Magnesium secondary battery electrolyte and magnesium secondary battery using the same

Abstract

To provide an electrolyte for a magnesium secondary battery that has electrochemical magnesium precipitation and dissolution activity, oxidation resistance, and non-corrosion properties, and a magnesium secondary battery using the same.SOLUTION: An electrolyte for a magnesium secondary battery according to the present invention includes at least a nonaqueous solvent and a supporting salt, the non-aqueous solvent includes oligoethylene glycol substituted at least at both ends with 2,2,2-trifluoroethyl groups, and the supporting salt is an aluminum-based magnesium salt.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an electrolyte for a magnesium secondary battery and a magnesium secondary battery using the same. [Background technology]

[0002] Magnesium metal has been actively researched as one of the next-generation secondary batteries because of its high theoretical capacity density, abundant resource availability, and low cost.

[0003] Electrochemical deposition and dissolution of magnesium metal proceeds with high efficiency only in ether solvents. However, ethers generally have poor oxidation resistance, and it is necessary to improve the oxidation resistance in order to increase the voltage of batteries. It has been reported that the oxidation resistance of electrolytes for lithium secondary batteries can be improved by fluorinating a part of the ether skeleton or introducing a fluoroalkyl group (see, for example, Non-Patent Documents 1 and 2).

[0004] On the other hand, in magnesium secondary batteries, it has been reported that electrolytes in which a part of the ether skeleton is fluorinated or a fluoroalkyl group is introduced have been used (see, for example, Patent Documents 1 and 2). However, this has not led to an improvement in oxidation resistance, and further improvement is desired. In fact, partial fluorination of ethers reduces the donor property, which tends to reduce the solubility of various metal salts, but magnesium salts often do not dissolve, making it difficult to even prepare the electrolyte, let alone evaluate its properties.

[0005] On the other hand, corrosion of the current collector caused by the electrolyte is an essential issue that must be overcome in the practical application of batteries. In lithium-ion secondary batteries, HF is generated by self-dissociation (or hydrolysis) from LiPF6 contained in the electrolyte, and reacts with the aluminum positive electrode current collector to form a coating mainly composed of aluminum fluoride on the surface, thereby suppressing corrosion. However, the electrolyte of magnesium secondary batteries cannot form a stable coating on the aluminum surface, so the current collector is always at risk of corrosion at high voltages (see, for example, Non-Patent Documents 3 and 4).

[0006] Alloys such as Inconel 625 and Hastelloy B described in Non-Patent Document 4 are not subject to corrosion because they form a coating in some electrolytes, but they are heavy alloys, making it difficult to increase the energy density of the battery, and therefore are not preferred. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2012-074135 A [Patent Document 2] JP 2014-232719 A [Non-patent literature]

[0008] [Non-Patent Document 1] Chibueze V. Amanchukwu et al., J. Am. Chem. Soc. 2020, 142, 7393-7403 [Non-Patent Document 2] Zhiao Yu et al.,Nature Energy,VOL 5,526,July 2020,526-533 [Non-Patent Document 3] John Muldoon et al.,Energy Environ.Sci.,2013,6,482 [Non-Patent Document 4] Clemens Wall,ECS Electrochemistry Letters,4 (1),C8-C10,2015 Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above, an object of the present invention is to provide an electrolyte for a magnesium secondary battery that has electrochemical magnesium deposition / dissolution activity, and is oxidation-resistant and non-corrosive, and a magnesium secondary battery using the electrolyte. [Means for solving the problem]

[0010] The electrolyte for a magnesium secondary battery according to the present invention contains at least a non-aqueous solvent and a supporting salt, wherein the non-aqueous solvent contains oligoethylene glycol substituted at both ends with 2,2,2-trifluoroethyl groups, and the supporting salt is an aluminum-based magnesium salt, thereby solving the above-mentioned problems. The oligoethylene glycol having both ends substituted with 2,2,2-trifluoroethyl groups is represented by the formula R 1 (OC2H4) n OR 1 It may be expressed as: (wherein, R 1 represents -CH2CF3, n represents an integer of 2 or more and 10 or less, and * represents the bonding position. The n may be an integer of 2 or more and 5 or less. The n may be an integer of 2 or more and 4 or less. The oligoethylene glycol having both ends substituted with 2,2,2-trifluoroethyl groups is represented by the formula R 1 (OC2H4) n1 OR 1 and R 1 (OC2H4) n2 OR 1 It may also be a mixture of. (wherein, R 1 represents -CH2CF3, n1 is 2, n2 represents an integer of 3 or more and 10 or less, and * represents a bonding position. The R in the mixture 1 (OC2H4) n1 OR 1 The volume fraction of may be in the range of 0.01 to 0.99. The aluminum-based magnesium salt is Mg[Al(OR 2 )4]2. (wherein, R 2 each independently represents a monovalent hydrocarbon group having an alkyl group which may be substituted with a halogen atom; R 2Any two or more of R may be bonded to each other to form a ring. 2 are the same group.) R 2 The group represented by the following formula may have 1 or more and 6 or less carbon atoms. R 2 At least one of the groups represented by *-LC(R 3 ) n (CX3) 3-n The group may be represented by the following formula: (In the formula, L represents a single bond or a divalent hydrocarbon group, and R 3 represents a hydrogen atom or a monovalent hydrocarbon group, X represents a halogen atom, n represents an integer of 0 to 3, and * represents a bonding position. R 2 At least one of the groups represented by the formula may be a group represented by -CH(CX3)2. The aluminum-based magnesium salt may be represented as Mg[Al(OCH(CX3)2)4]2. The X may be a fluorine atom. The molar concentration of the supporting salt in the non-aqueous solvent is 0.01 molar mol -3 More than 0.8moldm -3 It may be in the following range: The molar concentration of the supporting salt in the non-aqueous solvent is 0.05 molar d -3 More than 0.55moldm -3 The following ranges may be used: The magnesium secondary battery according to the present invention includes a positive electrode, a negative electrode, and an electrolyte, and the electrolyte is the above-mentioned electrolyte, thereby solving the above-mentioned problems. The positive electrode may further include a current collector, and the current collector may be selected from the group consisting of aluminum, copper, stainless steel, nickel, and carbon-based materials. The current collector may be aluminum. A separator may be provided between the positive electrode and the negative electrode. The positive electrode may be an air electrode. Effect of the Invention

[0011] The electrolyte for magnesium secondary batteries of the present invention contains a non-aqueous solvent containing an oligoethylene glycol with at least both ends substituted with 2,2,2-trifluoroethyl groups, and a supporting salt which is an aluminum-based magnesium salt. The above-mentioned specific oligoethylene glycol dissolves the aluminum-based magnesium salt, and can exhibit electrochemical magnesium deposition and dissolution activity. Furthermore, since it has excellent oxidation resistance and non-corrosiveness, the use of such an electrolyte can provide a magnesium secondary battery with improved battery characteristics.

[0012] Since the electrolyte of the present invention is non-corrosive, even when inexpensive and lightweight aluminum is used as the positive electrode current collector, corrosion of the aluminum can be suppressed. This makes it possible to provide a magnesium secondary battery with a high energy density, which is advantageous for practical use. [Brief description of the drawings]

