JP2020102374A - Method for producing lisicon-type crystal particle for lithium ion secondary battery solid electrolyte - Google Patents

Abstract

To provide a method for producing a LISICON-type crystal particle for a secondary battery solid electrolyte, the LISICON-type oxide particle being capable of developing excellent lithium ion conductivity by appropriately and effectively refining a LISICON-type crystal represented by Li4-xSi1-xPxO4.SOLUTION: A method for producing a LISICON-type crystal particle that is represented by the following formula (1): Li4-xSi1-xPxO4...(1) (in the formula (1), x represents a number satisfying 0<x<1.) includes the following steps (I) to (III): the step (I) of mixing a lithium compound, a phosphate compound, a silicic acid compound and water to prepare a mixed solution having a pH at 25°C of 9-14; the step (II) of, after subjecting an obtained mixed solution to a hydrothermal reaction at 100°C or above, cleaning a hydrothermal reaction product and then drying a cleaned hydrothermal reaction product to obtain a precursor mixture; and the step (III) of firing an obtained precursor mixture at 400-1,000°C.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing LISICON type crystal particles for a solid electrolyte for use in a secondary battery such as an all solid lithium ion secondary battery.

Lithium-ion secondary batteries currently in practical use use a flammable organic solvent as the electrolytic solution, so it is necessary to take sufficient safety measures against liquid leakage, ignition, etc. The difficulty level of conversion is also high. However, an all-solid-state lithium-ion secondary battery equipped with an oxide-based or sulfide-based solid electrolyte has a high energy density and can be manufactured without using combustible materials, so safety measures are taken. The burden is reduced, and the manufacturing cost and productivity can be easily increased. For these reasons, various developments have been activated for solid electrolyte materials, which are expected to be highly useful.

In particular, Li ₄ SiO ₄ —Li ₃ PO ₄ having a LIICON type crystal structure having a skeleton structure formed by γ-Li ₃ PO ₄ (high temperature phase) type SiO ₄ and PO ₄ tetrahedra and LiO ₆ octahedra. The solid solution has the characteristic that it is a solid electrolyte having a high lithium ion concentration in terms of its crystal structure, and also has the excellent property of exhibiting a high lithium ion conductivity of the order of 10 ⁻⁴ S/cm at room temperature. It is one of the highly anticipated solid electrolyte materials.

In order to put such LISICON-type crystal particles into practical use, it is important to realize high purification and finer particles. For example, in Non-Patent Document 1, Li _3.5 Si is obtained by pulverizing and firing raw materials. how to obtain a lithium ion conductive solid electrolyte is disclosed which is represented by _0.5 P _0.5 O _4.

D. Wang et al. / Solid State Ionics 283 (2015) 109-114.

However, the LISICON-type crystal particles obtained by the production method described in the above Non-Patent Document has a defect induced in the crystal structure of the LISICON-type crystal due to the pulverization treatment that must be performed in order to reduce the size, Lithium ion conductivity may be reduced.

Therefore, an object of the present invention is to appropriately and effectively refine the LISICON type crystal represented by Li _4-x Si _1-x P _x O ₄ to obtain excellent lithium ion conductivity, An object of the present invention is to provide a method for producing LISICON type crystal particles useful as a solid electrolyte of a secondary battery.

Then, the inventors of the present invention have conducted various studies, and found that by performing a specific step while preparing a specific mixed solution, high crystallinity LISICON crystal particles can be obtained while appropriately miniaturizing. The present invention has been completed.

That is, the present invention includes the following steps (I) to (III):
(I) A step of mixing a lithium compound, a silicic acid compound and a phosphoric acid compound with water to prepare a mixed solution having a pH of 9 to 14 at 25°C (II) The obtained mixed solution is 100°C or higher. The step of washing the hydrothermal reaction product, and then drying it to obtain a precursor mixture (III) firing the obtained precursor mixture at 400° C. to 1000° C. , The following formula (1):
Li _4-x Si _1-x P _x O ₄ (1)
(In the formula (1), x represents a number satisfying 0<x<1.)
The present invention provides a method for producing LISICON-type crystal particles for a solid electrolyte of a lithium ion secondary battery represented by.

