JP3052760B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a structure of a carbon material used for its negative electrode.

[0002]

2. Description of the Related Art In recent years, portable and cordless electronic devices have been rapidly advancing, and there is a high demand for a small and lightweight secondary battery having a high energy density as a drive power source for these devices. In this regard, various batteries have been proposed because non-aqueous secondary batteries, particularly lithium secondary batteries, have high voltage and high energy density. Among them, Li as a positive electrode active material is a compound containing Li.
CoO ₂ or LiMn ₂ O ₄ , or all or a part of these Co and Mn may be replaced with another element such as Fe, N
A device using a composite oxide substituted with i, Mn, or the like has been studied. Batteries using a carbon material capable of occluding and releasing Li as a substance have attained high reliability such as cycle characteristics, storage stability, and safety, and thus have been partially put to practical use.

On the other hand, as a material used for the negative electrode, a carbon material such as graphite capable of occluding and releasing lithium has been studied in order to prevent lithium from being deposited in a dendritic state during charging.

[0004]

However, when mesophase small sphere particles capable of realizing a high capacity are used as a graphite material, these particles are electrically connected only at point contact, so that the charge In particular, during high-rate charge / discharge, crystal particles expand and contract in the C-axis direction, making contact between the particles worse, reducing the electron conductivity of the negative electrode and depositing metallic lithium in a dendritic form, which may cause an internal short circuit in the battery. Or the cycle life characteristics were reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is intended to prevent a decrease in contact between particles due to expansion and shrinkage of powder particles when a high-capacity graphite powder is used. It is intended to prevent the capacity from dropping during the charge / discharge cycle due to the deterioration of the properties and the precipitation of metallic lithium in the form of dendrite.

[0006]

In order to solve the above-mentioned problems, a nonaqueous electrolyte secondary battery according to the present invention uses a mesophase spheroidal graphite powder and a graphite powder for a negative electrode, and obtains an average particle of the mesophase spheroidal graphite powder. The ratio of the average particle diameter of the graphite powder to the diameter was set to 1.3 to 4.0.

[0007] Further, carbon fibers are further added to the above configuration.

[0008]

When only the graphite powder is applied on the current collector, the graphite crystal has a property that its C axis is oriented in parallel with the vertical direction of the current collector. Since the graphite crystal has a smaller electron conductivity in the C-axis direction than the electron conductivity in the A-axis and B-axis directions, the electron conductivity in the thickness direction is reduced in this configuration.

In the structure of the negative electrode of the present invention, a mixture of mesophase small sphere powder and graphite powder is used as the negative electrode mixture, and the ratio of the average particle diameter of the graphite powder to the average particle diameter of the mesophase small sphere powder is 1.3 to 4. Therefore, when the mixture is coated on the current collector, the graphite powder particles are distorted between the powder particles of the mesophase spheres and exist in various directions,
The graphite powder contacts adjacent particles in a state where the C-axis direction is inclined from the vertical direction of the current collector.

Therefore, when the mesophase small sphere powder is mixed with graphite powder having a larger average particle diameter, the mesophase small sphere powder which is electrically connected only at point contact by utilizing the A-axis and B-axis electron conductivities of the graphite powder. The electronic conductivity between the spherical powders can be improved.

Further, by adding and dispersing carbon fibers to these, the bonding state between the particles can be enhanced, and the electron conductivity of the negative electrode can be enhanced.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a longitudinal sectional view of the cylindrical battery used in this example. In the figure, reference numeral 1 denotes a battery case processed from a stainless steel sheet having resistance to organic electrolyte, 2 denotes a sealing plate provided with a safety valve, and 3 denotes an insulating packing. Reference numeral 4 denotes an electrode group, in which the positive electrode and the negative electrode are spirally wound a plurality of times via a separator and housed in the case 1. A positive electrode lead 5 is drawn out from the positive electrode and connected to the sealing plate 2. Reference numeral 7 denotes an insulating ring provided on the upper and lower portions of the electrode plate group 4, respectively. Hereinafter, the positive and negative electrode plates will be described in detail.

The cathode mixes Li ₂ CO ₃ and Co ₃ O ₄ ,
3 parts by weight of acetylene black and 7 parts by weight of a fluororesin binder were mixed with 100 parts by weight of LiCoO ₂ powder synthesized by firing at 900 ° C. for 10 hours, and suspended in an aqueous solution of carboxymethyl cellulose to form a paste. . This paste was applied to both sides of an aluminum foil, dried and rolled to obtain a positive electrode plate having a predetermined size.

The negative electrode was made of mesophase small sphere powder of 2800.
Graphite at high temperature of ℃ (average particle size 5.4μm,
d002 = 3.37 °) (hereinafter referred to as mesophase graphite) and flaky natural graphite powder (average particle diameter of 9.8 μm, d
002 = 3.36 °) was mixed at a ratio of 60:40. 100 parts by weight of this mixture was mixed with 5 parts by weight of styrene / butadiene rubber, and suspended in an aqueous solution of carboxymethyl cellulose to form a paste. The paste was applied to both sides of a copper foil, dried and rolled to obtain a negative electrode plate having a predetermined size.

