Electrolyte salt for lithium-based energy stores

Abstract

The invention relates to lithium 1-trifluoromethoxy-1,2,2,2-tetra-fluoroethanesulphonate, the use of lithium 1-trifluoromethoxy-1,2,2,2-tetra-fluoroethanesulphonate as electrolyte salt in lithium-based energy stores and also ionic liquids comprising 1-trifluoro-methoxy-1,2,2,2-tetrafluoro-ethanesulphonate as anion.

Claims

1. A lithium-based energy storage means containing an electrolyte comprising lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate, an organic solvent and an ionic liquid and/or a polymer matrix.

2. A lithium-based energy storage means containing an electrolyte comprising lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate, an organic solvent and an additional ingredient selected from the group consisting of an ionic liquid, a polymer matrix and a combination of an ionic liquid and a polymer matrix.

3. A lithium-based energy storage means containing an electrolyte comprising lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate and an ionic liquid.

4. A lithium-based energy storage means containing an electrolyte comprising lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate and a polymer matrix.

5. The lithium-based energy storage means of claim 3, wherein the ionic liquid comprises 1-trifluoro-methoxy-1,2,2,2-tetrafluoroethanesulfonate and an organic cation.

6. The ionic liquid as claimed in claim 3, wherein the cation is selected from the group consisting of alkylammonium, pyridinium, pyrazolium, pyrrolium, pyrrolinium, piperidinium, pyrrolidinium, imidazolium and sulfonium compounds.

7. The ionic liquid as claimed in claim 3, wherein the cation is selected from the group consisting of N-butyl-N-methylpyrrolidinium, N-methyl-N-propyl-pyrrolidinium, 1-ethyl-3-methylimidazolium, 1-ethyl-2,3-dimethylimidazolium and 1-butyl-3-methyl-imidazolium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Examples and figures which serve to illustrate the present invention are adduced hereinafter.

(2) The figures here show:

(3) FIG. 1 shows the conductivity of a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate (LiB-8) in various solvent mixtures as a function of temperature.

(4) FIG. 2 shows the electrochemical stability window of a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate (LiB-8) in a solvent mixture of ethylene carbonate and diethyl carbonate (EC:DEC) in a ratio of 1:1 and of LiPF.sub.6 in EC:DEC in a ratio of 3:7. The current density is plotted against the potential.

(5) FIG. 3 shows the discharge capacity of a lithium-ion half-cell with 1 M lithium 1-trifluoro-methoxy-1,2,2,2-tetrafluoroethanesulfonate (LiB-8) in a solvent mixture of ethylene carbonate and diethyl carbonate (EC:DEC) in a ratio of 1:1 and graphite as working electrode compared to LiPF.sub.6. The discharge capacity is plotted against the cycle number.

(6) FIG. 4 shows the discharge capacity and efficiency of a lithium-ion half-cell with 1 M lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethane-sulfonate (LiB-8) in a solvent mixture of ethylene carbonate and diethyl carbonate (EC:DEC) in a ratio of 1:1. The working electrode used was the cathode material nickel manganese cobalt oxide. The discharge capacity is plotted against the cycle number.

(7) FIG. 5 shows the cyclic voltammetry of a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate (LiB-8) in a solvent mixture of ethylene carbonate and diethyl carbonate (EC:DEC) in a ratio of 1:1 on a graphite anode for three cycles (1), (2) and (3).

(8) FIG. 6 shows the breakdown products of the thermal aging at 95° C. of a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethane-sulfonate (LiB-8) in ethylene carbonate and diethyl carbonate (EC:DEC) in a ratio of 1:1 compared to LiPF.sub.6 in EC:DEC in a ratio of 3:7.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

Preparation of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate

a) Preparation of 1,2,2,2-tetrafluoro-1-(trifluoro-methoxy)ethanesulfonyl fluoride

(9) 10 mg of tetramethylammonium fluoride (ABCR) were suspended in 10 ml of dry bis(2-methoxyethyl) ether (diglyme, ABCR). At −197° C., 36 mmol of sulfuryl difluoride (ABCR) and 36 mmol of 1,1,2-trifluoro-2-(trifluoromethoxy)ethene (ABCR) were condensed in. The reaction mixture was heated to 60° C. for 12 h and then the product was distilled. 1,2,2,2-Tetrafluoro-1-(trifluoromethoxy)ethanesulfonyl fluoride was obtained as a colorless liquid in a yield of 92%.

