WO2025205482A1 - Electrode material - Google Patents

Abstract

Provided is an electrode material containing: at least one among a sulfur-based active material and a discharge product of a sulfur-based active material; and an organic redox compound.

Description

electrode material

The present invention relates to electrode materials used in lithium-ion secondary batteries, etc.

All-solid-state lithium-ion batteries using solid electrolytes are expected to be highly safe because they are less susceptible to electrolyte leakage and fire.
Sulfur-based active materials are expected to be high-capacity active materials, but they have low electronic and ionic conductivities. Therefore, when a positive electrode is produced using a sulfur-based active material, it is common to use an ionically conductive material and an electronically conductive material together with the active material (see, for example, Patent Documents 1 and 2).

In the case of liquid-type lithium-ion secondary batteries, it is known that part or all of the sulfur-based active material is converted into discharge products during the battery reaction. Therefore, discharge products of the sulfur-based active material exist. For example, discharge products of sulfur include _Li2S in a fully _discharged state and lithium polysulfides in _the intermediate stages _, such as _Li2S2 , _Li2S4 , _Li2S6 , and _{Li2S8 (} see, for example, Non-Patent Document 1).

Non-Patent Document 2 discloses that in a lithium-sulfur battery using a liquid electrolyte, the use of 2,6-dimethoxyanthraquinone or anthraquinone prevents polysulfides, which are intermediate products of the oxidation-reduction reaction of sulfur, from dissolving into the liquid electrolyte and prevents the precipitation of Li ₂ S.

On the other hand, Non-Patent Document 3 discloses that the electrochemical reaction of all-solid-state lithium-sulfur batteries is fundamentally different from that of lithium-sulfur batteries using liquid electrolytes. For example, it discloses that in all-solid-state lithium-sulfur batteries using solid electrolytes, long-chain lithium polysulfides (Li ₂ S _n , 4≦n≦8) are not produced during the redox reaction, but short-chain polysulfides (Li ₂ S ₂ ) are produced as intermediate products, and the presence of Li ₂ S ₂ reduces the sulfur utilization rate and battery performance.

When the active material, solid electrolyte (ionically conductive material), and electronically conductive material are all used in particulate form, the ionic and electronic conduction paths within the electrode are formed by point contact, which creates the problem of high overvoltage during charging and discharging, resulting in a large voltage difference during charging and discharging. Previously, in order to reduce overvoltage during charging and discharging, it was necessary to increase the number of contact points between the active material and solid electrolyte, and between the active material and electronically conductive material.

Furthermore, Non-Patent Document 4 discloses that sulfide solid electrolytes have poor chemical stability and decompose when they come into contact with polar substances such as water, N-methylpyrrolidone, ethanol, and acetonitrile. It was therefore necessary to search for auxiliary substances that would suppress decomposition upon contact and remain stable.

JP 2013-258079 A JP 2013-258080 A

Chem. Rev. 2014, 114, 11751-11787 Chem. Eng. J. ,2024,484,149611 Angew. Chem. Int. Ed. 2023,e202302363 Mater. Chem. Front. , 2023, 7, 5475-5499

One of the purposes of the present invention is to provide an electrode material that can reduce the voltage difference during charging and discharging.

In order to reduce the voltage difference during charging and discharging, it is possible to lower the voltage during charging and increase the voltage during discharging. As a result of extensive research, the inventors discovered that by adding an oxidation-reduction (redox) compound that does not decompose the sulfide-based solid electrolyte as an auxiliary substance to the sulfur-based active material, the voltage difference during charging and discharging can be reduced more than when no auxiliary substance is added, leading to the completion of this invention.

