Non-aqueous electrolyte secondary batteries

Abstract

To provide a non-aqueous electrolyte secondary battery capable of suppressing a resistance increase in a case where charge / discharge is repeated.SOLUTION: A non-aqueous electrolyte secondary battery comprises a cathode, an anode, and a non-aqueous electrolyte. The anode includes an anode active material layer containing an anode active material. The anode active material contains a first anode active material particle of which the aspect ratio is 1.5 or less and a second anode active material particle of which the aspect ratio exceeds 1.5. A mass ratio of the first anode active material particle and the second anode active material particle ranges from 50:50 to 95:5. The second anode active material particles of 50% or more are oriented in such a manner that an angle between a surface direction of the anode active material layer and a long axis of the second anode active material particle becomes 30° or more. The non-aqueous electrolyte contains a non-aqueous solvent and electrolyte salt. The non-aqueous solvent contains carboxylic acid ester, of which the carbon number is 4 or less, in 1-30 vol%.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] A typical negative electrode of a non-aqueous electrolyte secondary battery includes a negative electrode active material layer containing a negative electrode active material. Known techniques for improving the characteristics of non-aqueous electrolyte secondary batteries include a technique using two or more types of negative electrode active material particles (see, for example, Patent Document 1) and a technique for orienting negative electrode active material particles with a high aspect ratio (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-129212 [Patent Document 2] Patent No. 6057124 Summary of the Invention [Problem to be solved by the invention]

[0005] In applications as a power source for driving a vehicle, there is an increasing demand for higher-capacity non-aqueous electrolyte secondary batteries from the viewpoint of extending the vehicle's driving range. The inventors have found that, when the density of the negative electrode active material layer is increased or the dimensions of the negative electrode active material layer are increased in order to increase the capacity of the non-aqueous electrolyte secondary battery, the increase in resistance during repeated charge and discharge of the non-aqueous electrolyte secondary battery is likely to be significant. Therefore, in order to meet the demand for higher capacity non-aqueous electrolyte secondary batteries, it is necessary to develop a new technology that can suppress the increase in resistance during repeated charge and discharge.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary battery that can suppress an increase in resistance during repeated charge and discharge. [Means for solving the problem]

[0007] The nonaqueous electrolyte secondary battery disclosed herein comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode comprises a negative electrode active material layer containing a negative electrode active material. The negative electrode active material includes first negative electrode active material particles having an aspect ratio of 1.5 or less and second negative electrode active material particles having an aspect ratio of more than 1.5. The mass ratio of the first negative electrode active material particles to the second negative electrode active material particles is 50:50 to 95:5. At least 50% of the second negative electrode active material particles are oriented such that the angle between the plane direction of the negative electrode active material layer and the major axis of the second negative electrode active material particles is 30° or more. The nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte salt. The nonaqueous solvent contains 1 to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms.

[0008] With this configuration, it is possible to provide a nonaqueous electrolyte secondary battery that can suppress an increase in resistance when repeatedly charged and discharged. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view schematically showing the internal structure of a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 2] 1 is a schematic exploded view showing the configuration of a wound electrode body of a lithium ion secondary battery according to one embodiment of the present invention. [Figure 3] FIG. 4 is a schematic cross-sectional view of a negative electrode for illustrating the angle between the surface direction of a negative electrode active material layer and the major axis of a second negative electrode active material particle. [Figure 4] 1 is a cross-sectional SEM image of a negative electrode of a lithium ion secondary battery for evaluation in Example 1 before press processing. [Figure 5] 1 is a cross-sectional SEM image of a negative electrode of a lithium ion secondary battery for evaluation according to Example 1 after being subjected to a press treatment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for implementing the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In this specification, a numerical range expressed as "A to B" includes A and B.

[0011] In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

[0012] The present invention will be described in detail below using as an example a flat prismatic lithium ion secondary battery having a flat wound electrode body and a flat battery case, but it is not intended that the present invention be limited to the embodiment described above.

[0013] The lithium-ion secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat wound electrode assembly 20 and a nonaqueous electrolyte 80 in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, as well as a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 also has an inlet (not shown) for injecting the nonaqueous electrolyte 80. The positive terminal 42 is electrically connected to a positive current collector plate 42a. The negative terminal 44 is electrically connected to a negative current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum. Note that FIG. 1 does not accurately represent the amount of nonaqueous electrolyte 80.

