A binder for use in electrodes

Abstract

The application discloses a kind of binders for electrode, the binder includes binder matrix, sulfide graft modified polyimide, the mass ratio of binder matrix and sulfide graft modified polyimide additive is (10-100):1.The application is not only favorable to improve the glass transition temperature of polyimide by the graft modification of sulfide group, but also can be favorable to the commercialization of polyimide.Sulfide graft modified polyimide and binder matrix cooperate to generate three-dimensional crosslinking network, which can also overcome the volume change of silicon during battery cycle and reduce the occurrence of lithium precipitation.The binder of the application is particularly suitable for silicon-based batteries.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrode, in particular to a binder used in electrode. BACKGROUND

[0002] With the popularity of mobile phones, notebook computers, digital cameras and other electronic devices, China has become the largest consumer in the battery industry. The convenience and diversity brought by electronic products to life have made China's lithium ion battery market maintain a rapid growth trend in the past few years. Now, people have higher requirements for the energy density and service life of batteries, and the existing lithium ion battery system has gradually failed to meet this increasing demand. Silicon-based negative electrode has high theoretical specific capacity and low electrochemical lithium intercalation potential, and excellent fast charging performance, which is the urgent need for the development of a series of new technology fields such as 3C products, portable electronic products, unmanned aerial vehicles, new energy vehicles and energy storage battery systems.

[0003] However, during the charging and discharging cycle of the battery, the silicon-based negative electrode will expand in volume, resulting in rapid capacity decay; in addition, the volume expansion will lead to a decrease in the adhesion strength between the negative electrode slurry and the negative electrode current collector, which will further lead to a decrease in the chemical performance of the battery. Therefore, how to overcome the huge volume change of silicon during the charging and discharging cycle of the battery and the decrease in adhesion strength caused by the volume change is an important step in promoting the commercial development of silicon-based negative electrodes. SUMMARY

[0004] The present application discloses a binder used in electrode, which provides a binder particularly suitable for silicon-based electrodes, which can overcome the volume expansion effect of silicon-based electrodes during the charging cycle and enhance the adhesion strength between the negative electrode slurry and the negative electrode current collector.

[0005] Polyimide has good heat resistance and adhesion, but its high glass transition temperature and electrical insulation limit its commercial development. The negative electrode of the battery also bears the embedding and stripping of lithium during the charging and discharging process of the battery, and even in silicon-based electrodes, there will be a change in the volume of silicon. Based on these factors, how does polyimide improve its performance to be applied in silicon-based electrodes, while taking advantage of polyimide, improve the cycle performance and safety performance of the battery.

[0006] The present application is realized by the following technical solutions:

[0007] The binder provided by the present application for electrode includes a binder matrix and a sulfide grafted modified polyimide, and the mass ratio of the binder matrix and the sulfide grafted modified polyimide additive is (10-100):1.

[0008] The above design of the present application improves the glass transition temperature of polyimide by grafting modification of sulfide groups on polyimide. The sulfide grafting modification not only reduces the generation of by-products during grafting, but also improves the electrochemical activity of polyimide by cooperating with the C-N structure in polyimide. In addition, the introduction of sulfide can reduce the energy barrier of the three-dimensional cross-linked network formed by the grafting modified polyimide under the cooperation of the binder matrix. The three-dimensional cross-linked network makes the binder have good flexibility, high temperature resistance and bonding performance, and the three-dimensional cross-linked network can form Li + The sulfide grafting modification not only reduces the generation of by-products during grafting, but also improves the electrochemical activity of polyimide by cooperating with the C-N structure in polyimide. In addition, the introduction of sulfide can reduce the energy barrier of the three-dimensional cross-linked network formed by the grafting modified polyimide under the cooperation of the binder matrix. The three-dimensional cross-linked network makes the binder have good flexibility, high temperature resistance and bonding performance, and the three-dimensional cross-linked network can form Li

[0009] As a further scheme, the modification group of the sulfide grafting modified polyimide includes a sulfide group; the modified group of the sulfide grafting modified polyimide includes an imide group; and the mass ratio of the sulfide group to the imide group is 1:(2-30). When the mass ratio of the sulfide group to the imide group is greater than 1:(2-30), the grafting amount is too large, and repeated grafting may occur on the same imide group; when the mass ratio of the sulfide group to the imide group is less than 1:(2-30), the grafting amount may be insufficient, and it is difficult to achieve the expected requirements of inhibiting the expansion of the pole piece and improving the glass transition temperature.

