Non-aqueous electrolyte secondary battery

Abstract

A negative electrode (12) has a negative electrode current collector (30) and a negative electrode mixture layer (32) disposed on the surface of the negative electrode current collector (30). The negative electrode mixture layer (32) has a first negative electrode mixture layer (32a) facing the negative electrode current collector (30) and a second negative electrode mixture layer (32b) facing a positive electrode. The main component of at least the second negative electrode mixture layer (32b) is coated graphite particles covered with amorphous carbon, the second negative electrode mixture layer also including carbon nanotubes. When the inter-particle porosity of the negative electrode active material in the first negative electrode mixture layer (32a) is S1, and the inter-particle porosity of the negative electrode active material in the second negative electrode mixture layer 32b is S2, 3.5 ≤ S2 / S1 ≤ 5.2 is satisfied.

Description

Non-aqueous electrolyte secondary battery

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

[0002] Non-aqueous electrolyte secondary batteries are widely used as high-energy density secondary batteries. The negative electrode of a non-aqueous electrolyte secondary battery is composed of a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector. Graphite is generally used as the negative electrode active material contained in the negative electrode mixture layer. Patent Document 1 discloses a technology in which, from the viewpoint of increasing capacity and improving charge / discharge cycle characteristics, the negative electrode mixture layer has a two-layer structure, and the interparticle porosity of the negative electrode mixture layer on the positive electrode side is greater than that of the negative electrode mixture layer on the negative electrode current collector side.

[0003] International Publication No. 2023 / 149529

[0004] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been used as power sources for driving vehicles, and further improvements in charge-discharge cycle characteristics are required. The negative electrode of Patent Document 1 still has room for improvement in terms of improving charge-discharge cycle characteristics.

[0005] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure is a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, the negative electrode mixture layer has a first negative electrode mixture layer facing the negative electrode current collector and a second negative electrode mixture layer facing the positive electrode, and at least the second negative electrode mixture layer contains coated graphite particles coated with amorphous carbon as a main component and also contains carbon nanotubes, and wherein, when the interparticle porosity of the negative electrode active material in the first negative electrode mixture layer is S1 and the interparticle porosity of the negative electrode active material in the second negative electrode mixture layer is S2, the relationship 3.5≦S2 / S1≦5.2 is satisfied.

[0006] According to the nonaqueous electrolyte secondary battery of one aspect of the present disclosure, charge-discharge cycle characteristics can be improved.

[0007] 1 is a schematic diagram showing a cross section of a graphite particle according to an embodiment of the present invention; 2 is a schematic diagram showing a cross section of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention;

[0008] 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 respective components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

[0009] In the following, a cylindrical battery in which a wound electrode body is housed in a cylindrical, bottomed exterior body is exemplified as a nonaqueous electrolyte secondary battery; however, the exterior body of the battery is not limited to a cylindrical exterior body. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery with a prismatic exterior body, a coin-type battery with a coin-type exterior body, or a pouch-type battery with an exterior body made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrodes are not limited to wound type, and may be a laminated electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween. Furthermore, the design of the nonaqueous electrolyte secondary battery according to the present disclosure is not limited to the design of the exemplified nonaqueous electrolyte secondary battery, and known nonaqueous electrolyte secondary battery designs may also be applied.

[0010] FIG. 1 is an axial cross-sectional view of a cylindrical 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 exterior body 16 that accommodates 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, and 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 exterior body 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the exterior body 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the sealing body 17 side of the nonaqueous electrolyte secondary battery 10 will be referred to as the "top" and the bottom side of the exterior body 16 will be referred to as the "bottom."

[0011] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all rectangular, elongated bodies that are spirally wound in the longitudinal direction and stacked alternately in the radial direction of the electrode assembly 14. The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. The electrode assembly 14 includes 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. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the lateral direction of the positive electrode 11 and the negative electrode 12 is the axial direction. That is, the lateral end faces of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode assembly 14.

[0012] 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 exterior body 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 exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0013] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 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 body 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0014] 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.

[0015] The positive electrode 11, the negative electrode 12, the separator 13, and the nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below, with the negative electrode 12 being particularly described below.

[0016] [Positive Electrode] The positive electrode 11 is composed of a positive electrode current collector such as a metal foil and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode current collector 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 contains, for example, a positive electrode active material, a binder, a conductive agent, etc. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive agent, etc. to the positive electrode current collector, drying the slurry to form a positive electrode mixture layer, and then rolling the positive electrode mixture layer.

[0017] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. ｘ CoO ２ , Li ｘ NiO ２ , Li ｘ MnO ２ , Li ｘ Co ｙ Ni １－ｙ O ２ , Li ｘ Co ｙ M １－ｙ O ｚ , Li ｘ Ni １－ｙ Mｙ O ｚ , Li ｘ Mn ２ O ４ , Li ｘ Mn ２－ｙ M ｙ O ４ , LiMPO ４ , Li ２ MPO ４ F (M: at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, 0<x≦1.2, 0<y≦0.9, 2.0≦z≦2.3). These may be used alone or in combination of two or more.

[0018] In terms of increasing the capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material is Li ｘ NiO ２ , Li ｘ Co ｙ Ni １－ｙ O ２ , Li ｘ Ni １－ｙ M ｙ O ｚ It is preferable that the lithium-nickel composite oxide contains a lithium-nickel composite oxide such as (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B; 0<x≦1.2, 0<y≦0.9, 2.0≦z≦2.3).

[0019] Examples of the conductive agent contained in the positive electrode mixture layer include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes (CNT), graphene, and graphite. These may be used alone or in combination of two or more.

