JP2024118190A - Composite carbon particles, electricity storage device, and method for producing composite carbon particles. - Google Patents

Abstract

To enhance a high-rate characteristic of a power storage device.SOLUTION: A composite carbon particle is a composite carbon particle obtained by composing a graphite particle and an amorphous carbon, and has a pore diameter calculated by a nitrogen adsorption measurement of 40 μL/g or more and an average pore diameter calculated by the nitrogen adsorption measurement of 7 nm. A manufacturing method of the composite carbon particle, comprises: a granulation step of obtaining a granulation powder by using a raw material liquid containing a graphite particle and a metal carboxylate; a thermal processing step of generating a metal and/or a metal chemical compound, derived from the metal carboxylate while performing a thermal processing to the granulation powder to carbonize it; and a removing step of removing the metal and/or the metal chemical compound from the granulation power after the thermal processing step.SELECTED DRAWING: Figure 1

Description

This disclosure relates to composite carbon particles, electricity storage devices, and methods for producing composite carbon particles.

Conventionally, secondary batteries have been studied that use graphite negative electrodes such as MAG (Massive Artificial Graphite) and PLGL (Porous Lithiated Graphite Lamina) to utilize the lithium deposition and dissolution capacity on graphite in addition to the charge and discharge capacity of graphite (lithium insertion and desorption capacity into graphite) (see, for example, Non-Patent Documents 1-2). The present inventors have also proposed a secondary battery that uses a nonwoven carbon fiber aggregate as the negative electrode to utilize the lithium deposition and dissolution capacity on the carbon fiber in addition to the charge and discharge capacity of carbon fiber (lithium insertion and desorption capacity into carbon fiber) (see, for example, Patent Documents 1-4). Furthermore, a secondary battery has been proposed that uses a carbon-lithium composite powder that supports lithium metal as a skeleton of a carbon material as the negative electrode to utilize the lithium deposition and dissolution capacity of lithium in addition to the lithium insertion and desorption capacity into the carbon material (see, for example, Patent Document 5). In this way, by utilizing the lithium deposition and dissolution capacity at the negative electrode in addition to the lithium insertion and desorption capacity at the negative electrode, the energy density of the secondary battery can be increased. Secondary batteries that utilize the storage of lithium metal in the sub-nanopores of hard carbon are also being considered (see, for example, Non-Patent Document 3). In this way, it is said that by utilizing the sub-nanopores of hard carbon, it is possible to suppress the dendrite growth of lithium metal and improve the coulombic efficiency and cycle stability of secondary batteries.

JP 2019-96475 A JP 2019-135689 A JP 2019-160730 A JP 2020-17479 A JP 2020-13779 A

Chem, 1, 287-297 (2016) ACS Nano, 14, 1837-1845 (2020) Chem. Eng. J., 450, 138049 (2022)

However, the secondary batteries in Patent Documents 1 to 5 and Non-Patent Documents 1 to 3 utilize the precipitation and dissolution capacity of lithium, which is a charge carrier, but they sometimes have poor high-rate characteristics. Therefore, there has been a demand for further improving the high-rate characteristics of electricity storage devices.

This disclosure has been made in consideration of these issues, and its primary objective is to further improve the high-rate characteristics of electricity storage devices.

In order to achieve the above-mentioned object, the inventors have conducted extensive research. They have come up with the idea of producing a granulated powder using a raw material liquid containing graphite particles and a metal sugar carboxylate, carbonizing the powder by heat treatment and generating metals and/or metal compounds derived from the metal sugar carboxylate, and removing the generated metals and/or metal compounds to produce composite carbon particles. The composite carbon particles thus obtained are a composite of graphite particles and amorphous carbon, and have a predetermined pore structure. When used in an electrode for an electricity storage device, the composite carbon particles can exhibit the charge carrier insertion/desorption capacity into graphite as well as the charge carrier precipitation/dissolution capacity, and can further improve high-rate characteristics, leading to the completion of the present disclosure.

That is, the composite carbon particles disclosed herein are
The composite carbon particles are obtained by combining graphite particles and amorphous carbon, and have a pore volume calculated by nitrogen adsorption measurement of 40 μL/g or more and an average pore diameter calculated by nitrogen adsorption measurement of 7 nm or less.

The electricity storage device disclosed in the present specification is
The battery is provided with an electrode containing the above-mentioned composite carbon particles as an electrode active material.

The method for producing composite carbon particles disclosed herein includes the steps of:
A granulation step of obtaining a granulated powder using a raw material liquid containing graphite particles and a sugar carboxylate metal salt;
a heat treatment step of heat-treating the granulated powder to carbonize it and generate a metal and/or a metal compound derived from the sugar carboxylate metal salt;
a removing step of removing the metal and/or metal compound from the granulated powder after the heat treatment step;
It includes.

In the present disclosure, the high-rate characteristics of the power storage device can be improved. The reason for such an effect is presumed to be as follows. For example, when the composite carbon particles of the present disclosure are used in the electrodes of a power storage device, charge carriers are inserted and removed from the graphite particles, while the amorphous carbon reduces the deposition overvoltage of the charge carriers, contributing to the deposition and dissolution of the charge carriers. It is believed that this amorphous carbon is formed by, for example, carbonizing the sugar and/or sugar carboxylic acid of the sugar carboxylate metal salt, and removing the metal and/or metal compound derived from the sugar carboxylate metal salt, thereby forming pores. These pores also function as a deposition field for the charge carriers. It is presumed that such composite carbon particles have a suitable pore structure, which allows the charge carriers to be inserted and removed from the graphite smoothly, and the charge carriers to be deposited and dissolved, thereby further improving the high-rate characteristics.

FIG. 2 is a schematic cross-sectional view of a composite carbon particle 10. 1 is a schematic diagram showing an example of a method for producing a composite carbon particle 10. FIG. FIG. 3 is a schematic diagram of an electricity storage device 50. 13 is an SEM image of composite carbon particles of Experimental Example 3. _N2 adsorption isotherm of composite carbon particles of Experimental Example 3. TG curve of composite carbon particles of Experimental Example 3. 1 is an X-ray diffraction pattern of composite carbon particles of Experimental Example 3. 10 shows charge/discharge curves for the cell of Experimental Example 3 after 10 cycles.

[Composite carbon particles]
The composite carbon particles of the present disclosure are used, for example, as an electrode active material of an electrode of an electricity storage device. The electricity storage device may be, for example, a secondary battery such as an alkali metal ion secondary battery, a hybrid capacitor, an air battery, or the like. The electricity storage device may be one in which the charge carrier is an alkali metal or a Group 2 metal. Examples of the alkali metal include lithium, sodium, potassium, and the like, among which lithium is preferred. Examples of the Group 2 metal include magnesium, calcium, strontium, barium, and the like. The electrode for the electricity storage device is either a positive electrode or a negative electrode based on the potential of the counter electrode relative to the potential of the electrode active material, but when lithium is used as the carrier, it is preferable to use the negative electrode. In this embodiment, the case in which the composite carbon particles are used as a negative electrode active material of a lithium ion secondary battery will be mainly described.

The composite carbon particles are composite carbon particles in which graphite particles and amorphous carbon are composited, and the pore volume calculated by nitrogen adsorption measurement is 40 μL/g or more, and the average pore diameter calculated by nitrogen adsorption measurement is 7 nm or less. If the above-mentioned pore volume is 40 μL/g or more and the above-mentioned average pore diameter is 7 nm or less, the high-rate characteristics can be further improved. In this disclosure, the "pore volume", "average pore diameter", and "specific surface area" are values obtained by nitrogen adsorption measurement and values obtained by BET analysis, unless otherwise specified. In this disclosure, the "pore volume" indicates the pore volume per unit mass. The above-mentioned pore volume may be, for example, 200 μL/g or less, 150 μL/g or less, or 100 μL or less from the viewpoint of further improving the high-rate characteristics and further suppressing the decrease in the initial charge/discharge efficiency. The pore volume may be, for example, 45 μL/g or more, or 50 μL/g or more, from the viewpoint of further improving the cycle characteristics. The average pore diameter may be, for example, 1 nm or more, 2 nm or more, or 2.2 nm or more, from the viewpoint of further improving the high-rate characteristics and further suppressing the decrease in the initial charge/discharge efficiency. The average pore diameter may be, for example, 6 nm or less, 5 nm or less, or 4 nm or less, from the viewpoint of further improving the cycle characteristics. The composite carbon particles may have a specific surface area calculated by nitrogen adsorption measurement of 30 m ² /g or more and 250 m ² /g or less. The specific surface area may be, for example, 200 m ² /g or less, or 170 m ² /g or less, from the viewpoint of further improving the high-rate characteristics and further suppressing the decrease in the initial charge/discharge efficiency. In addition, the specific surface area may be 50 m ² /g or more, 70 m ² /g or more, or 100 m ² /g or more from the viewpoint of further improving cycle characteristics. The composite carbon particles may be such that no steps specific to graphite are confirmed in the Kr adsorption isotherm. Regarding the steps specific to graphite, according to "Gas Adsorption on Graphite Surface" by Horikawa et al. (Acc. Mater. Surf. Res., 3, 51-62 (2018)), the Kr adsorption behavior on the graphite surface is such that the amount of Kr adsorbed increases stepwise as the number of layers increases while Kr forms layers on the graphite surface, and the adsorption isotherm is a type VI adsorption isotherm according to the IUPAC classification. In the composite carbon particles of the present disclosure, amorphous carbon is formed on the crystal structure surface of graphite, so that there is almost no widely exposed portion of the graphite surface, and it is presumed that no steps specific to graphite are confirmed.

