Water-soluble polyimide as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of high polymer materials and lithium batteries, and particularly relates to water-soluble polyimide as well as a preparation method and application thereof. The water-soluble polyimide is prepared from the following raw materials: diamine containing sulfonic groups, diamine containing carboxyl, naphthalene-1, 4, 5, 8-tetracarboxylic dianhydride, a catalyst, an alkaline neutralizer and a solvent. The invention also relates to a negative electrode slurry containing the water-soluble polyimide, a negative electrode plate and a lithium battery. The water-soluble polyimide disclosed by the invention has excellent electrochemical performance of naphthalene-1, 4, 5, 8-tetracarboxylic dianhydride and excellent ion exchange capacity and proton conductivity of sulfonic acid groups at the same time, so that a battery prepared from a negative pole piece prepared from the polyimide water-based binder can show excellent charge-discharge performance and cycling stability.

Description

Technical Field

[0001] This application relates to the technical fields of polymer materials and lithium battery technologies, and particularly relates to a water-soluble polyimide, a preparation method thereof, and an application thereof. Background Art

[0002] In order to improve the energy density of lithium-ion batteries to meet the growing demands of portable electronic devices, electric vehicles, and large-scale energy storage systems, various electrode materials have been studied. Among these materials, silicon is considered an ideal anode material for next-generation lithium-ion batteries because its theoretical specific capacity is as high as 4200 mAh·g-1, which is more than 10 times higher than that of currently used graphite (372 mAh·g-1), and it is rich in natural content. However, the cycle life of silicon anodes is limited by the huge volume change during the charge-discharge cycle, resulting in the repeated formation of the solid electrolyte interface, electrode pulverization, and continuous capacity decay.

[0003] Although the binder accounts for a small proportion in the silicon electrode composite material, it has been reported that the binder has a significant impact on the cycle performance of silicon anodes. In recent years, various polymer binders have been developed, such as mussel-inspired binders, polyacrylic acid, polysaccharides, polyacrylonitrile, self-healing polymer binders, conductive binders, etc. Yao et al. synthesized a series of polyimides containing poly(ethylene glycol) (PEG) segments with different molecular weights in their polymer chains and characterized them by Fourier transform infrared spectroscopy and hydrogen nuclear magnetic resonance spectroscopy. The main components of the polyimides are derived from trimellitic anhydride chloride (TMAC) and 4,4′-methylenedianiline, and the PEG segments are introduced into them through an esterification reaction with TMAC. These polyimides have excellent water solubility after being neutralized with triethylamine and can be used as water-soluble binders for silicon anodes in lithium-ion batteries, significantly improving the electrochemical performance of silicon anodes.

[0004] Sulfonated polyimides have excellent thermal stability, high mechanical strength, good film-forming ability, and excellent chemical resistance, and thus have been widely used in many industrial fields. These advantages are exactly what are required for the polyelectrolyte membrane materials used in fuel cell systems. However, due to the easy hydrolysis of the imino ring, ordinary five-membered ring polyimides are usually unstable to acids, and sulfonated polyimides are expected to be more unstable than non-sulfonated polyimides. Six-membered ring polyimides derived from naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTDA) have been found to be quite stable to both acids and pure water. Sulfonated polyimides are almost the only advanced membranes that can be used in practical systems due to their high proton conductivity, good mechanical strength, high thermal stability, and chemical stability.

[0005] Therefore, there is a continuous need in this field to develop a water-soluble polyimide suitable for use as a binder for silicon-based lithium batteries and a preparation method thereof. Summary of the Invention

[0006] The purpose of this application is to provide a water-soluble polyimide containing a naphthalene ring structure, which is prepared by one-step polymerization of a sulfonated diamine monomer and 1,4,5,8-naphthalenetetracarboxylic dianhydride. The obtained water-soluble polyimide can be used as a binder for silicon-based anodes, improving the cyclic capacity retention rate of silicon-based anode materials.

[0007] This application also provides a method for preparing a water-soluble polyimide.

[0008] This application also provides a negative electrode slurry, a negative electrode plate and a lithium battery containing the water-soluble polyimide as described above.

[0009] This application also provides the use of the water-soluble polyimide as a binder for lithium batteries.

[0010] To solve the above technical problems, this application provides the following technical solutions.

[0011] In the first aspect, this application provides a water-soluble polyimide, which is characterized in that the water-soluble polyimide is made from the following raw materials: diamine containing sulfonic acid group, diamine containing carboxyl group, naphthalene-1,4,5,8-tetracarboxylic dianhydride, catalyst, basic neutralizer, solvent and optionally polyetheramine.

[0012] In an embodiment of the first aspect, the diamine containing sulfonic acid group includes one or more of 2,2'-benzidine disulfonic acid, 4,4-bis(4-aminophenoxy)biphenyl-3,3-disulfonic acid, 4,4′–diaminodiphenyl ether-2,2′–disulfonic acid.

[0013] The diamine containing carboxyl group includes one or more of 4,4'-diaminobiphenyl-2,2-dicarboxylic acid, 3,5-diaminobenzoic acid, 4,4′–diaminodiphenyl ether-2,2′–dicarboxylic acid.

[0014] The number average molecular weight of the polyetheramine is 210 - 810, and more preferably 210 - 410.

[0015] The basic neutralizer is one or more of triethylamine, pyridine and 1,2-dimethylimidazole.

