Low viscosity modified CMC for lithium battery separator and preparation method thereof

Abstract

This invention discloses a low-viscosity modified CMC for lithium-ion battery separators and its preparation method, belonging to the field of modified carboxymethyl cellulose technology. The modified CMC of this invention has a degree of polymerization (DP) of 50-200. The following side groups are introduced into the molecular chain of the modified CMC through free radical grafting: (a) zwitterionic side groups containing pyridinium-propanesulfonate or imidazolium-propanesulfonate; (b) unsaturated carboxylic acid side groups containing dicarboxyl groups; and (c) polyolefin-compatible side groups containing hydrophobic C4-C12 alkyl or aryl side chains. The modified CMC provided by this invention combines low viscosity processing characteristics, high adhesion strength, ion selectivity, and polyolefin interfacial compatibility, meeting the application requirements of next-generation high-safety lithium-ion battery separators.

Description

Technical Field

[0001] This invention belongs to the field of modified carboxymethyl cellulose technology, specifically relating to a low-viscosity CMC for lithium battery separators and its preparation method. Background Technology

[0002] Lithium-ion batteries, with their high energy density and excellent cycle life, have become the core power source for portable consumer electronics and are widely regarded as one of the most promising solutions for electric vehicles and smart grid energy storage. Inside the battery, the separator, as a key inner component, functions to isolate the positive and negative electrodes to prevent physical contact short circuits, while simultaneously providing a channel for electrolyte ion transport. Currently, commercially available lithium-ion batteries commonly use porous polyolefin membranes such as polyethylene (PE) and polypropylene (PP) as separators. While these polyolefin separators possess good mechanical strength and chemical stability, their inherent thermal shrinkage characteristics pose a serious safety hazard: when the battery's internal temperature rises abnormally due to overcharging, thermal shock, or external short circuits, the separator will shrink significantly or even melt, causing direct contact between the positive and negative electrodes and triggering an internal short circuit; the enormous heat released by the internal short circuit further triggers thermal runaway, potentially leading to fire or explosion. To mitigate these risks, the industry has developed PP / PE / PP multilayer composite shut-off separators, utilizing the melting and closing-cell effect of PE at approximately 130°C to cut off ion conduction and achieve primary safety protection. However, once the temperature continues to rise and exceeds the melting point of PP (approximately 160°C), the multilayer structure will also fail and melt, failing to block electrode contact, and its protective capability has a clear upper limit. Therefore, developing separator materials that maintain structural integrity, do not shrink, and do not melt at high temperatures is an urgent need to improve the intrinsic safety of lithium-ion batteries.

[0003] Ceramic-coated separators (CCS) are considered an effective solution to the aforementioned problems. These separators typically use a polyolefin-based membrane as a support, with inorganic heat-resistant particles such as Al2O3 and SiO2 coated onto the membrane surface using an adhesive, forming an organic-inorganic composite coating. The introduction of inorganic ceramic particles can significantly suppress the thermal shrinkage of the separator at high temperatures, while simultaneously improving electrolyte wettability and retention capacity. However, traditional ceramic coatings commonly use oily adhesives such as polymethyl methacrylate, polyvinylidene fluoride, or polyvinylidene fluoride-hexafluoropropylene copolymer. These adhesives have several drawbacks: firstly, the film-forming amount usually needs to exceed 20 wt%, and even high levels of organic adhesives can cause significant shrinkage at high temperatures, weakening the coating's heat resistance; secondly, these polymers are prone to swelling, gelation, and even dissolution in non-aqueous electrolytes, forming viscous fluids or gel-like substances, causing the ceramic coating to peel off from the membrane surface and lose its protective function; thirdly, oily systems require the use of organic solvents, resulting in high processing costs and a heavy environmental burden.

[0004] In recent years, waterborne bio-based binders, represented by carboxymethyl cellulose (CMC), have attracted attention. CMC is widely available, inexpensive, environmentally friendly, and exhibits excellent biocompatibility. Its molecular chain is rich in hydroxyl and carboxylate groups, which can achieve good particle dispersion and interfacial adhesion through hydrogen bonding. Waterborne hybrid binders formed by compounding CMC with styrene-butadiene rubber (SBR) have shown significant advantages in ceramic-coated membranes. Studies have shown that in ceramic-coated membranes prepared using Al2O3 as ceramic particles, PE as the base membrane, and CMC-SBR as the binder, the binder dosage can be as low as 5 wt%, far lower than the requirements for oil-based binders; furthermore, this system uses water as the dispersion medium, combining environmental and cost advantages. Performance tests in the study showed that the thermal shrinkage rate of the 3μm thick CCS-3 separator at 145°C decreased from 63.5% for the pure PE separator to 29.3%, while the thermal shrinkage rate of the 9μm thick CCS-9 separator could be further reduced to 2.5%. Simultaneously, the CCS separator exhibited excellent electrolyte wettability and absorbance, and the capacity retention of pouch cells using this separator after 50 cycles was superior to that of pure PE separators. However, the direct application of existing CMC to the ceramic coating of separators still faces significant technical bottlenecks that are difficult to reconcile. Conventional CMC has a high molecular weight and high aqueous solution viscosity, which limits the solid content of ceramic slurry (usually far below 30%), thus affecting coating efficiency and coating density. If the viscosity of CMC is reduced by means of degradation, the bonding strength and cohesion of the film after film formation will decrease significantly, and the coating is prone to powdering or peeling, forming a contradiction between low viscosity and high strength.

