Crosslinked polyimide adhesive for lithium ion battery, electrode and preparation method

Abstract

The invention relates to a cross-linked polyimide adhesive for a lithium ion battery and an electrode, and a preparation method of the polyimide adhesive comprises the following steps: mixing aromatic dianhydride, diamine and a first solvent for prepolymerization to obtain polyamide acid; fully mixing the polyamide acid and the cross-linking agent to obtain a mixed glue solution; coating the mixed glue solution on a glass slide to form a film, then heating to 200-350 DEG C, and carrying out imidization reaction and cross-linking reaction to obtain the cross-linked polyimide adhesive for the lithium ion battery. The diamine selected by the polyimide adhesive disclosed by the invention contains a plurality of ether bonds, so that the polyimide adhesive has good solubility. Meanwhile, the cross-linking agent also contains a plurality of ether bonds, so that the solubility of the cross-linking polyimide adhesive is further improved, the bonding effect is better played, and the battery performance is improved. The prepared electrode using the cross-linked polyimide adhesive is low in internal resistance, good in adhesive performance and good in electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and in particular to a cross-linked polyimide adhesive for lithium ion batteries and a preparation method thereof, and also to an electrode using the cross-linked polyimide adhesive and a preparation method thereof. Background Art

[0002] Batteries have become the core of the new energy technology field, and higher requirements have been placed on batteries, especially in terms of energy density and electrical cycle performance. Lithium-ion batteries are composed of positive electrodes, separators, negative electrodes and electrolytes. The positive electrodes and negative electrodes are the key components of the battery. The positive electrode material is generally made by coating the positive electrode slurry onto aluminum foil. The quality of the positive electrode sheet is related to the performance of the battery. In the preparation process of the positive electrode sheet, there are the following problems: (1) The use of a large amount of conductive agent and adhesive leads to an increase in the internal resistance of the battery and excessive side reactions. (2) The commercial adhesive polyvinylidene fluoride (PVDF) has low bonding strength, is easy to swell and decompose in the electrolyte, has poor thermal stability, weak ion diffusion ability, insufficient resistance to high voltage and many other problems. These problems not only seriously affect the electrochemical performance of the battery, but also fail to meet the safety requirements of high-performance batteries. (3) The volume change of the electrode active material is easy to fall off, and interfacial reactions are prone to occur during the cycle, resulting in phase change and decreased structural stability.

[0003] Therefore, it is necessary to develop a lithium-ion battery with low internal resistance, good bonding performance and good electrochemical performance. Summary of the invention

[0004] To this end, the present invention provides a cross-linked polyimide adhesive for lithium ion batteries and a preparation method thereof, which is expected to improve the performance and cycle life of the battery and meet the urgent demand of the electric vehicle industry for more advanced and efficient battery technology. The present invention also provides an electrode using the cross-linked polyimide adhesive with high cycle life, low internal resistance, good bonding performance and good electrochemical performance and a preparation method thereof.

[0005] To achieve the above object, the inventor provides a method for preparing a cross-linked polyimide adhesive for lithium ion batteries, which comprises the following steps: S1: In an inert atmosphere, an aromatic dianhydride, a diamine and a first solvent are mixed, and prepolymerized at 5-20° C. for 4-12 hours to obtain a polyamic acid; S2: fully mixing the polyamic acid and the cross-linking agent to obtain a mixed glue solution; the mass of the cross-linking agent added is 5-15% of the total mass of the aromatic dianhydride and the diamine; S3: coating the mixed adhesive on a glass slide to form a film, and then heating to 200-350° C. to perform imidization reaction and cross-linking reaction to obtain the cross-linked polyimide adhesive for lithium-ion batteries.

[0006] The cross-linked polyimide adhesive for lithium ion batteries prepared by the present invention, on the one hand, improves the solubility of the adhesive due to the introduction of multiple ether bonds, so that the adhesive can be better dissolved and play a bonding role; on the other hand, the multiple carbonyl structures in the structure have good affinity with lithium ions, which is beneficial to the diffusion of lithium ions, thereby improving the electrochemical performance.

[0007] In addition, the polyimide adhesive of the present invention adopts aromatic dianhydride, which contains a large number of benzene rings and a network structure formed by a cross-linking reaction, so that the adhesive has excellent mechanical properties and chemical stability. After the carboxyl cross-linking agent is introduced into the polyimide adhesive of the present invention, the polyamic acid undergoes a cross-linking reaction while imidization. The formation of the cross-linked network structure gives the adhesive excellent bonding properties, allowing the adhesive to better bond electrode active materials and conductive agents, stabilize electrode materials, reduce polarization and side reactions in redox reactions, and reduce internal resistance. Furthermore, the cross-linked PI segment also has more carbonyl groups, which also promotes the diffusion of lithium ions. The comprehensive design of the above structure improves the performance of lithium-ion batteries and has certain advantages over the commercial adhesive PVDF.

