Nonaqueous electrolyte secondary battery

Abstract

Provided is a nonaqueous electrolyte secondary battery which achieves both high output and good quick charge cycle characteristics. In the nonaqueous electrolyte secondary battery according to the present disclosure, a positive electrode mixture layer contains a positive electrode active material, single-walled carbon nanotubes, and multi-walled carbon nanotubes, and a negative electrode mixture layer contains a first negative electrode mixture layer that is disposed on the surface of a negative electrode and a second negative electrode mixture layer that is disposed between the first negative electrode mixture layer and a negative electrode core body. The ratio P1 of coated graphite to the total mass of graphite in the first negative electrode mixture layer and the ratio P2 of coated graphite to the total mass of graphite in the second negative electrode mixture layer satisfy the relationship P1 > P2. The void fraction Vc of the positive electrode mixture layer during discharge and the void fraction Va of the negative electrode mixture layer during discharge satisfy the relationship Va - Vc < 13%.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion batteries have been used in a variety of applications, including in-vehicle applications and power storage applications. In particular, in-vehicle applications require not only rapid charging performance but also the ability to maintain high battery capacity even after repeated charge and discharge. Patent Documents 1 to 4 disclose techniques related to negative electrodes aimed at improving rapid charging cycle characteristics.

[0003] Patent No. 6108166 Patent No. 6959010 Patent No. 7136017 Patent No. 7246810

[0004] The required characteristics of non-aqueous electrolyte secondary batteries are becoming higher every year, and it is difficult for the techniques described in Patent Documents 1 to 4 to meet these requirements. Furthermore, improving output is an important issue for non-aqueous electrolyte secondary batteries. The techniques described in Patent Documents 1 to 4 cannot adequately address the issue of achieving both high output and good rapid charge cycle characteristics, and there is still much room for improvement.

[0005] An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that combines high output with good rapid charge cycle characteristics.

[0006] The non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode has a positive electrode core and a positive electrode mixture layer disposed on the surface of the positive electrode core. The positive electrode mixture layer contains a positive electrode active material, single-walled carbon nanotubes, and multi-walled carbon nanotubes. The negative electrode has a negative electrode core and a negative electrode mixture layer disposed on the surface of the negative electrode core. The negative electrode mixture layer includes a first negative electrode mixture layer disposed on the surface of the negative electrode and a second negative electrode mixture layer disposed between the first negative electrode mixture layer and the negative electrode core. a negative electrode mixture layer, the negative electrode mixture layer containing graphite, part of the graphite being coated with amorphous graphite, a ratio P1 of the coated graphite to the total mass of graphite in the first negative electrode mixture layer and a ratio P2 of the coated graphite to the total mass of graphite in the second negative electrode mixture layer satisfy the relationship P1 > P2, and a porosity Vc of the positive electrode mixture layer during discharge and a porosity Va of the negative electrode mixture layer during discharge satisfy the relationship Va - Vc < 13%.

[0007] The nonaqueous electrolyte secondary battery according to the present disclosure has high output and excellent rapid charge cycle characteristics.

[0008] 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention;

[0009] As a result of extensive research by the present inventors, it was found that even if the negative electrodes described in Patent Documents 1 to 4 are used, it is not possible to obtain a nonaqueous electrolyte secondary battery that achieves both high output and good rapid charge cycle characteristics. In recent years, the characteristics required of nonaqueous electrolyte secondary batteries have become increasingly higher, and it is difficult to meet these requirements with the techniques described in Patent Documents 1 to 4. Furthermore, although improving the output of nonaqueous electrolyte secondary batteries is an important issue, Patent Documents 1 to 4 hardly consider increasing the output of batteries.

[0010] Therefore, the inventors conducted further research and succeeded in obtaining a nonaqueous electrolyte secondary battery that combines high output and excellent rapid charge cycle characteristics by optimizing the structures of both the positive and negative electrodes, rather than focusing solely on the negative electrode. The positive electrode mixture layer contains both single-walled carbon nanotubes and multi-walled carbon nanotubes as conductive agents, and the porosity Vc and Va between the positive electrode mixture layer and the negative electrode mixture layer are relatively large so that the relationship Va - Vc < 13% is satisfied during discharge. This allows the positive electrode mixture layer to absorb nonaqueous electrolyte discharged from the negative electrode during charge while ensuring a conductive path. Furthermore, the negative electrode mixture layer has a multilayer structure, and the proportion of coated graphite contained in the first negative electrode mixture layer on the surface side is greater than the proportion of coated graphite contained in the second negative electrode mixture layer on the core side. This ensures that the first negative electrode mixture layer has a high porosity but a conductive path. The coated graphite has higher conductivity and is harder than regular graphite. The first negative electrode mixture layer contains a higher proportion of coated graphite than the second negative electrode mixture layer, which makes the graphite less likely to crack during the rolling process performed in manufacturing the negative electrode and increases the porosity. On the other hand, the second negative electrode mixture layer has a lower porosity than the first negative electrode mixture layer, which contributes to maintaining the battery capacity.

