Diaphragm, preparation method thereof and lithium secondary battery

Abstract

The invention provides a diaphragm and a preparation method thereof and a lithium secondary battery, the diaphragm comprises a diaphragm base material and particle layers deposited on two surfaces of the diaphragm base material, the particle layers comprise inorganic nanoparticles and / or pre-lithiation agent particles, the particle size distribution width of the particle layers is 0.9-1.3, and the thickness difference of the particle layers is 0.5-1.2 [mu] m; the diaphragm has a surface contact angle of 40 DEG or less. A uniform particle layer containing inorganic nanoparticles and / or pre-lithiation agent particles is introduced on the surface of the diaphragm. The particle layer can improve the electrolyte wettability and heat resistance of the diaphragm, and meanwhile, the uniform particle layer can guide the uniform distribution of lithium ions, so that the uniform and compact deposition of lithium is realized, and the growth of lithium dendrites is effectively inhibited.

Description

TECHNICAL FIELD

[0001] The present application relates to a diaphragm and its preparation method and a lithium secondary battery, belonging to the technical field of lithium ion batteries. BACKGROUND

[0002] The diaphragm is an indispensable component of the lithium ion battery system, which can prevent short circuit between the positive and negative electrodes of the battery, ensure the safety of the battery, and provide an effective channel for the rapid migration of lithium ions. The formation and growth of lithium dendrites can penetrate the diaphragm to cause direct contact between the positive and negative electrodes, leading to internal short circuit of the battery, etc. Therefore, it is of great scientific significance and practical value to design a lithium dendrite-inhibiting diaphragm and improve the reliability and safety of the battery under harsh environments.

[0003] At present, the most common method for modifying the diaphragm to enhance its mechanical strength and prevent lithium dendrites from penetrating is to coat the diaphragm with ceramic nanoparticles such as Al2O3.

[0004] Some documents disclose a lithium battery diaphragm and battery integrated preparation method. This preparation method can significantly enhance the mechanical strength and thermal stability of the diaphragm by coating the diaphragm with inorganic ceramic materials with good thermal stability, thereby improving the safety of the battery. However, the coating method is prone to uneven coating, and uneven distribution of inorganic ceramic particles can cause uneven current density distribution, accelerate the growth of lithium dendrites, and affect the capacity, cycle performance and safety performance of the battery.

[0005] In addition, during the first charging of the lithium ion battery, a solid electrolyte interface film (SEI film) will be formed on the surface of the negative electrode, and this process will consume a large amount of lithium ions, resulting in irreversible capacity loss of the battery.

[0006] Pre-lithiation of the diaphragm can effectively improve the above problems. The pre-lithiated diaphragm introduces a pre-lithiation reagent on the surface of the diaphragm to supplement lithium to the negative electrode in advance before the battery works, offsetting the lithium loss caused by the formation of SEI film, etc., thereby improving the first cycle coulombic efficiency, total capacity and energy density of the battery. However, the current method for introducing the pre-lithiation reagent is still mainly coating, which still has the above problems of uneven coating and uneven distribution of pre-lithiation reagents. SUMMARY

[0007] Problems to be solved by the application

[0008] In order to solve the problems existing in the prior art as described above, the present application needs to provide a diaphragm with a uniform surface modification layer, excellent mechanical strength, ability to inhibit the formation and growth of lithium dendrites, and uniform introduction of pre-lithiation reagents, thereby improving the safety and cycle performance of the battery.

[0009] Solution for solving the problem

[0010] The present application provides a separator, which introduces a uniform particle layer containing inorganic nanoparticles and / or pre-lithiation agent particles on the surface. The particle layer can improve the electrolyte wettability and heat resistance of the separator, and the uniform particle layer can also guide the uniform distribution of lithium ions, achieving uniform and dense lithium deposition and effectively inhibiting lithium dendrite growth.

[0011] In addition, the addition of pre-lithiation agent also enables the negative electrode to form a protective SEI layer at all times, and the battery system can maintain a high lithium ion concentration, thereby improving the cycle performance of the battery.

[0012] The present application first provides a separator, which comprises a separator substrate and a particle layer deposited on both surfaces of the separator substrate,

[0013] The particle layer comprises inorganic nanoparticles and / or pre-lithiation agent particles, the particle size distribution width of the particle layer is 0.9-1.3, and the thickness difference of the particle layer is 0.5-1.2 μm.

[0014] The surface contact angle of the separator is 40° or less.

[0015] According to the separator of the present application, the porosity retention rate of the separator relative to the separator substrate is 90% or more.

[0016] According to the separator of the present application, the thickness of the particle layer is 2-5 μm.

[0017] According to the separator of the present application, the mass ratio of the inorganic nanoparticles to the pre-lithiation agent particles is 1:0.5-1:2.

[0018] According to the separator of the present application, the separator substrate comprises one or more of polyethylene, polypropylene, PP / PE composite separator, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyethylene terephthalate, cellulose membrane, glass fiber separator, aramid separator, and polyimide separator; and / or, the inorganic nanoparticles comprise one or more of SiO2, Al2O3, Si3N4, AlN, TiO2, MnO, NiO, CoO, Mo3N2, and FeO; and / or, the pre-lithiation agent comprises one or more of Li3N, Li2O, Li2S, LiF, and Li2CO3.

