Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery (10) includes a positive electrode (11), a negative electrode (12), and a nonaqueous electrolyte, and is characterized in that: the positive electrode (11) has a positive electrode core and a positive electrode mixture layer provided on the positive electrode core; the positive electrode mixture layer contains a positive electrode active material; the positive electrode active material includes a first positive electrode active material having 《MK1》the shape of a secondary particle《 / MK1》 formed by aggregation of primary particles, and a second positive electrode active material having 《MK1》the shape of a single particle《 / MK1》; the content of the second positive electrode active material is 10 mass% or more and less than 40 mass% with respect to the total mass of the positive electrode active material; the nonaqueous electrolyte contains a heterocyclic compound including at least one electron withdrawing group R and a heterocycle; the electron withdrawing group R contains oxygen and / or nitrogen; and 《RA》the heterocycle《 / RA》 contains nitrogen and sulfur.

Description

Nonaqueous electrolyte secondary battery

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

[0002] A non-aqueous electrolyte secondary battery such as a lithium ion secondary battery includes, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0003] For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery that uses a non-aqueous electrolyte containing a thiazole compound.

[0004] International Publication No. 2023 / 145896

[0005] By using a non-aqueous electrolyte containing a thiazole compound, it is possible to reduce the battery resistance. However, in order to further increase the capacity in the future, it is desired to achieve a further reduction in the battery resistance.

[0006] Therefore, an object of the present disclosure is to improve the reduction in battery resistance in a non-aqueous electrolyte secondary battery that uses a non-aqueous electrolyte containing a thiazole compound.

[0007] One aspect of the present disclosure is a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode composite layer provided on the positive electrode core, the positive electrode composite layer including a positive electrode active material, the positive electrode active material including a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles, and a second positive electrode active material having a single particle shape, the content of the second positive electrode active material being 10 mass% or more and less than 40 mass% with respect to the total mass of the positive electrode active material, the non-aqueous electrolyte including at least one heterocyclic compound including an electron-withdrawing group R and a heterocycle, the electron-withdrawing group R including oxygen and / or nitrogen, and the heterocycle including nitrogen and sulfur.

[0008] According to one aspect of the present disclosure, in a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte containing a thiazole compound, it is possible to improve reduction in battery resistance.

[0009] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention;

[0010] One aspect of the present disclosure is a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode core and a positive electrode composite layer disposed on the positive electrode core. The positive electrode composite layer includes a positive electrode active material. The positive electrode active material includes a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles and a second positive electrode active material having a single particle shape. The content of the second positive electrode active material is 10% by mass or more but less than 40% by mass, based on the total mass of the positive electrode active material. The non-aqueous electrolyte includes a heterocyclic compound including at least one electron-withdrawing group R and a heterocycle. The electron-withdrawing group R includes oxygen and / or nitrogen, and the heterocycle includes nitrogen and sulfur. According to the present disclosure, the heterocyclic compound (thiazole compound) is reductively decomposed on the negative electrode during charge and discharge of the battery, forming a coating on the negative electrode surface. Because the coating derived from the heterocyclic compound has high ionic conductivity, its formation on the surface of the negative electrode reduces battery resistance. Generally, the reductive decomposition of the heterocyclic compound, a side reaction, also occurs on the positive electrode. When the positive electrode active material expands and contracts during charging and discharging, particle cracking occurs, and new interfaces are formed on the particles of the positive electrode active material, new reductive decomposition of the heterocyclic compound occurs on the new interfaces. Therefore, when a positive electrode active material that is prone to forming new interfaces is used, the reductive decomposition of the heterocyclic compound on the positive electrode progresses, and accordingly, the reactivity of the reductive decomposition of the heterocyclic compound on the negative electrode decreases. As a result, the amount of the coating derived from the heterocyclic compound formed on the surface of the negative electrode decreases, and the battery resistance reduction effect may not be sufficiently achieved. However, because the second positive electrode active material having a single particle shape generates less new interfaces during charging and discharging, the use of a second positive electrode active material having a single particle shape as disclosed herein suppresses the reductive decomposition of the heterocyclic compound on the positive electrode, and accordingly, the reductive decomposition of the heterocyclic compound on the negative electrode proceeds preferentially. As a result, an appropriate amount of coating is formed on the negative electrode surface, which is thought to sufficiently reduce the battery resistance. Note that if the content of the second positive electrode active material having a single particle shape is too high, the resistance of the positive electrode itself increases, leading to an increase in battery resistance. Therefore, as disclosed herein, it is also necessary to appropriately content the second positive electrode active material having a single particle shape.