[0013] [Figure 1] Schematic diagram showing a magnesium secondary battery of the present invention. [Diagram 2] FIG. 1 shows a CV profile in a three-electrode beaker cell using the electrolyte of Example 7. [Diagram 3] FIG. 1 shows a CV profile in a three-electrode beaker cell using the electrolyte of Example 16. [Figure 4] FIG. 13 shows the CV profile in a three-electrode beaker cell using the electrolyte of Example 38. [Diagram 5] FIG. 3 shows the CV profile in a three-electrode beaker cell using the electrolyte of Example 39. [Figure 6] FIG. 1 shows a CV profile in a three-electrode beaker cell using the electrolyte of Example 41. [Figure 7] FIG. 1 shows a CV profile in a three-electrode beaker cell using the electrolyte of Example 42. [Figure 8] FIG. 1 shows a CV profile in a three-electrode beaker cell using the electrolyte of Example 43. [Figure 9]FIG. 1 shows a CV profile in a three-electrode beaker cell using the electrolyte of Example 8. [Figure 10] FIG. 1 shows the CV profile in a three-electrode beaker cell using the electrolyte of Example 17. [Figure 11] FIG. 1 shows the CV profile in a three-electrode beaker cell using the electrolyte of Example 13. [Figure 12] FIG. 1 shows a CV profile in a three-electrode beaker cell using the electrolyte of Example 10. [Figure 13] FIG. 1 shows LSV profiles in three-electrode beaker cells using the electrolytes of Examples 1, 7, 16, and 38. [Figure 14] FIG. 13 shows the potential-current response profile in a two-electrode cell using the electrolyte of Example 7 and an Al working electrode. [Figure 15] FIG. 1 shows the potential-current response profile in a two-electrode cell using the electrolyte of Example 7 and a CNF working electrode. [Figure 16] FIG. 13 shows the potential-current response profile in a two-electrode cell using the electrolyte of Example 38 and an Al working electrode. [Figure 17] FIG. 1 shows the potential-current response profile in a two-electrode cell using the electrolyte of Example 1 and an Al working electrode. [Figure 18] FIG. 13 shows an SEM image of the Al working electrode after an experiment on a two-electrode cell using the electrolyte and the Al working electrode of Example 7. [Figure 19] FIG. 1 shows an SEM image of the Al working electrode after an experiment on a two-electrode cell using the electrolyte and Al working electrode of Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are given like reference numerals and their description will be omitted. (Embodiment 1) In the first embodiment, the electrolyte solution for a magnesium secondary battery and the method for producing the same of the present invention will be described in detail.

[0015] The electrolyte for magnesium secondary batteries of the present invention contains at least a non-aqueous solvent and a supporting salt. The non-aqueous solvent contains an oligoethylene glycol substituted at both ends with 2,2,2-trifluoroethyl groups, and the supporting salt is an aluminum-based magnesium salt. The inventors of the present application have found that by using a specific oligoethylene glycol and an aluminum-based magnesium salt, the aluminum-based magnesium salt dissolves in the non-aqueous solvent, exhibits electrochemical magnesium deposition and dissolution activity, and provides an electrolyte that improves oxidation resistance. In addition, the electrolyte of the present invention is non-corrosive to various metals. Each component will be described in detail.

[0016] <Oligoethylene glycol substituted with 2,2,2-trifluoroethyl groups at both ends> In the present specification, the oligoethylene glycol is intended to have 2 to 10 consecutive oxyethylene units (OC2H4). If the number of oxyethylene units exceeds 10, the effect of the terminal 2,2,2-trifluoroethyl group may not be obtained. The oxyethylene unit may have a substituent within a range that does not impair the effect of the present invention. Examples of such a substituent include an alkyl group, an alkenyl group, an alkoxy group, a trialkylsilyl group, an amino group, an amide group, a sulfonyl group, a group in which some or all of the hydrogen atoms of these groups are substituted with halogen atoms (fluorine atom, chlorine atom, bromine atom, etc.), a halogeno group (fluorine atom, chlorine atom, bromine atom, fluorine atom), etc.

[0017] The oligoethylene glycol substituted at both ends with 2,2,2-trifluoroethyl groups (hereinafter, for the sake of simplicity, referred to as double-ended oligoethylene glycol) is preferably R 1 (OC2H4) n OR 1 In the formula, R 1 represents *-CH2CF3, n represents an integer of 2 or more and 10 or less, and * represents a bonding position. This can facilitate dissolution of aluminum-based magnesium salts, exhibit electrochemical magnesium precipitation / dissolution activity, and improve oxidation resistance.

[0018] In the above general formula, n is preferably an integer of 2 or more and 5 or less. This further improves the oxidation resistance. n is more preferably an integer of 2 or more and 4 or less. Within this range, it is possible to achieve a large current density and a small overvoltage while maintaining the oxidation resistance, and to obtain good reversibility.

[0019] The oligoethylene glycol substituted at both ends may be one type, or two or more types may be mixed. The smaller the number of repetitions of the oxyethylene unit, the greater the current density tends to be obtained in the deposition and dissolution activity of magnesium, and the larger the number of repetitions of the oxyethylene unit, the smaller the overvoltage tends to be, so by combining these, it is possible to provide an electrolyte with a large current density and a small overvoltage.

[0020] For example, when the double-ended oligoethylene glycol is composed of two types, R 1 (OC2H4) n1 OR 1 And, R 1 (OC2H4) n2 OR 1 It is a mixture of R 1 represents *-CH2CF3, n1 is 2, n2 represents an integer of 3 or more and 10 or less, and * represents a bonding position. By using such a combination, a large current density due to the both-end-substituted oligoethylene glycol satisfying n1=2, and a small overvoltage effect due to the both-end-substituted oligoethylene glycol satisfying n1=3 to 10 can be obtained. From the viewpoint of overvoltage, n2 is more preferably an integer satisfying the range of 3 or more and 5 or less.

[0021] Both ends substituted oligoethylene glycol is R 1 (OC2H4) n1 OR 1 And, R 1 (OC2H4) n2 OR 1 In the case of a mixture of R 1 (OC2H4) n1 OR1 The volume ratio of R in the mixture is preferably in the range of 0.01 to 0.99. Within this range, a large current density and a small overvoltage can be achieved. 1 (OC2H4) n1 OR 1 The volume ratio of is more preferably in the range of 0.4 to 0.6. Within this range, the magnesium deposition / dissolution activity is excellent, and a larger current density and a smaller overvoltage can be achieved.

[0022] The non-aqueous solvent is preferably composed of the above-mentioned double-substituted oligoethylene glycol alone, but may further contain a non-aqueous solvent such as oligoethylene glycol dialkyl ether, dialkyl sulfone, methoxyamine, etc., within a range that does not impair the characteristics. For example, if the solvent contains ethylene glycol dimethyl ether, it is advantageous for the current density. Note that the double-substituted oligoethylene glycol in the non-aqueous solvent has the above-mentioned effect as long as it is at least 50% by volume or more.

[0023] The oligoethylene glycol substituted at both ends may be commercially available or may be synthesized. The synthesis method is not particularly limited as long as the terminal hydroxyl groups are substituted with 2,2,2-trifluoroethanol, but for example, it can be synthesized as follows.

[0024] It is synthesized by replacing the hydroxyl groups at both ends of an oligoethylene glycol (having 2 to 10 consecutive oxyethylene units) with a good leaving group such as a sulfonyl group, and then replacing it with 2,2,2-trifluoroethanol. Those skilled in the art can understand and carry out such a reaction.

[0025] <Aluminum-based magnesium salt> In the present specification, the aluminum-based magnesium salt is not particularly limited as long as it is a magnesium salt of an organic complex ion coordinated to aluminum. Examples of the aluminum-based magnesium salt include Mg[Al(OR 2 )4]2 is an aluminum-based magnesium salt.

[0026] Here, R2 each independently represents a monovalent hydrocarbon group having an alkyl group which may be substituted with a halogen atom; R 2 Any two or more of R may be linked together to form a ring. 2 are all the same group.

[0027] R 2 is a monovalent hydrocarbon group having an alkyl group which may be substituted with a halogen atom. The halogen atom is not particularly limited and may be a fluorine atom (F), a chlorine atom (Cl), an iodine atom (I), etc., and a fluorine atom is preferred in that it provides a more excellent effect of the present invention.

[0028] R 2 The total number of carbon atoms in the hydrocarbon group is not particularly limited, but in general, it is preferably 1 or more and 20 or less, more preferably 10 or less, even more preferably 8 or less, particularly preferably 6 or less, and most preferably 4 or less. For example, the number of carbon atoms may be 1 or more and 6 or less.

[0029] Also, R 2 The hydrocarbon group may be any of linear, branched, and cyclic, but from the viewpoint of solubility in the above-mentioned double-substituted oligoethylene glycol, a linear or branched chain is preferred, and a linear or branched chain alkyl group is more preferred.

[0030] R 2The hydrocarbon group is preferably a linear or branched chain having 1 to 10 carbon atoms as a whole and having an alkyl group in which at least one hydrogen atom is substituted with a halogen atom (preferably a fluorine atom); more preferably a linear or branched chain having 1 to 8 carbon atoms as a whole and having an alkyl group in which at least one hydrogen atom is substituted with a halogen atom (preferably a fluorine atom); still more preferably a linear or branched chain having 1 to 5 carbon atoms as a whole and having an alkyl group in which at least one hydrogen atom is substituted with a halogen atom (preferably a fluorine atom); and particularly preferably a linear or branched chain having 1 to 4 carbon atoms as a whole and being an alkyl group itself substituted with a halogen atom (preferably a fluorine atom).

