Resin-bonded fiber, and active material layer, electrode, and nonaqueous electrolyte secondary battery using the same

Abstract

According to the present application, there is provided a resin-bonded fiber characterized by comprising: an electrically conductive fiber having an average fiber diameter of 10 to 5,000 nm and an average aspect ratio of 30 or more; and a thermoplastic resin integrated with the electrically conductive fiber by being in contact with at least a part of a surface of the electrically conductive fiber; the resin-bonded fiber having a powder volume resistivity of 10 Ω-cm or less at a density of 0.8 g / cm 3 or more.

Description

TECHNICAL FIELD

[0001] The present application relates to resin-bonded fibers that can be used as an electrically conductive material, and an active material layer, an electrode, and a nonaqueous electrolyte secondary battery configured using the resin-bonded fibers. BACKGROUND

[0002] An active material layer of a lithium secondary battery contains at least an active material capable of occluding and releasing lithium ions, and generally uses an electrically conductive material for improving electronic conductivity and a binder material for binding them. In the case of a lithium secondary battery that is a full solid lithium secondary battery, a solid electrolyte is further contained to configure the battery.

[0003] When a lithium secondary battery is charged and discharged, the strength of the active material layer needs to be improved and the contact between the particles contained in the active material layer needs to be maintained in order to repeat charging and discharging, because expansion and contraction of the active material occur. In addition, the strength of the active material layer also needs to be improved from the viewpoint of battery size. In particular, in the case of a full solid lithium secondary battery in which the electrolyte is a solid electrolyte, the contact between the particles configuring the active material layer and the electrolyte layer must be maintained, and the requirement for strength improvement becomes higher.

[0004] In order to improve the strength of the active material layer, the use of a binder material has been proposed (Patent Literature 1), but since the binder material generally used has no electronic conductivity and ionic conductivity, it leads to a decrease in electronic conductivity and ionic conductivity of the active material layer, and there is a tendency for battery characteristics to decrease.

[0005] In addition, the use of carbon fibers to improve electronic conductivity (Patent Literatures 1 and 2) and to improve the strength of the active material layer (Patent Literature 3) has also been proposed, but in order to obtain sufficient strength of the active material layer and to maintain the contact between the particles, a large amount of the binder material needs to be added, and a decrease in ionic conductivity becomes a problem.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURES

[0008] Patent Literature 1: Japanese Patent Application Publication No. 2010-262764

[0009] Patent Literature 2: Japanese Patent Application Publication No. 2016-9679

[0010] Patent Literature 3: WO 2008 / 115852 SUMMARY

[0011] PROBLEMS TO BE SOLVED BY THE INVENTION

[0012] When a lithium secondary battery, particularly a full solid lithium secondary battery is charged and discharged, cracks are generated in an active material layer due to expansion and shrinkage of the active material, or it is difficult to ensure contact of particles in the active material layer with each other. In order to increase the strength of the active material layer and ensure contact of the particles with each other, it is effective to use a binder material, but if a large amount of the binder material is used, the space into which an electrically conductive material and an electrolyte enter becomes small. Further, the surface of the active material particles is coated with the binder material, which sometimes hinders electron conductivity and ion conductivity of the active material layer.

[0013] An object of the present application is to provide a resin-bonded fiber which is a resin-bonded fiber that can be used as an electrically conductive material, which can produce an active material layer having high strength, and in which it is difficult to hinder electron conductivity and ion conductivity of the active material layer. Another object of the present application is to provide an active material layer, an electrode, and a nonaqueous electrolyte secondary battery produced using the resin-bonded fiber.

[0014] Means for solving the problem

[0015] The present inventors and others have intensively studied in view of the above-described prior art, and as a result, have found that the above-described problem can be solved by integrating an electrically conductive fiber and a binder material containing a thermoplastic resin by bonding them together, thereby completing the present application.

[0016] That is, by integrating the electrically conductive fiber and the thermoplastic resin by bonding them together, even if volume change of the active material occurs in the active material layer, the change in the structure of the active material layer is suppressed, and thus the physical strength of the active material layer is increased. Further, since the electrically conductive fiber and the thermoplastic resin are integrated, the thermoplastic resin as the binder material is suppressed from spreading in the active material layer in the form of a film, and the formation of an insulating layer is suppressed, and as a result, the ion conductivity and the electron conductivity in the active material layer can be maintained high, and thus the increase in the resistance of the battery can be suppressed.

[0017] The present application for solving the above-described problem is described below.

[0018] (1) A resin-bonded fiber characterized by comprising:

[0019] an electrically conductive fiber having an average fiber diameter of 10 to 5,000 nm and an average aspect ratio of 30 or more; and

[0020] a thermoplastic resin that is in contact with at least a part of the surface of the aforementioned electrically conductive fiber and is integrated with the aforementioned electrically conductive fiber;

[0021] The density of the resin-bonded fiber is 0.8 g / cm 3 or less, and the powder volume resistivity at that time is 10 Ω·cm or less.

[0022] 〔2〕 The resin-bonded fiber according to claim 1, wherein the content of the aforementioned thermoplastic resin is 1 to 70 mass% relative to the total amount of the aforementioned electrically conductive fiber and the aforementioned thermoplastic resin.

[0023] 〔3〕 The resin-bonded fiber according to claim 1 or 2, which has a tap density of 0.001 to 0.1 g / cm 3 .

[0024] 〔4〕 The resin-bonded fiber according to any one of claims 1 to 3, wherein the aforementioned thermoplastic resin is a thermoplastic resin having a melting point of 50 to 250°C.

[0025] 〔5〕 The resin-bonded fiber according to any one of claims 1 to 4, wherein the aforementioned electrically conductive fiber is a carbon fiber or a nickel fiber.

[0026] 〔6〕 The resin-bonded fiber according to claim 5, wherein the aforementioned carbon fiber is substantially free of metal elements.

[0027] 〔7〕 The resin-bonded fiber according to any one of claims 1 to 6, wherein the aforementioned thermoplastic resin is a thermoplastic resin containing a fluorine atom.

[0028] The resin-bonded fiber according to any one of claims 1 to 7 is formed by integrating an electrically conductive fiber of a prescribed shape with a thermoplastic resin. The resin-bonded fiber has a density of 0.8 g / cm 3 when filled and measured, a powder volume resistivity of 10 Ω-cm or less. Here, integration does not simply mean a state in which the electrically conductive fiber is mixed with the thermoplastic resin, but rather means a state in which the electrically conductive fiber is bonded in a manner that penetrates one thermoplastic resin particle, or a state in which the electrically conductive fiber is coated with the thermoplastic resin on a portion thereof.

[0029] 〔8〕 The resin-bonded fiber according to any one of claims 1 to 7, which contains at least the aforementioned thermoplastic resin in the form of particles.

[0030] 〔9〕 An active material layer for a nonaqueous electrolyte secondary battery, which contains the resin-bonded fiber according to any one of claims 1 to 8.

[0031] 〔10〕 An electrode for a nonaqueous electrolyte secondary battery, which is constituted by containing the active material layer according to claim 9.

[0032] 〔11〕 A nonaqueous electrolyte secondary battery, which is constituted by containing the electrode according to claim 10.

[0033] Effects of the Invention

[0034] The resin-bonded fiber of the present application, in which the electrically conductive fiber that functions as an electrically conductive material and the thermoplastic resin that functions as a binding material are integrated by being bonded together, can produce an active material layer having high strength. Furthermore, the active material layer produced using the resin-bonded fiber of the present application can maintain high ion conductivity and electron conductivity even when the volume of the active material changes due to charging and discharging. Therefore, the active material layer configured using the resin-bonded fiber of the present application can provide a nonaqueous electrolyte secondary battery that has reduced battery resistance and excellent cycle characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 : is a photomicrograph of the resin-bonded fiber produced in Example 4, obtained using a scanning electron microscope (SEM).

[0036] Figure 2 : is a photomicrograph of the resin-bonded fiber produced in Example 5, obtained using a scanning electron microscope (SEM).

