Non-aqueous electrolyte secondary batteries

Abstract

To provide a non-aqueous electrolyte secondary battery which is obtained by using positive electrode active material having small BET specific surface area and nonetheless has high output characteristic.SOLUTION: A non-aqueous electrolyte secondary battery disclosed here, comprises: a positive electrode; a negative electrode; and a non-aqueous electrolyte. The positive electrode includes a positive electrode active material layer containing a positive electrode active material. The positive electrode active material is a lithium-nickel-cobalt-manganese composite oxide. A BET specific surface area of the positive electrode active material is 1.8 m2 / g to 2.8 m2 / g. A ratio of the BET specific surface area to an average particle size (D50) of the positive electrode active material is 0.62 or larger. The non-aqueous electrolyte contains a non-aqueous electrolyte and an electrolyte salt. The non-aqueous solvent contains a methyl acetate.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] A positive electrode active material is generally used in the positive electrode of a non-aqueous electrolyte secondary battery, and a lithium nickel manganese composite oxide is known as a positive electrode active material (see, for example, Patent Document 1). It is also known that the characteristics of a non-aqueous electrolyte secondary battery change depending on the BET specific surface area of ​​the positive electrode active material, and the range of the BET specific surface area of ​​the positive electrode active material used is 0.1 m 2 / g to 10m 2 The range is wide, from 0.1g to over 0.1g. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-175721 Summary of the Invention [Problem to be solved by the invention]

[0005] Positive electrode active materials with a small BET specific surface area have the advantage of being highly stable and capable of imparting excellent durability to nonaqueous electrolyte secondary batteries. However, a small BET specific surface area of ​​a positive electrode active material leads to a trade-off: a smaller reaction area, resulting in a decrease in output characteristics. Therefore, achieving both of these is a difficult task, and there is a need for the development of nonaqueous electrolyte secondary batteries that have high output characteristics while using positive electrode active materials with a small BET specific surface area.

[0006] Therefore, an object of the present invention is to provide a non-aqueous electrolyte secondary battery that has high output characteristics even when using a positive electrode active material with a small BET specific surface area. [Means for solving the problem]

[0007] The non-aqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode active material layer containing a positive electrode active material. The positive electrode active material is a lithium-nickel-cobalt-manganese composite oxide. The BET specific surface area of ​​the positive electrode active material is 1.8 m. 2 / g~2.8m 2 / g. The ratio of the BET specific surface area to the average particle diameter (D50) of the positive electrode active material is 0.62 or more. The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt. The non-aqueous solvent contains methyl acetate.

[0008] According to this configuration, it is possible to provide a nonaqueous electrolyte secondary battery that has high output characteristics even when using a positive electrode active material with a small BET specific surface area. [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] 1 is a schematic cross-sectional view of an example of a hollow particle of a positive electrode active material used in a lithium ion secondary battery according to one embodiment of the present invention. [Figure 4] FIG. 3 is a schematic cross-sectional view of another example of hollow particles of a positive electrode active material used in a lithium ion secondary battery according to one embodiment of the present invention. 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] The non-aqueous electrolyte solution 80 contains a non-aqueous solvent and an electrolyte salt. In this embodiment, the non-aqueous solvent contains methyl acetate.

[0015] Methyl acetate acts to reduce the viscosity of the non-aqueous electrolyte solution 80. The volume percentage of methyl acetate in the non-aqueous solvent is not particularly limited as long as the effects of the present invention can be obtained. From the viewpoint of suitably reducing the viscosity of the non-aqueous electrolyte solution 80, the volume percentage of methyl acetate in the non-aqueous solvent is, for example, 1% by volume, preferably 2% by mass or more, and more preferably 3% by mass or more. On the other hand, the volume percentage of methyl acetate in the non-aqueous solvent is, for example, 40% by mass or less, preferably 35% by mass or less, and more preferably 30% by mass or less.

[0016] The non-aqueous solvent may contain an organic solvent other than ethyl acetate. Examples of such organic solvents include carbonates, ethers, nitriles, sulfones, lactones, etc., with carbonates being 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 may be used alone or in appropriate combination of two or more. The non-aqueous solvent typically contains, for example, ethyl acetate and a carbonate, and may contain only ethyl acetate and a carbonate.

[0017] 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.

