JP7246789B1 - Positive electrode active material, positive electrode mixture and secondary battery - Google Patents

Abstract

A positive electrode active material for a secondary battery that does not require lithium or a transition metal element is provided. A positive electrode active material containing a sulfur compound represented by the following formula (1) or (2). [Chemical Formula 1] MxSyOz (1) (wherein M is Na or K, x is in the range of more than 0 and 3 or less, y is 1 or more and 8 or less, and z is 1 or more and 8 or less ) [Chemical 2] MxHSyOz (2) (In formula (2), M is Na or K, x is greater than 0 and is 3 or less, y is 1 or more and 8 or less, and z is 1 or more and 8 or less.) [Selection] None

Description

TECHNICAL FIELD The present invention relates to a positive electrode active material containing a sulfur-containing compound of sodium or potassium, a positive electrode mixture containing the positive electrode active material, and a secondary battery.

Demand for lithium secondary batteries has been increasing in recent years as a small power source for mobile phones and the like, and medium- and large-sized power sources for electric vehicles, power storage, and the like. On the other hand, the production of lithium, which is a major constituent element of the positive electrode active material of lithium secondary batteries, from salt lakes, where it is profitable, is limited to some countries. There are concerns about supply instability and soaring prices. Therefore, research and development of positive electrode active materials for new secondary batteries to replace lithium secondary batteries are being conducted.

A sodium secondary battery is an alternative candidate for a lithium secondary battery because sodium compounds, which are abundant in terms of resources, can be used as positive electrode active materials. Various sodium-transition metal complex compounds have been proposed as positive electrode active materials for sodium secondary batteries. Examples of such sodium-transition metal composite compounds include sodium-iron-cobalt composite metal oxides (for example, Patent Document 1), sodium-manganese-cobalt-nickel composite metal oxides (for example, Patent Document 2), and sodium-manganese-titanium-nickel composite oxides. (For example, Patent Document 3) and the like have been reported.

In addition, as a positive electrode active material for a sodium secondary battery other than a sodium transition metal complex compound, Patent Document 4 discloses a carbon skeleton derived from a carbon source compound selected from special polycyclic aromatic hydrocarbons, and a carbon skeleton bonded to the carbon skeleton. A positive electrode active material composed of sulfur (S) and sulfur (S) has been reported.

Potassium secondary batteries can also be mentioned as alternative candidates for lithium secondary batteries other than sodium secondary batteries. Patent Document 5 reports a potassium secondary battery using a composite oxide containing potassium, a transition metal, and phosphorus as a positive electrode active material.

JP 2013-203565 A JP 2014-229452 A JP 2012-206925 A Japanese Patent No. 5737679 Japanese Patent No. 6800322

The positive electrode active materials for sodium secondary batteries disclosed in Patent Documents 1 to 3 and the like require transition metal elements such as cobalt and nickel, which are scarce in terms of resources. Although it does not contain metal elements, it requires a synthesis that combines a special carbon skeleton with sulfur, and the positive electrode active material itself does not contain alkali metals, so when used as a battery, alkali metal ions in the electrolyte There is room for improvement in terms of raw material supply, production cost, operating conditions, etc., such as the need to pre-dope the positive electrode active material with the process. In addition, although there are reports on potassium secondary batteries in Patent Document 5 and the like, development examples are scarce, and candidates for useful positive electrode active materials have been sought.

Under such circumstances, an object of the present invention is to provide a novel positive electrode active material, a positive electrode mixture containing the same, and a secondary battery that do not require lithium or a transition metal element.

As a result of earnest research to solve the above problems, the inventors of the present invention have found that the following inventions meet the above objects, and have completed the present invention.

That is, the present invention relates to the following inventions.
<1> A positive electrode active material containing a sulfur-containing compound represented by the following formula (1) or (2).

[Chemical 1]
_{MxSyOz ( 1} )
(In formula (1), M is Na or K, x is in the range of more than 0 and 3 or less, y is 1 or more and 8 or less, and z is 1 or more and 8 or less.)

