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Sodium metal battery (SMB) technology is one of the most promising candidates for next-generation rechargeable energy storage systems due to its high theoretical capacity and economical cost-effectiveness. Unfortunately, its practical implementation is hindered by several challenges including short life-span and fast capacity decay, which is closely related to the uncontrollable generation of the sodium dendrites. Herein, a nitrogen and oxygen co-doped three-dimensional carbon cloth with hollow tubular fiber units was constructed as the host material for Na plating (Na@CC) to tackle these challenges. The obtained composite electrode can effectively reduce the nucleation overpotential of Na, guide the homogeneous Na+ flux, increase the kinetics of Na electrodeposition, lower the effective current density and eventually suppress the formation of electrochemically inactive Na dendrites. As a result, batteries built with the Na@CC composites exhibited stable long-term cycling stability. To gain an in-depth and comprehensive understanding of such phenomena, non-destructive and three-dimensional synchrotron X-ray tomography was employed to investigate the cycled batteries. Moreover, the COMSOL Multiphysics simulation was further employed to reveal the Na electrodeposition mechanisms. The current work not only showcases the feasibility of currently proposed sodiophilic 3D Na@CC composite electrode but also provides fundamental insights into the underlying working mechanisms that govern its outstanding electrochemical performance.

Understanding how gas diffusion electrodes are working is crucial to improve their performance and cost efficiency. One key issue is the electrolyte distribution during operation. Here, operando synchrotron imaging of the electrolyte distribution in silver based gas diffusion electrodes is presented. For this purpose, a half cell compartment was designed for operando synchrotron imaging of chronoamperometric measurements. For the first time, the electrolyte distribution could be analyzed in real time 1 s time resolution even in individual pores as small as a few micrometers. The detailed analyses of dynamic filling processes are an important step for understanding and improving electrodes

Operando phase-contrast radiography combined with impedance spectroscopy and electron microscopy is applied to study the morphological changes in a lithium-silicon cell over several cycles. The single-crystal silicon (100) surface is employed as a working electrode. A checkerboard-like cracking pattern aligned with the crystallographic axis is formed in the 4th cycle during the second step of the delithiation. With the crack position kept fixed, this crack pattern vanishes during lithiation and gets more pronounced in the subsequent cycles. This pattern forms just after the first delithiation peak that corresponds to the decomposition of the highly lithiated phase. It vanishes during lithiation in the potentiostatic step. Therefore, capacity-limited cycling, which partly maintains the highly lithiated phase, may avoid the formation of fractures. The surface area of the crack cells remains almost constant with increasing cycle numbers, and no electrochemically inactive sites were observed.

Herein, we present a detailed investigation of the electrochemically triggered formation and dissolution processes of α- and β-sulfur crystals on a monolithic carbon cathode using operando high-res...

Lithium sulfur batteries are one of the most promising next-generation energy storage technologies, because of their impressive theoretical energy density, low materials cost, and relative safety. ...

Understanding the complicated interplay of the continuously evolving electrode materials in their inherent 3D states during the battery operating condition is of great importance for advancing rechargeable battery research. In this regard, the synchrotron X-ray tomography technique, which enables non-destructive, multi-scale, and 3D imaging of a variety of electrode components before/during/after battery operation, becomes an essential tool to deepen this understanding. The past few years have witnessed an increasingly growing interest in applying this technique in battery research. Hence, it is time to not only summarize the already obtained battery-related knowledge by using this technique, but also to present a fundamental elucidation of this technique to boost future studies in battery research. To this end, this review firstly introduces the fundamental principles and experimental setups of the synchrotron X-ray tomography technique. After that, a user guide to its application in battery research and examples of its applications in research of various types of batteries are presented. The current review ends with a discussion of the future opportunities of this technique for next-generation rechargeable batteries research. It is expected that this review can enhance the reader's understanding of the synchrotron X-ray tomography technique and stimulate new ideas and opportunities in battery research.

Inherently uncontrollable Li electrodeposition has significantly hindered the practical application of Li metal batteries largely due to a dendritic deposition which can initiate an internal short circuit and gives rise to severe safety issues. The understanding of the fundamental electrodeposition mechanism is, however, elusive and limited due to a lack of feasible in situ characterization techniques. Here synchrotron X-ray tomography was employed to noninvasively visualize Li deposition at the lithium/separator and lithium/carbon matrix interregion. A higher concentration of widely distributed deposition sites was observed under an increased current density. The 3D morphology and distribution of deposited Li within the widely used Celgard® 2325 polyolefin separator are, for the first time, visualized in situ, thus promoting the understanding of the short-circuiting process of Li metal batteries. In addition, we also visualized and quantified the spatial distribution of Li depositions inside a porous carbon host to unravel the deposition behavior that can hardly be probed by surface imaging techniques. The Li electrodeposition behavior found here could help to promote the understanding and development of surface modifications related to Li anodes, separators as well as novel 3D geometry electrode designs for accommodation of Li depositions and alleviation of volumetric changes.

A possible way to increase the energy density in lithium-ion batteries, and, at the same time, reduce the production costs, is to use thicker electrodes. However, transport limitations can occur in thick electrodes, leading to a drawback in performance. A way to mitigate this problem is a more sophisticated microstructure of the electrode, using, e.g., structural gradients. This can, for instance, be achieved by multi-layer casting, i.e., casting and drying of a first layer, and then adding a second layer. An important question is how the interface between the two layers is shaped and how the corresponding microstructure influences the electrochemical performance. In the present paper, two different two-layer cathodes are analyzed and compared to single-layer cathodes of the same thickness. The analysis involved tomographic imaging, a statistical analysis of the 3D microstructure of the active material particle systems with a focus on the interface between the layers, and electrochemical characterization of the active material systems using experimental measurements as well as electrochemical simulations. The analysis showed that at the interface the connectivity of active material particles decreases, which results in higher electric resistivity. This effect is stronger if an intermediate calendering step is performed, i.e., the first layer is calendered before casting the second layer.

