A method for preparing a graphene lithium battery cell and a graphene-based lithium-ion battery

Abstract

The present invention relates to the technical field of graphene lithium-ion batteries, and specifically provides a method for preparing a graphene lithium battery cell. After hydrophilic oxygen-containing functional groups are attached to the surface of a battery separator, a positive electrode slurry is first applied to one surface of the battery separator containing the hydrophilic oxygen-containing functional groups to obtain a positive electrode active material coating. A graphene conductive slurry is then sprayed onto the surface of the positive electrode active material coating under rotating conditions to form a positive electrode current collector with the positive electrode active material coating. A carbon negative electrode slurry is applied to the other side of the battery separator to obtain a negative electrode active material coating. The graphene conductive slurry is then sprayed onto the surface of the negative electrode active material coating under rotating conditions to form a negative electrode current collector, thereby obtaining a thin film composite thin film battery cell. By using graphene as a current collector and plasma bombardment to enhance the adhesion between the layers of the integrated flexible thin film battery, an integrated flexible graphite composite thin film battery cell is obtained.

Description

Technical Field

[0001] The present invention relates to the field of graphene conductive agents, and in particular to a method for preparing a graphene lithium battery core and a graphene-based lithium-ion battery. Background Art

[0002] With the rapid development of flexible devices, wearable devices, and smart biosensor technologies, smart, portable, wearable, and flexible electronic products are emerging. This also poses new challenges for flexible and adaptable power sources, and research on flexible thin-film lithium-ion batteries has garnered widespread attention. Compared to traditional pouch, cylindrical, and button batteries, flexible thin-film batteries can adapt to a variety of curved and deformable operating environments. Traditional lithium-ion batteries have separate positive and negative electrodes, and the separator is typically a coiled structure consisting of the negative electrode, separator, and positive electrode. The active material coatings on the positive and negative electrodes are not well suited to repeated deformation. This is because the aluminum current collector of the positive electrode and the copper current collector of the negative electrode rely solely on adhesives to support the active material coating, resulting in weak peel strength. Repeated deformation can cause the active material to detach, leading to a rapid decline in battery performance. Therefore, traditional lithium-ion batteries are generally not flexible and are unsuitable for powering flexible devices that require flexible, curved, and deformable energy. Summary of the Invention

[0003] In response to the above problems, the present invention provides a flexible thin-film graphene-based lithium-ion battery. The cell structure of this lithium-ion battery is a composite thin film that integrates a current collector, positive and negative electrodes, and a diaphragm. The composite thin-film battery cell is directly prepared by a high-speed rotary spraying process, which makes full use of the centrifugal force caused by high-speed rotation and the van der Waals force of rapid evaporation to obtain a tight interface bonding force. The composite thin-film battery cell directly integrates the current collector, positive electrode, negative electrode, and diaphragm into an integrated thin film, which can be used as a flexible lithium-ion battery after direct packaging. Compared with traditional "separate" lithium-ion batteries, the flexible thin-film graphene-based lithium-ion battery provided by the present invention not only has good flexibility, but also maintains stable electrochemical performance, cycle life and good structural stability after multiple bending. The composite thin-film battery cell also greatly shortens the lithium ion deintercalation and transport distance, and greatly improves the diffusion coefficient of lithium ions.

[0004] The primary purpose of the present invention is to provide a method for preparing a graphene lithium battery cell. Another purpose of the present invention is to provide a graphene-based lithium-ion battery.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] The main preparation process of a graphene lithium battery cell provided by the present invention includes:

[0007] 1) Prepare graphene conductive slurry: The graphene conductive slurry is configured with graphene powder as the conductive additive, PVP-K30 as the dispersant, and NMP as the solvent. The preferred specific surface area of ​​graphene is >400m 2 / g graphene powder, with a designed carbon content of 1.5-2.5% and a designed dispersant content of 0.6-1.2%, is obtained after being homogenized at a pressure of >800 bar for 5-10 times and then sand-milled at a speed of >1500 r / min for 3-5 hours.

