Sodium ion cylindrical battery baking method

Abstract

The invention discloses a sodium ion cylindrical battery baking method which comprises the following steps: S1, tray assembling and feeding: firstly, preparing a drying oven, arranging positive electrodes of sodium ion cylindrical batteries upwards on a material tray of the drying oven, and putting the sodium ion cylindrical batteries on the inner side of the drying oven at an interval of 5-10cm; s2, preheating the battery cell and introducing nitrogen, firstly introducing nitrogen into the inner side of the drying oven, raising the temperature of the drying oven to 90-100 DEG C, and preheating for 55-65 minutes. According to the invention, heating is carried out while nitrogen is introduced, so that the battery cell reaches the set temperature, meanwhile, water in the battery cell is vaporized, then the vaporization temperature of water is reduced through vacuumizing, and drying is accelerated; after the moisture is evaporated to a certain degree, nitrogen is charged for vacuum breaking, moisture is discharged, the dry environment is kept for heating for a certain period of time, vacuumizing is conducted again, and the cycle is conducted till drying is completed, the battery cell is baked through the method, the battery cell baking efficiency can be improved, the nitrogen usage amount is reduced, and the baked battery cell is low in moisture content.

Description

Technical Field

[0001] The invention relates to the technical field of battery baking, and in particular to a sodium ion cylindrical battery baking method. Background Art

[0002] At present, the assembly process of 32mm and 46mm large cylindrical batteries is: pole piece winding → full pole ear flattening → collector plate welding → cell shelling → bottom tapping → groove rolling → cell baking → liquid injection → cap welding → sealing. This process of baking after the cell assembly is completed is not conducive to the discharge of moisture. In the production of sodium-ion batteries, moisture is one of the important factors affecting battery performance. Since the positive electrode material of sodium-ion batteries uses layered oxides, there is a serious problem of water absorption. Therefore, the humidity of the workshop will be strictly controlled during the production process, and the battery cell will be baked before liquid injection to ensure that the moisture content of the battery cell reaches a certain standard. When the moisture content of the battery exceeds the standard before liquid injection, it will affect the performance of the sodium-ion battery such as the initial charge and discharge capacity, internal resistance, and battery cycle life.

[0003] The invention with announcement number: CN115790090A belongs to the field of battery production technology, and discloses a battery baking line and a battery baking method. The battery baking line includes a transfer device, a loading device, a drying box and a unloading device; the transfer device includes a transfer trolley, a handling device and a conveying track arranged along a first direction, the handling device is installed on the conveying track, and the handling device can carry the transfer trolley along the first direction and the second direction respectively; the loading device can carry the battery body to be baked from the loading position to the transfer trolley; the drying box can bake the battery body in the transfer trolley; the unloading device can carry the baked battery body from the transfer trolley to the unloading position; the loading device, the drying box and the unloading device are arranged on the side of the conveying track in sequence, and the handling device can move the transfer trolley into or out of the loading device, the drying box and the unloading device in sequence along the second direction. The battery baking line can automatically complete the loading, baking and unloading of the battery to be baked, thereby improving the efficiency of baking the battery.

[0004] The invention with announcement number: CN115751849A belongs to a method for rapid baking of square aluminum shell batteries. The present invention provides the following technical solution: a method for rapid baking of square aluminum shell batteries, comprising the following steps: taking out the pole coils that need to be baked inside the square aluminum shell battery, and placing the pole coils of the square aluminum shell battery that need to be baked into an oven through a clamp, and vacuumizing the inside of the oven, introducing a protective gas, and preheating the oven; heating the oven, stopping heating after reaching a specified temperature, and vacuumizing; continuing heating and vacuumizing until the humidity sensor set on the surface of the clamp and the pole coil of the square aluminum shell battery closely contacts the outer surface of the pole coil of the square aluminum shell battery, and stopping heating and vacuumizing. The present invention bakes the pole coils of the square aluminum shell battery by setting multiple vacuuming and multiple heating and cooling baking, and specifically removes the adsorbed water adsorbed on the positive and negative electrodes and the surface of the diaphragm in the battery and the moisture in the capillary pores of the material, so that the battery meets the use requirements.

