Method for producing nickel powder

Abstract

To provide a method for producing nickel powder that achieves high reaction yield and productivity, and provides fine and uniform particle size.SOLUTION: A method for producing nickel powder includes a step of initiating a nickel precipitation reaction in a reaction solution obtained by mixing a nickel salt, a metal or a salt thereof that is more noble than nickel, a reducing agent, an alkali metal hydroxide, and a solvent, wherein the solvent is a mixed solvent containing an aprotic polar solvent and water, and a content of the aprotic polar solvent in the solvent is less than 50 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing nickel powder. [Background technology]

[0002] As multilayer ceramic capacitors, which are electronic components that make up electronic circuits, become smaller and have larger capacitances, the electrodes are becoming thinner.

[0003] Nickel powder is used as a material for forming the internal electrodes of multilayer ceramic capacitors, and there is a demand for finer and more uniform particle size in order to achieve thinner electrodes.

[0004] Liquid phase synthesis and vapor phase synthesis are commonly used methods for producing nickel powder. Liquid phase synthesis, in particular, is a simple method for producing fine nickel powder.

[0005] Known methods for producing fine nickel powder by liquid phase synthesis include adding a nucleating agent with high catalytic activity to increase nucleation energy (for example, Patent Document 1). Patent Document 2 also discloses a method for obtaining fine nickel particles with a uniform particle size by using a sparingly soluble nickel compound in liquid phase synthesis. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-160964 [Patent Document 2] International Publication No. 2008 / 001741 Summary of the Invention [Problem to be solved by the invention]

[0007] The method disclosed in Patent Document 1 produces nickel powder with a fine particle size, but the particle size distribution is not always sufficiently narrow. In particular, when the amount of nucleating agent added is increased to increase the number of nuclei and obtain finer nickel powder, the particle size distribution of the resulting nickel powder tends to become broader due to factors such as a delay in the onset of the nucleation reaction between the nucleating agent and nickel ions and variations in the dispersion state of the nucleating agent in the reaction field. When nickel powder with such a wide particle size distribution is used in paste for internal electrodes of multilayer ceramic capacitors, problems arise such as a deterioration in electrode smoothness and non-uniform sintering behavior.

[0008] The method disclosed in Patent Document 2 is a manufacturing method in which the solubility of the nickel salt is reduced during the reaction process, leaving unreacted nickel, resulting in a reduced reaction rate (amount of nickel produced per hour) and reaction yield, and low productivity.

[0009] An object of the present invention is to provide a method for producing nickel powder having a fine and uniform particle size with high reaction yield and productivity. [Means for solving the problem]

[0010] The present invention includes the following. [1] A method for producing nickel powder, comprising: The method includes a step of initiating a nickel precipitation reaction in a reaction solution obtained by mixing a nickel salt, a metal nobler than nickel or a salt thereof, a reducing agent, an alkali metal hydroxide, and a solvent, the solvent is a mixed solvent containing an aprotic polar solvent and water, A method for producing nickel powder, wherein the content of the aprotic polar solvent in the solvent is less than 50 mass%. [2] The method for producing nickel powder according to [1], wherein the reaction solution contains the nickel salt, the metal nobler than nickel or its salt, the reducing agent, and the alkali metal hydroxide dissolved in the solvent. [3] The method for producing nickel powder according to [1] or [2], wherein the aprotic polar solvent is at least one selected from the group consisting of dimethyl sulfoxide, N,N-dimethylformamide, and N-methylpyrrolidone. [4] The method for producing nickel powder according to [1] or [2], wherein the content of the aprotic polar solvent in the solvent is greater than 15 mass%. [Effects of the Invention]

[0011] According to the present invention, a method for producing nickel powder having a fine and uniform particle size can be provided, which has a high reaction yield and productivity. DETAILED DESCRIPTION OF THE INVENTION

[0012] The method for producing nickel powder of the present invention may use a liquid-phase synthesis method, for example, by preparing a nickel salt solution containing a nickel salt, a metal nobler than nickel or its salt, and a solvent, and a reducing agent solution containing a reducing agent, an alkali metal hydroxide, and a solvent, and then mixing the nickel salt solution with the reducing agent solution to initiate a nickel precipitation reaction in the reaction solution. More specifically, the method may include a step of preparing the nickel salt solution, a step of preparing the reducing agent solution, and a step of mixing the nickel salt solution with the reducing agent solution, and initiates a nickel precipitation reaction in the reaction solution. As the reaction progresses, metallic nickel powder precipitates. Hereinafter, the "step of initiating a nickel precipitation reaction in the reaction solution and precipitating metallic nickel powder" will also be referred to as the "precipitation step."

