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Structural Analysis of Magnesium-Chloride Complexes in Dimethoxyethane Solutions in the Context of Mg Batteries Research Michael Salama, Ivgeni Shterenberg, Linda J. W. Shimon, Keren Keinan-Adamsky, Michal Afri, Yosef Gofer, and Doron Aurbach J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05452 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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The Journal of Physical Chemistry

Structural Analysis of Magnesium-Chloride Complexes in Dimethoxyethane Solutions in the Context of Mg Batteries Research Michael Salama†, Ivgeni Shterenberg†, Linda J.W. Shimon§, Keren KeinanAdamsky†, Michal Afri†, Yosef Gofer†, and Doron Aurbach†*

Department of Chemistry and the institute of Nano-technology and

†

advanced materials (BINA), Bar-Ilan University,Ramat Gan, 5290002, Israel §

Chemical research support unit, Weizmann Institute of Science, 76100 Rehovot, Israel

ACS Paragon Plus Environment


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Abstract

Recently, MgTFSI2/MgCl2 electrolyte solutions in dimethoxyethane (DME) have been shown to function as viable electrolyte solutions for secondary Mg batteries that can facilitate reversible magnesium deposition/dissolution. MgCl2 is a crucial component in these solutions. On its own, however, it is practically insoluble in DME. Therefore, the fact that it is readily dissolved in MgTFSI2/DME solution is remarkable. Addition of MgCl2 greatly improves the electrochemical performance of MgTFSI2/DME electrolyte solutions. Thus, identifying the species formed in MgTFSI2/MgCl2 solutions is intriguing. In this study, we implemented a wide variety of analytical tools, including single crystal X-ray diffraction, multinuclear NMR, and Raman spectroscopy, to elucidate the structure of these solutions. Various solution species were determined, and a suitable reaction scheme is suggested.


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Introduction Rechargeable magnesium batteries seem attractive and interesting as an electrical energy storage technology ‘beyond Li ion batteries’.1-2 Following the first magnesium battery prototype presented by Aurbach et. al. in 2000,3 comprising Chevrel phase cathode, metallic magnesium anode, and ethereal solutions with organometallic complex electrolytes, tremendous efforts and resources were invested toward the development of improved, rechargeable magnesium batteries which may have a practical importance.4 The use of organometallics-based electrolytes limits the possible operating voltage of these Mg battery prototypes due to their susceptibility to oxidation.5-6 This, in turn, limits the potential cathode materials to low voltage ones. The most notable electrolyte systems, with improved electrochemical stability windows, developed since the organometallic-based complex electrolytes are: MACC,7 borohydride-based electrolytes,8-9 APC,10 and MgTFSI2 (with or without additives).11-14 Practical magnesium batteries should make use of metallic magnesium anodes in order to exploit their high volumetric capacity (3833 mAh/cm3). The magnesium’s low standard redox potential (-2.37V vs SHE) is one of the most important electrochemical characteristics of this metal as an anode material. Magnesium as a very reactive metal is naturally covered by surface films which are formed by spontaneous reactions with atmospheric components (oxygen, water vapor) and with all protic solvents and many polar aprotic solvents as well. Unfortunately, the surface films formed naturally on Mg are not solid ionic conductors. They cannot behave as a solid electrolyte interphase (SEI), in contrast to the case of usual surface films formed on Li metal which always behaves like SEI, allowing effective transport of Li ions. This situation limits the spectrum of suitable solvents for rechargeable Mg batteries to ethereal solvents, which do not react spontaneously with magnesium metal. Also, due to the relatively high charge density of the bivalent Mg ions, the conceivable “simple” magnesium salts that form sufficiently conducting solutions in ethers have been limited, so far, to MgTFSI2 exclusively. MgTFSI2


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possesses many desired properties, including high oxidation stability, ready solubility in ethers, and high thermal stability. On the other hand, its ‘simple’ solutions glymes exhibit poor electrochemical performance as electrolyte solutions for Mg batteries: low coulombic efficiency and huge hysteresis for magnesium deposition/dissolution processes.12 Extensive research on the solution structure of MgTFSI2/DME solutions revealed that MgTFSI2 in DME completely dissociates to DME solvates of free Mg ions, while the TFSI anions remain uncoordinated, probably as a result of efficient charge dispersion.15 The poor coulombic efficiencies for magnesium deposition obtained with MgTFSI2/DME solutions can be drastically improved by incorporating chlorides into the solutions, mainly through the addition of MgCl2.13 MgTFSI2/ DME/ MgCl2 solutions exhibit reversible Mg deposition, growth of uniform deposits of metallic magnesium and high anodic stability. When coupling Mg metal anodes with Chevrel-phase cathodes using these solutions, proofof-concept for rechargeable Mg batteries prototypes could be demonstrated.13 In this paper, we report on detailed studies aiming at understanding the structure of MgTFSI2/MgCl2/DME at the same level we reached recently with the study of MgTFSI2/DME solutions, which thermodynamic behavior and structure were found to be very complicated.15 Our main goal in the present work was to identify the complex solvated ions formed in these solutions. The identification of the solution species may lead to an improved formulation of electrolyte solutions. In order to elucidate the solution structure, we used a variety of analytic tools such as Raman spectroscopy, and multinuclear NMR spectroscopy. In special cases, where the imperative spectral region for the detection of critical peaks was possibly overrun with uncritical compound’s bands, we made use of chemically similar model systems to alleviate the problem. The use of MgCl2/AlCl3 in DME, for instance, was implemented in order to simplify the Raman spectrum of the corresponding TFSI based solutions. The TFSI based solutions exhibit many strong peaks in the critical spectral domain of 200-350 cm-1. The



