JP7405342B2 - Negative electrode active material for sodium ion secondary battery and method for producing the same - Google Patents

Description

The present invention relates to a negative electrode active material for sodium ion secondary batteries used, for example, in portable electronic devices and electric vehicles, and a method for manufacturing the same.

In recent years, with the spread of portable electronic devices, electric vehicles, etc., development of lithium ion secondary batteries has become active. However, there are concerns about the depletion of Li resources used in lithium ion secondary batteries, and as a solution to this problem, sodium ion secondary batteries in which Li ions are replaced with Na ions are being considered.

Materials containing Si or Sn, which have a high theoretical capacity, have been proposed as negative electrode active materials for sodium ion secondary batteries. However, when a negative electrode active material containing Si or Sn is used for the negative electrode, there is a large volume change due to expansion and contraction of the negative electrode active material that occurs during the insertion/extraction reaction of sodium ions. There is a problem in that the decay of the carbon is severe and the cycle characteristics are likely to deteriorate. Therefore, Patent Document 1 proposes a negative electrode active material containing Bi ₂ O ₃ in order to improve cycle characteristics.

Japanese Patent Application Publication No. 2015-198000

The negative electrode active material described in Patent Document 1 has a problem that the initial charge capacity is larger than the initial discharge capacity and the initial irreversible capacity is large (that is, the initial charge/discharge efficiency is low).

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode active material for a sodium ion secondary battery having a low initial irreversible capacity and a method for producing the same.

The negative electrode active material for sodium ion secondary batteries of the present invention is made of crystallized glass in which metal Bi is precipitated in a matrix containing at least one selected from SiO ₂ , B ₂ O ₃ and P ₂ O ₅ . It is characterized by

The negative electrode active material containing Bi ₂ O ₃ undergoes the reactions of the following formulas (1) and (2) during charging and discharging. First, the reaction of formula (1) occurs irreversibly during initial charging. Thereafter, the reaction of formula (2) repeatedly occurs as the battery charges and discharges.

Bi ₂ O ₃ +6Na ⁺ +6e ^- → 2Bi+3Na ₂ O...(1)
Bi+3Na ⁺ +3e ^- ←→ BiNa ₃ ...(2)

The initial irreversible capacity that occurs during charging and discharging of a negative electrode active material containing Bi ₂ O ₃ is caused by Na ⁺ carrier ions and some electrons in the positive electrode active material discharging Bi ₂ O ₃ during the initial charging process. This is thought to be due to consumption in reaction (1) for reduction. On the other hand, since the negative electrode active material of the present invention has a structure in which metal Bi, which is already in a reduced state, is precipitated in a matrix, reaction (1) can be omitted, reducing the initial irreversible capacity. be able to. In addition, since the matrix containing at least one selected from SiO ₂ , B ₂ O ₃ and P ₂ O ₅ plays the role of a buffer material, it alleviates the expansion and contraction of the Bi component caused by repeating reaction (2). It also has the effect of having excellent cycle characteristics.

The negative electrode active material for sodium ion secondary batteries of the present invention contains Bi ₂ O ₃ 15 to 75%, P ₂ O ₅ 0 to 45%, SiO ₂ 0 to 60%, B ₂ O in terms of mol% in terms of oxides. It is preferable to contain ₃₀ to 60%, P ₂ O ₅ +SiO ₂ +B ₂ O ₃ 25 to 85%, and Na ₂ O 0 to 50%. In addition, " _P2O5 + _SiO2 + _{B2O3 "} means the _total amount _{of P2O5} , _SiO2 , _{and B2O3} .

The negative electrode active material for a sodium ion secondary battery of the present invention preferably further contains 0 to 25% of Fe ₂ O ₃ in terms of mol% in terms of oxide. By containing Fe ₂ O ₃ , the reduction of Bi ₂ O ₃ is promoted and metal Bi is easily precipitated.

