JPH0922696A - Secondary battery electrode and method for manufacturing the same - Google Patents

Abstract

(57) [Abstract] [Purpose] An electrode for a secondary battery and a method for producing the same, particularly to reduce the irreversible capacity while maintaining a high capacity when an amorphous carbon material is used for a negative electrode. . [Structure] An electrode for a secondary battery using an amorphous carbon material having a ratio of oxygen atoms to carbon atoms of 0.4 or less, a step of preparing a graphitizing material, and a step of preparing the graphitizing material.
Amorphous carbon material production step of producing an amorphous carbon material by performing a first heat treatment at 00 ° C. or higher, and a gas mixture of the amorphous carbon material in hydrogen gas or a mixture of hydrogen gas and an inert gas. A second heat treatment step of performing heat treatment in
It is a method of manufacturing an electrode for a secondary battery.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for a secondary battery and a manufacturing method thereof, and more particularly to an amorphous carbon negative electrode for a lithium secondary battery and a manufacturing method thereof.

[0002]

2. Description of the Related Art In recent years, the use of secondary batteries has expanded more and more, and the demand for higher capacity has increased accordingly. Such demands have been met mainly by means of improving the capacity of Ni-Cd (nickel-cadmium) secondary batteries. As a result, it is now 175 mAh
The capacity has been improved to about / g.

On the other hand, a non-aqueous electrolyte battery using an alkali metal such as lithium for its negative electrode has a high energy density and is widely used as a power source for electronic devices and the like.

Recently, research and development of secondary batteries have been in progress. However, in the case of a secondary battery in which lithium is used as the negative electrode, metal may be deposited in the form of dendrite on the negative electrode during overcharge, and as a result, a regular voltage may not be generated.

Therefore, a non-aqueous electrolyte secondary battery has been proposed in which a carbonaceous material or a compound obtained by doping or alloying with lithium is used as a negative electrode instead of using lithium metal as a negative electrode. .

This secondary battery is less likely to cause dendrites as compared with the case where lithium metal is directly used as the negative electrode.

Among them, in the case of using graphite, lithium is held in graphite in an ionic state rather than in a metallic state by taking the form of a lithium-graphite intercalation compound peculiar to graphite.

As a result, in terms of energy density, a capacity of 372 mAh / g is obtained in the first stage where lithium ions are held between the graphite layers.

This is about 1/10 of the capacity of 3700 mAh / g, which is the value when metallic lithium is used as the negative electrode, but it may occur when metallic lithium is used as the negative electrode. Precipitation does not occur.

Furthermore, when compared to conventional Ni-Cd ₂ battery is approximately twice as high capacity is grave.

Furthermore, recently, among carbon materials,
Attempts have been made to use amorphous carbon, which is obtained by treating at a heat treatment temperature of about 1000 ° C. (a range of about 800 ° C. to 1300 ° C.), which is relatively lower than graphitization of an organic substance, for a negative electrode.

In this case, the capacity varies greatly depending on the organic material used as the raw material. When a specific starting material is used, a material having a much higher capacity than that of graphite such as 500 mAh / g or more is obtained.

Further, attempts have been made to increase the capacity by using additives and foaming agents in such carbon materials.

[0014]

As described above, graphite,
Alternatively, a carbon material having a lower treatment temperature than that is used for the negative electrode instead of metallic lithium, whereby the performance is improved.

However, in the case of graphite,
It has various problems to be solved.

The graphite includes natural graphite and artificial graphite. As the natural graphite, graphite excavated from a mine is used.

However, since this natural graphite contains impurities such as iron in a considerable amount, it is necessary to remove impurities.

Impurities cannot be completely removed,
When natural graphite is used for the negative electrode of the secondary battery, it is advantageous in terms of cost, but there are variations in the characteristics of the electrode.

On the other hand, artificial graphite is energetically disadvantageous because it requires a heat treatment temperature of at least about 2800 ° C. when manufactured.

Moreover, no matter how the starting material is selected,
It is quite difficult to obtain something close to a graphite structure.

In any of the natural graphite and the artificial graphite, since they take the form of a lithium-graphite intercalation compound, the composition is limited to C ₆ Li and the energy density is limited to 372 mAh / g.

On the other hand, in the case of amorphous carbon, although the reason has not always been clear, an energy density of 372 mAh / g or more, which is the upper limit of the graphite material, is obtained.

However, amorphous carbon has two major problems. The first problem is that the voltage characteristics during charging / discharging are not flat.

When the above-mentioned graphite is used for the negative electrode, the electrochemical reaction is constant because the electrode reaction is the formation and decomposition of the lithium-graphite intercalation compound. It is flat.

