Carbonaceous material, method for producing carbonaceous material, negative electrode for nonaqueous electrolyte battery, and nonaqueous electrolyte battery

Abstract

The present invention pertains to a carbonaceous material having a silicon element content of at least 5 mass%, a specific surface area of at least 250 m2 / g, and an ash content of at least 40 mass%.

Description

Carbonaceous material, method for producing carbonaceous material, negative electrode for non-aqueous electrolyte battery, and non-aqueous electrolyte battery

[0001] The present invention relates to a carbonaceous material, a method for producing a carbonaceous material, a negative electrode for a nonaqueous electrolyte battery, and a nonaqueous electrolyte battery.

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, have high energy density and excellent output characteristics, and are therefore widely used in small portable devices such as mobile phones and laptop computers. In recent years, their application to automotive applications, such as hybrid vehicles and electric vehicles, has also been promoted. As a negative electrode material for lithium-ion secondary batteries, non-graphitizable carbons capable of doping (charging) and dedoping (discharging) lithium in amounts exceeding the theoretical capacity of graphite, 372 mAh / g, have been developed and used (see, for example, Patent Documents 1 and 2, and Non-Patent Document 1).

[0003] However, as further improvements in capacity are currently required, the battery capacity of graphite is insufficient, and proposals have been made to achieve higher battery capacity by coexisting it with a metal that has a high lithium absorption capacity. While silicon and tin are being investigated due to their high absorption capacity as metals alone, the short battery life caused by the expansion associated with Li absorption and contraction upon release has become a problem. One solution to this problem is to combine graphite with carbon.

[0004] Methods for effectively utilizing silicon contained in plants have been investigated for composites with carbon materials (Patent Document 2, Non-Patent Document 2).

[0005] JP 2021-116197 A JP 2017-91822 A

[0006] Electrochimica Acta 203 (2016) 9-20 Scientific Reports (2022) 12:975

[0007] Although various studies have been conducted on the application of lithium-ion secondary batteries to automotive applications, conventional carbonaceous materials have often had insufficient discharge capacity and low lithium ion utilization efficiency.

[0008] Therefore, an object of the present invention is to provide a carbonaceous material suitable as a negative electrode active material that provides a nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery, a sodium ion secondary battery, a lithium-sulfur battery, or a lithium-air battery) with high charge / discharge capacity and charge / discharge efficiency; a negative electrode containing the carbonaceous material; a nonaqueous electrolyte secondary battery having the negative electrode; and a method for producing the carbonaceous material.

[0009] The present inventors have found that the above object can be achieved by the carbonaceous material for a non-aqueous electrolyte secondary battery of the present invention, which will be described below. That is, the present invention encompasses the following preferred embodiments: [1] A carbonaceous material having a silicon element content of 5 mass % or more and a specific surface area of ​​250 m ２ / g or more and an ash content of 40 mass% or more. [2] A carbonaceous material having a specific surface area of ​​Am ２ / g and the ash content is B mass %, the value X calculated by X = {A / (100-B)} x 100 is 500 or more. [3] The carbonaceous material according to claim 1, wherein the value X is a value obtained by Raman spectroscopy of 100 to 600 cm －１ The intensity of the peak derived from silicon in the vicinity of １ and 1300 cm －１ The intensity of the peak derived from the carbon in the vicinity is D ２ When this is done, D ２ D against １ The ratio (D １ / D ２) is less than 0.2. [4] The carbonaceous material according to any of [1] to [3], wherein the R value by Raman spectroscopy is 0.9 to 1.1. [5] The carbonaceous material according to any of [1] to [4], wherein the potassium content is 1.0 mass% or less. [6] The carbonaceous material according to any of [1] to [5], wherein the carbonaceous material is derived from a grass plant. [7] A method for producing a carbonaceous material having an ash content of 40 mass% or more, comprising: (1) a step of heating a carbide precursor having a silicon content of 3 mass% or more at 300 to 900°C in an inert gas atmosphere to obtain a carbide; and (2) a step of activating the carbide obtained in step (1) at 800 to 1000°C in an oxidizing gas atmosphere to obtain a carbonaceous material. [8] The method according to [7], further comprising, between steps (1) and (2), a step of pulverizing the carbonized material obtained in step (1) and a step of washing the carbonized material obtained in step (1) with an acid. [9] A negative electrode for a non-aqueous electrolyte battery using the carbonaceous material according to any one of [1] to [6].

[10] A non-aqueous electrolyte battery using the negative electrode according to [9].

[0010] According to the present invention, it is possible to provide a carbonaceous material suitable as a negative electrode active material, which provides a nonaqueous electrolyte secondary battery having high charge / discharge capacity and charge / discharge efficiency, a negative electrode containing the carbonaceous material, a nonaqueous electrolyte secondary battery having the negative electrode, and a method for producing the carbonaceous material.

[0011] Hereinafter, embodiments of the present invention will be described in detail. Note that the scope of the present invention is not limited to the embodiments described here, and various modifications can be made without departing from the spirit of the present invention.

[0012] The carbonaceous material of the present invention has a silicon element content of 5 mass % or more and a specific surface area of ​​250 m ２ / g or more and an ash content of 40 mass% or more. If the silicon content in the carbonaceous material is less than 5 mass%, the number of sites for absorbing and desorbing lithium ions during charge and discharge is reduced, and sufficient charge and discharge capacity cannot be obtained. From the viewpoint of improving charge and discharge capacity, the silicon content in the carbonaceous material of the present invention is preferably 6 mass% or more, more preferably 8 mass% or more, even more preferably 10 mass% or more, still more preferably 12 mass% or more, and particularly preferably 13 mass% or more. Furthermore, the silicon content in the carbonaceous material is preferably 50 mass% or less, more preferably 45 mass% or less, and even more preferably 40 mass% or less. When the silicon atom content in the carbonaceous material is equal to or less than the above upper limit, this is preferable from the viewpoint of preventing distortion of the carbonaceous material's structure due to expansion of silicon upon Li adsorption and maintaining a stable structure. By preventing distortion of the carbonaceous material's structure, sites for absorbing and desorbing lithium ions are maintained, and a conductive path structure is maintained, thereby increasing the charge and discharge capacity. The silicon element content in the carbonaceous material of the present invention is preferably 5 to 50 mass%, more preferably 6 to 50 mass%, even more preferably 8 to 45 mass%, still more preferably 10 to 45 mass%, particularly preferably 12 to 40 mass%, and especially preferably 13 to 40 mass%. The silicon element content is measured, for example, by X-ray fluorescence analysis, and may be measured, for example, by the method described in the Examples.

