Composite, positive electrode for lithium-ion battery, and lithium-ion battery

Abstract

Provided is a composite comprising: electroconductive carbon; elemental sulfur and / or a discharge product of elemental sulfur; and a solid electrolyte, wherein the uniformity of the composite as determined by cross-sectional SEM is 15 or less.

Description

Composite, positive electrode for lithium-ion battery and lithium-ion battery

[0001] The present invention relates to a composite, a positive electrode for a lithium-ion battery, and a lithium-ion battery. Specifically, the present invention relates to a composite, a positive electrode for a lithium-ion battery, and a lithium-ion battery that have excellent initial capacity and improved cycle characteristics, especially when used as a positive electrode for an all-solid-state lithium-ion battery.

[0002] It has been proposed to use a sulfur-activated carbon composite as the positive electrode of an all-solid-state lithium-ion battery. On the other hand, it has also been proposed to reduce internal resistance by using a conductive material that satisfies specific pore volume conditions as the conductive material used for the positive electrode (Patent Document 1).

[0003] Japanese Patent Publication No. 2022-115375

[0004] However, conventional technologies, including those described in Patent Document 1, have room for further improvement in terms of improving the initial capacity and cycle characteristics (capacity retention rate after repeated charging and discharging) of batteries.

[0005] One of the objectives of the present invention is to provide a composite, a lithium-ion battery cathode, and a lithium-ion battery that offer excellent initial capacity and improved cycle characteristics, particularly when used in the cathode of an all-solid-state lithium-ion battery.

[0006] As a result of diligent research, the inventors have discovered that a composite material satisfying specific uniformity can exhibit superior initial capacity and improved cycle characteristics, particularly when used in the positive electrode of an all-solid-state lithium-ion battery, and have completed the present invention. According to the present invention, the following composite materials can be provided: 1. A composite material comprising conductive carbon, at least one of elemental sulfur and the discharge product of elemental sulfur, and a solid electrolyte, wherein the uniformity determined by cross-sectional SEM is 15 or less. 2. The specific surface area of ​​the conductive carbon is 1500 m². ２1. The composite according to 1, wherein the amount is 1 / g or more. 3. The composite according to 1 or 2, wherein the solid electrolyte comprises a sulfide solid electrolyte. 4. The composite according to 3, wherein the sulfide solid electrolyte comprises at least lithium atoms (Li), phosphorus atoms (P), sulfur atoms (S), and halogen atoms. 5. A positive electrode for a lithium-ion battery comprising the composite according to any one of 1 to 4. 6. A lithium-ion battery comprising the positive electrode for a lithium-ion battery according to 5.

[0007] According to the present invention, it is possible to provide a composite, a lithium-ion battery cathode, and a lithium-ion battery that have excellent initial capacity and improved cycle characteristics, especially when used in the cathode of an all-solid-state lithium-ion battery.

[0008] This figure illustrates the calculation of the "area ratio of each brightness region by brightness distribution analysis" using secondary electron image analysis of scanning electron microscope (SEM) images in an example of a positive electrode composite material (Comparative Example 2). Here, (a) is a secondary electron image, (b) is a cropped image, and (c) is an example of brightness distribution data processing.

[0009] The composite, positive electrode for lithium-ion battery, and lithium-ion battery of 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 arbitrarily. 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.

[0010] 1. Composite A composite according to one aspect of the present invention comprises conductive carbon, at least one of elemental sulfur and the discharge product of elemental sulfur, and a solid electrolyte, wherein the uniformity determined by cross-sectional SEM is 15 or less. The composite according to this aspect, in which sulfur, activated carbon, and solid electrolyte are mixed to satisfy a specific uniformity, has the effect of improving initial capacity and cycle characteristics, especially when used in the positive electrode of an all-solid-state lithium-ion battery.

[0011] "Uniformity determined by cross-sectional SEM" (hereinafter also simply referred to as "uniformity") is determined by the method described in the examples. In one embodiment, the smaller the uniformity, the more preferable it is, and it is 15 or less, 14 or less, 13 or less, 12 or less, 11 or less, 10 or less, 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, 3 or less, or 2 or less. The lower limit is not particularly limited and can be, for example, 0.1 or more.

[0012] (Conductive carbon) Examples of the conductive carbon include activated carbon, carbon black, and the like. In one embodiment, the conductive carbon has pores.

