A natural rubber / conductive polymer composite binder for dry electrode and a method for preparing the same, a dry electrode and a method for preparing the same

Abstract

The present application relates to the technical field of lithium ion batteries, and particularly relates to a natural rubber / conductive polymer composite binder for dry-process electrodes and a preparation method thereof, a dry-process electrode and a preparation method thereof.The present application comprises the following steps: S1, freeze-drying natural rubber latex to obtain natural rubber powder; low-temperature plasma treatment is performed on the natural rubber powder; S2, high-speed mixing the surface-activated natural rubber powder with a conductive monomer, an initiator and a crosslinking agent to obtain a premixed powder; S3, performing a crosslinking reaction, extruding, cooling and crushing to obtain a primary composite binder powder; and S4, screening the primary composite binder powder to obtain the natural rubber / conductive polymer composite binder for dry-process electrodes.The present application obtains a composite binder with high bonding strength, excellent conductivity and stable electrochemical performance through an in-situ polymerization-molecular chain entanglement-crosslinking synergistic modification mechanism, and the preparation process is a full dry process without solvent, which meets the requirements of green manufacturing.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a natural rubber / conductive polymer composite binder for dry electrode and its preparation method, and a dry electrode and its preparation method. Background Technology

[0002] Dry electrode technology has become an important development direction in lithium-ion battery manufacturing processes due to its advantages such as solvent-free operation, low energy consumption, environmental friendliness, and ability to fabricate high-load, thick electrodes. As a key component of dry electrodes, the binder must effectively bond the active material and conductive agent while ensuring a strong bond between the electrode and the current collector. Currently, the mainstream dry electrode binder is polytetrafluoroethylene (PTFE), which relies on fibrosis under high pressure and high shear conditions to achieve its bonding function. However, it has several drawbacks: First, PTFE is a fluorinated binder, and the CF bonds in its molecular structure are prone to irreversible reactions with the negative electrode material at low potentials, reducing the battery's initial efficiency and generating harmful byproducts, affecting the battery's cycle stability. Second, PTFE has strong chemical inertness, weak interfacial interaction with the active material and current collector, and insufficient viscoelasticity. During charge and discharge, it cannot adapt to the expansion and contraction of the active material, easily leading to active material detachment, electrode cracking, and consequently, rapid capacity decay. Third, PTFE lacks electronic conductivity and ion transport capabilities, which increases the ion tortuosity inside the electrode, reduces charge transport efficiency, and limits the battery's high-rate performance.

[0003] To address the aforementioned shortcomings of PTFE, existing technologies primarily employ two approaches: First, chemical modification of PTFE, such as the use of polycyclic aromatic hydrocarbon-alkali metal complexes in patent CN119351005 A. However, this method requires the use of aprotic polar solvents, posing a risk of solvent residue, and is complex and costly. Second, the development of non-fluorinated binders to replace PTFE, such as acrylate and acrylamide copolymers. However, these non-fluorinated binders generally suffer from a single bonding mechanism and insufficient interfacial bonding strength, and require solvent polymerization processes for preparation, contradicting the solvent-free production concept of dry electrodes. Furthermore, their electrochemical stability and high-temperature tolerance are insufficient to meet the requirements of high-energy-density batteries, making it difficult to achieve a complete replacement of PTFE.

[0004] Therefore, research on non-fluorinated binders remains a hot topic in the field. Summary of the Invention

[0005] This invention provides a natural rubber / conductive polymer composite binder for dry electrodes and its preparation method. Through an in-situ polymerization-molecular chain entanglement-crosslinking synergistic modification mechanism, a composite binder with high bonding strength, excellent conductivity and stable electrochemical performance is obtained. Moreover, the preparation process is entirely dry and solvent-free, which meets the requirements of green manufacturing.

[0006] To achieve the above objectives, the present invention provides a method for preparing a natural rubber / conductive polymer composite binder for dry-process electrodes, comprising the following steps: S1. Natural rubber pretreatment: Natural rubber latex is freeze-dried to obtain natural rubber powder; the natural rubber powder is then subjected to low-temperature plasma treatment to obtain surface-activated natural rubber powder; the surface-activated natural rubber powder has a hydroxyl group mass fraction of 0.8%-2.3% and a carbonyl group mass fraction of 0.5%-1.6%; S2. Preparation of premixed system: Surface-activated natural rubber powder is mixed with conductive monomer, initiator and crosslinking agent at a mass ratio of 100:5-20:0.5-3:0.1-0.5 at high speed to obtain premixed powder; S3. In-situ polymerization-crosslinking synergistic reaction: The premixed powder is transferred to a twin-screw extruder. Under inert gas protection, the screw temperature is controlled at 80-120℃ and the rotation speed is 200-400 r / min for crosslinking reaction, and the reaction time is 10-25 min. After the reaction is completed, the powder is extruded, cooled and pulverized to obtain primary composite binder powder. S4. Finished product preparation: The primary composite binder powder is sieved to obtain the natural rubber / conductive polymer composite binder for the dry electrode.

