Anode for all-solid-state secondary battery and all-solid-state secondary battery

Abstract

Provided are: a nonaqueous-electrolyte secondary battery negative electrode having excellent charging load characteristics; and a nonaqueous-electrolyte secondary battery using said negative electrode. The present invention pertains to goals 7 and 12 of the Sustainable Development Goals (SDGs).　The nonaqueous-electrolyte secondary battery negative electrode according to the present invention is characterized by: including a negative electrode active material and an electrically-conductive auxiliary agent; and containing, as the electrically-conductive auxiliary agent, carbon nanotubes having a fiber diameter of 0.8-20 nm and an aspect ratio of 5,000 or more. Further, the nonaqueous-electrolyte secondary battery according to the present invention is characterized by: having a positive electrode and a negative electrode; and having, as the negative electrode, the nonaqueous-electrolyte secondary battery negative electrode of the present invention.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery having excellent charging load characteristics, and a non-aqueous electrolyte secondary battery using the negative electrode. [Background technology]

[0002] In recent years, with the development of portable electronic devices such as mobile phones and laptop personal computers, and the practical application of electric vehicles, there has been a growing demand for small, lightweight secondary batteries with high capacity and high energy density. By providing society with non-aqueous electrolyte secondary batteries, we can contribute to achieving Goal 7 (Ensure access to affordable, reliable, sustainable and modern energy for all) and Goal 12 (Ensure sustainable consumption and production patterns) of the 17 Sustainable Development Goals (SDGs) established by the United Nations.

[0003] Currently, non-aqueous electrolyte secondary batteries that can meet the above-mentioned requirements use lithium-containing composite oxides such as lithium cobalt oxide (LiCoO) and lithium nickel oxide (LiNiO) as the positive electrode active material, graphite or the like as the negative electrode active material, and an organic electrolyte solution containing an organic solvent and a lithium salt as the non-aqueous electrolyte.

[0004] Furthermore, in non-aqueous electrolyte secondary batteries, improvements have been made not only to the positive electrode active materials and negative electrode active materials that are directly involved in the battery reaction, but also to conductive additives for ensuring conductivity in the electrodes. For example, Patent Documents 1 to 3 propose using a fibrous conductive additive such as carbon nanotubes in the electrodes of non-aqueous electrolyte secondary batteries. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-277128 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-243558 [Patent Document 3] Japanese Patent Application Publication No. 2018-181707 Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, a common method for charging a non-aqueous electrolyte secondary battery is to charge at a constant current until the battery voltage reaches a predetermined value (constant current charging), and then charge at a constant voltage until the current value decreases and reaches a predetermined value (constant voltage charging). However, in order to improve the rapid charging characteristics of a non-aqueous electrolyte secondary battery, for example, it is desirable that the negative electrode constituting the non-aqueous electrolyte secondary battery has a large capacity even when constant current charging is performed at a large current value, that is, has high charging load characteristics.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode for a non-aqueous electrolyte secondary battery that has excellent charge load characteristics, and a non-aqueous electrolyte secondary battery that uses the negative electrode. [Means for solving the problem]

[0008] The negative electrode for a non-aqueous electrolyte secondary battery of the present invention comprises a negative electrode active material and a conductive additive, and is characterized in that the conductive additive contains carbon nanotubes having a fiber diameter of 0.8 to 20 nm and an aspect ratio of 5000 or more.

[0009] The non-aqueous electrolyte secondary battery of the present invention comprises a positive electrode and a negative electrode, and is characterized in that the negative electrode comprises the negative electrode for non-aqueous electrolyte secondary batteries of the present invention. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a negative electrode for a non-aqueous electrolyte secondary battery having excellent charge load characteristics, and a non-aqueous electrolyte secondary battery using the negative electrode. [Brief explanation of the drawings]

[0011] [Figure 1]1 is a cross-sectional view schematically illustrating an example of a nonaqueous electrolyte secondary battery of the present invention. [Figure 2] FIG. 2 is a plan view schematically illustrating another example of a nonaqueous electrolyte secondary battery of the present invention. [Figure 3] FIG. 3 is a cross-sectional view taken along line II in FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Non-aqueous electrolyte secondary battery negative electrode> The negative electrode for a non-aqueous electrolyte secondary battery of the present invention comprises a negative electrode active material and a conductive additive, and the conductive additive contains carbon nanotubes having a fiber diameter of 0.8 to 20 nm and an aspect ratio of 5000 or more.

[0013] The negative electrode of a non-aqueous electrolyte secondary battery expands and contracts significantly as Li (lithium) ions enter and exit the battery during charging and discharging. Repeated charging and discharging, for example, gradually reduces the number of contact points between the negative electrode active material and the conductive additive, destroying the conductive pathway within the negative electrode and reducing its conductivity. This phenomenon becomes more pronounced when the current value during charging is increased.

[0014] Therefore, in the negative electrode for a non-aqueous electrolyte secondary battery of the present invention, carbon nanotubes (CNTs) with a fiber diameter of 0.8 to 20 nm and an aspect ratio of 5000 or more are used as the conductive additive. The use of CNTs with such a shape improves the strength of the negative electrode (a molded body of a negative electrode mixture containing a negative electrode active material and a conductive additive (including a negative electrode mixture layer formed on a current collector)) and suppresses breakdown of the conductive path during battery charge and discharge. Furthermore, the long length of the CNTs with such a shape maintains good conductivity between particles of the negative electrode active material. In the negative electrode for a non-aqueous electrolyte secondary battery of the present invention, these effects of the CNTs with such a shape allow the three-dimensional conductive network within the negative electrode to be well maintained, even during charging, for example, at a large current.

[0015] Furthermore, in all-solid-state secondary batteries in which Li ion conduction in the negative electrode is achieved by a solid electrolyte rather than a nonaqueous electrolyte (nonaqueous electrolytic solution), volume changes in the negative electrode associated with charging and discharging tend to gradually reduce the number of contact points between the negative electrode active material and the solid electrolyte, resulting in a decrease in Li ion conductivity in the negative electrode. However, in the negative electrode for a nonaqueous electrolyte secondary battery of the present invention, the use of CNTs having the above-described shape improves the strength of the negative electrode, and therefore good Li ion conductivity in the negative electrode is maintained even when charging at a large current, for example.

