Separator and method for manufacturing the same, and lithium secondary battery

Abstract

The present application provides a kind of diaphragm and its preparation method and lithium secondary battery, the diaphragm includes diaphragm base material and the particle layer deposited in the two surfaces of the diaphragm base material, wherein, the particle layer includes inorganic nanoparticles and / or prelithiation agent particles, the particle size distribution width of the particle layer is 0.9~1.3, the thickness difference of the particle layer is 0.5~1.2 μm;The surface contact angle of the diaphragm is 40 ° or less.The diaphragm of the present application introduces a layer of uniform particle layer containing inorganic nanoparticles and / or prelithiation agent particles on the surface.The particle layer can improve the electrolyte wettability and heat resistance of the diaphragm, at the same time, uniform particle layer can also guide lithium ion uniform distribution, realizes the uniform and dense deposition of lithium, effectively inhibits lithium dendrite growth.

Description

Technical Field

[0001] This invention relates to a separator, its preparation method, and a lithium secondary battery, belonging to the field of lithium-ion battery technology. Background Technology

[0002] The separator is an indispensable component of a lithium-ion battery system. It prevents short circuits between the positive and negative electrodes, ensuring battery safety, while providing an effective channel for the rapid migration of lithium ions. The formation and growth of lithium dendrites can penetrate the separator, causing direct contact between the positive and negative electrodes and leading to internal short circuits. Therefore, designing separators that suppress lithium dendrites and improve the reliability and safety of batteries operating in harsh environments has significant scientific and practical value.

[0003] Currently, the most common method for modifying separators is to use ceramic nanoparticles such as Al2O3 to coat the surface of the separator in order to enhance the mechanical strength of the separator and prevent lithium dendrite penetration.

[0004] Some literature discloses a method for integrating lithium-ion battery separators with batteries. This method significantly enhances the mechanical strength and thermal stability of the separator by coating it with a thermally stable inorganic ceramic material, thereby improving battery safety. However, the coating method is prone to uneven coating. The uneven distribution of inorganic ceramic particles can lead to uneven current density distribution, accelerate lithium dendrite growth, and affect battery capacity, cycle performance, and safety performance.

[0005] In addition, during the first charge of a lithium-ion battery, a solid electrolyte interphase (SEI) film forms on the surface of the negative electrode. This process consumes a large number of lithium ions, resulting in irreversible capacity loss of the battery.

[0006] Pre-lithiation of the separator can effectively improve the above-mentioned problems. Pre-lithiation of the separator involves introducing a pre-lithiation reagent onto the separator surface, replenishing the lithium source to the negative electrode before the battery operates, offsetting lithium loss caused by processes such as SEI film formation, thereby improving the battery's first-cycle coulombic efficiency, total capacity, and energy density. However, current methods for introducing the pre-lithiation reagent mainly employ coating, which still suffers from problems such as uneven coating and uneven distribution of the pre-lithiation reagent. Summary of the Invention

[0007] The problem the invention aims to solve

[0008] In order to solve the problems existing in the prior art as described above, the present invention provides a separator with a uniformly distributed surface modification layer, excellent mechanical strength, the ability to suppress the formation and growth of lithium dendrites, and the ability to uniformly introduce pre-lithiation reagents, thereby improving the safety performance and cycle performance of the battery.

[0009] Solution for solving the problem

[0010] This invention provides a separator with a uniform granular layer on its surface comprising inorganic nanoparticles and / or pre-lithiation agent particles. This granular layer improves the electrolyte wettability and heat resistance of the separator. Simultaneously, the uniform granular layer guides the uniform distribution of lithium ions, achieving uniform and dense lithium deposition and effectively suppressing lithium dendrite growth.

[0011] In addition, the addition of pre-lithiation agent ensures that a protective SEI layer is always formed on the negative electrode, and the battery system can maintain a high lithium-ion concentration, thereby improving the battery's cycle performance.

[0012] This invention first provides a diaphragm, which includes a diaphragm substrate and a particle layer deposited on two surfaces of the diaphragm 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 0.5~1.2μm;

[0014] The surface contact angle of the diaphragm is less than 40°.

[0015] According to the diaphragm of the present invention, the diaphragm has a porosity retention rate of 90% or more relative to the diaphragm substrate.

