High-wettability long-cycle lithium battery composite diaphragm as well as preparation method and application thereof

Abstract

The invention relates to a high-wettability long-circulation lithium battery composite diaphragm and a preparation method and application thereof.The method comprises the steps that a carbonic ester-based electrolyte formula is obtained, a composite film-forming agent is added, a mixed solution is formed, and the mixed solution is stirred to be in a uniform state; adding nanometer aluminum oxide particles for catalysis in the process of stirring the mixed solution, controlling the particle size range of the nanometer aluminum oxide particles to be 50-100 nanometers, adding cellulose into the mixed solution, and adjusting the viscosity of the solution until the mixed solution forms a stable suspension system; injecting and uniformly covering the mixed solution in the suspension system state on the surface of the battery cell diaphragm, and curing the mixed solution attached to the battery cell diaphragm at a high temperature of 150-200 DEG C to form a uniform dendrite-free diaphragm protection layer; the purposes of improving the wettability of the lithium battery diaphragm to electrolyte, prolonging the cycle life of a battery cell and keeping high coulombic efficiency are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a high-wettability long-cycle lithium-ion battery composite separator and its preparation method and application. Background Art

[0002] In the current field of lithium-ion batteries, as a key component, the performance of the separator plays a decisive role in the overall performance of the battery. Facing the increasing high-performance requirements, traditional lithium-ion battery separators expose many problems that need to be solved urgently. On the one hand, the wettability of ordinary separators to electrolytes is poor, resulting in low ion conduction efficiency and seriously affecting the charge and discharge performance of the battery. On the other hand, during the charge and discharge process of the battery, the growth of metallic lithium dendrites is likely to cause internal short circuits or overheating in the battery, greatly threatening the safety and service life of the battery. At the same time, in the existing separator preparation process, the problem of the dispersion of inorganic particles in the separator always exists. It is not only easy to have pore-blocking problems, hindering the normal shuttling of lithium ions, but also has a negative impact on the mechanical properties of the separator. In addition, under high-temperature environments, the thermal stability of the separator is insufficient and the thermal shrinkage rate is too high, further limiting the application of the battery under complex working conditions. Currently, the general cycle life of the battery cells in the industry is only about 2000 times, far from meeting the strict requirements for long life and high stability of batteries such as electric vehicles and energy storage power stations. Summary of the Invention

[0003] The first object of the present invention is to provide a high-wettability long-cycle lithium-ion battery composite separator and its preparation method and application to achieve the purpose of improving the wettability of the lithium-ion battery separator to the electrolyte, increasing the cycle life of the battery cell, and maintaining a high coulombic efficiency.

[0004] To achieve the above object, the present invention provides a preparation method of a high-wettability long-cycle lithium-ion battery composite separator, including the following steps: Obtain a carbonate-based electrolyte formula, add a composite film-forming agent to form a mixed solution, and stir until it is in a uniform state. The composite film-forming agent includes vinylene carbonate, ethylene vinylene carbonate, and fluoroethylene carbonate; Add nano-aluminum oxide particles for catalysis during the stirring of the mixed solution. The particle size range of the nano-aluminum oxide particles is controlled within 50-100 nanometers. Add cellulose to the mixed solution to adjust the solution viscosity until the mixed solution forms a stable suspension system; Inject the mixed solution in the state of a suspension system and uniformly cover it on the surface of the battery cell separator. Under high-temperature conditions of 150-200 °C, cure the mixed solution attached to the battery cell separator to form a uniform dendrite-free separator protective layer.

[0005] Further, the step of adding the composite film-forming agent to form a mixed solution includes: Add a composite film-forming agent to the carbonate-based electrolyte, and the total addition amount of the composite film-forming agent is 0.5-5 wt% of the mass of the carbonate-based electrolyte; In a dry environment with a dew point temperature < -40 °C, the stirring speed is 300-800 revolutions per minute, and the stirring time is 2-6 hours to form a mixed solution.

[0006] Further, the step of adding nano-aluminum oxide particles for catalysis during the stirring of the mixed solution includes: Add surface-hydroxylated nano-aluminum oxide particles to the stirred mixed solution, and the addition amount of the nano-aluminum oxide particles is 0.1-3 wt% of the total mass of the mixed solution; During the addition of the nano-aluminum oxide particles, ultrasonic-assisted dispersion treatment is carried out, and the ultrasonic frequency is 20-40 kHz.