[0019] According to the separator of the present application, the separator satisfies at least one of the following conditions:

[0020] i. The tensile strength of the separator is 1500 Kgf / cm 2 The above;

[0021] ii. the transverse heat shrinkage TD of the separator is less than 5% and the longitudinal heat shrinkage MD is less than 6% after the separator is heated at 150℃ for 1h.

[0022] The application also provides a preparation method of the separator according to the application, which comprises a step of pretreating a separator substrate, a step of preparing a target material and a step of magnetron sputtering.

[0023] The step of pretreating the separator substrate comprises:

[0024] S1: placing the separator substrate in a plasma cleaning machine, introducing oxygen and water vapor into a vacuum cavity, and performing plasma treatment on the separator substrate under an electric field;

[0025] S2: closing the gas input, and performing first annealing treatment on the surface of the separator substrate by plasma residual energy in a vacuum environment to obtain a pretreated separator substrate;

[0026] The step of preparing the target material comprises: pressing, sintering and cooling inorganic material powder to obtain a target material, wherein the inorganic material comprises inorganic nanoparticles and / or pre-lithiation agent.

[0027] The step of magnetron sputtering comprises: performing magnetron sputtering on the pretreated separator substrate by using the target material in an inert gas atmosphere to deposit a particle layer on both surfaces of the pretreated separator substrate to obtain a separator.

[0028] According to the preparation method, in step S1, the flow rate of the oxygen introduced is 10-50sccm; and / or the amount of water vapor introduced is 0.1-5% of the total volume of oxygen introduced; and / or the time of the plasma treatment is 10-40min, and the pulse frequency of the electric field is 10-500kHz.

[0029] According to the preparation method, step S1 comprises the following steps: placing the separator substrate in a plasma cleaning machine, introducing oxygen into a vacuum cavity, and performing rough cleaning under a low-frequency pulse; switching to a high-frequency pulse while introducing water vapor;

[0030] The frequency of the low-frequency pulse is 10-50kHz, and the time of the low-frequency pulse treatment is 5-20min; the frequency of the high-frequency pulse is 100-500kHz, and the time of the high-frequency pulse treatment is 5-20min.

[0031] According to the preparation method, in step S2, the time of the first annealing treatment is 5-30min; and / or the surface roughness Ra of the pretreated separator substrate is less than or equal to 50nm.

[0032] According to the preparation method, in the target material preparation step, the sintering temperature is 1500-1800 DEG C; the sintering time is 25-35h.

[0033] According to the preparation method, in the magnetron sputtering step, the vacuum degree is 5*10 -4 Pa or below; and / or,

[0034] The magnetron sputtering time is 10-60min; and / or,

[0035] The magnetron sputtering power is 100-500W; and / or,

[0036] The pretreated diaphragm substrate temperature is 20-80 DEG C; and / or,

[0037] The inert gas flow is 20-100sccm; and / or,

[0038] The magnetron sputtering pressure is 0.2-1.0Pa; and / or,

[0039] The particle layer thickness is 2-5um.

[0040] According to the preparation method, the preparation method further comprises a post-processing step, and the post-processing step comprises: performing a second annealing treatment on the diaphragm, wherein the second annealing treatment temperature is 80-120 DEG C, and the time is 10min-6h.

[0041] The application further provides a lithium secondary battery comprising a positive electrode, a negative electrode, an electrolyte and the diaphragm.

[0042] Effects of the application

[0043] 1. The diaphragm provided by the application has a controllable surface particle layer thickness, which can reach the nanometer level, and the particles are uniformly distributed, thereby avoiding the uneven coating problem caused by the traditional coating method. Meanwhile, the particles in the particle layer can be stacked and form gaps on the diaphragm surface, so that the contact area between the coating and Li + is larger, which is beneficial to the formation of uniform ion flux, and the uniform particle layer can induce uniform lithium ion deposition, thereby avoiding the safety hazards and capacity reduction problems of lithium batteries caused by lithium dendrites.

[0044] 2. The diaphragm provided by the application can retain the original porosity of the diaphragm base film, and ensure the free migration of lithium ions.

[0045] 3. The diaphragm provided by the application can provide a lithium supplement effect through the addition of a pre-lithiation agent, so that the battery system can maintain a high lithium ion concentration, thereby improving the cycle performance of the battery.

[0046] 4、The particle layer uniformly deposited in the separator provided by the application can also effectively increase the mechanical strength of the separator to prevent the penetration of lithium dendrites, further improving the safety of the battery.

[0047] 5、The preparation method provided by the application realizes the uniform deposition of the particle layer by using the magnetron sputtering technology. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 A schematic diagram of the separator obtained from Example 1 after the heat shrinkage test;

[0049] Figure 2 A schematic diagram of the separator obtained from Example 2 after the heat shrinkage test;

[0050] Figure 3 A schematic diagram of the separator obtained from Example 3 after the heat shrinkage test;

[0051] Figure 4 A schematic diagram of the separator obtained from Comparative Example 1 after the heat shrinkage test;

[0052] Figure 5 A schematic diagram of the separator obtained from Comparative Example 2 after the heat shrinkage test. DETAILED DESCRIPTION

[0053] Various exemplary embodiments, features, and aspects of the present application will be described in detail below. The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any implementation described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other implementations.