[0013] In one embodiment, the specific surface area of the conductive carbon is 1500 m ２ / g or more, 1600 m ２ / g or more, 1700 m ２ / g or more, 1800 m ２ / g or more, 1900 m ２ / g or more, 2000 m ２ / g or more, 2100 m ２ / g or more, 2200 m ２ / g or more, 2300 m ２ / g or more, 2400 m ２ / g or more, or 2500 m ２ / g or more. The upper limit is not particularly limited and can be, for example, 4000 m ２ / g or less, 3500 m ２ / g or less, 3300 m ２ / g or less, or 3000 m ２ / g or less. The larger the specific surface area of the conductive carbon, the more preferable it is, particularly 1500 m ２ / g or more. Thereby, the capacity (particularly the initial capacity) can be further improved. Note that the specific surface area of the conductive carbon is a value measured by the method described in the examples.

[0014] (Elemental sulfur and discharge products of elemental sulfur) Elemental sulfur (sulfur) is not particularly limited, but preferably has a purity of 95% by mass or more, more preferably 96% by mass or more, and particularly preferably 97% by mass or more. Examples of crystalline systems of elemental sulfur include α-sulfur (orthorhombic), β-sulfur (monoclinic), γ-sulfur (monoclinic), amorphous sulfur, etc. These may be used individually or in combination of two or more. Elemental sulfur becomes a melt when heated.

[0015] During a battery reaction, some or all of elemental sulfur is converted into discharge products. Therefore, in one embodiment of the composite, discharge products of elemental sulfur are present. When discharge products are present, the amount of sulfur in the composite is the sum of the amount of elemental sulfur and the amount of sulfur in the discharge products. As for the discharge products of elemental sulfur, Li in a completely discharged state... ２ Li as S and its intermediate stage lithium polysulfide ２ S ２ Li ２ S ４ Li ２ S ６ Li ２ S ８ These are some examples.

[0016] In one embodiment, in the composite, some or all of elemental sulfur is attached (impregnated) into the pores of conductive carbon. Furthermore, elemental sulfur that is not impregnated into the pores exists to cover some or all of the conductive carbon. Whether sulfur is impregnated into the pores of conductive carbon can be confirmed by analyzing the particle cross-section of the conductive carbon using an element-mapping analytical method such as SEM-EDS or TEM-EDX, and evaluating the overlap of elements derived from conductive carbon and sulfur elements. In one embodiment, due to a high elemental sulfur content in the composite, elemental sulfur is also present outside the pores of the conductive carbon. In this case, the composite may be a pellet-like mass, but it can be pulverized by mechanical crushing.

[0017] In one embodiment, the composite contains 200 parts by mass or more, 210 parts by mass or more, 220 parts by mass or more, or 230 parts by mass or more of elemental sulfur and discharge products of elemental sulfur in terms of sulfur per 100 parts by mass of conductive carbon. The upper limit is not particularly limited and may be, for example, 600 parts by mass or less, 500 parts by mass or less, 400 parts by mass or less, or 300 parts by mass or less. By having 200 parts by mass or more, a sufficient sulfur content is ensured, and the energy density can be further improved.

[0018] (Sulfide Solid Electrolytes) A ​​sulfide solid electrolyte is a solid electrolyte that contains at least sulfur atoms and exhibits ionic conductivity due to the contained metal atoms, preferably containing lithium atoms and phosphorus atoms in addition to sulfur atoms, and more preferably containing lithium atoms, phosphorus atoms and halogen atoms, and having ionic conductivity due to lithium atoms. In one embodiment, the solid electrolyte contains at least lithium atoms (Li), phosphorus atoms (P), sulfur atoms (S), and halogen atoms. In one embodiment, the solid electrolyte contains lithium atoms, phosphorus atoms, sulfur atoms, bromine atoms, and iodine atoms. The sulfide solid electrolyte may be an amorphous sulfide solid electrolyte or a crystalline sulfide solid electrolyte.

[0019] (Amorphous sulfide solid electrolyte) As an amorphous sulfide solid electrolyte, any material that contains at least sulfur atoms and exhibits ionic conductivity due to the contained metal atoms can be used without particular restrictions. Typical examples include, for example, 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.

[0020] 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 30-85:15-70, more preferably 40-80:20-60, and even more preferably 45-78:22-55, 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 ５In the case of -LiI-LiBr, the total content of lithium sulfide and phosphorus pentasulfide is preferably 30 to 95 mol%, more preferably 35 to 90 mol%, and even more preferably 40 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%.

