Method for preparing modified polymer composition for high compaction energy storage cell pole piece bonding
By grafting flexible spacer phosphate monomers onto the main chain of vinylidene fluoride-hexafluoropropylene copolymer, a gradient bonding network was constructed, which solved the problem of reduced peel strength of electrode bonding network under high pressure conditions and achieved stability and stress dissipation of high energy density battery.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- 益阳长天新能源科技有限公司
- Filing Date
- 2026-03-17
- Publication Date
- 2026-06-09
Smart Images
Figure CN121851939B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of energy storage cell electrode bonding technology, and particularly relates to a method for preparing a modified polymer composition for bonding high voltage energy storage cell electrodes. Background Technology
[0002] The high energy density design requirements in the current energy storage cell manufacturing field necessitate high compaction density of the electrode sheets. Currently, vinylidene fluoride copolymers are the mainstream binder system, relying on their chemical stability and coating effect on active material particles to achieve initial adhesion of the coating on the current collector surface. When the electrode compaction density exceeds 1.6 g / cm³, the electrode sheet is subjected to transient vertical pressure and horizontal shear force exceeding 100 MPa during the rolling process. Under the action of high-intensity mechanical field, the active material particles undergo drastic displacement, resulting in stress concentration within the binder network. Since the existing polymer compositions exhibit a uniform modulus distribution at the surface level, and the connection structure between polar groups and non-polar main chains lacks flexible buffer space, the binder network cannot absorb transient shear energy through conformational rearrangement of molecular chain segments. The stress is directly transmitted to the interface anchoring point, inducing the binder to desorb from the current collector surface, resulting in a decrease in peel strength.
[0003] Conventional improvement methods attempt to enhance interfacial adsorption by increasing the proportion of polar monomers. However, these methods do not change the inherently restricted movement of molecular chains; instead, the increased crosslinking density leads to increased coating brittleness. In the engineering environment of rapid drying and film formation, the kinetic locking effect of polymer chains restricts the orderly arrangement of functional groups. Besides hardware solutions such as optimizing the precision of the rolling equipment or adjusting the surface morphology of the rollers, attempts have been made to improve the polymer distribution using specific film-forming processes. However, these efforts focus on overall distribution uniformity while neglecting the interfacial surface mechanical response. For example, the patent with authorization announcement number CN106222728B... A Chinese invention patent discloses a method for preparing a polyvinylidene fluoride-hexafluoropropylene (PVDF-HFA) membrane. The method utilizes electrophoretic deposition to directionally stack polymer particles under an electric field, thereby improving the film density and pore uniformity. However, this type of film-forming mechanism driven by an external physical field is difficult to construct a gradient network with stress dissipation capabilities at the molecular level. When faced with transient shear forces of hundreds of megapascals, due to the lack of flexible branched conformation buffering and directional migration mechanism of interfacial polar groups, stress accumulates at the current collector interface, inducing physical fracture of the bonding network. Relying solely on adjusting the overall process parameters cannot meet the requirements of high-pressure compaction conditions for the intrinsic mechanical properties of the binder.
[0004] Therefore, how to construct a gradient bonding network with stress conduction characteristics, and utilize the dynamic differences between components to form a tough buffer layer at the interface, so that the composition can dissipate stress under high pressure and compaction conditions, is the technical problem to be solved by this invention. Summary of the Invention
[0005] This invention provides a method for preparing a modified polymer composition for bonding electrodes in high-voltage energy storage cells, comprising the following steps:
[0006] Step S1: Dissolve 75 to 85 parts by mass of vinylidene fluoride-hexafluoropropylene copolymer with a weight average molecular weight of 750,000 to 850,000 and a hexafluoropropylene content of 12% to 15% in N-methylpyrrolidone, and stir at 60°C to 70°C for 3 to 5 hours to prepare a base adhesive with a solid content of 10%.
[0007] Step S2: Add 18 parts by mass of acrylate mixed monomers and 0.2 parts by mass of azobisisobutyronitrile to the base adhesive solution, and heat to 75°C to 85°C to carry out in-situ grafting reaction to form a primary reaction system containing acrylate active branches. The acrylate mixed monomer system is composed of methyl methacrylate and butyl acrylate mixed in a mass ratio of 1:2.
[0008] Step S3: The conversion rate of the monomer in the primary reaction system is monitored by gas chromatography. When the conversion rate of the monomer reaches 85% to 90% and the viscosity of the primary reaction system reaches 4000 mPa·s to 5500 mPa·s, 4 parts by mass of phosphate ester monomer containing flexible spacer groups are added dropwise at a rate of 0.5 parts by mass / min, and the reaction continues for 1 to 2 hours to form an intermediate reaction system in which polar groups are grafted onto the ends of active branches. The phosphate ester monomer containing flexible spacer groups has methylene segments with a length of 4 to 10.
