A dynamic roll forming device and method for gradient porous separator for silicon-based negative electrode

Abstract

The application discloses a dynamic roller pressing forming device for a gradient-pore diaphragm for a silicon-based negative electrode, comprising a modular coating unit, a multi-shaft roller pressing machine group and a partitioned annealing cavity arranged in sequence, the modular coating unit comprises a slit coating mechanism, a spray coating mechanism and a micro-gravure coating mechanism arranged in sequence, the multi-shaft roller pressing machine group comprises a main pressing roller and a supporting roller matched with the main pressing roller, and a dynamic pressure feedback system is arranged on the main pressing roller; and the application further discloses a method for preparing a gradient-pore high-molecular diaphragm by using the forming device. The device is provided with the main pressing roller of the multi-shaft roller pressing machine group and the partitioned annealing cavity, and the dynamic pressure feedback system is arranged on the main pressing roller, so that the accurate matching between the pore structure and the expansion behavior of the silicon-based negative electrode is realized, the stress buffering capacity and the ion transmission efficiency of the diaphragm are improved, and the device is suitable for the technical field of lithium ion battery materials.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery materials, and particularly relates to a gradient-pore diaphragm dynamic roller forming device and method for a silicon-based negative electrode. BACKGROUND

[0002] Silicon-based negative electrode material has become a key material for breaking through the energy density bottleneck of lithium ion batteries due to its ultra-high theoretical specific capacity of 3578 mAh / g (about 10 times that of graphite negative electrode). However, the 300%-400% volume expansion effect generated during the charging and discharging process of the silicon-based negative electrode material leads to multiple failures of the electrode / electrolyte interface: (1) the anisotropic expansion causes local stress concentration (peak value up to 150 MPa), resulting in delamination and plastic deformation of the traditional diaphragm interface; (2) the silicon particles break and recombine, leading to a dramatic increase in the roughness of the electrode surface (Ra>5 μm), which increases the mechanical wear rate of the diaphragm by 300%.

[0003] The existing commercial diaphragm, such as Celgard® 2400, has the following technical defects when dealing with the above challenges: (1) structural mismatch: the homogeneous pore structure (monodisperse pore size distribution) cannot adapt to the multi-scale expansion of silicon particles, and the porosity (38±5%) is asynchronous with the expansion phase change; (2) mechanical imbalance: the high elastic modulus (>2 GPa) and the plastic deformation of the silicon negative electrode (strain rate >200%) form a rigid constraint, and the stress buffering efficiency is less than 40%; (3) process defects: the step-by-step coating and rolling process results in low pore control precision (pore size dispersion ±15%), making it difficult to build a gradient structure. Furthermore, experiments have shown that when the silicon content is >5%, the porosity decay rate of the traditional diaphragm is 58% after 200 cycles, and the existing technology cannot meet the needs of high-silicon content systems.

[0004] Although some patents have proposed improvement schemes, they still have significant deficiencies: for example, Chinese invention patent “Lithium battery diaphragm with gradient pore structure and preparation method thereof” (publication number CN119812677A) reserves expansion space through a double-layer structure, but its static pore distribution cannot adapt to the dynamic changes of local stress in the silicon negative electrode cycle, leading to pore blockage and interface failure later. The essence of the above technical defects lies in the fact that the existing diaphragm system cannot achieve the coordinated regulation of dynamic stress adaptation and multi-scale ion conduction, which is specifically manifested in: (a) insufficient spatiotemporal matching degree of pore structure and expansion phase change; (b) uncoupling of modulus gradient distribution and stress field evolution; (c) the preparation process cannot meet the precision structure control requirements.

[0005] The industry urgently needs to develop a new generation of intelligent diaphragm system, and the core breakthrough point should focus on spatiotemporal coupling pore engineering, mechanical gradient interface design, and precision manufacturing process innovation, breaking the core bottleneck of high-silicon negative electrode industrialization, and providing key material support for the development of 500 Wh / kg-level high-specific-energy batteries. SUMMARY

[0006] The present application aims to overcome the deficiencies in the prior art, and provides a gradient pore separator dynamic roller forming device for silicon-based negative electrode. The forming device is provided with a main compression roller of a multi-axis roller unit and a partitioned annealing cavity, and a dynamic pressure feedback system is arranged on the main compression roller, so as to realize accurate matching of the pore structure and the expansion behavior of the silicon-based negative electrode, improve the stress buffering capacity and ion transmission efficiency of the separator, and solve the contradictory requirements of the traditional homogeneous separator pores, i.e. buffering (high pore zone) and fast ion transmission (low tortuosity channel).

[0007] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is: a gradient pore separator dynamic roller forming device for silicon-based negative electrode, characterized in that it comprises a modular coating unit, a multi-axis roller unit and a partitioned annealing cavity arranged in sequence, the modular coating unit comprises a slit coating mechanism, a spray coating mechanism and a micro-gravure coating mechanism arranged in sequence, the multi-axis roller unit comprises a main compression roller and a support roller matched with the main compression roller, and a dynamic pressure feedback system is arranged on the main compression roller.