[0007] The preparation method of this invention first involves freeze-drying and low-temperature plasma pretreatment of natural rubber. Freeze-drying preserves the elastic structure of the rubber and prevents high-temperature embrittlement. Low-temperature plasma treatment introduces hydroxyl groups (hydroxyl groups) onto the surface of the natural rubber. The mechanism of the formation of oxygen-containing active groups such as OH and carbonyl (C=O) in the plasma is as follows: under an inert gas atmosphere, argon / nitrogen gas is excited in the plasma device to generate high-energy particles (electrons, ions, metastable atoms, and free radicals, etc.). These high-energy particles bombard the natural rubber molecular chain, causing CH bonds to break and generating surface free radicals. The free radicals further react with trace amounts of oxygen or water molecules in the system to form oxygen-containing active groups. Under a functional gas atmosphere, oxygen, carbon dioxide, water vapor, etc., directly participate in the plasma reaction, and through oxidation and functional group transfer, hydroxyl and carbonyl groups are efficiently generated in situ on the rubber surface. Under a small molecule monomer vapor atmosphere, volatile monomers such as acrylic acid and ethyl hydroxyacrylate undergo gas-phase graft polymerization under plasma initiation, covalently bonding hydroxyl and carbonyl groups to the rubber molecular chain. Under a macromolecular crosslinking agent blend system, macromolecules rich in oxygen-containing functional groups such as polyvinyl alcohol and polyacrylic acid are activated by plasma and bonded to the rubber molecular chain, simultaneously introducing hydroxyl and carbonyl groups. These active groups can serve as initiation or anchoring sites for the in-situ polymerization of conductive monomers (pyrrole / aniline), significantly improving the grafting uniformity and interfacial bonding strength of conductive polymers on the rubber skeleton. This invention prepares the product by mixing surface-activated natural rubber powder with conductive monomers, initiators, and crosslinking agents. The unsaturated double bonds in the natural rubber molecular chain undergo intermolecular crosslinking reactions initiated by free radicals generated from the thermal decomposition of the crosslinking agent, forming a continuous three-dimensional elastic skeleton. The conductive monomers undergo oxidative polymerization under the action of the initiator, forming conjugated conductive polymer chains. These conductive polymer chains interpenetrate with the natural rubber skeleton through molecular chain entanglement and form hydrogen bonds and partial covalent bonds with the hydroxyl and carbonyl groups on the surface of the natural rubber, thus embedding and filling the pores of the three-dimensional elastic skeleton of the natural rubber, forming an interpenetrating symbiotic structure of "elastic skeleton-conductive network".

[0008] Preferably, the freeze-drying conditions in S1 are: vacuum degree 0.01-0.05MPa, freezing temperature -40 ~ -60℃, and drying time 12-24h; the low-temperature plasma treatment is carried out under one or more of the following conditions: inert gas, functional gas atmosphere, small molecule monomer vapor or macromolecular crosslinking agent system, with a power of 10-30W for 5-15min.

[0009] The functional gas includes one or more of oxygen, carbon dioxide, water vapor, or compressed air, with a gas flow rate of 20-50 sccm. The functional gas plasma directly introduces hydroxyl and carbonyl groups onto the rubber surface through an oxidation reaction.

[0010] The inert gas includes argon and / or nitrogen, which can be used in combination with functional gases. The inert gas plasma generates free radicals by bombarding the surface of natural rubber with high-energy particles.

[0011] The small molecule monomers include one or more volatile monomers selected from acrylic acid, methacrylic acid, ethyl hydroxyacrylate, methyl hydroxymethacrylate, ethanol, and acetic acid, with a monomer vapor partial pressure of 5-20 Pa. The small molecule monomers undergo gas-phase graft polymerization initiated by plasma, covalently bonding hydroxyl and carbonyl groups to the rubber molecular chain.

[0012] The macromolecular crosslinking agent includes one or more of polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyacrylic acid (PAA), and carboxymethyl cellulose (CMC), and participates in plasma treatment in the form of powder blends or vapor. After being activated by plasma, the macromolecular crosslinking agent introduces hydroxyl and carbonyl groups onto the rubber surface.

[0013] Preferably, the conductive monomer in S2 includes one or more of pyrrole monomers, aniline monomers, or pyrrole-aniline complex monomers.

[0014] Preferably, the initiator in S2 includes one or more of ammonium persulfate or potassium persulfate, and the crosslinking agent includes one or more of dicumyl peroxide (DCP) or bis(tert-butylperoxyisopropyl)benzene (BIPB).

[0015] Preferably, the high-speed mixing described in S2 is carried out in a high-speed mixer with a mixing temperature of 50-80℃, a rotation speed of 500-1500 r / min, and a mixing time of 10-30 min.