The method for producing a negative electrode active material for a sodium ion secondary battery of the present invention is a method for producing the above-mentioned negative electrode active material for a sodium ion secondary battery, and is a method for producing a negative electrode active material for a sodium _{ion secondary} battery. , a step of reducing Bi ₂ O ₃ to metal Bi by performing heat treatment while supplying a reducing gas. In this way, Bi ₂ O ₃ in the oxide material can be efficiently reduced to metal Bi, and a negative electrode active material having desired characteristics can be easily produced.

In the method for producing a negative electrode active material for a sodium ion secondary battery of the present invention, it is preferable that the reducing gas contains 90 to 99.5% by volume of an inert gas and 0.5 to 10% of H ₂ . .

According to the manufacturing method of the present invention, it is possible to provide a negative electrode active material for a sodium ion secondary battery that has a low initial irreversible capacity.

3 shows powder X-ray diffraction patterns of samples of Example 2 and Comparative Example 2.

The negative electrode active material for sodium ion secondary batteries of the present invention (hereinafter also simply referred to as negative electrode active material) contains metal Bi in a matrix containing at least one selected from SiO ₂ , B ₂ O ₃ and P ₂ O ₅ . It is characterized by being made of crystallized glass formed by precipitating. Specifically, the negative electrode active material of the present invention contains Bi ₂ O ₃ 15-75%, P ₂ O ₅ 0-45%, SiO ₂ 0-60%, B ₂ O ₃ in terms of mol% of the oxide component. It preferably contains 0 to 60%, P ₂ O ₅ +SiO ₂ +B ₂ O ₃ 25 to 85%, and Na ₂ O 0 to 50%. The reason for limiting the composition in this way will be explained below. In the following description of the composition, "%" means "mol%" unless otherwise specified.

Bi ₂ O ₃ is an active material component that serves as a site for occluding and releasing sodium ions. The content of Bi ₂ O ₃ is preferably 15 to 75%, more preferably 20 to 70%, even more preferably 30 to 65%, and particularly preferably 40 to 55%. . If the content of Bi ₂ O ₃ is too small, the charge/discharge capacity per unit mass of the negative electrode active material tends to decrease. On the other hand, if the content of Bi ₂ O ₃ is too high, the amorphous component in the negative electrode active material will be relatively small, making it impossible to alleviate the volume change associated with storage and release of sodium ions during charging and discharging. , cycle characteristics tend to deteriorate.

P ₂ O ₅ is a network-forming oxide that encompasses the storage and release sites of sodium ions in Bi ₂ O ₃ and has the effect of improving cycle characteristics. The content of P ₂ O ₅ is preferably 0 to 45%, more preferably 0.1 to 42%, even more preferably 5 to 40%, and even more preferably 10 to 38%. Particularly preferred. When the content of P ₂ O ₅ is too high, water resistance tends to deteriorate.

SiO ₂ is also a network-forming oxide and has the effect of encompassing the storage and release sites of sodium ions in Bi ₂ O ₃ and improving cycle characteristics. The content of SiO ₂ is preferably 0 to 60%, preferably 0.1 to 55%, more preferably 1 to 50%, even more preferably 5 to 40%, A range of 10 to 30% is particularly preferred. If the content of SiO ₂ is too large, the ionic conductivity tends to decrease and the discharge capacity tends to decrease.

B ₂ O ₃ also acts as a network-forming oxide to encompass the storage and desorption sites of sodium ions in Bi ₂ O ₃ and improve the cycle characteristics. The content of B ₂ O ₃ is preferably 0 to 85%, more preferably 10 to 60%, even more preferably 20 to 55%, particularly 25 to 50%. Preferably, it is between 28 and 40%. If the content of B ₂ O ₃ is too large, the coordination bond to the Bi component becomes strong, the initial charge capacity increases, and as a result, the initial irreversible capacity tends to increase.