[0009] Step S4: Control the intermediate reaction system to continue reacting until the total conversion rate reaches more than 98%. Add 0.5% of a nonionic surfactant according to the total mass of the intermediate reaction system and stir evenly to obtain a modified polymer composition; wherein, the nonionic surfactant is polyoxyethylene ether.
[0010] Preferably, step S3 further includes: utilizing the polarity difference between the phosphate ester monomer containing flexible spacer groups and the vinylidene fluoride-hexafluoropropylene copolymer, to distribute polar groups on the side of the composition coating close to the metal substrate during the film formation process of the modified polymer composition.
[0011] Preferably, in step S1, the stirring speed is set to 800 rpm to 1000 rpm.
[0012] Preferably, in step S2, the mass ratio of methyl methacrylate in the acrylate mixed monomers is adjusted to create a difference between the Young's modulus of the generated active branched chain and the Young's modulus of the vinylidene fluoride-hexafluoropropylene copolymer.
[0013] Preferably, the length n of the methylene segment in the phosphate ester monomer containing the flexible spacer group satisfies the following relationship: , where n is a positive integer.
[0014] Preferably, in step S3, the viscosity is measured using a rotational viscometer at 25°C, and the peak value of the second derivative of the viscosity change rate over time is used as the signal to trigger the dropping action.
[0015] Preferably, in step S3, the temperature fluctuation range of the in-situ grafting reaction system is controlled within... Within 2℃.
[0016] Preferably, the modified polymer composition obtained by the preparation method is applied to electrode manufacturing conditions where the orthogonal rolling pressure is greater than 100 MPa.
[0017] Preferably, steps S1 to S4 are carried out under a nitrogen protective atmosphere, and the oxygen content in the reaction environment is not higher than 50 ppm.
[0018] Compared with existing technologies, the method for preparing modified polymer compositions for bonding high-voltage energy storage cell electrodes of the present invention has the following advantages:
[0019] 1. In the preparation of modified polymer compositions, due to the in-situ grafting of phosphate monomers with flexible spacer groups of a specific length onto the main chain of vinylidene fluoride copolymer, the composition constructs an anchor rope structure at the molecular level. When the electrode is subjected to a high-pressure transient shear force exceeding 100 MPa, the methylene segments of this specific length absorb and dissipate the shear energy at the interface through a conformational transformation from coiled to extended, avoiding stress directly acting on the chemical bonding points between the phosphate ester and the current collector surface, thus solving the problem of brittle drop in peel strength of the electrode bonding network under high-pressure conditions.
[0020] 2. By precisely controlling the conversion rate of acrylate mixed monomers within the range of 85% to 90%, phosphate monomers containing flexible spacer groups are introduced to ensure that polar groups are grafted to the ends of pre-constructed side chains rather than randomly distributed. Combined with the difference in solubility parameters between the vinylidene fluoride copolymer main chain and the grafted side chains, at the film-forming critical point when the solvent evaporates and the content drops below 30%, a controlled microphase separation kinetic repulsion force is triggered, causing the phosphate anchor to contact the metal substrate before the main chain, thus achieving physical locking of gradient distribution without the need for external flow control logic.
[0021] 3. This spontaneous migration process driven by the intrinsic polarity difference of the molecular structure enables the coating to form a low-modulus and highly flexible enriched layer on the side near the current collector, while maintaining strong mechanical support on the side near the active material, thus constructing a quasi-steady modulus gradient layer. This gradient distribution eliminates the interfacial stress concentration caused by abrupt changes in modulus, enhancing the adhesion stability of the electrode after multiple thermal expansion and contraction cycles. At the same time, the directional arrangement of phosphate groups and fluorocarbon backbone inhibits the penetration of electrolyte into the interface, reduces the swelling rate of the composition, and solves the problem of active material shedding during long-term cycling of high-voltage solid electrodes. Attached Figure Description
[0022] Figure 1 This is a flowchart illustrating the preparation process of the modified polymer composition and the construction of the gradient bonding network of the present invention.
[0023] Figure 2 This is a logic diagram of the interaction and monitoring between the operator and the control system during the preparation process of this invention. Detailed Implementation
[0024] The technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.
[0025] It should be noted that all directional and positional terms used in this invention, such as: up, down, left, right, front, back, vertical, horizontal, inner, outer, top, bottom, transverse, longitudinal, center, etc., are only used to explain the relative positional relationship and connection between components in a specific state (as shown in the accompanying drawings). They are only for the convenience of describing this invention and do not require that this invention be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention. In addition, the descriptions of "first," "second," etc., in this invention are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated.
