Positive electrode for all-solid-state battery

Abstract

The purpose of the present invention is to provide a positive electrode for an all-solid battery which is excellent in strength and can improve cycle durability of a battery.　According to the present invention, there is provided a positive electrode for an all-solid-state battery having a positive electrode active material layer containing a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene, and a fibrous conductive assistant, wherein the positive electrode active material layer contains a composite of a film made of the polytetrafluoroethylene and the fibrous conductive assistant.

Description

Positive electrode for all-solid-state batteries

[0001] The present invention relates to a positive electrode for an all-solid-state battery.

[0002] In recent years, research and development into all-solid-state batteries using oxide- or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials primarily composed of ionic conductors capable of conducting ions in a solid state. Therefore, all-solid-state batteries have the advantage that, in principle, they do not encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based batteries using nonaqueous electrolytes. Furthermore, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the output density and energy density of batteries.

[0003] In a typical all-solid-state battery, the positive electrode has a configuration in which a positive electrode active material layer is disposed on the surface of a positive electrode current collector. The positive electrode active material layer contains, in addition to the positive electrode active material, a conductive additive for improving the electrical conductivity in the positive electrode active material layer, a solid electrolyte for improving the lithium ion conductivity in the positive electrode active material layer, and a binder for binding particles of the positive electrode active material, the conductive additive, and the solid electrolyte to each other and to the positive electrode current collector.

[0004] Here, WO 2022 / 050252 discloses the use of a polytetrafluoroethylene resin having a predetermined fine fibrous structure as a binder contained in an all-solid-state battery mixture.

[0005] However, according to the investigations of the present inventors, it was found that even when the binder described in WO 2022 / 050252 was used, the durability was low and the strength was insufficient.

[0006] Therefore, an object of the present invention is to provide a positive electrode for an all-solid-state battery that has excellent strength and can improve the cycle durability of the battery.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that the above-mentioned problems can be solved by using a fibrous conductive additive and forming a specific composite structure between the fibrous conductive additive and polytetrafluoroethylene in a positive electrode active material layer. They have also found that a positive electrode active material layer containing this composite can be obtained by heating and rolling a positive electrode mixture containing fibrous polytetrafluoroethylene and a fibrous conductive additive, and have completed the present invention.

[0008] That is, one embodiment of the present invention is a positive electrode for an all-solid-state battery having a positive electrode active material layer including a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene, and a fibrous conductive additive, wherein the positive electrode active material layer includes a composite of a film made of the polytetrafluoroethylene and the fibrous conductive additive.

[0009] Another aspect of the present invention is a method for manufacturing a positive electrode for an all-solid-state battery, the method having a positive electrode active material layer including a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene, and a fibrous conductive additive, the method including a step of rolling a positive electrode mixture containing the positive electrode active material, the solid electrolyte, the fibrous conductive additive, and the fibrous polytetrafluoroethylene while heating it to 40°C or higher.

[0010] Fig. 1 is a perspective view showing the appearance of a flat laminate-type all-solid-state lithium secondary battery, which is one embodiment of the all-solid-state battery according to the present invention. Fig. 2 is a cross-sectional view taken along line 2-2 shown in Fig. 1. Figs. 3(a) and 3(b) are SEM images of the samples of the positive electrode active material layer prepared in Comparative Example 2 and Example 2 after hydrochloric acid treatment, respectively.

[0011] One aspect of the present invention is a positive electrode for an all-solid-state battery having a positive electrode active material layer including a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene, and a fibrous conductive additive, wherein the positive electrode active material layer includes a composite of a film made of the polytetrafluoroethylene and the fibrous conductive additive. According to this aspect, a positive electrode with excellent strength can be obtained. Furthermore, the cycle durability of an all-solid-state battery using the positive electrode for an all-solid-state battery of this aspect can be improved.

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention should be defined based on the description of the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0013] FIG. 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery according to one embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1. The layered structure allows the battery to be compact and have a high capacity. This specification will be described in detail using the flat-layered, non-bipolar all-solid-state lithium secondary battery shown in FIGS. 1 and 2 (hereinafter, also simply referred to as a "layered secondary battery"). However, in terms of the internal electrical connection configuration (electrode structure) of the secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

[0014] As shown in Fig. 1, the stacked secondary battery 10a has a flat rectangular shape, with a negative current collector 25 and a positive current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked secondary battery 10a, and the periphery is heat-sealed, sealing the power generating element 21 with the negative current collector 25 and the positive current collector 27 extending to the outside. Note that the extension of the current collectors (25, 27) shown in Fig. 1 may be such that the negative current collector 25 and the positive current collector 27 are extended from the same side, or the negative current collector 25 and the positive current collector 27 may each be divided into multiple pieces and extended from each side; the extension is not limited to that shown in Fig. 1.

[0015] As shown in FIG. 2 , during charging, the power generating element 21 of the stacked secondary battery 10 a of this embodiment has a configuration in which a negative electrode in which a negative electrode active material layer 13 containing lithium metal is disposed on both sides of a negative electrode current collector 11′, a solid electrolyte layer 17, and a positive electrode in which a positive electrode active material layer 15 containing a lithium transition metal composite oxide is disposed on both sides of a positive electrode current collector 11″ are laminated. Specifically, the negative electrode, solid electrolyte layer, and positive electrode are laminated in this order such that one negative electrode active material layer 13 and an adjacent positive electrode active material layer 15 face each other with the solid electrolyte layer 17 interposed therebetween. As a result, adjacent negative electrode, solid electrolyte layer, and positive electrode constitute one unit cell layer 19. Therefore, the stacked secondary battery 10 a shown in FIG. 2 can also be said to have a configuration in which a plurality of unit cell layers 19 are laminated and electrically connected in parallel.

