JP7714558B2 - Carbonaceous material suitable for negative electrode active material of electricity storage device, negative electrode for electricity storage device, electricity storage device - Google Patents

Description

This patent application claims priority under the Paris Convention to Japanese Patent Application No. 2020-154229 (filing date: September 15, 2020), the entire contents of which are incorporated herein by reference.
The present invention relates to a carbonaceous material suitable as a negative electrode active material for an electricity storage device, a negative electrode for an electricity storage device containing the carbonaceous material, and an electricity storage device having the negative electrode.

Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are widely used in small portable devices such as mobile phones and laptops due to their high energy density and excellent output characteristics. Nitrogen-containing non-graphitizable carbons capable of doping (charging) and dedoping (discharging) lithium in amounts exceeding the theoretical capacity of graphite, 372 mAh/g, have been disclosed as negative electrode materials for lithium-ion secondary batteries (see, for example, Patent Documents 1, 2, and 3).

Japanese Patent Application Laid-Open No. 2006-083012 International Publication No. 2019/009332 International Publication No. 2019/009333

In recent years, as lithium-ion secondary batteries have been considered for use in automobiles and other applications, there has been a growing demand for cycle durability in addition to the high capacity characteristics mentioned above. For example, automotive lithium-ion secondary batteries are large and expensive, making them difficult to replace mid-use. For this reason, lithium-ion secondary batteries are required to have at least the same level of durability as automobiles, and in particular, high cycle durability, meaning that discharge capacity does not decrease even after repeated charge and discharge.

In contrast, the nitrogen elements contained in the carbon materials described in Patent Documents 1 to 3 can act as lithium ion absorption sites and improve discharge capacity, but they can also act as reaction sites with lithium ions, which may prevent cycle durability from being ensured, making it difficult to achieve both high discharge capacity and high cycle durability.

The present invention therefore aims to provide a carbonaceous material suitable as a negative electrode active material for an electricity storage device (e.g., lithium ion secondary battery, sodium ion secondary battery, lithium-sulfur battery, lithium-air battery, all-solid-state battery, capacitor, etc.) that has a high discharge capacity and high cycle durability, a negative electrode for an electricity storage device that contains the carbonaceous material, and an electricity storage device that has the negative electrode.

As a result of extensive research, the present inventors have found that the above problems can be solved by using a negative electrode made of a specific carbonaceous material, and have arrived at the present invention.
That is, the present invention includes the following preferred embodiments.
[1] A carbonaceous material having a nitrogen element content of 1.0 mass% or more and 4.0 mass% or less, an oxygen element content of 1.1 mass% or more and 2.1 mass% or less, as determined by elemental analysis, and a carbon interplanar spacing (d ₀₀₂ ) of 3.70 Å or more, as observed by X-ray diffraction measurement.
[2] The carbonaceous material according to [1], wherein the oxygen element content determined by elemental analysis is 1.2 mass % or more and 2.0 mass % or less.
[3] The carbonaceous material according to [1] or [2], wherein the ratio of the oxygen content to the nitrogen content (R _O/N ) determined by elemental analysis is 0.30 or more and 1.0 or less.
[4] The carbonaceous material according to any one of [1] to [3], wherein the half width of a peak near 1360 cm ⁻¹ in a Raman spectrum observed by laser Raman spectroscopy is 250 cm ⁻¹ or more.
[5] The carbonaceous material according to any one of [1] to [4], wherein the half width of a peak near 1650 cm ⁻¹ in a Raman spectrum observed by laser Raman spectroscopy is 98 cm ⁻¹ or more.
[6] The carbonaceous material according to any one of [1] to [5] above, wherein the average particle diameter _D50 is 30 μm or less.
[7] The carbonaceous material according to any one of [1] to [6], wherein the carbonaceous material is derived from a sugar.
[8] The carbonaceous material according to any one of [1] to [7], which is used as a negative electrode active material for an electricity storage device.
[9] A negative electrode for an electricity storage device, comprising the carbonaceous material according to [8] above.
[10] An electricity storage device having the electricity storage device negative electrode according to [9] above.

Electricity storage devices using negative electrodes containing the carbonaceous material of the present invention have high discharge capacity and high cycle durability. Note that, in this specification, cycle durability refers to the capacity retention rate (cycle capacity retention rate) when the battery is repeatedly charged and discharged.

The following describes in detail an embodiment of the present invention. Note that the scope of the present invention is not limited to the embodiment described here, and various modifications can be made without departing from the spirit of the present invention. In addition, the following description uses an example in which the present invention is applied to a lithium-ion battery as an electricity storage device.

In the carbonaceous material of the present invention, the nitrogen element content determined by elemental analysis is 1.0% by mass or more. If the nitrogen element content in the carbonaceous material is less than 1.0% by mass, there will be fewer sites for absorbing and desorbing lithium ions during charge and discharge, and the discharge capacity will tend to decrease. In order to achieve such a high discharge capacity, the nitrogen element content in the carbonaceous material of the present invention must be 1.0% by mass or more, preferably 1.5% by mass or more, more preferably 1.75% by mass or more, and even more preferably 2.0% by mass or more.