[0005] The prior art uses a vacuum drying method to remove moisture from the battery cell, mainly by heating and baking the battery cell for a long time and continuously vacuuming and filling with nitrogen to remove moisture from the battery cell. However, the assembled battery cell only exchanges gas with the outside world through the injection hole, and the vacuuming and nitrogen filling processes take a certain amount of time to enter the battery cell and there is a certain deviation from the box, which is low in efficiency and high in energy consumption. Therefore, the present invention proposes a sodium ion cylindrical battery baking method to solve the above problems. Summary of the invention

[0006] Based on the background technology, there is a method of using vacuum drying to remove moisture from battery cells, mainly by heating and baking the battery cells for a long time and continuously vacuuming and filling with nitrogen to remove moisture from the battery cells. However, the assembled battery cells only exchange gas with the outside world through the injection holes, and the vacuuming and filling with nitrogen processes take a certain amount of time to enter the battery cells and there is a certain deviation from the box body, resulting in low efficiency and high energy consumption. The present invention proposes a baking method for sodium ion cylindrical batteries.

[0007] The present invention provides a sodium ion cylindrical battery baking method, comprising the following steps: S1: feeding of assembly trays; S2: preheat the battery cell and introduce nitrogen; S3: vacuum drying; S4: filling with nitrogen; S5: vacuum drying; S6: filling with nitrogen; S7: vacuum drying.

[0008] Preferably, in the step S1, an oven is prepared first, and the positive electrode of the sodium ion cylindrical battery is placed upward on the material tray of the oven, with each sodium ion cylindrical battery 5-10 cm apart, and then placed inside the oven. The oven is a device that uses an electric heating wire interlayer to heat and dry an object. The oven shell is generally made of thin steel plate, the surface is painted, the studio is made of high-quality structural steel plate, and aluminum silicate fiber is filled between the shell and the studio. The heater is installed at the bottom and can also be placed on the top or both sides. The temperature control instrument uses a digital display intelligent temperature control energy meter to make the operation of the oven simpler, faster and more effective. It is widely used in laboratories, laboratories, electronic communications, plastics, cables, electroplating, hardware, optoelectronics, plastic products, molds, shoe materials, spraying, printing, medical, aerospace and colleges and universities and other industries.

[0009] Preferably, in the step S2, nitrogen is first introduced into the inner side of the oven, and the temperature of the oven is raised to 90-100 degrees Celsius and preheated for 55-65 minutes. Nitrogen is a colorless, odorless, non-toxic gas. It exists in the form of a diatomic molecule (N2) at room temperature. It is the main component of air, occupying about 78% of the volume of air. Nitrogen has high chemical stability and is not easy to react with other elements. It usually reacts with other elements through high temperature and high pressure methods, or through arc discharge methods. One of the main uses of nitrogen is as an industrial chemical. It is used as an inert gas in many industrial processes to prevent oxidation, corrosion and explosion of substances. In addition, nitrogen is also widely used in the food processing industry for packaging and preserving food to extend the shelf life of food. In scientific laboratories, nitrogen is also used to prepare and preserve some special materials.

[0010] Preferably, in the step S3, the air inside the oven is evacuated to 225-240 Pa, and baking is continued for 35-45 minutes. After the time is up, the air inside the oven is continued to be evacuated to 140-160 Pa, and baking is continued for 10-25 minutes.

[0011] Preferably, in step S4, after the time in step S3 is over, nitrogen is filled into the oven and baking is continued for 15-25 minutes.

[0012] Preferably, in the step S5, the air inside the oven is again evacuated to 140-155 Pa, and baking is continued for 20-35 minutes. After the time is up, the air inside the oven is continued to be evacuated to 90-110 Pa, and baking is continued for 5-11 minutes, and then steps S4 and S5 are repeated 3-8 times.

[0013] Preferably, in step S6, nitrogen is continuously charged and baked for 15-25 minutes.

[0014] Preferably, in step S7, the air inside the oven is extracted again, evacuated to 140-160 Pa, and baking is continued for 25-35 minutes. After the time is up, the air inside the oven is continued to be extracted to 50-70 Pa, and baking is continued for 5-15 minutes, and then steps S6 and S7 are repeated 4-8 times.

