JP7196816B2 - Method for manufacturing positive electrode for all-solid-state sodium battery - Google Patents

Description

The present invention relates to a sodium all-solid-state battery.

Lithium-ion secondary batteries are used as power sources for mobile devices and vehicles, taking advantage of their high-capacity and light weight characteristics. Since the electrolytic solution used here may leak, the use of a solid electrolyte instead of the electrolytic solution is being studied.

However, there is a concern that the price of raw materials for lithium will soar. Therefore, sodium-ion all-solid-state batteries using sodium, which is an abundant resource, are drawing attention as a material to replace lithium. A sodium ion all-solid-state battery requires sodium ion conductivity.

Patent Literature 1 discloses that a positive electrode active material having a P2-type layered crystal structure and a solid electrolyte are used in a positive electrode of a sodium secondary battery.

Japanese Patent Publication No. 2018-514908

However, a reaction occurs at the contact portion between the positive electrode active material having the P2-type layered crystal structure and the solid electrolyte, and by-products are generated to increase interfacial resistance, resulting in insufficient battery output.

The present application has been made in view of this situation, and provides a method for manufacturing a positive electrode for an all-solid-state sodium battery that can suppress the interfacial resistance between a positive electrode active material having a P2-type layered crystal structure and a solid electrolyte. The main purpose is

As one means for solving the above problems, the present application provides a method for manufacturing a positive electrode for a sodium all-solid-state battery, which comprises mixing a positive electrode active material having a P2-type layered crystal structure and a NASICON-type phosphate compound. Disclosed is a method for manufacturing a positive electrode, which includes a step of performing heat treatment at 300° C. or higher and 600° C. or lower after heating.

According to the manufacturing method of the positive electrode of the sodium all-solid-state battery disclosed by the present application, the interfacial resistance between the positive electrode active material having the P2-type layered crystal structure and the solid electrolyte can be suppressed.

1 is a schematic cross-sectional view of a sodium-ion all-solid-state battery with a positive electrode disclosed by the present application; FIG. It is a figure showing the result of an Example and a comparative example.

[Production of positive electrode for all-solid-state sodium battery]
<Preparation of positive electrode active material>
Prepare a positive electrode active material. In the present disclosure, the positive electrode active material is a composite oxide containing Na having a P2-type layered crystal structure.
Here, the "complex oxide containing Na" is to contain, in addition to Na atoms, transition metal elements containing at least one selected from Mn, Ni, Co, Fe, and Cr, and O atoms. . For example, Na _x MO ₂ (0<x≦1, M is at least one of Mn, Ni, Co, Fe, and Cr) can be used.

It can be confirmed by X-ray diffraction (XRD) measurement or the like that the positive electrode active material has a P2-type layered crystal structure.

The positive electrode active material is prepared, for example, by mixing an acidic mixed liquid containing a transition metal element such as a Mn source, a Ni source, and a Co source and a basic mixed liquid containing a Na source, heating and stirring, and then filtering to obtain a precursor. It can be obtained by mixing this precursor with an additional Na source and heat-reacting it at 700°C or higher and 1000°C or lower, preferably 900°C. When the temperature of the heating reaction is lower than 700°C, an O3 type crystal structure is obtained.

The shape of the positive electrode active material is preferably particulate from the viewpoint of ease of handling. The average particle diameter (D50) of the particles of the positive electrode active material is not particularly limited, but can be 0.5 μm or more and 20 μm or less.
In the present application, unless otherwise specified, the average particle diameter of particles is the value of the volume-based median diameter (D50) measured by laser diffraction/scattering particle size distribution measurement. The median diameter (D50) is the diameter (volume average diameter) at which the cumulative volume of particles becomes half (50%) of the total when the particles are arranged in order from the smallest particle diameter.