Furthermore, in the carbonaceous material of the present invention, the nitrogen content determined by elemental analysis is 4.0% by mass or less. If the nitrogen content in the carbonaceous material is greater than 4.0% by mass, the charge/discharge efficiency decreases. This is thought to be because the nitrogen not incorporated into the carbon skeleton exists as a surface functional group such as an —NH ₂ group, and if the amount of such surface functional groups increases, it becomes impossible to suppress irreversible side reactions that may occur during charge/discharge.
In addition, it has been found that, unexpectedly, when the nitrogen element content in the carbonaceous material is 4.0% by mass or less, the discharge capacity tends to be less likely to decrease even with repeated charge and discharge, i.e., high cycle durability tends to be obtained. Although the reason for this is not clear, the following is thought to be the reason. That is, when the nitrogen element content in the carbonaceous material is high, it is thought that a combination of factors occurs, such as a decrease in discharge capacity due to the occurrence of irreversible side reactions during repeated charge and discharge, a decrease in discharge capacity due to the expansion and contraction of the electrodes during charge and discharge, and a decrease in discharge capacity due to structural changes in the carbonaceous material, particularly around the nitrogen element, which causes distortion due to repeated charge and discharge. It is thought that these factors that deteriorate cycle durability are eliminated by setting the nitrogen element content to 4.0% by mass or less.
In order to obtain such high cycle durability, the nitrogen element content in the carbonaceous material needs to be 4.0 mass% or less, and in one embodiment is 3.8 mass% or less, preferably 3.5 mass% or less, more preferably 3.3 mass% or less, and even more preferably 3.0 mass% or less.

The details of measuring the nitrogen element content are described below, and it is measured by elemental analysis (inert gas dissolution method). There are no limitations on the method for adjusting the nitrogen element content to the above range, but if the inert gas flow rate is too high or the heating rate is too low, the nitrogen element in the carbonaceous material will be more likely to be desorbed, reducing the amount of nitrogen element in the resulting carbonaceous material; therefore, it is advisable to adjust the inert gas flow rate, heating rate, and heat treatment temperature.

In the carbonaceous material of the present invention, the oxygen content determined by elemental analysis is 1.1% by mass or more and 2.1% by mass or less. Like nitrogen, oxygen in a carbonaceous material serves as a site for absorbing and desorbing lithium ions during charge and discharge. Therefore, a high oxygen content tends to increase discharge capacity. For this reason, the oxygen content determined by elemental analysis must be 1.0% by mass or more. In some embodiments, it is 1.1% by mass or more, preferably 1.2% by mass or more, more preferably 1.4% by mass or more, even more preferably 1.5% by mass or more (1.5% by mass or more), and even more preferably 1.6% by mass or more. On the other hand, a high oxygen content may result in a deterioration in cycle durability, similar to the nitrogen content described above. For this reason, in some embodiments, the oxygen content in the carbonaceous material of the present invention is 2.0% by mass or less, preferably 1.9% by mass or less, and more preferably 1.8% by mass or less. The details of the measurement of the oxygen element content are as described in the Examples, and the measurement is carried out by the elemental analysis method (inert gas dissolution method) described later.

In the carbonaceous material of the present invention, in order to obtain a high discharge capacity and high cycle durability, the ratio of the nitrogen element content to the hydrogen element content, R _N/H (nitrogen element content/hydrogen element content), is preferably 6 to 37, more preferably 8 to 25, even more preferably 10 to 20, and even more preferably 12 to 15. Furthermore, in the carbonaceous material of the present invention, in order to obtain a high discharge capacity and high cycle durability, the ratio of the oxygen element content to the nitrogen element content, R _O/N (oxygen element content/nitrogen element content), is preferably 0.30 to 1.0. This is believed to be due to the following reason. In other words, increasing both the oxygen element content and the nitrogen element content increases the discharge capacity while deteriorating the cycle durability. Furthermore, if either the oxygen element content or the nitrogen element content is too high or too low, the amount of oxygen or nitrogen present as surface functional groups, such as —OH groups or —NH ₂ groups, in the carbon structure increases, resulting in a decrease in discharge capacity and a decrease in cycle durability. In order to obtain a high discharge capacity and high cycle durability, the lower limit of R _O/N is preferably 0.30 or more, more preferably 0.35 or more, and even more preferably 0.40 or more. The upper limit of R _O/N is preferably 1.0 or less, more preferably 0.90 or less, and even more preferably 0.80 or less. The ratio of nitrogen element content to hydrogen element content R _N/H is calculated from the nitrogen element content and hydrogen element content measured as above by the formula R _N/H = nitrogen element content/hydrogen element content, and the ratio of oxygen element content R _O/N is calculated from the oxygen element content and nitrogen element content measured as above by the formula R _O/N = oxygen element content/nitrogen element content.

In the carbonaceous material of the present invention, a lower hydrogen element content in the carbonaceous material is preferable, as it reduces carbon edge portions, expands carbon planes, and facilitates electron movement. From this perspective, the hydrogen element content determined by elemental analysis in the carbonaceous material is preferably 0.50 mass% or less, more preferably 0.40 mass% or less, and even more preferably 0.35 mass% or less. Furthermore, a higher hydrogen element content in the carbonaceous material is preferable, as it facilitates lithium ion adsorption and desorption. From this perspective, the hydrogen element content in the carbonaceous material is preferably 0.10 mass% or more, more preferably 0.15 mass% or more. Details of measuring the hydrogen element content are described below, and it is measured by elemental analysis (inert gas dissolution method).