[0037] Figure 3 : is a photomicrograph of the case where the electrically conductive fiber and the thermoplastic resin are simply mixed (Comparative Example 3), obtained using a scanning electron microscope (SEM). DETAILED DESCRIPTION

[0038] (1) Resin-bonded fiber

[0039] The resin-bonded fiber of the present application contains an electrically conductive fiber and a thermoplastic resin, is preferably substantially formed of an electrically conductive fiber and a thermoplastic resin, and more preferably is formed of an electrically conductive fiber and a thermoplastic resin. The resin-bonded fiber is integrated (complexed) by direct bonding of the electrically conductive fiber and the thermoplastic resin. Here, integration does not mean a state in which the electrically conductive fiber and the thermoplastic resin are simply mixed, but means, for example, a state in which the electrically conductive fiber is adhered and / or bonded to the surface of one particle-shaped (spherical) thermoplastic resin, a state in which the electrically conductive fiber is bonded in such a manner as to penetrate one particle-shaped thermoplastic resin, a state in which the electrically conductive fiber is coated with a part of the thermoplastic resin, and the like. In particular, it is preferable that at least a part of the thermoplastic resin be adhered in a particle shape. Here, a particle shape means a particle having an aspect ratio of 5 or less, preferably 2 or less, and more preferably 1.5 or less. Specifically, the resin-bonded fiber of the present application is produced by a method in which the electrically conductive fiber dispersed in a solution of the thermoplastic resin is spray-dried, a method in which a monomer solution is mixed with the electrically conductive fiber and polymerized, a method in which the thermoplastic resin is precipitated in a solvent in which the electrically conductive fiber is dispersed, and the like. Furthermore, the resin-bonded fiber of the present application is integrated by direct bonding of the electrically conductive fiber and the thermoplastic resin, and is not a state in which the electrically conductive fiber and the particles of the thermoplastic resin are mixed and bonded by a third component. The state of integration can be confirmed, for example, by a SEM image.

[0040] The resin-bonded fiber of the present application is characterized in that the volume resistivity of the powder at the time of filling is 10 Ω-cm or less. As a prerequisite, the resin-bonded fiber cannot be one in which the density is 0.8 g / cm 3 The volume resistivity of the powder at the time of filling is 10 Ω-cm or less. As a prerequisite, the resin-bonded fiber cannot be one in which the density is 0.8 g / cm 3 The resin-bonded fiber of the present application is characterized in that the volume resistivity of the powder at the time of filling is 10 Ω-cm or less. As a prerequisite, the resin-bonded fiber cannot be one in which the density is 0.8 g / cm 3 The upper limit value of the volume resistivity of the powder at the time of filling is preferably 5 Ω-cm or less, more preferably 3 Ω-cm or less, further preferably 2.5 Ω-cm or less, more preferably 1 Ω-cm or less, and more preferably 0.5 Ω-cm or less. The density of the resin-bonded fiber of the present application is 0.8 g / cm 3 The lower limit value of the volume resistivity of the powder at the time of filling is not particularly limited and is 0.001 Ω-cm or more, and more specifically 0.01 Ω-cm or more.

[0041] The resin-bonded fiber of the present application is characterized in that the volume resistivity of the powder at the time of filling is 10 Ω-cm or less. As a prerequisite, the resin-bonded fiber cannot be one in which the density is 0.8 g / cm 3 The volume resistivity of the powder at the time of filling is 10 Ω-cm or less. As a prerequisite, the resin-bonded fiber cannot be one in which the density is 0.8 g / cm 3 The lower limit value of the volume resistivity of the powder at the time of filling is not particularly limited and is 0.001 Ω-cm or more, and more specifically 0.01 Ω-cm or more.

[0042] The resin-bonded fiber of the present application is characterized in that the volume resistivity of the powder at the time of filling is 10 Ω-cm or less. As a prerequisite, the resin-bonded fiber cannot be one in which the density is 0.8 g / cm 3 The volume resistivity of the powder at the time of filling is 10 Ω-cm or less. As a prerequisite, the resin-bonded fiber cannot be one in which the density is 0.8 g / cm 3 The lower limit value of the volume resistivity of the powder at the time of filling is not particularly limited and is 0.001 Ω-cm or more, and more specifically 0.01 Ω-cm or more.

[0043] In addition, the tap density of the resin-bonded fiber of the present application is preferably 0.001 to 0.1 g / cm 3 . The lower limit value of the tap density is more preferably 0.005 g / cm 3 or more, more preferably 0.010 g / cm 3 or more, more preferably 0.012 g / cm 3 or more. The upper limit value of the tap density is more preferably 0.070 g / cm 3 or less, more preferably 0.065 g / cm 3 or less, more preferably 0.050 g / cm 3 or less, more preferably 0.040 g / cm 3 or less, and more preferably 0.030 g / cm3 The following is true: less than 0.001 g / cm³. 3 In such cases, it is believed that excessive thermoplastic resin content or the circular shape of the conductive fibers will result in a smaller improvement in conductivity relative to the amount added. (Exceeding 0.1 g / cm³) 3 If so, it is considered that the content of thermoplastic resin is too low, or that the thermoplastic resin has been detached from the conductive fibers.

[0044] The average fiber length of the resin-bonded fibers is not particularly limited, but is preferably 10 μm or more. The lower limit of the average fiber length is preferably 11 μm or more, more preferably 12 μm or more. The upper limit of the average fiber length is not limited, but is preferably 100 μm or less, more preferably 80 μm or less, more preferably 60 μm or less, more preferably 50 μm or less, more preferably 40 μm or less, and more preferably 30 μm or less.

[0045] The content of thermoplastic resin in the resin-bonded fiber of the present invention is preferably 1 to 70% by mass relative to the total amount of conductive fiber and thermoplastic resin. The lower limit of the thermoplastic resin content is more preferably 5% by mass or more, more preferably 10% by mass or more, more preferably 15% by mass or more, and more preferably 20% by mass or more. The upper limit of the thermoplastic resin content is more preferably 65% ​​by mass or less, more preferably 60% by mass or less, more preferably 55% by mass or less, and more preferably 50% by mass or less. When it is less than 1% by mass, it is difficult to exert the reinforcing effect of the thermoplastic resin. When it exceeds 70% by mass, the amount of thermoplastic resin adhering is excessive, which easily increases the battery resistance.

[0046] The content of conductive fibers in the resin adhesive fiber of the present invention is preferably 30 to 99% by mass relative to the total amount of conductive fibers and thermoplastic resin. The lower limit of the conductive fiber content is more preferably 35% by mass or more, more preferably 40% by mass or more, more preferably 45% by mass or more, more preferably 50% by mass or more, and more preferably 55% by mass or more. The upper limit of the conductive fiber content is more preferably 95% by mass or less, more preferably 90% by mass or less, more preferably 85% by mass or less, and more preferably 80% by mass or less.

[0047] (2) Conductive fibers

[0048] The electrically conductive fiber used in the present application is not particularly limited as long as it is a fiber having electrical conductivity, and examples of the material of the electrically conductive fiber include carbon, nickel, copper, stainless steel, aluminum, and the like. Of these, carbon and nickel are preferred, and carbon is particularly preferred. When the material of the electrically conductive fiber is carbon, examples of the electrically conductive fiber include carbon nanotubes (CNT), vapor grown carbon fiber (VGCF (registered trademark)), PAN-based carbon fiber, pitch-based carbon fiber, and the like, but the pitch-based carbon fiber is more preferred because it has high crystallinity, a fine fiber diameter, and is less likely to aggregate and has excellent dispersibility. Hereinafter, the case where the electrically conductive fiber is a carbon fiber will be described as an example.

[0049] The average fiber diameter of the carbon fiber used in the present application is 10 to 5000 nm. The lower limit of the average fiber diameter is preferably 50 nm or more, more preferably 100 nm or more, more preferably 150 nm or more, more preferably 200 nm or more, more preferably more than 200 nm, and more preferably 250 nm or more. The upper limit of the average fiber diameter is preferably 3000 nm or less, more preferably 2000 nm or less, more preferably 1000 nm or less, more preferably 900 nm or less, more preferably 800 nm or less, more preferably 700 nm or less, more preferably 600 nm or less, more preferably 500 nm or less, more preferably 400 nm or less, and more preferably 350 nm or less. When the average fiber diameter is less than 10 nm, the fiber is likely to aggregate, and it is difficult to exert the function of the electrically conductive material. In addition, the carbon fiber having an average fiber diameter of less than 10 nm has a large specific surface area, and is likely to coat the surface of the active material within the active material layer. As a result, the contact between the solid electrolyte and the active material decreases, and the formation of ion conduction paths is hindered. The carbon fiber having an average fiber diameter of more than 5000 nm is likely to have gaps between the fibers within the active material layer, and it is sometimes difficult to increase the density of the active material layer.

