Graphene-polyimide composite lithium ion battery diaphragm and preparation method thereof

Abstract

The invention provides a composite lithium ion battery diaphragm. The composite lithium ion battery diaphragm is composed of rare earth doped graphene and polyimide. According to the invention, the problem that the existing composite lithium ion battery diaphragm is insufficient in conductivity, dendritic crystal inhibition, stability in long-term use under extreme conditions and the like is solved, and the preparation of the lithium ion battery diaphragm capable of keeping high ionic conductivity, mechanical strength, thermal stability and dendritic crystal inhibition is realized. Experiments prove that the ionic conductivity of the cerium nitrate doped graphene-polyimide composite lithium ion battery diaphragm obtained by the invention is 0.5-1.8 mS / cm; the mechanical strength is 50 to 79 MPa; the thermal shrinkage rate in thermal stability is less than or equal to 5%-12% (150 DEG C / 1h); and the cycle life of a dendritic crystal inhibition test is more than 80-200 times (1mA / cm).

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and specifically to a cerium nitrate-doped graphene-polyimide composite lithium-ion battery separator. Background Art

[0002] The separator of a lithium-ion battery is one of the key components of a lithium-ion battery and is located between the positive and negative electrodes. Its main function is to prevent the direct contact between the positive and negative electrodes from causing a short circuit, while allowing lithium ions to freely pass through during charge and discharge to ensure the normal electrochemical cycle of the battery. At the microscopic level, the separator has a large number of microporous structures, and the characteristics such as the size, distribution, and connectivity of these micropores have a crucial impact on the lithium-ion transport efficiency and the performance of the battery.

[0003] The development of lithium-ion battery separators can be traced back to the early stage of lithium-ion battery research and development. In the early days, people realized the need for a material to separate the positive and negative electrodes. The initial separator materials were mainly some simple porous materials, such as glass fiber filter paper, etc., but these materials had many deficiencies in performance, such as poor mechanical strength and chemical stability. With the development of materials science, polyolefin materials (such as polyethylene PE and polypropylene PP) have been widely used in separator manufacturing due to their good chemical stability, mechanical properties, and suitable processing properties. In the 1980s and 1990s of the 20th century, polyolefin separators gradually became the mainstream materials for lithium-ion battery separators. During this period, through continuous modification of polyolefin materials and optimization of processing techniques, the performance of the separators was significantly improved. Entering the 21st century, with the large-scale application of lithium-ion batteries in fields such as electric vehicles and energy storage systems, higher requirements have been put forward for the performance of lithium-ion battery separators. For example, the growth of lithium dendrites during battery cycling is a major safety issue, which may pierce the separator, cause a short circuit, and potential battery failures; to improve the safety of the battery, the separator is required to have better thermal stability and puncture resistance. To increase the energy density and charge-discharge efficiency of the battery, the separator is required to have higher ionic conductivity and a thinner thickness.

[0004] Chinese patent document CN118589142A discloses a production method of a polyimide lithium-ion battery separator, which includes the following steps: S1, preparing a substrate; S2, treating the substrate; S3, preparing a polyimide resin solution; S4, coating the polyimide resin solution on the substrate; S5, performing an immobilization treatment on the substrate coated with the polyimide resin solution; S6, performing a cutting and shaping treatment on the cured substrate; S7, post-treating and drying the separator; S8, performing an imidization treatment on the cut and shaped substrate to obtain a polyimide lithium-ion battery separator with excellent performance; S9, performing a quality inspection on the polyimide lithium-ion battery separator. This technology can obtain a polyimide lithium-ion battery separator with excellent performance by first preparing the substrate of the polyimide lithium-ion battery separator, then coating the polyimide solution on the substrate of the polyimide lithium-ion battery separator and performing a series of treatments.