[0008] Further, the aromatic dianhydride is one of the following: pyromellitic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylene) diphthalic anhydride, 4,4'(4,4-isopropylenediphenoxy)bis(naphthalic anhydride), biphenyltetracarboxylic dianhydride, 4,4-biphenyl ether dianhydride; The diamine is one or more of the following: 1,2-bis(2-aminoethoxy)ethane, 4,4-diaminodiphenyl ether, 1,2-bis(4-aminophenoxy)ethane, bis(3-aminopropyl) end-capped polydimethylsiloxane), 2,4-diaminobenzoic acid, 3,4-diaminobenzoic acid, 2,5-diaminoterephthalic acid, 3,5-diaminobenzoic acid; The molar ratio of the diamine added in step S1 to the aromatic dianhydride is 1:1.0-1.2.

[0009] Further, the cross-linking agent is one of the following: trimethylolpropane-tris[3-(2-methylaziridine-1-yl)propionate], N,N'-(methylenebis-P-phenyl)bis(aziridine-1-carboxamide), pentaerythritol tris(3-aziridine)propionate, 1,1'-(1,3-phthaloyl)bis(2-methyl-aziridine), tris(2-methylaziridine)phosphine oxide, carbodiimide hydrochloride, isocyanate or 3,5-diaminobenzoic acid.

[0010] The present invention also provides a cross-linked polyimide adhesive for lithium ion batteries. The cross-linked polyimide adhesive for lithium ion batteries is prepared by the above-mentioned preparation method.

[0011] The present invention also provides an application of a cross-linked polyimide adhesive for lithium ion batteries. The cross-linked polyimide adhesive is applied as a lithium ion battery adhesive.

[0012] The present invention also provides an electrode using a cross-linked polyimide adhesive, which comprises the following components in parts by weight: 80-90 parts of electrode active material; 5-10 parts of the cross-linked polyimide adhesive; Conductive agent 5-10 parts.

[0013] Furthermore, the electrode active material includes a positive electrode active material or a negative electrode active material.

[0014] Furthermore, the conductive agent is one of the following: conductive carbon black, acetylene black, carbon nanotubes, graphene, or Ketjen black.

[0015] Furthermore, the positive electrode active material includes one of the following: lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese oxide, lithium cobalt oxide, lithium nickel cobalt aluminum oxide, and lithium-rich manganese base; when the electrode is applied to the positive electrode, the positive electrode active material is added to the electrode; The negative electrode active material includes one of the following: nano silicon, micro silicon, graphite, silicon-carbon material; when the electrode is applied to the negative electrode, the negative electrode active material is added to the electrode.

[0016] The present invention also provides a method for preparing an electrode using the cross-linked polyimide adhesive, which comprises the following steps: S1: In an inert atmosphere, an aromatic dianhydride, a diamine and a first solvent are mixed, and prepolymerized at 5-20° C. for 4-12 hours to obtain a polyamic acid; S2: fully mixing the electrode active material, the polyamic acid, the conductive agent, the cross-linking agent, and the second solvent to prepare an electrode slurry; S3: coating the electrode slurry on the current collector to form a film, and then heating to 200-350° C. to perform imidization reaction and cross-linking reaction to prepare the electrode using the cross-linked polyimide adhesive.

[0017] Further, the step S3 is specifically as follows: coating the electrode slurry on the current collector to form a film with a thickness of 40-200 μm, and after the solvent of the film evaporates, placing the electrode piece in a tubular furnace for programmed heating under a nitrogen atmosphere, and the heating rate is 3-8°C / min, and the temperature is raised from room temperature to 200-350°C under a nitrogen atmosphere and kept warm for 1-4 hours.

[0018] Furthermore, the first solvent is N-methylpyrrolidone or N-ethylpyrrolidone; the second solvent is N-methylpyrrolidone or N-ethylpyrrolidone.

[0019] In the battery prepared by the present invention, only a small amount of cross-linked polyimide adhesive is needed to achieve a good bonding effect. The structural design containing multiple ether bonds in the polyimide improves its solubility, can well penetrate into the electrode active material to play a bonding role, and tightly combine the electrode active material and the conductive agent, and has good bonding performance. The polyimide forms a cross-linked network structure by cross-linking reaction with the cross-linking agent, which gives the battery excellent mechanical properties, enables the adhesive to better bond the electrode active material and the conductive agent, stabilizes the electrode, reduces the polarization and side reactions in the redox reaction, and reduces the internal resistance of the battery. The multiple carbonyl groups in the polyimide adhesive structure have strong lithium ion affinity, which promotes the diffusion of lithium ions. Therefore, the cross-linked polyimide adhesive in the present invention is expected to improve the performance and cycle life of the battery, and meet the urgent needs of the electric vehicle industry for more advanced and efficient battery technology.

