DE102024119236A1 - β-H-free tetraalkylammonium salts of substituted trifluoroborate-containing anions and their use in an electrolyte composition in an energy storage device - Google Patents

Abstract

The present disclosure relates to β-H-free tetraalkylammonium salts of trifluoroborates substituted with electron-withdrawing groups and their use in an electrolyte composition. It relates in particular to their use in an electrolyte composition in electrochemical supercapacitors and ultracapacitors, for example in double-layer capacitors in electric motors.

Description

This disclosure relates to β-H-free tetraalkylammonium salts of trifluoroborates substituted with electron-withdrawing groups (EWGs) and their use in an electrolyte composition. In particular, this disclosure relates to their use in an electrolyte composition in electrochemical supercapacitors and ultracapacitors, for example, in double-layer capacitors in electric motors.

Technical background

Electromobility requires high-performance energy storage systems. Rapid load changes in electromobility systems lead to a drop in performance and a reduction in the lifespan of electrochemical energy storage devices. Therefore, high-performance electrical storage buffers are needed to relieve the strain on electrochemical energy storage devices in electromobility systems. New materials for more efficient energy storage are required for vehicles with electric motors. The desired novel materials should guarantee a rapid and continuous availability of drive energy. In particular, novel electrolyte compositions in capacitors with increased storage capacity and power would be desirable. This would buffer the load, especially during rapid load changes caused by charging or discharging the electrochemical batteries, and thus extend the battery lifespan.

Efficient energy storage and conversion in electric motors is possible using electrochemical supercapacitors and ultracapacitors. In electromobility, electrochemical double-layer capacitors (EDLCs) are particularly common. The charge capacity and performance of supercapacitors and ultracapacitors are significantly influenced by the electrolyte composition. Currently available electrolyte compositions for supercapacitors and ultracapacitors are predominantly based on strongly acidic or basic aqueous salt solutions. Typically, 1 M solutions of tetraethylammonium _BF₄ are used. However, these existing electrolyte compositions exhibit limited charge capacity, a restricted electrochemical window, and undesirable thermal expansion upon heating.

Furthermore, when using salts as electrolytes, the viscosity of the solution has the dominant influence on the capacitor's resistance, as the ions migrate through the solution to the electrode. The viscosity of the salt solutions depends on the concentration and molecular weight of the salts, but also on the presence of weakly polarizable substituents on the anions. This precludes the generally attractive use of ionic liquids, as these exhibit far too high a viscosity. Another important factor influencing performance is the possible electrochemical window, which necessitates a compromise between these properties when selecting the solvent.

Furthermore, currently available electrolyte compositions for supercapacitors and ultracapacitors are characterized by a relatively high solvent content. Many electrolyte salts for double-layer capacitors are also limited to the use of acetonitrile. Consequently, the operating temperature of these double-layer capacitors is limited to the boiling point of acetonitrile. Additionally, commercial electrolyte salts such as tetraethylammonium tetrafluoroborate (TE-ABF ₄ ) decompose rapidly due to a small amount of water. This reduces the performance and lifespan of the energy storage device. To date, no alternative to TEABF ₄ is known that offers comparable performance and higher stability. The development of a new electrolyte composition to overcome these drawbacks would be highly desirable.

It is therefore an objective of the present disclosure to overcome the disadvantages of the prior art. In particular, it is an objective to provide further improved electrolyte salts and a process for their preparation for use in electrolyte compositions in energy storage devices, especially in supercapacitors and ultracapacitors.

Summary of Revelation

In a first aspect, the disclosure relates to a β-H-free tetraalkylammonium salt of substituted trifluoroborates with the structural formula R ² ₄ N ⁺ R'BF ₃ ^- with R ¹ = electron-withdrawing group, optionally selected from the group comprising partially fluorinated or perfluorinated alkyl group, partially fluorinated or perfluorinated aryl group, partially chlorinated or perchlorinated alkyl group, partially chlorinated or perchlorinated aryl group, mixed fluorinated and chlorinated alkyl group, mixed fluorinated and chlorinated aryl group and R ² = β-H-free alkyl group.

It should be noted that, by definition, the electron-withdrawing group cannot be a single fluorine atom, which, while technically an electron-withdrawing group, would then no longer be a (substituted) trifluoroborate anion, but rather a tetrafluoroborate anion. The electron-withdrawing group therefore does not include -F. Mixed fluorinated and chlorinated alkyl and aryl groups include both those that bear fluorine and chlorine on a single carbon atom, such as _-CF₂Cl , and those that bear fluorine and chlorine separately on different carbon _atoms , such as _-CCl₂CF₃ .

