Non-aqueous electrolyte secondary batteries

Abstract

To provide a nonaqueous electrolyte secondary battery having excellent output characteristics while using a ternary active material with low cobalt content.SOLUTION: A nonaqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode and a nonaqueous electrolyte. The positive electrode has a positive electrode active material layer including a positive electrode active material. The positive electrode active material is a lithium-nickel-cobalt-manganese composite oxide. In the composite oxide, a molar ratio of Ni to an all metal elements other than Li is 40-60 mol%, and a molar ratio of Co is 15-25 mol%. The composite oxide is hollow particles each including a shell part, a hollow part formed inside the shell part, and a through hole penetrating through the shell part. The BET specific surface area of the composite oxide is 3.0 m2 / g or more. The nonaqueous electrolyte contains a nonaqueous solvent, an electrolyte salt, and lithium bis(oxalate)borate. The nonaqueous solvent contains a carboxylic acid ester with a carbon number of 4 or less.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, from the perspective of decarbonization, demand for electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc. has been rapidly increasing. As a result, demand for nonaqueous electrolyte secondary batteries, which serve as the driving power sources for these vehicles, has been rapidly increasing.

[0003] Lithium-nickel-cobalt-manganese composite oxides, also known as ternary active materials, are known as typical positive electrode active materials used in nonaqueous electrolyte secondary batteries (see, for example, Patent Documents 1 to 4). Ternary active materials that contain approximately equimolar amounts of Ni, Co, and Mn (i.e., a molar ratio of approximately 1:1:1) are commonly used in practice. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-09722 [Patent Document 2] Japanese Patent Publication No. 2020-95842 [Patent Document 3] Special Publication No. 2018-530122 [Patent Document 4] International Publication No. 2021 / 186949 Summary of the Invention [Problem to be solved by the invention]

[0005] If demand for BEVs continues to expand due to decarbonization, there are concerns that the resources used in non-aqueous electrolyte secondary batteries will be depleted in the future. One of the resources that is of concern is cobalt (Co). One possible solution to this problem is to reduce the cobalt content of ternary active materials. However, while there is a demand for even higher output from non-aqueous electrolyte secondary batteries, ternary active materials with low cobalt content have the problem of insufficient output characteristics.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary battery that has excellent output characteristics while using a ternary active material with a low cobalt content. [Means for solving the problem]

[0007] The non-aqueous electrolyte secondary battery disclosed herein contains 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-based composite oxide. In the lithium nickel cobalt manganese-based composite oxide, the molar ratio of Ni to all metal elements other than Li is 40 mol % to 60 mol %, and the molar ratio of Co is 15 mol % to 25 mol %. The lithium nickel cobalt manganese-based composite oxide is in the form of hollow particles having a shell portion, a hollow portion formed inside the shell portion, and through-holes penetrating the shell portion. The BET specific surface area of ​​the lithium nickel cobalt manganese-based composite oxide is 3.0 m 2 / g or more. The non-aqueous electrolyte solution contains a non-aqueous solvent, an electrolyte salt, and lithium bis(oxalato)borate. The non-aqueous solvent contains a carboxylic acid ester having 4 or less carbon atoms.

[0008] According to this configuration, it is possible to provide a non-aqueous electrolyte secondary battery that has excellent output characteristics even when using a ternary active material with a low cobalt content. [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] 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] 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.

[0016] 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.

[0017] The positive electrode active material layer 54 contains a positive electrode active material. In this embodiment, at least a lithium nickel cobalt manganese based composite oxide is used as the positive electrode active material.

[0018] 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 non-metal element such as S, F, Cl, Br, or I.

[0019] In lithium nickel cobalt manganese composite oxides that are commonly used in practice, the molar ratio of Co to all metal elements other than Li is approximately 30 mol% to 33.3 mol%. In contrast, the lithium nickel cobalt manganese composite oxide used in this embodiment has a low Co content. Therefore, in this lithium nickel cobalt manganese composite oxide, the molar ratio of Co to all metal elements other than Li is 15 mol% to 25 mol%. In addition, the molar ratio of Ni to all metal elements other than Li is 40 mol% to 60 mol%. In this embodiment, as the lithium nickel cobalt manganese composite oxide, one that satisfies the above molar ratios of Ni and Co may be used alone, or two or more types that satisfy the above molar ratios of Ni and Co may be used in combination. The molar ratio of Co is preferably 15 mol% to 22 mol%, and more preferably 15 mol% to 20 mol%.

