A modified separator, its preparation method and application

Abstract

The application discloses a modified diaphragm and a preparation method and application thereof, relates to the technical field of electrochemical energy storage, and comprises a base film and a modified inorganic coating arranged on at least one side of the base film, wherein the modified inorganic coating comprises an inorganic material and a modifier, the modifier comprises a polyphenol compound, the polyphenol compound comprises an aromatic group and a hydrophilic group, and the hydrophilic group comprises at least one of a hydroxyl group, a carboxyl group and a carbonyl group. The modified diaphragm provided by the application comprises the inorganic material and the modifier, the polyphenol compound of the modifier has good dispersibility and excellent adhesion, the modified diaphragm obtained by combining the inorganic material has good thermal stability, mechanical strength, ion conductivity and electrolyte wettability, can significantly improve the cycle stability of a battery under a large current density condition, effectively inhibits the growth of lithium dendrites, and improves the overall safety and service life of a lithium battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemical energy storage, in particular to a modified separator and a preparation method and application thereof. BACKGROUND

[0002] Secondary battery technology plays a crucial role in driving the development of high-energy and high-power density devices, widely used in electric vehicles, hybrid electric vehicles, and grid energy storage systems. However, secondary batteries face serious safety challenges, especially under extreme conditions such as high temperature, high rate charging and discharging.

[0003] Secondary batteries are generally composed of three main functional components: electrodes, separators, and electrolytes. The main function of the separator is to prevent the positive and negative electrodes from making electrical contact and to ensure ion transport. Traditional commercial polyolefin separators (such as polyethylene PE and polypropylene PP) have low melting points, poor thermal stability, are prone to thermal shrinkage (greater than 65℃), and have low mechanical strength, making them susceptible to lithium dendrite puncture, which can lead to internal short circuits in the battery and cause safety accidents. SUMMARY

[0004] The main purpose of the present application is to provide a modified separator and a preparation method and application thereof, aiming to solve the shortcomings of existing modified separators in terms of thermal stability, mechanical strength, and electrolyte affinity, thereby improving the rate performance, long-term cycle stability, safety, and service life of secondary batteries.

[0005] To achieve the above-mentioned purpose, the present application provides a modified separator, which comprises a base film and a modified inorganic coating layer provided on at least one side of the base film, the modified inorganic coating layer comprising an inorganic material and a modifier, the modifier comprising a polyphenol compound, the polyphenol compound comprising an aromatic group and a hydrophilic group, the hydrophilic group comprising at least one of a hydroxyl group, a carboxyl group, and a carbonyl group.

[0006] In an embodiment, the polyphenol compound comprises at least one of tannic acid, catechin, procyanidin, epigallocatechin gallate, and quercetin.

[0007] In an embodiment, the inorganic material comprises at least one of SiO2, Al2O3, boehmite, Mg(OH)2, zeolite, ZrO2, TiO2, and BN.

[0008] The present application also provides a preparation method of the modified separator as described above, comprising the following steps:

[0009] Mixing inorganic material, modifier, and solvent to obtain a mixed solution, mixing the mixed solution with grinding beads, and performing ball milling to obtain modified inorganic material;

[0010] mixing the modified inorganic material with an aqueous solvent to obtain a modified inorganic slurry;

[0011] applying the modified inorganic slurry to at least one surface of a base film and drying to prepare a modified inorganic coating layer on the at least one surface of the base film, thereby obtaining a modified separator.

[0012] In an embodiment, the mass ratio of the inorganic material to the modifier is (3-20):1; and / or,

[0013] The solvent includes at least one of water, methanol and ethanol.

[0014] In an embodiment, the particle size of the grinding beads is 0.8-5 mm; and / or,

[0015] The ratio of the mass of the grinding beads to the sum of the mass of the inorganic material and the mass of the modifier is (30-300):1; and / or,

[0016] The ball milling time is 3-48 h; and / or,

[0017] The particle size of the modified inorganic material is 0.5-1.5 μm.

[0018] In an embodiment, the mass ratio of the modified inorganic material to the aqueous solvent is (4-10):40; and / or,

[0019] The aqueous solvent includes at least one of water, methanol, ethanol, acetonitrile, dimethyl sulfoxide and dimethyl formamide; and / or,

[0020] The step of mixing the modified inorganic material with an aqueous solvent to obtain a modified inorganic slurry includes ultrasonic stirring and mixing the modified inorganic material with an aqueous solvent for 10 nim-30 min to obtain a modified inorganic slurry.

[0021] In an embodiment, the drying time is 10-14 h; and / or,

[0022] The drying temperature is 25-60 °C; and / or,

[0023] The thickness of the modified inorganic coating layer is 1-8 μm.

[0024] The present application also provides an electrode assembly, which comprises a positive electrode sheet, a negative electrode sheet and a separator arranged between the positive electrode sheet and the negative electrode sheet, wherein the separator is the modified separator described above or prepared by the method described above.

[0025] The present application also provides a lithium ion battery, which comprises the electrode assembly described above.

[0026] In the technical solution of the present application, the above-mentioned polyphenol compound is used as a modifier, which has good dispersibility and excellent adhesion. The hydrophilic group in the above-mentioned polyphenol compound can form a hydrogen bond or a coordination bond with the surface of the inorganic particles or the base film, thereby enhancing the interfacial bonding, wherein the aromatic ring structure has a certain π-π stacking ability, which helps to adhere to the surface of the base film; in addition, the polyphenol compound can coat the surface of the inorganic particles, thereby reducing the surface energy between the particles and preventing agglomeration. Therefore, the modifier can not only uniformly coat the inorganic particles in the solution, reduce the specific surface energy, and prevent agglomeration of the inorganic particles, but also enhance the bonding force between the inorganic particles and the base film, thereby realizing good dispersibility and excellent adhesion, so as to improve the uniformity of the inorganic particles on the surface of the base film and the bonding force between the inorganic particles and the base film. Therefore, the modified separator obtained by modifying the inorganic material with the polyphenol compound has good thermal stability, mechanical strength, ionic conductivity, and electrolyte wettability, which can significantly improve the cycle stability of the battery under a large current density condition, effectively inhibit the growth of dendrites, and improve the overall safety and service life of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the drawings shown.