[0008] 2) Prepare lithium iron phosphate positive electrode slurry for standby use: select lithium iron phosphate (LFP) with a smaller particle size (D50 = 5 μm) as the positive electrode active material, use the graphene conductive slurry prepared in step 1) as the conductive agent, use PVDF-1810 (purchased from Shanghai Huiping Chemical Co., Ltd.) as the binder, and use NMP as the solvent to prepare the positive electrode slurry. The designed solid content ratio of LFP is 96%, and the designed solid content ratio of graphene conductive slurry is 2%; the PVDF-1810 binder addition ratio is 2%, and a vacuum planetary mixer is used to mix for 3-5 hours to obtain the LFP positive electrode slurry.

[0009] 3) Prepare negative electrode slurry for standby use: Use sieved artificial graphite as the main negative electrode material, with a designed addition ratio of 98%, use CMC-Li (purchased from Green Energy Fiber Technology Co., Ltd.) as a dispersant, with a designed addition ratio of 1.5%, use the graphene conductive slurry prepared in step 1) as a conductive additive, with a designed addition ratio of 0.5%, use NMP as a solvent, and use a vacuum planetary mixer to mix for 3-5 hours to obtain the negative electrode slurry.

[0010] 4) Battery separator pretreatment: Soak the separator in a 0.5-1.5M hydrochloric acid solution containing hydrogen peroxide for 24 hours. The hydrogen peroxide solution is added at a rate of one-tenth the mass of the hydrochloric acid solution. After soaking, remove the separator and rinse five times with deionized water. After acid washing and oxidation, the membrane surface contains a small amount of oxygen-containing functional groups and achieves a certain degree of wettability. After wettability, oxygen plasma bombardment is applied to the membrane surface to generate a large number of oxygen-containing functional groups. The membrane is then flattened and transferred to a plasma etcher where it is bombarded with oxygen plasma for 10-20 minutes before removal. This aims to enrich the separator with hydrophilic oxygen-containing functional groups. Hydrophilic oxygen-containing functional groups, such as hydroxyl and carboxyl groups, can react with other groups in the substrate to form chemical bonds or electrostatic adsorption, achieving an "anchoring" effect and enhancing chemical adhesion between the separator and the active material coating. The separator can be made of PP, PE, PVDF, or a PP / PE / PP composite film, with PP / PE / PP composite films being preferred.

[0011] 5) The battery separator after the pretreatment in step 4) is transferred and fixed on the spin coating disk of a flat high-speed centrifugal spray coater. Under the rotation condition of 3500-5000 r / min, the LFP positive electrode slurry prepared in step 2) is sprayed on the composite separator. The spraying time is designed to be 10-20 s. Step 5) is completed within 5 minutes after step 4) is completed to form a positive electrode active material layer to obtain an LFP / PP / PE / PP film.

[0012] 6) After step 5) is completed for 1 minute, a self-supporting graphene current collector layer is sprayed. The graphene conductive slurry prepared in step 1) is sprayed on the surface of the positive electrode active material layer under the rotation condition of 3500-5000 r / min. The spraying time is designed to be 10-20 s. The positive electrode current collector is formed with the positive electrode active material layer to obtain an RGO / LFP / PP / PE / PP film sample.

[0013] 7) Take out the thin film sample obtained in step 6) and transfer it to a plasma etcher, and use oxygen plasma to etch the unsprayed side of the uncomposite membrane with oxygen plasma for 10-20 minutes. After the end, transfer it to the spin coating disk of the high-speed centrifugal spray coater, with the unsprayed side of the uncomposite membrane facing up. Under the rotation condition of 3500-5000r / min, the carbon negative electrode slurry prepared in step 2) is sprayed on the unsprayed side of the composite membrane. The spraying time is designed to be 10-20s to form a negative electrode material layer to obtain an RGO / LFP / PP / PE / PP / C thin film sample. This step is completed within 5 minutes after the end of oxygen plasma etching.