[0020] Specifically, the lithium nickel cobalt manganese composite oxide has a composition represented by the following formula (I), for example. Li 1+x Ni y Co z Mn (1-y-z) M α O 2-β Q β (I)

[0021] In formula (I), x, y, z, α, and β satisfy the following conditions: -0.05≦x≦0.3, 0.4≦y≦0.6, 0.15≦z≦0.25, 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.

[0022] x preferably satisfies 0≦x≦0.3, and more preferably 0≦x≦0.20. y preferably satisfies 0.45≦y≦0.6. z preferably satisfies 0.15≦z≦0.22, and more preferably 0.15≦z≦0.20. α preferably satisfies 0≦α≦0.05, and more preferably 0≦α≦0.03. β preferably satisfies 0≦β≦0.1, and more preferably 0.

[0023] Regarding the particle shape of the positive electrode active material used in this embodiment, the lithium nickel cobalt manganese composite oxide is hollow particle-shaped, and the hollow particle has a shell portion, a hollow portion formed inside the shell portion, and through-holes that penetrate the shell portion. The number of hollow portions is not particularly limited and may be one or two or more, preferably one. The number of through-holes is not particularly limited and may be one or two or more.

[0024] 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 shell portion 16A. 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 shell portion 16A to form multiple layers. Shell portion 16A surrounds hollow portion 14A. Through holes 14Aa are formed in shell portion 16A, and the opening width h of through holes 14Aa is large enough to allow nonaqueous electrolyte solution 80 to penetrate.

[0025] In the example shown in Fig. 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 Fig. 4, some of the primary particles 12B are separated from one another because the drawing is a cross-sectional view; in reality, the primary particles 12B are in contact with other primary particles (not shown) in areas outside the drawing. In the hollow particle 10B, the multiple aggregated primary particles 12B form a shell portion 16B that surrounds the hollow portion 14B. A through-hole 14Ba is formed in the shell portion 16B, and the opening width h of the through-hole 14Ba is large enough to allow the nonaqueous electrolyte solution 80 to penetrate.

[0026] The BET specific surface area of ​​the lithium nickel cobalt manganese composite oxide is 3.0 m 2 / g or more. Such a high BET specific surface area can be achieved by forming the lithium nickel cobalt manganese composite oxide into hollow particles and increasing the internal surface area of ​​the particles. The BET specific surface area is preferably 3.1 m 2 / g or more. There is no particular upper limit to the BET specific surface area, and it is determined by technical limitations. The BET specific surface area is, for example, 4.0 m 2 / g or less, and 3.8m 2 / g or less, 3.6m 2 / g or less, or 3.4m 2 The BET specific surface area is preferably 3.0 m / g or less. 2 / g~3.6m 2 / g, more preferably 3.1m 2 / g~3.4m 2 / g. The BET specific surface area can be measured by a known method using nitrogen (N2) gas as an adsorbate. Specifically, for example, the BET specific surface area can be determined by performing a measurement based on the BET method using nitrogen (N2) gas and a commercially available specific surface area measuring device.

[0027] By combining this lithium-nickel-cobalt-manganese composite oxide with a nonaqueous electrolyte solution 80 containing a carboxylic acid ester having four or less carbon atoms and lithium bis(oxalato)borate, the lithium-ion secondary battery 100 exhibits excellent output characteristics despite using a ternary active material with a low cobalt content. This is for the following reason. Specific details of the nonaqueous electrolyte solution 80 will be described later.

[0028] Lithium nickel cobalt manganese composite oxide is 3.0m 2 The hollow particle structure has a specific surface area increased to a range of 1 / g or more, thereby increasing the surface area available for the battery reaction. In particular, the viscosity of the nonaqueous electrolyte solution 80 is reduced by the carboxylic acid ester having four or fewer carbon atoms, and the presence of through-holes in the shells of the hollow particles makes it easy for the nonaqueous electrolyte solution 80 to enter the hollow spaces of the hollow particles, thereby enabling the battery reaction to occur efficiently even on the inner surfaces of the particles. This therefore enables the lithium-ion secondary battery 100 to have low resistance during discharge (i.e., high output).

