JP7125302B2 - Manufacturing method of NASICON type negative electrode active material particles for sodium ion secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing negative electrode active material particles for sodium ion secondary batteries capable of effectively increasing the charge/discharge capacity of sodium ion secondary batteries.

Among the development of secondary batteries used in portable electronic devices, hybrid vehicles, electric vehicles, etc., lithium-ion secondary batteries are particularly widely used due to their high output characteristics and good cycle characteristics. . On the other hand, lithium, which is used as a raw material for lithium ion secondary batteries, is an expensive raw material because it is not necessarily abundant as a resource and its production areas are limited. Therefore, instead of such lithium, attention is being paid to sodium ion secondary batteries using sodium, which is an abundant resource that is not unevenly distributed in production areas.

However, compared to lithium, sodium has a standard electrode potential about 0.3 V higher, has more than twice the ion volume, and has more than three times the atomic weight. It is difficult to apply to material development of secondary batteries. For example, graphite, which is generally used as a negative electrode active material for lithium ion secondary batteries, cannot be used as a negative electrode active material for sodium ion secondary batteries because it cannot occlude and release sodium ions.

Under these circumstances, for example, metallic sodium is known as a material that can be used as a negative electrode active material for sodium ion secondary batteries. Therefore, there is a big problem in safety. In addition, Na-Pb alloy, which is a sodium alloy, is also known as a material that can be used, but there are concerns about environmental impact such as reduced energy density and toxicity caused by lead. is being attempted.

For example, Patent Document 1 discloses a negative electrode active material for a sodium ion secondary battery characterized by containing a crystalline phase of NaSn ₂ (PO ₄ ) ₃ , and Patent Document 2 discloses a crystalline Na _3-x M ₂ (PO ₄ ) ₃ (M is one or more metal elements selected from V, Ti and Fe, 0≦x≦3) as a main component. A structure is disclosed. Furthermore, Non-Patent Document 1 shows that _{NaTi 2} ( _{PO 4} ) ₃ coated with carbon has high reversible capacity and excellent rate characteristics _. It has been reported that (PO ₄ ) ₃ has excellent electrochemical properties as a solid electrolyte.
These _NaSn2 (PO4) ₃ , _{Na3 - xM2} ( _PO4 ) ₃ , _{NaTi2 ( PO4} ) _{3 , Na1.3Al0.3Zr1.7} ( _PO4 ) ₃ all have a NASICON crystal structure. These NASICON-type sodium phosphate compounds are highly expected materials as negative electrode active materials for sodium ion secondary batteries.

JP 2014-35986 A JP 2015-43283 A

"Carbon-coated NaTi2(PO4)3 composite: A promising anode material for sodium-ion batteries with superior Na-storage performance", Solid State Ionics 314 (2018), pp.61-65 "Characterization of Na1.3Al0.3Zr1.7(PO4)3solid electrolyte ceramics by impedance spectroscopy", Solid State Ionics 271 (2015), pp.128-133

However, in producing such a NASICON-type sodium phosphate compound, although the solid-phase method is used in Patent Document 1, such a method inevitably requires pulverization in order to make the reaction product finer. Since the obtained particles have a broad particle size distribution, there is a risk of deterioration in charge/discharge characteristics. In addition, although the above-mentioned Patent Document 2 uses an electrospinning method (electrospinning method), such a method involves the cumbersome task of applying a high voltage to the polymer solution, which is the raw material in the syringe, to charge it and cause an electrostatic explosion. Moreover, in order to use the obtained one-dimensional nanostructure as a secondary battery material, it is necessary to further perform a pulverization process, as in Patent Document 1.

On the other hand, in Non-Patent Documents 1 and 2, the citrate method (Pechini method) is used to obtain a reaction product having a particle size of about 30 nm to 100 nm. is dissolved in a mixed solution, a chelate such as citric acid and a polyalcohol such as ethylene glycol are subjected to a polyesterification reaction to form a precursor, which is then heat treated to obtain a powder. Moreover, for implementation on an industrial scale, it is also necessary to prepare a large amount of chelate and polyalcohol in advance.
For these reasons, as a means of obtaining highly useful NASICON-type sodium phosphate compound particles, it is strongly desired to realize a simpler and lower-cost production method.

Accordingly, an object of the present invention is to provide a production method capable of easily obtaining fine NASICON-type sodium phosphate compound particles that are highly useful as a negative electrode active material for sodium ion secondary batteries.

Accordingly, the present inventors conducted various studies and found that particles having a fine and highly crystalline NASICON-type crystal structure, that is, NASICON-type sodium phosphate, were produced by a production method using a so-called hydrothermal method, which involves a hydrothermal reaction. The present inventors have found that negative electrode active material particles for sodium ion secondary batteries comprising salt compound particles can be easily obtained, and have completed the present invention.

