JP2002246013A - Negative electrode for lithium secondary battery and its producing method, as well as lithium secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium secondary battery, having reduced voids near the surface of a collector by adhering the collector to an active material layer with sufficiently high adhesive force, and to provide its producing method. SOLUTION: The negative electrode for the lithium secondary battery comprises the active material layer containing an active material and a binder, laminated on the collector, 90% or more of the surface of the collector being covered with the binder. The producing method for the negative electrode for the lithium secondary battery comprises applying a binder solution on the surface of the collector and drying it to form a coating of the binder, then applying a slurry containing an active material and a binder on the coating of the binder, and drying it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a negative electrode for a lithium secondary battery, a method for producing the negative electrode, and a lithium secondary battery using the negative electrode.

[0002]

2. Description of the Related Art In general, a negative electrode of a lithium secondary battery is formed on a foil-like current collector such as a copper foil on an active material composed of a carbon material such as various graphite materials and carbon black and a binder (polymer material). )
An active material layer comprising a mixture with the active material layer is formed, and the active material layer is usually formed by applying a slurry containing an active material and a binder on the surface of a current collector and drying the slurry. I have.

[0003] Recently, the performance of lithium secondary batteries has been considerably improved. However, problems such as a rapid decrease in discharge capacity due to repeated charge / discharge cycles and a decrease in capacity during large-current discharge (high-rate discharge). Has not yet been sufficiently improved.

The inventors of the present invention have investigated the cause of the above problem. As a result, the decrease in discharge capacity due to repeated charge / discharge cycles was caused by the peeling of the active material layer from the current collector of the negative electrode due to repeated charge / discharge cycles. One of the main causes is the occurrence of partial peeling. The decrease in capacity during large-current discharge (high-rate discharge) is caused by voids existing near the surface of the current collector of the negative electrode. It has been found that one of the causes is that the interface resistance between the conductor and the active material layer is high.

[0005]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a method of bonding a current collector and an active material layer with a sufficiently high adhesive force,
Moreover, an object of the present invention is to provide a negative electrode for a lithium secondary battery in which the voids near the surface of the current collector are sufficiently reduced and a method of manufacturing the same, and a lithium secondary battery in which charge / discharge cycle characteristics and high-rate discharge characteristics are improved as compared with conventional ones. And

[0006]

In order to achieve the above object, the present invention has the following features. (1) A negative electrode for a lithium secondary battery in which an active material layer containing an active material and a binder is formed on a current collector, and 90% or more of the current collector surface is coated with a binder. A negative electrode for a lithium secondary battery, comprising: (2) A binder solution is applied to the surface of the current collector and dried to form a coating film of the binder, and then the active material and the binder are contained on the coating film of the binder. A method for producing a negative electrode for a lithium secondary battery, comprising a step of applying and drying a slurry. (3) A method for producing a negative electrode for a lithium secondary battery, comprising a step of applying a slurry containing an active material, a binder and an acid on the surface of the current collector and drying the slurry. (4) the acid is a solid organic acid, and the solid organic acid is
0.01 to 3 per 100 parts by weight of the total amount of the active material and the binder
(3) The method according to the above (3), wherein the slurry is used by mixing with parts by weight. (5) The above-mentioned method wherein the acid is an aqueous solution having an acid concentration of 0.1 mol / l or more, and a slurry in which the acid aqueous solution is mixed in an amount of 0.01 to 1 part by weight per 100 parts by weight of the total amount of the active material and the binder (3) The method according to the above. (6) A lithium secondary battery having the negative electrode according to (1).

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The negative electrode for a lithium secondary battery (hereinafter, also simply referred to as a negative electrode) of the present invention is characterized in that at least 90% of the surface of a current collector is covered with a binder in an active material layer.

In general, in a lithium secondary battery, the ratio of the active material and the binder constituting the active material layer of the negative electrode depends on the capacity of the battery, the binding (binding) between the active materials, and the electric resistance of the entire negative electrode. From the point of view, the weight ratio (active material: binder) is approximately 8
It is designed in the range of 0:20 to 98: 2. As described above, the active material layer is usually formed by applying a slurry containing the active material and the binder on the current collector and drying the slurry, and the specific gravity of the active material is higher than the specific gravity of the binder. Therefore, the active material tends to be deposited first on the current collector surface. For this reason, in the negative electrode of the conventional lithium secondary battery, the ratio of the portion of the current collector surface covered with the binder (the portion where the binder is attached) is small, and most of the current collector surface is The active material had been deposited. (In such a region, the binder did not adhere to the surface of the current collector, the active material particles contacted the surface of the current collector, and the gap due to the overlap of the active material particles Exists.).

