Non-aqueous electrolyte secondary batteries

Abstract

To provide a non-aqueous electrolyte secondary battery having excellent input characteristics and high-temperature storage characteristics.SOLUTION: A non-aqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a non-aqueous electrolyte solution. The negative electrode includes a negative electrode active material layer containing a negative electrode active material. The negative electrode active material includes carbon material particles having an average circularity of 0.86 or more. The negative electrode active material layer has an apparent density of 1.30 g / cm3 to 1.69 g / cm3. The non-aqueous electrolyte solution includes a non-aqueous solvent and an electrolyte salt. The non-aqueous solvent contains 1 vol.% to 30 vol.% of a carboxylate having 4 or less carbon atoms.SELECTED DRAWING: Figure 1

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] In applications as a power source for driving vehicles, there is an ever-increasing demand for higher input power and higher capacity for non-aqueous electrolyte secondary batteries. Regarding the increase in capacity of non-aqueous electrolyte secondary batteries, it is known that the battery capacity per volume can be increased by increasing the negative electrode density (i.e., the apparent density of the negative electrode active material layer) (see, for example, Patent Document 1). Patent Document 1 describes a negative electrode density of 1.3 to 1.7 g / cm 3 It is stated that this will be done. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-120217 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as a result of intensive research, the present inventors have found the following problem. When a non-aqueous electrolyte secondary battery is used as a driving power source for an HEV, the opportunity to charge the battery is limited to when the HEV is generating power while it is running. Therefore, from the perspective of further extending the driving range of an HEV, further improvement in the input characteristics of non-aqueous electrolyte secondary batteries is desired. In contrast, the input characteristics of non-aqueous electrolyte secondary batteries in the above-mentioned conventional technologies are still insufficient. Furthermore, in the above-mentioned conventional technologies, the capacity of non-aqueous electrolyte secondary batteries deteriorates significantly when they are left at high temperatures (for example, around 60°C) for long periods of time.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary battery that is excellent in input characteristics and high-temperature storage characteristics. [Means for solving the problem]

[0007] The nonaqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode includes a negative electrode active material layer containing a negative electrode active material. The negative electrode active material includes carbon material particles having an average circularity of 0.86 or more. The negative electrode active material layer has an apparent density of 1.30 g / cm. 3 ~1.69g / cm 3 The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt. The non-aqueous solvent contains 1% by volume 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 is excellent in both input characteristics and high-temperature storage characteristics. [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. 10 is a schematic cross-sectional view of a negative electrode for illustrating an example in which the carbon material particles have a low average circularity. [Figure 4] FIG. 10 is a schematic cross-sectional view of a negative electrode for illustrating an example in which the carbon material particles have a high average circularity. 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, at least carbon material particles having an average circularity of 0.86 or more are used as the negative electrode active material. Examples of carbon materials constituting the particles include 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 the graphite, amorphous carbon-coated graphite in which natural graphite is coated with an amorphous carbon material is particularly preferred.

[0031] Preferably 75% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass of the negative electrode active material are carbon material particles having an average circularity of 0.86 or more.

[0032] In this specification, circularity refers to the ratio of the perimeter of a perfect circle having the same area as the projected area of ​​the particle to the perimeter of the projected image of the particle (i.e., circularity = perimeter of a perfect circle having the same area as the projected area of ​​the particle / perimeter of the projected image of the particle). Therefore, the closer the circularity is to 1, the closer the projected image of the particle is to a perfect circle, and the closer the particle is to a perfect sphere. The average circularity can be determined, for example, by using a commercially available static automatic image analyzer (e.g., the Morphologi series manufactured by Malvern Panalytical) to determine the circularity of 1,000 or more carbon material particles and calculating the average value.

[0033] The average circularity of the carbon material particles is preferably 0.87 or more, more preferably 0.88 or more, and even more preferably 0.89 or more.

[0034] The carbon material particles used in this embodiment have a higher average circularity than that of general flake graphite. Therefore, carbon material particles with such a high average circularity can typically be obtained by subjecting flake graphite to a spheroidizing treatment according to a known method. As an example, carbon material particles with such a high average circularity can be obtained by granulating flake graphite into spherical particles. Specifically, for example, flake graphite is rolled and impacted to firmly adhere to each other, and granulation is performed until spherical particles of several μm to several tens of μm are obtained. For such granulation, 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. The average circularity can be adjusted by changing the treatment conditions (e.g., treatment time, number of treatments, etc.).