[0016] According to the diaphragm of the present invention, the thickness of the particulate layer is 2~5 μm.

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

[0018] According to the diaphragm of the present invention, the diaphragm substrate comprises one or more of polyethylene, polypropylene, PP / PE composite diaphragm, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyethylene terephthalate, cellulose membrane, glass fiber diaphragm, aramid diaphragm, and polyimide diaphragm; 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 diaphragm of the present invention, the diaphragm satisfies at least one of the following conditions:

[0020] i. The tensile strength of the diaphragm is 1500 kgf / cm². 2 above;

[0021] 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%.

[0022] The present invention also provides a method for preparing a diaphragm according to the present invention, which includes a step of pretreatment of the diaphragm substrate, a step of target preparation, and a step of magnetron sputtering;

[0023] The pretreatment steps for the diaphragm substrate include:

[0024] 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;

[0025] 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.

[0026] 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;

[0027] 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.

[0028] According to the preparation method of the present invention, 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 volume; and / or, the plasma treatment time is 10~40 min, and the electric field pulse frequency is 10~500 kHz.

[0029] According to the preparation method of the present invention, 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 and simultaneously introducing water vapor;

[0030] 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.

[0031] According to the preparation method of the present invention, in step S2, the time of the first annealing treatment is 5~30 min; and / or, the surface roughness Ra of the pretreated diaphragm substrate is ≤50 nm.

[0032] According to the preparation method of the present invention, in the target preparation step, the sintering temperature is 1500~1800℃; the sintering time is 25~35h.

[0033] According to the preparation method of the present invention, in the magnetron sputtering step, the vacuum degree is 5 × 10⁻⁶. -4 Below Pa; and / or,

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

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

[0036] The temperature of the pretreated diaphragm substrate is 20~80℃; and / or,

[0037] The flow rate of the inert gas is 20~100 sccm; and / or,

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

[0039] The thickness of the particle layer is 2~5μm.

[0040] According to the preparation method of the present invention, the preparation method further includes a post-processing step, the post-processing step including: subjecting the diaphragm to a second annealing treatment, wherein the temperature of the second annealing treatment is 80~120℃ and the time is 10min~6h.

[0041] The present invention also provides a lithium secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator according to the present invention.

[0042] The effects of the invention

[0043] 1. Compared with diaphragms obtained by traditional coating techniques, the diaphragm provided by this invention has a controllable surface particle layer thickness, reaching the nanometer level, and the particles are uniformly distributed, avoiding the uneven coating problem caused by traditional coating methods. Simultaneously, the particles in the particle layer can stack together and form voids on the diaphragm surface, allowing the coating to adhere more effectively to the Li coating. + The larger contact area between them is conducive to the formation of uniform ion flux. The uniform particle layer induces uniform lithium ion deposition, which can avoid the formation of lithium dendrites that cause safety hazards and capacity reduction in lithium batteries.

[0044] 2. The diaphragm provided by the present invention can retain the original porosity of the diaphragm base membrane, ensuring the free migration of lithium ions.

[0045] 3. The separator provided by the present invention can provide lithium replenishment effect by adding 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 uniformly deposited particulate layer in the separator provided by the present invention can also effectively increase the mechanical strength of the separator to prevent lithium dendrites from piercing it, thereby further improving the safety of the battery.

[0047] 5. The preparation method provided by the present invention achieves the deposition of a uniform particle layer by using magnetron sputtering technology. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the diaphragm obtained in Example 1 after undergoing a heat shrinkage test.

[0049] Figure 2 This is a schematic diagram of the diaphragm obtained in Example 2 after undergoing a heat shrinkage test;

[0050] Figure 3 This is a schematic diagram of the diaphragm obtained in Example 3 after undergoing a heat shrinkage test;

[0051] Figure 4 This is a schematic diagram of the diaphragm obtained in Comparative Example 1 after undergoing a heat shrinkage test.

[0052] Figure 5 This is a schematic diagram of the diaphragm obtained in Comparative Example 2 after undergoing a heat shrinkage test. Detailed Implementation

[0053] Various exemplary embodiments, features, and aspects of the present invention will be described in detail below. The term "exemplary" as used herein means "serving as an example, embodiment, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as superior to or better than other embodiments.