The negative electrode of the present invention has an interface structure between the current collector and the active material layer, which has not been obtained conventionally, that 90% or more of the current collector surface is coated with a binder. As a result, the active material layer is held on the current collector with a high degree of adhesion, and the amount of voids near the current collector surface is reduced. Therefore, even if charge / discharge cycles are repeated, the active material layer is hardly peeled off from the current collector due to the adhesion of the binder covering a wide area of the current collector surface. The interface resistance between the conductor and the active material layer is sufficiently low,
A decrease in capacity during large-current discharge (high-rate discharge) is also suppressed.

In the negative electrode of the present invention, from the viewpoint of only the bonding strength between the current collector and the active material layer, 95% or more of the current collector surface and further the entire current collector surface are covered with the binder. However, from the viewpoint of further reducing the interfacial resistance between the active material layer and the current collector, about 1 to 5% of the surface of the current collector is an active material deposition region (active material contact region). Is preferred.

In the present invention, the ratio of the portion of the current collector surface covered with the binder (binder coverage) is determined by peeling the active material layer from the current collector of the negative electrode, It can be obtained by measuring the ratio occupied by the binder. Specifically, it is performed by the following method.

First, the active material layer is separated from the current collector by immersing the negative electrode in a liquid in which the binder in the active material layer does not dissolve, and irradiating the negative electrode with ultrasonic waves through the liquid. Then, a scanning electron micrograph (SEM photograph) of the separated surface of the separated active material layer is visually observed, the area of the binder is measured, and the binder on the entire separated surface (entire photograph) is calculated by the following formula. Calculate the ratio of Note that the scanning electron micrograph is a photograph of a 200 μm × 300 μm area (magnification: 300 to 1000 times) at any three points on the peeled surface, and the binder occupies every three photographs (images). Calculate the percentages and take these averages.

[(Total area of binder part) / (area of entire photographic image)] × 100 (%)

The image taken by the microscope may be photographed on a television, scanned with an electron beam, and the result may be automatically measured using a computer.

In the negative electrode of the present invention, the active material is
Granular carbon materials used as active materials for negative electrodes of known lithium secondary batteries, such as various graphite materials, carbon black, amorphous carbon materials (hard carbon and soft carbon), and activated carbon, are used. Among these, graphitized carbon is preferable from the viewpoint of the charge / discharge cycle characteristics of the battery, the design of a high-capacity battery, and the improvement of the discharge voltage. The granular shape of the granular carbon material is not particularly limited, and may be any of a scale, a sphere, a pseudo sphere, a lump, and a whisker. The size is not particularly limited, but the average particle size is preferably about 0.1 to 100 μm, and particularly preferably 1 to 50 μm. Such granular carbon material (active material)
Is measured by the following method.

First, the particulate matter to be measured is introduced into an organic liquid such as water or ethanol, and the frequency is set to 35 kHz to 40 kHz.
Dispersion is performed for about 2 minutes by applying ultrasonic waves of about Hz.
Note that the amount of the particulate matter to be measured is such that the laser transmittance (the ratio of the output light amount to the incident light amount) of the dispersion liquid after the dispersion treatment is 70% to 95%. Next, this dispersion is applied to a Microtrac particle size analyzer, and the particle size of each particle (D ₁ , D ₂ , D ₃ ...) And the number of particles present (N ₁ , N _2, N ₃ ···) to measure. Note that the microtrack particle size analyzer calculates the particle size distribution of the spherical particles that has the theoretical intensity closest to the observed scattering intensity distribution. That is, the particle is assumed to be a sphere having a cross-sectional circle having the same area as the projected image obtained by the irradiation of the laser light, and the diameter (equivalent sphere diameter) of this cross-sectional circle is measured as the particle diameter. The average particle size (μm) is calculated from the particle size (D) of the individual particles obtained above and the number (N) of each particle size by the following formula. Average particle size (μm) = (ΣND ³ / ΣN) ^1/3

If the particle size is less than 1 μm, the particles tend to aggregate in the dispersion. For this reason, when the particle size is less than 1 μm, it is preferable to use an electron microscope. Specifically, an electron microscope photograph is first taken with the magnification set so that 20 or more particles enter the visual field. Next, the area of the image of each particle in the photograph is calculated, and the diameter of a circle having the same area is calculated from the calculated area. This diameter is the particle size of the active material. In this case, the average particle size is a number average.

[0018] Especially when using the graphitized carbon having a specific surface area of 2.0 m ² / g or less (particularly preferably 0.7 m ²
/ G to 1.5 m ² / g), and the inter-plane distance (d002) of the crystal lattice is 0.3380 nm or less (particularly preferably 0.3350 nm to 0.3370 n).
m) are preferred.