[0010] As a further scheme, the sulfide group is derived from a sulfur-containing salt. The sulfur-containing salt is more conducive to reducing by-products generated during grafting modification.

[0011] As a further scheme, the sulfur-containing salt includes one or more of anhydrous sodium sulfide, anhydrous ammonium sulfide, and anhydrous potassium sulfide.

[0012] As a further solution, the imide group is derived from a halogenated polyimide. It is easier to reduce side reactions during graft modification on the basis of easier graft modification with sulfur-containing salts.

[0013] As a further solution, the halogenated polyimide comprises one or more of a chlorinated polyimide, a brominated polyimide, an iodinated polyimide, a fluorinated polyimide.

[0014] As a further solution, the binder matrix comprises one or more of PAA (polyacrylic acid), CMC (carboxymethyl cellulose), PVDF (polyvinylidene fluoride).

[0015] As a further solution, the mass ratio of the binder matrix and the sulfide-grafted polyimide additive is (30-90): 1. Under this ratio, the prepared binder has the best inhibition of electrode expansion on the basis of more favorable formation of better bonding strength, so that the silicon-based battery has better cycle performance.

[0016] As a further solution, the weight average molecular weight of the sulfide-grafted polyimide is 10000-300000. When the weight average molecular weight of the sulfide-grafted polyimide is higher than 300000, the molecular weight is too high to be dispersed; and when the weight average molecular weight is lower than 10000, the melting point of the sulfide-grafted polyimide is too low to be processed; within this range of weight average molecular weight, the sulfide-grafted polyimide has more suitable physical properties on the basis of more favorable formation of a three-dimensional cross-linked network with the binder matrix.

[0017] As a further solution, the weight average molecular weight of the sulfide-grafted polyimide is 15000-150000. The sulfide-grafted polyimide with a weight average molecular weight within this range is more suitable for processing and has better cycle performance of the silicon-based battery on the basis of more favorable formation of better bonding strength.

[0018] As a further solution, the mass ratio of the binder matrix and the sulfide-grafted polyimide in the binder is 80%-100%. The binder can also contain dispersants and other substances to improve the dispersibility of the substances, which is conducive to the formation of a three-dimensional cross-linked network, thereby more conducive to obtaining a battery with better cycle performance and lower expansion rate.

[0019] The application also provides the use of the binder in an electrode sheet.

[0020] As a further solution, the electrode sheet comprises a negative electrode sheet.

[0021] As a further solution, the negative electrode sheet comprises a negative electrode with a silicon-based negative electrode material.

[0022] As a further solution, the negative electrode tab comprises one of a silicon-carbon negative electrode tab, a silicon-oxygen negative electrode tab.

[0023] As a further solution, the silicon-based negative electrode material comprises one of nano-silicon and silicon monoxide;

[0024] The silicon-carbon negative electrode adopts a mixture of nano-silicon and graphite material, and the particle size of the silicon-based material is reduced to the nano level. The silicon-oxygen negative electrode adopts a mixture of silicon monoxide (SiOx) and graphite material. Compared with silicon material, the volume expansion of silicon monoxide material during lithium intercalation is greatly reduced, and thus the cycle performance is greatly improved.

[0025] As a further solution, the binder accounts for 1-20% of the total mass of the negative active layer of the negative electrode tab. The specific selection of the binder can be 1%, 3%, 6%, 9%, 11%, 15%, 18%, 20%, etc. In the application, the binder is added to the negative electrode tab of the battery, and the addition amount in this range can achieve good binding effect. Within the required range of the binder, the cycle performance of the battery can be improved. When the content of the binder exceeds 1-20%, the binder is higher than 20%, which can cause the increase of internal resistance, limit the electrode capacity and electrode degradation. When the content of the binder is less than 1%, the adhesion effect is poor, which can cause powder falling and poor mechanical integrity of the electrode tab.

