Electrolyte for fluoride ion battery

Abstract

To provide an electrolyte for a fluoride ion battery that is highly safe and has a wide potential window.SOLUTION: The present invention provides a solution containing two or more alkali metal fluoride salts dissolved in water.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrolyte for a fluoride ion battery. [Background technology]

[0002] Fluoride-ion batteries are fluoride shuttle type storage batteries that have attracted attention as a new battery to replace lithium-ion batteries due to their high volumetric energy density.

[0003] Among halide-ion batteries, many conventionally known fluoride-ion batteries have been reported to operate at high temperatures using ionic liquids, organic electrolytes, or solid electrolytes. Therefore, fluoride-ion batteries that can be charged at lower temperatures are being sought.

[0004] For example, Non-Patent Document 1 reports a room temperature operating fluoride ion battery using an electrolyte consisting of an ether solution of tetraalkylammonium fluoride.

[0005] Furthermore, Non-Patent Document 2 reports an aqueous fluoride ion battery using 0.8 mol / L NaF as the electrolyte.

[0006] Furthermore, Patent Document 1 reports a fluoride ion battery that uses an aqueous solution containing 9.0 to 11.0 mol / kg of a quaternary ammonium halide salt or a hydrate thereof as an electrolyte. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2022 / 186394 [Non-patent literature]

[0008] [Non-Patent Document 1] Science.362,1144-1148(2018) [Non-patent document 2] J.Electrochem.Soc.,166,A2419-A2424(2019) Summary of the Invention [Problem to be solved by the invention]

[0009] However, in Non-Patent Documents 1 and 2, there is a possibility that free hydrogen fluoride may be produced as a reaction intermediate, which poses a safety problem.

[0010] Furthermore, in Patent Document 1, the width of the potential window is not sufficient, and the materials that can be used for the electrodes are limited. In other words, there is room for improvement in terms of the width of the potential window.

[0011] Therefore, there is a demand for an electrolyte solution for fluoride ion batteries that is excellent in safety and has a wide potential window. [Means for solving the problem]

[0012] The electrolyte solution for a fluoride ion battery according to the present invention is characterized in that it is a solution in which two or more kinds of alkali metal fluoride salts are dissolved in water.

[0013] As a result of extensive research aimed at achieving the above-mentioned object, the present inventors have found that the potential window of a fluoride ion battery electrolyte is widened when the electrolyte is a solution in which two or more alkali metal fluoride salts are dissolved in water. This is presumably because a hydrate melt is formed in a solution containing two or more alkali metal fluoride salts. This causes water molecules that would normally be present together through hydrogen bonds to exist in isolation, reducing the amount of water formed by hydrogen bonding between water molecules, thereby suppressing water decomposition. In other words, the electrolyte for a fluoride ion battery according to the present invention has a low water content and suppresses water decomposition, resulting in a wide potential window. Furthermore, a solution in which two or more alkali metal fluoride salts are dissolved in water is less flammable because it uses water as a solvent, and hydrogen fluoride is less likely to be generated during the electrochemical reaction. In other words, the electrolyte for a fluoride ion battery according to the present invention is highly safe and has a wide potential window. [Brief explanation of the drawings]

[0014] [Figure 1] 10 is a graph showing the results of potential window measurement for the electrolyte solution of Example 4. [Figure 2] 1 is a graph showing the results of potential window measurement for the electrolyte solution of Example 1. [Figure 3] 10 is a graph showing the results of potential window measurement for the electrolyte solution of Example 9. [Figure 4] 1 is a graph showing the results of potential window measurement for the electrolyte solution of Comparative Example 1. [Figure 5] 10 is a graph showing the results of potential window measurement for the electrolyte solution of Comparative Example 2. [Figure 6] 10 is a graph showing the results of a charge-discharge test using a half cell using the electrolyte solution of Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the electrolyte for a fluoride ion battery according to the present invention will be described. Note that the embodiments described below are merely examples for explaining the present invention, and the present invention is not limited to these embodiments. Therefore, the present invention can be implemented in various forms without departing from the gist of the present invention.

[0016] The electrolyte for a fluoride ion battery of the present invention is a solution in which two or more alkali metal fluoride salts are dissolved in water.

