Danielle Gonbeau

Systematic XPS studies of metal oxides, hydroxides and peroxides

The results of a systematic XPS study, under high controlled conditions, of different basic oxides of transition metals, alkali and alkaline-earth metals are presented; the XPS data of some hydroxides and peroxides are also reported. Variations of the O 1s binding energies are analysed and one point of interest is the large binding energy scale obtained for O 1s peaks all associated with a ‘‘2− ’’ formal charge. Through extended Huckel theory-tight binding (EHT-TB) calculations, attempts are made to rationalize the observed variations. The results illustrate the significant differences between real charges on oxygen atoms in transition metal and alkaline-earth oxides.

Reversible anionic redox chemistry in high-capacity layered-oxide electrodes

Li-ion batteries have contributed to the commercial success of portable electronics and may soon dominate the electric transportation market provided that major scientific advances including new materials and concepts are developed. Classical positive electrodes for Li-ion technology operate mainly through an insertion-deinsertion redox process involving cationic species. However, this mechanism is insufficient to account for the high capacities exhibited by the new generation of Li-rich (Li(1+x)Ni(y)Co(z)Mn(1-x-y-z)O₂) layered oxides that present unusual Li reactivity. In an attempt to overcome both the inherent composition and the structural complexity of this class of oxides, we have designed structurally related Li₂Ru(1-y)Sn(y)O₃ materials that have a single redox cation and exhibit sustainable reversible capacities as high as 230 mA h g(-1). Moreover, they present good cycling behaviour with no signs of voltage decay and a small irreversible capacity. We also unambiguously show, on the basis of an arsenal of characterization techniques, that the reactivity of these high-capacity materials towards Li entails cumulative cationic (M(n+)→M((n+1)+)) and anionic (O(2-)→O₂(2-)) reversible redox processes, owing to the d-sp hybridization associated with a reductive coupling mechanism. Because Li₂MO₃ is a large family of compounds, this study opens the door to the exploration of a vast number of high-capacity materials.

Cathode composites for Li-S batteries via the use of oxygenated porous architectures.

Li-S rechargeable batteries are attractive for electric transportation because of their low cost, environmentally friendliness, and superior energy density. However, the Li-S system has yet to conquer the marketplace, owing to its drawbacks, namely, soluble polysulfide formation. To tackle this issue, we present here a strategy based on the use of a mesoporous chromium trimesate metal-organic framework (MOF) named MIL-100(Cr) as host material for sulfur impregnation. Electrodes containing sulfur impregnated within the pores of the MOF were found to show a marked increase in the capacity retention of Li-S cathodes. Complementary transmission electron microscopy and X-ray photoelectron spectroscopy measurements demonstrated the reversible capture and release of the polysulfides by the pores of MOF during cycling and evidenced a weak binding between the polysulphides and the oxygenated framework. Such an approach was generalized to other mesoporous oxide structures, such as mesoporous silica, for instance SBA-15, having the same positive effect as the MOF on the capacity retention of Li-S cells. Besides pore sizes, the surface activity of the mesoporous additives, as observed for the MOF, appears to also have a pronounced effect on enhancing the cycle performance. Increased knowledge about the interface between polysulfide species and oxide surfaces could lead to novel approaches in the design and fabrication of long cycle life S electrodes.

XPS study of thin films of titanium oxysulfides

In this study thin films of titanium oxysulfides which could be used as positive electrode materials in microbatteries were analysed by XPS. In light of the binding energies obtained for reference compounds (different titanium oxides and Sulfides) the results have shown the existence of a new type of titanium. It presents an environment with S2−2 pairs, S2− and O2− ions, this latter in a proportion half that of sulfur ions.

Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries

Peering into cathode layered oxides The quest for better rechargeable batteries means finding ways to pack more energy into a smaller mass or volume. Lithium layered oxides are a promising class of materials that could double storage capacities. However, the design of safe and long-lasting batteries requires an understanding of the physical and chemical changes that occur during redox processes. McCalla et al. used a combination of experiments and calculations to understand the formation of O-O dimers, which are key to improving the properties of these cathode materials. Science, this issue p. 1516 A model lithium layered oxide is used to probe the enhanced charge storage capacity of this family of materials. Lithium-ion (Li-ion) batteries that rely on cationic redox reactions are the primary energy source for portable electronics. One pathway toward greater energy density is through the use of Li-rich layered oxides. The capacity of this class of materials (>270 milliampere hours per gram) has been shown to be nested in anionic redox reactions, which are thought to form peroxo-like species. However, the oxygen-oxygen (O-O) bonding pattern has not been observed in previous studies, nor has there been a satisfactory explanation for the irreversible changes that occur during first delithiation. By using Li2IrO3 as a model compound, we visualize the O-O dimers via transmission electron microscopy and neutron diffraction. Our findings establish the fundamental relation between the anionic redox process and the evolution of the O-O bonding in layered oxides.

