Preparation method of a kind of spheroid-like spinel lithium nickel manganese oxide cathode material

Abstract

The application provides a preparation method of a spherical-like spinel lithium nickel manganese oxide positive electrode material and belongs to the technical field of lithium ion batteries. Ammonia water and polyvinyl alcohol are mixed to obtain a bottom solution; the bottom solution is heated, and then a nickel manganese mixed solution, a sodium hydroxide solution and concentrated ammonia water are added to carry out a reaction to obtain a first nickel manganese hydroxide precursor particle; finally, the first nickel manganese hydroxide precursor particle is mixed with a lithium source, and two-stage sintering is carried out to obtain a lithium nickel manganese oxide positive electrode material. The application can effectively reduce cation mixing, increase the dispersity of the particle, stabilize the structure of the material and improve the electrochemical performance of the positive electrode material by adding polyvinyl alcohol. The preparation method is simple, easy to operate and easy to implement, and can achieve the purpose of mass production of high-voltage spinel lithium nickel manganese oxide positive electrode materials.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a preparation method of a kind of spherical spinel lithium nickel manganese oxide cathode material. BACKGROUND

[0002] At present, lithium ion batteries have been widely used in various mobile electronic products, electric tools and static energy storage and other fields. Developing low-cost, high-safety, high-working voltage lithium ion battery energy storage devices is the current research hotspot. The lithium ion battery cathode material is the key part of determining the battery capacity, and the energy density, cycle life and safety performance of the battery are basically determined by the electrochemical performance and physical and chemical properties of the cathode material. It accounts for more than 30% in the cost of lithium ion battery, and is the key component of determining the cost of battery. Therefore, breaking through the key technical bottleneck of low-cost and high-performance lithium ion battery cathode material is the key to solving the high-quality development of the current new energy vehicles, large-scale energy storage, 3C electric and other fields.

[0003] Spinel lithium nickel manganese oxide cathode material is concerned due to its stable structure, three-dimensional Li + diffusion channel, high working voltage. Especially, manganese element is rich in reserves, non-toxic and environmentally friendly, which meets the advantages of green environmental protection, so it has wide application prospect in 3C electronic products and electric vehicle batteries and other fields.

[0004] However, the main synthesis method of spinel manganese-based cathode material at present is solid phase sintering method, the secondary particles show irregular dispersion, the lithium ion diffusion and electron transport path in the charging and discharging process is long, which limits the discharge specific capacity and rate performance of lithium manganate, and the material also has the problem of manganese ion dissolution, which reduces the cycle performance of the material, and the solid phase sintering method has a certain amount of lithium nickel manganese oxygen ion mixing, so it is difficult to obtain pure phase spinel cathode material in batch. Therefore, the preparation method of a kind of spherical spinel lithium nickel manganese oxide cathode material can not only improve the electrochemical performance of the cathode material, but also achieve the purpose of batch production. SUMMARY

[0005] The purpose of the present application is to provide a preparation method of a kind of spherical spinel lithium nickel manganese oxide cathode material, to solve the problems of irregular secondary particle morphology, serious agglomeration, lithium nickel manganese oxygen ion mixing and long lithium ion diffusion and electron transport path in the charging and discharging process in the prior art.

[0006] In order to achieve the above-mentioned purpose of the application, the present application provides the following technical scheme: a preparation method of a kind of spherical spinel lithium nickel manganese oxide cathode material, comprising the following steps:

[0007] 1) mixing ammonia and polyvinyl alcohol to obtain a bottom solution;

[0008] 2) after heating the base solution, adding the nickel-manganese mixed solution, the sodium hydroxide solution and the concentrated ammonia water to react, obtaining the primary nickel-manganese hydroxide precursor particles;

[0009] 3) mixing the primary nickel-manganese hydroxide precursor particles and the lithium source, and then performing two-stage sintering to obtain the lithium nickel-manganese oxide positive electrode material.

[0010] Preferably, in the step 1), the concentration of the ammonia water is 1-3 mol / L; and the mass of the polyvinyl alcohol accounts for 8-12% of the mass of the ammonia water.

[0011] Preferably, in the step 1), the sodium hydroxide solution is added to adjust the pH value of the base solution to 10-12, and the concentration of the sodium hydroxide solution is 4-10 mol / L.

