JPH09320602A - Negative electrode for lithium secondary battery - Google Patents

Abstract

(57) [Problem] To provide a negative electrode for a lithium secondary battery capable of obtaining a lithium secondary battery having a large charge / discharge capacity. A negative electrode for a lithium secondary battery, which comprises a negative electrode active material that occludes lithium, wherein the negative electrode active material is a heat-treated coke obtained by heat-treating raw coke of petroleum or coal at 500 to 850 ° C. It consists of 1.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a negative electrode for a lithium secondary battery comprising a carbon-based negative electrode active material.

[0002]

2. Description of the Related Art In recent years, miniaturization and cordlessness of electronic devices such as mobile phones have been rapidly progressing. In addition, development and diffusion of electric vehicles are desired due to environmental problems and energy problems. Accordingly, a secondary battery having a high energy density is required. Conventionally, as a secondary battery,
Nickel cadmium batteries, nickel metal hydride batteries, and lead storage batteries are known. However, these batteries are heavy,
Low energy density. On the other hand, the lithium secondary battery is expected to be used as a high-performance mobile phone battery, electric vehicle battery, etc., because of its light weight and high energy density.

However, when lithium metal is used for the negative electrode in a lithium secondary battery, the lithium metal precipitates in the form of dendrites and powder on the surface of the negative electrode during charge and discharge, and the charge and discharge efficiency decreases. Therefore, conventionally, coke (Japanese Patent Laid-Open No. 1-2221859), which can occlude lithium and is low in cost, has been used as a negative electrode active material of a negative electrode for a lithium secondary battery. In addition, the above-mentioned coke means high temperature carbonization of oil or coal (1200 to 14
It is a grayish black porous solid obtained by (00 ° C.).

Further, a coke which has been subjected to a heat history at 400 to 700 ° C., which has been crushed and heat-treated at a temperature of 900 to 1500 ° C. in an inert gas atmosphere, has been developed as a negative electrode material. (JP-A-8-102324
issue).

[0005]

However, the negative electrode active material comprising coke has the following problems. That is, since the negative electrode active material made of the coke is produced at a high temperature of 900 ° C. or higher, the coke crystallites forming the coke become large, and the amount of storable lithium ions is smaller than the theoretical capacity. Therefore, a lithium secondary battery using the above coke cannot have a large charge / discharge capacity.

The above-mentioned theoretical capacity is based on the assumption that the above-mentioned negative electrode active material is made of graphite having a carbon limit structure. That is, lithium ions are inserted into the negative electrode active material by the intercalation (first stage) of lithium ions between the respective layers (see FIG. 3) of the graphite layer structure. In this case, one lithium atom can be stored for six carbon atoms, and the charge / discharge capacity per gram of the negative electrode active material is 372 mAhg ⁻¹ , which is the theoretical capacity.

In view of the above problems, the present invention aims to provide a negative electrode for a lithium secondary battery, which can obtain a lithium secondary battery having a large charge / discharge capacity.

[0008]

The invention of claim 1 is a negative electrode for a lithium secondary battery in which lithium is occluded in a negative electrode active material, wherein the negative electrode active material is raw coke of petroleum or coal.
A negative electrode for a lithium secondary battery, comprising a heat-treated coke heat-treated by heating at 0 to 850 ° C.

The negative electrode active material of the negative electrode for a lithium secondary battery according to the present invention is a heat-treated coke obtained by heat-treating the following raw coke. The raw coke is obtained, for example, by subjecting a heavy petroleum oil to dry distillation (heating the air off) for a certain period of time at a temperature of around 500 ° C., whereby a thermal decomposition polymerization reaction proceeds and a gas and a liquid fraction are obtained. Raw petroleum coke can be used. As the raw coke, one produced by carbonizing coal at a temperature of about 500 ° C. can be used.