[0008] The gradient pore separator dynamic roller forming device for silicon-based negative electrode is characterized in that a unwinder is arranged at the front end of the modular coating unit, a winding machine is arranged at the rear end of the partitioned annealing cavity, the slit coating mechanism, the spray coating mechanism and the micro-gravure coating mechanism are all equipped with a monitoring system, the monitoring system comprises a laser confocal displacement meter and a terahertz wave pore analyzer, and a plasma treatment device is arranged at the front end of the slit coating mechanism.

[0009] The gradient pore separator dynamic roller forming device for silicon-based negative electrode is characterized in that a micro-groove array is laser-engraved on the surface of the main compression roller, the ratio of the width to the depth of the micro-groove is 1.5-2.5, and the ratio of the center distance between adjacent micro-grooves to the depth of the micro-groove is 3-5.

[0010] The dynamic pressure feedback system comprises a piezoelectric ceramic stress sensor array embedded in the surface of the main compression roller, and a dynamic control hydraulic servo system connected with the piezoelectric ceramic stress sensor array, the dynamic control hydraulic servo system is used for controlling the pressure fluctuation on the surface of the main compression roller to be less than 0.3 MPa by adjusting the parameters by the Ziegler-Nichols method.

[0011] The gradient pore separator dynamic roller forming device for silicon-based negative electrode is characterized in that the partitioned annealing cavity has a plurality of independent temperature control zones provided with infrared heating devices, a laminar flow air curtain device is arranged on the top of each independent temperature control zone, and a vacuum and gas control system is connected therewith; a post-processing mechanism is arranged at the rear end of the partitioned annealing cavity.

[0012] The dynamic roller forming device for the gradient-pore separator for the silicon-based negative electrode has the features that the multi-shaft roller press unit further comprises a feeding roller arranged at the front end of the main press roller and a shaping roller arranged at the rear end of the main press roller, the feeding roller is internally provided with an electric heating pipe, the surface of the shaping roller is covered with a PTFE coating, and the internal part of the shaping roller is provided with a cooling circulating water pipe, and the diameter ratio of the feeding roller, the main press roller and the shaping roller is 1:2:4.

[0013] The application further discloses a method for preparing the gradient-pore high polymer separator by using the forming device, which has the features that the method comprises the following steps:

[0014] Step one, gradient coating: coating a base film with a dense layer, a transition layer and a buffer layer in sequence by using a modular coating unit to obtain a coated film;

[0015] Step two, dynamic roller pressing: sequentially passing the coated film obtained in step one through a multi-shaft roller press unit to obtain a coated film with a cavity array;

[0016] Step three, gradient annealing: annealing the coated film with the cavity array obtained in step two by using a partitioned annealing cavity to induce directional migration of the pore-forming agent, and obtaining a gradient-pore separator after cooling.

[0017] The method has the features that the base film in step one is made of PE or PP, the thickness of the base film is 5-15 microns, and the base film is subjected to plasma activation treatment before being passed through the modular coating unit;

[0018] The preparation slurry of the dense layer comprises, in terms of weight percentage, 5-15 parts of nano-SiO2 pore-forming agent, 2-10 parts of PVDF binder and 60-93 parts of NMP solvent, and the particle size of the nano-SiO2 is 50-200 nm; and the thickness of the dense layer is 3-5 microns.

[0019] The preparation slurry of the transition layer comprises, in terms of weight percentage, 20-40 parts of PMMA microsphere pore-forming agent, 0.5-5 parts of PEO binder and 50-80 parts of deionized water, and the particle size of the PMMA microsphere pore-forming agent is 500 nm-1 micron; and the thickness of the transition layer is 8-12 microns.

[0020] The preparation slurry of the buffer layer comprises, in terms of weight percentage, 30-60 parts of NaCl crystal pore-forming agent, 5-20 parts of PU binder and 20-70 parts of ethanol, and the particle size of the NaCl crystal pore-forming agent is 5-10 microns; and the thickness of the buffer layer is 10-15 microns.

[0021] The application has the advantages that the base film is first subjected to plasma activation treatment to improve the surface energy and improve the bonding force between the coating and the base film.

[0022] The method, characterized in that, the film-coating roller is preheated at 50 DEG C before pressing in step two, and the diameter of the film-coating cavity is 50 nm-200 nm.

[0023] The method, characterized in that, the temperature of the partitioned annealing cavity in step three is 50 DEG C-130 DEG C, the temperatures of the multiple independent temperature control zones of the partitioned annealing cavity are sequentially increased, the annealing treatment system comprises: 80 DEG C-100 DEG C for 10 s or more, 100 DEG C-110 DEG C for 20 s or more, and 115 DEG C-130 DEG C for 30 s or more, and the annealing treatment is followed by cooling at a rate of 5 DEG C / min.

[0024] The pore diameter of the compact layer in the gradient-pore separator is 10 nm-50 nm, and the porosity is 22%-40%; the pore diameter of the transition layer in the gradient-pore separator is 200 nm-1 mu m, and the porosity is 40%-55%; the pore diameter of the buffer layer in the gradient-pore separator is 500 nm-10 mu m, and the porosity is 50%-65%.

[0025] The present application can match the migration dynamics of the pore-forming agent in the compact layer, the transition layer and the buffer layer by designing the temperatures of the partitions of the partitioned annealing cavity to sequentially increase, linearly increasing the temperature in the range of 50 DEG C-130 DEG C, and controlling the residence time of the film coating in each partition, avoiding the warping of the separator caused by thermal stress by controlling the cooling rate, so that the warping degree is less than 0.1 mm / m.