[0011] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described below. The drawings referred to in the following embodiment are schematic, and the dimensional ratios of the components depicted in the drawings may differ from those of the actual battery.

[0012] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. The nonaqueous electrolyte secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical outer can 16 with a bottom and a sealing member 17 that closes the opening of the outer can 16. Note that, instead of the wound electrode assembly 14, other electrode assembly configurations may be used, such as a laminated electrode assembly formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include cylindrical, prismatic, coin-shaped, or button-shaped metal cases, and resin cases (so-called pouch-shaped cases) formed by laminating resin sheets.

[0013] The outer can 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its upper surface.

[0014] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in this order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except for the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the nonaqueous electrolyte secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, 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 ruptures, and gas is discharged from the opening of the cap 27.

[0015] 1 , a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the outer can 16. The positive electrode lead 20 is connected by welding or the like to the underside of a filter 23, which is the bottom plate of the sealing body 17, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner bottom surface of the outer can 16, and the outer can 16 serves as the negative electrode terminal.

[0016] The non-aqueous electrolyte, the positive electrode 11, the negative electrode 12, and the separator 13 will be described below.

[0017] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte salt, and a heterocyclic compound containing at least one electron-withdrawing group R and a heterocycle. Hereinafter, the heterocycle and the heterocyclic compound may be referred to as "heterocycle (H)" and "heterocyclic compound (C)," respectively. The electron-withdrawing group R contains oxygen and / or nitrogen. The heterocycle (H) contains nitrogen and sulfur. As described above, the inclusion of the heterocyclic compound (C) in the non-aqueous electrolyte forms a coating on the surface of the negative electrode, leading to a reduction in battery resistance.

[0018] The non-aqueous electrolyte may contain only one type of compound as the heterocyclic compound (C), or may contain multiple types of compounds.

[0019] The electron-withdrawing group R contained in the heterocyclic compound (C) may contain only one of oxygen and nitrogen, or may contain both. For example, the electron-withdrawing group R may be a carbonyl group (—C(═O)—), a nitrile group (—C≡N), a sulfonyl group (—S(═O)—), or a hydroxyl group (—S(═O)—) in order to further reduce the battery resistance. ２ The hydroxy group, which is the electron-withdrawing group R, may be bonded to a carbon atom constituting a saturated hydrocarbon group (e.g., an alkyl group or an alkylene group). The nitrile group may be contained in a thionitrile group. The sulfonyl group may be a sulfonate ester bond (-S(=O) ２ —O—). The C═O moiety contained in an isocyanate group and an isothiocyanate group is not usually considered to be a carbonyl group. Therefore, in this specification, the C═O moiety contained in an isocyanate group and an isothiocyanate group is not considered to be a carbonyl group.

[0020] The carbonyl group may be contained in at least one selected from the group consisting of an aldehyde group (-CHO), a ketone, an amide bond (C(=O)-N), an ester bond (COO), and a carboxy group (-COOH). That is, the electron-withdrawing group R may be at least one selected from the group consisting of an aldehyde group, a carbonyl group contained in a ketone, an amide bond, an ester bond, and a carboxy group.

[0021] The number of electron-withdrawing groups R contained in the heterocyclic compound (C) may be 1, 2 or more, 5 or more, or 5 or less. The number of heterocycles (H) contained in the heterocyclic compound (C) may be 1, 2, 3 or less, or 3 or more.

[0022] The heterocycle (H) contains nitrogen and sulfur. The heterocycle (H) may or may not have aromaticity. The number of atoms constituting the heterocycle (H) may be in the range of 5 to 8, or in the range of 5 to 7, or may be 5 or 6. That is, the heterocycle (H) may be a five-membered ring, a six-membered ring, a seven-membered ring, or an eight-membered ring.

[0023] The heterocycle (H) may satisfy the following conditions (1) and / or (2), or may satisfy the following conditions (1) and (3): (1) The heterocycle (H) is composed of one nitrogen atom, one sulfur atom, and multiple carbon atoms; (2) The nitrogen atom forms a double bond with one adjacent carbon atom that constitutes the heterocycle (H); and (3) The nitrogen atom forms a single bond with each of the two adjacent carbon atoms that constitute the heterocycle (H).

[0024] The heterocycle (H) may be a thiazole ring represented by the following formula:

[0025]

[0026] The heterocycle (H) may be a thiomorpholine ring represented below.

[0027]

[0028] The heterocycle (H) may be a thiazepine ring represented by the following formula: The thiazepine ring may be a 1,3-thiazepine ring or a 1,4-thiazepine ring.

[0029]

[0030] The heterocycle (H) may contain at least one ring selected from the group consisting of a thiazole ring, a thiomorpholine ring, and a thiazepine ring.