[0026] In the description of this invention, unless otherwise explicitly specified and limited, the terms installation, connection, and linking should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections; they can refer to direct connections or indirect connections through an intermediate medium; they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances.
[0027] In the description of this specification, references to the terms "an embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example, and the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.
[0028] This invention provides a method for preparing a modified polymer composition for bonding electrodes in high-voltage energy storage cells. The method involves in-situ grafting phosphate monomers with flexible spacer groups of specific lengths onto the main chain of a vinylidene fluoride-hexafluoropropylene copolymer to construct a gradient bonding network with stress dissipation characteristics. The entire preparation process includes core stages such as basic adhesive preparation, active branch grafting, polar anchoring group implantation, and phase locking treatment. In the basic adhesive preparation stage, 75 to 85 parts by weight of the vinylidene fluoride-hexafluoropropylene copolymer are dissolved in N-methylpyrrolidone, wherein the weight-average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer is... The molecular weight of the copolymer is 750,000 to 850,000, and the mass content of hexafluoropropylene is 12% to 15%. The dissolution process is carried out at a temperature of 60°C to 70°C, with a stirring speed of 800 rpm to 1000 rpm, and stirring is carried out continuously for 3 to 5 hours to obtain a base adhesive solution with a solid content of 10%. By controlling the molecular weight and component ratio of the copolymer, the basic mechanical strength and chemical stability boundaries of the adhesive system are determined. In the active branch grafting stage, 18 parts by mass of acrylate mixed monomers and 0.2 parts by mass of azobisisobutyronitrile are added to the base adhesive solution. The acrylate mixed monomers are composed of methyl methacrylate and butyl acrylate mixed in a mass ratio of 1:2. The reaction system is heated to 75°C to 85°C under a nitrogen protective atmosphere for in-situ grafting reaction. The oxygen content in the reaction environment is not higher than 50 ppm. In this stage, by controlling the incorporation of acrylate branches, non-polar flexible units with low solubility parameters are generated on the copolymer backbone, providing a kinetic carrier for the subsequent directional migration of polar groups.
[0029] During the polar anchoring group implantation stage, the conversion rate of the monomer in the reaction system was monitored using gas chromatography. When the monomer conversion rate reached 85% to 90% and the viscosity of the reaction system reached 4000 mPa·s to 5500 mPa·s, 4 parts by mass of phosphate ester monomer containing flexible spacer groups were added dropwise at a rate of 0.5 parts by mass / min. The viscosity was measured using a rotational viscometer at 25°C. The phosphate ester monomer containing flexible spacer groups has the following structural formula: ,in It is either hydrogen or methyl, and n is the number of atoms that satisfy the following conditions: After the addition of positive integers, the reaction continues for 1 to 2 hours to allow polar groups to bond to the ends of active side chains. During this process, the peak value of the second derivative of the viscosity-time rate of change is used as the signal to trigger the addition, ensuring the precise arrangement of phosphate anchors at the ends of the side chains and avoiding disordered random grafting. The calibration procedure for the peak value of the second derivative of viscosity in step S3 to trigger the addition is based on maintaining the shear rate of the reactor stirrer at a certain value. to Under the specified conditions, the viscosity η data of the primary reaction system is collected in real time using an online rotational viscosity sensor with a sampling frequency of 10Hz. The control system performs a moving average filtering process on the collected original viscosity sequence with a step size of 5 sampling points. The second-order rate of change of viscosity with respect to time is calculated according to the discretization difference formula. When the conversion rate of the acrylate mixed monomers is monitored to be in the range of 85% to 90%, and the first positive peak exceeding the preset noise threshold appears in the second-order rate of change of viscosity sequence and lasts for no less than 3 sampling cycles, the micro-drop pump is turned on to uniformly drop phosphate monomers containing flexible spacer groups. The positive peak corresponds to the kinetic inflection point of the transformation of acrylate branches from linear free growth to chain segment entanglement and branching point saturation. The deviation of the phosphate monomer at the active branching position is controlled within 2% of the length of the branch end.
