A separator and a lithium ion battery comprising the same

Abstract

This invention provides a separator and a lithium-ion battery comprising the same, relating to the field of lithium-ion battery technology. The separator comprises a polymer, the polymer monomers of which include a first monomer and a second monomer. The first monomer contains a cyano group, an ester group, a carbon-carbon double bond, and a tertiary amine group. The second monomer has the structural formula O=CH-Ar-NHCO-CH=CH2, where Ar is a substituted or unsubstituted aromatic ring, including benzene and / or naphthalene. This invention improves the chemical stability of the polymer, promotes lithium-ion conduction, enhances the mechanical strength of the separator and its adsorption capacity for electrolyte, improves the interfacial contact between the separator and the electrode, and improves battery performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more particularly to a separator and a lithium-ion battery comprising the separator. Background Technology

[0002] Currently, the driving range of electric vehicles is constantly improving, alleviating range anxiety but highlighting charging anxiety, making the development of fast charging a trend; long driving range (400+km) and fast charging (15~30min) are the two main demand directions; and to achieve the development of high energy density fast charging technology, it is necessary to improve the energy density, power density and other performance of lithium-ion batteries.

[0003] In response, improvement measures mainly focus on anode materials. Silicon-based anodes, with their high theoretical specific capacity and safety, are strong competitors to graphite anodes. Graphite anodes are approaching their theoretical upper limit of 372 mAh / g, while silicon-based anodes have a theoretical specific capacity as high as 4200 mAh / g, about 10 times that of graphite anodes. Moreover, silicon-based anode materials have a lower lithium insertion / extraction potential (~0.4V vs. Li / Li+), slightly higher than graphite (~0.05V vs. Li / Li+), which can avoid lithium plating on the surface during fast charging.

[0004] As lithium-ion batteries develop towards high-energy-density fast charging, commercial polyolefin separators and traditional coated films have revealed the following core problems:

[0005] First, insufficient thermal stability: Traditional polyolefin separators and coated films undergo significant thermal shrinkage (shrinkage rate > 10%) at temperatures above 150°C.

[0006] Second, there is an imbalance between mechanical strength and flame retardancy: rigid coatings (such as Al2O3 coatings) are puncture resistant but lack flame retardancy, while flexible coatings (such as polyacrylic acid) are easily torn by the expansion stress of silicon and are easily combustible at high temperatures, posing a high risk of thermal runaway.

[0007] Third, there is a contradiction between ion conduction and wettability: low porosity (<40%) leads to insufficient electrolyte wetting (liquid absorption rate <150%), and interfacial impedance >100mΩ·cm. 2 The lithium-ion conduction efficiency is low during 6C fast charging (constant current charge ratio <70%).

[0008] Fourth, lack of thermal runaway protection: Traditional diaphragms lack thermal runaway early warning mechanisms and cannot suppress the spread of flames at high temperatures, resulting in insufficient battery safety.

[0009] Therefore, it is necessary to develop membranes that combine high ion conductivity, high heat resistance, and high interfacial stability. Summary of the Invention

[0010] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a separator and a lithium-ion battery containing the separator, which can improve ion conductivity and interface stability and has broad application prospects.

[0011] To achieve this objective, the present invention adopts the following technical solution:

[0012] In a first aspect, the present invention provides a diaphragm comprising a polymer, wherein the polymer monomers comprise a first monomer and a second monomer;

[0013] The structural formula of the first monomer is shown in equation (1):

[0014] Equation (1);

[0015] The structural formula of R1 is shown in formula (2):

[0016] Equation (2);

[0017] Wherein, R3 is a group containing -NR5R6 and a carbon-carbon double bond, and R2, R5 and R6 are each independently a hydrocarbon group; R4 is an oxygen-containing group; the structural formula of the second monomer is O=CH-Ar-NHCO-CH=CH2, where Ar is a substituted or unsubstituted aromatic ring, and the aromatic ring includes benzene and / or naphthalene.

[0018] The diaphragm provided by this invention has the following special advantages:

[0019] The polymer of this invention uses two monomers copolymerized together, and the two components work synergistically to significantly improve the ion conductivity and mechanical strength of the membrane.