[0031] The structure of the heterocyclic compound (C) other than the heterocycle (H) and the electron-withdrawing group R is not limited as long as the effects of the present disclosure are obtained. The portion other than the heterocycle (H) and the electron-withdrawing group R may be composed only of hydrocarbons. Examples of hydrocarbons include hydrocarbon groups (including hydrocarbon chains). Examples of hydrocarbons include aliphatic hydrocarbons and aromatic hydrocarbons. The heterocyclic compound (C) may contain an ether bond, a thioether bond, nitrogen not included in the electron-withdrawing group R, etc.

[0032] The molecular weight of the heterocyclic compound (C) may be 100 or more, or 130 or more, and may be 400 or less, or 370 or less.

[0033] As the heterocyclic compound (C), a compound that dissolves in the non-aqueous solvent of the non-aqueous electrolyte is preferably used.

[0034] Examples of the heterocyclic compound (C) include 4,5-dimethyl-1,3-thiazole-2-carbaldehyde, 4,5,6,7-tetrahydro-1,3-benzothiazole-2-carbaldehyde, ethyl 2-ethynyl-4-methylthiazole-5-carboxylate, N-benzyl-2-ethynyl-4-methylthiazole-5-carboxamide, 4-methyl-5-(2-thiocyanatoethyl)thiazole, perfluorophenyl 4,5-dimethylthiazole-2-sulfonate, 2-(trimethylsilyl)thiazole-4-carbaldehyde, 4-(((tert-butyl Examples of the thiocyanate include 1-(5-(hydroxymethyl)-4-methylthiazol-2-yl)ethan-1-one, 5-isocyanato-4-methyl-2-phenylthiazole, 5-isocyanato-4-methyl-2-(pyrazin-2-yl)thiazole, 2-ethylthiomorpholine-4-carbaldehyde, 2,3-dimethylthiomorpholine-4-carbaldehyde, 2-methyl-4-thiocyanatobenzo[b][1,4]thiazepine, and 4-methyl-2-thiocyanatobenzo[b][1,4]thiazepine. Among these, it is preferable that the heterocyclic compound (C) contains at least one selected from the group consisting of 4,5-dimethyl-1,3-thiazole-2-carbaldehyde, 4,5,6,7-tetrahydro-1,3-benzothiazole-2-carbaldehyde, 2-ethylthiomorpholine-4-carbaldehyde, and 2,3-dimethylthiomorpholine-4-carbaldehyde, in that it is possible to further improve the reduction in battery resistance.

[0035] The heterocyclic compound (C) may be a commercially available compound, or may be synthesized according to a known synthesis method.

[0036] The content of the heterocyclic compound (C) in the non-aqueous electrolyte may be 0.1 mass % or more, 0.5 mass % or more, or 1.0 mass % or more, and may be 10.0 mass % or less, 5.0 mass % or less, or 3.0 mass % or less. By setting the content within the above range, it is possible to further reduce the battery resistance.

[0037] The content of the heterocyclic compound (C) in the non-aqueous electrolyte is determined using gas chromatography under the following conditions. Used equipment: GC-2010 Plus, manufactured by Shimadzu Corporation. Column: HP-1 (film thickness 1 μm, inner diameter 0.32 mm, length 60 m), manufactured by J&W. Column temperature: heated from 50°C to 90°C at a heating rate of 5°C / min, maintained at 90°C for 15 minutes, then heated from 90°C to 250°C at a heating rate of 10°C / min, and maintained at 250°C for 15 minutes. Split ratio: 1 / 50. Linear velocity: 30.0 cm / sec. Injection port temperature: 270°C. Injection volume: 1 μL. Detector: FID 290°C (sens. 10 １ )

[0038] Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous electrolyte may contain only one non-aqueous solvent, or may contain two or more non-aqueous solvents.

[0039] 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, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０Examples of the lithium salts include lithium carboxylates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of the borates include lithium difluorooxalate borate and lithium bis(oxalate) borate. Examples of the imide salts include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ The non-aqueous electrolyte may contain only one type of electrolyte salt or may contain two or more types of electrolyte salts. The concentration of the electrolyte salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0040] (Positive Electrode) FIG. 2 is a cross-sectional view of a positive electrode according to an embodiment. As shown in FIG. 2, the positive electrode 11 includes a positive electrode core 30 and a positive electrode composite layer 31 provided on the positive electrode core 30. In the positive electrode 11 shown in FIG. 2, the positive electrode composite layer 31 is provided on one side of the positive electrode core 30, but this is not limiting and the positive electrode composite layer 31 may be provided on both sides of the positive electrode core 30. The positive electrode composite layer 31 includes a positive electrode active material. The positive electrode composite layer 31 may include, for example, a conductive material, a binder, and the like. The positive electrode 11 is obtained, for example, by applying and drying a positive electrode composite slurry containing the positive electrode active material and the like onto the positive electrode core 30 to form the positive electrode composite layer 31 on the positive electrode core 30, and then rolling the positive electrode composite layer 31.

