Method for storing nonaqueous electrolytic solution

Abstract

To provide a storage method suitable for a nonaqueous electrolyte of low acidity.SOLUTION: A method for storing a non-aqueous electrolytic solution having an acidity of less than 1meq / kg, the method comprising storing the non-aqueous electrolytic solution in a sealed vessel at a temperature of 20 °C. or less, wherein the non-aqueous electrolytic solution has a water content of less than 10 ppm by mass, a lithium-based electrolyte content of 0.5 to 1.5 mol / L, and a lithium salt electrolyte-containing liquid obtained by dissolving a lithium-based electrolyte in a carbonate ester is treated by being brought into contact with a weakly basic anion-exchange resin.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for storing a non-aqueous electrolyte having a low acidity, particularly an acidity of less than 1 meq / kg. [Background technology]

[0002] In lithium ion batteries, a non-aqueous electrolyte solution is used in which a lithium-based electrolyte such as lithium hexafluorophosphate (LiPF6) is dissolved in an organic non-aqueous solvent. However, trace amounts of water remain in the solvent and lithium-based electrolyte constituting the electrolytic solution, and this water reacts with the lithium-based electrolyte such as LiPF to produce hydrogen fluoride (HF) and the like, for example, as shown in the following reaction formulas (1) to (3). (1) LiPF6 ⇔ LiF + PF5 (2) PF5 + H2O ⇒ POF3 + 2HF (3) POF3 + H2O ⇒ POF2(OH) + HF

[0003] When acidic impurities such as hydrogen fluoride (hydrofluoric acid) are present in the non-aqueous electrolyte, the battery capacity and charge / discharge cycle characteristics of the lithium ion battery decrease, and corrosion inside the battery becomes more likely to occur (see Patent Document 1, etc.). Therefore, non-aqueous electrolytes are generally stored in sealed containers to prevent water from entering the electrolyte. For example, Non-Patent Document 1 discloses that a non-aqueous electrolyte solution can maintain stable quality for about 4 to 12 weeks at 40° C. when stored in a sealed container made of stainless steel or aluminum. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-71111 [Non-patent literature]

[0005] [Non-Patent Document 1] Journal of Power Sources 81-82 (1999) 119-122 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the non-aqueous electrolyte discussed in Non-Patent Document 1 has a high acidity of about 2 to 4 meq / kg, and no consideration is given to the sealed storage of a non-aqueous electrolyte with a low acidity, particularly an acidity of less than 1 meq / kg. According to the investigations of the present inventors, it has been found that when a high-quality non-aqueous electrolyte solution with low acidity is sealed and stored using a general method such as that discussed in Non-Patent Document 1, the acidity increases over time even without the inclusion of moisture. Therefore, an object of the present invention is to provide a storage method suitable for a non-aqueous electrolyte with low acidity. [Means for solving the problem]

[0007] In view of the above problems, the present inventors have conducted extensive research and discovered that the quality of a non-aqueous electrolyte solution with low acidity can be maintained by storing the non-aqueous electrolyte solution at or below a certain temperature, leading to the completion of the present invention. That is, the present invention relates to the following [1] to [6]. [1] A method for storing a non-aqueous electrolyte, comprising storing the non-aqueous electrolyte having an acidity of less than 1 meq / kg in a sealed container at 20°C or below. [2] The method for storing a non-aqueous electrolyte according to [1] above, wherein the water content of the non-aqueous electrolyte is less than 10 ppm by mass. [3] The method for storing a non-aqueous electrolyte according to [1] or [2], wherein the non-aqueous electrolyte contains 0.5 to 1.5 mol / L of LiPF6. [4] The method for storing a non-aqueous electrolyte according to any one of [1] to [3], wherein the non-aqueous electrolyte is produced by contacting a lithium salt electrolyte-containing solution in which LiPF6 is dispersed in a carbonate ester with a weakly basic anion exchange resin. [5] The method for storing a non-aqueous electrolyte according to any one of [1] to [4] above, wherein the non-aqueous electrolyte contains 0.1 to 50 mass % of an organic fluorine compound. [6] The method for storing a non-aqueous electrolyte according to any one of [1] to [5] above, wherein the non-aqueous electrolyte contains 0.1 to 10 mass % of an organic sulfur compound. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method for storing a non-aqueous electrolyte that can maintain the acidity of the non-aqueous electrolyte at a low level. [Brief explanation of the drawings]