An important step in the analysis of liquid water distributions in high resolution tomographies of PEMFCs is the segmentation of water and cell materials. This is conducted via subtraction of the dry from the operated state of the cells. The morphology of the gas diffusion layer (GDL) slightly changes due to membrane swelling at high humidification that makes quantification of water amounts hardly possible and leads to artifacts and false water detection. Here we present a computational algorithm identifying the changes in GDL morphology via local shift detection of the tomographic data sets that allows for an optimal quantification of water amounts. We compare the technique with typical global alignment methods. The results reveal huge errors in water quantification of up to 50% for the global alignment method that can be mostly avoided by using the presented technique. The morphology correction algorithm can basically be applied to many research fields, where operando measurements are important. This applies for a wide variety of materials employed in most types of fuel cells, electrolyzer cells, batteries and hydrogen storage systems.

A pluri Gaussian model for three phase microstructures is presented and relationships between model parameters and microstructure characteristics are discussed. In particular, analytical formulas for two point coverage probability functions in terms of covariance functions of the underlying Gaussian random fields are considered, which allow for an efficient estimation of model parameters. The model is fitted to tomographic image data obtained by FIB tomography, which represent porous gas diffusion electrodes consisting of silver and polytetrafluorethylene. The considered type of electrode is used as oxygen depolarized cathode for the production of chlorine. In order to fit the microstructure model, the covariance functions of the Gaussian random fields are parameterized, which leads to a stochastic microstructure model with five parameters. It is shown that most microstructure characteristics of tomographic image data are well reproduced by the model despite the low number of model parameters. Finally, limitations of the model with respect to the fit of continuous phase size distributions are discussed. Combining stochastic microstructure modeling with numerical simulation of effective macroscopic properties will allow in future work for a model based investigation of microstructure property relationships for the considered gas diffusion electrodes

While X-ray based darkfield imaging with grating interferometers is already widely used, darkfield imaging with neutrons has still a relatively small user community focused mostly on magnetic materials. Here, we demonstrate the application of neutron darkfield imaging byTalbot-Lau type grating interferometry to fiber reinforced plastics. Common carbon and glass fiber composites have been investigated including characteristic damage structures. The darkfield images show a strong signal response caused by fiber delamination, suitable fiber direction, particles, pores and cracks. The basic principles of neutron darkfield imaging applied to fiber composites are highlighted.

Energy storage materials 38, 80 - 92 (2021). doi:10.1016/j.ensm.2021.02.029, Electrolyte filling takes place between sealing and formation in Lithium Ion Battery (LIB) manufacturing process. This step is crucial as it is directly linked to LIB quality and affects the subsequent time consuming electrolyte wetting process. Although having fast, homogeneous and complete wetting is of paramount importance, this process has not been sufficiently examined and fully understood. For instance, experimentally available data is insufficient to fully capture the complex interplay upon filling between electrolyte and air inside the porous electrode. We report here for the first time a 3D-resolved Lattice Boltzmann Method (LBM) model able to simulate electrolyte filling upon applied pressure of LIB porous electrodes obtained both from experiments (micro X-ray tomography) and computations (stochastic generation, simulation of the manufacturing process using Coarse Grained Molecular Dynamics and Discrete Element Method). The model allows obtaining advanced insights about the impact of the electrode mesostructures on the speed of electrolyte impregnation and wetting, highlighting the importance of porosity, pore size distribution and pores interconnectivity on the filling dynamics. Furthermore, we identify scenarios where volumes with trapped air (dead zones) appear and evaluate the impact of those on the electrochemical behavior of the electrodes., Published by Elsevier, Amsterdam

Nano energy 82, 105762 (1-5) (2021). doi:10.1016/j.nanoen.2021.105762, From non-destructive and cross-sectional visualization by synchrotron X-ray tomography, we suggest a novel capacity decaying mechanism of all-solid-state Na batteries built with closo-borate electrolytes caused by electrochemically generated inactive Na electrodeposits., Published by Elsevier, Amsterdam [u.a.]

Complex plant-soil interactions can be visualized and quantified by combined application of different non-invasive imaging techniques. Oxygen, carbon dioxide and pH gradients in the rhizosphere can be observed with fluorescent planar optodes, while neutron radiography detects small-scale heterogeneities in soil moisture and its dynamics. Respiration and exudation rates can vary between roots of different types, such as primary and lateral roots, as well as along single roots among the same plant. The 3D root system architecture is therefore a key information when studying rhizosphere processes. It can be captured in detail with neutron tomography, but so far only for plants grown in small, cylindrical containers.Combined non-invasive imaging of biogeochemical dynamics, soil moisture distribution and 3D root system architecture is a technical challenge. Thin, slab-shaped rhizotrons with relatively large vertical and lateral extension are well suited for optical fluorescence imaging, allowing for spatially extended observation of biogeochemical patterns. This rhizotron geometry is, however, unfavorable for standard 3D tomography due to reconstruction artefacts triggered by insufficient neutron transmission when the long side of the sample is aligned parallel to the beam direction.We therefore applied neutron laminography, a method where the rotational axis is tilted, to measure the root systems of maize and lupine plants grown in slab-shaped glass rhizotrons (length = 150 mm, width = 150 mm, depth = 15 mm) in 3D. In parallel, we investigated rhizosphere oxygen dynamics and pH value via fluorescence imaging and assessed soil moisture distribution with neutron radiography.Neutron laminography enabled the 3D reconstruction of the root systems with a nominal spatial resolution of 100 µm/pixel. Reconstruction quality strongly depended on root-soil contrast and hence soil moisture level. After reconstruction of the root system and co-registration with the fluorescence images, first results indicate that observed oxygen concentrations and pH gradients depend on root type and individual distance of the roots from the planar optode.In conclusion, neutron laminography is a novel 3D imaging method for root-soil systems grown in slab-shaped rhizotrons. The method allows for determining the precise 3D position of individual roots within the rhizotron and can be combined with 2D imaging approaches. Following experiments will address X-ray laminography as a possible attractive further application.