[0005] Conventional CMC molecules only have carboxymethyl anionic groups on their molecular chains, lacking the ability to selectively regulate specific ions. The coating only plays a physical isolation role and cannot improve electrode / electrolyte interface polarization or inhibit lithium dendrite growth through functional design.

[0006] Third, the interfacial compatibility between the existing CMC-SBR system and the polyolefin-based membrane mainly relies on physical adsorption, lacking chemical bonding or strong hydrophobic interactions, and there is a risk of interfacial delamination under long-term cycling or high-temperature storage conditions.

[0007] The aforementioned high viscosity leads to three major drawbacks: low solids content, lack of ion selectivity, and insufficient interfacial compatibility with polyolefin-based membranes. These constitute the core challenges restricting the application of CMC in high-end separators. Therefore, there is an urgent need to develop a specialized modified CMC that combines low viscosity processing characteristics, high bonding strength, ion selectivity, and polyolefin interfacial compatibility to meet the application requirements of next-generation high-safety lithium-ion battery separators. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the defects of viscosity-strength contradiction, lack of ion selectivity and insufficient interfacial compatibility with polyolefin-based membranes when CMC is used as a ceramic coating for lithium battery separators in the prior art. The invention provides a modified CMC that has the functions of low viscosity, high bonding strength, ion selective transport and polyolefin interface enhancement, as well as a high solids content ceramic coating separator containing the modified CMC.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A low-viscosity modified CMC for lithium battery separators, wherein the degree of polymerization (DP) of the modified CMC is 50~200; The modified CMC has the following side groups introduced into its molecular chain via free radical grafting: (a) Contains zwitterionic side groups of pyridinium-propanesulfonate or imidazolium-propanesulfonate; (b) Itaconic acid side group containing a dicarboxyl group; (c) Polyolefin compatible side groups containing hydrophobic C4~C12 alkyl side chains; The Brookfield viscosity of the 2wt% aqueous solution of the low-viscosity modified CMC at 25°C is 10~500 mPa·s.

[0010] Preferably, the zwitterionic side group is selected from at least one of 4-vinylpyridine propanesulfonate, 2-vinylpyridine propanesulfonate, or 1-vinyl-3-(3-sulfopropyl)imidazolium salt.

[0011] Preferably, the unsaturated carboxylic acid side group is selected from at least one of itaconic acid, citraconic acid, mesocarboxylic acid, fumaric acid, maleic acid, or 2-methyleneglutarate.

[0012] Preferably, the polyolefin-compatible side group is selected from at least one of butyl acrylate, isooctyl acrylate, lauryl acrylate, or styrene.

[0013] Preferably, the preparation method of the 4-vinylpyridine propanesulfonate, 2-vinylpyridine propanesulfonate, or 1-vinyl-3-(3-sulfopropyl)imidazolium lactone salt is as follows: 4-vinylpyridine, 2-vinylpyridine, or N-vinylimidazolium is dissolved in anhydrous acetonitrile at a molar ratio of 1:(0.95~1.2) with 1,3-propanesulfonic acid lactone, and a ring-opening quaternization reaction is carried out under nitrogen protection. After the reaction is completed, the mixture is filtered, washed successively with acetonitrile and anhydrous ethanol, and dried under vacuum. The reaction conditions for 4-vinylpyridine are 50~60℃ for 24 h; for 2-vinylpyridine, 55~65℃ for 30 h; and for N-vinylimidazolium, 40~50℃ for 12 h.

[0014] Preferably, the grafting rate of the zwitterionic side group is 3-15 mol; the grafting rate of the unsaturated carboxylic acid side group is 5-20 mol; and the grafting rate of the polyolefin compatible side group is 2-12 mol.

[0015] The grafting rates mentioned in this invention are all molar grafting rates (mol%), defined as the number of corresponding side groups attached to each mole of dehydrated glucose unit in the CMC main chain. It should be noted that the carboxyl content of the CMC raw material needs to be subtracted when calculating the grafting rate of unsaturated carboxylic acid side groups.

[0016] This invention also protects the preparation method of the low-viscosity modified CMC for lithium battery separators, comprising the following steps: (1) The CMC is subjected to oxidative degradation treatment, and the reaction conditions are controlled so that the degree of polymerization (DP) of the CMC is 50~200, so as to obtain a low viscosity CMC matrix; (2) Disperse the low viscosity CMC matrix obtained in step (1) in a solvent, add an initiator and a mixed grafting monomer composed of zwitterionic monomer, unsaturated carboxylic acid and polyolefin compatible monomer under nitrogen protection, and carry out free radical grafting reaction at 50~75℃. (3) After the reaction is complete, wash with ethanol / water mixed solvent to remove homopolymer and residual monomer, adjust pH to 6.5~7.5, dry to obtain the low viscosity modified CMC.

[0017] Preferably, the oxidative degradation treatment in step (1) is as follows: treating the CMC aqueous solution with hydrogen peroxide or sodium hypochlorite for 1 to 4 hours under conditions of pH 8 to 11 and temperature 40 to 80°C.

