Synchrotron X-ray Analytical Techniques for Studying Materials Electrochemistry in Rechargeable Batteries

Simultaneous fast XAS/SAXS measurements in an energy-dispersive mode

X-ray absorption spectroscopy (XAS) and small-angle X-ray scattering (SAXS) are common materials characterization tools at synchrotron radiation facilities used in many research fields. Since XAS can provide element-specific chemical states and local atomic structures and SAXS can provide nano-scale structural information, their complementary use is advantageous for a comprehensive understanding of multiscale phenomena. This paper presents a new method for simultaneous XAS/SAXS measurements with synchrotron radiation. The method employs a polychromatic X-ray beam as in the energy-dispersive XAS technique and captures both the transmission XAS spectrum and the SAXS intensity distribution with an area X-ray detector, which eliminates the energy scan in the conventional methods and realizes the simultaneous data acquisition in a shorter time. We succeeded in obtaining the atomic and nano-scale structures of Pt and Pt/Pd nanoparticles with a data acquisition time of 0.1 s, suggesting the potential for real-time observation of multiscale phenomena.

In situ/Operando Synchrotron Radiation Analytical Techniques for CO2/CO Reduction Reaction: From Atomic Scales to Mesoscales

Electrocatalytic carbon dioxide/carbon monoxide reduction reaction (CO(2)RR) has emerged as a prospective and appealing strategy to realize carbon neutrality for manufacturing sustainable chemical products. Developing highly active electrocatalysts and stable devices has been demonstrated as effective approach to enhance the conversion efficiency of CO(2)RR. In order to rationally design electrocatalysts and devices, a comprehensive understanding of the intrinsic structure evolution within catalysts and micro-environment change around electrode interface, particularly under operation conditions, is indispensable. Synchrotron radiation has been recognized as a versatile characterization platform, garnering widespread attention owing to its high brightness, elevated flux, excellent directivity, strong polarization and exceptional stability. This review systematically introduces the applications of synchrotron radiation technologies classified by radiation sources with varying wavelengths in CO(2)RR. By virtue of in situ/operando synchrotron radiationanalytical techniques, we also summarize relevant dynamic evolution processes from electronic structure, atomic configuration, molecular adsorption, crystal lattice and devices, spanning scales from the angstrom to the micrometer. The merits and limitations of diverse synchrotron characterization techniques are summarized, and their applicable scenarios in CO(2)RR are further presented. On the basis of the state-of-the-art fourth-generation synchrotron facilities, a perspective for further deeper understanding of the CO(2)RR process using synchrotron radiation analytical techniques is proposed.

Beam Effects in Synchrotron Radiation Operando Characterization of Battery Materials: X-Ray Diffraction and Absorption Study of LiNi0.33Mn0.33Co0.33O2 and LiFePO4 Electrodes

Operando synchrotron radiation-based techniques are a precious tool in battery research, as they enable the detection of metastable intermediates and ensure characterization under realistic cycling conditions. However, they do not come exempt of risks. The interaction between synchrotron radiation and samples, particularly within an active electrochemical cell, can induce relevant effects at the irradiated spot, potentially jeopardizing the experiment's reliability and biasing data interpretation. With the aim of contributing to this ongoing debate, a systematic investigation into these phenomena was carried out by conducting a root cause analysis of beam-induced effects during the operando characterization of two of the most commonly employed positive electrode materials in commercial Li-ion batteries: LiNi0.33Mn0.33Co0.33O2 and LiFePO4. The study spans across diverse experimental conditions involving different cell types and absorption and scattering techniques and seeks to correlate beam effects with factors such as radiation energy, photon flux, exposure time, and other parameters associated with radiation dosage. Finally, it provides a comprehensive set of guidelines and recommendations for assessing and mitigating beam-induced effects that may affect the outcome of battery operando experiments.

Developing high-power Li||S batteries via transition metal/carbon nanocomposite electrocatalyst engineering

The activity of electrocatalysts for the sulfur reduction reaction (SRR) can be represented using volcano plots, which describe specific thermodynamic trends. However, a kinetic trend that describes the SRR at high current rates is not yet available, limiting our understanding of kinetics variations and hindering the development of high-power Li||S batteries. Here, using Le Chatelier's principle as a guideline, we establish an SRR kinetic trend that correlates polysulfide concentrations with kinetic currents. Synchrotron X-ray adsorption spectroscopy measurements and molecular orbital computations reveal the role of orbital occupancy in transition metal-based catalysts in determining polysulfide concentrations and thus SRR kinetic predictions. Using the kinetic trend, we design a nanocomposite electrocatalyst that comprises a carbon material and CoZn clusters. When the electrocatalyst is used in a sulfur-based positive electrode (5 mg cm-2 of S loading), the corresponding Li||S coin cell (with an electrolyte:S mass ratio of 4.8) can be cycled for 1,000 cycles at 8 C (that is, 13.4 A gS-1, based on the mass of sulfur) and 25 °C. This cell demonstrates a discharge capacity retention of about 75% (final discharge capacity of 500 mAh gS-1) corresponding to an initial specific power of 26,120 W kgS-1 and specific energy of 1,306 Wh kgS-1.

Strategies to alleviate distortive phase transformations in Li-ion intercalation reactions: an example with vanadium pentoxide

Chemical and electrochemical Li-ion insertion in transition metal oxides, either via a phase transformation reaction (ions insert into specific crystallographic sites of the host lattice) or a solid solution insertion (ions distribute uniformly throughout the host lattice), enables high energy density electrochemical energy storage. Many phase transformation cathode materials, that undergo two-phase reactions, exhibit high theoretical capacities arising from multi-electron redox reactions. However, challenges in distortive phase transformations and uncontrolled phase nucleation, propagation, segregation, and co-existence continue to limit the energy density, (dis)charging rate performances, and cycling stability of available phase transformation cathode materials. Vanadium pentoxide (V2O5), a classical layered intercalation host material with high theoretical capacity, undergoes irreversible structural changes and capacity fading when intercalating more than one lithium ion per V2O5 unit in its thermodynamically stable phase. Here, we review recent synthetic strategies to alter the V-O connectivity, thereby alleviating distortive phase transformations and promoting solid solution-based Li-ion insertion in V2O5. We also summarize several widely accessible and classical molecular-based analytical tools that can provide local structural dynamics and phase transformation mechanism information on the lithiation of V2O5, including single-crystal X-ray diffraction, infrared and Raman spectroscopy, electron paramagnetic resonance, and nuclear magnetic resonance spectroscopy.