[0018] The nonaqueous electrolyte 80 may contain various additives other than the above-mentioned components, such as film-forming agents such as vinylene carbonate (VC) and oxalate complexes; gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); and thickeners, as long as the effects of the present invention are not significantly impaired.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, at least a lithium nickel cobalt manganese-based composite oxide is used. The lithium nickel cobalt manganese-based composite oxide has a crystal structure with a layered structure.

[0023] In addition, in this specification, the term "lithium nickel cobalt manganese-based composite oxide" includes, in addition to oxides composed of Li, Ni, Co, Mn, and O as constituent elements, oxides containing one or more additional elements other than these. 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, Sn, etc. Further, the additional element may be a semi-metal element such as B, C, Si, P, or a non-metal element such as S, F, Cl, Br, I.

[0024] From the viewpoint of the high energy density of the lithium ion secondary battery 100, in the lithium nickel cobalt manganese-based composite oxide, the ratio of nickel to the total of metal elements other than lithium is preferably 30 mol% or more, and more preferably 50 mol% or more.

[0025] Specifically, as the lithium nickel cobalt manganese-based composite oxide, those having a composition represented by the following formula (I) are preferable. Li 1+x Ni y Co z Mn (1-y-z) M α O 2-β Q β (I)

[0026] In formula (I), x, y, z, α, and β respectively satisfy -0.3 ≤ x ≤ 0.3, 0.1 < y < 0.9, 0 < z < 0.5, 0 ≤ α ≤ 0.1, and 0 ≤ β ≤ 0.5. M is at least one element selected from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Sn, B, and Al. Q is at least one element selected from the group consisting of F, Cl, and Br.

[0027] x preferably satisfies 0≦x≦0.3, and more preferably satisfies 0≦x≦0.15. From the viewpoint of a high energy density of the lithium ion secondary battery 100, y and z preferably satisfy 0.30≦y≦0.88 and 0.02≦z≦0.45, and more preferably satisfy 0.50≦y≦0.88 and 0.02≦z≦0.25, respectively. α preferably satisfies 0≦α≦0.01, and may be 0. β preferably satisfies 0≦β≦0.1, and more preferably is 0.

[0028] As the positive electrode active material, only one type of lithium nickel cobalt manganese based composite oxide may be used, or two or more types of lithium nickel cobalt manganese based composite oxides differing in composition, particle shape, etc. may be used in combination.

[0029] In this embodiment, the BET specific surface area of ​​the positive electrode active material is 1.8 m 2 / g~2.8m 2 The BET specific surface area of ​​the positive electrode active material used in lithium-ion secondary batteries is in the range of 0.1 m 2 / g to 10m 2 / g, a positive electrode active material having a small BET specific surface area is used in this embodiment. A positive electrode active material having a BET specific surface area in the above range is excellent in stability.

[0030] In addition, the ratio of the BET specific surface area to the average particle diameter (D50) of the positive electrode active material is 0.62 or more. By combining such a positive electrode active material with a non-aqueous electrolyte containing methyl acetate, the output characteristics of the lithium-ion secondary battery 100 can be improved for the following reasons.

[0031] A ratio of the BET specific surface area to the average particle diameter (D50) of a positive electrode active material of 0.62 or more means that a positive electrode active material having a large BET specific surface area relative to the average particle diameter (D50) is used. A configuration in which the ratio of the BET specific surface area to the average particle diameter (D50) is 0.62 or more can be achieved by using hollow particles as the positive electrode active material particles. In other words, hollow particles have internal voids, which result in a large internal surface area, making it possible to increase the ratio of the BET specific surface area to the average particle diameter (D50).

[0032] Therefore, in this embodiment, the positive electrode active material is usually hollow particles. In this embodiment, the viscosity of the nonaqueous electrolyte solution 80 is reduced by adding methyl acetate to the nonaqueous electrolyte solution 80. Therefore, by combining such a positive electrode active material and nonaqueous electrolyte solution 80, the reduced viscosity nonaqueous electrolyte solution 80 can easily enter the hollow portions (i.e., internal voids) of the hollow particles, thereby increasing the contact area between the nonaqueous electrolyte solution 80 and the positive electrode active material. As a result, the reaction area can be increased, and the output characteristics of the lithium ion secondary battery 100 can be improved.

[0033] From the viewpoint of higher output characteristics, the ratio of the BET specific surface area to the average particle diameter (D50) of the positive electrode active material is preferably 0.70 or more, more preferably 0.80 or more. The upper limit of this ratio is determined by technical limitations and may be 3.0 or less, 2.0 or less, 1.5 or less, or 1.0 or less.