b) Preparation of lithium-1,2,2,2-tetrafluoro-1-(trifluoromethoxy)ethanesulfonate

(10) 20 mmol of the 1,2,2,2-tetrafluoro-1-(trifluoro-methoxy)ethanesulfonyl fluoride prepared in step a) were dissolved in 10 ml of ethanol (ROTH). At 0° C., 40 mmol of lithium hydroxide (ROTH) were added. The suspension was stirred at room temperature (20±3° C.) for 2 h, then centrifuged for 15 min, and the liquid phase was decanted off. The solvent was drawn off and the product was dried under reduced pressure (0.001 mm) at 60° C. for 6 h. The yield was 65%.

(11) Lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethane-sulfonate was dried before use at 60° C. for 24 hours.

Example 2

Determination of the conductivity of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate

(12) The conductivity of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate was determined in different solvents within a temperature range from −40° C. to +60° C.

(13) Mixtures of 50% by weight of ethylene carbonate (EC) (Ferro Corporation, battery grade) and 50% by weight of diethyl carbonate (DEC) (Ferro Corporation, battery grade) (EC:DEC, 1:1), of 50% by weight of ethylene carbonate and 50% by weight of gamma-butyrolactone (γ-BL) (Ferro Corporation, battery grade) (EC:γ-BL, 1:1), and 45% by weight of ethylene carbonate, 45% by weight of gamma-butyrolactone and 10% by weight of fluoroethylene carbonate (Solvay GmbH) (EC:γ-BL:FEC, 4.5:4.5:1) were prepared. In these solvent mixtures were dissolved 217 mg per milliliter of the lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate prepared according to example 1, so as to give a concentration of 1 M. For comparison, a 1 M solution of LiPF.sub.6 (Sigma-Aldrich, battery grade) in a mixture of 30% by weight of ethylene carbonate and 70% by weight of diethyl carbonate (EC:DEC, 3:7) was prepared.

(14) The conductivity of the electrolytes was analyzed using platinum conductivity measurement cells (Amel Glassware, cell constant 1 cm.sup.−1) with a potentiostat (Solartron 1287A) in conjunction with an impedance measurement unit (Solartron 1260) within a temperature range from −40° C. to +60° C. (climate-controlled cabinet, Binder MK53). For this purpose, the conductivity measurement cells were first heated to 60° C. and then cooled in temperature intervals of 5° C. to −40° C.

(15) FIG. 1 shows the plot of the conductivity of the 1 M solutions of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate (LiB-8) in the various solvent mixtures within the temperature range from −40° C. to +60° C., and that of LiPF.sub.6. As can be inferred from FIG. 1, the conductivity in the solvent mixtures of ethylene carbonate and gamma-butyrolactone (EC:γ-BL, 1:1) and of ethylene carbonate, gamma-butyrolactone and fluoroethylene carbonate (EC:γ-BL:FEC) within the temperature range from −40° C. to +0° C. was much higher compared to the conductivity in ethylene carbonate and diethyl carbonate (EC:DEC, 1:1), and attained virtually the conductivity of LiPF.sub.6 in an ethylene carbonate and diethyl carbonate (EC:DEC, 3:7) over the entire temperature range.

(16) The conductivity of 1 M lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate at 25° C. in the various solvents was 1.8 mS cm.sup.−1 for a 1:1 mixture of ethylene carbonate and diethyl carbonate (EC:DEC, 1:1), 3.7 mS cm.sup.−1 for a 1:1 mixture of ethylene carbonate and gamma-butyrolactone (EC:γ-BL, 1:1), and 3.5 mS cm.sup.−1 for a mixture of ethylene carbonate, gamma butyrolactone and fluoroethylene carbonate (EC:γ-BL:FEC) in a ratio of 4.5:4.5:1.

(17) This shows that lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate has a sufficient conductivity at 25° C. in the customary carbonate solvents.

Example 3

Determination of the electrochemical stability of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethane-sulfonate

(18) The electrochemical stability of a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethane-sulfonate prepared according to example 1 in a solvent mixture of 50% by weight of ethylene carbonate and 50% by weight of diethyl carbonate (EC:DEC, 1:1) compared to the stability of a 1 M solution of LiPF.sub.6 (Sigma-Aldrich, battery grade) in a mixture of 30% by weight of ethylene carbonate and 70% by weight of diethyl carbonate (EC:DEC, 3:7) was determined by means of linear sweep voltammetry (LSV). In this method, there is a continuous change in the electrode voltage (linear sweep).

