Cathode composite material and lithium-ion battery

Abstract

To provide a positive electrode mixture material which can increase rate characteristics of a lithium ion battery.SOLUTION: A positive mixture material includes: a sulfide solid electrolyte; a material A having electronic conductivity with an average small hole diameter of 10 nm or smaller than 10 nm; a material B as conductive carbon for which a ratio (G / D) of an area of a G-band and an area of a D-band in Laman measurement is at least 0.6; and at least one of elemental sulfur and a discharge product of the elemental sulfur in at least a part of the small holes of the material A. A ratio [S / (A+B)] of total mass (S) of the sulfur of the elemental sulfur and the discharge product of the elemental sulfur to total mass (A+B) of the materials A and B is at least 2.5.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a positive electrode composite material and a lithium-ion battery. [Background technology]

[0002] Regarding the electrical capacity of all-solid-state lithium-ion batteries, methods using sulfur as the positive electrode are being considered due to its high theoretical capacity. However, sulfur has low electronic and lithium-ion conductivity, resulting in a problem of low discharge capacity at high current densities (rate characteristics).

[0003] To address the above issues, sulfur cathode composites, which are formed by impregnating activated carbon with sulfur and then compounding it with a solid electrolyte, have been investigated (see, for example, Patent Documents 1 and 2). In addition, cathode composites in which fibrous conductive additives such as vapor-grown carbon fibers (VGCF) are mixed with sulfur-containing compounds have been disclosed (see, for example, Patent Document 3). However, further improvement in the rate characteristics of lithium-ion batteries is required. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6243103 [Patent Document 2] Japanese Patent Publication No. 2020-161288 [Patent Document 3] Japanese Patent Publication No. 2021-26838 [Overview of the project] [Problems that the invention aims to solve]

[0005] The objective of the present invention is to provide a cathode composite material that can improve the rate characteristics of lithium-ion batteries. [Means for solving the problem]

[0006] According to the present invention, the following positive electrode composite materials are provided. 1. A sulfide solid electrolyte, Material A having electronic conductivity with an average pore radius of 10 nm or less, Material B which is conductive carbon having a ratio (G / D) of the area of the G band to the area of the D band in Raman measurement of 0.6 or more, including at least one of elemental sulfur and discharge products of elemental sulfur present in at least a part of the pores of the material A, A positive electrode composite material in which the ratio [S / (A + B)] of the total mass (S) of sulfur of the elemental sulfur and the discharge products of elemental sulfur to the total mass (A + B) of the materials A and B is 2.5 or more. 2. The positive electrode composite material according to 1, wherein the ratio (A:B:S) of the material A, the material B, and the total mass of sulfur of the elemental sulfur and the discharge products of elemental sulfur is from 5 to 15:1 to 10:30 to 50. 3. The positive electrode composite material according to 1 or 2, wherein the average pore radius of the material A is less than 5 nm. 4. The positive electrode composite material according to any one of 1 to 3, wherein the material A is a porous carbon material. 5. The positive electrode composite material according to any one of 1 to 4, wherein the micropore volume of the material B is 0.1 cc / g or less. 6. The positive electrode composite material according to any one of 1 to 5, wherein the material B is carbon fiber. 7. The positive electrode composite material according to any one of 1 to 6, wherein the ratio (G / D) of the area of the G band to the area of the D band in Raman measurement of the material A is less than 0.6. 8. The positive electrode composite material according to any one of 1 to 7, wherein the total pore volume of the material A and the material B is 1.0 cc / g or more. 9. The positive electrode composite material according to any one of 1 to 8, wherein the total content of sulfur of the elemental sulfur and the discharge products of elemental sulfur is 1.0 cc or more with respect to 1 g of the total mass of the material A and the material B. 10. The positive electrode composite material according to any one of 1 to 9, wherein the total content of sulfur of the elemental sulfur and the discharge products of elemental sulfur is 70% or more of the total pore volume of the material A and the material B. 11. A positive electrode for a lithium ion battery, including the positive electrode composite material according to any one of 1 to 10. A lithium-ion battery, including the positive electrode for lithium-ion batteries described in 12.11. 13. A step of heating an electronically conductive material A having an average pore radius of 10 nm or less and elemental sulfur at a temperature above the melting point of the elemental sulfur to obtain a material A-elemental sulfur composite powder, A method for producing a cathode composite, comprising the step of mixing the aforementioned material A - elemental sulfur composite powder, material B which is conductive carbon with a G-band area / D-band area of ​​0.6 or more in Raman measurement, and a sulfide solid electrolyte to produce a cathode composite. [Effects of the Invention]

[0007] According to the present invention, a cathode composite material that can improve the rate characteristics of lithium-ion batteries can be provided. [Brief explanation of the drawing]

[0008] [Figure 1] This is an SEM image of the cross-section of the cathode composite material prepared in Example 1. [Modes for carrying out the invention]

[0009] 1. Positive electrode composite material A positive electrode composite material according to one embodiment of the present invention comprises a sulfide solid electrolyte, an electronically conductive material A having an average pore radius of 10 nm or less, and a conductive carbon material B having a ratio of the area of ​​the G band to the area of ​​the D band in Raman measurement (G / D) of 0.6 or more. It also comprises at least one of elemental sulfur and elemental sulfur discharge products present in at least a portion of the pores of material A, and the ratio of the total mass of elemental sulfur and elemental sulfur discharge products in terms of sulfur (S / A+B) to the total mass of materials A and B (A+B) is 2.5 or more.

[0010] In this embodiment, material A is presumed to have the function of smoothly supplying electrons and ions by retaining elemental sulfur and its discharge products, and by keeping their particle sizes small. On the other hand, material B is presumed to contribute to improving rate characteristics by facilitating electron conduction to the entire positive electrode due to its high electron conductivity resulting from its high degree of graphitization.