[0016] A negative electrode current collector 25 and a positive electrode current collector 27 that are electrically connected to the respective electrodes (negative and positive electrodes) are attached to the negative electrode current collector 11′ and the positive electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and led out of the laminate film 29. The negative electrode current collector 25 and the positive electrode current collector 27 may be attached to the negative electrode current collector 11′ and the positive electrode current collector 11″ of the respective electrodes by ultrasonic welding, resistance welding, or the like, via a negative electrode terminal lead and a positive electrode terminal lead (not shown), respectively, as necessary.

[0017] In the stacked secondary battery 10a according to this embodiment, the power generating element 21 sealed in the laminate film 29 shown in FIG. 1 is preferably sandwiched between two plate-shaped members and further fastened using a fastening member. As a result, the plate-shaped members and fastening members function as pressure members that pressurize (restrain) the power generating element 21 in the stacking direction. Examples of the plate-shaped members include metal plates and resin plates. Examples of the fastening members include bolts and nuts. However, the pressure members are not particularly limited as long as they can pressurize the power generating element 21 in the stacking direction. A typical pressure member is a combination of a plate made of a rigid material, like the plate-shaped members, and the fastening members described above. Furthermore, the fastening members may be not only bolts and nuts, but also tension plates or the like that secure the ends of the plate-shaped members so as to restrain the power generating element 21 in the stacking direction. The lower limit of the load applied to the power generating element 21 (constraint pressure in the stacking direction of the power generating element) is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and even more preferably 5 MPa or more. The upper limit of the constraint pressure in the stacking direction of the power generating element is, for example, 100 MPa or less, preferably 70 MPa or less, more preferably 40 MPa or less, and even more preferably 10 MPa or less.

[0018] The main components of the stacked secondary battery 10a will be described below.

[0019] [Current Collectors] The current collectors (negative electrode current collector 11′ and positive electrode current collector 11″) are conductive members that function as a flow path for electrons that are released from the positive electrode toward an external load or that flow from a power source toward the positive electrode as the battery reaction (charge / discharge reaction) progresses. There are no particular restrictions on the material that constitutes the current collectors, and for example, metals or conductive resins can be used.

[0020] Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Foils in which aluminum is coated on the surface of a metal may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of active materials.

[0021] The latter conductive resin may be a conductive polymer material or a non-conductive polymer material to which a conductive filler is added as needed.

[0022] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector includes at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the negative electrode active material layer and the positive electrode active material layer are themselves electrically conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate member from these active material layers. In such a configuration, the negative electrode active material layer directly constitutes the negative electrode, and the positive electrode active material layer directly constitutes the positive electrode.

[0023] There is no particular limitation on the thickness of the current collector, but an example is 10 to 100 μm.

[0024] [Negative Electrode Active Material Layer] The negative electrode active material layer 13 contains a negative electrode active material and may optionally contain a solid electrolyte, a binder, and a conductive additive. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. A lithium-containing active material may also be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. The negative electrode active material preferably contains lithium metal or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and more preferably contains lithium metal or a lithium-containing alloy. When lithium metal or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery is preferably a so-called lithium deposition type in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during charging. In this case, the layer of lithium metal deposited on the negative electrode current collector during the charging process becomes the negative electrode active material layer, and the thickness of the negative electrode active material layer increases as the charging process progresses and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during full discharge, but in some cases, a negative electrode active material layer consisting of a certain amount of lithium metal may be present during full discharge. The thickness of the negative electrode active material layer (lithium metal layer) during full charge is not particularly limited, but is typically 0.1 to 1000 μm.

[0025] [Solid Electrolyte Layer] The solid electrolyte layer 17 is interposed between the positive electrode active material layer and the negative electrode active material layer, and is a layer that essentially contains a solid electrolyte.

[0026] Examples of the solid electrolyte include sulfide solid electrolytes and oxide solid electrolytes, and the sulfide solid electrolyte is preferred. In this specification, the term "solid electrolyte" refers to a material mainly composed of an ion conductor capable of conducting ions in a solid state, and in particular, a material having a lithium ion conductivity of 1×10 at room temperature (25° C.). －５ S / cm or more, and this lithium ion conductivity is preferably 1×10－４ The ionic conductivity is 200 S / cm or more. Here, the value of the ionic conductivity can be measured by an AC impedance method.

[0027] Examples of sulfide solid electrolytes include LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ , Li ２ S-P ２ S ５ , LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ , Li ２ S-P ２ S ５ -Li ２ O, Li ２ S-P ２ S ５ -Li ２ O-LiI, Li ２ S-SiS ２ , Li ２ S-SiS ２ - LiI, Li ２ S-SiS ２ - LiBr, Li ２ S-SiS ２ -LiCl, Li ２ S-SiS ２ -B ２ S ３ - LiI, Li ２ S-SiS ２ -P ２ S ５ - LiI, Li ２ S-B ２ S ３ , Li ２ S-P ２ S ５ -Z ｍ S ｎ (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li ２ S-GeS２ , Li ２ S-SiS ２ -Li ３ P.O. ４ , Li ２ S-SiS ２ -Li ｘ MO ｙ (where x and y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In). ２ S-P ２ S ５ " is written by Li ２ S and P ２ S ５ The same applies to other descriptions.

[0028] The sulfide solid electrolyte is, for example, Li ３ P.S. ４ It may have a Li framework. ４ P ２ S ７ It may have a Li framework. ４ P ２ S ６ It may have a Li skeleton. ３ P.S. ４ Examples of sulfide solid electrolytes having a skeleton include LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ In addition, Li ４ P ２ S ７ Examples of the sulfide solid electrolyte having a skeleton include a Li-P-S solid electrolyte called LPS (for example, Li ７ P ３ S １１ ) can be mentioned. In addition, examples of sulfide solid electrolytes include Li （４－ｘ） Ge （１－ｘ） P ｘ S ４(x satisfies 0<x<1), or the like. Among them, the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing a P element, and the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing Li ２ S-P ２ S ５ It is more preferable that the sulfide solid electrolyte is a material mainly composed of Li. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte is a material mainly composed of Li. ６ P.S. ５ X, where X is Cl, Br or I, preferably Cl.

