JP2000123826A - Negative electrode for nonaqueous electrolyte secondary battery and secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode and capable of increasing electrode density and attaining large capacity without breaking double-structured active material particles formed out of high crystalline carbon particles covered with low crystalline carbon a secondary battery using it. SOLUTION: Double structured graphite particles having a means grain diameter 1 to 50 μm, and the plane interval (d002) <=0.34 nm for a plane (002) by X-ray wide angle diffraction method, covered with a noncrystalline carbon layer having plane intervals >=0.34 nm is used as active material particles a resin is used as a bonding agent, and, metal is used as a current collector. This negative electrode for a nonaqueous electrolyte secondary battery is characterized by the porosity 20 to 35%, electrode density 1.20 to 1.60 g/cm3 and electrode capacity >=400 mAh/cm3, and a nonaqueous electrolyte secondary battery using the negative electrode is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary battery using a non-aqueous electrolyte, and more particularly to a novel negative electrode which significantly improves the performance of a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years, portable or cordless consumer electronic devices have been rapidly advanced. With this,
There is also a growing demand for improvements in functions / performance such as miniaturization, weight reduction, and high energy density of secondary batteries as power supplies for driving electronic devices. From such a viewpoint, there is great expectation for non-aqueous secondary batteries, particularly lithium secondary batteries having basically a high voltage / high energy density, and their practical use has been rushed.

That is, in a nonaqueous electrolyte secondary battery provided with a positive electrode made of a lithium ion-containing composite oxide, a nonaqueous electrolyte, and a rechargeable negative electrode, lithium ions are charged at the negative electrode during charging. It is doped between the layers, and is dedoped from the carbon layer during discharge. For this reason, even if the charge / discharge cycle proceeds, precipitation of dendrite-like crystals does not occur at the time of charging in the negative electrode, so that an internal short circuit hardly occurs and good charge / discharge cycle characteristics are exhibited. In addition, since the energy density is high and the weight is low, development is proceeding toward practical use.

JP-A-57-208079 and JP-A-63-24
Japanese Patent Publication No. 555 proposes the use of graphite as a negative electrode material which is excellent in flexibility and does not cause moss-like lithium to be deposited due to repeated charge / discharge cycles. Graphite has a unique layer structure and has the property of forming an interlayer compound, and therefore has been put to practical use as an electrode material for secondary batteries utilizing this property. When used in an electrolytic solution, it is known that a carbon material having low crystallinity, for example, a carbon material having a turbostratic structure obtained by thermally decomposing a hydrocarbon in a gas phase and a selective orientation is preferable. (Japanese
No. 63-24555). However, when a carbon material having low crystallinity is used as a negative electrode material, the change in potential accompanying the release of lithium ions increases, so that the capacity that can be used as a battery is reduced, and it is difficult to manufacture a high-capacity battery. is there.

On the other hand, when a highly crystalline carbon material having graphite as its apex is used as a negative electrode material, the change in potential accompanying the release of lithium ions is theoretically small, and the capacity that can be used as a battery is reduced. Is known to grow. However, as the crystallinity of the carbon material increases, the charging efficiency is reduced, which is considered to be caused by the decomposition of the electrolytic solution. Further, the carbon material is destroyed by expansion / contraction of the interplanar spacing of the crystal due to repeated charge and discharge. Leads to.

Japanese Patent Application Laid-Open No. 4-368778 discloses that the formation of a double structure in which carbon particles having high crystallinity are coated with carbon having low crystallinity can prevent the destruction of the carbon material due to repeated charge and discharge. ing. When using a carbon material prepared by this method as an active material, theoretically,
A high-capacity electrode with excellent potential smoothness can be obtained by preventing decomposition of the electrolyte solution. However, when attempting to create a practical electrode using the double-structured active material particles, for example, for a cylindrical battery, when the active material was applied on a copper foil to produce a 50 to 500 μm thick electrode Since the electrode density was hardly increased, the capacity per electrode volume did not increase. More specifically, it is difficult to increase the electrode density, and if the electrode density is 1.20 g / cm
^{If it is set to 3} or more, the active material particles having a double structure are destroyed, so that a high capacity of 400 mA / cm ³ or more cannot be obtained.

