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.

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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. Graphical abstractKeywords Oxygen reduction reaction · Gas diffusion electrode · Chlor-alkali electrolysis · Oxygen depolarized cathode · Silver

Commercially used LiNi1/3Mn1/3Co1/3O2 (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.

Limited understanding of the lithium (Li) nucleation and growth mechanism has hampered the implementation of Li-metal batteries. Herein, we unravel the evolution of the morphology and inner structure of Li deposits using focused ion beam scanning electron microscopy (FIB/SEM). Ball-shaped Li deposits are found to be widespread and stack up at a low current density. When the current density exceeds the diffusion-limiting current, bush-shaped deposition appears that consists of Li-balls, Li-whiskers, and bulky Li. Cryogenic transmission electron microscopy (cryo-TEM) further reveals that Li-balls are primarily amorphous, whereas the Li-whiskers are highly crystalline. Additionally, the solid electrolyte interface (SEI) layers of the Li-balls and whiskers show a difference in structure and composition, which is correlated to the underlying deposition mechanism. The revealed Li nucleation and growth mechanism and the correlation with the nanostructure and chemistry of the SEI provide insights toward the practical use of rechargeable Li-metal batteries.

The gap between its successful application and perspective promise of lithium-oxygen battery (LOB) technology should be filled by an in-depth and comprehensive understanding of its underlying working/degradation mechanisms. Herein, the correlation between the morphological evolution of 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 post-mortem SEM, XRD and FTIR characterizations. It has been disclosed with solid evidence that the irreversible transformation of anode Li associated with chemical/electrochemical sidereactions does link intimately to the observed electrochemical performance decay. The current discoveries have not only fundamentally enriched our knowledge on the underlying degradation/failure mechanisms of LOBs but also directions for future promising research activities aimed to further enhance their performance can be drawn therefrom.

Synchrotron X-ray tomography and scanning electron microscopy were applied to elucidate the spatial distribution of discharge product (NaO2) in the carbon cathode of sodium-oxygen batteries. Various batteries were discharged galvanostatically and their cathodes were analyzed. We observe a particle density gradient along the cathode that scales with the current density applied. Besides, we show that the particle size and shape of discharge product strongly depend on current density, and on whether the particles are deposited close to the oxygen reservoir or near the separator. We correlate our findings to transport limitations for the supplied oxygen and gain crucial information for optimal operation of sodium-oxygen batteries. Our findings imply that for low current densities pore clogging might occur, and that for elevated current densities small high surface area particles with limited electric conductivity form; both phenomena can decrease the available discharge and charge capacity significantly.

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