[Chemical 2]
_{MxHSyOz ( 2} )
(In formula (2), M is Na or K, x ranges from 0 to 3, y ranges from 0 to 8, and z ranges from 1 to 8.)
<2> _The sulfur-containing compound is _Na2SO3 , _NaHSO3 , _{Na2SO4 , NaHSO4 , Na2SO5 , Na2SO8 , Na2S2O3 , Na2S2O4 , Na 2} S ₂ O ₅ , Na ₂ S ₂ O ₆ , Na ₂ S ₂ O ₇ and Na ₂ S ₂ O ₈ , the cathode active material according to <1>.
<3> The sulfur-containing compound _{is K2SO3 , KHSO3 , K2SO4} , _{KHSO4 , K2SO5 , K2SO8 , K2S2O3 , K2S2O4 ,} K ₂ S ₂ O ₅ , K ₂ S ₂ O ₆ , K ₂ S ₂ O ₇ and K ₂ S ₂ O ₈ .
<4> The positive electrode active material according to any one of <1> to <3>, comprising a composite of the sulfur-containing compound and carbide.
<5> The positive electrode active material according to <4>, wherein the carbide is a tire-derived carbide.
<6> A positive electrode mixture containing the positive electrode active material according to any one of <1> to <5> and a conductive material.
<7> A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode uses the positive electrode mixture according to <6>.

According to the present invention, a positive electrode active material for a new secondary battery that does not contain lithium or a transition metal element and replaces the conventional sodium secondary battery or potassium secondary battery, a positive electrode mixture containing the same, and a secondary battery is provided.

4 shows measurement data of a charge/discharge test of the secondary battery (positive electrode active material: sodium thiosulfate pentahydrate (reagent)) of Experimental Example 1. FIG. 4 is an XRD pattern of the positive electrode active material of Experimental Example 2. FIG. XAFS spectrum (S K-edge) of the positive electrode active material, (a) of Example 1 (sodium thiosulfate pentahydrate (reagent)) and (b) of Experimental Example 2 (synthetic product). 4 is an SEM image of the positive electrode active material of Experimental Example 2. FIG. 4 is SEM-EDX mapping of the positive electrode active material of Experimental Example 2. FIG.

Hereinafter, the present invention will be described in detail with reference to examples, etc., but the present invention is not limited to the following examples, etc., and can be arbitrarily modified without departing from the scope of the present invention. In this specification, "~" is used as an expression including numerical values or physical quantities before and after it. Also, in this specification, the expression "A and/or B" includes "only A", "only B", and "both A and B".

<1. Positive electrode active material>
A first aspect of the positive electrode active material of the present invention contains a sulfur-containing compound represented by the following formula (1).

[Chemical 3]
_{MxSyOz ( 1} )

In formula (1), M is sodium (Na) or potassium (K). x indicates the ratio of M and ranges from over 0 to 3 or less.

In formula (1), y represents the ratio of sulfur (S) and ranges from 1 to 8.

In formula (1), z represents the proportion of oxygen (O) and ranges from 1 to 8.

Note that in formula (1), x, y, and z may be not integers. That is, the sulfur-containing compound represented by Formula (1) may be a non-stoichiometric oxide.

In formula (1), when x, y, and z are within the above ranges, the positive electrode active material can provide a positive electrode that can be repeatedly charged and discharged. The values of x, y, and z can be adjusted by controlling the amount of raw materials used, manufacturing conditions, and the like.

A first aspect of the positive electrode active material of the present invention contains a sulfur-containing compound represented by the following formula (2).

[Chemical 4]
_{MxHSyOz ( 2} )

In formula (2), M is Na or K. x indicates the ratio of M and ranges from over 0 to 3 or less.

In formula (2), y represents the ratio of sulfur (S) and is in the range of over 0 to 8 or less.

In formula (2), z represents the proportion of oxygen (O) and ranges from 1 to 8.

Note that in formula (2), x, y, and z may be non-integers. That is, the sulfur-containing compound represented by Formula (2) may be a non-stoichiometric oxide.

In formula (2), when x, y and z are within the above ranges, the positive electrode active material can provide a positive electrode that can be repeatedly charged and discharged. The values of x, y, and z can be adjusted by controlling the amount of raw materials used, manufacturing conditions, and the like.

Hereinafter, when it is not necessary to distinguish between the sulfur-containing compound represented by formula (1) and the sulfur-containing compound represented by formula (2), they are collectively referred to as "the sulfur-containing compound of the present invention". It shall be.