[0026] The compact layer with nano-scale pores is used to provide Li + fast transmission channels while inhibiting silicon particle penetration; the transition layer with sub-micron pores is used to buffer the expansion stress of the silicon negative electrode and balance the mechanical strength and ion transmission; the buffer layer with micron-scale pores is used to absorb macro-strain and prevent the separator from tearing; the porosities of the compact layer, the transition layer and the buffer layer are designed to control the modulus of each layer to form a gradient relationship, so as to realize the precise matching of the pore structure and the expansion behavior of the silicon-based negative electrode.

[0027] The method, characterized in that, the pressure P of the main pressing roller in step two is calculated by the following formula:

[0028] ;

[0029] Wherein, P is the pressure of the main pressing roller, in MPa; is the highest temperature of the partitioned annealing cavity in step three, in DEG C.

[0030] According to the porosity, thickness and other parameter design of the dense layer, transition layer and buffer layer coated on the surface of the base film, the interval temperature of high temperature annealing is adjusted, since high temperature annealing will soften the polymer chain, the main compression roller pressure needs to be reduced synchronously to avoid excessive compression of the pores, and the dynamic coupling of the main compression roller pressure and the annealing temperature is controlled through the above formula.

[0031] Compared with the prior art, the present application has the following advantages:

[0032] 1. The present application realizes the precise matching of the pore structure and the expansion behavior of the silicon-based negative electrode by setting the main compression roller of the multi-shaft roller press unit and the partition annealing cavity, and by setting the dynamic pressure feedback system on the main compression roller, thereby improving the stress buffering capacity and ion transmission efficiency of the diaphragm.

[0033] 2. The present application sets the slot coating mechanism, the spray coating mechanism and the micro-gravure coating mechanism for preparing a three-stage gradient pore structure, which can realize Li + Multi-path transmission, so that the ionic conductivity reaches 1.2 mS / cm or more.

[0034] 3. The present application can improve the pore control precision by 3 times by adjusting the dynamic roller pressing algorithm to adjust the dynamic control hydraulic servo system parameters, and the product yield is improved from 65% to 92%.

[0035] 4. The present application realizes the precise matching of the pore structure and the expansion behavior of the silicon-based negative electrode by using the dynamic roller pressing of the multi-shaft roller press unit, combining the modulus gradient and real-time pressure control of each layer of the gradient pore diaphragm, so that the gradient pore diaphragm can withstand 400% volume expansion of the silicon negative electrode without rupture, the cycle life is improved to 500 times and the capacity retention rate is greater than 80%.

[0036] The technical solutions of the present application will be further described in detail below with the aid of the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 It is a structure schematic view of the forming device in Example 1 of the present application.

[0038] Figure 2 It is a tensile strength result graph of the gradient pore diaphragm prepared in Example 2 of the present application.

[0039] Figure 3 It is a physical map after thermal shrinkage rate test of the gradient pore diaphragm prepared in Example 2 of the present application.

[0040] Figure 4 It is a cycle test curve graph of the Li / / Si@C button cell assembled by Example 2, Example 3, Example 4 and Comparative Example 1 of the present application.

[0041] Figure 5The graph shows the ionic conductivity results of the Li / / Si@C coin cells assembled in Examples 2, 4 and Comparative Example 1 of the present invention.

[0042] Explanation of reference numerals in the attached figures:

[0043] 1—Unwinder; 2—Base film; 3—Drive shaft; 4—Plasma treatment equipment; 5—Slit coating mechanism; 6—Monitoring system; 7—Spray coating mechanism; 8—Microgravure coating mechanism; 9—Feed roller; 10—Support roller; 11—Main pressure roller; 12—Microgroove; 13—Shaping roller; 14—Separate annealing chamber; 15—Post-processing mechanism; 16—Gradient pore diaphragm; 17—Winder. Detailed Implementation

[0044] Example 1

[0045] like Figure 1 As shown, the molding apparatus of this embodiment includes a modular coating unit, a multi-axis roller press, and a partitioned annealing chamber 14 arranged in sequence. The modular coating unit includes a slot coating mechanism 5, a spray coating mechanism 7, and a micro-gravure coating mechanism 8 arranged in sequence. The multi-axis roller press includes a main pressure roller 11 and a support roller 10 matched with the main pressure roller 11. A dynamic pressure feedback system is provided on the main pressure roller 11.

[0046] In practical use, in this embodiment, a modular coating unit, a multi-axis roller press, and a partitioned annealing chamber 14 are sequentially arranged. The modular coating unit includes a slit coating mechanism 5, a spray coating mechanism 7, and a microgravure coating mechanism 8 arranged sequentially, as well as a main pressure roller 11, which is used to sequentially coat the base film 2 with a dense layer, a transition layer, a buffer layer, roll pressing, and annealing treatment to prepare a gradient pore membrane 16. A dynamic pressure feedback system is set on the main pressure roller 11 to monitor the real-time pressure.