[0020] The first monomer of this invention contains a cyano group, which enhances the chemical stability and antioxidant properties of the copolymer; moreover, the tertiary amine group provides coordination sites for lone pair electrons, achieving a lithium-ion diffusion coefficient of 2.1 × 10⁻⁶. -10 m 2 Above / s, the conjugated olefin chain containing tertiary amine groups has a certain electron-donating ability, which can interact with lithium ions and promote lithium ion conduction. The conjugated structure can significantly improve conductivity, further improving the performance of lithium-ion batteries. It works in conjunction with the formyl group in the second monomer to synergistically enhance lithium ion conduction. At the same time, the ester group in the first monomer can improve the flexibility and solubility of the copolymer, and the acrylamide group in the second monomer can form a hydrogen bond network to improve the mechanical strength of the separator. The combination of the two is more conducive to obtaining a separator with excellent mechanical properties. Furthermore, the acrylamide group can improve the separator's adsorption capacity for electrolyte, thereby improving the interfacial contact effect between the separator and the electrode, and finally obtaining a separator that can balance high ion electron conduction and strong interfacial adhesion.

[0021] The role of the benzene ring and / or naphthalene structure in the second monomer in this invention is as follows:

[0022] 1. Rigid support and thermal stability: The aromatic conjugated structure of benzene ring / naphthalene provides a rigid skeleton for the polymer, inhibiting molecular chain creep at high temperatures and reducing the thermal shrinkage rate of the membrane;

[0023] 2. Synergistic ion transport: The π-π conjugated system of the aromatic ring can form a continuous charge transport channel with the conjugated alkene chain (containing tertiary amine group) of the first monomer, which assists lithium ion migration.

[0024] Preferably, R2, R5 and R6 in the first monomer are each independently an alkyl group.

[0025] Preferably, the number of carbon atoms in R2, R5 and R6 is independently 1 to 4, for example, 1, 2, 3 or 4.

[0026] Preferably, R4 contains an ether group.

[0027] Preferably, the oxygen atom content of R4 is 1 to 2.

[0028] Preferably, the number of carbon atoms in R4 is 1 to 3, for example, it can be 1, 2 or 3.

[0029] Preferably, R4 is an ethoxy group.

[0030] In this invention, R4 preferably contains an ether group, and more preferably an ethoxy group. The ethoxy group can cooperate with the ester group to improve the flexibility and solubility of the polymer, and the membrane is used for processing.

[0031] Preferably, the structural formula of R1 is shown in formula (3):

[0032] Equation (3).

[0033] Preferably, Ar in the second monomer is a benzene ring substituted with formyl and / or chlorine, and / or Ar in the second monomer is naphthalene substituted with formyl and / or chlorine.

[0034] In this invention, Ar in the second monomer is preferably benzene containing both formyl and chlorine substitution. The cyclic conjugated structure of the benzene ring prevents the polymer chain from breaking when immersed in the electrolyte or when the silicon anode expands. Moreover, the benzene ring can stably connect functional groups such as formyl (-CHO) and chlorine (-Cl), ensuring that the coordination effect of the formyl group with the tertiary amine group of the first monomer and the antioxidant synergistic effect of the chlorine atom with the cyano group are effectively exerted. Specifically, the formyl group can interact with lithium ions to promote lithium ion conduction, while the chlorine can coordinate with the cyano group in the first monomer to synergistically improve the chemical stability and antioxidant performance of the membrane.

[0035] Preferably, the O=CH- and -NHCO-CH=CH2 are meta, para, or ortho structures on Ar, with a meta structure being preferred.

[0036] Preferably, the formyl group and the chlorine are in a meta-position on the Ar.

[0037] Preferably, the structural formula of the second monomer is shown in formula (4):

[0038] Equation (4).

[0039] The preferred second monomer of this invention has the structural formula shown above, namely N-(2-chloro-4-formyl-5-methoxyphenyl)acrylamide (CAS: 1251456-87-7), which results in better electrochemical performance of the membrane.

[0040] Preferably, the molar ratio of the first monomer to the second monomer is 1:(3~6), for example, it can be 1:3, 1:3.2, 1:3.3, 1:3.4, 1:3.5, 1:3.8, 1:4.0, 1:4.2, 1:4.5, 1:4.8, 1:5.0, 1:5.2, 1:5.5 or 1:6.0, etc.

[0041] The present invention preferably controls the molar ratio of the first monomer and the second monomer within the above-mentioned range. Studies have found that when the molar content of the first monomer is too high, the excess of tertiary amine groups (-NR5R6) leads to excessively strong polymer polarity, excessive electrolyte adsorption, causing membrane swelling and a decrease in mechanical strength; at the same time, the dense cyano groups (-CN) increase the polymer brittleness and make it easy to be torn by the expansion stress of the silicon anode; when the molar content of the second monomer is too high, the excess of acrylamide groups (-NHCO-CH=CH2) leads to an excessively dense hydrogen bond network, a decrease in porosity, insufficient electrolyte wetting, a decrease in the 6C fast charging constant current charge ratio, and insufficient tertiary amine sites, resulting in a decrease in the lithium ion diffusion coefficient and a tendency to deteriorate rate performance.