On the other hand, in the case of amorphous carbon, the charge / discharge curve has a slope shape, that is, a curve having a certain slope with respect to time, and the slope of this curve is not constant.

In the case of a secondary battery, such a characteristic means that the voltage between the electrodes is not constant, which is an obstacle to the design of the device.

Recently, in order to remedy this drawback, a secondary battery package has been equipped with a constant voltage circuit.

The second problem is that the irreversible capacity is large. Here, the irreversible capacity means the difference between the capacity at the time of initial charging and the discharge capacity subsequent thereto.

For this reason, when manufacturing a secondary battery, it is necessary to put lithium ions in an amount corresponding to an irreversible capacity in advance. However, the source of lithium ions is not a metal state, but is usually a metal ion. Is supplemented by adding a large amount of positive electrode active material.

This causes a positive electrode active material, which is not used when used in a device, to exist in the battery case, which is disadvantageous in terms of space efficiency.

Also, since a part of the irreversible capacity is used for gas generation, a large amount of gas is generated when the irreversible capacity is large.

Therefore, it is necessary to seal the battery case after the initial charging, which complicates the manufacturing process.

The present invention was mainly made to reduce the irreversible capacity explained as the second problem of amorphous carbon, and is for a secondary battery having a high energy density and a low irreversible capacity. It is intended to provide an electrode.

[0034]

In order to solve the above problems, the secondary battery electrode of the present invention uses an amorphous carbon material having a ratio of oxygen atoms to carbon atoms of 0.4 or less. Is.

Then, the secondary battery electrode using this amorphous carbon material has a specific surface area of 1 in which a graphitizable material is prepared and the graphitizable material is heat treated at 600 ° C. or higher.
And an amorphous carbon material production step of producing an amorphous carbon material of not more than 00 m ² / g.

Alternatively, a step of preparing a graphitized material is generally performed without specifying the type of the graphitized material, and a first heat treatment is performed on the graphitized material at 600 ° C. or higher to produce an amorphous carbon material. Electrode for secondary battery comprising: an amorphous carbon material producing step; and a second heat treatment step of heat treating the amorphous carbon material in hydrogen gas or a mixed gas of hydrogen gas and an inert gas May be used.

The temperature range in this second heat treatment step is preferably 200 ° C. or higher and 800 ° C. or lower.

Alternatively, a step of preparing a graphitized material, a step of preparing the amorphous carbon material by heat-treating the graphitized material at 600 ° C. or higher, and a step of preparing the amorphous carbon material And a pulverization step of pulverizing in hydrogen gas or in a mixed gas of hydrogen gas and an inert gas. 2
It may be a method of manufacturing an electrode for a secondary battery.

Here, it is preferable that the temperature range in the pulverization process is 200 ° C. or higher and 800 ° C. or lower.

When heat treatment or pulverization treatment is performed using a mixed gas of hydrogen gas and an inert gas, the volume ratio of hydrogen gas to the inert gas is preferably 1% or more.

[0041]

With the above structure, an amorphous carbon material having a ratio of oxygen atoms to carbon atoms of 0.4 or less can be reliably realized.

Therefore, when this amorphous carbon material is used for an electrode of a secondary battery, an energy density of 372 mAh / g or more, which is the upper limit of a usual graphite material, can be obtained, and the irreversible capacity can be obtained. Effectively reduced.

[0043]

EXAMPLES The inventors of the present application first examined the irreversible capacity in the case of graphite in order to clarify the cause of the irreversible capacity, which is the difference between the charge capacity immediately after the secondary battery was made and the discharge capacity following it. went.

Since the surface of graphite has a layered structure, there are two types of surfaces, one of which is the ab plane.
The other is the end face perpendicular to the a-axis.

Of these, the ab plane is inactive because there is no chemical bond, but the plane perpendicular to the a axis is active because there is a chemical bond.

In general, the heat treatment for producing graphite is carried out in an inert atmosphere, but since this heat treatment is carried out at 2500 ° C. or higher, the carbon atoms at the end face have no carbon atoms bonded at the end of the heat treatment. Up to.

In such a state, when air is introduced, it is considered that bonds of oxygen atoms or functional groups containing oxygen atoms are formed and bonded, though the specific structure is unknown.

Similarly, in the case of amorphous carbon, it is considered that oxygen atoms or functional groups containing oxygen atoms are formed by introducing air after the heat treatment.