Gaining More Insights from Synchrotron-Based X-ray Spectroscopy for Alkali Ion Rechargeable Batteries

Alkali ion rechargeable batteries play a significant part in portable electronic devices and electronic vehicles. The rapid development of renewable energy technology nowadays demands batteries with even higher energy density for grid storage. To fulfill such demand, extensive research efforts have been devoted to optimizing electrochemical properties as well as developing novel energy storage schemes and designing new systems. In the investigation process, synchrotron-based X-ray spectroscopy plays a vital role in investigating the detailed degradation mechanism and developing novel energy storage schemes. Herein, we critically review the applications of synchrotron-based X-ray spectroscopy in battery research in recent years. This review begins with a discussion of the different scientific issues in alkali ion rechargeable batteries within various time and space scales. Subsequently, the principle of synchrotron-based X-ray spectroscopy is introduced, and the characteristics of various characterization techniques are summarized and compared. Typical application cases of synchrotron-based X-ray spectroscopy are then introduced into battery investigations. The final part presents perspectives in the development direction of both alkali ion rechargeable battery systems and synchrotron-based X-ray spectroscopy in the future.

Mechanisms of the Accelerated Li+ Conduction in MOF-Based Solid-State Polymer Electrolytes for All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) for lithium metal batteries have garnered considerable interests owing to their low cost, flexibility, lightweight, and favorable interfacial compatibility with battery electrodes. Their soft mechanical nature compared to solid inorganic electrolytes give them a large advantage to be used in low pressure solid-state lithium metal batteries, which can avoid the cost and weight of the pressure cages. However, the application of SPEs is hindered by their relatively low ionic conductivity. In addressing this limitation, enormous efforts are devoted to the experimental investigation and theoretical calculations/simulation of new polymer classes. Recently, metal-organic frameworks (MOFs) have been shown to be effective in enhancing ion transport in SPEs. However, the mechanisms in enhancing Li+ conductivity have rarely been systematically and comprehensively analyzed. Therefore, this review provides an in-depth summary of the mechanisms of MOF-enhanced Li+ transport in MOF-based solid polymer electrolytes (MSPEs) in terms of polymer, MOF, MOF/polymer interface, and solid electrolyte interface aspects, respectively. Moreover, the understanding of Li+ conduction mechanisms through employing advanced characterization tools, theoretical calculations, and simulations are also reviewed in this review. Finally, the main challenges in developing MSPEs are deeply analyzed and the corresponding future research directions are also proposed.

In Situ and Operando X-ray Scattering Methods in Electrochemistry and Electrocatalysis

Electrochemical and electrocatalytic processes are of key importance for the transition to a sustainable energy supply as well as for a wide variety of other technologically relevant fields. Further development of these processes requires in-depth understanding of the atomic, nano, and micro scale structure of the materials and interfaces in electrochemical devices under reaction conditions. We here provide a comprehensive review of in situ and operando studies by X-ray scattering methods, which are powerful and highly versatile tools to provide such understanding. We discuss the application of X-ray scattering to a wide variety of electrochemical systems, ranging from metal and oxide single crystals to nanoparticles and even full devices. We show how structural data on bulk phases, electrode-electrolyte interfaces, and nanoscale morphology can be obtained and describe recent developments that provide highly local information and insight into the composition and electronic structure. These X-ray scattering studies yield insights into the structure in the double layer potential range as well as into the structural evolution during electrocatalytic processes and phase formation reactions, such as nucleation and growth during electrodeposition and dissolution, the formation of passive films, corrosion processes, and the electrochemical intercalation into battery materials.

Atomic Distance Engineering in Metal Catalysts to Regulate Catalytic Performance

It is very important to understand the structure-performance relationship of metal catalysts by adjusting the microstructure of catalysts at the atomic scale. The atomic distance has an essential influence on the composition of the environment of active metal atom, which is a key factor for the design of targeted catalysts with desired function. In this review, we discuss and summarize strategies for changing the atomic distance from three aspects and relate their effects on the reactivity of catalysts. First, the effects of regulating bond length between metal and coordination atom at one single-atom site on the catalytic performance are introduced. The bond lengths are affected by the strain effect of the support and high-shell doping and can evolve during the reaction. Next, the influence of the distance between single-atom sites on the catalytic performance is discussed. Due to the space matching of adsorption and electron transport, the catalytic performance can be adjusted with the shortening of site distance. In addition, the effect of the arrangement spacing of the surface metal active atoms on the catalytic performance of metal nanocatalysts is studied. Finally, a comprehensive summary and outlook of the relationship between atomic distance and catalytic performance is given.

Imaging Voids and Defects Inside Li-Ion Cathode LiNi0.6Mn0.2Co0.2O2 Single Crystals

Li-ion battery cathode active materials obtained from different sources or preparation methods often exhibit broadly divergent performance and stability despite no obvious differences in morphology, purity, and crystallinity. We show how state-of-the-art, commercial, nominally single crystalline LiNi0.6Mn0.2Co0.2O2 (NMC-622) particles possess extensive internal nanostructure even in the pristine state. Scanning X-ray diffraction microscopy reveals the presence of interlayer strain gradients, and crystal bending is attributed to oxygen vacancies. Phase contrast X-ray nano-tomography reveals two different kinds of particles, welded/aggregated, and single crystal like, and emphasizes the intra- and interparticle heterogeneities from the nano- to the microscale. It also detects within the imaging resolution (100 nm) substantial quantities of nanovoids hidden inside the bulk of two-thirds of the overall studied particles (around 3000), with an average value of 12.5%v per particle and a mean size of 148 nm. The powerful combination of both techniques helps prescreening and quantifying the defective nature of cathode material and thus anticipating their performance in electrode assembly/battery testing.

Defective oxygen inert phase stabilized high-voltage nickel-rich cathode for high-energy lithium-ion batteries

Pushing layered cathode to higher operating voltage can facilitate the realization of high-energy lithium-ion batteries. However, the released oxygen species initiate materials surface upon highly delithiated states will react severely with electrolyte, accelerating the structure deterioration and triggering the thermal degradation. Here we propose an inert phase of La2Mo2O9 with abundant oxygen vacancies (about 41%) by regulating the annealing temperature to engineer the cathode interface beyond conventional modifications. By employing LiNi0.8Co0.1Mn0.1O2 as a model system and extending to higher voltage-operated LiCoO2 and Li-rich cathode, we demonstrate that the introduced lanthanum and molybdenum ions will transfer electrons to enhance the surface oxygen electronegativities, thus served as "oxygen anchor" to alleviate oxygen evolution. Furthermore, the possible released oxygen can be operando captured and reserved by β-phase La2Mo2O9 depositor for the intrinsic high oxygen vacancy formation energy. The reaction involving oxygen species with electrolyte is fundamentally diminished, thus effectively mitigate the structure deterioration and elevate the electrochemical performances, enabling a 1.5-Ah pouch-type full cell to exhibit negligible 6.0% capacity loss after 400 cycles.

