Three-dimensional conductive network carboxymethyl cellulose composite binder and preparation method thereof

Abstract

The invention discloses a three-dimensional conductive network carboxymethyl cellulose composite binder and a preparation method thereof, and belongs to the technical field of lithium ion batteries, the three-dimensional conductive network carboxymethyl cellulose composite binder is prepared by grafting carboxymethyl cellulose with an acrylamide monomer and a pyrenyl monomer and then compounding with a conductive material. An acrylamide monomer and a pyrenyl monomer copolymerized long chain are grafted on a main chain of carboxymethyl cellulose, pyrenyl in a double-site pyrenyl monomer and a conjugated structure of a conductive material can attract each other through pi-pi accumulation, and the solubility of the pyrenyl monomer is improved by an alkenyl-terminated long carbon chain; a chemical cross-linked network structure can be formed between side chains through double sites, an amide group and carboxyl on carboxymethyl cellulose can attract a silicon negative electrode through hydrogen bonds, the cohesiveness is improved, the capacity of coping with the volume expansion problem of silicon particles is improved through a three-dimensional conductive network structure, and the cycle performance of the battery is remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a three-dimensional conductive network carboxymethyl cellulose composite binder and a preparation method thereof. Background Art

[0002] Commercially available lithium-ion batteries generally use graphite as the anode material, but its theoretical capacity is relatively low. When applied to electric vehicles, it provides a small energy density. Silicon, on the other hand, has an extremely high theoretical capacity, a low voltage for lithium-ion deintercalation reaction, abundant natural reserves, mature mining and processing technologies, and is non-toxic and environmentally friendly. However, although silicon-based lithium batteries show great potential in electrochemical performance, there are still some factors restricting the application and development of silicon-based lithium batteries. Like other high-performance anode materials such as tin, germanium, and antimony, silicon exhibits a volume expansion effect during charge and discharge. During lithiation, the volume expands to more than 300% of the previous volume, but during delithiation, the volume decreases to different sizes. This volume expansion generates stress on the silicon particles, and cracks and fractures will occur in the anode material. This change will cause the silicon particles to detach from the anode material. The expansion of silicon particles will also cause the surrounding conductive materials to move away from the silicon particles, resulting in a lower conductivity of the anode material. To solve the limitations of silicon-based electrodes, an effective method is to develop advanced functional binders that can suppress their volume expansion while maintaining the integrity of the electrode and ensure the mechanochemical stability of the electrode SEI.

[0003] Carboxymethyl cellulose (CMC) has a high elastic modulus. When silicon undergoes volume expansion, it can ensure that the electrode does not deform, so that the specific capacity of the electrode does not decay too quickly and relatively stable cycle performance can be obtained. In addition, CMC also contains a large number of carboxyl groups that can form stable chemical bonds with the hydroxyl groups on the silicon surface to ensure the integrity of the electrode. However, the mechanical modulus of the CMC binder is relatively weak, and it is often difficult to maintain the integrity of the electrode structure when used alone as an electrode binder. Conductive polymer binders can be divided into structural conductive polymers and filled conductive polymers. Structural conductive polymers such as polyaniline and polythiophene have good electrical and mechanical properties. Studies have shown that conductive polymer binders used in lithium batteries have the functions of both binder and conductive substance, can provide good electrical conductivity and three-dimensional space framework, promote the transport of ions and electrons and provide good three-dimensional contact, can adapt to the volume expansion problem of silicon particles, and enable the electrode to have a higher specific capacity and cycle stability, but there are problems such as high stiffness, difficult melting, difficult dissolution, difficult forming, and high cost. The filled conductive polymer is a composite of inorganic conductive materials such as graphite and carbon nanotubes and a polymer binder. Its preparation process is relatively simple, the cost is relatively low, and the conductivity coverage range is wide, but there are problems such as weak binding force with the polymer matrix, poor dispersion, difficulty in forming a three-dimensional conductive network, and reduction of the mechanical properties and adhesiveness of the polymer. Summary of the invention

[0004] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a three-dimensional conductive network carboxymethyl cellulose composite binder and a preparation method thereof, wherein carboxymethyl cellulose is grafted with acrylamide monomer and pyrene monomer and then composited with a conductive material to obtain a carboxymethyl cellulose composite binder with a three-dimensional conductive network structure.

