A diaphragm, a method for preparing the same and a lithium ion battery

Abstract

This invention provides a separator, its preparation method, and a lithium-ion battery. The separator includes a base film and a first coating disposed on at least one side of the base film. The first coating comprises a polymer containing sterically hindered groups, aminosulfonyl groups, and urethane groups. By providing a first coating containing a polymer with specific groups on the surface of the base film, the problems of volume expansion and poor high-temperature performance faced by silicon-based anodes in lithium-ion batteries, as well as insufficient wettability of traditional separators, are effectively solved. This improves the lithium-ion transport efficiency, the rate capability and cycle performance of silicon-based anodes, and thus enhances the overall performance of the battery.

Description

Technical Field

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

[0002] As the global energy structure accelerates its transformation towards low-carbon and electrification, lithium-ion batteries have become the mainstream power and energy storage carrier for large-scale applications such as electric vehicles and renewable energy storage systems. In recent years, the driving range of electric vehicles has generally exceeded 400km, effectively alleviating "range anxiety," but "charging anxiety" is becoming increasingly prominent. The market urgently needs high-power fast-charging technology that can replenish more than 80% of the battery capacity in a short time. To meet the dual demands of long driving range and fast charging, power batteries must simultaneously improve both energy density and power density. On the one hand, the energy density of individual battery cells needs to be pushed to 300Wh / kg. -1 On the other hand, it is necessary to achieve continuous fast charging capability without sacrificing cycle life and safety.

[0003] The theoretical specific capacity of currently available commercial graphite anodes is only 372 mAh g. *1 It has already approached the physical limits and is difficult to meet the needs of the next generation of high specific energy systems. Silicon-based anodes, due to their high 4200mAh g⁻¹, are nearing their physical limits and cannot meet the demands of next-generation high-energy-density systems. *1 With its theoretical specific capacity (approximately 10 times that of graphite) and a slightly higher lithium insertion / extraction potential, silicon anodes can suppress surface lithium deposition during high-rate charging, making them a strong alternative to graphite anodes. However, silicon anodes experience significant volume expansion during lithium insertion, posing unprecedented challenges to the mechanical strength, thermal stability, and interface integrity of the separator.

[0004] The polyolefin (PP / PE) based separators and conventional ceramic / polymer coated films widely used in current power batteries have the following problems: insufficient thermal stability, contradiction between ion conduction and wettability, and lack of thermal runaway protection.

[0005] In summary, improvements are needed in the composition of the separator to address issues such as volume expansion and poor high-temperature performance of silicon-based anodes in lithium-ion batteries, as well as insufficient wettability of traditional separators, thereby meeting market demand for high-performance lithium-ion batteries. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a separator, its preparation method, and a lithium-ion battery. By setting a first coating containing a polymer with specific functional groups on the surface of the base film, the invention effectively solves the problems of volume expansion and poor high-temperature performance of silicon-based anodes in lithium-ion batteries, as well as the insufficient wettability of traditional separators. This improves the lithium-ion transport efficiency, the rate capability and cycle performance of silicon-based anodes, and thus enhances the overall performance of the battery.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a diaphragm, the diaphragm comprising a base membrane and a first coating disposed on at least one surface of the base membrane;

[0009] The first coating comprises a polymer, the polymer having a structure containing sterically hindered groups, aminosulfonyl groups, and carbamate groups.

[0010] The separator provided by this invention, by forming a first coating of a polymer containing specific groups on the surface of the base film, achieves high porosity and low thermal shrinkage. It effectively solves the problems of volume expansion and poor high-temperature performance faced by silicon-based anodes in lithium-ion batteries, as well as the insufficient wettability of traditional separators. This improves lithium-ion transport efficiency and the rate and cycle performance of silicon-based anodes, thereby enhancing the overall performance of the battery and meeting market demand for high-performance lithium-ion batteries. The specific reasons are as follows:

[0011] (1) The polymer contains steric hindrance groups, which can regulate the porosity of the coating to make the porosity > 50%, thereby solving the problem of insufficient wettability of the membrane, reducing inter-chain crosslinking of polymer chains, improving the transport efficiency of lithium ions, and reducing intermolecular packing density, thus improving the wettability and interfacial stability of the membrane.

[0012] (2) The polymer contains aminosulfonyl groups, which can provide strong polar sites, increase the lithium ion solubilization ability, and improve the lithium ion transport efficiency. At the same time, it has excellent thermal stability and rigid structure, thereby improving the thermal stability of the membrane.

[0013] (3) The polymer contains urethane groups, which have excellent flexibility, can enhance the adhesion between the first coating and the base film, prevent delamination, and thus improve the inhibitory effect of the separator on the expansion of the silicon-based negative electrode.

[0014] Preferably, the weight-average molecular weight of the polymer is 70,000 Da to 150,000 Da, for example, it can be 80,000 Da, 90,000 Da, 100,000 Da, 110,000 Da, 120,000 Da, 130,000 Da or 140,000 Da.

[0015] It should be noted that controlling the weight-average molecular weight range of the polymer is beneficial for improving the film-forming properties, interfacial adhesion, and mechanical strength of the coating. If the weight-average molecular weight is too low, the film-forming properties, interfacial adhesion, and mechanical strength of the coating will all decrease; if the weight-average molecular weight is too high, the viscosity of the slurry will increase, resulting in a decrease in coating uniformity. Due to its higher crystallinity, although the melting temperature is increased, the brittleness is also increased.

[0016] Preferably, the polymer is a polymer formed by polymerization of a first monomer.

