Negative electrode and secondary battery including the same

Abstract

To provide a negative electrode that includes graphite as a negative electrode active material and includes a carbon nanotube as a conductive material, and that can suppress change in thickness of the negative electrode at the time when charging and discharging to and from a secondary battery are repeated and can give an excellent high-temperature preservation characteristic to the secondary battery.SOLUTION: A negative electrode comprises a negative electrode current collector, and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material and a single-walled carbon nanotube. The negative electrode active material is essentially constituted of graphite. A mass ratio of the single-walled carbon nanotube to the negative electrode active material is 0.02 mass% to 0.08 mass%.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a negative electrode. The present invention also relates to a secondary battery including the negative electrode. [Background technology]

[0002] 2. Description of the Related Art In recent years, secondary batteries such as lithium-ion secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, and the like, and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), among others.

[0003] A negative electrode used in a secondary battery such as a lithium ion secondary battery generally has a structure in which a negative electrode active material layer is provided on a negative electrode current collector. A technique is known in which graphite is used as the negative electrode active material contained in the negative electrode active material layer and carbon nanotubes are used as a conductive material (for example, Patent Document 1). Patent Document 1 shows that the cycle characteristics of a secondary battery are improved by using 0.1 to 10 mass % of multi-walled carbon nanotubes relative to graphite in the negative electrode active material layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2005-4974 A Summary of the Invention [Problem to be solved by the invention]

[0005] As a result of intensive research by the present inventors, it was found that in a secondary battery using the above-mentioned conventional negative electrode, there is a problem in that it is not possible to simultaneously suppress the change in thickness of the negative electrode when repeatedly charged and discharged, and suppress the capacity deterioration when placed at high temperatures for a long period of time. If the thickness change of the negative electrode is large, the dimensional change of the electrode body including the negative electrode becomes large. If the dimensional change of the electrode body is large, a reaction force is generated against the maintenance of the interelectrode distance, which may adversely affect the battery characteristics. In addition, if the dimensional change of the electrode body is large, the problem of battery case swelling may also occur.

[0006] Therefore, an object of the present invention is to provide a negative electrode that uses graphite as the negative electrode active material and carbon nanotubes as the conductive material, which can suppress changes in the thickness of the negative electrode when the secondary battery is repeatedly charged and discharged, and can impart excellent high-temperature storage characteristics to the secondary battery. [Means for solving the problem]

[0007] The negative electrode disclosed herein includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material and single-walled carbon nanotubes. The negative electrode active material is essentially composed of graphite. The mass ratio of the single-walled carbon nanotubes to the negative electrode active material is 0.02 mass% or more and 0.08 mass% or less.

[0008] According to such a configuration, a negative electrode using graphite as the negative electrode active material and carbon nanotubes as the conductive material can be provided, which can suppress changes in the thickness of the negative electrode when the secondary battery is repeatedly charged and discharged, and can provide the secondary battery with excellent high-temperature storage characteristics (i.e., resistance to capacity degradation when stored at high temperatures for a long period of time). [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view illustrating a schematic diagram of a negative electrode according to one embodiment of the present invention. [Diagram 2]FIG. 1 is a cross-sectional view showing a schematic internal structure of a lithium-ion secondary battery using a negative electrode according to one embodiment of the present invention. [Diagram 3] 3 is a schematic exploded view showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for carrying out 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 carried out based on the contents disclosed in this specification and the technical common sense in the relevant field. In addition, in the following drawings, members and parts having the same function are described by using the same reference numerals. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

[0011] 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 the lithium ions between the positive and negative electrodes.

[0012] The negative electrode disclosed herein is typically used in a secondary battery, and is preferably used in a lithium ion secondary battery. Fig. 1 is a cross-sectional view showing a cross-sectional view along the thickness direction of a negative electrode 60 according to the present embodiment, which is an example of the negative electrode disclosed herein. The negative electrode 60 according to the present embodiment shown in Fig. 1 is a negative electrode for a lithium ion secondary battery.