Although a great variety of strategies to suppress Li dendrite have been proposed for lithium metal batteries (LMBs), a deeper understanding of the factors playing a crucial role during extended el...

ACS applied energy materials 3(12), 12565 - 12574 (2020). doi:10.1021/acsaem.0c02494, Commercially used LiNi$_{1/3}$Mn$_{1/3}$Co$_{1/3}$O$_2$ (NMC111) in lithium-ion batteries mainly consists of a large-grained nonporous active material powder prepared by coprecipitation. However, nanomaterials are known to have extreme influence on gravimetric energy density and rate performance but are not used at the industrial scale because of their reactivity, low tap density, and diminished volumetric energy density. To overcome these problems, the build-up of hierarchically structured active materials and electrodes consisting of microsized secondary particles with a primary particle scale in the nanometer range is preferable. In this paper, the preparation and detailed characterization of porous hierarchically structured active materials with two different median secondary particle sizes, namely, 9 and 37 μm, and primary particle sizes in the range 300–1200 nm are presented. Electrochemical investigations by means of rate performance tests show that hierarchically structured electrodes provide higher specific capacities than conventional NMC111, and the cell performance can be tuned by adjustment of processing parameters. In particular, electrodes of coarse granules sintered at 850 °C demonstrate more favorable transport parameters because of electrode build-up, that is, the morphology of the system of active material particles in the electrode, and demonstrate superior discharge capacity. Moreover, electrodes of fine granules show an optimal electrochemical performance using NMC powders sintered at 900 °C. For a better understanding of these results, that is, of process-structure–property relationships at both granule and electrode levels, 3D imaging is performed with a subsequent statistical image analysis. Doing so, geometrical microstructure characteristics such as constrictivity quantifying the strength of bottleneck effects and descriptors for the lengths of shortest transportation paths are computed, such as the mean number of particles, which have to be passed, when going from a particle through the active material to the aluminum foil. The latter one is at lowest for coarse-grained electrodes and seems to be a crucial quantity., Published by ACS Publications, Washington, DC

Most cathode materials for lithium-ion batteries exhibit a low electronic conductivity. Hence, a significant amount of conductive graphitic additives are introduced during electrode production. The mechanical stability and electronic connection of the electrode is enhanced by a mixed phase formed by the carbon and binder materials. However, this mixed phase, the carbon binder domain (CBD), hinders the transport of lithium ions through the electrolyte pore network. Thus, reducing the performance at higher currents. In this work we combine microstructure resolved simulations with impedance measurements on symmetrical cells to identify the influence of the CBD distribution. Microstructures of NMC622 electrodes are obtained through synchrotron X-ray tomography. Resolving the CBD using tomography techniques is challenging. Therefore, three different CBD distributions are incorporated via a structure generator. We present results of microstructure resolved impedance spectroscopy and lithiation simulations, which reproduce the experimental results of impedance spectroscopy and galvanostatic lithiation measurements, thus, providing a link between the spatial CBD distribution, electrode impedance, and half-cell performance. The results demonstrate the significance of the CBD distribution and enable predictive simulations for battery design. The accumulation of CBD at contact points between particles is identified as the most likely configuration in the electrodes under consideration., publishedVersion

The gap between the successful application and perspective promise of lithium–oxygen battery (LOB) technology should be filled by an in-depth and comprehensive understanding of the underlying working and degradation mechanisms. Herein, the correlation between the morphological evolution of the Li anode and the overall cell electrochemical performance of cycled LOBs has been revealed for the first time by complementary X-ray and neutron tomography, together with further postmortem scanning electron microscopy, X-ray diffraction, and Fourier transfrom infrared spectroscopy characterizations. It has been disclosed with solid evidence that the irreversible transformation of anode Li associated with chemical/electrochemical side reactions does link intimately to the observed electrochemical performance decay. The current discoveries not only fundamentally enrich our knowledge of the underlying degradation and failure mechanisms of LOBs but also provide direction for future promising research activities aimed at further enhancing their performance.

This manuscript presents a detailed characterization of the cellular structure of two types of polymeric foams by non-destructive multi-scale X-ray computed micro-tomography. This comparative study is conducted in two different polymeric materials –rigid polyurethane foams and cross-linked low density polyethylene foams-. Based on this technique and using 3D image analysis, conventional descriptors of the foams gaseous phase (cell size distribution, mean cell size and anisotropy ratio) are characterized at a standard voxel size of 5 µm. Complementarily, 0.4 µm voxel size synchrotron X-ray tomography data sets have been used to characterize, using a new image analysis approach, features of the solid phase of the foams (fraction of mass in the struts and thickness of the different entities of the solid architecture: struts and walls). The presented methodology, based on multi-scale tomographic analysis, allows obtaining a better understanding of the connection between foaming process and cellular structure and is important to gain knowledge on the structure-properties relationships for these materials.

