Negative electrode active material for sodium ion secondary batteries

Abstract

Provided is a negative electrode active substance for a sodium ion secondary battery having a low initial irreversible capacity.　The negative electrode active substance for a sodium ion secondary battery is characterized by being formed from at least one selected from Fe2O3 and CuO and a crystallized glass obtained by precipitation of Bi metal into a matrix containing SiO2.

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] Metallic Bi has a high theoretical capacity of 385 mAhg when alloyed with sodium, and is therefore known as a promising candidate for the negative electrode material in sodium-ion secondary batteries (see, for example, Patent Document 1). Metallic Bi converts to Bi+3Na during charge and discharge. + +3e - The reaction between Bi and BiNa3 is repeated. However, the volume of Bi metal changes by a factor of 2.4 during alloying during charging and discharging, which causes a decrease in capacity due to electrode destruction. As a method for alleviating the volume change during charging and discharging, a method of precipitating Bi metal in a glass matrix has been proposed (see, for example, Patent Document 2 and Non-Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-198000 A [Patent Document 2] Japanese Patent Publication No. 2020-077615 [Non-patent literature]

[0005] [Non-Patent Document 1] Y. Omori, et. al, Journal of the Ceramic Society of Japan, 126, 820-825(2018) Summary of the Invention [Problem to be solved by the invention]

[0006] In glass-ceramics made by precipitating metallic Bi, amorphous components such as SiO2, P2O5, and B2O3 contained in the glass matrix act as buffers to mitigate the expansion and contraction of the Bi component. However, these amorphous components absorb Na ions during the initial charge, which can easily cause initial irreversible capacity problems.

[0007] 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. [Means for solving the problem]

[0008] The negative electrode active material for a sodium ion secondary battery of the present invention is characterized by comprising a crystallized glass in which metallic Bi is precipitated in a matrix containing at least one selected from Fe2O3 and CuO, and SiO2.

[0009] The negative electrode active material for a sodium ion secondary battery of the present invention contains at least one selected from Fe2O3 and CuO in the matrix. Since Fe2O3 and CuO themselves function as active materials that absorb and release Na ions and electrons, the initial irreversible capacity caused by Na ion absorption by the matrix can be suppressed, and as a result, the initial charge-discharge efficiency can be improved. Furthermore, Fe2O3 and CuO are components that function as network-forming oxides and promote amorphization. As a result, Fe2O3 and CuO function as components that mitigate the expansion and contraction of the Bi component, and can also improve cycle characteristics. Furthermore, Fe2O3 acts as a component that binds Fe between Fe ions. 2+ -O-Fe 3+ ←→ Fe 3+-O-Fe 2+ As shown above, electrons hop over the Fe ions, helping to transfer electrons as metallic Bi absorbs and releases Na ions, improving the conductivity of the oxide matrix component. Furthermore, CuO absorbs Na ions and electrons during charging to form metallic Cu, improving the conductivity of the oxide matrix component. This also improves rapid charge / discharge characteristics.

[0010] The negative electrode active material for a sodium ion secondary battery of the present invention preferably contains, in mole percent calculated as oxides, 30 to 90% Bi2O3, 2 to 30% SiO2, and 4 to 50% Fe2O3+CuO.

[0011] The negative electrode active material for a sodium ion secondary battery of the present invention preferably further comprises metallic Cu precipitated in the matrix. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a negative electrode active material for a sodium ion secondary battery having a low initial irreversible capacity. DETAILED DESCRIPTION OF THE INVENTION

[0013] 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 comprising crystallized glass formed by depositing metallic Bi in a matrix containing at least one selected from Fe2O3 and CuO, and SiO2. Specifically, the negative electrode active material of the present invention preferably contains, in mole percent (oxide equivalent), 30 to 90% Bi2O3, 2 to 30% SiO2, and 4 to 50% Fe2O3 + CuO. The reasons for limiting the composition in this way are explained below. In the following description of the composition, "%" means "mol %" unless otherwise specified.

[0014] Bi2O3 is an active material component that serves as a site for absorbing and releasing sodium ions. The Bi2O3 content is preferably 30 to 90%, 40 to 80%, 50 to 75%, 60 to 70%, and particularly preferably 65 to 68%. If the Bi2O3 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 Bi2O3 content is too high, the amorphous component in the negative electrode active material becomes relatively small, making it difficult to alleviate the volume change associated with the absorption and release of sodium ions during charge / discharge, and therefore, the cycle characteristics are likely to deteriorate.

