Non-aqueous electrolyte and magnesium ion secondary battery

Abstract

To provide a non-aqueous electrolyte with excellent storage stability. [Solution] The present invention provides a non-aqueous electrolyte for a magnesium ion secondary battery, comprising an electrolyte represented by the following formula (1) and a non-aqueous solvent, characterized in that the sulfate ion content is 700 ppm by mass or less. Mg[N(FSO2)2]2Y a (1) (In equation (1), Y represents a ligand and a represents a positive number.)

Description

[Technical Field]

[0001] This disclosure relates to a non-aqueous electrolyte and a magnesium ion secondary battery. [Background technology]

[0002] Lithium-ion batteries are known as secondary batteries. A lithium-ion battery is a secondary battery that charges and discharges by the movement of lithium ions between the positive and negative electrodes. On the other hand, next-generation secondary batteries that do not use alkali metals such as lithium (for example, magnesium-ion secondary batteries in which magnesium ions move between the positive and negative electrodes) are being considered. Patent document 1 describes novel bis(fluorosulfonyl)imide compounds, such as magnesium bis(fluorosulfonyl)imide complexes, used as electrolytes in secondary batteries that do not use alkali metals. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2020 / 235336 [Overview of the project] [Problems that the invention aims to solve]

[0004] In secondary batteries equipped with a non-aqueous electrolyte containing a magnesium bis(fluorosulfonyl)imide complex, the present inventors have conducted extensive research and found that when the bis(fluorosulfonyl)imide (anion) decomposes over time, the sulfate ions in the non-aqueous electrolyte increase, leading to a problem of reduced charge-discharge performance of the secondary battery. Therefore, there is a need for a non-aqueous electrolyte that suppresses the decomposition of bis(fluorosulfonyl)imide (anion) and exhibits excellent storage stability. This disclosure aims to provide a non-aqueous electrolyte with excellent storage stability. Furthermore, this disclosure aims to provide a magnesium-ion secondary battery using such a non-aqueous electrolyte. [Means for solving the problem]

[0005] This disclosure provides the non-aqueous electrolyte described in [1] to

[11] below, and the magnesium ion secondary battery described in

[12] . [1] A non-aqueous electrolyte for a magnesium ion secondary battery comprising an electrolyte represented by the following formula (1) and a non-aqueous solvent, characterized in that the sulfate ion content is 700 ppm by mass or less. Mg[N(FSO2)2]2Y a (1) (In equation (1), Y represents a ligand and a represents a positive number.) [2] The non-aqueous electrolyte according to [1], wherein the sulfate ion content is 50 ppm by mass or less. [3] The non-aqueous electrolyte according to [1] or [2], wherein the content of amide sulfate ions is 200 ppm by mass or less. [4] The non-aqueous electrolyte according to [3], wherein the content of amide sulfate ions is 50 ppm by mass or less. [5] A non-aqueous electrolyte according to any one of [1] to [4], wherein the fluoride ion content is 500 ppm by mass or less. [6] A non-aqueous electrolyte according to any one of [1] to [5], wherein the water content is 30 ppm by mass or less. [7] A non-aqueous electrolyte according to any one of [1] to [6], wherein the ligand consists of a molecule of a coordinating solvent, and the coordinating solvent comprises at least one selected from the group consisting of nitrile solvents, carbonate solvents, ether solvents, ester solvents, sulfone solvents, and amide solvents. [8] The non-aqueous electrolyte according to [7], comprising at least one coordinating solvent selected from the group consisting of nitrile solvents and ether solvents. [9] The non-aqueous electrolyte according to [7], comprising at least one coordinating solvent selected from the group consisting of aliphatic nitrile solvents and linear ether solvents.

[10] The non-aqueous electrolyte according to [9], wherein the aliphatic nitrile solvent is acetonitrile.

[11] A non-aqueous electrolyte according to any one of [1] to

[10] , wherein the non-aqueous solvent comprises at least one selected from the group consisting of nitrile solvents, carbonate solvents, ether solvents, ester solvents, sulfone solvents, and amide solvents.

