Positive electrode mixture, lithium ion battery, and production method for positive electrode mixture

Abstract

A positive electrode mixture including: a conductivity aid that is a carbon material; a sulfur-based active material; and a solid electrolyte, wherein the variation in luminance intensity in the same field of view is 0.250 or less in a secondary electron image from a scanning electron microscope, and at least part of the solid electrolyte is crystalline with the crystallite diameter thereof being 40 nm or more.

Description

Positive electrode mixture, lithium ion battery, and method for manufacturing the positive electrode mixture

[0001] The present invention relates to a cathode composite, a lithium ion battery, and a method for manufacturing the cathode composite. Specifically, the present invention relates to a cathode composite, a lithium ion battery, and a method for manufacturing the cathode composite that can improve rate characteristics.

[0002] In sulfur-based positive electrode composites used in lithium-ion batteries and the like, sulfur is mixed with a conductive additive and a solid electrolyte to form a composite, which allows the insulating sulfur to react sufficiently (Patent Document 1).

[0003] JP 2013-258079 A

[0004] However, it has been found that solid electrolytes are susceptible to deterioration due to the mechanical mixing required for the above-mentioned composite formation, making it difficult to fully demonstrate their inherent lithium ion conductivity. On the other hand, weakening the mechanical mixing to prevent deterioration results in insufficient mixing. Therefore, sulfur-based positive electrode composites have room for further improvement in terms of improving the rate characteristics of batteries. Conventional technologies, including those described in Patent Document 1, have not been able to solve these problems.

[0005] An object of the present invention is to provide a positive electrode composite, a lithium ion battery, and a method for producing the positive electrode composite that can improve rate characteristics.

[0006] As a result of extensive research, the inventors have found that a specific cathode mixture can improve rate characteristics, and have completed the present invention. According to the present invention, the following cathode mixtures and the like can be provided. 1. A cathode mixture comprising a conductive additive that is a carbon material, a sulfur-based active material, and a solid electrolyte, wherein the brightness intensity variation within the same field of view in a secondary electron image of a scanning electron microscope is 0.250 or less, and at least a portion of the solid electrolyte is crystalline, and the crystallite size is 40 nm or more. 2. The cathode mixture according to 1, wherein the solid electrolyte comprises a sulfide solid electrolyte containing lithium atoms, phosphorus atoms, and sulfur atoms. 3. The cathode mixture according to 2, wherein the sulfide solid electrolyte further contains a halogen atom. 4. The cathode mixture according to any one of 1 to 3, wherein the solid electrolyte comprises a crystalline phase other than glass ceramics. 5. The cathode mixture according to 4, wherein the crystalline phase other than glass ceramics comprises an argyrodite-type crystal structure. 6. 6. The cathode mixture according to any one of 1 to 5, obtained by a manufacturing method including a first mixing step of adding a solid electrolyte to a sulfur-based active material and mixing the mixture to obtain a first mixture, and a second mixing step of adding a further solid electrolyte to the first mixture and mixing the mixture to obtain a second mixture, wherein the solid electrolyte added in the second mixing step includes solid electrolyte α, and at least a portion of the solid electrolyte α contained in the second mixture is crystalline and has a crystallite diameter of 40 nm or more. 7. The cathode mixture according to 6, wherein the second mixing step is performed with a lower energy than the first mixing step. 8. The cathode mixture according to 6 or 7, wherein the first mixing step is performed under conditions that cause the solid electrolyte added in the first mixing step to become amorphous, and the second mixing step is performed under conditions that do not cause the crystallinity of the solid electrolyte α added in the second mixing step to be lost and have a crystallite diameter of 40 nm or more. 9. A lithium ion battery including the cathode mixture according to any one of 1 to 8.10. A method for producing a cathode composite, comprising: a first mixing step of adding a solid electrolyte to a sulfur-based active material and mixing the resulting mixture to obtain a first mixture; and a second mixing step of adding a further solid electrolyte to the first mixture and mixing the resulting mixture to obtain a second mixture, wherein the solid electrolyte added in the second mixing step comprises solid electrolyte α, and at least a portion of the solid electrolyte α contained in the second mixture is crystalline and has a crystallite diameter of 40 nm or more. 11. The method for producing a cathode composite according to 10, wherein the second mixing step is performed with lower energy than the first mixing step. 12. The method for producing a cathode composite according to 10 or 11, wherein the first mixing step is performed under conditions that promote amorphization of the solid electrolyte added in the first mixing step, and the second mixing step is performed under conditions that prevent the crystallinity of the solid electrolyte α added in the second mixing step from disappearing and result in a crystallite diameter of 40 nm or more. 13. 13. The method for producing a positive electrode composite according to any one of 10 to 12, wherein the amount of the solid electrolyte α added in the second mixing step is 50 to 100 parts by mass when the total amount of the solid electrolytes added in the second mixing step is 100 parts by mass.

[0007] According to the present invention, it is possible to provide a positive electrode composite, a lithium ion battery, and a method for manufacturing the positive electrode composite that can improve rate characteristics.

[0008] 1 is a diagram for explaining calculation of "brightness intensity variation within the same visual field" by secondary electron image analysis of a scanning electron microscope (SEM) image in the positive electrode composite of Example 1 ((a) is a secondary electron image, (b) is its luminance distribution, and (c) is an example of the luminance distribution of a small section (similar to FIGS. 2 to 6)). FIG. 1 is a diagram for explaining calculation of "brightness intensity variation within the same visual field" in the positive electrode composite of Example 2. FIG. 2 is a diagram for explaining calculation of "brightness intensity variation within the same visual field" in the positive electrode composite of Comparative Example 1. FIG. 3 is a diagram for explaining calculation of "brightness intensity variation within the same visual field" in the positive electrode composite of Comparative Example 2. FIG. 4 is a diagram for explaining calculation of "brightness intensity variation within the same visual field" in the positive electrode composite of Comparative Example 3. FIG. 5 is a diagram for explaining calculation of "brightness intensity variation within the same visual field" in the positive electrode composite of Comparative Example 4. FIG. 6 is a diagram showing the results (diffraction spectra) of powder X-ray diffraction (XRD) of the positive electrode composite powders of Examples 1 and 2. FIG. 1 is a diagram showing the results (diffraction spectra) of powder X-ray diffraction (XRD) of the positive electrode composite powders of Comparative Examples 1 to 3.

[0009] The cathode composite, lithium ion battery, and method for producing the cathode composite of the present invention are described in detail below. In this specification, "x to y" represents a numerical range of "greater than or equal to x and less than or equal to y." The upper and lower limits of the numerical ranges can be arbitrarily combined. Furthermore, among the individual embodiments of the aspects of the present invention described below, two or more mutually exclusive embodiments can be combined, and an embodiment combining two or more embodiments is also an embodiment of the aspects of the present invention.

[0010] 1. Cathode Composite A cathode composite according to one aspect of the present invention includes a conductive additive that is a carbon material, a sulfur-based active material, and a solid electrolyte, wherein the brightness intensity variation within the same field of view in a secondary electron image taken with a scanning electron microscope is 0.250 or less, and at least a portion of the solid electrolyte is crystalline, and the crystallite diameter is 40 nm or more. The cathode composite of this aspect exhibits the effect of improving rate characteristics.

