EP1485928A1

EP1485928A1 - Electrolyte solution and use thereof

Info

Publication number: EP1485928A1
Application number: EP03720166A
Authority: EP
Inventors: Andree Schwake
Original assignee: Epcos AG
Current assignee: TDK Electronics AG
Priority date: 2002-03-21
Filing date: 2003-03-13
Publication date: 2004-12-15
Also published as: DE10212609A1; US20060024577A1; JP2005521257A; WO2003081620A1; DE10212609B4

Abstract

The invention relates to an electrolyte solution for electrochemical cells with a high boiling point > 86 DEG C at 1 bar and a high degree of conductivity > 40 mS/cm at 25 DEG C. Said solution contains acetonitrile as the first solvent, (component A), in a proportion of 40 - 90 wt % of the average weight of the solvent, in addition to at least one additional electrochemically stable solvent with a boiling point > 120 DEG C at 1 bar, a dielectricity constant > 10 at 25 DEG C and a viscosity < 6 mPas at 25 DEG C and additionally at least one support electrolyte as component C). Inventive electrolyte solutions of this type have a high degree of conductivity, which is comparable to electrolyte solutions that use acetonitrile as the sole solvent, whilst at the same time exhibiting an increased boiling point as a result of component B).

Description

Electrolyte solution and its use

Electrolyte solutions containing acetonitrile as a solvent are often used in electrochemical cells, for example capacitors or batteries. Acetonitrile has a high polarity (dielectric constant DK = 37.5 at 25 ° C) with a very low viscosity (0.325 mPas at 25 ° C). Due to the high polarity of the acetonitrile, this supports the dissociation of the conductive salts, which at the same time have a high mobility in the electrolyte solution due to the low viscosity of the acetonitrile, so that electrolyte solutions with acetonitrile as the sole solvent achieve a very high conductivity. An electrolyte consisting, for example, of 0.9 M tetraethylammonium tetrafluoroborate in 100% acetonitrile as a solvent has a conductivity of 55.1 mS / cm at 25 ° C. Electrolytes without acetonitrile as a solvent have a much lower conductivity. For example, an electrolyte solution consisting of 0.9 M tetraethylammonium tetrafluoroborate in 100% propylene carbonate has a conductivity of only 13.7 mS / cm at 25 ° C.

The disadvantage of electrolyte solutions that use acetonitrile as a solvent is the relatively low boiling point of acetonitrile (81.6 ° C at 1 bar). This boiling point is only slightly increased by the addition of the conductive salt, so that boiling points of about 84 ° C. result in electrolyte solutions containing acetonitrile. Due to these low boiling points, the upper operating temperature of electrochemical cells containing the acetonitrile-containing electrolytes is limited to a maximum of 70 ° C, since at higher temperatures the internal pressure of the electrochemical cells rises so much that it may cause the housing to deform and the pressure relief valve to respond or Predetermined breaking point can come. If the housing is deformed, the functionality of the electrochemical cell can no longer be guaranteed. If the pressure relief valve or the predetermined breaking point address, so acetonitrile vapors enter the atmosphere, which pose a high safety risk due to the potential fire and explosion hazard. Nowadays, electrochemical cells with an operating temperature of> 85 ° C are still required.

US Pat. No. 5,418,682 describes electrolyte solutions with operating temperatures up to 150 ° which contain glutaronitrile mixed with another dinitrile as the solvent. Although glutaronitrile and other dinitriles have high dielectric constants, they also have a high viscosity due to their high boiling points. For this reason, such electrolyte solutions have only low conductivities. For example, an electrolyte consisting of 1 M tetraethylammonium tetrafluoroborate with glutaronitrile and succinonitrile as a solvent has only a low conductivity of 7.19 mS / cm at room temperature.

It is also possible to use molten salts which do not require a solvent in electrochemical cells which are to be used at temperatures above 70.degree. These molten salts, for example 1-ethyl-3-methylimidazolium tetrafluoroborate, have high boiling points of, for example, 200 ° C., but also have only low conductivities, which in the case of the above-mentioned molten salt are about 13 mS / cm at 25 ° C. (Journal of the Electrochemical Society (1999), 146 (5), 1687-1695).