[0031] R 2 When the carbon number is 2 or more and 4 or less, the size of the anion of the aluminum-based magnesium salt is smaller, and this is preferable in that the mobility of the cation is improved.

[0032] R 2 When the hydrocarbon group has an alkyl group substituted with a halogen atom (halogenated alkyl group), the electron attracting property of the halogen atom causes the electrons in the anion of the aluminum-based magnesium salt to be further delocalized, thereby further improving the oxidation resistance.

[0033] R 2 The number of halogenated alkyl groups that the hydrocarbon group has is not particularly limited, and is preferably one or more, and it is more preferable that all of the hydrogen atoms bonded to carbon atoms are substituted with halogenated alkyl groups.

[0034] Examples of the linear or branched alkyl group having 1 to 10 carbon atoms include the following groups. A methyl group with one carbon atom; An ethyl group with two carbon atoms; Propyl and isopropyl groups with three carbon atoms; butyl, isobutyl, tert-butyl, and sec-butyl groups with four carbon atoms; C5 groups: pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1,1-dimethylpropyl, 2,2-dimethylpropyl, 1-ethylpropyl; C6 groups: hexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 1,4-dimethylbutyl, 2,3-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1-ethyl-2-methylpropyl, 1,1,2-trimethylpropyl; The following are carbon numbers of 7: heptyl, 1-methylhexyl, 2-methylhexyl, 3-methylhexyl, 4-methylhexyl, 5-methylhexyl, 1,1-dimethylpentyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl, 1,2-dimethylpentyl, 1,3-dimethylpentyl, 1,4-dimethylpentyl, 2,3-dimethylpentyl, 2,4-dimethylpentyl, 3,4-dimethylpentyl, 1-ethylpentyl, 2-ethylpentyl, 3-ethylpentyl, 1,2,2-trimethylbutyl, 1,1,2-trimethylbutyl, 1,3,3-trimethylbutyl, 1,1,3-trimethylbutyl, 2,2,3-trimethylbutyl, and 2,3,3-trimethylbutyl; C8 octyl, 1-methylheptyl, 2-methylheptyl, 3-methylheptyl, 4-methylheptyl, 5-methylheptyl, 6-methylheptyl, 1-ethylhexyl, 2-ethylhexyl, 3-ethylhexyl, 4-ethylhexyl, 1-propylpentyl, 2-propylpentyl, 1,1-dimethylhexyl, 2,2-dimethylhexyl, 3,3-dimethylhexyl, 4,4-dimethylhexyl, 5,5-dimethylhexyl, 3-ethyl-3-methylpentyl, 1,1-diethylbutyl, 2,2-diethylbutyl, 1,1,2,2-tetramethylbutyl, 1,1,3,3-tetramethylbutyl, 2,2,3,3-tetramethylbutyl, 1,1-dimethyl-2-ethylbutyl; Nonyl group, 2-methyloctyl group, 3-methyloctyl group, 4-methyloctyl group, 2,2-dimethylheptyl group, 2,3-dimethylheptyl group, 2,4-dimethylheptyl group, 2,6-dimethylheptyl group, 3,3-dimethylheptyl group, 3,4-dimethylheptyl group, 3,5-dimethylheptyl group, 4,4-dimethylheptyl group, 3-ethylheptyl group, 4-ethylheptyl group, 2,2,3-trimethylhexyl group, 2,2,4-trimethylhexyl group, 2,2,5-trimethylhexyl group, 2,3,3-trimethylhexyl group, 2,3,4-trimethylhexyl group, 2,3, 5-trimethylhexyl, 2,4,4-trimethylhexyl, 3,3,4-trimethylhexyl, 2-methyl-3-ethylhexyl, 3-methyl-3-ethylhexyl, 3-ethyl-4-methylhexyl, 3-ethyl-5-methylhexyl, 2,2,3,3-tetramethylpentyl, 2,2,3,4-tetramethylpentyl, 2,2,4,4-tetramethylpentyl, 2,3,3,4-tetramethylpentyl, 2,2-dimethyl-3-ethylpentyl, 2,3-dimethyl-3-ethylpentyl, 2,4-dimethyl-3-ethylpentyl, 3,3-diethylpentyl; Examples of the aryl group include a decyl group and an isodecyl group having 10 carbon atoms.

[0035] R 2Examples of the alkyl group substituted with a halogen atom that may be possessed by the alkyl group include groups in which at least one of the hydrogen atoms possessed by the alkyl group is substituted with a halogen atom (preferably a fluorine atom). Among these, a perfluoroalkyl group is preferable, a perfluoroalkyl group having 1 to 4 carbon atoms is more preferable, and a trifluoromethyl group is even more preferable.

[0036] R 2 At least one of the groups represented by *-LC(R 3 ) n (CX3) 3-n In the formula, L represents a single bond or a divalent hydrocarbon group (preferably an alkylene group having 1 to 4 carbon atoms), and R 3 represents a hydrogen atom or a monovalent hydrocarbon group, X represents a halogen atom (preferably a fluorine atom), and n represents an integer of 0 or more and 3 or less. In addition, * represents a bonding position.

[0037] In the formula, L is preferably a single bond. 3 Examples of the monovalent hydrocarbon group include an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, an aryl group having 3 to 10 carbon atoms, and an aralkyl group having 7 to 10 carbon atoms, and it is preferable that none of these groups is substituted with a halogen atom.

[0038] R 3 The alkyl group having 1 to 10 carbon atoms is, for example, R 2 Examples of the alkyl groups include the linear and branched alkyl groups exemplified in the description of R. 3 may be a cyclic alkyl group, such as a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, a cyclooctyl group, a bicyclo[2.2.1]heptyl group, a bicyclo[2.2.2]octyl group, a bicyclo[3.2.1]octyl group, a bicyclo[3.3.1]nonyl group, a bicyclo[3.2.2]nonyl group, or an adamantyl group.

[0039] R 3Examples of the alkenyl group having 1 to 10 carbon atoms include a 1-propynyl group, a 2-propynyl group, a 1-butynyl group, a 2-butynyl group, a 3-butynyl group, a 1-methyl-2-propynyl group, a 1-pentynyl group, a 2-pentynyl group, a 3-pentynyl group, a 4-pentynyl group, a 1-methyl-3-butynyl group, a 2-methyl-3-butynyl group, a 1-hexynyl group, a 2-hexynyl group, a 3-hexynyl group, a 4-hexynyl group, a 5-hexynyl group, a 6-hexynyl group, a 7-hexynyl group, a 8-hexynyl group, a 9-hexynyl group, a 10-hexynyl group, a 11-hexynyl group, a 22-hexynyl group, a 23-hexynyl group, a 24-hexynyl group, a 25-hexynyl group, a 26-hexynyl group, a 27-hexynyl group, a 28-hexynyl group, a 29-hexynyl group, a 30-hexynyl group, a 31-hexynyl group, a 32-hexynyl group, a 33-hexynyl group, a 34-hexynyl group, a 35-hexynyl group, a 36-hexynyl group, a 37-hexynyl group, a 38-hexynyl group, a 39-hexynyl group, a 41-hexynyl group, a 42-hexynyl group, a 43-hexynyl group, a 44-hexynyl group, a 45-hexynyl group, a 46-hexynyl group, a 47-hexynyl group, a 48-hexynyl group, a 49-hexynyl group, a 4 Examples of the aryl group include phenylene, 4-hexynyl, 5-hexynyl, 1-heptynyl, 2-heptynyl, 3-heptynyl, 4-heptynyl, 5-heptynyl, 6-heptynyl, 2-octynyl, 3-octynyl, 4-octynyl, 5-octynyl, 6-octynyl, 7-octynyl, 2-nonynyl, 8-nonynyl, 2-decynyl, and 9-decynyl groups.

[0040] R 3 Examples of the aryl group having 3 to 10 carbon atoms include phenyl, o-tolyl, m-tolyl, and p-tolyl.

[0041] R 3 Examples of the aralkyl group having 7 to 10 carbon atoms include a benzyl group, a 1-phenethyl group, a 2-phenethyl group, 1-methyl-2-phenethyl, 1-methyl-1-phenethyl, 1,1-dimethyl-2-phenethyl, 1-phenylpropyl, 2-phenylpropyl, 3-phenylpropyl, 1-methyl-2-phenylpropyl, 2-methyl-2-phenylpropyl, 1-phenylbutyl, 2-phenylbutyl, 3-phenylbutyl, and 4-phenylbutyl.