[0011] Furthermore, in this embodiment, the material A contains elemental sulfur or discharge products of elemental sulfur present in a portion of the pores, and the ratio of the total mass of elemental sulfur and discharge products of elemental sulfur in sulfur equivalent (S / A+B) to the total mass of materials A and B (A+B) is 2.5 or more. This requirement means that the amount of elemental sulfur contained in the positive electrode composite is large, resulting in a battery with high energy density.

[0012] In one embodiment, the aspect ratio of material B is 5 or greater. By using two materials A and B with different shapes, even with the addition of small amounts of materials A and B, it is possible to uniformly impart electron conductivity to sulfur, which does not possess electron conductivity or ionic conductivity. Furthermore, since the sulfur can be maintained in a finely atomized state that facilitates the supply of lithium ions, the rate characteristics of the battery can be improved while increasing the sulfur content in the positive electrode composite material.

[0013] Material A has pores, so it is presumed that elemental sulfur can be highly dispersed. On the other hand, since elemental sulfur is an insulator, a high content will inhibit electron transfer within the positive electrode. If the aspect ratio of material B is large, electron transfer over a longer distance can be achieved compared to material A, even with a high elemental sulfur content. From these effects, it is presumed that the rate characteristics will be improved in a lithium-ion battery using the positive electrode composite material of this embodiment as the positive electrode. The components of this embodiment will be described below.

[0014] [Material A] Material A is not particularly limited as long as it is an electronically conductive material with pores having an average pore radius of 10 nm or less. In one embodiment, the average pore radius of the pores in material A is preferably 7 nm or less, more preferably less than 5 nm, and even more preferably 4 nm or less. A smaller average pore radius tends to result in a greater improvement in rate characteristics. There is no particular lower limit to the average pore radius of the pores in material A, but it is usually 0.1 nm or more.

[0015] In one embodiment, material A is preferably a porous carbon material. Examples of porous carbon materials include carbon black such as Ketjenbrak®, activated carbon, and Knobel®. Among these, activated carbon is preferred. These may be used individually or in combination of two or more.

[0016] In one embodiment, the pore volume of material A is preferably 1.0 cc (mL) / g or more. This increases the amount of sulfur that can be retained, leading to an improvement in the energy density of the battery. The pore volume of material A is more preferably 1.5 cc / g or more, and even more preferably 2.0 cc / g or more. The pore volume of electronically conductive material A is usually 10 cc / g or less.

[0017] In this invention, the average pore radius and pore volume can be measured by the Brenauer-Emmet-Telle (BET) method or the BJH method (Barrett-Joyner-Halenda method). Specifically, this can be determined using a nitrogen adsorption isotherm obtained by adsorbing nitrogen gas onto an electronically conductive material A at liquid nitrogen temperature. For example, the specific surface area and pore size distribution can be measured using a Quantacrome Autosorb-3 measuring device.

[0018] [Elemental sulfur] There are no particular limitations on elemental sulfur (sulfur), but it is preferably 95% by mass or higher in purity, more preferably 96% by mass or higher, and particularly preferably 97% by mass or higher. Examples of elemental sulfur crystal systems include α-sulfur (orthorhombic), β-sulfur (monoclinic), γ-sulfur (monoclinic), and amorphous sulfur. These may be used individually or in combination of two or more. Elemental sulfur becomes a melt when heated.

[0019] During a battery reaction, some or all of elemental sulfur is converted into discharge products. Therefore, in one embodiment, discharge products of elemental sulfur are present in the positive electrode mixture (positive electrode). When discharge products are present, the amount of sulfur contained in the positive electrode mixture is the sum of the amount of elemental sulfur and the amount of sulfur contained in the discharge products. Examples of discharge products of elemental sulfur include Li2S, which is in a fully discharged state, and intermediate lithium polysulfides such as Li2S2, Li2S4, Li2S6, and Li2S8.

[0020] In the cathode composite material, some or all of elemental sulfur is impregnated into the pores of material A. Elemental sulfur that is not impregnated into these pores exists to cover some or all of materials A and B. Whether sulfur is impregnated into the pores of material A can be confirmed by analyzing the particle cross-section of material A using an element-mapping analytical method such as SEM-DES or TEM-EDX, and evaluating the overlap between elements originating from material A and sulfur elements. In one embodiment, because the positive electrode composite has a high content of elemental sulfur, elemental sulfur is also present outside the pores of material A. In this case, the composite of elemental sulfur and material A forms a pellet-like mass, but it can be pulverized by mechanical crushing.

[0021] [Material B] Material B is conductive carbon, and the ratio of the area of ​​the G band to the area of ​​the D band (G / D) in Raman measurement, which indicates the degree of graphitization of the carbon material, is 0.6 or higher. The area of ​​the above G band is 1800 cm². -1 ~1475cm -1 This is the area of ​​the peak located at [location], and the area of ​​the D band is 1475 cm². -1 ~700cm -1This is the area of ​​the peak located at [location]. The area of ​​the D band indicates the amount of defects in the carbon material. Carbon materials with a ratio (G / D) of 0.6 or higher are expected to have high electrical conductivity because graphitization is progressing. A ratio (G / D) of 0.7 or higher is more preferable, and 1.0 or higher, 1.5 or higher, and even more preferable to 2.0 or higher. In addition, the ratio (G / D) is infinite in graphite with no defects because there is no D band, but it is usually 100 or less, and preferably 10 or less. The areas of the G band and D band in Raman measurements are measured by the method described in the examples.