[0042] Preferably, the weight-average molecular weight M of the polymer is... w The range is 50,000 to 100,000 Da, for example, it can be 50,000 Da, 55,000 Da, 60,000 Da, 65,000 Da, 70,000 Da, 75,000 Da, 80,000 Da, 85,000 Da, 90,000 Da, or 100,000 Da, etc.

[0043] The present invention preferably controls the weight-average molecular weight of the polymer within the above-mentioned range. If the molecular weight is too low, the film-forming properties and interfacial adhesion are insufficient, and the mechanical strength is insufficient. If the molecular weight is too high, the viscosity increases, the coating uniformity decreases, and the high molecular weight polymer has higher crystallinity, higher melting temperature but increased brittleness.

[0044] Preferably, the preparation of the polymer includes: mixing a first monomer, a second monomer, an organic solvent and an initiator, and carrying out a polymerization reaction to prepare the polymer.

[0045] Preferably, the organic solvent includes any one or a combination of at least two of benzene, THF (tetrahydrofuran), NMP (N-methylpyrrolidone), or DMF (N,N-dimethylformamide), wherein typical but non-limiting combinations are combinations of benzene and THF, benzene and NMP, NMP and THF, DMF and THF, and benzene and DMF.

[0046] Preferably, the mass ratio of the polymer monomer to the organic solvent is 1:6 to 1:10, for example, it can be 1:6, 1:6.5, 1:7, 1:7.5, 1:8, 1:8.5, 1:9, 1:9.5 or 1:10, etc.

[0047] Preferably, the initiator comprises any one or a combination of at least two of azobisisobutyronitrile, azobisisoheptanenitrile, or benzoyl peroxide, wherein typical but non-limiting combinations are a combination of azobisisobutyronitrile and azobisisoheptanenitrile, a combination of benzoyl peroxide and azobisisoheptanenitrile, or a combination of azobisisobutyronitrile and benzoyl peroxide.

[0048] Preferably, the initiator accounts for 0.2 to 0.8 wt% of the total mass of the first monomer and the second monomer, for example, it can be 0.2 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, 0.5 wt%, 0.6 wt%, 0.65 wt%, 0.7 wt%, 0.75 wt%, or 0.8 wt%.

[0049] Preferably, the atmosphere for the polymerization reaction includes any one or a combination of at least two of argon, nitrogen, helium, or neon, wherein typical but non-limiting combinations are combinations of argon and nitrogen, helium and nitrogen, argon and helium, and neon and nitrogen.

[0050] Preferably, the polymerization reaction temperature is 60~100℃, for example, it can be 60℃, 62℃, 65℃, 68℃, 70℃, 72℃, 75℃, 80℃, 85℃, 90℃, 95℃ or 100℃, etc.

[0051] Preferably, the polymerization reaction time is 15-32 hours, for example, 15 hours, 16 hours, 20 hours, 25 hours, 28 hours, 29 hours, 30 hours, 31 hours or 32 hours.

[0052] Preferably, the preparation of the polymer further includes: mixing the material after the polymerization reaction with a precipitation solvent to precipitate a polymer, and washing and drying the polymer precipitate to obtain the polymer.

[0053] Preferably, the mass ratio of the material after polymerization to the precipitated solvent is (3~5):1, for example, it can be 3:1, 3.2:1, 3.5:1, 4:1, 4.2:1, 4.5:1, 4.8:1, 5:1, etc.

[0054] Preferably, the precipitation solvent includes any one or a combination of at least two of propanol, isopropanol, or acetone, wherein typical but non-limiting combinations are a combination of propanol and isopropanol, a combination of acetone and isopropanol, or a combination of propanol and acetone.

[0055] Preferably, the membrane includes a functional membrane layer, and the polymer is located in the functional membrane layer. The functional membrane layer further includes an inorganic material; wherein the inorganic material includes inorganic oxides.

[0056] Preferably, the inorganic oxide is zirconium oxide.

[0057] The present invention preferably uses zirconium oxide compounded with the above-mentioned polymer, wherein the zirconium oxide contains Zr 4+ It can form coordination bonds (Zr-O, Zr-N) with cyano (-CN) and ester (-COOR) groups in polymers, enhancing the organic-inorganic interface bonding force and preventing zirconium oxide particles from agglomerating. Moreover, zirconium oxide can also coordinate with the ester (-COOEt) group of the first monomer and the amide (-NHCO-) ​​group of the second monomer to form a "polymer-zirconia" interpenetrating network, improving the diaphragm puncture strength and thermal stability. The resulting diaphragm has excellent mechanical properties and reduced thermal shrinkage.

[0058] Preferably, the zirconium oxide has a particle size range of 20~48nm, for example, it can be 20nm, 25nm, 29nm, 34nm, 38nm, 43nm, 47nm or 48nm, etc.

