Browsing by Author "Häcker, Joachim"

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Open Access
Corrosion study of current collectors for magnesium batteries
(2024) Rademacher, Laurin; Häcker, Joachim; Blázquez, J. Alberto; Nojabaee, Maryam; Friedrich, K. Andreas
For rechargeable magnesium batteries, chlorine‐containing electrolytes are used because chlorine species reduce the energy barrier for the intercalation process at the cathode. However, these species can cause corrosion of the cathode‐side current collectors during polarization. In this study, carbon‐coated aluminum and Nickel metal substrates, as well as a graphite foil, were investigated using Linear Sweep Voltammetry, Chronoamperometry, and Electrochemical Impedance Spectroscopy to evaluate their potential as current collectors in APC electrolyte. The graphite‐based current collector withstood corrosive environments at polarization potentials up to 2 V, displaying passivating behavior comparable to platinum in Chronoamperometry measurements. During Electrochemical Impedance Spectroscopy measurements, the graphite foil exhibited exceptionally high polarization resistance of at least 4.5 MΩ cm 2 . Combined with its low areal density of 5 mg/cm -2 , this makes it an excellent current collector material for rechargeable magnesium batteries with chlorine‐containing electrolytes. In contrast, Al foil are instable towards corrosion – despite protective coatings.
Open Access
Electrolyte transport parameters and interfacial effects in calcium metal batteries : analogies and differences to magnesium and lithium c ounterparts
(2025) Häcker, Joachim; Rommel, Tobias; Rademacher, Laurin; Riedel, Sibylle; Zhao‐Karger, Zhirong; Blázquez, J. Alberto; Friedrich, K. Andreas; Nojabaee, Maryam
Magnesium and calcium metal batteries are promising emerging technologies. Their high capacity and low redox potential translate to a high theoretical energy density, making them attractive candidates for future energy storage solutions. Owing to their neighboring position and the diagonal relationship in the periodic table to lithium, Mg2+, Ca2+, and Li+ ions feature commonalities in terms of ionic radius, carried charge, and charge density. The present study aims to shed light on the similarities but also differences of Ca electrolytes and metal anodes in comparison to their Mg and Li counterparts in terms of transport properties and processes at the anode/electrolyte interface, respectively. To ensure comparability, an electrolyte comprising B(hfip)4- anions in monoglyme is applied in either case. By executing galvanostatic polarization and pulsing with different separator materials, the separator tortuosity, diffusion coefficient, and transference number are determined. Further, the charge transfer characteristics as well as the adsorption layer and solid electrolyte interphase formation are investigated by electrochemical impedance spectroscopy. The cation charge density was found to be crucial for diffusion and desolvation processes, yet surprisingly, also a cation‐dependent separator tortuosity was observed. The study concludes with a recommendation on suitable separators for each metal battery system.
Open Access
Fundamental understanding of inherent processes in magnesium-sulfur batteries
(2024) Häcker, Joachim; Friedrich, K. Andreas (Prof. Dr. rer. nat.)
In the face of climate change, the decarbonization of industry and everyday life has finally been declared a global goal in recent years, with energy storage in batteries playing a key role. These are needed for both, the decarbonization of vehicles and, in the long term, aviation, as well as in the stationary sector for grid stabilization due to day-night or seasonal fluctuations in renewable energy generation. On account of raw material shortages, lithium-based batteries alone, however, will not be capable to meet the global demand – thus alternative battery systems are tracking attention in the past decade. Among the various cell chemistries under research, magnesium-sulfur represent a promising electrochemical couple in terms of material abundance, high energy density, improved safety, good recyclability and low cost. Despite benefitting from the long-term research on lithium-sulfur batteries (Li-S), the magnesium-sulfur battery (Mg-S) is still in its infancy facing unique challenges and intrinsic limitations. This cumulative dissertation consists of five peer-reviewed scientific articles, which aim to shed light on different components and processes in a Mg-S battery constituting the main obstacles in its development, namely (i) the high ion charge density resulting in large desolvation energy, slow diffusion and impeded redox kinetics, (ii) the sulfur dissolution, self-discharge and polysulfide shuttle and (iii) the passivating surface layers on the Mg anode. Therefore, different attempts in terms of electrode manufacturing and operando characterization methods were pursued. Starting with an in-depth analysis of the first discharge cycle by means of electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS), the subsequent study applied operando UV/Vis spectroscopy and operando imaging to gain insights into the sulfur and polysulfide dissolution of different cathode compositions during the initial ten cycles. Identifying the magnesium anode and the processes at its electrolyte interface as crucial for the efficiency and capacity retention, the long-term cycling performance of pristine and coated Mg anodes was investigated over 150 cycles. Additionally, the influence of sulfur species on the interfacial processes of six different anode concepts could be determined in symmetrical and full cells applying operando EIS. In a concluding study, the transport properties of Mg cations in different separators were compared to their calcium and lithium counterparts. The main findings comprise a severe three-staged self-discharge governed by the sulfur reduction at the unprotected Mg surface and boosted by temperature. An artificial SEI coating is beneficial to not only mitigate the self-discharge, but also enhance the initial Coulombic efficiency and capacity retention. This is originated in mitigated parasitic reactions to form an in situ SEI, mainly consisting of MgF2, MgS and MgO, on the magnesium surface. Therein, hindering the reaction of sulfur species is particularly decisive to circumvent large interfacial resistances. On the cathode side, polar additives are beneficial to serve as adsorption and reaction centers, however with no long-term effect due to precipitates covering the surface. The kinetic of the sulfur redox reactions, which involve S8, S62- and S42- in the glyme-based Mg[B(hfip)4]2 electrolyte, are significantly enhanced by temperature indicating the sluggish MgS nucleation kinetics and Mg2+ solid diffusion. Its inherent high charge density further affects the magnesium cation transport in the electrolyte and its desolvation at the anode/electrolyte interface due to the rigid and strongly bound solvation shell. Consequently, in comparison to calcium and lithium, larger polarization overpotentials and separator tortuosities, respectively, were observed in the Mg system.
Open Access
Identification of the underlying processes in impedance response of sulfur/carbon composite cathodes at different SOC
(2022) Gerle, Martina; Wagner, Norbert; Häcker, Joachim; Nojabaee, Maryam; Friedrich, K. Andreas
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.