[0012] Preferably, in the step 2), the temperature of the heated base solution is 60-80℃.

[0013] Preferably, in the step 2), the concentration of the sodium hydroxide solution is 4-10 mol / L.

[0014] Preferably, in the step 2), the molar ratio of the manganese ions to the nickel ions in the nickel-manganese mixed solution is 2-4:1, and the total concentration of the metal ions is 1-2 mol / L; and the concentration of the concentrated ammonia water is 10-16 mol / L.

[0015] Preferably, in the step 2), the nickel-manganese mixed solution, the sodium hydroxide solution and the concentrated ammonia water are added in a parallel flow mode, wherein the flow rate of the nickel-manganese mixed solution is 10-2000 mL / min, and the feeding time is 20-50 h.

[0016] Preferably, in the step 2), the reaction temperature is 60-80℃.

[0017] Preferably, in the step 3), the molar ratio of the total nickel-manganese ions in the primary nickel-manganese hydroxide precursor particles to the lithium ions in the lithium source is 1:1.02-1.05.

[0018] Preferably, in the step 3), in the two-stage sintering, first sintering at 500-600℃ for 4-6 h, and then sintering at 800-900℃ for 10-14 h.

[0019] The present application has the following beneficial effects:

[0020] (1) The present application can reduce the ion mixing and the agglomeration of particles by adding PVA to interact with the cations in the reaction medium through ion-dipole and ion-ion interaction forces to anchor the metal salt cations in the water reaction medium and promote the growth of crystals to reduce agglomeration when preparing the lithium nickel-manganese oxide positive electrode material, which greatly reduces the ion mixing in the crystal and the agglomeration between particles, stabilizes the material structure, and improves the electrochemical performance of the lithium nickel-manganese oxide positive electrode material.

[0021] (2) The preparation method provided by the present application is simple and easy to operate, avoids the complicated and precise parameter control process of the co-precipitation process, has the advantages of green safety, easy implementation, simple process method, and is conducive to popularization and application.

[0022] (3) The present application can achieve the purpose of mass production of high-voltage spinel lithium nickel-manganese oxide positive electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 SEM image of the LiNi0.5Mn1.5(OH)4 precursor particles prepared in Example 1; 0.25 Mn 0.75 (OH)4 precursor particles prepared in Example 1;

[0024] Figure 2 SEM image of the LiNi0.5Mn1.5(OH)4 precursor particles prepared in Example 1; 0.5 Mn 1.5 O4 positive electrode material prepared in Example 1;

[0025] Figure 3 SEM image of the LiNi0.5Mn1.5(OH)4 precursor particles prepared in Example 2; 0.25 Mn 0.75 (OH)4 precursor particles prepared in Example 2;

[0026] Figure 4 SEM image of the LiNi0.5Mn1.5(OH)4 precursor particles prepared in Example 2; 0.5 Mn 1.5 O4 positive electrode material prepared in Example 2;

[0027] Figure 5 Cycling performance curve of the LiNi0.5Mn1.5(OH)4 positive electrode material prepared in Example 2; 0.5 Mn 1.5 O4 positive electrode material prepared in Example 2;

[0028] Figure 6 SEM image of the LiNi0.5Mn1.5(OH)4 positive electrode material prepared in Example 3; 0.5 Mn 1.5 O4 positive electrode material prepared in Example 3;

[0029] Figure 7 XRD image of the LiNi0.5Mn1.5(OH)4 positive electrode material prepared in Example 3; 0.5 Mn 1.5 O4 positive electrode material prepared in Example 3; DETAILED DESCRIPTION

[0030] The application provides a preparation method of a spherical spinel lithium nickel manganese oxide positive electrode material.

[0031] 1) mixing ammonia water and polyvinyl alcohol to obtain a bottom solution;

[0032] 2) adding a nickel manganese mixed solution, a sodium hydroxide solution and concentrated ammonia water to the bottom solution after heating to react to obtain a first nickel manganese hydroxide precursor particle;

[0033] 3) mixing the first nickel manganese hydroxide precursor particle and a lithium source and then performing two-stage sintering to obtain the lithium nickel manganese oxide positive electrode material.