[0016] For these reasons, the negative electrode for a non-aqueous electrolyte secondary battery of the present invention has high charging load characteristics.

[0017] Examples of negative electrodes for non-aqueous electrolyte secondary batteries include a molded body (e.g., a pellet) obtained by molding a negative electrode mixture containing a negative electrode active material and a conductive additive, and a structure in which a layer (negative electrode mixture layer) made of a molded negative electrode mixture is formed on a current collector.

[0018] Examples of the negative electrode active material include carbon materials; alloys containing elements that can be alloyed with lithium, such as Si and Sn; oxides of Si and Sn; metallic lithium; and lithium alloys (lithium-aluminum alloys). One or more of these may be used.

[0019] Examples of carbon materials that can be used as the negative electrode active material include graphite (natural graphite; artificial graphite obtained by graphitizing easily graphitizable carbon such as pyrolytic carbon, mesophase carbon microbeads, and carbon fiber at 2800°C or higher; etc.), easily graphitizable carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), pyrolytic carbon, cokes, glassy carbon, fired bodies of organic polymer compounds, mesophase carbon microbeads, carbon fiber, and activated carbon.

[0020] Among these negative electrode active materials, it is preferable to use hard carbon, which is advantageous for improving the charge load characteristics of a negative electrode for a nonaqueous electrolyte secondary battery, because it has high Li ion acceptability, large capacity, a relatively small amount of volume change during battery charge and discharge, high affinity with solid electrolytes (described below) that are sometimes used in negative electrodes, and is less likely to produce Li dendrites.

[0021] When hard carbon is used as the negative electrode active material, it may be used alone or in combination with other negative electrode active materials, but when hard carbon is used in combination with other negative electrode active materials, the proportion of hard carbon in the total negative electrode active materials is preferably 50 mass% or more. Note that, as described above, since only hard carbon may be used as the negative electrode active material, the preferred upper limit of the proportion of hard carbon in the total negative electrode active materials is 100 mass%.

[0022] The content of the negative electrode active material in the negative electrode mixture is preferably 10 to 80 mass %.

[0023] The conductive additive for the negative electrode of the non-aqueous electrolyte secondary battery uses CNTs having a specific fiber diameter and aspect ratio.

[0024] The fiber diameter of the CNT is preferably 20 nm or less, more preferably 15 nm or less. CNTs with such a fiber diameter can be dispersed more uniformly within the negative electrode mixture compact, improving the strength of the compact and effectively suppressing the decrease in conductivity caused by volumetric changes in the negative electrode during battery charging and discharging. However, since CNTs that are too thin are difficult to handle, the fiber diameter is preferably 0.8 nm or more, more preferably 2 nm or more.

[0025] Furthermore, the aspect ratio of the CNT (expressed as the value obtained by dividing the fiber length by the fiber diameter) is 5,000 or more, and preferably 10,000 or more. When the CNT satisfies the above-mentioned fiber diameter and has such an aspect ratio, a good three-dimensional conductive network can be formed within the negative electrode mixture molded body, resulting in good conductivity within the molded body. There is no particular upper limit to the aspect ratio of the CNT, but it is usually about 200,000, and preferably 50,000 or less.

[0026] The CNT fiber diameter and aspect ratio referred to in this specification are as follows. The CNT fiber diameter is a value obtained by measuring the width (minor axis) in the short-side direction of an observation image of the CNT surface for a total of 10 CNTs using a transmission electron microscope (TEM) and calculating the average value. The CNT aspect ratio is a value obtained by dividing the average CNT fiber length by the average CNT fiber diameter. The CNT fiber length referred to here is a value obtained by selecting 10 CNTs whose outline shape can be confirmed in an image obtained by observing a molded negative electrode mixture body with a scanning electron microscope (SEM) at a magnification of 100 to 4000 times, measuring the maximum distance between two points on the periphery, and averaging the values ​​obtained for the selected 10 CNTs.

[0027] The fiber length of the CNTs determined by the above method is preferably 50 μm or more, more preferably 100 μm or more, and is preferably 500 μm or less, more preferably 300 μm or less.

[0028] The negative electrode for a non-aqueous electrolyte secondary battery (a molded body of a negative electrode mixture) may contain only CNTs (hereinafter sometimes referred to as "CNTs of the aforementioned shape") whose fiber diameter and aspect ratio satisfy the above values ​​as a conductive additive, or may contain other conductive additives together with the CNTs of the aforementioned shape. Examples of other conductive additives that the negative electrode may contain together with the CNTs of the aforementioned shape include carbon black and graphene.

[0029] From the perspective of ensuring good effects by its use, the content of the CNTs of the above shape in the negative electrode binder is preferably 0.5% by mass or more, and more preferably 0.8% by mass or more. However, if the amount of CNTs satisfying the above shape is too large, there is a risk that the effect of improving the charging load characteristics of the negative electrode for non-aqueous electrolyte secondary batteries will be small. Therefore, the content of the CNTs of the above shape in the negative electrode binder is preferably 2% by mass or less, and more preferably 1.8% by mass or less.

[0030] Also, when the negative electrode contains the CNTs of the above shape and other conductive aids, it is preferable that the total amount of conductive aids in the negative electrode binder is 11% by mass or less.

[0031] When using the negative electrode for non-aqueous electrolyte secondary batteries as the negative electrode of all-solid-state secondary batteries, the molded body of the negative electrode binder contains a solid electrolyte.

[0032] The solid electrolyte is not particularly limited as long as it has lithium ion conductivity. For example, sulfide-based solid electrolytes, hydride-based solid electrolytes, oxide-based solid electrolytes, etc. can be used.

