Conductive carbon and its manufacturing method, manufacturing method of conductive carbon mixture containing the conductive carbon, and manufacturing method of electrode using the conductive carbon or conductive carbon mixture

Abstract

To provide a conductive carbon which enables achievement of a power storage device having high energy density and having improved charge / discharge cycle stability even in high-voltage application.SOLUTION: A conductive carbon is produced by a method comprising: an oxidation step of performing an oxidizing process on a carbon material, thereby obtaining an oxidation-processed carbon having a property of expanding like a paste when receiving a pressure; and a heat-processing step of performing a heat process on the oxidation-processed carbon in vacuum or in a non-oxidizing atmosphere. In the method, CO2 desorbs from the oxidation-processed carbon during the heat process and as such, an amount of CO2 generated from resultant conductive carbon in a range of 25-200°C is reduced in comparison with an amount of CO2 generated from the oxidation-processed carbon in the range of 25-200°C. It is preferable to perform the heat process in such a way that the amount of CO2 generated from resultant conductive carbon in the range of 25-200°C falls in a range of 0.015-0.050 mass% of a whole of the conductive carbon.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a conductive carbon used for an electricity storage device having a high energy density and suppressing deterioration of charge / discharge cycle characteristics due to application of a high voltage, a method for producing the conductive carbon, and a method for producing an electrode for an electricity storage device using the conductive carbon. The present invention also relates to a method for producing a conductive carbon mixture containing the conductive carbon, and a method for producing an electrode for an electricity storage device using the conductive carbon mixture. [Background technology]

[0002] Power storage devices such as secondary batteries, electric double layer capacitors, redox capacitors, and hybrid capacitors are devices whose applications are being considered widely as power sources for information devices such as mobile phones and notebook computers, motor drive power sources for low-pollution vehicles such as electric vehicles and hybrid vehicles, and energy regeneration systems. However, in order to meet the demand for higher performance and smaller size of these power storage devices, there is a demand for improving the energy density.

[0003] In these power storage devices, electrode active materials that exhibit capacity through a faradaic reaction involving the transfer of electrons with ions in an electrolyte (including an electrolytic solution) or a non-faradaic reaction not involving the transfer of electrons are used for energy storage. These active materials are generally used in the form of a composite material with a conductive agent. As the conductive agent, conductive carbons such as carbon black, natural graphite, artificial graphite, and carbon nanotubes are usually used. These conductive carbons are used in combination with active materials with low conductivity to impart conductivity to the composite material, but they also act as a matrix that absorbs volume changes associated with the reaction of the active material, and also play a role in ensuring an electronic conduction path even if the active material is mechanically damaged.

[0004] By the way, these composite materials of active material and conductive carbon are generally manufactured by a method of mixing active material particles with conductive carbon. Since conductive carbon does not basically contribute to improving the energy density of an electricity storage device, in order to obtain an electricity storage device with a high energy density, it is necessary to reduce the amount of conductive carbon per unit volume and increase the amount of active material. Therefore, studies have been conducted to increase the amount of active material per unit volume by improving the dispersibility of conductive carbon or reducing the structure of conductive carbon to reduce the distance between active material particles, but it is difficult to efficiently insert the conductive carbon as described above between active material particles, and therefore it is difficult to reduce the distance between active material particles to increase the amount of active material per unit volume.

[0005] In response to this problem, the applicant has proposed to use conductive carbon, which can be obtained by subjecting carbon raw materials to a strong oxidation treatment and has the property of spreading into a paste-like shape when subjected to pressure, for the electrodes of electricity storage devices (Patent Documents 1 to 15). Here, "paste-like" refers to a state in which the grain boundaries of primary carbon particles are not visible in an SEM photograph taken at a magnification of 25,000 times, and non-particulate, amorphous carbon is connected together.

[0006] Even when pressure is applied to conductive carbon such as carbon black, natural graphite, and carbon nanotubes, which are used as conductive agents in the electrodes of conventional power storage devices, the particle shape of the carbon is maintained. However, when these conductive carbons are subjected to an oxidation treatment, the surface of the particles is oxidized, and hydroxyl groups, carboxyl groups, carbonyl groups, ether bonds, etc. are introduced into the carbon, and the conjugated double bonds of the carbon are oxidized to form carbon single bonds, and carbon-carbon bonds are partially broken. By further increasing the intensity of the oxidation treatment, a conductive carbon can be obtained that has the property of spreading out like a paste under pressure.

[0007] The conductive carbon, which has the property of spreading like a paste under pressure, is characterized in that it easily adheres to the surface of the electrode active material particles, and when pressure is applied, it is compressed together and spreads like a paste, and is not easily broken apart. Therefore, when the conductive carbon and the electrode active material particles are mixed to obtain a mixture for an electrode of an electric storage device, the conductive carbon adheres to the surface of the active material particles during the mixing process, covering the surface, and improving the dispersibility of the active material particles. If the pressure applied to the conductive carbon during the production of the mixture is large, at least a part of the conductive carbon spreads like a paste and partially covers the surface of the active material particles. Then, when an active material layer is formed on the current collector of the electrode using this mixture and pressure is applied to the active material layer, the conductive carbon further spreads like a paste and densifies while covering the surface of the electrode active material particles, and the active material particles approach each other, and accordingly, the conductive carbon covers the surface of the active material particles and is pushed out into not only the gaps formed between adjacent active material particles but also the insides of the holes (including the gaps between primary particles recognized in secondary particles) present on the surface of the active material particles, and is densely filled. Therefore, the amount of electrode active material per unit volume in the electrode is increased, and the electrode density is increased. In addition, the densely packed conductive carbon paste has sufficient conductivity to function as a conductive agent and does not inhibit the impregnation of the electrolyte in the power storage device. As a result, the energy density of the power storage device is improved.

[0008] For example, Patent Document 6 shows the results of evaluating the content of hydrophilic parts in the conductive carbon. Here, the "hydrophilic part" of the conductive carbon has the following meaning. That is, 0.1 g of conductive carbon is added to 20 mL of an aqueous ammonia solution of pH 11, ultrasonic irradiation is performed for 1 minute, and the resulting liquid is left for 5 hours to precipitate the solid phase part. The part that does not precipitate and is dispersed in the aqueous ammonia solution of pH 11 is the "hydrophilic part". The content of the hydrophilic part relative to the entire conductive carbon is determined by the following method. After the precipitation of the solid phase part, the remaining part after removing the supernatant liquid is dried, and the weight of the solid after drying is measured. The weight obtained by subtracting the weight of the solid after drying from the initial weight of the conductive carbon of 0.1 g is the weight of the "hydrophilic part" dispersed in the aqueous ammonia solution of pH 11. The weight ratio of the weight of the "hydrophilic part" to the weight of the initial weight of the conductive carbon of 0.1 g is the content of the "hydrophilic part" in the conductive carbon.

