JP7191766B2 - Method for producing non-graphitizable carbon material - Google Patents

Description

The present invention relates to a method for producing a non-graphitizable carbon material.

2. Description of the Related Art In recent years, due to growing global awareness of global environmental protection, hybrid vehicles (HEV, PHEV) and electric vehicles (EV) that can reduce the use of fossil fuels and reduce CO ₂ emissions are attracting attention. 2. Description of the Related Art As a power source for driving hybrid vehicles and electric vehicles, lithium-ion secondary batteries (LIB) have high energy density per volume and mass and are capable of being miniaturized. Currently, carbon materials are generally used as negative electrode materials for lithium ion secondary batteries. In addition to carbon, metals such as Si, Sn, Ti, and V having high energy densities, lithium salts of metal oxides, hybrid materials of carbon and metals, and the like are said to be under research.

Among carbon materials, graphite-based materials have been widely used in mobile electronic devices and the like because they generally have a high capacity. Graphite materials, which have high energy density, are the main materials used as negative electrode materials for automotive batteries, but non-graphitizable carbon materials, which have high input/output characteristics and cycle characteristics, are attracting attention. In particular, batteries for hybrid vehicles require high input/output characteristics for starting the vehicle and for obtaining regenerative energy, and life characteristics that enable repeated charging and discharging over a long period of time. Non-graphitizable carbon materials are suitable. there is

Regarding non-graphitizable carbon materials as negative electrode materials for lithium ion secondary batteries, those using petroleum-based pitch or coal-based pitch as raw materials have been reported (see, for example, Patent Documents 1 to 4).

JP-A-3-252053 JP-A-6-89721 JP-A-8-115723 JP-A-9-153359

The process for producing a non-graphitizable carbon material using petroleum-based pitch or coal-based pitch as a raw material can be roughly divided into a pitch cross-linking (oxidation) process, an infusibilization process, a firing process, and the like. A solvent extraction step using an organic solvent may be included between the step and the infusibilizing step.

The present inventors have investigated a method for producing such a non-graphitizable carbon material. As a result, it was found that the charging/discharging capacity of the carbon material to be obtained may increase when the firing process is performed after the solvent extraction treatment and the re-infusibility treatment after the infusibilization treatment. rice field.

The present invention has been made in view of the above points, and an object of the present invention is to provide a method for producing a non-graphitizable carbon material that increases the charge/discharge capacity of the obtained non-graphitizable carbon material.

As a result of intensive studies by the present inventors in order to achieve the above object, the charge-discharge capacity is was found to increase, and the present invention was completed.

That is, the present invention is the following [1].
[1] a cross-linking step of cross-linking the raw material of the non-graphitizable carbon material;
an infusibilization step of infusibilizing the crosslinked product obtained in the crosslinking step;
a solvent extraction step of mixing the infusibilized product obtained in the infusibilization step with a solvent and performing solvent extraction;
a re-infusibilization step of subjecting the solvent-extracted product obtained in the solvent-extraction step to infusibilization treatment again;
and a baking step of obtaining a non-graphitizable carbon material by baking the re-infusibilized product obtained in the re-infusible step.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the non-graphitizable carbon material which increases the charge/discharge capacity of the non-graphitizable carbon material obtained can be provided.

FIG. 3 is a cross-sectional view showing a coin-type secondary battery for evaluation;

In the present invention, when a range is indicated using "-", the range includes both ends of "-". For example, the range AB includes A and B.

[Method for producing non-graphitizable carbon material]
The method for producing a non-graphitizable carbon material of the present invention (hereinafter also simply referred to as “the production method of the present invention”) comprises a cross-linking step of cross-linking a raw material of the non-graphitizable carbon material, and a solvent extraction step of mixing the infusible product obtained in the infusibilizing step with a solvent for solvent extraction; and a firing step of firing the re-infusibilized product obtained in the re-infusibilizing step to obtain a non-graphitizable carbon material.
The manufacturing method of the present invention will be described in detail below.

[Crosslinking step]
In the cross-linking step, the raw material for the non-graphitizable carbon material (hereinafter also simply referred to as "raw material") is cross-linked. A crosslinked product is thus obtained.

