Preparation method of adhesive

Abstract

The invention discloses a preparation method of an adhesive, and relates to the technical field of lithium batteries. According to the prepared adhesive, polytetrafluoroethylene particles serve as a core material, the surfaces of the polytetrafluoroethylene particles are covered with a first covering material through thermal spraying, the coverage rate is 30%-70%, then the surfaces of the polytetrafluoroethylene particles are covered with a second covering material through in-situ polymerization, and the second covering material makes contact with the polytetrafluoroethylene particles and the first covering material; gaps of the first covering material are filled with the second covering material, and the coverage rate is 80%-98%; wherein the first covering material is one or a combination of more of a lithium polyacrylate polymer, a lithium polyphosphate polymer and a polyethylene oxide lithium polymer; the second covering material is one or a combination of more of polyvinylidene fluoride, polyoxyethylene and a polystyrene-butadiene copolymer; the finally prepared adhesive is applied to a battery, and has good energy density and long cycle life.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, in particular to a method for preparing an adhesive. Background Art

[0002] Dry-process electrodes are currently a hot research and development topic in the lithium battery industry due to their higher energy density and environmental friendliness. Dry-process electrodes rely primarily on the fibrillation of the adhesive polytetrafluoroethylene (PTFE) under external shear forces to achieve adhesion with dry active material particles, thereby forming a conductive network. PTFE generally has a low reactivity with lithium, but in the electrochemical environment within the battery, PTFE reacts with the lithium ions embedded in the negative electrode during the initial charge and discharge process, weakening the bonding effect and consuming the active lithium within the battery, resulting in a decrease in battery energy density and a shortened cycle life. Summary of the Invention

[0003] The object of the present invention is to provide a method for preparing an adhesive to solve the problems raised in the prior art.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] An adhesive has a core-shell structure with polytetrafluoroethylene particles as the core and a first covering material and a second covering material sequentially provided on the surface as the shell layer;

[0006] Preferably, the first covering material is a lithium-containing polymer, specifically one or more combinations of lithium polyacrylate polymer, lithium polyphosphate polymer, and lithium polyethylene oxide polymer;

[0007] Preferably, the second covering material is one or more combinations of polyvinylidene fluoride, polyethylene oxide and polystyrene butadiene copolymer;

[0008] Preferably, the first covering material is coated on the polytetrafluoroethylene particles by thermal spraying, with a coverage rate of 30-70%;

[0009] Preferably, the second covering material is polymerized in situ on the first covering material and fills the gaps in the first covering material, with a coverage rate of 80-98%;

[0010] Preferably, the adhesive comprises the following preparation steps:

[0011] Step 1: Place the ball-milled and sieved polytetrafluoroethylene particles into a drum dryer, set the rotation speed to 8 r / min, prepare a first covering coating and spray it on the surface of the polytetrafluoroethylene particles, dry the material after discharge, and cool it to obtain the first covering polytetrafluoroethylene particles;

[0012] Step 2: The first covering polytetrafluoroethylene particles prepared in step 1 are added to the second covering coating and dispersed, and an initiator is added to initiate a polymerization reaction. After the polymerization reaction stops, the reaction is coagulated, washed, and dried to obtain second covering polytetrafluoroethylene particles, which are the adhesive.

[0013] Preferably, the first covering coating in step 1 is a 2-10 mol / L lithium polyacrylate aqueous solution, which is formed by dispersing lithium polyacrylate in deionized water;

[0014] Spraying is done in the form of atomization, applying 250-400kpa air pressure;

[0015] Drying is carried out by rotary evaporation at a temperature of 40-45°C for 20-30 minutes;

[0016] Preferably, the mass of the lithium polyacrylate covering in the first covering polytetrafluoroethylene particles is 1%-4% of the mass of the polytetrafluoroethylene particles;

[0017] Preferably, the preparation step of the second covering coating in step 2 comprises: using deionized water as a solvent, adding ammonium perfluorooctanoate and mixing to obtain a 0.3-0.5 g / L ammonium perfluorooctanoate solution, evacuating and filling with an inert gas, heating to 90° C., and then pressing into 100 g / L vinylidene fluoride to obtain the second covering coating;

[0018] Preferably, the mass of the first covered polytetrafluoroethylene particles prepared in step 2 is 20-30 times the mass of vinylidene fluoride; the initiator is potassium persulfate; and the working conditions of the polymerization reaction are: reaction temperature of 80° C., pressure of 3 MPa, and reaction time of 1-2 h;