[0009] [Figure 1] 1A and 1B are diagrams showing an example of a method for storing a non-aqueous electrolyte according to the present invention. [Figure 2] FIG. 2 is a diagram showing the configuration of a nonaqueous electrolyte solution manufacturing apparatus in a manufacturing example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in detail below based on preferred embodiments. The present inventors have been studying methods for removing acidic impurities such as hydrogen fluoride from non-aqueous electrolyte solutions, and as a result have succeeded in providing non-aqueous electrolyte solutions with extremely low content of acidic impurities such as hydrogen fluoride, i.e., low acidity, in which the acidity has been reduced to the utmost limit. The present invention solves a new problem that did not exist when conventional high-acidity nonaqueous electrolytes were stored in sealed containers; that is, when a low-acidity nonaqueous electrolyte with acidity reduced to the minimum level is stored in a sealed container, the acidity increases over time when the low-acidity nonaqueous electrolyte is stored at room temperature (25°C), even if no water is mixed into the container. Therefore, the method for storing a non-aqueous electrolyte of the present invention is intended for non-aqueous electrolytes with low acidity of less than 1 meq / kg. In the present invention, a non-aqueous electrolyte solution with an acidity of less than 1 meq / kg is stored at 20°C or below to prevent an increase in acidity over time. On the other hand, if the temperature exceeds 20°C, particularly above room temperature (25°C), the acidity of the non-aqueous electrolyte solution increases over time, and the effects of the present invention cannot be obtained. There is no particular lower limit for the storage temperature, but it is usually preferable to set it to 0°C or above in order to prevent solidification of the electrolyte solution and precipitation of the electrolyte. Furthermore, the storage method of the present invention is effective only for nonaqueous electrolytes with an acidity of less than 1 meq / kg. For nonaqueous electrolytes with a high acidity, for example, about 2 to 4 meq / kg, even if the nonaqueous electrolyte is stored at a temperature below 20°C or above 20°C, such as at room temperature (25°C) or about 40°C, the acidity does not increase over time, unlike nonaqueous electrolytes with low acidity, and therefore no effect can be obtained by setting the temperature to 20°C or below.

[0011] In the storage method of the present invention, the nonaqueous electrolyte solution is stored, for example, by preparing a sealed container as shown in Fig. 1, replacing the atmosphere in the container with an inert gas such as nitrogen gas, argon gas, or helium gas, placing the low-acidity nonaqueous electrolyte solution in the container, then sealing the container, and storing it at 20°C or below. The upper limit of the storage period is preferably within 100 days, and more preferably within 50 days.

[0012] The sealed container used does not require a special structure or material of construction, and can have a wide range of shapes and functions, as long as it can prevent moisture from entering from the outside air. Examples include pressure-resistant containers such as storage tanks, which are fixed storage containers, and canisters used for transportation. Furthermore, materials that can be used to construct the sealed container include carbon steel, manganese steel, chromium-molybdenum steel and other low-alloy steels, stainless steel, and aluminum alloys.

[0013] The nonaqueous electrolyte to which the storage method of the present invention is applied is not particularly limited except that its acidity is less than 1 meq / kg, but a nonaqueous electrolyte produced by preparing an alkali metal salt electrolyte-containing solution in which an alkali metal salt electrolyte such as a lithium-based electrolyte is dispersed in a carbonate ester and contacting this alkali metal salt electrolyte-containing solution with a weakly basic anion exchange resin is preferably used. In the following paragraphs of this specification, the nonaqueous electrolyte before treatment with the weakly basic anion exchange resin will be referred to as an alkali metal salt electrolyte-containing solution or a high-acidity nonaqueous electrolyte, and the nonaqueous electrolyte after treatment with the weakly basic anion exchange resin will be referred to as a low-acidity nonaqueous electrolyte or simply a nonaqueous electrolyte.