[0015] Beneficial effects of the present invention: The present invention heats the battery cell while passing nitrogen, so that the battery cell reaches a set temperature, and at the same time, water in the battery cell is vaporized, and then vacuuming is performed to reduce the vaporization temperature of the water, thereby accelerating drying. After the water evaporates to a certain extent, the vacuum is broken by filling with nitrogen to discharge the moisture, and the dry environment is maintained for heating for a certain period of time, and vacuuming is performed again, and this cycle is repeated until the drying is completed. The battery cell baking method can improve the battery cell baking efficiency and save the amount of nitrogen used, and the water content of the baked battery cell is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The working flow chart proposed by the present invention. DETAILED DESCRIPTION

[0017] The present invention will be further explained below in conjunction with specific embodiments.

[0018] Reference Figure 1 , Embodiment 1 This embodiment provides a sodium ion cylindrical battery baking method, comprising the following steps: S1: Feed the materials in the tray. Prepare the oven first. Place the positive electrode of the sodium ion cylindrical battery upward on the tray of the oven. Each sodium ion cylindrical battery is 5 cm apart and then placed inside the oven. The oven is a device that uses electric heating wire interlayers to heat and dry objects. The oven shell is generally made of thin steel plates with paint on the surface. The studio is made of high-quality structural steel plates. Aluminum silicate fiber is filled between the shell and the studio. The heater is installed at the bottom and can also be placed on the top or both sides. The temperature control instrument uses a digital display intelligent temperature control energy meter to make the operation of the oven easier, faster and more effective. It is widely used in laboratories, laboratories, electronic communications, plastics, cables, electroplating, hardware, optoelectronics, plastic products, molds, shoe materials, spraying, printing, medical, aerospace and colleges and universities. S2: Preheat the battery cell and introduce nitrogen. First, introduce nitrogen into the inner side of the oven and raise the temperature of the oven to 90 degrees Celsius. Preheat for 55 minutes. Nitrogen is a colorless, odorless, non-toxic gas. It exists in the form of a diatomic molecule (N2) at room temperature. It is the main component of air and occupies about 78% of the volume of air. Nitrogen has high chemical stability and is not easy to react with other elements. It usually reacts with other elements through high temperature and high pressure methods, or through arc discharge methods. One of the main uses of nitrogen is as an industrial chemical. It is used as an inert gas in many industrial processes to prevent oxidation, corrosion and explosion of substances. In addition, nitrogen is also widely used in the food processing industry to package and preserve food to extend the shelf life of food. In scientific laboratories, nitrogen is also used to prepare and preserve some special materials; S3: vacuum drying, then evacuate the air inside the oven to 230 Pa, and continue baking for 35 minutes. After the time is up, continue to evacuate the air inside the oven to 140 Pa and bake for 10 minutes; S4: Filling with nitrogen, after the time in step S3 is over, filling the oven with nitrogen and continuing baking for 15 minutes; S5: vacuum drying, the air inside the oven is again extracted, the vacuum is pumped to 140 Pa, and the baking is continued for 20 minutes. After the time is up, the air inside the oven is continued to be extracted, the vacuum is pumped to 90 Pa, and the baking is continued for 5 minutes, and then steps S4 and S5 are repeated 3 times; S6: Fill with nitrogen, continue filling with nitrogen, and bake for 15-25 minutes; S7: vacuum drying, extract the air inside the oven again, evacuate to 140 Pa, continue baking for 25-35 minutes, after the time is up, continue to extract the air inside the oven, evacuate to 50 Pa, bake for 5 minutes, and then repeat S6 and S7 steps 4 times.

