Methods for making magnesium salts

Abstract

A method of making a salt of the formula: Mg[Al(R).sub.4].sub.2, where R represents a compound selected from a deprotonated alcohol or thiol; an amine; or a mixture thereof. The method comprising the steps of; combining a Mg(AlH.sub.4).sub.2 precursor with an alcohol, thiol or amine of the general formula R—H to create a reaction liquor containing Mg[Al(R).sub.4].sub.2; and washing the reaction liquor in an organic solvent.

Claims

1. A method of making an electrolyte, the method comprising: forming a Mg[AlH.sub.4].sub.2 precursor by a one-step ball milling process of NaAlH.sub.4 and MgCl; combining the Mg[AlH.sub.4].sub.2 precursor with a deprotonated alcohol, thiol, amine, or a mixture thereof to create a reaction liquor containing Mg[Al(R).sub.4].sub.2, wherein R represents the deprotonated alcohol, thiol, amine, or the mixture thereof; washing the reaction liquor in an organic solvent selected from the group consisting of dry DME, 2-methyl-THF, diglyme, triglyme, and tetraglyme to obtain a salt; and combining the salt with an Mg(PF.sub.6).sub.2 additive.

2. The method of claim 1, comprising filtering the washed reaction liquor under an inert atmosphere.

3. The method of claim 1, wherein at least one of the deprotonated alcohol, thiol, amine, or mixture thereof is aromatic.

4. The method of claim 1, wherein the deprotonated alcohol, thiol, amine, or mixture thereof is fluorinated.

5. The method of claim 1, wherein an organic moiety of at least one of the deprotonated alcohol, thiol, amine, or a mixture thereof comprises tert-butyl, perfluoro-tert-butyl, hexafluoro-iso-propyl, phenyl, or pentafluorophenyl.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) In order that the present disclosure may be more readily understood, an embodiment of the disclosure will now be described, by way of example, with reference to the accompanying Figures, in which:

(2) FIG. 1 is a linear sweep voltammetry plot of a 0.25 M solution of magnesium tert-butoxyaluminate (1) in THF on stainless steel (ss-316), aluminum, copper, gold, and platinum electrodes, according to some embodiments;

(3) FIG. 2 is a linear sweep voltammetry plot of a 0.25 M solution of magnesium perfluoro-tert-butoxyaluminate (2) in DME on stainless steel (ss-316), aluminum, copper, gold, and platinum electrodes, according to some embodiments;

(4) FIG. 3 is a linear sweep voltammetry plot of a 0.25 M solution of magnesium hexafluoro-iso-propoxyaluminate (3) in DME on stainless steel (ss-316), aluminum, copper, gold, and platinum electrodes, according to some embodiments;

(5) FIG. 4 is a linear sweep voltammetry plot of a 0.25 M solution of magnesium phenoxyaluminate (4) in DME on stainless steel (ss-316), aluminum, copper, gold, and platinum electrodes, according to some embodiments;

(6) FIG. 5 is a linear sweep voltammetry plot of a 0.25 M solution of magnesium perfluorophenoxyaluminate (5) in DME on stainless steel (ss-316), aluminum, copper, gold, and platinum electrodes, according to some embodiments;

(7) FIG. 6 is a cyclic voltammogram and a columbic efficiency plot of a 0.25 M solution of magnesium perfluoro-tert-butoxyaluminate (2) in DME cycling at a rate of 10 mVs.sup.−1 over 50 cycles on a platinum working electrode, according to some embodiments;

(8) FIG. 7 is a cyclic voltammogram and a columbic efficiency plot of a 0.25 M solution of magnesium phenoxyaluminate (4) in DME cycling at a rate of 10 mVs.sup.−1 over 50 cycles on a platinum working electrode, according to some embodiments; and

(9) FIGS. 8A-8H show the charge-discharge behaviour of magnesium full cells containing Chevrel phase cathodes and magnesium anodes and 0.25 M solutions of magnesium aluminates (2) [a) and b)], (3) [c) and d)], (4) [e) and f)], and (5) [g) and h)] in DME at room temperature (rate: C/25) and 55° C. (rate: C/10), according to some embodiments.

DETAILED DESCRIPTION OF THE DISCLOSURE

(10) The present disclosure will now be illustrated with reference to the following examples.

Example 1—Synthesis of Mg(AlH.SUB.4.).SUB.2 .Precursor

(11) A mixture of sodium aluminum hydride from Acros Organics and magnesium chloride from Alfa Aesar in a ratio of 2:1 was ball-milled for an hour to produce a mixture of magnesium aluminum hydride and sodium chloride, containing a theoretical 42.5 wt % of magnesium aluminum hydride (scheme below).

(12) ##STR00001##

(13) The resulting magnesium aluminum hydride mixture offers a general platform for the synthesis of magnesium aluminates, as will be shown by the following examples.

Example 2—Synthesis of Magnesium Aluminates Using Alcohol

(14) Magnesium aluminates were synthesized by treating magnesium aluminum hydride with various fluorinated/non-fluorinated alkyl and aryl alcohols in dry THF or DME (Scheme below).

(15) ##STR00002##

(16) These reactions were followed by filtration under inert atmosphere to remove insoluble impurities (i.e. sodium chloride and aluminum-containing by-products). The resulting magnesium aluminates were retrieved, typically as THF or DME solvates, in moderate to high yields (77-94%). The particular alcohols that were used in the synthesis were (1) tert-butanol (Sigma-Aldrich); (2) perfluoro-tert-butanol (Alfa Aesar); (3) hexafluoro-iso-propanol (Fluorochem); (4) phenol (Sigma-Aldrich); (5) pentafluorophenol (Fluorochem).