According to the present invention, the following electrode materials and the like are provided.
1. An electrode material comprising at least one of a sulfur-based active material and a discharge product of the sulfur-based active material, and an organic redox compound.
2. The electrode material according to 1, further comprising a sulfide solid electrolyte.
3. The electrode material according to 2, wherein the sulfide solid electrolyte contains Li, P, S, and a halogen as constituent elements.
4. The electrode material according to any one of 1 to 3, further comprising an electron-conductive substance.
5. The electrode material according to 4, wherein the electron-conductive material is a carbon material.
6. The electrode material according to 4 or 5, wherein the electron conductive material has pores.
7. The electrode material according to any one of 1 to 6, wherein the organic redox compound is a quinone compound or a π-conjugated compound.
8. An electrode comprising the electrode material according to any one of 1 to 7.
9. The electrode according to 8, which is a positive electrode.
10. A lithium ion battery comprising the electrode according to 8 or 9.
11. A method for producing an electrode material, comprising the step of mixing a sulfur-based active material and an organic redox compound.
12. The method according to 11, further comprising mixing an electronically conductive material.
13. The manufacturing method according to 12, wherein the electron conductive material has pores, and the pores are impregnated with at least one of the sulfur-based active material and the organic redox compound.
14. The method according to 13, wherein at least one of the sulfur-based active material and the organic redox compound is heated and melted to be impregnated into the pores.
15. The manufacturing method according to 13, wherein at least one of the sulfur-based active material and the organic redox compound and the electron conductive material are subjected to a mechanical milling treatment.

The present invention provides an electrode material that can reduce the voltage difference during charging and discharging.

1 shows charge/discharge curves at the sixth cycle of all-solid-state lithium-ion batteries fabricated in Examples and Comparative Examples.

[Electrode material]
An electrode material according to one embodiment of the present invention includes at least one of a sulfur-based active material and a discharge product of the sulfur-based active material (hereinafter, "at least one of a sulfur-based active material and a discharge product of the sulfur-based active material" may be collectively referred to as "sulfur-based active material") and an organic redox compound. In this embodiment, it is presumed that the use of a sulfur-based active material in combination with an organic redox compound activates the electrochemical reaction between the sulfur-based active material and lithium ions or lithium, thereby reducing the voltage difference during charge and discharge.
The constituent members of the electrode material will be described below.

(Sulfur-based active material)
The sulfur-based active material is not particularly limited, but examples thereof include sulfur, lithium sulfide ( _Li2S ), lithium polysulfide ( _{Li2Sn :} n satisfies 1<n≦8), titanium sulfide ( _TiS2 ), molybdenum sulfide ( _MoS2 ), iron sulfide (FeS, _FeS2 ), copper sulfide (CuS), nickel _sulfide ( _Ni3S2 ), sulfur-containing polymer compounds, etc. Among these, sulfur is preferred.
Although there are no particular limitations on the sulfur, sulfur having a high purity is preferred. Specifically, the purity is preferably 95% by mass or more, more preferably 96% by mass or more, and particularly preferably 97% by mass or more.
Examples of the crystal system of sulfur include α sulfur (orthorhombic system), β (monoclinic system), γ (monoclinic system), amorphous sulfur, etc. These may be used alone or in combination of two or more.

The sulfur-based active material is partially or entirely converted into discharge products during the battery reaction. Therefore, in one embodiment, discharge products of the sulfur-based active material are present in the electrode material. For example, the discharge products of sulfur include Li ₂ S in a fully discharged state and lithium polysulfides in the intermediate stages thereof, such as Li ₂ S ₂ , Li ₂ S ₄ , Li ₂ S ₆ , and Li ₂ S ₈ .

(organic redox compounds)
The organic redox compound may be any compound that exhibits an oxidation-reduction effect on the sulfur-based active material. In one embodiment, the oxidation-reduction potential (charge/discharge potential) of the organic redox compound and the oxidation-reduction potential of the sulfur-based active material are preferably comparable. For example, when the sulfur-based active material is sulfur, the oxidation-reduction potential of sulfur relative to lithium is 2.1 to 2.6 V for charge potential and 1.7 to 2.1 V for discharge potential. In one embodiment, the oxidation-reduction potential of the organic redox compound is within ±1.0 V, ±0.5 V, or ±0.3 V of the oxidation-reduction potential of the sulfur-based active material.
The redox potential of the organic redox compound is sufficiently close to the redox potential of the sulfur-based active material, which is thought to exhibit a catalytic effect that facilitates the electrochemical reaction, thereby reducing the voltage difference during charging and discharging.

Examples of the organic redox compound include quinone compounds and π-conjugated compounds. Specific examples of the organic redox compound include benzoquinone, naphthoquinone, anthraquinone, naphthacenequinone, tetracenequinone, pentatetracenetetrone, and derivatives thereof.
In one embodiment, the organic redox compound is a compound represented by the following formulas (1-1) to (1-6).