[0054] In addition, in order to better illustrate the present application, numerous specific details are given in the following detailed description. Those skilled in the art will understand that the present application can be practiced without certain specific details. In some instances, well-known methods, means, instruments and steps have not been described in detail in order to highlight the principles of the present application.

[0055] Unless otherwise stated, the units used in the specification are international standard units, and the numerical values and numerical ranges appearing in the present application should be understood to include the systematic errors that are inevitable in industrial production.

[0056] In the specification, the meaning of "may" includes both the meaning of performing a certain process and the meaning of not performing a certain process.

[0057] In this specification, reference to "some specific / preferred embodiments", "other specific / preferred embodiments", "embodiments" etc. means that a particular feature, structure, property or characteristic described is included in at least one embodiment of the technology described herein, and can or can not be present in other embodiments. In addition, it is to be understood that the described features, structures, properties etc. can be combined in any suitable manner in the various embodiments.

[0058] In this specification, a numerical range expressed using "numerical value A ~ numerical value B" means a range including the end point values A, B.

[0059] In this specification, "room temperature" or "ambient temperature" means an indoor environmental temperature of "23 ± 2°C".

[0060] [First aspect]

[0061] The first aspect of the present application provides a separator comprising a separator substrate and a particle layer deposited on both surfaces of the separator substrate, wherein the particle layer comprises inorganic nanoparticles and / or pre-lithiation agent particles. The particle layer can improve the electrolyte wettability and heat resistance of the separator.

[0062] In the present application, the particle size distribution width of the particle layer is 0.9-1.3, for example, it can be 0.95, 1, 1.05, 1.1, 1.15, 1.2, 1.25, etc., wherein the particle size distribution width can be used to characterize the uniformity of the particle layer distribution, which is obtained by (D90-D10) / D50, wherein D10, D50 and D90 respectively represent the particle size corresponding to the cumulative distribution of 10%, 50% and 90% of the particles; the thickness difference of the particle layer is 0.5-1.2 μm, for example, it can be 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, etc., wherein the thickness difference is the thickness difference of the particle layer deposited on either side of the base film; by controlling the uniform distribution of particles, the present application avoids the problem of uneven coating caused by traditional coating methods.

[0063] In the present application, the porosity retention rate of the separator relative to the separator substrate is 90% or more, preferably 92% or more, more preferably 94% or more; by retaining the original porosity of the separator substrate, the free migration of lithium ions can be ensured.

[0064] In some specific embodiments, the D90 of the particles in the particle layer is 0.9-1.6 μm, for example, it can be 0.95 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, etc.; the D50 is 0.7-1.2 μm, for example, it can be 0.75 μm, 0.8 μm, 0.85 μm, 0.9 μm, 1 μm, 1.1 μm, etc.; and the D10 is 0.1-0.5 μm, for example, it can be 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.4 μm, etc.

[0065] In some specific embodiments, the porosity of the separator substrate can be 30-36%, for example, it can be 31%, 32%, 33%, 34%, 35%, etc.

[0066] Further, the surface contact angle of the separator is 40° or less, preferably 38° or less, and more preferably 35° or less. The contact angle is the angle between the tangent of the gas-liquid interface at the gas-liquid-solid three-phase intersection and the liquid-solid interface when 0.5-3 μL of electrolyte forms a droplet on the surface of the separator. The particles in the particle layer can be stacked on each other and form voids on the surface of the separator, which increases the surface roughness and reduces the contact angle of the surface of the separator, which is beneficial to the formation of uniform ion flux.

[0067] In some specific embodiments, the thickness of the particle layer (single-side thickening of the separator substrate) can be 2-5 μm, for example, it can be 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, etc.

[0068] For the separator substrate, the present application does not make special limitations, and the separator substrate commonly used in the art can be used, for example, it can include one or more of polyethylene (PE), polypropylene (PP), PP / PE composite separator, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), cellulose membrane, glass fiber separator, aramid separator, and polyimide (PI) separator.

[0069] In some specific embodiments, the thickness of the separator substrate can be 5-15 μm, for example, it can be 7 μm, 9 μm, 10 μm, 12 μm, 14 μm, etc.

[0070] In some specific embodiments, the inorganic nanoparticles include one or more of SiO2, Al2O3, Si3N4, AlN, TiO2, MnO, NiO, CoO, Mo3N2, and FeO.

[0071] In some specific embodiments, the pre-lithiation agent particles include one or more of Li3N, Li2O, Li2S, LiF, and Li2CO3.

[0072] In some specific embodiments, the mass ratio of the inorganic nanoparticles to the pre-lithiation agent particles is 1:0.5~1:2, for example, can be 1:0.8, 1:1, 1:1.2, 1:1.5, 1:1.8, etc.

[0073] In some specific embodiments, the separator satisfies at least one of the following conditions:

[0074] i. The tensile strength of the separator is 1500 Kgf / cm 2 Preferably, the above can be 1550 Kgf / cm 2 More preferably, the above can be 1600 Kgf / cm 2 ;

[0075] ii. After the separator is heated at 150℃ for 1h, its transverse thermal shrinkage rate TD<5%, and its longitudinal thermal shrinkage rate MD<6%. The thermal shrinkage rate is tested according to GB / T 36363-2018.