[0042] R 2 At least one of the groups represented by the formula is preferably a group represented by *-CH(CX3)2. Here, X represents a halogen atom (preferably a fluorine atom). This can promote dissolution in the double-ended oligoethylene glycol and the expression of electrochemical deposition / dissolution activity of magnesium.

[0043] Among them, Mg[Al(OR 2As the aluminum-based magnesium salt represented by formula 4]2, the compounds shown below are preferred from the viewpoints of solubility in both-substituted oligoethylene glycol, expression of magnesium precipitation dissolution activity, oxidation resistance and non-corrosion.

[0044] [ka]

[0045] Among these, the aluminum-based magnesium salt represented by Mg[Al(OCH(CX3)2)4] is preferred because of its excellent solubility in non-aqueous solvents, its ability to express electrochemical deposition / dissolution activity of magnesium, and its excellent oxidation resistance.

[0046] R 2 The ring formed by linking R is preferably an aliphatic ring, which may be either saturated or unsaturated, and the number of carbon atoms contained in the ring skeleton is preferably 2 to 6. 2 Examples of the ring formed by linking include those represented by the following formula:

[0047] [ka]

[0048] The aluminum-based magnesium salt may be commercially available or may be synthesized. There is no particular restriction on the synthesis method, but it can be synthesized, for example, as follows.

[0049] Mg(R 4 )2 and a compound represented by R 2 A magnesium source compound having a structure represented by the formula: 2 3, where R 2 is the above R 2 represents the same group as 4 each represents the same or different alkyl group (preferably an alkyl group having 1 to 10 carbon atoms).

[0050] Mg(R 4 )2 and a compound represented by R 2 When reacted with a compound represented by the formula Mg(OR 2 )2 is obtained. Mg(R 4 )2+2R 2 OH → Mg(OR 2 )2+2HR 4 The magnesium source compound thus obtained is reacted with an aluminum source compound.

[0051] In the electrolyte of the present invention, the molar concentration of the supporting salt in the nonaqueous solvent is not particularly limited, but is preferably 0.01 molar mol / mol. -3 More than 0.8moldm -3 Within this range, the aluminum-based magnesium salt can be easily dissolved, and the electrochemical deposition / dissolution activity of magnesium can be promoted with a small amount of supporting salt.

[0052] The molar concentration of the supporting salt in the non-aqueous solvent is more preferably 0.05 molar mol -3 More than 0.55moldm -3 and even more preferably 0.25 mouldm -3 More than 0.35moldm -3 Within this range, the expression of the precipitation and dissolution activity of magnesium is particularly promoted, and the oxidation resistance and non-corrosion properties are excellent.

[0053] The electrolytic solution of the present invention may contain components other than the above-mentioned double-substituted oligoethylene glycol and aluminum-based magnesium salt as necessary. Examples of such other compounds include biphenyl, alkyl biphenyl, terphenyl, terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, dibenzofuran, 2-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene, 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, 3,5-difluoroanisole, ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, thioanisole, diphenyl disulfide, and dipyridinium disulfide.

[0054] For example, when the battery contains ethylene sulfite, propylene sulfite, dimethyl sulfite, dimethyl sulfate, ethylene sulfate, etc., decomposition at high temperatures can be prevented and gas generation can be suppressed. For example, when the battery contains biphenyl, alkylbiphenyl, terphenyl, terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, dibenzofuran, 2-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene, etc., overcharging can be suppressed.

[0055] The content of the above components in the electrolytic solution is not particularly limited, but is preferably 0.01 to 5% by mass when the total mass of the electrolytic solution is taken as 100% by mass, in order to obtain a better effect of the present invention. The above components may be used alone or in combination of two or more. When two or more of the above components are used in combination, the total content is preferably within the above range.

[0056] The electrolyte of the present invention is prepared by mixing at least the above-mentioned oligoethylene glycol having both ends substituted with 2,2,2-trifluoroethyl groups and an aluminum-based magnesium salt, and does not require any special technology, making it advantageous for practical use.

[0057] (Embodiment 2) In the second embodiment, a magnesium secondary battery using the electrolyte for a magnesium secondary battery of the present invention described in the first embodiment will be described.

[0058] FIG. 1 is a schematic diagram showing a magnesium secondary battery of the present invention.

[0059] The magnesium secondary battery 1 of the present invention includes a positive electrode 11, a negative electrode 12, and an electrolyte solution 13, which are contained in a container 14. The electrolyte solution 13 is the electrolyte solution of the present invention described in detail in the first embodiment, and therefore a description thereof will be omitted.

[0060] Although not shown, the positive electrode 11 is composed of a positive electrode collector and a positive electrode active material held on the positive electrode collector. The material used as the positive electrode collector is not particularly limited, but examples thereof include aluminum, copper, stainless steel (SUS), nickel, and carbon-based materials such as carbon nanofibers. Among them, the electrolyte of the present invention is highly non-corrosive to aluminum, so that an inexpensive and lightweight secondary battery can be provided. Note that oxygen in the air may be used as the positive electrode active material. In this case, the positive electrode 11 functions as an air electrode, and a magnesium-air battery can be provided.

[0061] . The material used as the positive electrode active material is not particularly limited, but typically, it is preferable that the material is capable of inserting and extracting magnesium ions, and examples of such materials include MgFeSiO4, MgMn2O4, and V2O5.

[0062] The negative electrode 12 is not particularly limited as long as it dissolves and precipitates magnesium metal, and is, for example, magnesium metal or a magnesium alloy. The magnesium alloy is preferably an alloy of magnesium (Mg) and at least one metal selected from the group consisting of aluminum (Al), silicon (Si), gallium (Ga), zinc (Zn), tin (Sn), manganese (Mn), bismuth (Bi), and antimony (Sb). The negative electrode 12 may include a negative electrode current collector similar to the positive electrode current collector.

[0063] The magnesium secondary battery 1 may further include a separator (not shown) located between the positive electrode 11 and the negative electrode 12. The material of the separator is not particularly limited, and examples thereof include fluororesins such as polytetrafluoroethylene, polyolefin resins such as polyethylene and polypropylene, glass, and ceramics.

[0064] The present invention will now be described in detail with reference to specific examples, but it should be noted that the present invention is not limited to these examples. EXAMPLES

[0065] [Supporting salt] The following three types of supporting salts were prepared: ·Mg[Al(HFIP)4]2 (Magnesium tetrakis(1,1,1,3,3,3-hexafluoroisopropoxyl)aluminate) ·Mg[B(HFIP)4]2 (Magnesium tetrakis(1,1,1,3,3,3-hexafluoroisopropoxyl)borate) ·Mg[TFSI]2 (Magnesium bis(trifluoromethanesulfonyl)imide) Mg[Al(HFIP)4]2 and Mg[B(HFIP)4]2 were synthesized as described below, and Mg[TFSI]2 was obtained from Kishida Chemical Co., Ltd. The structural formulas of each are shown below.

[0066] [ka]

[0067]

Chem.

[0068]

Chem.

[0069] <Synthesis of Mg[Al(HFIP)4]2 In a glove box under an Ar (argon) atmosphere, a magnetic stir bar with a length of 2 cm was placed at the bottom of a 200 mL eggplant flask, and 5 mL (5 mmol) of a 1.0 M di-n-butylmagnesium heptane solution (manufactured by Sigma-Aldrich) was added using a syringe. Then, 1.04 mL (10.0 mmol; manufactured by Fujifilm Wako Pure Chemical Corporation) of 1,1,1,3,3,3-hexafluoro-2-propanol was added dropwise over 10 minutes, and the solution was continuously stirred at 25 °C during that time.

[0070] Butane gas was generated immediately after the dropwise addition. When the total amount of 1,1,1,3,3,3-hexafluoro-2-propanol was added dropwise, a white solid was obtained. The obtained white solid was dissolved in 20 mL of ethylene glycol dimethyl ether (manufactured by Kanto Chemical Co., Inc.) to prepare a homogeneous solution. To the prepared solution, 5.5 mL (10 mmol) of a 1.8 M toluene solution of trimethylaluminum (manufactured by Tokyo Chemical Industry Co., Ltd.) was added, and the mixture was stirred at 25 °C for 30 minutes.