In the carbonaceous material of the present invention, the carbon interplanar spacing (d ₀₀₂ ) calculated by the Bragg equation from the peak position (diffraction angle 2θ) observed by powder X-ray diffraction is 3.70 Å or more. For example, when the carbon planes are closest to each other, as in graphite, d ₀₀₂ is approximately 3.35 to 3.40 Å. However, when d ₀₀₂ is larger than that in the case of graphite, lithium ions can migrate efficiently, and micropores capable of occluding a large amount of clustered lithium are formed, resulting in a high discharge capacity. From this perspective, d ₀₀₂ must be 3.70 Å or more, preferably 3.74 Å or more, more preferably 3.76 Å or more. On the other hand, if d ₀₀₂ is too large, the volume of the carbonaceous material increases, which may result in a small effective capacity per volume. From this perspective, d ₀₀₂ is preferably 4.00 Å or less, more preferably 3.95 Å or less, and even more preferably 3.90 Å or less.

In the carbonaceous material of the present invention, the half-width of the peak near 1360 cm ⁻¹ in the Raman spectrum observed by laser Raman spectroscopy is preferably 250 cm ⁻¹ or more. Here, the peak near 1360 cm ⁻¹ is a Raman peak commonly referred to as the D band, which is a peak caused by disorder and defects in the graphite structure. The peak near 1360 cm ⁻¹ is usually observed in the range of 1345 cm ⁻¹ to 1375 cm ⁻¹ , preferably 1350 cm ⁻¹ to 1370 cm ⁻¹ . The half-width of the D band peak is related to the amount of disorder and defects in the graphite structure contained in the carbonaceous material. Such structural disorder can be caused by the introduction of nitrogen elements into the carbon skeleton. If the half-width of the Raman peak in the D band is less than 250 cm ⁻¹ , the structure will develop too much, and the development of the graphite structure will tend to prevent efficient entry and exit of lithium ions. Therefore, from the viewpoint of facilitating a reduction in resistance, the half-width of the peak near 1360 ^cm is preferably 250 ^cm or more, more preferably 260 cm ^or more, and even more preferably 270 ^cm or more. Furthermore, if the half-width is greater than 300 ^cm , the graphite structure contained in the carbonaceous material tends to be difficult to maintain, the amount of amorphous material increases, and the number of sites capable of absorbing lithium tends to decrease. This may result in a decrease in the amount of lithium ions absorbed, leading to a decrease in discharge capacity. From this viewpoint, the half-width of the peak near 1360 ^cm is preferably 300 ^cm or less.

Furthermore, in the carbonaceous material of the present invention, the half-width value of the peak near 1650 cm ⁻¹ in the Raman spectrum observed by laser Raman spectroscopy is preferably 98 cm ⁻¹ or more. Here, the peak near 1650 cm ⁻¹ is a Raman peak generally referred to as the G band, which is a peak attributable to the graphite structure. If the half-width of the Raman peak in the G band is in the range of less than 98 cm ⁻¹ , the structure develops too much, and the development of the graphite structure prevents efficient entry and exit of lithium ions, which tends to increase resistance. Therefore, from the viewpoint of easily reducing the resistance, the half-width of the peak near 1650 cm ⁻¹ is preferably 98 cm ⁻¹ or more, more preferably 100 cm ⁻¹ or more, and even more preferably 102 cm ⁻¹ or more. There is no particular upper limit to the half width of the peak near 1650 cm ⁻¹ , but it is usually 115 cm ⁻¹ or less, preferably 110 cm ⁻¹ or less, and more preferably 107 cm ⁻¹ or less.

The Raman spectrum is measured using a Raman spectrometer (for example, a Raman spectrometer "LabRAM ARAMIS (VIS)" manufactured by Horiba, Ltd.). Specifically, for example, the particles to be measured are placed on an observation stage, the magnification of the objective lens is set to 100 times, the focus is adjusted, and 532 nm argon ion laser light is irradiated into the measurement cell, with an exposure time of 1 second, an accumulation count of 100, and a measurement range of 50-2000 ^cm .

The carbonaceous material of the present invention preferably has an average particle diameter _D50 of 30 μm or less, which not only improves the coatability during electrode preparation but also reduces the free diffusion path of lithium ions within the carbonaceous material particles, facilitating rapid charge and discharge. Furthermore, in lithium-ion secondary batteries, it is important to increase the electrode area to improve input/output characteristics. To achieve this, it is necessary to reduce the coating thickness of the active material on the current collector plate during electrode preparation. From these perspectives, the smaller the average particle diameter _D50 , the more preferable it is, and it is preferably 30 μm or less, more preferably 25 μm or less, even more preferably 20 μm or less, particularly preferably 18 μm or less, and most preferably 15 μm or less. When the average particle diameter of the carbonaceous material serving as the active material is below the above upper limit, it is easy to reduce the coating thickness during electrode preparation. Furthermore, the carbonaceous material of the present invention preferably has an average particle diameter _D50 of 2 μm or more, which suppresses an increase in the specific surface area and an increase in reactivity with the electrolyte due to fine powder in the carbonaceous material, and thus facilitates suppression of an increase in irreversible capacity. Furthermore, when a negative electrode is produced using a carbonaceous material, it is easy to ensure that voids are formed between the carbonaceous material, and the migration of lithium ions in the electrolyte is less likely to be inhibited, making it easy to reduce the resistance of the electricity storage device. From these perspectives, the average particle diameter _D50 is preferably 2 μm or more, more preferably 3 μm or more, even more preferably 4 μm or more, and particularly preferably 5 μm or more. The average particle diameter _D50 is the particle diameter at which the cumulative volume becomes 50%, and can be determined, for example, by measuring the particle size distribution by a laser scattering method using a particle diameter/particle size distribution measuring device ("Microtrac MT3300EXII" manufactured by Microtrac-Bell Corporation).

