Gwenaëlle Rousse

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.

A comparative structural and electrochemical study of monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3

Pure monoclinic Li 3 M 2 (PO 4 ) 3 (M: Fe, V) powders (<1 μm in diameter) were obtained by an original route that involved initial homogenization of precursors in aqueous solution followed by slow evaporation and annealing under controlled atmosphere at moderate temperatures. The crystal structure of Li 3 V 2 (PO 4 ) 3 was determined for the first time through Rietveld refinements of neutron diffraction data. As for Li 3 Fe 2 (PO 4)3 , Li is distributed within three crystallographic sites, fully occupied at room temperature. The values of the temperature factors on Li(2) and Li(3) sites (five-fold coordination) were found significantly higher than that of Li(1) (four-fold coordination). Li 3 V 2 (PO 4 ) 3 shows four reversible redox phenomena upon insertion of two Li + (V 3+ /V 2+ couple), at 1.98, 1.88, 1.73 and 1.70 V vs. Li. By comparison, Li 3 Fe 2 (PO 4 ) 3 shows two reversible redox phenomena upon insertion of two Li(Fe 3+ /Fe 2+ couple), at 2.88 and 2.73 V vs. Li. Experimental capacities close to the theoretical ones were obtained after optimal composite electrode preparation through ball-milling. In situ X-ray diffraction showed very minor changes from Li 3 M 2 (PO 4 ) 3 to Li 5 M 2 (PO 4 ) 3 Additionally, Li is extracted from Li 3 V 2 (PO 4 ) 3 towards V 2 (PO 4 ) 3 (V 4+ /V 3+ and V 5+ /V 4+ couples) through four redox phenomena at 3.59, 3.67, 4.06 and 4.35 V vs. Li. Despite all these phase transitions, the [M 2 (PO 4 ) 3 ] framework is remarkably stable on cycling, particularly for M: Fe, while partial vanadium dissolution into the electrolyte occurs either on deep reduction to 1.5 V or deep oxidation to 4.6 V vs. Li.

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.

On the origin of the 3.3 and 4.5 V steps observed in LiMn{sub 2}O{sub 4}-based spinels

Different series of LiMn{sub 2}O{sub 4}-based spinels were studied, all presenting two reduction steps at 4.5 and 3.3 V in addition to the normal spinel plateaus around 4 V. A correlation was found between the capacity recovered on these additional steps, the manganese oxidation degree, and the cell parameter of a given spinel. By means of in situ synchrotron diffraction the authors were able to detect the appearance of a new set of diffraction peaks upon oxidation of the 3.3 V step at 3.95 V, that disappeared on subsequent reduction at 3.3 V. Electron diffraction and high resolution electron microscopy studies on partially delithiated samples revealed the formation of double hexagonal layers upon oxidation, consistent with the additional peaks observed in the in situ experiments. Finally, a model to explain the existence of the redox steps at 4.5 and 3.95/3.3 V based on the creation of these double hexagonal layers is proposed.

Understanding the roles of anionic redox and oxygen release during electrochemical cycling of lithium-rich layered Li4FeSbO6.

Li-rich oxides continue to be of immense interest as potential next generation Li-ion battery positive electrodes, and yet the role of oxygen during cycling is still poorly understood. Here, the complex electrochemical behavior of Li4FeSbO6 materials is studied thoroughly with a variety of methods. Herein, we show that oxygen release occurs at a distinct voltage plateau from the peroxo/superoxo formation making this material ideal for revealing new aspects of oxygen redox processes in Li-rich oxides. Moreover, we directly demonstrate the limited reversibility of the oxygenated species (O2(n-); n = 1, 2, 3) for the first time. We also find that during charge to 4.2 V iron is oxidized from +3 to an unusual +4 state with the concomitant formation of oxygenated species. Upon further charge to 5.0 V, an oxygen release process associated with the reduction of iron +4 to +3 is present, indicative of the reductive coupling mechanism between oxygen and metals previously reported. Thus, in full state of charge, lithium removal is fully compensated by oxygen only, as the iron and antimony are both very close to their pristine states. Besides, this charging step results in complex phase transformations that are ultimately destructive to the crystallinity of the material. Such findings again demonstrate the vital importance of fully understanding the behavior of oxygen in such systems. The consequences of these new aspects of the electrochemical behavior of lithium-rich oxides are discussed in detail.

Preparation, Structure, and Electrochemistry of Layered Polyanionic Hydroxysulfates: LiMSO4OH (M = Fe, Co, Mn) Electrodes for Li-Ion Batteries

The Li-ion rechargeable battery, due to its high energy density, has driven remarkable advances in portable electronics. Moving toward more sustainable electrodes could make this technology even more attractive to large-volume applications. We present here a new family of 3d-metal hydroxysulfates of general formula LiMSO4OH (M = Fe, Co, and Mn) among which (i) LiFeSO4OH reversibly releases 0.7 Li(+) at an average potential of 3.6 V vs Li(+)/Li(0), slightly higher than the potential of currently lauded LiFePO4 (3.45 V) electrode material, and (ii) LiCoSO4OH shows a redox activity at 4.7 V vs Li(+)/Li(0). Besides, these compounds can be easily made at temperatures near 200 °C via a synthesis process that enlists a new intermediate phase of composition M3(SO4)2(OH)2 (M = Fe, Co, Mn, and Ni), related to the mineral caminite. Structurally, we found that LiFeSO4OH is a layered phase unlike the previously reported 3.2 V tavorite LiFeSO4OH. This work should provide an impetus to experimentalists for designing better electrolytes to fully tap the capacity of high-voltage Co-based hydroxysulfates, and to theorists for providing a means to predict the electrochemical redox activity of two polymorphs.

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.

Preparation, structure and electrochemistry of LiFeBO3: a cathode material for Li-ion batteries

LiMBO3 (M = Fe, Co, Mn) has been identified as an interesting new cathode material for Li-ion batteries. It was shown to be difficult to synthesize the material as a pure phase and in a highly electrochemically active form. Here we report several methods for the successful preparation of LiFeBO3, including traditional ceramic and self-combustion reactions. By decreasing the particle size and introducing in situ carbon coating, conventionally ceramic-synthesized LiFeBO3/C yields a first discharge of 210 mA h g−1 within the 1.5–4.5 V voltage window at a C/20 rate, 55 °C. Using high-resolution synchrotron X-ray diffraction, neutron powder diffraction and single crystal X-ray diffraction in combination with 6Li NMR and 57Fe Mossbauer spectroscopies, we present a “1Fe 2Li” complex cation distribution model for LiFeBO3 powder.

Design of new electrode materials for Li-ion and Na-ion batteries from the bloedite mineral Na2Mg(SO4)2·4H2O

Mineralogy offers a large database to search for Li- or Na-based compounds having suitable structural features for acting as electrode materials, LiFePO4 being one example. Here we further explore this avenue and report on the electrochemical properties of the bloedite type compounds Na2M(SO4)2·4H2O (M = Mg, Fe, Co, Ni, Zn) and their dehydrated phases Na2M(SO4)2 (M = Fe, Co), whose structures have been solved via complementary synchrotron X-ray diffraction, neutron powder diffraction and transmission electron microscopy. Among these compounds, the hydrated and anhydrous iron-based phases show electrochemical activity with the reversible release/uptake of 1 Na+ or 1 Li+ at high voltages of ∼3.3 V vs. Na+/Na0 and ∼3.6 V vs. Li+/Li0, respectively. Although the reversible capacities remain lower than 100 mA h g−1, we hope this work will stress further the importance of mineralogy as a source of inspiration for designing eco-efficient electrode materials.

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.