[0029] Examples of oxide solid electrolytes include compounds having a NASICON structure. An example of a compound having a NASICON structure is a compound represented by the general formula Li １＋ｘ Al ｘ Ge ２－ｘ (P.O. ４ ) ３ (0≦x≦2) (LAGP), a compound represented by the general formula Li １＋ｘ Al ｘ Ti ２－ｘ (P.O. ４ ) ３ (0≦x≦2) (LATP) and the like. Another example of the oxide solid electrolyte is LiLaTiO (for example, Li ０．３４ La ０．５１ TiO ３ ), LiPON (e.g., Li ２．９ P.O. ３．３ N ０．４６ ), LiLaZrO (e.g., Li ７ La ３ Zr ２ O １２ ) etc.

[0030] Examples of the shape of the solid electrolyte include particulate shapes such as spherical and oval spheres, and thin films. When the solid electrolyte is particulate, its average particle diameter (D50) is not particularly limited, but is preferably 40 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. On the other hand, the average particle diameter (D50) is preferably 0.01 μm or more, and more preferably 0.1 μm or more.

[0031] The solid electrolyte layer may further contain a binder in addition to the above-described solid electrolyte. The binder is not particularly limited, and examples thereof include thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene (PTFE), polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (P Fluorine resins such as vinylidene fluoride (CTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc., vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride Examples of the fluororubber include vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber); epoxy resin; and carboxymethyl cellulose.Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

[0032] The thickness of the solid electrolyte layer varies depending on the intended configuration of the all-solid-state battery, but is preferably within the range of 0.1 to 1000 μm, and more preferably 10 to 40 μm.

[0033] (Positive Electrode Active Material Layer) The positive electrode active material layer includes a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene (PTFE), and a fibrous conductive additive. The positive electrode active material layer has a structure including a composite of a film made of PTFE and the fibrous conductive additive.

[0034] (Positive electrode active material) The positive electrode active material is not particularly limited as long as it can release lithium ions during the charging process of the all-solid-state battery and absorb lithium ions during the discharging process. An example of such a positive electrode active material is a material containing an M1 element and an O element, where the M1 element contains at least one element selected from the group consisting of Li, Mn, Ni, Co, Cr, Fe, and P. An example of such a positive electrode active material is LiCoO ２ , LiMnO ２ , LiNiO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi ０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ olivine-type active materials such as LiMnPO4, Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ , LiVO ２ Examples include:

[0035] Furthermore, the positive electrode active material may contain elemental sulfur. Examples of positive electrode active materials containing elemental sulfur include, but are not limited to, elemental sulfur (S), as well as particles or thin films of organic sulfur compounds or inorganic sulfur compounds. Any material may be used as long as it utilizes the oxidation-reduction reaction of sulfur to release lithium ions during charging and absorb lithium ions during discharging. Examples of organic sulfur compounds include disulfide compounds, sulfur-modified polyacrylonitrile, sulfur-modified polyisoprene, rubeanic acid (dithiooxamide), polycarbon sulfide, and the like. Among these, disulfide compounds, sulfur-modified polyacrylonitrile, and rubeanic acid are preferred, with sulfur-modified polyacrylonitrile being particularly preferred. As disulfide compounds, those having dithiobiurea derivatives, thiourea groups, thioisocyanates, or thioamide groups are more preferred. On the other hand, inorganic sulfur compounds are preferred due to their excellent stability. Specific examples include elemental sulfur (S), S-carbon composites, and Li ２ S, TiS ２ , TiS ３ , TiS ４ , NiS, NiS ２ , CuS, FeS ２ , MoS ２ , MoS ３ Among them, S, Li ２ S, S-carbon composite, TiS ２ , TiS ３ , TiS ４ , FeS ２ and MoS ２ are preferred, and elemental sulfur (S), Li ２ S, TiS ２ and FeS ２ is more preferable, and from the viewpoint of high capacity, elemental sulfur (S) or Li ２ S is particularly preferred. As the elemental sulfur (S), α-sulfur, β-sulfur, or γ-sulfur having an S structure can be used. These elemental sulfur (S) absorb lithium ions during discharge and exist in the positive electrode active material layer in the form of lithium (poly)sulfides.

[0036] In some cases, two or more positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those mentioned above may also be used.

[0037] The positive electrode active material is preferably in the form of particles. When the positive electrode active material is in the form of particles, the average particle diameter (D50) is preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. The average particle diameter (D50) of the positive electrode active material is determined as the average value of the particle diameters of the positive electrode active material observed in several to several tens of fields of view using a scanning electron microscope (SEM) on a cross section of the positive electrode active material layer.

[0038] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 30 to 99 mass %, more preferably in the range of 40 to 90 mass %, and even more preferably in the range of 50 to 85 mass %.

[0039] (Solid electrolyte) The specific form of the solid electrolyte contained in the positive electrode active material layer is not particularly limited, and the solid electrolytes exemplified in the section on the solid electrolyte layer and their preferred forms can be similarly adopted. In some cases, a solid electrolyte other than the above-mentioned solid electrolytes may be used in combination.

[0040] The content of the solid electrolyte in the positive electrode active material layer is, for example, preferably in the range of 1 to 60 mass %, and more preferably in the range of 10 to 50 mass %.