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identification of complicated solution species was, in appropriate cases, done by isolation of single crystals of selected structures, followed by SCXRD analysis. These, fully characterized compounds, were then measured by Raman spectroscopy to obtain a selection of relevant bands. These, if proved to be adequate for solution’s species identification, were added to the compilation of spectral database. The discussion and conclusions sections of this work are based on a variety of spectral evidence and a whole range of complimentary data. At certain cases this led to intricate and complex set of arguments that eventually distilled into our conclusions. We emphasize that what the structures/identifications we propose seem to be the most logical ones, within the limitations of the spectroscopic tools we used for this study. EXPERIMENTAL SECTION Materials. dimethoxyethane (99%), AlCl3 (99.999%) and MgCl2 (99.5%) were purchased from Sigma-Aldrich. MgTFSI2 99.95% was purchased from "Solvionic". The MgTFSI2 was dried under vacuum for 48h at 250°C. Dimethoxyethane was dried with 4Å activated molecular sieves; the dryness was measured via Karl Fischer titration, and was below 15 ppm. Material handling. Material storage, and all preparation of samples and electrolyte solutions, was carried out under argon atmosphere. An MBRAUN glovebox was used with less than 1 ppm of water and oxygen. All single crystals were grown from saturated solutions of MgTFSI2/MgCl2 or AlCl3/MgCl2. The crystals grown from MgTFSI2/MgCl2 solutions with 1:1 and 1:2 molar ratios were dried slowly at RT in an argon atmosphere. Analysis


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1

H, 13C, 35Cl and 25Mg NMR spectra were measured with a Bruker Avance-I- 400

spectrometer, using a 90° excitation pulse, without sweeping the lock channel since no deuterated solvent could be introduced into the DME samples. All measurements were performed at room temperature (25±2°C). 35Cl NMR spectra were referenced to 1M MgCl2 in D2O.

SCXRD measurements were carried out using a Bruker Kappa Apex II system equipped with a Mo Kα sealed tube radiation source. The crystals were mounted on MiTeGen loops, coated in Paratone oil from Hampton Research, flash frozen in liquid nitrogen, and then measured at 100 K in a nitrogen atmosphere. The diffraction data of [Mg2(μ-Cl)2(DME)4](AlCl4)2, [Mg3(μ-Cl)4(DME)5](TFSI)2, MgCl2(DME)4, and [Mg2(μ-Cl)2(DME)4](TFSI)2. Were collected with a Bruker Kappa APEX-II CCD diffractometer and processed with SAINT. Intensity data of these compounds were collected using Mo-Kα radiation (0.7107 Å). Data collection was performed under LN at 100 K. The structures were solved by direct methods using the SHELXT. All non-hydrogen atoms were further refined by SHELXL with anisotropic displacement coefficients. Hydrogen atoms were assigned with isotropic displacement coefficients, U(H) = 1.2U(C) or 1.5U (C-methyl), and their coordinates were allowed to ride on their respective carbons. Crystallographic data and refinement parameters are summarized in Table S1.

Raman spectroscopy was carried out using a JY Horiba spectrometer with a He−Ne 632.817 nm laser, an appropriate notch filter, and a high-density grating. The spectra were collected in segments, iteratively, until an acceptable S/N ratio was obtained. All solutions were measured with 40 sec exposure time and 20 repeats. The solid samples were measured for 60 sec exposure time 10 repeats. The spectrometer was equipped with a Peltier cooled