(In the formula, R1 to R50 each independently represent a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 10 carbon atoms, a hydroxy group, an alkoxy group having 1 to 6 carbon atoms, an amino group, or a cyano group.)

Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom.
The alkyl group may be any of a linear alkyl group, a branched alkyl group, and a cyclic alkyl group.
Examples of the aryl group include a phenyl group and a naphthyl group.
The alkoxy group may be linear, branched, or cyclic.
The amino group may be either a secondary amine or a tertiary amine.
In addition, some or all of the hydrogen atoms bonded to the carbon atoms of the alkyl group, aryl group, alkoxy group, and amino group may be substituted with a substituent, such as a halogen atom such as a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom.

Compounds represented by formulas (1-1) to (1-6) include 1,4-benzoquinone, methyl-p-benzoquinone, methoxybenzoquinone, tert-butyl-1,4-benzoquinone, 2-chloro-5-methyl-1,4-benzoquinone, 2,5-di-tert-butyl-1,4-benzoquinone, 2,5-dichloro-1,4-benzoquinone, 2,6-dichloro-1,4-benzoquinone, 2,5-dimethyl-1,4-benzoquinone, 2,6-dimethyl-1,4-benzoquinone, 2,5-dibromo-1,4-benzoquinone, 2,5-dimethoxy-1,4-benzoquinone, 2,6-di-tert-butyl-1,4-benzoquinone, tetrachloro-1,4-benzoquinone, and tetrabromo-1,4-benzoquinone. tetramethyl-1,4-benzoquinone, tetrafluoro-1,4-benzoquinone, 2,3,5-trimethyl-1,4-benzoquinone, 1,4-naphthoquinone, 2-chloro-1,4-naphthoquinone, 2,3-dichloro-1,4-naphthoquinone, 2-amino-3-chloro-1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone, 1,2-naphthoquinone, anthraquinone, 2-aminoanthraquinone, 1-aminoanthraquinone, 1-amino-2-methylanthraquinone, 1,4-anthraquinone, 1,2-benzanthraquinone, 1,5-dichloroanthraquinone, naphthacenequinone, pentacenequinone, pentacenetetrone, and the like. Preferred are anthraquinone, naphthacenequinone, pentacenequinone, and pentacenetetrone.

Organic redox compounds other than quinone compounds include tetracyanoquinodimethane, polythiophene, polyaniline, tetrathiafulvalene, rubeanic acid, and indigo.

In one embodiment, the mass ratio (A:B) of the sulfur-based active material A to the organic redox compound B in the electrode material is 10:90 to 90:10, 50:50 to 90:10, or 70:30 to 90:10.

(Optional components)
In one embodiment, the electrode material preferably contains a sulfide solid electrolyte in addition to the sulfur-based active material and the organic redox compound, which facilitates the formation of a conductive path for lithium ions in the electrode.
Furthermore, the electrode material preferably further contains an electron-conductive substance, which facilitates the formation of an electron-conductive path within the electrode.

(1) Sulfide Solid Electrolyte The sulfide solid electrolyte is a solid electrolyte that contains at least sulfur atoms and exhibits ionic conductivity due to the contained metal atoms. In addition to sulfur atoms, the sulfide solid electrolyte preferably contains lithium atoms and phosphorus atoms, and more preferably contains lithium atoms, phosphorus atoms, and halogen atoms, and has ionic conductivity due to lithium atoms.
The sulfide solid electrolyte may be an amorphous sulfide solid electrolyte or a crystalline sulfide solid electrolyte.

