Non-aqueous electrolyte secondary battery

Abstract

To provide a nonaqueous electrolyte secondary battery having excellent initial resistance characteristics and storage characteristics despite small pores of a positive electrode active material layer.SOLUTION: A nonaqueous electrolyte secondary battery disclosed herein includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode includes a positive electrode current collector, and a positive electrode active material layer supported on the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material and a conductive material. The positive electrode active material layer has a peak pore diameter of 0.50 μm to 0.70 μm. A content of the conductive material in the positive electrode active material layer is 0.2 mass% to 1.0 mass%. 80 mass% or more of the conductive material is carbon nanotubes. The nonaqueous electrolyte includes a nonaqueous solvent, an electrolyte salt, and a positive electrode film forming agent.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] Positive electrodes used in non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries generally have a configuration in which a positive electrode active material layer is provided on a positive electrode current collector. The positive electrode active material layer contains positive electrode active material particles, and the positive electrode active material layer has pores formed by voids between the positive electrode active material particles (see, for example, Patent Documents 1 to 3). To improve the performance of non-aqueous electrolyte secondary batteries, a technique is known in which the non-aqueous electrolyte contains an additive that forms a coating on the positive electrode (also known as a "positive electrode coating agent" or "positive electrode additive") (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-069822 [Patent Document 2] International Publication No. 2021 / 186949 [Patent Document 3] International Publication No. 2023 / 054308 Summary of the Invention [Problem to be solved by the invention]

[0005] Recently, demand for BEVs has been rapidly increasing, spurring a demand for even higher performance secondary batteries for BEV driving power sources. One method for achieving this is to densify the positive electrode active material layer. After extensive research, the present inventors have discovered the following problem when densifying the positive electrode active material layer. Specifically, densifying the positive electrode active material layer results in excessively small pore diameters, making it difficult for the positive electrode coating agent to penetrate into the fine pores of the positive electrode active material layer. As a result, it becomes difficult to form a uniform coating on the positive electrode active material layer, leading to a decrease in the durability of the nonaqueous electrolyte secondary battery. Specifically, the capacity of the nonaqueous electrolyte secondary battery deteriorates significantly during storage. Furthermore, high initial resistance characteristics are required for secondary batteries used as vehicle driving power sources.

[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 initial resistance characteristics and storage characteristics despite the small pores in the positive electrode active material layer. [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 includes a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material and a conductive material. The peak pore diameter of the positive electrode active material layer is 0.50 μm to 0.70 μm. The content of the conductive material in the positive electrode active material layer is 0.2 mass % to 1.0 mass %. 80 mass % or more of the conductive material is carbon nanotubes. The non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte salt, and a positive electrode coating-forming agent.

[0008] With this configuration, it is possible to provide a nonaqueous electrolyte secondary battery that is excellent in initial resistance characteristics and storage characteristics, despite the small pores in the positive electrode active material layer. [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. 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 a charge carrier and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes. Furthermore, in this specification, the term "electric vehicle (BEV)" refers to a vehicle that has a motor operated by a secondary battery as a power source, but does not have an internal combustion engine such as a gasoline engine.

[0012] Hereinafter, the present invention will be described in detail 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 electrode terminal 42 and a negative electrode 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 injection port (not shown) for injecting the nonaqueous electrolyte 80. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode 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 and a conductive 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.

[0018] 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, and lithium iron nickel manganese composite oxide.

[0019] 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.

[0020] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0021] These positive electrode active materials may be used alone or in combination of two or more. As the positive electrode active material, lithium nickel cobalt manganese composite oxide is particularly preferred because of its excellent properties such as initial resistance.

[0022] The positive electrode active material is in the form of particles, and 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.

[0023] 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.

[0024] In this embodiment, at least carbon nanotubes (CNTs) are used as the conductive material of the positive electrode active material layer 54, and the proportion of CNTs in the conductive material is 80 mass % or more. The CNTs are usually dispersed in the form of individual particles and / or aggregates together with the positive electrode active material within the positive electrode active material layer 54. When the conductive material contains 80 mass % or more of CNTs, the initial resistance of the lithium ion secondary battery 100 can be reduced. In other words, the initial resistance characteristics of the lithium ion secondary battery can be improved.

