JPH1125986A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the capacity and cycle characteristics of a non-aqueous lithium secondary battery by using expansion graphite as one kind graphite electro-conductive agent in a positive electrode mix and setting the mixing ratio of the expansion graphite and other electro-conductive agent properly. SOLUTION: A nonaqueous lithium secondary battery is provided with a positive electrode mix containing an composite oxide positive electrode substance containing lithium and an electro-conductive substance in a positive electrode 1. Expansion graphite is used in the electro-conductive substance as at least one kind thereof, and the same is contained by five sixths or less of the whole weight of the electro-conductive substance. As the other electro-conductive substance carbon black-based for instance, acetylene black is used, and the expansion graphite and the acetylene black are mixed in the weight ratio of (1:1) to (5:1).

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 using a positive electrode mixture containing a positive electrode material and a conductive material for a positive electrode, and more particularly, to a positive electrode material of a composite oxide containing lithium. The present invention relates to a non-aqueous lithium secondary battery in which a positive electrode mixture containing a carbon material and a conductive material is used for a positive electrode, and a carbon material is used as a negative electrode material for a negative electrode.

[0002]

2. Description of the Related Art A lithium ion secondary battery, which is a non-aqueous lithium secondary battery, uses a lithium-containing composite oxide of a transition metal as a positive electrode active material, that is, LiM having a layered structure.
O ₂ or LiM ₂ O ₄ having a spinel structure (where M is a transition metal such as cobalt, manganese, nickel,
Reversible reaction in which one of the positive and negative electrodes uses a carbon material that absorbs lithium ions and forms an intercalation compound, and the other absorbs lithium ions released between the positive and negative electrodes. Charge and discharge.

Since the lithium ion secondary battery has a large discharge capacity and a theoretically high voltage and high energy density, great expectations are placed on its development.

At present, the above-mentioned lithium ion secondary battery uses a carbon material (graphite, carbon) capable of retaining lithium ions as a negative electrode material, and uses this material as a cobalt oxide compound (LiCoO ₂ ) or a nickel oxide compound (LiNiO).
A combination of a lithium-containing composite oxide such as O ₂ ) with a positive electrode material has been put to practical use, and further studies are being made to increase the capacity and improve the charge / discharge cycle characteristics.

[0005] Here, since the lithium composite oxide used as the positive electrode material has low conductivity, it is common to use a lithium compound oxide containing a conductive material as the positive electrode.

[0006]

In order to increase the capacity and the cycle stability of the non-aqueous electrolyte secondary battery, the use efficiency of the lithium composite oxide contained in the positive electrode must be improved. It is necessary to increase the content.

However, carbon black conductive agents, such as acetylene black and Ketjen black, which have been widely used, are excellent in conductivity and liquid absorption, but are poor in moldability and large in bulk density. Therefore, when used alone as the conductive agent of the positive electrode mixture, a large volume loss occurs as compared with a graphite-based conductive agent. In other words, the charge / discharge cycle characteristics can be maintained well, but the density of the electrode mixture decreases, the electrodes become thick and bulky, and as a result, the amount of the positive electrode active material that can be put in a battery can of a fixed volume decreases, resulting in high capacity There is a problem that can not be achieved.

On the other hand, artificial graphite, which is a graphite-based conductive agent, has hexagonal plate-like bonds of carbon spread laterally (in the A-axis direction).
The crystal structure between the plates (length of the C axis) is very wide, and the conductivity in the crystal plane direction is excellent, but the conductivity in the C axis direction, that is, the carbon hexagonal plate stacking direction is low. Poor, therefore, when this is used alone, even if the initial capacity can be secured, the cycle characteristics are inferior to the former, and the capacity decreases when charging and discharging are repeated, especially when the current value during charging and discharging is reduced. When it is high, the capacity tends to be remarkable.

For this reason, conventionally, as the conductive agent constituting the positive electrode mixture, it has been used to mix acetylene black or Ketjen black, which is a carbon black conductive agent, with artificial graphite, which is a graphite conductive agent. In general, the mixing ratio of these two conductive agents is 1: 1:
It has been said that setting to 1 can maximize the conductivity.

However, even when a conductive agent in which acetylene black and artificial graphite are mixed at a weight ratio of 1: 1 is used, a volume loss still occurs as compared with a case where a graphite-based conductive agent is used alone. There was a problem.

The present invention has been made to solve such a conventional problem, and an object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of improving the capacity and charge / discharge cycle characteristics of the secondary battery. To provide.

[0012]

According to the present invention, there is provided a non-aqueous electrolyte secondary battery comprising a positive electrode mixture containing a positive electrode material and a conductive material. The conductive material is obtained by mixing a plurality of types, and at least one of them contains expanded graphite in an amount of 5/6 or less of the total weight of the conductive material.

