Method for manufacturing positive electrode mixture

Abstract

A method for manufacturing a positive electrode mixture, said method including: a step for mechanically mixing starting materials including a conductive aid that is a carbon material, a sulfur-based active material, and a solid electrolyte to form a composite; and a step for heating the composite at less than 160°C.

Description

Manufacturing method for positive electrode composite material

[0001] This invention relates to a method for manufacturing a positive electrode composite material used in lithium-ion secondary batteries and the like.

[0002] Lithium-ion batteries are required to have high capacity. To improve battery capacity, methods using sulfur as the positive electrode are being considered due to its large theoretical capacity. However, since sulfur has low lithium-ion conductivity and electronic conductivity, it is necessary to ensure lithium-ion conductivity and electronic conductivity within the positive electrode when using sulfur as the positive electrode.

[0003] To address the above issues, methods for compounding sulfur, a conductive additive, and a solid electrolyte by mechanical mixing have been investigated (see, for example, Patent Documents 1 to 3). In particular, a method using porous carbon as a conductive additive, which is pre-compounded with sulfur, and then compounded with a solid electrolyte by mechanical mixing is preferred because it allows for a state of high contact between sulfur and porous carbon (see Patent Documents 1 and 2).

[0004] Japanese Patent Publication No. 2014-11033, International Publication No. 2024 / 34499, International Publication No. 2022 / 230163

[0005] However, further performance improvements are required for cathode composite materials. One of the objectives of the present invention is to provide a method for manufacturing a cathode composite material that can improve the rate characteristics of a battery.

[0006] As a result of diligent research, the inventors discovered that the rate characteristics of a battery can be improved by mechanically mixing a conductive additive, a sulfur-based active material, and a solid electrolyte, and then heat-treating them at a predetermined temperature, thus completing the present invention.

[0007] The present invention provides the following methods for manufacturing a positive electrode composite: 1. A method for manufacturing a positive electrode composite comprising: a step of mechanically mixing raw materials, which include a conductive additive that is a carbon material, a sulfur-based active material, and a solid electrolyte, to form a composite; and a step of heating the composite at a temperature of less than 160°C. 2. The method according to 1, wherein the sulfur-based active material includes elemental sulfur. 3. The method according to 1 or 2, wherein the conductive additive is porous carbon. 4. The method according to any one of 1 to 3, wherein the sulfur-based active material and the conductive additive form a sulfur-based active material-conductive additive composite. 5. The method according to any one of 1 to 4, which includes a step of pre-compounding the sulfur-based active material and the conductive additive. 6. The method according to 5, wherein the sulfur-based active material is in a molten state in the compounding step. 7. The method according to any one of 1 to 6, wherein the temperature of the heating step is 110°C or higher. 8. The method according to any one of 1 to 6, wherein the temperature of the heating step is 115°C to 130°C. 9. A method for producing the product according to any one of 1 to 8, wherein the solid electrolyte comprises a sulfide solid electrolyte. 10. A method for producing the product according to 9, wherein the composite comprises an amorphous sulfide solid electrolyte. 11. A method for producing the product according to 9, wherein the sulfide solid electrolyte comprises an argyrodite type crystal structure. 12. A method for producing the product according to any one of 1 to 11, wherein the weight loss rate before and after the heating step is 3.0% or less.

[0008] According to the present invention, it is possible to provide a method for manufacturing a positive electrode composite material that can improve the rate characteristics of a battery.

[0009] These are the XRD patterns of the cathode composite materials obtained in Example 1 and Comparative Example 1. These are the XRD patterns of the cathode composite materials obtained in Example 2 and Comparative Example 2.

[0010] The present invention will be described in detail below. In this specification, "x to y" represents a numerical range of "x or more, and y or less". The upper and lower limits described for the numerical range can be combined in any way. Furthermore, it is possible to combine two or more non-conflicting embodiments of the embodiments of the present invention described below, and an embodiment that combines two or more embodiments is also an embodiment of the embodiments of the present invention.

[0011] A method for manufacturing a positive electrode composite according to one aspect of the present invention includes the steps of mechanically mixing a conductive additive, which is a carbon material, a sulfur-based active material, and a solid electrolyte to form a composite, and heating the composite at a temperature of less than 160°C. Through mechanical mixing, the conductive additive, the sulfur-based active material, and the solid electrolyte form a composite, but a portion of the sulfur-based active material remains isolated within the positive electrode. For example, in methods such as those described in Patent Document 1, in which porous carbon and sulfur are pre-composited, the porous carbon material and sulfur composite (S / C composite) and the solid electrolyte are mechanically mixed in a subsequent step. However, the inventors have found that a portion of the sulfur separates from the S / C composite and becomes isolated. It is believed that the isolated sulfur inhibits the conduction of ions and electrons within the positive electrode, thereby reducing the rate characteristics of the battery. In particular, when a sulfur-based active material-conductive additive complex (S / C complex) is formed from a conductive additive (C) and a sulfur-based active material (S), and the S / C complex is mechanically mixed with a solid electrolyte to form a complex, it is presumed that a large amount of sulfur-based active material will detach from the conductive additive and become isolated.

[0012] In this embodiment, after compounding by mechanical mixing, the material is heat-treated at a temperature of less than 160°C. By heat-treating under appropriate conditions, the molten sulfur-based active material is rearranged near the conductive additive, eliminating its isolated state, and thus it is presumed that the conductivity of ions and electrons within the positive electrode is improved. As a result, lithium-ion batteries using the positive electrode composite material of this embodiment exhibit improved rate characteristics.

