JPH07326343A - Negative electrode material for nonaqueous electrolytic secondary battery and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture an nonaqueous electrolytic secondary battery excellent in low temperature charging characteristic by combining a plurality of different carbon materials together as negative electrode material. CONSTITUTION:A working electrode 1 is formed by mixing, for example, 5wt% of styrene-butadiene rubber to a carbon material powder forming the main body as binder, and press-molding it onto a titanium net 3 fixed to the inside of a positive electrode case 2 by spot welding. A counter electrode 3 is formed by pressure welding metal lithium onto a stainless net 6 fixed to the inside of a sealing plate 5 by spot welding. A polypropylene separator 7 is arranged between the working electrode 1 and the counter electrode 3. As the electrolyte, a solution of lithium hexafluoride phosphate dissolved into a mixed solvent of ethylene carbonate and diethyl carbonate. Further, a case 2 and the sealing plate 3 are sealed through a polypropylene gasket 9 between them to provide a completed battery, for example, 20mm in diameter and 1.6mm in height. Thus, a battery excellent in low temperature charging characteristic is manufactured.

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 carbon material as a negative electrode.

[0002]

2. Description of the Related Art In recent years, portable electronic devices and cordless electronic devices have been rapidly developed, and there is a great demand for secondary batteries having small size, light weight, and high energy density as power sources for driving these electronic devices. From this point of view, non-aqueous electrolyte secondary batteries, especially lithium secondary batteries, are particularly expected as batteries having high voltage and high energy density.

Recently, LiCoO 2 has been used as a positive electrode active material.
Battery systems using a lithium composite oxide such as ₂ , LiNiO ₂ or the like and a carbon material for the negative electrode have been attracting attention. The characteristics of this battery system are that the battery voltage is high and that the positive and negative electrodes utilize the intercalation reaction of lithium. Since metallic lithium is not used for the negative electrode, the dendrite-like L
There is no short circuit due to the precipitation of i, and safety and rapid charging can be expected.

A battery using LiCoO ₂ as a positive electrode and a carbon material as a negative electrode has already been commercialized. In particular, in the case of a battery using graphite as the carbon material of the negative electrode, a high voltage can be obtained because the reaction potential is relatively close to that of metallic lithium. In addition, graphite has a high density, a high packing property, and an ordered crystal layer as compared with other carbon materials. Graphite is theoretically C ₆ Li (corresponding to a capacity of 372 Ah / g)
The active material lithium can be incorporated between the layers of the crystal. Therefore, if graphite is used as the negative electrode material, a battery having a high capacity and a high energy density can be obtained.

[0005]

The higher the crystallinity of graphite, the more ideal the crystal layer structure, and the capacity of the battery using the same approaches the theoretical value. Further, since the reaction potential is close to that of metallic lithium, it is considered that a high voltage can be expected. The charging system for these batteries currently commercialized uses a constant current-constant voltage charging system, and by using this method, charging is realized in a short time of about 1 to 2 hours.

When graphite, particularly highly crystalline graphite, is used,
When used at around room temperature (10 ° C to 30 ° C), problems such as deterioration of charge / discharge characteristics are not recognized. However, in a low temperature environment, the capacity decreases during repeated charging / discharging, and the capacity does not recover even if the temperature is returned to room temperature, resulting in performance deterioration. In order to solve these problems, various charging methods have been attempted, including a method of charging with an extremely low current, but an effective charging method that can satisfy both electric characteristics such as discharge capacity and practicality is currently available. Not obtained yet.

On the other hand, when graphite having a slightly lowered crystallinity is used, deterioration of performance during charging in a low temperature environment is alleviated. However, there is a problem that the capacity becomes smaller than that in the case where the graphite having high crystallinity is used.

[0008]

In order to solve these contradictory problems, the inventors of the present invention have conducted extensive studies and found that at least two kinds of carbon materials should be used as the carbon material of the negative electrode. .

One of them is graphite having a high crystallinity, in which the interplanar spacing d002 obtained by the wide-angle X-ray diffraction method is at least 3.37 Å and the unit crystallite size Lc in the C-axis direction is at least 1000 Å. The carbon material selected from the group.
Hereinafter, in the present specification, the carbon material having the above-mentioned physical properties will be referred to as highly crystalline graphite, or simply graphite.