Here, the expanded graphite and carbon black are preferably used as the conductive material, and the expanded graphite and carbon black are desirably mixed in a weight ratio of 1: 1 to 5: 1. .

The expanded graphite has a crystallite size in the C-axis direction of not less than 500 Å and not more than 100 Å.
It is desirable to use one having a thickness of 0 Å or less.

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that a non-aqueous solution provided with a positive electrode mixture containing a positive electrode material of a composite oxide containing lithium and a conductive material in the positive electrode. In the electrolyte secondary battery, when expanded graphite is contained as at least one kind of the conductive substance in an amount equal to or less than 5/6 of the total weight of the conductive substance, and particularly, the conductive substance contained in the positive electrode is the expanded graphite. And a mixture of a carbon black conductive agent such as acetylene black,
It has been found that when a conductive agent obtained by mixing the expanded graphite and the carbon black-based conductive agent in a weight ratio of 1: 1 to 5: 1 is used, the charge / discharge cycle characteristics are stabilized and the capacity can be increased. . Further, it was confirmed that it is better to use the expanded graphite having a crystallite size of 500 Å or more and 1000 Å or less in the C-axis direction.

Here, when expanded graphite is used as one kind of the conductive agent as in the present invention and is contained in an amount of not more than 5/6 of the total weight of the conductive material, the capacity of the battery is increased and the charge / discharge cycle characteristics are improved. The possible reasons are as follows.

That is, as described above, the graphite crystal has a structure in which hexagonal plate-like bonds of carbon are spread laterally (in the A-axis direction), and the distance between the plates (the length of the C-axis) is very wide. The graphite-based conductive agent has excellent conductivity in the crystal plane direction.
Poor conductivity in the axial direction, that is, in the direction in which the carbon hexagonal plates are stacked.

On the other hand, expanded graphite has a short conductivity in the C-axis direction because of its short length in the C-axis direction. For this reason,
When expanded graphite is used as the graphite-based conductive agent, the conductivity in the C-axis direction is better than that of artificial graphite, so the charge / discharge cycle characteristics do not decrease. Therefore, the content of expanded graphite is increased and the amount is increased accordingly. It is considered that the content of bulky acetylene black can be reduced, the density of the electrode mixture increases, and a higher capacity of the battery can be achieved.

On the other hand, if the content of the expanded graphite exceeds 5/6, the charge / discharge cycle characteristics are undesirably deteriorated.

[0020]

Embodiments of the present invention will be described below.

FIG. 1 shows the overall structure of the manufactured AA lithium ion secondary battery.

<Assembly of Battery> In FIG. 1, reference numeral 1 denotes a positive electrode plate, which represents the weight ratio of LiCoO _{2 as} a positive electrode active material, carbon powder as a conductive agent, and an aqueous dispersion of PTFE as a binder. The mixture was mixed at a ratio of 1 and kneaded into a paste with water, applied to both sides of a 30 μm-thick aluminum foil, dried, rolled, cut into a predetermined size,
It was prepared as a belt-like positive electrode sheet. A part of this positive electrode sheet is scraped off the mixture perpendicular to the longitudinal direction of the sheet,
A positive electrode lead plate made of titanium was mounted on the current collector by spot welding.

As the active material, LiCoO ₂ , a mixture of cobalt oxide (CoO) and lithium carbonate (Li ₂ CO ₃ ) at a molar ratio of 2: 1 and heating in air at 900 ° C. for 9 hours was used.

As the conductive agent for the positive electrode mixture, commercially available expanded graphite as a graphite-based conductive agent and commercially available acetylene black as a carbon black-based conductive agent were used as shown in Table 1.
A mixture in which the weight ratio was 1: 1 to 5: 1 was used. Here, the expanded graphite is obtained by heat-treating a graphite material as follows. That is, a graphite material with highly developed crystals, such as natural graphite, is immersed in a mixture of nitric acid and sulfuric acid. In this case, the mixture of nitric acid and sulfuric acid is not particularly limited, but the weight ratio is 1:10 to 10:
If it is 1, there is no particular problem. Then, the graphite material after immersion was calcined at 1000 ° C. for 3 hours to obtain the expanded graphite as a conductive graphite material. The expanded graphite used had a crystallite size in the C-axis direction of not less than 500 Å and not more than 1000 Å.

Of the above mixing ratios of the materials, PTFE
The ratio of the aqueous dispersion is the ratio of the solid content. At this time, the weight of the positive electrode active material was 4.7 g.