A preferred example of the sulfur-containing compound of the present invention is a sulfur-containing compound in which M is Na in Formula (1) or Formula (2). Specific examples of such sulfur-containing compounds include Na ₂ SO ₃ , NaHSO ₃ , Na ₂ SO ₄ , NaHSO ₄ , Na ₂ SO ₅ , Na ₂ SO ₈ , Na ₂ S ₂ O ₃ , Na ₂ S _{2 O4 , Na2S2O5 , Na2S2O6 , Na2S2O7 and Na2S2O8 . _ _ _} Among these, sodium thiosulfate (Na ₂ S ₂ O ₃ ) is a suitable example.

Another preferred example of the positive electrode active material of the present invention is a sulfur-containing compound in which M is K in Formula (1) or Formula (2). Specific examples of such sulfur-containing compounds include K ₂ SO ₃ , KHSO ₃ , K ₂ SO ₄ , KHSO ₄ , K ₂ SO ₅ , K ₂ SO ₈ , K ₂ S ₂ O ₃ , K ₂ S ₂ O ₄ , K ₂ S ₂ O ₅ , K ₂ S ₂ O ₆ , K ₂ S ₂ O ₇ and K ₂ S ₂ O ₈ . Among these, potassium thiosulfate (K ₂ S ₂ O ₃ ) is a suitable example.

The positive electrode active material of the present invention can contain one or more of the above sulfur-containing compounds.

In the positive electrode active material of the present invention, the sulfur-containing compound of the present invention is not limited to crystals, and may be amorphous, or may be a mixture of crystals and amorphous. Moreover, the sulfur-containing compound of the present invention may be non-hydrated, but may be used as a hydrate.

The positive electrode active material of the present invention may contain the sulfur-containing compound of the present invention, and may contain only the sulfur-containing compound of the present invention, but may contain other substances as long as the effects of the present invention are not impaired. may Other substances include, for example, carbides. In the present specification, the term “carbide” refers to a carbon-containing raw material (organic substance) carbonized, and may be carbon (C) only. may contain elements of
Elements other than carbon include, for example, nitrogen (N), sulfur (S), oxygen (O), metal elements such as Fe, and the like, but are not limited as long as the object of the present invention is not impaired.

The carbide can be made porous, depending on the carbon-containing raw material and firing conditions.
The conditions for producing the carbide are appropriately determined in consideration of the type of carbon-containing raw material and the physical properties (crystallinity, porosity, etc.) of the desired carbide. Typically, the carbon-containing raw material is heated and carbonized under a low oxygen partial pressure (for example, under the flow of an inert gas such as N ₂ ). The carbonization temperature is, for example, 500° C. or higher and 2000° C. or lower.

The carbon-containing raw material, which is the raw material for the carbide, is arbitrary as long as it does not impair the object of the present invention, and includes biomass raw materials (for example, wood, bamboo, rice husks, etc.), various resin materials, and plastics such as rubber materials.

The positive electrode active material of the present invention is preferably composed of a composite of the sulfur-containing compound of the present invention and a carbide (hereinafter sometimes referred to as "composite of the present invention" or simply "composite"). Here, the composite of the present invention is not simply a mixture of the sulfur-containing compound of the present invention and the carbide, but a composite obtained by physically and/or chemically combining the sulfur-containing compound of the present invention and the carbide. means body. A physical composite is a composite in which the sulfur-containing compound of the present invention is physically retained on a carbide. For example, a composite in which the sulfur-containing compound medium of the present invention is supported on a carbide, Examples include a composite having a sulfur-containing compound adsorbed thereon, and a composite having the sulfur-containing compound of the present invention deposited on a carbide. The chemical composite is a composite in which the sulfur-containing compound of the present invention and a carbide are chemically bonded. For example, the sulfur-containing compound of the present invention or a precursor compound thereof and a carbide are mixed, It can be manufactured by heat-treating in an inert gas atmosphere.

In the composite of the present invention, the sulfur-containing compound of the present invention composited with the carbide may be solid, liquid, or a mixture of solid and liquid.

Battery performance is improved by using the composite of the present invention for the positive electrode active material of the present invention. The reason for this is partially unclear at this stage, but by forming the composite of the sulfur-containing compound and carbide of the present invention, compared to the mixture of the sulfur-containing compound and carbide of the present invention, the carbide It is speculated that one of the reasons is that the electron transfer between the sulfur-containing compound and the sulfur-containing compound becomes smoother.