[0016] Preferably, the screw length-to-diameter ratio of the twin-screw extruder in S3 is 30-40:1, and the temperature of each section is set as follows: 80-90℃ for the feeding section, 90-120℃ for the reaction section, and 80-90℃ for the discharge section.

[0017] The degree of crosslinking is controlled by adjusting the screw speed and reaction temperature. The degree of crosslinking is controlled within the range of 30%-50%, which can avoid the weakening of adhesion caused by insufficient crosslinking or the embrittlement caused by excessive crosslinking.

[0018] Preferably, the sieving in S4 specifically includes: first removing large particles through a 15-25μm sieve, and then collecting the particles retained by a 3-7μm sieve.

[0019] Under the same technical concept, the present invention also provides a natural rubber / conductive polymer composite binder for dry-process electrodes, wherein the composite binder is prepared by the aforementioned preparation method for the natural rubber / conductive polymer composite binder for dry-process electrodes; the composite binder comprises a natural rubber elastic skeleton, a conductive polymer conductive network, and a cross-linked three-dimensional network structure, with a number-average molecular weight (Mn) of 1.2 × 10⁻⁶. 5 -2.5×10 5The conductive polymer has a mass fraction of 4.8%-16.9%, the crosslinking agent has a mass fraction of 0.1%-0.5%, the natural rubber has a mass fraction of 82.6%-95.1%, and the degree of crosslinking is 30%-50%.

[0020] The number-average molecular weight was calculated by gel permeation chromatography (GPC); the degree of crosslinking was tested by Soxhlet extraction, with the following steps: Weigh 0.5g of the composite adhesive product (accurate to 0.0001g, denoted as m0), wrap it in degreased cotton and place it in a Soxhlet extractor; use toluene as solvent and reflux in an 80℃ constant temperature water bath for 24h to remove uncrosslinked linear molecules (natural rubber oligomers, unbound conductive polymer monomers, etc.); place the residual gel sample after extraction in an 80℃ vacuum drying oven and dry for 12h to constant weight, cool to room temperature and weigh (denoted as m1); calculate the degree of crosslinking according to the formula: Degree of crosslinking (%) = (m1 / m0) × 100%.

[0021] Under the same technical concept, the present invention also provides a dry electrode, wherein the raw materials of the dry electrode include the composite binder prepared by the preparation method of the natural rubber / conductive polymer composite binder for dry electrodes, or the composite binder, active material and conductive agent, wherein the mass ratio of the composite binder, active material and conductive agent is 5-15:70-85:5-15.

[0022] Under the same technical concept, the present invention also provides a method for preparing a dry electrode, comprising the following steps: The composite binder, active material, and conductive agent are placed in a dry mixer and mixed for 20-40 minutes at a speed of 800-1200 r / min to obtain an electrode mixture. The electrode mixture is then pressed onto a current collector and calendered at a pressure of 100-300 bar and a temperature of 60-100℃ to obtain a dry electrode. The active material is one of lithium iron phosphate, lithium manganese iron phosphate, ternary materials, graphite or silicon-carbon composite materials, and the conductive agent is one or more of carbon black, graphene or carbon nanotubes. The current collector of the dry electrode is aluminum foil or copper foil.

[0023] The above-described solution of the present invention has the following beneficial effects: (1) The preparation of the natural rubber / conductive polymer composite adhesive of the present invention adopts a synergistic process of "low-temperature plasma pretreatment - in-situ polymerization - crosslinking - mechanical entanglement", which does not require the use of any solvent and realizes the preparation by a completely dry method, avoiding the defects of solvent residue and complicated steps in the existing modification process. The oxygen-containing active groups (such as -OH, C=O) introduced on the surface of natural rubber by low-temperature plasma pretreatment not only increase the surface energy, but also serve as adsorption and polymerization active sites for conductive monomers (pyrrole / aniline). Specifically, the hydroxyl group can act as a proton donor to form hydrogen bond interaction with the amino group (-NH2) in the conductive monomer (pyrrole / aniline) molecule, so that the conductive monomer molecule is directionally adsorbed on the surface of natural rubber, increasing the local monomer concentration; at the same time, the surface free radicals remaining at the active sites can act as polymerization initiation sites, and undergo addition reaction with the unsaturated double bond of the conductive monomer, initiating the in-situ oxidative polymerization reaction of the conductive monomer, so that the conductive polymer chain is directly grafted onto the surface of the natural rubber skeleton. This synergistic effect of directional adsorption and chemical bonding ensures that the conductive polymer is uniformly dispersed in the natural rubber matrix, forming a continuous conductive network and preventing the agglomeration of conductive components; the cross-linking reaction forms a stable three-dimensional network structure, which can significantly enhance the cohesive force of the binder and the interfacial bonding force between it and the active material and current collector; the mechanical entanglement further enhances the interfacial bonding force between natural rubber and conductive polymer, improving the stability of the composite system. (2) The natural rubber / conductive polymer composite adhesive prepared by the present invention is a non-fluorinated adhesive. Natural rubber provides excellent viscoelasticity and mechanical toughness, which can effectively adapt to the volume change of active material during charging and discharging and prevent material from falling off. The conductive polymer network formed by in-situ polymerization can significantly improve the electronic conductivity and ion transport efficiency of the adhesive. (3) The raw materials for the preparation of the composite adhesive of the present invention are widely available and environmentally friendly. Natural rubber is a renewable resource, and the conductive monomers and crosslinking agents are inexpensive. The preparation process does not require high temperature and high pressure equipment, and it is easy to scale up production, which has significant economic and environmental benefits. Detailed Implementation