[0015] SiO2 is a component that functions as a network-forming oxide and promotes amorphization. This encompasses sodium ion absorption / desorption sites in the Bi component, improving cycle characteristics. The SiO2 content is preferably 2 to 30%, 5 to 20%, and particularly 7 to 15%. If the SiO2 content is too low, it becomes difficult to achieve the above effects. On the other hand, if the SiO2 content is too high, ionic conductivity decreases and discharge capacity tends to decrease. Furthermore, since the Bi component becomes relatively small, charge / discharge capacity tends to decrease.

[0016] Fe2O3 and CuO are components that function as active materials that absorb and release sodium ions and electrons. Furthermore, Fe2O3 and CuO function as network-forming oxides and promote amorphization. This allows them to mitigate the expansion and contraction of the Bi component, improving cycle characteristics. Furthermore, they improve the conductivity of the oxide matrix component in the negative electrode active material, improving rapid charge / discharge characteristics. The content of Fe2O3+CuO is preferably 4 to 50%, 4 to 45%, 10 to 30%, and particularly 15 to 25%. If the content of Fe2O3+CuO is too low, it becomes difficult to achieve the above-mentioned effects. On the other hand, if the content of Fe2O3+CuO is too high, ionic conductivity decreases, tending to result in a decrease in discharge capacity.

[0017] The negative electrode active material of the present invention may contain the following components in addition to the above components.

[0018] Na2O is a component that improves the ionic conductivity of the oxide matrix other than the Bi component. The Na2O content is preferably 0 to 50%, 1 to 45%, 3 to 43%, 5 to 40%, and particularly preferably 7 to 35%. If the Na2O content is too high, a large amount of heterogeneous crystals (for example, crystals containing Na2O and SiO2) are formed, which tends to reduce cycle characteristics.

[0019] P2O5, like SiO2, functions as a network-forming oxide and promotes amorphization. This serves to occlude sodium ion absorption / desorption sites in the Bi component, improving cycle characteristics. The P2O5 content is preferably 0 to 30%, 2 to 30%, 5 to 20%, and particularly 7 to 15%. If the P2O5 content is too high, the water resistance of the negative electrode active material tends to decrease. Furthermore, the charge / discharge capacity tends to decrease due to the relative decrease in the Bi component.

[0020] Like SiO2, B2O3 also functions as a network-forming oxide and promotes amorphization. This allows it to occlude sodium ion storage and release sites in the Bi component, improving cycle characteristics. The B2O3 content is preferably 0-30%, 2-30%, 5-20%, and particularly 7-15%. If the B2O3 content is too high, the coordination bond to the Bi component becomes strong, increasing the initial charge capacity and, as a result, the initial irreversible capacity tends to increase. Furthermore, the relative decrease in the Bi component tends to decrease the charge / discharge capacity.

[0021] The content of P2O5+SiO2+B2O3 is preferably 2 to 30%, 5 to 20%, and particularly preferably 7 to 15%. If the content of P2O5+SiO2+B2O3 is too low, the volume change of the Bi component accompanying the absorption and release of sodium ions during charge and discharge cannot be alleviated, causing structural deterioration and making the cycle characteristics more likely to deteriorate. On the other hand, if the content of P2O5+SiO2+B2O3 is too high, the Bi component becomes relatively small, which tends to reduce the charge and discharge capacity. 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, and it is acceptable to include a component that is not contained (i.e., a content of 0%).

[0022] The negative electrode active material of the present invention may contain TiO2, MnO, ZnO, MgO, CaO, or Al2O3 in a total amount ranging from 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.

[0023] The negative electrode active material of the present invention has metallic Bi precipitated within it. The metallic Bi can be identified by powder X-ray diffraction (XRD) using CuKα radiation. Specifically, in the diffraction profile obtained by the measurement, diffraction lines having peak positions at 2θ values ​​of 27.2°, 37.9°, and 39.6° can be attributed to the crystalline phase of metallic Bi (hexagonal crystal system, space group R-3m(166)). The crystalline amount of metallic Bi is preferably 40% to 99.9%, 40% to 90%, 40% to 75%, 45% to 70%, or 50% to 65% by mass. If the crystalline amount of metallic Bi is too high, the volume expansion of the negative electrode active material increases when Na ions are absorbed during the initial charge, causing cracks in the electrode, which disrupts electronic conduction and increases the irreversible capacity. On the other hand, if the crystalline amount of metallic Bi is too low, the irreversible capacity tends to increase.

