WO2017056991A1

WO2017056991A1 - Sodium-ion secondary battery negative electrode carbonaceous material and sodium-ion secondary battery using thereof

Info

Publication number: WO2017056991A1
Application number: PCT/JP2016/077100
Authority: WO
Inventors: 哲広小林; 正太小林; 靖浩多田
Original assignee: 株式会社クレハ
Priority date: 2015-09-30
Filing date: 2016-09-14
Publication date: 2017-04-06
Also published as: TWI672855B; JPWO2017056991A1; TW201712931A

Abstract

The purpose of the present invention is to provide a sodium-ion secondary battery having a high discharge capacity and having excellent cycling and preservation characteristics. The problems can be solved by a sodium-ion secondary battery negative electrode carbonaceous material characterized in that the ratio H/C of a hydrogen atom to a carbon atom determined by element analysis is 0.05 or less.

Description

Carbonaceous material for negative electrode of sodium ion secondary battery and sodium ion secondary battery using the same

The present invention relates to a carbonaceous material for a negative electrode of a sodium ion secondary battery and a sodium ion secondary battery using the same. According to the present invention, it is possible to provide a sodium ion secondary battery having a high discharge capacity and exhibiting excellent cycle characteristics and storage characteristics.

In recent years, lithium ion secondary batteries can be used as a power source for automobiles and a large power source for stationary use. However, rare metals such as cobalt, nickel, and lithium are used as materials, and there is concern about their supply. On the other hand, sodium ion secondary batteries using abundant sodium resources have been studied in order to eliminate concerns about material supply.

The basic structure of a sodium ion secondary battery is similar to that of a lithium ion secondary battery, but differs in that sodium is used as a charge carrier instead of lithium. For this reason, the sodium ion secondary battery has different electrochemical characteristics from the lithium ion secondary battery.

In order for a sodium ion secondary battery to be used as a power source for automobiles or a large power source for stationary use, the battery must have a high discharge capacity and excellent cycle characteristics and storage characteristics. In order to have a high discharge capacity, a negative electrode material capable of electrically inserting (doping) and detaching (de-doping) a lot of sodium is required. Moreover, since it is a secondary battery, the negative electrode material excellent in cycling characteristics which can endure repeated dope and dedope is required. In stationary applications, the battery is held in a charged state in preparation for emergency power supply. Therefore, there is a need for a negative electrode material that can maintain the capacity after being fully charged with a high maintenance rate. In particular, storage characteristics at high temperatures are required.

Carbonaceous materials are being studied as candidates for negative electrode materials for sodium ion secondary batteries. It is known that graphite generally used in lithium ion secondary batteries cannot electrochemically dope and dedope sodium. Therefore, an amorphous carbon material that can be doped and dedoped with sodium ions has been proposed as a carbonaceous material for a sodium ion secondary battery negative electrode.

JP 2007-35588 A JP 2010-251283 A JP 2009-129741 A Japanese Patent Laying-Open No. 2015-5050

Patent Document 1 discloses a secondary battery using a non-graphitic carbonaceous material having d ₀₀₂ of 0.377 nm or more and Lc of 1.29 nm or less as a negative electrode material. However, the initial discharge capacity density was as low as 234 mAh / g.
In Patent Document 2, a carbonaceous material prepared using a plant as a raw material is used as the negative electrode material. However, the initial discharge capacity was as small as 223 mAh / g.
Patent Document 3 discloses a secondary battery using glassy carbon as a negative electrode material. However, the discharge capacity was as small as 265 mAh / g.
Patent Document 4 discloses a secondary battery using a carbonaceous material having a ratio of ρp / ρH of less than 0.950 as a negative electrode material. However, the negative electrode capacity per unit weight was 230 mAh / g.
Thus, although non-graphitic carbonaceous materials have been studied as an alternative negative electrode material for graphite materials, the discharge capacity of sodium secondary batteries using non-graphitic carbonaceous materials reported so far is It is about 200 to 260 mAh / g, and it cannot be said that it has a sufficient capacity.
Accordingly, an object of the present invention is to provide a sodium ion secondary battery having a high discharge capacity and excellent cycle characteristics and storage characteristics. Furthermore, the objective of this invention is providing the carbonaceous material for secondary battery negative electrodes used for the said battery.

As a result of earnest research on a sodium ion secondary battery having a high discharge capacity and excellent cycle characteristics and storage characteristics, the present inventor has surprisingly found that hydrogen atoms and carbon atoms required by elemental analysis. It has been found that a sodium ion secondary battery exhibiting excellent battery characteristics can be obtained by using a carbonaceous material having a ratio H / C of 0.05 or less as a negative electrode material of a sodium ion secondary battery.
The present invention is based on these findings.
Therefore, the present invention
[1] A carbonaceous material for a negative electrode of a sodium ion secondary battery, wherein the ratio H / C of hydrogen atoms to carbon atoms determined by elemental analysis is 0.05 or less,
[2] The carbonaceous material for a sodium ion secondary battery negative electrode according to [1], wherein the BET specific surface area is less than 20 m ² / g,
[3] The carbonaceous material for a sodium ion secondary battery negative electrode according to [1] or [2], wherein the true density determined by the butanol method is less than 1.53 g / cm ³ ;
[4] The carbonaceous material for a sodium ion secondary battery negative electrode according to any one of [1] to [3], wherein petroleum pitch or tar or coal pitch or tar is used as a carbon source.
[5] A negative electrode for a sodium ion secondary battery comprising the carbonaceous material according to any one of [1] to [4], and a sodium ion secondary battery comprising the electrode according to [6] [5],
About.

The sodium ion secondary battery using the carbonaceous material of the present invention has a large discharge capacity and exhibits excellent cycle characteristics and storage characteristics. In particular, when the ratio H / C of hydrogen atoms to carbon atoms is 0.05 or less, excellent cycle characteristics can be exhibited. Furthermore, when the BET specific surface area is less than 20 m ² / g, the storage characteristics can be improved. Furthermore, when the butanol true density is less than 1.53 g / cm ³ , a high discharge capacity can be exhibited.
Further, by combining the physical properties of H / C, BET specific surface area, and butanol true density in the above ranges, the sodium ion secondary battery using the carbonaceous material of the present invention has further excellent discharge capacity, cycle characteristics, And storage characteristics.

It is the graph which showed the cycling characteristics of the sodium ion secondary battery which used the carbonaceous material of this invention as negative electrode material. It is the graph which showed the storage characteristic at 40 degreeC of the sodium ion secondary battery using the carbonaceous material of this invention as negative electrode material.