That is, the present invention provides the following steps (I) to (II):
(I) A step of mixing a sodium compound, a titanium compound, a phosphate compound and water to prepare a mixed solution having a pH of 1 to 5 at 25°C (II) Hydrothermally treating the obtained mixed solution at 100°C or higher Provided is a method for producing negative electrode active material particles for a sodium ion secondary battery, comprising the steps of washing the hydrothermal reaction product after reacting it and then drying it to obtain particles.
Further, the present invention provides a method for producing negative electrode active material particles for a sodium ion secondary battery, comprising the steps (I) to (II) and then (III) supporting carbon on the surface of the obtained particles. may be

According to the production method of the present invention, negative electrode active material particles for sodium ion secondary batteries comprising NASICON-type sodium phosphate compound particles having an appropriate particle size can be easily obtained without performing a pulverization treatment. can be used to realize a sodium-ion secondary battery with excellent charge-discharge characteristics.

1 is an SEM image showing negative electrode active material particles for a sodium ion secondary battery obtained in Example 1. FIG. 2 is an X-ray diffraction pattern of the negative electrode active material particles for sodium ion secondary batteries obtained in Example 1. FIG. 4 is an SEM image showing negative electrode active material particles for a sodium ion secondary battery obtained in Comparative Example 1. FIG. 4 is an X-ray diffraction pattern of the negative electrode active material particles for a sodium ion secondary battery obtained in Comparative Example 1. FIG.

The present invention will be described in detail below.
The method for producing negative electrode active material particles for sodium ion secondary batteries of the present invention comprises the following steps (I) to (II):
(I) A step of mixing a sodium compound, a titanium compound, a phosphate compound and water to prepare a mixed solution having a pH of 1 to 5 at 25°C (II) Hydrothermally treating the obtained mixed solution at 100°C or higher After reacting, the hydrothermal reaction product is washed and then dried to obtain particles.

The negative electrode active material particles for a sodium ion secondary battery obtained by the production method of the present invention are composed of particles having a NASICON-type crystal structure, that is, NASICON-type sodium phosphate compound particles, and are specifically represented by the following formula (X): :
_{Na1 + aTibMc} (PO4) ₃ ... ₍ X)
(In formula (X), M is one or more selected from Al, Zr, Sc, In, Fe, Cr, Ga, Y, La, Zn, Si, Mn, Ge, Nd, Sr and V. and a, b, and c are numbers satisfying 0≦a≦2, 1≦b≦2, 0≦c≦1, and a+4b+(valence of M)×c=8.)
is represented by

The negative electrode active material particles for a sodium ion secondary battery represented by the above formula (X) contain at least sodium and titanium, and may contain a metal (M) which is an atom different from these sodium and titanium as a dope metal. good. Thus, the negative electrode active material particles for sodium ion secondary batteries represented by the above formula (X) which may contain a metal (M) are particles made of a compound obtained by the production method of the present invention that undergoes a hydrothermal reaction. It is possible to realize a sodium ion secondary battery exhibiting a high discharge capacity by using the negative electrode active material particles for a sodium ion secondary battery in which such a metal (M) can be interposed as the negative electrode active material.

In formula (X), M is one or more selected from Al, Zr, Sc, In, Fe, Cr, Ga, Y, La, Zn, Si, Mn, Ge, Nd, Sr and V. , preferably one or more selected from Al, Zr, Mn and Ge, more preferably Al or Zr. a is 0≤a≤2, preferably 0≤a≤1. b is 1≤b≤2, preferably 1.5≤b≤2. c satisfies 0≦c≦1, preferably 0<c≦0.5. These a, b and c satisfy a+4b+(valence of M)×c=8. Specific examples of the NASICON-type sodium phosphate compound represented by the above formula (X) include NaTi ₂ (PO ₄ ) ₃ , Na _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Na _1.5 Al _{0.5 Ti1.5 ( PO4} ) _{3 , Na1.3Al0.3TiZr0.7 (} PO4) ₃ and the like.

The step (I) provided in the production method of the present invention is a step of mixing a sodium compound, a titanium compound, a phosphate compound and water to prepare a mixed solution (A) having a pH of 1 to 5 at 25°C. . Thus, by using the mixed solution (A) obtained from the sodium compound, the titanium compound, the phosphoric acid compound and water for obtaining the negative electrode active material particles for the sodium ion secondary battery in the step (II) described later, , it is possible to effectively reduce the size of the resulting negative electrode active material particles for a sodium ion secondary battery.

Usable sodium compounds include one or more selected from hydroxides, chlorides, carbonates, sulfates, and organic acid salts. More specific examples include sodium hydroxide, sodium chloride, sodium carbonate, sodium hydrogen carbonate, sodium sulfate, sodium nitrate, sodium acetate, sodium oxalate and the like. Among them, sodium hydroxide is preferable from the viewpoint of easily adjusting the pH of the mixed solution (A) to be obtained while enhancing the reactivity.

