JPH11322323A - Carbon compound and its production, and electrode for secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a cathodic carbon material for Li ion secondary batteries having large charge or discharge capacities by adding a specific amount of silicon to a carbon substance obtained by firing a phenolic resin in an inert gas atmosphere at a specified temperature. SOLUTION: This carbon material is obtained by mixing a phenolic resin with a silicon compound, thermally treating the mixture in the temperature range of 150-220 deg.C for >=2 hr, subjecting the product to a pyrolytic reaction treatment in an inert bas atmosphere or in vacuo in the temperature range of 450-600 deg.C, and subsequently similarly subjecting the product to a thermal treatment firing in an inert gas atmosphere or in vacuo in the temperature range of 800-1,500 deg.C. The silicon compound is preferably a silane compound having a reactive group comprising either of amino group. vinyl group, epoxy group, chloro group and hydroxyl group, and the silicon is preferably contained in an amount of 1-100 wt.% based on the carbon. The phenolic resin is preferably a resol type phenolic resin produced from formaldehyde and phenol in a ratio of 1.0-2.0 in the presence of an alkali catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon electrode material mainly used in a lithium ion secondary battery, particularly to a carbon compound as a negative electrode material, a method for producing the same, and an electrode for a secondary battery.

[0002]

2. Description of the Related Art In recent years, the development of secondary batteries of small size, light weight and high energy density has become popular with miniaturization and high performance of information equipment and the like. As the small secondary battery, three types of Ni-Cd, Ni-H, and Li ions have been put to practical use. When a Li-ion battery with a feature of high energy density is intended to be small and lightweight,
It is being used effectively. It has been known that a carbon material is useful as a negative electrode material of a Li-ion secondary battery, and a carbon material is used as many negative electrodes of a Li-ion secondary battery that is actually put into practical use.

[0003] Initially, it was proposed to use lithium metal itself as the negative electrode. However, lithium was deposited in a dendrite shape after repeated charging and discharging or during charging, which caused an internal short circuit and a significant decrease in charging and discharging efficiency. Phenomenon may occur. Therefore, it is said that there is a problem in safety.

[0004] However, by using a carbon material having a function of electrochemically intercalating and deintercalating lithium ions, it is possible to avoid such a phenomenon, and the lithium ion is practically used. I have. On the other hand, there are still some issues and demands that need to be solved by using carbon materials, such as the magnitude of charge / discharge capacity, the loss of charge and discharge capacity, and the deterioration of battery characteristics due to deterioration of the electrolyte. And the like. It is said that the magnitude of the charge / discharge capacity is theoretically about 372 mAh / g in the graphite structure. Even those having a graphite structure are known to be spherical, fibrous, scaly, etc., depending on the shape. It is said that the practical capacity is 280 to 330 mAh / g. As for the capacity, the larger the capacity, the better for the battery. Therefore, a higher capacity is demanded. In addition, since it is a secondary battery, high repetition stability,
Further, there is a demand for higher reliability such as environmental resistance that can withstand the environment in which the equipment is used.

[0005]

Therefore, an amorphous carbon material has been proposed as a negative electrode material having a large charge / discharge capacity. However, although a high initial charge amount can be obtained, there is a problem that a discharge loss is large and a potential variation with respect to the discharge amount is large as compared with graphite-based carbon.

An object of the present invention is to provide a negative electrode material having a large discharge capacity per unit weight or volume among carbon materials used as a negative electrode used in the lithium ion secondary battery.

[0007]

Means for Solving the Problems The carbon material used for the negative electrode has a difference in the characteristics including the structure depending on the starting material and how to make it, and the battery characteristics greatly differ. So far, the definition of the carbon material has been determined from the structural aspect as an average lattice constant by the X-ray diffraction method and 1580 cm ^-1 from the Raman spectrum measurement.
There are an intensity ratio of 360 cm ^{-1 and} the like.
There has been proposed a carbon material defined by the atomic ratio of carbon / hydrogen in terms of chemical structure such as powder particle size and specific surface area.

As a carbon material, a crystal structure is known as a main factor affecting the battery capacity. Regarding the crystallinity, an amorphous carbon material can provide a material having a higher charge / discharge amount. Are known. The charge / discharge mechanism is 2000
Regarding graphite-based carbon, which is crystalline carbon produced by baking above ℃, the charge / discharge phenomenon is clear by examining the state of the presence of Li ions by NMR analysis and the like, and the transfer of Li ions into and out of the graphite layers It is believed to be due to. With respect to carbon having an amorphous structure formed by firing at 2000 ° C. or lower, the existence state of Li ions during charging is not clear.

