EP1395999A1

EP1395999A1 - Flame-retardant electrolyte solution for electrochemical double-layer capacitors

Info

Publication number: EP1395999A1
Application number: EP02732425A
Authority: EP
Inventors: Andree Schwake
Original assignee: Epcos AG
Current assignee: TDK Electronics AG
Priority date: 2001-06-13
Filing date: 2002-05-22
Publication date: 2004-03-10
Also published as: DE10128581A1; WO2003003393A1; JP2004531091A; US20040218347A1; KR20040010516A; CN1463455A; DE10128581B4

Abstract

The invention relates to flame-retardant electrolyte solutions with flash points in excess of 76 °C. Said solutions contain at least one support electrolyte which is dissolved in a solvent mixture consisting of at least one highly polar component and at least one flame-retardant, low-viscosity carbamate component.

Description

description

FLAME RETARDANT ELECTROLYTE SOLUTION FOR ELECTROCHEMICAL DOUBLE LAYER CONDENSORS

Electrochemical capacitors serve as power and

Energy storage, whereby they can emit or absorb high currents and the associated high powers and energies in relatively short periods of time. A low internal electrical resistance of the capacitors is required for this purpose. In addition to the material of the electrode layers, the separator and the cell structure, the internal resistance of an electrochemical capacitor is very much dependent on the conductivity of the electrolyte.

According to the current state of the art, electrochemical capacitors with non-aqueous electrolytes preferably use solvent mixtures of at least two or more components. The mixture must contain at least one strongly polar component which, due to its polarity, has a strongly dissociating effect on the conductive salts used. Ethylene carbonate and propylene carbonate are generally used as such polar components. The disadvantage of these highly polar solvents is their relatively high viscosity, which greatly reduces the mobility of the dissociated conductive salt ions and thus reduces the conductivity of the electrolyte. In order to reduce the viscosity of the electrolyte, in addition to the highly polar solvent, one or more low-viscosity solvents are generally used as thinners. Typical diluents are, for example, 1,2-dimethoxyethane, 1,3-dioxolane, 2-methyltetrahydrofuran or dimethyl carbonate. The proportion of thinners is usually between 20 and 60% by weight of the solvent mixture. The disadvantage of these low-viscosity thinners is their very high volatility and the very low flash points of these solvents: 1,2 dimethoxyethane (boiling point 85 ° C, flash point -6 ° C); 1.3 dioxolane (boiling point 78 ° C, flash point 2 ° C); 2-methyltetrahydrofuran (boiling point 80 ° C, flash point -12 ° C); Dimethyl carbonate (boiling point 90 ° C, flash point 18 ° C). As a result, the solvent mixtures corresponding to the prior art also have low flash points (for example 28.5 ° C. for the solvent mixture ethylene carbonate: dimethyl carbonate 2: 1). Since heating always occurs when using electrochemical capacitors, especially in the case of the above-mentioned high currents according to the current state of the art, when faults occur (overcharging, short circuit), this means, in particular when the capacitor housing bursts open and the associated leakage of electrolytes, a significant risk of inflammation with the corresponding serious consequences.

Another disadvantage of the low-viscosity thinners is their comparatively low dielectric constants

(DK) and associated low conductivities. They are between 3 and 7 for the mentioned thinners.

There are currently no equivalent substitutes for the above-mentioned thinners in electrolytes for electrochemical capacitors. According to the current state of the art, reduced flammability of the electrolyte solution is achieved primarily by increasing the viscosity of the electrolyte solution by means of binders or fillers or by using polymer electrolytes which are practically solid at room temperature. All these gel-like solid electrolytes have in common that, due to their high viscosity, the mobility of the ions of the salts dissolved in them is far less than in liquid electrolyte solutions, so that in part no longer the required conductivities for electrochemicals

High power density capacitors can be achieved.