[0011] (Conductive Aid) An electron-conductive carbon material can be used as the conductive aid. Carbon materials have high conductivity and are lighter than other conductive materials, which allows the battery's power density and capacity per weight to be increased. The conductive aid is preferably a carbon material having micropores. Examples of carbon materials include, but are not limited to, carbon blacks such as ketjen black, acetylene black, denka black, thermal black, and channel black; mesoporous carbon; activated carbon; amorphous carbon; carbon nanotubes; vapor-grown carbon fiber (VGCF); and carbon nanohorns. Examples of conductive carbon materials include fullerenes, carbon fibers, natural graphite, artificial graphite, graphene, graphene oxide, and reduced graphene oxide. These materials may be used alone or in combination of two or more. A composite of these materials may also be used.

[0012] (Sulfur-based active material) The sulfur-based active material is not particularly limited, but sulfur, lithium sulfide (Li ２ S), lithium polysulfide (Li ２ S ｎ : n satisfies 1<n≦8.), titanium sulfide (TiS ２ ), molybdenum sulfide (MoS ２ ), iron sulfide (FeS, FeS ２ ), copper sulfide (CuS), nickel sulfide (Ni ３ S ２ ), sulfur-containing polymer compounds, etc. The sulfur is not particularly limited, but 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 can be used alone or in combination of two or more types.

[0013] (Solid electrolyte) In this embodiment, at least a portion of the solid electrolyte is crystalline, and its crystallite diameter is 40 nm or more. Usually, in order to cause an electrode reaction in a sulfur-based positive electrode composite, it is considered necessary to finely mix an insulating sulfur-based active material and a conductive additive with a solid electrolyte to form a structure including a large number of interfaces. In contrast, in the positive electrode composite of this embodiment, the inclusion of a solid electrolyte with a large crystallite diameter reduces the number of interfaces, thereby reducing the interfacial resistance and demonstrating significantly excellent rate characteristics. In addition, the crystallite diameter of 40 nm or more and the luminance intensity variation of 0.250 or less, as described below, work synergistically to more significantly improve the rate characteristics.

[0014] In one embodiment, the crystallite size is 40 nm or more, 50 nm or more, 60 nm or more, 70 nm or more, 80 nm or more, 90 nm or more, more than 90 nm, 91 nm or more, 100 nm or more, 110 nm or more, 120 nm or more, 130 nm or more, 140 nm or more, 150 nm or more, or 160 nm or more. The upper limit is not particularly limited, and is, for example, 800 nm or less.

[0015] The presence or absence of a crystalline peak described in the Examples can confirm that at least a portion of the solid electrolyte contained in the positive electrode composite is crystalline. That is, if a crystalline peak is present, it is determined that at least a portion of the solid electrolyte is crystalline. The crystallite size is a value measured by the method described in the Examples.

[0016] In one embodiment, the solid electrolyte comprises a sulfide solid electrolyte containing lithium atoms, phosphorus atoms, and sulfur atoms, and in one embodiment, the sulfide solid electrolyte further comprises a halogen atom.

[0017] In one embodiment, the solid electrolyte includes a crystalline phase other than glass ceramic. In another embodiment, the crystalline phase other than glass ceramic includes an argyrodite-type crystalline structure. The solid electrolyte of this embodiment can be obtained, for example, by treating the solid electrolyte using the cathode composite manufacturing method described below. In this specification, a glass-ceramic solid electrolyte is a solid electrolyte in which peaks derived from the solid electrolyte are observed in the X-ray diffraction pattern in powder X-ray diffraction (XRD) measurement, regardless of whether or not peaks derived from the raw materials of the solid electrolyte are present. That is, a glass-ceramic solid electrolyte includes a crystalline structure derived from the solid electrolyte, and a portion of the crystalline structure may be derived from the solid electrolyte, or the entire crystalline structure may be derived from the solid electrolyte. Furthermore, as long as the glass-ceramic solid electrolyte has the X-ray diffraction pattern described above, it may also include an amorphous component (also referred to as a "glass component") in part. Crystalline solid electrolytes include so-called glass ceramics, which are obtained by heating an amorphous solid electrolyte (glass component) above its crystallization temperature. Examples of glass ceramics include known materials, mainly Li ２ S and P ２ S ５ Li synthesized from the raw material ３ P.S. ４ Crystal structure, Li ４ P ２ S ６ Crystal structure, Li ８ P ２ S ９ Crystal structure, Li ７ P ３ S １１ Examples of such glass ceramics include a crystalline structure, such as a thiolicon region II type crystalline structure, and a crystalline structure similar to that of thiolicon region II type. Furthermore, glass ceramics generally have a low degree of crystallinity, for example, a degree of crystallinity of 90% or less. Furthermore, glass ceramics generally have poor stability at high temperatures, and when heated to, for example, about 500°C, the crystalline phase decomposes or transitions to another crystalline phase.

[0018] In one embodiment, the positive electrode composite may or may not contain components other than the sulfur-based active material, the solid electrolyte, and the conductive additive. The other components are not particularly limited, and examples thereof include a binder, a solvent, a dispersant, and the like.

[0019] In the positive electrode mixture, the contents of the sulfur-based active material, solid electrolyte, and conductive additive are not particularly limited. In one embodiment, the content of the sulfur-based active material is 40 to 350 parts by mass per 100 parts by mass of the solid electrolyte. In one embodiment, the content of the conductive additive is 10 to 300 parts by mass per 100 parts by mass of the solid electrolyte. In one embodiment, the mass ratio of the sulfur-based active material to the conductive additive (sulfur-based active material:conductive additive) is 10:90 to 95:5 or 20 to 90:80 to 10. In one embodiment, the ratio of the content of the solid electrolyte to the content of the sulfur-based active material and conductive additive (solid electrolyte:sulfur-based active material + conductive additive) is 10:90 to 90:10, 15 to 70:85 to 30, or 20 to 60:80 to 40. In one embodiment, the positive electrode composite comprises 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 conductive additive, sulfur-based active material, and solid electrolyte. Note that "substantially 100% by mass" may contain inevitable impurities.

[0020] (Brightness Intensity Variation Within the Same Visual Field) As described above, the brightness intensity variation within the same visual field of the positive electrode composite of this embodiment is 0.250 or less in a secondary electron image of a scanning electron microscope. The smaller this brightness intensity variation is, the smaller the compositional unevenness of the positive electrode composite is, and the better the mixed state is. In one embodiment, the brightness intensity variation is 0.250 or less, 0.240 or less, 0.230 or less, 0.220 or less, 0.210 or less, 0.200 or less, 0.190 or less, 0.180 or less, or 0.170 or less. The lower limit is not particularly limited and may be, for example, 0.010 or more. The brightness intensity variation within the same visual field is a value measured by the method described in the examples.

[0021] In one embodiment, the positive electrode composite is obtained by a manufacturing method including a first mixing step of adding a solid electrolyte to a sulfur-based active material and mixing the mixture to obtain a first mixture, and a second mixing step of adding a further solid electrolyte to the first mixture and mixing the mixture to obtain a second mixture. The solid electrolyte added in the second mixing step includes solid electrolyte α (described in detail later). At least a portion of the solid electrolyte α contained in the second mixture is crystalline and has a crystallite diameter of 40 nm or more. In this embodiment, the second mixing step is preferably performed with less energy than the first mixing step. In this embodiment, the first mixing step is preferably performed under conditions that promote amorphization of the solid electrolyte added in the first mixing step, and the second mixing step is preferably performed under conditions that maintain the crystallinity of the solid electrolyte α added in the second mixing step and result in a crystallite diameter of 40 nm or more. Note that the description of "2. Method for Manufacturing Positive Electrode Composite" is incorporated herein by reference for the manufacturing method in this embodiment.