[0014] 8) Spray a self-supporting graphene current collector layer on the surface of the negative electrode material layer, and spray the graphene conductive slurry prepared in step 1) on the surface of the negative electrode material layer under the rotation condition of 3500-5000r / min. The spraying time is designed to be 10-20s. This step is completed 1 minute after the end of step 7), and the negative electrode current collector is formed with the negative electrode material layer to obtain an RGO / LFP / PP / PE / PP / C / RGO composite thin film battery cell sample.

[0015] Another object of the present invention is to provide a graphene-based lithium-ion battery, wherein the composite thin film battery cell obtained in step 8 is subjected to a bending test and is encapsulated using lithium-ion battery heat-sealed aluminum foil and injected with electrolyte to assemble into a flexible thin film battery.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. The present invention provides a method for preparing a graphene lithium battery cell. The difference from the traditional "separate" lithium-ion battery cell is that the cell prepared by this method is an "integrated" composite film. The bending of traditional battery electrodes will cause the active material to fall off. The composite film cell provided by the present invention is more flexible than traditional lithium-ion battery cells, and has a curved surface and deformation application range that traditional batteries do not have. Compared with traditional lithium-ion batteries, flexible thin-film lithium-ion batteries are more suitable for powering wearable electronic devices, smart biosensors, etc.

[0018] 2. The present invention produces abundant hydrophilic oxygen-containing functional groups on the surface of the composite membrane by acidification and oxygen plasma etching of the composite membrane, thereby helping to enhance the chemical adhesion between the membrane and the active material coating. By making full use of the centrifugal force caused by high-speed rotation and the van der Waals force of rapid evaporation, a composite thin-film battery cell with strong interface bonding is obtained. The composite thin-film battery cell shortens the free path of lithium ion transport during the intercalation and deintercalation process, greatly improving the ion diffusion coefficient of lithium ions.

[0019] 3. The present invention provides a flexible thin-film graphene-based lithium-ion battery, which is a flexible thin-film lithium-ion battery. The problem of weak contact strength between the active material coating and the current collector interface is solved by high-speed centrifugal spraying. The graphene layer is obtained by high-speed centrifugal spraying as a self-supporting positive and negative current collector. Compared with traditional aluminum and copper current collectors, the contact resistance between the graphene current collector and the active material coating is smaller, providing an important reference and reference for the research of flexible and thin-film batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic structural diagram of a graphene thin film battery according to the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described below in conjunction with specific examples, but should not be construed as limiting the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0022] The following is a specific embodiment:

[0023] Example 1

[0024] The present invention proposes a method for preparing a graphene lithium battery cell, and the specific steps are as follows:

[0025] 1) Prepare graphene conductive slurry for standby use: The graphene conductive slurry is configured with graphene powder as a conductive additive, PVP-K30 as a dispersant, and NMP as a solvent. The graphene preferably has a specific surface area of ​​450 m2 / g. Calculated by mass percentage, the designed carbon content is 2.0%, the designed dispersant content is 1.0%, and the balance is NMP solvent. The slurry is homogenized at a pressure of 1000 bar for 8 times and then sand-milled at a speed of 2000 r / min for 4 hours to obtain the slurry.

[0026] 2) Prepare lithium iron phosphate positive electrode slurry for standby use: select lithium iron phosphate (LFP) with a small particle size (D50 = 5 μm) as the positive electrode active material, use the graphene conductive slurry prepared in step 1) as the conductive agent, use PVDF-1810 (purchased from Shanghai Huiping Chemical Co., Ltd.) as the binder, and use NMP as the solvent to prepare the positive electrode slurry. Calculated by mass percentage, the designed solid content ratio of LFP is 96%, and the designed solid content ratio of graphene conductive slurry is 2%; the PVDF-1810 binder addition ratio is 2%, and a vacuum planetary mixer is used to mix for 4 hours to obtain the LFP positive electrode slurry.

