JP2020053282A - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

To provide a negative electrode having a good energy density.SOLUTION: The negative electrode for a lithium ion secondary battery includes: a current collector and an active material layer formed on the current collector. The electrode density of the active material layer is 1.4 g/cmor more. The electrolyte permeation rate into the active material layer is 0.04 cm/min or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery.

  A lithium ion secondary battery generally includes two electrodes each having an electrode active material layer formed on a surface of a metal foil, and a separator disposed between the two electrodes. Lithium-ion secondary batteries have been used as large stationary power supplies for power storage and power supplies for electric vehicles due to their high energy density.In recent years, research into battery miniaturization and thinning has progressed. I have. In such a situation, a higher capacity of the lithium ion secondary battery is further demanded with higher performance of the device, and it is important to further improve the energy density.

  As a means for improving the energy density of a lithium ion secondary battery, an active material layer on a current collector is thickened. However, when the thickness of the active material layer is increased, the distance from the active material layer to the current collector increases, so that the electrolyte does not easily penetrate from the surface of the active material layer to the vicinity of the current collector, and the charge / discharge characteristics are reduced. May not improve.

Therefore, in Patent Document 1, an electrode in which the thickness of the active material layer is 20 to ²⁰⁰ μm per one side of the current collector and the penetration rate of diethyl carbonate into the active material containing layer is 0.1 g / cm ² · min or more. Has been proposed.

JP 2013-62226 A

However, in Patent Literature 1, since the porosity of the negative electrode active material-containing layer is increased near the current collector, the negative electrode active material-containing layer is likely to peel off near the current collector, resulting in reduced durability. Is concerned.
In addition, when the energy density is high, the permeation rate of the electrolytic solution becomes slow, and it becomes difficult to obtain good charge / discharge characteristics. On the other hand, it is conceivable to lower the electrode density in order to increase the permeation rate of the electrolytic solution. However, when the electrode density decreases, the energy density also decreases.

  Therefore, an object of the present invention is to provide a negative electrode having a good energy density and excellent electrode characteristics such as charge / discharge characteristics and durability.

  The present inventors have conducted intensive studies and found that the above-mentioned problems can be solved by setting the electrode density of the active material layer in the negative electrode and the rate of penetration of the electrolyte into the active material layer in a specific range. The present invention has been completed. The gist of the present invention is as follows.

[1] A negative electrode for a lithium ion secondary battery including a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer has an electrode density of 1.4 g. / Cm ³ or more, and the electrolyte penetration rate into the negative electrode active material layer is 0.04 cm / min or more.
[2] The negative electrode for a lithium ion secondary battery according to [1], wherein the negative electrode active material contained in the negative electrode active material layer has a median diameter (D ₅₀ ) of 7 to 15 μm.
[3] The negative electrode for a lithium ion secondary battery according to [1] or [2], wherein the specific surface area of the negative electrode active material is 0.5 to 7 m ² / g.
[4] The negative electrode for a lithium ion secondary battery according to [2] or [3], wherein the negative electrode active material contains natural graphite and artificial graphite.
[5] A lithium ion secondary battery including the negative electrode for a lithium ion secondary battery according to any one of [1] to [4].

  According to the present invention, it is possible to provide a negative electrode having good energy density and excellent electrode characteristics such as charge / discharge characteristics and durability.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic sectional drawing which shows one Embodiment of the negative electrode for lithium ion secondary batteries of this invention.

<Negative electrode for lithium ion secondary battery>
Hereinafter, the negative electrode for a lithium ion secondary battery of the present invention will be described in detail.
As shown in FIG. 1, the negative electrode 10 for a lithium ion secondary battery includes a negative electrode current collector 13 and a negative electrode active material layer 11 formed on the current collector. In addition, the negative electrode active material layer 11 may be laminated on both surfaces of the negative electrode current collector 13.

Here, the electrode density of the negative electrode active material layer 11 is 1.4 g / cm ³ or more, and the electrolyte penetration rate into the negative electrode active material layer 11 is 0.04 cm / min or more.