[0013] In the process of mechanically mixing a conductive additive, a sulfur-based active material, and a solid electrolyte to form a composite, a carbon material is used as the conductive additive. (Carbon material) Because carbon materials have high electronic conductivity and are lighter than other conductive materials, the power density and capacity per unit weight of the battery can be increased. The carbon material is preferably porous carbon having pores. The pores of porous carbon facilitate the rearrangement of the sulfur-based active material due to heat treatment.

[0014] The carbon material is not particularly limited, and examples thereof include carbon blacks such as Ketjenblack, acetylene black, Denka black, thermal black and channel black, mesoporous carbon, activated carbon, amorphous carbon, carbon nanotubes, vapor grown carbon fiber (VGCF), carbon nanohorns, fullerenes, carbon fibers, natural graphite, artificial graphite, graphene, graphene oxide, reduced graphene oxide, and the like. These may be used alone, or two or more thereof may be used in combination. Composites of these may also be used.

[0015] In one embodiment, the carbon material is porous carbon. For example, the BET specific surface area of the carbon material is 50 m ２ / g to 6000 m ２ / g. This enables formation of a wide contact interface between the carbon material and the sulfur-based active material, and can improve the utilization rate of the sulfur-based active material. The BET specific surface area is preferably 70 m ２ / g to 5500 m ２ / g, more preferably 100 m ２ / g to 5000 m ２ / g, still more preferably 1000 m ２ / g to 5000 m ２ / g, and particularly preferably 1500 m ２ / g to 5000 m ２ / g.

[0016] Further, the pore volume of the carbon material is 0.5 cm ３ / g to 6 cm ３ / g. This allows the pores of the carbon material to be impregnated with the sulfur-based active material, and can further improve the capacity of the battery. The pore volume is preferably 0.7 cm ３ / g to 5.5 cm ３ / g, more preferably 1.0 cm ３ / g to 5.0 cm ３ / g.

[0017] In this invention, the BET specific surface area and pore volume can be determined using nitrogen adsorption isotherms obtained by adsorbing nitrogen gas onto a carbon material at liquid nitrogen temperature. Specifically, the BET specific surface area can be calculated using the Brunauer-Emmett-Teller (BET) multipoint method with respect to the nitrogen adsorption isotherms. The pore volume can be determined using the Barrett-Joyner-Halenda (BJH) method with respect to the nitrogen adsorption isotherms. As a measuring device, for example, the specific surface area and pore distribution analyzer (Autosorb-3) manufactured by Quantachrome can be used for measurement.

[0018] (Sulfur-based active material) There are no particular limitations on sulfur-based active materials, 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 Examples include sulfur-containing polymer compounds, etc. Among these, sulfur (elemental sulfur) is preferred. There are no particular limitations on the sulfur, but high purity is preferred. Specifically, a purity of 95% by mass or higher is preferred, more preferably 96% by mass or higher, and particularly preferably 97% by mass or higher. Examples of sulfur crystal systems include α-sulfur (orthorhombic), β (monoclinic), γ (monoclinic), amorphous sulfur, etc. These can be used individually or in combination of two or more.

[0019] (Solid Electrolytes) The solid electrolyte is not particularly limited, but an example is a sulfide solid electrolyte. A sulfide solid electrolyte is a solid electrolyte that contains at least a sulfur atom and exhibits ionic conductivity due to the contained lithium ions, and preferably contains a lithium atom and a phosphorus atom in addition to a sulfur atom, and more preferably contains a lithium atom, a phosphorus atom and a halogen atom, and has ionic conductivity due to the lithium atom. The sulfide solid electrolyte may be an amorphous sulfide solid electrolyte or a crystalline sulfide solid electrolyte.

[0020] (a) Amorphous sulfide solid electrolytes As amorphous sulfide solid electrolytes, any that contain at least sulfur atoms and exhibit ionic conductivity due to the contained lithium ions can be used without particular limitations, and typical examples include Li ２ S-P ２ S ５ A solid electrolyte containing sulfur atoms, lithium atoms, and phosphorus atoms, composed of lithium sulfide and phosphorus sulfide, etc. 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; further containing other elements such as oxygen and silicon, for example, Li ２ S-P ２ S ５ -Li ２ O-LiI, Li ２ S-SiS ２ -P ２ S ５ - Solid electrolytes such as LiI are preferred. From the viewpoint of obtaining higher ionic conductivity, 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 Solid electrolytes composed of lithium sulfide, phosphorus sulfide, and lithium halide, such as -LiI-LiBr, are preferred. The types of elements constituting the amorphous sulfide solid electrolyte can be confirmed, for example, by an ICP emission spectrometer.

[0021] The amorphous sulfide solid electrolyte contains at least Li ２ S-P ２ S ５ If it has Li ２ S and P ２ S ５ The molar ratio of is preferably 65-85:15-35, more preferably 70-80:20-30, and even more preferably 72-78:22-28, from the viewpoint of obtaining high chemical stability and higher ionic conductivity. The amorphous sulfide solid electrolyte is, for example, Li ２ S-P ２ S ５ In the case of -LiI-LiBr, the total content of lithium sulfide and phosphorus pentasulfide is preferably 60 to 95 mol%, more preferably 65 to 90 mol%, and even more preferably 70 to 85 mol%. Furthermore, 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%.

[0022] In amorphous sulfide solid electrolytes containing lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms, the mixing 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 mixing 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 mixing 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 with higher ionic conductivity having the thiolysicon region II type crystal structure described later.

