Negative electrode active material for secondary battery and secondary battery using the same

Abstract

The present invention addresses the problem of providing: a negative electrode active material that can achieve both, at high levels, the high capacity and the improvement in cycle characteristics of a secondary battery; and a secondary battery using the same.　This negative electrode active material is for a secondary battery and is formed of composite particles between carbon black particles as active materials and degradation reducing material particles that reduce degradation of active material characteristics of the carbon black particles caused by performing charging and discharging. The degradation reducing material particles have a particle diameter larger than the diameter of the carbon black particles, and do not act as an active material. The composite particles are mixture particles between the carbon black particles and the degradation reducing material particles.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material for a secondary battery and a secondary battery using the same. [Background technology]

[0002] In recent years, secondary batteries have been used in a wide range of fields and industries, including electric vehicles and mobile phones. Among these, lithium-ion batteries are particularly popular, boasting higher voltages and capacities than their predecessors, such as nickel-cadmium batteries. However, lithium, the primary material in lithium-ion batteries, is not cheap or readily available due to uneven regional distribution. To address this issue, various alternative elements are currently being explored. Among these, sodium has attracted attention as an inexpensive and readily available element with properties similar to those of lithium. However, sodium ions have a large ionic radius, making it difficult for them to penetrate between layers of graphite, a commonly used negative electrode active material. Therefore, further improvements are needed to achieve higher capacities.

[0003] Currently, cases have been reported in which amorphous carbon materials, especially hard carbon, are used as a countermeasure, but further improvements, including detailed investigation of their physical properties, are required (Non-Patent Document 1). Attempts have also been made to improve battery properties by using specific compounds as negative electrode active materials and obtaining composites with carbon materials (Patent Documents 1 and 2), but these technologies only address the electrical conductivity and related charge / discharge performance of batteries, and no studies have been conducted on increasing capacity.

[0004] In response to this issue, the inventors have achieved a certain goal by using carbon black with specific physical properties as the negative electrode active material (Patent Document 3). However, because of the carbon black's specific properties, design flexibility is limited. In particular, increasing the basis weight to increase capacity is difficult because carbon black has a larger specific surface area than conventional active materials, i.e., its small particle size, which significantly adsorbs binders and other substances, resulting in reduced electrode strength, a lower solids content, and severe shrinkage during the drying process. This makes it difficult to fabricate an electrode using carbon black in its raw form, making it impossible to increase the basis weight. Furthermore, because carbon black exhibits large volume fluctuations during charge and discharge, increasing the basis weight results in a deterioration in cycle performance. For these reasons, there is a need for improved handling to increase the basis weight, thereby increasing capacity per electrode area, and improving cycle performance by mitigating volume fluctuations during battery use. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5704143 [Patent Document 2] Patent No. 5850006 [Patent Document 3] Japanese Patent Application Publication No. 2019-117741 [Non-patent literature]

[0006] [Non-Patent Document 1] A. Kano et al., Potential of Sodium-Ion Batteries as a Post-Lithium-Ion Battery, Panasonic Technical Journal Vol. 63 No. 1 May 2017 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a negative electrode active material that can achieve high capacity and improved cycle characteristics of a secondary battery at the same time, and a secondary battery using the same. [Means for solving the problem]

[0008] The present inventors began research to solve the above-mentioned problems. As a result of their research, they discovered that when carbon black is used as the negative electrode active material for sodium-ion secondary batteries, composite particles prepared by mixing carbon black with particles that are larger in particle size than carbon black and do not themselves function as an active material, but are stable during charging and discharging, improve cycle performance. This is believed to be because the particles that are not involved in charging and discharging act as a buffer to mitigate the effects of volumetric fluctuations of the carbon black and maintain the electrode structure. When used as a negative electrode active material, the composite particles obtained in this way can suppress deterioration of the active material properties due to volumetric fluctuations of the carbon black, thereby improving the cycle performance of secondary batteries. Furthermore, because of its large specific surface area (i.e., small particle size), carbon black significantly adsorbs binders, etc., resulting in reduced electrode strength and solids content, and severe shrinkage during the drying process, making it difficult to fabricate electrodes. However, they found that increasing the basis weight of carbon black, which was previously thought to be impossible, can be used to increase the capacity of secondary batteries. To the best of the inventors' knowledge, no prior art attempts have been made to achieve both high capacity and excellent cycle characteristics by granulating and compounding carbon black with a material that has a larger particle size than carbon black and can form a stable skeleton without structural collapse during charge and discharge, when using carbon black as the negative electrode active material for sodium-ion secondary batteries as in the present invention. Furthermore, the negative electrode active material formed by the composite of the present invention is a completely novel material that possesses optimal properties as a negative electrode active material, and exhibits significant effects not achievable by prior art. Furthermore, the negative electrode active material of the present invention can be used not only in sodium-ion secondary batteries but also in other secondary batteries.

