Dominique Foix

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.

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.

Microsized Sn as Advanced Anodes in Glyme-Based Electrolyte for Na-Ion Batteries

Microsized Sn presents stable cyclic performance in a glyme-based electrolyte, which brings 19% increase in energy density of Sn/Na3 V2 (PO4 )3 cells as compared to the cells using a hard carbon anode. The NaSn intermediate phases are also clarified.

Evidence for anionic redox activity in a tridimensional-ordered Li-rich positive electrode [beta]-Li2IrO3

Lithium-ion battery cathode materials have relied on cationic redox reactions until the recent discovery of anionic redox activity in Li-rich layered compounds which enables capacities as high as 300 mAh g-1. In the quest for new high-capacity electrodes with anionic redox, a still unanswered question was remaining regarding the importance of the structural dimensionality. The present manuscript provides an answer. We herein report on a β-Li2IrO3 phase which, in spite of having the Ir arranged in a tridimensional (3D) framework instead of the typical two-dimensional (2D) layers seen in other Li-rich oxides, can reversibly exchange 2.5 e- per Ir, the highest value ever reported for any insertion reaction involving d-metals. We show that such a large activity results from joint reversible cationic (Mn+) and anionic (O2)n- redox processes, the latter being visualized via complementary transmission electron microscopy and neutron diffraction experiments, and confirmed by density functional theory calculations. Moreover, β-Li2IrO3 presents a good cycling behaviour while showing neither cationic migration nor shearing of atomic layers as seen in 2D-layered Li-rich materials. Remarkably, the anionic redox process occurs jointly with the oxidation of Ir4+ at potentials as low as 3.4 V versus Li+/Li0, as equivalently observed in the layered α-Li2IrO3 polymorph. Theoretical calculations elucidate the electrochemical similarities and differences of the 3D versus 2D polymorphs in terms of structural, electronic and mechanical descriptors. Our findings free the structural dimensionality constraint and broaden the possibilities in designing high-energy-density electrodes for the next generation of Li-ion batteries.

The structure of ionically conductive chalcogenide glasses : a combined NMR, XPS and ab initio calculation study

Abstract This paper reports on the structural investigation of lithium and sodium thiosilicate crystals and glasses by means of X-ray photoelectron spectroscopy and ab initio calculation. The results are analysed in conjunction with previously reported 29 Si NMR data. While NMR proved to be an effective tool for the quantitative discrimination of edge- and corner-sharing tetrahedra existing in these materials, X-ray photoelectron spectroscopy (XPS) gives information on the nature of SiS bonds, i.e. bridging and non-bridging bonds. The main result is the noticeable difference existing between the structures of lithium and sodium thiosilicate glasses, which, according to XPS data, is due to different electronic redistributions over the network when one or the other alkali is added, the sodium addition resulting in a change in the electronic distribution over the entire network.

Electronic structure of thiogermanate and thioarseniate glasses: experimental (XPS) and theoretical (ab initio) characterizations

Abstract The paper reports on structural investigation and electronic structure of thiogermanate and thioarseniate glasses by means of X-ray photoelectron spectroscopy (XPS) and ab initio calculation. Besides an increase of nonbridging sulfur (nbS) when increasing the amount of modifier (Ag 2 S), a limit in the breakdown between nbS and bridging sulfur (bS) atoms was observed for the intermediate Ag concentrations in the two families for the silver containing glasses. The influence of the modifier cation has been studied for the M 2 GeS 3 glass family (M=Ag, Li, Na). Compared to alkali glasses, a more homogeneous electronic distribution on sulfur atoms was observed for Ag 2 GeS 3 . Furthermore, the electronic redistribution along Ge–S bonds is the most important in the case of Na 2 GeS 3 . In agreement with XPS results, Mulliken population analysis showed only a small difference (more important for Na than for Ag) between the charges on bS and nbS. According to XPS data (valence band), ab initio calculations on cluster models showed important changes—compared to GeS 2 —in both Ge–S and Ge–Ge interactions, and support the concept of a nonlocalized effect of alkali-metal atoms.

Fundamental interplay between anionic/cationic redox governing the kinetics and thermodynamics of lithium-rich cathodes

Reversible anionic redox has rejuvenated the search for high-capacity lithium-ion battery cathodes. Real-world success necessitates the holistic mastering of this electrochemistry’s kinetics, thermodynamics, and stability. Here we prove oxygen redox reactivity in the archetypical lithium- and manganese-rich layered cathodes through bulk-sensitive synchrotron-based spectroscopies, and elucidate their complete anionic/cationic charge-compensation mechanism. Furthermore, via various electroanalytical methods, we answer how the anionic/cationic interplay governs application-wise important issues—namely sluggish kinetics, large hysteresis, and voltage fade—that afflict these promising cathodes despite widespread industrial and academic efforts. We find that cationic redox is kinetically fast and without hysteresis unlike sluggish anions, which furthermore show different oxidation vs. reduction potentials. Additionally, more time spent with fully oxidized oxygen promotes voltage fade. These fundamental insights about anionic redox are indispensable for improving lithium-rich cathodes. Moreover, our methodology provides guidelines for assessing the merits of existing and future anionic redox-based high-energy cathodes, which are being discovered rapidly.Anionic redox chemistry has enabled the design of high-capacity battery cathodes for energy storage. Here, the authors demonstrate reversible anionic redox in an archetypical lithium-rich oxide via bulk-sensitive spectroscopies, further revealing its crucial role in practically important properties.