[0014] As shown in Figures 1 and 2, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62. The positive electrode active material layer-free portion 52a (i.e., a portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer-free portion 62a (i.e., a portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20. The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

[0015] A load may be applied to the wound electrode body 20 in the thickness direction thereof. The load is preferably 0.1 kN / cm 2 More preferably, it is 0.4 kN / cm or more. 2That's all. A load in this range is particularly advantageous when used as a driving power source for an HEV. This load can be a restraint load applied to an assembled battery when the lithium-ion secondary battery 100 is configured as a battery.

[0016] The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current collector used in lithium ion secondary batteries, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0017] The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0018] The positive electrode active material layer 54 contains a positive electrode active material. The positive electrode active material may be a known positive electrode active material used in lithium-ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0019] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, etc. These positive electrode active materials may be used alone or in combination of two or more.

[0020] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0021] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0022] As the positive electrode active material, a lithium nickel cobalt manganese based composite oxide is particularly preferable.

[0023] The average particle diameter (median diameter: D50) of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm to 25 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 15 μm. The average particle diameter (D50) of the positive electrode active material can be determined, for example, by a laser diffraction scattering method.

[0024] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, etc. Suitable conductive materials include carbon materials such as carbon black (e.g., acetylene black (AB)), carbon nanotubes, and graphite. Suitable binders include polyvinylidene fluoride (PVdF), etc.

[0025] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass to 97% by mass, and even more preferably 85% by mass to 96% by mass. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 2% by mass to 12% by mass. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 3% by mass to 13% by mass. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, and more preferably 1.5% by mass to 10% by mass.

[0026] The thickness of the positive electrode active material layer 54 is not particularly limited, but is, for example, 10 μm or more and 400 μm or less, and preferably 20 μm or more and 300 μm or less.

[0027] The positive electrode sheet 50 may have an insulating layer (not shown) at the boundary between the positive electrode active material layer non-forming portion 52a and the positive electrode active material layer 54. The insulating layer contains, for example, ceramic particles.

[0028] The negative electrode current collector 62 constituting the negative electrode sheet 60 may be a known negative electrode current collector used in lithium ion secondary batteries, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil is preferred as the negative electrode current collector 62.

[0029] The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 6 μm to 20 μm.

[0030] The negative electrode active material layer 64 contains a negative electrode active material. In this embodiment, the negative electrode active material includes first negative electrode active material particles and second negative electrode active material particles having different aspect ratios.

[0031] Examples of materials for the first negative electrode active material particles and the second negative electrode active material particles include carbon materials such as graphite, hard carbon, and soft carbon, with graphite being preferred. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material. As graphite, natural graphite is preferred, and amorphous carbon-coated graphite in which natural graphite is coated with an amorphous carbon material is more preferred.

[0032] The aspect ratio of the first negative electrode active material particles is 1.5 or less, and the aspect ratio of the second negative electrode active material particles is greater than 1.5. Therefore, in this embodiment, first negative electrode active material particles with a low aspect ratio and second negative electrode active material particles with a high aspect ratio are used. Therefore, the first negative electrode active material particles have a spherical or nearly spherical shape, and the second negative electrode active material particles have a long axis, so they are easily oriented.

[0033] The most common graphite used in lithium ion secondary batteries is flake graphite, and the aspect ratio of flake graphite is usually at least 2. Therefore, flake graphite may be used as the second negative electrode active material particles.

[0034] On the other hand, flake graphite cannot be used as it is for the first negative electrode active material particles. Therefore, the first negative electrode active material particles are typically made of flake graphite that has been subjected to a spheroidizing treatment (so-called spheroidized graphite). This spheroidizing treatment is well known, and a specific example of a spheroidizing treatment method is a method in which flake graphite is rolled while being impacted so that the individual particles adhere firmly to each other, and then granulated into spherical particles of several μm to several tens of μm. For such granulation treatment, devices such as a ball mill, a bead mill, a hybridization system manufactured by Nara Machinery Works, Ltd., a Nobilta manufactured by Hosokawa Micron Corporation, an FM mixer manufactured by Nippon Coke and Engineering Co., Ltd., or a composite manufactured by Nippon Coke and Engineering Co., Ltd. can be used. In this case, the aspect ratio can be adjusted by changing the treatment conditions (particularly, the treatment time, the number of treatments, etc.).

[0035] By subjecting the flake graphite used for the second negative electrode active material particles to the above-mentioned granulation treatment, it is also possible to adjust the aspect ratio within a range in which the aspect ratio exceeds 1.5.