[0007] Further, the weight ratio of vinylene carbonate, ethylene vinylene carbonate and fluoroethylene carbonate in the composite film-forming agent is (1-3):(2-4):(1-2), and the specific surface area of the surface-hydroxylated nano-aluminum oxide particles after treatment is 50-150 m² / g, and the ultrasonic dispersion time is 10-60 minutes.

[0008] Further, the step of adding cellulose to the mixed solution to adjust the solution viscosity until a stable suspension system is formed for the mixed solution includes: Add cellulose to the mixed solution to which nano-aluminum oxide particles have been added. The cellulose is sodium carboxymethylcellulose, and the addition amount is 0.05-1.5 wt% of the total mass of the mixed solution; Stir the mixed solution evenly to adjust the solution viscosity to 300-1500 mPa·s measured at 25 °C to form a mixed solution in a suspension system state. The absolute value of the Zeta potential of the mixed solution in the suspension system state is ≥ 30 mV, and the sedimentation volume ratio after standing for 24 hours is ≤ 5%.

[0009] Further, the step of injecting the mixed solution in the suspension system state and uniformly covering the surface of the battery cell diaphragm, and curing the mixed solution attached to the battery cell diaphragm at a high temperature of 150-200 °C to form a uniform dendrite-free diaphragm protection layer includes: Uniformly coat the suspension system mixed solution on the surface of the battery cell diaphragm with a wet film thickness of 5-20 μm, and after coating, leave it standing in an environment with a humidity < 30% for 5-30 minutes to form a pre-cured layer; The process of forming the pre-cured layer adopts a gradient heating method. The temperature is raised to 80-120°C at a heating rate of 5-10°C / min. After maintaining the temperature for 10-30 minutes, it is raised to the final curing temperature of 150-200°C at a rate of 3-5°C / min. The curing pressure is controlled at 0.1-1 MPa, and the nano-aluminum oxide particles are evenly embedded in the polymer network to form a dendrite-free protective layer. The cured separator is placed in an argon protection environment and secondarily annealed at 120-150°C for 0.5-2 hours to eliminate internal stress and improve the uniformity of the protective layer. After annealing, it is cooled to room temperature at a rate of 2-5°C / min to ensure the formation of a dendrite-free protective layer.

[0010] Further, after the step of curing the mixed solution attached to the battery cell separator to form a uniform dendrite-free separator protective layer, the following steps are also included: Perform a wettability test on the surface of the separator protective layer. Drop the electrolyte onto the surface of the separator protective layer, and measure the static contact angle with a contact angle measuring instrument. When the contact angle ≥ 30°, increase the total addition amount of the composite film-forming agent in a gradient of 0.5-2 wt% and re-perform coating and curing. When the contact angle < 30°, it is determined that the wettability of the separator meets the standard. The contact angle measurement needs to be completed in an inert atmosphere glove box, the test temperature is 25 ± 2°C, the electrolyte dropping amount is 2 μL, and the measurement time is controlled to be completed within 10 seconds after dropping. Perform a lithium ion mobility test on the separator with qualified wettability. Use the alternating current impedance method to test the ionic conductivity of the separator in the frequency range of 1 MHz - 100 kHz and calculate the lithium ion mobility. Perform a high-temperature thermal shrinkage verification on the separator with qualified mobility. Finally, the separator needs to be verified for high-temperature thermal stability. Place the separator in an oven at 200°C and keep it for 1 hour, and measure the transverse and longitudinal shrinkage rates. When the shrinkage rate in any direction ≥ 0.3%, adjust the process and re-prepare the separator.

[0011] Further, the step of adjusting the process and re-preparing the separator includes: When the shrinkage rate in any direction ≥ 0.3%, perform at least one process adjustment and then re-prepare. The process adjustments include raising the curing pressure to the range of 0.5-1 MPa, adding 1-5 wt% polyimide fiber to cellulose, and replacing the nano-aluminum oxide particles with nano-zirconium oxide particles modified by a silane coupling agent.

