JP2015050050A - Negative electrode material for sodium secondary batteries, method for manufacturing the same, and sodium secondary battery arranged by use of negative electrode material for sodium secondary batteries - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte battery which has a large negative electrode capacity per unit weight and is preferable as a power source of a compact electronic device.SOLUTION: A negative electrode material for sodium secondary batteries comprises a carbonaceous material. Supposing that a true specific gravity measured by an immersion-substitution method using n-butanol as immersion liquid is ρp, and a true specific gravity measured by a gas substitution method using a helium gas is ρHe, ρp/ρHe is less than 0.950 in the negative electrode material. The negative electrode material can be obtained by a method including the step of baking a carbon precursor while exposing it to the current of an inert gas.

Description

  The present invention relates to a negative electrode material for sodium secondary battery that can be doped and dedoped with sodium, a method for producing the same, and a sodium secondary battery using the negative electrode material for sodium secondary battery.

  In recent years, downsizing and weight reduction have been raised as development issues in electronic devices, and higher energy density is also required for batteries serving as a power source in order to cope with them. Various non-aqueous electrolyte batteries such as so-called sodium batteries have been proposed to meet such demands. However, for example, in a battery using sodium metal for the negative electrode, the cycle life is shortened particularly when the secondary battery is used.

  This is due to sodium metal itself, which is caused by changes in sodium form, dendritic sodium formation, irreversible change of sodium, etc. that occur with repeated charge and discharge.

  Thus, as a technique for solving this problem, it has been proposed to use a non-graphitizable carbon material for the negative electrode. This is based on the fact that sodium is easily doped into voids in the non-graphitizable carbon material electrochemically. For example, a non-graphitizable carbon material is used as a negative electrode, and a compound containing sodium is used as a positive electrode. When the battery is charged in a non-aqueous electrolyte, sodium in the positive electrode is electrochemically doped into the negative electrode carbon voids. The sodium-doped carbon thus functions as a sodium electrode, and the sodium in the negative electrode is dedoped from the carbon gap along with the discharge and returns to the positive electrode.

  By the way, in order to cope with the weight reduction of electronic devices, the battery needs to have a large energy per unit weight and a negative electrode capacity.

  For this reason, even in the non-aqueous electrolyte battery using the above-described carbonaceous material for the negative electrode, various studies have been made to increase the amount of sodium dope per unit weight of the carbonaceous material and reduce the weight of the negative electrode.

  As a conventional technique of such a sodium secondary battery, for example, Non-Patent Document 1 is cited.

Abstracts of the 39th Annual Meeting of the Carbon Society of Japan p.170 Lecture number 2C12

  However, the actual situation is that a non-aqueous electrolyte battery (especially a sodium secondary battery) that can sufficiently contribute to downsizing of electronic equipment has not been obtained yet.

  Therefore, the present invention has been proposed in view of such conventional circumstances, and an object thereof is to provide a nonaqueous electrolyte battery having a large negative electrode capacity per unit weight and suitable as a power source for small electronic devices. And

  In order to achieve the above-mentioned object, the present inventors have conducted extensive research over a long period of time. As a result, the true specific gravity ρp measured by the liquid immersion substitution method and the true specific gravity ρHe measured by the gas substitution method are calculated. To obtain knowledge that a carbonaceous material having a ratio ρp / ρHe of less than 0.950 has a large sodium doping amount and dedoping amount per unit weight and is a suitable negative electrode active material for miniaturization of a battery. It came. This carbonaceous material is obtained, for example, by firing a carbon precursor under an inert gas stream.

That is, this invention includes the sodium secondary battery using the following negative electrode material for sodium secondary batteries, its manufacturing method, and this negative electrode material for sodium secondary batteries.
Item 1. A negative electrode material for a sodium secondary battery made of a carbonaceous material,
The negative electrode material is ρp / ρHe, where ρp is the true specific gravity measured by the liquid immersion substitution method using n-butanol as the immersion liquid, and ρHe is the true specific gravity measured by the gas substitution method using helium gas. A negative electrode material having an A of less than 0.950.
Item 2. Item 2. The negative electrode material according to Item 1, wherein the carbonaceous material is composed of a non-graphitizable carbon material obtained by firing a carbon precursor.
Item 3. The method for producing a negative electrode material for a sodium secondary battery according to Item 1 or 2,
3. A step of firing the carbon precursor in an inert gas stream or in a low-pressure atmosphere. Item 4. The method according to Item 3, wherein the flow of the inert gas is a flow rate of 0.1 ml / min or more per 1 g of the carbon precursor.
Item 5. Item 3. A sodium secondary battery comprising a negative electrode comprising the negative electrode material for a sodium secondary battery according to Item 1 or 2, a positive electrode, and a non-aqueous electrolyte.