[0033] Examples of sulfide-based solid electrolytes include particles such as Li2S-P2S5, Li2S-SiS2, Li2S-P2S5-GeS2, Li2S-B2S3-based glasses. In recent years, those of the LGPS system (Li 10 GeP2S 12 etc.) that have attracted attention for their high lithium ion conductivity, and those of the argyrodite type [such as Li6PS5Cl, Li 7-x+y PS 6-x Cl x+y (where 0.05 ≦ y ≦ 0.9, -3.0x + 1.8 ≦ y ≦ -3.0x + 5.7), Li 7-a PS 6-a Cl b Br c (where a = b + c, 0 < a ≦ 1.8, 0.1 ≦ b / c ≦ 10.0), etc.] can also be used.

[0034] Examples of hydride-based solid electrolytes include LiBH4, solid solutions of LiBH4 and the following alkali metal compounds (for example, those in which the molar ratio of LiBH4 to the alkali metal compound is 1:1 to 20:1). The alkali metal compound in the solid solution may be at least one selected from the group consisting of lithium halides (LiI, LiBr, LiF, LiCl, etc.), rubidium halides (RbI, RbBr, RbF, RbCl, etc.), cesium halides (CsI, CsBr, CsF, CsCl, etc.), lithium amide, rubidium amide, and cesium amide.

[0035] Examples of oxide-based solid electrolytes include Li7La3Zr2O 12 , LiTi(PO4)3, LiGe(PO4)3, LiLaTiO3, etc.

[0036] Among these solid electrolytes, sulfide-based solid electrolytes are preferred because of their high lithium ion conductivity, sulfide-based solid electrolytes containing lithium and phosphorus are more preferred, and argyrodite-type sulfide-based solid electrolytes are even more preferred because of their particularly high lithium ion conductivity and high chemical stability.

[0037] The average particle size of the solid electrolyte used in the negative electrode for a nonaqueous electrolyte secondary battery is preferably 0.1 μm or more, and more preferably 0.2 μm or more, from the viewpoint of reducing grain boundary resistance, and is preferably 10 μm or less, and more preferably 5 μm or less, from the viewpoint of forming a sufficient contact interface between the negative electrode active material and the solid electrolyte.

[0038] The average particle diameter of various particles (solid electrolyte, positive electrode active material, etc.) referred to in this specification is the 50% diameter value (D ) in the volume-based integrated fraction when calculating the integrated volume from small particles using a particle size distribution analyzer (e.g., a Microtrac particle size distribution analyzer "HRA9320" manufactured by Nikkiso Co., Ltd.). 50 ) means

[0039] The content of the solid electrolyte in the negative electrode mixture is preferably 4 to 70 mass %.

[0040] The negative electrode mixture may contain a resin binder, but it may not be necessary if good moldability can be ensured without the use of a binder, such as in the case of a negative electrode that also contains a sulfide-based solid electrolyte. Examples of resin binders include fluororesins such as polyvinylidene fluoride (PVDF). However, because the resin binder acts as a resistance component in the negative electrode mixture, it is desirable to keep its amount as small as possible. Therefore, if a resin binder is required in the negative electrode mixture, its content is preferably 5% by mass or less, and preferably 0.5% by mass or more. On the other hand, if a resin binder is not required in the negative electrode mixture from the perspective of moldability, its content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and even more preferably 0% by mass (i.e., no resin binder is contained).

[0041] When a current collector is used in the negative electrode of a non-aqueous electrolyte secondary battery, the current collector may be copper or nickel foil, punched metal, mesh, expanded metal, foamed metal; carbon sheet; or the like.

[0042] The compact of the negative electrode mixture can be formed, for example, by compressing a negative electrode mixture prepared by mixing a negative electrode active material, a conductive additive, and, if necessary, a solid electrolyte and a binder, by pressure molding, etc. A negative electrode for a non-aqueous electrolyte secondary battery composed only of a compact of the negative electrode mixture can be produced by the above-mentioned method.

[0043] In the case of a negative electrode for a non-aqueous electrolyte secondary battery having a current collector, it can be produced by bonding a molded body of the negative electrode mixture formed by the above-mentioned method to the current collector by, for example, pressure bonding.

[0044] Furthermore, in the case of a negative electrode for a non-aqueous electrolyte secondary battery having a current collector, it can also be produced by a method in which a negative electrode mixture-containing composition (paste, slurry, etc.) in which a negative electrode active material, a conductive additive, and further, if necessary, a solid electrolyte and a binder are dispersed in a solvent is applied to a current collector, dried, and then, if necessary, pressure-molded by calendering or the like, to form a molded body of the negative electrode mixture (negative electrode mixture layer) on the surface of the current collector.

[0045] The solvent for the negative electrode mixture-containing composition can be water or an organic solvent such as N-methyl-2-pyrrolidone (NMP). When a solid electrolyte is also included in the negative electrode mixture-containing composition, it is preferable to select a solvent that is less likely to deteriorate the solid electrolyte. In particular, sulfide-based and hydride-based solid electrolytes undergo chemical reactions with trace amounts of water, so nonpolar aprotic solvents such as hydrocarbon solvents such as hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene are preferred. Ultra-dehydrated solvents with a water content of 0.001% by mass (10 ppm) or less are particularly preferred. Fluorine-based solvents such as "Vertrel®" from DuPont-Mitsui Fluorochemicals, "Zeorolla®" from Nippon Zeon, and "Novec®" from Sumitomo 3M can also be used, as well as nonaqueous organic solvents such as dichloromethane and diethyl ether.

[0046] In addition, the long CNTs contained in the negative electrode mixture are usually cut and shortened during the manufacturing stage of the negative electrode for a non-aqueous electrolyte secondary battery (mainly when mixing the negative electrode mixture). Therefore, when manufacturing the negative electrode for a non-aqueous electrolyte secondary battery, it is desirable to adjust the conditions in each process to adjust the aspect ratio (fiber length) of the CNTs to the above value.

[0047] The thickness of the negative electrode mixture compact (in the case of a negative electrode having a current collector, the thickness of the negative electrode mixture compact per one side of the current collector; the same applies hereinafter) is preferably 200 μm or more from the viewpoint of increasing the capacity of the battery. Note that, although the load characteristics of a battery are generally improved by making the positive electrode or negative electrode thinner, according to the present invention, the load characteristics can be improved even when the negative electrode mixture compact is as thick as 200 μm or more. Therefore, in the present invention, the effect is more pronounced when the thickness of the negative electrode mixture compact is, for example, 200 μm or more. Furthermore, the thickness of the negative electrode mixture compact is usually 3000 μm or less.