[0034] As used herein, the term "specific surface area" refers to the surface area calculated by analyzing the gas adsorption amount measured by a gas adsorption method (constant volume adsorption method) using nitrogen (N) gas as the adsorbate using a BET method (e.g., a single-point BET method). The BET specific surface area can be measured using a commercially available specific surface area measuring device.

[0035] In this specification, the term "average particle size (D50)" refers to the median size (D50), which means the particle size corresponding to a cumulative frequency of 50 volume % from the smallest particle size side in a volume-based particle size distribution based on a laser diffraction / scattering method. Therefore, the average particle size (D50) can be determined using a commercially available laser diffraction / scattering particle size distribution analyzer.

[0036] The BET specific surface area of ​​the positive electrode active material is 1.8 m 2 / g~2.8m 2 / g, and the ratio of the BET specific surface area to the average particle diameter (D50) of the positive electrode active material is 0.62 or more, so the average particle diameter (D50) of the positive electrode active material is at most about 4.52 μm. The average particle diameter (D50) of the positive electrode active material is preferably 1.0 μm or more, and more preferably 2.0 μm or more.

[0037] As described above, the positive electrode active material is usually a hollow particle. In this specification, the term "hollow particle" refers to a particle having an internal void larger than the gap between primary particles in a secondary particle (so-called solid particle) formed by simple aggregation of primary particles. Therefore, a hollow particle typically has a shell portion made of a lithium-nickel-manganese-cobalt-based composite oxide and a hollow portion formed inside the shell portion. The hollow particle also typically further has through-holes penetrating the shell portion so that the nonaqueous electrolyte solution 80 can penetrate into the hollow portion. The number of hollow portions is not particularly limited and may be one or two or more. The number of through-holes is not particularly limited and may be one or two or more.

[0038] The porosity of hollow particles is not particularly limited, but is typically 20% to 50%. The porosity of hollow particles can be determined as follows: A cross-sectional electron microscope image of a hollow particle is obtained, and the ratio of the total area of ​​void portions to the area of ​​the entire particle (the sum of the area occupied by the particle and the total area of ​​void portions) is calculated as a percentage. This value is calculated for 100 or more hollow particles, and the average value is taken as the porosity of the hollow particle.

[0039] The DBP oil absorption of the hollow particles is not particularly limited, but is typically 35 mL / 100 g to 50 mL / 100 g. The DBP absorption can be measured using dibutyl phthalate (DBP) as a reagent liquid in accordance with the method described in JIS K6217-4:2008, and can be calculated as the average of three measurement results.

[0040] Examples of hollow particles are shown in Figures 3 and 4. Figures 3 and 4 are each a schematic cross-sectional view of an example of a hollow particle of a positive electrode active material. In the example shown in Figure 3, hollow particle 10A has one hollow portion 14A, and in hollow particle 10A, primary particles 12A made of a lithium-nickel-manganese-cobalt-based composite oxide are connected in a ring shape to form a shell portion. In the illustrated example, the primary particles 12A form a single layer, but multiple primary particles 12A may be stacked in the thickness direction of the shell portion to form multiple layers. The shell portion surrounds hollow portion 14A. Through holes 14Aa are formed in the shell portion, and the opening width h of through holes 14Aa is large enough to allow nonaqueous electrolyte solution 80 to penetrate.

[0041] In the example shown in Figure 4, the primary particles 12B are more loosely aggregated than usual, and therefore the hollow particle 10B has multiple, relatively large hollow portions 14B. Note that in Figure 4, some of the primary particles 12B are separated from one another because the figure is a cross-sectional view; in reality, they are in contact with other primary particles (not shown) in areas outside the figure. In the hollow particle 10B, the aggregated primary particles 12B form a shell portion surrounding the hollow portion 14B. Through-holes 14Ba are formed in the shell portion, and the opening width h of the through-holes 14Ba is large enough to allow the nonaqueous electrolyte solution 80 to penetrate.

[0042] Note that methods for producing hollow particulate positive electrode active materials with various particle sizes, various pore sizes, and various porosities are known, and therefore the positive electrode active material used in this embodiment can be produced according to a known method.

[0043] The positive electrode active material may contain a positive electrode active material other than the lithium nickel manganese cobalt-based composite oxide within a range that does not impair the effects of the present invention (for example, less than 20 mass % relative to the total mass of the positive electrode active material, preferably 10 mass % or less, more preferably 5 mass % or less). The positive electrode active material may be composed only of the lithium nickel manganese cobalt-based composite oxide.