According to the method of the present invention, it is possible to produce fine and highly crystalline LISICON type crystal particles only by preparing a predetermined liquid mixture, hydrothermally reacting the liquid mixture, and then calcining the mixture. Since it is not necessary to perform a pulverization process for achieving the above, it can be used as a solid electrolyte while having a high degree of crystallinity, whereby a secondary battery having excellent charge/discharge characteristics can be realized.

1 is an SEM image showing LISICON type crystal particles obtained in Example 1. 2 is an X-ray diffraction pattern of LISICON type crystal particles obtained in Example 1. 5 is an SEM image showing LISICON type crystal particles obtained in Comparative Example 1. 3 is an X-ray diffraction pattern of LISICON type crystal particles obtained in Comparative Example 1.

Hereinafter, the present invention will be described in detail.
The method of the present invention comprises the following steps (I) to (III):
(I) A step of mixing a lithium compound, a silicic acid compound and a phosphoric acid compound with water to prepare a mixed solution having a pH of 9 to 14 at 25°C (II) The obtained mixed solution is 100°C or higher. The step of washing the hydrothermal reaction product, and then drying it to obtain a precursor mixture (III) firing the obtained precursor mixture at 400° C. to 1000° C. , The following formula (1):
Li _4-x Si _1-x P _x O ₄ (1)
(In the formula (1), x represents a number satisfying 0<x<1.)
Is a method for producing LISICON-type crystal particles for a solid electrolyte of a lithium ion secondary battery represented by.

The step (I) is a step of mixing all the raw material compounds of the lithium compound, the silicic acid compound and the phosphoric acid compound with water to prepare a mixed solution (A) having a pH of 9 to 14 at 25°C. is there. As described above, by using the above-mentioned raw material compound for obtaining the LISICON type crystal particles and water, and obtaining the mixed solution (A) in which the pH is controlled within the above range, the hydrothermal reaction in the step (II) described later can be performed. In the hydrothermal reaction product obtained by adding, the homogeneity of the chemical composition can be enhanced, and it can greatly contribute to the miniaturization and high crystallinity of LISICON type crystal particles.

Examples of the lithium compound that can be used as one of the above-mentioned raw material compounds include one or more selected from hydroxides, chlorides, carbonates, sulfates, and organic acid salts. More specifically, for example, lithium hydroxide or a hydrate thereof, lithium peroxide, lithium chloride, lithium carbonate, lithium sulfate, lithium acetate, lithium oxalate and the like can be preferably used.

The silicic acid compound that can be used as one of the raw material compounds together with the lithium compound is not particularly limited as long as it is a reactive silica compound, and includes silicon dioxide, amorphous silica, Na ₄ SiO ₄ (for example, Na ₄ SiO ₄ · H ₂ O), and the like.

The silicic acid compound efficiently causes the hydrothermal reaction in the step (II) to obtain a hydrothermal reaction product having a high homogeneity of chemical composition and high crystallinity and a sufficiently reduced diameter. From the viewpoint, fine particles are preferable, and the average particle diameter is preferably 100 nm or less, more preferably 75 nm or less.

Examples of the phosphoric acid compound include orthophosphoric acid (H ₃ PO ₄ , phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, ammonium hydrogenphosphate and the like. Among them, phosphoric acid is preferably used from the viewpoint of being able to act also as a pH adjuster for controlling the pH of the resulting mixed liquid (A), and is used as an aqueous solution having a concentration of 70% by mass to 90% by mass. preferable.

The raw material compound is composed of three kinds of compounds of a lithium compound, a silicic acid compound and a phosphoric acid compound. In the step (I), all of these raw material compounds are used collectively.

In the step (I), specifically, a mixed solution (a-1) containing a lithium compound, a silicic acid compound and water and a mixed solution (a-2) containing a phosphoric acid compound and water are prepared, respectively. After that, these two kinds of mixed solutions are mixed to prepare a mixed solution (A). As a result, all the raw material compounds and water are present in the resulting mixed liquid (A), and the reaction product having a high homogeneity of the chemical composition and a sufficiently reduced diameter, that is, trilithium phosphate. Since the mixture of the silicic acid compound added as the raw material compound is contained, it is possible to effectively miniaturize the LISICON type crystal particles.

The mixed liquid (a-1) is prepared by mixing a lithium compound and a silicic acid compound with water. The total content of the lithium compound and the silicic acid compound in the mixed solution (a-1) is a mixed solution from the viewpoint of increasing the homogeneity of the chemical composition of the reaction product generated by mixing with the mixed solution (a-2). With respect to 100 parts by mass of water in (a-1), it is preferably 1 part by mass to 50 parts by mass, more preferably 5 parts by mass to 30 parts by mass, and further preferably 5 parts by mass to 20 parts by mass. is there.