[0020] The beneficial effects of the present invention are: The polyimide adhesive of the present invention has multiple ether bonds in the diamine, which makes the polyimide adhesive have good solubility. At the same time, the cross-linking agent also contains multiple ether bonds, which further improves the solubility of the cross-linked polyimide adhesive, allowing it to better penetrate into the electrode active material, better play the bonding effect, and improve the battery performance.

[0021] The carbonyl group in the structure of the polyimide adhesive has a strong affinity with lithium ions, which promotes the diffusion of lithium ions. Compared with commercial battery adhesives, the use of the cross-linked polyimide adhesive of the present invention, which is rarely used, can ensure good bonding performance, improve the electrochemical performance of the battery, and provide a feasible solution for the development of high-performance batteries.

[0022] The network structure formed by the cross-linking reaction between the cross-linked polyimide adhesive for lithium ion batteries and the cross-linking agent in the present invention gives the adhesive excellent bonding properties, stabilizes the electrode active material, reduces the polarization and side reactions in the redox reaction, and thus reduces the internal resistance of the battery.

[0023] The electrode prepared by the invention using the cross-linked polyimide adhesive has low internal resistance, good bonding performance and good electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a picture of the polyamic acid prepared in Example 1-3.

[0025] Figure 2 This is a picture of the polyamic acid prepared in Example 1-3 after heating and cross-linking.

[0026] Figure 3 This is the infrared spectrum of the polyimide adhesive prepared in Example 1-3.

[0027] Figure 4 The peeling performance diagram of the positive electrode sheets prepared in Examples 5-7 and Comparative Example 1.

[0028] Figure 5 This is a SEM scanning electron microscope image of the positive electrode sheet prepared in Example 5; Figure 6 This is a SEM scanning electron microscope image of the positive electrode sheet prepared in Example 6; Figure 7 This is a SEM image of the positive electrode sheet prepared in Comparative Example 1; Figure 8 The figure is a rate performance test diagram of the battery at different current densities in Examples 5-7 and Comparative Example 1; Fig. 9 It is a long cycle performance test diagram of the battery at 3C in Examples 5-7 and Comparative Example 1; Fig.10 A positive peak current diagram of the oxidation reaction of the lithium ion diffusion performance of the battery in Examples 5-7 and Comparative Example 1; Fig.11 It is the negative peak current of the reduction reaction of the lithium ion diffusion performance of the batteries in Examples 5-7 and Comparative Example 1. DETAILED DESCRIPTION

[0030] In order to explain the technical content, structural features, achieved purpose and effect of the technical solution in detail, the following is a detailed description in conjunction with specific embodiments and accompanying drawings. It should be pointed out that if there are processes that are not particularly described in detail below, they can be implemented or understood by those skilled in the art with reference to the prior art. The reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased commercially.

[0031] The present invention provides embodiments of a cross-linked polyimide adhesive and a lithium ion battery using the cross-linked polyimide adhesive.

[0032] Example 1 This embodiment prepares a cross-linked polyimide adhesive, comprising the following steps: S1, preparation of polyamic acid: 20 mmol of 1,2-bis(2-aminoethoxy)ethane diamine monomer was dissolved in 60 mL of N-methylpyrrolidone (NMP), nitrogen was continuously introduced, stirred in a water bath at 10°C for 30 minutes to dissolve it, and then 20.4 mmol of dianhydride monomer pyromellitic dianhydride was added in batches, and nitrogen was introduced in a vacuum to react for 6 hours to obtain a polyamic acid solution. After the reaction was completed, the polyamic acid solution was collected and refrigerated for later use.

[0033] S2, putting the above polyamic acid glue solution into a beaker, weighing and adding 5% of trimethylolpropane-tris[3-(2-methylaziridine-1-yl) propionate] crosslinking agent relative to the mass fraction of the polyamic acid solute, and stirring for half an hour to obtain a mixed glue solution.

[0034] S3, evenly coating the mixed adhesive on a glass slide, first placing it in a blast oven to remove the solvent, then placing the glass slide coated with the mixed adhesive into a quartz tube of a tubular furnace, introducing nitrogen, raising the temperature by 5 °C / min, and calcining at 300 °C for 2 hours to convert the polyamic acid into polyimide. At the same time, a cross-linking reaction occurs, and a cross-linking network is formed, thereby finally obtaining a cross-linked polyimide adhesive.