Solid-state hydrogen storage in metal hydrides offers highest volumetric energy storage densities and low working gas pressures at the same time. Recently developed metal hydride composites (MHC) consist of a hydride-forming metal alloy and a secondary phase, typically graphite, to realize form-stable composites and short loading and unloading times ( This work focuses on the in-operando characterization of the volume expansion of MHC that could trigger mechanical stresses acting on the walls and internal assemblies of the storage container. MHC with different metal particle shapes (flakes and powder) were studied. In-operando neutron imaging of axially freely expanding MHC was applied to analyze the time-resolved and spatial concentration of hydrogen, reaction fronts and the evolution of volume expansion and stability of the MHC. Stress measurements revealed that stresses of up to 330% of the respective operating gas pressure occur for confined MHC. Both techniques combined deliver crucial information and implications for the design of safe (according to ISO 16111), efficient and dynamic metal hydride storage systems.

Imaging techniques based on neutron beams are rapidly developing and have become versatile non-destructive analyzing tools in many research fields. Due to their intrinsic properties, neutrons differ strongly from electrons, protons or X-rays in terms of their interaction with matter: they penetrate deeply into most common metallic materials while they have a high sensitivity to light elements such as hydrogen, hydrogenous substances, or lithium. This makes neutrons perfectly suited probes for research on materials that are used for energy storage and conversion, e.g., batteries, hydrogen storage, fuel cells, etc. Moreover, their wave properties can be exploited to perform diffraction, phase-contrast and dark-field imaging experiments. Their magnetic moment allows for resolving magnetic properties in bulk samples. This review will focus on recent applications of neutron imaging techniques in both materials research and fundamental science illustrated by examples selected from different areas.

Efficient Li utilization is generally considered to be a prerequisite for developing next-generation energy storage systems (ESSs). However, uncontrolled growth of Li microstructures (LmSs) during electrochemical cycling has prevented its practical commercialization. Herein, we attempt to understand the correlation of morphological evolution of Li electrodes with degrading electrochemical performances of Li/LiCoO2 and Li/S systems by synchrotron X-ray phase contrast tomography technique. It was found that the continuous transformation of the initial dense Li bulk to a porous lithium interface (PLI) structure intimately correlates with the gradually degrading overall cell performance of these two systems. Additionally, the formation mechanism of the PLI and its correlation with previously reported inwardly growing LmS and the lithium-reacted region have been intensively discussed. The information that we gain herein is complementary to previous investigations and may provide general insights into understan...

Complementary in operando X-ray radiography and neutron radiography measurements were conducted to investigate and visualize the initial lithiation in silicon-electrode lithium-ion batteries. By means of X-ray radiography, a significant volume expansion of Si particles and the Si electrode during the first discharge was observed. In addition, many Si particles were found that never undergo electrochemical reactions. These findings were confirmed by neutron radiography, which, for the first time, showed the process of Li alloying with the Si electrode during initial lithiation. These results demonstrate that complementary X-ray and neutron radiography is a powerful tool to investigate the lithiation mechanisms inside Si-electrode based lithium-ion batteries.

Porous carbon felts (CFs) are widely used electrode materials for vanadium redox flow batteries (VRFBs). These materials differ in their precursor material, thickness, or graphitization degree and demonstrate broad differences in electrochemical performance. Prior to operation, an activation step, such as acid or heat treatment (HT), is commonly performed to improve their performance. A thermal treatment in air functionalizes the surface of the electrode and improves reaction kinetics as well as the wettability of the electrode. Herein, pristine and heat‐treated CFs are compared regarding their electrolyte wetting behavior for the use in VRFB. Contact angle (CA) measurements are conducted ex situ to investigate the effect of the HT. Furthermore, the porous CFs are examined in situ with an in‐house‐built flow cell regarding their invasion behavior with different types of electrolytes by X‐ray radiography. Additionally, the distribution of the electrolyte inside the felts is investigated by X‐ray tomography. The results demonstrate the effect of the HT and choice of electrolyte on the wetting behavior and electrolyte distribution.

Operando laboratory X-ray radiographies were carried out for imaging of two different silver-based gas diffusion electrodes containing an electroconductive Ni mesh structure, one gas diffusion electrode composed of 95 wt.% Ag and 5 wt.% polytetrafluoroethylene and one composed of 97 wt.% Ag and 3 wt.% polytetrafluoroethylene, under different operating parameters. Thereby, correlations of their electrochemical behavior and the transport of the 30 wt.% NaOH electrolyte through the gas diffusion electrodes were revealed. The work was divided into two parts. In the first step, the microstructure of the gas diffusion electrodes was analyzed ex situ by a combination of focused ion beam technology and synchrotron as well as laboratory X-ray tomography and radiography. In the second step, operando laboratory X-ray radiographies were performed during chronoamperometric measurements at different potentials. The combination of the ex situ microstructural analyses and the operando measurements reveals the impact of the microstructure on the electrolyte transport through the gas diffusion electrodes. Hence, an impact of the Ni mesh structure within the gas diffusion electrode on the droplet formation could be shown. Moreover, it could be observed that increasing overpotentials cause increasing electrolyte transport velocities and faster droplet formation due to electrowetting. In general, higher electrolyte transport velocities were found for the gas diffusion electrode with 97 wt.% Ag in contrast to that with 95 wt.% Ag.