[0007]

SUMMARY OF THE INVENTION Therefore, the present invention can increase the electrode density without destroying the double-structured active material particles in which high-crystalline carbon particles are coated with low-crystalline carbon, thereby increasing the capacity. An object of the present invention is to obtain an electrode that enables the above and a secondary battery using the same.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted various studies while paying attention to the state of the art as described above, and as a result, have found that a double-structured carbon material particle having specific parameters by the X-ray wide-angle diffraction method is obtained. Has been found to exhibit excellent effects as a negative electrode material for a non-aqueous electrolyte secondary battery. That is, the present invention provides the following negative electrode material for a non-aqueous electrolyte secondary battery.

1. X-ray wide-angle diffraction method for both distances between (002) planes
The surface spacing of the graphite-based particles in which (d002) is 0.34 nm or less has
Uses double-structure graphite particles with an average particle size of 1 to 50 μm coated with an amorphous carbon layer exceeding 0.34 nm as active material particles, uses resin as a binder, and uses metal as a current collector. It has a porosity of 20-35% and an electrode density of 1.
20 to 1.60 g / cm ³ (more preferably 1.35 to 1.60 g / cm ³ , particularly preferably 1.40 to 1.50 g / cm ³ ), and a non-aqueous electrolyte solution characterized by an electrode capacity of 400 mAh / cm ³ or more. Negative electrode for secondary battery.

[0010] 2. A non-aqueous electrolyte secondary battery using the negative electrode for a non-aqueous electrolyte secondary battery described in the above item 1.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Graphite-based particles used as an active material in the present invention have a double structure in which the surface of graphite particles is covered with amorphous carbon. By using such a double-structured graphite-based particle, a reduction in charging efficiency, which is considered to be caused by decomposition of the electrolytic solution, does not substantially occur, and destruction of the graphite structure is also prevented.

By using such a double-structured graphite-based particle as an active material and using a resin as a binder, it is stable even at a high temperature and has good adhesion to a metal as a current collector. A good negative electrode is obtained.

The porosity is obtained by using such a graphite-based material, the porosity is 20 to 35%, and the electrode density is 1.20 to 1.60 g / cm.
³ (more preferably 1.35 to 1.60 g / cm ³ , particularly preferably 1.4
The material having a capacity of 0 to 1.50 g / cm ³ ) has a high electrode capacity of 400 mA / cm ³ or more, since the electrolyte is easily impregnated and lithium ions and electrons move smoothly. Can be used to obtain a non-aqueous electrolyte secondary battery.

In the negative electrode for a non-aqueous electrolyte secondary battery according to the present invention, the spacing (d002) of the (002) plane of the graphite-based particles used as the active material by X-ray wide-angle diffraction is usually less than 0.34 nm, more preferably 0.3354 to 0.3380 nm, more preferably 0.33
It is about 54 nm to 0.3360 nm. If this value exceeds 0.34 nm, the crystallinity becomes low, so that the potential change accompanying the release of lithium ions increases, and the effective capacity that can be used as a battery decreases.

The plane spacing of the amorphous carbon layer covering the graphite-based particles is determined by the X-ray wide-angle diffraction method (d002).
Is about 0.34 nm or more, more preferably about 0.34 to 0.38 nm, and still more preferably about 0.34 to 0.36 nm. This value is 0.34 nm
If it is less than 1, the crystallinity is too high, and the charging efficiency is likely to decrease due to the decomposition of the electrolytic solution, and the carbon material is destroyed due to the expansion / contraction of the crystal plane spacing due to repeated charge / discharge. Is done. On the other hand, when the thickness exceeds 0.38 nm, the movement of lithium ions becomes difficult, and the effective capacity usable as a battery decreases.

The particle diameter of the double-structured active material particles composed of graphite particles and its coating layer is about 1 to 50 μm, more preferably 3 to 50 μm.
About 40 μm, more preferably about 5 to 35 μm.
If the particle diameter of the double structure is less than 1 μm, the electrode density cannot be increased, whereas if it exceeds 50 μm, when the electrode thickness is as thin as about 100 μm, press work is performed to increase the electrode density At this time, the double-structured active material particles are broken, and a high capacity cannot be obtained.