Here, the raw material used in the production method of the present invention is not particularly limited, and conventionally known raw materials can be used, for example, pitch such as coal-based pitch and petroleum-based pitch; resin; a mixture of pitch and resin;

Examples of the method for cross-linking the raw material described above include a method using an air blowing reaction; a dry method using an oxidizing gas (air, oxygen, ozone); a wet method using an aqueous solution of nitric acid, sulfuric acid, hypochlorous acid, mixed acid, or the like; etc., and among them, a method using an air blowing reaction is preferred.

The air blowing reaction is a reaction to raise the softening point by heating the raw material described above and blowing in an oxidizing gas (eg, air, oxygen, ozone, or a mixture thereof). According to the air blowing reaction, it is possible to obtain a crosslinked product (for example, air blown pitch) having a high softening point of 200° C. or higher.

According to Patent Document 4, the air blowing reaction is a reaction in a liquid phase state, and it is known that almost no oxygen atoms are taken into the carbon material compared to a cross-linking treatment in a solid phase state. ing.
In the air blowing reaction, a reaction mainly composed of oxidative dehydration proceeds, and polymerization proceeds due to biphenyl-type cross-linking. Then, the non-graphitizable carbon material having a non-oriented three-dimensional structure in which the crosslinked portion becomes dominant in the subsequent infusibilization step and the firing step, leaving many voids in which lithium is occluded. is obtained.

The conditions for the air blowing reaction are not particularly limited, but the reaction temperature is preferably 280 to 420°C, more preferably 320 to 380°C, because mesophase is generated when the temperature is too high, and the reaction rate becomes slow when the temperature is too low. more preferred. The amount of oxidizing gas to be blown is preferably 0.5 to 15 L/min, more preferably 1.0 to 10 L/min, per 1000 g of pitch as compressed air. The reaction pressure may be normal pressure, reduced pressure, or increased pressure, and is not particularly limited.

The softening point of the cross-linked product such as air blown pitch obtained by such a cross-linking treatment is preferably 200 to 400° C., more preferably 250 to 350° C., because of the ease of the infusibility treatment in the subsequent infusibility treatment. is more preferred.

The obtained crosslinked product may be coarsely pulverized using an atomizer or the like before the infusibilizing step is performed.

[Infusibilization step]
In the infusibilization step, the crosslinked product such as air-blown pitch obtained in the crosslinking step is infusibilized. As a result, an infusibilized product (for example, infusible pitch) is obtained. The infusibilization treatment is a kind of cross-linking treatment (oxidation treatment) performed in a solid phase state. As a result, oxygen is taken into the structure of the cross-linked product, and further cross-linking progresses, making it difficult to melt at high temperatures. Become.

The method of infusibility treatment is not particularly limited, and examples thereof include a dry method using an oxidizing gas (air, oxygen); a wet method using an aqueous solution of nitric acid, sulfuric acid, hypochlorous acid, mixed acid, etc.; However, a dry method using an oxidizing gas is preferred.

The treatment temperature for the infusible treatment must be selected below the softening point of the crosslinked product. In the case of a batch method, the heating rate is preferably 5 to 100° C./hour, more preferably 10 to 50° C./hour, from the viewpoint of further preventing fusion.

Other treatment conditions in the infusible treatment are not particularly limited, but for example, the amount of oxidizing gas blown is preferably 1.0 to 20 L / min, more preferably 2.0 to 10 L / min, as compressed air per 1000 g. preferable. The reaction pressure may be normal pressure, reduced pressure, or increased pressure, and is not particularly limited.

[Solvent extraction step]
In the solvent extraction step, the infusibilized products such as infusibilized pitch obtained in the infusibilization step are mixed with a solvent and subjected to solvent extraction. This removes the volatile matter. Therefore, it is easy to expand the pore volume of the finally obtained non-graphitizable carbon material and improve the discharge capacity.
It is presumed that this is because the volatile matter is removed in advance from the infusibilized product before the re-infusibilizing step and the baking step, thereby preventing the pores from being clogged with the volatile matter during the baking step.

The organic solvent used for the solvent extraction treatment is not particularly limited, and examples thereof include washing oil, benzene, ethanol, and toluene. The amount of the organic solvent relative to the pitch is not particularly limited, but is preferably 1 to 5 equivalents. Although the extraction temperature is not particularly limited, it is 20 to 300°C, preferably 200 to 250°C. The extraction time is not particularly limited, but is 2 hours or longer, preferably 4 hours or longer. The pressure may be normal pressure, reduced pressure, or increased pressure, and is not particularly limited.