Furthermore, “β-H-free” refers to the absence of hydrogen at the β-positions to the nitrogen of the tetraalkylammonium cations.

In various embodiments, R ² is selected from the group comprising methyl group, t-butyl group, benzyl group.

In various formulations, the anion is selected from the group comprising methyl trifluoroborate, ethyl trifluoroborate, propyl trifluoroborate, trifluoromethyl trifluoroborate, pentafluoroethyl trifluoroborate, heptafluoropropyl trifluoroborate, nonafluorobutyl trifluoroborate, undecafluoropentyl trifluoroborate, trichloromethyl trifluoroborate, pentachloroethyl trifluoroborate, heptachloropropyl trifluoroborate, nonachlorobutyl trifluoroborate, undecachlorpentyl trifluoroborate, pentafluorophenyl trifluoroborate, heptafluoronaphtyl trifluoroborate, pentachlorophenyl trifluoroborate and heptachloronaphtyl trifluoroborate.

In a second aspect, the disclosure relates to the use of a β-H-free tetraalkylammonium salt of substituted trifluoroborates according to aspect 1 in an electrolyte composition in an energy storage device, in particular a capacitor.

In a third aspect, the disclosure relates to an electrolyte composition for an energy storage device, in particular a capacitor, comprising a 0 < x ≤ 5 molar solution of a β-H-free tetraalkylammonium salt of substituted trifluoroborates according to aspect 1 in a solvent selected from the group comprising acetonitrile, propylene carbonate, diethyl ether, carbonic acid esters and γ-butyrolactone.

Detailed description of the revelation

As the inventors recognized, a combined problem exists when improving commercial electrolyte salts such as tetraethylammonium tetrafluoroborate (TEABF ₄ ). On the one hand, a degradation reaction occurs at the tetraethylammonium cation via Hofmann degradation, which is responsible for the rapid impairment of the electrolyte salt's performance and the limitation of the possible number of cycles. On the other hand, the replacement of the cation is limited because the salt, together with the tetrafluoroborate anion, is no longer sufficiently soluble in the common solvents (acetonitrile and propylene carbonate) in conceivable alternatives to achieve good capacitor capacitance. The performance gain from increased stability would thus be counteracted by a generally weaker performance. Furthermore, the tetrafluoroborate anion in TEABF ₄ also contributes to the performance degradation due to a certain susceptibility to hydrolysis when a small amount of water is present in the electrolyte solution.

In contrast to TEABF ₄ , the disclosed substituted β-H-free tetraalkylammonium trifluoroborate salts are stable in air and under hydrolysis. Furthermore, no Hofmann degradation of the β-H-free tetraalkylammonium cation occurs. In addition, the salts are readily soluble in solvents other than acetonitrile, which, besides maintaining equally good conductivity, also allows for the use of other solvents with higher boiling points and thus a wider operating temperature range. Essential to achieving these advantages is the use of a β-H-free tetraalkylammonium cation and an electron-withdrawing substituent on a trifluoroborate anion.

By using β-hydrogen-free tetraalkylammonium salts of trifluoroborates substituted with electron-withdrawing substituents, such as tetramethylammonium trifluoromethyltrifluoroborate, tetramethylammonium can be used as the cation, in contrast to the prior art. This would not be possible with _BF₄ as the anion, since tetramethylammonium tetrafluoroborate, as described above, is not sufficiently soluble in acetonitrile and propylene carbonate. In common acetonitrile, tetramethylammonium trifluoromethyltrifluoroborate is soluble to at least 1 molar. Using tetramethylammonium instead of tetraethylammonium prevents Hoffmann degradation at the cation, which is the only degradation mechanism with respect to the cation. These properties, illustrated using the methyl group as an example, apply generally to β-hydrogen-free tetraalkyl groups, especially to the t-butyl and benzyl groups. Furthermore, trifluoromethyltrifluoroborate is significantly more hydrolytically stable than tetrafluoroborate. This also minimizes the degradation mechanism on the anion side compared to the state of the art.

The same advantages as for the trifluoromethyl substituent as an electron-withdrawing substituent on the trifluoroborate anion are also achieved by other electron-withdrawing substituents, thus enabling optimization with regard to the desired solvent, for example. In various embodiments, the electron-withdrawing substituent can therefore be a partially fluorinated or perfluorinated alkyl group.

In other embodiments, the electron-withdrawing substituent can be a partially fluorinated or perfluorinated aryl group.

In other embodiments, the electron-withdrawing substituent can be a partially chlorinated or perchlorinated alkyl group.