[0028] Figure 1 A real object comparison diagram of the modified separator prepared in Example 1 of the present application and a polyethylene base film;

[0029] Figure 2 A bending test diagram of the modified separator prepared in Example 1 of the present application;

[0030] Figure 3 A liquid drop side view diagram of the electrolyte obtained after the electrolyte is added to the modified separator prepared in Example 1 of the present application and the polyethylene separator of Comparative Example 1, respectively;

[0031] Figure 4 A top view wetting distribution diagram of the electrolyte obtained after the same amount of electrolyte is added to the modified separator prepared in Example 1 of the present application and the polyethylene separator of Comparative Example 1, respectively;

[0032] Figure 5 A thermal stability test result diagram of the modified separator prepared in Example 1 of the present application and the polyethylene separator of Comparative Example 1;

[0033] Figure 6The charge-discharge test curve of the Li / separator / graphite battery prepared by using the modified separator in the embodiment 1 of the present application and the polyethylene separator in the comparative example 1 under different current densities is shown in the following figure:

[0034] Figure 7 The cycle performance test of the Li / separator / graphite battery prepared by using the modified separator in the embodiment 1 of the present application and the polyethylene separator in the comparative example 1 is shown in the following figure:

[0035] Figure 8 The cycle performance test of the Li / separator / graphite battery prepared by using the modified separator in the embodiment 1 of the present application and the polyethylene separator in the comparative example 1 is shown in the following figure.

[0036] The implementation, functional features and advantages of the present application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0038] It should be noted that if the embodiments of the present application involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative positional relationship, movement condition, etc. between the components in a certain posture, and if the certain posture changes, the directional indications will also change accordingly.

[0039] In addition, if the embodiments of the present application involve descriptions of “first”, “second”, etc., the descriptions of “first”, “second”, etc. are only for description purposes, and cannot be understood as indicating or implying the relative importance of the indicated technical features or implicitly indicating the number of the indicated technical features. Therefore, the features limited by “first” and “second” can explicitly or implicitly include at least one of the features. In addition, “and / or” or “and / or” appears throughout the text, which means that the three parallel solutions include A solution, or B solution, or A and B solutions. In addition, the technical solutions of each embodiment can be combined with each other, but it must be based on the realization of ordinary skilled in the art, when the combination of technical solutions appears contradictory or unachievable, it should be considered that the combination of technical solutions does not exist, nor within the scope of protection claimed by the present application.

[0040] Secondary batteries are generally composed of three main functional components: electrodes, separators, and electrolytes. Among them, the main function of the separator is to prevent the positive and negative electrodes from making electrical contact and to ensure ion transport. Traditional commercial polyolefin separators (such as polyethylene PE and polypropylene PP) have low melting points, poor thermal stability, are prone to thermal shrinkage (greater than 65℃), and have low mechanical strength, making them susceptible to lithium dendrite puncture, which can lead to internal short circuits in the battery and cause safety accidents.

[0041] The thermal stability of traditional polyolefin separators can be improved by coating inorganic solid particles. However, there are many problems with this inorganic modified separator in practical applications, such as the large specific surface energy of inorganic powders, which easily agglomerate and have poor uniformity of coating; the weak bonding force between the inorganic modified inorganic coating and the base film, and the poor adhesion between inorganic solid particles, so the modified inorganic coating is prone to powdering and peeling off. Coating a layer of polyvinylidene fluoride (PVDF) or other organic modified inorganic coating on the surface of the inorganic modified inorganic coating can improve the adhesion to some extent, but it will further increase the risk of internal resistance and pore blockage, and the process is complicated, increasing production costs. There are too many additives (dispersants, thickeners, binders, solvents, wetting agents, etc.) in the inorganic solid slurry, and the addition amount is large. In the electrolyte, it will dissolve and migrate for a long time, causing side reactions and affecting the cycle life of the battery.

[0042] In view of this, the present application proposes a modified separator, which comprises a base film and a modified inorganic coating provided on at least one side of the base film, the modified inorganic coating comprising an inorganic material and a modifier, the modifier comprising a polyphenol compound, the polyphenol compound comprising an aromatic group and a hydrophilic group, the hydrophilic group comprising at least one of a hydroxyl group, a carboxyl group, and a carbonyl group.

[0043] In the technical solution of the present application, the above-mentioned polyphenol compound is used as a modifier with good dispersibility and excellent adhesion. The hydrophilic group in the above-mentioned polyphenol compound can form hydrogen bonds or coordination bonds with the surface of inorganic particles or the base film, enhancing the interfacial bonding, and the aromatic ring structure has a certain π-π stacking ability, which helps to adhere to the surface of the base film; in addition, the polyphenol compound can coat the surface of inorganic particles, reducing the surface energy between particles and preventing agglomeration. Therefore, the modifier not only uniformly coats inorganic particles in the solution, reduces the specific surface energy, and prevents inorganic particle agglomeration, but also enhances the bonding force between inorganic particles and the base film, thereby achieving good dispersibility and excellent adhesion, thereby improving the uniformity of inorganic particles on the surface of the base film and the bonding force between inorganic particles and the base film. Therefore, the modified separator obtained by modifying inorganic materials with polyphenol compounds has good thermal stability, mechanical strength, ionic conductivity, and electrolyte wettability, can significantly improve the cycle stability of the battery under high current density conditions, effectively suppresses the growth of dendrites, and improves the overall safety and service life of the battery.

[0044] It should be noted that the modified inorganic coating in the present application can be composed of a modifier and an inorganic material, without adding other additives (such as adhesives / thickeners) to have the above-mentioned good effects, thereby solving the problem that the addition of a large amount of additives (such as dispersants, thickeners, adhesives, solvents, wetting agents, etc.) in the slurry during the coating of the inorganic material causes the additives to dissolve and migrate in the electrolyte for a long time, resulting in side reactions and affecting the cycle life of the battery.

[0045] In addition, compared with the scheme of recoating an organic modified inorganic coating such as polyvinylidene fluoride (PVDF) on the surface of the inorganic modified inorganic coating, the technical scheme of the present application not only reduces the risk of increased internal resistance and blocked pores, but also has simpler process and lower production cost.

[0046] Therefore, the technical scheme of the present application breaks through the problem of the traditional organic material system relying on an organic adhesive (such as PVDF), avoids the adverse effects of increased internal resistance, electrolyte side reactions, and blocked pores caused by the adhesive, and realizes a modified scheme without adding an adhesive / thickener.

[0047] It can be understood that the base film can adopt a polyolefin separator, and the polyolefin separator such as polyethylene PE and polypropylene PP can be obtained by purchase.