When the electrode density is less than 1.4 g / cm ³ , the amount of the negative electrode active material per unit volume decreases, and the energy density decreases due to a decrease in capacity. Electrode density is preferably 1.55 g / cm ³ or more, 1.56 g / cm ³ or more is more preferable. In addition, from the viewpoint of preventing damage to the negative electrode active material layer and maintaining a good initial capacity, 2.0 g / cm ³ or less is preferable, and 1.8 g / cm ³ or less is more preferable. The electrode density can be measured by the method described in Examples.

If the electrolyte permeation rate is less than 0.04 cm / min, the Li transfer rate is reduced and the electrode performance such as input characteristics is reduced. The electrolyte penetration rate is preferably 0.041 cm / min or more, and more preferably 0.043 cm / min or more. In addition, from the viewpoint of maintaining the electrode density at 1.4 g / cm ³ or more, the electrolyte permeation rate is preferably equal to or less than 0.1 cm / min, and more preferably equal to or less than 0.06 cm / min. The electrolyte permeation rate can be measured by the method described in Examples.

In order to set the electrode density and the permeation rate of the electrolytic solution in the above ranges, for example, the press pressure for forming the negative electrode active material layer may be adjusted in the range of 400 to 1200 N / m. Further, in order to set the permeation rate of the electrolyte solution in a desired range, for example, it is preferable to adjust the particle size (median diameter) of the negative electrode active material. In addition, you may adjust particle shape, orientation, etc.
Hereinafter, the configuration of the negative electrode for a lithium ion secondary battery will be described in more detail.

[Negative electrode active material layer]
The negative electrode active material layer typically includes a negative electrode active material and a negative electrode binder, and appropriately includes a conductive additive and the like.

(Negative electrode active material)
Examples of the negative electrode active material used in the negative electrode active material layer include graphite, a composite of a tin compound and silicon and carbon, lithium, and the like.In these, graphite is preferred, and natural graphite and artificial graphite are preferred as graphite. Although it is preferable to include at least natural graphite.

  Examples of the natural graphite include flaky graphite, flaky graphite, and soil graphite. Among these natural graphites, soil graphite generally has a small particle size and low purity. On the other hand, flaky graphite and flaky graphite have advantages such as a high degree of graphitization and a low impurity content, and can be preferably used in the present invention.

  Natural graphite can be appropriately selected from flaky, fibrous, amorphous particles and the like, and is preferably spherical. Graphite particles are generally flat, and therefore have a high specific surface area, making it difficult to achieve high packing. In addition, lithium ions are adsorbed and desorbed only on the edge surface. Therefore, a spheroidizing treatment is performed for the purpose of reducing the specific surface area and obtaining an isotropic crystal structure. By performing the spheroidizing treatment, the shape of the graphite particles can be controlled. It is preferable that the aspect ratio (minor axis / major axis) of the spheroidized graphite is 0.7 to 1.

The spheroidizing treatment may be a mechanical treatment or a method of performing granulation using pitch or the like.
Examples of a method of forming graphite granules by assembling a plurality of graphite, for example, a method of mixing a plurality of flaky graphite in the presence of a binder of graphite raw material, a method of applying a mechanical external force to a plurality of flaky graphite, There is also a method of using the above two methods in combination. Preferably, it is a method of granulating by applying a mechanical external force without using a binder component. As a device for applying a mechanical external force, for example, a counter jet mill AFG (registered trademark, manufactured by Hosokawa Micron Corporation), a current jet (registered trademark, manufactured by Nisshin Engineering Co., Ltd.), an ACM pulverizer (registered trademark, Hosokawa Micron Co., Ltd.) ), A hybridization system (registered trademark, manufactured by Nara Machinery Co., Ltd.), and a mechano hybrid (registered trademark, manufactured by Nippon Coke Industry Co., Ltd.).

  Examples of artificial graphite include, for example, graphitized mesophase calcined carbon (bulk mesophase) and cokes (raw coke, green coke, pitch coke, needle coke, petroleum coke, etc.) using coal tar pitch as a raw material, coal-based Heavy oil, atmospheric residual oil, petroleum heavy oil, aromatic hydrocarbon, nitrogen-containing cyclic compound, sulfur-containing cyclic compound, polyphenylene, polyvinyl chloride, polyvinyl alcohol, polyacrylonitrile, polyvinyl butyral, natural polymer, polyphenylene Organic materials such as sulfide, polyphenylene oxide, furfuryl alcohol resin, phenol, formaldehyde resin and imide resin are calcined and graphitized at a temperature of 2500 to 3200 ° C. Of these, mesophase calcined carbon and cokes are preferred.