[0024] To make the technical problems, solutions, and advantages of this invention clearer, a detailed description will be provided below with reference to specific embodiments. Obviously, the described embodiments are only a portion, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0025] In the description of this invention, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0026] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a locking connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0027] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] Example 1 Example 1 provides a natural rubber / conductive polymer composite binder for dry electrode and its preparation method, comprising the following steps: S1. Natural rubber pretreatment: Natural rubber latex was placed in a freeze dryer and dried for 18 hours under a vacuum of 0.03 MPa and a freezing temperature of -50℃ to obtain natural rubber powder; the natural rubber powder was placed in a low-temperature plasma treatment device, and a functional gas mixture of argon and oxygen with a volume ratio of 5:1 was introduced as the plasma treatment raw material (flow rate 30 sccm) and treated with a power of 20W for 10 minutes to obtain surface-activated natural rubber powder; XPS characterization revealed that the powder surface contained 1.5% hydroxyl groups and 0.9% carbonyl groups by mass. S2. Preparation of premixed system: 100g of surface-activated natural rubber powder, 10g of pyrrole monomer, 1.5g of ammonium persulfate and 0.3g of DCP were placed in a high-speed mixer and mixed for 20min at 65℃ and 1000r / min to obtain premixed powder; S3. In-situ polymerization-crosslinking synergistic reaction: The premixed powder is transferred to a twin-screw extruder (screw length-to-diameter ratio 35:1), argon gas is introduced for protection, and the temperature of each section of the screw is controlled as follows: feed section 85℃, reaction section 100℃, discharge section 85℃, screw speed 300 r / min, reaction time 18 min, and crosslinking degree controlled at 40%; after the reaction is completed, the powder is extruded, cooled, and pulverized to obtain primary composite binder powder; The composite adhesive M passed GPC testing. n 1.8×10 5 .

[0029] S4. Finished product preparation: The primary composite adhesive powder is sieved through a vibrating screen. First, large particles are removed by passing through a 20μm screen, and then the particles retained by a 5μm screen are collected, which is the natural rubber / conductive polymer composite adhesive.

[0030] This embodiment also provides a method for preparing a dry electrode, including the following steps: The above-mentioned composite binder, NCM811 active material, and conductive carbon black were placed in a dry mixer at a mass ratio of 10:80:10 and mixed for 30 minutes at a speed of 1000 r / min to obtain an electrode mixture. The electrode mixture was pressed onto an aluminum current collector and rolled into shape under a pressure of 200 bar and a temperature of 80°C to obtain a dry electrode sheet.

[0031] Example 2 Example 2 provides a natural rubber / conductive polymer composite binder for dry electrode and its preparation method, including the following steps: S1. Natural rubber pretreatment: Natural rubber latex was placed in a freeze dryer and dried for 24 hours under a vacuum of 0.01 MPa and a freezing temperature of -40℃ to obtain natural rubber powder; the natural rubber powder was placed in a low-temperature plasma treatment device, and a mixture of nitrogen and carbon dioxide functional gas with a volume ratio of 4:1 was introduced as the plasma treatment raw material (flow rate 20 sccm) and treated with a power of 10W for 15 minutes to obtain surface-activated natural rubber powder; XPS characterization revealed that the powder surface contained 0.8% hydroxyl groups and 0.5% carbonyl groups by mass. S2. Preparation of premixed system: 100g of surface-activated natural rubber powder, 5g of aniline monomer, 0.5g of potassium persulfate and 0.1g of DCP were placed in a high-speed mixer and mixed for 30min at 50℃ and 500r / min to obtain premixed powder; S3. In-situ polymerization-crosslinking synergistic reaction: The premixed powder is transferred to a twin-screw extruder (screw length-to-diameter ratio 30:1), nitrogen gas is introduced for protection, and the temperature of each section of the screw is controlled as follows: feed section 80℃, reaction section 90℃, discharge section 80℃, screw speed 200 r / min, reaction time 25 min, and crosslinking degree controlled at 30%; after the reaction is completed, the primary composite binder powder is obtained by extrusion, cooling and pulverization. The composite adhesive M passed GPC testing. n 1.2×10 5 ; S4. Finished product preparation: The primary composite adhesive powder is sieved through a vibrating screen. First, large particles are removed by passing through a 20μm screen, and then the particles retained by a 5μm screen are collected, which is the natural rubber / conductive polymer composite adhesive.