In other embodiments, the electron-withdrawing substituent can be a partially chlorinated or perchlorinated aryl group.

In other embodiments, the electron-withdrawing substituent can be a mixed fluorinated and chlorinated alkyl group.

In other embodiments, the electron-withdrawing substituent can be a mixed fluorinated and chlorinated aryl group.

In various embodiments, the electron-withdrawing substituents can be a perfluorinated alkyl group with a length of C ₁ -C ₅ or a perfluorinated aryl group.

In various embodiments, the electron-withdrawing substituents can be a partially fluorinated alkyl group with a length of C ₁ - C ₅ or a partially fluorinated aryl group.

In various embodiments, the electron-withdrawing substituents can be a mixed fluorinated and chlorinated alkyl group with a length of C ₁ - C ₅ or a mixed fluorinated and chlorinated aryl group.

The disclosed β-H-free tetraalkylammonium salts with substituted trifluoroborates can be used in an electrolyte composition for an energy storage device. In particular, the energy storage device can be a capacitor. In various embodiments, the capacitor is an electrochemical supercapacitor or ultracapacitor, especially an electrochemical double-layer capacitor (EDLC).

An electrolyte composition as disclosed may comprise a 0 < x ≤ 5 molar solution of a disclosed β-H-free tetraalkylammonium salt of substituted trifluoroborates. In embodiments, the solution may be a 0 < x ≤ 5 molar solution, a 0 < x ≤ 4 molar solution, a 0 < x ≤ 3 molar solution, a 0 < x ≤ 2 molar solution, or a 0 < x ≤ 1 molar solution. In various configurations, the solution can be a 0.5 ≤ x ≤ 5 molar solution, a 1.0 ≤ x ≤ 5 molar solution, a 1.5 ≤ x ≤ 5 molar solution, a 2.0 ≤ x ≤ 5 molar solution, a 2.5 ≤ x ≤ 5 molar solution, or a 3.0 ≤ x ≤ 5 molar solution. In further configurations, the solution can be a 0.5 ≤ x ≤ 4 molar solution, a 0.5 ≤ x ≤ 3 molar solution, a 0.5 ≤ x ≤ 2 molar solution, or a 0.5 ≤ x ≤ 1.5 molar solution.

In various embodiments, the solvent can be selected from the group comprising acetonitrile, propylene carbonate, diethyl ether, carbonic acid esters and γ-butyrolactone.

In various embodiments, the electrolyte composition can consist of a β-H-free tetraalkylammonium salt of substituted trifluoroborates dissolved in acetonitrile, as disclosed, in a concentration of 0 < x ≤ 5 molar solution, 0 < x ≤ 4 molar solution, 0 < x ≤ 3 molar solution, 0 < x ≤ 2 molar solution, 0 < x ≤ 1 molar solution, 0.5 ≤ x ≤ 5 molar solution, 1.0 ≤ x ≤ 5 molar solution, 1.5 ≤ x ≤ 5 molar solution, 2.0 ≤ x ≤ 5 molar solution, 2.5 ≤ x ≤ 5 molar solution, or 3.0 ≤ x ≤ 5 molar solution.

In embodiments, the electrolyte composition can be a β-H-free tetraalkylammonium salt of substituted trifluoroborates dissolved in acetonitrile in a concentration of a 0.5 ≤ x ≤ 5 molar solution, a 0.5 ≤ x ≤ 4 molar solution, a 0.5 ≤ x ≤ 3 molar solution, a 0.5 ≤ x ≤ 2 molar solution or a 0.5 ≤ x ≤ 1.5 molar solution.

In various embodiments, the electrolyte composition can consist of a β-H-free tetraalkylammonium salt of substituted trifluoroborates dissolved in propylene carbonate at a concentration of 0 < x ≤ 5 molar solution, 0 < x ≤ 4 molar solution, 0 < x ≤ 3 molar solution, 0 < x ≤ 2 molar solution, 0 < x ≤ 1 molar solution, 0.5 ≤ x ≤ 5 molar solution, 1.0 ≤ x ≤ 5 molar solution, 1.5 ≤ x ≤ 5 molar solution, 2.0 ≤ x ≤ 5 molar solution, 2.5 ≤ x ≤ 5 molar solution, or 3.0 ≤ x ≤ 5 molar solution.

In embodiments, the electrolyte composition can be a β-H-free tetraalkylammonium salt of substituted trifluoroborates dissolved in propylene carbonate in a concentration of a 0.5 ≤ x ≤ 5 molar solution, a 0.5 ≤ x ≤ 4 molar solution, a 0.5 ≤ x ≤ 3 molar solution, a 0.5 ≤ x ≤ 2 molar solution or a 0.5 ≤ x ≤ 1.5 molar solution.