[0011] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

[0012] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified as a nonaqueous electrolyte secondary battery, but the outer can of the battery is not limited to a cylindrical outer can. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery having a prismatic outer can, a coin battery having a coin-shaped outer can, or a pouch-type battery having an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, and may be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0013] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 housing the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The electrode assembly 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The diameter D and height H of the electrode assembly 14 satisfy, for example, H / D≧2. The larger the H / D ratio, the more non-uniform the distribution of the nonaqueous electrolyte contained in the positive electrode 11 and the negative electrode 12 becomes, which tends to deteriorate the rapid charge cycle characteristics. Therefore, the effects of the secondary battery according to the present disclosure, as described below, become more pronounced. The outer can 16 is a cylindrical metal container with a bottom and an open axial end. The opening of the outer can 16 is closed by a sealing member 17. In the following description, for convenience of explanation, the sealing body 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0014] The non-aqueous electrolyte has lithium ion conductivity. The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may include, for example, LiPF ６ Lithium salts such as

[0015] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length and width directions. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0016] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0017] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0018] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0019] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode assembly 14 will be described in detail below.

[0020] [Positive Electrode] The positive electrode 11 has, for example, a positive electrode core and a positive electrode mixture layer disposed on the surface of the positive electrode core. The positive electrode core can be a foil of a metal such as aluminum that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on its surface. The positive electrode mixture layer preferably contains, for example, a positive electrode active material, a conductive agent, and a binder, and is provided on both sides of the positive electrode core except for the portion to which the positive electrode lead 20 is connected. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to the surface of the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0021] An example of the positive electrode active material contained in the positive electrode mixture layer 31 is a lithium-containing composite oxide having a layered rock salt structure. Examples of the layered rock salt structure of the lithium-containing composite oxide include a layered rock salt structure belonging to the space group R-3m and a layered rock salt structure belonging to the space group C2 / m. From the viewpoints of increasing capacity and stability of the crystal structure, the lithium-containing composite oxide preferably has a layered rock salt structure belonging to the space group R-3m.

[0022] The lithium-containing composite oxide is, for example, a secondary particle formed by the aggregation of multiple primary particles. The volume-based median diameter (D50) of the composite oxide is not particularly limited, but is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm. When the composite oxide is a secondary particle formed by the aggregation of primary particles, the D50 of the composite oxide refers to the D50 of the secondary particles. D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the composite oxide (as well as that of the negative electrode active material) can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Corporation) using water as a dispersion medium.

[0023] The average particle size of the primary particles constituting the lithium-containing composite oxide is, for example, 0.05 μm or more and 1 μm or less, and is calculated by averaging the diameters of the circumscribed circles of the primary particles extracted by analyzing a scanning electron microscope (SEM) image of the cross section of the secondary particles.

[0024] The lithium-containing composite oxide is a composite oxide containing metal elements such as Ni, Co, Al, and Mn in addition to Li. Metal elements constituting the lithium-containing composite oxide are, for example, Ni and M (M is at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, Si, Nb, Mo, W, and Zn). Among these, it is preferable to contain at least one element selected from Ni, Co, Al, and Mn. Examples of suitable composite oxides include composite oxides containing Ni, Co, and Al, and composite oxides containing Ni, Co, and Mn.

[0025] From the viewpoint of increasing capacity, the lithium-containing composite oxide preferably contains 80 mol% or more of Ni relative to the total number of moles of metal elements excluding Li. The Ni content may be 87 mol% or more, or may be 90 mol% or more, relative to the total number of moles of metal elements excluding Li. The upper limit of the Ni content is, for example, 95 mol%.

[0026] The lithium-containing composite oxide is, for example, a compound represented by the general formula Li ａ Ni ｘ M ｙ O２－ｂ (wherein 0.95≦a≦1.2, 0.80≦x≦0.95, 0.05≦y≦0.20, 0≦b≦0.05, x+y=1, and M is at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, Si, Nb, Mo, W, and Zn.) In the above general formula, x preferably satisfies 0.87≦x≦0.95. The contents of the elements constituting the lithium-containing composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0027] Examples of conductive agents contained in the positive electrode mixture layer include carbon materials such as carbon black (e.g., acetylene black, Ketjen black), graphite, carbon nanotubes (CNT), carbon nanofibers, and graphene. Examples of CNT include single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). SWCNTs are carbon nanostructures in which a single layer of graphene sheets forms a cylindrical shape, while multi-walled carbon nanotubes are carbon nanostructures in which two or more layers of graphene sheets are concentrically stacked to form a cylindrical shape. A graphene sheet refers to a layer in which carbon atoms in the sp2 hybrid orbitals that form graphite crystals are located at the vertices of a regular hexagon. The shape of the carbon nanotube is not limited. Examples of such shapes include needles, cylindrical tubes, fishbone-shaped (fishbone or cup-stacked), playing card-shaped (platelet), and coil-shaped.

[0028] The positive electrode mixture layer preferably contains both SWCNTs and MWCNTs, which allows the conductive paths in the positive electrode mixture layer to be maintained even when the positive electrode mixture layer has a predetermined porosity, as will be described later.

[0029] The SWCNT content in the positive electrode mixture layer is, for example, 0.0001 mass % to 0.02 mass % or less, relative to the total mass of the positive electrode active material. The MWCNT content in the positive electrode mixture layer is, for example, 0.01 mass % to 0.5 mass % or less, relative to the total mass of the positive electrode active material.

[0030] The surface area per unit mass of SWCNT is, for example, 300 m ２ The surface area per unit mass of MWCNT is, for example, 200 m ２ The upper limit of the surface area per unit mass of SWCNT and MWCNT is not particularly limited, but is, for example, 10,000 m ２ / g.

