JPH08250118A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To provide a lithium secondary battery having large capacity, the low degree of shelf deterioration, and high practicability. CONSTITUTION: This lithium secondary battery is composed of a positive electrode 3, a carbonaceous-material-made negative electrode 7 capable of storing and emitting a lithium ion, and a lithium ion conductive electrolyte. The electrode 7 made of a carbonaceous material is a heat-treated mesophase pitch carbon fiber. A second lithium secondary battery is composed of a positive electrode 3, the electrode 7 made of the carbonaceous material and capable of storing and releasing a lithium ion, and the lithium ion conductive electrolyte. The electrode 7 is heat-treated under an air atmosphere, and is the mesophase pitch carbon fiber having a specific surface of 5m<2> /g or less.

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 in which the capacity is improved and storage deterioration is avoided by improving a negative electrode.

[0002]

2. Description of the Related Art In recent years, a lithium secondary battery using lithium as a negative electrode active material has been attracting attention as a high energy density battery as a non-aqueous electrolyte battery. In other words, the lithium primary battery property, which uses manganese dioxide (MnO ₂ ), fluorocarbon (CF ₂ ) _n , thionyl chloride (SOCl ₂ ) etc. as the positive electrode active material, is often used as a backup battery for calculators, watches, and memories. ing. On the other hand, nickel-hydrogen secondary batteries, or alkaline secondary batteries typified by nickel-cadmium secondary batteries have been widely put into practical use as operating power sources for various equipment systems such as mobile phones and portable image pickup devices. There is. And this type of secondary battery
Securing or storing electric power by a charging operation and driving (discharging) a load using the secured or stored electric power as a power source,
Since it can be repeatedly performed, it has been put into practical use by being incorporated into the above-mentioned various device systems.

With respect to these secondary batteries, along with the downsizing and weight saving of the above-mentioned portable telephones and portable image pickup devices, there is a demand for secondary batteries of high energy density as a power source. In response to this, development of lithium secondary batteries using lithium as a negative electrode active material is underway.

By the way, generally, in a lithium secondary battery, (a) lithium is used as a negative electrode active material, and (b) propylene carbonate, ethylene carbonate, 1,2-
In a non-aqueous solvent such as dimethoxyethane, γ-butyrolactone or tetrahydrofuran, for example, LiCl0 ₄ , LiB
A non-aqueous electrolytic solution prepared by dissolving a lithium salt such as F ₄ or LiAsF ₆ or a lithium ion conductive solid electrolyte is
(c) Furthermore, as positive electrode active materials, compounds such as vanadium oxide, cobalt oxide, and manganese oxide that are involved in the topochemical reaction with lithium are mainly used.

[0005]

However, the conventionally known lithium secondary battery has problems that the charge / discharge efficiency is low and the number of charge / discharge cycles (charge / discharge cycle) is short. It is considered that this is largely due to the deterioration of lithium due to the reaction between the negative electrode active material lithium and the non-aqueous electrolyte. That is, lithium that has been dissolved in the non-aqueous electrolyte as lithium ions during discharge reacts with the solvent during precipitation during charging, and its surface is partially inactivated. Therefore, when charge / discharge is repeated, dendrite-like (dendritic) lithium is deposited to cause an internal short circuit, and the charge / discharge cycle life is shortened.

[0006] In order to solve the problem of charge / discharge cycle life, it has been attempted to use a carbonaceous material capable of inserting and extracting lithium as a negative electrode incorporated in a lithium secondary battery. That is, coke, resin sintered body,
By incorporating a negative electrode having a structure in which lithium (negative electrode active material) is supported on carbon fiber, pyrolysis vapor-grown carbonaceous material, etc., deterioration of the negative electrode due to reaction between the lithium and the non-aqueous electrolytic solution or deposition of dentrite is prevented. It is suggested to prevent. Then, by adopting such a configuration, although it is possible to prevent the deterioration of the negative electrode, on the other hand, the charge and discharge capacity is lower than that of a secondary battery in which metal lithium is directly used as the negative electrode, and the degree of storage deterioration. The problem is that it is also large.

The present invention has been made in view of such circumstances, and an object thereof is to provide a lithium secondary battery having a high capacity, a low degree of storage deterioration, and a higher practicality.

[0008]

A first lithium secondary battery according to the present invention comprises a positive electrode, a negative electrode made of a carbonaceous material capable of inserting and extracting lithium ions, and a lithium ion conductive electrolytic solution. In the lithium secondary battery formed as described above, the carbonaceous material negative electrode is a heat-treated mesophase pitch carbon fiber.