[0005] Chinese patent document CN117810644A discloses a polyimide lithium-ion battery separator and its preparation method, which relates to the technical field of battery separators. When preparing the polyimide lithium-ion battery separator, first react 5-chloro-1-pentene with [(oxiran-2-yl)methyl]dipropylamine to obtain a double-bond epoxy quaternary ammonium salt; electrospin tetraethyl orthosilicate to prepare porous nano-silica fibers, and then react the porous nano-silica fibers with 3-aminopropyltriethoxysilane and the double-bond epoxy quaternary ammonium salt in sequence to obtain modified porous nano-silica fibers; react 2-allylbenzene-1,4-diol with 2-amino-5-chlorobenzoxazole to obtain a double-bond-containing benzoxazole diamine monomer; finally, react the modified porous nano-silica fibers, the double-bond-containing benzoxazole diamine monomer, p-phenylenediamine and 3,3',4,4'-biphenyltetracarboxylic dianhydride to prepare a polyimide lithium-ion battery separator. The polyimide lithium-ion battery separator prepared by this technology has excellent thermal stability and ionic conductivity.

[0006] The inventor has found through research that the lithium-ion battery separators disclosed in the above prior art need to add other materials to improve their ionic conductivity and electrochemical performance, and further inhibit the growth of lithium dendrites; when used in a long-term extreme environment, a single substrate is prone to degradation and other materials need to be added for compounding to improve its long-term mechanical properties and thermal stability. Generally speaking, there are still deficiencies in conductivity, dendrite inhibition and stability under long-term extreme conditions. Therefore, it is of great significance to develop a lithium-ion battery separator that can maintain high ionic conductivity, mechanical strength, thermal stability and dendrite inhibition. Summary of the Invention

[0007] The present invention provides a high-performance flexible graphene-based supercapacitor and a preparation method thereof, aiming to solve the problems of deficiencies in aspects such as conductivity, dendrite inhibition, and stability under long-term extreme conditions of existing composite lithium-ion battery separators. The preparation of a lithium-ion battery separator capable of maintaining high ionic conductivity, mechanical strength, thermal stability, and dendrite inhibition is realized.

[0008] The present invention specifically provides a composite lithium-ion battery separator, which is composed of rare-earth doped graphene and polyimide.

[0009] Preferably, the rare-earth doped graphene is obtained by mixing a graphene oxide suspension and a rare-earth precursor solution in a molar ratio of graphene oxide:rare-earth compound of 1:0.1 - 0.5.

[0010] More preferably, the graphene suspension is formed by ultrasonically dispersing graphene oxide powder with a specific surface area of 200 - 800 m 2 / g and an oxygen content of 40% - 50% in deionized water or ethanol at a concentration of 0.5 - 2 wt% for 1 - 2 hours with an output power of 100 - 300 w.

[0011] Preferably, the rare-earth precursor solution is a solution formed by dispersing a rare-earth compound in ethanol or deionized water at a concentration of 0.05 - 0.1 mol / L, and the rare-earth compound is selected from lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, or gadolinium.

[0012] More preferably, the polyimide is formed by imidization reaction of a polyamic acid precursor.

[0013] Further preferably, the polyamic acid precursor is formed by reacting 4,4'-diaminodiphenyl ether and phthalic anhydride in a molar ratio of 1:2 - 2:1 under an inert gas atmosphere.

[0014] The present invention also provides a method for preparing the composite lithium-ion battery separator, which is characterized by including the steps: S1) preparation of a rare-earth doped graphene dispersion; S2) preparation of a polyamic acid precursor; S3) mixing the solution in step S1) and the solution in step S2) to generate a pore structure; S4) carrying out an imidization reaction on a separator substrate.

[0015] Preferably, the method for generating the pore structure in step S3) is selected from: 1) adding 8 - 15 wt% of polyethylene glycol to the mixed solution and then carrying out in-situ mixing; 2) casting the mixed solution into a film by solvent evaporation; 3) making the mixed solution into electrospun fibers by electrospinning.

[0016] More preferably, the mass of the rare-earth doped graphene in step S3) is 5 - 20 wt% of the polyamic acid.