[0017] Preferably, the general formula of the first monomer is R1-OCONH-R2-(CH=CH)n-SO2NH-R3; wherein R1 is a sterically hindered group selected from any one of tert-butyl, neopentyl, adamantyl or triphenylmethyl; R2 is selected from substituted or unsubstituted C1-C4 alkylene groups; R3 is selected from hydrogen atoms or methyl groups; and n is selected from 1 or 2.

[0018] In this invention, the substituted or unsubstituted C1-C4 alkylene groups may be, for example, substituted or unsubstituted methylene, ethylene, isopropylene, or butylene; wherein, among the substituted alkyl groups, the substituted groups include methyl, ethyl, halogen, or cyano, etc.

[0019] Preferably, the structural formula of the first monomer is:

[0020] In this invention, when the first monomer adopts the structure of the above formula, the polymer and the separator formed by the polymer have better thermal stability and resilience, effectively alleviating the volume expansion problem of silicon-based anode, thereby improving the cycle performance and rate performance of the battery at high temperature.

[0021] Preferably, the polymer is prepared by the following method:

[0022] The first monomer and an organic solvent are mixed, and then an initiator is added to carry out a polymerization reaction to obtain the polymer.

[0023] Preferably, the organic solvent includes any one or a combination of at least two of benzene, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), or N,N-dimethylformamide (DMF).

[0024] Preferably, the initiator includes an azo initiator and / or benzoyl peroxide.

[0025] In this invention, the azo initiator includes azobisisobutyronitrile and / or azobisisoheptanenitrile.

[0026] Preferably, the amount of the initiator added is 0.2%-1% of the mass of the first monomer, for example, it can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8% or 0.9%, etc.

[0027] Preferably, the polymerization reaction is carried out under a protective atmosphere.

[0028] In this invention, the protective atmosphere includes argon.

[0029] Preferably, the polymerization reaction temperature is 80℃-130℃, for example, it can be 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃ or 125℃, etc., and the time is 15h-32h, for example, it can be 16h, 18h, 20h, 22h, 24h, 25h, 26h, 28h or 30h, etc.

[0030] It should be noted that by controlling the temperature and time parameters of the polymerization process, polymers with specific molecular weights can be obtained, and the resulting polymers have excellent bonding properties, thermal stability, and high porosity.

[0031] Preferably, the polymerization reaction is followed by post-processing.

[0032] In this invention, the post-processing includes precipitation, washing, and drying performed sequentially. The precipitation method is as follows: mixing the polymerization reaction solution and the precipitation solvent to perform precipitation; the precipitation solvent includes any one of propanol, isopropanol, or acetone.

[0033] Preferably, the first coating further includes nano-oxides.

[0034] It should be noted that by introducing nano-oxides to co-modify the membrane, a strong organic-inorganic network is formed. The polymer provides adhesion and stress buffering, while the nano-oxide particles provide rigid support and thermal barrier, synergistically resisting high-temperature thermal shrinkage and silicon expansion stress, thereby reducing the thermal shrinkage rate of the membrane.

[0035] Preferably, the nano-oxide comprises nano-TiO2.

[0036] Preferably, the D50 particle size of the nano-oxide is 10nm-18nm, for example, it can be 11nm, 12nm, 13nm, 14nm, 15nm, 16nm or 17nm, etc.

[0037] It should be noted that controlling the D50 particle size range of the nano-oxides is beneficial for forming a uniform coating slurry and avoiding agglomeration of the nano-oxides. If the D50 particle size of the nano-oxides is too large, their settling rate in the coating slurry is too fast, resulting in a coating that is thinner at the top and thicker at the bottom; if the D50 particle size of the nano-oxides is too small, it will spontaneously agglomerate into micron-sized clumps.

[0038] Preferably, in the first coating, the mass ratio of the polymer to the nano-oxide is (15-30):(10-23), for example, it can be 16:12, 18:15, 20:15, 25:18 or 28:20, etc.

[0039] Preferably, the thickness of the first coating is 1μm-4μm, for example, it can be 1.5μm, 2μm, 2.5μm, 3μm or 3.5μm.

[0040] It should be noted that by controlling the thickness range of the first coating, its bonding performance to the silicon-based anode can be achieved with a small amount of coating. If the thickness of the first coating is too thin, the base film coating will be uneven, resulting in insufficient buffering for the expansion of the silicon-based anode and limited improvement in thermal stability. If the thickness of the first coating is too thick, it will increase the total thickness and internal resistance of the separator, reduce the energy density and power density of the battery, and may lead to excessively high permeability, thereby affecting electrolyte wetting and ion transport.

[0041] Preferably, the base film is made of polyimide (PI).

[0042] Preferably, the porosity of the base membrane is 43%-57%, for example, it can be 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55% or 56%, etc.

[0043] Preferably, the thickness of the base film is 6μm-9μm, for example, it can be 6.2μm, 6.5μm, 6.8μm, 7μm, 7.2μm, 7.5μm, 7.8μm, 8μm, 8.2μm, 8.5μm, 8.6μm or 8.8μm, etc.

[0044] In this invention, if the thickness of the base film is too thin, the mechanical strength will be reduced; if the thickness of the base film is too thick, the energy density will be reduced.

[0045] Preferably, the air permeability of the base membrane is 80s / 100m-100s / 100m, for example, it can be 82s / 100m, 85s / 100m, 88s / 100m, 90s / 100m, 92s / 100m, 95s / 100m, 96s / 100m or 98s / 100m, etc.