[0013] The method for adjusting the silicon element content to fall within the above range is not particularly limited as long as it can produce a carbonaceous material having a silicon element content within the above range. For example, a method for producing a carbonaceous material using a plant-derived raw material containing a large amount of silicon as a carbon source, as described below, can be mentioned.

[0014] The specific surface area of ​​the carbonaceous material of the present invention is 250 m ２ / g or more. ２ If the specific surface area is less than 250 m / g, the electrolyte retention cannot be sufficiently increased, and therefore the battery resistance cannot be reduced. In addition, the battery capacity cannot be sufficiently increased. From the viewpoint of reducing the battery resistance and improving the battery capacity, the specific surface area of ​​the carbonaceous material is preferably 250 m / g. ２ / g or more, more preferably 270m ２ / g or more, more preferably 300m ２ / g or more. From the viewpoint of reducing the amount of water present in the carbonaceous material, it is preferably 1000 m ２ / g or less, more preferably 950m ２ / g or less, more preferably 900m ２ From the same viewpoint, the specific surface area of ​​the carbonaceous material is preferably 250 to 1000 m ２ / g, more preferably 270 to 950 m ２ / g, more preferably 300 to 900m ２ The specific surface area of ​​the carbonaceous material is measured by the nitrogen adsorption BET method, and may be measured by the method described in the examples, for example.

[0015] The method for adjusting the specific surface area of ​​a carbonaceous material measured by the nitrogen adsorption BET method to fall within the above range is not particularly limited. For example, in the method for producing a carbonaceous material described below, lowering the temperature or shortening the heating time in the step of obtaining a carbon precursor and / or the step of obtaining a carbide tends to increase the BET specific surface area of ​​the carbonaceous material obtained as a result of suppressing thermal structural contraction, etc. In the step of activating the carbide, increasing the activation temperature or lengthening the activation time tends to increase the BET specific surface area of ​​the carbonaceous material obtained as a result. Therefore, by adjusting the temperature and time of heating and activation, a carbonaceous material having a BET specific surface area within the desired range can be obtained.

[0016] The ash content of the carbonaceous material of the present invention is 40% by mass or more. The ash is ash remaining after burning the carbonaceous material in the presence of air, and is thought to be mainly composed of metal oxides contained in the carbonaceous material. For example, when a carbonaceous material is produced using a silicon-rich material such as rice husk as a carbon source, silicic acid formed by the oxidation of silicon is thought to be the main component of the ash in the carbonaceous material. If the ash content is less than 40% by mass, the silicon element content in the carbonaceous material is insufficient. The ash content of the carbonaceous material is preferably 40 to 80% by mass, more preferably 45 to 75% by mass, and even more preferably 50 to 70% by mass.

[0017] The ash content of the carbonaceous material of the present invention can be calculated by burning a dried carbonaceous material in the presence of air and dividing the mass of the ash remaining after combustion by the mass of the dried carbonaceous material, and may be measured, for example, by the method described in the Examples.

[0018] The method for adjusting the ash content of the carbonaceous material to fall within the above range is not limited in any way, but an example thereof includes a method for producing a carbonaceous material, which will be described later, in which a carbon precursor containing a large amount of a metal element, for example, silicon element, is used as a raw material, and the carbonaceous material is produced by carbonizing the carbon precursor in an inert gas atmosphere and then activating the carbon precursor in an oxidizing gas atmosphere.

[0019] The reason why a carbonaceous material having a silicon element content, specific surface area, and ash content within the above ranges can provide a nonaqueous electrolyte secondary battery with high charge / discharge capacity and charge / discharge efficiency is not clear; however, it is thought that because the silicon element content is within an appropriate range, it is possible to maintain sites for adsorbing and desorbing lithium ions and maintain a conductive path structure.

[0020] In order to further improve the charge / discharge capacity and charge / discharge efficiency, the carbonaceous material of the present invention has a specific surface area of ​​Am ２ / g, and the ash content is B mass %, the value X calculated by X = {A / (100-B)} x 100 is preferably 500 or more, more preferably 550 or more, and even more preferably 600 or more. The value X is preferably 500 to 1500, more preferably 550 to 1450, and even more preferably 600 to 1400. The value X represents the relationship between the specific surface area and ash content of the carbonaceous material. When the value X is equal to or greater than the above-mentioned lower limit, not only can the electrolyte retention ability be improved and an increase in electrical resistance be suppressed, but it is also effective in supporting and retaining other lithium ion active components such as phosphorus, silicon, and tin to achieve higher battery capacity. Furthermore, when the value X is equal to or less than the above-mentioned upper limit, the hygroscopicity of the carbonaceous material is likely to be reduced, and the amount of water present in the carbonaceous material is likely to be reduced. As a result, hydrolysis of the electrolyte due to moisture and electrolysis of water are suppressed, and the accompanying generation of acid and gas is likely to be suppressed.