[0026] As a further solution, the binder accounts for 5-10% of the total mass of the negative active layer of the negative electrode tab. The specific selection of the binder can be 5%, 7%, 8%, 10%, etc. Through the selection of the above-mentioned range of the binder, the adhesion and cohesion of the electrode can be further improved, the cycle life is improved, and the battery has good rate performance.

[0027] The application also provides a silicon-based battery or a digital battery with the negative electrode tab. The silicon-based battery or the digital battery can be applied to 3C products, including but not limited to, computers, tablet computers, mice, mobile phones, digital cameras, sound listening, electronic dictionaries, digital audio players, smart watches, MP3, MP4, radios, Bluetooth earphones. The binder of the application is particularly suitable for application in the negative electrode tab of 3C products, which can reduce the decrease of the adhesion strength caused by the expansion of the battery during the cycle process of the battery, and reduce the lithium precipitation, thereby improving the safety performance and service life of the battery.

[0028] The application also provides an electrochemical device with the negative electrode tab. The electrochemical device can be used in terminal consumer products, and the terminal consumer products include but are not limited to, mobile phones, notebook computers, pen input computers, mobile computers, electronic book players, portable phones, portable fax machines, portable copiers, and portable printers.

[0029] The present invention also provides an electrochemical device having the aforementioned negative electrode, which can be used in electrical equipment, including large and small electrical equipment. Small electrical equipment includes consumer products, wearable electronic devices, or portable electronic devices; large electrical equipment includes transportation equipment. Transportation equipment includes, but is not limited to, vehicles such as automobiles, motorcycles, electric bicycles, buses, subways, high-speed trains, airplanes, and ships; wearable electronic devices or portable electronic devices include, but are not limited to, headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, drones, motors, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors. The negative electrode of the battery of the present invention is used in an electrochemical device, which can be housed in an electrical device in the form of an electrochemical device. Typically, the electrochemical device includes a battery pack or / multiple battery modules or / single battery module or / single cell and a management system for controlling them.

[0030] The present invention also provides a method for preparing the thioether-grafted modified polyimide.

[0031] As a further embodiment, the preparation method includes: dissolving a sulfur-containing salt in organic solvent 1, adding a halopolyimide and sodium acetate to react, filtering after the reaction is complete, washing several times with hot deionized water and organic solvent 2 until no chloride ions are present in the system, obtaining a white powder product, and drying the product.

[0032] As a further embodiment, the ratio of the sulfur-containing salt: halopolyimide: sodium acetate by mass is 1:(5-40):(20-50).

[0033] As a further embodiment, the organic solvent 1 includes an NMP (N-methylpyrrolidone) solution, wherein the amount of NMP added is twice the total mass of the sulfur-containing salt, the halogenated polyimide, and the sodium acetate; and the organic solvent 2 includes acetone.

[0034] As a further refinement, the reaction conditions are: maintaining the temperature at 150℃-220℃ for 3-8 hours, followed by natural cooling to room temperature to complete the reaction; the drying temperature is 25℃-30℃. In this invention, excessively low reaction temperatures may significantly increase the reaction time, while excessively high reaction temperatures may lead to numerous side reactions and uncontrollable product development.

[0035] The features and beneficial effects of this invention are as follows:

[0036] (1) The graft modification of the sulfide group is not only beneficial to improve the glass transition temperature of the polyimide, but also beneficial to the commercialization of the polyimide.

[0037] (2) The sulfide graft modified polyimide and the adhesive matrix cooperate to form a three-dimensional crosslinked network, which is not only beneficial to improve the electrochemical performance of the battery, but also beneficial to improve the safety of the battery.

[0038] (3) The sulfide graft modified polyimide and the adhesive matrix cooperate to form a three-dimensional crosslinked network, which can overcome the volume change of silicon in the battery cycle process and reduce the occurrence of lithium precipitation.

[0039] (4) The adhesive has better flexibility, high temperature resistance and bonding performance.