[0017] Beneficial effects

[0018] Through the synergy of graphene and polyimide, the mechanical strength and flexibility of the lithium-ion electronic separator are enhanced in the present invention. As a filler material, graphene oxide sheets can distribute stress more evenly on the polyimide matrix, helping to prevent crack propagation. The doping of cerium nitrate further improves the stability and dispersibility of graphene, thereby enhancing the mechanical stability under stress and strengthening the structural integrity during battery cycling. This synergy endows the polyimide separator of the present invention with higher flexibility and mechanical strength, which is crucial for battery separators that must withstand continuous expansion, compression, and mechanical forces during charge and discharge cycles.

[0019] The lithium-ion battery separator of the present invention has extremely high thermal stability. Polyimide itself has excellent thermal stability; however, under extreme conditions, pure polyimide is prone to degradation over time. The high thermal conductivity of graphene helps to evenly distribute heat within the separator, preventing the formation of local hotspots during high-speed charge and discharge cycles; the doping of cerium nitrate further enhances the thermal stability of graphene oxide sheets by altering the microstructure and increasing the thermal resistance of the composite material. The rare earth element can also act as a catalyst for oxidation stability, helping the separator to resist degradation when exposed to high-temperature environments. This synergistic combination enables the lithium-ion battery separator of the present invention to operate at higher temperatures and withstand thermal cycling without loss of mechanical properties or structural integrity, thereby improving the safety and reliability of the battery system.

[0020] The lithium-ion battery separator of the present invention enhances ionic conductivity and electrochemical performance. Graphene is renowned for its excellent electrical conductivity, but the presence of graphene oxide in its oxidized form (epoxides, hydroxyl groups, and carboxyl groups) usually reduces its conductivity. The doping of cerium nitrate can not only enhance the dispersibility of graphene but also optimize the transport on the separator, facilitating the movement of ions through the electrolyte and the separator, thereby improving the electrochemical stability of the separator. The high surface area of polyimide also provides more active sites for ion absorption.

[0021] The lithium-ion battery separator of the present invention greatly improves battery safety by inhibiting dendrites and having high thermal stability. Graphene helps to form a structurally rigid framework, preventing electrolyte leakage and dendrite penetration, which is a crucial property for preventing short circuits in lithium-ion batteries. The high mechanical strength and thermal stability of polyimide ensure that the separator can withstand high temperatures and pressures without cracking; in addition, polyimide acts as an effective thermal barrier, providing additional protection against thermal runaway. Cerium nitrate can form a stable insulating layer to prevent the penetration of lithium dendrites during the charging cycle and improve the flame retardancy and antioxidant properties of the separator, further enhancing the safety of the separator.

[0022] Verified by experiments, the ionic conductivity of the cerium nitrate-doped graphene-polyimide composite lithium-ion battery separator obtained by the present invention is 0.5-1.8 mS / cm; the mechanical strength is 50-79 MPa; the thermal shrinkage rate in thermal stability is ≤5%-12% (150 °C / 1 h); the cycle life of the dendrite inhibition test is >80-200 times (1 mA / cm 2 ). Compared with the traditional PE / PP separator, the present invention has higher ionic conductivity, more stable thermal shrinkage rate, and longer cycle life under the same mechanical strength. It is proved that the present invention solves the problems of insufficient conductivity, dendrite inhibition, and stability under long-term extreme conditions of the existing lithium-ion battery separator, and realizes the preparation of a lithium-ion battery separator with high conductivity, dendrite inhibition, and thermal stability under long-term extreme conditions. Detailed Description of the Invention

[0023] The following is a further detailed description through specific embodiments. It should be noted that the embodiments given below are only for better explaining the content of the present invention, but do not represent that the content of the present invention is limited to the given embodiments. Therefore, those skilled in the art who make non-essential improvements and adjustments to the implementation solutions based on the above invention content still fall within the protection scope of the present invention, which is subject to the protection scope of the appended claims.