Nanoscale chemical imaging with structured X-ray illumination

High-resolution imaging with compositional and chemical sensitivity is crucial for a wide range of scientific and engineering disciplines. Although synchrotron X-ray imaging through spectromicroscopy has been tremendously successful and broadly applied, it encounters challenges in achieving enhanced detection sensitivity, satisfactory spatial resolution, and high experimental throughput simultaneously. In this work, based on structured illumination, we develop a single-pixel X-ray imaging approach coupled with a generative image reconstruction model for mapping the compositional heterogeneity with nanoscale resolvability. This method integrates a full-field transmission X-ray microscope with an X-ray fluorescence detector and eliminates the need for nanoscale X-ray focusing and raster scanning. We experimentally demonstrate the effectiveness of our approach by imaging a battery sample composed of mixed cathode materials and successfully retrieving the compositional variations of the imaged cathode particles. Bridging the gap between structural and chemical characterizations using X-rays, this technique opens up vast opportunities in the fields of biology, environmental, and materials science, especially for radiation-sensitive samples.

In situ X-ray spectroscopies beyond conventional X-ray absorption spectroscopy on deciphering dynamic configuration of electrocatalysts

Realizing viable electrocatalytic processes for energy conversion/storage strongly relies on an atomic-level understanding of dynamic configurations on catalyst-electrolyte interface. X-ray absorption spectroscopy (XAS) has become an indispensable tool to in situ investigate dynamic natures of electrocatalysts but still suffers from limited energy resolution, leading to significant electronic transitions poorly resolved. Herein, we highlight advanced X-ray spectroscopies beyond conventional XAS, with emphasis on their unprecedented capabilities of deciphering key configurations of electrocatalysts. The profound complementarities of X-ray spectroscopies from various aspects are established in a probing energy-dependent "in situ spectroscopy map" for comprehensively understanding the solid-liquid interface. This perspective establishes an indispensable in situ research model for future studies and offers exciting research prospects for scientists and spectroscopists.

Electrochemical Restoration of Battery Materials Guided by Synchrotron Radiation Technology for Sustainable Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have been ubiquitous in modern society, especially in the fields of electronic devices, electric vehicles and grid storage, while raising concerns about a tremendous number of spent batteries in the next five to ten years. As environmental awareness and resource security is gaining increasingly extensive attention, how to effectively deal with spent LIBs has become a challenging issue academically and industrially. Accordingly, the development of battery recycling has surfaced as a highly researched topic in the battery community. Recently, the structural and electrochemical restoration of recycled electrode materials have been proposed as a non-destructive method to save more energy and chemical agents compared with mature metallurgical methods. Such a refurbishment process of electrode materials is also regarded as a reverse process of their degradation in the working condition. Notably, synchrotron radiation technology, which is previously applied to diagnose battery degrade, has started to play major roles in gaining more insight into the structural restoration of electrode materials. Here, the contribution of synchrotron radiation technology to reveal the underlying degradation and regeneration mechanisms of LIBs cathodes is highlighted, providing a theoretical basis and guidance for the direct recycling and reuse of degraded cathodes.

Learning heterogeneous reaction kinetics from X-ray videos pixel by pixel

Reaction rates at spatially heterogeneous, unstable interfaces are notoriously difficult to quantify, yet are essential in engineering many chemical systems, such as batteries1 and electrocatalysts2. Experimental characterizations of such materials by operando microscopy produce rich image datasets3-6, but data-driven methods to learn physics from these images are still lacking because of the complex coupling of reaction kinetics, surface chemistry and phase separation7. Here we show that heterogeneous reaction kinetics can be learned from in situ scanning transmission X-ray microscopy (STXM) images of carbon-coated lithium iron phosphate (LFP) nanoparticles. Combining a large dataset of STXM images with a thermodynamically consistent electrochemical phase-field model, partial differential equation (PDE)-constrained optimization and uncertainty quantification, we extract the free-energy landscape and reaction kinetics and verify their consistency with theoretical models. We also simultaneously learn the spatial heterogeneity of the reaction rate, which closely matches the carbon-coating thickness profiles obtained through Auger electron microscopy (AEM). Across 180,000 image pixels, the mean discrepancy with the learned model is remarkably small (<7%) and comparable with experimental noise. Our results open the possibility of learning nonequilibrium material properties beyond the reach of traditional experimental methods and offer a new non-destructive technique for characterizing and optimizing heterogeneous reactive surfaces.

Physical Delithiation of Epitaxial LiCoO2 Battery Cathodes as a Platform for Surface Electronic Structure Investigation

We report a novel delithiation process for epitaxial thin films of LiCoO₂(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 < x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO₂ surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co⁴⁺ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtain.

Structural Transformation of Unconventional-Phase Materials

The structural transformation of materials, which involves the evolution of different structural features, including phase, composition, morphology, etc., under external conditions, represents an important fundamental phenomenon and has drawn substantial research interest. Recently, materials with unconventional phases that are different from their thermodynamically stable ones have been demonstrated to possess distinct properties and compelling functions and can further serve as starting materials for structural transformation studies. The identification and mechanism study of the structural transformation process of unconventional-phase starting materials can not only provide deep insights into their thermodynamic stability in potential applications but also offer effective approaches for the synthesis of other unconventional structures. Here, we briefly summarize the recent research progress on the structural transformation of some typical starting materials with various unconventional phases, including the metastable crystalline phase, amorphous phase, and heterophase, induced by different approaches. The importance of unconventional-phase starting materials in the structural modulation of resultant intermediates and products will be highlighted. The employment of diverse in situ/operando characterization techniques and theoretical simulations in studying the mechanism of the structural transformation process will also be introduced. Finally, we discuss the existing challenges in this emerging research field and provide some future research directions.