[0040] (5) The adhesive is used in the negative electrode of the battery, so that the battery has high capacity density, good cycle stability, high safety and long service life.

[0041] (6) The adhesive is particularly suitable for silicon-based batteries. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0043] Figure 1 Capacity retention rate results of the inventive examples and comparative examples.

[0044] Figure 2 Lithium precipitation of the inventive examples and comparative examples. DETAILED DESCRIPTION

[0045] In order to facilitate the understanding of the adhesive for electrode of the present application, the adhesive for electrode of the present application will be described more fully below, and the embodiments of the present application are given, but the scope of the present application is not limited thereto.

[0046] The adhesive for electrode of the present application comprises an adhesive matrix and a sulfide graft modified polyimide, and the mass ratio of the adhesive matrix and the sulfide graft modified polyimide additive is (10-100):1.

[0047] The modified group of the sulfide graft modified polyimide is a sulfide group, and the sulfide group is derived from a sulfur-containing salt, and the sulfur-containing salt comprises one or more of anhydrous sodium sulfide, anhydrous ammonium sulfide and anhydrous potassium sulfide.

[0048] The binder matrix comprises one or more of PAA (polyacrylic acid), CMC (carboxymethyl cellulose), PVDF (polyvinylidene fluoride).

[0049] The modified group of the sulfide graft-modified polyimide comprises a sulfide group; the modified group of the sulfide graft-modified polyimide comprises an imide group; the mass ratio of the sulfide group to the imide group is 1:(2-30).

[0050] The weight average molecular weight of the sulfide graft-modified polyimide is 10000-300000.

[0051] The mass ratio of the binder matrix and the sulfide graft-modified polyimide in the binder is 80%-100%.

[0052] The preparation method of the sulfide graft-modified polyimide: dissolve the sulfur-containing salt in the NMP solution, and add halogenated polyimide and sodium acetate for reaction, the reaction conditions are 150-220°C for 3-8h, natural cooling to room temperature, filtration, washing with hot deionized water and acetone several times until there is no chloride ion in the system, to obtain white powder product, drying the product, the drying temperature is 25-30°C. Among them, the mass ratio of sulfur-containing salt: halogenated polyimide: sodium acetate is 1:(5-40):(20-50); the addition amount of NMP is 2 times the total mass of sulfur-containing salt, halogenated polyimide and sodium acetate. The sulfide graft-modified polyimide obtained by the above method has less by-products, which is beneficial to reduce the influence of by-products on the chemical performance of the battery when the binder is applied in the negative electrode of the battery, and reduce the influence of by-products on the electrochemical performance and safety performance of the battery.

[0053] The binder can also include a binding substance and / or a dispersant:

[0054] The binding substance comprises one or more of styrene-butadiene rubber, polytetrafluoroethylene, fluorinated rubber, polyurethane, polyacrylic acid sodium, polyvinyl alcohol, sodium alginate, polyvinyl alcohol, carboxymethyl cellulose, polyethylene glycol, polylactic acid, polymethyl methacrylate, polystyrene, polyvinylidene fluoride, hexafluoropropylene polymer, styrene-butadiene rubber, sodium alginate, starch, cyclodextrin and polysaccharide. The above substances can be added to regulate the ratio of the binder matrix and the sulfide graft-modified polyimide, which is beneficial to obtain a binder with better adhesion.

[0055] The dispersing agent includes one or more of sodium carboxymethylcellulose (CMC), a non-ionic surfactant, polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), sodium polyacrylate, sodium polymetaphosphate, sodium silicate, and sodium dodecyl sulfate. It is advantageous to promote the dispersion of the molecular particles of the binder matrix, thereby promoting the better formation of the three-dimensional cross-linked network in the binder.

[0056] We also selected a sulfide grafting modified polyimide and a binder matrix to prepare a binder, as described in Examples A1-A5. We also applied the resulting binder to a battery, as described in Examples B1-B5.