The second lithium secondary battery according to the present invention is
In a lithium secondary battery comprising a positive electrode, a negative electrode made of a carbonaceous material capable of inserting and extracting lithium ions, and a lithium ion conductive electrolyte, the negative electrode made of the carbonaceous material is heated in an air atmosphere. Treated and specific surface area 10 m ²
/ G or less of mesophase pitch carbon fiber.

A third lithium secondary battery according to the present invention is
In Claim 1 or 2, the carbonaceous material-made negative electrode is characterized by being heat-treated at 400 to 900 ° C. for 30 minutes or more in an air atmosphere.

As described above, in the lithium secondary battery according to the present invention, in the structure of the electromotive element part built-in and provided, the carbonaceous material carrying the negative electrode active material is made of mesophase pitch and heated. It is characterized by the choice of treated mesophase pitch carbon fiber. Here, the heat-treated mesophase pitch carbon fiber was selected because it was experimentally confirmed that the specific surface area can be increased by the heat treatment (surface oxidation treatment) and the charge / discharge capacity can be improved. . That is, it is considered that the surface becomes uneven due to the heat treatment, the specific surface area becomes large, the amount of lithium intercalated and deintercalated in the carbonaceous material increases, and the charge / discharge capacity improves. The heat treatment temperature is generally in an air atmosphere.
The treatment is performed at a temperature of 400 ° C or higher (preferably 400 to 900 ° C), and when the treatment temperature is set high, the treatment time is selected to be short, and the treatment time is set to at least 30 minutes and a long time of about 50 hours. Then, the heat treatment time, but the specific surface area increases, also be obtained by improving the specific surface area is too large capacity, since the storage deterioration is observed, the specific surface area 10 m ²
It is preferable to selectively set the value to / g or less.

The mesophase pitch carbon fiber used in the present invention is a carbonaceous material obtained by firing and carbonizing petroleum pitch, coal tar, heavy oil or the like as a raw material in an inert gas stream or in the air. It is a fiber. More specifically, the carbon fiber used can be obtained as follows. That is, petroleum pitch, coal tar, heavy oil, organic resin, synthetic polymer materials, etc. are used as raw materials, and these are placed in an inert gas such as nitrogen or argon at a temperature of 800 to 1000 ° C and normal pressure or pressure. Carbonize under pressure conditions or even in an inert gas, 1000
It can be obtained by graphitizing at a temperature of ~ 3200 ° C and normal pressure or pressure. In particular, after melting the mesophased pitch-based raw material and infusibilizing the fiber created by spinning,
It can be produced by carbonization or graphitization. The carbonization firing temperature is 2000 ° C. or lower, preferably 600 to
The temperature is 1500 ° C, and the firing temperature for graphitization is 1000-3200 ° C, preferably 2500-3200 ° C.

Here, the carbon fiber may have a so-called orientation, a random orientation, or a mixture of a portion having orientation and a portion having random orientation. Good. The orientation and form of the carbon layer in the cross section of the carbon fiber include a radial structure, a radial structure on the fiber surface side and a random structure inside.
Examples include a strip structure or a lamella structure. The graphite structure of this carbon fiber is defined by the interplanar spacing (d ₀₀₂ ) of the (002) plane obtained by X-ray diffraction and the crystallite size (L _c ) in the c-axis direction, and the interplanar spacing (d
_It is desirable that the graphite structure has an average value of ₀₀₂ ) of 0.336 to 0.380 nm and an average value of crystallite size (L _c ) of 1 to 70 nm. That is, when the average value of the interplanar spacing (d ₀₀₂ ) and the average value of the crystallite size (L _c ) are out of the above ranges, the lithium ion storage / release amount in the negative electrode is reduced, the graphite structure is deteriorated,
This is because the reduction of the capacity of the secondary battery is recognized due to the generation of gas due to the reductive decomposition of the solvent in the non-aqueous electrolyte.

Further, the diameter of the carbon fiber is 1 to 100 μm.
m, preferably 2-40 μm, more preferably 4-20 μm
However, if the fiber diameter and the fiber length are less than 1 μm, they may easily pass through the pores of the separator and may cause a short circuit with the positive electrode. On the other hand, when the fiber diameter and the fiber length exceed 100 μm, the specific surface area decreases, and it may be difficult to increase the lithium ion storage / release amount. Therefore,
In use, it is effective to pulverize the carbon fibers and adjust the average particle diameter within the above range.