  Natural graphite and artificial graphite may be used alone, but it is preferable to use at least natural graphite. From the viewpoint of improving electrode performance including durability, it is preferable to use a mixture of natural graphite and artificial graphite. In this case, the mass ratio of natural graphite / artificial graphite is 90/10 to 50. / 50, more preferably from 85/15 to 60/40. Within the above range, good electrode characteristics and durability can be easily exhibited.

  Although the negative electrode active material is not particularly limited, in the particle size distribution obtained by the laser diffraction / scattering method, the median diameter (D50) at a volume integration of 50% is determined from the viewpoint of suppressing reduction in electrode density due to reduction in packing and increasing specific surface area. Is preferably from 7 to 15 μm from the viewpoint of suppressing the side reaction due to the above and improving the initial charge / discharge efficiency. From the above viewpoint, it is more preferably 14 μm or less, and more preferably 9 μm or more. When there are a plurality of types of negative electrode active materials, D50 preferably satisfies the above range as a whole including these.

The specific surface area of the negative electrode active material may be 0.5 to 7 m ² / g from the viewpoint of increasing the peel strength by obtaining the anchor effect and suppressing the side reaction due to the increase of the specific surface area and improving the initial charge / discharge efficiency. preferable. From the above viewpoint, it is more preferably 0.8 m ² / g or more, and further preferably 6 m ² / g or less. When there are a plurality of types of negative electrode active materials, it is preferable that the specific surface area satisfies the above range as a whole including these.

  The content of the negative electrode active material in the negative electrode active material layer is preferably 50 to 98.5% by mass, more preferably 60 to 98% by mass, based on the total amount of the negative electrode active material layer, from the viewpoint of efficiently exhibiting the function of the negative electrode active material. Is more preferred.

The bulk density of the negative electrode active material is preferably 0.35 g / cm ³ or more, and more preferably 0.4 g / cm ³ or more, from the viewpoint of improving the initial charge / discharge efficiency. In addition, from the viewpoint of suppressing a decrease in electrode density due to a decrease in packing, the amount is preferably 0.75 g / cm ³ or less, and more preferably 0.7 g / cm ³ or less. When there are a plurality of types of the negative electrode active materials, it is preferable that the bulk density including the above materials satisfy the above range. The bulk density can be measured by the method described in Examples.

The negative electrode active material layer may contain a conductive auxiliary. As the conductive assistant, a material having higher conductivity than the above-described negative electrode active material is used, and specific examples thereof include carbon materials such as carbon black, carbon nanofiber, carbon nanotube, and graphite particles.
In the case where the negative electrode active material layer contains a conductive auxiliary, the content thereof is preferably 1 to 30% by mass, more preferably 2 to 25% by mass, based on the total amount of the negative electrode active material layer. .

Examples of the negative electrode binder contained in the negative electrode active material layer include fluorine-containing resins such as polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and polytetrafluoroethylene (PTFE); Acrylic resin such as polymethyl acrylate (PMA) and polymethyl methacrylate (PMMA), polyvinyl acetate, polyimide (PI), polyamide (PA), polyvinyl chloride (PVC), polyether nitrile (PEN), polyethylene (PE) , Polypropylene (PP), polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene-butadiene rubber, poly (meth) acrylic acid, carboxymethylcellulose, hydroxyethylcellulose, polyvinyl alcohol, etc. It is below. These binders may be used alone or in combination of two or more. Further, carboxymethyl cellulose and the like may be used in the form of a salt such as a sodium salt.
Among the above examples, a combination of a styrene butadiene rubber and a salt of carboxymethyl cellulose is preferable. In this case, the mass ratio of the styrene butadiene rubber and the salt of carboxymethyl cellulose (styrene butadiene rubber / salt of carboxymethyl cellulose) is preferably 30/70 to 70/30, and is 40/60 to 60/40. Is more preferred.

  The content of the negative electrode binder in the negative electrode active material layer is preferably from 1.5 to 40% by mass, more preferably from 2.0 to 25% by mass, based on the total amount of the negative electrode active material layer.

