A three-dimensional conductive network carboxymethyl cellulose composite binder and its preparation method

Abstract

The present invention discloses a three-dimensional conductive network carboxymethyl cellulose composite binder and a preparation method thereof, belonging to the technical field of lithium-ion batteries. 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 acrylamide monomers and pyrene-based monomer copolymer long chains 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. The long carbon chain capped with alkenyl improves the solubility of the pyrene-based monomers, and the double-site enables the formation of a chemically cross-linked network structure between the side chains. At the same time, the amide groups and the carboxyl groups on the carboxymethyl cellulose can be attracted to the silicon anode through hydrogen bonds, increasing the adhesion. Its three-dimensional conductive network structure improves the ability to cope with the volume expansion problem of silicon particles and significantly improves the cycle performance of the battery.

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] Commercial lithium-ion batteries generally use graphite as the negative electrode material, but its theoretical capacity is relatively low, and the energy density provided when it is applied to electric vehicles is small. Silicon has an extremely high theoretical capacity, a low voltage for lithium-ion insertion and extraction reactions, rich 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 negative electrode materials, such as tin, germanium, and antimony, silicon exhibits a volume expansion effect during charge and discharge. When lithium is inserted, the volume will expand to more than 300% of the previous volume, but during the de-lithiation process, the volume decreases to different sizes. This volume expansion will generate stress on the silicon particles, and then cracks and fractures will occur in the negative electrode material. This change will cause the silicon particles to detach from the negative electrode material. The expansion of the silicon particles will also cause the surrounding conductive materials to move away from the silicon particles, resulting in a lower conductivity of the negative electrode 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 mechano-chemical 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 will not decay too quickly, and relatively stable cycling 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. Some studies have shown that conductive polymer binders used in lithium batteries have the functions of both a binder and a conductive substance, can provide good electrical conductivity and a three-dimensional spatial 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 make the electrode have a higher specific capacity and cycling 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 dispersibility, 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. After grafting carboxymethyl cellulose with acrylamide monomers and pyrene-based monomers, a carboxymethyl cellulose composite binder with a three-dimensional conductive network structure is prepared by compounding with conductive materials.

[0005] The technical solutions to achieve the object of the present invention are as follows:

[0006] A three-dimensional conductive network carboxymethyl cellulose composite binder, calculated by weight, includes 100 parts of modified carboxymethyl cellulose and 5 - 30 parts of conductive materials; the modified carboxymethyl cellulose is prepared by grafting pyrene-based monomers and acrylamide monomers onto carboxymethyl cellulose through free radical polymerization, and the mass ratio of the acrylamide monomers, pyrene-based monomers to carboxymethyl cellulose is 1:(5 - 10):(4 - 6); the structural formula of the pyrene-based monomer is as shown in Formula 1:

[0007] Formula 1, where m is a natural number between 6 and 20.

[0008] Preferably, the acrylamide monomer is selected from at least one of acrylamide, methacrylamide, hydroxymethylacrylamide, and N-hydroxyethylacrylamide.

[0009] More preferably, the acrylamide monomer is selected from at least one of hydroxymethylacrylamide and N-hydroxyethylacrylamide.

[0010] Preferably, the preparation method of the modified carboxymethyl cellulose includes the following steps: dissolving carboxymethyl cellulose in deionized water, then performing solvent exchange with N,N-dimethylacetamide and anhydrous methanol, and finally dissolving it in N,N-dimethylacetamide solvent to obtain a carboxymethyl cellulose solution; dissolving pyrene-based monomers and acrylamide monomers in N,N-dimethylacetamide, mixing them evenly with the carboxymethyl cellulose solution, adding an initiator to the solution, heating and reacting at 55 - 65°C in a nitrogen atmosphere for 18 - 30 hours, and purifying the product by ether precipitation method to obtain modified carboxymethyl cellulose.

[0011] Preferably, the initiator is azobisisobutyronitrile, and the addition amount of the initiator is 0.5 - 1 wt% of the total mass of the monomers.

[0012] 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.

[0013] Preferably, the carboxymethyl substitution degree of the carboxymethyl cellulose is 0.7-1.2.

[0014] Preferably, the conductive material is at least one of graphite, carbon nanotubes or graphene.

[0015] The present application also protects a preparation method of a three-dimensional conductive network carboxymethyl cellulose composite binder, comprising 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.

[0016] The present 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.

[0017] 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 acrylamide monomers and pyrene-based monomer copolymer long chains 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 groups 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 groups and the carboxyl groups on the 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 the Si particles can effectively inhibit the volume expansion of the 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 electrical conductivity of the electrode and reduce the amount of conductive agent used in the electrode. Description of the Drawings

[0018] Figure 1 is the synthesis route diagram of the pyrene-based monomer;

[0019] Figure 2 is the 1H NMR spectrum of pyrene-based monomer 1;

[0020] Figure 3 is the IR spectrum of modified carboxymethyl cellulose 1. Detailed Embodiments

[0021] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in combination 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.