XPS Identification of the Organic and Inorganic Components of the Electrode/Electrolyte Interface Formed on a Metallic Cathode

X-ray photoelectron spectroscopy (XPS) was used to determine the nature and composition of electrode/electrolyte interfaces forming during the 55°C cycling of Li-based cells in ethylene carbonate:dimethyl carbonate LiPF 6 electrolyte using a heat-treated stainless steel substrate as the positive electrode. From a classical analysis of the XPS C 1s, O 1s, F 1s, P 2p, and Li Is core peak spectra complemented by an unusual detailed interpretation of XPS valence spectra, we could follow, as a function of the cell cycling history, the evolution and nature of the species constituting the organic/inorganic layer as well as determine its approximate composition. We have shown that this surface layer mainly consists of PEO oligomers (-CH 2 -CH 2 -O-) n , carbonates Li 2 CO 3 and/or CH 3 OCO 2 Li, LiPF 6 salt, and of degradation products of the salt such as LiF and phosphates. Moreover, we give evidence that this layer does not only grow but also becomes richer in CH 3 OCO 2 Li and LiF species upon cycling.

XPS analysis of new lithium cobalt oxide thin-films before and after lithium deintercalation

Lithium cobalt oxides thin-films, prepared by radio frequency magnetron sputtering and which could be used as positive electrode materials in microbatteries, were studied by X-ray photoelectron spectroscopy. The results have shown two formal oxidation numbers (+III and +IV) for cobalt ions and two environments, octahedral and tetrahedral for lithium ions. A systematic overcontent of oxygen compared to crystalline LiCoO2 has been evidenced for the thin-films, this excess, attributed to unusual co-ordinations with metal and more covalent Co–O bonds, could be localised in the boundaries of the randomly oriented nanodomains observed by transmission electron microscopy. The evolution of these LixCoO2+y thin-films at different stages of their cycle in experimental microbatteries has also been studied by an analysis of both core peaks and valence bands. The deintercalation of lithium has led to an increase of the proportion of ‘Co4+’ ions; in parallel an implication of the anions has been observed.

Improved Performances of Nanosilicon Electrodes Using the Salt LiFSI: A Photoelectron Spectroscopy Study

Silicon is a very good candidate for the next generation of negative electrodes for Li-ion batteries, due to its high rechargeable capacity. An important issue for the implementation of silicon is the control of the chemical reactivity at the electrode/electrolyte interface upon cycling, especially when using nanometric silicon particles. In this work we observed improved performances of Li//Si cells by using the new salt lithium bis(fluorosulfonyl)imide (LiFSI) with respect to LiPF6. The interfacial chemistry upon long-term cycling was investigated by photoelectron spectroscopy (XPS or PES). A nondestructive depth resolved analysis was carried out by using both soft X-rays (100-800 eV) and hard X-rays (2000-7000 eV) from two different synchrotron facilities and in-house XPS (1486.6 eV). We show that LiFSI allows avoiding the fluorination process of the silicon particles surface upon long-term cycling, which is observed with the common salt LiPF6. As a result the composition in surface silicon phases is modified, and the favorable interactions between the binder and the active material surface are preserved. Moreover a reduction mechanism of the salt LiFSI at the surface of the electrode could be evidenced, and the reactivity of the salt toward reduction was investigated using ab initio calculations. The reduction products deposited at the surface of the electrode act as a passivation layer which prevents further reduction of the salt and preserves the electrochemical performances of the battery.

A photoelectron spectroscopic study of the electrochemical processes in polyaniline

The electronic structure of the base forms of electrochemically prepared polyaniline is studied by means of x‐ray photoelectron spectroscopy (XPS). Model compounds for the amine, imine and crosslinked nitrogen sites provide the references for the analysis of the core level spectra in the polymer. In particular, the analysis of the nitrogen 1s spectra represents a useful method for accurate determination of the amine to imine ratio. The evolution in the concentration of the different nitrogen species as a function of the electrochemical potential is shown to be in good agreement with the results of cyclic voltammetry measurements. Varying the pH leads to large modifications in the stability domain of the different oxidation states.

Characterisation of r.f. sputtered tungsten disulfide and oxysulfide thin films

Abstract Tungsten disulfide (WS2) and tungsten oxysulfide (WOySz) thin films were prepared by reactive radio frequency magnetron sputtering using a WS2 target and argon or a mixture of argon and oxygen as a discharge gas. For a total pressure of 1 Pa, a large range of composition, determined by Rutherford backscattering spectroscopy, can be obtained from WS2.05 when no oxygen gas is introduced in the sputtering chamber to WO3.04S0.09 when the oxygen partial pressure is 10−2 Pa. Scanning electron microscopy studies have shown that the film morphology depends on the sputtering conditions (oxygen partial pressure, total pressure and sputtering time). A structural analysis (X-ray diffraction and transmission electron microscopy) has highlighted that the tungsten oxysulfide thin films such as WO1.05S2.01, WO1.35S2.20 and WO3.04S0.09 are amorphous. Only the WS2 and the WO0.4S1.96 films are made of small crystallites (length ≤80 nm and width ≤10 nm) which grow with their c-axis parallel to the substrate. An X-ray photoelectron spectroscopy study on both the core levels (W4f and S2p peaks) and the valence bands has shown that three different environments of the tungsten atoms exist inside the tungsten oxysulfide thin films: an oxygen and a sulfur environment, respectively as in WO3 and WS2, and a mixed oxygen–sulfur environment constituted of O2−, S2− and S22−.