Sub-second pair distribution function using a broad bandwidth monochromator

Here the use of a broad energy bandwidth monochromator, i.e. a pair of B₄C/W multilayer mirrors (MLMs), is demonstrated for X-ray total scattering (TS) measurements and pair distribution function (PDF) analysis. Data are collected both on powder samples and from metal oxo clusters in aqueous solution at various concentrations. A comparison between the MLM PDFs and those obtained using a standard Si(111) double-crystal monochromator shows that the measurements yield MLM PDFs of high quality which are suitable for structure refinement. Moreover, the effects of time resolution and concentration on the quality of the resulting PDFs of the metal oxo clusters are investigated. PDFs of heptamolybdate clusters and tungsten α-Keggin clusters from X-ray TS data were obtained with a time resolution down to 3 ms and still showed a similar level of Fourier ripples to PDFs obtained from 1 s measurements. This type of measurement could thus open up faster time-resolved TS and PDF studies.

Recent advances in in situ and operando characterization techniques for Li7La3Zr2O12-based solid-state lithium batteries

Li₇La₃Zr₂O₁₂ (LLZO)-based solid-state Li batteries (SSLBs) have emerged as one of the most promising energy storage systems due to the potential advantages of solid-state electrolytes (SSEs), such as ionic conductivity, mechanical strength, chemical stability and electrochemical stability. However, there remain several scientific and technical obstacles that need to be tackled before they can be commercialised. The main issues include the degradation and deterioration of SSEs and electrode materials, ambiguity in the Li⁺ migration routes in SSEs, and interface compatibility between SSEs and electrodes during the charging and discharging processes. Using conventional ex situ characterization techniques to unravel the reasons that lead to these adverse results often requires disassembly of the battery after operation. The sample may be contaminated during the disassembly process, resulting in changes in the material properties within the battery. In contrast, in situ/operando characterization techniques can capture dynamic information during cycling, enabling real-time monitoring of batteries. Therefore, in this review, we briefly illustrate the key challenges currently faced by LLZO-based SSLBs, review recent efforts to study LLZO-based SSLBs using various in situ/operando microscopy and spectroscopy techniques, and elaborate on the capabilities and limitations of these in situ/operando techniques. This review paper not only presents the current challenges but also outlines future developmental prospects for the practical implementation of LLZO-based SSLBs. By identifying and addressing the remaining challenges, this review aims to enhance the comprehensive understanding of LLZO-based SSLBs. Additionally, in situ/operando characterization techniques are highlighted as a promising avenue for future research. The findings presented here can serve as a reference for battery research and provide valuable insights for the development of different types of solid-state batteries.

Synchrotron X-ray Studies of the Structural and Functional Hierarchies in Mineralised Human Dental Enamel: A State-of-the-Art Review

Hard dental tissues possess a complex hierarchical structure that is particularly evident in enamel, the most mineralised substance in the human body. Its complex and interlinked organisation at the Ångstrom (crystal lattice), nano-, micro-, and macro-scales is the result of evolutionary optimisation for mechanical and functional performance: hardness and stiffness, fracture toughness, thermal, and chemical resistance. Understanding the physical-chemical-structural relationships at each scale requires the application of appropriately sensitive and resolving probes. Synchrotron X-ray techniques offer the possibility to progress significantly beyond the capabilities of conventional laboratory instruments, i.e., X-ray diffractometers, and electron and atomic force microscopes. The last few decades have witnessed the accumulation of results obtained from X-ray scattering (diffraction), spectroscopy (including polarisation analysis), and imaging (including ptychography and tomography). The current article presents a multi-disciplinary review of nearly 40 years of discoveries and advancements, primarily pertaining to the study of enamel and its demineralisation (caries), but also linked to the investigations of other mineralised tissues such as dentine, bone, etc. The modelling approaches informed by these observations are also overviewed. The strategic aim of the present review was to identify and evaluate prospective avenues for analysing dental tissues and developing treatments and prophylaxis for improved dental health.

Synchrotron radiation based operando characterization of battery materials

Synchrotron radiation based techniques are powerful tools for battery research and allow probing a wide range of length scales, with different depth sensitivities and spatial/temporal resolutions. Operando experiments enable characterization during functioning of the cell and are thus a precious tool to elucidate the reaction mechanisms taking place. In this perspective, the current state of the art for the most relevant techniques (scattering, spectroscopy, and imaging) is discussed together with the bottlenecks to address, either specific for application in the battery field or more generic. The former includes the improvement of cell designs, multi-modal characterization and development of protocols for automated or at least semi-automated data analysis to quickly process the huge amount of data resulting from operando experiments. Given the recent evolution in these areas, accelerated progress is expected in the years to come, which should in turn foster battery performance improvements.

Electron and Ion Transport in Lithium and Lithium-Ion Battery Negative and Positive Composite Electrodes

Electrochemical energy storage systems, specifically lithium and lithium-ion batteries, are ubiquitous in contemporary society with the widespread deployment of portable electronic devices. Emerging storage applications such as integration of renewable energy generation and expanded adoption of electric vehicles present an array of functional demands. Critical to battery function are electron and ion transport as they determine the energy output of the battery under application conditions and what portion of the total energy contained in the battery can be utilized. This review considers electron and ion transport processes for active materials as well as positive and negative composite electrodes. Length and time scales over many orders of magnitude are relevant ranging from atomic arrangements of materials and short times for electron conduction to large format batteries and many years of operation. Characterization over this diversity of scales demands multiple methods to obtain a complete view of the transport processes involved. In addition, we offer a perspective on strategies for enabling rational design of electrodes, the role of continuum modeling, and the fundamental science needed for continued advancement of electrochemical energy storage systems with improved energy density, power, and lifetime.

Probing Distinctive Redox Mechanism in Ni-Rich Cathode Via Real-Time Quick X-Ray Absorption Spectroscopy

X-ray radiation damage on the measuring system has been a critical issue regularly for a long-time exposure to X-ray beam during the in operando characterizations, which is particularly severe when the applied X-ray energy is near the absorption edges (M, L, K, etc.) of the interest element. To minimize the negative effects raised by beam radiation, we employ quick X-ray absorption spectroscopy (QXAS) to study the electrochemical reaction mechanism of a Ni-rich layered structure cathode for lithium-ion batteries. With the advanced QXAS technique, the electronic structure and local coordination environment of the transition metals (TMs) are monitored in-operando with limited radiation damage. Compared to the conventional step-mode X-ray absorption spectroscopy, the QXAS can provide more reliable oxidation state change and more detailed local structure evolutions surrounding TMs (Ni and Co) in Ni-rich layered oxides. By leveraging these advantages of QXAS, we demonstrated that the Ni dominates the electrochemical process with the Co being almost electrochemically inactive. Reversible Ni ions movement between TMs sites and Li sites is also revealed by the time-resolved QXAS technique.

Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO₂ nanoparticles their phase transformation upon Mg²⁺ insertion leads to the formation of domains of similar chemical identity but different orientations at nanometre length scale, following the nucleation, growth and coalescence process. Electrolytes have a substantial impact on the transformation microstructure ('island' versus 'archipelago'). Further, large strain gradients build up from the development of phase domains across their boundaries with high impact on the chemical diffusion coefficient by a factor of ten or more. Our findings thus provide critical insights into the microstructure formation mechanism and its impact on the ion-insertion process, suggesting new rules of transformation structure control for energy storage materials.

Single-Crystal Nickel-Based Cathodes: Fundamentals and Recent Advances

Lithium-ion batteries (LIBs) represent the most promising choice for meeting the ever-growing demand of society for various electric applications, such as electric transportation, portable electronics, and grid storage. Nickel-rich layered oxides have largely replaced LiCoO2 in commercial batteries because of their low cost, high energy density, and good reliability. Traditional nickel-based oxide particles, usually called polycrystal materials, are composed of microsized primary particles. However, polycrystal particles tend to suffer from pulverization and severe side reactions along grain boundaries during cycling. These phenomena accelerate cell degradation. Single-crystal materials, which exhibit robust mechanical strength and a high surface area, have great potential to address the challenges that hinder their polycrystal counterparts. A comprehensive understanding of the growing body of research related to single-crystal materials is imperative to improve the performance of cathodes in LIBs. This review highlights origins, recent developments, challenges, and opportunities for single-crystal layered oxide cathodes. The synthesis science behind single-crystal materials and comparative studies between single-crystal and polycrystal materials are discussed in detail. Industrial techniques and facilities are also reviewed in combination with our group’s experiences in single-crystal research. Future development should focus on facile production with strong control of the particle size and distribution, structural defects, and impurities to fully reap the benefits of single-crystal materials.

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X-Ray absorption spectroscopy on airborne aerosols

Here we demonstrate a method for performing X-ray absorption spectroscopy (XAS) on airborne aerosols. XAS provides unique insight into elemental composition, chemical and phase state, local coordination and electronic structure of both crystalline and amorphous matter. The aerosol is generated from different salt solutions using a commercial atomizer and dried using a diffusion drier. Embedded in a carrier gas, the aerosol is guided into the experimental chamber for XAS analysis. Typical particle sizes range from some 10 to a few 100 nm. Inside the chamber the aerosol bearing gas is then confined into a region of about 1-2 cm³ in size, by a pure flow of helium, generating a stable free-flowing stream of aerosol. It is hit by a monochromatic X-ray beam, and the emitted fluorescent light is used for spectroscopic analysis. Using an aerosol generated from CaCl₂, KCl, and (NH₄)₂SO₄ salt solutions, we demonstrate the functionality of the system in studying environmentally relevant systems. In addition, we show that the detection limits are sufficient to also observe subtle spectroscopic signatures in XAS spectra with integration times of about 1-2 hours using a bright undulator beamline. This novel setup opens new research opportunities for studying the nucleation of new phases in multicomponent aerosol systems in situ, and for investigating (photo-) chemical reactions on airborne matter, as relevant to both atmospheric science and also for general chemical application.

New Degradable Semiconducting Polymers for Photoacoustic Imaging of λ-Carrageenan-Induced Arthritis Mouse Model

Semiconducting polymer has a high extinction coefficient and a long band absorption and can be used as a photoacoustic imaging contrast agent. However, nonbiodegradable semiconducting polymers may cause biosafety issues due to being retained in the body. Therefore, developing degradable semiconducting polymers is necessary for in vivo imaging. Herein, we developed three degradable semiconducting polymers with unique optical properties. We adjusted the optical properties of semiconducting polymers by designing the molecular structure of semiconducting polymers. Polymers with a donor-π-acceptor structure could easily improve the optical properties through adjusting the donor or acceptor units. Through adjusting the electron-donor and -acceptor units, three diketopyrrolopyrrole derivative polymers (DPPTz, DPPQu, and DPPWu) were synthesized and converted into nanosize particles. By introducing the degradable chemical groups in the main chain structure of semiconducting polymers, diketopyrrolopyrrole polymers could be degraded by ClO^-. Among these nanosize particles, DPPTz NPs and DPPQu NPs were used to achieve the in vivo photoacoustic imaging of λ-carrageenan-induced arthritis mouse model. This work provides a novel design idea for the designing of red-shifted semiconducting polymer with degradable properties.

Vacancy-enhanced oxygen redox and structural stability of spinel Li2Mn3O7-x

Vacancies have been proved effective in activating the oxygen redox and stabilizing the structure of the oxide cathode materials for the Na-ion batteries, but their effect on the cathode materials of the Li-ion batteries is unclear. We herein show that they have similar effect on spinel [Li_4/7Mn_2/7□_1/7]_8a[Li_4/7Mn_10/7]_16d[O_4-x']_32e.

Exact two-component Hamiltonians for relativistic quantum chemistry: Two-electron picture-change corrections made simple

Based on self-consistent field (SCF) atomic mean-field (amf) quantities, we present two simple yet computationally efficient and numerically accurate matrix-algebraic approaches to correct both scalar-relativistic and spin-orbit two-electron picture-change effects (PCEs) arising within an exact two-component (X2C) Hamiltonian framework. Both approaches, dubbed amfX2C and e(xtended)amfX2C, allow us to uniquely tailor PCE corrections to mean-field models, viz. Hartree-Fock or Kohn-Sham DFT, in the latter case also avoiding the need for a point-wise calculation of exchange-correlation PCE corrections. We assess the numerical performance of these PCE correction models on spinor energies of group 18 (closed-shell) and group 16 (open-shell) diatomic molecules, achieving a consistent ≈10^-5 Hartree accuracy compared to reference four-component data. Additional tests include SCF calculations of molecular properties such as absolute contact density and contact density shifts in copernicium fluoride compounds (CnF_n, n = 2,4,6), as well as equation-of-motion coupled-cluster calculations of x-ray core-ionization energies of 5d- and 6d-containing molecules, where we observe an excellent agreement with reference data. To conclude, we are confident that our (e)amfX2C PCE correction models constitute a fundamental milestone toward a universal and reliable relativistic two-component quantum-chemical approach, maintaining the accuracy of the parent four-component one at a fraction of its computational cost.