The carbonaceous material of the present invention is
(1) mixing the raw material with a compound containing nitrogen (hereinafter also referred to as "nitrogen-containing compound");
(2) heating the resulting mixture in an inert gas atmosphere at a temperature increase rate of 100°C/hour or more to a first predetermined temperature between 500°C and 1200°C;
The carbonaceous material can be produced by a method including the steps of (3) heat-treating the carbide in an inert gas atmosphere at a temperature of 500 to 1200°C, preferably a first predetermined temperature, to obtain a carbide; (4) heating the obtained carbide in an inert gas atmosphere at a heating rate of 100°C/hour or more to a second predetermined temperature between the first predetermined temperature and 1400°C; and (5) heat-treating the carbide in an inert gas atmosphere at a temperature of 800 to 1400°C, preferably the second predetermined temperature. In this specification, the above steps (2) and/or (3) may be referred to as "low-temperature firing" (step), and the above steps (4) and/or (5) may be referred to as "high-temperature firing" (step). Carbonaceous materials can be obtained by conventional methods, such as pulverizing the carbide using a ball mill or jet mill. Pulverization may be performed after step (3) and/or step (5).

The raw material for the carbonaceous material of the present invention is not limited, and known non-graphitizable carbonaceous materials such as petroleum pitch, coal pitch, and coconut shells can be used. However, sugars are preferably used as raw materials, as they are easy to use to obtain the carbonaceous material of the present invention having the above characteristics and are easy to modify the structure, such as to form a crosslinked structure. By using sugars as raw materials, it is possible to obtain a carbonaceous material derived from sugars, more specifically, derived from a substance having a sugar skeleton.

When sugars are used as raw materials, there are no particular limitations on the type. Examples include monosaccharides such as glucose, galactose, mannose, fructose, ribose, and glucosamine; disaccharides such as sucrose, trehalose, maltose, cellobiose, maltitol, lactobionic acid, and lactosamine; and polysaccharides such as starch, glycogen, agarose, pectin, cellulose, chitin, and chitosan. These sugars can be used alone or in combination of two or more. Among these sugars, starch is preferred because it is easily available in large quantities. By using such sugars as raw materials, a sugar-derived carbonaceous material can be obtained.

The nitrogen-containing compound is not particularly limited, but examples include inorganic ammonium salts such as ammonium chloride, ammonium sulfate, ammonium carbonate, and ammonium nitrate; organic ammonium salts such as ammonium formate, ammonium acetate, ammonium oxalate, and diammonium hydrogen citrate; aromatic amine hydrochlorides such as aniline hydrochloride and aminonaphthalene hydrochloride; and nitrogen-containing organic compounds such as melamine, pyrimidine, pyridine, pyrrole, imidazole, indole, and urea. Among these nitrogen-containing compounds, melamine and urea, which have a high intramolecular nitrogen content, are preferred, as they allow for greater incorporation of nitrogen elements into the carbonaceous material.

In addition to the nitrogen-containing compound, an acid catalyst may also be added. The acid catalyst to be added is not particularly limited, but examples include organic acids. Examples of organic acids include, but are not limited to, aliphatic carboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, palmitic acid, stearic acid, succinic acid, linoleic acid, oleic acid, and adipic acid; aromatic carboxylic acids such as benzoic acid, salicylic acid, phthalic acid, and terephthalic acid; hydroxycarboxylic acids such as lactic acid, tartaric acid, citric acid, and malic acid; carboxylic acids such as ethylenediaminetetraacetic acid; and sulfonic acids such as p-toluenesulfonic acid and methanesulfonic acid. Adding an acid catalyst promotes the reaction between the sugars and the nitrogen-containing compound, tending to produce a carbonaceous material with a high nitrogen content.

The method of mixing the raw material and the nitrogen-containing compound, or the raw material, the nitrogen-containing compound and the acid catalyst, is not particularly limited, but dry or wet mixing can be used.

In the case of dry mixing, a mixture of the nitrogen-containing compound or the nitrogen-containing compound and the acid catalyst can be obtained by adding them to the raw materials. In this case, in order to uniformly mix the nitrogen-containing compound or the nitrogen-containing compound and the acid catalyst with the raw materials, it is preferable to grind the nitrogen-containing compound or the nitrogen-containing compound and the acid catalyst with the raw materials in a mortar or mortar, or to grind them in a ball mill to form powder and then mix them.