[0034] In the application, in step 1), the concentration of the ammonia water is 1-3 mol / L, preferably 1 mol / L, 2 mol / L or 3 mol / L, and further preferably 1 mol / L or 2 mol / L; and the mass of the polyvinyl alcohol accounts for 8-12% of the mass of the ammonia water, preferably 9-11%, and further preferably 10%.

[0035] In the application, in step 1), the sodium hydroxide solution is added to adjust the pH value of the bottom solution to 10-12, preferably 10, 11 or 12, and further preferably 10; and the concentration of the sodium hydroxide solution is 4-10 mol / L, preferably 5-9 mol / L, and further preferably 6-8 mol / L.

[0036] In the application, in step 2), the temperature of the bottom solution after heating is 60-80℃, preferably 65-75℃, and further preferably 70℃.

[0037] In the application, in step 2), the concentration of the sodium hydroxide solution is 4-10 mol / L, preferably 5-9 mol / L, and further preferably 6-8 mol / L.

[0038] In the application, in step 2), the nickel manganese mixed solution is mixed by nickel sulfate, manganese sulfate and water, the molar ratio of manganese ions to nickel ions in the nickel manganese mixed solution is 2-4:1, preferably 3:1; the total concentration of metal ions is 1-2 mol / L, preferably 2 mol / L; and the concentration of the concentrated ammonia water is 10-16 mol / L, preferably 12-14 mol / L, and further preferably 12 mol / L, 13 mol / L or 14 mol / L.

[0039] In the present application, in the step 2), the nickel-manganese mixed solution, the sodium hydroxide solution and the concentrated ammonia water are added in a concurrent way, wherein the flow rate of the nickel-manganese mixed solution is 10-2000 mL / min, preferably 50-1500 mL / min, and further preferably 100-1000 mL / min; and the feeding time is 20-50 h, preferably 25-45 h, and further preferably 25 h, 30 h or 40 h.

[0040] In the present application, when the nickel-manganese mixed solution, the sodium hydroxide solution and the concentrated ammonia water are added for reaction, the reaction is preferably carried out in an inert atmosphere, and the present application is preferably carried out in an argon atmosphere.

[0041] In the present application, in the step 2), the reaction temperature is 60-80 °C, preferably 65-75 °C, and further preferably 70 °C.

[0042] In the present application, in the step 3), the molar ratio of the total nickel-manganese ions in the primary nickel-manganese hydroxide precursor particles to the lithium ions in the lithium source is 1:1.02-1.05, preferably 1:1.03-1.04, and further preferably 1:1.03.

[0043] In the present application, the lithium source is lithium hydroxide.

[0044] In the present application, in the step 3), in the two-stage sintering, first, sintering is carried out at 500-600 °C for 4-6 h, preferably at 520-580 °C for 4.5-5.5 h, and further preferably at 550 °C for 5 h; and then sintering is carried out at 800-900 °C for 10-14 h, preferably at 820-880 °C for 11-13 h, and further preferably at 850 °C for 12 h.

[0045] The technical solutions provided by the present application will be described in detail below in combination with the examples, but they should not be understood as limiting the scope of protection of the present application.

[0046] Example 1

[0047] In a reaction kettle, ammonia water with a concentration of 1 mol / L was prepared, then polyvinyl alcohol with a mass fraction of 8% in the ammonia water, a sodium hydroxide solution with a concentration of 4 mol / L were added to obtain a base solution with a pH value of 10; after the base solution was heated to 60℃, a nickel-manganese mixed solution with a total concentration of metal ions of 2 mol / L (the nickel-manganese mixed solution was prepared by mixing nickel sulfate, manganese sulfate and water, and the molar ratio of manganese ions to nickel ions was 1.5:0.5), 12 mol / L concentrated ammonia water and 4 mol / L sodium hydroxide solution were added to the base solution in parallel at a uniform speed under an argon atmosphere, and stirring was continuously performed at a rotation speed of 400 r / min; the flow rate of the nickel-manganese mixed solution was 10 mL / min, the pH value of the reaction system was maintained at 10, the concentration of ammonia was 1 mol / L, the temperature of the reaction system was 60℃, the feeding time was 25 h, and after the feeding was completed, the reaction system was cooled to 30℃ and maintained for 3 h, then the obtained mixed suspension was naturally cooled to room temperature, aged for 30 min, and then sequentially subjected to suction filtration, washing and drying; when washing, the sodium hydroxide solution with a mass fraction of 5% at 60℃ was used for cleaning 3 times, and deionized water was used for cleaning 2 times; the precipitate after washing was dried at 90℃ for 12 h to obtain Ni 0.25 Mn 0.75 (OH)4 precursor particles.