[0031] It is preferable that SWCNT and MWCNT contain substantially no metal catalyst in the tube. The metal catalyst may deteriorate the positive electrode 11 and reduce the characteristics of the secondary battery. Here, "substantially not containing a metal catalyst" means that the metal catalyst is contained at the lower limit of detection by ICP-AES.

[0032] The positive electrode mixture layer may contain at least one of hydrogenated nitrile butadiene rubber (HNBR) and a cellulose derivative, which functions as a dispersant and suppresses aggregation of CNTs in the positive electrode mixture slurry.

[0033] Examples of the binder contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, etc. Furthermore, these resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide, etc.

[0034] Area A [m ２ ] and the discharge capacity B [Ah] when discharged from a charged state of 4.2 V to 2.5 V at 0.05 C is B / A≧40 Ah / m ２ It is preferable to satisfy the relationship: The above results are from a temperature environment of 25°C, and the secondary battery was charged at a constant current of 1 C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 C, thereby achieving a state of charge of 4.2 V. Furthermore, when a positive electrode mixture layer is provided on both the surface side of the positive electrode core material and the back surface of the positive electrode mixture layer, the area A of the positive electrode mixture layer is the sum of both the surface area and the back surface area.

[0035] [Negative Electrode] FIG. 2 is a cross-sectional view of the negative electrode 12 according to an example embodiment. As shown in FIG. 2, the negative electrode 12 includes a negative electrode core 30 and a negative electrode mixture layer 32 disposed on the surface of the negative electrode 12. The negative electrode mixture layer 32 includes a first negative electrode mixture layer 32a disposed on the surface of the negative electrode 12 and a second negative electrode mixture layer 32b disposed between the first negative electrode mixture layer 32a and the negative electrode core 30. The negative electrode core 30 may be made of a foil of a metal, such as copper, that is stable within the potential range of the negative electrode 12, or a film having such a metal disposed on its surface. The first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are preferably provided on both sides of the negative electrode core 30, excluding the portion to which the negative electrode lead 21 is connected. The configuration of the negative electrode 12 is not limited to the example shown in FIG. 2. For example, a functional layer may be disposed between the second negative electrode mixture layer 32b and the negative electrode core 30.

[0036] The thickness W1 of the first negative electrode mixture layer 32a and the thickness W2 of the second negative electrode mixture layer 32b satisfy, for example, 0.3≦W1 / (W1+W2)≦0.7, thereby improving the battery capacity in addition to the output characteristics and rapid charge cycle characteristics.

[0037] The negative electrode mixture layer 32 contains, for example, a negative electrode active material and a binder. An example of the negative electrode active material contained in the negative electrode mixture layer 32 is graphite. Graphite includes natural graphite such as flake graphite, massive artificial graphite, and artificial graphite such as graphitized mesophase carbon microbeads. The volume-based D50 of the graphite is, for example, 1 μm or more and 30 μm or less. For the negative electrode active material, natural graphite and artificial graphite may be used in combination, or two or more types of graphite with different D50 may be mixed and used.

[0038] A portion of the graphite contained in the negative electrode mixture layer 32 is coated graphite, in which the surface of the graphite is coated with amorphous graphite. Amorphous graphite is a highly conductive material, and coating it with amorphous graphite improves the conductivity of the graphite. Furthermore, coated graphite is harder than uncoated regular graphite. Coated graphite can be formed, for example, by a CVD method using acetylene, methane, or the like, or by a method in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with graphite and heat-treated. The thickness of the conductive layer is not particularly limited, but is, for example, 1 nm or more and 200 nm or less, taking into consideration ensuring conductivity and the diffusibility of Li ions into the particles. The thickness of the conductive layer can be measured by observing the cross-section of the composite material using a SEM or a transmission electron microscope (TEM). Coated graphite may contain elemental sulfur due to its manufacturing method. Generally, regular graphite does not contain elemental sulfur. Therefore, an effective method for confirming the ratio of the amount of coated graphite to ordinary graphite is to confirm the presence of sulfur element by EDX (energy dispersive X-ray fluorescence spectroscopy) or EPMA (electron probe microanalyzer), etc. Furthermore, since the intensity ratio of the G band to the D band (G-D ratio) in Raman spectroscopy differs due to differences in crystallinity between the coated and uncoated portions depending on the thickness of the coated graphite, it may also be effective to perform mapping of the G-D ratio.

[0039] The ratio P1 of the coated graphite to the total mass of graphite in the first anode mixture layer 32a and the ratio P2 of the coated graphite to the total mass of graphite in the second anode mixture layer 32b satisfy the relationship P1 > P2. This allows both high output and good rapid charge cycle characteristics to be achieved. Because the coated graphite is less likely to crack during the rolling process performed in manufacturing the anode 12, the porosity of the first anode mixture layer 32a is high. Furthermore, the good conductivity of the coated graphite ensures a conductive path within the first anode mixture layer 32a. The second anode mixture layer 32b has a lower porosity than the first anode mixture layer 32a, allowing for improved battery capacity.

[0040] P1 is preferably 80% by mass or more. The upper limit of P1 is, for example, 100% by mass. P2 is preferably 20% by mass or less. The lower limit of P2 is, for example, 0% by mass.