[0018] Preferably, the initiator in step (2) is one or more of cerium ammonium nitrate / acid, potassium persulfate-sodium bisulfite, ammonium persulfate / tetramethylethylenediamine, azobisisobutyronitrile, azobisisoheptanenitrile, or benzoyl peroxide. The amount of the initiator is 0.1-5 wt% of the dry basis mass of the CMC matrix (redox system) or 0.5-2% of the total molar mass of the mixed grafted monomers (thermal initiator system). The solvent is a mixture of at least one of 1,4-dioxane, N,N-dimethylformamide, or isopropanol with water, wherein the water content in the solvent is 5-60 wt%.

[0019] The present invention also provides a lithium battery separator, comprising a polyolefin-based film and a ceramic coating coated on at least one side of the film, wherein the ceramic coating comprises, by solid content, 0.5-5 wt% modified CMC, 85-95 wt% ceramic particles, and 2-10 wt% aqueous binder, wherein the modified CMC is the aforementioned low-viscosity modified CMC for lithium battery separators or the modified CMC prepared by the aforementioned method for preparing low-viscosity modified CMC for lithium battery separators.

[0020] Preferably, the ceramic particles are at least one of alumina, silica, and boehmite, with a particle size D50 of 0.3~1.5μm; the water-based binder is styrene-butadiene rubber and / or polyacrylate binder; the polyolefin-based film is polyethylene, polypropylene, or a multilayer composite film thereof, with a thickness of 7~25μm; and the ceramic coating has a thickness of 3~6μm.

[0021] The present invention also provides a method for preparing the above-mentioned lithium battery separator, comprising the following steps: mixing modified CMC, ceramic particles, aqueous binder and deionized water to prepare a ceramic coating slurry with a solid content of 20-40 wt%; coating the slurry onto at least one side surface of a polyolefin-based film by microgravure coating or dip coating; drying with hot air at 50-80°C and winding to obtain the lithium battery separator.

[0022] Beneficial effects

[0023] This invention provides a low-viscosity modified CMC for lithium battery separators and its preparation method, which has the following beneficial effects: 1. Synergistic solution to the contradiction between low viscosity and high strength By precisely controlling the degree of polymerization of CMC to 50-200 through oxidative degradation, the viscosity of a 2wt% aqueous solution of modified CMC is reduced to 10-500 mPa·s, fundamentally reducing the internal friction of the slurry and allowing the solid content of the ceramic coating slurry to be increased to 20-40wt%. Simultaneously, the dicarboxyl groups of the unsaturated carboxylic acid side groups can form multi-point hydrogen bonds and coordination interactions with the hydroxyl groups on the surface of ceramic particles (such as Al2O3), enhancing the inorganic-organic interfacial adhesion; the hydrophobic alkyl / aryl chains of the polyolefin-compatible side groups can embed into the surface of the polyolefin-based film, significantly improving the peel strength between the coating and the base film through hydrophobic interactions and physical anchoring effects. The synergistic effect of these three factors enables modified CMC to achieve high bonding strength at extremely low dosages, resolving the technical challenge of balancing low viscosity and high bonding strength.

[0024] 2. Construction of ion-selective transport channels The zwitterionic side groups (pyridinium-propanesulfonate or imidazolium-propanesulfonate) introduce a cationic-alkyl-anionic dipole structure into the CMC molecular chain, which can form a locally ordered electric field in the electrolyte and promote the growth of Li through the space charge effect. + Desolvation transport, inhibiting anion (PF6) - The migration of lithium ions (such as lithium dendrites) reduces concentration polarization and homogenizes lithium ion flow, thus suppressing lithium dendrite growth. This functional design upgrades the membrane from a simple physical isolation layer to an active interface layer with ion regulation capabilities.

[0025] 3. Chemical enhancement of interfacial compatibility of polyolefins Polyolefin compatible side groups (C4~C12 alkyl esters or aryl groups) have similar hydrophobic structural parameters to polyolefin-based films (PE / PP). During the coating and drying process, they can be anchored to the surface of the base film through chain segment entanglement and hydrophobic aggregation, forming a gradient interface of ceramic particles-modified CMC-polyolefin-based film, which significantly improves the interface stability under long-term cycling or high-temperature storage conditions.

[0026] In summary, the ceramic-coated separator prepared using the modified CMC of this invention can reduce the thermal shrinkage rate of polyolefin separators and improve the ionic conductivity of polyolefin separators. At the same time, it meets the requirements of slurry processability, coating adhesion, heat resistance, ion transport performance and long-term cycle stability, thus meeting the application needs of the next generation of high-safety lithium-ion battery separators. Attached Figure Description

[0027] Figure 1 Fourier transform infrared (FTIR) spectra of CMC and the modified CMC prepared in Example 1 of this invention. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Unless otherwise specified, the experimental methods used in the embodiments are conventional methods, and the materials and reagents used are commercially available unless otherwise specified.