[0054] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art should understand that the present invention can be practiced without certain specific details. In other instances, methods, means, apparatus, and steps well known to those skilled in the art have not been described in detail in order to highlight the spirit of the present invention.

[0055] Unless otherwise stated, all units used in this specification are international standard units, and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors that are unavoidable in industrial production.

[0056] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0057] In this specification, references to "some specific / preferred embodiments," "other specific / preferred embodiments," "implementation," etc., refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to that embodiment, which are included in at least one of the embodiments described herein and may or may not be present in other embodiments. Furthermore, it should be understood that these elements may be combined in any suitable manner in various embodiments.

[0058] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0059] In this instruction manual, "normal temperature" or "room temperature" refers to an indoor ambient temperature of "23±2℃".

[0060] [First aspect]

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

[0062] In this invention, 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, and it is obtained by (D90-D10) / D50, where D10, D50 and D90 represent the particle size corresponding to the cumulative distribution of particles reaching 10%, 50% and 90%, respectively; the thickness difference of the particle layer is 0.5~1.2μm, for example, it can be 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.1μm, etc., wherein the thickness difference is the thickness difference of the particle layer deposited on any side of the base film; in this invention, by controlling the uniform distribution of particles, the problem of uneven coating caused by traditional coating methods is avoided.

[0063] In this invention, the porosity retention rate of the separator relative to the separator substrate is 90% or more, preferably 92% or more, and 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 implementations, the particle size distribution (D90) of the particles in the particle layer is 0.9~1.6μm, for example, it can be 0.95μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, etc.; the particle size distribution (D50) is 0.7~1.2μm, for example, it can be 0.75μm, 0.8μm, 0.85μm, 0.9μm, 1μm, 1.1μm, etc.; and the particle size distribution (D10) is 0.1~0.5μm, for example, it can be 0.15μm, 0.2μm, 0.25μm, 0.3μm, 0.4μm, etc.

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

[0066] Furthermore, the surface contact angle of the diaphragm is less than 40°, preferably less than 38°, and more preferably less than 35°. Specifically, the contact angle is the angle between the tangent of the gas-liquid interface at the gas-liquid-solid three-phase junction when 0.5~3 μL of electrolyte forms droplets on the diaphragm surface and the liquid-solid-liquid interface line. The particles in the granular layer can stack together and form voids on the diaphragm surface, increasing surface roughness and reducing the contact angle, which is beneficial for forming a uniform ion flux.

[0067] In some specific implementations, the thickness of the particle layer (the thickness of one side of the membrane substrate) can be 2~5μm, for example, it can be 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, etc.

[0068] The present invention does not impose any particular limitation on the membrane substrate, and commonly used membrane substrates in the art can be used, such as one or more of polyethylene (PE), polypropylene (PP), PP / PE composite membrane, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), cellulose membrane, glass fiber membrane, aramid membrane, and polyimide (PI) membrane.

[0069] In some specific implementations, the thickness of the diaphragm substrate can be 5~15μm, for example, 7μm, 9μm, 10μm, 12μm, 14μm, 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 implementations, the pre-lithiation agent particles include one or more of Li3N, Li2O, Li2S, LiF, and Li2CO3.

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

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

[0074] i. The tensile strength of the diaphragm is 1500 kgf / cm². 2 The preferred value is 1550 kgf / cm³. 2 The above, and more preferably 1600 kgf / cm 2 above;

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

[0076] [Second aspect]

[0077] A second aspect of the present invention provides a method for preparing a diaphragm, comprising a diaphragm substrate pretreatment step, a target material preparation step, and a magnetron sputtering step.

[0078] (Steps for pretreatment of diaphragm substrate)

[0079] In this invention, the pretreatment step of the diaphragm substrate includes:

[0080] 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;

[0081] 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.

[0082] Step S1

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

[0084] The present invention does not impose any particular limitation on the membrane substrate, and commonly used membrane substrates in the art can be used, such as one or more of polyethylene (PE), polypropylene (PP), PP / PE composite membrane, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), cellulose membrane, glass fiber membrane, aramid membrane, and polyimide (PI) membrane.