[0041] The negative electrode mixture layer 32 may contain a silicon-containing material as the negative electrode active material. This can increase the capacity of the battery. Because silicon-containing materials undergo a larger volume change during charge and discharge than carbon materials, when the negative electrode mixture layer 32 contains a silicon-containing material, non-uniform distribution of the non-aqueous electrolyte is likely to occur within the negative electrode mixture layer 32. However, as described above, optimizing the structures of the positive electrode 11 and the negative electrode 12 can suppress non-uniform distribution of the non-aqueous electrolyte and improve rapid charge cycle characteristics. The content of the silicon-containing material in the negative electrode mixture layer 32 is, for example, 5% by mass or more relative to the total mass of the negative electrode active material contained in the negative electrode mixture layer 32. The upper limit of the content of the silicon-containing material is not particularly limited, but is, for example, 30% by mass. The content of the silicon-containing material in the first negative electrode mixture layer 32a and the content of the silicon-containing material in the second negative electrode mixture layer 32b may be different from each other or the same.

[0042] The silicon-containing material may be any material containing Si, and examples thereof include Si particles, silicon alloys, silicon compounds, and Si-containing composite materials. The D50 of the Si-containing composite material is generally smaller than the D50 of graphite. The volume-based D50 of the composite material is, for example, 1 μm or more and 15 μm or less. The silicon-containing material may be used alone or in combination of two or more types.

[0043] A suitable silicon-containing material (the composite material) is a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicide phase is a phase of a compound consisting of Si and an element more electropositive than Si, such as NiSi, Mg ２ Si, TiSi ２ The Si phase is formed by dispersing Si in the form of fine particles, and the ion-conducting phase is a continuous phase formed by an aggregation of particles that are finer than the Si phase.

[0044] The average size of the Si phase is preferably 1 nm or more and 200 nm or less, more preferably 1 nm or more and 100 nm or less. The average size of the Si phase is calculated by taking an SEM image of the particle cross section of the silicon-containing material and averaging the diameters of the circumscribed circles of the Si phase extracted by image analysis. The average size of the Si phase may be, for example, 1 nm or more and 10 nm or less. By reducing the size of the Si phase, it is possible to suppress the particle expansion rate associated with charge and discharge while maintaining a high capacity.

[0045] The composite material may have a conductive layer covering the surface of the ion-conducting phase. The conductive layer is composed of a material with higher conductivity than the ion-conducting layer and forms a good conductive path in the negative electrode mixture layer. The conductive layer is, for example, a carbon coating composed of a conductive carbon material. Examples of conductive carbon materials that can be used include carbon black such as acetylene black and ketjen black, graphite, and amorphous carbon (amorphous carbon) with low crystallinity. The thickness of the conductive layer is preferably 1 nm or more and 200 nm or less, more preferably 5 nm or more and 100 nm or less, taking into consideration ensuring conductivity and the diffusibility of Li ions into the particles. The thickness of the conductive layer can be measured by observing the cross section of the composite material using a SEM or a transmission electron microscope (TEM).

[0046] An example of a suitable Si-containing composite material has a sea-island structure in which fine Si is dispersed almost uniformly in an amorphous silicon oxide phase, and the overall structure is represented by the general formula SiO ｘ (0.5≦x≦1.5). The main component of the silicon oxide may be silicon dioxide. The silicon oxide phase may be doped with Li.

[0047] Another example of a suitable Si-containing composite material is composite particles having a sea-island structure in which fine Si particles are uniformly dispersed in an amorphous silicate phase. The silicate phase contains, for example, at least one element selected from the group consisting of Group 1 and Group 2 elements of the periodic table. The silicate phase may further contain at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, V, La, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W.

[0048] A suitable silicate phase is a lithium silicate phase containing Li. The lithium silicate phase can be, for example, of the general formula Li ２ｚ SiO （２＋ｚ） (0<z<2). The lithium silicate phase contains Li ４ SiO ４ It is preferable that (Z=2) is not included. ４ SiO ４ is an unstable compound and reacts with water to become alkaline, which may cause Si to change and lead to a decrease in charge / discharge capacity. The lithium silicate phase is considered to be a suitable phase for Li, from the viewpoints of stability, productivity, Li ion conductivity, etc. ２ SiO ３ (Z=1) or Li ２ Si ２ O ５ It is preferable that (Z=1 / 2) is used as the main component.

[0049] Another example of a suitable Si-containing composite material is composite particles having a sea-island structure in which fine Si particles are substantially uniformly dispersed in a carbon phase. In at least a portion of the silicon-containing material, the ion-conducting phase is preferably a carbon phase. The carbon phase is preferably an amorphous carbon phase. The carbon phase may contain a crystalline phase component, but preferably contains more amorphous phase components. The amorphous carbon phase is, for example, composed of a carbon material having an average interplanar spacing of (002) planes of more than 0.34 nm as measured by X-ray diffraction. The composite material containing a carbon phase may or may not have a conductive layer separate from the carbon phase.

[0050] Examples of binders contained in the negative electrode mixture layer 32 include fluorine-containing resins such as styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF), as well as polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. Among these, SBR and NBR are preferred, and SBR is particularly preferred. These may be used alone or in combination of two or more. The binder contained in the first negative electrode mixture layer 32a and the binder contained in the second negative electrode mixture layer 32b may be different from each other or may be the same.

