# Patent

## Title
Process for preparing metal difluorochelatoborates and use as battery electrolytes or additives in electrochemical cells

## Abstract

The invention relates to a process for preparing metal difluorochelatoborates, in which a metal bis(chelato)borate of the formula M[BL2] is reacted with boron trifluoride and a metal fluoride (MF) and/or a metal salt of the chelating ligand (M2L) where M+ is a monovalent cation selected from the group consisting of lithium, sodium, potassium and ammonium NR4+, where R═H, alkyl (C1 to C8) and L is a chelating agent having two terminal oxygen atoms and having the general formula (II), where: when m=1 and Y1 and Y2 together with C1 form a carbonyl group, n=0 or 1 and o=0 or 1 and R1 and R2 are each, independently of one another, H or alkyl having from one to eight carbon atoms (C1-C8) and Y3, Y4 are each, independently of one another, OR3 (R3═C1-C8-alkyl), then n or o≠1: p=0 or 1 and when n and o=0, p=1; or Y1, Y2, Y3, Y4 are each, independently of one another, OR3 (R3═C1-C8-alkyl), m=1, n=0 or 1, o=1 and p=0; or C2 and C3 are members of a 5- or 6-membered aromatic or heteroaromatic ring (with N, O or S as heteroelement) which can optionally be substituted by alkyl, alkoxy, carboxy or nitrile, where R1, R2, Y3 and Y4 are absent, m=0 or in the case of 1, Y1 and Y2 together with C1 form a carbonyl group and p is 0 or 1, in an organic, aprotic solvent.

## Background
This application is a §371 of International Application No. PCT/EP2012/072603 filed Nov. 14, 2012, and claims priority from German Patent Application No. 10 2011 086 306.0 filed Nov. 14, 2011.
The invention relates to a process for preparing metal difluorochelatoborates, and their use as battery electrolytes or additives in electrochemical cells.
Mobile electronic devices require increasingly more powerful rechargeable batteries for independently supplying power. In addition to nickel/cadmium and nickel/metal hydride batteries, lithium batteries in particular, which have much higher energy densities than the first-mentioned systems, are suitable for this purpose. In the future, large-scale lithium batteries will also be used, for example, for stationary applications (power back-up) and in the automotive sector for traction purposes (hybrid drives or solely electric drives). Great importance is attached to reliability, in particular for the latter-mentioned applications.
The current generation of lithium-ion batteries presently in use utilizes as an electrolyte liquid, gel, or polymer electrolytes containing LiPF6 as the conductive salt. This salt starts to decompose when the temperature exceeds approximately 70° C., and forms the highly reactive Lewis acid PF5 according to the formula

LiPF6→LiF+PF5  (1).

The acid attacks the organic components of the electrolytes (alkyl carbonates, for example) used according to the prior art. This reaction is exothermic, and may result in “run-away” self-heating. Thus, at the minimum the functionality of the electrochemical cell is impaired, or the cell may be completely destroyed, with hazardous repercussions.
As an alternative electrolyte, solutions of lithium salts containing fluorochelatoborate anion, such as lithiumdifluorooxalatoborate (LiDFOB) (U.S. Pat. No. 6,849,752 Z. Chen, J. Liu, K. Amine, Electrochem. Solid State Lett. 10 (2007) A45-47) or lithium difluoro(1,2-benzenediolato(2-)-O,O′-borate (X. Zhao-Ming, J. Power Sources 196 (2011) 8710), among others, have been proposed.
The following discussions focus on the conductive salt LiDFOB. However, they also analogously apply for variations of this structure according to general formula I




where M+ is a monovalent cation selected from the group lithium, sodium, potassium, or ammonium NR4+, and R═H or alkyl (C1 to C8, acyclic or cyclic),
L is a chelating agent, having two terminal oxygen atoms, with the general formula




