Deep eutectic solvent and electrolyte

Abstract

To provide a deep eutectic solvent containing metal ions, the solvent exhibiting high stability over time and superior electrochemical stability even with use of an oxidative anion.SOLUTION: A deep eutectic solvent (A) comprising a metal salt and a hydrogen bond donor, the hydrogen bond donor comprising (B) a multidentate coordinating compound and (C) a monodentate coordinating compound, wherein the mass ratio (B) / (C) is 10 / 90 to 90 / 10, and the mass ratio (A) / ((B)+(C)) is 10 / 90 to 70 / 30.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to deep eutectic solvents and electrolytes containing metal ions. [Background technology]

[0002] Deep eutectic solvents consisting of hydrogen bond donors and hydrogen bond acceptors have characteristics similar to ionic liquids, such as low vapor pressure, flame retardancy, thermal and electrochemical stability, and the ability to dissolve a variety of compounds. Furthermore, since there are countless possible combinations of hydrogen bond acceptors and donors, it is possible to tailor deep eutectic solvents to suit specific applications.

[0003] Deep eutectic solvents generally use organic ammonium compounds such as choline chloride as hydrogen bond acceptors, and compounds such as urea, carboxylic acids, and alcohols as hydrogen bond donors. Deep eutectic solvents are expected to be used as electrolytes due to their high safety. Patent Document 1 proposes an electrolyte in which a deep eutectic solvent is used as a molten salt. Deep eutectic solvents have a structure consisting of organic cations and anions. On the other hand, when used as an electrolyte for secondary batteries, consideration must be given to not only electronic conductivity but also ionic conductivity. It is desirable to include metal ions in the electrolyte, but Patent Document 1 fails to achieve this. Patent Document 2 uses a deep eutectic solvent consisting of aluminum chloride and urea as an electrolyte for aluminum secondary batteries, and it can be used as an electrolyte containing metal ions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-164877 [Patent Document 2] Japanese Patent Publication No. 2023-156753 Summary of the Invention [Problem to be solved by the invention]

[0005] In the case of deep eutectic solvents containing metal ions, anions and hydrogen bond donors may form associations, potentially resulting in the precipitation of crystals over time. Furthermore, the Lewis acidity of metal ions can cause corrosion and oxidation-reduction reactions, particularly with oxidizing anions such as nitric acid, sulfuric acid, and perchloric acid, which can react with hydrogen bond donors to oxidize the anions, potentially generating gas. Patent Document 2 does not discuss the electrochemical stability of deep eutectic solvents.

[0006] Patent Document 1 uses zinc chloride or bromide, but does not disclose its stability over time or electrochemical stability. Patent Document 2 also uses aluminum halide salts, but does not discuss its stability over time or electrochemical stability.

[0007] The present invention has been made in view of the above problems, and aims to provide a deep eutectic solvent containing metal ions that has high stability over time and excellent electrochemical stability even when an oxidizing anion is used. [Means for solving the problem]

[0008] As a result of intensive research to solve the above problems, the present inventors have found that by using a specific amount of a multidentate compound and a monodentate compound in combination as hydrogen bond donors, a deep eutectic solvent having excellent temporal stability and electrochemical stability can be obtained, and have completed the present invention.

[0009] That is, the deep eutectic solvent of the present invention is a deep eutectic solvent containing (A) a metal salt and a hydrogen bond donor, wherein the hydrogen bond donor contains (B) a multidentate compound and (C) a monodentate compound, the mass ratio of (B) / (C) is 10 / 90 to 90 / 10, and the mass ratio of (A) / ((B)+(C)) is 10 / 90 to 70 / 30. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a deep eutectic solvent that contains metal ions and has excellent temporal stability and electrochemical stability. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described. In the present invention, a numerical range defined using the symbol "to" is intended to include the numerical values ​​on both ends (upper and lower limits) of the symbol "to." For example, "10 to 30" represents a range of 10 or more and 30 or less.

[0012] <Deep eutectic solvent> The deep eutectic solvent of the present invention comprises (A) a metal salt, and (B) a multidentate compound and (C) a monodentate compound as hydrogen bond donors, wherein the mass ratio of (B) / (C) is 10 / 90 to 90 / 10, and the mass ratio of (A) / ((B)+(C)) is 10 / 90 to 70 / 30.