The object of the present invention is to provide an electrolyte solution with high conductivity and at the same time a high boiling point which avoids the disadvantages mentioned of known electrolyte solutions and has an operating temperature> 85 ° C.

According to the invention, this object is achieved by an electrolyte solution having the features of claim 1. advantageous Embodiments of the electrolyte solution and their use are the subject of further claims.

An electrolyte solution according to the invention has a boiling point of greater than 86 ° C. at 1 bar pressure and a conductivity of greater than 40 mS / cm at 25 ° C. and comprises as component A) acetonitrile with a proportion of 40-90% by weight of the solvent weight as the first solvent and as component B) at least one second, electrochemically stable solvent with a boiling point> 120 ° C. at 1 bar pressure, a dielectric constant> 10 at 25 ° C. and a viscosity <6 mPas at 25 ° C. At least one conductive salt is added as component C).

The inventor has recognized that, surprisingly, electrolyte solutions with high conductivity and at the same time a high boiling point can be achieved by combining acetonitrile as component A) with at least one further solvent as component B) which has a boiling point of greater than 120 ° C. 1 bar. Because of the increased boiling point of this component B), the boiling point of the entire electrolyte solution is raised, so that a boiling point of greater than 86 ° C. results for the entire electrolyte solution.

Apart from the high boiling point greater than 120 ° C, component B) must also have a certain viscosity <6 mPas at 25 ° C and a dielectric constant> 10 at 25 ° C. Component B) thus has a higher viscosity than acetonitrile, so that a person skilled in the art would expect that electrolyte solutions with this solvent

Component have significantly lower conductivities than electrolyte solutions with acetonitrile as the sole solvent. Nevertheless, component B) in the electrolyte solutions according to the invention has a dissociating effect on the conducting salt due to its sufficient polarity and, at the same time, ensures that the ions formed remain mobile due to its relatively low viscosity the electrolyte solution, so that a surprisingly high conductivity of the electrolyte solutions according to the invention results. Surprisingly, the inventor has thus succeeded in obtaining electrolyte solutions which only show the desired positive characteristics of acetonitrile (high conductivity) and component B) (high boiling point), without, conversely, the undesired properties of the two components ( Acetonitrile = low boiling point; component B) = low conductivity) come into play too much. Electrolyte solutions according to the invention have a high conductivity, which is roughly in the range of electrolyte solutions that use acetonitrile as the sole solvent, but at the same time have a high boiling point, which until now could not be achieved with electrolyte solutions containing acetonitrile.

Furthermore, the solvent of component B) must be electrochemically stable, so that it is neither oxidatively nor reductively decomposed on the charged surfaces of the electrodes during the operation of the electrochemical cells. The electrochemical stability of electrolytes and their solvents can be determined, for example, by recording cyclovoltammograms. The precise determination of the electrochemical stability of electrolytes and solvents is described, for example, in the publication in the Journal Electrochemica Acta (2001), 46, 1823-1827, to which reference is hereby made in full.

The dielectric constant of a solvent can be determined in a decameter using methods known to the person skilled in the art. They are presented, for example, in the Römpp chemistry lexicon (9th edition) under the term "dielectric constant" (page 955-956), to which reference is also made here in full.

The viscosity of a solvent can be determined, for example, in a manner familiar to those skilled in the art using an Ubbeloh- Determine the de-viscometer. The boiling points of solvents can also be determined in a simple manner by determining the temperature of the boiling liquid.

Component B) is advantageously selected from the following solvents: ethylene carbonate, propylene carbonate, γ-butyrolactone, γ-valerolactone, butylene carbonate, sulfolane, 3-methylsulfolane, dimethyl sulfoxide, glutaronitrile, succinonitrile, 3-methoxypropionitrile, diethyl carbonate, ethyl methyl phosphate, trimethyl N-methylpyrrolidinone, N-

Methyloxazolidinone, N, N-dimethylimidazolidinone, dimethylformamide and dimethylacetamide.

