JP3237472B2 - Method of manufacturing electrode for secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the manufacture of an electrode for a secondary battery.
Method , especially amorphous carbon negative electrode for lithium secondary battery
And a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the use of secondary batteries has been increasingly widespread, and accordingly, demands for higher capacity have been increasing. Such demands have been mainly addressed by means of improving the capacity of Ni-Cd (nickel-cadmium) secondary batteries. As a result, 175 mAh
/ G has been improved.

On the other hand, non-aqueous electrolyte batteries using an alkali metal such as lithium for the negative electrode are widely used as power sources for electronic devices and the like because of their high energy density.

[0004] In recent years, research and development, especially of secondary batteries, have been promoted. However, in the case of a secondary battery using lithium as the negative electrode, at the time of overcharging, metal is deposited in a dendrite shape on the negative electrode, and as a result, a normal voltage may not be generated.

[0005] Therefore, a non-aqueous electrolyte secondary battery has been proposed in which lithium metal is doped or alloyed in a carbonaceous material or compound, instead of using lithium metal for the negative electrode. .

[0006] In this secondary battery, the possibility of dendrite generation is lower than when lithium metal is used as it is as a negative electrode.

[0007] Among them, in the case of using graphite, lithium is held in graphite in an ionic state instead of a metallic state by taking a form of 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 in which lithium ions are held between the layers of graphite.

This is about one-tenth the capacity of 3700 mAh / g, which is the value when lithium metal is used for the negative electrode, but it is also a dendrite that can be generated when lithium metal is used for the negative electrode. Does not precipitate.

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

Further, recently, among carbon materials,
Attempts have been made to use amorphous carbon as a negative electrode by heat treatment at a heat treatment temperature of around 1000 ° C. (about 800 ° C. to about 1300 ° C.), which is relatively lower than that of graphitizing organic substances.

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 when graphite is used, such as 500 mAh / g or more, is obtained.

[0013] Further, attempts have been made to increase the capacity by using additives and foaming agents for such carbon materials.

[0014]

As described above, graphite,
Alternatively, performance has been improved by using a carbon material having a lower processing temperature for the negative electrode in place of lithium metal.

However, in the case of graphite,
There are 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 natural graphite contains a considerable amount of impurities such as iron, a treatment for removing the impurities is required.

The impurities cannot be completely removed,
When natural graphite is used for the negative electrode of a secondary battery, it is advantageous in terms of cost, but the characteristics of the electrode vary.

On the other hand, artificial graphite is disadvantageous in terms of energy because it requires a heat treatment temperature of at least about 2800 ° C. during production.

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

In either case of natural graphite or artificial graphite, in order to 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, the energy density is not less than 372 mAh / g, which is the upper limit of the graphitic material, although the reason is not always clear.

However, amorphous carbon has two major problems. The first problem is that the voltage characteristics during charging and 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.

Such characteristics, when viewed as a secondary battery, mean that the voltage between the electrodes is not constant, which is an obstacle to the design of equipment.

Recently, in order to improve the disadvantage, a package of a secondary battery is provided with a constant voltage circuit.

The second problem is that the irreversible capacity is large. Here, the irreversible capacity refers to the difference between the capacity at the time of the first charge and the subsequent discharge capacity.

For this reason, when manufacturing a secondary battery, lithium ions for the irreversible capacity must be inserted in advance. Supplements by adding more positive electrode active material.

This means that a positive electrode active material that is not used when used in equipment is present in the battery case, which is disadvantageous in terms of space efficiency.

Further, since a part of the irreversible capacity is used for gas generation, if the irreversible capacity is large, more gas is generated.

For this reason, it is necessary to seal the battery case after the first charging, which complicates the manufacturing process.

The present invention has been made mainly for reducing the irreversible capacity described as the second problem of amorphous carbon, and is intended for a secondary battery having a low irreversible capacity while maintaining a high energy density. It is intended to provide an electrode.

[0034]

[0035]

[0036]

[Means for Solving the Problems] In order to solve the above-mentioned problems
In general, a step of preparing a graphitized material without specifying the type of the graphitized material, and a step of performing a first heat treatment on the graphitized material at 600 ° C. or higher to form an amorphous carbon material A method for manufacturing an electrode for a secondary battery, comprising: a carbon material producing step; and a second heat treatment step of performing a heat treatment on the amorphous carbon material in a hydrogen gas or a mixed gas of a hydrogen gas and an inert gas. It is.

Here, the temperature range in the second heat treatment step is preferably 200 ° C. or more and 800 ° C. or less.

Alternatively, a step of preparing a graphitized material, a step of subjecting the graphitized material to a heat treatment at a temperature of 600 ° C. or higher to produce an amorphous carbon material, A pulverizing treatment step of performing a pulverizing treatment in hydrogen gas or a mixed gas of hydrogen gas and inert gas.
A method for manufacturing an electrode for a secondary battery may be used.

