Positive electrode mixture for composite all-solid-state lithium-sulfur batteries

Abstract

The present invention provides: a positive electrode mixture that fully exploits the excellent physical properties of sulfur, has excellent charge-discharge capacity, and is suitable for use in a positive electrode mixture layer of an all-solid-state lithium sulfur battery; and a production method for the positive electrode mixture. The present invention pertains to a positive electrode mixture for a composite all-solid-state lithium sulfur battery, said positive electrode mixture including sulfur or a discharge product thereof (A), phosphorus pentasulfide (B), an electrically conductive carbon (C), and a lithium halide (D), at a weight ratio A : B : C : D of 40-60 : 15-35 : 5-20 : 16-30, wherein when subjected to 31P-MAS NMR, the 50 ppm peak relative intensity is 40% or less.

Description

[Technical Field]

[0001] The present invention relates to a composite positive electrode mixture for an all-solid-state lithium-sulfur battery and a method for producing the positive electrode mixture for an all-solid-state lithium-sulfur battery. [Background technology]

[0002] Lithium-ion batteries have a high energy density and are used in many portable devices, including communication devices, electric vehicles, etc. In recent years, lithium-sulfur batteries, which can achieve even higher energy density, have attracted attention. Lithium-sulfur batteries are broadly divided into two types: liquid-type lithium-sulfur batteries, which use an organic electrolyte, and all-solid-state lithium-sulfur batteries, which use a solid electrolyte.

[0003] While liquid-type lithium-sulfur batteries have a problem in that lithium polysulfides produced during the charge-discharge reaction dissolve into the electrolyte solution, adversely affecting the battery's charge-discharge capacity and lifespan, all-solid-state lithium-sulfur batteries do not have this problem, making them suitable for maintaining the battery's charge-discharge capacity and extending its lifespan.

[0004] However, in all-solid-state lithium-sulfur batteries, the negative electrode, solid electrolyte layer, and positive electrode composite layer are substantially solvent-free, resulting in solid-solid contact, and the sulfur contained in the positive electrode composite layer as a positive electrode active material is electrically insulating, resulting in very low electronic conductivity and lithium ion conductivity in the positive electrode composite layer. In particular, when a high proportion of sulfur is added to the positive electrode composite, there is a problem in that the reactivity of the charge-discharge reaction is poor, making it impossible to ensure sufficient charge-discharge capacity.

[0005] Patent Document 1 proposes an electrode material for the positive electrode of an all-solid-state lithium-sulfur battery, which uses sulfur and its discharge products as a positive electrode active material, conductive carbon, and a solid electrolyte containing lithium atoms, phosphorus atoms, and sulfur atoms with a high phosphorus content. Patent Document 2 proposes an electrode material using a solid electrolyte containing lithium atoms, phosphorus atoms, iodine atoms, and sulfur atoms. The electrode materials disclosed in these documents are said to be able to improve the battery performance of all-solid-state lithium batteries. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-072781 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-146281 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the methods for producing a positive electrode composite disclosed in Patent Documents 1 and 2 consist of two steps: first, synthesizing a solid electrolyte, and then synthesizing a positive electrode composite. In particular, the synthesis of the solid electrolyte requires a long time, making it difficult to mass-produce. The present invention aims to provide a positive electrode composite that makes the most of the excellent physical properties of sulfur and is suitable for use in a positive electrode composite layer of an all-solid-state lithium-sulfur battery that has excellent charge / discharge capacity, and a method for producing the same. [Means for solving the problem]

[0008] The present inventors have conducted extensive research into positive electrode composites for use in all-solid-state lithium-sulfur batteries and have found that a positive electrode composite containing sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a specific blending ratio has a high charge / discharge capacity. They have also found that mass productivity can be improved by producing the positive electrode composite by compounding it in a single step, and have completed the present invention.

[0009] That is, the present invention relates to a composite positive electrode mixture for an all-solid-state lithium-sulfur battery, which contains sulfur or a discharge product thereof (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D=40-60:15-35:5-20:16-30, and which has a relative intensity of a peak at 50 ppm in 31P-MAS NMR of 40% or less.

[0010] The present invention also relates to a method for producing a positive electrode composite for an all-solid-state lithium-sulfur battery, the method comprising a step of mechanically milling a mixture containing sulfur or a discharge product thereof (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D=40-70:10-50:5-20:1-30.

[0011] The present invention also relates to a method for producing a positive electrode composite for an all-solid-state lithium-sulfur battery, which includes a step of mechanically milling a mixture containing sulfur or a discharge product thereof (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D=40-70:10-50:5-20:0-30 at a gravitational acceleration of 20 G or less for 10 hours or less.

[0012] In the above-described production method, the relative intensity of the peak at 50 ppm in 31P-MAS NMR of the positive electrode mixture is preferably 40% or less.

[0013] The specific surface area of ​​the conductive carbon (C) is 1000 m 2 / g or more is preferable.

[0014] The sulfur or discharge product thereof (A) is preferably a mixture containing sulfur and lithium sulfide in a weight ratio of sulfur:lithium sulfide=100-70:0-30.