The content of P ₂ O ₅ +SiO ₂ +B ₂ O ₃ is preferably 20 to 85%, more preferably 25 to 60%, even more preferably 28 to 50%, and even more preferably 28 to 40%. % is particularly preferred. If the content of P ₂ O ₅ + SiO ₂ + B ₂ O ₃ is too small, the volume change of Bi atoms due to storage and release of sodium ions during charging and discharging cannot be alleviated, resulting in structural deterioration, resulting in a decrease in cycle characteristics. It becomes easier. On the other hand, if the content of P ₂ O ₅ +SiO ₂ +B ₂ O ₃ is too large, the Bi component becomes relatively small, and the charge/discharge capacity tends to decrease.

Na ₂ O is a component that improves the ionic conductivity of an oxide matrix other than the _three Bi ₂ O components (particularly an oxide matrix composed of P ₂ O ₅ , SiO ₂ or B ₂ O ₃ ). The content of Na ₂ O is preferably 0 to 50%, more preferably 1 to 45%, even more preferably 3 to 43%, particularly preferably 5 to 40%, Most preferably it is between 7 and 35%. If the content of Na ₂ O is too large, a large amount of heterogeneous crystals consisting of P ₂ O ₅ , SiO ₂ or B ₂ O ₃ and Na ₂ O is formed, and the cycle characteristics tend to deteriorate.

In addition to the above components, Fe ₂ O ₃ may also be contained. Fe ₂ O ₃ is a component that promotes reduction of Bi ₂ O ₃ and facilitates precipitation of metal Bi. The content of Fe ₂ O ₃ is preferably 0 to 25%, more preferably 0.1 to 20%, even more preferably 0.3 to 19%, and even more preferably 0.5 to 14%. is particularly preferred, and most preferably 1 to 9%. If the content of Fe ₂ O ₃ is too high, a large amount of heterogeneous crystals (σ-Bi _3.43 Fe _0.57 O ₆ ) consisting of Bi ₂ O ₃ and Fe ₂ O ₃ are formed, which tends to deteriorate the cycle characteristics. Become.

The negative electrode active material of the present invention may contain various components in addition to the above components. For example, it contains TiO ₂ , MnO, CuO, ZnO, MgO, CaO, Al ₂ O ₃ in a total amount of 0 to 25%, 0 to 23%, 0 to 21%, or even 0.1 to 20%. You may do so. By containing these components, the structure becomes disordered, making it easier to obtain an amorphous material. However, if the content is too large, the network consisting of the network-forming oxides (P ₂ O ₅ , SiO ₂ or B ₂ O ₃ ) described above will be easily cut, and as a result, the negative electrode active material will be damaged during charging and discharging. There is a risk that the cycle characteristics may deteriorate because the volume change cannot be alleviated.

In the negative electrode active material of the present invention, metal Bi particles are precipitated inside. Metallic Bi particles can be identified by powder X-ray diffraction measurement using CuKα rays. Specifically, in the diffraction line profile obtained by measurement, the diffraction lines having peak positions at 2θ values of 27.2°, 37.9°, and 39.6° correspond to the crystal phase (hexagonal system, It can belong to the space group R-3m (166)).

In the diffraction line profile, the half width of the diffraction line attributed to the crystal phase of metal Bi is preferably 0.01° or more, more preferably 0.05° or more, and 0.07° or more. More preferably, the angle is 0.1° or more. When the half-value width of the diffraction line is large, the crystallite size of metal Bi in the negative electrode active material is on the nano-order (for example, 0.1 to 100 nm), so even if sodium ions are occluded by the charging reaction, volume expansion will not occur. This is less likely to occur, and as a result, cycle characteristics tend to be excellent. On the other hand, if the half-width of the diffraction line is too small, the crystallite size of metal Bi becomes large (for example, on the order of microns or more), and a large local volume expansion occurs when sodium ions are occluded by a charging reaction. , cycle characteristics tend to deteriorate.

The degree of crystallinity of the negative electrode active material is preferably 5% or more, more preferably 30% or more, and even more preferably 50% or more. The greater the degree of crystallinity, the easier it is to reduce the initial irreversible capacity. However, if the degree of crystallinity is too high, cycle characteristics tend to deteriorate. Therefore, from the viewpoint of improving cycle characteristics, the degree of crystallinity is preferably 99% or less, particularly 95% or less.