[1] Carbonaceous material for sodium ion secondary battery negative electrode The carbonaceous material for a sodium ion secondary battery negative electrode of the present invention has a hydrogen atom to carbon atom ratio H / C determined by elemental analysis of 0.05 or less. . The BET specific surface area is preferably less than 20 m ² / g. Furthermore, the true density determined by the butanol method is preferably less than 1.53 g / cm ³ .

《Carbon source》
The carbon source of the carbonaceous material for the negative electrode of the sodium ion secondary battery of the present invention is not particularly limited. For example, petroleum-based pitch or tar, coal-based pitch or tar, thermoplastic resin (for example, ketone resin, polyvinyl Alcohol, polyester resin (eg, polyethylene terephthalate, polybutylene terephthalate, polyarylate), polyacetal resin, polyacrylonitrile, styrene / divinylbenzene copolymer, polyamide resin (nylon resin), polyimide resin, polycarbonate resin, modified polyphenylene ether, polysulfone Resin, polyphenylene sulfide resin, fluororesin, polyamideimide resin, aramid resin, or polyetheretherketone), thermosetting resin (eg, epoxy resin, urethane resin, A resin, diallyl phthalate resin, silicone resin, furan resin, phenol resin, melamine resin, an amino resin and amide resin). The carbon source is preferably petroleum-based pitch or tar, coal-based pitch or tar, and petroleum-based pitch is particularly preferable because it has few impurities and is inexpensive.

<< Atomic ratio of hydrogen atom to carbon atom (H / C) >>
H / C is measured by elemental analysis of hydrogen atoms and carbon atoms. Since the hydrogen content of the carbonaceous material decreases as the degree of carbonization increases, H / C tends to decrease. Therefore, H / C is effective as an index representing the degree of carbonization. H / C of the carbonaceous material of the present invention is 0.05 or less, more preferably 0.04 or less, and still more preferably 0.03 or less. When the ratio H / C of hydrogen atoms to carbon atoms exceeds 0.05, there are many functional groups in the carbonaceous material, and sodium doped into the negative electrode carbon by reaction with sodium is not completely dedope, There is a problem that a large amount of sodium remains in the negative electrode carbon, and sodium which is an active material is wasted. The lower limit of H / C is not particularly limited, but H may be below the detection limit. In this case, H / C is substantially zero. In particular, when the ratio H / C of hydrogen atoms to carbon atoms is within the above range, excellent cycle characteristics can be exhibited.

"Specific surface area"
The specific surface area can be obtained by an approximate expression derived from the BET expression by nitrogen adsorption. The specific surface area of the carbonaceous material of the present invention is not limited, but is preferably 20 m ² / g or less, more preferably 15 m ² / g or less. When the specific surface area exceeds 20 m ² / g, the reaction with the electrolytic solution increases, leading to an increase in irreversible capacity, and thus battery performance may be reduced. In particular, when the specific surface area is in the above range, the storage characteristics can be improved. That is, it is possible to prevent a decrease in capacity when stored at a high temperature with full charge. Further, when the specific surface area is 20 m ² / g or less, pores that do not contribute to storage of sodium are reduced, and pores having a size that can store sodium are increased, and thus an excellent discharge capacity can be exhibited.
Although the minimum of a specific surface area is not specifically limited, If a specific surface area is less than 0.5 m < ² > / g, input-output characteristics may fall. Therefore, the lower limit of the specific surface area is preferably 0.5 m ² / g or more.

《Butanol true density》
The butanol true density of the carbonaceous material of the present invention is not limited, but is preferably less than 1.53 g / cm ³ . The upper limit of butanol true density is more preferably 1.525g / cm ³ or less, further preferably 1.52 g / cm ³ or less, more preferably 1.515g / cm ³ or less. A carbonaceous material having a true density of more than 1.53 g / cm ³ has few pores of a size that can store sodium, and may have a small doping and dedoping capacity. In particular, when the butanol true density is within the above range, the sodium ion secondary battery using the carbonaceous material of the present invention can exhibit a high discharge capacity. The lower limit of the butanol true density is not limited, but is preferably 1.35 g / cm ³ or more, more preferably 1.39 g / cm ³ or more.
A carbonaceous material having a butanol true density of 1.48 g / cm ³ or less exhibits a further excellent discharge capacity. Accordingly, the butanol true density of the carbonaceous material of the present invention is most preferably 1.35 g / cm ³ to 1.45 g / cm ³ .

《Helium true density》
The true helium density of the carbonaceous material of the present invention is not particularly limited, but is preferably less than 2.20 g / cm ³ . A carbonaceous material having a helium true density exceeding 2.20 g / cm ³ may have fewer pores capable of storing sodium, and the doping and dedoping capacity may be reduced. The lower limit of the helium true density is preferably 1.35 g / cm ³ or more, more preferably 1.39 g / cm ³ or more.

《Average particle size》
The average particle diameter (D _v50 ) of the carbonaceous material of the present invention is 1 to 50 μm. The lower limit of the average particle diameter is preferably 1 μm or more, more preferably 1.5 μm or more, and particularly preferably 2.0 μm or more. When the average particle diameter is less than 1 μm, the specific surface area is increased by increasing the fine powder. Therefore, the irreversible capacity, which is a capacity that does not discharge even when charged due to increased reactivity with the electrolytic solution, is increased, and the proportion of wasted capacity of the positive electrode is increased. The upper limit of the average particle diameter is preferably 40 μm or less, more preferably 35 μm or less. When the average particle diameter exceeds 50 μm, the diffusion free path of sodium in the particles increases, so that rapid charge / discharge becomes difficult. Further, in the secondary battery, it is important to increase the electrode area in order to improve the input / output characteristics. For this reason, it is necessary to reduce the coating thickness of the active material on the current collector plate during electrode preparation. In order to reduce the coating thickness, it is necessary to reduce the particle diameter of the active material. From such a viewpoint, the upper limit of the average particle diameter is preferably 50 μm or less.