[0015] The lithium halide (D) is preferably lithium iodide. [Effects of the Invention]

[0016] The composite cathode mixture for an all-solid-state lithium-sulfur battery of the present invention has a high charge / discharge capacity. According to the manufacturing method of the present invention, the cathode mixture can be composited in one step, thereby improving productivity. DETAILED DESCRIPTION OF THE INVENTION

[0017] <<Composite cathode composite for all-solid-state lithium-sulfur batteries>> The composite positive electrode mixture for an all-solid-state lithium-sulfur battery of the present invention is a composite positive electrode mixture for an all-solid-state lithium-sulfur battery, and is characterized by containing sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D=40-60:15-35:5-20:16-30, and having a relative intensity of a peak at 50 ppm in 31P-MAS NMR of 40% or less.

[0018] The components (A) to (D) used to produce the positive electrode mixture will now be described. <Sulfur or its discharge products (A)> Sulfur or its discharge product (A) functions as a positive electrode active material in the positive electrode mixture. As the sulfur, elemental sulfur or the like can be used. The discharge product of sulfur is not particularly limited, but examples thereof include lithium polysulfides such as Li2S8, Li2S4, and Li2S2, and lithium sulfide (Li2S). These compounds may be used alone or in combination with two or more of them, or may even be used in combination with elemental sulfur.

[0019] The sulfur or discharge product thereof (A) is preferably a mixture of sulfur and lithium sulfide in a weight ratio of sulfur:lithium sulfide=100-70:0-30, more preferably 100-80:0-20. If the weight ratio of sulfur to lithium sulfide is less than 70:30, the capacity tends to decrease.

[0020] <Diphosphorus pentasulfide (B)> Diphosphorus pentasulfide (B) functions as a solid electrolyte in the cathode mixture, reducing the reaction resistance that occurs when sulfur, electrons, and lithium ions react at the reaction interface, thereby improving the charge / discharge capacity of all-solid-state lithium-sulfur batteries.

[0021] <Conductive carbon (C)> The conductive carbon (C) forms an electron network in the positive electrode mixture, compensating for the low electrical conductivity of sulfur or its discharge product (A). Specific examples of the conductive carbon (C) include carbon nanotubes, activated carbon, and graphene. Among these, activated carbon is preferred because it can significantly improve charge / discharge capacity.

[0022] The conductive carbon (C) preferably has a large surface area in order to build an electron network and improve the charge / discharge capacity. The specific surface area of ​​the conductive carbon (C) is 1000 m 2 / g or more is preferable, and 1500m 2 / g or more is more preferable, and 2000m 2 / g or more is more preferable. 2 If the specific surface area is less than 1 / g, the reaction sites between the sulfur and / or its discharge products (A) and the conductive carbon (C) cannot be increased sufficiently, and therefore the effect of improving the charge / discharge capacity tends to be insufficient. The upper limit of the specific surface area is not particularly limited, but is usually 6000 m 2 / g or less.

[0023] In the present invention, the specific surface area refers to the BET specific surface area determined by the Brenauer-Emmet-Telle (BET) method, and more specifically, refers to the specific surface area determined using a nitrogen adsorption isotherm obtained by placing a sample of the (C1) conductive material or the (C2) conductive material described below at liquid nitrogen temperature and allowing nitrogen gas to adsorb onto the sample. As a measuring device for determining the BET specific surface area, for example, an automatic specific surface area / pore distribution measuring device (BELSORP-mini II, manufactured by BEL Japan Co., Ltd.) can be used.

[0024] <Lithium halide (D)> The lithium halide (D) compensates for the low electrical conductivity of sulfur or its discharge product (A) in the positive electrode mixture. The solid lithium halide (D) mixes uniformly with the solid sulfur or its discharge product (A) to form a solid solution, and the halide ions partially replace the sulfide ions in the sulfur or its discharge product (A), improving the ionic conductivity of the sulfur or its discharge product (A). Examples of the lithium halide (D) include lithium iodide, lithium chloride, lithium bromide, and lithium fluoride. Among these, lithium iodide is preferred from the viewpoint of electrical conductivity.

[0025] As a combination of sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D), a combination of (A) sulfur, or lithium polysulfide or lithium sulfide, (B) diphosphorus pentasulfide, (C) carbon nanotubes, activated carbon, or graphene, and (D) lithium iodide, lithium chloride, lithium chloride, lithium bromide, or lithium fluoride is preferred, and a combination of (A) sulfur or lithium sulfide, (B) diphosphorus pentasulfide, (C) activated carbon, and (D) lithium iodide is more preferred.

[0026] <Mixing ratio> The positive electrode mixture of the present invention contains sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D=40-60:15-35:5-20:16-30.

[0027] If the blending ratio of sulfur or its discharge product (A) is less than the above range, the amount of lithium ions that can move to the positive electrode decreases, and sufficient charge / discharge capacity may not be obtained, whereas if the blending ratio is greater than the above range, the blending amount of conductive carbon (C) decreases, and the charge / discharge capacity per positive electrode mixture may decrease. The blending amount of sulfur or its discharge product (A) is preferably 45 to 60 wt %, more preferably 50 to 55 wt %, of the total amount of components (A), (B), (C), and (D).

[0028] If the blending ratio of diphosphorus pentasulfide (B) is less than the above range, the charge-discharge reaction does not proceed sufficiently, and the charge-discharge capacity tends to decrease, while if it is more than the above range, the amount of sulfur or its discharge products decreases, and the amount of lithium ions that can move to the positive electrode tends to decrease. The blending amount of diphosphorus pentasulfide (B) is preferably 15 to 30 wt % of the total amount of components (A), (B), (C), and (D).