[0025] The type of CNT used is not particularly limited, and examples thereof include single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT). These may be used alone or in combination of two or more. MWCNT is preferred as the CNT. The CNT may be produced by arc discharge, laser ablation, chemical vapor deposition, or the like.

[0026] The average length of the CNTs is not particularly limited. If the average length of the CNTs is too long, the CNTs may aggregate, reducing dispersibility and potentially reducing the effect of uniformly distributing the positive electrode coating agent within the positive electrode active material layer 54. Therefore, the average length of the CNTs is preferably 10 μm or less, more preferably 5.0 μm or less, even more preferably 3.0 μm or less, and most preferably 1.0 μm or less. On the other hand, if the average length of the CNTs is too short, the number of CNTs per unit weight may be too small, making it difficult to form conductive paths between the positive electrode active materials. Therefore, the average length of the CNTs is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more.

[0027] The average diameter of the CNTs is not particularly limited. If the average diameter of the CNTs is too large, the number of CNTs per unit mass will be too small, which may make it difficult to form a conductive path between the positive electrode active materials. Therefore, the average diameter of the CNTs is preferably 50 nm or less, more preferably 40 nm or less, and even more preferably 30 nm or less. On the other hand, if the average diameter of the CNTs is too small, the hollow diameter of the CNTs will be too small, which may reduce the effect of improving the impregnation of the nonaqueous electrolyte 80 due to capillary action. Therefore, the average diameter of the CNTs is preferably 1 nm or more, more preferably 5 nm or more, and even more preferably 8 nm or more.

[0028] Particularly preferably, the average length of the CNTs is 0.3 μm to 1.0 μm, and the average diameter of the CNTs is 8 nm to 30 nm.

[0029] 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.

[0030] Examples of conductive materials other than CNT include carbon materials (for example, carbon black such as acetylene black (AB), graphite, etc.) Among these, carbon black is preferred, and acetylene black is particularly preferred.

[0031] From the viewpoint of particularly high initial resistance characteristics of the lithium ion secondary battery 100, the proportion of CNT in the conductive material is preferably 90 mass % or more, and more preferably 100 mass % (ie, the conductive material is composed only of CNT).

[0032] The content of the conductive material in the positive electrode active material layer 54 is 0.2% by mass to 1.0% by mass. The significance of this will be described later. The content of the conductive material in the positive electrode active material layer 54 is preferably 0.4% by mass to 1.0% by mass. In this case, the lithium ion secondary battery 100 is one in which an increase in resistance during long-term storage is highly suppressed.

[0033] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a binder, a carbon nanotube dispersant (CNT dispersant), etc. As the binder, for example, polyvinylidene fluoride (PVdF) or the like can be used.

[0034] 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. CNT dispersants 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 that has a chemical structure in which a hydrophilic moiety and a lipophilic moiety are covalently bonded within its molecular structure.

[0035] 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.

[0036] The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably from 1 to 15% by mass, and more preferably from 2 to 12% by mass. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably from 0.1 to 10% by mass, more preferably from 0.2 to 5% by mass, and even more preferably from 0.3 to 2% by mass.

[0037] 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.

[0038] The positive electrode active material layer 54 has pores that interconnect the gaps between the positive electrode active material particles. In this embodiment, the peak pore size of the positive electrode active material layer 54 is 0.50 μm to 0.70 μm. In this embodiment, the pores of the positive electrode active material layer 54 are small. However, by adding 0.2 mass % to 1.0 mass % of a conductive material in which 80 mass % or more is CNT to the positive electrode active material layer 54, the storage characteristics of the lithium ion secondary battery 100 can be improved.

