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Item 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.