<Synthesis of Composite Active Material>
A composite active material is obtained by mixing a NASICON-type phosphoric acid compound containing Na, Zr, Si, P, and O atoms with the positive electrode active material prepared above and heat-treating the mixture.
In the present disclosure, this heat treatment is performed at 300° C. or higher and 600° C. or lower.
By heat-treating at a temperature within this range, the interfacial resistance can be reduced when the obtained composite active material is used for the positive electrode, and an all-solid-state battery with high output can be obtained. It is believed that this is because the heat treatment within this temperature range forms a film on the positive electrode active material that has the effect of suppressing the formation of by-products. It is presumed that this coating suppresses the formation of by-products and suppresses the increase in interfacial resistance.

NASICON _- type compounds are ₃ It is a structure arranged dimensionally. Specific NASICON-type phosphate compounds include Na ₃ Zr ₃ Si ₃ PO ₁₂ and the like.

The embodiment of the NASICON-type phosphate compound is preferably in the form of particles from the viewpoint of good handleability. Also, the average particle diameter (D50) of the particles is not particularly limited, but can be 0.5 μm or more and 2 μm or less.
In this case, the powder of the positive electrode active material and the powder of the NASICON-type phosphoric acid compound are mixed, sealed in a vacuum, and heat-treated within the temperature range described above to synthesize the composite active material.

However, both the positive electrode active material and the NASICON-type phosphoric acid compound do not have to be in the form of particles, and a composite active material can be synthesized by mixing one of them as a liquid phase and heat-treating it within the above temperature range. can.

<Preparation of solid electrolyte>
The solid electrolyte is not particularly limited as long as it has sodium ion conductivity, and various solid electrolytes can be applied.
For example, _Na3MX6 is _mentioned . where M is Yb, Y, In or La and X is Cl, Br or I; _More specifically, _{Na3YbCl6 , Na3YbBr6 , Na3YbI6 , Na3YCl6 , Na3YBr6 , Na3YI6 , Na3InCl6 , Na3InBr6 , Na3InI6} , _{Na3LaC16 , Na3LaBr6 , Na3LaI6 .}
In addition, Na ₃ PS ₄ , Na ₃ SbS ₄ , Na ₅ YSi ₄ O ₁₂ , Na ₆ Al ₆ Si ₁₀ O ₃₂ , Na ₈₆ Al ₈₆ Si ₁₆ O ₃₈₄ , and NaCB ₉ H ₁₀ and NaCB ₁₁ H ₁₂ and the like.

<Preparation of positive electrode active material layer>
The positive electrode active material layer is produced from the composite active material synthesized above, the solid electrolyte, and, if necessary, a conductive material and a binder. The method for producing the positive electrode active material layer is not particularly limited, and it can be produced by a dry method or a wet method. That is, the above components are added to a solvent to prepare a slurry, and the slurry is applied to the surface of a substrate (which may be a positive electrode current collector or a solid electrolyte layer described later) and then dried to obtain a predetermined A positive electrode active material layer having a thickness (for example, 0.1 μm or more and 1 mm or less) can be produced by a wet process. Alternatively, the positive electrode active material layer may be obtained by dry-mixing the above components and subjecting the mixture to press molding.

The volume ratio of the positive electrode active material (positive electrode active material having a P2 type crystal structure) in the positive electrode active material layer to the positive electrode active material layer is not particularly limited, but is preferably 45% by volume or more, and is preferably 60% by volume. % or more is more preferable.