[0046] It should be noted that the base film has high porosity, excellent high-temperature resistance, and good air permeability. The separator formed by combining it with the first coating can alleviate the volume expansion problem of the silicon-based anode and improve the battery's cycle performance and rate performance at high temperatures. Furthermore, the nitrogen-containing polar groups in the PI base film form hydrogen bonds with the electrolyte solvent, enhancing wettability and further improving the battery's rate performance.

[0047] Preferably, the base film is prepared by the following method:

[0048] The polyimide and the first solvent are mixed and subjected to a first ball milling. Then, a first wetting agent and a first dispersant are added and subjected to a second ball milling. Finally, a pore-forming agent is added and subjected to a third ball milling to obtain a mixed slurry.

[0049] The mixed slurry is sequentially coated and dried to obtain a first intermediate film. Then, the pore-forming agent in the first intermediate film is removed to obtain the base film.

[0050] In this invention, the first solvent includes water.

[0051] Preferably, the temperature of the first ball mill is 25℃-45℃, for example, it can be 26℃, 28℃, 30℃, 32℃, 35℃, 36℃, 38℃, 40℃, 42℃ or 44℃, etc., the rotation speed is 500rpm-900rpm, for example, it can be 550rpm, 600rpm, 650rpm, 700rpm, 750rpm, 800rpm or 850rpm, etc., and the time is 6h-9h, for example, it can be 6.5h, 7h, 7.5h, 8h or 8.5h, etc.

[0052] Preferably, the temperature of the second ball mill is 25℃-45℃, for example, it can be 26℃, 28℃, 30℃, 32℃, 35℃, 36℃, 38℃, 40℃, 42℃ or 44℃, etc., the rotation speed is 500rpm-700rpm, for example, it can be 520rpm, 550rpm, 580rpm, 600rpm, 620rpm, 650rpm or 680rpm, etc., and the time is 2h-5h, for example, it can be 2.5h, 3h, 3.5h, 4h or 4.5h, etc.

[0053] Preferably, the temperature of the third ball mill is 25℃-45℃, for example, it can be 26℃, 28℃, 30℃, 32℃, 35℃, 36℃, 38℃, 40℃, 42℃ or 44℃, etc., the rotation speed is 500rpm-700rpm, for example, it can be 520rpm, 550rpm, 580rpm, 600rpm, 620rpm, 650rpm or 680rpm, etc., and the time is 2h-4h, for example, it can be 2.2h, 2.5h, 2.6h, 2.8h, 3h, 3.2h, 3.5h, 3.6h or 3.8h, etc.

[0054] Preferably, the mass ratio of the polyimide, the first wetting agent, the first dispersant and the pore-forming agent is (250-350):(1-3):(3-6):(31-40).

[0055] In this invention, the first wetting agent comprises TIG 270 wetting agent. The first dispersant comprises any one or a combination of at least two of triethylhexylphosphate, sodium dodecyl sulfate, methylpentanol, cellulose derivatives, polyacrylamide, glucon, or fatty acid polyethylene glycol esters. The pore-forming agent comprises polyethylene glycol.

[0056] Preferably, the porosity of the membrane is 40%-50%, for example, it can be 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48% or 49%, etc.

[0057] In a second aspect, the present invention provides a method for preparing a diaphragm as described in the first aspect, the method comprising:

[0058] The first coating slurry is sprayed onto the surface of the base film by dot coating, and the diaphragm is obtained after drying.

[0059] The components of the first coating slurry include the polymer.

[0060] The preparation method provided by the present invention enables the slurry to be uniformly sprayed onto the base film through dot-coating, resulting in a separator with high air permeability and high porosity, which in turn results in lower impedance, greatly improving the lithium-ion transport rate and enhancing the overall performance of the battery.

[0061] Preferably, the first coating slurry further includes nano-oxides, an optional second solvent, a second wetting agent, a second dispersant, a third solvent, and a binder.

[0062] In this invention, the mixing method of the first coating slurry is not specifically limited. It can be mixed once or mixed multiple times. Those skilled in the art can determine the method according to actual needs.

[0063] Preferably, the mass ratio of the polymer, nano-oxide, second solvent, second wetting agent, second dispersant, third solvent, and binder is (15-30):(10-23):(40-66):(0.02-0.08):(0.1-0.3):(1-4):(3-7), for example, it can be 20:15:50:0.05:0.2:2.5:5, 25:18:55:0.07:0.3:2:6 or 30:22:60:0.05:0.3:4:5, etc.

[0064] In this invention, the second solvent comprises water. The second wetting agent comprises any one or a combination of at least two of sodium hexametaphosphate, sodium tripolyphosphate, or sodium pyrophosphate. The second dispersant comprises any one or a combination of at least two of silicates, sodium polyacrylate, or sodium citrate. The third solvent comprises an alcohol solvent, including isopropanol. The binder comprises any one or a combination of at least two of carboxymethyl cellulose, hydroxypropyl methylcellulose, or polyvinyl alcohol.

[0065] Preferably, the temperature of the dotted coating is 40℃-90℃, for example, it can be 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃ or 95℃, etc.

[0066] Preferably, the stretching speed difference of the dotted coating is 0.1%-10%, for example, it can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8% or 9%, etc.

[0067] Thirdly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode, a negative electrode, an electrolyte, and the separator described in the first aspect.

[0068] The lithium-ion battery prepared by this invention has a low silicon-based anode expansion rate and high rate and cycle performance.

