A polyimide binder for lithium ion battery cathodes and a method for preparing the same

Abstract

The application discloses a kind of polyimide binder for lithium ion battery positive pole and its preparation method, the polyimide binder contains cyclodextrin and sulfonic acid group;Cyclodextrin, sulfonated diamine and triethylamine are dissolved in organic solvent, after stirring to be fully dissolved, add binary acid anhydride, obtain the solution containing the sulfonated polyamide acid that is included by cyclodextrin;Benzoic acid and catalyst are added to it again, imidization is carried out, cooling is added to washing solvent and precipitate is separated, filtration is carried out, and polyimide binder is obtained;The polyimide binder of the application has excellent thermal stability, stronger mechanical property, and higher bonding strength effectively inhibits the separation between binder and active material particles and aluminum foil, and the electrode sheet prepared by using the obtained polyimide binder is used in lithium ion battery, effectively improves the electrochemical performance of lithium ion battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a polyimide binder for lithium ion battery cathodes and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries, as a new generation of green energy storage system, have the advantages of high specific energy, high energy density, high coulomb efficiency, high working voltage, long cycle life, wide working temperature range, etc. Therefore, lithium ion batteries are widely used in portable electronic products, electric vehicles and other fields, but there are still problems such as energy density, cost, safety in these fields. Developing lithium ion batteries with high energy density, low cost and high safety is still the focus of current research.

[0003] Lithium ion batteries are composed of positive electrodes, separators, electrolytes and negative electrodes, etc. The positive and negative electrodes are composed of powdered active materials (such as lithium metal oxide, carbon negative electrode material), conductive agent, electrode current collector and binder, among which the binder is to bond the active material, conductive agent and current collector together. For an ideal electrode, each active particle should be reasonably shaped, dispersed and connected to the current collector and electrolyte, with low resistance and continuous internal channels.

[0004] For a long time, in the large-scale production of lithium ion battery industry, polyvinylidene fluoride (PVDF) is mainly used as a binder, and organic solvents such as N-methyl pyrrolidone (NMP) are used as dispersants. However, PVDF has its own shortcomings, such as poor flexibility and adhesion, certain swelling in electrolyte, and poor compatibility with metal lithium, Li x C6exothermic reaction at high temperature, which has a great safety hazard.

[0005] Therefore, in view of the above problems, the present application provides a polyimide binder for lithium ion battery cathodes and a preparation method thereof. The molecular structure of cyclodextrin has multiple hydroxyl groups (-OH), which can form hydrogen bonds and other physical and chemical interactions with the surface of the electrode material, thereby providing good adhesion. The molecular structure of cyclodextrin has the characteristics of internal hydrophilicity and external hydrophobicity, which can promote the penetration of electrolyte and ion migration through its internal channel structure. This property helps to improve the conduction speed and efficiency of lithium ions in the electrode material during charging and discharging, thereby improving the rate performance and energy density of the battery. The sulfonate group can dissociate into negatively charged sulfonate ions, increasing the ionic conductivity in the electrode and improving the transmission efficiency of lithium ions in the electrode, thereby improving the overall electrochemical performance of the battery. Polyimide material has the characteristics of structural diversity, good thermal stability, chemical stability, excellent mechanical properties, etc. As a binder for lithium ion battery cathodes, it is of great significance to develop polyimide with high capacity, stable cycle performance and good safety performance. SUMMARY

[0006] The present application aims to provide a polyimide binder for lithium ion battery cathode and a preparation method thereof, which has the advantages of excellent thermal stability, stable chemical structure and strong adhesion. The binder improves the conduction speed and efficiency of lithium ions in the electrode material by inclusion of cyclodextrin and introduction of sulfonate groups, improves the rate performance and energy density of the battery, reduces the electrode impedance, increases the flexibility of the electrode sheet, and ensures the cycle stability of the lithium ion battery.

[0007] To achieve the above-mentioned application purposes, the technical solutions adopted by the present application are as follows:

[0008] A polyimide binder for lithium ion battery cathode, the polyimide binder for lithium ion battery cathode comprises polyimide containing cyclodextrin and sulfonic acid groups, and the structure formula of the polyimide is shown in structure formula 1:

[0009]

[0010] wherein m is any integer from 100 to 1000, and the weight average molecular weight is greater than 20000.