[0005] The technical solution for achieving the purpose of the present invention is as follows: A three-dimensional conductive network carboxymethyl cellulose composite binder comprises, by weight, 100 parts of modified carboxymethyl cellulose and 5-30 parts of conductive material; the modified carboxymethyl cellulose is prepared by grafting pyrene monomer and acrylamide monomer onto carboxymethyl cellulose through free radical polymerization, and the mass ratio of acrylamide monomer, pyrene monomer and carboxymethyl cellulose is 1: (5-10): (4-6); the structural formula of the pyrene monomer is shown in Formula 1: Formula 1, where m is a natural number between 6 and 20.

[0006] Preferably, the acrylamide monomer is selected from at least one of acrylamide, methacrylamide, hydroxymethyl acrylamide and N-hydroxyethyl acrylamide.

[0007] More preferably, the acrylamide monomer is selected from at least one of hydroxymethyl acrylamide and N-hydroxyethyl acrylamide.

[0008] Preferably, the preparation method of the modified carboxymethyl cellulose comprises the following steps: dissolving carboxymethyl cellulose in deionized water, then performing solvent exchange with N,N-dimethylacetamide and anhydrous methanol, and finally dissolving in N,N-dimethylacetamide solvent to obtain a carboxymethyl cellulose solution; dissolving pyrene monomer and acrylamide monomer in N,N-dimethylacetamide, uniformly mixing with the carboxymethyl cellulose solution, adding an initiator to the solution, heating the solution at 55-65° C. in a nitrogen atmosphere for reaction for 18-30 hours, and purifying the product by ether precipitation method to obtain modified carboxymethyl cellulose.

[0009] Preferably, the initiator is azobisisobutyronitrile, and the amount of the initiator added is 0.5-1 wt % of the total weight of the monomers.

[0010] Preferably, the preparation method of the pyrene-based monomer is as follows: pyrene, bis(pinacolato)diboron, methoxy(cyclooctadiene)iridium dimer, 4,4'-di-tert-butyl-2,2'-bipyridine and tetrahydrofuran are mixed and stirred, and refluxed at 70-90 °C for 15-20 hours, and the intermediate is obtained after purification; the intermediate, bromoalkenyl compound, palladium acetate, tri-o-tolylphosphine and saturated sodium bicarbonate solution are mixed and stirred, and refluxed at 70-90 °C for 15-20 hours, and the pyrene-based monomer is obtained after purification.

[0011] Preferably, the carboxymethyl substitution degree of the carboxymethyl cellulose is 0.7-1.2.

[0012] Preferably, the conductive material is at least one of graphite, carbon nanotubes or graphene.

[0013] This application also protects a preparation method of a three-dimensional conductive network carboxymethyl cellulose composite binder, which includes the following steps: dissolving the modified carboxymethyl cellulose in deionized water to prepare a solution, and then adding the conductive material to the modified carboxymethyl cellulose solution in batches under ultrasonic and stirring conditions to obtain the three-dimensional conductive network carboxymethyl cellulose composite binder.

[0014] This application also protects the application of the three-dimensional conductive network carboxymethyl cellulose composite binder in a binder for a silicon-based anode material, and the silicon-based anode material includes elemental silicon, silicon monoxide or a silicon-carbon composite material.