A preferred example of the carbon-containing raw material that is the raw material of the carbide in the composite is a tire. A tire is mainly made of rubber (for example, natural rubber (NR), styrene-butadiene rubber (SBR), isoprene rubber (IR)), and contains various compounding agents and structural materials. Thus, one example of the positive electrode active material of the present invention is a composite using tire-derived carbide.

The ratio of the sulfur-containing compound of the present invention in the positive electrode active material of the present invention is, for example, 5 to 100% by weight when the entire positive electrode active material is 100% by weight (dry weight basis).

The shape of the positive electrode active material of the present invention is arbitrary as long as it does not impair the purpose of the present invention. For example, it may be particulate, island-shaped, or film-shaped.

<2. Positive Electrode Mixture>
The positive electrode mixture of the present invention contains the positive electrode active material of the present invention and a conductive material.

Since the positive electrode active material of the present invention does not have a very high electrical conductivity, the internal resistance of the electrode increases when the positive electrode is formed using only the positive electrode active material. Therefore, it is usually mixed with other conductive materials (and other materials as necessary) to ensure conduction in the electrodes.

The conductive material has a role of improving electron conductivity when forming an electrode. The conductive material may be any material as long as it has conductivity (electronic conductivity) and does not impair the performance of the positive electrode active material of the present invention, and may be either an inorganic material or an organic material, but is typically a carbon-based material. is.

Any carbon-based material used in secondary batteries and fuel cells can be used as the carbon-based material. More specifically, graphite powder, carbon black (for example, acetylene black, ketjen black, furnace black, etc.), fibrous carbon materials (carbon nanotubes, carbon nanofibers, vapor-grown carbon fibers, etc.).
Among these, carbon black is fine and has a large surface area, and by adding it to the electrode material, it is possible to increase the conductivity inside the obtained electrode and improve the charge-discharge efficiency and high-current discharge characteristics.

The shape and size of the carbon-based material can be appropriately selected in consideration of the purpose of use of the electrode. , the diameter is about 2 nm to 20 μm and the total length is about 0.03 to 500 μm.

The conductive material used in the present invention may be one type, or two or more types of conductive materials having different sizes (particle diameter, fiber diameter and fiber length) and crystallinity, etc. may be used in an arbitrary ratio. good too.

The ratio of the conductive material in the electrode material can be appropriately selected in consideration of the purpose of use of the electrode, etc. When the conductive material is a carbon-based material and the carbon-based material in the electrode, it is usually 100 parts by weight of the electrode active material. 5 to 100 parts by weight.

The electrode material of the present invention may be composed only of the positive electrode active material and the conductive material of the present invention, but may contain other components as long as the object of the present invention is not impaired. Other ingredients typically include binders.

The binder acts as a binding agent that adheres other electrode constituent materials. When the electrode contains the above electrode-forming agent, the binder is used when the amount of the electrode-forming agent added is insufficient and the adhesiveness is insufficient.
Examples of binders include binders made of organic polymer compounds. Examples of organic polymer compounds as binders include polysaccharides such as methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, carboxymethylhydroxyethylcellulose, and nitrocellulose, and derivatives thereof;
Polyvinylidene fluoride (hereinafter sometimes referred to as PVDF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), ethylene tetrafluoride/propylene hexafluoride/vinylidene fluoride copolymer, hexafluoride Propylene/vinylidene fluoride copolymer, tetrafluoroethylene/perfluorovinyl ether copolymer; phenolic resin; melamine resin; polyurethane resin; urea resin; polyamide resin; etc.

As for the binder, a plurality of types of binders may be used as binders.

The amount of the constituent material of the binder electrode is, for example, usually about 0.5 to 50 parts by weight, preferably about 1 to 30 parts by weight, per 100 parts by weight in total of the positive electrode active material and the conductive material.

<3. Secondary battery>
A secondary battery of the present invention comprises a positive electrode, a negative electrode, and an electrolyte, and is characterized in that the positive electrode is formed using the positive electrode mixture of the present invention. The structure of the secondary battery of the present invention is not particularly limited, and conventionally known secondary battery structures can be employed. Examples include stacked (flat) batteries and wound (cylindrical) batteries.