[0029] However, carboxylic acid esters are easily decomposed, and the large specific surface area of ​​the lithium-nickel-cobalt-manganese composite oxide makes the decomposition reaction of the carboxylic acid ester more likely to occur. As a result, the resistance-reducing effect (power-improving effect) of the carboxylic acid ester cannot be fully realized. Therefore, in this embodiment, the nonaqueous electrolyte 80 contains lithium bis(oxalato)borate (LiBOB), a film-forming agent. This LiBOB forms a coating not only on the negative electrode 60 but also on the positive electrode 50 (particularly the positive electrode active material), suppressing the oxidative decomposition of the carboxylic acid ester and significantly improving the resistance-reducing effect (i.e., power-improving effect). In this way, the special particle shape and BET specific surface area of ​​the lithium-nickel-cobalt-manganese composite oxide, the carboxylic acid ester with four or fewer carbon atoms in the nonaqueous solvent, and the LiBOB film-forming agent work together to significantly improve the power characteristics of the lithium-ion secondary battery 100.

[0030] The porosity of hollow particles is not particularly limited, but is preferably 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.

[0031] The DBP oil absorption of the hollow particles is not particularly limited, but is preferably 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.

[0032] The average particle size (median size: D50) of the lithium nickel cobalt manganese composite oxide is not particularly limited, but is, for example, 0.05 μm to 25 μm, preferably 1 μm to 20 μm, and more preferably 2 μm to 15 μm. The average particle size (D50) of the positive electrode active material can be determined, for example, by a laser diffraction scattering method.

[0033] Various methods for producing hollow particulate lithium nickel cobalt manganese composite oxides are known, and the hollow particulate lithium nickel cobalt manganese composite oxide used in this embodiment can be produced according to a known method.

[0034] 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 10 mass % relative to the total mass of the positive electrode active material, preferably 5 mass % or less). The positive electrode active material may be composed only of the lithium nickel manganese cobalt-based composite oxide.

[0035] 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.

[0036] The positive electrode active material layer 54 may contain components other than the positive electrode active material (that is, optional components). Examples of the optional components include a conductive material and a binder.

[0037] Examples of the conductive material that can be used include carbon materials such as carbon black (e.g., acetylene black), carbon nanotubes (CNTs), and graphite, with CNTs being particularly preferred. CNTs have high conductivity, which can further improve the output characteristics of the lithium-ion secondary battery 100. Additionally, CNTs have a hollow structure, which allows carboxylic acid esters having four or fewer carbon atoms to penetrate into, thereby improving the wettability between the positive electrode active material layer 54 and the nonaqueous electrolyte solution 80 and further improving the output characteristics of the lithium-ion secondary battery 100. When the positive electrode active material layer 54 contains CNTs, the positive electrode active material layer 54 may further contain a dispersant for the CNTs (e.g., a surfactant-type dispersant, a polymer-type dispersant, an inorganic-type dispersant, etc.).

[0038] The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1 mass % or more and 15 mass % or less, and more preferably 0.5 mass % or more and 13 mass % or less.

[0039] 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 from 1% by mass to 15% by mass, and more preferably from 1.5% by mass to 10% by mass.

[0040] The thickness of the positive electrode active material layer 54 is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0041] 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:

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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).

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] In this embodiment, the non-aqueous electrolyte solution 80 contains a non-aqueous solvent, an electrolyte salt, and lithium bis(oxalato)borate. The non-aqueous solvent contains a carboxylic acid ester having four or less carbon atoms.

[0053] As described above, the carboxylic acid ester having 4 or less carbon atoms acts to reduce the viscosity of the non-aqueous electrolyte solution 80. Examples of the carboxylic acid ester having 4 or less carbon atoms include methyl acetate, ethyl acetate, and the like. Lu et al. Among these, methyl acetate is preferred.

[0054] If the content of the carboxylic acid ester in the non-aqueous solvent is too low, the output improvement effect may be reduced. Therefore, the volume ratio of the carboxylic acid ester in the non-aqueous solvent is preferably 1% by volume or more, more preferably 3% by volume or more, even more preferably 10% by volume or more, and particularly preferably 15% by volume or more. On the other hand, if the content of the carboxylic acid ester in the non-aqueous solvent is too high, the carboxylic acid ester may be easily decomposed. Therefore, the volume ratio of the carboxylic acid ester in the non-aqueous solvent is 50% by volume or less, preferably 40% by volume or less, and more preferably 30% by volume or less.

[0055] The non-aqueous solvent includes an organic solvent other than a carboxylic acid ester. Examples of the organic solvent 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. The non-aqueous solvent may contain only carbonates and a carboxylic acid ester.