[0014] The carbonate ester may be one or more selected from cyclic carbonate esters and chain carbonate esters. Examples of cyclic carbonates include one or more selected from ethylene carbonate (ethylene carbonate), propylene carbonate (propylene carbonate), etc., and examples of chain carbonates include one or more selected from dimethyl carbonate (dimethyl carbonate), diethyl carbonate (diethyl carbonate), ethyl methyl carbonate (ethyl methyl carbonate), etc.

[0015] Examples of the alkali metal salt electrolyte include lithium-based electrolytes, and examples of the lithium-based electrolyte include one or more selected from LiPF6, LiClO4, LiBF4, LiAsF6, LiSbF6, LiAlCl4, LiCF3SO3, etc., and LiPF6 is preferred when battery performance is taken into consideration.

[0016] The content of the alkali metal salt, for example, LiPF6, in the non-aqueous electrolyte is preferably 0.5 to 1.5 mol / L, more preferably 0.5 to 1.2 mol / L, and even more preferably 0.8 to 1.2 mol / L.

[0017] In order to improve cycle characteristics, the non-aqueous electrolyte preferably contains an organic fluorine compound, such as fluoroethylene carbonate (FEC), 4-(fluoromethyl)-1,3-dioxolan-2-one, or fluoromethyl methyl carbonate. The content of the organic fluorine compound in the non-aqueous electrolyte is preferably from 0.1 to 50 mass %, more preferably from 0.5 to 25 mass %, and even more preferably from 1 to 10 mass %.

[0018] Furthermore, in order to improve cycle characteristics, the non-aqueous electrolyte preferably contains an organic sulfur compound, such as 1,3-propane sultone (PS), sulfolane, ethyl methyl sulfone, or 1-ethanesulfonyl-2-methoxyethane. The content of the organic sulfur compound in the non-aqueous electrolyte is preferably 0.1 to 10 mass %, more preferably 0.5 to 5 mass %.

[0019] In the non-aqueous electrolyte according to the present invention, the lower the water content, the better, but from the viewpoint of the influence on the increase in acidity, it is preferable that the water content be less than 10 ppm by mass. The water content of the non-aqueous electrolyte can be measured by the method described in the Examples.

[0020] The method for preparing the alkali metal salt electrolyte-containing liquid is not particularly limited, but it can be prepared, for example, by adding and dissolving an alkali metal salt electrolyte, and optionally an organic fluorine compound and / or an organic sulfur compound, in a carbonate ester.

[0021] The alkali metal salt electrolyte-containing solution prepared by the above method can be brought into contact with a weakly basic anion exchange resin to reduce the acidity to less than 1 meq / kg, thereby obtaining a non-aqueous electrolyte solution with low acidity. The contact of the alkali metal salt electrolyte-containing liquid with the weakly basic anion exchange resin is carried out, for example, using an apparatus as shown in Fig. 2. The apparatus shown in Fig. 2 has an ion exchange unit containing a weakly basic anion exchange resin, and the alkali metal salt electrolyte-containing liquid is passed through this ion exchange unit, thereby coming into contact with the weakly basic anion exchange resin.

[0022] The weakly basic anion exchange resin used in the ion exchange section has a styrene-based resin as a base.

[0023] The styrene-based resin refers to a resin obtained by homopolymerizing or copolymerizing styrene or a styrene derivative, and containing 50% by mass or more of structural units derived from styrene or a styrene derivative.

[0024] The styrene derivative may be one or more selected from α-methylstyrene, vinyltoluene, chlorostyrene, ethylstyrene, i-propylstyrene, dimethylstyrene, bromostyrene, and the like.