[0009] That is, the present invention is specified by the following items. (1) A negative electrode active material for a secondary battery that is a composite particle of carbon black particles as an active material and degradation-reducing material particles that reduce the degradation of the active material properties of the carbon black particles due to charge and discharge, wherein the degradation-reducing material particles have a particle diameter larger than that of the carbon black particles and do not function as an active material, and the composite particle is a mixture particle of the carbon black particles and the degradation-reducing material particles. (2) The negative electrode active material for a secondary battery according to (1) above, wherein the composite particles contain nanofibers. (3) The negative electrode active material for a secondary battery according to (1) or (2) above, wherein the composite particles have an average particle size in the range of 0.5 to 30 μm, and the degradation reducing material particles have an average particle size in the range of 0.4 to 10 μm. (4) The negative electrode active material for a secondary battery according to any one of (1) to (3) above, wherein the degradation reducing material particles are metal oxide particles. (5) A secondary battery comprising a negative electrode having an active material layer containing the negative electrode active material for secondary batteries according to any one of (1) to (4) above. (6) The secondary battery according to (5) above, wherein sodium is used as a carrier. (7) An electrical device using the secondary battery described in (5) or (6) above. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a negative electrode active material that can highly simultaneously achieve both high capacity and improved cycle characteristics of a secondary battery, and a secondary battery using the same. In particular, it is possible to provide a negative electrode active material that can highly simultaneously achieve both high capacity and improved cycle characteristics of a sodium ion secondary battery, and a sodium ion secondary battery using the same. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a scanning electron microscope image (secondary electron image) of the composite particles obtained in Examples 1 to 4. [Figure 2] FIG. 2 is a scanning electron microscope image (backscattered electron image) of the composite particles obtained in Examples 1 to 4. DETAILED DESCRIPTION OF THE INVENTION

[0012] The negative electrode active material for a secondary battery of the present invention is a composite particle of carbon black particles as an active material and degradation-reducing material particles that reduce the degradation of the active material properties of the carbon black particles due to charge and discharge, where the degradation-reducing material particles have a particle diameter larger than that of the carbon black particles and do not function as an active material, and the composite particle is a mixture particle of the carbon black particles and the degradation-reducing material particles. The carbon black particles in the present invention are not particularly limited, and may be, for example, a carbon black particle having a nitrogen adsorption specific surface area of ​​5 to 2000 m2 measured by the method described in JIS K6217-2:2017. 2 / g. Carbon black with such specific surface area has an average particle diameter of about 10 to 200 nm. The degradation reducing material in the degradation reducing material particles of the present invention is an inorganic substance that does not itself act as an active material and is resistant to oxidation and reduction reactions during charging and discharging within the charge-discharge voltage range of the carbon black, and the degradation reducing material particles are particles made of a degradation reducing material and have a larger particle diameter and density than carbon black particles. If the degradation reducing material particles undergo oxidation and reduction reactions during charging and discharging, the irreversible capacity increases, which causes a decrease in the energy density of the battery. The degradation reducing material particles of the present invention are not particularly limited as long as they have these characteristics. Examples include metal oxides such as aluminum oxide (Al2O3), titanium oxide (TiO2), zirconium oxide (ZrO2), silicon oxide (SiO2), calcium oxide (CaO), magnesium oxide (MgO), cerium oxide (CeO), and yttrium oxide (Y2O3), nitrides such as aluminum nitride (AlN) and boron nitride (BN), and carbides such as tungsten carbide (WC), silicon carbide (SiC), and boron carbide (B4C).Of these, materials that are chemically and physically stable, water-resistant, highly heat-resistant, and have high thermal conductivity are preferred. Among these, oxide materials are preferred from the viewpoints of carbon black itself not acting as an active material within the charge / discharge voltage range, preventing oxidation and reduction reactions during charge / discharge, providing excellent thermal conductivity for the composite particles, and increasing the volumetric energy density of the active material. More specifically, aluminum oxide (Al2O3), titanium oxide (TiO2), zirconium oxide (ZrO2), or yttrium oxide (YO3) is preferred, with aluminum oxide (Al2O3) being particularly preferred due to its low cost. Aluminum oxide (Al2O3) may be any of α-alumina, β-alumina, β-double prime alumina, and γ-alumina. In the present invention, the particle size refers to the average particle size, and the average particle size of the degradation reducing material particles is preferably 0.4 to 10 μm. If the particle size is less than 0.4 μm, the specific surface area of ​​the degradation reducing material particles is large, requiring a large amount of binder to produce a high-area electrode. If the particle size exceeds 10 μm, the slurry is prone to settling.It is more preferably in the range of 0.6 to 7 μm. In the present specification, the expressions "A to B μm" or "within the range of A to B μm" both mean A μm or more and B μm or less.

