DE102022116245A1 - Electrolyte solution for high performance rechargeable batteries and capacitors - Google Patents

Abstract

The invention relates to batteries and capacitors and their production. What is described is the use of an electrolyte solution for batteries or capacitors, comprising a zinc complex, containing zinc, the ligand betaine and an anion selected from NTf2, OTf, CI and Br, and • the solvent acetonitrile, whereby the battery and the capacitor contain a graphite electrode. The production of such batteries or capacitors using this electrolyte solution is also described.

Description

The invention relates to batteries and capacitors and their manufacture.

State of the art

Zinc batteries combine various advantageous properties, such as high specific capacity, low electrochemical potential, low cost, environmental friendliness and good compatibility with aqueous and organic electrolytes. However, disadvantages include inadequate stability over high cycle numbers, low coulombic efficiency (ratio between charging and discharging capacity) and low battery voltage. This is caused, among other things, by dendrite growth.

Conventional alkaline zinc batteries with KOH electrolyte suffer from heavy dendritic growth on the electrode as well as the formation of solid ZnO or [Zn(OH) ₄ ] ²⁺ . Over time, all of this leads to a significant reduction in battery capacity and low Coulombic efficiency.

Organic solvents based on organic phosphates or mixtures of propylene carbonate with triethyl phosphate etc. were also tested. However, such solutions also lead to dendritic growth on the electrode.

Chen et al. (2021) describe electrolytes with a complex of ZnO and [Hbet][NTf ₂ ] for zinc-graphite batteries. EMC (ethyl methyl carbonate) was used as a solvent in the electrolyte.

Chen et al. (2022) disclose the use of a Zn-betaine complex in different solvents, such as acetonitrile, in zinc-air batteries.

Dong et al. (2021) describe the salt Zn(OTf) ₂ in aqueous electrolyte solutions with the addition of DMC (dimethyl carbonate).

All known batteries suffer from dendritic growth on an electrode, which significantly reduces long-term usability.

Task of the invention

The task is to overcome the disadvantages of the state of the art. A battery should be provided that has good performance characteristics. Dendritic growth on the electrode should be prevented as much as possible. The performance of the battery should be maintained over the highest possible number of charge/discharge cycles. In particular, it should still be >95% after 1000 cycles and >60% after 10,000 cycles.

The charge/discharge should take place as quickly as possible so that the structure can also be used as a capacitor.

Solution to the task

According to the invention the object is achieved by the features according to the independent claims. Advantageous embodiments of the invention are specified in the dependent claims.

• • Zink-Komplex, enthalten Zink, den Liganden Betain (abgekürzt durch bet) und ein Anion, ausgewählt aus NTf₂, OTf, CI und Br, und
• • das Lösungsmittel Acetonitril,

wobei die Batterie und der Kondensator eine Graphit-Elektrode enthält.The subject of the invention is the use of an electrolyte solution for batteries or capacitors, comprising a

• • Zinc complex, containing zinc, the ligand betaine (abbreviated to bet) and an anion selected from NTf ₂ , OTf, CI and Br, and
• • the solvent acetonitrile,

wherein the battery and capacitor contains a graphite electrode.

NTf ₂ is bis(trifluoromethyl(sulfonyl)imide and is known to carry a negative charge OTf is the anion of trifluoromethanesulfonic acid. [Hbet] ⁺ is the betainium cation with the formula Me ₃ N( ⁺ )-CH ₂ -COOH. When producing the zinc complex from ZnO, the proton of this COOH group reacts with the ZnO to form water, which is useful to evaporate, preferably at 175°C ± 15°C.

Surprisingly, the selection according to the invention showed a particularly good suitability for batteries in which an electrode comprises graphite. Due to the fast charging/discharging times, capacitors with graphite electrodes have surprisingly also been suitable.

The advantage of selecting the anions is that they do not attach directly to the zinc central atom of the zinc complex, but rather to the betaine. This results in a “linear coordination” on the zinc cation. The betaine is a monodentate ligand. It is also possible to use a mixture of the zinc complexes with various such anions.