[0016] The catalyst is one or more of benzoic acid, isoquinoline, 4-methylquinoline, 2-methylquinoline and 5,6,7,8-tetrahydroquinoline.

[0017] The solvent is an aprotic polar solvent. Preferably, the aprotic polar solvent includes one or more of N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, m-cresol.

[0018] In an embodiment of the first aspect, the molar ratio of the sulfonic acid group-containing diamine, the carboxyl group-containing diamine, and the polyetheramine is 1:(0-0.2):(0-0.2), preferably 1:(0.1-0.2):(0.05-0.1);

[0019] The molar ratio of the total molar amount of the sulfonic acid group-containing diamine, the carboxyl group-containing diamine, and the polyetheramine to the molar amount of naphthalene-1,4,5,8-tetracarboxylic dianhydride is 1:(0.9-1.1).

[0020] In the second aspect, the present application provides a method for preparing the water-dispersible polyimide as described above, which includes the following steps:

[0021] Step S1: Mix the sulfonic acid group-containing diamine, the carboxyl group-containing diamine, and the basic neutralizing agent in a solvent to obtain a clear solution;

[0022] Step S2: Add naphthalene-1,4,5,8-tetracarboxylic dianhydride to the clear solution obtained in Step S1 for reaction. After the naphthalene-1,4,5,8-tetracarboxylic dianhydride is completely dissolved, add the polyetheramine and the catalyst, and react at the first reaction temperature to obtain a polyimide precursor solution;

[0023] Step S3: Subject the polyimide precursor solution to an imidization reaction at the second reaction temperature to obtain the water-soluble polyimide.

[0024] In an embodiment of the second aspect, the reactions in Step S1, Step S2, and Step S3 are all carried out under the protection of a protective gas;

[0025] Preferably, the protective gas includes nitrogen or argon;

[0026] The first reaction temperature in Step S2 is 60-120 °C, more preferably 80-100 °C;

[0027] The reaction time in Step S2 is 2-8 h, more preferably 4-6 h;

[0028] After the reaction in Step S2, it further includes the steps of sedimenting, washing, and drying the reaction solution in ethanol and acetone.

[0029] The present application also provides a negative electrode paste, which includes:

[0030] (A) The water-soluble polyimide according to any one of claims 1-3;

[0031] (B) A silicon-based negative electrode material; and,

[0032] (C) A conductive agent.

[0033] In one embodiment, the content of the polyimide in the polyimide aqueous solution is preferably 1% by mass or more and 8% by mass or less, more preferably 2% by mass or more and 4% by mass or less, in the non-volatile components, and the viscosity of the aqueous solution is preferably in the range of 4000±2000 mPa·s, more preferably in the range of 3000±1000 mPa·s.

[0034] Preferably, the water-soluble polyimide needs to be prepared into aqueous solutions with different mass fractions in deionized water in advance for use, and an appropriate amount of lithium hydroxide monohydrate needs to be added during the preparation process for pre-lithiation.

[0035] The silicon-based anode material is a pure silicon anode material, a silicon-carbon alloy or silicon monoxide. Preferably, based on 100 parts by weight of the water-soluble polyimide, the amount of the silicon-based anode material is 300-900 parts by weight, preferably 400-800 parts by weight.

[0036] The conductive agent is Super P conductive carbon black. Preferably, based on 100 parts by weight of the water-soluble polyimide, the amount of the conductive agent is 50-200 parts by weight, preferably 50-100 parts by weight.

[0037] In one embodiment, the present application provides a negative electrode plate, which includes a copper foil and the above-mentioned water-soluble polyimide coated on the copper foil.

[0038] In one embodiment, the present application provides a lithium battery, which includes the above-mentioned negative electrode plate.

[0039] In one embodiment, the present application provides the use of the above-mentioned water-soluble polyimide as a silicon anode binder for a lithium battery.

[0040] Compared with the prior art, the positive effect of the present invention is that: the water-soluble polyimide described herein simultaneously has excellent electrochemical performance of naphthalene-1,4,5,8-tetracarboxylic dianhydride and excellent ion exchange capacity (IEC) and proton conductivity of the sulfonic acid group, so that the battery prepared from the negative electrode plate made of the polyimide aqueous binder can exhibit excellent charge and discharge performance and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The following drawings detail the exemplary embodiments disclosed in the present application. Wherein the same reference numerals represent similar structures in several views of the drawings. Those of ordinary skill in the art will understand that these embodiments are non-limiting, exemplary embodiments, and the drawings are only for the purpose of illustration and description, and are not intended to limit the scope of the present application. Embodiments in other ways may also achieve the inventive intent in the present application. It should be understood that the drawings are not drawn to scale.

[0042] Figure 1 Show the synthesis route of water-soluble polyimide.

[0043] Figure 2 Show the infrared spectrum of the water-soluble polyimide according to Example 1.

[0044] Figure 3 Show the mechanical properties of the water-soluble polyimides prepared in Examples 1-3 and Comparative Example 1.

[0045] Figure 4 Show the impedance performance of the button half-cells prepared in Examples 2-3 and Comparative Example 1 after 300 cycles.

[0046] Figure 5 Show the rate performance of the button half-cells prepared in Examples 1-2 and Comparative Example 1. See Figure 4 .

[0047] Figure 6 Show the schematic diagram of the button half-cell.