[0011] Preferably, the A group in structure formula 1 is selected from any one of the following structure formulas:

[0012]

[0013]

[0014] Preferably, the B group in structure formula 1 is selected from any one of the following structure formulas:

[0015]

[0016] The present application also claims to protect a preparation method of a polyimide binder for lithium ion battery cathode, comprising the following steps:

[0017] (1) Under the protection of nitrogen, cyclodextrin, sulfonated diamine and triethylamine are dissolved in an organic solvent, and after stirring to fully dissolve, a dibasic anhydride is added, and stirring is carried out at 60-100℃ for 6-12h to obtain a solution containing sulfonated polyamide acid which is included by cyclodextrin;

[0018] (2) The solution containing sulfonated polyamide acid which is included by cyclodextrin obtained in step (1) is added with benzoic acid and a catalyst, heated to 180-220℃, and kept at constant temperature for 10-18h for imidization, cooled to room temperature, added with a washing solvent to precipitate, filtered, and the polyimide binder for lithium ion battery cathode is obtained.

[0019] Preferably, in step (1), the molar ratio of the cyclodextrin to the sulfonated diamine is 1:0.5-1.5; and the molar ratio of the sulfonic acid group in the sulfonated diamine to the triethylamine is 1:0.9-1.1.

[0020] The cyclodextrin is selected from one of α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin;

[0021] The organic solvent is selected from one or more of m-cresol, N-methylpyrrolidone, dimethylacetamide, N,N-dimethylformamide, and p-chlorophenol;

[0022] The solid content of the sulfonated polyamic acid solution is 10-25 wt%.

[0023] Preferably, in step (2), the washing solvent is selected from one or more of ethyl acetate, methanol, ethanol, isopropanol, ethylene glycol, 2-butanol or cyclopentanol;

[0024] The catalyst is selected from one of quinoline, isoquinoline, or tertiary amine.

[0025] The method for preparing the lithium-ion battery positive electrode sheet using polyimide binder as described above includes the following steps:

[0026] S1. Take polyimide binder and stir it to dissolve it in solvent to obtain binder solution. Grind and mix the positive electrode active material and conductive agent evenly and add them to binder solution. Adjust the viscosity of the mixture to 2000-10000 mPa·s by adding solvent to obtain black slurry.

[0027] S2. The black slurry obtained in step S1 is uniformly coated onto aluminum foil, dried, and then compacted to obtain the positive electrode sheet of a lithium-ion battery.

[0028] Preferably, in step S1, the solvent is selected from one of N,N-dimethylformamide, N,N-dimethylacetamide, or N-methylpyrrolidone;

[0029] The positive electrode active material is selected from one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, or ternary positive electrode material lithium nickel cobalt manganese oxide.

[0030] The conductive agent is selected from one or more of acetylene black, superconducting carbon black (Super P), carbon nanotubes, graphene, and Ketjen black.

[0031] Preferably, in step S1, the mass concentration of the adhesive solution is 5%-20%; in step S2, the drying is vacuum drying at 80℃-120℃.

[0032] Preferably, in step S1, the content of the positive electrode active material is 2 mg·cm³.-2 -4mg·cm -2 ; in step S2, the lithium ion battery positive electrode plate is composed of an active material layer and an aluminum foil; the active material layer is composed of a positive electrode active material, a conductive agent and a polyimide binder; the content of the binder in the active material layer is 1%-15%; the content of the conductive agent in the active material layer is 2%-25%.

[0033] Due to the use of the above technical solution, the present application has the following beneficial effects compared with the prior art:

[0034] 1. The polyimide binder for lithium ion battery positive electrode of the present application has excellent thermal stability and strong mechanical properties, improves the bonding strength of the binder, and effectively inhibits the separation between the binder and the active material particles and the aluminum foil.

[0035] 2. The polyimide binder for lithium ion battery positive electrode of the present application has cyclodextrin wrapped on the polyimide backbone chain, and the molecular structure of cyclodextrin has multiple hydroxyl groups, which can form hydrogen bonds and other interactions with the surface of the electrode material to provide good adhesion; the internal pore structure of cyclodextrin can also promote the penetration of electrolyte and ion migration, thereby improving the cycle performance.

[0036] 3. The polyimide binder for lithium ion battery positive electrode of the present application introduces sulfonate groups, which can dissociate into negatively charged sulfonate ions, increase the ionic conductivity in the electrode, and improve the transmission efficiency of lithium ions in the electrode.

