Chimie du Solide et Energie

The production of sustainable hydrogen with water electrolyzers is envisaged as one of the most promising ways to match the continuously growing demand for renewable electricity storage. While so far regarded as fast when compared to the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER) regained interest in the last few years owing to its poor kinetics in alkaline electrolytes. Indeed, this slow kinetics not only may hinder the foreseen development of the anionic exchange membrane water electrolyzer (AEMWE), but also raises fundamental questions regarding the parameters governing the reaction. In this perspective, we first briefly review the fundamentals of the HER, emphasizing how studies performed on model electrodes allowed for achieving a good understanding of its mechanism under acidic conditions. Then, we discuss how the use of physical descriptors capturing the sole properties of the catalyst is not sufficient to describe the HER kinetics under alkaline conditions, thus forcing the catalysis community to adopt a more complex picture taking into account the electrolyte structure at the electrochemical interface. This work also outlines new techniques, such as spectroscopies, molecular simulations, or chemical approaches that could be employed to tackle these new fundamental challenges, and potentially guide the future design of practical and cheap catalysts while also being useful to a wider community dealing with electrochemical energy storage devices using aqueous electrolytes.

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.

Since their commercialization Li-ion batteries have relied on the use of layered oxides (LiMO2) as positive electrodes. Over the years, via skilful chemical substitution their performances have drastically improved in terms of safety and capacity, which has nearly doubled (280 mAh/g) with the recent arrival of Li-rich NMC, i.e. layered LiCoO2 in which Co has been simultaneously replaced by Mn, Ni and Li. This review will aim to describe the chemical rationale which has led to this material evolution prior to focus on Li-rich NMC phases which are sources of excitement but challenges as well. The benefits of going back to fundamentals to rationalize and understand the new science at work with these Li-rich NMC phases will be stressed and illustrated by the discovery of a new reversible anionic redox process. Issues regarding voltage fade and limited rate capability which are plaguing their present utilization in commercial Li-ion cells will be addressed as well and solutions proposed. Owing to such advances, layered oxides which are over performing spinel or polyanionic-based compounds have a bright future.

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.

Hard carbons are considered among the most promising anode materials for Na‐ion batteries. Understanding their structure is of great importance for optimizing their Na storage capabilities and therefore achieving high performance. Herein, carbon nanofibers (CNFs) are prepared by electrospinning and their microstructure, texture, and surface functionality are tailored through carbonization at various temperatures ranging from 650 to 2800 °C. Stepwise carbonization gradually removes the heteroatoms and increases the graphitization degree, enabling us to monitor the corresponding electrochemical performance for establishing a correlation between the CNFs characteristics and Na storage behavior. Outstandingly, it is found that for CNFs carbonized at above 2000 °C, a single voltage Na uptake plateau at ≈0.1 V with a capacity of ≈200 mAh g ‐1 . This specific performance may be nested in the higher degree of graphitization, lower active surface area, and different porous texture of the CNFs at such temperatures. It is demonstrated via the assembly of a CNF/Na 2 Fe 2 (SO 4 ) 3 cell the benefit of such CNFs electrode for enhancing the energy density of full Na‐ion cells. This finding sheds new insights in the quest for high performance carbon based anode materials.

This paper aims to identify robust descriptors to rationalize the anionic redox mechanism in layered Li-rich TM-oxides using conceptual tools, such as atomic charges, orbital interactions and crystal orbital overlap populations (COOP), based on first-principles DFT calculations.

This is a critical review of artificial intelligence/machine learning (AI/ML) methods applied to battery research. It aims at providing a comprehensive, authoritative, and critical, yet easily understandable, review of general interest to the battery community. It addresses the concepts, approaches, tools, outcomes, and challenges of using AI/ML as an accelerator for the design and optimization of the next generation of batteries─a current hot topic. It intends to create both accessibility of these tools to the chemistry and electrochemical energy sciences communities and completeness in terms of the different battery R&D aspects covered.

Selective electrochemical reduction of CO2 into energy-dense organic compounds is a promising strategy for using CO2 as a carbon source. Herein, we investigate a series of iron-based catalysts synthesized by pyrolysis of Fe-, N-, and C-containing precursors for the electroreduction of CO2 to CO under aqueous conditions and demonstrate that the selectivity of these materials for CO2 reduction over proton reduction is governed by the ratio of isolated FeN4 sites vs Fe-based nanoparticles. This ratio can be synthetically tuned to generate electrocatalysts producing controlled CO/H2 ratios. It notably allows preparing materials containing only FeN4 sites, which are able to selectively reduce CO2 to CO in aqueous solution with Faradaic yields of over 90% and at low overpotential.

