JP2015201388A - Cathode active material for non-aqueous secondary battery and manufacturing method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cathode active material that can implement a nonaqueous secondary battery that can endure use in a field where larger scale and longer-term use are assumed.SOLUTION: A cathode active material contains core particles formed of lithium transition metal composite oxide and coating layers formed on the surfaces of the core particles. The coating layer contains at least one kind of element M selected from the group consisting of boron and tungsten, and niobium. A manufacturing method comprises a first coating step of coating the surfaces of the core particles with first coating raw material comprising compounds containing the element M to obtain first coated particles, and a second coating step of coating the surfaces of the first coated particles with second coating raw material comprising compounds containing niobium to obtain second coated particles.

Description

  The present invention relates to a positive electrode active material for a non-aqueous secondary battery and a method for producing the same.

  A non-aqueous secondary battery represented by a lithium ion secondary battery has a positive electrode active material that can desorb / insert alkali metal ions, and a negative electrode active material that removes alkali metal alone or alkali metal ions such as metallic lithium. A battery that exchanges electric power with the outside by exchanging alkali metal ions between positive and negative electrodes through an alkali metal ion conductive medium using a non-aqueous electrolyte or the like as a separable / insertable substance. In lithium ion secondary batteries, lithium transition metal composite oxides such as lithium cobaltate are typically used as positive electrode active materials.

  By the way, as a technique for modifying the interface of the positive electrode active material, there is a technique for coating the surface of the positive electrode active material with a specific substance.

In Patent Document 1, for the purpose of suppressing an increase in impedance due to high-temperature storage or the like, after firing niobium oxide or the like on the surface of a lithium nickel composite oxide such as LiNi _0.82 Co _0.15 Al _0.03 O ₂ , firing is performed. Techniques to do this have been proposed.

In Patent Document 2, for the purpose of increasing capacity and improving charge and discharge efficiency, composite oxide particles such as Li _1.03 Ni _0.77 Co _0.20 Al _0.03 O ₂ are added to boron such as ammonium pentaborate. Techniques have been proposed in which an acid compound or the like is deposited and then fired in an oxidizing atmosphere.

In Patent Document 3, lithium such as Li _1.03 (Ni _0.8 Co _0.2 ) _0.9 Al _0.1 O ₂ is used for the purpose of improving thermal stability without greatly degrading the initial charge / discharge capacity. A technique for forming a surface layer containing W or the like and Li on the surface of the composite oxide powder has been proposed. As a specific example, a mixture of Li _1.03 (Ni _0.8 Co _0.2 ) _0.9 Al _0.1 O ₂ and Li ₄ WO ₅ and heat-treated at 752 ° C. is disclosed.

Japanese Patent Laid-Open No. 2004-253305 JP 2009-146739 A JP 2002-75367 A

  As non-aqueous secondary batteries have been improved so far, fields that are expected to be used for large-scale and long-term use, such as power sources for large equipment such as electric vehicles and storage batteries for power leveling, are being studied. Has been. With such expansion of application fields, non-aqueous secondary batteries are required to have higher performance in terms of charge / discharge capacity, output characteristics, thermal stability, life characteristics (storage characteristics, cycle characteristics, etc.). .

  In the case of an all-solid secondary battery that employs a solid electrolyte as the alkali metal ion conductive medium, the thermal and chemical stability is significantly improved as compared with the case of a non-aqueous electrolyte secondary battery that employs a non-aqueous electrolyte. However, since the alkali metal ion conductivity in the solid electrolyte is lower than that of the non-aqueous electrolyte, when the extraction current is the same, the discharge capacity of the all-solid secondary battery is compared to that of the non-aqueous electrolyte secondary battery. Lower.

  In the case of non-aqueous electrolyte secondary batteries, there is still room for improvement in view of the recent demand for rapid charging. This is related to the fact that when the current during charge / discharge increases, the crystal structure of the lithium transition metal composite oxide is easily destroyed, and the reaction with the electrolyte in the nonaqueous electrolytic solution is promoted.

  As described above, there is a point to be overcome in application to the field that has been studied recently, whether it is an all-solid secondary battery or a non-aqueous electrolyte secondary battery. The objective of this invention is providing the positive electrode active material which can implement | achieve the non-aqueous secondary battery which can endure use in the field | area which assumed larger scale and long-term use.