[0027] 3) Prepare negative electrode slurry for standby use: Use sieved artificial graphite as the main material for the negative electrode. Calculated by mass percentage, the designed addition ratio of artificial graphite is 98%. CMC-Li (purchased from Green Energy Fiber Technology Co., Ltd.) is used as a dispersant with a designed addition ratio of 1.5%. Use the graphene conductive slurry prepared in step 1) as a conductive additive with a designed addition ratio of 0.5%. Use NMP as a solvent and use a vacuum planetary mixer for 4 hours to obtain the negative electrode slurry.

[0028] 4) PP / PE / PP Battery Separator Pretreatment: PP / PE / PP battery separator 3 (20μm thick) was soaked in a 1.0M hydrochloric acid solution containing hydrogen peroxide for 24 hours. The hydrogen peroxide solution was added at a concentration of one-tenth the mass of the hydrochloric acid solution. After soaking, the separator was removed and washed five times with deionized water. After acid washing and oxidation, the film surface had a small amount of oxygen-containing functional groups and achieved a certain degree of wettability. After wettability, oxygen plasma bombardment was applied to the film surface to generate a large number of oxygen-containing functional groups. The separator was flattened and transferred to a plasma etcher and bombarded with oxygen plasma for 15 minutes before removal. This aims to enrich the PP / PE / PP polymer separator with hydrophilic oxygen-containing functional groups, which helps to enhance the chemical adhesion between the separator and the active material coating.

[0029] 5) The battery separator after the pretreatment in step 4) is transferred and fixed on the spin coating disk of a flat high-speed centrifugal spray coater, and a 1 cm area on the outer edge of the composite separator is covered. Under the rotation condition of 4000 r / min, the LFP positive electrode slurry prepared in step 2) is sprayed on the composite separator. The spraying time is designed to be 15 s. Step 5) is completed within 5 minutes after the completion of step 4) to form a positive electrode active material layer 2 and obtain an LFP / PP / PE / PP film, wherein the LFP positive electrode active material layer 2 is designed to have a thickness of 30±2 μm.

[0030] 6) One minute after step 5), a self-supporting graphene current collector layer was sprayed. The graphene conductive slurry prepared in step 1) was sprayed on the surface of the LFP coating at a rotation speed of 4000 r / min. The spraying time was designed to be 10 s to obtain an RGO / LFP / PP / PE / PP film sample, wherein the RGO spray layer served as a self-supporting positive electrode current collector 1 with a designed thickness of 10±2 μm.

[0031] 7) Take out the thin film sample obtained in step 6) and transfer it to a plasma etcher, and use oxygen plasma to etch the unsprayed side of the uncomposite membrane with oxygen plasma for 15 minutes. After the etching, transfer it to the spin coating disk of the high-speed centrifugal sprayer, with the unsprayed side of the uncomposite membrane on top. Under the rotation condition of 4000r / min, spray the carbon negative electrode slurry prepared in step 2) on the unsprayed side of the composite membrane. The spraying time is designed to be 15s to form a negative electrode material layer 4, and obtain an RGO / LFP / PP / PE / PP / C thin film sample. This step is completed within 5 minutes after the end of oxygen plasma etching. The designed thickness of the negative electrode material layer 4 is 30±2μm.

[0032] 8) Spray a self-supporting graphene current collector layer on the surface of the negative electrode coating. The graphene conductive slurry prepared in step 1) is sprayed on the surface of the carbon negative electrode coating under a rotation condition of 4000 r / min. The spraying time is designed to be 10 s. This step is completed 1 minute after the end of step 7) to obtain an RGO / LFP / PP / PE / PP / C / RGO film sample. The RGO spray layer serves as a self-supporting negative electrode current collector 5 with a designed thickness of 10±2 μm.