[0036] In this embodiment, it is preferable that the first negative electrode active material particles are spherical graphite particles and the second negative electrode active material particles are scaly graphite particles, and it is more preferable that the first negative electrode active material particles are spherical natural graphite particles and the second negative electrode active material particles are scaly natural graphite particles.

[0037] The average aspect ratio of the first negative electrode active material particles is 1.50 or less, preferably 1.45 or less, more preferably 1.40 or less, even more preferably 1.30 or less, and particularly preferably 1.25 or less, while the average aspect ratio of the first negative electrode active material particles is 1.00 or more, and may be 1.10 or more.

[0038] The average aspect ratio of the second negative electrode active material particles is greater than 1.50, preferably 1.75 or more, more preferably 2.00 or more, even more preferably 2.20 or more, and particularly preferably 2.40 or more, while the average aspect ratio of the second negative electrode active material particles may be 10.00 or less, 7.00 or less, 5.00 or less, or 4.00 or less.

[0039] The aspect ratio of the negative electrode active material particles is the ratio of the negative electrode active material particle's major axis (or major side) to its minor axis (or short side), and can be determined by a known method. For example, it can be determined by capturing a cross-sectional electron microscope image of the negative electrode active material layer 64 using a scanning electron microscope (SEM) or the like, determining the lengths of the minor and major axes of the negative electrode active material particles in the image, and calculating the ratio (major axis / minor axis). The determination of the minor and major axes of the negative electrode active material particles and calculation of the ratio can be easily performed using image analysis software (e.g., "ImageJ"). The average aspect ratio of the first negative electrode active material particles can be determined by randomly selecting 25 or more negative electrode active material particles with an aspect ratio of 1.5 or less from the cross-sectional electron microscope image and calculating the average aspect ratio of these particles. The average aspect ratio of the second negative electrode active material particles can be determined by randomly selecting 25 or more negative electrode active material particles with an aspect ratio of more than 1.5 from a cross-sectional electron microscope image and calculating the average aspect ratio of these particles. This can also be easily done using image analysis software (e.g., "ImageJ").

[0040] The mass ratio of the first negative electrode active material particles to the second negative electrode active material particles (first negative electrode active material particles:second negative electrode active material particles) is 50:50 to 95:5. If the mass ratio is outside this range, the suppression of resistance increase when the lithium ion secondary battery 100 is repeatedly charged and discharged becomes insufficient. From the viewpoint of a higher effect of suppressing resistance increase, the mass ratio is preferably 50:50 to 82:18, more preferably 50:50 to 70:30, and even more preferably 50:50 to 65:35.

[0041] The average particle diameters (median diameter: D50) of the first and second negative electrode active material particles are not particularly limited, but are, for example, 0.1 μm to 50 μm, preferably 1 μm to 30 μm, and more preferably 5 μm to 20 μm. The average particle diameter (D50) of the negative electrode active material can be determined, for example, by a laser diffraction scattering method.

[0042] In this embodiment, 50% or more of the second negative electrode active material particles are oriented so that the angle between the surface direction of the negative electrode active material layer 64 and the major axis of the second negative electrode active material particles is 30° or more. Preferably, 50% or more of the second negative electrode active material particles are oriented so that the angle between the surface direction of the negative electrode active material layer 64 and the major axis of the second negative electrode active material particles is 45° or more. Preferably, 55% or more (particularly, 58% or more) of the second negative electrode active material particles are oriented so that the angle between the surface direction of the negative electrode active material layer 64 and the major axis of the second negative electrode active material particles is 30° or more. The technical significance of this will be described later.

[0043] This angle will be explained using FIG. 3. FIG. 3 is a schematic cross-sectional view of an anode 60, showing an anode current collector 62 and an anode active material layer 64. A first anode active material particle 66 and a second anode active material particle 68 are also shown within the anode active material layer 64. For ease of explanation, one first anode active material particle 66 and one second anode active material particle 68 are shown, but in reality, a plurality of these particles are contained in the anode active material layer 64. The second anode active material particle 68 has a long axis L. The plane direction of the anode active material layer 64 is the direction of the main surface of the anode active material layer 64, which is the direction of arrow P in FIG. 3. Here, the anode active material layer 64 has two (a pair of) main surfaces: an outer surface and a surface in contact with the anode current collector 62. Therefore, the plane direction P of the anode active material layer 64 may be the plane direction of the anode current collector 62. The angle θ between this arrow P and the major axis L is the angle between the surface direction of the negative electrode active material layer and the major axis of the second negative electrode active material particles, and an acute angle is adopted as this angle.