[0039] By setting the content of the conductive material to 0.2% by mass or more and using CNTs as 80% by mass or more of the conductive material, the impregnation of the nonaqueous electrolyte 80 into the positive electrode active material layer 54 is improved due to capillary action caused by the CNTs in the pores of the positive electrode active material layer 54. As a result, the positive electrode film-forming agent can be supplied to the fine details of the pores of the positive electrode active material layer 54 along with the nonaqueous electrolyte 80. As a result, the positive electrode film-forming agent can be uniformly distributed within the positive electrode active material layer 54, and a uniform film can be formed in the positive electrode active material layer 54 when the lithium ion secondary battery 100 is initially charged. This improves the storage characteristics of the lithium ion secondary battery 100. On the other hand, if the content of the conductive material is too high, i.e., exceeds 1.0% by mass, the excess conductive material makes it difficult for the nonaqueous electrolyte 80 to flow through the pores. Therefore, by making the content of the conductive material 1.0 mass % or less, the positive electrode film-forming agent can be uniformly distributed within the positive electrode active material layer 54, and the effect of improving the storage characteristics of the lithium ion secondary battery 100 described above can be highly obtained.

[0040] Furthermore, since the conductive material contains 80 mass % or more of CNT, a good conductive path is formed within the positive electrode active material layer 54, which reduces the initial resistance of the lithium ion secondary battery 100 and thereby improves the initial resistance characteristics.

[0041] The peak pore diameter of the positive electrode active material layer 54 can be measured by mercury intrusion porosimetry. Specifically, the peak pore diameter of the positive electrode active material layer 54 can be measured using a mercury porosimeter according to a known method.

[0042] The pores in the positive electrode active material layer 54 are formed by gaps between the positive electrode active material particles. Therefore, the peak pore diameter of the positive electrode active material layer 54 can be adjusted by controlling the porosity of the positive electrode active material layer 54. The smaller the porosity of the positive electrode active material layer 54, the smaller the peak pore diameter of the positive electrode active material layer 54 tends to be. The porosity of the positive electrode active material layer 54 can be adjusted by changing the pressing conditions for the positive electrode active material layer 54 when fabricating the positive electrode 50, controlling the particle diameter of the positive electrode active material particles, or the like. Alternatively, the pore diameter can be reduced by disposing a conductive material in the gaps between the positive electrode active material particles. Therefore, the peak pore diameter of the positive electrode active material layer 54 can also be controlled by the amount of conductive material used.

[0043] The porosity of the positive electrode active material layer 54 is preferably 15 to 40% by volume, more preferably 15 to 30% by volume, and even more preferably 15 to 25% by volume. The porosity of the positive electrode active material layer 54 can be measured by mercury intrusion porosimetry. Specifically, the porosity of the positive electrode active material layer 54 can be measured using a mercury porosimeter according to a known method.

[0044] 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.

[0045] The density of the positive electrode active material layer 54 is not particularly limited, and is, for example, 2.00 g / cm 3 ~4.00g / cm 3 From the viewpoint of a high volumetric energy density, the density of the positive electrode active material layer 54 is preferably 3.00 g / cm 3 ~4.00g / cm 3 and more preferably 3.40 g / cm 3 ~4.00g / cm 3 and more preferably 3.50 g / cm 3 ~4.00g / cm 3 and particularly preferably 3.60 g / cm 3 ~4.00g / cm 3 is.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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).

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] The non-aqueous electrolyte 80 contains a non-aqueous solvent, an electrolyte salt (in other words, a supporting salt), and a positive electrode film-forming agent.

[0057] As the nonaqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones commonly used in electrolytes for lithium ion secondary batteries can be used without any particular limitation. Among these, carbonates and esters are preferred, and specific examples thereof 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), methyl acetate, and methyl propionate. One of these nonaqueous solvents can be used alone, or two or more can be used in appropriate combination. The nonaqueous solvent may contain, for example, only carbonates. The nonaqueous solvent may contain, for example, only carbonates and esters.

[0058] As the electrolyte salt, for example, a lithium salt such as LiPF, LiBF, or lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF) can be suitably used. The concentration of the electrolyte salt in the non-aqueous electrolyte 80 is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0059] The positive electrode film-forming agent is a component that forms a film on the positive electrode active material layer 54 (particularly on the surface of the positive electrode active material particles) when the lithium ion secondary battery 100 is initially charged, and is also called a positive electrode additive. As the positive electrode film-forming agent, at least one selected from the group consisting of lithium difluorophosphate (LiPO2F2) and lithium fluorosulfate (LiSO3F) can be suitably used. Use of these agents can particularly improve the storage characteristics of the lithium ion secondary battery 100.