Each component will be described below.

The positive electrode has a current collector and a positive electrode active material layer formed on the surface of the current collector, and the positive electrode active material layer is the positive electrode mixture (positive electrode active material, conductive material, and binder) of the present invention. It is formed.

The current collector is not particularly limited, and conventionally known ones can be used. Examples thereof include foils, meshes, expanded grids (expanded metals), punched metals, and the like using conductive materials such as nickel, aluminum, and stainless steel (SUS).

The size and thickness of the current collector are determined according to the application of the battery, and a suitable size can be selected according to the size of the positive electrode to be used.

A method for producing a positive electrode typically involves coating a slurry prepared by mixing a positive electrode mixture (positive electrode active material, conductive material, and binder) and a solvent on a current collector, drying the slurry, and then pressing. A positive electrode active material layer is formed on the surface of the current collector to obtain a positive electrode.

Examples of the slurry solvent include amines such as N,N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; -Aprotic polar solvents such as methyl-2-pyrrolidone.
The method for applying the slurry onto the current collector is not limited as long as it does not impair the object of the present invention. An electric spray method and the like can be mentioned.

The negative electrode has a current collector and a negative electrode active material layer formed on the surface of the current collector, and the negative electrode active material layer is typically formed of a negative electrode mixture containing a negative electrode active material.

As the negative electrode active material, carbon materials such as natural graphite, artificial graphite, cokes, hard carbon, carbon black, pyrolytic carbons, carbon fiber, organic polymer compound sintered body, etc. that can occlude and desorb sodium ions. are mentioned. The shape of the carbon material may be flaky such as natural graphite, spherical such as mesocarbon microbeads, fibrous such as graphitized carbon fiber, or aggregates of fine powder. Here, the carbon material may also serve as a conductive material.

As described above, in the present invention, the negative electrode active material is not particularly limited, but hard carbon may be used.

Hard carbon is a carbon material whose stacking order hardly changes even when heat-treated at a high temperature of 2000° C. or higher, and is also called non-graphitizable carbon. As hard carbon, carbon fibers obtained by carbonizing infusible threads, which are intermediate products in the manufacturing process of carbon fibers, at about 1000 to 1400 ° C., organic compounds are air-oxidized at about 150 to 300 ° C., and then dried to about 1000 to 1400 ° C. can be exemplified by carbonized carbon materials. The method for producing hard carbon is not particularly limited, and hard carbon produced by a conventionally known method can be used.

Although the content of the negative electrode active material in the negative electrode active material layer is not particularly limited, it is preferably 80 to 95% by mass.

As the binder, the same binders as those usable for the positive electrode can be used, so the description thereof will be omitted. As the current collector, a conductive material such as nickel, copper, stainless steel (SUS) is used. The current collector is made of foil, mesh, expanded grid (expanded metal), punched metal, or the like, like the current collector for the positive electrode.

Moreover, as a method of forming the negative electrode active material layer on the current collector, the same method as the method of forming the positive electrode active material layer on the current collector can be employed.

The electrolyte is not particularly limited, and may be in either liquid or solid form.
A solution obtained by dissolving an electrolyte sodium salt or potassium salt in an electrolytic solution (non-aqueous solvent) is used. The electrolyte in the secondary battery of the present invention is not particularly limited, but the electrolyte disclosed in the examples is a suitable example.

As the electrolyte salt, electrolyte salts commonly used in sodium secondary batteries or potassium secondary batteries can be used. One type of electrolyte salt may be used, or two or more types may be used in combination.

As the electrolytic solution, an electrolytic solution used in a sodium secondary battery or a potassium secondary battery can be used, and carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate; ethers such as dimethoxyethane and 1,3-dimethoxypropane; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; Phosphates such as 2-trifluoroethyl)phosphate; or the above organic solvents further introduced with a fluorine substituent can be used.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these. In addition, "%" below indicates "% by weight" unless otherwise specified.

(Experimental example 1)
As the positive electrode active material in Experimental Example 1, the reagent sodium thiosulfate pentahydrate (Na ₂ S ₂ O _{3.5H 2} O, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and the conductive material acetylene black (Denki Kagaku Kogyo Co., Ltd., hereinafter referred to as "AB"), and polyvinylidene fluoride (Daikin Industries, Ltd., hereinafter referred to as "PVDF") was used as a binder. Incidentally, PVDF is a binder dissolved in an N-methyl-2-pyrrolidone solution containing 0.5% carbon nanotubes (CNT) (“TUBALL ^™ BATT NMP PVDF” manufactured by Kusumoto Kasei Co., Ltd., hereinafter referred to as “NMP solvent”). used as a solution.