  In the present invention, when the true specific gravity measured by the liquid immersion substitution method using n-butanol as an immersion liquid as the negative electrode active material is ρp, and the true specific gravity measured by the gas substitution method using helium gas is ρHe, Since a carbonaceous material having ρp / ρHe of less than 0.950 is used, the negative electrode capacity per unit weight is excellent, so that sufficient battery capacity and energy can be obtained even when the negative electrode weight is designed to be small. Therefore, downsizing is possible, which greatly contributes to downsizing of electronic devices.

1. Negative electrode material for sodium secondary battery and manufacturing method thereof The negative electrode material for sodium secondary battery of the present invention is a negative electrode material used for a sodium secondary battery made of a carbonaceous material, and n-butanol is used as an immersion liquid. When the true specific gravity measured by the liquid immersion substitution method is ρp and the true specific gravity measured by the gas substitution method using helium gas is ρHe, ρp / ρHe is less than 0.950.

That is, the true specific gravity of the carbonaceous material can be measured based on JIS R7212: 1995 in order to measure by the liquid immersion substitution method using n-butanol as the immersion liquid. Specifically, the weight x of the carbonaceous material is measured, and then the carbonaceous material is immersed in n-butanol filled in the container, and is excluded by the immersion of the carbonaceous material. - measuring the volume y _l of butanol, the ratio x / y _l is true specific gravity in the liquid immersion displacement process, corresponding to the above .rho.p.

On the other hand, in order to measure the true specific gravity of a carbonaceous material by a gas substitution method using helium, a value measured by a dry densimeter can be used. Examples of the dry density meter include the dry automatic density meter Accupic II 1340 series manufactured by Shimadzu Corporation. Specifically, the weight x of the carbonaceous material is measured, and then this carbonaceous material is left in a container filled with helium gas, and the volume y of helium eliminated by leaving the carbonaceous material is left. _g is measured, and this ratio x / y _g is the true specific gravity in the gas displacement method, and corresponds to the above ρHe.

  The ρp and ρHe of the carbonaceous material measured in this way are usually because n-butanol and helium have different bulks at the molecular level, and there are large and small pores on the surface of the carbonaceous material. Different.

That is, when a carbonaceous material is immersed in n-butanol, the molecular length of the n-butanol is about 0.7 nm along the straight chain. It is considered difficult to diffuse into pores (so-called sub-micropores) having a diameter of about 0.8 nm or less. Hereinafter, when the term “sub-micropore” is simply used, it means a pore having a diameter of 0.8 nm or less. In other words, the volume y _l of n- butanol is eliminated by immersion of the carbonaceous material, since the n- butanol is unable to diffuse into Sabumikuropoa, substantially to a value obtained by adding the volume of Sabumikuropoa to the volume of itself carbonaceous material It can be said that it is equivalent.

On the other hand, when a carbonaceous material is left in helium gas, helium atoms have an atomic radius of about 0.15 nm, and enter into pores having a diameter of about 0.3 to 0.4 nm so that n-butanol cannot enter. Is also possible. That is, the volume y _{g of the} helium gas that is excluded by leaving the carbonaceous material is a value that takes into account the presence of the submicropores on the surface of the carbonaceous material because helium can diffuse into the submicropores, that is, the submicropores. It can be said that the volume substantially equals the volume of the carbonaceous material itself.

Therefore, since the ratio ρp / ρHe between ρp and ρHe is (x / y ₁ ) / (x / y _g ) = y _g / y ₁ , the carbonaceous material excluding the sub-micropore volume relative to the sub-micropore volume Reflecting the ratio of its own volume, a carbonaceous material having a smaller value means more sub-micropores per unit volume. Similarly, a small value means that there are many submicropores per unit weight.

  In the present invention, the negative electrode capacity per unit weight is improved by regulating such ρp / ρHe to less than 0.950.