In the negative electrode according to the present invention, graphite-based particles are used as a negative electrode active material capable of doping / dedoping an alkali metal such as lithium. Examples of the raw material for producing graphite-based particles include pitch coke, cokes such as needle coke, polymers, carbon fibers, and the like.These are fired at a temperature of about 1500 ° C. to 3000 ° C. according to a conventional method to obtain a desired material. Can be obtained.

Examples of the material for forming the coating layer of the graphite-based particles include organic materials such as pitches and polymers. The amorphous coating carbon material, according to a conventional method, for example,
The surface of the graphite-based particle material obtained by the above method is coated with a liquid organic material.
(E.g., a molten pitch), and the coated organic material is calcined at a temperature of about 500 ° C. to 2000 ° C. to be carbonized.

The negative electrode for a non-aqueous electrolyte secondary battery according to the present invention is obtained by coating the above-mentioned double structure active material particles on a metal as a current collector using an organic solvent solution of a resin as a binder. Wear
It is obtained by drying.

The resin as the binder used in the present invention binds the double-structured active material particles together,
The active material particles are bound and fixed on the metal foil. The type of the binder resin is not particularly limited, but specifically, polyvinylidene fluoride, a fluorinated resin such as polytetrafluoroethylene, a fluorinated rubber, SBR, an acrylic resin,
Examples thereof include polyolefins such as polyethylene and polypropylene. Among these, especially general-purpose organic solvents (N-
Polyvinylidene fluoride, which is soluble in methylpyrrolidone, toluene, styrene, etc.) and has excellent electrolytic solution resistance and withstand voltage, is preferred.

The amount of the binder mixed in the negative electrode depends on the type, particle size, shape, target electrode thickness,
The ratio may be appropriately determined according to the strength and the like, and is not particularly limited. However, it is usually preferable to set the ratio to about 1 to 30% of the weight of the active material particles.

In the present invention, the metal used as the current collector is not particularly limited, and examples thereof include copper foil, stainless steel foil, and titanium foil. Furthermore, an electrode can be formed on a metal foil or in a gap between metals,
For example, expanded metal, steel material, or the like can be used. Among them, a copper foil having a thickness of about 1 to 50 μm is more preferable because it is easy to manufacture a negative electrode by a coating method described later and is excellent in strength, electric resistance, and the like.

As an example of a specific method for producing the negative electrode for a non-aqueous secondary battery according to the present invention, a method using polyvinylidene fluoride as a binder resin and using a copper foil as a current collector will be described below. Of course, the invention is not limited by this approach.

First, a double-structured active material particle is uniformly dissolved in a binder resin solution in which polyvinylidene fluoride is dissolved in N-methylpyrrolidone to prepare a slurry. At this time, if necessary, a conductive material such as acetylene black, a molding aid such as polyvinylpyrrolidone, and the like can be added. Next, using a coater, the obtained slurry is applied on a copper foil and dried, and after forming an electrode layer on the copper foil, pressing is performed to a thickness of 50 μm to 500 μm.
A negative electrode for a non-aqueous secondary battery of about m can be obtained. The electrode layers are formed on both sides or one side of the copper foil as required.

The negative electrode for a non-aqueous secondary battery of the present invention thus obtained has a density of about 1.20 to 1.60 g / cm ³ , preferably
1.35~1.60g / cm ^3, more preferably about density 1.40~1.50g
/ cm ³ or so, a porosity of 20-35%, the electrode capacity, 400 mAh
/ cm ³ or more. These densities and porosity are values for the electrode layer itself formed on the metal foil, and can be calculated from the true density and the electrode density of the double structure active material particles in the electrode layer, the binder resin. . Regarding the electrode capacity,
The capacitance is based on the volume of the electrode layer.

When the density of the negative electrode for a non-aqueous secondary battery is too low, sufficient electrode capacity cannot be obtained. On the other hand, when the density is too high, the capacity is reduced due to the destruction of the double-structured active material. Is not preferred. If the porosity is too low, sufficient rate characteristics cannot be obtained, while if it is too high, sufficient electrode capacity cannot be obtained.

The negative electrode for a non-aqueous secondary battery according to the present invention is a high-density electrode with an electrode capacity of 400 mAh / cm ³ or more and without capacity reduction.