(19) The cathodic stability limit, the potential at which reduction sets in, was defined as that potential at which the current density falls below −0.1 mA cm.sup.−2. The anodic stability limit, the potential at which oxidation sets in, was defined as that potential at which the current density goes above +0.1 mA cm.sup.−2. The anodic stability in particular depends crucially on the stability of the electrolyte used.

(20) The experiments were conducted in a 3-electrode arrangement in modified Swagelok® T-pieces (tube connector, stainless steel body) with a platinum electrode (eDAQ, model: ET075, diameter 1 mm) as working electrode and lithium foil (diameter 12 mm and 7 mm, respectively, Chemetall) as counterelectrode and reference electrode. In addition, the cell body was lined with a polyester film siliconized on one side (Mylar®, PPI-SP 914, 100 μm) and the electrodes were introduced into the cell body. The electrodes were separated by a nonwoven fabric (Freudenberg®, FS2226E, 6 plies) which had been impregnated with the corresponding electrolyte. The scan rate was 1 mV s.sup.−1.

(21) As shown in FIG. 2, in the case of the 1 M lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate (LiB-8) electrolyte in a 1:1 mixture of ethylene carbonate and diethyl carbonate, the cathodic stability limit was attained at 0.03 V. The anodic stability of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethane-sulfonate was 5.7 V and is entirely sufficient for use of the electrolyte in combination with high-voltage cathode materials.

(22) This result shows that lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate has a sufficiently good electrochemical stability for all electrochemical applications in the customary carbonate solvents.

Example 4

Determination of the cycling performance of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate with a graphite electrode

(23) The cycling performance of a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate prepared according to example 1 was determined in a mixture of 50% by weight of ethylene carbonate and 50% by weight of diethyl carbonate (EC:DEC, 1:1) compared to the cycling of a 1 M solution of LiPF.sub.6 in a mixture of 30% by weight of ethylene carbonate and 70% by weight of diethyl carbonate (EC:DEC, 3:7).

(24) The experiments were conducted in a 3-electrode arrangement in modified Swagelok® T-pieces (tube connector, stainless steel body) with a graphite electrode (Timcal T44 graphite material) as working electrode and lithium foil (diameter 12 mm and 5 mm, respectively, Chemetall) as counterelectrode and reference electrode. In addition, the cell body was lined with a polyester film siliconized on one side (Mylar®, PPI-SP 914, 100 μm) and the electrodes were introduced into the cell body. The electrodes were separated by a nonwoven fabric (Freudenberg®, FS2226E, 6 plies) which had been impregnated with the corresponding electrolyte.

(25) The test of the cycling performance comprised several phases. In the first phase, the forming of the graphite (SEI formation) was ensured by three cycles with a constant current C rate of C/5. Thereafter, in the second phase, the cycling performance was tested over 20 cycles at a charge and discharge rate of 1 C. The cell system was kept here at a voltage of 0.025 V for one hour after charging. In the third phase, the graphite was always charged at C/2 and, thereafter, kept at 0.025 V for one hour before the graphite was discharged at different rates. The D rates (discharge rates) used were D/5, D/3, D/2, 1D, 2D, 3D, 5D and 10D. The D rate test was followed by five cycles with charge and discharge rates of C/5, in order to check whether the graphite had been damaged by the stress test. The last phase involved the same cycling parameters as phase 2, but was conducted for 30 cycles.

(26) FIG. 3 shows the discharge capacity or lithium deintercalation capacity of the 1 M solutions of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethane-sulfonate and LiPF.sub.6 against the number of cycles of the lithium-ion battery half-cell charged at a C rate of 1 C. As shown in FIG. 3, the half-cell with 1 M lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethane-sulfonate had a starting capacity of about 373 mAh g.sup.−1 after the forming of the cell, which rose with the number of cycles to about 374 mAh g.sup.−1 in the 15th cycle. This shows the excellent cycling stability of the cell, which corresponds to that of LiPF.sub.6.