[0021] The carbonaceous material of the present invention has a Raman spectrum of 100 to 600 cm －１ The peak detected around 500 cm is a peak derived from silicon, especially －１ The peak detected around 500 cm is a peak derived from crystalline silica. Crystalline silica is a non-electron conductor and does not intercalate Li ions, so it is a component that increases resistance and is also electrochemically inactive. When a carbonaceous material contains crystalline silica in an amount that can be detected as this peak, it is thought that the number of sites in the carbon material where lithium ions can be intercalated decreases, resulting in a decrease in charge / discharge capacity. Therefore, in Raman spectroscopy, －１ It is preferable that no steep peak is detected near 0.055 mm. In order to prevent such a peak from being detected, it is preferable not to use a carbonaceous material containing crystalline silica.

[0022] The carbonaceous material of the present invention has a Raman spectrum of 100 to 600 cm －１ Between ｘ It has a peak due to Li ion insertion. ｘ If the crystallinity of is too high, it is difficult for Li ions to be inserted and it is difficult for them to contribute to the charge / discharge capacity, which is not preferable. －１ The intensity of the peak near １ and 1300 cm －１ The intensity of the peak derived from the carbon in the vicinity is D ２ When this is done, D ２ D against １ The ratio (D １ / D ２ ) is preferably less than 0.2, more preferably 0.18 or less, and even more preferably 0.15 or less in terms of effectiveness in charging and discharging.

[0023] In the carbonaceous material of the present invention, the R value in the Raman spectrum of the carbonaceous material is 0.9 to 1.1. Here, the R value is the value at 1360 cm in the Raman spectrum observed by laser Raman spectroscopy. －１ The peak intensity (I Ｄ ) and 1580 cm －１The peak intensity (I Ｇ ) intensity ratio (R value = I Ｄ / I Ｇ ) where 1360 cm －１ The peak around 1580 cm is a Raman peak generally known as the D band, which is a peak caused by disorder and defects in the graphite structure. －１ The peak around this range is a Raman peak generally known as the G band, which is a peak derived from the graphite structure. The R value of the carbonaceous material in the Raman spectrum is preferably greater than 0.9 and less than 1.1, more preferably 0.95 to 1.05. An R value within this range is preferable because it can accept Li ion insertion and improve battery capacity while ensuring electrical conductivity.

[0024] From the viewpoint of improving charge / discharge capacity and charge / discharge efficiency, the potassium element content in the carbonaceous material of the present invention is preferably 1.2 mass% or less, more preferably 1.1 mass% or less, and even more preferably 1.0 mass% or less. The smaller the potassium element content, the better, and the lower limit is 0 mass% or more. When the potassium element content in the carbonaceous material is equal to or less than the above upper limit, a non-aqueous electrolyte secondary battery using this carbonaceous material tends to have a large undoped capacity and a small undoped capacity. Furthermore, when the potassium element content in the carbonaceous material is equal to or less than the above upper limit, the occurrence of a short circuit due to the elution and reprecipitation of these metal elements in the electrolyte is suppressed, thereby ensuring the safety of the non-aqueous electrolyte secondary battery. It is particularly preferable that the carbonaceous material does not substantially contain potassium element. The potassium element content in the carbonaceous material is typically 0 mass% or more. The potassium element content is measured by fluorescent X-ray analysis. For example, the measurement may be performed by the method described in the Examples.

[0025] The average particle diameter D of the carbonaceous material of the present invention ５０is preferably 30 μm or less, more preferably 25 μm or less, even more preferably 20 μm or less, even more preferably 18 μm or less, particularly preferably 16 μm or less, and most preferably 15 μm or less. When the average particle size is below the above upper limit, not only is the coating property during electrode preparation good, but the free diffusion path of lithium ions within the carbonaceous material particles is reduced, making it easier to achieve rapid charge and discharge. Furthermore, in lithium ion secondary batteries, it is important to increase the electrode area to improve input / output characteristics, and to achieve this, it is necessary to reduce the coating thickness of the active material on the current collector plate during electrode preparation. When the average particle size of the carbonaceous material to be used as the active material is below the above upper limit, it is easy to reduce the coating thickness during electrode preparation. In addition, the average particle size D of the carbonaceous material of the present invention is ５０ is preferably 2 μm or more, more preferably 3 μm or more, even more preferably 4 μm or more, and even more preferably 5 μm or more. ５０ When the average particle diameter D is equal to or greater than the above lower limit, when a negative electrode is produced using the carbonaceous material, voids formed between the carbonaceous material particles are easily secured, the movement of lithium ions in the electrolyte is less likely to be inhibited, and the resistance of the nonaqueous electrolyte secondary battery is easily reduced. ５０ is the particle size at which the cumulative volume becomes 50%, and can be determined by measuring the particle size distribution by a laser scattering method using, for example, a particle size / particle size distribution measuring device ("Microtrac MT3300EXII" manufactured by Microtrac Bell Co., Ltd.).

[0026] The method for producing a carbonaceous material of the present invention is not particularly limited as long as it can produce a carbonaceous material having the above-mentioned properties. From the viewpoint of increasing the silicon element content, it is preferable to produce the carbonaceous material using a carbide precursor that contains a relatively large amount of silicon.

[0027] Examples of carbide precursors containing relatively large amounts of silicon include plant-derived materials, such as bamboo, rice straw, wheat straw, barley straw, rice straw, buckwheat straw, soybean straw, bran, rice husks, wheat husks, buckwheat husks, corn stalks, sugarcane tops, peanut husks, red cedar sawdust, larch bark, and fallen ginkgo leaves. From the viewpoint of increasing the silicon content in the final carbonaceous material, bamboo, rice straw, wheat straw, barley straw, rice straw, buckwheat straw, bran, rice husks, wheat husks, buckwheat husks, and peanut husks are preferred as carbide precursors. Considering the generation scale, accumulation, availability, and stability of the plant raw materials, such as decay, the carbonaceous material is preferably derived from a grass family plant, and more preferably from rice husks. Note that "a carbonaceous material derived from a grass family plant" means that the carbon source of the carbonaceous material is derived from a grass family plant.