The specific surface area of the graphitized carbon is described in pages 178 to 184 of "Material Chemistry of Powder" (Yasuo Arai, first edition, 9th edition, published by Baifukan (Tokyo), 1995). Among the adsorption methods, a gas-phase adsorption method using nitrogen as an adsorbent (one-point method) can be used. The measurement of the specific surface area using such a gas-phase adsorption method using nitrogen as an adsorbent can be suitably performed using, for example, a specific surface area meter Monosorb (manufactured by Quantachrome).

The interplanar distance (d002) between the crystal lattices of graphitized carbon can be measured by the Japan Society for the Promotion of Science described below.

First, high-purity silicon for X-ray standard is pulverized with an agate mortar to a size of 325 mesh standard sieve or less to prepare a standard substance, and this standard substance and the graphitized carbon of the sample to be measured are mixed in an agate mortar ( Mixing ratio: 10 parts by weight of standard substance per 100 parts by weight of graphitized carbon) to prepare an X-ray sample. This X-ray sample was prepared using an X-ray diffractometer (Rigaku R
The sample plate of INT2000, X-ray source: CuKα ray) is uniformly filled. Next, the applied voltage to the X-ray tube was 40 kV,
The applied current was set to 50 mA, and the scanning range was 2θ = 2.
The 002 peak of carbon and the 111 peak of a standard substance are measured at 3.5 to 29.5 degrees and a scan speed of 0.25 degrees / min. Subsequently, from the obtained peak position and its half-value width, using the graphitization degree calculation software attached to the X-ray diffractometer, the interplanar distance (d) of the crystal lattice is used.
002) is calculated.

The graphitized carbon may be a fibrous material other than the above-mentioned granular material. In this case, the carbon material may be linear or curled. The size of the fibrous graphitized carbon is not particularly limited, but the average fiber length is preferably 1 to 100 μm, and particularly preferably 3 to 50 μm. Further, the average fiber diameter is preferably 0.5 to 15 μm,
1 to 15 μm is particularly preferred, and 5 to 10 μm is particularly preferred. In addition, the aspect ratio (average fiber length /
(Average fiber diameter) is preferably from 1 to 5, particularly preferably from 3 to 5.

The size (fiber diameter, fiber length) of the fibrous graphitized carbon can be determined by using an electron microscope as in the case of the granular conductive material having a particle size of 1 μm or less. That is, it can be performed by setting a magnification so that 20 or more fibers are included in the visual field, taking an electron micrograph, and measuring the fiber diameter and the fiber length of each fiber in the photograph with a caliper or the like. The fiber length may be measured by measuring the shortest distance between one end and the other end if the fiber is linear. However, if the fiber is curled or the like, any two points that are most distant from each other on the fiber are taken, the distance between these two points is measured, and this is set as the fiber length. The average fiber diameter and the average fiber length are the number average values of the number of measurement.

Examples of the binder include those conventionally used for the active material layer of the negative electrode of a lithium secondary battery, for example, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF). , Ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (C
A polymer material such as MC) is used.

The amount of the active material in the active material layer (the amount of the active material present on one side of the current collector) is usually 1 to 50.
mg / cm ² or so, preferably 5~20mg / cm ² about. The ratio of the active material and the binder in the active material layer is in the above-described conventional general range, that is, the weight ratio (active material: binder) is 80:20 to 98: 2, preferably 9
0:10 to 98: 2.

The method for producing the negative electrode of the present invention is not particularly limited, but the following method is preferred. A binder solution is applied to the surface of the current collector and dried to form a coating film of the binder, and then a slurry containing an active material and a binder is further applied on the coating film of the binder. A method of forming a coating film by processing and drying, and further performing a rolling treatment as necessary. That is, a coating film of a binder is formed on the surface of the current collector, and a coating film containing an active material and a binder is formed on the coating film of the binder to form the entire active material layer. In this method, first, a coating film of the binder is formed on the surface of the current collector, and then a coating film containing the active material and the binder is formed. Can be coated. The process of forming a coating film containing an active material and a binder to be formed later (coating of a slurry,
In the drying process), a part of the active material in the slurry and at least a part of the previously formed binder coating film are mixed. Therefore, when the thickness of the coating film of the binder is small, a portion where the active material faces the current collector surface (a portion in direct contact with the current collector surface) is finally formed. Therefore, in this method, by adjusting the thickness (adhesion amount) of the previously formed coating film,
It can be formed with good reproducibility in a state where 90% or more of the current collector surface is covered with the binder. The thickness of the coating film of the binder is preferably about 0.01 to 3 μm, particularly preferably 0.1 μm.
03 to 2 μm. With such a preferable thickness, 90% or more of the current collector surface can be surely covered with the binder without greatly increasing the interface resistance between the current collector and the active material layer.

