JP6741390B2 - Positive electrode material, paste and sodium ion battery - Google Patents

Description

The present invention relates to a positive electrode material, a paste and a sodium ion battery.

In recent years, stationary secondary batteries have been studied in efforts to reduce energy costs and reduce environmental load such as power load leveling, use as an emergency power source, and stabilization of renewable energy. In particular, in such efforts, sodium ion secondary batteries using inexpensive materials have been studied (see, for example, Patent Documents 1 and 2).
Further, there is known a positive electrode material in which the surface of the particles of the positive electrode active material is uniformly coated with a carbonaceous material by thermal decomposition of an organic substance in order to enhance the electron conductivity, and the current density of the positive electrode active material is improved. (For example, see Patent Document 3).

Sodium represented by Na _x M _y (SO ₄ ) _y having a M ₂ O ₁₀ dimer crystal structure in which two MO ₆ in the structure share edges, which is a positive electrode active material for a novel sodium ion battery A positive electrode active material containing a sulfate compound (Na ₂ M ₂ (SO ₄ ) ₃ (where M is a transition metal element such as Fe, Mn, Co, or Ni)) has been proposed (for example, see Non-Patent Document 1). ).
This positive electrode active material is different from the conventional sodium sulfate compound having a monoclinic structure or a Nasicon type structure, and when used as a positive electrode material, 3.6 V vs. Since it is a low-cost material composed of elements with abundant resources such as Na, Fe, S, and O in addition to causing a discharge reaction with Na or more, it is a very promising material as a positive electrode active material for sodium ion batteries. Is expected.

Japanese Patent Publication No. 2000-509193 JP, 2009-266821, A JP 2001-015111 A

https://ecs.confex.com/ecs/imlb2014/webprogram/Paper34490.html 216.Materials Exploration in Na-Fe-S-O System: Crystal Structure (17th International Meeting on Lithium Batteries, 2014) https://ecs.confex.com/ecs/imlb2014/webprogram/Paper35323.html 217.Comparative Studies of Li Versus Na System: Phase Diagram and New Polyanion Cathodes (17th International Meeting on Lithium Batteries, 2014) https://ecs.confex.com/ecs/imlb2014/webprogram/Paper34514.html 218.Materials Exploration in Na-Fe-S-O System: Synthesis and Electrochemistry (17th International Meeting on Lithium Batteries, 2014) https://ecs.confex.com/ecs/imlb2014/webprogram/Paper35663.html 219.Na-Fe-S-O System As Cathode Materials for Na-Ion Batteries: An Ab Initio Study (17th International Meeting on Lithium Batteries, 2014)

However, a positive electrode active material containing a sodium sulfate compound having a novel crystal structure has low electron conductivity and has a problem of solubility in water, and therefore battery characteristics such as discharge capacity at a fast charge/discharge rate and In order to improve the cycle characteristics, it is necessary to uniformly coat the surface of the sodium sulfate compound particles with the carbonaceous material.
Further, conventionally, the surface of the particles of the positive electrode active material is uniformly coated with the carbonaceous material by the thermal decomposition of the organic matter, but the positive electrode active material containing the sodium sulfate compound has low thermal stability, so that the low temperature process It is necessary to coat the carbonaceous material.

The present invention has been made in view of the above circumstances, and provides a positive electrode material, a paste, and a sodium ion battery that are excellent in water resistance, have high electron conductivity, and are preferably used in a sodium ion secondary battery. To aim.

The positive electrode material of the present _{invention, Na x M y (SO 4} ) z having two _M 2 MO ₆ were formed edge-sharing to dimers _{O 10} dimeric crystal structure in the structure (where, M is One or more selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag; 1.6≦x≦2.4, 1.6≦y≦2.4, 2.4≦z≦3.6), and positive electrode active material particles containing a sodium sulfate compound, A carbonaceous electrode active material composite particle comprising a carbonaceous coating covering the surface of the positive electrode active material particles, wherein the carbonaceous coating has a coverage of 60% or more on the surface of the positive electrode active material particles. Has an average thickness of 2.1 nm or more and 16.3 nm or less, and the carbon content of the carbonaceous electrode active material composite particles is 0.71% by mass or more and 7.12% by mass or less. To do.

The paste of the present invention is characterized by containing the positive electrode material of the present invention, a conductive auxiliary agent, and a binder.

The sodium ion battery of the present invention is characterized by comprising an electrode plate having a positive electrode formed on one main surface of a current collector using the paste of the present invention.

According to the positive electrode material of the present invention, two MO _{6 in the} structure are represented by Na _x M _y (SO ₄ ) _z having an M ₂ O ₁₀ dimer crystal structure in which a ridge is shared to form a dimer. The high coverage of the carbonaceous coating on the surface of the primary particles of the positive electrode active material particles containing the sodium sulfate compound, which is excellent in water resistance and electron conductivity, makes it suitable for sodium ion secondary batteries. Can be used. Thereby, cycle deterioration of battery capacity can be significantly improved.

Since the paste of the present invention contains the positive electrode material of the present invention, when this paste is applied to one main surface of the current collector to form a positive electrode and used as an electrode plate, it has a high capacity and high capacity. An electrode plate having an energy density can be provided.

According to the sodium ion battery of the present invention, since the positive electrode is formed on one main surface of the current collector by using the paste containing the positive electrode material of the present invention, a high capacity and a high energy density are obtained. It is possible to provide a sodium ion battery.