[0047] It should be noted that in this embodiment, the slit coating mechanism 5, the spray coating mechanism 7, and the microgravure coating mechanism 8 are all connected to the corresponding slit, spray, and microgravure coating heads by independent slurry supply systems through metering pumps and pipelines; the slurry flow rate is controlled in real time by the metering pump to ensure the coating thickness accuracy; the independent slurry supply system contains preparation slurries for the dense layer, transition layer, and buffer layer respectively.

[0048] Furthermore, such as Figure 1 As shown, in this embodiment, the modular coating unit is provided with an unwinder 1 at the front end, and the partitioned annealing chamber 14 is provided with a winding machine 17 at the rear end; the slit coating mechanism 5, the spray coating mechanism 7 and the microgravure coating mechanism 8 are all equipped with a monitoring system 6, the monitoring system 6 includes a laser confocal displacement meter and a terahertz wave porosity analyzer, and the slit coating mechanism 5 is provided with a plasma treatment device 4 at the front end.

[0049] In actual use, in the embodiment, the unwinder 1 is arranged at the front end, and the winding machine 17 is arranged at the rear end, so that the preparation of the diaphragm and the collection of the finished product are continuously carried out, and the continuous production is realized; and preferably, in the embodiment, the transmission shaft 3 is arranged between each unit, so that the stable transmission is realized, and the wrinkles or deviation in the dynamic roller pressing and the annealing process are avoided;

[0050] The laser confocal displacement meter and the terahertz wave pore analyzer are arranged for real-time monitoring of the thickness and surface pore size distribution of the base film 2; the plasma treatment equipment 4 is arranged at the front end of the slit coating mechanism 5, so as to improve the surface energy of the base film 2, and the plasma treatment equipment 4 is a radio frequency plasma cleaning machine; and preferably, the rear end of each coating mechanism in the embodiment is provided with a drying equipment.

[0051] It should be noted that the laser confocal displacement meter in the embodiment is arranged at the outlet of each coating mechanism, and the terahertz wave pore analyzer is arranged at the rear end of the drying equipment.

[0052] Further, as shown in Figure 1 In the embodiment, the surface of the main pressure roller 11 is laser engraved with an array of micro-grooves 12, wherein the ratio of the width to the depth of the micro-groove 12 is 1.5-2.5, and the ratio of the center distance between adjacent micro-grooves 12 to the depth of the micro-groove 12 is 3-5.

[0053] The dynamic pressure feedback system comprises a piezoelectric ceramic stress sensor array embedded in the surface of the main pressure roller 11, and a dynamic control hydraulic servo system connected with the piezoelectric ceramic stress sensor array, wherein the dynamic control hydraulic servo system is used for controlling the pressure fluctuation on the surface of the main pressure roller 11 to be less than 0.3 MPa by Ziegler-Nichols method.

[0054] In actual use, in the embodiment, the surface of the main pressure roller 11 is laser engraved with an array of micro-grooves 12, so as to cause local stress concentration during roller pressing and induce the diaphragm to form a cavity array on the surface of the base film 2; the ratio of the width to the depth of the micro-groove 12 is 1.5-2.5, and the ratio of the center distance between adjacent micro-grooves 12 to the depth of the micro-groove 12 is 3-5, so as to ensure that the stress concentration factor is controlled in the range of 1.2-1.8; and preferably, in the embodiment, the width of the micro-groove 12 is 20-50 μm, the depth is 10-30 μm, and the distance between adjacent micro-grooves 12 is 50-100 μm, so as to induce the diaphragm to form a cavity array with a diameter of 50-200 nm inside, and the energy dissipation efficiency is improved to 82%.

[0055] By embedding a piezoelectric ceramic stress sensor array on the surface of the main pressure roller 11, and a dynamic control hydraulic servo system connected to the piezoelectric ceramic stress sensor array, local stress is collected in real time and transmitted to the dynamic control hydraulic servo system for adjustment. Then, the parameters are tuned using the Ziegler-Nichols method to control the pressure fluctuation on the surface of the main pressure roller 11 to be less than 0.3MPa. Preferably, in this embodiment, the piezoelectric ceramic stress sensor is located 200μm from the edge of the microgroove 12, which is more conducive to pressure control.

[0056] Furthermore, such as Figure 1 As shown, in this embodiment, the partitioned annealing chamber 14 has multiple independent temperature control zones equipped with infrared heating devices. Each independent temperature control zone is equipped with a laminar flow air curtain device at its top and is connected to a vacuum and gas control system. A post-processing mechanism 15 is provided at the rear end of the partitioned annealing chamber 14.

[0057] In practical use, this embodiment uses an infrared heating device, which has the advantages of high thermal efficiency, non-contact heating, and avoidance of mechanical damage. The infrared wavelength of this infrared heating device is 2μm~5μm. By setting a laminar flow air curtain device at the top, a uniform airflow can be blown downwards to ensure the temperature uniformity within the partitioned annealing chamber 14, with a temperature fluctuation of less than 1.5℃. By setting multiple independent temperature control zones, the required reaction temperature environment and holding time for the buffer layer, transition layer, and dense layer can be achieved. By connecting to the vacuum and gas control system, an inert atmosphere environment is provided for annealing to prevent the oxidative degradation of polymers. By setting a post-treatment mechanism 15, including impregnation tanks containing ethanol and deionized water respectively, ethanol soaking and deionized water rinsing can be performed. After cooling, ethanol soaking is used to remove residual NaCl and PMMA and increase porosity. Deionized water rinsing is used to control the conductivity of the gradient pore membrane 16.

