Method for producing positive electrode active material for alkali ion secondary battery

Abstract

The present invention provides a method for producing a positive electrode active material for alkali ion secondary batteries, the positive electrode active material containing a large amount of a transition metal, while enabling a battery to operate.　A method for producing a positive electrode active material for alkali ion secondary batteries, the positive electrode active material containing 34% by mole or more of CrO + FeO + MnO + CoO + NiO. This method for producing a positive electrode active material for alkali ion secondary batteries is characterized by comprising: a step for preparing a positive electrode active material precursor that contains crystals; and a step wherein at least some of the crystals are melted and amorphized by irradiating the positive electrode active material precursor with light.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a positive electrode active material used as an electrode material for alkali ion secondary batteries such as sodium ion secondary batteries. [Background technology]

[0002] In recent years, polyanion-based materials as well as layered rock salt-based oxides have been investigated as positive electrode active materials for alkali-ion secondary batteries such as sodium-ion secondary batteries (see, for example, Non-Patent Document 1). To achieve high energy density in batteries, it is important to improve the capacity of the materials. For example, maricite-type NaFePO4 crystals are known to have a relatively high theoretical capacity (approximately 155 mAh / g) due to the high proportion of transition metal elements in the crystals. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Prabeer Barpanda et al., Solid State Ionics, 2014(DOI:10.1016 / j.ssi.2014.03.011) Summary of the Invention [Problem to be solved by the invention]

[0004] However, maricite crystals do not have a diffusion path for sodium ions and are electrochemically inactive, so they do not function as a positive electrode active material and cannot operate the battery.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a positive electrode active material for an alkali ion secondary battery that has a high transition metal content and is capable of operating the battery. [Means for solving the problem]

[0006] The method for producing a positive electrode active material for an alkali ion secondary battery of the present invention is a method for producing a positive electrode active material for an alkali ion secondary battery containing, by mole, 34% or more of CrO+FeO+MnO+CoO+NiO. The method includes the steps of preparing a crystalline positive electrode active material precursor and irradiating the positive electrode active material precursor with light to melt the crystalline and at least partially amorphize it. By irradiating the crystalline positive electrode active material precursor with light to melt it, the crystalline structure collapses and the positive electrode active material becomes amorphous, thereby increasing the interatomic distance within the positive electrode active material and forming a three-dimensional conduction path for alkali ions. This allows the positive electrode active material to function as a positive electrode active material and operate a battery. Note that materials containing large amounts of transition metal element components, as described above, are difficult to vitrify using conventional melting methods using a melting vessel. In contrast, the production method of the present invention uses a crystalline positive electrode active material precursor as a raw material and melts the positive electrode active material precursor by irradiating it with light, so that even materials containing large amounts of transition metal element components can be easily amorphized.

[0007] In this specification, "x + y +..." means the total amount of each component. Here, each component does not necessarily need to be contained as an essential component, and it is acceptable for some components not to be contained (i.e., the content is 0%).

[0008] In the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention, the positive electrode active material for an alkali ion secondary battery preferably contains, in mole % calculated as the following oxides, 20 to 55% of Li2O+Na2O, 34 to 70% of CrO+FeO+MnO+CoO+NiO, and 35 to 55% of P2O5+SiO2+B2O.

[0009] In the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention, the positive electrode active material for an alkali ion secondary battery preferably further contains 40 to 60% by mole of LiF+NaF in an external ratio.

[0010] In the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention, the crystal is preferably a maricite-type crystal, which is preferred because it has a high ratio of transition metal elements in the crystal and can provide a positive electrode active material with a high discharge capacity.

[0011] In the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention, the peak wavelength of the light is preferably 0.1 to 30 μm, which makes it possible to easily convert a crystalline positive electrode active material precursor into an amorphous state.

[0012] In the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention, the light source for irradiating light is preferably at least one selected from a near-infrared heater, a far-infrared heater, a halogen lamp, a CO laser, a YAG laser, a Yb:YVO laser, a laser diode, and a xenon lamp.

[0013] In the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention, a layer containing a positive electrode active material for an alkali ion secondary battery may be formed on the solid electrolyte layer by irradiating the positive electrode active material precursor with light while the positive electrode active material precursor is disposed on the solid electrolyte layer.

[0014] The method for producing a positive electrode active material for an alkali ion secondary battery of the present invention may include a step of amorphizing at least a portion of a positive electrode active material precursor to obtain an amorphous phase, and then crystallizing at least a portion of the amorphous phase by heat treatment. [Effects of the Invention]

[0015] According to the present invention, it is possible to produce a positive electrode active material for an alkali ion secondary battery that contains a high amount of transition metal and is capable of operating the battery. DETAILED DESCRIPTION OF THE INVENTION

[0016] (Positive electrode active material for alkaline ion secondary batteries) First, the positive electrode active material for an alkali ion secondary battery (hereinafter simply referred to as "positive electrode active material") produced by the method of the present invention will be described. In the following description of the content of each component, "%" means "mol %" unless otherwise specified.