Modes for carrying out the positive electrode material, paste, and sodium ion battery of the present invention will be described.
It should be noted that this mode is specifically described for better understanding of the gist of the invention, and does not limit the invention unless otherwise specified.

[Cathode material]
The positive electrode material of the present embodiment has a Na _x M _y (SO ₄ ) _z (provided that M ₂ O ₁₀ dimer crystal structure has a structure in which two MO ₆ in the structure share edges to form a dimer) Is one or more selected from the group of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag. , 1.6≦x≦2.4, 1.6≦y≦2.4, 2.4≦z≦3.6), and positive electrode active material particles containing the sodium sulfate compound. It is characterized in that it contains carbonaceous electrode active material composite particles comprising a carbonaceous coating film covering the surface of the positive electrode active material particles.

In the present _{embodiment, Na x M y (SO 4} ) particles of sodium sulfate compounds represented by _z, the non-patent document 3 (https://ecs.confex.com/ecs/imlb2014/webprogram/Paper34514.html ), which is produced based on the production method (synthesis method) using Na ₂ SO ₄ and FeSO ₄ (anhydrous) as raw materials.

Regarding M, Fe, Mn, Co, and Ni are preferable from the viewpoint of high discharge potential and the like.
_{Further, Na x M y (SO 4} ) z _is preferably Na ₂ M _{2 (SO 4)} is _{3, Na 2 Fe 2 (SO} 4) and more preferably _3.

In the present embodiment, it is preferable that the carbonaceous film is formed by a low temperature process of 500° C. or lower described later. Since the carbonaceous film is formed by a low temperature process of 500° C. or lower, the positive electrode active material particles containing the above sodium sulfate compound, which have low thermal stability, do not deteriorate due to heat and have a high carbon coverage. A quality coating can be formed.
Here, the carbonaceous material means a carbon simple substance or a carbon material containing carbon as a main component.
Further, "the surface of the positive electrode active material particles is covered with a carbonaceous film" means that the surface of the positive electrode active material particles is covered with a film made of carbonaceous material (carbonaceous film). A state in which a plurality of particles made of simple carbon or particles made of a carbon material containing carbon as a main component are attached or bonded to the surface of the substance particles, particles made of simple carbon or a main component of carbon on the surface of the positive electrode active material particles That is, a state in which a plurality of agglomerates formed by aggregating a plurality of particles of a carbon material are attached or bonded to each other is provided.

The average particle diameter of the primary particles of the positive electrode active material particles (carbonaceous electrode active material composite particles) coated with the carbonaceous film is preferably 0.05 μm or more and 20 μm or less, more preferably 0.1 μm or more and 5 μm or less. is there.
Here, the average particle size of the primary particles of the carbonaceous electrode active material composite particles is preferably 0.05 μm or more and 20 μm or less because the average particle size of the primary particles of the carbonaceous electrode active material composite particles is less than 0.05 μm. Then, not only does the required carbon mass increase due to the increase in the specific surface area of the carbonaceous electrode active material composite particles, the charge/discharge capacity is reduced, but also carbon coating becomes difficult, and primary particles with a sufficient coverage ratio are obtained. This is because it cannot be obtained, and the effect of suppressing transition metal ion elution into the electrolytic solution cannot be sufficiently obtained. On the other hand, when the average particle size of the primary particles of the carbonaceous electrode active material composite particles exceeds 20 μm, it takes time for the sodium ions or electrons to move within the carbonaceous electrode active material composite particles, so that the internal resistance increases. Not only unfavorably, the output characteristics are deteriorated and the output characteristics are deteriorated.However, when a coating film is formed using the paste of the present embodiment, lump-like or lump-like foreign matter or streak-shaped dents are generated in the coating film, and the coating is uniform. This is because a coating film having a different thickness cannot be obtained.

In the positive electrode material of the present embodiment, the carbonaceous film suppresses the elution of transition metal ions from the positive electrode material into the electrolytic solution, and improves the ionic conductivity between the positive electrode active material particles. It is preferable that the composite electrode active material particles are agglomerated particles obtained by aggregating a plurality of primary particles of the positive electrode active material particles coated with a carbonaceous coating, and the carbonaceous coating is interposed between the primary particles. ..
When the carbonaceous electrode active material composite particles are aggregated particles obtained by aggregating a plurality of primary particles of the positive electrode active material particles coated with a carbonaceous film, the average aggregated particle size of the aggregated particles is 0.5 μm or more and 100 μm or more. The following is preferable, and more preferably 1 μm or more and 20 μm or less.

Here, the reason for setting the average agglomerated particle diameter of the agglomerated particles to the above range is that when the average agglomerated particle diameter is less than 0.5 μm, the agglomerated particles are too fine and thus easily fly, and when the positive electrode forming paste is produced. It is difficult to handle. On the other hand, when the average agglomerated particle diameter exceeds 100 μm, there is a high possibility that agglomerated particles having a size exceeding the thickness of the dried positive electrode will be present when the positive electrode for a battery is produced, and therefore, the thickness of the positive electrode This is because it becomes impossible to maintain the uniformity of.