[0041] (Fibrous conductive additive) The fibrous conductive additive can contribute to improving the conductivity (reducing resistance) in the active material layer. In this specification, the term "fibrous conductive additive" refers to a conductive additive having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less in an image obtained by observing the cross section of the positive electrode active material layer using a scanning electron microscope (SEM). The aspect ratio is calculated by dividing the maximum Feret diameter by the minimum Feret diameter. The maximum Feret diameter is the maximum distance between two parallel lines when the contour of the conductive additive is sandwiched between the lines, and the minimum Feret diameter is the minimum distance between the lines when the contour of the conductive additive is sandwiched between two parallel lines. The electronic conductivity of the fibrous conductive additive is preferably 1 S / m or more, and 1×10 ２ S / m or more is more preferable, and 1×10 ４ It is more preferable that the density is 1×10 S / m or more. ５ The upper limit of the electronic conductivity of the fibrous conductive additive is not particularly limited, but is usually 1×10 ７ S / m or less.

[0042] As the fibrous conductive additive, fibrous carbon is preferred because it is lightweight and has excellent conductivity. The type of fibrous carbon is not particularly limited as long as it has the above-mentioned shape, but examples include carbon fiber (carbon nanofiber), graphene, and carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes). Of these, carbon fiber (carbon nanofiber) and carbon nanotubes are preferred. Only one type of fibrous carbon may be used alone, or two or more types may be used in combination.

[0043] The average fiber length of the fibrous conductive additive contained in the positive electrode active material layer is not particularly limited, but is, for example, 5 μm or more, more preferably 7 μm or more, and even more preferably 10 μm or more. Within the above range, a stronger composite can be obtained. Furthermore, this is preferable because a sufficient electron conduction path can be obtained, the utilization rate of the positive electrode active material can be improved, and the capacity retention rate can be improved. The upper limit of the average fiber length is not particularly limited, but is, for example, 100 μm or less. The average fiber length of the fibrous conductive additive can be determined as the average value of the fiber lengths of the fibrous conductive additive observed by photographing approximately 10 fields of view of the cross section of the positive electrode active material layer using a scanning electron microscope (SEM).

[0044] In addition, in the positive electrode active material layer, the average fiber length of the fibrous conductive additive is preferably greater than the average particle diameter (D50) of the positive electrode active material. This can result in a stronger composite. Furthermore, it is possible to prevent the electron conduction path of the positive electrode active material from being broken due to expansion and contraction of the positive electrode active material during charging, and it is possible to maintain electronic contact between the positive electrode active materials. Therefore, the effects of the present invention can be more significantly obtained. The ratio of the average fiber length of the fibrous conductive additive to the average particle diameter (D50) of the positive electrode active material is, for example, 1.1 or more, preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more. The upper limit of the ratio of the average fiber length of the fibrous conductive additive to the average particle diameter (D50) of the positive electrode active material is not particularly limited, but is, for example, 100 or less.

[0045] The content of the fibrous conductive additive in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 2% by mass or more and 5% by mass or less.

[0046] The positive electrode active material layer may further contain a conductive additive (non-fibrous conductive additive) other than the fibrous conductive additive. The particle shape of the non-fibrous conductive additive is not particularly limited, and may be any shape, such as powder, sphere, rod, needle, plate, column, irregular shape, flaky, or spindle-shaped. By further using such a conductive additive, the number of electronic contact points increases, thereby further reducing the resistance of the positive electrode. Examples of such conductive additives include carbon blacks such as acetylene black, Ketjen Black (registered trademark), furnace black, channel black, and thermal black.

[0047] When a non-fibrous conductive additive is contained, the proportion of the non-fibrous conductive additive is, for example, more than 0 and less than 50 mass%, preferably 1 to 30 mass%, and more preferably 1 to 10 mass%, relative to the total mass of the conductive additive contained in the positive electrode active material layer.

[0048] (Binder) The positive electrode active material layer contains polytetrafluoroethylene (PTFE) as a binder. PTFE is a polymer of tetrafluoroethylene, and can be fibrillated (fiberized) by the shear force of kneading. This allows the positive electrode active material, solid electrolyte, and fibrous conductive additive to become entangled and held in the fibrous structure of the fibrillated PTFE. Meanwhile, it has been reported that PTFE undergoes a crystalline phase transition around 19°C, softening the crystals. Around this transition temperature, molded articles undergo dimensional changes and mechanical properties also change (Thermal Analysis of Powder-Molded Polytetrafluoroethylene Films, Kobunshi Ronbunshu, Vol. 66, No. 12, pp. 585-590 (Dec., 2009)).

[0049] The polytetrafluoroethylene may include those in which the terminals or part of the side chains are substituted (modified) with other substituents. In the case where the terminals or part of the side chains are substituted (modified) with other substituents, the proportion of the structural units in which the terminals or side chains are substituted (modified) with other substituents relative to 100 mol % of all structural units is preferably 10 mol % or less, more preferably 5 mol % or less.

[0050] The positive electrode active material layer may contain a binder other than PTFE. For example, the same binder as the binder exemplified in the solid electrolyte layer section can be used as the other binder. However, from the viewpoint of further improving the mechanical strength of the positive electrode active material layer, the proportion of the other binder is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, particularly preferably 1% by mass or less, and most preferably 0% by mass (i.e., no other binder is included) relative to the total mass of the binder contained in the positive electrode active material layer.

[0051] The content of PTFE in the positive electrode active material layer is not particularly limited, but is, for example, 0.1 to 5 mass %, preferably 0.1 to 3 mass %, more preferably 0.2 to 2 mass %, even more preferably 0.3 to 1.2 mass %, and still more preferably 0.4 to 1 mass %, relative to the total solid content contained in the positive electrode active material layer. If it is within the above range, an excellent balance between energy density and strength is achieved.

[0052] (Composite of Polytetrafluoroethylene Film and Fibrous Conductive Aid) The positive electrode of this embodiment has a composite of polytetrafluoroethylene film and fibrous conductive aid in the positive electrode active material layer.