[0009] Patent Document 6 shows that when the intensity of the oxidation treatment is increased so that the content of the hydrophilic portion exceeds 8 mass% of the entire conductive carbon, the electrode density of the electrode manufactured using the obtained conductive carbon begins to increase, and when the content of the hydrophilic portion exceeds 9 mass% of the entire conductive carbon, the electrode density begins to increase rapidly, and when the content of the hydrophilic portion exceeds 10 mass% of the entire conductive carbon, an extremely high electrode density is obtained (see Figure 2 of this document). This improvement in electrode density corresponds to the fact that the conductive carbon is given a property of spreading like a paste when pressure is applied as the intensity of the oxidation treatment is increased. In addition, it has been shown that when a lithium ion secondary battery having a positive electrode containing acetylene black as conductive carbon and a positive electrode active material and a lithium counter electrode and a lithium ion secondary battery having a positive electrode in which half or all of the acetylene black is replaced with the conductive carbon obtained by the above-mentioned strong oxidation treatment are constructed and the charge and discharge characteristics are evaluated, the latter shows better rate characteristics and charge and discharge cycle characteristics than the former (see Figures 6 to 11, 13, and 14 of this document). This improved charge-discharge cycle characteristic is believed to be due to the fact that the surface of the electrode active material is coated with conductive carbon spread in a paste-like form, thereby suppressing the dissolution of the active material in the electrolyte (see Table 1 in this document). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2015-079678 A [Patent Document 2] JP 2015-079680 A [Patent Document 3] JP 2015-079681 A [Patent Document 4] WO2015 / 056759 issue [Patent Document 5] WO2015 / 056760 issue [Patent Document 6] WO2015 / 133586 [Patent Document 7] JP 2015-181089 A [Patent Document 8] JP 2015-181090 A [Patent Document 9] JP 2016-001592 A [Patent Document 10] JP 2016-096125 A [Patent Document 11] JP 2019-019014 A [Patent Document 12] JP 2019-021420 A [Patent Document 13] JP 2019-021421 A [Patent Document 14] JP 2019-021427 A [Patent Document 15] JP 2019-200866 A [Non-patent literature]

[0011] [Non-Patent Document 1] Carbon 37(1999) 1379-1389 Summary of the Invention [Problem to be solved by the invention]

[0012] As can be seen from FIG. 7 of Patent Document 6, LiNi 0.5 Mn 0.3 Co 0.2Regarding a lithium ion secondary battery having a positive electrode containing O2 and acetylene black as conductive carbon, a 1:1 solution of 1M LiPF6 as an electrolyte, and lithium as a counter electrode, and a lithium ion secondary battery having a positive electrode in which half of the acetylene black is replaced with conductive carbon obtained by the above-mentioned strong oxidation treatment, when charging and discharging are repeated in the range of 4.6 to 3.0 V under conditions of 60°C and a charge / discharge rate of 0.5 C, the latter shows better charge / discharge cycle stability than the former. However, in this evaluation, lithium is used as the counter electrode, and it can be said that it is an evaluation of a half cell. Therefore, in order to evaluate the performance as a practical lithium ion secondary battery, LiNi as a positive electrode active material (details will be described later). 0.5 Mn 0.3 Co 0.2 A lithium ion secondary battery was constructed having a positive electrode containing O2 and conductive carbon, an electrolyte in which 1M LiPF6 and 1 wt% vinylene carbonate as an additive were dissolved in a 1:1 mixed solvent of ethylene carbonate / dimethyl carbonate, and a negative electrode containing graphite, and a charge / discharge cycle test was performed in the range of 4.5V to 2.5V under the conditions of 45°C and a charge / discharge rate of 0.6C. In a general lithium ion secondary battery using the above positive electrode active material, the upper cutoff value is generally set to about 4.2V, but in consideration of the recent demand for high energy density in lithium ion secondary batteries, the upper cutoff value was set to 4.5V, which is higher than the general value, in the above test to evaluate the charge / discharge cycle performance.

[0013] FIG. 1 shows the results of the above-mentioned charge-discharge cycle test. The conductive carbon used for the manufacture of the positive electrode of the battery of Comparative Example 2 is acetylene black, and in the manufacture of the positive electrode of the battery of Comparative Example 1, half of the acetylene black is replaced with the conductive carbon obtained by the above-mentioned strong oxidation treatment. As is clear from FIG. 1, the battery of Comparative Example 1 had a higher capacity retention rate than the battery of Comparative Example 2 until 25 cycles of charge and discharge were repeated, but from about 30 cycles of charge and discharge, it showed a capacity retention rate equivalent to that of the battery of Comparative Example 2, and from about 60 cycles of charge and discharge, it showed a capacity retention rate lower than that of the battery of Comparative Example 2. Therefore, when an electrode is formed using conductive carbon having a property of spreading like a paste under pressure obtained by a strong oxidation treatment, a power storage device having a high energy density can be obtained, but in order to provide a practical power storage device, it is desired to improve it so that it shows stable charge-discharge cycle characteristics even when a high voltage is applied.

[0014] Therefore, an object of the present invention is to provide a conductive carbon that leads to an electricity storage device having a high energy density and improved charge / discharge cycle stability even when a high voltage is applied, and a method for producing the same. [Means for solving the problem]

[0015] The inventors considered that the cause of the decrease in the capacity retention rate of the lithium-ion secondary battery of Comparative Example 1 was leakage current that may occur in the positive electrode when a high voltage was applied. If leakage current occurs in the positive electrode, it is believed that the leakage current will cause an SEI film to form on the surface of the graphite of the negative electrode, thereby decreasing the capacity of the battery. It is predicted that leakage current in the positive electrode is more likely to occur when there are many substituents on the surface of the conductive carbon.

[0016] Therefore, the inventors next tried to reduce the number of substituents on the surface of the conductive carbon used in the positive electrode of the battery of Comparative Example 1 by heat-treating the conductive carbon in a nitrogen atmosphere at a temperature in the range of 200 to 900°C. Then, a positive electrode was constructed using the conductive carbon obtained by the heat treatment, and the performance of a battery equipped with this positive electrode was evaluated. It was found that even the conductive carbon heat-treated at 200°C provided an improved capacity retention rate in a charge-discharge cycle test under high voltage application (see FIG. 2). The inventors further investigated the change in the amount of substituents on the surface of the conductive carbon that provided the improved capacity retention rate using a temperature-programmed desorption photoionization mass spectrometry (TPD-PIMS) device (see FIG. 3). As a result, it was found that the amount of CO2 generated from the conductive carbon in the range of 25 to 200°C also varies greatly depending on the heat treatment at 200°C. It is known that CO2 generation from carbon in the range of 25 to 200°C is mainly due to detachment from the carboxyl groups on the surface (see Non-Patent Document 1), and this detachment from the carboxyl groups on the surface was almost completed by heat treatment at 250°C. In consideration of these results, it was determined that in order to obtain improved charge-discharge cycle stability when a high voltage is applied, it would be useful to specify conductive carbon focusing on the amount of CO2 generation in the range of 25 to 200°C.

[0017] Therefore, the present invention first provides a method for producing conductive carbon to be used as a conductive agent in an electrode of an electricity storage device, comprising the steps of: An oxidation step in which a carbon raw material is oxidized to obtain an oxidized carbon having a property of spreading like a paste under pressure; and a heat treatment step in which the oxidized carbon is heat-treated in a vacuum or a non-oxidizing atmosphere to reduce the amount of CO2 generated from the resulting conductive carbon in the range of 25 to 200°C to less than the amount of CO2 generated from the oxidized carbon in the range of 25 to 200°C; The present invention relates to a method for producing conductive carbon, comprising the steps of:

[0018] Here, "the amount of CO2 generated from carbon in the range of 25 to 200°C" has the following meaning. An experiment is carried out to measure the amount of CO2 generated while heating 30 mg of the carbon to be measured using a TPD-PIMS device under conditions of a helium atmosphere at atmospheric pressure (helium flow rate 300 mL / min), a temperature range of room temperature to 1200°C, and a heating rate of 20°C / min. The total amount (integral value) of CO2 generated in the temperature range of 25 to 200°C calculated from the obtained experimental results is the "amount of CO2 generated from carbon in the range of 25 to 200°C."

[0019] In the above-mentioned oxidation step, it is preferable to carry out the oxidation treatment so that the content of hydrophilic portions in the obtained oxidation treated carbon is 10 mass% or more of the entire oxidation treated carbon, since this significantly enhances the property of the oxidation treated carbon of spreading into a paste-like form under pressure.