When a conductive material is used, its type is not particularly limited, and any known conductive material for sodium ion all-solid-state batteries can be employed. For example, a carbon material is preferable, and a highly crystalline carbon material is particularly preferable. This is because if the carbon material has high crystallinity, it becomes difficult for sodium ions to be inserted into the carbon material, and the irreversible capacity due to sodium ion insertion can be reduced. As a result, a sodium-ion all-solid-state battery with even better cycle characteristics can be obtained. The crystallinity of the carbon material can be defined, for example, by the interlayer distance d002 and the D/G ratio. The interlayer distance d002 refers to the interplanar distance between (002) planes in the carbon material, and specifically corresponds to the distance between graphene layers. The interlayer distance d002 can be determined from peaks obtained by, for example, an X-ray diffraction (XRD) method using CuKα rays. The D / G ratio is observed in Raman spectrometry (wavelength 532 nm), relative to the peak intensity of the G-band derived from the graphite structure near 1590 cm ^-1 , the D-band derived from the defect structure near 1350 cm ^-1 is the peak intensity of In the present invention, for example, the upper limit of d002 is preferably 3.54 Å or less, more preferably 3.50 Å or less. The lower limit is usually 3.36 Å or more. Also, the upper limit of the D/G ratio is preferably 0.90 or less, more preferably 0.80 or less, still more preferably 0.50 or less, and particularly preferably 0.20 or less. The content of the conductive material in the positive electrode active material layer is not particularly limited.

When a binder is used, it is not particularly limited as long as it is chemically and electrically stable. binding materials, rubber-based binders such as styrene-butadiene rubber (SBR), olefin-based binders such as polypropylene (PP) and polyethylene (PE), and cellulose-based binders such as carboxymethyl cellulose (CMC); can be done. The content of the binder in the positive electrode is not particularly limited.

<Preparation of positive electrode>
A positive electrode current collector is laminated on the positive electrode active material layer to form a positive electrode. However, depending on the material contained in the positive electrode active material layer, the positive electrode current collector may be omitted in some cases. In this case, the positive electrode active material layer itself becomes the positive electrode.
Examples of materials for the positive electrode current collector include stainless steel, aluminum, nickel, iron, titanium and carbon. Examples of the shape of the positive electrode current collector include a foil shape, a mesh shape, and a porous shape.

[Sodium-ion all-solid-state battery]
The positive electrode manufactured as described above can be applied to the positive electrode of an all-solid battery. The positive electrode manufactured by the positive electrode manufacturing method of the present application can keep the interfacial resistance between the positive electrode active material and the solid electrolyte low, and as a result, can increase the battery output.

FIG. 1 shows a schematic cross-sectional view of a sodium ion all-solid-state battery 100 including a positive electrode 20 (a positive electrode active material layer 22 and a positive electrode current collector 24) manufactured by the positive electrode manufacturing method of the present disclosure. The sodium ion all-solid-state battery 100 shown in FIG. It has a positive current collector 24 that collects current for the material layer 22 and a negative current collector 34 that collects current for the negative electrode active material layer 32 . As described above, the cathode active material layer 22 and the cathode current collector 24 constitute the cathode 20 . In addition, the negative electrode active material layer 32 and the negative electrode current collector 34 constitute the negative electrode 30 .

<Solid electrolyte layer 10>
In this embodiment, the solid electrolyte layer 10 can be made of the same solid electrolyte as the solid electrolyte contained in the positive electrode. The thickness of the solid electrolyte layer is appropriately adjusted depending on the configuration of the battery and is not particularly limited, and is usually 0.1 μm or more and 1 mm or less.

<Positive electrode 20>
The positive electrode 20 has a positive electrode active material layer 22 and a positive electrode current collector 23, and a positive electrode manufactured by the above-described method for manufacturing a positive electrode is applied.

<Negative electrode active material layer 32>
The negative electrode active material layer 32 contains a negative electrode active material. More specifically, in addition to the negative electrode active material, it may optionally contain a solid electrolyte, a conductive material, and a binder. The solid electrolyte can be considered in the same manner as the solid electrolyte described in the manufacturing method of the positive electrode.

(Negative electrode active material)
The negative electrode active material is not particularly limited, and any known negative electrode active material for sodium ion all-solid-state batteries can be employed. For example, metal materials containing sodium such as sodium metal and sodium alloys; carbon materials such as graphite, hard carbon and carbon black; sodium-transition metal composite oxides such as sodium titanate; oxides composed of elements other than sodium such as SiOx things; and the like. The negative electrode active material is preferably particulate like the positive electrode active material.