Titanium compounds that can be used include, for example, one or more selected from sulfates, nitrates, carbonates, acetates, oxalates, oxides, hydroxides and halides of titanium. Specific examples include titanyl sulfate, titanium sulfate, titanium acetate, and titanium chloride. Among them, titanyl sulfate or titanium sulfate is preferable from the viewpoint of enhancing reactivity.

Usable phosphoric acid compounds include orthophosphoric acid (H ₃ PO ₄ , phosphoric acid), metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, ammonium phosphate, and ammonium hydrogen phosphate. Among them, it is preferable to use phosphoric acid from the viewpoint that it can also act as a pH adjuster for controlling the pH of the resulting mixed solution (A), and it is preferable to use it as an aqueous solution with a concentration of 70% by mass to 90% by mass. preferable.

In preparing the mixed solution (A), a metal (M) compound is further mixed as a compound other than the sodium compound, titanium compound and phosphoric acid compound, and the obtained negative electrode active material particles for sodium ion secondary batteries are added with a metal ( M) may be included as a doping metal. Here, M is synonymous with M in the above formula (X), Al, Zr, Sc, In, Fe, Cr, Ga, Y, La, Zn, Si, Mn, Ge, Nd, Sr and V One or two or more selected from Such metal (M) compounds include halides, sulfates, organic acid salts, hydroxides, chlorides, sulfides, oxides, and hydrates thereof. Among them, one or more selected from sulfates and organic acid salts are preferable from the viewpoint of efficiently proceeding the reaction in the mixed solution (A).

In step (I), specifically, a mixed solution (a-1) containing a titanium compound and water, and optionally a metal (M) compound, a sodium compound, a phosphate compound and water are mixed. It is preferable to prepare each of the mixtures (a-2) and then mix these mixtures to prepare the mixture (A). As a result, the obtained mixed solution (A) contains a reaction product with a highly homogeneous chemical composition and sufficiently finely divided, that is, a mixture of sodium phosphate and titanium oxide hydrate. As a result, it is possible to effectively refine the NASICON-type sodium phosphate compound particles obtained as the hydrothermal reaction product in the next step (II).

Mixed solution (a-1) is prepared by mixing a titanium compound and, if necessary, a metal (M) compound with water. The total content of the titanium compound and the metal (M) compound in the mixed liquid (a-1) is the mixed liquid (a It is preferably 1 part by mass to 50 parts by mass, more preferably 5 parts by mass to 30 parts by mass, and still more preferably 5 parts by mass to 20 parts by mass with respect to 100 parts by mass of water in -1).

Further, in preparing the mixed solution (a-1), the mixing ratio of the titanium compound and the metal (M) compound is the molar ratio (Ti:M) of the amount of titanium and the amount of metal (M), preferably 100:1. ~1:1, more preferably 50:1 to 2:1, still more preferably 19:1 to 3:1.

The liquid mixture (a-1) may be stirred in advance before being mixed with the liquid mixture (a-2) from the viewpoint of dispersing each component more uniformly. The time for stirring the mixed solution (a-1) is preferably 5 minutes to 3 hours, more preferably 10 minutes to 2 hours, still 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 still more preferably 20 cm/sec to 100 cm/sec.

Mixed solution (a-2) is prepared by mixing a sodium compound, a phosphate compound and water. The content of the sodium compound in the mixed liquid (a-2) is 100 It is preferably 1 part by mass to 50 parts by mass, more preferably 5 parts by mass to 30 parts by mass, even more preferably 5 parts by mass to 20 parts by mass.

Further, the content of the phosphoric acid compound in the mixed solution (a-2) is preferably 3 parts by mass to 20 parts by mass, more preferably 5 parts by mass with respect to 100 parts by mass of water in the mixed solution (a-2). It is from 7 parts by mass to 15 parts by mass, more preferably from 7 parts by mass to 15 parts by mass.

The pH of the mixed solution (a-2) at 25°C is preferably 3 to 10, more preferably 4 to 10, from the viewpoint of adjusting the pH of the resulting mixed solution (A) to 1 to 5 at 25°C. and more preferably 5-10. A pH adjuster, which is a compound other than the above components, such as potassium hydroxide or ammonium hydroxide, may be added so that the pH of the mixed solution (a-2) falls within the above range.

The mixed solution (a-2) may be stirred in advance before being mixed with the mixed solution (a-1) from the viewpoint of dissolving or dispersing each component more uniformly. The time for stirring the mixed solution (a-2) is preferably 1 to 30 minutes, more preferably 2 to 20 minutes, still more preferably 2 to 10 minutes. The stirring speed is preferably 10 cm/sec to 200 cm/sec, more preferably 15 cm/sec to 150 cm/sec in terms of the flow speed of the mixture (a-2) on the inner wall surface of the reaction vessel, More preferably, it is 20 cm/sec to 100 cm/sec.

