JP7600190B2 - Positive electrode and secondary battery using the same - Google Patents

Description

The present invention relates to a positive electrode. The present invention also relates to a secondary battery using the positive electrode.

In recent years, secondary batteries such as lithium-ion secondary batteries have been used effectively 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).

The positive electrodes used in 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 typically contains a positive electrode active material, a conductive material, a binder, and the like. It is known that the conductivity of the positive electrode can be improved by using carbon nanotubes as this conductive material (see, for example, Patent Documents 1 to 4). In addition, polyvinylidene fluoride is known as a binder contained in the positive electrode active material layer (see, for example, Patent Documents 1 to 3).

Special table 2018-501602 publication JP 2015-153714 A Special Publication No. 2021-506064 JP 2013-036021 A

The use of carbon nanotubes as a conductive material in the positive electrode active material layer is advantageous from the viewpoint of reducing the initial resistance of the secondary battery. However, it is desirable for secondary batteries to suppress heat generation in the event of a severe internal short circuit in which the metal member penetrates the electrode body.

After extensive research, the inventors have found the following: Immediately after a serious internal short circuit occurs, such as when a metal member penetrates the electrode body, the temperature of the secondary battery exceeds 500°C due to heat generation. If the resistance of the positive electrode active material layer is high during this heat generation, it is possible to suppress the temperature rise (i.e., further heat generation). However, the inventors have found that the above-mentioned conventional technology, which uses carbon nanotubes as a conductive material, has a problem in that the resistance of the positive electrode active material layer during heat generation is too low.

The present invention aims to provide a positive electrode that uses carbon nanotubes and in which the resistance of the positive electrode active material layer increases when heat is generated due to a severe internal short circuit in which the metal member penetrates the electrode body.

The positive electrode disclosed herein 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, carbon nanotubes, polyvinylidene fluoride, and polyamic acid.

This configuration makes it possible to provide a positive electrode using carbon nanotubes in which the resistance of the positive electrode active material layer increases when heat is generated due to a severe internal short circuit in which the metal member penetrates the electrode body.

From another perspective, the secondary battery disclosed herein comprises the above-mentioned positive electrode, negative electrode, and electrolyte. With this configuration, it is possible to provide a secondary battery in which heat generation due to a severe internal short circuit, such as when a metal member penetrates the electrode body, is suppressed.

FIG. 1 is a cross-sectional view that illustrates a positive electrode according to one embodiment of the present invention. 1 is a cross-sectional view showing a schematic internal structure of a lithium-ion secondary battery according to one embodiment of the present invention. FIG. 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.

Below, the embodiments of the present invention will be described with reference to the drawings. Note that 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. In addition, in the following drawings, the same reference numerals are used to describe members and parts that perform the same function. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships. Note that the numerical ranges expressed as "A to B" in this specification include A and B.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged, and includes so-called storage batteries and electricity storage elements such as electric double-layer capacitors. 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 realizes charging and discharging by the transfer of charge associated with lithium ions between the positive and negative electrodes.

The present invention will be described in detail below using a positive electrode used in a lithium ion secondary battery as an example, but it is not intended to limit the present invention to the embodiment described therein. Figure 1 is a schematic cross-sectional view of the positive electrode according to this embodiment taken along the thickness and width directions.

As shown in the figure, the positive electrode 50 includes a positive electrode current collector 52 and a positive electrode active material layer 54 supported by the positive electrode current collector 52. In the illustrated example, the positive electrode active material layer 54 is provided on both sides of the positive electrode current collector 52. However, the positive electrode active material layer 54 may be provided on one side of the positive electrode current collector 52. Preferably, the positive electrode active material layer 54 is provided on both sides of the positive electrode current collector 52.

As shown in the illustrated example, a positive electrode active material layer non-forming portion 52a where the positive electrode active material layer 54 is not provided may be provided at one end in the width direction of the positive electrode 50. In the positive electrode active material layer non-forming portion 52a, the positive electrode current collector 52 is exposed, and the positive electrode active material layer non-forming portion 52a can function as a current collector. However, the configuration for collecting current from the positive electrode 50 is not limited to this.