[0033] 9) The composite thin-film cell obtained in step 8 was subjected to a bending test and encapsulated using heat-sealed aluminum foil for lithium-ion batteries, injected with electrolyte, and assembled into a flexible thin-film battery. Related electrical performance tests were also conducted; the results showed that the composite thin-film cell had a thickness of 117.3 μm and remained intact after 1600 bends (at a 90° bend angle), with no damage, powder loss, or cracking. The film cell structure remained intact. The encapsulated battery exhibited a discharge capacity of 142.2 mAh / g under 0.1C / 0.1C charge / discharge conditions, with an initial efficiency of 87.3%. The discharge capacity was 117.3 mAh / g under 1C / 1C charge / discharge conditions, with a capacity retention rate of 88.6% after 200 cycles. The low initial efficiency can be attributed to the large amount of irreversible lithium loss formed in the SEI film of the negative electrode during the first cycle due to the addition of graphene and smaller-sized artificial graphite particles to the negative electrode.

[0034] Example 2

[0035] The difference between this embodiment and embodiment 1 is that in step 7), when spraying the negative electrode material layer, the pre-loaded composite separator surface is not subjected to oxygen plasma etching, but is directly sprayed. The other steps are the same as those in embodiment 1. The purpose is to explore the effect of oxygen plasma etching on the strength of the negative electrode material coating. The specific steps are as follows:

[0036] 1) Prepare graphene conductive slurry for standby use. The specific steps are the same as those in Example 1.

[0037] 2) Prepare lithium iron phosphate positive electrode slurry for standby use, and the specific steps are the same as those in Example 1.

[0038] 3) Prepare negative electrode slurry for standby use. The specific steps are the same as those in Example 1.

[0039] 4) Pretreatment of PP / PE / PP battery separator: the specific steps are the same as those in Example 1.

[0040] 5) Spraying the positive electrode active material layer, the specific steps are the same as those in Example 1.

[0041] 6) Spraying the positive electrode current collector, the specific steps are the same as those in Example 1.

[0042] 7) Take out the thin film sample obtained in step 6) and transfer it to the spin coating disk of a high-speed centrifugal spray coater, with the unsprayed side of the composite membrane facing up. Under the rotation condition of 4000r / min, the carbon negative electrode slurry prepared in step 2) is sprayed on the unsprayed side of the composite membrane. The spraying time is designed to be 15s to form a negative electrode material layer to obtain an RGO / LFP / PP / PE / PP / C thin film sample. This step is completed within 5 minutes after the end of oxygen plasma etching. The designed thickness of the negative electrode material layer is 30±2μm.

[0043] 8) Spraying the negative electrode current collector: the specific steps are the same as those in Example 1 to obtain a composite thin film battery cell.

[0044] The results showed that after 727 bends, cracks appeared at the interface between the negative electrode coating and the composite separator. This was attributed to the fact that oxygen plasma etching created a large number of oxygen-containing functional groups on the composite separator surface, which enhanced the bonding between the active material coating and the composite separator.

[0045] Example 3

[0046] This example differs from Example 1 in that the membrane surface is not bombarded with oxygen plasma in step 4). The remaining steps are identical to those in Example 1 and are not described here. The purpose of this example is to investigate the effect of oxygen plasma etching on the strength of the active material coating. The specific steps are as follows:

[0047] 1) Prepare graphene conductive slurry for standby use. The specific steps are the same as those in Example 1.

[0048] 2) Prepare lithium iron phosphate positive electrode slurry for standby use, and the specific steps are the same as those in Example 1.

[0049] 3) Prepare negative electrode slurry for standby use. The specific steps are the same as those in Example 1.

[0050] 4) Wash the PP / PE / PP battery separator 5 times with deionized water.

[0051] The following steps are the same as those in Example 1 and will not be repeated here.

[0052] Example 4

[0053] This example differs from Example 1 in that the separator is not pickled in step 4), and the other steps are the same as in Example 1. The purpose is to explore the effect of pickling on the enhancement of chemical adhesion between the separator and the active material coating.

[0054] 1) Prepare graphene conductive slurry for standby use. The specific steps are the same as those in Example 1.

[0055] 2) Prepare lithium iron phosphate positive electrode slurry for standby use, and the specific steps are the same as those in Example 1.

[0056] 3) Prepare negative electrode slurry for standby use. The specific steps are the same as those in Example 1.