[0019] Preferably, the covering mass of the in-situ polymer in the second covered polytetrafluoroethylene particles is 5%-10% of the mass of the polytetrafluoroethylene particles;

[0020] Preferably, the particle size D50 of the polytetrafluoroethylene particles after ball milling and screening is 15 μm, and the particle size D50 of the second covering polytetrafluoroethylene particles is 16 μm-17 μm.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention uses core-shell coated PTFE particles as an adhesive, which includes a PTFE core and a first covering material provided on the surface of the PTFE core. The first covering material is a lithium-containing polymer material. The first covering material partially covers the surface of the PTFE core with a coverage rate of 30% to 70%. After being prepared into a battery, in an electrolyte environment, the lithium in the lithium-containing polymer forms lithium ions under electrochemical action and undergoes a pre-lithiation reaction with PTFE, thereby reducing the surface activity of PTFE and preventing the loss of active lithium caused by the side reaction of lithium ions migrating from the positive electrode to the negative electrode with PTFE. At the same time, because the lithium-containing polymer material can be simultaneously fiberized with PTFE during the PTFE fiberization process, the decrease in PTFE adhesion caused by pre-lithiation can be compensated.

[0023] 2. The core-shell coated PTFE particles used in the present invention also include a second covering material, which is in contact with both the PTFE core and the first covering material and fills the gaps in the first covering material, further compensating for the decreased adhesion of PTFE caused by pre-lithiation; at the same time, it can passivate PTFE, forming a barrier between PTFE and the electrolyte in the battery, reducing the possibility of side reactions between active lithium ions migrating from the positive electrode to the negative electrode and PTFE. DETAILED DESCRIPTION

[0024] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0025] In the experiment, polytetrafluoroethylene was purchased from Daikin, Japan, model F-208, with a particle size D50 = 200 μm; lithium polyacrylate LiPAA was purchased from Green Energy Fiber (Chongqing) Technology Co., Ltd., with a polyacrylate content of 99.9% and a lithium content of 2.5-3.5%;

[0026] Vinylidene fluoride was purchased from Hubei Maotu Runda Gas Co., Ltd. with a purity of 99.9%;

[0027] Example 1: This example provides a method for preparing an adhesive, and the specific steps are as follows:

[0028] Step 1: High-speed ball milling of polytetrafluoroethylene is performed, and polytetrafluoroethylene particles having a particle size D50 of 15 μm are obtained after screening. The particles are placed in a rotary drum dryer at a speed of 8 r / min, and a lithium polyacrylate covering coating is prepared by dispersing lithium polyacrylate in deionized water to form a 5 mol / L dispersed coating. The coating is then sprayed onto the surface of the polytetrafluoroethylene particles in an atomized form under an air pressure of 300 kPa. After the material is discharged, it is rotary evaporated at 40° C. for 30 minutes and then air-cooled to obtain lithium polyacrylate-covered polytetrafluoroethylene particles. The coverage of the lithium polyacrylate is 48%, and the coverage mass is 2.5% of the polytetrafluoroethylene.

[0029] Step 2: Deionized water was added as a solvent to the reactor, and ammonium perfluorooctanoate was added to form a 0.5 g / L emulsion. The mixture was evacuated and filled with nitrogen. The temperature was raised to 90° C., and 100 g / L of vinylidene fluoride was pressed into the reactor at a pressure of 3 MPa. The lithium polyacrylate-covered polytetrafluoroethylene particles prepared in step 1 were fully dispersed, and 0.1 g / L of potassium persulfate initiator was added. A polymerization reaction occurred at 80° C. and 3 MPa. The reaction was stopped after 2 hours. The emulsion obtained by the polymerization reaction was condensed, washed, and dried to obtain polyvinylidene fluoride-covered polytetrafluoroethylene particles with a particle size D50 of 17 μm. The mass of the lithium polyacrylate-covered polytetrafluoroethylene particles was 25 times the mass of the vinylidene fluoride particles. The coverage rate of the polyvinylidene fluoride was 95%, and the coverage mass was 10% of the polytetrafluoroethylene particles.