[0057] Example A1: A binder matrix and a sulfide grafting modified polyimide were used in a mass ratio of 100:1. The preparation method of the sulfide grafting modified polyimide was as follows: 2 g of anhydrous sodium sulfide and 121 mL of NMP solution were placed in a three-necked flask equipped with a nitrogen inlet tube, a stirrer, a reflux condenser, and a thermometer, and the temperature was slowly raised to 180°C. Then, 20 g of chlorinated polyimide and 40 g of sodium acetate were added, and the temperature was maintained at 180°C for 5 h. The reaction was completed after natural cooling to room temperature. The product was filtered and washed several times with hot deionized water and acetone until there was no chloride ion in the system. A white powdery product was obtained, which was dried at 25°C. The weight average molecular weight of the sulfide grafting modified polyimide was 10,000 ± 1,000. The binder matrix was a 10 wt% PAA (weight average molecular weight of 150,000) aqueous solution.

[0058] Example A2: A binder matrix and a sulfide grafting modified polyimide were used in a mass ratio of 80:1. The preparation method of the sulfide grafting modified polyimide was as follows: 2 g of anhydrous sodium sulfide and 121 mL of NMP solution were placed in a three-necked flask equipped with a nitrogen inlet tube, a stirrer, a reflux condenser, and a thermometer, and the temperature was slowly raised to 180°C. Then, 20 g of chlorinated polyimide and 40 g of sodium acetate were added, and the temperature was maintained at 180°C for 5 h. The reaction was completed after natural cooling to room temperature. The product was filtered and washed several times with hot deionized water and acetone until there was no chloride ion in the system. A white powdery product was obtained, which was dried at 25°C. The weight average molecular weight of the sulfide grafting modified polyimide was 50,000 ± 1,000. The binder matrix was a 10 wt% CMC (weight average molecular weight of 120,000) aqueous solution.

[0059] Example A3: Adhesive matrix and thioether graft-modified polyimide in a mass ratio of 50:1. The preparation method of the thioether graft-modified polyimide is as follows: 2 g of anhydrous sodium sulfide and 121 mL of NMP solution are placed in a three-necked flask equipped with a nitrogen inlet tube, a stirring rod, a reflux condenser and a thermometer, and slowly warmed to 150°C, the temperature is kept, and 20 g of chlorinated polyimide and 40 g of sodium acetate are added, and kept at 150°C for 5 h, and then naturally cooled to room temperature, filtered, and washed with hot deionized water and acetone several times until there is no chloride ion in the system, to obtain a white powder product, which is dried at 25°C. The weight average molecular weight of the thioether graft-modified polyimide is 80000±1000. The adhesive matrix is a 10wt% PAA (weight average molecular weight 240,000) aqueous solution.

[0060] Example A4: Adhesive matrix and thioether graft-modified polyimide in a mass ratio of 40:1. The preparation method of the thioether graft-modified polyimide is as follows: 2 g of anhydrous ammonium sulfide and 121 mL of NMP solution are placed in a three-necked flask equipped with a nitrogen inlet tube, a stirring rod, a reflux condenser and a thermometer, and slowly warmed to 200°C, the temperature is kept, and 20 g of chlorinated polyimide and 40 g of sodium acetate are added, and kept at 200°C for 5 h, and then naturally cooled to room temperature, filtered, and washed with hot deionized water and acetone several times until there is no chloride ion in the system, to obtain a white powder product, which is dried at 25°C. The weight average molecular weight of the thioether graft-modified polyimide is 100000±1000. The adhesive matrix is a 10wt% PVDF (weight average molecular weight 300,000) aqueous solution.

[0061] Example A5: Adhesive matrix and thioether graft-modified polyimide in a mass ratio of 30:1. The preparation method of the thioether graft-modified polyimide is as follows: 2 g of anhydrous potassium sulfide and 121 mL of NMP solution are placed in a three-necked flask equipped with a nitrogen inlet tube, a stirring rod, a reflux condenser and a thermometer, and slowly warmed to 220°C, the temperature is kept, and 20 g of chlorinated polyimide and 40 g of sodium acetate are added, and kept at 220°C for 5 h, and then naturally cooled to room temperature, filtered, and washed with hot deionized water and acetone several times until there is no chloride ion in the system, to obtain a white powder product, which is dried at 25°C. The weight average molecular weight of the thioether graft-modified polyimide is 200000±1000. The adhesive matrix is a 10wt% PVDF (weight average molecular weight 300,000) aqueous solution.