  The thickness of the negative electrode active material layer is not particularly limited, but is preferably from 10 to 200 μm, more preferably from 50 to 150 μm, and even more preferably from 70 to 110 μm.

[Negative electrode current collector]
Examples of the material constituting the negative electrode current collector include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, aluminum or copper is preferable, and copper is more preferable. The negative electrode current collector is generally made of a metal foil, and its thickness is not particularly limited, but is preferably 1 to 50 μm.

[Method of manufacturing negative electrode]
The negative electrode for a lithium ion secondary battery of the present invention comprises a negative electrode active material, a negative electrode binder, and a conductive auxiliary compound, if necessary, a negative electrode active material layer-containing composition containing a solvent, on the negative electrode current collector. And dried, and then press-pressed.

  As the solvent in the composition for the negative electrode active material layer, water, N-methylpyrrolidone, N-ethylpyrrolidone, dimethylacetamide, dimethylformamide, and the like are used. The solid concentration of the composition for a negative electrode active material layer is preferably 5 to 75% by mass, more preferably 20 to 65% by mass.

  The pressure press may be performed by a roll press or the like. From the viewpoint of obtaining a desired electrode density and electrolyte permeation rate, as described above, the pressing pressure is in the range of 400 to 1200 N / m, preferably in the range of 500 to 1000 N / m, and more preferably in the range of 550 to 900 N / m. It is preferable to adjust with.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention includes the negative electrode for a lithium ion secondary battery of the present invention. Specifically, a positive electrode, the negative electrode disposed to face the positive electrode, and a positive electrode and a negative electrode And a separator to be disposed.

(Positive electrode)
The positive electrode in the lithium ion secondary battery of the present invention has a positive electrode active material layer, and preferably has a positive electrode current collector and a positive electrode active material layer laminated on the positive electrode current collector.

Examples of the positive electrode active material used in the positive electrode active material layer include a lithium metal oxide compound. Examples of the lithium metal oxide compound include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and the like. Further, olivine-type lithium iron phosphate (LiFePO ₄ ) may be used. Further, a plurality of metals other than lithium may be used, and an NCM (nickel-cobalt-manganese) -based oxide or an NCA (nickel-cobalt-aluminum-based) oxide called a ternary system may be used. Among them, NCA is preferred.
The content of the positive electrode active material in the positive electrode active material layer is preferably 50 to 98.5% by mass, more preferably 70 to 98% by mass, and still more preferably 85 to 96% by mass, based on the total amount of the positive electrode active material layer.

  The positive electrode active material is preferably in the form of particles. The positive electrode active material is not particularly limited, but preferably has a median diameter of 0.5 to 50 μm, more preferably 1 to 30 μm.

(Binder for positive electrode active material)
The positive electrode active material layer is formed by binding a positive electrode active material and a conductive auxiliary with a positive electrode binder. Specific examples of the positive electrode binder are the same as those exemplified for the negative electrode binder contained in the negative electrode active material layer.

  The content of the positive electrode binder in the positive electrode active material layer is preferably 1 to 40% by mass, more preferably 2 to 20% by mass, and still more preferably 3 to 10% by mass, based on the total amount of the positive electrode active material layer. 3 to 7% by weight is particularly preferred.

  The thickness of the positive electrode active material layer is not particularly limited, but is preferably from 10 to 200 μm, and more preferably from 50 to 150 μm.

  Examples of the material constituting the positive electrode current collector include metals having conductivity, such as copper, aluminum, titanium, nickel, and stainless steel. Preferably, aluminum or copper, and more preferably, aluminum is used. The positive electrode current collector is generally made of a metal foil, and its thickness is not particularly limited, but is preferably 1 to 50 μm.

  The positive electrode is formed by applying a positive electrode active material, a binder for the positive electrode, and a composition for a positive electrode active material layer containing a conductive auxiliary agent and a solvent, if necessary, onto a positive electrode current collector, followed by drying. It can be manufactured by pressing. The pressure press can be performed by a known method such as a roll press. The pressure (linear pressure) at the time of pressurization is preferably 100 to 500 kN / m.

(Separator)
The lithium ion secondary battery of the present invention includes, for example, a separator disposed between a negative electrode and a positive electrode. The separator effectively prevents a short circuit between the positive electrode and the negative electrode. Further, the separator may hold an electrolyte described later.
Examples of the separator include a porous polymer film, a nonwoven fabric, and a glass fiber. Among these, a porous polymer film is preferable. Examples of the porous polymer film include an olefin-based porous film such as an ethylene-based porous film.

