JP7340995B2 - Method for producing chain-like lithium lanthanum titanate crystal particle aggregate for solid electrolyte - Google Patents

Description

The present invention relates to a method for producing a chain lithium lanthanum titanate crystal particle aggregate for a solid electrolyte for use in a lithium ion secondary battery.

Lithium-ion secondary batteries currently in practical use use flammable organic solvents for the electrolyte, so it is necessary to take sufficient safety measures against leakage and ignition. It is also highly difficult to convert. However, all-solid-state lithium-ion secondary batteries equipped with oxide-based or sulfide-based solid electrolytes have a high energy density and can be manufactured without using flammable materials, so safety measures must be taken. The burden is reduced and manufacturing costs and productivity can be easily increased.
Among these, solid electrolyte materials that are expected to be highly useful, especially high lithium ion materials such as lithium lanthanum titanate Li _3x La _2/3-x TiO ₃ (0<x<0.3), which has a perovskite crystal structure Regarding solid electrolyte materials having conductivity, various developments are gradually becoming more active.

Although lanthanum lithium titanate is often molded into a desired shape by compression molding, it is extremely hard, so contact between lanthanum lithium titanate particles is insufficient, resulting in high grain boundary resistance and molded products. lithium ion conductivity tends to decrease. On the other hand, if sintering is performed at a temperature of 1000°C or higher after compression molding, grain boundary resistance between lithium lanthanum titanate particles can be improved, but lithium will volatilize from lithium lanthanum titanate and the lithium ion conductivity will decrease. There is a risk.
In order to improve these problems, attempts have been made to improve the contact between particles by making lithium lanthanum titanate particles finer.

For example, Patent Document 1 includes a step of adding a chelating agent, a water-soluble salt containing lithium, and a crosslinking agent to a peroxotitanic acid aqueous solution to gel the aqueous solution, and heating at a temperature of 300 ° C. or higher and 500 ° C. or lower. A method for manufacturing a solid electrolyte is disclosed, in which the refinement of Li _3x La _2/3-x TiO ₃ crystal particles is attempted. Further, Patent Document 2 discloses a method for producing composite metal oxide particles using a flame spray pyrolysis method in which a precursor solution of a composite metal oxide is reacted at 2000°C to 4000°C, and the resulting composite Attempting to make metal oxide nanoparticles.

Japanese Patent Application Publication No. 2012-188312 Korean Published Patent No. 10-2015-0055799

However, the manufacturing method described in Patent Document 1 is a very complicated manufacturing method, and there is a possibility that good ionic conductivity may not be imparted to the obtained lanthanum lithium titanate. Further, even the manufacturing method described in Patent Document 2 requires firing at an extremely high temperature range of 2,000° C. to 4,000° C., which necessitates implementation using special equipment.

Therefore, an object of the present invention is to obtain a lanthanum lithium titanate crystal particle aggregate useful for solid electrolytes using a simple method using lanthanum lithium titanate crystal particles that exhibit excellent lithium ion conductivity. The purpose is to provide a manufacturing method.

Therefore, after various studies, the present inventor found that by using cellulose nanofibers and/or lignocellulose nanofibers together with predetermined raw material compounds and going through a process that utilizes a hydrothermal reaction, a plurality of lanthanum lithium titanate crystal particles can be produced. It was discovered that a lanthanum lithium titanate crystal particle aggregate that is assembled in a chain shape and has excellent properties as a solid electrolyte for a lithium ion secondary battery can be easily obtained, and the present invention was completed.

Therefore, the present invention provides the following steps (I) to (V):
(I) A lanthanum compound, a titanium compound, a cellulose nanofiber, and/or a lignocellulose nanofiber are mixed with water, and 0.1 mass of cellulose nanofiber and/or lignocellulose nanofiber is added to 100 parts of water by mass. A step of preparing a mixed solution (a-1) containing 1 to 40 parts by mass,
(II) a step of mixing a lithium compound and water to prepare a mixed solution (a-2);
(III) mixing the obtained mixture (a-1) and mixture (a-2) to prepare a mixture (A) having a pH of 8 to 11 at 25°C;
(IV) A step of subjecting the obtained mixed solution (A) to a hydrothermal reaction at 100° C. or higher, washing the obtained hydrothermal reaction product and then drying it to obtain a precursor mixture; V) Provides a method for producing a chain-like lithium lanthanum titanate crystal particle aggregate for a solid electrolyte, which comprises a step of firing the obtained precursor mixture at 400°C to 1000°C.

According to the manufacturing method of the present invention, although it is a simple method that does not require excessive temperature load, it exhibits a unique shape in which lanthanum lithium titanate crystal particles are aggregated in a chain shape, and has excellent ion conductivity. By using this as a solid electrolyte, it is possible to realize a lithium ion secondary battery with excellent charge and discharge characteristics.

1 is a TEM image showing a chain-like lithium lanthanum titanate crystal particle aggregate obtained in Example 1. 1 is an X-ray diffraction pattern of lithium lanthanum titanate crystal particles constituting the chain-like lithium lanthanum titanate crystal particle aggregate obtained in Example 1. 1 is a SEM image showing lithium lanthanum titanate crystal particles obtained in Comparative Example 1.