[0071] Then, 3.42 mL (33.0 mmol) of 1,1,1,3,3,3-hexafluoro-2-propanol was added dropwise over 30 minutes, and the solution was continuously stirred at 25 °C during that time. After the total amount of 1,1,1,3,3,3-hexafluoro-2-propanol was added dropwise, the solution was further stirred at 25 °C for 12 hours.

[0072] Next, a three-way cock was attached to the eggplant flask containing the reaction solution, and the flask was taken out of the glove box while maintaining the inside of the eggplant flask in an Ar atmosphere. The eggplant flask was depressurized using an oil rotary vacuum pump and dried at 45 °C for 8 hours to remove the solvent. By the drying treatment, a reaction product containing the target compound Mg[Al(HFIP)4]2 was obtained. The structure of Mg[Al(HFIP)4]2 was confirmed by nuclear magnetic resonance spectrum. Mg[Al(HFIP)4]2 corresponds to an aluminum-based magnesium salt.

[0073] <Synthesis of <Mg[B(HFIP)4]2>> In a glove box under an Ar (argon) atmosphere, a magnetic stir bar with a length of 2 cm was placed at the bottom of a 200 mL eggplant flask, and 5 mL (5 mmol) of a 1.0 M solution of di-n-butylmagnesium in heptane (manufactured by Sigma-Aldrich) was put in using a syringe. 1.04 mL (10.0 mmol; manufactured by Fujifilm Wako Pure Chemical Corporation) of 1,1,1,3,3,3-hexafluoro-2-propanol was added dropwise thereto over 10 minutes, and the solution was continuously stirred at 25 °C during that time.

[0074] Butane gas was generated immediately after the dropwise addition, and when the total amount of 1,1,1,3,3,3-hexafluoro-2-propanol was added dropwise, a white solid was obtained. The obtained white solid was dissolved in 20 mL of ethylene glycol dimethyl ether (manufactured by Kanto Chemical Co., Inc.) to prepare a uniform solution. 12.2 mL (11 mmol; manufactured by Fujifilm Wako Pure Chemical Corporation) of a tetrahydrofuran solution of a tetrahydrofuran-borane complex was added to the prepared solution, and the mixture was stirred at 25 °C for 30 minutes.

[0075] Therein, 3.42 mL (33.0 mmol) of 1,1,1,3,3,3-hexafluoro-2-propanol was added dropwise over 30 minutes, and the solution was continuously stirred at 25 °C during that time. After the total amount of 1,1,1,3,3,3-hexafluoro-2-propanol was added dropwise, the solution was further stirred at 25 °C for 12 hours.

[0076] Next, a three-way cock was attached to the recovery flask containing the reaction solution, and the recovery flask was taken out of the glove box while maintaining the inside of the recovery flask under Ar atmosphere. The recovery flask was depressurized using an oil rotary vacuum pump, and the contents were dried at 50°C for 8 hours to remove the solvent. The drying process yielded a crude product, a white solid containing Mg[B(HFIP)4]2 and excess B(HFIP)3.

[0077] Next, the eggplant flask containing the white solid was introduced back into the glove box, the three-way stopcock was removed, and the white solid was washed three times with 30 mL of 1,4-dioxane (dehydrated) (Kanto Chemical Co., Ltd.) (a total of 90 mL of 1,4-dioxane was used). The three-way stopcock was attached to the eggplant flask again, and the eggplant flask was taken out of the glove box while maintaining the inside of the eggplant flask in an Ar atmosphere. The pressure was reduced using an oil rotary vacuum pump, and the reaction product containing the target compound Mg[B(HFIP)4]2 was obtained by drying at 50°C for 24 hours. The structure of Mg[B(HFIP)4]2 was confirmed by nuclear magnetic resonance spectroscopy.

[0078] [Non-aqueous solvent] The following 12 types of non-aqueous solvents were prepared: · G2 (Diethylene glycol dimethyl ether; 1-methoxy-2-(2-methoxyethoxy)ethane) ·G2TFM (Diethylene glycol bis(trifluoromethyl) ether; 1-(trifluoromethoxy)-2-[2-(trifluoromethoxy)ethoxy]ethane) G2TFE (Diethylene glycol bis(2,2,2-trifluoroethyl) ether; 1,1,1-trifluoro-2-{2-[2-(2,2,2-trifluoroethoxy)ethoxy]ethoxy}ethane) ·G2TFP (Diethylene glycol bis(3,3,3-trifluoropropyl) ether; 1,1,1-trifluoro-3-{2-[2-(3,3,3-trifluoropropoxy)ethoxy]ethoxy}propane) ·G3TFM (Triethylene glycol bis(trifluoromethyl) ether; 1,1,1,12,12,12-hexafluoro-2,5,8,11-tetraoxadodecane) ·G3TFE (Triethylene glycol bis(2,2,2-trifluoromethyl) ether; 1,1,1,14,14,14-hexafluoro-3,6,9,12-tetraoxatetradecane) ·G3TFP (Triethylene glycol bis(3,3,3-trifluoropropyl) ether; 1,1,1,16,16,16-hexafluoro-4,7,10,13-tetraoxahexadecane) ·G4TFM (Tetraethylene glycol bis(trifluoromethyl) ether; 1,1,1,15,15,15-hexafluoro-2,5,8,11,14-pentaoxapentadecane) ·G4TFE (Tetraethylene glycol bis(2,2,2-trifluoroethyl) ether; 1,1,1,17,17,17-hexafluoro-3,6,9,12,15-pentaoxaheptadecane) ·G4TFP (Tetraethylene glycol bis(3,3,3-trifluoropropyl) ether; 1,1,1,19,19,19-hexafluoro-4,7,10,13,16-pentaoxanonadecane) ·G5TFM (Pentaethylene glycol bis(trifluoromethyl) ether; 1,1,1,18,18,18-hexafluoro-2,5,8,11,14,17-hexaoxaoctadecane) ·2,2,3,3-Tetrafluoro-1,4-dimethoxybutane G2 was obtained from Kanto Chemical Co., Inc., and the others were synthesized as described below. Their structural formulas are shown.

[0079]

Chemical formula

[0080] <Synthesis of G2TFM> A magnetic stir bar with a length of 2 cm was placed in a 500 mL three-necked flask, and 175 mL of ethyl acetate and 8.36 g (0.14 mol; manufactured by FUJIFILM Wako Pure Chemical Corporation) of potassium fluoride were added, and the mixture was stirred at room temperature.

[0081] To this, 38.0 g (0.11 mol) of 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (manufactured by Tokyo Chemical Industry Co., Ltd.) and 27.7 g (0.11 mol; manufactured by FUJIFILM Wako Pure Chemical Corporation) of silver trifluoromethanesulfonate were added, and 1.9 g (0.018 mmol; manufactured by Tokyo Chemical Industry Co., Ltd.) of diethylene glycol was added dropwise. Further, 10.5 g (0.11 mol; manufactured by Tokyo Chemical Industry Co., Ltd.) of 2-fluoropyridine and 15.4 g (0.11 mol; manufactured by Tokyo Chemical Industry Co., Ltd.) of trimethyl(trifluoromethyl)silane were added dropwise, and the mixture was stirred at room temperature for 3 days.

[0082] The reaction solution was filtered to remove solid components, and the filtrate was concentrated using a rotary evaporator to obtain a crude product. Thereafter, the crude product was dissolved in 200 mL of dichloromethane (manufactured by FUJIFILM Wako Pure Chemical Corporation) and washed 3 times with 100 mL of distilled water. The organic layer was dried over magnesium sulfate (manufactured by FUJIFILM Wako Pure Chemical Corporation), filtered, and concentrated. Column chromatography was performed using a column filled with silica gel with ethyl acetate as the developing solvent to extract only the target product, which was concentrated using a rotary evaporator and then purified by distillation to obtain the target product.

[0083] <Synthesis of G2TFE> A magnetic stir bar with a length of 2 cm was placed in a 1 L three-necked flask, and 25.1 g (0.24 mol) of diethylene glycol, 100 mL (0.72 mol; manufactured by FUJIFILM Wako Pure Chemical Corporation) of triethylamine, and 100 mL of dichloromethane were added, and the mixture was stirred under ice cooling.

