Manufacturing method for cathode composites for all-solid-state lithium-ion batteries

Abstract

To provide a positive electrode complex which improves a charge / discharge capacity of a battery when it is used for all-solid lithium ion battery.SOLUTION: A manufacturing method of a positive electrode complex for all-solid lithium ion battery includes a composition step of performing composition by giving mechanical energy to a mixture which contains S, P2S5, a conductive material and lithium salt of oxo acid and of which the content of Li2S is equal to or less than 10 mass%. Examples of lithium salt of oxo axis include lithium oxide, lithium peroxide, lithium borate, lithium carbonate, lithium nitrate, lithium silicate, lithium phosphate, lithium sulfate, lithium chlorate, lithium chlorite and lithium perchlorate. A raw material mixture may also further contain one or more of lithium chloride, lithium bromide and lithium iodide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This application relates to a method for producing a cathode composite used in all-solid-state lithium-ion batteries, comprising applying mechanical energy to a mixture containing a lithium salt of oxoacid and sulfur, and containing little to no Li2S, in order to obtain a cathode composite. [Background technology]

[0002] Lithium-ion rechargeable batteries have high energy density and are used in information-related devices such as personal computers and mobile phones. In recent years, development of high-power and high-capacity lithium-ion rechargeable batteries for electric vehicles and hybrid vehicles has been progressing. Lithium-ion rechargeable batteries that use flammable organic electrolytes may ignite if they overheat. For this reason, all-solid-state lithium-ion batteries, which replace the organic electrolyte with an inorganic solid electrolyte, are attracting attention.

[0003] Furthermore, sulfur (S) has a larger theoretical capacity compared to lithium transition metal oxides. For this reason, lithium-ion secondary batteries using positive electrode active materials containing S can be expected to achieve a dramatic improvement in energy density. Patent Document 1 proposes an electrode material for an all-solid-state lithium-sulfur battery in which sulfur and its discharge products as positive electrode active materials, conductive carbon, and a solid electrolyte containing lithium atoms, phosphorus atoms, and sulfur atoms, with a high phosphorus content.

[0004] Patent Document 2 proposes an electrode material for the positive electrode of an all-solid-state lithium-sulfur battery that uses a solid electrolyte containing lithium atoms, phosphorus atoms, iodine atoms, and sulfur atoms. According to the electrode materials of Patent Documents 1 and 2, the battery performance of an all-solid-state lithium-sulfur battery can be improved. However, in Patent Documents 1 and 2, the positive electrode composite material is manufactured after the solid electrolyte has been synthesized. Since the synthesis of the solid electrolyte takes a long time, the positive electrode composite materials of Patent Documents 1 and 2 are not suitable for mass production.

[0005] Patent Document 3 discloses a method for producing a positive electrode composite material by mechanically milling a raw material mixture containing a positive electrode active material containing Li2S, P2S5, and elemental sulfur, and a conductive additive containing a carbon material. Although Patent Document 3 proposes a one-step method for producing a positive electrode composite material, it requires a long mechanical milling process. Therefore, the positive electrode composite material of Patent Document 3 also has poor mass production capabilities. Furthermore, the all-solid-state lithium-sulfur battery of Patent Document 3 does not achieve sufficient energy density. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-072781 [Patent Document 2] Japanese Patent Publication No. 2015-146281 [Patent Document 3] Japanese Patent Publication No. 2019-033067 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The objective of this invention is to provide a cathode composite that utilizes the excellent physical properties of S to improve charge and discharge capacity when used in an all-solid-state lithium-ion battery. [Means for solving the problem]

[0008] The above-mentioned problems of this invention are solved by the following means. (1) A method for producing a positive electrode composite for an all-solid-state lithium-ion battery, comprising a composite step of applying mechanical energy to a mixture containing S, P2S5, a conductive material, and a lithium salt of an oxo acid, wherein the Li2S content is 10% by mass or less. (2) A method for producing a cathode composite for an all-solid-state lithium-ion battery according to (1) above, wherein the lithium salt of the oxoacid is one or more of lithium oxide, lithium peroxide, lithium borate, lithium carbonate, lithium nitrate, lithium silicate, lithium phosphate, lithium sulfate, lithium chlorate, lithium chlorite, and lithium perchlorate.