[0062] Example B1: The binder obtained in Example A1 was applied to the silicon-based negative electrode sheet, and a negative electrode slurry was prepared with a ratio of 80:10:10:100 by mass of the silicon-based negative electrode material: conductive agent: binder: deionized water, and coated on a copper foil to obtain a negative electrode active layer, and a silicon-based negative electrode sheet was prepared, which was baked in a vacuum oven at 150°C for 4h to obtain the negative electrode sheet. The negative electrode active material in the silicon-based negative electrode material included silicon monoxide (SiOx) and graphite, wherein the mass of the silicon monoxide accounted for 20% of the total mass of the silicon-based negative electrode material.

[0063] Positive electrode: a positive electrode slurry was prepared with a ratio of 95:2:3:100 by mass of LiFePO4: conductive agent SP: binder PVDF: NMP, coated on an aluminum foil and dried to prepare a positive electrode sheet, which was baked in a vacuum oven at 100°C to remove trace water to obtain the positive electrode sheet;

[0064] Separator: a polyethylene (PE) separator was used;

[0065] Electrolyte: the electrolyte was a 1 mol / L LiPF6 solution, and the solvent was composed of EC (ethylene carbonate) and DEC (diethyl carbonate) at a volume ratio of 1:1;

[0066] Assembly: the above-mentioned positive electrode sheet, negative electrode sheet, electrolyte and separator were assembled into a lithium ion soft package battery according to the assembly requirements of a lithium ion battery.

[0067] According to the preparation method of the negative electrode sheet and the battery of Example B1, the binders obtained in Examples A2-A5 were applied to the battery negative electrode sheet, respectively, and then the batteries were obtained. The batteries obtained in Examples A2-A5 corresponded to Examples B2-B5, respectively.

[0068] Comparative Example A1: the binder was a 10wt% PAA (weight average molecular weight 150,000) aqueous solution.

[0069] Comparative Example A2: the binder was a 10wt% CMC (weight average molecular weight 120,000) aqueous solution.

[0070] Comparative Example A3: the binder was a 10wt% PAA (weight average molecular weight 240,000) aqueous solution.

[0071] Comparative Example A4: the binder was prepared with a mass ratio of 100:1 of the binder matrix and polyimide, wherein the binder matrix was a 10wt% PAA (weight average molecular weight 150,000) aqueous solution.

[0072] The binders obtained in Comparative Examples A1-A4 were applied to the negative electrode sheet, and the negative electrode sheet and the battery were prepared according to the preparation method of Example B1 to obtain the comparative batteries. The batteries obtained in Comparative Examples A1-A4 corresponded to Comparative Examples B1-B4, respectively.

[0073] We also obtained the battery for the relevant performance test:

[0074] (1) Negative electrode peel strength test: the negative electrode is divided into 20 mm x 100 mm, and the pressure-sensitive 3M double-sided tape is pasted on the surface of the negative electrode sheet, and the other side is pasted on the stainless steel plate. Roll back and forth with a pressure roller 3 times; peel off the free end of the current collector from the negative electrode material layer, and fold the free end of the current collector 180°, and clamp the free end of the current collector and the stainless steel plate on the upper and lower clamps of the tensile testing machine, respectively, at a speed of 100 mm / min, at an angle of 180°. The average value of the stable peeling stage is taken as the peel strength value.

[0075] (2) 0.1C cycle performance test: the battery is stored at 25°C, and the 0.1C current is used for charge and discharge cycling in the charge and discharge voltage range of 3.0-4.4V. The initial capacity is Q0, and the capacity of 50 cycles is Q1. The 0.1C cycle 50 cycle capacity retention rate is calculated by the following formula:

[0076] (3) 0.1C cycle 50 cycle capacity retention rate (%) = Q1 / Q0 x 100%. 0.5C cycle performance test: the charge and discharge current is 0.5C, and the other operations are the same as the 0.1C cycle performance test.