The degree of crystallinity is determined from a diffraction line profile with a 2θ value of 10 to 60° obtained by powder X-ray diffraction measurement using CuKα radiation. Specifically, the integrated intensity obtained by peak-separating the broad diffraction line (amorphous halo) at 10 to 45° from the total scattering curve obtained by subtracting the background from the diffraction line profile is Ia, 10 If Ic is the sum of the integrated intensities obtained by peak-separating each crystalline diffraction line detected at ~60°, the crystallinity Xc can be calculated from the following formula.

Xc=[Ic/(Ic+Ia)]×100(%)

Here, the average particle diameter and maximum particle diameter are the median diameter of primary particles, and are expressed as D ₅₀ (50% volume cumulative diameter) and D ₉₀ (90% volume cumulative diameter), respectively, and are measured using a laser diffraction particle size distribution analyzer. is the value given.

A common crusher or classifier is used to obtain powder of a predetermined size. For example, a mortar, ball mill, vibrating ball mill, satellite ball mill, planetary ball mill, jet mill, sieve, centrifugal separation, air classification, etc. are used.

Although the shape of the negative electrode active material is not particularly limited, it is usually in the form of powder. The average particle diameter of the negative electrode active material is preferably 0.1 to 20 μm, more preferably 0.2 to 15 μm, even more preferably 0.3 to 10 μm, and even more preferably 0.5 to 5 μm. It is particularly preferable. Further, the maximum particle size of the negative electrode active material is preferably 150 μm or less, more preferably 100 μm or less, even more preferably 75 μm or less, and particularly preferably 55 μm or less. If the average particle size or maximum particle size is too large, volume changes of the negative electrode active material due to storage and release of sodium ions during charging and discharging cannot be alleviated, and cycle characteristics tend to deteriorate significantly. On the other hand, if the average particle diameter is too small, the powder will tend to be poorly dispersed when made into a paste, making it difficult to manufacture uniform electrodes. Further, the precipitated metal Bi is easily oxidized by oxygen in the atmosphere.

Next, a method for producing the negative electrode active material of the present invention will be explained. The method for producing a negative electrode active material of the present invention includes a step of reducing Bi ₂ O ₃ to metal Bi by heating an oxide material containing Bi ₂ O ₃ while supplying a reducing gas; It is characterized by containing.

The oxide material containing Bi ₂ O ₃ contains Bi ₂ O ₃ and at least one selected from SiO ₂ , B ₂ O ₃ and P ₂ O ₅ . Specifically, in terms of mol%, Bi ₂ O ₃ 15-75%, P ₂ O ₅ 0-45%, SiO ₂ 0-60%, B ₂ O ₃ 0-60%, P ₂ O ₅ +SiO ₂ +B Preferably, it contains 25 to 85% of ₂ O ₃ and 0 to 50% of Na ₂ O. The reason why the composition is limited in this way is as already stated, and the explanation will be omitted.

The oxide material is produced, for example, by heating and melting a raw material powder prepared to have the above-mentioned composition at, for example, 600 to 1200° C. to form a homogeneous melt, and then cooling and solidifying it. The obtained molten solidified product is subjected to post-processing such as pulverization and classification as necessary.

The oxide material is preferably amorphous, so that metal Bi is precipitated in an amorphous matrix containing at least one selected from SiO ₂ , B ₂ O ₃ and P ₂ O ₅ . , it becomes easier to obtain the negative electrode active material of the present invention.

The shape of the oxide material is usually powder like the negative electrode active material. The average particle size of the oxide material is preferably 0.1 to 20 μm, more preferably 0.2 to 15 μm, even more preferably 0.3 to 10 μm, and even more preferably 0.5 to 5 μm. It is particularly preferable. Further, the maximum particle diameter of the oxide material is preferably 150 μm or less, more preferably 100 μm or less, even more preferably 75 μm or less, and particularly preferably 55 μm or less. If the average particle size or maximum particle size is too large, the particle size of the obtained negative electrode active material will also become large, which tends to cause the above-mentioned problems. Furthermore, there is a possibility that Bi ₂ O ₃ cannot be sufficiently reduced to metal Bi due to the reducing gas. On the other hand, if the average particle size is too small, the particle size of the obtained negative electrode active material will also be small, which tends to cause the above-mentioned problems.