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

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

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

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

[0089] In some preferred embodiments, to further improve the cleaning and activation effect, step S1 may include 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 frequency of the low-frequency pulses can be 10~50kHz, for example, 15kHz, 20kHz, 25kHz, 30kHz, 35kHz, 40kHz, 45kHz, etc., and the low-frequency pulse treatment time can be 5~20 minutes; the frequency of the high-frequency pulses can be 100~500kHz, for example, 150kHz, 200kHz, 250kHz, 300kHz, 350kHz, 400kHz, 450kHz, etc., and the high-frequency pulse treatment time can be 5~20 minutes.

[0090] Introducing oxygen under low-frequency pulses can generate initial plasma for rough cleaning of the diaphragm surface. Switching to high-frequency pulses while introducing water vapor can utilize the synergistic reaction of oxygen free radicals (O·) and hydroxyl groups (·OH) to avoid excessive oxidation, while simultaneously cleaning the diaphragm substrate surface and introducing oxygen-containing functional groups (-OH, -COOH), significantly improving surface energy.

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

[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 using the residual energy of the plasma to eliminate stress and stabilize the functional group structure.

[0094] In some specific implementations, the time for the first annealing process can be 5 to 30 minutes, for example, 10 minutes, 15 minutes, 20 minutes, 25 minutes, etc.

[0095] In some specific embodiments, the surface roughness Ra of the pretreated membrane substrate is ≤50nm, preferably 45nm or less, and more preferably 40nm or less. When the surface roughness Ra of the pretreated membrane substrate is ≤50nm, the uniformity of the particle layer thickness can be improved, the local retention of electrolyte on the membrane surface can be reduced, and the rapid penetration of electrolyte into the internal pores of the membrane can be promoted to form a uniform ion conduction network.

[0096] (Steps for target preparation)

[0097] In this invention, the target preparation steps include: pressing, sintering and cooling an inorganic material to obtain a target, wherein the inorganic material includes inorganic nanoparticles and / or a pre-lithiation agent.

[0098] By forming an inorganic nanoparticle coating on the surface of the separator substrate using inorganic nanoparticles, the electrolyte wettability and heat resistance of the separator can be improved. By adding a pre-lithiation agent, a protective SEI layer can always be formed on the negative electrode, 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 compression molding may include filling an analytically pure inorganic material powder into a mold and using a hydraulic press to press the powder into a target blank with a diameter of 60 mm. The specific operation is as follows: the mold filled with powder is placed in the center of the hydraulic press's worktable; the hydraulic press is started, the upper male mold descends at a set rate while the lower male mold rises simultaneously until the target pressure is reached; the male mold is made of cemented carbide; the target blank is an injection-molded blank from a polymer material blank. There is no particular limitation on the type of target blank; it can be any material commonly used in the art.

[0100] In some specific implementations, 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] The present invention does not impose any particular limitation on the equipment used for sintering, and the equipment can be selected as needed, for example, it can be carried out in a resistance furnace.

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

[0103] (Steps of magnetron sputtering)

[0104] In this invention, 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.

[0105] The present invention does not particularly limit the type of inert gas, which can be a commonly used inert gas in the art, such as argon, helium, etc.

[0106] In some specific implementations, the vacuum level during the magnetron sputtering step is 5 × 10⁻⁶. -4 Below Pa.

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

[0108] In some specific implementations, 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 implementations, the temperature of the pretreated diaphragm substrate is 20~80℃, for example, it can be 30℃, 40℃, 50℃, 60℃, 70℃, etc.

[0110] In some specific implementations, the thickness of the particle layer (the thickness of one side of the membrane substrate) is 2~5μm, for example, it can be 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, etc.

[0111] (Other steps)

[0112] The preparation method of the present invention may further include a post-processing step, specifically, the post-processing step may include: performing a second annealing treatment on the diaphragm. The second annealing treatment can eliminate internal stress.

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

[0114] [Third aspect]

[0115] A third aspect of the present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator according to the first aspect.

[0116] Example

[0117] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0118] Example 1

[0119] The steps for pretreatment of the diaphragm substrate are as follows:

[0120] S1: Place a commercially available 9μm PE diaphragm in a plasma cleaner. Introduce oxygen into the vacuum chamber at a flow rate of 25 sccm. Apply a low-frequency pulse power supply (frequency 20kHz) to generate initial plasma for coarse cleaning of the diaphragm surface. The low-frequency treatment time is 10 minutes.