[0060] The concentration of the positive electrode film-forming agent in the non-aqueous electrolyte 80 is not particularly limited and is, for example, 0.05 to 2.0 mass %, preferably 0.1 to 1.8 mass %, and more preferably 0.3 to 1.5 mass %.

[0061] The nonaqueous electrolyte 80 may contain various additives other than the above-mentioned components, such as a negative electrode film-forming agent such as vinylene carbonate (VC) or an oxalate complex; a gas generating agent such as biphenyl (BP) or cyclohexylbenzene (CHB); or a thickener, as long as the effects of the present invention are not significantly impaired.

[0062] In the lithium ion secondary battery 100, capacity degradation during long-term storage is suppressed despite the small pores in the positive electrode active material layer. Furthermore, in the lithium ion secondary battery 100, an increase in resistance during long-term storage is also effectively suppressed. Therefore, the lithium ion secondary battery 100 has excellent storage characteristics. Furthermore, the lithium ion secondary battery 100 also has excellent input characteristics. Furthermore, the reaction distribution in the positive electrode 50 tends to be uniform, which makes it difficult for lithium to deposit on the surface of the negative electrode 60, and thus makes it difficult for capacity degradation to occur.

[0063] The lithium ion secondary battery 100 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices and the like. A particularly suitable application of the lithium ion secondary battery 100 is as a driving power source for BEVs. The lithium ion secondary battery 100 can also be used in the form of an assembled battery, typically consisting of a plurality of batteries connected in series and / or parallel.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] [Examples 1 to 3 and Comparative Examples 1 to 8] LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2, a conductive material, and PVdF as a binder were mixed in a mass ratio of active material:conductive material:PVdF = 99.5-x:x:0.5. MWCNT (average diameter 13 nm, average length 0.5 μm) and acetylene black were used as the conductive materials. The MWCNT was used in the form of a dispersion, which contained pyrene ammonium salt as a CNT dispersant. In Table 1, x1 is the mass ratio of MWCNT, and in Table 2, x2 is the mass ratio of AB. The sum of x1 and x2 is the mass ratio x of the conductive material.

[0068] An appropriate amount of N-methyl-2-pyrrolidone was added to the resulting mixture to prepare a positive electrode slurry. The positive electrode slurry was applied to both sides of a 13 μm-thick aluminum foil serving as a positive electrode current collector. At this time, a portion of the aluminum foil not coated with the positive electrode slurry was provided as a lead connection portion. The amount of positive electrode slurry applied was adjusted so that the basis weight of the positive electrode active material layer formed on both sides was 45 mg / cm in total. 2 It was adjusted so that

[0069] The coated slurry was dried to form a positive electrode active material layer. The obtained sheet was pressed with a roller to increase the density of the positive electrode active material layer and adjust the porosity of the positive electrode active material layer to 21% by volume. The porosity was measured using a commercially available mercury porosimeter. The sheet was cut to a predetermined size to obtain a positive electrode in which a positive electrode active material layer was formed on both sides of the positive electrode current collector.

[0070] Graphite as a carbon-based negative electrode active material, carboxymethylcellulose sodium salt (CMC-Na), and a dispersion of styrene butadiene rubber (SBR) were mixed in a solids mass ratio of graphite:CMC-Na:CMC = 98:1:1. An appropriate amount of ion-exchanged water was added to prepare a negative electrode slurry. The negative electrode slurry was applied to both sides of an 8 μm-thick copper foil serving as a negative electrode current collector. An uncoated portion of the copper foil was left as a lead connection.

[0071] The applied paste was dried to form a negative electrode active material layer. The obtained sheet was pressed with a roller and then cut to a predetermined size to obtain a negative electrode in which a negative electrode active material layer was formed on both sides of the negative electrode current collector. The packing density of the negative electrode active material layer was 1.50 g / cm. 3 It was.

[0072] A lead was attached to each of the positive and negative electrodes prepared above. A single-layer polypropylene separator was prepared. The positive and negative electrodes were alternately stacked one by one with the separator interposed therebetween to prepare a laminated electrode assembly.

