JPH05290836A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To provide a lithium secondary battery excellent in a cycle characteristic, by using a binding body sheet, composed by integrating crystalline carbon powder and positive electrode active material powder with a binding agent, for a positive electrode. CONSTITUTION:A binding body sheet, composed by integrating crystalline carbon powder 2 and positive electrode active material powder 3 capable of storing and emitting lithium with binding agents 4, is used for a positive electrode 1. The use of the binding body sheet for the positive electrode 1 in this way, causes the possibility, in which a positive electrode mixture layer is fallen from a collecting body, to be utterly eliminated. Consequently a lithium secondary battery, having an excellent cycle characteristic and the less lowering of battery capacity following the repetition of a charge/discharge cycle, can be obtained. Moreover the crystalline carbon powder of graphite is preferable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to improvement of a positive electrode for the purpose of improving cycle characteristics.

[0002]

2. Description of the Related Art Conventionally, the problems to be solved by the invention
As a positive electrode of a lithium secondary battery, a slurry in which a mixture of a positive electrode active material powder, a conductive agent powder, and a binder is dispersed in a solvent is applied to both surfaces of an aluminum foil by a doctor blade method, and dried. Laminated sheets and the like were used.

In such a conventional positive electrode of a lithium secondary battery, as shown in the sectional view of FIG. 5, a positive electrode mixture layer 51 made of a mixture of a positive electrode active material powder, a conductive agent powder and a binder is collected. It was not adhered to the electric body 52, but merely pressed.

Therefore, the conventional lithium secondary battery is
There was a problem that a part of the positive electrode mixture layer 51 was detached from the current collector 52 while charging and discharging were repeated, and the battery capacity was reduced.

It has also been pointed out that a Schottky barrier is generated at the interface between the positive electrode mixture layer 51 and the current collector 52, which makes charging difficult.

The present invention has been made to solve these problems, and its object is to repeat charging and discharging cycles as compared with a conventional lithium secondary battery using a laminate sheet as a positive electrode. In order to provide a lithium secondary battery with less deterioration in battery capacity due to the above, that is, excellent in cycle characteristics.

[0007]

In order to achieve the above object, a lithium secondary battery according to the present invention (hereinafter, referred to as "invention battery") according to the present invention has a positive electrode which absorbs and releases crystalline carbon powder and lithium. It is composed of a binder sheet obtained by integrating a possible positive electrode active material powder with a binder.

Examples of the crystalline carbon powder include graphite and coke. Amorphous carbons such as acetylene black and carbon black that do not show a clear crystalline state are excluded. There are two reasons (1) and (2) below that the carbon powder according to the present invention is limited to those exhibiting crystallinity as described above and excludes amorphous carbon and the like. by.

(1) It is necessary to use crystalline carbon powder having a high electric conductivity in order to allow the carbon powder to have two functions of a conductive agent and a current collector. (2) When overcharging is performed, the battery voltage rises abnormally and the electrolytic solution is decomposed. However, if crystalline carbon powder capable of anion doping such as graphite is used, anion doping phenomenon occurs during overcharging. An abnormal rise in battery voltage is suppressed, and it is possible to withstand a certain amount of overcharge. For the reason (2) above, graphite is preferable to coke in which the amount of anion capable of doping is larger than that of coke.

A positive electrode active material capable of inserting and extracting lithium
As an inorganic compound, Li₂FeO₃, TiO₂,
V₂O_FiveOxides with tunnel-shaped holes such as
iS ₂, MoS₂Chalcogens having a layered structure such as
Compounds are exemplified, but the composition formula Li_xMO₂Or Li_yM
₂O_Four(However, M is a transition element, 0 ≦ x ≦ 1, 0 ≦ y ≦
The complex oxide represented by 2) is preferable,
For LiCoO₂, LiMnO₂, LiNiO₂, L
iCrO₂, LiMn₂O_FourIs exemplified.

Binders used for producing the positive electrode include polypropylene resin (PP), polyethylene resin (PE), polyethylene terephthalate resin (PET).
Is exemplified.