[0020] Furthermore, in the above heat treatment step, if the heat treatment is carried out so that the amount of CO2 generated from the obtained conductive carbon in the range of 25 to 200°C is in the range of 0.015 to 0.050 mass% of the entire conductive carbon, the charge / discharge cycle stability during application of a high voltage is particularly improved, which is preferable. Therefore, the present invention also relates to a conductive carbon to be used as a conductive agent in an electrode of an electricity storage device, which can be produced by undergoing the above heat treatment step, and which has the property of spreading in a paste-like form under pressure, and is characterized in that the amount of CO2 generated from the conductive carbon in the range of 25 to 200°C is in the range of 0.015 to 0.050 mass% of the entire conductive carbon.

[0021] The oxidized carbon obtained in the oxidation step can be used as a conductive agent in the form of a conductive carbon mixture in which it is mixed with another conductive carbon having a higher conductivity than that of the oxidized carbon, and it has been found that the use of this conductive carbon mixture further improves the conductivity of the electrode (see Patent Document 10). In obtaining a conductive carbon mixture, it is convenient to mix the oxidized carbon with the other conductive carbon and then subject the mixture to heat treatment. Therefore, the present invention also relates to a method for producing a conductive carbon mixture to be used as a conductive agent in an electrode of an electricity storage device, comprising: An oxidation step in which a carbon raw material is oxidized to obtain an oxidized carbon having a property of spreading like a paste under pressure; a mixing step of mixing the oxidized carbon with another conductive carbon having a higher conductivity than that of the oxidized carbon to obtain a preliminary carbon mixture in which at least a part of the oxidized carbon spreads in a paste-like form and coats a surface of the other conductive carbon; and a heat treatment step in which the amount of CO2 generated from the obtained conductive carbon mixture in the range of 25 to 200°C is reduced by heat treating the preliminary carbon mixture in a vacuum or a non-oxidizing atmosphere, compared with the amount of CO2 generated from the preliminary carbon mixture in the range of 25 to 200°C; The oxidation treatment in the oxidation step is preferably carried out so that the content of hydrophilic parts in the obtained oxidized carbon is 10 mass % or more of the total oxidized carbon.

[0022] The pressure applied to the oxidized carbon in the mixing step causes at least a portion of the oxidized carbon to spread into a paste-like form, and this paste-like spread form is maintained even after the heat treatment step. The portion of the conductive carbon mixture obtained after the heat treatment step that is derived from the oxidized carbon is referred to in this specification as "carbon derived from the oxidized carbon."

[0023] By constructing an electrode of an electricity storage device using the conductive carbon or conductive carbon mixture produced by the production method of the present invention, it is possible to obtain an electricity storage device having a high energy density and improved charge / discharge cycle stability even when a high voltage is applied.

[0024] Therefore, the present invention also provides a method for producing an electrode for an electricity storage device, comprising the steps of: a mixing step of mixing the electrode active material particles with the conductive carbon obtained by the above-mentioned method for producing conductive carbon to obtain an electrode mixture in which at least a part of the conductive carbon spreads in a paste-like form to cover the surfaces of the electrode active material particles; and a pressurizing step in which the electrode mixture is applied onto a current collector for the electrode to form an active material layer, and pressure is applied to the active material layer to further spread the conductive carbon into a paste-like form and densify it; The present invention relates to a method for producing an electrode, comprising the steps of:

[0025] The present invention further provides a method for producing an electrode for an electricity storage device, comprising the steps of: electrode active material particle a mixing step of mixing the conductive carbon mixture obtained by the above-mentioned method for producing a conductive carbon mixture with the above-mentioned oxidized carbon to obtain a mixture for an electrode in which at least a part of the carbon that is spread in a paste-like form and originates from the oxidized carbon covers not only the surface of the separate conductive carbon but also the surface of the electrode active material particles; and a pressurizing step of applying the electrode mixture onto a current collector for the electrode to form an active material layer, and applying pressure to the active material layer to further spread and densify at least a portion of the carbon derived from the oxidized carbon, the carbon being spread in a paste-like form; The present invention relates to a method for producing an electrode, comprising the steps of: Effect of the Invention

[0026] Since the conductive carbon obtained by the present invention has the property of easily adhering to the surface of active material particles and being compressed integrally and spread into a paste-like form when pressure is applied, when pressure is applied to a mixture containing active material particles and the conductive carbon in the manufacture of electrodes for an electricity storage device, the pressure causes the conductive carbon to spread into a paste-like form and densify while covering the surfaces of the active material particles, thereby improving the energy density of the electricity storage device. Furthermore, an electricity storage device equipped with an electrode containing the conductive carbon obtained by the present invention exhibits improved charge-discharge cycle stability even when a high voltage is applied. [Brief description of the drawings]

[0027] [Figure 1] FIG. 1 shows the results of a charge-discharge cycle stability test performed on a lithium ion secondary battery having a positive electrode in which acetylene black is used as a conductive agent, and a lithium ion secondary battery having a positive electrode in which a mixture in which half of the acetylene black is replaced with oxidized carbon is used as a conductive agent. [Diagram 2] FIG. 1 shows the results of a charge-discharge cycle stability test performed on a lithium ion secondary battery having a positive electrode in which acetylene black is used as a conductive agent, and a lithium ion secondary battery having a positive electrode in which a mixture in which half of the acetylene black is replaced with conductive carbon obtained by subjecting oxidized carbon to a heat treatment is used as a conductive agent. [Diagram 3] This figure shows the results of measuring the amount of CO2 generated from a preliminary carbon mixture and a conductive carbon mixture obtained by subjecting the preliminary carbon mixture to heat treatment, using a TPD-PIMS device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] (I) Conductive carbon and its manufacturing method The conductive carbon for the electrode of the electricity storage device of the present invention is An oxidation step in which a carbon raw material is oxidized to obtain an oxidized carbon having a property of spreading like a paste under pressure; and a heat treatment step in which the oxidized carbon is heat-treated in a vacuum or a non-oxidizing atmosphere to reduce the amount of CO2 generated from the resulting conductive carbon in the range of 25 to 200°C to less than the amount of CO2 generated from the oxidized carbon in the range of 25 to 200°C; Each step will be described in detail below.

[0029] (I-1) Oxidation stage In the oxidation step, the conductive carbon raw material is subjected to a relatively strong oxidation treatment. The carbon raw material used may be any conductive carbon used as a conductive agent for electrodes of conventional power storage devices, such as carbon blacks, such as Ketjen black, acetylene black, furnace black, and channel black, fullerene, carbon nanotubes, carbon nanofibers, graphene, amorphous carbon, carbon fibers, natural graphite, artificial graphite, graphitized Ketjen black, mesoporous carbon, and vapor-phase grown carbon fibers. From the viewpoint of ease of oxidation, however, porous carbon powder, Ketjen black, furnace black with voids, carbon nanofibers, and carbon nanotubes with voids are preferred. Among them, conductive carbons having a specific surface area of ​​300 m2 measured by the BET method are preferred. 2 Conductive carbon having voids of 1000 nm or more per 1000 nm is preferred, and spherical conductive carbon such as Ketjen black and furnace black having voids is particularly preferred.

[0030] For the oxidation treatment of the carbon raw material, a known oxidation method can be used without any particular limitation. For example, the carbon raw material can be treated in a solution of an acid or hydrogen peroxide to obtain the oxidation-treated carbon. As the acid, nitric acid, a mixture of nitric acid and sulfuric acid, an aqueous solution of hypochlorous acid, etc. can be used. The carbon raw material can be heated in an oxygen-containing atmosphere, water vapor, or carbon dioxide to obtain the oxidation-treated carbon. Furthermore, the carbon raw material can be mixed with an alkali metal hydroxide, heated in an oxygen-containing atmosphere, and the alkali metal can be removed by washing with water or the like to obtain the oxidation-treated carbon. The carbon raw material can be subjected to a plasma treatment, ultraviolet light irradiation, corona discharge treatment, and glow discharge treatment in an oxygen-containing atmosphere, a treatment with ozone water or ozone gas, or an oxygen bubbling treatment in water to obtain the oxidation-treated carbon.