[0035] The average particle diameter (median diameter: D50) of the negative electrode active material is not particularly limited, but is, for example, 0.1 μm to 50 μm, preferably 1 μm to 25 μm, more preferably 5 μm to 20 μm, and even more preferably 7 μm to 15 μm. The average particle diameter (D50) of the negative electrode active material can be determined, for example, by a laser diffraction scattering method.

[0036] 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).

[0037] 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.

[0038] 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.

[0039] In this embodiment, the apparent density of the negative electrode active material layer 64 (so-called negative electrode density) is 1.30 g / cm 3 ~1.69g / cm 3 The apparent density is 1.69 g / cm 3 If the apparent density exceeds 1.30 g / cm, the effects of improving input characteristics and high-temperature storage characteristics cannot be obtained. 3 ~1.50g / cm 3 Moreover, from the viewpoint of particularly high input characteristics and particularly high high-temperature storage characteristics, the apparent density is preferably 1.30 g / cm. 3 ~1.40g / cm 3 The apparent density of the negative electrode active material layer 64 can be increased by pressing the negative electrode active material layer 64 when the negative electrode 60 is produced, and can be adjusted by changing the conditions of the pressing process.

[0040] 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 calculating the ratio by dividing the basis weight of the negative electrode active material layer 64 by the thickness of the negative electrode active material layer 64.

[0041] The weight of the negative electrode active material layer 64 is not particularly limited, but is preferably 8.5 mg / cm 2 More preferably, it is 10 mg / cm or more. 2 More preferably, it is 15 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 2or less. Such a weight is particularly advantageous when used as a driving power source for HEVs. The above weight is the weight of the negative electrode active material layer 64 per side of the negative electrode current collector 62.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] From the viewpoint of higher input characteristics, the non-aqueous solvent preferably contains 3% by volume to 30% by volume, more preferably 6% by volume to 30% by volume, and even more preferably 15% by volume to 30% by 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.

[0048] 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.

[0049] 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 a film-forming agent in the nonaqueous electrolyte solution 80, the high-temperature storage characteristics of the lithium-ion secondary battery 100 can be further 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.

[0050] Other optional components include various additives such as thickeners and gas generating agents (eg, biphenyl (BP), cyclohexylbenzene (CHB), etc.).

[0051] As described above, the negative electrode active material layer 64 contains carbon material particles having an average circularity of 0.86 or more, and the apparent density of the negative electrode active material layer 64 is 1.30 g / cm 3 ~1.69g / cm 3 By including 1% by volume to 30% by volume of a carboxylic acid ester having four or less carbon atoms in the nonaqueous solvent of nonaqueous electrolyte solution 80, it is possible to significantly improve the input characteristics and high-temperature storage characteristics of lithium ion secondary battery 100. The reason for this will be explained below with reference to FIGS. 3 and 4.

[0052] FIG. 3 shows a schematic cross-sectional view of an example of a negative electrode in which the average circularity of the carbon material particles of the negative electrode active material is low (i.e., the average circularity is less than 0.86). As shown in FIG. 3, when the negative electrode density is high, the pressing process for increasing the density orients the long axes of the carbon material particles 166 so that they are parallel to the negative electrode current collector 162. In this case, lithium ions (Li +) into the negative electrode active material layer 164. This reduces and narrows the gaps through which the non-aqueous electrolyte solution containing Li ions penetrates into the negative electrode active material layer 164. This makes it difficult for the non-aqueous electrolyte solution to penetrate into the negative electrode active material layer 164. In addition, as shown by the arrow in FIG. 3, the negative electrode active material layer 164 is filled with Li ions at the deepest part (i.e., in the vicinity of the negative electrode current collector 162). + The path to reach it becomes longer.

[0053] In contrast, FIG. 4 shows a cross-sectional view of an example of a negative electrode in which the circularity of the carbon material of the negative electrode active material is high (i.e., 0.86 or more). When the circularity of the carbon material particles 66 is high, lithium ions (Li + ) into the negative electrode active material layer 64, and the negative electrode active material layer 64 has a large number of gaps through which the non-aqueous electrolyte solution containing Li + The path to reach the target is also short, improving the input characteristics.