  As described above, a carbonaceous material having a small ρp / ρHe of less than 0.950 is considered to have a lot of submicropores, and has a large amount of doped sodium and dedope per unit weight, and is used with a small weight. However, a sufficient negative electrode capacity can be exhibited. Therefore, by using such a carbonaceous material as the negative electrode active material, it is possible to reduce the size, and it is possible to realize a sodium secondary battery that greatly contributes to the downsizing of electronic devices. Note that ρp / ρHe of the carbonaceous material is preferably less than 0.940, more preferably less than 0.920, for the reasons described above. The lower limit of ρp / ρHe of the carbonaceous material is not particularly limited, but is usually about 0.5.

  The carbonaceous material with more sub-micropores per unit weight, the larger the amount of sodium doping and dedoping, is presumed that the sub-micropores probably function as sodium doping sites from the following knowledge. .

  In the carbonaceous material, the site considered to be doped with sodium is not particularly limited, but includes pores present in the carbonaceous material. There are pores that are open outside the system and pores that are closed outside the system. Of these, the open pores are easier to diffuse sodium than closed pores and are effective as doping sites. It is likely that it is functioning.

  Of the open pores, only a few sodium ions can be arranged in the radial direction in the sub-micropores having a diameter of 0.8 nm or less. In such a space, the influence of van der Waals force from the inner wall of the pore cannot be ignored, and it is considered that sodium is adsorbed and desorbed by such van der Waals force, and doping and dedoping are repeated.

  The carbonaceous material that constitutes the negative electrode material in which many sub-micropores exist per unit weight can cause changes in sodium form, dendrite-like sodium formation, irreversible change in sodium, etc. that occur with repeated charge and discharge. In order to suppress more, it is preferable to consist of the non-graphitizable carbon material obtained by baking a carbon precursor.

  In the present invention, the non-graphitizable carbon material constituting the negative electrode material for a sodium secondary battery usually means a carbon material that does not easily graphitize even when subjected to a high-temperature heat treatment such as 2000 ° C.

  Such a carbonaceous material, for example, by firing the carbon precursor to produce a carbonaceous material, the firing atmosphere is an atmosphere that facilitates the escape of volatile components generated from the carbon precursor, It can be obtained efficiently.

  Usually, when a carbon precursor is calcined, volatile components such as methane, carbon dioxide, and hydrogen gas are generated as a gas and diffused out of the system. When this gas diffuses out of the system, fine pores of molecular size order are generated along the gas diffusion path. These pores remain on the carbonaceous material obtained by firing and are observed as sub-micropores. When such a carbon precursor is baked in an atmosphere that actively removes the volatile components, the generation of submicropores due to diffusion of the volatile components is further promoted, and there are more submicropores per unit weight. The carbonaceous material to be obtained is obtained.

  In addition to this, as a method for introducing pores into the carbon material, the carbonaceous material before and after firing is subjected to treatment such as pulverization and pressure molding to mechanically generate cracks, or the carbon material before and after firing. A method of introducing pores by performing an oxidation treatment called activation is widely known. It is preferable to adjust the firing atmosphere from the viewpoint of introducing more very fine pores of the order of submicropores and introducing the submicropores more uniformly into the carbonaceous particles.

  In order to improve the escape of volatile components from the carbon precursor during firing, for example, the firing of the carbon precursor is preferably performed in a stream of inert gas.

  More specifically, it is preferably performed in an inert gas atmosphere at a flow rate of 0.1 ml / min or more per 1 g of the carbon precursor. The upper limit of the flow rate of the inert gas is not particularly limited, but is usually about 100 ml / min. The firing temperature is preferably 900 ° C. or higher, and more preferably 1020 ° C. or higher. The upper limit of the firing temperature is not particularly limited, but is usually about 1700 ° C. When the carbon precursor is calcined in such an inert gas stream atmosphere, volatile components are efficiently removed by the inert gas stream, and there are many submicropores per unit weight, and ρp / ρHe is A carbonaceous material of less than 0.950 can be obtained more efficiently.

  The reason why the flow rate of the inert gas is regulated per unit weight of the carbon precursor is that the ease of volatile matter removal depends on the amount of the carbon precursor used for carbonization together with the flow rate of the atmosphere. .

  The amount of carbon precursor refers to the total amount in the furnace when firing is performed in a batch type carbonization furnace. The carbon precursor is continuously introduced into the furnace over time and the carbonaceous material is taken out each time. When performed in a type carbonization furnace, it refers to the amount of carbon precursor that is preferably heated to a temperature of 700 ° C. or higher.

  Moreover, as an inert gas which flows in into a baking atmosphere, it is necessary to use the gas which does not react with a carbonaceous material, for example at the carbonization temperature of 900-1500 degreeC. If illustrated, the gas which has nitrogen, argon, or those mixed gas as a main component is preferable.