[0044] The above-described orientation state of the second negative electrode active material particles can be obtained, for example, by the following method: A slurry containing first negative electrode active material particles and second negative electrode active material particles in a mass ratio of 50:50 to 95:5 is prepared. This slurry is applied to a negative electrode current collector 62 and dried to form a negative electrode active material layer 64. The negative electrode active material layer 64 is then pressed to a thickness of 0.95 g / cm. 3 Above, especially 1.00 g / cm 3 Do this so that the result is as above.

[0045] Typically, negative electrode active material particles with a high aspect ratio are likely to be oriented along the surface direction of the negative electrode active material layer 64 during slurry coating, and further oriented along the surface direction of the negative electrode active material layer 64 by pressing. Therefore, for most negative electrode active material particles, the angle between the surface direction of the negative electrode active material layer 64 (i.e., the direction of the main surface) and the long axis of the negative electrode active material particle is typically less than 30°.

[0046] However, the presence of an appropriate amount of the first negative electrode active material particles with a low aspect ratio creates appropriate spaces between the negative electrode active material particles after the slurry is applied (i.e., before the negative electrode active material layer is pressed). The pressing process then causes the negative electrode active material particles to move so as to fill these spaces, resulting in an orientation state in which the second negative electrode active material particles are tilted relative to the in-plane direction of the negative electrode active material layer 64. In this way, a state is achieved in which 50% or more of the second negative electrode active material particles are oriented such that the angle between the in-plane direction of the negative electrode active material layer 64 and the major axis of the second negative electrode active material particles is 30° or greater (for reference, scanning electron microscope (SEM) images of the cross sections of the negative electrode active material layer before and after the pressing process in Example 1, which will be described later, are shown in FIGS. 4 and 5, respectively).

[0047] The orientation state of the second active material particles can be confirmed as follows. A cross-sectional electron microscope image of the negative electrode active material layer 64 is taken using a scanning electron microscope (SEM) or the like, and the aspect ratio of each negative electrode active material particle is calculated. This allows second negative electrode active material particles with an aspect ratio of more than 1.5 to be identified. Note that the aspect ratio can be easily calculated using image analysis software (e.g., "ImageJ" or the like).

[0048] Next, the angle between the surface direction of the negative electrode active material layer 64 and the major axis of the second negative electrode active material particles is determined. This angle can also be easily determined using image analysis software (e.g., "ImageJ"). The surface direction of the negative electrode active material layer 64 usually coincides with the surface direction of the negative electrode current collector 62. When the surface direction of the negative electrode active material layer 64 coincides with the surface direction of the negative electrode current collector 62, a cross-sectional electron microscope image of the negative electrode active material layer 64 is obtained so that the negative electrode current collector 62 is included, and the surface direction of the negative electrode current collector 62 can be determined. This makes it easy to identify the surface direction of the negative electrode active material layer 64 and measure the degree of orientation.

[0049] The angle is measured for 20 or more randomly selected second negative electrode active material particles. The number of second negative electrode active material particles oriented so that the angle is 30° or more is counted, and the percentage (%) is calculated by (number of second negative electrode active material particles with the angle of 30° or more / number of second negative electrode active material particles for which the angle is measured)×100.

[0050] The negative electrode active material layer 64 may contain components other than the active material, such as a binder, a thickener, etc. Examples of binders that may be used include styrene butadiene rubber (SBR) and polyvinylidene fluoride (PVdF). Examples of thickeners that may be used include carboxymethyl cellulose (CMC).

[0051] The content of the negative electrode active material in the negative electrode active material layer 64 is preferably 90% by mass or more, and more preferably 95% by mass or more and 99% by mass or less. The content of the binder in the negative electrode active material layer 64 is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer 64 is preferably 0.3% by mass or more and 3% by mass or less, and more preferably 0.5% by mass or more and 2% by mass or less.

[0052] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 400 μm or less, and preferably 20 μm or more and 300 μm or less.