Upgraded imaging capabilities at the BAMline (BESSY II)

The BAMline at the BESSY II synchrotron X-ray source has enabled research for more than 20 years in widely spread research fields such as materials science, biology, cultural heritage and medicine. As a nondestructive characterization method, synchrotron X-ray imaging, especially tomography, plays a particularly important role in structural characterization. A recent upgrade of key equipment of the BAMline widens its imaging capabilities: shorter scan acquisition times are now possible, in situ and operando studies can now be routinely performed, and different energy spectra can easily be set up. In fact, the upgraded double-multilayer monochromator brings full flexibility by yielding different energy spectra to optimize flux and energy resolution as desired. The upgraded detector (based on an sCMOS camera) also allows exploiting the higher flux with reduced readout times. Furthermore, an installed slip ring allows the sample stage to continuously rotate. The latter feature enables tomographic observation of processes occurring in the time scale of a few seconds.

Learning from the Heterogeneity at Electrochemical Interfaces

Understanding the structure-activity relationship at electrochemical interfaces is crucial in improving the performance of practical electrochemical devices, ranging from fuel cells, electrolyzers, and batteries to electrochemical sensors. However, functional electrochemical interfaces are often complex and contain various surface structures, creating heterogeneity in electrochemical activity. In this Perspective, we highlight the role of heterogeneity in electrochemistry, especially in the context of electrocatalysis. Current methods for revealing the heterogeneity at electrochemical interfaces, including nanoelectrochemistry tools and single-entity approaches, are discussed. Lastly, we provide perspectives on what one can learn by studying heterogeneity and how one can use heterogeneity to design more efficient electrochemical devices.

Crystallographically Textured Electrodes for Rechargeable Batteries: Symmetry, Fabrication, and Characterization

The vast of majority of battery electrode materials of contemporary interest are of a crystalline nature. Crystals are, by definition, anisotropic from an atomic-structure perspective. The inherent structural anisotropy may give rise to favored mesoscale orientations and anisotropic properties whether the material is in a rest state or subjected to an external stimulus. The overall perspective of this review is that intentional manipulation of crystallographic anisotropy of electrochemically active materials constitute an untapped parameter space in energy storage systems and thus provide new opportunities for materials innovations and design. To that end, we contend that crystallographically textured electrodes, as opposed to their textureless poly crystalline or single-crystalline analogs, are promising candidates for next-generation storage of electrical energy in rechargeable batteries relevant to commercial practice. This perspective is underpinned first by the fundamental─to a first approximation─uniaxial, rotation-invariant symmetry of electrochemical cells. On this basis, we show that a crystallographically textured electrode with the preferred orientation aligned out-of-plane toward the counter electrode represents an optimal strategy for utilization of the crystals' anisotropic properties. Detailed analyses of anisotropy of different types lead to a simple, but potentially useful general principle that "P_ec//P_c" textures are optimal for metal anodes, and "P_ec//S_c" textures are optimal for insertion-type electrodes.

Hysteresis Induced by Incomplete Cationic Redox in Li-Rich 3d-Transition-Metal Layered Oxides Cathodes

Activation of oxygen redox during the first cycle has been reported as the main trigger of voltage hysteresis during further cycles in high-energy-density Li-rich 3d-transition-metal layered oxides. However, it remains unclear whether hysteresis only occurs due to oxygen redox. Here, it is identified that the voltage hysteresis can highly correlate to cationic reduction during discharge in the Li-rich layered oxide, Li_1.2 Ni_0.4 Mn_0.4 O₂ . In this material, the potential region of discharge accompanied by hysteresis is apparently separated from that of discharge unrelated to hysteresis. The quantitative analysis of soft/hard X-ray absorption spectroscopies discloses that hysteresis is associated with an incomplete cationic reduction of Ni during discharge. The galvanostatic intermittent titration technique shows that the inevitable energy consumption caused by hysteresis corresponds to an overpotential of 0.3 V. The results unveil that hysteresis can also be affected by cationic redox in Li-rich layered cathodes, implying that oxygen redox cannot be the only reason for the evolution of voltage hysteresis. Therefore, appropriate control of both cationic and anionic redox of Li-rich layered oxides will allow them to reach their maximum energy density and efficiency.

Chemomechanics of Rechargeable Batteries: Status, Theories, and Perspectives

Chemomechanics is an old subject, yet its importance has been revived in rechargeable batteries where the mechanical energy and damage associated with redox reactions can significantly affect both the thermodynamics and rates of key electrochemical processes. Thanks to the push for clean energy and advances in characterization capabilities, significant research efforts in the last two decades have brought about a leap forward in understanding the intricate chemomechanical interactions regulating battery performance. Going forward, it is necessary to consolidate scattered ideas in the literature into a structured framework for future efforts across multidisciplinary fields. This review sets out to distill and structure what the authors consider to be significant recent developments on the study of chemomechanics of rechargeable batteries in a concise and accessible format to the audiences of different backgrounds in electrochemistry, materials, and mechanics. Importantly, we review the significance of chemomechanics in the context of battery performance, as well as its mechanistic understanding by combining electrochemical, materials, and mechanical perspectives. We discuss the coupling between the elements of electrochemistry and mechanics, key experimental and modeling tools from the small to large scales, and design considerations. Lastly, we provide our perspective on ongoing challenges and opportunities ranging from quantifying mechanical degradation in batteries to manufacturing battery materials and developing cyclic protocols to improve the mechanical resilience.

Tracking the Oxygen Dynamics of Solid-Liquid Electrochemical Interfaces by Correlative In Situ Synchrotron Spectroscopies