[0023] In one embodiment, the molar ratio of lithium atoms to phosphorus atoms in the positive electrode composite material (lithium atoms / phosphorus atoms) is preferably less than 3.5, more preferably 2 to 3.3, even more preferably 2.8 to 3.1, and particularly preferably 3. In one embodiment, the molar ratio of phosphorus atoms to halogen atoms in the sulfide solid electrolyte (X / P) is preferably 0.33 or less. The molar ratio (X / P) is more preferably 0.1 or less, and particularly preferably 0.05 or less. In one embodiment, the sulfide solid electrolyte does not contain halogen atoms. For example, the composition of the sulfide solid electrolyte is Li ３ PS ４ In this case, since additives such as LiX mentioned above, which enhance ionic conductivity, are not required, the cost of such additives and the energy required to incorporate them into the solid electrolyte can be eliminated.

[0024] There are no particular restrictions on the shape of the amorphous sulfide solid electrolyte, and for example, a particulate shape can be mentioned. The average particle diameter (D ５０ ) can, for example, be in the range of 0.01 μm to 500 μm, or 0.1 μm to 200 μm. In the present specification, the average particle diameter (D ５０ ) refers to the particle diameter at which the cumulative integration of particles starting from the particle with the smallest particle diameter reaches 50% of the total when a cumulative particle diameter distribution curve is drawn, and is an average particle diameter that can be measured, for example, using a laser diffraction / scattering particle size distribution analyzer.

[0025] (b) Crystalline Sulfide Solid Electrolyte As the crystalline sulfide solid electrolyte, for example, so-called glass-ceramics obtained by heating the aforementioned amorphous sulfide solid electrolyte to a temperature equal to or higher than its crystallization temperature may be used, and a sulfide solid electrolyte having the following crystal structure can be employed. Examples of crystal structures that may be possessed by a crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, and phosphorus atoms include Li ３ PS ４ crystal structure, Li ４ P ２ S ６ crystal structure, Li ７ PS ６ crystal structure, Li ７ P ３ S １１ crystal structure, and a crystal structure having peaks near 2θ=20.2° and near 23.6° (see, for example, Japanese Patent Application Laid-Open No. 2013-16423).

[0026] Further, examples of crystal structures that may be possessed by a crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms include Li ４－ｘ Ge １－ｘ P ｘ S ４ -based thio-LISICON Region II type crystal structure (see Kanno et al., Journal of The Electrochemical Society, 148(7) A742-746 (2001)), Li ４－ｘ Ge １－ｘ P ｘ S ４System thio-LISICON Region II-type (thio-LISICON Region II) similar crystal structure (see Solid State Ionics, 177 (2006), 2721-2725), etc. Here, "thio-LISICON Region II-type crystal structure" refers to Li ４－ｘ Ge １－ｘ P ｘ S ４ system thio-LISICON Region II (thio-LISICON Region II)-type crystal structure, Li ４－ｘ Ge １－ｘ P ｘ S ４ it indicates any of the crystal structures similar to the system thio-LISICON Region II (thio-LISICON Region II) type.

[0027] In the X-ray diffraction measurement using CuKα radiation, for Li ３ PS ４ crystal structure, diffraction peaks appear, for example, around 2θ=17.5°, 18.3°, 26.1°, 27.3°, and 30.0°. For Li ４ P ２ S ６ crystal structure, diffraction peaks appear, for example, around 2θ=16.9°, 27.1°, and 32.5°. For Li ７ PS ６ crystal structure, diffraction peaks appear, for example, around 2θ=15.3°, 25.2°, 29.6°, and 31.0°. For Li ７ P ３ S １１ crystal structure, diffraction peaks appear, for example, around 2θ=17.8°, 18.5°, 19.7°, 21.8°, 23.7°, 25.9°, 29.6°, and 30.0°. For Li ４－ｘ Ge １－ｘ P ｘ S ４ system thio-LISICON Region II (thio-LISICON Region II)-type crystal structure, diffraction peaks appear, for example, around 2θ=20.1°, 23.9°, and 29.5°. For Li ４－ｘ Ge １－ｘ P ｘ S ４Diffraction peaks for crystal structures similar to the thio-LISICON Region II type appear, for example, around 2θ = 20.2° and 23.6°. Note that these peak positions may vary within a range of ±0.5°.

[0028] Furthermore, argyrodite crystal structures can also be cited as crystalline sulfide solid electrolytes. For example, Li ７ PS ６ Crystal structure; Li ７ PS ６ Composition formula Li has a structural framework ７－ｘ P １－ｙ Si ｙ S ６ and Li ７＋ｘ P １－ｙ Si ｙ S ６ Crystal structure represented by (x is -0.6 to 0.6, y is 0.1 to 0.6); Li ７－ｘ－２ｙ PS ６－ｘ－ｙ Cl ｘ Crystal structure shown by (0.8 ≤ x ≤ 1.7, 0 < y ≤ -0.25x + 0.5); Li ７－ｘ PS ６－ｘ Ha ｘ Examples of crystal structures include those represented by (Ha is Cl or Br, and x is preferably 0.2 to 1.8).

[0029] Among the crystal structures described above, Li is an example of a crystal structure found in crystalline sulfide solid electrolytes. ３ PS ４ A crystal structure, a thiolysicon region II type crystal structure, and an argyrodite type crystal structure are preferred.