[0044] 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, for example, 80 mass% or more, preferably 87 mass% or more, more preferably 90 mass% or more, even more preferably 95 mass% or more, and most preferably 97 mass% or more.

[0045] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a binder, a conductive material, and the like.

[0046] The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably from 1% by mass to 15% by mass, and more preferably from 2% by mass to 12% by mass.

[0047] The binder may be, for example, polyvinylidene fluoride (PVdF), etc. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass to 10% by mass, more preferably 0.2% by mass to 5% by mass, and even more preferably 0.3% by mass to 2% by mass.

[0048] Suitable conductive materials include carbon materials such as carbon black (e.g., acetylene black (AB)), carbon nanotubes (CNT), and graphite. CNT is preferred as the conductive material. CNT has high conductivity and a hollow structure. Therefore, when CNT is used as the conductive material, the nonaqueous electrolyte solution 80, the viscosity of which has been reduced by methyl acetate, can flow through the hollow portion of the CNT, improving the wettability between the positive electrode active material layer 54 and the nonaqueous electrolyte solution 80. As a result, the output characteristics of the lithium-ion secondary battery 100 can be further improved.

[0049] The type of CNT is not particularly limited, and examples thereof include single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs). These may be used alone or in combination of two or more. CNTs may be produced by arc discharge, laser ablation, chemical vapor deposition, or the like. In general, MWCNTs have a larger inner diameter than SWCNTs. Therefore, MWCNTs are preferred as CNTs because they allow the nonaqueous electrolyte solution 80, whose viscosity has been reduced by methyl acetate, to flow more easily through the hollow portion of the CNTs.

[0050] The average length of the CNTs is not particularly limited. If the average length of the CNTs is too long, the CNTs tend to aggregate and reduce dispersibility. Furthermore, Li ions diffusing inside the CNTs are less likely to escape from the CNTs. Therefore, the average length of the CNTs is preferably 15 μm or less, more preferably 8.0 μm or less, and even more preferably 5.0 μm or less. On the other hand, if the average length of the CNTs is too short, it becomes difficult for the CNTs to cover the surface of the positive electrode active material, and it tends to be difficult to form a conductive path between the positive electrode active materials. Therefore, the average length of the CNTs is preferably 0.1 μm or more.

[0051] The average diameter of the CNTs is not particularly limited and is, for example, 0.1 nm to 150 nm. Because the nonaqueous electrolyte solution 80 can easily flow through the hollow portions of the CNTs, the average diameter of the CNTs is preferably 1.0 nm or more, and more preferably 2.0 nm or more. On the other hand, if the average diameter of the CNTs is too large, the flexibility of the CNT particles decreases, and they become more rod-like, making it difficult for the CNTs to coat the positive electrode active material. As a result, the degree of improvement in wettability of the positive electrode active material surface may be reduced. Therefore, the average diameter of the CNTs is preferably 100 nm or less, and more preferably 50 nm or less.

[0052] The average length and average diameter of CNTs can be determined, for example, by taking an electron microscope photograph of the CNTs and averaging the lengths and diameters of 100 or more CNTs. Specifically, for example, a CNT dispersion is diluted and then dried to prepare a measurement sample. This sample is observed with a scanning electron microscope (SEM), and the lengths and diameters of 100 or more CNTs are determined and the average values ​​are calculated. If the CNTs have re-aggregated, the length and diameter of the aggregated CNT bundle are determined.

[0053] When CNTs are used as the conductive material, there are no particular restrictions on the CNT content in the positive electrode active material layer 54. If the CNT content in the positive electrode active material layer 54 is too low, the above-mentioned effects may be reduced. On the other hand, if the CNT content is too high, problems such as increased viscosity of the positive electrode slurry and reduced impregnation of the positive electrode active material layer 54 with the nonaqueous electrolyte solution 80 may occur during production of the lithium-ion secondary battery 100. Therefore, the CNT content in the positive electrode active material layer 54 is preferably 0.1% by mass or more and 3.0% by mass or less, more preferably 0.3% by mass or more and 2.5% by mass or less, and even more preferably 0.5% by mass or more and 2.0% by mass or less.

[0054] When a conductive material other than CNT is used, its content in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass to 15% by mass, more preferably 3% by mass to 13% by mass.