[0048] Figure 7 Are the structural formulas of the lithiated polyimide polymer materials in Examples 1-2.

[0049] Figure 8 Are the structural formulas of the lithiated polyimide polymer materials in Example 3. Detailed implementation manners

[0050] The existing binders for battery anodes cannot adapt to the severe volume expansion problem of silicon-based anode materials during lithium insertion and extraction. During long-term charge and discharge cycles, the electrode surface will show phenomena such as pulverization and cracking, resulting in a rapid decline in battery capacity and loss of electrolyte. Currently, although the existing polyimide binders have high mechanical strength, their elongation at break is relatively low, and they can only be dissolved in high-boiling-point strong polar organic solvents to meet the requirements of green production. The present invention prepares a water-soluble polyimide containing a naphthalene ring structure as a binder for the silicon-based anode of a battery, improving the cycle capacity retention rate of the silicon-based anode material.

[0051] Water-soluble polyimide

[0052] As Figure 1 shown, the present invention provides a preparation method and application of a polyimide aqueous binder.

[0053] The raw materials for preparing the polyimide aqueous binder include a combination of diamines containing sulfonic acid groups, diamines containing carboxyl groups, polyetheramines, naphthalene-1,4,5,8-tetracarboxylic dianhydride, and a catalyst. By adjusting the ratio of diamines containing sulfonic acid groups to other diamines, polyimide with excellent solubility in water is directly synthesized by a one-step high-temperature method, while retaining the excellent mechanical properties of the polyimide material, and having excellent adhesion properties to silicon-based anode sub-materials on copper foil. Since this material simultaneously has the excellent electrochemical properties of naphthalene-1,4,5,8-tetracarboxylic dianhydride and the excellent ion exchange capacity (IEC) and proton conductivity of sulfonic acid groups, the battery prepared from the anode electrode sheet made of the polyimide aqueous binder can exhibit excellent charge and discharge performance and cycle stability.

[0054] In a specific embodiment, the water-soluble polyimide described herein may have a structure as shown in Structural Formula I:

[0055]

[0056] Wherein, the value ranges of x, y, and z are: x = 0.2 - 0.4; y = 0.4 - 0.6; z = 0.1 - 0.2; x + y + z = 1.0. In this embodiment, the raw materials for preparing the water-soluble polyimide include polyetheramine.

[0057] In another specific embodiment, the water-soluble polyimide has a structure as shown in Structural Formula II:

[0058]

[0059] In Structural Formula II, the value ranges of x and y are x = 0.4 - 0.6, y = 0.4 - 0.6, and x + y = 1.0. In this embodiment, the raw materials for preparing the water-soluble polyimide do not include polyetheramine.

[0060] In a preferred embodiment, the weight-average molecular weight of the water-soluble polyimide is 120,000 - 180,000, such as 120,000, 130,000, 140,000, 150,000, 160,000, 170,000, 180,000 or the range or sub-range between any two of them. Only water-soluble polyimide with a specific molecular weight can obtain the mechanical properties required in this application.

[0061] In one embodiment, the raw materials for preparing the water-soluble polyimide include a diamine containing a sulfonic acid group, a diamine containing a carboxyl group, a polyetheramine, naphthalene-1,4,5,8-tetracarboxylic dianhydride, and a catalyst. Preferably, the diamine containing a sulfonic acid group includes any one or a combination of at least two of 2,2'-benzidine disulfonic acid, 4,4-bis(4-aminophenoxy)biphenyl-3,3-disulfonic acid, and 4,4′–diaminodiphenyl ether-2,2′–disulfonic acid. Preferably, the diamine containing a carboxyl group includes any one or a combination of at least two of 4,4'-diaminobiphenyl-2,2-dicarboxylic acid, 3,5-diaminobenzoic acid, and 4,4′–diaminodiphenyl ether-2,2′–dicarboxylic acid. Preferably, the number-average molecular weight of the polyetheramine is 210 to 810, more preferably 210 to 410. For example, the number-average molecular weight of the polyetheramine is 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, 600, 610, 620, 630, 640, 650, 660, 670, 680, 690, 700, 710, 720, 730, 740, 750, 760, 770, 780, 790, 800, 810, or the range or sub-range between any two of these values.

[0062] Preferably, the molar ratio of the diamine containing a sulfonic acid group, the diamine containing a carboxyl group, and the polyetheramine is 1:(0 to 0.2):(0 to 0.2), preferably 1:(0.1 to 0.2):(0.05 to 0.1). Preferably, the molar ratio of the total molar amount of the diamine containing a sulfonic acid group, the diamine containing a carboxyl group, and the polyetheramine to naphthalene-1,4,5,8-tetracarboxylic dianhydride is 1:(0.9 to 1.1).

[0063] In another embodiment, the raw materials for preparing the water-soluble polyimide further include a solvent. Preferably, the solvent is an aprotic polar solvent. Preferably, the aprotic polar solvent includes any one or a combination of at least two of N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and m-cresol, more preferably m-cresol.

[0064] In another embodiment, the present application provides a method for preparing the polyimide aqueous binder as described above, which includes the following steps: Step (1): Add triethylamine to a diamine containing a sulfonic acid group and a diamine containing a carboxyl group in a solvent and mix and stir to obtain a clear solution; Step (2): Add naphthalene-1,4,5,8-tetracarboxylic dianhydride to the clear solution in Step (1) for reaction. After complete dissolution, add benzoic acid and polyetheramine and then heat for reaction; Step (3): After heating and polymerizing the system in Step (2) for a certain period of time, continue to raise the temperature for reaction to carry out a one-pot high-temperature imidization reaction to obtain the polyimide aqueous binder.