[0037] 4. The electrode plate prepared by using the obtained polyimide binder in the present application effectively enhances the electrochemical performance of lithium ion batteries. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or the prior art description. Obviously, some of the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0039] Figure 1 The battery assembly process of Example 1 of the present application is shown in the figure;

[0040] Figure 2 The structure of the electrode plate for preparing the positive electrode of Example 1 of the present application is shown in the figure;

[0041] Figure 3 The initial morphology SEM of the electrode plate for preparing the positive electrode of Comparative Example 1 of the present application is shown in the figure, and the magnification is 1000 times.

[0042] Figure 4 The initial morphology SEM image of the electrode sheet for preparing the positive electrode of the battery in Example 1 of the present invention is shown at a magnification of 500x.

[0043] Figure 5 This is a graph showing the bonding strength of the electrode sheets used to prepare the positive electrode of the battery in Example 1 and Comparative Example 1 of the present invention.

[0044] Figure 6 The images show the initial impedance diagram and the impedance diagram after 100 cycles of the assembled battery in Comparative Example 1 of this invention.

[0045] Figure 7 The diagram shows the cycle performance of the assembled batteries in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention. Detailed Implementation

[0046] To provide a clearer understanding of the technical features, objectives, and effects of this invention, specific implementation schemes are now described in detail.

[0047] The present invention will be further described below with reference to embodiments, but the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0048] Example 1

[0049] See appendix Figure 1 Appendix Figure 2 Appendix Figure 4 Appendix Figure 5 and appendix Figure 7 This embodiment provides a polyimide binder for lithium-ion battery cathodes and its preparation method, including the following steps:

[0050] (1) Under nitrogen protection, 1.794 g of 4,4'-bis(4-aminophenoxy)-[1,1'-biphenyl]-3-sulfonic acid, 4.54 g of β-cyclodextrin and 0.85 g of triethylamine were dissolved in 20 ml of m-cresol and stirred until fully dissolved. Then, 1.2533 g of 4,4'-oxydiphthalic anhydride was added and stirred at 80 °C for 8 h to obtain a sulfonated polyamic acid solution containing cyclodextrin.

[0051] (2) To the sulfonated polyamide acid solution containing the inclusion of cyclodextrin obtained in step (1), 0.0844 g of isoquinoline and 0.7327 g of benzoic acid were added, and the temperature was raised to 180°C, and imidization was performed for 18 h. After the reaction was completed, the temperature was cooled to room temperature, the polyimide solution was dropped into ethyl acetate to precipitate, and filtration was performed to obtain the polyimide binder for the lithium ion battery cathode;

[0052] The preparation method of the lithium ion battery cathode electrode sheet of the above-mentioned polyimide binder for the lithium ion battery cathode includes the following steps:

[0053] S1, 0.1 g of the binder was dissolved in 1.5 ml of N-methylpyrrolidone to obtain a binder solution, 0.8 g of lithium cobalt oxide (LiCoO2) cathode active material and 0.1 g of conductive agent Super P were thoroughly ground and mixed, and then added to the binder solution in batches and stirred to mix uniformly to obtain a black slurry;

[0054] S2, the black slurry obtained in step S1 was uniformly coated on a clean aluminum foil using a four-sided preparation device, and vacuum drying was performed at 120°C for 12 h to obtain a LiCoO2 lithium ion battery cathode electrode sheet.

[0055] Example 2

[0056] The present embodiment provides a polyimide binder for a lithium ion battery cathode and a preparation method thereof, including the following steps:

[0057] (1) Under the protection of nitrogen, 1.794 g of 4,4'-di(4-aminophenoxy)-[1,1'-biphenyl]-3-sulfonic acid, 4.54 g of β-cyclodextrin and 0.85 g of triethylamine were dissolved in 20 ml of m-cresol, and after stirring to dissolve thoroughly, 1.3018 g of 3,3',4,4'-benzophenone tetracarboxylic dianhydride was added, and stirring was performed at 80°C for 8 h to obtain a sulfonated polyamide acid solution containing the inclusion of cyclodextrin;

[0058] (2) To the sulfonated polyamide acid solution containing the inclusion of cyclodextrin obtained in step (1), 0.0844 g of isoquinoline and 0.7327 g of benzoic acid were added, and the temperature was raised to 180°C, and imidization was performed for 18 h. After the reaction was completed, the temperature was cooled to room temperature, the polyimide solution was dropped into ethyl acetate to precipitate, and filtration was performed to obtain the polyimide binder for the lithium ion battery cathode;