[0012] The second object of the present invention is to provide a highly wettable long-cycle lithium battery composite separator obtained by the above method for preparing a highly wettable long-cycle lithium battery composite separator.

[0013] The third objective of the present invention is to provide an application of a highly wettable long-cycle lithium battery composite separator. In some specific embodiments, the composite separator is used in lithium-ion batteries.

[0014] In a specific embodiment, the composite separator is used in power batteries for electric vehicles, which can enhance the thermal stability of the battery under high-temperature conditions (such as fast charging and high-load operation), avoid internal short circuits or thermal runaway caused by separator shrinkage, and at the same time, through a uniform and dendrite-free SEI film, effectively block the risk of lithium dendrites penetrating the separator, greatly reducing safety hazards such as short circuits and fires caused by dendrites in electric vehicle batteries.

[0015] In a specific embodiment, the composite separator is used in energy storage system batteries. The ultra-long cycle life (>10,000 times) of the composite separator meets the stringent requirements of the energy storage system for battery life, reduces system maintenance costs, and can also enable the battery to operate stably in extreme temperature environments (such as outdoor energy storage power stations), avoiding capacity attenuation or failure caused by thermal deformation. In addition, it can also reduce the risk of chain reactions caused by local short circuits in the energy storage system by inhibiting dendrites and forming a uniform SEI film, especially suitable for large-scale battery pack integration scenarios.

[0016] In a specific embodiment, the composite separator is used in batteries for high-rate charge and discharge of portable electronic devices. The high ionic conductivity and porous structure of the composite separator significantly reduce the migration resistance of lithium ions, meeting the high-rate (such as 5C) charge and discharge requirements of devices such as mobile phones and laptops. During high-current density charge and discharge, the uniformity of the SEI film and the catalytic effect of nano-aluminum oxide can effectively prevent the growth of lithium dendrites, avoiding battery swelling or sudden capacity drop. It can increase the battery capacity in a limited space and extend the device's battery life. At the same time, the ultra-long cycle life (>10,000 times) reduces the need for users to frequently replace the battery, especially suitable for wearable devices (such as smart watches and wireless earphones) that need to be charged multiple times a day.

[0017] The highly wettable long-cycle lithium battery composite separator, its preparation method and application provided by the present invention have the following beneficial effects: (1) Through the self-developed carbonate-based electrolyte and the addition of a composite film-forming agent (FEC / VEC / VC), the present invention constructs a uniform and dendrite-free separator protection layer (SEI film), significantly improving the wettability of the separator to the electrolyte. The dendrite-free separator protection layer formed by the present invention can effectively prevent the growth of metallic lithium dendrites, eliminating battery short circuits or overheating phenomena caused by the growth of lithium dendrites. The excellent performance of the separator with a thermal shrinkage efficiency <0.3% at 200°C greatly improves the safety and stability of the battery in high-temperature environments; (2) By adding nano-aluminum oxide particles during the preparation process and combining specific process conditions, the problem of easy pore blockage of inorganic particles in the separator is solved, enabling lithium ions to shuttle quickly and unobstructed inside the separator, promoting the uniform distribution and migration of lithium ions throughout the membrane layer, and enhancing the ion transport efficiency inside the battery. At the same time, by adding cellulose and optimizing the process, the tensile resistance of the separator is improved, which not only enhances the structural stability of the separator in the complex internal environment of the battery but also provides a solid physical support for the long-term reliable operation of the battery. (3) Verified by a large number of experiments, the lithium-ion battery composite separator prepared by the present invention can achieve a cycle life of more than 10,000 times for the battery cell, which is several times that of the industry average of 2,000 times. The ultra-long cycle life means that the battery does not need to be replaced frequently during use, greatly reducing the use cost, and at the same time improving the use efficiency and reliability of related equipment. Description of the Drawings

[0018] Figure 1 is a schematic flow chart of a method for preparing a highly wettable and long-cycle lithium-ion battery composite separator according to an embodiment of the present invention.