[0019] Reference Figure 1 , Embodiment 2 This embodiment provides a sodium ion cylindrical battery baking method, comprising the following steps: S1: Feed the materials in the tray. Prepare the oven first. Place the positive electrode of the sodium ion cylindrical battery upward on the tray of the oven. Each sodium ion cylindrical battery is 6 cm apart and then placed inside the oven. The oven is a device that uses electric heating wire interlayers to heat and dry objects. The oven shell is generally made of thin steel plates with paint on the surface. The studio is made of high-quality structural steel plates. Aluminum silicate fiber is filled between the shell and the studio. The heater is installed at the bottom and can also be placed on the top or both sides. The temperature control instrument uses a digital display intelligent temperature control energy meter to make the operation of the oven easier, faster and more effective. It is widely used in laboratories, laboratories, electronic communications, plastics, cables, electroplating, hardware, optoelectronics, plastic products, molds, shoe materials, spraying, printing, medical, aerospace and colleges and universities. S2: Preheat the battery cell and introduce nitrogen. First, introduce nitrogen into the inner side of the oven and raise the temperature of the oven to 95 degrees Celsius. Preheat for 60 minutes. Nitrogen is a colorless, odorless, non-toxic gas. It exists in the form of a diatomic molecule (N2) at room temperature. It is the main component of air and occupies about 78% of the volume of air. Nitrogen has high chemical stability and is not easy to react with other elements. It usually reacts with other elements through high temperature and high pressure methods, or through arc discharge methods. One of the main uses of nitrogen is as an industrial chemical. It is used as an inert gas in many industrial processes to prevent oxidation, corrosion and explosion of substances. In addition, nitrogen is also widely used in the food processing industry for packaging and preserving food to extend the shelf life of food. In scientific laboratories, nitrogen is also used to prepare and preserve some special materials. S3: vacuum drying, then evacuate the air inside the oven to 230 Pa, and continue baking for 40 minutes. After the time is up, continue to evacuate the air inside the oven to 155 Pa and bake for 20 minutes; S4: Filling with nitrogen, after the time in step S3 is over, filling the oven with nitrogen and continuing baking for 18 minutes; S5: vacuum drying, the air inside the oven is again extracted to 450 Pa, and the baking is continued for 30 minutes. After the time is up, the air inside the oven is continued to be extracted to 100 Pa, and the baking is continued for 6 minutes, and then steps S4 and S5 are repeated 7 times; S6: Fill with nitrogen, continue filling with nitrogen, and bake for 21 minutes; S7: vacuum drying, extract the air inside the oven again, evacuate to 150 Pa, continue baking for 30 minutes, after the time is up, continue to extract the air inside the oven, evacuate to 65 Pa, bake for 10 minutes, and then repeat S6 and S7 steps 6 times.

[0020] Reference Figure 1 , Embodiment 3 This embodiment provides a sodium ion cylindrical battery baking method, comprising the following steps: S1: Feed the materials in the tray. Prepare the oven first. Place the positive electrode of the sodium ion cylindrical battery upward on the tray of the oven. Each sodium ion cylindrical battery is 5 cm apart and then placed inside the oven. The oven is a device that uses electric heating wire interlayers to heat and dry objects. The oven shell is generally made of thin steel plates with paint on the surface. The studio is made of high-quality structural steel plates. Aluminum silicate fiber is filled between the shell and the studio. The heater is installed at the bottom and can also be placed on the top or both sides. The temperature control instrument uses a digital display intelligent temperature control energy meter to make the operation of the oven easier, faster and more effective. It is widely used in laboratories, laboratories, electronic communications, plastics, cables, electroplating, hardware, optoelectronics, plastic products, molds, shoe materials, spraying, printing, medical, aerospace and colleges and universities. S2: Preheat the battery cell and introduce nitrogen. First, introduce nitrogen into the inner side of the oven and raise the temperature of the oven to 95 degrees Celsius. Preheat for 60 minutes. Nitrogen is a colorless, odorless, non-toxic gas. It exists in the form of a diatomic molecule (N2) at room temperature. It is the main component of air and occupies about 78% of the volume of air. Nitrogen has high chemical stability and is not easy to react with other elements. It usually reacts with other elements through high temperature and high pressure methods, or through arc discharge methods. One of the main uses of nitrogen is as an industrial chemical. It is used as an inert gas in many industrial processes to prevent oxidation, corrosion and explosion of substances. In addition, nitrogen is also widely used in the food processing industry for packaging and preserving food to extend the shelf life of food. In scientific laboratories, nitrogen is also used to prepare and preserve some special materials. S3: vacuum drying, then evacuate the air inside the oven to 230 Pa, and continue baking for 40 minutes. After the time is up, continue to evacuate the air inside the oven to 150 Pa and bake for 20 minutes; S4: Filling with nitrogen, after the time in step S3 is over, filling the oven with nitrogen and continuing baking for 20 minutes; S5: vacuum drying, the air inside the oven is again extracted to 150 Pa, and the baking is continued for 30 minutes. After the time is up, the air inside the oven is continued to be extracted to 100 Pa, and the baking is continued for 10 minutes, and then steps S4 and S5 are repeated 5 times; S6: Fill with nitrogen, continue filling with nitrogen, and bake for 20 minutes; S7: vacuum drying, extract the air inside the oven again, evacuate to 150 Pa, continue baking for 30 minutes, after the time is up, continue to extract the air inside the oven, evacuate to 60 Pa, bake for 10 minutes, and then repeat S6 and S7 steps 6 times.