The proportion of component B) in the solvent weight is advantageously about 10 to 60% by weight, preferably 10 to 50% by weight (without conductive salt). This means that acetonitrile is present at the same time with a preferred proportion of 50 to 90% by weight. This can ensure that electrolyte solutions according to the invention on the one hand have high conductivity due to a sufficiently high proportion of acetonitrile, but at the same time also have a high boiling point due to a high proportion of component B).

The conductive salts as component C) are selected from combinations of certain anions and cations. Anions include borate, for example tetrafluoroborate, fluroalkylphosphate, PF ₆ ^~ , AsF ₆ ^" , SbF ₆ -, fluoroalkylarsenate, fluoroalkylantimonate, trifluoromethylsulfonate, bis (trifluoromethanesulfon) imide, tris (trifluoromethanesulfonyl) methide, perchlorate, tetrachloro, tetrachloride, and OR) ₄ ^~ , for example oxalatoborate, where R is an alkyl group which can also be bridged with other OR groups. The cations are generally the ammonium cation, for example tetraalkyammonium cation, the phosphonium Cation and its tetraalkyl cations, the pyridinium cation, morpholinium, lithium,

Imidazolium and pyrrolidinium cations used. The salts can also be melted at room temperature. In the electrolytes according to the invention, tetraethylammonium tetrafluoroborate is often used as component C), that is to say as the conductive salt, since it is particularly readily soluble in the solvents of the electrolyte solutions according to the invention, is readily available and guarantees high conductivity.

In the following, the invention will be explained in more detail by means of exemplary embodiments. In the associated table 1, the composition of 21 electrolyte solutions according to the invention are given together with their respective boiling temperatures at 1 bar and their conductivity at 25 ° C. and compared with a conventional electrolyte solution. For the two solvent components A) and B), the% by weight are given after the colon, the weight of the conductive salt being disregarded. A conventional electrolyte solution which contains acetonitrile as the sole solvent serves as comparative example 1. In all of the exemplary embodiments of the invention and also in the case of the conventional electrolyte solution, tetraethylammonium tetrafluoroborate is used as the conductive salt in a concentration of 1.2 moles per liter. The conductive salt can also be exchanged for the other conductive salts mentioned above without major changes in conductivity:

Abbreviations:

AC = acetonitrile, PC = propylene carbonate, EC = ethylene carbonate, γ-B. = γ-butyrolactone, DMSO = dimethyl sulfoxide, MPN = 3-methoxy propionitrile, GN = glutaronitrile, TEATFB = tetraethylammonium tetrafluoroborate. Table 1 :

The electrolyte solutions according to the invention in the exemplary embodiments comprise, as component B), a whole series of solvents, for example γ-butyrolactone, propylene carbo- nat, ethylene carbonate, glutaronitrile, dimethyl sulfoxide, 3-methoxy propionitrile, or a mixture of γ-butyrolactone and 3-methoxy propionitrile or a mixture of γ-butyrolactone and ethylene carbonate.

A particularly high boiling point of 101 ° C combined with a high conductivity of 42.9 mS / cm at 25 ° C can be achieved with approximately equal proportions by weight of acetonitrile and γ-butyrolactone as component B) and tetraethylammonium tetrafluoroborate in a concentration of about 0. 9 to 1.2

Reach mol per liter as component C). The proportion of acetonitrile can vary between 50 to 60 percent by weight and the proportion of γ-butyrolactone between 40 to 50 percent by weight.

To determine the electrical data of double-layer capacitors with electrolyte solutions according to the invention, electrochemical double-layer capacitors were impregnated with an electrolyte solution according to Example 6, the electrical data of which were determined and compared with those of the known comparative electrolyte solution No. 1. The corresponding data are shown in Table 2:

Table 2:

It can be seen that capacitors with electrolyte solutions according to the invention still have an acceptable series resistance (ESR) with high capacitance at the same time, which are comparable to values of conventional capacitors. In contrast to the conventional capacitors, capacitors with the electrolyte solutions according to the invention, however, have significantly higher operating temperatures. The electrolyte solutions according to the invention can also be used in primary and secondary Li batteries or Li-ion batteries. These then also have higher operating temperatures due to the electrolyte solutions.