The other is selected from the group of carbons having low crystallinity, in which the interplanar spacing d002 by the wide-angle X-ray diffraction method is 3.50 Å or more and the unit crystallite size Lc in the C-axis direction is 100 Å or less. The carbon material will be referred to as low crystalline carbon hereinafter. This is a raw material pitch in an inert atmosphere, 6
It is obtained by performing heat treatment at 00 ° C to 1000 ° C and carbonizing. Preferably, the raw material pitch is an isotropic pitch, the mesophase carbon content is 50% or less, and the softening point is 320 ° C. or less.

The mesophase carbon heats coal tar pitch or pyrolysis heavy oil to 350 ° C. to 550 ° C., and escapes gas and light fractions in the pyrolysis reaction. After that, the mesophase spheres that precipitate in the molten pitch, or aggregates thereof, are separated and collected. In this method, after removing the mesophase spheres or aggregates thereof from the above-mentioned raw material pitch, the residue of the molten pitch can be used as a pitch having a low mesophase content, so that the by-product can be effectively utilized.

The above-mentioned two kinds of carbon materials can be mixed by simply mixing the powder of high crystalline graphite and the powder of low crystalline carbon to obtain a sufficient effect, but the powder of highly crystalline graphite is preferably used. The powder of the raw material pitch is mixed in advance and heat-treated at 600 ° C. to 1000 ° C. in an inert atmosphere, and the carbon material in which low crystalline carbon is fused to the powder of highly crystalline graphite obtained by this is crushed to obtain a negative electrode. Use carbon material.

The mixing ratio of these carbon materials is at least 70% by weight of the carbon materials selected from the group of highly crystalline graphite.
It contains at least 15% by weight of a carbon material selected from the group of low crystalline carbon.

By the way, there are some reports on the technique of mixing two different kinds of carbon materials. For example,
Although it relates to amorphous carbon different from the present invention, JP-A-4-368778, in which the surface of the main carbon is covered with amorphous carbon to suppress decomposition of the electrolytic solution, and lithium is previously doped. There are JP-A-4-332465 in which non-graphitizable carbon is mixed with graphitized carbon material, and JP-A-4-155576 in which the shape of the carbon material is limited. Further, it has a combination of powdered carbon and carbon fiber disclosed in JP-A-4-196055, and has a high crystallinity portion and a low crystallinity portion in a single particle presented in US Pat. No. 5,028,500. There is a technique of using a carbon material and mixing carbon black.

However, the present invention is not characterized by mixing two kinds of different carbon materials as described above, but is rather characterized by the limited physical properties of the carbon materials and the combination thereof, and the battery. It is intended to provide a non-aqueous electrolyte secondary battery having excellent capacity and low-temperature charging characteristics.

[0016]

When the highly crystalline graphite is used as the negative electrode material, the decrease in charge / discharge capacity under low temperature environment is due to the deposition of metallic lithium on the graphite surface during charging.
Since the reaction potential is close to that of metallic lithium, it polarizes in a low temperature environment and easily reaches the deposition potential of lithium.
Further, lithium deposited on the surface of the graphite falls off from the electrode plate and is lost each time charging and discharging are repeated. Therefore, the battery capacity decreases as the number of charge and discharge increases.

On the other hand, when the crystallinity of graphite is lowered, the reaction potential shifts to a noble level and it becomes difficult to reach the lithium deposition potential, but the battery capacity is significantly reduced. In the present invention, precipitation of lithium is suppressed during charging of a battery in a low temperature range while maintaining the high capacity characteristics of graphite.

The low crystalline carbon in the present invention is not an amorphous carbon, but the interplanar spacing d002 is 3.50 Å or more and the unit crystallite size Lc in the C-axis direction is 100 Å or less, so rather it is a crystal. It can be seen that it is an aggregate of fine particles having properties. This carbon has a pitch of 600 ° C to 1000
Obtained by firing at ℃. Especially when an isotropic pitch is used as a raw material, the orientation of fine particles becomes isotropic,
More effective.

These low crystalline carbons have an extremely high rate of intercalating lithium during charging. It is considered that this carbon takes up lithium and rapidly moves in the solid phase to supply lithium directly into the highly crystalline graphite before the lithium is deposited. Therefore, the precipitation of lithium on the surface of the highly crystalline graphite is suppressed, which leads to the improvement of the low temperature charging characteristics.