Furthermore, in the case of amorphous carbon, there is a tendency that a part of the functional group containing an oxygen atom, which is already contained in the raw material organic material or formed during the thermal decomposition reaction, remains. is there.

Further, in the case of amorphous carbon, since the heat treatment temperature is low, the graphite structure is not sufficiently developed, and there are considerable defects in the crystal. It is also considered that a functional group including is bonded.

That is, in the case of amorphous carbon, it is estimated that more oxygen atoms and functional groups containing oxygen atoms are present than in ordinary graphite.

Then, paying attention to the fact that the oxygen atom and the functional group containing the oxygen atom are contained in a relatively large amount, the ratio is changed by using amorphous carbon for the negative electrode of the secondary battery. As a result, even when amorphous carbon obtained by heat-treating the same raw material has an O / C value of 0.4 or less, which is the ratio of carbon atoms and oxygen atoms, the O / C value is 0.4. It finds a novel fact that the irreversible capacity is effectively reduced as compared with the one exceeding.

For identification of the types of these functional groups,
In general, infrared spectroscopy is used, but when used as the negative electrode of a secondary battery, the type of functional groups does not matter because the amount of the functional groups as a whole becomes a problem. Only the value is the important factor.

In order to obtain the value of O / C, it is necessary to obtain the amounts of carbon and oxygen.
Since the purpose is to obtain the / C value, X-ray photoelectron spectroscopy (XPS) was used.

The method of obtaining the O / C value by this XPS is a general method, for example, "technical analysis method".
(Kagaku Doujin, Vol. 3, P83).

The procedure for obtaining the O / C value by this method is as follows:
It is as follows. First, carbon 1 near 283 eV
Peak due to S orbit and 1S of oxygen near 532 eV
The peak due to the orbit is measured.

From the chart obtained by this measurement, the areas of the peaks of carbon and oxygen (area intensity) are obtained.

After correcting the area intensity with the sensitivity of each element to X-rays, the O / C value can be obtained by calculating the ratio of carbon and oxygen.

Further, the inventor of the present application examined how to adjust the O / C value. As a result, the O / C value tends to be different depending on the starting material of amorphous carbon and the heat treatment temperature. It was

For example, regarding the starting material, there are classifications of easily graphitizable material and non-graphitizable material.
/ C value is greatly different.

For example, in a graphitizable material having a graphite structure after heat treatment at 3000 ° C., if the heat treatment temperature is 600 ° C. or higher and the specific surface area is 100 m ² / g or less, O / C
The value can be 0.4 or less.

This is because the graphitizable material is from 600 ° C. to 1
It is considered that the heat treatment at 200 ° C. has already formed a planar structure and there are few defects existing inside the crystal.

On the other hand, in the case of the non-graphitizable material, the O / C value becomes 0.4 or more. This is from 600 ℃ to 12
It is considered that the crystal has many defects inside the crystal by heat treatment at 00 ° C. as compared with the graphitizable material, and a functional group containing oxygen is bonded to this portion.

Regardless of the kind, the O / C value can be increased by further performing heat treatment in hydrogen gas or in a mixed gas of hydrogen gas and an inert gas after the heat treatment for making amorphous carbon. It could be made 0.4 or less.

When a mixed gas of hydrogen gas and an inert gas is used, the volume ratio of hydrogen gas may be 1% or more.

The temperature of this treatment depends on the magnitude of the O / C value, but it is 200 ° C. to 800 ° C., and the higher the amorphous carbon having a large O / C value, the higher the temperature must be. .

First, at 200 ° C. or lower, water is mainly desorbed from amorphous carbon, and it is difficult for a chemical reaction due to hydrogen to occur. At 800 ° C. or higher, amorphous carbon is more likely to react than hydrogen. It is considered that this is not effective because the release of hydrogen contained therein is the main factor.

After the heat treatment for changing to amorphous carbon,
O / C can also be obtained by pulverizing amorphous carbon in hydrogen gas or a mixed gas of hydrogen gas and an inert gas.
The value could be 0.4 or less.

This is because some carbon bonds are broken when a new surface is formed by pulverization, but since the pulverization treatment in air is avoided, a functional group containing oxygen in this bond is used. It is considered that this is because the formation of the group is prevented.

Further, this crushing treatment is performed at 200 ° C. or higher for 8 hours.
By carrying out at a temperature of 00 ° C. or lower, formation of a functional group containing oxygen can be suppressed more effectively, and when used as an electrode of a secondary battery, irreversible capacity can be suppressed.

When a mixed gas of hydrogen gas and an inert gas is used, the volume ratio of hydrogen gas may be 1% or more.