[0031] When a carbon raw material, preferably a carbon raw material having the above-mentioned voids, is subjected to oxidation treatment, the surface of the carbon particles is oxidized, hydroxyl groups, carboxyl groups, and ether bonds are introduced into the carbon, and the conjugated double bonds of the carbon are oxidized to form carbon single bonds, and carbon-carbon bonds are partially cut, resulting in the formation of hydrophilic parts on the particle surface. Then, by increasing the intensity of the oxidation treatment, the proportion of hydrophilic parts in the carbon particles increases, and it is possible to obtain oxidation-treated carbon that has the property of spreading like a paste under pressure. The content of hydrophilic parts in the oxidation-treated carbon is preferably 10% by mass or more of the entire oxidation-treated carbon. Such oxidation-treated carbon is easily compressed as a whole and spreads like a paste under pressure, and is easily densified.

[0032] Oxidized carbon containing 10% or more by mass of hydrophilic moieties is (a1) treating a porous carbon raw material with an acid; (b1) mixing the product after the acid treatment with a transition metal compound; (c1) grinding the mixture to cause a mechanochemical reaction; (d1) heating the product of the mechanochemical reaction in a non-oxidizing atmosphere; and (e1) A step of removing the transition metal compound and / or its reaction product from the product after heating. The compound can be suitably obtained by the first production method comprising the steps of:

[0033] In step (a1), the carbon raw material having voids is immersed in acid and left to stand. Ultrasonic irradiation may be used during this immersion. The acid may be an acid normally used in the oxidation treatment of carbon, such as nitric acid, a mixture of nitric acid and sulfuric acid, or an aqueous solution of hypochlorous acid. The immersion time depends on the concentration of the acid and the amount of the carbon raw material to be treated, but is generally in the range of 5 minutes to 5 hours. After the acid treatment, the carbon is thoroughly washed with water and dried, and then mixed with a transition metal compound in step (b1).

[0034] As the transition metal compound added to the carbon raw material in the (b1) step, inorganic metal salts such as halides, nitrates, sulfates, and carbonates of transition metals, organic metal salts such as formates, acetates, oxalates, methoxides, ethoxides, and isopropoxides, or mixtures thereof can be used. These compounds may be used alone or in combination of two or more. Compounds containing different transition metals may be mixed and used in a predetermined amount. In addition, as long as they do not adversely affect the reaction, compounds other than the transition metal compound, such as alkali metal compounds, may be added together. Since the conductive carbon produced by the present invention is used by mixing with active material particles in the production of electrodes for electricity storage devices, it is preferable to add compounds of elements constituting the active material to the carbon raw material, since this can prevent the inclusion of elements that may become impurities in the active material.

[0035] In step (c1), the mixture obtained in step (b1) is pulverized to cause a mechanochemical reaction. Examples of pulverizers for this reaction include a Raikai mill, a ball mill, a bead mill, a rod mill, a roller mill, an agitator mill, a planetary mill, a vibration mill, a hybridizer, a mechanochemical composite device, and a jet mill. The pulverization time depends on the pulverizer used and the amount of carbon to be treated, and is not strictly limited, but is generally in the range of 5 minutes to 3 hours. Step (d1) is performed in a non-oxidizing atmosphere such as a nitrogen atmosphere or an argon atmosphere. The heating temperature and heating time are appropriately selected depending on the transition metal compound used. In the subsequent step (e1), the transition metal compound and / or its reaction product are removed from the product after heating by means of dissolving it in an acid, etc., and then the product is thoroughly washed and dried to obtain an oxidation-treated carbon.

[0036] In the first manufacturing method, in the step (c1), the transition metal compound acts to promote the oxidation of the carbon raw material through a mechanochemical reaction, and the oxidation of the carbon raw material proceeds rapidly. By this oxidation, an oxidized carbon containing hydrophilic portions of 10 mass % or more of the entire conductive carbon is obtained.

[0037] The oxidized carbon containing 10% by mass or more of a hydrophilic portion is also (a2) mixing a void-containing carbon raw material with a transition metal compound; (b2) heating the resulting mixture in an oxidizing atmosphere; and (c2) removing the transition metal compound and / or its reaction product from the product after heating; It can also be suitably obtained by the second production method including the steps of:

[0038] As the transition metal compound added to the carbon raw material in the (a2) step, inorganic metal salts such as transition metal halides, nitrates, sulfates, and carbonates, organic metal salts such as formates, acetates, oxalates, methoxides, ethoxides, and isopropoxides, or mixtures thereof can be used. These compounds may be used alone or in combination of two or more. Compounds containing different metals may be mixed and used in a predetermined amount. In addition, as long as they do not adversely affect the reaction, compounds other than the transition metal compound, such as alkali metal compounds, may be added together. Since this conductive carbon is mixed with active material particles in the manufacture of electrodes for power storage devices, it is preferable to add compounds of elements constituting the active material to the carbon raw material, since this can prevent the active material from being contaminated with elements that may become impurities.

[0039] Step (b2) is carried out in an oxygen-containing atmosphere, for example, air, at a temperature at which carbon is partially but not completely lost, preferably at a temperature of 200 to 350° C. In the subsequent step (c2), the transition metal compound and / or its reaction product is removed from the product after heating by means of dissolving it in an acid or the like, and then the product is thoroughly washed and dried to obtain oxidized carbon.

[0040] In the second manufacturing method, the transition metal compound acts as a catalyst to oxidize the carbon raw material in the heating step in an oxidizing atmosphere, and the oxidation of the carbon raw material proceeds rapidly. This oxidation produces oxidized carbon containing hydrophilic portions that account for 10 mass % or more of the entire conductive carbon.

[0041] A suitable oxidized carbon containing hydrophilic portions of 10 mass% or more of the total amount can be obtained by subjecting a carbon raw material to a strong oxidation treatment, and it is also possible to promote the oxidation of the carbon raw material by a method other than the first production method and the second production method.

[0042] (I-2) Heat treatment stage In the heat treatment step, the oxidized carbon obtained in the oxidation step is heat treated in a vacuum or in a non-oxidizing atmosphere. The non-oxidizing atmosphere may be composed of an inert gas such as nitrogen or argon, a reducing gas such as hydrogen or carbon monoxide, or a mixed gas of an inert gas and a reducing gas. During the heat treatment, CO2, which is mainly due to carboxyl groups, is released from the surface of the oxidized carbon, and the amount of CO2 generated from the conductive carbon in the range of 25 to 200°C is reduced compared to the amount of CO2 generated from the oxidized carbon in the range of 25 to 200°C.

[0043] The temperature and time of the heat treatment vary depending on the amount of oxidized carbon to be treated, the type of heat treatment atmosphere, etc. For example, when the heat treatment atmosphere is a nitrogen atmosphere, the treatment conditions can be a temperature of 200°C or higher and lower than 500°C for 2 to 8 hours, or a temperature of 500°C or higher and 900°C or lower for 0.5 to 2 hours.

[0044] It is preferable to carry out the heat treatment so that the amount of CO2 generated from the conductive carbon obtained after the treatment at a temperature range of 25 to 200°C is in the range of 0.015 to 0.050 mass% of the total conductive carbon, since this particularly improves the charge-discharge cycle stability when a high voltage is applied.