[0022] In one embodiment, the ratio (G / D) in Raman measurement of material A is less than 0.6, and preferably less than 0.55. A ratio of less than 0.6 means that material A has a moderate defect structure, and Li ion conduction within the material through the defects can be expected. It can also be expected that sulfur will be more easily impregnated from the defects. For example, in carbon materials that do not have a graphite structure, such as diamond, the G band is not present, so the ratio (G / D) is 0. However, for conductive materials, a ratio of 0.1 or higher is preferable, and 0.2 or higher is more preferable. A ratio of 0.1 or higher means that a moderately developed graphite structure is contained in material A, resulting in suitable electronic conductivity.

[0023] In one embodiment, the aspect ratio of material B is 5 or more, preferably 10 or more, and more preferably 15 or more. The higher the aspect ratio, the more electronic conductivity can be imparted to the entire electrode with a small amount of material. The aspect ratio of material B is preferably 5000 or less, more preferably 3000 or less, and even more preferably 2000 or less. If the aspect ratio is too high, it leads to a decrease in the bulk density of the powder, which increases transportation costs and also leads to a decrease in mixability.

[0024] The aspect ratio of material B is calculated using the following formula. Aspect ratio of material B = Average fiber length of material B / Average fiber diameter of material B The aspect ratio is evaluated using an appropriate method depending on the size of the material. For example, a scanning electron microscope (SEM) can be used.

[0025] As material B, carbon fibers are preferred. Examples of carbon fibers include carbon nanotubes and vapor-processed carbon fibers (VGCF). In addition, materials such as acetylene black, Denka Black (registered trademark), thermal black, and channel black, which have a structural form and function as conductive additives with substantially high aspect ratios, as well as flat electron conductors such as graphene, reduced graphene, and flake graphite, can also be used as material B. These can be used individually, or two or more can be used in combination.

[0026] The average fiber length of material B is preferably 3 μm or more, and more preferably 5 μm or more. Furthermore, the average fiber length of material B is preferably 500 μm or less, and more preferably 100 μm or less. By adjusting the average fiber length in this way, electronic conductivity can be imparted to the entire electrode with a small amount of material.

[0027] The average fiber diameter of material B is preferably 2 nm or more, and more preferably 5 nm or more. Furthermore, the average fiber diameter of material B is preferably 1000 nm or less, and more preferably 300 nm or less. By adjusting the average fiber diameter in this way, it is possible to impart electronic conductivity to the entire electrode even with a small amount of mixing, while maintaining sufficient electronic conductivity.

[0028] In one embodiment, the micropore content of material B is 0.1 cc / g or less. Here, micropores generally refer to pores with a diameter of 2 nm or less. When the micropore content is 0.1 cc / g, material B is considered to have virtually no micropores. It is more preferable that the micropore content of material B is 0.05 cc / g or less. Because material B has no internal pores, it has high electronic conductivity, and therefore, electronic conductivity can be imparted to the entire positive electrode with a small amount of additive. The number of micropores can be analyzed using a t-plot from the nitrogen adsorption isotherm of material B. Furthermore, the absence of sulfur-containing micropores can be analyzed by evaluating the presence or absence of particles without overlap between carbon and sulfur using elemental mapping analytical techniques such as SEM-EDS or TEM-EDX.

[0029] [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 metal atoms, preferably containing a lithium atom and a phosphorus atom in addition to the sulfur atom, and more preferably containing a lithium atom, a phosphorus atom and a halogen atom, and having ionic conductivity due to the lithium atom. The sulfide solid electrolyte may be an amorphous sulfide solid electrolyte or a crystalline sulfide solid electrolyte.

[0030] (Amorphous sulfide solid electrolyte) As amorphous sulfide solid electrolytes, any electrolyte that contains at least a sulfur atom and exhibits ionic conductivity due to the contained metal atoms can be used without particular limitations. Typical examples include solid electrolytes containing sulfur, lithium, and phosphorus atoms, such as Li2S-P2S5, which is composed of lithium sulfide and phosphorus sulfide; solid electrolytes composed of lithium sulfide, phosphorus sulfide, and lithium halide, such as Li2S-P2S5-LiI, Li2S-P2S5-LiCl, Li2S-P2S5-LiBr, and Li2S-P2S5-LiI-LiBr; and solid electrolytes that further contain other elements such as oxygen and silicon, such as Li2S-P2S5-Li2O-LiI and Li2S-SiS2-P2S5-LiI. From the viewpoint of obtaining higher ionic conductivity, solid electrolytes composed of lithium sulfide, phosphorus sulfide, and lithium halide, such as Li2S-P2S5-LiI, Li2S-P2S5-LiCl, Li2S-P2S5-LiBr, and Li2S-P2S5-LiI-LiBr, are preferred. The types of elements constituting amorphous sulfide solid electrolytes can be determined, for example, by an ICP emission spectrometer.

[0031] When the amorphous sulfide solid electrolyte contains at least Li2S-P2S5, the molar ratio of Li2S to P2S5 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. When the amorphous sulfide solid electrolyte is, for example, Li2S-P2S5-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%.

[0032] 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 1.0~1.8:1.0~2.0:0.1~0.8:0.01~0.6, more preferably 1.1~1.7:1.2~1.8:0.2~0.6:0.05~0.5, and even more preferably 1.2~1.6:1.3~1.7:0.25~0.5:0.08~0.4. 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 1.0~1.8:1.0~2.0:0.1~0.8:0.01~0.3:0.01~0.3, more preferably 1.1~1.7:1.2~1.8:0.2~0.6:0.02~0.25:0.02~0.25, more preferably 1.2~1.6:1.3~1.7:0.25~0.5:0.03~0.2:0.03~0.2, and even more preferably 1.35~1.45:1.4~1.7:0.3~0.45:0.04~0.18:0.04~0.18. 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.