[0069] In this invention, the raw materials for preparing the negative electrode include silicon-carbon negative electrode material. The silicon content in the silicon-carbon negative electrode material is 40%-55%, for example, it can be 42%, 44%, 45%, 46%, 48%, 50%, 52%, or 54%, etc. The D50 particle size of the silicon-carbon negative electrode material is 4μm-8μm, for example, it can be 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, or 7.5μm, etc.

[0070] In this invention, by controlling the silicon content and D50 particle size range in the silicon-carbon anode material, it is beneficial to improve the rate performance of the battery and suppress the expansion rate of the silicon-based anode.

[0071] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0072] Compared with the prior art, the present invention has the following beneficial effects:

[0073] (1) The separator provided by the present invention has a high porosity and a low thermal shrinkage rate by providing a first coating of polymer containing specific groups on the surface of the base film. It can effectively solve the problems of volume expansion and poor high-temperature performance of silicon-based anode in lithium-ion batteries, as well as the insufficient wettability of traditional separators, thereby improving the lithium-ion transport efficiency and the rate and cycle performance of silicon-based anode, and thus improving the overall performance of the battery.

[0074] (2) The separator provided by this invention, by introducing nano-oxides and polymers to jointly modify the separator, forms a strong organic-inorganic network that synergistically resists high-temperature thermal shrinkage and silicon expansion stress, further reducing the thermal shrinkage rate of the separator and thus improving the thermal stability of the battery. By using a PI-based membrane in conjunction with the first coating, the air permeability and porosity of the separator are improved, thereby improving the rate capability and cycle performance of the battery.

[0075] (3) The method for preparing the separator provided by the present invention enables the slurry to be uniformly sprayed onto the base film through dot-coating. The resulting separator has high air permeability and high porosity, which in turn makes it have lower impedance, greatly improves the lithium-ion transport rate, and improves the overall performance of the battery. Detailed Implementation

[0076] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0077] All materials used in the specific embodiments of the present invention can be purchased commercially or prepared using conventional methods in the prior art.

[0078] The structural formulas of the first monomer A in the following preparation examples 1-1, 1-2, 1-4 to 1-7 are all shown below:

[0079]

[0080] The structural formula of the first monomer B in the following preparation examples 1-3 is shown below:

[0081] The structural formula of the first monomer C in the following preparation examples 1-8 is shown below:

[0082] The structural formula of the first monomer D in the following preparation examples 1-9 is shown below:

[0083]

[0084] Preparation Example

[0085] Polymer preparation

[0086] Preparation Example 1-1

[0087] This preparation example provides a polymer and a method for preparing the same, wherein the polymer has a weight-average molecular weight of 110,000 Da;

[0088] The preparation method includes:

[0089] The first monomer A and tetrahydrofuran were mixed at a mass ratio of 1:4, and azobisisobutyronitrile was added to the mixture. The amount of azobisisobutyronitrile added was 0.5% of the mass of the first monomer A. The polymerization reaction was carried out for 24 hours under an argon atmosphere and at a temperature of 100°C. Then, acetone and the polymerization reaction solution were mixed, and a polymer precipitate was obtained after precipitation. The precipitate was then washed and dried to obtain the polymer.

[0090] Preparation Examples 1-2

[0091] This preparation example provides a polymer and a method for preparing the same, wherein the polymer has a weight-average molecular weight of 140,000 Da;

[0092] The preparation method includes:

[0093] The first monomer A and N-methyl-2-pyrrolidone were mixed at a mass ratio of 1:5. Benzoyl peroxide was added to the mixture at a mass ratio of 0.3% of the first monomer A. The polymerization reaction was carried out at 90°C under an argon atmosphere for 30 hours. Then, isopropanol and the polymerization reaction solution were mixed. After precipitation, a polymer precipitate was obtained. The precipitate was then washed and dried to obtain the polymer.

[0094] Preparation Examples 1-3

[0095] This preparation example provides a polymer and a method for preparing the same, wherein the polymer has a weight-average molecular weight of 80,000 Da;

[0096] The preparation method includes:

[0097] The first monomer B and N,N-dimethylformamide were mixed at a mass ratio of 1:4.5, and azobisisobutyronitrile was added to the mixture at a mass of 0.8% of the first monomer B. The polymerization reaction was carried out at 130°C under an argon atmosphere for 18 hours. Then, propanol and the polymerization reaction solution were mixed, and a polymer precipitate was obtained after precipitation. The precipitate was then washed and dried to obtain the polymer.

[0098] Preparation Examples 1-4

[0099] This preparation example provides a polymer and its preparation method. Except that the weight-average molecular weight of the polymer is 60,000 Da, all other conditions are the same as in Preparation Example 1-1.

[0100] Preparation Examples 1-5

[0101] This preparation example provides a polymer and its preparation method. Except that the weight-average molecular weight of the polymer is 170,000 Da, all other conditions are the same as in Preparation Example 1-1.

[0102] Preparation Examples 1-6

[0103] This preparation example provides a polymer and its preparation method, except that the polymerization reaction temperature is 60°C, and all other conditions are the same as in Preparation Example 1-1.

[0104] Preparation Examples 1-7

[0105] This preparation example provides a polymer and its preparation method, except that the polymerization reaction temperature is 150°C, and all other conditions are the same as in Preparation Example 1-1.

[0106] Preparation Examples 1-8

[0107] This preparation example provides a method for preparing a polymer, except that the first monomer A is replaced with the first monomer C, and all other conditions are the same as in Preparation Example 1-1.