In the present invention, examples of the material forming the positive electrode include lithium-manganese composite oxide, manganese dioxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel-cobalt oxide, and lithium-containing amorphous pentaoxide. Examples thereof include oxides such as vanadium oxide and chalcogen compounds such as titanium disulfide and molybdenum disulfide.

Examples of the lithium ion conductive electrolyte usable in the present invention include ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, sulfolane, acetonitrile, 1,2-dimethoxymethane, 1,3-dimethoxypropane. ，
Lithium perchlorate (LiCl) is added to a non-aqueous solvent consisting of at least one selected from the group of dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate.
O ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF
₆ ), lithium trifluoromethanesulfonate (LiCF ₃
0.5 to 1.5 mol / of lithium salt (electrolyte) such as SO ₃ )
Generally, a non-aqueous electrolytic solution in which about 1 is dissolved is used. Instead of the non-aqueous electrolyte solution, a lithium ion conductive solid electrolyte, for example, a polymer solid electrolyte obtained by compounding a lithium salt with a polymer compound can also be used. Further, as the separator that insulates and separates the negative electrode and the positive electrode, for example, a nonwoven fabric of polyolefin resin such as polyethylene or polypropylene or a porous film can be used.

[0017]

According to the lithium secondary battery of the present invention, the heat-treated mesophase pitch carbon fiber system is particularly selected and used for the constitution of the negative electrode which absorbs and releases lithium ions. The heat-treated mesophase pitch carbon fiber system has an increased specific surface area due to surface oxidation, and as the specific surface area increases, the intercalating and deintercalating ability of lithium also increases. hand,
It exhibits high charge and discharge capacity and retains good storage resistance (low storage deterioration).

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG.

Example 1 Preparation of Positive Electrode First, powder of lithium manganese composite oxide prepared from lithium hydroxide and manganese dioxide as active materials was prepared as follows.
Artificial graphite powder was prepared as the conductive material, and polytetrafluoroethylene powder was prepared as the binder. Then, the active material, the conductive material, and the binder were mixed and kneaded in a mass ratio of 90: 10: 5 to prepare a mixture. Then, using a pressure press, 2 ton /
By pressure molding at a pressure of cm ² , a pellet-shaped positive electrode having a diameter of 15 mm and a thickness of 0.8 mm was prepared.

Preparation of Negative Electrode Carbon fibers obtained from mesophase pitch as a raw material were finely pulverized and fired at a temperature of 2800 ° C. to obtain carbon powder. Next, the carbon powder was placed in an air atmosphere at 500 ° C. for 24 hours.
After heat-treatment for an hour, this heat-treated carbon powder 95
5 parts by weight of styrene-butadiene rubber as a binder
A mixture was prepared by adding, mixing and kneading in a proportion of parts by mass. Then, using a pressure press machine, the mixture is mixed with 3 ton /
After pressure molding with a pressure of cm ² to form a pellet having a diameter of 15 mm and a thickness of 0.96 mm, lithium was contained (supported) in the pellet-shaped compact by an electrolytic impregnation method to prepare a negative electrode.

Preparation of separator An electrolytic solution prepared by dissolving lithium perchlorate in a solvent in which ethyl methyl carbonate, ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 65:25:10 to a concentration of 1 mol / l was prepared. Prepared. The thickness of this electrolyte is 0.2
A disc-shaped polypropylene non-woven fabric having a diameter of 17.6 mm and a diameter of 17.6 mm was impregnated to prepare a separator.

Assembly of Secondary Battery A conventional non-aqueous solvent secondary battery was assembled by conventional assembling means as shown in FIG. First, the pellet-shaped positive electrode 3 prepared as described above is housed and mounted on the inner surface of the positive electrode can 1 made of stainless steel via the positive electrode current collector 2.
Further, the separator 4 prepared above was placed on the pellet-shaped positive electrode 3.

On the other hand, the pellet-shaped negative electrode 7 prepared above was housed and mounted on the inner surface of the negative electrode can 5 also made of stainless steel, with the negative electrode current collector 6 having a diameter of 15 mm and a thickness of 1.0 mm interposed therebetween. After that, the insulating packing 8 is attached to the opening of the positive electrode can 1, the opening of the negative electrode can 5 is fitted into the opening of the positive electrode can 1, the positive electrode can 1 is caulked, and the positive electrode can 1 and A coin-shaped non-aqueous solvent secondary battery having an outer diameter of 20 mm and a thickness of 2.5 mm, which was formed by hermetically sealing the positive electrode 3, the separator 4, the negative electrode 7, etc. in the negative electrode can 5, was assembled.