[0021] Furthermore, there are no particular restrictions on the shape of the amorphous sulfide solid electrolyte, but for example, particulate form can be given. The average particle size (D) of the particulate amorphous sulfide solid electrolyte. ５０ For example, the average particle size (D) can be exemplified by being in the range of 0.01 μm to 500 μm or 0.1 to 200 μm. In this specification, the average particle size (D) ５０ The volume distribution is the particle size at which the accumulation of particle diameters reaches 50% of the total when plotting a particle diameter distribution integral curve, starting from the smallest particle. The volume distribution is the average particle size, which can be measured, for example, using a laser diffraction / scattering particle diameter distribution analyzer.

[0022] (Crystalline sulfide solid electrolyte) As a crystalline sulfide solid electrolyte, for example, it may be a so-called glass ceramic obtained by heating the above amorphous sulfide solid electrolyte above the crystallization temperature, and a sulfide solid electrolyte having the following crystal structure can be used. The crystal structure that a crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms and phosphorus atoms may have is 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 Examples include crystal structures, and crystal structures having peaks near 2θ = 20.2° and 23.6° (for example, Japanese Patent Publication No. 2013-16423).

[0023] Furthermore, the crystal structure that a crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms may have is as described above. ４－ｘ Ge１－ｘ P ｘ S ４ is a thio-LISICON Region II (thio-LISICON Region II) type crystal structure, Li ４－ｘ Ge １－ｘ P ｘ S ４ Examples of crystal structures similar to the thio-LISICON Region II (thio-LISICON Region II) type include

[0024] In the X-ray diffraction measurement using CuKα rays, Li ３ PS ４ Diffraction peaks of the crystal structure appear, for example, around 2θ = 17.5°, 18.3°, 26.1°, 27.3°, 30.0°, Li ４ P ２ S ６ Diffraction peaks of the crystal structure appear, for example, around 2θ = 16.9°, 27.1°, 32.5°, Li ７ PS ６ Diffraction peaks of the crystal structure appear, for example, around 2θ = 15.3°, 25.2°, 29.6°, 31.0°, Li ７ P ３ S １１ Diffraction peaks of the crystal structure appear, for example, around 2θ = 17.8°, 18.5°, 19.7°, 21.8°, 23.7°, 25.9°, 29.6°, 30.0°, Li ４－ｘ Ge １－ｘ P ｘ S ４ Diffraction peaks of the thio-LISICON Region II (thio-LISICON Region II) type crystal structure appear, for example, around 2θ = 20.1°, 23.9°, 29.5°, Li ４－ｘ Ge １－ｘ P ｘ S ４ Diffraction peaks of crystal structures similar to the thio-LISICON Region II (thio-LISICON Region II) type appear, for example, around 2θ = 20.2°, 23.6°. Note that these peak positions may shift within a range of ±0.5°.

[0025] 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 include a crystal structure represented by (Ha is Cl or Br, and x is preferably 0.2 to 1.8).

[0026] 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.

[0027] 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, examples include the ranges of 0.01 μm to 500 μm and 0.1 to 200 μm.

[0028] In one embodiment, the content of the sulfide solid electrolyte in the composite is 50 parts by mass or more, 60 parts by mass or more, 70 parts by mass or more, or 80 parts by mass or more, and also 300 parts by mass or less, 250 parts by mass or less, 200 parts by mass or less, 150 parts by mass or less, or 130 parts by mass or less, based on 100 parts by mass of the total of conductive carbon, elemental sulfur, and discharge products of elemental sulfur (in terms of sulfur).

[0029] In one embodiment, 80% or more by mass of the composite is 90% or more by mass, 95% or more by mass, 98% or more by mass, 99% or more by mass, 99.5% or more by mass, 99.8% or more by mass, 99.9% or more by mass, or substantially 100% or more by mass. This composite consists of at least one of conductive carbon, elemental sulfur, and discharge products of elemental sulfur, as well as a sulfide solid electrolyte. In the case of "substantially 100% or more by mass," unavoidable impurities may be included.