The influences of the polytetrafluoroethylene PTFE content in silver based gas diffusion electrodes on the resulting physical properties and the electrochemical performance during oxygen reduction in concentrated sodium hydroxide electrolyte were investigated through half cell measurements. A systematic variation of the pore system was achieved by application of different silver PTFE ratios during the production of the gas diffusion electrodes GDE . In all electrodes, a silver skeleton structure with relatively constant properties was formed, while the PTFE fills up part of the open pore space. The resulting structures were characterized with a variety of methods for the physical properties supported by focused ion beam milling and scanning electron microscope FIB SEM tomography. It could be shown that variations in the obtained pore system strongly influence the electrochemical performance of the electrodes. Determination of the Tafel slopes revealed that this is not due to changes in the electrocatalytic activity but rather caused by variations in the electrolyte uptake. While too small amounts of PTFE 1 wt lead to decreased performance through electrolyte flooding, higher PTFE contents above about 5 wt also deteriorate the electrode performance because the extent of the three phase boundary diminishes. The decisive role of the electrolyte intrusion was confirmed by measurements at higher electrolyte pressure. While the best electrochemical performance was achieved with an electrode containing 98 wt silver, a slightly higher PTFE content is advisable to prevent breakthrough of the electrolyte

Producción Científica, This manuscript presents the effect of the solid phase corrugation of low densityflexible cellular polymers onboth their mechanical and thermal properties. First, a detailed quantification of solid phase corrugation hasbeen carried out by means of high resolution synchrotron micro-tomography in a collection of polyethylenefoams. Subsequently, the collapse stress in compression has been analysed both from the theoretical and exper-imental points of view achieving a clear relation between the solid phase corrugation and the ratio of theoreti-cally calculated and experimentally measured collapse stress. Finally, solid phase corrugation has beencorrelated with the thermal expansion coefficient at room temperature., Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (project MAT2015-69234-R), Junta de Castilla y Leon (project VA011U16)

Materials today 27, 21 - 32 (2018). doi:10.1016/j.mattod.2018.11.003, An intrinsic knowledge gap between current understandings obtained experimentally and the underlying working or degradation mechanisms of rechargeable lithium batteries still remains, giving direct rise to application challenges, e.g., safety issues, predicaments in identifying performance-aging factors and dilemmas in guiding further research directions. Against this background, non-destructive and three-dimensional (synchrotron) X-ray tomography that guarantees a direct visual access to inner electrodes has been employed herein to: in-situ record the evolution of internal short circuits; characterize the behaviors of widely employed separators; investigate the morphological evolution of Li electrodes under different cycling conditions; and study the degradation mechanisms of Li/carbon cells. By incorporating the currently presented results with the previously published studies on those topics, a complete picture of the degradation mechanism of rechargeable lithium batteries has been painted. This advancement of mechanistic understanding supplies the missing pieces of information to bridge fundamental R&D research activities and practical applications., Published by Elsevier Science, Amsterdam [u.a.]

Nano energy 83, 105841 (2021). doi:10.1016/j.nanoen.2021.105841, Rechargeable potassium (K) batteries are a promising next-generation technology for low-cost grid scale energy storage applications. Nevertheless, the undesirable interfacial instabilities originating from the interplay between the employed separators and electrodes largely compromise the battery’s performance, and the underlying mechanism of which remains elusive. Herein, the interfacial stability between three types of commercial separators (Celgard 2325, Celgard 2400 and GF/D) and the K electrodeposits is investigated in K|K symmetric cells via in-situ Synchrotron X-ray tomography technique. It is demonstrated that the cell built with a Celgard 2400 separator can achieve a stable cycling performance due to its high mechanical strength and integrity along the thickness direction, thus alleviating the K dendrites growth. In contrast, a GF/D membrane of low mechanical cohesion and excessive porosity is found to be easily deformed and filled with deciduous potassium dendritic aggregates during battery cycling. Similarly, the tri-layer Celgard 2325 separators, which are weakly bonded by interlaminar forces, are found to be severely delaminated by the overgrowth of K dendrites. Furthermore, it is revealed that the delamination failure behaviors of Celgard 2325 is driven by the local stress induced by the spatially and heterogeneously formed "dead" K dendrites. Our work provides direct visualization of morphological evolvement of the separators in presence of potassium dendrites in K|K symmetric cells and highlights the significance of mechanical cohesion, porosity distribution and mechanical integrity of separators in dictating the battery’s performance under realistic battery operation conditions. As a result, these discoveries provide an in-depth understanding that is needed to design next-generation high performance separators to mitigate the formation of potassium dendrite in KMBs., Published by Elsevier, Amsterdam [u.a.]

Combined three-dimensional (3D) mapping of (micro-)structures with elemental and crystalline phase variations is of significant importance for the characterization of materials. Neutron wavelength selective imaging is a spectral imaging technique that exploits unique contrast differences e.g. for mapping dissimilar elemental, isotope, or phase compositions, and has the particular advantage of being applicable to sample volumes on the meso- and macroscale. While being mostly applied as radiography (2D) so far, we herein report that the extension to tomography allows for the display of the full spectral information for every voxel and in 3D. The development is supported by example data from a continuous as well as a pulsed neutron source. As a practical example, we collected 4D data sets (3D + spectral) of plastically deformed metastable stainless steel and herein demonstrate an improved quantification strategy for crystalline phase fractions. These exemplary results illustrate that localized phase transformations can be quantified even in complex geometries within centimeter-sized samples, and we will discuss the limits and future prospects of the technique that is not limited to crystalline materials.