• Ein oktaedrischer Komplex aus Zn als Zentralatom mit vier Acetonitril-Liganden und zwei Betain-Liganden, sowie ein tetraedrischer Komplex mit Zink als Zentralatom und zwei Acetonitril-Liganden sowie zwei Betain-Liganden.

According to quantum chemical calculations, acetonitrile is itself part of the zinc complex, but is mentioned separately here as a solvent. The acetonitrile most likely also attaches itself as a ligand to the zinc central atom of the zinc complex. According to the invention, various zinc complexes are formed, the two main complexes being the following:

• An octahedral complex with Zn as the central atom with four acetonitrile ligands and two betaine ligands, as well as a tetrahedral complex with zinc as the central atom and two acetonitrile ligands and two betaine ligands.

1. a) Bereitstellen einer Graphit-Elektrode in einem Gehäuse
2. b) Einbringen einer Elektrolyt-Lösung (insbesondere der erfindungsgemäß verwendeten Elektrolyt-Lösung), umfassend einen
   • • Zink-Komplex, enthaltend Zink, Betain und ein Anion, ausgewählt aus NTf₂, OTf, CI und Br, und
   • • das Lösungsmittel Acetonitril,

in das Gehäuse.The invention furthermore relates to a method for producing a battery or a capacitor, with the steps:

1. a) Providing a graphite electrode in a housing
2. b) introducing an electrolyte solution (in particular the electrolyte solution used according to the invention), comprising a
   • • Zinc complex containing zinc, betaine and an anion selected from NTf ₂ , OTf, CI and Br, and
   • • the solvent acetonitrile,

into the housing.

Statements regarding the process therefore also apply in mirror image to the use of the electrolyte solution and vice versa.

Advantages

Advantageously, with the combination according to the invention in the electrolyte solution of zinc-betaine complex with the corresponding anions in acetonitrile in the metal-graphite battery (such as zinc-graphite batteries as button cells) a voltage of 2.6 V can be achieved an average discharge voltage of 2.4 V can be achieved. The charge retention at 3 A/g (150C) is 97% after 1000 cycles and 68% after 1000 cycles. Overall, the battery or capacitor that can be produced with this electrolyte solution has excellent characteristics, as shown in Table 1 at the end of exemplary embodiment 1.

It has surprisingly been shown that this complex with acetonitrile as a solvent for a battery or capacitor with a graphite electrode offers these particular advantages.

The invention is suitable for applications where the charging/discharging speed is important (such as in capacitors), because this is very fast.

The discharge voltage is stable and the electrolyte solution can be produced cost-effectively due to the availability of the ingredients. The electrolyte solution is advantageously difficult or non-flammable, depending on the concentration of the zinc complex.

Preferred Embodiments

In a preferred embodiment of the invention, the second electrode (of the battery or capacitor) is a zinc electrode, so that the battery is a zinc-graphite battery and the capacitor is a zinc-graphite capacitor. Accordingly, a second electrode is preferred in the process in step a). provided, which is a zinc electrode, so that the battery is a zinc-graphite battery and the capacitor is a zinc-graphite capacitor.

In a further preferred embodiment of the invention, the electrolyte solution also comprises a metal salt selected from LiPF ₆ , MnSO ₄ , Na ₂ SO ₄ and Zn[NTf ₂ ] ₂ .

In one embodiment of the invention, the electrolyte solution comprises as a solvent a mixture of acetonitrile with a further solvent selected from acetone, sulfolanes, methanol, dimethylformamide and propylene carbonate, or mixtures thereof.

In a preferred embodiment of the invention, the concentration of the zinc complex in the electrolyte solution is 0.3-1 mol/L in the solvent. It is particularly preferred 0.5 mol/L ±0.1 mol/L. In particular, the solvent can also only comprise acetonitrile. In this embodiment, the characteristics of the battery or capacitor are particularly good.

In a likewise preferred embodiment of the invention, the zinc complex contains zinc, the ligand betaine and the anion NTf ₂ . This has proven to be particularly beneficial for graphite electrodes.

In a preferred embodiment of the production method according to the invention, in step b) before introducing (the electrolyte solution) into the housing, the zinc complex is produced from zinc oxide ZnO and [Hbet][NTf ₂ ] and/or [Hbet][OTf] and/or [Hbet]CI and/or [Hbet]Br. It makes sense to remove any water that forms, preferably at a temperature of 175°C ±20°C.