The composite carbon particles may be aggregated particles that contain multiple graphite particles and are formed by agglomerating multiple graphite particles with amorphous carbon. In the aggregated particles, amorphous carbon may be present between the graphite particles, and the graphite particles may be bonded together by the amorphous carbon. In the aggregated particles, the graphite particles are preferably flat, such as flaky.

The composite carbon particles may be spherical. When spherical composite carbon particles are used in the electrodes of an electricity storage device, it is believed that the degree of curvature in the electrode film thickness direction in the lithium migration path is less likely to increase, current concentration on the electrode surface can be suppressed, and lithium precipitation on the electrode surface can be further suppressed. The average particle size of the composite carbon particles may be, for example, 8 μm or more, 10 μm or more, or 12 μm or more. The average particle size of the composite carbon particles may be, for example, 50 μm or less, 30 μm or less, or 20 μm or less. In this disclosure, the "average particle size" may refer to the median diameter D50 measured by a laser diffraction method.

The composite carbon particles may contain amorphous carbon in a ratio of 1% by mass to 50% by mass, 1.5% by mass to 30% by mass, or 2% by mass to 20% by mass. The ratio of amorphous carbon (coating amount) may be a value calculated from the yield in the heat treatment process, the removal process, and the firing process in the manufacturing method of composite carbon particles described below. Specifically, first, the masses Xh1 [g] and Xh2 [g] of the granulated powder before and after the heat treatment process, the masses Xr1 [g] and Xr2 [g] of the granulated powder before and after the removal process, and the masses Xf1 [g] and Xf2 [g] of the granulated powder before and after the firing process are measured. Then, using the ratio g [%] of the graphite particles in the raw material liquid and the ratio c [%] of the coating source (sugar carboxylate metal salt and sugar compound) (where g + c = 100), the mass of the graphite particles in the granulated powder before the heat treatment step Gh1 [g] = Xh1 × g / 100 and the mass of the coating source Ch1 [g] = Xh1 × c / 100 are calculated. Next, using the yield s [%] in the heat treatment step when the graphite particles alone are subjected to the heat treatment step, the mass of the graphite particles in the granulated powder after the heat treatment step Gh2 [g] = Gh1 × s / 100 is calculated, and the mass of the coating source Ch2 [g] = Xh2 - Gh2 is calculated. Next, the mass of the graphite particles in the granulated powder before the removal treatment step Gr1 [g] = Xr1 × Gh2 / Xh2 and the mass of the coating source Cr1 [g] = Xr1 × Ch2 / Xh2 are calculated. Then, the mass of the remaining coating source Cr2' [g] = Cr1 x t/100 is calculated from the theoretical yield t [%] when the entire amount of MgO is removed from the coating source, and assuming that the losses due to filtration and washing are equal between the graphite particles and the remaining coating source, the mass of the graphite particles in the granulated powder after the removal treatment step Gr2 [g] = Xr2 x Gr1 / (Gr1 + Cr2') and the mass of the coating source Cr2 [g] = Xr2 x Cr2' / (Gr1 + Cr2') are calculated. Next, the mass of the graphite particles in the granulated powder before the firing step Gf1 [g] = Xf1 x Gr2 / Xr2 and the mass of the coating source Cf1 [g] = Xf1 x Cr2 / Xr2 are calculated. Next, the mass of the graphite particles in the granulated powder (composite carbon particles) after the firing process, Gf2 [g] = Gf1 × u/100, is calculated using the yield u [%] in the firing process when the graphite particles alone after the heat treatment process are subjected to the firing process, and the mass of the coating, Cf2 [g] = Xf2 - Gf2, is calculated. The value of Cf2/Xf2 × 100 is the proportion of amorphous carbon in the composite carbon particles (coating amount). In thermogravimetry, the composite carbon particles may have a mass loss of 1% to 50%, 1.5% to 30%, or 2% to 20% at 600°C. This mass loss has some correlation with the proportion of amorphous carbon calculated from the yields in the heat treatment process, removal process, and firing process.

The composite carbon particles may be, for example, graphite particles in which at least a portion of the surface is coated with amorphous carbon, graphite particles in which amorphous carbon is interposed between the graphite particles to hold them together, or graphite particles in which amorphous carbon is inserted between the graphite layers, or may satisfy a combination of these. Since the composite carbon particles are considered to have at least a portion of the surface of the graphite particles coated with amorphous carbon, in this disclosure, the amorphous carbon is also referred to as the "coat" and the proportion of amorphous carbon is also referred to as the "coat amount."

The composite carbon particles may contain lithium in a mass of more than 8.8% of the mass of the composite carbon particles when the SOC of the power storage device is 100%, or may contain lithium in a mass of 10% or more. When graphite is used as the negative electrode active material, C ₆ Li in a state of 100% SOC contains lithium in a mass of 8.8% of the graphite. In contrast, when the composite carbon particles of the present disclosure are used as the negative electrode active material, lithium metal can be precipitated/dissolved to utilize the charge/discharge capacity, and in this case, the composite carbon particles may contain lithium in a mass of more than 8.8% of the mass of the composite carbon particles, or may contain Li in a mass of 10% or more.

The graphite particles may be natural graphite (scale graphite, flake graphite) or artificial graphite, but natural graphite is more preferable. Natural graphite is generally cheaper than artificial graphite, which requires graphitization. The graphite may be mechanically shape-controlled, for example, by removing the corners of the graphite, rounding it into a spherical shape, or crushing it. It is believed that such shape control can suppress selective orientation and suppress the inhibition of lithium ion insertion and desorption. The graphite particles may be modified from graphite particles before compounding by heat treatment or the like. The average particle size of the graphite particles may be, for example, 1 μm or more and 20 μm or less, or 5 μm or more and 10 μm or less. The average particle size may be a value measured for graphite particles before compounding.

The graphite particles may have the following pore structure. For example, the pore volume calculated by nitrogen adsorption measurement may be smaller than the pore volume of the composite carbon particles, and may be, for example, less than 40 μL/g. The average pore diameter calculated by nitrogen adsorption measurement may be larger than the average pore diameter of the composite carbon particles, and may be, for example, more than 7 nm. The specific surface area calculated by nitrogen adsorption measurement may be smaller than the specific surface area of the composite carbon particles, and may be less than 30 m ² /g. A step specific to graphite may be confirmed in the Kr adsorption isotherm. Such a pore structure may be determined by performing nitrogen adsorption measurement or Kr adsorption measurement on the graphite particles before composite formation.

Amorphous carbon, which will be described in detail later, may be, for example, a carbonized sugar and/or sugar carboxylic acid of a sugar carboxylate metal salt, or a carbonized sugar compound. In the present disclosure, the sugar carboxylate metal salt and the sugar compound are also referred to as amorphous carbon sources. The sugar of the sugar carboxylate metal salt and the sugar compound may be a monosaccharide, an oligosaccharide in which 2 to 20 monosaccharide molecules are bonded, or a polysaccharide in which even more monosaccharides are bonded. Of these, monosaccharides and oligosaccharides are preferred, and monosaccharides and disaccharides are more preferred. More specifically, the monosaccharides and disaccharides are preferably one or more selected from the group consisting of monosaccharides glucose and fructose, disaccharides sucrose, lactose, maltose, trehalose, turanose, cellobiose, and derivatives thereof. These have a small molecular weight (180 to 350) and a small molecular size, and are therefore considered to be easily distributed over the entire surface of the graphite particles. Of these, glucose is more preferred. The amorphous carbon may be, for example, non-graphitizable carbon.