[0033] In addition, the shape of the amorphous sulfide solid electrolyte is not particularly limited, and examples thereof include particulate form. The average particle diameter (D 50 ) of the particulate amorphous sulfide solid electrolyte can be exemplified within the range of, for example, 0.01 μm to 500 μm, 0.1 to 200 μm. In this specification, the average particle diameter (D 50 ) is the particle diameter at which, when a particle diameter distribution integration curve is drawn, 50% of the whole is reached by sequentially integrating from the particles with the smallest particle diameter, and the volume distribution is, for example, the average particle diameter that can be measured using a laser diffraction / scattering type particle diameter distribution measuring device.

[0034] (Crystalline sulfide solid electrolyte) As the crystalline sulfide solid electrolyte, for example, so-called glass ceramics obtained by heating the above amorphous sulfide solid electrolyte to a temperature not lower than the crystallization temperature may be used, and a sulfide solid electrolyte having the following crystal structure may be adopted. Examples of the crystal structure that a crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, and phosphorus atoms may have include a Li3PS4 crystal structure, a Li4P2S6 crystal structure, a Li7PS6 crystal structure, a Li7P3S 11 crystal structure, a crystal structure having peaks in the vicinity of 2θ = 20.2° and 23.6° (for example, see Japanese Unexamined Patent Application Publication No. 2013 - 16423), and the like.

[0035] In addition, examples of the crystal structure that a crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms may have include Li 4-x Ge 1-x P x S4 - type thio - LISICON Region II crystal structure (see Kanno et al., Journal of The Electrochemical Society, 148(7) A742 - 746(2001)), Li 4-x Ge 1-x P xExamples include crystal structures similar to the S4-type thio-LISICON region II (see Solid State Ionics, 177 (2006), 2721-2725). Here, "thio-LISICON region II type crystal structure" refers to Li 4-x Ge 1-x P x S4-type thio-LISICON Region II crystal structure, Li 4-x Ge 1-x P x This indicates that the crystal structure is one of the S4-type thio-LISICON Region II types.

[0036] In X-ray diffraction measurements using CuKα rays, diffraction peaks for the Li3PS4 crystal structure appear, for example, around 2θ = 17.5°, 18.3°, 26.1°, 27.3°, and 30.0°, diffraction peaks for the Li4P2S6 crystal structure appear, for example, around 2θ = 16.9°, 27.1°, and 32.5°, diffraction peaks for the Li7PS6 crystal structure appear, for example, around 2θ = 15.3°, 25.2°, 29.6°, and 31.0°, and Li7P3S 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°, and 30.0°, Li 4-x Ge 1-x P x The diffraction peaks of the S4-type thio-LISICON Region II crystal structure appear, for example, around 2θ = 20.1°, 23.9°, and 29.5°, and Li 4-x Ge 1-x P x Diffraction peaks for crystal structures similar to the S4-type thio-LISICON Region II appear, for example, around 2θ = 20.2° and 23.6°. Note that these peak positions may vary within a range of ±0.5°.

[0037] In addition, as the crystal structure of the crystalline sulfide solid electrolyte, an argyrodite-type crystal structure can also be mentioned. As the argyrodite-type crystal structure, for example, the Li7PS6 crystal structure; the composition formula Li having the structural framework of Li7PS6 7-x P 1-y Si y S6 and Li 7+x P 1-y Si y The crystal structure represented by S6 (x is -0.6 to 0.6, y is 0.1 to 0.6); Li 7-x-2y PS 6-x-y Cl x (0.8 ≤ x ≤ 1.7, 0 < y ≤ -0.25x + 0.5); Li 7-x PS 6-x Ha x (Ha is Cl or Br, x is preferably 0.2 to 1.8).

[0038] Among the above crystal structures, as the crystal structure of the crystalline sulfide solid electrolyte, the Li3PS4 crystal structure, the thiosilicon region II-type crystal structure, and the argyrodite-type crystal structure are preferable.

[0039] The shape of the crystalline sulfide solid electrolyte is not particularly limited, and for example, particulate form can be mentioned. The average particle diameter (D 50 [[ID=�4]]) of the particulate crystalline sulfide solid electrolyte can be exemplified within the range of, for example, 0.01 μm to 500 μm, 0.1 to 200 μm, similar to the average particle diameter (D 50 ) of the aforementioned amorphous sulfide solid electrolyte.

[0040] [Mixing ratio] In the positive electrode mixture of the present embodiment, the content of material B is preferably 20 to 200 parts by mass, and more preferably 30 to 100 parts by mass when the amount of material A is 100 parts by mass. By satisfying the above range, the conductivity in the positive electrode mixture can be maintained high, and the rate characteristics are likely to be improved.

[0041] Furthermore, the ratio [S / (A+B)] of the total mass of elemental sulfur and the sulfur equivalent of the discharge products of elemental sulfur to the total mass of materials A and B (A+B) is 2.5 or greater. A ratio [S / (A+B)] of 3 or greater is preferable. In the positive electrode composite, the higher the proportion of elemental sulfur, the greater the expected improvement in the energy density of the battery, assuming the same sulfur utilization rate. On the other hand, it becomes difficult to adequately impart electron conduction within the positive electrode, so a ratio [S / (A+B)] of 10 or less is preferable, and a ratio of 5 or less is even more preferable.