[0013] The composite particles of the present invention are mixture particles of carbon black particles and degradation-reducing material particles. When the total of the carbon black particles and degradation-reducing material particles is taken as 100% by mass, the composite particles may contain 10 to 90% by mass of carbon black particles and 10 to 90% by mass of degradation-reducing material particles. A higher carbon black particle content results in a higher capacity active material, while a higher degradation-reducing material particle content results in a longer life. Furthermore, when the degradation-reducing material particles are metal oxides, the active material has even greater thermal conductivity and volumetric energy density. The mixture particles of the present invention include particles in which carbon black particles are attached to the surface of aggregates of degradation-reducing material particles, particles in which carbon black particles are present between the degradation-reducing material particles within the aggregates, and particles in which carbon black particles are attached to the surface of aggregates of degradation-reducing material particles and between the degradation-reducing material particles within the aggregates. The composite particles of the present invention are preferably mixed particles in which carbon black particles are attached to the surface of aggregates of degradation-reducing material particles, and carbon black particles are further present between the degradation-reducing material particles within the aggregates. The average particle size of the composite particles is not particularly limited as long as it is within a range that does not interfere with the preparation of an electrode slurry and application to a current collector, etc., but a preferred range is, for example, 0.5 to 30 μm. Composite particles with an average particle size within this range enable the production of high-basis-weight electrodes with a small amount of electrode binder. The preferred average particle size is 0.6 μm to 20 μm. The composite particles may contain a granulation binder in addition to the carbon black particles and the degradation-reducing material particles. The binder is not particularly limited as long as it bonds the carbon black particles and the degradation-reducing material particles together and between the individual particles, and examples thereof include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide, styrene butadiene rubber (SBR), styrene-ethylene-butylene-styrene copolymer (SEBS), polyvinyl alcohol (PVA), acrylic binders, etc. The composite particles may contain components such as additives such as carboxymethyl cellulose (CMC) and nanofibers.Incorporating a binder into the composite particles has the effect of preventing powder separation in electrode slurries that mix powders with low specific gravity, such as carbon black, with powders with high specific gravity, such as metal oxides. Without a binder, the composite particles may lack sufficient mechanical strength. In this case, the composite particles are prone to being crushed during the slurry mixing process, and separation occurs due to differences in the specific gravities of the composite particle raw materials, making it difficult to obtain a normal slurry. The composite particles of the present invention can be used as a negative electrode active material for secondary batteries. The amount of binder contained in the composite particles is not particularly limited, but is preferably 20% by mass or less of the total composite particles. That is, the binder is included as needed. If the amount exceeds 20% by mass, the proportion of active material contributing to the charge / discharge reaction is low, resulting in a low electrode capacity density.

[0014] The composite particles of the present invention may further contain nanofibers instead of or in addition to the binder. Nanofibers are fibrous materials with a diameter of 1 to 1,000 nm. The nanofibers of the present invention are not particularly limited, but examples include polymer nanofibers made of polypropylene, polyethylene terephthalate, etc., bionanofibers made of cellulose, etc., carbon fibers made of carbon nanotubes, vapor-grown carbon fibers, etc. Adding nanofibers to the composite particles can better maintain the structure of the composite particles. The content of the nanofibers in the composite particles is preferably 0.1 to 5% by mass relative to the total mass of the carbon black particles and the degradation-reducing material particles. The fiber diameter of the nanofibers is preferably within the range of 1 to 500 nm. The fiber length is preferably within the range of 0.5 μm to 10 mm.