[0020] Examples of the binder contained in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyimide-based resins, acrylic-based resins, polyolefin-based resins, polyacrylonitrile (PAN), etc. These may be used alone or in combination of two or more.

[0021] [Negative Electrode] Fig. 2 is a cross-sectional view of the negative electrode 12 according to one embodiment. As shown in Fig. 2, the negative electrode 12 includes a negative electrode current collector 30 and a negative electrode mixture layer 32 disposed on the surface of the negative electrode current collector 30. The negative electrode mixture layer 32 includes a first negative electrode mixture layer 32a facing the negative electrode current collector 30 and a second negative electrode mixture layer 32b facing the positive electrode 11 with the separator 13 interposed therebetween. In other words, the first negative electrode mixture layer 32a is disposed between the negative electrode current collector 30 and the second negative electrode mixture layer 32b.

[0022] The first and second negative electrode mixture layers 32a and 32b may have the same thickness or different thicknesses. The thickness of the second negative electrode mixture layer 32b is, for example, smaller than the thickness of the first negative electrode mixture layer 32a. The ratio of the thickness of the second negative electrode mixture layer 32b to the thickness of the first negative electrode mixture layer 32a is preferably 2:8 to 5:5, and more preferably 2:8 to 4:6. In this case, the effect of improving the charge-discharge cycle characteristics, which will be described later, becomes significant.

[0023] The negative electrode current collector 30 may be, for example, a foil of a metal such as copper that is stable in the potential range of the negative electrode 12, or a film having such a metal disposed on its surface. The thickness of the negative electrode current collector 30 is, for example, 5 μm or more and 30 μm or less.

[0024] The negative electrode mixture layer 32 contains a negative electrode active material capable of reversibly absorbing and releasing lithium ions. The negative electrode active material includes graphite particles such as natural graphite and artificial graphite (hereinafter, sometimes referred to as graphite particles 40). From the standpoint of ease of adjusting the internal porosity, which will be described later, it is preferable that the graphite particles contain artificial graphite.

[0025] Of the negative electrode mixture layers 32, at least the second negative electrode mixture layer 32b is primarily composed of coated graphite particles coated with amorphous carbon. That is, a large number of coated graphite particles coated with amorphous carbon are disposed on the surface side of the negative electrode 12. Coated graphite particles coated with amorphous carbon have superior lithium ion diffusibility compared to graphite particles not coated with amorphous carbon. Therefore, by disposing a large number of coated graphite particles on the surface side of the negative electrode 12, favorable lithium ion diffusibility can be obtained. Furthermore, as will be described in detail later, by controlling the interparticle porosity between the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b to a predetermined ratio and using coated graphite particles as the primary component of the second negative electrode mixture layer 32b, charge-discharge cycle characteristics can be specifically improved.

[0026] Here, taking the second negative electrode mixture layer 32 b as an example, the term “main component” refers to the component that accounts for the largest proportion by mass among the components that make up the second negative electrode mixture layer 32 b. The coated graphite particles are contained in an amount of preferably 50 mass % or more, more preferably 75 mass % or more, and even more preferably 75 mass % or more and 90 mass % or less, relative to the total mass of the second negative electrode mixture layer 32 b.

[0027] The first anode mixture layer 32a may contain coated graphite particles coated with amorphous carbon as the anode active material, but preferably contains graphite particles not coated with amorphous carbon as its main component. Graphite particles not coated with amorphous carbon are more easily crushed during the rolling process than coated graphite particles. Therefore, by disposing a large number of graphite particles not coated with amorphous carbon in the first anode mixture layer 32a disposed on the anode current collector 30 side, good adhesion between the anode mixture layer 32 and the anode current collector 30 can be ensured. Furthermore, by disposing a large number of graphite particles not coated with amorphous carbon in the first anode mixture layer 32a, it becomes easier to increase the packing density of the anode mixture layer 32, thereby facilitating the realization of a high-capacity battery.

[0028] The coated graphite particles are core-shell particles having graphite particles and an amorphous carbon coating formed on the surface of the graphite particles. The amorphous carbon coating is a carbon coating in an amorphous or microcrystalline turbostratic state in which the graphite crystal structure is not developed, and is composed of carbon having a d(002) spacing of greater than 0.340 nm as determined by X-ray diffraction, for example.

[0029] The amorphous carbon coating is preferably formed on the entire surface of the graphite particles. The amorphous carbon coating has functions, for example, of reducing decomposition of non-aqueous electrolytes and improving the hardness of the carbon-coated graphite. The coated graphite particles are harder than graphite particles not coated with amorphous carbon and are less likely to be crushed during the rolling process.

[0030] Specific examples of amorphous carbon include hard carbon (hardly graphitizable carbon), soft carbon (easily graphitizable carbon), carbon black such as acetylene black, ketjen black, thermal black, and furnace black, carbon fiber, and activated carbon.

[0031] The amount of amorphous carbon coating is preferably 0.5% by mass or more, and more preferably 1% by mass or more, relative to the mass of the coated graphite particles. By setting the amount of amorphous carbon coating to 0.5% by mass or more, relative to the mass of the coated graphite particles, the lithium ion diffusibility of the graphite particles can be improved. As a result, the lithium ion diffusibility in the negative electrode mixture layer 32 is further improved, and the charge / discharge cycle characteristics can be further improved.

