JPH10241686A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery in which cycle performances are superior, even at the temperature of a room temperature or more as keeping high capacity having such a structure as lithium containing metal oxide is spinel lithium manganese oxide, an X-ray diffraction peak exists in specified ranges, and the half-value width of the X-ray diffraction peak exists in a specified range. SOLUTION: In a nonaqueous electrolyte secondary battery, lithium-containing metal oxide is spinel lithium manganese oxide which is shown by a general formula Li[Lix Mn2-x ]O4 (where 0.05<=x<=0.18), no X-ray diffraction peak exists in a range of 2.75±0.02Å, at least the X-ray diffraction peaks exist in ranges of 4.74±0.02Å, 2.47±0.02Å, 2.05±0.02Å, and the half-value widths of the X-ray diffraction peak exist in a range of 0.1±0.05 respectively. Therein, the initial capacity is high, no occurrence of Mn elusion does not occur even at the temperature of a room temperature or more, and good cycle performances are maintained. Thereby, using the low-cost lithium manganese oxide, a high- performance nonaqueous electrolytic secondary battery, which is not inferior to the one using high-cost lithium cobalt oxide, can be supplied at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of cycle performance at high temperature of a non-aqueous electrolyte secondary battery using lithium manganese oxide as a positive electrode active material.

[0002]

2. Description of the Related Art With the recent development of electronic technology, mobile communication devices and portable computers have become widespread. There is a demand for a high energy density secondary battery as a power source for these portable devices. In particular, since a non-aqueous electrolyte secondary battery can be expected to have a high voltage, further reduction in size and weight of the device can be expected. However, in a non-aqueous electrolyte secondary battery using lithium metal and lithium alloy as the negative electrode material, repetition of charge and discharge tends to cause lithium dendrites on the negative electrode metal, resulting in cycle performance and reliability at high temperatures. There was a problem with the sex, and it was not easily put to practical use.

As means for solving these problems, a non-aqueous electrolyte secondary battery using a carbon material capable of inserting and extracting lithium as a negative electrode active material and a composite oxide of lithium and cobalt as a positive electrode active material (Japanese Patent No. 1989293) was developed and has a voltage of 4 V or more in a charged state.
It has become widely used as a power source for mobile devices. However, current nonaqueous electrolyte secondary batteries are expensive because they contain a large amount of cobalt, and attempts to replace cobalt with other transition metals are active. Among the transition metals, cheap manganese is expected to replace cobalt.

However, lithium manganese oxide having a stoichiometric composition has poor cycle performance, and as a method for improving this, for example, as described in JP-A-5-205744, a part of manganese is replaced with lithium. Has been proposed. The conventional method of synthesizing a lithium manganese oxide in which a part of manganese is replaced with lithium is obtained by mixing a Mn raw material and a Li raw material at a desired ratio and performing a heat treatment at a relatively low temperature of 700 ° C. or less. Met.
However, since the heat treatment is performed at a low temperature, the crystallinity does not increase, and as a result, there is a problem that the reversible capacity around about 4 V with respect to the oxidation-reduction potential of lithium metal decreases. Further, when heat treatment is performed at a high temperature of 700 ° C. or more,
When a secondary phase such as Li ₂ MnO ₃ is generated, manganese is not sufficiently replaced by lithium, so when charging and discharging at a temperature higher than room temperature, manganese is eluted, and the cycle performance at high temperatures is reduced. There was a problem.

[0005]

Lithium manganese oxide in which part of manganese has been replaced by lithium has been able to improve the cycle performance near room temperature, but still has a high temperature and in more severe conditions. There is a problem that manganese is eluted and the cycle performance is reduced. Further, there is a problem that the crystallinity is low and the capacity is greatly reduced. An object of the present invention is to replace a part of manganese with lithium, and by using a lithium manganese oxide having good crystallinity, while maintaining high capacity, particularly good cycle performance even at room temperature or higher. To provide a nonaqueous electrolyte secondary battery.