[0015] Beneficial effects: The present invention provides a lithium battery anode binder of a three-dimensional conductive network carboxymethyl cellulose composite binder, which has the following beneficial effects: The three-dimensional conductive network carboxymethyl cellulose composite binder of the present invention is prepared by grafting acrylamide monomers and pyrene-based monomers onto carboxymethyl cellulose and then compounding with conductive materials. By grafting copolymer long chains of acrylamide monomers and pyrene-based monomers onto the main chain of carboxymethyl cellulose, the pyrene groups in the double-site pyrene-based monomers can be attracted to the conjugated structure of the conductive material through π-π stacking, so that the conductive material can be evenly dispersed on the surface of carboxymethyl cellulose, improving the dispersion performance of the conductive material. The long carbon chain improves the solubility of the pyrene-based monomer, and the double-site with vinyl end-capping enables it to form a chemically cross-linked network structure between the side chains, increasing the mechanical stress of the binder on the silicon-based particles, improving the toughness and elongation at break of carboxymethyl cellulose, ensuring that the binder can withstand the huge stress caused by the volume change of the silicon-based particles during charge and discharge, and maintaining the integrity of the electrode structure. At the same time, the amide group and the carboxyl group on carboxymethyl cellulose can be attracted to the silicon negative electrode through hydrogen bonds, increasing the adhesiveness. Its three-dimensional conductive network structure can improve the transport ability of electrons and ions in its spatial structure, provide good electrical conductivity and a three-dimensional space framework, promote the transport of ions and electrons, and provide good three-dimensional contact. Among them, the electrical contact with Si particles can effectively inhibit the volume expansion of Si particles, making the battery have excellent cycle stability. In addition, adding conductive groups to the binder to form a conductive network can also improve the conductivity of the electrode and reduce the amount of conductive agent used in the electrode. Description of the drawings

[0016] Figure 1 is the synthesis route diagram of the pyrene-based monomer; Figure 2 is the 1H NMR spectrum of pyrene-based monomer 1; Figure 3 is the infrared spectrum of modified carboxymethyl cellulose 1. Detailed implementation manners

[0017] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0018] In the embodiments, the experimental methods used are all conventional methods unless otherwise specified, and the materials, reagents, etc. used can be obtained from commercial channels unless otherwise specified.

[0019] Now, the raw materials and equipment used in the examples and comparative examples are described as follows: Carboxymethyl cellulose: M.W. 250000 (DS = 0.9), 1500 - 3100 mPa·s, Changshu Weiyi Technology Co., Ltd.; Carbon nanotubes: XFS30, highly conductive single-walled carbon nanotubes, purchased from Xianfeng Nano; Graphene: XF020, monolayer graphene oxide dispersion, sheet diameter 50 - 200 nm, concentration: 2 mg / ml, solvent: water, purchased from Xianfeng Nano; N-Hydroxyethyl acrylamide: purity 98%, purchased from Shanghai Macklin Biochemical Co., Ltd.; Methacrylamide, analytical pure, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; Pyrene: purity 98%, purchased from Tianjin Xiensi Biochemical Technology Co., Ltd.; Bis(pinacolato)diboron: CAS: 73183 - 34 - 3, purity 98%, purchased from Shanghai Macklin Biochemical Co., Ltd.; Methoxy(cyclooctadiene)iridium(III) dimer: CAS: 12148 - 71 - 9, purity 96%, purchased from Shanghai Macklin Biochemical Co., Ltd.; Palladium acetate: 99.9%, Shanghai Merck Biochemical Technology Co., Ltd.; 8-Bromo-1-octene: 97%, purchased from Sigma-Aldrich; 16-Bromo-1-hexadecene: CAS: 118625 - 56 - 2, purity 97%, purchased from Chengdu Carmel Pharmaceutical Technology Co., Ltd.; Pyrene-based monomer 1: self-made, the preparation method is as follows: 2.0g pyrene, 5.524g bis(pinacol)diboron, 0.327g methoxy(cyclooctadiene)iridium dimer, 0.265g 4,4'-di-tert-butyl-2,2'-bipyridine and 25mL anhydrous tetrahydrofuran were added to a reaction bottle, the mixture was degassed and stirred continuously under argon protection, and then refluxed at 80°C for 16 hours. After the reaction, it was cooled to room temperature, washed with distilled water and extracted with chloroform, the organic phase was collected and the solvent was removed by vacuum concentration, the remaining organic phase was poured into methanol, filtered through an alkaline activated alumina column, the precipitate was filtered out and washed with methanol to obtain a gray pure product; under argon gas protection, 1.0mmol of the above product and 2.15mmol of 8-bromo-1-octene compound were dissolved in 10 mL of anhydrous tetrahydrofuran, slowly add 0.01g of palladium acetate and 0.02g of tri-o-tolylphosphine to the solution, then add 4.0 mL of saturated sodium bicarbonate solution, stir the suspension and reflux to react overnight. After the reaction, extract the mixture with 50 mL of chloroform, wash the organic phase with distilled water several times, dry it over anhydrous magnesium sulfate and filter it. Distill off the solvent under reduced pressure to obtain a crude product, which is then purified by silica gel column chromatography (eluent: petroleum ether / dichloromethane = 10:0.2) to finally obtain a pyrene monomer with a yield of 87%. The synthetic route is as follows Figure 1 As shown, the structural formula is shown in formula 2, and the nuclear magnetic hydrogen spectrum of the pyrene monomer 1 1 H NMR Figure 2 As shown, the solvent is CDCl3, Figure 2 The assigned 1 The chemical shift and integration of H indicated that the pyrene-based monomer 1 was successfully prepared.

