CN107086323B - Magnesium salt - Google Patents

Abstract

A salt of the formula: mg (magnesium)²⁺(L)_x(PF₆)₂Wherein each L is a ligand selected from one of the following compounds: methyl halide, cyclic crown ether; or a nitrile of the formula R-C.ident.N. The method for preparing the salt comprises the following steps: providing Mg metal in a solution (L) containing a first ligand₁) In a first non-aqueous solution of activated Mg metal with NOPF₆In a solution containing a second ligand (L)₂) Treating the activated Mg metal and L in a second anhydrous solution of₁Heating the treated Mg metal solution to remove residual solvent under vacuum, and recrystallizing the remaining solid to form a salt, wherein L_xComprises L₁And L₂A mixture of (a). The salt may be used as a salt in an electrolyte, or added as an additive to the electrolyte of a unit cell or battery.

Description

Magnesium salt

Technical Field

The present invention relates to magnesium hexafluorophosphate salts. In addition, the invention relates to a process for the preparation of magnesium salts of hexafluorophosphate and the use of magnesium salts of hexafluorophosphate as an electrolyte in a cell or battery (battery).

Background

Currently, lithium ion batteries are used in various electronic devices. Lithium ion cells are more popular than other battery technologies because of their ability to be recharged for short periods of time without significant loss of capacity. Furthermore, the energy density of lithium ion batteries enables their use in portable products such as portable computers and mobile phones. However, lithium batteries are known to lose capacity over time. And, heat dissipation and overheating risk problems are widely reported.

Many lithium ion electrolyte systems have been developed and the use of a wide range of lithium salts, including LiBF, has been investigated₄、LiClO₄、LiNTF₂、LiPF₆、LiAsF₆And LiSbF₆And the like. LiPF₆Is a preferred electrolyte in lithium ion cells because several of its properties are balanced, and other lithium salts are not found to have such a balance. However, given the relatively low abundance of lithium in the earth's crust, lithium is currently at a high price relative to alkali and alkaline earth metals, and there is concern over long-term use of lithium batteries.

Disclosure of Invention

In a first aspect, the present invention provides a salt of the general formula:

Mg²⁺(L)_x(PF₆)₂(i)

wherein x represents a number from 1 to 6; each L represents a ligand selected from one of the following compounds: methyl halide, cyclic crown ether; or a nitrile of the formula R-C.ident.N.

It is theorized that alkaline earth metals, such as magnesium, may be used in electrochemical cells and electrolyte solutions in batteries. Magnesium has a high abundance in the crust of the earth and is therefore cheaper per ton than other alkali and alkaline earth metals. In addition, magnesium has a higher capacity than lithium. Also, in magnesium-ion unit cells, magnesium metal can be used as a metal anode without the risk of thermal runaway, since dendrites are not formed on the magnesium metal. However, despite this, magnesium has not been widely selected as an electrolyte or anode material because it is difficult to form an electrolyte that is stable over a wide voltage range and compatible with multiple electrodes.

As described above, lithium hexafluorophosphate is a preferred electrolyte in lithium ion unit cells. However, the obstacles to using magnesium hexafluorophosphate-based electrolytes in magnesium ion batteries are in fact: the synthesis of alkaline earth metal hexafluorophosphate salts is more expensive and more problematic (often resulting in lower purity materials) than the synthesis of lithium hexafluorophosphate salts. Moreover, MgPF₆PF of₆ ^-The anion is said to react with the magnesium metal anode to form passivated MgF₂And (3) a layer. However, it has been found that the magnesium hexafluorophosphate salt of the present invention can be easily synthesized in solution under relatively mild conditions, and the resulting salt can also be used as an electrolyte in coin cells, and no passivation occurs at the anode.

The term "salt" as used in the present specification is intended to cover a complex magnesium salt with a ligand (L), which falls within the scope of the above general formula. The choice of ligand or mixture of ligands can make the reaction mixture more stable in the synthesis of magnesium hexafluorophosphate salts. When x is greater than 1, each ligand may be independently selected from a methyl halide, or a cyclic crown ether or nitrile compound. In view of simplifying the reaction mixture during synthesis, when x is greater than 1, L may represent a ligand selected from only one of the following compounds: methyl halide, cyclic crown ether; or a nitrile of the formula R-C.ident.N. That is, L may comprise two or more methyl halides, a plurality of cyclic crown ethers or two or more nitriles of the formula R-C.ident.N.