[0028] The method for producing the carbonaceous material of the present invention is not particularly limited as long as it can produce the carbonaceous material of the present invention having the above-described properties. For example, the carbonaceous material can be produced using a carbonaceous material production method that includes at least the steps described below.

[0029] The present invention also provides a method for producing a carbonaceous material that has high charge / discharge capacity and charge / discharge efficiency and is suitable as a negative electrode active material or conductive material for nonaqueous electrolyte secondary batteries (e.g., lithium ion secondary batteries, sodium ion batteries, lithium-sulfur batteries, and lithium-air batteries).

[0030] In one aspect of the present invention, the method for producing a carbonaceous material of the present invention includes at least the following steps: (1) heating a carbide precursor having a silicon content of 3% by mass or more in an inert gas atmosphere at 300 to 900°C to obtain a carbide; and (2) activating the carbide obtained in step (1) in an oxidizing gas atmosphere at 800 to 1000°C to obtain a carbonaceous material. The present invention also provides a method for producing a carbonaceous material having an ash content of 40% by mass or more, which includes the above steps. The above production method is not particularly limited as long as it includes each of steps (1) and (2). It may include only steps (1) and (2), or it may include additional steps before or after each of the above steps. Furthermore, the carbide to be activated in step (2) may be the carbide obtained in step (1) as is, or may be the carbide that has been further treated. Furthermore, the carbide to be activated in step (2) may be the activated product obtained in step (2) that has been further treated.

[0031] First, the method for producing a carbonaceous material of the present invention, which includes steps (1) and (2), will be described.

[0032] Step (1) involves heating a carbide precursor having a silicon content of 3% by mass or more in an inert gas atmosphere at 300 to 900°C to obtain a carbide. Examples of carbide precursors having a silicon content of 3% by mass or more include the above-described carbide precursors containing relatively high amounts of silicon and carbide precursors obtained by calcining the carbide precursors. The inert gas is not particularly limited as long as it is a gas that does not contain oxygen, and examples include nitrogen gas, argon gas, helium gas, carbon dioxide gas, carbon monoxide gas, and fuel exhaust gas. The inert gas may be one of the above gases or a mixture of two or more of the above gases. In the present invention, the inert gas refers to a gas that does not oxidize and erode carbon at the heat treatment temperature. Nitrogen gas and carbon dioxide gas are preferred as the inert gas from the viewpoint of reducing production costs, and nitrogen gas is preferred from the viewpoint of operability. Step (1) is preferably performed under an inert gas supply. When step (1) is carried out under supply of an inert gas, the supply amount of the inert gas is preferably 0.01 to 20 L / min, more preferably 0.02 to 15 L / min, and even more preferably 0.03 to 10 L / min, per 10 g of the carbide precursor, from the viewpoint of the removal of volatile components.

[0033] The heating temperature in step (1) is 300 to 900°C, preferably 310 to 850°C, more preferably 320 to 800°C, and even more preferably 330 to 750°C. The heating temperature may be constant or may vary within the above range. The heat treatment time is preferably 0.2 to 8 hours, more preferably 0.5 to 4 hours, and even more preferably 0.5 to 2 hours. It is believed that the heat treatment at the above heating temperature in step (1) dries the physically adsorbed water contained in the carbide precursor, and a dehydration reaction occurs at the molecular level in the carbide precursor, thereby promoting carbonization. By performing this heat treatment in an inert gas atmosphere, preferably while supplying an inert gas, it is believed that structural loss due to carbon oxidation does not occur when water is removed from the carbide precursor by drying and dehydration, and a carbide that maintains a fine structure can be obtained. The above heating conditions are believed to facilitate efficient and sufficient removal of water from the carbon precursor raw material by drying and dehydration, and also facilitate the final production of a carbonaceous material having the above characteristics.

[0034] After the heat treatment, the carbonized material may be extracted by cooling to, for example, 200°C or less, or the next step (2) may be carried out without the cooling step. When the cooling step is carried out, the cooling temperature is preferably 200°C or less, more preferably 150°C or less, even more preferably 100°C or less, still more preferably 50°C or less, and extremely preferably room temperature (25°C) or less, to obtain the carbonized material.

[0035] Step (2) is a step of activating the carbide obtained in step (1) in an oxidizing gas atmosphere at 800 to 1000°C to obtain a carbonaceous material. In step (2), the obtained carbide can be activated by reacting it with an oxidizing gas (for example, water vapor, carbon dioxide gas, etc.).

[0036] In terms of efficiently promoting activation, a mixture of water vapor and an inert gas similar to that used in carbonization is preferred, and the partial pressure of the water vapor in this case is preferably in the range of 3 to 60%. When the partial pressure of water vapor is 3% or more, activation can be easily promoted sufficiently, and when it is 60% or less, rapid activation can be suppressed, making it easy to control the reaction.

[0037] The total amount of oxidizing gas supplied during activation is preferably 50 to 10,000 parts by mass, more preferably 100 to 5,000 parts by mass, and even more preferably 200 to 3,000 parts by mass, relative to 100 parts by mass of the carbon precursor. When the total amount of oxidizing gas supplied is within the above range, the activation reaction can proceed more efficiently.

[0038] The activation temperature is preferably 800 to 1000°C. The activation time and temperature rise rate are not particularly limited and vary depending on the type, shape, size, and desired pore size distribution of the selected carbon precursor. Increasing the activation temperature or lengthening the activation time during activation tends to increase the BET specific surface area of ​​the resulting carbonaceous material. Therefore, the activation temperature and activation time can be adjusted to obtain a carbonaceous material having a BET specific surface area within the desired range.