《Manufacture of carbonaceous material for negative electrode of sodium ion secondary battery》
The carbonaceous material for a sodium ion secondary battery negative electrode of the present invention is not limited, but a forming step of a porous molded body, an infusibilization step, an alkali deposition step, a pulverization step, a preliminary firing step, a main firing step, And a plurality of processes selected from the group consisting of a pyrolytic carbon coating process.
The carbonaceous material for the negative electrode of the nonaqueous electrolyte secondary battery of the present invention is not limited. For example, as shown in Examples 1 to 3, petroleum-based pitch or tar, coal-based pitch or tar is used as the carbon source. When used, (1) a step of obtaining a porous pitch molded body by adding an additive to a carbon source, heating and molding, and (2) oxidizing the porous pitch molded body at 120 to 400 ° C. An infusibilization step, (3) a preliminary firing step of firing at 400 ° C. or more and less than 800 ° C. in a non-oxidizing gas atmosphere, and (4) a main firing step of firing at 800 ° C. to 1500 ° C. in a non-oxidizing gas atmosphere, It can manufacture with the manufacturing method containing. In the following manufacturing method, pitch is described as an example, but it is possible to manufacture a carbonaceous material using tar in the same manner.

[Porous pitch molding process (1)]
A 2- or 3-ring aromatic compound having a boiling point of 200 ° C. or higher or a mixture thereof is added to a petroleum-based or coal-based pitch or the like as an additive, mixed by heating, and then molded to obtain a pitch molded body. Next, the additive is extracted and removed from the pitch molded body with a solvent having low solubility with respect to pitch and high solubility with respect to the additive to obtain a porous pitch. The purpose of the aromatic additive is to extract and remove the additive from the molded pitch molded body to make the molded body porous, to facilitate crosslinking treatment by oxidation, and to obtain a carbonaceous material obtained after carbonization. To make it porous. Such additives can be selected from one or a mixture of two or more such as naphthalene, methylnaphthalene, phenylnaphthalene, benzylnaphthalene, methylanthracene, phenanthrene, or biphenyl. The amount added to the pitch is preferably in the range of 30 to 70 parts by weight with respect to 100 parts by weight of the pitch. The pitch and additive are mixed in a molten state by heating in order to achieve uniform mixing. The mixture of pitch and additive is preferably formed into particles having a particle size of 1 mm or less so that the additive can be easily extracted from the mixture. Molding may be performed in a molten state, or may be performed by pulverizing the mixture after cooling. Solvents for extracting and removing the additive from the mixture of pitch and additive include aliphatic hydrocarbons such as butane, pentane, hexane, or heptane, mixtures mainly composed of aliphatic hydrocarbons such as naphtha or kerosene, methanol, Aliphatic alcohols such as ethanol, propanol or butanol are preferred. By extracting the additive from the pitch and additive mixture molded body with such a solvent, the additive can be removed from the molded body while maintaining the shape of the molded body. At this time, it is presumed that a through hole for the additive is formed in the molded body, and a pitch molded body having uniform porosity is obtained.

[Infusibilization step (2)]
In order to crosslink the resulting porous pitch, it is then oxidized with an oxidizing agent, preferably at a temperature of 120 to 400 ° C. As the oxidizing agent, O ₂ , O ₃ , NO ₂ , a mixed gas obtained by diluting them with air or nitrogen, or an oxidizing gas such as air, or an oxidizing liquid such as sulfuric acid, nitric acid, or hydrogen peroxide water is used. be able to. It is simple and economical to oxidize at 120 to 400 ° C. and carry out a crosslinking treatment using a gas containing oxygen such as air or a mixed gas of air and other gas such as combustion gas as an oxidizing agent. Is also advantageous. In this case, if the softening point such as pitch is low, the pitch melts during oxidation and it becomes difficult to oxidize. Therefore, the pitch or the like used preferably has a softening point of 150 ° C. or higher.

(Oxygen crosslinking degree)
The oxygen crosslinking degree when the carbonaceous precursor is infusible by oxidation is not particularly limited as long as the effect of the present invention is obtained. That is, when the infusibilization treatment by oxygen crosslinking is not performed, the oxygen crosslinking degree may be 0% by weight, but the lower limit of the oxygen crosslinking degree is preferably 1% by weight or more, more preferably 2% by weight or more, More preferably, it is 3% by weight or more. If it is less than 1% by weight, the true density becomes large and the void for storing sodium becomes small, which is not preferable. In particular, in order to obtain an optimum butanol true density carbonaceous material, the oxygen content is preferably 10% by weight or more. The upper limit of the oxygen crosslinking degree is preferably 25% by weight or less, more preferably 20% by weight or less, and still more preferably 18% by weight or less. Exceeding 25% by weight is not preferable because the true density decreases and the charge / discharge capacity per volume may decrease.

(Alkali attachment process)
A carbonaceous material having an optimal pore structure can be obtained by performing alkali addition on the carbonaceous precursor and then performing a heat treatment such as preliminary firing. The carbonaceous precursor to be alkali-added is not limited, and examples thereof include petroleum pitch or tar, coal pitch or tar, thermoplastic resin, or thermosetting resin.

The alkali deposition step is a step of adding an alkali metal element-containing compound to the carbonaceous precursor and heat-treating it in a non-oxidizing gas atmosphere at 500 ° C. to 1000 ° C. to obtain an alkali-treated carbonaceous precursor. As the alkali metal element, an alkali metal element such as lithium, sodium, or potassium can be used. The alkali metal element may be attached to the carbonaceous precursor in a metal state, but a compound containing an alkali metal element such as a hydroxide, carbonate, hydrogencarbonate, or halogen compound (hereinafter referred to as an alkali metal compound). May be attached). The alkali metal compound is not limited, but is preferably a hydroxide because it has high permeability and can be uniformly impregnated into the carbonaceous precursor.

(Alkali-added carbonaceous precursor)
An alkali-added carbonaceous precursor can be obtained by adding an alkali metal element or an alkali metal compound to the carbonaceous precursor. The addition method of an alkali metal element or an alkali metal compound is not limited. For example, a predetermined amount of an alkali metal element or an alkali metal compound may be mixed in a powder form with respect to the carbonaceous precursor. Moreover, an alkali metal compound is dissolved in an appropriate solvent to prepare an alkali metal compound solution. After mixing the alkali metal compound solution with the carbonaceous precursor, the solvent may be volatilized to prepare a carbonaceous precursor to which the alkali metal compound is attached. The addition amount of the alkali metal compound attached to the carbonaceous precursor is not particularly limited, but the upper limit of the addition amount is preferably 70.0% by weight or less, more preferably 60.0% by weight or less. More preferably, it is 50.0% by weight or less. When the amount of the alkali metal element or alkali metal compound added is too large, the alkali activation occurs excessively. For this reason, the specific surface area increases, which increases the irreversible capacity, which is not preferable. Further, the lower limit of the addition amount is not particularly limited, but is preferably 5.0% by weight or more, more preferably 10.0% by weight or more, and further preferably 15.0% by weight or more. is there. If the amount of the alkali metal compound added is too small, it becomes difficult to form a pore structure for doping and dedoping, which is not preferable.