[0013] As shown in the figure, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes the negative electrode current collector 62 and the negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided on only one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. The negative electrode active material layer 64 is preferably provided on both sides of the negative electrode current collector 62.

[0014] As shown in the illustrated example, a negative electrode active material layer non-forming portion 62a where no negative electrode active material layer 64 is provided may be provided at one end in the width direction of the negative electrode 60. In the negative electrode active material layer non-forming portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-forming portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0015] In the illustrated example, the shape of the negative electrode collector 62 is a foil (or sheet), but is not limited thereto. The negative electrode collector 62 may have various forms such as a rod, a plate, a mesh, etc. As with conventional lithium ion secondary batteries, the material of the negative electrode collector 62 may be a metal with good electrical conductivity (e.g., copper, nickel, titanium, stainless steel, etc.), and among these, copper is preferable. Copper foil is particularly preferable as the negative electrode collector 62.

[0016] The dimensions of the negative electrode current collector 62 are not particularly limited and may be appropriately determined depending on the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 6 μm or more and 20 μm or less.

[0017] The negative electrode active material layer 64 contains a negative electrode active material and single-walled carbon nanotubes (SWCNTs). The SWCNTs are typically dispersed in the negative electrode active material layer 64. The SWCNTs come into contact with a plurality of particles of the negative electrode active material, or come into contact with the particles of the negative electrode active material and the negative electrode current collector 62, thereby forming a conductive path and functioning as a conductive material.

[0018] In this embodiment, the negative electrode active material is essentially composed of graphite. In this specification, "the negative electrode active material is essentially composed of graphite" means that the negative electrode active material essentially contains graphite and may contain a negative electrode active material other than graphite, but the negative electrode active material other than graphite is present to an extent that does not adversely affect the effects of the present invention. The proportion of graphite in the negative electrode active material is preferably 99% by mass or more, more preferably 99.5% by mass or more, and most preferably 100% by mass (i.e., the negative electrode active material is composed only of graphite).

[0019] The graphite may be natural graphite or artificial graphite. The graphite may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

[0020] The average particle diameter (D50) of the negative electrode active material is not particularly limited, and is typically 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 average particle diameter (D50) of the negative electrode active material can be obtained by measuring the particle size distribution of the negative electrode active material on a volume basis by a laser diffraction scattering method and determining the particle diameter (D50) at which the cumulative frequency in the particle size distribution is 50% by volume percentage.

[0021] The BET specific surface area of ​​the negative electrode active material is not particularly limited and is usually 1.5 m 2 / g or more, preferably 2.5m 2 / g or more. On the other hand, the BET specific surface area is usually 10 2 / g or less, preferably 6m 2 / g or less. In this specification, the term "BET specific surface area" refers to a value obtained by analyzing the amount of gas adsorption measured by a gas adsorption method (constant volume adsorption method) using nitrogen (N2) gas as an adsorbate, using the BET method.

[0022] In this embodiment, SWCNTs are used. The type of SWCNT is not particularly limited, and the SWCNTs may be produced by an arc discharge method, a laser ablation method, a chemical vapor deposition method, or the like.

[0023] By using a specific amount of SWCNTs as CNTs, it is possible to suppress the change in thickness of the negative electrode 60 when the secondary battery using the negative electrode 60 is repeatedly charged and discharged, and to improve the resistance to capacity degradation when the secondary battery using the negative electrode 60 is left at high temperatures for a long period of time. This is believed to be due to the following reasons.

[0024] The graphene sheet structure of the CTN interacts with the graphene sheet structure of the graphite, and since the CNT has a fibrous shape, it entangles the graphite particles and holds them. Therefore, the CNT has a certain degree of function of bonding between the graphite particles. Since the SWCNT has a much smaller diameter than the multi-walled carbon nanotube (MWCNT) used in the conventional technology, the number of CNTs is much larger when the same amount is used. Therefore, even when a small amount is used, the SWCNT can sufficiently bond between the graphite particles, and the change in thickness of the negative electrode 60 when the secondary battery is repeatedly charged and discharged can be suppressed. In addition, the intercalation of lithium ions into the CNT structure of the SWCNT is significantly less likely to occur than that of the MWCNT, and this makes it difficult for the electrolyte to decompose when the secondary battery is placed under high temperature for a long period of time. As a result, it is possible to suppress the capacity deterioration when the secondary battery is placed under high temperature for a long period of time.