[0108] Preparation Examples 1-9

[0109] This preparation example provides a method for preparing a polymer, except that the first monomer A is replaced with the first monomer D, and all other conditions are the same as in Preparation Example 1-1.

[0110] Preparation of base film

[0111] Preparation Example 2-1

[0112] This preparation example provides a base membrane and its preparation method. The base membrane is a PI base membrane with a porosity of 50%, a thickness of 8 μm, and an air permeability of 88 s / 100 m.

[0113] The preparation method includes:

[0114] Mix 300g PI and 60mL pure water and sonicate for 80min. Then, ball mill for 7h at 35℃ and 800rpm. Next, add 2g of DIG 270 wetting agent and 5g of sodium dodecyl sulfate and ball mill for 3h at 35℃ and 600rpm. Then, add 33g of polyethylene glycol and ball mill for 3h at 35℃ and 600rpm to obtain a mixed slurry.

[0115] The mixed slurry was coated using a casting coating machine and dried to obtain a first intermediate film. Then, the first intermediate film was soaked in water at a temperature of 110°C. After repeating the soaking process three times, the polyethylene glycol in the first intermediate film was removed to obtain a PI-based film.

[0116] Preparation Example 2-2

[0117] This preparation example provides a base membrane and its preparation method. The base membrane is a PI base membrane with a porosity of 45%, a thickness of 7 μm, and an air permeability of 85 s / 100 m.

[0118] The preparation method includes:

[0119] Mix 260g PI and 50mL pure water and sonicate for 60min. Then, ball mill for 6h at 25℃ and 900rpm. Next, add 1g of DIG 270 wetting agent and 4g of sodium dodecyl sulfate and ball mill for 2h at 25℃ and 700rpm. Then, add 31g of polyethylene glycol and ball mill for 2h at 25℃ and 700rpm to obtain a mixed slurry.

[0120] The mixed slurry was coated using a casting coating machine and dried to obtain a first intermediate film. Then, the first intermediate film was soaked in water at a temperature of 100°C. After repeating the soaking process three times, the polyethylene glycol in the first intermediate film was removed to obtain a PI-based film.

[0121] Preparation Examples 2-3

[0122] This preparation example provides a base membrane and its preparation method. The base membrane is a PI base membrane with a porosity of 56%, a thickness of 9 μm, and an air permeability of 95 s / 100 m.

[0123] The preparation method includes:

[0124] Mix 320g PI and 90mL pure water and sonicate for 90min. Then, ball mill for 9h at 45℃ and 500rpm. Next, add 3g of DIG 270 wetting agent and 6g of sodium dodecyl sulfate and ball mill for 5h at 45℃. Then, add 35g of polyethylene glycol and ball mill for 4h at 45℃ and 500rpm to obtain a mixed slurry.

[0125] The mixed slurry was coated using a casting coating machine and dried to obtain a first intermediate film. Then, the first intermediate film was soaked in water at a temperature of 110°C. After repeating the soaking process three times, the polyethylene glycol in the first intermediate film was removed to obtain a PI-based film.

[0126] Preparation Examples 2-4

[0127] This preparation example provides a base film and its preparation method, except that the porosity of the PI base film is 40%; other conditions are the same as in preparation example 2-1.

[0128] Preparation Examples 2-5

[0129] This preparation example provides a base film and its preparation method. Except that the thickness of the PI base film is 4 μm, other conditions are the same as in Preparation Example 2-1.

[0130] Preparation Examples 2-6

[0131] This preparation example provides a base film and its preparation method, except that the thickness of the PI base film is 11 μm; other conditions are the same as in preparation example 2-1.

[0132] Example

[0133] Example 1

[0134] This embodiment provides a diaphragm and its preparation method, the diaphragm comprising a PI-based membrane obtained in Preparation Example 2-1 and a first coating disposed on one side surface of the PI-based membrane;

[0135] The first coating comprises a polymer prepared in Preparation Example 1-1 at a mass ratio of 20:16 and nano-TiO2; the nano-TiO2 has a D50 particle size of 15 nm; and the thickness of the first coating is 2 μm.

[0136] The preparation method includes:

[0137] The first coating slurry is sprayed onto the surface of the base film by dot coating, and the diaphragm is obtained after drying.

[0138] The first coating slurry comprises the polymer prepared in Preparation Example 1-1 with a mass ratio of 20:16:52:0.05:0.2:2.5:5, nano-TiO2, water, sodium hexametaphosphate, sodium polyacrylate, isopropanol, and carboxymethyl cellulose.

[0139] The temperature of the dot-coating is 60°C, and the stretching speed difference is 5%.

[0140] Example 2

[0141] This embodiment provides a diaphragm and its preparation method, wherein the diaphragm includes a PI-based membrane prepared in Preparation Example 2-2 and a first coating disposed on one side surface of the PI-based membrane;

[0142] The first coating comprises a polymer prepared in Preparation Examples 1-2 at a mass ratio of 15:10 and nano-TiO2; the nano-TiO2 has a D50 particle size of 11 nm; and the thickness of the first coating is 4 μm.

[0143] The preparation method includes:

[0144] The first coating slurry is sprayed onto the surface of the base film by dot coating, and the diaphragm is obtained after drying.

[0145] The first coating slurry comprises the polymer prepared in Preparation Examples 1-2 with a mass ratio of 15:10:45:0.03:0.1:1.5:4, nano TiO2, water, sodium tripolyphosphate, sodium citrate, isopropanol, and hydroxypropyl methylcellulose.