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

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

The positive electrode active material layer 54 contains a positive electrode active material, carbon nanotubes (CNTs), polyvinylidene fluoride (PVdF), and polyamic acid.

As the positive electrode active material, for example, a lithium composite oxide, a lithium transition metal phosphate compound, etc. can be used. 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, etc.

As the lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferable, 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.

In this specification, the term "lithium nickel cobalt manganese composite oxide" includes oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements other than those. 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 semimetal 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 above-mentioned 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.

Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ ), and lithium manganese iron phosphate.

The above positive electrode active materials can be used alone or in combination of two or more. Lithium nickel cobalt manganese composite oxide is preferred as the positive electrode active material because of its excellent properties such as low initial resistance. The higher the ratio of nickel to metal elements other than lithium contained in the lithium nickel cobalt manganese composite oxide, the greater the benefit of the heat suppression effect during a severe internal short circuit. Therefore, in the lithium nickel cobalt manganese composite oxide, the ratio of nickel to metal elements other than lithium is preferably 50 mol% or more, more preferably 55 mol% or more, and even more preferably 60 mol% or more.

The average particle diameter 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. In this specification, the term "average particle diameter" refers to the median diameter (D50), and therefore means the particle diameter corresponding to a cumulative frequency of 50 volume% from the fine particle side with a small particle diameter in the volume-based particle size distribution based on the laser diffraction/scattering method. Therefore, the average particle diameter (D50) can be determined using a known laser diffraction/scattering type particle size distribution measuring device, etc.

The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 87% by mass or more, more preferably 90% by mass or more, and even more preferably 96% by mass or more.

In this embodiment, a combination of PVdF and polyamic acid is used as the binder component of the positive electrode active material layer 54. The combination of this binder component with CNT as a conductive material ensures the performance of the positive electrode and increases the resistance of the positive electrode active material layer 54 when heat is generated due to a severe internal short circuit.

PVdF contributes to ensuring the various characteristics (particularly cycle characteristics) of the secondary battery using the positive electrode according to this embodiment. The PVdF used in this embodiment may be a known binder used in the positive electrode of a secondary battery. Therefore, the molecular weight of PVdF may be the same as that of the known binder PVdF used in the positive electrode of a secondary battery. PVdF is typically a homopolymer, but may be copolymerized with monomers other than vinylidene fluoride within a range that does not impair the effects of the present invention (for example, 10 mol% or less of all monomer units).

Polyamic acid, also called polyamic acid, is a compound obtained by the reaction of tetracarboxylic dianhydride and diamine, and functions as a polyimide precursor. Therefore, when polyamic acid is heated to about 200°C to 300°C, it is usually imidized. Therefore, the temperature of the secondary battery exceeds 500°C due to heat generation immediately after a severe internal short circuit occurs, such as when a metal member penetrates the electrode body, but the polyamic acid causes imidization in the positive electrode active material layer 54, which can increase the resistance of the positive electrode active material layer 54. As a result, heat generation during a severe internal short circuit can be suppressed. This effect is obtained by the combination of CNT and polyamic acid. This is thought to be because the CNT adsorbs polyamic acid due to the interaction between the surface functional group of the CNT and the carboxyl group of the polyamic acid, and the adsorbed polyamic acid is imidized, thereby increasing the resistance of the conductive path.

The tetracarboxylic dianhydride may be an aliphatic tetracarboxylic dianhydride, an alicyclic tetracarboxylic dianhydride, or an aromatic tetracarboxylic dianhydride.

Examples of aliphatic tetracarboxylic dianhydrides include 1,2,3,4-butanetetracarboxylic dianhydride.

Examples of alicyclic tetracarboxylic dianhydrides include 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 1,2,4,5-cyclopentane tetracarboxylic dianhydride, 1,2,4,5-cyclohexane tetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic dianhydride, and 2,3,5-tricarboxycyclopentylacetic dianhydride.

Examples of aromatic tetracarboxylic dianhydrides include pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, and 2,3,6,7-naphthalenetetracarboxylic dianhydride.

These tetracarboxylic dianhydrides can be used alone or in combination of two or more. As the tetracarboxylic dianhydride, aromatic tetracarboxylic dianhydrides are preferred, pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 4,4'-oxydiphthalic anhydride are more preferred, and pyromellitic dianhydride is even more preferred.