[0050] The average aspect ratio of the carbon fiber used in the present application is 30 or more, preferably 35 or more, and preferably 40 or more. The upper limit of the average aspect ratio is not limited, and is preferably 1000 or less, more preferably 500 or less, more preferably 300 or less, more preferably 200 or less, more preferably 150 or less, and more preferably 100 or less. When the average aspect ratio is less than 30, the formation of electrically conductive paths using the carbon fiber in the active material layer is likely to be insufficient when the active material layer is manufactured, and the resistance value in the film thickness direction of the active material layer is sometimes not sufficiently reduced. In addition, the mechanical strength of the active material layer is insufficient, and thus when stress is applied to the active material layer upon volume change of the active material accompanying charge and discharge, the active material layer is likely to crack.

[0051] The average fiber length of the carbon fiber is not particularly limited, and is preferably 10 μm or more. The lower limit of the average fiber length is preferably 11 μm or more, more preferably 12 μm or more. The upper limit of the average fiber length is not limited, and is preferably 100 μm or less, more preferably 80 μm or less, more preferably 60 μm or less, more preferably 50 μm or less, more preferably 40 μm or less, more preferably 30 μm or less.

[0052] The carbon fiber used in the present application preferably has a linear structure substantially free of branching. Here, substantially free of branching means that the degree of branching is 0.01 / μm or less. Branching refers to a granular portion in which the carbon fiber is bonded to another carbon fiber at a position other than the end portion, and refers to a case where the main axis of the carbon fiber branches in the middle, and a case where the main axis of the carbon fiber has a branched secondary axis. As a carbon fiber having branching, for example, vapor-phase growth (vapor-phase method) carbon fiber (for example, VGCF (registered trademark) manufactured by Showa Denko K.K.) manufactured by a vapor-phase method in which a hydrocarbon such as benzene is vaporized in a high-temperature atmosphere in the presence of a metal such as iron as a catalyst is known. The carbon fiber having a substantially linear structure has good dispersibility compared to the carbon fiber having branching, and it is easy to form a long-distance conductive path.

[0053] Here, the degree of branching of the carbon fiber used in the present application means a value determined from a photograph taken at a magnification of 5,000 times by a field emission type scanning electron microscope.

[0054] Note that the carbon fiber can have a fibrous shape as a whole, and also includes, for example, those in which carbon fibers having an average aspect ratio less than the above-mentioned preferable range are in contact or bonded to each other to have a fibrous shape as a whole (for example, those in which spherical carbon is connected in a beaded shape, those in which at least one or a plurality of very short fibers are connected by fusion or the like, and the like).

[0055] The distance (d002) between adjacent graphite sheets of the carbon fiber used in the present application measured by wide-angle X-ray measurement is not particularly limited, and is preferably 0.3365 nm or more, more preferably 0.3380 nm or more, more preferably 0.3390 nm or more, more preferably 0.3400 nm or more, more preferably more than 0.3400 nm, more preferably 0.3410 nm or more, and further preferably 0.3420 nm or more. In addition, d002 is preferably 0.3450 nm or less, more preferably 0.3445 nm or less. In particular, in the case where d002 is 0.3400 nm or more, the carbon fiber is less likely to be brittle. Therefore, the fiber is less likely to be broken during crushing, processing such as preparation of a kneaded slurry, and the like, and there is a tendency that the fiber length is maintained. As a result, it is easy to form a long-distance conductive path. In addition, there is a tendency that the conductive path is easily maintained along with the volume change of the active material accompanying charge and discharge of the all-solid-state lithium secondary battery.

[0056] The crystallite size (Lc002) of the carbon fiber used in the present application is not particularly limited and is preferably 120 nm or less, more preferably 100 nm or less, more preferably 80 nm or less, more preferably 60 nm or less, more preferably 50 nm or less, more preferably 40 nm or less, more preferably 30 nm or less, and further preferably 25 nm or less. The larger the crystallite size (Lc002) is, the higher the crystallinity is, and the more excellent the electrical conductivity is. However, in the case where the crystallite size (Lc002) is small, the carbon fiber is less likely to be brittle. Therefore, the fiber is less likely to be broken during the breaking or the processing such as the production of a kneaded slurry, and the fiber length is maintained. As a result, it is easy to form a long-distance conductive path. In addition, it is easy to maintain the conductive path along with the volume change of the active material accompanying the charge and discharge of the all-solid lithium secondary battery. The lower limit value of the crystallite size (Lc002) is greater than 0, and is usually 5.0 nm or more of the detection limit of the measuring device.

[0057] In the present application, the crystallite size (Lc002) means a value measured by Japanese Industrial Standards JIS R 7651 (2007 edition) "Method of measuring lattice constant and crystallite size of carbon materials".

[0058] The carbon fiber used in the present application is preferably substantially free of metal elements. Specifically, the total content of the metal elements is preferably 50 ppm or less, more preferably 30 ppm or less, and further preferably 20 ppm or less. When the content of the metal elements exceeds 50 ppm, the battery is likely to be deteriorated due to the catalytic action of the metal. In the present application, the content of the metal elements means the total content of Li, Na, Ti, Mn, Fe, Ni, and Co. In particular, the content of Fe is preferably 5 ppm or less, more preferably 3 ppm or less, and further preferably 1 ppm or less. When the content of Fe exceeds 5 ppm, the battery is particularly likely to be deteriorated, and thus is not preferred. Note that the aforementioned vapor-grown (vapor-phase method) carbon fiber (for example, VGCF (registered trademark) manufactured by Showa Denko K.K.) contains a metal such as iron as a catalyst.

[0059] The carbon fiber used in the present application is preferably 0.5% by mass or less, and more preferably 0.3% by mass or less, of any one of hydrogen, nitrogen, and ash in the fiber. When any one of hydrogen, nitrogen, and ash in the carbon fiber is 0.5% by mass or less, the structural defects of the graphite layer are further suppressed, and thus it is preferred that the side reaction in the battery be suppressed.

[0060] The carbon fibers used in this invention, other than carbon nanotubes (CNTs) and vapor-grown carbon fibers (VGCF (registered trademark)), exhibit particularly excellent dispersion in the active material layer. While the reasons are not yet clear, it can be assumed that: they possess the aforementioned structure; they use artificial graphite, non-graphitizable carbon, and easily graphitizable carbon, etc., produced by heat treatment of natural graphite, petroleum-based and coal-based coke, as raw materials; and they are manufactured using resin-composite fibers, etc. Within the active material layer, even without spherical particles, the dispersion is excellent, thus forming long-distance conductive pathways. It can be considered that excellent battery performance can be achieved with a small amount of these fibers.

[0061] The carbon fibers used in this invention can be porous or hollow, but in the manufacturing process, they are preferably produced via resin composite fibers obtained through melt blending and spinning. Therefore, the carbon fibers of this invention are preferably substantially solid, with a generally smooth surface, and, as described above, do not have a branched linear structure.

[0062] The carbon fiber used in this invention can be manufactured by, for example, the method described in WO2009 / 125857. An example is shown below.

[0063] First, an mesophase pitch composition is prepared by dispersing mesophase pitch within a thermoplastic polymer. Then, the mesophase pitch composition is molten and formed into filaments or films. Spinning is particularly preferred. Through spinning, the mesophase pitch dispersed within the thermoplastic polymer extends within the polymer, and the mesophase pitch composition is fibrous to obtain resin composite fibers. These resin composite fibers have an island structure, where the thermoplastic polymer is the sea component and the mesophase pitch is the island component.

[0064] Next, the obtained resin composite fiber is brought into contact with oxygen-containing gas to stabilize the mesophase pitch, thereby obtaining resin composite stabilized fiber. This resin composite stabilized fiber has an island structure with thermoplastic polymer as the sea component and stabilized mesophase pitch as the island component.

[0065] Next, the thermoplastic polymer, which is the marine component of the resin composite stabilized fiber, is removed to obtain the carbon fiber precursor.

[0066] Furthermore, the carbon fiber precursor was heated at high temperature to obtain ultrafine carbon fibers as carbon fibers.

[0067] (3) Thermoplastic resin

[0068] The thermoplastic resin constituting the resin-bonded fiber of the present application is not particularly limited as long as it is a thermoplastic resin capable of electrode formation and having sufficient electrochemical stability. As the thermoplastic resin, one or more selected from the group consisting of polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (P-(VDF-HFP)), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), styrene butadiene rubber (SBR), fluorine-olefin copolymer, polyimide, polyamide-imide, aromatic polyamide, phenol resin, and the like are preferably used, and a thermoplastic resin containing a fluorine atom such as polyvinylidene fluoride (PVDF) and vinylidene fluoride-hexafluoropropylene copolymer (P-(VDF-HFP)) is particularly preferred.