[0030] The raw materials and equipment used in the embodiments and comparative examples are described below: Carboxymethyl cellulose: degree of substitution (DS) of 0.70~1.2, Changshu Weiyi Technology Co., Ltd. Polyolefin membrane: Commercial polyolefin membrane, polypropylene / polyethylene / polypropylene three-layer microporous composite structure, 20μm thickness, Celgard 2320, purchased from Shenzhen Kejing Zhida Technology Co., Ltd. Polyethylene diaphragm: Hipore 16μm, Asahi; 4-Vinylpyridine propanesulfonate: Prepared in-house, as follows: In a 250 mL three-necked flask equipped with a mechanical stirrer, reflux condenser, nitrogen inlet tube, and drying tube, 0.1 mol of 4-vinylpyridine, 0.12 mol of 1,3-propanesulfonic acid lactone, and 100 mL of anhydrous acetonitrile were added sequentially. After purging the system with high-purity nitrogen for 30 min, a slight positive pressure of nitrogen was maintained. The reaction system was heated to 60 °C and stirred continuously for 24 h under light-protected conditions. A white solid gradually precipitated during the reaction. After the reaction was complete, the mixture was allowed to cool naturally to room temperature and stirred for another 1 h to allow for complete crystallization. The reaction mixture was filtered under reduced pressure, and the white solid was collected. The filter cake was washed three times with anhydrous acetonitrile to remove unreacted 1,3-propanesulfonic acid lactone, and then washed twice with anhydrous ethanol to remove residual 4-vinylpyridine and oligomers. The obtained solid was placed in a vacuum drying oven and dried at 50 °C and -0.095 MPa for 12 h to obtain 4-vinylpyridine propanesulfonate, with a yield of 92.6%. The product structure was confirmed by ¹H NMR spectroscopy: ¹H NMR (400 MHz, DMSO-d6): δ2.29 (m, 2H, -CH2-, methylene middle of propanesulfonic acid chain); δ2.81 (t, 2H, -CH2SO3). - (methylene group attached to sulfonate group); δ 4.55 (t, 2H, N) + -CH2-, methylene group attached to pyridinium nitrogen; δ5.83 (d, 1H, =CH2, vinyl cis hydrogen); δ6.28 (d, 1H, =CH2, vinyl trans hydrogen); δ6.86 (dd, 1H, =CH-, vinyl methine hydrogen); δ7.85 (d, 2H, Py-H) 3,5 (hydrogens at positions 3 and 5 of the pyridine ring); δ 8.59 (d, 2H, Py-H) 2,6 (2,6 hydrogens of the pyridine ring).

[0031] 2-Vinylpyridine propanesulfonate: The preparation method differs from that of 4-vinylpyridine propanesulfonate in that 4-vinylpyridine is replaced with an equimolar amount of 2-vinylpyridine. The reaction temperature is increased to 55℃, and the reaction time is extended to 30 h, while the remaining post-treatment and purification steps remain unchanged. The yield is 89.3%. The product structure was confirmed by 1H NMR: ¹H NMR (400 MHz, DMSO-d6): δ2.47 (m, 2H, -CH2-, methylene middle of propanesulfonic acid chain); δ2.96 (t, 2H, -CH2SO3). - (methylene group attached to sulfonate group); δ 4.48 (t, 2H, N) +-CH2-, methylene group attached to pyridine nitrogen; δ5.80 (d, 1H, =CH2, vinyl cis hydrogen); δ5.93 (d, 1H, =CH2, vinyl trans hydrogen); δ7.00 (dd, 1H, =CH-, vinyl methine hydrogen); δ7.65 (d, 2H, Py-H3, pyridine ring 3-hydrogen); δ7.91 (d, 2H, Py-H5, pyridine ring 5-hydrogen); δ8.13 (d, 2H, Py-H4, pyridine ring 4-hydrogen); δ8.70 (d, 2H, Py-H6, pyridine ring 6-hydrogen).

[0032] 1-Vinyl-3-(3-sulfopropyl)imidazolium salt: The preparation method differs from that of 4-vinylpyridine propanesulfonate in that 4-vinylpyridine is replaced with an equimolar amount of N-vinylimidazolium. The reaction temperature is reduced to 45℃, and the reaction time is shortened to 12 h. After the reaction, most of the acetonitrile is removed by vacuum distillation, and then anhydrous ethanol is added for recrystallization purification. The yield is 94.3%, and the product structure is confirmed by 1H NMR as follows: ¹H NMR (DMSO-d6): δ2.37 (m, 2H, -CH2-, methylene group in the propanesulfonic acid chain); δ2.91 (t, 2H, -CH2SO3-, methylene group attached to the sulfonate group); δ4.57 (t, 2H, N-methyl group). + -CH2-, methylene group attached to pyridinium nitrogen; δ5.11 (d, 1H, =CH2, imidazolium vinyl cis); δ5.28 (d, 1H, =CH2, imidazolium vinyl trans); δ7.08 (dd, 1H, =CH-, vinyl methine hydrogen); δ7.92 (s, 1H, Im-H) 4,5 (4, 5 hydrogens of the imidazole ring); δ 9.61 (s, 1H, Im-H2, 2 hydrogen of the imidazole ring).

[0033] Unless otherwise specified, all components and raw materials used in the embodiments and comparative examples of this invention are commercially available, and the same type of components and raw materials are used in each parallel experiment.

[0034] Example 1

[0035] A low-viscosity modified CMC for lithium battery separators is prepared as follows: (1) Oxidative degradation: Sodium carboxymethyl cellulose (CMC) with a degree of substitution (DS) of 0.90 was dissolved in deionized water to prepare a 3wt% aqueous solution. The pH was adjusted to 9.0 with 0.1 mol / L NaOH. A 30wt% aqueous solution of hydrogen peroxide (5wt% of the dry weight of CMC) was added, and the mixture was stirred at 60°C for 2 h. After the reaction was completed, the mixture was cooled to room temperature, precipitated with ethanol, washed, and vacuum dried to obtain a low-viscosity CMC matrix. The degree of polymerization (DP) was 120 as determined and calculated by the Ubbelohde viscometer method; the Brookfield viscosity of a 2wt% aqueous solution at 25°C was 180 mPa·s.