The average agglomerated particle size of the agglomerated particles is the same as the method for measuring the average particle size of the positive electrode active material particles coated with a carbonaceous film, and may be observed using an SEM or the like, or dispersed in water or the like. The number average particle diameter may be measured using a laser diffraction/scattering type particle size distribution measuring device or the like.

The volume density of the aggregate can be measured using a mercury porosimeter, and is preferably 40% by volume or more and 95% by volume or less, more preferably 60% by volume or more of the volume density when the aggregated particles are solid. And 90% by volume or less.

As described above, by setting the volume density of the aggregated particles to 40% by volume or more, the aggregated particles are densified to increase the strength of the aggregated particles, and, for example, the positive electrode material of the present embodiment includes the binder, the conductive additive, and the solvent. Aggregated particles are less likely to collapse when preparing an electrode slurry by mixing with, as a result, increase in viscosity of the electrode slurry is suppressed, and by maintaining fluidity, coatability is improved and the electrode is It is also possible to improve the filling property of the electrode material in the slurry coating film.

The coverage of the carbonaceous film on the surface of the positive electrode active material particles can be measured using a transmission electron microscope (TEM), an energy dispersive X-ray spectrometer (EDX), or the like.
In the positive electrode active material particles in the present embodiment, 60% or more of the surface of the positive electrode active material particles is used in order to uniformly carry out the reaction relating to the insertion and removal of sodium ions when used as the positive electrode material of a sodium ion battery. More preferably, 80% or more, and even more preferably 90% or more, is covered with a carbonaceous film.
Here, the reason why the coverage of the carbonaceous coating on the surface of the positive electrode active material particles is 60% or more is that when the coverage is less than 60%, the surface of the positive electrode active material particles is exposed and water is easily adsorbed, The hydrofluoric acid generated by the adsorbed water may deteriorate the battery constituent members, and the decomposition of water by charging/discharging may generate gas to increase the internal pressure in the battery, which may damage the battery. This is because it is not preferable.
In the present embodiment, the coverage of the carbonaceous coating on the surface of the positive electrode active material particles is the carbonaceous coating with respect to the particle surface length of the cross section of the positive electrode active material particles obtained by using a transmission electron microscope (TEM). The ratio of the known surface length is measured, and 50 particles are similarly measured and averaged.

The amount of oxygen in this carbonaceous material contains oxygen containing at least one of a hydroxyl group (-OH), a carbonyl group (>C=O), a carboxyl group (-COOH), an ether bond, and an ester bond at or near the end. It depends on the amount of functional groups.
Therefore, the oxygen content in the carbonaceous material is preferably 5.0% by mass or less, more preferably 3.0% by mass or less.

Here, the reason why the oxygen content in the carbonaceous material is 5.0% by mass or less is that when the oxygen content in the carbonaceous material exceeds 5.0% by mass, the oxygen-containing functional groups present in the carbonaceous material during charging. This is because the gas generated by the oxidation of the groups increases the internal pressure in the sodium ion battery, and as a result, the sodium ion battery may be destroyed, which is not preferable.
When the oxygen content in the carbonaceous material exceeds 5.0% by mass, the amount of water adsorbed on the oxygen-containing functional group in the carbonaceous material increases, and the water content in the sodium ion battery causes an electrolyte. This is because NaPF ₆ is decomposed and hydrofluoric acid generated by this decomposition may deteriorate the battery constituent members, which is not preferable.

The thickness (average value) of the carbonaceous coating is preferably 0.1 nm or more and 20 nm or less.
The reason for setting the thickness of the carbonaceous coating within the above range is that when the thickness of the carbonaceous coating is less than 0.1 nm, the thickness of the carbonaceous coating is too thin to form a film having a desired resistance value. This is because it becomes impossible, and as a result, the conductivity decreases, and the conductivity as the positive electrode material cannot be secured. On the other hand, when the thickness of the carbonaceous coating exceeds 20 nm, the battery activity, for example, the battery capacity per unit mass of the positive electrode material, decreases.

In the positive electrode material of the present embodiment, the carbonaceous film suppresses the elution of transition metal ions from the positive electrode material into the electrolytic solution, and improves the ionic conductivity between the positive electrode active material particles. It is preferable that the composite electrode active material particles are agglomerated particles obtained by aggregating a plurality of primary particles of the positive electrode active material particles coated with a carbonaceous coating, and the carbonaceous coating is interposed between the primary particles. ..

The specific surface area of the carbonaceous electrode active material composite particles is preferably 0.1 m ² /g or more and 30 m ² /g or less, and more preferably 0.2 m ² /g or more and 20 m ² /g or less.
Here, the reason why the specific surface area of the carbonaceous electrode active material composite particles is limited to 0.1 m ² /g or more and 30 m ² /g or less is that the specific surface area of the carbonaceous electrode active material composite particles is 0.1 m ² /g. If it is less than the above range, it takes time for the sodium ions or electrons to move within the carbonaceous electrode active material composite particles, which increases the internal resistance and deteriorates the output characteristics, which is not preferable. On the other hand, when the specific surface area of the carbonaceous electrode active material composite particles exceeds 30 m ² /g, the required carbon mass increases due to the increase in the specific surface area of the carbonaceous electrode active material composite particles, and the charge/discharge capacity decreases. Is not preferred.