[0030] In the phase-locking stage, the reaction system is controlled to continue reacting until the total conversion rate reaches over 98%. A nonionic surfactant, accounting for 0.5% of the total mass of the reaction system, is added and stirred until homogeneous, yielding a modified polymer composition. The nonionic surfactant is polyoxyethylene ether. The surface tension of the system is adjusted to 32 mN / m to 35 mN / m to maintain the phase stability of the composition during storage and coating. The modified polymer composition obtained by this preparation method is applied to electrode manufacturing under orthogonal rolling pressure greater than 100 MPa. During film formation, the polarity difference between the phosphate monomer containing flexible spacer groups and the vinylidene fluoride-hexafluoropropylene copolymer is utilized to distribute polar groups on the side of the composition coating closest to the metal substrate. Due to the difference in solubility parameters between the main chain and the grafted side chains of the vinylidene fluoride-hexafluoropropylene copolymer... Greater than 1.5 When the solvent evaporates to a content below 30%, the system generates a controlled microphase separation kinetic repulsive force, causing the phosphate anchor to migrate towards the current collector interface and form a gradient distribution. This gradient distribution structure, under high-pressure transient shear stress on the electrode, undergoes a conformational transformation through methylene segments of length 4 to 10, changing from a coiled state to an extended state. This absorbs and dissipates the shear energy at the interface. The dissipated energy is determined by the increase in the conformational entropy of the methylene segments. Under orthogonal roller pressing pressure of 100 MPa to 120 MPa, increasing the length of the methylene segments from 4... The coefficient of performance was increased to 10, which increased the conformational entropy increment of a single molecular chain segment from 12 J / mol·K to 35 J / mol·K. The resulting energy dissipation per unit volume was sufficient to offset more than 15% of the interfacial shear work, preventing stress from acting directly on the chemical bonding points between the phosphate ester and the current collector surface. This suppressed the brittle drop in electrode peel strength under high-pressure working conditions. The resulting electrode coating exhibited low modulus and high flexibility on the side near the current collector and maintained mechanical support on the side near the active material, thus solving the problem of active material shedding during cycling in high-energy-density batteries.
[0031] Environmental adaptive adjustment during coating operations involves adjusting the dry-bulb temperature of the heating zone within the coating oven. With wind speed, the solvent partial pressure on the coating surface was kept within the relative saturation range of 0.85 to 0.95. When the solvent content inside the coating dropped to 30% and component segregation was driven by the solubility parameter difference Δδ, the coating line speed was adjusted to keep the thickness of the enrichment interface gradient layer between 1.1 μm and 1.3 μm. The length n of the methylene chain in the flexible spacer phosphate monomer satisfies the positive integer requirement of 4 ≤ n ≤ 10. It absorbs shear energy from a roll load exceeding 100 MPa through the conformational transition from coiling to extension. Within this n value range, the active material shedding rate of the electrode after 500 charge-discharge cycles is less than 0.8%. The solubility parameter difference Δδ and the number of methylene groups n work together to make the peel strength exhibit nonlinear stability during compaction. To determine the solubility parameter difference between the main chain and the grafted side chain of the vinylidene fluoride-hexafluoropropylene copolymer. This scheme uses the Fedors group contribution method for calculation, which obtains the solubility parameter of each component by taking the square root of the ratio of the sum of the cohesive energies of the repeating units in each component chain segment to the sum of their molar volumes. According to the formula Calculate the difference between the two, where Due to poor solubility parameters, The solubility parameter for the backbone of the vinylidene fluoride-hexafluoropropylene copolymer is... These are solubility parameters for grafted acrylate side chains; all units are... This calculation procedure establishes the numerical basis for the source of the driving force for microphase separation.
[0032] Example 1: In an application environment where the electrode design compaction density is 1.65 g / cm³ and the transient rolling pressure is 115 MPa, the active material particles are displaced due to mechanical compression, generating transient shear stress within the binder system that is transmitted to the current collector interface. Using the aforementioned preparation method, 80 parts by mass of weight-average molecular weight... A 13.5% hexafluoropropylene copolymer containing 800,000 KJ of vinylidene fluoride was dissolved in N-methylpyrrolidone and stirred at 900 rpm for 4 hours at 65°C to prepare a base adhesive with a solid content of 10%. 18 parts by mass of a methyl methacrylate and butyl acrylate mixed monomer mixture (1:2 by mass) was added. Under the action of 0.2 parts by mass of azobisisobutyronitrile initiator, the mixture was heated to 80°C in a nitrogen-protected environment with an oxygen content of 35 ppm to trigger an in-situ grafting reaction. When the conversion rate of the acrylate mixed monomers reached 88% and the system viscosity was measured to be 4850 mPa·s at 25°C using a rotational viscometer, the polar group implantation procedure was triggered. 4 parts by mass of a phosphate ester monomer containing flexible spacer groups were added dropwise at a rate of 0.5 parts by mass / min. The structural formula of this phosphate ester monomer is [insert structural formula here]. ,in The methyl group is 8, and the solubility parameter difference between the main chain and the grafted side chain of the vinylidene fluoride-hexafluoropropylene copolymer is significant. It is 1.8 During the film-forming stage when the solvent content drops to 25%, the system generates a repulsive force of microphase separation kinetics, causing the phosphate anchor grafted at the end of the side chain to contact the current collector interface before the fluorocarbon main chain. When the electrode is rolled under the pressure of 115 MPa, the methylene segment with n=8 at the interface undergoes a conformational transformation, changing from a coiled state to an extended state, absorbing the shear energy generated by particle displacement. The experimentally measured electrode peel strength is stable above 26 N / m during the period when the compaction density increases from 1.55 g / cm³ to 1.65 g / cm³. The dynamic distribution of molecular chain segments and the conformational flexibility of the spacer groups enable the bonding network to maintain the chemical stability of the matrix while having mechanical response characteristics that adapt to high compaction conditions.