[0048] The polyphenol compound can select a synthetic polyphenol compound or a natural polyphenol compound. In some embodiments, the polyphenol compound includes a natural polyphenol compound, and the natural polyphenol compound includes at least one of tannic acid, catechin, procyanidine, epigallocatechin gallate, and quercetin. The natural polyphenol compound derived from plants is rich in hydrophilic groups, has the advantages of natural environmental protection, non-toxicity, harmlessness, low cost, and strong adhesion compared with other modifiers, not only promotes the uniform dispersion of inorganic solid particles, but also enhances the bonding of inorganic solid particles and the base film.

[0049] In some embodiments, the inorganic material includes at least one of SiO2, Al2O3, boehmite, Mg(OH)2, zeolite, ZrO2, TiO2, and BN. By using the above-mentioned inorganic material, the thermal stability of the polyolefin base film can be effectively improved.

[0050] It should be noted that the above conditions can be satisfied alternatively, or simultaneously. When satisfied simultaneously, the modified inorganic coating in the application can significantly improve the affinity and wettability of the electrolyte, has a higher ionic conductivity, and can improve the rate performance and long-term cycle stability of the battery. Compared with the traditional commercial separator, the modified separator of the application has excellent thermal stability, thermal conductivity and mechanical strength, reduces the thermal shrinkage of the separator under high temperature or high charge-discharge current of the battery, and can inhibit the growth of lithium dendrites in the metal anode of the battery to a certain extent, thereby improving the safety and service life of the battery.

[0051] The application further provides a preparation method of the modified separator.

[0052] The inorganic material, the modifier and the solvent are mixed to obtain a mixed solution, the mixed solution is mixed with grinding beads, ball milling is performed to obtain a modified inorganic material;

[0053] The modified inorganic material is mixed with an aqueous solvent to obtain a modified inorganic slurry;

[0054] The modified inorganic slurry is coated on at least one surface of the base film, dried, and a modified inorganic coating layer is prepared on the at least one surface of the base film to obtain a modified separator.

[0055] In the technical solution of the application, the liquid phase environment and the continuous shearing collision effect of ball milling promote the in-situ adsorption and coating of the modifier on the surface of the inorganic material particles, forming a stable particle dispersion system. Therefore, not only the uniformity, stability and dispersibility of the particles can be improved, but also the coating quality of the inorganic solid particles on the base film can be improved, greatly reducing the problems of particle agglomeration, sedimentation and defects of the modified inorganic coating in the traditional slurry, and providing high-quality precursors for subsequent film formation.

[0056] It can be understood that the inorganic material particles coated with the modifier (modified inorganic material) have hydrophilic groups on the surface, so that they have good dispersibility in the aqueous solvent, and the obtained modified inorganic slurry is more uniform and stable. In addition, there is no residue after the aqueous solvent is dried, and there is no need for complex recovery process, which can reduce energy consumption and cost.

[0057] Further, in the step of mixing the inorganic material, the modifier and the solvent, the mass ratio of the inorganic material to the modifier is (3-20): 1; specifically, the mass ratio of the inorganic material to the modifier can be 3:1, 5:1, 10:1, 15:1, 20:1, etc. Although the modifier can improve dispersibility and affinity, when the ratio is too high, the modifier can cover the surface of the particles, form an organic dense layer, and hinder the free migration of ions. Polyphenols do not have ion-conducting function, and excessive accumulation of polyphenols can dilute the ion-conducting contribution of the inorganic phase. The organic layer can absorb moisture and swell, causing local occlusion or distortion of the pore structure. Differences in surface energy can lead to non-uniform film formation and the formation of "high resistance areas". The mass ratio of the inorganic material to the modifier in the above range can better coat the surface of the inorganic particles, reduce the surface energy between the particles, and have the effect of preventing agglomeration, so that the modified separator has lower internal resistance. In some embodiments, the mass ratio of the inorganic material to the modifier is (5-15): 1. In this range, the coating slurry has better dispersibility and stronger particle bonding, and the obtained separator has better performance in internal resistance, mechanical strength, thermal stability, wettability, electrical conductivity and cycle life.

[0058] The solvent includes at least one of water, methanol and ethanol. The use of the above solvent has good solubility and good dispersibility, is beneficial to subsequent wet ball milling, and can be removed in the subsequent drying process. For example, in an embodiment, 50% ethanol aqueous solution can be used as the solvent.

[0059] It can be understood that in the step of mixing the inorganic material, the modifier and the solvent to obtain a mixed solution, mixing the mixed solution with grinding beads, ball milling, and obtaining the modified inorganic material, the modifier and the solvent can be mixed first, and then the inorganic material is added for ball milling.

[0060] Further, in the step of mixing the mixed solution with the grinding beads and performing ball milling to obtain the modified inorganic material, the particle size of the modified inorganic material obtained is 0.5 μm to 1.5 μm. Specifically, the particle size of the modified inorganic material is 0.5 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, or within a range formed by any two of the above values. The modified inorganic material particles within the above range have a moderate specific surface area, a small agglomeration tendency, and can be stably dispersed in a slurry, which is conducive to forming a continuous, uniform, and dense modified inorganic coating structure, thereby improving the overall consistency of the modified separator. The particles within the above particle size range can synergize with the environmentally friendly modifier (natural polyphenol compound) to effectively form hydrogen bonds or interface polarity with the surface of the base film, thereby improving adhesion, reducing powdering and peeling, and improving the structural stability of the separator. Controlling the particle size to be below microns can obtain moderate porosity, which is conducive to the rapid penetration and storage of electrolyte, improves the electrolyte absorption rate of the separator, and enhances the connectivity of ion channels, thereby improving the ionic conductivity and rate capability. The inorganic ceramic modified inorganic coating structure formed within the above particle size range is dense and has high mechanical strength, which can effectively block the growth path of lithium dendrites under high rate or long cycle conditions, thereby improving the thermal stability and safety of the battery.

[0061] The particle size of the modified inorganic material can be controlled within the above range by controlling the particle size of the grinding beads and / or the ball milling time, etc.

[0062] In some embodiments, the particle size of the grinding beads is 0.8 mm to 5 mm. Specifically, the particle size of the grinding beads can be 0.8 mm, 1.5 mm, 2.5 mm, 3.5 mm, 4.5 mm, 5 mm, etc. The grinding beads within the above particle size range can have better grinding effect.