The positive electrode of Example 1 was produced as follows.
Contains 0.8 g (74% of the total) of sodium thiosulfate pentahydrate as a positive electrode active material, 0.2 g of AB (19% of the total) as a conductive material, and 0.076 g of PVDF (7.01% of the total) A positive electrode mixture of Experimental Example 1 was obtained by uniformly mixing (kneading) 0.747 g of NMP solvent (CNT is substantially 0.004 g, solid content ratio is 0.37%).
Next, the obtained positive electrode mixture was applied to an aluminum sheet (thickness 16 μm) serving as a positive electrode current collector, and dried at 50° C. for 3 hours to obtain a positive electrode sheet. A positive electrode of Experimental Example 1 was obtained by cutting out a positive electrode sheet with a diameter of 15 mm from the obtained positive electrode sheet.

A negative electrode was produced as follows.
Waste tire-derived carbide “Lunesis A1 (product number)” was used as the negative electrode active material.
Waste tire-derived carbide “Lunesis A1 (product number)” is produced by heating waste tires to 400 ° C in 2 hours in an inert atmosphere container, gasifying and cooling the components in the waste tires to oil (gasification and about 55% of total oil conversion), and the residue that did not gasify or oilify was recovered. Carbide derived from waste tire pyrolysis residue obtained by further heat-treating the recovered waste tire pyrolysis residue, pulverizing it to an average particle size of 20 μm, and completely removing the magnetic material with a 16000 gauss electromagnetic separator. be.
1.5 g (92.71% of the total) of the negative electrode active material (carbide derived from waste tires) and 0.106 g (6 of the total) of polyacrylic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name 20CLPAH) as a binder .55%) is mixed (kneaded) with 1.2 g of an aqueous solution containing 0.007 g of carboxymethyl cellulose (CMC) and 0.005 g of CNT as a solvent (“TUBALL ^™ BATT H ₂ O CMC” manufactured by Kusumoto Kasei Co., Ltd.). By doing so, a negative electrode mixture was obtained.
Next, the obtained negative electrode mixture was applied to an aluminum sheet (thickness: 16 μm) serving as a negative electrode current collector, and dried at 70° C. for 3 hours to obtain a negative electrode sheet. A negative electrode having a diameter of 11.3 mm was cut out from the obtained negative electrode sheet.

The resulting positive electrode and negative electrode were mixed with tris (2,2,2-trifluoroethyl) phosphate (TFEP) as an electrolyte and sodium bis (fluorosulfonyl) imide (NaFSI) as a sodium salt so that the concentration was 2 mol/liter. (hereinafter referred to as "NaFSI/TFEP"), Celgard 2400 as a separator, and these were combined to form the battery of Experimental Example 1 (coin battery (cylindrical, outer diameter 20 mm / height 3.2 mm ), hereinafter referred to as “coin-type battery (R2032)”).

Using the battery of Example 1 above, a charge/discharge test was performed under the conditions shown below while maintaining the temperature at 25°C.
(Cell configuration) Bipolar positive electrode: electrode containing positive electrode active material Negative electrode: electrode containing negative electrode active material Electrolyte: 2M NaFSI/TFEP
(Charging and discharging conditions)
Voltage range: 0.5-3.9V
Charging was carried out by CVCC charging in which a constant current was kept flowing up to 3.9 V, and then a current sufficient to maintain the voltage was passed for 5 hours, and the current value decreased.