(Insulating layer)
The lithium ion secondary battery of the present invention may include an insulating layer on the negative electrode active material layer or the positive electrode active material layer. The short circuit between the positive electrode and the negative electrode is effectively prevented by the insulating layer. The insulating layer preferably includes an insulating fine particle and a binder for the insulating layer, and is a layer having a porous structure formed by binding the insulating fine particle with the binder for the insulating layer.

The insulating fine particles are not particularly limited as long as they are insulating, and may be organic particles or inorganic particles. Specific organic particles include, for example, cross-linked polymethyl methacrylate, cross-linked styrene-acrylic acid copolymer, cross-linked acrylonitrile resin, polyamide resin, polyimide resin, poly (lithium 2-acrylamido-2-methylpropanesulfonate), Examples include particles composed of an organic compound such as a polyacetal resin, an epoxy resin, a polyester resin, a phenol resin, and a melamine resin. Examples of the inorganic particles include silicon dioxide, silicon nitride, alumina, boehmite, titania, zirconia, boron nitride, zinc oxide, tin dioxide, niobium oxide (Nb ₂ O ₅ ), tantalum oxide (Ta ₂ O ₅ ), potassium fluoride, and fluoride. Examples include particles composed of inorganic compounds such as lithium chloride, clay, zeolite, and calcium carbonate. The inorganic particles may be particles composed of a known composite oxide such as a niobium-tantalum composite oxide or a magnesium-tantalum composite oxide. One kind of the insulating fine particles may be used alone, or a plurality of kinds may be used in combination.
The median diameter of the insulating fine particles is not particularly limited as long as it is smaller than the thickness of the insulating layer, and is, for example, 0.001 to 1 μm, preferably 0.05 to 0.8 μm, and more preferably 0.1 to 0.6 μm. .
The content of the insulating fine particles contained in the insulating layer is preferably 15 to 95% by mass, more preferably 40 to 90% by mass, and still more preferably 60 to 85% by mass, based on the total amount of the insulating layer. When the content of the insulating fine particles is within the above range, the insulating layer can form a uniform porous structure and impart appropriate insulating properties.

As the binder for the insulating layer, the same kind as the binder for the negative electrode described above can be used. The content of the binder for the insulating layer in the insulating layer is preferably from 5 to 50% by mass, more preferably from 10 to 45% by mass, and still more preferably from 15 to 40% by mass, based on the total amount of the insulating layer.
The thickness of the insulating layer is preferably 1 to 10 μm, more preferably 2 to 8 μm, and still more preferably 3 to 7 μm.

(Electrolytes)
The lithium ion secondary battery of the present invention includes an electrolyte. The electrolyte is not particularly limited, and a known electrolyte used in a lithium ion secondary battery may be used. For example, an electrolyte is used as the electrolyte.

Examples of the electrolyte include an electrolyte containing an organic solvent and an electrolyte salt. Examples of the organic solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, and 1,2-diethoxyethane. And polar solvents such as tetrohydrafuran, 2-methyltetrahydrofuran, dioxolane, and methyl acetate, or a mixture of two or more of these solvents.
Examples of electrolyte salts include LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₆ , LiCF ₃ CO ₂ , LiPF ₆ SO ₃ , LiN (SO ₃ CF ₃ ) ₂ , Li (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ , a salt containing lithium such as LiN (COCF ₂ CF ₃ ) ₂ , lithium bisoxalate borate (LiB (C ₂ O ₄ ) _{2, and the} like; Examples include complexes such as boron hydride complexes and complex hydrides such as LiBH _4. These salts or complexes may be used alone or in a mixture of two or more.

  Further, the electrolyte may be a gel electrolyte further containing a polymer compound in the above-mentioned electrolytic solution. Examples of the polymer compound include a fluorine-based polymer such as polyvinylidene fluoride and a polyacryl-based polymer such as poly (methyl meth) acrylate. Note that the gel electrolyte may be used as a separator.

  The electrolyte may be disposed between the negative electrode and the positive electrode. For example, the electrolyte is filled in the battery cell in which the above-described negative electrode, positive electrode, and separator are housed. Further, the electrolyte may be, for example, applied on the negative electrode or the positive electrode and disposed between the negative electrode and the positive electrode.