[12] A magnesium ion secondary battery comprising a non-aqueous electrolyte as described in any one of [1] to

[11] above. [Effects of the Invention]

[0006] According to this disclosure, it is possible to provide a non-aqueous electrolyte with excellent storage stability. Furthermore, according to this disclosure, it is possible to provide a magnesium-ion secondary battery using such a non-aqueous electrolyte. [Modes for carrying out the invention]

[0007] Embodiments of the present disclosure are described below. However, the present disclosure is not limited to the embodiments described below. The upper or lower limits of the numerical ranges expressed in the present disclosure may be replaced with any of the values ​​shown in the examples. In addition, the upper and lower limits described individually may be combined in any way. Unless otherwise specified, the materials or components exemplified in the present disclosure may be used individually or in combination of two or more.

[0008] A non-aqueous electrolyte according to one embodiment is a non-aqueous electrolyte for a magnesium ion secondary battery comprising an electrolyte represented by the following formula (1) and a non-aqueous solvent, characterized in that the sulfate ion content is 700 ppm by mass or less. Mg[N(FSO2)2]2Y a (1) (In equation (1), Y represents a ligand and a represents a positive number.)

[0009] The electrolyte shown in equation (1) is Mg 2+ The ions are two [N(FSO2)2] - This is a complex compound having Mg[N(FSO2)2]2 ion-bonded to an ion, and ligand Y. Here, [N(FSO2)2] -is an anion called bis(fluorosulfonyl)imide (anion), and is sometimes called the FSI anion. Also, Mg[N(FSO2)2]2 is sometimes called Mg(FSI)2.

[0010] In a non-aqueous electrolyte containing the electrolyte shown in formula (1), when the FSI anion contained in the non-aqueous electrolyte decomposes, sulfate ions are generated as decomposition products. Since sulfate ions are an acid component, the sulfate ions generated by the decomposition of the FSI anion promote the decomposition of the FSI anion. Thus, when the FSI anion decomposes and the sulfate ions increase in the non-aqueous electrolyte, the charge-discharge performance of the secondary battery deteriorates. Further, when the decomposition of the FSI anion further proceeds and the content of sulfate ions reaches the saturation concentration, the sulfate ions combine with magnesium ions and precipitate in the non-aqueous electrolyte as magnesium sulfate crystals. The non-aqueous electrolyte according to the present embodiment can suppress the decomposition of the FSI anion during storage of the non-aqueous electrolyte by reducing the content of sulfate ions. Therefore, the storage stability of the non-aqueous electrolyte can be improved.

[0011] From the viewpoint of suppressing the deterioration of the charge-discharge performance of the secondary battery and further improving the storage stability, the content of sulfate ions may be 400 mass ppm or less, or 350 mass ppm or less. From the viewpoint of further improving the storage stability of the non-aqueous electrolyte, the content of sulfate ions may be 50 mass ppm or less, or 40 mass ppm or less. The content of sulfate ions may be, for example, 1 mass ppm or more, 5 mass ppm or more, 10 mass ppm or more, or 15 mass ppm or more.

[0012] The upper and lower limits of the content of sulfate ions may be arbitrarily combined. For example, it may be 1 mass ppm or more and 700 mass ppm or less, 5 mass ppm or more and 400 mass ppm or less, 10 mass ppm or more and 350 mass ppm or less, 15 mass ppm or more and 50 mass ppm or less, or 15 mass ppm or more and 40 mass ppm or less.

[0013] In a non-aqueous electrolyte, the content of amide sulfate ions may be 200 ppm by mass or less, 150 ppm by mass or less, or 100 ppm by mass or less. Amidosulfate ions are acidic components produced by the decomposition of FSI anions and promote the decomposition of FSI anions. On the other hand, amide sulfate ions are decomposed into sulfate ions. Therefore, by keeping the content of amide sulfate ions within the above range, the decomposition of FSI anions can be further suppressed, and the storage stability of the non-aqueous electrolyte can be further improved. From the viewpoint of further improving the storage stability of the non-aqueous electrolyte, the content of amide sulfate ions may be 50 ppm by mass or less. The content of amide sulfate ions may be, for example, 1 ppm by mass or more, or 5 ppm by mass or more.