[0006]

As a result of intensive studies on the heat treatment conditions and the replacement method of a spinel lithium manganese oxide in which a part of manganese is replaced by lithium, a desired replacement amount can be obtained while maintaining high crystallinity. The present inventors have found that the substituted product is suitable as a positive electrode material for a non-aqueous electrolyte secondary battery, and have reached the present invention.

That is, the present invention provides: (1) a negative electrode active material capable of inserting and extracting lithium ions, a non-aqueous electrolyte having lithium ion conductivity, and a lithium-containing metal capable of inserting and extracting lithium ions. In a nonaqueous electrolyte secondary battery provided with a cathode active material composed of an oxide, the lithium-containing metal oxide is a spinel-based lithium manganese oxide represented by the following general formula, and has an X-ray diffraction peak of 2.75. ± 0.02 ° and at least 4.74 ± 0.02 °, 2.47
A non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte secondary battery has Li ± 2.0% and 2.05 ± 0.02 °, and the X-ray diffraction peaks each have a half width of 0.1 ± 0.05. Li _x Mn _2-x ] O ₄ (provided that 0.05 ≦ x ≦
0.18)

(2) General formula Li [Li _x Mn _2-x ] O ₄
(Provided that the lattice constant of the spinel-based lithium manganese oxide represented by (0.05 ≦ x ≦ 0.18) is not less than 8.20 ° and not more than 8.24 °), 3) General formula Li [Li _x Mn _2-x ] O ₄ (however,
0.05 ≦ x ≦ 0.18) A spinel-based lithium manganese oxide is heat-treated at a temperature of 700 ° C. or more in a mixture of electrolytic manganese dioxide and lithium carbonate in an air atmosphere, and then heat-treated at a temperature of 700 ° C. or less. A non-aqueous electrolyte secondary battery, characterized by being obtained by:

Hereinafter, the present invention will be described in detail.
The manganese raw material of the lithium manganese oxide used in the present invention is, for example, EMD (Electrolytic M).
angiose Dioxide, CMD (Che
medical Manganise Dioxide),
γ-MnOOH and MnCO ₃ can be mentioned,
EMD having a high tetravalent Mn content is preferred. In addition, lithium materials are also used, for example, Li ₂ CO ₃ , LiOH, LiC
1, LiNO ₃ , Li ₂ SO ₄ , and CH ₃ COOLi, but Li ₂ CO ₃ is preferable.

[0010] The lithium manganese oxide used in the present invention can be prepared as follows. For example, EM pulverized to have an average particle size of 5 to 25 μm
After mixing D and Li ₂ CO _{3 so} that the Mn / Li ratio becomes 0.5, heat treatment is performed at 800 to 900 ° C. in the air.
After cooling to around room temperature, L is adjusted to a desired Li amount.
added i ₂ CO _3, mixed and 400 to 700 ° C., preferably can be obtained by heat treatment at 500 to 650 ° C..

As another example, pulverized EMD and Li
It is also possible to mix ₂ CO ₃ at a desired Mn / Li ratio in advance and perform heat treatment. Heat treatment is 800 ~
It is necessary to carry out at 900 ° C. and then at 400-700 ° C., preferably 500-650 ° C. In this case, it is important to perform the second heat treatment at 400 to 700 ° C. If the second heat treatment is not performed, Li2 MnO3
Such a phase other than spinel is not preferable.
The substitution amount of Mn by Li is preferably in the range of 0.05 to 0.18 atomic%, and more preferably in the range of 0.07 to 0.16 atomic%. If the content is less than 0.05 atomic%, the effect of replacing manganese with lithium is small, and the cycle performance at room temperature is reduced. On the other hand, if it exceeds 0.18 atomic%, the capacity is greatly reduced, which is not preferable.

Next, a method of measuring an X-ray diffraction pattern according to the present invention will be described. The measurement of the X-ray diffraction pattern used RINT2500 manufactured by Rigaku Corporation. C for X-ray source
The measurement was performed using u-Kα1 (wavelength: 1.5405 °) under the following instrument conditions. The tube voltage and current are 50 kV and 160 kV, respectively.
mA, divergence slit 0.5 ゜, scattering slit 0.5 ゜,
The light receiving slit width was 0.15 mm, and a monochromator was used. The measurement was performed at a scanning speed of 2 ° / min.
The scanning was performed under the condition of 2θ / θ at 01 °. Further, the half width was obtained by subtracting the background from the measured value of the diffraction pattern expressed on the 2θ axis, and was defined as the width of the peak at half the height (h / 2) of the diffraction peak intensity (h).