[0032] Furthermore, the amount of amorphous carbon coating is preferably 15% by mass or less, and more preferably 10% by mass or less, relative to the mass of the coated graphite particles. If the amount of amorphous carbon coating exceeds 15% by mass relative to the mass of the coated graphite particles, the amount of lithium ions that can be occluded by the graphite particles may decrease, resulting in a decrease in battery capacity. Therefore, the amount of amorphous carbon coating is preferably 0.5% by mass or more and 15% by mass or less, and more preferably 1% by mass or more and 10% by mass or less, relative to the mass of the coated graphite particles. The amount of amorphous carbon coating can be measured by the method described in the Examples below.

[0033] An example of a suitable range for the thickness of the amorphous carbon coating is 1 nm or more and 200 nm, taking into consideration ensuring electrical conductivity and the diffusibility of lithium ions into the particles. The thickness of the amorphous carbon coating can be measured by observing the cross section of the particle of the coated graphite particle using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) to form a cross section of the composite material.

[0034] The amorphous carbon coating can be formed by mixing coal tar, tar pitch, naphthalene, anthracene, phenanthrolene, or the like with graphite particles and heat treating the mixture at a temperature of 800°C or higher and 1200°C or lower, or by chemical vapor deposition (CVD) using hydrocarbon gas, or the like.

[0035] Fig. 3 is a schematic diagram showing a cross section of a graphite particle 40. As shown in Fig. 3, in a cross section of the graphite particle 40, the graphite particle 40 has closed voids 42 (hereinafter referred to as internal voids 42) that do not connect the interior of the particle to the particle surface, and voids 44 (hereinafter referred to as external voids 44) that connect the interior of the particle to the particle surface.

[0036] The lattice spacing (d ００２ ) is, for example, preferably 0.3354 nm or more, more preferably 0.3357 nm or more, and preferably less than 0.340 nm, more preferably 0.338 nm or less. The crystallite size (Lc(002)) of the graphite particles 40 determined by X-ray diffraction is, for example, preferably 5 nm or more, more preferably 10 nm or more, and preferably 300 nm or less, more preferably 200 nm or less. ００２ When the porosity (Lc(002)) and the crystallite size (Lc(002)) satisfy the above ranges, the battery capacity tends to be larger than when the porosity (Lc(002)) and the crystallite size (Lc(002)) do not satisfy the above ranges.

[0037] The negative electrode active material may further contain a metal that can be alloyed with lithium, such as Si or Sn, or an alloy or oxide containing such a metal. These can occlude more lithium ions than graphite, thereby enabling the battery to have a higher capacity.

[0038] The negative electrode active material preferably contains a Si-based material. Examples of the Si-based material include Si, an alloy containing Si, and SiO ｘ Si oxide represented by (0.5≦x≦1.6), Li ２ｙ SiO （２＋ｙ） Examples of such a material include a Si-containing material in which fine particles of Si are dispersed in a lithium silicate phase represented by (0<y<2), and a Si-containing material in which fine particles of Si are dispersed in a carbon phase.

[0039] From the viewpoint of improving the battery capacity and suppressing the deterioration of the charge-discharge cycle characteristics, the ratio of the Si-based material to the total mass of the negative electrode active material in the negative electrode mixture layer 32 is preferably 1 mass % or more and 20 mass % or less, and more preferably 3 mass % or more and 15 mass % or less. ｘ It is preferable that (0.5≦x≦1.6) is included.

[0040] Here, when the interparticle porosity of the negative electrode active material in the first negative electrode mixture layer 32a is S1 and the interparticle porosity of the negative electrode active material in the second negative electrode mixture layer 32b is S2, the relationship 3.5≦S2 / S1≦5.2 is satisfied. The interparticle porosity of the negative electrode active material is a two-dimensional value calculated from the ratio of the area of ​​the interparticle voids of the negative electrode active material to the cross-sectional area of ​​the negative electrode mixture layer 32. S2 / S1 can be calculated by calculating the interparticle porosity S1 of the negative electrode active material in the first negative electrode mixture layer 32a and the interparticle porosity S2 of the negative electrode active material in the second negative electrode mixture layer 32b using the following procedure.

[0041] <Method for Measuring Interparticle Porosity of Negative Electrode Active Material> (1) Exposing a cross section of the negative electrode mixture layer 32. For example, a method for exposing the cross section includes cutting out a portion of the negative electrode 12 and processing it with an ion milling device (e.g., IM4000PLUS, manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode mixture layer 32. (2) Using a scanning electron microscope, backscattered electron images of the cross sections of the exposed negative electrode mixture layer 32 are taken for each of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b. The backscattered electron images are taken at a magnification of, for example, 800x. (3) The cross-sectional images obtained above are 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 images of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the voids converted to white, excluding the internal voids 42 of the graphite particles 40 and the external voids 44 with a width of 3 μm or less, are regarded as the interparticle voids of the negative electrode active material, and the area of ​​the interparticle voids of the negative electrode active material is calculated. The interparticle void ratio of the negative electrode active material can be calculated based on the following formula: Interparticle void ratio of negative electrode active material (%) = Area of ​​interparticle voids of negative electrode active material / Area of ​​cross section of negative electrode mixture layer × 100 (5) S1 and S2 are each calculated as the average value of the three measurements.

[0042] As a result of investigations by the present inventors, it was found that by using coated graphite particles as the main component of the second negative electrode mixture layer 32b while the interparticle porosities S1 and S2 satisfy the relationship 3.5≦S2 / S1≦5.2, the charge-discharge cycle characteristics are significantly improved. It is presumed that when the interparticle porosities S1 and S2 satisfy the relationship 3.5≦S2 / S1≦5.2, the permeability of the non-aqueous electrolyte in the negative electrode mixture layer 32 is improved. Furthermore, by arranging a large number of coated graphite particles on the surface side of the negative electrode 12, favorable diffusibility of lithium ions can be obtained. As a result, the synergistic effect of improved permeability of the non-aqueous electrolyte and improved diffusibility of lithium ions can significantly improve the charge-discharge cycle characteristics.