The "internal resistance" used here is mainly a sum of electronic resistance and sodium ion transfer resistance. Electronic resistance is proportional to the amount of carbon, carbon density and crystallinity, and sodium ion transfer resistance is carbon. Inversely proportional to amount, carbon density and crystallinity.
As a method of evaluating the internal resistance, for example, a current rest method or the like is used. In this current quiescent method, the internal resistance is wiring resistance, contact resistance, charge transfer resistance, sodium ion transfer resistance, sodium reaction resistance at positive and negative electrodes, interelectrode resistance determined by the distance between positive and negative electrodes, solvation of sodium ions, desolvation. It is measured as the sum of the resistance related to the sum and the SEI (Solid Electrolyte Interface) migration resistance of sodium ions.

The water content of the carbonaceous electrode active material composite particles is preferably 5% by mass or less, and more preferably 2.5% by mass or less.
Here, the reason why the water content of the carbonaceous electrode active material composite particles is 5% by mass or less is that when the water content exceeds 5% by mass, when the carbonaceous electrode active material composite particles are used in a sodium ion battery. In addition, the residual water decomposes NaPF ₆ which is an electrolyte, and hydrofluoric acid generated thereby deteriorates the battery constituent member, which is not preferable.

The carbon content of the carbonaceous electrode active material composite particles is preferably 0.3% by mass or more and 8.0% by mass or less, more preferably 0.5% by mass or more and 5.0% by mass or less. is there.
Here, the reason why the carbon content of the carbonaceous electrode active material composite particles is 0.3% by mass or more and 8.0% by mass or less is that the carbon content of the carbonaceous electrode active material composite particles is 0.3% by mass. If it is less than mass %, the discharge capacity at a high charge/discharge rate becomes low when a battery is formed, and it becomes difficult to realize sufficient charge/discharge rate performance, which is not preferable. On the other hand, when the carbon content of the carbonaceous electrode active material composite particles exceeds 8.0% by mass, the distance that sodium ions move in the carbon increases, and when sodium ions diffuse in the carbonaceous film, sodium is diffused. This is because the distance that the ions move in the carbonaceous coating film having a slow diffusion rate becomes long, and hence the voltage drop at a high charge/discharge rate cannot be ignored, which is not preferable.

[Method of manufacturing electrode material]
The method for producing the electrode material of the present embodiment is not particularly limited. For example, a step of coating the surface of the Na _x M _y (SO ₄ ) _z particles (primary particles) with a carbonaceous film by a low temperature process of 500° C. or lower is performed. The method of having is mentioned.

Na _x M _y (SO ₄ ) _z particles are described in Non-Patent Document 3 (https://ecs.confex.com/ecs/imlb2014/webprogram/Paper34514.html), Na ₂ SO ₄ and FeSO. ₄ (anhydrous) is used as a raw material and is produced based on a production method (synthesis method).

As a method for coating the surface of the _{Na x M y (SO 4)} z particles carbonaceous coating, by 500 ° C. or less of a low temperature _process to form a carbonaceous coating on the surface of the _{Na x M y (SO 4)} z particles If possible, it is not particularly limited. using a bead mill or a planetary _mill, a method of mixing the _{Na x M y (SO 4)} z particles and carbon _source, to collide with the _{Na x M y (SO 4)} z particles and a carbon source, a carbonaceous coating A hybridization method for forming the mixture, a method of mixing Na _x M _y (SO ₄ ) _z particles and a carbon source, and then heating the mixture to carbonize the mixture are exemplified.

Examples of the carbon source used in the CVD include alcohols such as methanol, ethanol and propanol, and gases such as methane and acetylene.
Examples of carbon sources used in PVD and sputtering include graphite and graphite target materials.
Examples of the carbon source used in the method using a bead mill or a planetary mill and the hybridization method include carbon powder such as acetylene black, conductive furnace, Ketjen black, graphite, graphene, graphene oxide, fullerene, and carbon nanotube.
By heating a mixture of _{Na x M y (SO 4)} z particles and the carbon source, the carbon source used in the method of carbonization, for example, glucose, lactose and sugars such as sucrose, organic solvents such as glycerin, ethylene glycol Polymers such as polyvinylpyrrolidone, polyvinyl alcohol and polyacrylic acid, monomers such as vinylpyrrolidone and vinyl alcohol, carbon powder such as acetylene black, graphite and carbon nanotubes.

According to these methods for forming a carbonaceous film, a carbonaceous film having a low thermal stability and having a high coverage can be formed without degrading the positive electrode active material particles containing the sodium sulfate compound by heat. You can

[paste]
The paste of this embodiment is characterized by containing the positive electrode material of this embodiment, a conductive auxiliary agent, and a binder.

The positive electrode material is preferably contained in an amount of 70% by mass or more and 98% by mass or less, and 85% by mass or more and 95% by mass, when the total mass of the positive electrode material, the conductive additive and the binder is 100% by mass. More preferably, it is contained below.
By containing the positive electrode material in the above range, a positive electrode having excellent battery characteristics can be obtained.

The conductive additive is not particularly limited, but is selected from the group of fibrous carbon such as acetylene black, Ketjen black, furnace black, vapor grown carbon fiber (VGCF), carbon nanotube, etc. 1 One kind or two or more kinds can be used.