[0042] The mass ratio (A:B:S) of material A, material B, and the total sulfur equivalent mass (S) of elemental sulfur and the discharge product of elemental sulfur is preferably 5-15:1-10:30-50, more preferably 5-10:2-8:35-45, and particularly preferably 7-10:3-8:35-45. This allows the effects of improved ion and electron conduction efficiency due to sulfur filling the pores of material A and holding it as small particles, and the effect of imparting electron conductivity to the entire electrode of material B, to function in a balanced manner, resulting in good battery characteristics.

[0043] The content of the sulfide solid electrolyte is preferably 5 to 200 parts by mass, and more preferably 10 to 120 parts by mass, when the total amount of material A, material B, and elemental sulfur is 100 parts by mass. If the solid electrolyte content is 5 parts by mass or less, it becomes difficult to obtain sufficient ion conduction, and if the solid electrolyte content is 200 parts by mass or more, the active material content decreases, making it difficult to improve the energy density.

[0044] In one embodiment, the total sulfur content of elemental sulfur and the discharge products of elemental sulfur in the positive electrode mixture is preferably 1.0 cc or more, more preferably 1.5 cc or more, and even more preferably 2.0 cc or more, per 1 g of the total of materials A and B. Furthermore, the total sulfur content of elemental sulfur and the discharge products of elemental sulfur is preferably 10 cc or less, more preferably 5.0 cc or less, and even more preferably 4.0 cc or less, per 1 g of the total of materials A and B. By setting the content within the above range, the elemental sulfur content becomes sufficient, and the rate characteristics tend to improve. For conversion purposes, the density of elemental sulfur is assumed to be 2 g / cc.

[0045] In one embodiment, the total sulfur content of elemental sulfur and the discharge products of elemental sulfur in the positive electrode composite is preferably 70% or more, more preferably 100% or more, and even more preferably 120% or more of the total pore volume of the combined material A and material B. This ensures a sufficient elemental sulfur content and allows for a high energy density. The total sulfur content of elemental sulfur and the discharge products of elemental sulfur is preferably 300% or less, more preferably 200% or less, and even more preferably 170% or less of the total pore volume of the combined material A and material B. The total pore volume of material A and material B can be calculated from the amount of material A and B charged (g) and the above-mentioned pore volume (cc / g).

[0046] In one embodiment, the positive electrode mixture may or may not contain other components besides the sulfur-based active material, solid electrolyte, and conductive additive. The other components are not particularly limited, but examples include binders, solvents, and dispersants. The positive electrode composite material of this embodiment can be manufactured, for example, by the manufacturing method of the present invention described below.

[0047] 2. Method for manufacturing positive electrode composite material A method for manufacturing a cathode composite material according to one embodiment of the present invention comprises the following steps (1) and (2). Step (1): A process to obtain a material A-elemental sulfur composite powder by heating an electronically conductive material A having an average pore radius of 10 nm or less and elemental sulfur at a temperature above the melting point of the elemental sulfur. Step (2): A process of mixing material A - elemental sulfur composite powder, material B - conductive carbon with a G-band area / D-band area of ​​0.6 or more in Raman measurement, and a sulfide solid electrolyte to form a positive electrode composite material.

[0048] [Process (1)] In step (1), for example, material A and elemental sulfur are mixed, sealed, and then the mixture is heated to melt the elemental sulfur and impregnate the pores with elemental sulfur, thereby producing a material A-elemental sulfur composite powder.

[0049] In step (1), the mixing ratio of material A and elemental sulfur can be adjusted as appropriate according to the materials used. In one embodiment, as described above, it is preferable that the elemental sulfur content be 1.0 cc or more per 1 g of material A. It is also preferable that the elemental sulfur content be 10 cc or less per 1 g of material A. Furthermore, in one embodiment, it is preferable that the content of elemental sulfur be 70% or more of the total pore volume of material A. It is also preferable that it be 300% or less of the total pore volume.

[0050] A mixture of material A and elemental sulfur is heated to a temperature above the melting point of elemental sulfur (approximately 115°C). The heating temperature is adjusted according to material A and elemental sulfur, but is preferably 130°C or higher, and more preferably 150°C or higher. The upper limit of the heating temperature is below the boiling point of elemental sulfur (approximately 445°C). The heating time is preferably 0.1 to 24 hours. By heating and then cooling, a composite powder of material A (elementary sulfur) is obtained. If necessary, a grinding step may be performed after cooling.

[0051] [Process (2)] In step (2), material A - elemental sulfur composite powder, material B - conductive carbon with a G-band area / D-band area ratio of 0.6 or more in Raman measurement, and a sulfide solid electrolyte are mixed to form a positive electrode composite material. For example, after step (1) above, material A - a single sulfur composite powder and material B are mixed by applying mechanical stress to form a positive electrode composite. Here, "applying mechanical stress" means applying shear force, impact force, etc. mechanically. Means of applying mechanical stress include, for example, pulverizers such as planetary ball mills, vibratory mills, and rolling mills, as well as kneaders. In this process, some of materials A and B may be crushed.

[0052] In one embodiment, the amount of material B is preferably 20 to 200 parts by mass per 100 parts by mass of material A, as described above. Furthermore, the amount of sulfide solid electrolyte added is preferably 5 to 200 parts by mass per 100 parts by mass of the total of material A, material B, and elemental sulfur.

[0053] For mixing conditions, for example, when using a planetary ball mill, the rotation speed should be set to tens to hundreds of revolutions per minute, and the process should be carried out for 0.5 to 100 hours. More specifically, in the case of the planetary ball mill used in the embodiment of this application (manufactured by Fritsch: model number P-5), the rotation speed of the planetary ball mill is preferably 100 rpm to 600 rpm, and more preferably 150 rpm to 400 rpm. If zirconia balls are used as the grinding media, their diameter is preferably 0.2 to 20 mm.