In the case of wet mixing, for example, a solution can be prepared by dissolving the raw materials in a solvent, and then mixing can be performed by adding the nitrogen-containing compound, or the nitrogen-containing compound and the acid catalyst, to this solution. Mixing can also be performed by spraying (e.g., spraying) the solution onto the nitrogen-containing compound. Mixing can also be performed by adding the raw materials to a solution prepared by dissolving the nitrogen-containing compound, or the nitrogen-containing compound and the acid catalyst, in a solvent. After mixing, the solvent can be evaporated if necessary. This process can produce a mixture of the raw materials and the nitrogen-containing compound, or the nitrogen-containing compound and the acid catalyst. The solvent used is not particularly limited, but examples thereof include water, alcohol solvents (ethanol, methanol, ethylene glycol, isopropyl alcohol, etc.), ester solvents (ethyl acetate, butyl acetate, ethyl lactate, etc.), ether solvents (tetrahydrofuran, dimethoxyethane, 1,4-dioxane, etc.), ketone solvents (acetone, 2-butanone, cyclopentanone, cyclohexanone, etc.), aliphatic hydrocarbon solvents (pentane, hexane, heptane, etc.), aromatic hydrocarbon solvents (toluene, xylene, mesitylene, etc.), nitrile solvents (acetonitrile, etc.), chlorinated hydrocarbon solvents (dichloromethane, chloroform, chlorobenzene, etc.), and mixtures thereof. In order to uniformly mix the raw material and the nitrogen-containing compound, or the raw material, the nitrogen-containing compound, and the acid catalyst, it is effective that both are easily soluble in the solvent. Therefore, water, alcohol solvents, and mixtures thereof are preferred as the solvent. The method for evaporating the solvent is not particularly limited, but examples thereof include heat treatment, reduced pressure treatment, and combinations thereof. The temperature for the heat treatment may be any temperature at which thermal decomposition of the nitrogen-containing compound or the sugar is unlikely to occur, and varies depending on the type of solvent, but is preferably 40 to 150°C, more preferably 50 to 120°C, and even more preferably 60 to 100°C.

The amount of nitrogen contained in the nitrogen-containing compound mixed with the raw materials is preferably 0.1 molar equivalents or more, more preferably 0.2 molar equivalents or more, even more preferably 0.3 molar equivalents or more, and preferably 2.0 molar equivalents or less, more preferably 1.0 molar equivalents or less, and even more preferably 0.8 molar equivalents or less, based on the mass of the resulting mixture. Mixing the amount of nitrogen-containing compound at or above the lower limit above is preferred because nitrogen elements are efficiently incorporated into the resulting carbonaceous material. Mixing the amount of nitrogen-containing compound at or below the upper limit above suppresses excessive nitrogen element incorporation and prevents excessive disruption of the carbon structure, making it easier for an energy storage device containing the resulting carbonaceous material to exhibit high discharge capacity and high cycle durability.

In the production method of the present invention, the mixture obtained in step (1) of mixing the raw material with the nitrogen-containing compound, or the nitrogen-containing compound and the acid catalyst, is heated to perform a first calcination, and the resulting carbonized product is then subjected to a second calcination.
In the first firing, the mixture obtained from step (1) is preferably heated to a temperature between 500 and 1200°C, preferably 600 and 1150°C, more preferably 700 and 1100°C, and even more preferably 800 and 1100°C, and is preferably subjected to a heat treatment in an inert gas, preferably an inert gas atmosphere, at a first predetermined temperature described below to obtain a carbide [step (3)]. Examples of inert gases include nitrogen gas. The temperature at which the heat treatment is performed using an inert gas is preferably the first predetermined temperature described below, but is not particularly limited as long as it is within the above range. Furthermore, the temperature at which the heat treatment is performed using an inert gas may be a constant temperature, but is not particularly limited as long as it is within the above range.

In this process, the mixture obtained in step (1), in which the raw materials are mixed with a nitrogen-containing compound or a nitrogen-containing compound and an acid catalyst, is preferably heated in an inert gas atmosphere such as nitrogen gas at a heating rate of 100°C/hour or more, preferably 100 to 600°C/hour, to a first predetermined temperature between 500 to 1200°C, preferably 600 to 1150°C, more preferably 700 to 1130°C, and even more preferably 800 to 1100°C [step (2)]. The heating in step (2) is carried out in an inert gas atmosphere, but heat treatment may also be carried out while an inert gas is being supplied.

In the manufacturing method of the present invention, the carbide obtained by the first firing is then subjected to a second firing. In the second firing, the carbide obtained from step (3) is preferably heated to a temperature between 800 and 1400°C, preferably 1050 and 1380°C, more preferably 1100 and 1370°C, even more preferably 1150 and 1360°C, and particularly preferably 1200 and 1350°C, and is then heat-treated in an inert gas atmosphere, such as nitrogen gas, preferably an inert gas atmosphere, at a temperature between 800 and 1400°C, preferably 1050 and 1380°C, more preferably 1100 and 1370°C, even more preferably 1150 and 1360°C, and particularly preferably 1200 and 1350°C, preferably at the second predetermined temperature described below [step (5)]. The temperature at which the inert gas heat treatment is performed is preferably the second predetermined temperature described below, but is not particularly limited as long as it is within the above range. The temperature at which the inert gas heat treatment is performed may also be a constant temperature, but is not particularly limited as long as it is within the above range.

In this case, a volatile organic compound may be added to the carbonized material obtained in step (3) and then subjected to step (4). Volatile organic compounds are organic compounds that undergo heat treatment (e.g., at 500°C or higher) with an inert gas such as nitrogen, but are rarely carbonized (e.g., 80% or more, preferably 90% or more) and volatilize (vaporize or pyrolyze into gas). Volatile organic compounds include, but are not limited to, thermoplastic resins and low-molecular-weight organic compounds. Specific examples of thermoplastic resins include polystyrene, polyethylene, polypropylene, poly(meth)acrylic acid, and poly(meth)acrylic acid esters. In this specification, (meth)acrylic is a general term for methacrylic and acrylic. Examples of low-molecular-weight organic compounds include toluene, xylene, mesitylene, styrene, naphthalene, phenanthrene, anthracene, and pyrene. Thermoplastic resins that volatilize at firing temperatures and do not oxidize and activate the surface of the carbon precursor when pyrolyzed are preferred, and therefore polystyrene, polyethylene, and polypropylene are preferred. Further, from the viewpoint of safety, the low-molecular-weight organic compound is preferably one that is less volatile at room temperature (e.g., 20°C), and preferred examples include naphthalene, phenanthrene, anthracene, pyrene, etc. Addition of such a volatile organic compound is preferred in that it allows the specific surface area to be further reduced while maintaining the characteristic structure of the carbonaceous material of the present invention.