[0077] Verification result analysis:

[0078] Table 1 battery test results of examples and comparative examples

[0079] Group Peeling strength (N / m) 0.1 C cycle 50 cycles capacity retention 0.5 C cycle 50 cycles capacity retention Example B1 40.2 93.1％ 90.3％ Example B2 34.1 92.6％ 91.1％ Example B3 36.4 94.0％ 91.3％ Example B4 32.3 93.2％ 90.4％ Example B5 39.4 92.8％ 90.8％ Comparative Example B1 19.5 89.0％ 78.3％ Comparative Example B2 21.2 85.2％ 76.5％ Comparative Example B3 20.5 86.9％ 79.1％ Comparative Example B4 30.3 89.9％ 85.6％

[0080] Table 2 negative electrode sheet expansion rate of the battery of examples and comparative examples

[0081]

[0082]

[0083] We prepared a silicon-based negative electrode binder by matching the sulfide grafted modified polyimide and the binder matrix, and used it in a silicon-based battery. We found that the adhesion strength of the binder of the present application was better than that of Comparative Examples B1-B3 (traditional binders in silicon-based batteries) by comparing Examples B1-B5 with Comparative Examples B1-B4 in Tables 1 and 2, and the binder of the present application was used in a silicon-based negative electrode. The sulfide grafted modified polyimide forms a three-dimensional cross-linked network structure in the silicon-based negative electrode in cooperation with the binder matrix, which is beneficial to the Li +and the migration of electrons, thereby reducing the occurrence of lithium precipitation and improving the electrochemical performance of silicon-based batteries; and can also effectively inhibit the volume expansion effect of silicon in the carbon negative electrode, thereby solving the pulverization of the negative active material and the change in the thickness of the battery cell, thereby improving the safety performance of the battery. We found through Figure 1 , Figure 2 and Table 2 that the electrochemical performance and safety performance of the embodiments of the present application are better than the comparative examples, and the binder of the present application is used in silicon-based batteries, which significantly reduces the occurrence of lithium precipitation (as shown in Figure 2 ). The capacity retention rate of the battery in the embodiments is maintained at about 95% after 100 cycles at a rate of 0.2C (as shown in Figure 1 ).

[0084] On this basis, we found that although polyimide itself has strong adhesion strength, when the polyimide is grafted and modified with a sulfide group, the sulfide grafted and modified polyimide not only has enhanced electrochemical activity, but also forms a three-dimensional cross-linked network with reduced energy barrier, thereby facilitating the improvement of the electrochemical performance and safety performance of the battery and the adhesion strength of the binder. As found in the comparison of Example B1-Example B5 and Comparative Example B4, this can be verified in Figure 1 , Figure 2 and Table 2.

[0085] When the mass ratio of the binder matrix to the sulfide grafted and modified polyimide increases, it is more conducive to reducing the energy barrier of the three-dimensional cross-linked network structure, thereby promoting the formation of the three-dimensional cross-linked network structure in the binder, and thus more conducive to improving the adhesion strength of the binder, but the cycle performance of the battery at a rate of 0.5C will decrease, which we believe may be because the migration of Li + and electrons will be hindered, as found in the comparison of Example B1-Example B3. Therefore, we further prefer that the mass ratio of the binder matrix to the sulfide grafted and modified polyimide is (90-30):1.

[0086] We can also choose to increase the weight average molecular weight of the sulfide grafted and modified polyimide, as found in the comparison of Example B4 and Example B5, which we believe may be because the intramolecular molecular chains of the sulfide grafted and modified polyimide are entangled to a certain extent, thereby promoting the enhancement of the adhesion strength. Therefore, we further prefer that the weight average molecular weight of the sulfide grafted and modified polyimide is 15000-150000.

[0087] In summary, the binder used in the electrode of the present application, by the cooperation of the binder matrix and the polyimide grafted with sulfide, not only can improve the strength of the binder, but also form a three-dimensional cross-linked network structure for lithium ion and electron migration, overcome the volume change of silicon in the silicon-based battery, thereby improve the cycle performance of the battery, and reduce the occurrence of lithium precipitation and improve the safety performance of the battery.

