Mechanisms of Magnesium Ion Transport in Pyrrolidinium Bis

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Mechanisms of Magnesium Ion Transport in Pyrrolidinium Bis(trifluoromethanesulfonyl)imide-Based Ionic Liquid Electrolytes Sebastian Jeremias, Guinevere A. Giffin, Arianna Moretti, Sangsik Jeong, and Stefano Passerini J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 18 Nov 2014 Downloaded from http://pubs.acs.org on November 18, 2014

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The Journal of Physical Chemistry

Mechanisms of Magnesium Ion Transport in Pyrrolidinium Bis(trifluoromethanesulfonyl)imideBased Ionic Liquid Electrolytes Sebastian Jeremias,1 Guinevere A. Giffin,*1,2,3 Arianna Moretti,1,2,3 Sangsik Jeong,1,2,3 Stefano Passerini*1,2,3 1. Institute of Physical Chemistry and MEET Battery Research Center, University of Muenster, Corrensstrasse 28, 48149 Muenster, Germany. 2. Helmholtz Institute Ulm (HIU),3 Electrochemistry I, Helmholtzstrasse 11, 89081 Ulm, Germany.

3. Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany.

ABSTRACT Inert polar aprotic electrolytes based on pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids were investigated for Mg battery applications. On a molecular-scale, there are two TFSI- populations coordinating Mg2+ ions: one in a bidentate coordination to a single Mg2+ and one in a bridging geometry between two Mg2+ ions. On average each Mg2+ cation is surrounded

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by 3 – 4 TFSI- anions. The electrolytes, in general, remain amorphous far below ambient conditions, which results to a wide useable temperature range in practical devices. There is a change in the ratio of bidentate:bridging TFSI- and in the conductivity, viscosity and diffusion behavior at a salt mole fraction of 0.12 – 0.16. At concentrations above this threshold, there is a more dramatic decrease of the diffusion coefficients and the conductivity with increasing salt concentration due to slower exchange of the more strongly coordinated bidentate TFSI-. The mechanism of ion transport likely proceeds via structural diffusion is exchange of the bridging and “free” TFSI- anions within adjacent [Mgn(TFSI)m](m-2n)- clusters and exchange of bidentate anions via a bidentate to bridging mechanism. The vehicular mechanism likely makes only a small contribution. At concentrations above approximately 0.16 mole fraction, the structural diffusion is more closely related to the tightly bound bidentate anions.

KEYWORDS Ionic liquids, ion transport, magnesium, electrolyte, batteries


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INTRODUCTION Magnesium-based battery technology is an interesting alternative to the lithium battery chemistry due to its high natural abundance and high volumetric density.1-2 Most of the electrolytes under investigation are based on Grignard reagents or organohaloaluminate complexes in organic solvents.1 While these electrolytes demonstrate a good ability to strip and plate magnesium, they all present serious safety concerns due to the highly nucleophilic and reactive nature of the magnesium complexes used. Recent studies on alternatives to conventional organic electrolytes for lithium-ion batteries have demonstrated that ionic liquids (ILs) show significant promise to replace the highly volatile and flammable organic electrolytes.3 Ionic liquids have also been used as electrolytes for magnesium batteries but always with a nucleophilic and reactive salt such as a Grignard reagent or a Lewis acid-base.2, 4-6 There have been a few studies on magnesium stripping and deposition in ionic liquid-based electrolytes that do not contain highly reactive salts, but the results could not be reproduced.4-5 Inert polar aprotic salts with significantly lower reactivity than those described above have been used with organic electrolytes and although magnesium plating was not observed, reversible intercalation of Mg into Mo6S8 with lower irreversible capacities than Grignard-based electrolytes was demonstrated.7 There are significant advantages if inert polar aprotic electrolytes based on ILs can be employed, not only in terms of safety as noted above, but also in terms of chemical and electrochemical stability.3-4 To this end, electrolytes based on N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) and N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr12O1TFSI) ionic liquids combined with magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2) are investigated for application in magnesium


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ion battery technology. In a previous publication,8 the ionic coordination of Mg2+ with the bis(trifluoromethanesulfonyl)imide (TFSI-) anion was investigated by means of Raman spectroscopy combined with density functional theory calculations. These results were used to deduce possible Mg2+ charge-carrying species. It was concluded that the Mg2+ charge carriers are quite complex as there are likely a combination of single Mg2+ ion solvate species and larger complexes containing two or more cations.8 Despite the large and complex structures, the solvates contain Mg2+ cations that are mobile in the IL-based electrolytes.8