[0029] If the blending ratio of the conductive carbon (C) is less than the above range, the electron conduction becomes insufficient and the charge-discharge reaction tends to not proceed, whereas if the blending ratio is more than the above range, the conduction of lithium ions tends to be hindered and the charge-discharge reaction tends to not proceed. The blending amount of the conductive carbon (C) is preferably 5 to 20 wt %, more preferably 8 to 15 wt %, of the total amount of the components (A), (B), (C), and (D).

[0030] If the blending ratio of the lithium halide (D) is less than the above range, the lithium ion conductivity tends to decrease, while if it is more than the above range, the amount of component (A) decreases, and the amount of lithium ions that can move to the positive electrode tends to decrease. The blending amount of the lithium halide (D) is preferably 17 to 28 wt %, more preferably 20 to 25 wt %, of the total amount of components (A), (B), (C), and (D).

[0031] The blending amounts of sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) are preferably in a weight ratio of A:B:C:D=40-60:15-35:5-20:20-25.

[0032] In the present invention, "composite" does not simply mean mixing of predetermined components, but means adding mechanical, thermal or chemical energy to a mixture of predetermined components, causing a chemical reaction in all or part of the predetermined components.

[0033] In the positive electrode composite of the present invention, components (A) to (D) are sufficiently composited, and diphosphorus pentasulfide (B) becomes a substance with lithium ion conductivity, resulting in a reduced amount of residual diphosphorus pentasulfide (B). Therefore, the relative intensity of the peak at 50 ppm derived from diphosphorus pentasulfide in 31P-MAS NMR is 40% or less. The relative intensity is preferably 35% or less, more preferably 30% or less. A relative intensity exceeding 40% indicates insufficient composite formation.

[0034] The relative intensity is calculated under the following conditions: 31P MAS NMR of the prepared positive electrode composite is measured using ammonium phosphate (1.0 ppm) as an external standard with a Bruker Avance III HD 600WB, and the maximum intensity is set to 100 and the minimum intensity is set to 0, and the relative intensity at 50 ppm, where the peak of diphosphorus pentasulfide appears, is calculated.

[0035] <Optional ingredients> The positive electrode mixture of the present invention may be a composite of the above components (A) to (D) and optional components such as a binder, a solvent, an ion-conductive material, and a conductive material.

[0036] <Binder> The binder is not particularly limited, but thermoplastic resins, thermosetting resins, etc. can be used, for example, 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 Examples of binders include polymers (ETFE resins), polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymers, propylene-tetrafluoroethylene copolymers, ethylene-chlorotrifluoroethylene copolymers (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymers, ethylene-acrylic acid copolymers, polyacrylic acid, sodium polyacrylate, lithium polyacrylate, polymethacrylic acid, sodium polymethacrylate, and lithium polymethacrylate. These binders may be used alone or in combination of two or more.

[0037] When the positive electrode mixture of the present invention contains a binder, the content thereof is not particularly limited, but is preferably 0.01 to 10% by weight in the positive electrode mixture.

[0038] <Solvent> The positive electrode composite obtained by mixing the solvent facilitates the preparation of a positive electrode composite layer. The solvent is removed by drying when preparing the positive electrode composite layer. The solvent is not particularly limited, but examples thereof include amine-based solvents such as N,N-dimethylaminopropylamine and diethylenetriamine, ether-based solvents such as tetrahydrofuran, ketone-based solvents such as methyl ethyl ketone, ester-based solvents such as methyl acetate, amide-based solvents such as dimethylacetamide and 1-methyl-2-pyrrolidone, and hydrocarbon-based solvents such as toluene, xylene, n-hexane, and cyclohexane. These solvents may be used alone or in combination of two or more.

[0039] When the positive electrode mixture of the present invention is obtained by mixing the above-mentioned solvent, the content thereof is not particularly limited, but is preferably 10 to 10,000 parts by weight per 100 parts by weight of the solid content of the positive electrode mixture.

[0040] <Ion-conductive materials> The positive electrode composite of the present invention may contain an ion-conductive material in addition to diphosphorus pentasulfide (B). Examples of the ion-conductive material include a composite of Li, S, and P. Specifically, a composite obtained by mechanical milling Li2S, S, and P, or a composite of Li2S and P x S y (where x and y are independently integers that give a stoichiometric ratio) by mechanical milling. Mechanical milling allows for easy rearrangement of bonds, and also produces an amorphous ion-conducting material.

[0041] The ion-conductive material preferably contains phosphorus in a weight ratio of 0.2 to 0.55, more preferably 0.2 to 0.45. If the weight ratio of phosphorus is less than 0.2 or exceeds 0.55, sufficient charge / discharge capacity tends to be unavailable when used in an all-solid-state sodium-sulfur battery.

[0042] The above Li2S and P x S yThe composite with the lithium ion may further contain a lithium salt or a lithium nitride. The lithium salt is not particularly limited, but examples thereof include Li3PO4, Li4SiO4, Li2O, and LiBH4. The lithium nitride is not particularly limited, but examples thereof include Li3N.

[0043] <Conductive material> In addition to the conductive carbon (C), the positive electrode mixture may contain at least one conductive material selected from the group consisting of graphite, acetylene black, furnace black (e.g., furnace black having a hollow shell structure), carbon nanotubes, and carbon fibers. This is because, when the conductivity of the conductive carbon (C) is low, the addition of a conductive material can improve the electronic conductivity in the positive electrode mixture, thereby further improving the charge / discharge capacity.