[0056] The non-aqueous electrolyte solution 80 may contain an electrolyte salt (in other words, a supporting salt). Examples of suitable electrolyte salts include lithium salts such as LiPF, LiBF, and lithium bis(fluorosulfonyl)imide (LiFSI), preferably LiPF. The concentration of the electrolyte salt in the non-aqueous electrolyte solution 80 is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0057] The nonaqueous electrolyte 80 contains lithium bis(oxalato)borate (LiBOB). As described above, LiBOB forms a coating on the surface of the positive electrode active material, thereby suppressing the decomposition of the carboxylic acid ester, which contributes to lowering the resistance (i.e., improving the output) of the lithium-ion secondary battery 100. If the concentration of LiBOB in the nonaqueous electrolyte 80 is too low, the output improvement effect may be reduced. Therefore, the concentration of LiBOB in the nonaqueous electrolyte 80 is preferably 0.15% by mass or more, more preferably 0.20% by mass or more, and even more preferably 0.30% by mass or more. On the other hand, if the concentration of LiBOB in the nonaqueous electrolyte 80 is too high, an excessive coating is formed, thereby reducing the output improvement effect. Therefore, the concentration of LiBOB in the nonaqueous electrolyte 80 is preferably 0.80% by mass or less, more preferably 0.70% by mass or less, and even more preferably 0.60% by mass or less.

[0058] The nonaqueous electrolyte solution 80 may contain various additives, such as gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB), thickeners, and the like, in addition to the above-mentioned components, as long as the effects of the present invention are not significantly impaired.

[0059] The lithium-ion secondary battery 100 has excellent output characteristics despite using a ternary active material with a low cobalt content. Because the lithium-ion secondary battery 100 uses a ternary active material with a low cobalt content, it can significantly alleviate the problem of cobalt resource depletion that accompanies decarbonization.

[0060] 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 small-sized 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] [Examples 1 to 12 and Comparative Examples 1 to 6] The positive electrode active material was LiNi with a BET specific surface area shown in Table 1. 0.5 Co 0.2 Mn 0.3O2 was prepared. The BET specific surface area was determined using a commercially available specific surface area measuring device with nitrogen (N2) gas as the adsorbate. The positive electrode active materials of each example and comparative example were hollow particles having a shell portion, a hollow portion formed inside the shell portion, and through-holes penetrating the shell portion.

[0065] This positive electrode active material, a carbon nanotube (CNT) dispersion as a conductive material, and PVdF as a binder were mixed in N-methyl-2-pyrrolidone at a solid mass ratio of active material:CNT:PVdF = 97.5:1.5:1.0 to prepare a positive electrode slurry. The positive electrode slurry was applied to an aluminum foil as a positive electrode current collector and dried to obtain a positive electrode sheet.

[0066] 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 obtain a negative electrode sheet.

[0067] A porous polyolefin sheet was prepared as a separator. The positive electrode sheet and the negative electrode sheet were stacked together with the separator sandwiched between them to prepare an electrode assembly. In this electrode assembly, the electrode facing area was approximately 20 cm. 2 It was.

[0068] A mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and a carboxylic acid ester shown in Table 1 in a volume ratio (volume %) of 30:30-x:40:x was prepared. The value of x was set to the value shown in Table 1. LiPF6 as a supporting electrolyte was dissolved in this mixed solvent at a concentration of 1.1 mol / L, and lithium bis(oxalate)borate was also dissolved at the concentration shown in Table 1. This resulted in a nonaqueous electrolyte solution.

[0069] Terminals were attached to the electrode body, and the electrode body was housed in a laminate case together with a non-aqueous electrolyte. The laminate case was then sealed to obtain a lithium-ion secondary battery for evaluation with a capacity of approximately 20 mAh. This lithium-ion secondary battery for evaluation was initially charged at a current value of 1 / 3 C, and then activated by aging at 60°C for 24 hours.

[0070] <Output evaluation - Output resistance measurement> Each activated lithium-ion secondary battery for evaluation was adjusted to a SOC (State of charge) of 50% in a thermostatic bath at 25°C. Next, each lithium-ion secondary battery for evaluation was discharged for 10 seconds at current values ​​of 5C to 45C in a thermostatic bath at 25°C, and the battery voltage after discharge at each current value was measured. Each current value and each battery voltage were plotted to determine the IV characteristics during discharge, and the IV resistance (Ω) during discharge was calculated as the output resistance from the slope of the resulting straight line. When the output resistance of the lithium-ion secondary battery for evaluation in Comparative Example 1 was set to "1.00," the ratio of the output resistance of the other lithium-ion secondary batteries for evaluation to that of the lithium-ion secondary battery for evaluation in Comparative Example 1 was calculated. The results are shown in Table 1.