[0036] (2) Free radical grafting: 10g of the above low-viscosity CMC matrix was dispersed in 150g of a mixed solvent of N,N-dimethylformamide (DMF) containing 30wt% water, and the mixture was deoxygenated by purging with nitrogen for 30min. A mixed monomer solution was prepared by dissolving 0.05mol of 4-vinylpyridine propanesulfonate, 0.075mol of itaconic acid, and 0.025mol of butyl acrylate in 20g of DMF. Separately, an initiator solution was prepared by dissolving 0.25g of cerium ammonium nitrate in 5mL of 1% dilute nitric acid. Under nitrogen protection, the monomer solution was first added to the CMC dispersion, followed by the slow dropwise addition of the initiator solution. The temperature was raised to 50℃, and the reaction was allowed to proceed for 6h.

[0037] (3) Post-treatment: After the reaction is completed, the mixture is cooled, filtered, and washed three times with a 3:1 volume ratio of ethanol / water mixed solvent to remove homopolymer and residual monomer. Then, it is washed with deionized water until the filtrate is neutral. Finally, the pH is adjusted to 7.0 with 0.1 mol / L hydrochloric acid / sodium hydroxide and dried under vacuum at 50°C for 12 h to obtain white powdery low viscosity modified CMC.

[0038] Elemental analysis (sulfur content), acid-base titration (carboxyl group content), and 1 The ¹H NMR spectroscopy revealed grafting rates of 8.5 mol% for zwitterionic side groups (VPPS), 10.2 mol% for itaconic acid side groups, and 6.3 mol% for butyl acrylate side groups. The FTIR spectrum of the product showed a high 1726 cm⁻¹ density. -1 An enhanced peak for the C=O stretching vibration of butyl acrylate appears at ~1590 cm⁻¹. -1 and ~1410 cm -1 The relative absorbance of the carboxylate characteristic peak at 1040 cm⁻¹ was significantly enhanced compared to ordinary CMC, which is attributed to the introduction of the itaconic acid dicarboxyl group. -1 and 1180cm -1 The characteristic peak of sulfonate O=S=O appears at 2960~2850 cm⁻¹. -1 The intensity of the CH stretching vibration peak of butyl acrylate increased significantly, confirming that all three side groups had been successfully grafted.

[0039] Application Example 1 A lithium battery separator is prepared as follows: 0.3g of modified CMC from Example 1, 0.6g of styrene-butadiene rubber (SBR) emulsion (50% solid content) (0.3g on a dry basis), and 9.4g of alumina (Al2O3) particles with an average particle size D50 of 0.5μm were mixed with deionized water to prepare a ceramic coating slurry with a total solid content of 35wt% (based on solid content, modified CMC accounts for 3.0wt%, Al2O3 accounts for 94.0wt%, and SBR accounts for 3.0wt%). The slurry was coated onto one side of a polyethylene separator using a microgravure coating machine, and the wet coating thickness was controlled so that the ceramic coating thickness after drying was 3μm. After drying with hot air at 60℃ for 6 min, the membrane was wound up to obtain lithium battery separator 1.

[0040] Example 2

[0041] A low-viscosity modified CMC for lithium battery separators is prepared as follows: (1) Oxidative degradation: CMC with a degree of substitution (DS) of 1.00 was dissolved in deionized water to prepare a 4 wt% aqueous solution. The pH was adjusted to 10.0 with 0.1 mol / L NaOH. A sodium hypochlorite solution (10 wt% available chlorine) at 3 wt% relative to the dry weight of the CMC was added, and the mixture was stirred at 50 °C for 3 h. After the reaction was completed, the mixture was cooled to room temperature, precipitated with ethanol, washed, and vacuum dried to obtain a low-viscosity CMC matrix. The DP of the obtained low-viscosity CMC matrix was measured and calculated to be 80, and the Brookfield viscosity of a 2 wt% aqueous solution was 80 mPa·s.

[0042] (2) Free radical grafting: 10g of the above low-viscosity CMC matrix was dispersed in 120g of a mixed solvent containing 20wt% water and 1,4-dioxane. 0.04mol of 1-vinyl-3-(3-sulfopropyl)imidazolium, 0.04mol of citralic acid, and 0.02mol of lauryl acrylate were dissolved in 15g of 1,4-dioxane to prepare a monomer solution. Separately, 0.25g of cerium ammonium nitrate was dissolved in 5mL of 1% dilute nitric acid to prepare an initiator solution. Under nitrogen protection, the monomer solution was first added to the CMC dispersion, followed by the slow dropwise addition of the initiator solution. The temperature was raised to 50℃, and the reaction was allowed to proceed for 6 hours.

[0043] (3) Post-treatment: Same as in Example 1, adjust pH to 6.8.

[0044] The grafting rates of zwitterionic side groups (VPIS) were determined to be 7.8 mol, citrate side groups 8.5 mol, and lauryl acrylate side groups 6.0 mol.