[0030] There are no particular restrictions on the shape of the crystalline sulfide solid electrolyte, but for example, particulate form can be given. The average particle size (D) of the particulate crystalline sulfide solid electrolyte. ５０ ) is the average particle size (D) of the amorphous sulfide solid electrolyte described above. ５０ Similarly, for example, ranges of 0.01 μm to 500 μm and 0.1 μm to 200 μm can be exemplified.

[0031] In one embodiment, instead of a solid electrolyte, raw materials for solid electrolytes such as lithium sulfide, phosphorus sulfide, or lithium halide may be used. In this case, the raw materials do not contain a solid electrolyte, but a solid electrolyte can be obtained by mechanically mixing a sulfur-based active material with a conductive additive.

[0032] In this embodiment, a conductive additive (carbon material), a sulfur-based active material, and a solid electrolyte are mechanically mixed to form a composite. The starting materials may be a mixture of the conductive additive, the sulfur-based active material, and the solid electrolyte. Alternatively, a composite (S / C composite) of the sulfur-based active material and the conductive additive may be formed in advance, and a mixture of this composite and the solid electrolyte may be used. In this application, "composite formation" refers to actions such as heating, mixing, bonding, or contacting two or more substances.

[0033] In one embodiment, the starting materials may or may not contain components other than the conductive additive, sulfur-based active material, and solid electrolyte. The other components are not particularly limited, but examples include binders, solvents, and dispersants.

[0034] The content of the conductive additive, sulfur-based active material, and solid electrolyte in the starting materials is not particularly limited. For example, the content of the sulfur-based active material is 40 to 350 parts by mass per 100 parts by mass of solid electrolyte. More preferably, it is 50 to 200 parts by mass, even more preferably 60 to 180 parts by mass, and particularly preferably 100 to 170 parts by mass. The content of the conductive additive is 10 to 300 parts by mass per 100 parts by mass of solid electrolyte. More preferably, it is 20 to 90 parts by mass, even more preferably 30 to 75 parts by mass, and particularly preferably 45 to 70 parts by mass.

[0035] In one embodiment, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, 95% or more, 99% or more, 99.5% or more, or substantially 100% by mass of the starting materials of the positive electrode composite are a conductive additive, a sulfur-based active material, and a solid electrolyte. In the case of "substantially 100% by mass", unavoidable impurities may be included.

[0036] In one embodiment, in the process of compounding a sulfur-based active material with a conductive additive, the sulfur-based active material is brought into a molten state. By melting the sulfur-based active material, the impregnation of the sulfur-based active material into the pores of the conductive additive can be promoted, and it can be dispersed to a high degree.

[0037] The heating temperature can be appropriately set according to the sulfur-based active material used. For example, if the sulfur-based active material is sulfur, the temperature should be above the melting point of sulfur (approximately 115°C). Preferably, it should be 130°C or higher, and more preferably 150°C or higher. Heating may be carried out in two or more stages.

[0038] In one embodiment, a sulfur-based active material may be impregnated into the pores of a carbon material by mechanical mixing to form an S / C composite.

[0039] The starting materials for the cathode composite are mechanically mixed to form a composite. Known equipment can be used for mechanical mixing. The equipment is not particularly limited as long as it can apply mechanical stress to the starting materials to form a composite. Specific examples of equipment include pulverizers such as planetary ball mills, vibratory mills, and rolling mills, as well as kneaders.

[0040] In one embodiment, a sulfur-based active material and a conductive additive are compounded in a mixing step. The mixing in the above mixing step includes mixing a solid electrolyte into a treated product obtained by heating and / or mixing the conductive additive and the sulfur-based active material. The treated product is preferably obtained by heating the conductive additive and the sulfur-based active material, and it is even more preferable to heat it above the melting point of the sulfur-based active material. Heating can improve the fluidity of the sulfur-based active material, so that the sulfur-based active material is uniformly dispersed. Heating above the melting point can further improve the fluidity. If the conductive additive is activated carbon (a conductive additive having pores), the sulfur-based active material can be impregnated into the pores, and the sulfur-based active material can cover at least one part of the surface of the conductive additive.

[0041] In this case, of the conductive additive, sulfur-based active material, and sulfide solid electrolyte to be used in the mixing process, the conductive additive and sulfur-based active material can be heated and / or mixed as a pretreatment for the mixing process. Here, it is preferable not to subject the sulfide solid electrolyte to pretreatment (pretreatment in the absence of the sulfide solid electrolyte), and it is preferable to subject only the conductive additive and sulfur-based active material to pretreatment, however, this does not preclude pretreatment of a portion of the total amount of sulfide solid electrolyte to be finally blended together with the conductive additive and sulfur-based active material. The raw material composition to be used in the mixing process can be prepared by adding the sulfide solid electrolyte to the pretreated product (containing the conductive additive and sulfur-based active material) and mixing it. By performing such pretreatment, good conductive paths are formed in the final positive electrode mixture.

[0042] The heating in the pretreatment can be carried out, for example, by melting the sulfur-based active material while the conductive additive and sulfur-based active material are in the same presence. The heating temperature can be set to a temperature above the melting point of the sulfur-based active material. When using elemental sulfur, it is preferable to heat to 110°C or higher. The mixing in the pretreatment can be carried out by crushing the conductive additive and / or sulfur-based active material while the conductive additive and sulfur-based active material are in the same presence. This mixing is preferably done by mechanical mixing. The means for mechanical mixing in the pretreatment and the means for mechanical mixing in the mixing step may be the same or different.