[0084] To this, 350 mL of a dichloromethane solution in which 99.6 g (0.52 mol; Tokyo Chemical Industry Co., Ltd.) of paratoluenesulfonyl chloride was dissolved was added dropwise, and the mixture was stirred at room temperature overnight. Thereafter, 200 mL of 2M HCl (manufactured by Fujifilm Wako Pure Chemical Corporation) was added, and the mixture was stirred at room temperature for 1 hour. The aqueous layer of the separated reaction solution was extracted twice with 100 mL of dichloromethane (100 mL × 2). The extracts and the organic layer were combined, washed with a 5% aqueous sodium hydrogen carbonate solution (manufactured by Fujifilm Wako Pure Chemical Corporation), pure water, and saturated brine (100 mL × 3 each), dried over magnesium sulfate, filtered, and concentrated. The obtained solid was recrystallized from ethanol (manufactured by Fujifilm Wako Pure Chemical Corporation), filtered, and dried to obtain diethylene glycol bis(p-toluenesulfonate) as an intermediate.

[0085] Subsequently, a magnetic stir bar with a length of 2 cm was placed in a 1 L three-necked flask, and 20.8 g (0.52 mol) of sodium hydride (60%, dispersion in paraffin liquid; manufactured by Tokyo Chemical Industry Co., Ltd.) was added and washed three times with 50 mL of hexane (manufactured by Fujifilm Wako Pure Chemical Corporation). 80 mL of tetrahydrofuran (manufactured by Fujifilm Wako Pure Chemical Corporation) was added thereto, and the mixture was stirred under ice cooling. After 80 mL of a tetrahydrofuran solution of 37 mL (0.51 mol; manufactured by Tokyo Chemical Industry Co., Ltd.) of 2,2,2-trifluoroethanol was added dropwise, the mixture was stirred at room temperature for 2 hours. It was cooled again with ice, and after 350 mL of a tetrahydrofuran solution of 53.3 g (0.13 mol) of the diethylene glycol bis(p-toluenesulfonate) synthesized earlier was added dropwise, the mixture was stirred under reflux overnight.

[0086] The reaction solution was returned to room temperature, an aqueous ammonium chloride solution (manufactured by Fujifilm Wako Pure Chemical Corporation) was added, and the mixture was stirred at room temperature. The aqueous layer of the separated reaction solution was extracted three times with 100 mL of ethyl acetate. The extracts and the organic layer were combined and washed with water and saturated brine. It was dried over magnesium sulfate, filtered, and concentrated using a rotary evaporator. The obtained crude product was distilled to obtain the target product.

[0087] <Synthesis of G2TFP> It was synthesized in the same manner as G2TFE. Specifically, 45 mL (0.51 mol; manufactured by Tokyo Chemical Industry Co., Ltd.) of 3,3,3-trifluoro-1-propanol was reacted instead of 2,2,2-trifluoroethanol in G2TFE, and the target product was obtained by distillation purification.

[0088] <Synthesis of G3TFM> It was synthesized in the same manner as G2TFM. Specifically, 2.7 g of triethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd.) was reacted instead of diethylene glycol in G2TFM, and the target product was obtained by distillation purification.

[0089] <Synthesis of G3TFE> A magnetic stir bar with a length of 2 cm was placed in a 1 L three-necked flask, 25.1 g (0.17 mol) of triethylene glycol and 125 mL of a dichloromethane solution of 70 mL (0.51 mol) of triethylamine were added, and the mixture was stirred under ice cooling.

[0090] To this, 260 mL of a dichloromethane solution of 70.3 g (0.37 mol) of p-toluenesulfonyl chloride was added dropwise, and the mixture was stirred at room temperature overnight. After adding 2 M HCl and stirring at room temperature, the two layers were separated, and the aqueous layer was extracted twice with 50 mL of dichloromethane (50 mL × 2). The extracts and the organic layer were combined, washed with a 5% aqueous sodium hydrogen carbonate solution, water, and saturated brine (50 mL × 3 each), dried over magnesium sulfate, filtered, and concentrated using a rotary evaporator. The obtained crude product was recrystallized from ethanol, filtered, and dried to obtain triethylene glycol bis(p-toluenesulfonate) as an intermediate.

[0091] Subsequently, a magnetic stir bar with a length of 2 cm was placed in a 1 L three-necked flask, 26.9 g (0.67 mol) of sodium hydride (60%, dispersion in paraffin liquid) was added, and it was washed three times with 50 mL of hexane. 100 mL of tetrahydrofuran was added, and the mixture was stirred under ice cooling. After dropping 100 mL of a THF solution of 48 mL (0.67 mol) of 2,2,2-trifluoroethanol, the mixture was stirred at room temperature for 2 hours.

[0092] It was cooled again, and after dropping 380 mL of a tetrahydrofuran solution of 78.6 g (0.17 mol) of triethylene glycol bis(p-toluenesulfonate), it was refluxed with stirring overnight. The reaction solution was returned to room temperature, 200 mL of an aqueous ammonium chloride solution was added, and after stirring at room temperature, the two layers were separated. The aqueous layer was extracted three times with 100 mL of ethyl acetate, and the extracts and the organic layer were combined and washed with water and saturated brine (100 mL × 3 each). It was dried over magnesium sulfate, filtered, and concentrated using a rotary evaporator. The obtained crude product was purified by distillation to obtain the target product.

[0093] <Synthesis of G3TFP> It was synthesized in the same manner as G3TFE. Instead of 2,2,2-trifluoroethanol in G3TFE, 59 mL (0.67 mol) of 3,3,3-trifluoro-1-propanol was reacted, and the target product was obtained by distillation purification.

[0094] <Synthesis of G4TFM> It was synthesized in the same manner as G2TFM. Instead of diethylene glycol in G2TFM, 3.5 g (0.018 mol; manufactured by Tokyo Chemical Industry Co., Ltd.) of tetraethylene glycol was reacted, and the target product was obtained by distillation purification.

[0095] <Synthesis of G4TFE> A magnetic stir bar with a length of 2 cm was placed in a 1 L three-necked flask, 15.7 g (0.39 mol) of sodium hydride (60% oil dispersion) was added, and it was washed three times with 50 mL of hexane. 60 mL of tetrahydrofuran was added, and it was stirred under ice cooling. After dropping 60 mL of a tetrahydrofuran solution of 28 mL (0.39 mol) of 2,2,2-trifluoroethanol, it was stirred at room temperature for 2 hours.

[0096] It was cooled again, and after dropping 200 mL of a tetrahydrofuran solution containing 50.6 g (0.10 mol; manufactured by Tokyo Chemical Industry Co., Ltd.) of tetraethylene glycol bis(p-toluenesulfonate), it was stirred under reflux overnight. The reaction solution was returned to room temperature, 100 mL of an aqueous ammonium chloride solution was added, and after stirring at room temperature, the separated aqueous layer was extracted 3 times with 100 mL of ethyl acetate. The extract and the organic layer were combined and washed with water and saturated brine (100 mL × 3 each). It was dried over magnesium sulfate, filtered, and concentrated using a rotary evaporator. The obtained crude product was purified by distillation to obtain the target product.

[0097] <Synthesis of G4TFP> It was synthesized in the same manner as G4TFE. Specifically, instead of 2,2,2-trifluoroethanol in G4TFE, 34 mL (0.39 mol) of 3,3,3-trifluoro-1-propanol was reacted, and the target product was obtained by distillation purification.

[0098] <Synthesis of G5TFM> It was synthesized in the same manner as G2TFM. Specifically, instead of diethylene glycol in G2TFM, 4.3 g (0.018 mol; manufactured by Tokyo Chemical Industry Co., Ltd.) of pentaethylene glycol was reacted, and the target product was obtained by distillation purification.

[0099] <Synthesis of 2,2,3,3-tetrafluoro-1,4-dimethoxybutane> A magnetic stir bar with a length of 2 cm was placed in a 1 L three-necked flask, 5.23 g (0.13 mol) of sodium hydride (60% oil dispersion) was added, and it was washed 3 times with 30 mL of hexane. 50 mL of tetrahydrofuran was added, and it was stirred under ice cooling. After dropping 50 mL of a tetrahydrofuran solution containing 21.1 g (0.13 mol) of 2,2,3,3-tetrafluoro-1,4-butanediol, it was stirred at room temperature for 2 hours.