In the positive electrode of the present invention, for example, a binder powder, a crystalline carbon powder and a positive electrode active material powder are uniformly mixed, and the melting temperature of the binder (for example, in the case of polypropylene resin, usually 180 ° C.). Temperature) to form a slurry, and then using an extruder equipped with a slit having a predetermined thickness (usually 0.1 to 0.3 mm) according to the thickness of the target positive electrode. It is produced by extrusion, quenching, and quenching.

The mixing ratio of the binder powder, the crystalline carbon powder, and the positive electrode active material powder in producing the positive electrode varies depending on the type of each powder used, but the binder powder is generally 5 to 5.
0% by weight, crystalline carbon powder 1 to 20% by weight, and positive electrode active material powder 30 to 94% by weight are preferable. Above all, it is more preferable not to add the binder powder in an amount exceeding 20% by weight, because if the binder powder is added in a large amount, the conductivity is lowered.

In order to improve the cycle characteristics and the like, the battery of the present invention has the problem that the positive electrode material mixture layer separates from the current collector, which the conventional positive electrode made of a laminated sheet has.
It is characterized in that a positive electrode made of a binder sheet that does not cause such detachment is used. Therefore, negative electrode material, non-aqueous electrolyte, separator (when using liquid electrolyte)
As for the above, various materials conventionally used for lithium secondary batteries or proposed have been usable without limitation.

For example, as the negative electrode material, a material capable of inserting and extracting Li such as lithium metal, lithium alloy, or carbon material can be used. A powder material such as a carbon material is used as a negative electrode mixture by kneading it with a binder and optionally a conductive agent.

As the non-aqueous electrolyte, various non-aqueous electrolytes such as a solution of a solute such as LiPF ₆ or LiClO ₄ in ethylene carbonate, dimethyl carbonate or a mixed solvent thereof can be used.

[0017]

In the battery of the present invention, since the positive electrode is not the laminate sheet but the binder sheet, the problem that the positive electrode mixture layer is separated from the current collector, which has been a problem in the conventional battery, and the Schottky barrier. The problem that charging is difficult due to generation does not occur.

[0018]

EXAMPLES The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited to the following examples, and various modifications can be made without departing from the scope of the invention. Is possible.

Example 1 [Preparation of Positive Electrode] To 65 parts by weight of LiCoO ₂ as a positive electrode active material, 15 parts by weight of polypropylene resin as a binder and 20 parts by weight of natural graphite powder were added, and the mixture was heated to 180 ° C. After uniformly mixing to form a slurry, the mixture was heated and maintained at the same temperature. Next, this heated and held slurry was extruded by an extruder (slit thickness: 0.2 mm, slit width: 30 cm) and rapidly cooled to produce a positive electrode. FIG. 1 is a schematic cross-sectional view of the positive electrode 1 manufactured in this way, and shows a state in which the natural graphite powder 2 and the positive electrode active material powder 3 are integrated by the binder 4.

[Production of Negative Electrode] Fluororesin (PTFE) dispersion as a binder was mixed with natural graphite powder in a weight ratio of 95: 5 to obtain a negative electrode mixture. This negative electrode mixture was rolled into an aluminum foil as a current collector, and 2
A negative electrode was produced by heat treatment under vacuum at 50 ° C. for 2 hours.

[Preparation of Electrolyte Solution] LiPF 6 was added to an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate.
_An electrolytic solution was prepared by dissolving ₆ at 1 mol / liter.

[Preparation of Battery BA1 of the Present Invention] A cylindrical lithium secondary battery BA1 according to the present invention was prepared using the above-described positive and negative electrodes and an electrolytic solution (battery size: diameter 14.2 m).
m; length 50.0 mm). As the separator, an ion-permeable polypropylene microporous thin film was used.

FIG. 2 is a cross-sectional view of the manufactured battery BA1. The battery BA1 shown in FIG.
Separator 23 that separates these two electrodes, positive electrode lead 2
4, a negative electrode lead 25, a positive electrode external terminal 26, a negative electrode can 27, and the like. The positive electrode 21 and the negative electrode 22 are housed in a negative electrode can 27 in a state of being spirally wound via a separator 23 in which a nonaqueous electrolytic solution is injected, and the positive electrode 21 is connected to a positive electrode external terminal via a positive electrode lead 24. 26, and the negative electrode 22
Is connected to a negative electrode can 27 via a negative electrode lead 25, and chemical energy generated inside the battery BA1 can be taken out as electric energy to the outside.