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 of 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 mixed solution (a-1) being stirred. preferably. Thus, the mixture (a-2) containing the sodium compound and the phosphate compound is added dropwise little by little to the mixture (a-1) containing the titanium compound and optionally the metal (M) compound. Thereby, the homogeneity of the liquid mixture (A) can be further improved.

At this time, the rate of dropping the mixed liquid (a-2) into the mixed liquid (a-1) is preferably 0.1 part by mass/min to 0.1 part by mass/min with respect to 10 parts by mass of the mixed liquid (a-1). It is 4 parts/minute, more preferably 0.15 parts/minute to 0.4 parts/minute, still more preferably 0.2 parts/minute to 0.35 parts/minute. The stirring speed of the mixed solution (a-1) when the mixed solution (a-2) is added dropwise is preferably 10 cm/sec or more in terms of the flow speed 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, 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, still more preferably 5:1 to 2:3.

The temperature of the mixture (a-1) when mixing the mixture (a-1) and the mixture (a-2) is preferably 10°C to 80°C, more preferably 15°C to 70°C. and more preferably 20°C to 60°C. The temperature of the mixture (a-2) is preferably 10°C to 80°C, more preferably 15°C to 70°C, still more preferably 20°C to 60°C.

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

A mixture (A) containing sodium phosphate and titanium oxide hydrate is obtained by mixing the mixture (a-1) and the mixture (a-2). Titanium oxide hydrate is fine particles having an average particle size of 100 nm or less, and is subjected to a hydrothermal reaction in step (II) described later to obtain NASICON-type sodium phosphate compound particles having a high degree of crystallinity. to generate The total content of sodium phosphate and titanium oxide hydrate in the mixture (A) is preferably 0.5% by mass to 40% by mass, more preferably 1 % to 35% by mass, more preferably 1.5% to 30% by mass. The pH of the mixture (A) at 25° C. is 1-5, preferably 2.5-5, more preferably 3-5. In adjusting the pH of the mixed solution (A) so that it falls within this range, the pH of the mixed solution (a-2) is adjusted as necessary.
In addition, in the mixed liquid (A), these sodium phosphates and titanium oxide hydrates are produced and contained as a mixture, which is confirmed by the X-ray diffraction method of the evaporated and dried mixed liquid (A). can be confirmed by

Step (II) is a step of subjecting the mixture (A) obtained in step (I) to a hydrothermal reaction at 100° C. or higher, then washing and then drying the hydrothermal reaction product. The hydrothermal reaction in step (II) is performed under pressure by storing the reaction vessel filled with the mixture (A) in a pressure vessel or the like. The resulting hydrothermal reaction product consists of the NASICON-type sodium phosphate compound particles produced.

When the mixed solution (A) obtained in step (I) is stirred before proceeding to step (II), such stirring may be performed in a reaction vessel, and then the reaction vessel is placed in a pressure vessel or the like. It should be stored. From the viewpoint of efficiently producing NASICON-type sodium phosphate compound particles, such stirring is preferably continued until the hydrothermal reaction in step (II) is completed in a reaction vessel such as a pressure vessel. At this time, the stirring speed of the mixed solution (A) is preferably 15 cm/sec or more, more preferably 15 cm/sec to 80 cm/sec in terms of the flow speed of the mixed solution (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 is achievable. For example, a method using a stirring blade or a pump described in JP-A-2014-118328 is used to stir the slurry in the synthesis vessel. methods and the like can be suitably used.
Furthermore, when using the above-mentioned stirring method, from the viewpoint of uniformly causing a hydrothermal reaction in the entire mixed liquid (A), a baffle plate is installed in the reaction vessel, the rotation direction of the stirring blade, or the liquid feeding direction of the pump. It is effective to disturb the flow of the mixture (A) by intermittently reversing the flow of the liquid mixture (A).

The temperature (reaction temperature) of the mixed solution (A) during the hydrothermal reaction may be 100°C or higher, preferably 130°C to 250°C, 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 higher, and 0.3 MPa to 2.0 MPa for 10 minutes when the reaction is performed at 130° C. to 250° C. Up to 24 hours is preferable, and when the reaction is carried out at 140° C. to 230° C., 0.4 MPa to 1.8 MPa and 10 minutes to 16 hours are preferable.

Moreover, the atmosphere in the reaction vessel in the hydrothermal reaction is not limited, but from the viewpoint of easiness of adjusting the atmosphere, it is preferably under air or under water vapor.

Then, the mixed liquid (A) after the hydrothermal reaction is subjected to solid-liquid separation to recover the hydrothermal reaction product. Apparatuses used for solid-liquid separation include, for example, a filter press and a centrifugal filter. Among them, it is preferable to use a filter press from the viewpoint of obtaining a hydrothermal reaction product efficiently.