[0035] Example 2 This embodiment prepares a cross-linked polyimide adhesive. The operation is different from that of the embodiment 1 only in step S1, and the other steps are the same. Step S1 of this embodiment is specifically as follows: S1, preparation of polyamic acid: 20 mmol of 4,4'-diaminodiphenyl ether diamine monomer was dissolved in 60 mL of N-methylpyrrolidone, nitrogen was continuously introduced, stirred in a water bath at 15°C for 30 minutes to dissolve it, and then 20.4 mmol of dianhydride monomer pyromellitic dianhydride was added in batches, and nitrogen was introduced in a vacuum to react for 4 hours to obtain a polyamic acid polymer solution. After the reaction was completed, the polyamic acid solution was collected and refrigerated for later use.

[0036] Example 3 This embodiment prepares a cross-linked polyimide adhesive. The operation is different from that of the embodiment 1 only in step S1, and the other steps are the same. Step S1 of this embodiment is specifically as follows: S1, preparation of polyamic acid: 20 mmol of 1,2-bis(4-aminophenoxy)ethane diamine monomer was dissolved in 60 mL of N-methylpyrrolidone, nitrogen was continuously introduced, stirred in a water bath at 10°C for 30 minutes to dissolve it, and then 20.4 mmol of dianhydride monomer pyromellitic dianhydride was added in batches, and nitrogen was introduced in a vacuum to react for 6 hours to obtain a polyamic acid polymer solution. After the reaction was completed, the polyamic acid solution was collected and refrigerated for later use.

[0037] Example 4 This embodiment prepares a cross-linked polyimide adhesive. The operation is different from that of the embodiment 1 only in step S2, and the other steps are the same. Step S2 of this embodiment is specifically as follows: S2, putting the polyamic acid glue solution into a beaker, weighing and adding a pentaerythritol tris (3-aziridinyl) propionate crosslinker in an amount of 10% by mass fraction relative to the polyamic acid solute, and stirring for half an hour to obtain a mixed glue solution.

[0038] The above-mentioned Examples 1-4 are preparation examples of cross-linked polyimide adhesives, and the following Examples 5-13 are preparation examples of lithium-ion batteries using the cross-linked polyimide adhesives.

[0039] Example 5 This embodiment provides a lithium ion battery using a cross-linked polyimide adhesive, which is prepared by the following preparation method: S1, preparation of polyamic acid: 20 mmol of 1,2-bis(2-aminoethoxy)ethane diamine monomer was dissolved in 60 mL of N-methylpyrrolidone (NMP), nitrogen was continuously introduced, stirred in a water bath at 10°C for 30 minutes to dissolve it, and then 20.4 mmol of dianhydride monomer pyromellitic dianhydride was added in batches, and nitrogen was introduced in a vacuum to react for 6 hours to obtain a polyamic acid solution. After the reaction was completed, the polyamic acid solution was collected and refrigerated for later use.

[0040] S2, preparation of positive electrode sheet: lithium iron phosphate (LFP), conductive carbon black, polyamic acid solution (polyamic acid solution prepared in step S1 × solid content) are mixed in a mass ratio of 8:1:1. Weigh 160 mg of lithium iron phosphate and 20 mg of conductive carbon black respectively and place them in a mortar, then add 200 mg of the polyamic acid glue prepared above, and finally add 5% of the mass of polyamic acid solute trimethylolpropane-tri[3-(2-methylaziridine-1-yl) propionate] crosslinker, grind thoroughly for half an hour, add N-methylpyrrolidone to adjust the slurry viscosity, and finally obtain a uniformly mixed electrode slurry. The electrode slurry is evenly coated on the current collector with a thickness of 100 μm. First place it in a blast drying oven at 60°C for 2 hours to remove most of the organic solvent, and then move it to a vacuum drying oven at 80°C for 12 hours to fully volatilize the solvent to obtain the positive electrode sheet to be treated.

[0041] S3, imidization of polyamic acid: punch the positive electrode sheet to be treated into small discs of 12 mm, then put the small discs into a quartz boat of a tubular furnace, introduce nitrogen, program the temperature to rise by 5 °C / min, and calcine at 300 °C for 2 hours to convert the polyamic acid into polyimide. At the same time, a cross-linking reaction occurs and a cross-linking network is formed, finally obtaining a positive electrode sheet containing a cross-linked polyimide adhesive.

[0042] S4, battery assembly: The battery assembly is carried out in a glove box filled with argon gas, and is assembled into CR2032 button cells, which are lithium-ion batteries in this example.

[0043] Example 6

[0044] The preparation method of the lithium ion battery in this embodiment is different from the operation in Embodiment 5 only in step S1, and the other steps are the same. Step S1 in this embodiment is specifically as follows: S1, preparation of polyamic acid: 20 mmol of 4,4'-diaminodiphenyl ether diamine monomer was dissolved in 60 mL of N-methylpyrrolidone, nitrogen was continuously introduced, stirred in a water bath at 10°C for 30 minutes to dissolve it, and then 20.4 mmol of dianhydride monomer pyromellitic dianhydride was added in batches, and nitrogen was introduced in a vacuum to react for 6 hours to obtain a polyamic acid polymer solution. After the reaction was completed, the polyamic acid solution was collected and refrigerated for later use.