In preparing the mixed liquid (a-1), the mixing ratio of the lithium compound and the silicic acid compound is a molar ratio (Li:Si), preferably 3.1:0.1 to 4:1. It is more preferably 3.1:0.1 to 3.9:0.9, and even more preferably 3.2:0.2 to 3.8:0.8.

The mixed solution (a-1) may be previously stirred before being mixed with the mixed solution (a-2) from the viewpoint of more uniformly dispersing each component. The time for stirring the mixed solution (a-1) is preferably 5 minutes to 3 hours, more preferably 10 minutes to 2 hours, and further 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, even more preferably converted to the flow rate of the mixed liquid (a-1) on the inner wall surface of the reaction vessel. 20 cm/sec to 100 cm/sec.

The mixed solution (a-2) is prepared by mixing the phosphoric acid compound and water. The content of the phosphoric acid compound in the mixed solution (a-2) is preferably 3 parts by mass to 20 parts by mass, and more preferably 5 parts by mass, relative to 100 parts by mass of water in the mixed solution (a-2). Parts to 15 parts by mass, more preferably 7 parts to 15 parts by mass.

The pH of the mixed liquid (a-2) at 25°C is preferably 12 to 14, and more preferably 12.5, from the viewpoint of adjusting the pH of the resulting mixed liquid (A) at 25°C to 9 to 14. It is -14, More preferably, it is 13-14. In adjusting the pH of the mixed solution (a-2) within the above range, a pH adjustor may be further used. The pH adjuster is not particularly limited, but from the viewpoint of easily adjusting the pH of the mixed solution (a-2), water such as lithium hydroxide, sodium hydroxide, potassium hydroxide or ammonium hydroxide is used. Preference is given to using oxides.

The mixed solution (a-2) may be previously stirred before being mixed with the mixed solution (a-1) from the viewpoint of more uniformly dissolving or dispersing each component. The time for stirring the mixed solution (a-2) is preferably 1 minute to 30 minutes, more preferably 2 minutes to 20 minutes, and further preferably 2 minutes to 10 minutes. 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 converted to the flow rate of the mixed liquid (a-2) on the inner wall surface of the reaction vessel. Is 20 cm/sec to 100 cm/sec.

Next, in the step (I), the mixed solution (a-1) and the mixed solution (a-2) are mixed to obtain a mixed solution (A). The method for mixing the mixed solution (a-1) and the mixed solution (a-2) is not particularly limited, but the mixed solution (a-2) is added dropwise to the stirred mixed solution (a-1). Preferably. As described above, the mixture liquid (a-1) containing the lithium compound and the silicic acid compound was added dropwise to the mixture liquid (a-2) containing the phosphoric acid compound to add all of the raw material compounds and water. While containing, the homogeneity of the chemical composition of the reaction product can be further enhanced.
At this time, the dropping rate of the mixed solution (a-2) to the mixed solution (a-1) is preferably 0.1 part by mass/min to 0. It is 4 parts by mass/min, more preferably 0.15 parts by mass/min to 0.4 parts by mass/min, and even 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 the mixed solution (a-2) is dropped is preferably 10 cm/sec or more in terms of the flow rate of the mixed solution (a-1) on the inner wall surface of the reaction vessel. It is 200 cm/sec, more preferably 15 cm/sec to 150 cm/sec, and even more preferably 20 cm/sec to 100 cm/sec.

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

The temperature of the mixed solution (a-1) when the mixed solution (a-1) and the mixed solution (a-2) are mixed is preferably 10°C to 80°C, more preferably 15°C to 70°C. Yes, and more preferably 20°C to 60°C. The temperature of the mixed solution (a-2) is preferably 10°C to 80°C, more preferably 15°C to 70°C, and further 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 further preferably 30 minutes to 12 hours. Is. In addition, at this time, stirring may be performed in order to dissolve or disperse each component more uniformly and enhance homogeneity of the chemical composition of the reaction product. 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 20 cm/, in terms of the flow rate of the mixed solution (A) on the inner wall surface of the reaction vessel. Second to 100 cm/sec.