[0146] The temperature of the dot-coating is 40°C, and the stretching speed difference is 1%.

[0147] Example 3

[0148] This embodiment provides a diaphragm and its preparation method, wherein the diaphragm includes a PI-based membrane prepared in preparation examples 2-3 and a first coating disposed on one side surface of the PI-based membrane;

[0149] The first coating comprises a polymer prepared in Preparation Examples 1-3 at a mass ratio of 30:18 and nano-TiO2; the nano-TiO2 has a D50 particle size of 18 nm; and the thickness of the first coating is 1.5 μm.

[0150] The preparation method includes:

[0151] The first coating slurry is sprayed onto the surface of the base film by dot coating, and the diaphragm is obtained after drying.

[0152] The first coating slurry comprises the polymer prepared in Preparation Examples 1-3 with a mass ratio of 30:18:60:0.06:0.3:3:6, nano-TiO2, water, sodium tripolyphosphate, sodium citrate, isopropanol, and hydroxypropyl methylcellulose.

[0153] The temperature of the dot-coating is 90°C, and the stretching speed difference is 10%.

[0154] Examples 4-13 and Comparative Examples 1-2

[0155] The types of base film, polymer, nano oxides, and the thickness of the first coating were changed, as shown in Table 1. All other conditions were the same as in Example 1.

[0156] Table 1

[0157]

[0158]

[0159] Diaphragm performance testing

[0160] The performance testing methods for the diaphragms prepared in the above embodiments and comparative examples are as follows:

[0161] (I) Porosity test: Cut three diaphragm pieces at 150mm intervals along the longitudinal direction. If the diaphragm width is ≥100mm, the sample size should be 100mm×100mm; if the diaphragm width is <100mm, the sample size should be 100mm×diaphragm width. The specific test and calculation methods shall refer to the provisions of GB / T 6672-2001, and the porosity of the diaphragm shall be calculated.

[0162] (II) Heat shrinkage rate test: Take a 100mm×100mm diaphragm and place it flat on a quartz glass plate. Place it in a high-temperature furnace at 180℃ for 30 minutes, then remove it and cool it at room temperature for 10 minutes. The specific test method and measurement method shall be in accordance with GB / T 36363-2018. Calculate the heat shrinkage rate of the diaphragm in the longitudinal direction (MD) and transverse direction (TD).

[0163] The test results are shown in Table 2.

[0164] Battery manufacturing

[0165] The separators prepared in the above embodiments and comparative examples were used to prepare lithium-ion batteries.

[0166] (1) Preparation of positive electrode sheet

[0167] The ternary material NCM811(LiNi) 0.8 Co 0.1 Mn 0.05 O2) positive electrode active material, binder PVDF (polyvinylidene fluoride), conductive agent SP (conductive carbon black Super-P), SWCNT (single-walled carbon nanotubes) are mixed and stirred evenly in a mass ratio of 94:3:2.9:0.1 to obtain positive electrode slurry. Then, the positive electrode slurry is coated onto aluminum foil through a coating process, and after drying and cold pressing, a positive electrode sheet is obtained.

[0168] (2) Preparation of negative electrode sheet

[0169] Silicon-carbon anode material (silicon content 50wt%, D50 particle size 6μm), conductive agent SP (Super-P conductive carbon black), SWCNT (single-walled carbon nanotubes), and binders PAA (polyacrylic acid) and SBR (styrene-butadiene rubber) were mixed and stirred evenly in a mass ratio of 92:2:0.5:3:2.5 to obtain a negative electrode slurry. The solid content was controlled at 30%. The negative electrode slurry was then coated onto a copper foil current collector through a coating process. After vacuum drying and cold pressing, the negative electrode sheet was obtained.

[0170] (3) Selection of electrolyte

[0171] The electrolyte consisted of 1 mol / L LiPF6 and EC+PC+DMC+DEC+FEC in a volume ratio of 15:20:25:30:10.

[0172] (4) Selection of diaphragm

[0173] The diaphragms were prepared using Examples 1-13 and Comparative Examples 1-2 of the present invention.

[0174] (5) Preparation of lithium-ion batteries

[0175] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrode sheets to provide isolation. The resulting bare cell is then wound up. The bare cell is then placed in an outer packaging shell, dried, and injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0176] Battery performance test

[0177] The batteries prepared in the above examples and comparative examples were subjected to electrochemical performance tests, and the test methods are as follows:

[0178] 1) First Coulomb efficiency

[0179] At 25℃, the lithium-ion battery was charged at a constant current and constant voltage rate of 0.33C to 4.2V, allowed to stand for 10 minutes, and then discharged at a constant current rate of 0.33C to 2.5V, allowed to stand for 10 minutes. The initial coulombic efficiency of the lithium-ion battery was calculated. Initial coulombic efficiency (%) = (Total capacity of lithium-ion battery during initial discharge at 0.33C) / (Total capacity of lithium-ion battery during initial charge at 0.33C) × 100%.

[0180] 2) Capacity retention rate after 1000 cycles at room temperature with a temperature of 1°C / 2°C

[0181] At 25°C, the lithium-ion battery was charged at a 1C rate with constant current and constant voltage to 4.2V, with a cutoff current of 0.05C. After resting for 10 minutes, the lithium-ion battery was discharged at a 2C rate with constant current to 2.5V and then rested for 10 minutes. This constitutes one charge-discharge cycle. The lithium-ion battery was subjected to 1000 charge-discharge cycles using the above method. The capacity retention rate of the lithium-ion battery after 1000 charge-discharge cycles at 1C / 2C was calculated.