[0025] The styrene-based resin may be a copolymer with other copolymerizable vinyl monomers, so long as it is primarily composed of a homopolymer or copolymer of styrene or a styrene derivative. Examples of such vinyl monomers include one or more selected from polyfunctional monomers such as divinylbenzenes, such as o-divinylbenzene, m-divinylbenzene, and p-divinylbenzene, alkylene glycol di(meth)acrylates, such as ethylene glycol di(meth)acrylate and polyethylene glycol di(meth)acrylate, (meth)acrylonitrile, methyl (meth)acrylate, and the like.

[0026] The other copolymerizable vinyl monomers are more preferably ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate having an ethylene polymerization number of 4 to 16, and divinylbenzene, more preferably divinylbenzene and ethylene glycol di(meth)acrylate, and even more preferably divinylbenzene.

[0027] The weakly basic anion exchange resin used in the ion exchange section has a tertiary amino group as the weakly basic anion exchange group.

[0028] The tertiary amino group is a group represented by the following general formula (I): [ka] (However, R 1 Groups and R 2 The groups are hydrocarbon groups having 1 to 3 carbon atoms and may be the same or different, and * indicates the bonding site with the substrate or the bonding group for bonding to the substrate. ) can be mentioned.

[0029] In the weakly basic anion exchange group represented by the general formula (I), R 1 Groups and R 2 The group is a hydrocarbon group having 1 to 3 carbon atoms. R 1 group or R 2 The group may be at least one selected from an alkyl group and an alkenyl group, and is preferably an alkyl group. R 1 group or R 2 Specific examples of the group include one or more selected from a methyl group, an ethyl group, a propyl group, and a propylene group, with a methyl group being preferred. In the weakly basic anion exchange group represented by the general formula (I), R 1 Groups and R 2 The groups may be the same or different from one another.

[0030] Examples of the weakly basic anion exchange group represented by the above general formula (I) include a dimethylamino group, a diethylamino group, and a dipropylamino group, with the dimethylamino group being preferred.

[0031] In the above general formula (I), * indicates the bonding site between the weakly basic anion exchange group represented by the above general formula (I) and the substrate or the bonding group for bonding to the substrate.

[0032] The weakly basic anion exchange group represented by the general formula (I) is bonded to a substrate made of a styrene-based resin by an appropriate bonding group R 3 It is preferred that the bond is via a group. [ka] (However, R 1 Groups and R 2 groups are hydrocarbon groups having 1 to 3 carbon atoms and may be the same or different from each other, and R 3 The group is a hydrocarbon group having 1 to 3 carbon atoms, and * indicates the bonding site with the substrate.

[0033] Above R 1 Groups and R 2 Examples of the group include the same groups as those mentioned above. Above R 3 The group is a hydrocarbon group having 1 to 3 carbon atoms, and R 3 The group may be at least one selected from an alkylene group and an alkenylene group, and is preferably an alkylene group. R 3 Specific examples of the group include one or more selected from a methylene group (-CH2-), an ethylene group (-CH2CH2-), a propylene group (-CH2CH2CH2-), and the like, with a methylene group being preferred.

[0034] The weakly basic anion-exchange group represented by the above general formula (I) can be introduced into a styrene-based resin by introducing it as a substituent into styrene or a styrene derivative.

[0035] The weakly basic anion exchange resin contained in the ion exchange section may have any of a gel structure, a macroporous (MP) structure, and a porous structure, and preferably has a macroporous structure.

[0036] The size of the weakly basic anion exchange resin is not particularly limited, but the harmonic mean diameter is preferably 300 to 1000 μm, more preferably 400 to 800 μm, and even more preferably 500 to 700 μm.

[0037] Such a weakly basic anion exchange resin may be a commercially available product, and examples thereof include one or more selected from Diaion WA30 manufactured by Mitsubishi Chemical Corporation and ORLITE DS-6 manufactured by Organo Corporation.

[0038] The form of the weakly basic anion exchange resin contained in the ion exchange unit is not particularly limited as long as it allows contact between the alkali metal salt electrolyte-containing liquid and the weakly basic anion exchange resin. For example, the ion exchange section may be a column or tank filled with a weakly basic anion exchange resin through which the alkali metal salt electrolyte-containing liquid can pass. The ion exchange unit may also be provided with a pump for passing the alkali metal salt electrolyte-containing liquid.