[0020]

[0021] Formula 2.

[0022] Pyrenyl monomer 2: Compared with pyrenyl monomer 1, the difference is that the 8-bromo-1-octene compound is replaced by 16-bromo-1-hexadecene; Modified carboxymethyl cellulose 1: Pour 250ml of deionized water into a 500ml beaker, add 30g of carboxymethyl cellulose under stirring with a magnetic stirrer, stir for half an hour to form a slurry, let stand to separate, filter, disperse the solid in 250ml N,N-dimethylacetamide and stir for 6h, let stand to separate and filter, repeat the same operation once. Disperse the solid in 250ml of anhydrous methanol and repeat the above steps twice to complete the whole process. Place the final solid in a vacuum drying oven at 80℃ for 48h to obtain solvent exchange cellulose; add 125ml of N,N-dimethylacetamide and 4g of solvent exchange cellulose to a 250ml three-necked flask protected by nitrogen, stir at room temperature for 12h, the solution becomes transparent, and a carboxymethyl cellulose solution is obtained; Dissolve 5.0 g of pyrene-based monomer and 0.5 g of N-hydroxyethyl acrylamide in 20 ml of freshly distilled N,N-dimethylacetamide, mix it evenly with the above carboxymethyl cellulose solution, and add 30 mg of azobisisobutyronitrile (AIBN) to the solution. The mixture was subjected to three freeze-pump-thaw cycles to degas, and then heated at 60 °C for 24 hours under a nitrogen atmosphere. The final product was purified by precipitation with diethyl ether. Then, it was soaked in distilled water for 24 hours to remove the homopolymer and dried in vacuo to obtain modified carboxymethyl cellulose 1. The modified carboxymethyl cellulose was tested by infrared spectroscopy using an infrared spectrometer of model IR / Nicolet 6700. When preparing the sample, a small amount of sample powder was mixed and ground with KBr, and the ground sample should be transparent to ensure that light can pass through the sample. In the experiment, the wavelength range of the spectral analyzer was set to 4000 cm -1 ~500 cm -1 ; The results are as Figure 3 shown. Among them, the modified carboxymethyl cellulose showed stretching vibration peaks of methyl and methylene C-H of the grafted chain segment at 2921 cm -1 and 2853 cm -1 . The stretching vibration peak of C=O in the acrylamide monomer appeared at 1647 cm -1 . The stretching vibration peak of C=C in the pyrene-based monomer appeared at 1592 cm -1 . The in-plane bending vibration peaks of C-H in the pyrene-based monomer appeared at 1488 cm -1 and 1458 cm -1 . The bending vibration absorption peak of C -1 -H of the polypyrene chain segment also appeared at 722 - 845 cm Ar , proving that the acrylamide monomer and the pyrene-based monomer were successfully grafted onto the carboxymethyl cellulose.