[0017] In the electrolyte for a fluoride ion battery of the present invention, a hydrate melt (room-temperature molten hydrate) consisting of two or more alkali metal fluoride salts and water is formed. Specifically, in the electrolyte for a fluoride ion battery, two or more alkali metal fluoride salts are mixed and dissolved in water, and a hydrate melt that is a stable liquid at room temperature is formed as the water molecules and alkali metal ions interact with each other. As an example, when two alkali metal fluoride salts, cesium fluoride (CsF) and rubidium fluoride (RbF), are dissolved in water, a hydrate melt is formed in which water molecules and fluorine ions are coordinated with the alkali metal ions.

[0018] Here, compared with organic solvents, water has a lower voltage tolerance and is electrolyzed into hydrogen and oxygen even at low voltages, so the presence of water in the electrolyte solution tends to narrow the potential window. However, in the electrolyte solution for fluoride ion batteries of the present invention, a hydrate melt is formed and water molecules exist in isolation. Therefore, the content of water formed by hydrogen bonding between water molecules is low. Therefore, in the electrolyte solution for fluoride ion batteries of the present invention, water decomposition is suppressed and the potential window is widened.

[0019] The alkali metal fluoride salt is not particularly limited and may be a composite salt containing two or more alkali metals and fluorine. According to the findings of the present inventors, the use of two or more alkali metal fluoride salts having a solubility in water (g / 100g-H2O) at 20°C of 100 or more (preferably 200 or more, more preferably 300 or more) facilitates the formation of a hydrate melt and facilitates the generation of electricity when used as an electrolyte. In particular, the use of cesium fluoride (solubility: 322) and rubidium fluoride (solubility: 300) as the alkali metal fluorides provides an electrolyte for a fluoride ion battery with a wide potential window and excellent charge / discharge characteristics.

[0020] The fluoride ion battery electrolyte of the present invention preferably has a mass molar concentration of the alkali metal fluoride salt of 30 mol / kg or more, more preferably 40 mol / kg or more, and even more preferably 45 mol / kg or more. In the fluoride ion battery electrolyte of the present invention, a hydrate melt is formed as described above. Therefore, if the mass molar concentration of the alkali metal fluoride salt is high, the content of the alkali metal fluoride salt increases while the water content remains low. In other words, if the mass molar concentration of the alkali metal fluoride salt is high, the potential window becomes wider, and the charge / discharge cycle performance of a battery using the fluoride ion battery electrolyte of the present invention also improves. Therefore, it is preferable that the concentration of the alkali metal fluoride salt be as high as possible.

[0021] When the electrolyte for a fluoride ion battery of the present invention is a solution in which two alkali metal fluoride salts, CsF and RbF, are dissolved in water, the molar ratio of CsF to RbF is preferably 9:1 to 1:9. Within this range, a hydrate melt is formed, the water content is reduced, and the potential window is widened. From the perspective of further reducing the water content and widening the potential window, the molar ratio of CsF to RbF is more preferably 7:3 to 4:6, and even more preferably 6:4. The molar concentration of the alkali metal fluoride salt is 40 mol / kg or higher when the molar ratio of CsF to RbF is 7:3 to 4:6, and 45 mol / kg or higher when the molar ratio is 6:4. Therefore, the water content is further reduced when the molar ratio of CsF to RbF is 7:3 to 4:6, and even further reduced when the molar ratio is 6:4, resulting in a widened potential window. In this specification, when a numerical range is expressed as "X to Y", it means X or more and Y or less.

[0022] For example, when cesium fluoride and rubidium fluoride are used as alkali metal fluoride salts, the potential window at room temperature (25°C) is approximately 3.0 V when the molar ratio of CsF to RbF is 9:1, approximately 2.81 V when the molar ratio is 1:9, and approximately 3.5 V when the molar ratio is 6:4.

[0023] The electrolyte for a fluoride ion battery of the present invention may contain an electrolyte that has been conventionally usable in fluoride ion batteries. However, from the viewpoints of safety, the width of the electrolytic window, capacity, etc., it is preferable that the content of the conventional electrolyte is as small as possible, for example, 0 to 1 mol / kg is preferable, and 0 to 0.1 mol / kg is more preferable. The conventional electrolyte is, for example, TEAF.