[0048] In the case of a negative electrode for a non-aqueous electrolyte secondary battery produced by forming a negative electrode mixture layer on a current collector using a negative electrode mixture-containing composition containing a solvent, the thickness of the negative electrode mixture layer is preferably 50 to 1000 μm.

[0049] <Nonaqueous electrolyte secondary battery> The non-aqueous electrolyte secondary battery of the present invention has a positive electrode and a negative electrode, and the negative electrode is the negative electrode for a non-aqueous electrolyte secondary battery of the present invention.

[0050] A cross-sectional view schematically illustrating one example of the nonaqueous electrolyte secondary battery of the present invention is shown in Fig. 1. The nonaqueous electrolyte secondary battery 1 shown in Fig. 1 has a positive electrode 10, a negative electrode 20, a separator (a solid electrolyte layer in the case of an all-solid-state secondary battery) 30 interposed between the positive electrode 10 and the negative electrode 20, and a nonaqueous electrolyte (in the case of a nonaqueous electrolyte secondary battery other than an all-solid-state secondary battery) sealed in an exterior body formed by an exterior can 40, a sealing can 50, and a resin gasket 60 interposed between them.

[0051] The sealing can 50 is fitted into the opening of the outer can 40 via a gasket 60, and the open end of the outer can 40 is tightened inward, causing the gasket 60 to abut against the sealing can 50, thereby sealing the opening of the outer can 40 and creating an airtight structure inside the battery.

[0052] The outer can and sealing can can be made of stainless steel or other materials. Materials such as polypropylene and nylon can be used for the gasket. If heat resistance is required for the battery's intended use, heat-resistant resins with melting points exceeding 240°C, such as fluororesins such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA), polyphenylene ether (PEE), polysulfone (PSF), polyarylate (PAR), polyethersulfone (PES), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK), can also be used. If the battery is intended for use in applications requiring heat resistance, a glass hermetic seal can also be used for the sealing.

[0053] 2 and 3 are diagrams schematically showing other examples of the nonaqueous electrolyte secondary battery of the present invention, in which Fig. 2 is a plan view of the nonaqueous electrolyte secondary battery, and Fig. 3 is a cross-sectional view taken along line II in Fig. 2.

[0054] The nonaqueous electrolyte secondary battery 100 shown in FIGS. 2 and 3 houses an electrode assembly 200 in a laminate film casing 500 made of two metal laminate films, and the laminate film casing 500 is sealed at its outer periphery by heat-sealing the upper and lower metal laminate films.

[0055] When the nonaqueous electrolyte secondary battery 100 is an all-solid-state secondary battery, the electrode assembly 200 is configured by laminating a positive electrode, a negative electrode for a nonaqueous electrolyte secondary battery of the present invention, and a solid electrolyte layer interposed therebetween. On the other hand, when the nonaqueous electrolyte secondary battery 100 is a nonaqueous electrolyte secondary battery other than an all-solid-state secondary battery, the electrode assembly 200 is configured by laminating a positive electrode, a negative electrode for a nonaqueous electrolyte secondary battery of the present invention, and a separator interposed therebetween, and the electrode assembly 200 and a nonaqueous electrolyte are enclosed in the laminate film exterior body 500.

[0056] In FIG. 3, in order to avoid complication of the drawing, each layer constituting the laminate film exterior body 500 and each component (positive electrode, negative electrode, etc.) forming the electrode body 200 are not shown separately.

[0057] The positive electrode of the electrode body 200 is connected to the positive electrode external terminal 300 in the battery 100. Although not shown, the negative electrode of the electrode body 200 is also connected to the negative electrode external terminal 400 in the battery 100. The positive electrode external terminal 300 and the negative electrode external terminal 400 are drawn out to the outside of the laminate film exterior body 500 at one end side so as to be connectable to external devices and the like.

[0058] (Positive electrode) Examples of the positive electrode of the non-aqueous electrolyte secondary battery include a molded body (such as a pellet) formed by molding a positive electrode mixture containing a positive electrode active material and a conductive auxiliary agent, and a structure in which a layer (positive electrode mixture layer) made of a molded body of the positive electrode mixture is formed on a current collector.

[0059] The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in a conventionally known non-aqueous electrolyte secondary battery, that is, an active material capable of occluding and releasing Li ions. Specific examples of the positive electrode active material include LiM x Mn 2-x O4 (where M is at least one element selected from the group consisting of Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, Sn, Sb, In, Nb, Mo, W, Y, Ru and Rh, and 0.01 ≦ x ≦ 0.5), spinel type lithium manganese composite oxide represented by Li x Mn (1-y-x) Ni y M z O (2-k) F l (where M is at least one element selected from the group consisting of Co, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr and W, 0.8 ≦ x ≦ 1.2, 0 < y < 0.5, 0 ≦ z ≦ 0.5, k + l < 1, -0.1 ≦ k ≦ 0.2, 0 ≦ l ≦ 0.1), layered compound represented by LiCo 1-xM x Lithium cobalt composite oxide represented by LiNiO2 (wherein M is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5), 1-x M x Lithium nickel composite oxide represented by LiM02 (wherein M is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Fe, Co, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5) 1-x N x olivine-type composite oxides represented by Li4Ti5O4 (wherein M is at least one element selected from the group consisting of Fe, Mn, and Co, and N is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0≦x≦0.5); 12 Among these, only one kind may be used, or two or more kinds may be used in combination.

[0060] When the nonaqueous electrolyte secondary battery is an all-solid-state secondary battery, the average particle size of the positive electrode active material is preferably 1 μm or more, more preferably 2 μm or more, and preferably 10 μm or less, more preferably 8 μm or less. The positive electrode active material may be primary particles or secondary particles formed by aggregation of primary particles. When a positive electrode active material having an average particle size within the above range is used, a large interface with the solid electrolyte contained in the positive electrode can be secured, thereby further improving the load characteristics of the battery.