[0017] The positive electrode active material for alkali ion secondary batteries produced by the manufacturing method of the present invention contains at least one selected from CrO, FeO, MnO, CoO, and NiO as an essential component. The transition metal oxides CrO, FeO, MnO, CoO, and NiO change their valence during charge and discharge, thereby increasing the redox potential of the positive electrode active material. Among these, MnO and NiO are particularly effective in increasing the redox potential. Furthermore, FeO also stabilizes the structure of the positive electrode active material during charge and discharge, improving cycle characteristics. Therefore, it is preferable to appropriately select transition metal oxides, and in some cases, mix them, depending on the desired characteristics.

[0018] It is preferable that Cr, Fe, Mn, Co, and Ni have a low valence, particularly a divalent valence. When alkali ions are released from the positive electrode active material during the initial charge, the alkali ions are oxidized by the oxidation reaction of transition metal ions (e.g., Fe 2+ →Fe 3+ The greater the proportion of low-valent (especially divalent) transition metal ions that contribute to this charge compensation, the greater the amount of alkali ions released from the positive electrode active material, making it easier to achieve a high charge / discharge capacity.

[0019] The content of CrO+FeO+MnO+CoO+NiO is 34% or more, preferably 35% or more, 40% or more, 45% or more, and particularly preferably 48% or more. If the content of CrO+FeO+MnO+CoO+NiO is too low, the transition metal elements that undergo redox reactions are reduced, resulting in fewer alkali ions involved in absorption and desorption, which tends to reduce the charge / discharge capacity. On the other hand, if the content of CrO+FeO+MnO+CoO+NiO is too high, it becomes difficult to form an amorphous material, which tends to reduce the discharge capacity. Therefore, the content of CrO+FeO+MnO+CoO+NiO is preferably 70% or less, 60% or less, 55% or less, and particularly preferably 53% or less. Note that when any two or more of CrO, FeO, MnO, CoO, and NiO are contained, it is preferable that the total amount thereof also satisfies the above range.

[0020] The content of each of the components CrO, FeO, MnO, CoO, and NiO is preferably 0% or more, 10% or more, 20% or more, 30% or more, 34% or more, 35% or more, 40% or more, 45% or more, and particularly preferably 48% or more, and is preferably 70% or less, 60% or less, 55% or less, and particularly preferably 53% or less. In the present invention, the content of non-divalent transition metal oxides (e.g., Cr2O3, Fe2O3, MnO2, etc.) is expressed in terms of divalent transition metal oxide.

[0021] Li2O and Na2O serve as sources of alkali ions that migrate between the positive electrode active material and the negative electrode active material during charge and discharge. The content of Li2O + Na2O is preferably 20% or more, 23% or more, and particularly 25% or more, and is preferably 55% or less, 40% or less, 32% or less, 29% or less, and particularly 27% or less. If the content of Li2O + Na2O is too low, the amount of alkali ions involved in occlusion and release decreases, tending to reduce the charge and discharge capacity. On the other hand, if the content of Li2O + Na2O is too high, heterogeneous crystals such as Li3PO4 and Na3PO4 that are not involved in charge and discharge tend to precipitate, tending to reduce the charge and discharge capacity. Note that when the positive electrode active material for an alkali ion secondary battery of the present invention is used as a positive electrode active material for a sodium ion secondary battery, it is preferable to contain Na2O as the alkali metal oxide in the above range. When the positive electrode active material for an alkali ion secondary battery of the present invention is used as a positive electrode active material for a lithium ion secondary battery, it is preferable that Li2O is contained as the alkali metal oxide in the above range.

[0022] P2O5, SiO2, and B2O3 are components that form a three-dimensional network structure and stabilize the structure of the positive electrode active material. The inclusion of these components facilitates the formation of an amorphous phase, improving alkali ion conductivity. P2O5 is particularly preferred due to its excellent alkali ion conductivity. The content of P2O5 + SiO2 + B2O3 is preferably 5% or more, 10% or more, 20% or more, and particularly 23% or more, and is preferably 55% or less, 40% or less, 35% or less, 30% or less, and particularly 26% or less. If the content of P2O5 + SiO2 + B2O3 is too low, the above effects are difficult to achieve. On the other hand, if the content of P2O5 + SiO2 + B2O3 is too high, heterogeneous crystals such as P2O5 that are not involved in charge and discharge are likely to precipitate, resulting in a decrease in charge and discharge capacity. When any two of P2O5, SiO2, and B2O3 are included, the total amount of each is preferably within the above range.

[0023] The content of each of the P2O5, SiO2 and B2O3 components is preferably 0% or more, 5% or more, 10% or more, 20% or more, particularly preferably 23% or more, and is preferably 55% or less, 40% or less, 35% or less, 30% or less, particularly preferably 26% or less.

