In the search for post-lithium battery systems, magnesium-sulfur batteries have attracted research attention in recent years due to their high potential energy density, raw material abundance, and low cost. Despite significant progress, the system still lacks cycling stability mainly associated with the ongoing parasitic reduction of sulfur at the anode surface, resulting in the loss of active materials and passivating surface layer formation on the anode. In addition to sulfur retention approaches on the cathode side, the protection of the reductive anode surface by an artificial solid electrolyte interphase (SEI) represents a promising approach, which contrarily does not impede the sulfur cathode kinetics. In this study, an organic coating approach based on ionomers and polymers is pursued to combine the desired properties of mechanical flexibility and high ionic conductivity while enabling a facile and energy-efficient preparation. Despite exhibiting higher polarization overpotentials in Mg-Mg cells, the charge overpotential in Mg-S cells was decreased by the coated anodes with the initial Coulombic efficiency being significantly increased. Consequently, the discharge capacity after 300 cycles applying an Aquivion/PVDF-coated Mg anode was twice that of a pristine Mg anode, indicating effective polysulfide repulsion from the Mg surface by the artificial SEI. This was backed by operando imaging during long-term OCV revealing a non-colored separator, i.e. mitigated self-discharge. While SEM, AFM, IR and XPS were applied to gain further insights into the surface morphology and composition, scalable coating techniques were investigated in addition to ensure practical relevance. Remarkably therein, the Mg anode preparation and all surface coatings were prepared under ambient conditions, which facilitates future electrode and cell assembly. Overall, this study highlights the important role of Mg anode coatings to improve the electrochemical performance of magnesium-sulfur batteries.

Numerical simulations are a powerful tool for the development and improvement of Li-ion batteries[1]. Modelling the mass transport of the involved electrolytic solutions requires precise determination of the corresponding electrolyte parameter. However, the determination methods are still controversially discussed. In this work, we attempt to measure the complete set of electrolyte parameter for a system of 0.5 M LiPF6 dissolved in a blend of ethylene carbonate and ethyl methyl carbonate (EC:EMC, 3:7 weight) at 20°C and 50°C. In order to determine the conductivity, the diffusion coefficient, the transference number and the thermodynamic factor we conduct a combination of concentration cell measurements, galvanostatic polarization experiments and electrochemical impedance spectroscopy[2]. While the conductivity and concentration cell measurements show comparable results to the literature, we find the potential response in the polarization experiments deviating from theoretical expectations. Applying galvanostatic polarization experiments to symmetrical metal Li | electrolyte + separator | Li metal cells result in slower, impeded diffusion processes. This indicates the presence of additional, undesired porous structures on the Li electrodes, preventing a reliable evaluation of the electrolyte parameters[3]. To spectrally resolve the diffusive processes, we conduct very low frequency impedance spectroscopy in an attempt to identify the impedance for the sole diffusion through the separator[4]. 1. Max Schammer et al 2021 J. Electrochem. Soc. 168 026511 2. Johannes Landesfeind and Hubert A. Gasteiger 2019 J. Electrochem. Soc. 166 A3079 3. Talian et al. The Journal of Physical Chemistry C 2019 123 (46), 27997-28007 4. F. Wohde et al 2016 J. Electrochem. Soc. 163 A714 Figure 1: VLF-IS measurements with amplitudes of 60µA, 500µA and 10µA together wit the corresponding equivalent circuits, fits and Kramers-Kronig transformations at 20° C and 50°C respectively Figure 1

To enable the shift towards renewable energy, the use of low-cost energy storage systems is necessary. Magnesium (Mg) is an abundant and safer raw material than lithium for electrochemical energy storage systems such as batteries, due to its reduced sensitivity to air. Promising intercalation materials for cathodes include Chevrel phase Mo6S8 and Prussian green FeFe(CN)6. Although magnesium perchlorate-based electrolytes are practical for early-stage cathode research, they are not compatible with Mg metal due to corrosion. The organo-metallic all phenyl complex (APC) based electrolyte is a potential candidate for magnesium full cells, but both systems contain reactive chloride species that cause severe corrosion of the current collector. This study utilizes linear sweep voltammetry, chronoamperometry, and electrochemical impedance spectroscopy to identify suitable materials for current collectors, including carbon-coated Al and Ni. The selected materials must withstand a polarization potential of 2 V vs. Mg/Mg2+. A Graphite-based current collector is shown to be the most promising candidate due to its low areal density, which can increase the energy density of practical Mg batteries.