[0045] Application Example 2 0.2g of modified CMC, 0.4g of polyacrylate-based waterborne binder (dry basis), and 9.4g of boehmite particles (D50=0.8μm) from Example 2 were mixed with deionized water to prepare a slurry with a solid content of 38wt% (modified CMC 2.0wt%, boehmite 94.0wt%, binder 4.0wt%). This slurry was coated onto one side of a 20μm thick PP / PE / PP three-layer composite polyolefin separator base membrane. After drying, the coating thickness was 4.5μm, yielding lithium battery separator 2.

[0046] Example 3

[0047] A low-viscosity modified CMC for lithium battery separators is prepared as follows: (1) Oxidative degradation: CMC with a degree of substitution (DS) of 0.70 was prepared into a 2wt% aqueous solution, the pH was adjusted to 8.5 with 0.1mol / L NaOH, 4wt% hydrogen peroxide was added, and the reaction was carried out at 70℃ for 1.5h. The resulting low-viscosity CMC matrix had a DP of 180, and the Brookfield viscosity of the 2wt% aqueous solution was 350 mPa·s.

[0048] (2) Free radical grafting: 10g of low-viscosity CMC matrix was dispersed in 180g of a mixed solvent containing 10wt% water in isopropanol. 0.06mol of 2-vinylpyridine propanesulfonate, 0.12mol of fumaric acid, and 0.03mol of styrene were dissolved in 25g of isopropanol to prepare a monomer solution. 0.0017mol of azobisisobutyronitrile (AIBN, accounting for 0.8% of the total molar amount of monomer) was added. Under nitrogen protection, the monomer solution was first added to the CMC dispersion, and then the initiator solution was slowly added dropwise. The temperature was raised to 80℃ and the reaction was carried out for 3h.

[0049] (3) Post-treatment: Same as in Example 1, adjust pH to 7.2.

[0050] The grafting rates of zwitterionic side groups were determined to be 4.5 mol, fumaric acid side groups 12.0 mol, and styrene side groups 3.2 mol.

[0051] Application Example 3 A lithium battery separator is prepared as follows: 0.5 g of modified CMC, 0.5 g of SBR (dry basis), and 9.0 g of magnesium hydroxide particles (D50 = 1.2 μm) from Example 3 were mixed with deionized water to prepare a slurry with a solid content of 30 wt% (modified CMC 5.0 wt%, magnesium hydroxide 90.0 wt%, and SBR 5.0 wt%). This slurry was coated onto one side of a 20 μm thick polyethylene separator. After drying, the coating thickness was 6 μm, resulting in lithium battery separator 3.

[0052] Comparative Example 1 Compared with Example 1, the difference is that the low viscosity modified CMC was replaced with sodium carboxymethyl cellulose (DS=0.90, DP=800, Brookfield viscosity of 5000 mPa·s for 2wt% aqueous solution).

[0053] Application Comparative Example 1 Compared to Application Example 1, the difference lies in replacing the low-viscosity modified CMC with sodium carboxymethyl cellulose. Because ordinary CMC has excessively high viscosity, the slurry becomes gelled at a solid content of 22 wt%, making coating impossible. Therefore, a slurry with a solid content of 20 wt% was actually prepared. After drying, the coating thickness was 3 μm, resulting in lithium-ion battery separator 4.

[0054] Comparative Example 2 Compared with Example 1, the difference is that the low viscosity modified CMC is replaced with the low viscosity CMC matrix (DP=120, viscosity 180 mPa·s) obtained in step (1) of Example 1, that is, no grafting modification is performed.

[0055] Comparative Application Example 2 Compared with Application Example 1, the difference is that the low viscosity modified CMC is replaced with the low viscosity CMC matrix obtained in step (1) of Example 1 to obtain lithium battery separator 5.

[0056] Comparative Example 3 Compared with Example 1, the difference is that butyl acrylate was not added in the free radical grafting of step (2). That is, 10g of low-viscosity CMC matrix was reacted with 0.05 mol VPPS and 0.075 mol itaconic acid under the same conditions to obtain modified CMC containing only zwitterionic side groups and itaconic acid side groups. The VPPS grafting rate was measured to be 10.5 mol%, and the itaconic acid grafting rate was 13.2 mol%.

[0057] Comparative Application Example 3 Compared with Application Example 1, the difference is that the modified CMC in Example 1 was replaced with the modified CMC in Comparative Example 3, resulting in lithium battery separator 6.

[0058] Comparative Example 4 Compared with Example 1, the difference is that itaconic acid was not added in the free radical grafting of step (2). That is, 10g of low viscosity CMC matrix was reacted with 0.05mol of 4-vinylpyridine propanesulfonate and 0.025mol of butyl acrylate under the same conditions to obtain modified CMC containing only zwitterionic side groups and butyl acrylate side groups. The grafting rate of zwitterionic side groups was measured to be 12.5mol%, and the grafting rate of butyl acrylate side groups was 7.8mol%.

[0059] Comparative Application Example 4 Compared with Application Example 1, the difference is that the modified CMC in Example 1 was replaced with the modified CMC in Comparative Example 4, resulting in lithium battery separator 7.

[0060] Comparative Example 5 Compared with Example 1, the difference is that 4-vinylpyridine propanesulfonate was not added in the free radical grafting of step (2). That is, 10g of low viscosity CMC matrix was reacted with 0.075mol itaconic acid and 0.025mol butyl acrylate under the same conditions to obtain modified CMC containing only itaconic acid side groups and butyl acrylate side groups. The grafting rate of itaconic acid side groups was determined to be 15.5mol%, and the grafting rate of butyl acrylate side groups was 7.2mol%.

