JPH1116570A - Lithium secondary battery and electronic equipment using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material in which the reduction of discharge capacity accompanying the increase of charge and discharge cycle frequency is minimized by forming a coating layer mainly composed of an amorphous carbon material on the surface of a granular or fibrous crystalline carbon material contained in a negative electrode in mesh so as partially expose the substrate. SOLUTION: The covering ratio of an amorphous carbon covering the surface of a crystalline carbon material in mesh is 20-70 % by area ratio, and the thickness of the amorphous carbon is 1-10 nm. The granular or fibrous crystalline carbon material contained in a negative electrode preferably has an organic compound of aromatic cyclic structure contained in the amorphous carbon covering the surface in mesh. To form the film, the particle or fiber of the crystalline carbon material is dispersed into an aqueous solution in which an organic compound having an aromatic ring in the structure such as phthalic acid is dissolved. The aromatic ring having a geometrically flat structure is adsorbed onto the flat crystal surface having a developed graphite structure with the cyclic surface being substantially parallel to the flat surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a long-life lithium secondary battery using a lithium secondary battery negative electrode material which has a high lithium diffusion rate and can withstand a long charge / discharge cycle, and an electronic apparatus using the same. About.

[0002]

2. Description of the Related Art A lithium secondary battery uses a material capable of reversibly occluding and releasing lithium for each of positive and negative electrodes. A typical commercially available lithium secondary battery is α-NaC
LixCoyO ₂ having a layered structure of rO ₂ is used as a positive electrode material, and graphite capable of intercalating lithium ions between crystal layers or amorphous carbon capable of occluding lithium in voids is used as a negative electrode material.

[0003] These electrode materials are required to be able to reversibly occlude and release more lithium. Also,
In order to realize high-power discharge, it is necessary for lithium to diffuse quickly. In this respect, graphite, which is a crystalline carbon material, is more advantageous than an amorphous carbon material as a negative electrode material. However, in the case of crystalline carbon material, when inserting and extracting lithium,
The distance between the crystal layers changes, and the change in volume due to the change causes the deterioration and destruction of the material in a long-term charge / discharge cycle.

On the other hand, an amorphous carbon material has the advantage that the change in volume due to insertion and extraction of lithium is smaller than that of a crystalline carbon material, and the deterioration in a long-term charge / discharge cycle is small. Graphite, which is a crystalline carbon material, includes natural graphite and artificial graphite made from organic substances. Artificial graphite contains 50 organic substances
After carbonization at 0 to 1500 ° C., it is graphitized at a high temperature of 2000 ° C. or higher, and the crystal structure is developed. In observation of a high-resolution surface of artificial graphite by a scanning electron microscope, a flat surface of graphite crystals stacked in layers developed by graphitization is observed.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a lithium battery having a high lithium diffusion rate capable of coping with high-output charging and discharging.
Another object of the present invention is to provide a negative electrode material having a small discharge capacity reduction rate with an increase in the number of charge / discharge cycles, and to provide a lithium secondary battery having a long cycle life using the same.

[0006]

The object of the present invention is to use, as a negative electrode material, a crystalline carbon material having a high lithium diffusion rate as a base, and an amorphous carbon material as a main component on the surface of the crystalline carbon material. The problem can be solved by forming the coating layer in a net-like form in which a part of the underlying amorphous carbon material is exposed.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION As the crystalline carbon material used for the base, natural graphite, artificial graphite having a particle size of 10 to several tens of μm, granular or fibrous carbon material formed by firing mesophase pitch at a high temperature, and the like are preferable. As a method of forming a film on these crystalline carbon materials, the particles or fibers of the above-mentioned crystalline carbon materials are dispersed in an aqueous solution in which an organic compound having an aromatic ring such as phthalic acid in the structure is dissolved.

An aromatic ring having a geometrically flat structure is adsorbed on a flat crystal surface having a developed graphite structure, with the ring surface being substantially parallel to the flat surface. The organic compound to be adsorbed is not limited to phthalic acid as long as it has a geometrically flat structure.