[0024] LiF and NaF function as fluxes to promote amorphization and also serve as sources of alkali ions that migrate between the positive and negative electrode active materials during charge and discharge. Because fluoride ions have a smaller ionic radius than oxide ions, the inclusion of these components tends to improve the alkali ion diffusion capacity within the positive electrode active material.

[0025] The total amount of LiF and NaF is preferably 40 to 60%, 45 to 55%, and particularly 47.5 to 52.5% in terms of mol % of the external ratio (specifically, the external ratio to the total amount of oxide components). If the total amount of LiF and NaF is too high, the amount of alkali ions not involved in charge and discharge increases, which tends to reduce the charge and discharge capacity. On the other hand, if the total amount of LiF and NaF is too low, the rapid charge and discharge characteristics tend to deteriorate. Both LiF and NaF may be contained, or only one of them may be contained.

[0026] The content of the amorphous phase in the positive electrode active material is preferably, in mass %, 50% or more, 70% or more, 80% or more, 85% or more, 95% or more, and particularly 100%. If the content of the amorphous phase is too low, the alkali ion conductivity is likely to decrease, and the charge / discharge characteristics (particularly high-speed charge / discharge characteristics) and cycle characteristics are likely to decrease.

[0027] The amorphous phase content of a positive electrode active material can be determined by peak separation into crystalline diffraction lines and amorphous halo in a diffraction line profile from 10 to 60° at 2θ values ​​obtained by powder X-ray diffraction measurement using CuKα radiation. Specifically, the total scattering curve obtained by subtracting the background from the diffraction line profile is used to obtain a peak separation curve for broad diffraction lines (amorphous halo) from 10 to 45°. The integrated intensity Ia is determined by peak separation of the broad diffraction lines (amorphous halo) from 10 to 45°, and the sum of the integrated intensities Ic is determined by peak separation of the crystalline diffraction lines detected from 10 to 60°. The amorphous phase content Xg can be calculated using the following formula:

[0028] Xg=[1-{Ic / (Ic+Ia)}]×100(mass%)

[0029] The shape of the positive electrode active material is not particularly limited, but a powder form is preferred. A powder form increases the specific surface area, providing more sites for occlusion and release of alkali ions, which facilitates improved charge / discharge capacity. The average particle diameter of the positive electrode active material is preferably 0.1 to 20 μm, 0.3 to 15 μm, 0.5 to 10 μm, and particularly 0.6 to 5 μm. The maximum particle diameter is preferably 150 μm or less, 100 μm or less, 75 μm or less, and particularly 55 μm or less. If the average particle diameter or maximum particle diameter is too large, occlusion and release of alkali ions during charge / discharge tends to be difficult, resulting in a decrease in charge / discharge capacity. On the other hand, if the average particle diameter is too small, the powder tends to be poorly dispersed when made into a paste, making it difficult to produce a uniform electrode.

[0030] Here, the average particle size and maximum particle size refer to the median diameters of primary particles, D50 (50% volume cumulative diameter) and D99 (99% volume cumulative diameter), respectively, and are values ​​measured using a laser diffraction particle size distribution analyzer.

[0031] (Method of manufacturing a positive electrode active material for an alkali ion secondary battery) Next, the method for producing the positive electrode active material for an alkali ion secondary battery of the present invention will be described in detail.

[0032] First, a positive electrode active material precursor containing a crystal is prepared. For example, the positive electrode active material precursor can be obtained by calcining and reacting (solid-phase reaction) raw materials prepared so as to obtain a positive electrode active material having a desired composition. The composition of the positive electrode active material precursor is the same as that of the positive electrode active material for alkali ion secondary batteries described above, and therefore a detailed description thereof will be omitted. Examples of the crystal contained in the positive electrode active material precursor include maricite-type crystals (NaMPO4; M is at least one selected from Cr, Fe, Mn, Co, and Ni). Maricite-type crystals are preferred because they contain a high proportion of transition metal elements in the crystals, allowing the production of a positive electrode active material with a high discharge capacity.

[0033] The positive electrode active material precursor may be composed solely of crystals (i.e., the crystal content may be 100%), or may contain a partial amorphous phase. The positive electrode active material precursor may be a bulk body, for example, pulverized into a powder.

[0034] When the positive electrode active material contains LiF or NaF, examples of the positive electrode active material precursor include Li2MPO4F crystals and Na2MPO4F crystals. Alternatively, a mixture of LiMPO4 crystals and LiP, or a mixture of NaMPO4 crystals and NaF may be used as the positive electrode active material precursor. In particular, using Li2MPO4F crystals or Na2MPO4F crystals as the positive electrode active material precursor is preferable because it is easier to obtain a homogeneous positive electrode active material. Note that in the above Li2MPO4F crystals, Na2MPO4F crystals, LiMPO4 crystals, and NaMPO4 crystals, M is at least one selected from Cr, Fe, Mn, Co, and Ni.