[0073] A mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:40:30 was prepared. Vinylene carbonate was dissolved in this mixed solvent at a concentration of 1 mass%, lithium bis(oxalato)borate was dissolved at a concentration of 0.8 mass%, lithium fluorosulfate, which serves as a positive electrode film-forming agent, was dissolved at a concentration of 1.5 mass%, and LiPF6, which serves as a supporting electrolyte, was dissolved at a concentration of 1.15 mol / L. However, in Comparative Example 8, no positive electrode film-forming agent was added. A nonaqueous electrolyte was thus obtained.

[0074] The laminated electrode assembly and the non-aqueous electrolyte were placed in a rectangular battery case and sealed to obtain a rectangular lithium ion secondary battery for evaluation. The amount of non-aqueous electrolyte injected was 2.05 g / Ah.

[0075] <Peak pore size measurement> Using a commercially available mercury porosimeter (AutoPore V9620 manufactured by Micromeritics), the peak pore diameter (μm) of the positive electrode active material layer of the above-prepared positive electrode was determined. The results are shown in Table 1.

[0076] <PC Impregnation Rate Evaluation> 3 μL of propylene carbonate (PC) solvent was dropped onto the positive electrode active material layer of the above-prepared positive electrode using a micropipette. Visually, the time (seconds) from the dropping until all of the PC solvent was absorbed into the positive electrode active material layer was measured. This measurement was performed 20 times, and the average value was adopted as the PC impregnation time. The results are shown in Table 1. Note that the shorter this PC impregnation time, the faster the PC impregnation rate, which means that the non-aqueous electrolyte is more likely to impregnate the positive electrode active material layer.

[0077] <Initial Resistance Characteristic Evaluation - Input Resistance Measurement> Each lithium-ion secondary battery for evaluation was adjusted to SOC 50% by constant current constant voltage (CC-CV) charging and then placed in an environment at 25°C. Charging was performed at a current value of 4C for 10 seconds, and the voltage rise amount ΔV at this time was obtained. Using this voltage rise amount ΔV and the current value, the input resistance value (initial input resistance) of each secondary battery for evaluation was calculated. The results are shown in Table 1.

[0078] <Storage Characteristic Evaluation> Each lithium-ion secondary battery for evaluation was placed in an environment at 25°C and charged up to 4.25V by CC-CV charging (0.01C cut-off) to adjust to SOC 100%. Then, CC-CV discharge (0.01C cut-off) was performed at a current value of 1 / 3C down to 3V. The discharge capacity at this time was measured and taken as the initial capacity.

[0079] Next, each lithium-ion secondary battery for evaluation was adjusted to SOC 95% and stored in a constant temperature bath at 60°C for 45 days. Then, the discharge capacity was measured in the same manner as the initial capacity. The capacity retention rate (%) was calculated from (Discharge capacity after storage / Initial capacity) × 100. The results are shown in Table 1.

[0080] The input resistance value of each evaluation lithium ion secondary battery after storage was measured in the same manner as above. The resistance increase rate was calculated as (input resistance after storage / initial input resistance). The results are shown in Table 1. Note that in Comparative Example 7, the initial input resistance was high, so the storage characteristics were not evaluated.

[0081] <Measurement of remaining concentration of positive electrode film forming agent> Each lithium-ion secondary battery for evaluation was placed in a 25°C environment and charged to 4.25 V using CC-CV charging, adjusting the SOC to 100%. It was then CC-CV discharged to 3 V at a current of 1 / 3C. Each lithium-ion secondary battery for evaluation was then disassembled, and the non-aqueous electrolyte was recovered. The remaining concentration of the positive electrode film-forming agent in the non-aqueous electrolyte was measured using an Oxford Instruments benchtop NMR spectrometer "X-Pulse" (with an autosampler "X-Auto"). The results are shown in Table 1.