[0059] The present invention preferably controls the particle size of zirconium oxide within a reasonable range, resulting in a more uniform coating and less tendency to agglomerate into micron-sized clumps.

[0060] Preferably, the mass ratio of inorganic material to polymer in the diaphragm is (10~23):(15~30), wherein the number of parts of inorganic material can be, for example, 10, 12, 13, 15, 16, 18, 19, 21, 22 or 23; and the number of parts of polymer can be, for example, 15, 17, 19, 20, 22, 24, 25, 27, 29 or 30.

[0061] In this invention, the mass ratio of inorganic materials to polymers is preferably controlled within the above-mentioned range. When the content of inorganic materials is too low, the thermal shrinkage rate of the diaphragm increases. When the content of inorganic materials is too high, zirconium oxide particles agglomerate, blocking the pores of the diaphragm, reducing the porosity, increasing the interfacial resistance, increasing the brittleness of the coating, and making it prone to cracking during the coating process.

[0062] Preferably, the functional film layer further includes a dispersant.

[0063] Preferably, the dispersant comprises any one or a combination of at least two of silicates, sodium polyacrylate, or sodium citrate, wherein typical but non-limiting combinations are combinations of silicates and sodium polyacrylate, combinations of sodium citrate and sodium polyacrylate, and combinations of silicates and sodium citrate.

[0064] Preferably, the mass ratio of the dispersant to the polymer is (0.1~0.3):(15~30), wherein the number of parts of the dispersant can be, for example, 0.1, 0.13, 0.15, 0.17, 0.19, 0.22, 0.24, 0.26, 0.28 or 0.3, etc.; and the number of parts of the polymer can be, for example, 15, 17, 19, 20, 22, 24, 25, 27, 29 or 30, etc.

[0065] Preferably, the functional film layer further includes a wetting agent.

[0066] Preferably, the wetting agent comprises any one or a combination of at least two of sodium hexametaphosphate, sodium tripolyphosphate, or sodium pyrophosphate, wherein typical but non-limiting combinations are a combination of sodium hexametaphosphate and sodium tripolyphosphate, a combination of sodium pyrophosphate and sodium tripolyphosphate, or a combination of sodium hexametaphosphate and sodium pyrophosphate.

[0067] Preferably, the mass ratio of the wetting agent to the polymer is (0.02~0.08):(15~30), wherein the number of parts of the wetting agent can be, for example, 0.02, 0.027, 0.034, 0.04, 0.047, 0.054, 0.06, 0.067, 0.074 or 0.08, etc.; and the number of parts of the polymer can be, for example, 15, 17, 19, 20, 22, 24, 25, 27, 29 or 30, etc.

[0068] Preferably, the functional membrane layer further includes a membrane solvent and an adhesive.

[0069] Preferably, the film solvent includes water and isopropanol.

[0070] Preferably, the mass ratio of water, isopropanol, and polymer in the functional membrane layer is (40~66):(1~4):(15~30), wherein the number of parts of water can be, for example, 40, 43, 46, 49, 52, 55, 58, 61, 64, or 66; the number of parts of isopropanol can be, for example, 1, 1.4, 1.7, 2, 2.4, 2.7, 3, 3.4, 3.7, or 4; and the number of parts of polymer can be, for example, 15, 17, 19, 20, 22, 24, 25, 27, 29, or 30.

[0071] Preferably, the adhesive comprises any one or a combination of at least two of CMC (sodium carboxymethyl cellulose), hydroxypropyl methylcellulose (HPMC), or polyvinyl alcohol (PVA), wherein typical but non-limiting combinations are combinations of CMC and hydroxypropyl methylcellulose, combinations of polyvinyl alcohol and hydroxypropyl methylcellulose, and combinations of CMC and polyvinyl alcohol.

[0072] Preferably, the mass ratio of the adhesive to the polymer is (3~7):(15~30), wherein the number of parts of the adhesive can be, for example, 3, 3.5, 4, 4.5, 5, 5.2, 5.5, 5.8, 6, 6.2, 6.5, 6.8 or 7; and the number of parts of the polymer can be, for example, 15, 17, 19, 20, 22, 24, 25, 27, 29 or 30.

[0073] Preferably, the preparation method of the coating slurry of the functional film layer includes: mixing a dispersant and water for a first stirring, then adding inorganic materials for a second stirring, then adding isopropanol, binder, polymer and wetting agent, and then vacuum shaking to form the coating slurry of the functional film layer.

[0074] Preferably, the first and second stirring are carried out in a planetary stirring and dispersing apparatus.