[0032] The method for preparing a dry electrode includes the following steps: The above-mentioned composite binder, LFP active material, and conductive carbon black were placed in a dry mixer at a mass ratio of 5:85:10 and mixed for 40 minutes at a speed of 800 r / min to obtain an electrode mixture. The electrode mixture was pressed onto an aluminum current collector and rolled into shape at a pressure of 100 bar and a temperature of 60°C to obtain a dry electrode sheet.

[0033] Example 3 Example 3 provides a natural rubber / conductive polymer composite binder for dry-process electrodes and its preparation method, including the following steps: S1. Natural rubber pretreatment: Natural rubber latex is placed in a freeze dryer and dried for 12 hours under a vacuum of 0.05 MPa and a freezing temperature of -60℃ to obtain natural rubber powder; the natural rubber powder is placed in a low-temperature plasma treatment device, and acrylic acid small molecule monomer vapor is introduced as plasma treatment raw material. The monomer vapor partial pressure is controlled at 10 Pa, and the treatment is carried out at a power of 30 W for 5 minutes to obtain surface-activated natural rubber powder. XPS characterization revealed that the powder surface contained 2.3% hydroxyl groups and 1.6% carbonyl groups by mass. S2. Preparation of premixed system: 100g of surface-activated natural rubber powder, 20g of pyrrole monomer, 3g of ammonium persulfate and 0.5g of DCP were placed in a high-speed mixer and mixed for 10min at 80℃ and 1500r / min to obtain premixed powder; S3. In-situ polymerization-crosslinking synergistic reaction: The premixed powder is transferred to a twin-screw extruder (screw length-to-diameter ratio 40:1), argon gas is introduced for protection, and the temperature of each section of the screw is controlled as follows: feed section 90℃, reaction section 120℃, discharge section 90℃, screw speed 400r / min, reaction time 10min, and crosslinking degree controlled at 50%; after the reaction is completed, the powder is extruded, cooled, and pulverized to obtain primary composite binder powder; The composite adhesive M passed GPC testing. n 2.5×10 5 ; S4. Finished product preparation: The primary composite adhesive powder is sieved through a vibrating screen. First, large particles are removed by passing through a 20μm screen, and then the particles retained by a 5μm screen are collected, which is the natural rubber / conductive polymer composite adhesive.

[0034] The method for preparing a dry electrode includes the following steps: The above-mentioned composite binder, NCM622 active material, and conductive carbon black were placed in a dry mixer at a mass ratio of 15:70:15 and mixed for 20 minutes at a speed of 1200 r / min to obtain an electrode mixture. The electrode mixture was pressed onto an aluminum current collector and rolled into shape under a pressure of 300 bar and a temperature of 100 °C to obtain a dry electrode sheet.

[0035] Example 4 Example 4 provides a natural rubber / conductive polymer composite binder for dry-process electrodes and its preparation method, comprising the following steps: S1. Natural rubber pretreatment: Natural rubber latex is placed in a freeze dryer and dried for 20 hours under a vacuum of 0.02 MPa and a freezing temperature of -55℃ to obtain natural rubber powder; Natural rubber powder is placed in a low-temperature plasma treatment device, and nitrogen is introduced as the plasma treatment raw material (flow rate 40 sccm) and treated with a power of 25W for 8 minutes to obtain surface-activated natural rubber powder. XPS characterization revealed that the powder surface contained 1.9% hydroxyl groups and 1.2% carbonyl groups by mass. S2. Preparation of premixed system: 100g of surface-activated natural rubber powder, 15g of pyrrole-aniline composite monomer (mass ratio 1:1), 2.0g of ammonium persulfate and 0.4g of DCP were placed in a high-speed mixer and mixed for 15min at 70℃ and 1200r / min to obtain premixed powder; S3. In-situ polymerization-crosslinking synergistic reaction: The premixed powder is transferred to a twin-screw extruder (screw length-to-diameter ratio 38:1), nitrogen gas is introduced for protection, and the temperature of each section of the screw is controlled as follows: feed section 88℃, reaction section 110℃, discharge section 88℃, screw speed 350 r / min, reaction time 14 min, and crosslinking degree controlled at 45%; after the reaction is completed, the primary composite binder powder is obtained by extrusion, cooling and pulverization. The composite adhesive M passed GPC testing. n 2.1×10 5 ; S4. Finished product preparation: The primary composite adhesive powder is sieved through a vibrating screen. First, large particles are removed by passing through a 20μm screen, and then the particles retained by a 5μm screen are collected, which is the natural rubber / conductive polymer composite adhesive.

[0036] The method for preparing a dry electrode includes the following steps: The above-mentioned composite binder, NCM900 high-nickel active material, and conductive carbon black-graphene composite conductive agent (mass ratio 1:1) were placed in a dry mixer at a mass ratio of 12:75:13 and mixed for 25 minutes at a speed of 1100 r / min to obtain an electrode mixture. The electrode mixture was pressed onto an aluminum current collector and rolled into shape under a pressure of 250 bar and a temperature of 90 °C to obtain a dry electrode sheet.