[0054] In one embodiment, the positive electrode composite material can be manufactured by mixing the above-mentioned material A-single sulfur composite powder with material B, and then adding a sulfide solid electrolyte and mixing again.

[0055] In one embodiment, the positive electrode composite material can be heated after the mixing of material A-single sulfur composite powder and material B. By recrystallizing the solid electrolyte whose crystallinity has decreased in the mixing process, or by the effect of interface formation through heating, it is possible to improve ion conductivity and thereby improve battery characteristics.

[0056] The heating temperature and time can be adjusted as needed depending on the type of solid electrolyte. For example, if the crystallization temperature of the solid electrolyte is 200°C or lower, the heating treatment should be performed in accordance with the crystallization temperature of the solid electrolyte. If the intercrystalline temperature of the solid electrolyte is high, such as 300°C or higher, heating to around 200°C, where excessive sulfur evaporation does not occur, can cause the interface to be reformed, although crystallization may not be achieved.

[0057] As mentioned above, the heating temperature is not limited as it varies depending on the type of solid electrolyte, but for example, it is 250°C or lower, and preferably 200°C or lower. The lower the temperature, the more effectively sulfur evaporation can be suppressed. The heating temperature is 50°C or higher, preferably 100°C or higher, and more preferably 120°C or higher. The heating time is not particularly limited, but for example, it is 1 minute to 99 hours, preferably 10 minutes to 10 hours, and especially preferably 30 minutes to 3 hours.

[0058] Furthermore, heating can be performed immediately after mixing, or after processing the mixture into electrode sheets using methods such as slurry casting with a solvent or electrostatic coating. In the case of manufacturing by slurry casting, it is also possible to reduce the number of steps by performing crystallization heating simultaneously with solvent removal.

[0059] 3. Lithium-ion batteries A lithium-ion battery according to one embodiment of the present invention includes the positive electrode composite material of the present invention described above. For example, an all-solid-state lithium-ion battery can be manufactured by using a solid electrolyte instead of a liquid electrolyte. By using the positive electrode composite material of the present invention, an all-solid-state lithium-ion battery with good rate characteristics can be manufactured.

[0060] All-solid-state lithium-ion batteries mainly consist of a positive electrode layer, a negative electrode layer, and an electrolyte layer, and the positive electrode composite material of the present invention is suitable as a constituent material for the positive electrode layer. The negative electrode layer and electrolyte layer can be manufactured by known methods. In addition to the positive electrode layer, negative electrode layer, and electrolyte layer, it is preferable that a current collector is used, and known current collectors can also be used. The solid electrolyte is not particularly limited, but examples include the sulfide solid electrolytes mentioned above. [Examples]

[0061] The present invention will be described in detail below based on examples. The present invention is not limited to these examples. The ratio of the area of ​​the G band to the area of ​​the D band (G / D) in Raman measurements of materials A and B was determined under the following conditions. <Measurement conditions> Equipment: DXR2 (Thermo Fisher Scientific Co., Ltd.) Exposure time: 10 seconds Total number of times: 20 Background: 20 times Laser wavelength: 532nm Laser power: 3.0mW Pinhole: 25 μm pinhole Resolution:HIGH RES GRATING Measurement range: 50~1800cm -1 Magnification: 50x objective lens

[0062] 700 and 1800 cm -1 A straight line passing through two points without nearby structural peaks was used as the baseline. If there is a peak at the above wavenumber, a wavenumber without nearby peaks can be used as the starting point for the baseline. 1475cm, which separates the G band and the D band. -1 1400~1550cm -1 The wavenumber at which each Raman spectrum takes its minimum value was calculated, and its average value was adopted.

[0063] (Example 1) (1) Preparation of activated carbon-elemental sulfur composite powder Activated carbon (manufactured by Kansai Thermal Chemical Co., Ltd., MSC-30, pore volume 1.67 cc / g, average pore radius 1.12 nm) and elemental sulfur were placed in a glass bottle as material A in a mass ratio of 1:5 and sealed inside a SUS pipe 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 an activated carbon-elemental sulfur composite powder. The pore volume of activated carbon is 1.67 cc / g. Assuming a density of 2 g / cc for elemental sulfur, the amount of elemental sulfur per gram of activated carbon is 2.5 cc. Therefore, the amount of elemental sulfur (charged) is 150% (volume) of the total pore volume of the activated carbon. Regarding the activated carbon-elemental sulfur composite powder, particle cross-sections were formed by ion milling, and the cross-sections were observed using SEM-EDS. The results showed that sulfur elements were observed within the carbon, confirming that elemental sulfur was impregnated into the pores of the activated carbon. The average pore radius and pore volume of the activated carbon were measured using a pore distribution analyzer (Autosorb-3) manufactured by Quantacrome.

[0064] (2) Preparation of solid electrolytes 0.4127 g of lithium sulfide, 0.6655 g of phosphorus pentasulfide, 0.2137 g of lithium iodide, and 0.2080 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.

[0065] (3) Preparation of positive electrode composite material 0.45 g of the activated carbon-elemental sulfur composite powder obtained in (1) above, 0.5 g of the solid electrolyte obtained in (2) above, and 0.05 g of vapor-phase grown multilayer carbon nanotubes (VGCF®, manufactured by Showa Denko K.K., average fiber length 6 μm, average fiber diameter 150 nm, aspect ratio 40, micropore count 0 cc / g by t-plot method) were placed together with 10 zirconia balls with a diameter of 10 mm in a 45 mL zirconia pot and sealed (Material A + Material B:Sulfur = 1:3 (mass ratio), Material A:Material B:Sulfur = 7.5:5:37.5 (mass ratio)). Using a planetary ball mill (manufactured by Fritsch, model P-7), the mixture was processed at a rotational speed of 370 rpm for 20 hours at room temperature to obtain a positive electrode composite powder.