[0065] In a specific embodiment, the reactions in Step (1), Step (2), and Step (3) are all carried out under the protection of a protective gas. Preferably, the protective gas includes nitrogen or argon. Preferably, the temperature of the reaction in Step (2) is 60-120°C, more preferably 80-100°C. For example, the temperature of the reaction in Step (2) is 60, 70, 80, 90, 100°C or the range or sub-range between any two of these values. Preferably, the reaction time in Step (2) is 2-8h, more preferably 4-6h. Preferably, after the reaction in Step (3), it further includes the steps of sedimenting, washing, and drying the reaction solution in ethanol and acetone.

[0066] Negative electrode slurry containing aqueous polyimide binder

[0067] In another embodiment, the present application provides a negative electrode slurry, which contains: (A) the above-mentioned water-soluble polyimide, (B) a silicon-based negative electrode material, and (C) a conductive agent.

[0068] Next, the components included in the negative electrode slurry of the present application will be described in detail.

[0069] (A) Water-soluble polyimide

[0070] The negative electrode slurry of the present invention contains the aforementioned polyimide fiber containing a carboxyl group and a sulfonic acid group as a water-soluble binder, so that an aqueous negative electrode slurry meeting the requirements of green chemistry can be prepared.

[0071] It should be noted that the water-soluble polyimide of the present invention needs to be prepared into aqueous solutions with different mass fractions in deionized water in advance for use, and an appropriate amount of lithium hydroxide monohydrate needs to be added for lithiumation during the preparation process.

[0072] The content of polyimide in the polyimide aqueous solution is preferably 1% by mass or more and 8% by mass or less, more preferably 2% by mass or more and 4% by mass or less in the non-volatile components, and the viscosity of the aqueous solution is preferably in the range of 4000 ± 2000 mPa·s, and more preferably in the range of 3000 ± 1000 mPa·s. Through this range of content, the effects of the present invention can be fully obtained.

[0073] (B) Silicon-based anode material

[0074] The negative electrode paste containing the aqueous polyimide binder of the present invention contains a silicon-based negative electrode material, and examples thereof include a pure silicon negative electrode material and a silicon-carbon alloy. Among them, from the viewpoint of charge-discharge capacity, a pure silicon negative electrode material is preferred.

[0075] The silicon-based negative electrode material can be compounded in a commonly known ratio. For example, for the pure silicon negative electrode material, relative to 30 parts by mass of the water-soluble polyimide, it is preferably compounded in a ratio of 100 to 270 parts by mass, and preferably in a ratio of 120 to 240 parts by mass.

[0076] It should be noted that there are various types of such silicon-based negative electrode materials to choose from.

[0077] Elemental silicon is usually used as a semiconductor material, but in addition, it can also be used as a negative electrode material for lithium-ion batteries. The capacity of the negative electrode material containing a silicon component is much larger than that of the traditional graphite negative electrode material. Examples of the negative electrode material containing a silicon component include, for example: silicon monoxide, micron silicon, nano silicon, silicon-carbon alloy, and silicon-carbon coated graphite material. Among the above, from the viewpoint of the size of the charge-discharge capacity, silicon dioxide and nano silicon materials are preferred.

[0078] As the silicon monoxide material, silicon and silicon dioxide are compounded into silicon monoxide (SiOx). Since the silicon material particles are small and the structure is dense and stable, the expansion during lithium insertion and extraction is small, and the cycle performance is better. However, its first efficiency is low, and it is necessary to improve the capacity by lithium supplementation or improve the first efficiency by magnesium modification. The silicon monoxide selected in the present invention has a measured capacity of 1400 - 1600.

[0079] The maximum theoretical specific capacity of pure silicon is 4200 mAh / g, and the molecular formula is Li22Si5. At room temperature, at a low charging potential <0.5V, the theoretical specific capacity is 3579 mAH / g. Due to the unique surface effect and size effect of silicon nanoparticles, the stress can be quickly released during the process of lithium insertion and extraction, so it is less likely to break than large particles. The cycle performance of the silicon negative electrode battery can be improved. Since nano silicon has a large specific surface area and generates more SEI films and the nanoparticles are easy to agglomerate, nano silicon is usually used after being compounded with other materials. The nano silicon material with a size of 80 - 100 nm is selected in the present invention.

[0080] (C) Conductive agent and other components

[0081] In the present invention, in order to ensure the electrical conductivity of the negative electrode plate, a certain amount of conductive agent needs to be added. Super P conductive carbon black is used as the conductive agent in the present invention. Relative to 10 parts by mass of water-soluble polyimide, it is preferably compounded in a ratio of 5 to 20 parts by mass, more preferably 5 to 10 parts by mass. Other components are for improving the adhesion performance, and other water-soluble binders such as lithiated polyacrylic acid or sodium carboxymethylcellulose are added for blending, or for improving the viscosity and solubility, a small amount of ethanol or acetone is added as a mixed solvent.