[0033] Example 2: To address the challenge of bonding failure of energy storage cell electrodes under rolling pressure exceeding 100 MPa, the performance of modified polymer compositions was verified on a test platform equipped with a zoned temperature-controlled reactor and a peel strength tester. The peel strength tester has a measurement range of 0 to 100 N / m and a sampling frequency of 100 Hz. During the test, raw mechanical data were directly obtained through the physical experimental platform. The dropping acceleration rate of the phosphate ester groups was set based on a trade-off between monomer dispersion uniformity and side chain grafting probability. Under the condition that the viscosity of the primary reaction system was 4850 mPa·s, the dropping acceleration rate was set to 0.5 parts by mass / min to ensure the contact frequency between the functional monomer and the active end of the side chain.
[0034] The sample groups of the present invention were prepared according to the preparation method. Control group A consisted of a physical blend of vinylidene fluoride-hexafluoropropylene copolymer and acrylate; control group B consisted of a composition grafted only with acrylate side chains and without phosphate groups; and control group C consisted of a phosphate monomer modified with a flexible spacer group n of 4. Surface tension fluctuations with an amplitude of 5% were introduced during the film formation process to simulate interference. It was observed that when the solvent content decreased to 30%, the sample groups of the present invention exhibited a 1.8 μm difference between the vinylidene fluoride-hexafluoropropylene copolymer main chain and the grafted side chains. The difference in solubility parameter Δδ generates a repulsive force in the microphase separation kinetics. Under a roller pressing pressure of 115 MPa, the measured peel strength of the sample group of this invention is 26.3 N / m, while the measured value of control group A is 12.4 N / m, control group B is 14.8 N / m, and control group C is 18.2 N / m. Moreover, the strength decay rate of control group C increases with the compaction density from 1.55 g / cm³ to 1.65 g / cm³. Here, n is the length of the methylene segment in the phosphate ester monomer structure, and Δδ is the value of vinylidene fluoride-hexafluoroethylene. The difference in solubility parameters between propylene copolymer and acrylate side chains; experimental data show that the performance gain of the modified polymer composition originates from the distribution of polar anchoring points at the end of the side chains and the flexible buffering effect of the methylene segments. With the increase of compaction pressure, the energy absorbed by the conformational transformation of the methylene segments is positively correlated with the interfacial shear energy, confirming that the range of flexible spacer groups with n of 4 to 10 is the working window to ensure the adhesion stability of the electrode under high compaction conditions. This bonding network solves the adhesion failure problem of the electrode under high compaction conditions while maintaining the chemical stability of the matrix.
[0035] Example 3: This example combines Figures 1 to 2 The preparation method of the modified polymer composition used for bonding electrode sheets in high-voltage energy storage cells is described, such as... Figure 1 As shown, step S1, the preparation of the basic adhesive solution, is carried out by dissolving the vinylidene fluoride-hexafluoropropylene copolymer to prepare an adhesive solution with a solid content of 10%. Acrylic ester mixed monomers and initiators are added in the raw material addition step, and the reaction proceeds to step S2, the in-situ grafting reaction, to generate a primary reaction system containing acrylate active branches. The reaction status is then monitored to determine whether the monomer conversion rate reaches the threshold and the system viscosity meets the standard. If not, the reaction continues; if yes, the drop addition is triggered, and the drop addition of raw materials, namely phosphate monomers containing flexible spacer groups, proceeds to step S3, the implantation of polar groups. The polar groups are bonded to the ends of the active branches by the uniform drop addition of phosphate monomers containing flexible spacer groups. Then, step S4, phase locking and post-treatment, is carried out. After the total conversion rate reaches the standard, a surfactant is added, and finally, a gradient bonding network with stress dissipation characteristics, i.e., a modified polymer composition, is obtained.