The electrode capacity in the present invention is the electrode capacity when lithium is sufficiently doped and undoped. For example, an electrochemical cell using lithium metal as a counter electrode and a reference electrode is assembled, and in a non-aqueous electrolyte described below, a constant voltage is applied at a potential of 1 mV with respect to the lithium metal potential, and the current value is sufficiently small (for example, After doping with lithium to 0.01 mA / cm ² ), a sufficiently slow rate (for example, 0.2
The undoping capacity is measured by undoping up to 2 V with respect to the lithium metal potential at 5 mA / cm ² ). By dividing the undoped capacity by the electrode volume, the electrode capacity according to the present invention can be obtained.

In producing the negative electrode according to the present invention, care must be taken not to destroy the double-structured active material particles.
That is, when the dual-structure active material particles according to the present invention are used, the electrode density is less likely to increase than when other known negative electrode materials (eg, graphite, MCMB, etc.) are used. In the production example, it is necessary to pay attention to various conditions in the pressing step. As these conditions,
More specifically, the pressing speed, tension, roller curvature when pressing the electrode layer formed on the metal foil by a roller, or the dry state (solvent remaining amount) of the electrode layer before pressing,
Further, a press temperature and the like can be mentioned.

Dry state of electrode layer before pressing (solvent remaining)
Is usually about 1 to 10%, preferably about 1 to 8%, and more preferably about 2 to 5%. When such a solvent remains, the density of the electrode layer can be improved by pressing without breaking the double structure active material particles. In other words, when a certain amount of solvent remains, the solvent exists on the surface of the double-structured active material particles, the binder, and the conductive material, so that the slip between these materials becomes good at the time of pressing. As a result, it is considered that the density of the electrode layer can be improved without breaking the dual structure active material particles. In the conventional common sense, it has been considered that the solvent is regarded as an impurity, and the remaining amount thereof should be suppressed as much as possible (the remaining amount of the solvent should be 0.2% or less). However, according to the study of the present inventor, when adjusting the remaining amount of the solvent within a predetermined range, the electrode density is unexpectedly high, which is suitable as a negative electrode material for a high capacity non-aqueous electrolyte secondary battery. It has been found that a new material can be obtained.

Although the pressing temperature of the electrode layer is related to the remaining amount of the solvent, it is usually from room temperature (25 ° C.) to about 140 ° C., preferably from room temperature to about 100 ° C., and more preferably from room temperature to about 70 ° C.

By preliminarily adjusting the above conditions (especially, the remaining amount of the solvent) on a trial basis, the capacity does not decrease without breaking the double-structured active material particles, that is, even if the electrodes are made denser. The electrode of the present invention can be manufactured.

With respect to the negative electrode for a non-aqueous secondary battery of the present invention, the battery to be used is not limited. For example, a lithium secondary battery is manufactured by combining with a positive electrode and a non-aqueous electrolyte. be able to. In this case, in order to obtain high voltage and high capacity, Li
It is preferable to use a lithium composite oxide such as CoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , and LiNiO ₂ . As the non-aqueous electrolyte, a non-aqueous electrolyte containing a lithium salt is used. The type of the electrolytic solution is appropriately determined depending on the type of the positive electrode material, the properties of the dual-structure active material particles, the use conditions such as the charging voltage, and the like. Examples of the electrolyte include LiPF ₆ , LiBF ₄ , LIClO ₄
A lithium salt such as propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane, γ-butyl lactone, methyl acetate, methyl formate, and the like are dissolved in one or more organic solvents. Is preferred.

[0034]

The following examples are provided to further clarify the features of the present invention.

Examples 1 to 7 I. Preparation of Electrode A double-structured active material particle in which the surface of graphite particles is coated with amorphous carbon is used as a negative electrode active material, and acetylene plaque (trade name: Denka Black: manufactured by Denki Kagaku Kogyo Co., Ltd.) is used as a conductive material. An electrode was prepared using a solution in which polyvinylidene fluoride (trade name: KF # 1100: manufactured by Kureha Chemical Industry Co., Ltd.) was dissolved in N-methylpyrrolidone as a binder.