In this process, the carbide obtained from step (3) is preferably heated to a second predetermined temperature in the range of the first predetermined temperature to 1400°C at a heating rate of 100°C/hour or more, preferably 100 to 600°C/hour, in an inert gas atmosphere such as nitrogen gas [step (4)]. The heating in step (4) is carried out in an inert gas atmosphere, but heat treatment may also be carried out while an inert gas is being supplied.

The second firing is typically carried out at a temperature equal to or higher than the temperature of the first firing, preferably at a temperature higher than the temperature of the first firing. Therefore, in this specification, the first firing and the second firing are sometimes referred to as low-temperature firing and high-temperature firing, respectively. The temperature of the second firing, i.e., the second predetermined temperature in step (4), is preferably equal to or higher than the temperature of the first firing, i.e., the first predetermined temperature in step (2), in order to obtain a carbonaceous material that provides high discharge capacity and charge/discharge efficiency when used in an electrode. The second predetermined temperature is more preferably 50 to 700°C higher than the first predetermined temperature, even more preferably 100 to 500°C higher than the first predetermined temperature, even more preferably 150 to 400°C higher than the first predetermined temperature, and particularly preferably 200 to 300°C higher than the first predetermined temperature. The second predetermined temperature is preferably 1400°C or less, more preferably 1380°C or less, even more preferably 1360°C or less, and particularly preferably 1350°C or less.

The carbonaceous material of the present invention can be suitably used as a negative electrode active material for an electricity storage device. The present invention also provides a negative electrode for an electricity storage device comprising the carbonaceous material of the present invention.

The following describes in detail a method for manufacturing a negative electrode for an electricity storage device of the present invention. The negative electrode of the present invention can be manufactured by adding a binder to the carbonaceous material of the present invention, adding an appropriate amount of a suitable solvent, and kneading the mixture to form an electrode mixture, which is then applied to a current collector plate made of a metal plate or the like, dried, and then pressure-molded.

In addition, lithium pre-doping can be performed using known methods to reduce the irreversible capacity of the resulting negative electrode.

By using the carbonaceous material of the present invention, an electrode (negative electrode) having high conductivity can be produced without adding a conductive additive. However, to further enhance conductivity, a conductive additive can be added during preparation of the electrode mixture, as needed. 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). PVDF is preferred because the PVDF attached to the active material surface does not significantly inhibit lithium ion migration, resulting in good input/output characteristics. Polar solvents such as N-methylpyrrolidone (NMP) are preferably used to dissolve PVDF and form a slurry, but aqueous emulsions of SBR or CMC dissolved in water can also be used. 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, but for example, for PVDF-based binders, it is preferably 3 to 13 mass %, more preferably 3 to 10 mass %. On the other hand, for binders that use water as a solvent, a mixture of multiple binders, such as a mixture of SBR and CMC, is often used, and the total amount of all binders used is preferably 0.5 to 5 mass %, more preferably 1 to 4 mass %. Furthermore, the carbonaceous material of the present invention in the electrode mixture is preferably 80 mass % or more, more preferably 90 mass % or more.
The carbonaceous material of the present invention in the electrode mixture preferably accounts for 100 mass % or less, more preferably 97 mass % or less.

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. The thicker the electrode active material layer, the higher the capacity. 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 perspective of achieving both high capacity and input/output characteristics, 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.

The electricity storage device of the present invention includes the electricity storage device negative electrode of the present invention. An electricity storage device having an electricity storage device negative electrode containing the carbonaceous material of the present invention has a high discharge capacity and high cycle durability.

When the carbonaceous material of the present invention is used to form a negative electrode for an electricity storage device, there are no particular restrictions on the other materials that make up the battery, such as the positive electrode material, separator, and electrolyte, and it is possible to use various materials that have traditionally been used or proposed for non-aqueous solvent secondary batteries.

For example, the positive electrode material is preferably a composite metal chalcogen compound such as a layered oxide ₍ represented as _LiMO2 , where _M is a metal, e.g., _LiCoO2 , _LiNiO2 , _LiMnO2 , or _{LiNixCoyMozO2} (where x, y, and z represent _the composition ratio)), an olivine ₍ represented as _LiMPO4 , where M is a metal, e.g., _LiFePO4 ), or a spinel (represented as _LiM2O4 , where M is _a metal, e.g., _LiMn2O4 ). These chalcogen compounds may be mixed as needed. The positive electrode is formed by molding these positive electrode materials together with a suitable binder and a carbon material that imparts conductivity to the electrode, and then forming a layer on a conductive current collector.

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

An electricity storage device is generally formed by arranging the positive and negative electrodes formed as described above, with a liquid-permeable separator interposed between them as needed, 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.

The carbonaceous material of the present invention is suitable as a carbonaceous material for batteries (typically, vehicle drive electricity storage devices) mounted on vehicles such as automobiles. In the present invention, the term "vehicle" refers to, without limitation, what is commonly known as an electric vehicle, a hybrid vehicle with a fuel cell or internal combustion engine, or the like, and includes at least a power supply unit equipped with the above-mentioned battery, an electric drive mechanism driven by power supplied from the power supply unit, and a control device for controlling the same. The vehicle may further include a dynamic brake or regenerative brake, and a mechanism for converting braking energy into electricity and charging the electricity storage device.