[Re-infusibilization step]
In the re-infusibilization step, the extracted product obtained in the solvent extraction step is again infusibilized. Specifically, a re-infusible product (for example, re-infusible pitch) is obtained by firing while circulating compressed air . The temperature reached in the re-infusibilization is not particularly limited, but is 150 to 300°C, preferably 200 to 250°C.

The oxygen content of the re-infusible product obtained by the re-infusibilizing treatment is preferably 3 to 20% by mass, more preferably 5 to 15% by mass, for the purpose of preventing fusion during firing.
The oxygen content of the re-infusibilized product can be measured, for example, by quantitative analysis using an elemental analyzer (FLASH2000, manufactured by Thermo Fisher Scientific).

[Baking process]
In the firing step, the re-infusible product obtained in the re-infusibilizing step is fired to obtain a non-graphitizable carbon material. Specifically, the non-graphitizable carbon material is obtained by firing under reduced pressure or in an inert gas atmosphere such as nitrogen. The attained temperature (firing temperature) in the firing step is 900 to 1300°C, preferably 1000 to 1200°C. At this time, the heating rate is preferably 50 to 150° C./hour, more preferably 80 to 120° C./hour.

[Non-graphitizable carbon material]
The non-graphitizable carbon material obtained by the production method of the present invention as described above (hereinafter also referred to as "non-graphitizable carbon material of the present invention") is suitable as a negative electrode material for lithium ion secondary batteries. Available.

In the non-graphitizable carbon material of the present invention, the specific surface area (BET) obtained by the BET method by adsorption of nitrogen gas varies depending on the particle size, so it cannot be said unconditionally, but it is said that the reactivity with the electrolyte is suppressed. For this reason, it is preferably 2 m ² /g or less, more preferably 1.5 m ² /g or less.

Next, a lithium ion secondary battery using the non-graphitizable carbon material of the present invention as a negative electrode material (hereinafter also referred to as "lithium ion secondary battery of the present invention") will be described.

[Lithium ion secondary battery]
Lithium-ion secondary batteries usually have a negative electrode, a positive electrode, and a non-aqueous electrolyte as the main battery components. Lithium ions move in and out in the process between the layers. This is a battery mechanism in which lithium ions are doped into the negative electrode during charging and dedoped from the negative electrode during discharging.
The lithium ion secondary battery of the present invention is not particularly limited except that the non-graphitizable carbon material of the present invention is used as the negative electrode material, and the other battery components are those of general lithium ion secondary batteries. comply.

[Negative electrode]
The method for producing a negative electrode from the non-graphitizable carbon material of the present invention is not particularly limited, and can be carried out according to a normal production method. When manufacturing a negative electrode, a negative electrode mixture obtained by adding a binder to the non-graphitizable carbon material of the present invention can be used. As the binder, it is preferable to use one having chemical stability and electrochemical stability with respect to the electrolyte, and it is usually preferable to use the binder in an amount of about 1 to 20% by mass based on the total amount of the negative electrode mixture. Polyvinylidene fluoride, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR) and the like can be used as binders.

Specifically, for example, the non-graphitizable carbon material of the present invention is mixed with a binder to prepare a paste-like negative electrode mixture paint, and this negative electrode mixture paint is usually applied to one side of a current collector. Alternatively, a negative electrode mixture layer is formed by coating both surfaces. In this case, a usual solvent can be used for paint preparation. The shape of the current collector used for the negative electrode is not particularly limited, and examples thereof include a foil shape; a net shape such as mesh and expanded metal; and the like. Examples of current collectors include copper, stainless steel, and nickel.

[Positive electrode]
As the positive electrode material (positive electrode active material), it is preferable to select a material capable of doping/dedoping a sufficient amount of lithium ions. Such positive electrode active materials include, for example, transition metal oxides, transition metal chalcogenides, vanadium oxides and their lithium-containing compounds, and the general formula MXMo ₆ S _8-y (wherein X is 0≤X≤4, Y is a numerical value in the range of 0 ≤ Y ≤ 1, and M represents a metal such as a transition metal). may be used, or two or more may be used in combination. For example, a carbonate such as lithium carbonate can be added to the positive electrode.