The methyl halide may be methyl chloride, such as CH₂Cl₂，CHCl₃，CCl₄. The chlorinated methane represents a stable and inexpensive anhydrous solvent for the synthesis. Dichloromethane (CH)₂Cl₂) It is particularly suitable as ligand and solvent for magnesium salt synthesis due to its low boiling point and solvation characteristics.

The cyclic crown ether may comprise a generally cyclic crown ether selected from one of the following: [12]-crown-4, [18]]-crown-6, [24]]-crown-8. The cyclic crown ethers may be used to chelate magnesium cations. The use of polydentate ligands is advantageous because magnesium cations remain in solution, but have lower reactivity and can also inhibit the plating of magnesium on the electrode surface. These crown polyethers may be reacted with halomethyl solvents (e.g. CH)₂Cl₂，CHCl₃，CCl₄) Used in combination without interfering with the desired synthesis of the magnesium salt.

In the general formula of the nitrile, and when x is 6, each R represents an organic group and is independently selected from: methyl, ethyl, propyl, butyl, tert-butyl, pentyl, ethylene (ethylene), propylene (propylene), butylene (butylene), pentene (pentylene), toluene (tolumene), naphthalene (naphthalene) or phenyl. Sterically bulky ligands may prevent solvation of the magnesium cation. Thus, for this formula, R may preferably represent a nitrile providing group, which is believed to have low steric hindrance.

Each L may be the same nitrile. This makes the synthesis of the salt simpler and clearer, since the same nitrile solution can be used for the activation step as well as for the treatment step. For this salt, L may be acetonitrile, which is the least sterically hindered nitrile. As an additional advantage, the use of acetonitrile provides good solvation of the magnesium cation, as well as low production costs, since it is easier to effect the solvent under high vacuum than other solvents. The desolvated salt can then be redissolved with, for example, an ether (e.g., THF, diethyl ether) or other donor solvent.

In a second aspect, the present invention provides a process for the preparation of a salt of the general formula:

Mg²⁺(L_y)_x(PF₆)₂(ii)

wherein x represents a number from 1 to 6; l is_yRepresents a ligand independently selected from any one of the following compounds: methyl halide, cyclic crown ether; or a nitrile of the formula R-C.ident.N; l is_yComprising a compound L₁And L₂A mixture of (a); the method comprises the following steps: providing Mg metal in a mixture containing a first compound (L)₁) In a first dry solution (dry solution) of (A) with NOPF₆In the presence of a second compound (L)₂) Treating the activated Mg metal and the first compound L in a second anhydrous solution of₁Removing residual solvent, and recrystallizing the remaining solid to form the salt of formula (ii).

The residual solvent can be removed by evaporation, for example in vacuo or by heating.

In a third aspect, the present invention provides an electrolyte comprising a salt according to formula (i) or formula (ii) above. The salt may be included in the electrolyte as an additive to conventional electrolytes, or the salt may be used in neat solution to form the electrolyte itself with a suitable solvent.

In a fourth aspect, the invention provides a cell or battery comprising an electrolyte according to formula (i) or formula (ii) above. The salts of the present invention do not suffer from some of the same disadvantages observed when using lithium salts in electrochemical cells or batteries. In addition, the salts of the present invention may be used in the electrolyte of a number of unit cells or battery systems. More specifically, the unit cell or battery may be, for example, a lithium unit cell or a lithium ion unit cell. However, a cell or battery using the salts of the present invention may be more generally described as a metal-based or metal ion-based cell or battery. Examples of other metal-based or metal ion-based cells or batteries may include magnesium, calcium, or aluminum metal or metal ions. When the salt of the present invention is used in a metal unit cell or an electrolyte of a battery, a metal such as magnesium, calcium or aluminum can be used as a metal anode without the risk of salt decomposition. Another advantage is that the salts of the present invention are useful in reducing or limiting corrosion of metal or metal ion based unit cells or aluminum current collectors used in batteries.