Further, when carbonization of the above-mentioned pitch is carried out after mixing with graphite, reduction of electrode plate resistance, improvement of filling property, etc. were observed. This will be described later in detail, but this is because a form in which the low crystalline carbon is in close contact with graphite was obtained.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

As an example of the present invention, a coin-type battery was manufactured and evaluated. FIG. 1 is a vertical sectional view of a coin-type battery. Working electrode 1 is mainly composed of carbon material powder,
Styrene butadiene rubber (SBR) as a binder 5
The mixture was mixed by weight%, and this was press-molded on the titanium net 3 fixed to the inside of the positive electrode case 2 by spot welding. Further, the counter electrode 4 is obtained by crimping metallic lithium on a stainless steel net 6 fixed by spot welding on the inside of the sealing plate 5. In addition, a polypropylene separator 7 was placed between the working electrode 1 and the counter electrode 4. The electrolyte contains
A solution prepared by dissolving 1 mol / l lithium hexafluorophosphate (LiPF ₆ ) in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used. Case 2 and sealing plate 5
Was sealed with a polypropylene gasket 9 between them to obtain a completed battery having a diameter of 20 mm and a height of 1.6 mm.

By the way, the test battery used in this example is
A carbon material for the negative electrode was used for the working electrode. Therefore, charging is performed in the direction in which the battery voltage decreases (reaction in which lithium intercalates in the carbon material), and discharge is performed in the direction in which the battery voltage increases (reaction in which lithium is deintercalated from the carbon material).

First, in order to select graphite, artificial graphites SP10 and SP20 manufactured by Nippon Graphite Co., Ltd. were obtained as highly crystalline graphite, test batteries were prepared, and the capacity was confirmed. At room temperature (20 ° C.), the charge end voltage was 0 V and the discharge end voltage was 1.5 V at a constant current of 0.5 mA / cm ² . As a result, the surface distance d002 is 3.37 Å or less and Lc
It was found that graphite having a crystal parameter of 1000 Å or higher stably exhibits a high capacity of 320 mAh / g or higher. At least in graphite, a high capacity can be obtained by using a material having high crystallinity. In addition, as the graphite, which has a promising capacity, SP10 and SP manufactured by Nippon Graphite Co., Ltd.
20 and KS44, KS25, KS1 manufactured by Lonza Co., Ltd.
There was 5 mag.

A test battery was prepared using the above-mentioned KS25 manufactured by Lonza Co., Ltd. as the highly crystalline graphite and a low crystalline carbon material described later, and a charge / discharge test was conducted in an environment of 0 ° C. At this time, charging was performed with a constant amount of electricity corresponding to the capacity obtained at room temperature, and discharging was performed by setting the final voltage to 1.5 V and charging / discharging with a constant current of 0.5 mA / cm ^2. It was

FIG. 2 shows a charging voltage curve of a battery using KS25. The battery voltage dropped below 0 V during charging and showed a negative value thereafter. By repeating charging and discharging, as shown in FIG. 3, the discharge capacity of graphite became half or less of the initial value after repeating 10 times. When this was decomposed, metallic lithium was deposited in such a form as to cover the entire surface of the working electrode of graphite.

Next, a study was conducted on a low crystalline carbon material. Several pitch materials were obtained as raw materials for low crystalline carbon, and carbonized at firing temperatures of 500 ° C. to 2000 ° C., respectively. For comparison, polyacrylonitrile (PAN) -based resin and polyamide (PA) -based resin were similarly carbonized at a firing temperature of 500 ° C to 2000 ° C.
So-called amorphous carbon was also examined. A test battery was prepared in the same manner as in the case of the above-mentioned highly crystalline graphite, and the capacity was evaluated at room temperature. In the firing of the pitch material, a carbon material having a relatively low mesophase content called isotropic pitch or a material having a low softening point fired at 600 ° C. to 1000 ° C. showed a high capacity of 300 mAh / g or more.

Therefore, the mesophase content and the softening point of the pitch materials having these high capacities were measured. As a result, the mesophase content is 50% or less, or the softening point is 320.
It was found that those having a temperature of ℃ or less are particularly excellent. In addition,
When the mesophase content exceeds 50% and the softening point exceeds 320 ° C, the capacity is 100 to 200 m.
It was about Ah / g. On the other hand, the capacities of the carbon materials obtained by firing the resin were all 100 mAh / g or less.

Further, a charge / discharge test was performed on these materials under an environment of 0 ° C. under the same conditions as in the case of the above highly crystalline graphite. As a result, both materials have 0 during charging.
V was turned off and showed a negative voltage.