[0030] The composite according to one aspect of the present invention described above can be manufactured by the method described below. First, conductive carbon and sulfur (S) are placed in a stirring and heating mixing apparatus, nitrogen gas is circulated through the apparatus to create a nitrogen atmosphere, and then the mixture is stirred under heating to obtain a composite powder of activated carbon and sulfur. Next, the obtained composite powder and a solid electrolyte are placed in a zirconia pot together with zirconia balls, sealed, and pulverized using a planetary ball mill apparatus for an extremely long time (processing time). For example, the processing time is 20 hours or more if the rotation speed (rotational speed) is 370 rpm, and 90 hours or more if the rotation speed (rotational speed) is 180 rpm. In this way, a composite according to one aspect of the present invention (activated carbon-sulfur-solid electrolyte composite) is obtained. Conventionally, from the viewpoint of composite energy, the processing conditions of a ball mill, such as 20 hours or more at 370 rpm (or 90 hours or more at 180 rpm), were considered unnecessary (excessive), and it was thought that a uniform composite could be obtained in a shorter time. In response to this, the inventors conducted thorough research and found that by performing the treatment for a long period of time, improving the method of compounding sulfur and conductive carbon, and devising the type of conductive carbon to create a more uniform structure, the durability when repeatedly charging and discharging can be significantly improved.

[0031] 2. Positive electrode for lithium-ion battery and lithium-ion battery A positive electrode for a lithium-ion battery according to one aspect of the present invention includes a composite according to one aspect of the present invention. According to the positive electrode for a lithium-ion battery of this aspect, when used as the positive electrode of a lithium-ion battery, particularly an all-solid-state lithium-ion battery, it can have excellent initial capacity and improved cycle characteristics.

[0032] A lithium-ion battery according to one aspect of the present invention includes a positive electrode for a lithium-ion battery according to one aspect of the present invention. According to this aspect of the lithium-ion battery, excellent initial capacity and improved cycle characteristics can be achieved.

[0033] A composite according to one aspect of the present invention can be used, for example, as a positive electrode composite material, and more specifically, it can be suitably used as a positive electrode layer in a lithium-ion battery. In this case, other components of the lithium-ion battery can be those known in the art, and a negative electrode layer that does not contain lithium ions can be selected for the negative electrode active material. Furthermore, the negative electrode active material contained in the negative electrode layer of a lithium-ion battery can be a "negative electrode active material containing lithium ions." In addition, the negative electrode active material contained in the negative electrode layer of a lithium-ion battery may be a "negative electrode active material that supplies lithium ions to the positive electrode."

[0034] The negative electrode of a lithium-ion battery is not particularly limited as long as it can be used in a normal battery. The negative electrode may consist of a negative electrode mixture, which is a mixture of a negative electrode active material and a solid electrolyte.

[0035] Commercially available materials can be used as the negative electrode active material. For example, carbon materials, Sn metal, In metal, Si metal, Li metal, and alloys of these metals can be used. Specifically, natural graphite, various types of graphite, lithium titanate, metal powders such as Si, Sn, Al, Sb, Zn, Bi, SiAl, Sn ５ Cu ６ , Sn ２ Co, Sn ２ Examples include metal alloys such as Fe, as well as other amorphous alloys and plated alloys. There are no particular restrictions on particle size, but alloys with an average particle size of several μm to 80 μm are preferably used.

[0036] The electrolyte layer is not particularly limited, and known electrolytes can be used. For example, oxide solid electrolytes, sulfide solid electrolytes, and polymer-based electrolytes are preferred, with sulfide solid electrolytes being more preferred from the viewpoint of ionic conductivity. The sulfide solid electrolyte used is preferably the same as that used in the positive electrode composite material described above.

[0037] The method for manufacturing a lithium-ion battery is not particularly limited. For example, one method involves forming a solid electrolyte layer on a sheet in which a positive electrode layer made of a composite according to one aspect of the present invention is formed on a positive electrode current collector, laminating a sheet in which a negative electrode layer is formed on a pre-formed negative electrode current collector, and then pressing it.

[0038] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments. In the following embodiments and comparative examples, the handling of substances that degrade by reacting with air and moisture, such as sulfide solid electrolytes, and the evaluation of batteries were carried out using glove boxes or sealed containers under an argon atmosphere. Unless otherwise specified, substances such as sulfur and activated carbon were pre-dried at a temperature that does not alter the substance before being introduced into the glove box and handled.