By mixing the mixed solution (a-1) and the mixed solution (a-2), all the raw material compounds are mixed together, and the lithium phosphate and the silicic acid compound added as the raw material compound are contained. A mixed solution (A) is obtained. The trilithium phosphate and the silicic acid compound are fine particles having an average particle size of 100 nm or less, and by undergoing a step of subjecting to a hydrothermal reaction in the step (II) described later, a precursor mixture (B ) Is generated well. The total content of the lithium phosphate and the silicic acid compound in the mixed solution (A) is preferably 0.5% by mass to 40% by mass, more preferably 1% by mass, in the mixed solution (A). Is 35% by mass, more preferably 1.5% by mass to 30% by mass. Further, the pH of the mixed liquid (A) at 25° C. is 9 to 14, preferably 10 to 14, and more preferably 10.5 to 14. When adjusting the pH of the mixed solution (A) so as to be within such a range, the above pH adjuster may be used if necessary.
In addition, it can be confirmed by X-ray diffraction that lithium phosphate is produced in the mixed solution (A) and is contained as a mixture.

In the step (II), the mixed solution (A) obtained in the step (I) is subjected to a hydrothermal reaction at 100° C. or higher, the hydrothermal reaction product is washed, and then dried to obtain a precursor mixture ( This is the step of obtaining B). The hydrothermal reaction in the step (II) is carried out under pressure by storing a reaction container filled with the mixed liquid (A) containing all the raw material compounds and water in a pressure container or the like.

When the mixed solution (A) obtained in the step (I) is stirred before proceeding to the step (II), such stirring may be performed in the reaction vessel, and then the reaction vessel is changed to a pressure vessel or the like. Just store it. From the viewpoint of efficiently producing the precursor mixture of LISICON type crystal particles, such stirring is preferably continued until the hydrothermal reaction in step (II) is completed in the reaction vessel stored in a pressure vessel or the like. The stirring speed of the mixed liquid (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 liquid (A) on the inner wall surface of the reaction vessel. 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 realized, but for example, a method using a stirring blade or a slurry described in JP-A-2014-118328 is used to stir the slurry in the synthesis container. A method etc. can be used conveniently.
Furthermore, when using the above stirring method, from the viewpoint of causing a uniform hydrothermal reaction in the entire liquid mixture (A), a baffle plate is installed in the reaction vessel, the rotating direction of the stirring blades, or the liquid feeding direction of the pump. It is effective to cause a disturbance in the flow of the mixed liquid (A) by intermittently reversing.

The temperature (reaction temperature) of the mixed solution (A) when subjected to the hydrothermal reaction may be 100° C. or higher, preferably 130° C. to 250° C., and more preferably 140° C. to 230° C. The pressure and reaction time at this time are preferably 0.3 MPa or more and 10 minutes or more when the reaction temperature is 100° C. or more, and 0.3 MPa to 2 MPa and 10 minutes to 24 when the reaction is performed at 130° C. to 250° C. Time is preferable, and when the reaction is performed at 140° C. to 230° C., 0.4 MPa to 1.8 MPa and 10 minutes to 16 hours are preferable.

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

Then, the mixed solution (A) is subjected to the hydrothermal reaction, and then the hydrothermal reaction product is washed and then dried. Such a hydrothermal reaction product may be obtained by solid-liquid separation of the mixed solution (A) after the hydrothermal reaction, and the resulting hydrothermal reaction product is a mixture of lithium phosphate and lithium silicate, that is, a LIICON type crystal. A precursor mixture (B) of particles is contained. Examples of the apparatus used for solid-liquid separation include a filter press machine and a centrifugal filter machine. Above all, it is preferable to use a filter press machine from the viewpoint of efficiently obtaining the hydrothermal reaction product.

Next, the recovered hydrothermal reaction product 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, and more preferably 7 parts by mass to 50 parts by mass with respect to 1 part by mass of dry mass of the hydrothermal reaction product. It is a mass part, More preferably, it is 9 mass parts-50 mass parts. 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 washed hydrothermal reaction product is then dried. Examples of the drying means include spray drying, box drying, fluidized bed drying, external heating drying, freeze drying, vacuum drying, and the like. From the viewpoint of effectively controlling the growth of the shaped crystal particles more than necessary and achieving sufficient miniaturization, spray drying is preferable.