[0024] The electrolyte solution for a fluoride ion battery of the present invention can be prepared, for example, by mixing predetermined amounts of cesium fluoride and rubidium fluoride with a predetermined amount of water in the atmosphere, subjecting the mixture to ultrasonic treatment, and leaving the mixture to stand for a predetermined time (for example, 4 to 5 days) while stirring at a predetermined temperature (for example, 40°C).

[0025] The electrolyte for a fluoride ion battery of the present invention is unlikely to generate liberated hydrogen fluoride during the electrochemical reaction, and is highly flammable and safe because it uses water as a solvent. Furthermore, the electrolyte for a fluoride ion battery of the present invention has a wide potential window, which increases the variety of materials that can be used for electrode active materials, enabling the realization of a high-capacity fluoride ion battery. Furthermore, by using the electrolyte for a fluoride ion battery of the present invention, a fluoride ion battery that has excellent charge / discharge characteristics and can operate at room temperature can be realized.

[0026] The electrolyte for a fluoride ion battery of the present invention can be used, for example, in a fluoride ion battery including, in a battery case, a positive electrode active material layer, a negative electrode active material layer, an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer and containing the electrolyte for a fluoride ion battery, a positive electrode current collector that collects current from the positive electrode active material layer, and a negative electrode current collector that collects current from the negative electrode active material layer.

[0027] The positive electrode active material layer of the fluoride ion battery contains a positive electrode active material. The positive electrode active material can be an active material that defluorinates during discharge. Examples of the positive electrode active material include metals, alloys, metal oxides, carbon materials, fluorides of these, and polymer materials, which can be used alone or in combination of two or more.

[0028] Examples of metal elements contained in the positive electrode active material include copper, silver, nickel, cobalt, lead, cerium, manganese, gold, platinum, rhodium, vanadium, osmium, ruthenium, iron, chromium, bismuth, niobium, antimony, titanium, tin, and zinc.

[0029] Examples of carbon materials include graphite, coke, and carbon nanotubes.

[0030] Examples of polymer materials include polyaniline, polypyrrole, polyacetylene, and polythiophene.

[0031] The positive electrode active material layer may contain a conductive material or a binder. The conductive material may be any material having the desired electronic conductivity, such as a carbon material such as carbon black. The binder may be any material that is chemically and electrically stable, such as a fluorine-based binder such as polyvinylidene fluoride.

[0032] The negative electrode active material layer of the fluoride ion battery contains a negative electrode active material. The negative electrode active material can be an active material that is fluorinated during discharge. Examples of the negative electrode active material include metals, alloys, metal oxides, carbon materials, fluorides of these materials, and polymer materials, which can be used alone or in combination of two or more.

[0033] Examples of metal elements contained in the negative electrode active material include lanthanum, calcium, aluminum, europium, lithium, silicon, germanium, tin, indium, vanadium, cadmium, chromium, iron, zinc, gallium, titanium, niobium, manganese, ytterbium, zirconium, samarium, cerium, magnesium, barium, and lead.

[0034] As for the carbon material and polymer material, the same materials as those described above for the positive electrode active material layer can be used.

[0035] The negative electrode active material layer may contain a conductive material and a binder, similar to the positive electrode active material layer. The conductive material and the binder may be the same materials as those described for the positive electrode active material layer.

[0036] [Example] Examples of the present invention will be described below, but the present invention is not limited to the descriptions of these examples.

[0037] In the following examples and comparative examples, cesium fluoride (manufactured by Sigma-Aldrich Co.) and rubidium fluoride (manufactured by Sigma-Aldrich Co.) were used as alkali metal fluoride salts.

[0038] First, the electrolyte solutions of Examples 1 to 9 and Comparative Examples 1 and 2 were prepared as follows: First, predetermined amounts of cesium fluoride and rubidium fluoride (only one of cesium fluoride or rubidium fluoride was used in the Comparative Examples) were weighed in an argon glove box and placed in a vial. Second, the vial was removed from the argon glove box, and a predetermined amount of water was added in the air. Third, the vial was subjected to ultrasonic treatment for 15 minutes. Fourth, the vial was left at 40°C for 4 to 5 days with stirring.