The magnesium-sulfur battery represents a promising post-lithium system with potentially high energy density and improved safety. However, just as all metal-sulfur systems, it is plagued with the polysulfide shuttle leading to active material loss and surface layer formation on the anode. To gain further insights, the present study aims to shed light on the dissolution characteristics of sulfur and polysulfides in glyme-based electrolytes for magnesium-sulfur batteries. Therefore, operando UV/vis spectroscopy and imaging were applied to survey their concentration in solution and the separator coloration during galvanostatic cycling. The influence of conductive cathode additives (carbon black and titanium nitride) on the sulfur retention and cycling overpotentials were investigated. Thus, valuable insights into the system's reversibility and the benefit of additional reaction sites are gained. On the basis of these findings, a reduction pathway is proposed with S-8, S-6(2-), and S-4(2-) being the present species in the electrolyte, while the dissolution of S-8(2-) and S-3(center dot-) is unfavored. In addition, the evolution of the sulfur species concentration during an extended rest at open-circuit voltage was investigated, which revealed a three-staged self-discharge.

Knowing the phase evolution behavior of battery materials is important for optimizing the operating conditions and understanding the aging behavior of the material. Phase changes of state-of-the-art battery materials at room temperature are well understood, but the influence of temperature on the phase evolution is barely investigated. In this study, operando X-ray diffraction (XRD) is applied for the observation of phase changes of graphite during cycling at different temperatures. The phase evolution is altered at lower temperatures (0 degrees C) compared to room temperature due to slower kinetics and diffusion rates even at low current rates of C/20. Differences in phase evolution between charge and discharge are more pronounced at 0 degrees C compared to higher temperatures. During delithiation at 43 degrees C, the formation of stage 2L is more distinct than at 25 degrees C whereas at 0 degrees C, no stage 2L formation is observed. However, at 0 degrees C, several phases co-exist, supporting the theory of the shrinking annuli mechanism for phase evolution during (de-)lithiation of graphite.

Lithium-sulfur (Li-S) batteries, which in recent years have been aimed at a high gravimetric energy density of 500-600 Wh kg-1, indicate particular potential in aerospace applications such as aircraft, satellites and drones.1,2 In light of this, the German Aerospace Center (DLR) has been pursuing a strategy of developing and researching metal-sulfur batteries for a decade. Herewith, we present the results of a fundamental study on the investigation of structural and compositional parameters affecting the electrochemical performance of sulfur-carbon composite cathode. Highly porous carbon materials are widely employed in composite cathodes in Li-S batteries to compensate for the non-conductive property of the sulfur element. Indeed, this would have significant effects not only on the conductivity of the cathode, but also on the diffusion and charge transfer parameters. Herewith, we have systematically studied porous carbon-based electrodes employing electrochemical impedance spectroscopy. Using a suitable transmission-line model peculiar to the porous electrodes, the resistance of bulk and pore electrolyte, inter-particles, and charge transfer is defined and quantified. The dependence of the identified processes at the open circuit voltage on the fraction of the active material, the porosity of utilized carbon and thickness of the electrode is investigated elaborately. It is indicated that the electrolyte resistance in the pore increases on increasing the content of active material, electrode thickness and on reducing the pore size distribution of the C component. Additionally, an innovative symmetrical three-electrode cell configuration with an integrated Li-ring serving as counter electrode is implemented, enabling the investigation of the cathode impedance at different depths of discharge to shed light on the kinetics of the charge transfer depending on the infiltration method. To this end, various porous carbon materials such as Ketjenblack® (mesoporous) and carbon aerogel (microporous) in combination with different sulfur infiltration techniques including gas, melt and mechanical mixing have been employed as model systems3. The thermodynamic and kinetics of charge transfer mechanisms as a function of infiltration method as well as the variation of the charge transfer resistances upon discharge/charge are discussed in details4. A. Fotouhi, D. J. Auger, L. O’Neill, T. Cleaver and S. Walus, Energies, 2017, 10, 1937. Sripad S., Viswanathan V. (2017), ACS. Energy. Lett.2, 1669-1673 M. Nojabaee, B. Sievert, M. Schwan, J. Schettler, F. Warth, N. Wagner, B. Milow, K. Andreas Friedrich, J. Mater. Chem. A, 2021,9, 6508-6519. Martina Gerle, Norbert Wagner, Joachim Häcker, Maryam Nojabaee, Andreas Friedrich J. Electrochem. Soc., 2022, 169, 030505.