[0061] Comparative Application Example 5 Compared with Application Example 1, the difference is that the modified CMC in Example 1 was replaced with the modified CMC in Comparative Example 5, resulting in lithium battery separator 8.

[0062] Comparative Application Example 6 Commercially available polyethylene diaphragm.

[0063] The modified CMC and slurries prepared in the examples and comparative examples were subjected to viscosity performance tests, and the results are shown in Table 1: Table 1. Performance results of the modified CMC and slurry prepared in the examples and comparative examples.

[0064] The slurry viscosity in Table 1 refers to the viscosity at 25°C and a shear rate of 10 s⁻¹, under their respective maximum configurable solid content. -1 Brookfield viscosity measured at that time.

[0065] The thermal shrinkage performance of the lithium battery separators prepared in the application examples and comparative application examples was tested, and the results are shown in Table 2: Table 2. Results of thermal shrinkage performance of lithium battery separators in application examples and comparative application examples.

[0066] The thickness was determined according to the method in GB / T 451.3-2002.

[0067] The heat shrinkage rate is determined by cutting the diaphragm into round pieces with a diameter of 15.8 mm using a cutting machine, heating the round pieces in an electric constant temperature drying oven, and measuring the shrinkage area of ​​the diaphragm.

[0068] The hot rod puncture test conditions are as follows: a 2mm diameter metal rod is heated to 200℃ and placed 3mm above the diaphragm for 1 minute.

[0069] The adhesion performance of the ceramic coatings on the lithium battery separators prepared in the application examples and comparative application examples was tested, and the results are shown in Table 3: Table 3. Adhesion performance results of ceramic coatings on lithium-ion battery separators in application examples and comparative application examples.

[0070] Peel strength testing was performed according to ASTM D903. The ceramic coating side of the diaphragm was placed upwards, and after being attached with double-sided tape, it was peeled at a rate of 100 mm / min on a universal testing machine.

[0071] The electrochemical performance of the lithium battery separators prepared in the application examples and comparative application examples was tested, and the results are shown in Table 4: Table 4 Electrochemical performance results of lithium battery separators in application examples and comparative application examples.

[0072] Ionic conductivity was determined using the AC impedance method with a stainless steel blocking electrode.

[0073] The electrolyte absorption rate was determined by immersing the weighed diaphragm in the electrolyte for 2 hours, then removing it. Excess electrolyte was wiped off the surface with filter paper, and the diaphragm was weighed before and after immersion.

[0074] The contact angle is measured using a contact angle tester.

[0075] The lithium battery separators prepared in the application examples and comparative application examples were assembled with LiCoO2 positive electrode and graphite negative electrode, respectively, into 280 mAh pouch batteries with the same electrolyte injection amount. The batteries were tested under 0.5C / 0.5C charge and discharge conditions and a voltage range of 3.0~4.2V. The results are shown in Table 5.

[0076] Table 5 Performance results of the lithium-ion battery separators prepared in the application examples and comparative application examples in the battery.

[0077] Unless otherwise specified, all electrochemical performance tests were conducted using the following electrolyte system: Battery-grade lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) to prepare an electrolyte with a concentration of 1.0 mol / L, wherein the volume ratio of EC:DMC:EMC was 1:1:1. The water content of the mixed solvent and lithium salt was controlled below 20 ppm, and the hydrogen fluoride (HF) content was controlled below 80 ppm. The electrolyte injection rate for the pouch cell was 3.5 g / Ah, and after injection, the cells were allowed to stand at 25°C for 24 h to allow the separator and electrodes to be fully wetted.

[0078] The systematic comparison between Examples 1-3 and Comparative Examples 1-6 shows that the present invention, by simultaneously introducing zwitterionic side groups, itaconic acid side groups, and polyolefin-compatible side groups onto a low-viscosity CMC matrix, achieves a significant synergistic enhancement effect from these three side groups, while simultaneously satisfying the advantages of slurry processability, coating adhesion, heat resistance, ion transport performance, and long-term cycling stability. Specifically, the slurry solid content of Examples 1-3 reaches 30-38 wt%, significantly higher than the 20 wt% of ordinary CMC in Comparative Example 1, solving the problem of low solid content caused by high viscosity. The peel strength of Example 1 under a 3 μm coating is significantly higher than that of Comparative Examples 4 and 5; the thermal shrinkage rate at 145°C is also significantly better than that of Comparative Example 4, proving that the dicarboxyl groups of unsaturated carboxylic acids play a dominant role in the anchoring of ceramic particles and the cohesiveness of the coating. The ionic conductivity of Example 1 is significantly higher than that of Comparative Example 5, and the conductivity of Comparative Example 5 is even lower than that of the ungrafted Comparative Example 2. This result demonstrates that when itaconic acid coexists with compatible side groups of polyolefins but lacks zwitterions, ion transport performance not only fails to improve but actually deteriorates; only with the synergy of the three side groups can ion transport be achieved through the space charge effect of zwitterions with Li. + Selective channels achieve a breakthrough in conductivity. The capacity retention rate of Example 1 after 50 cycles is significantly higher than that of Comparative Examples 4 and 5, demonstrating the synergistic maintenance effect of the three side groups on interface stability, ion transport uniformity, and coating structural integrity during cycling.