The amorphous carbon may have pores. The pore volume of the amorphous carbon may be larger than the pore volume of the graphite particles, for example, 50 μL/g or more, 60 μL/g or more, or 70 μL/g or more. This pore volume may be 500 μL/g or less, 300 μL/g or less, or 100 μL/g or less. The average pore diameter of the amorphous carbon may be smaller than the average pore diameter of the graphite particles, for example, 10 nm or less, 5 nm or less, or 2 nm or less. This average pore diameter may be 0.5 nm or more, 1 nm or more, or 1.5 nm or more. The specific surface area of the amorphous carbon may be larger than the specific surface area of the graphite particles, for example, 50 m ² /g or more, 100 m ² /g or more, or 200 m ² /g or more. The specific surface area may be ¹⁰⁰⁰ m2/g or less, 700 ^m2 /g or less, or 500 ^m2 /g or less. The pore volume, average pore diameter and specific surface area of the amorphous carbon may be values measured for particles produced in a method for producing composite carbon particles described below, in which graphite particles are omitted from the raw material liquid.

The amorphous carbon may assume that the pore volume Vc [μL/g] of a composite carbon particle having an amorphous carbon ratio of Ca [%], an amorphous carbon pore volume of Va [μL/g], a graphite particle ratio of (100-Ca) [%], and a graphite particle pore volume of Vg [μL/g] is expressed as Vc = Va × Ca/100 + Vg × (100-Ca)/100, and the theoretical pore volume Va calculated by substituting the measured or calculated values of Vc, Vg, and Ca into this formula satisfies the following. For example, the theoretical pore volume Va may be 100 μL/g or more, 200 μL/g or more, or 400 μL/g or more. The theoretical pore volume Va may be, for example, 3000 μL/g or less, 1000 μL/g or less, or 900 μL/g or less. Alternatively, the amorphous carbon may have a specific surface area Sc [m ² /g] of a composite carbon particle having an amorphous carbon ratio of Ca [%], a specific surface area of the amorphous carbon of Sa [m ^{2 /g], a ratio of graphite particles of (100 - Ca) [%], and a specific surface area of the graphite particles of Sg [m 2 /} g], which is assumed to be expressed as Sc = Sa x Ca/100 + Sg x (100 - Ca)/100, and the theoretical specific surface area calculated by substituting the measured or calculated values of Sc, Sg, and Ca into this formula may satisfy the following: For example, the theoretical specific surface area Sa may be 200 m ² /g or more, 500 m ² /g or more, or 1500 m ² /g or more. The theoretical specific surface area Sa may be 5000 ^m2 /g or less, 4000 ^m2 /g or less, or 3000 ^m2 /g or less. The pore volume Vg and the specific surface area Sg of the graphite particles may be values measured for particles produced in the same manner as in the production method of the composite particles of the present disclosure, except that no coating source is used.

1 is a schematic cross-sectional view of a composite carbon particle 10, which is an example of the composite carbon particle of the present disclosure. The composite carbon particle 10 is a particle in which graphite particles 20 and amorphous carbon 30 are composited, and the pore volume calculated by nitrogen adsorption measurement is 40 μL/g or more, and the average pore diameter calculated by nitrogen adsorption measurement is 7 nm or less. The composite carbon particle 10 includes a plurality of scaly graphite particles 20, and amorphous carbon 30 is present between the graphite particles 20, and is an aggregate particle in which the plurality of graphite particles 20 are aggregated by the amorphous carbon 30. The composite carbon particle 10 is approximately spherical. The graphite particles 20 may be, for example, scaly graphite. The amorphous carbon 30 may be, for example, sugar and/or sugar carboxylic acid carbonized, or may be non-graphitizable carbon. The amorphous carbon 30 may have pores 31.

[Method of manufacturing composite carbon particles]
The method for producing composite carbon particles according to the present disclosure includes a granulation step of obtaining a granulated powder using a raw material liquid containing graphite particles and a sugar metal carboxylate, a heat treatment step of carbonizing the granulated powder by heat treatment and generating a metal and/or metal compound derived from the sugar metal carboxylate, and a removal step of removing the metal and/or metal compound derived from the sugar metal carboxylate from the granulated powder after the heat treatment step. The method for producing composite carbon particles may also include a firing step of firing the granulated powder after the removal step. The method for producing composite carbon particles may be, for example, a method for producing the composite carbon particles described above.

(Granulation process)
In the granulation step, a granulated powder is produced using a raw material liquid containing graphite particles and a sugar metal carboxylate. The raw material liquid may contain a sugar compound in addition to the graphite particles and the sugar metal carboxylate. The raw material liquid may be a slurry in which the graphite particles are dispersed in a solvent. The solvent may be a solvent capable of dissolving the sugar metal carboxylate, or a solvent capable of dissolving the sugar compound. The raw material liquid may also contain a thickener.

The graphite particles may be natural graphite (scale graphite, flake graphite) or artificial graphite, with natural graphite being more preferable. The graphite may be one that has been subjected to mechanical shape control, for example, graphite that has had its corners removed, that has been rounded into a spherical shape, or that has been crushed. The average particle size of the graphite particles may be, for example, 1 μm or more and 20 μm or less, or 5 μm or more and 10 μm or less.

The graphite particles may have the following pore structure. For example, the pore volume calculated by nitrogen adsorption measurement may be smaller than the pore volume of the composite carbon particles, and may be, for example, less than 40 μL/g. The average pore diameter calculated by nitrogen adsorption measurement may be larger than the average pore diameter of the composite carbon particles, and may be, for example, more than 7 nm. The specific surface area calculated by nitrogen adsorption measurement may be smaller than the specific surface area of the composite carbon particles, and may be less than 30 m ² /g. Furthermore, a step specific to graphite may be confirmed in the Kr adsorption isotherm.

The sugar of the sugar carboxylate metal salt may be a monosaccharide, an oligosaccharide in which 2 to 20 molecules of monosaccharides are bonded, or a polysaccharide in which even more monosaccharides are bonded. Of these, monosaccharides and oligosaccharides are preferred, and monosaccharides and disaccharides are more preferred. More specifically, the monosaccharides and disaccharides are preferably one or more selected from the group consisting of monosaccharides such as glucose and fructose, disaccharides such as sucrose, lactose, maltose, trehalose, turanose, cellobiose, and derivatives thereof, and glucose is more preferred. The sugar carboxylic acid has a structure in which one or more of the oxygen functional groups of these sugars are oxidized to become a carboxyl group, and gluconic acid is more preferred. The metal of the sugar carboxylate metal salt may be, for example, an alkali metal or a Group 2 metal. Examples of alkali metals include lithium, sodium, and potassium. Examples of Group 2 metals include magnesium, calcium, strontium, and barium. Of these, Group 2 metals are preferred, and magnesium is more preferred. As the sugar carboxylate metal salt, for example, magnesium gluconate is preferable. The sugar carboxylate metal salt may be a hydrate.

The sugar compound is different from the sugar carboxylate metal salt and may be a monosaccharide, an oligosaccharide in which 2 to 20 monosaccharide molecules are bonded, or a polysaccharide in which even more monosaccharide molecules are bonded. Of these, monosaccharides and oligosaccharides are preferred, and monosaccharides and disaccharides are more preferred. More specifically, the monosaccharides and disaccharides are preferably one or more selected from the group consisting of monosaccharides such as glucose and fructose, and disaccharides such as sucrose, lactose, maltose, trehalose, turanose, cellobiose, and derivatives thereof, and glucose is more preferred. The sugar compound may be the same or different from the sugar of the sugar carboxylate metal salt, but is preferably the same.

The solvent is not particularly limited, but may be, for example, an aqueous solvent or an organic solvent such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, or tetrahydrofuran, either alone or as a mixture of two or more of them. Of these, the solvent is preferably an aqueous solvent, and more preferably water.