[0013] In particular, the nickel powder manufacturing method of the present invention is characterized in that in the precipitation process, the solvent of the reaction liquid (mixture of nickel salt solution and reducing agent solution) is a mixed solvent containing an aprotic polar solvent and water.

[0014] [Nickel salt solution] The nickel salt solution contains a nickel salt, a metal more noble than nickel or a salt thereof, and a solvent.

[0015] The nickel salt is not particularly limited as long as it is a nickel salt that is easily soluble in water, and examples thereof include nickel chloride, nickel sulfate, nickel nitrate, nickel acetate, etc. From the viewpoint of being inexpensive and easily procurable, nickel chloride, nickel sulfate, or a mixture thereof is preferred as the nickel salt.

[0016] A metal more noble than nickel or its salt is added to the nickel salt solution to function as a nucleating agent for generating crystal nuclei in the precipitation step. The metal ions of the metal more noble than nickel are reduced before nickel to form initial nuclei, and these initial nuclei grow into particles, thereby producing a fine nickel powder.

[0017] Examples of metals more noble than nickel or salts thereof include copper, silver, gold, platinum, palladium, rhodium, iridium, etc., or salts of these metals, and specific examples include copper sulfate, silver nitrate, sodium palladium(II) chloride, ammonium palladium(II) chloride, palladium(II) nitrate, palladium(II) sulfate, hexachloroplatinic acid, or hydrates of these salts. One or more types of metals more noble than nickel or salts thereof can be used.

[0018] The amount of the metal or salt thereof more noble than nickel is not particularly limited, but the amount of the metal serving as a nucleating agent is preferably 0.4 ppm by mass or more and 1000 ppm by mass or less relative to the amount of nickel powder produced, i.e., the amount of nickel contained in the nickel salt solution. If the amount is less than 0.4 ppm by mass, the particle size of the nickel powder tends to be large, and if the amount is more than 1000 ppm by mass, the particle size becomes finer but the particle size distribution tends to be broader.

[0019] In addition to the nickel salt and the metal or salt thereof, the nickel salt solution preferably contains a complexing agent. The complexing agent stabilizes the nickel ion concentration by forming a complex with nickel ions in the nickel salt solution. Therefore, by adding the complexing agent, the reaction rate of the reduction reaction in the precipitation step can be controlled.

[0020] As the complexing agent, it is preferable to use a hydroxycarboxylic acid, a salt thereof, or a derivative thereof, or a carboxylic acid, a salt thereof, or a derivative thereof, and specific examples thereof include tartaric acid, citric acid, malic acid, ascorbic acid, formic acid, acetic acid, pyruvic acid, and salts and derivatives thereof.

[0021] A dispersant may be further blended into the nickel salt solution to control the particle size and particle size distribution of the nickel powder. Known dispersants may be used, and specific examples include amines such as triethanolamine, diethanolamine, and oxyethylene alkylamine, as well as salts and derivatives thereof, and amino acids such as alanine and glycine, as well as salts and derivatives thereof.

[0022] In the method for producing nickel powder of the present invention, the order in which the components to be mixed in the nickel salt solution are mixed is not particularly limited.

[0023] [Reducing agent solution] The reducing agent solution includes, for example, a reducing agent, an alkali metal hydroxide, and a solvent.

[0024] Examples of the reducing agent that can be used include hydrazine, sodium borohydride, alcohol, ascorbic acid, or a salt thereof. Hydrazine is preferred as a reducing agent because it has high reducing power, does not produce by-products in the reaction solution during the reduction reaction, contains few impurities, and is easily available. Hydrazine includes anhydrous hydrazine and hydrazine hydrate, which is a hydrazine hydrate, and either may be used. Specifically, commercially available industrial-grade 60% by mass hydrazine hydrate can be used as the hydrazine.