The present invention will be explained in detail below.
The method for producing a chain lithium lanthanum titanate crystal particle aggregate for a solid electrolyte of the present invention includes the following steps (I) to (V):
(I) A lanthanum compound, a titanium compound, a cellulose nanofiber, and/or a lignocellulose nanofiber are mixed with water, and 0.1 mass of cellulose nanofiber and/or lignocellulose nanofiber is added to 100 parts of water by mass. A step of preparing a mixed solution (a-1) containing 1 to 40 parts by mass,
(II) a step of mixing a lithium compound and water to prepare a mixed solution (a-2);
(III) mixing the obtained mixture (a-1) and mixture (a-2) to prepare a mixture (A) having a pH of 8 to 11 at 25°C;
(IV) A step of subjecting the obtained mixed solution (A) to a hydrothermal reaction at 100° C. or higher, washing the obtained hydrothermal reaction product and then drying it to obtain a precursor mixture; V) a step of firing the obtained precursor mixture at 400°C to 1000°C.

The chain-like lithium lanthanum titanate crystal particle aggregate obtained by the production method of the present invention is made up of a plurality of lithium lanthanum titanate crystal particles aggregated in a chain, as if the lithium lanthanum titanate crystal particles were continuous in a straight line. It has a unique shape, as if it were arranged in a row. The lanthanum lithium titanate (hereinafter also referred to as "LLTO") crystal particles constituting the chain lanthanum lithium titanate crystal particle aggregate are particles represented by the following formula (1).
Li _3x La _2/3-x TiO ₃ ...(1)
(In formula (1), x represents a number satisfying 0<x<0.3.)

The LLTO crystal particles represented by the above formula (1) are titanate compounds containing lithium and lanthanum, and more specifically, such particles include, for example, Li _0.33 La _0.56 TiO ₃ , Li _0.81 La _0.40 TiO ₃ , Li _0.12 La _0.63 TiO ₃ .

Step (I) is to mix a lanthanum compound, a titanium compound, a cellulose nanofiber, and/or a lignocellulose nanofiber, and water, so that 0 of the cellulose nanofiber and/or the lignocellulose nanofiber is added to 100 parts by mass of water. This is a step of preparing a mixed solution (a-1) containing 1 part by mass to 40 parts by mass. The obtained mixed liquid (a-1) is mixed with the mixed liquid (a-2) obtained in step (II) described later to obtain a mixed liquid (A) to be subjected to a hydrothermal reaction later. . That is, the present invention aims to improve the solubility or dispersibility of each compound in the mixed liquid, particularly the dispersibility of cellulose nanofibers and/or lignocellulose nanofibers, and to improve the homogeneity of the chemical composition of LLTO crystal particles. From the viewpoint of greatly contributing to the crystal refinement of LLTO particles, the mixed solution (a-1) and the mixed solution (a-2) are prepared separately.
Note that step (I) for obtaining the mixed solution (a-1) and step (II) for obtaining the mixed solution (a-2) described below may be performed in this order, and step (II) After that, step (I) may be performed.

The lanthanum compound used in step (I) is a raw material compound for forming LLTO crystal particles together with the titanium compound and the lithium compound. Such lanthanum compounds are not particularly limited as long as they are water-soluble, and include one or more lanthanum compounds selected from halides, hydroxides, chlorides, carbonates, sulfates, and organic acid salts. More specifically, for example, lanthanum hexaboride, lanthanum hydroxide, lanthanum chloride, lanthanum carbonate, lanthanum sulfate, lanthanum selenate, lanthanum formate, lanthanum acetate, lanthanum oxalate, lanthanum bromate, lanthanum nitrate, etc. are preferable. More preferred are lanthanum nitrate, lanthanum sulfate, and lanthanum chloride.

The titanium compound used in step (I) is a raw material compound for forming LLTO crystal particles together with the titanium compound and the lithium compound. Such titanium compounds are not particularly limited, and include, for example, titanium sulfates, nitrates, carbonates, acetates, oxalates, oxides, hydroxides, and halides. Specifically, for example, titanyl sulfate, titanium sulfate, titanium chloride, titanium acetate, etc. are preferable, and titanyl sulfate and titanium chloride are more preferable.

In step (I), the cellulose nanofibers (x1) and/or lignocellulose nanofibers (x2) (hereinafter also collectively referred to as "CNF(X)") used together with the above raw material compound are used in step (IV) described below. ) functions as a site for nucleation and crystal growth of hydrothermal reaction products. In step (IV), after producing a fine hydrothermal reaction product (precursor of lithium lanthanum titanate crystal) having a highly homogeneous chemical composition, the step (V) described below is performed to produce a fine hydrothermal reaction product with a highly homogeneous chemical composition. Since all CNF(X) is burnt out, almost no CNF(X) remains in the chained lithium lanthanum titanate crystal particle aggregate obtained by the present invention. In addition, the fact that the lithium lanthanum titanate crystal particles forming the chain-like lithium lanthanum titanate crystal particle aggregate obtained by the present invention is fine particles can be confirmed by using CNF(X). This is because a crystal precursor is obtained and CNF(X) acts as a barrier to suppress sintering of lithium lanthanum titanate crystal particles.

In addition, since almost all of CNF(X) is burned out in the step (V) described later, the chain-like lithium lanthanum titanate crystal particle aggregate obtained in the present invention does not have electronic conductivity, so lithium ion When used as a solid electrolyte in secondary batteries, no short circuit occurs between the positive and negative electrodes.