[0014] The upper and lower limits of the amide sulfate ion content may be any combination, for example, 1 ppm to 200 ppm by mass, 1 ppm to 150 ppm by mass, 5 ppm to 100 ppm by mass, or 5 ppm to 50 ppm by mass.

[0015] In a non-aqueous electrolyte, the fluoride ion content may be 500 ppm by mass or less, or 300 ppm by mass or less. Fluoride ions are acidic components produced by the decomposition of FSI anions and promote the decomposition of FSI anions. Therefore, by keeping the fluoride ion content within the above range, the decomposition of FSI anions can be further suppressed, thereby improving the storage stability of the non-aqueous electrolyte. From the viewpoint of further improving the storage stability of the non-aqueous electrolyte, the fluoride ion content may be 100 ppm by mass or less, or 50 ppm by mass or less. The fluoride ion content may be, for example, 1 ppm by mass or more, 5 ppm by mass or more, 10 ppm by mass or more, or 15 ppm by mass or more.

[0016] The upper and lower limits of the fluoride ion content may be any combination, for example, 1 ppm to 500 ppm by mass, 5 ppm to 300 ppm by mass, 10 ppm to 100 ppm by mass, or 15 ppm to 50 ppm by mass.

[0017] Sulfate ions, amidosulfate ions, and fluoride ions in the non-aqueous electrolyte can be measured, for example, by ion chromatography.

[0018] In the non-aqueous electrolyte, the water content may be 30 mass ppm or less, or 20 mass ppm or less. The FSI anion is hydrolyzed by water into decomposition products such as sulfate ions, amidosulfate ions, and fluoride ions. Therefore, by keeping the water content within the above range, the decomposition of the FSI anion can be further suppressed and the storage stability of the non-aqueous electrolyte can be further improved. The water content may be, for example, 1 mass ppm or more, or 5 mass ppm or more. The water content contained in the non-aqueous electrolyte can be determined, for example, using a Karl Fischer moisture measuring device.

[0019] The upper and lower limits of the water content may be arbitrarily combined. For example, it may be 1 mass ppm or more and 30 mass ppm or less, or 5 mass ppm or more and 20 mass ppm or less.

[0020] An example of the reaction mechanism in which the FSI anion decomposes into sulfate ions, amidosulfate ions, and fluoride ions due to the hydrolysis reaction of Mg(FSI)2 is shown in the following reaction formulas (2) to (5). The following reaction formulas are examples, and the decomposition of the FSI anion may occur due to reactions other than the following reaction formulas (2) to (5). In the following formulas (2) to (5), HSO4 - represents sulfate ions. Also, H2NSO3H represents amidosulfuric acid, and HF represents hydrogen fluoride. Mg[N(FSO2)2]2 + 2H2O → 2FSO2NH2 + Mg(FSO3)2 (2) Mg(FSO3)2 + 2H2O → Mg 2+ + 2HSO4 - + 2HF (3) FSO2NH3H + H2O → H2NSO3H + HF (4) H2NSO3H ++ H2O → NH4 + + HSO4 - (5)

[0021] Ligand Y consists of a molecule of a coordinating solvent, and the coordinating solvent may include at least one selected from the group consisting of nitrile solvents, carbonate solvents, ether solvents, ester solvents, sulfone solvents, and amide solvents.

[0022] As a nitrile solvent, an aliphatic nitrile solvent [for example, cyanoalkanes (e.g., mono or dicyanoalkanes) such as acetonitrile, propionitrile, butyronitrile, isobutyronitrile, valeronitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, preferably mono or dicyanoC] 1-4 Examples include alkanes, aromatic nitriles (e.g., benzonitrile, tolunitrile), etc.