[0063] In some embodiments, the ball milling time is 3 h to 48 h. Specifically, the ball milling time can be 3 h, 8 h, 15 h, 24 h, 32 h, 48 h, etc. The ball milling time within the above range can have better grinding effect.

[0064] Further, in some embodiments, the ratio of the mass of the grinding beads to the sum of the masses of the inorganic material and the modifier is (30 to 300): 1. Specifically, the ratio of the mass of the grinding beads to the sum of the masses of the inorganic material and the modifier can be 30:1, 50:1, 80:1, 150:1, 200:1, 250:1, 300:1, etc. The ratio of the mass of the grinding beads to the sum of the masses of the inorganic material and the modifier within the above range can promote the in-situ adsorption and coating of the modifier on the surface of the inorganic material particles, forming a stable particle dispersion system.

[0065] It should be noted that the slurry after ball milling can be centrifuged, suction filtered, and dried to obtain the powder-like modified inorganic material.

[0066] The above conditions can be set alternatively or simultaneously, and the application does not make any limitation as long as the particle size of the modified inorganic material is controlled within the range of 0.5 μm to 1.5 μm.

[0067] Further, in some embodiments, the above conditions can be set simultaneously, that is, the inorganic material solid particles and the modifier natural polyphenol compound can be dispersed in the mixed solvent system of water and ethanol, and the grinding beads with a diameter of 0.8 mm to 5 mm are added; by controlling the ball powder ratio to be (30-300):1 and the sand milling time to be 3 h to 48 h, the particle size is precisely controlled within the range of 0.5 μm to 1.5 μm; at the same time, the liquid phase environment and the continuous shearing collision effect promote the in-situ adsorption and coating of the modifier on the particle surface, forming a stable particle dispersion system. The uniformity, stability and dispersibility of the particles are greatly improved, the coating quality of the inorganic solid particles on the base film is significantly improved, and the problems of particle agglomeration, sedimentation and modified inorganic coating defects in the traditional slurry are avoided, providing high-quality precursors for subsequent film formation.

[0068] Further, the mass ratio of the modified inorganic material to the aqueous solvent is 1:(4-10); the mass ratio of the modified inorganic material to the aqueous solvent is 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, etc. Within the above range, the modified inorganic slurry has good fluidity and stability, the modified coating after coating is uniform and dense, has good adhesion and thermal stability, and the electrolyte wettability and ion conductivity are significantly enhanced. In some embodiments, the mass ratio of the modified inorganic material to the aqueous solvent is 1:(5-6), and the modified separator prepared within this range has more excellent performance in terms of electrolyte absorption rate, ion conductivity, thermal stability and mechanical properties, and is suitable for harsh electrochemical environments such as high rate cycling.

[0069] Further, in the step of mixing the modified inorganic material with the aqueous solvent to obtain the modified inorganic slurry, the aqueous solvent includes at least one of methanol, ethanol, acetonitrile, dimethyl sulfoxide and dimethyl formamide; it should be noted that the aqueous solvent is a solvent system in which the mass fraction of water is greater than 50%, such as a mixture of water and polar solvents such as methanol, ethanol, acetonitrile, dimethyl sulfoxide and dimethyl formamide.

[0070] Further, the step of "mixing the modified inorganic material with the aqueous solvent to obtain a modified inorganic slurry" comprises: mixing the modified inorganic material with the aqueous solvent by ultrasonic stirring for 10 nim~30 min to obtain a modified inorganic slurry. The particles can be better dispersed and the mixing efficiency can be improved by mixing for 10 nim~30 min by ultrasonic stirring.

[0071] Further, the thickness of the modified inorganic coating layer is 1 μm~8 μm. When the thickness of the modified inorganic coating layer is 1 μm~8 μm, the particles with a particle size of 0.5 μm~1.5 μm have good fluidity and film-forming performance, and it is easier to realize a stable and controllable industrial preparation process by the blade coating process. In some embodiments, the thickness of the modified inorganic coating layer is 1 μm~2 μm. Within this range, the obtained separator has more excellent performance in terms of internal resistance, mechanical strength, thermal stability, wettability, electrical conductivity and cycle life.

[0072] It can be understood that the modified inorganic slurry can be coated on the surface of the battery base film by using a doctor blade coating method, the thickness of the modified inorganic coating layer is controlled by adjusting the height of the doctor blade, and the modified separator is obtained after drying.

[0073] Further, the drying time is 10 h~14 h; and the drying temperature is 25℃~60℃. Specifically, the drying time can be 10 h, 11 h, 12 h, 13 h, 14 h, etc., the drying temperature can be 25℃, 30℃, 40℃, 45℃, 50℃, 60℃, etc., and the drying temperature and time within the above range can make the obtained modified separator have better electrolyte absorption rate.

[0074] The modified separator developed in the present application has the following effects in practical application:

[0075] (1) The thermal stability of the modified separator is significantly enhanced, and the modified separator has no obvious shrinkage at 120℃, which is much better than the commercial PE separator;

[0076] (2) The wettability and ion conductivity of the modified separator to the electrolyte are improved, the contact angle is reduced from 54° to 16°, and the ion conductivity is increased to 0.70 mS / cm;

[0077] (3) The mechanical properties of the modified separator are better, and the tensile strength and puncture strength are both improved by more than 20%;

[0078] (4) The cycle stability is better, and the capacity retention rate under high rate (5C) is much higher than that of the traditional separator, and the cycle life of the lithium symmetric battery is increased to more than 1000 cycles;

[0079] (5) It has the beneficial effects of green environmental protection and low cost scalable preparation.

[0080] Therefore, the application takes natural environment-friendly polyphenol modifier and inorganic solid particles as core materials, and realizes particle dispersion and particle size control through wet sand grinding as core process, so that a modified separator with high thermal stability, high mechanical strength, high electrical conductivity, low internal resistance and green environmental protection and high safety is prepared.

[0081] The application also provides an electrode assembly, which comprises a positive electrode sheet, a negative electrode sheet and a separator arranged between the positive electrode sheet and the negative electrode sheet, wherein the separator comprises a base film and a modified inorganic coating arranged on at least one side of the base film, and the modified inorganic coating comprises an inorganic material and a modifier, and the modifier comprises a polyphenol compound, the polyphenol compound comprises an aromatic group and a hydrophilic group, and the hydrophilic group comprises at least one of a hydroxyl group, a carboxyl group and a carbonyl group.

[0082] The application also provides a lithium ion battery comprising the electrode assembly.

