JP3301786B2 - Lithium 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 lithium secondary battery, and more particularly to an object of the present invention is to obtain a highly reliable (safety) lithium secondary battery having a low risk of explosion even when an abnormal temperature rise occurs. To improved electrolyte solutions.

[0002]

2. Description of the Related Art In recent years,
As a negative electrode material of a lithium secondary battery, a carbon material capable of occluding and releasing lithium such as graphite and coke (hereinafter, referred to as a graphite material or coke) because of its excellent flexibility and no risk of depositing mossy lithium. Is sometimes simply referred to as “carbon material”) is being studied as a negative electrode material to replace the conventional metallic lithium (foil or plate).

However, a lithium secondary battery using a carbon material as a negative electrode material has an abnormal battery temperature (for example, 16
(0 ° C. or higher), it sometimes burst.

[0004] As a result of studies by the present inventors, this rupture may occur due to a rapid reaction between air that has entered the battery from a relief valve opened due to an increase in internal pressure and C ₆ Li generated during charging. Do you get it.

[0005] The present invention has been made based on such findings, and its object is to reduce the risk of rupture even when the temperature is abnormally increased, and to provide reliability (safety).
To provide a high rechargeable lithium battery.

[0006]

In order to achieve the above object, a lithium secondary battery according to the present invention (hereinafter, referred to as "battery of the present invention") uses a carbon material capable of inserting and extracting lithium for a negative electrode. A lithium secondary battery,
The electrolyte is characterized in that polymethacrylate having an average molecular weight of 10,000 or less is added in an amount not exceeding 10% by weight .

[0007] Examples of the carbon material capable of inserting and extracting lithium used in the negative electrode of the present invention include graphite and coke. Above all, graphite is a carbon material whose reliability is greatly improved by the present invention because a large amount of C ₆ Li is generated during charging. It does not matter whether the graphite is natural graphite or artificial graphite.

[0008] These carbon materials are kneaded with a binder such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) by a conventional method and used as a negative electrode mixture.

The carbon material in the present invention absorbs lithium ions Li ⁺ in the electrolyte as lithium during charging,
This serves as a host for so-called lithium ion exchange, which is discharged into the electrolyte as lithium ions Li ⁺ during discharge.

Here, at the time of completion of charging, most of graphite and part of coke are
It becomes C ₆ Li. This C ₆ Li does not react even if it comes into contact with air unless the battery temperature becomes abnormally high. However, when the battery temperature comes into contact with air at a high temperature of about 160 ° C. or more, it reacts rapidly and the battery ruptures. You do it.

In the battery of the present invention, in order to solve this problem, an electrolyte prepared by adding a polymethacrylate (methacrylic acid ester) having an average molecular weight (number average molecular weight) of 10,000 or less to the electrolyte is used. You.

Examples of the polymethacrylate include alkyl polymethacrylates such as polymethyl methacrylate and polyethyl methacrylate.

The polymethacrylate has an average molecular weight of 1
Although it is not particularly limited in view of the effect of preventing rupture, the battery characteristics are impaired when the average molecular weight of the polymethacrylate to be added exceeds 10,000 and the viscosity of the electrolytic solution is increased. This is because there is a tendency to cause problems in practical use.

For the same reason, the addition ratio of polymethacrylate to the electrolytic solution is also limited to 10% by weight based on the total amount of the electrolytic solution .

The electrolyte solvent in the battery of the present invention is not particularly limited. In addition to ethylene carbonate, dimethyl carbonate, and a mixed solvent thereof, there are various electrolyte solvents that have been proposed or practically used as electrolyte solvents for conventional lithium secondary batteries. Can be used.

There is no particular limitation on the solute when preparing the electrolytic solution. For example, LiPF ₆ , LiBF ₄ , Li
_{CF 3 SO 3, LiAsF 6,} LiSbF 6, LiC
Various solutes conventionally used or proposed for lithium secondary batteries, such as fluorine-containing lithium salts such as (CF ₃ SO ₂ ) ₃ and LiN (CF ₃ SO ₂ ) _2, can be used.

The reason why bursting at an abnormally high temperature is eliminated by adding polymethacrylate to the electrolytic solution is not necessarily clear to the present inventors, but is presumed as follows.

That is, when the battery temperature is, for example, 160 ° C.
When the temperature becomes higher than that, the solvent evaporates in order from the solvent having a relatively low boiling point and escapes from the relief valve to the outside of the battery, and as a result,
When polymethacrylate is not added, a part of the surface of C ₆ Li generated by charging comes into contact with air that has entered from the relief valve. However, in the present invention, since polymethacrylate is added to the electrolytic solution, even after the low-boiling solvent evaporates, the polymethacrylate coats the surface of C ₆ Li to prevent contact with air. Isn't it?

As described above, the present invention solves the problem of the rupture under abnormally high temperatures that the lithium secondary battery using the carbon material for the negative electrode has by adding polymethacrylate as a rupture inhibitor to the electrolytic solution. Therefore, the other members constituting the battery are not particularly limited.