Knowledge about degradation and failure of Li ion batteries LIBs is of paramount importance, especially because failure can be accompanied by severe hazards. To contribute to the understanding of such phenomena, synchrotron in line phase contrast X ray tomography was employed to investigate internal cell deformation and degradation caused by an internal short circuit ISC . The tomographic images taken from an uncycled Li Li cell and a short circuited Li Li cell reveal how lithium microstructures LmSs develop during electrochemical stripping and plating during discharge and charge and how the three layer separator used is damaged by growing LmSs and delaminates and melts as a consequence of an ISC. Previously unknown insights into the internal cell degradation and deformation mechanisms caused by an ISC are obtained and provide hints of how the properties of the separator could be modified to improve the reliability and safety of current and next generation LIBs

Gas generation within lithium ion batteries (LIBs) gives rise to safety concerns that question their applicability. By employing synchrotron X-ray imaging, the gas and channel evolution occurring in an operating LIB have been directly visualized in their inherent 3D state as a function of discharge and charge. Using the spatial 3D distribution of gas bubbles and channels, the active particles that dictate the performance of a functional LIB were identified and visualized in 3D. Delithiation and lithiation are interpreted as the process of activating particles continuously in a step-by-step way. The present work not only demonstrates the generation and evolution of gas within LIB in 3D, but also reveals the distribution of active particles for the first time. These fundamentally findings presented here shed light on a range of processes that could not previously be characterized in 3D and can provide practical guidance for the design of next-generation LIBs with improved safety.

Decorating carbon felt electrodes with bismuth and bismuth oxide nanoparticles is proposed to be a promising strategy for enhancing the sluggish electron transfer kinetics of the negative half-cell reaction in vanadium redox flow batteries. However, regarding the highly corrosive electrolyte solution, major concerns on the stability of the solid bismuth phase and thus on the local catalyst distribution and catalyst functionality emerge. With this study we present a novel in-operando cell design allowing for X-ray imaging during battery tests in laboratory scale dimensions. In this manner, we verify a dissolution of the bismuth/bismuth oxide particles in open circuit potential condition at SOC = 0 (SOC: state of charge) as well as a re-deposition during the charging process of the battery. No dissolution during the discharging process can be detected. With this knowledge, the effect of the bismuth/bismuth oxide distribution on the electrochemical performance of the battery is investigated. By performing the deposition during the charging process at different electrolyte flow rates, different deposition patterns are obtained. However, independent of the deposition pattern, similar cell performances are achieved that put the enhancement effect by bismuth as heterogeneous catalyst into question.

A thin membrane (≈47 μm) of a Celgard separator-reinforced cross-linked LiTFSI-PEO-Pyr14TFSI-LLZO composite polymer electrolyte (CgCPE) was prepared and investigated for its ability to suppress the formation and propagation of high surface area lithium (HSAL) with dendritic morphology. The membrane shows superior cycling stability in Li||Li-symmetric cells under high capacity utilization (5 mAh/cm2) for up to 2000 h. Using a combination of conventional laboratory techniques and nondestructive synchrotron X-ray tomography, the synergetic effect of the membrane components towards homogenous lithium electrodeposition/dissolution was revealed. The modulated ion migration in CgCPE and decreased interfacial/interphasial resistance ensures an evenly regulated Li-ion flux during deposition. While, the good mechanical properties of the CgCPE membrane were maintained up to a capacity utilization of 15 mAh/cm2. Based on the results, an approach for a hybrid solid electrolyte to control lithium electrodeposition for Li metal electrodes with high capacity utilization is provided, and furthermore, such an approach could be further used for lithium metal batteries with high energy density and safety.

A novel four-step strategy for real-space simulation of cyclic voltammetry (CV) at carbon felt electrodes is presented, circumventing the diffusion domain approximation approach used so far for CV simulation at porous electrodes. At first, the three-dimensional template of the internal electrode structure is constructed from micro X-ray tomography measurements. Subsequently, by exploiting the Douglas–Gunn modification of the three-dimensional Crank–Nicolson algorithm to Cottrellian boundary conditions, the mass transfer controlled current of this ”true” network is obtained. Based on this current, the third step is to compute the mass transfer functions related to the electrode under investigation by an inverse convolution algorithm. In this manner, the spatial dimensionality of the system is reduced from three to one, resulting in significant savings in computation time. The fourth and final step is then to simulate CV experiments via classical convolution methods, featuring the great advantage that any degree of electrochemical reversibility, coupled homogeneous reactions, electrolyte resistances and double layer capacities can be implemented readily. As a proof of concept, the simulations are supported by experimental data acquired for the oxidation of VO2+ in carbon felt electrodes.

Cyclic voltammetry (CV) is the prevalently used technique for investigating the kinetic performance of carbon felt electrodes for application as porous electrocatalysts in vanadium redox-flow batteries. However, the theoretical treatment of internal diffusion-reaction phenomena of such electrodes, allowing for quantitative data evaluation, is still unexplored and mostly relies on diffusion domain approximations assuming the felt as an array of either planar, spherical or cylindrical microelectrodes in a randomly distributed and finite diffusion domain. With this study, we present a novel three-step strategy for real-space CV simulation of felt electrodes. By using micro x-ray tomography (CT) data of the felts, we obtain a locally resolved 3D template of the diffusion space making any diffusion domain approximations obsolete. CV simulation involving this real diffusion domain template is performed by combining the implicit Crank-Nicolson technique in 3D with convolutive modelling. At first, by utilizing the Crank-Nicolson method, we obtain the chronoamperometric current of the 3D porous network for Cottrell-like Dirichlet boundary conditions. Subsequently, the mass transfer functions, usually generated with the aid of Laplace integral transformation techniques, are calculated from these chronoamperometric currents. Then, CV simulation is performed on the basis of these mass transfer functions involving classical convolution integrals. This strategy offers three significant advantages. First, we avoid the difficulty of implementing non-Dirichlet boundaries in the matrices of the Crank-Nicolson scheme. Second, the difficulty of performing an inverse Laplace transformation is avoided since no direct Laplace transformation step is involved. Third, once the mass transfer functions are obtained the simulation can be done with significant savings in time since the 3D network is converted into a one-dimensional problem that can be solved via classical convolution integrals. Our simulation is supported by experimental data acquired for the VO2+-oxidation reaction underlining the validity of our approach. Figure 1