[0082] Negative electrode plate

[0083] In another embodiment, the present application also provides a negative electrode plate made of the negative electrode paste as described above, and its preparation method is as follows. The electrode plate of the present invention has: a negative electrode plate coated on a copper foil prepared from the above-mentioned negative electrode paste containing water-soluble polyimide. Its thickness is scraped on a 12-nm-thick copper foil using a coater of 50 or 100 nm, dried at room temperature for 12 h, and then dried under vacuum at 80 °C for 12 h. After taking out, it is cut into circular pieces with a diameter of 12 mm by a cutting machine, weighed, and then placed in a vacuum oven and dried at 120 °C for 2 h to remove excess moisture.

[0084] Lithium battery

[0085] In another embodiment, the present application also provides a lithium battery, which includes the negative electrode plate as described above. The assembly process of the negative electrode half-cell is as follows.

[0086] The assembly of the button half-cell is carried out in a glove box. Among them, the active material of the button half-cell is a lithium sheet, the separator is a Celgard 2320 separator, and the electrolyte is 1 M LiFP6 (EC:DEC:FEC = 9:9:2 v / v). The assembly sequence is the positive electrode shell, the negative electrode plate, the electrolyte, the separator, the lithium sheet, the nickel foam, and the negative electrode shell.

[0087] Five batteries are made from each negative electrode plate, and the button half-cells prepared above are connected to a potential tester for charge and discharge experimental tests. Among them, the potential range is 0 to 1.5 V, and cyclic tests are carried out at a constant current of 0.5 C to test the initial charge capacity, the initial charge efficiency, and the capacity retention rate after 200 cycles of testing.

[0088] Example

[0089] Next, the technical solutions of the present application will be clearly and completely described in conjunction with the embodiments of the present application. Unless otherwise specified, the reagents and raw materials used can be purchased through commercial channels. The experimental methods without specific conditions indicated in the following examples are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0090] Example 1

[0091] This example relates to a silicon-based anode slurry prepared from a water-soluble polyimide containing a naphthalene ring structure, which comprises the following components in parts by weight: 15 parts of a water-soluble polyimide containing a naphthalene ring structure, 15 parts of a conductive agent, 70 parts of a silicon-based anode material, and 200 parts of a solvent (water).

[0092] The preparation method of the silicon-based anode half-cell of this example comprises the following steps:

[0093] (1) Add 0.2863 g of 6,6'-diamino-3,3'-methylenedibenzoic acid (MBAA) and 0.5165 g of 2,2'-disulfonic acid benzidine (BDSA) to a three-necked flask, then add 5 ml of m-cresol and 0.75 ml of triethylamine. After stirring at room temperature for 15 min, add 0.6703 g of naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTDA), 0.293 g of benzoic acid, 0.3102 g of isoquinoline, and 5 ml of m-cresol. Then heat to 80 °C and react for 4 h, and continue to heat to 180 °C and react for 20 h to obtain a viscous black solution A;

[0094] (2) After diluting solution A with 15 ml of m-cresol, slowly pour solution A into acetone to precipitate light brownish-white fibrous filaments. Soak them in acetone for 24 h, and then dry them in a vacuum at 80 °C for 12 h to obtain solid fibrous filaments B;

[0095] (3) Put the solid fibrous filaments B into a 0.2 mol / L hydrochloric acid aqueous solution, soak for 24 h, then take them out and put them into a new 0.2 mol / L hydrochloric acid solution and continue to soak for 24 h, then filter, and dry them in a vacuum at 120 °C for 12 h to obtain solid fibrous filaments C;

[0096] (4) Add 0.4 g of solid fibrous filaments C and 56.0 mg of lithium hydroxide monohydrate with an equimolar amount of sulfonic acid groups and carboxyl groups to a bottle, add 9.5 g of deionized water, and stir at 50 °C until dissolved to obtain a black aqueous solution D;

[0097] (5) Add 1.125 g of the black aqueous solution D, 45.0 mg of Super P conductive carbon black, and 210.0 mg of the SiOx anode material to a slurry mixing box, and stir evenly with a stirrer at a speed of 2000 rpm / min to obtain a silicon-based anode slurry E;

[0098] (6) Uniformly coat the silicon-based anode slurry E on a copper foil current collector, dry it in a vacuum at 80 °C for 10 h, cut it into small round pieces with a diameter of 12 mm, weigh them, and then dry them in a vacuum at 120 °C for 4 h to obtain a silicon-based anode sheet F;

[0099] (7) In a glove box, under an environment with a water content < 0.1 ppm and an oxygen content < 0.1 ppm, assemble a button half-cell in the order of a CR2016 positive electrode case, a silicon-based negative electrode sheet F, a drop of an electrolyte of 1.0 M in FEC:EC:DEC = 2:9:9, a PE battery separator, a lithium sheet, nickel foam, and a CR2016 negative electrode case.

[0100] Example 2

[0101] This example relates to a silicon-based negative electrode slurry prepared from a water-soluble polyimide containing a naphthalene ring structure, including the following components in parts by weight: 15 parts of a water-soluble polyimide containing a naphthalene ring structure, 15 parts of a conductive agent, 70 parts of a silicon-based negative electrode material, and 200 parts of a solvent (water).