[0039] The liquid passing speed (liquid hourly space velocity) of the alkali metal salt electrolyte-containing liquid through the weakly basic anion exchange device in the ion exchange section may be appropriately selected from the speed at which acidic impurities in the alkali metal salt electrolyte-containing liquid can be removed.

[0040] In the contact treatment with the weakly basic anion exchange resin, for example, the weakly basic anion exchange resin is first washed with the carbonate ester solvent constituting the alkali metal salt electrolyte-containing liquid to be treated, and then dried under reduced pressure at about 40 to 80°C. Next, the weakly basic anion exchange resin is swelled again with the carbonate ester solvent constituting the alkali metal salt electrolyte-containing liquid to be treated, and then packed into a column. After that, backwashing and extrusion operations are carried out according to the usual method, and the electrolyte to be treated is preferably treated at an SV (flow rate / ion exchange resin volume ratio) of 1 to 100 hours. -1 , more preferably SV2 to 50 hr -1 , and more preferably SV5 to 20 hr-1 This can be done by passing the solution through the column. [Example]

[0041] (Production Example 1) <Manufacture of high acidity non-aqueous electrolyte 1> Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:3, LiPF6 was dissolved therein to give a concentration of 1 mol / L, and fluoroethylene carbonate (FEC) was added to give a concentration of 5 mass% to prepare an alkali metal salt electrolyte-containing solution (hereinafter referred to as "high acidity nonaqueous electrolyte 1"). The acidity and water content of the high acidity nonaqueous electrolyte immediately after preparation were measured using the methods described below, and the acidity was 1.9 meq / kg and the water content was less than 10 ppm.

[0042] (Production Example 2) <Production of low acidity non-aqueous electrolyte 1> As the ion exchange section of the production apparatus shown in Figure 2, a column packed with a weakly basic anion exchange resin based on styrene-divinylbenzene and having dimethylamino groups as weakly basic anion exchange groups was prepared. Next, high acidity nonaqueous electrolyte 1 produced in Production Example 1 was passed through the column at a space velocity (SV) of 10 (L / L-resin) / hr using pump P, and brought into contact with the weakly basic anion exchange resin in the column, thereby preparing low acidity nonaqueous electrolyte 1 having an acidity of less than 1 meq / kg.

[0043] Example 1 A sealed container (material: stainless steel, volume: 1 L) shown in FIG. 1 was prepared, and the inside of the sealed container was replaced with nitrogen. The low acidity nonaqueous electrolyte 1 prepared in Production Example 2 was then placed in the sealed container, which was then stored at 5°C.

[0044] Example 2 In the same manner as in Example 1, low acidity nonaqueous electrolyte solution 1 was stored at 15°C. (Comparative Example 1) In the same manner as in Example 1, low acidity nonaqueous electrolyte solution 1 was stored at 25°C. (Comparative Example 2) Highly acidic nonaqueous electrolyte solution 1 was stored at 5°C in the same manner as in Example 1. (Comparative Example 3) Highly acidic nonaqueous electrolyte solution 1 was stored at 15°C in the same manner as in Example 1. Comparative Example 4 Highly acidic nonaqueous electrolyte solution 1 was stored at 25°C in the same manner as in Example 1.

[0045] (Production Example 3) <Preparation of Highly Acidic Non-Aqueous Electrolyte Solution 2> Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:3, LiPF was dissolved therein to a concentration of 1 mol / L, and 1,3-propane sultone (PS) was added thereto to a concentration of 1 mass % to prepare an alkali metal salt electrolyte-containing solution (hereinafter referred to as "high acidity nonaqueous electrolyte solution 2").