The patents DE 197 24 709 and EP 0 575 191 describe fluorine- or alkyl-substituted carbamates with high flash points and low viscosity as diluent components for flame-retardant electrolyte solutions of lithium batteries. You can in combination with conventional highly viscous and polar solvents such as ethylene or propylene carbonate can be used. However, the conductivities of these electrolyte solutions with values of 2.8 to 6.7 mS / cm at room temperature are far too low to be considered for use in double-layer capacitors. For double-layer capacitors with a high power density, electrolytes with a conductivity of more than 20 mS / cm are required. Among other things, active carbon electrodes with very large surfaces are used in electrochemical capacitors, which are much more difficult to wet with an electrolyte than the metal oxide electrodes of the lithium batteries. For this reason, it is advantageous to optimize electrolyte solutions for use in electrochemical capacitors so that good wetting of the electrodes is ensured. In addition, the lithium conductive salts used in lithium batteries are unsuitable for electrochemical capacitors, since metallic lithium can be deposited in the active carbon electrodes of the capacitor and thus impair its function.

The object of the present invention is to provide a flame-retardant electrolyte solution which avoids the disadvantages of known electrolyte solutions.

This object is achieved by a. Solved electrolyte solution according to claim 1. Advantageous embodiments of the invention and electrochemical capacitors in which the electrolyte solution according to the invention can be used are the subject of further claims.

An electrolyte solution according to the invention has flash points greater than 76 ° C. at 1 bar pressure and conductivities> 20 mS / cm at 25 ° C. and comprises the three components A, B and C.

Component A comprises at least one high polarity solvent. Due to its polarity, it has a strongly dissociating effect on the conductive salt. Solvent with high polarity in the sense of the invention have a dielectric constant (DK)> 20. 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 made here in full.

In contrast to conventional electrolyte solutions, a low-viscosity thinner (component B) with a high flash point is used according to the invention, so that a flame-retardant electrolyte solution is formed. For the purposes of the invention, low-viscosity solvents are to be regarded which have a viscosity of <2 cP at 25 ° C. The viscosity of a solvent can be determined, for example, using an Ubbelohde viscometer.

Alkyl or fluorine-substituted carbamates are used for solvent component B. The carbamates are characterized by the general formula 1

wherein

Rl and R2 are independently the same or different, a linear C1-C6-alkyl- (Rl and R3 = CH _{2j +} ι with j = 1-6), a branched C3-C6-alkyl- (Rl and R3 = C _k H _{2k +} ι with k

= 3-6) or a C3-C7 cycloalkyl group (Rl and R3 = CjH _2j _ι with j = 3-7) or

R1 and R2 according to formula 2

directly or via one or more additional N and / or O atoms to form a ring with 3 to 7 ring members, so that X is represented by the empirical formula (CR'R "") _m O _n (NR "") ₀ with 2 <(m + n + o) <6 can be described, wherein any additional N atoms present in the ring are saturated with C1-C3-alkyl (R "'= C _p H _{2p + 1} with p = 0-3) and also the ring carbons can carry C1-C3 alkyl groups, it being possible for one or more hydrogen atoms in the radicals R1 and R2 to be replaced by fluorine atoms (R 'and R "= C _r H ( _{2r +} i) -sFs with r = 0-3 and s = 0- (2r + l)),

R3 in both formulas is a linear Cl-C6-alkyl- (R3 = C _t H ₂ t ₊ ι with t = 1-6), branched C3-C6-alkyl- (R3 = C _u H _{2u +} ι with u = 3- 6), C3-C7-cycloalkyl- (R3 = C _v H _2v -ι with v = 3-7) or a partially or perfluorinated straight-chain alkyl group with 3 to 7 carbon atoms, which may be one or more times with Cl-C6- Alkyl can be substituted is (R3 = C _w H ( _{2w +} ι) - χF _x (C _y H _{2y +} ι) _z with w = 3-7 and x = 0- (2w + l) and y = 1-6 with z = 0- (2w + l) where x + z = (2w + l)).