[0083] The technical solutions of the application are further described in detail below in combination with specific embodiments and the drawings, and it should be understood that the following embodiments are only used to explain the application and do not limit the application.

[0084] Embodiment 1

[0085] A modified separator is provided and prepared by the following method:

[0086] 1g of a modifier (tannic acid) powder is dissolved in a mixed solvent (water: ethanol = 50vol%: 50vol%), 9g of hexagonal boron nitride powder is added, and the mixture is put into a sand mill for sand grinding at 400rpm for 5h (the particle size of the grinding beads is 1mm), and then centrifuged, suction filtered and dried to obtain modified boron nitride powder (fBN) with a particle size of 1μm. 6g of the modified boron nitride powder is added into 40mL of N,N-dimethylformamide (DMF) solvent, and then ultrasonic stirring is performed for 10min to obtain a uniformly and stably dispersed fBN modified slurry; the fBN modified slurry is coated on a polyethylene (PE) separator, and the coating thickness is controlled by a doctor blade. After vacuum drying at 40℃, a modified separator with a coating thickness of about 2μm is prepared.

[0087] Embodiment 2

[0088] A modified separator is provided and prepared by the following method:

[0089] Take 1 g of modifier (tannic acid) powder dissolved in mixed solvent (water: ethanol = 50vol%: 50vol%), add 9 g of hexagonal boron nitride powder, put into sand mill 400 rpm sand mill 5h (the particle size of grinding beads is 1mm), after centrifugation, suction filtration, drying to get the particle size of 1μm modified boron nitride powder (fBN). Weigh 4g of modified boron nitride powder into 40mL N,N-dimethylformamide (DMF) solvent, after ultrasonic stirring for 10min, get uniform and stable dispersion of fBN modified slurry; The fBN modified slurry is coated on the polyethylene (PE) base film, and the coating thickness is controlled by scraper. After vacuum drying at 40℃, the modified separator with coating thickness of about 2μm is prepared.

[0090] Example 3

[0091] A modified separator is provided, which is prepared by the following method:

[0092] Take 1 g of modifier (tannic acid) powder dissolved in mixed solvent (water: ethanol = 50vol%: 50vol%), add 9 g of hexagonal boron nitride powder, put into sand mill 400 rpm sand mill 5h (the particle size of grinding beads is 1mm), after centrifugation, suction filtration, drying to get the particle size of 1μm modified boron nitride powder (fBN). Weigh 4g of modified boron nitride powder into 40mL N,N-dimethylformamide (DMF) solvent, after ultrasonic stirring for 10min, get uniform and stable dispersion of fBN modified slurry; The fBN modified slurry is coated on the polyethylene (PE) base film, and the coating thickness is controlled by scraper. After vacuum drying at 40℃, the modified separator with coating thickness of about 2μm is prepared.

[0093] Examples 4 to 11 are prepared by similar steps as example 1, the difference is shown in table 1.

[0094] The difference between example 4 and example 1 is that the average particle size of the modified inorganic material is different, and the thickness of the modified inorganic coating is different, the specific parameters are shown in table 1.

[0095] The preparation method of the modified separator provided in example 4 is:

[0096] Take 1 g of modifier (tannic acid) powder dissolved in mixed solvent (water: ethanol = 50vol%: 50vol%), add 9 g of hexagonal boron nitride powder, put into sand mill 600 rpm sand mill 10 h (the particle size of grinding beads is 1 mm), after centrifugation, suction filtration, drying to get the particle size of 0.7 μm modified boron nitride powder (fBN). Weigh 6 g of modified boron nitride powder into 40 mL of N,N-dimethylformamide (DMF) solvent, and get a uniform and stable dispersion of fBN modified slurry after ultrasonic stirring for 10 min; the fBN modified slurry is coated on the polyethylene (PE) separator, and the coating thickness is controlled by scraper. After vacuum drying at 40℃, the modified separator with a coating thickness of about 1 μm is prepared.

[0097] Examples 5 to 6 are different from Example 1 in that the average particle size of the modified inorganic material is different, and the specific parameters are shown in Table 1.

[0098] The preparation method of the modified separator provided in Example 5 is:

[0099] Take 1 g of modifier (tannic acid) powder dissolved in mixed solvent (water: ethanol = 50vol%: 50vol%), add 9 g of hexagonal boron nitride powder, put into sand mill 300 rpm sand mill 3 h (the particle size of grinding beads is 2 mm), after centrifugation, suction filtration, drying to get the particle size of 1.5 μm modified boron nitride powder (fBN). Weigh 6 g of modified boron nitride powder into 40 mL of N,N-dimethylformamide (DMF) solvent, and get a uniform and stable dispersion of fBN modified slurry after ultrasonic stirring for 10 min; the fBN modified slurry is coated on the polyethylene (PE) separator, and the coating thickness is controlled by scraper. After vacuum drying at 40℃, the modified separator with a coating thickness of about 2 μm is prepared.

[0100] The preparation method of the modified separator provided in Example 6 is:

[0101] Take 1 g of modifier (tannic acid) powder dissolved in mixed solvent (water: ethanol = 50vol%: 50vol%), add 9 g of hexagonal boron nitride powder, put into sand mill 650 rpm sand mill 12 h (the particle size of grinding beads is 1 mm), after centrifugation, suction filtration, drying to get the particle size of 0.5 μm modified boron nitride powder (fBN). Weigh 6 g of modified boron nitride powder into 40 mL of N,N-dimethylformamide (DMF) solvent, and get a uniform and stable dispersion of fBN modified slurry after ultrasonic stirring for 10 min; the fBN modified slurry is coated on the polyethylene (PE) separator, and the coating thickness is controlled by scraper. After vacuum drying at 40℃, the modified separator with a coating thickness of about 1 μm is prepared.

[0102] Example 7 differs from Example 1 in that the modifier is different, and the modified inorganic coating layer has a different thickness. The specific parameters are shown in Table 1.

[0103] The method for preparing the modified separator provided in Example 7 is as follows:

[0104] Dissolve 1 g of the modifier (catechin) powder in a mixed solvent (water: ethanol = 50 vol%: 50 vol%), add 9 g of hexagonal boron nitride powder, and put it into a sand mill for sanding at 350 rpm for 4 h (the particle size of the grinding beads is 1.5 mm). After centrifugation, suction filtration, and drying, the modified boron nitride powder (fBN) with a particle size of 1.2 μm is obtained. Weigh 6 g of the modified boron nitride powder and add it to 40 mL of N,N-dimethylformamide (DMF) solvent. After ultrasonic stirring for 10 min, a uniformly and stably dispersed fBN modified slurry is obtained. The fBN modified slurry is coated on a polyethylene (PE) separator, and the coating thickness is controlled by a doctor blade. After vacuum drying at 40°C, a modified separator with a coating thickness of about 2 μm is prepared.