[0019] The realization, functional characteristics, and advantages of the object of the present invention will be further described in conjunction with the embodiments with reference to the drawings. Detailed Embodiments

[0020] In order to make the object, technical solution, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0021] Refer to Figure 1 , which is a schematic flow chart of a method for preparing a highly wettable and long-cycle lithium-ion battery composite separator proposed by the present invention, and includes the following steps: S1. Obtain a carbonate-based electrolyte formulation, add a composite film-forming agent to form a mixed solution, and stir until it reaches a uniform state. The composite film-forming agent includes vinylene carbonate, ethylene vinylene carbonate, and fluoroethylene carbonate. S2. Add nano-aluminum oxide particles for catalysis during the stirring of the mixed solution. The particle size range of the nano-aluminum oxide particles is controlled within 50 - 100 nanometers. Add cellulose to the mixed solution to adjust the solution viscosity until the mixed solution forms a stable suspension system. S3. Inject the mixed solution in the suspension state and evenly cover it on the surface of the battery cell separator. Under high-temperature conditions of 150 - 200 °C, cure the mixed solution attached to the battery cell separator to form a uniform dendrite-free separator protection layer.

[0022] As described in the above step S1, the composite film-forming agent includes vinylene carbonate (VC), ethylene vinylene carbonate (VEC), and fluoroethylene carbonate (FEC) added in a weight ratio of (1-3):(2-4):(1-2), and the total addition amount is 0.5-5 wt% of the mass of the carbonate-based electrolyte. Among them, VC is used to preferentially decompose to form a dense SEI layer to inhibit the continuous decomposition of the electrolyte, VEC is used to enhance the toughness of the SEI film through vinyl cross-linking, and FEC is used to improve the wettability of the electrolyte to the separator (contact angle < 30°). The mixing solution is controlled to operate in a dry environment with a dew point temperature < -40°C to avoid side reactions caused by moisture. The stirring parameters are 300-800 revolutions per minute for 2-6 hours to ensure the full compatibility of the film-forming agent and the electrolyte.

[0023] As described in the above step S2, nano-aluminum oxide is added for catalysis and dispersion. The surface-hydroxylated nano-aluminum oxide particles (specific surface area 50-150 m² / g) are added in an amount of 0.1-3 wt%. Through ultrasonic-assisted dispersion (20-40 kHz), particle agglomeration is destroyed (agglomeration rate < 5%), and the absolute value of the Zeta potential ≥ 30 mV. The Zeta potential refers to the potential of the shear plane, also known as the electrokinetic potential or electrokinetic potential, which is an important index characterizing the stability of the colloidal dispersion system. The nano-aluminum oxide particles are used to catalyze the cross-linking reaction between cellulose and the film-forming agent, shorten the curing time, and at the same time serve as the skeleton of the lithium-ion transport channel, increasing the mobility to ≥ 1.2×10 -3 S / cm. Cellulose, sodium carboxymethyl cellulose (CMC-Na), with a viscosity-average molecular weight of 9×10 4 -7×10 5 ; The viscosity of the mixed solution is adjusted to 300-1500 mPa·s (25°C) to form a stable suspension system (sedimentation volume ratio ≤ 5% after standing for 24 hours). The cellulose can prevent the sedimentation of nanoparticles, ensure the coating uniformity (thickness deviation ≤ ±5%), and cooperate with nano-aluminum oxide to construct a three-dimensional porous structure (porosity 15-30%).

[0024] As described in the above step S3, the dip coating method or the slot-die coating method is adopted, with a wet film thickness of 5-20 μm. The mixed solution is evenly coated on the surface of the battery cell separator, and the environment is controlled to stand for 5-30 minutes at a humidity <30% to form a pre-crosslinked network (the thickness of the pre-cured layer is 3-15 μm). A gradient curing and forming process is adopted. In the first stage (80-120 °C), the temperature is increased at a rate of 5-10 °C / min and held for 10-30 minutes to promote the crosslinking of cellulose and the film-forming agent. In the second stage (150-200 °C), the temperature is increased at a rate of 3-5 °C / min and a pressure of 0.1-1 MPa is applied to embed nano-aluminum oxide into the polymer matrix to form a dendrite-free structure (the surface roughness Ra ≤ 50 nm). Post-treatment and performance verification are carried out on the surface of the cured separator, including argon annealing at 120-150 °C for 0.5-2 hours to eliminate internal stress (the bonding strength ≥ 8 N / m); cooling control, cooling at a rate of 2-5 °C / min to inhibit thermal shrinkage (the shrinkage rate at 200 °C < 0.3%); calculating the mobility. If the lithium ion mobility is lower than 0.8×10 -3 S / cm, the addition amount of nano-aluminum oxide particles is increased in a gradient of 0.1-0.5 wt% until the mobility ≥ 1.2×10 -3 S / cm; the alternating current impedance method test uses a symmetric stainless steel blocking electrode, applies a 10 mV sinusoidal wave perturbation voltage, and the test data is calculated after fitting with an equivalent circuit model to obtain the ionic conductivity; closed-loop regulation, if the contact angle ≥ 30° or the thermal shrinkage rate ≥ 0.3%, at least one process adjustment is performed and then re-prepared, such as increasing the curing pressure to the range of 0.5-1 MPa, adding 1-5 wt% polyimide fiber to the cellulose, or replacing the nano-aluminum oxide particles with nano-zirconium oxide particles modified by a silane coupling agent.