[0043] In addition, when the interparticle porosities S1 and S2 satisfy the relationship 3.5≦S2 / S1≦5.2 but the second negative electrode mixture layer 32b does not primarily contain coated graphite particles, the charge-discharge cycle characteristics are improved, but the effect is limited compared to the effect of the present disclosure. This is presumably because, even when the permeability of the non-aqueous electrolyte in the negative electrode mixture layer 32 is improved, the lithium ion diffusivity of the graphite particles arranged on the surface side of the negative electrode 12 is low, resulting in insufficient lithium ion diffusivity in the thickness direction of the negative electrode mixture layer 32. Furthermore, when the second negative electrode mixture layer 32b primarily contains coated graphite particles but the interparticle porosities S1 and S2 do not satisfy the relationship 3.5≦S2 / S1≦5.2, the charge-discharge cycle characteristics are slightly improved, but the effect is limited compared to the effect of the present disclosure. This is presumably because the permeability of the non-aqueous electrolyte in the negative electrode mixture layer 32 is insufficient, resulting in uneven charge-discharge reactions in the thickness direction of the negative electrode mixture layer 32.

[0044] The interparticle porosity S1 of the negative electrode active material in the first negative electrode mixture layer 32a and the interparticle porosity S2 of the negative electrode active material in the second negative electrode mixture layer 32b may satisfy 3.5≦S2 / S1≦5.2, preferably 3.8≦S2 / S1≦5.0, and more preferably 4.0≦S2 / S1≦4.8, in which case the effect of improving the charge-discharge cycle characteristics of the present disclosure becomes more pronounced.

[0045] Examples of means for adjusting the interparticle porosity of the negative electrode active material in the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b include a means for adjusting the packing density of the negative electrode mixture layer 32 and a means for adjusting the internal porosity of the graphite particles 40. The latter means reduces the internal porosity of the graphite particles 40, thereby increasing the interparticle porosity of the negative electrode active material without reducing the packing density of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b. In other words, by making the packing densities of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b equivalent and reducing the internal porosity of the second graphite particles mainly contained in the second negative electrode mixture layer 32b compared to the first graphite particles mainly contained in the first negative electrode mixture layer 32a, the S2 / S1 ratio can be increased.

[0046] The first graphite particles mainly contained in the first negative electrode mixture layer 32a can be prepared, for example, as follows: The main raw material, coke (precursor), is crushed to a predetermined size, agglomerated with a binder, and then press-molded into a block. The block-shaped body is then fired at a temperature of 2600°C or higher to graphitize it. The graphitized block-shaped body is crushed and sieved to obtain first graphite particles of a desired size. The internal porosity can be adjusted to be larger than that of the second graphite particles described below by adjusting the amount of volatile components added to the block-shaped body. The internal porosity of the first graphite particles is preferably 8% or more and 20% or less, more preferably 10% or more and 18% or less, and particularly preferably 12% or more and 16% or less. When a portion of the binder added to the coke (precursor) volatilizes during firing, the binder can be used as a volatile component. Pitch is an example of such a binder.

[0047] The second graphite particles mainly contained in the second negative electrode mixture layer 32b can be produced, for example, as follows. The coke (precursor) serving as the main raw material is crushed to a predetermined size, agglomerated with a binder, and then fired at a temperature of 2600°C or higher to graphitize the particles. The resulting graphite particles are then sieved to obtain second graphite particles of a desired size. The internal porosity of the second graphite particles can be adjusted by adjusting the particle size of the crushed precursor or the particle size of the agglomerated precursor. For example, the internal porosity can be reduced by increasing the particle size of the crushed precursor. The average particle size (median diameter in volume terms, hereinafter sometimes referred to as D50) of the crushed precursor may be in the range of 12 μm or more and 20 μm or less. The internal porosity of the second graphite particles is preferably 5% or less, more preferably 1% or more and 5% or less, and particularly preferably 3% or more and 5% or less.

[0048] At least the second negative electrode mixture layer 32b of the negative electrode mixture layer 32 contains carbon nanotubes (hereinafter, sometimes referred to as CNTs) as a conductive agent. By incorporating CNTs into the second negative electrode mixture layer 32b, which has a high interparticle porosity of the negative electrode active material, the conductive paths between the particles of the negative electrode active material can be maintained even after repeated charge and discharge. As a result, by keeping S2 / S1 within a predetermined range, the conductive paths can be maintained while improving the permeability of the nonaqueous electrolyte, thereby further improving the charge and discharge cycle characteristics.

[0049] When the CNT content in the first negative electrode mixture layer 32a is A1 and the CNT content in the second negative electrode mixture layer is A2, it is preferable that A2 / A1 > 1. When the total amount of CNT contained in the negative electrode mixture layer 32 is constant, the charge / discharge cycle characteristics are further improved when A2 is larger than A1. Note that the first negative electrode mixture layer 32a may or may not contain CNT.

[0050] The CNT content in the second negative electrode mixture layer 32b is preferably 0.01 mass% or more and 0.1 mass% or less, and more preferably 0.01 mass% or more and 0.05 mass% or less, relative to the mass of the second negative electrode mixture layer 32b. If the CNT content in the second negative electrode mixture layer 32b is within the above range, the electrical conductivity of the second negative electrode mixture layer 32b can be ensured while the content of the negative electrode active material in the second negative electrode mixture layer 32b can be sufficiently ensured.

