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Electrochemical stability and speciation of a magnesocene / THF electrolyte
ZSW - Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg.
Chalmers University of Technology, Department of Physics.
Chalmers University of Technology, Department of Physics.
Honda R&D Europe (Deutschland) GmbH.
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2017 (English)Conference paper, Poster (with or without abstract) (Refereed)
Abstract [en]

Magnesium batteries are currently attracting a lot of interest as potential post Li-ion battery technology. One of the critical issues is to find a suitable electrolyte. Classical electrolytes based on simple salts, such as Mg(ClO4)2 or Mg(TFSI)2 (TFSI = bis(trifluoromethylsulfonyl)imide), result in a strong passivation of the Mg metal anode, which prevents reversible plating and stripping of Mg. With very few exemptions the typical Mg electrolyte is composed of metal chloride complexes or reactive chlorine-containing metal-organic compounds [1], and the contained chloride can cause problems with corrosion for several of the cell components.

Recently, we have introduced bis(cyclopentadienyl)magnesium (magnesocene, MgCp2) in tetrahydrofuran (THF) as a new chlorine-free electrolyte for low-voltage Mg batteries [2]. Unlike ferrocene, magnesocene behaves as a weak electrolyte and shows some minor dissociation in electron-donating solvents, thus exhibiting low conductivities in the order of 10-5 S/cm [3,4]:

MgCp2 ↔ MgCp+ + Cp; MgCp2 + Cp ↔ MgCp3; MgCp2 + MgCp+ ↔ Mg2Cp3+

Despite the low conductivity, high Mg plating / stripping current densities can be observed (Figure 1), which are comparable to those of other Mg electrolytes, such as Mg(BH4)2 / THF [2]. The plating / stripping process shows high reversibility for many cycles, which suggests that the Mg metal anode is not overly passivated in the MgCp2-based electrolyte. X-ray photoelectron spectroscopy (XPS) characterization of the surface of the cycled Mg electrode does not reveal any significant amounts of decomposition products or passivation films (Figure 2), and indicates that the electrolyte is rather stable at the working potential of the Mg metal electrode. The anodic stability has been investigated by linear sweep voltammetry, and values of approx. 1.5, 1.7, and 1.8 V vs. Mg2+/Mg0 have been observed on Pt, Cu, and stainless steel, respectively (Figure 3).

The experimental findings are corroborated by density functional theory (DFT) calculations performed for various hypothetical Mg-Cp-THF species, using implicit (C-PCM) THF solvation. The cathodic stability of these species is well below 0 V vs. Mg2+/Mg0, indicating that they should not be reduced at the potentials of the Mg plating / stripping reactions. The overall anodic stability of the electrolyte is limited by the Cp species, which is oxidized at 1.86 V vs. Mg2+/Mg0. The DFT calculations, furthermore, show MgCp2THF2 to be the most stable neutral species, wherein one Cp moiety is η5-coordinated, the other one is η1-coordinated, and the THF molecules are coordinated via their oxygen atoms (Figure 4). The structure previously resolved for solid MgCp2THF2 crystals by X-ray diffraction [5] is thus preserved in THF solution.

Place, publisher, year, edition, pages
Kokkola, 2017.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-337739OAI: oai:DiVA.org:uu-337739DiVA, id: diva2:1170739
Conference
3rd Nordic Battery Conference - Nordbatt
Available from: 2018-01-04 Created: 2018-01-04 Last updated: 2018-01-04

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