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charge-coupled device. All spectra were collected at stable RT (25°C), unless noted otherwise. Before and after every measurement, a spectrum of silicon wafer was acquired (major peak at 520 cm−1) for spectral window stability monitoring and recalibration. Liquid samples were measured with airtight-capped quartz cuvettes (1 cm) that were loaded in the glovebox. Automated baseline subtraction was done with LabSpec software package (provided with the Raman instrument). Results The results section is divided into several topics. Each is focused on a specific analysis technique. Each of these sections is further divided to each of the individual chemical systems. Single crystals analysis (by SCXRD) SCXRD have been shown10, 15 to be highly valuable in constructing a Raman bands database for new, unknown materials. It is also important as first guesses for the assignment of spectral features of possible species in the solution phase, relevant to the moieties that were measured by XRD as single crystals. Single crystals were obtained from DME solutions with various reactants ratios and total concentration. We isolated single crystals from solutions containing the following salts MgCl2, MgTFSI2/MgCl2 at 1:1 and 1:2 molar ratios, and AlCl3/MgCl2 in 1:1 and 1:1.5 molar ratios. The reactions were carried out by the dissolution of the reactants in DME with vigorous stirring and heating for an appropriate time. In general, the solubility limits of MgCl2/AlCl3 in DME were much lower compared to that of DME/MgTFSI2/MgCl2 solutions and the preparation of the former solutions required longer time and higher temperatures. These results reflect much higher heats of formation of the latter solutions


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Thermal ellipsoids and crystal data are presented in figures S1-S4 and table S1. MgCl2 is only slightly soluble in DME, even at elevated temperatures. Nonetheless we were able to obtain crystals from a saturated MgCl2/DME solution. The crystal’s refined structure is presented in figure 1. MgCl2 acquires a V- shape with an equilibrium angle of 99.5 degrees between the two chlorine atoms.



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Figure 1 - A refined structure (from SCXRD) of the solid recrystallized from MgCl2 solution in DME. For clearer view, the hydrogen atoms are omitted. Separate cif files and full structure are provided in the SI.

Figure 2 shows the refined structure of the Mg based cation in the moiety precipitated from MgTFSI2/MgCl2 1:1 in DME solution (elucidated by SCXRD). The structure consists of a [Mg2 (μ-Cl) 2(DME) 4] (TFSI) 2 complex; the anion (TFSI, not shown here) is positioned at a relatively large distance, ca. 5.5 nm, at closest approach to the Mg atoms.

Figure 2 - A refined structure (from SCXRD) of the complex cation obtained by recrystallized of a solution containing MgTFSI2/MgCl2 at a 1:1 ratio. The full structure formula is [Mg2 (μ-Cl)


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2(DME) 4]

(TFSI) 2. For clearer view, the anions and hydrogen atoms are omitted. The full

crystallographic structure and cif files can be found in SI. The complex [Mg3(μ-Cl) 4(DME) 5] (TFSI) 2 structure was obtained from the single crystals grown from MgTFSI2/MgCl2 1:2 in DME solution (figure 3). Here also, the distance between the magnesium and the TFSI is higher than 5.5 nm at closest approach.

Figure 3 - A refined structure of Mg3 [(μ-Cl) 4(DME) 5] (TFSI) 2 recrystallized from solutions of MgTFSI2/MgCl2 at a 1:2 molar ratio in DME. The anions and hydrogen atoms are omitted in order to improve the visibility. The full structures and the cif files can be found in the SI. Single crystals were also obtained from AlCl3/MgCl2 1:1 DME solutions. The refined structure is presented in figure S5. The chemical formula of the single crystal is [Mg2 (μ-Cl) 2(DME) 4] (AlCl4)2.



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The structure of the single crystals precipitated from DME solutions containing AlCl3/MgCl2, 1:1.5 molar ratios, is presented in figure S6. The elucidated crystal's formula is [Mg3 (μ-Cl) 4(DME) 5]

(AlCl3) 2. This crystal cation is identical to the one precipitated from 1:2

MgTFSI2/MgCl2 /DME solutions.

Raman spectroscopic studies The following DME based solutions were analyzed via Raman scattering spectroscopy: MgCl2, MgTFSI2/MgCl2 with 1:1 and 1:2 molar ratios, and AlCl3/MgCl2 with 1:1: ratio. Additional, solid state Raman measurements, were conducted on the single crystals obtained from these solutions. A table containing all Raman bands and assignment (when available) can be found in SI (Table S2). MgCl2 solutions MgCl2 has a very low solubility in DME. No MgCl2 related peaks could be detected in saturated MgCl2 /DME solutions. Regardless, we have managed to produce single crystals from a DME/MgCl2 solution by heating a saturated MgCl2/DME solution, filtering it while it is still hot and slowly removing the solvent by letting it evaporate under argon atmosphere without further heating or applying vacuum. Owing to the sensitivity of the obtained crystals to ambient atmosphere, separate measurements were performed with crystals immersed in DME and completely dry crystals (figure 4). Ignoring the peaks that relate to free DME vibrations,15-16 the “wet” crystals show the following peaks: 217cm-1,305, 332, and 390. Vigorous drying by heating to 700 under vacuum for 30 minutes and measuring the crystal at RT reveals that the 217, 305, and 392 cm-1 peaks remains. This indicates that these peaks relate to the species which structure is presented in figure 1 and are not related to residual "free" DME molecules.