[0051] The negative electrode mixture layer 32 may further contain a thickener. Examples of thickeners include carboxymethyl cellulose (CMC) or a salt thereof (CMC-Na, etc.), polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), and polyvinyl alcohol (PVA). These may be used alone or in combination of two or more. The thickener contained in the first negative electrode mixture layer 32a and the thickener contained in the second negative electrode mixture layer 32b may be different from each other or may be the same.

[0052] The negative electrode mixture layer 32 may further contain a conductive agent. The conductive agent is preferably single-walled carbon nanotubes (SWCNTs). The negative electrode mixture layer 32 has relatively large voids. In particular, the first negative electrode mixture layer 32a, which has larger voids than the second negative electrode mixture layer 32b, is prone to breaking of conductive paths. However, the addition of SWCNTs makes it possible to maintain the conductive paths. From the viewpoint of both maintaining the conductive paths and suppressing SWCNT aggregation, the content of SWCNTs in the negative electrode mixture layer 32 is preferably 0.001% by mass to 0.03% by mass relative to the total mass of the negative electrode active material.

[0053] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0054] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0055] [Secondary Battery During Discharge] In the present disclosure, a secondary battery is charged at a constant current of 1 C in a temperature environment of 25° C. until the battery voltage reaches 4.2 V, then charged at a constant voltage of 4.2 V until the current value reaches 0.02 C, and then discharged at a constant current of 1 C until the battery voltage reaches 2.5 V, and this secondary battery in this state is defined as a secondary battery during discharge. The characteristics of the positive electrode 11, the negative electrode 12, and the separator 13 can be obtained by disassembling the secondary battery during discharge and measuring the positive electrode 11, the negative electrode 12, and the separator 13 taken out.

[0056] The porosity Vc of the positive electrode mixture layer during discharge and the porosity Va of the negative electrode mixture layer during discharge satisfy the relationship Va-Vc<13%. Vc is, for example, 22% or more. The upper limit of Vc is not particularly limited, but is, for example, 40%. Va is, for example, 28% or more. The upper limit of Va is not particularly limited, but is, for example, 45%.

[0057] The porosity of the positive electrode mixture layer is a two-dimensional value calculated from the ratio of the area of ​​voids to the cross-sectional area of ​​the positive electrode mixture layer in the cross section of the positive electrode mixture layer. More specifically, the porosity of the positive electrode mixture layer can be calculated by the following procedure. The porosity of the negative electrode mixture layer can also be calculated by the same procedure.

[0058] <Method for Measuring Porosity> (1) Exposing the cross section of the positive electrode mixture layer. For example, a method for exposing the cross section includes cutting out a portion of the positive electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the positive electrode mixture layer. (2) Using a scanning electron microscope, a backscattered electron image of the cross section of the exposed positive electrode mixture layer is captured. The backscattered electron image is captured at a magnification of, for example, 800x. (3) The cross-sectional image obtained above is imported into a computer and binarized using image analysis software (e.g., ImageJ manufactured by the National Institutes of Health) to obtain a binarized image in which particle cross sections in the cross-sectional image are colored black and voids present in the particle cross sections are colored white. (4) In the binarized image obtained above, of the voids converted to white, voids inside the graphite particles (pores not connected to the particle surface) and pores connected to the graphite particle surface and having a width of 3 μm or less are excluded from the voids, and the area of ​​the voids is calculated. The porosity is calculated based on the following formula: Porosity (%) = Area of ​​voids / Area of ​​cross section of negative electrode mixture layer × 100 (5) The calculation of the porosity according to (3) and (4) above was performed three times, and the average value was used as the porosity of the positive electrode mixture layer.

[0059] The tortuosity τ of the negative electrode mixture layer during discharge is preferably 2.5 or less. The tortuosity of the negative electrode mixture layer is an index showing the degree of curvature of the voids (pores) formed in the negative electrode mixture layer through which the non-aqueous electrolyte passes, and the smaller the tortuosity, the less the curvature of the void path. The tortuosity is the value obtained by dividing the path (path length) from the start point to the end point of the void in the mixture layer by the linear distance from the start point to the end point of the void in the mixture layer. When the path length is the same as the linear distance from the start point to the end point of the void in the mixture layer, the tortuosity is 1.

[0060] The tortuosity of the negative electrode mixture layer is calculated by the following formula: τ is the tortuosity, f is the path length of the medial axis passing through the opposing surfaces in the thickness direction (abbreviated as path length), and s is the length of the straight line connecting the start point and end point of the path of f (abbreviated as straight line distance between the start and end points). τ = f / s

[0061] The path length and linear distance are determined by cross-sectional observation and image analysis of the negative electrode mixture layer using a 3D scanning electron microscope (3D SEM, for example, Ethos NX-5000 manufactured by Hitachi High-Technologies Corporation). A specific method for calculating the tortuosity is as follows.

[0062] <Method for measuring tortuosity> (1) The negative electrode mixture layer is placed on the sample stage of a 3D SEM, and continuous cross-sectional slicing and cross-sectional observation are alternately performed. Observation is performed at an acceleration voltage of 5 kV. The obtained two-dimensional continuous images are binarized using three-dimensional image analysis software (e.g., EXFACT VR manufactured by Nippon Visual Science Co., Ltd.), and then connected to construct a three-dimensional structure. The three-dimensional structure is preferably 100 μm × 100 μm × 100 μm or more. (2) The image of the three-dimensional structure obtained in (1) is binarized to extract voids. The obtained voids are thinned, and the axis passing through the center of the void (medial axis) is determined. From the medial axis present in the cube, those that penetrate in a direction perpendicular to the core surface are extracted, and for those paths that have branches, the shortest path is determined as the path length (f) of the negative electrode mixture layer. (3) Using the average path length (f) of the negative electrode mixture layer obtained in (2) and the average linear distance (s) connecting the paths, the tortuosity of the negative electrode mixture layer is calculated according to the above formula.