  The flow rate of the inert gas is preferably an amount discharged to the outside of the carbonization furnace by touching the carbon precursor heated to 700 ° C. or higher. Therefore, the airflow in an inert atmosphere for the purpose of replacing the atmosphere in the system before the temperature of the carbonization furnace or the carbon precursor is raised to the above temperature is not included in the above-described airflow.

When the carbon precursor is baked in the inert gas stream, the contact area per kg of the carbon precursor with respect to the atmosphere is 10 cm ² or more in a rough surface shape. It becomes easier to touch and volatile components are removed more efficiently. However, the contact area in the rough surface shape mentioned here does not include a rough surface of the material and minute irregularities and a minute specific surface area in the particles.

  In order to increase the contact area of the carbon precursor, for example, the carbon precursor is divided and placed on multiple stages or stirred (in this case, the specific surface area of the carbon precursor is an area in contact with the atmosphere). .

  In addition, as a method for efficiently removing volatile components from the carbon precursor during firing, a method of firing the carbon precursor at a temperature of 600 ° C. or higher in an atmosphere having a pressure of 50 kPa or less is also effective.

  When the carbon precursor is calcined in a low pressure atmosphere having a pressure of 50 kPa or less (particularly 40 kPa or less), diffusion and desorption of volatile components from the carbon precursor are further promoted, and volatile components are removed more efficiently. The lower limit of the pressure is not particularly limited, but is usually about 0.5 kPa. Thereby, a lot of submicropores exist per unit weight, and a carbonaceous material having ρp / ρHe of less than 0.950 can be obtained more efficiently.

  Here, the pressure in the firing atmosphere may be maintained at 50 kPa or less at the time of carbonization attainment or at one time during the temperature raising process. The evacuation in the carbonization furnace may be performed before the carbonization furnace or the carbon precursor is heated, or in the temperature raising process or the reached temperature holding time.

  As described above, the carbonaceous material having ρp / ρHe of less than 0.950 can be obtained by firing the carbon precursor in an inert gas stream atmosphere or a low-pressure atmosphere, but in any atmosphere. In this case, the heating method of the carbonization furnace is not particularly limited, and any of induction heating method, resistance heating method, and the like may be adopted.

  Moreover, the ultimate temperature at the time of carbonizing a carbon precursor and a temperature increase rate are not particularly limited. For example, after carbonizing a carbon precursor at 300 to 700 ° C. in an inert atmosphere, the temperature rising rate is 1 ° C./min or more, the ultimate temperature is 900 to 1700 ° C., and the retention time at the ultimate temperature is about 0 to 5 hours. Can be fired. Of course, the carbonization operation may be omitted in some cases.

  Furthermore, when the obtained carbonaceous material is used as a negative electrode material, it needs to be formed into a molded body. However, it may be formed in advance before or during the heat treatment of the carbon precursor. The carbonaceous material obtained as a molded body is crushed and / or classified and used for the negative electrode material, or is used as the molded body for the negative electrode material. Thereafter, it may be performed after firing.

  The carbon precursor that is fired in the firing atmosphere as described above is not limited as long as it can generate a volatile component upon firing. Specifically, it is not particularly limited, heavy coal oil (tar, pitch, etc.), heavy petroleum oil (tar, pitch, etc.), molten polymer (polymers include phenol resin, aramid resin, polyamide, etc. And other thermoplastic resins). Further, the above heavy oil may be used as it is by removing light components by performing a distillation operation in advance, or a product obtained by polycondensation in the presence of a bifunctional crosslinking agent can be used.

  However, carbon precursors containing nitrogen-containing functional groups and oxygen-containing functional groups, which are generally regarded as precursors of non-graphitizable carbon, generate a large amount of ammonia, carbon monoxide, carbon dioxide gas, etc. during firing. Sub-micropores are more easily formed, and it is more preferable to produce a carbonaceous material having a large amount of sodium doping and dedoping. However, even a carbon precursor that does not contain a functional group, which is a precursor of graphitizable carbon, generates hydrogen, methane gas, and the like during firing, so that sub-micropores are formed, and ρp / It is possible to produce a carbonaceous material in which ρHe satisfies the above conditions.