[0021] Reference Figure 1 , Embodiment 4 This embodiment provides a sodium ion cylindrical battery baking method, comprising the following steps: S1: Feed the materials in the tray. Prepare the oven first. Place the positive electrode of the sodium ion cylindrical battery upward on the tray of the oven. Each sodium ion cylindrical battery is 7 cm apart and then placed inside the oven. The oven is a device that uses electric heating wire interlayers to heat and dry objects. The oven shell is generally made of thin steel plates with paint on the surface. The studio is made of high-quality structural steel plates. Aluminum silicate fiber is filled between the shell and the studio. The heater is installed at the bottom and can also be placed on the top or both sides. The temperature control instrument uses a digital display intelligent temperature control energy meter to make the operation of the oven easier, faster and more effective. It is widely used in laboratories, laboratories, electronic communications, plastics, cables, electroplating, hardware, optoelectronics, plastic products, molds, shoe materials, spraying, printing, medical, aerospace and colleges and universities. S2: Preheat the battery cell and introduce nitrogen. First, introduce nitrogen into the inner side of the oven and raise the temperature of the oven to 90 degrees Celsius. Preheat for 55 minutes. Nitrogen is a colorless, odorless, non-toxic gas. It exists in the form of a diatomic molecule (N2) at room temperature. It is the main component of air and occupies about 78% of the volume of air. Nitrogen has high chemical stability and is not easy to react with other elements. It usually reacts with other elements through high temperature and high pressure methods, or through arc discharge methods. One of the main uses of nitrogen is as an industrial chemical. It is used as an inert gas in many industrial processes to prevent oxidation, corrosion and explosion of substances. In addition, nitrogen is also widely used in the food processing industry to package and preserve food to extend the shelf life of food. In scientific laboratories, nitrogen is also used to prepare and preserve some special materials; S3: vacuum drying, then evacuate the air inside the oven to 230 Pa, and continue baking for 40 minutes. After the time is up, continue to evacuate the air inside the oven to 150 Pa and bake for 21 minutes; S4: Filling with nitrogen, after the time in step S3 is over, filling the oven with nitrogen and continuing baking for 21 minutes; S5: vacuum drying, the air inside the oven is again extracted to 140 Pa, and the baking is continued for 30 minutes. After the time is up, the air inside the oven is continued to be extracted to 100 Pa, and the baking is continued for 8 minutes, and then steps S4 and S5 are repeated 6 times; S6: Fill with nitrogen, continue filling with nitrogen, and bake for 18 minutes; S7: vacuum drying, extract the air inside the oven again, evacuate to 150 Pa, continue baking for 30 minutes, after the time is up, continue to extract the air inside the oven, evacuate to 60 Pa, bake for 8 minutes, and then repeat S6 and S7 steps 5 times.