[0075] Preferably, the rotation speed of the first stirrer is 1000~2000 r / min, for example, it can be 1000 r / min, 1100 r / min, 1200 r / min, 1300 r / min, 1400 r / min, 1500 r / min, 1600 r / min, 1700 r / min, 1880 r / min or 2000 r / min, etc.

[0076] Preferably, the revolution speed of the first stirrer is 40~60 r / min, for example, it can be 40 r / min, 43 r / min, 45 r / min, 47 r / min, 49 r / min, 52 r / min, 54 r / min, 56 r / min, 58 r / min or 60 r / min, etc.

[0077] Preferably, the first stirring time is 46 to 70 minutes, for example, it can be 46 minutes, 49 minutes, 52 minutes, 54 minutes, 57 minutes, 60 minutes, 62 minutes, 65 minutes, 68 minutes or 70 minutes.

[0078] Preferably, the rotation speed of the second stirrer is 3200~3800 r / min, for example, it can be 3200 r / min, 3250 r / min, 3300 r / min, 3400 r / min, 3460 r / min, 3500 r / min, 3600 r / min, 3650 r / min, 3700 r / min or 3800 r / min, etc.

[0079] Preferably, the revolution speed of the second stirrer is 10~30 r / min, for example, it can be 10 r / min, 13 r / min, 15 r / min, 17 r / min, 19 r / min, 22 r / min, 24 r / min, 26 r / min, 28 r / min or 30 r / min, etc.

[0080] Preferably, the second stirring time is 30 to 50 minutes, for example, it can be 30 minutes, 33 minutes, 35 minutes, 37 minutes, 39 minutes, 42 minutes, 44 minutes, 46 minutes, 48 ​​minutes or 50 minutes.

[0081] Preferably, the diaphragm includes a base membrane, and the functional membrane layer is disposed on the surface of the base membrane. The base membrane is made of PI (polyimide) film.

[0082] In this invention, the PI-based film has high temperature resistance (film breakage temperature up to 430℃), which can improve battery safety and increase the pass rate of needle penetration and hot box safety tests. The nitrogen-containing polar groups of PI form hydrogen bonds with the electrolyte solvent (ethylene carbonate EC or other solvents), which improves wettability and rate performance.

[0083] Those skilled in the art will understand that the preparation method of the diaphragm of the present invention includes coating, rewinding and slitting in sequence.

[0084] Preferably, the coating comprises dotted coating.

[0085] The present invention preferably employs dot-matrix coating, which has high air permeability and high porosity, thereby resulting in lower impedance, thus greatly improving the lithium-ion transport rate and improving the rate performance of the battery.

[0086] Preferably, the dot coating is applied using ultrasonic dot spraying.

[0087] Preferably, the temperature of the dotted coating is 40~90℃, for example, it can be 40℃, 46℃, 52℃, 57℃, 63℃, 68℃, 74℃, 79℃, 85℃ or 90℃, etc.

[0088] Preferably, the stretching speed difference of the dot-coating is 0.1% to 10%, for example, it can be 0.1%, 1.2%, 2.3%, 3.4%, 4.5%, 5.6%, 6.7%, 7.8%, 8.9% or 10%, etc.

[0089] Preferably, the rewinding temperature is 60~100℃, for example, it can be 60℃, 65℃, 69℃, 74℃, 78℃, 83℃, 87℃, 92℃, 96℃ or 100℃, etc.

[0090] Preferably, the winding and unwinding tension during the rewinding process is 0.1~50N, for example, it can be 0.1N, 5N, 10N, 15N, 20N, 25N, 30N, 38N, 40N or 50N.

[0091] Preferably, the unwinding and rewinding tension of the slitting is 0.1~20N, for example, it can be 0.1N, 1N, 4N, 6N, 8N, 10N, 12N, 14N, 16N, 18N or 20N, etc.

[0092] Preferably, the contact pressure for the cutting is 0.01~16N, for example, it can be 0.01N, 1N, 3N, 5N, 6N, 8N, 10N, 11N, 13N, 15N or 16N, etc.

[0093] Preferably, the porosity of the base membrane is 40-50%, for example, it can be 40%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49% or 50%, etc.

[0094] Preferably, the air permeability of the base membrane is 70~120s / 100mL, for example, it can be 70s / 100mL, 76s / 100mL, 82s / 100mL, 87s / 100mL, 93s / 100mL, 98s / 100mL, 104s / 100mL, 109s / 100mL, 115s / 100mL or 120s / 100mL, etc.

[0095] Preferably, the thickness of the base film is 6~8μm, for example, it can be 6μm, 6.3μm, 6.5μm, 6.7μm, 6.9μm, 7.2μm, 7.4μm, 7.6μm, 7.8μm or 8μm, etc.