[0035] Next, the positive electrode active material precursor is irradiated with light. This melts the crystals, which are then rapidly cooled and converted into an amorphous phase. As a result, a positive electrode active material for an alkali ion secondary battery having an amorphous phase can be obtained. Note that the entire crystal may be converted into an amorphous phase, or only a portion of the crystal may be converted into an amorphous phase.

[0036] The peak wavelength of the irradiated light is preferably 0.1 μm or more, 0.3 μm or more, 0.5 μm or more, particularly preferably 0.9 μm or more, and is preferably 30 μm or less, 20 μm or less, 10 μm or less, 5 μm or less, particularly preferably 3 μm or less. This tends to increase the energy density of the light, making it possible to amorphize the positive electrode active material precursor with shorter irradiation time.

[0037] Examples of light sources used for light irradiation include near-infrared heaters, far-infrared heaters, halogen lamps, CO2 lasers, YAG lasers, Yb:YVO4 lasers, laser diodes, and xenon lamps. Among these, halogen lamps are preferred because they can irradiate a wide area. These light sources may be used alone or in combination.

[0038] The light irradiation time may be appropriately selected so that the crystals in the positive electrode active material precursor are sufficiently melted and amorphous, and is preferably, for example, 1 second or more, 3 seconds or more, and particularly 5 seconds or more. There are no particular limitations on the upper limit of the light irradiation time, but if it is too long, problems may occur, such as the alkali components evaporating and making it impossible to obtain a positive electrode active material having the desired composition. Therefore, it is preferable that the light irradiation time be 1000 seconds or less, 500 seconds or less, 200 seconds or less, and particularly 100 seconds or less. In particular, when the light source is a laser, the irradiated area tends to be localized, so it is preferable to irradiate while scanning as necessary.

[0039] It is preferable to mix conductive carbon with the positive electrode active material precursor and irradiate it with light. Because conductive carbon has light absorption properties, it can efficiently amorphize the positive electrode active material precursor in a short time when irradiated with light. Furthermore, by doing so, the positive electrode active material is composited with conductive carbon, which can impart conductivity to the positive electrode active material and improve discharge capacity and high-speed charge / discharge characteristics. Furthermore, because conductive carbon has reducing properties, it can suppress oxidation (change in valence) of the transition metal element contained in the positive electrode active material precursor during light irradiation, making it easier to obtain a positive electrode active material with high charge / discharge capacity and good cycle characteristics. Examples of conductive carbon that can be used include highly conductive carbon black such as acetylene black and ketjen black, carbon powder such as graphite, and carbon fiber. Among these, acetylene black, which has high conductivity, is preferred.

[0040] Instead of conductive carbon, an organic compound serving as a conductive carbon source may be mixed with the cathode active material precursor and then irradiated with light. In this manner, the organic compound is carbonized by light irradiation, resulting in a cathode active material composited with conductive carbon. Any organic compound that remains as carbon during light irradiation (heat treatment) may be used as the organic compound. Preferred organic compounds include glucose, citric acid, ascorbic acid, phenolic resins, and surfactants. Surfactants that are easily adsorbed to the cathode active material surface are particularly preferred. Surfactants may include cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants. Nonionic surfactants, which have excellent adsorption properties to the cathode active material surface, are particularly preferred. Examples of nonionic surfactants include polyethylene oxide nonyl phenyl ether.

[0041] In the positive electrode active material composited with conductive carbon, the mass ratio of the positive electrode active material to the conductive carbon is preferably 80-99.5:0.5-20, and more preferably 85-98:2-15. If the content of conductive carbon is too low, the conductivity tends to be poor. On the other hand, if the content of conductive carbon is too high, the content of the positive electrode active material becomes relatively small, and the discharge capacity tends to decrease.

[0042] Alternatively, at least a portion of the positive electrode active material precursor may be amorphized to obtain an amorphous phase, and then at least a portion of the amorphous phase may be crystallized by heat treatment. This method is effective for compositions that function as an active material after crystallization. Examples of such crystalline compositions include Na4Ni3(PO4)2(P2O7) and Na4Ni5(PO4)2(P2O7)2. The heat treatment temperature may be appropriately selected so long as it is above the temperature at which crystallization begins. For example, 400°C or higher, 450°C or higher, and particularly 500°C or higher are preferred. There is no particular upper limit to the heat treatment temperature, but if it is too high, alkali components and other components of the positive electrode active material may evaporate, resulting in a decrease in discharge capacity. Therefore, it is preferable that the heat treatment temperature be 800°C or lower, 700°C or lower, and particularly 600°C or lower.

[0043] The positive electrode active material produced by the method of the present invention can be used in alkali ion secondary batteries (sodium ion secondary batteries and lithium ion secondary batteries) that use electrolytes such as aqueous solvents, non-aqueous solvents, ionic liquids, etc. It can also be used in all-solid-state alkali ion secondary batteries (all-solid-state sodium ion secondary batteries and all-solid-state lithium ion secondary batteries) that use solid electrolytes.

