Electrolyte-additive for lithium-ion battery systems

Abstract

The invention relates to the use of compounds according to general formula (1), in particular 1,4,2-dioxoazol-5-on-derivatives, as additives in electrolytes for electrochemical energy sources such as lithium-ion-batteries, and compounds containing electrolytes according to general formula (1), in particular 1,4,2-dioxoazol-5-on-derivatives.

Claims

1. An electrolyte for an electrochemical energy store comprising an electrolyte salt and a solvent, characterized in that the electrolyte comprises at least one compound of the general formula (1) as indicated below: ##STR00004## where: X is C, S or S═O; R.sup.1 is selected from the group consisting of CN, C.sub.1-C.sub.10-alkyl, C.sub.1-C.sub.10-alkoxy, C.sub.3-C.sub.7-cycloalkyl, C.sub.6-C.sub.10-aryl, —CO—O—R.sup.2 and combinations thereof, wherein the alkyl, alkoxy, cycloalkyl and aryl groups are each unsubstituted or monosubstituted or polysubstituted by at least one substituent selected from the group consisting of F, C.sub.1-4-alkyl, CN and mixtures thereof; and R.sup.2 is selected from the group consisting of C.sub.1-C.sub.10-alkyl, C.sub.3-C.sub.7-cycloalkyl, C.sub.6-C.sub.10-aryl and mixtures thereof.

2. The electrolyte as claimed in claim 1, characterized in that R.sup.1 is selected from the group consisting of CN, C.sub.1-C.sub.5-alkyl, C.sub.1-C.sub.5-phenyl and mixtures thereof, wherein the C.sub.1-C.sub.5-alkyl and/or C.sub.1-C.sub.5-phenyl are unsubstituted or monosubstituted or polysubstituted by fluorine.

3. The electrolyte as claimed in claim 1, characterized in that X is carbon and R.sup.1 is selected from the group consisting of CH.sub.3, CF.sub.3, CN, tert-butyl, phenyl and mixtures thereof.

4. The electrolyte as claimed in claim 1, characterized in that the electrolyte contains the compound of the general formula (1) in an amount in the range from ≥0.1% by weight to ≤10% by weight based on the total weight of the electrolyte.

5. The electrolyte as claimed in claim 1, characterized in that the electrolyte contains the compound of the general formula (1) in an amount in the range from ≥0.5% by weight to ≤7% by weight, based on the total weight of the electrolyte.

6. The electrolyte as claimed in claim 1, characterized in that the electrolyte contains the compound of the general formula (1) in an amount in the range from ≥3% by weight to ≤5% by weight, based on the total weight of the electrolyte.

7. The electrolyte as claimed in claim 1, characterized in that the solvent is selected from the group consisting of an unfluorinated organic solvent, a partially fluorinated organic solvent, an ionic liquid, a polymer matrix and mixtures thereof.

8. The electrolyte as claimed in claim 1, characterized in that the solvent is an organic solvent selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, acetonitrile, propionitrile, 3-methoxypropionitrile, glutaronitrile, adiponitrile, pimelonitrile, gamma-butyrolactone, gamma-valerolactone, dimethoxyethane, 1,3-dioxolane, methyl acetate, ethyl acetate, ethyl methanesulfonate, dimethyl methylphosphonate, linear sulfone, cyclic sulfone, symmetrical alkyl phosphates, unsymmetrical alkyl phosphates and mixtures thereof.

9. The electrolyte as claimed in claim 1, characterized in that the electrolyte salt is selected from the group consisting of LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiSbF.sub.6, LiClO.sub.4, LiPtCl.sub.6, LiN(SO.sub.2F).sub.2, LiN(SO.sub.2CF.sub.3).sub.2, LiN(SO.sub.2C.sub.2F.sub.5).sub.2, LiC(SO.sub.2CF.sub.3).sub.3, LiB(C.sub.2O.sub.4).sub.2, LiBF.sub.2(C.sub.2O.sub.4), LiSO.sub.3CF.sub.3 and mixtures thereof.

10. The electrolyte as claimed in claim 1 characterized in that the electrolyte salt is LiPF.sub.6.

11. An electrochemical energy store comprising an electrolyte as claimed in claim 1.

12. An electrochemical energy store, wherein the electrochemical store is a supercapacitor comprising an electrolyte as claimed in claim 1.

13. An electrochemical energy store based on lithium, comprising an electrolyte as claimed in 1.

14. A method for forming a solid electrolyte interphase on an electrode of an electrochemical cell comprising an anode, a cathode and an electrolyte, wherein the cell is operated using an electrolyte as claimed in claim 1.