[0048] Figure 1 The SEM image of the Ni 0.25 Mn 0.75 (OH)4 precursor particles can be seen from Figure 1 It can be seen that the precursor particles are in a flaky shape and have a concentrated particle size distribution.

[0049] The LiNi 0.25 Mn 0.75 O4 positive electrode material was obtained by uniformly mixing the Ni 0.5 Mn 1.5 (OH)4 precursor particles and lithium hydroxide (the molar ratio of the total nickel-manganese ions to lithium ions was 1:1.03), performing two-stage sintering in a muffle furnace under an air atmosphere, first sintering at a temperature increasing rate of 2℃ / min to 550℃ for 6 h, then sintering at a temperature increasing rate of 2℃ / min to 850℃ for 12 h, cooling to room temperature in the furnace, and finally crushing and grinding through a 400-mesh sieve. Figure 2 .

[0050] The LiNi 0.5 Mn 1.5 O4 positive electrode material was assembled into a battery, and was charged and discharged in a voltage window of 3-4.9 V, and the initial discharge capacity was 128 mAh / g at 0.1C, and the capacity retention rate was >60% after 300 cycles.

[0051] Example 2

[0052] A 2 mol / L ammonia solution was prepared in a reactor. Then, 10% polyvinyl alcohol (polyvinyl alcohol by mass) and an 8 mol / L sodium hydroxide solution were added to obtain a base solution with a pH of 11. The base solution was heated to 70°C. Under an argon atmosphere, a nickel-manganese mixed solution with a total metal ion concentration of 2 mol / L (the manganese-nickel mixed solution was composed of nickel sulfate, manganese sulfate, and water, with a molar ratio of manganese ions to nickel ions of 1.5:0.5), 16 mol / L concentrated ammonia solution, and 7 mol / L sodium hydroxide solution were added to the base solution in a parallel, uniform flow while continuous stirring. The stirring speed was [not specified in the original text]. The flow rate of the nickel-manganese mixed solution was 50 mL / min at 600 r / min, the pH of the reaction system was maintained at 11, the ammonia concentration was 2 mol / L, the temperature of the reaction system was 70℃, and the feeding time was 30 h. After the feeding was completed, the reaction system was cooled to 40℃ and stirred for 4 h. The resulting mixed suspension was naturally cooled to room temperature and aged for 30 min. Then, it was sequentially filtered, washed, and dried. The washing process involved washing three times with a 5% sodium hydroxide solution at 60℃, followed by washing twice with deionized water. The precipitate after washing was dried at 90℃ for 12 h to obtain Ni. 0.25 Mn 0.75 (OH)4 precursor particles.

[0053] Figure 3 For Ni 0.25 Mn 0.75 SEM images of (OH)4 precursor particles, from Figure 3 It can be seen that the precursor particles are plate-shaped, with uniform particle size distribution and obvious spherical shape.

[0054] Ni 0.25 Mn 0.75 After uniformly mixing (OH)4 precursor particles and lithium hydroxide (with a molar ratio of total nickel-manganese ions to lithium ions of 1:1.02), the mixture was placed in a muffle furnace and sintered in two stages under an air atmosphere. First, the temperature was increased to 500℃ at a rate of 2℃ / min and sintered for 5 hours. Then, the temperature was increased to 900℃ at a rate of 2℃ / min and sintered for 110 hours. The mixture was then cooled to room temperature in the furnace. Finally, the mixture was pulverized and ground through a 400-mesh sieve to obtain LiNi. 0.5 Mn 1.5 O4 cathode material, morphology shown in [reference needed] Figure 4 .

[0055] LiNi 0.5 Mn 1.5 When O4 cathode material is assembled into a battery, the initial discharge capacity is 126 mAh / g under 0.1C charge-discharge in the 3-4.9V voltage window, and the capacity retention rate is >90% after 100 cycles at 1C.

