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Anionic redox in Li-rich and Na-rich transition metal oxides (A-rich-TMOs) has emerged as a new paradigm to increase the energy density of rechargeable batteries. Ever since, numerous electrodes delivering extra anionic capacity beyond the theoretical cationic capacity have been reported. Unfortunately, most often the anionic capacity achieved in charge is partly irreversible in discharge. A unified picture of anionic redox in A-rich-TMOs is designed here to identify the electronic origin of this irreversibility and to propose new directions to improve the cycling performance of the electrodes. The electron localization function is introduced as a holistic tool to unambiguously locate the oxygen lone pairs in the structure and follow their participation in the redox activity of A-rich-TMOs. The charge-transfer gap of transition metal oxides is proposed as the pertinent observable to quantify the amount of extra capacity achievable in charge and its reversibility in discharge, irrespective of the material chemical composition. From this generalized approach, we conclude that the reversibility of the anionic capacity is limited to a critical number of O holes per oxygen, hO ≤ 1/3.Although anionic redox in Li- and Na-rich transition metal oxides can enhance energy density of rechargeable batteries, anionic capacity is partly irreversible in discharge. A unified picture to clarify this irreversibility and to improve cycling performance is proposed.

The oxygen evolution reaction (OER) is of prime importance in multiple energy storage devices; however, deeper mechanistic understanding is required to design enhanced electrocatalysts for the reaction. Current understanding of the OER mechanism based on oxygen adsorption on a metallic surface site fails to fully explain the activity of iridium and ruthenium oxide surfaces, and the drastic surface reconstruction observed for the most active OER catalysts. Here we demonstrate, using La 2 LiIrO 6 as a model catalyst, that the exceptionally high activity found for Ir-based catalysts arises from the formation of active surface oxygen atoms that act as electrophilic centres for water to react. Moreover, with the help of transmission electron microscopy, we observe drastic surface reconstruction and iridium migration from the bulk to the surface. Therefore, we establish a correlation between surface activity and surface stability for OER catalysts that is rooted in the formation of surface reactive oxygen. Electrocatalytic water oxidation is key in energy storage technologies, but deeper mechanistic understanding is still required. Grimaud  et al.  show that surface oxygen atoms in a model oxide catalyst act as electrophilic centres for reactions and observe drastic reconstruction of the catalyst surface.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The Li-rich rocksalt oxides Li 2 MO 3 (M = 3 d /4 d /5 d transition metal) are promising positive-electrode materials for Li-ion batteries, displaying capacities exceeding 300 mAh g –1 thanks to the participation of the oxygen non-bonding O(2 p ) orbitals in the redox process. Understanding the oxygen redox limitations and the role of the O/M ratio is therefore crucial for the rational design of materials with improved electrochemical performances. Here we push oxygen redox to its limits with the discovery of a Li 3 IrO 4 compound (O/M = 4) that can reversibly take up and release 3.5 electrons per Ir and possesses the highest capacity ever reported for any positive insertion electrode. By quantitatively monitoring the oxidation process, we demonstrate the material’s instability against O 2 release on removal of all Li. Our results show that the O/M parameter delineates the boundary between the material’s maximum capacity and its stability, hence providing valuable insights for further development of high-capacity materials. Anionic redox provides extra capacity for battery electrodes, but it is challenging to realize its full potential. Tarascon and colleagues report a record-high reversible capacity of 3.5 electrons per Ir in a Li 3 IrO 4 phase, and discuss the importance of increasing the ratio of oxygen versus transition metal.

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.

Quantum embedding is a divide and conquer strategy that aims at solving the electronic Schrödinger equation of sizeable molecules or extended systems. We establish in the present work a clearer and in-principle-exact connection between density matrix embedding theory (DMET) and density-functional theory (DFT) within the simple but nontrivial one-dimensional Hubbard model. For that purpose, we use our recent reformulation of single-impurity DMET as a Householder transformed density-matrix functional embedding theory (Ht-DMFET). On the basis of well-identified density-functional approximations, a self-consistent local potential functional embedding theory (LPFET) is formulated and implemented. Combining both LPFET and DMET numerical results with our formally exact density-functional embedding theory reveals that a single statically embedded impurity can in principle describe the density-driven Mott–Hubbard transition, provided that a complementary density-functional correlation potential (which is neglected in both DMET and LPFET) exhibits a derivative discontinuity (DD) at half filling. The extension of LPFET to multiple impurities (which would enable to circumvent the modeling of DDs) and its generalization to quantum chemical Hamiltonians are left for future work.

The performance of embedding methods is directly tied to the quality of the bath orbital construction. In this paper, we develop a versatile framework, enabling the investigation of the optimal construction of the orbitals of the bath. As of today, in state-of-the-art embedding methods, the orbitals of the bath are constructed by performing a Singular Value Decomposition (SVD) on the impurity-environment part of the one-body reduced density matrix, as originally presented in Density Matrix Embedding Theory. Recently, the equivalence between the SVD protocol and the use of unitary transformation, the so-called Block-Householder transformation, has been established. We present a generalization of the Block-Householder transformation by introducing additional flexible parameters. The additional parameters are optimized such that the bath-orbitals fulfill physically motivated constraints. The efficiency of the approach is discussed and exemplified in the context of the half-filled Hubbard model in one-dimension.

Mechanical degradation in electrode materials during successive electrochemical cycling is critical for battery lifetime and aging properties. A common strategy to mitigate electrode mechanical degradation is to suppress the volume variation induced by Li/Na intercalation/deintercalation, thereby designing strain-less electrodes. In this study, we investigate the electrochemically-induced volume variation in layered and spinel compounds used in Li-ion and Na-ion battery electrode materials through density functional theory computations. Specifically, we propose to decompose the volume variation into electronic, ionic, and structural contributions. Based on this analysis, we suggest methods to separately influence each contribution through strategies such as chemical substitution, doping, and polymorphism. Altogether, we conclude that volume variations can be controlled by designing either mechanically hard or compact electrode materials.

The voltage delivered by rechargeable Lithium- and Sodium-ion batteries is a key parameter to qualify the device as promising for future applications. Here we report a new formulation of the cell voltage in terms of chemically intuitive quantities that can be rapidly and quantitatively evaluated from the alkaliated crystal structure with no need of first-principles calculations. The model, which is here validated on a wide series of existing cathode materials, provides new insights into the physical and chemical features of a crystal structure that influence the material potential. In particular, we show that disordered materials with cationic intermixing must exhibit higher potentials than their ordered homologues. The present method is utilizable by any solid-state chemist, is fully predictive and allows rapid assessement of material potentials, thus opening new directions for the challenging project of material design in rechargeable batteries. Cell voltage is a key indicator of battery performance, but its prediction often involves first-principles calculations. Here the authors propose a simple approach that allows rapid assessment of the cell potential as well as provides insights into factors influencing potential amplitude.

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.