[0079] In summary, this invention provides a novel bio-based binder solution for high-performance, high-safety lithium-ion battery separators, which combines low viscosity processability, high interfacial adhesion, ion selective transport, and polyolefin interface reinforcement, and has good application prospects.

[0080] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A low-viscosity modified CMC specifically for lithium battery separators, characterized in that, The degree of polymerization (DP) of the modified CMC is 50~200; The modified CMC has the following side groups introduced into its molecular chain via free radical grafting: (a) Contains zwitterionic side groups of pyridinium-propanesulfonate or imidazolium-propanesulfonate; (b) Unsaturated carboxylic acid side groups containing dicarboxyl groups; (c) Polyolefin compatible side groups containing hydrophobic C4~C12 alkyl or aryl side chains.

2. The low-viscosity modified CMC for lithium battery separators as described in claim 1, characterized in that, The zwitterionic side group is selected from at least one of 4-vinylpyridine propanesulfonate, 2-vinylpyridine propanesulfonate, or 1-vinyl-3-(3-sulfopropyl)imidazolium salt; the unsaturated carboxylic acid side group is selected from at least one of itaconic acid, citraconic acid, medaconic acid, fumaric acid, maleic acid, or 2-methyleneglutarate; the polyolefin compatible side group is selected from at least one of butyl acrylate, isooctyl acrylate, lauryl acrylate, or styrene.

3. The low-viscosity modified CMC for lithium battery separators as described in claim 1, characterized in that, The grafting rate of the zwitterionic side group is 3-15 mol; the grafting rate of the unsaturated carboxylic acid side group is 5-20 mol; and the grafting rate of the polyolefin-compatible side group is 2-12 mol.

4. The low-viscosity modified CMC for lithium battery separators as described in claim 1, characterized in that, The preparation method of the 4-vinylpyridine propanesulfonate, 2-vinylpyridine propanesulfonate or 1-vinyl-3-(3-sulfopropyl)imidazolium lactone salt is as follows: 4-vinylpyridine, 2-vinylpyridine or N-vinylimidazolium and 1,3-propanesulfonic acid lactone are dissolved in anhydrous acetonitrile at a molar ratio of 1:(0.95~1.2), and the ring-opening quaternization reaction is carried out under inert gas protection.

5. The method for preparing low-viscosity modified CMC for lithium battery separators as described in any one of claims 1 to 4, characterized in that, Includes the following steps: (1) The CMC is subjected to oxidative degradation treatment, and the reaction conditions are controlled so that the degree of polymerization (DP) of the CMC is 50~200, so as to obtain a low viscosity CMC matrix; (2) Disperse the low viscosity CMC matrix obtained in step (1) in a solvent, add an initiator and a mixed grafting monomer composed of zwitterionic monomer, unsaturated carboxylic acid and polyolefin compatible monomer under nitrogen protection, and carry out free radical grafting reaction at 50~75℃. (3) After the reaction is complete, wash with ethanol / water mixed solvent to remove homopolymer and residual monomer, adjust pH to 6.5~7.5, dry to obtain the low viscosity modified CMC.

6. The preparation method of low-viscosity modified CMC for lithium battery separators as described in claim 5, characterized in that, The oxidative degradation treatment in step (1) is as follows: under conditions of pH 8-11 and temperature 40-80°C, CMC aqueous solution is treated with hydrogen peroxide or sodium hypochlorite for 1-4 hours.

7. The preparation method of low-viscosity modified CMC for lithium battery separators as described in claim 5, characterized in that, The initiator in step (2) is one or more of cerium ammonium nitrate / acid, potassium persulfate-sodium bisulfite, ammonium persulfate / tetramethylethylenediamine, azobisisobutyronitrile, azobisisoheptanenitrile, or benzoyl peroxide; the solvent is a mixture of at least one of 1,4-dioxane, N,N-dimethylformamide, or isopropanol with water.

8. A lithium battery separator, characterized in that, The invention comprises a polyolefin-based film and a ceramic coating coated on at least one surface thereon. The ceramic coating, based on solid content, comprises 0.5-5 wt% modified CMC, 85-95 wt% ceramic particles, and 2-10 wt% aqueous binder. The modified CMC is the low-viscosity modified CMC for lithium battery separators as described in any one of claims 1-4 or the modified CMC prepared by the method described in any one of claims 5-7.

9. The lithium battery separator as described in claim 8, characterized in that, The ceramic particles are at least one of alumina, silica, and boehmite, with a particle size D50 of 0.3~1.5μm; the water-based binder is at least one of styrene-butadiene rubber or polyacrylate binder; the polyolefin-based film is polyethylene, polypropylene, or a multilayer composite film thereof, with a thickness of 7~25μm; and the ceramic coating has a thickness of 3~6μm.

10. A method for preparing a lithium battery separator as described in any one of claims 8 or 9, characterized in that, The process includes the following steps: mixing modified CMC, ceramic particles, water-based binder and deionized water to prepare a ceramic coating slurry with a solid content of 20-40 wt%; coating the slurry onto at least one side of a polyolefin-based film by microgravure coating or dip coating; drying with hot air at 50-80°C and winding it up to obtain the lithium battery separator.

Images