Component B according to the invention comprises at least one flame-retardant carbamate of the general formulas shown above with low viscosity. Since a number of highly polar category A solvents have a high viscosity, which prevent the electrolyte solution from achieving a sufficiently high conductivity, component B serves to reduce the viscosity of the solvent mixture. The result is a solvent mixture according to the invention with a flash point greater than 76 ° C. at 1 bar and a conductivity comparable to or even greater than that of conventional capacitors with a high power density (> 20 mS / cm at 25 ° C.). The solvent mixture of components A and B with maximum Conductivity does not have a maximum polarity, expressed by a large to maximum dielectric constant (DK), but coordinated with one another a non-maximum DK and a non-minimum viscosity, which in combination result in maximum conductivity. Since component B has a high flash point according to the invention, solvent mixtures with high conductivity are formed which, in contrast to solvent mixtures which correspond to the prior art, are flame-retardant. According to the invention, conductive salts which do not contain lithium are used as component C. This can prevent metallic lithium from being deposited in the activated carbon electrodes and impairing their function. Furthermore, Li-free electrolyte solutions have higher conductivities than electrolyte solutions that contain Li conductive salts.

Highly polar, highly viscous solvents for component A can be selected from the group of cyclic carbonates, such as, for example, ethylene or propylene carbonate. Component A can also be selected from the following nitriles: acetonitrile, 3-methoxypropionitrile, glutaronitrile and succinonitrile. Component A can also be lactones, for example γ-butyrolactone and / or γ-valerolactone. It is also possible to mix any of the solvents mentioned in the electrolyte solutions according to the invention, so that, for example, component A can also be a mixture of propylene carbonate and acetonitrile.

Preferably 2, 2, 2-trifluoroethyl-N, N-dimethylcarbamate, 2,2,2 trifluoroethyl-N, N-diethylcarbamate, methyl-N, N-

Dimethyl carbamate, ethyl-N, N-dimethyl carbamate and methyl-N, N-diethyl carbamate can be used. In contrast to conventional low-viscosity thinners, they have much higher flash points, but with almost the same low viscosity they have higher dielectric constants (for example methyl-N, N-dimethyl carbamate 12.5 compared to a dielectric constant of 2.1 for dimethyl carbonate). The Carbamates used according to the invention can generally be used in concentration ranges between 10 to 60% by weight, solvent mixtures with maximum conductivities and at the same time high flash points above 76 ° C. at 1 bar preferably containing 30 to 50% by weight of the carbamates mentioned.

Component C can be used, inter alia, as lithium-free conducting salts based on onium salts with nitrogen or phosphorus as the central atom. For example, quaternary ammonium or phosphonium cations such as, for example, tetraethylammonium or methyltriethylammonium in combination with anions such as tetrafluoroborate, hexafluoroarsenate, hexafluoroantimonate, borate, for example oxalatoborate, bis (trifluoromethylsulfonyl) trifluoride (trifluoromethylsulfonyl), trifluoromethyl trachloroaluminate can be used. It is also possible to use molten conductive salts with organic cations, for example based on imidazolium or pyrrolidinium cations in conjunction with the anions mentioned above. Furthermore, pyridinium and morpholinium cations and also as anions borates of the general formula B (OR) ι ^~ can be used, where R is selected from the following substituents: Cl to C6 alkyl groups,

- -OC- (Rl) _x with x = 0.1, where Rl is a Cl to C6 alkyl group.

It is also possible for two substituents to be connected to one another via the carbon atoms, so that the two

Oxygen atoms, which each contact the boron atom, are bridged. If, for example, all four substituents R are an -OC- (Rl) _x group with x = 0 and two of the substituents are bonded to one another via the carbon atoms of the keto group, the above-mentioned oxalatoborate B (OOCCOO) ₂ ^" results. The use of mixtures of several conductive salts is also possible. Good results with sufficiently high conductivities are also achieved with standard conductive salts, for example tetraalkylammonium tetrafluoroborates. In contrast to the lithium salts mentioned, these conductive salts do not accumulate in activated carbon electrodes and are therefore particularly suitable for use in electrochemical capacitors. The conductive salts are advantageously used in the electrolyte solutions according to the invention in a concentration> 0.7 mol / 1, preferably in a concentration> 1 mol / 1.

When the various components are added, the viscosity of the electrolyte solution is adjusted so that good wetting of the carbon electrodes can be ensured at the same time.