[0023] Examples of carbonate-based solvents include linear carbonates [for example, dialkyl carbonates (e.g., dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc.)]. 1-4 Alkyl carbonate, preferably diC 1-2 Alkyl carbonates), alkylaryl carbonates (e.g., C120 1-4 Alkylphenyl carbonates, diaryl carbonates (e.g., diphenyl carbonate), cyclic carbonates {e.g., saturated cyclic carbonates [e.g., alkylene carbonates such as ethylene carbonate, propylene carbonate, 2,3-dimethylethylene carbonate, 1,2-butylene carbonate (e.g., C 2-6 Alkylene carbonate, preferably C 2-4 Examples include alkylene carbonates, erythritol carbonate, etc., unsaturated cyclic carbonates (e.g., alkenylene carbonates such as vinylene carbonate, methylvinylene carbonate, and ethylvinylene carbonate; 2-vinyl ethylene carbonate), fluorine-containing cyclic carbonates (e.g., fluoroethylene carbonate, 4,5-difluoroethylene carbonate, trifluoropropylene carbonate), etc.

[0024] Examples of ether-based solvents include linear ethers {for example, linear aliphatic ethers [for example, alkanediolic dialkyl ethers (for example, 1,2-dimethoxyethane (ethylene glycol dimethyl ether), ethylene glycol diethyl ether, etc. C 2-4 Alkandiol C 1-4 Alkyl ethers), polyalkanediol dialkyl ethers (for example, diethylene glycol methyl ether (diglyme), triethylene glycol dimethyl ether (tridiglyme), tetraethylene glycol dimethyl ether (tetradiglyme), etc., di to tetra C 2-4 Alkandiol C 1-4 Examples include alkyl ethers, cyclic ethers, tetrahydrofurans (e.g., tetrahydrofuran, 2-methyltetrahydrofuran, 2,6-dimethyltetrahydrofuran), tetrahydropyrans (e.g., tetrahydropyran), dioxanes (e.g., 1,4-dioxane), dioxolanes (e.g., 1,3-dioxolane), crown ethers, etc.).

[0025] Examples of ester solvents include linear esters {for example, aliphatic esters [for example, methyl acetate, ethyl acetate, ethyl propionate, propyl propionate, and other alkanate esters (e.g., C 1-5 Alkanic Acid C 1-4 Examples include alkyl esters, aromatic esters (e.g., methyl benzoate, ethyl benzoate), cyclic esters, or lactones (e.g., γ-butyrolactone, valerolactone (γ-valerolactone, δ-valerolactone, etc.)).

[0026] Examples of sulfone-based solvents include linear sulfones (or linear sulfone-based solvents, such as dialkyl sulfones like dimethyl sulfone, ethylmethyl sulfone, and diethyl sulfone), and cyclic sulfones [for example, sulfolanes (or sulfolane-based solvents, such as sulfolane, 3-methylsulfolane, and 2,4-dimethylsulfolane)].

[0027] Examples of amide solvents include linear amides [e.g., linear aliphatic amides (e.g., alkanate amides such as dimethylformamide and dimethylacetamide)], cyclic amides (or lactams, e.g., N-methylpyrrolidone (NMP)), etc.

[0028] From the viewpoint of improving the stability of the electrolyte, the coordinating solvent may include at least one selected from the group consisting of nitrile solvents and ether solvents, and from the viewpoint of further improving the stability of the electrolyte, it may include at least one selected from the group consisting of aliphatic nitrile solvents and linear ether solvents. From the viewpoint of improving the safety and handling of the electrolyte, the aliphatic nitrile solvent may be acetonitrile. In addition, the linear ether solvent may be at least one selected from the group consisting of 1,2-dimethoxyethane, diglyme, triglyme, and tetraglyme, and may be triglyme.

[0029] The above-mentioned coordinating solvents may be included in the electrolyte shown in formula (1) either alone or in combination of two or more.

[0030] The non-aqueous solvent may include at least one selected from the group consisting of nitrile solvents, carbonate solvents, ether solvents, ester solvents, sulfone solvents, and amide solvents, and may also include at least one selected from the group consisting of aliphatic nitrile solvents and linear ether solvents, from the viewpoint of further improving the stability of the electrolyte. The non-aqueous solvent refers to the solvent for dissolving the electrolyte shown in formula (1). The non-aqueous solvent may be the same solvent as the coordinating solvent constituting ligand Y, or it may be a different solvent. The types of non-aqueous solvents can be the same as those listed in the description of the coordinating solvent above.