[0121] S2: Switch to high-frequency pulse power supply (frequency 100kHz), and simultaneously introduce a trace amount of water vapor (1% by volume). Utilize the synergistic reaction of oxygen free radicals (O·) and hydroxyl groups (·OH) to achieve membrane surface cleaning and the introduction of oxygen-containing functional groups. After 10 minutes of high-frequency treatment, the average surface roughness Ra measured by AFM was 5nm.

[0122] S3: Gas input is shut off, a vacuum environment is maintained, and the surface is annealed using residual plasma energy to eliminate stress and stabilize the functional group structure. The process lasts for 5 minutes, resulting in an average surface roughness Ra of 50 nm for the diaphragm.

[0123] The target preparation steps are as follows: Analytically pure Al2O3 powder and Li3N powder are filled into a mold in a specific ratio, and a hydraulic press is used to press the powder into a 60mm diameter target blank. Specifically, the mold filled with powder is placed in the center of the hydraulic press's worktable; the hydraulic press is started, the upper male mold descends at a set rate while the lower male mold rises simultaneously until the target pressure is reached; the male mold is made of cemented carbide; the target blank is an injection-molded blank from a polymer material blank. The mass ratio of Al2O3 powder to Li3N powder is 1:1. The target is sintered at high temperature in a resistance furnace at 1600℃ for 30 hours, and then naturally cooled to room temperature to obtain the target material.

[0124] The steps of magnetron sputtering are as follows: The pretreated diaphragm and inorganic target are placed in an argon atmosphere at a vacuum of 5 × 10⁻⁶. -4 Magnetron sputtering was performed under the following conditions: Pa, sputtering time 10 min, sputtering power 100 W, substrate temperature 45℃, argon flow rate 20 sccm, and sputtering pressure 0.2 Pa. A particle layer was deposited on both sides of the membrane, and the particle layer thickness was controlled to 2 μm (single-sided thickening) by controlling the sputtering time.

[0125] Post-treatment: The diaphragm was annealed in an 80℃ vacuum drying oven for 30 minutes to eliminate internal stress. The resulting diaphragm had a porosity of 33-34%.

[0126] Example 2

[0127] The steps for pretreatment of the diaphragm substrate are as follows:

[0128] S1: Place a commercially available 9μm PE diaphragm in a plasma cleaner. Introduce oxygen into the vacuum chamber at a flow rate of 25 sccm. Apply a low-frequency pulse power supply (frequency 20kHz) to generate initial plasma for coarse cleaning of the diaphragm surface. The low-frequency treatment time is 10 minutes.

[0129] S2: Switch to high-frequency pulse power supply (frequency 100kHz), and simultaneously introduce a trace amount of water vapor (1% by volume). Utilize the synergistic reaction of oxygen free radicals (O·) and hydroxyl groups (·OH) to achieve membrane surface cleaning and the introduction of oxygen-containing functional groups. After 10 minutes of high-frequency treatment, the average surface roughness Ra measured by AFM was 5nm.

[0130] S3: Gas input is shut off, a vacuum environment is maintained, and the surface is annealed using residual plasma energy to eliminate stress and stabilize the functional group structure. The process lasts for 5 minutes, resulting in an average surface roughness Ra of 50 nm for the diaphragm.

[0131] The target preparation steps are as follows: Analytical-grade Al2O3 powder is filled into a mold, and a hydraulic press is used to press the powder into a 60mm diameter target blank. Specifically, the mold filled with powder is placed in the center of the hydraulic press's worktable; the hydraulic press is started, the upper male mold descends at a set rate while the lower male mold rises simultaneously until the target pressure is reached; the male mold is made of cemented carbide; the target blank is an injection-molded blank from a polymer material blank. The target is sintered at high temperature in a resistance furnace at 1600℃ for 30 hours, and then naturally cooled to room temperature to obtain the target material.

[0132] The steps of magnetron sputtering are as follows: The pretreated diaphragm and inorganic target are placed in an argon atmosphere at a vacuum of 5 × 10⁻⁶. -4 Magnetron sputtering was performed under the following conditions: Pa, sputtering time 10 min, sputtering power 100 W, substrate temperature 45℃, argon flow rate 20 sccm, and sputtering pressure 0.2 Pa. A particle layer was deposited on both sides of the membrane, and the particle layer thickness was controlled to 2 μm (single-sided thickening) by controlling the sputtering time.