While there are many studies on the development of electrolytes for magnesium electrolytes, there are very few studies that present an in-depth investigation of the mechanism of Mg2+ ion transport. The few studies that do exist examine polymer or polymer-like, i.e. oligomers such as polyethylene glycol 400 (PEG400), electrolytes.9-12 In all of these studies, the Mg conduction mechanism was closely tied to the host polymer system and in particular, ether moieties present in the polymeric chains. The magnesium cations were in 5-, 6-, or 7-coordinate geometries depending on the ligand.13-14 The cumulative results of these studies illustrate two main modes of Mg conduction:9-12 1) an intrachain mechanism, where the Mg2+ ion moves between coordination sites of a single chain, and 2) an interchain mechanism, where the Mg2+ is transferred between coordination sites of adjacent chains. Unfortunately, the information learned from these systems, where the ether groups preferentially coordinate the Mg2+ ions, is not as transferable to the ILbased electrolytes as here TFSI- is the only coordinating species. As a result, it is expected that the mechanism will be closely tied to the movement of the anions. This manuscript describes the material and transport properties of these Mg2+-IL electrolytes and ties these properties to the molecular organization of the mixtures. Using these results and those previously reported,8 a mechanism of Mg transport involving the TFSI- anions is proposed.


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EXPERIMENTAL Materials. N-methylpyrrolidine (>99%, Fluka), 1-bromobutane (99%, Acros), ethylacetate (analytical reagent grade, Fluka) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 99.9 wt% battery grade, 3M) were used without further purification. 2-Bromoethyl methyl ether (>95%, TCI Japan) was distilled at 35 °C and 2 kPa before being used in the IL synthesis. Magnesium bis(trifluoromethanesulfonyl)imide (Aldrich) was dried 24 h at 90 °C under vacuum to remove residual water and then stored in a dry room (dew point < -50°C). Ionic Liquid Synthesis and Electrolyte Preparation. The ionic liquids Pyr12O1TFSI and Pyr14TFSI were synthesized by a two-step synthesis route, via direct alkylation of Nmethylpyrrolidine and followed by anion exchange with LiTFSI in aqueous solution, which is in detail explained in the literature.15-16 The ILs and the magnesium salt had water contents of less than 20 ppm water as measured by Karl Fischer titration. Binary mixtures (Pyr1RTFSI(1-x)[Mg(TFSI)2]x, R=4 or 2O1), where x is between 0 and 0.32 mol percent for Pyr14TFSI and between 0 and 0.2 mol percent for Pyr12O1TFSI were prepared by adding an appropriate amount of Mg(TFSI)2 to the respective ionic liquids. The resulting mixtures were stirred over night at 50°C and then again dried under vacuum conditions to remove dissolved oxygen/air. The samples were stored in a dry room in an evacuated desiccator. Characterization Methods. DSC measurements were made using a TA Instruments Q2000 differential scanning calorimeter with liquid N2 cooling. The samples were hermetically sealed in Al pans in a dry room. The samples were cycled between -150 and 80°C at a rate of 5°C·min-1. A sub-ambient annealing procedure was used in an attempt to crystallize the samples. Thermogravimetric analysis was carried out using a Q5000 (TA Instruments). Samples of


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approximately 20-30 mg were heated at a rate of 5°C·min-1 to 600°C under a nitrogen purge. The samples were hermetically sealed in aluminum crucibles, which were punched immediately prior to measurement. Raman spectra were recorded on a RAM II FT-Raman-module coupled to a VERTEX 70 FT-IR spectrometer (Bruker Optics, Germany). The FT-Raman-module is equipped with a Nd:YAG laser, which operates at 1064 nm. The spectra are the average of 500 scans at an optical resolution of 2 cm-1 collected at room temperature. The samples were sealed in glass tubes under vacuum. The resultant spectra were fit in the spectral region from 860 to 680 cm-1 with the multipeak fitting package in IGOR PRO 6.22A using a Voigt function with a fixed Lorentzian:Gaussian ratio (shape factor=1.2). The NMR experiments were measured on a Bruker Avance III spectrometer (constant field = 4.7 T) equipped with a gradient probe head (Bruker, “Diff 30”) with exchangeable radiofrequency inserts for 1H and

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F with a maximum gradient

strength of 1880 G/cm. The PGSTE- pulse sequence from the Bruker library (Topspin 3.0) was used for the diffusion coefficient measurements. Samples of 70 µL were sealed under vacuum in 5 mm NMR tubes. Impedance data was collected on a Novocontrol Alpha A impedance spectrometer in the frequency range between 10-2 Hz to 107 Hz. The samples were contained in a sealed sample cell with parallel plate geometry where the electrolytes were sandwiched between stainless steel electrodes. The cell was subjected to sub-ambient annealing, as in the DSC measurements, to promote sample crystallization. The conductivity values were determined from the plateau in the plots of the real component of the conductivity versus frequency.17-18 In addition,

the

ionic

conductivity

of

PYR14TFSI0.76[Mg(TFSI)2]0.24

and

PYR14TFSI0.68[Mg(TFSI)2]0.32 electrolytes was determined by an automated conductimeter equipped with a frequency analyzer and a thermostatic bath (MMates Italia). The ILs were housed in sealed, glass conductivity cells (mounted in a dry room, dew point

Mechanisms of Magnesium Ion Transport in Pyrrolidinium Bis

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