[0053] In this embodiment, there is no particular limitation on the apparent density (so-called negative electrode density) of the negative electrode active material layer 64. Since the second negative electrode active material particles are oriented by the pressure applied during pressing when the negative electrode 60 is produced, the apparent density of the negative electrode active material layer 64 is usually 0.95 g / cm 3 or more, preferably 1.00 g / cm 3 More preferably, it is 1.10 g / cm or more. 3 More preferably, 1.30 g / cm 3 The apparent density of the negative electrode active material layer 64 is 2.00 g / cm 3 Below 1.70g / cm 3 or less than 1.50g / cm 3 A larger apparent density is advantageous for use as a power source for driving a vehicle (particularly for use as a power source for driving an HEV).

[0054] The apparent density of the negative electrode active material layer 64 is the apparent volume (cm ) of the negative electrode active material layer 64 including the voids. 3 The ratio of the weight (g) of the negative electrode active material layer 64 to the weight (g) of the negative electrode active material layer 64 can be easily calculated by measuring the basis weight of the negative electrode active material layer 64 and the thickness of the negative electrode active material layer 64, for example, and expressing the ratio as the basis weight of the negative electrode active material layer 64 / the thickness of the negative electrode active material layer 64.

[0055] The weight of the negative electrode active material layer 64 is not particularly limited, but is preferably 4 mg / cm 2 More preferably, it is 8 mg / cm or more.2 More preferably, it is 10 mg / cm or more. 2 More preferably, it is 20 mg / cm or more. 2 The weight of the negative electrode active material layer 64 is 50 mg / cm 2 or less, or 40 mg / cm 2 A larger weight per unit area is advantageous for use as a power source for driving a vehicle (particularly for use as a power source for driving an HEV).

[0056] Examples of separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such porous sheets may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) containing ceramic particles or the like may be provided on the surface of separator 70.

[0057] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability of the separator 70 measured by the Gurley test method is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0058] The non-aqueous electrolyte solution 80 contains a non-aqueous solvent and an electrolyte salt (also called a supporting salt). In this embodiment, the non-aqueous solvent contains 1% to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms.

[0059] A carboxylic acid ester having four or fewer carbon atoms acts to reduce the viscosity of the non-aqueous electrolyte solution 80. Therefore, if the carboxylic acid ester has five or more carbon atoms, the viscosity of the non-aqueous electrolyte solution 80 will not be sufficiently reduced. Examples of carboxylic acid esters having four or fewer carbon atoms include methyl acetate, ethyl acetate, vinyl acetate, and methyl propionate. Of these, methyl acetate is preferred because it has a particularly high viscosity-reducing effect on the non-aqueous electrolyte solution 80.

[0060] The non-aqueous solvent includes organic solvents other than carboxylic acid esters. Examples of such organic solvents include carbonates, ethers, nitriles, sulfones, lactones, etc., and among these, carbonates are preferred. Examples of carbonates include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), etc. Such organic solvents can be used alone or in appropriate combination of two or more.

[0061] From the viewpoint of achieving a higher resistance increase suppression effect, the non-aqueous solvent preferably contains 15 to 30 volume % of a carboxylic acid ester having 4 or less carbon atoms. The non-aqueous solvent preferably contains a carboxylic acid ester having 4 or less carbon atoms and a carbonate, or may contain only a carboxylic acid ester having 4 or less carbon atoms and a carbonate. The carbonate preferably contains both a chain carbonate and a cyclic carbonate.

[0062] As the electrolyte salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be suitably used. The concentration of the electrolyte salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0063] The nonaqueous electrolyte solution 80 may contain components other than the nonaqueous solvent and the electrolyte salt (hereinafter also referred to as optional components). Suitable examples of the optional components include film-forming agents such as vinylene carbonate and oxalato complexes. Of these, oxalato complexes are preferred, and examples thereof include lithium bis(oxalato)borate (LiBOB) and lithium difluorooxalatoborate (LiDFOB). By including the film-forming agent in the nonaqueous electrolyte solution 80, the durability of the lithium ion secondary battery 100 can be improved. The concentration of the film-forming agent in the nonaqueous electrolyte solution 80 is not particularly limited, but is, for example, 0.05% by mass to 1.2% by mass, and preferably 0.1% by mass to 1.0% by mass.

[0064] Other optional components include various additives such as thickeners and gas generating agents (eg, biphenyl (BP), cyclohexylbenzene (CHB), etc.).