[0053] FIG. 3(a) shows an SEM image of the positive electrode active material layer of a conventional positive electrode, and FIG. 3(b) shows an SEM image of the positive electrode active material layer of the positive electrode of the present embodiment. In these images, components other than PTFE and the fibrous conductive additive have been removed to observe the structure of the composite. As shown in FIG. 3(a), in the conventional positive electrode active material layer, PTFE is primarily present in a fibrous form. The fibrous PTFE and the fibrous conductive additive are entangled with each other, but the fibrous conductive additive is not embedded in the PTFE, and the spatial position of the fibrous conductive additive is not fixed by the PTFE. Furthermore, no film-like PTFE is observed. In contrast, as shown in FIG. 3(b), in the positive electrode active material layer of the present embodiment, a film-like PTFE is observed in addition to the fibrous PTFE. This PTFE film is formed in the in-plane direction of the positive electrode active material layer, and the PTFE film and the fibrous conductive additive form a composite. In a preferred embodiment, a fibrous conductive additive is embedded in a film made of PTFE. In a preferred embodiment, in the composite, the spatial position of the fibrous conductive additive is fixed by PTFE. In a preferred embodiment, the composite is formed by kneading the fibrous conductive additive into softened PTFE and then curing it. In a preferred embodiment, the composite has an integrated structure in which the fibrous conductive additive is entangled while the softened PTFE is crushed. Note that in the positive electrode of this embodiment, it is sufficient that at least a portion of the PTFE contained in the positive electrode active material layer is in the form of a film to form a composite with the fibrous conductive additive.

[0054] Fibrous binders such as fibrillated PTFE are used to fabricate electrode active material layers and solid electrolyte layers for all-solid-state secondary batteries using a dry process without using a solvent. In secondary batteries such as lithium secondary batteries, the positive electrode active material expands and contracts during charge and discharge reactions. Generally, the positive electrode active material, solid electrolyte, and conductive additive in the positive electrode active material layer are maintained in contact with each other. However, large expansion and contraction of the positive electrode active material can create voids between the positive electrode active material and each component, potentially isolating the positive electrode active material from the electronic and / or ionic conduction paths, resulting in reduced utilization of the positive electrode active material. This can result in insufficient battery cycle durability. Compared to the use of particulate or other conductive additives, the use of fibrous conductive additives facilitates maintaining contact between the positive electrode active material and the electronic conduction path even when the positive electrode active material contracts during charging. On the other hand, because all-solid-state batteries lack the electrolyte fluidity of liquid-based batteries, maintaining the ionic conduction path when the positive electrode active material contracts is difficult. Therefore, even when a fibrous conductive additive is used, sufficient improvement in cycle durability may not be obtained. Also, it was found that the mechanical strength of a positive electrode active material layer prepared by a dry process using fibrous PTFE as a binder is insufficient.

[0055] In contrast, the positive electrode for an all-solid-state secondary battery according to the present embodiment has a composite of a PTFE film and a fibrous conductive additive in the positive electrode active material layer. This configuration allows the fibrous conductive additive to function as a filler that further strengthens the PTFE structure, resulting in a high-strength structure. The positive electrode for an all-solid-state secondary battery according to the present embodiment has high mechanical strength sufficient to exist as a freestanding film, resulting in excellent handleability. Furthermore, this high strength allows the amount of PTFE in the positive electrode active material layer to be reduced, thereby increasing energy density. Furthermore, the formation of a composite of a film of PTFE and a fibrous conductive additive makes it easier to ensure contact between particles of the positive electrode active material and the solid electrolyte, even when the positive electrode active material shrinks during charging, compared to when the fibrous binder and the fibrous conductive additive are present in an uncomplexed state. Therefore, the composite of the PTFE film and the fibrous conductive additive forms not only an electronic conduction path but also a high-strength ionic conduction path that can accommodate the expansion and contraction of the positive electrode active material during charging and discharging. As a result, cycle durability is believed to be further improved.

[0056] Preferably, at least one of the positive electrode active material and the solid electrolyte is embedded in the composite, and more preferably, both the positive electrode active material and the solid electrolyte are embedded in the composite, which allows electron conduction paths and ion conduction paths to be more effectively secured, thereby enabling the effects of the present invention to be more pronounced.

[0057] <Method for manufacturing a positive electrode for an all-solid-state battery> The method for manufacturing a positive electrode for an all-solid-state battery of this embodiment is not particularly limited as long as it can form the above-mentioned specific composite in the positive electrode active material layer. For example, it preferably includes a step of heating and rolling a mixture containing a positive electrode active material, a solid electrolyte, a fibrous conductive additive, and fibrous polytetrafluoroethylene to 40 ° C or higher. In this step, the fibrous PTFE can be softened by heating to 40 ° C or higher. Then, by rolling in a heated state, the softened PTFE is crushed into a film-like form and hardened. At this time, the PTFE hardens in a state in which the fibrous conductive additive is kneaded into the PTFE, thereby obtaining a structure of a composite of a film made of PTFE and a fibrous conductive additive.

[0058] That is, one aspect of the present invention is a method for manufacturing a positive electrode for an all-solid-state battery, the method having a positive electrode active material layer including a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene, and a fibrous conductive additive, the method including a step of rolling a mixture containing the positive electrode active material, the solid electrolyte, the fibrous conductive additive, and the fibrous polytetrafluoroethylene while heating it to 40°C or higher.

[0059] In a preferred embodiment, the manufacturing method includes: (1) a mixing step of mixing a positive electrode active material, a fibrous conductive additive, and a solid electrolyte; (2) a kneading step of kneading the mixture obtained in the mixing step with polytetrafluoroethylene (PTFE) to fibrillate the PTFE; and (3) a heating and rolling step of heating and rolling the mixture containing the positive electrode active material, the solid electrolyte, the fibrous conductive additive, and the fibrous polytetrafluoroethylene to 40°C or higher.