[0045] (II) Conductive carbon mixture and its manufacturing method The oxidized carbon obtained in the oxidation step can be used as a conductive agent in the form of a conductive carbon mixture in which the oxidized carbon is mixed with another conductive carbon having a higher conductivity than that of the oxidized carbon. An oxidation step in which a carbon raw material is oxidized to obtain an oxidized carbon having a property of spreading like a paste under pressure; a mixing step of mixing the oxidized carbon with another conductive carbon having a higher conductivity than that of the oxidized carbon to obtain a preliminary carbon mixture in which at least a part of the oxidized carbon spreads in a paste-like form and coats a surface of the other conductive carbon; and a heat treatment step in which the amount of CO2 generated from the obtained conductive carbon mixture in the range of 25 to 200°C is reduced by heat treating the preliminary carbon mixture in a vacuum or a non-oxidizing atmosphere, compared with the amount of CO2 generated from the preliminary carbon mixture in the range of 25 to 200°C; The method can be suitably obtained by a production method including the steps of:

[0046] (II-1) Oxidation stage The oxidation step is the same as that described in (I-1) above for the preparation of conductive carbon, and therefore will not be described further.

[0047] (II-2) Mixing stage In the mixing step, the oxidized carbon obtained in the oxidation step is mixed with another conductive carbon having a higher conductivity than that of the oxidized carbon to obtain a preliminary carbon mixture.

[0048] Examples of the other conductive carbon include carbon blacks such as ketjen black, acetylene black, furnace black, and channel black, which are used for electrodes of conventional electricity storage devices, fullerene, carbon nanotubes, carbon nanofibers, graphene, amorphous carbon, carbon fibers, natural graphite, artificial graphite, graphitized ketjen black, mesoporous carbon, and vapor grown carbon fibers. The mass ratio of the oxidation-treated carbon to the other conductive carbon is generally in the range of 3:1 to 1:3.

[0049] The mixing method for carrying out the mixing step is not particularly limited, and any known mixing method can be used, but it is preferable to mix by dry mixing. For dry mixing, a Raikai mill, a ball mill, a bead mill, a rod mill, a roller mill, an agitator mill, a planetary mill, a vibration mill, a hybridizer, a mechanochemical composite device, and a jet mill can be used. The mixing time varies depending on the total amount of the oxidation-treated carbon and the other conductive carbon to be mixed and the mixing device used, but is generally between 10 minutes and 1 hour.

[0050] Since the oxidized carbon has the property of spreading out like a paste when subjected to pressure, the pressure applied to the oxidized carbon during the mixing step causes at least a portion of the oxidized carbon to spread out like a paste and coat the surface of the other conductive carbon.

[0051] (II-3) Heat treatment stage The heat treatment step is the same as the heat treatment step (I-2) for producing conductive carbon, except that the preliminary carbon mixture is heat-treated instead of the oxidation-treated carbon, and so no further explanation will be given. The paste-like spread form of the oxidation-treated carbon contained in the preliminary carbon mixture is maintained in the carbon derived from the oxidation-treated carbon contained in the conductive carbon mixture obtained after the heat treatment.

[0052] Although a preferred method for performing heat treatment after obtaining a preliminary carbon mixture has been described, it is also possible to heat treat the oxidized carbon and then mix it with the other conductive carbon, or it is also possible to heat treat the oxidized carbon and then mix it with the other conductive carbon, and then subject the mixture to an additional heat treatment.

[0053] (III) Manufacturing method of electrodes The conductive carbon and conductive carbon mixture obtained by the above-mentioned manufacturing method are mixed with particles of an electrode active material that exhibits capacity by a Faraday reaction involving the transfer of electrons with ions in the electrolyte of the electricity storage device or a non-Faraday reaction not involving the transfer of electrons, and are used for electrodes of electricity storage devices such as secondary batteries, electric double layer capacitors, redox capacitors, and hybrid capacitors. An electricity storage device essentially includes a pair of electrodes (positive electrode, negative electrode) and an electrolyte disposed between them, and at least one of the positive electrode and negative electrode is manufactured using a mixture containing the conductive carbon or conductive carbon mixture obtained by the above-mentioned method and electrode active material particles. The obtained electricity storage device has a high energy density and improved charge / discharge cycle stability even when a high voltage is applied.

[0054] The electrolyte placed between the positive and negative electrodes in the power storage device may be an electrolyte solution held in a separator, a solid electrolyte, or a gel electrolyte, and any electrolyte used in conventional power storage devices can be used without any particular limitation. Representative electrolytes are shown below. For lithium ion secondary batteries, an electrolyte solution in which a lithium salt such as LiPF6, LiBF4, LiCF3SO3, or LiN(CF3SO2)2 is dissolved in a solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, or dimethyl carbonate is used in a state held by a separator such as a polyolefin fiber nonwoven fabric or a glass fiber nonwoven fabric. In addition, Li5La3Nb2O 12 , Li 1.5 Al 0.5 Ti 1.5 (PO4)3, Li7La3Zr2O 12 , Li7P3S 11 Inorganic solid electrolytes such as ethylene glycol, organic solid electrolytes consisting of a complex of lithium salt and polymer compounds such as polyethylene oxide, polymethacrylate, and polyacrylate, and gel electrolytes in which electrolyte is absorbed in polyvinylidene fluoride, polyacrylonitrile, etc. are also used. For electric double layer capacitors and redox capacitors, electrolytes in which quaternary ammonium salts such as (C2H5)4NBF4 are dissolved in solvents such as acrylonitrile and propylene carbonate are used. For hybrid capacitors, electrolytes in which lithium salts are dissolved in propylene carbonate, etc. and electrolytes in which quaternary ammonium salts are dissolved in propylene carbonate, etc. are used.

[0055] The positive or negative electrode of an electricity storage device using the conductive carbon produced by the above-mentioned method is generally a mixing step of mixing the electrode active material particles and the conductive carbon together with a solvent in which a binder is dissolved as necessary to obtain an electrode mixture in which at least a part of the conductive carbon spreads in a paste-like form to cover the surfaces of the electrode active material particles; and A pressurizing step in which the electrode mixture is applied onto a current collector for the electrode to form an active material layer, and pressure is applied to the active material layer to further spread the conductive carbon into a paste-like form and densify it. The present invention is produced by a method comprising the steps of:

[0056] In the mixing step, the conductive carbon adheres to the surface of the active material particles to cover the surface, so that the aggregation of the active material particles can be suppressed. In addition, when the pressure applied to the conductive carbon in the mixing step is large, at least a part of the conductive carbon spreads in a paste-like form to partially cover the surface of the active material particles. In addition, the pressure applied to the active material layer in the pressurizing step causes the conductive carbon to further spread in a paste-like form to cover the surface of the active material particles while densifying, and the active material particles approach each other, and the conductive carbon is pushed out not only into the gaps formed between the adjacent active material particles while covering the surface of the active material particles, but also into the inside of the holes present on the surface of the active material particles, and densely filled. Therefore, the amount of active material per unit volume in the electrode increases, and the electrode density increases. In addition, the densely filled paste-like conductive carbon has sufficient conductivity to function as a conductive agent and does not suppress the impregnation of the electrolyte in the electricity storage device. As a result, the energy density of the electricity storage device is improved. In addition, since the conductive carbon is heat-treated, the electricity storage device has improved charge-discharge cycle stability even when a high voltage is applied.