[0054] However, when the negative electrode active material layer 64 is densified, the improvement in input characteristics becomes insufficient, and the high-temperature storage characteristics also become insufficient. The inventors have found that this is because the gaps between the carbon material particles 66 become narrower, resulting in the Li + The inventors have discovered that this is because the non-aqueous electrolyte solution 80 containing the above-mentioned component is less likely to permeate the negative electrode active material layer 64.

[0055] Therefore, in this embodiment, the viscosity of the nonaqueous electrolyte solution 80 is reduced by adding a predetermined amount of a carboxylic acid ester having four or less carbon atoms. Therefore, even when the negative electrode active material layer 64 is densified and the gaps between the carbon material particles 66 are narrow, the nonaqueous electrolyte solution 80 easily enters the gaps and can sufficiently impregnate the negative electrode active material layer 64. As a result, the input characteristics of the lithium-ion secondary battery 100 can be further improved. Furthermore, by allowing the nonaqueous electrolyte solution 80 to sufficiently impregnate the negative electrode active material layer 64, a uniform coating can be formed over the entire negative electrode active material layer 64, thereby suppressing capacity degradation during long-term storage at high temperatures. In other words, the high-temperature storage characteristics can be improved. Furthermore, since the carbon material particles 66 have a high circularity and are substantially spherical in shape, the coating formed on the surfaces of the carbon material particles 66 is also uniform. This further improves the high-temperature storage characteristics. In other words, the input characteristics and high-temperature storage characteristics of the lithium-ion secondary battery 100 can be significantly improved. The above coating is formed by the decomposition of the non-aqueous electrolyte solution 80, but if the non-aqueous electrolyte solution 80 contains a coating-forming agent, the quality of the coating is improved, and the high-temperature storage characteristics can be further enhanced.

[0056] As explained above, the lithium ion secondary battery 100 is excellent in both input characteristics and high-temperature storage characteristics. The lithium ion secondary battery 100 can be used for a variety of applications. 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-sized power storage devices and the like.

[0057] In an HEV, the driving power supply is charged only when generating power while the HEV is running. For this reason, the lithium-ion secondary battery 100, which has particularly excellent input characteristics, is particularly advantageous for HEV use. Therefore, a particularly suitable application of the lithium-ion secondary battery 100 is as a driving power supply for an HEV. 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] [Examples 1 to 11 and Comparative Examples 1 to 4] <Preparation of Lithium-ion Secondary Batteries for Evaluation> The following three types of carbon materials (natural graphite particles) were prepared as negative electrode active materials. Note that these natural graphite particles were coated with amorphous carbon.

[0062] [Table 1]

[0063] The average circularity of natural graphite particles was determined using a commercially available static automatic image analyzer (Malvern Panalytical's "Morphologi G3 / G3SE") as follows. First, natural graphite particles were dispersed on a substrate so as not to aggregate, and particle projection images were obtained using an optical microscope. These were converted into digital data using the software provided with the device, and the circularity of 1,000 or more particles was calculated. The average value was then calculated to obtain the average circularity. The average particle size was determined by measuring the median diameter D50 of the natural graphite particles using a commercially available laser diffraction / scattering particle size distribution analyzer.

[0064] 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 10 mg / cm. 2 At this time, the negative electrode density (apparent density of the negative electrode active material layer) was adjusted to the values ​​shown in Table 2 by changing the pressing conditions.

[0065] 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.

[0066] 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.

[0067] 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 2, 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 2. 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.

[0068] 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.

[0069] <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.

[0070] <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.

[0071] With the initial capacity defined as 100% SOC, each evaluation lithium-ion secondary battery was charged at a current of 0.3 C in a 25°C thermostatic chamber until the SOC reached 50%. Next, the battery was charged for 10 seconds at currents of 1 C, 3 C, 5 C, and 10 C in a -10°C thermostatic chamber, and the battery voltage after charging at each current was measured. The IV characteristics during charging were determined by plotting each current value versus each battery voltage, and the IV resistance (Ω) during discharge was calculated as the initial resistance from the slope of the resulting line. The ratio of the initial resistance of the evaluation lithium-ion secondary battery of Comparative Example 1 to that of the evaluation lithium-ion secondary battery of Comparative Example 1 was calculated, assuming the initial resistance of the evaluation lithium-ion secondary battery of Comparative Example 1 as 100. The results are shown in Table 2. Note that the smaller the initial resistance ratio, the better the input characteristics.