(Amorphous sulfide solid electrolyte)
The amorphous sulfide solid electrolyte can be used without any particular limitation as long as it contains at least sulfur atoms and exhibits ionic conductivity due to the contained metal atoms. Typical examples include solid electrolytes containing sulfur atoms, lithium atoms, and phosphorus atoms, which are composed of lithium sulfide and phosphorus sulfide, such as Li ₂ S-P ₂ S ₅ ; solid electrolytes composed of lithium sulfide, phosphorus sulfide, and lithium halide, such _as Li ₂ S _- P _{2 S 5} -LiI, Li ₂ S-P ₂ S ₅ -LiCl, Li ₂ S-P ₂ S 5 -LiBr, and Li 2 S-P 2 S ₅ -LiI-LiBr; and solid electrolytes further containing other elements such as oxygen and silicon, such as Li ₂ S-P ₂ S ₅ -Li ₂ O-LiI and Li ₂ S-SiS ₂ -P ₂ S ₅ From the viewpoint of obtaining higher ionic conductivity, a solid electrolyte composed of lithium sulfide, phosphorus sulfide, and a lithium halide, such as Li ₂ S—P ₂ S ₅ —LiI, Li ₂ S—P ₂ S ₅ —LiCl, Li ₂ S—P ₂ S ₅ —LiBr, or Li ₂ S—P ₂ S ₅ —LiI-LiBr, is preferred.
The types of elements constituting the amorphous sulfide solid electrolyte can be confirmed, for example, by an ICP emission spectrometer.

When the amorphous sulfide solid electrolyte has at least Li ₂ S—P ₂ S ₅ , the molar ratio of Li ₂ S to P ₂ S ₅ is preferably 30 to 85:15 to 70, more preferably 40 to 80:20 to 60, and even more preferably 45 to 78:22 to 55, from the viewpoint of obtaining high chemical stability and higher ionic conductivity.
When the amorphous sulfide solid electrolyte is, for example, Li ₂ S—P ₂ S ₅ —LiI—LiBr, the total content of lithium sulfide and diphosphorus pentasulfide is preferably 30 to 95 mol %, more preferably 35 to 90 mol %, and even more preferably 40 to 85 mol %. The ratio of lithium bromide to the total of lithium bromide and lithium iodide is preferably 1 to 99 mol %, more preferably 20 to 90 mol %, even more preferably 40 to 80 mol %, and particularly preferably 50 to 70 mol %.

When the amorphous sulfide solid electrolyte contains lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms, the compounding ratio (molar ratio) of these atoms is preferably 1.0 to 1.8: 1.0 to 2.0: 0.1 to 0.8: 0.01 to 0.6, more preferably 1.1 to 1.7: 1.2 to 1.8: 0.2 to 0.6: 0.05 to 0.5, and even more preferably 1.2 to 1.6: 1.3 to 1.7: 0.25 to 0.5: 0.08 to 0.4.
Furthermore, when bromine and iodine are used in combination as halogen atoms, the compounding ratio (molar ratio) of lithium atoms, sulfur atoms, phosphorus atoms, bromine atoms, and iodine atoms is preferably 1.0 to 1.8: 1.0 to 2.0: 0.1 to 0.8: 0.01 to 0.3: 0.01 to 0.3, more preferably 1.1 to 1.7: 1.2 to 1.8: 0.2 to 0.6: 0.02 to 0.25: 0.02 to 0.25, more preferably 1.2 to 1.6: 1.3 to 1.7: 0.25 to 0.5: 0.03 to 0.2: 0.03 to 0.2, and even more preferably 1.35 to 1.45: 1.4 to 1.7: 0.3 to 0.45: 0.04 to 0.18: 0.04 to 0.18. By setting the compounding ratio (molar ratio) of lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms within the above range, it becomes easier to obtain a solid electrolyte having a thiolisiconregion II type crystal structure described below and having higher ionic conductivity.

The shape of the amorphous sulfide solid electrolyte is not particularly limited, but may be, for example, particulate. The average particle size (D ₅₀ ) of the particulate amorphous sulfide solid electrolyte may be, for example, within the range of 0.01 μm to 500 μm, or 0.1 μm to 200 μm.
In this specification, the average particle size (D ₅₀ ) is the particle size at which 50% of the total particle size is reached when the particle size distribution integral curve is drawn and the particle size is integrated sequentially from the smallest particle size, and the volume distribution is the average particle size that can be measured using, for example, a laser diffraction/scattering particle size distribution measuring device.

(Crystalline sulfide solid electrolyte)
The crystalline sulfide solid electrolyte may be, for example, a so-called glass ceramic obtained by heating the above-mentioned amorphous sulfide solid electrolyte to a temperature equal to or higher than the crystallization temperature, and a sulfide solid electrolyte having the following crystal structure may be used.
Examples of the crystal structure that the crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, and phosphorus atoms may have include a Li ₃ PS ₄ crystal structure, a Li ₄ P ₂ S ₆ crystal structure, a Li ₇ PS ₆ crystal structure, a Li ₇ P ₃ S ₁₁ crystal structure, and a crystal structure having peaks at 2θ = approximately 20.2° and approximately 23.6° (for example, JP 2013-16423 A).