[0025] The average length of SWCNT (in other words, the average fiber length) is not particularly limited. If the average length of SWCNT is too long, SWCNT tends to aggregate and the dispersibility tends to decrease. Therefore, the average length of SWCNT 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 SWCNT is too short, it tends to be difficult to form a conductive path between the negative electrode active materials. Therefore, the average length of SWCNT 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.

[0026] The average diameter of the SWCNTs (in other words, the average fiber diameter) is not particularly limited, but is preferably 0.5 nm or more, more preferably 1.0 nm or more. The average diameter of the SWCNTs is preferably 3.0 nm or less, more preferably 2.5 nm or less, and even more preferably 2.0 nm or less.

[0027] The average length and average diameter of CNTs can be obtained, for example, by taking an electron microscope photograph of 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 obtain the lengths and diameters of 100 or more CNTs and calculate the average value. At this time, if the CNTs are re-aggregated, the length and diameter are obtained for the bundle of aggregated CNTs.

[0028] In this embodiment, the mass ratio of SWCNT to the negative electrode active material is 0.02 mass% or more and 0.08 mass% or less. If the mass ratio of SWCNT is less than 0.02 mass%, the dimensional change of the negative electrode 60 increases when the charge and discharge are repeated. On the other hand, if the mass ratio of SWCNT exceeds 0.08 mass%, the resistance to capacity degradation when placed under high temperature for a long period of time decreases.

[0029] The negative electrode active material layer 64 may contain components other than the negative electrode active material and SWCNTs (hereinafter also referred to as "optional components"). Examples of the optional components include a binder, a thickener, a carbon nanotube (CNT) dispersant, etc.

[0030] Examples of binders include styrene butadiene rubber (SBR) and polyvinylidene fluoride (PVDF). Examples of thickeners include carboxymethyl cellulose (CMC) and its salts. Specific examples of CNT dispersants include polycondensation aromatic surfactants such as sodium salt of naphthalene sulfonic acid formalin condensate, ammonium salt of naphthalene sulfonic acid formalin condensate, and sodium salt of methylnaphthalene sulfonic acid formalin 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 such as pyrene and anthracene in 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 the CNT dispersant, those containing polynuclear aromatics are preferable. Specifically, as the CNT dispersant, a polymer having a polynuclear aromatic on the side chain and a polynuclear aromatic ammonium derivative are preferable.

[0031] 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 amount of the CNT dispersant used 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, relative to 100 parts by mass of the SWCNTs.

[0032] The thickness of each surface of the negative electrode active material layer 64 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 400 μm or less, and preferably 300 μm or less.

[0033] The negative electrode 60 may include a member other than the negative electrode current collector 62 and the negative electrode active material layer 64 (for example, an insulating layer).

[0034] The negative electrode 60 according to this embodiment can suppress the change in thickness of the negative electrode 60 when the secondary battery using the negative electrode 60 is repeatedly charged and discharged, and can also suppress capacity degradation. In addition, the secondary battery using the negative electrode 60 can be endowed with excellent high-temperature storage characteristics (particularly, resistance to capacity degradation when stored at high temperatures for a long period of time).

[0035] From another perspective, the secondary battery disclosed herein includes a positive electrode, the negative electrode 60 according to the present embodiment, and an electrolyte. Hereinafter, one embodiment of the secondary battery disclosed herein will be described with reference to Figs. 2 and 3, taking a lithium ion secondary battery as an example.

[0036] The lithium ion secondary battery 100 shown in FIG. 2 is a sealed lithium ion secondary battery 100 constructed by housing a flat wound electrode body 20 and a nonaqueous electrolyte (not shown) in a flat rectangular battery case (i.e., an outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode 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. The positive electrode terminal 42 is electrically connected to the positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to the negative electrode current collector plate 44a. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum.