[0069] The melting point of the thermoplastic resin is preferably 50 to 250°C. The lower limit of the melting point of the thermoplastic resin is more preferably 60°C or higher, more preferably 70°C or higher, more preferably 80°C or higher, more preferably 90°C or higher, and more preferably 100°C or higher. The upper limit of the melting point of the thermoplastic resin is more preferably 220°C or lower, more preferably 200°C or lower, more preferably 180°C or lower, more preferably 160°C or lower, and more preferably 150°C or lower.

[0070] When the melting point is less than 50°C, the particles of the thermoplastic resin are likely to aggregate during dispersion into the electrode. In addition, the heat resistance of the battery is reduced. When the melting point exceeds 250°C, the active material and the solid electrolyte can be degraded.

[0071] The glass transition temperature of the thermoplastic resin contained in the resin-bonded fiber of the present application is not particularly limited and is preferably 250°C or lower. The upper limit of the glass transition temperature is preferably 200°C or lower, more preferably 150°C or lower, more preferably 120°C or lower, more preferably 100°C or lower, more preferably 80°C or lower, more preferably 50°C or lower, more preferably 40°C or lower, more preferably 30°C or lower, more preferably 20°C or lower, more preferably 10°C or lower, and more preferably 0°C or lower.

[0072] (4) Method for producing resin-bonded fiber

[0073] The method for directly integrating the conductive fiber and the thermoplastic resin in the resin-bonded fiber of the present application is not particularly limited. Examples include a method in which the thermoplastic resin is dissolved in a solvent, the conductive fiber is dispersed in the solution, and spray drying is performed; a method in which the thermoplastic resin is dissolved in a solvent, the conductive fiber is dispersed in the solution, and then another solvent is added to cause the thermoplastic resin to precipitate in a state of being bonded to the conductive fiber; a method in which the conductive fiber is wetted with a monomer solution of the thermoplastic resin, and the monomer is polymerized; and the like.

[0074] Examples of spray drying methods include the following.

[0075] First, the thermoplastic resin used as a binder is dissolved in a solvent. The thermoplastic resin may be completely dissolved, or partially dissolved and the remainder dispersed. There are no particular limitations on the solvent, as long as it can dissolve the thermoplastic resin used. Preferably, it is a low-boiling-point solvent such as ethanol, propanol, ketones such as acetone, ethers, or water.

[0076] Next, conductive fibers are dispersed in a solution containing dissolved thermoplastic resin. The amount of dissolved (dispersed) thermoplastic resin and the amount of dispersed conductive fibers can be appropriately determined by taking into account the spray drying efficiency.

[0077] The resulting slurry is then spray-dried using a spray dryer. Since reducing the droplet size is necessary to form a finely dispersed, small-particle-size complex, a nozzle-type spray dryer is preferred over a disc-type dryer. The nozzle diameter and drying temperature can be appropriately determined considering the spray drying efficiency and the powder characteristics of the resulting resin-bonded fibers.

[0078] By spray drying the above slurry, a resin-bonded fiber can be obtained by directly bonding conductive fibers and thermoplastic resin into one piece.

[0079] As a method for precipitating thermoplastic resin in a state of being bonded to conductive fibers (reprecipitation method), the following methods are examples.

[0080] First, the thermoplastic resin used as a binder is dissolved in a solvent. The thermoplastic resin can be completely dissolved, or partially dissolved and the remainder dispersed. There are no particular limitations on the solvent, as long as it can dissolve the thermoplastic resin used. Low-boiling-point aqueous solvents such as ethanol, propanol, and acetone are preferred.

[0081] Next, conductive fibers are dispersed in a solution containing dissolved thermoplastic resin. A solvent different from the one described above is added to the dispersion to precipitate the dissolved thermoplastic resin. The solvent is not particularly limited as long as it has low solubility in thermoplastic resin; examples include toluene, xylene, and water.

[0082] In addition to the methods mentioned above, conductive fibers can be pre-dispersed in solvents such as toluene, and the dispersion can be added dropwise to a solution containing thermoplastic resin.

[0083] By precipitating temporarily dissolved thermoplastic resin in the presence of conductive fibers, resin-bonded fibers that are directly bonded to thermoplastic resin can be obtained. The resin-bonded fibers precipitated from the solvent are separated, washed, and dried using known methods.

[0084] As a method of polymerizing the monomer after the conductive fiber is wetted with a monomer solution of the thermoplastic resin (polymerization method), for example, the following method is exemplified.

[0085] First, a monomer of a thermoplastic resin (polymer) is dissolved in a solvent such as water. As the solvent, there is no particular limitation as long as it is a solvent capable of dissolving the monomer used. A low-boiling water-based solvent such as ethanol, propanol, acetone, or the like is preferred.

[0086] Next, the solution in which the monomer is dissolved is sprayed to the conductive fiber, and the monomer solution is attached to the conductive fiber.

[0087] Then, the monomer is polymerized to become a thermoplastic resin (polymer) by heating or light irradiation or the like to the conductive fiber to which the monomer solution is attached. At this time, a known polymerization initiator or the like can be added.

[0088] By polymerizing the monomer in a state in which a droplet of the monomer solution is attached to the conductive fiber, a resin-bonded fiber in which the conductive fiber and the thermoplastic resin (polymer) are directly bonded and integrated can be obtained.

[0089] (5) Active material layer

[0090] The resin-bonded fiber of the present application can be used for an active material layer of a lithium ion secondary battery, a full solid secondary battery, or the like. In a nonaqueous electrolyte secondary battery configured to contain an electrode having an active material layer, the resin-bonded fiber functions as an electrically conductive aid by virtue of its electrical conductivity. In addition, since the junctions of active materials to each other are ensured by the thermoplastic resin in the resin-bonded fiber, it contributes to the performance of the nonaqueous electrolyte secondary battery.

[0091] The active material layer of the present application can be either of a positive electrode active material layer or a negative electrode active material layer. The active material layer is configured to contain at least an active material, the resin-bonded fiber of the present application, and can contain a solid electrolyte.

[0092] The active material layer has voids. The void ratio thereof is preferably 5.0% by volume or more and 50% by volume or less. If the void ratio is within this range, the phenomenon of cracks occurring in the active material layer is particularly suppressed even if charge and discharge cycles that involve a change in the volume of the active material are repeated. By using an active material layer having such voids, a full-solid-state lithium secondary battery having high electronic and ionic conductivity and high output can be configured. The lower limit of the void ratio is more preferably 7.0% by volume or more, more preferably 9.0% by volume or more, more preferably 10% by volume or more, more preferably 11% by volume or more, further preferably 12% by volume or more, further more preferably 15% by volume or more, and particularly preferably 18% by volume or more. The upper limit of the void ratio is more preferably 48% by volume or less, more preferably 45% by volume or less, further preferably 42% by volume or less, further more preferably 37% by volume or less, further more preferably 35% by volume or less, and particularly preferably 30% by volume or less.

[0093] The void ratio of the active material layer can be adjusted by controlling the material, size, content of the positive or negative active material used, and the molding conditions of pressure molding as needed when forming the active material layer, in addition to the average fiber diameter and average fiber length of the resin-bonded fiber of the present application.

[0094] The method for calculating the void ratio is not particularly limited, and for example, there are a method of calculating from the true density and the apparent density of the active material layer based on the following formula (1), a method of calculating from a three-dimensional image obtained by tomography such as X-ray CT, and the like.

[0095] Void ratio (% by volume) = (true density - apparent density of active material layer) / true density x 100 Formula (1)

[0096] In the case of calculating based on formula (1), the true density and the apparent density of the active material layer are measured separately. The method for measuring the true density includes, for example, a method of calculating from the true density and the mass ratio of each material constituting the active material layer, and a method of measuring using a gas displacement method (pycnometer method) or a liquid method (Archimedes method) after crushing the active material layer. The apparent density of the active material layer can be calculated from the mass and the volume of the active material layer by the following formula (2), for example.