The open architecture of cathodes in Li–air batteries implies the need for open porosity with adequate pore size distribution and surface energy optimization with regard to the electrolyte. The interaction of liquid and cathode material, especially the wetting properties, which depend on cathode material, roughness and porosity, and electrolyte properties, needs to be understood properly to avoid flooding and assure high active areas. In this work, contact angle goniometry, capillary rise method, and pressure saturation curves are used to investigate the wetting properties of dimethyl sulfoxide (DMSO), tetraethylene glycol dimethyl ether (Tetraglyme), a 1:1 mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) and water on a gas diffusion layer (GDL) Sigracet 39BC, and a pure flat polytetrafluorethylene (PTFE) foil. Contact angle measurement shows that all three organic solvents wet the GDL hydrophobic agent PTFE. Capillary rise measurements show that all sample liquids slowly imbibe into the porous network. While for Tetraglyme an efficient penetration is limited by the high viscosity, water flow rate is slowed down by the hydrophobic pore network of the GDL. Pressure saturation curves for DMSO, Tetraglyme, and EC:DMC can be obtained for the first time and are compared with the water pressure saturation curve.

For lithium-sulfur batteries, porous carbon/sulfur composite cathodes are the primary solution to compensate the non-conductive nature of sulfur. The composition and structure of this class of cathodes are crucial to the electrochemical performance, achieved energy density and the stability of the cell. Electrochemical impedance spectroscopy is employed to investigate and correlate the electrochemical performance of lithium-sulfur batteries to the composition and microstructure of differently fabricated carbon/sulfur composite cathodes. A transmission line model is applied to identify different underlying electrochemical processes appearing in the impedance response of a range of porous carbon/sulfur cathodes. The integration of a lithium ring serving as a counter electrode coupled with advanced wiring has allowed an artifact-free recording of the cathode impedance at different states of charge with the aim to investigate the evolution of impedance during discharge/charge and the kinetics of charge transfer depending on the infiltration method and the utilized carbon host. It is shown that impedance response of this class of cathodes is highly diverse and the plausible underlying processes are discussed in details. To this end, quasi-solid-state and various polysulfide-based charge transfer mechanisms are identified and their time constants are reported.

The understanding of the wetting behavior of gas diffusion electrodes (GDE) during operation is the key to developing gas diffusion electrodes with a stable triple-phase boundary and high electrochemical performance. In metal-air batteries, this is even more complex as the wide potential range during charge and discharge leads to several oxidation states with different molar volumes and wetting characteristics. In our work, we showed with a segmented cell the shift of current density to areas with less hydrostatic pressure for the oxygen reduction reaction and analyzed the hydrostatic pressure influence further with a cell enabling us to set the electrolyte pressure. We could also show the decrease of pore sizes and the influence of these smaller pores and higher wettability of the oxidized silver surface by using a pressure saturation setup. The knowledge of these findings offers the opportunity to design GDEs with lower tendency for flooding and therefore longer operation times.