In addition, examples of the crystal structure that the crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms may have include a Li _4-x Ge _1-x P _x S ₄ based thio-LISICON Region II (thio-LISICON Region II) type crystal structure (see Kanno et al., Journal of The Electrochemical Society, 148 (7) A742-746 (2001)), a crystal structure similar to Li _4-x Ge _1-x P _x S ₄ based thio-LISICON Region II (thio-LISICON Region II) type (see Solid State Ionics, 177 (2006), 2721-2725), and the like. Here, the term "thio-LISICON Region II crystal structure" refers to either a Li4 _{-xGe1 - xPxS4} -based thio- _LISICON Region II (thio-LISICON Region II) crystal structure or a crystal structure similar to a Li4 _{-xGe1 - xPxS4 -} based thio-LISICON Region II (thio-LISICON Region II) type.

In X-ray diffraction measurements using CuKα radiation, the diffraction peaks of the Li ₃ PS ₄ crystal structure appear, for example, at 2θ=17.5°, 18.3°, 26.1°, 27.3°, and 30.0°; the diffraction peaks of the Li ₄ P ₂ S ₆ crystal structure appear, for example, at 2θ=16.9°, 27.1°, and 32.5°; the diffraction peaks of the Li ₇ PS ₆ crystal structure appear, for example, at 2θ=15.3°, 25.2°, 29.6°, and 31.0°; the diffraction peaks of the Li ₇ P ₃ S ₁₁ crystal structure appear, for example, at 2θ=17.8°, 18.5°, 19.7°, 21.8°, 23.7°, 25.9°, 29.6°, and 30.0°; and the diffraction peaks of the Li _4-x Ge _1-x P Diffraction peaks of the _Li4 - _xGe1 -xPxS4 thiolicon region II (thio-LISICON Region II) crystal structure appear, for example, at 2θ = 20.1°, 23.9°, and 29.5°, and diffraction peaks of a crystal structure similar to the Li4 _- xGe1 _{-xPxS4 thiolicon region} II (thio-LISICON Region II) crystal structure appear, for example, at 2θ = 20.2° and 23.6°. These peak positions may vary within a range of ±0.5°.

The crystal structure of the crystalline sulfide solid electrolyte also includes an argyrodite-type crystal structure. Examples of the argyrodite-type crystal structure include the Li ₇ PS ₆ crystal structure, crystal structures represented by the composition formula Li _{7 -x} P _{1 -y} Si _y S ₆ and Li _7+x P _1-y Si _y S ₆ (x is −0.6 to 0.6, y is 0.1 to 0.6) having a Li 7 PS 6 structural skeleton, the crystal structure represented by Li _7-x-2y PS _6-x-y Cl _x (0.8≦x≦1.7, 0<y≦−0.25x+0.5), and the crystal structure represented by Li _7-x PS _6-x Ha _x (Ha is Cl or Br, and x is preferably 0.2 to 1.8).

Among the above crystal structures, the crystal structure possessed by the crystalline sulfide solid electrolyte is preferably a Li ₃ PS ₄ crystal structure, a thiolicon region II type crystal structure, or an argyrodite type crystal structure.

The shape of the crystalline sulfide solid electrolyte is not particularly limited, but may be, for example, particulate. The average particle size (D ₅₀ ) of the particulate crystalline sulfide solid electrolyte may be, for example, within the range of 0.01 μm to 500 μm, or 0.1 μm to 200 μm, similar to the average particle size (D ₅₀ ) of the amorphous sulfide solid electrolyte described above.

(2) Electronically Conductive Material The electronically conductive material is not particularly limited as long as it has electronic conductivity and can be composited with a sulfur-based active material. It is preferable that the electronically conductive material contains a carbon material, since it is lighter than other materials and can increase the output density and capacity per unit mass of the battery. Furthermore, it is preferable that the electronically conductive material has pores, since this material has a large surface area and is highly capable of dispersing and retaining the sulfur-based active material and the organic redox compound. In order for the organic redox compound to exert its catalytic effect, it is important that the organic redox compound is dispersed in the electrode material.