[0031] The combination of the coordinating solvent constituting ligand Y and the non-aqueous solvent can be determined as appropriate. For example, the coordinating solvent constituting ligand Y may include at least one selected from the group consisting of nitrile solvents and ether solvents, and the non-aqueous solvent may include at least one selected from the group consisting of nitrile solvents and ether solvents. Alternatively, the coordinating solvent constituting ligand Y may include at least one selected from the group consisting of aliphatic nitrile solvents and linear ether solvents, and the non-aqueous solvent may include at least one selected from the group consisting of aliphatic nitrile solvents and linear ether solvents. Furthermore, the coordinating solvent constituting ligand Y may be acetonitrile, 1,2-dimethoxyethane, diglyme, triglyme, or tetraglyme, and the non-aqueous solvent may be acetonitrile, 1,2-dimethoxyethane, diglyme, triglyme, or tetraglyme. Alternatively, the coordinating solvent constituting ligand Y may be acetonitrile or triglyme, and the non-aqueous solvent may be acetonitrile or triglyme.

[0032] The electrolyte shown in formula (1) can be obtained as crystals by a recrystallization process in which the magnesium salt of bis(fluorosulfonyl)imide is recrystallized in the presence of a coordinating solvent. The electrolyte obtained after recrystallization has the coordinating solvent as ligand Y.

[0033] Magnesium salts of bis(fluorosulfonyl)imide can be synthesized by known methods. By performing a recrystallization step, the crystals of the magnesium salt of bis(fluorosulfonyl)imide can be purified, and impurities such as sulfate ions contained in the crystals can be reduced. After the recrystallization step, a non-aqueous electrolyte can be prepared by performing a dissolution step in which the obtained electrolyte crystals are dissolved in the non-aqueous solvent described above to obtain a solution. If a solvent different from the coordinating solvent used in the recrystallization step is used as the non-aqueous solvent, some or all of the coordinating solvent in ligand Y may be replaced by the non-aqueous solvent.

[0034] The electrolyte concentration in the non-aqueous electrolyte may be 0.2 mol / L or more and 5.0 mol / L or less, 0.25 mol / L or more and 1.0 mol / L or less, or 0.3 mol / L or more and 0.7 mol / L or less.

[0035] The recrystallization step may be, for example, a method of precipitation by cooling a coordinating solvent containing the magnesium salt of bis(fluorosulfonyl)imide. Before cooling, the coordinating solvent may be removed by heating and vacuum concentration, etc., to concentrate the magnesium salt of bis(fluorosulfonyl)imide.

[0036] In the recrystallization process, recrystallization may be performed two or more times in the presence of a coordinating solvent. By performing recrystallization two or more times, impurities such as sulfate ions contained in the electrolyte crystals can be further reduced. Therefore, the non-aqueous electrolyte obtained by dissolving the electrolyte crystals obtained by recrystallization two or more times in a non-aqueous solvent has further improved storage stability. From the viewpoint of further improving the storage stability of the non-aqueous electrolyte, recrystallization may be performed, for example, two or more times, three or more times, or four or more times. Alternatively, recrystallization may be performed seven or fewer times, or six or fewer times.

[0037] As a procedure for performing recrystallization twice, for example, the crystal obtained in the first recrystallization step may be used as the first crystal, and the first crystal may be dissolved in a coordinating solvent and recrystallized in the presence of the coordinating solvent to obtain the second crystal in a second recrystallization step. In this case, the first recrystallization step becomes the first recrystallization step. The second recrystallization step can be performed in the same manner as the first recrystallization step. The coordinating solvent used in the second recrystallization step may be the same solvent as the coordinating solvent used in the first recrystallization step, or it may be a different solvent. If the coordinating solvent used in the first recrystallization step is designated as the first coordinating solvent, and the coordinating solvent used in the second recrystallization step is a second coordinating solvent different from the first coordinating solvent, then some or all of the first coordinating solvent in ligand Y contained in the second crystal may be replaced by the second coordinating solvent. By performing the second recrystallization step, impurities such as sulfate ions contained in the second crystal can be further reduced. Therefore, the non-aqueous electrolyte obtained by dissolving the second crystal in a non-aqueous solvent exhibits even greater storage stability.