In various embodiments, the electrolyte composition can consist of a β-H-free tetraalkylammonium salt of substituted trifluoroborates dissolved in diethyl ether, carbonic acid ester, or γ-butyrolactone, as disclosed, in a concentration of 0 < x ≤ 5 molar solution, 0 < x ≤ 4 molar solution, 0 < x ≤ 3 molar solution, 0 < x ≤ 2 molar solution, 0 < x ≤ 1 molar solution, 0.5 ≤ x ≤ 5 molar solution, 1.0 ≤ x ≤ 5 molar solution, 1.5 ≤ x ≤ 5 molar solution, 2.0 ≤ x ≤ 5 molar solution, or 2.5 ≤ x ≤ 5 molar solution. solution or a 3.0 ≤ x ≤ 5 molar solution.

In embodiments, the electrolyte composition can be a tetramethylammonium salt of substituted trifluoroborates dissolved in diethyl ether, carbonic acid ester or γ-butyrolactone, as disclosed, in a concentration of a 0.5 ≤ x ≤ 5 molar solution, a 0.5 ≤ x ≤ 4 molar solution, a 0.5 ≤ x ≤ 3 molar solution, a 0.5 ≤ x ≤ 2 molar solution or a 0.5 ≤ x ≤ 1.5 molar solution.

Synthesis of tetramethylammonium salts with substituted trifluoroborates (Me ₄ N ⁺ R ¹ BF ₃ ^- )

The R ² ₄ N ⁺ R ¹ BF ₃ ^products as disclosed can be produced according to the following general rule.

K ^{+ R₁BF₃⁻} reacts with ^R₁ = EWG and ^{R₂ = 4N +} _Cl⁻ ^{in an} aprotic solvent or an alcohol to form ^R₂ _{= 4N} ^{+ R₁BF₃⁻} , as shown below for ^R₂ = Me. The same applies to ^R₂ = t-butyl ^or _{benzyl .}

The reaction can be carried out at a temperature in the range of 20 °C to 125 °C, preferably in the range of 20 °C to 75 °C, particularly preferably in the range of 20 °C to 45 °C.

Brief description of the character

• 1 shows the conductivity of tetramethylammonium trifluoromethyl trifluoroborate and tetraethylammonium tetrafluoroborate in comparison (each 1 M dissolved in acetonitrile) as a function of temperature.

Examples

The present invention is illustrated by the following examples, although the invention is not limited to these. For the sake of simplicity, tetramethylammonium cations were chosen for the examples. These apply analogously to other β-H-free tetraalkylammonium cations, such as tetra-t-butylammonium cations or tetra-benzylammonium cations.

Unless otherwise stated, for the purposes of this disclosure, ‘room temperature’ means a temperature in the range of 20 °C - 25 °C, in particular 22 °C.

1. Synthesis of tetramethylammonium salts with substituted trifluoroborates

1.1 Example 1 - Tetramethylammonium trifluoromethyl trifluoroborate (Me ₄ N ⁺ F ₃ CBF ₃ ^- )

K ⁺ F ₃ CBF ₃ ^- and Me ₄ N ⁺ Cl ^- were dissolved in acetonitrile as an aprotic solvent. The solution was then filtered and the filtrate concentrated. The resulting solid was again dissolved in acetonitrile as an aprotic solvent. The solution was then again filtered and the filtrate concentrated.

1.2 Example 2 - Tetramethylammonium pentafluoroethyltrifluoroborate (Me ₄ N ⁺ F ₅ C ₂ BF ₃ ^- )

K ⁺ F ₅ C ₂ BF ₃ ^- and Me ₄ N ⁺ Cl ^- were dissolved in acetonitrile as an aprotic solvent. The solution was then filtered and the filtrate concentrated. The resulting solid was again dissolved in acetonitrile as an aprotic solvent. The solution was then again filtered and the filtrate concentrated.

1.3 Example 3 - Tetramethylammonium trichloromethyl trifluoroborate (Me ₄ N ⁺ Cl ₃ CBF ₃ ^- )

K₂ ⁺ _{Cl₃CBF₃⁻} and _Me₄N ^{+ Cl⁻} were dissolved in _acetonitrile as an aprotic solvent. The solution was then filtered and the filtrate ^concentrated . The resulting solid was dissolved again in acetonitrile as an aprotic solvent. The solution was then filtered again and the filtrate concentrated.

