiCoO2 surface, while NMC materials exhibit sulfate and sulfite species at the surface. On the basis of XPS analysis of bare materials and previous theoretical work, we propose that the acid-base adsorption mode involving the Ni(2+) cation is responsible for the sulfite species on the NMC surface. After Al2O3 coating, the surface reactivity was clearly decreasing for both LiCoO2 and NMC materials. In addition, for LiCoO2, the coating modifies the surface reactivity with the identification of both sulfate and sulfite species. This result is in line with a change in the adsorption mode from redox toward acid-base after Al/Co substitution. In the case of NMC materials, the coating induced a decrease of the sulfite species content at the surface. This phenomenon can be related to the cation mixing effect in the NMC.

Anisotropic susceptibility of the geometrically frustrated spin-chain compound Ca3Co2O6

Ca3Co2O6 is a system exhibiting a series of fascinating properties, including magnetization plateaus and remarkably slow dynamics at low-T. These properties are intimately related to the geometrical frustration, which results from a particular combination of features: (i) the chains are arranged on a triangular lattice; (ii) there is a large uniaxial anisotropy; (iii) the intrachain and interchain couplings are ferromagnetic and antiferromagnetic, respectively. The uniaxial anisotropy is thus an issue of crucial importance for the analysis of the physical properties of Ca3Co2O6. However, it turns out that no precise investigation of this magnetic anisotropy has been performed so far. On the basis of susceptibility data directly recorded on single crystals, the present study reports on quantitative information about the anisotropy of Ca3Co2O6.

Morphology and Surface Reactivity Relationship in the Li1+xMn2–xO4 Spinel with x = 0.05 and 0.10: A Combined First-Principle and Experimental Study

This article focuses on the surface reactivity of two spinel samples with different stoichiometries and crystal morphologies, namely Li1+xMn2-xO4 with x = 0.05 and 0.10. LiMn2O4 compounds are good candidates as positive electrode of high-power lithium-ion batteries for portable devices. The samples were investigated using both experimental and theoretical approaches. On the experimental point of view, they were characterized in depth from X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS) analyses. Then, the reactivity was investigated through the adsorption of (SO2) gaseous probes, in controlled conditions, followed by XPS characterization. First-principle calculations were conducted simultaneously to investigate the electronic properties and the reactivity of relevant surfaces of an ideal LiMn2O4 material. The results allow us to conclude that the reactivity of the samples is dominated by an acido-basic reactivity and the formation of sulfite species. Nonetheless, on the x = 0.05 sample, both sulfite and sulfate species are obtained, the later, in lesser extent, corresponding to a redox reactivity. Combining experimental and theoretical results, this redox reactivity could be associated with the presence of a larger quantity of Mn4+ cations on the last surface layers of the material linked to a specific surface orientation.

Surface Reactivity of Li2MnO3: First-Principles and Experimental Study

This article deals with the surface reactivity of (001)-oriented Li2MnO3 crystals investigated from a multitechnique approach combining material synthesis, X-ray photoemission spectroscopy (XPS), scanning electron microscopy, Auger electron spectroscopy, and first-principles calculations. Li2MnO3 is considered as a model compound suitable to go further in the understanding of the role of tetravalent manganese atoms in the surface reactivity of layered lithium oxides. The knowledge of the surface properties of such materials is essential to understand the mechanisms involved in parasitic phenomena responsible for early aging or poor storage performances of lithium-ion batteries. The surface reactivity was probed through the adsorption of SO2 gas molecules on large Li2MnO3 crystals to be able to focus the XPS beam on the top of the (001) surface. A chemical mapping and XPS characterization of the material before and after SO2 adsorption show in particular that the adsorption is homogeneous at the micro- and nanoscale and involves Mn reduction, whereas first-principles calculations on a slab model of the surface allow us to conclude that the most energetically favorable species formed is a sulfate with charge transfer implying reduction of Mn.