[0076] [Second aspect]

[0077] The second aspect of the present application provides a preparation method of a separator, which includes a step of pretreating a separator substrate, a step of preparing a target material, and a step of magnetron sputtering.

[0078] (Pretreatment of separator substrate)

[0079] In the present application, the step of pretreating the separator substrate includes:

[0080] S1: placing the separator substrate in a plasma cleaning machine, introducing oxygen and water vapor into the vacuum chamber, and performing plasma treatment on the separator substrate under an electric field;

[0081] S2: closing the gas input, and performing a first annealing treatment on the surface of the separator substrate by plasma residual energy in a vacuum environment to obtain a pretreated separator substrate.

[0082] Step S1

[0083] In step S1, the plasma treatment on the separator substrate is performed by introducing oxygen and water vapor into the vacuum chamber to generate plasma under an electric field, which can clean and activate the surface of the separator substrate, introduce oxygen-containing functional groups (-OH, -COOH), and significantly improve the surface energy of the separator substrate.

[0084] For the diaphragm substrate, the present application is not particularly limited, and a diaphragm substrate commonly used in the art can be used, for example, can include one or more of polyethylene (PE), polypropylene (PP), PP / PE composite diaphragm, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), cellulose membrane, glass fiber diaphragm, aramid diaphragm, polyimide (PI) diaphragm.

[0085] In some specific embodiments, the thickness of the diaphragm substrate can be 5-15 μm, for example, can be 7 μm, 9 μm, 10 μm, 12 μm, 14 μm, etc.

[0086] In some specific embodiments, the flow rate of the oxygen introduced can be 10-50 sccm, for example, can be 15 sccm, 20 sccm, 25 sccm, 30 sccm, 35 sccm, 40 sccm, 45 sccm, etc.

[0087] In some specific embodiments, the amount of water vapor introduced is 0.1-5% of the total volume of oxygen introduced, for example, can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, etc.

[0088] In some specific embodiments, the pulse frequency of the electric field can be 10-500 kHz; the time of the plasma treatment is 10-40 min.

[0089] In some preferred embodiments, in order to further improve the cleaning and activation effect, step S1 can include the following steps: placing the diaphragm substrate in a plasma cleaning machine, introducing oxygen in the vacuum cavity, and performing rough cleaning under low-frequency pulse; switch to high-frequency pulse while introducing water vapor. Among them, the frequency of the low-frequency pulse can be 10-50 kHz, for example, can be 15 kHz, 20 kHz, 25 kHz, 30 kHz, 35 kHz, 40 kHz, 45 kHz, etc., and the time of low-frequency pulse treatment can be 5-20 min; the frequency of the high-frequency pulse can be 100-500 kHz, for example, can be 150 kHz, 200 kHz, 250 kHz, 300 kHz, 350 kHz, 400 kHz, 450 kHz, etc., and the time of high-frequency pulse treatment can be 5-20 min.

[0090] The oxygen gas is introduced under low-frequency pulse to generate initial plasma for rough cleaning of the surface of the separator. The water vapor is introduced while switching to high-frequency pulse to utilize the synergistic reaction of oxygen radicals (O·) and hydroxyl radicals (·OH) to avoid excessive oxidation and to realize surface cleaning and introduction of oxygen-containing functional groups (-OH, -COOH) of the separator substrate, thereby significantly improving the surface energy.

[0091] The separator substrate obtained after the plasma treatment has a surface roughness Ra≤5nm, a contact angle ≤30°, and preferably a contact angle <20°, as detected by AFM.

[0092] Step S2

[0093] In step S2, after the gas input is turned off, the vacuum environment is maintained, and the surface is subjected to a first annealing treatment by the residual energy of the plasma to eliminate stress and stabilize the functional group structure.

[0094] In some specific embodiments, the first annealing treatment can be performed for 5-30min, for example, for 10min, 15min, 20min, 25min, etc.

[0095] In some specific embodiments, the surface roughness Ra of the pretreated separator substrate is ≤50nm, preferably ≤45nm, and more preferably ≤40nm. When the surface roughness Ra of the pretreated separator substrate is ≤50nm, the thickness uniformity of the particle layer can be improved, the local retention of the electrolyte on the surface of the separator can be reduced, the electrolyte can be quickly penetrated into the internal pores of the separator, and a uniform ion conduction network can be formed.

[0096] (Step of target preparation)

[0097] In the present application, the step of target preparation comprises: pressing, sintering and cooling inorganic materials to obtain a target, wherein the inorganic materials comprise inorganic nanoparticles and / or pre-lithiation agents.

[0098] The inorganic nanoparticles form an inorganic nanoparticle coating on the surface of the separator substrate, which can improve the electrolyte wettability and heat resistance of the separator. The addition of the pre-lithiation agent can enable the negative electrode to form a protective SEI layer at all times, and the battery system can maintain a high lithium ion concentration, thereby improving the cycle performance of the battery.

[0099] In some specific embodiments, the press forming can include packing the analytically pure inorganic material powder in a mold and using a hydraulic machine to press the powder into a 60mm diameter target blank. The specific operation mode is as follows: the mold packed with the powder is placed in the center of the hydraulic machine workbench; the hydraulic machine is started, the upper punch is lowered at a set rate, while the lower punch is raised until the target pressure is reached; the punch material is hard alloy; the target blank is an injection molded blank in a high polymer material blank. The type of target blank is not particularly limited and can be a material commonly used in the art.