In the present invention, the heat treatment temperature is 500
When the temperature is lower than ℃, the conductivity of the heat-treated coke becomes small, and the IR drop of the electrode causes the phenomenon that the closed-circuit voltage (terminal voltage) decreases compared to the open-circuit voltage. As a result, charging and discharging of the lithium secondary battery occurs. It may be insufficient. The IR drop refers to a voltage drop generated when a current flows through the carbon electrode.

When the heat treatment temperature is higher than 850 ° C., H / C described later is 0.06 and O / C is 0.0.
It is smaller than 03, and the amount of lithium clusters generated in the cavities between the ends of each carbon crystallite (see FIG. 1) during charging may be small. Further, the preferable lower limit of the temperature of the heat treatment is 600 ° C, and the more preferable upper limit of the temperature is 800 ° C. Further, the preferable time of the heat treatment is not particularly limited. Among them, a range of 30 minutes to 3 hours is particularly preferable because a very large charge / discharge capacity can be obtained.

The negative electrode for a lithium secondary battery according to the present invention may use a binder, a current collector, etc. in addition to the heat-treated coke as the negative electrode active material for absorbing lithium. . For example, the negative electrode for a lithium secondary battery according to the present invention can be obtained by molding a mixture of the heat-treated coke and a binder together with a current collector.

Next, the operation of the present invention will be described below. The negative electrode active material of the negative electrode for a lithium secondary battery of the present invention is composed of a heat-treated coke in which lithium ions and lithium clusters composed of the lithium ions are stored, as shown in FIG.

The heat-treated coke is composed of coke crystallites as shown in FIG. 1 described later. The above-mentioned coke crystallite is larger than the coke crystallite in the raw coke as a raw material, but the normal coke (120
(Manufactured at 0 to 1400 ° C.) or coke heat-treated at a temperature of 900 to 1500 ° C., the ends of the coke crystallites are increased and a large number of cavities are formed between the ends.

The coke crystallite is mainly composed of hydrocarbon and has a 6-membered ring network planar structure, and a part thereof has a layered structure similar to crystalline graphite. The end of the coke crystallite is in a state where hydrogen is bonded to carbon.

In the coke crystallite, lithium ions are occluded between the layers in the layered structure similar to that of the crystalline graphite in the state of lithium ions. Further, lithium ions are absorbed into the cavities formed between the ends of the coke crystallites while forming lithium clusters. For this reason, as shown in FIG. 3 described below, the negative electrode active material according to the present invention is more excellent in comparison with a conventional negative electrode active material composed of crystalline graphite or coke containing more crystalline graphite. Many lithium ions and lithium clusters can be stored.

Further, in the above heat-treated coke, the number of coke crystallites is increased and the number of coke crystallites is further increased as compared with the conventionally used ordinary coke and the like. Is increasing. Therefore, the negative electrode active material according to the present invention can store more lithium clusters. The above-mentioned lithium cluster indicates a state in which lithium ions are aggregated by an interaction between lithium ions to form one lump.

Further, the negative electrode active material according to the present invention comprises heat treated coke as described above. Accordingly, lithium metal is prevented from being precipitated in the negative electrode in the form of dendrite or powder, and a decrease in charge / discharge efficiency in the lithium secondary battery can be prevented.

As described above, according to the present invention, it is possible to provide a negative electrode for a lithium secondary battery which can obtain a lithium secondary battery having a large charge / discharge capacity.

Next, as in the invention of claim 2, claim 1
In the above heat treatment coke, the value of the atomic ratio H / C between hydrogen atoms and carbon atoms is 0.06 or more, and the atomic ratio O / O between the oxygen atoms and carbon atoms in the heat treatment coke is O /
The value of C is preferably 0.003 or more. The increase in the values of the atomic ratios H / C and O / C corresponds to the increase in the ends of the coke crystallites, and the increase in the ends of the coke crystallites increases the cavities formed between the ends. It means to do.

As described above, since the number and volume of cavities capable of forming lithium clusters increase, more lithium ions can be stored in the negative electrode active material, so that a lithium secondary battery having a large charge / discharge capacity can be obtained. be able to. The value of the atomic ratio H / C is 0.
When it is less than 06 and the value of O / C is less than 0.003, the number of cavities and the volume are small, and only a lithium secondary battery having a small charge / discharge capacity may be obtained.