[0024] Those skilled in the art are aware that the lithium-ion battery involved in the present invention is a secondary battery (rechargeable battery), generally abbreviated as a lithium battery. It mainly relies on the movement of lithium ions between the positive electrode and the negative electrode to work. During the charging process, lithium ions are removed from the positive electrode material, migrate through the electrolyte solution to the negative electrode, and are embedded in the crystal lattice of the negative electrode material; during discharge, on the contrary, lithium ions are removed from the negative electrode material and return to the positive electrode through the electrolyte. During this process, electrons flow in the external circuit, thereby generating current and realizing the conversion between chemical energy and electrical energy. The concept of lithium-ion batteries can be traced back to the 1970s. The initial research focused on batteries with metallic lithium as the negative electrode. However, due to the generation of lithium dendrites during the charge and discharge process of metallic lithium, resulting in battery short circuits and safety problems, it is difficult to actually apply such batteries. In 1991, Sony successfully launched the first commercial lithium-ion battery. This battery uses carbon materials (such as graphite) as the negative electrode and layered lithium cobalt oxide (LiCoO 2)As the positive electrode, and an organic liquid electrolyte. This battery structure effectively solves the dendrite problem of the lithium metal negative electrode, and has a high energy density and good cycling performance, providing strong power support for the development of portable electronic devices (such as laptops, mobile phones, etc.). With the continuous progress of technology, the application scope of lithium-ion batteries has been continuously expanded. In terms of materials, the positive electrode materials have expanded from the initial lithium cobalt oxide to lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LiFePO 4 ) and other systems. These new materials have the advantages of higher energy density, better safety and lower cost, etc.; the negative electrode materials are also constantly being improved. In addition to graphite, new negative electrode materials such as silicon-based materials are also under research and application. In the application field, lithium-ion batteries not only dominate the consumer electronics field, but also have been widely used in emerging fields such as electric vehicles and energy storage systems.

[0025] Those skilled in the art are aware that the lithium-ion battery separator involved in the present invention is one of the key components of a lithium-ion battery. Its main function is to prevent the positive and negative electrodes from directly contacting and causing a short circuit, while allowing lithium ions to freely pass through during charge and discharge, ensuring the normal electrochemical cycle of the battery. The initial separator materials were mainly some simple porous materials, such as glass fiber filter paper, etc., but these materials had many deficiencies in performance, such as poor mechanical strength and poor chemical stability. With the development of materials science, polyolefin materials (such as polyethylene PE and polypropylene PP) have been widely used in separator manufacturing due to their good chemical stability, mechanical properties and suitable processing properties. In the 1980s - 1990s of the 20th century, polyolefin separators gradually became the mainstream materials for lithium-ion battery separators. During this period, through continuous modification of polyolefin materials and optimization of processing technologies, the performance of the separators was significantly improved.

[0026] Those skilled in the art are aware that the graphene involved in the present invention is a new material with a single-layer two-dimensional honeycomb lattice structure formed by carbon atoms bonded with sp 2 hybrid bonds. Graphene is one of the materials with the highest known strength, and at the same time has good toughness and can be bent. The carrier mobility of graphene is more than twice that of indium antimonide (InSb), which is the substance with the highest known carrier mobility. In addition, the carriers in graphene follow a special quantum tunneling effect and do not produce backscattering when encountering impurities, having excellent electrical conductivity. Graphene has very good thermal conductivity, and the thermal conductivity coefficient of pure defect-free single-layer graphene is as high as 5300 W / mK. Graphene can be used as electrode materials, conductive additives, etc. for supercapacitors and lithium-ion batteries, which can improve the charge and discharge efficiency, cycle life and energy density of the battery; it can also be used in fuel cells, solar cells, etc.

[0027] The rare earths involved in the present invention refer to the collective name of 17 elements, namely lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), as well as scandium (Sc) and yttrium (Y), and its English name is Rare Earth. The present invention preferably uses light rare earths, including lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, and most preferably uses cerium.