[0051] The G / D ratio obtained by Raman spectroscopy of the CNTs contained in the second negative electrode mixture layer 32b is preferably 40 or more and 130 or less, and more preferably 50 or more and 120 or less. This significantly improves the charge / discharge cycle characteristics. Although the detailed mechanism is unclear, since the G / D ratio represents the crystalline ratio, it is presumed that an appropriate amount of structural defects is required in the CNTs to ensure a conductive path in the second negative electrode mixture layer 32b.

[0052] The G / D ratio can be calculated by the D-Band (1300 cm) in the Raman spectrum. －１ ~1350cm －１ ) peak intensity relative to the G-Band (1550 cm －１ ~1600cm－１ ) and CNTs with a high G / D ratio have high crystallinity.

[0053] The Raman spectrum of CNTs can be measured using a Raman spectrometer (for example, the NRS-5500 manufactured by JASCO Corporation). For example, a sample is prepared by dispensing CNTs onto a slide and flattening it with a spatula, and then the measurement is performed. The measurement conditions are, for example, as follows: Measurement time: 5 seconds Number of integrations: 2 Neutral density filter OD: 0.3 Objective lens magnification: 100x Measurement range: 950 cm －１ ~1900cm －１

[0054] Examples of CNTs contained in the negative electrode mixture layer 32 include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The CNTs contained in the negative electrode mixture layer 32 are preferably SWCNTs. Alternatively, the CNTs contained in the negative electrode mixture layer 32 may be a combination of SWCNTs and MWCNTs.

[0055] The diameter of the SWCNT is, for example, 0.1 nm or more and 2 nm or less. The length of the SWCNT is, for example, 0.1 μm or more and 200 μm or less. Here, the diameter of the SWCNT is calculated by measuring the diameters of 10 SWCNTs using a transmission electron microscope (TEM) and averaging the measured values. The length of the SWCNT is calculated by measuring the lengths of 10 SWCNTs using a scanning electron microscope (SEM) and averaging the measured values. The diameter of the MWCNT is, for example, 3 nm or more and 100 nm or less. The length of the MWCNT is, for example, 0.1 μm or more and 200 μm or less. The diameter and length of the MWCNT can be calculated in the same manner as for the SWCNT.

[0056] The negative electrode mixture layer 32 may contain a conductive agent other than CNT. Examples of conductive agents other than CNT include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black, and graphite. These may be used alone or in combination of two or more.

[0057] The negative electrode mixture layer 32 may further contain a binder. Examples of binders include fluorine-based resins, polyimide-based resins, acrylic-based resins, polyolefin-based resins, polyacrylonitrile (PAN), styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), and the like. These may be used alone or in combination of two or more.

[0058] Next, a method for forming the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b will be described. For example, first, a negative electrode active material containing the first graphite particles described above, a binder, and a solvent such as water are mixed to prepare a first negative electrode mixture slurry. Separately, a negative electrode active material containing the second graphite particles described above coated with amorphous carbon, a binder, and a solvent such as water are mixed to prepare a second negative electrode mixture slurry. Then, the first negative electrode mixture slurry is applied to both sides of the negative electrode current collector 30 and dried. After that, the second negative electrode mixture slurry is applied to both sides of the coating of the first negative electrode mixture slurry and dried. Furthermore, the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are rolled using a rolling roller to form the negative electrode mixture layer 32. In the above method, the second anode mixture slurry is applied after the first anode mixture slurry is applied and dried, but the second anode mixture slurry may be applied after the first anode mixture slurry is applied and before drying. Alternatively, the second anode mixture slurry may be applied onto the first anode mixture layer 32a after the first anode mixture slurry is applied, dried, and rolled.

[0059] By changing the rolling conditions of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the packing densities of the respective layers can be adjusted more freely. Note that even if the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are rolled simultaneously, the interparticle porosity of the negative electrode active material of each layer will not be the same. For example, by changing the particle size distribution of the first graphite particles and the second graphite particles, the interparticle porosity of the negative electrode active material of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b can be adjusted.

[0060] [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.

[0061] 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.

[0062] [Non-aqueous electrolyte] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0063] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0064] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0065] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0066] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２ Examples of the borate salt include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFFOB). Examples of the imide salt include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ Among these, LiPF is preferred from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ The concentration of the lithium salt may be, for example, 4 mol or less, or 3 mol or less, preferably 1.8 mol or less, and more preferably 0.8 mol or more and 1.8 mol or less, per 1 L of the non-aqueous solvent.

[0067] The non-aqueous electrolyte may contain an additive such as an unsaturated carbonate ester, an acid anhydride, a phenol compound, a benzene compound, a nitrile compound, an isocyanate compound, a sultone compound, a sulfate compound, a borate ester compound, a phosphate ester compound, or a phosphite ester compound.

[0068] Examples of unsaturated cyclic carbonates include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. One type of unsaturated cyclic carbonate may be used alone, or two or more types may be used in combination. Some of the hydrogen atoms in the unsaturated cyclic carbonate may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by intermolecular condensation of multiple carboxylic acid molecules, but is preferably an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0069] Examples of phenolic compounds include phenol, hydroxytoluene, etc. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, cyclohexylbenzene (CHB), etc.

[0070] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanatomethylcyclohexane (BIMCH). Examples of sultone compounds include propane sultone and propene sultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethyl borate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethyl phosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethyl phosphite and tris(trimethylsilyl)phosphite.