[0030] Example 2: Referring to the preparation method of Example 1, the difference is that some parameters in step 1 are adjusted to change the coverage of lithium polyacrylate. The specific steps are as follows:

[0031] Step 1: High-speed ball milling of polytetrafluoroethylene is performed, and polytetrafluoroethylene particles having a particle size D50 of 15 μm are obtained after screening. The particles are placed in a rotary drum dryer at a speed of 12 r / min, and a lithium polyacrylate covering coating is prepared. The lithium polyacrylate is dispersed in deionized water to form a 10 mol / L dispersed coating. The coating is sprayed onto the surface of the polytetrafluoroethylene particles in an atomized form under an air pressure of 400 kPa. After discharge, the material is rotary evaporated at 40° C. for 30 minutes and then air-cooled to obtain lithium polyacrylate-covered polytetrafluoroethylene particles. The coverage of the lithium polyacrylate is 70%, and the coverage mass is 4% of the polytetrafluoroethylene particles.

[0032] Step 2: Deionized water was added as a solvent to the reactor, and ammonium perfluorooctanoate was added to form a 0.5 g / L emulsion. The mixture was evacuated and filled with nitrogen. The temperature was raised to 90° C., and 100 g / L of vinylidene fluoride was pressed into the reactor at a pressure of 3 MPa. The lithium polyacrylate-covered polytetrafluoroethylene particles prepared in step 1 were added and fully dispersed. Then, 0.1 g / L of potassium persulfate initiator was added, and a polymerization reaction was carried out at 80° C. and 3 MPa. The reaction was stopped after 2 hours. The emulsion obtained by the polymerization reaction was condensed, washed, and dried to obtain polyvinylidene fluoride-covered polytetrafluoroethylene particles with a particle size D50 of 17 μm. The mass of the lithium polyacrylate-covered polytetrafluoroethylene particles was 25 times the mass of the vinylidene fluoride particles; the coverage rate of the polyvinylidene fluoride was 95%, and the coverage mass was 10% of the polytetrafluoroethylene particles.

[0033] Example 3: Referring to the preparation method of Example 1, the difference is that some parameters in step 1 are adjusted to change the coverage of lithium polyacrylate. The specific steps are as follows:

[0034] Step 1: High-speed ball milling of polytetrafluoroethylene is performed, and polytetrafluoroethylene particles having a particle size D50 of 15 μm are obtained after screening. The particles are placed in a rotary drum dryer at a speed of 6 r / min, and a lithium polyacrylate covering coating is prepared. The lithium polyacrylate is dispersed in deionized water to form a 2 mol / L dispersed coating. The coating is sprayed onto the surface of the polytetrafluoroethylene particles in an atomized form under an air pressure of 250 kPa. After the material is discharged, it is rotary evaporated at 40° C. for 30 minutes and then air-cooled to obtain lithium polyacrylate-covered polytetrafluoroethylene particles. The coverage of the lithium polyacrylate is 30%, and the coverage mass is 1% of the polytetrafluoroethylene particles.

[0035] Step 2: Deionized water was added as a solvent to the reactor, and ammonium perfluorooctanoate was added to form a 0.5 g / L emulsion. The mixture was evacuated and then filled with nitrogen. After the temperature was raised to 90° C., 100 g / L of vinylidene fluoride and the lithium polyacrylate-covered polytetrafluoroethylene particles prepared in step 1 were added and fully dispersed. Then, 0.1 g / L of potassium persulfate initiator was added, and a polymerization reaction was carried out at 80° C. and 3 MPa. The reaction was stopped after 2 hours. The emulsion obtained by the polymerization reaction was condensed, washed, and dried to obtain polyvinylidene fluoride-covered polytetrafluoroethylene particles with a particle size D50 of 17 μm, wherein the mass of the lithium polyacrylate-covered polytetrafluoroethylene particles was 25 times the mass of the vinylidene fluoride; the coverage rate of the polyvinylidene fluoride was 95%, and the coverage mass was 10% of the polytetrafluoroethylene particles.