For example, as the positive electrode material, any compound capable of inserting and extracting lithium can be used. As the inorganic compound, an oxide having a so-called tunnel-like vacancy such as TiO ₂ or V ₂ O ₅ can be used. And metal chalcogenides having a layered structure such as TiS ₂ and MoS ₂ , and the composition formula Li _x MO ₂ or Li _y M ₂ O ₄ (where M is a transition element, 0 ≦ x ≦ 1, 0 ≦ y ≦ 2) is preferred, and specific examples thereof include LiCoO ₂ , Li
MnO ₂ , LiNiO ₂ , LiCrO ₂ , LiMn ₂ O
₄ is mentioned.

The above-mentioned positive electrode material is made of a conductive agent such as acetylene black or carbon black, polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF).
Is used as a positive electrode mixture after kneading with such a binder.

[0022]

In the battery of the present invention, since polymethacrylate is added to the electrolytic solution, even if the relief valve is opened at the time of abnormal temperature rise and air enters the battery, the air and C ₆ generated at the time of charging are charged. Contact with Li is prevented by the polymethacrylate.

[0023]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples, and may be carried out by appropriately changing the scope of the present invention. Is possible.

(Example 1) [Preparation of positive electrode] LiCoO ₂ , acetylene black as a conductive agent, and a fluororesin dispersion as a binder were mixed at a weight ratio of 85: 10: 5 to form a positive electrode. Agent was obtained. This positive electrode mixture was rolled into an aluminum foil as a current collector, and heat-treated at 250 ° C. for 2 hours under vacuum to produce a positive electrode.

[Preparation of Negative Electrode] A natural graphite was mixed with a fluororesin dispersion as a binder at a weight ratio of 95: 5 to obtain a negative electrode mixture. This negative electrode mixture was rolled into a copper foil as a current collector, and heat-treated under vacuum at 250 ° C. for 2 hours to produce a negative electrode.

[Preparation of Electrolyte] An electrolyte was prepared by dissolving 1 mol / liter of LiPF ₆ as a solute in a mixed solvent of equal volumes of ethylene carbonate and dimethyl carbonate. Next, polymethyl methacrylate (PMMA; average molecular weight 10,000) was dissolved in the electrolyte solution to prepare an electrolyte solution containing 10% by weight of polymethyl methacrylate.

[Preparation of Lithium Secondary Battery] Using the above positive and negative electrodes and the electrolyte, five cylindrical batteries BA of the present invention were used.
1 (battery dimensions: diameter 14.2 mm; length 5)
0.0 mm). As the separator, a microporous thin film made of polypropylene (manufactured by Polyplastics, trade name "Celgard 3401") was used.

FIG. 1 is a cross-sectional view of the battery BA1 of the present invention produced. The battery BA1 of the present invention comprises a positive electrode 1 and a negative electrode 2, a separator 3 separating these electrodes, a positive electrode lead 4, a negative electrode lead 5, and a positive electrode. It comprises an external terminal 6, a negative electrode can 7, and the like. The positive electrode 1 and the negative electrode 2 are housed in a negative electrode can 7 while being spirally wound through a separator 3 into which an electrolytic solution has been injected. The positive electrode 1 is connected to a positive electrode external terminal 6 through a positive electrode lead 4. The negative electrode 2 is connected to a negative electrode can 7 via a negative electrode lead 5, so that chemical energy generated inside the battery BA1 can be taken out as electric energy.

Comparative Example 1 Five comparative batteries BC1 were prepared in the same manner as in Example 1 except that polymethyl methacrylate was not added in the preparation of the electrolytic solution.

(Burst Test) For each of the five batteries of the present invention BA1 and the comparative battery BC1, after charging, each battery was housed in a heating furnace, and the temperature in the furnace was raised to around 160 ° C. and then 1
When the presence or absence of rupture was examined after holding for a time, none of the battery BA1 of the present invention ruptured, whereas all the comparative batteries BC1 ruptured within 5 minutes.

In the above embodiment, an example in which polymethyl methacrylate is used as the polymethacrylate has been described, but other examples include polyethyl methacrylate (PEMA).
Similarly, excellent results can be obtained by using other polymethacrylic esters.

[0032]

According to the battery of the present invention, C ₆ generated during charging can be used.
Since the contact between Li and air is prevented by the polymethacrylate added to the electrolyte, the present invention has a low risk of rupture even if the battery temperature rises abnormally, and has high reliability (safety). Has an excellent specific effect.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a cylindrical battery BA1 of the present invention.

[Explanation of symbols]

 BA1 cylindrical battery of the present invention 1 positive electrode 2 negative electrode 3 separator

Claims (1)

(57) [Claims] 1. A lithium secondary battery in which a carbon material capable of inserting and extracting lithium is used for a negative electrode, wherein polymethacrylate having an average molecular weight of 10,000 or less is contained in an electrolytic solution.
A lithium secondary battery characterized by being added in an amount not exceeding 10% by weight .