[0100] In some specific embodiments, the sintering temperature can be 1500-1800℃, for example, 1550℃, 1600℃, 1650℃, 1700℃, 1750℃, etc.; the sintering time can be 25-35h, for example, 27h, 29h, 30h, 32h, 34h, etc.

[0101] For the equipment used for sintering, the present application is not particularly limited and can be selected as needed, for example, in a resistance furnace.

[0102] In some specific embodiments, the cooling includes naturally cooling the sintered diaphragm substrate to room temperature.

[0103] (Magnetron sputtering step)

[0104] In the present application, the magnetron sputtering step includes: using a target to perform magnetron sputtering on the pretreated diaphragm substrate in an inert gas atmosphere, depositing a particle layer on both surfaces of the pretreated diaphragm substrate, and obtaining a diaphragm.

[0105] For the type of inert gas, the present application is not particularly limited and can be an inert gas commonly used in the art, for example, argon, helium, etc.

[0106] In some specific embodiments, the vacuum degree in the magnetron sputtering step is 5×10 -4 Pa or less.

[0107] In some specific embodiments, the magnetron sputtering time is 10-60min, for example, 15min, 20min, 25min, 30min, 35min, 40min, 45min, 50min, 55min, etc.; the magnetron sputtering power is 100-500W, for example, 150W, 200W, 250W, 300W, 350W, 400W, 450W, etc.; the magnetron sputtering pressure is 0.2-1.0Pa, for example, 0.3Pa, 0.4Pa, 0.5Pa, 0.6Pa, 0.7Pa, 0.8Pa, 0.9Pa, etc.

[0108] In some specific embodiments, the flow rate of the inert gas is 20-100 sccm, for example, it can be 30 sccm, 40 sccm, 50 sccm, 60 sccm, 70 sccm, 80 sccm, 90 sccm, etc.

[0109] In some specific embodiments, the temperature of the pretreated separator substrate is 20-80℃, for example, it can be 30℃, 40℃, 50℃, 60℃, 70℃, etc.

[0110] In some specific embodiments, the thickness of the particle layer (thickening of a single side of the separator substrate) is 2-5 μm, for example, it can be 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, etc.

[0111] (Other steps)

[0112] The preparation method of the present application can further include a post-treatment step, specifically, the post-treatment step can include: subjecting the separator to a second annealing treatment. Internal stress can be eliminated by the second annealing treatment.

[0113] In some specific embodiments, the temperature of the second annealing treatment can be 80-120℃, for example, it can be 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, etc.; the time can be 10 min-6 h, for example, it can be 0.5 h, 1 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, etc.

[0114] [Third aspect]

[0115] The third aspect of the present application provides a lithium secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator according to the first aspect.

[0116] Examples

[0117] Embodiments of the present application will be described in detail below with reference to examples, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. The specific conditions not noted in the examples are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments not noted by the manufacturer are all conventional products that can be obtained by purchase on the market.

[0118] Example 1

[0119] Step of pretreating the separator substrate:

[0120] S1: Commercially available 9 pm PE separator was placed in a plasma cleaning machine, oxygen was introduced into the vacuum chamber, the flow rate of oxygen was 25 sccm, and an initial plasma was generated by applying a low-frequency pulse power (frequency 20 kHz) to coarsely clean the surface of the separator. The low-frequency treatment time was 10 min.

[0121] S2: Switch to high-frequency pulse power (frequency 100 kHz), while introducing a small amount of water vapor (volume fraction 1%), using the synergistic reaction of oxygen free radicals (O·) and hydroxyl radicals (·OH) to realize the cleaning and oxygen-containing functional group introduction of the surface of the separator. After high-frequency treatment for 10 min, the AFM detected that the average roughness Ra of the surface was 5 nm.

[0122] S3: Turn off the gas input and maintain the vacuum environment, and perform annealing treatment on the surface by plasma residual energy to eliminate stress and stabilize the functional group structure. The time lasted for 5 min, and the average roughness Ra of the surface of the separator was 50 nm.

[0123] Target preparation steps: Analytically pure Al2O3 powder and Li3N powder were filled in the mold according to the proportion, and the powder was pressed into a 60 mm diameter target blank using a hydraulic machine. The specific operation method is as follows: place the powder-filled mold in the center of the hydraulic machine workbench; start the hydraulic machine, and lower the upper punch at a set rate while raising the lower punch until the target pressure is reached; the punch material is hard alloy; the target blank is an injection molded blank in a high polymer material blank. The mass ratio of Al2O3 powder and Li3N powder is 1:1, and the sintering temperature is 1600°C, the sintering time is 30h, and then it is naturally cooled to room temperature as a target.

[0124] Magnetron sputtering steps: The pretreated separator and inorganic material target were placed in an argon atmosphere, and magnetron sputtering was carried out under the conditions of vacuum degree 5x10 -4 Pa, magnetron sputtering time 10 min, magnetron sputtering power 100 W, substrate temperature 45°C, argon flow rate 20 sccm, and sputtering pressure 0.2 Pa. The particle layer was deposited on both sides of the separator, and the particle layer thickness was controlled to be 2 pm (single-side thickening) by controlling the sputtering time.