Generally, several models have been proposed. For example, as in the case of crystallinity, it may be trapped in a layer state in an irregular layer structure, pores or structural defects in the layer in a cluster state, or may exist in an adsorbed state on the surface of the layer. It is estimated.

[0010] As a factor for determining the capacity, the above factors are said to be applied to the amorphous type as well as the crystalline carbon. There has also been proposed a material in which an additive is contained in carbon as a material. Japanese Patent Application Laid-Open No. 3-245438 discloses that the addition of phosphorus or boron has the effect of improving the capacity. JP-A-8-259213, JP-A-31
No. 5822 discloses an example in which a silicon-containing carbon compound has improved capacity characteristics. In addition, it discloses a method for producing a carbon compound, in which a polymer precursor is composed of a mixture of a silicon-containing polymer and a curing agent, and a curing reaction is performed before thermal decomposition. As an example, a material having an epoxy group is disclosed.

In the present invention, a phenol resin, which is a non-graphite material, is selected as a raw material. The phenolic resin includes a novolak type and a resol type, but the preferable type is a resol type. Many carbons using phenolic resin as a raw material have been proposed so far.However, in the present invention, the carbon containing silicon atoms is dispersed in the carbon material in the form of atoms or ultrafine particles in a uniform state. It is doing. Regarding the carbon material itself, firing conditions and treatments have been proposed on the basis of consciously controlling the surface structure, pores, structural defects, and the like, which particularly affect the improvement of capacity in battery characteristics.

The basic characteristics required for a Li-ion secondary battery are the size of the battery capacity and the stability when the capacity and potential are repeatedly used. The pattern of the graphite-based discharge curve has a small potential variation with respect to the discharge capacity, but as described above, it is characterized in that the capacity is limited. When amorphous carbon is used, there are two types of discharge curve patterns. One is that the discharge capacity per unit weight can be increased, but the potential fluctuation with respect to the capacity is large. The other has a small potential fluctuation region similar to the graphite system with respect to the capacity,
There is still a problem that the capacity is small. The type provided here is this type.

[0013] Since the true density of amorphous carbon is lower than that of graphite carbon, it is disadvantageous when comparing the capacity per unit volume. Due to the high true density of the graphite system, it is 20% to 50% as compared with the amorphous system after printing into a powder after printing.
%, The bulk density of the graphite system is larger. Therefore, in order to obtain the superior capacity in the amorphous system, the capacity per unit weight must be large by 50% or more. Regarding the amorphous carbon in general, the structure of the Li ion storage site has some unclear points, but it is estimated that the layer structure, pores, surface structure, and the like have an influence.

In the present invention, a carbon compound is synthesized from a composite of phenolic resin and silicon as a raw material, which is a non-graphite material. In this carbon compound, the structure of layers, pores, surfaces, and the like is controlled to increase the number of Li ion storage sites and to increase the capacity.

The phenolic resin includes a novolak type and a resol type. It is preferable that the resin does not melt in the process of performing the high-temperature heat treatment and that the resin has a suitable non-graphitic property. Therefore, the curing conditions for infusibility are also important conditions. In the case of a novolak resin, a crosslinking agent must be added to cause a thermosetting reaction.

The resole type resin causes a thermosetting reaction by itself. In order for the curing reaction to occur uniformly within the resin, it is necessary to perform a sufficient heat treatment under conditions that do not cause thermal oxidation or decomposition reaction. For this purpose, it is necessary to carry out a curing reaction by heat treatment at 80 ° C. to 200 ° C. before pulverization, then to carry out pulverization treatment, and to further sufficiently perform heat treatment at 180 ° C. to 220 ° C. In order to form a state in which silicon is dispersed in an atomic state in a carbon compound state, it is necessary to mix a phenol resin and a silicon compound in a molecular state in a raw material state. For this purpose, it is desirable that both the phenol resin and the silicon compound can be mixed in a solution. The phenol resin can be obtained in a solution state by mixing with an aqueous solution or an appropriate organic solvent.