[Pre-baking step (3)]
The pre-baking step is a step of removing a volatile component such as CO ₂ , CO, CH ₄ , H _2, and the tar component by performing a heat treatment on the carbon precursor. The pre-baking temperature is preferably 400 ° C. or higher and lower than 800 ° C., more preferably 500 ° C. or higher and lower than 800 ° C. When the pre-baking temperature is less than 400 ° C., detarring becomes insufficient, and there is a large amount of tar and gas generated in the main baking process after pulverization, which may adhere to the particle surface. If not, battery performance may be degraded. On the other hand, when the pre-baking temperature is 800 ° C. or higher, the tar generation temperature region is exceeded, and the energy efficiency to be used may be reduced. Furthermore, the generated tar may cause a secondary decomposition reaction, which may adhere to the carbon precursor and cause a decrease in performance.

[Crushing process]
In the carbon material, it is preferable to reduce the particle diameter in order to increase the region where sodium can be doped and dedoped. Although the timing of pulverization is not particularly limited, it is preferably before calcination. It can pulverize the carbonaceous precursor before pre-firing, but the carbonaceous precursor may melt during pre-firing. Moreover, although it can carry out after this baking, since a carbon precursor will become hard if a carbonization reaction advances, control of the particle size distribution by grinding | pulverization will become difficult. By grinding, the average particle diameter of the carbonaceous material of the present invention can be reduced to 1 to 50 μm. The pulverizer used for pulverization is not particularly limited, and for example, a jet mill, a rod mill, a vibrating ball mill, or a hammer mill can be used. A jet mill equipped with a classifier is preferable.

(Washing of alkali metals and alkali metal compounds)
In the firing step of the present invention, it is preferable to remove the alkali metal and the alkali metal compound (washing the alkali compound). When a large amount of alkali metal and alkali metal compound remain in the carbonaceous material, the carbonaceous material becomes strongly alkaline. For example, when a negative electrode is produced using PVDF (polyvinylidene fluoride) as a binder, PVDF may gel if the carbonaceous material exhibits strong alkalinity. In addition, when the alkali metal remains in the carbonaceous material, it is considered that the alkali metal moves to the counter electrode during the discharge of the secondary battery and adversely affects the charge / discharge characteristics. Therefore, it is preferable to remove the alkali metal compound from the carbonaceous precursor. Although washing of the alkali compound is not limited, it can be performed before or after the main baking. The alkali metal and the alkali compound can be washed according to a usual method. Specifically, the alkali metal and the alkali compound can be washed in a gas phase or a liquid phase. In the case of a gas phase, it is performed by volatilizing an alkali metal element or an alkali metal compound at a high temperature.

[Main firing step (4)]
The main calcination in the production method of the present invention can be performed according to a normal main calcination procedure, and a carbonaceous material for a negative electrode of a sodium ion secondary battery can be obtained by performing the main calcination. The temperature of the main firing is not limited, but is, for example, 800 to 1500 ° C. However, the lower limit of the firing temperature of the present invention is preferably more than 1000 ° C, more preferably 1050 ° C or more, further preferably 1100 ° C or more, and particularly preferably 1150 ° C or more. If the heat treatment temperature is too low, carbonization is insufficient, many functional groups remain in the carbonaceous material and the H / C value increases, and the irreversible capacity may increase due to reaction with sodium. On the other hand, the upper limit of the main firing temperature of the present invention is 1450 ° C. or lower. When the main baking temperature exceeds 1450 ° C., voids formed as sodium storage sites may decrease, and the doping and dedoping capacity may decrease. That is, the selective orientation of the carbon hexagonal plane is increased and the discharge capacity may be reduced.

The main firing is preferably performed in a non-oxidizing gas atmosphere. Examples of the non-oxidizing gas include helium, nitrogen, and argon, and these can be used alone or in combination. Further, the main calcination can be performed in a gas atmosphere in which a halogen gas such as chlorine is mixed with the non-oxidizing gas. Moreover, this baking can also be performed under reduced pressure, for example, can also be performed at 10 kPa or less. Although the time for the main baking is not particularly limited, it can be performed, for example, in 0.1 to 10 hours, preferably 0.3 to 8 hours, and more preferably 0.4 to 6 hours.

(Coating process with pyrolytic carbon)
The CVD method can be used for coating with pyrolytic carbon. Specifically, the fired product is brought into contact with a linear or cyclic hydrocarbon gas, and carbon purified by thermal decomposition is deposited on the fired product. This method is well known as a so-called chemical vapor deposition method (CVD method). The specific surface area of the obtained carbonaceous material can be controlled by the coating process with pyrolytic carbon. The pyrolytic carbon used in the present invention is not limited as long as it can be added as a hydrocarbon gas and can reduce the specific surface area of the carbonaceous material. The hydrocarbon gas is preferably mixed with a non-oxidizing gas and brought into contact with the carbonaceous material.
The carbon source of the hydrocarbon gas is not limited, for example, methane, ethane, propane, butane, pentane, hexane, octane, nonane, decane, ethylene, propylene, butene, pentene, hexene, acetylene, cyclopentane, cyclohexane , Cycloheptane, cyclooctane, cyclononane, cyclopropene, cyclopentene, cyclohexene, cycloheptene, cyclooctene, decalin, norbornene, methylcyclohexane, norbornadiene, benzene, toluene, xylene, mesitylene, cumene, butylbenzene or styrene. Further, as the carbon source of the hydrocarbon gas, it is possible to use a hydrocarbon gas generated by heating a gaseous organic material and a solid or liquid organic material.