A method in which a slurry containing an active material, a binder, and an acid is applied onto a current collector, dried to form a coating film, and further subjected to a rolling treatment as necessary. In this method, the coating film forming step is substantially performed once, but the surface of the current collector is etched by the acid mixed into the slurry during the coating and drying of the slurry, and the surface of the current collector (metal ), The binder is easily deposited, a wide area of the current collector surface is covered with the binder, and a state where 90% or more of the current collector surface of the present invention is covered with the binder is formed. . In this method, by adjusting the composition of the slurry, the ratio of the portion of the current collector surface covered with the binder can be controlled. Incidentally, the above current collector surface is etched, is that the oxide film of the current collector surface is removed (e.g., if the current collector is a copper foil, the surface of the copper foil of the CuO ₂ layer , Which is removed by etching.) By removing the oxide film, the resin as a binder is easily deposited (adhered) on the surface of the current collector.

In the above and the above methods, examples of the solvent used for preparing the binder solution and the slurry include N-methyl-2-pyrrolidone (NMP), water and the like. In addition, polyvinylidene fluoride (P
When VdF) is used, NMP (N-methyl-
When 2-pyrrolidone) is used, polyvinylidene fluoride (PVdF) may degrade by causing a hydrofluoric acid reaction to alkaline NMP. However, in the above method of adding an acid to the slurry, such polyvinylidene fluoride is used. There is an advantage that deterioration of (PVdF) can be suppressed.

In the above method, the amount of the solvent in the slurry to be applied later is generally about 50 to 200 parts by weight based on 100 parts by weight of the total amount of the active material and the binder, preferably, 50 to 150 parts by weight.

The amount of the solvent in the slurry in the above method is generally about 50 to 200 parts by weight based on 100 parts by weight of the total amount of the active material and the binder. As an acid to be added to the slurry, a solid organic acid such as oxalic acid, maleic acid, or citric acid (these are added as solids), or an inorganic acid such as hydrochloric acid, nitric acid, sulfuric acid, or an aqueous solution of acetic acid (an aqueous solution). To be added). When a solid organic acid is used, its addition amount is 0.01 to 3 parts by weight, preferably 0.01 to 1 part by weight, per 100 parts by weight of the total amount of the active material and the binder. If the amount is less than 0.01 part by weight, the effect of depositing the binder preferentially on the current collector surface is difficult to obtain, and if it is more than 3 parts by weight, the acid is evaporated together with the solvent in the slurry drying step. And the acid remains in the active material layer, is electrolyzed during charge and discharge of the battery, and lowers the charge and discharge efficiency. On the other hand, when an acid aqueous solution is added, the concentration of the solution is 0.1 mol / l or more, and the amount of addition is 0.01 mole per 100 parts by weight of the total amount of the active material and the binder.
To 1 part by weight, preferably 0.01 to 0.5 part by weight. If the concentration of the solution is less than 0.1 mol / l or the amount added is less than 0.01 part by weight, it is difficult to obtain the effect of preferentially depositing (adhering) the binder on the current collector surface. If the amount is more than 1 part by weight, the slurry gels due to the added water, and the slurry cannot be coated on the current collector.

In the above and the above, when the rolling treatment is performed, a known apparatus such as a rolling press machine is used. The rolling conditions are not particularly limited, and are conventional general rolling conditions. For example, the rolling is performed at a temperature of about room temperature to 150 ° C. and a rolling reduction of about 30 to 40%.

In the negative electrode of the present invention, the current collector generally includes a conductive metal foil such as copper, nickel, silver, and stainless steel, a perforated foil, and an expanded metal. In the case of foil or perforated foil, the thickness is 5 μm to 100 μm.
μm, preferably about 8 μm to 50 μm. In the case of expanded metal, the thickness is about 20 μm to 30 μm.
It is about 0 μm, preferably about 25 μm to 100 μm.

When a negative electrode of the present invention is used to form a lithium secondary battery, the components of the battery other than the negative electrode, such as the positive electrode, electrolyte, and separator, are not particularly limited, and known components may be used in accordance with ordinary methods. can do.