(Comparative Example 1) 90 parts by weight of LiCoO ₂ and
6 parts by weight of acetylene black as a conductive agent and 4 parts by weight of polyvinylidene fluoride as a binder are mixed and dispersed in N-methylpyrrolidone as a solvent to form a slurry, which is a doctor on both sides of an aluminum foil. It was applied by a blade method and dried to prepare a laminate sheet-shaped positive electrode. A comparative battery BC1 was produced in the same manner as in Example 1 except that this laminated sheet was used as the positive electrode.

(Cycle characteristics of each battery) Battery BA of the present invention
1 and the comparative battery BC1 were charged at a charging current of 200 mA to a charge end voltage of 4.1 V, and then discharged at a discharge current of 200
A cycle test in which the process of discharging to a discharge end voltage of 3 V at mA was defined as one cycle was performed to examine the cycle characteristics of each battery.

FIG. 3 is a graph showing the cycle characteristics of each battery, with the vertical axis representing the battery capacity (mAh) and the horizontal axis representing the number of cycles (times). From the figure, the battery BA of the present invention
While No. 1 retains the initial battery capacity (500 mAh) even after 400 cycles, the comparative battery BC1 has a sharp decrease in battery capacity from the beginning of the cycle, and the comparative battery BC1 has less than 250 mAh at 250 cycles. It can be seen that the battery capacity will decrease until.

(Voltage characteristics of each battery when overcharged) Battery capacity 500 for the battery BA1 of the present invention and the comparative battery BC1
The voltage characteristics when charged (overcharged) over mAh were examined.

FIG. 4 is a graph showing the voltage characteristics of each battery, with the vertical axis representing the battery voltage (V) and the horizontal axis representing the charging capacity (mAh). From the figure, it can be seen that in the battery BA1 of the present invention, even if the battery capacity exceeds 500 mAh and is overcharged, the battery voltage rises up to about 4.3 V and does not rise further. This is because the doping of graphite with anions is started near the battery voltage of 4.3V. By this doping reaction, decomposition of the electrolytic solution is suppressed, and even in the subsequent discharge, no adverse effect due to overcharge is observed, and normal discharge is possible.

On the other hand, in the comparative battery BC1, it can be seen that the battery voltage rises sharply when overcharged. The rise of the battery voltage causes decomposition of the electrolytic solution, and as a result, the adverse effect of the previous overcharge appears at the time of discharging and the battery capacity decreases.

In the above embodiment, a specific example in which the present invention is applied to a cylindrical battery is described, but the shape of the battery is not particularly limited, and the present invention has various shapes such as flat type and square type. It is applicable to a lithium secondary battery.

[0031]

In the battery of the present invention, since the positive electrode made of the binder sheet is used, the present invention has excellent unique effects such as excellent cycle characteristics.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a positive electrode manufactured in an example.

FIG. 2 is a sectional view of a cylindrical battery BA1 of the present invention.

FIG. 3 is a cycle characteristic diagram.

FIG. 4 is a voltage characteristic diagram.

FIG. 5 is a cross-sectional view of a positive electrode of a conventional battery.

[Explanation of symbols]

 1 Positive Electrode 2 Natural Graphite Powder (Crystalline Carbon Powder) 3 Positive Electrode Active Material Powder 4 Binder

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kazuro Moriwaki 2-18 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Koji Nishio 2-18-2 Keihanhondori, Moriguchi-shi, Osaka Sanyo Denki Within the corporation

Claims (2)

[Claims] 1. A lithium secondary battery, wherein the positive electrode comprises a binder sheet in which crystalline carbon powder and positive electrode active material powder capable of occluding and releasing lithium are integrated with a binder. 2. The crystalline carbon powder is graphite.
The lithium secondary battery described.