[0065] As described above, the negative electrode active material contains first negative electrode active material particles having an aspect ratio of 1.5 or less and second negative electrode active material particles having an aspect ratio of more than 1.5 in a mass ratio of 50:50 to 95:5, 50% or more of the second negative electrode active material particles are oriented such that the angle between the plane direction of the negative electrode active material layer 64 and the major axis of the second negative electrode active material particles is 30° or more, and the non-aqueous solvent of the non-aqueous electrolyte solution 80 contains 1% by volume to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms, thereby making it possible to suppress an increase in resistance when the lithium ion secondary battery 100 is repeatedly charged and discharged. The reason for this will be explained below.

[0066] When the lithium ion secondary battery 100 is repeatedly charged and discharged, expansion and contraction of the negative electrode active material causes the nonaqueous electrolyte solution 80 to flow out and in from the ends of the wound electrode body 20. This repeated flow-out and flow-in of the nonaqueous electrolyte solution 80 causes unevenness in the concentration of the electrolyte salt within the wound electrode body 20, which causes an increase in resistance.

[0067] In this embodiment, 50% or more of the second negative electrode active material particles are oriented such that the angle between the surface direction of the negative electrode active material layer 64 and the major axis of the second negative electrode active material particles is 30° or more (see the SEM image in FIG. 5). Therefore, the second negative electrode active material particles make it difficult for the nonaqueous electrolyte solution 80 to move in the surface direction of the negative electrode active material layer 64 (the left-right direction in the SEM image in FIG. 5), thereby making it possible to suppress the outflow of the nonaqueous electrolyte solution 80 from the end of the wound electrode body 20. Note that this orientation state is obtained when the negative electrode active material contains first negative electrode active material particles having an aspect ratio of 1.5 or less and second negative electrode active material particles having an aspect ratio of more than 1.5 in a mass ratio of 50:50 to 95:5.

[0068] On the other hand, in this embodiment, the nonaqueous solvent of the nonaqueous electrolyte solution 80 contains 1% by volume to 30% by volume of a carboxylic acid ester having four or less carbon atoms, thereby reducing the viscosity of the nonaqueous electrolyte solution 80. Therefore, the nonaqueous electrolyte solution 80 discharged from the wound electrode body 20 is likely to return into the wound electrode body 20 even when the negative electrode active material layer 64 is densified.

[0069] As a result, the impregnation state of the nonaqueous electrolyte solution 80 throughout the wound electrode body 20 is improved, and the occurrence of uneven electrolyte salt concentration within the wound electrode body 20 is suppressed. Therefore, an increase in resistance when the lithium ion secondary battery 100 is repeatedly charged and discharged is suppressed.

[0070] As explained above, the lithium ion secondary battery 100 can suppress an increase in resistance when repeatedly charged and discharged. The lithium ion secondary battery 100 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices and the like.

[0071] The lithium-ion secondary battery 100 can achieve high input / output performance over a long period of time because the increase in resistance during repeated charge / discharge is suppressed. Therefore, the lithium-ion secondary battery 100, which has excellent input / output characteristics, is suitable as a power source for driving vehicles, which require high power generation efficiency, and is particularly suitable for use as a power source for driving HEVs. The lithium-ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of a plurality of batteries connected in series and / or parallel.

[0072] The above describes, as an example, a rectangular lithium ion secondary battery 100 equipped with a flat wound electrode assembly 20. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery equipped with a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are stacked alternately). The lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery, a laminated case lithium ion secondary battery, etc.

[0073] The secondary battery according to this embodiment can be constructed as a non-aqueous electrolyte secondary battery other than a lithium ion secondary battery according to a known method.

[0074] Examples of the present invention will be described in detail below, but it is not intended that the present invention be limited to those shown in these examples.

[0075] [Examples 1 to 10 and Comparative Examples 1 to 3] <Preparation of Lithium-ion Secondary Batteries for Evaluation> Spherical natural graphite with an average aspect ratio of 1.2 was prepared as the first negative electrode active material particles. Scale-like natural graphite with an average aspect ratio of 2.5 was prepared as the second negative electrode active material particles. The first negative electrode active material particles and the second negative electrode active material particles were coated with amorphous carbon. The first negative electrode active material particles and the second negative electrode active material particles were mixed in the mass ratio shown in Table 1 to prepare the negative electrode active material.

[0076] The negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed with ion-exchanged water in a mass ratio of active material:SBR:CMC=99:0.5:0.5 to prepare a negative electrode slurry. This slurry was applied in strips to both sides of a long copper foil with a thickness of 8 μm, dried, and then pressed to produce a negative electrode sheet. The basis weight of the negative electrode active material layer per side was 4 mg / cm. 2 The pressing process was carried out so that the negative electrode density (apparent density of the negative electrode active material layer) was 1.0 g / cm 3 The above was carried out.