[0100] The mixture was cooled again on ice, and 100 mL of a solution of 2,2,2-trifluoroethyl paratoluenesulfonate (8.47 g, 0.03 mol; Tokyo Chemical Industry Co., Ltd.) in tetrahydrofuran was added dropwise, followed by stirring at reflux overnight. The reaction solution was returned to room temperature, 100 mL of an aqueous solution of ammonium chloride was added, and the mixture was stirred at room temperature. The aqueous layer separated into two layers was extracted three times with 50 mL of ethyl acetate, and the extracted fraction and the organic layer were combined and washed with water and saturated saline (100 mL x 3 each). The mixture was dried over magnesium sulfate, filtered, and concentrated using a rotary evaporator. The resulting crude product was purified by distillation to obtain the target product.

[0101] [Example 1 to Example 36] In Examples 1 to 36, compositions were prepared by mixing various non-aqueous solvents and supporting salts based on Tables 1 to 4.

[0102] The obtained compositions of Examples 1 to 36 were observed to visually confirm the solubility of the supporting salt. In Tables 1 to 4, those in which the supporting salt was not visually confirmed are marked with "◯", and those in which the supporting salt was visually confirmed are marked with "X".

[0103] [Table 1]

[0104] [Table 2]

[0105] [Table 3]

[0106] [Table 4]

[0107] [Example 37 to Example 39] In Examples 37 to 39, compositions were prepared by mixing a combination of G2TFE and G3TFE as a non-aqueous solvent and Mg[Al(HFIP)4]2 as a supporting salt, as shown in Table 5. In Examples 37 to 39, G2TFE and G3TFE were mixed so that the volume ratio of G2TFE / (G2TFE+G3TFE) was 0.1, 0.5, and 0.9, and the concentration of the supporting salt was 0.3 mol / cm3 in all cases. 3 It was confirmed that in the compositions of Examples 37 to 39, the supporting salt was entirely dissolved in the non-aqueous solvent.

[0108] [Example 40~Example 41] In Examples 40 and 41, as shown in Table 5, G2TFE was used as the nonaqueous solvent and Mg[Al(HFIP)4]2 was used as the supporting salt, and the supporting salt concentration was 0.1 mol / dm 3 and 0.5 mol / dm 3 In the compositions of Examples 40 and 41, it was confirmed that the supporting salt was entirely dissolved in the non-aqueous solvent.

[0109] [Example 42] In Example 42, as shown in Table 5, G3TFE was used as the nonaqueous solvent and Mg[Al(HFIP)4]2 was used as the supporting salt, and the supporting salt concentration was 0.5 mol / dm 3 A composition was prepared by mixing the above ingredients so as to obtain the following composition: In the composition of Example 42, it was confirmed that the supporting salt was entirely dissolved in the non-aqueous solvent.

[0110] [Example 43] In Example 43, another composition was prepared by mixing a combination of G2TFE and G3TFE as a non-aqueous solvent and Mg[Al(HFIP)4]2 as a supporting salt, as shown in Table 5. In Example 43, G2TFE and G3TFE were mixed so that the volume ratio of G2TFE / (G2TFE+G3TFE) was 0.5, and the concentration of the supporting salt was 0.5 mol / dm 3 It was confirmed that in the composition of Example 43, the supporting salt was entirely dissolved in the non-aqueous solvent.

[0111] [Table 5]

[0112] Of Examples 1 to 43, the compositions in which the supporting salt was dissolved were used as electrolytes for magnesium secondary batteries to evaluate electrochemical activity. In detail, 0.6 mL of electrolyte was poured into each cell, and a three-electrode beaker cell was assembled using a platinum plate as the working electrode, a magnesium ribbon as the counter electrode, and a silver wire as the reference electrode. The electrochemical dissolution and deposition activity was evaluated by cyclic voltammetry (CV), and the oxidation resistance was evaluated by linear sweep voltammetry (LSV). These results are shown in Figures 2 to 13.

[0113] Next, 0.6 mL of electrolyte was poured into the cell, and a two-electrode cell was assembled using an aluminum (Al) foil current collector and carbon nanofiber (CNF) as the working electrode, and magnesium foil as the counter electrode and reference electrode. A constant-potential polarization test was performed, and the potential-current characteristics were measured. The results are shown in Figures 14 to 17. The state of the working electrode after the constant-potential polarization test was observed with a scanning electron microscope (SEM). The results are shown in Figures 18 and 19.

[0114] The above results will be summarized. As shown in Examples 34 to 36 in Table 4, none of the magnesium salts were dissolved in partially fluorinated ethers such as 2,2,3,3-tetrafluoro-1,4-dimethoxybutane shown in Non-Patent Document 2. Tables 1 to 4 show that a dissolved composition cannot be obtained unless a fluorinated non-aqueous solvent and a magnesium salt are appropriately selected, and as shown in Examples 7, 16, and 25, it was confirmed that oligoethylene glycols substituted at both ends with 2,2,2-trifluoroethyl groups dissolve aluminum-based magnesium salts.

[0115] FIG. 2 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 7. FIG. 3 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 16. FIG. 4 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 38. FIG. 5 shows a CV profile in a three-electrode beaker cell using the electrolyte of Example 39. FIG. 6 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 41. FIG. 7 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 42. FIG. 8 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 43. FIG. 9 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 8. FIG. 10 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 17. FIG. 11 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 13. FIG. 12 is a diagram showing a CV profile in a three-electrode beaker cell using the electrolyte of Example 10.

[0116] As shown representatively in FIG. 2, the potential was changed in the order indicated by the numbers (1) to (6) and the arrows. As shown in FIGS. 2 to 8, when the electrolytes of Examples 7, 16, 38, and 41 to 43 were used, the oxidation-reduction potential of Mg (0 V vs. Mg 2+ When the potential was scanned negatively with respect to the potential of the electrolyte (I) / Mg, a reduction current corresponding to the deposition of Mg was observed. When the potential was reversed and scanned positively, an oxidation current corresponding to the dissolution of Mg was observed. Although not shown, the electrolytes of Examples 25, 37, and 40 also showed similar profiles. This indicates that the electrolytes of Examples 7, 16, and 37 to 43 exhibit magnesium deposition and dissolution activity and function as electrolytes for magnesium secondary batteries.

[0117] On the other hand, as shown in Figs. 9 to 12, when the electrolyte solutions of Examples 8, 10, 13, and 17 were used, the oxidation-reduction potential of Mg (0 V vs. Mg 2+A reduction current was observed even before the potential was scanned toward the noble side for Mg / Mg. This reduction current is due to a side reaction other than magnesium precipitation. Even when the potential was scanned toward the noble side, no oxidation current corresponding to the dissolution of Mg was observed.

[0118] Comparisons between Examples 7, 8, and 10, and between Examples 13, 16, and 17, show that the combination of an oligoethylene glycol substituted at both ends with a 2,2,2-trifluoroethyl group and an aluminum-based magnesium salt is essential.

[0119] In addition, when the electrolytes of Examples 1 to 3, 11, 14, 19, 20, 22, 26, 28, 29, 31, and 32 were used, profiles similar to those in Figures 9 to 12 were observed and no precipitation / dissolution activity was exhibited.

[0120] Referring again to Figures 2 to 8, a comparison between Figures 2 and 3 reveals that an electrolyte having a smaller number of repeating oxyethylene units (repeat number is 2), such as the electrolyte of Example 7, provides a large current density in the deposition and dissolution activity of magnesium, while an electrolyte having a larger number of repeating oxyethylene units (repeat number is 3), such as the electrolyte of Example 16, provides a small overvoltage.

[0121] As shown in Figures 2 and 6, and Figures 3 and 7, it was shown that both a large current density and a small overvoltage can be achieved by changing the molar concentration of the supporting salt in the electrolyte. More specifically, a comparison of the CV profiles of the electrolytes of Examples 7, 40, and 41, and the CV profiles of the electrolytes of Examples 16 and 42 showed that when the molar concentration of the supporting salt was 0.3 mol dm -3 It was found that the precipitation and dissolution activity was most promoted when the electrolyte solutions of Examples 7 and 16, which have a molar concentration of 0.25 molar or less, were used. -3 More than 0.35moldm -3 It was shown that the range is

[0122] On the other hand, as shown in FIG. 4, FIG. 5 and FIG. 8, it was shown that by combining the electrolyte of Example 7 and the electrolyte of Example 16 to obtain the electrolytes of Examples 38 to 39 and Example 43, both a large current density and a small overvoltage can be achieved. More specifically, when the CV profiles of the electrolytes of Examples 37 to 39 were compared, it was found that the electrolyte of Example 38 can achieve both a large current density and a small overvoltage. From this, it was shown that when the electrolytes are used in combination, the volume ratio of G2TFE in the mixture is more preferably in the range of 0.4 to 0.6.