15. A method for preparing a stable solid electrolyte interphase for protecting graphite anodes against exfoliation comprising: preparing a compound of the general formula (1) as indicated below: ##STR00005## where: X is C, S or S═O; R.sup.1 is selected from the group consisting of CN, C.sub.1-C.sub.10-alkyl, C.sub.1-C.sub.10-alkoxy, C.sub.3-C.sub.7-cycloalkyl, C.sub.6-C.sub.10-aryl, and/or —CO—O—R.sup.2 and mixtures thereof, wherein the alkyl, alkoxy, cycloalkyl and aryl groups are each unsubstituted or monosubstituted or polysubstituted by at least one substituent selected from the group consisting of F, C.sub.1-4-alkyl, CN and mixtures thereof and R.sup.2 is selected from the group consisting of C.sub.1-C.sub.10-alkyl, C.sub.3-C.sub.7-cycloalkyl, C.sub.6-C.sub.10-aryl and mixtures thereof, using the compound in an electrochemical energy store.

16. The method of claim 15, wherein the electrochemical energy store is a supercapacitor or an electrochemical energy store based on lithium ions.

17. The method of claim 15, wherein the electrochemical energy store is based on lithium ions.

Description

(1) Examples and figures which serve to illustrate the present invention are described below.

(2) Here, the figures show:

(3) FIG. 1 shows the first cycle of a graphite anode in a graphite/lithium half cell using an electrolyte solution as per one embodiment of the invention containing 1 M LiPF.sub.6 and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one (additive) in propylene carbonate (PC) and also comparative electrolytes containing 1 M LiPF.sub.6 in propylene carbonate or a 1:1 mixture of ethylene carbonate and dimethyl carbonate (EC:DMC). The potential is plotted against the specific capacity.

(4) FIG. 2 shows the oxidative stability window in LiMn.sub.2O.sub.4/Li half cells of the electrolyte containing 1 M LiPF.sub.6 and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one in propylene carbonate (PC) and also of comparative electrolytes containing 1 M LiPF.sub.6 in a 1:1 mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) with or without 5% by weight of VC.

(5) FIG. 3 shows the constant current cycling of an NMC/graphite full cell using the electrolyte solution containing 1 M LiPF.sub.6 and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one (additive) in propylene carbonate (PC) and also a comparative electrolyte containing 1 M LiPF.sub.6 in a 1:1 mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) over 50 cycles. The specific discharging capacity in mAh g.sup.−1 is plotted against the number of cycles. Only every third cycle is shown.

(6) FIG. 4 shows in each case the Coulomb efficiency plotted against the number of cycles for the NMC/graphite full cell using the electrolyte solution containing 1 M LiPF.sub.6 and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one (additive) in propylene carbonate (PC) and also a comparative electrolyte containing 1 M LiPF.sub.6 in a 1:1 mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) over 50 cycles. Only every third cycle is shown.

(7) FIG. 5 shows the specific discharging capacity against the number of cycles of a constant current cycling of an NMC622/graphite full cell at −20° C. (after electrochemical forming at 25° C.) using an electrolyte containing 1 M LiPF.sub.6 in a 30:5:65% by weight mixture of ethylene carbonate, propylene carbonate and diethyl carbonate (EC:PC:DEC) and 2% by weight of 3-methyl-1,4,2-dioxoazol-5-one (additive) and also a comparative electrolyte containing 2% by weight of vinylene carbonate (VC).

EXAMPLE 1

Preparation of 3-methyl-1,4,2-dioxoazol-5-one

(8) The synthesis was carried out as described by S. Chang et al. in J. Am. Chem. Soc. 2015, 137, pages 4534-4542. For this purpose, 50 mmol of acetohydroxamic acid (Sigma-Aldrich) were dissolved in 500 ml of dichloromethane. 50 mmol of 1,1′-carbonyldiimidazole (Combi-Blocks) were added thereto all at once at room temperature (20° C.±2° C.). After stirring for 16 hours, the reaction mixture was quenched with 300 ml of 1 M HCl, extracted three times with 150 ml in each case of dichloromethane and dried over magnesium sulfate. The solvent was removed under reduced pressure and 3-methyl-1,4,2-dioxoazol-5-one was obtained as a colorless, slightly yellowish oil.

(9) The reaction product obtained was examined by means of .sup.1H- and .sup.13C-NMR, and the .sup.1H and .sup.13C signals corresponded to the expected values for 3-methyl-1,4,2-dioxoazol-5-one.

EXAMPLE 2

Determination of the Specific Capacity in a Graphite/Li Half Cell

(10) The determination of the specific capacity in half cells was carried out using a three-electrode cell (Swagelok® type). Graphite electrodes (SFG6L, Imerys SA) were utilized as electrodes. A polymer nonwoven (Freudenberg SE, FS2226) was used as separator. Lithium foil (Rockwood Lithium, battery unit) served as reference electrode and counterelectrode.