As a typical result of the low crystalline carbon material, the change in the number of times of charging and discharging and the change in the discharge capacity for the 800 ° C. carbonized product having an isotropic pitch are also shown in FIG. No significant change in capacity is observed even after repeated charging and discharging. As described above, when high crystalline graphite was used, lithium was precipitated, but no low crystalline carbon material could be confirmed. The fact that no lithium deposition was observed despite showing a negative voltage was due to the high overvoltage of the lithium deposition reaction,
In addition, it is considered that the rate of the lithium occlusion reaction is high. In particular, the carbon material obtained by firing the isotropic pitch at 600 ° C. to 1000 ° C. has a high capacity of 300 mAh / g or more as described above.
When combined with an oO ₂ positive electrode, a high capacity, practical battery is obtained.

FIG. 4 shows discharge curves of low crystalline carbon and graphite at room temperature. As you can see, compared to graphite,
The discharge reaction potential of low crystalline carbon is noble and has no flatness. For this reason, a battery in which low crystalline carbon is used for the negative electrode and this is combined with the positive electrode has a drawback that the operating voltage becomes low.

However, when mixed with the graphite material of the present invention, from the viewpoint of ensuring the capacity of the battery, and considering that even if a large amount of lithium is contained, lithium precipitation does not occur, at least the low crystalline carbon having a large capacity is used. Clearly, the choice of material is advantageous.

Further, a study was conducted to prove the effect of using the above two kinds of carbon materials together as the negative electrode material of the present invention. As described above, since highly crystalline graphite is excellent in battery capacity, in this example, KS25 manufactured by Lonza Co., Ltd. among them was used for the study. As the low crystalline carbon material, a material obtained by carbonizing isotropic pitch at 800 ° C. (discharge capacity at room temperature 315 mAh / g) was used. As is clear from FIG. 4, highly crystalline graphite and low crystalline carbon have greatly different voltage profiles.

In general, when materials having different voltage profiles are made to coexist, the voltage profiles are synthesized according to the mixing ratio. Therefore, it is expected that a higher ratio of graphite is preferable from the viewpoint of voltage characteristics at room temperature of the battery, and a higher ratio of low crystalline carbon material is preferable from the viewpoint of low temperature charging characteristics.

Samples having different mixing ratios were prepared using the above-mentioned two kinds of carbon materials, and the above-mentioned test battery was produced. The capacity, voltage profile and low temperature charging characteristics of these batteries at room temperature were evaluated, and the influence of the negative electrode characteristics on the mixing ratio was examined. As a result, the discharge capacity at room temperature was an average value according to the mixing ratio of the two types of materials, based on the capacity of each of graphite and low crystalline carbon used alone.

Next, FIG. 5 shows discharge voltage profiles for materials having different mixing ratios. In this figure, the value shown in parentheses on each curve is the proportion of graphite in the mixed carbon material, and the rest (difference from 100%) corresponds to the weight ratio of low crystalline carbon. As is clear from FIG. 5, when the proportion of graphite is less than 70% by weight, the influence of the voltage profile of the low crystalline carbon material becomes apparent. As the amount of graphite decreases, the voltage profile approaches the shape of low crystalline carbon alone. On the other hand, when the ratio of graphite is 70% by weight or more, the voltage profile becomes very similar to that of graphite, and the difference due to the change in the ratio of graphite and low crystalline carbon is small. Thus, the voltage profile can be classified into a graphite-dependent type and a low crystalline carbon-dependent type with the graphite ratio of 70% by weight as a boundary. From the viewpoint of voltage characteristics, it is desirable that the graphite ratio is at least 70% by weight or more when the application to an actual battery is assumed.

Next, the low temperature charging characteristics of each of the above samples were examined by the method of charging the capacity determined at room temperature with a constant capacity. FIG. 6 shows the influence of the graphite ratio on the number of times of charge and discharge and the discharge capacity. As is clear from FIG. 6, when the ratio of graphite exceeds 85% by weight, the phenomenon that the capacity decreases as the charge and discharge are repeated, and when the ratio of graphite further increases, the rate of decrease of the capacity increases. On the other hand, when the content is 85% by weight or less, the capacity hardly decreases. Therefore, the content ratio of the low crystalline carbon material having the effect of suppressing the capacity decrease is preferably at least 15% by weight or more. From these results, it was found that there is an optimum mixing ratio of graphite and low crystalline carbon that satisfies at least the discharge voltage profile and the low temperature charging characteristics.