Example 3—Use of Magnesium Aluminates as an Electrolyte Salt

(17) All cyclic voltammetry (CV) and linear sweep voltammetry (LSV) experiments reported below were performed in a glovebox (MBraun) under an atmosphere of dry argon using dry solvents. Cyclic voltammetry and linear sweep voltammetry were performed using an IVIUM CompactStat.

(18) A solution of each of the magnesium aluminates (1)-(5) in dry organic solvent was prepared at a concentration of 0.25 M. A solution of magnesium tert-butoxyaluminate (1) in THF was found to exhibit poor oxidative stability on stainless steel (ss-316), aluminum, copper, gold, and platinum electrodes, with the onset of oxidation occurring at around 1 V vs magnesium on each electrode, as shown in FIG. 1.

(19) In contrast to magnesium tertbutoxyaluminate (1), magnesium aluminates 2-5 are soluble in DME. The solution of magnesium perfluoro-tert-butoxyaluminate (2) in DME exhibits an extended stability window on the five electrodes tested above, exhibiting onsets of oxidation between 1.9 V (copper) and 2.6 V (platinum) vs magnesium, as shown in FIG. 2. The LSV of aluminate (2) in DME on platinum exhibited a minor anodic process beginning at approximately 1.8 V vs magnesium. Without wishing to be bound by theory, this can be attributed to platinum-catalyzed decomposition of small amounts of residual alcohol starting material.

(20) A 0.25 M solution of magnesium hexafluoro-iso-propoxyaluminate (3) made by the presently claimed method exhibits an onset of oxidation of around 2.2 V, 2.5 V, and 2.9 V vs magnesium on copper, aluminum, and gold, respectively, as shown in FIG. 3. On platinum and stainless steel, minor anodic processes are observed to begin around 1.5 V and 1.8 V vs magnesium, respectively, with more significant processes beginning around 2.8 to 3 V vs magnesium on both electrodes. As these onsets of oxidation are typically lower than the previously reported values, it is possible that the two electrolyte preparation methods result in different impurities or by-products (i.e. chlorides or Mg alkoxides) that enhance or limit the stability of the solution, and/or passivate the current collectors.

(21) A solution of magnesium phenoxyaluminate (4) in DME exhibits moderate oxidative stability with the electrodes that were tested, showing onsets of oxidation between 1.5 V (aluminum, gold and platinum) and 2.2 V ss-316 vs magnesium, as shown in FIG. 4. A minor anodic process beginning around 1 V vs magnesium is observed on copper, followed by a larger process at approximately 2.3 V vs magnesium.

(22) The solution of magnesium perfluorophenoxyaluminate (5) in DME exhibits an onset of oxidation below 2 V vs magnesium on all electrodes tested, with ss-316 and aluminum exhibiting the lowest onset, as shown in FIG. 5.

(23) CV was used to examine the ability of these 0.25 M magnesium aluminate solutions to facilitate magnesium plating and stripping using a platinum working electrode.

(24) CV measurements of magnesium aluminate (1) in THF as well as magnesium aluminates (3) and (5) in DME did not show evidence of magnesium plating/stripping behaviour between −0.5 V and 1 V vs Mg.

(25) CV of magnesium aluminate (2) in DME shows that this solution facilitates plating and stripping of magnesium using a platinum working electrode over fifty cycles between −0.55 V and 1 V vs magnesium, as shown in FIG. 6. Plating overpotentials decrease over the 50 cycles from approximately −0.45 to −0.15 V vs magnesium. However, Coulombic efficiencies (CE) of the plating-stripping process peak at around 85% around cycle 15 and drops to 60% through cycle 50. This gradual decrease in CE suggests that the electrolyte decomposes during cycling and partially passivates the electrodes.

(26) CV of magnesium aluminate (4) in DME shows clear plating and stripping behaviour on platinum between −0.5 V and 1 V vs magnesium over 50 voltammetric cycles, as shown in FIG. 7. Again, plating overpotentials are observed to decrease from −0.41 V to −0.29 V vs magnesium over the 50 cycles. The CEs of magnesium plating and stripping facilitated by magnesium aluminate (4) increase over the 50 cycles to roughly 95%.

(27) The electrochemical behaviour of 0.25 M DME solutions of magnesium aluminates (2)-(5) was further examined in magnesium full cells constructed using Chevrel phase (Mo6S8) cathodes, magnesium ribbon anodes, and stainless steel current collectors both at room temperature and 55° C.

(28) Generally, the magnesium aluminate electrolytes exhibited better reversibility, maintained higher capacities over more charge-discharge cycles, and could be cycled at higher rates at 55° C. than at room temperature, as shown in FIG. 8. At room temperature, full cells containing magnesium aluminates (2)-(4) typically reached a maximum gravimetric capacity of around 80 mAh.Math.g-1 (FIGS. 8A, 8C, and 8E). However, at 55° C., full cells containing the same electrolytes maintained gravimetric capacities at around 100 mAh.Math.g-1 over 10 charge-discharge cycles with small to moderate overpotentials (FIGS. 8B, 8D, and 8F).

(29) Full cells containing magnesium aluminate (5) exhibited very poor charge-discharge behaviour and significant capacity fade within five cycles at room temperature and 55° C. The full cell performance of magnesium aluminate (5) in DME is consistent with its apparent instability as observed by LSV measurements. Without wishing to be bound by theory, the low stability of the magnesium pentafluorophenyl aluminate (5) may result from the stability of the pentafluorophenoxy anion, which could render it more labile and more easily removed from aluminum.