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The 217cm-1 is consistent with Mg-Cl symmetric stretching's.10, 17-18 Furthermore this spectral region (200 – 300 cm-1) is not associated with any DME vibration band .15-19 The peaks at

~305 and 392 cm-1 have not been assigned unambiguously and cannot be linked directly to DME bands. One unique DME related peak at 878cm-1 15 could be assigned to a moiety comprising DME and Mg ion (Figure S7).15



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Figure 4 - Raman spectra for [MgCl2 (DME) 4] recrystallized from MgCl2/DME solution. a) The 100-500 cm-1 spectral region for a vacuum dried recrystallized solid (completely dry crystals) b) A Raman spectrum for MgCl2 (DME) 4 crystals immersed in DME, recrystallized from


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MgCl2/DME solution. The complete Raman spectrum for the crystal appears in the supporting information (figure S7).



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a

b

Figure 5 A Raman spectra of a) MgTFSI2:MgCl2 (1:1) solution in DME b) single crystal [Mg2(μCl)2(DME)4](TFSI) 2 obtained from DME solution comprising 1:1 MgTFSI2/MgCl2.


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The Raman spectra of both 1:1 MgTFSI2/MgCl2 /DME solution (figure 5a) and the [Mg2(μ-Cl) 2(DME) 4]

(TFSI) 2 crystals obtained from this solution (figure 5b), exhibit similar DME and TFSI

related peaks in the range of 900-1600 cm-1 15. Both spectra show a strong and sharp peak at ca. 880 cm-1 (879 cm-1 for the solution, 880.5 cm-1 for the crystal). Both the crystal and the solution exhibit a clear peak at ca. 220 cm-1 associated with Mg-Cl vibrations.10, 17-21



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a

b

Figure 6 A Raman spectra of a) solutions of MgTFSI2/MgCl2 (1:2) in DME ,b) [Mg3 (μ-Cl) 4(DME) 5]

(TFSI) 2 single crystal, obtained from DME solutions of 1:2 MgTFSI2/MgCl2.


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Similarly to the spectra related to the 1:1 MgTFSI2/MgCl2/ DME solutions, the vast majority of the peaks of both the [Mg3 (μ-Cl) 4(DME) 5] (TFSI) 2 crystal and the 1:2 MgTFSI2/MgCl2/DME solutions can be assigned to TFSI or DME bands. A peak associated with DME-solvate is observed around 880 cm-1 (880.1 cm-1 for the [Mg3 (μ-Cl) 4(DME) 5] (TFSI) 2 crystal and 878.5 cm-1 for the solution). The peak at ca. 220 cm-1 is present in the spectra of both the crystal and the solution. 1:1 AlCl3 / MgCl2 /DME solutions. Figure 7 shows the Raman spectrum of a 1:1 AlCl3/MgCl2 /DME solution. Such Lewis acid – Lewis base reactions between AlCl3 and MgCl2 in THF were studied well and documented. 7, 10 The anion [AlCl4]- exists in the solution and is identified by the strong peak at 348 cm-1 .10 There are no peaks at 309 and 430 cm-1 that can be associated with [Al2Cl7]- . 22 A clear peak at 220 cm-1 is identified, assigned as an Mg-Cl related band.


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Intensity (a.u)


200

400

600

800

Wavenumber (cm-1) Figure 7 – A Raman spectrum of a 1:1 MgCl2/AlCl3 /DME solution.


1000

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Comparative Raman spectroscopic studies

a

b

Figure 8 – Raman spectra for MgTFSI2 and MgTFSI2/MgCl2 solutions with various ratios in DME. a) 150-500 cm-1 spectral region, and b) 800-900 cm-1 spectral region. All peaks heights are normalized in relation to the 740 cm-1 band, associated with free TFSI- . The black curve



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represents 0.35M MgTFSI2 in DME. The red curve represents 0.5M MgTFSI2/ 1M MgCl2 in DME. The green curve represents 0.5M MgTFSI2/ 0.7M MgCl2 in DME. The pink curve represents 0.5M MgTFSI2/ 0.5M MgCl2 in DME. The yellow curve represents 0.5M MgTFSI2/ 0.25M MgCl2 in DME. The blue curve represents 0.5M MgTFSI2/ 0.05M MgCl2 in DME. Figure 8 compares the Raman spectra of MgTFSI2/DME solutions with varying ratios of MgCl2. The MgTFSI2 concentration was held constant as 0.5M. Individual Raman spectra are presented in figures S8-S12. Three major trends are observed with increasing MgCl2 concentration: First, the peak at 868 cm-1 diminishes. This peak is associated with the strong solvate formed between DME and bare Mg2+ ions.15 The second trend is a red shift in the 880 cm-1 band. This peak represents the well-documented vibration band associated with the interaction of the solvent with cations in the solvate ensemble.23-24 A red shift suggests that weaker interactions takes place. Finally, a new peak appears at 220 cm-1. Although there is a wave like feature/band in this spectral region, a trustworthy and clear formation of a peak is undeniable. This peak, based on a wealth of previous data, can be confidently associated with Mg-Cl bonds’ vibrations.17-18, 20-21, 25-26