[0096] It is worth noting that the preparation method of the base film in this invention includes: mixing 250-350 parts of PI binder with water (the ratio of PI binder to water is 250-350:40-90 g / mL) and sonicating for 60-90 min, then placing it in a ball mill jar and ball milling for 6-9 h at a temperature of 25-45℃ and a speed of 500-900 rpm; then adding 1-3 parts of base film wetting agent and 3-6 parts of base film dispersant to the ball mill jar, adjusting the ball milling speed to 500-700 rpm, and ball milling for 2-5 h; then adding 31-35 parts of base film pore-forming agent and continuing ball milling for 2-4 h to obtain a base film slurry; using a casting coating machine to obtain the base film PI; after drying in a drying zone, the production line then passes through a hot water bath zone at a temperature of 100-120℃, and washes away the pore-forming agent by three hot water soaks to obtain the PI base film. The above parts are by weight.

[0097] Preferably, the PI adhesive is DuPont PI-2540.

[0098] Preferably, the base film wetting agent includes DIGIC 270.

[0099] Preferably, the base film dispersant comprises any one or a combination of at least two of triethylhexylphosphate, sodium lauryl sulfate, methylpentanol, cellulose derivatives, polyacrylamide, guru gum, or fatty acid polyethylene glycol esters, wherein typical but non-limiting combinations are the combination of triethylhexylphosphate and sodium lauryl sulfate, the combination of methylpentanol and sodium lauryl sulfate, the combination of triethylhexylphosphate and methylpentanol, the combination of cellulose derivatives and fatty acid polyethylene glycol esters, and the combination of guru gum and polyacrylamide.

[0100] Preferably, the pore-forming agent of the base film includes polyethylene glycol.

[0101] Preferably, the thickness of the functional film layer is 2~5μm, for example, it can be 2.0μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3μm, 3.2μm, 3.5μm, 4μm, 4.5μm or 5μm, etc.

[0102] It's worth noting that a thinner separator makes it more susceptible to puncture, while a thicker separator increases ion transport distance but reduces rate performance. High porosity reduces mechanical strength and leads to high-temperature shrinkage, while low porosity results in low electrolyte absorption and poor rate performance. An excessively thin functional coating can lead to uneven coverage, insufficient buffering / protection against silicon anode expansion, and limited improvement in thermal stability. An excessively thick functional coating increases the total separator thickness and internal resistance, reducing battery energy density and power density, and may result in excessively high permeability (Gurley value), affecting electrolyte wetting and ion transport, thus increasing costs.

[0103] Preferably, the porosity of the diaphragm is 35-45%, for example, it can be 35%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44% or 45%, etc.

[0104] In a second aspect, the present invention provides a lithium-ion battery, the lithium-ion battery comprising the separator described in the first aspect.

[0105] Preferably, the lithium-ion battery includes a negative electrode, a positive electrode, and the separator. The positive electrode comprises a ternary material, and / or the negative electrode comprises a silicon-carbon negative electrode material.

[0106] Preferably, the ternary material is a high-nickel ternary material, wherein the nickel content in the high-nickel ternary material is 80~95wt%.

[0107] Preferably, the silicon content in the silicon-carbon anode material is 45-55 wt%, for example, it can be 45 wt%, 47 wt%, 48 wt%, 49 wt%, 50 wt%, 51 wt%, 52 wt%, 53 wt%, 54 wt%, or 55 wt%.

[0108] The separator of the present invention is preferably suitable for silicon content of 45-55 wt% in silicon-carbon anode materials, which can optimize the expansion problem and improve volumetric energy and rate performance.

[0109] Preferably, the particle size D50 of the silicon particles and / or carbon particles in the silicon-carbon anode material is 4~10μm, for example, it can be 4μm, 4.7μm, 5.4μm, 6μm, 6.7μm, 7.4μm, 8μm, 8.7μm, 9.4μm or 10μm, etc.

[0110] In this invention, the preferred silicon-carbon anode material has a particle size D50 of 4~10μm for silicon particles and / or carbon particles, which can improve rate performance and optimize dispersion.

[0111] Preferably, the lithium-ion battery further includes an electrolyte. The electrolyte includes additives, which include any one or a combination of at least two of ethylene sulfate (DTD), LiPO2F2, or tris(trimethylsilyl)phosphate (TMSP), wherein typical but non-limiting combinations are combinations of DTD and LiPO2F2, combinations of TMSP and LiPO2F2, and combinations of DTD and TMSP.

[0112] The present invention adds the above-mentioned additives to the electrolyte, which can further reduce the interfacial impedance and have a synergistic effect with the membrane.