[0097] Apparent density of active material layer = mass of active material layer / (film thickness of active material layer x area) Formula (2)

[0098] The electrical conductivity in the film thickness direction of the active material layer is preferably 1.0 x 10 -3 S / cm or more, more preferably 5.0 x 10 - 3 S / cm or more, further preferably 1.0 x 10 -21.6 x 10-4S / cm or higher, particularly preferably 1.6 x 10-2S / cm or higher -2 S / cm or higher. Such conductivity can be achieved by containing the resin-bonding fiber of the present application as a conductive aid.

[0099] (5-1) Positive electrode active material layer

[0100] The positive electrode active material layer of the present application contains at least a positive electrode active material and the resin-bonding fiber of the present application, and can further contain a solid electrolyte, a binder material, and the like.

[0101] As the positive electrode active material, a material conventionally known can be used. For example, a lithium-containing metal oxide capable of occluding and releasing lithium ions is preferred. As the lithium-containing metal oxide, a composite oxide containing lithium and at least one element selected from Co, Mg, Mn, Ni, Fe, Al, Mo, V, W, and Ti, and the like can be given.

[0102] Specific examples can be given as a composite oxide containing lithium and at least one element selected from Co, Mg, Mn, Ni, Fe, Al, Mo, V, W, and Ti, and the like. x CoO2, Li x NiO2, Li x MnO2, Li x Co a Ni 1- aO2, Li x Co b V 1-b O z , Li x Co b Fe 1- b O2, Li x Mn2O4, Li x Mn c Co 2-c O4, Li x Mn c Ni 2-c O4, Li x Mn c V 2-c O4, Li x Mn c Fe 2-c O4, Li x Ni a Mn d Co 1-a―d O2, Li x Ni a Co d Al 1-a―dat least one of O2(wherein, x = 0.02 to 1.2, a = 0.1 to 0.9, b = 0.8 to 0.98, c = 1.2 to 1.96, d = 0.1 to 0.9, and z = 2.01 to 2.3), etc. As the preferable lithium-containing metal oxide, Li x CoO2, Li x NiO2, Li x MnO2, Li x Co a Ni 1-a O2, Li x Mn2O4, Li x Mn c Co 2-c O4, Li x Mn c Ni 2-c O4, Li x Co b V 1-b O z , Li x Ni a Mn d Co 1-a―d O2, Li x Ni a Co d Al 1-a―d O2(wherein, x, a, b, c, d, and z are the same as described above). The positive electrode active material can be used alone or in combination of two or more. Note that the value of x is the value before the start of charge and discharge, and varies depending on charge and discharge.

[0103] The surface of the positive electrode active material can be coated with a coating layer. By the coating layer, the reaction of the positive electrode active material with the solid electrolyte (particularly, sulfide solid electrolyte) can be suppressed. As the coating layer, a Li-containing oxide such as LiNbO3, Li3PO4, LiPON, etc. can be given. The average thickness of the coating layer is, for example, 1 nm or more. On the other hand, the average thickness of the coating layer is, for example, 20 nm or less, and can be 10 nm or less.

[0104] The average particle diameter of the positive electrode active material is preferably 20 μm or less, more preferably 0.05 to 15 μm, and further preferably 1 to 12 μm. If the average particle diameter exceeds 20 μm, the efficiency of charge and discharge reaction under a large current can be reduced in some cases.

[0105] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, and is preferably 30 to 99% by mass, more preferably 40 to 95% by mass, and further preferably 50 to 90% by mass. When less than 30% by mass, it is sometimes difficult to apply to power sources where the energy density is required to be high. When more than 99% by mass, the content of the positive electrode active material becomes small, and the performance of the positive electrode active material layer is sometimes reduced.

[0106] The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, and is preferably 5 to 60% by mass, more preferably 10 to 50% by mass, and further preferably 20 to 40% by mass. When less than 5% by mass, the ion conductivity of the positive electrode active material layer is sometimes insufficient. When more than 60% by mass, the content of the positive electrode active material becomes small, and it is sometimes difficult to apply to power sources where the energy density is required to be high.

[0107] In the positive electrode active material layer, a small amount of a binder material can be contained within a range that does not hinder the electronic conductivity and the ion conductivity.

[0108] The thickness of the positive electrode active material layer is usually 10 to 1000 μm.

[0109] (5-2) Negative electrode active material layer

[0110] The negative electrode active material layer constituting the all-solid-state lithium secondary battery of the present application contains at least a negative electrode active material, and can further contain a solid electrolyte, the resin binder fiber of the present application, a binder material, and the like.

[0111] As the negative electrode active material, a material known in the art can be selected and used. For example, any one of Li metal, a carbon material, lithium titanate (Li4Ti5O 12 ), Si, Sn, In, Ag, and Al, or an alloy, an oxide, or the like containing at least one of these, can be used. Among them, from the viewpoint of improving the energy density, Li metal is preferred.

[0112] As the negative electrode active material other than Li metal, a carbon material is widely used. As the carbon material, natural graphite, artificial graphite produced by heat-treating petroleum-based or coal-based coke, hard carbon obtained by carbonizing a resin, mesophase pitch-based carbon material, and the like can be mentioned.

[0113] As the carbon material selected as the negative electrode active material of the all-solid-state battery, from the viewpoint that the interlayer spacing of the crystal is wide and the expansion and contraction during charge and discharge are not large, hard carbon is preferred. Hard carbon has a structure in which fine crystalline graphene layers are arranged irregularly, and the occlusion of lithium ions is performed by the insertion of lithium ions into the graphene layers and the aggregation of lithium (lithiation) in the space formed between the graphene layers.

[0114] When natural graphite or artificial graphite is used, from the viewpoint of increasing the capacity of the battery, those in which the interplanar spacing d(002) of the (002) plane of the graphite structure obtained by powder X-ray diffraction is in the range of 0.335 to 0.337 nm are preferred. Natural graphite refers to a graphite material that is naturally produced as a mineral. Natural graphite is classified into two types, i.e., scaly graphite having high crystallinity and earthy graphite having low crystallinity, depending on its appearance and properties. The scaly graphite is further classified into flaky graphite having a leaf-like appearance and massive scaly graphite. The origin, properties, and type of the natural graphite as a graphite material are not particularly limited. In addition, the natural graphite or particles produced using the natural graphite as a raw material can be subjected to heat treatment and used.

[0115] Artificial graphite refers to graphite and a graphite material close to completely crystallized graphite that is produced by a wide range of artificial processes. As representative examples, there are those obtained using, as a raw material, tar, coke, or the like obtained from the dry distillation of coal, the distillation of crude oil, or the like, and subjected to a firing process at around 500 to 1000°C and a graphitization process at 2000°C or higher. In addition, coagulated graphite obtained by reprecipitating carbon from a dissolved iron is also one type of artificial graphite.

[0116] As the negative electrode active material, in addition to the carbon material, an alloy containing at least one of Si and Sn is used, and is effective in reducing the capacity compared to the case where Si and Sn are used as a single body, and the case where the respective oxides are used. Among them, a Si-based alloy is preferred. As the Si-based alloy, there are alloys of at least one element selected from B, Mg, Ca, Ti, Fe, Co, Mo, Cr, V, W, Ni, Mn, Zn, and Cu, and the like, and Si. Specifically, there are at least one selected from SiB4, SiB6, Mg2Si, Ni2Si, TiSi2, MoSi2, CoSi2, NiSi2, CaSi2, CrSi2, Cu5Si, FeSi2, MnSi2, VSi2, WSi2, ZnSi2, and the like.

[0117] In the active material layer for a full solid-state lithium secondary battery of the present application, as the negative electrode active material, one of the above-described materials can be used alone, or two or more can be used in combination.

[0118] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, and is preferably 30 to 100% by mass, more preferably 40 to 99% by mass, and further preferably 50 to 95% by mass. When it is less than 30% by mass, it is sometimes difficult to apply to a power source use where the energy density is required to be high.

[0119] The content of the solid electrolyte in the negative electrode active material layer is not particularly limited, and is preferably 0 to 60 mass%, more preferably 5 to 50 mass%, and further preferably 10 to 40 mass%. When the content of the solid electrolyte exceeds 60 mass%, the content of the positive electrode active material becomes small, and sometimes it is difficult to apply to power sources requiring high energy density.

[0120] In the negative electrode active material layer, a small amount of a binder can be contained within a range not impairing electronic conductivity and ionic conductivity.

[0121] The thickness of the negative electrode active material layer is usually 1 to 1000 μm.