[0071] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0072] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0073] Example 1 [Fabrication of Positive Electrode] Powdered LiCo was used as the positive electrode active material. ０．９７９ Zr ０．００１ Mg ０．０１ Al ０．０１ O ２A lithium transition metal oxide represented by the formula (I) was used. 95 parts by mass of the above positive electrode active material, 2.5 parts by mass of acetylene black (AB) as a conductive agent, and 2.5 parts by mass of polyvinylidene fluoride powder as a binder were mixed together, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode mixture slurry. This slurry was applied to both sides of a positive electrode current collector made of aluminum foil (thickness 15 μm) by a doctor blade method, and after drying the coating, the coating was rolled with a rolling roller to produce a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector.

[0074] [Preparation of Graphite Particles A] Coke was pulverized to an average particle size (D50) of 17 μm, and pitch was added as a binder to the pulverized coke to aggregate the coke. Isotropic pressure of 1.6 g / cm was applied to the aggregate. ３ ~1.9 g / cm ３ This block-shaped compact was graphitized by firing at a temperature of 2800°C, and the graphitized block-shaped compact was then pulverized and sieved using a 250 mesh sieve to obtain graphite particles A having an average particle size (D50) of 23 µm.

[0075] [Preparation of Graphite Particles B] Coke was pulverized to an average particle size (D50) of 13 μm, and pitch was added as a binder to the pulverized coke to aggregate to an average particle size (D50) of 18 μm. The aggregate was graphitized by firing at a temperature of 2800° C. The graphitized block-shaped compact was then pulverized and sieved using a 250-mesh sieve to obtain graphite particles B having an average particle size (D50) of 23 μm.

[0076] [Preparation of Graphite Particles C and D] Graphite particles C were prepared by mixing graphite particles A with pitch, adhering the pitch to the surfaces of graphite particles A, and then calcining the mixture at 1000°C. Graphite particles D were prepared by mixing graphite particles B with pitch, adhering the pitch to the surfaces of graphite particles B, and then calcining the mixture at 1000°C. The amount of amorphous carbon coated on graphite particles C and D was measured using a simultaneous differential thermal and thermogravimetric analyzer, and was found to be 3.0% by mass. In this measurement method, samples were heated in an air atmosphere from room temperature to 450°C at a heating rate of 15°C / min and from 450°C to 600°C at a heating rate of 5°C / min, and the amount of amorphous carbon coated was calculated using the formula [mass loss rate at 600°C (%) - mass loss rate at 100°C (%)].

[0077] [Fabrication of Negative Electrode] Graphite particles A and SiO were mixed in a mass ratio of 95:5 to prepare a first negative electrode active material. 100 parts by mass of the first negative electrode active material, 1 part by mass of carboxymethyl cellulose sodium salt (CMC-Na), and 1 part by mass of styrene-butadiene copolymer rubber (SBR) were mixed, and the mixture was kneaded in water to prepare a first negative electrode mixture slurry.

[0078] A second negative electrode active material was prepared by mixing 25 parts by mass of graphite particles A and 75 parts by mass of graphite particles D to obtain a mixed graphite, and then mixing this mixed graphite with SiO at a mass ratio of 95:5. 100 parts by mass of the second negative electrode active material, 1 part by mass of CMC-Na, 1 part by mass of SBR, and 0.02 parts by mass of single-walled carbon nanotubes (SWCNT) (G / D ratio: 90) were mixed, and the mixture was kneaded in water to prepare a second negative electrode mixture slurry.

[0079] The first negative electrode mixture slurry was applied to both sides of a copper foil negative electrode current collector by a doctor blade method and dried to form a first negative electrode mixture layer. The second negative electrode mixture slurry was then applied to the first negative electrode mixture layer and dried to form a second negative electrode mixture layer. The applied mass ratio (thickness ratio) per unit area of ​​the first negative electrode mixture slurry to the second negative electrode mixture slurry was 3:7. The first negative electrode mixture layer and the second negative electrode mixture layer were rolled using a rolling roller to produce a negative electrode.

[0080] [Preparation of non-aqueous electrolyte] LiPF 6 was added to 100 parts by mass of a non-aqueous solvent prepared by mixing ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) in a volume ratio of 10:10:80. ６ was dissolved in a solution at a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte.

[0081] [Fabrication of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] An aluminum positive electrode lead was attached to the positive electrode current collector, and a nickel negative electrode lead was attached to the negative electrode current collector. The positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, and then pressed radially to form a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the non-aqueous electrolyte was poured into it. The opening of the exterior body was then sealed to obtain a test cell measuring 62 mm in height, 35 mm in width, and 3.6 mm in thickness.

[0082] Example 2 A test cell was produced in the same manner as in Example 1, except that in the production of the negative electrode, a mixed graphite obtained by mixing 25 parts by mass of graphite particles C and 75 parts by mass of graphite particles D was mixed with SiO in a mass ratio of 95:5 to form a second negative electrode active material.

[0083] Example 3 A test cell was produced in the same manner as in Example 1, except that in the production of the negative electrode, a mixed graphite obtained by mixing 34 parts by mass of graphite particles A and 66 parts by mass of graphite particles D was mixed with SiO in a mass ratio of 95:5 to form a second negative electrode active material.

[0084] Example 4 A test cell was produced in the same manner as in Example 1, except that in the production of the negative electrode, a mixed graphite obtained by mixing 34 parts by mass of graphite particles C and 66 parts by mass of graphite particles D was mixed with SiO in a mass ratio of 95:5 to form a second negative electrode active material.

[0085] Example 5 A test cell was produced in the same manner as in Example 1, except that in the production of the negative electrode, graphite particles D and SiO were mixed in a mass ratio of 95:5 to form a second negative electrode active material.

[0086] Comparative Example 1 A test cell was fabricated in the same manner as in Example 1, except for the following changes in the fabrication of the negative electrode: (1) A second negative electrode active material was prepared by mixing SiO at a mass ratio of 95:5 with mixed graphite obtained by mixing 60 parts by mass of graphite particles A and 40 parts by mass of graphite particles B. (2) A second negative electrode mixture slurry was prepared without mixing SWCNTs.

