Negative electrode active material for sodium ion secondary batteries

Abstract

To provide a negative electrode active material for a sodium ion secondary battery that exhibits stable battery characteristics when charging and discharging are repeatedly performed.SOLUTION: A negative electrode active material for a sodium ion secondary battery contains an amorphous phase containing SiO2, and an Fe-Sn based alloy.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material for sodium ion secondary batteries used in, for example, portable electronic devices and electric vehicles. [Background technology]

[0002] In recent years, the development of lithium-ion secondary batteries has been active with the spread of portable electronic devices and electric vehicles, etc. However, there are concerns about the depletion of the Li resource used in lithium-ion secondary batteries, and as a solution to this, sodium-ion secondary batteries, in which Li ions are replaced with Na ions, are being considered.

[0003] Among them, metallic Sn has a high theoretical capacity of 847 mAhg when alloyed with sodium, and is therefore known as a promising candidate for the negative electrode material in sodium-ion secondary batteries. + +15e - ←→Na 15 The reaction of Sn+ with Sn4 is repeated. Here, metallic Sn undergoes a large volume change of 4.2 times due to alloying during charging and discharging, which causes a decrease in capacity due to electrode destruction, which is an issue. As a method for mitigating the volume change during charging and discharging, a method of precipitating metallic Sn in a glass matrix has been proposed (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-229539 Summary of the Invention [Problem to be solved by the invention]

[0005] In crystallized glass made by precipitating metallic Sn, amorphous components such as SiO2, P2O5, and B2O3 contained in the glass matrix act as buffers to mitigate the expansion and contraction of the Sn component. However, because they cannot fully mitigate the volume change during charge and discharge, the battery characteristics tend to become unstable when repeatedly charged and discharged.

[0006] 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 that exhibits stable battery characteristics when repeatedly charged and discharged. [Means for solving the problem]

[0007] The negative electrode active material for a sodium ion secondary battery of the present invention is characterized by containing an amorphous phase containing SiO2 and an Fe-Sn-based alloy.

[0008] As described above, the negative electrode active material for a sodium ion secondary battery of the present invention contains an Fe—Sn alloy. FeSn2, a type of Fe—Sn alloy, converts to 2FeSn2+15Na during charge and discharge. + +15e - ←→2Fe+Na 15 The negative electrode active material for a sodium ion secondary battery of the present invention repeats the reaction of forming FeSn2 with Sn4. Here, FeSn2 undergoes a smaller volume change associated with alloying during charge and discharge than metallic Sn. Furthermore, the negative electrode active material for a sodium ion secondary battery of the present invention contains an amorphous phase containing SiO2. This amorphous phase serves as a buffer that alleviates the expansion and contraction of Fe-Sn alloys such as FeSn2. The small volume change associated with alloying during charge and discharge, and the presence of the amorphous phase as a buffer that alleviates this volume change, allow the battery to exhibit stable battery characteristics, especially when repeatedly charged and discharged.

[0009] The negative electrode active material for a sodium ion secondary battery of the present invention preferably contains, in mole percent calculated as oxide, 30% to 90% of SnO, 2% to 69% of SiO2, and 1% to 20% of Fe2O3.

[0010] In the negative electrode active material for a sodium ion secondary battery of the present invention, the Fe—Sn-based alloy is preferably FeSn 2 .

[0011] The negative electrode active material for a sodium ion secondary battery of the present invention preferably comprises crystallized glass in which the Fe—Sn alloy is precipitated in a matrix made of the amorphous phase.

[0012] The negative electrode active material for a sodium ion secondary battery of the present invention preferably has a crystallinity of 30% by mass or more and 99% by mass or less.

[0013] In the negative electrode active material for a sodium ion secondary battery of the present invention, the amount of crystals of the Fe—Sn-based alloy is preferably 0.5 mass % or more and 70 mass % or less.

[0014] The negative electrode active material for a sodium ion secondary battery of the present invention may further contain β-Sn. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a negative electrode active material for a sodium ion secondary battery that exhibits stable battery characteristics even when repeatedly charged and discharged. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a graph showing the battery characteristics of the test battery fabricated in Example 3 at each cycle. [Figure 2] FIG. 2 is a graph showing the battery characteristics of the test battery prepared in Comparative Example 1 at each cycle. DETAILED DESCRIPTION OF THE INVENTION

[0017] Preferred embodiments will be described below. However, the following embodiments are merely examples, and the present invention is not limited to the following embodiments. In addition, in each drawing, components having substantially the same functions may be referred to by the same reference numerals.