The lithium manganese oxide synthesized by the method described above has at least a (111) plane and a (311) plane.
X-ray diffraction peaks derived from the (222) plane
± 0.02 °, 2.47 ± 0.02 °, 2.05 ± 0.
02 °, and the half width as an index of crystallinity of these diffraction peaks is 0.1 ± 0.05. When the half width is 0.1 ± 0.05 or more, Mn is easily eluted, which is not preferable. Further, it is characterized by having no diffraction peak of 2.75 ± 0.02 ° due to Mn ₂ O ₃ or Mn ₃ O ₄ .

The conventional stoichiometric composition LiMn ₂ O ₄
Is JCPDS (The Joint Committee)
on Power Diffraction Sta
According to the ndards card 35-782, the lattice constant is 8.248 °. The lattice constant of the lithium manganese oxide of the present invention is preferably from 8.20 ° to 8.24 °. The reason why the lattice constant of the lithium manganese oxide of the present invention is small is considered that a part of Mn in the crystal unit cell was replaced with Li having an ionic radius smaller than Mn. When the lattice constant is 8.20 ° or less, the capacity is significantly reduced. This is probably because the average valence of Mn is too close to 4. On the other hand, at 8.24 ° or more, the effect of substituting a part of Mn with Li cannot be obtained, and Mn is eluted to lower the cycle performance. As the negative electrode material used in the present invention,
There is no particular limitation as long as lithium can be inserted and extracted in an ionic state. For example, carbon materials such as coke, natural graphite, artificial graphite and non-graphitizable carbon, metal oxides such as SiSnO,
Metal nitrides such as LiCoN ₂ can be mentioned,
Preferably, it is a carbon material.

In the present invention, when the active material is formed into an electrode, a conductive agent can be added as necessary, and the active material can be fixed to the current collector with a binder. Examples of the conductive agent include natural graphite, artificial graphite, carbon black, Ketjen black, and acetylene black, and graphite or a combination of graphite and acetylene black is preferable.
Although the addition amount is not particularly limited, it is preferably 1 to 20% by weight, and more preferably 3 to 10% by weight. If it is less than 1% by weight, the conductivity will not be uniform,
If it exceeds 0% by weight, the capacity per unit volume will decrease.
As the binder, polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene terpolymer, carboxymethylcellulose, styrene-butadiene rubber, fluororubber and the like are usually used alone or in a mixture, but there is no particular limitation. Not done. The amount of these added is preferably 1 to 20% by weight, more preferably 1 to 10% by weight.
% By weight. If it is less than 1% by weight, the binding strength is weak,
If it exceeds 20% by weight, the movement of Li ions is inhibited, and the performance as a battery is reduced.

As the electrolytic solution, a mixture obtained by dissolving a lithium salt as an electrolyte in various organic solvents is used. The electrolyte is not particularly limited.
_{O 4, LiBF 4, LiPF 6} , LiAsF 6, LiC
A single or a mixture such as F ₃ SO ₃ can be used. The organic solvent is not particularly limited, but, for example, propylene carbonate, ethylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2
-Dimethoxyethane, tetrahydrofuran, etc., alone or in combination of two or more.

[0017]

In the lithium manganese oxide according to the present invention, X
Mn ₂ O ₃ which is an unreacted product and a by-product without having a peak at 2.75 ± 0.02 ° in the diffraction pattern of the line.
And Mn ₃ O ₄ is not contained, so that the elution of Mn can be prevented. Further, heat treatment is always performed at a temperature of 700 ° C. or more, and therefore, the crystallinity is extremely high as shown by the fact that the half width of the peak in the X-ray diffraction pattern is 0.1 ± 0.05 °. And the elution of Mn can be prevented. Therefore, if the lithium manganese oxide of the present invention is used, good cycle characteristics can be obtained even at a high temperature at which the conventional lithium manganese oxide causes leaching of Mn to lower the cyclability.

