Atom-efficient synthesis of a benchmark electrolyte for magnesium battery applications

The benchmark magnesium electrolyte, [Mg2Cl3]+ [AlPh4]-, can be prepared in a 100% atom-economic fashion by a ligand exchange reaction between AlCl3 and two molar equivalents of MgPh2. NMR and vibrational spectroscopy indicate that the reported approach results in a simpler ionic composition than the more widely adopted synthesis route of combining PhMgCl with AlCl3. Electrochemical performance has been validated by polarisation tests and cyclic voltammetry, which demonstrate excellent stability of electrolytes produced via this atom-efficient approach.

FTIR and Raman Spectroscopy FTIR and Raman spectra of all samples were recorded using a commercial spectroscopic cell (ECC-Opto-Std, EL-CELL® GmbH). For FTIR measurements, this cell was interfaced with a ZnSe internal reflection element and fitted to a variable angle ATR-FTIR accessory (VeeMAX™ III, Pike Technologies, USA). The incident light was polarised at 90° and an incidence angle of 45° was employed. The ATR accessory was connected to a Thermo Fisher Nicolet iS50 FTIR spectrometer, equipped with a liquid nitrogen cooled MCT (mercury cadmium telluride) detector. Each spectrum was averaged over 495 acquisitions. Raman scattering measurements were performed using a confocal Raman microscope (inVia Qontor, Renishaw) equipped with a CCD detector, using a 830 nm laser at 5% nominal power (0.46 mW at the sample) and a 1200 lines/mm grating. Each spectrum was acquired with 10 s exposure and 10 accumulations. The background was subtracted using WiRE version 5.1 using an intelligent polynomial fitting. FTIR and Raman spectra were collected for electrolyte samples prepared according to equations 1 or 3 (see main text for discussion) THF. Sample preparation and cell assembly were performed in an Ar filled glove box (MBraun LabMaster Pro SP; O 2 < 0.1 ppm; H 2 O < 0.1 ppm) prior to the measurements.

Synthesis of MgPh 2 ·2THF
Dioxane (5 mL) was added to a solution of PhMgCl (10 mL, 20 mmol, 2 M in THF from Sigma-Aldrich) and stirred at room temperature overnight. The resulting white suspension (dioxane·MgCl 2 ) was then filtered and the THF and excess dioxane were removed in vacuo to concentrate the solution; crystals could then be grown from this solution of the product MgPh 2 ·2THF, confirmed by X-Ray diffraction. Isolated crystalline yield 39 % from a maximum of 50 %. MgPh 2 ·2THF (0.064 g, 0.2 mmol) was added to an NMR tube fitted with a J Youngs tap and dissolved in 0.5 mL of d 8 -THF. 1 H and 13 C NMR was then carried out and the data are shown in Figures S1 and S2, below.

Synthesis of [Mg 2 Cl 3 ·6THF] + [AlPh 4 ] -
MgPh 2 ·2THF (0.646 g, 2 mmol) and AlCl 3 (0.133 g, 1 mmol) were added to the same Schlenk flask in an argon atmosphere glove box. 7 mL of THF was then added slowly at 0°C and the solution slightly heated to dissolve the reactants, the solution was then stirred at room temperature overnight. Colourless crystals were obtained by slowly layering hexane on top of the solution (0.288 g, 31 % crystalline yield). Excess hexane was then added to the filtrate after collection of the crystals resulting in another white precipitate. The precipitate was then collected via filtration and dried in vacuo and found to be [Mg 2 Cl 3 ·6THF] + [AlPh 4 ]by NMR spectroscopy (0.631 g, 68 % yield). Total yield 99 %.

In situ preparation
In an argon filled glove box AlCl 3 (0.013 g, 0.1 mmol) was weighed out in a small vial. The MgPh 2 ·2THF solution from the NMR tube was then syringed into the vial and stirred until all reactants had dissolved. The solution was then transferred back into the NMR tube and 1 H, 13 C and 27 Al NMR spectroscopic measurements were performed. The data are shown in Figures S3-S8 Figure S9a and Figure S10. A Voigt waveform in Origin was used to fit the absorption peaks, iterating until R 2 > 0.99.

Deconvolution of FTIR spectra FTIR absorption spectra were deconvoluted into individual component peaks as shown in
For the pure THF solvent (Figure S9a), the following peaks were identified:

Vibration (major contributor marked in bold)
P1 1066 C-O-C asymmetric stretch + C α C β asymmetric stretch + ring bend (type B vibrations) P2 1027 βCH 2 wag + βCH 2 twist + C-O-C symmetric stretch + C α C β symmetric stretch + C β C β stretch + ring bend (type A vibrations) P3 907 βCH 2 twist + αCH 2 rock + C α C β asymmetric stretch (type B vibrations) P4 875 βCH2 rock + ring bend + C-O-C asymmetric stretch (type B vibrations) Note that the 33 normal modes of THF are classified into A and B modes according to the irreducible representation [Γ(C 2 )=17A + 16B] of the C 2 point group which represents the symmetry of the THF molecule in equilibrium in solution. [2] The ratio of the peak area of the satellite peaks (P 2 and P 4 ) compared to the primary peaks (P 1 and P 3 , respectively) was calculated from the fit, and for THF these were determined as: For the electrolyte synthesised in this paper ( Figure S10a) the major peaks were identified as:

Vibration (major contributor marked in bold)
The ratio of the peak area of the satellite peaks (P 2 +P 3 and P 5 +P 6 ) compared to the primary peaks (P 1 and P 4 , respectively) was calculated from the fit, and for the electrolyte synthesised without stoichiometric control these were determined as: Comparing ratios (iii) to (vi) with (i) and (ii) indicates that presence of the electrolyte increases the relative absorbance associated with satellite peaks of THF at around 1027 cm -1 and between 841 cm -1 and 886 cm -1 . This arises as a consequence of coordination of THF to species present in solution (generally the solvent is known to coordinate to the cationic species, i.e. Mg 2+ , present in solution; the other cation, Al 3+ , that gives rise to the anionic species in solution, is preferentially coordinated by organic ligands i.e. phenyl rings) which distorts the overall symmetry of the molecule in solution, thus affecting the normal modes of vibration. Comparison of (iii) and (iv) with (v) and (vi), respectively, indicates a higher absorbance associated with satellite peaks for the stoichiometrically non-controlled synthesis. We believe that this is due to (a) presence of multiple solvent coordinated species and (b) increase in concentration of solvent coordinated species in the solution obtained from nonstoichiometrically controlled synthesis.

Figure S11
Nyquist plots obtained from electrolyte synthesised by traditional method using Grignard reagent (1A), atom efficient method prepared in situ (1B), and crystallized pure electrolyte re-dissolved in THF (1C). The intercept of each trace at the high frequency regime, where Z" = 0, is used to calculate the conductivity (the cell constant for the set-up is 1 cm -1 )