[0036] like Figure 2As shown, the operator, as the main executor, is associated with the operation modules for preparing the base adhesive solution, performing in-situ grafting reaction, and adding nonionic surfactants, and is connected to the optional process dashed line for performing dynamic response calibration. In the step of monitoring monomer conversion rate, the drop-addition operation is controlled by the control system after the addition of phosphate ester monomers containing flexible spacer groups, and is associated with monitoring the viscosity of the system, ultimately leading to the data processing step of calculating the peak value of the second derivative of viscosity.
[0037] Example 4: In an application scenario where the electrode design compaction density is 1.7 g / cm³ and the active material particles undergo a combined displacement in the normal and tangential directions towards the current collector under pressure, the aforementioned preparation method is used to select 85 parts by mass of weight-average molecular weight... Using a vinylidene fluoride-hexafluoropropylene copolymer with a mass content of 15% and a mass of 750,000 as the matrix, the copolymer was dissolved in N-methylpyrrolidone at 70°C to prepare a base adhesive. 18 parts by mass of a mixture of methyl methacrylate and butyl acrylate monomers in a mass ratio of 1:2 were added, along with 0.2 parts by mass of azobisisobutyronitrile initiator. The mixture was heated to 85°C in a reactor with a nitrogen flow rate of 0.5 L / min and an oxygen content not exceeding 20 ppm. The shear rate of the reactor stirrer was set to 120. In the in-situ grafting reaction stage, real-time viscosity data of the system is collected by an online rotating viscosity sensor integrated on the reactor. The sampling frequency of the sensor is set to 10Hz. The control system performs a moving average filter with a step size of 5 sampling points on the collected original viscosity sequence and calculates the second-order rate of change of viscosity with respect to time according to the discretized difference formula.
[0038] When the conversion rate of the acrylate mixed monomers enters the 85% to 90% range and the first positive peak exceeding the preset noise threshold appears in the second viscosity change rate sequence, the control system activates the micro-drop pump to inject 4 parts by mass of a phosphate monomer with n=10 at a constant rate of 0.5 parts by mass / min. The structural formula of this phosphate monomer is as follows: ,in The methyl group is 10, and the solubility parameter difference between the main chain of the vinylidene fluoride-hexafluoropropylene copolymer and the grafted acrylate side chain is significant. It is 1.6 As the solvent diffuses from the inner layer of the coating to the surface within a 120°C oven, a component segregation driving force is generated within the system, pointing towards the current collector interface. This causes the side chains with phosphate ester groups at their ends to accumulate towards the metal substrate, forming a gradient transition layer with a thickness of 1.2 μm. When the electrode is subjected to a 120 MPa rolling transient stress, the methylene segments enriched at the interface change from a random coiled conformation to a tangentially extended conformation. The energy absorbed through this conformational transformation is given by the formula... It is determined that E is the dissipated energy in J, n is the number of methylene groups, and ΔS is the change in conformational entropy. This method makes the transient load acting on the chemical bonds between the phosphate ester and the current collector surface below the fracture threshold. The experiment shows that the active material shedding rate of the electrode after 500 charge-discharge cycles is less than 0.8%, and the peel strength exhibits nonlinear stability during the compaction process.
[0039] Example 5: When the semi-crystallinity of the vinylidene fluoride-hexafluoropropylene copolymer deviates from the median value by 5% due to batch-to-batch variations, the swelling and diffusion rate of the copolymer in the solvent fluctuates. To determine the engineering baseline of the current batch of raw materials, a dissolution equilibrium calibration is performed before preparing the base adhesive. Ten parts by mass of the copolymer are weighed and placed in a constant-temperature stirring container containing 90 parts by mass of N-methylpyrrolidone. The temperature is set at 65°C and the rotation speed at 900 rpm. The value of the system transitioning from a turbid state to a transparent state is monitored using an online transmittance sensor, and the characteristic time corresponding to the transmittance reaching the baseline value of 95% is recorded. , and by formula Determine the stirring time, among which This refers to the total stirring time during the preparation of the base adhesive solution, expressed in hours (h). The time required for the transmittance to reach the standard is expressed in hours. This procedure eliminates the influence of differences in the crystal morphology of polymer chain segments on the uniformity of the system and limits the solid content deviation of the base adhesive to within ±0.1%.