[0039] (Example 1) "Preparation of composite powder" Activated carbon-1 (MSC30 manufactured by Kansai Thermal Chemical Co., Ltd., specific surface area: 2992 m²) ２ A composite powder A of activated carbon and sulfur was obtained by placing conductive carbon (in this case, activated carbon) and sulfur (S) in a mass ratio of 3:7 into a stirring and heating mixing apparatus (an apparatus configured to be able to be heated and stirred under a specific atmosphere such as a nitrogen atmosphere), circulating nitrogen gas through the apparatus to create a nitrogen atmosphere, and then heating at 150°C for 3 hours while stirring. The specific surface area of ​​conductive carbon (in this case, activated carbon) can be measured by the Brenauer-Emmet-Telle (BET) method. Specifically, it can be determined using a nitrogen adsorption isotherm obtained by adsorbing nitrogen gas onto the sample at liquid nitrogen temperature. As a measuring device, for example, it can be measured using a specific surface area and pore distribution analyzer (Autosorb-3) manufactured by Quantachrome.

[0040] "Preparation of Solid Electrolyte" 0.4398 g of lithium sulfide, 0.7084 g of phosphorus pentasulfide, 0.2133 g of lithium iodide, and 0.1384 g of lithium bromide, along with 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 a solid electrolyte.

[0041] "Preparation of Cathode Compound Powder" 0.5 g of composite powder A and 0.5 g of solid electrolyte 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 ground at a rotation speed of 370 rpm for 20 hours at room temperature (25°C) to obtain a cathode compound (activated carbon-sulfur-solid electrolyte composite) powder.

[0042] (Comparative Example 1) "Preparation of Composite Powder" Activated Carbon-1 (MSC30 manufactured by Kansai Thermal Chemical Co., Ltd., specific surface area: 2992 m²) ２ A mixture of activated carbon ( / g) and sulfur (S) was placed in a mass ratio of 3:7 and sealed in a stainless steel tube container. It was heated in an electric furnace at 150°C for 6 hours and then at 300°C for 2.75 hours to obtain a composite powder B of activated carbon and sulfur.

[0043] "Preparation of Cathode Compound Powder" A cathode compound was obtained in the same manner as in Example 1, except that composite powder B was used instead of composite powder A.

[0044] (Example 2) In Example 1, when preparing the cathode composite material, 16.5 g of composite powder, 16.5 g of solid electrolyte, and 590 g of zirconia balls with a diameter of 5 mm were placed in a 500 mL zirconia pot and sealed. Cathode composite powder was obtained in the same manner as in Example 1, except that the mixture was ground using a planetary ball mill (Fritsch, model P-5) at a rotation speed of 180 rpm for 90 hours at room temperature.

[0045] (Example 3) "Preparation of conductive carbon" Carbon black-1 (Lion Specialty Chemicals "EC-600JD", specific surface area: 1357 m²) ２ 500g of carbon black (1 / g) was placed in a rotary kiln with an internal volume of 32L, and the furnace was evacuated and replaced with nitrogen. Next, while circulating nitrogen at 10L / min, the temperature was raised to 950°C at a rate of 10°C / min, and then further raised to 1000°C at a rate of 5°C / min, followed by heat treatment at 1000°C for 1 hour. Next, the gas circulating in the furnace was switched to carbon dioxide at a rate of 10L / min, and the material was activated at 1000°C for 6 hours. The specific surface area of ​​the obtained carbon black-2 was 1890m². ２ It was / g.

[0046] "Preparation of Composite Powder C" Carbon black-2 and sulfur (S) were placed in a mass ratio of 3:7 and sealed in a stainless steel tube 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 a composite powder C of activated carbon and sulfur.

[0047] "Preparation of Cathode Compound Powder" A cathode compound was obtained in the same manner as in Example 1, except that composite powder C was used instead of composite powder A.

[0048] (Comparative Example 2) In the "Preparation of Cathode Compound Powder" of Example 1, 16.5 g of composite powder A and 16.5 g of solid electrolyte were placed together with 590 g of zirconia balls with a diameter of 5 mm in a 500 mL zirconia pot and sealed. Cathode compound powder was obtained in the same manner as in Example 1, except that it was ground using a planetary ball mill (Fritsch, model P-5) at a rotation speed of 180 rpm for 30 hours at room temperature.

[0049] 2. Measurement and Test Methods The following measurements and tests were performed on the composites (cathode composite powders) obtained in the examples and comparative examples.