Cellulose nanofiber (x1) is a skeletal component that accounts for about 50% of all plant cell walls, and is a lightweight, high-strength fiber that can be obtained by fibrillating the plant fibers that make up the cell wall to nano-size. be. Moreover, lignocellulose nanofibers (x2) are cellulose nanofibers obtained without performing purification to remove lignin, and are formed by covering cellulose nanofibers (x1) with lignin.
Both cellulose nanofiber (x1) and lignocellulose nanofiber (x2) have excellent dispersibility in water. In step (I), only one of these cellulose nanofibers (x1) and lignocellulose nanofibers (x2) may be used, or both may be used in combination.

The average fiber diameter of CNF(X) is preferably 50 nm or less, more preferably 20 nm or less, and still more preferably 10 nm or less. There is no particular restriction on the lower limit, but it is usually 1 nm or more. Further, from the viewpoint of handling, the average length of CNF(X) is preferably 100 nm to 100 μm, more preferably 1 μm to 100 μm, and still more preferably 5 μm to 100 μm.

The mixed solution (a-1) is a mixture of a lanthanum compound, a titanium compound, CNF(X), and water, so that CNF(X) becomes 0.1 parts by mass to 40 parts by mass per 100 parts by mass of water. Prepare as follows. The content of CNF(X) is determined from the viewpoint of effectively reducing the size of LLTO crystal particles and promoting the formation of chain-like lithium lanthanum titanate crystal particle aggregates. The amount is from 0.1 parts by weight to 40 parts by weight, preferably from 0.5 parts by weight to 30 parts by weight, and more preferably from 1 part by weight to 20 parts by weight, based on 100 parts by weight.

The total content of the lanthanum compound and the titanium compound in the mixed solution (a-1) is determined from the viewpoint of improving the homogeneity of the chemical composition in the reaction product produced by mixing with the mixed solution (a-2) described below. It is preferably 0.1 parts by mass to 40 parts by mass, more preferably 0.5 parts by mass to 30 parts by mass, and even more preferably 1 part by mass, relative to 100 parts by mass of water in the liquid mixture (a-1). parts to 20 parts by mass.
Further, in preparing the mixed solution (a-1), the mixing ratio of the lanthanum compound and the titanium compound is preferably 0.38:1 to 0.66:1 in terms of molar ratio (La:Ti), and more preferably The ratio is preferably 0.41:1 to 0.63:1, more preferably 0.44:1 to 0.60:1.

The liquid mixture (a-1) may be stirred in advance before being mixed with the liquid mixture (a-2) in step (III) described later, from the viewpoint of dispersing each component more uniformly. The time for stirring the liquid mixture (a-1) is preferably 5 minutes to 3 hours, more preferably 10 minutes to 2 hours, and even more preferably 15 minutes to 90 minutes. The stirring speed is preferably 10 cm/sec to 200 cm/sec, more preferably 15 cm/sec to 150 cm/sec, and further Preferably it is 20 cm/sec to 100 cm/sec.

Step (II) is a step of preparing a mixed solution (a-2) by mixing a lithium compound and water.
The lithium compound used in step (II) is a raw material compound for forming LLTO crystal particles together with the titanium compound and lithium compound used in step (I). Such lithium compounds include one or more selected from hydroxides, chlorides, carbonates, sulfates, and organic acid salts. More specifically, for example, lithium hydroxide or its hydrate, lithium peroxide, lithium chloride, lithium carbonate, lithium sulfate, lithium acetate, lithium oxalate, etc. are preferable, and lithium hydroxide or its hydrate, chloride, etc. Lithium and lithium sulfate are more preferred.

The content of the lithium compound in the mixed solution (a-2) is preferably 0.1 parts by mass to 20 parts by mass, more preferably 0.1 parts by mass to 100 parts by mass of water in the mixed solution (a-2). The amount is 5 parts by weight to 15 parts by weight, more preferably 1 part to 10 parts by weight.

The pH of the mixed solution (a-2) at 25° C. is preferably 10 to 13 from the viewpoint of adjusting the pH at 25° C. of the mixed solution (A) obtained in step (III) described later to 8 to 11. , more preferably 10.5 to 12.5, still more preferably 11 to 12. A pH adjuster may be further used to adjust the pH of the mixed solution (a-2) to the above range. Such a pH adjuster is not particularly limited, but from the viewpoint of easily adjusting the pH of the mixed solution (a-2), it is preferable to use lithium hydroxide or an aqueous ammonia solution.

The liquid mixture (a-2) may be stirred in advance before being mixed with the liquid mixture (a-1) in step (III) described later, from the viewpoint of dissolving or dispersing each component more uniformly. The time for stirring the mixed solution (a-2) is preferably 1 minute to 30 minutes, more preferably 2 minutes to 20 minutes, and still more preferably 2 minutes to 10 minutes. In addition, the stirring speed is preferably 10 cm/sec to 200 cm/sec, more preferably 15 cm/sec to 150 cm/sec, and even more preferably is 20 cm/sec to 100 cm/sec.