Since the carbamates have improved electrochemical stability compared to conventional low-viscosity thinners, electrochemical capacitors can be operated with electrolyte solutions according to the invention containing the above-mentioned carbamates at higher line voltages> 2.5 V, preferably 2.7 V, most preferably 3 V. Due to the increased line voltages, these electrochemical capacitors also have an advantageous increased power and energy density. Due to the flame retardancy of the electrolyte solutions, capacitors with electrolyte mixtures according to the invention can be used at higher temperatures than conventional capacitors. If electrolytes escape from the capacitor housing, the risk of the capacitor electrolyte igniting is also lower than in the case of capacitors according to the current state of the art.

The invention is explained in more detail below on the basis of exemplary embodiments. The associated Table 1 shows the physical properties of some selected carbamates. The dielectric conformity constant, the viscosity and the boiling point, which correlates directly with the flash point (higher boiling point means higher flash point).

Table 1

Example 1:

Production of a flame-retardant electrolyte solution based on methyl-N, N-dimethyl carbamate.

As components A and B, propylene carbonate and methyl - N, N-dimethyl carbamate are mixed in a weight ratio of 1: 1. 1.4 M tetraethylammonium tetrafluoroborate is added as the conductive salt (component C). This electrolyte solution has a conductivity of more than 21 mS / cm and a boiling point of more than 131 ° C at room temperature.

Example 2:

Production of a flame-retardant electrolyte solution based on 2, 2, 2-trifluoroethyl-N, N-dimethyl carbamate. Components A and B are ethylene carbonate and 2,2,2-

Trifluoroethyl-N, N-dimethyl carbamate mixed in a weight ratio of 2: 1. 1 M tetraethylammo- nium tetrafluoroborate added. This electrolytic solution has a flash point greater than 85 ° C.

Example 3:

Production of a flame-retardant electrolyte solution based on ethyl-N, N-dimethyl carbamate.

As components A and B, propylene carbonate and ethyl N, N-dimethyl carbamate are mixed in a weight ratio of 1.5: 1. 1.4 M methyltriethylammonium tetrafluoroborate is added as the conductive salt. At room temperature, this electrolyte solution has a conductivity greater than 21 mS / cm and a flash point greater than 90 ° C.

The electrolyte solutions according to the invention can advantageously be used in electrochemical double-layer capacitors with low internal resistance, as disclosed in US Pat. No. 6094788, to which reference is hereby made in full.

The electrodes of these double-layer capacitors are preferably each composed of a large number of metal-impregnated active carbon wipes, each active carbon wipe being contacted in an electrically conductive manner by one end of a current collector foil. These foils are used to reduce the internal resistance of the activated carbon electrodes. The other ends of the current collector foils of electrodes of the same polarity are each connected to form a bundle, so that an electrode terminal is formed for both the anode and the cathode, to which an electrical potential can be applied. The electrodes of opposite polarity are separated in an electrically insulating manner by porous separators impregnated with operating electrolyte. The resulting stack of alternating activated carbon Cloths and porous separators are packaged in a housing under slight pressure so that the largest possible contact area between the current collector foils and the activated carbon electrodes is created. It is also possible to use carbon powder electrodes in the embodiment described above.

Furthermore, the electrolyte solutions according to the invention can be used in hybrid capacitors with metal oxide electrodes or combinations of metal oxide electrodes with electrodes in the form of activated carbon cloths or activated carbon powder. Pseudo-capacitors which contain conductive polymers or combinations of conductive polymers with electrodes in the form of active carbon cloths or active carbon powder or conductive polymers in combination with metal oxide electrodes are also suitable. Embodiments of the electrochemical capacitors can include components with a cylindrical or prismatic shape. Radial capacitors in which the electrode connections are arranged on one side are also possible, or axial capacitors in which one connection each is located on the top and bottom of the component.

The invention also relates to an aluminum electrolytic capacitor with the electrolytic solutions according to the invention, which can be used at higher operating temperatures due to these electrolytic solutions. Aluminum electrolytic capacitors have electrodes that comprise aluminum foils. Porous separators are arranged between the electrodes, so that layer sequences of electrodes and separators are formed. Both the electrodes and the separator are in contact with the electrolyte solution. The layer sequences of electrodes and separators are often rolled up to form capacitor coils.