  In order to achieve the above object, the present inventors have conducted intensive studies and have completed the present invention. The inventor of the present invention can increase the discharge capacity in an all-solid-state secondary battery by forming a coating layer containing a plurality of specific elements on the surface of the core particle made of lithium transition metal composite oxide, and can be charged with a high current. It has been found that the reaction between the electrolyte in the non-aqueous electrolyte and the positive electrode active material is suppressed even when the discharge is repeated.

  The positive electrode active material of the present invention includes core particles made of a lithium transition metal composite oxide and a coating layer present on the surface of the core particles, and the coating layer is at least selected from the group consisting of boron and tungsten. It is a coating layer containing a kind of element M and niobium.

  Since the positive electrode active material of the present invention has the above characteristics, the discharge capacity in the all-solid secondary battery is increased. Moreover, since the positive electrode active material of this invention is equipped with said characteristic, even if it repeats charging / discharging with a high electric current, reaction with the electrolyte in a non-aqueous electrolyte is suppressed. For this reason, the high current cycle characteristic is improved in the non-aqueous electrolyte secondary battery.

FIG. 1 is a conceptual diagram relating to an example of a preferred form for producing the positive electrode active material of the present invention.

  Hereinafter, the positive electrode active material and the manufacturing method thereof of the present invention will be described in detail using embodiments and examples.

  The positive electrode active material of the present invention includes core particles made of a lithium transition metal composite oxide and a coating layer present on the surface of the core particles. Hereinafter, these will be mainly described.

[Core particles]
For the core particles, a known lithium transition metal composite oxide may be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate having a spinel structure (LiMn ₂ O ₄ ), lithium nickel cobalt manganate (Li (Ni, Co, Mn) O ₂ ), olivine structure And lithium iron phosphate (LiFePO ₄ ).

A lithium transition metal composite oxide having a layered structure such as lithium cobaltate is preferable because a non-aqueous secondary battery having a good balance of charge / discharge capacity, energy density, and the like can be easily obtained. In particular, lithium transition metal composite oxides containing nickel, cobalt and manganese as transition metals are preferred. The nickel content in the three elements is about 30 to 70 mol% in terms of charge / discharge capacity and ease of synthesis, and the cobalt content is about 10 to 35 mol% in terms of cost and output characteristics, and contains manganese. The amount is preferably about 20 mol% to about 35 mol% in view of thermal stability and charge / discharge capacity. As transition metals other than nickel, cobalt, and manganese, tungsten, zirconium, molybdenum, niobium, or the like may be contained up to about 2 mol% of the total transition metals depending on the purpose. Expressed as a composition, the general formula Li _a Ni _1-xy Co _x Mn _y L _z O ₂ (0.95 ≦ a ≦ 1.2, 0.10 ≦ x ≦ 0.35, 0.20 ≦ y ≦ A lithium transition metal composite oxide represented by 0.35, 0 ≦ z ≦ 0.02, and L is at least one element selected from the group consisting of W, Zr, Mo and Nb is particularly preferable.

[Coating layer]
The coating layer includes at least one element M selected from the group consisting of boron and tungsten, and niobium. Since the positive electrode active material has a coating layer containing at least two kinds of these specific elements, the interface resistance between the solid electrolyte and the positive electrode active material is drastically reduced. As a result, the discharge capacity of the all-solid secondary battery is reduced. To increase. Moreover, the crystal structure of the whole positive electrode active material is stabilized, and the reaction between the electrolyte in the non-aqueous electrolyte and the positive electrode active material is suppressed even when charging / discharging with a high current.

  The element M in the coating layer mainly contributes to stabilization of the crystal structure of the entire positive electrode active material. Note that if the amount of the element M is too small, the above-described effect becomes insufficient, and if the amount is too large, the capacity decreases. If the ratio of the amount of the element M to the core particles is 0.1 or more and 5.0 or less, it is preferable because the above-mentioned effects can be sufficiently exhibited by coexisting with niobium.

  Niobium in the coating layer is considered to contribute mainly to reducing the interface resistance between the solid electrolyte and the positive electrode active material, and as a result, the discharge capacity in the all-solid secondary battery increases. Note that if the amount is too small, the above-described effects become insufficient, and if the amount is too large, the capacity decreases. If the ratio of the amount of niobium to the core particles is 0.1 or more and 7.0 or less, the coexistence with the element M can sufficiently exhibit the above-described effects, which is preferable.