The diamine may be an aliphatic diamine, an alicyclic diamine, or an aromatic diamine.

Examples of aliphatic diamines include 1,3-propanediamine, tetramethylenediamine, pentamethylenediamine, and hexamethylenediamine.

Examples of alicyclic diamines include 1,4-diaminocyclohexane and 4,4'-methylenebis(cyclohexylamine).

Examples of aromatic diamines include p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, 4,4'-diaminobiphenyl, 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl, 3,3'-diaminodiphenylsulfone, 4,4'-diaminodiphenylsulfone, 4,4'-diaminodiphenylsulfide, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylether, 3,4'-diaminodiphenylether, and 3,3'-diaminodiphenyl ether. ether, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, etc.

These diamines can be used alone or in combination of two or more. As the diamine, aromatic diamines are preferred, with p-phenylenediamine, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and 4,4'-diaminodiphenyl ether being more preferred, and 4,4'-diaminodiphenyl ether being even more preferred.

Since the positive electrode according to this embodiment increases the resistance of the positive electrode active material layer by imidizing the polyamic acid, it is preferable that the imidization rate of the polyamic acid is low. Specifically, the imidization rate of the polyamic acid is preferably 0% to 30%, more preferably 0% to 15%, and even more preferably 0% to 10%. The imidization rate of the polyamic acid can be obtained by measurement using a Fourier transform infrared spectrophotometer (FT-IR). For example, when the polyamic acid contains an aromatic ring, the imidization rate is obtained by measuring the intensity of the peak derived from the imide group (for example, a peak at a wavelength of about 1775 cm ⁻¹ ) and the intensity of the peak derived from the aromatic ring group (for example, a peak at a wavelength of about 1519 cm ⁻¹ ), and obtaining the ratio of the peak intensity derived from the imide group to the peak intensity derived from the aromatic ring group. Then, when the polyamic acid is completely imidized (for example, when heat-treated at 350° C. for 3 hours under vacuum), the ratio of these peak intensities is obtained, and the imidization rate is calculated based on this as the standard (i.e., 100%).

The mass ratio of polyamic acid to PVdF (polyamic acid/PVdF) is not particularly limited, but the larger it is, the greater the effect of increasing the resistance of the positive electrode active material layer when heat is generated. Therefore, the mass ratio (polyamic acid/PVdF) is preferably 20/80 or more. On the other hand, from the viewpoint of particularly good battery characteristics, the mass ratio (polyamic acid/PVdF) is preferably 80/20 or less. From the viewpoint of a higher effect of increasing the resistance of the positive electrode active material layer when heat is generated, the mass ratio (polyamic acid/PVdF) is preferably 40/60 to 80/20, more preferably 50/50 to 80/20, and even more preferably 65/35 to 80/20.

The content of polyamic acid in the positive electrode active material layer 54 is not particularly limited, but from the viewpoint of a higher resistance increase effect of the positive electrode active material layer during heat generation, it is preferably 0.2 mass %, and more preferably 0.5 mass % or more. On the other hand, the content of polyamic acid in the positive electrode active material layer 54 may be 5 mass % or less, 2 mass % or less, or 0.8 mass % or less. The content of polyamic acid in the positive electrode active material layer 54 is particularly preferably 0.2 mass % to 0.8 mass %.

The total content of PVdF and polyamic acid in the positive electrode active material layer 54 is not particularly limited, and is, for example, 0.3% by mass to 10% by mass, preferably 0.4% by mass to 5% by mass, and more preferably 0.5% by mass to 2% by mass.

The CNTs function as a conductive material in the positive electrode active material layer 54. Examples of CNTs that can be used include single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs). These can be used alone or in combination of two or more types. The CNTs may be manufactured by an arc discharge method, a laser ablation method, a chemical vapor deposition method, or the like.