In a very preferred variant of this embodiment with production from ZnO etc., in step b) before introduction into the housing, the zinc complex is produced from zinc oxide ZnO, betaine and an acid selected from HNTf ₂ , HOTf, HCl and HBr, includes.

In particular, it is preferred if the ratio of ZnO to acid is 1:1.5 to 1:2.5, in particular 1:2.

• 1 zeigt eine Ausführungsform (Ausführungsbeispiel 1) der Erfindung anhand einer Batterie-Knopfzelle (Zink-Graphit) mittels SEM (Rasterelektronenmikroskopie). Die Bilder a) bis c) zeigen die glatte Oberfläche der Zn-Elektrode nach Zyklisierung bei 1,0 mA/cm² nach 1235 h Zyklen, in verschiedenen Vergrößerungsstufen. Die Oberfläche erscheint noch immer glatt ohne dendritische Formen.
• 2 dagegen zeigt das Vergleichsbeispiel 2, analog wie Ausführungsbeispiel 1, allerdings ohne Betain-Ligand am Zink-Komplex. Die Teile d) bis f) zeigen die gleichen verschiedenen Vergrößerungsstufen, wie 1. Die Unebenheiten an der Zink-Elektrode sind gut zu erkennen.
• 3 zeigt die elektrochemischen Eigenschaften der Zink-Graphit-Batterie mit dem entsprechenden Elektrolyten aus Ausführungsbeispiel 1 zwischen 0,25 V und 2,6 V mit einem PVDF-Binder bei 3,0 A/g.
• 4 zeigt die Überlegenheit der erfindungsgemäßen Kombination aus dem Zink-Komplex mit Betain, den Anionen, dem Lösungsmittel Acetonitril bei einer Zinkbatterie mit Graphit als Material für die zweite Elektrode. Gezeigt werden die Ladungshaltefähigkeiten (capacity retention) in % abhängig von der Zyklenzahl. Graphit als Elektrodenmaterial zeigt im Zusammenhang mit der erfindungsgemäßen Elektrolyt-Lösung kaum übersehbare Vorteile.
• 5 und 6 zeigen die elektrochemischen Eigenschaften der Zink-Graphit-Batterien aus Ausführungsbeispiel 4 bei 3 A/g, bei dem andere Binder für die Graphit-Elektrode eingesetzt wurden.

To implement the invention, it is also expedient to combine the above-described inventive configurations, embodiments and features of the claims with one another.

• 1 shows an embodiment (embodiment 1) of the invention using a battery button cell (zinc graphite) using SEM (scanning electron microscopy). Images a) to c) show the smooth surface of the Zn electrode after cycling at 1.0 mA/cm ² after 1235 h cycles, in different magnification levels. The surface still appears smooth without dendritic shapes.
• 2 In contrast, comparative example 2 shows analogous to exemplary embodiment 1, but without betaine ligand on the zinc complex. Parts d) to f) show the same different magnification levels as 1 . The unevenness on the zinc electrode can be clearly seen.
• 3 shows the electrochemical properties of the zinc-graphite battery with the corresponding electrolyte from embodiment 1 between 0.25 V and 2.6 V with a PVDF binder at 3.0 A / g.
• 4 shows the superiority of the combination according to the invention of the zinc complex with betaine, the anions, the solvent acetonitrile in a zinc battery with graphite as the material for the second electrode. The charge retention capabilities (capacity retention) are shown in % depending on the number of cycles. Graphite as an electrode material shows advantages that can hardly be overlooked in connection with the electrolyte solution according to the invention.
• 5 and 6 show the electrochemical properties of the zinc-graphite batteries from exemplary embodiment 4 at 3 A/g, in which other binders were used for the graphite electrode.

Table 1 shows the characteristics of the Zn-graphite battery produced according to embodiment 1.

Examples of embodiments

The invention will be explained in more detail below using an exemplary embodiment, without being limited thereto.