[0037] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that: commercially available PTFE powder (particle size 5-20 μm) was used directly as a binder, and its M... n It is 6.2×10 5 It has no cross-linked structure.

[0038] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the natural rubber latex in step S1 is replaced with an equal mass of acrylate resin (PAA, M). n 1.5×10 5 The remaining steps (freeze-drying conditions, low-temperature plasma treatment parameters, premixing ratio and conditions, in-situ polymerization-crosslinking parameters, finished product sieving and dry electrode preparation process) were completely consistent with Example 1. XPS characterization showed that the acrylate resin had a surface hydroxyl mass fraction of 0.7% and a carbonyl mass fraction of 1.1%; GPC testing showed that the composite binder M... n 1.6×10 5 .

[0039] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that in step S1, the natural rubber latex is not freeze-dried, but instead dried in a forced-air dryer at 60°C and normal pressure for 24 hours to obtain rubber powder. The remaining steps (low-temperature plasma treatment parameters, premixing ratio and conditions, in-situ polymerization-crosslinking parameters, finished product sieving, and dry electrode preparation process) are completely consistent with Example 1. XPS characterization showed that the surface hydroxyl content of the rubber powder was 0.6% and the carbonyl content was 0.3%. GPC testing showed that the composite binder M... n 1.4×10 5 .

[0040] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the in-situ polymerization reaction temperature in step S3 is 70°C, while the remaining steps (natural rubber pretreatment, premixing ratio and conditions, screw speed, reaction time, finished product sieving, and dry electrode preparation process) are completely consistent with Example 1. The composite binder was tested and found to have a crosslinking degree of 22%; through GPC testing, M... n 1.0×10 5 .

[0041] Detection methods and results Number-average molecular weight (Mn) test: Test method: Weigh 10 mg of the composite adhesive product (accurate to 0.0001 g), place it in a 10 mL volumetric flask, add tetrahydrofuran (THF, chromatographic grade, water content ≤0.05%) as solvent, seal and shake in a constant temperature shaker at 25 °C for 24 h to completely dissolve the sample; filter the solution through a 0.22 μm organic phase filter membrane to remove insoluble impurities, and obtain a test sample solution with a concentration of 1 mg / mL; A Waters 1515 gel permeation chromatograph equipped with Waters Styragel HR1, HR3, and HR4E tandem columns (300 mm × 7.8 mm) was used. THF was used as the mobile phase, the flow rate was 1.0 mL / min, the column temperature was 35 °C, and the detection temperature was 35 °C using a refractive index detector (RID). A series of polystyrene (PS) standards with known number average molecular weights (molecular weight range 5 × 10³ - 1 × 10⁻⁶) were used. 6 A calibration curve was plotted and fitted using GPC software. The sample solution was injected into the syringe at a volume of 20 μL, and the chromatogram was recorded. The M value of the sample was calculated using the calibration curve. n .

[0042] Electrode-current collector bonding strength test: Test method: The polyester tape is flatly attached to the electrode plate and cut into a rectangular sample of 100mm×25mm. The peel force is tested at 180° using a universal testing machine at a speed of 50mm / s.

[0043] Electrode cohesive force test: Test method: Cut the electrode sheet into 50mm×50mm square samples and weigh the initial mass m0 of the sample using an electronic balance. Cut polyester tape of the same size as the sample, remove the protective film from the tape surface, and flatly adhere the tape to the surface of the active material layer of the sample. Holding the free end of the tape, peel the tape off rapidly at a speed of 50mm / s in a 180° direction, ensuring that the peeling process is continuous and uninterrupted. After peeling, immediately weigh the remaining mass m1 of the sample using an electronic balance. The cohesive force of the electrode is calculated using the following formula: Cohesion (N / mm) = (m0-m1) / m0×K, where K is a correction factor (K=0.5).

[0044] Ion tortuosity test: Test method: Assemble symmetrical coin cells and perform electrochemical impedance spectroscopy (EIS) using an electrochemical workstation. The test frequency range was 0.01~100000Hz, and the amplitude was 5mV. The Nyquist curve was fitted using ZView software, employing an equivalent circuit model R. s -(R int / / CPE), where R s R is the solution impedance. int Let R be the ion transport impedance within the electrode, and CPE be a constant-phase element. The ion tortuosity (τ) is calculated using the Bruggeman equation and impedance data, with the formula: τ = (R / τ) int ×ρ e ×L) / d, where: ρ e Electrolyte resistivity (ρ) e =80Ω·cm), L is the thickness of the active material layer of the electrode sheet, and d is the packing density of the active material layer of the electrode (calculated from the sample mass, area and thickness, d=m / (S×L)).