The results of the charge/discharge test of the battery of Experimental Example 1 are as follows. FIG. 1 shows measurement data of the charge/discharge test.
133.5 μA/1.766 cm ² (positive electrode 15 mmφ)
Charge/discharge times: 20 times Discharge time: about 1 hour (133 μAh)

(Experimental example 2)
The positive electrode active material of Experimental Example 2 was obtained by the following procedure.
As the carbide, a waste tire-derived carbide “Lunesis A1 (product number)” (same as the negative electrode in Experimental Example 1) was used. 5.5 g (24.2% of the total raw material) of carbide (carbide derived from waste tires), 8.44 g of sodium percarbonate (37.2% of the total raw material), 2.75 g of coal tar (12.1% of the total raw material) ) and 6 g of sulfur (26.4% of the total raw material) were kneaded in a stainless steel container, put into a heating furnace, and nitrogen gas was flowed to make the inside of the container an inert atmosphere. After 10 minutes, heating was started. The temperature rise was set to 300°C in 50 minutes, continued heating after reaching 300°C, and stopped at 350°C. After that, when the temperature dropped to 200° C., it was removed from the heating furnace and cooled naturally. After the internal temperature of the container became 50°C or less, the synthesized product was taken out. The composite weight was 15.2 g. The synthesized product was pulverized with a mixer to obtain a positive electrode active material of Experimental Example 2 consisting of a powder having an average particle size of 20 μm.

The obtained positive electrode active material of Experimental Example 2 was evaluated by powder X-ray diffraction measurement (XRD) and X-ray absorption fine structure measurement (XAFS). The measurement equipment and measurement conditions are as follows.

<Powder X-ray diffraction measurement>
Powder X-ray diffraction measurement (XRD) was performed using a model number: D8ADVANCE manufactured by Bruker axs.
For the measurement, a powder sample was filled on a dedicated substrate, and a powder X-ray diffraction pattern was obtained using a CuKα ray source.
<X-ray absorption fine structure measurement>
X-ray absorption fine structure (XAFS, transmission method) was evaluated using BL11 at the Kyushu Synchrotron Light Research Center.

FIG. 2 shows the XRD pattern of the positive electrode active material of Experimental Example 2, and FIG. 3 shows the XAFS spectrum (SK-edge).
When the XRD profile of the positive electrode active material of Experimental Example 2 in FIG. 2 was compared with COD (Crysyallography Open Database) data, the signal of sodium percarbonate disappeared and a signal consistent with sodium thiosulfate pentahydrate was confirmed. rice field.

Moreover, in FIG. 3, it was confirmed that the XAFS spectra of the positive electrode active material of Experimental Example 2 and sodium thiosulfate pentahydrate (reagent) almost matched.
From the above results, it was confirmed that the positive electrode active material of Experimental Example 2 contained crystals of sodium thiosulfate pentahydrate.

FIG. 4 shows an SEM image of the positive electrode active material of Experimental Example 2, and FIG. 5 shows the results of SEM-EDX mapping corresponding to FIG. In SEM-EDX mapping, Na and S are distributed throughout the carbide, so it was determined that the positive electrode active material of Experimental Example 2 was a composite in which sodium thiosulfate was uniformly dispersed in the carbide.

A positive electrode of Experimental Example 2 was obtained as follows.
2 g of the positive electrode active material of Experimental Example 2 (94.49% of the total) and 0.056 g of AB as the conductive material (2.64% of the total) were mixed with 0.058 g of PVDF (2.71% of the total) in NMP solvent 0 A positive electrode mixture of Experimental Example 2 was obtained by uniformly mixing (kneading) 0.673 g (CNT: 0.003 g, solid content ratio: 0.08%). Next, the obtained positive electrode mixture was applied to an aluminum sheet (thickness 16 μm) serving as a positive electrode current collector, and dried at 50° C. for 3 hours to obtain a positive electrode sheet. A positive electrode of Experimental Example 2 was obtained by cutting out a positive electrode sheet having a diameter of 15 mm from the obtained positive electrode sheet.

A battery of Experimental Example 2 (coin-type battery (R2032)) was obtained in the same manner as in Experimental Example 1 except that the positive electrode of Experimental Example 2 was used.
The results of the charge/discharge test of the battery of Experimental Example 2 are as follows.
73.5 μA/1.766 cm ² (positive electrode 15 mmφ)
Charge/discharge times: 20 times Discharge time: about 4 hours (73.5 μAh)

(Experimental example 3)
A positive electrode of Experimental Example 3 was obtained as follows.
As the positive electrode active material of Example 3, the reagent potassium thiosulfate anhydride was used. 2.453 g of NMP solvent containing 1.003 g of anhydrous potassium thiosulfate (86% of total) and 0.063 g of AB as conductive material (5.41% of total) and 0.073 g of PVDF (6.27% of total) The positive electrode mixture of Experimental Example 3 was obtained by uniformly mixing (kneading) the CNTs (substantially 0.025 g, solid content ratio 2.14%).
Next, the obtained positive electrode mixture was applied to an aluminum sheet (thickness 16 μm) serving as a positive electrode current collector, and dried at 50° C. for 3 hours to obtain a positive electrode sheet. A positive electrode of Experimental Example 3 was obtained by cutting out a positive electrode sheet with a diameter of 15 mm from the obtained positive electrode sheet.