Based on the work by Boukamp (Boukamp, 2017), the method of Fuoss and Kirkwood (Fuoss and Kirkwood, 1941) is applied to derive an analytical distribution function of relaxation times for physics based porous electrode impedance cases. These impedance models are typically described by transcendental transfer functions. The porous electrode impedance treated here reflects a balance of the effective ionic and electronic impedances inside a porous electrode consisting of particles. Therefore, first the DFRT of the single particle interface impedance is derived. This includes treatment of charge transfer, double layer charging, solid state diffusion inside the particles, open-circuit voltage variations due to solid-state concentration, and insulating layers sur-rounding the particles. The resulting single particle DFRT relations are then incorporated into a mathematical description of the porous electrode DFRT. The results show that the DFRT of the porous electrode can be clearly separated into distributions of time constants corresponding to charge transfer, solid state diffusion and in case of intercalating particles, like in lithium-ion batteries, a third distribution of time constants is identified. A novelty of this work is the explicit treatment of the low-frequency capacitance and the resulting distribution of time constants in porous electrode systems. Analytical relations for the individual time constants are derived and reported. Since the ideal distribution of time constants can be represented by a series of R||C circuit elements, validation is performed by reconstruction of the impedance spectra, based on the analytical results.

Electric vehicles are an important element for a sustainable environmentally and climate-friendly mobility. But charging of these takes considerably more time than refueling of a conventional one. This can be an obstacle for many consumers. For an increased acceptance of electric vehicles, the general objective is to charge the battery up to 80% SoC in less than 15 minutes. To reduce the common charging time, higher charging currents are needed. In order to avoid accelerated aging of the battery due to this higher currents, advanced charging profiles have to be applied. During charging, the anode potential should always be above 0 V vs. Li/Li+ to prevent lithium plating which is a major degradation phenomenon and safety concern in the case of fast charging. Not only the anode potential and charge cut-off voltage but also the increased heat generation and growth of solid electrolyte interface (SEI) need to be considered for an optimal charging profile. In order to investigate different charging profiles for cylindrical lithium ion batteries, a multicriteria optimization was conducted considering the charging time tch, maximum temperature Tmax and capacity loss due to SEI growth Closs,SEI. For this, an electrochemical model developed at DLR was used together with a thermal model and SEI growth model from literature [1,2]. Two different 21700 cells were investigated, one with a high energy density and one with a high power capability. A pareto frontier of possible multi-stage constant current (MSCC) profiles was calculated for each cell to evaluate the correlation between tch, Tmax and Closs,SEI. By applying a weighted objective function defined optimized charging profiles are obtained depending on the importance of the three variables. The optimized profiles were validated with the 21700 cells and laboratory test cells with reference electrode. With this, it could be ensured that the simulated temperature and voltage progression matches the experiment and the given limits are not exceeded. [1] R. Richardson, S. Zhao, D. Howey, J. Power Sources 326 (2016) 377-388 [2] M. Safari, M. Morcrette, A. Teyssot, C. Delacourt, J. Electrochem. Soc. 156 (3) (2009) A145-A153

A stable solid-electrolyte interphase (SEI) is of crucial essence for realization of lithium (Li) metal batteries. This article provides an overview of attempts undertaken to understand the nature of the natural SEI, including growth behavior at the open circuit potential and under cycling conditions as well as underlying causes of instabilities. Additionally, the influence of features such as morphology and composition of the SEI and electrolyte properties on the charge transport and transfer mechanism, resulting in distinct growth behavior and overall stability, is elaborated. Finally, it will be discussed how already at the fundamental stage of research, it is crucial to take into account the implications that progressing from coin-cell level to more realistic cell and cycling conditions has on SEI properties and stability.

Metal-sulfur (Me-S) batteries are one of the most promising battery technologies and might even outperform future Li-ion batteries. However, these systems generally show a relatively fast capacity loss, low power density, and fast self-discharge. Furthermore, the underlying mechanisms are insufficiently understood, making it difficult to systematically improve cell performance. In the present study, we use a pseudo-two-dimensional continuum model to analyze the degradation behavior during cycling of Li-S and Mg-S batteries. The simulations reproduce charge and discharge curves of both systems and allow detailed investigations on the influence of material properties on degradation mechanisms. Special attention is paid to the redistribution of active sulfur during cycling and its effect on long-term stability. Finally, we demonstrate the application of the model for qualitative predictions of battery performance and lifetime of cathode designs with improved sulfur loading.