[0040] When electromagnetic interference in the production environment causes high-frequency random fluctuations with a signal-to-noise ratio of 15dB to superimpose on the raw signal of the online rotary viscosity sensor, in order to suppress false signals triggering the dripping action, the sensor is calibrated before the in-situ grafting reaction is started. A standard sample oil with a viscosity of 5000 mPa·s is injected into the reaction vessel, and the voltage signal output by the sensor is read. Determine the gain correction factor Satisfy the formula ,in This is the viscosity-voltage conversion factor, with units of mPa·s / V. The original voltage value corresponding to the standard sample oil is in V. During the reaction process, the corrected viscosity data is input into the second-order difference calculation loop. The micro-drop pump is started when the second derivative of the viscosity-time rate of change reaches a peak value at three consecutive sampling points. This procedure controls the deviation of the influence of environmental disturbance on the timing of dropping within 1.5% and maintains the consistency of the grafting position of phosphate ester monomer in the side chain topology.
[0041] Example 6: Weight-average molecular weight of vinylidene fluoride-hexafluoropropylene copolymer Under conditions where the viscosity evolution rate fluctuates between 750,000 and 850,000 in the initial stage of the in-situ grafting reaction varies, a dynamic response calibration procedure is performed before preparing the base adhesive to eliminate the interference of the physical properties of the raw materials on the determination of the timing of addition. One part by mass of the copolymer from the current batch is dissolved in nine parts by mass of N-methylpyrrolidone to prepare the calibration adhesive. The second-order viscosity change rate characteristic value after the addition of the acrylate mixed monomers is monitored at 80°C. The first positive peak intensity is then recorded. As a calibration benchmark, set the logic trigger threshold for the production system. ,in The second-order viscosity change rate threshold used to trigger the drip pump to start, in units of , The intensity of the characteristic peak of the second-order viscosity change rate obtained from calibration, in units of This procedure limits the grafting position deviation of phosphate ester monomers to within 2% of the length of the active branch ends by quantitatively compensating for the physical properties of raw materials.
[0042] When the relative humidity of the electrode coating environment fluctuates within the range of 15% to 40%, causing the solvent evaporation rate to deviate from the baseline value, an environmentally adaptive thermal balance procedure is implemented during the coating stage to maintain the layer thickness of the gradient bonding network in the electrode. This involves monitoring the real-time humidity H using a humidity sensor and adjusting the dry-bulb temperature of the heating zone in the oven. This ensures that the solvent partial pressure on the coating surface is within the relative saturation range of 0.85 to 0.95. When the solvent content inside the coating drops to 30% and the solubility parameter is poor... During the driving of component segregation, the coating line speed was adjusted to keep the thickness of the gradient layer enriched at the interface between 1.1 μm and 1.3 μm, thus correcting the segregation kinetic deviation caused by ambient relative humidity, where H is the ambient relative humidity. The oven temperature is set in °C to ensure that when the electrode is subjected to a transient stress of 115 MPa under rolling pressure, the energy absorbed by the conformational transformation of the methylene chain segments offsets the interfacial shear energy, and the electrode peel strength exhibits nonlinear stability during the process of increasing compaction density.
[0043] To further quantitatively verify the influence of the preparation method and key process parameters of this invention on the bonding performance of high-voltage energy storage cell electrodes, this invention constructs a complete performance evaluation system by setting up multiple sets of parallel and comparative experiments. Lithium iron phosphate was selected as the active material, and conductive carbon black as the conductive agent. Modified polymer compositions obtained from various embodiments of this invention and specific comparative examples were used as binders. Slurries were prepared at a mass ratio of 96:2:2 and coated onto 15-micron thick aluminum foil current collectors. The comparative example design logic defined in this invention is as follows: Comparative example D aims to verify the influence of the monomer conversion rate threshold in step S3 on the grafting position of polar groups; Comparative example E aims to verify the limitation of the flexible spacer length n on the interfacial stress dissipation capability; Comparative example F serves as a benchmark reference for conventional technologies. Specific comparison schemes and measured data are shown in Table 1.
[0044] Table 1: Comparison of Key Process Parameters and Electrode Peel Strength Properties of Modified Polymer Compositions
[0045]
[0046] Technical analysis of the above experimental data reveals that, comparing the sample group of this invention with Comparative Example D, when the phosphate monomer is added too early in step S3 (conversion rate is only 60%), the active side chains are still in a linear rapid growth phase. The polar phosphate monomer tends to randomly embed into the side chain backbone rather than bond to the side chain ends. This results in the polar groups being unable to effectively migrate to the current collector interface through the microphase separation kinetics repulsion during film formation due to the embedding effect, causing a sharp drop in peel strength of about 40% under high-pressure compaction conditions. Comparing the sample group of this invention with Comparative Example E, when the methylene segment length n is less than 4 (e.g., n=2), the conformational space of the segment is extremely limited, making it impossible to effectively absorb the transient shear energy generated by the high-intensity mechanical field through the transformation from coiling to extension. At this time, the shear stress... It directly acts on the chemical bonding points at the interface between the phosphate ester and the current collector, inducing physical breakage of the bonding network. Experiments show that only when n is in the range of 4 to 10 can the conformational entropy increase of the molecular chain segments be sufficient to offset the interfacial work generated by high compaction, thereby stabilizing the peel strength at above 26 N / m. Comparative example F shows that, due to the lack of the in-situ grafting structure and kinetic control mechanism of this invention, the polar groups of the traditional physical blend system are randomly distributed. When the high compaction density reaches 1.68 g / cm³, its peel strength is less than 10 N / m, which cannot meet the manufacturing requirements of high energy density energy storage cells. However, this invention utilizes the polarity difference of the components to drive the polar groups to be directionally enriched at the interface, spontaneously forming a modulus gradient buffer layer without external intervention, completely solving the problem of brittle drop in peel strength.