[0057] In cases where it is particularly desired to improve the electrical conductivity of the electrodes, the electrical conductivity of the electrodes can be further improved by using the conductive carbon mixture produced by the above-mentioned method instead of the above-mentioned conductive carbon. electrode active material particle a mixing step of mixing the conductive carbon mixture with a solvent having a binder dissolved therein as necessary, to obtain a mixture for an electrode in which at least a part of the carbon originating from the oxidized carbon spreads in a paste-like form and covers not only the surface of the separate conductive carbon but also the surface of the electrode active material particles; and a pressurizing step of forming an active material layer by applying the electrode mixture onto a current collector for the electrode, and applying pressure to the obtained active material layer to further spread and densify the carbon, at least a part of which is derived from the oxidized carbon and has spread in a paste-like form; The present invention is produced by a method comprising the steps of:

[0058] In this embodiment, in the mixing step, the carbon originating from the oxidation-treated carbon, which has at least a part thereof spread in a paste-like form, adheres to and covers not only the surface of the other conductive carbon but also the surface of the electrode active material particles, thereby suppressing the aggregation of the electrode active material particles. Furthermore, in the pressurizing step, the pressure applied to the active material layer formed on the current collector causes the carbon originating from the oxidation-treated carbon, which has at least a part thereof spread in a paste-like form, to be densely filled, together with the other conductive carbon, in the gaps formed by the adjacent electrode active material particles, thereby improving the conductivity of the entire electrode, and thus further improving the energy density of the electricity storage device. In addition, since the conductive carbon mixture has been subjected to a heat treatment, the electricity storage device has improved charge-discharge cycle stability even when a high voltage is applied.

[0059] In the mixing step, the positive electrode active material and the negative electrode active material to be mixed with the conductive carbon or the conductive carbon mixture may be any electrode active material used in conventional power storage devices without any particular limitation. The active material may be a single compound or a mixture of two or more compounds.

[0060] Examples of positive electrode active materials for secondary batteries include layered rock salt LiMO2, layered Li2MnO3-LiMO2 solid solution, and spinel LiM2O4 (wherein M is Mn, Fe, Co, Ni, or a combination thereof). Specific examples of these include LiCoO2, LiNiO2, LiNi 4 / 5 Co 1 / 5 O2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 1 / 2 Mn 1 / 2O2, LiFeO2, LiMnO2, Li2MnO3-LiCoO 2、 Li2MnO3-LiNiO2, Li2MnO3-LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, Li2MnO3-LiNi 1 / 2 Mn 1 / 2 O2, Li2MnO3-LiNi 1 / 2 Mn 1 / 2 O2-LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiMn2O4, LiMn 3 / 2 Ni 1 / 2 O4. Sulfur and Li2S, TiS2, MoS2, FeS2, VS2, Cr 1 / 2 V 1 / 2 Sulfides such as S2, selenides such as NbSe3, VSe2, NbSe3, Cr2O5, Cr3O8, VO2, V3O8, V2O5, V6O 13 In addition to oxides such as LiNi 0.8 Co 0.15 Al 0.05 O2, LiVOPO4, LiV3O5, LiV3O8, MoV2O8, Li2FeSiO4, Li2MnSiO4, LiFePO4, LiFe 1 / 2 Mn 1 / 2 Examples include complex oxides such as PO4, LiMnPO4, and Li3V2(PO4)3.

[0061] Examples of negative electrode active materials for secondary batteries include oxides such as Fe2O3, MnO, MnO2, Mn2O3, Mn3O4, CoO, Co3O4, NiO, Ni2O3, TiO, TiO2, SnO, SnO2, SiO2, RuO2, WO, WO2, and ZnO, metals such as Sn, Si, Al, and Zn, LiVO2, Li3VO4, and Li4Ti5O 12 Complex oxides such as Li 2.6 Co 0.4 Nitrides such as N, Ge3N4, Zn3N2, and Cu3N.

[0062] Examples of the active material in the polarizable electrode of the electric double layer capacitor include carbon materials with a large specific surface area, such as activated carbon, carbon nanofibers, carbon nanotubes, phenolic resin carbide, polyvinylidene chloride carbide, and microcrystalline carbon. In the hybrid capacitor, the positive electrode active material exemplified for the secondary battery can be used for the positive electrode, and in this case, the negative electrode is composed of a polarizable electrode using activated carbon or the like. Also, the negative electrode active material exemplified for the secondary battery can be used for the negative electrode, and in this case, the positive electrode is composed of a polarizable electrode using activated carbon or the like. Examples of the positive electrode active material of the redox capacitor include metal oxides such as RuO2, MnO2, and NiO, and the negative electrode is composed of an active material such as RuO2 and a polarizable material such as activated carbon.

[0063] There is no limitation on the shape or particle size of the active material particles, but it is preferable that the average particle size is greater than 2 μm and not more than 25 μm. Such active material particles having a relatively large particle size improve the electrode density by themselves, and in the mixing step, the pressing force of the active material particles promotes the pasting of the conductive carbon or carbon derived from the oxidized carbon in the conductive carbon mixture. In addition, in the process of applying pressure to the active material layer on the current collector in the electrode production, the active material particles having such a relatively large particle size further press the conductive carbon or carbon derived from the oxidized carbon that has at least partially turned into a paste, and the carbon derived from the conductive carbon or the oxidized carbon is further spread into a paste and densified. As a result, the electrode density is further increased, and the energy density of the electricity storage device is further improved.

[0064] In addition, it is preferable that the active material particles are composed of fine particles having an average particle size of 0.01 to 2 μm and coarse particles having an average particle size of more than 2 μm and not more than 25 μm, which can act as an active material of the same polarity as the fine particles. In the mixing step, the carbon derived from the conductive carbon or the oxidized carbon adheres to and covers not only the surfaces of the fine particles but also the surfaces of the coarse particles, so that the aggregation of the active material particles can be suppressed and the mixed state of the active material particles and the conductive carbon or the conductive carbon mixture can be made uniform. In addition, as described above, the coarse particles promote the pasting and densification of the carbon derived from the conductive carbon or the oxidized carbon, thereby increasing the electrode density and improving the energy density of the electric storage device. Furthermore, by the pressure applied to the active material layer formed on the current collector by the rolling process in the electrode production, the fine particles are pushed out and filled into the gaps formed between the adjacent coarse particles together with the conductive carbon or the conductive carbon mixture while pressing them, so that the electrode density is further increased and the energy density of the electric storage device is further improved.

[0065] There is no particular limitation on the mixing method for carrying out the mixing step, and any known mixing method can be used, but it is preferable to mix the active material particles and the conductive carbon or the conductive carbon mixture by dry mixing, and then mix them together with a solvent in which a binder is dissolved as necessary. For dry mixing, a Raikai mill, a ball mill, a bead mill, a rod mill, a roller mill, an agitator mill, a planetary mill, a vibration mill, a hybridizer, a mechanochemical composite device, and a jet mill can be used. In order to obtain a power storage device having a high energy density, the ratio of the active material particles to the conductive carbon or the conductive carbon mixture is preferably in the range of 90:10 to 99.5:0.5 by mass ratio, and more preferably in the range of 95:5 to 99:1. If the ratio of the conductive carbon or the conductive carbon mixture is less than the above range, the conductivity of the active material layer tends to be insufficient, and the coverage rate of the active material particles by the conductive carbon or the conductive carbon mixture tends to be reduced. Also, if the ratio of the conductive carbon or the conductive carbon mixture is more than the above range, the electrode density tends to be reduced, and the energy density of the power storage device tends to be reduced.

[0066] When a solid electrolyte or a gel electrolyte is used as the electrolyte between the positive electrode and the negative electrode, the solid electrolyte is added to an electrode mixture containing the conductive carbon or the conductive carbon mixture and electrode active material particles in order to ensure an ion conduction path in the active material layer, and these are thoroughly kneaded together with a solvent in which a binder is dissolved as necessary, and the obtained mixture is used to form an active material layer on a current collector.