The invention is not restricted to the exemplary embodiments shown here. Other electrolyte compositions with other components B) and other conductive salts in different mixing ratios are also within the scope of the invention.

Claims

claims

1. Electrolyte solution for electrochemical cells with a boiling point> 86 ° C at 1 bar and a conductivity of> 40 mS / cm at 25 ° C, which comprises the following components:

A) acetonitrile with a proportion of 40-90% by weight of the solvent weight as the first solvent,

B) At least one second electrochemically stable solvent with a boiling point> 120 ° C at 1 bar, a DK> 10 at 25 ° C and a viscosity <6 mPas at 25 ° C,

C) At least one conductive salt.

2. Electrolyte solution according to the preceding claim, - in component B) is selected from the following solvents:

Ethylene carbonate, propylene carbonate, γ-butyrolactone, γ-valerolactone, butylene carbonate, sulfolane, 3-methylsulfolane, dimethyl sulfoxide, glutaronitrile, succinonitrile, 3-methoxyproprionitrile, diethyl carbonate, ethyl methyl carbonate, trimethyl phosphate, N-methyl pyrrolidinone, Dimethylimidazolidinone, dimethylformamide and dimethylacetamide.

3. Electrolyte solution according to one of the preceding claims, in which component B) is present in a proportion of about 10-60% by weight of the solvent weight.

4. Electrolyte solution according to one of the preceding claims, - in which component C) is a conductive salt, which is selected from pairs of combinations of the following anions and cations:

Anions: borate, tetrafluoroborate, fluoroalkyl phosphate, PF ₆ ^" , AsF ₆ ^" , SbF _s -, fluoroalkyl arsenate, fluoroalkyl antimonate, Trifluoromethyl sulfonate, bis (trifluoromethanesulfon) imide, tris (trifluoromethanesulfonyl) methide, perchlorate, tetrachloroaluminate, oxalatoborate and anions with B (OR) ₄ ^" , where R is an alkyl group which can also be bridged with other OR groups,

Cations: ammonium cation, tetraalkylammonium cation, phosphonium cation, tetraalkylphosphonium cation, pyridinium cation, morpholinium cation, lithium cation, imidazolium and pyrrolidinium.

5. Electrolyte solution according to one of the preceding claims, in which component C) is tetraethylammonium tetrafluoroborate.

6. Electrolytic solution according to one of the preceding claims, in which component B) is γ-butyrolactone.

7. Electrolyte solution according to one of claims 1 to 5, - in component B) is propylene carbonate.

8. Electrolyte solution according to one of claims 1 to 5, in which component B) is ethylene carbonate.

9. Electrolytic solution according to one of claims 1 to 5, in which component B) is glutaronitrile.

10. Electrolyte solution according to one of claims 1 to 5, in which component B) is dimethyl sulfoxide.

11. Electrolyte solution according to one of claims 1 to 5, in which component B) is 3-methoxy propionitrile.

12. Electrolyte solution according to one of claims 1 to 5, - in component B) is a mixture of γ-butyrolactone and 3-methoxy propionitrile.

13. Electrolyte solution according to one of claims 1 to 5, in which component B) is a mixture of γ-butyrolactone and ethylene carbonate.

14. Electrolyte solution according to one of claims 1 to 6, in which acetonitrile is present as component A) in a proportion of approximately 50-60% by weight, in component B) γ-butyrolactone in a proportion of approximately 40-50% by weight is - in component C) is tetraethylammonium tetrafluoroborate with a concentration of about 0.9 to 1.2 mol / 1.

15. Use of an electrolytic solution according to one of the preceding claims in capacitors.

16. Use of an electrolyte solution according to one of claims 1 to 14 in electrochemical double-layer capacitors.

17. Use of an electrolyte solution according to one of claims 1 to 14 in primary and secondary Li batteries or Li-ion batteries.