Then, these were spirally wound between the positive and negative electrodes through a polyethylene porous film via a porous film made of polyethylene to form an electrode group, which was delivered to a battery case. As the electrolyte, a solution obtained by dissolving 1 mol / liter of LiPF ₆ in a solvent in which EC, DEC, and MP were mixed at a volume ratio of 30:50:20 was used. did. Here, the battery specification is the nominal specification 3.6V550mAH
And

Also, as shown in Table 1, negative electrode plates and batteries were prepared in the same manner as above except that the average particle size of the flaky natural graphite powder was changed, and these were designated as batteries B to H. .

[0017]

[Table 1]

A charge / discharge cycle life test was performed using these batteries. The charge and discharge conditions were as follows: at 20 ° C., a current of 1320 mA was charged to a voltage of 4.2 V for 30 minutes;
The discharge was performed at a current of 550 mA to a voltage of 3.0 V.

FIG. 2 shows the result. As shown in FIG. 2, when the ratio of the average particle diameter of the flaky natural graphite powder to the average particle diameter of the mesophase graphite powder is set to 1.3 or more,
The flaky natural graphite powder exists between the mesophase graphite powders in various directions, and the C axis of the natural graphite powder is inclined with respect to the vertical direction of the copper foil of the current collector. The contact state between the mesophase graphite powder particles can be improved by utilizing the electron conductivity of the B axis. However, when the ratio of the average particle size of the flaky natural graphite powder to the average particle size of the mesophase graphite powder exceeds 4.0 and the flaky natural graphite powder particles become large, the Li in the particles becomes large.
Since the diffusion rate of ions was slow, when rapid charging was performed, lithium metal was precipitated on the surface of the particles, and as a result, the charge / discharge cycle life characteristics were reduced.

Further, when the ratio is less than 1, the flaky natural graphite powder, even if inclined between mesophase graphite powders, has a small degree of inclination, so that the electron conductivity of the A and B axes of the crystal cannot be utilized. The charge-discharge cycle life characteristics decreased.

Therefore, the ratio of the average particle diameter of the flaky natural graphite powder to the average particle diameter of the mesophase graphite powder is preferably 1.3 or more and 4.0 or less.

Next, a battery similar to the battery A was prepared except that a vapor-grown carbon fiber was added as a carbon fiber to the negative electrode.

The vapor-grown carbon fiber has an average diameter of 0.5 μm,
The average length was 15 μm, d002 = 3.37 °, and the mixing ratio of mesophase graphite, natural graphite and carbon fiber was 60:30:10 by weight.

FIG. 2 shows the result of the same charge / discharge cycle life test performed on the battery I as described above.

As shown in FIG. 2, when the carbon fibers are mixed and dispersed in the negative electrode, the holding power of the negative electrode mixture can be increased, the contact between the particles can be kept good, and the capacity reduction accompanying the cycle progress can be prevented. Could be prevented.

In this embodiment, flaky natural graphite powder is used as the graphite powder, but flaky artificial graphite powder may be used. The same effect can be obtained as long as the carbon fiber is a conventionally known carbon fiber such as a pitch-based carbon fiber.

[0027]

As described above, the non-aqueous electrolyte secondary battery of the present invention uses the mesophase small sphere graphite powder and a mixture of the graphite powder for the negative electrode, and the average of the graphite powder with respect to the average particle diameter of the mesophase small sphere graphite powder. The ratio of particle diameter is 1.3 to
Since it is 4.0, the graphite powder can be arranged in various directions between the mesophase small sphere graphite powders. For this reason, the electrical contact between the mesophase spheroidal graphite powder can be improved by utilizing the electron conductivity of the A and B axes of the graphite powder crystal, and the charge / discharge cycle life characteristics of the battery can be improved.

Further, by mixing and dispersing carbon fibers in these, the holding power of the negative electrode mixture can be increased and the charge / discharge cycle life characteristics of the battery can be further improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery used in this example.

FIG. 2 is a diagram showing charge / discharge cycle life characteristics of a battery.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Insulation packing 4 Electrode group 5 Positive electrode lead 6 Negative electrode lead 7 Insulation ring

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akitatsu Morita 1006 Kazuma Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-4-332465 (JP, A) JP-A-5- 234584 (JP, A) JP-A-5-307958 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (4)

(57) [Claims] A mixture of mesophase small sphere graphite powder and graphite powder is used for the negative electrode, and the ratio of the average particle diameter of the graphite powder to the average particle diameter of the mesophase small sphere graphite powder is 1.
A non-aqueous electrolyte secondary battery having a value of 3 to 4.0. 2. A negative electrode comprising mesophase small sphere graphite powder, a mixture of graphite powder and carbon fiber, wherein the ratio of the average particle diameter of the graphite powder to the average particle diameter of the mesophase small sphere powder is 1.3 to 4.0. Non-aqueous electrolyte secondary battery. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the graphite powder is artificial graphite or natural graphite. 4. The non-aqueous electrolyte secondary battery according to claim 2, wherein the carbon fiber is a vapor growth type carbon fiber or a pitch type carbon fiber.