[0061] When the non-aqueous electrolyte secondary battery is an all-solid-state secondary battery, the positive electrode active material preferably has a reaction suppression layer on its surface to suppress reaction with the solid electrolyte contained in the positive electrode.

[0062] If the positive electrode active material and the solid electrolyte come into direct contact within the positive electrode mixture compact, the solid electrolyte may oxidize to form a resistance layer, resulting in a decrease in ionic conductivity within the compact. By providing a reaction suppression layer on the surface of the positive electrode active material that suppresses reaction with the solid electrolyte and preventing direct contact between the positive electrode active material and the solid electrolyte, it is possible to suppress a decrease in ionic conductivity within the compact due to oxidation of the solid electrolyte.

[0063] The reaction suppression layer may be made of any material that has ion conductivity and can suppress the reaction between the positive electrode active material and the solid electrolyte. Examples of materials that can form the reaction suppression layer include oxides containing Li and at least one element selected from the group consisting of Nb, P, B, Si, Ge, Ti, and Zr, more specifically, Nb-containing oxides such as LiNbO3, Li3PO4, Li3BO3, Li4SiO4, Li4GeO4, LiTiO3, and LiZrO3. The reaction suppression layer may contain only one of these oxides, or may contain two or more of them, or may even form a composite compound of two or more of these oxides. Among these oxides, Nb-containing oxides are preferred, and LiNbO3 is more preferred.

[0064] The reaction suppression layer is preferably present on the surface in an amount of 0.1 to 1.0 part by mass per 100 parts by mass of the positive electrode active material, which allows for effective suppression of the reaction between the positive electrode active material and the solid electrolyte.

[0065] Examples of methods for forming a reaction suppression layer on the surface of a positive electrode active material include the sol-gel method, mechanofusion method, CVD method, and PVD method.

[0066] The content of the positive electrode active material in the positive electrode mixture is preferably 60 to 95 mass %.

[0067] Examples of the conductive additive for the positive electrode include carbon materials such as graphite (natural graphite, artificial graphite), graphene, carbon black, carbon nanofiber, carbon nanotube, etc. The content of the conductive additive in the positive electrode mixture is preferably 1 to 10 mass %.

[0068] When the non-aqueous electrolyte secondary battery is an all-solid-state secondary battery, the positive electrode mixture contains a solid electrolyte.

[0069] The positive electrode solid electrolyte may be one or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, and oxide-based solid electrolytes listed above as examples of those usable for the negative electrode. To improve battery characteristics, it is preferable to include a sulfide-based solid electrolyte, and it is more preferable to include an argyrodite-type sulfide-based solid electrolyte.

[0070] The content of the solid electrolyte in the positive electrode mixture is preferably 4 to 30 mass %.

[0071] The positive electrode mixture may contain a resin binder, but it may not be necessary if good moldability can be ensured without the use of a binder, such as in the case of a positive electrode that also contains a sulfide-based solid electrolyte. Examples of resin binders include fluororesins such as polyvinylidene fluoride (PVDF). However, because the resin binder acts as a resistance component in the positive electrode mixture, it is desirable to keep its amount as small as possible. Therefore, if a resin binder is required in the positive electrode mixture, its content is preferably 5% by mass or less, and preferably 0.5% by mass or more. On the other hand, if a resin binder is not required in the positive electrode mixture from the perspective of moldability, its content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and even more preferably 0% by mass (i.e., no resin binder is contained).

[0072] When a current collector is used for the positive electrode, the current collector may be made of a metal foil such as aluminum or stainless steel, a punched metal, a mesh, an expanded metal, a foamed metal, a carbon sheet, or the like.

[0073] The compact of the positive electrode mixture can be formed, for example, by compressing a positive electrode mixture prepared by mixing a positive electrode active material, a conductive additive, and optionally a solid electrolyte and a binder, by pressure molding, etc. A positive electrode composed only of a compact of the positive electrode mixture can be produced by the above-mentioned method.

[0074] In the case of a positive electrode having a current collector, it can be produced by bonding a molded body of the positive electrode mixture formed by the above-mentioned method to the current collector by, for example, pressing.

[0075] Furthermore, in the case of a positive electrode having a current collector, the positive electrode can also be produced by a method in which a positive electrode mixture-containing composition (paste, slurry, etc.) in which a positive electrode active material, a conductive additive, and further, if necessary, a solid electrolyte or binder, etc. are dispersed in a solvent is applied to the current collector, dried, and then, if necessary, pressure-molded by calendaring or the like, to form a molded body of the positive electrode mixture (positive electrode mixture layer) on the surface of the current collector.

[0076] The solvent for the positive electrode mixture-containing composition can be an organic solvent such as NMP. When the positive electrode mixture-containing composition also contains a solid electrolyte, it is desirable to select a solvent that is unlikely to deteriorate the solid electrolyte, and it is preferable to use the same solvents as those exemplified above for the negative electrode mixture-containing composition containing the solid electrolyte.

[0077] The thickness of the positive electrode mixture compact (in the case of a negative electrode having a current collector, the thickness of the positive electrode mixture compact per one side of the current collector; the same applies hereinafter) is preferably 200 μm or more from the viewpoint of increasing the capacity of the battery. The thickness of the positive electrode mixture compact is usually 2000 μm or less.

[0078] In the case of a positive electrode produced by forming a positive electrode mixture layer on a current collector using a positive electrode mixture-containing composition containing a solvent, the thickness of the positive electrode mixture layer is preferably 50 to 1000 μm.

[0079] (solid electrolyte layer) When the nonaqueous electrolyte secondary battery is an all-solid-state secondary battery, the solid electrolyte in the solid electrolyte layer interposed between the positive electrode and the negative electrode can be one or more of the various sulfide-based solid electrolytes, hydride-based solid electrolytes, and oxide-based solid electrolytes listed above as examples usable for negative electrodes for nonaqueous electrolyte secondary batteries. However, to improve battery characteristics, it is preferable to include a sulfide-based solid electrolyte, and it is more preferable to include an argyrodite-type sulfide-based solid electrolyte. It is even more preferable to include a sulfide-based solid electrolyte in all of the positive electrode, negative electrode, and solid electrolyte layer, and it is even more preferable to include an argyrodite-type sulfide-based solid electrolyte.