[0022] 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.

[0023] Now, the raw materials and equipment used in the examples and comparative examples are described as follows:

[0024] Carboxymethyl cellulose: M.W. 250000 (DS = 0.9), 1500 - 3100 mPa·s, Changshu Weiyi Technology Co., Ltd.;

[0025] Carbon nanotubes: XFS30, highly conductive single-walled carbon nanotubes, purchased from Xianfeng Nano;

[0026] Graphene: XF020, monolayer graphene oxide dispersion, sheet diameter 50 - 200 nm, concentration: 2 mg / ml, solvent: water, purchased from Xianfeng Nano;

[0027] N-Hydroxyethylacrylamide: purity 98%, purchased from Shanghai Macklin Biochemical Co., Ltd.;

[0028] Methacrylamide, analytical pure, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0029] Pyrene: purity 98%, purchased from Tianjin Xiensi Biochemical Technology Co., Ltd.;

[0030] Bis(pinacolato)diboron: CAS: 73183 - 34 - 3, purity 98%, purchased from Shanghai Macklin Biochemical Co., Ltd.;

[0031] Methoxy(cyclooctadiene)iridium dimer: CAS: 12148 - 71 - 9, purity 96%, purchased from Shanghai Macklin Biochemical Co., Ltd.;

[0032] Palladium acetate: 99.9%, Shanghai Merck Chemical Technology Co., Ltd.;

[0033] 8-Bromo-1-octene: 97%, purchased from Sigma-Aldrich;

[0034] 16-Bromo-1-hexadecene: CAS: 118625 - 56 - 2, purity 97%, purchased from Chengdu Camel Pharmaceutical Technology Co., Ltd.;

[0035] 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.

[0036]

[0037] Formula 2.

[0038] 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;

[0039] Modified carboxymethyl cellulose 1: Pour 250 ml of deionized water into a 500 ml beaker. With stirring on a magnetic stirrer, add 30 g of carboxymethyl cellulose and stir for half an hour to form a slurry. Let it stand for stratification, filter, disperse the solid in 250 ml of N,N-dimethylacetamide and stir for 6 h. Let it stand for stratification and filter, and repeat the same operation once. Then disperse the solid in 250 ml of anhydrous methanol and repeat the above steps twice to complete the whole process. Place the finally obtained solid in a vacuum drying oven at 80 °C for 48 h to obtain solvent-exchanged cellulose; Add 125 ml of N,N-dimethylacetamide and 4 g of solvent-exchanged cellulose to a 250 ml three-necked flask with nitrogen protection, stir at room temperature for 12 h, and the solution becomes transparent to obtain a carboxymethyl cellulose solution;

[0040] Dissolve 5.0 g of pyrene 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. Carry out three freeze-pump-thaw cycles on the mixture to degas, and then heat it at 60 °C for 24 h in a nitrogen atmosphere. The final product is purified by the ether precipitation method. Then soak it in distilled water for 24 h to remove homopolymers and dry it in vacuo to obtain modified carboxymethyl cellulose 1. Use an infrared spectrometer of model IR / Nicolet 6700 to perform infrared spectroscopy tests on the modified carboxymethyl cellulose. When preparing the sample, mix a small amount of sample powder with KBr and grind it. The ground sample should be transparent to ensure that light can pass through the sample. In the experiment, set the wavelength range of the spectral analyzer to 4000 cm -1 ~500 cm -1 ; The results are as Figure 3 shown. Among them, the modified carboxymethyl cellulose shows stretching vibration peaks of methyl and methylene C-H of the grafted chain segment at 2921 cm -1 and 2853 cm -1 , shows a stretching vibration peak of C=O in the acrylamide monomer at 1647 cm -1 , shows a stretching vibration peak of C=C in the pyrene monomer at 1592 cm -1 , shows in-plane bending vibration peaks of C-H in the pyrene monomer at 1488 cm -1 , 1458 cm -1 , and shows bending vibration absorption peaks of C -1 -H of the polypyrene chain segment at 722 - 845 cm Ar as well, proving that the acrylamide monomer and the pyrene monomer are successfully grafted onto the carboxymethyl cellulose.

[0041] Modified carboxymethyl cellulose 2: Compared with the preparation method of modified carboxymethyl cellulose 1, the difference is that N-hydroxyethyl acrylamide is replaced by methacrylamide;

[0042] Modified carboxymethyl cellulose 3: The preparation method is different from that of modified carboxymethyl cellulose 1 in that the pyrene monomer 1 is replaced by pyrene monomer 2;

[0043] Modified carboxymethyl cellulose 4: The preparation method is different from that of modified carboxymethyl cellulose 1 in that no pyrene monomer is added;

[0044] Modified carboxymethyl cellulose 5: The preparation method is different from that of modified carboxymethyl cellulose 1 in that N - hydroxyethyl acrylamide is not added;

[0045] 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 the same kind.