[0059] The preparation method of the lithium ion battery cathode electrode sheet of the above-mentioned polyimide binder for the lithium ion battery cathode includes the following steps:

[0060] S1, 0.1 g of the binder was dissolved in 1.5 ml of N-methyl pyrrolidone to obtain a binder solution, 0.8 g of lithium cobalt oxide (LiCoO2) positive electrode active material and 0.1 g of conductive agent Super P were fully ground and mixed, then the binder solution was added in batches and stirred to mix uniformly to obtain a black slurry;

[0061] S2, the black slurry obtained in step S1 was uniformly coated on a clean aluminum foil using a four-sided preparation device, vacuum dried at 120°C for 12h to obtain a LiCoO2 lithium ion battery positive electrode sheet.

[0062] Example 3

[0063] The present embodiment provides a polyimide binder for lithium ion battery positive electrode and a preparation method thereof, comprising the following steps:

[0064] (1) Under the protection of nitrogen, 1.794 g of 4,4'-bis(4-aminophenoxy)-[1,1'-biphenyl]-3-sulfonic acid, 4.54 g of β-cyclodextrin and 0.85 g of triethylamine were dissolved in 20 ml of m-cresol, stirred until completely dissolved, then 2.1028 g of bisphenol A type diether dianhydride was added, stirred at 80°C for 8h to obtain a solution containing cyclodextrin-included sulfonated polyamide acid;

[0065] (2) 0.0929 g of isoquinoline and 0.7327 g of benzoic acid were added to the solution containing cyclodextrin-included sulfonated polyamide acid obtained in step (1), heated to 180°C, and reacted for 18h to perform imidization. After the reaction was completed, the polyimide solution was cooled to room temperature, dropped into ethyl acetate to precipitate, filtered to obtain the polyimide binder for lithium ion battery positive electrode;

[0066] The above-mentioned preparation method of the lithium ion battery positive electrode sheet of the polyimide binder for lithium ion battery positive electrode comprises the following steps:

[0067] S1, 0.1 g of the binder was dissolved in 1.5 ml of N-methyl pyrrolidone to obtain a binder solution, 0.8 g of lithium cobalt oxide (LiCoO2) positive electrode active material and 0.1 g of conductive agent Super P were fully ground and mixed, then the binder solution was added in batches and stirred to mix uniformly to obtain a black slurry;

[0068] S2, the black slurry obtained in step S1 was uniformly coated on a clean aluminum foil using a four-sided preparation device, vacuum dried at 120°C for 12h to obtain a LiCoO2 lithium ion battery positive electrode sheet.

[0069] Example 4

[0070] The present embodiment provides a polyimide binder for lithium ion battery positive electrode and a preparation method thereof, comprising the following steps:

[0071] (1) 2.1142 g of 4,4'-bis(4-aminophenoxy)-[1,1'-biphenyl]-3,3'-disulfonic acid, 4.54 g of β-cyclodextrin and 0.85 g of triethylamine were dissolved in 20 ml of m-cresol under nitrogen protection, after stirring to complete dissolution, 1.2533 g of 4,4'-oxybisphthalic anhydride was added, and stirring was carried out at 80°C for 8 h to obtain a solution containing a cyclodextrin-included sulfonated polyamic acid;

[0072] (2) 0.0876 g of isoquinoline and 0.7327 g of benzoic acid were added to the solution containing the cyclodextrin-included sulfonated polyamic acid obtained in step (1), and the temperature was raised to 180°C, and imidization was carried out for 18 h, after the reaction was completed, the temperature was cooled to room temperature, and the polyimide solution was dropped into ethyl acetate to precipitate, and filtration was carried out to obtain the polyimide binder for lithium ion battery cathodes;

[0073] The preparation method of the lithium ion battery cathode pole piece using the above-mentioned polyimide binder for lithium ion battery cathodes comprises the following steps:

[0074] S1, 0.1 g of the binder was dissolved in 1.5 ml of N-methylpyrrolidone to obtain a binder solution, 0.8 g of lithium cobalt oxide (LiCoO2) cathode active material and 0.1 g of conductive agent Super P were thoroughly ground and mixed, and then added to the binder solution in batches and stirred to mix uniformly to obtain a black slurry;

[0075] S2, the black slurry obtained in step S1 was uniformly coated on a clean aluminum foil using a four-sided preparation device, and vacuum drying was carried out at 120°C for 12 h to obtain a LiCoO2 lithium ion battery cathode pole piece.