[0041] The positive electrode core 30 can be made of a foil of a metal such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface. The thickness of the positive electrode core 30 is, for example, 10 μm or more and 50 μm or less.

[0042] The positive electrode active material includes a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles, and a second positive electrode active material having a single particle shape.

[0043] The first and second positive electrode active materials may be, for example, lithium transition metal composite oxides containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al. One type of lithium transition metal composite oxide may be used alone, or multiple types may be used in combination.

[0044] The first and second positive electrode active materials have, for example, a layered rock salt structure. Examples of layered rock salt structures 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. Among these, a layered rock salt structure belonging to the space group R-3m is preferred from the viewpoints of high capacity and stability of the crystal structure. The total amount of the positive electrode active material including the first and second positive electrode active materials is, for example, 90% by mass or more and 99% by mass or less with respect to the mass of the positive electrode mixture layer.

[0045] As described above, the first positive electrode active material is a secondary particle (polycrystalline particle) formed by aggregation of a large number of primary particles. The inclusion of the first positive electrode active material in the form of secondary particles is advantageous in terms of increasing the capacity of the battery.

[0046] The first positive electrode active material is, for example, a secondary particle formed by aggregation of 1,000 or more primary particles. The average particle size of the primary particles in the first positive electrode active material may be, for example, 30 nm or more, 50 nm or more, or 100 nm or more. The average particle size of the primary particles in the first positive electrode active material may be, for example, 3 μm or less, 2 μm or less, or 1 μm or less. Such ranges may lead to, for example, higher battery capacity. The average particle size of the primary particles is determined by observing the particle surface of the active material using a scanning electron microscope (SEM). The average particle size of the primary particles can be calculated by selecting any 100 primary particles from an SEM image of the particle surface, measuring the diameters of the circumscribed circles, and averaging the measured values.

[0047] The volume-based median diameter (hereinafter referred to as "D50") of the first positive electrode active material (secondary particles) may be, for example, 8 μm or more, 10 μm or more, or 12 μm or more, and may be 40 μm or less, 30 μm or less, or 25 μm or less. Such a range may lead to, for example, a higher capacity battery. D50 refers to the particle size at which the cumulative frequency of particles in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the positive electrode active material is measured using a laser diffraction particle size distribution measuring device, using water as a dispersion medium.

[0048] As described above, the second positive electrode active material has a single particle shape. In this specification, a single particle does not refer to a secondary particle formed by agglomeration of a large number of primary particles, but refers to a single crystal particle formed substantially of one or 10 or less primary particles. For example, the particle interface of the primary particles is substantially absent within the particle of the second positive electrode active material. Note that a particle formed by agglomeration of 10 or less primary particles approximates a single particle shape and can be considered to be substantially a single particle.

[0049] The D50 of the second positive electrode active material may be, for example, smaller than the D50 of the first positive electrode active material, and may be 1 μm or more, 2 μm or more, or 3 μm or more, and may be 15 μm or less, 10 μm or less, or 8 μm or less. This range may, for example, further suppress particle cracking of the second positive electrode active material due to charge and discharge, thereby further suppressing the generation of new particle interfaces and further improving the reduction of battery resistance. The crystallite size of the second positive electrode active material may also be, for example, 370 Å or more and 1500 Å or less, or 370 Å or more and 1000 Å or less. The crystallite size is calculated from the half-width of the diffraction peak of the (104) plane in the X-ray diffraction pattern obtained by X-ray diffraction using the Scherrer equation shown below. In the formula below, s is the crystallite size, λ is the wavelength of the X-ray, B is the half-width of the diffraction peak of the (104) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this specification, K is set to 0.9. s = Kλ / B cos θ

[0050] The X-ray diffraction pattern was obtained by powder X-ray diffraction using a powder X-ray diffractometer (RINT-TTR manufactured by Rigaku Corporation, Cu-Kα source) under the following conditions: Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background: B-spline Profile function: Split pseudo-Voigt function Constraint conditions: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = α (α is the Ni content of each element) ICSD No.: 98-009-4814

[0051] The content of the second positive electrode active material is 10% by mass or more and less than 45% by mass, preferably 10% by mass or more and 35% by mass or less, and more preferably 15% by mass or more and 30% by mass or less, based on the total mass of the positive electrode active material. As described above, the combination of the predetermined amount of the second positive electrode active material and the heterocyclic compound (C) can improve the reduction of battery resistance. The content of the first positive electrode active material may be, for example, 65% by mass or more and 90% by mass or less, based on the total mass of the positive electrode active material.