[0122] (5-3) Solid electrolyte

[0123] The solid electrolyte used in the present application can be selected from conventionally known materials. Examples include sulfide-based solid electrolytes, oxide-based solid electrolytes, hydride-based solid electrolytes, and polymer electrolytes. In the present application, since the lithium ion conductivity is high, it is preferable to use a sulfide-based solid electrolyte.

[0124] As the sulfide-based solid electrolyte, a sulfide-based solid electrolyte containing Li, A, and S (Li-A-S) can be mentioned. A in the above-mentioned sulfide-based solid electrolyte Li-A-S is at least one selected from P, Ge, B, Si, Sb, and I. As such a sulfide-based solid electrolyte Li-A-S, Li7P3S 11 , 70Li2S-30P2S5, LiGe 0.25 P 0.75 S4, 75Li2S-25P2S5, 80Li2S-20P2S5, Li 10 GeP2S 12 , Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 , Li2S-SiS2, Li6PS5Cl, and the like, and among them, Li7P3S 11 is particularly preferable because of high ionic conductivity.

[0125] As the hydride-based solid electrolyte, a complex hydride of lithium borohydride, and the like can be mentioned. As the complex hydride, LiBH4-LiI-based complex hydride and LiBH4-LiNH2-based complex hydride, LiBH4-P2S5, LiBH4-P2I4, and the like can be mentioned.

[0126] The above-mentioned solid electrolyte can be used alone, or two or more kinds can be used in combination as needed.

[0127] (5-4) Conductive aid

[0128] The conductive aid contained in the active material layer of the present application contains the resin-bonded fiber of the present application. In addition to the resin-bonded fiber, a carbon-based conductive aid can also be contained.

[0129] The proportion of the resin-bonded fiber contained in the active material layer is less than 0.1 mass% or more and less than 5 mass%. The lower limit of the proportion of the resin-bonded fiber is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, further preferably 1.2 mass% or more, particularly preferably 1.5 mass% or more. In addition, the upper limit of the proportion of the resin-bonded fiber is preferably 4.5 mass% or less, more preferably 4.0 mass% or less, further preferably 3.5 mass% or less, more further preferably 3.0 mass% or less, particularly preferably 2.5 mass% or less. By the proportion of the resin-bonded fiber being in the above range, the balance between electronic conductivity and lithium ion conductivity is good, the rate characteristic value is high, and the reaction resistance value can be reduced. In addition, since the amount of the resin-bonded fiber in the active material layer is small, the amount of the active material can be increased.

[0130] (5-5) Binder

[0131] In the active material layer of the present application, a binder can also be contained in order to further improve the strength of the active material layer. The binder is not limited, and the aforementioned thermoplastic resin constituting the resin-bonded fiber of the present application can be cited.

[0132] As the content of the binder, in the active material layer, it is preferably in the range of 5 mass% or less, more preferably 1 to 3 mass%.

[0133] (5-6) Method for manufacturing active material layer for all-solid-state lithium secondary battery

[0134] The active material layer of the present application can be manufactured, for example, by preparing a slurry in which the aforementioned active material, solid electrolyte, resin-bonded fiber, and the like are mixed with a solvent. The slurry is caused to adhere to a current collector by coating or the like, and then dried to remove the solvent, and pressure-molded as necessary by pressing to manufacture. Alternatively, the aforementioned active material, solid electrolyte, and resin-bonded fiber, and the like can be powder-mixed, and then pressure-molded by pressing to manufacture.

[0135] (6) Electrode

[0136] The electrode for a nonaqueous electrolyte secondary battery of the present application is constituted by containing the aforementioned active material layer.

[0137] The current collector used in the electrode of the present application can be formed of any electrically conductive material. For example, the current collector can be formed of a metal material of aluminum, nickel, iron, stainless steel, titanium, or copper. Aluminum, stainless steel, and copper are particularly preferred. Aluminum or aluminum on which a carbon coating layer is formed is more preferably used for the positive electrode, and copper is more preferably used for the negative electrode.

[0138] The thickness of the current collector is preferably 10 to 50 μm.

[0139] (7) Non-aqueous electrolyte secondary battery

[0140] A non-aqueous electrolyte secondary battery of the present application will be described. The non-aqueous electrolyte secondary battery of the present application is a battery comprising the aforementioned electrode for non-aqueous electrolyte secondary batteries.

[0141] The non-aqueous electrolyte secondary battery of the present application can be exemplified by, for example, a lithium ion secondary battery, a lithium battery, a lithium ion polymer battery, a full solid lithium secondary battery, and the like. Among them, the full solid lithium secondary battery described later is preferred in view of the effects of the present application.

[0142] (8) Full solid lithium secondary battery

[0143] The full solid lithium secondary battery has the aforementioned positive electrode active material layer, a solid electrolyte layer containing a solid electrolyte, and the aforementioned negative electrode active material layer, and the positive electrode active material layer and the negative electrode active material layer are disposed so as to sandwich the solid electrolyte layer. Typically, a positive electrode current collector is provided on the positive electrode active material layer so as to sandwich them, a negative electrode current collector is provided on the negative electrode active material layer, and a battery case is further disposed so as to cover the entire positive electrode current collector and negative electrode current collector.

[0144] In particular, according to the present application, since the resin-bonded fibers are three-dimensionally and randomly oriented within the active material layer, even when volume changes due to expansion and contraction of the active material occur during charge and discharge, ion conduction paths and electron conduction paths can be maintained. Thus, ion conductivity and electron conductivity can be both taken into account. By doing so, a high-output full solid lithium secondary battery having excellent rate characteristics and cycle characteristics can be provided.

[0145] The full solid lithium secondary battery of the present application is not particularly limited as long as it has at least an active material layer and a solid electrolyte layer, and typically has a positive electrode current collector, a negative electrode current collector, a battery case, and the like as described above.

[0146] In the full solid lithium secondary battery, the active material layer and the solid electrolyte layer can not have a clear interface. When there is no clear interface, a layer in which 10% by volume or more of the active material exists within 10 μm in the thickness direction can be regarded as the active material layer.

[0147] Example

[0148] The present application is further specifically explained by Examples below, but the present application is not limited to these Examples. The various measurements and analyses in the Examples were respectively performed in accordance with the following methods.

[0149] (Confirmation of shape of fibrous carbon)

[0150] The fiber length of the fibrous carbon was measured using an image analysis particle size distribution meter (manufactured by Jasco International Corporation, Model IF-200 nano) for a thin dispersion liquid of the fibrous carbon (sample) dispersed in 1-methyl-2-pyrrolidone. The average fiber length of the fibrous carbon was the average value based on the number.

[0151] The fiber diameter of the fibrous carbon was observed and photographed using a scanning electron microscope (manufactured by Hitachi, Ltd., S-2400), and the fiber diameter was measured at 300 points selected at random from the obtained electron microscope photograph, and the average value of all the measurement results (n = 300) was taken as the average fiber diameter.

[0152] Further, the CV value was calculated from these average values and standard deviations. Further, the average aspect ratio was calculated from the average fiber length and the average fiber diameter.

[0153] (X-ray diffraction measurement of carbon fiber)

[0154] The X-ray diffraction measurement was performed using RINT-2100 manufactured by Rigaku Corporation in accordance with the JIS R7651 method to measure the interplanar spacing (d002) and the crystallite size (Lc002).

[0155] (Composite ratio)

[0156] The content ratio of the conductive fiber / thermoplastic resin was calculated from the weight reduction ratio by thermogravimetric analysis (TGA).

[0157] (Measurement method of volume resistivity of powder)

[0158] The measurement of the volume resistivity of the powder was performed using a powder resistance system (MCP-PD51) manufactured by Mitsubishi Chemical Analytech Corporation using a four-probe type electrode unit under a load of 0.02 to 2.50 kN. The volume resistivity was taken as the value of the volume resistivity at a packing density of 0.5 g / cm 3 , 0.8 g / cm 3 , and 1.0 g / cm 3 at the time of measurement of the volume resistivity of the sample.

[0159] (Melting point of thermoplastic resin)

[0160] The melting point and glass transition temperature were measured by differential scanning calorimetry (DSC) according to the measurement method of ISO 3146 (Plastics - Determination of the temperature of transition by differential scanning calorimetry, JIS K7121).

[0161] (tensile strength at break)

[0162] In a low-humidity environment at a dew point temperature of -60°C or lower, 40 parts by mass of LPS, 50 parts by mass of a positive electrode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), and 10 parts by mass of a resin-bonded fiber aggregate were mixed using an agate mortar. The mixture was filled in a jig for press molding, and the product of heat press molding (150°C, 100 MPa) was cut into a size of 5 mm in width x 7 mm in length, thereby producing a test piece for adhesion evaluation. Using the produced test piece, a tensile test was performed, and the results are described in Table 1. It was found that the tensile strength at break was improved by using the resin-bonded fiber aggregate.