[0038] After the second recrystallization step, a third recrystallization step may be performed using the same procedure as the second recrystallization step. After the third recrystallization step, a fourth recrystallization step may be performed, and the recrystallization steps may be repeated more times. In this way, recrystallization can be performed two or more times in the recrystallization process.

[0039] After the dissolution step, a dehydration step may be performed to dehydrate the non-aqueous electrolyte. The dehydration step may be carried out by contacting the non-aqueous electrolyte with a desiccant. As a desiccant, for example, an alumina silicate (zeolite) such as molecular sieves can be used. By performing the dehydration step, the water content in the non-aqueous electrolyte can be reduced. This further suppresses the decomposition of FSI anions in the non-aqueous electrolyte and further improves storage stability. Other methods may be used to dehydrate the non-aqueous electrolyte; for example, it may be dehydrated by distillation.

[0040] A method for producing a non-aqueous electrolyte for a magnesium ion secondary battery according to one embodiment may be any one of the following

[13] to

[15] .

[13] A recrystallization step to obtain crystals of the electrolyte shown in formula (1) below by recrystallizing the magnesium salt of bis(fluorosulfonyl)imide in the presence of a coordinating solvent, A method for producing a non-aqueous electrolyte for a magnesium ion secondary battery, comprising a dissolution step of dissolving the electrolyte crystals in a non-aqueous solvent to obtain a solution. Mg[N(FSO2)2]2Y a (1) (In equation (1), Y represents a ligand and a represents a positive number.)

[14] A method for producing a non-aqueous electrolyte for a magnesium ion secondary battery according to

[13] , wherein the recrystallization step is performed two or more times in the presence of a coordinating solvent.

[15] A method for producing a non-aqueous electrolyte for a magnesium ion secondary battery according to

[13] or

[14] , comprising a dehydration step of dehydrating the solution after the dissolution step.

[0041] A magnesium-ion secondary battery according to one embodiment comprises the above-described non-aqueous electrolyte. Because such a magnesium-ion secondary battery comprises the above-described non-aqueous electrolyte, the decomposition of FSI anions in the non-aqueous electrolyte is suppressed, resulting in excellent storage stability. Furthermore, the charge and discharge performance of the magnesium-ion secondary battery can be maintained even when stored for a long period of time. [Examples]

[0042] The present disclosure will be described in more detail below with reference to examples and comparative examples. However, the present disclosure is not limited to the examples.

[0043] (Comparative Example 1) [Synthesis of unrefined product (unrefined Mg(FSI)2(MeCN))] In a 500 mL reaction vessel equipped with a stirrer, thermometer, and dropper, 61.97 g (650.9 mmol, 1.1 eq.) of magnesium chloride (MgCl2) and 1 L (777 g, water content 7.8 ppm) of super-dehydrated acetonitrile (MeCN) were added under a nitrogen stream and cooled to 4°C in an ice bath. Then, 217.93 g (1183.7 mmol, 1.0 eq.) of bis(fluorosulfonyl)imide (HFSI) was added dropwise from the dropper over 1 hour to obtain the reaction mixture. After filtering off the white solid using a Kiriyama funnel, the resulting colorless, transparent solution was concentrated under reduced pressure using an evaporator (55°C, 70 hPa) to obtain the unpurified electrolyte product. The crystals thus obtained are crystals of the electrolyte coordinated with acetonitrile.

[0044] The obtained unpurified product was dissolved in triethylene glycol dimethyl ether (G3) (manufactured by Kanto Chemical Co., Ltd.: for electrochemistry), a non-aqueous solvent, to prepare a non-aqueous electrolyte with a Mg(FSI)2 concentration of 0.5 mol / L. The obtained non-aqueous electrolyte was passed through a column packed with molecular sieves (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: 3A) to obtain the non-aqueous electrolyte of Comparative Example 1, which had a reduced water content.