[0052] 2 , when the positive electrode mixture layer 31 is divided into two equal parts in the thickness direction into a lower half region 31a on the positive electrode core 30 side and an upper half region 31b on the outer surface side, the second positive electrode active material may be contained in a larger amount in the upper half region 31b on the outer surface side than in the lower half region 31a on the positive electrode core 30 side. For example, the ratio of the content of the second positive electrode active material in the upper half region 31b on the outer surface side to the content of the second positive electrode active material in the lower half region 31a on the positive electrode core 30 side may be 60:40 or more, 80:20 or more, or 100:0. In the present disclosure, dividing the positive electrode mixture layer 31 into two equal parts in the thickness direction means that, when the stacking direction of the positive electrode core 30 and the positive electrode mixture layer 31 is the thickness direction of the positive electrode mixture layer 31, the positive electrode mixture layer 31 is divided in half at the middle Z of the thickness. In this way, by disposing a large amount of the second positive electrode active material having a single particle shape in the upper half region 31b on the outer surface side, the reductive decomposition of the heterocyclic compound on the positive electrode 11 is further suppressed. As a result, film formation by reductive decomposition of the heterocyclic compound (C) occurs more preferentially on the negative electrode 12, making it possible to further reduce the battery resistance. The positive electrode composite layer 31 in which the second positive electrode active material is contained in a large amount in the upper half region 31b on the outer surface side can be obtained, for example, by preparing two types of positive electrode composite slurries with different concentrations of the second positive electrode active material and applying them to the positive electrode core 30 in two layers.

[0053] The content of the second positive electrode active material in the lower half region 31 a on the positive electrode substrate 30 side and the content of the second positive electrode active material in the upper half region 31 b on the outer surface side are determined by image analysis of an SEM photograph of a cross section of the positive electrode 11. Specifically, this is as follows.

[0054] First, the areas enclosed by the outlines of the particles of the first positive electrode active material and the particles of the second positive electrode active material are determined from outline images of the particles of the first positive electrode active material in an SEM image of a cross section of the positive electrode 11. The diameter of a circle (equivalent circle) having the same area as the outline area of ​​the particles of the first positive electrode active material is determined and used as the particle size of each particle i, and the volume of a sphere having the same diameter is regarded as the volume V of each particle i. Similarly, the diameter of a circle (equivalent circle) having the same area as the outline area of ​​the particles of the second positive electrode active material is determined and used as the particle size of each particle j, and the volume of a sphere having the same diameter is regarded as the volume V of each particle j. When the positive electrode active material is substantially composed of two types of materials, a first positive electrode active material and a second positive electrode active material, the density of the first positive electrode active material is ρ1, the density of the second positive electrode active material is ρ2, and trace amounts of components such as binders are ignored. The content C1 of the second positive electrode active material in a predetermined region can be calculated by deriving Vi or Vj for each particle present in a predetermined region of the SEM image of the cross section of the positive electrode 11, using the following formula: C1=Σ ｊ ρ2Vj / (Σ ｉ ρ1Vi+Σ ｊ ρ2Vj)

[0055] The content of the second positive electrode active material in the lower half region 31 a is derived by arbitrarily selecting a plurality of predetermined regions (e.g., 20 or more) within the lower half region 31 a, calculating the C1 for each predetermined region, and averaging the values. Similarly, the content of the second positive electrode active material in the upper half region 31 b is derived by arbitrarily selecting a plurality of predetermined regions (e.g., 20 or more) within the upper half region 31 b, calculating the C1 for each predetermined region, and averaging the values.

[0056] Examples of conductive materials include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, and conductive whiskers. One type of conductive material may be used alone, or multiple types may be used in combination. The content of the conductive material may be, for example, 0.1% by mass or more and 5% by mass or less with respect to the total mass of the positive electrode mixture layer 31.

[0057] Examples of binders include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer, acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer, styrene-butadiene copolymer (SBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), and polyethylene oxide (PEO). One type of binder may be used alone, or multiple types may be used in combination. The content of the binder may be 0.6% by mass or more and 1.5% by mass or less with respect to the total mass of the positive electrode mixture layer 31.