Examples of carbon materials with micropores include carbon blacks such as ketjen black, acetylene black, denka black, thermal black, and channel black, as well as graphite and activated carbon. These may be used alone or in combination of two or more.

In one embodiment, the BET specific surface area of the carbon material is 50 m ² /g or more and 6000 ^{m 2} /g or less, which allows a wide contact interface to be formed between the carbon material and the sulfur-based active material, thereby improving the utilization rate of the sulfur-based active material.
The BET specific surface area is preferably 70 m ² /g or more, more preferably 100 m ² /g or more, 1000 m ² /g or more, or 1500 ^{m 2} /g or more, and is preferably 5500 m ² /g or less, more preferably 5000 m ² /g or less.

The pore volume of the carbon material is 0.5 cm ³ /g or more and 6 cm ³ /g or less, which allows the sulfur-based active material to be impregnated into the pores of the carbon material, thereby further improving the capacity of the battery.
The pore volume is preferably 0.7 cm ³ /g or more, more preferably 1.0 cm ³ /g or more, and is preferably 5.5 cm ³ /g or less, more preferably 5.0 cm ³ /g or less.

In the present invention, the BET specific surface area and pore volume can be determined using a nitrogen adsorption isotherm obtained by adsorbing nitrogen gas onto a carbon material at liquid nitrogen temperature. Specifically, the BET specific surface area can be calculated by the Brenauer-Emmet-Telle (BET) multipoint method using the nitrogen adsorption isotherm. The pore volume can be determined by the Barret-Joyner-Halenda (BJH) method using the nitrogen adsorption isotherm.
The measurement can be carried out using, for example, a specific surface area/pore distribution measuring device (Autosorb-3) manufactured by Quantacrome.

(3) Others In one embodiment, the electrode material may or may not contain components other than the above-described sulfur-based active material, organic redox compound, sulfide solid electrolyte, and electron conductive material. The other components are not particularly limited, and examples thereof include a binder, a solvent, and a dispersant.

In the electrode material, the contents of the sulfur-based active material, the organic redox compound, the sulfide solid electrolyte, and the electron conductive material are not particularly limited.
For example, when the electrode material contains a sulfide solid electrolyte, the content of the sulfur-based active material is 40 to 200 parts by mass per 100 parts by mass of the sulfide solid electrolyte.
The content of the electron conductive material is 10 to 150 parts by mass per 100 parts by mass of the sulfide solid electrolyte.

In one embodiment, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, 99% by mass or more, 99.5% by mass or more, or substantially 100% by mass of the electrode material is a sulfur-based active material, an organic redox compound, a sulfide solid electrolyte, and an electronic conductive material. Note that "substantially 100% by mass" may contain inevitable impurities.

[Method of manufacturing electrode material]
The electrode material of the present invention can be produced by mixing the above-mentioned sulfur-based active material, an organic redox compound, and optionally a sulfide solid electrolyte, an electron-conductive material, etc. The mixing method is not particularly limited, and can be carried out by known methods and devices.
Examples of mixing devices used for mixing include a planetary ball mill, a tumbling mill, a bead mill, Filmics, a Nauta mixer, a tornado mixer, a twin-screw extruder, a multi-screw roller, and a solid-phase shear kneader.

In one embodiment, at least one of the sulfur-based active material and the organic redox compound is heated and melted, and then impregnated into the pores of the electron-conductive material. Melting the sulfur-based active material and the organic redox compound promotes impregnation into the pores. Furthermore, the sulfur-based active material and the organic redox compound can be highly dispersed in the electron-conductive material.

The heating temperature can be appropriately set depending on the sulfur-based active material and organic redox compound used. For example, when the sulfur-based active material is sulfur, the heating temperature is equal to or higher than the melting point of sulfur (about 115°C), preferably equal to or higher than 130°C, and more preferably equal to or higher than 150°C.
Heating may be carried out in two or more stages. For example, the heating temperature in the first stage may be set to a temperature equal to or higher than the melting point of sulfur, and the heating temperature in the second stage may be set to a temperature equal to or higher than the melting point of the organic redox compound.