[0082] [Table 1]

[0083] The results of Comparative Examples 2 to 6 show that the larger the peak pore diameter of the positive electrode active material layer, the shorter the PC impregnation time, i.e., the higher the impregnation of non-aqueous electrolyte. Comparative Examples 2 to 5 are examples in which only AB was used as the conductive material. In Comparative Examples 2 to 5, the smaller the amount of AB, the larger the peak pore diameter, the higher the impregnation of non-aqueous electrolyte, and the more positive electrode film-forming agent was consumed in film formation. Furthermore, in Comparative Examples 2 to 5, the smaller the amount of AB, the worse both the input characteristics and storage characteristics became.

[0084] On the other hand, Examples 1 to 3 and Comparative Example 1 are examples in which only CNT was used as the conductive material. Comparison with Comparative Examples 2 to 6 reveals that when CNT was used as the conductive material, a different tendency was obtained from when AB was used. Specifically, when the CNT content was 0.2% by mass to 1.0% by mass, the residual concentration of the positive electrode film-forming agent was specifically reduced, resulting in low input resistance and high storage characteristics. This is thought to be because the CNT allowed the positive electrode film-forming agent to penetrate into the fine pores of the positive electrode active material layer, allowing a uniform coating to be formed within the positive electrode active material layer.

[0085] Furthermore, a comparison of Comparative Example 7 with Examples 1 to 3 reveals that only when the proportion of CNTs in the conductive material is high, it is possible to reduce input resistance and improve storage characteristics.

[0086] From the above, it can be seen that the nonaqueous electrolyte secondary battery disclosed herein can provide a nonaqueous electrolyte secondary battery that is excellent in input characteristics and storage characteristics, despite the small pores in the positive electrode active material layer.

[0087] 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.

[0088] That is, the nonaqueous electrolyte secondary battery disclosed herein has the following features [1] to [6]. [1] Positive electrode, a negative electrode, and non-aqueous electrolyte A non-aqueous electrolyte secondary battery comprising: the positive electrode comprises a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector; the positive electrode active material layer contains a positive electrode active material and a conductive material, the peak pore diameter of the positive electrode active material layer is 0.50 μm to 0.70 μm, the content of the conductive material in the positive electrode active material layer is 0.2% by mass to 1.0% by mass, 80% by mass or more of the conductive material is carbon nanotubes, the non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte salt, and a positive electrode film-forming agent; Nonaqueous electrolyte secondary battery. [2] The nonaqueous electrolyte secondary battery according to item [1], wherein the conductive material consists solely of the carbon nanotubes. [3] The nonaqueous electrolyte secondary battery according to item [1] or [2], wherein the positive electrode film-forming agent is at least one selected from the group consisting of lithium difluorophosphate and lithium fluorosulfate. [4] The nonaqueous electrolyte secondary battery according to any one of items [1] to [3], wherein the content of the conductive material in the positive electrode active material layer is 0.4 mass % to 1.0 mass %. [5] The nonaqueous electrolyte secondary battery according to any one of items [1] to [4], wherein the carbon nanotubes have an average length of 0.3 μm to 1.0 μm and an average diameter of 8 nm to 30 nm. [6] The nonaqueous electrolyte secondary battery according to any one of items [1] to [5], which is used as a vehicle driving power source for an electric vehicle. [Explanation of symbols]

[0089] 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 Non-aqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. positive electrode, a negative electrode, and non-aqueous electrolyte A non-aqueous electrolyte secondary battery comprising: the positive electrode comprises a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector; the positive electrode active material layer contains a positive electrode active material and a conductive material, the peak pore diameter of the positive electrode active material layer is 0.50 μm to 0.70 μm, the content of the conductive material in the positive electrode active material layer is 0.2% by mass to 1.0% by mass, 80 mass % or more of the conductive material is carbon nanotubes, the non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte salt, and a positive electrode film-forming agent; the positive electrode active material is a lithium composite oxide having a layered structure, The positive electrode film-forming agent is at least one selected from the group consisting of lithium difluorophosphate and lithium fluorosulfate. Nonaqueous electrolyte secondary battery.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the conductive material consists solely of the carbon nanotubes.

3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of said conductive material in said positive electrode active material layer is 0.4% by mass to 1.0% by mass.

4. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the carbon nanotubes have an average length of 0.3 μm to 1.0 μm and an average diameter of 8 nm to 30 nm.

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

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