That is, the above-mentioned polyvinylidene fluoride solution was applied to a copper foil having a thickness of 14 μm, dried at 80 ° C. for 15 minutes, and with a radius of curvature of 30 with N-methylpyrrolidone remaining.
The roll was continuously pressed by a roll press having a thickness of 100 cm to produce a negative electrode having an electrode thickness of 100 μm.

Particle size (μm) of the obtained double-structure active material particles
And the spacing (d002) between the (002) planes of the graphite-based particles and the carbon layer coated thereon by X-ray wide-angle diffraction (unit: n
m) is shown in Table 1, and the composition ratio of each component of the electrode layer is shown in Table 2,
Table 3 shows the electrode density, the initial capacity, and the remaining amount of the solvent.

Comparative Example 1 The procedure of Example 1 was repeated except that the remaining amount of the solvent was 0.8%.
An electrode was prepared.

Comparative Example 2 The procedure of Example 1 was repeated except that the remaining amount of the solvent was 0.6%.
An electrode was prepared.

Comparative Example 3 The procedure of Example 1 was repeated except that the remaining amount of the solvent was changed to 0.2%.
An electrode was prepared.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

As is clear from the results shown in Tables 1 to 3, when the negative electrode according to the present invention is used, a high discharge capacity is obtained despite a high electrode density.

[0045]

According to the present invention, an inexpensive material having an amorphous carbon layer formed on the surface of graphite-based particles serving as nuclei is used as an active material, so that a negative electrode and a non-aqueous electrolyte solution using the same are used. It can contribute to cost reduction of secondary batteries.

Further, the particle diameter of the active material, the porosity of the electrode,
By optimizing the electrode density and the like, a negative electrode having a higher density and a higher capacity than a conventional negative electrode material can be obtained. As a result, a high performance non-aqueous electrolyte secondary battery can be obtained.

Continued on the front page (72) Inventor Shizukuni Yada 17 Kando-ji Minamimachi, Shimogyo-ku, Kyoto-shi, Kyoto Prefecture F-term in Kansai New Technology Research Institute (reference) 4G046 EA02 EA03 EA05 EB02 EB06 EC05 EC06 5H029 AJ03 AK03 AL07 AM03 AM04 AM05 AM07 CJ22 DJ07 DJ08 DJ16 EJ01 EJ12 HJ08 HJ09 HJ13 HJ19

Claims (4)

[Claims] (1) Both distances (d00) of the (002) plane by X-ray wide-angle diffraction
2) is 0.34 nm or less graphite-based particles have a surface spacing of 0.34
Average particle size 1 covered with amorphous carbon layer exceeding nm
5050 μm double-layered graphite-based particles are used as active material particles, a resin is used as a binder, and a metal is used as a current collector. The porosity is 20 to 35%, and the electrode density is 1.20.
~1.60g / cm ^3, the electrode capacity 400 mAh / cm ³ or more in a negative electrode for a nonaqueous electrolyte secondary battery, characterized in that. 2. The two distances (d00) of the (002) plane determined by the X-ray wide-angle diffraction method.
2) is 0.34 nm or less graphite-based particles have a surface spacing of 0.34
Average particle size 1 covered with amorphous carbon layer exceeding nm
5050 μm double-layer graphite-based particles are used as active material particles, a resin is used as a binder, and a metal is used as a current collector. The porosity is 20 to 35%, and the electrode density is 1.35.
~1.60g / cm ^3, the electrode capacity 400 mAh / cm ³ or more in a negative electrode for a nonaqueous electrolyte secondary battery, characterized in that. 3. The two distances between the (002) planes (d00
2) is 0.34 nm or less graphite-based particles have a surface spacing of 0.34
Average particle size 1 covered with amorphous carbon layer exceeding nm
5050 μm double-structured graphite-based particles are used as active material particles, a resin is used as a binder, and a metal is used as a current collector. The porosity is 20 to 35%, and the electrode density is 1.40.
~1.50g / cm ^3, the electrode capacity 400 mAh / cm ³ or more in a negative electrode for a nonaqueous electrolyte secondary battery, characterized in that. 4. A non-aqueous electrolyte secondary battery using the negative electrode for a non-aqueous electrolyte secondary battery according to claim 1.