Reference numeral 2 denotes a negative electrode using a carbon material, which is obtained by kneading a commercially available coal-based pitch coke and an aqueous dispersion of PTFE as a binder at a weight ratio of 100: 5, and pressing the mixture into a nickel expanded metal. Dry, cut,
It was prepared as a strip-shaped negative electrode sheet. A part of this negative electrode sheet is scraped off the mixture perpendicular to the longitudinal direction of the sheet,
A nickel negative electrode lead was spot-welded on the current collector. In addition, the ratio of PTFE is a ratio of a solid content similarly to the above. The weight of the carbonaceous powder in the negative electrode 2 is 2.1 g.
It is.

The positive electrode 1 and the negative electrode 2 are spirally wound through a separator 3 made of a porous film made of polyethylene and inserted into an outer can 4. After the insertion, the titanium lead 5 is spot welded to the stainless sealing plate 6.

Reference numeral 7 denotes a positive electrode cup / positive terminal made of aluminum, which is spot-welded to the sealing plate 6 in advance. The negative electrode lead plate 11 is spot-welded to the center of the circular bottom surface of the outer can 4 also serving as the negative electrode terminal. Reference numeral 8 denotes a polypropylene insulating gasket. 10, the battery has an abnormality,
This safety valve is installed so that the internal gas is released to the outside when the internal pressure of the battery rises. Reference numeral 12 denotes an insulating bottom plate made of polypropylene, which has a hole so as to have the same area as the space 13 generated at the time of winding.

After the above operation, ethylene carbonate,
An electrolyte was prepared by adding LiPF ₆ (1 mol / l) as an electrolyte to an organic solvent having a ratio of 1: 1 of diethyl carbonate, and 2.3 ml of the electrolyte was injected and sealed.

The size of the completed battery is AA type (diameter 1).
4.5 mm x 50 mm).

<Test of Battery> The assembled battery was charged at a constant current and a constant voltage of 250 mA, 4.1 V and 5 hr, discharged at a constant current of 500 mA to 3.0 V, and a charge / discharge cycle was repeated. .

<Battery Test Results> FIG. 2 and FIG. 3 show the discharge capacity and cycle characteristics of each specification battery together with comparative examples. FIG. 3 and FIG. 4 show the capacity deterioration rates for each cycle together with comparative examples.

[0033]

[Table 1]

As can be seen from Table 1, Example 1 is 500
3% by weight of expanded graphite and 3% by weight of acetylene black of angstrom or more and 1000 angstrom or less, that is, a material using a conductive agent in which expanded graphite and carbon black are mixed at a weight ratio of 1: 1. It contains graphite at ／ of the total weight of the conductive material.

In Example 2, 4 wt% of expanded graphite having a thickness of 500 Å to 1,000 Å was used.
%, Acetylene black being 2 wt%, that is, a conductive agent obtained by mixing expanded graphite and carbon black at a weight ratio of 2: 1 was used, and expanded graphite was 4% of the total weight of the conductive substance. / 6.

Similarly, in Example 3, expanded graphite having a thickness of 500 Å to 1,000 Å was used for 5 watts.
% of acetylene black and 1 wt% of acetylene black, that is, the weight ratio of expanded graphite to carbon black is 5: 1.
In which the conductive agent is mixed at a ratio of 5%, and contains expanded graphite at 5/6 of the total weight of the conductive substance.

In Examples 1 to 3, the capacity of the battery was large as shown by the curve in FIG. 4 or FIG.
Alternatively, the charge / discharge cycle characteristics are stable as shown by the curve in FIG. Here, artificial graphite was not included in Examples 1 to 3, and the content was 0 wt%.

On the other hand, in Comparative Example 1, 6 wt% of expanded graphite and 0 wt% of both acetylene black and artificial graphite were used.
Comparative Example 2 was made of only acetylene black with 0% by weight of expanded graphite and artificial graphite, 6% by weight of acetylene black, and Comparative Example 3 was made of 0% by weight of both expanded graphite and acetylene black. %, Artificial graphite was 6 wt% and only artificial graphite was used. In Comparative Example 1, the weight ratio of expanded graphite to carbon black was 5: 1.
And a ratio of 6: 0 was obtained, and Comparative Example 2 was a case where the weight ratio was lower than 1: 1 and became a ratio of 0: 6.

