ABSTRACT
In many Mg-based battery systems, the reversibility of Mg deposition and dissolution is lowered by parasitic formation processes of the electrolyte. Therefore, high Coulombic efficiencies of Mg deposition and dissolution are only achieved after several "conditioning" cycles. As this phenomenon is especially reported for AlCl3-containing solutions, this study focuses on the "conditioning" mechanisms of MgCl2/AlCl3 and MgHMDS2/AlCl3 (HMDS = hexamethyldisilazide) in tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes. Electrochemical (cyclic voltammetry) and spectroscopic investigations (27Al nuclear magnetic resonance spectroscopy, Raman spectroscopy, inductively coupled plasma optical emission spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy) reveal that cationic AlCl2+ species in TEGDME-based electrolytes with an AlCl3/MgCl2 ratio higher than 1:1 corrode the Mg metal. According to a cementation reaction mechanism, the corrosion of Mg is accompanied with Al deposition. In effect, the consumption of Mg results in low Coulombic efficiencies of Mg deposition and dissolution during the electrolyte "conditioning". After understanding the mechanism of this process, we demonstrate that a careful adjustment of the stoichiometry in MgCl2/AlCl3 and MgHMDS2/AlCl3 in TEGDME formulations prevents Mg corrosion and results in "conditioning"-free, highly efficient Mg deposition and dissolution.
ABSTRACT
In order to gain a deeper understanding of Li and Mg polysulphides in Li/S and Mg/S batteries, respectively, this work investigates the impact of the two different cations as well as the influence of the electrolyte solvents' relative dielectric permittivity and Gutmann's donor number on the solubility and relative stability of different Li and Mg polysulphide species. Therefore, the disproportionation and dissociation equilibria of chemically prepared "Li2S8" and "MgS8" solutions in DMSO, DMF, ACN, THF, DME, TEGDME, and Pyr14TFSI are characterized by UV/Vis spectroscopy. Varying the cation and the solvent reveals their mutual interplay in stabilizing different polysulphide species. To our knowledge, this is the first time that chemically synthesized Mg polysulphides in solutions are studied. The results of this work provide essential knowledge for further development of the economically, ecologically, and also in terms of energy density and safety, attractive Mg/S batteries.
ABSTRACT
This comparative work studies the self-enforcing heterogeneity of lithium deposition and dissolution as the cause for dendrite formation on the lithium metal anode in various liquid organic solvent based electrolytes. In addition, the ongoing lithium corrosion, its rate and thus the passivating quality of the SEI are investigated in self-discharge measurements. The behavior of the lithium anode is characterized in two carbonate-based standard electrolytes, 1 M LiPF6 in EC/DEC (3 : 7) and 1 M LiPF6 in EC/DMC (1 : 1), and in two alternative electrolytes 1 M LiPF6 in TEGDME and 1 M LiTFSI in DMSO, which have been proposed in the literature as promising electrolytes for lithium metal batteries, more specifically for lithium/air batteries. As a result, electrolyte decomposition, SEI and dendrite formation at the lithium electrode as well as their mutual influences are understood in the development of overpotentials, surface resistances and lithium electrode surface morphologies in subsequent lithium deposition and dissolution processes. A general model of different stages of these processes could be elaborated.