The mixing of graphite and low crystalline carbon in the above examination was carried out by using a mixer equipped with rotary blades (mill, household mixer, etc.). It is a well-known fact that the mixing method affects the characteristics when mixing different kinds of powders. Therefore, in order to study the mixing method, highly crystalline graphite and low crystalline carbon powder were mixed using various methods. However, simply changing the mixing method did not affect the subsequent characteristics by performing the mixing for a sufficient time. Therefore, graphite and isotropic pitch were mixed in advance, and the isotropic pitch was carbonized in the presence of graphite. Although the graphitization process of pitch is carried out at a high temperature, the physical properties of graphite do not change at about the carbonization temperature of the pitch of the present invention, so that finally, graphite and low crystalline carbon are mixed, and It has been found that when used as a material, its electrical resistance becomes low. The reduction of the electric resistance of the electrodes leads to the reduction of the internal resistance of the battery, which is effective in terms of voltage characteristics. This is because the pitch adheres to the graphite when it becomes a molten state in the carbonization process, and in that state carbonizes and fuses as it is, so the contact between the graphite and the low crystalline carbon becomes strong, and It is considered that this contributed to the reduction of the contact resistance. Further, the carbon material prepared by this method tends to have a higher bulk density than that obtained by simply mixing the respective powders. As a result, the filling efficiency of the negative electrode plate is increased, and more active material can be used.

Next, the same test as above was conducted using the carbon material obtained by this method. As a result, the same result as above was obtained with high reproducibility. Especially in the low temperature charging characteristics, it is considered that the influence of the electrode plate resistance, but the polarization was reduced and the battery characteristics were further improved.

[0040]

As is apparent from the above description, the present invention can provide not only the discharge characteristics but also the charging characteristics, particularly a practically very effective non-aqueous electrolyte battery that can be easily charged even in a low temperature environment.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a coin-type battery used in an example of the present invention.

FIG. 2 is a graph showing the charging voltage behavior of graphite.

FIG. 3 is a diagram showing the relationship between the number of low temperature charge / discharge and the discharge capacity.

FIG. 4 is a diagram showing discharge voltage behavior of graphite and a low crystalline carbon material.

FIG. 5 is a comparison diagram of discharge voltage behavior of a negative electrode made of a carbon material according to the present invention.

FIG. 6 is a graph showing the relationship between the low temperature charge / discharge frequency and the discharge capacity of the carbon material negative electrode according to the present invention.

[Explanation of symbols]

 1 Working electrode 2 Positive electrode case 3 Titanium net 4 Counter electrode 5 Sealing plate 6 Stainless steel net 7 Separator 8 Electrolyte 9 Gasket

Claims (5)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode using a lithium composite oxide as an active material, a non-aqueous electrolyte, and a negative electrode using a carbon material capable of intercalating and deintercalating lithium. In the above-mentioned negative electrode material, the negative electrode material contains two or more kinds of carbon materials, and at least one of the carbon materials has a surface spacing d002 obtained by a wide-angle X-ray diffraction method of 3.37 Å or less, and a size of a unit crystallite in the C-axis direction Lc is 1000 Å or more and is selected from a group of highly crystalline graphites, and at least the other one is a surface spacing d00 measured by a wide-angle X-ray diffraction method.
2 is 3.50 Å or more and Lc is 100 Å or less, and is selected from the group of low crystalline carbon. The above non-aqueous electrolysis in which low crystalline carbon is fused to highly crystalline graphite. Liquid secondary battery. 2. The carbon material contained in the negative electrode contains at least 70% by weight or more of a carbon material selected from the group of highly crystalline graphite and a carbon selected from the group of low crystalline carbon. The non-aqueous electrolyte secondary battery according to claim 1, which contains at least 15% by weight of the material. 3. Highly crystalline graphite and pitch powder are premixed, and this is mixed in an inert atmosphere at 600 ° C. to 1000 ° C.
A method for producing a negative electrode material for a non-aqueous electrolyte secondary battery, which comprises crushing a composite of the above-mentioned highly crystalline graphite and low crystalline carbon obtained by heat treatment in the temperature range of 1. 4. The pitch is an isotropic pitch, and its mesophase content is 50% or less, or its softening point is 3.
The method for producing a negative electrode material for a non-aqueous electrolyte secondary battery according to claim 3, wherein the temperature is 20 ° C. or lower. 5. Coal tar pitch or pyrolysis heavy oil is pyrolyzed at 350 ° C. to 550 ° C. to escape gas and light fractions, and then the mesophase spheres and aggregates precipitated in the molten pitch are removed. Thus, the method for producing a negative electrode material for a non-aqueous electrolyte secondary battery, the content of which is controlled to 50% or less.