[0043] In pretreatment, when both heating and mixing are performed, heating and mixing may be performed simultaneously, heating followed by mixing, or mixing followed by heating. During the transition from heating to mixing, or from mixing to heating, a period of simultaneous heating and mixing may or may not be provided. As an example of pretreatment, if the conductive additive is a carbon material having pores, a sulfur-based active material may be heated and melted to impregnate the pores of the conductive additive. By melting the sulfur-based active material, impregnation into the pores can be promoted. In addition, the sulfur-based active material can be highly dispersed in the conductive additive. It is preferable that the conductive additive is porous carbon, and the sulfur-based active material contains sulfur, with at least a portion of the sulfur impregnated into the pores of the porous carbon. For example, the treated product can be obtained by melting the sulfur-based active material in a system in which the conductive additive and the sulfur-based active material coexist.

[0044] Pretreatment allows for the compounding of the conductive additive and the sulfur-based active material, thereby forming a sulfur-based active material-conductive additive composite. In the sulfur-based active material-conductive additive composite, it is preferable that one or more states selected from the group consisting of the following are formed: the conductive additive and the sulfur-based active material are attached to each other; the sulfur-based active material covers at least one part of the surface of the conductive additive; the sulfur-based active material is impregnated into the pores of the conductive additive; and the sulfur-based active material and the conductive additive are chemically bonded, particularly covalently bonded.

[0045] The mixing ratio of elemental sulfur to conductive additive (carbon material) can be appropriately adjusted according to the materials used. For example, the mass ratio (S / C) of elemental sulfur (S) to carbon material (C) in the sulfur-based active material-conductive additive composite is 0.5 or higher. When the utilization rate of sulfur is the same, increasing the sulfur content in the positive electrode composite can be expected to improve the energy density of the battery. Furthermore, the mass ratio (S / C) is preferably 0.5 to 6.0, more preferably 1.0 to 4.0, and even more preferably 2.0 to 2.5.

[0046] In one embodiment, the above-described mixing (mixing a conductive additive, a sulfur-based active material, and a sulfide solid electrolyte (mixing step)) includes first bringing the conductive additive and the sulfur-based active material into contact (first contact) to produce a first mixture (which may be a treated product or a sulfur-based active material-conductive additive composite), and then bringing the solid electrolyte into contact with the first mixture (second contact) to produce a second mixture. Here, it is preferable to heat the conductive additive and the sulfur-based active material before the second contact. The first contact may be bringing the conductive additive into contact with the heated sulfur-based active material.

[0047] In one embodiment, the sulfide solid electrolyte after mechanical mixing is amorphous. Alternatively, the composite contains an amorphous sulfide solid electrolyte. For example, before mechanical mixing, a solid electrolyte having a thiolysicon region II type crystal structure or an argyrodite type crystal structure may be amorphous (vitrified) by mechanical mixing. Amorphousness can be confirmed by X-ray diffraction measurement, where no diffraction peaks originating from crystals are present. In one embodiment, the raw material sulfide solid electrolyte is amorphous.

[0048] The composite material obtained by mechanically mixing the starting materials is heated at a temperature below 160°C. This rearranges the sulfur-based active material, which has become isolated due to mechanical mixing, near the conductive additive, for example, within the pores of the carbon material. Heating at a temperature below 160°C suppresses the decrease in the amount of sulfur-based active material in the cathode composite material due to vaporization, etc.

[0049] In one embodiment, the weight loss rate before and after heating [(weight loss after heating) × 100 / (weight of the composite before heating)] is preferably 3.0% or less, more preferably 2.0% or less, and particularly preferably 1.0% or less. The lower limit is not limited, but for example, it is 0.1% or more. In one embodiment, the weight loss rate before and after heating (based on sulfur) [(weight loss after heating) × 100 / (weight of sulfur before heating)] is preferably 9.5% or less, more preferably 5.0% or less, and particularly preferably 3.0% or less. The lower limit is not limited, but for example, it is 0.1% or more. In one embodiment, the heating temperature is 30°C to 155°C, 70°C to 150°C, or 100°C to 140°C. Preferably, it is 110°C or higher, for example, 110°C to 135°C or 115°C to 130°C.

[0050] The heating time is 1 second to 100 hours, 1 minute to 20 hours, or 1 hour to 5 hours. Even if a sulfur-based active material-conductive additive composite has not been formed in advance by pretreatment, heating impregnates the pores of the carbon material with sulfur-based active material, thus allowing the sulfur-based active material-conductive additive composite to be formed.

[0051] The atmosphere during heat treatment is not particularly limited and can be an air atmosphere or an inert atmosphere (nitrogen, argon atmosphere), etc. If a sulfide solid electrolyte is included, it is preferable to use an inert atmosphere to suppress hydrolysis. Although the heat treatment can be carried out under atmospheric pressure, it can also be carried out under pressurized or reduced pressure. In the case of pressurized or reduced pressure, the heating conditions can be appropriately adjusted according to the state of the sulfur-based active material used. Furthermore, the heat treatment may be carried out when the positive electrode mixture is in the form of a powder or slurry, or after it has been processed into an electrode sheet.

[0052] The positive electrode composite material obtained by the manufacturing method of the present invention can be suitably used, for example, as a component material for secondary batteries. For example, it can be used as the positive electrode of a lithium-ion battery.