Porous polymer films, nonwovens, felts can advantageously be used as separators in all the capacitor types mentioned above or fabrics made of polymers or fiberglass or absorbent papers can be used.

The exemplary embodiments are only exemplary examples. Variations are possible both with regard to the composition of the electrolytes and with regard to the electrochemical capacitors used.

Claims

claims

1. Flame retardant electrolyte solution for electrochemical capacitors with conductivities> 20 mS / cm at 25 ° C and flash points higher than 76 ° C at 1 bar, comprising the following components:

A) At least one solvent of high polarity with a DK> 20,

B) at least one further solvent for lowering the viscosity, which is selected from carbamates of the general formula 1,

wherein

Rl and R2 are independently the same or different, a linear Cl-C6-alkyl- (Rl and R3 = C _j H _{2j +} ι with j = 1-6), a branched C3-C6-alkyl- (Rl and R3 = C _k H _{2 +1} with k ^• = 3-6) or a C3-C7 cycloalkyl group (Rl and R3 = C _j H _2j -ι with j = 3-7) or

R1 and R2 according to formula 2

are connected directly or via one or more additional N and / or O atoms to form a ring with 3 to 7 ring members, so that X has the molecular formula (CR'R ^' ") _m O _n (NR ^' '"") ₀ with 2 <

(m + n + o) <6 can be described where R "^""= C _p H _{2p + 1} with p = 0-3, and wherein one or more in the residues Rl and R2 Hydrogen atoms can be replaced by fluorine atoms: R 'and R "= C _r H _{(2r +} i) - _s F _s with r = 0-3 and s = 0- (2r + l),

R3 in both formulas is a linear Cl-C6-alkyl- (R3 = C _t H _{2 +} ι with t = 1-6), branched C3-C6-alkyl- (R3 = C _u H _{2u + 1} with u = 3- 6), C3-C7-cycloalkyl- (R3 = C _v H _2v -ι with v = 3-7) or a partially or perfluorinated straight-chain alkyl group having 3 to 7 carbon atoms, optionally one or more times with C1-C6-alkyl can be substituted is: R3 = C _w H _{(2W +} ι) - _x F _x (C _y H ₂ y ₊ ι) _z with w = 3-7 and x = 0- (2w + l) and y = 1- 6 with z = 0- (2w + l) where x + z = (2w + l).

C) At least one conductive salt that does not contain lithium.

2. electrolyte solution according to the preceding claim,

- In which the carbamate is contained in a proportion of 10 to 60 percent by weight.

3. Electrolyte solution according to the preceding claim, - in which the carbamate is contained in a proportion of 30 to 50 percent by weight.

4. Electrolyte solution according to one of the preceding claims, in which component A comprises one or more cyclic carbonates.

5. Electrolytic solution according to the preceding claim, in which ethylene or propylene carbonate are contained as component A.

6. Electrolytic solution according to one of claims 1 to 3, in which nitriles are contained as component A.

7. Electrolytic solution according to the preceding claim, in which component A is selected from the following nitriles:

- acetonitrile, 3-methoxy propionitrile,

- glutaronitrile, succinonitrile.

8. electrolyte solution according to one of claims 1 to 3,

- Containing lactones as component A.

9. electrolytic solution according to the preceding claim,

- Component A is selected from the following lactones:

- γ-butyrolactone,

- γ-valerolactone.

10. Electrolytic solution according to one of claims 1 to 5 with the feature that component A comprises propylene carbonate and component B methyl N, N-dimethyl carbamate.

11. Electrolytic solution according to the preceding claim, - in which propylene carbonate and methyl N, N-dimethyl carbamate are contained in approximately equal parts by weight.

12. Electrolyte solution according to one of claims 1 to 5 with the feature that component A comprises ethylene carbonate and component B 2,2,2-trifluoroethyl-N, N-dimethyl carbamate.

13. Electrolyte solution according to the preceding claim, in which the solvent mixture of components A and B ethylene carbonate and 2, 2, 2-trifluoroethyl-N, N-

Dimethyl carbamate in a ratio of about 2: 1 parts by weight.