[0044] The furnace black having the hollow shell structure is a type of conductive furnace black that has a hollow shell-like structure with a porosity of approximately 60 to 80%. Here, the "hollow shell structure" refers to a structure in which graphite crystals are thinly gathered together to form a particle-shaped outer shell, with voids inside the outer shell. An example of the furnace black having the hollow shell structure is Ketjen Black (manufactured by Lion Corporation).

[0045] When the conductive material is contained, the weight ratio of the conductive carbon (C) to the conductive material is preferably conductive carbon (C):conductive material = 9.5:0.5 to 5:5. The reason for this is that the specific surface area is large, and by increasing the amount of the conductive carbon (C) blended, many reaction sites with sulfur or its discharge product (A) are available, thereby accelerating the charge / discharge reaction.

[0046] <Method for producing positive electrode mixture> The positive electrode mixture of the present invention can be obtained by mixing optional components, as required, with the above-mentioned components (A) to (D). The method for mixing these materials is not particularly limited, and conventionally known methods can be used. Examples include mixing methods using a planetary ball mill (manufactured by Fritsch), a hybridization system (manufactured by Nara Machine Works), Cosmos (manufactured by Kawasaki Heavy Industries, Ltd.), a Mechanofusion system (manufactured by Hosokawa Micron Corporation), Nobilta NOB (manufactured by Hosokawa Micron Corporation), Mechanomill (manufactured by Okada Seiko Co., Ltd.), Theta Composer (manufactured by Tokuju Kogyosho Co., Ltd.), Nanosonic Mill (manufactured by Inoue Seisakusho Co., Ltd.), a kneader (manufactured by Inoue Seisakusho Co., Ltd.), a Supermass Colloider (manufactured by Masuko Sangyo Co., Ltd.), Nanomec Reactor (manufactured by Techno-eye Co., Ltd.), Cornell Despa (manufactured by Asada Iron Works Co., Ltd.), a planetary mixer (manufactured by Asada Iron Works Co., Ltd.), Miracle KCK (manufactured by Asada Iron Works Co., Ltd.), a vibration mill (manufactured by Matsubo Co., Ltd.), or the like.

[0047] In preparing the positive electrode composite, a heat treatment may be performed after mixing the components. This is because the contact interface between the sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) contained in the positive electrode composite can be strengthened, thereby reducing interfacial resistance. The heat treatment is not particularly limited, and can be performed, for example, in an atmosphere of argon, nitrogen, air, or the like, at 80 to 250°C, preferably 100 to 200°C, for 1 second to 10 hours. The heat treatment may be performed using a conventionally known heating device, specifically, for example, a constant temperature dryer, a blower dryer, a reduced pressure dryer, an infrared dryer, or the like.

[0048] <<Method 1 for manufacturing cathode composite for all-solid-state lithium-sulfur batteries>> The method 1 for producing a positive electrode composite for an all-solid-state lithium-sulfur battery of the present invention includes a step of mechanically milling a mixture containing sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D=40-70:10-50:5-20:1-30.

[0049] The contents of the components (A) to (D) and the preferred weight ratio of the component (C) are as described above for the positive electrode mixture.

[0050] In production method 1, if the blending ratio of sulfur or its discharge product (A) is smaller than the above range, the amount of lithium ions that can move to the positive electrode decreases, and sufficient charge / discharge capacity may not be obtained, while if it is larger than the above range, the blending amount of conductive carbon (C) decreases, and the charge / discharge capacity per positive electrode mixture may decrease. The blending amount of sulfur or its discharge product (A) is preferably 45 to 60 wt %, more preferably 50 to 55 wt %, of the total amount of components (A), (B), (C), and (D).

[0051] In manufacturing method 1, if the compounding ratio of diphosphorus pentasulfide (B) is less than the above range, the charge-discharge reaction does not proceed sufficiently, and the charge-discharge capacity tends to decrease, while if it is more than the above range, the amount of sulfur or its discharge products decreases, and the amount of lithium ions that can move to the positive electrode tends to decrease. The compounding amount of diphosphorus pentasulfide (B) is preferably 15 to 30 wt % of the total amount of components (A), (B), (C), and (D).

[0052] In Production Method 1, if the blending ratio of the lithium halide (D) is greater than the above range, the amount of component (A) decreases, and therefore the amount of lithium ions that can migrate to the positive electrode tends to decrease. The blending amount of the lithium halide (D) is preferably 17 to 28 wt %, more preferably 20 to 25 wt %, of the total amount of components (A), (B), (C), and (D).

[0053] In manufacturing method 1, mechanical milling is performed under conditions that include lithium halide (D) as an essential element, which allows for easy rearrangement of the bonds between the components and produces an amorphous cathode composite. Conventional methods can be used for mechanical milling, including methods using devices such as planetary ball mills, vibration mills, and high-shear, high-compression rotary devices. When using a planetary ball mill, the processing conditions include a rotation speed of 225 to 500 rpm and a revolution speed of 450 to 1000 rpm (rotation counter to the rotation) for 0.5 to 10 hours. Specific examples of high-shear, high-compression rotary devices include Nara Kikai's Mirolla and Hosokawa Micron's Nobilta.

[0054] In Production Method 1, a heat treatment may be carried out after mixing the components. The heat treatment is not particularly limited, but can be carried out, for example, in an atmosphere of argon, nitrogen, air, or the like, at 80 to 250°C, preferably 100 to 200°C, for 1 second to 10 hours. The heat treatment may be carried out using a conventionally known heating device, and specifically, for example, a constant temperature dryer, a blower dryer, a reduced pressure dryer, an infrared dryer, or the like.