[0088] It should be noted that the above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A binder for use in electrodes, characterized in that The adhesive comprises an adhesive matrix and a sulfide-grafted polyimide, wherein the mass ratio of the adhesive matrix to the sulfide-grafted polyimide is (10-100):

1.

2. A binder for use in electrodes according to claim 1, characterized in that The mass ratio of the binder matrix to the sulfide-grafted modified polyimide is (30-90):

1.

3. A binder for use in electrodes according to claim 1, characterized in that The modifying group of the thioether-grafted polyimide includes a thioether group; the modified group of the thioether-grafted polyimide includes an imide group; the mass ratio of the thioether group to the imide group is 1:(2-30).

4. A binder for use in electrodes according to claim 3, characterized in that The thioether group is derived from a sulfur-containing salt.

5. A binder for use in electrodes according to claim 4, characterized in that The sulfur-containing salt includes one or more of anhydrous sodium sulfide, anhydrous ammonium sulfide, and anhydrous potassium sulfide.

6. A binder for use in electrodes according to claim 3, characterized in that The imide group is derived from halogenated polyimide.

7. A binder for use in electrodes according to claim 6, characterized in that The halogenated polyimide includes one or more of chlorinated polyimide, bromopolyimide, iodopolyimide, and fluorinated polyimide.

8. A binder for use in electrodes according to claim 1, characterized in that The adhesive matrix includes one or more of PAA, CMC, and PVDF.

9. A binder for use in electrodes according to claim 1, characterized in that The weight-average molecular weight of sulfide-grafted modified polyimide is 10,000-300,000.

10. A binder for use in electrodes according to claim 1, characterized in that The weight-average molecular weight of sulfide-grafted modified polyimide is 15,000-150,000.

11. A binder for use in electrodes according to claim 1, characterized in that The adhesive matrix and the sulfide-grafted modified polyimide account for 80%-100% of the mass of the adhesive.

12. A pole piece characterized by, The electrode comprises the binder as described in any one of claims 1-11.

13. The pole piece of claim 12, wherein The electrode includes a negative electrode.

14. The pole piece of claim 13, wherein The binder accounts for 1%-20% of the total mass of the negative electrode active layer of the negative electrode sheet by weight.

15. An electrode sheet according to claim 13, characterized in that, The binder accounts for 5%-10% of the total mass of the negative electrode active layer of the negative electrode sheet by weight.

16. An electrode sheet according to claim 13, characterized in that, The negative electrode includes one of silicon-carbon negative electrode and silicon-oxygen negative electrode.

17. An electrode sheet according to claim 16, characterized in that, The silicon-carbon anode or silicon-oxygen anode includes silicon-based anode materials.

18. An electrode sheet according to claim 17, characterized in that, The silicon-based anode material includes nano-silicon or silicon suboxide.

19. An electrochemical device, characterized in that, The electrochemical device includes the electrode as described in any one of claims 12-18.

20. An electrical appliance, characterized in that, The electrical equipment includes the electrochemical device as described in claim 19.

21. A method for preparing the sulfide-grafted modified polyimide of the adhesive according to any one of claims 1-11, characterized in that, The preparation method includes: dissolving a sulfur-containing salt in organic solvent 1, adding a halopolyimide and sodium acetate to react, filtering after the reaction, washing several times with hot deionized water and organic solvent 2 until no halogen ions are present in the system, obtaining a white powder product, and drying the product.

22. The preparation method according to claim 21, characterized in that, By mass, the ratio of the sulfur-containing salt, the halogenated polyimide to sodium acetate is 1:(5-40):(20-50).

23. The preparation method according to claim 21, characterized in that, The organic solvent 1 includes an NMP solution, wherein the amount of NMP added is twice the total mass of the sulfur-containing salt, the halogenated polyimide, and the sodium acetate; the organic solvent 2 includes acetone.

24. The preparation method according to claim 21, characterized in that, The reaction conditions are: maintaining the temperature at 150℃-220℃ for 3-8 hours, and then naturally cooling to room temperature to complete the reaction; the drying temperature is 25℃-30℃.

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