[0055] When CNTs are used as the conductive material, the positive electrode active material layer may further contain a carbon nanotube dispersant (CNT dispersant). Examples of CNT dispersants that can be used include surfactant-type dispersants (also called low-molecular-weight dispersants), polymer-type dispersants, and inorganic-type dispersants. The CNT dispersant may be anionic, cationic, amphoteric, or nonionic. Therefore, the CNT dispersant may have at least one functional group selected from the group consisting of anionic groups, cationic groups, and nonionic groups in its molecular structure. A surfactant is an amphiphilic substance having a chemical structure in which a hydrophilic moiety and a lipophilic moiety are covalently bonded within its molecular structure.

[0056] Specific examples of CNT dispersants include polycondensation-based aromatic surfactants such as sodium salt of naphthalenesulfonic acid-formaldehyde condensate, ammonium salt of naphthalenesulfonic acid-formaldehyde condensate, and sodium salt of methylnaphthalenesulfonic acid-formaldehyde condensate; polycarboxylic acids and their salts, such as polyacrylic acid and its salts, polymethacrylic acid and its salts; triazine derivative dispersants (preferably those containing a carbazolyl group or a benzimidazolyl group); polyvinylpyrrolidone (PVP); polymers having polynuclear aromatics in the side chain, such as pyrene and anthracene; polynuclear aromatic ammonium derivatives, such as pyrene ammonium derivatives (e.g., compounds in which an ammonium bromide group is introduced into pyrene) and anthracene ammonium derivatives; and the like. These CNT dispersants can be used alone or in combination. CNT dispersants containing polynuclear aromatics are preferred. Specifically, polymers having polynuclear aromatics in the side chain and polynuclear aromatic ammonium derivatives are preferred.

[0057] The amount of CNT dispersant may be determined appropriately depending on the type of CNT and CNT dispersant. If the proportion of CNT dispersant is too small, dispersibility may be insufficient. On the other hand, if the proportion of CNT dispersant is too large, excessive CNT dispersant may adhere to the CNT surface, causing an increase in resistance. When the CNTs are SWCNTs, the amount of CNT dispersant used is, for example, 1 to 400 parts by mass, preferably 20 to 200 parts by mass, relative to 100 parts by mass of CNTs. When the CNTs are MWNTs, the amount of CNT dispersant used is, for example, 1 to 100 parts by mass, preferably 4 to 40 parts by mass, relative to 100 parts by mass of CNTs.

[0058] 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.

[0059] The basis weight of the positive electrode active material layer 54 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 positive electrode active material layer 54 is 50 mg / cm 2 or less, or 40 mg / cm 2 It may be the following:

[0060] The positive electrode sheet 50 may contain 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.

[0061] 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.

[0062] 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.

[0063] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, for example, a carbon material such as graphite, hard carbon, or soft carbon can be used. 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.

[0064] 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, 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.

[0065] 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).

[0066] 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.

[0067] 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.

[0068] 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 It may be the following:

[0069] 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.

[0070] 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.

[0071] The lithium-ion secondary battery 100 has excellent output characteristics. Furthermore, since a positive electrode active material with a small BET specific surface area is used, the stability of the positive electrode active material is excellent, and therefore the lithium-ion secondary battery 100 also has excellent durability. The lithium-ion secondary battery 100 can be used in 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 compact power storage devices and the like. Because the lithium-ion secondary battery 100 has excellent output characteristics, a particularly suitable application of the lithium-ion secondary battery 100 is as a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) (particularly, a driving power source for 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 9 and Comparative Examples 1 to 5] The positive electrode active material was LiNi with the BET specific surface area and average particle size (D50) shown in Table 1. 0.50 Co0.20 Mn 0.30 O2 was prepared. The positive electrode active material of Comparative Example 3 was solid particles, and the positive electrode active materials of each Example and other Comparative Examples were hollow particles.

[0076] This positive electrode active material, carbon nanotubes as a conductive material, and PVdF as a binder were mixed in N-methyl-2-pyrrolidone (NMP) in a mass ratio of active material:conductive material:PVdF=97.5:1.5:1.0 to prepare a positive electrode slurry. This positive electrode slurry was applied to an aluminum foil and dried to prepare a positive electrode sheet.

[0077] A negative electrode slurry was prepared by mixing natural graphite (C) as the negative electrode active material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener in a mass ratio of C:SBR:CMC = 98:1:1 in pure water. This negative electrode slurry was applied to copper foil and dried to prepare a negative electrode sheet.