A negative electrode was prepared in the same manner as in Example 1.

The resulting positive electrode and negative electrode were mixed with tris (2,2,2-trifluoroethyl) phosphate (TFEP) as an electrolyte and potassium bis (fluorosulfonyl) imide (KFSI) as a potassium salt so that the concentration was 2 mol/liter. (hereinafter referred to as "KFSI/TFEP"), Celgard 2400 as a separator, and these were combined to obtain a battery of Experimental Example 3 (coin-type battery (R2032)).

Using the battery of Example 3, a charge/discharge test was performed under the following conditions while maintaining at 25°C.
(Cell configuration) Bipolar positive electrode: electrode containing positive electrode active material Negative electrode: electrode containing negative electrode active material Electrolyte: 2M KFSI/TFEP
(Charging and discharging conditions)
Voltage range: 2.0-4.2V
Charging was performed by CVCC charging in which a constant current continued to flow up to 4.2 V, and then a current sufficient to maintain the voltage was passed for 5 hours, and the current value decreased.

The results of the charge/discharge test of the battery of Experimental Example 3 are as follows.
62.4 μA/1.766 cm ² (positive electrode 15 mmφ)
Charge/discharge times: 5 times Discharge time: about 4 hours (62.4 μAh)

(Experimental example 4)
The positive electrode active material of Experimental Example 4 was obtained by the following procedure.
As the carbide, a waste tire-derived carbide “Lunesis A1 (product number)” (same as the negative electrode in Experimental Example 1) was used. 10 g of carbide (carbide derived from waste tires) (20% of the total raw material), 25 g of potassium hydrogen carbonate (51% of the total raw material), 3.8 g of coal tar (8% of the total raw material), 10 g of sulfur (20% of the total raw material) ) was kneaded in a stainless steel container and put into a heating furnace, nitrogen gas was flowed to make the inside of the container an inert atmosphere, and heating was started after 10 minutes. The temperature rise was set to 300°C in 50 minutes, continued heating after reaching 300°C, and stopped at 350°C. After that, when the temperature dropped to 200° C., it was removed from the heating furnace and cooled naturally. After the internal temperature of the container became 50°C or less, the synthesized product was taken out. The composite weight was 32.5g with a yield of 67%. The synthesized product was pulverized with a mixer to obtain a positive electrode active material of Experimental Example 4 consisting of a powder having an average particle size of 20 μm. When the positive electrode active material of Experimental Example 4 was evaluated by XRD, it was confirmed that crystals of potassium thiosulfate were contained.

A positive electrode mixture and a positive electrode of Experimental Example 4 were obtained in the same manner as in Experimental Example 3 except that the positive electrode active material of Experimental Example 4 was used. A battery of Experimental Example 4 (coin-type battery (R2032)) was obtained in the same manner as in Experimental Example 3 except that the positive electrode of Experimental Example 4 was used.

Using the battery of Example 4 above, a charge/discharge test was performed under the same conditions as in Example 3 at 25°C.
The results of the charge/discharge test are as follows.
64.7 μA/1.766 cm ² (positive electrode 15 mmφ)
Charge/discharge times: 5 times Discharge time: about 5 hours (64.7 μAh)

The positive electrode active material of the present invention can provide a positive electrode for secondary batteries using an inexpensive material without using lithium, so the present invention is industrially extremely useful.

Claims (4)

A positive electrode active material comprising a composite of a sulfur-containing compound containing Na ₂ S ₂ O ₃ or K ₂ S ₂ O ₃ and a carbide. The positive electrode active material according to claim 1 , wherein the carbide is a tire-derived carbide. A positive electrode mixture containing the positive electrode active material according to claim 1 or 2 and a conductive material. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode uses the positive electrode mixture according to claim 3 .

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