[0125] Post-processing: The separator was placed in an 80°C vacuum drying box for annealing treatment for 30 min to eliminate internal stress. The obtained separator porosity was 33-34%.

[0126] Example 2

[0127] Separator substrate pretreatment steps:

[0128] S1: Commercially available 9 pm PE separator was placed in a plasma cleaning machine, oxygen was introduced into the vacuum chamber, the flow rate of oxygen was 25 sccm, and the initial plasma was generated by applying a low-frequency pulse power (frequency 20 kHz) to coarsely clean the surface of the separator. The time of low-frequency treatment was 10 min.

[0129] S2: Switch to high-frequency pulse power (frequency 100 kHz), while introducing a small amount of water vapor (volume fraction 1%), using the synergistic reaction of oxygen free radicals (O·) and hydroxyl radicals (·OH) to realize the surface cleaning and oxygen-containing functional group introduction of the separator. After high-frequency treatment for 10 min, the AFM detected that the average roughness Ra of the surface was 5 nm.

[0130] S3: Turn off the gas input and maintain the vacuum environment, and perform annealing treatment on the surface by plasma residual energy to eliminate stress and stabilize the functional group structure. The time lasted for 5 min, and the average roughness Ra of the surface of the separator was 50 nm.

[0131] The steps of preparing the target material: The analytical pure Al2O3 powder was filled in the mold, and the powder was pressed into a target blank with a diameter of 60 mm using a hydraulic machine. The specific operation method is as follows: place the mold filled with powder in the center of the workbench of the hydraulic machine; start the hydraulic machine, and lower the upper punch at a set rate while raising the lower punch until the target pressure is reached; the punch material is hard alloy; the target blank is an injection molded blank in a high polymer material blank. Sintering in a resistance furnace at a high temperature of 1600°C for 30 h, and then naturally cooled to room temperature as a target material.

[0132] The steps of magnetron sputtering: The pretreated separator and inorganic material target were placed in an argon atmosphere, and magnetron sputtering was carried out under the conditions of vacuum degree 5x10 -4 Pa, magnetron sputtering time 10 min, magnetron sputtering power 100 W, substrate temperature 45°C, argon flow rate 20 sccm, and sputtering pressure 0.2 Pa. The particle layer was deposited on both sides of the separator, and the particle layer thickness was controlled to be 2 pm (single side thickening) by controlling the sputtering time.

[0133] Post-processing: The separator was annealed in a vacuum drying oven at 80°C for 30 min to eliminate internal stress. The obtained separator had a porosity of 33-34%.

[0134] Example 3

[0135] The steps of pretreating the separator substrate:

[0136] S1: Commercially available 9 pm PE separator was placed in a plasma cleaning machine, oxygen was introduced into the vacuum chamber at a flow rate of 25 sccm, and an initial plasma was generated by applying a low-frequency pulsed power source (frequency 20 kHz) to coarsely clean the surface of the separator. The low-frequency treatment time was 10 min.

[0137] S2: Switch to a high-frequency pulsed power source (frequency 100 kHz) while introducing a small amount of water vapor (volume fraction 1%), and use the synergistic reaction of oxygen radicals (O·) and hydroxyl radicals (·OH) to achieve surface cleaning and oxygen-containing functional group introduction. After 10 min of high-frequency treatment, the AFM detected that the average surface roughness Ra was 5 nm.

[0138] S3: Turn off the gas input and maintain a vacuum environment, and perform annealing treatment on the surface by plasma residual energy to eliminate stress and stabilize the functional group structure. The time lasted for 5 min, and the average roughness Ra of the surface of the separator was 50 nm.

[0139] Target preparation steps: Analytically pure Li3N powder was filled in a mold, and the powder was pressed into a 60 mm diameter target blank using a hydraulic machine. The specific operation method is as follows: place the mold filled with powder in the center of the hydraulic machine workbench; start the hydraulic machine, lower the upper punch at a set rate, while the lower punch rises until the target pressure is reached; the punch material is hard alloy; the target blank is an injection molded blank in a high polymer material blank. Sintering in a resistance furnace at a high temperature of 1600°C for 30 h, and then naturally cooled to room temperature as a target.

[0140] Magnetron sputtering steps: The pretreated separator and inorganic material target were placed in an argon atmosphere, and magnetron sputtering was carried out under the conditions of vacuum degree 5 x 10 -4 Pa, magnetron sputtering time 10 min, magnetron sputtering power 100 W, substrate temperature 45°C, argon flow rate 20 sccm, and sputtering pressure 0.2 Pa. The particle layer was deposited on both sides of the separator, and the particle layer thickness was controlled to be 2 pm (single-side thickening) by controlling the sputtering time.

[0141] Post-processing: The separator was annealed in a vacuum drying oven at 80°C for 30 min to eliminate internal stress. The obtained separator had a porosity of 33-34%.

[0142] Comparative Example 1

[0143] 9 pm PE base film, without any treatment. The porosity of the base film was 35%.

[0144] Comparative Example 2

[0145] The analytical pure Al2O3 powder and Li3N powder were prepared into a water-based slurry with a solid content of 40% at a ratio of 1:1, coated on a commercially available 9 μm PE separator using a wire bar, thickened by 2 μm on both sides, and baked at 90°C for 12 h for standby. The obtained separator had a porosity of 28-30%.