The present invention will be explained in more detail below using examples, but these are not intended to limit the scope of the present invention. Furthermore, methods for measuring 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.

(Elemental analysis)
Elemental analysis was carried out based on the inert gas dissolution method using an oxygen, nitrogen and hydrogen analyzer EMGA-930 manufactured by Horiba Ltd.
The detection method of the device is as follows:
Oxygen: inert gas fusion - non-dispersive infrared (NDIR);
Nitrogen: inert gas fusion - thermal conduction method (TCD),
Hydrogen: Inert gas fusion - Non-dispersive infrared (NDIR)
Calibration was performed using a (oxygen/nitrogen) Ni capsule, TiH ₂ (H standard sample), and SS-3 (N, O standard sample). As a pretreatment, 20 mg of the sample whose moisture content had been measured at 250°C for approximately 10 minutes was placed in a Ni capsule and degassed for 30 seconds in the elemental analyzer before measurement.
The test was carried out on three samples, and the average value was used as the analytical value.

(X-ray diffraction)
The carbonaceous material powder was filled into a sample holder and subjected to X-ray diffraction measurement using a MiniFlex II manufactured by Rigaku Corp. The CuKα (λ=1.5418 Å) radiation source was used, and the scanning range was 10°<2θ<35°.

(Raman spectrum)
Using a Raman spectrometer ("LabRAM ARAMIS (VIS)" manufactured by Horiba, Ltd.), the measurement target particles, which were carbonaceous materials, were set on the observation stage, the magnification of the objective lens was set to 100 times, the particles were focused, and measurements were performed while irradiating them with argon ion laser light. Details of the measurement conditions are as follows.
Wavelength of argon ion laser light: 532 nm
Laser power on sample: 15 mW
Resolution: 5-7cm ^-1
Measurement range: 50-2000 ^cm
Exposure time: 1 second Number of integrations: 100 Peak intensity measurement: Baseline correction Automatic correction with Polynom-3
Peak search and fitting processing GaussLoren

(Average particle diameter _D50 measured by laser scattering method)
The average particle size (particle size distribution) of the carbonaceous material was measured by the following method. A sample was placed in an aqueous solution containing 5% by mass of a surfactant ("Toriton X100" manufactured by Wako Pure Chemical Industries, Ltd.), treated in 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 Corporation). _D50 is the particle size at which the cumulative volume becomes 50%, and this value was used as the average particle size.

Example 1
10 g of starch, 0.29 g of melamine (0.037 mol per mol of starch monosaccharide units), and 0.76 g of adipic acid (0.084 mol per mol of starch monosaccharide units) were placed in a 100 ml container and mixed by shaking at 2 Hz for 5 minutes. The resulting mixture was heated to 600°C in a nitrogen gas atmosphere. The heating rate to 600°C was 600°C/hour (10°C/min). Next, the mixture was carbonized by heat treatment (low-temperature firing) at 600°C for 60 minutes under a nitrogen gas flow to obtain a carbonized product (first firing). The nitrogen gas supply rate was 1 L/min per 10 g of starch. The resulting carbonized product was then pulverized in a ball mill to obtain a pulverized carbonized product. Next, the pulverized carbide was heated to 1000°C and subjected to a high-temperature calcination treatment at 1000°C for 60 minutes to obtain a carbonaceous material (second calcination). The rate of temperature increase to 1000°C was 600°C/hour (10°C/min). The temperature increase and heat treatment were carried out under a nitrogen gas flow. The nitrogen gas supply rate was 3 L/min per 5 g of pulverized carbide.

Example 2
A carbonaceous material was obtained by treating in the same manner as in Example 1, except that the amount of melamine was 0.045 mol per mol of starch monosaccharide unit and the amount of adipic acid was 0.025 mol per mol of starch monosaccharide unit.

Example 3
A carbonaceous material was obtained by the same treatment as in Example 1, except that the amount of melamine was 0.075 moles per mole of starch monosaccharide unit, the amount of adipic acid was 0.042 moles per mole of starch monosaccharide unit, and the second firing temperature was 1100°C.

Example 4
A carbonaceous material was obtained by carrying out the same treatment as in Example 1, except that the amount of melamine was 0.15 mol per mol of starch monosaccharide unit and the second baking temperature was 1100°C.

(Example 5 : Reference Example )
A carbonaceous material was obtained by treating in the same manner as in Example 1, except that the amount of melamine was 0.045 mol per mol of starch monosaccharide unit and the amount of adipic acid was 0.042 mol per mol of starch monosaccharide unit.

(Comparative Example 1)
A carbonaceous material was obtained in the same manner as in Example 1, except that the amount of melamine was 0.15 mol per 1 mol of starch monosaccharide unit.

(Comparative Example 2)
A carbonaceous material was obtained by carrying out the same treatment as in Comparative Example 1, except that the second firing temperature was set to 1200°C.

(Comparative Example 3)
A carbonaceous material was obtained by carrying out the same treatment as in Comparative Example 1, except that the second firing temperature was set to 800°C.

(Comparative Example 4)
A carbonaceous material was obtained in the same manner as in Example 1, except that the mixture in Example 1 was used as starch.