[0039] Example 1 An electrolyte solution containing 0.9 mol of CsF and 0.1 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0040] Example 2 An electrolyte solution containing 0.8 mol of CsF and 0.2 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0041] Example 3 An electrolyte solution containing 0.7 mol of CsF and 0.3 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0042] Example 4 An electrolyte solution containing 0.6 mol of CsF and 0.4 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0043] Example 5 An electrolyte solution containing 0.5 mol of CsF and 0.5 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0044] Example 6 An electrolyte solution containing 0.4 mol of CsF and 0.6 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0045] Example 7 An electrolyte solution containing 0.3 mol of CsF and 0.7 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0046] Example 8 An electrolyte solution containing 0.2 mol of CsF and 0.8 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0047] Example 9 An electrolyte solution containing 0.1 mol of CsF and 0.9 mol of RbF per mol of alkali metal fluoride salt was obtained.

[0048] Comparative Example 1 An electrolyte solution containing only CsF was obtained.

[0049] Comparative Example 2 An electrolyte solution containing only RbF was obtained.

[0050] The ratio of cesium fluoride to rubidium fluoride contained per mole of alkali metal fluoride salt (CsF:RbF) and the molar concentration of alkali metal fluoride salt for the electrolyte solutions of Examples and Comparative Examples are summarized in Table 1. The molar concentration of each electrolyte solution was calculated based on the amounts of cesium fluoride, rubidium fluoride, and water used in preparing each electrolyte solution.

[0051] [Table 1]

[0052] As shown in Table 1, when cesium fluoride and rubidium fluoride are used, the molar concentration tends to be high. In particular, in Examples 3 to 6, where the CsF:RbF ratio is 0.7:0.3 to 0.4:0.6, the molar concentration is 40 mol / kg or higher. In particular, in Example 4, where the CsF:RbF ratio is 0.6:0.4, the molar concentration is 45.2 mol / kg, which is significantly higher than in Comparative Examples 1 and 2, where cesium fluoride or rubidium fluoride was used alone.

[0053] [Test Example 1: Measurement of Potential Window] A glassy carbon electrode (manufactured by EC Frontier Co., Ltd.) having a diameter of 3 mm was used as the working electrode (positive electrode), a platinum wire was used as the counter electrode, and a silver / silver chloride electrode was used as the reference electrode. These were immersed in the electrolyte solutions of Examples 1, 4, and 9 and Comparative Examples 1 and 2 to prepare cells for measuring the potential window.

[0054] The potential window measurement cell was fabricated using a potentiostat (Hokuto Denko Corporation) at room temperature (25°C), and the current flowing through the working electrode was measured by sweeping the potential of the working electrode against the counter electrode at a constant rate (sweep rate 0.5 mV / sec) (LSV measurement). The potential when the current reached a constant value (0 mA) was defined as the limiting redox potential, and the potential window was determined.

[0055] 1 to 5 are graphs showing the results. As shown in FIG. 1, the potential window was 3.5 V for the electrolyte of Example 4. As shown in FIGS. 2 and 3, the potential window was 3.0 V for the electrolyte of Example 1, and 2.81 V for the electrolyte of Example 9. In contrast, as shown in FIGS. 4 and 5, the potential windows for Comparative Examples 1 and 2 were 2.34 V and 2.80 V, respectively. These results demonstrate that the potential window is significantly wider when two alkali metal fluoride salts (CsF and RbF) are contained than when only one alkali metal fluoride salt (CsF or RbF) is contained. Furthermore, even when two alkali metal fluoride salts are contained, there is an optimum molar ratio at which the potential window is widest.

[0056] [Test Example 2: Charge / Discharge Test] In the charge-discharge test, a three-electrode electrolytic cell, VC-4 voltammetry cell for electrochemical measurements (manufactured by BAS Inc.), was assembled as follows and used.

[0057] Copper nanoparticles (positive electrode active material) with an average particle diameter of 100 nm, acetylene black, and polytetrafluoroethylene powder were mixed so that the positive electrode active material content was 85% by mass, the acetylene black content was 10% by mass, and the polytetrafluoroethylene content was 5% by mass. The resulting mixture was molded using a punch to a diameter of 8 mm to obtain a positive electrode.