[0024] The negative electrode active material of the present invention may have metallic Cu precipitated therein. The metallic Cu improves the conductivity of the oxide matrix component and has the effect of improving the discharge capacity and rapid charge / discharge characteristics. The metallic Cu can be identified by powder X-ray diffraction measurement (XRD) using CuKα radiation. Specifically, in the diffraction profile obtained by the measurement, the diffraction lines having peak positions at 2θ values ​​of 43.6° and 50.7° can be attributed to the metallic Cu crystalline phase (cubic system, space group Fm-3m). The amount of metallic Cu crystals is preferably 0% to 20%, 3% to 20%, 5% to 15%, or 7% to 12% by mass. If the metallic Cu crystal content is too high, the ionic conductivity decreases, tending to reduce the discharge capacity.

[0025] The negative electrode active material of the present invention may have Bi2O3 crystals or CuBi2O4 precipitated therein, which function as active materials and can further improve the discharge capacity.

[0026] The crystallinity of the negative electrode active material is preferably 30% or more, 40% or more, and particularly preferably 50% 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% or less, and particularly preferably 95% or less.

[0027] The degree of crystallinity is determined from the diffraction line profile at 2θ values ​​of 10 to 60° obtained by powder X-ray diffraction measurement using CuKα radiation. Specifically, from the total scattering curve obtained by subtracting the background from the diffraction line profile, the integrated intensity obtained by peak separation of the broad diffraction line (amorphous halo) at 10 to 45° is defined as Ia, and the sum of the integrated intensities obtained by peak separation of each crystalline diffraction line detected at 10 to 60° is defined as Ic. The degree of crystallinity, Xc, can be calculated from the following formula:

[0028] Xc = [Ic / (Ic+Ia)] x 100(%)

[0029] The shape of the negative electrode active material is not particularly limited, but is usually powder. The average particle size of the negative electrode active material is preferably 0.1 to 20 μm, 0.2 to 15 μm, 0.3 to 10 μm, and particularly preferably 0.5 to 5 μm. The maximum particle size of the negative electrode active material is preferably 150 μm or less, 100 μm or less, 75 μm or less, and particularly preferably 55 μm or less. If the average particle size or maximum particle size is too large, the volume change of the negative electrode active material accompanying the absorption and release of sodium ions during charge and discharge cannot be alleviated, and cycle characteristics tend to deteriorate significantly. On the other hand, if the average particle size is too small, the powder will be poorly dispersed when made into a paste, making it difficult to produce a uniform electrode. In addition, the deposited metallic Bi is easily oxidized by oxygen in the atmosphere.

[0030] 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.

[0031] 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, centrifugal separator, or air classifier.

[0032] 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 Bi2O3 contained in the oxide material to metallic Bi.

[0033] The oxide material is produced by heating and melting raw material powder prepared to have the above-mentioned composition at, for example, 600 to 1200°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.

[0034] The oxide material is preferably amorphous, which facilitates the production of the negative electrode active material of the present invention, which is made of a crystallized glass in which metallic Bi is precipitated in a matrix containing at least one selected from Fe2O3 and CuO, and SiO2. Note that crystals of Bi2O3, Cu2O, etc. may be precipitated inside the oxide material.

[0035] The oxide material is usually in powder form, similar to the negative electrode active material. The average particle size of the oxide material is preferably 0.1 to 20 μm, 0.2 to 15 μm, 0.3 to 10 μm, and particularly preferably 0.5 to 5 μm. The maximum particle size of the oxide material is preferably 150 μm or less, 100 μm or less, 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 resulting negative electrode active material will also be large, which tends to cause the above-mentioned problems. Furthermore, there is a risk that Bi2O3 may not be sufficiently reduced to metallic Bi by the reducing gas. On the other hand, if the average particle size 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.

[0036] The temperature during the heat treatment is preferably 250°C or higher, 300°C or higher, and particularly 400°C or higher. If the heating temperature is too low, the applied thermal energy is small, making it difficult for Bi2O3 in the oxide material to be reduced to metallic Bi. There is no particular upper limit to the heating temperature, but if it is too high, the reduced metallic Bi particles tend to become coarse, 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.

[0037] The heating time 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 Bi2O3 in the oxide material to be reduced to metallic Bi. On the other hand, if the heating time is too long, the reduced metallic Bi particles tend to become coarse, which may significantly reduce the cycle characteristics of the negative electrode active material.

[0038] For the heat treatment, an electric heating furnace, a rotary kiln, a microwave heating furnace, a high-frequency heating furnace, or the like can be used.

[0039] The reducing gas may be at least one gas selected from H2, NH3, CO, H2S, and SiH4. From the viewpoint of ease of handling, at least one gas selected from H2, NH3, and CO is preferred, with H2 being particularly preferred.