[0077] LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (LNCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed with N-methylpyrrolidone (NMP) in a mass ratio of LNCM:AB:PVdF = 92:5:3 to prepare a positive electrode slurry. This slurry was applied in strips to both sides of a 15 μm thick long aluminum foil, dried, and then pressed to prepare a positive electrode sheet.

[0078] A separator was prepared using a porous polyolefin sheet (20 μm thick) with a three-layer structure of PP / PE / PP, on which an HRL (4 μm thick) was provided. The positive electrode sheet, negative electrode sheet, and two of the separator sheets prepared above were stacked and wound to produce a wound electrode assembly. At this time, the HRL of the separator faced the positive electrode sheet.

[0079] A mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and a carboxylic acid ester in a volume ratio of 30:30:40-x:x was prepared. As shown in Table 1, methyl acetate (MA) or methyl propionate (MP) was used as the carboxylic acid ester, with the value of x (volume ratio; also volume %) shown in Table 1. Lithium bis(oxalate)borate was dissolved in this mixed solvent at a concentration of 1.0 mass %, and LiPF6 was dissolved as the electrolyte salt at a concentration of 1.0 mol / L. This produced a nonaqueous electrolyte solution.

[0080] Terminals were attached to the electrode body produced above, and the electrode body was housed in a battery case together with a non-aqueous electrolyte solution and sealed to obtain a lithium ion secondary battery for evaluation.

[0081] <Evaluation of the orientation of second active material particles> The negative electrode of each evaluation lithium-ion secondary battery was subjected to ion milling to prepare a cross-sectional sample of the negative electrode active material layer. These samples were then observed using a scanning electron microscope (SEM) to obtain cross-sectional SEM images. The cross-sectional SEM images included the copper foil (negative electrode current collector). The image analysis software "ImageJ" was used to measure the aspect ratios of the negative electrode active material particles in the cross-sectional SEM images, identifying first negative electrode active material particles with an aspect ratio of 1.5 or less and second negative electrode active material particles with an aspect ratio of more than 1.5. The average aspect ratios of the first and second negative electrode active material particles were then calculated using the image analysis software "ImageJ."

[0082] Next, the image analysis software "ImageJ" was used to determine the angle between the surface direction of the negative electrode active material layer and the long axis of the second negative electrode active material particle for 20 or more randomly selected second negative electrode active material particles. At this time, the surface direction of the copper foil in the cross-sectional SEM image was used to determine the surface direction of the copper foil as the surface direction of the negative electrode active material layer.

[0083] The number of second negative electrode active material particles oriented at this angle of 30° or more was counted, and the percentage (%) was calculated by (number of second negative electrode active material particles with the angle of 30° or more / number of second negative electrode active material particles for which the angle was measured) × 100. The values ​​are shown in Table 2.

[0084] <Activation process> Each of the lithium-ion secondary batteries for evaluation prepared above was placed in a thermostatic chamber at 25° C. Each of the lithium-ion secondary batteries for evaluation was charged at a constant current of 0.3 C up to 4.10 V. Thereafter, each of the lithium-ion secondary batteries was discharged at a constant current of 0.3 C down to 3.00 V.

[0085] <Initial characteristic evaluation> Each activated lithium-ion secondary battery for evaluation was charged at a constant current of 0.2 C to 4.10 V, and then charged at a constant voltage until the current reached 1 / 50 C, thereby fully charging the battery. The battery was then discharged at a constant current of 0.2 C to 3.00 V. The discharge capacity at this time was measured and used as the initial capacity.

[0086] With the initial capacity defined as SOC 100%, each evaluation lithium-ion secondary battery was charged at a current of 1 C in a thermostatic chamber at 25°C until the SOC reached 60%. Next, charging was performed at a current of 30 C for 10 seconds, and the voltage drop ΔV at this time was obtained. The initial resistance of each evaluation lithium-ion secondary battery was calculated using this voltage drop ΔV and the current value.