[0022] A positive electrode composite according to another aspect of the present invention includes a conductive additive that is a carbon material, a sulfur-based active material, and a solid electrolyte, wherein the luminance intensity variation within the same field of view in a secondary electron image of a scanning electron microscope is 0.750 or less, and at least a portion of the solid electrolyte is crystalline, with a crystallite diameter of 40 nm or more. The positive electrode composite according to this aspect also achieves excellent rate characteristics and cycle characteristics, similar to the positive electrode composite according to the above-described aspect of the present invention. The positive electrode composite according to this aspect is similar to the positive electrode composite according to the other aspect of the present invention, except that the range of luminance intensity variation is different (including a range in which the luminance intensity variation exceeds 0.250), and the description of the positive electrode composite according to the other aspect of the present invention is applicable. In this aspect, the smaller the luminance intensity variation, 0.750 or less, the smaller the compositional unevenness of the positive electrode composite and the better the mixed state. In one embodiment, the luminance intensity variation is 0.750 or less, 0.700 or less, 0.600 or less, 0.500 or less, 0.400 or less, 0.300 or less, 0.250 or less, 0.2300 or less, or 0.200 or less. The lower limit is not particularly limited and may be, for example, 0.010 or more.

[0023] 2. Manufacturing Method of Cathode Composite A manufacturing method of a cathode composite according to one aspect of the present invention includes a first mixing step of adding a solid electrolyte to a sulfur-based active material and mixing the resulting mixture to obtain a first mixture, and a second mixing step of adding a further solid electrolyte to the first mixture and mixing the resulting mixture to obtain a second mixture, wherein the solid electrolyte added in the second mixing step includes solid electrolyte α, and at least a portion of the solid electrolyte α contained in the second mixture is crystalline and has a crystallite diameter of 40 nm or more. According to the manufacturing method of the cathode composite according to this aspect, the cathode composite according to one aspect of the present invention can be suitably manufactured. Furthermore, the cathode composite according to other aspects of the present invention can also be manufactured.

[0024] First, the solid electrolyte used as a raw material will be described. As described above, in this embodiment, the solid electrolyte α is used as the solid electrolyte. The solid electrolyte α may be used in combination with a solid electrolyte other than the solid electrolyte α (described in detail later).

[0025] (Solid Electrolyte α) When the solid electrolyte α is used as a positive electrode composite, at least a part of the solid electrolyte α is crystalline and has a crystallite diameter of 40 nm or more.

[0026] The solid electrolyte α before being subjected to the mixing process to obtain the positive electrode composite is at least partially crystalline and has a crystallite diameter of 40 nm or more. The crystallite diameter of the solid electrolyte α before being subjected to the mixing process may be, for example, 40 nm to 800 nm, preferably 60 nm to 700 nm, and more preferably 80 nm to 600 nm, but is not limited thereto.

[0027] In one embodiment, the solid electrolyte α includes a sulfide solid electrolyte containing lithium atoms, phosphorus atoms, and sulfur atoms. In another embodiment, the sulfide solid electrolyte further includes a halogen atom.

[0028] In one embodiment, the solid electrolyte α includes a crystalline phase other than a glass ceramic. In another embodiment, the crystalline phase other than a glass ceramic includes an argyrodite-type crystalline structure. Examples of the argyrodite-type crystalline structure include Li ７ P.S. ６Crystal structure; Li ７ P.S. ６ The structural skeleton of 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); Li ７－ｘ－２ｙ P.S. ６－ｘ－ｙ Cl ｘ (0.8≦x≦1.7, 0<y≦−0.25x+0.5); Li ７－ｘ P.S. ６－ｘ Ha ｘ (Ha is Cl or Br, and x is preferably 0.2 to 1.8).

[0029] In one embodiment, the solid electrolyte α contains lithium atoms, phosphorus atoms, and sulfur atoms, and further contains one or more atoms selected from halogen atoms. Examples of halogen atoms include fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms. Among these, it is preferable to contain one or more atoms (e.g., one atom) selected from chlorine atoms, bromine atoms, and iodine atoms, and it is particularly preferable to contain one or more atoms selected from chlorine atoms and bromine atoms. In one embodiment, the molar ratio (Li:P:S:Ha) of lithium atoms (Li), phosphorus atoms (P), sulfur atoms (S), and halogen atoms (Ha) in the solid electrolyte α is 5.0-6.0:1.0:4.0-5.0:1.0-2.0.

[0030] In one embodiment, the solid electrolyte α contains lithium atoms, phosphorus atoms, and sulfur atoms, and further contains two or more atoms selected from halogen atoms. Examples of halogen atoms include fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms. It is preferable to contain two or more atoms (e.g., two atoms) selected from chlorine atoms, bromine atoms, and iodine atoms, and it is particularly preferable to contain a combination of chlorine atoms and bromine atoms. In one embodiment, the molar ratio of lithium atoms (Li), phosphorus atoms (P), sulfur atoms (S), chlorine atoms (Cl), and bromine atoms (Br) in the solid electrolyte α (Li:P:S:Cl+Br) is preferably 5.0-6.0:1.0:4.0-5.0:1.0-2.0. The molar ratio of chlorine atoms (Cl) to the total halogens (Cl+Br) is preferably greater than 0.25 and less than 1.0, and more preferably greater than 0.5 and less than 0.8.

[0031] The method for producing the solid electrolyte α is not particularly limited. For example, the method for producing an argyrodite-type solid electrolyte is not particularly limited, and known methods can be used. As starting materials, a combination of two or more compounds or simple substances containing lithium atoms, phosphorus atoms, sulfur atoms, chlorine atoms, bromine atoms, etc. as constituent elements can be used, and any starting material can be used without particular limitation as long as it exhibits ionic conductivity due to the contained metal atoms.

[0032] Examples of raw materials containing lithium (Li) include lithium sulfide (Li ２ S), lithium oxide (Li ２ O), lithium carbonate (Li ２ CO ３ Among these, lithium compounds are preferred, and lithium sulfide is more preferred.

[0033] Examples of raw materials containing phosphorus (P) and sulfur (S) include diphosphorus trisulfide (P ２ S ３ ), diphosphorus pentasulfide (P ２ S ５ ) and other phosphorus sulfides, sodium phosphate (Na ３ P.O. ４Examples of suitable phosphorus compounds include phosphorus compounds such as phosphorus pentasulfide, phosphorus elemental compounds, and sulfur elemental compounds. Among these, phosphorus sulfide is preferred, and diphosphorus pentasulfide is more preferred. Phosphorus compounds such as diphosphorus pentasulfide, phosphorus elemental compounds, and sulfur elemental compounds can be used without any particular limitation as long as they are industrially produced and commercially available.

[0034] Examples of raw materials containing halogen include lithium halides such as lithium chloride (LiCl), lithium bromide (LiBr), and lithium iodide (LiI), and phosphorus pentachloride (PCl ５ ), phosphorus trichloride (PCl ３ ), phosphorus pentabromide (PBr ５ ), phosphorus tribromide (PBr ３ Among them, lithium halides such as LiCl, LiBr, and LiI, PBr ３ is preferred, and lithium halides such as LiCl, LiBr, and LiI are more preferred, with LiCl and LiBr being more preferred.