[0046] (Production Example 4) <Preparation of low acidity non-aqueous electrolyte 2> As the ion exchange section of the production apparatus shown in Figure 2, a column packed with a weakly basic anion exchange resin based on styrene-divinylbenzene and having dimethylamino groups as weakly basic anion exchange groups was prepared. Next, high acidity nonaqueous electrolyte 2 produced in Production Example 3 was passed through the column at a space velocity (SV) of 10 (L / L-resin) / hr using pump P, and brought into contact with the weakly basic anion exchange resin in the column, thereby preparing low acidity nonaqueous electrolyte 2 having an acidity of less than 1 meq / kg.

[0047] Example 3 In the same manner as in Example 1, low acidity nonaqueous electrolyte solution 2 was stored at 5°C. Example 4 In the same manner as in Example 1, low acidity nonaqueous electrolyte solution 2 was stored at 15°C. (Comparative Example 5) In the same manner as in Example 1, low acidity nonaqueous electrolyte solution 2 was stored at 25°C. (Comparative Example 6) Highly acidic nonaqueous electrolyte solution 2 was stored at 5°C in the same manner as in Example 1. (Comparative Example 7) Highly acidic nonaqueous electrolyte solution 2 was stored at 15°C in the same manner as in Example 1. (Comparative Example 8) Highly acidic nonaqueous electrolyte solution 2 was stored at 25°C in the same manner as in Example 1.

[0048] The changes in acidity and water content over time after storage of each non-aqueous electrolyte solution under each condition were measured, and the results are shown in Tables 1 and 2. The acidity and water content were measured by the following methods.

[0049] <Method for measuring acidity> The sample was dissolved in ice water and neutralized with 0.01N aqueous sodium hydroxide solution while stirring to quantify the amount of acid (meq) in the sample. The acidity (meq / kg) was calculated by dividing the amount by the sample weight (kg).

[0050] <Method for measuring moisture content> The sample was introduced into a Hiranuma Sangyo trace moisture analyzer AQ-2200AF, and the moisture content (ppm by mass) in the sample was measured by Karl Fischer titration.

[0051] [Table 1]

[0052] [Table 2]

[0053] The results in Table 1 show that in Examples 1 and 2, the acidity was maintained at less than 0.1 meq / kg even after 8 weeks. On the other hand, in Comparative Example 1, the acidity rose to 1.0 meq / kg after 8 weeks. These results demonstrate the effectiveness of the present invention, which allows the quality of non-aqueous electrolyte to be maintained by storing the non-aqueous electrolyte at 20°C or below. Furthermore, in Comparative Examples 2 to 4, the acidity remained stable at 1.8 to 1.9 meq / kg regardless of the storage temperature, demonstrating that the present invention is only effective for non-aqueous electrolytes with an acidity of less than 1 meq / kg. The results in Table 2 showed the same tendency as in Table 1, demonstrating that the effects of the present invention can be obtained regardless of the composition of the electrolyte solution. [Explanation of symbols]

[0054] 1 Nonaqueous electrolyte 2. Airtight containers 3. Non-aqueous electrolyte manufacturing equipment 4. Ion exchange section (containing weakly basic anion exchange resin) S Highly acidic non-aqueous electrolyte (containing alkali metal salt electrolyte) T Low acidity non-aqueous electrolyte P pump

Claims

1. A method for storing a non-aqueous electrolyte, comprising storing the non-aqueous electrolyte having an acidity of less than 1 meq / kg in a sealed container at 20°C or lower.

2. The method for storing a non-aqueous electrolyte according to claim 1 , wherein the water content of the non-aqueous electrolyte is less than 10 ppm by mass.

3. The non-aqueous electrolyte is LiPF ６ The method for storing a non-aqueous electrolyte according to claim 1 or 2, wherein the non-aqueous electrolyte contains 0.5 to 1.5 mol / L of the compound.

4. The non-aqueous electrolyte solution contains LiPF in carbonate ester. ６ 4. The method for storing a non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte is produced by contacting a lithium salt electrolyte-containing solution in which the above-mentioned compound is dispersed with a weakly basic anion exchange resin.

5. 5. The method for storing a non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte contains 0.1 to 50% by mass of an organic fluorine compound.

6. 6. The method for storing a non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte contains 0.1 to 10% by mass of an organic sulfur compound.

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