The hydrothermal reaction product recovered from the mixed solution (A) after the hydrothermal reaction is washed from the viewpoint of effectively removing water-soluble impurities such as anionic 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, more preferably 7 parts by mass to 50 parts by mass, relative to 1 part by mass of the dry mass of the hydrothermal reaction product. parts by mass, more preferably 9 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 washed hydrothermal reaction product is then dried. Drying means include spray drying, constant temperature drying, fluidized bed drying, external heat drying, freeze drying, vacuum drying, etc. Among them, constant temperature drying or spray drying is preferable from the viewpoint of convenience. .

The average particle size of the NASICON-type sodium phosphate compound particles obtained after drying is preferably 5 nm to 150 nm, more preferably 5 nm to 100 nm, in terms of the _D50 value in the particle size distribution based on a scanning electron microscope (SEM). Yes, more preferably 5 nm to 80 nm. Here, the _D50 value in a scanning electron microscope (SEM) is a value obtained by measuring and averaging the primary particle diameters of 100 NASICON-type sodium phosphate compound particles, and the _D50 value is a cumulative 50%. means the particle size (median diameter) of

Since the NASICON-type sodium phosphate compound particles obtained in step (II) have electrical conductivity, they may be used as the negative electrode active material particles for sodium ion secondary batteries as they are. Furthermore, by passing through the next step (III), carbon may be supported on the surface of such particles to be used as negative electrode active material particles for sodium ion secondary batteries.
That is, step (III) is a step of supporting carbon on the surface of the particles obtained in step (II). In the step (III), in order to support carbon on the surface of the negative electrode active material particles for sodium ion secondary batteries obtained in step (II), a carbon source is added to the negative electrode active material particles for sodium ion secondary batteries. do it.

Examples of carbon sources that can be used include water-soluble carbon materials such as glucose, fructose, sucrose, polyethylene glycol, polyvinyl alcohol, carboxymethylcellulose, saccharose, starch, dextrin, and citric acid; acetylene black, ketjen black, channel black, furnace Carbon black such as black, lamp black, and thermal black can be used. Among them, the water-soluble carbon material is preferably glucose, fructose, sucrose, dextrin, or citric acid, and glucose and citric acid are preferable from the viewpoint of increasing solubility and dispersibility in a solvent and effectively functioning as a carbon material. more preferred. As for carbon black, ketjen black is preferable from the viewpoint of effectively increasing the tap density of the produced negative electrode active material particles for sodium ion secondary batteries. The water-soluble carbon material means an organic compound that dissolves in 100 g of water at 25° C. in terms of carbon atom equivalent of 0.4 g or more, preferably 1.0 g or more.

These carbon sources (water-soluble carbon material or carbon black) in step (III) preferably have a supported amount of carbon in 100% by mass of the negative electrode active material particles for a sodium ion secondary battery, in terms of carbon atoms, of 0.5%. It may be added in an amount of 1 to 15% by mass, more preferably 0.2 to 10% by mass, still more preferably 0.3 to 8% by mass. For example, the amount of the carbon source to be added is preferably 0.1 to 17.7 parts by mass in terms of carbon atoms with respect to 100 parts by mass of the NASICON-type sodium phosphate compound particles obtained in step (II), It is more preferably 0.2 to 11.1 parts by mass, still more preferably 0.3 to 8.7 parts by mass.

When a water-soluble carbon material is selected as the carbon source in step (III), in step (III), a water-soluble carbon material and water are added to the NASICON-type sodium phosphate compound particles obtained in step (II). After that, they are spray-dried to obtain a mixture (B) of the NASICON-type sodium phosphate compound particles and the water-soluble carbon material.

Next, the obtained mixture (B) is fired. Such firing is preferably carried out in a reducing atmosphere or an inert atmosphere. The firing temperature is preferably 500 from the viewpoint of effectively supporting the carbon derived from the water-soluble carbon material on the surface of the NASICON-type sodium phosphate compound particles and from the viewpoint of improving the crystallinity of the NASICON-type sodium phosphate compound particles. °C to 800 °C, more preferably 600 °C to 770 °C, still more preferably 650 °C to 750 °C. The firing time is preferably 10 minutes to 3 hours, more preferably 30 minutes to 1.5 hours.