[0102] The preparation method of the silicon-based negative electrode sheet half-cell in this example includes the following steps:

[0103] (1) Add 0.4294 g of 6,6'-diamino-3,3'-methylenedibenzoic acid (MBAA) and 0.3444 g of 2,2'-disulfonic acid benzidine (BDSA) to a three-necked flask, then add 5 ml of m-cresol and 0.6 ml of triethylamine. After stirring at room temperature for 15 min, add 0.6703 g of naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTDA), 0.293 g of benzoic acid, 0.3102 g of isoquinoline, and 5 ml of m-cresol, and then heat to 80 °C and react for 4 h. Continue to heat to 180 °C and react for 20 h to obtain a viscous black solution A;

[0104] (2) After diluting solution A with 15 ml of m-cresol, slowly pour solution A into acetone to precipitate light brownish-white fibrous filaments. Immerse them in acetone for 24 h, and then dry them in a vacuum at 80 °C for 12 h to obtain solid fibrous filaments B;

[0105] (3) Put solid fibrous filaments B into a 0.2 mol / L hydrochloric acid aqueous solution, soak for 24 h, then take them out and put them into a new 0.2 mol / L hydrochloric acid for continued soaking for 24 h, then filter, and dry them in a vacuum at 120 °C for 12 h to obtain solid fibrous filaments C;

[0106] (4) Add 0.5 g of solid fibrous filaments C and 66.5 mg of lithium hydroxide monohydrate with an equimolar amount of sulfonic acid and carboxyl groups to a bottle, add 9.5 g of deionized water, and stir at 50 °C until dissolved to obtain a black aqueous solution D;

[0107] (5) Add 0.9 g of black aqueous solution D, 45.0 mg of Super P conductive carbon black, and 210.0 mg of SiOx negative electrode material to a slurry mixing box, and stir evenly with a stirrer at a speed of 2000 rpm / min to obtain a silicon-based negative electrode slurry E;

[0108] (6) Evenly coat the silicon-based anode slurry E on the copper foil current collector, dry it in a vacuum environment at 80 °C for 10 h, cut it into small round pieces with a diameter of 12 mm, weigh them, and then dry them in a vacuum environment at 120 °C for 4 h to obtain the silicon-based anode sheet F.

[0109] (7) In a glove box under an environment with a water content < 0.1 ppm and an oxygen content < 0.1 ppm, assemble a button half-cell in the order of a CR2016 positive electrode case, the silicon-based anode sheet F, a drop of 1.0 M in FEC:EC:DEC = 2:9:9 electrolyte solution, a PE battery separator, a lithium sheet, a nickel foam, and a CR2016 negative electrode case.

[0110] Example 3

[0111] This example relates to a silicon-based anode slurry prepared from a water-soluble polyimide containing a naphthalene ring structure, and includes the following components in parts by weight: 15 parts of a water-soluble polyimide containing a naphthalene ring structure, 15 parts of a conductive agent, 70 parts of a silicon-based anode material, and 200 parts of a solvent (water).

[0112] The preparation method of the silicon-based anode sheet half-cell in this example includes the following steps:

[0113] (1) Add 0.3435 g of 6,6'-diamino-3,3'-methylenedibenzoic acid (MBAA) and 0.2755 g of 2,2'-disulfonic acid benzidine (BDSA) to a three-necked flask, then add 5 ml of m-cresol and 0.75 ml of triethylamine. After stirring at room temperature for 15 min, add 0.5899 g of naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTDA), 0.293 g of benzoic acid, 0.3102 g of isoquinoline, polyetheramine (M w = 210) 0.042 g and 5 ml of m-cresol, then heat to 80 °C and react for 4 h, continue to heat to 180 °C and react for 20 h to obtain a viscous black solution A;

[0114] (2) After diluting solution A with 15 ml of m-cresol, slowly pour solution A into acetone, precipitate light brownish-white fibrous filaments, soak them in acetone for 24 h, and then dry them in a vacuum at 80 °C for 12 h to obtain solid fibrous filaments B;

[0115] (3) Put the solid fibrous filaments B into a 0.2 mol / L hydrochloric acid aqueous solution, soak them for 24 h, take them out and put them into a new 0.2 mol / L hydrochloric acid and continue to soak for 24 h, then filter, and dry them in a vacuum at 120 °C for 12 h to obtain solid fibrous filaments C;

[0116] (4) Add 0.5 g of solid fiber C and 65.0 mg of lithium hydroxide monohydrate, which is equimolar to sulfonic acid groups and carboxyl groups, into a bottle. After adding 9.5 g of deionized water, stir at 50 °C until dissolved to obtain a black aqueous solution D;

[0117] (5) Add 0.9 g of black aqueous solution D, 45.0 mg of Super P conductive carbon black, and 210.0 mg of nmSi anode material into a slurry mixing box, and stir evenly with a stirrer at a speed of 2000 rpm / min to obtain a silicon-based anode slurry E;

[0118] (6) Coat the silicon-based anode slurry E evenly on a copper foil current collector, dry it in a vacuum environment at 80 °C for 10 h, cut it into small round pieces with a diameter of 12 mm, weigh it, and then dry it in a vacuum environment at 120 °C for 4 h to obtain a silicon-based anode plate F;

[0119] (7) In a glove box, under an environment with a water content < 0.1 ppm and an oxygen content < 0.1 ppm, assemble a button half-cell in the order of a CR2016 positive electrode case, a silicon-based anode plate F, a drop of 1.0 M in FEC:EC:DEC = 2:9:9 electrolyte solution, a PE battery separator, a lithium sheet, a nickel foam, and a CR2016 negative electrode case.