[0080] The solid electrolyte layer may have a porous body such as a resin nonwoven fabric as a support.

[0081] The solid electrolyte layer can be formed by a method of compressing the solid electrolyte by pressure molding or the like; or a method of applying a solid electrolyte layer-forming composition prepared by dispersing the solid electrolyte in a solvent onto a substrate, a positive electrode, or a negative electrode, drying the composition, and, if necessary, performing pressure molding such as pressing.

[0082] It is desirable to select a solvent that is unlikely to deteriorate the solid electrolyte as the solvent used in the solid electrolyte layer-forming composition, and it is preferable to use the same solvents as those exemplified above as the solvents for the negative electrode mixture-containing composition containing the solid electrolyte.

[0083] The thickness of the solid electrolyte layer is preferably 100 to 300 μm.

[0084] (separator) When the non-aqueous electrolyte secondary battery is a battery other than an all-solid-state secondary battery, the separator interposed between the positive electrode and the negative electrode should have sufficient strength and be able to hold a large amount of non-aqueous electrolyte. From this perspective, a microporous film or nonwoven fabric containing polyethylene, polypropylene, or an ethylene-propylene copolymer, having a thickness of 10 to 50 μm and an opening ratio of 30 to 70%, is preferred.

[0085] (non-aqueous electrolyte) When the nonaqueous electrolyte secondary battery is a battery other than an all-solid-state secondary battery, a nonaqueous liquid electrolyte (hereinafter referred to as "electrolyte") is usually used as the nonaqueous electrolyte. The electrolyte is prepared by dissolving an electrolyte salt such as a lithium salt in an organic solvent. The organic solvent is not particularly limited, but examples thereof include chain esters such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propyl carbonate; cyclic esters with high dielectric constants such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; and mixed solvents of chain esters and cyclic esters. Mixed solvents of chain esters as the main solvent and cyclic esters are particularly suitable.

[0086] Examples of electrolyte salts to be dissolved in an organic solvent when preparing an electrolyte solution include LiPF6, LiBF4, LiAsF6, LiSbF6, LiCF3SO3, LiC4F9SO3, LiCF3CO2, Li2C2F4(SO3)2, LiC n F 2n+1 SO3 (n≧2), LiN(RfSO2)(Rf'SO2), LiC(RfSO2)3, LiN(RfOSO2)2 (where Rf and Rf' are fluoroalkyl groups), etc. may be used alone or in combination. The concentration of the electrolyte salt in the electrolytic solution is not particularly limited, but is preferably 0.3 mol / L or more, more preferably 0.4 mol / L or more, and is preferably 1.7 mol / L or less, more preferably 1.5 mol / L or less.

[0087] In the non-aqueous electrolyte secondary battery of the present invention, in addition to the above-mentioned electrolytic solution, a gel electrolyte obtained by gelling the above-mentioned electrolytic solution with a gelling agent made of a polymer or the like can also be used as the non-aqueous electrolyte.

[0088] (electrode body) The positive electrode and the negative electrode can be used in a battery in the form of a laminated electrode body in which the positive electrode and the negative electrode are laminated with a solid electrolyte layer or a separator interposed therebetween, or in the form of a wound electrode body in which this laminated electrode body is wound.

[0089] When forming an electrode body having a solid electrolyte layer, it is preferable to pressure-mold the positive electrode, negative electrode, and solid electrolyte layer in a stacked state, from the viewpoint of increasing the mechanical strength of the electrode body.

[0090] (Battery type) The nonaqueous electrolyte secondary battery may have a configuration having an exterior body composed of an exterior can, a sealing can, and a gasket as shown in FIG. 1, i.e., a configuration generally referred to as a coin-type battery or a button-type battery, or a configuration having an exterior body composed of a resin film or a metal-resin laminate film as shown in FIGS. 2 and 3, or may have an exterior body having a metallic, bottomed, tubular (cylindrical or rectangular) exterior can and a sealing structure that seals the opening of the can. [Example]

[0091] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0092] Example 1 [Preparation of negative electrode made from negative electrode mixture molded body] Hard carbon (HC), CNT (fiber diameter: 10 nm, fiber length: 2 mm), graphene, and a sulfide-based solid electrolyte (Li6PS5Cl) were mixed in a mass ratio of 20:1.5:9.5:69 and thoroughly kneaded to prepare a negative electrode mixture (1). Next, 80 mg of the negative electrode mixture (1) was placed in a powder molding die with a diameter of 10 mm and pressed at room temperature at 4 t / cm using a press. 2The mixture was pressed for 5 minutes under the above conditions to prepare a negative electrode mixture compact.

[0093] The fiber diameter, fiber length and aspect ratio of the CNTs measured by the above-mentioned method for a part of the negative electrode were 10 nm, 200 μm and 20,000, respectively.

[0094] [Production of model cells] (Formation of solid electrolyte layer) 80 mg of sulfide-based solid electrolyte (Li6PS5Cl) was placed in a powder molding die with a diameter of 10 mm, and pressure molding was carried out using a press to form a solid electrolyte layer.

[0095] (Preparation of negative electrode) Negative electrode mixture (1): 15 mg was placed on the solid electrolyte layer in the powder molding die, and pressure molding was performed using a press to form a negative electrode made of a negative electrode mixture compact on the solid electrolyte layer.

[0096] (Formation of laminated electrode body) The counter electrode was made by bonding together Li metal and In metal, each formed into a cylindrical shape, and placed on the side of the solid electrolyte layer in the powder molding die opposite the negative electrode. This counter electrode was then pressed using a press to produce a laminated electrode body.

[0097] (Model cell assembly) Using the laminated electrode body, an all-solid-state battery (model cell) with a planar structure similar to that shown in FIG. A battery was fabricated using a laminate film exterior. A negative electrode current collector foil (SUS foil) and a counter electrode current collector foil (SUS foil) were attached side by side with a certain amount of space between them on the inner surface of the aluminum laminate film that constitutes the exterior of the laminate film. Each current collector foil was cut into a shape that included a main body portion facing the negative electrode surface or counter electrode surface of the laminated electrode body, and a portion that protruded from the main body toward the outside of the battery to become the negative electrode external terminal 400 and the counter electrode external terminal 300.