Further, when carbon black is selected as the carbon source in the step (III), in the step (III), the carbon is densely and uniformly dispersed on the surface of the NASICON-type sodium phosphate compound particles to effectively support it. From the standpoint of making the particles of the NASICON-type sodium phosphate compound obtained in step (II), after adding carbon black, they are preferably mixed while applying compressive force and shearing force to form a composite. The process of mixing while applying compressive force and shear force is preferably carried out in a closed container equipped with an impeller. good. The circumferential speed of the impeller is preferably 25 m/s to 40 m/s from the viewpoint of effectively increasing the conductivity of the resulting negative electrode active material particles for a sodium ion secondary battery and improving the discharge capacity of the battery. More preferably, it is 27 m/s to 40 m/s. Also, the mixing time is preferably 5 to 90 minutes, more preferably 10 to 80 minutes. The peripheral speed of the impeller means the speed of the outermost end of the rotary stirring blade (impeller), and can be expressed by the following formula (1). The processing time may also vary with the impeller peripheral speed, such that the slower the impeller peripheral speed, the longer it takes.
Peripheral speed of impeller (m/s) =
Radius of impeller (m) x 2 x π x number of revolutions (rpm) ÷ 60 (1)

In the step (III), the processing time and/or the peripheral speed of the impeller when performing the processing of mixing while applying the compressive force and the shear force are determined according to the ratio of the NASICON-type sodium phosphate compound particles and the carbon black to be put into the container. It is necessary to adjust accordingly according to the total amount. Then, by operating the container, it becomes possible to perform a process of mixing the mixture while applying compressive force and shear force to the mixture between the impeller and the inner wall of the container, thereby obtaining the NASICON-type sodium phosphate compound particles. Carbon can be densely and uniformly dispersed on the surface. For example, when the above-described mixing process is performed in a closed container equipped with an impeller rotating at a peripheral speed of 25 m/s to 40 m/s for 6 minutes to 90 minutes, NASICON-type sodium phosphate compound particles and carbon black are introduced into the container. The total amount of is preferably 0.1 g to 0.7 g, more preferably 0.15 g per 1 cm ³ of the effective container (the container corresponding to the part of the closed container equipped with the impeller that can accommodate the composite). ~0.4g.

Examples of equipment equipped with a closed container that can easily perform the mixing process while applying such compressive force and shear force include a high-speed shear mill, a blade type kneader, etc. Specifically, for example, Fine particle compounding device Nobilta (manufactured by Hosokawa Micron Corporation) can be suitably used.

The average particle size of the negative electrode active material particles for sodium ion secondary batteries obtained by the production method of the present invention is preferably 5 nm to 150 nm, more preferably 5 nm to 100 nm, and still more preferably 5 nm to 80 nm. Furthermore, the average crystallite size is preferably 5 nm to 150 nm, more preferably 5 nm to 100 nm, and even more preferably 5 nm to 80 nm.
Here, the average crystallite size of the NASICON-type sodium phosphate compound particles constituting the negative electrode active material particles for sodium ion secondary batteries is X-rays with a diffraction angle 2θ range of 10° to 80° by Cu-kα rays. For the diffraction profile, it means a value obtained by applying Scherrer's formula.

In addition, the BET specific surface area of the obtained negative electrode active material particles for sodium ion secondary batteries is preferably 10 m ² /g or more, more preferably 15 m ² /g, from the viewpoint of obtaining a secondary battery having excellent charge-discharge characteristics. g or more, more preferably 20 m ² /g or more.

As described above, the production method of the present invention can obtain negative electrode active material particles for sodium ion secondary batteries, which are composed of NASICON-type sodium phosphate compound particles with an effectively finer average particle size and a high degree of crystallinity. can. Therefore, it is possible to easily manufacture a sodium-ion secondary battery capable of exhibiting excellent charge-discharge characteristics.

A secondary battery to which the negative electrode active material particles for a sodium ion secondary battery obtained by the production method of the present invention can be suitably applied includes a positive electrode, a negative electrode, a separator and an electrolytic solution, or a battery essentially comprising a positive electrode, a negative electrode and a solid electrolyte. If so, it is not particularly limited.

Here, regarding the positive electrode active material layer, the material configuration is not particularly limited as long as it can release sodium ions during charging and absorb sodium ions during discharging, and known materials can be used. For example, a positive electrode active material layer made of various polyanion-type positive electrode active materials obtained by hydrothermally reacting raw material compounds can be suitably used.

Moreover, the method for manufacturing a secondary battery using the negative electrode active material particles for a sodium ion secondary battery obtained by the present invention is not particularly limited, and a known method can be used.

The shape of the secondary battery having the above configuration is not particularly limited, and may be coin-shaped, cylindrical, square, or any other shape, or may be an irregular shape enclosed in a laminated outer package.

EXAMPLES The present invention will be specifically described below based on examples.

[Example 1]
3.74 g of TiOSO ₄ ·1.5H ₂ O was mixed with 40 mL of water to obtain a mixture (a-1). Further, 3.6 g of NaOH and 3.46 g of 85 mass % H ₃ PO ₄ were mixed with 40 mL of water to obtain a mixed solution (a-2). The pH of the mixture (a-2) at 25°C was 7.0.
Next, while the resulting mixture (a-1) was maintained at a temperature of 25° C., it was stirred at a stirring speed of 200 rpm for 60 minutes. Mixed solution (a-2) stirred at 200 rpm for 5 minutes was added dropwise at 50 mL/min and mixed, and then stirred at a stirring speed of 200 rpm for 30 minutes to obtain mixed solution (A). The pH of the mixture (A) at 25°C was 4.0.