[0039] The production method of the present invention includes steps (1) and (2), and is not particularly limited as long as it can produce a carbonaceous material with an ash content of 40% by mass or more. However, from the viewpoint of producing a carbonaceous material having an elemental silicon content, specific surface area, and ash content within the above ranges, it is preferable to perform the heat treatment in steps (1) and (2) while thoroughly removing decomposition products that volatilize between each stage. This is thought to be because, if such decomposition products are present during the heat treatment, the decomposition products may generate amorphous carbon portions, which may deactivate Li ions in the carbon portions. For example, the generation of amorphous carbon portions can be suppressed by performing a step of cooling the obtained carbide to 200°C or below after step (1), or by performing the heat treatment in steps (1) and (2) while removing the volatilized decomposition products together with nitrogen gas under a fast nitrogen gas flow. In particular, for example, step (2) is also a step of removing volatiles at a high temperature, which increases the edge portions of the carbon, and there is a concern that the Li ion efficiency will decrease due to the deactivation of Li ions in the edge portions. On the other hand, the volatile products may oxidize Si contained in the raw material to form SiO . ２ The formation and crystallization of SiO can be suppressed, and the battery capacity can be expressed. ｘIt is believed that this promotes reduction to the (1<x<2) form, overcomes the deactivation that may occur at the crystal ends, and improves charge / discharge efficiency. In non-aqueous electrolyte secondary batteries such as Li-ion batteries, a decrease in this efficiency often leads to Li deficiency on the positive electrode side, which makes battery operation unstable, which is undesirable. Therefore, not only high battery capacity but also high initial Li-ion efficiency contributes significantly to battery performance.

[0040] In addition, the ratio of the D band to the G band (D / G) in the Raman spectrum represents the ratio of crystalline to amorphous, and in the present invention, it is 0.9<D / G<1.1. In the above activation step, removing volatiles makes the crystalline G band stronger, leading to improved conductivity, but a high G band intensity is undesirable because it increases the amount of microcrystals and reduces mechanical strength, and a high D band intensity is undesirable because it reduces conductivity.

[0041] The production method of the present invention preferably further includes a step of removing metal components and the like that may be contained in a carbide precursor (e.g., a plant-derived carbide precursor) having a silicon content of 3 mass% or more. Examples of the metal components and the like to be removed include potassium, sodium, calcium, and the like. The step of removing metal components and the like may be carried out either before or after step (1) or after step (2). The method for removing metal components and the like may be any method, such as washing with an acid or washing with an alkali, or a combination of both. However, washing with an acid is preferred because it does not destroy the structure of the carbide precursor and also because it reduces the difficulty of treating metal-removal wastewater by dissolving the organic matter that constitutes the carbide precursor in water.

[0042] In a preferred embodiment of the present invention, the production method of the present invention further includes, between step (1) and step (2), a step of pulverizing the carbide obtained in step (1) and a step of washing the carbide obtained in step (1) with acid. The order of the acid washing step and the pulverization step is not particularly limited. The acid washing step may be performed after the pulverization step, or may be performed after the acid washing step. However, it is more preferable to perform the acid washing step after the pulverization step. Furthermore, this embodiment is not particularly limited as long as it includes each of the pulverization step and the washing step at least once. The pulverization step and the washing step may each be performed once or two or more times. Furthermore, the pulverization step and / or the washing step may be performed at a step other than between step (1) and step (2).

[0043] The acid used may be any of mineral acids such as hydrochloric acid, sulfuric acid, and nitric acid, or water-soluble organic acids such as formic acid, acetic acid, butyric acid, and citric acid. They may be used alone or in combination. Considering the metal removal efficiency and residual properties, hydrochloric acid is preferred. The concentration of the acidic water used is not particularly limited, but when using hydrochloric acid, it is preferable to use acidic water in the range of 0.01N to 2N, more preferably 0.05N to 1.8N, and even more preferably 0.1N to 1.5N.

[0044] The temperature of the acid washing is not particularly limited and is naturally complementary to the washing time, but it is usually carried out at a temperature ranging from room temperature to 80° C. for a period ranging from 0.5 to 6 hours, more preferably from 1 to 5 hours. This operation may be carried out once, or may be carried out multiple times by repeating the dewatering, as long as the metal content can be reduced to the target level.

[0045] After acid washing, the carbonized material may be washed with ion-exchanged water or the like to deoxidize it.

[0046] In a preferred embodiment, the production method of the present invention preferably does not include a step of alkali washing, particularly before or after step (1) or after step (2), until the ash content of the finally obtained carbonaceous material is less than 40 mass%.

[0047] Average particle diameter D of the carbonaceous material after pulverization５０ is preferably 2 μm or more, more preferably 3 μm or more, even more preferably 4 μm or more, and even more preferably 5 μm or more. ５０ is preferably 30 μm or less, more preferably 25 μm or less, even more preferably 22 μm or less, and particularly preferably 20 μm or less. ５０ When the average particle diameter D ５０ is easily adjusted to the range described above for the carbonaceous material, and the charge / discharge capacity and charge / discharge efficiency of the non-aqueous electrolyte secondary battery are easily increased. ５０ can be measured in the same manner as described for the carbonaceous material.

[0048] The method for producing the carbonaceous material of the present invention is not limited to the production method of the present invention described above, and any production method may be used as long as the carbonaceous material satisfies the above characteristics. In the production method of the present invention, in order to sufficiently remove the volatile matter generated in steps (1) and (2), the carbide or carbon precursor is cooled to 200°C or less in each of steps (1) and (2) before the next step is performed. However, for example, in particular in step (1), if a step is performed to sufficiently remove the volatile matter generated by the heat treatment by injecting a large amount of inert gas, it is considered possible to perform the heat treatments of steps (1) and (2) consecutively without cooling to 200°C or less.

[0049] The carbonaceous material of the present invention or the carbonaceous material obtained by the production method of the present invention can be suitably used as a negative electrode active material for a nonaqueous electrolyte secondary battery. The present invention also provides a negative electrode for a nonaqueous electrolyte secondary battery containing the carbonaceous material of the present invention, and a nonaqueous electrolyte secondary battery having the negative electrode.