[0045] Example 7 The preparation method of the lithium ion battery in this embodiment is different from the operation in Embodiment 5 only in step S1, and the other steps are the same. Step S1 in this embodiment is specifically as follows: S1, preparation of polyamic acid: 20 mmol of 1,2-bis(4-aminophenoxy)ethane diamine monomer was dissolved in 60 mL of N-methylpyrrolidone, nitrogen was continuously introduced, stirred in a water bath at 10°C for 30 minutes to dissolve it, and then 20.4 mmol of dianhydride monomer pyromellitic dianhydride was added in batches, and nitrogen was introduced in a vacuum to react for 6 hours to obtain a polyamic acid polymer solution. After the reaction was completed, the polyamic acid solution was collected and refrigerated for later use.

[0046] Example 8 The only difference between the preparation method of the lithium ion battery in this example and that in Example 5 is that step S2 is different, specifically: S2, preparation of positive electrode sheet: lithium iron phosphate (LFP), conductive carbon black, and mixed glue (mixed glue prepared in step S1 × solid content) are mixed in a mass ratio of 90:5:5. Weigh 180 mg of lithium iron phosphate and 10 mg of conductive carbon black respectively and place them in a mortar, then add 100 mg of the polyamic acid glue prepared above, and finally add 5% of the mass of polyamic acid solute N, N'-(methylene bis-P-phenyl) bis(aziridine-1-formamide) crosslinker to it, grind it thoroughly for half an hour, add N-methylpyrrolidone to adjust the viscosity of the slurry, and finally obtain a uniformly mixed electrode slurry. The electrode slurry is evenly coated on the current collector with a thickness of 100 μm. First place it in a blast drying oven at 60°C for 2 hours to remove most of the organic solvent, and then move it to a vacuum drying oven at 80°C for 12 hours to fully volatilize the solvent to obtain the positive electrode sheet to be treated.

[0047] Example 9 The only difference between the preparation method of the lithium ion battery in this example and that in Example 5 is that step S2 is different, specifically: S2, preparation of positive electrode sheet: lithium iron phosphate (LFP), conductive carbon black, polyamic acid (mixed glue prepared in step S1 × solid content) are mixed in a mass ratio of 8:1:1. Weigh 160 mg of lithium iron phosphate and 20 mg of conductive carbon black respectively and place them in a mortar, then add 200 mg of the polyamic acid glue prepared above, and finally add 5% of the polyamic acid solute mass of pentaerythritol tris (3-aziridine) propionate crosslinker, grind it thoroughly for half an hour, add N-methylpyrrolidone to adjust the slurry viscosity, and finally obtain a uniformly mixed electrode slurry. The electrode slurry is evenly coated on the current collector with a thickness of 100 μm. First place it in a blast drying oven at 60°C for 2 hours to remove most of the organic solvent, and then move it to a vacuum drying oven at 80°C for 12 hours to fully volatilize the solvent to obtain the positive electrode sheet to be treated.

[0048] Example 10 The preparation method of the lithium ion battery in this embodiment is different from the operation in Embodiment 5 only in step S1, and the other steps are the same. Step S1 in this embodiment is specifically as follows: S1, preparation of polyamic acid: 20 mmol of 4,4'-diaminodiphenyl ether diamine monomer was dissolved in 60 mL of N-methylpyrrolidone, nitrogen was continuously introduced, stirred in a water bath at 5°C for 30 minutes to dissolve it, and then 24 mmol of dianhydride monomer 4,4-biphenyl ether dianhydride was added in batches, and nitrogen was introduced in a vacuum to react for 12 hours to obtain a polyamic acid polymer solution. After the reaction was completed, the polyamic acid solution was collected and refrigerated for later use.

[0049] Embodiment 11 The preparation method of the lithium ion battery in this embodiment is different from the operation in Embodiment 5 only in step S1, and the other steps are the same. Step S1 in this embodiment is specifically as follows: S1, preparation of polyamic acid: 20 mmol of 4,4'-diaminodiphenyl ether diamine monomer was dissolved in 60 mL of N-methylpyrrolidone, nitrogen was continuously introduced, stirred in a water bath at 10°C for 30 minutes to dissolve it, and then 20 mmol of dianhydride monomer 4,4'-(hexafluoroisopropylene) diphthalic anhydride was added in batches, and nitrogen was introduced in a vacuum to react for 6 hours to obtain a polyamic acid polymer solution. After the reaction was completed, the polyamic acid solution was collected and refrigerated for later use.