[0055] <<Method 2 for manufacturing cathode composite for all-solid-state lithium-sulfur batteries>> The second method of the present invention for producing a positive electrode composite for an all-solid-state lithium-sulfur battery includes a step of mechanically milling a mixture containing sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D=40-70:10-50:5-20:0-30 at a gravitational acceleration of 20 G or less for 10 hours or less.

[0056] The contents of the components (A) to (D) and the preferred weight ratio of the component (C) are as described above for the positive electrode mixture.

[0057] In production method 2, if the blending ratio of sulfur or its discharge product (A) is smaller than the above range, the amount of lithium ions that can move to the positive electrode decreases, and sufficient charge / discharge capacity may not be obtained, while if it is larger than the above range, the blending amount of conductive carbon (C) decreases, and the charge / discharge capacity per positive electrode mixture may decrease. The blending amount of sulfur or its discharge product (A) is preferably 45 to 60 wt %, more preferably 50 to 55 wt %, of the total amount of components (A), (B), (C), and (D).

[0058] In Production Method 2, if the compounding ratio of diphosphorus pentasulfide (B) is less than the above range, the charge-discharge reaction does not proceed sufficiently, and the charge-discharge capacity tends to decrease, while if it is more than the above range, the amount of sulfur or its discharge products decreases, and the amount of lithium ions that can move to the positive electrode tends to decrease. The compounding amount of diphosphorus pentasulfide (B) is preferably 10 to 40 wt %, more preferably 15 to 30 wt %, of the total amount of components (A), (B), (C), and (D).

[0059] In Production Method 2, if the blending ratio of the lithium halide (D) is greater than the above range, the amount of component (A) decreases, and therefore the amount of lithium ions that can migrate to the positive electrode tends to decrease. The blending amount of the lithium halide (D) is preferably 0 to 25 wt %, more preferably 10 to 25 wt %, of the total amount of components (A), (B), (C), and (D).

[0060] In production method 2, examples of mechanical milling include treatment with a bead mill and treatment with a ball mill. A planetary ball mill or other device can be used as the ball mill. An Imex batch-type sandblinder or other device can be used as the bead mill. The mechanical milling is performed under conditions of a gravitational acceleration of 20 G or less for 10 hours or less. The gravitational acceleration and treatment time are preferably 16 G or less for 10 hours or less, more preferably 15 G or less for 8 hours or less, and even more preferably 12 G or less for 4 hours or less. If the gravitational acceleration exceeds 20 G or the treatment time exceeds 10 hours, the conductive material tends to become over-dispersed, resulting in a decrease in electronic conductivity.

[0061] In Production Method 2, a heat treatment may be carried out after mixing the components. The heat treatment is not particularly limited, but can be carried out, for example, in an atmosphere of argon, nitrogen, air, or the like, at 80 to 250°C, preferably 100 to 200°C, for 1 second to 10 hours. The heat treatment may be carried out using a conventionally known heating device, and specifically, for example, a constant temperature dryer, a blower dryer, a reduced pressure dryer, an infrared dryer, or the like.

[0062] <<All-solid-state lithium-sulfur battery>> The all-solid-state lithium-sulfur battery includes a positive electrode mixture layer containing the positive electrode mixture of the present invention, a solid electrolyte layer, a negative electrode, and a current collector.

[0063] In this specification, the term "all-solid-state" refers to a battery that uses a polymer solid electrolyte and / or an inorganic solid electrolyte as the electrolyte and that contains substantially no solvent in the negative electrode, solid electrolyte layer, and positive electrode composite layer. Note that in this specification, "substantially no solvent" means that a trace amount of solvent may remain.

[0064] An all-solid-state lithium-sulfur battery has a structure in which a negative electrode, a solid electrolyte layer, and a positive electrode composite layer are stacked in this order, with current collectors (negative electrode current collector and positive electrode current collector) disposed on both sides. Below, we will explain the current collectors (negative electrode current collector and positive electrode current collector), negative electrode, solid electrolyte layer, and positive electrode composite layer in order.

[0065] <Current collector> The current collector is not particularly limited, but may be, for example, Al, Cu, Ni, stainless steel, etc. The negative electrode current collector is preferably made of Cu because it is difficult to form an alloy with lithium and is easy to process into a thin film. The positive electrode current collector is preferably made of Al because it is easy to process into a thin film and is inexpensive.

[0066] <Negative electrode> The negative electrode is not particularly limited as long as it contains a material that absorbs and releases lithium ions as the negative electrode active material. Examples of materials that absorb and release lithium ions include metallic lithium, lithium alloys, metal oxides, metal sulfides, and carbonaceous materials that absorb and release lithium ions. Examples of lithium alloys include alloys of lithium with aluminum, silicon, tin, magnesium, indium, calcium, and the like. Examples of metal oxides include tin oxide, silicon oxide, lithium titanium oxide, niobium oxide, and tungsten oxide. Examples of metal sulfides include tin sulfide and titanium sulfide. Examples of carbonaceous materials that absorb and release lithium ions include graphite, coke, mesophase pitch-based carbon fiber, spherical carbon, and resin-fired carbon.