[0025] Since the reducing power of the reducing agent is particularly enhanced in an alkaline solution, an alkali metal hydroxide is added to the reducing agent solution as a pH adjuster. Examples of alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide. One or more alkali metal hydroxides can be used. Alternatively, an alkali metal hydroxide can be added to a nickel salt solution.

[0026] The reducing agent solution may contain a complexing agent, a dispersing agent, etc., similar to the nickel salt solution. The mixing order of the components contained in the reducing agent solution is not particularly limited.

[0027] The nickel salt solution and the reducing agent solution are prepared using the solvents described below, and the preparation of each solution may be carried out either first or simultaneously.

[0028] [solvent] In the method for producing nickel powder of the present invention, the solvent in the reaction solution obtained by mixing a nickel salt, a metal nobler than nickel or its salt, a reducing agent, an alkali metal hydroxide, and a solvent is a mixed solvent containing an aprotic polar solvent and water. In this specification, the aprotic polar solvent refers to a polar solvent that does not have proton-donating properties, and the polar solvent refers to a solvent that is miscible with water in the reaction solution without undergoing phase separation. By adding the aprotic polar solvent, the stability of the water-soluble nickel ion complex in the reaction solution can be reduced, thereby lowering the activation energy of the nickel reduction reaction. As a result, the reaction rate of the reduction reaction can be improved and uniform nucleation can be promoted, resulting in good reaction yield and productivity, and nickel powder with fine particle size and a narrow particle size distribution can be obtained.

[0029] In the method for producing nickel powder of the present invention, for example, by preparing at least one of the nickel salt solution and the reducing agent solution using a mixed solvent containing an aprotic polar solvent and water, the solvent for the reaction solution in the precipitation step can be a mixed solvent containing an aprotic polar solvent and water, but it is preferable to prepare both the nickel salt solution and the reducing agent solution using a mixed solvent containing an aprotic polar solvent and water. When both the nickel salt solution and the reducing agent solution are prepared using a mixed solvent of an aprotic polar solvent and water, it is preferable that the type of aprotic polar solvent in each mixed solvent is the same, and it is more preferable that the type of aprotic polar solvent in each mixed solvent and the content of the aprotic polar solvent in each mixed solvent are the same.

[0030] Examples of aprotic polar solvents include dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dichloromethane, tetrahydrofuran, acetone, acetonitrile, etc. From the viewpoints of improving productivity and achieving finer and more uniform particle sizes, at least one solvent selected from the group consisting of dimethyl sulfoxide, N,N-dimethylformamide, and N-methylpyrrolidone is preferred.

[0031] As the water, it is preferable to use pure water from the viewpoint of reducing the amount of impurities in the nickel powder.

[0032] The content of the aprotic polar solvent in the mixed solvent is the percentage of the mass ratio of the aprotic polar solvent to the total amount of solvent in a reaction solution obtained by mixing a nickel salt, a metal nobler than nickel or its salt, a reducing agent, an alkali metal hydroxide, and a solvent; for example, it is the percentage of the mass ratio of the aprotic polar solvent to the total amount of solvent in a reaction solution obtained by mixing a nickel salt solution and a reducing agent solution.

[0033] The content of the aprotic polar solvent in the mixed solvent must be less than 50% by mass, and from the viewpoints of improving productivity and making the particle size finer and more uniform, it is preferably 15% by mass or more and less than 50% by mass, more preferably 20% by mass or more and less than 50% by mass, even more preferably 30% by mass or more and less than 50% by mass, and particularly preferably 30% by mass or more and less than 45% by mass. If the content of the aprotic polar solvent is 50% by mass or more, the nickel precipitation reaction will not be completed, making it impossible to produce nickel powder, or the reaction yield will be low.

[0034] [Precipitation process] In the method for producing nickel powder of the present invention, a nickel precipitation reaction is initiated in a reaction solution obtained by mixing a nickel salt, a metal or its salt nobler than nickel, a reducing agent, an alkali metal hydroxide, and a solvent. Specifically, nickel powder is precipitated by reducing the nickel salt (nickel ions) dissolved in the solvent in the reaction solution with the reducing agent. Therefore, it is preferable that all of the components in the reaction solution are dissolved in the solvent at the time of initiating the precipitation reaction. Furthermore, although only a portion of each component may be dissolved in the solvent at the time of initiating the precipitation reaction, it is preferable that all of the components are dissolved in the solvent as the precipitation reaction progresses.