  It is preferable to appropriately adjust the ratio of the element M and niobium in the coating layer because the effects of the element M and niobium are amplified. A preferable range of the ratio is a range in which the mass ratio of the element M to niobium is 0.01 to 50.0.

  Next, the manufacturing method of the positive electrode active material of this invention is demonstrated. In the positive electrode active material of the present invention, a coating layer may be formed by a known method on core particles obtained by a known method.

  A preferred method for forming the coating layer will be described with reference to FIG. FIG. 1 is for explaining the concept, and each element in the figure is exaggerated or omitted.

[First coating step]
First, the first coated raw material 2 made of a compound containing the element M is attached to the surface of the core particle 1 (FIG. 1 (a)) to obtain the first coated particle 3 (FIG. 1 (b)). . In FIG. 1B, the first coating raw material 2 uniformly coats the entire surface of the core particle 1, but such a form is only an example and does not necessarily need to be such a form. .

  The first coating material may be any compound as long as it contains the element M. Oxides, halides, oxoacid salts, hydroxides and the like can be taken. Further, in this specification, the “compound containing the element M” includes the element M alone. When a plurality of elements M are selected, an alloy of a plurality of elements M is also included.

  The technique for attaching the first coating material to the surface of the core particles is not particularly limited. For example, the core particles and the first coating raw material are stirred at high speed to adhere to the mechanochemical, the core particles and the first coating raw material are dispersed in a dispersion medium, mixed and then dried to physically adhere. For example, a method may be used in which an aqueous solution serving as a precursor of one coating raw material and core particles are mixed and the first coating raw material is deposited on the surface of the core particles.

[First heat treatment process]
The obtained first coated particles may be further subjected to heat treatment. By the heat treatment, the first coating raw material reacts with a part of the core particle, and a larger area of the surface of the core particle is coated with the first coating raw material or the like. As a result, the discharge capacity is further increased in the all solid state secondary battery. Therefore, the first heat treatment step is particularly preferable when obtaining a positive electrode active material for an all-solid secondary battery.

  Note that if the heat treatment temperature in the first heat treatment step is too high, the reaction between the first coating material and the core particles proceeds, and the original properties of the core particles may be impaired. If the heat treatment temperature is 800 ° C. or lower, there is usually no problem. The effect of the heat treatment temperature appears from about 150 ° C. For this reason, the heat treatment temperature is preferably 150 ° C. or higher and 500 ° C. or lower.

[Second coating step]
A second coated raw material 4 made of a compound containing niobium is attached to the surface of the obtained first coated particle 3 ((c) in FIG. 1) to obtain a second coated particle 5 ((( d) A positive electrode active material. In FIG. 1 (d), the second coating raw material 2 uniformly coats the entire surface of the first coated particles 3, but such a form is only an example and is not necessarily such a form. There is no need. Further, in FIG. 1 (d), the second coated particle 5 has a coating layer composed of two layers, but such a form is only an example, and it is not always necessary to have such a form. . Moreover, it is not necessary for the coating layer to clearly distinguish the region derived from the first coating material from the region derived from the second coating material.

  About the method which makes the compound which can be taken as a 2nd coating raw material, and the 2nd coating raw material adhere to the surface of the 1st coating particle, it applies to those of the 1st coating process.

[Second heat treatment step]
The obtained second coated particles may be further subjected to heat treatment. It is preferable because the coating is strengthened by heat treatment.

  Note that if the heat treatment temperature in the second heat treatment step is too high, the reaction between the second coating material and the first coated particles proceeds, and the original properties of the coating layer may be impaired. If the heat treatment temperature is 500 ° C. or lower, there is usually no problem. The effect of the heat treatment temperature appears from about 250 ° C. For this reason, the heat treatment temperature is preferably 250 ° C. or higher and 800 ° C. or lower.

  Hereinafter, it demonstrates more concretely using an Example etc.

A lithium transition metal composite oxide represented by the general formula Li _1.12 Ni _0.33 Co _0.33 Mn _0.33 W _0.005 O ₂ is used as a core particle, and 0.5 mol% as tungsten with respect to the core particle. Of tungsten oxide (VI) and core particles were mixed with a high-speed shear mixer to obtain a first mixture. After mixing, the first mixture was heat-treated in the atmosphere at 400 ° C. for 10 hours to obtain first coated particles.