[0015] The method for producing the composite particles of the present invention is not particularly limited, but can be, for example, by mixing carbon black particles and degradation-reducing material particles in a liquid dispersion medium to prepare a suspension, and then granulating the prepared suspension by spraying using a spray dryer to obtain composite granules. The dispersion medium used to prepare the suspension is not particularly limited, but examples thereof include water, alcohol, and organic solvents. A binder may be added when preparing the suspension, and examples of binders include PVdF, PI, SBR, and acrylic binders. Furthermore, additives such as CMC, alginate, and xanthan gum may be added when preparing the suspension. Nanofibers may also be added. The binders, additives, and nanofibers may be used alone or in combination.

[0016] The secondary battery of the present invention is characterized by comprising a negative electrode having an active material layer containing the negative electrode active material for a secondary battery of the present invention. The active material layer in the negative electrode of the secondary battery of the present invention may contain a binder and other additive components in addition to the negative electrode active material for a secondary battery of the present invention. In other words, in this disclosure, the active material layer refers to a composite portion containing an active material provided on the surface of a current collector. The negative electrode of the present invention can be obtained, for example, by dispersing the negative electrode active material for a secondary battery of the present invention and a binder in a solvent to prepare an electrode slurry, applying the prepared electrode slurry to a current collector, and drying it. Current collectors commonly used in electrodes, such as aluminum foil, can be used as current collectors. The binder used in preparing the electrode slurry is not particularly limited, but examples include polyvinylidene fluoride, polyimide, SBR, and acrylic binders. The solvent used in preparing the electrode slurry is not particularly limited, but examples include N-methyl-2-pyrrolidone (NMP), acetone, water, and the like. The secondary battery of the present invention has a structure that is common to secondary batteries, such as a positive electrode, a separator, and an electrolyte layer (liquid or solid), except for the negative electrode of the present invention, and these components can be those used in common secondary batteries. The secondary battery of the present invention can be suitably used as a sodium ion secondary battery using sodium metal as the counter electrode.

[0017] The structure of the secondary battery is not particularly limited, and existing forms and structures such as stacked and wound types can be adopted. That is, an electrode group in which a positive electrode and a negative electrode are stacked or wound facing each other with a separator interposed therebetween is immersed in an electrolyte and sealed to form an electricity storage device. Alternatively, an electrode group in which a positive electrode and a negative electrode are stacked or wound facing each other with a solid electrolyte interposed therebetween is sealed to form a secondary battery.

[0018] The electrolyte used in this battery may be any liquid or solid that can transfer alkali metal ions from the positive electrode to the negative electrode or from the negative electrode to the positive electrode. That is, the same electrolytes as those used in known secondary batteries that use nonaqueous electrolytes can be used. Examples include electrolytic solutions, gel electrolytes, solid electrolytes, ionic liquids, and molten salts.

[0019] In other words, in the present disclosure, a secondary battery refers to a device or element that has at least a positive electrode and a negative electrode and that can repeatedly extract chemically or physicochemically stored energy in the form of electric power. Examples include secondary batteries that use non-aqueous electrolytes, such as lithium ion batteries, sodium ion batteries, and potassium ion batteries, and ion capacitors that use secondary battery electrodes. Among these, batteries that use sodium as the carrier (sodium ion batteries and sodium ion capacitors) are preferred, as they are inexpensive and easily available.