Achieving high current densities in the electrochemical reduction of CO2 is one of the critical issues keeping this technology from commercialization. Although in the past few years, gas diffusion electrode-based electrolyzers have frequently been reported to reach a few hundred milliamperes per square centimeter, higher reaction rates are still endeavored to lower the capital costs and increase the process flexibility necessary for peak-shaving of fluctuating, renewable energies. Here, we report a series of optimizations that allow for the operation of the presented tin oxide nanoparticle-based, homogeneous single-layer gas diffusion electrode at current densities of up to 1.8 A cm(-2). Up to this current density, formate faradic efficiency can be kept above 70%. Individual single parameter optimizations, namely, the type of cation contained in the electrolyte, the catalyst loading of the electrode, and the hydrophobicity of the electrode, are investigated separately and afterward combined to achieve a maximized current density.

A transimpedance amplifier circuit as well as an instrumental amplifier circuit were used to measure current densities of a zinc-air battery with an integrated segmented current collector foil. Error calculation showed that the transimpedance amplifier is superior to the used instrumental amplifier, but both methods provide valuable and consistent results. They both showed comparable results with operando insight into the current distribution of the battery. The knowledge about those distributions is essential to avoid fast degradation of battery materials and irreversible capacity loss due to heterogeneous dissolution of the anode during discharge. In this work we showed that oxygen starvation as well as gas flow rate leads to large current gradients. It was also demonstrated that heterogeneous current distributions on cathode side induces also a heterogenous dissolution behavior on the anode, resulting in irreversible capacity loss.

Der Ausbau von erneuerbaren Energien und der damit verbundene steigende Bedarf an station¨ aren Energiespeichern, das Vordringen der Elektromobilit¨ at sowie die kontinuierliche Weiterentwicklung von elektronischen Ger¨ aten fuhrt zu einer steigenden Nachfrage an kleinen, leichten und g ¨ unstigen ¨ Energiespeichern mit einer hohen Ladungskapazit¨ at. In der Forschung und Entwicklung werden daher Zelltypen mit hoher spezifischer und volumetrischer Energiedichte sowie gunstigen Kompo- ¨ nenten angestrebt. Die sekund¨ are Zink-Luft Batterie stellt einen aussichtsvollen Kandidaten dar. Damit sich der Zelltyp jedoch durchsetzen kann, muss ein gewisser Grad an technologischem Level (engl. Technical Readiness Level) (TRL), Wirtschaftlichkeit und Nachhaltigkeit erreicht sein. Hierfur ist die Degradationsgeschwindigkeit beziehungsweise die Zyklenfestigkeit der Zelle eine ¨ wichtige charakteristische Gr¨ ose. Um diese zu maximieren, muss eine homogene Stromverteilung gew¨ ahrleistet sein. Dazu muss determiniert werden, in welcher Art und Gr¨ ose die Stromdichteverteilung durch verschiedene Einflusse ver ¨ ¨ andert wird. Zur Untersuchung wird folglich eine spezifische Messdiagnostik ben¨ otigt. In der vorliegenden Thesis wird die Entwicklung, Validierung und Anwendung einer Segmentierungsdiagnostik in einer Zink-Luft-Sekund¨ arzelle beschrieben. Mit dem entwickelten Messsystem werden Stromdichteeinflusse von Elektrolytf ¨ ullst ¨ ¨ anden, Variation des Gaseingangsortes und des Volumenstroms sowie Zellzyklisierung und Anodenaufl¨ osung untersucht. Die entwickelte Segmentationsdiagnostik hebt sich durch die modifizierte und ausgelagerte Messschaltung von den bestehenden Diagnostiken aus dem Brennstoffzellenbereich ab. Dadurch wird zur Messung lediglich eine segmentierte Stromkollektorfolie in der Messzelle ben¨ otigt, wodurch die Messdiagnostik fur verschieden Zelltypen und -geometrien angewendet werden kann. ¨ Die Messungen haben gezeigt, dass die entwickelte Segmentationsdiagnostik ortsaufgel¨ oste sowie zeitabh¨ angige Stromdichteverteilungen bis in den Zehntel-Milliampere-Bereich determinieren kann. Des Weiteren wurden Messergebnisse mittels Impedanzspektroskopie und zyklischer Voltammetrie in einer Steigrohrzelle erzielt, die stark auf einen hydrostatischen Einfluss des Elektrolyten auf die Stromdichteverteilung hinweisen. Die Messdiagnostik sowie die resultierenden Stromdichteverteilungen stellen einen grundlegenden Baustein fur weitere Untersuchungen zur Steigerung der Zyklenfestigkeit von Zink-Luft-Sekund ¨ ¨ arbatterien dar.