The precursor mixture (B) obtained after drying is a mixture of lithium phosphate and lithium silicate. The average particle size of the precursor mixture (B) is a D ₅₀ value in the particle size distribution based on the laser diffraction/scattering method, and is preferably 5 nm to 250 nm, more preferably 10 nm to 200 nm. Here, the D ₅₀ value in the particle size distribution measurement is a value obtained by a volume-based particle size distribution based on a laser diffraction/scattering method, and the D ₅₀ value means a particle size (median diameter) at a cumulative 50%. To do. Therefore, when spray drying is adopted as the drying means, the particle size of the precursor mixture (B) may be adjusted by appropriately optimizing the operating conditions of the spray dryer used.

The step (III) is a step of firing the precursor mixture (B) obtained in the step (II) at 400°C to 1000°C. By passing through the step (III), it is possible to obtain LISICON type crystal particles which are fine particles having extremely high crystallinity and high purity and which are very useful as a solid electrolyte of a lithium ion secondary battery.

The firing temperature in step (III) is 400° C. to 1000° C., preferably 500° C. to 900° C., from the viewpoint of effectively refining while increasing the crystallinity of the obtained LISICON type crystal particles. More preferably, it is 600°C to 800°C. From the same viewpoint, the firing time is preferably 0.5 hours to 30 hours, more preferably 1 hour to 24 hours, and further preferably 1 hour to 18 hours.

The atmosphere in the above firing is not particularly limited, and may be any of an air atmosphere, an inert gas atmosphere or a reducing condition, but from the viewpoint of simplicity, the air atmosphere is preferable. The apparatus used for such firing is not particularly limited as long as the temperature can be adjusted, and batch type, continuous type, and heating type (indirect or direct) types can be used. , An external heat kiln, a roller hearth, and other firing furnaces.

The average particle size of the LISICON type crystal particles produced by the method of the present invention is preferably 70 nm to 300 nm, more preferably 70 nm to 250 nm, and further preferably 70 nm to 200 nm.
Here, the average particle size of the LISICON type crystal particles means the average value of the measured values of the particle size (long axis length) of several tens of particles in the observation using an electron microscope of SEM or TEM. ..

Further, the BET specific surface area of the obtained LISICON type crystal particles is preferably 3 m ² /g or more, more preferably 5 m ² /g or more from the viewpoint of obtaining a lithium ion secondary battery having excellent charge/discharge characteristics. And more preferably 7 m ² /g or more.

The LISICON crystal particles produced by the method of the present invention are particles having a very small content of water-soluble impurities such as SO ₃ . For example, when a sulfate is used as a raw material compound to be used, the content of water-soluble impurities (SO ₃ amount) in the obtained LISICON type crystal particles is preferably 500 ppm or less, more preferably 300 ppm or less.

The LISICON type crystal particles produced by the method of the present invention are oxides having a LISICON type crystal structure, and are specifically represented by the following formula (1).
Li _4-x Si _1-x P _x O ₄ (1)
(In the formula (1), x represents a number satisfying 0<x<1.)
More specifically, for example, Li _3.5 Si _0.5 P _0.5 O ₄ , Li _3.9 Si _0.9 P _0.1 O ₄ , and Li _3.1 Si _0.1 P _0.9 O ₄ can be mentioned.

As described above, according to the method of the present invention, it is possible to produce LISICON-type crystal particles having a high crystallinity and effectively reducing the particle size. Therefore, it is possible to easily obtain a solid electrolyte for an all-solid-state lithium-ion secondary battery that can exhibit excellent charge/discharge characteristics.

Thus, the lithium ion secondary battery to which the LISICON type crystal particles produced by the 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 constituents. There is no particular limitation as long as it forms a laminate in which the electrolyte layer and the negative electrode active material layer are laminated in this order.

Here, the material composition of the positive electrode active material layer is not particularly limited as long as lithium ions can be released during charging and can be occluded during discharging, and known material compositions can be used. .. For example, a positive electrode active material layer made of various polyanion-type positive electrode active materials obtained by hydrothermally reacting a raw material compound can be preferably used.

The material structure of the negative electrode active material layer is not particularly limited as long as it can absorb lithium ions during charging and release lithium ions during discharging, and a known material structure can be used. For example, a negative electrode active material layer made of titanium niobium oxide or sodium lithium titanate composite oxide obtained by hydrothermally reacting raw materials can be preferably used.