(Comparative Example 5)
Glucose and ammonium chloride (1.1 moles per mole of glucose) were mixed in a mortar. The resulting mixture was heated to 1000°C in a nitrogen gas atmosphere. The heating rate to 1000°C was 240°C/hour (4°C/min). Next, a carbonization treatment was performed by heat-treating the mixture at 1000°C for 60 minutes under a nitrogen gas flow, thereby obtaining a carbide (first firing). The nitrogen gas supply rate was 1 L/min per 5 g of glucose. The resulting carbide was then pulverized in a ball mill to obtain a pulverized carbide. Next, the pulverized carbide was heated to 1200°C and then heat-treated at 1200°C for 60 minutes, thereby obtaining a carbonaceous material (second firing). The heating rate to 1200°C was 600°C/hour (10°C/min). The heating and heat treatment were performed under a nitrogen gas flow. The nitrogen gas was supplied at a rate of 3 L/min per 5 g of pulverized carbide.

(Comparative Example 6)
A carbonaceous material was obtained by carrying out the same treatment as in Example 1, except that the amount of melamine was 0.015 mol per mol of starch monosaccharide unit and the second baking temperature was 800°C.

(Preparation of electrodes)
Using the carbonaceous materials obtained in each of the Examples and Comparative Examples, negative electrodes were fabricated according to the following procedure.
A slurry was obtained by mixing 95 parts by mass of a carbonaceous material, 2 parts by mass of conductive carbon black (Super-P (registered trademark) manufactured by TIMICAL), 1 part by mass of CMC, 2 parts by mass of SBR, and 90 parts by mass of water. The obtained slurry was applied to a copper foil having a thickness of 18 μm, dried, and pressed to obtain an electrode having a thickness of 45 μm.

(Battery initial 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. A 1:1 volumetric mixture of propylene carbonate and ethylene glycol dimethyl ether was used as the solvent. _LiClO4 was dissolved in this solvent at 1 mol/L and used as the electrolyte. A polypropylene membrane was used as the separator. A coin cell was prepared in a glove box under an argon atmosphere.
For the lithium secondary battery having the above configuration, a charge/discharge test was performed using a charge/discharge tester (manufactured by Toyo Systems Co., Ltd., "TOSCAT") 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 terminate the doping. The capacity (mAh/g) at this time was taken as the charge capacity (initial charge capacity) (evaluation of initial charge). Next, undoping was performed at a rate of 70 mA/g relative to the active material mass until the doping reached 1.5 V relative to the lithium potential, and the discharged capacity at this time was taken as the discharge capacity (initial discharge capacity) (evaluation of initial discharge). The percentage of discharge capacity/charge capacity was taken as the charge/discharge efficiency (initial charge/discharge efficiency) and was used as an index of the utilization efficiency of lithium ions in the battery.

<Cycle capacity retention rate>
The discharge capacity obtained after repeating the above initial charge/discharge evaluation conditions 10 times was defined as the discharge capacity at the 10th cycle. The 10-cycle capacity retention rate was calculated using the following formula.
10-cycle capacity retention rate (%)=10th cycle discharge capacity (mAh/g)/initial discharge capacity (mAh/g)×100

The production conditions for the carbonaceous materials in each Example and Comparative Example, the evaluation results for the physical properties of the obtained carbonaceous materials, and the evaluation results for the battery characteristics are shown in the following tables.
The batteries fabricated using the carbonaceous materials of each Example exhibited high discharge capacities and high cycle capacity retention rates, whereas the batteries fabricated using the carbonaceous materials of each Comparative Example, which did not have the specified nitrogen element content or oxygen element content, did not exhibit sufficiently high discharge capacities or cycle capacity retention rates.

Claims (10)

A carbonaceous material having a nitrogen element content of 1.0 mass % or more and 3.49 mass % or less, an oxygen element content of 1.1 mass % or more and 2.1 mass % or less, and a hydrogen element content of more than 0.10 mass % and 0.50 mass % or less, as determined by elemental analysis, a ratio R _N/H of the nitrogen element content to the hydrogen element content (nitrogen element content/hydrogen element content) of 6 or more and 37 or less, and a carbon interplanar spacing (d ₀₀₂ ) of 3.70 Å or more as measured by X-ray diffraction measurement. The carbonaceous material according to claim 1, wherein the oxygen element content determined by elemental analysis is 1.2% by mass or more and 2.0% by mass or less. 3. The carbonaceous material according to claim 1, wherein the ratio of the oxygen content to the nitrogen content (RO _/N ), determined by elemental analysis, is 0.30 or more and 1.0 or less. 4. The carbonaceous material according to claim 1, wherein the half-width value of the peak near 1360 cm ⁻¹ in the Raman spectrum observed by laser Raman spectroscopy is 250 cm ⁻¹ or more. 5. The carbonaceous material according to claim 1, wherein the half-width value of a peak near 1650 cm ⁻¹ in a Raman spectrum observed by laser Raman spectroscopy is 98 cm ⁻¹ or more. The carbonaceous material according to any one of claims 1 to 5, having an average particle size _D50 of 30 µm or less. The carbonaceous material according to any one of claims 1 to 6, wherein the carbonaceous material is derived from a sugar. The carbonaceous material according to any one of claims 1 to 7, which is used as a negative electrode active material for an electricity storage device. A negative electrode for an electricity storage device comprising the carbonaceous material described in claim 8. An electricity storage device having the electricity storage device negative electrode described in claim 9.