The resulting mixed solution (A) was put into an autoclave and hydrothermally reacted at 140° C. and 0.3 MPa for 1 hour. The resulting solid content is suction-filtered, and then the obtained solid content is washed with 12 parts by mass of water per 1 part by mass of the solid content, followed by constant temperature drying at 80° C. for 12 hours to obtain a NASICON-type sodium phosphate compound. A negative electrode active material particle (X1-1) for a sodium ion secondary battery composed of particles was obtained. 1.0 g of the obtained negative electrode active material particles for sodium ion secondary batteries (X1-1), 0.5 g of citric acid and 5 mL of water are mixed and dried, and then baked at 700 ° C. for 1 hour in a nitrogen atmosphere, A negative electrode active material particle (X2-1) for a sodium ion secondary battery having carbon supported on the particle surface was obtained (amount of carbon supported: 1.5% by mass). The obtained negative electrode active material particles for sodium ion secondary batteries (X2-1) had a single phase of NaTi ₂ (PO ₄ ) ₃ , had an average particle size of 70 nm, an average crystallite size of 70 nm, and a BET specific surface area of 30 m. ² /g. A SEM photograph of the obtained negative electrode active material particles (X2-1) for a sodium ion secondary battery is shown in FIG. 1, and an X-ray diffraction pattern is shown in FIG.

[Example 2]
Particles were prepared in the same manner as in Example 1, except that the amount of TiOSO ₄ .1.5H ₂ O added was 3.37 g and 0.63 g of Al ₂ (SO ₄ ) ₃ .14-18H ₂ O was additionally added. A negative electrode active material particle (X2-2) for a sodium ion secondary battery having carbon supported on the surface was obtained (amount of carbon supported: 1.5% by mass). The obtained negative electrode active material particles for sodium ion secondary batteries (X2-2) had a Na _1.2 Al _0.2 Ti _1.8 (PO ₄ ) ₃ single phase, an average particle size of 70 nm, an average crystallite size of 70 nm, and a BET The specific surface area was 30 m ² /g.

[Comparative Example 1]
0.53 g of Na ₂ CO ₃ and 10.35 g of NH ₄ H ₂ PO ₄ were mixed with 40 mL of water to obtain a mixture (b-1). The obtained mixture (b-1) was mixed with 1.60 g of TiO ₂ to obtain a mixture (b-2). The obtained mixed solution (b-2) was mixed with 0.5 g of citric acid to obtain a mixed solution (b-3). Water was distilled off from the resulting (b-3) using an evaporator to obtain a NASICON-type sodium phosphate compound precursor (B). The obtained NASICON-type sodium phosphate compound precursor (B) was baked at 800° C. for 6 hours in a nitrogen atmosphere to obtain a negative electrode active material for a sodium ion secondary battery in which carbon derived from citric acid is supported on the particle surface. Substance particles (Y-1) were obtained (amount of carbon supported: 1.5% by mass). The obtained negative electrode active material particles for sodium ion secondary batteries (Y-1) had a single phase of NaTi ₂ (PO ₄ ) ₃ , the average primary particle diameter was 250 nm, the average crystallite diameter was 200 nm, and the BET ratio was The surface area was 10 m ² /g.
A SEM photograph of the obtained negative electrode active material particles (Y-1) for a sodium ion secondary battery is shown in FIG. 3, and an X-ray diffraction pattern is shown in FIG.

[Comparative Example 2]
A negative electrode active material particle for a sodium ion secondary battery (Y-2) was obtained in the same manner as in Comparative Example 1, except that the amount of TiO ₂ added was 1.44 g and 0.10 g of Al ₂ O ₃ was additionally added. (the amount of carbon carried: 1.5% by mass). The obtained negative electrode active material particles for sodium ion secondary batteries (Y-2) had a Na _1.2 Al _0.2 Ti _1.8 (PO ₄ ) ₃ single phase, an average particle size of 250 nm, an average crystallite size of 200 nm, and a BET The specific surface area was 10 m ² /g.

《Evaluation of rate characteristics》
Using the negative electrode active material particles for sodium ion secondary batteries obtained in Examples 1 and 2 and Comparative Examples 1 and 2, negative electrodes for sodium ion secondary batteries were produced.
Specifically, the obtained negative electrode active material particles for sodium ion secondary batteries, Ketjenblack, and polyvinylidene fluoride were mixed at a mass ratio of 90:5:5, and N-methyl-2-pyrrolidone was added thereto. was added and sufficiently kneaded to prepare a negative electrode slurry. The negative electrode slurry was applied to a current collector made of aluminum foil having a thickness of 20 μm using a coating machine, and vacuum-dried at 80° C. for 12 hours. Next, after punching into a disk shape of φ14 mm, it was pressed at 16 MPa for 2 minutes using a hand press to obtain a negative electrode.
Next, a coin-type secondary battery was constructed using the above negative electrode. A sodium foil punched to φ15 mm was used as the counter electrode. The electrolytic solution used was obtained by dissolving NaPF ₆ at a concentration of 1 mol/L in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3:7. A polypropylene film was used as the separator. These battery parts were incorporated and housed in an atmosphere with a dew point of −50° C. or lower by a conventional method to produce a coin-type secondary battery (CR-2032).