[0013] Hereinafter, in the present invention, a metal salt may be simply referred to as "(A) metal salt" or "(A)", a polydentate compound may be simply referred to as "(B) polydentate compound" or "(B)", and a monodentate compound may be simply referred to as "(C) monodentate compound" or "(C)".

[0014] In the deep eutectic solvent of the present invention, both the (B) multidentate compound and the (C) monodentate compound form a eutectic with the metal ion, resulting in a deep eutectic solvent with high temporal stability and excellent electrochemical stability. The multidentate compound forms multiple coordinate bonds with the metal ion to form a stable complex. This allows the metal ion to maintain an electrochemically stable state. Furthermore, the monodentate compound weakly interacts with the metal ion, and in the solution, an associated state and a dissociated state exist in equilibrium. This prevents the metal ion from recrystallizing, maintaining the temporal stability of the deep eutectic solvent. From the above, it is believed that the multidentate compound contributes to electrochemical stability, and the monodentate compound contributes to temporal stability. The present invention can achieve both temporal stability and electrochemical stability by mixing the multidentate compound and the monodentate compound in a specific ratio.

[0015] The deep eutectic solvent of the present invention is used for various substrates such as electrolytes, solvents, catalysts, and sintered materials, and is suitably used, for example, as an electrolyte, solvent, catalyst, or coating agent in fields such as batteries, chemistry, and ceramics.

[0016] <(A) Metal Salt> There are no particular limitations on the metal ions and counter anions of the metal salts, but they are preferably water-soluble or solvent-soluble. They may also be hydrated or solvated. They may also contain multiple types of metal ions or counter anions.

[0017] Examples of metal ions include alkali metal ions, alkaline earth metal ions, typical metal ions, transition metal ions, and lanthanoid metal ions, and specific examples include sodium ions, lithium ions, magnesium ions, calcium ions, aluminum ions, lead ions, gallium ions, indium ions, tin ions, scandium ions, titanium ions, vanadium ions, chromium ions, manganese ions, iron ions, cobalt ions, nickel ions, copper ions, zinc ions, yttrium ions, zirconium ions, niobium ions, molybdenum ions, ruthenium ions, rhodium ions, palladium ions, silver ions, cadmium ions, rhenium ions, iridium ions, platinum ions, gold ions, mercury ions, lanthanum ions, and gadolinium ions. In particular, alkali metal ions such as lithium ions, transition metal ions such as manganese ions, iron ions, copper ions, nickel ions, ruthenium ions, platinum ions, and gold ions, typical metal ions such as aluminum ions and tin ions, and lanthanoid metal ions such as cerium and gadolinium ions are preferred in the present invention.

[0018] Examples of counter anions include halogen ions such as fluoride ions and chloride ions, carboxylate ions such as acetate ions and formate ions, hydroxide ions, oxoacid ions such as sulfate ions, nitrate ions, phosphate ions, perchlorate ions and permanganate ions, and non-coordinating anions such as tetrafluoroborate ions, hexafluorophosphate ions and tetrakis(pentafluorophenyl)borate ions. Anions such as halogen ions, oxoacid ions including nitrate ions and sulfate ions, and hexafluorophosphate ions are particularly preferred in the present invention. Oxoacid ions are particularly preferred because of their improved electrochemical stability.

[0019] The content of (A) metal salt contained in the deep eutectic solvent of the present invention is not particularly limited, but in consideration of the use of the deep eutectic solvent of the present invention as an electrolyte, catalyst, etc., it is preferably 10 mass% or more, more preferably 20 mass% or more, and even more preferably 40 mass% or more, relative to 100 mass% of the deep eutectic solvent; and in consideration of improving the stability over time of the deep eutectic solvent of the present invention, it is preferably 70 mass% or less, more preferably 60 mass% or less, and even more preferably 50 mass% or less.

[0020] <(B) Multidentate compound> In this application, a multidentate compound is defined as a ligand capable of forming a chelate. In other words, a multidentate compound is defined as a ligand capable of forming multiple coordinate bonds with one metal ion. A multidentate compound may form coordinate bonds with one or more metal ions. There are no particular limitations on the multidentate compound, but a structure capable of coordinating multiple coordination sites with one metal ion is desirable.