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NMR spectroscopic data 1

H and 13C NMR spectra



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a

b

Figure 9 – a) 1H NMR and b) 13C NMR spectra of DME/MgTFSI2 solutions with different concentration of MgCl2, as indicated. 1

H and 13C NMR spectra of MgTFSI2/DME solutions, with varying ratios of MgCl2, are

presented in figure 9 a and b, respectively. in the 1H spectra the peaks associated with DME, as [Mg(DME)3]2+,solvate15 appear at 4 and 4.4 ppm. The corresponding 13C peaks appear at the 61 and 70.5 ppm (marked by the blue line in figure 9). Both these interrelated peaks


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decrease with increasing MgCl2 ratio. This trend continues until the peaks are no longer detectable at 0.25M of MgCl2 . 25

Mg and 35Cl NMR spectra

25

Mg and 35Cl NMR measurements (presented in figures S13 and S14) were also performed on all the solutions we studied. 35Cl is a spin 3/2 quadrupolar nucleus and its signals linewidth depends on its symmetry. Free Cl- in case of MgCl2 in D2O shows a narrow singlet since the solvated ion has a very high symmetry. Since no 35Cl NMR peak was observed at the same position in the DME solutions spectra it leads us to the conclusion that free chloride ions do not exist in these solutions, at least at the detection level of these measurements.

Discussion Introductory remarks. In our previous study, we showed that magnesium deposition and dissolution from MgTFSI2 /DME solutions require very high overpotential for both processes. 13 The addition of MgCl2 to the solutions dramatically improves the poor electrochemical performance: considerably lowers deposition and dissolution overpotentials and enables Mg deposition/dissolution cycling at efficiencies approaching 100%.

MgTFSI2/MgCl2/DME solutions reach the

electrochemical performance of the Mg organo-haloaluminates complex electrolyte solutions of previous generations. Unfortunately, this advantageous performance is maintained for a limited number of cycles. The rigorous analysis of these solutions and the full identification of their actual components are of great interest. A deep understanding of the solutions structures and their correlation to their electrochemical characteristics may help in formulating electrolytic systems for Mg batteries with optimal electrochemical performance.



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The nature of the different species and their relative ratios in DME/MgTFSI2/MgCl2 solutions are determined by two main factors: the ratio between MgCl2 and MgTFSI2, and the ability of the solvent, DME, to stabilize the MgXCly (X and Y are integers) complex cations. It is important to note that the solubility of pure and dry MgCl2 in DME is rather low, below 0.001M. This suggests that DME cannot solvate MgCl2 efficiently, nor DME-MgCl2 interactions can overcome the crystal energy and promote dissociation. However, MgCl2 can be dissolved in MgTFSI2/DME solutions, up to a MgCl2/MgTFSI2 2:1 ratio. It was possible to dissolve up to 2 M of MgCl2 in 1 M MgTFSI2 / DME solution. We believe that this remarkable dissolution of MgCl2 is actually a consequence of a reaction between the [Mg (DME)3]

2+

solvate, formed in MgTFSI2 /DME solutions,15 with MgCl2, forming Mg–Cl complexes that are efficiently solvated by DME. These complexes constitute a solvate ensemble with DME molecules. The determination of the solution species requires measurements carried at "normal" conditions.10,

15

Hence, analyses carried out at conditions other than those of the

electrochemical experiments in terms of temperature, concentration etc. considered authentic and relevant.

cannot

be

Consequently, we applied Raman and NMR

spectroscopic measurements of authentic, relevant solutions, with efforts to identify responses based on libraries of assigned spectra which were measured with the most logical model compounds (single crystals precipitated from the authentic solutions).

FTIR

spectroscopy can also provide intimate structural/compositional information on composite materials; however, due to problems of masking by dominant solvent peaks, this spectroscopy could not be used herein. 25

Mg and 35Cl NMR analyses of the solutions explored herein also suffers from shortcomings:

limited spectral information and lack of data on 25Mg and 35Cl NMR spectroscopic response of similar complexes.


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Another shortcoming that should be mentioned is the lack of computational data on the vibrational behavior and spectral response, related to these solutions. Compiling such data may be possible but it was beyond the scope of this work and our capabilities. The various solutions explored in the present study are discussed below separately, based on the above described results. MgCl2/DME solutions. Although the solubility of MgCl2 in DME is low, as mentioned before, single crystals of the formula [MgCl2 (DME) 2] were obtained from a saturated solution as shown in figure 1. We believe based on these results, that MgCl2 indeed interacts with DME to form