[0163] (dispersibility)

[0164] The resin-bonded fiber was dispersed in 500 parts by mass of toluene, and the dispersion state was visually evaluated.

[0165] O: Dispersion was possible by shaking the dispersion liquid.

[0166] Δ: Dispersion was not possible by shaking the dispersion liquid alone, but was possible by ultrasonic treatment.

[0167] X: Dispersion was not possible even by shaking the dispersion liquid or performing ultrasonic treatment.

[0168] (tapped density measurement)

[0169] A resin-bonded fiber was added to a glass measuring cylinder having an inner diameter of 31 mm and a capacity of 150 ml, and the tapped density was measured by a tapped density measuring machine (Suzuki Rikagaku Kikai Co., Ltd., TPM-1A type) under conditions of a tapping speed of 40 times / min, a tapping stroke range of 60 mm, and a number of taps of 500.

[0170] (method for producing mesophase pitch)

[0171] A coal tar pitch having a softening point of 80°C from which quinoline-insoluble components were removed was hydrogenated in the presence of a Ni-Mo catalyst at a pressure of 13 MPa and a temperature of 340°C to obtain a hydrogenated coal tar pitch. The hydrogenated coal tar pitch was heat-treated at normal pressure at 480°C, and low-boiling components were removed under reduced pressure to obtain a mesophase pitch. The mesophase pitch was filtered at a temperature of 340°C using a filter to remove foreign matter in the pitch, and a refined mesophase pitch was obtained.

[0172] (Method (i) for producing carbon fiber (CNF))

[0173] A mesophase pitch composition was prepared by melt-kneading 60 parts by mass of a linear low-density polyethylene (EXCEED (registered trademark) 1018HA, manufactured by ExxonMobil, MFR = 1 g / 10 min) as a thermoplastic resin and 40 parts by mass of a mesophase pitch (mesophase ratio 90.9%, softening point 303.5°C) obtained by the above-mentioned (Method for producing mesophase pitch) using a uniaxial extruder (TOSHIBA MACHINE CO., LTD. "TEM-26SS", barrel temperature 300°C, under nitrogen flow).

[0174] Next, the mesophase pitch composition was melt-spun by setting the spinneret temperature to 360°C, whereby long fibers having a fiber diameter of 90 μm were formed.

[0175] The fiber bundle containing the mesophase pitch obtained by the above-mentioned operation was used in an amount of 0.1 kg, and was kept in air at 215°C for 3 hours, whereby the mesophase pitch was stabilized, and a fiber bundle containing the stabilized mesophase pitch was obtained. The above-mentioned fiber bundle containing the stabilized mesophase pitch was subjected to nitrogen substitution in a vacuum gas substitution furnace, and then depressurized to 1 kPa, and was heated to 500°C at a temperature increase rate of 5°C / min in this depressurized state, and kept at 500°C for 1 hour, whereby the thermoplastic resin was removed, and a stabilized fiber was obtained.

[0176] Next, the stabilized fiber was carbonized by keeping it at 1000°C for 30 minutes under a nitrogen atmosphere, and was graphitized by further heating it to 1500°C under an argon atmosphere, and keeping it at 1500°C for 30 minutes.

[0177] Next, the graphitized carbon fiber aggregate was pulverized, and a powder-like carbon fiber aggregate was obtained. The carbon fiber was a linear structure having no branch.

[0178] From the SEM photograph of the obtained carbon fiber, no branch could be confirmed in the carbon fiber (branching degree less than 0.01 / μm). The interlamellar spacing d002 was 0.3441 nm, the crystallite size Lc002 was 5.4 nm, the average fiber diameter was 270 nm, the average fiber length was 15 μm, the average aspect ratio was 56, the powder volume resistivity at 0.5 g / cm 3 was 0.0677 Ω-cm, the powder volume resistivity at 0.8 g / cm 3 was 0.0277 Ω-cm, the compression recovery degree was 59%, and the specific surface area was 10 m 2 / g. The metal content was less than 20 ppm.

[0179] The obtained carbon fiber is an excellent fibrous carbon having a large d002, a large average aspect ratio, a long average fiber length, and a high electrical conductivity. Hereinafter, this fibrous carbon is sometimes abbreviated as "CNF(i)".

[0180] (Method for producing carbon fiber (CNF(i)))

[0181] The carbon fiber was obtained in the same manner as in the aforementioned method for producing fibrous carbon (CNF(i)), except that the graphitization temperature was set to 1700°C.

[0182] From the SEM photograph of the obtained carbon fiber, no branch (branching degree: less than 0.01 / μm) was confirmed in the carbon fiber. The interlamellar spacing d002 was 0.3432 nm, the crystallite size Lc002 was 10.1 nm, the average fiber diameter was 299 nm, the average fiber length was 14 μm, the average aspect ratio was 47, the powder volume resistivity at 0.5 g / cm 3 at 0.8 g / cm 3 The small powder volume resistivity was 0.0205 Ω-cm, the compression recovery was 73%, and the specific surface area was 9 m 2 / g. The metal content was less than 20 ppm.

[0183] The obtained carbon fiber is an excellent fibrous carbon having a large d002, a large average aspect ratio, a long average fiber length, and a high electrical conductivity. Hereinafter, this fibrous carbon is sometimes abbreviated as "CNF(ii)".

[0184] (Method for producing resin-bonded fiber)

[0185] (Examples 1, 3 to 6, Comparative Example 2) (Spray drying (SD) method)

[0186] A VDF-HFP copolymer (Kynar 2500-20 manufactured by Arkema) was dissolved in acetone, and the conductive fiber was dispersed to prepare a dispersion liquid. The aforementioned dispersion liquid was spray-dried by using a spray dryer (SB39 manufactured by Prisma), and a resin-bonded fiber was obtained. The evaluation results of this resin-bonded fiber are shown in Table 1. Note that the SEM photograph of Example 4 is shown in Figure 1 , and the SEM photograph of Example 5 is shown in Figure 2 .

[0187] (Example 2) (Reprecipitation method)

[0188] A VDF-HFP copolymer (Kynar 2500-20 manufactured by Arkema) was dissolved in acetone at 1 part by mass to prepare a resin solution. An electrically conductive fiber was dispersed in toluene at 3 parts by mass, and the resin solution was added dropwise while stirring to precipitate the resin. Note that the amount of liquid was adjusted so that the mass ratio of acetone to toluene was 1:2. After the addition was completed, stirring was continued for 60 minutes, and then the stirring was stopped after the resin was completely precipitated. The resin-bonded fiber was obtained by filtration and drying. The evaluation results of the resin-bonded fiber are shown in Table 1.

[0189] (Comparative Example 3) (Simple Mixture)

[0190] A VDF-HFP copolymer (Kynar 2500-20 manufactured by Arkema) and CNF (i) were dispersed in toluene, and filtration and drying were performed to prepare a simple mixture. An SEM photograph of the simple mixture is shown in FIG. 2. Figure 3 .

[0191] The comprehensive evaluation of Examples 1 to 6 and CNF (i) (Comparative Example 1), Comparative Examples 2 and 3 is shown in Table 1.

[0192] : The tensile breaking strength is high (more than 3.0 MPa), and the powder volume resistivity at 1.0 g / cc is low (less than 0.1 Ω-cm)

[0193] : The tensile breaking strength is high (more than 3.0 MPa), and the powder volume resistivity at 1.0 g / cc is slightly low (more than 0.1 Ω-cm and less than 1 Ω-cm)

[0194] : The tensile breaking strength is slightly high (more than 0.1 MPa and less than 3.0 MPa), and the powder volume resistivity at 1.0 g / cc is low (less than 1.0 Ω-cm)

[0195] : The tensile breaking strength is low (less than 0.1 MPa), or the powder volume resistivity at 1.0 g / cc is high (more than 1.0 Ω-cm), or the dispersibility evaluation is x

[0196] In the obtained resin-bonded fiber, the integration of the electrically conductive fiber and the thermoplastic resin was confirmed by the following method. That is, each resin-bonded fiber was dispersed in toluene using an ultrasonic wave, and after being sufficiently shaken, it was left to stand for 5 minutes, and the height of the settled solid component was measured. As a result, the height of the settled solid component of each resin-bonded fiber was about 40 mm.