A charge/discharge test was performed using the manufactured sodium ion secondary battery. Specifically, the charging conditions are a current density of 13 mA/g (0.1 CA) and a constant current charge of a voltage of 2.8 V, and the discharge conditions are a constant current discharge of 13 mA/g (0.1 CA) and a final voltage of 1.2 V. and the discharge capacity at a current density of 13 mA/g was determined. Furthermore, charging is performed under the same charging conditions, the discharging conditions are constant current discharge at 1300 mA / g and the final voltage is 1.2 V, the discharge capacity at the current density of 1300 mA / g is calculated, and the capacity ratio is calculated from the following formula (2). rice field. Note that the charge/discharge test was performed at 30°C. Table 1 shows the results.
Capacity ratio (%) = (discharge capacity at current density of 1300 mA / g) /
(Discharge capacity at a current density of 13 mA / g) × 100 (2)

From the above results, the negative electrode active material particles for sodium ion secondary batteries obtained by the production methods of Examples 1 and 2 are the negative electrode active material particles for sodium ion secondary batteries obtained by the production methods of Comparative Examples 1 and 2. Although it can be obtained by a simple method, it can be seen that a sodium ion secondary battery using this shows excellent discharge capacity. It is believed that this is because the negative electrode active material particles for sodium ion secondary batteries obtained by the production methods of Examples 1 and 2 were able to effectively reduce both the particle size and the crystallite size.

Claims (7)

The following steps (I)-(II):
(I) Titanium compounds, metal (M) compounds (M is one selected from Al, Zr, Sc, In, Fe, Cr, Ga, Y, La, Zn, Si, Mn, Ge, Nd, Sr and V or two or more.) and water, and a mixed solution (a-1 ) and a mixed solution (a-2) containing a sodium compound, a phosphate compound and water, and having a sodium compound content of 1 part by mass to 50 parts by mass with respect to 100 parts by mass of water ( (a-1): (a-2)) are mixed so as to be 20:1 to 1:4 to prepare a mixed solution (A) having a pH of 1 to 5 at 25 ° C. (II) After subjecting the obtained mixture (A) to a hydrothermal reaction at 130 to 250 ° C., the hydrothermal reaction product is washed and then dried to obtain particles, having an average particle size of 5 nm to 80 nm. A method for producing NASICON-type negative electrode active material particles for sodium ion secondary batteries. 2. The method for producing NASICON negative electrode active material particles for a sodium ion secondary battery according to claim 1, wherein the sodium compound is sodium hydroxide. The NASICON negative electrode active material particles for a sodium ion secondary battery according to claim 1 or 2 , further comprising a step (III) of supporting carbon on the surface of the obtained particles after steps (I) to (II). manufacturing method. 4. A titanium compound according to any one of claims 1 to 3 , wherein the titanium compound used in step (I) is a sulfate, nitrate, carbonate, acetate, oxalate, oxide, hydroxide or halide. A method for producing NASICON-type negative electrode active material particles for sodium ion secondary batteries. 5. The metal (M) compound according to any one of claims 1 to 4 , wherein the metal (M) compound is a halide, sulfate, organic acid salt, hydroxide, chloride, sulfide, oxide or hydrate thereof. A method for producing NASICON-type negative electrode active material particles for a sodium ion secondary battery. NASICON-type negative electrode active material particles for sodium ion secondary batteries are represented by the following formula (X):
_{Na1 + aTibMc} (PO4) ₃ ... ₍ X)
(In formula (X), M is one or more selected from Al, Zr, Sc, In, Fe, Cr, Ga, Y, La, Zn, Si, Mn, Ge, Nd, Sr, or V. and a, b, and c are numbers satisfying 0≦a≦2, 1≦b≦2, 0≦c≦1, and a+4b+(valence of M)×c=8.)
The method for producing NASICON-type negative electrode active material particles for a sodium ion secondary battery according to any one of claims 1 to 5 , comprising particles having a NASICON-type crystal structure represented by: The sodium ion secondary battery according to any one of claims 3 to 6 , wherein the NASICON-type negative electrode active material particles for sodium-ion secondary batteries are particles in which carbon is supported on particles having a NASICON-type crystal structure. Method for producing NASICON-type negative electrode active material particles for

Images