Here, the CNT can adsorb polyamic acid. In this embodiment, it is preferable that the CNT adsorbs polyamic acid. If the specific surface area of the CNT is large, a large amount of polyamic acid can be adsorbed to the CNT, and imidization occurs mainly around the CNT due to heat generation during a severe internal short circuit. Therefore, it is advantageous for the resistance to be increased if the specific surface area of the CNT is relatively large. Therefore, the specific surface area of the CNT is preferably 60 m ² /g or more, more preferably 120 m ² /g or more, and even more preferably 160 m ² /g or more. On the other hand, the upper limit of the specific surface area of the CNT is not particularly limited, and may be 2000 m ² /g or less, 1500 m ² /g or less, or 1200 m ² /g or less.

The specific surface area of CNT refers to the BET specific surface area of CNT, and can be measured using nitrogen as an adsorbate according to the BET method.

The average length of the CNTs is not particularly limited. If the average length of the CNTs is too long, the CNTs tend to aggregate and the dispersibility decreases. Therefore, the average length of the CNTs is preferably 15 μm or less, more preferably 8.0 μm or less, even more preferably 5.0 μm or less, and most preferably 3.0 μm or less. On the other hand, if the average length of the CNTs is too short, it tends to be difficult to form a conductive path between the positive electrode active materials. Therefore, the average length of the CNTs is preferably 0.3 μm or more, more preferably 0.5 μm or more, even more preferably 0.8 μm or more, and most preferably 1.0 μm or more.

The average diameter of the CNTs is not particularly limited, and is, for example, 0.1 nm to 150 nm, preferably 0.5 nm to 60 nm, and more preferably 0.5 nm to 30 nm.

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) to determine the lengths and diameters of 100 or more CNTs and calculate the average value. At this time, if the CNTs have re-aggregated, the length and diameter are determined for the bundle of aggregated CNTs.

The CNT content in the positive electrode active material layer 54 is not particularly limited, and is, for example, 0.1% by mass to 10% by mass, preferably 0.2% by mass to 5% by mass, and more preferably 0.3% by mass to 2% by mass.

The positive electrode active material layer 54 may further contain components other than those described above (i.e., optional components). Examples of optional components include a carbon nanotube dispersant and trilithium phosphate.

Specific examples of CNT dispersants include polycondensation aromatic surfactants such as sodium salt of naphthalenesulfonic acid formalin condensate, ammonium salt of naphthalenesulfonic acid formalin condensate, and sodium salt of methylnaphthalenesulfonic acid formalin condensate; polycarboxylic acids and their salts, such as polyacrylic acid and its salts, and polymethacrylic acid and its salts; triazine derivative dispersants (preferably containing a carbazolyl group or a benzimidazolyl group); polyvinylpyrrolidone (PVP); polymers having polynuclear aromatics such as pyrene and anthracene on the side chain; 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 of two or more. As CNT dispersants, those containing polynuclear aromatics are preferred. Specifically, as CNT dispersants, polymers having polynuclear aromatics on the side chain and polynuclear aromatic ammonium derivatives are preferred.

The amount of CNT dispersant used is not particularly limited, and is, for example, 1 part by mass or more and 400 parts by mass or less, and preferably 20 parts by mass or more and 200 parts by mass or less, per 100 parts by mass of CNT.

The thickness of each side of the positive electrode active material layer 54 is not particularly limited, but is usually 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is usually 300 μm or less, and preferably 200 μm or less.

In the positive electrode 50, the positive electrode active material layer non-forming portion 52a may be provided with an insulating protective layer (not shown) containing insulating particles (e.g., ceramic particles) adjacent to the positive electrode active material layer 54. This protective layer can prevent a short circuit between the positive electrode active material layer non-forming portion 52a and the negative electrode.

The positive electrode 50 according to this embodiment can be produced by preparing a positive electrode slurry containing a positive electrode active material, CNT, PVdF, polyamic acid, optional components, and a solvent (dispersion medium), applying the slurry onto the positive electrode current collector 52, drying it, and then pressing it as necessary.

The positive electrode according to this embodiment can increase the resistance of the positive electrode active material layer when heat is generated during a severe internal short circuit in which the metal member penetrates the electrode body of the secondary battery. This can therefore suppress heat generation during the severe internal short circuit. Furthermore, the positive electrode according to this embodiment can impart good resistance characteristics and cycle characteristics to the secondary battery.