[0067] The method for obtaining the negative electrode is not particularly limited, but examples thereof include a method of pressing the material that absorbs and releases lithium ions, and a method of applying a negative electrode precursor dispersion containing the material that absorbs and releases lithium ions and a solvent to a negative electrode current collector, drying the dispersion, and then pressing the resulting mixture. The solvent contained in the negative electrode precursor dispersion can be the same as that used for the positive electrode mixture. The solvent is used to facilitate application of the negative electrode precursor dispersion, and is removed by drying after application.

[0068] <Solid electrolyte layer> The solid electrolyte layer may be made of a polymer solid electrolyte and / or an inorganic solid electrolyte. For example, the inorganic solid electrolyte may be a solid electrolyte having a conductivity of 0.1 mS / cm or more. Specific examples of the solid electrolyte include, but are not limited to, lithium salt, lithium sulfide, lithium oxide, and lithium nitride, as long as the solid electrolyte has a conductivity of 0.1 mS / cm or more.

[0069] The solid electrolyte is preferably a lithium salt, a lithium sulfide, or a combination thereof, because of its high electrical conductivity and low grain boundary resistance.

[0070] The lithium salt is not particularly limited, but examples thereof include LiBH4, LiI, etc. The lithium sulfide is not particularly limited, but examples thereof include the above-mentioned P x S y and composites, specifically, the above Li2S and P x S y and Li2S and P x S y The solid electrolyte may further comprise GeS2, SiS2, Li3PO4, Li4SiO4, etc. The lithium oxide is not particularly limited, but examples thereof include Li2O and Li2O2. The lithium nitride is not particularly limited, but examples thereof include Li3N. These solid electrolytes may be used alone or in combination of two or more.

[0071] The solid electrolyte layer made of the inorganic solid electrolyte can be obtained, for example, by a method of pressurizing the solid electrolyte, a method of dispersing the solid electrolyte in a solvent and then coating and drying the dispersion, or the like. The method of pressurizing the solid electrolyte is not particularly limited, but examples include a method of sandwiching the solid electrolyte between a negative electrode current collector and a positive electrode current collector and pressing it, or a method of pressing it with a pressure molding jig. When the solid electrolyte layer is obtained by a method of dispersing the solid electrolyte in a solvent and then coating and drying it, the dried solid electrolyte layer may be pressed in the same manner as above. The solvent used to disperse the solid electrolyte may be the same as that used for the cathode composite. When obtaining a solid electrolyte layer by these methods, a heat treatment may be performed at any time to reduce the interfacial resistance of the solid electrolyte layer and improve its density. Furthermore, examples of the solid electrolyte layer made of the polymer solid electrolyte include polyethylene oxide-based polymers containing lithium salts such as lithium perchlorate and lithium bistrifluoromethanesulfonylamide.

[0072] <Positive electrode mixture layer> The positive electrode composite layer can be obtained, for example, by supporting the positive electrode composite on a positive electrode current collector or by pressure-molding the positive electrode composite. The method for supporting the positive electrode composite on a positive electrode current collector is not particularly limited, but examples include pressure-molding the positive electrode composite, applying a paste of the positive electrode composite to the positive electrode current collector using an organic solvent or the like, drying, and then pressing to adhere the paste. The method for pressure-molding the positive electrode composite is not particularly limited, but examples include sandwiching the positive electrode composite between a solid electrolyte layer and a positive electrode current collector and pressing, or pressing with a pressure-molding jig. The method for applying the positive electrode composite to the positive electrode current collector is not particularly limited, but examples include slit-die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying. When obtaining the positive electrode composite layer by these methods, a heat treatment may be performed at any time to reduce the interfacial resistance of the positive electrode composite layer and improve its density.

[0073] The all-solid-state lithium-sulfur battery may include a separator, etc. in addition to the above-mentioned negative electrode current collector, negative electrode, solid electrolyte layer, positive electrode composite layer, and positive electrode current collector. The shape of the all-solid-state lithium-sulfur battery is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a prismatic type.

[0074] <Method for manufacturing all-solid-state lithium-sulfur batteries> The method for producing the all-solid-state lithium-sulfur battery is not particularly limited, but examples thereof include the following method. First, a solid electrolyte is sandwiched between an anode current collector and a cathode current collector and pressed to form a solid electrolyte layer. Next, a cathode composite is deposited on one side of the solid electrolyte layer, and both ends of the cathode composite are sandwiched between current collectors (the anode current collector on the solid electrolyte layer side and the cathode current collector on the cathode composite side) and pressed to form a cathode composite layer and a cathode current collector on one side of the solid electrolyte layer, and a cathode current collector on the other side of the solid electrolyte layer. Finally, the anode current collector is temporarily removed, and a cathode is inserted on the side of the solid electrolyte layer opposite the cathode composite layer. Then, a cathode current collector is inserted on the anode side and pressed to form a cathode and anode current collector on the other side of the solid electrolyte layer. Furthermore, each layer may be pressed individually as described above, or two or more layers may be deposited and pressed together to form a laminate. This method allows the fabrication of an all-solid-state lithium-sulfur battery.