  The lithium ion secondary battery may have a multilayer structure in which a plurality of negative electrodes and a plurality of positive electrodes are stacked. In this case, the negative electrode and the positive electrode may be provided alternately along the laminating direction. The separator may be disposed between each negative electrode and each positive electrode. When an insulating layer is provided, it may be disposed between the negative electrode and the separator or between the positive electrode and the separator.

  Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

[Measurement and evaluation]
The obtained negative electrode for a lithium ion secondary battery and the like were measured and evaluated by the following methods. The results are shown in Table 1 below.

(Electrode density of negative electrode active material layer)
The mass of the negative electrode prepared in each example is A (g), the mass of the negative electrode current collector is B (g), the surface area of the negative electrode active material layer in the negative electrode is C (cm ² ), The electrode density was determined from the following equation, where the thickness of the layer was D (cm).
Formula: Electrode density (g / cm ³ ) = (A−B) / (C × D)

(Electrolyte penetration rate)
The negative electrode manufactured in each example was cut into 10 × 70 mm, and the surface of the negative electrode active material layer of the negative electrode was brought into contact with the liquid surface of the beaker in which the electrolyte prepared by the procedure described below entered 10 mm from the bottom. Fixed to.
Immediately after the surface of the negative electrode active material layer of the electrode was brought into contact with the electrolytic solution, the time was set to 0 second, and then kept for 30 minutes.
The part where the electrolyte (a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7 (EC: DEC)) is measured above the liquid level, and the permeation rate is calculated by the following equation. did.
(Electrolyte permeation rate) = (Rise distance of electrolyte from liquid level (cm)) / (Immersion time (min))

(Bulk density of negative electrode active material)
The bulk density of the negative electrode active material was determined according to JIS K6219-2.

(Initial charge / discharge efficiency)
Using the test coin cells manufactured in each of the examples and comparative examples, in the first charge / discharge, the battery was charged to 0 V at a constant current of 0.25 mA, and then charged at 0 V (V vs Li / Li ⁺ ) at a constant voltage of 0 V (V vs Li / Li ⁺ ). The battery was charged until it reached 05 mA. Thereafter, the battery was discharged to 1.5 V (V vs Li / Li ⁺ ) at a constant current of 0.25 mA. The charge / discharge efficiency at that time was calculated by the following equation (charge / discharge efficiency (%)) = (discharge capacity (mAh)) / (charge capacity (mAh))

Initial charge / discharge efficiency evaluation result (%)
A: 90% or more B: 88% or more and less than 90% C: 85% or more and less than 88% D: less than 85% A and B pass.

(Input characteristics)
Using the test coin cells manufactured in Examples and Comparative Examples, charging was performed to 0 V at a constant current of 0.25 mA or 1.25 mA, and then to 0.05 mA at a constant voltage of 0 V (V vs Li / Li ⁺ ). Charged up to. Thereafter, discharge is completed at a point where the battery is discharged to 1.5 V (V vs Li / Li ⁺ ) at a constant current of 0.25 mA. The charge capacity ratio at this time was calculated, and the input characteristics were evaluated based on the following criteria.
(Input characteristics (%)) = (Charging capacity at 1.25 mA) / (Charging capacity at 0.25 mA (mAh))

Input characteristics evaluation result (%)
A: 60% or more B: 45% or more and less than 60% C: 30% or more and less than 45% D: less than 30% A and B pass.

(capacity)
The battery was charged to 0 V at a constant current of 0.25 mA, and then charged to 0.05 mA at a constant voltage of 0 V (V vs Li / Li ⁺ ). Thereafter, the battery was discharged to 1.5 V (V vs Li / Li ⁺ ) at a constant current of 0.25 mA. The above charge / discharge cycle was repeated three times, and the third discharge capacity was defined as the capacity value.

Evaluation result of capacity (mAh) A: 6.5 mAh or more B: 5 mAh or more and less than 6.5 mAh C: 4 mAh or more and less than 5 mAh D: less than 4 mAh A and B are acceptable.