ConspectusOxygen-involved electrocatalytic processes, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), are central to a series of advanced modern energy and conversion technologies, such as water electrolyzers, fuel cells, and CO₂ reduction or N₂ fixation devices. A comprehensive and in-depth understanding of the charge transfer and energy conversion process that ubiquitously occurs over solid-liquid electrochemical interfaces during oxygen electrocatalysis is crucial for understanding the key essence of oxygen-related electrochemistry. The huge challenges for dynamic studies over solid-liquid interfaces during oxygen electrocatalysis lie in the all-embracing electrochemical processes of the catalytic reactions, associated with both structural and reactive intermediates evolution on the electrode surface, and in the significant influence of the aqueous environments of electrolytes used. Hence, overcoming these challenges intrinsically calls for a great cooperation of multiple cutting-edge in situ technologies. Synchrotron radiation (SR) X-ray absorption fine structure (SR-XAFS) spectroscopy is highly sensitive to the local atomic structure of nanomaterials, and SR-based Fourier transform infrared (SR-FTIR) spectroscopy features unique molecular fingerprint identification to determine active species on the surface of electrodes. One can imagine that the correlative in situ SR-XAFS/FTIR spectroscopic investigations will potentially provide sufficient, reliable, and complementary information at the atomic/molecular level to depict vivid and comprehensive "dynamic movies" of solid-liquid electrochemical interfaces during oxygen electrocatalysis, which will help effectively promote/simplify the complicated screening process of advanced oxygen electrocatalysts for efficient high-energy-density energy systems.In this Account, starting with some fundamentals of SR-based spectroscopic technologies, tips for obtaining high-quality SR-XAFS and SR-FTIR spectroscopy results during the electrocatalytic process are comprehensively specified. Subsequently, the latest research achievements of dynamic investigations mainly from our group based on in situ SR-XAFS and/or SR-FTIR spectroscopies will be systematically scrutinized and properly emphasized in detail, where the currently attractive metal-organic-framework (MOF) nanomaterials and single-atom catalysts (SACs) are selected as the main object of research. Moreover, the vital contributions of correlative in situ SR-XAFS/FTIR studies on new discoveries of the dynamic evolution of solid-liquid interfaces during oxygen electrocatalysis are highlighted. In particular, our pioneering research found that the potential-dependent dynamically coupled oxygen formed in the precatalytic stage was a very useful promoter in SACs to promote efficient OER kinetics under acidic conditions. In addition, the in situ generated metastable Ni₁-N₂ centers with more structural degrees of freedom in SACs could potentially facilitate the fast 4e^- ORR kinetics. This Account is anticipated to stimulate broad interest in dynamic explorations in various catalytic processes of interest in the material science and electrochemistry communities using correlative SR-based technologies.

Synchrotron radiation based X-ray techniques for analysis of cathodes in Li rechargeable batteries

Li-ion rechargeable batteries are promising systems for large-scale energy storage solutions. Understanding the electrochemical process in the cathodes of these batteries using suitable techniques is one of the crucial steps for developing them as next-generation energy storage devices. Due to the broad energy range, synchrotron X-ray techniques provide a better option for characterizing the cathodes compared to the conventional laboratory-scale characterization instruments. This work gives an overview of various synchrotron radiation techniques for analyzing cathodes of Li-rechargeable batteries by depicting instrumental details of X-ray diffraction, X-ray absorption spectroscopy, X-ray imaging, and X-ray near-edge fine structure-imaging. Analysis and simulation procedures to get appropriate information of structural order, local electronic/atomic structure, chemical phase mapping and pores in cathodes are discussed by taking examples of various cathode materials. Applications of these synchrotron techniques are also explored to investigate oxidation state, metal-oxygen hybridization, quantitative local atomic structure, Ni oxidation phase and pore distribution in Ni-rich layered oxide cathodes.

Are Operando Measurements of Rechargeable Batteries Always Reliable? An Example of Beam Effect with a Mg Battery

Operando and in situ techniques are becoming mandatory to study Li-ion, post Li-ion, and solid-state batteries. They are essential for monitoring the (electro)chemical and dynamic processes in the battery environment and for providing understanding at different spatial and temporal scales. While operando measurements are becoming more and more routine, scientists have to keep in mind that such experiments are not always harmless for the battery operation, especially when using synchrotron techniques. This is the case in the example described herein with Mg batteries. We show that the electrode reactivity in a InSb/organohaluminate electrolyte/Mg cell is strongly retarded during operando synchrotron X-ray absorption acquisition. Through comparison of ex situ, operando, and in situ data, we demonstrate that this effect occurred only on the samples' volumes exposed to the X-ray radiation. This study illustrates how incorrect conclusions might be drawn from operando measurements, especially when looking at new battery chemistries, and calls for extreme caution when designing and interpreting operando data.

Atom-Resolved Investigation on Dynamic Nucleation and Growth of Platinum Nanocrystals

Understanding the mechanism of nucleation and growth of nanocrystals is crucial for designing and regulating the structure and properties of nanocrystals. However, the process from molecules to nanocrystals remains unclear because of the rapid and complicated dynamics of evolution under reaction conditions. Here, the complete evolution process of solid-phase chloroplatinic acid during the electron beam irradiation triggered reduction and nucleation of platinum nanocrystals is recorded. Aberration-corrected environmental transmission electron microscopy is used for direct visualization of the dynamic evolution from H₂ PtCl₆ to Pt nanocrystals at the atomic scale, including the formation and growth of amorphous clusters, crystallization, and growth of clusters, and the ripening of Pt nanocrystals. At the first two stages, there exists a critical size of ≈2.0 nm, which represents the start of crystallization. Crystallization from the center and density fluctuation are observed in the second stage of the crystallization of a few clusters with a size obviously larger than the critical size. The work provides valuable information to understand the kinetics of the early stage of nanocrystal nucleation and crystallization at atomic scale.

Origin of structural degradation in Li-rich layered oxide cathode

Li- and Mn-rich (LMR) cathode materials that utilize both cation and anion redox can yield substantial increases in battery energy density^1-3. However, although voltage decay issues cause continuous energy loss and impede commercialization, the prerequisite driving force for this phenomenon remains a mystery^3-6 Here, with in situ nanoscale sensitive coherent X-ray diffraction imaging techniques, we reveal that nanostrain and lattice displacement accumulate continuously during operation of the cell. Evidence shows that this effect is the driving force for both structure degradation and oxygen loss, which trigger the well-known rapid voltage decay in LMR cathodes. By carrying out micro- to macro-length characterizations that span atomic structure, the primary particle, multiparticle and electrode levels, we demonstrate that the heterogeneous nature of LMR cathodes inevitably causes pernicious phase displacement/strain, which cannot be eliminated by conventional doping or coating methods. We therefore propose mesostructural design as a strategy to mitigate lattice displacement and inhomogeneous electrochemical/structural evolutions, thereby achieving stable voltage and capacity profiles. These findings highlight the significance of lattice strain/displacement in causing voltage decay and will inspire a wave of efforts to unlock the potential of the broad-scale commercialization of LMR cathode materials.