[0036] Example 4: Referring to the preparation method of Example 1, the difference is that some parameters in step 2 are adjusted to change the coverage of polyvinylidene fluoride. The specific steps are as follows:

[0037] Step 1: High-speed ball milling of polytetrafluoroethylene is performed, and polytetrafluoroethylene particles having a particle size D50 of 15 μm are obtained after screening. The particles are placed in a rotary drum dryer at a speed of 8 r / min, and a lithium polyacrylate covering coating is prepared by dispersing lithium polyacrylate in deionized water to form a 5 mol / L dispersed coating. The coating is then sprayed onto the surface of the polytetrafluoroethylene particles in an atomized form under an air pressure of 300 kPa. After discharge, the material is rotary evaporated at 40° C. for 30 minutes and then air-cooled to obtain lithium polyacrylate-covered polytetrafluoroethylene particles. The coverage of the lithium polyacrylate is 48%, and the coverage mass is 2.5% of the polytetrafluoroethylene particles.

[0038] Step 2: Deionized water was added as a solvent to the reactor, and ammonium perfluorooctanoate was added to form a 0.3 g / L emulsion. The mixture was evacuated and then filled with nitrogen. After the temperature was raised to 90° C., 100 g / L of vinylidene fluoride and the lithium polyacrylate-covered polytetrafluoroethylene particles prepared in step 1 were added and fully dispersed. Then, 0.1 g / L of potassium persulfate initiator was added, and a polymerization reaction was carried out at 80° C. and 3 MPa. The reaction was stopped after 1 hour. The emulsion obtained by the polymerization reaction was condensed, washed, and dried to obtain polyvinylidene fluoride-covered polytetrafluoroethylene particles with a particle size D50 of 16 μm, wherein the mass of the lithium polyacrylate-covered polytetrafluoroethylene particles was 40 times the mass of the vinylidene fluoride; the coverage rate of the polyvinylidene fluoride was 80%, and the coverage mass was 5% of the polytetrafluoroethylene particles.

[0039] Comparative Example 1: As a control experiment of Example 1, some parameters of step 1 were adjusted to change the coverage of lithium polyacrylate. The specific steps are as follows:

[0040] Step 1: High-speed ball milling of polytetrafluoroethylene is performed, and polytetrafluoroethylene particles having a particle size D50 of 15 μm are obtained after screening. The particles are placed in a rotary drum dryer at a speed of 6 r / min, and a lithium polyacrylate covering coating is prepared. The lithium polyacrylate is dispersed in deionized water to form a 1 mol / L dispersed coating. The coating is sprayed onto the surface of the polytetrafluoroethylene particles in an atomized form under an air pressure of 200 kPa. After the material is discharged, it is rotary evaporated at 40° C. for 30 minutes and then air-cooled to obtain lithium polyacrylate-covered polytetrafluoroethylene particles. The coverage of the lithium polyacrylate is 15%, and the coverage mass is 0.6% of the polytetrafluoroethylene particles.

[0041] Step 2: Deionized water was added as a solvent to the reactor, and ammonium perfluorooctanoate was added to form a 0.5 g / L emulsion. The mixture was evacuated and then filled with nitrogen. After the temperature was raised to 90° C., 100 g / L of vinylidene fluoride and the lithium polyacrylate-covered polytetrafluoroethylene particles prepared in step 1 were added and fully dispersed. Then, 0.1 g / L of potassium persulfate initiator was added, and a polymerization reaction was carried out at 80° C. and 3 MPa. The reaction was stopped after 2 hours. The emulsion obtained by the polymerization reaction was condensed, washed, and dried to obtain polyvinylidene fluoride-covered polytetrafluoroethylene particles with a particle size D50 of 16 μm, wherein the mass of the lithium polyacrylate-covered polytetrafluoroethylene particles was 25 times the mass of the vinylidene fluoride; the coverage rate of the polyvinylidene fluoride was 95%, and the coverage mass was 10% of the polytetrafluoroethylene particles.

[0042] Comparative Example 2: As a control experiment of Example 1, the polytetrafluoroethylene core is not covered with polyvinylidene fluoride, and the specific steps are as follows:

[0043] Polytetrafluoroethylene was subjected to high-speed ball milling and sieved to obtain polytetrafluoroethylene particles with a particle size D50 of 15 μm. The particles were placed in a rotary drum dryer with a rotation speed set at 8 r / min. A lithium polyacrylate covering coating was prepared by dispersing lithium polyacrylate in deionized water to form a 5 mol / L dispersed coating. The coating was sprayed on the surface of the polytetrafluoroethylene particles in the form of an atomized coating under an air pressure of 300 kPa. After discharge, the material was rotary evaporated at 40° C. for 30 minutes and then air-cooled to obtain lithium polyacrylate-covered polytetrafluoroethylene particles. The coverage rate of the lithium polyacrylate was 48%, and the coverage mass was 2.5% of the polytetrafluoroethylene particles.

