MAGNESIUM SALTS

Abstract

A salt of the formula: Mg[Al(R).sub.4].sub.2, where R represents a halogen-free compound selected from a deprotonated alcohol or thiol; or an amine; or a mixture thereof.

Claims

1. A salt comprising: Mg[Al(R).sub.4].sub.2, wherein R represents a halogen-free compound selected from a deprotonated alcohol, a thiol, an amine or a mixture thereof.

2. The salt of claim 1, wherein one of the halogen-free alcohol, thiol, amine, or mixture thereof is aromatic.

3. The salt of claim 1, wherein an organic moiety of the halogen-free alcohol, thiol, amine, or mixture thereof comprises tert-butyl or phenyl.

4. The salt of claim 1, wherein the salt is crystallised from an organic solvent.

5. The salt of claim 4, wherein the organic solvent is dry DME, 2-methyl-THF, diglyme, triglyme, tetraglyme, or THF.

6. The salt of claim 1, wherein R represents a halogen-free deprotonated alcohol.

7. An electrolyte comprising a salt of claim 1.

8. The electrolyte of claim 7, wherein the electrolyte comprises an Mg(PF.sub.6).sub.2 additive.

9. A cell or battery comprising an electrolyte of claim 7.

10. The cell or battery of claim 9, wherein the cell or battery is a magnesium cell or battery or a magnesium-ion cell or battery.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0012] 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:

[0013] FIG. 1 is an X-ray single crystal structure of a salt, according to some embodiments;

[0014] FIG. 2 is an X-ray single crystal structure of a salt, according to some embodiments;

[0015] FIG. 3 is a .sup.1H NMR spectrum of magnesium tertbutoxyaluminate (1), according to some embodiments;

[0016] FIG. 4 is a .sup.13C NMR spectrum of magnesium tertbutoxyaluminate (1), according to some embodiments;

[0017] FIG. 5 is a .sup.27Al NMR spectrum of magnesium tertbutoxyaluminate (1), according to some embodiments;

[0018] FIG. 6 is a .sup.1H NMR spectrum of magnesium phenoxyaluminate (2), according to some embodiments;

[0019] FIG. 7 is a .sup.13C NMR spectrum of magnesium phenoxyaluminate (2), according to some embodiments;

[0020] FIG. 8 is a .sup.27Al NMR spectrum of magnesium phenoxyaluminate (2), according to some embodiments;

[0021] FIG. 9 shows LSV measurements of an electrolyte solution of magnesium tertbutoxyaluminate (1) in THF, according to some embodiments;

[0022] FIG. 10 shows LSV measurements of an electrolyte solution of magnesium phenoxyaluminate (2) in DME, according to some embodiments;

[0023] FIG. 11 shows CV measurements of an electrolyte solution of magnesium phenoxyaluminate (2) in DME using a Pt working electrode, according to some embodiments;

[0024] FIG. 12 shows the cycling behaviour of an electrolyte solution of magnesium phenoxyaluminate (2) in DME in a coin cell constructed using a magnesium ribbon anode and a Chevrel phase cathode, cycling at room temperature, according to some embodiments; and

[0025] FIG. 13 shows the cycling behaviour of an electrolyte solution of magnesium phenoxyaluminate (2) in DME in a coin cell constructed using a magnesium ribbon anode and a Chevrel phase cathode, cycling at 55 C., according to some embodiments.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0026] The present disclosure will now be illustrated with reference to the following examples.

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

[0027] A mixture of sodium aluminium hydride from XXX and magnesium chloride from XXX in a ratio of 2:1 was ball-milled for an hour to produce a mixture of magnesium aluminium hydride and sodium chloride at a theoretical 42.5 wt % of magnesium aluminium chloride (scheme below).

##STR00001##

[0028] The resulting magnesium aluminium hydride mixture offers a general platform for the synthesis of magnesium aluminates, as will be shown by the following examples.

Example 2Synthesis of Magnesium Aluminates Using Alcohol

[0029] Magnesium aluminates were synthesized by treating magnesium aluminium hydride with various fluorinated/non-fluorinated alkyl and aryl alcohols in dry THF or DME (Scheme below).

##STR00002##

[0030] These reactions were followed by filtration under inert atmosphere to remove insoluble impurities (i.e. sodium chloride and aluminium-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; and (2) phenol.

Example 3Characterisation of Magnesium Aluminates

[0031] A single crystal was obtained from THF containing magnesium tertbutoxyaluminate (1), and magnesium phenoxyaluminate (2), as shown in FIGS. 1 and 2, respectively. X-ray analysis was carried out on data collected with a Bruker D8-Quest PHOTON-100 diffractometer equipped with an Incoatec IS Cu microsource (=1.5418 ). and confirmed the complex to be the desired salt.

[0032] Multinuclear NMR spectra of the powder of the two magnesium aluminates is shown in FIGS. 3 to 8. Magnesium tertbutoxyaluminate (1) exhibits the following NMR signals: .sup.1H NMR (C.sub.6D.sub.6) 1.48 (s, 1H), 1.46 (s, 1H) ppm; 13C NMR (C.sub.6D.sub.6) 71.82, 68.62, 34.20, 33.24 ppm; .sup.27Al NMR (DME) 49.17 ppm. Magnesium phenoxyaluminate (2) exhibits the following NMR signals: .sup.1H NMR (C.sub.6D.sub.6) 7.08-6.99 (m, 32H), 6.76 (t, J=7.2 Hz, 8H), 3.64 (s, THF), 1.27 (s, THF) ppm; .sup.13C NMR (C.sub.6D.sub.6) 156.89, 129.99, 121.16, 120.44, 69.99, 25.11 ppm; .sup.27Al NMR (DME). NMR spectra were recorded at 298.0 K on a Bruker 400 MHz AVIII HD Smart Probe spectrometer CH at 400 MHz, .sup.13C 101 MHz, .sup.27Al 104 MHz) unless otherwise specified. Chemical shifts (, ppm) are given relative to residual solvent signals for .sup.1H and .sup.13C and to external Al(NO.sub.3).sub.3 for .sup.27Al.

Example 3Use of Magnesium Aluminates as an Electrolyte Salt

[0033] 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.

[0034] A solution of the magnesium aluminates above (1) and (2) 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), aluminium, copper, gold, and platinum electrodes, with the onset of oxidation occurring at around 1 V vs magnesium on each electrode, as shown in FIG. 9.

[0035] In contrast to magnesium tertbutoxyaluminate (1), a solution of magnesium phenoxyaluminate (2) in DME exhibits moderate oxidative stability with the electrodes that were tested, showing onsets of oxidation between 1.5 V (aluminium, gold and platinum) and 2.2 V ss-316 vs magnesium, as shown in FIG. 10. 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.

[0036] 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.

[0037] CV measurements of magnesium aluminate (1) in THF did not show evidence of magnesium plating/stripping behaviour between 0.5 V and 1 V vs Mg.

[0038] CV of magnesium aluminate (2) 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. 11. Plating overpotentials are observed to decrease from 0.41 V to 0.29 V vs magnesium over the 50 cycles.

[0039] The electrochemical behaviour of 0.25 M DME solutions of magnesium aluminate (2) 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.

[0040] 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 FIGS. 12 and 13. At room temperature, full cells containing magnesium aluminate (2) typically reached a maximum gravimetric capacity of around 80 mAh-g1 (FIG. 12). However, at 55 C., full cells containing the same electrolytes maintained gravimetric capacities at around 100 mAh-g1 over 10 charge-discharge cycles with small to moderate overpotentials (FIG. 13).