Here, it is preferable that the temperature range in the pulverizing step is 200 ° C. or more and 800 ° C. or less.

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

[0041]

According to the above configuration, 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 normal graphite material, can be obtained, and the irreversible capacity can be increased. Effectively reduced.

[0043]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors of the present invention first examined the irreversible capacity of graphite in order to clarify the cause of the occurrence of irreversible capacity, which is the difference between the charge capacity immediately after the production of a secondary battery and the subsequent discharge capacity. went.

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

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

In general, the heat treatment for producing graphite is performed in an inert atmosphere. However, since this heat treatment is performed at 2500 ° C. or higher, the bonding of carbon atoms on the end face is stopped at the end of the heat treatment. Up to.

In this state, when air is introduced, the specific structure is unknown, but it is considered that a bond of an oxygen atom or a functional group containing an oxygen atom is formed and bonded.

Similarly, in the case of amorphous carbon, it is considered that an oxygen atom or a functional group containing an oxygen atom is formed by introducing air after the heat treatment.

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

Further, in the case of amorphous carbon, the graphite structure is not sufficiently developed due to the low heat treatment temperature, and considerable defects exist in the crystal. It is also considered that a function containing

That is, in the case of amorphous carbon, it is estimated that there are more oxygen atoms and functional groups containing oxygen atoms as compared with ordinary graphite.

Focusing on the fact that oxygen atoms and functional groups containing oxygen atoms are contained in a relatively large amount as described above, this ratio is changed by using amorphous carbon for the negative electrode of the secondary battery. As a result, as a result, even if the same raw material is heat-treated amorphous carbon, if the O / C value which is the ratio of carbon atoms to oxygen atoms is 0.4 or less, the O / C value becomes 0.4. The present inventors have found a novel fact that the irreversible capacity is effectively reduced as compared with those exceeding the limit.

For identification of the types of these functional groups,
In general, infrared spectroscopy is used. However, when used as a negative electrode of a secondary battery, the type of the functional group does not matter because the total amount of the functional group is a problem. The value is the only important factor.

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

The method of obtaining the O / C value by the XPS is a general method, for example, a method of “instrument analysis”.
(Kagaku Doujin, Vol. 3, p. 83).

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 orbital and 1S of oxygen near 532 eV
Measure the peak due to the orbit.

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

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

Further, the present inventor examined how to adjust the O / C value, and found that the O / C value tended to be different depending on the starting material of the amorphous carbon and the heat treatment temperature. Was.

For example, starting materials are classified into graphitizable materials and non-graphitizable materials.
/ C values differ greatly.

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 more and the specific surface area is 100 m ² / g or less, the O / C
The value can be 0.4 or less.

This is because the graphitizable material can be heated from 600 ° C. to 1
It is considered that the planar structure was already formed by the heat treatment at 200 ° C., and that few defects exist inside the crystal.

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

Regardless of the type, after the heat treatment for forming amorphous carbon is completed, the heat treatment is further performed in a hydrogen gas or a mixed gas of a hydrogen gas and an inert gas to reduce the O / C value. 0.4 or less could be achieved.

When a mixed gas of hydrogen gas and 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 is from 200 ° C. to 800 ° C., and the higher the amorphous carbon having a higher O / C value, the higher the temperature must be. .

At first, at 200 ° C. or lower, the main component is the elimination of water from the amorphous carbon, and a chemical reaction with hydrogen is unlikely to occur. This is considered to be because there is no effect because the elimination of hydrogen contained therein is mainly performed.

After completion of the heat treatment for forming amorphous carbon,
O / C can also be achieved by grinding amorphous carbon in hydrogen gas or a mixed gas of hydrogen gas and inert gas.
The value could be reduced to 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 the air is avoided, the bond containing oxygen contains It is considered that the formation of the group was prevented.

Further, this pulverization treatment is performed at a temperature of 200 ° C.
By performing the reaction at a temperature of 00 ° C. or lower, the formation of the functional group containing oxygen can be suppressed more effectively, and the irreversible capacity can be suppressed when used as an electrode of a secondary battery.

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

Now, the effects will be described in more detail with reference to typical examples and comparative examples.

In each of the examples and comparative examples, the heat treatment of the graphitizable carbon material was performed in a nitrogen atmosphere using a quartz glass furnace core 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 alumina balls, and pulverized using a planetary ball mill.

Since the alumina pot can be hermetically sealed, the atmosphere at the time of pulverizing the carbon material can be changed by changing the atmosphere when the alumina balls are put into the pot.