[0113] Preferably, the content of additives in the electrolyte is 0.2~0.8wt%, for example, it can be 0.2wt%, 0.27wt%, 0.34wt%, 0.4wt%, 0.47wt%, 0.54wt%, 0.6wt%, 0.67wt%, 0.74wt%, or 0.8wt%, etc.

[0114] Compared with the prior art, the present invention has at least the following beneficial effects:

[0115] The separator provided by the present invention is made by copolymerizing a polymer obtained by copolymerizing a first monomer and a second monomer. It has excellent ion conductivity, heat resistance and interfacial stability, which enhances the mechanical strength of the separator and its ability to adsorb electrolyte, improves the interfacial contact between the separator and the electrode, and improves the performance of the battery. Detailed Implementation

[0116] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0117] Polymer A1

[0118] Polymer A1 is formed by polymerization of a first monomer and a second monomer, wherein the structural formula of the first monomer is shown in formula (5):

[0119] Equation (5);

[0120] The structural formula of the second monomer is shown in equation (4):

[0121] Equation (4);

[0122] The molar ratio of the first monomer to the second monomer is 1:4, and the weight-average molecular weight M of the polymer is... w It is 80000Da.

[0123] The preparation methods of polymer A1 include:

[0124] A first monomer, a second monomer, an organic solvent (benzene), and an initiator (benzoyl peroxide) were mixed, with the initiator accounting for 0.5 wt% of the total mass of the first and second monomers. The mass ratio of polymer monomer to organic solvent (benzene) was 1:8. The polymer was prepared by polymerization at 75°C for 30 h under an argon atmosphere. The polymer was then mixed with the precipitating solvent (propanol) at a mass ratio of 4.3:1, resulting in polymer precipitation. The polymer precipitate was washed and dried to obtain polymer A1.

[0125] The peak positions of the infrared spectrum (KBr tablet) of polymer A1 in this invention are shown below: Cyano-CN: 2232 cm⁻¹ -1 Left and right; ester group -C=O: 1735cm -1 Left and right; amide-C=O: 1662cm -1 Left and right; tertiary amine group -N-: 1265cm -1 Left and right; Ether-COC: 1115cm -1 Left and right; Aromatic ring -C=C: 1592cm -1 Left and right; formyl-CHO: 1715cm -1 Left and right; Amide-NH bending: 1540cm -1 Left and right; carbon-hydrogen bond -CH: 2935cm -1 Left and right; Zirconia-Zr-O: 468cm -1 about.

[0126] In this invention, polymer A1 1 The peak elution of the HNMR spectrum is as follows: First monomer tertiary amine group -CH3 (-N(CH3)2): 2.85 (s, 6H); First monomer ethoxy group -O-CH2-CH3 (-OEt): 3.52 (q, 2H, J=7.2Hz, -O-CH2-), 1.28 (t, 3H, J=7.2Hz, -CH3); First monomer ester group -O-CH2-CH3 (-COOEt): 4.21 (q, 2H, J=7.2Hz, -O-CH2-), 1.39 (t, 3H, J=7.2Hz, -CH3); First monomer carbon-carbon double bond -H: 6.32 (s, 1H); Second monomer methoxy group -O-CH3: 3.91 ( s, 3H); second monomer aromatic ring -H: 7.35 (d, 1H, J=8.4Hz), 7.58 (s, 1H), 7.72 (d, 1H, J=8.4Hz); second monomer formyl-CHO: 9.92 (s, 1H); second monomer amide-NH-: 8.15 (s, 1H); second monomer carbon-carbon double bond -H (-CH=CH2): 5.31 (d, 1H, J=10.8Hz, cis=CH2), 5.72 (d, 1H, J=17.4Hz, trans=CH2), 6.45 (dd, 1H, J=10.8, 17.4Hz, -CH=); polymer backbone -CH2-CH-: 2.05 (m, 2H).

[0127] Polymer A2

[0128] Polymer A2 is formed by polymerization of a first monomer and a second monomer, wherein the first and second monomers are the same as those in polymer A1, the molar ratio of the first monomer to the second monomer is 1:3, and the weight-average molecular weight M of the polymer is... w It is 50000Da.

[0129] The preparation methods of polymer A2 include:

[0130] A first monomer, a second monomer, an organic solvent (THF), and an initiator (azobisisobutyronitrile) were mixed, with the initiator accounting for 0.2 wt% of the total mass of the first and second monomers. The mass ratio of polymer monomer to organic solvent (THF) was 1:6. The polymer was prepared by polymerization at 60°C for 15 h under an argon atmosphere. The polymer was then mixed with the precipitating solvent (acetone) at a mass ratio of 5:1 to precipitate the polymer. The polymer precipitate was washed and dried to obtain polymer A2.