[0105] Examples 8 to 11 differ from Example 1 in that the mass ratio of the inorganic material to the modifier is different. The specific parameters are shown in Table 1.

[0106] The polyethylene (PE) separators in Examples 1 to 11 are purchased from Bik Battery Co., Ltd.

[0107] Table 1. Preparation parameters of the modified separators in Examples 1 to 11

[0108]

[0109] Comparative Example 1

[0110] A polyethylene separator is provided, which is purchased from Bik Battery Co., Ltd.

[0111] Comparative Example 2

[0112] A modified separator is provided, which is prepared by the following method:

[0113] Mix 6 g of boron nitride powder with 0.5 g of the binder PVDF and 40 g of the solvent NMP, add 0.5 g of the dispersant, and shear disperse at 1000 rpm for 1 h to obtain a slurry.

[0114] Coat the slurry on the surface of a polyethylene separator (purchased from Bik Battery Co., Ltd.), control the coating thickness by a doctor blade, and after vacuum drying at 60°C, a composite separator with a coating thickness of about 2 μm is prepared.

[0115] Comparative Example 3

[0116] A modified separator is provided, which is prepared by the following method:

[0117] Take 1 g of modifier (tannic acid) powder dissolved in a mixed solvent (water: ethanol = 50vol%: 50vol%), add 9 g of hexagonal boron nitride powder, shear dispersion at 500 rpm for 5 h, after centrifugation, suction filtration, drying, the particle size of the modified boron nitride powder (fBN) is 1.6 μm. Weigh 6 g of modified boron nitride powder into 40 mL of N,N-dimethylformamide (DMF) solvent, and get a uniform and stable dispersion of fBN modified slurry after ultrasonic stirring for 10 min; The fBN modified slurry is coated on the polyethylene (PE) separator (purchased from Bik Battery Co., Ltd.), and the coating thickness is controlled by a doctor blade. After vacuum drying at 40°C, a modified separator with a coating thickness of about 2 μm is prepared.

[0118] Separator performance test

[0119] (1) Bending test

[0120] The modified separator prepared in Example 1 was bent as shown in Figure 2 It can be seen that the modified separator prepared in Example 1 still maintains the good flexibility of the PE-based film and can be bent arbitrarily without obvious damage, and the modified coating is not easy to peel off and crack.

[0121] (2) Electrolyte affinity and wettability test

[0122] Preparation of electrolyte: Dissolve lithium hexafluorophosphate (LiPF6) in a mixed solvent of EC and DEC in a volume ratio of 1:1 to obtain an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L.

[0123] The modified separator prepared in Example 1 and the polyethylene separator of Comparative Example 1 were cut into the same size (diameter of 17 mm) by a punching machine for contact angle test, and the state of the electrolyte droplet on the surface of the separator was recorded at the moment and after 5 s, respectively, as shown in Figure 3 . Figure 3 The side view of the droplet obtained after adding the electrolyte to the modified separator prepared in Example 1 and the polyethylene separator of Comparative Example 1, respectively. Figure 3 (a) in the side view of the droplet is the side view of the droplet at the moment of 0 s after adding the above electrolyte to the surface of the polyethylene separator of Comparative Example 1; Figure 3 (b) in the side view of the droplet is the side view of the droplet after 5 s after adding the above electrolyte to the surface of the polyethylene separator of Comparative Example 1; Figure 3 (c) in the side view of the droplet is the side view of the droplet at the moment of 0 s after adding the above electrolyte to the surface of the modified separator of Example 1; Figure 3 (d) in the side view of the droplet is the side view of the droplet after 5 s after adding the above electrolyte to the surface of the modified separator of Example 1. From Figure 3It can be seen that the contact angles of the polyethylene separator of Comparative Example 1 and the modified separator of Example 1 are 54° and 16°, respectively, at the moment when the electrolyte contacts the separators; after 5 seconds, the contact angle of the polyethylene separator slightly decreases to 49°, while the electrolyte droplet on the modified separator of Example 1 has completely spread out. This shows that the modified separator of Example 1 exhibits better affinity with the electrolyte than the polyethylene separator of Comparative Example 1.

[0124] Figure 4 are top view wetting distribution diagrams of the polyethylene separator of Comparative Example 1 and the modified separator of Example 1, respectively, after the same amount of electrolyte is added to the separators; Figure 4 (e) in FIG. 1 is a top view wetting distribution diagram of the polyethylene separator of Comparative Example 1 before the electrolyte is added to the surface of the polyethylene separator; Figure 4 (f) in FIG. 1 is a top view wetting distribution diagram of the polyethylene separator of Comparative Example 1 after the electrolyte is added to the surface of the polyethylene separator for 5 minutes; Figure 4 (g) in FIG. 1 is a top view wetting distribution diagram of the polyethylene separator of Example 1 before the electrolyte is added to the surface of the modified separator; Figure 4 (h) in FIG. 1 is a top view wetting distribution diagram of the polyethylene separator of Example 1 after the electrolyte is added to the surface of the modified separator for 5 minutes; and Figure 4 It can be seen that after the electrolyte is added for 5 minutes, the electrolyte on the PE separator still appears in the form of large droplets and only shows a small range of wetting. In contrast, the electrolyte on the surface of the modified separator of Example 1 has penetrated and diffused in a large area and has been completely absorbed by the modified separator, showing excellent electrolyte wettability. This is because the polar hydroxyl groups of the modified inorganic coating improve the compatibility of the coating with the polar electrolyte, thereby exhibiting better wettability and thus having a higher ionic conductivity.

[0125] (3) Thermal stability test

[0126] The modified separator of Example 1 and the polyethylene separator of Comparative Example 1 were respectively cut into multiple circular pieces of the same size (diameter of 17 mm) by a puncher, and were respectively placed at 60°C, 80°C, 100°C, 120°C and 130°C for 5 minutes, and the deformation thereof was observed, to obtain Figure 5 . Figure 5 is a diagram of the thermal stability test results of the modified separator of Example 1 and the polyethylene separator of Comparative Example 1.