[0044] As the solid electrolyte, for example, a sodium ion conductive oxide can be used. Examples of the sodium ion conductive oxide include compounds containing at least one selected from Al, Y, Zr, Si, and P, Na, and O. Specific examples include β-alumina, β"-alumina, and NASICON-type crystals. Preferably, the sodium ion conductive oxide is β-alumina or β"-alumina. These have even better sodium ion conductivity.

[0045] When the solid electrolyte is β-alumina or β"-alumina, examples include those containing, in mole percent, 65% to 98% Al2O3, 2% to 20% Na2O, and 0.3% to 15% MgO+Li2O.

[0046] In addition to the above components, the solid electrolyte preferably contains ZrO2 and Y2O3. ZrO2 and Y2O3 have the effect of suppressing abnormal grain growth of β-alumina and / or β"-alumina when the raw materials are fired to produce the solid electrolyte, and further improving the adhesion of the respective particles of β-alumina and / or β"-alumina. The ZrO2 content is preferably 0% to 15%, more preferably 1% to 13%, and even more preferably 2% to 10%. The Y2O3 content is preferably 0% to 5%, more preferably 0.01% to 4%, and even more preferably 0.02% to 3%. If the amount of ZrO2 or Y2O3 is too large, the amount of β-alumina and / or β"-alumina produced decreases, and sodium ion conductivity is likely to decrease.

[0047] When the solid electrolyte is a NASICON-type crystal, the general formula is Na s A1 t A2 u O v (A1 is at least one selected from Al, Y, Yb, Nd, Nb, Ti, Hf, and Zr; A2 is at least one selected from Si and P; s = 1.4 to 5.2, t = 1 to 2.9, u = 2.8 to 4.1, v = 9 to 14). A preferred form of the above crystal is where A1 is at least one selected from Y, Nb, Ti, and Zr; s = 2.5 to 3.5, t = 1 to 2.5, u = 2.8 to 4, and v = 9.5 to 12. In this case, a crystal with even better sodium ion conductivity can be obtained. In particular, a monoclinic or trigonal NASICON-type crystal is preferred because it has even better sodium ion conductivity.

[0048] The above general formula Na s A1 t A2 u O v A specific example of the crystal represented by the formula is Na3Zr2Si2PO 12 , Na 3.2 Zr 1.3 Si 2.2 P 0.8 O 10.5 , Na3Zr 1.6 Ti 0.4SiPO 12 , Na3Hf2Si2PO 12 , Na 3.4 Zr 0.9 Hf 1.4 Al 0.6 Si 1.2 P 1.8 O 12 , Na3Zr 1.7 Nb 0.24 SiPO 12 , Na 3.6 Ti 0.2 Y 0.8 Si 2.8 O9, Na3Zr 1.88 Y 0.12 SiPO 12 , Na 3.12 Zr 1.88 Y 0.12 SiPO 12 , Na 3.6 Zr 0.13 Yb 1.67 Si 0.11 P 2.9 O 12 etc.

[0049] The cathode active material precursor may be disposed on the surface of the solid electrolyte layer and then irradiated with light to form a layer containing the cathode active material (cathode layer) on the solid electrolyte layer. In this way, the cathode active material precursor becomes amorphous due to the light irradiation and fuses with the solid electrolyte layer, thereby forming a conduction path for alkali ions.

[0050] Furthermore, after forming a positive electrode layer containing a positive electrode active material on the solid electrolyte by light irradiation, heat treatment may be performed to crystallize the positive electrode active material. The heat treatment temperature may be appropriately selected as long as it is above the temperature at which crystallization begins, for example, 400°C or higher, 450°C or higher, and particularly 500°C or higher. There is no particular upper limit to the heat treatment temperature, but if it is too high, alkaline components and other components of the positive electrode active material may evaporate, resulting in a decrease in discharge capacity. Furthermore, the positive electrode active material may react with the solid electrolyte, resulting in the precipitation of heterogeneous crystals at the interface between the two, which may increase the internal resistance of the battery. Therefore, the heat treatment temperature is preferably 800°C or lower, 700°C or lower, and particularly 600°C or lower. [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] (I) Liquid sodium ion secondary battery Table 1 shows examples (Nos. 1 to 4) and comparative examples (Nos. 5 and 6) in which the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention is applied to a liquid sodium ion secondary battery.

[0053] [Table 1]

[0054] (1) Raw material preparation Various oxide raw materials, carbonate raw materials, and phosphate raw materials were weighed and mixed to prepare batches with the compositions shown in Table 1. The resulting batches were fired in nitrogen at 1200°C to cause a reaction, producing positive electrode active material precursors. The crystal structure of the positive electrode active material precursors was examined using an XRD (powder X-ray diffractometer). It was confirmed that maricite-type NaFePO4 crystals were synthesized in Nos. 1 to 3, and maricite-type NaMnPO4 crystals were synthesized in No. 4. The positive electrode active material precursors were pulverized and classified to obtain positive electrode active material precursor powders with an average particle size of 0.8 μm.