[0058] (Negative Electrode) The negative electrode 12 has a negative electrode core and a negative electrode composite layer disposed on the negative electrode core. The negative electrode core can be a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface layer. The thickness of the negative electrode core is, for example, 5 μm or more and 50 μm or less. The negative electrode composite layer contains, for example, a negative electrode active material, a binder, etc. The negative electrode composite layer may be formed on only one surface of the negative electrode core, or on both surfaces of the negative electrode core. The negative electrode 12 can be obtained, for example, by applying and drying a negative electrode composite slurry containing, for example, a negative electrode active material, etc., onto the negative electrode core to form a negative electrode composite layer on the negative electrode core, and then rolling the negative electrode composite layer.

[0059] The negative electrode active material is not particularly limited as long as it is a material that reversibly absorbs and releases ions such as lithium ions, and examples thereof include carbon-based materials. Examples of carbon-based materials include natural graphite such as flake graphite, lump graphite, and earthy graphite, and graphite such as lump artificial graphite and artificial graphite such as graphitized mesophase carbon microbeads. The negative electrode active material may also be a metal that alloys with lithium, such as Si or Sn, an alloy containing such a metal, or a compound containing such a metal. For example, in terms of increasing the capacity of the battery, the negative electrode active material preferably contains a compound containing Si (hereinafter referred to as a Si-containing material).

[0060] The Si-containing material includes, for example, a lithium ion conductive phase and a silicon phase (silicon particles in one aspect) dispersed within the lithium ion conductive phase. The lithium ion conductive phase includes, for example, at least one of a silicon oxide phase, a silicate phase, and a carbon phase.

[0061] The silicate phase preferably contains at least one element selected from the group 2 elements of the periodic table, including alkali metal elements such as lithium, sodium, potassium, rubidium, cesium, and francium, and elements of Group 2 of the periodic table, including beryllium, magnesium, calcium, strontium, barium, and radium, in terms of high lithium ion conductivity, etc. Among these, a silicate phase containing lithium (hereinafter sometimes referred to as a lithium silicate phase) is preferred in terms of high lithium ion conductivity, etc.

[0062] The lithium silicate phase can be, for example, a compound of the formula: Li ２ｚ SiO ２＋ｚ (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, and the like, z preferably satisfies the relationship 0<z<1, and more preferably z=1 / 2.

[0063] The Si-containing material in which the silicon phase is dispersed in the silicon oxide phase can be, for example, a material having the general formula SiO ｘ (The range of 0<x<2 is preferred, and the range of 0.5≦x≦1.6 is more preferred.) The Si-containing material in which the silicon phase is dispersed in the carbon phase can be represented by, for example, the general formula Si ｘ C ｙ (The ranges of 0<x≦1 and 0<y≦1 are preferred).

[0064] The content ratio of the Si-containing material in the negative electrode active material may be 0.5% by mass or more, 3% by mass or more, or 5% by mass or more, and may be 20% by mass or less, 18% by mass or less, or 15% by mass or less, from the viewpoint of increasing the capacity of the battery, etc.

[0065] Examples of the binder include the same materials as those used in the case of positive electrode 11. Note that negative electrode mixture layer 41 may also contain a conductive material.

[0066] (Separator) The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Preferred materials for the separator 13 include polyolefin resins such as polyethylene and polypropylene, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 13.

[0067] The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[0068] Example 1 [Fabrication of Positive Electrode] A positive electrode active material having the composition formula LiNi ０．９０ Co ０．０５ Mn ０．０５ O ２ and a first positive electrode active material (D50: 13 μm) having a secondary particle shape (polycrystalline) formed by aggregation of primary particles having an average particle size of 5 μm, and a composition formula LiNi ０．８５ Co ０．０５ Mn ０．１０ O ２ A mixture of the first positive electrode active material (D50:5 μm) and a second positive electrode active material (D50:5 μm) having a single particle shape (single crystal) was used in a mass ratio of 80:20. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 100:1:1, and a positive electrode composite slurry was prepared using N-methylpyrrolidone (NMP) as a dispersion medium. The slurry was applied to both sides of a positive electrode core made of aluminum foil, and the coating was dried and rolled to obtain a positive electrode having a positive electrode composite layer formed on both sides of the positive electrode core.

[0069] [Fabrication of Negative Electrode] Graphite, carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a solid content mass ratio of 100:1:1, and an appropriate amount of water was added to prepare a negative electrode composite slurry. The slurry was applied to both sides of a copper foil negative electrode core, and the coating was dried and rolled to obtain a negative electrode having a negative electrode composite layer formed on both sides of the negative electrode core.