[0018]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to specific examples, but the present invention is not limited to these examples. Example 1 EMD having an average particle size of 20 μm as a starting material
And Li ₂ CO ₃ were mixed at a composition ratio of Li / Mn = 0.5 (atomic ratio), heat-treated in air at 850 ° C. for 20 hours, and then cooled to around room temperature. From the result of comparing the X-ray diffraction pattern of this lithium manganese oxide with the JCPDS card, this lithium manganese oxide was found to be LiMn ₂ O ₄
Was confirmed. Then, the obtained LiMn ₂ O
₄ and Li ₂ CO ₃ were mixed at a composition ratio of Li / Mn = 0.6 (atomic ratio) and heat-treated in air at 650 ° C. for 12 hours to obtain a lithium manganese oxide of the present invention.

The X-ray diffraction pattern of this lithium manganese oxide has (111) plane, (311) plane,
Each diffraction peak derived from the (222) plane was taken to be 4.738.
{2.475} and 2.052}, and their half-value widths are 0.118, 0.106 and 0.118, respectively, and have high crystallinity, Mn ₂ O ₃ , Mn ₃ O ₄ (Mn
O _y ) and Li ₂ MnO ₃ do not have diffraction peaks of sub-phases, so it was confirmed that the spinel was a single cubic spinel. When the lattice constant was accurately determined using Si as a standard material, it was 8.214 °. Further, from the result of elemental analysis, it was confirmed that the substance was Li [Li _0.12 Mn _1.83 ] O 4. As a result of observing the surface of the lithium manganese oxide thus prepared with a scanning electron microscope, it was found that the secondary particles were formed by an aggregate of primary particles having a diameter of about 0.3 to 1.0 μm. It could be confirmed.

A description will now be given of a specific battery production in the present invention. Lithium manganese oxide 10 obtained above
After mixing 3 parts by weight of acetylene black and 3 parts by weight of scale-like natural graphite as conductive agents with respect to 0 parts by weight, a fluororubber was mixed at a ratio of 3 parts by weight with respect to the total weight, and acetic acid as a solvent of the fluororubber was mixed. A mixed solvent of ethyl / ethyl cellosolve was added to perform wet mixing to obtain a paste. Next, this paste was uniformly applied to both sides of a 20-μm-thick aluminum foil serving as a positive electrode current collector, dried, and then pressure-formed by a roller press to form a belt-shaped positive electrode. Next, 1 part by weight of carboxymethylcellulose, 2 parts by weight of styrene-butadiene rubber, and purified water as a solvent were added to a mixture of 95 parts by weight of mesocarbon fiber graphitized at 3000 ° C. and 5 parts by weight of scale-like natural graphite, followed by wet mixing. To obtain a paste. Next, this paste was uniformly applied to both sides of a copper foil having a thickness of 12 μm as a negative electrode current collector, dried, and then pressed and formed by a roller press to form a strip-shaped negative electrode. Furthermore, a 25-μm-thick polyethylene microporous membrane was sandwiched between the positive electrode and the negative electrode as a separator to form a roll.

An insulating film was inserted into the bottom of a nickel-plated iron cylindrical can, and the wound body was inserted. Next, the negative electrode tab taken out from the wound body was welded to the bottom of the can, and the positive electrode tab was welded to a closing lid made of a gasket, an explosion-proof disk, and a PTC element. An electrolyte in which LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate was injected into the battery can, and an insulating film was inserted into the upper part of the wound body. A cylindrical non-aqueous electrolyte secondary battery having an outer shape of 17 mm and a height of 500 mm was prepared by inserting the lid and crimping the end of the battery can.

Example 2 A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the method of synthesizing lithium manganese oxide was changed as follows. As starting materials, EMD having an average particle size of 15 μm and Li ₂ CO ₃ were mixed with Li / Mn.
= 0.565 (atomic ratio) in air, 90
After heat treatment at 0 ° C for 20 hours, the temperature was lowered to 650 ° C and
Heat treatment was performed for 2 hours to obtain a lithium manganese oxide of the present invention.