[0076] Example 5

[0077] The present embodiment provides a polyimide binder for lithium ion battery cathodes and a preparation method thereof, comprising the following steps:

[0078] (1) 2.1142 g of 4,4'-bis(4-aminophenoxy)-[1,1'-biphenyl]-3,3'-disulfonic acid, 4.54 g of β-cyclodextrin and 0.85 g of triethylamine were dissolved in 20 ml of m-cresol under nitrogen protection, after stirring to complete dissolution, 1.2533 g of 4,4'-oxybisphthalic anhydride was added, and stirring was carried out at 80°C for 8 h to obtain a solution containing a cyclodextrin-included sulfonated polyamic acid;

[0079] (2) to the sulfonated polyamide acid solution containing cyclodextrin inclusion obtained in step (1), 0.0881 g of isoquinoline and 0.7327 g of benzoic acid were added, and the temperature was raised to 180°C, and imidization was performed for 18 h. After the reaction was completed, the temperature was cooled to room temperature, the polyimide solution was dropped into ethyl acetate to precipitate, and filtration was performed, to obtain the polyimide binder for lithium ion battery cathodes;

[0080] The preparation method of the lithium ion battery cathode pole piece of the above-mentioned polyimide binder for lithium ion battery cathodes comprises the following steps:

[0081] S1, 0.1 g of the binder was dissolved in 1.5 ml of N-methylpyrrolidone to obtain a binder solution, 0.8 g of lithium cobalt oxide (LiCoO2) cathode active material and 0.1 g of conductive agent Super P were thoroughly ground and mixed, and then added to the binder solution in batches and stirred to mix uniformly, to obtain a black slurry;

[0082] S2, the black slurry obtained in step S1 was uniformly coated on a clean aluminum foil using a four-sided preparation device, and vacuum drying was performed at 120°C for 12 h, to obtain a LiCoO2 lithium ion battery cathode pole piece.

[0083] Example 6

[0084] The present embodiment provides a polyimide binder for lithium ion battery cathodes and a preparation method thereof, comprising the following steps:

[0085] (1) 2.1142 g of 4,4'-bis(4-aminophenoxy)-[1,1'-biphenyl]-3,3'-disulfonic acid, 4.54 g of β-cyclodextrin and 0.85 g of triethylamine were dissolved in 20 ml of m-cresol under nitrogen protection, and after stirring to completely dissolve, 2.1028 g of bisphenol A type diether dianhydride was added, and stirring was performed at 80°C for 8 h, to obtain a sulfonated polyamide acid solution containing cyclodextrin inclusion;

[0086] (2) to the sulfonated polyamide acid solution containing cyclodextrin inclusion obtained in step (1), 0.0961 g of isoquinoline and 0.7327 g of benzoic acid were added, and the temperature was raised to 180°C, and imidization was performed for 18 h. After the reaction was completed, the temperature was cooled to room temperature, the polyimide solution was dropped into ethyl acetate to precipitate, and filtration was performed, to obtain the polyimide binder for lithium ion battery cathodes;

[0087] The preparation method of the lithium ion battery cathode pole piece of the above-mentioned polyimide binder for lithium ion battery cathodes comprises the following steps:

[0088] S1, 0.1 g of the binder was dissolved in 1.5 ml of N-methyl pyrrolidone to obtain a binder solution, 0.8 g of lithium cobalt oxide (LiCoO2) positive electrode active material and 0.1 g of conductive agent Super P were fully ground and mixed, then the binder solution was added in batches and stirred to mix uniformly to obtain a black slurry;

[0089] S2, the black slurry obtained in step S1 was uniformly coated on a clean aluminum foil using a four-sided preparation device, vacuum dried at 120°C for 12 h to obtain a LiCoO2 lithium ion battery positive electrode sheet.