[0021] Examples of polydentate compounds include polyamines such as ethylenediamine and 2,2'-bipyridine, polyalcohols such as ethylene glycol, propylene glycol, glycerin, and catechol, polyethers such as polyethylene glycol, polycarboxylic acids such as oxalic acid, malic acid, and ethylenediaminetetraacetic acid, chelating compounds such as crown ethers, acetylacetone, cyclodextrin, porphyrin, and phthalocyanine, and amino acids such as 1,2-bis(diphenylphosphino)ethane, glycine, and alanine. In particular, polyamines, polyalcohols, polycarboxylic acids, and chelating compounds, specifically ethylene glycol, propylene glycol, and polyethers, and dicarboxylic acids such as glycerin, oxalic acid, and malic acid are preferred, with polyalcohols and polyethers such as ethylene glycol and propylene glycol, and glycerin being particularly preferred.

[0022] <(C) Monodentate coordination compound> In the present application, a monodentate compound refers to a ligand capable of forming one coordinate bond with one metal ion. A monodentate compound may form coordinate bonds with one or more metal ions. Examples of monodentate compounds include urea and other carbamide derivatives, ammonia, monoethers such as tetrahydrofuran, water, alcohols such as methanol and ethanol, monoamines such as pyridine, piperidine, morpholine, and benzylamine, mercaptans such as methanethiol, monocarboxylic acids such as acetic acid and oleic acid, and ketones such as acetone and 2-butanone. Carbamide derivatives, alcohols, monoamines, monocarboxylic acids, and ketones are preferred.

[0023] In the deep eutectic solvent of the present invention, the mass ratio of (B) / (C) is 10 / 90 to 90 / 10. From the viewpoint of electrochemical stability, the mass ratio of (B) / (C) is preferably 20 / 80 or more, more preferably 30 / 70 or more, and from the viewpoint of stability over time, it is preferably 85 / 15 or less, more preferably 70 / 30 or less.

[0024] In the deep eutectic solvent of the present invention, the mass ratio of (A) / ((B)+(C)) is 10 / 90 to 70 / 30. If the mass ratio of (A) / ((B)+(C)) is less than 10 / 90, the stability over time will be insufficient. On the other hand, if the mass ratio of (A) / ((B)+(C)) is greater than 70 / 30, the electrochemical stability will be insufficient. The lower limit of the mass ratio of (A) / ((B)+(C)) is preferably 20 / 80 or more, more preferably 30 / 70 or more, from the viewpoint of improving the stability over time of the deep eutectic solvent of the present invention. The upper limit is preferably 65 / 35 or less, more preferably 60 / 40 or less, and even more preferably 55 / 45 or less, from the viewpoint of improving the electrochemical stability of the deep eutectic solvent of the present invention.

[0025] The deep eutectic solvent of the present invention may contain either one type of polydentate compound (B) alone or a combination of two or more types thereof, and the deep eutectic solvent of the present invention may contain either one type of monodentate compound (C) alone or a combination of two or more types thereof.

[0026] <(D)Other> The deep eutectic solvent of the present invention can contain various additives such as thickeners and antioxidants as needed, as long as the effects of the present invention are not impaired. Examples of thickeners include polyisobutylene, ethyl cellulose, and nitrocellulose. Examples of antioxidants include sodium hypophosphite, ascorbic acid, and tocopherol.

[0027] The content of the components such as the additives other than (A), (B), and (C) contained in the deep eutectic solvent of the present invention is not particularly limited, but from the viewpoint of not impairing the effects of the present invention, it is preferably 10.0% by mass or less, more preferably 5.0% by mass or less, even more preferably 1.0% by mass or less, and particularly preferably 0.5% by mass or less, relative to 100% by mass of the deep eutectic solvent. Note that the deep eutectic solvent of the present invention can contain, as components other than (A), (B), and (C), compounds such as the additives described above, either alone or in combination of two or more. [Example]

[0028] The present invention will be described in more detail below with reference to examples and comparative examples, in which % is by mass unless otherwise specified.

[0029] Examples 1 to 6 The compounds were mixed in the ratios shown in Table 1 in a 300 ml Erlenmeyer flask and heated to 80°C in a water bath to obtain a deep eutectic solvent.

[0030] (Comparative Examples 1 to 4) The compounds were mixed in the ratios shown in Table 2 in a 300 ml Erlenmeyer flask and heated to 80°C in a water bath to obtain a deep eutectic solvent.