[0058] Furthermore, such as Figure 1 As shown, in this embodiment, the multi-axis roller press unit further includes a feed roller 9 disposed at the front end of the main pressure roller 11 and a shaping roller 13 disposed at the rear end of the main pressure roller 11. The feed roller 9 is equipped with an electric heating tube inside, and the surface of the shaping roller 13 is covered with a PTFE coating and is equipped with a cooling circulating water pipe inside. The diameter ratio of the feed roller 9, the main pressure roller 11 and the shaping roller 13 is 1:2:4.

[0059] In practical use, in this embodiment, a feed roller 9 is set at the front end of the main pressure roller 11 for preheating and reducing the feeding stress; a shaping roller 13 with internal cooling circulating water pipes and a PTFE coating is set at the rear end of the main pressure roller 11 to control the friction coefficient to be less than 0.1, prevent adhesion, and stabilize the pore structure; by controlling the diameter ratio to 1:2:4, the dynamic process of the expansion phase change of silicon-based negative electrode can be matched and the stress transmission path can be optimized.

[0060] Examples 2-6 are all methods for preparing gradient porous polymer separators using the forming device in Example 1.

[0061] Example 2

[0062] The base film 2 of this example is a polypropylene (PP) base film with a thickness of 5-15 μm and a crystallinity greater than 70%, and the preparation method includes the following steps:

[0063] Step 1, gradient coating: the base film 2 is unwound by the unwinder 1, passed through a radio frequency plasma cleaning machine, and plasma treated at a power of 50 W and an argon gas flow of 20 sccm for 30 s, so that the surface energy of the base film 2 is increased to greater than 50 mN / m, and then passed through the slot coating mechanism 5, the spray coating mechanism 7, and the micro-gravure coating mechanism 8 in sequence to coat a dense layer, a transition layer, and a buffer layer, to obtain a coated film;

[0064] The coating speed of the dense layer is 10 m / min and the thickness is 4 μm, and then hot air drying at 80°C for 5 min is performed to obtain a dense dry film with a thickness of 2.5 μm; the coating speed of the transition layer is 8 m / min and the thickness is 10 μm, and then hot air drying at 100°C for 8 min is performed to obtain a transition dry film with a thickness of 6 μm; the coating speed of the buffer layer is 5 m / min and the thickness is 12 μm, and then hot air drying at 120°C for 10 min is performed to obtain a buffer dry film with a thickness of 8 μm;

[0065] The preparation slurry of the dense layer includes, by weight fraction: 10 parts of nano-SiO2 pore-forming agent with a particle size of 80 nm, 6 parts of PVDF binder, and 84 parts of NMP solvent; the preparation slurry of the transition layer includes, by weight fraction: 30 parts of PMMA microspheres pore-forming agent with a particle size of 800 nm, 3 parts of PEO binder, and 67 parts of deionized water; and the preparation slurry of the buffer layer includes, by weight fraction: 40 parts of NaCl crystal pore-forming agent with a particle size of 8 μm, 12 parts of PU binder, and 48 parts of ethanol;

[0066] Step 2, dynamic roller pressing: the coated film obtained in step 1 is passed through the feeding roller 9 at a temperature of 50°C, the main pressing roller 11, and the setting roller 13 at a temperature of 25°C in sequence, to obtain a coated film with a cavity array with a diameter of 50-200 nm; the pressure of the main pressing roller 11 is 3.5 MPa;

[0067] Step three, gradient annealing: the coating film with cavity array obtained in step two is annealed in a N2-protected partitioned annealing cavity 14, the temperature of each independent temperature control zone is 50°C, 70°C, 80°C, 100°C, 120°C in turn, wherein the coating film is kept at 80°C for more than 20s, at 100°C for more than 40s, and at 120°C for more than 60s; then the temperature is decreased at a rate of 5°C / min, and after that, the coating film is soaked in ethanol for 24h, rinsed in deionized water until the conductivity is less than 5μS / cm, and vacuum dried at 60°C for 2h, to obtain a gradient porous separator 16 with a thickness of 28.5μm; the O2 concentration in the partitioned annealing cavity 14 is less than 100ppm.

[0068] The PP-based film in this embodiment can also be replaced by a PE-based film.

[0069] It is found by inspection that the average pore size of the dense layer of the gradient porous separator 16 prepared in this embodiment is 25nm, the Weibull modulus is >8, and the pore size distribution is excellent; the average pore size of the transition layer is 800nm, and the transition radius of curvature is smooth (45μm~50μm); the average pore size of the buffer layer is 8μm, and the pore connectivity rate is >95%. The gradient porous separator 16 is tested by mercury injection method, and the porosities of the dense layer, the transition layer, and the buffer layer are measured to be 25%, 45%, and 60% respectively, and the specific surface area reaches 120m 2 / g (only 30m 2 / g for a traditional PP separator).