[0090] Comparative Example 1

[0091] Referring to the accompanying Figure 3 , the accompanying Figure 5 , the accompanying Figure 6 and the accompanying Figure 7 , the embodiment provides a preparation method of a lithium ion battery positive electrode sheet, comprising the following steps:

[0092] S1, 0.1 g of polyvinylidene fluoride (PVDF) was dissolved in 2.0 ml of N-methyl pyrrolidone to obtain a binder solution, 0.8 g of lithium cobalt oxide (LiCoO2) positive electrode active material and 0.1 g of conductive agent Super P were fully ground and mixed, then the binder solution was added in batches and stirred to mix uniformly to obtain a black slurry;

[0093] S2, the black slurry obtained in step S1 was uniformly coated on a clean aluminum foil using a four-sided preparation device, vacuum dried at 120°C for 12 h to obtain a LiCoO2 lithium ion battery positive electrode sheet.

[0094] Comparative Example 2

[0095] Referring to the accompanying Figure 7 , the embodiment provides a polyimide binder for lithium ion battery positive electrode and a preparation method thereof, comprising the following steps:

[0096] (1) Under the protection of nitrogen, 1.794 g of 4,4'-bis(4-aminophenoxy)-[1,1'-biphenyl]-3-sulfonic acid and 0.85 g of triethylamine were dissolved in 20 ml of m-cresol, stirred until completely dissolved, then 1.2533 g of 4,4'-oxybisphthalic anhydride was added, stirred at 80°C for 8 h to obtain a solution containing a sulfonated polyamide acid encapsulated by cyclodextrin;

[0097] (2) 0.039 g of isoquinoline and 0.7327 g of benzoic acid were added to the solution containing the sulfonated polyamide acid occluded by the cyclodextrin obtained in step (1), and the temperature was raised to 180°C, and the imidization was performed for 18 h. After the reaction was completed, the temperature was cooled to room temperature, and the polyimide solution was dropped into ethyl acetate to precipitate, and was filtered to obtain the polyimide binder for the lithium ion battery cathode;

[0098] The method for preparing the lithium ion battery cathode electrode sheet of the polyimide binder for the lithium ion battery cathode described above comprises the following steps:

[0099] S1, 0.1 g of the binder was dissolved in 1.5 ml of N-methylpyrrolidone to obtain a binder solution, 0.8 g of lithium cobalt oxide (LiCoO2) cathode active material and 0.1 g of conductive agent Super P were thoroughly ground and mixed, and then were added to the binder solution in batches and stirred to mix uniformly to obtain a black slurry;

[0100] S2, the black slurry obtained in step S1 was uniformly coated on a clean aluminum foil using a four-side preparation device, and was vacuum dried at 120°C for 12 h to obtain a LiCoO2 lithium ion battery cathode electrode sheet.

[0101] The lithium ion battery cathode electrode sheets obtained in the above examples and comparative examples were cut into 14 mm diameter discs, were rolled, were weighed, and then were detected, and the detection results are shown in Table 1.

[0102] The lithium ion battery cathode electrode sheets obtained in the above examples and comparative examples were detected by the following method:

[0103] After the cathode electrode sheet was dried and weighed, it was placed in a glove box, and a 2032 type button cell shell was assembled: the cathode shell, the cathode electrode sheet, the separator, the lithium sheet, the gasket, and the spring were sequentially placed, 60 μL of electrolyte was added, the button cell was capped, and the button cell was sealed on a capper. The cathode sheet preparation and battery assembly are suitable for all examples and comparative examples.

[0104] Electrochemical long cycle test: the battery was tested for cycle performance at room temperature (25°C) using a battery test system of Blue Electric Co., Ltd. First, 3 cycles were performed, and then long cycle test was performed at a current density of 0.5C, and the test voltage range was 2.5-4.5V.

[0105] AC impedance test: battery test conditions, amplitude of 1 mV, frequency range of 10 -2 -10 6 Hz, initial interfacial impedance and interfacial impedance after 100 cycles of the test battery were tested.

[0106] Bonding strength test: cut the prepared dry complete positive electrode sheet into a long strip with a width of 25 mm and a length of 150 mm; then paste one side of the 3M double-sided tape on a clean aluminum plate, smooth it with force to ensure that the double-sided tape is tightly attached to the aluminum plate, remove the double-sided tape, and paste the electrode sheet coated with slurry on the side of the tape, making sure that the electrode sheet and the double-sided tape are completely matched and attached; insert the aluminum plate with the fixed electrode sheet into the lower clamp and fix it vertically; insert the electrode sheet without tape into the upper clamp and fix it, so that the electrode sheet attached to the double-sided tape is 180° to the electrode sheet fixed by the upper clamp; after fixing the test sample, first calibrate and zero, then start the test at a peeling speed of 100 mm / min, and the peeling strength curve and average value can be obtained.