[0045] (Example 1) [Preparation of refined product A] To 309.23 g of unrefined product, 168.53 g of super-dehydrated acetonitrile (MeCN) (water content 7.8 ppm by mass) was added, and after heating to 50°C, it was cooled overnight in a -10°C freezer to obtain 213.44 g of refined product A, which was a white crystalline product.

[0046] The purified product A obtained was dissolved in triethylene glycol dimethyl ether (G3), a non-aqueous solvent, to prepare a non-aqueous electrolyte with a Mg(FSI)2 concentration of 0.5 mol / L. The obtained non-aqueous electrolyte was passed through a column packed with molecular sieves to obtain the non-aqueous electrolyte of Example 1 with reduced water content.

[0047] (Example 2) [Preparation of refined product B] To 180.23 g of purified product A, 70.74 g of super-dehydrated acetonitrile (MeCN) (water content 7.8 ppm by mass) was added, the mixture was heated to 50°C, and then cooled overnight in a -10°C freezer to obtain 82.32 g of purified product B, which was a white crystalline product.

[0048] The purified product B obtained was dissolved in triethylene glycol dimethyl ether (G3), a non-aqueous solvent, to prepare a non-aqueous electrolyte with a Mg(FSI)2 concentration of 0.5 mol / L. The obtained non-aqueous electrolyte was passed through a column packed with molecular sieves to obtain the non-aqueous electrolyte of Example 2, in which the water content was reduced.

[0049] (Example 3) [Preparation of refined product C] After recovering purified product B by filtration, the filtrate was concentrated under reduced pressure using an evaporator (55°C, 70 hPa), and then cooled overnight in a -10°C freezer to obtain 79.65 g of purified product C, which was a white crystalline product.

[0050] The purified product C obtained was dissolved in triethylene glycol dimethyl ether (G3), a non-aqueous solvent, to prepare a non-aqueous electrolyte with a Mg(FSI)2 concentration of 0.5 mol / L. The obtained non-aqueous electrolyte was passed through a column packed with molecular sieves to obtain the non-aqueous electrolyte of Example 3, in which the water content was reduced.

[0051] (Example 4) A non-aqueous electrolyte with a Mg(FSI)2 concentration of 0.5 mol / L was prepared by dissolving purified product B in super-dehydrated acetonitrile, a non-aqueous solvent. The obtained non-aqueous electrolyte was passed through a column packed with molecular sieves to obtain the non-aqueous electrolyte of Example 4, which had a reduced water content.

[0052] (Example 5) A non-aqueous electrolyte with a Mg(FSI)2 concentration of 0.5 mol / L was prepared by dissolving purified product B in propylene carbonate, a non-aqueous solvent. The obtained non-aqueous electrolyte was passed through a column packed with molecular sieves to obtain the non-aqueous electrolyte of Example 5, which had a reduced water content.

[0053] (Example 6) A non-aqueous electrolyte with a Mg(FSI)2 concentration of 0.5 mol / L was prepared by dissolving purified product B in N-methylpyrrolidone, a non-aqueous solvent. The obtained non-aqueous electrolyte was passed through a column packed with molecular sieves to obtain the non-aqueous electrolyte of Example 6, which had a reduced water content.

[0054] <Storage Stability Test> The non-aqueous electrolytes of Comparative Example 1 and Examples 1-6 were stored at 25°C in sealed containers made of PFA (fluoropolymer). The sulfate ions, amide sulfate ions, fluoride ions, and water content in the non-aqueous electrolytes were measured at the start of storage and after 1 and 3 weeks of storage using the following procedure.

[0055] <Measurement of sulfate ion, amide sulfate ion, and fluoride ion content> The content of sulfate ions, amide sulfate ions, and fluoride ions in the non-aqueous electrolyte at the start of storage and after 1 and 3 weeks of storage in the storage stability test was measured by ion chromatography. Specifically, the non-aqueous electrolyte was diluted 100-fold with ultrapure water (greater than 18.2 Ω·cm) to prepare the measurement solution, and the content of sulfate ions, amide sulfate ions, and fluoride ions in the electrolyte was measured using an ion chromatography system (Nippon Dionex Co., Ltd.: ICS-3000). The measurement conditions were as follows. • Separation mode: Ion exchange • Eluent: 7-18 mM KOH aqueous solution • Detector: Electrical conductivity detector • Column: IonPAC AS-17C anion analysis column (manufactured by Nippon Dionex Co., Ltd.)