[0050] Example 12 The preparation method of the lithium ion battery in this embodiment is compared with the operation of Example 5, in which the electrode slurry coating film thickness in step S2 is 40 μm, and step S3 is also different from that in Example 5, and the other steps are the same. Step S3 of this embodiment is specifically as follows: S3, imidization of polyamic acid: punch the positive electrode sheet to be treated into small discs of 12 mm, then put the small discs into a quartz boat of a tubular furnace, introduce nitrogen, program the temperature to rise at 8 °C / min, and calcine at 350 °C for 1 hour to convert the polyamic acid into polyimide. At the same time, a cross-linking reaction occurs and a cross-linking network is formed, finally obtaining a positive electrode sheet containing a cross-linked polyimide adhesive.

[0051] Embodiment 13 The preparation method of the lithium ion battery in this embodiment is compared with the operation of Example 5, in which the electrode slurry coating thickness in step S2 is 200 μm, and step S3 is also different from that in Example 5, and the other steps are the same. Step S3 in this embodiment is specifically as follows: S3, imidization of polyamic acid: punch the positive electrode sheet to be treated into small discs of 12 mm, then put the small discs into a quartz boat of a tubular furnace, introduce nitrogen, program the temperature to rise by 3 °C / min, and calcine at 200 °C for 4 hours to convert the polyamic acid into polyimide. At the same time, a cross-linking reaction occurs and a cross-linking network is formed, finally obtaining a positive electrode sheet containing a cross-linked polyimide adhesive.

[0052] In step S2 of Example 5-13, imidization and crosslinking reaction of the crosslinked polyamic acid are carried out, and the reaction equation is shown in Formula I below, where AZ represents the crosslinking agent component: The reaction equations for preparing the cross-linked polyimide in Examples 8 and 9 differ from those in the figure below in that the cross-linking agents AZ are different, and the reaction temperature and the amount of raw materials are adjusted.

[0053]

[0054] Formula I

[0055] Comparative Example 1 This comparative example provides a lithium ion battery, and its specific preparation method is as follows: S1, preparation of positive electrode sheet: lithium iron phosphate (LFP), conductive carbon black, and adhesive (PVDF) were mixed in a mass ratio of 8:1:1, NMP was added to adjust the slurry to a suitable viscosity, and then the slurry was evenly coated on aluminum foil with a thickness of 100μm. First, it was placed in a blast drying oven at 60℃ for 2 hours to remove most of the organic solvent, and then moved to a vacuum drying oven at 80℃ for 12 hours to obtain an LFP-PVDF positive electrode sheet.

[0056] S2, battery assembly: The battery is assembled in a glove box filled with argon gas into CR2032 button cells, which are lithium-ion batteries in this example.

[0057] The performance tests were performed on the polyimide adhesives prepared in Examples 1-4, the batteries prepared in Examples 5-13, and the battery prepared in Comparative Example 1: (1) Polyimide adhesive morphology observation experiment like Figure 1 As shown, before the cross-linking reaction with the cross-linking agent, the polyamic acid in step S1 of Example 1, Example 2, and Example 3 is a polymer glue, and no cross-linking phenomenon occurs; The polyamic acid obtained in step S1 of Examples 1-3 was further heated and stirred, and cross-linking occurred, and the glue liquid was transformed into a gel. This phenomenon can directly prove that the polyamic acid has undergone a cross-linking reaction. The generated product is as follows Figure 2 as shown in .

[0058] (2) Infrared spectrum test of polyimide adhesive The infrared spectra of the cross-linked polyimide polymers prepared in Examples 1-3 were tested respectively. The specific test results are as follows: Figure 3 As shown, Figure 3 The characteristic functional groups of polyimide exist in the -1 and 1780 cm -1 The symmetric and asymmetric stretching vibration peaks at 1370 cm -1 The peak at 730 cm corresponds to the stretching vibration of the CN bond. -1 The peak at corresponds to the out-of-plane bending vibration of the imide ring. The appearance of these characteristic groups indicates the successful preparation of the polyimide in Examples 1-3.

[0059] (3) Peel strength test The peel strength of the positive electrode sheet prepared in step S2 of Examples 5-7 and the positive electrode sheet in Comparative Example 1 were tested respectively. The specific test results are as follows: Figure 4 As shown. Figure 4 It can be seen that the peel strength of the positive electrode sheets of Examples 5-7 and Comparative Example 1 are 0.108 KN / m, 0.932 KN / m, 0.596 KN / m, and 0.051 KN / m, respectively. This shows that the electrode sheet peeling force of the cross-linked polyimide adhesive prepared in Examples 5-7 is higher than that of Comparative Example 1. In addition, in Examples 5-7, as the rigidity of the molecular chain increases, the peel strength of the electrode sheet also increases, which may be because the rigid chain segment can bear more mechanical loads. It can also be clearly seen that PVDF cannot provide strong adhesion only by weak van der Waals force.