The conductive additive is preferably contained in an amount of 0.5% by mass or more and 15% by mass or less and 1% by mass or more and 10% by mass or less, based on the total mass of the positive electrode material, the conductive auxiliary agent and the binder. The content is more preferably 10% by mass or less, further preferably 2% by mass or more and 8% by mass or less.
Here, the reason for setting the content of the conductive additive in the above range is that when the content of the conductive additive is less than 0.5% by mass, when the positive electrode is formed using the paste of the present embodiment, the electron conductivity is reduced. This is not preferable because the battery does not have sufficient properties and the battery capacity and charge/discharge rate decrease. On the other hand, if the content of the conductive additive exceeds 15% by mass, the positive electrode material in the positive electrode is relatively decreased, and the battery capacity of the sodium ion battery per unit volume is reduced, which is not preferable.

The binder is not particularly limited, but for example, one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyethylene, polypropylene, or the like, or Two or more types may be mentioned.

The binder is preferably contained in an amount of 0.5% by mass or more and 10% by mass or less, preferably 1% by mass or more and 10% by mass or less, when the total mass of the positive electrode material, the conductive additive and the binder is 100% by mass. It is more preferable that the content is 7% by mass or less.
Here, the reason for setting the content of the binder to the above range is that when the content of the binder is less than 0.5% by mass, when the coating film is formed using the paste of the present embodiment, This is because the binding property between the film and the current collector is not sufficient, and the coating film may be cracked or fallen off during rolling formation of the positive electrode, which is not preferable.
Further, it is not preferable because the coating film may be peeled off from the current collector during the charge/discharge process of the battery, which may reduce the battery capacity and the charge/discharge rate.
On the other hand, if the content of the binder exceeds 10% by mass, the internal resistance of the positive electrode material may increase and the battery capacity at a high charge/discharge rate may decrease, which is not preferable.

The solvent may be appropriately mixed in order to facilitate the application of the paste. The type of solvent is not particularly limited, and examples thereof include water, methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol, pentanol, hexanol, octanol, diacetone alcohol and the like, ethyl acetate. , Butyl acetate, ethyl lactate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, esters such as γ-butyrolactone, diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve ), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, diethylene glycol monoethyl ether and other ethers, acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone, cyclohexanone and other ketones, dimethylformamide, N , N-dimethylacetoacetamide, amides such as N-methylpyrrolidone, and glycols such as ethylene glycol, diethylene glycol and propylene glycol. These may be used alone or in combination of two or more.

The solvent is preferably mixed in the paste in an amount of 30% by mass or more and 70% by mass or less. In other words, the total mass of the positive electrode material, the conductive additive, and the binder is preferably contained in the paste so as to be 30% by mass or more and 70% by mass or less, and 40% by mass or more and 60% by mass. More preferably, it is contained below. By mixing in such a range, it is possible to obtain a paste for forming a positive electrode, which is excellent in forming a positive electrode and excellent in battery characteristics.

The method for producing the paste of the present embodiment is not particularly limited as long as it is a method capable of uniformly mixing the positive electrode material, the conductive auxiliary agent, the binder, and the solvent, and examples thereof include a ball mill, a sand mill, a planetary (planetary type) mixer, A method using a kneading machine such as a paint shaker or a homogenizer can be used.

[Electrode plate]
The electrode plate of the present embodiment is characterized in that a positive electrode is formed on one main surface of a current collector using the paste of the present embodiment. This electrode plate is used for the positive electrode of a sodium ion battery.

The method for manufacturing the electrode plate of the present embodiment is not particularly limited as long as it is a method capable of forming a positive electrode on one surface of the current collector using the positive electrode material of the present embodiment.
For example, the paste of the present embodiment is applied to one surface of the current collector to form a coating film, the coating film is dried, and then pressure-bonded to obtain a positive electrode on one surface of the current collector. The formed electrode plate can be obtained.

[Sodium ion battery]
The sodium ion battery of the present embodiment is characterized by including the electrode plate of the present embodiment.

The sodium ion battery according to the present embodiment includes a positive electrode including the electrode plate according to the present embodiment, a negative electrode, a separator, and an electrolytic solution.

In the sodium ion battery of this embodiment, the negative electrode, the electrolytic solution, the separator and the like are not particularly limited.
As the negative electrode, for example, a negative electrode material such as metallic Na, a carbon material, a Na alloy, Li ₄ Ti ₅ O ₁₂ , Na ₄ Ti ₅ O ₁₂ , Na ₂ Ti ₃ O ₇ , a tin-based alloy, or a silicon compound can be used. it can.
A solid electrolyte may be used instead of the electrolytic solution and the separator.

As the electrolytic solution, for example, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 1:1 and the obtained mixed solvent is mixed with sodium hexafluorophosphate (Na PF 4 ). ₆ ) can be prepared by, for example, dissolving so that the concentration becomes 1 mol/dm ³ .
As the separator, for example, porous propylene can be used.

In the sodium ion battery of this embodiment, since the electrode plate of this embodiment is used as the positive electrode, it has high capacity and high energy density.