[0050] A method for producing a negative electrode for a nonaqueous electrolyte secondary battery according to the present invention will be specifically described below. The negative electrode of the present invention can be produced as follows. For example, a binder is added to the carbonaceous material of the present invention, and an appropriate amount of a suitable solvent is added, and then these are kneaded to prepare an electrode mixture. The obtained electrode mixture is applied to a current collector plate made of a metal plate or the like, dried, and then pressure-molded.

[0051] By using the carbonaceous material of the present invention, an electrode (negative electrode) having high conductivity can be produced without the addition of a conductive additive. To further enhance conductivity, a conductive additive can be added as needed during the preparation of the electrode mixture. Examples of conductive additives that can be used include conductive carbon black, vapor-grown carbon fiber (VGCF), and nanotubes. The amount of conductive additive added varies depending on the type of conductive additive used. Adding too little may result in the desired conductivity not being achieved, while adding too much may result in poor dispersion in the electrode mixture. From this perspective, the preferred proportion of conductive additive added is 0.5 to 10% by mass (where the amount of active material (carbonaceous material) + the amount of binder + the amount of conductive additive = 100% by mass), more preferably 0.5 to 7% by mass, and particularly preferably 0.5 to 5% by mass. The binder is not particularly limited as long as it does not react with the electrolyte, such as PVDF (polyvinylidene fluoride), polytetrafluoroethylene, or a mixture of SBR (styrene butadiene rubber) and CMC (carboxymethyl cellulose). A mixture of SBR and CMC is particularly preferred because the SBR and CMC attached to the active material surface do not significantly inhibit lithium ion migration, resulting in good input / output characteristics. A polar solvent such as water is preferably used to disperse an aqueous emulsion of SBR or CMC to prepare a slurry, but a resin such as PVDF can also be dissolved in N-methylpyrrolidone or the like. Adding too much binder increases the resistance of the resulting electrode, which can increase the internal resistance of the battery and degrade battery performance. Adding too little binder can result in insufficient bonding between the particles of the negative electrode material and with the current collector. The preferred amount of binder added varies depending on the type of binder used. For example, when using a binder that uses water as a solvent, a mixture of multiple binders, such as a mixture of SBR and CMC, is often used. The total amount of binder used is preferably 0.5 to 5 mass%, more preferably 1 to 4 mass%. On the other hand, for PVDF-based binders, the amount is preferably 3 to 13 mass%, more preferably 3 to 10 mass%. The amount of the carbonaceous material of the present invention in the electrode mixture is preferably 80 mass% or more, more preferably 90 mass% or more.The amount of the carbonaceous material of the present invention in the electrode mixture is preferably 100 mass % or less, more preferably 97 mass % or less.

[0052] The electrode active material layer is generally formed on both sides of the current collector plate, but may be formed on one side if necessary. A thicker electrode active material layer is preferable for achieving higher capacity because fewer current collector plates, separators, etc. are required. However, since a larger electrode area facing the counter electrode is advantageous for improving input / output characteristics, if the electrode active material layer is too thick, the input / output characteristics may deteriorate. From the viewpoint of output during battery discharge, the thickness of the active material layer (per side) is preferably 10 to 80 μm, more preferably 20 to 75 μm, and even more preferably 30 to 75 μm.

[0053] The nonaqueous electrolyte secondary battery of the present invention includes the negative electrode for a nonaqueous electrolyte secondary battery of the present invention. A nonaqueous electrolyte secondary battery having a negative electrode for a nonaqueous electrolyte secondary battery containing the carbonaceous material of the present invention has high charge / discharge capacity and charge / discharge efficiency.

[0054] When the carbonaceous material of the present invention is used to form a negative electrode for a non-aqueous electrolyte secondary battery, other materials constituting the battery, such as a positive electrode material, a separator, and an electrolyte solution, are not particularly limited, and various materials that have been conventionally used or proposed for non-aqueous solvent secondary batteries can be used.

[0055] For example, the positive electrode material is a layered oxide (LiMO ２ where M is a metal, for example, LiCoO ２ , LiNiO ２ , LiMnO ２ , or LiNi ｘ Co ｙ Mo ｚ O ２ (where x, y, and z represent the composition ratio), olivine-based (LiMPO ４ where M is a metal, for example LiFePO ４ etc.), spinel type (LiM ２ O ４ where M is a metal, for example LiMn ２ O ４These chalcogen compounds may be mixed as needed. These positive electrode materials are molded together with a suitable binder and a carbon material for imparting electrical conductivity to the electrode, and the molded product is layered on a conductive current collector to form a positive electrode.

[0056] The non-aqueous electrolyte used in combination with these positive and negative electrodes is generally formed by dissolving an electrolyte in a non-aqueous solvent. As the non-aqueous solvent, for example, organic solvents such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, diethoxyethane, γ-butyllactone, tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, or 1,3-dioxolane can be used singly or in combination. In addition, as the electrolyte, LiClO ４ , LiPF ６ , LiBF ４ , LiCF ３ SO ３ , LiAsF ６ , LiCl, LiBr, LiB(C ６ H ５ ) ４ , or LiN(SO ３ CF ３ ) ２ etc. are used.

[0057] A nonaqueous electrolyte secondary battery is generally formed by arranging the positive electrode and negative electrode formed as described above facing each other, optionally with a liquid-permeable separator interposed therebetween, and immersing them in an electrolyte solution. Such separators can be nonwoven fabrics commonly used in secondary batteries or other permeable or liquid-permeable separators made of porous materials. Alternatively, a solid electrolyte made of a polymer gel impregnated with an electrolyte solution can be used instead of or together with the separator.

[0058] The carbonaceous material of the present invention is suitable as a carbonaceous material for batteries (typically, non-aqueous electrolyte secondary batteries for driving vehicles) mounted on vehicles such as automobiles. The vehicle referred to in the present invention is not particularly limited to those generally known as electric vehicles, hybrid vehicles with fuel cells or internal combustion engines, and the like, and is equipped with at least a power supply device including the battery, an electric drive mechanism driven by power supplied from the power supply device, and a control device for controlling the same. The vehicle may further be equipped with a dynamic brake or a regenerative brake, and may be equipped with a mechanism for converting braking energy into electricity and charging the non-aqueous electrolyte secondary battery.