Carbon-based, high surface area gas diffusion electrodes (GDEs) are commonly used as cathodes in Li-O2 batteries. These GDEs provide an sufficient amount of active sites to deposit the main discharge product, Li2O2. Despite the fast electron transfer at the electrode surface to form Li2O2, experimental results show that the very high theoretical energy densities of Li-O2 batteries cannot be achieved at the moment. Instead of the desired formation of high amounts of soluble LiO2, predominantly insoluble Li2O2 precipitates at the active sites of the cathode during discharge. Unfortunately Li2O2 is poorly conductive for electrons, which leads to a low amount of deposited Li2O2 per active site and a fast passivation of the cathode. Thus, only the cathode surface area and not the pore volume is utilized by Li2O2, which explains the discrepancy between theory and experiments. Furthermore, the precipitation leads to continuous changes in porosity, available active surface area and predominant reaction pathway. For this reason a detailed analysis of the distribution of Li2O2 at all points in the GDE and of the factors that influence the Li2O2 morphology is needed to overcome performance limitations. [1] In this work, we elucidate how the local distribution of Li2O2 inside the GDE and its particle size evolve as a function of discharge current density. We apply a powerful combination of experimental and model-based analysis. To the best of our knowledge, this is the first study on the cathode surface utilization by Li2O2 and its particle sizes performed for different locations and states of discharge (SOD). The impact of the distribution and the resulting changes in transport resistance and active surface area on the battery performance are derived thereof. In the experimental part, decreasing capacities and Li2O2 particle sizes are observed for increasing discharge current densities (see SEM analysis of discharged battery cathodes in fig. 1 a)). Furthermore, the experimental results show that particle precipitation starts mainly at the side of the cathode that faces the oxygen reservoir of the battery (see fig. 1 b)). Discharging a battery at low current densities leads to a uniform cathode surface coverage by Li2O2 even at low SOD whereas discharging at high current densities yields a gradient of blocked active sites through the cathode. Simulations based on a physical GDE model show that a two-step reaction mechanism with the soluble species LiO2 as reaction intermediate can quantitatively explain the experimental findings. Depending on the current density, either a chemical or an electrochemical particle growth process predominates, which leads to the distinct different particle size distributions (see fig. 1 c) and d)). The strong dependence of capacity on the current density is related to different capacities per carbon cathode surface area at the end of discharge. At high discharge rates and thereby lowered potential more nucleation takes place. As a result, the particle number is higher and the average size smaller. Due to the surface volume ratio, smaller particles entail lower capacities per cathode surface area. The results point out that even at moderate current densities the battery capacity is limited by the surface area of the GDE and not by the concentration of dissolved O2 in the liquid electrolyte. Therefore, the solubility and reaction kinetics of the reaction intermediate LiO2 play a crucial role to enhance Li2O2 particle size and with it the obtainable discharge capacity. The presented results provide detailed insight into the cathode surface utilization and the underlying processes that limit the performance of GDEs in Li-O2 batteries. In the end, the study will help to achieve higher discharge capacities for this type of battery and thus will propel the ongoing research and the efforts in commercialization of metal-oxygen batteries. Acknowledgement: The authors are thankful for financial support by the BMBF (Federal Ministry of Education and Research) within the project “MeLuBatt” (03XP0110A). References: [1] A. C. Luntz et al., “Nonaqueous Li-air batteries: A status report”, Chem. Rev., 114(23), 2014. Figure 1

In the present work, a combined study with polarized neutron imaging (PNI) and neutron-grating interferometry based dark field imaging (DFI) experiments on grain oriented high permeability steel sample with 3% Si and 0.35 mm thickness is reported. With the combination of these two experimental techniques it was possible to obtain the complex picture of magnetic domain distribution and domain walls in the electric steel samples. With the PNI technique, the observed fringe pattern contrast may be correlated with basic magnetic domains. These magnetic domains are oriented anti-parallel with respect to each other and the different magnetic field in each domain may lead to fringe pattern contrast. The magnetic domains are disrupted at the grain boundaries, observed as discontinuities in the fringe pattern contrast. In comparison, with DFI measurements, we visualized scattering contrast from magnetic domain walls. A clear correlation between basic domains (from PNI) and domain walls (using DFI) is observed. In the current study we also demonstrate the potential of combining two differential experimental techniques to visualize and obtain collective information about magnetic domains and domain walls.

Dynamic and direct visualization of interfacial evolution is helpful in gaining fundamental knowledge of all solid state lithium battery working degradation mechanisms and clarifying future research directions for constructing next generation batteries. Herein, in situ and in operando synchrotron X ray tomography and energy dispersive diffraction were simultaneously employed to record the morphological and compositional evolution of the interface of InLi anode sulfidesolid electrolyte during battery cycling. Compelling morphological evidence of interfacial degradation during all solid state lithium battery operation has been directly visualized by tomographic measurement. The accompanying energy dispersive diffraction results agree well with the observed morphological deterioration and the recorded electrochemical performance. It is concluded from the current investigation that a fundamental understanding of the phenomena occurring at the solid solid electrode electrolyte interface during all solid state lithium battery cycling is critical for future progress in cell performance improvement andmay determine its final commercial viability

The internal microstructure of a silicon electrode in a lithium ion battery was visualized by operando synchrotron X-ray radioscopy during battery cycling. The silicon particles were found to change their sizes upon lithiation and delithiation and the changes could be quantified. It was found that volume change of a particle is related to its initial size and is also largely determined by the changing surrounding electron-conductive network and internal interface chemical environment (e.g., electrolyte migration, solid-electrolyte interphase propagation) within fractured particles. Moreover, an expansion prolongation phenomenon was discovered whereby some particles continue expanding even after switching the battery current direction and shrinkage would be expected, which is explained by assuming different expansion characteristics of particle cores and outer regions. The study provides new basic insights into processes inside Si particles during lithiation and delithiation and also demonstrates the unique possibilities of operando synchrotron X-ray imaging for studying degradation mechanisms in battery materials.