[0020] Secondary batteries (especially sodium ion batteries) using the negative electrode material of the present disclosure do not contain rare metals and have high capacity, and when the degradation-reducing material particles are metal oxides, the batteries have even better thermal conductivity and volumetric energy density. For this reason, examples of such items include air conditioners, washing machines, televisions, refrigerators, freezers, cooling equipment, laptop computers, tablets, smartphones, computer keyboards, computer displays, desktop computers, CRT monitors, computer racks, printers, all-in-one computers, mice, hard disks, computer peripherals, irons, clothes dryers, window fans, walkie-talkies, blowers, ventilation fans, televisions, music recorders, music players, ovens, microwave ovens, toilet seats with cleaning functions, heated air heaters, car stereos, car navigation systems, flashlights, humidifiers, portable karaoke machines, ventilation fans, dryers, air purifiers, mobile phones, emergency lights, game consoles, blood pressure monitors, coffee grinders, coffee makers, kotatsu tables, copy machines, disc changers, radios, shavers, juicers, shredders, water purifiers, lighting equipment, dehumidifiers, dish dryers, rice cookers, stereos, stoves, speakers, trouser presses, vacuum cleaners, body fat meters, weighing scales, health meters, movie players, electric carpets, electric rice cookers, rice cookers, and electric scalpels. desk lamps, electric kettles, electronic game consoles, portable game consoles, electronic dictionaries, electronic organizers, microwave ovens, induction cookers, calculators, electric carts, electric wheelchairs, power tools, electric toothbrushes, foot warmers, haircutting equipment, telephones, watches, intercoms, air circulators, electric insect killers, copiers, hot plates, toasters, hair dryers, electric drills, water heaters, panel heaters, grinders, soldering irons, video cameras, VCRs, fax machines, fan heaters, food processors, futon dryers, headphones, It can be used as a power source for a variety of electrical devices, such as air pots, heated carpets, microphones, massage machines, miniature light bulbs, mixers, sewing machines, rice cake makers, floor heating panels, lanterns, remote controls, refrigerators / heaters, water coolers, freezers, air coolers, word processors, whisks, GPS, electronic musical instruments, motorcycles, toys, lawn mowers, floats, bicycles, two-wheeled motor vehicles, automobiles, hybrid cars, plug-in hybrid cars, electric cars, trains, ships, airplanes, submarines, aircraft, artificial satellites, and emergency power supply systems. [Example]

[0021] EXAMPLES The present invention will be explained in more detail below by showing examples and comparative examples, but the present invention is not limited to these examples in any way.

[0022] [Examples 1 to 6, Comparative Examples 1 and 2] (Production of composite particles) Carbon black (manufactured by Asahi Carbon Co., Ltd.), a degradation reducing agent, cellulose nanofibers (CeNF: fiber diameter 3-5 nm, fiber length 2-5 μm), and a water-soluble acrylic binder were weighed out in the blending ratios shown in Table 1 and mixed in ion-exchanged water. In Examples 1-4 and 6, α-alumina particles (low soda alumina LS-719A, manufactured by Nippon Light Metal Co., Ltd.) with an average particle diameter of 1 μm were used as the degradation reducing agent, and in Example 5, yttrium oxide particles (manufactured by Kanto Chemical Co., Inc.) with an average particle diameter of 1.8 μm were used. In Examples 1-6 and Comparative Example 1, carbon black with an oil absorption capacity of 193 ml / 100 g and a nitrogen adsorption specific surface area of ​​339 m was used. 2 In Comparative Example 2, 90% by mass of carbon black was used, the oil absorption amount was 196 ml / 100 g, and the nitrogen adsorption specific surface area was 53 m 2 2% by mass of carbon black (1000 ppm / g) was blended. Subsequently, the mixture was diluted with ion-exchanged water to a viscosity suitable for spraying, and the diluted solution was spray-dried to granulate, producing composite particles. Images of the composite particles obtained in Examples 1 to 4 observed with a scanning electron microscope are shown in Figures 1 and 2. The average particle diameters of the composite particles obtained were 14.7 μm in Examples 1 to 4, 4.2 μm in Example 5, 10.3 μm in Example 6, 15.0 μm in Comparative Example 1, and 26.5 μm in Comparative Example 2. The average particle diameters were measured using a particle measuring device (laser diffraction / scattering particle size distribution measuring device, Horiba, Ltd.).

[0023] [Table 1]

[0024] Each of the composite particles of Examples and Comparative Examples manufactured according to the formulations in Table 1 was used as the negative electrode active material to fabricate a secondary battery for evaluation in the following procedure. (Battery manufacturing method) Electrodes were fabricated using a conventional method. A slurry was prepared using the composite particles produced in the Examples and Comparative Examples as the negative electrode active material, polyvinylidene fluoride (PVdF) or polyimide (PI) as the electrode binder, acetylene black (AB) as the conductive material, and N-methyl-2-pyrrolidone as the solvent. Table 2 shows the blending ratio of each component, assuming the total mass of the composite particles, binder, and conductive material to be 100%. This slurry was applied to aluminum foil, dried, and pressed to form an electrode. Sodium metal was used for the counter electrode. A coin-shaped cell was fabricated by combining the above electrode and counter electrode, glass filter paper (GA-100, ADVANTEC), a separator (Celgard #2325, Celgard), and the electrolyte listed in Table 3 to obtain a secondary battery. In Table 3, NaPF6 represents sodium hexafluorophosphate, EC represents ethylene carbonate, DEC represents diethyl carbonate, PC represents propylene carbonate, and EMC represents ethyl methyl carbonate.