[0067] As the binder mixed with the active material particles and the conductive carbon or the conductive carbon mixture, known binders such as polytetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, polyvinyl fluoride, carboxymethyl cellulose, etc. are used. The content of the binder is preferably 1 to 30% by mass with respect to the total amount of the mixed materials. If it is 1% by mass or less, the strength of the active material layer is insufficient, and if it is 30% by mass or more, problems such as a decrease in the discharge capacity of the electrode and excessive internal resistance occur. As the solvent mixed with the active material particles and the conductive carbon or the conductive carbon mixture, a solvent that does not adversely affect other materials, such as N-methylpyrrolidone, can be used without any particular limitation.

[0068] The current collector used in the pressing step may be made of a conductive material such as platinum, gold, nickel, aluminum, titanium, steel, carbon, etc. The current collector may have any shape such as a film, foil, plate, net, expanded metal, or cylinder.

[0069] As a modified embodiment, instead of mixing the other conductive carbon with the conductive carbon to obtain a conductive carbon mixture in advance and then mixing it with the electrode active material particles, the electrode active material particles, the conductive carbon, and the other conductive carbon can be mixed simultaneously to obtain a mixture for an electrode, which can then be applied onto a current collector. EXAMPLES

[0070] The present invention will be described with reference to the following examples, but the present invention is not limited to the following examples.

[0071] (1) Charge / discharge cycle test Comparative Example 1 10g of Ketjen Black (product name EC300J, Ketjen Black International) was added to 300mL of 40% nitric acid, and the resulting solution was irradiated with ultrasonic waves for 10 minutes, then filtered to recover Ketjen Black. The recovered Ketjen Black was washed with water three times and dried to obtain acid-treated Ketjen Black. 1.8g of this acid-treated Ketjen Black, 0.5g of Fe(CH3COO)2, 0.19g of Li(CH3COO), 0.28g of C6H8O7·H2O, 0.33g of CH3COOH, 0.33g of H3PO4, and 250mL of distilled water were mixed, and the resulting mixture was stirred with a stirrer for 1 hour, then evaporated to dryness at 100℃ in air, and the mixture was collected. The resulting mixture was then introduced into a vibration ball mill and milled at 20Hz for 10 minutes. The ground powder was heated in nitrogen at 700°C for 3 minutes to obtain a composite of LiFePO4 supported on Ketjen Black.

[0072] 1 g of the resulting composite was added to 100 mL of a 30% aqueous hydrochloric acid solution, and the resulting solution was irradiated with ultrasonic waves for 15 minutes to dissolve the LiFePO4 in the composite, and the remaining solid was filtered, washed with water, and dried. A part of the dried solid was heated to 900°C in air by TG analysis to measure the weight loss. The above-mentioned process of dissolving LiFePO4 in an aqueous hydrochloric acid solution, filtering, washing with water, and drying was repeated until it was confirmed that the weight loss was 100%, that is, no LiFePO4 remained, to obtain LiFePO4-free oxidation-treated carbon.

[0073] Next, 0.1 g of the obtained oxidized carbon was added to 20 mL of an aqueous ammonia solution of pH 11, and ultrasonic irradiation was performed for 1 minute. The obtained liquid was left to stand for 5 hours to allow the solid phase to precipitate. After the solid phase precipitated, the supernatant liquid was removed and the remaining part was dried, and the weight of the dried solid was measured. The weight ratio of the weight of the dried solid subtracted from the weight of the initial oxidized carbon (0.1 g) to the weight of the initial oxidized carbon (0.1 g) was determined as the content of the "hydrophilic portion" in the oxidized carbon. The content of the hydrophilic portion was 10.2% of the total oxidized carbon.

[0074] The above-mentioned oxidized carbon and acetylene black, which is another conductive carbon, were mixed in a mass ratio of 1:1 to obtain a preliminary carbon mixture.

[0075] In addition, 97 parts by mass of commercially available LiNi 0.5 Mn 0.3 Co 0.2 O2 particles (average particle size 5 μm), 2 parts by mass of the above preliminary carbon mixture, and 1 part by mass of polyvinylidene fluoride were mixed with an appropriate amount of N-methylpyrrolidone to form a slurry. This slurry was applied onto an aluminum foil and dried, and then pressed three times under a condition of 1.5 t / cm to obtain a positive electrode for a lithium-ion secondary battery.

[0076] In addition, 93 parts by mass of graphite, 1 part by mass of acetylene black, and 6 parts by mass of polyvinylidene fluoride were mixed with an appropriate amount of N-methylpyrrolidone to form a slurry. This slurry was applied onto a copper foil, dried, and then pressed to obtain a negative electrode for a lithium ion secondary battery having an electrode density of 1.5 g / cc. The ratio of the positive electrode capacity to the negative electrode capacity was 1:1.2.

[0077] The obtained positive and negative electrodes were dried in vacuum at 80°C for 12 hours, and then a lithium-ion secondary battery was fabricated using a 1:1 solution of 1M LiPF6 containing 1 wt% vinylene carbonate as an additive as an electrolyte, in an ethylene carbonate / dimethyl carbonate mixture.

[0078] The resulting battery was subjected to a charge / discharge cycle test under conditions of 45° C. and a charge / discharge rate of 0.6 C in the range of 4.5 V to 2.5 V.

[0079] Example 1 The preliminary carbon mixture obtained in Comparative Example 1 was heat-treated in a nitrogen atmosphere electric furnace at 200°C for 4 hours to obtain a conductive carbon mixture. A lithium ion secondary battery was produced using this conductive carbon mixture instead of the preliminary carbon mixture in the same manner as in Comparative Example 1, and a charge-discharge cycle test was performed on the obtained battery in the same manner as in Comparative Example 1.

[0080] Example 2 The preliminary carbon mixture obtained in Comparative Example 1 was heat-treated in a nitrogen atmosphere electric furnace at 400° C. for 4 hours to obtain a conductive carbon mixture. A lithium ion secondary battery was produced using this conductive carbon mixture instead of the preliminary carbon mixture in the same manner as in Comparative Example 1, and a charge-discharge cycle test was performed on the obtained battery in the same manner as in Comparative Example 1.

[0081] Example 3 The preliminary carbon mixture obtained in Comparative Example 1 was heat-treated in a nitrogen atmosphere electric furnace at 600° C. for 1 hour to obtain a conductive carbon mixture. A lithium ion secondary battery was produced using this conductive carbon mixture instead of the preliminary carbon mixture in the same manner as in Comparative Example 1, and a charge-discharge cycle test was performed on the obtained battery in the same manner as in Comparative Example 1.

[0082] Example 4 The preliminary carbon mixture obtained in Comparative Example 1 was heat-treated in a nitrogen atmosphere electric furnace at 900° C. for 4 hours to obtain a conductive carbon mixture. A lithium ion secondary battery was produced using this conductive carbon mixture instead of the preliminary carbon mixture in the same manner as in Comparative Example 1, and a charge-discharge cycle test was performed on the obtained battery in the same manner as in Comparative Example 1.

[0083] Comparative Example 2 A lithium ion secondary battery was produced in the same manner as in Comparative Example 1, except that acetylene black was used instead of the preliminary carbon mixture. The resulting battery was subjected to a charge-discharge cycle test in the same manner as in Comparative Example 1.