The method for producing a lithium ion secondary battery using the LISICON type crystal particles produced by the method of the present invention is not particularly limited, and a known method can be used. For example, as described in JP-A-2017-10816, the LISICON crystal particles of the present invention are used as the solid electrolyte particles contained in the positive electrode active material layer and the negative electrode active material layer, and the solid electrolyte layer is Solid electrolytes other than such solid electrolyte particles may be used.

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, and a square shape, or an indefinite shape enclosed in a laminate outer package. ..

Hereinafter, the present invention will be specifically described based on Examples. Unless otherwise specified in the table, the content of each component is% by mass.

[Production Example 1] (Production of LiCoPO ₄ positive electrode active material particles)
12.72 g of LiOH.H ₂ O and 40 mL of water were mixed to obtain a slurry (c-1). While maintaining the obtained slurry (c-1) at a temperature of 25° C. for 3 minutes while stirring, 85% by mass of H ₃ PO ₄ was added dropwise at 35 mL/min at 11.53 g, and the stirring speed was 400 rpm for 1 hour. by stirring, to obtain a Li ₃ PO ₄ slurry (c-2). Next, 21.08 g of CoSO ₄ .7H ₂ O was added to the total amount of the obtained Li ₃ PO ₄ slurry (c-2) to prepare a slurry (c-3), which was then placed in an autoclave. The hydrothermal reaction was carried out at 170° C. for 1 hour. The pressure inside the autoclave was 0.8 MPa. The generated hydrothermal reaction product was filtered, and then washed with 12 parts by mass of water with respect to 1 part by mass of the hydrothermal reaction product. The washed hydrothermal reaction product was freeze-dried at −50° C. for 12 hours to obtain LiCoPO ₄ positive electrode active material particles (particle diameter 100 nm).

[Example 1]
4.5 g of LiOH.H ₂ O and 0.9 g of SiO ₂ were mixed with 40 mL of water to obtain a mixed liquid (a-1) ¹ . The pH of the mixed solution (a-1) ¹ at 25° C. was 13.5. Next, while maintaining the obtained mixed liquid (a-1) ¹ at a temperature of 25° C. and stirring at a stirring speed of 200 rpm for 10 minutes, the mixed liquid (a-1) ¹ which was continuously stirred was After the entire amount of 1.73 g of 85% H ₃ PO ₄ was dropped and mixed at 50 mL/min, the mixture was further stirred at a stirring speed of 200 rpm for 30 minutes to obtain a mixed liquid (A) ¹ . The pH of this mixed solution (A) ¹ at 25° C. was 12, and the total content of the produced Li ₃ PO ₄ and SiO ₂ was 6 mass %.

The obtained mixed liquid (A) ¹ was put into an autoclave, and a hydrothermal reaction was carried out at 170° C. and 1.0 MPa for 6 hours. The produced solid content is suction-filtered, and the obtained solid content is washed with 5 parts by mass of water per 1 part by mass of the solid content, and then dried at 80° C. for 12 hours under constant temperature to obtain a precursor mixture (B) ¹ Got From the qualitative analysis by the X-ray diffraction method, the mass ratio of the mixture Li ₃ PO ₄ and Li ₄ SiO _{4 in} the obtained precursor mixture (B) ¹ was 1:1.04. The obtained precursor mixture (B) ¹ was calcined in an air atmosphere at 700° C. for 4 hours to obtain LIICON type crystal particles X ¹ .
The obtained LISICON type crystal particles X ¹ were Li _3.5 Si _0.5 P _0.5 O ₄ single phase, the average particle diameter was 80 nm, and the BET specific surface area was 25 m ² /g. An SEM photograph of the obtained LISICON type crystal particles X ¹ is shown in FIG. 1, and an X-ray diffraction pattern is shown in FIG.

[Example 2]
Lisicon type oxidation was performed in the same manner as in Example 1 except that the added amount of LiOH.H ₂ O was 5.01 g, the added amount of SiO ₂ was 1.62 g, and the added amount of 85% H ₃ PO ₄ was 0.35 g. Object particle X ² was obtained.
The obtained LISICON type oxide particles X ² were Li _3.9 Si _0.9 P _0.1 O ₄ single phase, had an average particle diameter of 80 nm, and had a BET specific surface area of 25 m ² /g.