[0127] From Figure 5It can be seen that the modified separator of Example 1 has excellent thermal stability at a temperature up to 120 °C without any shrinkage deformation, and finally maintains its original state. The PE separator of Comparative Example 1 starts to curl at the edge from 80 °C, and the thermal shrinkage deformation becomes more obvious as the temperature increases. At a temperature of 120 °C, the PE separator is completely deformed and shrinks and starts to melt, while the modified separator of Example 1 still maintains dimensional stability. When the temperature is increased to 130 °C, the edge of the modified separator starts to shrink, while the PE separator is almost completely melted, with an area shrinkage of one twelfth of the original. The excellent thermal stability of the modified separator of Example 1 is due to the high temperature resistance and high thermal conductivity of the inorganic solid material itself, which resists the melting and shrinkage of the PE-based film at high temperatures.

[0128] The following tests were performed on the separators of Examples 1 to 11 and Comparative Examples 1 to 3, and the test results are recorded in Table 2.

[0129] (4) Mechanical properties

[0130] The separators of Examples 1 to 11 and Comparative Examples 1 to 3 were each prepared into a rectangular separator sample with a size of 3 mm x 10 mm, and tensile strength tests were performed on an AllroundLine universal testing machine Zwick Roell Z010, and puncture strength tests were performed on a lithium battery separator puncture strength tester. The test data are recorded in Table 2.

[0131] (5) Thermal diffusivity

[0132] The thermal diffusivity of the separators was tested by a laser thermal conductivity instrument (Netzsch, LFA467), and the test data are recorded in Table 2.

[0133] (6) Electrolyte absorption rate

[0134] In the electrolyte absorption test, the separators were soaked in electrolyte for 2 h, and the excess electrolyte was wiped off with filter paper, and then each separator was weighed. The electrolyte absorption amount was calculated by the following formula:

[0135]

[0136] wherein w0 and w1 are the weights of the dry separator and the separator after absorbing the electrolyte, respectively.

[0137] The following tests were performed on the separators of Examples 1 and Comparative Examples 2 and 3, and the test results are recorded in Table 3.

[0138] Impedance and ionic conductivity tests:

[0139] The ion conductivity of the separator was calculated by electrochemical impedance spectroscopy (EIS) measured by the Chenhua CHI760E electrochemical workstation. The test method was to assemble a coin cell with two parallel stainless steel disc electrodes, with the separator soaked in the same volume of electrolyte (1.0 M LiPF, EC / DEC (1 / 1 v / v)) in the middle, and then assemble the coin cell in an argon atmosphere glove box after overnight aging. The assembled steel cell was clamped on the electrode clamp of the electrochemical workstation, and an alternating current amplitude of 10 mV was applied in the frequency range of 0.1 Hz to 1 MHz. The Nyquist curve was obtained, and the electrolyte impedance value R corresponding to the high frequency region intercept was measured, and the ion conductivity (σ) was calculated by the following formula:

[0140]

[0141] where t is the thickness of the separator, R is the resistance measured by EIS, and A is the area of the stainless steel disc electrode.

[0142] Table 2 Performance data of the separators obtained in Examples 1-11 and Comparative Examples 1-3

[0143]

[0144] As can be seen from Table 2, compared with the polyethylene separator of Comparative Example 1, the modified separator provided by the present application has higher tensile strength, puncture strength, thermal diffusion coefficient and electrolyte absorption rate.

[0145] Compared with the polyethylene separator modified by the adhesive and boron nitride powder of Comparative Example 2, the modified separator provided by the present application has higher tensile strength, puncture strength, thermal diffusion coefficient and electrolyte absorption rate.

[0146] Compared with the modified polyethylene separator prepared by shearing dispersion of particles of Comparative Example 3, the modified separator provided by the present application has higher tensile strength, puncture strength, thermal diffusion coefficient and electrolyte absorption rate.

[0147] Table 3 Performance data of the separators obtained in Example 1 and Comparative Examples 2 and 3

[0148]

[0149] As can be seen from Table 2, the impedance of the polyethylene separator modified by the adhesive and boron nitride powder of Comparative Example 2 is 2.4 Ω, which is greater than the impedance of the modified polyethylene separator in Example 1, indicating that the technical solution of the present application can reduce the internal resistance.

[0150] The particle distribution is uneven and the agglomeration is serious in Comparative Example 3 without sanding treatment but with shear dispersion, resulting in irregular or non-through pore structure of the coating, weak binding force between large particles, easy to appear cracks and local shedding, forming "high resistance points" of the resistance, loose coating structure, large interface contact resistance, although the electrolyte absorption rate is acceptable (175%), but the proportion of "non-effective channels" in the internal structure is high. Therefore, the modified inorganic material particles obtained by sanding in the present application are uniform, so that the modified separator obtained has good comprehensive performance and lower impedance.

[0151] In order to verify the performance of the modified battery separator in the present application, the battery separator in Example 1 and the unmodified PE separator are assembled into batteries respectively, and the electrochemical performance test is carried out, and the specific test method is as follows:

[0152] Preparation of battery

[0153] (1) Li / separator / Graphite battery (half battery)

[0154] The commercial graphite negative electrode material, conductive carbon black and binder (PVDF mass fraction of 7 wt%) are mixed in a mass ratio of 8:1:1, then an appropriate amount of NMP solvent is added to adjust the viscosity of the slurry to semi-flowing state, and then stirred for 8 h to obtain a uniform electrode slurry. The slurry is uniformly coated on a copper foil and dried in a vacuum oven at 60 ℃ for 12 h. The dried electrode sheet is cut into a diameter of 13 mm by a sheet punching machine, and then the battery assembly is carried out in an argon atmosphere glove box. When assembling the button half battery, 1.0 M LiPF6 is selected as the electrolyte (volume ratio of ethylene carbonate EC and dimethyl carbonate DEC is 1:1 vol%), the separator uses the PE separator in Comparative Example 1 and the modified separator in Example 1, and the lithium sheet is used as the counter electrode, and the electrode sheet is used as the working electrode, to obtain a CR2032 type button half battery. The assembled button half battery is tested by a new battery test system for charge-discharge test and cycle performance test under different current densities, and the results are shown in Figure 6 、 Figure 7 .