[0060] (4) SEM image of the positive electrode The positive electrode sheets prepared in step S1 of Examples 5-6 and Comparative Example 1 were subjected to SEM scanning electron microscopy. The SEM scanning electron microscopy results of the positive electrode sheet prepared in Example 5 are shown in FIG. Figure 5 The SEM scanning electron microscopy results of the positive electrode sheet prepared in Example 6 are shown in Figure 6 The SEM scanning electron microscopy results of the positive electrode sheet prepared in Comparative Example 1 are shown in Figure 7 . Combined with the attached Figure 5-7 It can be seen that the surface of the electrode after cross-linking shows good bonding and smoothness, and there are no obvious cracks on the surface. The positive electrode material and the conductive agent can be well bonded together under the action of the adhesive and are evenly distributed on the current collector. However, due to insufficient bonding force, the material in comparative example 1 is unevenly distributed and cracked, which also causes the battery performance prepared with this adhesive to be inferior to that of the battery in examples 5-6.

[0061] (5) Battery cycle performance test The rate performance of the lithium ion batteries in Examples 5-7 and Comparative Example 1 at different current densities and the long cycle performance at 3C were tested respectively. The rate performance test conditions were as follows: first, the battery was cycled 5 times at a current density of 0.2C (1C = 170 mAh g -1 ), and then cycled five times at 1C, 2C, 3C, and 0.2C respectively, with a voltage range of 2.5-3.7V. The specific test results are as follows Figure 8 As shown, Figure 8 It is the rate performance test diagram under different current densities; Fig. 9 This is the long cycle performance test diagram under 3C. Fig. 9 The vertical axis “Specific Capacity” represents the specific capacity, the “Coulombic effiency” represents the coulombic efficiency, and the horizontal axis “Cycle number” represents the cycle number.

[0062] Depend on Figure 8 , Fig. 9 It can be seen that at a low rate of 0.2C, the specific capacities of the batteries in Examples 6-7 and Comparative Example 1 are similar, but the specific capacity of the battery in Example 5 is larger than that of the other batteries. As the current density increases from 0.2C to 3C, the specific capacities of the batteries in Examples 5, 6, and 7 reach 117 mA h g -1 , 92 mA hg -1 and 87 mA hg -1 , which is significantly higher than the 82 mA hg of the battery in Comparative Example 1. -1When the current density decreases, the specific capacity of the battery of Examples 5-7 can also recover rapidly, showing better rate performance and reversibility. In addition, in the 500-cycle performance test at 3C current density, Examples 5-7 all showed good cycle stability, with capacities of 88 mAh g -1 、102 mAhg -1 , 92 mAh g -1 , which is higher than 81 mAh g of the battery using PVDF adhesive in Comparative Example 1. -1 This further verifies the application potential of polyimide as an adhesive.

[0063] (6) Lithium ion diffusion performance diagram Depend on Fig.10 and Fig.11 The diffusion rate of lithium ions in the battery can be observed. The lithium ion diffusion behavior of different adhesive electrodes can be studied by using CV with different scan rates. As the scan rate increases from 0.1 to 0.5 mV·s -1 , the peak current of the battery and the potential at which it appears also change. According to the Randles-Sevcik formula, the peak current (Ip) and the square root of the scan rate (ω 1 / 2 ) has a linear relationship, and its slope is k=d(Ip) / d(ω 1 / 2 ) reflects the kinetic properties of lithium ion diffusion. The larger the absolute value of the slope k, the faster the lithium ion diffusion rate. Fig.10 The positive peak current of the oxidation reaction is shown. The absolute values ​​of the slopes of the positive peak currents and the square root of the scan rate of Example 5, Example 6, and Example 7 are 3.216, 2.818, and 2.6, respectively, which are greater than 1.929 of the comparative example. Fig.11 is the negative peak current of the reduction reaction, and the absolute values ​​of the slopes of the negative peak current and the square root of the scan rate of Examples 5, 6, and 7 are 2.724, 2.428, and 2.134, respectively, which are greater than 1.674 of the comparative example. It can be observed that the slopes of Examples 5-7 are all greater than those of Comparative Example 1, indicating that the battery using the cross-linked polyimide adhesive has a faster lithium ion diffusion behavior, which also confirms the feasibility and superiority of the present invention as a battery adhesive.

[0064] It should be noted that, although the above embodiments have been described in this article, the patent protection scope of the present invention is not limited thereby. Therefore, based on the innovative concept of the present invention, changes and modifications made to the embodiments described herein, or equivalent structures or equivalent process changes made using the contents of the present invention specification and drawings, directly or indirectly applying the above technical solutions to other related technical fields, are all included in the patent protection scope of the present invention.