[0046] Examples and comparative examples

[0047] 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 a three - dimensional conductive network carboxymethyl cellulose composite binder, wherein the composition and weight parts of the modified carboxymethyl cellulose and the conductive material are shown in Table 1.

[0048] Table 1 Composition and parts (by weight) of the binders in examples and comparative examples

[0049]

[0050] 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 a negative electrode paste. Place the negative electrode on a 12 - mm - diameter copper foil and evenly coat it with a fixed thickness using a scraper, with a loading of 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 an electrode sheet. In an inert gas atmosphere, use a lithium metal sheet as a 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:

[0051] (1)Peeling test: The adhesion performance is usually evaluated by the peel strength test. When conducting the peel strength test, first, the pre-prepared electrode sheet is pasted onto the aluminum plate substrate, and then it is firmly pasted onto the surface of the electrode coating with 3M tape. The specific operation is to paste a 120×25 mm electrode sample onto the 3M transparent tape, paste it onto the coating, and pull it at a 180° angle. The force required to pull the tape at a fixed speed of 100 mm / min is recorded.

[0052] (2)Mechanical properties: The mechanical properties of the polymer binder are detected through nanoindentation experiments. At an indentation depth of 2400 nm, the indentation force corresponding to the binder is recorded. 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.

[0053] (3)Cyclic capacity retention rate: The charge-discharge performance of the half-cells assembled with the binders of the examples and comparative examples is tested by a LAND-CT3002A tester. The current density is set to 2000 mA·g -1 , and the current density with a voltage range of 0.01~1.5 V is set, and the capacity retention rate after 500 cycles is calculated.

[0054] Table 2 Performance test of examples and comparative examples

[0055]

[0056] 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 can return to the initial state with its volume contraction, effectively maintaining the integrity and stability of the electrode structure. And the three-dimensional conductive network formed by chemical cross-linking has an appropriate cross-linking density, has good adaptability to the volume change of silicon particles during the lithiation / delithiation process, and the polymer network with conductivity, high viscoelasticity, and strong adhesion tightly wraps the surface of each silicon particle and maintains the overall interface dynamic stability between silicon and the binder during the repeated volume expansion and contraction process, 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, thus effectively improving the electrochemical performance.

[0057] 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 principles and practical applications 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 By weight, it includes 100 parts of modified carboxymethyl cellulose and 5 - 30 parts of conductive material; the modified carboxymethyl cellulose is prepared by grafting pyrene-based monomer and acrylamide monomer onto carboxymethyl cellulose through free radical polymerization, and the mass ratio of acrylamide monomer, pyrene-based monomer to carboxymethyl cellulose is 1:(5 - 10):(4 - 6); the structural formula of the pyrene-based 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, hydroxymethylacrylamide, and N-hydroxyethylacrylamide.

3. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, wherein The preparation method of the modified carboxymethyl cellulose includes the following steps: Dissolve carboxymethyl cellulose in deionized water, then perform solvent exchange with N,N-dimethylacetamide and anhydrous methanol, and finally dissolve it in N,N-dimethylacetamide solvent to obtain a carboxymethyl cellulose solution; Dissolve the pyrene-based monomer and acrylamide monomer in N,N-dimethylacetamide, mix them evenly with the carboxymethyl cellulose solution, add an initiator to the solution, and heat and react at 55 - 65 °C in a nitrogen atmosphere for 18 - 30 hours. The product is purified by the 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 addition amount of the initiator is 0.5 - 1 wt% of the total mass of the monomers.

5. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, wherein The preparation method of the pyrene-based monomer is as follows: Mix pyrene, bis(pinacolato)diboron, methoxy(cyclooctadiene)iridium dimer, 4,4'-di-tert-butyl-2,2'-bipyridine, and tetrahydrofuran and stir, reflux and react at 70 - 90 °C for 15 - 20 hours, and obtain an intermediate after purification; Mix the intermediate, bromoalkenyl compound, palladium acetate, tri-o-tolylphosphine, and saturated sodium bicarbonate solution and stir, reflux and react at 70 - 90 °C for 15 - 20 hours, and obtain the pyrene-based monomer after purification.

6. The three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, wherein 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, wherein, The conductive material is at least one of graphite, carbon nanotubes, or graphene.

8. The preparation method of the three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1, characterized in that, Dissolve the modified carboxymethyl cellulose in deionized water to prepare a solution, and then add the conductive material to the modified carboxymethyl cellulose solution in batches under ultrasonic and stirring conditions to obtain a three-dimensional conductive network carboxymethyl cellulose composite binder.

9. The application of the three-dimensional conductive network carboxymethyl cellulose composite binder according to claim 1 as a binder for silicon-based anode materials.

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