[0050] "Uniformity" (1) Calculation of "area ratio of each brightness region" by secondary electron image analysis of scanning electron microscope (SEM) images (1-1) Preparation of positive electrode composite pellets 50 mg of positive electrode composite powder was placed in a 10 mm diameter Macol cylinder and pressurized to 185 MPa in terms of surface pressure. The pressurized positive electrode composite powder (sample) was then flipped over and pressurized again at 185 MPa. Next, the sample was flipped over again and pressed at 185 MPa, 370 MPa, and 370 MPa at each rotation position while rotating it 120° in the direction along the circumference of the cylinder. Next, the sample was flipped over again and pressed at 185 MPa, 370 MPa, and 370 MPa at each rotation position while rotating it 120° in the direction along the circumference of the cylinder. In this way, the positive electrode composite powder was formed and positive electrode composite pellets were obtained.

[0051] (1-2) Observation of secondary electron image with SEM The cathode composite pellet obtained was split vertically, and the exposed surface (cut surface) was subjected to ion milling (Hitachi High-Tech Corporation, IM4000) to obtain a cross-section of the cathode composite pellet. A secondary electron image was obtained from the obtained cross-section using an SEM (Hitachi High-Tech Corporation, SU8200). The observation magnification was 5000x, the acceleration voltage was 2kV, the emission was 10μA, the probe current was High, the condenser lens was 5, the W.D (working distance) was 2mm, and the scan area was 1280×960 [pixels].

[0052] (1-3) Area ratio of each luminance region by luminance distribution analysis ImageJ (1.53k) was used for image processing obtained by the above procedure. First, the image was loaded in grayscale, the lower part of the image (the black caption area) was removed (cropped to 1280 x 896 [pixels]), and only the positive electrode composite observation area was extracted. A luminance distribution of 256 levels (0 to 255) was output from the obtained image. The obtained luminance distribution is denoted as F(x), and the luminance as x. The luminance and luminance intensity when F(x) is at its maximum (this can also be described as "the luminance peak containing both light and heavy element components as the main peak by SEM-EDS, and the luminance and luminance intensity when this main peak is at its maximum") were denoted as A and F(A), respectively. The luminance width of two points where the luminance intensity is half from F(A) to the low luminance side and half to the high luminance side was denoted as W. The reference B on the low luminance side and the reference C on the high luminance side were as follows. B = A - W C = A + W Let S be the total value of the total brightness intensity. The area ratio SL on the low brightness side, the area ratio SM of the main peak, and the area ratio SH on the high brightness side were defined as follows: S = ΣF(x) (0 ≤ x ≤ 255) SL (%) = ΣF(x) / S × 100 (0 ≤ x ≤ B) SM (%) = ΣF(x) / S × 100 (B + 1 ≤ x ≤ C) SH (%) = ΣF(x) / S × 100 (C + 1 ≤ x ≤ 255) The area ratio SL on the low brightness side mainly consists of components containing relatively more light elements and voids. The area ratio SH on the high brightness side mainly consists of components containing relatively more heavy elements and edge scattering. Two fields of view were observed for each sample, and W, SL, SM, and SH were calculated for each image. The mean and standard deviation of the two fields of view were then calculated. The brightness distribution of SEM secondary electron images is affected by surface irregularities and contamination, but these effects can be significantly reduced by creating a smooth surface through ion milling, allowing for a brightness distribution that reflects compositional information. Here, the average value of the area ratio SL of low-brightness regions is defined as "uniformity." A smaller value indicates a more uniformly mixed state.

[0053] Referring to Figure 1, an example of uniformity measurement will be explained. Figure 1 is a diagram illustrating the calculation of the "area ratio of each brightness region by brightness distribution analysis" by secondary electron image analysis of scanning electron microscope (SEM) images in an example of a positive electrode composite material (Comparative Example 2). Here, (a) is the secondary electron image, (b) is a cropped image, and (c) is an example of brightness distribution data processing.