14. Electrolyte solution according to one of claims 1 to 5 with the feature

- That component A comprises propylene carbonate and component B ethyl N, N-dimethyl carbamate.

15. electrolytic solution according to the preceding claim,

- in which the solvent mixture of components A and B comprises propylene carbonate and ethyl N, N-dimethyl carbamate in a ratio of approximately 1.5: 1 parts by weight.

16. Electrolytic solution according to one of the preceding claims,

- Component C is selected from combinations of quaternary ammonium, phosphonium, imidazolium, pyridinium, morpholinium and / or pyrrolidinium cations with anions, including tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate, hexafluoroantimonate, borate, bis (trifylylomide) bis (trifyl) imide , Trifluoromethyl sulfonate, tris (trifluoromethylsulfonyl) methide, tetrachloroaluminate, fluoroalkyl phosphates, fluoroalkyl arsenates, fluoroalkyl antimonates, oxalatoborate and B (OR) ₄ ^" , where R is selected from the following groups:

C1 to C6 alkyl groups, it being possible for two alkyl groups to be linked to one another so that two oxygen atoms 0 are bridged,

--OC- (Rl) _x with x = 0.1, where Rl is a Cl to C6 alkyl group, it being possible for two -0C- (R1) _X groups to be connected to one another via the carbon atoms, so that two oxygen atoms which are the Contact boron atom, be bridged.

17. Electrolytic solution according to the preceding claim with the feature

- That component C is tetraethylammonium tetrafluoroborate or methyltriethylammonium tetrafluoroborate or a mixture of both salts.

18. Electrolytic solution according to the preceding claim, in which component C is present in a concentration> 0.7 mol / 1.

19. Electrolyte solution according to claims 11 and 13 with the feature that tetraethylammonium tetrafluoroborate is present as component C in a concentration of greater than 0.7 mol / 1.

20. Electrolytic solution according to claim 15 with the feature that methyltriethylammonium tetrafluoroborate is contained in a concentration of greater than 1 mol / 1.

21. Electrochemical double-layer capacitor with electrodes consisting of activated carbon cloths or activated carbon powder and porous separators located therebetween with the feature that they comprise a flame-retardant electrolytic solution according to one of the preceding claims.

22. Electrochemical double-layer capacitor according to the preceding claim, with the features that it consists of alternating layers of electrodes, built up from metal-impregnated activated carbon cloths with current collector foils therebetween, one end of each current collector film contacting the electrically conductive areas of an activated carbon cloth, - that the other end each current collector foil is connected to the ends of the other current collector foils of electrodes of the same polarity to form a bundle, so that they form an electrode terminal to which an electrical potential can be applied - that electrodes of different polarity are separated from porous separators in an electrically insulating manner so that the alternating ones Layers of electrodes are packaged in a housing under slight pressure, so that there is a large contact area between the current collector foils and the active carbon cloths.

23. Hybrid capacitor with metal oxide electrodes or combinations of metal oxide electrodes and electrodes made of activated carbon cloths or activated carbon powder and porous separators located therebetween, comprising a flame-retardant electrolytic solution according to one of claims 1 to 20.

24. Pseudocapacitor with electrodes made of conductive polymers and / or electrodes made of activated carbon cloths or activated carbon powder and porous separators located therebetween with the feature that it comprises a flame-retardant electrolyte solution according to one of claims 1 to 20.

25. Electrochemical capacitor according to one of claims 21 to 24 with the feature that it is shaped as a cylindrical, prismatic, radial or axial component.

26. Electrochemical capacitor according to one of claims 21 to 25 with the feature

- That it includes polymer films, nonwovens, felts, woven fabrics made of polymers or fiberglass or papers as separators.

27. Aluminum electrolytic capacitor, with electrodes consisting of aluminum foils and porous separators arranged between the electrodes, which comprises a flame-retardant electrolyte solution according to one of claims 1 to 20.

28. Use of a capacitor according to one of Claims 21 to 26 for line voltages> 2.5 V.