[0107] Table 1

[0108]

[0109] As can be seen from Table 1, the average bonding strength of Examples 1-6 is higher than that of Comparative Example 1, which shows that the polyimide binder used in the present application has strong adhesion to the electrode sheet, and there is strong interaction between the polyimide and the positive electrode components, effectively inhibiting the separation of the binder from the active material particles and the aluminum foil; the molecular chain of Comparative Example 1 (PVDF) is simple, and the interaction between the active material is mainly through the weak van der Waals force formed by F atoms and H atoms, so it is easy to fall off from the electrode sheet; as can be seen from Examples 1 and Comparative Example 2, the bonding capacity of the sulfonated polyimide binder containing cyclodextrin is stronger, because the molecular structure of cyclodextrin has multiple hydroxyl groups, which can form hydrogen bonds and other interactions with the surface of the electrode material, providing good bonding force;

[0110] As can be seen from Table 1, the impedance values of Examples 1-3 are generally larger than those of Examples 4-6, because the sulfonated diamine monomer of Examples 1-3 has fewer sulfonic acid groups, and the sulfonic acid groups can dissociate into negatively charged sulfonate ions, increasing the ion conductivity in the electrode and reducing the electrode impedance;

[0111] As can be seen from Table 1, the initial specific capacity and capacity retention rate of Examples 1-6 are higher than those of Comparative Example 1, which shows that the electrochemical performance of the polyimide binder used in the present application is more excellent than that of the commercial polyvinylidene fluoride;

[0112] From the SEM images of the cross-section of the positive electrode sheet of Example 1 and Comparative Example 1 in the accompanying drawings, Figure 3 and the SEM images of the cross-section of the positive electrode sheet of Example 2 and Comparative Example 2 in the accompanying drawings, Figure 4 It can be seen that the polyimide binder of Example 1 uniformly coats the positive electrode active material lithium cobaltate and conductive carbon black, and the positive electrode components are tightly connected to form a complete and tight conductive structure, and the adhesion performance of the positive electrode components is very strong, with good coating and adhesion effect; the active material of the electrode sheet prepared by the polyvinylidene fluoride binder of Comparative Example 1 is basically exposed on the surface, because the polyvinylidene fluoride binder relies only on van der Waals force with the active material, and the adhesion capacity is poor;

[0113] The positive electrode plate prepared in Example 1, Comparative Example 1 and Comparative Example 2 was assembled into a button cell, and a charge-discharge cycle test was carried out, and the test results are shown in Figure 2. Figure 7 As shown in Figure 2, the discharge specific capacity of the button cell assembled in Example 1 remained almost unchanged after 200 cycles, and the capacity retention rate reached 99%; the capacity of Comparative Example 1 and Comparative Example 2 attenuated rapidly after 200 cycles, the discharge specific capacity of Comparative Example 2 was reduced to half of the original, and the discharge specific capacity of Comparative Example 1 was only 30% of the original, which shows that the electrode plate prepared by the polyimide binder of the application can effectively improve the electrochemical performance of the lithium ion battery.

[0114] In summary, the polyimide binder for lithium ion battery positive electrode of the application has excellent thermal stability and strong mechanical properties, improves the bonding strength of the binder, and effectively inhibits the separation between the binder and the active material particles and the aluminum foil; the polyimide binder for lithium ion battery positive electrode of the application has cyclodextrin wrapped on the polyimide skeleton chain, and the molecular structure of cyclodextrin has multiple hydroxyl groups, which can form hydrogen bonds and other interactions with the surface of the electrode material to provide good bonding force; the internal pore structure of cyclodextrin can also promote the penetration of electrolyte and ion migration, thereby improving the cycle performance; the polyimide binder for lithium ion battery positive electrode of the application introduces sulfonate groups, which can dissociate into negatively charged sulfonate ions, increase the ionic conductivity in the electrode, and improve the transmission efficiency of lithium ions in the electrode; the electrode plate prepared by the polyimide binder of the application is used in lithium ion battery, which effectively enhances the electrochemical performance of lithium ion battery.

[0115] The above-described examples only express several embodiments of the application, which are described in more detail and in more detail, but should not be construed as limiting the scope of the patent. It should be noted that for ordinary skilled in the art, without departing from the concept of the application, a number of modifications and improvements can be made, which are within the scope of the application. Therefore, the scope of protection of the patent of the application should be subject to the appended claims.