Therefore, from another perspective, the secondary battery according to this embodiment includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode is the positive electrode according to this embodiment.

The secondary battery according to this embodiment will be described in detail below, taking as an example a flat rectangular lithium ion secondary battery having a flat wound electrode body and a flat battery case. However, the secondary battery according to this embodiment is not limited to the example described below.

The lithium ion secondary battery 100 shown in FIG. 2 is a sealed battery constructed by housing a flat wound electrode body 20 and a nonaqueous electrolyte 80 in a flat rectangular battery case (i.e., an outer container) 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher. The battery case 30 is also provided with an injection port (not shown) for injecting the nonaqueous electrolyte 80. The positive terminal 42 is electrically connected to the positive electrode current collector 42a. The negative terminal 44 is electrically connected to the negative electrode current collector 44a. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum. Note that FIG. 2 does not accurately represent the amount of the nonaqueous electrolyte 80.

As shown in Figures 2 and 3, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are stacked with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction.

The positive electrode sheet 50 used is the positive electrode 50 according to the present embodiment described above. 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 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 collector 62.

The positive electrode 50 has a positive electrode active material layer non-forming portion 52a where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed. The negative electrode 60 has a negative electrode active material layer non-forming portion 62a where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed. The positive electrode active material layer non-forming portion 52a and the negative electrode active material layer non-forming portion 62a are formed so as to protrude outward from both ends of the wound electrode body 20 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode active material layer non-forming portion 52a and the negative electrode active material layer non-forming portion 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

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, examples of which include a sheet or foil made of a metal with good electrical conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil is preferred as the negative electrode current collector 62.

The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately according to the battery design. When copper foil is used as the negative electrode current collector 62, the thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

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 the graphite is coated with an amorphous carbon material.

The average particle diameter (D50) of the negative electrode active material is not particularly limited, but is, for example, 0.1 μm or more and 50 μm or less, preferably 1 μm or more and 25 μm or less, and more preferably 5 μm or more and 20 μm or less.

The negative electrode active material layer 64 may contain components other than the active material, such as a binder or a thickener. 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).

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.

The thickness of the negative electrode active material layer 64 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.

The separator 70 may be a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which a PP layer is laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

The nonaqueous electrolyte 80 typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). As the nonaqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones that are used in the electrolyte of general lithium ion secondary batteries can be used without any particular limitation. Specific examples 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), and trifluorodimethyl carbonate (TFDMC). Such nonaqueous solvents can be used alone or in appropriate combination of two or more.

As the supporting salt, for example, a lithium salt such as LiPF ₆ , LiBF ₄ , or lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF ₆ ) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol/L or more and 1.3 mol/L or less.

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

The lithium ion secondary battery 100 constructed as described above suppresses heat generation in the event of a severe internal short circuit in which the metal member penetrates the electrode body 20. The lithium ion secondary battery 100 has low initial resistance and good cycle characteristics.

The lithium ion secondary battery 100 can be used for a variety of purposes. Specific applications include portable power sources for personal computers, portable electronic devices, portable terminals, etc.; power sources for driving vehicles such as electric vehicles (BEVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs); and storage batteries for small power storage devices. Of these, a power source for driving vehicles is preferred. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of multiple batteries connected in series and/or parallel.

As an example, a rectangular lithium ion secondary battery 100 having a flat wound electrode body 20 has been described. However, the lithium ion secondary battery disclosed herein can also be configured as a lithium ion secondary battery having a stacked electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked). In addition, the nonaqueous electrolyte secondary battery disclosed herein can also be configured as a cylindrical lithium ion secondary battery, a laminated case type lithium ion secondary battery, a coin type lithium ion secondary battery, etc.

In addition, a secondary battery other than a lithium-ion secondary battery can be constructed using the above-mentioned positive electrode according to a known method. Furthermore, an all-solid-state secondary battery (particularly an all-solid-state lithium-ion secondary battery) can be constructed using a solid electrolyte instead of the non-aqueous electrolyte 80 according to a known method. Note that, since there is a large benefit in suppressing heat generation during a severe internal short circuit, it is advantageous for the secondary battery to be a secondary battery that includes a non-aqueous electrolyte.