  While stirring the first coated particles with a high-speed shear mixer, a commercially available niobium oxide sol (containing 5% as niobium) was dropped in an amount of 2.0 mol% as niobium with respect to the core particles, and the second mixture was added. Obtained. After the dropping, the second mixture was heat-treated at 350 ° C. in the atmosphere for 5 hours to obtain the desired positive electrode composition.

  Instead of tungsten oxide (VI), 0.5 mol% of boric acid and core particles as boron with respect to the core particles were mixed with a high speed shear mixer to obtain a first mixture. After mixing, the first mixture was heat treated at 250 ° C. in the atmosphere for 10 hours to obtain first coated particles. Thereafter, the same procedure as in Example 1 was performed to obtain a target positive electrode composition.

  The lithium transition metal composite oxide in Example 1 was used as a core particle, 0.5 mol% tungsten oxide (VI) as tungsten with respect to the core particle, and 0.5 mol% boric acid and the core particle with respect to the core particle. The first mixture was obtained by mixing with a high-speed shear mixer. Thereafter, the same procedure as in Example 1 was performed to obtain a target positive electrode composition.

  The target positive electrode composition was obtained in the same manner as in Example 2 except that the amount of boric acid mixed was 4.0 mol% as boron with respect to the core particles.

[Comparative Example 1]
The core particles in Example 1 were used for comparison.

A lithium transition metal composite oxide represented by the general formula Li _1.05 Ni _0.6 Co _0.2 Mn _0.2 Zr _0.005 O ₂ is used as a core particle, and 0.5 mol% as boron with respect to the core particle. Was mixed with a high-speed shearing mixer to obtain a first mixture.

  While stirring the first coated particles with a high-speed shear mixer, a commercially available niobium oxide sol (containing 5% as niobium) was dropped in an amount of 0.5 mol% as niobium with respect to the core particles, and the second mixture was added. Obtained. After the dropping, the second mixture was heat-treated at 450 ° C. for 10 hours in the atmosphere to obtain the desired positive electrode composition.

[Comparative Example 2]
The core particles in Example 5 were used for comparison.

[Comparative Example 3]
The same lithium transition metal composite oxide as in Example 5 was used as core particles, and a commercially available niobium oxide sol (containing 5% as niobium) was stirred with a high-speed shear mixer, and 0.5 mol% as niobium with respect to the core particles. Was added dropwise to obtain a second mixture. After the dropping, the second mixture was heat-treated at 450 ° C. for 10 hours in the atmosphere to obtain the desired positive electrode composition.

<3. Evaluation of all-solid-state secondary battery>
All-solid secondary batteries were produced using the positive electrode active materials of Examples 1 to 4 and Comparative Example 1, and the battery characteristics were evaluated.

[3-1. Preparation of solid electrolyte]
Under an argon atmosphere, lithium sulfide and phosphorus pentasulfide were weighed so that the mass ratio was 7: 3. The weighed product was pulverized and mixed in an agate mortar to obtain sulfide glass. This was used as a solid electrolyte.

[3-2. Preparation of positive electrode]
A positive electrode mixture was obtained by mixing 60 parts by weight of a positive electrode active material, 36 parts by weight of a solid electrolyte, and 4 parts by weight of VGCF (vapor phase grown carbon fiber).

[3-3. Production of negative electrode]
A 0.05 mm thick indium foil was cut into a circular shape with a diameter of 11.00 mm to form a negative electrode.