Abstract Producing hydrogen by water electrolysis suffers from the kinetic barriers in the oxygen evolution reaction (OER) that limits the overall efficiency. With spin-dependent kinetics in OER, to manipulate the spin ordering of ferromagnetic OER catalysts (e.g., by magnetization) can reduce the kinetic barrier. However, most active OER catalysts are not ferromagnetic, which makes the spin manipulation challenging. In this work, we report a strategy with spin pinning effect to make the spins in paramagnetic oxyhydroxides more aligned for higher intrinsic OER activity. The spin pinning effect is established in oxide FM /oxyhydroxide interface which is realized by a controlled surface reconstruction of ferromagnetic oxides. Under spin pinning, simple magnetization further increases the spin alignment and thus the OER activity, which validates the spin effect in rate-limiting OER step. The spin polarization in OER highly relies on oxyl radicals (O∙) created by 1 st dehydrogenation to reduce the barrier for subsequent O-O coupling.

Abstract Developing highly active electrocatalysts for oxygen evolution reaction (OER) is critical for the effectiveness of water splitting. Low‐cost spinel oxides have attracted increasing interest as alternatives to noble metal–based OER catalysts. A rational design of spinel catalysts can be guided by studying the structural/elemental properties that determine the reaction mechanism and activity. Here, using density functional theory (DFT) calculations, it is found that the relative position of O p‐band and M Oh (Co and Ni in octahedron) d‐band center in ZnCo 2− x Ni x O 4 ( x = 0–2) correlates with its stability as well as the possibility for lattice oxygen to participate in OER. Therefore, it is testified by synthesizing ZnCo 2− x Ni x O 4 spinel oxides, investigating their OER performance and surface evolution. Stable ZnCo 2− x Ni x O 4 ( x = 0–0.4) follows adsorbate evolving mechanism under OER conditions. Lattice oxygen participates in the OER of metastable ZnCo 2− x Ni x O 4 ( x = 0.6, 0.8) which gives rise to continuously formed oxyhydroxide as surface‐active species and consequently enhances activity. ZnCo 1.2 Ni 0.8 O 4 exhibits performance superior to the benchmarked IrO 2 . This work illuminates the design of highly active metastable spinel electrocatalysts through the prediction of the reaction mechanism and OER activity by determining the relative positions of the O p‐band and the M Oh d‐band center.

Water reduction products catalyze the formation of a passivating layer that protects negative electrodes for batteries in aqueous superconcentrated electrolytes.

A comparative study of the electrode/electrolyte interface was carried out for lithium and sodium metal anodes in electrolytes consisting in 1 M LiPF6 in EC0.5:DMC0.5 (LP30) and 1 M NaPF6 in both EC0.5:DMC0.5 and EC0.45PC0.45DMC0.1. Symmetric Li/Li cells exhibited low polarization and smooth charge discharge curves with current densities of 0.1 and 1 mA/cm2. In contrast, large overpotentials were observed even at 0.1 mA/cm2 for Na/Na cells. Such differences cannot be related to ionic conductivity of the electrolytes but are rather due to an enhanced interfacial resistance (Rct + RSEI) as deduced from impedance measurements. The composition of the SEI layer was investigated by FTIR and found to be stable for Li electrodes but to evolve upon cycling for Na electrodes which is also in agreement with differences in surface morphology detected by SEM. A lower stability (partial solubility) of the SEI would also enable to understand the differences in the impedance of identical hard carbon (HC) electrodes in cells with either Li or Na counterelectrodes. These results cast some concerns on the reliability of the so termed half cell characterization and call for caution when interpreting the results of potential electrode materials for sodium ion batteries.

Abstract The growing need to store an increasing amount of renewable energy in a sustainable way has rekindled interest for sodium-ion battery technology, owing to the natural abundance of sodium. Presently, sodium-ion batteries based on Na 3 V 2 (PO 4 ) 2 F 3 /C are the subject of intense research focused on improving the energy density by harnessing the third sodium, which has so far been reported to be electrochemically inaccessible. Here, we are able to trigger the activity of the third sodium electrochemically via the formation of a disordered Na x V 2 (PO 4 ) 2 F 3 phase of tetragonal symmetry ( I 4 /mmm space group). This phase can reversibly uptake 3 sodium ions per formula unit over the 1 to 4.8 V voltage range, with the last one being re-inserted at 1.6 V vs Na + /Na 0 . We track the sodium-driven structural/charge compensation mechanism associated to the new phase and find that it remains disordered on cycling while its average vanadium oxidation state varies from 3 to 4.5. Full sodium-ion cells based on this phase as positive electrode and carbon as negative electrode show a 10–20% increase in the overall energy density.

Batteries for electrical storage are central to any future alternative energy paradigm. The ability to probe the redox mechanisms occurring at electrodes during their operation is essential to improve battery performances. Here we present the first report on Electron Paramagnetic Resonance operando spectroscopy and in situ imaging of a Li-ion battery using Li2Ru0.75Sn0.25O3, a high-capacity (>270 mAh g(-1)) Li-rich layered oxide, as positive electrode. By monitoring operando the electron paramagnetic resonance signals of Ru(5+) and paramagnetic oxygen species, we unambiguously prove the formation of reversible (O2)(n-) species that contribute to their high capacity. In addition, we visualize by imaging with micrometric resolution the plating/stripping of Li at the negative electrode and highlight the zones of nucleation and growth of Ru(5+)/oxygen species at the positive electrode. This efficient way to locate 'electron'-related phenomena opens a new area in the field of battery characterization that should enable future breakthroughs in battery research.