[0035] In the method for producing nickel powder of the present invention, for example, a nickel salt solution and a reducing agent solution are mixed to prepare a reaction solution, and nickel is precipitated in the reaction solution by the reducing agent, thereby obtaining nickel powder. The order in which the solutions are mixed when preparing the reaction solution is not particularly limited. That is, the reducing agent solution may be added to the nickel salt solution, or conversely, the nickel salt solution may be added to the reducing agent solution. Furthermore, the solutions may be added all at once, in multiple installments, or continuously dropwise.

[0036] The temperature of the reaction solution at the time when nickel precipitation begins after preparation, i.e., the reaction initiation temperature, is preferably 40°C or higher and 95°C or lower, and more preferably 50°C or higher and 90°C or lower. Because nickel precipitation begins immediately after preparation of the reaction solution, the reaction initiation temperature can be considered the temperature of the reaction solution at the time of preparation. The higher the reaction initiation temperature, the faster the reduction reaction rate. However, if the reaction initiation temperature exceeds 95°C, problems such as difficulty in controlling the particle size of the nickel powder or the reaction solution boiling over from the reaction vessel due to an inability to control the reaction rate may occur. Furthermore, if the reaction initiation temperature is lower than 40°C, the reduction reaction rate decreases, the time required for the precipitation process increases, and productivity decreases. For these reasons, if the reaction initiation temperature is set in the range of 40°C or higher and 95°C or lower, nickel powder can be produced with easy control of particle size and particle size distribution while maintaining high productivity.

[0037] For the reasons described above, it is preferable to heat at least one of the nickel salt solution and the reducing agent solution to a reduction temperature by the reducing agent before preparing the reaction solution in the precipitation step. The "reduction temperature by the reducing agent" refers to the temperature at which nickel is precipitated by reducing the nickel salt with the reducing agent, and is preferably 40°C or higher and 95°C or lower.

[0038] If the nickel salt solution and the reducing agent solution are heated to the reduction temperature before mixing, a sudden temperature change does not occur during mixing, and the temperature at which the precipitation reaction begins immediately after mixing can be kept at approximately the same temperature as the reduction temperature.

[0039] In the precipitation step, it is preferable to set the pH of the prepared reaction solution to 11 or higher. As mentioned above, the reducing power of the reducing agent increases in a more alkaline solution. When the pH of the reaction solution is 11 or higher, nuclei of a metal more noble than nickel are generated in the reaction solution with the increased reducing power of the reducing agent, allowing many initial nuclei to be formed uniformly, resulting in nickel powder with a fine particle size and a narrow particle size distribution.

[0040] The pH can be adjusted using a reducing agent solution containing the above-mentioned alkali metal hydroxide, and if necessary, a pH adjuster can be added when preparing the reaction solution.

[0041] The amount of reducing agent added to the reaction solution in the precipitation step can be adjusted as appropriate, as long as the amount is such that the entire amount of nickel contained in the reaction solution is reduced. For example, when the reducing agent is hydrazine, the molar ratio relative to the nickel contained in the reaction solution may be in the range of 0.5 to 3.25. If the amount of hydrazine added relative to the nickel contained in the reaction solution is less than 0.5, there is a possibility that the entire amount of nickel in the reaction solution will not be reduced. On the other hand, if the amount of hydrazine added relative to the nickel contained in the reaction solution is more than 3.25, an excess of hydrazine will be used, which is economically disadvantageous.

[0042] When mixing the nickel salt solution, reducing agent solution, reaction solution, and other solutions, it is preferable to stir these solutions. Stirring can make the precipitation reaction uniform, allowing nickel powder with a narrow particle size distribution to be obtained. Any known stirring method can be used, and it is preferable to use a stirring blade from the standpoint of controllability and equipment manufacturing costs. As the stirring blade, commercially available products such as paddle blades, turbine blades, Max Blend blades, and Full Zone blades can be used, and measures such as installing baffles or baffle rods in the reaction vessel can also be taken to improve stirring and mixing.