[0035] The combination of raw materials used is preferably lithium sulfide, diphosphorus pentasulfide, lithium chloride, and lithium bromide, and in this case, the molar ratio of the raw materials used is preferably lithium sulfide:diphosphorus pentasulfide:total of two lithium halides=30-60:10-25:15-50.

[0036] The above raw materials are subjected to mechanical stress to form an intermediate. Here, "applying mechanical stress" means mechanically applying shear force, impact force, or the like. Examples of means for applying mechanical stress include grinders such as planetary ball mills, vibration mills, and tumbling mills, and kneaders. The intermediate is heat-treated to obtain a crystalline solid electrolyte α (argyrodite-type solid electrolyte). The heat treatment temperature is preferably 350 to 650°C, more preferably 360 to 500°C, and even more preferably 380 to 450°C.

[0037] In one embodiment, the above raw materials are roughly mixed and then dispersed in a solvent (e.g., a mixed solvent of dehydrated toluene and dehydrated isobutyronitrile) to prepare a slurry. This slurry is mixed and pulverized using a mixer / pulverizer such as a bead mill. The solvent is then removed, and the mixture is heated to 400 to 430°C in an electric furnace and slowly cooled to obtain a raw sulfide solid electrolyte. The raw sulfide solid electrolyte is dispersed in a solvent (e.g., dehydrated toluene) in a nitrogen atmosphere and atomized using a planetary ball mill to obtain a slurry again. This slurry is dried to remove the solvent, thereby obtaining solid electrolyte α.

[0038] (Solid Electrolyte Other than Solid Electrolyte α) In this embodiment, a solid electrolyte other than the solid electrolyte α may be used as the solid electrolyte. The solid electrolyte other than the solid electrolyte α is not particularly limited, and examples thereof include sulfide solid electrolytes. 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, it preferably contains lithium atoms and phosphorus atoms, more preferably lithium atoms, phosphorus atoms, and halogen atoms, and has ionic conductivity due to lithium atoms. The solid electrolyte may contain metal atoms (e.g., lithium atoms) that contribute to the expression of ionic conductivity, or it may not yet contain metal atoms that contribute to the expression of ionic conductivity. A solid electrolyte that does not yet contain metal atoms that contribute to the expression of ionic conductivity may contain metal atoms that contribute to the expression of ionic conductivity, for example, depending on the progress of the electrode reaction. Therefore, a substance that exhibits ionic conductivity by containing metal atoms that contribute to the expression of ionic conductivity is also a solid electrolyte. Examples of such substances (solid electrolytes) include phosphorus sulfide and boron sulfide. The phosphorus sulfide includes phosphorus trisulfide (P ４ S ３ ), diphosphorus pentasulfide (P ２ S ５ ), phosphorus heptasulfide (P ４ S ７ ), tetraphosphorus pentasulfide (P ４ S ５) and the like. The phosphorus sulfide may have a dimer or polysulfide structure, or may be a mixture. Particularly preferred is diphosphorus pentasulfide, which is expected to react with lithium to form a sulfide solid electrolyte exhibiting high ionic conductivity. In one embodiment, the solid electrolyte contains lithium atoms, phosphorus atoms, sulfur atoms, bromine atoms, and iodine atoms. In this case, the compounding ratio (molar ratio) of lithium atoms, sulfur atoms, phosphorus atoms, bromine atoms, and iodine atoms is not particularly limited, but 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. The sulfide solid electrolyte may be an amorphous sulfide solid electrolyte or a crystalline sulfide solid electrolyte.

[0039] (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. Representative examples include Li ２ S-P ２ S ５ a solid electrolyte containing sulfur atoms, lithium atoms, and phosphorus atoms, which is composed of lithium sulfide and phosphorus sulfide 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, Li ２ S-P ２ S ５ -LiI-LiBr, etc., a solid electrolyte composed of lithium sulfide, phosphorus sulfide, and lithium halide; ２ S-P ２S ５ -Li ２ O-LiI, Li ２ S-SiS ２ -P ２ S ５ In order to obtain higher ionic conductivity, a solid electrolyte 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, Li ２ S-P ２ S ５ A solid electrolyte composed of lithium sulfide, phosphorus sulfide, and lithium halide, such as LiI-LiBr, is preferred. The types of elements constituting the amorphous sulfide solid electrolyte can be confirmed, for example, by an ICP emission spectrometer.

[0040] The amorphous sulfide solid electrolyte contains at least Li ２ S-P ２ S ５ When Li ２ S and P ２ S ５ From the viewpoint of obtaining high chemical stability and higher ionic conductivity, the molar ratio of Li to Li 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. ２ S-P ２ S ５ In the case of -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%.

[0041] (Crystalline sulfide solid electrolyte) The crystalline sulfide solid electrolyte may be, for example, a so-called glass ceramic obtained by heating the 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 crystal structures that the crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, and phosphorus atoms may have include Li ３ P.S. ４ Crystal structure, Li ４ P ２ S ６ Crystal structure, Li ７ P.S. ６ Crystal structure, Li ７ P ３ S １１ Examples of such structures include a crystal structure having peaks at 2θ=approximately 20.2° and 23.6° (for example, JP 2013-16423 A).

[0042] In addition, a crystalline structure that the crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms may have is a thiolicon region II type crystalline structure. Here, the "thiolicon region II type crystalline structure" refers to a structure in which Li ４－ xGe １－ｘ P ｘ S ４ Thio-LISICON Region II crystal structure (Kanno et al., Journal of the Electrochemical Society, 148(7)A742-746(2001)), Li ４－ xGe １－ｘ P ｘ S ４ Examples include a crystal structure similar to the thio-LISICON Region II type (see Solid State Ionics, 177 (2006), 2721-2725).

[0043] In X-ray diffraction measurement using CuKα radiation, Li ３ P.S. ４ Diffraction peaks of the crystal structure appear, for example, at 2θ=17.5°, 18.3°, 26.1°, 27.3°, and 30.0°. ４ P２ S ６ Diffraction peaks of the crystal structure appear, for example, at 2θ=16.9°, 27.1°, and 32.5°. ７ P.S. ６ Diffraction peaks of the crystal structure appear, for example, at 2θ=15.3°, 25.2°, 29.6°, and 31.0°. ７ P ３ S １１ Diffraction peaks of the 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 Li ４－ｘ Ge １－ｘ P ｘ S ４ The diffraction peaks of the thio-LISICON Region II crystal structure appear, for example, at 2θ=20.1°, 23.9°, and 29.5°, and Li ４－ｘ Ge １－ｘ P ｘ S ４ Diffraction peaks of a crystal structure similar to thio-lisicon region II type appear, for example, at 2θ = 20.2° and 23.6°. These peak positions may vary within a range of ±0.5°. The solid electrolyte may have the thio-lisicon region II type crystal structure, or may have it as the main crystal. In this specification, "having it as the main crystal" means that the proportion of the target crystal structure in the crystal structure is 80% or more, preferably 90% or more, and more preferably 95% or more.

[0044] Among the above crystal structures, the crystal structure of the crystalline sulfide solid electrolyte is Li ３ P.S. ４ The crystalline structure, thioliconregion type II crystalline structure, is preferred.

[0045] The method for producing the solid electrolytes other than the solid electrolyte α is not particularly limited, and known methods can be used.