Example 1:

In exemplary embodiment 1, a button cell was produced, ie a graphite electrode with a zinc electrode was provided and an electrolyte concentration of 0.5 mol/l of the zinc complex of zinc, betaine and the anion NTf ₂ in acetonitrile was set. The voltage achieved was 2.6 V with an average discharge voltage of 2.4 V. The charge retention at 3 A/g (150C) is 97% after 1000 cycles and 68% after 1000 cycles. The charge/discharge time was approximately 24 seconds at 3 A/g with an energy density of 46 Wh/kg at a power density of 4485 W/kg, based on the cathode.

The zinc electrode remains flat and no dendritic growth is observed - after 1235 h. This is shown in 1 . 3 shows the electrochemical properties. 4 shows, among other things, the charge retention compared to the charge/discharge cycles for graphite as an electrode material. Accordingly, the charge retention was 97% after 1000 cycles and 68% after 10,000 cycles. The charge/discharge time was less than 24 seconds at 3 A/g with an energy density of 46 Wh/kg at a power density of 4485 W/kg (cathode based).

The electrolyte was prepared by dissolving ZnO in [Hbet][NTf ₂ ] in a ratio (in this order) of 1:2.

A PVDF (polyvinylidene fluoride) binder was used for the graphite electrode.

log(i_p) = log(a) + b · log(v) , wobei i_p der Peak-Strom des Oxidationspeaks in der CV-Kurve ist; v ist die Scanrate des CV-tests und a und b sind die einstellbaren Parameter. Bei einem b-Wert von nahe 1,0 ist die Redoxreaktion hauptsächlich durch die Kapazität bestimmt. Bei einem Wert nahe 0,5 wäre der Prozess vorrangig durch die Faradaysche lonen-Einlagerung bestimmt. Der b-Wert in diesem Ausführungsbeispiel betrug 0,69, so dass man von einer ultraschnellen Zink-Graphit-Batterie sprechen kann. Die Erfindung eignet sich für Anwendungen, bei denen es auf die Lade-/Entladegeschwindigkeit ankommt.The time for loading and unloading was also examined. The capacitive effect in battery chemistry follows the following two equations: $$i_p=a\cdot v^b$$

log(i _p ) = log(a) + b · log(v) , where i _p is the peak current of the oxidation peak in the CV curve; v is the scan rate of the CV test and a and b are the adjustable parameters. At a b value close to 1.0, the redox reaction is mainly determined by capacity. At a value close to 0.5, the process would be primarily determined by Faraday ion incorporation. The b value in this exemplary embodiment was 0.69, so that one can speak of an ultra-fast zinc-graphite battery. The invention is suitable for applications where charging/discharging speed is important.

In summary, the battery from exemplary embodiment 1 provides the characteristics summarized in Table 1. Table 1: Characteristic values of the Zn-graphite battery from exemplary embodiment 1: Stable discharge voltage 2.4 - 2.6 V (vs. Zn/Zn ²⁺ ) Reversible loading capacity 43mAh/g @ 0.1A/g (2C) 20mAh/g @ 3.0A/g (150C) Cargo holding 97% @ 103 cycles 68% @ 104 cycles Coulombic efficiency 93% @ 103 cycles 73% @ 104 cycles Charge/discharge time 24 sec. @ 3.0 A/g Energy density 118 Wh/kg @ 0.1 A/g 46 Wh/kg @ 3.0 A/g Power density (cathode) 157 W/kg @ 0.1 A/g 4485 W/kg @ 3.0 A/g Cost (prototype) $0.30 (including $0.13 battery case) Capacity per button cell 1-2mAh

Comparative example 2 without betaine:

For this purpose, a betaine-free electrolyte was tested: 0.5 mol/L Zn[NTf ₂ ] ₂ in acetonitrile. Here the zinc electrode was significantly rougher after 1235 h cycles and showed a more inhomogeneous morphology. This was also confirmed using SEM.

Comparative example 3 for different electrode materials:

Comparative example 3 shows the surprising suitability of the electrolyte solution according to the invention with betaine and acetonitrile for batteries with a graphite electrode. The measurement was carried out at 3 A/g (150C).

The electrolyte solution, as in exemplary embodiment 1, was used in a zinc battery, with the material of the second electrode being varied. Graphite showed noticeably better charge retention.