Next, various characteristics of the non-aqueous solvent secondary battery having the above structure were evaluated.

(A) Battery Aging After assembling the non-aqueous solvent secondary battery, the battery is aged at room temperature of 20 ° C.
After aging for ~ 14 days, the battery circuit voltage was measured and found to be 3.4 V.

(B) Discharge test 250 μA of the non-aqueous solvent secondary battery after the assembly
The battery capacity was measured by conducting a discharge test up to 2.0 V with the constant current in Table 1 and the results are shown in Table 1 as the capacity before storage. (C)
Storage test After assembling the non-aqueous solvent secondary battery, store it at room temperature of 60 ° C for 20
After aging for a day, the battery circuit voltage was measured and found to be 3.4 V. Further, the discharge capacity was measured up to 2.0 V at a constant current of 250 μA, and the residual capacity of the battery was measured. The results are shown in Table 1 as the capacity after storage.

Example 2 Instead of heating the carbon powder used for preparing the negative electrode in the air atmosphere at 500 ° C. for 24 hours in the case of Example 1, except that the carbon powder was heated at 500 ° C. for 36 hours. ,
A coin type non-aqueous solvent secondary battery was assembled under the same condition setting as in the case of Example 1.

Table 1 also shows the results of various characteristic evaluations of this non-aqueous solvent secondary battery under the same conditions as in Example 1.

Example 3 In the case of Example 1, instead of heat-treating the carbon powder used for preparing the negative electrode in the air atmosphere at 500 ° C. for 24 hours, heat treatment was performed in the air atmosphere at 500 ° C. for 48 hours. ,
A coin type non-aqueous solvent secondary battery was assembled under the same condition setting as in the case of Example 1.

Table 1 also shows the results of various characteristic evaluations of this non-aqueous solvent secondary battery under the same conditions as in Example 1.

Comparative Example 1 In the same manner as in Example 1, except that the carbon powder used for preparing the negative electrode was not heat-treated at 500 ° C. for 24 hours in the air atmosphere, this heat-treatment was not performed. A coin type non-aqueous solvent secondary battery was assembled under the same condition setting as the case.

Table 1 also shows the results of various characteristic evaluations of this non-aqueous solvent secondary battery under the same conditions as in Example 1.

[0033] The present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the invention. For example, a mesophase-pitch-based carbon fiber that is a constituent material of the negative electrode may be used even when the constituent components of the positive electrode and the separator are changed to known positive electrodes and separators in lithium secondary batteries. The same action and effect can be obtained by using the heat treatment conditions of the temperature and the time exemplified above. Also, instead of the coin shape, the shape is, for example, cylindrical, flat,
Square type can also be adopted.

[0034]

As described above, according to the lithium secondary battery of the present invention, the mesophase pitch carbon fiber, which is heat-treated as the negative electrode constituent material and whose specific surface area is increased to some extent by surface oxidation, is used. By adopting the configuration, it is possible to easily increase the capacity and improve the storage resistance of the secondary battery. Therefore, it becomes possible to provide a portable power source that is compatible with the reduction in size and weight of the power source due to the recent trend toward compact electronic devices and the like.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing a configuration example of a main part of a lithium secondary battery according to the present invention.

[Explanation of symbols]

1 ... Positive electrode can 2 ... Positive electrode collector 3 ... Positive electrode 4 ... Separator 5 ... Negative electrode can 6 ...
… Negative electrode collector 7 …… Negative electrode 8 …… Insulating packing

Claims (3)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode made of a carbonaceous material capable of inserting and extracting lithium ions, and a lithium ion conductive electrolyte, wherein the negative electrode made of a carbonaceous material is heated. A lithium secondary battery, which is a treated mesophase pitch carbon fiber. 2. A lithium secondary battery comprising a positive electrode, a negative electrode made of a carbonaceous material capable of inserting and extracting lithium ions, and a lithium ion conductive electrolyte, wherein the negative electrode made of a carbonaceous material is A lithium secondary battery, which is a mesophase pitch carbon fiber having a specific surface area of 10 m ² / g or less, which is heat-treated in an air atmosphere. 3. The negative electrode made of a carbonaceous material according to claim 1 or 2, wherein the negative electrode is 400 to 900 in an air atmosphere.
A lithium secondary battery characterized by being heat-treated at ℃ for 30 minutes or more.