<Manufacture of negative electrode>
In the negative electrode using the carbonaceous material of the present invention, a binder (binder) is added to the carbonaceous material, and an appropriate solvent is added and kneaded to form an electrode mixture. It can be produced by pressure molding after coating and drying. By using the carbonaceous material of the present invention, it is possible to produce an electrode having high conductivity without adding a conductive auxiliary agent. When preparing the agent, a conductive aid can be added. As the conductive auxiliary agent, acetylene black, ketjen black, carbon nanofiber, carbon nanotube, carbon fiber or the like can be used, and the addition amount varies depending on the type of conductive auxiliary agent used, but the added amount is small. If the amount is too large, the expected conductivity cannot be obtained, and this is not preferable. If the amount is too large, the dispersion in the electrode mixture deteriorates. From this point of view, the preferred proportion of the conductive auxiliary agent to be added is 0.5 to 15% by weight (where the amount of active material (carbonaceous material) + the amount of binder + the amount of conductive auxiliary agent = 100% by weight). More preferably 0.5 to 7% by weight, particularly preferably 0.5 to 5% by weight. The binder is not particularly limited as long as it does not react with an electrolytic solution such as PVDF (polyvinylidene fluoride), polytetrafluoroethylene, and a mixture of SBR (styrene-butadiene rubber) and CMC (carboxymethylcellulose). Among these, PVDF is preferable because PVDF attached to the surface of the active material hardly inhibits sodium ion migration and obtains favorable input / output characteristics. In order to dissolve PVDF and form a slurry, a polar solvent such as N-methylpyrrolidone (NMP) is preferably used, but an aqueous emulsion such as SBR or CMC can be dissolved in water. When the amount of the binder added is too large, the resistance of the obtained electrode is increased, which is not preferable because the internal resistance of the battery is increased and the battery characteristics are deteriorated. Moreover, when there is too little addition amount of a binder, the coupling | bonding with negative electrode particle | grains and a current collection material becomes inadequate, and is unpreferable. The preferred addition amount of the binder varies depending on the type of binder used, but is preferably 3 to 13% by weight, more preferably 3 to 10% by weight for the PVDF-based binder. On the other hand, a binder using water as a solvent is often used by mixing a plurality of binders such as a mixture of SBR and CMC, and the total amount of all binders used is preferably 0.5 to 5% by weight. The amount is preferably 1 to 4% by weight. The electrode active material layer is basically formed on both sides of the current collector plate, but may be on one side if necessary. A thicker electrode active material layer is preferable for increasing the capacity because fewer current collectors and separators are required. However, the wider the electrode area facing the counter electrode, the better the input / output characteristics. Too much is not preferable because the input / output characteristics deteriorate. The thickness of the active material layer (per one side) is not limited and is in the range of 10 μm to 1000 μm, preferably 10 to 80 μm, more preferably 20 to 75 μm, and particularly preferably 20 to 60 μm. is there.
The negative electrode usually has a current collector. As the negative electrode current collector, for example, SUS, copper, aluminum, nickel, or carbon can be used, and among these, copper or SUS is preferable.

[3] Sodium ion secondary battery When the negative electrode material of the present invention is used to form a negative electrode of a sodium ion secondary battery, other materials constituting the battery such as the positive electrode material, the separator, and the electrolyte are particularly limited. Without limitation, it is possible to use various materials conventionally used or proposed for sodium ion secondary batteries.

(Positive electrode)
The positive electrode includes a positive electrode active material, and may further include a conductive additive, a binder, or both. The mixing ratio of the positive electrode active material and other materials in the positive electrode active material layer is not limited as long as the effect of the present invention is obtained, and can be determined as appropriate.
As the positive electrode active material, a positive electrode active material that can be doped and dedoped with sodium ions can be used without limitation. For example, as a positive electrode active material, NaFeO ₂ , NaNiO ₂ , NaCoO ₂ , NaMnO ₂ , NaFe _1-x M ¹ _x O ₂ , NaNi _1-x M ¹ _x O ₂ , NaCo _1-x M ¹ _x O ₂ , NaMn _1-x M ¹ _x O ₂ (where M ¹ is one or more elements selected from the group consisting of trivalent metals, and 0 ≦ x <0.5), NabM2cSi ₁₂ O ₃₀ (M2 is 1 More than one kind of transition metal element, 2 ≦ b ≦ 6, 2 ≦ c ≦ 5: For example, Na ₆ Fe ₂ Si ₁₂ O ₃₀ or Na ₂ Fe ₅ Si ₁₂ O ₃₀ ), NadM3eSi ₆ O ₁₈ (M3 is one or more types) transition metal elements, 3 ≦ d ≦ 6,1 ≦ e ≦ 2: for _{_{_{_{example, Na 2 Fe 2 Si 6 O}}}} 18 or _{_{_{Na 2 MnFeSi 6 O 18),}}} NafM4gSi 2 O 6 (M4 is a transition metal element One or more elements selected from the group consisting of Mg and Al, 1 ≦ f ≦ 2,1 ≦ g ≦ 2: For _{_{_{example, Na 2 FeSi 2 O 6)}}} , phosphates (e.g., NaFePO _4, _Na 3 Fe ₂ (PO ₄ ) ₃ ), borate (eg, NaFeBO ₄ or Na ₃ Fe ₂ (BO ₄ ) ₃ ), NahM5F6 (M5 is one or more transition metal elements, 2 ≦ h ≦ 3: for example, Na ₃ And compounds represented by FeF ₆ and Na ₂ MnF ₆ ).

The positive electrode can further contain a conductive additive and / or a binder. As a conductive support agent, acetylene black, ketjen black, or carbon fiber can be mentioned, for example. The content of the conductive assistant is not limited, but is, for example, 0.5 to 15% by weight. Moreover, as a binder, fluorine-containing binders, such as PTFE or PVDF, can be mentioned, for example. The content of the conductive assistant is not limited, but is, for example, 0.5 to 15% by weight. Further, the thickness of the positive electrode active material layer is not limited, but is in the range of 10 μm to 1000 μm, for example.
The positive electrode active material layer usually has a current collector. As the negative electrode current collector, for example, SUS, aluminum, nickel, iron, titanium, and carbon can be used, and among these, aluminum or SUS is preferable.