Electrochemical impedance spectroscopy (EIS) is a powerful tool for investigating electrochemical systems such as fuel-cells and batteries. But because of the overlapping of processes due to similar time constants, it is necessary to understand the system well to set up a correct equivalent circuit for analysis of the measured spectra. Distribution of relaxation times (DRT) offers a model-free approach for impedance analysis. In this work DRT was used to analyze a Ni/Co3O4 gas diffusion electrode (GDE) for metal-air batteries. To identify the corresponding process for the identified peaks temperature, current density, gas composition, electrolyte concentration and electrode material composition were varied. In total five processes could be identified. Four were observed both during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). A fifth could only be recognized during OER. In particular the charge transfer coupled with an adsorption process and the porous structure could be identified. Furthermore, it could be shown that having a bimodal pore size distribution, consisting of two different materials, the correct calculation of the DRT spectra is inhibited, resulting in two peaks instead of the characteristic number of peaks that are getting smaller towards smaller time constants.

In the presented study, a sulfur infiltrated ultra-microporous carbon aerogel as a composite cathode for lithium sulfur batteries is developed and investigated.

This work investigated the degradation of tin – based gas-diffusion electrodes (GDE) and also a promising Bi2O3 GDE in electrochemical CO2 reduction in highly alkaline media which has not been studied before. The contributions of the electrode wetting (or flooding, if excessively) and catalyst leaching on the degradation were analyzed. Therefore, electrochemical impedance spectroscopy was used to monitor the wetted surface area of the GDE in combination with post-mortem analysis of the penetration depth by visualizing the electrolyte’s cation in the GDE cross-section. Furthermore, to reveal a possible degradation of the electrocatalyst, its distribution was mapped in the GDEs cross-section after operation while the catholyte was additionally analyzed via ICP-MS. The results clearly demonstrate that the SnO2 catalyst dissolves in the reaction zone inside the GDE and might be partially redeposited near the GDEs surface. Since the redeposition process occurs only partially a steady loss of catalyst was observed impeding a clear distinction of the two degradation phenomena. Nevertheless, the deterioration of the electrode performance measured as faraday efficiency (FE) of the parasitic hydrogen evolution reaction (HER) qualitatively correlates with the differential double layer capacitance (Cdl). A significant difference of the rate of increase for the hydrogen FE and Cdl can be ascribed to the superposition of both above-mentioned degradation mechanisms. The demonstrated instability of SnO2 contrasts with the behavior of Bi2O3 GDE which is stabilized during CO2 conversion by redeposition of the diluted dissolved species as metallic Bi which is active for the CO2 reduction reaction.

The Cover Feature illustrates the solid–electrolyte interphase (SEI) formed on lithium metal in contact with the liquid electrolyte and the consequential influence of this interlayer on the regulation of lithium dissolution and deposition. The stable SEI is of essence for the realization of practical lithium metal cells with enhanced cycle life. More information can be found in the Minireview by M. Nojabaee and co-workers.