Example 4 for different graphite electrodes:

Example 4 was carried out analogously to Example 1. Other binders were only used for the graphite electrode, namely carboxymethyl cellulose or the alginic acid sodium salt. The electrochemical properties are in 5 (for carboxymethylcellulose) and 6 (for alginic acid sodium salt).

For the synthesis of the zinc complex, containing zinc, the ligand betaine and the anion NTf ₂ ^-

This complex is described by the formula [Zn(bet) ₂ ][NTf ₂ ] ₂ .

0.1 mol of [Hbet]Cl and a solution corresponding to 0.1 mol of Li[NTf ₂ ] were dissolved in 250 mL of deionized water and stirred for 1 h. The precipitated [Hbet][NTf ₂ ] was then separated using a dispensing funnel and washed five times with ice water until detection with silver nitrate solution indicated the absence of Cl- ions. Subsequently, [Hbet][NTf ² ] was dried in a dynamic vacuum at 130 °C overnight using a Schlenk line. The synthesis of the complex (abbreviated ZbN) was carried out by stirring ZnO and [Hbet][NTf ₂ ] (abbreviated IL, for ionic liquid) in the molar ratio n(ZnO): n(IL) = 1: 2 and heating 175 °C in an air-permeable flask for 12 hours and then drying in a dynamic vacuum at 175 °C for 12 hours using a Schlenk apparatus. The solid was then quickly poured onto weighing paper at 175 °C and cooled to room temperature for use.

The solubility of the complex (abbreviated as ZbN) in acetonitrile (abbreviated as AN), acetone, dimethylformamide, methanol, propylene carbonate, sulfolane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, _H2O , ethanol, n-butanol and ethylene glycol glycol was determined at a concentration of 0.5 mol/L tested. In addition, the ZbN/AN mixture according to the invention were prepared by dissolving ZbN in AN and stirring for 5 hours to obtain solutions with concentrations of 1.0, 2.0, 3.0, 4.0, 4.5 and 5.0 to obtain moVL.

Electrochemical measurements

Linear sweep voltammetry (LSV) and cyclic voltammogram (CV), operational optical characterization, open-circuit voltage (OCV) and electrochemical impedance spectrum (EIS) of batteries were performed on a Biologic SAS VMP-3 model operated by the EC- LAB electrochemistry software controlled. The EIS was performed in the frequency range from 100 mHz to 1 MHz at a noise voltage of 10 mV. All batteries were allowed to rest for 12 hours prior to cycling stability testing. The cycling stability of the batteries was tested using the NEWARE BTS4000-5 V 10 mA Battery Testing System (XIAMEN AOT ELECTRONICS TECHNOLOGY CO., LTD) at room temperature. All samples were washed with AN and ethanol, then air dried and stored in an argon glovebox before further characterization. For cyclic zinc electrodes, in addition to the cleaning step mentioned above, ultrasound was performed in ethanol for 10-15 minutes to remove as much of GF as possible. The electrochemical stability window of the ZbN/AN electrolyte was estimated using LSV over the first cycle. Zinc foil, punched and pressed into disks with a diameter of 12 mm, was placed in the glove box in CR2032 ZnllSS button cell batteries (Manual Coin Cell Crimper AOT-HCM-20, encapsulation pressure approx. 50 kg/cm ² ) with GF separator in a potential window of 1 V to approx. 5.5 V (vs. Zn/Zn2+) with a sweep rate of 2 mV-s-1. Zn||SS and balanced Zn||Zn button cell batteries with ZbN/AN electrolyte and medium Celgard 2400 and double-sided GF separator were also assembled to test CE and cycling stability. The Zn||SS cells were charged at 0.2 mA/cm ² for the discharging process and a cut-off potential of 0.5 V or 1.0 V at 0.2 mA/cm ² for the charging process.