[MgCl2

(DME) 2] solution species. Such solution species can be viewed as contact ion pairs. Interestingly, in these moieties, MgCl2 exists as a V-shaped structure, with an equilibrium angle of 99.5 degrees between the two chlorine atoms. In this form, the MgCl2 molecules attain a higher dipole moment than its linear form, presumably facilitating its interaction with the high charge density [Mg(DME)3]2+ solvates (from solutions of MgTFSI2 / DME), to form various MgxCly complexes. The Raman spectrum of [MgCl2 (DME) 2] (figure 4) reveals that this crystal exhibit a distinct peak at 217 cm-1, which can be associated with the Mg-Cl symmetric stretching. 10, 17-18, 20-21 Peaks in this region have been identified for THF coordinated Mg-Cl, complexes.10, 27 The peaks at ~305 and 392 cm-1 have not been assigned unambiguously yet. We hypothesize that they are related to DME ligands, bound to MgCl2. MgTFSI2/MgCl2 1:1/DME solutions The solid precipitated from these solutions contains (in addition to TFSI- anions) doubly charged Mg dimer cations, [Mg2Cl2]2+, with two Mg atoms bridged by two Cl atoms and coordinated by four DME molecules (figure 2). This structure may be regarded as the dimerization of the [MgCl]+ cation.10 The anions (TFSI-) are positioned at a relatively large



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distance, ca. 5.5 nm at closest approach to the Mg ions. This suggests that the 1:1 solution comprise free (fully unpaired) ions. A strong support to this assessment is obtained from the Raman measurements. Raman spectra of 1:1 MgTFSI2/MgCl2/DME solutions (figure 5a) show a distinct peak at ca. 220cm-1, which is the same peak that appears in the spectrum of [Mg2(μ-Cl) 2(DME) 4] (TFSI) 2 crystals (figure 5b). We assign this band to Mg-Cl containing molecules based on two main reasons: previous studies showed that the 220 cm-1 region is associated with complex Mg-Cl vibrations,17-18, 20, 26 .Also, this peak only exists when MgCl2 is present in the solution(figure 8a). The 190-277 cm-1 spectral region show bands associated with Mg-Cl bonds (in the range of the compounds under study) .10, 17-18, 28 The 740 cm-1 , associated with free TFSI- anions ,15 supports the assertion that contact ion pairs (figure 5a) are not present as a major constituent in these solutions. In the three analyses by Raman, 1H NMR and 13C NMR spectroscopies, the peaks related to DME as a solvate (which ultimately represent [Mg(DME)3]2+ ), diminish as the concentration of MgCl2 increases. Based on these results, it is logical to conclude that [Mg (DME) 3]2+ serve as precursor for a reaction between MgCl2 and the Mg2+ to form the [Mg2 (μ-Cl) 2(DME) 4]2+ dimer.

Some strong TFSI Raman bands are located in the 277-450 cm-1 region. This may interfere with the detection of potential peaks associated with Mg-Cl and DME peaks, thus hampering the ability for reliable analysis. In order to alleviate this problem we synthesized and analyzed a model system by reacting AlCl3 with MgCl2 in a 1:1 ratio, in DME. The resulting solution contains the same [Mg2(μ-Cl) 2(DME) 4]2+ cations that are formed in 1:1 MgTFSI2/MgCl2/DME

solutions , as evidenced from SCXRD measurements of the solid

precipitated from the AlCl3/MgCl2/DME solutions. The Raman spectrum of the AlCl3/MgCl2/ DME solution (figure 7) showed no traces of unreacted AlCl3 peaks. It consists of only one anion related band 348 cm-1, associated with the vibration of [AlCl4]-. 22 We are also ruling


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out the presence of [Mg2Cl3]+ and [Al2Cl7]- complex ions as major species in any of these solutions. Those ions , if existed, should show distinct Raman bands that were not present in any of our measurements of DME solutions.10, 22 35Cl NMR measurements showed that these solutions contain no free chloride ions (Figure S13). In addition, no free Mg2+ solvates are present, as concluded from the entire Raman spectral studies. Thus the MgxCly complex must comprise an equal amount of Mg and Cl (X=Y) in solutions containing 1:1 MgTFSI2/MgCl2 . Complexes such as [Mg3Cl3]3+ or [Mg4Cl4]4+ are unlikely to form due to their high charge density. Based on all of these conclusions which come-up very clearly from the spectral studies presented herein, we suggest the following reaction schemes: For the MgTFSI2/MgCl2 1:1 molar ratio solutions: . 1. MgTFSI2 +3DME Mg2+ 3DME +2TFSI. 2. MgCl2 + 2DME MgCl2 2DME

3. Mg2+.3DME +MgCl2.2DME Mg2Cl22+.4DME +DME 4. Mg2Cl22+.4DME 2MgCl+.xDME

For AlCl3:MgCl2 1:1 ratio (neglecting the DME ligands): 5. MgCl2 +AlCl3 MgCl+ + AlCl46. 2MgCl+ + 2AlCl4- Mg2Cl3+ +Al2Cl77. 2MgCl+ + AlCl4- Mg2Cl22+ + AlCl4-

We conclude that the Mg solution species in the 1:1 solution is [Mg2Cl2]2+, which might exist in equilibrium with [MgCl]+. Since [MgCl]+ is not detectable by Raman spectroscopy, we cannot estimate the ratios between [MgCl]+ and [Mg2Cl2]2+.28 We can say however, with



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utmost certainty that [Mg2Cl2]2+ is indeed present due to the presence of the 220cm-1 peak which relates to Mg-Cl vibration and isn't associated with [MgCl]+.