[0028] The basic method steps for preparing the cerium nitrate-doped graphene-polyimide composite lithium-ion battery separator of the present invention are as follows:

[0029] 1. Preparation of cerium nitrate-doped graphene dispersion

[0030] Use ultrasonic waves (200w) to disperse 0.5 - 2 wt% of graphene oxide (oxygen content is 40% - 50%, specific surface area is 200 - 800 m 2 / g) in deionized water or ethanol for 1 - 2 hours to ensure the uniform dispersion of graphene oxide. At the same time, prepare a precursor solution of 0.05 - 0.1 mol / L of cerium nitrate (Ce(NO 3 ) 3 ) in ethanol or deionized water. Under continuous stirring, slowly add the rare earth precursor solution to the graphene oxide suspension (GO). The molar ratio of graphene oxide to the rare earth compound should be 1:0.1 - 0.5. Stir for 4 - 6 hours, and use ammonia water to adjust the pH value to about 9 to uniformly dope the rare earth nanoparticles on the surface of graphene oxide. Then filter, wash with deionized water, and dry the cerium nitrate-doped graphene oxide mixture (GO-RE) in a vacuum oven at 60 °C for 12 hours to remove any residual solvents or unreacted substances.

[0031] 2. Preparation of polyamic acid (PAA) precursor

[0032] Dissolve 4,4'-diaminodiphenyl ether (ODA) in NMP under a nitrogen or argon atmosphere. Slowly add phthalic anhydride (PMDA) at a molar ratio of 1:2 - 2:1 while stirring, and stir at room temperature for 12 - 24 hours to form polyamic acid (PAA).

[0033] 3. In-situ mixing of polyamic acid and cerium nitrate-doped graphene

[0034] Slowly add cerium nitrate-doped graphene oxide (GO-RE) to the polyamic acid (PAA) solution, and continuously stir at a speed of 600 r / min. The mass of the dilute cerium nitrate-doped graphene oxide (GO-RE) is between 5 - 20 wt% of the polyamic acid (PAA) to balance the mechanical strength and conductivity.

[0035] 4. Preparation of Battery Separator

[0036] Polyethylene glycol (PEG) is added to the above-mentioned mixed dispersion to adjust its concentration to 8-15 wt% to ensure that the finally prepared separator has sufficient microporous structure. Subsequently, the above mixture is cast on a clean metal substrate with a thickness of about 20-50 μm. Subsequently, the mixture is heated with nitrogen in an oven. In the first step, it is left standing at 70-90 °C for about 1 hour to remove the solvent. In the second step, it is placed at 130-170 °C for about 1 hour. In the third step, it is kept at 200-300 °C for about 1 hour to cause imidization reaction. Finally, it is kept at 300-400 °C for about 12 hours to completely imidize the material. Subsequently, the composite is gradually cooled to room temperature to obtain the required battery separator, and the thickness of the finally formed film should be controlled within the range of 20-50 μm.

[0037] Example 1

[0038] 1. Preparation of Cerium Nitrate-Doped Graphene Dispersion

[0039] Using ultrasound (200 w), 1 wt% of graphene oxide (oxygen content is 45%, specific surface area is 500 m 2 / g) is dispersed in ethanol for 2 hours to ensure uniform dispersion of graphene oxide. At the same time, a 0.1 mol / L cerium nitrate (Ce(NO 3 ) 3 ) precursor solution is prepared in ethanol. Under continuous stirring, the rare earth precursor solution is slowly added to the graphene oxide suspension (GO). The molar ratio of graphene oxide to rare earth compound should be 1:0.5. Stir for 5 hours and adjust the pH value to about 9 with ammonia water to uniformly dope rare earth nanoparticles on the surface of graphene oxide. Subsequently, it is filtered, washed with deionized water, and dried in a vacuum oven at 60 °C for 12 hours for the cerium nitrate-doped graphene oxide mixture (GO-RE) to remove any residual solvent or unreacted substances.

[0040] 2. Preparation of Polyamic Acid (PAA) Precursor

[0041] 4,4'-Diaminodiphenyl ether (ODA) is dissolved in NMP under a nitrogen or argon atmosphere. While stirring, phthalic anhydride (PMDA) is slowly added in a molar ratio of 1:1 and stirred at room temperature for 20 hours to form polyamic acid (PAA).