In one embodiment, at least one of a sulfur-based active material and an organic redox compound and an electronically conductive material are subjected to mechanical milling. Various mills, such as a planetary ball mill, can be used for mechanical milling. Mechanical milling allows the sulfur-based active material, organic redox compound, and electronically conductive material to be composited.

In one embodiment, an organic redox compound and an electronically conductive material may be composited, and then the composite may be mixed and composited with a sulfur-based active material. Alternatively, the organic redox compound, sulfur-based active material, and electronically conductive material may be mixed and composited simultaneously.

In one embodiment, the sulfur-based active material, organic redox compound, and electronically conductive material are formed into a composite, and then the composite and sulfide solid electrolyte are subjected to mechanical milling. This allows for a combination of composite formation by heating and composite formation by mechanical milling.

The electrode material of the present invention can be suitably used, for example, as a constituent material of a secondary battery, for example, as a positive electrode of a lithium ion battery.
A lithium ion battery according to one embodiment of the present invention includes the electrode material of the present invention described above. For example, an all-solid-state lithium ion battery can be manufactured by using a solid electrolyte as the electrolyte. By using the electrode material of the present invention, an all-solid-state lithium ion battery with a reduced voltage difference during charging and discharging can be manufactured.
A lithium ion battery mainly comprises a positive electrode layer, a negative electrode layer, and an electrolyte layer. The negative electrode layer and the electrolyte layer can be manufactured by known methods. For example, the above-mentioned sulfide solid electrolyte can be used for the electrolyte layer. In addition to the positive electrode layer, the negative electrode layer, and the electrolyte layer, a current collector is preferably used, and a known current collector can also be used.

The present invention will be specifically explained below based on examples. The present invention is not limited to these examples.

[Preparation of solid electrolyte]
Production Example 1
0.4398 g of lithium sulfide, 0.7084 g of diphosphorus pentasulfide, 0.2133 g of lithium iodide, 0.1384 g of lithium bromide, and ten zirconia balls with a diameter of 10 mm were placed in a 45 mL zirconia pot and sealed. Using a planetary ball mill (manufactured by Fritsch, model number P-7), the mixture was mixed (mechanical milled) at a rotation speed of 370 rpm for 40 hours to obtain a powder. The obtained powder was heated at 195 ° C for 3 hours to obtain a solid electrolyte.

[Preparation of electrode material]
Example 1
425 mg of anthraquinone and 1275 mg of activated carbon (MSC-30, manufactured by Kansai Thermal Chemicals) were placed in a glass bottle, which was then sealed in an SUS tubular container. The bottle was heated in an electric furnace at 150°C for 6 hours and then at 300°C for 2.75 hours to obtain a composite powder A of activated carbon and anthraquinone.
Next, 600 mg of composite powder A and 1,050 mg of sulfur were placed in a glass bottle, which was then sealed in an SUS tubular container and heated in an electric furnace at 150°C for 6 hours and then at 300°C for 2.75 hours to obtain an electrode material, which was a composite powder of activated carbon, sulfur, and anthraquinone.

Example 2
200 mg of anthraquinone, 1,400 mg of sulfur, and 600 mg of activated carbon were placed in a glass bottle, which was then sealed in an SUS tubular container. The bottle was heated in an electric furnace at 150°C for 6 hours and then at 300°C for 2.75 hours to obtain an electrode material, which was a composite powder of activated carbon, sulfur, and anthraquinone.

Comparative Example 1
1,400 mg of sulfur and 600 mg of activated carbon were placed in a glass bottle, which was then sealed in an SUS tubular container. The bottle was heated in an electric furnace at 150° C. for 6 hours and then at 300° C. for 2.75 hours to obtain a composite powder of activated carbon and sulfur.

[evaluation]
All-solid-state lithium ion batteries were fabricated using the electrode materials and composite powders fabricated in the examples and comparative examples in the positive electrode, and the charge/discharge characteristics were evaluated.
(1) Preparation of Positive Electrode Composites 550 mg of the electrode material of Example 1 or 2 and 450 mg of the solid electrolyte of Production Example 1 were placed in a 45 mL zirconia pot together with ten zirconia balls having a diameter of 10 mm, and the pot was sealed. Using a planetary ball mill (manufactured by Fritsch, model number P-7), the mixture was pulverized at a rotation speed of 370 rpm for 20 hours at room temperature, to obtain positive electrode composites A and B.
A positive electrode composite C was obtained in the same manner as above, except that 500 mg of the composite powder of Comparative Example 1 and 500 mg of the solid electrolyte of Production Example 1 were used.