(Peel test)
The negative electrodes obtained in Examples and Comparative Examples were cut into a width of 10 mm and a length of 150 mm. The negative electrode active material layer was attached to the jig for peeling test with a double-sided tape, and one end in the length direction was pinched in parallel with the surface to which the negative electrode was attached, and the electrode was pulled at 5 cm / min. The peeling strength (peel strength) was measured.

Peel test measurement result (N / m)
A: 7 or more B: 5 or more to less than 7 C: 3.5 or more to less than 5 D: Less than 3.5 A and B are acceptable.

[Material used]
(1) Negative electrode active material Natural graphite A: specific surface area: 4.9 m ² / g, D50: 12 μm, bulk density: 0.5 g / cm ³
Natural graphite B: specific surface area: 0.9 m ² / g, D50: 13 μm, bulk density: 0.46 g / cm ³
Natural graphite C: specific surface area: 1.5 m ² / g, D50: 10 μm, bulk density: 0.6 g / cm ³
Natural graphite D: specific surface area: 4.5 m ² / g, D50: 13 μm, bulk density: 0.65 g / cm ³

Artificial graphite A: specific surface area: 0.9 m ² / g, D50: 10 μm, bulk density: 0.7 g / cm ³
Artificial graphite B: specific surface area: 0.3 m ² / g, D50: 6 μm, bulk density: 0.8 g / cm ³

(2) Conductive auxiliary agent super P: carbon black

[Example 1]
(Preparation of negative electrode)
98 parts by mass of natural graphite A as a negative electrode active material, 1 part by mass of styrene butadiene rubber, 1 part by mass of a sodium salt of carboxymethyl cellulose (CMC) as a binder for a negative electrode, and water as a solvent were mixed with a solid content of 55%. A composition for a negative electrode active material layer adjusted to mass% was obtained. This composition for a negative electrode active material layer was applied to both surfaces of a 10 μm-thick copper foil as a negative electrode current collector, and dried at 100 ° C. Thereafter, the negative electrode current collector having both sides coated with the negative electrode active material layer composition was pressed with a roller at a linear pressure of 600 kN / m to form a negative electrode. The density of the negative electrode active material layer was 1.6 g / cc. The produced negative electrode was punched out and used for cell production. In addition, the dimension of the negative electrode was φ16 mm, and of the dimensions, the dimension of the negative electrode active material layer applied was φ16 mm. The thickness of the negative electrode active material layers formed on both surfaces was 80 μm per one surface.

(Preparation of electrolyte solution)
LiPF ₆ as an electrolyte salt was dissolved in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7 (EC: DEC) so as to have a concentration of 1 mol / liter, and the electrolytic solution was dissolved. Prepared.

(Manufacture of lithium ion secondary battery (coin cell for test))
Using the prepared negative electrode, a lithium metal foil punched into φ16 mm as a counter electrode, a porous PP film having a thickness of 25 μm as a separator, and an electrolytic solution, a test coin cell (CR2032 type) was prepared.

[Examples 2 to 5, Comparative Examples 1 to 4]
A negative electrode was produced in the same manner as in Example 1 except that the composition and the pressing pressure were as shown in Table 1 below. D50, specific surface area, and bulk density when two types of the negative electrode active materials are used are values of the entire negative electrode active material.
Thereafter, test coin cells were prepared in the same manner as in Example 1, and various evaluations were performed. Table 1 shows the results.

DESCRIPTION OF SYMBOLS 10 Electrode for lithium ion secondary batteries 11 Electrode active material layer 13 Electrode current collector

Claims (5)

A negative electrode current collector, a negative electrode for a lithium ion secondary battery including a negative electrode active material layer formed on the negative electrode current collector,
An electrode density of the negative electrode active material layer is 1.4 g / cm ³ or more;
A negative electrode for a lithium ion secondary battery, wherein an electrolyte permeation rate into the negative electrode active material layer is 0.04 cm / min or more. The negative active median size of the negative electrode active material contained in the material layer (D ₅₀₎ is a negative electrode for a lithium ion secondary battery according to claim 1 which is 7 to 15 m. The negative electrode for a lithium ion secondary battery according to claim 1, wherein a specific surface area of the negative electrode active material is 0.5 to 7 m ² / g.   The negative electrode for a lithium ion secondary battery according to claim 2 or 3, wherein the negative electrode active material includes natural graphite and artificial graphite. A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 1.