[0098] The laminated electrode body was placed on the negative electrode current collecting foil of the laminated film casing, and the laminated electrode body was wrapped in the laminated film casing so that the counter electrode current collecting foil was positioned on the counter electrode of the laminated electrode body.The remaining three sides of the laminated film casing were sealed by heat sealing under vacuum to obtain a model cell.

[0099] Comparative Example 1 A negative electrode mixture (2) was prepared in the same manner as in Example 1, except that no CNT was used and the ratio of HC, graphene, and solid electrolyte was set to 20:11:69 by mass, and a negative electrode composed of a negative electrode mixture molded body was produced in the same manner as in Example 1, except that this negative electrode mixture (2) was used. In addition, a model cell was produced in the same manner as in Example 1, except that the negative electrode mixture (2) was used.

[0100] Comparative Example 2 A negative electrode mixture (3) was prepared in the same manner as in Example 1, except that the CNTs were changed to those with a fiber diameter of 2 nm and a fiber length of 9.5 μm, and a negative electrode made of a negative electrode mixture molded body was fabricated in the same manner as in Example 1, except that this negative electrode mixture (3) was used. For a part of the obtained negative electrode, the fiber diameter, fiber length, and aspect ratio of the CNTs determined by the above-mentioned methods were 2 nm, 9.0 μm, and 4500, respectively.

[0101] A model cell was also produced in the same manner as in Example 1, except that the negative electrode mixture (3) was used.

[0102] Comparative Example 3 A negative electrode mixture (4) was prepared in the same manner as in Example 1, except that the CNTs were changed to those with a fiber diameter of 13 nm and a fiber length of 24 μm, and a negative electrode composed of a negative electrode mixture molded body was fabricated in the same manner as in Example 1, except that this negative electrode mixture (4) was used. The fiber diameter, fiber length, and aspect ratio of the CNTs of a part of the obtained negative electrode, determined by the above-mentioned methods, were 13 nm, 19.6 μm, and 1500, respectively.

[0103] A model cell was also produced in the same manner as in Example 1, except that the negative electrode mixture (4) was used.

[0104] Comparative Example 4 A negative electrode active material (5) was prepared in the same manner as in Example 1, except that vapor-grown carbon fibers (fiber diameter: 150 nm, fiber length: 10 μm) were used instead of CNT. A negative electrode composed of a negative electrode active material molded body was produced in the same manner as in Example 1, except that the negative electrode active material (5) was used. For a part of the obtained negative electrode, the fiber diameter, fiber length, and aspect ratio of the vapor-grown carbon fibers determined in the same manner as in the case of CNT were 150 nm, 8.9 μm, and 60, respectively.

[0105] A model cell was produced in the same manner as in Example 1, except that the negative electrode active material (5) was used.

[0106] The following evaluations were performed on the negative electrodes of Example 1 and Comparative Examples 1 to 4, and model cells having these negative electrodes.

[0107] <CC Capacity Evaluation> For each model cell, under a pressurized state (1 t / cm 2 ) at a temperature of 23 °C, constant current charging was performed at a current value of 0.5C until the voltage reached -0.62V, followed by constant voltage charging at a voltage of -0.62V until the current value reached 0.01C. Subsequently, a series of steps of discharging at a current value of 0.1C until the voltage reached 1.88V was repeated twice. The capacity at the second constant current charging and the capacity at the second discharging (initial capacity) were determined. Then, the value obtained by dividing the capacity at the second constant current charging of each model cell by the initial capacity was expressed as a percentage to evaluate the CC capacity (0.5C capacity).

[0108] Also, for each model cell, a series of operations consisting of constant current charging - constant voltage charging and discharging under the same conditions as when measuring the 0.5C capacity were repeated twice, except that the current value during constant current charging and the current value during discharging were changed to 1.0C. The capacity at the second constant current charging and the capacity at the second discharging (initial capacity) were determined, and the value obtained by dividing the capacity at the second constant current charging by the initial capacity was expressed as a percentage to evaluate the CC capacity (1.0C capacity).

[0109] If the CC capacities (0.5C capacity and 1.0C capacity) are large, it can be said that the charge load characteristics of the negative electrode used in the model cell are excellent.

[0110] <Measurement of negative electrode strength> The strength of each negative electrode (a negative electrode made of a negative electrode mixture molded body) in the examples and comparative examples was measured using a force gauge ("Digital Force Gauge ZTS-500N" manufactured by Imada Co., Ltd.). Each negative electrode was placed on a table with the flat surface facing upward, and stress was applied from this flat surface in the thickness direction of the negative electrode using the force gauge. The load (kgf) at which the negative electrode broke was read from the force gauge, and this value was taken as the strength of the negative electrode. The strength of each negative electrode was evaluated as a relative value when the strength of the negative electrode of Comparative Example 1 was taken as 100 (%).

[0111] The fiber diameter, aspect ratio, and content in the negative electrode mixture of the CNTs for the negative electrode are shown in Table 1, and the evaluation results are shown in Table 2. Note that, although vapor-grown carbon fibers were used instead of CNTs in the negative electrode of Comparative Example 4, these values ​​are shown in parentheses in the CNT column in Table 1 to facilitate comparison.

[0112] [Table 1]

[0113] [Table 2]

[0114] As shown in Tables 1 and 2, the model cell prepared in Example 1 used a negative electrode containing CNTs with appropriate fiber diameters and aspect ratios, and had a higher CC capacity than the model cell of Comparative Example 1, which used a negative electrode without CNTs. Therefore, these negative electrodes can be said to have excellent charge load characteristics. Furthermore, the negative electrode of Example 1 also had greater strength than the negative electrode of Comparative Example 1.

[0115] On the other hand, the model cells of Comparative Examples 2 and 3, which used negative electrodes containing CNTs with inappropriate aspect ratios, and the model cell of Comparative Example 4, which used a negative electrode using vapor-grown carbon fiber instead of CNTs, had small CC capacities, and the charging load characteristics of these negative electrodes were inferior to those of Example 1.