[0054] "Evaluation of Battery Characteristics" (Initial Capacity and Cycle Characteristics (Capacity Retention Rate)) Using the composites obtained in each of the Examples and Comparative Examples, all-solid-state batteries were prepared by the following method. ・Preparation of Lithium-ion Battery (All-Solid) 100 mg of the solid electrolyte prepared above was placed in an electrically insulating ceramic cylinder with a diameter of 10 mm and pressurized. On the pressurized surface, the positive electrode composite powder prepared above was placed so that the sulfur content was 3.5 mg, and it was pressurized again. On the pressurized surface opposite the positive electrode composite, lithium titanate (Ishihara Sangyo Co., Ltd. "LT-112") and a conductive additive (Denka Co., Ltd. "Li-100", powdered acetylene black) and Li ２ S-P ２ S ５ A solid-state battery was fabricated by adding 166 mg of a negative electrode mixture (also called "LTO (lithium titanate) negative electrode mixture") prepared by mixing a LiCl-LiBr type solid electrolyte B with LiCl-LiBr in a mass ratio of 60:5:35 in a mortar for 5 minutes, then pressurizing the mixture, and finally adding lithium foil and pressurizing again. Subsequently, a constant current charge-discharge test was performed on the obtained solid-state battery. The cutoff potential for the constant current test was set to -0.4 to +1.3 (V vs. Li-LTO), and the current density was set to the conditions shown in Table 1 below.

[0055]

[0056] Table 2 shows (a) the discharge capacity (initial capacity) at the 4th cycle (current density during discharge: 0.586 mA) and (b) the capacity retention rate (%) obtained by dividing the discharge capacity at the 30th cycle by the discharge capacity at the 10th cycle for the examples and comparative examples. Note that the initial capacity was calculated as the value per gram of sulfur. Furthermore, under these test conditions, a capacity retention rate of 96% or higher is preferable.

[0057]

[0058] <Evaluation> Comparing the characteristics of the batteries in the example and the comparative example, it was found that the battery in the example had good battery characteristics (initial capacity and cycle characteristics (capacity retention rate)). Although Comparative Example 1 showed a higher value than Example 3 when focusing only on initial capacity, its capacity retention rate was low, so there is room for improvement in terms of slowing down degradation.

[0059] (Example 4) "Preparation of Cathode Compound Powder" 0.7 g of composite powder A and 0.3 g of solid electrolyte 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 ground at a rotation speed of 370 rpm for 100 hours at room temperature (25°C) to obtain a cathode compound (activated carbon-sulfur-solid electrolyte composite) powder.

[0060] (Comparative Example 3) "Preparation of Cathode Compound Powder" 0.7 g of composite powder A and 0.3 g of solid electrolyte 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 ground at a rotation speed of 370 rpm for 20 hours at room temperature (25°C) to obtain a cathode compound (activated carbon-sulfur-solid electrolyte composite) powder.

[0061] The composites (cathode composite powders) obtained in Example 4 and Comparative Example 3 were subjected to the same measurements and tests as in Examples 1-3 and Comparative Examples 1 and 2. Table 3 shows the results for uniformity, initial volume, and volume retention rate.

[0062]

[0063] <Evaluation> In order to make it easier to observe the effect of "uniformity," in Example 4 and Comparative Example 3, the proportion of sulfur in the positive electrode composite was increased compared to Examples 1-3, thereby increasing the expansion and contraction of sulfur during charging and discharging, and setting conditions that made it difficult to maintain a high capacity retention rate (consequently, the initial capacity calculated as the value per gram of sulfur tends to be lower than in Examples 1-3). Even under such harsh conditions, it was found that Example 4, with its enhanced uniformity, could maintain a high capacity retention rate. Here again, comparing the characteristics of the batteries in the examples and comparative examples, it was found that the battery in the examples had good battery characteristics (initial capacity and cycle characteristics (capacity retention rate)).

[0064] 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

conductive carbon and At least one of elemental sulfur and a discharge product of elemental sulfur, Solid electrolyte and Includes, A composite material whose uniformity, as determined by cross-sectional SEM, is 15 or less. 　 The specific surface area of ​​the conductive carbon is 1500 m². ２ The composite according to claim 1, wherein the amount is 1 / g or more. 　 The composite according to claim 1 or 2, wherein the solid electrolyte includes a sulfide solid electrolyte. 　 The composite according to claim 3, wherein the sulfide solid electrolyte comprises at least lithium atoms (Li), phosphorus atoms (P), sulfur atoms (S), and halogen atoms. 　 A positive electrode for a lithium-ion battery comprising the composite according to any one of claims 1 to 4. 　 A lithium-ion battery comprising the positive electrode for a lithium-ion battery described in claim 5.

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