(11) An electrolyte containing 1 M LiPF.sub.6 (BASF, battery purity) and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one obtained from example 1 was produced by dissolving the required amount of LiPF.sub.6 in propylene carbonate (PC, BASF, battery purity) and adding the appropriate amount of 3-methyl-1,4,2-dioxoazol-5-one. Comparative electrolytes containing 1 M LiPF.sub.6 in propylene carbonate and also a mixture of ethylene carbonate (BASF, battery purity) and dimethyl carbonate (BASF, battery purity) (EC:DMC) in a weight ratio of 1:1 were produced in the same way.

(12) The first charging process and the first discharging process in the range from 0.025 V to 1.5 V vs. Li/Li.sup.+ at a C rate of 0.2C was studied. In addition, the potential was kept constant at 0.025 V vs. Li/Li.sup.+ for one hour. FIG. 1 shows the first cycle of the graphite/lithium half cell for the electrolytes examined. As can be seen from FIG. 1, the comparative electrolyte 1 M LiPF.sub.6 in propylene carbonate did not show any reversible capacity. The addition of 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one to this electrolyte based on propylene carbonate made reversible cycling possible, corresponding to the positive control of the comparative electrolyte containing 1 M LiPF.sub.6 in a 1:1 mixture of ethylene carbonate and dimethyl carbonate.

EXAMPLE 3

Determination of the Oxidative Electrochemical Stability in an LMO/Li Half Cell

(13) The determination of the oxidative stability of the electrolytes in half cells was carried out by means of linear sweep voltammetry. In this method, the electrode voltage is continuously changed (linear sweep). A three-electrode cell (Swagelok® type) using lithium-manganese oxide (LMO, Customcells GmbH) as working electrode was utilized for this purpose. Lithium foil (Rockwood Lithium, battery purity) served as reference electrode and counterelectrode. A polymer nonwoven (Freudenberg SE, FS2226) was used as separator. To determine the oxidative stability, the potential between working electrode and reference electrode was increased from the no-load voltage to 6 V. The advance rate of the potential was 0.05 mV s.sup.−1.

(14) The electrolyte containing 1 M LiPF.sub.6 and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one obtained from example 1 was produced by dissolving the necessary amount of LiPF.sub.6 in propylene carbonate and adding the appropriate amount of 3-methyl-1,4,2-dioxoazol-5-one, as described in example 2. Comparative electrolytes were produced in the same way, with one comparative electrolyte containing 1 M LiPF.sub.6 in a mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) in a weight ratio of 1:1 and a second comparative electrolyte additionally containing 5% by weight of the additive vinylene carbonate (VC, UBE Industries).

(15) FIG. 2 shows the oxidative stability window of the electrolytes for a potential vs. Li/Li.sup.+ in the range from 3 V to 6 V. As can be seen from FIG. 2, the electrolyte containing 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one in propylene carbonate was stable up to 5.6 V vs.

(16) Li/L.sup.+, while the electrolytes in ethylene carbonate/dimethyl carbonate with or without addition of 5% by weight of vinylene carbonate were stable up to 4.7 V and 5.3 V vs. Li/Li.sup.+, respectively.

(17) The electrolyte according to the invention containing 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one thus shows a significantly higher oxidative stability limit than the electrolyte based on vinylene carbonate. This indicates that 3-methyl-1,4,2-dioxoazol-5-one is oxidized later than vinylene carbonate and thus has a better high-voltage stability. The electrolyte according to the invention containing 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one in propylene carbonate also displays a significantly higher oxidative stability than the reference electrolyte 1 M LiPF.sub.6 in a mixture of ethylene carbonate and dimethyl carbonate (EC:DMC). Accordingly, 3-methyl-1,4,2-dioxoazol-5-one is particularly useful for applications in which high-voltage electrode materials are present because of its high oxidative stability.

(18) Effective passivation of graphite can thus be achieved by the use of the dioxazolone derivatives according to the invention in small proportions by weight in the electrolyte. Due to the fact that only 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one in propylene carbonate is sufficient for the formation of a solid electrolyte interphase (SEI) on graphite electrodes, the use of dioxazolones as additive for electrolytes based on propylene carbonate is also economical.