Claims

1. A method for preparing a cross-linked polyimide adhesive for lithium ion batteries, characterized in that: It includes the following steps: S1: In an inert atmosphere, an aromatic dianhydride, a diamine and a first solvent are mixed, and prepolymerized at 5-20° C. for 4-12 hours to obtain a polyamic acid; S2: fully mixing the polyamic acid and the cross-linking agent to obtain a mixed glue solution; the mass of the cross-linking agent added is 5-15% of the total mass of the aromatic dianhydride and the diamine; S3: coating the mixed adhesive on a glass slide to form a film, and then heating to 200-350° C. to perform imidization reaction and cross-linking reaction to obtain the cross-linked polyimide adhesive for lithium-ion batteries.

2. The method for preparing a cross-linked polyimide adhesive for lithium ion batteries according to claim 1, characterized in that: The aromatic dianhydride is one of the following: pyromellitic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropylene) diphthalic anhydride, 4,4'(4,4-isopropylenediphenoxy)bis(naphthalic anhydride), biphenyltetracarboxylic dianhydride, 4,4-biphenyl ether dianhydride; The diamine is one or more of the following: 1,2-bis(2-aminoethoxy)ethane, 4,4-diaminodiphenyl ether, 1,2-bis(4-aminophenoxy)ethane, bis(3-aminopropyl) end-capped polydimethylsiloxane), 2,4-diaminobenzoic acid, 3,4-diaminobenzoic acid, 2,5-diaminoterephthalic acid, 3,5-diaminobenzoic acid; The molar ratio of the diamine added in step S1 to the aromatic dianhydride is 1:1.0-1.

2.

3. The method for preparing a cross-linked polyimide adhesive for lithium ion batteries according to claim 1, characterized in that: The crosslinking agent is one of the following: trimethylolpropane-tris[3-(2-methylaziridine-1-yl)propionate], N,N'-(methylenebis-P-phenyl)bis(aziridine-1-carboxamide), pentaerythritol tris(3-aziridine)propionate, 1,1'-(1,3-phthaloyl)bis(2-methyl-aziridine), tris(2-methylaziridine)phosphine oxide, carbodiimide hydrochloride, isocyanate, or 3,5-diaminobenzoic acid.

4. A cross-linked polyimide adhesive for lithium ion batteries, characterized in that: The cross-linked polyimide adhesive for lithium-ion batteries is prepared by the preparation method according to any one of claims 1 to 3.

5. An application of a cross-linked polyimide adhesive for lithium ion batteries, characterized in that: The cross-linked polyimide adhesive is used as a lithium-ion battery adhesive.

6. An electrode using a cross-linked polyimide adhesive, characterized in that: It includes the following components in parts by weight: 80-90 parts of electrode active material; 5-10 parts of the cross-linked polyimide adhesive according to claim 4; Conductive agent 5-10 parts.

7. The electrode using a cross-linked polyimide adhesive according to claim 6, characterized in that: The electrode active material includes a positive electrode active material or a negative electrode active material.

8. The electrode using a cross-linked polyimide adhesive according to claim 6, characterized in that: The conductive agent is one of the following: conductive carbon black, acetylene black, carbon nanotubes, graphene, or Ketjen black.

9. The electrode using a cross-linked polyimide adhesive according to claim 7, characterized in that: The positive electrode active material includes one of the following: lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese oxide, lithium cobalt oxide, lithium nickel cobalt aluminum oxide, and lithium-rich manganese base; when the electrode is applied to the positive electrode, the positive electrode active material is added to the electrode; The negative electrode active material includes one of the following: nano silicon, micro silicon, graphite, silicon-carbon material; when the electrode is applied to the negative electrode, the negative electrode active material is added to the electrode.

10. The method for preparing an electrode using a cross-linked polyimide adhesive according to any one of claims 6 to 9, characterized in that: It includes the following steps: S1: In an inert atmosphere, an aromatic dianhydride, a diamine and a first solvent are mixed, and prepolymerized at 5-20° C. for 4-12 hours to obtain a polyamic acid; S2: fully mixing the electrode active material, the polyamic acid, the conductive agent, the cross-linking agent, and the second solvent to prepare an electrode slurry; S3: The electrode slurry is coated on the current collector to form a film with a thickness of 40-200 μm. After the solvent of the film is evaporated, the electrode is placed in a tubular furnace for programmed heating under a nitrogen atmosphere. The heating rate is 3-8°C / min. The temperature is raised from room temperature to 200-350°C under a nitrogen atmosphere and kept warm for 1-4 hours to perform imidization reaction and cross-linking reaction to prepare the electrode using a cross-linked polyimide adhesive.

Images