As described above, according to the positive electrode material of the present embodiment, Na _x M _y (having a M ₂ O ₁₀ dimer crystal structure in which two MO ₆ in the structure share edges to form a dimer). SO ₄ ) _z, which has a high coverage of the carbonaceous coating on the surface of the primary particles of the positive electrode active material particles containing the sodium sulfate compound, is excellent in water resistance and has high electron conductivity. It can be suitably used for an ion secondary battery. Further, it is possible to a _{Na x M y (SO 4)} z and electrolyte contacting area is reduced directly, the transition metal ions can be inhibited from eluting into the electrolyte.
Furthermore, it is possible to transition metal ions eluted from the _{Na x M y (SO 4)} z are captured in the carbonaceous coating, a transition metal ion to the negative electrode can be inhibited from migration. Due to these effects, the amount of transition metal ions reaching the negative electrode can be reduced, and cycle deterioration of battery capacity can be significantly improved.

The paste of the present embodiment contains the positive electrode material of the present embodiment. Therefore, when the paste is applied to one main surface of the current collector to form a positive electrode and used as an electrode plate, a high capacity Moreover, an electrode plate having a high energy density can be provided.

Since the electrode plate of the present embodiment contains the positive electrode material of the present embodiment, it is possible to provide an electrode plate having a high capacity and a high energy density.

According to the sodium ion battery of the present embodiment, since the positive electrode is formed on the one main surface of the current collector using the paste containing the positive electrode material of the present embodiment, it has a high capacity and high capacity. An energy density sodium ion battery can be provided.

Hereinafter, the present invention will be specifically described with reference to Examples 1 to 6 and Comparative Examples 1 and 2, but the present invention is not limited to these examples.

[Example 1]
(Preparation of positive electrode material)
According to the method described in Non-Patent Document 3 (https://ecs.confex.com/ecs/imlb2014/webprogram/Paper34514.html), Na ₂ Fe ₂ (SO ₄ ) having an average primary particle size of 5.84 μm. ₃ was produced.
Na ₂ SO ₄ as a raw material and FeSO ₄ (anhydrous) were put in a pot together with zirconia beads having a diameter of 5 mm, and mixed for 1 hour at a rotation speed of 300 rpm using a ball mill device. The mixed powder thus obtained was placed in an alumina board and heated in an argon (Ar) stream at 350° C. for 12 hours in a tubular heating furnace to obtain the desired Na ₂ Fe ₂ (SO ₄ ) Got ₃ .
A carbonaceous film having an average thickness of 2.1 nm is formed on the surface of the Na ₂ Fe ₂ (SO ₄ ) ₃ by the plasma CVD method, and Na ₂ Fe ₂ (SO ₄ ) ₃ particles and Na ₂ Fe ₂ ( A positive electrode material of Example 1 containing carbonaceous electrode active material composite particles composed of a carbonaceous coating film covering the surfaces of SO ₄ ) ₃ particles was obtained. The coverage of the carbonaceous film on the surface of the carbonaceous electrode active material composite particles was 93%.
In the obtained positive electrode material, the carbon content of the carbonaceous electrode active material composite particles was 0.71% by mass.
Average primary particle diameter of Na ₂ Fe ₂ (SO ₄ ) ₃ in the positive electrode material of Example 1, average thickness of carbonaceous coating, coverage of carbonaceous coating on the surface of carbonaceous electrode active material composite particles, carbonaceous electrode Tables 1 and 2 show the carbon content and the like of the active material composite particles.

(Preparation of sodium ion battery)
The above positive electrode material, polyvinylidene fluoride (PVdF) as a binder, and acetylene black (AB) as a conductive additive are mixed in a mass ratio of 85:10:5, and N-methyl-as a solvent. 2-Pyrrolidinone (NMP) was added to impart fluidity to prepare a slurry.
Next, this slurry was applied onto an aluminum (Al) foil having a thickness of 15 μm and dried. Then, the positive pressure of the sodium ion battery of Example 1 was produced by applying a pressure of 600 kgf/cm ² .

Sodium metal was placed as a negative electrode on the positive electrode of this sodium ion battery, and a separator made of porous polypropylene was placed between the positive electrode and the negative electrode to obtain a battery member.
On the other hand, ethylene carbonate and diethyl carbonate were mixed at a ratio of 1:1 (mass ratio), and a 1M NaPF ₆ solution was further added to prepare an electrolyte solution having sodium ion conductivity.
Then, the battery member was immersed in the electrolyte solution to prepare the sodium ion battery of Example 1.

(Evaluation of sodium ion battery)
The charge/discharge characteristics of the sodium ion battery described above were evaluated.
Here, the above-mentioned sodium ion battery was subjected to constant current charging at 60° C. until the charging voltage was 4.2 V at a 1 C current value, and then switched to constant voltage charging, and the current value was 0.01 C. The charging was terminated at the point of time. Then, discharging was performed at a discharging current of 1 C, and the discharging was terminated when the battery voltage reached 2.5 V. The discharge capacity at that time was measured and defined as the initial discharge capacity.
Further, charging and discharging were repeated under the same conditions, the discharge capacity at the 50th cycle was measured, and the capacity retention ratio with respect to the initial discharge capacity was calculated. The evaluation results are shown in Table 2.