For example, cellulose derivatives (polysaccharides) such as carboxymethyl cellulose (CMC) and carboxyethyl cellulose, polyacrylic acid, etc. can be used alone or as a mixture of two or more types as the thickening material. Of these, carboxymethyl cellulose is preferred. Carboxymethyl cellulose may be, for example, an inorganic salt in which the terminal of the carboxymethyl group is sodium or calcium, or an ammonium salt in which the terminal of the carboxymethyl group is ammonium. The same applies to carboxyethyl cellulose. The content of the thickening material may be 0.1% or more and 2% or less, or 0.5% or more and 1.5% or less, relative to the graphite particles.

The raw material liquid may contain 0.1 to 100 parts by mass of sugar carboxylate metal salt per 10 parts by mass of graphite particles, 0.5 to 70 parts by mass, or 1 to 50 parts by mass. The raw material liquid may contain 0.1 to 100 parts by mass of sugar compound per 10 parts by mass of graphite particles, 0.2 to 30 parts by mass, or 0.3 to 20 parts by mass. The raw material liquid may contain 0.01 to 10 times the sugar compound in mass ratio to the sugar carboxylate metal salt, 0.1 to 1 times, or 0.2 to 0.7 times.

The raw material liquid may be prepared, for example, by mixing a graphite slurry in which graphite particles are dispersed in a solvent with a sugar metal carboxylate solution in which a sugar metal carboxylate is dissolved in a solvent. The graphite slurry may be prepared by mixing graphite particles with a thickener, and then adding a solvent and mixing. The sugar metal carboxylate solution may be prepared by dissolving a sugar metal carboxylate and a sugar compound in a solvent. In this disclosure, the raw material liquid containing a sugar metal carboxylate or a sugar compound is also referred to as a coating liquid. The coating liquid is the portion of the raw material liquid excluding the graphite particles.

The granulation method is preferably, for example, a spray drying method. The spray drying method is suitable for producing spherical composite carbon particles because it can obtain spherical granulated powder. Spray drying may be performed using a spray dryer. The spray drying conditions may be appropriately adjusted depending on, for example, the scale of the device and the amount of composite carbon particles to be produced. The drying temperature is preferably, for example, in the range of 100°C to 250°C. At 100°C or higher, the solvent can be sufficiently removed, and at 250°C or lower, it is preferable because energy consumption can be further reduced. The drying temperature is more preferably 120°C or higher, and more preferably 180°C or lower. The amount of liquid supplied may be, for example, in the range of 0.1 L/h to 2 L/h, although it depends on the scale of production. The nozzle size for spraying the raw material liquid may be, for example, in the range of 0.5 mm to 5 mm in diameter, although it depends on the scale of production.

(Heat treatment process)
In the heat treatment step, the granulated powder is heat-treated to carbonize it and generate a metal and/or metal compound derived from the sugar carboxylate metal salt. In the heat treatment step, a carbon material in which the sugar and/or sugar carboxylate of the sugar carboxylate metal salt is carbonized may be generated. The carbon material may include a carbonized sugar compound. The carbon material may be amorphous carbon or a precursor of amorphous carbon. In the heat treatment step, fine particles of a metal and/or metal compound derived from the sugar carboxylate metal salt may be generated. The average particle size of the fine particles may be, for example, 7 nm or less, 6 nm or less, 5 nm or less, or 4 nm or less. The average particle size of the fine particles may be, for example, 1 nm or more, 2 nm or more, or 2.2 nm or more. The fine particles may be dispersed, or may be partially or entirely connected. The metal compound generated in the heat treatment step may be, for example, an oxide or hydroxide, and an oxide is preferable. The heat treatment temperature may be, for example, 400° C. or more, 600° C. or more, 800° C. or more, or 850° C. or more. The heat treatment temperature may be, for example, 1300° C. or less, 1100° C. or less, 1000° C. or less, or 950° C. or less. The heat treatment atmosphere may be an inert atmosphere such as an Ar atmosphere or a N ₂ atmosphere, or may be a superheated steam atmosphere. The heat treatment time may be, for example, 30 minutes or more and 120 minutes or less.

(Removal process)
In the removal step, the metal and/or metal compound derived from the sugar carboxylate metal salt is removed from the granulated powder after the heat treatment step. In the removal step, the metal and/or metal compound derived from the sugar carboxylate metal salt may be selectively removed by acid treatment or alkali treatment. Examples of the acid or alkali include hydrochloric acid, sulfuric acid, sodium hydroxide, and ammonia. The acid or alkali may be an aqueous solution. The concentration of the acid or alkali may be, for example, in the range of 0.1 mol/L or more and 5 mol/L or less. The acid treatment or alkali treatment may be a treatment in which the granulated powder after the heat treatment step is immersed in an acid or alkali solution, and stirring may be performed at that time. The time for the acid treatment or alkali treatment may be, for example, 60 minutes or more and 600 minutes or less. It is desirable to remove the acid or alkali from the granulated powder after the acid treatment or alkali treatment by filtration or washing.

(Firing process)
In the firing step, the granulated powder after the removal step is fired. In the firing step, the metal and/or metal compound derived from the sugar carboxylate metal salt may be fired at a high temperature capable of removing the metal and/or metal compound. In this case, the firing step can also serve as the removal step. In addition, the firing step may further promote carbonization of the granulated powder. In addition, in the firing step, for example, when the carbon material generated in the heat treatment step is a precursor of amorphous carbon, the precursor of amorphous carbon may be converted into amorphous carbon in the firing step. The firing temperature may be higher than the heat treatment temperature in the heat treatment step, for example, 1000°C or higher, 1200°C or higher, or 1400°C or higher. In addition, the firing temperature may be, for example, 2000°C or lower, 1800°C or lower, or 1600°C or lower. The firing atmosphere may be an inert atmosphere such as an Ar atmosphere or a _N2 atmosphere, or a superheated steam atmosphere. The baking time may be, for example, 30 minutes or more and 120 minutes or less.

An example of a method for producing composite carbon particles according to the present disclosure is shown in FIG. 2. FIG. 2 is a schematic diagram showing an example of a method for producing composite carbon particles 10. This production method includes a granulation step, a heat treatment step, a removal step, and a firing step. In the granulation step, first, a raw material liquid 12 containing graphite particles 20 and a coating liquid 32 containing a sugar carboxylate metal salt 33 is prepared (FIG. 2A). As the graphite particles 20, for example, flake graphite, which is natural graphite, is suitable. As the sugar carboxylate metal salt 33, for example, magnesium gluconate is suitable. The coating liquid 32 may contain a sugar compound not shown. As the sugar compound, for example, glucose is suitable. Then, the raw material liquid 12 is spray-dried to obtain a granulated powder 14 (FIG. 2B). By this granulation step, a granulated powder 14 containing a plurality of graphite particles 20 and a solid content 34 remaining after the coating liquid 32 is dried is obtained. The solid content 34 includes the sugar carboxylate metal salt 33 and sugar carboxylic acid contained in the coating liquid 32. The solid content 34 may be formed so as to cover at least a part of the surface of the graphite particles 20, for example. In the subsequent heat treatment step, the granulated powder 14 is heat treated to obtain the granulated powder 16 after the heat treatment step (FIG. 2C). This heat treatment step obtains the granulated powder 16 including the graphite particles 20 and the carbon material 36 in which the solid content 34 is carbonized. The carbon material 36 may be amorphous carbon or a precursor of amorphous carbon. The carbon material 36 has a metal and/or metal compound 37 derived from the sugar carboxylate metal salt 33. When the sugar carboxylate metal salt is magnesium gluconate, the metal and/or metal compound 37 is, for example, magnesium oxide. In the subsequent removal step, the metal and/or metal compound 37 is removed from the granulated powder 16 after the heat treatment to obtain the granulated powder 18 after the removal step (FIG. 2D). The removal method may be, for example, an acid treatment using hydrochloric acid or the like. This removal step removes part or all of the metal and/or metal compound 37 from the granulated powder 16 after the heat treatment step, and the granulated powder 18 after the removal step is obtained in which the pores 31 are formed in the removed parts. In the final firing step, the granulated powder 18 after the removal step is fired to obtain the composite carbon particles 10. This firing step may advance the carbonization of the carbon material 36 to form amorphous carbon 30. This firing step may also remove the metal and/or metal compound 37 that was not completely removed in the removal step, and expand the pores 31. Note that if the carbon material 36 produced in the heat treatment step is amorphous carbon, this firing step may be omitted. In this manner, the composite carbon particles 10 are obtained.