[0123] In addition, when the CV profiles of the electrolytes of Examples 38 and 43 were compared, it was found that the electrolyte of Example 38 could achieve both a higher current density and a lower overvoltage. From this, it is also found that the molar concentration of the supporting salt in the nonaqueous solvent is more preferably 0.25 molar dm -3 More than 0.35moldm -3 It was shown that the range is

[0124] FIG. 13 is a diagram showing LSV profiles in three-electrode beaker cells using the electrolytes of Examples 1, 7, 16 and 38.

[0125] According to Figure 13, 0.1 mA / cm 2 is defined as the oxidation edge (a measure of oxidation resistance) of the electrolyte. It was found that the oxidation resistance of the electrolytes of Examples 7, 16 and 38 was improved by 0.5 V or more compared to that of the electrolyte of Example 1.

[0126] Although not shown, the oxidation resistance of the electrolytes of Examples 2, 3, 8, 10, 11, 13, 14, 17, 19, 20, 22, 26, 28, 29, 31, and 32 was the same as that of the electrolyte of Example 1. On the other hand, the oxidation resistance when the electrolytes of Examples 25, 37, 39 to 43 were used was improved by 0.5 V or more, similar to that of the electrolytes of Examples 7, 16, and 38. From this, it was shown that the electrolyte of the present invention containing an oligoethylene glycol substituted with a 2,2,2-trifluoroethyl group at both ends and an aluminum-based magnesium salt is advantageous in improving oxidation resistance in a magnesium secondary battery.

[0127] FIG. 14 is a graph showing the potential-current response profile in a two-electrode cell using the electrolyte of Example 7 and an Al working electrode. FIG. 15 shows the potential-current response profile in a two-electrode cell using the electrolyte of Example 7 and a CNF working electrode. FIG. 16 is a graph showing the potential-current response profile in a two-electrode cell using the electrolyte of Example 38 and an Al working electrode. FIG. 17 is a graph showing the potential-current response profile in a two-electrode cell using the electrolyte of Example 1 and an Al working electrode.

[0128] 14 to 16, when the electrolytes of Examples 7 and 38 were used, no current response (oxidation current) was observed even when a potential of 3.5 V vs. Mg or more was applied for a long period of time, regardless of the type of working electrode. This shows that the electrolytes of Examples 7 and 38 are stable and do not decompose or corrode the working electrode. Although not shown, the electrolytes of Examples 16, 25, 37, and 39 to 43 also showed similar profiles.

[0129] On the other hand, according to FIG. 17, when the electrolyte of Example 1 was used, an oxidation current was observed when a potential of 3.0 V vs. Mg or more was applied for a long time. In particular, as the potential increased, the oxidation current also increased, causing decomposition of the electrolyte and corrosion of the working electrode. Although not shown, the potential-current response profiles of the electrolytes of Examples 2, 3, 8, 10, 11, 13, 14, 17, 19, 20, 22, 26, 28, 29, 31, and 32 were the same as that of the electrolyte of Example 1, and were poor in stability.

[0130] FIG. 18 is a diagram showing an SEM image of the Al working electrode after an experiment of a two-electrode cell using the electrolyte of Example 7 and the Al working electrode. FIG. 19 is a diagram showing an SEM image of the Al working electrode after an experiment of a two-electrode cell using the electrolyte of Example 1 and the Al working electrode.

[0131] According to Fig. 18, no pitting corrosion was observed on the Al working electrode when the electrolyte of Example 7 was used. On the other hand, according to Fig. 19, many pitting corrosion was observed on the Al working electrode when the electrolyte of Example 1 was used. These results were consistent with the results of Figs. 14 and 17.

[0132] From the above, it has been shown that the electrolyte solution of the present invention containing an oligoethylene glycol having both terminals substituted with 2,2,2-trifluoroethyl groups and an aluminum-based magnesium salt can function as an electrolyte solution for a magnesium secondary battery and has excellent oxidation resistance and non-corrosion properties. [Industrial Applicability]

[0133] The electrolyte for a magnesium secondary battery of the present invention uses a specific nonaqueous solvent to dissolve an aluminum-based magnesium salt, improves oxidation resistance, and is non-corrosive, so that a magnesium secondary battery with excellent characteristics can be provided. [Explanation of symbols]

[0134] 1. Magnesium secondary battery 11 Positive electrode 12 Negative electrode 13 Electrolyte 14 Container

Claims

1. An electrolyte solution for a magnesium secondary battery containing at least a non-aqueous solvent and a supporting salt, the non-aqueous solvent contains an oligoethylene glycol substituted at both ends with 2,2,2-trifluoroethyl groups; The electrolyte for a magnesium secondary battery, wherein the supporting salt is an aluminum-based magnesium salt.

2. The oligoethylene glycol having both ends substituted with 2,2,2-trifluoroethyl groups is represented by R １ (OC ２ H ４ ) ｎ OR １ The electrolyte solution according to claim 1, wherein (wherein, R １ is *-CH ２ CF ３ where n is an integer of 2 or more and 10 or less, and * represents the bonding position.

3. The electrolyte solution according to claim 2 , wherein n is an integer of 2 or more and 5 or less.

4. The electrolyte solution according to claim 3 , wherein n is an integer of 2 or more and 4 or less.

5. The oligoethylene glycol having both ends substituted with 2,2,2-trifluoroethyl groups is represented by R １ (OC ２ H ４ ) ｎ１ OR １ and R １ (OC ２ H ４ ) ｎ２ OR １ The electrolyte of claim 1 , which is a mixture of (wherein, R １ is *-CH ２ CF ３ wherein n1 is 2, n2 is an integer of 3 or more and 10 or less, and * represents a bonding position.

6. The R １ (OC ２ H ４ ) ｎ１ OR １ The electrolyte solution according to claim 5 , wherein the volume ratio of is in the range of 0.01 to 0.

99.

7. The aluminum-based magnesium salt is Mg[Al(OR ２ ) ４ ] ２ The electrolyte solution according to claim 1, wherein (wherein, R ２ each independently represents a monovalent hydrocarbon group having an alkyl group which may be substituted with a halogen atom, R ２ Any two or more of R may be bonded to each other to form a ring. ２ are the same group.)

8. The R ２ The electrolyte solution according to claim 7, wherein the group represented by the formula (I) has 1 or more and 6 or less carbon atoms.

9. The R ２ At least one of the groups represented by *-LC(R ３ ) ｎ (CX ３ ) ３－ｎ The electrolyte solution according to claim 8 , wherein the group is represented by the formula: (wherein, in the formula, L represents a single bond or a divalent hydrocarbon group, R ３ represents a hydrogen atom or a monovalent hydrocarbon group, X represents a halogen atom, n represents an integer of 0 to 3, and * represents a bonding position.

10. The R ２ At least one of the groups represented by *—CH(CX ３ ) ２ The electrolyte solution according to claim 9 , wherein the group is represented by the formula:

11. The aluminum-based magnesium salt is Mg[Al(OCH(CX ３ ) ２ ) ４ ] ２ The electrolyte solution according to claim 10,

12. The electrolyte solution according to claim 9 , wherein X is a fluorine atom.

13. The molar concentration of the supporting salt in the non-aqueous solvent is 0.01 mol dm －３ Over 0.8 moldm －３ 2. The electrolyte solution of claim 1, wherein the electrolyte is in the range of:

14. The molar concentration of the supporting salt in the non-aqueous solvent is 0.05 mol dm －３ More than 0.55 moldm －３ 14. The electrolyte solution of claim 13, wherein the pH is in the range of:

15. A magnesium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, A magnesium secondary battery, wherein the electrolyte solution is the electrolyte solution according to any one of claims 1 to 14.

16. The positive electrode further comprises a current collector, The magnesium secondary battery according to claim 15 , wherein the current collector is selected from the group consisting of aluminum, copper, stainless steel, nickel, and carbon-based materials.

17. The magnesium secondary battery according to claim 16 , wherein the current collector is aluminum.

18. 16. The battery of claim 15, further comprising a separator between the positive electrode and the negative electrode. Magnesium secondary battery.

19. The magnesium secondary battery according to claim 15 , wherein the positive electrode is an air electrode.

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