[0037] 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 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 here) 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 here) 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.

[0038] 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 having good electrical conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). The positive electrode current collector 52 is preferably aluminum foil.

[0039] The dimensions of the positive electrode current collector 52 are not particularly limited and may be appropriately determined depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 7 μm or more and 20 μm or less.

[0040] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a positive electrode active material having a known composition used in a lithium ion secondary battery may be used. Specifically, for example, as the positive electrode active material, a lithium composite oxide, a lithium transition metal phosphate compound, or the like may 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, or the like.

[0041] 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-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel manganese-based composite oxides, lithium nickel cobalt manganese-based composite oxides, lithium nickel cobalt aluminum-based composite oxides, and lithium iron nickel manganese-based composite oxides.

[0042] 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 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.

[0043] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0044] 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 based composite oxide is particularly preferable because of its excellent properties such as initial resistance characteristics.

[0045] 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.

[0046] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, etc. As the conductive material, for example, carbon black such as acetylene black (AB), carbon fibers such as vapor grown carbon fiber (VGCF) and carbon nanotubes (CNT), and other carbon materials (e.g., graphite, etc.) can be suitably used. As the binder, for example, polyvinylidene fluoride (PVdF) can be used.

[0047] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70 mass% or more, more preferably 80 mass% or more, and even more preferably 85 mass% or more and 99 mass% or less. The content of the trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1 mass% or more and 15 mass% or less, and more preferably 0.2 mass% or more and 10 mass% or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1 mass% or more and 20 mass% or less, and more preferably 0.3 mass% or more and 15 mass% or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.4 mass% or more and 15 mass% or less, and more preferably 0.5 mass% or more and 10 mass% or less.

[0048] The thickness of the positive electrode active material layer 54 per side 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.

[0049] As the negative electrode sheet 60, the above-mentioned negative electrode 60 is used.

[0050] 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.

[0051] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability of the separator 70 obtained by the Gurley test method is not particularly limited, but is preferably 350 sec / 100 cc or less.

[0052] The non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones, which are used in electrolytes of general lithium ion secondary batteries, can be used without any particular limitation. Among them, carbonates are preferable, 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), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combination of two or more. As an example, the non-aqueous solvent is composed of only carbonates.

[0053] As the supporting salt, for example, a lithium salt such as LiPF6, LiBF4, lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) 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.

[0054] The nonaqueous electrolyte may contain various additives other than the above-mentioned components, such as film-forming agents such as vinylene carbonate (VC) and oxalate complexes; gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); and thickeners, as long as the effects of the present invention are not significantly impaired.

[0055] In the lithium ion secondary battery 100, the change in thickness of the negative electrode 60 and the electrode body 20 during repeated charging and discharging is suppressed, and the battery characteristics are less susceptible to adverse effects due to dimensional changes in the electrode body 20. In addition, the lithium ion secondary battery 100 is also suppressed from capacity degradation during repeated charging and discharging. Furthermore, the lithium ion secondary battery 100 is also suppressed from capacity degradation when placed under high temperatures for a long period of time. The lithium ion secondary battery 100 can be used for various applications. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices and the like. The lithium ion secondary battery 100 can also be used in the form of a battery pack typically consisting of a plurality of batteries connected in series and / or parallel.

[0056] Above, a rectangular lithium ion secondary battery 100 including a flat wound electrode assembly 20 has been described as an example. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery including a laminated electrode assembly (i.e., an electrode assembly in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). The lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery, a laminated case type lithium ion secondary battery, or the like. Note that rectangular lithium ion secondary batteries are most susceptible to adverse effects on battery characteristics due to dimensional changes in the electrode assembly. Therefore, in rectangular lithium ion secondary batteries, the advantageous effects brought about by suppressing changes in the thickness of the negative electrode when charging and discharging are repeated are maximized.

[0057] Furthermore, the lithium ion secondary battery 100 can also be constructed as an all-solid-state lithium ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte according to known methods.