  Various bifunctional compounds capable of crosslinking organic substances can be used as the bifunctional crosslinking agent used for polycondensation of heavy oil. More specifically, aromatic dimethylene halides such as xylene dichloride; aromatic dimethanols such as xylene glycol; aromatic dimethylenes such as terephthalic acid chloride, isophthalic acid chloride, phthalic acid chloride, and 2,6-naphthalenedicarboxylic acid chloride. Examples include carbonyl halides; aromatic aldehydes such as benzaldehyde, p-hydroxybenzaldehyde, p-methoxybenzaldehyde, 2,5-dihydroxybenzaldehyde, benzaldehyde dimethyl acetal, terephthalaldehyde, isophthalaldehyde, and salicylaldehyde. These crosslinking agents may be used alone or in combination of two or more as required.

  The amount of the crosslinking agent used can be selected from a wide range depending on the characteristics of the organic substance (heavy oil), the type of the crosslinking agent, and the like. The amount of the crosslinking agent used is usually preferably in the range of 0.01 to 30% based on the weight of the raw organic material.

  The crosslinking reaction of the organic substance is usually performed in the presence of an acid catalyst. As the acid catalyst, for example, conventional acids such as Lewis acid and Bronsted acid are used.

Examples of the Lewis acid include ZnCl ₂ , BF ₃ , AlCl ₃ , SnCl ₄ , TiCl _{4 and the} like, and examples of the Bronsted acid include organic acids such as p-toluenesulfonic acid, fluoromethanesulfonic acid and xylenesulfonic acid; hydrochloric acid And mineral acids such as sulfuric acid and nitric acid.

  As the acid catalyst, a Bronsted acid is more preferable.

  These acid catalysts may be used alone or in combination of two or more if necessary.

  Although the usage-amount of an acid catalyst is not specifically limited, About 0.01-10 mol is preferable normally with respect to 1 mol of crosslinking agents, and about 0.5-3 mol is more preferable. When tar and / or pitch is used as the raw material organic material, a polycondensation reaction product (carbon) is usually obtained by reacting the raw material with the bifunctional cross-linking agent while blowing air in the presence of an acid catalyst and heating. It is preferable to obtain a precursor. The temperature during the polycondensation reaction is usually preferably about 200 to 400 ° C (particularly about 250 to 350 ° C).

  The non-aqueous electrolyte battery of the present invention is configured by combining a negative electrode including a negative electrode material made of a carbonaceous material as described above with a positive electrode and a non-aqueous electrolyte.

Any positive electrode can be used as long as it can supply sodium to the negative electrode. However, the non-aqueous electrolyte battery of the present invention (especially a sodium secondary battery) is intended to achieve a particularly high capacity. It is necessary to be able to supply sodium corresponding to a charge / discharge capacity of 150 mAh or more per 1 g of the negative electrode carbonaceous material in a state (for example, after repeated charging and discharging about 5 times), preferably 180 mAh or more, more preferably 200 mAh. It is desirable to use one that can supply sodium equivalent to the above charge / discharge capacity. For example, a composite metal oxide represented by the general formula NaMO ₂ or Na ₄ M ₃ (PO ₄ ) ₂ P ₂ O ₇ (wherein M is the same or different and each represents Co or Ni), Na An intercalation compound containing is preferable, and better characteristics can be obtained particularly when NaCrO ₂ , Na ₄ Co ₃ (PO ₄ ) ₂ P ₂ O ₇ or the like is used.

  It is not always necessary to supply all of the sodium from the positive electrode material. In short, it is sufficient that sodium corresponding to a charge / discharge capacity of 150 mAh or more per 1 g of the negative electrode carbonaceous material exists in the battery system. The amount of sodium is determined by measuring the discharge capacity of the battery.

  On the other hand, the nonaqueous electrolytic solution is prepared by appropriately combining an organic solvent and an electrolyte, and any of these organic solvents and electrolytes can be used as long as they are used in this type of battery.

  Illustrative examples of the organic solvent include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3- Dioxolane, 4-methyl-1.3-dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, anisole, acetate ester, butyrate ester, propionate ester and the like are preferable.

As the electrolyte, NaClO ₄ , NaAsF ₆ , NaPF ₆ , NaBF ₄ , NaB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Na, CF ₃ SO ₃ Na, NaCl, NaBR and the like are preferable.

  Hereinafter, the present invention will be described based on specific experimental results.