[0047] The embodiments of this application have been described above with reference to the accompanying drawings. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. This application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of this application without departing from the spirit of this application and the scope of protection of this invention, and all of these forms are within the protection scope of this application.
Claims
1. A method for preparing a modified polymer composition for bonding electrodes in high-voltage energy storage cells, characterized in that, Includes the following steps: Step S1: Dissolve 75 to 85 parts by mass of vinylidene fluoride-hexafluoropropylene copolymer with a weight average molecular weight of 750,000 to 850,000 and a hexafluoropropylene content of 12% to 15% in N-methylpyrrolidone, and stir at 60°C to 70°C for 3 to 5 hours to prepare a base adhesive with a solid content of 10%. Step S2: Add acrylate mixed monomers and azobisisobutyronitrile to the base adhesive solution, and heat to 75°C to 85°C to carry out in-situ grafting reaction to form a primary reaction system containing acrylate active branches. The acrylate mixed monomer system is composed of methyl methacrylate and butyl acrylate mixed in a mass ratio of 1:
2. Step S3: The conversion rate of monomers in the primary reaction system is monitored using a gas chromatograph. When the conversion rate of monomers reaches 85% to 90% and the viscosity of the primary reaction system reaches 4000 mPa·s to 5500 mPa·s, phosphate ester monomers containing flexible spacer groups are added dropwise at a uniform rate, and the reaction continues for 1 to 2 hours to form an intermediate reaction system in which polar groups are grafted onto the ends of the active branches of acrylate. The phosphate ester monomers containing flexible spacer groups have methylene segments with a length of 4 to 10. Step S4: Control the intermediate reaction system to continue reacting until the total conversion rate reaches more than 98%. Add a nonionic surfactant and stir evenly to obtain a modified polymer composition.
2. The method for preparing a modified polymer composition for bonding high-voltage energy storage cell electrodes according to claim 1, characterized in that, Step S3 further includes: utilizing the polarity difference between the phosphate ester monomer containing flexible spacer groups and the vinylidene fluoride-hexafluoropropylene copolymer, the polarity difference being determined by the solubility parameter difference between the main chain and the grafted side chains of the vinylidene fluoride-hexafluoropropylene copolymer. Characterization, in which During the film formation process of the modified polymer composition, polar groups are distributed on the side of the composition coating closest to the metal substrate.
3. The method for preparing a modified polymer composition for bonding high-voltage energy storage cell electrodes according to claim 1, characterized in that, In step S1, the stirring speed is set to 800 rpm to 1000 rpm.
4. The method for preparing a modified polymer composition for bonding high-voltage energy storage cell electrodes according to claim 1, characterized in that, In step S2, by adjusting the mass ratio of methyl methacrylate in the acrylate mixed monomers, a difference in solubility parameters is created between the generated active acrylate branches and the main chain of the vinylidene fluoride-hexafluoropropylene copolymer. .
5. The method for preparing a modified polymer composition for bonding high-voltage energy storage cell electrodes according to claim 1, characterized in that, Length of methylene segment in phosphate ester monomers containing flexible spacer groups The following relationship must be satisfied: , where n is a positive integer.
6. The method for preparing a modified polymer composition for bonding electrode sheets of a high-voltage energy storage cell according to claim 1, characterized in that, In step S3, the viscosity is measured at 25°C using a rotational viscometer, and the peak value of the second derivative of the viscosity change rate over time is used as the signal to trigger the dropping action.
7. The method for preparing a modified polymer composition for bonding high-voltage energy storage cell electrodes according to claim 1, characterized in that, In step S3, the temperature fluctuation range of the primary reaction system and the intermediate reaction system is controlled within... Within 2℃.
8. The method for preparing a modified polymer composition for bonding high-voltage energy storage cell electrodes according to claim 1, characterized in that, In step S4, the nonionic surfactant is polyoxyethylene ether.