[0182] The capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity of the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0183] 3) Room temperature 6C rate performance - constant current charge ratio

[0184] At 25℃, the lithium-ion battery was discharged at a constant current rate of 1C to 2.5V, left to stand for 10 minutes, and then charged at a constant current and constant voltage rate of 6C to 4.2V with a cutoff current of 0.05C. After standing for 10 minutes, the constant current charging capacity Q1 and the total constant current and constant voltage charging capacity Q2 of the lithium-ion battery were recorded. The constant current charge ratio of the 6C rate charging was calculated according to the following formula: 6C rate charging constant current charge ratio = (constant current charging capacity Q1 / total constant current and constant voltage charging capacity Q2) × 100%.

[0185] 4) Capacity retention rate at room temperature with 1C / 8C discharge

[0186] The lithium-ion battery, after capacity gradation, was charged at 1C rate to 4.2V under constant current and constant voltage conditions at 25℃, with a cutoff current of 0.05C, and allowed to stand for 10 minutes. Then, the battery was discharged at 1C rate to 2.5V under constant current conditions, and its discharge capacity Q1C was recorded as the initial discharge capacity. Next, the battery was charged at 1C rate to 4.2V under constant current and constant voltage conditions at 25℃, with a cutoff current of 0.05C, and allowed to stand for 10 minutes. Finally, the fully charged battery was discharged at 8C rate to 2.5V under constant current conditions, and its discharge capacity Q8C was recorded. The discharge capacity retention rate (%) of the lithium-ion battery at 1C / 8C rate was calculated as: discharge capacity Q8C at 8C rate / discharge capacity Q1C at 1C rate × 100%.

[0187] 5) Cell thermal runaway ARC test

[0188] The ARC adiabatic thermal runaway test begins (the test sample is heated from room temperature to 45±2℃ inside the chamber, and after being left to stand for 90 minutes, the change in the battery temperature rise rate is monitored. If the temperature rise exceeds 0.2℃ within 10 minutes (i.e., SHR>0.02℃ / min), it is considered that a self-heating reaction has occurred inside the battery, and the adiabatic environment is maintained until the battery experiences thermal runaway; if the temperature rise does not exceed 0.2℃ within 10 minutes (i.e., SHR≤0.02℃ / min), the next temperature rise step test continues; each temperature step is 5℃, and the steps are repeated at each step. The ARC test temperature range is 45℃-300℃, the self-heating initiation temperature is T1 (temperature rise rate SHR>0.02℃ / min), and the thermal runaway initiation temperature is T2 (temperature rise rate SHR>1℃ / min). The self-heating rate (SHR) is used to define the temperature rise rate.

[0189] The test results are shown in Tables 2 and 3.

[0190] Table 2

[0191]

[0192]

[0193] Table 3

[0194]

[0195]

[0196] As can be seen from Tables 2 and 3:

[0197] Comparing Examples 1-13 and Comparative Examples 1-2, the separator provided in this application exhibits higher porosity and lower thermal shrinkage. Lithium-ion batteries manufactured using this separator demonstrate superior performance in initial coulombic efficiency, room-temperature cycling, 6C constant-current charge ratio, and safety. This is because the coating of the separator provided in this application contains polymers with specific functional groups, which possess excellent thermal stability and rigidity, significantly improving the separator's suppression of silicon-based anode expansion. Combined with a PI-based membrane with specific porosity, this enhances the separator's permeability and porosity, thereby improving the battery's rate performance and cycle life. In contrast, the polymers in Comparative Examples 1-2 lack urethane or aminosulfonyl groups, leading to a decline in the performance of both the separator and the battery. This demonstrates that polymers containing specific functional groups play a crucial role in improving separator performance. Furthermore, the absence of certain functional groups in the polymer significantly impacts its performance, resulting in a marked decrease in the performance of the polymer-based separator. Consequently, it fails to effectively suppress the silicon-based anode expansion rate or improve the separator's thermal stability, ultimately degrading battery performance.

[0198] As can be seen from the comparison between Examples 1 and Examples 4-5, if the weight-average molecular weight of the polymer is too low or too high, the performance of the separator will decrease slightly, which in turn will cause the performance of the battery to decrease. If the weight-average molecular weight is too low, the film-forming properties, interfacial adhesion and mechanical strength of the coating will decrease. If the weight-average molecular weight is too high, the viscosity of the slurry will increase, which will reduce the coating uniformity. Because of its higher crystallinity, although the melting temperature increases, the brittleness will also increase.

[0199] As can be seen from the comparison between Examples 1 and Examples 6-7, when the polymerization temperature of the polymer preparation reaction is not within the preferred range, it will have a negative impact on the separator, resulting in an increase in the expansion rate of the silicon-based negative electrode in the battery and a decrease in the battery's rate and cycle performance.

[0200] As can be seen from the comparison between Example 1 and Example 8, if the first coating does not contain nano-oxides, it cannot provide effective rigid support and thermal barrier for the separator, which leads to an increase in the thermal shrinkage rate of the separator and a decrease in the performance of the battery.

[0201] As can be seen from the comparison between Examples 1 and Examples 9-10, if the thickness of the first coating is too thin or too thick, the performance of the separator prepared by it will decrease. If the thickness of the first coating is too thin, it will result in uneven coating, insufficient buffering of the expansion of the silicon-based negative electrode, and limited improvement in thermal stability. If the thickness of the first coating is too thick, it will increase the total thickness and internal resistance of the separator, reduce the energy density and power density of the battery, and may lead to excessively high permeability, thereby affecting electrolyte wetting and ion transport.