[0197] On the other hand, when the conductive fiber was simply dispersed at the same concentration, the height of the settled solid component was about 53 mm, and when the conductive fiber was mixed at the same concentration without being compounded with the thermoplastic resin, the height of the settled solid component was about 52 mm. Thus, it was confirmed that the resin-bonded fiber obtained in the present application was integrated with the conductive fiber and the thermoplastic resin.

[0198] The resin-bonded fiber obtained in Example 4 Figure 1 ) was compared with the case where CNF (i) and the thermoplastic resin were simply mixed (Comparative Example 3, Figure 3 ) and it was found that, in Comparative Example 3 where the carbon fiber was simply attached to the surface with respect to the thermoplastic resin, there was no difference in dispersibility, but the value of the powder volume resistivity was low and the electrical conductivity was high. In addition, the tensile breaking strength was high and the adhesion was excellent, and it was presumed that the carbon fiber was integrated with the thermoplastic resin.

[0199] (Evaluation of battery)

[0200] (Method for producing solid electrolyte (LPS))

[0201] Li2S and P2S5 were mixed at a molar ratio of 75:25, and a ball mill treatment (100 cycles of rotation at 500 rpm for 12 min and stop for 8 min) was performed, whereby a sulfide-based solid electrolyte (LPS) was produced. Hereinafter, this sulfide-based solid electrolyte is sometimes abbreviated as "LPS".

[0202] (Example 9) (Method for producing positive electrode mixture)

[0203] In an argon atmosphere, 35.8 parts by mass of LPS, 61.6 parts by mass of a positive electrode active material, and 2 parts by mass of a resin-bonded fiber (the resin-bonded fiber produced in Example 4) were mixed in an agate mortar. As the positive electrode active material, LiNi0.8Co0.1Mn0.1O2 (average particle diameter: 10.18 μm, D 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (average particle diameter: 10.18 μm, D 50 : 10.26 μm, powder conductivity: 5.46 x 10 -7 @ 2.47 g / cm 3 , hereinafter abbreviated as "surface-coated NCM").

[0204] (Method for producing battery for evaluation of all-solid-state battery)

[0205] A solid electrolyte layer was formed by filling 10 parts by mass of LPS in a battery container for evaluation of all-solid-state batteries, and performing 100 MPa x 3 times pressing. An anode active material layer was formed on one face of the solid electrolyte layer by adding 1 part by mass of the anode composite agent, and pressing at 150°C under a pressure of 100 MPa for 10 minutes. On the opposite face of the solid electrolyte layer, a Li foil (thickness: 47 μm) and an In foil (thickness: 50 μm) were disposed as negative electrode active materials, and were pressed at 80 MPa, and finally, a constant voltage battery was bolted, thereby producing an all-solid-state battery for evaluation of batteries, which maintained a state of being pressurized at 2 N.

[0206] (Rate characteristic evaluation)

[0207] Using the battery produced as described above, measurement of the discharge rate characteristics was performed. The charge-discharge test was generally performed at 70°C. The measurement conditions of the discharge rate characteristics were as follows. As the charging condition, after 0.05 C constant current charging to 3.7 V, the discharge was switched. As the discharging condition, the lower limit voltage was set to 2.0 V, and constant current discharging was performed at each discharging rate. The discharging rate was set to be phased up in the order of 0.1 C→0.2 C→0.5 C→1 C. The discharge capacity per unit active material weight (mAh / g) at each discharging rate is shown in the table. The greater the discharge capacity, the higher the output of the all-solid-state lithium secondary battery.

[0208] (Cycle characteristic)

[0209] Using the battery after the rate characteristic evaluation, cycle characteristic evaluation was performed by repeatedly performing charge-discharge. The charge-discharge test for the cycle characteristic evaluation was generally performed at 70°C. As the charging condition, after 0.1 C constant current charging to 3.7 V and CV constant voltage charging (cut-off 0.05 C), the discharge was switched. As the discharging condition, the lower limit voltage was set to 2.0 V, and 0.1 C constant current discharging was performed. The discharge capacity retention rate after 30 cycles was evaluated.

[0210] (Examples 10 to 11, Comparative Examples 4, 5)

[0211] The resin binder fiber was changed as described in Table 2, and otherwise, the same operation as in Example 9 was performed, and an active material layer and a battery for evaluation of all-solid-state batteries were produced. The evaluation results of the rate characteristics and the cycle characteristics thereof are shown in Tables 2 and 3.

[0212] Note that, as the spherical particles, acetylene black (hereinafter, sometimes abbreviated as "AB". "Denka Black" (registered trademark) manufactured by Denka Corporation, 75% pressed product, average particle diameter: 0.036 μm, specific surface area: 65 m 2 / g) was used.

[0213] (Example 12) (Method of producing anode composite layer)

[0214] An adhesive solution in which 24 parts by mass of LPS, 70 parts by mass of a positive electrode active material, and 2 parts by mass of an acrylic adhesive (polystyrene-butyl acrylate copolymer) were dissolved in 10 parts by mass of butyl butyrate was stirred using a "Bubble Dispersion Mill" (manufactured by THINKY) under an argon atmosphere. Then, 4 parts by mass of the resin adhesive fiber (Example 4) and 15 parts by mass of butyl butyrate were added, and the mixture was stirred again, whereby a slurry for a positive electrode mixture was prepared.

[0215] The obtained slurry for a positive electrode mixture was coated on an aluminum foil, and vacuum dried at 50°C for 5 hours, whereby butyl butyrate was removed. Then, a positive electrode mixture layer was prepared by heating and pressing at 150°C for 10 minutes. The results of the electrode evaluation are described in Table 2.

[0216] (Method for producing battery for evaluation of all-solid-state battery)

[0217] An evaluation battery for an all-solid-state battery was produced in the same manner as in Example 9, except that the positive electrode mixture layer produced as described above was used as a positive electrode, and a graphite electrode sheet was used as a negative electrode.

[0218] The rate characteristics and the cycle characteristics were evaluated in the same manner as in Example 9, and the results are described in Tables 2 and 3.

[0219] (Examples 13, 14, Comparative Examples 6, 7)

[0220] The production conditions of the positive electrode mixture layer were changed as described in Table 2, and a positive electrode mixture layer and an evaluation battery for an all-solid-state battery were produced in the same manner as in Example 12. The results of the evaluation of the rate characteristics and the cycle characteristics are described in Tables 2 and 3.

[0221] [Table 1]

[0222]

[0223] [Table 2]

[0224]

[0225] [Table 2 (continued)]

[0226]

[0227] [Table 3]

[0228]

Claims

1. Resin-bonded fibres, characterised in that comprising: electrically conductive fibers having an average fiber diameter of 10 to 5,000 nm and an average aspect ratio of 30 or more and 1,000 or less; and a thermoplastic resin; at least a part of the thermoplastic resin is attached to a part of the surface of the aforementioned electrically conductive fibers in a particle form having an aspect ratio of 5 or less and is integrated with the aforementioned electrically conductive fibers, the content of the aforementioned thermoplastic resin is 1 to 70 mass% with respect to the total amount of the aforementioned electrically conductive fibers and the aforementioned thermoplastic resin, The resin-bonded fiber has a packing density of 0.8 g / cm 3 The powder has a volume resistivity of 0.001 Ω-cm or more and 10 Ω-cm or less at 0.8 g / cm 2. The resinbonded fiber according to claim 1, having a tap density of 0.001 to 0.1 g / cm 3 .

3. The resinbonded fiber according to claim 1 or 2, wherein, the aforementioned thermoplastic resin is a thermoplastic resin having a melting point of 50 to 250°C.

4. The resin-bonded fiber according to claim 1 or 2, wherein, the aforementioned electrically conductive fibers are carbon fibers or nickel fibers.

5. The resin-bonded fiber according to claim 4, wherein, the aforementioned carbon fibers are substantially free of metal elements.

6. The resin-bonded fiber according to claim 1 or 2, wherein, the aforementioned thermoplastic resin is a thermoplastic resin containing a fluorine atom.

7. An active material layer for a nonaqueous electrolyte secondary battery, comprising the resin-bonded fiber according to claim 1 or 2.

8. An electrode for a nonaqueous electrolyte secondary battery, which is constituted by comprising the active material layer according to claim 7.

9. A nonaqueous electrolyte secondary battery, which is constituted by comprising the electrode according to claim 8.

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