[0045] Electrochemical performance testing: Test method: In an argon-protected glove box (H2O<0.1ppm, O2<0.1ppm), using the dry electrode prepared above as the positive electrode, graphite as the negative electrode, Celgard 2400 as the separator, and a 1M LiPF6 solution dissolved in ethylene carbonate / diethyl carbonate / fluoroethylene carbonate (EC / DMC / EMC, volume ratio 1:1:1) as the electrolyte, a CR2032 coin cell was assembled.

[0046] Perform constant current and constant voltage charging test, charge to 4.3V at a rate of 0.1C, cut off current of 0.01C, and record the charging capacity C1; then discharge to 3.0V at a constant current rate of 0.1C and record the discharge capacity C2; calculate the first efficiency using the following formula: First efficiency (%) = (C2 / C1) × 100%.

[0047] A constant current charge-discharge cycle test was performed, charging to 4.3V at a constant current rate of 0.3C, with a cutoff current of 0.05C, and discharging to 3.0V at a constant current rate of 0.3C; 1000 cycles were performed. The capacity retention rate was calculated using the following formula: Capacity retention rate (%) = (Discharge capacity in the nth cycle / Discharge capacity in the first cycle) × 100%.

[0048] Table 1 Summary of test results for examples and comparative examples

[0049] Test Result Analysis: (1) The bonding force between the electrode and the current collector prepared in Examples 1-4 (0.35-0.40 N / mm) was increased by 34.6%-53.8% compared with Comparative Example 1 (0.26 N / mm), and the cohesive force of the electrode (0.22-0.26 N / mm) was increased by 22.2%-44.4% compared with Comparative Example 1 (0.18 N / mm). This indicates that the excellent viscoelasticity and three-dimensional network structure of the binder of the present invention significantly improve the cohesive force of the electrode and the interfacial bonding force between it and the current collector.

[0050] (2) The ion tortuosity (1.22-1.28) of Examples 1-4 is much lower than that of Comparative Example 1 (2.1), indicating that the polypyrrole / polyaniline conductive network formed by the in-situ polymerization of the binder of the present invention is uniformly distributed in the natural rubber skeleton, constructing a continuous ion / electron transport channel, reducing the transport path of ions inside the electrode, and thus improving the electronic conductivity and ion transport efficiency of the electrode.

[0051] (3) The first-cycle efficiency (95.0%-96.2%) of the batteries in Examples 1-4 was higher than that of Comparative Example 1 (89.6%). This is because the composite binder of the present invention is a non-fluorinated binder, which reduces the occurrence of irreversible side reactions compared to PTFE, thus improving the charge utilization rate during the first charge and discharge cycle. In addition, the capacity retention rate (88.1%-90.5%) of the batteries in Examples 1-4 after 1000 cycles at 0.3C rate was much higher than that of Comparative Example 1 (72.2%), indicating that the binder of the present invention can stabilize the long-term cycle performance of the battery.

[0052] (4) The bonding force between the electrode and the current collector (0.23 N / mm), the cohesive force of the electrode (0.16 N / mm), and the cycle capacity retention rate (78.5%) prepared in Comparative Example 2 (natural rubber replaced with acrylate resin) were significantly lower than those in Example 1. This indicates that the elastic skeleton structure of natural rubber is the key to improving the bonding force and cycle stability of the binder interface. The unsaturated double bonds and flexible structure in its molecular chain can form a more stable interpenetrating network with the conductive polymer. In contrast, acrylate resin is rigid and cannot adapt to changes in the volume of the active material, resulting in a decrease in performance.

[0053] (5) The surface hydroxyl (0.6%) and carbonyl (0.3%) mass fractions of natural rubber (without freeze drying) in Comparative Example 3 were lower than those in Example 1. The corresponding electrode bonding force, cohesion and cycling performance were not as good as those in Example 1. This verifies that freeze drying can preserve the elastic structure of natural rubber, providing a basis for the formation of active sites and subsequent graft polymerization. Conventional blower drying will lead to cross-linking and embrittlement of rubber molecular chains, a reduction in surface active sites, and thus affect the overall performance of the composite adhesive.

[0054] (6) The degree of crosslinking (22%) of Comparative Example 4 (in-situ polymerization temperature 70℃) was lower than that of Example 1 (40%), the degree of ion tortuosity (1.5) was higher than that of Example 1 (1.25), and the cycle capacity retention rate (84.1%) was lower than that of Example 1 (89.2%). This indicates that when the in-situ polymerization temperature is lower than the range specified in this application, the initiator decomposes incompletely, the polymerization efficiency of conductive monomers decreases, and the crosslinking reaction is insufficient, resulting in discontinuous conductive network and insufficient stability of three-dimensional network structure, which in turn affects the ion transport efficiency and cycle stability of the electrode. This verifies the rationality of the in-situ polymerization temperature range of 80-120℃ specified in this application.