[0055] (2) Preparation of positive electrode active material powder The positive electrode active material powder No. 1 was prepared as follows. A positive electrode active material precursor powder was deposited on a Si substrate and irradiated from above with a continuous wave Yb:YVO4 laser while scanning (power output 0.35 W, laser diameter 40 μm, peak wavelength 1080 nm, scanning speed 1 mm / s). This caused the positive electrode active material precursor powder to become amorphous. The obtained sample was pulverized to obtain a positive electrode active material powder with an average particle size of 2.5 μm.

[0056] Positive electrode active material powder No. 2 was prepared as follows. 100 parts by weight of the positive electrode active material precursor powder were thoroughly mixed with 21 parts by weight (12 parts by weight in terms of carbon) of polyethylene oxide nonylphenyl ether (weight average molecular weight 660), a nonionic surfactant, as a carbon source, and 10 parts by weight of ethanol, and then dried at 100°C for approximately 1 hour. The resulting mixture was irradiated with near-infrared light (output 200 W, irradiation time 60 seconds) using a halogen lamp under vacuum to simultaneously carbonize the nonionic surfactant and amorphize the positive electrode active material precursor powder. The resulting sample was pulverized to obtain a carbon-coated positive electrode active material powder with an average particle size of 2.5 μm.

[0057] Positive electrode active material powders Nos. 3 and 4 were prepared as follows. 100 parts by mass of the positive electrode active material precursor powder were thoroughly mixed with 1 part by mass of acetylene black as a conductive additive and 30 parts by mass of ethanol, and then dried at 100°C for approximately 1 hour. The resulting mixture was irradiated with near-infrared light (output 800 W, irradiation time 10 seconds) using a halogen lamp in a vacuum atmosphere to amorphize the positive electrode active material precursor powder. The resulting sample was pulverized to obtain a positive electrode active material powder with an average particle size of 2.5 μm and conductive carbon dispersed therein.

[0058] XRD analysis of the resulting positive electrode active material powders revealed a significant decrease in the peak intensity of the diffraction pattern of maricite-type crystals. It is believed that the crystals were melted by light irradiation, and then rapidly cooled, suppressing recrystallization and resulting in amorphization. The amorphous content was calculated from the resulting XRD charts. The results are shown in Table 1.

[0059] In Nos. 5 and 6, the positive electrode active material precursor powders obtained in Nos. 1 and 4, respectively, were used as they were as the positive electrode active material.

[0060] (3) Preparation of the positive electrode To the positive electrode active material powder obtained above, acetylene black (Super C65 manufactured by Timcal Corp.) as a conductive additive and polyvinylidene fluoride as a binder were weighed out so that the positive electrode active material powder:conductive additive:binder ratio was 90:5:5 (mass ratio), and the mixture was dispersed in N-methylpyrrolidone (NMP), and then thoroughly stirred in a planetary centrifugal mixer to form a slurry, thereby obtaining a positive electrode material.

[0061] The resulting positive electrode material was coated onto a 20 μm thick aluminum foil positive electrode current collector using a doctor blade with a gap of 125 μm, and after vacuum drying in a dryer at 70°C, it was 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 and dried under reduced pressure at 150°C for 8 hours to obtain a circular positive electrode.

[0062] (4) Preparation of test battery The cathode obtained above was placed on the bottom lid of a coin cell with the aluminum foil side facing down. A 16 mm diameter separator made of a porous polypropylene membrane dried under reduced pressure at 70°C for 8 hours, metallic sodium as the counter electrode, and the top lid of the coin cell were then stacked on top of it to prepare a test battery. The electrolyte used was a 1M NaPF6 solution / EC:DEC = 1:1 (EC = ethylene carbonate, DEC = diethyl carbonate). The test battery was assembled in an environment with a dew point temperature of -70°C or below.

[0063] (5) Charge / discharge test The test battery prepared above was subjected to CC (constant current) charging at 30°C from the open circuit voltage to 4 V, followed by CC discharging from 4 V to 2 V, and the amount of electricity discharged from the positive electrode active material per unit mass (initial discharge capacity) was determined. The C rate was 0.1 C. The results are shown in Table 1.

[0064] As shown in Table 1, the discharge capacities of Examples Nos. 1 to 4 were 48 to 102 mAh / g. On the other hand, the batteries of Comparative Examples Nos. 5 and 6 did not work.

[0065] (II) All-solid-state sodium-ion secondary battery (containing FeO as the positive electrode active material) Table 2 shows an example (No. 7) and a comparative example (No. 8) in which the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention is applied to an all-solid-state sodium ion secondary battery.