[3-4. Assembly of evaluation battery]
A cylindrical lower mold having an outer diameter of 11.00 mm was inserted into a cylindrical outer mold having an inner diameter of 11.00 mm from the lower part of the outer mold. The upper end of the lower mold was fixed in the middle of the outer mold. In this state, 80 mg of solid electrolyte was charged from the upper part of the outer mold to the upper end of the lower mold. After the introduction, a cylindrical upper mold having an outer shape of 11.00 mm was inserted from the upper part of the outer mold. After the insertion, a solid electrolyte was formed by applying a pressure of 90 MPa from above the upper mold to form a solid electrolyte layer. After molding, the upper mold was pulled out from the upper part of the outer mold, and 20 mg of the positive electrode mixture was put into the upper part of the solid electrolyte layer from the upper part of the outer mold. After the addition, the upper mold was inserted again, and this time a pressure of 360 MPa was applied to form a positive electrode mixture to form a positive electrode layer. After molding, the upper die was fixed, the lower die was released and pulled out from the lower portion of the outer die, and the negative electrode was introduced from the lower portion of the lower die to the lower portion of the solid electrolyte layer. After the charging, the lower mold was inserted again, and a negative electrode was formed by applying a pressure of 150 MPa from below the lower mold to obtain a negative electrode layer. The lower mold was fixed in a state where pressure was applied, the positive terminal was attached to the upper mold, and the negative terminal was attached to the lower mold to obtain an all-solid-state secondary battery.

[3-5. Initial discharge capacity]
Constant current and constant voltage charging was performed at a current density of 0.195 μA / cm ² and a charging voltage of 4.0 V. After charging, the current density 0.195μA / cm ^2, a constant current discharge at a discharge voltage 1.9V, the discharge capacity was measured _{Q d.}

<4. Evaluation of non-aqueous electrolyte secondary battery>
A non-aqueous electrolyte secondary battery was produced using the positive electrode active materials of Example 5 and Comparative Examples 2 and 3, and the battery characteristics were evaluated.

[4-1. Preparation of positive electrode]
A positive electrode slurry was prepared by dispersing 85 parts by weight of the positive electrode composition, 10 parts by weight of acetylene black, and 5.0 parts by weight of PVDF (polyvinylidene fluoride) in NMP (normal methyl-2-pyrrolidone). The obtained positive electrode slurry was applied to an aluminum foil, dried, compression-molded with a roll press, and cut into a predetermined size to obtain a positive electrode.

[4-2. Production of negative electrode]
A negative electrode slurry was prepared by dispersing 97.5 parts by weight of artificial graphite, 1.5 parts by weight of CMC (carboxymethylcellulose), and 1.0 part by weight of SBR (styrene butadiene rubber) in water. The obtained negative electrode slurry was applied to a copper foil, dried, compression-molded with a roll press, and cut into a predetermined size to obtain a negative electrode.

[4-3. Preparation of non-aqueous electrolyte]
EC (ethylene carbonate) and MEC (methyl ethyl carbonate) were mixed at a volume ratio of 3: 7 to obtain a solvent. Lithium hexafluorophosphate (LiPF ₆ ) was dissolved in the obtained mixed solvent so that the concentration thereof was 1 mol / l to obtain a nonaqueous electrolytic solution.

[4-4. Assembly of evaluation battery]
After the lead electrodes were attached to the positive and negative electrode current collectors, vacuum drying was performed at 120 ° C. Next, a separator made of porous polyethylene was disposed between the positive electrode and the negative electrode, and these were stored in a bag-like laminate pack. After storage, the moisture adsorbed on each member was removed by vacuum drying at 60 ° C. After vacuum drying, the above-mentioned non-aqueous electrolyte solution was injected into the laminate pack and sealed to obtain a laminate-type non-aqueous electrolyte secondary battery.

[4-5. High current cycle characteristics]
The non-aqueous electrolyte secondary battery was aged with a weak current, and the electrolyte was sufficiently applied to the positive electrode and the negative electrode. After aging, the battery is placed in a thermostatic chamber set at 45 ° C., charged with a charging voltage of 4.4 V, a charging current of 2.0 C (1 C: current that discharges in 1 hour), a discharging voltage of 2.75 V, The discharge at a discharge current of 2.0 C was defined as one cycle, and charging / discharging was repeated. A value obtained by dividing the discharge capacity at the nth cycle by the discharge capacity at the first cycle was defined as the discharge capacity retention rate Qs (n) at the nth cycle. A high Qs (n) means that the cycle characteristics are good.

[4-6. Output characteristics]
A weak current was passed through the non-aqueous electrolyte secondary battery for aging, and the electrolyte was sufficiently applied to the positive electrode and the negative electrode. Thereafter, discharging with a high current and charging with a weak current were repeated. The charge capacity in the 10th charge was defined as the total charge capacity of the battery. After the 11th discharge, the battery was charged to 40% of the total charge capacity. After the 11th charge, the battery was placed in a thermostatic chamber set at −25 ° C., left for 6 hours, then discharged at a discharge current of 0.02A, 0.04A, 0.06A, and the voltage at each discharge was measured. did. The intersection is plotted with the current on the horizontal axis and the voltage on the vertical axis, and the absolute value of the slope of the straight line connecting the intersections is defined as the DC internal resistance R. Low R means that the output characteristics are good.