[0034] As the Li salts used in the electrolyte solution, for _{example, LiClO 4, LiBF 4, LiPF} 6, LiAs
F ₆ , LiAlCl ₄ and Li (CF ₃ SO ₂ ) ₂ N
And these may be used alone or in combination of two or more. Further, as the organic solvent, for example,
Ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl sulfoxide, sulfolane,
γ-butyrolactone, 1,2-dimethoxyethane, N,
N-dimethylformamide, tetrahydrofuran, 1,
Examples thereof include 3-dioxolan, 2-methyltetrahydrofuran, diethyl ether, and the like, and these are used alone or in combination of two or more. Among them,
From the viewpoint of improving the charge / discharge cycle characteristics and high-rate discharge characteristics aimed at by the present invention, ethylene carbonate (E
Mixed solvent of C) and diethyl carbonate (DEC) (EC: DEC (volume ratio) is 40:60 to 60:40)
It is preferable to use The concentration of the Li salt in the electrolytic solution is generally about 0.1 to 2 mol / l,
0.5 to 1.8 m from the viewpoint of battery charge / discharge cycle characteristics
ol / l is preferable, and particularly preferably 0.8 to 1.
It is about 5 mol / l.

The positive electrode comprises an active material, a conductive material and
An active material layer containing a binder (a positive electrode active material layer) is formed.
The active material is a known lithium secondary battery.
Uses various materials used as active materials for positive electrodes
Yes, but not limited, charge-discharge cycle characteristics, high
From the viewpoint of rate characteristics and safety, LiCoO_TwoAnd Li _A
Co_1-XMe_XO_TwoLi-Co-based composite oxide represented by
Are preferred. Li of the latter_ACo_1-XMe_XO_Two
In the formula, A is 0.05 to 1.5 (preferably 0.1 to 1.5).
X is 0.01 to 0.5 (preferably 0.0 to 1.1).
2 to 0.2), Me is Zr, V, Cr, Mo, Mn, F
from e, Ni, B, Al, Ge, Pb, Sn and Sb
One or more selected elements. Note that Me
X is a combination of two or more elements when X is two or more elements.
It is weighing.

The particle size and specific surface area of the Li-Co-based composite oxide particles are not particularly limited, but the average particle size is preferably 10 μm or more from the viewpoint of safety and the like.
From the viewpoint of the electric resistance of the positive electrode (reducing the energy density per unit deposition of the battery), the average particle size is preferably 25 μm or less. Further, from the viewpoints of charge / discharge cycle characteristics, high-rate discharge characteristics, safety, etc., the specific surface area is 0.1 m ² / g or more.
Those having a range of 0.3 m ² / g are preferred.

The average particle size of the particles of the Li—Co-based composite oxide is measured by the same method as the above-mentioned method of measuring the average particle size of the active material used for the negative electrode. Further, the specific surface area of the Li—Co-based composite oxide can be measured by the same method as that of the graphitized carbon used for the active material of the negative electrode.

Examples of the binder include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (P
VdF), polyethylene, ethylene-propylene-diene copolymer (EPDM) and the like.

As the conductive material, a granular carbon material used as a conductive material for a known lithium secondary battery, such as graphite (natural and / or artificial) or conductive carbon black, is used.

When the total amount of the active material, the binder and the conductive material is 100 parts by weight, the amount of the binder is 1 part by weight. ~
Generally, the amount is about 10 parts by weight (preferably about 2 to 5 parts by weight) and the amount of the conductive material is about 3 to 15 parts by weight (preferably 4 to 10 parts by weight).

Examples of the current collector include a foil made of a conductive metal such as aluminum, an aluminum alloy, and titanium, a perforated foil, and an expanded metal. When the current collector is a foil or a perforated foil, the thickness is usually 1 unit.
About 0 to 100 μm, preferably 15 to 50 μm
It is about. When the current collector is an expanded metal, its thickness is usually about 25 to 300 μm, preferably 30 to 300 μm.
It is about 150 μm.

The active material layer in the positive electrode is formed according to a conventional method. For example, a slurry containing an active material, a conductive material and a binder is prepared, and this is coated on a current collector, dried, and Rolling process.

As the separator, a known separator such as a polyolefin separator conventionally used in lithium secondary batteries is used. Here, the separator may be a porous separator or a solid separator having substantially no pores formed. Further, the polyolefin separator may be a single polyethylene layer or a single polypropylene layer, but a type in which a polyethylene layer and a polypropylene layer are laminated is preferable. A more preferred embodiment is a laminated type of a polyethylene layer and a polypropylene layer, which is porous.

The form of the battery is not particularly limited. Known ones conventionally used in lithium secondary batteries can be used. For example, a sheet-shaped exterior material such as a cylindrical can, a square tubular can, a button-shaped can, or a laminated film is used. As the laminate film, a film in which a thermoplastic resin laminate layer such as polyester or polypropylene is formed on at least one surface of a metal foil such as copper or aluminum is preferable.

[0045]

The present invention will be described in more detail with reference to the following examples, and comparative examples will also be described to show the remarkable effects of the present invention.