(Electrolyte)
The nonaqueous solvent electrolyte used in combination of these positive electrode and negative electrode is generally formed by dissolving an electrolyte in a nonaqueous solvent. However, the electrolytic solution is not limited as long as the effect of the present invention is obtained, and for example, an ionic liquid may be used. Examples of the non-aqueous solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, diethoxyethane, γ-butyllactone, tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, and 1,3-dioxolane. These can be used alone or in combination of two or more. As electrolytes, NaClO ₄ , NaPF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (CF ₃ SO ₂ ) ₂ , NaN (FSO ₂ ) ₂ , NaN (C ₂ F ₅ SO ₂ ) ₂ , NaC (CF _{_{_{_{3 SO 2) 3, NaAsF 6}}}} , NaPF 6, NaB (C 6 H 5) 4, CH 3 SO 3 Na, CF 3 SO 3 Na, may be mentioned NaCl, or NaBr. In a secondary battery, the positive electrode layer and the negative electrode layer formed as described above are generally immersed in an electrolytic solution so that they face each other through a liquid-permeable separator made of a nonwoven fabric or other porous material as necessary. It is formed by. As the separator, a non-woven fabric usually used for a secondary battery or a permeable separator made of another porous material can be used. Alternatively, a solid electrolyte made of a polymer gel impregnated with an electrolytic solution can be used instead of or together with the separator.

EXAMPLES Hereinafter, the present invention will be specifically described by way of examples, but these do not limit the scope of the present invention.
The physical properties of carbonaceous materials (“hydrogen / carbon atomic ratio (H / C)”, “butanol true density”, “helium true density”, “average particle diameter”, “specific surface area”, “carbon material” The average layer surface spacing d ₀₀₂ ”and“ Lc ”) are described below. However, these physical property values described in this specification including the examples are based on values obtained by the following methods. is there.

<< Atomic ratio of hydrogen atom to carbon atom (H / C) >>
Measurement was performed in accordance with the method defined in JIS M8819. From the mass ratio of hydrogen and carbon in the sample obtained by elemental analysis with a CHN analyzer, the hydrogen / carbon atom number ratio was obtained.

《Butanol true density》
In accordance with the method defined in JIS R7212, measurement was performed using butanol. The outline is described below. Note that both the carbonaceous precursor and the carbonaceous material were measured by the same measurement method. The mass (m ₁ ) of a specific gravity bottle with a side tube having an internal volume of about 40 mL is accurately measured. Next, the sample is put flat on the bottom so as to have a thickness of about 10 mm, and then its mass (m ₂ ) is accurately measured. Gently add 1-butanol to this to a depth of about 20 mm from the bottom. Next, light vibration is applied to the specific gravity bottle, and it is confirmed that large bubbles are not generated. Then, the bottle is put in a vacuum desiccator and gradually exhausted to 2.0 to 2.7 kPa. Keep at that pressure for 20 minutes or more, take out after bubble generation stops, fill with 1-butanol, plug and immerse in a constant temperature water bath (adjusted to 30 ± 0.03 ° C) for 15 minutes or more. Adjust the liquid level of 1-butanol to the marked line. Next, this is taken out, the outside is well wiped off and cooled to room temperature, and then the mass (m ₄ ) is accurately measured. Next, the same specific gravity bottle is filled with only 1-butanol, immersed in a constant temperature water bath in the same manner as described above, and after aligning the marked lines, the mass (m ₃ ) is measured. Moreover, distilled water excluding the gas that has been boiled and dissolved immediately before use is placed in a specific gravity bottle, immersed in a constant temperature water bath as before, and the mass (m ₅ ) is measured after aligning the marked lines. The true density (ρ _Bt ) is calculated by the following formula.

(Where d is the specific gravity of water at 30 ° C. (0.9946))

《Average particle size》
Three drops of a dispersing agent (cationic surfactant “SN Wet 366” (manufactured by San Nopco)) are added to about 0.1 g of the sample, and the sample is made to conform to the dispersing agent. Next, after adding 30 mL of pure water and dispersing with an ultrasonic cleaner for about 3 minutes, particles having a particle size in the range of 0.02 to 2000 μm are measured with a particle size distribution measuring instrument (“Microtrac MT3300EXII” manufactured by Nikkiso Co., Ltd.). The diameter distribution was determined.
From the obtained particle size distribution, the average particle size Dv50 (μm) was defined as the particle size with a cumulative volume of 50%.

《Helium true density》
The measurement of the true density ρ _He using helium as a substitution medium was performed after the sample was vacuum-dried at 200 ° C. for 12 hours using a multi-volume pycnometer (Accumic 1330) manufactured by Micromeritics. The ambient temperature during measurement was constant at 25 ° C. The pressures in this measurement method are all gauge pressures and are the pressures obtained by subtracting the ambient pressure from the absolute pressure.
A multi-volume pycnometer manufactured by Micromerix, Inc. has a sample chamber and an expansion chamber, and the sample chamber has a pressure gauge for measuring the pressure in the chamber. The sample chamber and the expansion chamber are connected by a connecting pipe having a valve. A helium gas introduction pipe having a stop valve is connected to the sample chamber, and a helium gas distribution pipe having a stop valve is connected to the expansion chamber.
The measurement was performed as follows. The volume of the sample chamber (V _CELL ) and the volume of the expansion chamber (V _EXP ) were measured in advance using a standard sphere. A sample was put into the sample chamber, and helium gas was passed through the helium gas inlet tube, the connecting tube in the sample chamber, and the helium gas discharge tube in the expansion chamber for 2 hours to replace the inside of the apparatus with helium gas. Next, the valve between the sample chamber and the expansion chamber and the valve of the helium gas discharge pipe from the expansion chamber are closed (helium gas having the same pressure as the ambient pressure remains in the expansion chamber), and helium is introduced from the helium gas introduction tube of the sample chamber. After introducing the gas to 134 kPa, the stop valve of the helium gas introduction pipe was closed. The pressure (P ₁ ) in the sample chamber was measured 5 minutes after closing the stop valve. Next, the valve between the sample chamber and the expansion chamber was opened, and helium gas was transferred to the expansion chamber, and the pressure (P ₂ ) at that time was measured.
The volume of the sample (VSAMP) was calculated by the following formula.
V _SAMP = V _CELL −V _EXP / [(P ₁ / P ₂ ) −1]
Therefore, if the weight of the sample is W _SAMP , the true helium density is ρ _He = W _SAMP / V _SAMP .
The equilibrium speed was set to 0.010 psig / min.

"Specific surface area"
The specific surface area was measured according to the method defined in JIS Z8830. The outline is described below.
Approximate formula derived from BET formula

Was used to calculate v _m by the one-point method by nitrogen adsorption (relative pressure x = 0.2) at liquid nitrogen temperature, and the specific surface area of the sample was calculated by the following formula: specific surface area = 4.35 × v _m ( m ² / g) (where v _m is the amount of adsorption necessary to form a monolayer on the sample surface (cm ³ / g), v is the amount of adsorption actually measured (cm ³ / g), and x is relative Pressure.)
Specifically, using a “Flow Sorb II2300” manufactured by Micromeritics, the amount of nitrogen adsorbed on the carbonaceous material at the liquid nitrogen temperature was measured as follows.
The sample tube is filled with a carbon material, and the sample tube is cooled to −196 ° C. while flowing a helium gas containing nitrogen gas at a concentration of 20 mol%, and nitrogen is adsorbed on the carbon material. The test tube is then returned to room temperature. At this time, the amount of nitrogen desorbed from the sample was measured with a thermal conductivity detector, and the amount of adsorbed gas v was obtained.