The temperature during the heat treatment is preferably 250°C or higher, more preferably 300°C or higher, and particularly preferably 400°C or higher. If the heating temperature is too low, less thermal energy is applied, making it difficult for Bi ₂ O ₃ in the oxide material to be reduced to metal Bi. Note that the upper limit of the heating temperature is not particularly limited, but if it is too high, the reduced metal Bi particles tend to become coarse, which may significantly deteriorate the cycle characteristics of the negative electrode active material. Therefore, the heating temperature is preferably 700°C or lower, more preferably 600°C or lower.

The heating time is preferably 20 to 1000 minutes, more preferably 60 to 500 minutes. If the heating time is too short, less thermal energy is applied, making it difficult for Bi ₂ O ₃ in the oxide material to be reduced to metal Bi. On the other hand, if the heating time is too long, the reduced metal Bi particles tend to become coarse, which may significantly deteriorate the cycle characteristics of the negative electrode active material.

For the heat treatment, an electric heating furnace, rotary kiln, microwave heating furnace, high frequency heating furnace, etc. can be used.

Examples of the reducing gas include at least one gas selected from H ₂ , NH ₃ , CO, H ₂ S, and SiH ₄ . From the viewpoint of handling properties, at least one gas selected from H ₂ , NH ₃ and CO is preferred, and H ₂ is particularly preferred.

When using H ₂ as the reducing gas, it is preferable to mix it with an inert gas such as N ₂ or Ar in order to suppress the risk of explosion. The mixing ratio of the inert gas and H ₂ is preferably 90 to 99.5% by volume and _0.5 to 10% by volume. For example, in the case of a mixed gas of N ₂ and H ₂ , N ₂ 90-99.5%, H ₂ 0.5-10%, N ₂ 92-99%, H ₂ 1-8%, especially N It is preferable to contain ₉₆ to 99% of H ₂ and 1 to 4% of H 2 . In the case of a mixed gas of Ar and H ₂ , Ar 90-99.5%, H ₂ 0.5-10%, Ar 92-99%, H ₂ 1-8%, especially Ar 95-98% by volume. %, preferably containing 2 to 5% H ₂ .

Note that in the heat treatment step, the oxide material (oxide material powder) tends to soften and flow to form aggregates. When the oxide material forms aggregates, it becomes difficult for the reducing gas to spread throughout the oxide material, so reduction of the oxide material tends to take a long time. Alternatively, the generated negative electrode active material particles may become coarse and the battery characteristics may deteriorate. Therefore, it is preferable to add an agglomeration inhibitor when heat-treating the oxide material. In this way, aggregation of the oxide material during heat treatment can be suppressed, and Bi ₂ O ₃ in the oxide material can be reduced to metal Bi in a short time.

Examples of the anti-aggregation agent include carbon materials such as conductive carbon and acetylene black. Since the carbon material also has electronic conductivity, it can also impart conductivity to the negative electrode active material. Among these, acetylene black, which has excellent electron conductivity, is preferred.

The oxide material and the agglomeration inhibitor are preferably mixed in a ratio of 80 to 99.5% by mass for the oxide material and 0.5 to 20% for the aggregation inhibitor. In this way, it becomes easier to obtain a negative electrode active material having good initial charge characteristics and stable cycle characteristics.

The negative electrode active material of the present invention is used as a negative electrode material by adding a binder or a conductive additive.