[0060] (Mixing step) In the mixing step, the positive electrode active material, the fibrous conductive additive, and the solid electrolyte are mixed. The specific forms of these components are as described above. The order in which each component is added is not particularly limited. The mixing means is also not particularly limited, and known methods can be used as appropriate.

[0061] In the mixing step, if necessary, materials other than the positive electrode active material, the solid electrolyte, and the fibrous conductive additive (hereinafter also referred to as "other materials") may be added and mixed. Examples of other materials include a non-fibrous conductive additive.

[0062] The mixing conditions are not particularly limited, and the mixing time is not particularly limited, but is, for example, 1 to 120 minutes.

[0063] The mixing step is preferably carried out in the absence of a solvent, which eliminates the need for solvent removal when forming the positive electrode active material layer. This also prevents a reaction between the solid electrolyte and the solvent. The mixing step is also preferably carried out in an inert gas atmosphere.

[0064] (Kneading Step) In the manufacturing method according to this embodiment, it is preferable to have a kneading step after the mixing step, in which the mixture obtained in the mixing step is kneaded with polytetrafluoroethylene (PTFE). The PTFE can be fibrillated (fibrous) by the application of shear force due to the kneading. The positive electrode active material, solid electrolyte, and fibrous conductive additive are entangled and held in the fibrous structure of the fibrillated PTFE. Here, the timing of adding polytetrafluoroethylene (PTFE) is not particularly limited. For example, it may be added when the positive electrode active material, solid electrolyte, and fibrous conductive additive are placed in a mixing container during the mixing step, or it may be added after the mixing step and before the kneading step. In particular, it is preferable to add polytetrafluoroethylene (PTFE) after the mixing step and before the kneading step, which can prevent the fibrous conductive additive from being cut.

[0065] The kneading device used for kneading is not particularly limited as long as it can apply a shear force to PTFE, and an extruder, a Banbury mixer, a roller, a kneader, etc. may be appropriately used. Kneading may also be carried out using a mortar.

[0066] The above steps produce a mixture containing a positive electrode active material, a solid electrolyte, a fibrous conductive additive, and fibrous PTFE. The mixture is preferably a powder mixture substantially free of liquid components. The liquid component content in the mixture is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, even more preferably 0.5% by mass or less, particularly preferably 0.1% by mass or less, and most preferably 0% by mass, based on 100% by mass of the mixture.

[0067] (Hot Rolling Step) In the hot rolling step, the mixture containing the positive electrode active material, solid electrolyte, fibrous conductive additive, and fibrous PTFE obtained in the kneading step is rolled while being heated to 40° C. or higher.

[0068] The rolling means is not particularly limited, and known rollers and the like can be used. A rolling device such as a roll press may also be used. The heating means is also not particularly limited, and known heating means can be appropriately employed, although a roll press equipped with a heating mechanism can be suitably used. Here, if the heating temperature is below 40°C, the PTFE in the mixture will not soften sufficiently, and a composite will not be formed. While the upper limit of the heating temperature is not particularly limited, a temperature of 100°C or less is preferred because, when the mixture is sandwiched between two substrates such as SUS foil and rolled, reactions between the components in the mixture and the SUS foil are unlikely to occur. More preferably, the heating temperature is 40 to 90°C, even more preferably 50 to 90°C, even more preferably 55 to 85°C, and particularly preferably 60 to 80°C.

[0069] According to a preferred embodiment, the powder (kneaded product) obtained by the above kneading step is sandwiched between two substrates such as SUS foil and rolled while being heated to form a sheet having a thickness of 400 to 800 μm. It is then preferable to perform multiple rolling processes while heating so that the thickness of the sheet gradually decreases until the desired thickness of the positive electrode active material layer is achieved. For example, the rolling process can be performed so that the thickness of the sheet decreases in increments of 50 to 100 μm. By performing multiple rolling processes so that the thickness of the sheet gradually decreases, it is possible to prevent the density of the positive electrode from becoming excessively high and improve handleability. This heating and rolling process can produce a sheet-shaped positive electrode active material layer having a predetermined composite structure.

[0070] The thickness of the positive electrode active material layer varies depending on the intended configuration of the all-solid-state battery, but is preferably within the range of 0.1 to 1000 μm, and more preferably 40 to 100 μm, for example.

[0071] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material or different materials.

[0072] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collector and the current collector plate may be electrically connected via a positive electrode lead or a negative electrode lead. Materials used in known secondary batteries may be used as the constituent materials of the positive electrode and the negative electrode lead. It is preferable that the portion removed from the exterior be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, causing electrical leakage and affecting products (e.g., automotive parts, particularly electronic devices).

[0073] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power generating element can be used, as shown in Figures 1 and 2 . The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior body because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0074] Although one embodiment of the all-solid-state battery of the present invention has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0075] The following embodiments are also included within the scope of the present invention: the positive electrode for an all-solid-state battery according to claim 1 having the features of claim 2; the positive electrode for an all-solid-state battery according to claim 1 or 2 having the features of claim 3; the positive electrode for an all-solid-state battery according to any one of claims 1 to 3 having the features of claim 4; and an all-solid-state battery comprising the positive electrode for an all-solid-state battery according to any one of claims 1 to 4.

[0076] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the instruments and devices used in the glove box were thoroughly dried beforehand.

[0077] <Preparation of Positive Electrode Active Material Layer> [Comparative Example 1] In a glove box with an argon atmosphere having a dew point of -68°C or lower, an NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D50): 6 μm), carbon nanotubes with an average fiber length of 20 μm as a conductive additive, and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ Cl, average particle diameter (D50): 1 μm) were weighed to a mass ratio of 70:5:25 and mixed using an agate mortar. 0.5 parts by mass of polytetrafluoroethylene (PTFE) as a binder was added to 100 parts by mass of the obtained mixed powder, and after lightly kneading, the mixture was kneaded for 2 minutes while applying shear force in the mortar. Here, the content of PTFE in the positive electrode active material layer was 0.5% by mass relative to the total solid content contained in the positive electrode active material layer. After confirming that PTFE had fibrillated, the obtained kneaded product was formed into a sheet using a hand roller in an atmosphere of 25 ° C. to obtain a positive electrode active material layer (thickness 150 μm) of this comparative example.