Example 1 [Negative electrode] Active material: Graphitized carbon fiber (specific surface area: 1 m ² / g, average fiber diameter: 8 μm, aspect ratio: 3, d002 = 0.3360)
nm) The specific surface area was measured using a specific surface area meter Monosorb (manufactured by Quantachrome). The average fiber diameter and aspect ratio were measured by measuring an image shown in a scanning electron microscope photograph with a caliper. The distance between planes of the crystal lattice (d00
2) X-ray diffractometer RINT2000 (manufactured by Rigaku Corporation)
(X-ray source: CuKα ray). Binder: PVdF Solvent: NMP Acid: Oxalic acid The compounding ratio of graphitized carbon fiber, PVdF, oxalic acid and NMP (graphitized carbon fiber / PVdF / oxalic acid / NMP) is 92 /
An 8 / 0.1 / 90 (weight ratio) slurry was prepared, and the slurry was applied to both surfaces of a copper foil (thickness: 14 μm), dried, and subjected to a rolling treatment to produce a negative electrode. Next, the negative electrode prepared above was immersed in water, and the negative electrode was irradiated with ultrasonic waves having a frequency of 40 kHz to peel off the active material layer from the surface of the copper foil.
Next, 200 μm × 300 μm at any three points on the peeled surface
The SEM photograph was taken (500 × magnification) of the area of, the total area of the binder portion on the image was measured for each photographic image, and the ratio of the total area to the area of the entire photographic image was calculated. The average of the values obtained from the photographic images was obtained. as a result,
The ratio of the binder on the release surface (that is, the coverage of the binder on the current collector surface) was 99%.

[Positive electrode] Active material: LiCoO ₂ (average particle size: 20 μm, specific surface area: 0.12 m ² / g) Conductive material: spherical graphite (average particle size: 5 μm) Oil furnace black (average particle size: 40 nm) Binder: PVdF solvent: NMP LiCoO ₂ , spherical graphite, oil furnace black,
A slurry having a mixing ratio of PVdF and NMP (LiCoO ₂ / spheroidal graphite / oil furnace black / PVdF / NMP) of 91/5/1/3/50 (weight ratio) was prepared, and the slurry was coated on an Al foil having a thickness of 70 μm. Coating on both sides,
After drying and then rolling, a positive electrode was prepared.

[Electrolyte] Ethylene carbonate (EC)
In a volume ratio of 1: 1 to diethyl carbonate (DEC)
In a mixed solvent of, were formulated LiPF ₆ to give a concentration 1 mol / liter.

The negative electrode and the positive electrode are wound with a porous composite separator (PP / PE / PP) interposed therebetween, and the above-mentioned electrolytic solution is permeated between the negative electrode and the positive electrode to have an outer diameter of 18 mm and a height of 650.
mm cylindrical can type lithium secondary battery (discharge capacity: 180
0 mAh).

Regarding the lithium secondary battery produced above,
As a charge / discharge test, the first charge / discharge efficiency measurement and the following 1
C and 2C high rate discharge tests and 1C charge / discharge cycle tests were performed. The charging was performed by a constant current constant voltage method with a constant current value of 1.8 A and a constant voltage value of 4.2 V.
Time. The cut-off voltage was 3 V.

[Measurement of Initial Charge / Discharge Efficiency] The charge capacity at the time of the charge is defined as C _1, and 0.2 C (360 m
The constant current discharge of A) is performed, and the discharge capacity at this time is D ₁
And Then, (D ₁ / C ₁ ) × 100 (%) is calculated, and this is set as the initial charge / discharge efficiency. The low initial charge / discharge efficiency indicates that Li is consumed for a side reaction in the initial charge / discharge, and that Li is greatly wasted. The charge and discharge efficiency after the second time is generally about 100%.

[High Rate Discharge Test] A 1 C (1800 mA constant current) discharge was performed at room temperature (20 ° C.), and the ratio of the discharge capacity to the discharge capacity at the time of 0.2 C (360 mA constant current) discharge (capacity maintenance) Rate) was calculated. Also, 2
C (constant current of 3600 mA) discharge was performed, and the ratio (capacity maintenance ratio) of the discharge capacity to the discharge capacity at the time of 0.2 C (constant current of 360 mA) discharge was calculated.

[Charge / Discharge Cycle Test] The conditions described above (constant current value 1.8 A, constant voltage value 4.2 V, total charge time 3)
Time), a pause of 30 minutes, a discharge at 1 C (constant current of 1800 mA) until the voltage becomes 3 V, and a pause of 1 hour after discharge as one cycle, and the room temperature (20 C.), and the discharge capacity (mAh) is calculated from the discharge current value and the discharge time in each cycle. The ratio of the discharge capacity in each cycle to the initial discharge capacity is defined as a discharge capacity maintenance ratio (%).