[0056] Table 1 shows the measurement results for sulfate ions, Table 2 shows the measurement results for amide sulfate ions, and Table 3 shows the measurement results for fluoride ions.

[0057] <Measurement of water content> The water content in the non-aqueous electrolyte at the start of storage and after 1 and 3 weeks of storage in the storage stability test was measured using a Karl Fischer moisture analyzer (Hiranuma Sangyo Co., Ltd.: AQ-2000). Specifically, Aqualight RS-A (Hiranuma Sangyo Co., Ltd.) was used as the generating solution and Aqualight CN (Hiranuma Sangyo Co., Ltd.) was used as the counter electrode solution, and the water content in each non-aqueous electrolyte was measured. The measurement results for water content are shown in Table 4. Note that in Tables 1 to 4, each measurement result at the start of storage is shown as representing a storage period of 0 weeks.

[0058] [Table 1]

[0059] [Table 2]

[0060] [Table 3]

[0061] [Table 4]

[0062] As shown in Table 1, Comparative Example 1's sulfate ion content exceeded 700 ppm by mass from the start of storage until 3 weeks had elapsed. Furthermore, white crystals precipitated in the non-aqueous electrolyte of Comparative Example 1 after 3 weeks of storage. Therefore, it is considered that the sulfate ions in Comparative Example 1 reached a saturated state, and magnesium sulfate crystals precipitated. Thus, the non-aqueous electrolyte of Comparative Example 1 showed poor storage stability. On the other hand, the non-aqueous electrolytes of Examples 1 to 6 had lower sulfate ion content than Comparative Example 1, and no white crystal precipitation was observed after 3 weeks of storage, indicating superior storage stability.

Claims

1. A non-aqueous electrolyte for a magnesium ion secondary battery, comprising an electrolyte represented by the following formula (1) and a non-aqueous solvent, A non-aqueous electrolyte characterized by having a sulfate ion content of 700 ppm by mass or less. [#][##] ２ ) ２ ] ２ ﹹ ａ () (In equation (1), Y represents a ligand, and a represents a positive number.)

2. The non-aqueous electrolyte according to claim 1, wherein the sulfate ion content is 50 ppm by mass or less.

3. The non-aqueous electrolyte according to claim 1 or 2, wherein the content of amide sulfate ions is 200 ppm by mass or less.

4. The non-aqueous electrolyte according to claim 3, wherein the content of the amide sulfate ion is 50 ppm by mass or less.

5. The non-aqueous electrolyte according to claim 1 or 2, wherein the fluoride ion content is 500 ppm by mass or less.

6. The non-aqueous electrolyte according to claim 1 or 2, wherein the water content is 30 ppm by mass or less.

7. The ligand consists of molecules of a coordinating solvent. The non-aqueous electrolyte according to claim 1 or 2, wherein the coordinating solvent comprises at least one selected from the group consisting of nitrile solvents, carbonate solvents, ether solvents, ester solvents, sulfone solvents, and amide solvents.

8. The non-aqueous electrolyte according to claim 7, wherein the coordinating solvent comprises at least one selected from the group consisting of nitrile solvents and ether solvents.

9. The non-aqueous electrolyte according to claim 7, wherein the coordinating solvent comprises at least one selected from the group consisting of aliphatic nitrile solvents and linear ether solvents.

10. The non-aqueous electrolyte according to claim 9, wherein the aliphatic nitrile solvent is acetonitrile.

11. The non-aqueous electrolyte according to claim 1 or 2, wherein the non-aqueous solvent comprises at least one selected from the group consisting of nitrile solvents, carbonate solvents, ether solvents, ester solvents, sulfone solvents, and amide solvents.

12. A magnesium ion secondary battery comprising the non-aqueous electrolyte according to claim 1 or 2.

Images