[0078] [Comparative Examples 2 and 3] Positive electrode active material layers of Comparative Examples 2 and 3 were obtained in the same manner as in Comparative Example 1, except that the amounts of PTFE added per 100 parts by mass of the mixed powder were 0.8 parts by mass and 1.0 parts by mass, respectively. Here, the contents of PTFE in the positive electrode active material layer were 0.8% by mass and 1.0% by mass, respectively, relative to the total solid content contained in the positive electrode active material layer.

[0079] [Example 1] In a glove box with an argon atmosphere having a dew point of -68°C or less, an NMC composite oxide (LiNi ０．８ Mn ０．１ Co ０．１ O ２ , average particle diameter (D50): 6 μm), carbon nanotubes with an average fiber length of 20 μm as a conductive additive, and an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ Cl, average particle diameter (D50): 1 μm) were weighed to a mass ratio of 70:5:25 and mixed using an agate mortar. 0.5 parts by mass of polytetrafluoroethylene (PTFE) as a binder was added to 100 parts by mass of the obtained mixed powder, and after lightly kneading, the mixture was kneaded for 2 minutes while applying shear force in a mortar. Here, the content of PTFE in the positive electrode active material layer was 0.5% by mass with respect to the total solid content contained in the positive electrode active material layer. After confirming that the PTFE had fibrillated, the obtained kneaded product was heated on a hot plate at 80 ° C. and formed into a sheet using a hand roller, and stretched to a thickness of 600 μm. Thereafter, the heated and stretched sheet-shaped molded body was stretched in a tabletop roll press with the roll surface temperature at 80 ° C., in 9 steps, while reducing the gap by 50 μm each time, until it finally reached a thickness of 150 μm, to obtain the positive electrode active material layer of this example.

[0080] [Examples 2 and 3] Positive electrode active material layers of Examples 2 and 3 were obtained in the same manner as in Example 1, except that the amounts of PTFE added per 100 parts by mass of the mixed powder were 0.8 parts by mass and 1.4 parts by mass, respectively. Here, the contents of PTFE in the positive electrode active material layer were 0.8% by mass and 1.4% by mass, respectively, relative to the total solid content contained in the positive electrode active material layer.

[0081] Example 4 A positive electrode active material layer of this example was produced in the same manner as in Example 2, except that carbon nanotubes with an average fiber length of 16 μm were used as the conductive additive.

[0082] Example 5 A positive electrode active material layer of this example was produced in the same manner as in Example 2, except that carbon nanotubes having an average fiber length of 25 μm were used as the conductive additive.

[0083] Example 6 A positive electrode active material layer of this example was produced in the same manner as in Example 2, except that carbon nanotubes having an average fiber length of 2 μm were used as the conductive additive.

[0084] Comparative Example 4 A positive electrode active material layer of this comparative example was prepared in the same manner as in Comparative Example 2, except that carbon nanotubes having an average fiber length of 2 μm were used as the conductive additive.

[0085] <Observation of a composite of a PTFE film and a fibrous conductive additive> For the positive electrode active material layers prepared in each example and comparative example, in order to observe the composite of the PTFE film and the fibrous conductive additive, the positive electrode active material layer was treated with hydrochloric acid to remove the positive electrode active material and the solid electrolyte. Specifically, the positive electrode active material layer was immersed in 35% hydrochloric acid at 25 ° C for 4 hours. The hydrochloric acid was then removed by rinsing with pure water three times, air-dried, and vacuum-dried at 45 ° C for 4 hours. The hydrochloric acid-treated sample obtained was then subjected to SEM observation.

[0086] FIG. 3 shows SEM images of samples of the positive electrode active material layer prepared in (a) Comparative Example 2 and (b) Example 2 after hydrochloric acid treatment. FIGS. 3(a) and (b) are photographs taken perpendicular to the surface of the positive electrode active material layer. As shown in FIG. 3(a), a fibrous conductive additive and fibrous PTFE are observed in the positive electrode active material layer prepared in Comparative Example 2. However, in the positive electrode active material layer prepared in Comparative Example 2, no composite of a PTFE film and a fibrous conductive additive was observed. Furthermore, a relatively large number of dark spaces were observed in the image. On the other hand, as shown in FIG. 3(b), in the positive electrode active material layer prepared in Example 2, PTFE was densely present on the surface (in a film-like form). A composite of a PTFE film and a fibrous conductive additive was confirmed. Similarly, a composite of a PTFE film and a fibrous conductive additive was confirmed in the positive electrode active material layers prepared in Examples 1 and 3 to 6. On the other hand, in the positive electrode active material layers prepared in Comparative Examples 1, 3, and 4, the composite was not observed.

[0087] <Tensile Test of Positive Electrode Active Material Layer> The positive electrode active material layers prepared in each Example and Comparative Example were subjected to a tensile test in accordance with JIS K 6251:2017 to measure their breaking strength. Dumbbell-shaped test pieces were punched out of the positive electrode active material layers, and a tensile test was performed at room temperature using a universal testing machine manufactured by Imada Co., Ltd., at a tensile speed of 10 mm / min. The width and thickness of the dumbbell portion of the test piece were measured, and the breaking strength of the positive electrode active material layer was determined. The results are shown in Table 1 below.

[0088] <Charge-Discharge Test> Evaluation cells were prepared using the positive electrode active material layers prepared in Examples 4 to 6 and Comparative Example 4 as follows, and a charge-discharge test was carried out.