[0053] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0054] [Solid Electrolyte] Production Example 1 0.4127 g of lithium sulfide, 0.6655 g of phosphorus pentasulfide, 0.2137 g of lithium iodide, 0.2080 g of lithium bromide, and 10 zirconia balls with a diameter of 10 mm were placed in a 45 mL zirconia pot and sealed. Using a planetary ball mill (Fritsch, model 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 solid electrolyte A. XRD measurements showed diffraction peaks at 2θ = 20.1°, 23.7°, and 40.9°, confirming that solid electrolyte A is a glass ceramic with a crystal structure similar to thiolysicon region II.

[0055] Manufacturing Example 2: Lithium sulfide (Li) under a nitrogen atmosphere ２ S), diphosphorus pentasulfide (P ２ S ５ ), lithium bromide (LiBr) and lithium chloride (LiCl), in molar ratio (Li ２ S:P ２ S ５ The raw material mixture was prepared by weighing the LiBr:LiCl in a ratio of 47.5:12.5:15.0:25.0 and roughly mixing them. 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. The bead mill was operated at a peripheral speed of 12 m / s and a flow rate of 500 mL / min, the slurry was introduced into the mill, and the mixture was obtained by circulating it for 1 hour.

[0056] After removing the solvent from the obtained mixture, it was heated in an electric furnace at 400-430°C for 2 hours. Then, it was slowly cooled to obtain the raw material sulfide solid electrolyte. Under a nitrogen atmosphere, the raw material sulfide solid electrolyte was dispersed in dehydrated toluene and placed together with 0.3 mm diameter zirconia balls in a zirconia pot of a planetary ball mill (Fritsch: model P-7), creating an inert atmosphere inside the pot. The planetary ball mill was operated at a rotation speed of 150 rpm for 2 hours to obtain a slurry containing the finely particulated sulfide solid electrolyte. The slurry was transferred to a nitrogen-purged Schlenk bottle, dried at room temperature for 1 hour using a vacuum pump, and then heated to 80-100°C to further remove the solvent contained in the finely particulate sulfide solid electrolyte (vacuum drying) to obtain solid electrolyte B.

[0057] XRD measurements revealed diffraction peaks at 2θ = 25.5°, 30.0°, and 31.3°, confirming that solid electrolyte B is an argyrodite-type solid electrolyte.

[0058] Manufacturing example 3 Inside the glove box, Li ２ S 3.8307g, P ２ S ５ 6.1693 g and 600 g of 10 mm diameter zirconia balls were weighed and placed in a 500 mL zirconia pot, and the lid was screwed on. This was set in a planetary ball mill (Fritsch, model P-5) and operated at 220 rpm for 1 hour, followed by a 10-minute rest. This cycle was repeated for 40 times (the rotation direction was reversed after each cycle). The resulting powder was collected in a glove box and sieved through a 53 μm sieve to obtain solid electrolyte C. Solid electrolyte C is Li ３ PS ４ It is an amorphous solid electrolyte having the following composition.

[0059] [Cathode composite material] Example 1 (1) Preparation of composite powder A Activated carbon (MSC-30, manufactured by Kansai Thermal Chemical Co., Ltd.) and sulfur were placed in a glass bottle in a mass ratio of 3:7 and sealed in a SUS tube container. The mixture was heated in an electric furnace at 150°C for 6 hours and at 300°C for 2.75 hours to obtain a composite powder A (S / C composite) of activated carbon and sulfur.

[0060] (2) Preparation of Cathode Composite Material 0.4500 g of composite powder A and 0.4500 g of solid electrolyte A were placed in a 45 mL zirconia pot together with 10 zirconia balls with a diameter of 10 mm and sealed. Using a planetary ball mill (Fritsch, model P-7), the mixture was mechanically mixed at a rotation speed of 370 rpm for 20 hours at room temperature to obtain a powdered composite. 0.5000 g of the obtained composite was weighed into an aluminum foil petri dish (Tokyo Glass Instruments, volume 12 mL) and the powder was leveled so that it was distributed as uniformly as possible on the bottom surface of the petri dish. The aluminum foil petri dish was placed on a hot plate (AS ONE, ND-1A) and double-lidded with two types of stainless steel petri dishes (φ50 mm × height 15 mm, φ60 mm × height 20 mm), and the temperature was raised to 120 °C, after which it was kept at a constant temperature for 2 hours. After heat treatment, the powder was slowly cooled, and the powder on the aluminum foil petri dish was collected to obtain the cathode composite powder.

[0061] Example 2 A positive electrode composite material was obtained in the same manner as in Example 1, except that solid electrolyte B was used instead of solid electrolyte A.

[0062] Comparative Example 1 A positive electrode composite material was obtained in the same manner as in Example 1, except that heating of the composite material (at 120°C for 2 hours) was not performed.

[0063] Comparative Example 2: A cathode composite powder was obtained in the same manner as in Example 2, except that the composite was not heated (at 120°C for 2 hours).

[0064] Example 3 A positive electrode composite was obtained in the same manner as in Example 2, except that the heating temperature after mechanical mixing was changed to 130°C.

[0065] Example 4 0.3150 g of sulfur, 0.1350 g of activated carbon (MSC-30, manufactured by Kansai Thermal Chemical Co., Ltd.), and 0.4500 g of solid electrolyte C were placed in a 45 mL zirconia pot along with 10 zirconia balls with a diameter of 10 mm, and the pot was sealed. Using a planetary ball mill (Fritsch, model P-7), the mixture was mechanically mixed at a rotation speed of 370 rpm for 20 hours at room temperature to obtain a powdered composite. 0.5000 g of the obtained composite was weighed into an aluminum foil petri dish (Tokyo Glass Instruments, volume 12 mL), and the powder was leveled so that it was distributed as uniformly as possible on the bottom surface of the petri dish. An aluminum foil petri dish was placed on a hot plate (AS ONE, ND-1A) and double-lidded with two stainless steel petri dishes (φ50 mm x 15 mm high, φ60 mm x 20 mm high). The temperature was raised to 120°C and then kept constant for 2 hours. After the heat treatment, it was slowly cooled, and the powder on the aluminum foil petri dish was collected to obtain the cathode composite powder.