Now, the effects will be described in more detail below by using representative examples and comparative examples.

In each of the examples and comparative examples, the heat treatment of the graphitizable carbon raw material was carried out in a nitrogen atmosphere by using a quartz glass furnace core tube in an electric furnace using a Kanthal wire.

Then, the carbon material obtained by the heat treatment was put into an alumina pot together with the alumina balls and pulverized using a planetary ball mill.

Since the alumina pot can be hermetically closed, the atmosphere for crushing can be changed by changing the atmosphere when the alumina balls are filled with the carbon material.

With respect to the carbon powder obtained by the above method, the O / C value, which is the ratio of carbon to oxygen, was measured using the above-mentioned X-ray photoelectron spectroscopy (XPS).

Specifically, from the chart obtained by XPS, the area intensities of the peak due to the 1S orbital of carbon seen at 283 eV and the peak due to the 1S orbital of oxygen seen at 532 eV were obtained, and then, The O / C value was determined by correcting the sensitivity to X-rays and determining the ratio of the two peaks.

The specific surface area of the carbon powder was determined by the amount of nitrogen gas adsorbed at the liquid nitrogen temperature (hereinafter referred to as the BET method).

The specific contents of this method are described in detail in Carbon Material Experimental Techniques (edited by Japan Society of Carbon Materials, p. 120).

The characteristics of the obtained carbon powder as a secondary battery were measured by preparing a 2016 type coin battery, charging it to 4 V at a constant current of 0.5 mA, and then measuring 0. The discharge capacity was obtained by discharging to 3.2 V at 5 mA, and the irreversible capacity was obtained as the difference between the initial charge capacity and the initial discharge capacity.

Example 1 In this example, coal tar pitch was raised to 800 ° C. at a temperature rising rate of 5 ° C./min in a nitrogen atmosphere and kept for 1 hour to obtain an amorphous carbon material.

The amorphous carbon material was pulverized for 1 hour using a planetary ball mill to obtain an amorphous carbon material powder.

Ten samples of this powder were prepared and
The specific surface area by T method is measured and averaged to be 24.1 m ² /
g.

When the O / C value of these powders was measured by XPS, the average value was 0.3.

Then, tetrafluoroethylene (PTFE) as a binder was added to these amorphous carbon material powders for 10 w.
% And mixed on a copper foil having a thickness of 20 μm to obtain 10 negative electrodes.

On the other hand, LiCoO ₂ powder was prepared as the positive electrode in the same manner as the negative electrode. Further, 1 mol / l of LiPF ₆ was dissolved in an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a ratio of 1: 1 to prepare an electrolytic solution.

Then, between the positive electrode and the negative electrode thus formed, the electrolytic solution impregnated with porous polypropylene was sandwiched to produce ten coin batteries.

The average capacity and the average irreversible capacity of the battery thus obtained were measured, and the results are shown below (Table 1).
Shown in

[0089]

[Table 1]

Comparative Example 1 For comparison, the coal tar pitch used as a raw material in Example 1 was heated to 450 ° C. in air and then to 800 ° C. in a nitrogen atmosphere.
Then, it was kept at 800 ° C. for 1 hour to obtain an amorphous carbon material.

Then, 10 samples of the amorphous carbon material thus obtained were prepared and pulverized in the same manner as in Example 1 to obtain a powder.

Average O / C of these amorphous carbon material powders
The value was 0.55 when measured by XPS, and the average specific surface area measured by the BET method was 14.7 m ² / g.

Using these amorphous carbon material powders, a coin type battery was prepared in the same manner as in Example 1, the capacity and the irreversible capacity were measured, and the average value thereof is shown in (Table 1) above. .

Comparative Example 2 In this comparative example, a novolac type phenol resin was used and the same heat treatment and crushing treatment as in Example 1 were performed to obtain a powder of an amorphous carbon material.

O measured by XPS, which is the average value of 10
/ C value is 0.6, BET specific surface area is 8.3 m ²
/ G.

Then, using these amorphous carbon material powders, ten coin batteries were produced in the same manner as in Example 1.

The average capacity and average irreversible capacity of these batteries are shown in (Table 1) above. (Example 2) In this example, the amorphous carbon material powder obtained in Comparative Example 2 was used, and this amorphous carbon powder was mixed with hydrogen 4
The treatment was carried out for 1 hour at 500 ° C. in a mixed gas of 0% and 60% argon.

The average O / C value of the 10 amorphous carbon materials thus obtained was 0.2.

Then, using this amorphous carbon, ten coin batteries were produced in the same manner as in Example 1.