[0133] Post-treatment: The diaphragm was annealed in an 80℃ vacuum drying oven for 30 minutes to eliminate internal stress. The resulting diaphragm had a porosity of 33-34%.

[0134] Example 3

[0135] The steps for pretreatment of the diaphragm substrate are as follows:

[0136] S1: Place a commercially available 9μm PE diaphragm in a plasma cleaner. Introduce oxygen into the vacuum chamber at a flow rate of 25 sccm. Apply a low-frequency pulse power supply (frequency 20kHz) to generate initial plasma for coarse cleaning of the diaphragm surface. The low-frequency treatment time is 10 minutes.

[0137] S2: Switch to high-frequency pulse power supply (frequency 100kHz), and simultaneously introduce a trace amount of water vapor (1% by volume). Utilize the synergistic reaction of oxygen free radicals (O·) and hydroxyl groups (·OH) to achieve membrane surface cleaning and the introduction of oxygen-containing functional groups. After 10 minutes of high-frequency treatment, the average surface roughness Ra measured by AFM was 5nm.

[0138] S3: Gas input is shut off, a vacuum environment is maintained, and the surface is annealed using residual plasma energy to eliminate stress and stabilize the functional group structure. The process lasts for 5 minutes, resulting in an average surface roughness Ra of 50 nm for the diaphragm.

[0139] The target preparation steps are as follows: Analytical-grade Li3N powder is filled into a mold, and a hydraulic press is used to press the powder into a 60mm diameter target blank. Specifically, the mold filled with powder is placed in the center of the hydraulic press's worktable; the hydraulic press is started, the upper male mold descends at a set rate while the lower male mold rises simultaneously until the target pressure is reached; the male mold is made of cemented carbide; the target blank is an injection-molded blank from a polymer material blank. The target is then sintered at high temperature in a resistance furnace at 1600℃ for 30 hours, and then naturally cooled to room temperature to obtain the target material.

[0140] The steps of magnetron sputtering are as follows: The pretreated diaphragm and inorganic target are placed in an argon atmosphere at a vacuum of 5 × 10⁻⁶. -4 Magnetron sputtering was performed under the following conditions: Pa, sputtering time 10 min, sputtering power 100 W, substrate temperature 45℃, argon flow rate 20 sccm, and sputtering pressure 0.2 Pa. A particle layer was deposited on both sides of the membrane, and the particle layer thickness was controlled to 2 μm (single-sided thickening) by controlling the sputtering time.

[0141] Post-treatment: The diaphragm was annealed in an 80℃ vacuum drying oven for 30 minutes to eliminate internal stress. The resulting diaphragm had a porosity of 33-34%.

[0142] Comparative Example 1

[0143] A 9μm PE base film, without any treatment. The porosity of this base film is 35%.

[0144] Comparative Example 2

[0145] Analytical-grade Al₂O₃ powder and Li₃N powder were mixed in a 1:1 ratio to prepare an aqueous slurry with a solid content of 40%. This slurry was then coated onto a commercially available 9μm PE membrane using a wire rod, thickening both sides by 2μm. After baking at 90℃ for 12 hours, the membrane was ready for use. The resulting membrane had a porosity of 28-30%.

[0146] Performance testing

[0147] The membranes obtained in the examples and comparative examples were subjected to the following performance tests, and the test results are shown in Table 1.

[0148] 1. Diaphragm heat shrinkage test: The test was conducted according to GB / T 36363-2018. A 10.0cm × 10.0cm line was drawn on the diaphragm along the MD×TD direction. The diaphragm was sandwiched between two sheets of A4 paper and placed in an oven. It was heated at 150℃ for 1 hour. Three samples were measured at the same temperature, and the average value was taken as the test result. The heat shrinkage rate of the diaphragm was measured. The diaphragms obtained after the heat shrinkage test of Examples 1-3 and Comparative Examples 1-2 are shown below. Figures 1-5 As shown.

[0149] 2. Electrochemical Performance Testing of Button Cell Batteries: The separator was cut into 18mm diameter discs for later use. The sputtered coated separator was assembled into a button cell in an argon-filled glove box. Assembly was performed in the following order: negative electrode casing - gasket - stainless steel sheet - lithium sheet - separator - 20μL electrolyte - NCM positive electrode - positive electrode casing. The assembled button cells were allowed to stand at room temperature for 12 hours before electrochemical performance testing. The charge-discharge performance and cycle performance of the lithium-ion battery were characterized using a battery testing system within an electrochemical window of 2.7–4.35 V. 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: Cut the diaphragm to standard size (e.g., 20mm×20mm), clean the surface impurities with deionized water, and fix it on the test platform after drying.