The method of dispersing the organic compound and adsorbing it on the crystalline carbon material may be performed in a non-aqueous solvent instead of in an aqueous solution. The amount of adsorption on the surface of the crystalline carbon material can be controlled by the type, concentration and temperature of the organic compound. The crystalline carbon material having the organic compound adsorbed on the surface is filtered, and further dried by heating under vacuum to obtain a crystalline carbon material having an amorphous carbon film formed on the surface.

[0010] By controlling the amount of organic compound adsorbed, the film can be coated so as not to completely cover the surface of the crystalline carbon material but to partially expose the underlayer. Observation with a high-sensitivity scanning electron microscope revealed that the coating film covered the underlying crystalline carbon material surface in a network-like manner, like streaks on the surface of the muskmelon.

Using the carbon material thus produced as a negative electrode material, a monopolar test is performed, as compared with before the film forming treatment.
The characteristics of the high-power discharge are not impaired, and the cycle deterioration rate is greatly improved to 1/10 or less. The charge / discharge efficiency of the crystalline carbon material before the film formation during the cycle test is almost 100% as in the case of the film-formed carbon material, and the amount of active material decreases with each cycle rather than deterioration due to the inactivation reaction. It looks like deterioration to go. From this, when the volume of the crystalline carbon material that has expanded during charging and before the film is formed contracts during discharging, the active material, particularly the edges, fall off or break down, making it impossible to collect current and reducing the capacity. It is presumed that by covering with a net of crystalline carbon, the net-like coating suppressed falling off and destruction of the active material.

Also, it is presumed that the exposed surface functions as an entrance and exit for the crystalline carbon material of lithium ions because the underlayer is partially exposed instead of the entire surface, and high output discharge characteristics are not impaired. When the coverage is 70% or more, the high-output discharge characteristics deteriorate.

The cause is that the amount of lithium ions that can enter and exit the crystalline carbon material cannot be caught by comparing the edge portion of the crystalline carbon where the amorphous carbon enters and exits the lithium ions with the lithium diffusion ability inside the crystalline carbon material. Because it was closed
It is considered that the high output discharge characteristics were impaired.

On the other hand, if the coverage is 20% or less, the cycle deterioration rate cannot be improved. This is presumed to be due to the small amount of coating, which prevented the active material from falling or breaking down sufficiently. . From the above, the coverage of the amorphous carbon material film is desirably 20 to 70%.

Hereinafter, the present invention will be described in detail with reference to examples.

(Example) Preparation of negative electrode material: An artificial graphite powder having an average particle size of 15 µm was put into distilled water of a beaker on a hot stirrer, phthalic acid was added, and the mixture was stirred for 30 minutes. The mixture is suction filtered. The graphite powder remaining on the filter paper was vacuum dried in a vacuum drying furnace at 80 ° C. for 2 hours to obtain a crystalline carbon material having an amorphous carbon coating on the surface. By changing the amount of phthalic acid added, samples 1 to 4 having different coverages were obtained.

Observation of the produced negative electrode material with a high-resolution scanning electron microscope confirmed that a net-like film was formed on the surface. The area of the underlying portion exposed from between the net-like films was 100 square nanometers on average. Further, observation and measurement with a transmission electron microscope equipped with an electron energy loss spectrometer confirmed that a film of amorphous carbon was formed on the crystalline carbon material. The thickness of this coating was approximately 1-10 nanometers in the range of 20-70% coverage as shown in FIG.

Unipolar test: A slurry obtained by adding and kneading polyvinylidene fluoride as a binder in the form of an n-methylpyrrolidone solution to each of the above-prepared samples 1 to 4 was applied to a copper foil with a coating knife, and then dried. It pressed and was set as the test pole.