The raw material of the silicon compound may be any material as long as it becomes liquid. A silane coupling agent can be used as a suitable compound. Although the silane coupling agent has a reactive functional group, the functional group is not particularly limited here, but the amino group and the vinyl group are easily dispersed with the phenol resin. , An epoxy group, a chloro group, a hydroxyl group and the like. After mixing and dispersing such a compound with an appropriate composition,
After the above-described curing, pulverization, and infusibilization treatments, a heat treatment for carbonization is performed. The heat treatment is performed by optimizing in an inert gas in a range of 800 ° C to 1500 ° C to obtain a desired carbon. A powder is obtained.

Heat treatment is performed at 450 in an inert gas atmosphere.
First, a preliminary heat treatment is performed at a temperature of from about 600 ° C. to about 600 ° C., and then the above-described high-temperature treatment may provide a battery having more excellent battery characteristics. Since the range of 450 ° C. to 600 ° C. is the range in which the phenol resin undergoes the thermal decomposition reaction, by controlling the conditions such as the temperature increase and the temperature decrease rate, it is possible to obtain different pores and surface structures. .

The carbon structure can be evaluated by specific surface area measurement using N 2 gas or CO 2 gas, X-ray measurement, TEM observation and the like. The specific surface area does not always have a correlation with the capacity of the battery in the case of N2 gas. Rather, the specific surface area measured with CO2 gas is more correlated. This is presumably because the evaluation of the pore size is limited to 2 nm or more in the case of N2 gas. With CO2 gas, pores of 1 nm or less can be evaluated. However, just because this value is small does not necessarily mean that the amount of pores and structural defects is small even inside the powder. The inside of the powder was evaluated by measurement such as X-ray diffraction measurement, small angle scattering measurement, and TEM. As a result, it was found that when silicon was dispersed in an atomic state, the battery characteristics were better.
When silicon is present as an oxide or another compound, battery characteristics such as capacity are rather deteriorated. It has been found that the atomic dispersion of silicon affects the structure of carbon, particularly the size and distribution of pores. It was found that the factors affecting the surface structure, pores, and the state of formation of structural defects of the carbon powder required in the present patent are the structure of the starting material and the firing conditions.

As described above, a carbon compound obtained by dispersing silicon atoms in carbon in a carbon fired from a phenol resin is provided as a negative electrode of a Li-ion secondary battery.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The invention according to claim 1 of the present invention relates to a method in which phenol resin is used as a raw material, and silicon is converted to carbon in a carbon material fired at a temperature of 800 to 1400 ° C. in an inert gas atmosphere. On the other hand, it is contained in an amount of 1 to 100% by weight, and has an effect that the capacity can be increased.

According to a second aspect of the present invention, there is provided the carbon compound according to the first aspect, wherein the phenol resin is mixed with a silicon compound which is liquid at room temperature and then calcined. It has the effect that it can be measured.

According to a third aspect of the present invention, there is provided a carbon compound according to the first or second aspect, wherein a phenol resin and a silicon compound are mixed, and the mixture is mixed at a temperature in the range of 150 ° C. to 220 ° C. and 2H or more. In the heat treatment, a silane compound having any of the above-mentioned reactive groups is used as the silicon compound, and has an effect that the capacity can be increased.

[0024] The invention described in claim 4 is the first to third aspects of the present invention.
In the carbon compound according to any of the above, as a silicon compound, an amino group, a vinyl group, an epoxy group, a chloro group,
In this case, a silane compound having any one of the reactive groups of hydroxyl groups is used, and has an effect of increasing the capacity.

[0025] The invention according to claim 5 is the invention according to claims 1 to 4.
In the method for producing a carbon compound according to any one of the above, after mixing the phenol resin and the silicon compound,
A heat treatment of 2H or more is performed at a temperature in the range of 20 ° C., and then a pulverizing process is performed, and thereafter, a temperature of 150 to 220 ° C.
By performing this heat treatment and repeating this treatment one or more times, it is possible to increase the capacity.

According to a sixth aspect of the present invention, in the method for producing a carbon compound according to the fifth aspect, after mixing the phenol resin and the silicon compound, the mixture is subjected to a heat treatment at 150 ° C. to 220 ° C. for 2H or more. After that, a pyrolysis reaction treatment is performed in a temperature range from 450 ° C. to 600 ° C. in an inert gas atmosphere or vacuum, and then a pulverization treatment is performed.
It is fired by a heat treatment at a temperature of from 1500C to 1500C, and has the effect of increasing the capacity.