[0072] <High-temperature storage characteristic evaluation> Each activated lithium-ion secondary battery for evaluation was charged at a current of 0.3 C to an SOC of 80%. Each lithium-ion secondary battery for evaluation was placed in a thermostatic chamber set at 60°C and stored for 60 days. It was then discharged at a constant current of 0.2 C to 3.00 V. Each lithium-ion secondary battery for evaluation was then charged at a constant current of 0.2 C to 4.10 V, followed by constant voltage charging until the current reached 1 / 50 C, thereby fully charging the battery. It was then discharged at a constant current of 0.2 C to 3.00 V, and the discharge capacity at this point was measured and used as the recovered capacity. The capacity retention rate (%) was calculated using the formula: (recovered capacity / initial capacity) × 100. The results are shown in Table 2. Note that the higher the capacity retention rate, the better the high-temperature storage characteristics.

[0073] [Table 2]

[0074] As shown in the results in Table 2, the negative electrode active material layer contained carbon material particles having an average circularity of 0.86 or more, and the apparent density of the negative electrode active material layer was 1.30 g / cm 3 ~1.69g / cm 3When the nonaqueous solvent of nonaqueous electrolyte 80 contained 1% by volume to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms, the lithium ion secondary battery had excellent input characteristics and high-temperature storage characteristics. These results show that the nonaqueous electrolyte secondary battery disclosed herein is excellent in both input characteristics and high-temperature storage characteristics.

[0075] 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.

[0076] That is, the nonaqueous electrolyte secondary battery disclosed herein has the following features [1] to [6]. [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 contains carbon material particles having an average circularity of 0.86 or more, The apparent density of the negative electrode active material layer is 1.30 g / cm 3 ~1.69g / cm 3 and The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt, The nonaqueous electrolyte secondary battery, wherein the nonaqueous solvent contains 1% by volume to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms. [2] The apparent density of the negative electrode active material layer is 1.30 g / cm 3 ~1.50g / cm 3 The non-aqueous electrolyte secondary battery according to item [1], [3] The non-aqueous electrolyte secondary battery according to item [1] or [2], wherein the non-aqueous solvent contains the carboxylic acid ester in an amount of 6 to 30% by volume. [4] The nonaqueous electrolyte secondary battery according to any one of items [1] to [3], wherein the carboxylic acid ester is methyl acetate. [5] The nonaqueous electrolyte secondary battery according to any one of items [1] to [4], wherein the nonaqueous electrolyte further contains a film-forming agent. [6] The coating weight of the negative electrode active material layer is 10 mg / cm 2 The nonaqueous electrolyte secondary battery according to any one of items [1] to [5] above. [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]

[0077] 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 contains carbon material particles having an average circularity of 0.86 or more, The apparent density of the negative electrode active material layer is 1.30 g / cm ３ ~1.50 g / cm ３ and The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt, In the non-aqueous electrolyte secondary battery, the non-aqueous solvent contains 1% by volume to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms.

2. 2. The nonaqueous electrolyte secondary battery in accordance with claim 1, wherein the nonaqueous solvent contains the carboxylic acid ester in an amount of 6 to 30% by volume.

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

4. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the nonaqueous electrolyte further contains a film-forming agent.

5. The negative electrode active material layer has a basis weight of 10 mg / cm ２ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein:

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

7. 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 contains carbon material particles having an average circularity of 0.86 or more, the apparent density of the negative electrode active material layer is 1.30 g / cm 3 to 1.69 g / cm 3 ; The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt, the non-aqueous solvent contains 1% by volume to 30% by volume of a carboxylic acid ester having 4 or less carbon atoms; the carboxylic acid ester is methyl acetate; Nonaqueous electrolyte secondary battery.

8. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous solvent contains 6% to 30% by volume of the carboxylic acid ester.

9. The non-aqueous electrolyte secondary battery of claim 1, wherein the non-aqueous electrolyte further contains a film-forming agent.

10. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material layer has a basis weight of 10 mg / cm 2 or more.

11. A non-aqueous electrolyte secondary battery as described in claim 1, which is used as a vehicle driving power source for a hybrid vehicle.

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