Step (III) is to mix the mixture (a-1) obtained in step (I) and the mixture (a-2) obtained in step (II) until the pH at 25°C is 8. This is a step of preparing a mixed solution (A) of 1 to 11. That is, the mixed liquid (a-1) and the mixed liquid (a-2) are mixed to form a mixed liquid (A). The method of mixing the mixed liquid (a-1) and the mixed liquid (a-2) is not particularly limited, but the mixed liquid (a-2) may be dropped into the stirred mixed liquid (a-1). It is preferable to do so. In this way, by adding dropwise the mixture (a-2) containing the lithium compound little by little to the mixture (a-1) containing the lanthanum compound, titanium compound, and CNF(A), the reaction product can be produced. The homogeneity of the chemical composition can be further improved.
At this time, the dropping rate of the mixed solution (a-2) to the mixed solution (a-1) is preferably 0.1 parts by mass/min to 0.1 parts by mass/min to 10 parts by mass of the mixed solution (a-1). 4 parts by mass/min, more preferably 0.15 parts by mass/min to 0.4 parts by mass/min, still more preferably 0.2 parts by mass/min to 0.35 parts by mass/min. The stirring speed of the mixed solution (a-1) when dropping the mixed solution (a-2) is preferably 10 cm/sec to 10 cm/sec in terms of the flow rate of the mixed solution (a-1) on the inner wall surface of the reaction vessel. 200 cm/sec, more preferably 15 cm/sec to 150 cm/sec, still more preferably 20 cm/sec to 100 cm/sec.

In obtaining the mixed liquid (A), the mass ratio ((a-1):(a-2)) of the mixed liquid (a-1) and the mixed liquid (a-2) to be mixed is preferably 20:1. ~1:4, more preferably 10:1~1:3, still more preferably 5:1~2:3.

The temperature of the mixed liquid (a-1) when mixing the mixed liquid (a-1) and the mixed liquid (a-2) is preferably 10°C to 80°C, more preferably 15°C to 70°C. The temperature is more preferably 20°C to 60°C. Further, the temperature of the mixed solution (a-2) is preferably 10°C to 80°C, more preferably 15°C to 70°C, even more preferably 20°C to 60°C.

The time for mixing the mixed solution (a-1) and the mixed solution (a-2) is preferably 10 minutes to 24 hours, more preferably 15 minutes to 18 hours, and even more preferably 30 minutes to 12 hours. It is. Further, at this time, stirring may be performed from the viewpoint of dissolving or dispersing each component more uniformly. The stirring speed is preferably 10 cm/sec to 200 cm/sec, more preferably 15 cm/sec to 150 cm/sec, still more preferably 20 cm/sec, in terms of the flow rate of the mixed solution (A) on the inner wall surface of the reaction container. seconds to 100 cm/second.

In step (III), by mixing the mixed solution (a-1) and the mixed solution (a-2), a mixed solution (A) containing CNF(X) as well as lithium titanate and La(OH) ₃ is obtained. is obtained. These lithium titanate and La(OH) ₃ are fine particles with an average particle size of 100 nm or less, and by subjecting them to a hydrothermal reaction in step (IV) described below, they are converted into precursors of lithium lanthanum titanate crystal particles. and cellulose nanofibers. The total content of lithium titanate and La(OH) ₃ in the mixed liquid (A) is preferably 0.1% by mass to 60% by mass, more preferably 0.1% by mass to 60% by mass. It is 5% by mass to 45% by mass, more preferably 1% by mass to 30% by mass. The content of CNF(X) in the mixed liquid (A) is preferably 0.01% by mass to 40% by mass, more preferably 0.1% by mass to 30% by mass, and even more preferably 0. .2% to 20% by mass. Further, the pH of the mixed solution (A) at 25° C. is 8 to 11, preferably 8.5 to 10.5, and more preferably 9 to 10. In adjusting the pH of the liquid mixture (A) to fall within this range, the above-mentioned pH adjuster may be used as necessary.

Step (IV) is to subject the mixed solution (A) obtained in step (III) to a hydrothermal reaction at 100°C or higher, and then wash the hydrothermal reaction product containing CNF (X), This is then a step of drying to obtain a precursor mixture (B) containing CNF(X). The hydrothermal reaction in step (IV) is carried out under pressure by storing the reaction vessel filled with the mixed solution (A) in a pressure vessel or the like.

In addition, when stirring the mixed liquid (A) obtained in step (III) before proceeding to step (IV), such stirring may be performed in the reaction container, and then the reaction container is placed in a pressure container or the like. Just store it. From the viewpoint of efficiently producing the precursor mixture (B) containing CNF(X), such stirring is performed until the hydrothermal reaction in step (IV) is completed in the reaction vessel stored in a pressure vessel or the like. It is preferable to continue until The stirring speed of the mixed solution (A) at this time is preferably 15 cm/sec or more, more preferably 15 cm/sec to 80 cm/sec, in terms of the flow rate of the mixed solution (A) on the inner wall surface of the reaction container. and more preferably 15 cm/sec to 70 cm/sec. The stirring method is not particularly limited as long as this stirring speed can be achieved, but for example, a method using a stirring blade or a pump described in JP 2014-118328A is used to stir the slurry in the synthesis container. method etc. can be suitably used.

Furthermore, when using the above stirring method, from the viewpoint of uniformly generating a hydrothermal reaction throughout the mixed liquid (A), it is necessary to install a baffle plate in the reaction vessel, or to install a baffle plate in the rotational direction of the stirring blade or the liquid feeding direction of the pump. It is effective to create disturbances in the flow of the mixed liquid (A) by intermittently reversing the flow.

The temperature (reaction temperature) of the mixed solution (A) when subjected to the hydrothermal reaction may be 100°C or higher, preferably 150°C to 250°C, more preferably 160°C to 240°C. The pressure and reaction time at this time are preferably 0.5 MPa or more for 10 minutes or more when the reaction temperature is 100°C or higher, and 0.5 MPa to 4.0 MPa for 10 minutes when the reaction is carried out at 160°C to 240°C. ~24 hours is preferred, and when the reaction is carried out at 160°C ~ 240°C, 0.6 MPa ~ 3.4 MPa is preferred for 10 minutes ~ 18 hours.