Comparative Example 1 A negative electrode was produced in the same manner as in Example 1 except that oxalic acid was not added to the slurry. Then, the coverage of the binder on the surface of the current collector in this negative electrode was measured and found to be 20%. A cylindrical can type lithium secondary battery (discharge capacity: 1800 mAh) was prepared using this negative electrode, and using the same positive electrode, separator and electrolyte solution as in Example 1.
The same test was performed.

Comparative Example 2 The compounding ratio of the graphitized carbon fiber, PVdF and oxalic acid (graphitized carbon fiber, PVdF / oxalic acid) in the slurry was 92/8 /
Except having changed to 0.005 (weight ratio), it carried out similarly to Example 1, and produced the negative electrode. Then, the coverage of the binder on the current collector surface of the negative electrode was measured and found to be 30%. A cylindrical can type lithium secondary battery (discharge capacity: 1800 mAh) was prepared using this negative electrode, and the same positive electrode, separator, and electrolyte as those in Example 1, and subjected to the same test as in Example 1. Provided.

Example 2 Instead of oxalic acid, a 1 mol / l hydrochloric acid aqueous solution was used, and the composition of the slurry was graphitized carbon fiber / PVdF / hydrochloric acid aqueous solution / NMP = 92/8 / 0.1 / 90 (weight ratio). Except having changed, it carried out similarly to Example 1, and produced the negative electrode. Then, the coverage of the binder on the current collector surface of the negative electrode was measured to be 97%. Using this negative electrode, the same positive electrode, separator and electrolytic solution as in Example 1 were used, and a cylindrical can type lithium secondary battery (discharge capacity: 1800
mAh) and subjected to the same test as in Example 1.

Comparative Example 3 Mixing ratio of graphitized carbon fiber, PVdF and aqueous hydrochloric acid in slurry (graphitized carbon fiber / PVdF / aqueous hydrochloric acid)
Was changed to 92/8/5 (weight ratio), except that the negative electrode was produced in the same manner as in Example 2. However, the slurry gelled and could not be applied to the copper foil, and the negative electrode could not be produced.

Example 3 Instead of graphitized carbon fiber, flaky graphite (average particle size: 15 μm) was used.
m, d002 = 0.3354 nm)
A negative electrode was produced in the same manner as in Example 1. Then, the coverage of the binder on the current collector surface of the negative electrode was measured, and was 98%. A cylindrical can type lithium secondary battery (discharge capacity: 1800 mAh) was prepared using this negative electrode, and the same positive electrode, separator, and electrolyte as those in Example 1, and subjected to the same test as in Example 1. Provided.

Comparative Example 4 The compounding ratio of flaky graphite, PVdF and oxalic acid (flaky graphite / PVdF / oxalic acid) in the slurry was 92/8 / 0.00.
Except having changed to 5 (weight ratio), it carried out similarly to Example 3, and produced the negative electrode. Then, the coverage of the binder on the surface of the current collector in the negative electrode was 10%. Using this negative electrode, the positive electrode, separator, and electrolyte were
A cylindrical can type lithium secondary battery (discharge capacity: 1800 mAh) was manufactured using the same battery as in Example 1 and subjected to the same test as in Example 1.

Example 4 [Negative electrode] Active material: Graphitized carbon fiber same as in Example 1 Binder: PVdF Solvent: NMP A Nd solution of PVdF (PVdF concentration: 10% by weight) was applied to both surfaces of a copper foil having a thickness of 14 μm. Work, dry, thickness 1μm
Is formed, and the compounding ratio of the graphitized carbon fiber, PVdF and NMP (graphitized carbon fiber / PVdF / NMP) is 93/7/9 on this PVdF coating.
The slurry of No. 0 was applied, dried, and then subjected to a rolling treatment to prepare a negative electrode. The composition of the entire coating film (active material layer) formed by two coatings (graphitized carbon fiber / PVdF)
Was 92/8. The coverage of the binder on the current collector surface was measured and found to be 100%. That is, the entire surface of the current collector was covered with the binder. Using this negative electrode, the same positive electrode, separator and electrolytic solution as in Example 1 were used, and a cylindrical can type lithium secondary battery (discharge capacity: 1800
mAh) and subjected to the same test as in Example 1.