  The manufacturing conditions for Examples 1 to 4 and Comparative Example 1 are shown in Table 1, and the characteristics of the positive electrode active material and the characteristics of the all-solid-state secondary battery using the positive electrode active material are shown in Table 2.

  As can be seen from Tables 1 and 2, the discharge capacity of the all-solid secondary battery using the positive electrode active materials of Examples 1 to 4 in which both the element M and niobium are contained in the coating layer is extremely high. This is considered to be a result of dramatically reducing the interface resistance between the positive electrode active material and the solid electrolyte due to the presence of both the element M and niobium in the coating layer.

  The production conditions for Example 5 and Comparative Examples 2 and 3 are shown in Table 3, and the characteristics of the positive electrode active material and the characteristics of the nonaqueous electrolyte secondary battery using the positive electrode active material are shown in Table 4.

  As can be seen from Tables 3 and 4, the high current cycle characteristics and output characteristics of the nonaqueous electrolyte secondary battery using the positive electrode active material of Example 5 in which both the element M and niobium are contained in the coating layer are good. In particular, the improvement of the high current cycle characteristics is considered to be a result of obtaining a stable crystal structure that can withstand rapid charge / discharge due to the presence of the element M and niobium in the coating layer.

  When the positive electrode active material for a non-aqueous secondary battery of the present invention is used, an all-solid secondary battery having a high discharge capacity can be obtained even when a large current is taken out. Alternatively, it is possible to obtain a non-aqueous electrolyte secondary battery that can be used for a long time even when charging and discharging are repeated with a large current. The non-aqueous secondary battery obtained in this way can be particularly suitably used as a power source for large equipment such as an electric vehicle.

1 Core particle 2 First coated raw material 3 First coated particle 4 Second coated raw material 5 Second coated particle

Claims (10)

A core particle comprising a lithium transition metal composite oxide, and a coating layer present on the surface of the core particle,
The coating layer is a coating layer containing at least one element M selected from the group consisting of boron and tungsten, and niobium.
Positive electrode active material for non-aqueous secondary batteries.   2. The positive electrode active material according to claim 1, wherein a substance amount ratio of the element M to the core particle in the coating layer is 0.1 or more and 5.0 or less.   3. The positive electrode active material according to claim 1, wherein a material amount ratio of the niobium to the core particles in the coating layer is 0.1 or more and 5.0 or less.   4. The positive electrode active material according to claim 1, wherein a material amount ratio of the element M to the niobium in the coating layer is 0.01 or more and 50.0 or less. The lithium transition metal composite oxide has a general formula of Li _a Ni _1-xy Co _x Mn _y L _z O ₂ (0.95 ≦ a ≦ 1.2, 0.10 ≦ x ≦ 0.35,. 20 ≦ y ≦ 0.35, 0 ≦ z ≦ 0.02, L is represented by at least one element selected from the group consisting of W, Zr, Mo and Nb). The positive electrode active material according to one item. A non-aqueous two-layer coating comprising: core particles made of a lithium transition metal composite oxide; and a coating layer present on the surface of the core particles and containing at least one element M selected from the group consisting of boron and tungsten and niobium. A method for producing a positive electrode active material for a secondary battery, comprising:
A first coating step of obtaining a first coated particle by attaching a first coated raw material comprising a compound containing the element M to the surface of the core particle;
A second coating step of obtaining a second coated particle by attaching a second coated raw material comprising a compound containing niobium to the surface of the first coated particle;
Manufacturing method.   The manufacturing method according to claim 6, further comprising a first heat treatment step of heat treating the first coated particles.   The manufacturing method according to claim 6 or 7, further comprising a second heat treatment step of heat treating the second coated particles.   The manufacturing method of Claim 7 whose heat processing temperature in said 1st heat processing process is 150 degreeC or more and 800 degrees C or less.   The manufacturing method of Claim 8 whose heat processing temperature in said 2nd heat processing process is 250 degreeC or more and 500 degrees C or less.

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