Drawings

In order that the invention may be more readily understood, embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

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

FIG. 2 is a diagram of a salt of the present invention¹H NMR spectrum;

FIG. 3 is a diagram of a salt of the present invention¹³A C NMR spectrum;

FIG. 4 is a diagram of a salt of the present invention¹⁹F NMR spectrum;

FIG. 5 is a drawing of a salt of the invention³¹A P NMR spectrum;

FIG. 6 is Mg (CH) at 25 ℃ in a three-electrode unit cell comprising a glassy carbon working electrode and Mg reference and counter electrodes₃CN)₆(PF₆)₂At 1: 1THF-CH₃0.12M solution in CN at 25mVs^-1Cyclic voltammogram of rate cycles of (a);

FIG. 7 shows Mg (CH) on platinum, stainless steel, glassy carbon and aluminum working electrodes₃CN)₆(PF₆)₂At 1: 1THF-CH₃0.12M solution in CN at 25mVs^-1A linear scan voltammogram of a rate scan of (a);

FIG. 8 is a graph of Mg (CH) in a symmetrical three-electrode Mg | Mg | Mg flooded cell at 25 deg.C₃CN)₆(PF₆)₂At 1: 1THF-CH₃0.12M solution in CN at 50mVs^-1Cyclic voltammogram cycled at rate (inset: showing the expansion of the Mg plated area);

FIG. 9 shows Mg (CH) in a symmetrical three-electrode Mg | Mg | Mg flooded cell at 25 deg.C₃CN)₆(PF₆)₂At 1: 1THF-CH₃0.71M solution in CN at 50mVs^-1Cyclic voltammogram of rate cycles of (a);

FIG. 10 shows cycling at C/100 rate, containing Mg (CH)₃CN)₆(PF₆)₂At 1: 1THF-CH₃0.71M solution in CN, Mg anode, Mo₃S₄Three constant current discharge-charge cycles of button cells with cathode and Al current collectors; and

FIG. 11 shows cycling at C/100 rate, containing Mg (CH)₃CN)₆(PF₆)₂At 1: 1THF-CH₃0.71M solution in CN, Mg anode, Mo₃S₄Three constant current discharge-charge cycles for coin cells with cathode and carbon film current collectors.

Detailed Description

The invention is described with reference to the following examples.

Example 1 Mg (CH)₃CN)₆(PF₆)₂Synthesis of (2)

Magnesium metal in the form of magnesium powder (> 99%) from Sigma Aldrich was used with approximately 10mg of I₂Washed and activated until the solution becomes colorless. The resulting mixture was dried at room temperature under N₂In an atmosphere of CH₃CN and NOPF₆(from ACROS Organics) was solvated drop wise in anhydrous solution. In the presence of NOPF₆After solution, the reaction mixture produces a colorless gas (NO), which is removed from the dry N₂The lower reaction flask was drained. The solution was gently heated to 45 ℃ overnight. The equation for the reaction is as follows (1).

After removal of the solvent, the off-white solid was recrystallized twice with hot acetonitrile to give Mg (CH)₃CN)₆(PF₆)₂White crystalline powder, yield 52%.

Example 2 Mg (CH)₃CN)₆(PF₆)₂Is characterized by

From Et₂O in Mg (CH)₃CN)₆(PF₆)₂CH (A) of₃Diffusion in CN solution resulted in single crystals. X-ray analysis of the data collected using a Bruker D8 quest ccd diffractometer confirmed that the complex was the desired salt (fig. 1).

Mg(CH₃CN)₆(PF₆)₂Of white crystalline powder of¹H、¹³C、¹⁹F and³¹the P nmr spectra are shown in figures 2-5, respectively. It is to be noted that it is preferable that,¹⁹f and³¹the P NMR spectrum respectively shows a bimodal peak and a heptad peak, and the PF is characterized₆ ^-An anion. NMR spectra were obtained from a Bruker 500MHz A VIII HD Smart Probe Spectrometer (R) unless otherwise stated¹The frequency of the H is lower than that of the H at 500MHz,³¹P 202MHz，¹³C 125MHz，¹⁹f471 MHz) or Bruker 400MHz A VIII HD Smart Probe spectrometer (¹The frequency of H is under the frequency of 400MHz,³¹P 162MHz，¹³C 101MHz，¹⁹f376 MHz) was recorded at 298.0K. Chemical shifts (. delta.,. ppm) for¹H and¹³c signal relative to residual solvent is given for³¹P relative to the outer 85% H₃PO₄Given, and to¹⁹F vs CCl₃F is given.