[0063] The thickness of the separator during discharge is 15 μm or less. This increases the volume occupied by the positive electrode and negative electrode in the outer can, thereby improving the battery capacity. The lower limit of the separator thickness during discharge is, for example, 10 μm. If the thickness is less than 10 μm, the strength of the separator decreases, making it difficult to ensure safety in terms of preventing short circuits.

[0064] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0065] Example 1

[0066] [Preparation of Positive Electrode] As the positive electrode active material, LiNi ０．８８ Co ０．０９ Al ０．０３ O ２A lithium-containing composite oxide represented by the formula (I) was used. This positive electrode active material, single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), polyvinylidene fluoride (PVDF), and hydrogenated nitrile butadiene rubber (HNBR) were mixed in a mass ratio of 100:0.01:0.1:1:0.1, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was disposed on both sides of the positive electrode core. Note that an exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode core. Neither the SWCNT nor the MWCNT used substantially contained a metal catalyst. The surface area per unit mass of SWCNT is 1000 m ２ / g, and the surface area per unit mass of MWCNT is 400 m ２ / g.

[0067] [Negative Electrode Preparation] A coated graphite surface coated with amorphous graphite and Si particles were mixed in a mass ratio of 95:5 to form a first negative electrode active material. This first negative electrode active material was mixed with styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and SWCNT in a mass ratio of 100:1:1:0.05, and a first negative electrode mixture slurry was prepared using water as a dispersion medium. The coated graphite contained S on the surface. A second negative electrode mixture slurry was prepared in the same manner as the first negative electrode mixture slurry, except that a second negative electrode active material obtained by mixing normal graphite and Si particles in a mass ratio of 95:5 was used instead of the first negative electrode active material.

[0068] The second negative electrode mixture slurry was applied to both sides of a copper foil negative electrode core by the doctor blade method and dried to form a second negative electrode mixture layer. Furthermore, the first negative electrode mixture slurry was applied to the second negative electrode mixture layer and dried to form a first negative electrode mixture layer. The application mass ratio per unit area of ​​the first negative electrode mixture slurry to the second negative electrode mixture slurry was 4:6. The first negative electrode mixture layer and the second negative electrode mixture layer were rolled using a rolling roller to produce a negative electrode. The value of T1 / (T1+T2) calculated from the thickness W1 of the first negative electrode mixture layer and the thickness W2 of the second negative electrode mixture layer was 0.4. An exposed portion was provided on a portion of the negative electrode, exposing the surface of the negative electrode core.

[0069] [Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)] LiPF 6 was dissolved in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4 (25° C.). ６ was dissolved in the solution at a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0070] [Preparation of Test Cell (Secondary Battery)] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator between them to prepare a wound electrode body. The diameter of the electrode body was 21 mm, and the height of the electrode body was 70 mm. Insulating plates were placed on the top and bottom of the electrode body, and the electrode body was housed in an outer can. The negative electrode lead was welded to the bottom of a cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member. An electrolyte was poured into the outer can, and the opening of the outer can was sealed with a sealing member via a gasket to prepare a secondary battery as a test cell. In this secondary battery, the positive electrode area A [m ２ ] and the discharge capacity B [Ah] when discharged from a charged state of 4.2 V to 2.5 V at 0.05 C, the value of B / A obtained from this is 42.4 h / m ２ It was.

[0071] [Evaluation of DC Resistance (DCR)] The test cell was charged at a constant current of 1 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 C. Thereafter, the test cell was discharged at a constant current of 1 C until the battery voltage reached 2.5 V, and the discharge capacity was measured. From the results of this discharge capacity, the state of charge (SOC) of the battery was adjusted to 50%, and the battery was discharged at a current of 1 C for 10 seconds, and the voltage change ΔV after 10 seconds had elapsed was measured. The DC resistance DCR was calculated from the voltage change ΔV due to the discharge and the current amount. Note that C (A) = rated capacity (Ah) / 1 (h). DCR = ΔV / 1 C

[0072] [Evaluation of rapid charge cycle characteristics (capacity retention rate after cycle test)] The test cell was charged at a constant current of 1 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 C. Subsequently, the test cell was discharged at a constant current of 1 C until the battery voltage reached 2.5 V. This charge / discharge cycle constituted one cycle, and 150 cycles were performed. The capacity retention rate was calculated from the discharge capacity at the first cycle and the discharge capacity at the 150th cycle using the following formula. A higher capacity retention rate indicates better charge / discharge cycle characteristics. Capacity retention rate (%) = Discharge capacity at 150th cycle / Discharge capacity at 1st cycle × 100

[0073] [Evaluation of Secondary Battery Characteristics During Discharge] The test cell was charged at a constant current of 1 C in a temperature environment of 25° C. until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 C. Thereafter, the test cell was discharged at a constant current of 1 C until the battery voltage reached 2.5 V, thereby obtaining a secondary battery during discharge. This secondary battery was disassembled, and the positive electrode 11, negative electrode 12, and separator 13 were analyzed.