According to a seventh aspect of the present invention, in the method for producing a carbon compound according to the fifth or sixth aspect, the phenolic resin has a ratio of formaldehyde to phenol of 1.0 to 2.0. It uses a resol type which is made of an alkali catalyst in a ratio, and has an effect that a high capacity can be achieved.

[0028] The invention according to claim 8 provides the invention according to claims 5 to 7.
In the method for producing a carbon compound according to any one of the above, silicon is mixed with a phenol resin, and after being further calcined, the dispersed state is in an atomic state, and it is possible to increase the capacity. Has an action.

The invention according to claim 9 is the invention according to claims 1 to 8
And an electrode for a secondary battery using the carbon compound described in any one of the above or the carbon compound produced by the production method, and has an effect of increasing the capacity.

[0030]

Next, specific examples of the present invention will be described.

An example of the production of carbon powder using a resol type as a phenol resin is shown below.

The phenol resin was synthesized using sodium hydroxide as a catalyst and kept in an aqueous solution. A silane coupling agent having an epoxy group, γ-glycidoxypropyltrimethoxysilane (Toray Silicone Co., Ltd.) was added to the phenol resin in an amount of 0, 10, 20, 30 w.
Then, the mixture was sufficiently stirred and mixed. Then 8
After drying the water at 0 ° C, 1H at 150 ° C,
A heat treatment of 2H was performed at 0 ° C. Next, a pulverization process was performed on the resin. The average size of the particles was 10 μm. Thereafter, a 5H heat treatment was performed at 200 ° C. to sufficiently advance the curing reaction. Thereafter, a heat treatment was performed at 500 ° C. for 1 H in a nitrogen gas atmosphere, and the temperature was returned to room temperature once. Thereafter, the temperature is further increased at a rate of 3 ° C./min,
Heat treatment was performed at 1100 ° C. for 1 H, and the temperature was lowered at a rate of 5 ° C./min. When the carbon compound thus produced was evaluated, the following results were obtained.

First, regarding the specific surface area, N 2 gas and CO 2
The results when measured with gas are shown in (Table 1). The specific surface area due to N2 gas tends to be smaller than the added amount of the silane coupling agent, and tends to be larger with CO2 gas. Therefore, it can be considered that pores having small pores are increasing in proportion to the amount of addition.

[0034]

[Table 1]

When X-ray diffraction measurement was performed, only a broad peak due to the carbon layer structure was observed, but no peak due to the silicon compound was observed. When X-ray fluorescence analysis was performed to confirm the presence of silicon atoms, the presence of silicon in the carbon corresponding to the amount of addition was confirmed. From this, it is considered that silicon is atomically dispersed in carbon.

Further, when the size of the pores was estimated from the model formula based on the small-angle scattering measurement of X-rays, the size of the pores tended to become smaller in accordance with the added amount.

The carbon powder compounds having the above characteristics can have similar characteristics even if the type of the silane coupling agent and the type of the phenol resin are different from each other as long as mixing is good. In this case, it is necessary to optimize the heat treatment conditions and control the amount of the layer structure, pores, structural defects, and the like as the carbon structure that matches the desired battery characteristics.

The battery prepared for evaluating the battery characteristics of the carbon compound of the present invention is a coin type. For examining the electrode characteristics of the negative electrode material, a Li foil was used as a counter electrode.
The carbon electrode was carbonized and pulverized, and formed into a sheet shape on a copper foil using polyvinylidene fluoride as a binder. The particle size was adjusted so that the average particle size was 10 μm. The particle size distribution was measured by a laser diffraction method. As the basic composition of the electrolytic solution, a mixed solvent of ethylene carbonate and diethylene carbonate was used as a solvent, and LiPF ₆ was used as an electrolyte. A trial production of a battery was performed using the electrode thus manufactured, and the charge / discharge capacity was particularly evaluated. The charge / discharge amount was evaluated at the second cycle. Charging operation is constant current constant and constant voltage 1
0 mV was applied and the sample powder was sufficiently charged to perform the charging reaction. The discharge amount was measured at a constant current of 0.2 mA / cm ^2.
I went in.