[Example 2]
In the same manner as in Example 1, Na ₂ Fe ₂ (SO ₄ ) ₃ having an average primary particle size of 7.63 μm was produced.
A carbonaceous film having an average thickness of 5.9 nm is formed on the surface of this Na ₂ Fe ₂ (SO ₄ ) ₃ by a thermal CVD method to obtain a positive electrode material of Example 2 containing carbonaceous electrode active material composite particles. It was The coverage of the carbonaceous coating on the surface of the carbonaceous electrode active material composite particles was 85%.
In the obtained positive electrode material, the carbon content of the carbonaceous electrode active material composite particles was 1.34% by mass.
Average primary particle diameter of Na ₂ Fe ₂ (SO ₄ ) ₃ in the positive electrode material of Example 2, average thickness of carbonaceous coating, coverage of carbonaceous coating on the surface of carbonaceous electrode active material composite particles, carbonaceous electrode Tables 1 and 2 show the carbon content and the like of the active material composite particles.
Also, a sodium ion battery was produced using the positive electrode material of Example 2 in the same manner as in Example 1, and the sodium ion battery was evaluated. The evaluation results are shown in Table 2.

[Example 3]
In the same manner as in Example 1, Na ₂ Fe ₂ (SO ₄ ) ₃ having an average primary particle size of 12.73 μm was produced.
A carbonaceous coating having an average thickness of 7.1 nm was formed on the surface of this Na ₂ Fe ₂ (SO ₄ ) ₃ by a sputtering method to obtain a positive electrode material of Example 3 containing carbonaceous electrode active material composite particles. .. The coverage of the carbonaceous film on the surface of the carbonaceous electrode active material composite particles was 96%.
In the obtained positive electrode material, the carbon content of the carbonaceous electrode active material composite particles was 2.98% by mass.
Average primary particle diameter of Na ₂ Fe ₂ (SO ₄ ) ₃ in the positive electrode material of Example 3, average thickness of carbonaceous coating, coverage of carbonaceous coating on the surface of carbonaceous electrode active material composite particles, carbonaceous electrode Tables 1 and 2 show the carbon content and the like of the active material composite particles.
Further, in the same manner as in Example 1, a sodium ion battery was produced using the positive electrode material of Example 3, and the sodium ion battery was evaluated. The evaluation results are shown in Table 2.

[Example 4]
In the same manner as in Example 1, Na ₂ Fe ₂ (SO ₄ ) ₃ having an average primary particle size of 18.76 μm was produced.
Then, Na ₂ Fe ₂ (SO ₄ ) ₃ particles and sodium carboxymethyl cellulose powder as a carbon source were mixed in a mass ratio of 96.5:3.5.
Then, this mixture was treated with a planetary ball mill for 2 hours to obtain a precursor of a positive electrode material.
Next, this precursor is heat-treated in an argon (Ar) gas stream at 450° C. for 12 hours to carbonize the sodium carboxymethylcellulose powder and to form an average thickness of 12 on the surface of Na ₂ Fe ₂ (SO ₄ ) _3. A carbonaceous film having a thickness of 0.6 nm was formed, and a positive electrode material of Example 4 containing carbonaceous electrode active material composite particles was obtained. The coverage of the carbonaceous film on the surface of the carbonaceous electrode active material composite particles was 76%.
In the obtained positive electrode material, the carbon content of the carbonaceous electrode active material composite particles was 4.33% by mass.
Average primary particle diameter of Na ₂ Fe ₂ (SO ₄ ) ₃ in the positive electrode material of Example 4, average thickness of carbonaceous coating, coverage of carbonaceous coating on the surface of carbonaceous electrode active material composite particles, carbonaceous electrode Tables 1 and 2 show the carbon content and the like of the active material composite particles.
Also, in the same manner as in Example 1, a sodium ion battery was produced using the positive electrode material of Example 4, and the sodium ion battery was evaluated. The evaluation results are shown in Table 2.

[Example 5]
In the same manner as in Example 1, Na ₂ Fe ₂ (SO ₄ ) ₃ having an average primary particle size of 14.92 μm was produced.
Then, Na ₂ Fe ₂ (SO ₄ ) ₃ particles and acetylene black powder as a carbon source were mixed in a mass ratio of 96.5:3.5.
Then, this mixture was treated with a planetary ball mill for 2 hours to obtain a precursor of a positive electrode material.
Next, this precursor is heat-treated at 450° C. for 12 hours in an argon (Ar) gas flow to form a carbonaceous film having an average thickness of 8.7 nm on the surface of Na ₂ Fe ₂ (SO ₄ ) _3. Then, a positive electrode material of Example 5 containing carbonaceous electrode active material composite particles was obtained. The coverage of the carbonaceous film on the surface of the carbonaceous electrode active material composite particles was 64%.
In the obtained positive electrode material, the carbon content of the carbonaceous electrode active material composite particles was 2.26% by mass.
Average primary particle diameter of Na ₂ Fe ₂ (SO ₄ ) ₃ in the positive electrode material of Example 5, average thickness of carbonaceous coating, coverage of carbonaceous coating on the surface of carbonaceous electrode active material composite particles, carbonaceous electrode Tables 1 and 2 show the carbon content and the like of the active material composite particles.
Also, in the same manner as in Example 1, a sodium ion battery was produced using the positive electrode material of Example 5, and the sodium ion battery was evaluated. The evaluation results are shown in Table 2.