[0009] (3) A method for producing a cathode composite for an all-solid-state lithium-ion battery according to (1) or (2) above, wherein the mixture further comprises one or more of lithium chloride, lithium bromide, and lithium iodide. (4) The conductive material has a specific surface area of ​​10 m² 2 A method for producing a cathode composite for an all-solid-state lithium-ion battery according to any one of (1) to (3) above, wherein the carbon material is of a carbon content of 1 / g or more. (5) The specific surface area of ​​the carbon material is 1000 m 2 A method for producing a cathode composite for all-solid-state lithium-ion batteries as described in (4) above, wherein the composite is 1 / g or more. (6) A method for producing a cathode composite for an all-solid-state lithium-ion battery according to any one of (1) to (5) above, wherein the lithium salt of S:P2S5:conductive material:oxoacid in the mixture is 40-70:5-40:3-20:3-40 by mass. [Effects of the Invention]

[0010] According to the manufacturing method of the present invention, a cathode composite can be easily obtained by applying mechanical energy to a mixture containing S and a lithium salt of oxoacid, without using any or very much expensive Li2S. This makes it possible to improve the productivity and reduce the cost of cathode composites. [Modes for carrying out the invention]

[0011] The method for manufacturing a cathode composite for an all-solid-state lithium-ion battery (hereinafter, "cathode composite for an all-solid-state lithium-ion battery" may be simply referred to as "cathode composite") according to the embodiments of the present invention comprises a composite step. In the composite step, mechanical energy is applied to a mixture of raw materials (hereinafter, "raw material mixture") to composite them. This mixture contains S, P2S5, a conductive material, and a lithium salt of an oxoacid.

[0012] S functions as the positive electrode active material in the positive electrode composite. P2S5 functions as a lithium-ion conductive solid electrolyte by compounding with the lithium salt of the oxo acid in the positive electrode composite. The conductive material builds an electron network in the positive electrode composite, compensating for the low electrical conductivity of S. Examples of conductive materials include acetylene black, Ketjenblack, carbon nanotubes, activated carbon, and conductive carbon materials such as graphene. Here, conductivity refers to 1 × 10⁻¹⁶. -2 This refers to having an electronic conductivity of S / cm or higher. From the viewpoint of improving charge and discharge capacity, a large specific surface area of ​​the conductive carbon material is preferable.

[0013] Furthermore, from the viewpoint of sufficiently increasing the reaction sites with S, the specific surface area of ​​the conductive carbon material is 10 m². 2 It is preferable that it be 100m or more per gram. 2 It is more preferable that it be 1000m or more per gram. 2 It is even more preferable that the concentration is 1 / g or higher. There is no particular upper limit on the specific surface area of ​​the conductive carbon material, but it is usually 6000 m². 2 The specific surface area is less than or equal to / g. This specific surface area refers to the BET specific surface area determined by the Brenauer-Emmet-Telle (BET) method. More specifically, it is the specific surface area determined using the nitrogen adsorption isotherm obtained by adsorbing nitrogen gas onto a conductive carbon material at liquid nitrogen temperature. An example of an instrument for determining this BET specific surface area is the automatic specific surface area / pore distribution analyzer (BELSORP-mini II, manufactured by Nippon Bell Co., Ltd.).

[0014] Lithium salts of oxoacids function as lithium-ion conductive solid electrolytes by complexing with P2S5 in the cathode complex. Examples of lithium salts of oxoacids include one or more of lithium oxide, lithium peroxide, lithium borate, lithium carbonate, lithium nitrate, lithium silicate, lithium phosphate, lithium sulfate, lithium chlorate, lithium chlorite, and lithium perchlorate. The raw material mixture may further contain one or more of lithium chloride, lithium bromide, and lithium iodide.

[0015] The Li2S content of the raw material mixture is 10% by mass or less. Even a cathode composite obtained from a raw material mixture with a Li2S content of 10% by mass or less, that is, a raw material mixture containing little to no expensive Li2S, has a high charge-discharge capacity. The Li2S content in the raw material mixture is preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably no Li2S is present other than unavoidable impurities. The raw material mixture may also contain optional components such as binders, solvents, ion-conducting substances, or other conductive materials.

[0016] Examples of the binder include, but are not particularly limited to, thermoplastic resins and thermosetting resins. Specifically, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene rubber, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE resin), polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, ethylene-acrylic acid copolymer, polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polymethacrylic acid, sodium polymethacrylate, and lithium polymethacrylate are exemplified. The content of the binder in the raw material mixture is preferably 0.01% to 10% by mass, but is not particularly limited.