[0057] 4) Pretreatment of PP / PE / PP battery separator: The PP / PE / PP battery separator (thickness 20 μm) was washed 5 times with deionized water, flattened and transferred to a plasma etcher and bombarded with oxygen plasma for 15 min.

[0058] The following steps are the same as those in Example 1 and will not be repeated here.

[0059] Comparative Example 1

[0060] This example differs from Example 1 in that the diaphragm surface is not bombarded with oxygen plasma in steps 4) and 7). All other steps are identical to those in Example 1 and are not described here. The purpose of this example is to investigate the effect of the absence of oxygen plasma bombardment on the diaphragm surface loading. Results showed that the composite thin-film cell exhibited cracking and powder loss after 228 bends.

[0061] Comparative Example 2

[0062] This example differs from Example 1 in that, in steps 5) and 7), the active material spraying was performed 30 minutes after the separator was subjected to oxygen plasma bombardment. All other steps were identical to those in Example 1 and are not further described. The purpose of this experiment was to investigate the long-term durability of oxygen-containing functional groups generated on the composite separator surface by oxygen plasma bombardment. Results showed that the composite thin-film cell began to crack after 922 flex cycles.

[0063] Comparative Example 3

[0064] This example differs from Example 1 in that copper is used instead of graphene as the current collector. All other steps are identical to Example 1. The purpose of this experiment was to investigate the differences in the bending and electrical performance of a non-integrated thin-film cell compared to the present invention. Results showed that after 13 bends, the composite thin-film cell began to experience detachment of the active material coating, cracking of the coating, and irreversible creases.

[0065] Electrical performance test

[0066] The composite thin-film cells prepared in Examples 1-5 and Comparative Examples 1-2 were subjected to electrical performance testing. After the bending test, the composite thin-film cells were encapsulated with heat-sealed aluminum foil for lithium-ion batteries and injected with electrolyte to form flexible thin-film batteries. Specific test data is shown in the table below.

[0067]

[0068] From the experimental results of the above electrical performance tests, it can be seen that after the composite thin film cell obtained in Example 1 is assembled into a flexible thin film battery, after 200 cycles of testing, the cycle capacity retention rate is 88.6%, and the battery has good cycle performance. After research and analysis, it is believed that due to the chemical adhesion between the diaphragm and the active material coating after oxygen plasma bombardment treatment, since both the positive and negative electrode material layers can be stably loaded on the surface of the diaphragm, the stability is strong after the composite thin film cell is prepared, and the phenomenon is that the cycle capacity retention rate of the composite thin film cell after being prepared into a flexible thin film battery is high. One side of Example 2 and Example 3 is not plasma bombarded, so the negative electrode material layer of Example 2 has poor adhesion and is easy to fall off after multiple charge and discharge, reducing the sites for the positive electrode active material to be embedded. After 87 cycles, the cycle capacity retention rate is less than 80%; the positive electrode active material layer of Example 3 is not treated, so the positive electrode active material layer has poor adhesion. When the positive electrode active material layer falls off, the overall capacity decreases. Therefore, the cycle capacity retention rate after 72 weeks has dropped to below 80%, which is a greater decline than that of Example 2. Example 4 uses deionized water washing instead of acid washing, and the cycle capacity retention rate is not as good as Example 1. After analysis, it is believed that the acid washing step can make the diaphragm surface have a small amount of oxygen-containing functional groups and achieve a certain infiltration effect. After infiltration, oxygen plasma bombardment is conducive to the generation of a large number of oxygen-containing functional groups on the film surface. Example 4 does not undergo acid washing, and the diaphragm is not fully infiltrated, which affects the effect of subsequent oxygen plasma bombardment, causing the adhesion between the positive and negative electrode material layers and the diaphragm to decrease. After 155 cycles, the cycle capacity retention rate is reduced to below 80%. Comparative Example 1 does not undergo plasma bombardment treatment. Therefore, after 67 weeks of cycle capacity retention, it is less than 80%. Comparative Example 2 is sprayed after 30 minutes after oxygen plasma bombardment treatment. Compared with Example 1, the cycle stability of the battery is also significantly reduced. After analysis, it is believed that the oxygen-containing functional groups generated in the diaphragm after oxygen plasma bombardment are in an active state. After a period of time, the oxygen-containing functional groups will gradually decrease and eventually disappear. Spraying of the positive and negative electrode materials must be performed as soon as possible after plasma bombardment treatment. Comparative Example 3 uses copper instead of the high-speed rotating sprayed graphene layer as the current collector. At the same time, the interfacial bonding strength between the positive and negative electrodes and the diaphragm is similar to that of Example 1. Under static test conditions, it shows good cycle performance. However, compared with the sprayed graphene layer, the interface between the copper current collector and the negative electrode relies on adhesive coating. Therefore, the interfacial bonding strength between the copper current collector and the negative electrode is weak, and the cycle performance is lower than that of Example 1 with higher bonding strength.