The upper limit of the atomic ratio H / C is 0.3,
The upper limit of O / C is preferably 0.02. When the values of atomic ratio H / C and O / C exceed the above upper limits,
The resistance of the heat-treated coke increases and the conductivity is 10 ^-7 Scm.
^Since it becomes less than ^-1, there is a possibility that charging and discharging cannot be performed sufficiently due to IR drop.

Next, as in the invention of claim 3, it is preferable that the heat-treated coke has an electric conductivity of 10 ^-7 Scm ^-1 or more. As a result, the above-mentioned IR drop can be prevented and a lithium secondary battery having a large charge / discharge capacity can be obtained.

When the conductivity is less than 10 ^-7 Scm ^-1 , the number of cavities and the volume are small, and only a lithium secondary battery having a small charge / discharge capacity may be obtained. The upper limit of the conductivity is preferably 2 × 10 ^-2 Scm ^-1 . If the conductivity exceeds the upper limit, H / C is 0.06 and O / C is less than 0.003, which may reduce the number of cavities and the volume.

Further, in producing the negative electrode for a lithium secondary battery according to the present invention, raw coke of petroleum or coal is heated at 500 to 850 ° C. to form a negative electrode active material composed of heat-treated coke, and then, It is preferable that the negative electrode active material absorb lithium ions to form a negative electrode.

Since the heat-treated coke heated in the above temperature range has a large number of cavities formed between the ends of the coke crystallites and has a large volume, more lithium ions are converted into lithium clusters in the cavities. Can be accommodated. Therefore, the negative electrode active material produced from the heat-treated coke can store more lithium ions, and thus a lithium secondary battery having a large charge / discharge capacity can be obtained. The temperature range for the heating is the same as above.

The heating is preferably performed in an inert atmosphere. As a result, the oxidation of raw coke can be prevented, and as shown in FIG. 1, a heat-treated coke composed of coke crystallites can be obtained. The inert atmosphere may be, for example, a vacuum atmosphere, a rare gas, or N 2.
_An atmosphere of ₂ etc. can be mentioned. The preferable heating time is not particularly limited, but as described above, the range of 30 minutes to 3 hours is particularly preferable.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment Example A negative electrode for a lithium secondary battery according to an embodiment of the present invention,
The performance and the manufacturing method are shown in FIGS.
This will be described with reference to Table 3. The lithium secondary battery negative electrode of this example is a negative electrode for a lithium secondary battery in which lithium is contained in a negative electrode active material, and the negative electrode active material heats raw coke of petroleum or coal at 500 to 850 ° C. It consists of heat-treated coke which has been heat treated.

As shown in FIG. 1, the heat-treated coke 1 has the coke crystallites forming the raw coke,
It is composed of a larger coke crystallite 10 which is partially decomposed by heat and generated. In the negative electrode for the lithium secondary battery, lithium ions are occluded in the heat-treated coke 10 as described below.

That is, the coke crystallite 10 has a layered structure similar to that of graphite in a part thereof, and the lithium ions 2 are directly intercalated between the layers 13 in the layered structure. Then, a cavity 11 is formed between the terminal 12 of the coke crystallite 10 and the terminal 12 of another coke crystallite 10, and the lithium cluster 2 formed from the lithium ions 2 is formed in the cavity 11.
0 is stored.

Next, the performance and the like of the negative electrode for a lithium secondary battery according to the present invention will be described together with a comparative sample. First,
The method for producing the heat-treated coke used for producing the negative electrodes according to Samples 1 to 19 will be described. First, heavy petroleum oil was pyrolyzed at 500 ° C. to obtain raw coke. Thereafter, the raw coke was pulverized to an average particle size of 30 μm to obtain particulate raw coke.