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more transition metals. Specifically, the lithium-containing transition metal oxide is LiM(1) _1-p M(2) _p O ₂ (wherein P is a numerical value in the range of 0≦P≦1, M(1), M (2) consists of at least one transition metal element), or LiM(1) _2-q M(2) _q O ₄ (wherein Q is a numerical value in the range of 0≤Q≤1, M(1 ) and M(2) consists of at least one transition metal element). Here, the transition metal elements represented by M include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc. Co, Fe, Mn, Ti, Cr, V , Al are preferred.
Such lithium-containing transition metal oxides are produced by, for example, using Li, transition metal oxides or salts as starting materials, mixing these starting materials according to the composition, and firing in an oxygen atmosphere at a temperature range of 600 to 1000 ° C. can be obtained by The starting materials are not limited to oxides or salts, and can be synthesized from hydroxides and the like.

As a method for forming a positive electrode using such a positive electrode material, for example, a paste-like positive electrode material mixture coating composed of a positive electrode material, a binder, and a conductive agent is applied to one or both surfaces of a current collector to form a positive electrode. form a layer. As the binder, those exemplified for the negative electrode can be used. As the conductive agent, for example, fine carbon materials, fibrous carbon materials, graphite, carbon black, and VGCF (vapor growth carbon fiber) can be used. The shape of the current collector is not particularly limited, and one having the same shape as the negative electrode is used. As the material of the current collector, aluminum, nickel, stainless steel foil, etc. can be usually used.

When forming the negative electrode and the positive electrode described above, conventionally known various additives such as a conductive agent and a binder can be appropriately used.

〔Electrolytes〕
As the electrolyte, a normal non-aqueous electrolyte containing a lithium salt such as LiPF ₆ or LiBF ₄ as an electrolyte salt is used.
The nonaqueous electrolyte may be a liquid nonaqueous electrolyte, or may be a polymer electrolyte such as a solid electrolyte or a gel electrolyte.

When a non-aqueous liquid electrolyte is used, a non-aqueous solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, or other aprotic organic solvent can be used.

When the polymer electrolyte is used, it contains a matrix polymer gelled with a plasticizer (non-aqueous electrolyte). Examples of the matrix polymer include ether-based polymers such as polyethylene oxide and its crosslinked products, polymethacrylate-based polymers, polyacrylate-based polymers, fluorine-based polymers such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymers. can be used alone or in combination, and among them, fluorine-based polymers are preferable from the viewpoint of oxidation-reduction stability and the like.
As the electrolyte salt and the non-aqueous solvent that constitute the plasticizer (non-aqueous electrolyte) contained in the polymer electrolyte, those that can be used in liquid electrolytes can be used.

In the lithium-ion secondary battery of the present invention, a separator such as a microporous body of polypropylene or polyethylene, or a layered structure thereof; a non-woven fabric; or the like is usually used. It is also possible to use gel electrolytes. In this case, for example, a negative electrode containing the non-graphitizable carbon material of the present invention, a gel electrolyte, and a positive electrode are stacked in this order and housed in a battery exterior material.
The structure of the lithium ion secondary battery of the present invention is arbitrary, and its shape and form are not particularly limited.

EXAMPLES The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these.

<Example 1>
(Crosslinking step)
A coal-based QI-less pitch (QI: 0.1 to 0.5% by mass, softening point: 82.5°C) was used as a raw material and crosslinked to obtain a crosslinked product. Specifically, an air blowing reaction was carried out at a temperature of 340° C. under reduced pressure to obtain a crosslinked product.
(Infusibilization step)
After the crosslinked product was pulverized to an average particle size of 20 to 25 μm, it was subjected to infusibilization treatment at a maximum temperature of 350° C. in a rotary kiln to obtain infusibilized pitch (infusibilized product).
(Solvent extraction step)
300 g of the obtained infusibilized pitch (infusibilized product) was mixed with 900 g (3 equivalents) of washing oil as an organic solvent, and subjected to solvent extraction in an autoclave under normal pressure conditions at a maximum temperature of 240° C. for 6 hours, Filtration was performed to obtain an extracted pitch (solvent extraction processed product).
(Re-infusibilization step)
The obtained extracted pitch (solvent-extracted product) was again subjected to an infusibilization treatment. Specifically, with respect to 100 g of the obtained extracted pitch (solvent extraction processed product), the temperature was raised at 25 ° C./hr while circulating compressed air at 5 L / min and 1000 g, and held at 250 ° C. for 6 hours. A re-infusible pitch (a re-infusible product) was obtained by applying the infusible treatment (re-infusible treatment).
(Baking process)
The obtained re-infusible pitch (re-infusible product) is placed in a graphite container, heated to 1100° C. at a rate of 140° C./hour under a nitrogen stream, and fired at 1100° C. for 2 hours. A non-graphitizable carbon material was obtained.