[0025] In one embodiment, the preparation materials include: a carbonate-based electrolyte, ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1, and 1 M lithium hexafluorophosphate (LiPF6) is added; a composite film-forming agent is added, including vinylene carbonate (VC), ethylene vinylene carbonate (VEC), and fluoroethylene carbonate (FEC), with a purity ≥ 99.9%. Nano-aluminum oxide particles, surface hydroxylation treatment, particle size 60-80 nm, specific surface area 80-120 m² / g; cellulose, sodium carboxymethyl cellulose (CMC-Na, viscosity-average molecular weight 3×10 5 ); a base film, a polyethylene / polypropylene (PE / PP) composite separator, with a thickness of 20 μm and a porosity of 45%.

[0026] In a dry glove box at a dew point temperature of -50 °C, a composite film-forming agent (VC:VEC:FEC = 2:3:1, total addition amount 3 wt%) was added to 100 g of a carbonate-based electrolyte; it was stirred at a speed of 500 revolutions per minute for 4 hours to form a homogeneous mixed solution. 1.5 wt% of nano-alumina particles were added to the stirred mixed solution, and at the same time, an ultrasonic disperser (frequency 30 kHz) was turned on and processed for 30 minutes; 0.8 wt% of CMC-Na was added, and stirring was continued for 2 hours to adjust the solution viscosity to 800 mPa·s (25 °C); the Zeta potential of the suspension was measured to be -35 mV, and the sedimentation volume ratio after standing for 24 hours was 3%. The suspension was uniformly coated onto the surface of a PE / PP base film with a wet film thickness of 15 μm using a slot coater; after coating, the separator was left standing in an environment with a humidity of 25% for 20 minutes to form a pre-cured layer; it was heated to 100 °C at a rate of 8 °C / min and held for 20 minutes; it was heated to 180 °C at a rate of 4 °C / min and cured under a pressure of 0.5 MPa for 40 minutes; after curing, the separator was transferred to an argon annealing furnace (argon purity 99.999%) and annealed at 135 °C for 1 hour, and then cooled to room temperature at a rate of 3 °C / min.

[0027] Test 1: Wettability verification. 2 μL of electrolyte was dropped in the glove box, and the contact angle was measured to be 28° (meeting the standard, no adjustment required); Test 2: Lithium ion mobility test. The ionic conductivity was measured to be 1.3×10 -3 S / cm (meeting the standard) by the AC impedance method. Test 3: High-temperature thermal shrinkage rate. After maintaining a constant temperature of 200 °C for 1 hour, the transverse shrinkage rate was 0.22% and the longitudinal shrinkage rate was 0.25% (meeting the standard). Test 4: Dendrite inhibition effect. Constant current charge and discharge test (1C rate, 4.5V cut-off), after 10,000 cycles, the capacity retention rate was 86.5% and the Coulomb efficiency was 99.6%. SEM observation showed no dendritic protrusions on the surface of the protective layer.

[0028] A comparative experiment was added (process parameters were adjusted). The simulation of the adjustment after the thermal shrinkage rate exceeded the standard (0.35%) was to increase the curing pressure to 0.8 MPa and add 3 wt% of polyimide fibers; the test results after re-preparation were that the thermal shrinkage rate decreased to 0.18%; the ion mobility remained at 1.25×10 -3 S / cm; the contact angle was 26°.