[electrode]
The electrode of the present disclosure is an electrode for an electric storage device, and contains the above-mentioned composite carbon particles as an electrode active material. The composite carbon particles function as an electrode active material that absorbs and releases lithium. In this electrode, the composite carbon particles exhibit the insertion/desorption capacity of lithium, which is a charge carrier, into graphite, and also exhibit the deposition/dissolution capacity of lithium, which is a charge carrier. This electrode may be, for example, a mixture of the above-mentioned composite carbon particles, a binder, and optionally a conductive material, and a suitable solvent is added to form a paste-like electrode mixture, which is then applied to the surface of a current collector, dried, and compressed to increase the electrode density as necessary. The binder plays a role in binding the composite carbon particles and the conductive material particles, and may be, for example, a fluorine-containing resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or fluorine-containing rubber, or a thermoplastic resin such as polypropylene or polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR), or the like, used alone or as a mixture of two or more kinds. In addition, a cellulose-based aqueous binder or an aqueous dispersion of styrene butadiene rubber (SBR) can be used. The conductive material is not particularly limited as long as it is an electron-conductive material that does not adversely affect the battery performance, and for example, a mixture of one or more of acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.) can be used. Among these, carbon black and acetylene black are preferred as the conductive material from the viewpoint of electron conductivity and coatability. As a solvent for dispersing the composite carbon particles, the binder, and the conductive material, for example, an organic solvent such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran can be used. In addition, a dispersant, a thickener, etc. may be added to water, and the active material may be slurried with a latex such as SBR. As the thickener, for example, polysaccharides such as carboxymethylcellulose and methylcellulose can be used alone or as a mixture of two or more kinds. Examples of the coating method include roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc., and any of these can be used to form the desired thickness and shape. As the current collector, in addition to copper, nickel, stainless steel, titanium, aluminum, baked carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., for the purpose of improving adhesion, conductivity, and reduction resistance, for example, copper, etc., whose surface is treated with carbon, nickel, titanium, silver, etc. can also be used. For these, the surface can also be oxidized. As for the shape of the current collector, foil, film, sheet, net, punched or expanded, lath, porous body, foam, fiber group formation, etc. can be mentioned. The thickness of the current collector is, for example, 1 to 500 μm.

[Electricity storage device]
The power storage device of the present disclosure includes the above-described electrodes. This power storage device may include, for example, a negative electrode, which is the above-described electrode, a positive electrode, and an ion conductive medium that is interposed between the negative electrode and the positive electrode and conducts lithium ions.

The positive electrode may include a positive electrode active material capable of absorbing and releasing lithium. The positive electrode may be formed, for example, by mixing a positive electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like positive electrode mixture, applying it to the surface of a current collector, drying it, and compressing it to increase the electrode density as necessary. As the positive electrode active material, a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like may be used. Specifically, transition metal sulfides such as _TiS2 , _TiS3 , _MoS3 , and _FeS2 , lithium manganese composite oxides having a basic composition formula such as Li _{(1-x) MnO2} (0<x<1, etc., the same applies below) and Li _{(1-x) Mn2O4 ,} lithium cobalt composite oxides having a basic composition formula such as Li _{(1-x ) CoO2} , lithium nickel composite oxides having a basic composition formula such as Li(1-x) _NiO2 , lithium _{nickel cobalt} manganese composite oxides having a basic composition formula such as _{Li (1-x) NiaCobMncO2} (a+b+c=1), lithium vanadium composite oxides having a basic composition formula such as _LiV2O3 , _and transition metal oxides having _{a basic composition formula such as V2O5 can} be used. Among these, lithium transition metal composite oxides, such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiV ₂ O ₃ are preferred. The term "basic composition formula" means that other elements (such as Al and Mg) may be included. The binder, conductive material, solvent, and coating method used in the positive electrode can be the same as those exemplified in the above-mentioned electrodes. As the current collector, in addition to aluminum, titanium, stainless steel, nickel, iron, baked carbon, conductive polymer, conductive glass, and the like, those in which the surface of aluminum or copper is treated with carbon, nickel, titanium, silver, or the like for the purpose of improving adhesion, conductivity, and oxidation resistance can be used. The surfaces of these can also be oxidized. The shape of the current collector can be the same as that exemplified in the above-mentioned electrodes.

The ionically conductive medium may be a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, etc. Examples of the solvent for the non-aqueous electrolyte solution include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, a combination of cyclic carbonates and chain carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charging and discharging, but also provides a well-balanced electrolyte viscosity, the capacity of the resulting battery, and the battery output. It is considered that the cyclic carbonates have a relatively high relative dielectric constant and increase the dielectric constant of the electrolyte, while the chain carbonates suppress the viscosity of the electrolyte. The supporting salt may be, for example, an inorganic salt such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , or LiClO ₄ , or an organic salt such as LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , or LiC(CF ₃ SO ₂ ) ₃ . The concentration of the supporting salt in the non-aqueous electrolyte is preferably 0.1 mol/L or more and 5 mol/L or less, more preferably 0.5 mol/L or more and 2 mol/L or less. When the concentration of the supporting salt is 0.1 mol/L or more, a sufficient current density can be obtained, and when the concentration is 5 mol/L or less, the electrolyte can be more stable. A flame retardant such as a phosphorus-based or halogen-based flame retardant may be added to the ion-conducting medium. In addition, instead of a liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. As the ion-conducting polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, or vinylidene fluoride and a supporting salt can be used. Furthermore, an ion-conducting polymer can be used in combination with a non-aqueous electrolyte. As the ion-conducting medium, in addition to an ion-conducting polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound by an organic binder can be used.

The electricity storage device may have a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the electricity storage device, but examples include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

In this power storage device, when fully charged, lithium may be precipitated in the negative electrode (more preferably, inside the composite carbon particles). In this case, lithium may be precipitated as lithium metal or as a lithium alloy, or both. Whether or not the power storage device is fully charged may be determined by whether or not the voltage of the power storage device has reached a preset full charge voltage.

It is preferable that this storage device has a higher charge/discharge efficiency at a high rate, preferably 85% or more, more preferably 90% or more, and even more preferably 95% or more. The charge/discharge efficiency at a high rate is the charge/discharge efficiency when charging at a high rate of 0.25C and then discharging at 0.05C. In this disclosure, "charging" refers to the insertion/precipitation of charge carriers into the composite carbon particles, and "discharging" refers to the desorption/elution of charge carriers from the composite carbon particles. It is also preferable that this storage device has a higher initial charge/discharge efficiency, preferably 60% or more, more preferably 70% or more, and even more preferably 75% or more. It is also preferable that this storage device has a higher charge/discharge efficiency after cycling, preferably 90% or more, more preferably 95% or more, and even more preferably 96% or more.

The shape of the electric storage device is not particularly limited, but examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, and square type. It may also be applied to large devices used in electric vehicles and the like. FIG. 3 is a schematic diagram of an electric storage device 50, which is an example of an electric storage device of the present disclosure. The electric storage device 50 includes a cup-shaped case 51, a positive electrode 52 having a positive electrode active material and provided at the bottom of the case 51, a negative electrode 53 having a negative electrode active material and provided at a position facing the positive electrode 52 via a separator 54, a gasket 55 formed of an insulating material, and a sealing plate 56 disposed at the opening of the case 51 and sealing the case 51 via the gasket 55. In the electric storage device 50, an ion conductive medium 57 is filled in the space between the positive electrode 52 and the negative electrode 53. The negative electrode 53 contains the above-mentioned composite carbon particles 10.

The power storage device of the present disclosure is preferably used by charging until the negative electrode potential becomes lower than the redox potential of lithium, which is the charge carrier. When the negative electrode potential becomes lower than the redox potential of lithium (for example, until fully charged), lithium precipitates at the negative electrode and is stored in the negative electrode. When the negative electrode potential is then made higher than the redox potential of lithium, lithium dissolves at the negative electrode, and the energy can be extracted as discharge capacity. In this way, when the negative electrode potential is charged until it becomes lower than the redox potential of lithium, the lithium precipitation and dissolution capacity can be utilized, and the energy density of the power storage device can be further increased.