[0046] (First Mixing Step) In the first mixing step, a solid electrolyte is added to a sulfur-based active material and mixed to obtain a first mixture. The solid electrolyte added in the first mixing step is not particularly limited, and examples thereof include solid electrolyte α and solid electrolytes other than solid electrolyte α. As the solid electrolyte other than solid electrolyte α, it is preferable to use a solid electrolyte exhibiting crystallinity with a crystallite diameter of 40 nm or less or an amorphous solid electrolyte in order to form a strong interface. Alternatively, Li ２ S and P ２ S ５ Li ２ S:P ２ S ５ It is preferable to use a solid electrolyte containing the solid electrolyte α in a ratio of 78:22 to 0:100. In one embodiment, the solid electrolyte added in the first mixing step includes a solid electrolyte other than the solid electrolyte α.

[0047] In one embodiment, when the total amount of the solid electrolytes added in the first mixing step is 100 parts by mass, the amount of solid electrolyte α added in the first mixing step is 0 to 100 parts by mass, 0 to 50 parts by mass, 0 to 40 parts by mass, 0 to 30 parts by mass, 0 to 20 parts by mass, 0 to 10 parts by mass, 0 to 5 parts by mass, or 0 parts by mass.

[0048] (Second Mixing Step) In the second mixing step, a second mixture (cathode composite) is obtained by adding a further solid electrolyte to the first mixture and mixing the resulting mixture. The solid electrolyte added in the second mixing step includes solid electrolyte α. In the second mixing step, a solid electrolyte other than solid electrolyte α may be added together with solid electrolyte α.

[0049] In one embodiment, when the total amount of the solid electrolytes added in the second mixing step is 100 parts by mass, the amount of solid electrolyte α added in the second mixing step is 50 to 100 parts by mass, 60 to 100 parts by mass, 70 to 100 parts by mass, 80 to 100 parts by mass, 90 to 100 parts by mass, 95 to 100 parts by mass, or 100 parts by mass.

[0050] In one embodiment, when the solid electrolyte α contained in the cathode composite produced by the method for producing a cathode composite according to the present aspect is taken as 100 parts by mass, the proportion of the solid electrolyte α added in the second mixing step is 50 to 100 parts by mass, 60 to 100 parts by mass, 70 to 100 parts by mass, 80 to 100 parts by mass, 90 to 100 parts by mass, 95 to 100 parts by mass, or 100 parts by mass.

[0051] In one embodiment, the second mixing step is performed with a lower energy level than the first mixing step. In other words, in one embodiment, the energy levels of the first and second mixing steps satisfy the condition that the energy level of the first mixing step is higher than the energy level of the second mixing step.

[0052] In one embodiment, the first mixing step is performed under conditions in which the solid electrolyte added in the first mixing step undergoes amorphization, and the second mixing step is performed under conditions in which the crystallinity of the solid electrolyte α added in the second mixing step is not lost and the crystallite diameter is 40 nm or more. For example, the first mixing step can be performed under conditions in which the crystalline peak intensity of the solid electrolyte added in the first mixing step decreases, the peak half-width increases, or the peak disappears. On the other hand, the second mixing step can be performed under conditions in which the crystalline peak of the solid electrolyte α added in the second mixing step does not disappear, preferably under conditions in which the change in the crystalline peak intensity or the peak half-width is small. Here, "small change" can mean, for example, that the rate of change is smaller than the rate of change in the first mixing step. The condition "crystallite diameter becomes 40 nm or more" can also be rephrased as "maintaining the crystallite diameter at 40 nm or more."

[0053] Examples of mixing devices used in the first mixing step include planetary ball mills, tumbling mills, bead mills, Filmix mixers, Nauta mixers, tornado mixers, twin-screw extruders, multi-screw rollers, and solid-phase shear mixers. The mixing devices used in the second mixing step can be those exemplified for the first mixing step. In this case, when selecting a mixing device and setting operating conditions, it is preferable that the energy required for the mixing process be greater than that required for the first mixing step. The devices used in the first and second mixing steps may be the same or different. When using the same device, for example, the processing time or integrated power can be set to be greater than that required for the second mixing step, or, in the case of devices that utilize rotational power, such as planetary ball mills and tumbling mills, the rotation speed can be set to be greater than that required for the first mixing step. In one embodiment, a planetary ball mill is used in the first mixing step. In one embodiment, a tumbling mill is used in the second mixing step.

[0054] In one embodiment, the sulfur-based active material subjected to the first mixing step is pre-composited with a conductive additive (preferably a carbon material). For example, prior to the first mixing step, the sulfur-based active material can be composited with the conductive additive by heating the sulfur-based active material together with the conductive additive. The heating temperature is not particularly limited and may be, for example, a temperature exceeding 120°C, such as 150°C or higher, 170°C or higher, 200°C or higher, 150°C or higher, 200°C or higher, 250°C or higher, or 300°C or higher. The upper limit is, for example, 350°C or lower. In one embodiment, the mass ratio of the sulfur-based active material to the conductive additive (sulfur-based active material:conductive additive) is 10:90 to 95:5 or 20 to 90:80 to 10.

[0055] In one embodiment, in the cathode composite produced by the cathode composite production method according to the present aspect, the ratio of the total mass of the solid electrolyte to the total mass of the sulfur-based active material and the conductive additive (solid electrolyte:sulfur-based active material+conductive additive) is 10:90 to 90:10, 15 to 70:85 to 30, or 20 to 60:80 to 40.

[0056] In one embodiment, at least a portion of the solid electrolyte contained in the second mixture is crystalline, and the crystallite size thereof is 40 nm or more. Regarding the crystallite size, the description in "1. Positive electrode composite" is used.

[0057] In one embodiment, the cathode composite produced by the method for producing a cathode composite according to this aspect is the cathode composite according to the aspect of the present invention described above, and the description of the cathode composite according to the aspect of the present invention is incorporated herein by reference.

[0058] In one embodiment, the cathode composite produced by the method for producing a cathode composite according to the present aspect is the cathode composite according to the other aspect of the present invention described above, and the description of the cathode composite according to the other aspect of the present invention is applicable thereto.

[0059] 3. Lithium-ion battery A lithium-ion battery according to an aspect of the present invention includes the positive electrode composite according to an aspect of the present invention. The lithium-ion battery of this aspect can exhibit excellent rate characteristics.

[0060] The positive electrode composite can be used as the positive electrode layer of a lithium-ion battery. In this case, other components of the lithium-ion battery known in the art can be used, and the negative electrode layer can be selected so that the negative electrode active material does not contain lithium ions. The negative electrode active material contained in the negative electrode layer of the lithium-ion battery can be a "negative electrode active material containing lithium ions." Alternatively, the negative electrode active material contained in the negative electrode layer of the lithium-ion battery can be a "negative electrode active material that supplies lithium ions to the positive electrode."

[0061] The negative electrode of the lithium ion battery is not particularly limited as long as it is one that can be used in ordinary batteries. The negative electrode may be made of a negative electrode mixture that is a mixture of a negative electrode active material and a solid electrolyte.

[0062] As the negative electrode active material, commercially available materials can be used. For example, carbon materials, Sn metal, In metal, Si metal, alloys of these metals, etc. can be used. Specifically, natural graphite, various graphites, metal powders of Si, Sn, Al, Sb, Zn, Bi, etc., SiAl, Sn ５ Cu ６ , Sn ２ Co, Sn２ Examples include metal alloys such as Fe, amorphous alloys, and plated alloys. There are no particular restrictions on the particle size, but particles with an average particle size of several μm to 80 μm are preferably used.