[0075] <Applications of all-solid-state lithium-sulfur batteries> The applications of the all-solid-state lithium-sulfur battery are not particularly limited, but it can be suitably used in electrical products that require high energy density, such as hybrid vehicles and electric vehicles. [Example]

[0076] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0077] 1.Raw materials used In the examples and comparative examples, the following materials were used. 1-1. Sulfur or its discharge products (A) Sulfur (Fujifilm Wako Pure Chemical Industries, Ltd.) Lithium sulfide (Mitsuwa Chemical Co., Ltd.) 1-2. Diphosphorus pentasulfide (B) Diphosphorus pentasulfide (Sigma-Aldrich) 1-3.Conductive carbon (C) Activated carbon (Kansai Thermochemical Co., Ltd., specific surface area 3000 m 2 / g) Acetylene black (Denka Co., Ltd., specific surface area 68m 2 / g) 1-4. Lithium Halide (D) Lithium iodide (Sigma-Aldrich) 1-5. Red phosphorus Red phosphorus (Sigma-Aldrich)

[0078] 2. Manufacturing of cathode composite material Example 1 As sulfur and / or its discharge product (A), 81 mg of sulfur and 17 mg of lithium sulfide were weighed out so that the weight ratio of sulfur to lithium sulfide was 83:17, 50 mg of diphosphorus pentasulfide (B), 20 mg of activated carbon as conductive carbon (C), and 32 mg of lithium iodide as lithium halide (D). The weight ratios of components (A) to (D) are shown in Table 1.

[0079] A positive electrode composite for an all-solid-state lithium-sulfur battery was obtained by mixing the mixture in a planetary ball mill (Frilsch Premium Line P-7, revolution radius 0.07 m, rotation radius 0.0235 m, rotation to revolution ratio = -2) together with approximately 40 g of 5 mm zirconia balls in a 45 ml pot at a revolution speed of 370 rpm for 2 hours.

[0080] (Comparative Example 1) Components (A) to (D) were weighed out in the same raw materials and charged amounts as in Example 1. Instead of ball mill treatment, the mixture was mixed in a mortar for 30 minutes to obtain a positive electrode mixture for an all-solid-state lithium-sulfur battery.

[0081] (Comparative Example 2) To obtain the same element ratio as in Example 1, red phosphorus was used instead of diphosphorus pentasulfide (B), and 117 mg of sulfur and 17 mg of lithium sulfide were weighed out as sulfur and / or its discharge product (A) so that the weight ratio of sulfur to lithium sulfide was 87:13, as well as 14 mg of red phosphorus, 20 mg of activated carbon as conductive carbon (C), and 32 mg of lithium iodide as lithium halide (D).

[0082] A positive electrode composite for an all-solid-state lithium-sulfur battery was obtained by mixing the mixture in a planetary ball mill (Frilsch Premium Line P-7, revolution radius 0.07 m, rotation radius 0.0235 m, rotation to revolution ratio = -2) together with approximately 40 g of 5 mm zirconia balls in a 45 ml pot at a revolution speed of 370 rpm for 2 hours.

[0083] Example 2 A positive electrode composite for an all-solid-state lithium-sulfur battery was obtained in the same manner as in Example 1, except that 100 mg of sulfur and 0 mg of lithium sulfide were weighed as the sulfur and / or its discharge product (A), 40 mg of diphosphorus pentasulfide (B), 20 mg of activated carbon as the conductive carbon (C), and 40 mg of lithium iodide as the lithium halide (D).

[0084] Example 3 A positive electrode composite for an all-solid-state lithium-sulfur battery was obtained in the same manner as in Example 1, except that 90 mg of sulfur and 0 mg of lithium sulfide were weighed as sulfur and / or its discharge products (A), 50 mg of diphosphorus pentasulfide (B), 20 mg of activated carbon as conductive carbon (C), and 40 mg of lithium iodide as lithium halide (D).

[0085] Example 4 A positive electrode composite for an all-solid-state lithium-sulfur battery was obtained in the same manner as in Example 1, except that 85 mg of sulfur and 15 mg of lithium sulfide were weighed out as sulfur and / or its discharge product (A) so that the weight ratio of sulfur to lithium sulfide was 83:17, 50 mg of diphosphorus pentasulfide (B), 20 mg of activated carbon as conductive carbon (C), and 30 mg of lithium iodide as lithium halide (D).

[0086] Example 5 A positive electrode composite for an all-solid-state lithium-sulfur battery was obtained in the same manner as in Example 1, except that 90 mg of sulfur and 20 mg of lithium sulfide were weighed out as sulfur and / or its discharge product (A) so that the weight ratio of sulfur to lithium sulfide was 82:18, 50 mg of diphosphorus pentasulfide (B), 20 mg of activated carbon as conductive carbon (C), and 20 mg of lithium iodide as lithium halide (D).

[0087] Example 6 A positive electrode composite for an all-solid-state lithium-sulfur battery was obtained in the same manner as in Example 1, except that 100 mg of sulfur and 19 mg of lithium sulfide were weighed out as sulfur and / or its discharge product (A) so that the weight ratio of sulfur to lithium sulfide was 84:16, 61 mg of diphosphorus pentasulfide (B), and 20 mg of activated carbon were weighed out as conductive carbon (C).

[0088] (Comparative Example 3) The same raw materials and amounts as those in Example 6 were mixed in a mortar for 30 minutes instead of being subjected to a ball mill treatment, to obtain a positive electrode mixture for an all-solid-state lithium-sulfur battery.

[0089] Comparative Example 4 A positive electrode mixture for an all-solid-state lithium-sulfur battery was obtained in the same manner as in Example 1, except that 20 mg of acetylene black was used as the conductive material instead of activated carbon.