The average capacity and average irreversible capacity of these batteries are shown in (Table 1) above. (Example 3) In this example, as in Comparative Example 2, 10 samples of an amorphous carbon material were prepared by using a novolac type phenol resin and performing heat treatment.

Next, these amorphous carbon materials were filled in an alumina pot together with alumina balls in an atmosphere of hydrogen gas 40% and argon gas 60%.

Then, this alumina pot was attached to a planetary ball mill, and a carbon material was pulverized.

The resulting carbon material powder had an average specific surface area by the BET method of 9.3 m ² / g and an average O / C value of 0.31 measured by XPS.

Then, using these amorphous carbon material powders, ten coin type batteries were produced in the same manner as in Example 1.

The average capacity and average irreversible capacity of these batteries are shown in (Table 1) above. (Example 4) In this example, polyacrylonitrile (P
(AN) was heat-treated in the same manner as in Example 1 to obtain 10 samples of amorphous carbon material.

The average O / C value of these measured by XPS was 0.05. Ten coin batteries were produced from this amorphous carbon in the same manner as in Example 1.

The average capacity and average irreversible capacity of these batteries are shown in (Table 1) above. As can be understood from the above examples, the O / C value can be reliably reduced to 0.4 or less by such a process, and the irreversible capacity value can be effectively reduced while keeping the capacity value high. It has been possible to realize a secondary battery that can be put to practical use.

Further, as can be understood from each comparative example, the O / C value cannot be made 0.4 or less by using these steps, and when the O / C value is 0.4 or more. In some cases, the irreversible capacity value is not reduced.

[0109]

As described above, the irreversible capacity value of the secondary battery electrode is improved by using the electrode for a secondary battery using an amorphous carbon material in which the ratio of oxygen atoms to carbon atoms is 0.4 or less. Can be reduced.

In addition, the step of preparing the graphitizable material and the heat treatment of the graphitizable material at 600 ° C. or higher have a specific surface area of 1
A method for producing an electrode for a secondary battery, comprising an amorphous carbon material producing step of producing an amorphous carbon material having an amount of 00 m ² / g or less,
A step of preparing a graphitized material, a step of preparing the amorphous carbon material by subjecting the graphitized material to a first heat treatment at 600 ° C. or higher, and a hydrogen gas for the amorphous carbon material. Or a method of manufacturing an electrode for a secondary battery having a second heat treatment step of performing heat treatment in a mixed gas of hydrogen gas and an inert gas, or a step of preparing a graphitizing material, and 600 ° C. for the graphitizing material. Amorphous carbon material production step of producing the amorphous carbon material by the heat treatment described above, and pulverization treatment step of pulverizing the amorphous carbon material in hydrogen gas or a mixed gas of hydrogen gas and an inert gas By using the method for producing an electrode for a secondary battery having the above, the ratio of oxygen atoms to carbon atoms that can be suitably used for the electrode for a secondary battery is 0.
It is possible to reliably provide an amorphous carbon material having a number of 4 or less.

Claims (7)

[Claims] 1. The ratio of oxygen atoms to carbon atoms is 0.
An electrode for a secondary battery using an amorphous carbon material of 4 or less. 2. A step of preparing a graphitizable material, and a heat treatment of the graphitizable material at 600.degree.
A method for producing an electrode for a secondary battery, comprising: an amorphous carbon material producing step of producing an amorphous carbon material having an amount of 00 m ² / g or less. 3. A step of preparing a graphitized material, a step of preparing the amorphous carbon material by subjecting the graphitized material to a first heat treatment at 600 ° C. or higher, and the amorphous material. A second battery heat treatment step of heat-treating a carbon material in hydrogen gas or in a mixed gas of hydrogen gas and an inert gas. 4. The temperature range in the second heat treatment step is
The method for producing an electrode for a secondary battery according to claim 3, wherein the temperature is 200 ° C. or higher and 800 ° C. or lower. 5. A step of preparing a graphitizing material, a step of preparing an amorphous carbon material by heat-treating the graphitizing material at 600 ° C. or higher, and a step of preparing the amorphous carbon material. A method of manufacturing an electrode for a secondary battery, comprising a pulverization treatment step of pulverizing treatment in hydrogen gas or a mixed gas of hydrogen gas and an inert gas. 6. The temperature range in the pulverization process step is 20.
The method for producing an electrode for a secondary battery according to claim 5, wherein the temperature is 0 ° C or higher and 800 ° C or lower. 7. The 2 according to claim 3, wherein the volume ratio of hydrogen gas to inert gas is 1% or more.
Manufacturing method of secondary battery electrode.