[0044] Comparative Example 3: As a control experiment of Example 1, polytetrafluoroethylene was subjected to high-speed ball milling, and polytetrafluoroethylene particles with a particle size D50 of 15 μm were obtained after sieving without any covering treatment.

[0045] Application Example: Lithium batteries were prepared using the adhesives prepared in Examples 1-4 and Comparative Examples 1-3, all of which were prepared according to the following preparation method:

[0046] S1: artificial graphite and Ketjen black were mixed for 5 minutes, deionized water was added and the mixture was continued for 5 minutes, and then a binder was added to form an electrode membrane mixture; wherein the mass fraction ratio of artificial graphite, Ketjen black, binder and deionized water was 92%:3%:2%:3%;

[0047] S2: The electrode film mixture obtained in S1 was extruded into a self-supporting film, attached to the surface of the copper current collector, and compacted after drying to obtain a compaction density of 1.8 g / cm 3 The dry electrode is used as the negative electrode;

[0048] S3: 98.5 wt% nickel cobalt manganese 811 (NCM811) was mixed with 1 wt% carbon nanotubes and 0.5 wt% super P by mass fraction to obtain a pre-mixed material; 98 wt% of the pre-mixed material was then mixed with 2 wt% of a binder to obtain a positive electrode dry mix; the positive electrode mix was roll-pressed to obtain a positive electrode active material layer with a compaction density of 3.7 g / cm 3 , as the positive electrode;

[0049] S4: Ethylene carbonate and ethyl methyl carbonate are mixed in a volume ratio of 4:6, and lithium salt LiPF6 with a concentration of 1 mol / L is added as an electrolyte;

[0050] S5: After stacking the above-mentioned positive electrode sheet, polypropylene ceramic diaphragm, and negative electrode sheet in order, winding them to obtain a battery cell, placing them in a battery shell, adding the above-mentioned electrolyte, and after packaging, standing, formation, capacity division and other processes, a sample lithium battery is obtained.

[0051] Detection test

[0052] 1. Energy density test: The lithium ion batteries prepared in Examples 1-4 and Comparative Examples 1-3 were installed in a testing instrument, and the first charge and discharge were performed at 20°C with a current of 0.5C. The constant current-constant voltage charge and constant current discharge modes were adopted, with the termination voltage being 4.2V, the termination current being 0.05C, the charging was stopped, and after standing for 5 minutes, the battery was discharged at a constant current of 1C until the termination voltage was 3.0V. The battery was then left for 24 hours and the charge and discharge test was continued under the same conditions. The constant current and constant voltage charge was performed at a current of 1C, the termination voltage was 4.2V, the termination current was 0.05C, and the constant current discharge was performed at a current of 1C until the termination voltage was 3.0V. The discharge capacity of the battery cell was recorded, and the corresponding weight of the battery cell was weighed with a balance. The battery weight energy density was calculated according to "energy density = discharge capacity / battery cell weight". The data are recorded in Table 1.

[0053] 2. Cycle performance test: The lithium-ion batteries prepared in Examples 1-4 and Comparative Examples 1-3 were installed in a test instrument and charged and discharged for the first time at 20°C with a current of 0.5C. The constant current-constant voltage charging and constant current discharge modes were adopted. The termination voltage was 4.2V and the cut-off current was 0.05C. Charging was stopped. After standing for 5 minutes, the battery was discharged at a constant current of 1C until the termination voltage reached 3.0V. The battery was then left for 24 hours and the charge and discharge test was continued under the same conditions. The constant current and constant voltage charging was performed at a current of 1C and the termination voltage was 4.2V. 4.2V, the cut-off current is 0.05C, and then the constant current discharge is carried out at a current of 1C to a termination voltage of 3.0V, and the discharge capacity Cb of the battery cell during the first cycle is recorded. Then, the cycle life test is carried out under the same conditions, and the 1C / 1C charge and discharge cycle is continued. The voltage range is 3.0-4.2V, and a 5-minute rest is made between charge and discharge. The capacity is checked every 50 times, for a total of 1000 cycles. The discharge capacity Ce after 1000 cycles is recorded, and the capacity retention rate during the cycle is calculated according to Ce / Cb (100%). The data are recorded in Table 1.