[0090] 3. Battery Fabrication A SUS304 cylindrical jig (10 mm diameter, 10 mm height) was inserted from the bottom of a polycarbonate cylindrical tube (inner diameter 10 mm, outer diameter 23 mm diameter, height 20 mm) as a negative electrode current collector. 70 mg of a solid electrolyte (a 90:10 mixture by weight of compound 1, prepared by calcining 5LiS-GeS-P2S5 at 510°C for 8 hours, and compound 2, prepared by treating 80LiS-20P2S5 in a ball mill at 500 rpm for 10 hours) was placed in the top of the polycarbonate cylindrical tube. Another SUS304 cylindrical jig (10 mm diameter, 15 mm height) was inserted from the top of the polycarbonate cylindrical tube to sandwich the solid electrolyte. The solid electrolyte was then pressed under a pressure of 200 MPa for 3 minutes to form a solid electrolyte layer with a diameter of 10 mm and a thickness of approximately 0.6 mm.

[0091] Next, the SUS304 cylindrical jig (positive electrode current collector) inserted from above was temporarily removed, and 7.5 mg of each of the positive electrode composites prepared in Examples 1 to 6 and Comparative Examples 1 to 4 was placed on top of the solid electrolyte layer in the polycarbonate cylindrical tube.The SUS304 cylindrical jig (positive electrode current collector) was then inserted from above again, and pressed at a pressure of 200 MPa for 3 minutes to form a positive electrode composite layer with a diameter of 10 mmΦ and a thickness of approximately 0.1 mm.

[0092] Next, the SUS304 cylindrical jig (negative electrode current collector) inserted from the bottom was removed, and a 0.25 mm thick lithium sheet (manufactured by Furuuchi Chemical Co., Ltd.) punched to a diameter of 8 mm with a hole punch and a 0.3 mm thick indium sheet (manufactured by Furuuchi Chemical Co., Ltd.) punched to a diameter of 9 mm with a hole punch were stacked and inserted from the bottom of a polycarbonate cylindrical tube jig, and another SUS304 cylindrical jig (negative electrode current collector) was inserted from the bottom again and pressed at a pressure of 80 MPa for 3 minutes to form a lithium-indium alloy negative electrode. In this way, an all-solid-state lithium-sulfur battery was produced in which, from bottom to top, the negative electrode current collector, lithium-indium alloy negative electrode, solid electrolyte layer, positive electrode composite layer, and positive electrode current collector were stacked.

[0093] 4. Evaluation Method 4-1. Calculation of composite index of cathode composite by 31P-MAS NMR The 31P-MAS NMR of the prepared positive electrode composite was measured using a Bruker Avance III HD 600WB spectrometer with ammonium phosphate (1.0 ppm) as an external standard. Table 1 shows the relative intensity (including spinning side bands) at 50 ppm, where the diphosphorus pentasulfide peak appears, with the highest intensity being 100 and the lowest intensity being 0. The remaining amount of diphosphorus pentasulfide, the raw material, is used as the composite index, and the composite reaction proceeds until the relative intensity of the 50 ppm peak is 40% or less, resulting in a positive electrode composite that exhibits high battery performance.

[0094] 4-2. Evaluation of battery charge / discharge characteristics The fabricated all-solid-state lithium-sulfur battery was used in a charge-discharge device (ACD-M01A, manufactured by Asuka Electronics Co., Ltd.) at a current of 0.64 mA / cm. 2 The capacity per positive electrode mixture was measured when the battery was charged and discharged at a constant current density of 1.0 V and discharged to 1.0 V. The results are shown in Table 1.

[0095] [Table 1]

[0096] In Comparative Examples 1 and 3, the relative peak intensity at 50 ppm in 31P-MAS NMR exceeded 40%, resulting in low charge / discharge capacities. In Comparative Example 4, acetylene black with a small specific surface area was used instead of activated carbon as the conductive material, resulting in the relative peak intensity at 50 ppm in 31P-MAS NMR exceeding 40%, resulting in low charge / discharge capacities. In Comparative Example 2, red phosphorus was used instead of diphosphorus pentasulfide as the raw material for the solid electrolyte, resulting in low charge / discharge capacities. Examples 1 to 6 showed high charge / discharge capacities.

Claims

1. The method includes subjecting a mixture containing sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D = 40 to 70:10 to 50:5 to 20:1 to 30 to mechanical milling. A method for producing a positive electrode mixture for an all-solid-state lithium-sulfur battery, comprising: The relative intensity of the peak at 50 ppm in 31P-MAS NMR of the positive electrode composite is 40% or less; Manufacturing method.

2. The method includes subjecting a mixture containing sulfur or its discharge product (A), diphosphorus pentasulfide (B), conductive carbon (C), and lithium halide (D) in a weight ratio of A:B:C:D = 40 to 70:10 to 50:5 to 20:0 to 30 to mechanical milling at a gravitational acceleration of 20 G or less for 10 hours or less, A method for producing a positive electrode mixture for an all-solid-state lithium-sulfur battery, comprising: The relative intensity of the peak at 50 ppm in 31P-MAS NMR of the positive electrode composite is 40% or less; Manufacturing method.

3. The specific surface area of ​​the conductive carbon (C) is 1000 m ２ The method according to claim 1 or 2, wherein the SiO2 content is 1 / g or more.

4. The sulfur or its discharge product (A) is a mixture containing sulfur and lithium sulfide in a weight ratio of sulfur:lithium sulfide = 100 to 70: 0 to 30. The production method according to any one of claims 1 to 3.

5. The method according to any one of claims 1 to 4, wherein the lithium halide (D) is lithium iodide.

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