As a binder, cellulose derivatives such as carboxymethylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose, hydroxyethylcellulose, ethylcellulose, hydroxymethylcellulose, or water-soluble polymers such as polyvinyl alcohol; thermosetting polyimide, phenolic resin, epoxy resin, Thermosetting resins such as urea resin, melamine resin, unsaturated polyester resin, and polyurethane; polyvinylidene fluoride, and the like.

Examples of the conductive aid include highly conductive carbon black such as acetylene black and Ketjen black, carbon powder such as graphite, and carbon fiber.

The negative electrode material for a power storage device can be used as a negative electrode for a power storage device by applying it to the surface of a metal foil or the like that serves as a current collector.

The negative electrode active material for a sodium ion secondary battery of the present invention can also be applied to a hybrid capacitor, etc., which is a combination of a negative electrode active material used for a sodium ion secondary battery and a positive electrode material for a non-aqueous electric double layer capacitor.

A sodium ion capacitor, which is a hybrid capacitor, is a type of asymmetric capacitor whose positive and negative electrodes have different charging and discharging principles. A sodium ion capacitor has a structure that combines a negative electrode for a sodium ion secondary battery and a positive electrode for an electric double layer capacitor. Here, the positive electrode forms an electric double layer on its surface and is charged and discharged using physical action (electrostatic action), whereas the negative electrode uses the chemical reaction (occlusion and absorption) of Na ions, similar to sodium ion secondary batteries. and discharge).

For the positive electrode of a sodium ion capacitor, a positive electrode active material made of carbonaceous powder with a high specific surface area such as activated carbon, polyacene, mesophase carbon, etc. is used. On the other hand, the negative electrode active material of the present invention can be used for the negative electrode.

Hereinafter, the negative electrode active material of the present invention will be explained in detail based on Examples, but the present invention is not limited to these Examples.

(1) Preparation of oxide material Raw material powders were prepared using various oxides, carbonate raw materials, phosphate raw materials, etc. so as to have the compositions shown in Table 1. The raw material powder was put into a platinum crucible and melted in an air atmosphere using an electric furnace. Note that in Example 1 and Comparative Example 1, melting was performed at 750° C. for 30 minutes, and in Examples 2 to 5 and Comparative Examples 2 to 4, melting was performed at 1100° C. for 20 minutes.

Next, the melt was poured between a pair of cooling rotating rollers to form it while rapidly cooling it to obtain a glass film having a thickness of 0.1 to 2 mm. This film-like glass was pulverized at 100 rpm for 3 hours using a ball mill containing zirconia balls with a diameter of 1 to 3 cm, and then passed through a resin sieve with an opening of 120 μm. A raw material (coarse powder) was obtained. Next, the coarse glass powder was air classified to obtain glass powder (oxide material powder) having an average particle size of 2 μm and a maximum particle size of 28 μm.

When the structure of the oxide material powder was identified by powder X-ray diffraction measurement, it was found to be amorphous and no crystals were detected.

(2) Preparation of negative electrode active material The obtained oxide material was heat-treated under the conditions listed in Table 1. In Example 2, conductive carbon black powder (Super C65, manufactured by Timcal) was added as an anti-agglomeration agent to the oxide material powder in a ball mill in advance at a ratio of oxide material: anti-agglomeration agent = 95:5 (mass%). A mixture of these was used. Comparative Examples 1 and 2 were not subjected to heat treatment.

Next, the oxide material after the heat treatment was crushed in an agate mortar, and then passed through a resin sieve with an opening of 20 μm to obtain a negative electrode active material powder with an average particle size of 3 μm.

The structure of the obtained negative electrode active material was identified by powder X-ray diffraction measurement. The results are shown in Table 1 and FIG.

(3) Preparation of negative electrode For Example 1 and Comparative Example 1, negative electrodes were prepared as follows. The obtained negative electrode active material, conductive aid (conductive carbon black), and binder (polyvinylidene fluoride) were weighed and mixed at a mass ratio of 85:10:5, and then placed on a stainless steel mesh at 40 MPa. A negative electrode was prepared by pressure bonding.