In the X-ray diffraction pattern of the obtained substance, at least diffraction peaks derived from the (111), (311) and (222) planes were 4.741 °, 2.479 °, and 2.77 °.
055Å and its half-value width is 0.09 each
4, 0.094, and 0.106, indicating a single cubic spinel having high crystallinity and having no subphase peak.
When the lattice constant was accurately determined using Si as a standard material, it was 8.229 °. Furthermore, from the result of elemental analysis, it was confirmed that the _{substance was} Li [Li _0.07 Mn _1.93 ] O ₄ . As a result of observing the surface of the lithium manganese oxide thus produced with a scanning electron microscope, it was found that the secondary particles were formed by an aggregate of primary particles having a diameter of about 0.5 to 1.5 μm. It could be confirmed.

Example 3 Example 1 was repeated except that the method of synthesizing lithium manganese oxide was changed as follows, and that mesocarbon microbeads heat-treated at 2800 ° C. were used as the negative electrode active material instead of mesocarbon fibers. A non-aqueous electrolyte secondary battery was prepared in the same manner as described above.

EMD having an average particle size of 10 μm as a starting material
And Li ₂ CO ₃ at a composition ratio of Li / Mn = 0.635 (atomic ratio), and heat-treated in air at 850 ° C. for 12 hours, then cooled to 600 ° C. and heat-treated for 12 hours. The lithium manganese oxide of the invention was obtained.
The X-ray diffraction pattern of the obtained substance has at least (111)
Diffraction peaks derived from the (1,311) and (222) planes at 4.746, 2.477, and 2.054, respectively, and their half-value widths are 0.106, 0.10, respectively.
6, a single cubic spinel having a high crystallinity and no diffraction peak of a subphase. 7. The lattice constant was accurately determined using Si as a standard material.
It was 212Å. Furthermore, from the results of elemental analysis, Li
[Li _0.14 Mn _1.86 ] O ₄ was confirmed. As a result of observing the surface of the lithium manganese oxide thus prepared with a scanning electron microscope, it was found that the secondary particles were formed by an aggregate of primary particles having a diameter of about 0.3 to 1.0 μm. I was able to confirm.

Comparative Example 1 A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that the method of synthesizing lithium manganese oxide was changed as follows. As starting materials, EMD having an average particle size of 20 μm and Li ₂ CO ₃ were converted into Li / Mn.
= 0.6 (atomic ratio) in air at 600 ° C
And heat up to 900 ° C for 12 hours.
Heat treatment was performed for a period of time to obtain a lithium manganese oxide. The X-ray diffraction pattern of the obtained lithium manganese oxide has (111) plane, (311) plane, (2
22) each diffraction peak derived from the plane at 4.741 °;
2.483Å, 2.059Å and their half-value widths are 0.129, 0.129, 0.118, respectively.
Although the crystallinity is high, Mn ₂ O ₃ or Mn
_There were a diffraction peak attributed to ₃ O ₄ (MnO _y ) and a diffraction peak attributed to Li ₂ MnO ₃ . Further, when the lattice constant was accurately determined using Si as a standard substance, it was 8.249 °. From these results, LiMn ₂ O ₄ and Mn ₂ O ₃ , Mn ₃ O ₄
And a mixture of Li ₂ MnO ₃ and a uniform cubic spinel single phase.

Comparative Example 2 A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1 except that the method of synthesizing lithium manganese oxide was changed as follows. As a starting material, EMD having an average particle size of 10 μm and Li ₂ CO ₃ were mixed with Li / Mn.
= 0.635 (atomic ratio) in air and 65
Heat treatment was performed at 0 ° C. for 12 hours to obtain lithium manganese oxide (E). In the X-ray diffraction pattern of the obtained substance, the diffraction peaks derived from the (111) plane, the (311) plane, and the (222) plane were 4.743４, 2.475Å, and 2.
051Å, but its half-value width is 0.15 each.
2, 0.160 and 0.152, which were low in crystallinity but did not include a diffraction peak of a subphase, and thus it was confirmed that the single phase was a cubic spinel.