Focus on using nanopore technology for societal health, environmental, and energy challenges

With an increasing global population that is rapidly ageing, our society faces challenges that impact health, environment, and energy demand. With this ageing comes an accumulation of cellular changes that lead to the development of diseases and susceptibility to infections. This impacts not only the health system, but also the global economy. As the population increases, so does the demand for energy and the emission of pollutants, leading to a progressive degradation of our environment. This in turn impacts health through reduced access to arable land, clean water, and breathable air. New monitoring approaches to assist in environmental control and minimize the impact on health are urgently needed, leading to the development of new sensor technologies that are highly sensitive, rapid, and low-cost. Nanopore sensing is a new technology that helps to meet this purpose, with the potential to provide rapid point-of-care medical diagnosis, real-time on-site pollutant monitoring systems to manage environmental health, as well as integrated sensors to increase the efficiency and storage capacity of renewable energy sources. In this review we discuss how the powerful approach of nanopore based single-molecule, or particle, electrical promises to overcome existing and emerging societal challenges, providing new opportunities and tools for personalized medicine, localized environmental monitoring, and improved energy production and storage systems.

High-energy and durable lithium metal batteries using garnet-type solid electrolytes with tailored lithium-metal compatibility

Lithium metal batteries using solid electrolytes are considered to be the next-generation lithium batteries due to their enhanced energy density and safety. However, interfacial instabilities between Li-metal and solid electrolytes limit their implementation in practical batteries. Herein, Li-metal batteries using tailored garnet-type Li_7-xLa_3-aZr_2-bO₁₂ (LLZO) solid electrolytes is reported, which shows remarkable stability and energy density, meeting the lifespan requirements of commercial applications. We demonstrate that the compatibility between LLZO and lithium metal is crucial for long-term stability, which is accomplished by bulk dopant regulating and dopant-specific interfacial treatment using protonation/etching. An all-solid-state with 5 mAh cm^-2 cathode delivers a cumulative capacity of over 4000 mAh cm^-2 at 3 mA cm^-2, which to the best of our knowledge, is the highest cycling parameter reported for Li-metal batteries with LLZOs. These findings are expected to promote the development of solid-state Li-metal batteries by highlighting the efficacy of the coupled bulk and interface doping of solid electrolytes.

A top-down strategy for amorphization of hydroxyl compounds for electrocatalytic oxygen evolution

Amorphous materials have attracted increasing attention in diverse fields due to their unique properties, yet their controllable fabrications still remain great challenges. Here, we demonstrate a top-down strategy for the fabrications of amorphous oxides through the amorphization of hydroxides. The versatility of this strategy has been validated by the amorphizations of unitary, binary and ternary hydroxides. Detailed characterizations indicate that the amorphization process is realized by the variation of coordination environment during thermal treatment, where the M-OH octahedral structure in hydroxides evolves to M-O tetrahedral structure in amorphous oxides with the disappearance of the M-M coordination. The optimal amorphous oxide (FeCoSn(OH)₆-300) exhibits superior oxygen evolution reaction (OER) activity in alkaline media, where the turnover frequency (TOF) value is 39.4 times higher than that of FeCoSn(OH)₆. Moreover, the enhanced OER performance and the amorphization process are investigated with density functional theory (DFT) and molecule dynamics (MD) simulations. The reported top-down fabrication strategy for fabricating amorphous oxides, may further promote fundamental research into and practical applications of amorphous materials for catalysis.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Synchrotron X-ray Spectroscopic Investigations of In-Situ-Formed Alloy Anodes for Magnesium Batteries

Magnesium batteries present high volumetric energy density and dendrite-free deposition of Mg, drawing wide attention in energy-storage devices. However, their further development remains stagnated due to relevant interfacial issues between the Mg anode and the electrolyte and sluggish solid-state diffusion kinetics of Mg²⁺ ions. Herein, an in situ conversion chemistry to construct a nanostructured Bi anode from bismuth selenide driven by Li⁺ is proposed. Through the combination of operando synchrotron X-ray diffraction, ex situ synchrotron X-ray absorption spectroscopy, and comprehensive electrochemical tests, it is demonstrated that the nanosize of the in-situ-formed Bi crystals contributes to the fast Mg²⁺ diffusion kinetics and highly efficient Mg-Bi alloingy/de-alloying. The resultant Bi anodes exhibit superior long-term cycling stability with over 600 cycles under a high current density of 1.0 A g^-1 . This work provides a new approach to construct alloy anode and paves the way for exploring novel electrode materials for magnesium batteries.

Oxygen Loss in Layered Oxide Cathodes for Li-Ion Batteries: Mechanisms, Effects, and Mitigation

Layered lithium transition metal oxides derived from LiMO₂ (M = Co, Ni, Mn, etc.) have been widely adopted as the cathodes of Li-ion batteries for portable electronics, electric vehicles, and energy storage. Oxygen loss in the layered oxides is one of the major factors leading to cycling-induced structural degradation and its associated fade in electrochemical performance. Herein, we review recent progress in understanding the phenomena of oxygen loss and the resulting structural degradation in layered oxide cathodes. We first present the major driving forces leading to the oxygen loss and then describe the associated structural degradation resulting from the oxygen loss. We follow this analysis with a discussion of the kinetic pathways that enable oxygen loss, and then we address the resulting electrochemical fade. Finally, we review the possible approaches toward mitigating oxygen loss and the associated electrochemical fade as well as detail novel analytical methods for probing the oxygen loss.

Antiperovskite Electrolytes for Solid-State Batteries

Solid-state batteries have fascinated the research community over the past decade, largely due to their improved safety properties and potential for high-energy density. Searching for fast ion conductors with sufficient electrochemical and chemical stabilities is at the heart of solid-state battery research and applications. Recently, significant progress has been made in solid-state electrolyte development. Sulfide-, oxide-, and halide-based electrolytes have been able to achieve high ionic conductivities of more than 10^-3 S/cm at room temperature, which are comparable to liquid-based electrolytes. However, their stability toward Li metal anodes poses significant challenges for these electrolytes. The existence of non-Li cations that can be reduced by Li metal in these electrolytes hinders the application of Li anode and therefore poses an obstacle toward achieving high-energy density. The finding of antiperovskites as ionic conductors in recent years has demonstrated a new and exciting solution. These materials, mainly constructed from Li (or Na), O, and Cl (or Br), are lightweight and electrochemically stable toward metallic Li and possess promising ionic conductivity. Because of the structural flexibility and tunability, antiperovskite electrolytes are excellent candidates for solid-state battery applications, and researchers are still exploring the relationship between their structure and ion diffusion behavior. Herein, the recent progress of antiperovskites for solid-state batteries is reviewed, and the strategies to tune the ionic conductivity by structural manipulation are summarized. Major challenges and future directions are discussed to facilitate the development of antiperovskite-based solid-state batteries.