The above raw coke in particulate form was placed on an alumina boat, and the flow rate of argon was 11 / mi in an electric furnace.
n, temperature rising rate 20 ° C./min, ultimate temperature 600 to 850
Heat treatment was performed at ℃ (see Table 1 and Table 2) and holding time of 1 hour to obtain heat treated coke. After cooling the heat-treated coke, it was crushed in a mortar and classified to 30 μm or less with a mesh, and the binder and collector described later were used to prepare samples 1 to 1.
It was a negative electrode for No. 9.

Next, the heat-treated cokes used for the comparative samples C1 to C4, C7 and C8 were also manufactured in the same manner as the above samples 1 to 19. However, the ultimate temperature in heat treatment is 900
-1400 ° C. (see Table 3). Also, comparative sample C
In C5 and C6, artificial graphite (MCMB-25-28) manufactured by Osaka Gas Chemicals was used as the negative electrode instead of the heat-treated coke.

Next, each of the samples 1 to 19 and the comparative sample C
Fabrication of negative electrodes for lithium secondary batteries according to 1 to C8 and fabrication of test cells using the negative electrodes for lithium secondary batteries will be described. The charge / discharge capacity of the test cell is measured and evaluated. As shown in FIG. 2, the test cell 30 has a pair of electrolytic solutions 4 arranged around the separator 3 so as to sandwich the separator 3, and a carbon electrode 6 and a counter electrode 5 and a counter electrode 5 facing the carbon electrode 6 around the electrolytic solution 4. The current collector 7 is arranged. The integrated bodies 7 on both sides are connected to a charging / discharging device 8.

The counter electrode 5 applied to the test cell 30 is made of a tablet-shaped lithium metal having a diameter of 15 mm and a thickness of 0.8 mm. The carbon electrode 6 is 1% of a mixture in which 4 wt% of polytetrafluoroethylene as an adhesive is mixed with 96 wt% of each of Samples 1 to 19 and Comparative Samples C1 to C8.
0 mg or 20 mg (see Tables 1 to 3) is used as the current collector 7
It is a tablet formed to have a diameter of 15 mm together with SUS304 mesh.

The separator 3 provided between the carbon electrode 6 and the counter electrode 5 is made of porous polyethylene,
The size was 20 mm in diameter and 75 μm in thickness. The electrolytic solution 4 used for the test cell 30 was prepared by adding LiPF ₆ to a mixed solution of ethylene carbonate and diethyl carbonate (EC / DEC) (1: 1 in volume ratio).
What was melt | dissolved by the ratio of mol / liter was used.

The charge / discharge capacity of the test cell 30 was measured by the charge / discharge test of the test cell 30. First, in charging the test cell 30,
It was charged to 0V at a constant current of a 5 mA / cm ^-2 or 0.05 mA / cm ^-2 (see Tables 1-3). The discharge was terminated when the battery voltage of the test cell 30 exceeded 1.5V. In the above test, the charging capacity was calculated from the amount of electricity flowing until the potential of the carbon electrode 6 changed from about 3 V to 0 V by charging, and the electrode potential was changed from 0 V to 1.5 V by discharging.
The discharge capacity was determined from the amount of electricity that flowed until it changed to V.

The samples 1 to 19 and comparative sample C1 to
1.4 mg of heat-treated coke or graphite for C8
For 1.6 mg, H / C and O were measured using an element analyzer (Model 240C) manufactured by Perkin-Elmer.
/ C was measured. In addition, the above samples 1 to 19 and comparative sample C
100m of heat-treated coke or graphite from 1 to C8
Polytetrafluoroethylene was mixed at a ratio of 4 wt% with respect to g and molded into a tablet (diameter: 10 mm, thickness: about 1 mm), and the conductivity was determined by measuring the resistance between both ends of the tablet. The above results are shown in Tables 1 to 3.

From the table, it was found that the test cell 30 having Samples 1 to 19 according to the present invention had a larger charge capacity and discharge capacity than Comparative Samples C1 to C8. And
Among the samples 1 to 19, the charge / discharge capacity was 372, which is the theoretical capacity.
It was found that there were some that were able to obtain a large capacity test cell 30 that exceeded mAhg ^-1 .