Further, the atmosphere in the reaction vessel in the hydrothermal reaction is not limited, but from the viewpoint of ease of adjusting the atmosphere, it is preferably under air or under steam.

Next, after subjecting the mixed solution (A) to the above hydrothermal reaction, the hydrothermal reaction product containing CNF(X) is washed and then dried. The hydrothermal reaction product mixed with CNF(A) can be obtained by solid-liquid separation of the mixed liquid (A) after the hydrothermal reaction, and the resulting hydrothermal reaction product mixed with CNF(X) can be obtained by solid-liquid separation. The material is a mixture of lithium titanate (Li _1.81 , H _0.19 )Ti ₂ O ₅ .2H ₂ O), a lanthanum compound (amorphous), and CNF (X), that is, a precursor of LLTO crystal particles and CNF ( The precursor mixture (B) is a mixture of X) and X). Examples of devices used for solid-liquid separation include filter presses, centrifugal filters, and the like. Among these, it is preferable to use a filter press machine from the viewpoint of efficiently obtaining a hydrothermal reaction product containing cellulose nanofibers.

Next, the recovered hydrothermal reaction product mixed with CNF(X) is washed from the viewpoint of effectively removing water-soluble impurities such as anion components derived from the raw materials. Water may be used for such washing, and the amount of water is preferably 5 parts by mass to 50 parts by mass per 1 part by mass of the dry mass of the hydrothermal reaction product containing CNF (X). , more preferably 7 parts by mass to 50 parts by mass, still more preferably 9 parts by mass to 50 parts by mass. The temperature of such water is preferably 5°C to 70°C, more preferably 20°C to 70°C from the viewpoint of effectively removing water-soluble impurities.

The hydrothermal reaction product containing the washed CNF(X) is then dried. Examples of drying methods include spray drying, constant temperature drying, fluidized bed drying, external heating drying, freeze drying, vacuum drying, etc. Among them, aggregates are formed through calcination in step (V) described below. From the viewpoint of effectively controlling the excessive growth of the lithium lanthanum titanate crystal particles and achieving sufficient miniaturization, it is preferable to use spray drying or constant temperature drying.

The precursor mixture (B) containing CNF(X) obtained after drying contains lithium titanate (Li _1.81 , H _0.19 )Ti ₂ O ₅ .2H ₂ O) and a lanthanum compound (amorphous), It is a mixture with CNF(X). The average particle size of the precursor mixture (B) in which _such CNF(X) is mixed is preferably 5 nm to 100 nm, more preferably 5 nm to It is 80 nm. Here, the D ₅₀ value in particle size distribution measurement is a value obtained by volume-based particle size distribution based on laser diffraction/scattering method, and the D ₅₀ value means the particle size at 50% cumulative (median diameter). do. Therefore, when spray drying is employed as a drying method, the particle size of the precursor mixture (B) containing such CNF(X) may be adjusted by appropriately optimizing the operating conditions of the spray dryer used. .

Step (V) is a step of firing the precursor mixture (B) containing CNF (X) obtained in step (IV) at 400° C. to 1000° C. By going through this step (V), almost all of the CNF(X) is burnt out, and extremely fine particles of high purity lithium lanthanum titanate crystal particles are aggregated in a chain shape, which is extremely effective as a solid electrolyte. A useful chain-like lithium lanthanum titanate crystal particle aggregate can be obtained.

The firing temperature in step (V) is 400° C. to 1000° C., preferably 450° C. to 950° C., from the viewpoint of effectively refining the particles while increasing the crystallinity of the LLTO crystal particles aggregated in a chain shape. ℃, more preferably 500°C to 900°C. From the same viewpoint, the firing time is preferably 0.5 hours to 48 hours, more preferably 1 hour to 24 hours, and still more preferably 1 hour to 18 hours.

The atmosphere in the above firing is preferably an air atmosphere or an oxygen atmosphere from the viewpoint of burning out almost all of the CNF(X). The equipment used for such firing is not particularly limited as long as the temperature can be adjusted in the above atmosphere, and any of the batch type, continuous type, and heating type (indirect or direct) may be used. For example, a firing furnace such as an external heat kiln or a roller hearth can be used.
The fact that almost all of the CNF(X) was burnt out can be confirmed by the amount of carbon measured using a carbon/sulfur analyzer. The carbon content of the lithium lanthanum titanate crystal particles of the present invention is determined from the viewpoint of reducing the electronic conductivity and preventing a short circuit between the positive electrode and the negative electrode even when used as a solid electrolyte of a lithium ion secondary battery. The measured value is preferably 0% by mass to 0.5% by mass, more preferably 0% by mass to 0.3% by mass, and even more preferably 0% by mass to 0.1% by mass.

As described above, according to the production method of the present invention, titanium with high crystallinity, which has a high BET specific surface area while effectively reducing the particle size, and has a sufficiently reduced content of water-soluble impurities, can be produced. It is possible to obtain a chain-like lithium lanthanum titanate crystal particle aggregate in which lithium lanthanum titanate crystal particles are collected in a chain shape and arranged continuously in a straight line. Therefore, a solid electrolyte for an all-solid lithium ion secondary battery that can exhibit excellent charge and discharge characteristics can be easily produced.