The present disclosure described above in detail can further improve the high-rate characteristics of the power storage device. The reason for obtaining such an effect is presumed to be as follows. For example, when the composite carbon particles of the present disclosure are used in the electrodes of a power storage device, charge carriers are inserted and removed from the graphite particles, while the amorphous carbon reduces the deposition overvoltage of the charge carriers, contributing to the deposition and dissolution of the charge carriers. It is considered that the amorphous carbon is formed by, for example, carbonizing the sugar and/or sugar carboxylic acid of the sugar carboxylate metal salt, and removing the metal and/or metal compound derived from the sugar carboxylate metal salt. The pores also function as a deposition field for the charge carriers. It is presumed that such composite carbon particles have a suitable pore structure, which allows the charge carriers to be inserted and removed from the graphite smoothly, and the high-rate characteristics can be further improved.

It goes without saying that the present disclosure is in no way limited to the above-described embodiments, and may be implemented in various forms as long as they fall within the technical scope of the present disclosure.

For example, in the above-described embodiment, the charge carrier of the electricity storage device is lithium, but this is not particularly limited, and may be an alkali metal ion such as sodium or potassium, or a Group 2 element ion such as calcium ion or magnesium ion. The positive electrode active material may contain a carrier ion. The electrolyte is a non-aqueous electrolyte, but may be an aqueous electrolyte.

In the above-described embodiment, the positive electrode active material is a transition metal composite oxide or the like, but is not particularly limited thereto, and may be, for example, a carbon material used in a capacitor. The carbon material is not particularly limited, but may be, for example, activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, polyacenes, and the like. Among these, activated carbons exhibiting a high specific surface area are preferred. The activated carbon as the carbon material preferably has a specific surface area of 1000 m ² /g or more, more preferably 1500 m ² /g or more. When the specific surface area is 1000 m ² /g or more, the discharge capacity can be further increased. The specific surface area of this activated carbon is preferably 3000 m ² /g or less, and more preferably 2000 m ² /g or less, from the viewpoint of ease of preparation. In addition, the positive electrode is considered to store electricity by adsorbing and desorbing at least one of anions and cations contained in the ion conductive medium, but it may also store electricity by inserting and desorbing at least one of anions and cations contained in the ion conductive medium.

The present disclosure may be any one of the following [1] to [11].
[1] Composite carbon particles in which graphite particles and amorphous carbon are composited, the composite carbon particles having a pore volume calculated by nitrogen adsorption measurement of 40 μL/g or more and an average pore diameter calculated by nitrogen adsorption measurement of 7 nm or less.
[2] The composite carbon particle according to [1], which comprises a plurality of the graphite particles, the amorphous carbon being present between the graphite particles, and which is an aggregate particle in which the plurality of graphite particles are aggregated by the amorphous carbon.
[3] The composite carbon particles according to [1] or [2], wherein the pore volume is 50 μL/g or more and 100 μL/g or less.
[4] The composite carbon particles according to any one of [1] to [3], wherein the average pore diameter is 2.2 nm or more and 5 nm or less.
[5] The composite carbon particles according to any one of [1] to [4], which satisfy one or more of the following (1) to (2):
(1) The amorphous carbon is contained in an amount of 2% by mass or more and 20% by mass or less.
(2) The graphite particles are natural graphite.
[6] An electricity storage device comprising an electrode containing the composite carbon particle according to any one of [1] to [5] as an electrode active material.
[7] A granulation step of obtaining a granulated powder using a raw material liquid containing graphite particles and a sugar carboxylate metal salt;
a heat treatment step of heat-treating the granulated powder to carbonize it and generate a metal and/or a metal compound derived from the sugar carboxylate metal salt;
a removing step of removing the metal and/or metal compound from the granulated powder after the heat treatment step;
A method for producing composite carbon particles, comprising:
[8] The method for producing composite carbon particles according to [7], wherein the raw material liquid contains a sugar compound.
[9] The method for producing composite carbon particles according to [7] or [8], which satisfies one or more of the following (3) to (5):
(3) The metal salt of a sugar carboxylate is a gluconate.
(4) The metal salt of a sugar carboxylate is a magnesium salt.
(5) The raw material liquid contains a sugar compound, and the sugar compound is glucose.
[10] In the removing step, the metal and/or metal compound is removed by acid treatment.
A method for producing composite carbon particles according to any one of [7] to [9].
[11] An electrode comprising the composite carbon particles according to any one of [1] to [5] as an electrode active material.

Below, specific examples of the fabrication of the above-mentioned composite carbon particles, electrodes, and electricity storage devices are described as examples. Note that Experimental Examples 1 to 6 correspond to examples, and Experimental Examples 7 to 11 correspond to comparative examples.

[Experimental Example 1]
(Preparation of composite carbon particles)
After mixing 10 g of flake-like natural graphite (Fuji Graphite Industry, BF-8A) and 0.1 g of carboxymethylcellulose (Daiichi Kogyo Seiyaku, BSH-6), 20 g of water was added, and a slurry was obtained using a mixer (Thinky, ARE-310). 0.26 g of glucose (Tokyo Chemical Industry) and 0.6 g of magnesium gluconate (Tokyo Chemical Industry) were dissolved in 130 g of water, and then mixed with the above slurry to prepare a raw material liquid in which graphite, glucose, and magnesium gluconate were uniformly mixed. The raw material liquid was sprayed from a 0.7 mm nozzle at a flow rate of 0.5 L/h under a dry air flow at 140 ° C. using a spray dryer (Nihon Buchi, B-290) to obtain a granulated powder (granulation process). The granulated powder was heat-treated at 900 ° C. in an Ar atmosphere for 1 hour (heat treatment process). The mixture was then mixed in 100 ml of 1 mol/L hydrochloric acid for 3 hours to dissolve the Mg component (removal step). After filtration and washing, the mixture was vacuum dried at 120°C. Finally, the mixture was heat-treated at 1500°C in an Ar atmosphere for 1 hour (sintering step). In this way, the composite carbon particles of Experimental Example 1 were obtained. The amorphous carbon ratio (coating amount) in the composite carbon particles was calculated from the yields in the heat treatment step, removal treatment step, and firing step, and was 1% in Experimental Example 1.

(Morphological Observation of Composite Carbon Particles)
The particle morphology of the composite carbon particles was confirmed by SEM observation at a magnification of 5000 times using a scanning electron microscope (Regulus 8230 manufactured by Hitachi High-Technologies Corporation). Then, it was determined whether the particles were mainly spherical or scaly.

(Pore analysis of composite carbon particles)
Using a pore analyzer (Microtrack Bell BELSORP-max II),
The N ₂ adsorption isotherm measured at ° C. was subjected to BET analysis to calculate the specific surface area, average pore diameter, and pore volume of the composite carbon particles.

(Study on a method for measuring the amount of amorphous carbon coating)
A thermogravimetric (TG) measurement device (ThermoPlusTG8120 manufactured by Rigaku Corporation) was used to obtain a TG curve by increasing the temperature from room temperature to 500° C. at 20° C./min, and then increasing the temperature from 500° C. to 800° C. at 2° C./min. The measurement atmosphere was air at 250 cc/min.

(X-ray diffraction measurement of composite carbon particles)
X-ray diffraction measurements were carried out using an X-ray diffraction measurement device (Ultima IV manufactured by Rigaku Corporation).

(Preparation of electrodes)
The composite carbon particles were mixed in a ratio of 90 parts by mass and 10 parts by mass of polyvinylidene fluoride (PVdF, manufactured by Kureha, #9300), and then coated on copper foil so that the composite weight was 4 mg/cm ^2. After vacuum drying, the mixture was compressed with a roll press machine to adjust the composite density to 1.1 g/cm ³ , thereby producing a sheet-like electrode for the negative electrode.

(Charge/Discharge Evaluation)
The above electrode was punched out to 16 mmφ (2 cm ² ), and was placed opposite a lithium metal counter electrode via a separator, and then an electrolyte was injected to prepare a charge/discharge evaluation cell. The electrolyte was a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in which LiPF ₆ was dissolved at a concentration of 1 M. Then, constant current-constant voltage charge/discharge evaluation was performed at a current density of 0.15 mA/cm ² (0.05 C) and a voltage range of 0.005-1.5 V to calculate the initial charge/discharge efficiency in the insertion/desorption of lithium into and from the electrode containing the composite carbon particles. Note that "charging" refers to the insertion/precipitation of Li into the composite carbon particles, and "discharging" refers to the desorption/elution of Li from the composite carbon particles. In addition, the battery was charged at a constant current of 0.75 mA/ ^cm2 (0.25 C) to a capacity of 3 mAh/ ^cm2 , and then discharged at a current of 0.15 mA/ ^cm2 to 1.5 V at a constant current and constant voltage, and the charge and discharge efficiency at this time was determined as the high-rate charge and discharge efficiency to evaluate the high-rate characteristics. In addition, the battery was charged at a constant current of 0.15 mA/ ^cm2 to a capacity of 3 mAh/ ^cm2 (20 h) and then discharged at a constant current and constant voltage of 0.15 mA/ ^cm2 to 1.5 V for 10 cycles, and the charge and discharge efficiency at the 10 cycles was calculated to evaluate the charge and discharge cycle characteristics.