[0069] Bending test

[0070] The composite thin film battery cells prepared in Examples 1-5 and Comparative Examples 1-2 were subjected to a bending test using the three-point bending method with reference to the relevant conditions of the national standard GB / T35465.5-2020.

[0071]

[0072] From the experimental results of the above-mentioned bending test, it can be seen that the composite thin film battery cell obtained in Example 1 is still intact, without powder loss or cracking after being bent 1,600 times, and the thin film battery cell structure remains intact; the difference between Example 2 and Example 1 is that the surface of the pre-loaded composite diaphragm in Example 2 is directly sprayed without oxygen plasma etching, so the adhesion between the diaphragm and the negative electrode material deteriorates, thereby affecting the bonding strength of the composite thin film battery cell, and the phenomenon manifested is that in the bending test, when the number of bends is 647 times, cracking occurs at the interface between the negative electrode coating and the composite diaphragm. Obviously, since oxygen plasma bombardment treatment was not performed before spraying the negative electrode material, the bonding strength between the negative electrode material and the diaphragm is significantly reduced, and the result is that cracking occurs at the interface between the negative electrode coating and the diaphragm after multiple bending. Similarly, Example 3 differs from Example 1 in that plasma bombardment treatment is not performed before spraying the positive electrode active material. The final result is cracking at the interface between the positive electrode active material layer and the separator after repeated bending. This demonstrates that plasma bombardment can enhance the chemical adhesion between the separator and the active material coating.

[0073] Example 4 is to verify the effect of the pickling step. The difference from Example 1 is that before spraying, the diaphragm is only cleaned with deionized water. Although the bending resistance is better than that of Examples 2 and 3, cracks will appear at the interface between the positive electrode active material layer and the diaphragm and at the interface between the negative electrode material layer and the diaphragm after bending thousands of times. Analysis shows that after pickling and oxidation, the surface of the film has a small amount of oxygen-containing functional groups and achieves a certain wetting effect. After wetting, oxygen plasma bombardment is beneficial to the generation of a large number of oxygen-containing functional groups on the surface of the film. Direct oxygen plasma bombardment without treatment can also make the surface of the film have oxygen-containing functional groups, but the number will not be large. When the number of oxygen-containing functional groups decreases, it directly affects the adhesion between the spray layer and the diaphragm.