Specifically, from the viewpoint of exhibiting excellent lithium ion conductivity, the average particle size of the lithium lanthanum titanate crystal particles constituting the chained lithium lanthanum titanate crystal particle aggregate obtained by the production method of the present invention is as follows: The thickness is preferably 5 nm to 50 nm, more preferably 5 nm to 40 nm, and even more preferably 5 nm to 30 nm. Here, the average particle size of lithium lanthanum titanate crystal particles is the average value of the measured particle sizes (long axis lengths) of several dozen particles observed using an SEM or TEM electron microscope. means.

In addition, the BET specific surface area of the lithium lanthanum titanate crystal particles constituting the chain-like lithium lanthanum titanate crystal particle aggregate obtained by the production method of the present invention is determined between the lithium lanthanum titanate crystal particles and the lithium lanthanum titanate crystal particles. From the viewpoint of ensuring a sufficient probability of contact between electrode materials, the area is preferably 40 m ² /g or more, more preferably 45 m ² /g or more, and still more preferably 50 m ² /g or more.

The tap density of the lithium lanthanum titanate crystal particles constituting the chained lithium lanthanum titanate crystal particle aggregate obtained by the production method of the present invention is 0.5 g/cm ³ to 3.0 g/cm ³ , preferably It is 0.6g/cm ³ to 2.8g/cm ³ , more preferably 0.7g/cm ³ to 2.6g/cm ³ . The fact that lithium lanthanum titanate crystal particles, which are used as a solid electrolyte by forming an aggregate, has such a tap density means that when forming a solid electrolyte phase in an all-solid lithium ion secondary battery, lithium lanthanum titanate crystal particles are used as a solid electrolyte. It is possible to efficiently and effectively improve the filling properties of lanthanum crystal particles, which makes it possible to obtain a dense solid electrolyte layer and to develop excellent charge-discharge characteristics as an all-solid-state lithium-ion secondary battery. It is thought that this will make a significant contribution.

A lithium ion secondary battery to which the chain-like lithium lanthanum titanate crystal particle aggregate obtained by the production method of the present invention can be appropriately applied as a solid electrolyte has a positive electrode, a negative electrode, and a solid electrolyte as essential components, and There is no particular limitation as long as a laminate is formed in which a positive electrode active material layer, a solid electrolyte layer, and a negative electrode active material layer are stacked in this order.

Here, the material composition of the positive electrode active material layer is not particularly limited as long as it can release lithium ions during charging and absorb them during discharging, and any known material composition can be used. . For example, a positive electrode active material layer made of various polyanionic positive electrode active materials obtained by subjecting raw material compounds to a hydrothermal reaction can be suitably used.

Further, the material composition of the negative electrode active material layer is not particularly limited as long as it can occlude lithium ions during charging and release them during discharging, and any known material composition can be used. For example, a negative electrode active material layer made of titanium niobium oxide or sodium lithium titanate composite oxide obtained by subjecting raw materials to a hydrothermal reaction can be suitably used.

Further, the method for manufacturing a lithium ion secondary battery using the chain-like lithium lanthanum titanate crystal particle aggregate obtained by the manufacturing method of the present invention is not particularly limited, and any known method can be used. For example, as described in JP-A No. 2017-10816, chain lithium lanthanum titanate crystal particle aggregates obtained according to the present invention are used as solid electrolyte particles included in a positive electrode active material layer and a negative electrode active material layer. A solid electrolyte other than such solid electrolyte particles may be used in the solid electrolyte layer.

The shape of the lithium ion secondary battery having the above configuration is not particularly limited, and may be various shapes such as a coin shape, a cylindrical shape, a square shape, or an irregular shape enclosed in a laminate exterior body. .

Hereinafter, the present invention will be specifically explained based on Examples, but the present invention is not limited to these Examples.

[Production Example 1] (Production of LiMn _1.5 Ni _0.5 O ₄ positive electrode active material particles)
25.35 g of MnSO ₄ .H ₂ O, 13.14 g of NiSO ₄ .6H ₂ O, and 100 mL of water were mixed to obtain slurry (x-1). To the obtained slurry (x-1), while stirring and maintaining the temperature at 25°C, 48% by mass NaOH was added dropwise to the slurry (x-1) until the pH of the slurry (x-1) became 11. ) was obtained. Next, the obtained slurry (x-2) was stirred for 1 hour to obtain a precursor (x-3), and then 12 parts by mass was added to 1 part by mass of the obtained precursor (x-3). Washed with water.
After mixing the entire amount of precursor (x-3), 4.20 g of LiOH・H ₂ O, and 100 mL of water, they were mixed for 15 minutes in a planetary ball mill (P-5, manufactured by Fritsche Japan Co., Ltd.) to form a slurry (x -4) was obtained. Next, the obtained slurry (x-4) was dried at 80°C for 12 hours, and then the obtained dried product was fired at 800°C for 1 hour in the air to form LiMn _1.5 Ni _0.5 O ₄ positive electrode active material particles. (The average particle diameter of primary particles was 100 nm).