[0025] [Table 2]

[0026] The characteristics of the obtained secondary battery were measured by the following method using a charge / discharge tester (BLS5500, Measuring Instrument Center). The results are shown in Table 3. The basis weight in Table 3 is the mass of the composite particles present on the area of ​​a circle with a diameter of 11 mm. (1) Charge capacity, discharge capacity Both the charge capacity and the discharge capacity were measured at a temperature of 30°C, a rate of 0.13 mA / g, and a cutoff voltage of 0.001 to 3.0V. (2) Cycle characteristics (capacity retention rate) An index of the degree of decrease from the initial battery capacity after 10 charge / discharge cycles was calculated (capacity retention rate = 10th discharge capacity / 1st discharge capacity × 100). The larger the capacity retention rate, the smaller the degree of decrease, which is preferable.

[0027] [Table 3]

[0028] The electrode slurries prepared using the composite particles obtained in Examples 1 to 6 as negative electrode active materials exhibited viscosities suitable for coating. The surfaces were smooth after application and drying to aluminum foil current collectors, and the bonding strength between the aluminum foil and the active material layer was also sufficiently strong. On the other hand, the composite particles of Comparative Example 1 caused the active material layer to peel or fall off from the current collector during the drying process, making it impossible to fabricate an electrode. Since no granulation binder was used in Comparative Example 1, the composite particles were crushed and pulverized by the shear force generated during the kneading process of the slurry, which increased the cohesive stress of the slurry during drying, presumably making them more susceptible to peeling or falling off from the current collector. Furthermore, as is clear from the results in Table 3, the secondary battery using the negative electrode active material of the present invention exhibited better values ​​in both discharge capacity and cycle characteristics than the secondary battery of Comparative Example 2, demonstrating the excellent characteristics of achieving both high capacity and excellent cycle characteristics.

[0029] As described above, preferred embodiments of the present invention have been described with reference to the drawings, but various additions, modifications, or deletions are possible without departing from the spirit of the present invention. For example, the types and proportions of nanofibers, granulation binders, etc. are not limited to the numerical values ​​in the above-described embodiments. Therefore, such additions, modifications, or deletions are also included within the scope of the present invention. [Industrial Applicability]

[0030] The negative electrode active material for a secondary battery of the present invention can achieve both high capacity and improved cycle characteristics of the secondary battery despite the use of carbon black, and therefore can be suitably used as a negative electrode active material for a secondary battery, particularly as a negative electrode active material for a sodium ion secondary battery. Furthermore, since the secondary battery of the present invention can achieve both high capacity and improved cycle characteristics, it can be suitably used in fields where various secondary batteries are used. The secondary battery of the present invention can be suitably used, particularly as a sodium ion secondary battery.

Claims

1. A negative electrode active material for a sodium ion secondary battery, which is a composite particle of carbon black particles as an active material and degradation reducing material particles that reduce degradation of the active material properties of the carbon black particles due to charge and discharge, the degradation-reducing material particles are aluminum oxide particles and / or yttrium oxide particles, which are metal oxide particles having a particle diameter larger than that of the carbon black particles and do not function as an active material; The composite particles are a mixture of the carbon black particles and the degradation reducing material particles.

2. 2. The negative electrode active material for a sodium ion secondary battery according to claim 1, wherein the composite particles contain nanofibers.

3. 3. The negative electrode active material for a sodium ion secondary battery according to claim 1, wherein the composite particles have an average particle size in the range of 0.5 to 30 μm, and the degradation reducing material particles have an average particle size in the range of 0.4 to 10 μm.

4. A sodium ion secondary battery comprising a negative electrode having an active material layer containing the negative electrode active material for sodium ion secondary batteries according to claim 1 or 2.

5. An electrical device using the secondary battery according to claim 4.

Images