[0074] The porosity Vc of the positive electrode mixture layer during discharge was 26.6%, the porosity Va of the negative electrode mixture layer during discharge was 36.5%, the tortuosity τ of the negative electrode mixture layer during discharge was 1.8, and the thickness of the separator during discharge was 15 μm.

[0075] Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that in preparing the negative electrode, the graphite used as the second negative electrode active material was changed to a mixture of coated graphite and regular graphite in a mass ratio of 50:50.

[0076] Example 3 A test cell was produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, the graphite used as the first negative electrode active material was changed to a mixture of coated graphite and normal graphite in a mass ratio of 75:25.

[0077] Example 4 A test cell was prepared and evaluated in the same manner as in Example 1, except that in preparing the negative electrode, the graphite used as the first negative electrode active material was changed to a mixture of coated graphite and normal graphite in a mass ratio of 50:50.

[0078] Example 5 A test cell was produced and evaluated in the same manner as in Example 1, except that in preparing the positive electrode active material, the positive electrode active material, SWCNT, MWCNT, PVDF, and HNBR were mixed in a mass ratio of 100:0.1:0.0001:1:0.1.

[0079] Example 6 A test cell was produced and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, the positive electrode active material, SWCNT, MWCNT, PVDF, and HNBR were mixed in a mass ratio of 100:0.5:0.01:1:0.1.

[0080] Example 7 A test cell was produced and evaluated in the same manner as in Example 1, except that in preparing the positive electrode active material, the positive electrode active material, SWCNT, MWCNT, PVDF, and HNBR were mixed in a mass ratio of 100:0.1:0.02:1:0.1.

[0081] Example 8 A test cell was produced and evaluated in the same manner as in Example 1, except that in preparing the positive electrode active material, the positive electrode active material, SWCNT, MWCNT, PVDF, and HNBR were mixed in a mass ratio of 100:0.01:0.01:1:0.1.

[0082] Comparative Example 1 A test cell was produced and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, the conductive agent was changed to acetylene black (AB), and the positive electrode active material, AB, and PVDF were mixed in a mass ratio of 100:1:1.

[0083] Comparative Example 2 A test cell was produced and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, SWCNT was not used, and the positive electrode active material, MWCNT, PVDF, and HNBR were mixed in a mass ratio of 100:0.5:1:0.2.

[0084] <Comparative Example 3> A test cell was produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, the graphite used in the first negative electrode active material was changed to ordinary graphite and the graphite used in the second negative electrode active material was changed to coated graphite.

[0085] Comparative Example 4 A test cell was produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, the graphite used as the second negative electrode active material was changed to coated graphite.

[0086] Comparative Example 5 A test cell was produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, the graphite used in the first negative electrode active material was changed to ordinary graphite.

[0087] The evaluation results of the test cells of the examples and comparative examples are shown in Table 1. In Table 1, the DCR and capacity retention ratio of the test cells of Examples 1 to 8 and Comparative Examples 2 to 5 are relative values ​​when the DCR and capacity retention ratio of the test cell of Comparative Example 1 are each set to 100. The Va-Vc value of each test cell is also shown in Table 1. Whether or not P1>P2 is established for each test cell is indicated by "◯" or "×".

[0088]

[0089] In Table 1, the test cells of the examples all have smaller DCRs and higher capacity retention rates than the test cells of the comparative examples. The above results demonstrate that the nonaqueous electrolyte secondary battery according to the present disclosure can achieve both high output and good rapid charge cycle characteristics.

[0090] The present disclosure will be further described by the following embodiments. Configuration 1: A battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer disposed on the surface of the positive electrode core, wherein the positive electrode mixture layer contains a positive electrode active material, single-walled carbon nanotubes, and multi-walled carbon nanotubes, the negative electrode has a negative electrode core and a negative electrode mixture layer disposed on the surface of the negative electrode core, wherein the negative electrode mixture layer includes a first negative electrode mixture layer disposed on the surface of the negative electrode and a second negative electrode mixture layer disposed between the first negative electrode mixture layer and the negative electrode core, wherein the negative electrode mixture layer contains graphite, and a portion of the graphite is coated graphite in which the surface of the graphite is coated with amorphous graphite, A nonaqueous electrolyte secondary battery, wherein a ratio P1 of the coated graphite to the total mass of the graphite in the first negative electrode mixture layer and a ratio P2 of the coated graphite to the total mass of the graphite in the second negative electrode mixture layer satisfy the relationship P1 > P2, and a porosity Vc of the positive electrode mixture layer during discharge and a porosity Va of the negative electrode mixture layer during discharge satisfy the relationship Va - Vc < 13%. Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein Vc is 22% or more. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein Va is 28% or more. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the tortuosity of the negative electrode mixture layer during discharge is 2.5 or less. Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the thickness of the separator during discharge is 15 μm or less. Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the coated graphite contains elemental sulfur. Configuration 7: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the content of the single-walled carbon nanotubes in the positive electrode mixture layer is 0.0001% by mass or more and 0.02% by mass or less, relative to the total mass of the positive electrode active material, and the content of the multi-walled carbon nanotubes in the positive electrode mixture layer is 0.01% by mass or more and 0.5% by mass or less, relative to the total mass of the positive electrode active material.Configuration 8: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein the negative electrode mixture layer contains a silicon-containing material, and the content of the silicon-containing material in the negative electrode mixture layer is 5 mass % or more relative to the total mass of the negative electrode active material contained in the negative electrode mixture layer. Configuration 9: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein P1 is 80 mass % or more. Configuration 10: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 9, wherein P2 is 20 mass % or less. Configuration 11: The area A [m] of the positive electrode mixture layer. ２ ] and the discharge capacity B [Ah] when discharged from a charged state of 4.2 V to 2.5 V at 0.05 C is B / A≧40 Ah / m ２ The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 10, wherein the thickness W1 of the first negative electrode mixture layer and the thickness W2 of the second negative electrode mixture layer satisfy the relationship 0.3≦W1 / (W1+W2)≦0.7. Configuration 12: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 11, wherein the thickness W1 of the first negative electrode mixture layer and the thickness W2 of the second negative electrode mixture layer satisfy the relationship 0.3≦W1 / (W1+W2)≦0.7. Configuration 13: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 12, wherein the positive electrode mixture layer contains at least one of hydrogenated nitrile butadiene rubber and a cellulose derivative. Configuration 14: The single-walled carbon nanotubes have a surface area per unit mass of 300 m ２ / g or more, and the surface area per unit mass of the multi-walled carbon nanotubes is 200 m ２ / g or more. Configuration 15: The nonaqueous electrolyte secondary battery of any one of Configurations 1 to 14, wherein the single-walled carbon nanotubes and the multi-walled carbon nanotubes are substantially free of a metal catalyst. Configuration 16: The nonaqueous electrolyte secondary battery of any one of Configurations 1 to 15, wherein the positive electrode and the negative electrode form an electrode assembly wound with the separator interposed therebetween. Configuration 17: The nonaqueous electrolyte secondary battery of Configuration 16, wherein a diameter D of the electrode assembly and a height H of the electrode assembly satisfy H / D≧2.