[0063] The electrolyte layer is not particularly limited, and known electrolytes can be used. For example, oxide-based solid electrolytes, sulfide-based solid electrolytes, and polymer-based electrolytes are preferred, and sulfide-based solid electrolytes are more preferred from the viewpoint of ionic conductivity. The sulfide-based solid electrolyte is preferably the one used in the above-mentioned positive electrode composite.

[0064] The method for producing a lithium ion battery is not particularly limited, and examples thereof include a method in which a sheet is formed by forming a positive electrode layer made of the positive electrode composite according to one embodiment of the present invention on a positive electrode current collector, forming a solid electrolyte layer on the sheet, and laminating the sheet on which the negative electrode layer is formed on a previously formed negative electrode current collector, followed by pressing.

[0065] In the above description of the lithium ion battery, the positive electrode composite material according to one aspect of the present invention may be replaced with a positive electrode composite material according to another aspect of the present invention.

[0066] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0067] 1. Preparation of Positive Electrode Composite Material (Example 1) (1) Preparation of Composite Powder A Activated carbon (MSC-30 manufactured by Kansai Thermal Chemical Industries, Ltd.) and sulfur were placed in a glass bottle in a weight ratio of 3:7, and the bottle was sealed in an SUS tubular container. The mixture was heated in an electric furnace at 150°C for 6 hours and then at 300°C for 2.75 hours to obtain composite powder A of activated carbon and sulfur.

[0068] (2) Preparation of Solid Electrolyte A 0.4127 g of lithium sulfide, 0.6655 g of diphosphorus pentasulfide, 0.2137 g of lithium iodide, 0.2080 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 milling) 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 A. In X-ray diffraction measurements using CuKα radiation, diffraction peaks appeared at 2θ = 20.2 °, 23.7 °, and 41.2 °, confirming that solid electrolyte A has a crystalline structure similar to thiolicon region II type. The crystallite diameter was 20.3 nm.

[0069] (3) Preparation of Solid Electrolyte B Lithium sulfide (Li ２ S), diphosphorus pentasulfide (P ２ S ５ ), lithium bromide (LiBr) and lithium chloride (LiCl) in a molar ratio of Li ２ S:P ２ S ５The raw materials were roughly mixed to obtain a ratio of 47.5:12.5:15.0:25.0 of LiBr:LiCl. The raw material mixture was dispersed in a mixed solvent of dehydrated toluene and 2% by mass of dehydrated isobutyronitrile relative to the raw material mixture to obtain a slurry of approximately 10% by mass. A bead mill was operated at a peripheral speed of 12 m / s and a flow rate of 500 mL / min, and the slurry was introduced into the mill and circulated for 1 hour to obtain a mixture. After removing the solvent from the obtained mixture, the mixture was heated at 400 to 430 °C for 2 hours in an electric furnace. The mixture was then slowly cooled to obtain a raw material sulfide solid electrolyte. The raw material sulfide solid electrolyte was dispersed in dehydrated toluene under a nitrogen atmosphere and placed in a zirconia pot of a planetary ball mill (manufactured by Fritsch: model number P-7) together with 0.3 mm diameter zirconia balls, and the pot was filled with an inert atmosphere. The planetary ball mill was rotated at 150 rpm for 2 hours to obtain a slurry containing a finely divided sulfide solid electrolyte. The slurry was transferred to a nitrogen-purged Schlenk flask and then dried at room temperature for 1 hour using a vacuum pump. It was then heated to 80-100°C, and the solvent contained in the finely divided sulfide solid electrolyte was further removed (vacuum drying) to obtain solid electrolyte B. X-ray diffraction measurement using CuKα radiation revealed diffraction peaks at 2θ = 25.5° and 30.0°, confirming that solid electrolyte B had an argyrodite-type crystal structure. The crystallite size was 108.7 nm.

[0070] (4) Preparation of Positive Electrode Composite Powder First Mixing Step 0.7500 g of composite powder A and 0.2500 g of solid electrolyte A were placed in a 45 mL zirconia pot along with 10 10 mm diameter zirconia balls and sealed. Using a planetary ball mill (manufactured by Fritsch, model number P-7), the mixture was mixed at a rotation speed of 370 rpm for 20 hours at room temperature to obtain composite powder B. Second Mixing Step 0.7200 g of composite powder B and 0.1800 g of solid electrolyte B were placed in a 45 mL zirconia pot along with 34 g of 2 mm diameter zirconia balls and sealed. Using a tumbling mill ("Small Ball Mill Stand", manufactured by Asahi Rika Seisakusho, model number AV-1), the mixture was mixed at a rotation speed of 600 rpm for 1 hour at room temperature to obtain a positive electrode composite powder.

[0071] (Example 2) Steps (1) to (3) were carried out in the same manner as in Example 1. (4) Preparation of Positive Electrode Composite Powder First Mixing Step 0.8000 g of composite powder A and 0.2000 g of solid electrolyte A were placed in a 45 mL zirconia pot together with ten 10 mm diameter zirconia balls and sealed. Using a planetary ball mill (manufactured by Fritsch, model number P-7), mixing was carried out at a rotation speed of 370 rpm for 20 hours at room temperature to obtain composite powder C. Second Mixing Step 0.6750 g of composite powder C and 0.2250 g of solid electrolyte B were placed in a 45 mL zirconia pot together with 34 g of 2 mm diameter zirconia balls and sealed. Using a tumbling mill ("small ball mill stand", manufactured by Asahi Rika Seisakusho, model AV-1), mixing was carried out at a rotation speed of 600 rpm for 1 hour at room temperature to obtain a positive electrode composite powder.

[0072] (Comparative Example 1) (1) to (2) were carried out in the same manner as in Example 1. (3) Preparation of Positive Electrode Composite Powder 0.5400 g of composite powder A obtained in the same manner as in Example 1 and 0.3600 g of solid electrolyte B obtained in the same manner as in 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), mixing was carried out at a rotation speed of 370 rpm for 20 hours at room temperature to obtain a positive electrode composite powder.

[0073] (Comparative Example 2) (1) and (2) were carried out in the same manner as in Example 1. (3) Preparation of Solid Electrolyte C 15.3 g of lithium sulfide and 24.7 g of diphosphorus pentasulfide were placed in a 1 L reactor equipped with a stirring blade under a nitrogen atmosphere. After operating the stirring blade, 400 mL of tetrahydrofuran cooled to -20 °C was added to the vessel. After allowing the temperature to rise naturally to room temperature, stirring was continued for 72 hours. The resulting reaction solution slurry was placed in a glass filter (pore size: 40-100 μm) to obtain a solid content, which was then dried at 90 °C to obtain Li3PS4 powder (purity: 90% by mass). Under a nitrogen atmosphere, 1.70 g of the resulting Li3PS4 powder, 0.19 g of lithium bromide, and 0.28 g of lithium iodide were placed in a Schlenk flask (volume: 100 mL) equipped with a stirring bar. After rotating the stirrer, 20 mL of the complexing agent tetramethylethylenediamine (TMEDA) was added, and stirring was continued for 12 hours. The resulting electrolyte precursor content was dried under vacuum at room temperature to obtain a powdered electrolyte precursor. The resulting electrolyte precursor was heated under vacuum at 120°C for 2 hours, and then heated under vacuum at 140°C for 2 hours to obtain solid electrolyte C. X-ray diffraction measurement using CuKα radiation revealed diffraction peaks at 2θ = 20.2°, 23.7°, and 41.0°, confirming that solid electrolyte C has a crystalline structure similar to thiolicon region type II. The crystallite diameter was 34.4 nm.