[0056] Example 3

[0057] In a reaction kettle, ammonia water with a concentration of 3 mol / L was prepared, then polyvinyl alcohol with a mass fraction of 12% of the ammonia water was added, and a sodium hydroxide solution with a concentration of 10 mol / L was added to obtain a bottom solution with a pH value of 12; after the bottom solution was heated to 80℃, a nickel-manganese mixed solution with a total metal ion concentration of 2 mol / L (the manganese-nickel mixed solution was prepared by mixing nickel sulfate, manganese sulfate and water, and the molar ratio of manganese ions to nickel ions was 1.5:0.5), 10 mol / L concentrated ammonia water and 10 mol / L sodium hydroxide solution were added to the bottom solution in parallel at a uniform speed under an argon atmosphere, and stirring was continued, wherein the stirring speed was 800 r / min, the flow rate of the nickel-manganese mixed solution was 1000 mL / min, the pH value of the reaction system was maintained at 12, the concentration of ammonia was 3 mol / L, the temperature of the reaction system was 80℃, the feeding time was 30 h, and when the reaction system was cooled to 50℃, the temperature was maintained for 5 h after the feeding was completed, the obtained mixed suspension was naturally cooled to room temperature, aged for 30 min, then sequentially subjected to suction filtration, washing and drying, wherein the washing was performed by using a 60℃ sodium hydroxide solution with a mass fraction of 5% for 3 times, and then deionized water for 2 times, the precipitate after washing was dried at 80℃ for 12 h to obtain Ni 0.25 Mn 0.75 (OH)4precursor particles.

[0058] The Ni 0.25 Mn 0.75 (OH)4precursor particles and lithium hydroxide (the molar ratio of the total nickel-manganese ions to lithium ions was 1:1.05) were uniformly mixed, then two-stage sintering was performed in a muffle furnace under an air atmosphere, first sintering at a temperature increasing rate of 2℃ / min to 600℃ for 4 h, then sintering at a temperature increasing rate of 2℃ / min to 800℃ for 14 h, cooling to room temperature in the furnace, and finally crushing and grinding through a 400 mesh sieve to obtain LiNi 0.5 Mn 1.5 O4cathode material, and the morphology is shown in Figure 6 .

[0059] The LiNi 0.5 Mn 1.5 O4cathode material was assembled into a battery, and the capacity retention rate was >80% after 100 cycles under 1C charging and discharging in a voltage window of 3-4.9V.

[0060] Example 4

[0061] In a reaction kettle, ammonia water with a concentration of 3 mol / L was prepared, then polyvinyl alcohol with a mass fraction of 12% of the ammonia water was added, and a sodium hydroxide solution with a concentration of 10 mol / L was added to obtain a bottom solution with a pH value of 12; after the bottom solution was heated to 80°C, a nickel-manganese mixed solution with a total metal ion concentration of 1 mol / L (the nickel-manganese mixed solution was prepared by mixing nickel sulfate, manganese sulfate and water, and the molar ratio of manganese ions to nickel ions was 1.5:0.5), 10 mol / L concentrated ammonia water and 10 mol / L sodium hydroxide solution were added to the bottom solution in parallel at a uniform speed under an argon atmosphere, and stirring was continuously performed at a speed of 700 r / min; the flow rate of the nickel-manganese mixed solution was 200 mL / min, the pH value of the reaction system was maintained at 12, the concentration of ammonia was 3 mol / L, the temperature of the reaction system was 80°C, the feeding time was 30 h, and after the feeding was completed, the reaction system was cooled to 50°C and then maintained at this temperature for 5 h while stirring; the obtained mixed suspension was naturally cooled to room temperature, aged for 30 min, then sequentially subjected to suction filtration, washing and drying; during the washing, the precipitate was washed with a 60°C sodium hydroxide solution with a mass fraction of 5% for 3 times and then with deionized water for 2 times, and the washed precipitate was dried at 80°C for 12 h to obtain Ni 0.25 Mn 0.75 (OH)4 precursor particles.

[0062] The Ni 0.25 Mn 0.75 (OH)4 precursor particles and lithium hydroxide (the molar ratio of the total nickel-manganese ions to lithium ions was 1:1.05) were uniformly mixed, then placed in a muffle furnace and subjected to two-stage sintering in an air atmosphere; first, the temperature was increased to 550°C at a rate of 2°C / min and sintered for 4 h, then the temperature was increased to 800°C at a rate of 2°C / min and sintered for 14 h, the furnace was cooled to room temperature, and finally the product was crushed and ground through a 400-mesh sieve to obtain LiNi 0.5 Mn 1.5 O4 positive electrode material.