[0042] 3. In-situ Mixing of Polyamic Acid and Cerium Nitrate-Doped Graphene

[0043] Slowly add the cerium nitrate-doped graphene oxide (GO-RE) obtained in Step 1 into the polyamic acid (PAA) solution obtained in Step 2, while continuously stirring at a speed of 600 r / min. The mass of the dilute cerium nitrate-doped graphene oxide (GO-RE) is 10 wt% of the polyamic acid (PAA) to balance the mechanical strength and conductivity.

[0044] 4. Preparation of battery separator

[0045] Add polyethylene glycol (PEG) to the mixed dispersion obtained in Step 3 to adjust its concentration to 12 wt% to ensure that the finally prepared separator has sufficient microporous structure. Subsequently, cast the above mixture on a clean metal substrate with a thickness of about 50 μm. Then the mixture is heated in an oven with nitrogen. In the first step, it is left standing at 80 °C for 1 hour to remove the solvent. In the second step, it is placed at 150 °C for 1 hour. In the third step, it is kept at 250 °C for 1 hour to cause imidization reaction. Finally, it is kept at 350 °C for 12 hours to completely imidize the material. Then the composite is gradually cooled to room temperature to obtain the required battery separator, and the thickness of the finally formed film should be controlled at about 50 μm.

[0046] Example 2

[0047] 1. Preparation of cerium nitrate-doped graphene dispersion

[0048] Use ultrasonic waves (200 w) to disperse 0.5 wt% of graphene oxide (with an oxygen content of 50% and a specific surface area of 200 m 2 / g) in deionized water for 1 hour to ensure uniform dispersion of graphene oxide. At the same time, prepare a 0.05 mol / L cerium nitrate (Ce(NO 3 ) 3 ) precursor solution in ethanol. Under continuous stirring, slowly add the rare earth precursor solution to the graphene oxide suspension (GO). The molar ratio of graphene oxide to the rare earth compound should be 1:0.1. Stir for 4 hours and adjust the pH value to about 9 with ammonia water to uniformly dope the rare earth nanoparticles on the surface of graphene oxide. Then filter, wash with deionized water, and dry the cerium nitrate-doped graphene oxide mixture (GO-RE) in a vacuum oven at 60 °C for 12 hours to remove any residual solvent or unreacted substances.

[0049] 2. Preparation of polyamic acid (PAA) precursor

[0050] Dissolve 4,4'-diaminodiphenyl ether (ODA) in NMP under a nitrogen or argon atmosphere. Slowly add phthalic anhydride (PMDA) at a molar ratio of 1:2 while stirring, and stir at room temperature for 12 hours to form polyamic acid (PAA).

[0051] 3. In-situ mixing of polyamic acid and cerium nitrate-doped graphene

[0052] The cerium nitrate-doped graphene oxide (GO-RE) obtained in Step 1 was slowly added to the polyamic acid (PAA) solution obtained in Step 2, while continuously stirring at a speed of 600 r / min. The mass of the dilute cerium nitrate-doped graphene oxide (GO-RE) was 5 wt% of the polyamic acid (PAA) to balance mechanical strength and conductivity.

[0053] 4. Preparation of battery separator

[0054] Polyethylene glycol (PEG) was added to the mixed dispersion obtained in Step 3 to adjust its concentration to 8 wt% to ensure that the finally prepared separator had sufficient microporous structure. Subsequently, the above mixture was cast on a clean metal substrate with a thickness of about 20 μm. Subsequently, the mixture was heated in an oven with nitrogen. In the first step, it was left standing at 70 °C for 1 hour to remove the solvent. In the second step, it was placed at 130 °C for 1 hour. In the third step, it was kept at 200 °C for 1 hour to cause imidization reaction. Finally, it was kept at 300 °C for 12 hours to completely imidize the material. Subsequently, the composite was gradually cooled to room temperature to obtain the required battery separator, and the thickness of the finally formed film should be controlled at about 20 μm.