(Average layer spacing d _{002 of} carbon material)
The carbonaceous material powder was filled in the sample holder and measured by a symmetrical reflection method using X'Pert PRO manufactured by PANalytical. The scanning range was 8 <2θ <50 °, and the applied current / applied voltage was 45 kV / 40 mA. An X-ray diffraction pattern was obtained using a CuKα ray (λ = 1.5418Å) monochromated by a Ni filter as a radiation source. . Correction was performed using the diffraction peak of the (111) plane of the high-purity silicon powder for standard substances. The wavelength of the CuKα ray was 0.15418 nm, and d ₀₀₂ was calculated according to the Bragg formula.

<< Calculation of Lc (002) by X-ray diffraction method >>
Lc (002) is calculated by substituting into the Scherrer equation.
L = Kλ / (β · cos θ) (Scherrer equation)
K: Form factor (0.9), λ: X-ray wavelength (CuKαm = 0.15418 nm), θ: Diffraction angle, β: Half width

Example 1
A 70 kg petroleum pitch having a softening point of 205 ° C., an H / C atomic ratio of 0.65, and a quinoline insoluble content of 0.4%, and 30 kg of naphthalene are charged into a 300 liter pressure vessel equipped with a stirring blade and an outlet nozzle. The mixture was heated and melted. Thereafter, the heat-mixed petroleum pitch was cooled and pulverized, and the resulting pulverized product was poured into water at 90 to 100 ° C., stirred and dispersed, and cooled to obtain a spherical pitch molded body. After most of the water was removed by filtration, the spherical pitch molded body was extracted and removed with n-hexane. The porous spherical pitch obtained in this manner was heated and oxidized while passing heated air to obtain a porous spherical oxidized pitch that was infusible to heat. The oxygen crosslinking degree of the porous spherical oxide pitch was 17% by weight. Next, it was heat-treated at 600 ° C. in a nitrogen atmosphere and pulverized with a pulverizer to obtain a carbonaceous precursor having an average particle size of 10 to 15 μm. The obtained powdered carbon precursor was heated to 1200 ° C. at a temperature rising rate of 250 ° C./h in a nitrogen atmosphere, held at 1200 ° C. for 1 hour, and subjected to main firing to obtain a carbonaceous material 1. The main firing was performed in a nitrogen atmosphere with a flow rate of 10 L / min.

Example 2
A carbonaceous material 2 was obtained in the same manner as in Example 1 except that the main firing temperature was 1350 ° C.

Example 3
A carbonaceous material 3 was obtained in the same manner as in Example 1 except that the main firing temperature was 1450 ° C.

<< Comparative Example 1 >>
A carbonaceous material 5 was obtained in the same manner as in Example 1 except that the main firing temperature was 800 ° C.

<< Comparative Example 2 >>
A carbonaceous material 6 was obtained in the same manner as in Example 1 except that the main firing temperature was set to 1000 ° C.

Using the carbonaceous materials of Examples and Comparative Examples, negative electrodes and sodium ion secondary batteries were produced as follows, and the electrode performance was evaluated.

(A) Electrode preparation NMP was added to 94 parts by weight of the carbon material and 6 parts by weight of polyvinylidene fluoride (“KF # 9100” manufactured by Kureha Co., Ltd.) to form a paste, which was uniformly applied on the copper foil. After drying, it was punched out from a copper foil into a disk shape having a diameter of 15 mm and pressed to obtain an electrode. The amount of the carbon material in the electrode was adjusted to be about 10 mg.

(B) Production of test battery The carbon material of the present invention is suitable for constituting the negative electrode of a non-aqueous electrolyte secondary battery, but the discharge capacity (de-doping amount) and irreversible capacity (non-de-doping) of the battery active material. In order to accurately evaluate the quantity) without being affected by variations in the performance of the counter electrode, a sodium secondary battery is constructed using the electrode obtained above with sodium metal having stable characteristics as the counter electrode, Characteristics were evaluated.
The sodium electrode was prepared in a glove box in an Ar atmosphere. A 16 mm diameter stainless steel mesh disk is spot-welded to the outer lid of a 2016 coin-sized battery can, and then a 0.8 mm thick metal sodium sheet is punched into a 15 mm diameter disk shape. To be an electrode (counter electrode).
A pair of electrodes produced in this manner was used. As the electrolytic solution, propylene carbonate added with NaPF ₆ at a rate of 1.0 mol / L, a borosilicate glass fiber microporous membrane having a diameter of 19 mm was used. As a separator, a 2016-size coin-type non-aqueous electrolyte sodium ion secondary battery was assembled in an Ar glove box using a polyethylene gasket.

(C) Measurement of battery capacity About the sodium ion secondary battery of the said structure, the charge / discharge test was done using the charge / discharge test apparatus ("TOSCAT" by Toyo System). The sodium doping reaction to the carbon electrode was performed by the constant current constant voltage method, and the dedoping reaction was performed by the constant current method. Here, for the sake of convenience, the sodium doping reaction on the carbon electrode is described as “charging”. Conversely, “discharge” is a charge reaction in the test battery, but it is described as “discharge” for convenience because it is a dedoping reaction of sodium from the carbon material. The charging method adopted here is a constant current constant voltage method. Specifically, after the terminal voltage reaches 0 mV, constant current charging is performed at a current density of 0.1 mA / cm ² until the terminal voltage reaches 0 mV. Then, constant voltage charging was performed at a terminal voltage of 0 mV, and charging was continued until the current value reached 20 μA. A value obtained by dividing the supplied amount of electricity by the mass of the carbon material of the electrode is defined as a charge capacity (mAh / g) per unit weight of the carbon material. After charging, the battery circuit was opened for 30 minutes, and then sodium was dedopeed from the carbonaceous material. De-doping was performed at a current density of 0.1 mA / cm ² , and the final voltage was 1.5V. A value obtained by dividing the amount of electricity at this time by the mass of the carbon material of the electrode is defined as a discharge capacity (mAh / g) per unit weight of the carbon material. The irreversible capacity is then defined as the difference between the charge capacity and the discharge capacity. The value obtained by dividing the discharge capacity by the charge capacity was multiplied by 100 to obtain the efficiency (%). The measurement was performed at 25 ° C., and the charge / discharge capacity, the irreversible capacity, and the efficiency were determined by averaging the measured values of n = 3 for the test batteries prepared using the same sample.