[0091] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Negative electrode core, 32 Negative electrode mixture layer, 32a First negative electrode mixture layer, 32b Second negative electrode mixture layer

Claims

1. A battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode mixture layer arranged on a surface of the positive electrode core, the positive electrode mixture layer containing a positive electrode active material, single-walled carbon nanotubes, and multi-walled carbon nanotubes, the negative electrode has a negative electrode core and a negative electrode mixture layer arranged on the surface of the negative electrode core, the negative electrode mixture layer including a first negative electrode mixture layer arranged on the surface of the negative electrode and a second negative electrode mixture layer arranged between the first negative electrode mixture layer and the negative electrode core, the negative electrode mixture layer containing graphite, and a portion of the graphite is coated graphite in which a surface of the graphite is coated with amorphous graphite, a ratio P1 of the coated graphite to the total mass of the graphite in the first negative electrode mixture layer and a ratio P2 of the coated graphite to the total mass of the graphite in the second negative electrode mixture layer satisfy a relationship of P1>P2, and a porosity Vc of the positive electrode mixture layer during discharge and a porosity Va of the negative electrode mixture layer during discharge satisfy a relationship of Va-Vc<13%.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein said Vc is 22% or more.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein Va is 28% or more.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the tortuosity τ of the negative electrode mixture layer during discharge is 2.5 or less.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thickness of the separator during discharge is 15 μm or less.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the coated graphite contains elemental sulfur.

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the single-walled carbon nanotubes in the positive electrode mixture layer is 0.0001 mass % or more and 0.02 mass % or less, relative to the total mass of the positive electrode active material, and the content of the multi-walled carbon nanotubes in the positive electrode mixture layer is 0.01 mass % or more and 0.5 mass % or less, relative to the total mass of the positive electrode active material.

8. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer contains a silicon-containing material, and the content of the silicon-containing material in the negative electrode mixture layer is 5 mass % or more relative to the total mass of the negative electrode active material contained in the negative electrode mixture layer.

9. The nonaqueous electrolyte secondary battery according to claim 1, wherein P1 is 80 mass % or more.

10. The nonaqueous electrolyte secondary battery according to claim 1, wherein the P2 is 20 mass % or less.

11. Area A [m ２ ] and the discharge capacity B [Ah] when discharged from a charged state of 4.2 V to 2.5 V at 0.05 C is B / A≧40 Ah / m ２ The nonaqueous electrolyte secondary battery according to claim 1 , which satisfies the following relationship:

12. The nonaqueous electrolyte secondary battery according to claim 1, wherein the thickness W1 of the first negative electrode mixture layer and the thickness W2 of the second negative electrode mixture layer satisfy the relationship: 0.3≦W1 / (W1+W2)≦0.

7.

13. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode mixture layer contains at least one of hydrogenated nitrile butadiene rubber and a cellulose derivative.

14. The surface area per unit mass of the single-walled carbon nanotube is 300 m ２ / g or more, and the surface area per unit mass of the multi-walled carbon nanotube is 200 m ２ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the molecular weight of the nonaqueous electrolyte is 1 / g or more.

15. The nonaqueous electrolyte secondary battery according to claim 1, wherein the single-walled carbon nanotubes and the multi-walled carbon nanotubes contain substantially no metal catalyst within the tubes.

16. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode and the negative electrode are wound with the separator interposed therebetween to form an electrode assembly.

17. The nonaqueous electrolyte secondary battery according to claim 16, wherein the diameter D of the electrode body and the height H of the electrode body satisfy H / D≧2.

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