[0018] The negative electrode active material for a sodium ion secondary battery of the present invention (hereinafter also simply referred to as negative electrode active material) is characterized by containing an amorphous phase containing SiO2 and an Fe-Sn-based alloy. Specifically, the negative electrode active material of the present invention preferably contains, in mole percent oxide equivalent, 30% to 90% SnO, 2% to 69% SiO2, and 1% to 20% Fe2O3. The reasons for limiting the composition in this way are explained below. In the following description of the composition, "%" means "mol %" unless otherwise specified.

[0019] SnO is an active material component that serves as a site for occluding and releasing sodium ions. The SnO content, in mole percent (oxide equivalent), is preferably, for example, 30% or more, 35% or more, 40% or more, 45% or more, particularly 50% or more, and preferably 90% or less, 80% or less, 75% or less, 70% or less, particularly 68% or less. If the SnO content is too low, the charge / discharge capacity per unit mass of the negative electrode active material is likely to decrease. On the other hand, if the SnO content is too high, the amorphous component in the negative electrode active material is relatively reduced, making it difficult to alleviate volume changes associated with the occlusion and release of sodium ions during charge / discharge, resulting in unstable battery characteristics such as reduced cycle characteristics.

[0020] SiO2 functions as a network-forming oxide and promotes amorphization. This encompasses sites for occluding and releasing sodium ions in the Sn component, improving cycle characteristics. The SiO2 content, in mole percent oxide equivalent, is preferably, for example, 2% or more, 5% or more, 10% or more, 15% or more, and particularly 20% or more, and is preferably 69% or less, 65% or less, 55% or less, 45% or less, and particularly 35% or less. If the SiO2 content is too low, it becomes difficult to achieve the above-mentioned effects. On the other hand, if the SiO2 content is too high, ionic conductivity decreases, and discharge capacity tends to decrease. Furthermore, the charge / discharge capacity tends to decrease due to the relatively low Sn component.

[0021] Fe2O3 is a component that forms an Fe-Sn alloy, such as FeSn2, upon subsequent heat treatment and functions as an active material that absorbs and releases sodium ions and electrons. Fe2O3 also functions as a network-forming oxide, promoting amorphization. This reduces the expansion and contraction of the Sn component, improving cycle performance. It also improves the conductivity of the oxide matrix component in the negative electrode active material, improving rapid charge / discharge performance. The Fe2O3 content, in mole percent (oxide equivalent), is preferably, for example, 1% or more, 1.5% or more, 2% or more, 2.5% or more, and particularly 3% or more, and is preferably 20% or less, 17% or less, 15% or less, 12% or less, and particularly 10% or less. If the Fe2O3 content is too low, the above effects are difficult to achieve. On the other hand, if the Fe2O3 content is too high, ionic conductivity and discharge capacity tend to decrease.

[0022] The negative electrode active material of the present invention may contain the following components in addition to the above components.

[0023] Na2O is a component that improves the ionic conductivity of oxide matrix components other than the Sn component. The Na2O content, in mole percent converted to oxide, is, for example, preferably 1% or more, 3% or more, 5% or more, 7% or more, and particularly preferably 10% or more, and is preferably 50% or less, 40% or less, 30% or less, 25% or less, and particularly preferably 20% or less. If the Na2O content is too high, a large amount of heterogeneous crystals (e.g., crystals containing Na2O and SiO2) are formed, which tends to deteriorate the cycle characteristics.

[0024] Like SiO2, P2O5 functions as a network-forming oxide and promotes amorphization. This serves to encapsulate sites for occluding and releasing sodium ions in the Sn component, improving cycle characteristics. The P2O5 content, in mole percent oxide equivalent, is preferably, for example, 1% or more, 3% or more, 5% or more, and particularly 7% or more, and is preferably 30% or less, 25% or less, 20% or less, and particularly 15% or less. If the P2O5 content is too high, the water resistance of the negative electrode active material is likely to decrease. Furthermore, the charge / discharge capacity tends to decrease due to the relative decrease in the Sn component.