[0074] In order to further verify the effect of plasma bombardment on the reliability of composite thin film batteries, comparative example 1 was not subjected to plasma bombardment treatment, and the bonding strength between the positive and negative electrode material layers and the diaphragm could not pass the test. After 578 bends, cracking began to appear between the positive and negative electrode material layers and the diaphragm. After analysis, it is believed that without plasma bombardment treatment, the small amount of oxygen-containing functional groups after pickling cannot allow the positive and negative electrode material layers to be stably attached to the surface of the diaphragm. Comparative example 2 carried out active material spraying 30 minutes after the diaphragm was plasma treated, and a bending test result different from that of Example 1 appeared. Cracks began to appear at the interface between the positive and negative electrode materials and the diaphragm after less than 1,000 bends. After analysis, it is believed that when the functional groups connected by plasma bombardment treatment are in an unstable state, if they are not sprayed in time, the oxygen-containing functional groups on the surface of the diaphragm will automatically fall off over time, thereby reducing the bonding strength between the positive and negative electrode material layers and the diaphragm. Comparative Example 3 is to verify whether the traditional copper current collector can be used to prepare an integrated composite thin film battery cell. Since copper is loaded on the active material coating only by relying on an adhesive, the peel strength is very weak. Repeated deformation will cause cracking and peeling at the interface between the copper current collector and the positive and negative active materials, and it cannot pass the bending test.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a graphene lithium battery cell, characterized in that: After the hydrophilic oxygen-containing functional groups are connected to the surface of the battery separator, the positive electrode slurry is first coated on one surface of the battery separator containing the hydrophilic oxygen-containing functional groups to obtain a positive electrode active material coating, and the graphene conductive slurry is sprayed on the surface of the positive electrode active material coating under the condition of rotation to form a positive electrode current collector with the positive electrode active material coating; the carbon negative electrode slurry is coated on the other side of the battery separator to obtain a negative electrode active material coating, and the graphene conductive slurry is sprayed on the surface of the negative electrode active material coating under the condition of rotation to form a negative electrode current collector, so as to achieve a thin film Composite thin film battery cell; the specific steps of connecting hydrophilic oxygen-containing functional groups on the surface of the battery separator are: after the battery separator is placed flat, it is transferred to a plasma etcher and bombarded with oxygen plasma for 10-20 minutes and then taken out; before the plasma bombardment treatment, the battery separator is immersed in a hydrochloric acid aqueous solution containing hydrogen peroxide for 16-24 hours, and after the soaking is completed, it is taken out and washed 5 times with deionized water; the positive electrode slurry and the negative electrode slurry need to be coated within 5 minutes after the hydrophilic oxygen-containing functional groups are connected to the surface of the battery separator.

2. A method for preparing a graphene lithium battery cell according to claim 1, characterized in that: The mass ratio of hydrogen peroxide solution to hydrochloric acid solution is 1:10; the specific surface area of ​​the graphene powder used to prepare the graphene conductive slurry is greater than 400m 2 / g.

3. A method for preparing a graphene lithium battery cell according to claim 2, characterized in that: The preparation steps of the graphene conductive slurry are as follows: using PVP-K30 as a dispersant, NMP as a solvent, and graphene powder as a conductive agent, wherein, calculated by mass percentage, the designed carbon content in the components is 1.5-2.5%, the designed dispersant content is 0.6-1.2%, and the balance is NMP; after mixing the above components, homogenizing them 5-10 times and then sand milling them for 3-5 hours to obtain the graphene conductive slurry, wherein the pressure of the homogenization step is greater than 800 bar, and the speed of the sand milling treatment is greater than 1500 r / min.

4. The method for preparing a graphene lithium battery cell according to claim 1, wherein: The preparation step of the positive electrode slurry is: according to the mass percentage, 96% of lithium iron phosphate, 2% of PVDF adhesive and 2% of graphene conductive slurry solid content are put into a planetary mixer for mixing for 3-5 hours to obtain the positive electrode slurry.

5. The method for preparing a graphene lithium battery cell according to claim 1, wherein: The preparation step of the negative electrode slurry is: according to the mass percentage, 98% of artificial graphite, 1.5% of CMC-Li dispersant, and 0.5% of graphene conductive slurry solid content are put into a planetary mixer for mixing for 3-5 hours to obtain the negative electrode slurry.

6. The method for preparing a graphene lithium battery cell according to claim 1, wherein: The battery separator is selected from PP film, PE film, PVDF film or PP / PE / PP composite film.

7. The method for preparing a graphene lithium battery cell according to claim 6, wherein: The battery separator is a PP / PE / PP composite film.

8. A graphene-based lithium-ion battery, characterized in that: The graphene lithium battery cell prepared by the method according to any one of claims 1 to 7 is packaged with lithium-ion battery heat-sealed aluminum foil and injected with electrolyte to assemble into a graphene-based lithium-ion battery.

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