[0071] [Table 1]

[0072] As shown in the results in Table 1, in the lithium nickel cobalt manganese based composite oxide, the molar ratio of Ni to all metal elements other than Li was 40 mol % to 60 mol %, and the molar ratio of Co was 15 mol % to 25 mol %, the lithium nickel cobalt manganese based composite oxide was in the form of hollow particles having a shell portion, a hollow portion, and through-holes penetrating the shell portion, and the BET specific surface area of ​​the lithium nickel cobalt manganese based composite oxide was 3.0 m 2 / g or more, and it can be seen that the output resistance is significantly small when the nonaqueous electrolyte contains a carboxylic acid ester having four or less carbon atoms and lithium bis(oxalato)borate. Therefore, it can be seen that the nonaqueous electrolyte secondary battery disclosed herein has excellent output characteristics even when using a ternary active material with a low cobalt content.

[0073] 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.

[0074] 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, In the lithium nickel cobalt manganese based composite oxide, the molar ratio of Ni to all metal elements other than Li is 40 mol % to 60 mol %, and the molar ratio of Co is 15 mol % to 25 mol %, the lithium nickel cobalt manganese-based composite oxide is in the form of hollow particles having a shell portion, a hollow portion formed inside the shell portion, and through-holes penetrating the shell portion; The BET specific surface area of ​​the lithium nickel cobalt manganese composite oxide is 3.0 m 2 / g or more, the non-aqueous electrolyte solution contains a non-aqueous solvent, an electrolyte salt, and lithium bis(oxalato)borate; the non-aqueous solvent contains a carboxylic acid ester having 4 or less carbon atoms; Nonaqueous electrolyte secondary battery. [2] The nonaqueous electrolyte secondary battery according to item [1], wherein the concentration of lithium bis(oxalato)borate in the nonaqueous electrolyte is 0.20% by mass to 0.80% by mass. [3] The non-aqueous electrolyte secondary battery according to item [1] or [2], wherein the volume ratio of the carboxylic acid ester in the non-aqueous solvent is 1% by volume to 50% by volume. [4] The non-aqueous electrolyte secondary battery according to item [1] or [2], wherein the volume ratio of the carboxylic acid ester in the non-aqueous solvent is 15% by volume to 30% by volume. [5] The nonaqueous electrolyte secondary battery according to any one of items [1] to [4], wherein the carboxylic acid ester is methyl acetate. [6] The BET specific surface area of ​​the lithium nickel cobalt manganese composite oxide is 3.0 m 2 / g~3.6m 2 / g. / g. [7] The nonaqueous electrolyte secondary battery according to any one of items [1] to [6], which is used as a vehicle driving power source. [Explanation of symbols]

[0075] 10A,10B hollow particles 12A,12B Primary particles 14A,14B Hollow part 14Aa,14Ba through hole 16A,16B shell part 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 negative electrode has a negative electrode active material layer containing a negative electrode active material, the positive electrode active material is a lithium-nickel-cobalt-manganese composite oxide, In the lithium nickel cobalt manganese-based composite oxide, the molar ratio of Ni to all metal elements other than Li is 40 mol% to 60 mol%, and the molar ratio of Co is 15 mol% to 25 mol%, the lithium nickel cobalt manganese-based composite oxide is in the form of hollow particles having a shell portion, a hollow portion formed inside the shell portion, and through-holes penetrating the shell portion; The BET specific surface area of ​​the lithium nickel cobalt manganese composite oxide is 3.0 m ２ / g or more, the negative electrode active material is a carbon material, the non-aqueous electrolyte solution contains a non-aqueous solvent, an electrolyte salt, and lithium bis(oxalato)borate; the concentration of lithium bis(oxalato)borate in the nonaqueous electrolyte is 0.20% by mass to 0.80% by mass; the non-aqueous solvent contains a carboxylic acid ester having 4 or less carbon atoms; Nonaqueous electrolyte secondary battery.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the volume ratio of the carboxylic acid ester in the nonaqueous solvent is 1% by volume to 50% by volume.

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

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

5. The BET specific surface area of ​​the lithium nickel cobalt manganese composite oxide is 3.0 m ２ / g to 3.6m ２ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ionic strength is 0.1 / g.

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

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