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 by using the above-mentioned X-ray photoelectron spectroscopy (XPS).

Specifically, the peak intensities attributable to the 1S orbital of carbon at 283 eV and the peaks attributable to the 1S orbital of oxygen at 532 eV are respectively obtained from the chart obtained by XPS. The O / C value is determined by correcting the sensitivity to X-rays and determining the ratio of two peaks.

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

The specific contents of this method are described in detail in the Experimental Technique for Carbon Materials (edited by the 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 the battery to 4 V at a constant current of 0.5 mA, and then charging the battery. 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 first charge capacity and the first discharge capacity.

Example 1 In this example, an amorphous carbon material was obtained by raising the coal tar pitch to 800 ° C. at a rate of 5 ° C./min in a nitrogen atmosphere and holding it for 1 hour.

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

[0102] Ten samples of this powder were prepared, and BE
The specific surface area measured by the T method was measured and averaged to 24.1 m ² /
g.

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

Next, ethylene tetrafluoride (PTFE) as a binder was added to these amorphous carbon material powders for 10 watts.
%, And applied 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 a 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 formed in this way, ten electrolytic cells impregnated with porous polypropylene were sandwiched to produce ten coin batteries.

The average capacity and the average irreversible capacity of the battery thus obtained were measured, and the results were shown in the following (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.
Thereafter, the temperature was maintained 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 by the same method as in Example 1 to obtain a powder.

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

Using these amorphous carbon material powders, a coin-type battery was produced in the same manner as in Example 1, the capacity and irreversible capacity were measured, and their average values are shown in Table 1 above. .

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

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

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

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

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

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

The average capacity and the average irreversible capacity of these batteries are shown in the above (Table 1). Example 3 In this example, as in Comparative Example 2, novolak-type phenol resin was used, and heat treatment was performed to produce 10 samples of an amorphous carbon material.

Then, the operation of sealing these amorphous carbon materials in an alumina pot together with alumina balls in an atmosphere of 40% hydrogen gas and 60% argon gas was performed.

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 according to the BET method of 9.3 m ² / g and an average O / C value measured by XPS of 0.31.

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

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

The average O / C value 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 the average irreversible capacity of these batteries are shown in the above (Table 1). As can be understood from the above embodiments, the O / C value can be reliably reduced to 0.4 or less by such a process, and the irreversible capacitance value can be effectively reduced while maintaining the capacitance value high. Thus, a secondary battery which can be sufficiently used practically was realized.

As can be understood from each comparative example, the O / C value cannot be reduced to 0.4 or less by using these steps. In some cases, the irreversible capacitance value is not reduced.

[0109]

[0110]

As described above, the step of preparing the graphitized material and the step of preparing the amorphous carbon material by subjecting the graphitized material to the first heat treatment at 600 ° C. or higher. Heat-treating the amorphous carbon material in a hydrogen gas or a mixed gas of a hydrogen gas and an inert gas.
A method of manufacturing a secondary battery electrode having a heat treatment step, or a step of preparing a graphitized material;
Amorphous carbon material production process of producing an amorphous carbon material by heat treatment at a temperature of not less than ℃, and pulverization treatment of pulverizing the amorphous carbon material in hydrogen gas or a mixed gas of hydrogen gas and inert gas By using the method for manufacturing a secondary battery electrode having the steps described above, the amorphous carbon material having a ratio of oxygen atoms to carbon atoms of 0.4 or less, which can be suitably used for the secondary battery electrode, is surely obtained. Can be provided.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-100009 (JP, A) JP-A-6-132032 (JP, A) JP-A-6-20690 (JP, A) (58) Investigation Field (Int.Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/58 H01M 10/40

Claims (5)

(57) [Claims] A step of preparing a graphitized material; a step of performing a first heat treatment on the graphitized material at a temperature of 600 ° C. or higher to form an amorphous carbon material; A second heat treatment step of subjecting the carbon material to heat treatment in hydrogen gas or in a mixed gas of hydrogen gas and an inert gas. 2. The temperature range in the second heat treatment step is as follows:
The method for producing an electrode for a secondary battery according to claim 1, wherein the temperature is 200C or higher and 800C or lower. 3. A step of preparing a graphitized material, a step of heat-treating the graphitized material at 600 ° C. or higher to form an amorphous carbon material, and a step of preparing the amorphous carbon material. A pulverizing step of performing a pulverizing process in a hydrogen gas or a mixed gas of a hydrogen gas and an inert gas. 4. The temperature range in the pulverizing treatment step is 20.
The method for producing an electrode for a secondary battery according to claim 3 , wherein the temperature is from 0C to 800C. 5. A 2 according any one of the claims 1 to at least 1% by volume ratio of inert gas in the hydrogen gas 4
Method for producing electrode for secondary battery.