[0059] The carbonaceous material of the present invention preferably has low resistance, and therefore can also be used, for example, as an additive that imparts conductivity to battery electrode materials. The type of battery is not particularly limited, but nonaqueous electrolyte secondary batteries and lead-acid batteries are preferred. Adding the carbonaceous material to such battery electrode materials can form a conductive network, increasing the conductivity and suppressing irreversible reactions, thereby extending the battery's lifespan.

[0060] The present invention will be described in detail below with reference to examples, but these examples are not intended to limit the scope of the present invention. Measurement methods for the physical properties of carbonaceous materials are described below, but the physical properties described in this specification, including the examples, are based on values ​​determined by the following methods.

[0061] (Elemental Analysis) (Silicon and Potassium Contents) The silicon and potassium contents were measured, for example, by the following method. A carbon sample containing predetermined silicon and potassium elements was prepared in advance, and a calibration curve was created using an X-ray fluorescence analyzer to show the relationship between the silicon Kα ray intensity and the silicon content, and the relationship between the potassium Kα ray intensity and the potassium content. Next, the silicon Kα ray and potassium Kα ray intensities were measured for the sample in X-ray fluorescence analysis, and the silicon and potassium contents were determined from the previously prepared calibration curves. X-ray fluorescence analysis was performed using a "ZSX Primus II" manufactured by Rigaku Corporation under the following conditions. 5.0 g of the sample to be measured was weighed onto a nylon filter sheet, the measurement surface was wrapped in polypropylene film, and the sheet was placed in a sample holder for measurement. The X-ray source was set to 30 kV and 100 mA for silicon element measurement and 40 kV and 75 mA for potassium element measurement, and measurements were performed.

[0062] (Ash content) The carbonaceous material was dried to a constant weight in a thermostatic dryer adjusted to 110-120°C, and then allowed to cool in a desiccator. 1-2 g of the cooled carbonaceous material was placed in a crucible of known mass, which had been weighed out in advance, and weighed accurately to the nearest 1 mg. The crucible containing the carbonaceous material was placed in an electric furnace and, in the presence of air, initially heated weakly, then the temperature was gradually increased until complete ashing, and then ignited at 800-900°C for 1 hour. After allowing the crucible to cool in the desiccator, the mass of the residue was measured, and the ash content was calculated using the following formula: Ash content (mass%) = (mass of residue / mass of carbonaceous material) x 100

[0063] (Specific Surface Area by Nitrogen Adsorption BET Method) An approximate formula derived from the BET formula is shown below.

[0064] Using the above approximation formula, a predetermined relative pressure (p / p) is calculated by the multipoint method using nitrogen adsorption at liquid nitrogen temperature. ０ ) and substitute the measured adsorption amount (v) to obtain v ｍ The specific surface area (SSA: unit is m ２ g －１ ) was calculated.

[0065]

[0066] In the above formula, v ｍ is the amount of adsorption required to form a monolayer on the sample surface (cm ３ / g), v is the measured adsorption amount (cm ３ / g), p ０ is the saturated vapor pressure, p is the absolute pressure, c is a constant (reflecting the heat of adsorption), and N is Avogadro's number 6.022 × 10 ２３ , a (nm ２ ) is the area occupied by the adsorbate molecule on the sample surface (molecular occupied cross-sectional area).

[0067] Specifically, the amount of nitrogen adsorbed to a carbonaceous material at liquid nitrogen temperature was measured using a "BELL Sorb Mini" manufactured by Nippon BELL Co., Ltd., as follows. The measurement sample was filled into a sample tube, and the sample tube was cooled to -196°C. The pressure was then reduced once, and nitrogen (purity 99.999%) was then adsorbed onto the measurement sample at a desired relative pressure. The amount of nitrogen adsorbed onto the sample when equilibrium pressure was reached at each desired relative pressure was defined as the amount of adsorbed gas v.

[0068] (X value) The specific surface area of ​​the carbonaceous material measured as above is expressed as Am ２ / g, and the ash content is B mass %, and the value X was calculated by the formula: X = {A / (100-B)} x 100.

[0069] (Average particle diameter D by laser scattering method ５０ ) The average particle size (particle size distribution) of the carbonaceous material or carbide was measured by the following method. The sample was placed in an aqueous solution containing 5 mass% of a surfactant ("Toriton X100" manufactured by Wako Pure Chemical Industries, Ltd.), treated with an ultrasonic cleaner for 10 minutes or more, and dispersed in the aqueous solution. The particle size distribution was measured using this dispersion. The particle size distribution measurement was performed using a particle size / particle size distribution measuring device ("Microtrac MT3300EXII" manufactured by Microtrac Bell Co., Ltd.). D ５０ is the particle size at which the cumulative volume becomes 50%, and this value was used as the average particle size.

[0070] (Raman Spectrum) Raman spectra were measured using LabRAM ARAMIS manufactured by Horiba Ltd., with a laser light source of 532 nm wavelength. For each sample, particles were randomly sampled at three locations, and measurements were taken at two locations within each sampled particle. The measurement conditions were a wavelength range of 50 to 2000 cm －１ The number of measurements was 1000, and the average value of six points was calculated as the measurement value. The G-band half width was calculated by subtracting the D-band (1360 cm) from the D-band (1360 cm) for the spectrum obtained under the above measurement conditions. －１ around 1590cm －１ The R value was the intensity ratio D / G (D band peak intensity / G band peak intensity) of the D band and G band peaks.