[0087] Comparative Example 2 A test cell was prepared in the same manner as in Comparative Example 1, except that in preparing the negative electrode, a mixed graphite obtained by mixing 60 parts by mass of graphite particles A and 40 parts by mass of graphite particles D was mixed with SiO in a mass ratio of 95:5 to prepare a second negative electrode active material.

[0088] Comparative Example 3 A test cell was prepared in the same manner as in Comparative Example 1, except that in preparing the negative electrode, a mixed graphite obtained by mixing 40 parts by mass of graphite particles B and 60 parts by mass of graphite particles C was mixed with SiO in a mass ratio of 95:5 to prepare a second negative electrode active material.

[0089] Comparative Example 4 A test cell was prepared in the same manner as in Comparative Example 1, except that in preparing the negative electrode, a mixed graphite obtained by mixing 60 parts by mass of graphite particles C and 40 parts by mass of graphite particles D was mixed with SiO in a mass ratio of 95:5 to prepare a second negative electrode active material.

[0090] Comparative Example 5 A test cell was prepared in the same manner as in Comparative Example 1, except that in preparing the negative electrode, a mixed graphite obtained by mixing 25 parts by mass of graphite particles A and 75 parts by mass of graphite particles B was mixed with SiO in a mass ratio of 95:5 to prepare a second negative electrode active material.

[0091] Comparative Example 6 A test cell was fabricated in the same manner as in Example 1, except for the following changes in the fabrication of the negative electrode: (1) A second negative electrode active material was prepared by mixing 60 parts by mass of graphite particles A and 40 parts by mass of graphite particles B to obtain a mixed graphite, and then mixing this mixed graphite with SiO at a mass ratio of 95:5. (2) In preparing the first negative electrode mixture slurry, 0.02 parts by mass of SWCNT was further mixed to prepare a mixture.

[0092] Comparative Example 7 A test cell was produced in the same manner as in Example 1, except that in the production of the negative electrode, a mixed graphite obtained by mixing 25 parts by mass of graphite particles A and 75 parts by mass of graphite particles B was mixed with SiO in a mass ratio of 95:5 to form a second negative electrode active material.

[0093] Comparative Example 8 A test cell was prepared in the same manner as in Comparative Example 7, except that in preparing the negative electrode, a second negative electrode active material was prepared by mixing 34 parts by mass of graphite particles A and 66 parts by mass of graphite particles B to obtain a mixed graphite, and SiO in a mass ratio of 95:5.

[0094] Comparative Example 9 A test cell was produced in the same manner as in Comparative Example 7, except that in the production of the negative electrode, graphite particles B and SiO were mixed in a mass ratio of 95:5 to form a second negative electrode active material.

[0095] <Comparative Example 10> A test cell was produced in the same manner as in Comparative Example 7, except that in the production of the negative electrode, the application mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was changed to 8:2.

[0096] <Comparative Example 11> A test cell was produced in the same manner as in Comparative Example 7, except that in producing the negative electrode, the application mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was changed to 5:5.

[0097] [Evaluation of Interparticle Porosity of Negative Electrode Active Material] At an ambient temperature of 25°C, the test cells of each Example and Comparative Example were charged at a constant current of 0.2 C to 4.2 V, and then charged at a constant voltage of 1 / 50 C at 4.2 V. Subsequently, the test cells were discharged at a constant current of 0.2 C to 2.5 V. This charge / discharge cycle was counted as one cycle, and five cycles were performed. After the five cycles, the negative electrodes were removed from the test cells of each Example and Comparative Example, and the interparticle porosity of the negative electrode active material was calculated by the method described above.

[0098] [Evaluation of Capacity Retention Rate] At an ambient temperature of 25°C, the test cells of each Example and Comparative Example were charged at a constant current of 1 C to 4.2 V, and then charged at a constant voltage of 4.2 V to 1 / 50 C. Thereafter, the test cells were discharged at a constant current of 0.5 C to 2.5 V. This charge / discharge cycle was counted as one cycle, and 300 cycles were performed. The capacity retention rate of the test cells of each Example and Comparative Example after each charge / discharge cycle was calculated using the following formula: Capacity retention rate [%] = (discharge capacity at 300th cycle / discharge capacity at 1st cycle) × 100

[0099] The evaluation results of the capacity retention rates of the test cells of each example and each comparative example are summarized in Table 1. Table 1 also shows the ratios of graphite particles A to D in each of the first and second negative electrode mixture layers, the CNT content in the first and second negative electrode mixture layers, the thickness ratios of the first and second negative electrode mixture layers, and the S2 / S1 value in the negative electrode mixture layers.

[0100]

[0101] As shown in Table 1, the test cells of the examples have improved capacity retention rates compared to the test cells of the comparative examples. In other words, it can be said that the charge-discharge cycle characteristics can be improved by using coated graphite particles as the main component of at least the second negative electrode mixture layer and by including carbon nanotubes in the second negative electrode mixture layer while satisfying the relationship S2 / S1 3.5≦S2 / S1≦5.2.

[0102] Furthermore, the capacity retention rates of the test cells of Comparative Examples 3 and 4, in which the second negative electrode mixture layer contained coated graphite particles as the main component but the interparticle porosities S1 and S2 did not satisfy 3.5≦S2 / S1≦5.2, were smaller than the capacity retention rates of the test cells of Examples 1 and 2. This is presumably because the permeability of the nonaqueous electrolyte in the negative electrode mixture layer was insufficient, causing unevenness in the charge / discharge reaction in the thickness direction of the negative electrode mixture layer.