[0055] Example 3

[0056] 1. Preparation of cerium nitrate-doped graphene dispersion

[0057] Using ultrasound (200 w), 2 wt% of graphene oxide (oxygen content 50%, specific surface area 800 m 2 / g) was dispersed in deionized water for 2 hours to ensure uniform dispersion of graphene oxide. At the same time, a 0.1 mol / L cerium nitrate (Ce(NO 3 ) 3 ) precursor solution was prepared in ethanol. Under continuous stirring, the rare earth precursor solution was slowly added to the graphene oxide suspension (GO). The molar ratio of graphene oxide to rare earth compound should be 1:0.5. Stir for 6 hours and adjust the pH value to about 9 with ammonia water to uniformly dope rare earth nanoparticles on the surface of graphene oxide. Subsequently, it was filtered, washed with deionized water, and dried in a vacuum oven at 60 °C for 12 hours for the cerium nitrate-doped graphene oxide mixture (GO-RE) to remove any residual solvent or unreacted substances.

[0058] 2. Preparation of polyamic acid (PAA) precursor

[0059] 4,4'-Diaminodiphenyl ether (ODA) was dissolved in NMP under a nitrogen or argon atmosphere. While stirring, phthalic anhydride (PMDA) was slowly added in a molar ratio of 2:1 and stirred at room temperature for 24 hours to form polyamic acid (PAA).

[0060] 3. In-situ mixing of polyamic acid and cerium nitrate-doped graphene

[0061] Slowly add the cerium nitrate-doped graphene oxide (GO-RE) obtained in Step 1 to the polyamic acid (PAA) solution obtained in Step 2, and continuously stir at a speed of 600 r / min. The mass of the dilute cerium nitrate-doped graphene oxide (GO-RE) is 20 wt% of the polyamic acid (PAA) to balance the mechanical strength and conductivity.

[0062] 4. Preparation of battery separator

[0063] Add polyethylene glycol (PEG) to the mixed dispersion obtained in Step 3 to adjust its concentration to 15 wt% to ensure that the finally prepared separator has sufficient microporous structure. Subsequently, cast the above mixture on a clean metal substrate with a thickness of about 50 μm. Then, heat the mixture in an oven with nitrogen. In the first step, keep it static at 90 °C for 1 hour to remove the solvent. In the second step, place it at 170 °C for 1 hour. In the third step, keep it at 280 °C for 1 hour to cause imidization reaction. Finally, keep it at 400 °C for 12 hours to completely imidize the material. Then, gradually cool the composite to room temperature to obtain the required battery separator, and control the thickness of the finally formed film to be about 50 μm.

[0064] The measurement of the ionic conductivity of the battery separator of the present invention is carried out in accordance with the provisions of Section 6.6.2 of the national standard GB / T 36363-2018 "Polyolefin Separator for Lithium-Ion Batteries".

[0065] The measurement of the mechanical strength of the present invention is carried out in accordance with the provisions of the tensile strength in Section 6.5.1 of the national standard GB / T 36363-2018 "Polyolefin Separator for Lithium-Ion Batteries".

[0066] The measurement method of the thermal stability (thermal shrinkage rate at 150 °C for 1 h) of the present invention is carried out in accordance with the provisions of Section 6.2.6 of the national standard GB / T 31485-2015 "Safety Requirements and Test Methods for Power Batteries for Electric Vehicles", and the calculation is carried out in accordance with the provisions of GB / T 27761-2011 "Test Methods for Weight Loss and Remaining Amount of Thermogravimetric Analyzer".

[0067] The measurement of dendrite inhibition (cycle life) of the present invention is carried out in accordance with the national standard GB / T 31485-2015 "Safety Requirements and Test Methods for Power Batteries for Electric Vehicles" and Section 5.3.2.8 of GB / T 18287-2013 "General Specification for Lithium-Ion Batteries and Battery Packs for Mobile Phones".

[0068] The technical parameters of the cerium nitrate-doped graphene-polyimide composite lithium-ion battery separators obtained in Examples 1-3 are shown in Table 1 below.