[0155] (2) Li / separator / Li battery (lithium symmetric battery)

[0156] The positive and negative electrodes of the lithium ion battery are both Li sheets, and the positive electrode, negative electrode and PE separator in Comparative Example 1 / modified separator in Example 1 are assembled into a Li / Li battery for cycle test, and the results are shown in Figure 8 .

[0157] Electrochemical test

[0158] (1) The Li / separator / Graphite batteries (half-batteries) prepared from the modified separator in Example 1 and the polyethylene separator in Comparative Example 1 were subjected to charge-discharge tests at different current densities, and the current densities were 0.1 C, 0.5 C, 1 C, 2 C, 3 C, 5 C, and 0.1 C in turn, and the results are shown in Table 1. Figure 6 Figure 6 As can be seen, at different current densities, both batteries showed stable cycles. With the increase of current density, the discharge specific capacity of the batteries gradually decreased. Since the modified separator in Example 1 had higher ion conductivity and electrolyte absorption rate, the discharge specific capacity of the half-battery using the modified separator was higher than that of the PE separator at all current densities. Especially at a large current density of 5 C, the advantage of the modified separator in Example 1 was more obvious, and the capacity was maintained at 59 mAh g -1 , while the capacity of the PE separator decayed to 33 mAh g -1 . This means that the modified separator in Example 1 can withstand large current by absorbing additional heat, and transfer heat through the modified inorganic coating during the cycle process. If the ordinary PE film cannot diffuse these additional heat in time, it will reduce the cycle performance and pose a potential danger to the battery, especially it will accelerate the thermal shrinkage of the separator.

[0159] (2) The Li / separator / Graphite batteries (half-batteries) prepared from the modified separator in Example 1 and the polyethylene separator in Comparative Example 1 were subjected to cycle performance tests at a current density of 3 C, and the results are shown in Table 2. Figure 7 As shown in Table 2, the performance of the modified separator in Example 2 exceeded that of the commercial PE separator, showing good stability, maintaining a capacity of 98.6 mAh g -1 after 400 cycles (3 C), and a retention rate of 89%, which was better than that of the PE separator (66.5 mAh g -1 , 83%).

[0160] (3) The Li / separator / Li batteries (lithium symmetric batteries) prepared from the modified separator in Example 1 and the polyethylene separator in Comparative Example 1 were subjected to cycle performance tests at a current density of 5 mA cm -2 and a capacity of 1 mAh cm -2 , and the results are shown in Table 3. Figure 8 During the cycle process of the lithium symmetric battery, due to the symmetry of the battery, the voltage signal came from the overpotential difference of lithium dissolution and deposition between the positive and negative electrodes. At a current density of 5 mA cm -2 and a capacity of 1 mAh cm -2 ​In the case of the above, the overpotential of Comparative Example 1 at the 100th cycle was 57.3 mV, while the overpotential of Example 1 was only 9.8 mV. The overpotentials of Comparative Example 1 and Example 1 at the 400th cycle were 40.5 and 9.1 mV, respectively. The results show that the lithium nucleation barrier of the battery using the PE separator is higher than that of the battery using Example 2, which is mainly due to the higher affinity and wettability of the modified separator of Example 2 to the electrolyte. After 512 cycles, the voltage signal of the PE battery suddenly dropped to 0, indicating that the lithium dendrites pierced the separator and caused a short circuit. In contrast, the overpotential of the battery of Example 2 remained stable at ~10 mV after 1000 cycles, showing good cycle stability, which is due to the modified inorganic coating with excellent mechanical properties and thermal stability, which is conducive to blocking the growth of lithium dendrites and prolonging the service life of the battery.

[0161] The above merely describes the preferred embodiments of the present application, and is not intended to limit the patent scope of the present application. The present application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the patent protection scope of the present application.

Claims

1. A method for producing a modified separator, characterized by, The method comprises the following steps: mixing an inorganic material, a modifier and a solvent to obtain a mixed solution, mixing the mixed solution with grinding beads, and performing ball milling to obtain a modified inorganic material; mixing the modified inorganic material with an aqueous solvent to obtain a modified inorganic slurry; coating the modified inorganic slurry to at least one surface of a base film, and drying to prepare a modified inorganic coating layer on the at least one surface of the base film, thereby obtaining a modified separator; the modifier comprises a polyphenol compound, the polyphenol compound comprises an aromatic group and a hydrophilic group, and the hydrophilic group comprises at least one of a hydroxyl group, a carboxyl group and a carbonyl group; the polyphenol compound comprises at least one of tannic acid, catechin, procyanidin, epigallocatechin gallate and quercetin; the mass ratio of the inorganic material to the modifier is (5-15):1; the mass ratio of the grinding beads to the sum of the mass of the inorganic material and the mass of the modifier is (30-300):1; the particle size of the modified inorganic material is 0.5-1.5 μm.

2. The modified separator of claim 1, wherein the inorganic material comprises at least one of SiO2, Al2O3, boehmite, Mg(OH)2, zeolite, ZrO2, TiO2 and BN.

3. The method of claim 1, wherein the modified separator is prepared by the steps of: the solvent comprises at least one of water, methanol and ethanol.

4. The method for preparing the modified separator according to claim 1, wherein the particle size of the grinding beads is 0.8-5 mm; and / or the ball milling time is 3-48 h.

5. The method for preparing the modified separator according to claim 1, wherein the mass ratio of the modified inorganic material to the aqueous solvent is (4-10):40; the aqueous solvent comprises at least one of water, methanol, ethanol, acetonitrile, dimethyl sulfoxide and dimethylformamide; and / or the step of mixing the modified inorganic material with the aqueous solvent to obtain the modified inorganic slurry comprises ultrasonic stirring and mixing the modified inorganic material with the aqueous solvent for 10-30 min to obtain the modified inorganic slurry.

6. The method of claim 1, wherein the modified separator is prepared by the steps of: the drying time is 10-14 h; and / or the drying temperature is 25-60 °C; and / or the thickness of the modified inorganic coating layer is 1-8 μm.

7. A modified separator, characterized by, The modified separator is prepared by the method for preparing the modified separator according to any one of claims 1-6.

8. An electrode assembly characterized by, The electrode assembly comprises a positive electrode sheet, a negative electrode sheet and a separator arranged between the positive electrode sheet and the negative electrode sheet, wherein the separator is the modified separator according to claim 7 or is prepared by the method for preparing the modified separator according to any one of claims 1-6.

9. A lithium-ion battery, characterized by The lithium ion battery comprises the electrode assembly according to claim 8.

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