[0146] Performance test

[0147] The following performance tests were performed on the separators obtained in the examples and comparative examples, and the test results are shown in Table 1.

[0148] 1. Heat shrinkage test of the separator: The test was performed according to GB / T 36363-2018. A 10.0 cm x 10.0 cm line was drawn on the separator along the MD x TD direction. After the separator was sandwiched between two A4 papers, it was placed in an oven. The heat shrinkage rate of the separator was measured by heating at 150°C for 1 h, and the average value of three samples was taken as the test result. The separators obtained after the heat shrinkage test of the separators of Examples 1-3 and Comparative Examples 1-2 are shown in Figures 1-5

[0149] 2. Electrochemical performance test of the button cell: The separator was cut into a 18 mm diameter disc for standby. The sputtered coated separator was assembled into a button cell in an argon-filled glove box. The button cell was assembled in the order of negative shell-gasket-stainless steel sheet-lithium sheet-separator-20 μL electrolyte-NCM positive electrode-positive shell. After the assembled button cell was left to stand at room temperature for 12 h, the corresponding electrochemical performance test was performed. Within the electrochemical window of 2.7-4.35 V, the charge-discharge performance and cycle performance of the lithium ion battery were characterized using a battery test system. The electrolyte was 1M LiPF6 dissolved in a 1:1 mixture of ethylene carbonate / diethyl carbonate (EC / DEC). The test results are shown in Table 1.

[0150] 3. Surface contact angle: Sample preparation: The separator was cut into a standard size (e.g. 20 mm x 20 mm), the surface impurities were washed with deionized water, and then fixed on the test platform after drying.

[0151] Drop control: 0.5-3 μL of electrolyte was added to the surface of the separator using a micropipette to ensure uniform droplet size.

[0152] Image acquisition: The contact image of the droplet and the separator was vertically photographed by a high-speed camera or a microscope.

[0153] Data analysis: The contact angle was calculated using an elliptical fitting method. The test results are shown in Table 1.

[0154] ​4. Uniformity: The uniformity of the particle layer was characterized by the particle size distribution width (Span value) and thickness difference. The particle size distribution was tested by a laser particle size analyzer to obtain D10, D50 and D90, and the Span value = (D90-D10) / D50; the thickness difference: the probe of the thickness gauge was vertically pressed on the surface of the separator, and a constant pressure (usually 2 kPa, to avoid crushing the separator) was applied. After the reading was stable, the thickness value was recorded, and each point was measured 3 times to take the average value. The test results are shown in Table 1.

[0155] 5. Tensile strength: Sample preparation: using a high-precision cutter to cut into a strip-shaped sample with a width of 15±0.1 mm and a length of ≥150 mm. The sample was clamped vertically between the upper and lower clamps to avoid wrinkles or deflection. Test speed: 250 mm / min (lithium ion battery separator standard). Clamp spacing: 100 mm. Start the testing machine and record the force-displacement curve during the tensile process. Observe the deformation of the sample until the sample breaks. Calculate the tensile strength (maximum force value / cross-sectional area of the sample) according to the force-displacement curve. The test results are shown in Table 1.

[0156] Table 1. Performance test results of examples and comparative examples

[0157]

[0158] From Table 1 and Figures 1-5 It can be seen that the untreated separator (Comparative Example 1) and the separator prepared by the conventional coating method (Comparative Example 2) have poor heat shrinkage resistance, while the separator prepared by the preparation method of the present application has low heat shrinkage at 150℃, excellent heat resistance, and the battery prepared by using the separator of the present application has high cycle performance.

[0159] The battery prepared by the separator of Example 1 has a specific capacity retention rate of 94% after 50 cycles, and the attenuation rate per cycle is 0.12%, while the PE separator battery (Comparative Example 1) has a low specific capacity retention rate after 50 cycles. Due to the introduction of nanoparticles, the electrolyte absorption rate of the separator is improved, and the Li3N particles effectively inhibit the decomposition of the electrolyte and the growth of lithium dendrites, so that more lithium ions that can move freely are retained in the electrolyte solution, so that the lithium ion battery can still maintain a high discharge capacity after multiple charge-discharge cycles. In addition, the battery prepared by the separator prepared by the conventional coating method (Comparative Example 2) has only a slight improvement in specific capacity retention rate after 50 cycles compared to Comparative Example 1, and the uniformity of the nanoparticles is poor, which has a very limited effect on the improvement of the performance of the battery.

[0160] The particle size distribution width and thickness difference of the separators prepared in Examples 1-3 are significantly lower than those of the separator prepared by the conventional coating method (Comparative Example 2), which shows that the coating of the separator prepared by the preparation method of the present application is more uniform.

[0161] The porosity of the separator prepared in Examples 1-3 has little change compared to the porosity of the base film (Comparative Example 1), and the porosity retention is high, while the porosity retention of the separator prepared by the conventional coating method (Comparative Example 2) is obviously lower. The high porosity retention of the separator of the present application can ensure the free migration of lithium ions.

[0162] The tensile strength of the separator prepared in Examples 1-3 is obviously improved compared to the separators obtained in Comparative Examples 1 and 2. It can be seen that the mechanical properties of the separator can be effectively improved by the uniform particle layer to prevent the penetration of lithium dendrites, and the safety of the battery is further improved.