[0025] Like SiO2, B2O3 also functions as a network-forming oxide and promotes amorphization. This allows it to occlude sites for sodium ion absorption and desorption in the Sn component, improving cycle characteristics. The B2O3 content, in mole percent oxide equivalent, is preferably, for example, 1% or more, 3% or more, 5% or more, and particularly 7% or more, and is preferably 30% or less, 25% or less, 20% or less, and particularly 15% or less. If the B2O3 content is too high, the coordination bond to the Sn component becomes strong, increasing the initial charge capacity and, as a result, the initial irreversible capacity tends to increase. Furthermore, the charge / discharge capacity tends to decrease due to the relative decrease in the Sn component.

[0026] The content of P2O5+SiO2+B2O3, in mole percent (oxide equivalent), is preferably 2% or more, 3% or more, 4% or more, 5% or more, 10% or more, 15% or more, and particularly preferably 20% or more, and is preferably 69% or less, 65% or less, 55% or less, 45% or less, and particularly preferably 35% or less. If the content of P2O5+SiO2+B2O3 is too low, the volume change of the Sn component associated with the absorption and release of sodium ions during charge and discharge cannot be alleviated, causing structural deterioration and resulting in a deterioration in cycle performance. On the other hand, if the content of P2O5+SiO2+B2O3 is too high, the Sn component becomes relatively small, tending to reduce charge and discharge capacity. Note that in this specification, "x+y+···" refers to the total content of each component. Here, each component does not necessarily need to be contained as an essential component; it is acceptable to include a component that is not contained (i.e., a 0% content).

[0027] The negative electrode active material of the present invention may contain TiO2, MnO, CuO, ZnO, MgO, CaO, or Al2O3 in a total amount, calculated as oxide mole percent, of, for example, 0% to 25%, 0% to 23%, 0% to 21%, or even 0.1% to 20%. The inclusion of these components facilitates the production of an amorphous material. However, if the content is too high, the SiO2 network is easily broken, which may result in a failure to mitigate the volumetric change of the negative electrode active material during charge and discharge, resulting in a deterioration in cycle performance.

[0028] The negative electrode active material of the present invention preferably has an Fe—Sn alloy such as FeSn2 precipitated therein. FeSn2 can be identified by powder X-ray diffraction (XRD) using CuKα radiation. Specifically, in the diffraction profile obtained by powder X-ray diffraction (XRD), the diffraction lines having peak positions at 2θ values ​​of 35.1°, 43.9°, 61.2°, 33.7°, and 67.3° can be assigned to the FeSn2 crystalline phase (tetragonal system, space group I4 / mcm(140)).

[0029] The crystalline amount of FeSn2 is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 4% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 55% by mass or less. If the crystalline amount of FeSn2 is too high, the amorphous component in the negative electrode active material becomes relatively small, which makes it difficult to alleviate the volume change associated with the absorption and release of sodium ions during charge and discharge, tending to result in unstable battery characteristics. On the other hand, if the crystalline amount of FeSn2 is too low, the discharge capacity tends to decrease and battery characteristics tend to become unstable when repeatedly charged and discharged.

[0030] The negative electrode active material of the present invention may have β-Sn precipitated therein. β-Sn serves as a site for absorbing and releasing sodium ions, further improving battery capacity. β-Sn can be identified by powder X-ray diffraction (XRD) using CuKα radiation. Specifically, in the diffraction profile obtained by powder X-ray diffraction (XRD), the diffraction lines having peak positions at 2θ values ​​of 30.63° and 32.02° can be assigned to the β-Sn crystalline phase (tetragonal system, space group I41 / amd(141)).

[0031] The β-Sn crystalline content is preferably 1% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less. If the β-Sn crystalline content is too high, the amount of amorphous components in the negative electrode active material will be relatively small, which will make it difficult to alleviate the volume change associated with the absorption and release of sodium ions during charge and discharge, resulting in unstable battery characteristics. On the other hand, if the β-Sn crystalline content is too low, the discharge capacity will tend to decrease.

[0032] The negative electrode active material of the present invention may have FeNaSnO4 precipitated inside, which can further improve the stability of battery characteristics.