<Comparative Example 1>
In Example 1, the infusible pitch (infusibilized product) obtained in the infusible step was not subjected to the solvent extraction step and the re-infusibilizing step, but the firing step was performed under the same conditions as in Example 1. A graphitizable carbon material was obtained.

<Comparative Example 2>
In Example 1, the extracted pitch (solvent extraction treated product) obtained in the solvent extraction step was not subjected to the re-infusibilization step, but was subjected to the firing step under the same conditions as in Example 1 to obtain a non-graphitizable carbon material. got

<Comparative Example 3>
In Example 1, the infusible pitch (infusibilized product) obtained in the infusibilizing step was subjected to the re-infusibilizing step without performing the solvent extraction step. Specifically, the infusible pitch (infusibilized product) was placed in a rotary furnace, and the temperature was raised at 600°C/hour while circulating compressed air at 5L/min and 1000g, and held at 300°C for 10 hours. A re-infusible pitch (a re-infusible product) was obtained by applying the infusibilizing treatment (re-infusible treatment) again with the The obtained re-infusible pitch (re-infusible product) was subjected to the firing process under the same conditions as in Example 1 to obtain a non-graphitizable carbon material.

<Evaluation>
(Evaluation of non-graphitizable carbon material)
First, the non-graphitizable carbon materials obtained in Example 1 and Comparative Examples 1 to 3 were measured for average particle size (unit: μm) and specific surface area (unit: m ² /g) by the following procedure. The results are shown in Table 1 below.
In addition, in Example 1 and Comparative Examples 1 to 3, the oxygen content (unit :% by mass) was measured by the following procedure. The results are shown in Table 1 below.

<Average particle size>
The cumulative frequency of the particle size distribution measured by a laser diffraction particle size distribution meter (LMS-2000e, manufactured by Seishin Enterprise Co., Ltd.) was defined as the particle size (median size, 50% particle size) at which the volume percentage is 50%.

<Specific surface area>
Using a powder analyzer (MONOSORB (registered trademark), manufactured by Quantachrome), it was determined by the BET one-point method by nitrogen gas adsorption.

<Oxygen content>
It was measured by quantitative analysis using an elemental analyzer (FLASH2000, manufactured by Thermo Fisher Scientific).

Next, using the non-graphitizable carbon materials obtained in Example 1 and Comparative Examples 1 to 3 as negative electrode materials, coin-type secondary batteries for evaluation (see FIG. 1) were produced, and various evaluations were performed. rice field.

(Preparation of negative electrode mixture paste)
First, a negative electrode mixture paste was prepared using the obtained non-graphitizable carbon material as a negative electrode material. Specifically, carbon powder (96 parts by mass) and a 12% N-methylpyrrolidinone solution of polyvinylidene difluoride (4 parts by mass in terms of solid content) were placed in a planetary mixer and stirred at 100 rpm for 15 minutes. Furthermore, N-methylpyrrolidinone was added to adjust the solid content ratio to 60% by mass, followed by stirring for 15 minutes to prepare a negative electrode mixture paste.

(Preparation of working electrode (negative electrode))
The prepared negative electrode mixture paste was applied on a copper foil to a uniform thickness, and placed in an air drier to volatilize the solvent at 120° C. to form a negative electrode mixture layer. Next, the negative electrode mixture layer was pressurized with a hand press, and then punched out into a circular shape with a diameter of 15.5 mm to fabricate a working electrode (negative electrode) having a negative electrode mixture layer adhered to a current collector made of copper foil. did. Before the evaluation, drying was performed at 100° C. in vacuum for 5 hours or longer.
The electrode density of the produced working electrode (negative electrode) was measured by the following procedure.

<Electrode density>
After drying, the electrode mass and thickness were measured, and the electrode density (unit: g/cm ³ ) was determined from the negative electrode mixture layer volume determined from the mass and thickness excluding the copper foil mass.