(Conductive material and binder)
In the negative electrode active material layer 32, a conductive material and a binder that can be used in the positive electrode active material layer 22 can be used. The conductive material and the binder are optional components, and their contents are not particularly limited.

The method for producing the negative electrode active material layer 32 is not particularly limited, and it can be produced by a dry method or a wet method as in the case of the positive electrode active material layer 22 .

(Negative electrode current collector 34)
The negative electrode active material layer 32 is usually provided with a negative electrode current collector 34 . Examples of materials for the negative electrode current collector 34 include stainless steel, aluminum, nickel, copper, and carbon. Examples of the shape of the negative electrode current collector 34 include a foil shape, a mesh shape, a porous shape, and the like. By laminating the negative electrode current collector 34 on the negative electrode active material layer 32 described above, the negative electrode 30 can be easily manufactured. However, depending on the material contained in the negative electrode active material layer 32, the negative electrode current collector 34 may be omitted in some cases. In this case, the negative electrode active material layer 32 itself becomes the negative electrode 30 .

<Manufacturing of all-solid-state battery>
Although the production of the all-solid-state battery is not particularly limited, for example, the following method can be used.
One method is to add the components to be the positive electrode active material layer obtained by the above-described manufacturing method to a solvent to prepare a slurry, apply the slurry to one surface of the solid electrolyte layer, and then dry it. For example, a negative electrode component is added to a solvent to form a slurry, and the slurry is applied to the other surface of the fixed electrolyte layer and then dried. After that, the positive electrode current collector layer and the negative electrode current collector layer are laminated.
As another method, after producing a solid electrolyte layer, the positive electrode active material layer produced as described above is laminated on one surface of the molded body, and the negative electrode active material layer is laminated on the other surface to form a laminate. For example, sintering is performed by applying pressure to the body. After that, the positive electrode current collector layer and the negative electrode current collector layer are laminated.

<Other configurations>
A general battery case can be used as the battery case, and the battery case is not particularly limited. For example, a battery case made of stainless steel can be mentioned. Moreover, examples of the shape of the sodium ion all-solid-state battery include a coin type, a laminate type, a cylindrical type, a rectangular type, and the like.

Hereinafter, the method for manufacturing the positive electrode of the present disclosure will be described using examples.

[Preparation of positive electrode active material]
<Preparation of first solution (acidic mixed solution)>
Manganese nitrate tetrahydrate (Mn(NO ₃ ) 2.4H ₂ O, Sigma-Aldrich Japan) 5.02 g, nickel nitrate hexahydrate (Ni(NO ₃ ) _{2.6H 2} O, Nacalai Tesque Co., Ltd.) ₂ .40 g and 3.60 g of cobalt nitrate hexahydrate (Co(NO ₃ ) _{2.6H 2 O} , Sigma-Aldrich Japan) were dissolved in 33.0 g of pure water to prepare an acidic mixed solution as the first solution. Obtained.
<Preparation of second solution (basic mixed solution)>
4.25 g of sodium carbonate (Na ₂ CO ₃ , Sigma-Aldrich Japan) is dissolved in 40.0 g of pure water, and 2 mL of ammonia water (NH ₄ OH, Kishida Chemical Co., Ltd.) is added and stirred to form a second solution. A basic mixture was obtained.
<Synthesis of precursor>
44.0 g of the first solution and 45.0 g of the second solution were coprecipitated and heated and stirred overnight. The synthesized precursor was washed by suction filtration and dried.
<Synthesis of positive electrode active material>
The obtained precursor and sodium carbonate (Na ₂ CO ₃ , Sigma-Aldrich Japan) were mixed and reacted at 900° C. in an air atmosphere to form Na _0.7 Mn _0.5 Ni 0.7 Mn 0.5 Ni 0.5 having a P2-type layered crystal structure _{. A cathode} active material of _2Co0.3O2 was obtained.