[0033] The crystallinity of the negative electrode active material is preferably 30% by mass or more, more preferably 40% by mass or more, and particularly preferably 50% by mass or more. The higher the crystallinity, the easier it is to reduce the initial irreversible capacity. However, if the crystallinity is too high, the cycle characteristics tend to deteriorate. Therefore, from the viewpoint of improving the cycle characteristics, the crystallinity is preferably 99% by mass or less, more preferably 95% by mass or less, and particularly preferably 90% by mass or less.

[0034] The degree of crystallinity is determined from the diffraction line profile between 10° and 60° in 2θ obtained by powder X-ray diffraction measurement (XRD) using CuKα radiation. Specifically, the total scattering curve obtained by subtracting the background from the diffraction line profile obtained by powder X-ray diffraction measurement (XRD) is used to obtain a curve. The integrated intensity obtained by peak separation of the broad diffraction line (amorphous halo) between 10° and 45° is defined as Ia, and the sum of the integrated intensities obtained by peak separation of each crystalline diffraction line detected between 10° and 60° is defined as Ic. The degree of crystallinity, Xc, can be calculated from the following formula:

[0035] Xc = [Ic / (Ic+Ia)] x 100(%)

[0036] The shape of the negative electrode active material is not particularly limited, but powder form is preferred. The average particle size of the negative electrode active material is preferably 0.1 μm or more, more preferably 0.2 μm or more, even more preferably 0.3 μm or more, and particularly preferably 0.5 μm or more, and is preferably 20 μm or less, more preferably 15 μm or less, even more preferably 10 μm or less, and particularly preferably 5 μm or less. 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 of the negative electrode active material is too large, the volume change of the negative electrode active material associated with the absorption and release of sodium ions during charge and discharge cannot be alleviated, and cycle performance tends to be significantly reduced. On the other hand, if the average particle size of the negative electrode active material is too small, the powder dispersion state when made into a paste tends to be poor, making it difficult to produce a uniform electrode. In addition, the precipitated Fe-Sn-based alloy, such as FeSn2, is easily oxidized by oxygen in the atmosphere.

[0037] Here, the average particle size and maximum particle size are the median diameters of the primary particles, D 50 (50% volume cumulative diameter) and D 90 (90% volume cumulative diameter) and is a value measured using a laser diffraction particle size distribution analyzer.

[0038] To obtain powder of a predetermined size, a general pulverizer or classifier is used, such as a mortar, ball mill, vibration ball mill, satellite ball mill, planetary ball mill, jet mill, sieve, centrifuge, air classification, etc.

[0039] The negative electrode active material of the present invention can be produced by subjecting a raw oxide material to a heat treatment while supplying a reducing gas, thereby reducing SnO and Fe2O3 contained in the oxide material to Fe-Sn-based alloys such as FeSn2.

[0040] The oxide material is produced by heating and melting raw material powder prepared to have the above-mentioned composition at, for example, 600 to 1500°C to form a homogeneous melt, which is then cooled and solidified. The resulting melt-solidified material is then subjected to post-processing such as pulverization and classification as necessary.

[0041] For heating and melting, a furnace such as an electric heating furnace, a rotary kiln, a microwave heating furnace, or a high-frequency heating furnace, or laser irradiation, etc. can be used.

[0042] The oxide material is produced by heating and melting a portion of the composition to form a homogeneous melt, followed by cooling and solidifying it. Alternatively, the oxide material may be produced by mixing a powder of the melt-solidified composition, which has been subjected to post-processing such as pulverization and classification, with a powder of the remaining composition, and then subjecting the mixture to mechano-milling.

[0043] The oxide material is preferably amorphous, which facilitates the production of the negative electrode active material of the present invention, which is made of crystallized glass in which an Fe-Sn alloy such as FeSn is precipitated in a matrix containing SiO. Crystals such as SnO may be precipitated within the oxide material. The matrix may also contain FeO.