[0070] The gradient porous separator 16 prepared in this embodiment is tested for longitudinal tensile strength according to “Polyolefin Separator for Lithium Ion Battery” (GB / T36363-2018), and the result is shown in Figure 2 , which shows that the tensile strength of the gradient porous separator 16 is 40.5N, about 30% higher than that of Celgard® 2400 (31.3N); the thermal shrinkage rate of the gradient porous separator 16 with a size of 10.0cm×10.0cm is tested, and the result is shown in Figure 3 , which shows that the thermal shrinkage rate at 200°C is 5.0%, which is greatly reduced compared with the thermal shrinkage rate of Celgard® 2400 at 150°C (20%).

[0071] The gradient porous separator 16 prepared in this embodiment is assembled into a Li / / Si@C button cell, wherein the negative electrode silicon content is 20wt.% (capacity about 1000mAh / g), and the cycle test is performed (0.5C charge and discharge, voltage range 0.005V~1.5V), and the result is shown in Figure 4 , which shows that the cycle capacity retention rate of the Li / / Si@C button cell at the 500th cycle is 89%; the EIS test is performed, and the frequency is 100kHz~0.1Hz, and the result Figure 5 shows that the ionic conductivity is 1.2mS / cm, which is increased by 140% compared with the traditional separator.

[0072] Comparative Example 1

[0073] The difference between this comparative example and Example 2 is that the gradient annealing is cancelled, and a uniform temperature of 90℃ is used for annealing, to obtain a separator with a thickness of 20μm.

[0074] The separator prepared in this comparative example is tested by mercury intrusion method, and the pore gradient is blurred (the difference between the transition layer and the buffer layer is <10%).

[0075] The separator prepared in this comparative example is tested for longitudinal tensile strength according to “Polyolefin Separator for Lithium Ion Battery” (GB / T 36363-2018), and the tensile strength of the separator is 34.3N, which is lower than that of Example 2; the separator with a size of 10.0cm×10.0cm is tested for thermal shrinkage, and the thermal shrinkage at 200℃ is 7.5%, which is larger than that of Example 2.

[0076] The separator prepared in this comparative example is assembled into a Li / / Si@C button cell, where the negative electrode silicon content is 20wt.% (capacity about 1000mAh / g), and a cycle test is performed (0.5C charge and discharge, voltage range 0.005V~1.5V), and the results are shown in Figure 4 , the capacity retention rate at the 500th cycle is 72%; EIS test is performed, and the frequency is 100kHz~0.1Hz, and the results are shown in Figure 5 , the ionic conductivity is 0.8mS / cm; compared with the capacity retention rate and ionic conductivity of Example 2, both are reduced.

[0077] In summary, it is shown that the gradient annealing of the present application is beneficial to the improvement of the tensile strength of the separator and the reduction of the thermal shrinkage, and can improve the capacity retention rate and ionic conductivity of the battery.

[0078] Example 3

[0079] The difference between this example and Example 2 is that the pressure of the main press roller 11 in step two is 3.0MPa, and the temperatures of the independent temperature control zones of the partitioned annealing cavity 14 in step three are 55℃, 65℃, 85℃, 100℃, and 110℃ in turn, wherein the coating film is kept at 85℃ for more than 20s, at 100℃ for more than 40s, and at 110℃ for more than 60s.

[0080] The gradient pore separator 16 prepared in this example is tested for longitudinal tensile strength according to “Polyolefin Separator for Lithium Ion Battery” (GB / T 36363-2018), and the tensile strength of the gradient pore separator 16 is 48.4N; the gradient pore separator 16 with a size of 10.0cm×10.0cm is tested for thermal shrinkage, and the thermal shrinkage at 200℃ is 3.8%.

[0081] Example 4

[0082] The difference between this example and Example 2 is that the slurry for preparing the dense layer in Step 1 includes, in parts by weight, 15 parts of nano-SiO2 pore-forming agent with a particle size of 50 nm, 10 parts of PVDF binder, and 75 parts of NMP solvent, the thickness of the dense layer is 3 μm, and the coating speed is adjusted to 8 m / min; the slurry for preparing the transition layer includes, in parts by weight, 40 parts of PMMA microspheres pore-forming agent with a particle size of 500 nm, 5 parts of PEO binder, and 55 parts of deionized water, the thickness of the transition layer is 8 μm, and the drying temperature after coating is adjusted to 110°C; the slurry for preparing the buffer layer includes, in parts by weight, 60 parts of NaCl crystal pore-forming agent with a particle size of 5 μm, 20 parts of PU binder, and 20 parts of ethanol, and the thickness of the buffer layer is 10 μm.

[0083] In Step 2, the pressure of the main pressure roller 11 is 4.0 MPa, and in Step 3, the temperatures of the zones in the zone annealing cavity 14 are 60°C, 80°C, 90°C, 110°C, and 130°C in sequence, wherein the coated film is kept at 90°C for 10 s, at 110°C for 20 s, and at 130°C for 30 s.

[0084] After inspection, the average pore diameter of the dense layer of the gradient-pore separator 16 prepared in this example is 10 nm, the average pore diameter of the transition layer is 200 nm, and the average pore diameter of the buffer layer is 500 nm; the gradient-pore separator 16 is tested by mercury intrusion method, and the porosities of the dense layer, the transition layer, and the buffer layer are measured to be 35%, 55%, and 65%, respectively, and the specific surface area is 150 m 2 / g.