[0070] [Preparation of Non-Aqueous Electrolyte] 1 mass % of 4,5-dimethyl-1,3-thiazole-2-carbaldehyde, which is a heterocyclic compound (C), was added to a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 to prepare a non-aqueous electrolyte solution containing lithium hexafluorophosphate (LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.0 mol / L of ammonium hydroxide in water.

[0071] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] (1) A positive electrode lead was attached to the positive electrode, and a negative electrode lead was attached to the negative electrode. A separator made of a polyethylene microporous film was then interposed between the positive and negative electrodes, and the resulting structure was wound to produce a wound electrode assembly. (2) Insulating plates were placed on the top and bottom of the electrode assembly, respectively, and the negative electrode lead was welded to the outer can, and the positive electrode lead was welded to a sealing member, and the electrode assembly was then housed in the outer can. (3) A non-aqueous electrolyte was injected into the outer can using a reduced pressure method, and the opening of the outer can was sealed with a sealing member via a gasket. This resulted in a non-aqueous electrolyte secondary battery.

[0072] Example 2 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that 2-ethylthiomorpholine-4-carbaldehyde was added to the mixed solvent.

[0073] Comparative Example 1 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that, as the positive electrode active material, the second positive electrode active material having a single particle shape was not used, but only the first positive electrode active material having a secondary particle shape formed by aggregation of primary particles was used, and 4,5-dimethyl-1,3-thiazole-2-carbaldehyde was not added to the mixed solvent.

[0074] Comparative Example 2 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the second positive electrode active material having a single particle shape was not used as the positive electrode active material, and only the first positive electrode active material having a secondary particle shape formed by aggregation of primary particles was used.

[0075] Comparative Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that, as the positive electrode active material, the second positive electrode active material having a single particle shape was not used, but only the first positive electrode active material having a secondary particle shape formed by aggregation of primary particles was used, and 2-ethylthiomorpholine-4-carbaldehyde was added to the mixed solvent.

[0076] Comparative Example 4 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that, as the positive electrode active material, the second positive electrode active material having a single particle shape was not used, but only the first positive electrode active material having a secondary particle shape formed by aggregation of primary particles was used, and thiazole was added to the mixed solvent.

[0077] Comparative Example 5 A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that 4,5-dimethyl-1,3-thiazole-2-carbaldehyde was not added to the mixed solvent.

[0078] Comparative Example 6 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that a positive electrode active material was used that was a mixture of a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles and a second positive electrode active material having a single particle shape in a mass ratio of 60:40.

[0079] Comparative Example 7 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that thiazole was added to the mixed solvent.

[0080] [Measurement of Battery Resistance] The nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged to 50% SOC at a constant current of 0.5 C in an environment of 25°C. The voltage at this time was designated as V0. Next, the batteries were discharged for 10 seconds at a constant current of 0.5 C. The voltage at this time was designated as V1. The direct current resistance (DCR) was then calculated using the following formula. This was designated as the battery resistance, and the battery resistances of each Example and Comparative Example are summarized in Table 1. However, in Table 1, the battery resistance of Comparative Example 1 is set as the reference (100), and the battery resistances of the other Examples and Comparative Examples are shown relative to each other. DCR = (V0 - V1) / 0.5 C

[0081]

[0082] As in Examples 1 and 2, a cathode active material including a first cathode active material having a secondary particle shape formed by aggregation of primary particles and a second cathode active material having a single particle shape was used, the content of the second cathode active material was set to 10 mass % or more and less than 40 mass % with respect to the total mass of the cathode active material, and a heterocyclic compound including an electron-withdrawing group R containing oxygen and / or nitrogen and a heterocycle containing nitrogen and sulfur was added to the non-aqueous electrolyte, whereby a sufficient effect of reducing battery resistance could be obtained. On the other hand, as in Comparative Examples 2 to 7, when a cathode active material containing a first cathode active material having a secondary particle shape formed by aggregation of primary particles and a second cathode active material having a single particle shape was not used, when the content of the second cathode active material was not set to 10 mass % or more and less than 40 mass % with respect to the total mass of the cathode active material, or when a heterocyclic compound containing an electron-withdrawing group R containing oxygen and / or nitrogen and a heterocycle containing nitrogen and sulfur was not added to the non-aqueous electrolyte, the effect of reducing battery resistance could not be sufficiently obtained.