A polyethylene porous membrane serving as a separator is sandwiched between the test electrode and a lithium metal foil serving as a counter electrode. One mole equivalent of hexafluoride in a mixed solvent of ethylene carbonate and dimethyl carbonate is provided between the electrodes and the separator. A charge / discharge test was performed using a bipolar charge / discharge cell impregnated with an electrolytic solution in which lithium phosphate was dissolved.
In addition, a test electrode was prepared in the same manner as above using untreated artificial graphite, and a charge / discharge test was performed in the same manner as a comparative example. In the charge / discharge test, charge / discharge was performed at a constant current of 80 mA / g of active material, charging was stopped at 10 mV, and discharging was stopped at 1 V. Table 1 summarizes the coverage, the capacity deterioration rate after 50 cycles, and the capacity reduction rate at the time of 1C discharge with respect to the 1 / 8C discharge capacity in Table 1 and Comparative Example.

[0020]

[Table 1]

As is clear from the table, Examples 1 to 4 having a net coverage showed better characteristics than Comparative Examples.
FIG. 2 shows the discharge capacity of each cycle of the test electrode as a rate of decrease from the initial capacity of 1.0.

Charge / discharge of lithium secondary battery: A slurry obtained by adding a solution of polyvinylidene fluoride in n-methylpyrrolidone as a binder to lithium cobaltate LiCoO ₂ and kneading the mixture is applied to an aluminum foil with a coating knife, followed by drying and pressing. The positive electrode was used. The positive electrode and the test electrode (negative electrode) prepared in the above-described monopolar test were sandwiched between polyethylene porous membranes serving as separators to prepare test batteries. The charging was stopped at 4.1 V and the discharging was stopped at 3.0 V.

[0023]

As described above, although the lithium diffusion rate is high, it is advantageous as a negative electrode material for a high-output lithium secondary battery, but the volume of the crystalline carbon material changes greatly during charge and discharge, and the cycle deterioration is large. By covering the coating of the amorphous carbon material in a net shape and forming the coating so that the base is partially exposed, without impairing the advantage of high lithium diffusion rate,
It is possible to increase the resistance to cycle deterioration.

A lithium secondary battery is obtained by covering the surface of the crystalline carbon material with a coating of an amorphous carbon material in a net-like manner, and using a material having a coating formed so that a base is partially exposed as a negative electrode material. The service life can be extended.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the coverage of amorphous carbon and the film thickness.

FIG. 2 is a graph showing a change in discharge capacity with respect to the number of cycles in charging and discharging of the lithium secondary batteries of Examples and Comparative Examples.

[Explanation of symbols]

1 ... film thickness of amorphous carbon, 2 ... cycle characteristics of Examples 1, 2 and 3, 3 ... cycle characteristics of comparative example.

Claims (5)

[Claims] In a lithium secondary battery using a negative electrode containing a crystalline carbon material capable of electrochemically storing and releasing lithium and a positive electrode containing a metal oxide, granular or fibrous crystals contained in the negative electrode A lithium secondary battery characterized in that a coating layer mainly composed of an amorphous carbon material is formed in a mesh shape on a surface of a high quality carbon material such that a base layer is partially exposed. 2. The lithium secondary battery according to claim 1, wherein the carbon material contained in the negative electrode has an amorphous carbon having a crystalline carbon material surface coated in a mesh pattern with an area ratio of 20%. % To 70% of a lithium secondary battery. 3. The lithium secondary battery according to claim 1, wherein the carbon material contained in the negative electrode has a thickness of 1 to 10 nanometers of amorphous carbon in which the surface of the crystalline carbon material is coated in a mesh pattern. Meters, a lithium secondary battery. 4. The lithium secondary battery according to claim 1, wherein the carbon material contained in the negative electrode has an aromatic cyclic structure in amorphous carbon in which the surface of the crystalline carbon material is covered in a mesh pattern. A lithium secondary battery comprising an organic compound having the formula: 5. A rechargeable lithium battery according to claim 1, wherein said rechargeable battery is a notebook computer, word processor, mobile phone, cordless phone handset, portable facsimile, portable printer, headphone stereo, video movie, liquid crystal television, Electronic equipment characterized by being used as a power source for driving portable equipment such as handy cleaners, portable CDs, electric shavers, electronic translators, car phones, transceivers, power tools, and memory cards.