The following describes examples of the present invention, but is not intended to limit the present invention to those examples.

<Examples 1 to 5 and Comparative Examples 1 to 5>
A lithium transition metal oxide _{represented by LiNi0.6Co0.2Mn0.2O2 was} prepared as the positive electrode active material. CNT having a specific surface area shown in Table 1 and acetylene black (AB) were prepared as the conductive material. Polyamic acid (imidization rate: 8%), which is a reaction product of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, was prepared as the polyamic acid. The positive electrode active material, the conductive material shown in Table 1, polyamic acid, and PVdF were mixed in a mass ratio of 97.5:1.5:x:1-x, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the obtained mixture to prepare a positive electrode slurry.

The positive electrode slurry was applied to both sides of an aluminum foil having a thickness of 12 μm as a positive electrode current collector. At this time, a positive electrode slurry uncoated portion was provided on the aluminum foil as a lead connection portion. The applied slurry was dried to form a positive electrode active material layer. The obtained sheet was pressed using a roller, and then 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. The basis weight of the positive electrode was 45 mg/cm ² , and the packing density of the positive electrode active material layer was 3.50 g/cm ^3. Here, only the positive electrode of Comparative Example 5 was further heat-treated at 350° C. under vacuum for 3 hours to imidize the polyamic acid.

Graphite as a carbon-based negative electrode active material, sodium salt of carboxymethylcellulose (CMC-Na), and a dispersion of styrene butadiene rubber (SBR) were mixed in a solid mass ratio of graphite:CMC-Na:CMC=98:1:1. An appropriate amount of ion-exchanged water was further added to prepare a negative electrode slurry. The negative electrode slurry was applied to both sides of a copper foil having a thickness of 8 μm as a negative electrode current collector. At this time, a negative electrode slurry uncoated portion was provided on the copper foil as a lead connection portion. The applied slurry was dried to form a negative electrode active material layer. The obtained sheet was pressed using 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 .

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 separators in between to produce a laminated electrode body.

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 2 mass%, lithium bis(oxalate)borate was dissolved in a concentration of 0.8 mass%, and LiPF ₆ as a supporting salt was dissolved in a concentration of 1.15 mol/L. This resulted in a nonaqueous electrolyte solution.

The laminated electrode body and nonaqueous electrolyte prepared above were placed in a square battery case and sealed to obtain a square lithium-ion secondary battery for evaluation.

<Output resistance measurement>
Each evaluation lithium-ion secondary battery was adjusted to a state of charge (SOC) of 50% by constant current constant voltage (CC-CV) charging, and then placed in an environment of 25°C. Discharging was performed for 10 seconds at a current value of 5C, and the voltage drop amount ΔV at this time was obtained. The output resistance value of each evaluation secondary battery was calculated using the voltage drop amount ΔV and the current value. The results are shown in Table 1.

<Cycle characteristic evaluation>
Each evaluation lithium ion secondary battery was placed in an environment of 25° C. and subjected to CC-CV charging for 1.5 hours at a charging voltage of 4.25 V and a charging current of 1/3 C. Thereafter, the battery was discharged at a constant current (CC) of 1/3 C to 3.0 V. The discharge capacity at this time was measured and used as the initial capacity.

Next, each evaluation lithium-ion secondary battery was placed in an environment of 45°C. Each evaluation lithium-ion secondary battery was subjected to 500 cycles of charge and discharge, with the above-mentioned CC-CV charge and CC discharge being one cycle. Thereafter, the discharge capacity was measured in the same manner as the initial capacity. The capacity retention rate was calculated by (discharge capacity after 500 cycles/initial capacity) x 100. The results are shown in Table 1.

<Positive electrode resistance change>
The resistance of the positive electrode active material layer was measured using an electrode resistance measurement system "RM2610" manufactured by HIOKI EE Corporation. Each evaluation lithium ion secondary battery was disassembled to remove the positive electrode, and the resistance of the positive electrode was measured using the electrode resistance measurement system, which was taken as the initial resistance. The measurement conditions were a measurement current of 10 mA, a measurement speed of Normal, a voltage range of 0.5 V, and a volume resistivity of the aluminum foil current collector of 3.8E-06 Ω cm.