[0163] It should be noted that although the technical solutions of the present application are described with specific examples, those skilled in the art can understand that the present application should not be limited thereto.

[0164] The above has described various embodiments of the present application, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes are obvious to those skilled in the art without departing from the scope and spirit of the described embodiments. The choice of terms used herein is intended to best explain the principles, practical applications or technical improvements in the market of the embodiments, or to enable other ordinary skilled persons in the art to understand the embodiments disclosed herein.

Claims

1. A diaphragm, characterized in that, It includes a membrane substrate and a particle layer deposited on both surfaces of the membrane substrate. The particle layer comprises inorganic nanoparticles and / or pre-lithiation agent particles, the particle size distribution width of the particle layer is 0.9~1.3, and the thickness difference of the particle layer is 0.5~1.2μm; The surface contact angle of the diaphragm is less than 40°.

2. The diaphragm according to claim 1, characterized in that, The diaphragm retains more than 90% of the porosity relative to the diaphragm substrate.

3. The diaphragm according to claim 1 or 2, characterized in that, The thickness of the particle layer is 2~5μm.

4. The diaphragm according to claim 1 or 2, characterized in that, The mass ratio of the inorganic nanoparticles to the pre-lithiating agent particles is 1:0.5 to 1:

2.

5. The diaphragm according to claim 1 or 2, characterized in that, The membrane substrate includes one or more of polyethylene, polypropylene, PP / PE composite membrane, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyethylene terephthalate, cellulose membrane, glass fiber membrane, aramid membrane, and polyimide membrane; and / or, the inorganic nanoparticles include one or more of SiO2, Al2O3, Si3N4, AlN, TiO2, MnO, NiO, CoO, Mo3N2, and FeO; and / or, the pre-lithiation agent includes one or more of Li3N, Li2O, Li2S, LiF, and Li2CO3.

6. The diaphragm according to claim 1 or 2, characterized in that, The diaphragm satisfies at least one of the following conditions: i. The tensile strength of the diaphragm is 1500 kgf / cm². 2 above; ii. After the diaphragm is heated at 150°C for 1 hour, its transverse thermal shrinkage rate TD < 5% and its longitudinal thermal shrinkage rate MD < 6%.

7. A method for preparing a diaphragm according to any one of claims 1 to 6, characterized in that, This includes steps such as membrane substrate pretreatment, target preparation, and magnetron sputtering. The pretreatment steps for the diaphragm substrate include: S1: Place the diaphragm substrate in a plasma cleaner, introduce oxygen and water vapor into the vacuum chamber, and perform plasma treatment on the diaphragm substrate under an electric field; S2: Turn off the gas input and perform a first annealing treatment on the surface of the diaphragm substrate in a vacuum environment using the residual energy of plasma to obtain a pretreated diaphragm substrate. The steps for preparing the target material include: pressing inorganic material powder into shape, sintering and cooling to obtain the target material, wherein the inorganic material includes inorganic nanoparticles and / or pre-lithiation agents; The magnetron sputtering step includes: in an inert gas atmosphere, using a target to perform magnetron sputtering on a pretreated diaphragm substrate to deposit a particulate layer on both surfaces of the pretreated diaphragm substrate to obtain a diaphragm.

8. The preparation method according to claim 7, characterized in that, In step S1, the oxygen flow rate is 10~50 sccm; and / or, the water vapor flow rate is 0.1~5% of the total oxygen flow rate; and / or, the plasma treatment time is 10~40 min, and the electric field pulse frequency is 10~500 kHz.

9. The preparation method according to claim 7 or 8, characterized in that, Step S1 includes the following steps: placing the diaphragm substrate in a plasma cleaner, introducing oxygen into the vacuum chamber, and performing coarse cleaning under low-frequency pulses; switching to high-frequency pulses while simultaneously introducing water vapor; The low-frequency pulse has a frequency of 10~50kHz and a processing time of 5~20min; the high-frequency pulse has a frequency of 100~500kHz and a processing time of 5~20min.

10. The preparation method according to claim 7 or 8, characterized in that, In step S2, the first annealing treatment takes 5 to 30 minutes; and / or the surface roughness Ra of the pretreated diaphragm substrate is ≤ 50 nm.

11. The preparation method according to claim 7 or 8, characterized in that, In the target preparation step, the sintering temperature is 1500~1800℃; the sintering time is 25~35h.

12. The preparation method according to claim 7 or 8, characterized in that, In the magnetron sputtering process, the vacuum level is 5 × 10⁻⁶. -4 Below Pa; and / or, The magnetron sputtering time is 10-60 min; and / or, The magnetron sputtering power is 100~500W; and / or, The temperature of the pretreated diaphragm substrate is 20~80℃; and / or, The flow rate of the inert gas is 20~100 sccm; and / or, The magnetron sputtering pressure is 0.2~1.0 Pa; and / or, The thickness of the particle layer is 2~5μm.

13. The preparation method according to claim 7 or 8, characterized in that, The preparation method further includes a post-processing step, which includes: performing a second annealing treatment on the diaphragm, wherein the temperature of the second annealing treatment is 80~120℃ and the time is 10min~6h.

14. A lithium secondary battery, characterized in that, It includes a positive electrode, a negative electrode, an electrolyte, and a diaphragm according to any one of claims 1 to 6.

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