MAGNESIUM SALTS

Abstract

Described is a salt of the general formula: Mg.sup.2+(L.sub.x).sub.6(PF.sub.6).sub.2 wherein each L is a ligand selected from dichloromethane, a cyclic ether, or a nitrile of the general formula R—C≡N. The method of making the salt comprises the steps: providing Mg metal, activating the Mg metal in a first dry solution comprising a first ligand solution (L.sub.1), treating the dry solution of activated Mg metal and L.sub.1 with NOPF.sub.6 in a second dry solution comprising a second ligand solution (L.sub.2), heating the treated Mg metal solution removing residual solvent under vacuum, and recrystallizing the remaining solid to form the salt wherein L.sub.x comprises a mixture of L.sub.1 and L.sub.2. The salt can be used as the salt in an electrolyte, or as an additive to an electrolyte, in a cell or battery.

Claims

1. A salt of the general formula:
Mg.sup.2+(L).sub.x(PF.sub.6).sub.2  (i) wherein x represent a number between 1 and 6; and each L represents a ligand selected from one of the following compounds: a halomethane, a cyclic crown ether; or a nitrile of the general formula R—C≡N.

2. The salt of claim 1, wherein x is greater than 1, and L represents a ligand selected from only one of the following compounds: a halomethane, a cyclic crown ether; or a nitrile of the general formula R—C≡N.

3. The salt of claim 1, wherein x is equal to 6, ligand L is a nitrile and R represents an organic group independently selected from the following: methyl, ethyl, propyl, butyl, .sup.tbutyl, pentyl, ethylene, propylene, butylene, pentylene, toluene, naphthalene, or phenyl.

4. The salt of claim 3, wherein R is the same for each ligand represented by L.

5. The salt of claim 2, wherein each ligand L is acetonitrile.

6. The salt of claim 1, wherein x is equal to 1 and L is a cyclic crown ether selected from one of the following: [12]-crown-4, [18]-crown-6, [24]-crown-8.

7. The salt of claim 1, wherein the halomethane is dichloromethane.

8. A method of making a salt of the general formula:
Mg.sup.2+(L.sub.y).sub.x(PF.sub.6).sub.2  (i) wherein x represent a number between 1 and 6, L.sub.y represents a ligand independently selected from any one of the following compounds: a halomethane, a cyclic crown ether; or a nitrile of the general formula R—C≡N; and L.sub.y comprises a mixture of compounds L.sub.1 and L.sub.2; the method comprising: providing Mg metal, washing and activating the Mg metal in a first dry solution comprising a first compound (L.sub.1), treating the solution of activated Mg metal and first compound L.sub.1 with NOPF.sub.6 in a second dry solution comprising a second compound (L.sub.2), removing the residual solvent, and recrystallizing the remaining solid to form the salt of Formula (i).

9. The method of claim 8, wherein x is greater than 1, and L.sub.y represents a ligand selected from only one of the following compounds: a halomethane, a cyclic crown ether; or a nitrile of the general formula R—C≡N.

10. The method of claim 8, wherein x is equal to 6, L.sub.1 and L.sub.2 are each nitriles, and for L.sub.1 and L.sub.2 R independently represents an organic group selected from the following: methyl, ethyl, propyl, butyl, .sup.tbutyl, pentyl, ethylene, propylene, butylene, pentylene, toluene, naphthalene, or phenyl.

11. The method of claim 10, wherein L.sub.1 and L.sub.2 are the same nitrile.

12. The method of claim 11, wherein L.sub.1 and L.sub.2 are both acetonitrile.

13. The method of claim 8, wherein x is equal to 1 and L is a cyclic crown ether selected from one of the following: [12]-crown-4, [18]-crown-6, [24]-crown-8.

14. The method of claim 8, wherein the halomethane is dichloromethane.

15. An electrolyte comprising the salt of claim 1.

16. A cell comprising the electrolyte of claim 15.

17. A battery comprising the electrolyte of claim 15.

18. The cell of claim 16, wherein the cell is a magnesium cell or a magnesium-ion cell.

19. The battery of claim 17, wherein the battery is a magnesium battery or a magnesium-ion battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order that the present invention may be more readily understood, an embodiment of the invention will now be described, by way of example, with reference to the accompanying Figures, in which:

[0017] FIG. 1 is an X-ray crystal structure of a salt of the present invention;

[0018] FIG. 2 is a .sup.1H NMR spectrum of a salt of the present invention;