[0058] Furthermore, although the negative electrode 60 according to this embodiment is suitable for use as a negative electrode for a lithium ion secondary battery, it can also be used as a negative electrode for other secondary batteries, and the other secondary batteries can be constructed according to known methods.

[0059] Examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples.

[0060] <Examples 1 to 3 and Comparative Examples 1 to 9> Natural graphite (C) with an average particle size (D50) of 15 μm, silicon oxide (SiOx), an aqueous solution of sodium salt of carboxymethylcellulose (CMC-Na), a dispersion of styrene butadiene rubber (SBR), and a carbon nanotube (CNT) dispersion were mixed in a solid content mass ratio of graphite:SiOx:CMC-Na:SBR:CNT=100-α:α:1:1:β. Note that α and β are the values ​​shown in Table 1. As the carbon nanotubes, multi-walled carbon nanotubes (average fiber length 1.2 μm, average diameter 13 nm) were used in Comparative Examples 1 to 4, and single-walled carbon nanotubes (average fiber length 1.2 μm, average diameter 2 nm) were used in Examples 1 to 3 and Comparative Examples 5, 6, 8, and 9.

[0061] An appropriate amount of ion-exchanged water was added to the mixture to prepare a negative electrode slurry. The prepared negative electrode slurry was applied to both sides of a copper foil 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 amount of the negative electrode slurry applied was set to the basis weight (total of both sides) shown in Table 1.

[0062] 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.

[0063] LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2, acetylene black (AB) as a conductive material, and PVdF as a binder were mixed in a mass ratio of active material:AB:PVdF=97.0:2.0:1.0. An appropriate amount of N-methyl-2-pyrrolidone was added to this to prepare a positive electrode slurry. The positive electrode slurry was applied to both sides of an aluminum foil 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 packing density of the positive electrode active material layer was 3.50 g / cm 3 It was.

[0064] 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 interposed therebetween to prepare a laminated electrode body.

[0065] 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%, and LiPF6 was dissolved as a supporting electrolyte at a concentration of 1.15 mol / L. This resulted in a nonaqueous electrolyte.

[0066] The laminated electrode body and the non-aqueous electrolyte solution prepared above were housed in a square battery case and sealed to obtain lithium ion secondary batteries for evaluation in each of the Examples and Comparative Examples.

[0067] <Charge / discharge cycle evaluation> Each lithium ion secondary battery for evaluation was moved into a glove box under an argon atmosphere and disassembled. The electrode body was taken out and its thickness was measured, which was defined as the initial thickness.

[0068] On the other hand, each evaluation lithium-ion secondary battery was placed in an environment of 25° C. and subjected to CC-CV charging for 3 hours at a charging voltage of 4.2 V and a charging current of 0.5 C. Then, the battery was discharged at a constant current (CC) of 0.5 C to 3.0 V. The discharge capacity at this time was measured and used as the initial capacity.

[0069] Next, the charge / discharge cycle of the above CC-CV charge and CC discharge was repeated 1000 times for each evaluation lithium ion secondary battery. Thereafter, the discharge capacity was measured in the same manner as the initial capacity. The capacity retention rate (%) was calculated by (discharge capacity at the 1000th cycle / initial capacity)×100. The results are shown in Table 1.

[0070] Furthermore, each evaluation lithium-ion secondary battery after 1000 charge / discharge cycles was moved into a glove box under an argon atmosphere and disassembled. The electrode body was taken out and its thickness was measured. The difference between the thickness of the electrode body after 1000 cycles and the initial thickness was calculated as the thickness change (μm). The results are shown in Table 1. The thickness change of this electrode body is mainly due to the thickness change of the negative electrode.

[0071] <High temperature storage evaluation> The initial capacity of each evaluation lithium ion secondary battery was determined in the same manner as above. Next, each evaluation lithium ion secondary battery was placed in an environment of 25°C, and CC-CV charging was performed for 3 hours at a charging voltage of 4.2V and a charging current of 0.5C. Next, each evaluation lithium ion secondary battery was stored in a high-temperature chamber at 60°C for 180 days, and then the discharge capacity was measured in the same manner as the initial capacity. The capacity retention rate (%) was calculated by (discharge capacity after storage / initial capacity) x 100. The results are shown in Table 1. In this test, a capacity retention rate of 92.0% or more was considered to be acceptable.