Example 5 [Negative electrode] Active material: Graphitized carbon fiber same as in Example 1 Binder: SBR + CMC Solvent: Water SBR + CMC aqueous solution (SBR concentration 2% by weight, CMC
A concentration of 2% by weight) on both sides of a copper foil having a thickness of 14 μm,
After drying to form a mixed coating film of SBR and CMC having a thickness of 2 μm, graphitized carbon fiber, SBR, CMC
Of water and water (graphitized carbon fiber / SBR / CMC /
(Water) was coated with a slurry having a weight ratio of 95/2/2/90, dried, and then rolled to prepare a negative electrode. The overall composition (graphitized carbon fiber / SBR / CMC) of the coating film (active material layer) formed by the two coatings was 95 / 2.5.
/2.5, and the coverage of the current collector surface with the binder was measured and found to be 98%. Using this negative electrode, the same positive electrode, separator and electrolytic solution as in Example 1 were used, and a cylindrical can type lithium secondary battery (discharge capacity: 1800 mA) was used.
h) was prepared and subjected to the same test as in Example 1.

Comparative Example 5 The compounding ratio of graphitized carbon fiber, SBR, CMC and water (graphitized carbon fiber / SBR / CMC / water) was 95 / 2.5 / 2.5 / 90 (both sides of the copper foil). (Weight ratio) was applied, dried, and then rolled to produce a negative electrode. Then, the coverage of the binder on the current collector surface of the negative electrode was measured in the same manner as in Example 1, and was found to be 98%. Then, a cylindrical can type lithium secondary battery (discharge capacity: 1800 mAh) was manufactured using this negative electrode, and using the same positive electrode, separator, and electrolyte as those of Example 1, and was similar to that of Example 1. Tested.

Example 6 Instead of graphitized carbon fiber, flaky graphite (average particle size: 15 μm) was used.
m, d002 = 0.3354 nm)
A negative electrode was produced in the same manner as in Example 5. Then, the coverage of the binder on the surface of the current collector in the negative electrode was measured in the same manner as in Example 1, and it was 96%. Then, using this negative electrode, the positive electrode, the separator, and the electrolytic solution were used in Example 1.
A cylindrical can type lithium secondary battery (discharge capacity: 1800 mAh) was manufactured using the same battery as in Example 1 and subjected to the same test as in Example 1.

Comparative Example 6 Instead of graphitized carbon fiber, flaky graphite (average particle size: 15
A negative electrode was produced in the same manner as in Comparative Example 5, except that μm, d002 = 0.3354 nm) was used. And
The coverage of the current collector with the binder on the negative electrode was measured to be 0%. Then, using this negative electrode, the same positive electrode, separator, and electrolyte as those in Example 1 were used, and a cylindrical can type lithium secondary battery (discharge capacity: 1800
mAh) and subjected to the same test as in Example 1.

Tables 1 and 2 below show the test results of Examples 1 to 6 and Comparative Examples 1 to 6.

[0066]

[Table 1]

[0067]

[Table 2]

From the test results in Tables 1 and 2, the negative electrode of the present invention
That is, if a negative electrode in which 90% or more of the current collector surface is coated with a binder in the active material layer is used, a conventional negative electrode (most of the current collector surface has an active material deposition area) It can be seen that both the charge-discharge cycle characteristics and the high-rate discharge characteristics of the battery are greatly improved as compared with the case where the negative electrode is used.

[0069]

As is apparent from the above description, according to the present invention, the lithium secondary battery having a higher adhesive strength between the current collector and the active material layer than the conventional one and having no voids near the surface of the current collector. A negative electrode for a secondary battery can be provided, and by using the negative electrode, a lithium secondary battery with improved charge / discharge cycle characteristics and high-rate discharge characteristics can be obtained.

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) HA07 HA10

Claims (6)

[Claims] 1. A negative electrode for a lithium secondary battery in which an active material layer containing an active material and a binder is formed on a current collector, wherein 90% or more of the current collector surface is covered with a binder. A negative electrode for a lithium secondary battery, comprising: 2. A binder solution is applied to the surface of the current collector and dried to form a coating film of the binder, and then the active material and the binder are coated on the coating film of the binder. A method for producing a negative electrode for a lithium secondary battery, comprising a step of applying and drying a slurry containing: 3. A method for producing a negative electrode for a lithium secondary battery, comprising a step of applying a slurry containing an active material, a binder and an acid to the surface of a current collector and drying the slurry. 4. The method according to claim 1, wherein the acid is a solid organic acid, and the solid organic acid is added in an amount of 0.1 parts by weight per 100 parts by weight of the total amount of the active material and the binder.
4. The method according to claim 3, wherein a slurry containing 0 to 1 part by weight is used. 5. A slurry in which the acid is an aqueous solution having an acid concentration of 0.1 mol / l or more, and the acid aqueous solution is mixed with 0.01 to 1 part by weight per 100 parts by weight of the total amount of the active material and the binder. A method according to claim 3 for use. 6. A lithium secondary battery having the negative electrode according to claim 1.