Determination of Mg (CH) by elemental analysis (C, H and N)₃CN)₆(PF₆)₂Bulk purity (bulk purity). Elemental microanalysis data was obtained from the microanalysis service of the chemical institute of cambridge university. Furthermore, the infrared spectrum of 1 is 2299cm^-1The expected C ≡ N stretch peak is shown. FT-IR spectroscopy was performed using a Perkinelmer universal ATR sampling accessory.

Example 3 Mg (CH)₃CN)₆(PF₆)₂Application as electrolyte salt

All cyclic voltammograms and linear scanning voltammogram experiments recorded below were performed in a glove box (MBraun) under an atmosphere of dry argon using anhydrous solvents. Cyclic voltammograms and linear scan voltammograms were performed using IVIUM CompactStat.

FIG. 6 shows Mg (CH)₃CN)₆(PF₆)₂At 1: 1THF-CH₃Cyclic voltammogram of a 0.12M solution in CN. The electrolyte is between-0.5 and 1.5V (relative to Mg) at 25mVs^-1Reversibly cycling at a rate of at least more than 20 cycles using a glassy carbon working electrode (available from Alvatek Limited) and an Mg reference and counter electrode (magnesium strips from Sigma Aldrich (99.9%)). The electrolyte can be cycled for at least 20 cycles with only a modest loss of plating/stripping current, exhibiting small stripping overvoltage (about 0.25V vs. magnesium) and at 0V (vs. Mg/Mg)²⁺) The electroplating of (1) is started. For Mg/Mg²⁺The wide range of features observed near 0V back to positive potential is believed to be a result of the capacitive effect caused by the high surface area glassy carbon electrode.

Optimum Mg (PF)₆)₂Electrochemical stability of the electrolyte was further investigated by performing a Linear Scanning Voltammogram (LSV) using platinum (Pt) (platinum wire (99.95%) from Alfa Aesar), Glassy Carbon (GC), stainless steel (ss) (stainless steel 316 from additive Research materials) and aluminum (Al) working electrode (from Dexmet corp.) (results are shown in fig. 7). On the Pt and GC electrodes, the initial oxidation of the electrolyte occurred at 3V (vs. Mg/Mg)²⁺) Whereas over ss, the oxidation occurs at a potential of 1.5V (vs Mg/Mg)²⁺) Nearby. When scanning with Al working electrode to over 4V (vs. Mg/Mg)²⁺) When the surface was passivated, almost no current was observed, suggesting that the surface of Al was passivated. Found in pure CH₃1: 1THF-CH to 0.12M electrolyte solution in CN₃The CN electrolyte solvent mixture showed excellent electrochemical stability and plating-dissolution reversibility on GC under the same conditions.

Example 4 Mg (CH)₃CN)₆(PF₆)₂Application of electrolyte in Mg ion unit cell

Mg(CH₃CN)₆(PF₆)₂The use of salts as electrolytes in symmetrical unit cells (Mg | Mg) using Mg as the working electrode was investigated. Battery cycling was performed on an Arbin BT2043 battery test system. Mg (CH)₃CN)₆(PF₆)₂THF-CH at 1: 1 ratio₃0.12M solution and 0.71M solution in CN,between-0.5 and 0.5V (relative to Mg) at 50mVs^-1At a rate of 10 cycles. FIGS. 8 and 9 show voltammograms of the 0.12M and 0.71M electrolyte solutions. These solutions showed reversible Mg plating and dissolution in 10 cycles with little loss in plating current density. The reversibility of these processes shows almost no decay of the current density, suggesting that the Mg electrode remains free of insulating or passivation films.

As understood from the above results, Mg (PF)₆)₂The base electrolyte may facilitate reversible plating and stripping of Mg using GC and Mg electrodes. Further investigations of the novel electrolyte were carried out in prototype button cells. The 0.71M electrolyte solution was used in coin cells constructed using Mg anodes and cut friery phase cathodes. Due to the observed reactivity of the electrolyte on stainless steel, a non-reactive carbon film current collector was used to limit possible side reactions. Coin cells at C/100 cycles showed reversible charge-discharge curves (shown in fig. 10 and 11 with Al current collector and carbon film current collector, respectively, (carbon-charged polyethylene from Goodfellow cambridge limited)). For Al and carbon film cells, although experiencing modest capacity reduction, these cells can be cycled at least 3 times to 51 and 53mAhg, respectively^-1Cycling of the maximum reversible capacity of. It is understood that this observed reduction is common to many Mg ionic systems.