[0072] [Table 1]

[0073] In Comparative Examples 1 to 4, multi-walled carbon nanotubes (MWCNTs) were used as in the conventional technology. When MWCNTs were used, the thickness change after charge-discharge cycles was large when the content was small, and the high-temperature storage characteristics were poor when the content was large. Therefore, it is found that when MWCNTs are used, it is not possible to simultaneously suppress the thickness change of the negative electrode after charge-discharge cycles and achieve excellent high-temperature storage characteristics.

[0074] On the other hand, single-walled carbon nanotubes (SWCNTs) were used in Examples 1 to 3 and Comparative Examples 5 to 6. These results show that when SWCNTs are used in an amount of 0.02 mass % to 0.08 mass % relative to the negative electrode active material, it is possible to achieve both suppression of thickness change after charge / discharge cycles and excellent high-temperature storage characteristics.

[0075] In Comparative Example 8, graphite and a small amount of silicon oxide were used in combination as the negative electrode active material, and in Comparative Example 9, silicon oxide alone was used as the negative electrode active material. These results show that the effect of suppressing thickness change after charge / discharge cycles and the effect of improving high-temperature storage characteristics can be obtained when the negative electrode active material is essentially composed of graphite.

[0076] From the above results, it can be seen that the negative electrode disclosed herein can suppress changes in the thickness of the negative electrode when the secondary battery is repeatedly charged and discharged, and can impart excellent high-temperature storage characteristics to the secondary battery.

[0077] Although the specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples exemplified above.

[0078] That is, the negative electrode and secondary battery disclosed herein include the following items [1] to [5]. [1] A negative electrode current collector; a negative electrode active material layer supported on the negative electrode current collector; A negative electrode comprising: The negative electrode active material layer contains a negative electrode active material and a single-walled carbon nanotube, the negative electrode active material consists essentially of graphite, The mass ratio of the single-walled carbon nanotubes to the negative electrode active material is 0.02 mass% or more and 0.08 mass% or less. Negative electrode. [2] The negative electrode according to item [1], wherein the single-walled carbon nanotubes have an average diameter of 0.5 nm or more and 3.0 nm or less. [3] The negative electrode according to item [1] or [2], wherein the proportion of graphite in the negative electrode active material is 99 mass% or more. [4] The negative electrode according to any one of items [1] to [3], which is a negative electrode for a lithium ion secondary battery. [5] A positive electrode; The negative electrode according to any one of items [1] to [4], An electrolyte; A secondary battery comprising: [Explanation of symbols]

[0079] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector 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) 100 Lithium-ion secondary battery

Claims

1. A negative electrode current collector; a negative electrode active material layer supported on the negative electrode current collector; A negative electrode comprising: The negative electrode active material layer contains a negative electrode active material, single-walled carbon nanotubes, and a carbon nanotube dispersant, The negative electrode active material is composed only of graphite, A mass ratio of the single-walled carbon nanotubes to the negative electrode active material is 0.02 mass% or more and 0.08 mass% or less, The carbon nanotube dispersant is a polymer having a polynuclear aromatic group on a side chain or a polynuclear aromatic ammonium derivative. Negative electrode.

2. The negative electrode according to claim 1 , wherein the single-walled carbon nanotubes have an average diameter of 0.5 nm or more and 3.0 nm or less.

3. 2. The negative electrode according to claim 1, wherein the single-walled carbon nanotubes have an average length of 3.0 μm or less.

4. 2. The negative electrode according to claim 1, wherein the single-walled carbon nanotubes have an average length of 1.0 μm or more.

5. The negative electrode according to claim 1 , which is a negative electrode for a lithium ion secondary battery.

6. A positive electrode and The negative electrode according to claim 1 , An electrolyte; A secondary battery comprising:

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