Several emerging battery technologies are currently on endeavour to take a share of the dominant position taken by Li-ion batteries in the field of energy storage. Among them, sodium-based batteries offer a combination of attractive properties i.e., low cost, sustainable precursors and secure raw material supplies. Na-based batteries include related battery concepts, such as Na-ion, all solid-state Na batteries, Na/O2 and Na/S, that differ in key components and in redox chemistry, and therefore result in separate challenges and metrics. Na-ion batteries represent an attractive solution which is almost ready to challenge Li-ion technology in certain applications; the other cell concepts represent a more disruptive innovation, with a higher performance gain, provided that major hurdles are overcome. The present review aims at highlighting the most promising materials in the field of Na-based batteries and challenges needed to be addressed to make this technology industrially appealing, by providing an in-depth analysis of performance metrics from recent literature. To this end, half-cell reported metrics have been extrapolated to full cell level for the more mature Na-ion technology to provide a fair comparison with existing technologies.

Sodium-ion batteries have been considered as potential candidates for stationary energy storage because of the low cost and wide availability of Na sources. However, their future commercialization depends critically on control over the solid electrolyte interface formation, as well as the degree of sodiation at the positive electrode. Here we report an easily scalable ball milling approach, which relies on the use of metallic sodium, to prepare a variety of sodium-based alloys, insertion layered oxides and polyanionic compounds having sodium in excess such as the Na4V2(PO4)2F3 phase. The practical benefits of preparing sodium-enriched positive electrodes as reservoirs to compensate for sodium loss during solid electrolyte interphase formation are demonstrated by assembling full C/P'2-Na1[Fe0.5Mn0.5]O2 and C/'Na3+xV2(PO4)2F3' sodium-ion cells that show substantial increases (>10%) in energy storage density. Our findings may offer electrode design principles for accelerating the development of the sodium-ion technology.

Abstract The revival of the Na‐ion battery concept has prompted intense research activities toward new sustainable Na‐based insertion compounds and their implementation in full Na‐ion cells. Efforts are parted between Na‐based polyanionic and layered compounds. For the latter, there has been a specific focus on Na‐deficient layered phases that show cationic and anionic redox activity similar to a Na 0.67 Mn 0.72 Mg 0.28 O 2 phase. Herein, a new alkali‐deficient P2‐Na 2/3 Mn 7/9 Zn 2/9 O 2 phase using a more electronegative element (Zn) than Mg is reported. Like its Mg counterpart, this phase shows anionic redox activity and no O 2 release despite evidence of cationic migration. Density functional theory (DFT) calculations show that it is the presence of an oxygen nonbonding state that triggers the anionic redox activity in this material. The phase delivers a reversible capacity of 200 mAh g −1 in Na‐half cells with such a value be reduced to 140 mAh g −1 in full Na‐ion cells which additionally shows capacity decay upon cycling. These findings establish Na‐deficient layered oxides as a promising platform to further explore the underlying science behind O 2 release in insertion compounds based on anionic redox activity.

Microsized Sn presents stable cyclic performance in a glyme-based electrolyte, which brings 19% increase in energy density of Sn/Na3V2(PO4)3 cells as compared to the cells using a hard carbon anode. The NaSn intermediate phases are also clarified. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Metal-ion batteries are key enablers in today's transition from fossil fuels to renewable energy for a better planet with ingeniously designed materials being the technology driver. A central question remains how to wisely manipulate atoms to build attractive structural frameworks of better electrodes and electrolytes for the next generation of batteries. This review explains the underlying chemical principles and discusses progresses made in the rational design of electrodes/solid electrolytes by thoroughly exploiting the interplay between composition, crystal structure and electrochemical properties. We highlight the crucial role of advanced diffraction, imaging and spectroscopic characterization techniques coupled with solid state chemistry approaches for improving functionality of battery materials opening emergent directions for further studies.

Abstract A rational design for oxygen evolution reaction (OER) catalysts is pivotal to the overall efficiency of water electrolysis. Much work has been devoted to understanding cation leaching and surface reconstruction of very active electrocatalysts, but little on intentionally promoting the surface in a controlled fashion. We now report controllable anodic leaching of Cr in CoCr 2 O 4 by activating the pristine material at high potential, which enables the transformation of inactive spinel CoCr 2 O 4 into a highly active catalyst. The depletion of Cr and consumption of lattice oxygen facilitate surface defects and oxygen vacancies, exposing Co species to reconstruct into active Co oxyhydroxides differ from CoOOH. A novel mechanism with the evolution of tetrahedrally coordinated surface cation into octahedral configuration via non‐concerted proton‐electron transfer is proposed. This work shows the importance of controlled anodic potential in modifying the surface chemistry of electrocatalysts.