[0066] Comparative Example 3: A cathode composite material was obtained in the same manner as in Example 4, except that heating of the composite (at 120°C for 2 hours) was not performed.

[0067] Example 5 A positive electrode composite was obtained in the same manner as in Example 4, except that the amount of sulfur was changed to 0.3780 g, the amount of activated carbon to 0.1620 g, and the amount of solid electrolyte C to 0.3600 g.

[0068] Comparative Example 4: A cathode composite material was obtained in the same manner as in Example 5, except that heating of the composite (120°C for 2 hours) was not performed.

[0069] Example 6 A positive electrode composite was obtained in the same manner as in Example 1 (2), except that the amount of composite powder A was changed to 0.6300 g, solid electrolyte C was added instead of solid electrolyte A, and the heating temperature after mechanical mixing was changed to 130°C.

[0070] Comparative Example 5: A positive electrode composite was obtained in the same manner as in Example 6, except that heating of the composite (at 120°C for 2 hours) was not performed.

[0071] [Evaluation] XRD measurements, ion resistance measurements, and electronic conductivity measurements were performed on the cathode composite material. In addition, an all-solid-state lithium-ion battery was fabricated using the cathode composite material, and its rate characteristics were evaluated.

[0072] (1) X-ray diffraction (XRD) measurement: The solid electrolyte was packed into a groove with a diameter of 20 mm and a depth of 0.2 mm, and leveled with glass to prepare the sample. This sample was sealed with Kapton film for XRD and measured without exposure to air. XRD measurements were performed using a powder X-ray diffraction analyzer D2 PHASER from BRUKER Corporation under the following measurement conditions: Tube voltage: 30 kV Tube current: 10 mA X-ray wavelength: Cu-Kα line (1.5418 Å) Optical system: Focusing method Slit configuration: Solar slit 4° (both incident and receiving sides), diverging slit 1 mm, Kβ filter (Ni plate 0.5%), air scatter screen 3 mm Detector: Semiconductor detector Measurement range: 2θ = 10⁻⁶⁰° Step width, scan speed: 0.05°, 0.05° / sec

[0073] (2) Ion resistance measurement (a) Preparation of ion resistance measurement cell 100 mg of solid electrolyte A was pressure-molded in a 10 mm diameter Macol cylinder. 20 mg of positive electrode material powder was added to the pressure side and pressure-molded again. 20 mg of positive electrode material powder was added to the pressure side opposite the positive electrode material and pressurized to prepare an ion resistance measurement cell.

[0074] (b) Ion resistance measurement Using the ion resistance measurement cell prepared according to the procedure described above, AC impedance measurements were performed under the following conditions: • Frequency range: 1 MHz - 1 m Hz • Amplitude: 10 mV

[0075] (3) Measurement of electronic conductivity (a) Preparation of electronic conductivity measurement cell An electronic conductivity measurement cell was prepared by pressure molding 50 mg of the positive electrode composite material prepared by the above procedure in a 6 mm diameter Macol cylinder.

[0076] (b) Electronic Conductivity Measurement Using the electronic conductivity measurement cell prepared according to the procedure described above, the electronic conductivity was measured by the DC polarization method under the following conditions. The average value of the current after a steady state was reached was used to calculate the electronic conductivity. • Applied voltage: 0.01V, 0.02V, 0.03V, 0.04V, 0.05V • Application time: 10 minutes

[0077] (4) Evaluation of battery characteristics (a) Preparation of negative electrode composite Lithium titanate ("LT-112" manufactured by Ishihara Sangyo Co., Ltd.), conductive additive ("Li-100" manufactured by Denka Co., Ltd., powdered acetylene black), and solid electrolyte B were mixed in a mortar in a mass ratio of 60:5:35 for 5 minutes to obtain a negative electrode composite (also called "LTO (lithium titanate) negative electrode composite").

[0078] (b) Fabrication of a lithium-ion battery A solid electrolyte layer (a layer of solid electrolyte A) was formed by placing 100 mg of solid electrolyte A into a 10 mm diameter Macol cylinder and pressurizing it. Next, positive electrode material powder was placed on one pressurized surface of the solid electrolyte layer so that the sulfur content was 3.5 mg, and it was pressurized again. Next, 166 mg of LTO negative electrode material was placed on the other pressurized surface of the solid electrolyte layer (the pressurized surface opposite to the positive electrode), and it was pressurized. A lithium-ion battery was then fabricated by placing a 9 mm diameter, 0.1 mm thick Li foil on top of that and pressurizing it again.

[0079] (c) Charge and Discharge Tests Constant current charge and discharge tests were performed on lithium-ion batteries using the positive electrode composite materials of each example and comparative example. The voltage range for the constant current charge and discharge tests was set to -0.4 to 1.3V, and the current value was set as shown in Table 1 using the C rate determined based on the theoretical capacity of sulfur, 1672 mAh / g. For charging, CC-CV charging was performed, which involves constant current charging followed by constant voltage charging with a termination condition of 0.02C. For discharging, constant current discharge (CC discharge) was performed. The evaluation results are shown in Tables 2 to 4.