[0087] <Cycle characteristic evaluation> Each lithium-ion secondary battery for evaluation was charged at a current of 1 C in a thermostatic chamber at 25°C until the SOC reached 60%. Next, 2000 charge-discharge cycles were performed, each cycle consisting of charging at a current of 30 C for 10 seconds, pausing for 5 seconds, discharging at a current of 3 C for 100 seconds, and pausing for 5 seconds. The battery resistance after 2000 cycles was then measured using the same method as for measuring the initial resistance. The resistance increase rate (%) was calculated by multiplying (battery resistance after 2000 charge-discharge cycles / initial resistance) by 100. The results are shown in Table 1.

[0088] [Table 1]

[0089] As shown in the results in Table 1, when the negative electrode active material contains first negative electrode active material particles having an aspect ratio of 1.5 or less and second negative electrode active material particles having an aspect ratio of more than 1.5 in a mass ratio of 50:50 to 95:5, when 50% or more of the second negative electrode active material particles are oriented such that the angle between the plane direction of the negative electrode active material layer and the major axis of the second negative electrode active material particles is 30° or more, and when the nonaqueous solvent of the nonaqueous electrolyte contains 1% by volume to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms, the increase in resistance during repeated charge and discharge of the lithium ion secondary battery is small. These results demonstrate that the nonaqueous electrolyte secondary battery disclosed herein can suppress the increase in resistance during repeated charge and discharge.

[0090] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0091] That is, the nonaqueous electrolyte secondary battery disclosed herein has the following features [1] to [7]. [1] A positive electrode, a negative electrode; a nonaqueous electrolyte; A non-aqueous electrolyte secondary battery comprising: the negative electrode includes a negative electrode active material layer containing a negative electrode active material, the negative electrode active material includes first negative electrode active material particles having an aspect ratio of 1.5 or less and second negative electrode active material particles having an aspect ratio of more than 1.5; a mass ratio of the first negative electrode active material particles to the second negative electrode active material particles is 50:50 to 95:5; 50% or more of the second negative electrode active material particles are oriented such that the angle between the surface direction of the negative electrode active material layer and the major axis of the second negative electrode active material particles is 30° or more, The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt, The non-aqueous electrolyte secondary battery comprises the non-aqueous solvent containing 1 to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms. [2] The nonaqueous electrolyte secondary battery according to item [1], wherein the first negative electrode active material particles have an average aspect ratio of 1.0 to 1.4, and the second negative electrode active material particles have an average aspect ratio of 2.0 to 7.0. [3] The nonaqueous electrolyte secondary battery according to item [1] or [2], wherein the first negative electrode active material particles are spherical natural graphite particles, and the second negative electrode active material particles are scaly natural graphite particles. [4] The nonaqueous electrolyte secondary battery according to any one of items [1] to [3], wherein a mass ratio of the first negative electrode active material particles to the second negative electrode active material particles is 50:50 to 65:45. [5] The nonaqueous electrolyte secondary battery according to any one of items [1] to [4], wherein the carboxylic acid ester is methyl acetate. [6] The nonaqueous electrolyte secondary battery according to any one of items [1] to [5], wherein the nonaqueous solvent contains the carboxylic acid ester in an amount of 15 to 30% by volume. [7] The nonaqueous electrolyte secondary battery according to any one of items [1] to [6], which is used as a vehicle driving power source for a hybrid vehicle. [Explanation of symbols]

[0092] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator sheet (separator) 80 Nonaqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. A positive electrode and a negative electrode; a nonaqueous electrolyte; A non-aqueous electrolyte secondary battery comprising: the negative electrode includes a negative electrode active material layer containing a negative electrode active material, the negative electrode active material includes first negative electrode active material particles having an average aspect ratio of 1.0 to 1.4 and second negative electrode active material particles having an average aspect ratio of 2.0 to 7.0; a mass ratio of the first negative electrode active material particles to the second negative electrode active material particles is 50:50 to 95:5; 50% or more of the second negative electrode active material particles are oriented such that the angle between the surface direction of the negative electrode active material layer and the major axis of the second negative electrode active material particles is 30° or more, The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt, The non-aqueous solvent contains 1 to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first negative electrode active material particles are spherical natural graphite particles, and the second negative electrode active material particles are scaly natural graphite particles.

3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a mass ratio of the first negative electrode active material particles to the second negative electrode active material particles is 50:50 to 65:

35.

4. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the carboxylic acid ester is methyl acetate.

5. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous solvent contains 15 to 30% by volume of the carboxylic acid ester.

6. 10. The nonaqueous electrolyte secondary battery according to claim 1, which is used as a vehicle driving power source for a hybrid vehicle.

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