The cycle characteristics of Comparative Examples 1 to 3 are shown by curves a to c in FIG. 2, and the capacity deterioration rate is shown by curves a to c in FIG.
Indicated by In Comparative Examples 1 and 3 (curves a and c), the initial capacity is large, but the capacity deterioration in each cycle is large. If the mixing ratio of the expanded graphite and the carbon black is out of the range of 1: 1 to 5: 1 as described above, the desired stable cycle characteristics and high discharge capacity cannot be obtained. Further, as shown in Table 1, Comparative Example 2 in which the electrode mixture density was small
(Curve b) shows that the cycle characteristics are stable as shown in FIG. 2, but the original initial discharge capacity is smaller than Examples 1 to 3 because the density of the electrode mixture is small.

In Comparative Examples 4 to 6, artificial graphite was used in a corresponding amount instead of using the expanded graphite in Examples 1 to 3 above. These comparative examples 4 to 6
3 are shown by curves df in FIG. 3, and the capacity deterioration rate is shown by curves df in FIG. In each case, the initial capacity is large, but the capacity is greatly deteriorated in each cycle.

Comparative Example 7 is the same as Example 5
Instead of expanded graphite having a thickness of not less than 00 Å and not more than 1000 Å, expanded graphite having a thickness of less than 500 Å is used. As shown by the curve h in FIG. 3, satisfactory expanded cycle characteristics cannot be obtained when expanded graphite of less than 500 Å is used. This is equivalent to the curve h for the conventional product shown for reference in FIG.

As is clear from the above results, when the expanded graphite and carbon black (acetylene black) are mixed and used as the conductive agent contained in the positive electrode of the non-aqueous lithium secondary battery, Examples 1 to 3 are used. The weight ratio is 1: 1
When the content of the expanded graphite is set to be not less than 1/2 and not more than 5/6 of the total weight of the conductive substance at a mixing ratio of ~ 5: 1, a battery having a large discharge capacity and stable cycle characteristics can be obtained. understood. As in Comparative Examples 4 to 6, when commercially available artificial graphite was used as the conductive agent and compared at the same weight ratio, cycle deterioration occurred.

[0043]

As described above, according to the present invention, the following excellent effects can be obtained.

(1) In a non-aqueous electrolyte secondary battery provided with a positive electrode mixture containing a positive electrode material of a composite oxide containing lithium and a conductive material on the positive electrode, at least one of the conductive materials may be C By including expanded graphite having better conductivity in the axial direction than artificial graphite in an amount of 5/6 or less of the total weight of the conductive material, it is possible to suppress deterioration of charge / discharge cycle characteristics as much as possible and to increase bulkiness. Since the content of the carbon black acetylene black can be reduced, the density of the electrode mixture can be increased and the capacity of the battery can be increased.

(2) As a conductive substance contained in the positive electrode,
In particular, by using a mixture of acetylene black, which is a carbon black conductive agent, and expanded graphite, which is a graphite conductive agent, the weight ratio of the mixture is in the range of 1: 1 to 5: 1. Compared to those in the range of the mixing ratio, stabilization of charge / discharge cycle characteristics and increase in capacity can be remarkably achieved.

(3) The expanded graphite of the conductive material has a crystallite size in the C-axis direction of 500 Å or more and 1
By using a battery having a size of 000 angstroms or less, a battery having higher capacity and stable cycle characteristics can be obtained as compared with the case of using expanded graphite having a crystallite size outside this range.

[Brief description of the drawings]

FIG. 1 is a sectional view of a wound nonaqueous electrolyte secondary battery to which the present invention is applied.

FIG. 2 is a diagram showing cycle characteristics of an example of the present invention together with a comparative example.

FIG. 3 is a diagram showing cycle characteristics of an example of the present invention together with another comparative example.

FIG. 4 is a diagram showing a capacity deterioration rate of an example of the present invention together with a comparative example.

FIG. 5 is a diagram showing cycle characteristics of an example of the present invention together with another comparative example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 positive electrode plate 2 negative electrode 3 separator 4 outer can 5 lead 6 sealing plate 7 positive electrode cup and positive terminal 8 insulating gasket 10 safety valve 11 negative electrode lead plate 12 polypropylene insulating bottom plate 13 space

Claims (3)

[Claims] 1. A non-aqueous electrolyte secondary battery provided with a positive electrode mixture containing a positive electrode material and a conductive material in a positive electrode, wherein the conductive material is a mixture of a plurality of types. A non-aqueous electrolyte secondary battery, wherein expanded graphite is contained in an amount of 5/6 or less of the total weight of the conductive substance. 2. The method according to claim 1, wherein the conductive substance comprises the expanded graphite and carbon black, and the expanded graphite and carbon black are mixed in a weight ratio of 1: 1 to 5: 1. The non-aqueous electrolyte secondary battery according to claim 1, wherein 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the expanded graphite has a crystallite size in the C-axis direction of not less than 500 angstroms and not more than 1000 angstroms.