Example 1
An isotropic pitch (H / C atomic ratio 0.6 to 0.8, softening point 280 ° C.) manufactured by Osaka Gas Chemical Co., Ltd. was used as a carbon precursor. The carbon precursor was carbonized at 500 ° C. for 5 hours in a nitrogen stream at 10 ml / min. Then, beads obtained by carbonization were pulverized with a mill to obtain a carbonized raw material, of which about 10 g was charged in a crucible. The carbonized raw material charged in this crucible was baked in an electric furnace under a nitrogen flow of 10 l / min under conditions of a heating rate of 5 ° C./min, an ultimate temperature of 1300 ° C., and a holding time of 1 hour. Obtained.

  The obtained carbonaceous material was cooled, pulverized in a mortar, and classified to 38 μm or less with a mesh.

Comparative Example 1
A carbonaceous material powder was produced in the same manner as in Example 1 except that the carbonization raw material was not fired under a nitrogen stream but in a sealed nitrogen atmosphere.

(1) Measurement of true specific gravity The true specific gravity (ρp) measured by the liquid immersion substitution method was measured by the same method as described above using the method of JIS R 7212: 1995.

  On the other hand, in order to measure the true specific gravity (ρHe) of the carbonaceous material powder by the gas substitution method, a dry automatic densimeter Accupick II 1340 series manufactured by Shimadzu Corporation is used, and the measurement method described above is used. It was measured.

(2) Measurement of negative electrode capacity Next, the negative electrode capacity per unit weight of the carbonaceous material powder was measured as follows.

  First, an amount corresponding to 10% by weight of polyvinylidene fluoride was added to the carbonaceous materials of Example 1 and Comparative Example 1, and dimethylformamide was mixed as a solvent.

Then, after apply | coating to the copper foil which is a collector, it dried at 150 degreeC for 1 hour. Then, it punched into diameter 15.5mm and created the negative electrode. And the produced negative electrode was built in the coin-type battery of the structure shown below, charging / discharging was performed at 1 mA (current density 0.53 mA / cm < ² >), and the discharge capacity per 1g of negative electrode carbonaceous materials was measured. The configuration and charge / discharge conditions of the coin-type battery are shown below.

[Cell configuration]
Dimensions: Diameter 20mm, thickness 2.5mm
Counter electrode: Sodium metal separator: Porous membrane (polypropylene)
Electrolytic solution: a solution obtained by dissolving NaClO ₄ at a ratio of 1 mol / l in a mixed solvent obtained by mixing propylene carbonate and dimethoxyethane at a volume ratio of 1: 1. Current collector: copper foil.

The coin-type cell in which the negative electrode pellet was incorporated as described above was charged / discharged at a constant current of 1 mA (current density 0.53 mA / cm ² ) to determine the discharge capacity. The charge / discharge conditions are as follows.

  Charging was terminated when the potential reached 2 mV to sodium. The end of the discharge was when the potential reached 1.5V versus sodium.

  The measured discharge capacity is multiplied by the true specific gravity ρp determined by the liquid immersion substitution method using n-butanol as an immersion liquid, and the calculated value is used as the negative electrode capacity per unit weight of the carbonaceous material (Ah / kg). did.

  The negative electrode capacity per unit weight of the carbonaceous material thus determined is shown in Table 1 together with ρp, ρHe, and ρp / ρHe.

  As can be seen from Table 1, the carbonaceous material of Example 1 in which ρp / ρHe is less than 0.950 is less negative than the carbonaceous material of Comparative Example 1 in which ρp / ρHe is 0.950 or more. The capacity is as high as 150 Ah / kg or more. From this, it was found that regulating ρp / ρHe of the carbonaceous material used as the negative electrode active material is effective in increasing the capacity per unit weight of the negative electrode and reducing the size of the battery.

Claims (5)

A negative electrode material for a sodium secondary battery made of a carbonaceous material,
The negative electrode material is ρp / ρHe, where ρp is the true specific gravity measured by the liquid immersion substitution method using n-butanol as the immersion liquid, and ρHe is the true specific gravity measured by the gas substitution method using helium gas. A negative electrode material having an A of less than 0.950. The negative electrode material according to claim 1, wherein the carbonaceous material is made of a non-graphitizable carbon material obtained by firing a carbon precursor. It is a manufacturing method of the negative electrode material for sodium secondary batteries according to claim 1 or 2,
A production method comprising a step of firing a carbon precursor in an inert gas stream or in a low-pressure atmosphere. The manufacturing method according to claim 3, wherein the flow of the inert gas is a flow rate of 0.1 ml / min or more per 1 g of the carbon precursor. A sodium secondary battery comprising a negative electrode comprising the negative electrode material for a sodium secondary battery according to claim 1, a positive electrode, and a non-aqueous electrolyte.