[0074] (4) Preparation of Positive Electrode Composite Powder First Mixing Step 0.7500 g of composite powder A and 0.2500 g of solid electrolyte A were placed in a 45 mL zirconia pot along with 10 10 mm diameter zirconia balls and sealed. Using a planetary ball mill (manufactured by Fritsch, model number P-7), the mixture was mixed at a rotation speed of 370 rpm for 20 hours at room temperature to obtain composite powder B. Second Mixing Step 0.7200 g of composite powder B and 0.1800 g of solid electrolyte C were placed in a 45 mL zirconia pot along with 34 g of 2 mm diameter zirconia balls and sealed. Using a tumbling mill ("Small Ball Mill Stand", manufactured by Asahi Rika Seisakusho, model number AV-1), the mixture was mixed at a rotation speed of 600 rpm for 1 hour at room temperature to obtain a positive electrode composite powder.

[0075] (Comparative Example 3) (1) to (3) were carried out in the same manner as in Comparative Example 2. (4) Preparation of Positive Electrode Composite Powder First Mixing Step 0.8000 g of composite powder A and 0.2000 g of solid electrolyte A were placed in a 45 mL zirconia pot together with 10 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), mixing was carried out at a rotation speed of 370 rpm for 20 hours at room temperature to obtain composite powder C. Second Mixing Step 0.6750 g of composite powder C and 0.2250 g of solid electrolyte C were placed in a 45 mL zirconia pot together with 34 g of zirconia balls having a diameter of 2 mm, and the pot was sealed. Using a tumbling mill ("small ball mill stand", manufactured by Asahi Rika Seisakusho, model AV-1), mixing was carried out at a rotation speed of 600 rpm for 1 hour at room temperature to obtain a positive electrode composite powder.

[0076] (Comparative Example 4) (1) and (2) were carried out in the same manner as in Example 1. (3) Preparation of Positive Electrode Composite Powder 0.5400 g of composite powder A obtained in the same manner as in Example 1 and 0.3600 g of solid electrolyte A obtained in the same manner as in Example 1 were placed in a 45 mL zirconia pot together with 34 g of zirconia balls having a diameter of 2 mm, and the pot was sealed. Using a tumbling mill ("Small Ball Mill Stand", manufactured by Asahi Rika Seisakusho, model number AV-1), mixing was carried out at a rotation speed of 600 rpm for 1 hour at room temperature to obtain a positive electrode composite powder.

[0077] 2. Evaluation Method and Test Method (1) Calculation of "Brightness Intensity Variation Within the Same Field of View" by Secondary Electron Image Analysis of Scanning Electron Microscope (SEM) Images (1-1) Preparation of Positive Electrode Composite Pellets 100 mg of solid electrolyte A was placed in a Macol cylinder with a diameter of 10 mm and pressure-molded. 20 mg of the obtained positive electrode composite powder was placed on the pressure-pressed surface and pressure-molded again. Subsequently, 20 mg of the positive electrode composite powder was placed on the pressure-pressed surface on the opposite side and pressure-molded again, and the molded body was extracted from the cylinder to obtain a positive electrode composite pellet.

[0078] (1-2) SEM Secondary Electron Image Observation The obtained positive electrode composite pellet was split vertically, and the exposed surface was subjected to ion milling (Hitachi High-Tech Corporation, IM4000) to expose the cross section of the positive electrode composite pellet. A secondary electron image was obtained for the obtained cross section using an SEM (Hitachi High-Tech Corporation, SU8220). The observation magnification was 5000x, the acceleration voltage was 2 kV, the emission was 10 μA, the probe current was High, the condenser lens was 5, W.D2 mm, and the scan area was 1024 × 768.

[0079] (1-3) Luminance Intensity Variation within the Same Field of View (Compartmental Method) The images obtained using the above procedure were processed using Python (3) OpenCV (4.5.1). First, the image was read in grayscale, and the luminance distribution of the entire image was determined. When creating the luminance distribution, the histogram width was set to 3, and the distribution was created using 256 gradations (0 to 255). Here, the luminance at the maximum luminance distribution was defined as "x." Next, for one field of view, the side with the largest number of pixels was defined as horizontal, and the side with the smallest number as vertical. The image was divided vertically into two and horizontally into five, creating a total of 10 rectangular subdivisions. Similarly, a luminance distribution was created for each subdivision image using 256 gradations, with a histogram width of 3. For each subdivision, the maximum luminance distribution in the range of x - 30 to x + 30 was calculated and defined as "y." The average value and standard deviation of y were calculated for 10 small sections, and the value obtained by normalizing the standard deviation by the average value (standard deviation / average value) was defined as the brightness intensity variation within the same field of view. Note that the brightness distribution of the secondary electron image obtained by ion milling cross-section processing can be considered to be negligible due to unevenness, and the difference in contrast can be considered to be due to the composition. In other words, if the variation in brightness intensity is small, it can be determined that the compositional unevenness of the positive electrode composite is small and the mixed state is good. Furthermore, this method of evaluating the variation in each small section of a single image is considered to be capable of evaluating the mixed state in the entire structure, not just the local structure, from an image of one field of view within the positive electrode composite used for evaluation.

[0080] For reference, Fig. 1 shows a diagram illustrating the calculation of "brightness intensity variation within the same field of view" by secondary electron image analysis of a scanning electron microscope (SEM) image of the positive electrode composite of Example 1. Here, (a) is a secondary electron image, (b) is its brightness distribution, and (c) is an example of the brightness distribution of a small section (the same applies to Figs. 2 to 6 below). Fig. 2 corresponds to Example 2, Fig. 3 corresponds to Comparative Example 1, Fig. 4 corresponds to Comparative Example 2, Fig. 5 corresponds to Comparative Example 3, and Fig. 6 corresponds to Comparative Example 4, respectively.

[0081] (2) XRD Measurement and Crystallite Size Calculation (2-1) Powder X-ray Diffraction (XRD) of Positive Electrode Composite Powder XRD measurement was performed on the obtained positive electrode composite powder. Specifically, the positive electrode composite powder was filled into a groove with a diameter of 20 mm and a depth of 0.2 mm, and leveled with glass to prepare a sample. This sample was sealed with a Kapton film for XRD and measured without being exposed to air. The XRD measurement was performed under the following measurement conditions using a powder X-ray diffraction measurement device D2 PHASER from BRUKER Corporation. [Measurement conditions] Tube voltage: 30 kV Tube current: 10 mA X-ray wavelength: Cu-Kα ray (1.5418 Å) Optical system: focusing method Slit configuration: Soller slit 4° (on both the incident and receiving sides), divergence slit 1 mm, Kβ filter (Ni plate 0.5%), air scatter screen 3 mm) Detector: semiconductor detector Measurement range: 2θ = 10-60 deg Step width, scan speed: 0.05 deg, 0.05 deg / sec

[0082] Fig. 7 shows the diffraction spectra of the positive electrode composite powders obtained in Examples 1 and 2. Fig. 8 shows the diffraction spectra of the positive electrode composite powders obtained in Comparative Examples 1 to 3.