Further, H / of Samples 1 to 19 according to the present invention
C is 0.06 or more, and O / C is 0.003 or more, and has many coke crystallite ends, and the cavity formed by these is where lithium ions are stored as lithium clusters. It turned out that it has many.

The electrical conductivity of Samples 1 to 19 according to the present invention is 10 Scm ^-1 or more, and even the largest value is 2
It is smaller than × 10 ^-2 Scm ^-1 . However, the conductivity of the comparative samples C1 to C8 is larger than this value. This corresponds to the H / C of the comparative sample being less than 0.06 and the O / C being less than 0.003, which indicates that the comparative sample has a smaller amount of cavities.

Next, the operation and effect of this embodiment will be described. As shown in FIG. 1, the negative electrode active material relating to the negative electrode for a lithium secondary battery of the present example comprises lithium ions 2 and heat-treated coke 1 that occludes lithium clusters 20 produced from the lithium ions 2.

The heat-treated coke 1 comprises a coke crystallite 10 as shown in FIG. The coke crystallite 10 is smaller than the coke crystallite in the raw coke as a raw material. Therefore, in the heat-treated coke 1, the end 12 of the coke crystallite 10 is increased, and A large number of cavities 11 are formed between 12. Also, coke crystallite 1
Part of 0 has a layered structure similar to that of crystalline graphite.

In the coke crystallite 10, the lithium ions 2 are separated from the interlayer 1 in the layered structure.
3 and form a lithium cluster 20 in the cavity 11.

By the way, as shown in FIG. 3, a conventional negative electrode for a lithium secondary battery (comparative sample shown in Table 3) using crystalline graphite 9 or coke containing a large amount of such graphite 9 is used. In the negative electrode active material, the lithium ions 2 are only occluded by the intercalation of the lithium ions 2 with respect to the interlayer 93 of the layered structure of the graphite 9.

As described above, and as is known from the above Tables 1 to 3, the heat-treated coke 1, which is the negative electrode active material of the negative electrode according to this example, occludes more lithium ions 2 and lithium clusters 20 composed of the same. Therefore, the lithium secondary battery having the negative electrode has a larger charge / discharge capacity.

Further, the negative electrode active material of the negative electrode for a lithium secondary battery according to this example comprises the heat-treated coke 1 as described above, which prevents lithium metal from dendrite-like or powder-like depositing on the negative electrode. It is possible to prevent a decrease in charge / discharge efficiency of the lithium secondary battery. Furthermore, the above-mentioned heat-treated coke can be easily produced without the need for other additives or the like and without the need for special equipment.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

[Table 3]

[0051]

As described above, according to the present invention, it is possible to provide a negative electrode for a lithium secondary battery capable of obtaining a lithium secondary battery having a large charge / discharge capacity.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a state where lithium ions and lithium clusters are occluded in a heat-treated coke as a negative electrode active material according to an embodiment.

FIG. 2 is a cross-sectional view of a test cell according to the embodiment.

FIG. 3 is an explanatory view of a conventional negative electrode active material in a state in which lithium ions are occluded in graphite.

[Explanation of symbols]

 1. . . Heat treated coke,

Claims (3)

[Claims] 1. A negative electrode for a lithium secondary battery, comprising a negative electrode active material containing lithium stored therein, wherein the negative electrode active material is heat treated by heating raw coke of petroleum or coal at 500 to 850 ° C. A negative electrode for a lithium secondary battery, which is made of coke. 2. The atomic ratio H / C between hydrogen atoms and carbon atoms in the heat-treated coke according to claim 1, wherein the value is 0.0.
A negative electrode for a lithium secondary battery, which has a value of 6 or more and an atomic ratio O / C of oxygen atoms and carbon atoms in the heat-treated coke of 0.003 or more. 3. The negative electrode for a lithium secondary battery according to claim 1, wherein the heat-treated coke has an electric conductivity of 10 ⁻⁷ Scm ⁻¹ or more.