(D) Measurement of cycle characteristics A ratio of 1.5 mol / L in a mixed solvent in which ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate were mixed in a volume ratio of 1: 2: 2 for the electrolyte of the sodium ion secondary battery having the above-described configuration. In which NaPF ₆ was added. The first charge was performed by the constant current constant voltage method. Specifically, constant current charging is performed at a current density of 0.1 mA / cm ² until the terminal voltage reaches 0 mV, and after the terminal voltage reaches 0 mV, constant voltage charging is performed at a terminal voltage of 0 mV. Charging was continued until 20 μA was reached. After completion of charging, the battery circuit was opened for 10 minutes, and discharging was performed with a constant current of 0.1 mA / cm ² and a final voltage of 1.5V. After the first charge / discharge reaction, a repeated charge / discharge test was conducted. Specifically, constant current charging is performed at a current density of 0.2 mA / cm ² until the terminal voltage reaches 0 mV, and after the terminal voltage reaches 0 mV, constant voltage charging is performed at a terminal voltage of 0 mV. Charging was continued until 20 μA was reached. After the completion of charging, the battery circuit was opened for 10 minutes, and discharging was performed with a constant current of 0.2 mA / cm ² and a final voltage of 1.5V. Such a charge / discharge test was repeated 50 times. The ratio of the discharge capacity at the 50th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate. The measurement was performed at 25 ° C., and the discharge capacity and the capacity retention rate (%) were determined by averaging the measured values of n = 3 for the test batteries manufactured using the same sample.

(E) Storage characteristics NaPF at a rate of 1.5 mol / L in a mixed solvent in which ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate were mixed at a capacity ratio of 1: 2: 2 for the electrolyte solution of the sodium ion secondary battery having the above configuration. What added ₆ was used. As the first charge / discharge, constant current charging is performed at a current density of 0.1 mA / cm ² until the terminal voltage reaches 0 mV, and after the terminal voltage reaches 0 mV, constant voltage charging is performed at a terminal voltage of 0 mV. Charging was continued until the current reached 20 μA. After completion of charging, the battery circuit was opened for 10 minutes, and discharging was performed with a constant current of 0.1 mA / cm ² and a final voltage of 1.5V. As the second charge / discharge, constant current charging is performed at a current density of 0.1 mA / cm ² until the terminal voltage reaches 0 mV, and after the terminal voltage reaches 0 mV, constant voltage charging is performed at a terminal voltage of 0 mV. Charging was continued until the value reached 20 μA. After completion of charging, the battery circuit is opened for 10 minutes, and constant current discharge is performed at a current density of 0.1 mA / cm ^2. After the terminal voltage reaches 1.5 V, constant voltage discharge is performed at a terminal voltage of 1.5 V. The discharge was continued until the current value reached 20 μA. Thereafter, as a charge before storage, constant current charge is performed at a current density of 0.1 mA / cm ² , the terminal voltage reaches 0 mV, then constant voltage charge is performed at a terminal voltage of 0 mV, and the current value reaches 20 μA. Until charged. The charged test battery was stored at 40 ° C. for 168 hours. After storage, constant current discharge is performed at a current density of 0.1 mA / cm ² , terminal voltage reaches 1.5 V, constant voltage discharge is performed at terminal voltage 1.5 V, and discharge is performed until the current value reaches 20 μA. did. A value obtained by dividing the discharge capacity after storage by the charge capacity before storage was multiplied by 100 to obtain a capacity retention rate (%). The charge / discharge test was conducted at 25 ° C., and the capacity retention rate (%) was determined by averaging the measured values of n = 3 for the test batteries prepared using the same sample.

The carbonaceous materials having H / C of 0.05 or less obtained in Examples 1 to 3 showed excellent retention rate (%) and discharge amount (mAh / g) at 50 cycles (Table 1 and FIG. 1). That is, the sodium ion secondary battery using the carbonaceous material of the present invention exhibited excellent cycle characteristics. On the other hand, the secondary battery using the carbonaceous material with H / C of 0.1 and 0.06 obtained in Comparative Examples 1 and 2 was inferior in cycle characteristics.
Furthermore, the sodium ion secondary battery using the carbonaceous material with a small specific surface area obtained in the example showed an excellent capacity retention rate (Table 1 and FIG. 2).
In addition, the test battery which measured battery performance is a half cell using the carbon electrode containing sodium metal (counter electrode) and the carbonaceous material of this invention. Therefore, the test battery (half cell) does not have the configuration of a real cell (full cell) described in the section “[3] Sodium ion secondary battery”. However, those skilled in the art can manufacture a full cell using the carbonaceous material of the present invention from the description of “[3] Sodium ion secondary battery”. In addition, the battery performance of the half cell obtained in this example correlates with the battery performance of the full cell.

The sodium ion secondary battery using the carbonaceous material according to the present invention has improved discharge capacity. A sodium ion secondary battery using abundant sodium ions can be manufactured at low cost, and the present invention is industrially useful. The obtained sodium ion secondary battery can be effectively used for hybrid vehicles (HEV), plug-in hybrids (PHEV), and electric vehicles (EV).

Claims

A carbonaceous material for a negative electrode of a sodium ion secondary battery, wherein the ratio H / C of hydrogen atoms to carbon atoms determined by elemental analysis is 0.05 or less.
The carbonaceous material for a sodium ion secondary battery negative electrode according to claim 1, wherein the BET specific surface area is less than 20 m 2 / g.
The carbonaceous material for a sodium ion secondary battery negative electrode according to claim 1 or 2, wherein a true density obtained by a butanol method is less than 1.53 g / cm 3 .
The carbonaceous material for a negative electrode of a sodium ion secondary battery according to any one of claims 1 to 3, wherein petroleum pitch or tar or coal pitch or tar is used as a carbon source.
A negative electrode for a sodium ion secondary battery comprising the carbonaceous material according to any one of claims 1 to 4.
A sodium ion secondary battery comprising the electrode according to claim 5.