1:2 MgTFSI2/ MgCl2 /DME solutions In the case of 1:2 MgTFSI2/MgCl2/DME solutions, we applied the same logic as above: the resulting MgxCly complexes in solutions must retain a Mg:Cl 3:4 ratio, similar to the atomic ratio in the crystals precipitated from these solutions , as elucidated by SCXRD (figure 3). These solutions may be less easy for analysis due to the larger range of options, as suggested by the relevant reactions scheme below. The plausible reaction pathway entails the dissolution of MgTFSI2 in DME to form the [Mg(DME)3]2+ solvates. These solvates, in turn, acts as a Lewis acid to produce the [Mg3 (μ-Cl) 4(DME) 5] 2+ cation. 5. MgTFSI2 +3DME Mg2+.3DME +2TFSI6. MgCl2 + 2DME MgCl2.2DME 7. Mg2+.3DME +MgCl2.2DME Mg2Cl22+.4DME +DME 8. Mg2Cl22+.4DME + MgCl2.2DME Mg3Cl42+.5DME +DME

9. Mg3Cl42+ MgCl2 + Mg2Cl22+ 10. Mg3Cl42+ MgCl++ Mg2Cl3+ Reaction 9 can be ruled out (as a main route) since MgCl2 has low solubility in DME, while the solutions containing the [Mg3 (μ-Cl) 4(DME) 5] 2+cations are colorless and transparent at concentrations as high as 1M (based on total magnesium concentration). Reaction 10 predicts the presence of two equimolar species: [MgCl] +, which is undetectable in Raman spectroscopy 28 and [Mg2Cl3]+ which has a very distinct peak at 241 cm-1(in THF solutions).10 The latter cations were not detected in the solutions we explored. Analysis of 1:1 MgTFSI2/MgCl2 /DME solutions suggest that [MgCl]+ is expected to exist in equilibrium with [Mg2Cl2]2+. We have shown that [Mg2Cl3] + Raman fingerprint peak at 241 cm-1 is easily identified in all THF based solutions in which it is formed, like in 1:2 MgTFSI2/MgCl2 /THF (figure S15). In turn, this band is absent in the DME solutions, what suggests that [Mg2Cl3] +


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is not a major species in DME solutions. Hence, the above discussed spectral studies converge to the conclusion that reaction 8 that forms [Mg3 (μ-Cl) 4(DME) 5] 2+ as the major cationic species in these solutions is the most plausible option. Route 10, that predicts the existence of [MgCl]+ + [Mg2Cl3]+,cannot be ruled out altogether, but may only reside as minor option, undetectable by Raman spectroscopy of solution species. Furthermore the fact that we managed to obtain the [Mg3(μ-cl)4(DME)5] 2+ complex (figure S6) as a cationic species in the analogue 1:1.5 AlCl3 /MgCl2 /DME solutions strengthen our claim that at least in DME solutions the formation of these complex cations (containing 3 Mg atoms) are thermodynamically favorable. CONCLUSIONS AND FUTURE PROSPECTS The addition of Cl- anions to MgTFSI2/DME solutions is critically important for the preparation of complex electrolyte solutions with a wide electrochemical window, from which Mg can be deposited reversibly. The addition of MgCl2 as the source of chlorides involves reactions that lead to Mg-Cl based complexes that were never identified before, with mono-dentate ether based solutions (e.g. based on THF). In this study we have shown that DME, as a ligand, strongly promotes the formation of multivalent cations, such as the [Mg2Cl2]2+ and [Mg3Cl4]2+, stabilized by intrinsically bound DME molecules. This tendency is quite surprising, as the dissolution and dissociation of multivalent cations in ethers, which have a relatively low polarity, is not ubiquitous. This phenomenon is, presumably, the consequence of the bi-dentate nature of DME. It is expected that stabilization of complex cations, comprising several Mg ions, may be even more pronounced in multi-dentate, longer glyme solvents. One of the most important features that longer glyme solvents may possess is the capability to form exceptionally stable solvates with magnesium ions, as well as with the Mg-Cl bound moieties. We believe that these solvates, particularly those that contain more than one Mg ion at the core, are responsible for the great improvement obtained by the addition of Cl- ions to the pure MgTFSI2 solutions (bulk effect). However, the extreme



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improvement in the electrochemical properties, due to chloride anions addition to neat MgTFSI2 solutions in DME, must be related to either surface or near surface phenomena. This topic should be the focus of future research aiming to elucidate the surface and near surface electrochemical processes in these solutions.