[0089] (Preparation of a laminate of a solid electrolyte layer and a negative electrode current collector) In a glove box with an argon atmosphere having a dew point of −68° C. or less, an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５95 parts by mass of styrene-butadiene rubber (SBR) (average particle size (D50) 1 μm) and 5 parts by mass of styrene-butadiene rubber (SBR) as a binder were dispersed using 1,3,5-trimethylbenzene as a solvent to a solids content of 47% by mass. The binder was previously adjusted to a concentration of 6% by mass using 1,3,5-trimethylbenzene. The resulting dispersion was stirred at 1000 rpm for 3 minutes using a planetary mixer to prepare a slurry. The resulting slurry was applied to a negative electrode current collector (made of SUS430, thickness 10 μm) so that the film thickness after drying would be 40 μm. The solvent was then dried on a hot plate at 80°C for 30 minutes to obtain a laminate of a solid electrolyte layer and a negative electrode current collector.

[0090] (Joining of a positive electrode current collector, a positive electrode active material layer, and a laminate of a solid electrolyte layer and a negative electrode current collector) Using a 25 mm x 25 mm (corner R = 1 mm) punching machine manufactured by Nogami Giken Co., Ltd., the laminate of the solid electrolyte layer and the negative electrode current collector prepared above was punched into a 25 mm x 25 mm square. Using a 20 mm x 20 mm (corner R = 1 mm) punching machine manufactured by Nogami Giken Co., Ltd., the positive electrode active material layer prepared above was punched into a 20 mm x 20 mm square. Also, using a 25 mm x 25 mm (corner R = 1 mm) punching machine manufactured by Nogami Giken Co., Ltd., aluminum serving as a positive electrode current collector was punched into a 25 mm x 25 mm square. A positive electrode active material layer was placed on the positive electrode current collector, and a laminate of the solid electrolyte layer and the negative electrode current collector was stacked on top of this so that the solid electrolyte layer and the positive electrode active material layer were adjacent to each other to form a power generating element. The power generating element was placed in a 35 mm x 35 mm laminate film bag and sealed with a vacuum sealer. The above was subjected to isostatic pressing (CIP) at 80 °C and 500 MPa for 1 minute. The thickness of the solid electrolyte layer after CIP was 20 μm. This bonded the interfaces of the positive electrode current collector, positive electrode active material layer, solid electrolyte layer, and negative electrode current collector, yielding an evaluation cell that was a lithium deposition-type all-solid-state battery.

[0091] (Charge / Discharge Test) The following charge / discharge test was performed on the evaluation cell prepared above. The charge / discharge test was performed while applying a restraining pressure of 3 MPa to the evaluation cell in the stacking direction using a pressure member. First, the evaluation cell was charged at 0.01 C for 5 seconds and then placed in a 60°C thermostatic chamber for 5 hours to bring the cell temperature to 60°C. The cell was charged at a constant current equivalent to 0.05 C. When the cell voltage reached 4.2 V, the charging mode was switched to a constant voltage mode. After the current value reached 0.01 C, a 0.5-hour pause was performed. The cell was then discharged at a constant current equivalent to 0.05 C until the cell voltage reached 2.5 V, after which a 0.5-hour pause was performed. This cycle constitutes one cycle, and 10 charge / discharge cycles were performed. The discharge capacity up to the 10th cycle was measured, and the percentage (%) of the discharge capacity at the 1st cycle relative to the discharge capacity at the 10th cycle was calculated. The obtained value was taken as the capacity retention rate. The results are shown in Table 1 below.

[0092]

[0093] From the results shown in Table 1, the positive electrodes of Examples 1 to 6, which have a composite structure of a PTFE film and a fibrous conductive additive, have higher strength than the positive electrodes of Comparative Examples 1 to 4, which do not contain the composite. It was also found that the batteries using the positive electrodes of Examples 4 to 6 have improved cycle durability compared to the case where the positive electrode of Comparative Example 4 was used. Furthermore, from the comparison of Examples 4 to 6, it was found that Examples 4 and 5, in which the average fiber length of the fibrous conductive additive is greater than the average particle diameter (D50) of the positive electrode active material, have higher strength and more excellent cycle durability than Example 6.

[0094] 10a: laminated battery, 11': negative electrode current collector, 11'': positive electrode current collector, 13: negative electrode active material layer, 15: positive electrode active material layer, 17: solid electrolyte layer, 19: single cell layer, 21: power generating element, 25: negative electrode current collector, 27: positive electrode current collector, 29: laminate film.

Claims

1. A positive electrode for an all-solid-state battery having a positive electrode active material layer including a positive electrode active material, a solid electrolyte, a binder including polytetrafluoroethylene, and a fibrous conductive additive, wherein the positive electrode active material layer includes a composite of a film made of the polytetrafluoroethylene and the fibrous conductive additive.

2. The positive electrode for an all-solid-state battery according to claim 1, wherein at least one of the positive electrode active material and the solid electrolyte is embedded in the composite.

3. The positive electrode for an all-solid-state battery according to claim 1 or 2, wherein an average fiber length of the fibrous conductive assistant is greater than an average particle diameter (D50) of the positive electrode active material.

4. The positive electrode for an all-solid-state battery according to claim 1 or 2, wherein the positive electrode active material layer further contains a non-fibrous conductive assistant.

5. An all-solid-state battery comprising the positive electrode for an all-solid-state battery according to claim 1 or 2.

6. A method for manufacturing a positive electrode for an all-solid-state battery having a positive electrode active material layer containing a positive electrode active material, a solid electrolyte, a binder containing polytetrafluoroethylene, and a fibrous conductive additive, the method comprising the step of rolling a mixture containing the positive electrode active material, the solid electrolyte, the fibrous conductive additive, and the fibrous polytetrafluoroethylene while heating it to 40°C or higher.

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