[0066] [Table 2]

[0067] (1) Raw material preparation NaH2PO4 (manufactured by Nacalai Tesque, Inc.), FeO (manufactured by Kojundo Chemical Laboratory Co., Ltd.), and NaF (manufactured by Junsei Chemical Co., Ltd.) were weighed as raw materials and blended to obtain the composition shown in Table 2. The resulting batch was wet-mixed in ethanol using a planetary ball mill at 700 rpm for 2 hours. After drying, the mixture was fired at 600°C in a H2 / N2 atmosphere to produce a positive electrode active material precursor. The crystal structure of the positive electrode active material precursor was examined by XRD, confirming that Na2FePO4F crystals had been synthesized. The positive electrode active material precursor was then pulverized and classified to obtain a positive electrode active material precursor powder with an average particle size of 0.8 μm.

[0068] (2) Preparation of the positive electrode The cathode active material precursor powder obtained above and NASICON crystalline solid electrolyte powder (Na3Zr2PSi2O 12 A positive electrode composite was obtained by mixing cellulose nitrate (average particle size 2 μm) and acetylene black as a conductive additive in a mass ratio of 71:25:4. To the obtained positive electrode composite, polypropylene carbonate as a binder was added so that the external ratio was 10 mass%, and N-methylpyrrolidone as a solvent was further added so that the solid content concentration was 60 mass%. The obtained mixture was kneaded using a planetary mixer to produce a positive electrode composite paste.

[0069] NASICON crystal (Na3Zr2PSi2O 12The cathode composite paste was applied to a solid electrolyte substrate (aluminum alloy) to a thickness of 150 μm and then vacuum-dried at 300°C to form a cathode composite layer. For No. 7, a continuous-wave Yb:YVO4 laser was irradiated onto the cathode composite layer while scanning (output: 0.35 W, laser diameter: 90 μm, peak wavelength: 1080 nm, scanning speed: 1 mm / s). This caused the cathode active material precursor powder in the cathode composite to become amorphous and become the cathode active material. Furthermore, the cathode composites themselves and the cathode composite layer and the solid electrolyte substrate were fusion-bonded. For No. 8, instead of laser irradiation, firing was performed in a N2 atmosphere at 550°C for 1 hour.

[0070] XRD analysis of each of the obtained positive electrode composite layers revealed a decrease in the peak intensity of the diffraction pattern of the NaFePOF crystals in No. 7. This is thought to be due to the crystals being melted by light irradiation, and then rapidly cooled, suppressing recrystallization and partially converting them to amorphous form. The amorphous content was calculated from the obtained XRD chart. The results are shown in Table 2.

[0071] (3) Preparation of test battery A thin film of metallic aluminum was formed as a current collector on the surface of the positive electrode composite layer by sputtering. Metallic sodium was attached to the surface of the solid electrolyte layer opposite to the surface on which the positive electrode composite layer was formed, and the resulting solid electrolyte layer was placed in a coin cell to produce a CR2032 coin cell.

[0072] (4) Charge / discharge test The obtained test battery was subjected to CC (constant current) charging at 80°C from the open circuit voltage to 5 V, and then CC discharging from 5 V to 2 V to determine the amount of electricity discharged from the positive electrode active material per unit mass (initial discharge capacity). The C rate was set to 0.02 C.

[0073] As shown in Table 2, the discharge capacity of Example No. 7 was 96 mAh / g, while the battery of Comparative Example No. 8 did not work.

[0074] (III) All-solid-state sodium-ion secondary battery (containing NiO as the positive electrode active material) Table 3 shows examples (Nos. 9 to 13) and a comparative example (No. 14) in which the method for producing a positive electrode active material for an alkali ion secondary battery of the present invention is applied to an all-solid-state sodium ion secondary battery.

[0075] [Table 3]

[0076] (1) Raw material preparation NaH2PO4 and NiO were weighed as raw materials and mixed to prepare a batch so as to obtain the composition shown in Table 3. The resulting batch was mixed in a planetary ball mill at 700 rpm for 2 hours. It was then fired at 800°C to produce a positive electrode active material precursor. The crystal structure of the positive electrode active material precursor was examined by XRD, and it was confirmed that the crystals shown in Table 3 had been synthesized. The positive electrode active material precursor was then pulverized and classified to obtain a positive electrode active material precursor powder with an average particle size of 2 μm.

[0077] (2) Preparation of the positive electrode The positive electrode active material precursor powder obtained above and β”-alumina crystalline solid electrolyte powder (Na 1.6 Li 0.34 Al 10.66 O 17 A positive electrode composite was obtained by mixing cellulose nitrate (average particle size 2 μm) and acetylene black as a conductive additive in a mass ratio of 71:25:4. To the obtained positive electrode composite, 10 mass% of polypropylene carbonate was added as a binder, and N-methylpyrrolidone was further added as a solvent so that the solid content concentration was 60 mass%. The obtained mixture was kneaded using a planetary mixer to produce a positive electrode composite paste.