[0022] Reference Figure 1 Embodiment 5 This embodiment provides a sodium ion cylindrical battery baking method, comprising the following steps: S1: Feed the materials in the tray. Prepare the oven first. Place the positive electrode of the sodium ion cylindrical battery upward on the tray of the oven. Each sodium ion cylindrical battery is 10 cm apart and then placed inside the oven. The oven is a device that uses electric heating wire interlayers to heat and dry objects. The oven shell is generally made of thin steel plates with painted surfaces. The studio is made of high-quality structural steel plates. Aluminum silicate fibers are filled between the shell and the studio. The heater is installed at the bottom and can also be placed on the top or both sides. The temperature control instrument uses a digital display intelligent temperature control energy meter to make the operation of the oven easier, faster and more effective. It is widely used in laboratories, laboratories, electronic communications, plastics, cables, electroplating, hardware, optoelectronics, plastic products, molds, shoe materials, spraying, printing, medical, aerospace and colleges and universities. S2: Preheat the battery cell and introduce nitrogen. First, introduce nitrogen into the inner side of the oven and raise the temperature of the oven to 100 degrees Celsius. Preheat for 65 minutes. Nitrogen is a colorless, odorless, non-toxic gas. It exists in the form of a diatomic molecule (N2) at room temperature. It is the main component of air and occupies about 78% of the volume of air. Nitrogen has high chemical stability and is not easy to react with other elements. It usually reacts with other elements through high temperature and high pressure methods, or through arc discharge methods. One of the main uses of nitrogen is as an industrial chemical. It is used as an inert gas in many industrial processes to prevent oxidation, corrosion and explosion of substances. In addition, nitrogen is also widely used in the food processing industry for packaging and preserving food to extend the shelf life of food. In scientific laboratories, nitrogen is also used to prepare and preserve some special materials; S3: vacuum drying, then evacuate the air inside the oven to 240 Pa, and continue baking for 45 minutes. After the time is up, continue to evacuate the air inside the oven to 160 Pa and bake for 25 minutes; S4: Filling with nitrogen, after the time in step S3 is over, filling the oven with nitrogen and continuing baking for 25 minutes; S5: vacuum drying, the air inside the oven is again extracted to 155 Pa, and the baking is continued for 20-35 minutes. After the time is up, the air inside the oven is continued to be extracted to 110 Pa, and the baking is continued for 11 minutes, and then steps S4 and S5 are repeated 8 times; S6: Fill with nitrogen, continue filling with nitrogen, and bake for 25 minutes; S7: vacuum drying, extract the air inside the oven again, evacuate to 160 Pa, continue baking for 35 minutes, after the time is up, continue to extract the air inside the oven, evacuate to 70 Pa, bake for 15 minutes, and then repeat S6 and S7 steps 8 times.

[0023] Comparing the conventional baking method with the baking method obtained in Examples 1 to 5, the baking methods obtained in Examples 1 to 5 are as follows: Example one two three Four five Moisture reduction 15% 18% 20% 16% 17% Improved efficiency 20% 21% 22% 19% 20% It can be seen from the above table that the sodium ion cylindrical battery baking method proposed in the present invention has obvious improvements, and implementation three is the best embodiment.

[0024] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A sodium ion cylindrical battery baking method, characterized in that: The following steps are involved: S1: feeding of assembly trays; S2: preheat the battery cell and introduce nitrogen; S3: vacuum drying; S4: filling with nitrogen; S5: vacuum drying; S6: filling with nitrogen; S7: vacuum drying.

2. A sodium ion cylindrical battery baking method according to claim 1, characterized in that: In the step S1, first prepare an oven, place the positive electrodes of the sodium ion cylindrical batteries upward on a tray of the oven, with each sodium ion cylindrical battery spaced 5-10 cm apart, and then place them inside the oven.

3. A sodium ion cylindrical battery baking method according to claim 1, characterized in that: In the step S2, nitrogen is first introduced into the inner side of the oven, and the temperature of the oven is raised to 90-100 degrees Celsius and preheated for 55-65 minutes.

4. A sodium ion cylindrical battery baking method according to claim 1, characterized in that: In the step S3, the air inside the oven is evacuated to 225-240 Pa, and the baking is continued for 35-45 minutes. After the time is up, the air inside the oven is continued to be evacuated to 140-160 Pa, and the baking is continued for 10-25 minutes.

5. A sodium ion cylindrical battery baking method according to claim 1, characterized in that: In step S4, after the time in step S3 is over, nitrogen is filled into the oven and baking is continued for 15-25 minutes.

6. A sodium ion cylindrical battery baking method according to claim 1, characterized in that: In the step S5, the air inside the oven is again evacuated to 140-155 Pa, and baking is continued for 20-35 minutes. After the time is up, the air inside the oven is continued to be evacuated to 90-110 Pa, and baking is continued for 5-11 minutes, and then steps S4 and S5 are repeated 3-8 times.

7. A sodium ion cylindrical battery baking method according to claim 1, characterized in that: In the step S6, nitrogen is continuously charged and the baking is performed for 15-25 minutes.

8. A sodium ion cylindrical battery baking method according to claim 1, characterized in that: In the step S7, the air inside the oven is again extracted to a vacuum of 140-160 Pa, and baking is continued for 25-35 minutes. After the time is up, the air inside the oven is continued to be extracted to a vacuum of 50-70 Pa, and baking is continued for 5-15 minutes, and then steps S6 and S7 are repeated 4-8 times.

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