(Example 1) As a negative electrode material, a composite of the above-mentioned resol type phenol resin and a silane coupling agent was used. Four types were prepared in which the added amount of the silane coupling agent was 0, 10, 20, and 30 wt%. Battery samples were prepared using these powders, and battery characteristics were evaluated. The results are shown in (Table 2).

[0040]

[Table 2]

Example 2 In a resol type phenol resin used as a raw material for a negative electrode material, the ratio of formaldehyde to phenol was 0.8, 1.0, 1.2, 1.4,
Eight types of studies were performed for 1.6, 1.8, 2.0, and 2.2. As the silane coupling material, one having the above-mentioned epoxy group and 20 wt% based on phenol was used. NaOH was used as a polymerization catalyst. After heat treatment at 600 ° C. in an N 2 gas atmosphere, pulverization was performed.
Further, a heat treatment at 1000 ° C. for 1 hour was performed in a N 2 gas. A battery sample was prepared using the carbon powder thus prepared, and the battery characteristics were measured in the same manner. The results are shown in (Table 3).

[0042]

[Table 3]

(Example 3) In the above phenol resin, the amount of the silane coupling agent added was 30% and the ratio of formaldehyde to phenol was 1.4.
In which no heat treatment was performed, in which 2H was performed at 180 ° C., and further in which 2H was performed at 180 ° C. after pulverization, 18
After performing 5 H at 0 ° C., heat treatment was performed at 500 ° C., and heat treatment was performed at 1200 ° C. for 1 H, respectively. A battery sample was prepared using the carbon powder thus prepared, and the battery characteristics were also measured. The results are shown in (Table 4).

[0044]

[Table 4]

As described above, the respective embodiments have been described for the purpose of explaining the effects of the claim factors, and are not limited to these conditions. In order to obtain the maximum capacity of each battery, it is necessary to find out the optimum conditions such as the composition ratio of each raw material, the amount of the additive, and the firing conditions.

[0046]

As described above, when carbonizing carbon powder used as a negative electrode material from an organic raw material by firing, control in a specific temperature range, selection of the raw material structure, control of the surface and pores of the carbon powder. By performing the above, a negative electrode carbon material having a large charge / discharge capacity for a Li-ion secondary battery can be provided. By using this negative electrode, a Li-ion secondary battery having a high energy density can be provided.

Claims (9)

[Claims] 1. A carbon material fired at 800 ° C. to 1500 ° C. in an inert gas atmosphere using a phenol resin as a raw material, wherein silicon is contained in an amount of 1 to 100% by weight based on carbon.
A carbon compound characterized by containing. 2. The carbon compound according to claim 1, wherein the phenol resin and the liquid silicon compound are mixed at room temperature and then calcined. 3. A silane compound having a reactive group of the silicon compound is used in a heat treatment at a temperature in the range of 150 ° C. to 220 ° C. for 2H or more after mixing the phenol resin and the silicon compound. 1 or Claim 2
The carbon compound as described above. 4. The silicon compound according to claim 1, wherein the silicon compound is a silane compound having any one of an amino group, a vinyl group, an epoxy group, a chloro group and a hydroxyl group. Carbon compounds. 5. A phenolic resin and a silicon compound are mixed, heat-treated at a temperature in the range of 150 ° C. to 220 ° C. for 2H or more, followed by pulverization, and then at 150 ° C. to 220 ° C. The method for producing a carbon compound according to any one of claims 1 to 4, wherein the heat treatment is repeated one or more times. 6. A phenol resin and a silicon compound are mixed and heat-treated at a temperature in the range of 150 ° C. to 220 ° C. for 2H or more, and then from 450 ° C. to 600 ° C. in an inert gas atmosphere or vacuum. The carbon according to claim 5, characterized in that the carbon is subjected to a thermal decomposition reaction treatment in the temperature range described above, followed by a pulverization treatment, and then calcined by a heat treatment at 800 ° C to 1500 ° C in an inert gas atmosphere or vacuum. A method for producing a compound. 7. The phenolic resin according to claim 5, wherein the resin is a resin type produced by an alkali catalyst at a ratio of formaldehyde to phenol of 1.0 to 2.0. A method for producing a carbon compound according to claim 6. 8. The method for producing a carbon compound according to claim 5, wherein the dispersion state of the silicon after mixing with the phenol resin and after the calcination is atomic. 9. An electrode for a secondary battery using the carbon compound according to claim 1 or a carbon compound produced by the production method.