[Test Results] The batteries prepared in Examples 1 to 3 and Comparative Examples 1 and 2 were all charged for 24 hours after the aging period of 24 hours for the purpose of stabilizing the inside of the batteries. Charging was performed in 5 hours at 2 V. Next, the battery was discharged at a constant current of 500 mA to 2.7 V, the initial capacity of each battery was measured, and the capacity per unit positive electrode active material in the battery was determined.

Next, a cycle test was performed in which the battery was placed in a constant temperature bath adjusted to 60 ° C., the charging voltage was set to 4.2 V, the battery was charged in 3 hours, and the battery was repeatedly discharged at a constant current of 1 A to 2.7 V. The discharge capacity at the 50th cycle was measured, and the capacity per unit positive electrode active material in the battery was determined. Further, based on these results, the deterioration rate of the discharge capacity (Y) at the 50th cycle with respect to the initial capacity (X) was calculated according to the following equation. Deterioration rate (%) = [(XY) / X] × 100

Table 1 shows the initial discharge amount, the discharge amount after 50 cycles, and the deterioration rate per unit positive electrode active material calculated from these. As shown in Table 1, Examples 1 to 3
The battery of Comparative Example 1 has a lower initial discharge capacity than that of Comparative Example 1,
The deterioration rate after 0 cycles is small. In addition, compared to Comparative Example 2, the initial discharge capacity is higher, and the deterioration rate after 50 cycles is smaller. This is because heat treatment is performed at 700 ° C. or more, so that a uniform spinel with high crystallinity can be formed,
It is considered that the elimination of the subphase allows a part of manganese to be accurately replaced according to the amount of lithium charged.

[0031]

[Table 1]

[0032]

As described above, the X-ray diffraction peak of the present invention is not present at least at 2.75 ± 0.02 ° but at 4.74 ± 0.02 °, 2.47 ± 0.02 °,
2.05 ± 0.02Å, the half width of each is 0.1
A non-aqueous electrolyte secondary battery using a spinel lithium manganese oxide represented by Li [Li _x Mn _2-x ] O ₄ of ± 0.05 has a high initial capacity and a temperature above room temperature. However, Mn does not elute and good cycle performance is maintained. Also, there is no need to add expensive other elements. As a result, it is possible to provide a non-aqueous electrolyte secondary battery using lithium manganese oxide, which is an inexpensive material, which is comparable to the case using expensive lithium cobalt oxide. A high-performance nonaqueous electrolyte secondary battery can be supplied at low cost, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction pattern of Example 1 of the present invention.

FIG. 2 is an X-ray diffraction pattern of Comparative Example 1 of the present invention.

Claims (3)

[Claims] 1. A negative electrode active material capable of inserting and extracting lithium ions, a lithium ion conductive non-aqueous electrolyte,
And a non-aqueous electrolyte secondary battery comprising a positive electrode active material comprising a lithium-containing metal oxide capable of inserting and extracting lithium ions, wherein the lithium-containing metal oxide is represented by the following general formula: A manganese oxide, having no X-ray diffraction peak at 2.75 ± 0.02 ° and at least 4.74 ± 0.02 °;
47 ± 0.02 °, 2.05 ± 0.02 °,
A non-aqueous electrolyte secondary battery, wherein the half-widths of the line diffraction peaks are each 0.1 ± 0.05. Li [Li _x Mn _2-x ] O4 (provided that 0.05 ≦ x ≦
0.18) 2. The spinel-based lithium manganese oxide represented by the general formula Li [Li _x Mn _2-x ] O ₄ (where 0.05 ≦ x ≦ 0.18) has a lattice constant of at least 8.20 °. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the angle is 24 [deg.] Or less. 3. A spinel lithium manganese oxide represented by the general formula Li [Li _x Mn _2-x ] O ₄ (where 0.05 ≦ x ≦ 0.18) is a mixture of electrolytic manganese dioxide and lithium carbonate. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte secondary battery is obtained by heat-treating at a temperature of 700 ° C. or more in an air atmosphere and then heat-treating again at a temperature of 700 ° C. or less.