We speculate, based to some extent on previous studies,13 that three important characteristics play a major role in the great improvement in the electrochemical properties for these complexes. The first one pertains to the energetics of desolvation during to the reduction of magnesium ions at the electrode surface. We suggest that the exceptionally strong solvate formed between Mg ions and DME 15 , may be relieved by the interactions between Mg cations and chlorides. The weaker solvates formed with MgxCly complexes is evidenced by the red shift in the vibrational modes of the Mg based solvates, associated with the band at ~880 cm-1. The second one pertains to the electron transfer process accompanied by subsequent rearrangement reactions between the ligands (Cl- and DME). The [Mg3Cl4]2+ cations may undergo reduction by two electron transfer, to form Mg0 atom and neutral intermediate Mg2Cl4 (stabilized by DME molecules). Such an intermediate can either react with [Mg(DME2)]2+ cations to reform the [Mg3Cl4]2+ complex cation, or break down to two [MgCl2](DME)2 moieties. The latter species are also intermediates, as they are only slightly soluble in DME. Yet, they are stable intermediates that can readily react with solution components to yield stable solution species, including [Mg3(μ-Cl)4(DME)5]2+ cations. The third role we speculate for the critically important presence of chlorides in these solutions, involves surface phenomena related to possible surface film formation and/or corrosion of the metallic magnesium due to reactions with TFSI ions. Although not yet proven, we believe that the bare metallic magnesium electrode can react spontaneously with TFSI ions. This reaction, apparently does not lead to stable passivation layers. Interestingly, in the Cl-free solutions, magnesium deposition frequently necessitates over-


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potential in excess of 400 mV, which are by all means extreme. We speculate that an excess of the Cl-containing complexes resides near or at the negative electrode, possibly acting as adsorbates. We hypothesize that this adsorption layer, acts as an effective (not completely stable) barrier that keeps the TFSI- anion from reaching the reducing Mg surface. Also, the chloride containing complexes facilitate the removal of passivating oxides in case they form, for instance, by reaction with trace water and oxygen in solutions. It is possible that reactions similar to corrosion mediated by Cl- anions, help to dissolve away Mg metal passivating surface films, such as MgO/Mg(OH)2 layers.

Supporting information thermal ellipsoid of crystal obtained from 1:1 MgTFSI2/MgCl2 solution in DME (figure S1),thermal ellipsoid of crystal obtained from MgCl2 solution in DME (figure S2), thermal ellipsoid of crystal obtained from 1:1 AlCl3/MgCl2 solution in DME (figure S3), thermal ellipsoid of crystal obtained from 1:2 MgTFSI2/MgCl2 solution in DME (figure S4), Refined structure of [Mg2(μ-Cl)2(DME)4]2+cations of the solid which was recrystallized from solutions of AlCl3/MgCl2 at a 1:1 ratio in DME, provided by single crystal XRD (figure S5), refined structure of [Mg3 (μ-Cl) 4(DME) 5] (AlCl4) 2 recrystallized from solutions of AlCl3/MgCl2 at a 1:1.5 molar ratio in DME (figure S6), Full spectrum of untreated MgCl2 recrystallized from MgCl2/DME solution (figure S7), Raman spectrum of 0.35M MgTFSI2 solution in DME (Figure S8), Raman spectrum of 0.5M MgTFSI2 0.05M MgCl2 solution in DME (Figure S9) , Raman spectrum of 0.5M MgTFSI2 0.7M MgCl2 solution in DME(Figure S10), Raman spectrum of 0.5M MgTFSI2 0.25M MgCl2 solution in DME (Figure S11), Raman spectrum of 0.5M MgTFSI2 1 M MgCl2 solution in DME (Figure S12), 35Cl NMR spectra of: 1M MgCl2 in D2O, 0.25M MgTFSI2 0.25MgCl2 in DME, and 0.25M MgTFSI2 0.5MgCl2 in DME (figure S13), 25Mg NMR of



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solution with different MgCl2/MgTFS2 ratios (Figure S14), Raman spectrum of 0.5M MgTFSI2 1 M MgCl2 solution in THF(figure s15), Crystallographic data and refinement parameters of : [Mg2(μ-Cl)2(DME)4](AlCl4)2, [Mg3(μ-Cl)4(DME)5](TFSI)2, [MgCl2(DME)4], [Mg2(μCl)2(DME)4](TFSI)2, and [Mg3(μ-Cl)4(DME)5]( AlCl4)2 (Table S1), raman peak assigmnet table (Table S2).


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Author information Corresponding Author *E-mail [email protected]; phone +9723-5318317 (D.A.). Notes The authors declare no competing financial interest

Acknowledgment

Partial support for this work was obtained from the Israel Science Foundation (ISF) in the framework of the INREP project.



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Structural Analysis of Magnesium Chloride ... - ACS Publications

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