[0023] Modified carboxymethyl cellulose 2: Compared with the preparation method of modified carboxymethyl cellulose 1, the difference is that N-hydroxyethyl acrylamide is replaced by methyl acrylamide; Modified carboxymethyl cellulose 3: Compared with the preparation method of modified carboxymethyl cellulose 1, the difference is that pyrene-based monomer 1 is replaced by pyrene-based monomer 2; Modified carboxymethyl cellulose 4: Compared with the preparation method of modified carboxymethyl cellulose 1, the difference is that no pyrene-based monomer is added; Modified carboxymethyl cellulose 5: Compared with the preparation method of modified carboxymethyl cellulose 1, the difference is that no N-hydroxyethyl acrylamide is added; Unless otherwise specified, the component raw materials used in each example and comparative example of the present invention are all commercially available raw materials, and the component raw materials used in each parallel experiment are all of the same kind.

[0024] Examples and Comparative Examples A three-dimensional conductive network carboxymethyl cellulose composite binder, comprising modified carboxymethyl cellulose and a conductive material. The modified carboxymethyl cellulose is dissolved in deionized water to prepare a 1 mg / ml solution, and then under ultrasonic and stirring conditions, where the ultrasonic frequency is 350 kHz and the stirring speed is 500 rpm; the conductive material is added to the modified carboxymethyl cellulose solution in batches and stirred for 30 min to obtain the three-dimensional conductive network carboxymethyl cellulose composite binder. Among them, the composition and weight parts of the modified carboxymethyl cellulose and the conductive material are shown in Table 1.

[0025] Table 1 Composition and parts (by weight) of the binders in the examples and comparative examples

[0026] Take 9 mg of the binders prepared in the examples and comparative examples and dissolve them in deionized water to prepare a 9 wt% solution. Weigh 80 mg of 30 nm silicon powder and place it in a mortar with the binder solution and grind it thoroughly for 30 min to obtain the negative electrode paste. Place the negative electrode on a copper foil with a diameter of 12 mm and uniformly coat it with a fixed thickness using a scraper, and the loading amount is 0.8 - 0.9 mg cm -2 , and place it in a vacuum drying oven at 80 °C for vacuum drying for 12 h to obtain the electrode sheet. In an inert gas atmosphere, use a lithium metal sheet as the reference electrode, 1 M LiPF6 (DMC:FEC = 4:1 Vol%) as the electrolyte, and a separator produced by Gelgard company to assemble a half-cell, and conduct the following tests. The results are shown in Table 2: (1) Peel test: The evaluation of the adhesion performance is usually carried out through the peel strength test. When conducting the peel strength test, first paste the pre-prepared electrode sheet on the aluminum plate substrate, and then firmly paste it on the surface of the electrode coating with 3M tape. The specific operation is to paste a 120×25 mm electrode sample on the 3M transparent tape, paste it on the coating and pull it at a 180° angle, and record the force required to pull the tape at a fixed speed of 100 mm / min.

[0027] (2) Mechanical properties: The mechanical properties of the polymer binder are detected through nanoindentation experiments. At an indentation depth of 2400 nm, record the indentation force corresponding to the binder. The greater the indentation force, the better the mechanical properties of the binder; release the pressure, measure the indentation depth after the binder rebounds and calculate the rebound rate.

[0028] (3) Cycle capacity retention rate: The charge and discharge performance of the half-cells assembled with the binders in the examples and comparative examples is tested by a LAND-CT3002A tester. Set the current density to 2000 mA·g -1 , set the current density in the voltage range of 0.01 - 1.5 V, and calculate the capacity retention rate after 500 cycles.