[0151] Droplet control: Use a microsyringe to drop 0.5~3μL of electrolyte onto the membrane surface to ensure that the droplets are of uniform size.

[0152] Image acquisition: Images of the contact between the droplet and the membrane are captured vertically using a high-speed camera or microscope.

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

[0154] 4. Uniformity: The uniformity of the particle layer is characterized by the particle size distribution width (Span value) and thickness difference. The particle size distribution (D10, D50, and D90) is measured using a laser particle size analyzer, and the Span value is calculated as (D90 - D10) / D50. For thickness difference, a thickness gauge probe is pressed vertically onto the diaphragm surface under constant pressure (typically 2 kPa, to avoid crushing the diaphragm). After the readings stabilize, the thickness values ​​are recorded, with each point measured three times and the average value taken. The test results are shown in Table 1.

[0155] 5. Tensile Strength: Sample Preparation: Cut strip specimens with a width of 15±0.1 mm and a length ≥150 mm using a high-precision cutter. Clamp the specimens vertically between the upper and lower fixtures, avoiding wrinkles or skewing. Test speed: 250 mm / min (lithium-ion battery separator standard). Fixture spacing: 100 mm. Start the testing machine and record the force-displacement curve during the tensile process. Observe the specimen deformation until the specimen breaks. Calculate the tensile strength (maximum force / specimen cross-sectional area) based on the force-displacement curve. The test results are shown in Table 1.

[0156] Table 1 Performance test results of the 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 obtained by the preparation method of the present invention has a low heat shrinkage rate at 150°C and excellent heat resistance. Furthermore, the battery prepared using the separator of the present invention has high cycle performance.

[0159] The battery made with the separator prepared in Example 1 maintained a specific capacity retention of 94% after 50 cycles, with a capacity decay rate of 0.12% per cycle. In contrast, the PE separator battery (Comparative Example 1) showed a lower specific capacity retention after 50 cycles. This is attributed to the fact that the introduction of nanoparticles improved the electrolyte absorption rate of the separator, and the Li3N particles effectively inhibited electrolyte decomposition and lithium dendrite growth, resulting in a higher retention of freely mobile lithium ions in the electrolyte solution. This allowed the lithium-ion battery to maintain a high discharge capacity even after multiple charge-discharge cycles. Furthermore, the battery made with the separator prepared by the conventional coating method (Comparative Example 2) showed only a slight improvement in specific capacity retention after 50 cycles compared to Comparative Example 1, indicating that the uniformity of the nanoparticles was poor, and their effect on improving battery performance was very limited.

[0160] The particle size distribution width and thickness difference of the diaphragm particles prepared in Examples 1-3 are significantly lower than those of the diaphragm prepared by the traditional coating method (Comparative Example 2), indicating that the diaphragm coating obtained by the preparation method of the present invention is more uniform.

[0161] The porosity of the membranes prepared in Examples 1-3 is almost unchanged compared to the porosity of the base membrane (Comparative Example 1), and the porosity retention rate is high. In contrast, the porosity retention rate of the membranes prepared by the conventional coating method (Comparative Example 2) is significantly lower. The high porosity retention rate of the membranes of the present invention can ensure the free migration of lithium ions.

[0162] The tensile strength of the separators prepared in Examples 1-3 is significantly improved compared with that of the separators obtained in Comparative Examples 1 and 2. It can be seen that a uniform particle layer can effectively improve the mechanical properties of the separator, prevent lithium dendrite puncture, and further improve the safety of the battery.

[0163] It should be noted that although the technical solution of the present invention has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0164] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled 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-lithiating agent particles, the particle size distribution width of the particle layer is 0.9~1.3 μm, and the thickness difference of the particle layer is 0.5~1.2 μm; the particle size distribution width is obtained by (D90-D10) / D50, where D10, D50, and D90 represent the particle size corresponding to when the cumulative particle distribution reaches 10%, 50%, and 90%, respectively. 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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