[Experimental Examples 2 to 5]
Experimental Example 2 was carried out in the same manner as Experimental Example 1, except that the amounts of glucose and magnesium gluconate mixed with 10 g of scaly natural graphite were 0.5 g and 1.2 g (coating amount 2%), respectively. Experimental Example 3 was carried out in the same manner as Experimental Example 1, except that the amounts of glucose and magnesium gluconate mixed with 10 g of scaly natural graphite were 1.0 g and 2.5 g (coating amount 4%), respectively. Experimental Example 4 was carried out in the same manner as Experimental Example 1, except that the amounts of glucose and magnesium gluconate mixed with 10 g of scaly natural graphite were 6.5 g and 15 g (coating amount 20%), respectively. Experimental Example 5 was carried out in the same manner as Experimental Example 1, except that the amounts of glucose and magnesium gluconate mixed with 10 g of scaly natural graphite were 26 g and 60 g (coating amount 50%), respectively.

[Experimental Example 6]
Experimental Example 6 was carried out in the same manner as Experimental Example 1, except that glucose was omitted and the amount of magnesium gluconate mixed per 10 g of flake natural graphite was 5 g (coating amount 4%).

[Experimental Example 7]
Experimental Example 7 was carried out in the same manner as Experimental Example 1, except that magnesium gluconate was omitted, the amount of glucose mixed per 10 g of flake natural graphite was 2 g (coating amount 4%), and the heat treatment step was omitted.

[Experimental Example 8]
Instead of the granulation step, a mixing step was carried out in which 1.0 g of glucose and 2.5 g of magnesium gluconate (coating amount 4%) were mixed with 10 g of flake natural graphite in a mortar. Except for this, Experimental Example 8 was carried out in the same manner as Experimental Example 1.

[Experimental Example 9]
Experimental Example 9 was carried out in the same manner as Experimental Example 1, except that flake natural graphite was used as it was instead of the composite carbon particles.

[Experimental Example 10]
Experimental Example 10 was carried out in the same manner as Experimental Example 1, except that flake natural graphite that had been heat-treated at 1500° C. in an Ar atmosphere was used instead of the composite carbon particles.

[Experimental Example 11]
10 g of the flake natural graphite and 0.1 g of the carboxymethyl cellulose were mixed, and then 20 g of water was added to obtain a slurry using the mixer. 130 g of water was added to dilute the slurry, and the mixture was spray-dried to obtain a powder. The powder was then heat-treated in an Ar atmosphere at 1500° C. Experimental Example 11 was carried out in the same manner as Experimental Example 1, except that the particles thus prepared were used instead of the composite carbon particles.

[Results and Discussion]
As an example of the morphology of the composite carbon particles, SEM images of the composite carbon particles of Experimental Example 3 are shown in Fig. 4 (Fig. 4A, B). SEM images of the graphite particles of Experimental Example 9 are also shown in Fig. 4 (Fig. 4C, D). As shown in Fig. 4A, B, in the experimental example in which the graphite particles were mixed with the amorphous carbon source and the spray-drying process was performed, it was revealed that a plurality of scaly graphite particles were aggregated to form aggregated particles, and the particle shape became spherical. This was presumed to be because the amorphous carbon source became a "binder" and formed secondary particles during the process of drying the droplets of the solution sprayed to contain the graphite particles and the amorphous carbon source during the spray-drying process. On the other hand, even in the case of the sample that was sprayed with the spray-drying process, the sample that did not contain the amorphous carbon source remained scaly, so it was presumed that the amorphous carbon source was necessary from the viewpoint of obtaining aggregated particles such as spherical particles.

FIG. 5 shows the N ₂ adsorption isotherm of the composite carbon particles of Experimental Example 3 as an example of the N ₂ adsorption isotherm (FIG. 5A). FIG. 5 also shows the N ₂ adsorption isotherm of the graphite particles of Experimental Example 9. It was found that the N ₂ adsorption amount was significantly increased in the composite carbon particles composited with amorphous carbon. FIG. 5 also shows the value calculated for the pore volume of the particle surface using the BET method. From FIG. 5, it was found that the pore volume was significantly increased by the composite treatment. It is known that glucose, which is the raw material of the amorphous carbon component, becomes difficult-to-graphitize carbon by heat treatment. It is also known that its magnesium salt, magnesium gluconate, generates magnesium oxide microcrystals in the structure when similarly heat-treated, and that the magnesium oxide microcrystals can be removed by acid treatment to obtain a porous carbon material (Kamiyama et al., Angew. Chem. Int. Ed. 2021, 60, 5114-5120). It was found that the composite carbon particles of Experimental Examples 1 to 6 had surfaces with moderately increased pore volumes. Of these, it was found that the pore volumes increased significantly in Experimental Examples 5 and 6. For reference, FIG. 5 shows Kr adsorption isotherms of the composite carbon particles of Experimental Example 3 and the graphite particles of Experimental Example 9 (FIG. 5B). As shown in FIG. 5B, steps specific to graphite were confirmed in Experimental Example 9, whereas such steps were not confirmed in Experimental Example 3.

FIG. 6 shows the TG curve of the composite carbon particles of Experimental Example 3 as an example of TG measurement. In the TG curve, a two-stage weight loss behavior was confirmed, and it was inferred that the weight loss up to around 600°C was due to the amorphous carbon component, and the weight loss after 650°C was due to the graphite itself. In Experimental Example 3 (the coating amount calculated from the yield in the heat treatment process, the removal treatment process, and the firing process was 4%), the weight loss up to 600°C was about 4%, which was the same value as the coating amount calculated from the yield. From this, it was inferred that the amorphous carbon coating amount can be predicted to some extent based on the weight loss up to 600°C in the TG curve. For reference, the calculation process of the coating amount in Experimental Example 3 is shown below.
Mass before heat treatment process Granulated powder Xh1: 2.9691g (graphite Gh1: 2.0763g, coating source Ch1: 0.8928g)
Mass after heat treatment process Granulated powder Xh2: 2.2018g (graphite Gh2: 2.0690g, coating source Ch2: 0.1328g)
*The graphite mass was calculated based on the yield (s) of 99.65% when the heat treatment process is performed on graphite alone.
The remainder was calculated as the coating source mass.
Mass before removal treatment (acid treatment) process Granulated powder Xr1: 2.1163 g (graphite Gr1: 1.9887 g, coating source Cr1: 0.1276 g)
Mass after removal treatment (acid treatment) process Granulated powder Xr2: 1.9420 g (graphite Gr2: 1.8438 g, coating source Cr2: 0.0982 g)
*Theoretical yield t = 83% when MgO is removed from the coating source.
Calculations were made assuming that losses due to filtration, etc. were equal for graphite and the coating source from which MgO has been removed.
Mass before firing process Granulated powder Xf1: 1.8773 g (graphite Gf1: 1.7824 g, coating source Cf1: 0.0949 g)
Mass after firing process Granulated powder Xf2: 1.8537g (graphite Gf2: 1.7783g, coat Cf2: 0.0754g)
*The graphite mass was calculated from the yield u = 99.77% when the firing process is carried out using graphite alone.
The remainder was calculated as the coating source mass.
Calculation of the proportion of amorphous carbon in the composite carbon particle (coating amount) Coating amount = Cf2 / Xf2 x 100 ≒ 4

Figure 7 shows the X-ray diffraction pattern of the composite carbon particles of Experimental Example 3 as an example of X-ray diffraction measurement. Figure 7 also shows the X-ray diffraction pattern of the raw graphite particles of Experimental Example 9. Compared to Experimental Example 9, which is the raw graphite particles, the background intensity on the low angle side of Experimental Example 3 was increased. This was presumably due to the presence of amorphous carbon components, which increases the intensity in the low angle region with large interplanar spacing, as well as the effect of X-ray scattering due to the pore structure contained in the amorphous carbon components. From this, it was confirmed that graphite particles and amorphous carbon were composited in Experimental Example 3.