[0044] The shape of the oxide material is not particularly limited, but is preferably powdered, similar to the negative electrode active material. The average particle size of the oxide material is preferably 0.1 μm to 20 μm, more preferably 0.2 μm to 15 μm, even more preferably 0.3 μm to 10 μm, and particularly preferably 0.5 μm to 5 μm. The maximum particle size of the oxide material is, for example, 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 of the oxide material is too large, the particle size of the resulting negative electrode active material will also be large, which tends to cause the above-mentioned problems. Furthermore, there is a risk that SnO and Fe2O3 may not be sufficiently reduced to Fe-Sn-based alloys such as FeSn2 by the reducing gas. On the other hand, if the average particle size of the oxide material is too small, the particle size of the resulting negative electrode active material will also be small, which tends to cause the above-mentioned problems.

[0045] The heating temperature during the heat treatment is, for example, 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, the applied thermal energy is small, making it difficult for SnO and Fe2O3 in the oxide material to be reduced to Fe-Sn alloys such as FeSn2. While there is no particular upper limit for the heating temperature, if it is too high, the reduced Fe-Sn alloy particles such as FeSn2 tend to coarsen, which may significantly reduce the cycle characteristics of the negative electrode active material. Therefore, the heating temperature is preferably 700°C or lower, particularly 600°C or lower.

[0046] The heating time during the heat treatment is preferably 20 to 1000 minutes, particularly 60 to 500 minutes. If the heating time is too short, the applied heat energy is small, making it difficult for SnO and Fe2O3 in the oxide material to be reduced to Fe-Sn alloys such as FeSn2. On the other hand, if the heating time is too long, the particles of the reduced Fe-Sn alloys such as FeSn2 tend to become coarse, which may significantly reduce the cycle characteristics of the negative electrode active material.

[0047] For the heat treatment, a furnace such as an electric heating furnace, a rotary kiln, a microwave heating furnace, or a high-frequency heating furnace, or laser irradiation, etc., can be used.

[0048] The reducing gas used in the heat treatment may be at least one gas selected from H, NH, CO, HS, and SiH. From the viewpoint of ease of handling, the reducing gas is preferably at least one gas selected from H, NH, and CO, and particularly preferably H.

[0049] When H2 is used as the reducing gas, it is preferable to use it mixed with an inert gas such as N2 or Ar to reduce the risk of explosion, etc. The mixture ratio of the inert gas and H2 is preferably 85% to 99.5% inert gas and 0.5% to 15% H2 by volume, more preferably 87% to 99% inert gas and 1% to 13% H2, and even more preferably 89% to 99% inert gas and 1% to 11% H2.

[0050] During the heat treatment process, the oxide material (oxide material powder) tends to soften and flow, forming aggregates. When the oxide material forms aggregates, it becomes difficult for the reducing gas to reach the entire oxide material, which tends to prolong the reduction process. Alternatively, the resulting negative electrode active material particles may become coarse, potentially resulting in a deterioration in battery performance. Therefore, it is preferable to add an anti-aggregation agent when heat-treating the oxide material. This can suppress the aggregation of the oxide material during the heat treatment, enabling the SnO and Fe2O3 in the oxide material to be reduced to Fe-Sn alloys such as FeSn2 in a short time.

[0051] Examples of the anti-aggregation agent include carbon materials such as conductive carbon and acetylene black. Carbon materials also have electronic conductivity, so they can impart electrical conductivity to the negative electrode active material. Among these, acetylene black, which has excellent electronic conductivity, is preferred as the carbon material.

[0052] The oxide material and anti-aggregation agent are preferably mixed in a ratio of 80% to 99.5% by mass of the oxide material and 0.5% to 20% by mass of the anti-aggregation agent, which makes it easier to obtain a negative electrode active material with good initial charge characteristics and stable cycle characteristics.

[0053] The negative electrode active material of the present invention can be used as a negative electrode material for an electricity storage device by adding a binder and a conductive additive.

[0054] Examples of binders include cellulose derivatives such as carboxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, ethyl cellulose, and hydroxymethyl cellulose, and water-soluble polymers such as polyvinyl alcohol; thermosetting resins such as thermosetting polyimide, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; and polyvinylidene fluoride.

[0055] Examples of the conductive aid include highly conductive carbon black such as acetylene black and ketjen black, carbon powder such as graphite, and fibrous carbon such as carbon fiber and carbon nanotube.

[0056] The negative electrode material for an electricity storage device can be used as a negative electrode for an electricity storage device by applying it to the surface of a metal foil or the like that serves as a current collector.