[0017] The raw material mixture containing a solvent is easy to produce the positive electrode composite layer. The solvent in the raw material mixture is removed by drying after the positive electrode composite layer is produced. Examples of the solvent include, but are not particularly limited to, amine solvents such as N,N-dimethylaminopropylamine and diethylenetriamine, ether solvents such as tetrahydrofuran, ketone solvents such as methyl ethyl ketone, ester solvents such as methyl acetate, amide solvents such as dimethylacetamide and 1-methyl-2-pyrrolidone, and hydrocarbon solvents such as toluene, xylene, n-hexane, and cyclohexane. The mass of the solvent in the raw material mixture is preferably 0.1 to 100 times the total mass of the solid content, but is not particularly limited.

[0018] As the ionic conductive material, a material having a lithium ion conductivity of 10 -5 S / cm or more at room temperature is preferable, and a material having a lithium ion conductivity of 10 -4 S / cm or more is more preferable, but there is no particular limitation. Specifically, examples include lithium aluminum titanium phosphate (LATP), lithium aluminum germanium phosphate (LAGP), lithium lanthanum zirconium oxide (LLZ), lithium lanthanum titanate (LLT), lithium germanium phosphorus sulfide (LGPS), lithium silicon sulfide (LSS), lithium phosphorus halogen sulfide (LPSX), and lithium boron hydride.

[0019] By further containing other conductive materials in the raw material mixture, the electron conductivity in the positive electrode composite may be improved, and the charge-discharge capacity of the positive electrode composite may be further enhanced. Examples of such other conductive materials include graphite, acetylene black, furnace black, carbon nanotubes, carbon fibers, and two or more kinds of these carbon materials. Note that the other conductive materials have a smaller specific surface area and a larger electron conductivity than the conductive material.

[0020] The furnace black may have a hollow shell structure. The furnace black having a hollow shell structure is a kind of conductive furnace black and has a hollow shell structure with a porosity of about 60% to 80%. This hollow shell structure is formed by thin graphite crystals gathering together to form an outer shell in the particle form, and has voids inside the outer shell. Examples of the furnace black having a hollow shell structure include Ketjen black (manufactured by Lion Corporation). It is preferable that the mass ratio of the conductive material to the other conductive material is 9.5:0.5 to 5:5. This is because by increasing the content of the conductive material with a large specific surface area, more reaction points with S can be obtained.

[0021] The ratio of lithium salts of S:P2S5:conductive material:oxoacid in the raw material mixture is preferably 40-70:5-40:3-20:3-40 by mass. Note that this mass ratio is not the mass percentage of each component relative to the total mass of S, P2S5, conductive material, and oxoacid. Therefore, as long as the mass ratio of each component satisfies this numerical range, the sum of the mass ratios of each component may be less than 100 or greater than 100.

[0022] In the compounding process, which involves applying mechanical energy to a raw material mixture to create a compound, conventionally known methods can be used. Specifically, the raw material mixture can be mechanically milled using a planetary ball mill (manufactured by Fritsch), a hybridization system (manufactured by Nara Machine Works), Cosmos (manufactured by Kawasaki Heavy Industries), a mechanofusion system (manufactured by Hosokawa Micron), Nobilta NOB (manufactured by Hosokawa Micron), a mechanomill (manufactured by Okada Seikou), a Theta Composer (manufactured by Tokuju Kogyosho), a nanosonic mill (manufactured by Inoue Seisakusho), a kneader (manufactured by Inoue Seisakusho), a super mascolloider (manufactured by Masuko Sangyo), a nanomec reactor (manufactured by Techno-i), a Cornell Despa (manufactured by Asada Iron Works), a planetary mixer (manufactured by Asada Iron Works), a Miracle KCK (manufactured by Asada Iron Works), and a vibratory mill (manufactured by Matsubo).

[0023] From the viewpoint of strengthening the contact interface between S and the conductive material or between P2S5 and the lithium salt of the oxoacid and reducing interfacial resistance, a heat treatment may be performed after the composite formation process. This heat treatment can be carried out using a conventionally known heating device in an atmosphere of argon, nitrogen, or air at 80°C to 250°C, preferably 100°C to 200°C, for 1 second to 10 hours, but there are no particular limitations. Examples of heating devices include constant temperature dryers, forced-air dryers, vacuum dryers, and infrared dryers. The all-solid-state lithium-ion battery of this embodiment comprises a negative electrode, a solid electrolyte, and a positive electrode containing the positive electrode composite obtained by the method for manufacturing the positive electrode composite of this invention. The all-solid-state lithium-ion battery of this embodiment has a large charge and discharge capacity. [Examples]