[0202] A comparison of Examples 1 and 11 shows that if the porosity of the PI base film is too low, it will negatively affect the pore structure of the prepared separator, hindering sufficient contact between the separator and the electrolyte, thus causing a decrease in battery performance. A comparison of Examples 1 and 12-13 shows that if the thickness of the PI base film is too thin or too thick, it will negatively affect the performance of the prepared separator. If the base film is too thin, the mechanical strength will decrease; if the base film is too thick, the energy density will decrease.

[0203] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A diaphragm, characterized in that, The diaphragm includes a base membrane and a first coating disposed on at least one surface of the base membrane; The first coating comprises a polymer, wherein the polymer contains sterically hindered groups, aminosulfonyl groups and carbamate groups in its structure; The polymer is a polymer formed by polymerization of a first monomer; The first monomer has the general formula R1-OCONH-R2-(CH=CH)n-SO2NH-R3; wherein R1 is a sterically hindered group selected from any one of tert-butyl, neopentyl, adamantyl or triphenylmethyl; R2 is selected from substituted or unsubstituted C1-C4 alkylene groups; R3 is selected from hydrogen atoms or methyl groups; and n is selected from 1 or 2.

2. The diaphragm according to claim 1, characterized in that, The weight-average molecular weight of the polymer is 70,000 Da to 150,000 Da.

3. The diaphragm according to claim 1, characterized in that, The structural formula of the first monomer is 。 4. The diaphragm according to claim 1, characterized in that, The polymer is prepared by the following method: The first monomer and an organic solvent are mixed, and then an initiator is added to carry out a polymerization reaction to obtain the polymer.

5. The diaphragm according to claim 4, characterized in that, The amount of initiator added is 0.2%-1% of the mass of the first monomer.

6. The diaphragm according to claim 4, characterized in that, The polymerization reaction is carried out under a protective atmosphere.

7. The diaphragm according to claim 4, characterized in that, The polymerization reaction is carried out at a temperature of 80℃-130℃ for a time of 15h-32h.

8. The diaphragm according to claim 4, characterized in that, The polymerization reaction was followed by post-processing.

9. The diaphragm according to claim 1, characterized in that, The first coating also includes nano-oxides.

10. The diaphragm according to claim 9, characterized in that, The nano-oxides include nano-TiO2.

11. The diaphragm according to claim 9, characterized in that, The D50 particle size of the nano-oxide is 10nm-18nm.

12. The diaphragm according to claim 9, characterized in that, In the first coating, the mass ratio of the polymer to the nano-oxide is (15-30):(10-23).

13. The diaphragm according to claim 1, characterized in that, The thickness of the first coating is 1μm-4μm.

14. The diaphragm according to claim 1, characterized in that, The base film is made of polyimide.

15. The diaphragm according to claim 1, characterized in that, The porosity of the base membrane is 43%-57%.

16. The diaphragm according to claim 1, characterized in that, The thickness of the base film is 6μm-9μm.

17. The diaphragm according to claim 1, characterized in that, The air permeability of the base membrane is 80s / 100m-100s / 100m.

18. The diaphragm according to claim 1, characterized in that, The base film is prepared by the following method: The polyimide and the first solvent are mixed and subjected to a first ball milling. Then, a first wetting agent and a first dispersant are added and subjected to a second ball milling. Finally, a pore-forming agent is added and subjected to a third ball milling to obtain a mixed slurry. The mixed slurry is sequentially coated and dried to obtain a first intermediate film. Then, the pore-forming agent in the first intermediate film is removed to obtain the base film.

19. The diaphragm according to claim 18, characterized in that, The temperature of the first ball mill is 25℃-45℃, the rotation speed is 500rpm-900rpm, and the time is 6h-9h.

20. The diaphragm according to claim 18, characterized in that, The temperature of the second ball mill is 25℃-45℃, the rotation speed is 500rpm-700rpm, and the time is 2h-5h.

21. The diaphragm according to claim 18, characterized in that, The temperature of the third ball mill is 25℃-45℃, the rotation speed is 500rpm-700rpm, and the time is 2h-4h.

22. The diaphragm according to claim 18, characterized in that, The mass ratio of the polyimide, the first wetting agent, the first dispersant and the pore-forming agent is (250-350):(1-3):(3-6):(31-40).

23. The diaphragm according to claim 1, characterized in that, The porosity of the diaphragm is 40%-50%.

24. A method for preparing a diaphragm as described in any one of claims 1-23, characterized in that, The preparation method includes: The first coating slurry is sprayed onto the surface of the base film by dot coating, and the diaphragm is obtained after drying. The components of the first coating slurry include the polymer.

25. The preparation method according to claim 24, characterized in that, The first coating slurry further includes nano-oxides, a second solvent, a second wetting agent, a second dispersant, a third solvent, and a binder.

26. The preparation method according to claim 25, characterized in that, The mass ratio of the polymer, nano-oxide, second solvent, second wetting agent, second dispersant, third solvent and binder is (15-30):(10-23):(40-66):(0.02-0.08):(0.1-0.3):(1-4):(3-7).

27. The preparation method according to claim 24, characterized in that, The temperature for the dotted coating is 40-90℃.

28. The preparation method according to claim 24, characterized in that, The stretching speed difference of the dot-coating is 0.1%-10%.

29. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator as described in any one of claims 1-23.

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