The mesostructure (micro to millimeter scale) of clay rocks was considered as a two-component mixture consisting of impermeable non-clayey sand grains embedded in a permeable clay matrix. Provided that representative diffusion properties can be defined, the clay matrix can be treated as a continuum. Under these conditions diffusion at larger scales will depend on geometric properties of the mesostructure. The objective of this study, then, is to analyze geometric parameters, which control diffusion at larger scales. In a first step a set of different clay matrix mesostructures were reconstructed on the base of synchotron X-ray computed microtomography applied to clay rock samples from northern Switzerland (e.g. Opalinus Clay). In a second step mesostructural effects on diffusion were quantified by applying diffusion simulations to reconstructed mesostructures. Further analysis revealed that constrictivity is the most dominant parameter, which controls diffusion on the millimeter scale in Opalinus Clay. Regarding diffusion, the mesostructure of the clay matrix is near isotropic. Hence, the reason for anisotropic diffusion in Opalinus Clay must be searched on the nanometer to micrometer scale and it is caused by anisotropic pore path tortuosity related to shape preferred orientation of clay platelets.

We investigated the hydrogen distribution spatially and temporally in technical iron at room temperature. Samples were charged electrochemically and subsequently analysed by means of neutron radiography and tomography. The radiographic images allowed for a time-resolved analysis of hydrogen fluxes. The three-dimensional distribution of hydrogen measured by neutron tomography delivered valuable information for the damage analysis of hydrogen-induced cracks. For the first time hydrogen concentration gradients inside the material could be detect directly together with the cracks. The neutron radiography and tomography results were gained at the Research Reactor BER II of the HZB in Berlin.

We present a synchrotron X-ray tomographic study on the morphology of carbon fiber-based gas diffusion layer (GDL) material under compression. A dedicated compression device is used to provide well-defined compression conditions. A flat compression punch is employed to study the fiber geometry at different degrees of compression. Transport relevant geometrical parameters such as porosity, pore size and tortuosity distributions are calculated. The geometric properties notably change upon compression which has direct impact on transport conditions for gas and fluid flow. The availability of broad 3D paths, which are most important for the transport of liquid water from the catalyst layer through the GDL, is markedly reduced after compression. In a second experiment, we study the influence of the channel-land-pattern of the flow-field on shape and microstructure of the GDL. A flow-field compression punch is employed to reproduce the inhomogeneous compression conditions found during fuel cell assembly. While homogenously compressed underneath the land the GDL is much less and inhomogeneously compressed under the channel. The GDL material extends far into the channel volume where it can considerably influence gas and fluid flow. Loose fiber endings penetrate deeply into the channel and form obstacles for the discharge of liquid water droplets.

Nondestructive 3D mapping of crystallographic phases is introduced providing distribution of phase fractions within the bulk (centimeter range) of samples with micrometer-scale resolution. The novel neutron tomography based technique overcomes critical limitations of existing techniques and offers a wide range of potential applications. It is demonstrated for steel samples exhibiting phase transformation after being subjected to tensile and torsional deformation.

Predicting and increasing the expected battery lifetime is one of the major objectives in state-of-the-art battery research. Until now, the highly complex mechanisms taking place during cyclic aging are only understood to a certain extent. In the present paper, pristine and cyclically aged battery electrodes are considered and the relationship between their microstructure and functionality is investigated. For this purpose, three-dimensional image data obtained by synchrotron tomography is preprocessed by a novel data-driven trinarization approach based on k-means clustering. This allows us to explicitly distinguish between active material, pore space and the phase consisting of binder and conductive additives. This three-phase reconstruction is completed by a segmentation of active material particles, which enables a comprehensive statistical analysis of the electrode morphology. In addition, the investigation of numerous image characteristics together with electrochemical measurements contributes to a deeper understanding of the underlying aging mechanisms.

Nano-structuring of active materials is a promising way to enhance the performance of Li-ion batteries regarding electronic properties, charge transfer during cycling as well as the resistance to mechanical stress. Moreover, producing thicker electrodes, i.e. thicker than 100 μm, leads to an increase in energy density of the cell and to a reduction of costs. In the present paper, thin and thick electrodes are compared, each manufactured with nano-structured and bulk active material, respectively. The focus is on their 3D microstructure, which is, in turn, important for the performance of the battery. Experimental approaches are combined with statistical image analysis based on synchrotron tomography. In general the results obtained by experiments are in good accordance with the ones obtained by image analysis. Moreover, image analysis allows for the computation of characteristics that are experimentally not accessible. In particular, the system of particle midpoints, the local volume fraction of active material at certain distances to the current collector foil, as well as the windedness and narrow constrictions of transportation pathways are analyzed using tools from spatial statistics and mathematical morphology. The main result is that the considered microstructure characteristics depend only slightly on the thickness of the electrode, while significant differences can be observed between electrodes manufactured with nano-structured and bulk active material.