As an example of charge/discharge evaluation, Figure 8 shows the charge/discharge curves of the cell of Experimental Example 3 after 10 cycles. From the charge/discharge curves, it was confirmed that in addition to the Li insertion/extraction capacity, Li precipitation/dissolution capacity was also expressed. It was found that by utilizing the Li precipitation/dissolution capacity, it is possible to further increase the charge/discharge capacity and energy density.

Tables 1 and 2 summarize the composite conditions, the physical properties of the composite carbon particles, and the battery performance for Experimental Examples 1 to 11. As shown in Table 1, it was found that Experimental Examples 1 to 6 had high high-rate charge and discharge efficiency. This is presumed to be because, for example, the pore structure of the composite carbon particles in Experimental Examples 1 to 6 was favorable, so that the insertion and desorption of the charge carriers into the graphite and the precipitation and dissolution of the charge carriers proceeded smoothly. In addition, it was presumed that, for example, the particle shape was spherical, which improved ion conduction in the electrode, thereby promoting uniform charging (Li precipitation). Note that, although the composite carbon particles in Experimental Example 7 also had a spherical particle shape, the pore volume was small and there were few reaction sites for the insertion or precipitation of lithium ions, so it was presumed that the effects of Experimental Examples 1 to 6 were not obtained. As shown in Table 2, the theoretical specific surface area and theoretical pore volume of the amorphous carbon in Experimental Examples 1 to 5 and 8, which used glucose and magnesium gluconate as the coating source, were larger than the specific surface area and pore volume of the particles in the Reference Example, which was made using glucose and magnesium gluconate to be amorphous carbon alone. This is presumed to reflect the influence of the pores formed at the junctions between the graphite particles and the amorphous carbon.

Of Experimental Examples 1 to 6, Experimental Examples 2 to 4 in particular were found to exhibit favorable performance in terms of charge/discharge efficiency, high-rate charge/discharge efficiency, and charge/discharge efficiency after cycling. Below, we have considered the favorable conditions from each perspective.

Regarding the initial charge/discharge efficiency, in Experimental Examples 9 to 11 in which no amorphous carbon coating was applied, Experimental Examples 10 and 11 in which heat treatment at 1500°C was performed tended to have a higher initial charge/discharge efficiency than Experimental Example 9 in which no such heat treatment was performed. On the other hand, in Examples 1 to 8 in which an amorphous carbon coating was applied, heat treatment at 1500°C was performed, but the initial charge/discharge efficiency was slightly lower than that of Experimental Examples 10 and 11 in which no amorphous carbon coating was applied. This was presumed to be due to irreversible lithium trapping in the amorphous carbon coating. However, in all of Experimental Examples 1 to 8, the initial charge/discharge efficiency was 60% or more, which was acceptable. Among Experimental Examples 1 to 8, Experimental Examples 1 to 4 and 7 to 8 in which the pore volume was 100 μL/g or less and the average pore diameter was 2.2 nm or more showed a small decrease in the initial charge/discharge efficiency. From this, it was inferred that it is preferable for the pore volume to be 100 μL/g or less and the average pore diameter to be 2.2 nm or more in order to suppress a decrease in the initial charge/discharge efficiency.

In experimental examples 1 to 6, high charge/discharge efficiencies of over 90% were obtained during high-rate charging. In experimental examples 1 to 6, the pore volume was relatively large at 40 μL/g or more, and the average pore diameter was small at 7 nm or less, and it was inferred that such a pore structure is particularly suitable for improving high-rate characteristics.

The charge-discharge efficiency at 10 cycles was high in Experimental Examples 2 to 4 and 6. This was presumably because, like the charge-discharge efficiency at high-rate charging, the ion conduction path in the electrode was secured, and the lithium deposition/dissolution reaction proceeded smoothly due to the presence of appropriate pores as the lithium deposition field on the graphite particle surface. In Experimental Examples 2 to 4 and 6, the pore volume was 50 μL/g or more, the average pore diameter was 5 nm or less, and the specific surface area was 50 m ² /g or more and 200 m ² /g or less. From this, it was presumed that it is preferable to satisfy the following conditions from the viewpoint of improving cycle characteristics: pore volume is 50 μL/g or more, average pore diameter is 5 nm or less, specific surface area is 50 m ² /g or more and 200 m ² /g or less.

From these results, it was found that when the composite carbon particles of Experimental Examples ₂ to 4, which are composited with an amorphous carbon component on the surface of the graphite particles, have a pore volume of 50 μL/g or more and 100 μL/g or less, an average pore diameter of 2.2 nm or more and 5 nm or less, and are spherical, are used as a negative electrode, the graphite negative electrode is particularly suitable for charging and discharging using the capacity of lithium deposition/dissolution in addition to lithium insertion/extraction into/from the graphite particles. The following reasons were inferred for the reasons why good results were obtained in Experimental Examples 2 to 4. For example, it was inferred that the graphite particles were covered with an amorphous carbon layer, so that the lithium deposition overvoltage was reduced and lithium could be uniformly deposited. In addition, it was inferred that the graphite particles were spherical, so that the reaction uniformity in the film thickness direction was high when the charge rate was increased, and the subsequent discharge capacity was high. In addition, the pore volume measured by _N2 adsorption measurement was 50 μL/g or more and 100 μL/g or less, and it was inferred that lithium metal was suitably stored in such pores. In addition, the average pore diameter was 2.2 nm or more and 5 nm or less, and it was inferred that the initial charge/discharge efficiency was improved by suppressing the amount of film formed in the pores, and lithium metal was efficiently stored. In addition, it was inferred that the proportion of amorphous carbon was 2% or more and 20% or less, and while exerting the effect of composite with amorphous carbon, the increase in irreversible capacity during the initial charge/discharge due to amorphous carbon was suppressed.

This disclosure can be used in the technical field of electricity storage devices.

10 Composite carbon particles, 12 Raw material liquid, 14 Granulated powder, 16 Granulated powder after heat treatment, 18 Granulated powder after acid treatment, 20 Graphite particles, 30 Amorphous carbon, 31 Pore, 32 Coating liquid, 33 Sugar carboxylate metal salt, 34 Solid content, 36 Carbon material, 37 Metal and/or metal compound, 50 Electric storage device, 51 Case, 52 Positive electrode, 53 Negative electrode, 54 Separator, 55 Gasket, 56 Sealing plate, 57 Ion conductive medium.

Claims (10)

Composite carbon particles in which graphite particles and amorphous carbon are composited, the pore volume calculated by nitrogen adsorption measurement being 40 μL/g or more, and the average pore diameter calculated by nitrogen adsorption measurement being 7 nm or less.
Composite carbon particles. The amorphous carbon is present between the graphite particles, and the graphite particles are aggregated by the amorphous carbon.
The composite carbon particle of claim 1 . The composite carbon particles according to claim 1 or 2, wherein the pore volume is 50 μL/g or more and 100 μL/g or less. The composite carbon particles according to claim 1 or 2, wherein the average pore diameter is 2.2 nm or more and 5 nm or less. The composite carbon particles according to claim 1 or 2, which satisfy one or more of the following (1) to (2):
(1) The amorphous carbon is contained in an amount of 2% by mass or more and 20% by mass or less.
(2) The graphite particles are natural graphite. An electrode comprising the composite carbon particle according to claim 1 or 2 as an electrode active material.
Energy storage device. A granulation step of obtaining a granulated powder using a raw material liquid containing graphite particles and a sugar carboxylate metal salt;
a heat treatment step of heat-treating the granulated powder to carbonize it and generate a metal and/or a metal compound derived from the sugar carboxylate metal salt;
a removing step of removing the metal and/or metal compound from the granulated powder after the heat treatment step;
A method for producing composite carbon particles, comprising: The raw material solution contains a sugar compound.
A method for producing the composite carbon particles according to claim 7. The method for producing composite carbon particles according to claim 7, which satisfies one or more of the following (3) to (5):
(3) The metal salt of a sugar carboxylate is a gluconate.
(4) The metal salt of a sugar carboxylate is a magnesium salt.
(5) The raw material liquid contains a sugar compound, and the sugar compound is glucose. In the removing step, the metal and/or metal compound is removed by an acid treatment.
The method for producing the composite carbon particles according to any one of claims 7 to 9.

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