[0071] (Example 1) Step 1: 8,000 g of rice husks were heated to 500°C in a nitrogen atmosphere in a batch-type atmosphere furnace. At this time, the temperature was increased to 500°C at a rate of 200°C / hour (3.3°C / min). Next, the husks were carbonized by heat treatment at a low firing temperature of 500°C for 60 minutes in a nitrogen gas flow, to obtain a charcoal. At this time, the nitrogen gas supply rate was 200 L / min. The charcoal was cooled to 50°C in a nitrogen atmosphere to obtain the desired charcoal.

[0072] Step 2: 4.5 g of rice husk carbonized material was placed in a tubular furnace heated to 900°C, and nitrogen gas and water vapor (water vapor partial pressure: 3%) were introduced at a rate of 1.03 L / min, followed by activation for 120 minutes. The activated product was then pulverized in a sample mill to obtain a powder with an average particle size D ５０ The target carbonaceous material having a particle size of 10 μm was obtained. Table 1 shows the Si element and K element contents, ash content, specific surface area, value X, and results of Raman analysis of the obtained carbonaceous material.

[0073] (Example 2) A carbonaceous material was obtained in the same manner as in Example 1, except that the activation time in step 2 of Example 1 was changed to 150 minutes. Table 1 shows the contents of Si element and K element, ash content, specific surface area, value X, and results of Raman analysis of the obtained carbonaceous material.

[0074] (Example 3) A carbonaceous material was obtained in the same manner as in Example 1, except that the activation time in step 2 of Example 1 was changed to 90 minutes. Table 1 shows the contents of Si element and K element, ash content, specific surface area, value X, and results of Raman analysis of the obtained carbonaceous material.

[0075] (Comparative Example 1) The carbide obtained in step 1 of Example 1 was pulverized in a sample mill to obtain a powder with an average particle size D ５０ The target carbonaceous material having a particle size of 10 μm was obtained. Table 1 shows the Si element and K element contents, ash content, specific surface area, value X, and results of Raman analysis of the obtained carbonaceous material.

[0076] (Comparative Example 2) Coconut shell activated carbon "Kuraray Coal" GW48 / 100 manufactured by Kuraray Co., Ltd. was pulverized in a sample mill to obtain a powder with an average particle size D ５０ The target carbonaceous material having a particle size of 10 μm was obtained. Table 1 shows the Si element and K element contents, ash content, specific surface area, value X, and results of Raman analysis of the obtained carbonaceous material.

[0077] (Electrode Fabrication) Using the carbonaceous materials obtained in each Example and Comparative Example, negative electrodes were fabricated according to the following procedure. 87 parts by mass of the carbonaceous material, 10 parts by mass of conductive carbon black ("Super-P (trademark)" manufactured by TIMCAL), 1 part by mass of CMC, 2 parts by mass of SBR, and 90 parts by mass of water were mixed to obtain a slurry. The obtained slurry was applied to a copper foil with a thickness of 18 μm, dried, and pressed to obtain an electrode with a thickness of 45 μm. The density of the obtained electrode was as shown in Table 2.

[0078] (Battery Capacity and Charge / Discharge Efficiency) The electrode prepared above was used as the working electrode, and metallic lithium was used as the counter electrode and reference electrode. As the solvent, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were mixed in a volume ratio of 1:1:1. LiPF ６ was dissolved at 1 mol / L and used as the electrolyte. A polypropylene membrane was used as the separator. A coin cell was fabricated in a glove box under an argon atmosphere.

[0079] For the lithium secondary battery having the above configuration, a charge / discharge test was performed using a charge / discharge tester ("TOSCAT" manufactured by Toyo Systems Co., Ltd.) to measure the DC resistance before initial charging, followed by a charge / discharge test. Lithium doping was performed at a rate of 70 mA / g relative to the active material mass until the doping reached 1 mV relative to the lithium potential. A constant voltage of 1 mV relative to the lithium potential was applied for 8 hours to complete the doping. The capacity (mAh / g) at this time was taken as the charge capacity. Next, undoping was performed at a rate of 70 mA / g relative to the active material mass until the doping reached 2.5 V relative to the lithium potential, and the capacity at this time was taken as the discharge capacity. The percentage of the discharge capacity / charge capacity was taken as the charge / discharge efficiency, which was used as an index of the utilization efficiency of lithium ions in the battery.

[0080]

[0081]

Claims

1. The silicon content is 5% by mass or more and the specific surface area is 250 m ２ / g or more and an ash content of 40 mass% or more.

2. Specific surface area: Am ２ / g and an ash content of B mass%, the value X calculated by X = {A / (100-B)} x 100 is 500 or more.

3. Raman spectroscopy, 100-600 cm －１ The intensity of the peak derived from silicon in the vicinity of １ and 1300 cm －１ The intensity of the peak derived from the carbon in the vicinity is D ２ When this is done, D ２ D against １ The ratio (D １ / D ２ 3. The carbonaceous material according to claim 1, wherein σ is less than 0.

2.

4. The carbonaceous material according to any one of claims 1 to 3, having an R value of 0.9 to 1.1 by Raman spectroscopy.

5. The carbonaceous material according to any one of claims 1 to 4, wherein the potassium content is 1.0 mass% or less.

6. The carbonaceous material according to any one of claims 1 to 5, which is derived from a grass family plant.

7. A method for producing a carbonaceous material having an ash content of 40% by mass or more, comprising: (1) heating a carbide precursor having a silicon content of 3% by mass or more at 300 to 900°C in an inert gas atmosphere to obtain a carbide; and (2) activating the carbide obtained in step (1) at 800 to 1000°C in an oxidizing gas atmosphere to obtain a carbonaceous material.

8. The manufacturing method according to claim 7, further comprising, between steps (1) and (2), a step of pulverizing the carbide obtained in step (1) and a step of washing the carbide obtained in step (1) with an acid.

9. A negative electrode for a non-aqueous electrolyte battery using the carbonaceous material according to any one of claims 1 to 6.

10. A non-aqueous electrolyte battery using the negative electrode according to claim 9.

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