Claims (20)

1. A salt of the formula:

Mg²⁺(L)_x(PF₆)₂(i)

wherein x represents a number from 1 to 6; and

each L represents a ligand selected from one of the following compounds:

the halogenated methane is obtained by reacting halogenated methane with water,

a cyclic crown ether; or

Nitriles of the general formula R-C.ident.N,

r represents an organic group independently selected from: methyl, ethyl, propyl, butyl, tert-butyl, pentyl, ethylene, propylene, butene, pentene, toluene, naphthalene or phenyl.

2. A salt according to claim 1, wherein x is greater than 1,

l represents a ligand selected from only one of the following compounds:

the halogenated methane is obtained by reacting halogenated methane with water,

a cyclic crown ether; or

Nitriles of the general formula R-C.ident.N.

3. A salt according to claim 1 or 2, wherein x is 6 and ligand L is a nitrile.

4. A salt according to claim 3, wherein R is the same for each ligand represented by L.

5. A salt according to claim 1 or 2, wherein each ligand L is acetonitrile.

6. A salt according to claim 3, wherein each ligand L is acetonitrile.

7. A salt according to claim 4, wherein each ligand L is acetonitrile.

8. The salt according to claim 1 or 2, wherein x is 1 and L is a cyclic crown ether selected from the group consisting of: [12] -crown-4, [18] -crown-6, [24] -crown-8.

9. The salt according to claim 1 or 2, wherein the methyl halide is dichloromethane.

10. A process for preparing a salt of the formula:

Mg²⁺(L_y)_x(PF₆)₂(i)

wherein x represents a number from 1 to 6,

L_yrepresents a ligand independently selected from any one of the following compounds:

the halogenated methane is obtained by reacting halogenated methane with water,

a cyclic crown ether; or

A nitrile of the general formula R-C ≡ N, R representing an organic group independently selected from: methyl, ethyl, propyl, butyl, tert-butyl, pentyl, ethylene, propylene, butene, pentene, toluene, naphthalene or phenyl; or

L_yComprising a compound L₁And L₂A mixture of (a); the compound L₁And L₂Independently selected from any one of the following compounds:

the halogenated methane is obtained by reacting halogenated methane with water,

a cyclic crown ether; or

A nitrile of the general formula R-C ≡ N, R representing an organic group independently selected from: methyl, ethyl, propyl, butyl, tert-butyl, pentyl, ethylene, propylene, butene, pentene, toluene, naphthalene or phenyl; the method comprises the following steps:

the provision of the Mg metal,

in the presence of a first compound (L)₁) The first non-aqueous solution of (a) cleans and activates Mg metal,

by NOPF₆In the presence of a second compound (L)₂) Treating the activated Mg metal and the first compound L in a second anhydrous solution of₁The solution (a) of (b) is,

removing residual solvent, and

recrystallizing the remaining solid to form the salt of formula (i).

11. The method according to claim 10, wherein x is greater than 1,

L_yrepresents a ligand selected from only one of the following compounds:

the halogenated methane is obtained by reacting halogenated methane with water,

a cyclic crown ether; or

Nitriles of the general formula R-C.ident.N.

12. A process according to claim 10 or 11, wherein x is 6, L₁And L₂Each of which is a nitrile,

for L₁And L₂And R independently represents an organic group selected from: methyl, ethyl, propyl, butyl, tert-butyl, pentyl, ethylene, propylene, butene, pentene, toluene, naphthalene or phenyl.

13. The method according to claim 12, wherein L₁And L₂Is the same nitrile.

14. The method according to claim 12, wherein L₁And L₂Are all acetonitrile.

15. The method according to claim 13, wherein L₁And L₂Are all acetonitrile.

16. The process according to claim 10 or 11, wherein x is 1 and L is a cyclic crown ether selected from the group consisting of: [12] -crown-4, [18] -crown-6, [24] -crown-8.

17. The process according to claim 10 or 11, wherein the methyl halide is dichloromethane.

18. An electrolyte comprising a salt according to any one of claims 1 to 9 or a salt prepared according to the method of any one of claims 10 to 17.

19. A cell or battery comprising an electrolyte according to claim 18.

20. A cell or battery as claimed in claim 19 wherein the cell or battery is a magnesium cell or battery, or a magnesium ion cell or battery.

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