[0043] [Recovery of nickel powder] Only the nickel powder is separated from the nickel powder slurry containing the nickel powder obtained in the above precipitation step by a known procedure such as washing, solid-liquid separation, and drying.

[0044] To separate the nickel powder from the nickel powder slurry, solid-liquid separation is performed using known means such as a Denver filter, filter press, centrifuge, or decanter, and the nickel powder is thoroughly washed with high-purity water such as pure water or ultrapure water having a conductivity of 1 μS / cm or less. Here, "sufficient washing" refers to, for example, using pure water with a conductivity of about 1 μS / cm, filtering and washing the nickel powder until the conductivity of the filtrate obtained is 10 μS / cm or less. After solid-liquid separation and washing, nickel powder is obtained by drying at a temperature in the range of 50°C to 200°C, preferably 80°C to 150°C, using a general-purpose drying device such as an atmospheric dryer, hot air dryer, inert gas atmosphere dryer, or vacuum dryer.

[0045] If necessary, a sulfur-surface-treated nickel powder can be obtained by adding a sulfur coating agent, which is a water-soluble sulfur compound, to the nickel powder slurry. Examples of the sulfur coating agent include water-soluble sulfur compounds containing either a mercapto group or a disulfide group, such as thiomalic acid, L-cysteine, thioglycerol, and dithiodiglycolic acid.

[0046] [Crushing process (post-processing process)] The nickel powder obtained in the precipitation step can be used as it is as the final nickel powder product, but it is more preferable to subject it to a crushing treatment as necessary to reduce coarse particles and connected particles formed during the nickel precipitation process. As the crushing treatment, dry crushing methods such as spiral jet crushing and counter jet mill crushing, wet crushing methods such as high-pressure fluid collision crushing, and other general-purpose crushing methods can be applied. [Example]

[0047] The present invention will be explained in more detail below by showing examples and comparative examples, but the present invention is not limited to these examples.

[0048] [Evaluation method] (1) Judgment of reaction completion The color of the reaction solution was visually confirmed 60 minutes after the start of the precipitation reaction (the time when the reaction solution was prepared), and the presence of nickel ions in the reaction solution was confirmed using ion detection paper (Nickel Check, manufactured by ADVANTEC). Completion of the reaction was judged according to the following evaluation criteria. If the evaluation was ○, it can be said that the reaction yield of nickel powder was high. ○: The reaction solution is colorless and transparent, and the ion detection paper does not discolor. ×: The reaction solution is colored or the ion detection paper is discolored.

[0049] (2) Average particle size and CV value The obtained nickel powder was observed at a magnification of 20,000 times using a scanning electron microscope (SEM: JSM-7100F, manufactured by JEOL Ltd.), and the particle sizes of 1,000 particles were measured by image analysis of the observed image. The number-averaged average particle size and its standard deviation were calculated, and the CV value, which is an index of particle size distribution, was obtained using the following formula. CV value (%) = [standard deviation of average particle size / average particle size] x 100

[0050] (3) Judgment (overall evaluation) Based on the results of (1) and (2) above, a comprehensive evaluation was made according to the following evaluation criteria.

[0051] ○: The reaction is judged to be complete, the average particle size is 70 nm or less, and the CV value is 15% or less △: Reaction completion judged as "○", average particle size is 70 nm or less, and CV value is 20% or less (excluding cases where the overall evaluation is ○) ×: The reaction completion judgment is "×", or the average particle size is more than 70 nm, or the CV value is more than 20%

[0052] Example 1 [Preparation of nickel salt solution] A nickel salt solution was prepared by stirring and mixing 105.0 g of nickel sulfate hexahydrate as a nickel salt, 15.6 g of 0.1% palladium sulfate as a metal salt (nucleating agent) of a metal more noble than nickel, and 30.0 g of trisodium citrate as a complexing agent in a mixed solvent of 199.4 g (70 mass%) of pure water and 94.0 g (30 mass%) of dimethyl sulfoxide.

[0053] [Preparation of reducing agent solution] A reducing agent solution was prepared by mixing 64.8 g of 60% hydrazine hydrate, which had been purified by removing organic impurities such as pyrazole, as a reducing agent, and 57.0 g of sodium hydroxide as a pH adjuster, with stirring in a mixed solvent of 190.3 g (70% by mass) of pure water and 89.7 g (30% by mass) of dimethyl sulfoxide.