[0019] FIG. 3 is a .sup.13C NMR spectrum of a salt of the present invention;

[0020] FIG. 4 is a .sup.19F NMR spectrum of a salt of the present invention;

[0021] FIG. 5 is a .sup.31P NMR spectrum of a salt of the present invention;

[0022] FIG. 6 is a cyclic voltammogram of a 0.12 M solution of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 in 1:1 THF—CH.sub.3CN cycling at a rate of 25 mVs.sup.−1 in a three electrode cell containing a glassy carbon working electrode and Mg reference and counter electrodes at 25° C.;

[0023] FIG. 7 is a linear sweep voltammogram of 0.12 M Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 in 1:1 THF—CH.sub.3CN scanning at a rate of 25 mVs.sup.−1 on platinum, stainless steel, glassy carbon, and aluminium working electrodes;

[0024] FIG. 8 is a cyclic voltammogram of 0.12 M solution of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 in 1:1 THF—CH.sub.3CN cycling at a rate of 50 mVs.sup.−1 in a symmetric three electrode Mg|Mg|Mg flooded cell at 25° C. (insets: expansion of the region showing Mg plating);

[0025] FIG. 9 is a cyclic voltammogram of 0.71 M solution of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 in 1:1 THF—CH.sub.3CN cycling at a rate of 50 mVs.sup.−1 in a symmetric three electrode Mg|Mg|Mg flooded cell at 25° C.;

[0026] FIG. 10 shows three galvanostatic discharge-charge cycles of a coin cell containing a 0.71 M solution of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 in 1:1 THF—CH.sub.3CN, an Mg anode, an Mo.sub.3S.sub.4 cathode, and Al current collectors, cycling at a rate of C/100; and

[0027] FIG. 11 shows three galvanostatic discharge-charge cycles of a coin cell containing a 0.71 M solution of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 in 1:1 THF—CH.sub.3CN, an Mg anode, an Mo.sub.3S.sub.4 cathode, and carbon film current collectors, cycling at a rate of C/100.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention will now be illustrated with reference to the following examples.

EXAMPLE 1

Synthesis of Mg(CH.SUB.3.CN).SUB.6.(PF.SUB.6.).SUB.2

[0029] Magnesium metal in the form of Magnesium powder (>99%) from Sigma Aldrich was washed and activated with approximately 10 mg of I.sub.2 until the solution became colourless. The resulting mixture was solvated dropwise in dry solution of CH.sub.3CN and NOPF.sub.6 (purchased from ACROS Organics) under an atmosphere of dry N.sub.2 at room temperature. After adding the NOPF.sub.6 solution, the reaction mixture evolved a colourless gas (NO) which was vented from the reaction flask under dry N.sub.2. The solution was heated gently to 45° C. overnight. The equation for the reaction is given below (1).

##STR00001##

After removal of solvent, an off-white solid was recrystallized twice from hot acetonitrile, affording a white crystalline powder of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 with a yield of 52%.

EXAMPLE 2

Characterisation of Mg(CH.SUB.3.CN).SUB.6.(PF.SUB.6.).SUB.2

[0030] A single crystal obtained from the diffusion of Et.sub.2O in to a CH.sub.3CN solution of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2. X-ray analysis was carried out on data collected with a Bruker D8 Quest CCD diffractometer and confirmed the complex to be the desired salt (FIG. 1).

[0031] The .sup.1H, .sup.13C, .sup.19F and .sup.31P NMR spectra of the white crystalline powder of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 are shown in FIGS. 2 to 5, respectively. Notably, the .sup.19F and .sup.31P NMR spectra exhibited a doublet and heptet, respectively, characteristic of the PF.sub.6.sup.− anion. NMR spectra were recorded at 298.0 K on a Bruker 500 MHz AVIII HD Smart Probe Spectrometer (.sup.1H at 500 MHz, .sup.31P 202 MHz, .sup.13C 125 MHz, .sup.19F 471 MHz) or a Bruker 400 MHz AVIII HD Smart Probe spectrometer (.sup.1H at 400 MHz, .sup.31P 162 MHz, .sup.13C 101 MHz, .sup.19F 376 MHz) unless otherwise specified. Chemical shifts (δ, ppm) are given relative to residual solvent signals for .sup.1H and .sup.13C, to external 85% H.sub.3PO.sub.4 for .sup.31P and to CCl.sub.3F for .sup.19F.