[0120] Comparative Example 1

[0121] The silicon-based anode slurry prepared with a commercial polyacrylic acid binder includes the following components in parts by weight: 15 parts of polyacrylic acid binder, 15 parts of conductive agent, 70 parts of silicon-based anode material, and 200 parts of solvent (water).

[0122] The preparation method of the silicon-based anode plate half-cell in this comparative example includes the following steps:

[0123] (1) Add 0.5 g of polyacrylic acid powder solid into a bottle. After adding 9.5 g of deionized water, stir at 50 °C until dissolved to obtain a colorless and transparent aqueous solution A;

[0124] (2) Add 0.9 g of colorless aqueous solution D, 45.0 mg of Super P conductive carbon black, and 210.0 mg of SiOx anode material into a slurry mixing box, and stir evenly with a stirrer at a speed of 2000 rpm / min to obtain a silicon-based anode slurry E;

[0125] (3) Coat the silicon-based anode slurry E evenly on a copper foil current collector, dry it in a vacuum environment at 80 °C for 10 h, cut it into small round pieces with a diameter of 12 mm, weigh it, and then dry it in a vacuum environment at 120 °C for 4 h to obtain a silicon-based anode plate F;

[0126] (4) In a glove box under an environment with a water content < 0.1 ppm and an oxygen content < 0.1 ppm, assemble a button half-cell in the order of a CR2016 positive electrode case, a silicon-based negative electrode sheet F, a drop of an electrolyte of 1.0 M in FEC:EC:DEC = 2:9:9, a PE battery separator, a lithium sheet, a nickel foam, and a CR2016 negative electrode case.

[0127] Table 1: First charge efficiency and retention performance of Examples 1-3 and Comparative Examples

[0128] Initial charge efficiency Retention rate after 200 cycles Retention rate after 300 cycles Example 1 67.84％ 46.8％ 32.9％ Example 2 68.08％ 49.6％ 43.2％ Example 3 69.81％ 55.8％ 50.2％ Comparative Example 1 70.86％ 17.2％ ≈0

[0129] For the mechanical properties of the water-soluble polyimide prepared in Examples 1-3 and Comparative Example 1, see Figure 2 . For the impedance performance of the button half-cells prepared in Examples 2-3 and Comparative Example 1 after 300 cycles, see Figure 3 . For the rate performance of the button half-cells prepared in Examples 1-2 and Comparative Example 1, see Figure 4 . The schematic diagram of the button half-cell is as shown in Figure 5 . For the first charge efficiency and retention performance of the button half-cells prepared in Examples 1-3 and Comparative Example 1, see Table 1.

[0130] From Figure 3 - Figure 6 and the data in Table 1, it can be seen that with the increase in the content of the 6,6'-diamino-3,3'-methylenedibenzoic acid (MBAA) monomer and the addition of polyetheramine (M w = 210), the mechanical properties are enhanced to a maximum of an elongation at break of 17.6%, a breaking strength of 84.7 Mpa, and a tensile modulus of 2.8 Gpa. The charge retention performance at 200 and 300 cycles increases significantly with the improvement of the mechanical properties, and the highest 200-cycle retention rate reaches 55.8%.

[0131] Figure 7 is the structural formula of the lithiated polyimide polymer material in Examples 1-2, Figure 8 is the structural formula of the lithiated polyimide polymer material in Example 3. In Figure 7 , the weight-average molecular weight of the lithiated polyimide polymer material is about 150,000, and X:Y is equal to 4:6. In Figure 8 , the weight-average molecular weight of the lithiated polyimide polymer material is about 150,000, and X:Y:Z is equal to 4:6:1.

[0132] Examples 4-10

[0133] The applicant prepared other water-soluble polyimides, anode slurries, and coin half-cells by following the same process as in Example 1, while changing the types and weight ratios of the monomers. The formulation of the anode slurry was also: 15 parts of a water-soluble polyimide containing a naphthalene ring structure, 15 parts of a conductive agent, 70 parts of a silicon-based anode material, and 200 parts of a solvent (water).

[0134] The types and amounts of raw materials used to prepare the water-soluble polyimides in Examples 4-10 are shown in Table 2. The mechanical properties of the water-soluble polyimides prepared in Examples 4-10, as well as the first charge efficiency and retention rate performance of the coin half-cells, are shown in Table 3.

[0135] Table 2: Raw materials and amounts used to prepare the water-soluble polyimides in Examples 4-10

[0136]

[0137]

[0138]

[0139]

[0140] Table 3: Mechanical properties of the water-soluble polyimides prepared in Examples 4-10, as well as the first charge efficiency and retention rate performance of the coin half-cells

[0141] Initial charge efficiency Retention rate after 200 cycles Retention rate after 300 cycles Example 4 69.04％ 40.2％ 30.4％ Example 5 69.13％ 45.8％ 40.2％ Example 6 68.61％ 45.2％ 42.6％ Example 7 69.04％ 36.3％ 25.4％ Example 8 70.23％ 50.2％ 46.3％ Example 9 68.12％ 45.1％ 41.7％ Example 10 67.02％ 23.2％ 15.3％ Example 11 69.94％ 48.3％ 45.6％

[0142] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and apply the present application. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present application is not limited to the embodiments herein, and all improvements and modifications made by those skilled in the art within the scope and spirit of the present application without departing from the disclosure of the present application are within the scope of the present application.