[0054] [Precipitation process] The nickel salt solution and the reducing agent solution were each heated to a temperature of 60°C, and then the reducing agent solution was added dropwise to the nickel salt solution at a rate of approximately 150 ml / min to initiate the precipitation reaction in the reaction solution. Due to heat generated during stirring and mixing of the nickel salt solution and the reducing agent solution, the temperature of the reaction solution rose to 67°C, and the reaction initiation temperature was 67°C. Approximately 2 to 3 minutes after the start of the reaction, the reaction solution changed color (from green to gray) due to nucleation by the nucleating agent. However, stirring was continued to allow the reduction reaction to proceed, yielding nickel precipitate powder. The supernatant of the reaction solution was colorless and transparent, and no discoloration was observed on ion detection paper, confirming that all nickel components in the reaction solution had been reduced to metallic nickel. The reaction solution containing the resulting nickel precipitate powder was filtered and washed using pure water with a conductivity of 1 μS / cm until the conductivity of the filtrate filtered from the reaction solution reached 10 μS / cm or less. After solid-liquid separation, the filtrate was dried in a vacuum dryer set at 150°C to obtain nickel powder.

[0055] (Examples 2 to 4, Comparative Examples 1 to 4) The reduction reaction was carried out in the same manner as in Example 1, except that the type of organic solvent in the mixed solvent and the content of the organic solvent in the mixed solvent were changed as shown in Table 1. In Comparative Example 4, the solvent was water only.

[0056] (Comparative Examples 5 to 7) The reduction reaction was carried out in the same manner as in Comparative Example 4, except that the amount of nucleating agent was changed as shown in Table 1.

[0057] The completion of the reaction was determined by the above evaluation method, and the average particle size and CV value of the resulting nickel powder were determined. The results are shown in Table 1.

[0058] In Examples 1 to 4, the reaction was completed within 60 minutes, and nickel powder with a fine particle size and a narrow particle size distribution was obtained. In particular, in Examples 1 to 3, nickel powder with an average particle size of 70 nm or less and a CV value of 15% or less was obtained. By using a predetermined amount of aprotic polar solvent, nickel powder with a fine and uniform particle size could be produced while maintaining high reaction yield and productivity.

[0059] In Comparative Example 1, the content of the aprotic polar solvent was 50 mass %, so the reaction liquid remained green even 60 minutes after the start of the reaction, the reaction was not completed, and nickel powder was not obtained.

[0060] In Comparative Examples 2 to 4, no aprotic polar solvent was used, and therefore nickel powders with satisfactory average particle size and CV value were not obtained.

[0061] In Comparative Example 5, since no nucleating agent was added, the reaction solution remained green even 60 minutes after the start of the reaction, the reaction was not completed, and nickel powder was not obtained. Furthermore, as shown in Comparative Examples 4 to 7, unless an aprotic polar solvent was added, nickel powder with satisfactory average particle size and CV value could not be obtained even if only the amount of nucleating agent was increased.

[0062] [Table 1]

[0063] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

Claims

1. A method for producing nickel powder, comprising: The method includes a step of initiating a nickel precipitation reaction in a reaction solution obtained by mixing a nickel salt, a metal nobler than nickel or a salt thereof, a reducing agent, an alkali metal hydroxide, and a solvent, the solvent is a mixed solvent containing an aprotic polar solvent and water, A method for producing nickel powder, wherein the content of the aprotic polar solvent in the solvent is less than 50 mass%.

2. 2. The method for producing nickel powder according to claim 1, wherein in the reaction solution, the nickel salt, the metal nobler than nickel or its salt, the reducing agent, and the alkali metal hydroxide are dissolved in the solvent.

3. 3. The method for producing nickel powder according to claim 1, wherein the aprotic polar solvent is at least one selected from the group consisting of dimethyl sulfoxide, N,N-dimethylformamide, and N-methylpyrrolidone.

4. The method for producing nickel powder according to claim 1 or 2, wherein the content of the aprotic polar solvent in the solvent is greater than 15 mass%.

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