[0057] In the present invention, the negative electrode material for an electricity storage device containing the oxide material that has not been subjected to the heat treatment may be applied to the surface of a metal foil or the like, and then the heat treatment may be performed.

[0058] For example, the oxide material that has not been subjected to the heat treatment and a negative electrode material for an electricity storage device containing a binder and a conductive additive as needed may be applied to the surface of a metal foil or the like, and then the material may be subjected to a heat treatment by irradiation with laser light.

[0059] In this case, the laser beam irradiation is preferably performed while supplying a reducing gas, as in the heat treatment without using the laser beam described above. In this case, the oxide material can be crystallized by the laser beam irradiation to obtain the negative electrode active material.

[0060] However, in the present invention, after irradiating with laser light in a non-reducing atmosphere, the oxide material may be crystallized by further performing a heat treatment while supplying a reducing gas, thereby obtaining a negative electrode active material.

[0061] The wavelength of the laser beam is preferably in the near-infrared to infrared region. Specifically, the wavelength of the laser beam is preferably 750 nm to 1600 nm, more preferably 900 nm to 1400 nm, even more preferably 950 nm to 1200 nm, and particularly preferably 1000 nm to 1100 nm, which is preferred because the laser beam is easily absorbed by the laser-beam-absorbing component.

[0062] Therefore, the laser light is preferably one that can irradiate light in the wavelength range described above, and examples thereof include semiconductor lasers, YAG lasers, Yb fiber lasers, and YVO4 lasers.

[0063] The negative electrode active material for a sodium ion secondary battery of the present invention can also be applied to a hybrid capacitor in which a negative electrode active material used in a sodium ion secondary battery is combined with a positive electrode material for a non-aqueous electric double layer capacitor.

[0064] A sodium ion capacitor, a hybrid capacitor, is a type of asymmetric capacitor in which the positive and negative electrodes charge and discharge using different principles. A sodium ion capacitor combines the negative electrode of a sodium ion secondary battery with the positive electrode of an electric double layer capacitor. Here, the positive electrode forms an electric double layer on its surface and charges and discharges using a physical action (electrostatic action), while the negative electrode charges and discharges through a chemical reaction (storage and release) of sodium ions, just like a sodium ion secondary battery.

[0065] The positive electrode of the sodium ion capacitor uses a positive electrode active material made of a carbonaceous powder having a high specific surface area such as activated carbon, polyacene, mesophase carbon, etc. On the other hand, the negative electrode active material of the present invention can be used for the negative electrode. [Example]

[0066] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0067] Table 1 shows Examples 1 to 4 and Comparative Example 1.

[0068] [Table 1]

[0069] (1) Preparation of oxide materials A raw material powder was prepared using various oxide raw materials, carbonate raw materials, etc. so that the three components Na2O, Fe2O3, and SiO2, among the compositions listed in Table 1, had the compositions shown in Table 1. The obtained raw material powder was placed in a melting vessel and melted at 1400°C in an electric heating furnace in the atmosphere, after which it was poured between a pair of cooling rollers and formed into a film. The obtained film-like molded product was pulverized in a ball mill to produce a matrix material powder with an average particle size of 2 μm.

[0070] The obtained matrix material powder and SnO raw material powder were mixed to obtain the composition shown in Table 1, and the mixture and stainless steel beads were placed in a 45 mL planetary ball mill (device name: Fritch PULVERISETTE7 Classic Line) and subjected to mechano-milling at an orbital rotation speed of 700 rpm for 1 hour in an air atmosphere to produce an oxide material powder. The structure of the obtained oxide material powder was identified by powder X-ray diffraction measurement (XRD), which revealed that it was amorphous and no crystals were detected.

[0071] (2) Preparation of negative electrode active material The obtained oxide material powder was heat-treated in a mixed gas atmosphere of 90% by volume of N2 and 10% by volume of H2 at an ambient temperature of 450°C for 10 hours. The heat-treated oxide material was crushed using a mortar and pestle to obtain a negative electrode active material powder with an average particle size of 2 μm. The structure of the negative electrode active material powder was examined by XRD, and crystals were precipitated with the crystal types and amounts shown in Table 1.