[0040] 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 mixing ratio of the inert gas and H2 is preferably 90 to 99.5% inert gas and 0.5 to 10% H2 by volume, more preferably 92 to 99% inert gas and 1 to 8% H2, and even more preferably 96 to 99% inert gas and 1 to 4% H2.

[0041] 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 aggregation of the oxide material during heat treatment, enabling the Bi2O3 in the oxide material to be reduced to metallic Bi in a short time.

[0042] 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 also impart electrical conductivity to the negative electrode active material. Among these, acetylene black, which has excellent electronic conductivity, is preferred.

[0043] 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.

[0044] The negative electrode active material of the present invention can be used as a negative electrode material by adding a binder and a conductive additive.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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]

[0051] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0052] Tables 1 and 2 show Examples 1 to 18 and Comparative Examples 1 and 2.

[0053] [Table 1]

[0054] [Table 2]

[0055] (1) Preparation of oxide materials Raw material powders were prepared using various oxide raw materials, carbonate raw materials, etc. to obtain the compositions shown in Tables 1 and 2. The obtained raw material powders were placed in a melting vessel and melted in an electric heating furnace at 1100°C in air, after which they were cast 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 oxide material powders with an average particle size of 2 μm. The amorphous content and precipitated crystals were examined by XRD, and the results are shown in Tables 1 and 2.

[0056] (2) Preparation of negative electrode active material The obtained oxide material powder was subjected to heat treatment under the conditions shown in Tables 1 and 2. In Tables 1 and 2, "N2:H2 = 97:3" means a mixed gas atmosphere of 97% by volume of N2 and 3% by volume of H2. The oxide material after the heat treatment 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 was examined by XRD, and the crystals shown in Tables 1 and 2 were found to have precipitated.

[0057] (3) Preparation of the negative electrode The negative electrode active material powder, conductive additive (acetylene black), and binder (carboxymethyl cellulose) were weighed out to a mass ratio of 78:5:17, and pure water was added to prepare a slurry. The resulting slurry was coated onto aluminum foil, vacuum dried in a dryer at 70°C, and then pressed between a pair of rotating rollers to obtain an electrode sheet. This electrode sheet was punched out to a diameter of 11 mm using an electrode punching machine to prepare a negative electrode.

[0058] (4) Preparation of test battery The resulting negative electrode was combined with a 16 mm diameter porous polypropylene membrane separator dried under reduced pressure at 70°C for 8 hours, and a sodium counter electrode. The stack was then impregnated with an electrolyte solution to prepare a test battery. The electrolyte solution was a 1M NaPF6 solution / EC:DEC mixture (1:1, EC = ethylene carbonate, DEC = diethyl carbonate). The test battery was assembled in an argon atmosphere with a dew point below -70°C.

[0059] (5) Charge / discharge test The fabricated test batteries were subjected to CC (constant current) charging (sodium ion absorption into the negative electrode active material) at 30°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 (initial charge capacity) was determined. Next, CC discharging (sodium ion release from the negative electrode active material) from 0 V to 3 V was performed, and the amount of electricity discharged from a unit mass of the negative electrode active material (initial discharge capacity) was determined. The C rate was 0.1 C. From these results, the initial irreversible capacity (= initial charge capacity - initial discharge capacity) was calculated. The results are shown in Tables 1 and 2.

[0060] As shown in Tables 1 and 2, in Examples 1 to 18, metallic Bi was precipitated in a matrix containing at least one selected from Fe2O3 and CuO, and SiO2, and therefore the initial discharge capacity was high at 302 to 352 mAh / g and the initial irreversible capacity was low at 70 to 190 mAh / g. On the other hand, in Comparative Examples 1 and 2, the composition did not contain either Fe2O3 or CuO, and therefore the initial discharge capacity was low at 180 to 210 mAh / g and the initial irreversible capacity was high at 238 to 308 mAh / g. [Industrial Applicability]

[0061] The negative electrode active material of the present invention is suitable for use in 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

1. Fe ２ O ３ and CuO, and SiO ２ and the matrix is ​​made of a glass-ceramic material in which metal Bi is precipitated in the matrix. A negative electrode active material for a sodium ion secondary battery, characterized by containing, in mole percent calculated as oxides, 30 to 90% Bi 2 O 3 , 2 to 30% SiO 2 , and 4 to 50% Fe 2 O 3 +CuO.

2. 2. The negative electrode active material for a sodium ion secondary battery according to claim 1, further comprising metallic Cu precipitated in the matrix.

3. A negative electrode active material for a sodium ion secondary battery, comprising crystallized glass in which metallic Bi and metallic Cu are precipitated in a matrix containing CuO and SiO 2 .

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