Claims

1. A water-soluble polyimide, characterized in that: The water-soluble polyimide is prepared from the following raw materials: a diamine containing a sulfonic acid group, a diamine containing a carboxyl group, naphthalene-1,4,5,8-tetracarboxylic dianhydride, a catalyst, an alkaline neutralizer and a solvent.

2. The water-soluble polyimide according to claim 1, characterized in that The sulfonic acid group-containing diamine includes one or more of 2,2'-benzidine disulfonic acid, 4,4-bis(4-aminophenoxy)biphenyl-3,3-disulfonic acid, and 4,4'-diaminodiphenyl ether-2,2'-disulfonic acid; The carboxyl-containing diamine includes one or more of 6,6'-diamino-3,3'-methylene dibenzoic acid, 4,4'-diaminobiphenyl-2,2-dicarboxylic acid, 3,5-diaminobenzoic acid, and 4,4'-diaminodiphenyl ether-2,2'-dicarboxyl; Preferably, the raw material further comprises polyetheramine, more preferably, the number average molecular weight of the polyetheramine is 210-810, further preferably 210-410; The alkaline neutralizing agent is one or more of triethylamine, pyridine and 1,2-dimethylimidazole; The catalyst is one or more of benzoic acid, isoquinoline, 4-methylquinoline, 2-methylquinoline and 5,6,7,8-tetrahydroquinoline; The solvent is a non-protonated polar solvent, preferably, the non-protonated polar solvent includes one or more of N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and m-cresol; Preferably, the water-soluble polyimide has a structure as shown in structural formula I: In the structural formula I, the value ranges of x, y and z are: x = 0.2 to 0.4; y = 0.4 to 0.6; z = 0.1 to 0.2; x + y + z = 1.0; Preferably, the water-soluble polyimide has a structure as shown in structural formula II: In structural formula II, the value ranges of x and y are x=0.4-0.6, y=0.4-0.6, x+y=1.0; Preferably, the weight average molecular weight of the water-soluble polyimide is 120,000-180,000.

3. The water-soluble polyimide according to claim 1 or 2, characterized in that The molar ratio of the sulfonic acid group-containing diamine, the carboxyl group-containing diamine, and the polyetheramine is 1: (0-0.2): (0-0.2), preferably 1: (0.1-0.2): (0.05-0.1); The molar ratio of the total molar number of the sulfonic acid group-containing diamine, the carboxyl group-containing diamine and the polyetheramine to naphthalene-1,4,5,8-tetracarboxylic dianhydride is 1:(0.9-1.1).

4. The method for preparing a water-soluble polyimide according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following steps: Step S1: mixing a sulfonic acid group-containing diamine, a carboxyl group-containing diamine and an alkaline neutralizing agent in a solvent to obtain a clear solution; Step S2: adding naphthalene-1,4,5,8-tetracarboxylic acid dianhydride to the clarified solution obtained in step S1 to carry out a reaction, and after the naphthalene-1,4,5,8-tetracarboxylic acid dianhydride is completely dissolved, adding a polyetheramine and a catalyst to carry out a reaction at a first reaction temperature to obtain a polyimide precursor solution; Step S3: subjecting the polyimide precursor solution to an imidization reaction at a second reaction temperature to obtain the water-soluble polyimide.

5. The preparation method according to claim 4, characterized in that: The reactions in step S1, step S2 and step S3 are all carried out under the protection of protective gas; Preferably, the protective gas comprises nitrogen or argon; Step S2: the first reaction temperature is 60-120°C, more preferably 80-100°C; The reaction time of step S2 is 2 to 8 hours, more preferably 4 to 6 hours; After the reaction in step S2 is completed, the method further comprises the steps of: settling the reaction solution in ethanol or acetone, washing and drying.

6. A negative electrode slurry, characterized in that: The negative electrode slurry comprises: (A) the water-soluble polyimide according to any one of claims 1 to 3; (B) a silicon-based negative electrode material; and, (C) Conductive agent.

7. The negative electrode slurry according to claim 1, characterized in that: The content of polyimide in the polyimide aqueous solution is preferably 1 mass % or more and 8 mass % or less, more preferably 2 mass % or more and 4 mass % or less in the non-volatile components, and the viscosity of the aqueous solution is preferably in the range of 4000±2000 mPa·s, more preferably in the range of 3000±1000 mPa·s; Preferably, the water-soluble polyimide needs to be prepared in deionized water in advance into aqueous solutions of different mass fractions for use, and an appropriate amount of lithium hydroxide monohydrate needs to be added during the preparation process for early lithiation; The silicon-based negative electrode material is a pure silicon negative electrode material, a silicon-carbon alloy or silicon monoxide. Preferably, based on 100 parts by weight of the water-soluble polyimide, the amount of the silicon-based negative electrode material is 300-900 parts by weight, preferably 400-800 parts by weight. The conductive agent is Super P conductive carbon black. Preferably, based on 100 parts by weight of the water-soluble polyimide, the amount of the conductive agent is 50-200 parts by weight, preferably 50-100 parts by weight.

8. A negative electrode plate, characterized in that: The negative electrode plate comprises a copper foil and the water-soluble polyimide according to any one of claims 1 to 3 coated on the copper foil.

9. A lithium battery, characterized in that: The lithium battery comprises the negative electrode sheet as claimed in claim 8.

10. Use of the water-soluble polyimide according to any one of claims 1 to 3 as a silicon negative electrode binder for lithium batteries.

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