[0083] (2-2) Determination of the presence or absence of a crystalline peak An arbitrary point N (2θ=n°) is defined in the obtained diffraction spectrum (10°≦2θ≦60°). For the arbitrary point N, the average value of the X-ray intensity (counts) at 2θ=(n°−0.8°)±0.2° and 2θ=(n°+0.8°)±0.2° is calculated as the background (BG) intensity I ｂｇ The maximum value of the X-ray intensity (counts) at 2θ=n°±0.8° is defined as the peak intensity I ｐｅａｋ When the ratio (I ｐｅａｋ / Iｂｇ ) is 1.20 or more, it is determined that a crystalline peak is present.

[0084] (2-3) Calculation of Crystallite Size The crystallite size was determined for samples determined to have a crystalline peak. The crystallite size (L) was determined according to the method of P. Scherrer et al. Specifically, using the results of measurements performed in the same manner as in (2-1), it was determined by calculation using the following formula. Crystallite size (L) = Kλ / (β cos θ) K: constant, 0.9 was used. λ: 1.5418 Å (Cu-Kα radiation) β: calculated from β = w-B. w: half-width of the peak derived from the solid electrolyte obtained by measurement B: instrument constant (a standard material (silicon) was measured in the same manner as in (1-1), and B = 0.1269° was determined from the peak at 2θ = 23.6°.)

[0085] The w (half width obtained by measurement) was calculated as follows. That is, a linear baseline was set for the peak shape obtained by XRD measurement, and the difference between the intensity at each point and the baseline was calculated to obtain an XRD curve. The XRD curve was set as an equation consisting of a Lorentz function L(x) and a Gaussian function G(x) (f(x) = (1 - α) × L(x) + α × G(x)), and the parameters A, w, and x were calculated by curve fitting. ０ The crystallite size was calculated using the peak attributable to the solid electrolyte that had the maximum intensity in the range of 10°≦2θ≦60°.

[0086]

[0087] (3) Evaluation of Battery Characteristics (3-1) Preparation of Negative Electrode Composite Lithium titanate ("LT-112" manufactured by Ishihara Sangyo Kaisha), a conductive additive ("Li-100" manufactured by Denka Co., Ltd., powdered acetylene black), and solid electrolyte C were mixed in a mortar in a mass ratio of 60:5:35 for 5 minutes to obtain a negative electrode composite (also referred to as "LTO (lithium titanate) negative electrode composite").

[0088] (3-2) Preparation of Lithium Ion Battery 100 mg of solid electrolyte A was placed in a 10 mm diameter Macol cylinder and pressure molded to form a solid electrolyte layer (layer of solid electrolyte A). Next, 12 mg of positive electrode composite powder was placed on one pressure surface of the solid electrolyte layer and pressure molded again. Next, 166 mg of LTO negative electrode composite was placed on the other pressure surface of the solid electrolyte layer (the pressure surface opposite the positive electrode) and pressure was applied. A Li foil with a diameter of 9 mm and a thickness of 0.1 mm was placed on top of it and pressure was applied again to prepare a lithium ion battery.

[0089] (3-3) Charge / Discharge Test A constant current charge / discharge test was performed on the lithium ion batteries obtained in each Example and Comparative Example to determine the discharge capacity. Specifically, the voltage range of the constant current charge / discharge test was set to -0.4 to +1.3 V, and the current value was set as shown in Table 1 at a C rate determined based on the theoretical capacity of sulfur, 1672 mAh / g. Charging was performed as CC-CV charging, in which constant current charging was followed by constant voltage charging with a termination condition of 0.02 C. Discharging was performed as constant current discharge (CC discharge).

[0090]

[0091] The results are shown in Table 2.

[0092]

[0093] As shown in Table 2, Examples 1 and 2 had higher capacities at high rates (0.5 C) than Comparative Examples 1 to 3, and were found to have significantly superior rate characteristics. The reason for the significantly superior rate characteristics is the high Li content resulting from the good mixing state and the crystallinity of the solid electrolyte. ＋ The conductivity is considered to be a factor, but the large crystallite size is also considered to have a strong influence. That is, in order to cause an electrode reaction in a sulfur-based positive electrode composite, it is generally considered necessary to finely mix an insulating sulfur-based active material and a conductive additive with a solid electrolyte to form a structure including a large number of interfaces. In contrast, in the present invention, the use of a solid electrolyte with a large crystallite size reduces the number of interfaces, thereby reducing the interfacial resistance and demonstrating significantly excellent rate characteristics.

[0094] 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. Accordingly, these numerous modifications are within the scope of the present invention. The contents of all documents cited in this specification and of the applications from which this application claims priority under the Paris Convention are incorporated by reference in their entirety.

Claims

1. A positive electrode composite comprising a conductive additive which is a carbon material, a sulfur-based active material, and a solid electrolyte, wherein the brightness intensity variation within the same field of view in a secondary electron image taken with a scanning electron microscope is 0.250 or less, and at least a portion of the solid electrolyte is crystalline, and the crystallite diameter is 40 nm or more.

2. The cathode mixture of claim 1, wherein the solid electrolyte comprises a sulfide solid electrolyte containing lithium atoms, phosphorus atoms, and sulfur atoms.

3. The positive electrode mixture according to claim 2, wherein the sulfide solid electrolyte further contains a halogen atom.

4. The positive electrode mixture according to any one of claims 1 to 3, wherein the solid electrolyte contains a crystalline phase other than glass ceramics.

5. The positive electrode mixture according to claim 4, wherein the crystal phase other than the glass ceramic includes an argyrodite-type crystal structure.

6. The cathode composite according to any one of claims 1 to 5, obtained by a manufacturing method including a first mixing step of adding a solid electrolyte to a sulfur-based active material and mixing the resulting mixture to obtain a first mixture, and a second mixing step of adding a further solid electrolyte to the first mixture and mixing the resulting mixture to obtain a second mixture, wherein the solid electrolyte added in the second mixing step comprises solid electrolyte α, and at least a portion of the solid electrolyte α contained in the second mixture is crystalline and has a crystallite diameter of 40 nm or more.

7. The positive electrode mixture according to claim 6, wherein the second mixing step is performed with less energy than the first mixing step.

8. The cathode composite according to claim 6 or 7, wherein the first mixing step is performed under conditions that promote amorphization of the solid electrolyte added in the first mixing step, and the second mixing step is performed under conditions that prevent the crystallinity of the solid electrolyte α added in the second mixing step from disappearing and result in a crystallite diameter of 40 nm or more.

9. A lithium ion battery comprising the positive electrode mixture according to any one of claims 1 to 8.

10. A method for producing a positive electrode composite, comprising: a first mixing step of adding a solid electrolyte to a sulfur-based active material and mixing the resulting mixture to obtain a first mixture; and a second mixing step of adding a further solid electrolyte to the first mixture and mixing the resulting mixture to obtain a second mixture, wherein the solid electrolyte added in the second mixing step comprises solid electrolyte α, and at least a portion of the solid electrolyte α contained in the second mixture is crystalline and has a crystallite diameter of 40 nm or more.

11. The method for producing a positive electrode mixture according to claim 10, wherein the second mixing step is performed with less energy than the first mixing step.

12. The method for producing a positive electrode composite according to claim 10 or 11, wherein the first mixing step is performed under conditions that cause the solid electrolyte added in the first mixing step to become amorphous, and the second mixing step is performed under conditions that cause the solid electrolyte α added in the second mixing step to not lose its crystallinity and to have a crystallite diameter of 40 nm or more.

13. A method for producing a positive electrode composite according to any one of claims 10 to 12, wherein the amount of solid electrolyte α added in the second mixing step is 50 to 100 parts by mass when the total amount of solid electrolytes added in the second mixing step is 100 parts by mass.

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