[Example 3]
Lisicon oxidation was performed in the same manner as in Example 1 except that the amount of LiOH.H ₂ O added was 8.10 g, the amount of SiO ₂ added was 0.18 g, and the amount of 85% H ₃ PO ₄ added was 3.11 g. Object particle X ³ was obtained.
The obtained LISICON type oxide particles X ³ were a single phase of Li _3.1 Si _0.1 P _0.9 O ₄ , the average particle size was 80 nm, and the BET specific surface area was 25 m ² /g.

[Comparative Example 1]
40 mL of water was mixed with 11.38 g of Li ₂ CO _3, 2.30 g of (NH ₄ )H ₂ PO _{4, and} 1.20 g of SiO ₂ and pulverized with a planetary ball mill at 400 rpm for 4 hours to prepare a precursor mixture (B ) Got. The obtained precursor mixture (B) was calcined in an air atmosphere at 900° C. for 10 hours to obtain LISISON type crystal particles Y ¹ .
The obtained LISICON-type crystal particles Y ¹ were a Li _3.5 Si _0.5 P _0.5 O ₄ single phase, had an average particle diameter of 500 nm, and had a BET specific surface area of 1 m ² /g. An SEM photograph of the obtained LISICON type crystal particles Y ¹ is shown in FIG. 3, and an X-ray diffraction pattern is shown in FIG.

"Evaluation test"
Using the LISICON type crystal particles X ^{1 to} X ³ and Y ¹ obtained in Examples 1 to 3 and Comparative Example 1, an all solid lithium ion secondary battery was produced.
Specifically, the LiCoPO ₄ positive electrode active material particles obtained in Production Example 1 were used for the positive electrode, and the positive electrode active material:solid electrolyte (mass ratio) was mixed at a mixing ratio of 75:25, and then charged into a pressing jig. Then, a positive electrode active material layer was formed. Furthermore, only the LISICON-type crystal particles obtained in Examples 1 to 3 and Comparative Example 1 were further added onto the layer to be laminated as a solid electrolyte layer, and then pressed at 16 MPa for 2 minutes using a hand press. , A disk-shaped positive electrode having a diameter of 14 mm was obtained. Then, a lithium foil was attached to the solid electrolyte layer side as a negative electrode to produce an all solid lithium ion secondary battery.

Using the manufactured all-solid-state lithium-ion secondary battery, when the charging conditions were 16 mA/g, constant current charging with a voltage of 5.0 V, and the discharging conditions were 16 mA/g, constant current discharging with a final voltage of 3.5 V, The discharge capacity at 16 mA/g was determined. All charge/discharge tests were performed at 45°C. The results are shown in Table 1.

From the above results, it can be seen that the LISICON type crystal particles obtained by the method of Example 1 exhibit an excellent discharge capacity as compared with the LISICON type crystal particles obtained by the method of Comparative Example 1. It is considered that this is because, as can be seen from FIGS. 1 and 2, the LISICON type crystal particles obtained by the method of Example 1 are fine particles having high crystallinity.

Claims (4)

Next steps (I) to (III):
(I) A step of mixing a lithium compound, a silicic acid compound and a phosphoric acid compound with water to prepare a mixed solution having a pH of 9 to 14 at 25°C (II) The obtained mixed solution is 100°C or higher. The step of washing the hydrothermal reaction product, and then drying it to obtain a precursor mixture (III) firing the obtained precursor mixture at 400° C. to 1000° C. , The following formula (1):
Li _4-x Si _1-x P _x O ₄ (where 0<x<1) (1)
(In the formula (1), x represents a number satisfying 0<x<1.)
A method for producing LISICON-type crystal particles for a solid electrolyte of a lithium ion secondary battery represented by: In washing the hydrothermal reaction product in the step (II), 5 parts by mass to 50 parts by mass of water is used for 1 part by mass of the dry mass of the hydrothermal reaction product, and the lithium ion ion according to claim 1. A method for producing LISICON type crystal particles for a solid electrolyte of a secondary battery. The method for producing LISICON type crystal particles for a solid electrolyte of a lithium ion secondary battery according to claim 1, wherein the firing in the step (III) is performed in an air atmosphere, an inert gas atmosphere or a reducing condition. The LISICON type crystal particles for a solid electrolyte of a lithium ion secondary battery according to claim 1, wherein x in the formula (1) is 0.1≦x≦0.9. Method.