[0058] Next, the positive electrode was laminated on a titanium mesh (100 mesh) larger in size than the positive electrode as a positive electrode current collector, and the laminate was immersed in the electrolytic solution of Example 4.

[0059] In addition, copper fluoride (CuF2) was used as the negative electrode active material, and a negative electrode and a negative electrode current collector were also prepared in the same manner as the above positive electrode and positive electrode current collector. The negative electrode was stacked on the negative electrode current collector, and then immersed in the electrolyte solution of Example 4.

[0060] A silver / silver chloride electrode serving as a reference electrode was immersed in the electrolyte of Example 4.

[0061] A charge / discharge test was carried out using the three-electrode electrolytic cell assembled as described above.

[0062] The charge / discharge conditions were −1.0 to +4.0 V versus a silver / silver chloride electrode, the charge / discharge rate was 0.01 C in both charge and discharge modes, and the measurement temperature was room temperature (30° C.), and the charge / discharge test was carried out for three cycles.

[0063] Fig. 6 is a graph showing the results of the charge-discharge test. As shown in Fig. 6, the electrolyte solution of Example 4 has good charge-discharge efficiency (Coulomb efficiency) at room temperature and has excellent charge-discharge characteristics.

[0064] In the above-described embodiment, the following configurations are envisioned. (1) An electrolyte for a fluoride ion battery, which is a solution in which two or more types of alkali metal fluoride salts are dissolved in water.

[0065] As a result of extensive research aimed at achieving the above-mentioned object, the present inventors have found that the potential window of a fluoride ion battery electrolyte is widened when the electrolyte is a solution in which two or more alkali metal fluoride salts are dissolved in water. This is presumably because a hydrate melt is formed in a solution containing two or more alkali metal fluoride salts. This causes water molecules that would normally be present together through hydrogen bonds to exist in isolation, reducing the amount of water formed by hydrogen bonding between water molecules, thereby suppressing water decomposition. In other words, the electrolyte for a fluoride ion battery according to the present invention has a low water content and suppresses water decomposition, resulting in a wide potential window. Furthermore, a solution in which two or more alkali metal fluoride salts are dissolved in water is less flammable because it uses water as a solvent, and hydrogen fluoride is less likely to be generated during the electrochemical reaction. In other words, the electrolyte for a fluoride ion battery according to the present invention is highly safe and has a wide potential window.

[0066] (2) In the electrolyte for a fluoride ion battery of (1), it is preferable that the molar concentration of the alkali metal fluoride salt is 30 mol / kg or more.

[0067] According to this configuration, the electrolyte for a fluoride ion battery has a high content of alkali metal fluoride salt while keeping the water content low, which widens the potential window and improves battery performance such as charge / discharge cycle performance.

[0068] (3) In the electrolyte for a fluoride ion battery of (1) or (2), the alkali metal fluoride salt is preferably cesium fluoride and rubidium fluoride.

[0069] According to this configuration, the electrolyte for a fluoride ion battery is excellent in safety and has a wide potential window.

[0070] (4) In the electrolyte for a fluoride ion battery of (3), it is preferable that the molar ratio of cesium fluoride to rubidium fluoride is 1:9 to 9:1.

[0071] According to this configuration, a hydrate melt is formed in the electrolyte solution for a fluoride ion battery, and the water content is reduced, thereby widening the potential window. [Industrial Applicability]

[0072] The present invention can be used in electrolytes for fluoride ion batteries.

Claims

1. An electrolyte for a fluoride ion battery, which is a solution in which two or more alkali metal fluoride salts are dissolved in water.

2. 2. The electrolyte solution for a fluoride ion battery according to claim 1, wherein the alkali metal fluoride salt has a molar concentration of 30 mol / kg or more.

3. 3. The electrolyte for a fluoride ion battery according to claim 1, wherein the alkali metal fluoride salt is cesium fluoride and rubidium fluoride.

4. 4. The electrolyte for a fluoride ion battery according to claim 3, wherein the molar ratio of the cesium fluoride to the rubidium fluoride is 1:9 to 9:1.

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