[0029] In this example, the specific ratio (2:3:1) of the composite film-forming agent VC / VEC / FEC formed a gradient SEI film during curing. The fluorine-rich layer formed by the decomposition of FEC on the surface layer reduced the interfacial impedance; the cross-linking of VEC in the middle layer enhanced the mechanical strength; and VC at the bottom layer reacted with cellulose to form through pores. Hydroxylation treatment + ultrasonic dispersion evenly embedded the nano-alumina particles into the polymer network, and the ion channel density was increased to 1.5×10 4Channels / μm². By real-time monitoring and parameter adjustment (such as pressure increase, fiber addition), the yield rate has been increased from 80% of the traditional process to 98%.

[0030] In summary, the present invention obtains a carbonate-based electrolyte formulation, adds a composite film-forming agent to form a mixed solution, and stirs it until it is in a uniform state; during the stirring of the mixed solution, nano-aluminum oxide particles are added for catalysis, and the particle size range of the nano-aluminum oxide particles is controlled within 50-100 nanometers. Cellulose is added to the mixed solution to adjust the solution viscosity until the mixed solution forms a stable suspension system; the mixed solution in the suspension system state is injected and evenly covered on the surface of the battery cell separator, and the mixed solution attached to the battery cell separator is cured under high temperature conditions of 150-200 °C to form a uniform dendrite-free separator protection layer, so as to achieve the purpose of improving the wettability of the lithium battery separator to the electrolyte, increasing the cycle life of the battery cell, and maintaining a high coulomb efficiency.

[0031] It should be noted that in this article, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, device, article or method including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, device, article or method. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element in the process, device, article or method including that element.

[0032] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be equally included in the patent protection scope of the present invention.

Claims

1. A preparation method of a highly wettable long-circulation lithium battery composite separator, characterized in that It includes the following steps: Obtain a carbonate-based electrolyte formulation, add a composite film-forming agent to form a mixed solution, and stir until it reaches a homogeneous state. The composite film-forming agent includes vinylene carbonate, ethylene vinylene carbonate, and fluoroethylene carbonate; Add nano-aluminum oxide particles for catalysis during the stirring of the mixed solution. The particle size range of the nano-aluminum oxide particles is controlled within 50 - 100 nanometers. Add cellulose to the mixed solution to adjust the solution viscosity until a stable suspension system is formed; Inject the mixed solution in a suspension state and evenly cover the surface of the cell diaphragm. Under high-temperature conditions of 150 - 200 °C, cure the mixed solution attached to the cell diaphragm to form a uniform dendrite-free diaphragm protection layer.

2. The preparation method of the highly wettable long-circulation lithium battery composite separator according to claim 1, characterized in that The step of adding the composite film-forming agent to form a mixed solution includes: Add the composite film-forming agent to the carbonate-based electrolyte. The total addition amount of the composite film-forming agent is 0.5 - 5 wt% of the mass of the carbonate-based electrolyte; In a dry environment with a dew point temperature < -40 °C, the stirring speed is 300 - 800 revolutions per minute, and the stirring time is 2 - 6 hours to form a mixed solution.

3. The preparation method of the highly wettable long-circulation lithium battery composite separator according to claim 1, characterized in that, The step of adding nano-aluminum oxide particles for catalysis during the stirring of the mixed solution includes: Add surface-hydroxylated nano-aluminum oxide particles to the stirring mixed solution. The addition amount of the nano-aluminum oxide particles is 0.1 - 3 wt% of the total mass of the mixed solution; During the addition of the nano-aluminum oxide particles, perform ultrasonic-assisted dispersion treatment with an ultrasonic frequency of 20 - 40 kHz.

4. The preparation method of the highly wettable long-circulation lithium battery composite separator according to claim 1, characterized in that, The weight ratio of vinylene carbonate, ethylene vinylene carbonate, and fluoroethylene carbonate in the composite film-forming agent is (1 - 3):(2 - 4):(1 - 2), and the specific surface area of the surface-hydroxylated nano-aluminum oxide particles is 50 - 150 m² / g, and the ultrasonic dispersion time is 10 - 60 minutes.