[0085] The gradient-pore separator 16 prepared in this example is assembled into a Li / / Si@C button cell, wherein the content of the negative electrode silicon is 35 wt.% (the capacity is about 1000 mAh / g), and a cycle test is performed (0.5C charge and discharge, voltage range 0.005V~1.5V), and the results are shown in Figure 4 , the capacity retention rate of the Li / / Si@C button cell is 95% after 400 cycles; EIS test, frequency is 100 kHz~0.1 Hz, and the results are shown in Figure 5 , the ionic conductivity is 1.5 mS / cm.

[0086] Example 5

[0087] The difference between this embodiment and embodiment 2 is that the preparation slurry of the dense layer in step one includes 5 parts of nano-SiO2 pore-forming agent with a particle size of 200 nm, 2 parts of PVDF binder, and 93 parts of NMP solvent in terms of weight fraction, the thickness of the dense layer is 5 μm, and the coating speed is adjusted to 15 m / min; the preparation slurry of the transition layer includes 20 parts of PMMA microsphere pore-forming agent with a particle size of 1 μm, 0.5 parts of PEO binder, and 79.5 parts of deionized water in terms of weight fraction, the thickness of the transition layer is 12 μm; the preparation slurry of the buffer layer includes 30 parts of NaCl crystal pore-forming agent with a particle size of 10 μm, 5 parts of PU binder, and 65 parts of ethanol in terms of weight fraction, and the thickness of the buffer layer is 15 μm.

[0088] In step two, the pressure of the main pressure roller 11 is 3.25 MPa; in step three, the temperatures of the zones in the zone annealing cavity 14 are 60℃, 80℃, 100℃, 110℃, and 115℃ in sequence, and the coating film is kept at 100℃ for more than 20 s, at 110℃ for more than 40 s, and at 115℃ for more than 60 s.

[0089] After inspection, the average pore diameter of the dense layer of the gradient porous separator 16 prepared in this embodiment is 15 nm, the average pore diameter of the transition layer is 1 μm, and the average pore diameter of the buffer layer is 7 μm; the gradient porous separator 16 prepared in this embodiment is tested by the mercury injection method, and the porosities of the dense layer, the transition layer, and the buffer layer are 22%, 40%, and 55%, respectively.

[0090] The gradient porous separator 16 prepared in this embodiment is assembled into a Li / / Si@C button cell, in which the content of the negative electrode silicon is 20 wt.% (the capacity is about 1000 mAh / g), and a cycle test (0.5C charge and discharge, voltage range 0.005V~1.5V) is performed, and the capacity retention rate of the Li / / Si@C button cell at the 550th cycle is 87%.

[0091] The gradient porous separator 16 prepared in this embodiment with a size of 10.0 cm×10.0 cm is tested for thermal shrinkage rate, and the thermal shrinkage rate at 200℃ is 1.8%, which is better than Celgard® 2400.

[0092] Embodiment 6

[0093] The difference between this embodiment and embodiment 2 is that the annealing temperature is adjusted to 55℃, 75℃, 85℃, 105℃, and 125℃, and the coating film is kept at 85℃ for 20 s (to activate the PVDF molecular chain in the dense layer zone), at 105℃ for 40 s (to fully soften the PMMA in the transition layer zone), and at 125℃ for 60 s (to completely migrate the NaCl in the buffer layer zone).

[0094] The gradient porous diaphragm 16 prepared in this embodiment is tested, and the average pore size of the dense layer is 40 nm, the average pore size of the transition layer is 1 μm, and the average pore size of the buffer layer is 10 μm; the gradient porous diaphragm 16 is tested by the mercury injection method, and the porosities of the dense layer, the transition layer and the buffer layer are 30%, 50% and 63%, respectively. The thermal shrinkage rate of the gradient porous diaphragm 16 with a size of 10.0 cm x 10.0 cm is tested, and the thermal shrinkage rate at 200°C is 6.2%, which is higher than that of Example 2. The low binder content leads to a decrease in mechanical strength, but the high pore-forming agent content increases the porosity.

[0095] The gradient porous diaphragm 16 prepared in this embodiment is assembled into a Li / / Si@C button cell with a negative electrode silicon content of 20 wt.% (a capacity of about 1000 mAh / g), and a cycle test (0.5C charge and discharge, a voltage range of 0.005V~1.5V) is performed. The capacity retention rate of the Li / / Si@C button cell at the 500th cycle is 84%; an EIS test is performed, and the frequency is 100kHz~0.1Hz. The ionic conductivity is 0.95mS / cm.

[0096] The above is only a preferred embodiment of the present application, and does not limit the present application. Any simple modification, change and equivalent structural transformation of the above embodiment according to the technical essence of the present application are still within the protection scope of the technical solution of the present application.