[0078] Composition formula Na 1.6 Li 0.34 Al 10.66 O 17The above positive electrode composite paste was applied to a thickness of 150 μm on a 100 μm-thick β″-alumina solid electrolyte substrate represented by the formula (1), and then vacuum dried at 300°C to form a positive electrode composite layer. For Nos. 9 to 13, the positive electrode composite layer was irradiated with a continuous wave Yb:YVO laser while scanning (output 0.9 W, peak wavelength 1080 nm, scanning speed 5 mm / s). This caused the positive electrode active material precursor powder in the positive electrode composite to become amorphous and become the positive electrode active material. At this time, the positive electrode composites themselves and the positive electrode composite layer (positive electrode layer) and the solid electrolyte substrate were fusion-bonded.

[0079] Furthermore, for Nos. 12 and 13, after the laser irradiation, the positive electrode active material was crystallized by heat treatment for 30 minutes in a N2 atmosphere at the temperature shown in Table 3. The crystal structure of the positive electrode active material was examined by XRD, and it was confirmed that the same crystals as those of the positive electrode active material precursor had precipitated.

[0080] For No. 14, instead of laser irradiation, heat treatment was performed in an N2 atmosphere at 600°C for 30 minutes.

[0081] (3) Preparation of test battery A thin film of metallic aluminum was formed as a current collector on the surface of the positive electrode composite layer by sputtering. Metallic sodium was attached to the surface of the solid electrolyte layer opposite to the surface on which the positive electrode composite layer was formed, and the resulting solid electrolyte layer was placed in a coin cell to produce a CR2032 coin cell.

[0082] (4) Charge / discharge test The obtained test battery was subjected to CC (constant current) charging at 80°C from the open circuit voltage to 5.5 V, and then CC discharging from 5.5 V to 2 V to determine the amount of electricity discharged from the positive electrode active material per unit mass (initial discharge capacity). The C rate was set to 0.05C.

[0083] As shown in Table 3, the discharge capacities of Examples Nos. 9 to 13 were 63 mAh / g or more, whereas the battery of Comparative Example No. 14 did not work. [Industrial Applicability]

[0084] The positive electrode active material for alkali ion secondary batteries produced by the present invention is suitable as a constituent material of secondary batteries used as the main power sources for, for example, mobile communication devices, portable electronic devices, electric bicycles, electric motorcycles, electric vehicles, etc.

Claims

1. A method for producing a positive electrode active material for an alkali ion secondary battery, the positive electrode active material containing 34% or more of CrO+FeO+MnO+CoO+NiO in mole percent, comprising: preparing a positive electrode active material precursor containing crystals; and a step of irradiating the positive electrode active material precursor with light to melt the crystals and make at least a portion of the crystals amorphous; Including, the positive electrode active material for an alkali ion secondary battery contains, in mole percent in terms of oxide, 20 to 55% of Li 2 O + Na 2 O, 34 to 70% of CrO + FeO + MnO + CoO + NiO, and 5 to 55% of P 2 O 5 + SiO 2 + B 2 O 3 ; The method for producing a positive electrode active material for an alkali ion secondary battery, wherein the content of an amorphous phase in the positive electrode active material for an alkali ion secondary battery is 80% by mass or more.

2. The positive electrode active material for an alkali ion secondary battery further comprises, in mole percent, LiF+NaF 2. The method for producing a positive electrode active material for an alkali ion secondary battery according to claim 1, wherein the positive electrode active material contains 40 to 60% of the alkali metal ion.

3. 3. The method for producing a positive electrode active material for an alkali ion secondary battery according to claim 1, wherein the crystal is a maricite-type crystal.

4. 4. The method for producing a positive electrode active material for an alkali ion secondary battery according to claim 1, wherein the peak wavelength of the light is 0.1 to 30 μm.

5. The light source for irradiating the light is a near-infrared heater, a far-infrared heater, a halogen lamp, or a CO ２ Laser, YAG laser, Yb:YVO ４ 5. The method for producing a positive electrode active material for an alkali ion secondary battery according to claim 1, wherein the light source is at least one selected from the group consisting of a laser, a laser diode, and a xenon lamp.

6. The method for producing a positive electrode active material for an alkali ion secondary battery according to any one of claims 1 to 5, wherein the positive electrode active material precursor is disposed on a solid electrolyte layer, and the positive electrode active material precursor is irradiated with light to form a layer containing the positive electrode active material for an alkali ion secondary battery on the solid electrolyte layer.

7. 7. The method for producing a positive electrode active material for an alkali ion secondary battery according to claim 1, wherein the method does not include a step of amorphizing at least a portion of the positive electrode active material precursor to obtain an amorphous phase, and then crystallizing at least a portion of the amorphous phase by heat treatment.

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