[Synthesis of Composite Active Material]
The positive electrode active material thus obtained was mixed with powder of Na ₃ Zr ₂ Si ₂ PO ₁₂ , which is a NASICON type phosphoric acid compound, and the mixture was sealed in a vacuum and heat-treated at a predetermined temperature to obtain a composite active material. . Here, the determined temperatures are 300°C for Example 1, 400°C for Example 2, 500°C for Example 3, 600°C for Example 4, 200°C for Comparative Example 1, and 700°C for Comparative Example 2. is.
The crystal structure of all the examples was examined with an X-ray diffraction device (Ultima IV, Rigaku Corporation), and it was confirmed that all the positive electrode active materials had a P2-type layered crystal structure.

[Synthesis of solid electrolyte]
Sodium sulfide (Na ₂ S, Sigma-Aldrich Japan), antimony sulfide (Sb ₂ S ₃ Sigma-Aldrich Japan), sulfur (S, Alpha Acer) in a molar ratio of 3:2:1 under an inert atmosphere. This was mixed after weighing. This mixture was subjected to mechanical milling and synthesized under an inert atmosphere to obtain a Na ₃ SbS ₄ solid electrolyte.

[Preparation of positive electrode]
The synthesized composite active material, the synthesized solid electrolyte, and the conductive material (carbon in this example) were weighed out at a mass ratio of 50:45:5, mixed in a mortar, and formed into a cylindrical shape having a diameter of φ11 and a thickness of 50 μm. A positive electrode active material layer was obtained.
A cylindrical current collector made of stainless steel was laminated on the obtained positive electrode active material layer to form a positive electrode.

[Preparation of negative electrode]
Sodium metal (Sigma-Aldrich Japan) immersed in oil was processed into a disk shape, and a cylindrical current collector made of stainless steel was layered on this to form a negative electrode.

[Preparation of Solid Electrolyte Layer]
The solid electrolyte layer was made from the solid electrolyte used for the positive electrode active material layer. Specifically, a cylindrical stainless steel piece is fitted to the lower side of an annular (doughnut-shaped) evaluation cell, and a jig for protecting the cylindrical stainless steel piece is placed below it. The solid electrolyte was placed in the ring (inside the hole) of the toroidal evaluation cell, the upper side was covered with a cylindrical stainless steel, and a jig for protecting the cylindrical stainless steel was fitted on top, and pressed at 6 tons. It was made by pressing and hardening.

[Fabrication of all-solid-state battery]
A positive electrode was laminated on one side of the solid electrolyte layer obtained above, and a negative electrode was laminated on the other side to form an all-solid-state battery. This was sealed in a glass desiccator.

[evaluation]
Interfacial resistance was measured for each example. This measurement applied a voltage of ±10 mV with respect to the open circuit voltage to the all-solid-state battery of each example, and measured the interfacial resistance in the range of 0.01 Hz or more and 1 MHz or less. The results are shown in Table 1 and FIG.

表１、図２からわかるように、複合活物質を作製する際の熱処理温度が界面抵抗に大きく影響していることがわかった。

As can be seen from Table 1 and FIG. 2, it was found that the heat treatment temperature in producing the composite active material greatly affects the interfacial resistance.

100 Sodium ion all-solid battery 10 Solid electrolyte layer 22 Positive electrode active material layer 24 Positive electrode current collector 32 Negative electrode active material layer 34 Negative electrode current collector

Claims (1)

A method for manufacturing a positive electrode for a sodium all-solid-state battery, comprising:
A positive electrode active material having a P2-type layered crystal structure containing Na atoms, a transition metal element containing at least one selected from Mn, Ni, Co, Fe, and Cr, and O atoms; After mixing with a phosphoric acid compound, heat treatment is performed at 300 ° C. or higher and 600 ° C. or lower to synthesize a composite active material, and the composite active material is mixed with a solid electrolyte having sodium ion conductivity . A method for manufacturing a positive electrode.

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