[0029] Table 2 Performance Tests of Examples and Comparative Examples

[0030] It can be seen from the examples and comparative examples that the carboxymethyl cellulose composite binder with a three-dimensional conductive network structure has the advantages of high peel strength, high indentation force, and good resilience, indicating that the binder has good mechanical properties and viscoelasticity. It can be stretched with the volume expansion of silicon particles and restored to the initial state with its volume contraction, effectively maintaining the integrity and stability of the electrode structure. Moreover, the three-dimensional conductive network formed by chemical cross-linking has an appropriate cross-linking density and has good adaptability to the volume change of silicon particles during the lithiation / delithiation process. The polymer network with conductivity, high viscoelasticity, and strong adhesion tightly wraps around the surface of each silicon particle and maintains the overall interface dynamic stability between silicon and the binder during repeated volume expansion and contraction processes, providing a continuous and stable electrical connection for the electrode reaction. At the same time, the improved mechanical properties are beneficial to maintaining the integrity and stability of the electrode, thereby effectively improving the electrochemical performance.

[0031] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and variations can be made according to the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principle and practical application of the present invention, so that those skilled in the art can well understand and utilize the present invention. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A three-dimensional conductive network carboxymethyl cellulose composite binder, characterized in that: The invention comprises 100 parts of modified carboxymethyl cellulose and 5-30 parts of conductive material by weight; the modified carboxymethyl cellulose is prepared by grafting pyrene monomer and acrylamide monomer on carboxymethyl cellulose by free radical polymerization, and the mass ratio of acrylamide monomer, pyrene monomer and carboxymethyl cellulose is 1: (5-10): (4-6); the structural formula of the pyrene monomer is shown in Formula 1: Formula 1, where m is a natural number between 6 and 20.

2. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, characterized in that: The acrylamide monomer is selected from at least one of acrylamide, methacrylamide, hydroxymethyl acrylamide and N-hydroxyethyl acrylamide.

3. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, characterized in that: The preparation method of the modified carboxymethyl cellulose comprises the following steps: dissolving carboxymethyl cellulose in deionized water, then performing solvent exchange with N,N-dimethylacetamide and anhydrous methanol, and finally dissolving in N,N-dimethylacetamide solvent to obtain a carboxymethyl cellulose solution; dissolving a pyrene monomer and an acrylamide monomer in N,N-dimethylacetamide, uniformly mixing with the carboxymethyl cellulose solution, adding an initiator to the solution, heating the solution at 55-65° C. for reaction for 18-30 hours in a nitrogen atmosphere, and purifying the product by an ether precipitation method to obtain the modified carboxymethyl cellulose.

4. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 3, characterized in that: The initiator is azobisisobutyronitrile, and the amount of the initiator added is 0.5-1wt% of the total weight of the monomers.

5. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, characterized in that: The preparation method of the pyrene-based monomer comprises the following steps: mixing pyrene, bis(pinacol)diboron, methoxy(cyclooctadiene)iridium dimer, 4,4'-di-tert-butyl-2,2'-bipyridine and tetrahydrofuran, reacting the mixture under reflux at 70-90° C. for 15-20 hours, and purifying the mixture to obtain an intermediate; mixing the intermediate, a bromine-containing olefinic compound, palladium acetate, tri-o-tolylphosphine and a saturated sodium bicarbonate solution, reacting the mixture under reflux at 70-90° C. for 15-20 hours, and purifying the mixture to obtain a pyrene-based monomer.

6. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, characterized in that: The carboxymethyl substitution degree of the carboxymethyl cellulose is 0.7-1.

2.

7. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, characterized in that: The conductive material is at least one of graphite, carbon nanotubes or graphene.

8. The method for preparing the three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, characterized in that: The modified carboxymethyl cellulose is dissolved in deionized water to prepare a solution, and then the conductive material is added into the modified carboxymethyl cellulose solution in batches under ultrasonic and stirring conditions to prepare a three-dimensional conductive network carboxymethyl cellulose composite binder.

9. Use of the three-dimensional conductive network carboxymethyl cellulose composite binder as claimed in claim 1 in a silicon-based negative electrode material binder.

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