(Preparation of electrolytic solution)
LiPF ₆ was dissolved in a mixed solvent obtained by mixing ethylene carbonate (33% by volume) and methyl ethyl carbonate (67% by volume) at a concentration of 1 mol/dm ³ to prepare a non-aqueous electrolyte.

(Production of evaluation battery)
Next, using the prepared working electrode (negative electrode), a coin-type secondary battery for evaluation (also referred to simply as “evaluation battery”) shown in FIG. 1 was prepared. FIG. 1 is a cross-sectional view showing a coin-type secondary battery for evaluation.
First, a lithium metal foil was pressed against a nickel net and punched out into a circular shape with a diameter of 15.5 mm to prepare a disc-shaped counter electrode 4 made of lithium foil and in close contact with the current collector 7a made of nickel net.
Next, the separator 5 impregnated with the electrolyte solution is sandwiched between the working electrode (negative electrode) 2 in close contact with the current collector 7b and the counter electrode 4 in close contact with the current collector 7a. is housed in the outer cup 1, the counter electrode 4 is housed in the outer can 3, the outer cup 1 and the outer can 3 are put together, and the peripheral portions of the outer cup 1 and the outer can 3 are crimped via the insulating gasket 6. , to produce an evaluation battery.
In the fabricated evaluation battery, the outer cup 1 and the outer can 3 are crimped at their peripheral edges via an insulating gasket 6 to form a sealed structure. Inside the sealed structure, as shown in FIG. 1, a current collector 7a, a counter electrode 4, a separator 5, a working electrode (negative electrode) 2, and a A current collector 7b is laminated.

(Charging and discharging test)
The following charging/discharging test was performed at 25° C. on the produced evaluation battery. In this test, the process of doping lithium ions into the carbon powder was defined as "charging", and the process of dedoping the carbon powder was defined as "discharging".
First, constant-current charging was performed with a current value of 0.39 mA until the circuit voltage reached 0 mV. When the circuit voltage reached 0 mV, the charging was switched to constant-voltage charging, and charging was continued until the current value reached 20 μA. . The first charge capacity (unit: mAh/g) was obtained from the amount of electricity supplied during that time. Then rested for 120 minutes. Next, constant-current discharge was performed at a current value of 0.39 mA until the circuit voltage reached 1.5 V, and the first discharge capacity (unit: mAh/g) was obtained from the amount of electricity supplied during this period.

<Initial efficiency>
From the results of the charge/discharge test, the initial efficiency (unit: %) was determined based on the following formula.
Initial efficiency = (first discharge capacity/first charge capacity) x 100

<Irreversible capacity>
From the results of the charge/discharge test, the irreversible capacity (unit: mAh/g) was determined according to the following formula.
Irreversible capacity = (first discharge capacity) - (first charge capacity)

As is clear from the results shown in Table 1 above, in Example 1, the charge capacity and discharge capacity are increased as compared with the normal fired product of Comparative Example 1 through the solvent extraction step. In Comparative Example 2, calcination was performed after the solvent extraction process without going through the re-infusibilization process, but no increase in charge capacity and discharge capacity was confirmed. In Comparative Example 3, only the re-infusibilization step was performed without the solvent extraction step to increase the oxygen concentration of the firing raw material, but the charge capacity and discharge capacity were lower than those of Example 1. Therefore, it was found that the charge/discharge capacity was increased by performing the solvent extraction step and the re-infusibilization step after the infusibilization step.

REFERENCE SIGNS LIST 1 outer cup 2 working electrode 3 outer can 4 counter electrode 5 separator 6 insulating gasket 7a current collector 7b current collector

Claims (1)

a cross-linking step in which an oxidizing gas is blown into the raw material of the non-graphitizable carbon material in a liquid phase state and reacted at 280 to 420° C. to cross-link the raw material of the non-graphitizable carbon material;
an infusibilization step of infusibilizing the crosslinked product obtained in the crosslinking step by blowing an oxidizing gas into the solid state of the crosslinked product to take in oxygen ;
a solvent extraction step of mixing the infusibilized product obtained in the infusibilization step with a solvent and performing solvent extraction;
a re-infusibilization step in which the solvent-extracted product obtained in the solvent-extraction step is again infusibilized while circulating compressed air at 150 to 300°C ;
and a firing step of obtaining a non-graphitizable carbon material by firing the re-infusibilized product obtained in the re-infusibilization step at 900 to 1300° C. in an inert gas atmosphere .

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