Claims

1. A polyimide binder for lithium ion battery cathodes, characterized by, The polyimide binder for the lithium ion battery cathode comprises a polyimide containing a cyclodextrin and a sulfonic acid group, and a structural formula of the polyimide is shown in structural formula 1: Formula (1); wherein m is any integer from 100 to 1000, and the weight average molecular weight is greater than 20,000; the A group in the structural formula 1 is selected from any one of the following structural formulas: Structure (2); Structure (3); Structure (4); Formula (5); Formula (6); Formula (7); the B group in the structural formula 1 is selected from any one of the following structural formulas: Structure (8); Structure (9).

2. A method for preparing the polyimide binder for lithium ion battery cathodes according to claim 1, characterized by, comprising the following steps: (1) under nitrogen protection, the cyclodextrin, sulfonated diamine and triethylamine are dissolved in an organic solvent, after stirring to fully dissolve, the binary acid anhydride is added, and the stirring reaction is carried out at 60-100℃ for 6-12h to obtain a solution containing sulfonated polyamide acid wrapped by cyclodextrin; (2) the solution containing sulfonated polyamide acid wrapped by cyclodextrin obtained in step (1) is added with benzoic acid and a catalyst, heated to 180-220℃, and the imidization is carried out for 10-18h at constant temperature, cooled to room temperature, added with a washing solvent to precipitate, filtered to obtain the polyimide binder for the lithium ion battery cathode.

3. The method for preparing the polyimide binder for the positive electrode of a lithium-ion battery according to claim 2, characterized in that, In step (1), the molar ratio of the cyclodextrin to the sulfonated diamine is 1:0.5-1.5; the molar ratio of the sulfonic acid group in the sulfonated diamine to triethylamine is 1:0.9-1.1; the cyclodextrin is selected from one of α-cyclodextrin, β-cyclodextrin and γ-cyclodextrin; the organic solvent is selected from one or a mixture of two or more of m-cresol, N-methyl pyrrolidone, dimethylacetamide, N,N-dimethylformamide and p-chlorophenol; the solid content of the sulfonated polyamide acid solution is 10-25wt%.

4. The method for preparing the polyimide binder for the positive electrode of a lithium-ion battery according to claim 2, characterized in that, In step (2), the washing solvent is selected from one or more of ethyl acetate, methanol, ethanol, isopropanol, ethylene glycol, 2-butanol or cyclopentanol; the catalyst is selected from one of quinoline, isoquinoline or tertiary amine.

5. The polyimide binder for lithium-ion battery cathodes according to claim 1, characterized by, The preparation method of the lithium ion battery cathode pole piece using the polyimide binder for the lithium ion battery cathode comprises the following steps: S1, the polyimide binder is stirred and dissolved in a solvent to obtain a binder solution, the cathode active material and the conductive agent are uniformly mixed and added into the binder solution, the viscosity of the mixed substances is adjusted to 2000-10000mPa·s by adding a solvent to obtain a black slurry; S2, the black slurry obtained in step S1 is uniformly coated on an aluminum foil, dried and compacted to obtain a lithium ion battery cathode pole piece.

6. The polyimide binder for lithium-ion battery cathodes according to claim 5, characterized in that, In step S1, the solvent is selected from one of N,N-dimethylformamide, N,N-dimethylacetamide or N-methyl pyrrolidone; the cathode active material is selected from one or more of lithium iron phosphate, lithium cobaltate, lithium manganate, lithium nickelate or ternary cathode material lithium nickel cobalt manganate; the conductive agent is selected from one or more of acetylene black, super conductive carbon black Super P, carbon nanotube, graphene and ketchen black.

7. The polyimide binder for lithium-ion battery cathodes according to claim 5, characterized in that, In step S1, the mass concentration of the binder solution is 5%-20%; in step S2, the drying is vacuum drying at 80℃-120℃.

8. The polyimide binder for lithium-ion battery cathodes according to claim 5, characterized in that, In step S1, the cathode active material content is 2 mg·cm -2 -4 mg·cm -2 In step S2, the lithium ion battery cathode electrode sheet is composed of an active material layer and an aluminum foil; the active material layer is composed of a cathode active material, a conductive agent, and a polyimide binder; the binder mass content in the active material layer is 1%-15%; the conductive agent mass content in the active material layer is 2%-25%.

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