1.4 Example 4 - Tetramethylammonium pentafluorophenyltrifluoroborate (Me ₄ N ⁺ F ₅ C ₆ BF ₃ ^- )

K ⁺ F ₅ C ₆ BF ₃ ^- and Me ₄ N ⁺ Cl ^{- -} were dissolved in acetonitrile as an aprotic solvent. The solution was then filtered and the filtrate concentrated. The resulting solid was again dissolved in acetonitrile as an aprotic solvent. The solution was then again filtered and the filtrate concentrated.

1.5 Example 5 - Tetramethylammoniumheptachloronaphtyltrifluoroborate (Me ₄ N ⁺ Cl ₇ C ₁₀ BF ₃ ^- )

K₂ ⁺ _{Cl₇C₁₀BF₃⁻} and _{Me₄N +} ^Cl⁻ were dissolved in acetonitrile as an aprotic solvent. The solution was then filtered and the filtrate ^concentrated . The resulting solid was dissolved ^again in acetonitrile as an _aprotic solvent. The solution was then filtered again and the filtrate concentrated.

2. Electrolyte composition for a capacitor

As an example of a suitable electrolyte composition _, a 1 M solution of tetramethylammonium trifluoromethyl ^{trifluoroborate} ( _Me₄N ⁺ _{F₃CBF₃⁻} ) from Example 1 in acetonitrile (Sigma Aldrich; 99.8%) was prepared. This was compared in an electrochemical characterization with a 1 M solution of the commercially used _TEABF₄ in acetonitrile.

For this purpose, a comparative measurement of the conductivity as a function of temperature was carried out. This was performed in a closed measuring cell “TSC 1600 Closed” from rhd Instruments with a sample volume of 1 ml.

The result of this comparative measurement is in 1 plotted. As can be seen, the characteristic curve of Me ₄ N ⁺ F ₃ CBF ₃ ^lies above that of the reference salt TEABF ₄ from the prior art, which proves that the conductivity is better than that of TEABF ₄ in the relevant temperature range (10 - 55 °C).

Furthermore ^, higher capacity-enhancing concentrations can be achieved with Me ₄ N ⁺ F ₃ CBF _3. For this purpose, solutions were prepared in acetonitrile in a test series with concentrations of 0.5 M, 1 M, 1.5 M, 2 M and 5 M.

Furthermore, alternative solvents were tested. For this purpose, 1 M solutions of Me ₄ N ^{+ -} F ₃ CBF ₃ were prepared in propylene carbonate, diethyl ether, carbonic acid ester and γ-butyrolactone.

Claims (5)

β-H-free tetraalkylammonium salt of substituted trifluoroborates with the structural formula R ² ₄ N ⁺ R ¹ BF ₃ ^- with R ¹ = electron-withdrawing group, optionally selected from the group comprising partially fluorinated or perfluorinated alkyl group, partially fluorinated or perfluorinated aryl group, partially chlorinated or perchlorinated alkyl group, partially chlorinated or perchlorinated aryl group, mixed fluorinated and chlorinated alkyl group, mixed fluorinated and chlorinated aryl group and R ² = β-H-free alkyl group. β-H-free tetraalkylammonium salt of substituted trifluoroborates according to Claim 1 , where R ² is selected from the group comprising methyl group, t-butyl group, benzyl group. β-H-free tetraalkylammonium salt of substituted trifluoroborates according to Claim 1 or 2 , wherein the anion is selected from the group comprising methyl trifluoroborate, ethyl trifluoroborate, propyl trifluoroborate, trifluoromethyl trifluoroborate, pentafluoroethyl trifluoroborate, heptafluoropropyl trifluoroborate, nonafluorobutyl trifluoroborate, undecafluoropentyl trifluoroborate, trichloromethyl trifluoroborate, pentachloroethyl trifluoroborate, heptachloropropyl trifluoroborate, nonachlorobutyl trifluoroborate, undecachloropentyl trifluoroborate, pentafluorophenyl trifluoroborate, heptafluoronaphtyl trifluoroborate, pentachlorophenyl trifluoroborate and heptachloronaphtyl trifluoroborate. Use of a β-H-free tetraalkylammonium salt of substituted trifluoroborates according to one of the preceding claims in an electrolyte composition in an energy storage device, in particular a capacitor. Electrolyte composition for an energy storage device, in particular a capacitor, comprising a 0 < x ≤ 5 molar solution of a β-H-free tetraalkylammonium salt of substituted trifluoroborates according to one of the Claims 1 until 3 in a solvent selected from the group comprising acetonitrile, propylene carbonate, diethyl ether, carbonic acid esters and γ-butyrolactone.