[0066] (4) Fabrication of all-solid-state lithium-ion batteries 100 mg of the solid electrolyte prepared in (2) above was placed in a 10 mm diameter cylinder made of Macol® and pressurized. The positive electrode mixture powder prepared in (3) above was placed on the pressurized side so that the elemental sulfur content was 1.75 mg, and it was pressurized again. Indium foil and lithium foil were placed on the pressurized side opposite the positive electrode mixture, and the cylinder was pressurized to produce an all-solid-state battery.

[0067] (Example 2) A cathode composite powder was obtained in the same manner as in Example 1, except that the activated carbon (MSC-30) in Example 1 was replaced with a porous carbon material (CNovel®, manufactured by Toyo Tanso Co., Ltd., pore volume 3.09 cc / g, average pore radius 4.27 nm). The sulfur content is 81% of the total pore volume. In the porous carbon material-elemental sulfur composite powder, the cross-section was observed using SEM-EDS, similar to Example 1, and it was confirmed that elemental sulfur was impregnated into the pores of the porous carbon material. An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained positive electrode composite powder was used.

[0068] (Example 3) A cathode composite powder was obtained in the same manner as in Example 1, except that the activated carbon (MSC-30) in Example 1 was replaced with carbon black (Ketjenblack®, KB, pore volume 4.33 cc / g, average pore radius 6.64 nm). The content of elemental sulfur was 58% of the pore volume. In the case of the carbon black-elemental sulfur composite powder, the cross-section was observed using SEM-EDS, similar to Example 1, and it was confirmed that elemental sulfur was impregnated into the pores of the carbon black. An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained positive electrode composite powder was used.

[0069] (Comparative Example 1) In Example 1(3), without adding vapor-grown multilayer carbon nanotubes (VGCF®), 0.5 g of activated carbon-elemental sulfur composite powder, 0.5 g of solid electrolyte, and 10 zirconia balls with a diameter of 10 mm were placed in a zirconia pot to obtain a cathode composite powder in the same manner as in Example 1 (Material A + Material B:Sulfur = 1:5 (mass ratio), Material A:Material B:Sulfur = 8.3:0:41.7 (mass ratio)). An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained cathode composite powder was used.

[0070] (Comparative Example 2) In Example 1(3), instead of using activated carbon-single sulfur composite powder, a positive electrode composite powder was obtained in the same manner as in Example 1, except that 0.125 g of VGCF, 0.375 g of sulfur, and 0.5 g of solid electrolyte were placed in a zirconia pot along with 10 zirconia balls with a diameter of 10 mm (Material A + Material B:Sulfur = 1:3 (mass ratio), Material A:Material B:Sulfur = 0:12.5:37.5 (mass ratio)). An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained positive electrode composite powder was used.

[0071] (Comparative Example 3) In Example 1(3), instead of using activated carbon-elemental sulfur composite powder, 0.075 g of acetylene black (AB, a non-porous carbon material, Denka Black granular manufactured by Denka Co., Ltd.), 0.050 g of VGCF, and 0.375 g of sulfur were placed in a zirconia pot along with 10 zirconia balls with a diameter of 10 mm to obtain a cathode composite powder in the same manner as in Example 1 (Material A + Material B:Sulfur = 1:3 (mass ratio), Material A:Material B:Sulfur = 0:12.5:37.5 (mass ratio)). An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained cathode composite powder was used.

[0072] (Comparative Example 4) In Example 1(1), a carbon black-sulfur composite powder was prepared in the same manner, except that activated carbon (MSC-30) was replaced with carbon black (Ketjenblack®, KB, pore volume 4.33 cc / g, average pore radius 6.64 nm). The elemental sulfur content was 58% of the total pore volume. In Example 1(3), a cathode composite powder was obtained in the same manner as in Example 1(3), except that 0.5 g of carbon black-elemental sulfur composite powder and 0.5 g of solid electrolyte were placed in a zirconia pot along with 10 zirconia balls with a diameter of 10 mm, without adding vapor-evolved multilayer carbon nanotubes (VGCF®). (Material A + Material B:Sulfur = 1:5 (mass ratio), Material A:Material B:Sulfur = 8.3:0:41.7 (mass ratio)). An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained cathode composite powder was used.

[0073] (Example 4) In Example 1(3), instead of vapor-grown multilayer carbon nanotubes (VGCF®), 0.05 g of acetylene black (AB, a carbon material without pores, Denka Black granular manufactured by Denka Co., Ltd., with a micropore count of 0 cc / g by t-plot method), 0.45 g of activated carbon-elemental sulfur composite powder, and 0.5 g of solid electrolyte were placed in a zirconia pot along with 10 zirconia balls with a diameter of 10 mm, to obtain a cathode composite powder in the same manner as in Example 1 (Material A + Material B:Sulfur = 1:3 (mass ratio), Material A:Material B:Sulfur = 7.5:5:37.5 (mass ratio)). An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained cathode composite powder was used. Acetylene black is said to have a structural structure in which carbon particles without pores are linked together, and it can be expected to have essentially the same effect as adding carbon with a high aspect ratio.

[0074] (Example 5) In Example 1(3), instead of vapor-evolved multilayer carbon nanotubes (VGCF®), 0.05 g of carbon nanofibers (Merck KGaA, Carbon nanofibers, width 100 nm, length 20-200 μm, nominal aspect ratio 200-2000, PR-25-XT-HHT, micropore count 0 cc / g by t-plot method), 0.45 g of activated carbon-elemental sulfur composite powder, and 0.5 g of solid electrolyte were placed in a zirconia pot along with 10 zirconia balls with a diameter of 10 mm, except that a cathode composite powder was obtained in the same manner as in Example 1 (Material A + Material B:Sulfur = 1:3 (mass ratio), Material A:Material B:Sulfur = 7.5:5:37.5 (mass ratio)). An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained cathode composite powder was used.