(2) Preparation of all-solid-state lithium-ion battery 100 mg of the solid electrolyte of Preparation Example 1 was placed in a 10 mm diameter Macol cylinder and pressure-molded. The positive electrode composite prepared in (1) above was placed on the pressurized surface so that the sulfur content was 3.5 mg, and pressure-molded again. 166 mg of a negative electrode composite (LTO (lithium titanate) negative electrode composite) was placed on the pressurized surface opposite the positive electrode composite and pressurized, and then lithium foil was placed and pressurized to prepare an all-solid-state lithium-ion battery.
The LTO negative electrode composite was prepared by placing lithium titanate ("LT-112" manufactured by Ishihara Sangyo Kaisha), a conductive additive ("Li-100" manufactured by Denka Co., Ltd., powdered acetylene black), and a Li ₂ S-P ₂ S ₅ -LiCl-LiBr type solid electrolyte in a mass ratio of 60:5:35 into a mortar and mixing in the mortar for 5 minutes.

(3) Constant Current Charge/Discharge Test For the all-solid-state lithium ion battery prepared in (2) above, the cutoff potential of the constant current test was set to −0.4 V to +1.3 V vs. Li-LTO. The current density in the charge/discharge cycle is shown in Table 1.

Figure 1 shows the charge/discharge curves for the sixth cycle of the all-solid-state lithium-ion batteries produced in the examples and comparative examples. The charge/discharge voltage difference was determined as half the maximum value of the electrical charge (horizontal axis) in Figure 1, i.e., the voltage difference (difference between charge voltage and discharge voltage) at around 700 to 750 mAh/g. Table 2 shows the voltage difference during charge/discharge.

Table 2 confirms that Examples 1 and 2 have a smaller charge/discharge voltage difference than Comparative Example 1. This demonstrates that the internal resistance of the all-solid-state lithium-ion battery can be reduced in the examples.

The electrode material of the present invention can be suitably used as a constituent material of lithium-ion batteries, for example, as a positive electrode. Furthermore, the lithium-ion battery of the present invention can be suitably used as a battery for use in information-related devices and communication devices such as personal computers, video cameras, and mobile phones, as well as in vehicles such as electric cars.

Although several embodiments and/or examples of the present invention have been described in detail above, those skilled in the art will readily be able to make numerous modifications to these exemplary embodiments and/or examples without substantially departing from the novel teachings and advantages of the present invention, and therefore, these numerous modifications are within the scope of the present invention.
The contents of all documents cited in this specification and of the application from which this application claims priority under the Paris Convention are incorporated by reference in their entirety.

Claims (15)

An electrode material comprising at least one of a sulfur-based active material and a discharge product of the sulfur-based active material, and an organic redox compound. The electrode material according to claim 1, further comprising a sulfide solid electrolyte. The electrode material according to claim 2, wherein the sulfide solid electrolyte contains Li, P, S, and a halogen as constituent elements. The electrode material according to any one of claims 1 to 3, further comprising an electronically conductive substance. The electrode material according to claim 4, wherein the electronically conductive material is a carbon material. The electrode material according to claim 4 or 5, wherein the electronically conductive material has pores. The electrode material according to any one of claims 1 to 6, wherein the organic redox compound is a quinone compound or a π-conjugated compound. An electrode comprising the electrode material described in any one of claims 1 to 7. The electrode described in claim 8, which is a positive electrode. A lithium ion battery comprising the electrode described in claim 8 or 9. A method for manufacturing an electrode material, comprising the step of mixing a sulfur-based active material and an organic redox compound. The manufacturing method described in claim 11, further comprising mixing an electronically conductive material. The manufacturing method described in claim 12, wherein the electronically conductive material has pores, and the pores are impregnated with at least one of the sulfur-based active material and the organic redox compound. The manufacturing method described in claim 13, wherein at least one of the sulfur-based active material and the organic redox compound is heated and melted to impregnate the pores. The manufacturing method described in claim 13, wherein at least one of the sulfur-based active material and the organic redox compound and the electron conductive material are subjected to a mechanical milling treatment.