[0078] An electrode assembly was prepared by sandwiching a polyolefin porous film as a separator between the positive electrode sheet and the negative electrode sheet. In the electrode assembly, the opposing area of ​​the electrodes was approximately 20 cm. 2 This electrode assembly was placed in a laminate case having an opening at one end.

[0079] A mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and methyl acetate in a volume ratio of 30:30-x:40:x was prepared. The value of x is shown in Table 1. A nonaqueous electrolyte solution was prepared by dissolving LiPF6 as an electrolyte salt in this mixed solvent at a concentration of 1.1 mol / L. This nonaqueous electrolyte solution was poured into the laminate case, and the opening of the laminate case was heat-sealed to seal the laminate case. In this way, a single-layer laminate cell with a capacity of approximately 20 mAh was obtained as a lithium-ion secondary battery for evaluation.

[0080] <Output evaluation> Each evaluation lithium-ion secondary battery was initially charged at a current value of 1 / 3C, and then aged at 60°C for 24 hours. Each evaluation lithium-ion secondary battery was then adjusted to a 50% SOC (State of Charge) and placed in an environment at 25°C. Each evaluation lithium-ion secondary battery was discharged at various current values ​​to a predetermined cutoff voltage, and the time (seconds) for the battery voltage to reach the cutoff voltage was measured. Based on these measurement results, the output required to reach the cutoff voltage in 10 seconds (10-second output) was calculated. The output ratio of each Example and other Comparative Examples to Comparative Example 1 was calculated, assuming the output of Comparative Example 1 to be "1.00." The results are shown in Table 1.

[0081] [Table 1]

[0082] From the results in Table 1, it can be seen that a lithium nickel cobalt manganese composite oxide is used as the positive electrode active material, and the BET specific surface area of ​​the positive electrode active material is 1.8 m 2 / g~2.8m 2 / g, the ratio of the BET specific surface area to the average particle size (D50) of the positive electrode active material is 0.62 or more, and the nonaqueous solvent of the nonaqueous electrolyte contains methyl acetate, it can be seen that the output is particularly high. Therefore, it can be seen that the nonaqueous electrolyte secondary battery disclosed herein can improve the output characteristics even when using a positive electrode active material with a small BET specific surface area.

[0083] 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.

[0084] 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 positive electrode has a positive electrode active material layer containing a positive electrode active material, the positive electrode active material is a lithium-nickel-cobalt-manganese composite oxide, The BET specific surface area of ​​the positive electrode active material is 1.8 m 2 / g~2.8m 2 / g, the ratio of the BET specific surface area to the average particle diameter (D50) of the positive electrode active material is 0.62 or more; The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt, The non-aqueous electrolyte secondary battery, wherein the non-aqueous solvent contains methyl acetate. [2] The nonaqueous electrolyte secondary battery according to item [1], wherein the nonaqueous solvent contains 3% by volume to 30% by volume of methyl acetate. [3] The nonaqueous electrolyte secondary battery according to item [1] or [2], wherein the ratio of the BET specific surface area to the average particle diameter (D50) of the positive electrode active material is 0.80 or more. [4] The nonaqueous electrolyte secondary battery according to any one of items [1] to [3], wherein the positive electrode active material layer further contains carbon nanotubes as a conductive material. [5] The nonaqueous electrolyte secondary battery according to any one of items [1] to [4], wherein the positive electrode active material is hollow particles. [6] The nonaqueous electrolyte secondary battery according to any one of items [1] to [5], wherein in the lithium nickel cobalt manganese based composite oxide, the ratio of nickel to the total of metal elements other than lithium is 50 mol % or more. [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]

[0085] 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 positive electrode has a positive electrode active material layer containing a positive electrode active material, the positive electrode active material is a lithium-nickel-cobalt-manganese composite oxide, The BET specific surface area of ​​the positive electrode active material is 1.8 m ２ / g to 2.8m ２ / g, the ratio of the BET specific surface area to the average particle diameter (D50) of the positive electrode active material is 0.62 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 3% to 30% by volume of methyl acetate.

2. 2. The nonaqueous electrolyte secondary battery in accordance with claim 1, wherein the ratio of the BET specific surface area to the average particle diameter (D50) of said positive electrode active material is 0.80 or more.

3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material layer further contains carbon nanotubes as a conductive material.

4. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material is a hollow particle.

5. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein in said lithium-nickel-cobalt-manganese composite oxide, the ratio of nickel to the total of metal elements other than lithium is 50 mol % or more.

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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