[0075] (Example 6) In Example 1(3), a cathode composite powder was obtained in the same manner as in Example 1, except that 0.025 g of vapor-evolved multilayer carbon nanotubes (VGCF®), 0.475 g of activated carbon-elemental sulfur composite powder, and 0.5 g of solid electrolyte were placed in a zirconia pot along with 10 zirconia balls with a diameter of 10 mm (Material A + Material B:Sulfur = 1:3.2 (mass ratio), Material A:Material B:Sulfur = 9.5:2.5:38 (mass ratio)). An all-solid-state battery was fabricated in the same manner as in Example 1, except that the obtained cathode composite powder was used.

[0076] (evaluation) [Scanning Microscope (SEM)] Figure 1 is an SEM image of the cathode composite obtained in Example 1(3). It was confirmed that VGCF was well dispersed in the cathode composite.

[0077] [Evaluation of battery characteristics] Constant current charge-discharge tests were performed on the all-solid-state batteries fabricated in each example. The cutoff potential for the constant current test was set to 0.8-2.2V vs. Li-In, and the current density was as shown in Table 1.

[0078] [Table 1]

[0079] Table 2 shows the physical properties of materials A and B. Table 3 shows the discharge capacity (mAh / g) per gram of sulfur for Examples 1-6 and Comparative Examples 1-4 during the second cycle (current density during discharge: 0.147 mA) and the fifth cycle (current density during discharge: 0.586 mA).

[0080] [Table 2]

[0081] [Table 3]

[0082] Table 3 shows that when materials A and B are used in combination as in the example, the discharge capacity is large and the rate characteristics are excellent. It can also be seen that the presence or absence of pores in material A affects the rate characteristics. Furthermore, it can be seen that even when the type or content of material B is changed, the discharge capacity is larger and the rate characteristics are better than in Comparative Examples 1 and 2. [Industrial applicability]

[0083] The positive electrode composite material of the present invention is suitable for use as the positive electrode of a lithium-ion battery. Furthermore, the lithium-ion battery of the present invention is suitable for use 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.

Claims

1. Sulfide solid electrolytes, A material A having electronic conductivity with an average pore radius of 10 nm or less, Material B is a conductive carbon in which the ratio of the area of ​​the G band to the area of ​​the D band (G / D) in Raman measurement is 0.6 or higher, The material A comprises at least one of elemental sulfur and a discharge product of elemental sulfur, which are present in at least a portion of the pores of the material A. A positive electrode composite material in which the ratio [S / (A+B)] of the total mass of elemental sulfur and the sulfur from the discharge product of elemental sulfur to the total mass of materials A and B (A+B) is 2.5 or more.

2. The positive electrode composite material according to Claim 1, wherein the ratio of the area of ​​the G band to the area of ​​the D band (G / D) in the Raman measurement of material B is 0.7 or more.

3. The positive electrode composite material according to Claim 1, wherein the ratio of the area of ​​the G band to the area of ​​the D band (G / D) in the Raman measurement of material B is 2 or more and 10 or less.

4. The positive electrode composite material according to claim 1, wherein the ratio (A:B:S) of the total mass of material A, material B, elemental sulfur, and sulfur from the discharge product of elemental sulfur is 5 to 15:1 to 10:30 to 50.

5. The positive electrode composite material according to Claim 1, wherein the ratio (A:B:S) of the total mass of material A, material B, elemental sulfur, and sulfur from the discharge product of elemental sulfur is 7 to 10:3 to 8:35 to 45.

6. The positive electrode composite material according to claim 1, wherein the content of material B is 30 to 100 parts by mass when material A is 100 parts by mass.

7. The positive electrode composite material according to claim 1, wherein the average pore radius of material A is less than 5 nm.

8. The positive electrode composite material according to claim 1, wherein material A is a porous carbon material.

9. The positive electrode composite material according to claim 1, wherein the micropore count of material B is 0.1 cc / g or less.

10. The positive electrode composite material according to claim 1, wherein material B is carbon fiber.

11. The positive electrode composite material according to claim 1, wherein the ratio of the area of ​​the G band to the area of ​​the D band (G / D) in the Raman measurement of material A is less than 0.

6.

12. The positive electrode composite material according to claim 1, wherein the total pore volume of material A and material B is 1.0 cc / g or more.

13. The positive electrode composite material according to claim 1, wherein the total sulfur content of elemental sulfur and the discharge product of elemental sulfur is 1.0 cc or more per 1 g of the total mass of material A and material B.

14. The positive electrode composite material according to claim 1, wherein the total sulfur content of elemental sulfur and the discharge products of elemental sulfur is 70% or more of the total pore volume of material A and material B.

15. A positive electrode for a lithium-ion battery comprising the positive electrode composite material described in any one of claims 1 to 14.

16. A lithium-ion battery comprising a positive electrode for a lithium-ion battery as described in claim 15.

17. A process to obtain a composite powder of material A and elemental sulfur by heating an electronically conductive material A having an average pore radius of 10 nm or less and elemental sulfur at a temperature above the melting point of the elemental sulfur, A method for producing a cathode composite, comprising the step of mixing material A - elemental sulfur composite powder, material B which is conductive carbon with a G-band area / D-band area of ​​0.6 or more in Raman measurement, and a sulfide solid electrolyte to produce a cathode composite.

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