EXAMPLE 4

Examination of the Long-Term Cycling Stability at Constant Current

(19) The examination of the long-term cycling stability was carried out in full cells having a button cell structure (Hohsen Corp., CR2032) using lithium-nickel.sub.1/3-manganese.sub.1/3-cobalt.sub.1/3 oxide (NMC(111), Customcells GmbH) and graphite electrodes (Customcells GmbH). A polymer nonwoven (Freudenberg SE, FS2226) was used as separator. Cycling was carried out in a voltage window of from 4.2 V to 2.8 V. 2 forming cycles were carried out at 0.1C (including holding of the voltage at 4.2 V until the current is <0.05C), followed by 3 conditioning cycles at 0.33C (including holding of the voltage at 4.2 V until the current is <0.05C and a delay step of one hour), followed by 95 1.0C charging/discharging cycles. The measurements at constant current were carried out on a battery tester series 4000 (Maccor) at 20° C.±0.1° C.

(20) The electrolyte containing 1 M LiPF.sub.6 and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one obtained from example 1 was produced by dissolving the necessary amount of LiPF.sub.6 in propylene carbonate and adding the appropriate amount of 3-methyl-1,4,2-dioxoazol-5-one, as described in example 2. As comparative electrolyte, a solution of 1 M LiPF.sub.6 in a mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) in a weight ratio of 1:1 was produced.

(21) FIG. 3 shows the specific discharging capacity of the NMC/graphite full cell versus the number of cycles and FIG. 4 shows the Coulomb efficiency versus the number of cycles using the respective electrolytes. As FIG. 3 shows, the electrolyte containing 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one displayed only a small decrease in capacity over 50 cycles. The maintenance of capacity in cycle 50 based on the first cycle at 1.0C (cycle 6) was 98.7%. The comparative electrolyte 1 M LiPF.sub.6 in a mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) displayed a capacity retention of 98.1%.

(22) Furthermore, it can be seen from FIG. 4 that the Coulombic efficiency in the first cycle was 85.3% for the electrolyte containing 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one. The comparative electrolyte 1 M LiPF.sub.6 in a mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) displayed a Coulombic efficiency in the first cycle of 86.3%. The full cell with the electrolyte containing 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one thus displayed good cycling stability, corresponding to the comparative electrolyte 1 M LiPF.sub.6 in a mixture of ethylene carbonate and dimethyl carbonate (EC:DMC).

(23) Overall, the results show that 3-methyl-1,4,2-dioxoazol-5-one forms a passivating protective layer which conducts lithium ions on the surface of graphite, is suitable for high-voltage applications and enables lithium ion batteries to be operated with good cycling stability.

EXAMPLE 5

Examination of the Cycling Behavior at Low Temperatures

(24) The examination of the cycling behavior at low temperatures was carried out in full cells having a button cell structure (Hohsen Corp., CR2032) with lithium-nickel-manganese-cobalt oxide (NMC622, Customcells GmbH) and graphite electrodes (Customcells GmbH) using an S240/P20 Separion separator. Cycling was carried out in a voltage window of 2.8-4.2 V. The electrochemical forming at 2×0.1C and the subsequent charging process at 0.1C were carried out at a temperature of 25° C., and the subsequent charging/discharging cycles at 0.3333C were carried out at a temperature of −20° C. The measurements were carried out on a battery tester series 4000 (Maccor).

(25) 90 μl of a solution containing 1 M LiPF.sub.6 in a mixture of ethylene carbonate, propylene carbonate and diethyl carbonate (EC:PC:DEC) in a weight ratio of 30:5:65 and 2% by weight of 3-methyl-1,4,2-dioxoazol-5-one obtained from example 1 were used as electrolyte. A solution of 1 M LiPF.sub.6 in EC:PC:DEC containing 2% by weight of the additive vinylene carbonate (VC, UBE Industries) was used as comparative electrolyte. The electrolytes were produced by dissolving the necessary amount of LiPF.sub.6 in the solvent mixture and adding the appropriate amount of additive, as described in example 2.

(26) FIG. 5 shows the specific discharging capacity at −20° C., after prior forming at 25° C. for 2 cycles, plotted against the number of cycles for the electrolyte containing 2% by weight of 3-methyl-1,4,2-dioxoazol-5-one (additive) and also the comparative electrolyte containing 2% by weight of vinylene carbonate (VC).

(27) As FIG. 5 shows, the electrolyte containing 2% by weight of 3-methyl-1,4,2-dioxoazol-5-one displayed good capacity retention at low temperatures, with the capacity after 100 cycles still being more than 80% of the capacity of cycle 6. In contrast, the comparative electrolyte displays only a very short cycling life of only 20 cycles to the EOL (end of life) criterion of 80% of the capacity of cycle 6 is reached.

(28) This shows that 3-methyl-1,4,2-dioxoazol-5-one is, in contrast to vinylene carbonate, well suited to low-temperature applications and lithium ion batteries having 3-methyl-1,4,2-dioxoazol-5-one as additive can be operated with good cycling stability even at low temperatures.