where the following apply:
when m=1 and Y1 and Y2 together with C1 stand for a carbonyl group, n=0 or 1 and o=0 or 1, and R1 and R2 are each independently H or alkyl containing one to eight carbon atoms (C1-C8), and Y3, Y4 are each independently OR3 (R3═C1-C8 alkyl), and when n or o≠1, p=0 or 1, and when n and o=0, p=1;
or
Y1, Y2, Y3, Y4 are each independently OR3 (R3═C1-C8 alkyl), m=1, n=0 or 1, o=1, and p=0;
or
C2 and C3 are members of a 5- or 6-membered aromatic or heteroaromatic ring (with N, O, or S as heteroelement) which may optionally be substituted with alkyl, alkoxy, carboxy, or nitrile, where R1, R2, Y3, and Y4 are absent when m=0, or in the case of [m=] 1, Y1 and Y2 together with C1 stand for a carbonyl group, and p is 0 or 1.
The conductive salt LiDFOB (M+=Li and L=C2O42−) may be prepared in various ways.
In the reaction of lithium tetrafluoroborate (LiBF4) with 2 equivalents lithium hexafluoroisopropanolate in acetonitrile, initially lithium fluoride (LiF) is eliminated (EP 1195834). The alkoxy ligands of the intermediate product, which are only relatively weakly bonded, are replaced by the better chelate donor, oxalate, in a second step:




In this method it is disadvantageous that LiF remains in the product, the ligand 1,1,1,3,3,3-hexafluoroisopropanol is costly, and the process is complicated due to having two steps.
In another production method, lithium tetrafluoroborate is reacted with anhydrous oxalic acid and SiCl4 as auxiliary reagent (EP 1308449):

2LiBF4+2H2Ox+SiCl4→Li[F2BOx]+4HCl+SiF4  (3)

A disadvantage of this synthesis is the formation of the acidic, toxic co-products SiF4 and HCl. In addition, traces of chloride remain in the product. It is known that chloride is corrosive to aluminum, so that a conductive salt which is thus contaminated with chloride corrodes the cathode current collector, generally an aluminum foil, used in Li-ion batteries.
It is also known that in heat aging of equimolar mixtures of LiBF4 and LiBOB in ethylene carbonate/ethylmethyl carbonate (EC/EMC), LiDFOB forms in a very slow reaction (B. Lucht, Electrochem. Solid-State Lett. 14 (11) A161-A164 (2011)). Thus, when the mixed salt LiDFOB is stored at 100° C., an approximately 80% yield is obtained within 10 weeks. The disadvantages of this method are that the reaction is much too slow for commercial use, and the raw material LiBF4 is costly.
In another method, boron trifluoride, usually in the form of an ether addition product, is reacted with lithium oxalate (S. S. Zhang, Electrochem. Commun. 8 (2006) 1423-1428):




A disadvantage of this method is that the target product LiDFOB is formed in a yield of only 50%, and LiBF4 forms to the same extent. Namely, the by-product LiF which results according to Equation (4) reacts immediately with boron trifluoride to form LiBF4, so that overall, the following reaction equation applies:

Li2C2O4+2BF3→Li[F2BC2O4]+LiBF4  (4a)

Lastly, LiDFOB may be prepared from lithium tetrafluoroborate and bis(trimethylsilyl)oxalate in acetonitrile solution (C. Schreiner, M. Amereller, H. Gores, Chem. Eur. J. 13 (2009) 2270-2272):




Disadvantages of this method are the high costs, the unavailability of the silyl ester, and occurrence of the by-product trimethylsilyl fluoride.
The object of the invention is to provide a process which, starting from commercially available, easily handled raw materials, forms metal difluorochelatoborates, in particular LiDFOB, in a simple one-step reaction.
The object is achieved by reacting a metal bis(chelato)borate of formula M[BL2] with boron trifluoride and a metal fluoride (MF) and/or a metal salt of the chelate ligand (M2L) in an organic aprotic solvent, where M+ is a monovalent cation selected from the group lithium, sodium, potassium, or ammonium NR4+, where R═H or alkyl (C1 to C8, acyclic or cyclic), and
L is a chelating agent having two terminal oxygen atoms having the general formula




where the following apply:

    
    
        when m=1 and Y1 and Y2 together with C1 stand for a carbonyl group, n=0 or 1 and o=0 or 1, and R1 and R2 are independently H or alkyl containing one to eight carbon atoms (C1-C8), and Y3, Y4 are each independently OR3 (R3═C1-C8 alkyl), and when n or o≠1, p=0 or 1, and when n and o=0, p=1;
        or
        Y1, Y2, Y3, Y4 are each independently OR3 (R3═C1-C8 alkyl), m=1, n=0 or 1, o=1, and p=0; or
        C2 and C3 are members of a 5- or 6-membered aromatic or heteroaromatic ring (with N, O, or S as heteroelement) which may optionally be substituted with alkyl, alkoxy, carboxy, or nitrile, where R1, R2, Y3 and Y4 are absent when m=0, or in the case of [m=] 1, Y1 and Y2 together with C1 stand for a carbonyl group and p is 0 or 1.
    
    


The reactions may be described by the following general equations:

M2L+M[BL2]+2BF3→3M[F2BL]  (6)

MF+M[BL2]+BF3→2M[F2BL]  (7)

For the case of preparation of the particularly preferred conductive salt LiDFOB, lithium bis(oxalato)borate (LiBOB) is reacted with lithium fluoride or lithium oxalate and boron trifluoride:

Li2C2O4+LiBOB+2BF3→3Li[F2BC2O4]  (8)

LiF+LiBOB+BF3→2Li[F2BC2O4]  (9)

Similarly, the likewise particularly preferred conductive salt lithium difluoromalonatoborate (LiDFMB) is prepared from lithium-bis(malonato)borate and BF3 as well as LiF or lithium malonate (Li2C3H2O4). Further preferred products are the following: lithium difluorolactatoborate, lithium difluoroglycolatoborate, lithium difluorosalicylatoborate, lithium difluorocatecholatoborate, and the corresponding sodium salts. Aprotic organic solvents, preferably ethers, esters, nitriles, lactones, or carbonates, are used either in pure form or in any given mixture. In addition, hydrocarbons (aromatics or saturated compounds) may be used in mixtures with the above-mentioned functionalized solvents.
The use of solvents which are suitable for use in lithium batteries is very particularly preferred. Such solvents include the following: carboxylic acid esters (dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, propylene carbonate, ethylene carbonate), cyclic ethers such as tetrahydropyran or tetrahydrofuran, polyethers such as 1,2-dimethoxyethane or diethylene glycol dimethyl ether, as well as nitriles such as acetonitrile, adiponitrile, malodinitrile, and glutaronitrile, and lactones such as γ-butyrolactone.
The reaction is carried out at temperatures between 0 and 250° C., preferably between 20 and 150° C., particularly preferably between 30 and 130° C.
The sparingly soluble raw materials, i.e., the metal fluorides and/or metal chelate salts, are used in pulverized form, preferably ground. The average particle size is <100 μm, particularly preferably <50 μm.
All raw materials, in particular the metal salts and the solvents, are used in anhydrous form; i.e., the water content of the raw materials is <1000 ppm, preferably <300 ppm.
In one particularly preferred embodiment, a reaction acceleration catalyst is used. Lewis acids or substances which release or are able to release Lewis acids in the reaction mixture are used as catalyst. Preferred catalysts are compounds of elements Groups 2 through 15 of the periodic table, particularly preferably molecular halides, perfluoroalkyls, perfluoroaryls, and oxo compounds of boron, aluminum, and phosphorus. Examples include the following: aluminum alcoholates (Al(OR)3), boric acid esters (B(OR)3), phosphorus oxides, and phosphorus halides. Very particularly preferred are superacid boron compounds such as B(C6F5)3(BARF), C6F5BO2C6F4, and boric acid esters of trivalent oxygen-based chelate ligands such as the following:




In addition, very particularly preferred is the catalytic use of LiPF6, which under the above-mentioned reaction conditions is in equilibrium with the strong Lewis acid PF5 (Equation 1). The mentioned catalysts are used in quantities of 20 mol-% maximum, preferably up to 10 mol-% and particularly preferably up to 5 mol-%, based on the boron trifluoride used.
The process according to the invention is described in general terms below. The metal salts are placed in the anhydrous solvent. With stirring, boron trifluoride is then either introduced or condensed in the gaseous state, or added in the form of standard solvate complexes such as BF3×diethyl ether, BF3×THF, or BF3×acetonitrile. The use of gaseous BF3 or a solution prepared beforehand with BF3 gas in the desired solvent (for example, a carbonate such as dimethyl carbonate or propylene carbonate) is particularly preferred. The introduction of a solvent, such as diethyl ether, which is uncommon or even detrimental in battery electrolytes, is thus avoided. BF3 is added in a temperature range between 0° C. and 150° C., preferably between 10° C. and 100° C. After the BF3 has been added, stirring is performed until the reaction is complete. The progress of the reaction may be conveniently monitored by 11B NMR measurements, for example.
The process according to the invention may also deviate slightly from the theoretical stoichiometry (Equations 6 through 9). The stoichiometries are preferably selected which result in complete consumption of the raw material BF3, which has a detrimental effect in the battery. For this purpose, the metal salts MF and/or M2L are used in excess. The mentioned metal salts are preferably used in an excess of 0.1 to 100% by weight, particularly preferably in an excess of 1 to 20% by weight.
After the reaction is complete, the reaction solution is clarified by membrane filtration, for example. The reaction solution is then directly usable as such as a battery electrolyte or additive, if no solvents which are detrimental to the battery performance have been used. In the event that detrimental solvents are contained, the synthesized metal difluorochelatoborate according to the invention is obtained in pure form by means of a concentration or crystallization process.
The invention is explained with reference to the following seven examples.


EXAMPLE 1
Preparation of LiDFOB from LiBOB, Lithium Oxalate, and BF3 in Dimethyl Carbonate (DMC)
37.5 g LiBOB and 19.8 g Li2C2O4 in 229 g anhydrous DMC were placed in a 0.5-L double shell reactor equipped with a reflux cooler and dropping funnel, and heated to an internal temperature of 70° C. 55.0 g boron trifluoride etherate was then metered in over a period of one hour. The jacket temperature was set so that the reaction mixture was boiling lightly the entire time. After metering was complete, refluxing was continued with occasional withdrawal of samples.
The samples were checked for progress of the reaction by 11B NMR:















−1.1 ppm*
0 ppm*
3.1 ppm*
7.6 ppm*


Time of sampling
LiBF4
BF3
LiDFOB
LiBOB



















End of metering
41.5
4.7
42.9
10.8


2 h after the reaction
43.0
3.0
45.9
8.2


7.5 h after the reaction
37.8
3.9
47.6
10.7


9 h after the reaction
36.9
2.9
48.4
11.8


11.5 h after the reaction
37.7
2.6
55.3
4.5


18.5 h after the reaction
17.6
2.2
66.7
13.6


26 h after the reaction
9.2
1.9
78.9
8.9


30 h after the reaction
8.8
2.1
81.4
7.7





*Listed values represent the chemical shift of the particular product in the 11B NMR spectrum






After a 30-hour reaction time, >80% of theoretical LiDFOB had formed, and the composition did not change significantly with continued stirring. Thus, a thermodynamic equilibrium mixture was formed. The reaction mixture was filtered, and was used in this form (clear solution) as an electrolyte for lithium batteries.
EXAMPLE 2
Preparation of LiDFOB from LiBOB, Lithium Oxalate, and BF3 in Dimethyl Carbonate (DMC), 5 mol-% LiPF6 Catalyst
1.50 g LiBOB and 0.91 g lithium oxalate were dissolved or suspended in 9.16 g DMC in an inerted GC septum glass equipped with a magnetic stirrer, and 0.59 g of a 10% LiPF6 solution in DMC was added. 2.20 g boron trifluoride etherate was injected into the stirred suspension, which was then heated to 70° C. Samples were withdrawn at specified time intervals and checked for progress of the reaction by 11B NMR:















−1.1 ppm*
0 ppm*
3.1 ppm*
7.6 ppm*


Time of sampling
LiBF4
BF3
LiDFOB
LiBOB



















1 h, 70° C.
41.5
0.5
46
8


5 h, 70° C.
16
1.1
72
11


12 h, 70° C.
14
1.0
79
6





*Listed values represent the chemical shift of the particular product in the 11B NMR spectrum






EXAMPLE 3
Preparation of LiDFOB from LiBOB, Lithium Oxalate, and BF3 in Propylene Carbonate (PC), with and without 5 Mol-% LiPF6 Catalyst
1.50 g LiBOB and 0.79 g lithium oxalate were dissolved or suspended in 9.2 g PC in each of two inerted GC septum glasses equipped with a magnetic stirrer. 0.59 g of a 10% solution of LiPF6 in PC was injected into one of the glasses. 2.20 g boron trifluoride etherate was injected into each of the stirred suspensions, which were then heated to 70° C. Samples were withdrawn at specified time intervals and checked for progress of the reaction by 11B NMR:















LiBF4
LiDFOB
LiBOB













Reaction time
Without
With
Without
With
Without
With


70° C. (h)
catalyst
catalyst
catalyst
catalyst
catalyst
catalyst
















1 h
32
22
40
62
28
16


2 h
31
18
41
71
27
11


9 h
28
10
54
83
18
6









EXAMPLE 4
Preparation of LiDFOB from LiBOB, Lithium Fluoride and BF3 in Propylene Carbonate (PC) with 5 mol-% LiPF6 Catalyst
1.50 g LiBOB and 0.23 g ground lithium fluoride were dissolved or suspended in 6.8 g PC in an inerted GC septum glass equipped with a magnetic stirrer, and 0.59 g of a 10% LiPF6 solution in PC was added. 1.10 g boron trifluoride etherate was injected into the stirred suspension, which was then heated to 70° C. Samples were withdrawn at specified time intervals and checked for progress of the reaction by 11B NMR:















−1.1 ppm*
0 ppm*
3.1 ppm*
7.6 ppm*


Time of sampling
LiBF4
BF3
LiDFOB
LiBOB



















1 h, 70° C.
19
0.4
68
13


2 h, 70° C.
15
0.5
74
11


9 h, 70° C.
9
1
86
4





*Listed values represent the chemical shift of the particular product in the 11B NMR spectrum






EXAMPLE 5
Preparation of LiDFOB from LiBOB, Lithium Fluoride, and BF3 in Tetrahydropyran (THP), without Catalyst
1.50 g LiBOB and 0.25 g ground lithium fluoride were dissolved or suspended in 8.5 g THP in an inerted GC septum glass equipped with a magnetic stirrer. 1.10 g boron trifluoride etherate was injected into the stirred suspension, which was then heated to 70° C. Samples were withdrawn at specified time intervals and checked for progress of the reaction by 11B NMR:















−1.1 ppm*
0 ppm*
3.1 ppm*
7.6 ppm*


Time of sampling
LiBF4
BF3
LiDFOB
LiBOB



















15 min, 70° C.
28
0
47
25


2 h, 70° C.
16
0.5
79
4





*Listed values represent the chemical shift of the particular product in the 11B NMR spectrum






EXAMPLE 6
Preparation of LiDFMB from Lithium-bis(malonato)borate (LiBMB), Lithium Fluoride, and BF3 in Propylene Carbonate (PC), without Catalyst
1.78 g LiBMB and 0.21 g lithium fluoride were suspended in 11 g PC in an inerted GC septum glass equipped with a magnetic stirrer. 1.14 g boron trifluoride etherate was injected into the stirred suspension, which was then stirred at 100° C. Samples were withdrawn at specified time intervals and checked for progress of the reaction by 11B NMR:















−1.2 ppm*
0 ppm*
1.4 ppm*
3.6 ppm*


Time of sampling
LiBF4
BF3
LiDFMB
LiBMB



















30 min, 100° C.
29
4
54
11


2.5 h, 100° C.
29
4
61
5


7 h, 100° C.
24
3
70
3





*Listed values represent the chemical shift of the particular product in the 11B NMR spectrum






EXAMPLE 7
Preparation of LiDFMB from Lithium-bis(malonato)borate (LiBMB), Lithium Fluoride, and BF3 in Dimethylsulfoxide (DMSO), without Catalyst
1.78 g LiBMB and 0.21 g lithium fluoride were dissolved or suspended in 10.5 g DMSO in an inerted GC septum glass equipped with a magnetic stirrer. 1.14 g boron trifluoride etherate was injected into the stirred suspension, which was then stirred at 100° C. An almost clear reaction solution formed after a short time. Samples were withdrawn at specified time intervals and checked for progress of the reaction by 11B NMR:















−1.2 ppm*
0 ppm*
1.4 ppm*
3.6 ppm*


Time of sampling
LiBF4
BF3
LiDFMB
LiBMB



















30 min, 100° C.
19
2
45
34


2.5 h, 100° 0
11
0.9
67
20


7 h, 100° C.
6
approx. 0
83
11





*Listed values represent the chemical shift of the particular product in the 11B NMR spectrum

## Claims

1. A process for preparing a metal difluoroborate of formula



wherein M+ is a monovalent cation selected from the group consisting of lithium, sodium, and potassium; and
L is a chelating agent selected from the group consisting of lactate, glycolate, salicylate, catecholate, oxalate and malonate;
comprising the steps of reacting a metal bis(chelato)borate of formula M[BL2] with boron trifluoride and a metal fluoride (MF) or a metal salt of the chelate ligand (M2L) in an organic aprotic solvent.

2. A process according to claim 1, wherein M+ is lithium and wherein L is oxalate or malonate, and wherein the product is lithium difluorooxalatoborate.

3. A process according to claim 1, wherein the solvent comprises at least one member selected from the group consisting of an ether, an ester, a nitrile, a lactone and an alkyl carbonate.

4. A process according to claim 1, wherein the solvent comprises at least one member selected from the group consisting of a carboxylic acid ester, a cyclic ether, a polyether, a nitrile and a lactone.

5. A process according to claim 1, wherein the reaction is carried out at temperatures of 0 to 250° C.

6. A process according to claim 1, wherein at least one of the metal fluorides and metal chelate salts are in pulverized form.

7. A process according to claim 1, wherein the metal salts of at least one of MF and M2L are used in excess.

8. A process according to claim 1, wherein the metal salts MF and/or M2L are used in an excess of 0.1 to 100% by weight.

9. A process according to claim 1, wherein a reaction acceleration catalyst is used during the process.

10. A process according to claim 9, wherein the catalyst contains or is composed of a Lewis acid or a substance which releases a Lewis acid in the reaction mixture.

11. A process according to claim 9, where the catalyst is selected from the group consisting of an element from Groups 2 through 15 of the periodic table, a perfluoroalkyl, a perfluoroaryl, an oxo compound of boron, an oxo compound of aluminum and an oxo compound of phosphorus.

12. A process according to claim 9, wherein the catalyst is LiPF6.

13. A process according to claim 9, wherein the catalyst is used in quantities of 20 mol-% maximum based on the boron trifluoride used.

14. An electrochemical cell comprising the metal difluoroboarate prepared according to the process of claim 1.

15. A process according to claim 1, wherein the solvent is selected from the group consisting of dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, propylene carbonate, ethylene carbonate, tetrahydropyran, tetrahydrofuran, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, acetonitrile, adiponitrile, malodinitrile, glutaronitrile and γ-butyrolactone.

16. A process according to claim 9, wherein the catalyst is selected from the group consisting of an aluminum alcoholate, a boric acid ester, a phosphorus oxide and a phosphorus halide.