Next, this positive electrode was heat-treated at 350°C for 3 hours in a vacuum. After that, the resistance of the positive electrode was measured again using an electrode resistance measuring system, and this was taken as the resistance after heat treatment. The magnification of the resistance change of the positive electrode was calculated by dividing the resistance after heat treatment by the initial resistance. The results are shown in Table 1.

From the results in Table 1, it can be seen that in Examples 1 to 5, while the initial resistance (output resistance) is sufficiently small, the resistance of the positive electrode after heat treatment increases significantly. On the other hand, from the results of Comparative Examples 1 and 2, it can be seen that when acetylene black (AB), which is commonly used in lithium ion secondary batteries, is used as the conductive material, the resistance increase effect is not obtained. Furthermore, from the results of Comparative Example 3, it can be seen that the resistance increase effect is not obtained when only PVdF is used as the binder. Therefore, it can be seen that a unique effect is obtained by combining CNT and polyamic acid.

On the other hand, Comparative Example 4 shows that when only PVdF is used as the binder, the cycle characteristics are insufficient for the positive electrode of a secondary battery. Comparative Example 5 shows that when polyamic acid is imidized in advance, the initial resistance is high and the resistance characteristics are insufficient for the positive electrode of a secondary battery.

In addition, in the heat treatment at 350°C when evaluating the change in the positive electrode resistance, the resistance increase due to imidization occurs sufficiently. On the other hand, in the case of a severe internal short circuit in which the metal member penetrates the electrode body, the temperature reaches over 500°C. Therefore, the heat generated during such an internal short circuit can sufficiently promote imidization to increase the resistance. Therefore, it can be seen that the positive electrode disclosed herein can provide a positive electrode in which the resistance of the positive electrode active material layer increases when heat is generated due to a severe internal short circuit in which the metal member penetrates the electrode body.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

That is, the positive electrode and secondary battery disclosed herein are the following items [1] to [5].
[1] A positive electrode current collector,
a positive electrode active material layer supported on the positive electrode current collector;
A positive electrode comprising:
The positive electrode active material layer contains a positive electrode active material, carbon nanotubes, polyvinylidene fluoride, and polyamic acid.
Positive electrode.
[2] The positive electrode according to item [1], wherein the mass ratio of polyvinylidene fluoride to polyamic acid is 20/80 to 80/20.
[3] The positive electrode according to item [1], wherein the mass ratio of polyvinylidene fluoride to polyamic acid is 50/50 to 80/20.
[4] The positive electrode according to any one of items [1] to [3], wherein the specific surface area of the carbon nanotubes is 160 m ² /g or more and 1200 m ² /g or less.
[5] The positive electrode according to any one of items [1] to [4], wherein the content of the polyamic acid in the positive electrode active material layer is 0.2% by mass to 0.8% by mass.
[6] The positive electrode according to any one of items [1] to [5],
A negative electrode;
An electrolyte;
A secondary battery comprising:

20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode)
52 Positive electrode current collector 52a Positive electrode active material layer non-forming portion 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-forming portion 64 Negative electrode active material layer 70 Separator sheet (separator)
80 Non-aqueous electrolyte 100 Lithium ion secondary battery

Claims (5)

A positive electrode current collector;
a positive electrode active material layer supported on the positive electrode current collector;
A positive electrode comprising:
the positive electrode active material layer contains a positive electrode active material, carbon nanotubes, polyvinylidene fluoride, and polyamic acid ;
The content of the polyamic acid in the positive electrode active material layer is 0.2% by mass to 0.8% by mass.
Positive electrode. 2. The positive electrode according to claim 1, wherein a mass ratio of the polyamic acid to the polyvinylidene fluoride is from 20/80 to 80/20. 2. The positive electrode according to claim 1, wherein a mass ratio of the polyamic acid to the polyvinylidene fluoride is 50/50 to 80/20. 2. The positive electrode according to claim 1, wherein the carbon nanotubes have a specific surface area of 160 m ² /g or more and 1200 m ² /g or less. The positive electrode according to claim 1 ;
A negative electrode;
An electrolyte;
A secondary battery comprising:

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