[0063] The LiNi 0.5 Mn 1.5 O4 positive electrode material was assembled into a battery, and the capacity retention rate was >90% after 100 cycles under 1C charging and discharging in a voltage window of 3-4.9 V.

[0064] Comparative Example 1

[0065] The difference from Example 1 was that polyvinyl alcohol was not added, and the other conditions were the same, to obtain a LiNi 0.5 Mn 1.5 O4 positive electrode material.

[0066] The LiNi 0.5 Mn 1.5The O4 positive electrode material is assembled into a battery, and the first discharge capacity is 110 mAh / g under 0.1C in a voltage window of 3-4.9V, and the capacity retention rate is >75% after 100 cycles under 1C.

[0067] From the above embodiment, the application provides a preparation method of a kind of spherical spinel lithium nickel manganese oxide positive electrode material, first ammonia and polyvinyl alcohol are mixed to obtain bottom solution;Then after heating bottom solution, nickel manganese mixed solution, sodium hydroxide solution and concentrated ammonia are added to carry out reaction, to obtain first nickel manganese hydroxide precursor particle;Finally, first nickel manganese hydroxide precursor particle and lithium source are mixed to carry out two-stage sintering, to obtain lithium nickel manganese oxide positive electrode material.The application can effectively reduce cation disordering by adding polyvinyl alcohol, increase the dispersity of particles, stabilize the structure of material, and improve the electrochemical performance of positive electrode material.The preparation method provided by the application is simple, easy to operate and easy to implement, and can achieve the purpose of mass production of high-voltage spinel lithium nickel manganese oxide positive electrode material.

[0068] The above only describes the preferred embodiments of the present application, and it should be noted that those skilled in the art can make several improvements and refinements without departing from the principles of the present application, and these improvements and refinements should also be considered within the scope of protection of the present application.

Claims

1. A method for preparing a spherical-like spinel lithium nickel manganese oxide cathode material, characterized in that, The method comprises the following steps: 1) mixing ammonia water and polyvinyl alcohol to obtain a bottom solution; 2) adding a nickel-manganese mixed solution, a sodium hydroxide solution and concentrated ammonia water to the bottom solution after heating to react, to obtain a first nickel-manganese hydroxide precursor particle; 3) mixing the first nickel-manganese hydroxide precursor particle and a lithium source, and then performing two-stage sintering to obtain a lithium nickel-manganese oxide positive electrode material. In the step 1), the concentration of the ammonia water is 1-3 mol / L; and the mass of the polyvinyl alcohol accounts for 8-12% of the mass of the ammonia water. In the step 1), the sodium hydroxide solution is added to adjust the pH value of the bottom solution to 10-12, and the concentration of the sodium hydroxide solution is 4-10 mol / L.

2. The production method according to claim 1, characterized by, In the step 2), the temperature of the bottom solution after heating is 60-80℃.

3. The production method according to claim 1 or 2, characterized by, In the step 2), the concentration of the sodium hydroxide solution is 4-10 mol / L.

4. The production method according to claim 3, characterized by, In the step 2), the molar ratio of manganese ions to nickel ions in the nickel-manganese mixed solution is 2-4:1, and the total concentration of metal ions is 1-2 mol / L; and the concentration of the concentrated ammonia water is 10-16 mol / L.

5. The production method according to claim 1 or 2 or 4, characterized by, In the step 2), the nickel-manganese mixed solution, the sodium hydroxide solution and the concentrated ammonia water are added in a parallel flow mode, wherein the flow rate of the nickel-manganese mixed solution is 10-2000 mL / min, and the feeding time is 20-50 h.

6. The production method according to claim 5, wherein In the step 2), the reaction temperature is 60-80℃.

7. The production method according to claim 1 or 2 or 4 or 6, characterized by, In the step 3), the molar ratio of the total nickel-manganese ions in the first nickel-manganese hydroxide precursor particle to lithium ions in the lithium source is 1:1.02-1.

05.

8. The preparation method according to claim 7, characterized in that, In the step 3), during the two-stage sintering, first sintering is performed at 500-600℃ for 4-6 h, and then sintering is performed at 800-900℃ for 10-14 h.

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