Claims

1. A method for preparing a gradient-porous polymeric separator, characterized by, The method comprises the following steps: Step one, gradient coating: coating the base film (2) with a dense layer, a transition layer and a buffer layer in sequence through a modular coating unit to obtain a coated film; the preparation slurry of the dense layer comprises, in parts by weight: nano-SiO2 pore former 5-15 parts, PVDF binder 2-10 parts, NMP solvent 60-93 parts, the particle size of the nano-SiO2 is 50-200 nm; the thickness of the dense layer is 3-5 μm; The preparation slurry of the transition layer comprises, in parts by weight: PMMA microsphere pore former 20-40 parts, PEO binder 0.5-5 parts, deionized water 50-80 parts, the particle size of the PMMA microsphere pore former is 500-1 μm; the thickness of the transition layer is 8-12 μm; The preparation slurry of the buffer layer comprises, in parts by weight: NaCl crystal pore former 30-60 parts, PU binder 5-20 parts, ethanol 20-70 parts, the particle size of the NaCl crystal pore former is 5-10 μm; the thickness of the buffer layer is 10-15 μm; Step two, dynamic roller pressing: the coated film obtained in step one is sequentially passed through a multi-axis roller pressing machine to obtain a coated film with a cavity array; Step three, gradient annealing: the coated film with a cavity array obtained in step two is subjected to annealing treatment through a partitioned annealing cavity (14) to induce directional migration of the pore former, and a gradient porous separator (16) is obtained after cooling; the temperature of the partitioned annealing cavity (14) is 50-130℃, the temperatures of the multiple independent temperature control zones of the partitioned annealing cavity (14) increase in sequence; the annealing treatment regime comprises: 80-100℃ for 10 s or more, 100-110℃ for 20 s or more, and 115-130℃ for 30 s or more; the cooling rate after the annealing treatment is 5℃ / min; the pressure P of the main pressing roller (11) of the multi-axis roller pressing machine is calculated according to the following formula: ； P is the pressure of the main pressure roller (11) in MPa; Tmax is the maximum temperature of the zone annealing chamber (14) in step three in °C.

2. The method of claim 1, wherein, The base film (2) in step one is made of PE or PP, the thickness of the base film (2) is 5-15 μm, and the base film (2) is subjected to plasma activation treatment before being passed through the modular coating unit.

3. The method of claim 1, wherein, The coated film is preheated at 50℃ before being rolled in step two, and the diameter of the cavities of the coated film is 50-200 nm.

4. The method of claim 1, wherein, In step three, the pore diameter of the dense layer in the gradient porous separator (16) is 10-50 nm, and the porosity is 22-40%; the pore diameter of the transition layer in the gradient porous separator (16) is 200 nm-1 μm, and the porosity is 40-55%; the pore diameter of the buffer layer in the gradient porous separator (16) is 500 nm-10 μm, and the porosity is 50-65%.

5. A dynamic roll-pressing device for forming a gradient-pore separator for a silicon-based anode according to any one of claims 1 to 4, characterized by, The application relates to a multi-axle roller press unit and a partitioned annealing cavity (14) arranged in sequence, wherein the multi-axle roller press unit comprises a slotted coating mechanism (5), a spray coating mechanism (7) and a micro-gravure coating mechanism (8) arranged in sequence, and the partitioned annealing cavity (14) comprises a main compression roller (11) and a supporting roller (10) matched with the main compression roller (11), and a dynamic pressure feedback system is arranged on the main compression roller (11). 6.The dynamic roll-pressing forming device for a gradient-porous separator for a silicon-based negative electrode according to claim 5, characterized in that, The front end of the multi-axle roller press unit is provided with an unwinder (1), the rear end of the partitioned annealing cavity (14) is provided with a winding machine (17), the slotted coating mechanism (5), the spray coating mechanism (7) and the micro-gravure coating mechanism (8) are all equipped with a monitoring system (6), the monitoring system (6) comprises a laser confocal displacement meter and a terahertz wave pore analyzer, and the front end of the slotted coating mechanism (5) is provided with a plasma treatment device (4). 7.The dynamic roll-pressing forming device for a gradient-porous separator for a silicon-based negative electrode according to claim 5, characterized in that, The surface of the main compression roller (11) is laser-engraved with an array of micro-grooves (12), wherein the ratio of the width to the depth of the micro-grooves (12) is 1.5-2.5, and the ratio of the center distance between adjacent micro-grooves (12) to the depth of the micro-grooves (12) is 3-5. The dynamic pressure feedback system comprises a piezoelectric ceramic stress sensor array embedded on the surface of the main compression roller (11) and a dynamic control hydraulic servo system connected with the piezoelectric ceramic stress sensor array, the dynamic control hydraulic servo system is used for controlling the pressure fluctuation on the surface of the main compression roller (11) to be less than 0.3 MPa by Ziegler-Nichols method. 8.The dynamic roll-pressing forming device for a gradient-porous separator for a silicon-based negative electrode according to claim 5, characterized in that, The partitioned annealing cavity (14) has a plurality of independent temperature control zones provided with infrared heating devices, the top of each of the independent temperature control zones is provided with a laminar flow air curtain device, and the partitioned annealing cavity (14) is connected with a vacuum and gas control system; and the rear end of the partitioned annealing cavity (14) is provided with a post-processing mechanism (15). 9.The gradient-porous separator dynamic roll-pressing forming device for silicon-based negative electrodes according to claim 5, characterized in that, The multi-axle roller press unit further comprises a feeding roller (9) arranged at the front end of the main compression roller (11) and a shaping roller (13) arranged at the rear end of the main compression roller (11), the feeding roller (9) is internally provided with an electric heating pipe, the surface of the shaping roller (13) is covered with a PTFE coating, and the inside of the shaping roller (13) is provided with a cooling circulating water pipe, and the diameter ratio of the feeding roller (9), the main compression roller (11) and the shaping roller (13) is 1:2:4.

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