[0103] Furthermore, the capacity retention rates of the test cells of Comparative Examples 5 and 7 to 11 in which the interparticle porosities S1 and S2 satisfied the relationship 3.5≦S2 / S1≦5.2 but in which the main component of the second negative electrode mixture layer was not coated graphite particles were smaller than the capacity retention rates of the test cells of Examples. This is presumably because, even when the permeability of the nonaqueous electrolyte in the negative electrode mixture layer was improved, the lithium ion diffusibility in the graphite particles arranged on the surface side of the negative electrode was low, resulting in insufficient lithium ion diffusibility in the thickness direction of the negative electrode mixture layer.

[0104] Furthermore, the improvement effects of the capacity retention rate of the test cells of Examples 1 and 2 relative to the test cell of Comparative Example 1 were +20% and +22%, respectively, whereas the improvement effects of the capacity retention rate of the test cells of Comparative Examples 3 and 4 (in which the second negative electrode mixture layer mainly contains coated graphite particles coated with amorphous carbon but S2 / S1 is less than 3.5) relative to the test cell of Comparative Example 1 were +2% and +5%, respectively. Furthermore, the improvement effect of the capacity retention rate of the test cell of Comparative Example 7 (in which the second negative electrode mixture layer does not mainly contain coated graphite particles coated with amorphous carbon but S2 / S1 satisfies 3.5≦S2 / S1≦5.2) was +10%. In other words, the improvement effect of the capacity retention rate of the test cells of Examples 1 and 2 relative to the test cell of Comparative Example 1 was greater than the combined improvement effect of the capacity retention rates of Comparative Examples 3, 4, and 7. Therefore, when a negative electrode having the configuration of the present invention is used, it can be said that the charge-discharge cycle characteristics can be specifically improved due to the synergistic effect of improved permeability of the non-aqueous electrolyte and improved diffusibility of lithium ions.

[0105] The present disclosure will be further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on a surface of the negative electrode current collector, the negative electrode mixture layer has a first negative electrode mixture layer facing the negative electrode current collector and a second negative electrode mixture layer facing the positive electrode with the separator interposed therebetween, at least the second negative electrode mixture layer mainly containing coated graphite particles coated with amorphous carbon and containing carbon nanotubes, and wherein, when an interparticle porosity of the negative electrode active material in the first negative electrode mixture layer is S1 and an interparticle porosity of the negative electrode active material in the second negative electrode mixture layer is S2, the non-aqueous electrolyte secondary battery satisfies 3.5≦S2 / S1≦5.2. Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein the first negative electrode mixture layer is primarily composed of graphite particles that are not coated with amorphous carbon. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the amount of amorphous carbon coated on the coated graphite particles is 0.5 mass % or more and 15 mass % or less, relative to the mass of the coated graphite particles. Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the negative electrode active material includes a Si-based material. Configuration 5: The nonaqueous electrolyte secondary battery according to Configuration 4, wherein the proportion of the Si-based material with respect to the total mass of the negative electrode active material is 1 mass % or more and 20 mass % or less. Configuration 6: The Si-based material is SiO ｘ (0.5≦x≦1.6). Configuration 7: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the G / D ratio obtained by Raman spectroscopy of the carbon nanotubes is 40 or more and 130 or less. Configuration 8: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein, when the content of the carbon nanotubes in the first negative electrode mixture layer is A1 and the content of the carbon nanotubes in the second negative electrode mixture layer is A2, A2 / A1>1 is satisfied. Configuration 9: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein the ratio of the thickness of the second negative electrode mixture layer to the thickness of the first negative electrode mixture layer is 2:8 to 5:5.

[0106] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Exterior body, 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 current collector, 32 Negative electrode mixture layer, 32a First negative electrode mixture layer, 32b Second negative electrode mixture layer, 40 Graphite particles, 42 Internal void (void), 44 External void (void)

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, the negative electrode mixture layer has a first negative electrode mixture layer facing the negative electrode current collector and a second negative electrode mixture layer facing the positive electrode, at least the second negative electrode mixture layer is mainly composed of coated graphite particles coated with amorphous carbon and contains carbon nanotubes, and wherein, when the interparticle porosity of the negative electrode active material in the first negative electrode mixture layer is S1 and the interparticle porosity of the negative electrode active material in the second negative electrode mixture layer is S2, the non-aqueous electrolyte secondary battery satisfies 3.5≦S2 / S1≦5.

2.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first negative electrode mixture layer is primarily composed of graphite particles that are not coated with amorphous carbon.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the amount of amorphous carbon coated on the coated graphite particles is 0.5 mass % or more and 15 mass % or less with respect to the mass of the coated graphite particles.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material includes a Si-based material.

5. The nonaqueous electrolyte secondary battery according to claim 4, wherein the proportion of said Si-based material relative to the total mass of said negative electrode active material is 1 mass % or more and 20 mass % or less.

6. The Si-based material is SiO ｘ The nonaqueous electrolyte secondary battery of claim 4 , comprising: (0.5≦x≦1.6).

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the G / D ratio of the carbon nanotubes obtained by Raman spectroscopy is 40 or more and 130 or less.

8. The nonaqueous electrolyte secondary battery according to claim 1, wherein, when the content of the carbon nanotubes in the first negative electrode mixture layer is A1 and the content of the carbon nanotubes in the second negative electrode mixture layer is A2, A2 / A1>1 is satisfied.

9. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the thickness of the second negative electrode mixture layer to the thickness of the first negative electrode mixture layer is 2:8 to 5:5.

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