[Example 1] (Production of Li _0.33 La _0.56 TiO ₃ crystal particles)
2.24 g of TiSO ₄ .1.5H ₂ O, _2.89 g of La(NO ₃ ) 3.6H ₂ O, and cellulose nanofiber (KY-100G, manufactured by Sudaicel Finechem, water content 10%) in 40 mL of water.3. 89 g were mixed to obtain a mixed solution (a-1) ¹ .
Next, 2.08 g of LiOH·H ₂ O was dissolved in 20 mL of water to obtain a mixed solution (a-2) ¹ . The pH of the obtained mixed solution (a-2) ¹ at 25° C. was 13.
The obtained mixed solution (a-1) ¹ was stirred at a stirring speed of 200 rpm for 10 minutes while maintaining the temperature at 25°C, and then the mixed solution was added to the mixed solution (a-1) ¹ while stirring was continued. The entire amount of mixed solution (a-2) ¹ was added dropwise at a rate of 0.3 parts by mass/min to 10 parts by mass of (a-2) ¹ and mixed. Thereafter, the mixture was further stirred for 10 minutes at a stirring speed of 200 rpm to obtain a mixed solution (A) ¹ . The pH of the mixture (A) ¹ at 25° C. was 10.

The obtained mixed solution (A) ¹ was put into an autoclave, and a hydrothermal reaction was carried out at 170° C. and 0.8 MPa for 10 hours. The generated solid content was filtered by suction, and then the obtained solid content was washed with 5 parts by mass of water per 1 part by mass of solid content, and then dried at a constant temperature of 80° C. for 12 hours to obtain a precursor mixture (B) ¹ I got it. The obtained precursor mixture (B) ¹ was a mixture of lithium titanate (Li _1.81 , H _0.19 )Ti ₂ O ₅ .2H ₂ O), a lanthanum compound (amorphous), and CNF (X). Ta.

The obtained precursor mixture (B) ¹ in which cellulose nanofibers are mixed is fired at 750°C for 4 hours in an air atmosphere to completely burn out the cellulose nanofibers and remove the titanium constituting the chain aggregates. Lithium lanthanum oxide crystal particles X ¹ (Li _0.33 La _0.56 TiO ₃ ) were obtained.

The LLTO crystal particles X ¹ constituting the obtained chain aggregate were a single phase exhibiting an X-ray diffraction pattern corresponding to a perovskite phase, with an average particle size of 30 nm and a BET specific surface area of 49 m ² /g. . FIG. 1 shows a TEM photograph of the chain aggregate formed by the obtained LLTO crystal particles X ¹ , and FIG. 2 shows an X-ray diffraction pattern.

[Example 2] (Production of Li _0.81 La _0.40 TiO ₃ crystal particles)
Same as Example 1 except that the amount of La(NO ₃ ) ₃ ·6H ₂ O added was 2.07 g, the amount of LiOH·H ₂ O was 2.10 g, and the amount of cellulose nanofibers was 2.33 g. A chain aggregate formed of LLTO oxide particles X ² was obtained.
The LLTO oxide particles X ² constituting the obtained chain aggregate were Li _0.81 La _0.40 TiO ₃ single phase, had an average particle diameter of 28 nm, and a BET specific surface area of 52 m ² /g.

[Example 3] (Production of Li _0.12 La _0.63 TiO ₃ crystal particles)
Same as Example 1 except that the amount of La(NO ₃ ) ₃ ·6H ₂ O added was 3.26 g, the amount of LiOH·H ₂ O was 1.89 g, and the amount of cellulose nanofibers was 5.45 g. A chain aggregate formed of LLTO oxide particles X ³ was obtained.
The LLTO oxide particles X ³ constituting the obtained chain aggregate were Li _0.12 La _0.63 TiO ₃ single phase, had an average particle diameter of 33 nm, and a BET specific surface area of 47 m ² /g.

[Comparative Example 1] (Production of Li _0.81 La _0.40 TiO ₃ crystal particles)
Aqueous solution (y-1) ¹ was obtained by mixing 31.2 g of Cl ₃ La.7H ₂ O and 19.09 g of TiCl ₄ with 100 mL of water. 370g of 28% ammonia water was added dropwise to the obtained aqueous solution (y-1) ¹ , the obtained precipitate was separated, washed with water, and dried at 200°C for 12 hours to form a precipitate (y-2). Got ¹ .
Next, ¹ whole amount of the obtained precipitate (y-2) and 3.49 g of LiOH were mixed in 10 mL of water, and then the mixture was charged into an autoclave and a hydrothermal reaction was performed at 150° C. and 0.5 MPa for 5 hours. . The generated solid content was suction-filtered, and then the obtained solid content was washed with 5 parts by mass of water per 1 part by mass of solid content, and then dried at a constant temperature of 80°C for 12 hours to obtain a precursor mixture (y-3). ) got ¹ .
The obtained precursor mixture (y-3) ¹ was fired at 850° C. for 12 hours in an air atmosphere to obtain LLTO crystal particles Y ¹ (Li _0.81 La _0.40 TiO ₃ ). The obtained LLTO crystal particles Y ¹ (Li _0.81 La _0.40 TiO ₃ ) were a single phase exhibiting an X-ray diffraction pattern corresponding to a perovskite phase, with an average particle size of 67 nm and a BET specific surface area of 29 m ² /g. Ta.
A SEM photograph of the obtained LLTO crystal particles Y ¹ is shown in FIG.

[Comparative Example 2] (Production of Li _0.12 La _0.63 TiO ₃ crystal particles)
The same procedure as Comparative Example 1 was carried out except that Cl ₃ La 7H ₂ O was changed from 31.2 g to 22.41 g, 10 mL of water as the material to be charged into the autoclave was changed to 20 mL, and LiOH was changed from 2.07 g to 5.08 g. LLTO oxide particles Y ² (Li _0.12 La _0.63 TiO ₃ ) were obtained. The obtained LLTO crystal particles Y ² (Li _0.12 La _0.63 TiO ₃ ) were a single phase exhibiting an X-ray diffraction pattern corresponding to a perovskite phase, with an average particle size of 61 nm and a BET specific surface area of 31 m ² /g. Ta.