5. The preparation method of the highly wettable long-circulation lithium battery composite separator according to claim 1, characterized in that, The step of adding cellulose to the mixed solution to adjust the solution viscosity until a stable suspension system is formed includes: Add cellulose, which is sodium carboxymethyl cellulose, to the mixed solution added with nano-aluminum oxide particles. The addition amount is 0.05 - 1.5 wt% of the total mass of the mixed solution; Stir the mixed solution evenly to adjust the solution viscosity to 300 - 1500 millipascal-seconds measured at 25 °C to form a mixed solution in a suspension state. The absolute value of the Zeta potential of the mixed solution in the suspension state is ≥ 30 mV, and the sedimentation volume ratio after standing for 24 hours is ≤ 5%.

6. The preparation method of the highly wettable long-circulation lithium battery composite separator according to claim 1, wherein The step of injecting the mixed solution in a suspension state and evenly covering the surface of the cell diaphragm, and curing the mixed solution attached to the cell diaphragm under high-temperature conditions of 150 - 200 °C to form a uniform dendrite-free diaphragm protection layer includes: Evenly coat the suspension mixed solution onto the surface of the cell diaphragm with a wet film thickness of 5 - 20 μm. After coating, let it stand in an environment with a humidity < 30% for 5 - 30 minutes to form a pre-cured layer; The process of forming the pre-cured layer adopts a gradient heating method. The temperature is raised to 80 - 120 °C at a heating rate of 5 - 10 °C / min. After maintaining the temperature for 10 - 30 minutes, it is raised to the final curing temperature of 150 - 200 °C at a rate of 3 - 5 °C / min. The curing pressure is controlled at 0.1 - 1 MPa, and the nano-aluminum oxide particles are evenly embedded in the polymer network to form a dendrite-free protective layer; The cured separator is placed in an argon protection environment and annealed for 0.5 - 2 hours at 120 - 150 °C to eliminate internal stress and improve the uniformity of the protective layer; After annealing, it is cooled to room temperature at a rate of 2 - 5 °C / min to ensure the formation of a dendrite-free protective layer.

7. The preparation method of the highly wettable long-circulation lithium battery composite separator according to claim 1, characterized in that, After the step of curing the mixed solution attached to the battery cell separator to form a uniform dendrite-free separator protective layer, it further includes: Performing a wettability test on the surface of the separator protective layer. Drop the electrolyte onto the surface of the separator protective layer, and measure the static contact angle with a contact angle measuring instrument. When the contact angle ≥ 30°, increase the total addition amount of the composite film-forming agent in a gradient of 0.5 - 2 wt% and re-perform coating and curing. When the contact angle < 30°, it is determined that the wettability of the separator meets the standard. The contact angle measurement needs to be completed in an inert atmosphere glove box. The test temperature is 25 ± 2 °C, the electrolyte drop amount is 2 μL, and the measurement time is controlled to be completed within 10 seconds after dropping; Performing a lithium ion mobility test on the separator with qualified wettability. Using the AC impedance method to test the ionic conductivity of the separator in the frequency range of 1 MHz - 100 kHz and calculate the lithium ion mobility; Performing a high-temperature thermal shrinkage verification on the separator with qualified mobility. Finally, the separator needs to be verified for high-temperature thermal stability. Place the separator in an oven at 200 °C for 1 hour and measure the transverse and longitudinal shrinkage rates. When the shrinkage rate in any direction ≥ 0.3%, adjust the process and re-prepare the separator.

8. The preparation method of the highly wettable long-circulation lithium battery composite separator according to claim 7, characterized in that, The step of adjusting the process and re-preparing the separator includes: When the shrinkage rate in any direction ≥ 0.3%, perform at least one process adjustment and then re-prepare. The process adjustments include raising the curing pressure to the range of 0.5 - 1 MPa, adding 1 - 5 wt% polyimide fiber to cellulose, and replacing the nano-aluminum oxide particles with nano-zirconium oxide particles modified by a silane coupling agent.

9. A long-circulating lithium battery composite separator with high wettability, characterized in that Prepared by the method of the high-wettability long-cycle lithium-ion battery composite separator according to any one of claims 1 - 8.

10. Application of a highly wettable long-circulation lithium battery composite separator, characterized in that, The composite separator is used in a lithium-ion battery.

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