[0031] [evaluation] The deep eutectic solvents were evaluated for the following items.

[0032] (1) Liquefaction 100 ml of the deep eutectic solvent was placed in a screw cap bottle (Maruem No. 8, 110 ml, inner mouth diameter: 2.03 cm, body diameter: 4.0 cm) and left to stand in a thermostatic bath at 25°C for 3 hours, after which solidification was confirmed by visual inspection for crystal precipitation and by tilting the screw cap to check for fluidity. The evaluation criteria are as follows: 〇: No crystallization or solidification was observed ×: Crystallization or solidification is confirmed

[0033] (2) Stability over time 100 ml of the deep eutectic solvent was placed in a screw cap vial (Maruem No. 8, 110 ml, inner mouth diameter: 2.03 cm, body diameter: 4.0 cm), and left in a thermostatic chamber at 40°C and 60% humidity with the lid on for 7 days. The solution was then visually inspected under fluorescent light to ensure transparency, and to check for the presence of precipitation or turbidity. The evaluation criteria were as follows: ◎: Stable after 7 days ○: Precipitation or turbidity occurs after 6 to 7 days △: Precipitation or turbidity occurs after 3 to 5 days ×: Precipitation or turbidity occurs within 2 days

[0034] (3) Electrochemical stability 50 ml of deep eutectic solvent was placed in a 100 ml beaker and two carbon electrodes were inserted. The carbon electrodes were connected to a Bio Logic multichannel potentiostat / galvanostat: VSP3, the potential was fixed at -5.0 V, and the time until gas generation was measured. The evaluation criteria are as follows: ◎: 3 hours or more ○: Less than 3 hours, 1 hour or more △: Less than 1 hour, 30 minutes or more ×: Less than 30 minutes

[0035] [Table 1]

[0036] [Table 2]

[0037] As shown in Table 1, the deep eutectic solvents of Examples 1 to 6 remained liquid at 25° C. and were excellent in stability over time and electrochemical stability. On the other hand, the deep eutectic solvent of Comparative Example 1 did not contain (B) a multidentate compound, and although it remained liquid at 25°C, crystals formed within a short period of time, and it had poor stability over time. The deep eutectic solvent of Comparative Example 2 did not contain the (C) monodentate coordination compound, and although it similarly maintained a liquid state, it had poor electrochemical stability and was confirmed to release gas within a short period of time. Furthermore, the deep eutectic solvent of Comparative Example 3 had a mass ratio of (A) / ((B)+(C)) exceeding 70 / 30, and was unable to maintain a liquid state at room temperature. The deep eutectic solvent of Comparative Example 4 had a mass ratio of (A) / ((B)+(C)) below 10 / 90, and was poor in stability over time.

[0038] From the above results, it was revealed that a deep eutectic solvent containing (A) a metal salt, (B) a multidentate compound, and (C) a monodentate compound, in which the mass ratio of (B) / (C) is 10 / 90 to 90 / 10 and the mass ratio of (A) / ((B)+(C)) is 10 / 90 to 70 / 30, has excellent temporal stability and electrochemical stability. [Industrial Applicability]

[0039] The deep eutectic solvent of the present invention is used for various substrates such as electrolytes, solvents, catalysts, and sintered materials, and is useful as an electrolyte, solvent, catalyst, or coating agent in the fields of batteries, ceramics, and the like.

Claims

1. (A) a deep eutectic solvent comprising a metal salt and a hydrogen bond donor; the hydrogen bond donor comprises (B) a polydentate compound and (C) a monodentate compound; A deep eutectic solvent in which the mass ratio of (B) / (C) is 10 / 90 to 90 / 10 and the mass ratio of (A) / ((B)+(C)) is 10 / 90 to 70 / 30.

2. The deep eutectic solvent according to claim 1, wherein the polydentate compound (B) is at least one selected from the group consisting of polyamines, polyalcohols, polyethers, and polycarboxylic acids.

3. The deep eutectic solvent according to claim 1, wherein (C) the monodentate compound is one or more selected from the group consisting of carbamide derivatives, alcohols, amines, ethers, and ketones.

4. An electrolyte using the deep eutectic solvent according to any one of claims 1 to 3.

Images