[0024] (Fabrication of positive electrode composite 1) The raw materials include 100 mg of S (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (hereinafter the same)), 60 mg of P2S5 (manufactured by Sigma-Aldrich Corporation (hereinafter the same)), and activated carbon (specific surface area 3000 m²). 2 20 mg of MSC30 (same as below), manufactured by Kansai Thermal Chemical Co., Ltd., and 20 mg of lithium oxide (same as below), manufactured by Furuuchi Chemical Co., Ltd. (same as below) (mass ratio S:P2S5:activated carbon:lithium oxide = 50:30:10:10) were placed in a 45 mL pot together with approximately 40 g of zirconia balls with a diameter of 5 mm. The mixture in this pot was mixed for 2 hours at an orbital speed of 370 rpm using a planetary ball mill (Frilsch, Premium Line P-7) to compound and obtain cathode composite 1.

[0025] (Fabrication of positive electrode composite 2) Cathode composite 2 was obtained using the same manufacturing process as for cathode composite 1, except that lithium carbonate (manufactured by Rare Metallic Co., Ltd.) was used instead of lithium oxide.

[0026] (Fabrication of positive electrode composite 3) Cathode composite 3 was obtained using the same manufacturing process as for cathode composite 1, except that lithium phosphate (manufactured by Sigma-Aldrich) was used instead of lithium oxide.

[0027] (Fabrication of positive electrode composite 4) Cathode composite 4 was obtained using the same manufacturing process as for cathode composite 1, except that lithium sulfate (manufactured by Sigma-Aldrich) was used instead of lithium oxide.

[0028] (Fabrication of positive electrode composite 5) Cathode composite 5 was obtained using the same manufacturing process as for cathode composite 1, except that lithium borate (manufactured by Toyoshima Seisakusho Co., Ltd.) was used instead of lithium oxide.

[0029] (Fabrication of positive electrode composite 6) Cathode composite 6 was obtained in the same manner as the manufacturing process for cathode composite 1, except that 100 mg of S, 48 mg of P2S5, 20 mg of activated carbon, 16 mg of lithium oxide, and 16 mg of lithium chloride (manufactured by Sigma-Aldrich (hereinafter the same)) were used as raw materials (mass ratio of S:P2S5:activated carbon:lithium oxide:lithium chloride = 50:24:10:8:8).

[0030] (Fabrication of positive electrode composite 7) Cathode composite 7 was obtained in the same manner as the manufacturing process for cathode composite 1, except that 100 mg of S, 550 mg of P2S, 20 mg of activated carbon, 10 mg of lithium oxide, and 20 mg of lithium chloride were used as raw materials (S:P2S5:activated carbon:lithium oxide:lithium chloride = 50:25:10:5:10 by mass ratio).

[0031] (Fabrication of positive electrode composite 8) Cathode composite 8 was obtained in the same manner as the manufacturing process for cathode composite 1, except that 100 mg of S, 550 mg of P2S, 20 mg of activated carbon, 10 mg of lithium oxide, and 20 mg of lithium bromide (manufactured by Sigma-Aldrich (hereinafter the same)) were used as raw materials (mass ratio of S:P2S5:activated carbon:lithium oxide:lithium bromide = 50:25:10:5:10).

[0032] (Fabrication of comparative cathode composite 1) A comparative cathode composite 1 was obtained using the same manufacturing process as cathode composite 1, except that 100 mg of S, 580 mg of P2S, and 20 mg of activated carbon (mass ratio of S:P2S5:activated carbon = 50:40:10) were used as raw materials.

[0033] (Fabrication of comparative cathode composite 2) Comparative cathode composite 2 was obtained using the same manufacturing process as cathode composite 1, except that lithium bromide was used instead of lithium oxide.

[0034] (Fabrication of comparative cathode composite 3) Comparative cathode composite 3 was obtained using the same manufacturing process as cathode composite 1, except that lithium chloride was used instead of lithium oxide.

[0035] (Fabrication of evaluation batteries for cathode composites) A solid electrolyte was obtained by mixing composite 1, which was prepared by calcining 5Li2S-GeS2-P2S5 at 510°C for 8 hours, and composite 2, which was prepared by processing 80Li2S-20P2S5 in a ball mill at 500 rpm for 10 hours, in a mass ratio of composite 1:compound 2 = 90:10. Then, an all-solid-state lithium-ion battery was fabricated in an argon gas atmosphere glove box using the following procedure.