[Comparative Example 3] (When the pH of the mixed solution (A) is outside the range)
LLTO oxide particles Y ³ were obtained in the same manner as in Example 1, except that the amount of LiOH·H ₂ O added was 1.38 g. At this time, the pH of the mixed solution (A) ² at 25°C was 7.
The obtained LLTO oxide particles Y ³ contained impurity phases (such as La ₂ Ti ₂ O ₇ ) in addition to Li _0.33 La _0.56 TiO ₃ corresponding to the perovskite phase. The average particle size of these mixed phases was 25 nm, and the BET specific surface area was 57 m ² /g.

"Evaluation test"
All-solid-state lithium ion secondary batteries were produced using the chain aggregates or LLTO crystal particles obtained in Examples 1 to 3 and Comparative Examples 1 to 3.
Specifically, the LiMn _1.5 Ni _0.5 O ₄ positive electrode active material particles obtained in Production Example 1 were used for the positive electrode, and after mixing the positive electrode active material:solid electrolyte (mass ratio) at a mixing ratio of 75:25, the particles were pressed. The mixture was placed in a jig to form a positive electrode active material layer. Furthermore, only the chain aggregates or LLTO crystal particles obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were further added onto the layer to form a solid electrolyte layer, and then a pressure of 16 MPa was applied using a hand press. The mixture was pressed for 2 minutes to obtain a disk-shaped positive electrode with a diameter of 14 mm. Next, a lithium foil was attached as a negative electrode to the solid electrolyte layer side, thereby producing an all-solid lithium ion secondary battery.

Using the fabricated all-solid-state lithium ion secondary battery, charging conditions were 13 mA/g, voltage 5.0 V constant current charging, discharging conditions were 13 mA/g, final voltage 3.0 V constant current discharging. The capacity was determined. Note that all charge/discharge tests were conducted at 60°C. The results are shown in Table 1.

From the above results, it can be seen that the chain aggregate obtained by the production method of the example exhibits an excellent discharge capacity compared to the LLTO crystal particles obtained by the production method of the comparative example. This is because, as can be seen from the comparison between FIGS. 1 and 3, the chain aggregates obtained by the production method of Example 1 are particles with a small average particle size and a large BET specific surface area, and have a high degree of crystallinity. Since it is an aggregate made up of a plurality of LLTO crystal particles, which are fine particles, it is thought that it contributed to improving the performance efficiently and effectively.

Claims (5)

Next steps (I) to (V):
(I) A lanthanum compound, a titanium compound, a cellulose nanofiber, and/or a lignocellulose nanofiber are mixed with water, and 0.1 mass of cellulose nanofiber and/or lignocellulose nanofiber is added to 100 parts of water by mass. A step of preparing a mixed solution (a-1) containing 1 to 40 parts by mass,
(II) a step of mixing a lithium compound and water to prepare a mixed solution (a-2);
(III) mixing the obtained mixture (a-1) and mixture (a-2) to prepare a mixture (A) having a pH of 8 to 11 at 25°C;
(IV) A step of subjecting the obtained mixed solution (A) to a hydrothermal reaction at 100° C. or higher, washing the obtained hydrothermal reaction product and then drying it to obtain a precursor mixture; V) A method for producing a chain-like lithium lanthanum titanate crystal particle aggregate for a solid electrolyte, comprising a step of firing the obtained precursor mixture at 400° C. to 1000° C. in an air atmosphere or an oxygen atmosphere. ,
In step (I), the lanthanum compound is lanthanum nitrate, lanthanum sulfate or lanthanum chloride, and the titanium compound is titanyl sulfate or titanium chloride,
The lanthanum lithium titanate crystal particles constituting the chained lanthanum lithium titanate crystal particle aggregate for solid electrolyte have the following formula (1):
Li _3x La _2/3-x TiO ₃ ...(1)
(In formula (1), x represents a number that satisfies 0<x<0.3.)
A method for producing a chain-like lithium lanthanum titanate crystal particle aggregate for solid electrolyte, represented by: In the mixing in step (III), the mass ratio ((a-1):(a-2)) of mixed liquid (a-1) and mixed liquid (a-2) is 20:1 to 1:4. The method for producing a chain-like lithium lanthanum titanate crystal particle aggregate according to claim 1. The chain lanthanum titanate according to claim 1 or 2, wherein the mixed solution (A) obtained in step (III) contains 0.01% by mass to 40% by mass of cellulose nanofibers and/or lignocellulose nanofibers. A method for producing a lithium crystal particle aggregate. Chained lithium lanthanum titanate crystal according to any one of claims 1 to 3, wherein the mixing in step (III) is performed by dropping the mixed solution (a-2) into the mixed solution (a-1). Method for producing particle aggregates. A chain-like lithium lanthanum titanate crystal particle aggregate formed by a plurality of lithium lanthanum titanate crystal particles having an average particle size of 5 nm to 50 nm and a BET specific surface area of 40 m ² /g or more. A method for producing a chain-like lithium lanthanum titanate crystal particle aggregate according to any one of claims 1 to 4 , for obtaining the following.