Claims

1. A method for preparing a natural rubber / conductive polymer composite binder for dry-process electrodes, characterized in that, Includes the following steps: S1. Natural rubber pretreatment: Natural rubber latex is freeze-dried to obtain natural rubber powder; the natural rubber powder is then subjected to low-temperature plasma treatment to obtain surface-activated natural rubber powder; the surface-activated natural rubber powder has a hydroxyl group mass fraction of 0.8%-2.3% and a carbonyl group mass fraction of 0.5%-1.6%; S2. Preparation of premixed system: Surface-activated natural rubber powder is mixed with conductive monomer, initiator and crosslinking agent at a mass ratio of 100:5-20:0.5-3:0.1-0.5 at high speed to obtain premixed powder; S3. In-situ polymerization-crosslinking synergistic reaction: The premixed powder is transferred to a twin-screw extruder. Under inert gas protection, the screw temperature is controlled at 80-120℃ and the rotation speed is 200-400 r / min for crosslinking reaction, and the reaction time is 10-25 min. After the reaction is completed, the powder is extruded, cooled and pulverized to obtain primary composite binder powder. S4. Finished product preparation: The primary composite binder powder is sieved to obtain the natural rubber / conductive polymer composite binder for the dry electrode.

2. The preparation method according to claim 1, characterized in that, The freeze-drying conditions described in S1 are: vacuum degree 0.01-0.05MPa, freezing temperature -40 ~ -60℃, and drying time 12-24h; the low-temperature plasma treatment is carried out under one or more of the following conditions: inert gas, functional gas atmosphere, small molecule monomer vapor or macromolecular crosslinking agent system, with a power of 10-30W for 5-15min. The functional gas includes one or more of oxygen, carbon dioxide, water vapor, or compressed air, and the gas flow rate is 20-50 sccm; The inert gas includes argon and / or nitrogen; The small molecule monomers include one or more volatile monomers selected from acrylic acid, methacrylic acid, ethyl hydroxyacrylate, methyl hydroxymethacrylate, ethanol, and acetic acid, with a monomer vapor partial pressure of 5-20 Pa. The macromolecular crosslinking agent includes one or more of polyvinyl alcohol, polyethylene glycol, polyacrylic acid, or carboxymethyl cellulose, and participates in plasma treatment in the form of powder blend or vapor.

3. The preparation method according to claim 1, characterized in that, The conductive monomer described in S2 includes one or more of pyrrole monomers, aniline monomers, or pyrrole-aniline complex monomers.

4. The preparation method according to claim 1, characterized in that, The initiator in S2 includes one or more of ammonium persulfate or potassium persulfate, and the crosslinking agent includes one or more of dicumyl peroxide or bis(tert-butylperoxyisopropyl)benzene.

5. The preparation method according to claim 1, characterized in that, The high-speed mixing described in S2 is carried out in a high-speed mixer at a mixing temperature of 50-80℃, a rotation speed of 500-1500 r / min, and a mixing time of 10-30 min.

6. The preparation method according to claim 1, characterized in that, The twin-screw extruder described in S3 has a screw length-to-diameter ratio of 30-40:1, and the temperature settings for each section are: 80-90℃ for the feeding section, 90-120℃ for the reaction section, and 80-90℃ for the discharge section.

7. The preparation method according to claim 1, characterized in that, The sieving process described in S4 specifically includes: first removing large particles through a 15-25μm sieve, and then collecting the particles retained by a 3-7μm sieve.

8. A natural rubber / conductive polymer composite binder for dry-process electrodes, characterized in that, The composite binder is prepared using the method described in any one of claims 1-7 for preparing a natural rubber / conductive polymer composite binder for dry-process electrodes; the composite binder comprises a natural rubber elastic skeleton, a conductive polymer conductive network, and a cross-linked three-dimensional network structure, with a number-average molecular weight of 1.2 × 10⁻⁶. 5 -2.5×10 5 The conductive polymer has a mass fraction of 4.8%-16.9%, the crosslinking agent has a mass fraction of 0.1%-0.5%, the natural rubber has a mass fraction of 82.6%-95.1%, and the degree of crosslinking is 30%-50%.

9. A dry electrode, characterized in that, The raw materials for the dry electrode include a composite binder prepared by the method for preparing a natural rubber / conductive polymer composite binder for a dry electrode as described in any one of claims 1-7, or a composite binder, active material and conductive agent as described in claim 8, wherein the mass ratio of the composite binder, active material and conductive agent is 5-15:70-85:5-15.

10. A method for preparing a dry electrode as described in claim 9, characterized in that, Includes the following steps: The composite binder, active material, and conductive agent are placed in a dry mixer and mixed for 20-40 minutes at a speed of 800-1200 r / min to obtain an electrode mixture. The electrode mixture is then pressed onto a current collector and calendered at a pressure of 100-300 bar and a temperature of 60-100℃ to obtain a dry electrode. The active material is one of lithium iron phosphate, lithium manganese iron phosphate, ternary materials, graphite or silicon-carbon composite materials, and the conductive agent is one or more of carbon black, graphene or carbon nanotubes. The current collector of the dry electrode is aluminum foil or copper foil.

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