[0032] Bulk purity of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 was confirmed by elemental analysis (C, H, and N). Elemental microanalytical data were obtained from the University of Cambridge, Department of Chemistry microanalytical service. Additionally, the IR spectrum of 1 exhibits the expected C≡N stretching band at 2299 cm.sup.−1. FT-IR spectroscopic measurements were conducted using a PerkinElmer universal ATR sampling accessory.

EXAMPLE 3

Use of Mg(CH.SUB.3.CN).SUB.6.(PF.SUB.6.).SUB.2 .as an Electrolyte Salt

[0033] All Cyclic voltammetry and linear sweep voltammetry 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] FIG. 6 shows the cyclic voltammetry of a 0.12 M solution of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 in a 1:1 ratio with THF—CH.sub.3CN. The electrolyte was cycled reversibly between −0.5 and 1.5 V vs Mg over at least 20 cycles at a rate of 25 mVs.sup.−1 using a glassy carbon working electrode (purchased from Alvatek Limited) and an Mg reference and counter electrodes (Magnesium ribbon from Sigma Aldrich (99.9%)). The electrolyte could be cycled for at least 20 cycles with only moderate loss in plating/stripping current, exhibiting a small stripping overpotential (ca. 0.25 V vs Mg) and a plating onset at 0 V vs Mg/Mg.sup.2+. Broad features observed around 0 V vs Mg/Mg.sup.2+ returning to positive potentials are thought to be the result of capacitive effects arising from the high surface area glassy carbon electrode.

[0035] The electrochemical stability of the optimized Mg(PF.sub.6).sub.2 electrolyte was further studies by performing linear sweep voltammetry (LSV) using platinum (Pt) (Platinum wire (99.95%) purchased from Alfa Aesar), glassy carbon (GC), stainless steel (ss) (Stainless steel 316 purchased from Advent Research materials), and aluminium (Al) working electrodes (Purchased from Dexmet corp.) (results shown in FIG. 7). On the Pt and GC electrodes the onset of electrolyte oxidation occurs around 3 V vs Mg/Mg.sup.2+ while on ss the oxidation occurs at potentials around 1.5 V vs Mg/Mg.sup.2+. Virtually no current is observed when scanning with the Al working electrode out to 4 V vs Mg/Mg.sup.2+, suggesting that the Al surface is passivated. The 1:1 THF—CH.sub.3CN electrolyte solvent mixture was found to exhibit superior electrochemical stability and plating-stripping reversibility on GC than the 0.12 M electrolyte solution in pure CH.sub.3CN under the same conditions.

EXAMPLE 4

Use of Mg(CH.SUB.3.CN).SUB.6.(PF.SUB.6.).SUB.2 .as an Electrolyte in an Mg-Ion Cell

[0036] A study was conducted into the use of the Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 salt as an electrolyte in a symmetrical cell (Mg|Mg|Mg) using Mg as the working electrode. Battery cycling was conducted on an Arbin BT2043 battery test system. Both the 0.12 and 0.71 M solutions of Mg(CH.sub.3CN).sub.6(PF.sub.6).sub.2 in 1:1 ratio with THF—CH.sub.3CN were cycled between −0.5 and 0.5 V vs. Mg at a rate of 50 mV.Math.s.sup.−1 for 10 cycles. FIGS. 8 and 9 show the voltammograms of the 0.12 and 0.71 M electrolyte solutions. These solutions have exhibited reversible plating and stripping of Mg over 10 cycles with little to no loss in plating current density. The reversibility of these processes, showing virtually no attenuation of current density, suggests that the Mg electrode remains free of insulating or passivating films.

[0037] From the above results, it is understood that an Mg(PF.sub.6).sub.2 based electrolyte can facilitate the reversible plating and stripping of Mg using GC as well as Mg electrodes. A further investigation of this new electrolyte was conducted in prototype coin cell batteries. The 0.71 M electrolyte solution was used in coin cells constructed using an Mg anode and a Chevrel phase cathode (Mo.sub.3S.sub.4). Owing to the observed reactivity of this electrolyte on stainless steel, unreactive carbon film current collectors were employed to limit possible side reactions. The coin cells, cycled at C/100, showed reversible charge-discharge profiles (shown with Al current collectors and carbon film (Carbon filled polyethylene purchased from Goodfellow Cambridge Limited) current collectors in FIGS. 10 and 11, respectively). These cells could be cycled for at least three cycles reaching a maximum reversible capacity of 51 and 53 mAhg.sup.−1 for the Al and carbon film cells, respectively, while suffering a moderate fade in capacity. It is understood that this observed fade is common for many Mg-ion systems.