[0080]

[0081]

[0082]

[0083]

[0084] Comparing Example 1 with Comparative Example 1, and Example 2 with Comparative Example 2, it was confirmed that when using the same solid electrolyte, heat treatment increased the battery capacity under both low-rate conditions (0.05C) and high-rate conditions (1C). Furthermore, comparing Comparative Example 1 with Comparative Example 2, Comparative Example 2 had a lower capacity, indicating that using solid electrolyte B resulted in inferior battery characteristics under these conditions. However, in Example 2, despite using solid electrolyte B, the capacity at 1C was greater than that of Comparative Example 2. These results are considered to be the effect of heat treatment under appropriate conditions.

[0085] Figure 1 shows the XRD patterns of Example 1 and Comparative Example 1, and Figure 2 shows the XRD patterns of Example 2 and Comparative Example 2. From the XRD measurement results, it can be confirmed that the diffraction pattern hardly changes before and after the heat treatment. This fact suggests that no changes occur in the solid electrolyte itself or the carbon material itself during the heat treatment in the examples.

[0086] Regarding ionic resistance, it can be confirmed that it decreases with heat treatment regardless of the type of solid electrolyte, while electronic conductivity improves with heat treatment. As mentioned above, since the changes in the ionic conductivity of the solid electrolyte itself and the electronic conductivity of the carbon material itself are expected to be small from XRD, it can be estimated that the cause of the change lies in the structure of the positive electrode composite. Specifically, it is estimated that the arrangement of sulfur, which is the active material, has changed. In the composite before heating, there is a large amount of sulfur that has become isolated due to mechanical mixing, and this sulfur does not function as an active material and is thought to be a component that hinders ion and electron conduction in the positive electrode. On the other hand, with heat treatment, the isolated sulfur becomes molten and is rearranged near the carbon material, and as a result the isolation of sulfur improves, so it is thought that the ion and electron conduction in the positive electrode improves.

[0087] [Weight loss rate before and after heating process] Comparative Example 6 A cathode composite powder was obtained in the same manner as in Example 1, except that the heating conditions for the composite were 190°C for 2 hours.

[0088] Comparative Example 7: A cathode composite powder was obtained in the same manner as in Example 1, except that the heating conditions for the composite were changed to 170°C for 2 hours.

[0089] The weight loss rate of the cathode composite materials of Examples 1 to 6 and Comparative Examples 6 and 7 was measured before and after the heating process. The weight loss rate was calculated by weighing the composite material (cathode composite material) before and after heating. The results are shown in Table 5. Weight loss rate (%) = (weight loss) × 100 / (weight of composite material before heating) Weight loss rate (sulfur basis, %) = (weight loss) × 100 / (sulfur weight of composite material before heating)

[0090]

[0091] Table 5 shows that if the heating temperature is too high, the active material, sulfur, will vaporize and be lost; therefore, the heat treatment must be carried out under appropriate conditions. The loss of the active material, sulfur, is undesirable because it reduces the maximum volume that can be obtained as a composite material and causes corrosion of manufacturing equipment (especially metal components) due to the generated sulfur-containing gas.

[0092] The positive electrode composite material obtained by the manufacturing method of the present invention can be suitably used as the positive electrode of a lithium-ion battery. Furthermore, lithium-ion batteries can be suitably used in information-related equipment and communication equipment such as personal computers, video cameras, and mobile phones, as well as batteries used in vehicles such as electric vehicles.

[0093] Although several embodiments and / or examples of the present invention have been described in detail above, those skilled in the art will find it easy to make many modifications to these exemplary embodiments and / or examples without substantially departing from the novel teachings and effects of the present invention. Accordingly, many of these modifications fall within the scope of the present invention. All references to the documents described in this specification and the contents of the application on which the priority claim under the Paris Convention of this application is based are incorporated herein by reference.

Claims

1. A method for producing a positive electrode composite, comprising the steps of: mechanically mixing raw materials containing a conductive additive which is a carbon material, a sulfur-based active material, and a solid electrolyte to form a composite; and heating the composite at a temperature of less than 160°C.

2. The manufacturing method according to claim 1, wherein the sulfur-based active material contains elemental sulfur.

3. The manufacturing method according to claim 1 or 2, wherein the conductive additive is porous carbon.

4. The manufacturing method according to any one of claims 1 to 3, wherein the sulfur-based active material and the conductive additive form a sulfur-based active material-conductive additive composite.

5. A manufacturing method according to any one of claims 1 to 4, comprising the step of pre-compounding the sulfur-based active material and the conductive additive.

6. The manufacturing method according to claim 5, wherein the sulfur-based active material is in a molten state during the compounding step.

7. The manufacturing method according to any one of claims 1 to 6, wherein the temperature of the heating step is 110°C or higher.

8. The manufacturing method according to any one of claims 1 to 6, wherein the temperature of the heating step is 115°C to 130°C.

9. The manufacturing method according to any one of claims 1 to 8, wherein the solid electrolyte includes a sulfide solid electrolyte.

10. The manufacturing method according to claim 9, wherein the composite comprises an amorphous sulfide solid electrolyte.

11. The manufacturing method according to claim 9, wherein the sulfide solid electrolyte includes an argyrodite-type crystal structure.

12. The manufacturing method according to any one of claims 1 to 11, wherein the weight loss rate before and after the heating step is 3.0% or less.

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