[0083] The present disclosure is 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 positive electrode has a positive electrode core and a positive electrode composite layer provided on the positive electrode core, the positive electrode composite layer includes a positive electrode active material, the positive electrode active material includes a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles, and a second positive electrode active material having a single particle shape, the content of the second positive electrode active material being 10 mass % or more and less than 40 mass % with respect to the total mass of the positive electrode active material, the non-aqueous electrolyte includes at least one heterocyclic compound including an electron-withdrawing group R and a heterocycle, the electron-withdrawing group R includes oxygen and / or nitrogen, and the heterocycle includes nitrogen and sulfur. Configuration 2: The nonaqueous electrolyte secondary battery according to Configuration 1, wherein the average particle size of the primary particles in the first positive electrode active material is 50 nm or more and 2 μm or less, the volume-based median diameter of the first positive electrode active material is 10 μm or more and 30 μm or less, and the volume-based median diameter of the second positive electrode active material is 2 μm or more and 10 μm or less.Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the content of the second positive electrode active material is 10 mass % or more and 35 mass % or less with respect to the total mass of the positive electrode active material.Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein, when the positive electrode mixture layer is divided into two equal halves in the thickness direction, the second positive electrode active material is contained in a larger amount in an upper half region on the outer surface side than in a lower half region on the positive electrode current collector side. Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the electron-withdrawing group R comprises at least one selected from the group consisting of a carbonyl group, a nitrile group, a sulfonyl group, an isocyanate group, an isothiocyanate group, and a hydroxy group, and the hydroxy group is bonded to a carbon constituting a saturated hydrocarbon group.Configuration 6: The nonaqueous electrolyte secondary battery according to Configuration 5, wherein the carbonyl group is contained in at least one selected from the group consisting of an aldehyde group, a ketone, an amide bond, an ester bond, and a carboxy group.Configuration 7: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the heterocycle comprises at least one selected from the group consisting of a thiazole ring, a thiomorpholine ring, and a thiazepine ring.Configuration 8: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein the heterocyclic compound comprises at least one selected from the group consisting of 4,5-dimethyl-1,3-thiazole-2-carbaldehyde, 4,5,6,7-tetrahydro-1,3-benzothiazole-2-carbaldehyde, 2-ethylthiomorpholine-4-carbaldehyde, and 2,3-dimethylthiomorpholine-4-carbaldehyde. Configuration 9: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 8, wherein the content of the heterocyclic compound in the nonaqueous electrolyte is 0.1% by mass or more and 3% by mass or less.

[0084] REFERENCE SIGNS LIST 10 nonaqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 battery case, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 protruding portion, 23 filter, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode core body, 31 positive electrode composite layer, 31a lower half region, 31b upper half region.

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode has a positive electrode core and a positive electrode composite layer provided on the positive electrode core, the positive electrode composite layer containing a positive electrode active material, the positive electrode active material including a first positive electrode active material having a secondary particle shape formed by aggregation of primary particles, and a second positive electrode active material having a single particle shape, the content of the second positive electrode active material being 10 mass% or more and less than 40 mass% with respect to the total mass of the positive electrode active material, the non-aqueous electrolyte containing at least one electron-withdrawing group R and a heterocycle, the electron-withdrawing group R containing oxygen and / or nitrogen, and the heterocycle containing nitrogen and sulfur.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the average particle size of the primary particles in the first positive electrode active material is 50 nm or more and 2 μm or less, the volume-based median diameter of the first positive electrode active material is 10 μm or more and 30 μm or less, and the volume-based median diameter of the second positive electrode active material is 2 μm or more and 10 μm or less.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the second positive electrode active material is 10 mass % or more and 35 mass % or less with respect to the total mass of the positive electrode active material.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode mixture layer is divided into two equal parts in the thickness direction, and the second positive electrode active material is contained in a larger amount in an upper half region on the outer surface side than in a lower half region on the positive electrode current collector side.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electron-withdrawing group R comprises at least one selected from the group consisting of a carbonyl group, a nitrile group, a sulfonyl group, an isocyanate group, an isothiocyanate group, and a hydroxy group, and the hydroxy group is bonded to a carbon constituting a saturated hydrocarbon group.

6. The nonaqueous electrolyte secondary battery according to claim 5, wherein the carbonyl group is contained in at least one selected from the group consisting of an aldehyde group, a ketone, an amide bond, an ester bond, and a carboxy group.

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the heterocycle includes at least one selected from the group consisting of a thiazole ring, a thiomorpholine ring, and a thiazepine ring.

8. The nonaqueous electrolyte secondary battery according to claim 1, wherein the heterocyclic compound includes at least one selected from the group consisting of 4,5-dimethyl-1,3-thiazole-2-carbaldehyde, 4,5,6,7-tetrahydro-1,3-benzothiazole-2-carbaldehyde, 2-ethylthiomorpholine-4-carbaldehyde, and 2,3-dimethylthiomorpholine-4-carbaldehyde.

9. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the heterocyclic compound in the nonaqueous electrolyte is 0.1% by mass or more and 3% by mass or less.

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