For the production of graphite cathodes for zinc-graphite batteries:

Graphite powder was mixed with PVDF binder in a weight ratio of 180 mg:20 mg using N-methyl-2-pyrrolidone (1-2 ml) as a solvent. The resulting slurry was then poured onto a stainless steel current collector sheet (12 mm diameter). The graphite electrodes were first placed on a hot table at 80 °C for 3 hours and then in an oven at 90 °C overnight. The graphite loading of the electrodes was in a range between 1.12 and 7.88 mg/cm ² . Similar to the method described above, carboxymethyl cellulose and alginic acid sodium salt were used as binders. Salt binders, both with deionized water as solvent, two separate graphite cathodes were prepared with a graphite charge of 1.19-2.47 mg/cm ² and 1.27-2.15 mg/cm ² respectively. The prepared graphite electrodes and zinc disks were assembled into button cells using a double-layer GF separator and optimal 0.5 M ZbN/AN to ensure their cycle and rate performance between 0.25-2.6 V and 0.25-2.65, respectively V to test. The specific capacity and current density refer to active graphite. Zn||graphite batteries were initially activated at 0.1 A/g ¹ for three cycles before cycling with other currents. To test the first fully charged/discharged graphite electrodes, cycling was carried out directly at 0.05 A/g.

Non-patent literature cited:

• Chen, P.; Richter, J.; Wang, G.; Li, D.; Pietsch, T.; Ruck, M. Ionometallurgical Step-Electrodeposition of Zinc and Lead and its Application in a Cycling-Stable High-Voltage Zinc-Graphite Battery, Small 2021, 17, 2102058.
• Dong, Y.; Miao, L.; Like.; Di, S.; Wang, Y.; Wang, L.; Xu, J.; Zhang, N. Non-concentrated aqueous electrolytes with organic solvent additives for stable zinc batteries, Chem. Sci. 2021, doi: 10.1039/d0sc06734b.
• Chen, P.; Wang, X.; Li, D.; Pietsch, T.; Ruck, M. A Kinetically superior rechargeable zinc-air battery derived from efficient electroseparation of zinc, lead and copper in concentrated solutions, ChemSusChem 2022, e202200039.

Claims (12)

Use of an electrolyte solution for batteries or capacitors, comprising a • zinc complex, containing zinc, the ligand betaine and an anion selected from NTf ₂ , OTf, CI and Br, and • the solvent acetonitrile, the battery and the capacitor contain a graphite electrode. Use after Claim 1 , wherein the second electrode is a zinc electrode, so that the battery is a zinc-graphite battery and the capacitor is a zinc-graphite capacitor. Use after one of the Claims 1 or 2 , wherein the electrolyte solution also comprises a metal salt selected from LiPF ₆ , MnSO ₄ , Na ₂ SO ₄ , Zn[NTf ₂ ] ₂ . Use after one of the Claims 1 until 3 , wherein the electrolyte solution comprises as a solvent a mixture of acetonitrile with a further solvent selected from acetone, sulfolanes, methanol, dimethylformamide and propylene carbonate. Use after one of the Claims 1 until 4 , where the concentration of the zinc complex in the electrolyte solution is 0.3-1 mol/L. Use after one of the Claims 1 until 5 , where the zinc complex contains zinc, the ligand betaine and the anion NTf ₂ . Method for producing a battery or a capacitor, comprising the steps: a) Providing a graphite electrode in a housing b) introducing an electrolyte solution comprising a • zinc complex containing zinc, betaine and an anion selected from NTf ₂ , OTf, CI and Br, and • the solvent acetonitrile into the housing. Procedure according to Claim 7 , wherein in step a) a second electrode is provided, which is a zinc electrode, so that the battery is a zinc-graphite battery and the capacitor is a zinc-graphite capacitor. Procedure according to one of the Claims 7 or 8th , where the concentration of the zinc complex in the electrolyte solution in step b) is 0.3-1 mol/L. Procedure according to one of the Claims 7 until 9 , wherein step b) before introduction into the housing involves the production of the zinc complex from zinc oxide ZnO and [Hbet][NTf ₂ ] and/or [Hbet][OTf] and/or [Hbet]CI and/or [Hbet] Br includes. Procedure according to Claim 10 , wherein step b) comprises, before introduction into the housing, the production of the zinc complex from zinc oxide ZnO, betaine and an acid selected from HNTf ₂ , HOTf, HCl and HBr. Procedure according to one of the Claims 7 until 11 , where the zinc complex contains zinc, the ligand betaine and the anion NTf ₂ .

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