[0072] (3) Preparation of the negative electrode A negative electrode active material powder, carbon black as a conductive additive, and thermosetting polyimide resin as a binder were weighed out in a mass ratio of 80:5:15, and dehydrated N-methylpyrrolidone was added to prepare a slurry. The resulting slurry was coated onto copper foil, vacuum dried at 80°C for 1 hour and then at 200°C for 6 hours, and then pressed between a pair of rotating rollers to obtain an electrode sheet. This electrode sheet was punched out to a diameter of 16 mm using an electrode punching machine to prepare a negative electrode.

[0073] (4) Preparation of test battery A test battery was fabricated by stacking the resulting negative electrode with a 16 mm diameter polypropylene porous membrane separator dried under reduced pressure at 70°C for 8 hours and metallic sodium as the counter electrode, and then impregnating the electrode with an electrolyte. The electrolyte was a 1M NaPF6 solution / EC:DEC = 1:1 (EC = ethylene carbonate, DEC = diethyl carbonate). The test battery was assembled in an argon atmosphere with a dew point below -70°C.

[0074] (5) Charge / discharge test The fabricated test battery was subjected to CC (constant current) charging (sodium ion absorption into the negative electrode active material) at 25°C from the open circuit voltage to 0 V, and the amount of electricity charged to a unit mass of the negative electrode active material (charge capacity) was calculated. Next, CC discharging (sodium ion release from the negative electrode active material) was performed from 0 V to 2 V, and the amount of electricity discharged from a unit mass of the negative electrode active material (discharge capacity) was calculated. The C rate was 0.1 C. These charge / discharge cycles were counted as one cycle, and 50 charge / discharge cycles were performed. From these results, the coulombic efficiency (discharge capacity / charge capacity) of each cycle was calculated.

[0075] Fig. 1 is a diagram showing the battery characteristics at each cycle of the test battery prepared in Example 3. Fig. 2 is a diagram showing the battery characteristics at each cycle of the test battery prepared in Comparative Example 1. As shown in Fig. 1, the test battery prepared in Example 3 exhibits stable battery characteristics such as charge capacity, discharge capacity, and coulomb efficiency from 2 to 50 cycles. On the other hand, the test battery prepared in Comparative Example 1 exhibits unstable battery characteristics such as charge capacity, discharge capacity, and coulomb efficiency, particularly from 15 to 30 cycles.

[0076] Furthermore, the maximum and minimum values ​​of the coulombic efficiency at 2 cycles to 50 cycles for each example and comparative example are shown in Table 1. As shown in Table 1, examples 1 to 4 contained Fe2O3 in their compositions and precipitated FeSn2 at 2 mass % to 53 mass %, so the variation in the coulombic efficiency at 2 cycles to 50 cycles was small. On the other hand, comparative example 1 did not contain Fe2O3 in its composition and did not precipitate Fe-Sn alloys such as FeSn2, so the variation in the coulombic efficiency at 2 cycles to 50 cycles was large. [Industrial Applicability]

[0077] The negative electrode active material of the present invention can be suitably used for sodium ion secondary batteries used as main power sources for mobile communication devices, portable electronic devices, electric bicycles, electric motorcycles, electric vehicles, etc.

Claims

1. SiO ２ and an Fe—Sn-based alloy.

2. In terms of oxide mole percentage, SnO 30% to 90%, SiO ２ 2% to 69%, Fe ２ O ３ 2. The negative electrode active material for a sodium ion secondary battery according to claim 1, wherein the negative electrode active material contains 1% to 20% of sodium ion.

3. The Fe—Sn alloy is FeSn ２ 3. The negative electrode active material for a sodium ion secondary battery according to claim 1, wherein

4. 3. The negative electrode active material for a sodium ion secondary battery according to claim 1, characterized in that it is made of crystallized glass in which the Fe—Sn alloy is precipitated in a matrix made of the amorphous phase.

5. 3. The negative electrode active material for a sodium ion secondary battery according to claim 1, wherein the degree of crystallinity is 30% by mass or more and 99% by mass or less.

6. 3. The negative electrode active material for a sodium ion secondary battery according to claim 1, wherein the amount of crystals of the Fe—Sn-based alloy is 0.5 mass % or more and 70 mass % or less.

7. 3. The negative electrode active material for a sodium ion secondary battery according to claim 1, further comprising β-Sn.

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