[0116] Example 2 A negative electrode mixture (6) was prepared in the same manner as in Example 1, except that the ratio of HC, CNT, graphene, and solid electrolyte was changed to 20:0.5:10.5:69 by mass, and a negative electrode composed of a negative electrode mixture molded body was fabricated in the same manner as in Example 1, except that this negative electrode mixture (6) was used. For a part of the obtained negative electrode, the fiber diameter, fiber length, and aspect ratio of the CNTs determined by the above-mentioned methods were 10 nm, 200 μm, and 20,000, respectively.

[0117] A model cell was also produced in the same manner as in Example 1, except that the negative electrode mixture (6) was used.

[0118] Example 3 A negative electrode mixture (7) was prepared in the same manner as in Example 1, except that the ratio of HC, CNT, graphene, and solid electrolyte was changed to 20:1:10:69 by mass, and a negative electrode composed of a negative electrode mixture molded body was fabricated in the same manner as in Example 1, except that this negative electrode mixture (7) was used. For a part of the obtained negative electrode, the fiber diameter, fiber length, and aspect ratio of the CNTs determined by the above-mentioned methods were 10 nm, 200 μm, and 20,000, respectively.

[0119] A model cell was also produced in the same manner as in Example 1, except that the negative electrode mixture (7) was used.

[0120] Example 4 A negative electrode mixture (8) was prepared in the same manner as in Example 1, except that the ratio of HC, CNT, graphene, and solid electrolyte was changed to 20:2:9:69 by mass ratio, and a negative electrode composed of a negative electrode mixture molded body was fabricated in the same manner as in Example 1, except that this negative electrode mixture (8) was used. For a part of the obtained negative electrode, the fiber diameter, fiber length, and aspect ratio of the CNTs determined by the above-mentioned methods were 10 nm, 200 μm, and 20,000, respectively.

[0121] A model cell was also produced in the same manner as in Example 1, except that the negative electrode mixture (8) was used.

[0122] For the negative electrodes of Examples 2 to 4 and model cells having these negative electrodes, CC capacity measurements and negative electrode strength measurements were carried out in the same manner as in Example 1. The fiber diameter, aspect ratio, and content in the negative electrode mixture of the CNTs for the negative electrodes of Examples 2 to 4 are shown in Table 3, and the results of each of the evaluations are shown in Table 4. Tables 3 and 4 also list Example 1 and Comparative Example 1.

[0123] [Table 3]

[0124] [Table 4]

[0125] In the negative electrodes used in the model cells of Examples 1 to 4, the content of CNTs with appropriate fiber diameters and aspect ratios in the negative electrode mixture was varied, but these model cells all had higher CC capacities than the model cell of Comparative Example 1, which used a negative electrode containing no CNTs. Therefore, it can be said that the negative electrodes of Examples 2 to 4 also have excellent charge load characteristics, similar to the negative electrode of Example 1. Furthermore, the negative electrodes of Examples 2 to 4, like the negative electrode of Example 1, also had greater strength than the negative electrode of Comparative Example 1.

[0126] The strength of the negative electrodes of Comparative Examples 2 to 4 shown in Table 2 was equal to or greater than that of the negative electrodes of Examples 2 and 3, which had a low content of CNTs with appropriate fiber diameters and aspect ratios in the negative electrode mixture. However, the CNTs used in the negative electrodes of Comparative Examples 2 and 3 had inappropriate aspect ratios, and the negative electrode of Comparative Example 4 used vapor-grown carbon fibers with inappropriate fiber diameters and aspect ratios. Therefore, it is presumed that the model cells using these negative electrodes had smaller CC capacities than the model cells using the negative electrodes of Examples 2 and 3 (the negative electrodes of Comparative Examples 2 to 4 had inferior charging load characteristics than the negative electrodes of Examples 2 and 3).

[0127] The present invention can be implemented in other forms without departing from the spirit of the present invention. The embodiments disclosed in this application are merely examples, and the present invention is not limited to these embodiments. The scope of the present invention shall be interpreted in accordance with the appended claims rather than the description in the above specification, and all modifications within the scope of the claims are included in the scope of the claims. [Industrial Applicability]

[0128] The nonaqueous electrolyte secondary battery of the present invention can be used in the same applications as secondary batteries having conventionally known nonaqueous electrolytes (nonaqueous electrolytic solutions or gel electrolytes) and all-solid-state secondary batteries. Furthermore, the negative electrode for a nonaqueous electrolyte secondary battery of the present invention can constitute the nonaqueous electrolyte secondary battery of the present invention. [Explanation of symbols]

[0129] 1,100 Non-aqueous electrolyte secondary battery 10 positive electrode 20 negative electrode 30 Solid electrolyte layer or separator 40 outer can 50 sealed cans 60 gaskets 200 Electrode body 300 Positive external terminal 400 Negative external terminal 500 Laminated film exterior

Claims

1. A negative electrode for an all-solid-state secondary battery comprising a negative electrode active material, a conductive additive, and a solid electrolyte, The negative electrode for an all-solid-state secondary battery contains, as the conductive additive, carbon nanotubes having a fiber diameter of 0.8 to 20 nm and an aspect ratio of 5000 or more.

2. The negative electrode for an all-solid-state secondary battery according to claim 1 , wherein the negative electrode mixture containing the negative electrode active material and the conductive additive is formed into a pellet-like compact.

3. 3. The negative electrode for an all-solid-state secondary battery according to claim 1, wherein the negative electrode active material contains hard carbon.

4. The negative electrode for an all-solid-state secondary battery according to any one of claims 1 to 3, wherein the solid electrolyte is a sulfide-based solid electrolyte.

5. An all-solid-state secondary battery having a positive electrode and a negative electrode, and a solid electrolyte layer between the positive electrode and the negative electrode, An all-solid-state secondary battery comprising the anode for an all-solid-state secondary battery according to any one of claims 1 to 4 as the anode.

6. 6. The all-solid-state secondary battery according to claim 5, wherein the solid electrolyte layer contains a sulfide-based solid electrolyte.

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