[Example 6]
In the same manner as in Example 1, Na ₂ Fe ₂ (SO ₄ ) ₃ having an average primary particle size of 9.96 μm was produced.
Next, Na ₂ Fe ₂ (SO ₄ ) ₃ particles were mixed with sodium carboxymethyl cellulose powder as a carbon source and acetylene black powder so that the mass ratio was 96:2.5:1.5.
Then, this mixture was treated with a planetary ball mill for 2 hours to obtain a precursor of a positive electrode material.
Then, this precursor is heat-treated at 450° C. for 12 hours in an argon (Ar) stream to carbonize sodium carboxymethyl cellulose powder, and to form an average thickness of 16 on the surface of Na ₂ Fe ₂ (SO ₄ ) _3. A carbonaceous film having a thickness of 0.3 nm was formed to obtain a positive electrode material of Example 6 containing carbonaceous electrode active material composite particles. The coverage of the carbonaceous film on the surface of the carbonaceous electrode active material composite particles was 87%.
In the obtained positive electrode material, the carbon content of the carbonaceous electrode active material composite particles was 7.12% by mass.
Average primary particle diameter of Na ₂ Fe ₂ (SO ₄ ) ₃ in the positive electrode material of Example 6, average thickness of carbonaceous coating, coverage of carbonaceous coating on the surface of carbonaceous electrode active material composite particles, carbonaceous electrode Tables 1 and 2 show the carbon content and the like of the active material composite particles.
Also, in the same manner as in Example 1, a sodium ion battery was produced using the positive electrode material of Example 6, and the sodium ion battery was evaluated. The evaluation results are shown in Table 2.

[Comparative Example 1]
The surface of Na ₂ Fe ₂ (SO ₄ ) ₃ particles having an average primary particle size of 5.84 μm obtained in the same manner as in Example 1 was used as it is without being coated with a carbonaceous film, and Example 1 was used. Similarly to the above, a sodium ion battery was produced and the sodium ion battery was evaluated. The evaluation results are shown in Table 2.
Tables 1 and 2 show the average primary particle diameter of Na ₂ Fe ₂ (SO ₄ ) ₃ and the carbon content of the carbonaceous electrode active material composite particles in the positive electrode material of Comparative Example 1.

[Comparative Example 2]
In the same manner as in Example 1, Na ₂ Fe ₂ (SO ₄ ) ₃ having an average primary particle size of 5.99 μm was produced.
Then, Na ₂ Fe ₂ (SO ₄ ) ₃ particles and sodium carboxymethyl cellulose powder as a carbon source were mixed in a mass ratio of 96.5:3.5.
Then, this mixture was treated with a planetary ball mill for 2 hours to obtain a precursor of a positive electrode material.
Next, this precursor was heat-treated at 550° C. for 12 hours in an argon (Ar) stream.
Then, the crystal structure changed from Na ₂ Fe ₂ (SO ₄ ) ₃ and the desired positive electrode material could not be obtained.

From the results shown in Table 2, the positive electrode materials of Examples 1 to 6 were heat-treated at 450° C., and thus Na ₂ Fe ₂ (SO ₄ ) ₃ particles were coated with the carbonaceous film, so that the transition metals were added to the electrolytic solutions. It was found that the elution of ions was suppressed and excellent charge/discharge cycle characteristics were exhibited.

On the other hand, in the positive electrode material of Comparative Example 1, since the Na ₂ Fe ₂ (SO ₄ ) ₃ particles were not covered with the carbonaceous film, the transition metal ions could not be suppressed from eluting into the electrolytic solution, and the positive electrode material was not charged. It was found that the discharge cycle characteristics were significantly reduced.

In the positive electrode material of the present invention, since the Na ₂ Fe ₂ (SO ₄ ) ₃ particles are coated with the carbonaceous film by a low temperature process of 500° C. or less, the coverage of the carbonaceous film is high and the water resistance is excellent. Since the electron conductivity is high, the high capacity and high energy effect possessed by Na ₂ Fe ₂ (SO ₄ ) ₃ particles can be sufficiently exerted, resulting in higher voltage, high energy density, high load characteristics and high speed charging. It can also be applied to a next-generation secondary battery that is expected to have discharge characteristics, and in the case of the next-generation secondary battery, its effect is extremely large.

Claims (5)

Na _x M _y (SO ₄ ) _z (where M is Sc, Ti, V, Cr) having an M ₂ O ₁₀ dimer crystal structure in which two MO ₆ in the structure share edges to form a dimer. , Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, 1 or 2 or more, 1.6≦x≦2 .4, 1.6≦y≦2.4, 2.4≦z≦3.6), the positive electrode active material particles containing the sodium sulfate compound, and the surface of the positive electrode active material particles. Including carbonaceous electrode active material composite particles consisting of a carbonaceous coating to cover,
The coverage of the carbonaceous coating on the surface of the positive electrode active material particles is 60% or more,
The average thickness of the carbonaceous coating is 2.1 nm or more and 16.3 nm or less,
The positive electrode material, wherein the carbon content of the carbonaceous electrode active material composite particles is 0.71% by mass or more and 7.12% by mass or less. The positive electrode material according to claim 1, wherein the carbonaceous film is formed by a low temperature process of 500°C or lower. The carbonaceous electrode active material composite particles are agglomerated particles obtained by aggregating a plurality of primary particles of the positive electrode active material particles coated with the carbonaceous coating, and the carbonaceous coating is interposed between the primary particles. The positive electrode material according to claim 1 or 2, wherein A paste comprising the positive electrode material according to any one of claims 1 to 3, a conductive auxiliary agent, and a binder. A sodium ion battery comprising an electrode plate having a positive electrode formed on one main surface of a current collector using the paste according to claim 4.