For Examples 2 to 5 and Comparative Examples 2 to 4, negative electrodes were produced as follows. The obtained negative electrode active material, conductive aid (conductive carbon black), and binder (polyvinylidene fluoride) were weighed so that the mass ratio was 85:5:10, and added to dehydrated N-methylpyrrolidone. After being dispersed, the mixture was thoroughly stirred using a rotation/revolution mixer to form a slurry. Next, using a doctor blade with a gap of 75 μm, the obtained slurry was coated on a 20 μm thick copper foil serving as a negative electrode current collector, and after vacuum drying in a dryer at 70°C, the slurry was coated between a pair of rotating rollers. An electrode sheet was obtained by pressing through. This electrode sheet was punched out to a diameter of 11 mm using an electrode punching machine, and dried under reduced pressure at a temperature of 200° C. for 8 hours to obtain a circular negative electrode.

(4) Preparation of test battery The obtained negative electrode, a separator consisting of a polypropylene porous membrane with a diameter of 16 mm (Celgard #2400 manufactured by Hoechst Celanese) dried under reduced pressure at 70°C for 8 hours, and a counter electrode consisting of metallic sodium Test cells were prepared by laminating and impregnating with electrolyte. As the electrolytic solution, 1M NaPF ₆ solution/EC:DEC=1:1 (EC=ethylene carbonate, DEC=diethyl carbonate) was used. The test battery was assembled in an argon environment with a dew point temperature of -70°C or lower.

(5) Charge/discharge test The prepared test battery was CC (constant current) charged (occlusion of sodium ions into the negative electrode active material) from the open circuit voltage to 0 V at 30°C, and charged to unit mass of the negative electrode active material. The amount of electricity (initial charging capacity) was calculated. Next, CC discharge (release of sodium ions from the negative electrode active material) was performed from 0 V to 3 V, and the amount of electricity discharged from the negative electrode active material per unit mass (initial discharge capacity) was determined. Note that the C rate was 0.1C. From these results, the initial irreversible capacity (=initial charge capacity−initial discharge capacity) was determined. The results are shown in Table 1.

As shown in Table 1, the negative electrode active materials of Examples 1 to 5 have a small initial irreversible capacity of 238 mAh/g or less, whereas the negative electrode active materials of Comparative Examples 1 to 4 have an initial irreversible capacity of 254 mAh/g or more. It was big.

The negative electrode active material of the present invention is suitable for use in, for example, sodium ion secondary batteries used as main power sources for mobile communication devices, portable electronic devices, electric bicycles, electric motorcycles, electric vehicles, and the like.

Claims (5)

A negative electrode active material for a sodium ion secondary battery, which is used for manufacturing a negative electrode constituting a sodium ion secondary battery,
A negative electrode active material for a sodium ion secondary battery, characterized in that it is made of crystallized glass in which metal Bi is precipitated in a matrix containing at least one selected from SiO ₂ , B ₂ O ₃ and P ₂ O ₅ . In terms of mol% in terms of oxides, Bi ₂ O ₃ 15-75%, P ₂ O ₅ 0-45%, SiO ₂ 0-60%, B ₂ O ₃ 0-60%, P ₂ O ₅ +SiO ₂ +B ₂ The negative electrode active material for a sodium ion secondary battery according to claim 1, containing 25 to 85% of O ₃ and 0 to 50% of Na _{2 O.} The negative electrode active material for a sodium ion secondary battery according to claim 2, further comprising 0 to 25% of Fe ₂ O ₃ in terms of mol% in terms of oxide. A method for producing a negative electrode active material for a sodium ion secondary battery according to any one of claims 1 to 3, comprising:
A sodium ion secondary battery comprising the step of reducing Bi ₂ O ₃ to metal Bi by heating an oxide material containing Bi ₂ O ₃ while supplying a reducing gas. A method for producing a negative electrode active material for use. 5. The negative electrode active material for a sodium ion secondary battery according to claim 4, wherein the reducing gas contains, in volume percent, 90 to 99.5% inert gas and 0.5 to 10% H ₂ . manufacturing method.

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