[0054] Table 1

[0055] Battery Source Energy density Wh / kg Capacity retention rate after 1000 cycles (%) Example 1 368 94.6 Example 2 361 94.1 Example 3 371 93.8 Example 4 359 92.2 Comparative Example 1 346 88.7 Comparative Example 2 333 85.6 Comparative Example 3 305 82.5

[0056] Conclusion: All the embodiments have relatively excellent energy density and cycle life; Comparative Example 1 is a control experiment of Example 1, and the process parameters are adjusted so that the coverage of the first covering material is 15%, <30%, and the measured energy density and cycle life decrease accordingly; Comparative Example 2 does not directly perform the second layer of covering, and the performance decreases significantly; Comparative Example 3 does not perform any coating treatment. The measured data show that the adhesive coated with the first and second covering materials used in the present invention can effectively reduce the decrease in battery energy density and mitigate the adverse effects caused by the shortened battery cycle life.

[0057] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.

Claims

1. An adhesive, characterized in that: The adhesive has a core-shell structure with polytetrafluoroethylene particles as the core and a first covering material and a second covering material sequentially arranged on the surface as shell layers; the first covering material is a lithium-containing polymer, specifically one or more combinations of polyacrylic acid lithium polymer, polyphosphate lithium polymer, and polyethylene oxide lithium polymer; the second covering material is one or more combinations of polyvinylidene fluoride, polyethylene oxide, and polystyrene butadiene copolymer.

2. An adhesive according to claim 1, characterized in that: The first covering material is covered on the surface of the polytetrafluoroethylene particles by thermal spraying, with a coverage rate of 30-70%.

3. The adhesive according to claim 1, characterized in that: The second covering material is polymerized on the first covering material in situ and fills the gaps in the first covering material, with a coverage rate of 80%-98%.

4. A method for preparing the adhesive according to any one of claims 1 to 3, characterized in that: The method comprises the following preparation steps: Step 1: Place the ball-milled and sieved polytetrafluoroethylene particles into a drum dryer, set the rotation speed to 8 r / min, prepare a first covering coating and spray it on the surface of the polytetrafluoroethylene particles, dry the material after discharge, and cool it to obtain the first covering polytetrafluoroethylene particles; Step 2: The first covering polytetrafluoroethylene particles prepared in step 1 are put into the second covering coating and dispersed, and then an initiator is added to initiate a polymerization reaction. After the polymerization reaction stops, the reaction is coagulated, washed, and dried to obtain second covering polytetrafluoroethylene particles, which are the adhesive.

5. The method for preparing an adhesive according to claim 4, characterized in that: In step 1, the first covering coating is a 2-10 mol / L lithium polyacrylate aqueous solution, which is formed by dispersing lithium polyacrylate in deionized water; spraying is performed in the form of atomization, applying an air pressure of 250-400 kPa; drying is performed by rotary evaporation at a temperature of 40-45° C. and evaporation for 20-30 minutes.

6. The method for preparing an adhesive according to claim 4, characterized in that: The mass of the lithium polyacrylate covered in the first covered polytetrafluoroethylene particles is 1%-4% of the mass of the polytetrafluoroethylene particles.

7. The method for preparing an adhesive according to claim 4, characterized in that: In step 2, the preparation steps of the second covering coating include: using deionized water as a solvent, adding ammonium perfluorooctanoate to mix to form a 0.3-0.5g / L ammonium perfluorooctanoate solution, vacuuming and filling with inert gas, heating to 90°C and then pressing in 100g / L vinylidene fluoride to obtain the second covering coating.

8. The method for preparing an adhesive according to claim 4, characterized in that: In step 2, the mass of the first covered polytetrafluoroethylene particles is 20-50 times the mass of vinylidene fluoride; the initiator is potassium persulfate, and the working conditions of the polymerization reaction are: reaction temperature of 80° C., pressure of 3 MPa, and reaction time of 1-2 h.

9. The method for preparing an adhesive according to claim 4, characterized in that: The covering mass of the in-situ polymer in the second covered polytetrafluoroethylene particles is 5%-10% of the mass of the polytetrafluoroethylene particles.

10. The method for preparing an adhesive according to claim 4, characterized in that: The particle size D50 of the inner polytetrafluoroethylene core after ball milling and sieving is 15 μm, and the particle size D50 of the second covering polytetrafluoroethylene particles is 16 μm-17 μm.