[0084] FIG. 1 shows the results of the charge-discharge cycle test for the batteries of Comparative Example 1 and Comparative Example 2, and FIG. 2 shows the results of the charge-discharge cycle test for the batteries of Examples 1 to 4 and Comparative Example 2. As is clear from FIG. 1, the battery of Comparative Example 1 had a higher capacity retention rate than the battery of Comparative Example 2 until 25 charge-discharge cycles were repeated, but from about 30 charge-discharge cycles, it showed a capacity retention rate equivalent to that of the battery of Comparative Example 2, and from about 60 charge-discharge cycles, it showed a capacity retention rate lower than that of the battery of Comparative Example 2. In contrast, the batteries of Examples 1 to 4 using the conductive carbon mixture obtained by applying heat treatment to the preliminary carbon mixture showed improved charge-discharge cycle stability, as is clear from FIG. 2.

[0085] (2) Investigation of the effects of heat treatment In order to investigate the effect of heat treatment, the preliminary carbon mixture obtained in Comparative Example 1 was heat-treated in a nitrogen atmosphere electric furnace at 200°C for 4 hours, 250°C for 4 hours, 300°C for 4 hours, 400°C for 4 hours, 500°C for 4 hours, 600°C for 1 hour, and 900°C for 1 hour to obtain a conductive carbon mixture. An experiment was carried out to measure the amount of CO2 generated while heating 30 mg of the obtained conductive carbon mixture and the preliminary carbon mixture that was not subjected to heat treatment under conditions of a helium atmosphere at atmospheric pressure (helium flow rate 300 mL / min), a temperature range from room temperature to 1200°C, and a heating rate of 20°C / min, using a TPD-PIMS device (TPD type R made by Rigaku Corporation).

[0086] The measurement results are shown in FIG. 3. As shown in FIG. 2, improved charge-discharge cycle stability was obtained even with heat treatment at 200°C. However, as is clear from FIG. 3, it was found that the amount of CO2 generated from the conductive carbon mixture in the range of 25 to 200°C varies greatly depending on the heat treatment at 200°C. It is known that CO2 generation from carbon in the range of 25 to 200°C is mainly due to detachment from the carboxyl groups on the surface (see Non-Patent Document 1). However, the amount of CO2 generated in the range of 25 to 200°C is almost the same in the conductive carbon mixture heat-treated at 250 to 400°C, and it was considered that detachment from the carboxyl groups on the surface was almost completed by heat treatment at 250°C. Considering the results of this experiment and the results of the charge-discharge cycle test, it was determined that a method for identifying conductive carbon focusing on the amount of CO2 generated in the range of 25 to 200°C is useful for obtaining improved charge-discharge cycle stability when a high voltage is applied. Table 1 shows the results of calculating the ratio (mass %) of the total amount (integral value) of CO2 generated from each of the conductive carbon mixture and preliminary carbon mixture to the mass of the measurement object (30 mg) in the temperature range of 25 to 200°C. [Table 1]

[0087] (3) CO2 generation from conductive carbon The oxidized carbon obtained in Comparative Example 1 was heat-treated in a nitrogen atmosphere electric furnace at 200°C for 4 hours, 400°C for 4 hours, 600°C for 1 hour, and 900°C for 1 hour, respectively, to obtain conductive carbon. An experiment was carried out to measure the amount of CO2 generated by heating 30 mg of the obtained conductive carbon and the oxidized carbon that was not subjected to heat treatment under conditions of a helium atmosphere at atmospheric pressure (helium flow rate 300 mL / min), a temperature range of room temperature to 1200°C, and a temperature rise rate of 20°C / min, using a TPD-PIMS device (TPD type R manufactured by Rigaku Corporation). Table 2 shows the results of calculating the ratio (mass%) of the total amount (integral value) of CO2 generated from each of the conductive carbon and oxidized carbon to the mass (30 mg) of the measurement target in a temperature range of 25 to 200°C. [Table 2]

[0088] From Table 2, it was determined that stable charge / discharge cycle performance can be obtained even when a high voltage is applied when the amount of CO2 generated from the conductive carbon in the range of 25 to 200°C is in the range of 0.015 to 0.050 mass% of the total conductive carbon. [Industrial Applicability]

[0089] By using the conductive carbon or conductive carbon mixture produced according to the present invention, it is possible to obtain an electricity storage device having a high energy density and improved charge-discharge cycle stability even when a high voltage is applied.

Claims

1. A conductive carbon to be used as a conductive agent in an electrode of an electricity storage device, It has the property of spreading like a paste when subjected to pressure, and CO generated from the conductive carbon in the range of 25 to 200°C ２ The amount is in the range of 0.015 to 0.050 mass % of the total conductive carbon.

1. A conductive carbon comprising:

2. A method for producing conductive carbon to be used as a conductive agent in an electrode of an electricity storage device, comprising the steps of: An oxidation step in which a carbon raw material is oxidized to obtain an oxidized carbon having a property of spreading like a paste under pressure; and The oxidized carbon is heat-treated in a vacuum or in a non-oxidizing atmosphere, and the CO generated from the resulting conductive carbon is in the range of 25 to 200°C. ２ The amount of CO generated from the oxidized carbon in the range of 25 to 200° C. ２ The amount is reduced in the heat treatment step. Including, In the heat treatment step, the heat treatment is performed so that the amount of CO 2 generated from the obtained conductive carbon at a temperature range of 25 to 200° C. is in the range of 0.015 to 0.050% by mass of the total conductive carbon. A method for producing conductive carbon comprising the steps of:

3. The method for producing conductive carbon according to claim 2 , wherein in the oxidation step, the oxidation treatment is carried out so that the content of hydrophilic portions in the obtained oxidized carbon is 10 mass % or more of the entire oxidized carbon.

4. A method for producing a conductive carbon mixture to be used as a conductive agent in an electrode of an electrical storage device, comprising the steps of: An oxidation step in which a carbon raw material is oxidized to obtain an oxidized carbon having a property of spreading like a paste under pressure; a mixing step of mixing the oxidized carbon with another conductive carbon having a higher conductivity than that of the oxidized carbon to obtain a preliminary carbon mixture in which at least a part of the oxidized carbon spreads in a paste-like form and coats a surface of the other conductive carbon; and The preliminary carbon mixture is heat-treated in a vacuum or in a non-oxidizing atmosphere, and the CO generated from the resulting conductive carbon mixture is in a range of 25 to 200° C. ２ The amount of CO generated from the preliminary carbon mixture at a temperature in the range of 25 to 200° C. ２ The amount is reduced in the heat treatment step. A method for producing a conductive carbon mixture, comprising:

5. The method for producing a conductive carbon mixture according to claim 4 , wherein in the oxidation step, the oxidation treatment is carried out so that the content of hydrophilic parts in the obtained oxidized carbon is 10 mass % or more of the total oxidized carbon.

6. A method for producing an electrode for an electricity storage device, comprising the steps of: a mixing step of mixing the electrode active material particles with the conductive carbon obtained by the method for producing the electrode active material particles according to claim 2 or 3 to obtain a mixture for an electrode in which at least a part of the conductive carbon spreads in a paste-like form to cover the surfaces of the electrode active material particles; and a pressurizing step in which the electrode mixture is applied onto a current collector for the electrode to form an active material layer, and pressure is applied to the active material layer to further spread the conductive carbon into a paste-like form and densify it; A method for producing an electrode, comprising:

7. A method for manufacturing an electrode of an electricity storage device, comprising the steps of: a mixing step of mixing the electrode active material particles with the conductive carbon mixture obtained by the method of claim 4 or 5 to obtain an electrode mixture in which the carbon originating from the oxidized carbon, at least a part of which has spread in a paste-like form, covers not only the surface of the separate conductive carbon but also the surface of the electrode active material particles; and a pressurizing step of forming an active material layer by applying the electrode mixture onto a current collector for the electrode, and applying pressure to the obtained active material layer to further spread and densify the carbon, at least a part of which is derived from the oxidized carbon and has spread in a paste-like form; A method for producing an electrode, comprising:

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