Example 5

Determination of the cycling performance of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate with NCM cathode

(27) The cycling performance on NCM cathodes in a half-cell was conducted as described in example 4 in a 3-electrode arrangement, using a nickel cobalt manganese oxide electrode (NCM electrode, Toda Kogyo Europe GmbH) as working electrode and lithium foil (diameter 12 mm and 5 mm, respectively, Chemetall) as counterelectrode and reference electrode. In this example, a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate prepared according to Example 1 in a mixture of 50% by weight of ethylene carbonate and 50% by weight of diethyl carbonate (EC:DEC, 1:1) was used.

(28) FIG. 4 shows the discharge capacity or lithium deintercalation efficiency and the efficiency of the lithium-ion half-cell. The discharge capacity and efficiency are plotted against the number of cycles. As FIG. 4 shows, the half-cell with 1 M lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate had a starting capacity of about 138 mAh g.sup.−1 after the successful current rate stress test (C rate test) of the cell (I). After the subsequent cycling (II), the capacity in the 80th cycle was still about 137 mAh g.sup.−1. In addition, the efficiency of the cell after 2 standard cycles rose to more than 99.6% of a maximum efficiency of 100%. It was found that the cell exhibited excellent cycling stability with the NCM cathode too.

Example 6

Cyclic Voltammetry

(29) The cyclic voltammetry of a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate prepared according to example 1 was conducted in a mixture of 50% by weight of ethylene carbonate and 50% by weight of diethyl carbonate (EC:DEC, 1:1).

(30) The experiments were conducted in a 3-electrode arrangement in modified Swagelok® T-pieces (tube connector, stainless steel body) with a graphite electrode (Timcal T44 graphite material) as working electrode and lithium foil (diameter 12 mm and 4 mm, respectively, Chemetall) as counterelectrode and reference electrode. In addition, the cell body was lined with a polyester film siliconized on one side (Mylar®, PPI-SP 914, 100 μm) and the electrodes were introduced into the cell body. The electrodes were separated by a nonwoven fabric (Freudenberg®, FS2226E, 6 plies) which had been impregnated with the corresponding electrolyte.

(31) FIG. 5 shows the results of the cyclic voltammetry for three cycles (1), (2) and (3). In FIG. 5, the intercalation and deintercalation phases identifiable by the increased current densities in the particular cycles are recognizable in the range between 0 and 0.3 V. This demonstrates the reversibility of the system.

(32) The enlarged section of the range from 0.5 V to 1 V shows the formation of the solid electrolyte interphase (SEI) of the lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate-carbonate electrolyte. In comparison is shown, the formation potential of the SEI for a 1 M solution of LiPF.sub.6 in a mixture of 30% by weight of ethylene carbonate and 70% by weight of diethyl carbonate (EC:DEC, 3:7).

Example 7

Thermal Stability

(33) The breakdown products of a 1 M solution of lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate prepared according to example 1 in a mixture of 50% by weight of ethylene carbonate and 50% by weight of diethyl carbonate (EC:DEC, 1:1) were determined in comparison with the breakdown products of a 1 M solution of LiPF.sub.6 (Sigma-Aldrich, battery grade) in a mixture of 30% by weight of ethylene carbonate and 70% by weight of diethyl carbonate (EC:DEC, 3:7).

(34) The electrolytes were stored in a climate-controlled chamber at 95° C. for two weeks and subsequently analyzed by means of gas chromatography-mass spectrometry (Clarus GC 600 from Perkin Elmer).

(35) FIG. 6 shows the breakdown products of the thermal aging at 95° C. of a 1 M solution of lithium 1-trifluoro-methoxy-1,2,2,2-tetrafluoroethanesulfonate (LiB-8) in ethylene carbonate and diethyl carbonate (EC:DEC) in a ratio of 1:1 compared to 1 M LiPF.sub.6 in EC:DEC in a ratio of 3:7. As FIG. 6 shows, for the 1 M lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate electrolyte in EC:DEC, no carbonate breakdown products were found by means of GC-MS in the thermally aged electrolyte. In contrast, for the 1 M LiPF.sub.6 electrolyte in EC:DEC, carbonate breakdown products were detected on the basis of the signals between 2.5 min and 3.5 min. The signal after 12 min is attributable to diethyl carbonate. This shows that the thermal stability of 1 M lithium 1-trifluoromethoxy-1,2,2,2-tetrafluoroethanesulfonate exceeded the stability of the LiPF.sub.6 electrolyte.

(36) These results show overall that lithium 1-trifluoro-methoxy-1,2,2,2-tetrafluoroethanesulfonate is one possible substitute for LiPF.sub.6 as conductive salt in lithium-ion batteries.