[0069] Table 1 Technical Parameters of the Cerium Nitrate-Doped Graphene-Polyimide Composite Lithium-Ion Battery Separator of the Present Invention

[0070]

[0071] As can be seen from Table 1, the ionic conductivity of the cerium nitrate-doped graphene-polyimide composite lithium-ion battery separator obtained in the present invention is 0.5-1.8 mS / cm; the mechanical strength is 50-79 MPa; the thermal stability is that the thermal shrinkage rate ≤ 5%-12% (150 °C / 1 h); for the dendrite inhibition test, the cycle life > 80-200 times (1 mA / cm 2 ). Compared with the traditional PE / PP separator, the present invention has a higher ionic conductivity, a more stable thermal shrinkage rate, and a longer cycle life under the same mechanical strength. It is proved that the present invention solves the problems of the existing composite lithium-ion battery separator in terms of conductivity, dendrite inhibition, and stability under long-term extreme conditions, etc., and realizes the preparation of a cerium nitrate-doped graphene-polyimide composite lithium-ion battery separator with high ionic conductivity, mechanical strength, thermal stability, and dendrite inhibition.

[0072] The above are only the embodiments of the present invention. The well-known technical knowledge in the solution is not described in detail here. Those of ordinary skill in the art know all the common technical knowledge in the technical field to which the invention belongs before the application date, can know all the existing technologies in this field, and have the ability to apply the conventional experimental means before this date. Those of ordinary skill in the art can, under the inspiration given in this application, combine their own abilities to improve and implement this solution. Some typical well-known technologies should not become an obstacle for those of ordinary skill in the art to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several adjustments and improvements can still be made, and these should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be subject to the content of its claims, and the specific implementation manners and the like recorded in the specification can be used to interpret the content of the claims.

Claims

1. A composite lithium-ion battery separator, characterized in that: The diaphragm is composed of rare earth-doped graphene and polyimide.

2. The composite lithium-ion battery separator according to claim 1, characterized in that: The rare earth-doped graphene is obtained by mixing a graphene oxide suspension with a rare earth precursor solution in a molar ratio of graphene oxide to rare earth compound of 1:0.1-0.

5.

3. The composite lithium-ion battery separator according to claim 2, characterized in that: The graphene suspension is formed by ultrasonically dispersing graphene oxide powder with a specific surface area of ​​200-800 m2 / g and an oxygen content of 40%-50% in deionized water or ethanol at a concentration of 0.5-2wt% at an output power of 100-300w for 1-2 hours.

4. The composite lithium-ion battery separator according to claim 2, characterized in that: The rare earth precursor solution is a solution with a concentration of 0.05-0.1 mol / L formed by dispersing a rare earth compound in ethanol or deionized water, and the rare earth compound is selected from lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium or gadolinium.

5. The composite lithium-ion battery separator according to claim 1, characterized in that: The polyimide is formed by an imidization reaction of a polyamic acid precursor.

6. The composite lithium-ion battery separator according to claim 5, characterized in that: The polyamic acid precursor is formed by reacting 4,4'-diaminodiphenyl ether and phthalic anhydride in a molar ratio of 1:2-2:1 under an inert gas atmosphere.

7. A method for preparing the composite lithium-ion battery separator according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: S1) preparing a rare earth doped graphene dispersion; S2) preparing a polyamic acid precursor; S3) mixing the solution of step S1) and the solution of step S2) to generate a pore structure; and S4) performing an imidization reaction on a diaphragm substrate.

8. The preparation method according to claim 7, characterized in that: The method for generating the pore structure in step S3) is selected from: 1) adding 8-15wt% polyethylene glycol to the mixed solution and then mixing in situ; 2) casting the mixed solution into a film by solvent evaporation; 3) preparing electrospinning fibers from the mixed solution by electrospinning.

9. The preparation method according to claim 7, characterized in that: In the step S3), the mass of the rare earth-doped graphene is 5-20wt% of the polyamic acid.

10. The preparation method according to claim 7, characterized in that: The imidization reaction in step S4) includes multi-step heating and heat preservation in the order of 70-90°C for 50-70min, 130-170°C for 50-70min, 200-300°C for 50-70min, and 300-400°C for 10-15h.

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