[0036] First, a cylindrical jig made of SUS304 stainless steel (10 mm in diameter, 10 mm in height) was inserted into the bottom of a cylindrical polycarbonate jig (10 mm in inner diameter, 23 mm in outer diameter, 20 mm in height) to serve as the negative electrode current collector. Next, 80 mg of solid electrolyte was placed into the top of this cylindrical jig. Then, a cylindrical jig made of SUS304 stainless steel (10 mm in diameter, 15 mm in height) was inserted into the top of this cylindrical jig to serve as the positive electrode current collector. In this way, the solid electrolyte was sandwiched between the negative and positive electrode current collectors and pressed at a pressure of 80 MPa for 3 minutes to form a solid electrolyte layer with a diameter of 10 mm and a thickness of approximately 0.6 mm.

[0037] Next, the positive electrode current collector was temporarily removed from the top of the cylindrical jig, and 7.5 mg of the positive electrode composite prepared above was placed on top of the solid electrolyte layer. The positive electrode current collector was then reinserted into the cylindrical jig from the top, and pressed at a pressure of 200 MPa for 3 minutes to form positive electrode composite layers with a diameter of 10 mm and a thickness of approximately 0.1 mm. Then, the negative electrode current collector was temporarily removed from the bottom of the cylindrical jig, and a circular piece with a diameter of 8 mm, punched out from a 0.2 mm thick lithium sheet (manufactured by Honjo Metal Co., Ltd.), and a circular piece with a diameter of 9 mm, punched out from a 0.3 mm thick indium sheet (manufactured by Furuuchi Chemical Co., Ltd.), were stacked and inserted into the cylindrical jig from the bottom.

[0038] Next, the negative electrode current collector was inserted again into the cylindrical jig from below and pressed at a pressure of 80 MPa for 3 minutes to form a lithium-indium alloy negative electrode. In this way, all-solid-state lithium-ion batteries were fabricated, with the negative electrode current collector, lithium-indium alloy negative electrode, solid electrolyte layer, positive electrode composite layer, and positive electrode current collector stacked in that order from bottom to top. These all-solid-state lithium-ion batteries were sealed to obtain 11 types of evaluation batteries, from positive electrode composite 1 to positive electrode composite 8 and from comparative positive electrode composite 1 to comparative positive electrode composite 3.

[0039] (Evaluation of charge / discharge characteristics of batteries for evaluating positive electrode composites) These 11 types of evaluation batteries were subjected to repeated constant current discharge and constant current-constant voltage charging tests at 25°C, a voltage range of 0.5V-2.5V, and 0.2mA using a charge / discharge device (Asuka Electronics Co., Ltd., ACD-M01A). The discharge capacity per unit mass of these 11 types of positive electrode composites in the second cycle was as follows. This second-cycle discharge capacity was used as the evaluation target for the charge / discharge capacity of the positive electrode composite or all-solid-state lithium-ion battery. Cathode composite 1: 762mAh / g Cathode composite 2: 744mAh / g Cathode composite 3: 740mAh / g Cathode composite 4: 755mAh / g Cathode composite 5: 755mAh / g Cathode composite 6: 671mAh / g Cathode composite 7: 713mAh / g Cathode composite 8: 715mAh / g Comparative positive electrode composite 1: 544mAh / g Comparative positive electrode composite 2: 627mAh / g Comparative positive electrode composite 3: 514mAh / g

[0040] Thus, the charge and discharge capacities of all-solid-state lithium-ion batteries using cathode complexes 1 through 8 were greater than those of all-solid-state lithium-ion batteries using cathode complexes 1 through 3.

Claims

1. S, P ２ S ５ It contains a conductive carbon material with a specific surface area of ​​1000 m² / g or more, a lithium salt of an oxo acid, and one or more of lithium chloride, lithium bromide, and lithium iodide, in addition to unavoidable impurities. ２ A method for producing a cathode composite for an all-solid-state lithium-ion battery, comprising a composite step of applying mechanical energy to a sulfur-free mixture to form a composite.

2. A method for producing a cathode composite for an all-solid-state lithium-ion battery according to claim 1, wherein the lithium salt of the oxoacid is one or more of lithium oxide, lithium peroxide, lithium borate, lithium carbonate, lithium nitrate, lithium silicate, lithium phosphate, lithium sulfate, lithium chlorate, lithium chlorite, and lithium perchlorate.

3. S:P of the above mixture ２ S ５ A method for producing a cathode composite for an all-solid-state lithium-ion battery according to claim 1 or 2, wherein the carbon material and lithium salt of the oxo acid are in a mass ratio of 40-70:5-40:3-20:3-40.