[0131] Polymer A3

[0132] Polymer A3 is formed by polymerization of a first monomer and a second monomer, wherein the first and second monomers are the same as those in polymer A1, the molar ratio of the first monomer to the second monomer is 1:6, and the weight-average molecular weight M of the polymer is... w It is 100,000 Da.

[0133] The preparation methods of polymer A3 include:

[0134] A first monomer, a second monomer, an organic solvent (DMF), and an initiator (azobisisobutyronitrile) were mixed, with the initiator accounting for 0.8 wt% of the total mass of the first and second monomers. The mass ratio of polymer monomer to organic solvent (THF) was 1:10. The polymer was prepared by polymerization at 100°C for 32 hours under an argon atmosphere. The polymer was then mixed with the precipitation solvent (isopropanol) at a mass ratio of 3:1, and a polymer precipitate was formed. The polymer precipitate was washed and dried to obtain polymer A3.

[0135] Polymer A4

[0136] Polymer A4 is identical to polymer A1 except that the molar ratio of the first monomer to the second monomer is 1:8, and will not be repeated here.

[0137] Polymer A5

[0138] Polymer A5 is identical to polymer A1 except that the molar ratio of the first monomer to the second monomer is 1:2, and will not be repeated here.

[0139] Polymer A6

[0140] Polymer A6 is identical to polymer A1 except that the second monomer is replaced by N-(2-chlorophenyl)acrylamide (CAS No. 17090-09-4), and will not be described again here.

[0141] Polymer A7

[0142] Polymer A7 is identical to polymer A1 except that the second monomer is replaced with N-phenylacrylamide, and the CAS number is 2210-24-4. It will not be described again here.

[0143] Polymer D1

[0144] Polymer D1 is identical to polymer A1 except that the second monomer is replaced by the first monomer in equal molar amounts, and will not be described again here.

[0145] Polymer D2

[0146] Polymer D2 is identical to polymer A1 except that the first monomer is replaced by the second monomer in equal molar amounts, and will not be described again here.

[0147] Polymer D3

[0148] Polymer D3 is identical to polymer A1 except that the first monomer is ethyl 2-cyano-3,3-diethoxyprop-2-enoate (CAS No. 1146943-46-5), and its structural formula is shown in formula (6). It will not be described again here.

[0149] Equation (6).

[0150] Polymer D4

[0151] Polymer D4 is identical to polymer A1 except that the first monomer is replaced with ethyl 2-cyano-5-(dimethylamino)-3-methylpentane-2,4-dienoate (CAS No. 65996-11-4), which will not be repeated here.

[0152] Polymer D5

[0153] Polymer D5 is identical to polymer A1 except that the second monomer is replaced with N-(2-chlorophenyl)acrylamide (CAS No.: 17090-09-4), and will not be described again here.

[0154] For ease of experimental comparison, the preparation methods of the diaphragms in the following embodiments and comparative examples all adopted the following steps: A coating slurry for the functional film layer was ultrasonically dot-coated onto the base film at 50°C with a dot-coating speed difference of 5%, followed by rewinding at 70°C and a winding / unwinding tension of 30N. Then, the film was slit with a winding / unwinding tension of 15N and a contact pressure of 10N to obtain the diaphragm. However, this does not mean that the diaphragm provided by this invention can only be prepared using the above process parameters; other process parameters are also applicable.

[0155] Example 1

[0156] This embodiment provides a diaphragm, which, by weight, includes a base membrane and a functional membrane layer disposed on the surface of the base membrane; the base membrane is made of PI base membrane, has a porosity of 45%, and an air permeability of 100s / 100mL; the thickness of the base membrane is 7μm; and the thickness of the functional membrane layer is 3μm.

[0157] The coating slurry for the functional film layer comprises: polymer A1, zirconium oxide with a particle size range of 20~30nm, water, wetting agent (sodium hexametaphosphate), dispersant (sodium citrate), isopropanol and binder (CMC) = 26:15:50:0.06:0.2:3:5.

[0158] The preparation method of the coating slurry for the functional film layer includes: mixing a dispersant and water for a first stirring, wherein the rotation speed of the first stirring is 1500 r / min, the revolution speed is 50 r / min, and the time is 50 min; then adding inorganic materials for a second stirring, wherein the rotation speed of the second stirring is 3500 r / min, the revolution speed is 20 r / min, and the time is 40 min; then adding isopropanol, binder, polymer and wetting agent, and subjecting the mixture to vacuum oscillation to form the coating slurry for the functional film layer.

[0159] The preparation method of the base film includes: mixing 300 parts by weight of PI adhesive (brand name DuPont PI-2540) with water (PI adhesive to water ratio of 300:60 g / mL) and sonicating for 70 min, then placing it in a ball mill jar and ball milling for 7 h at 30℃ and 700 rpm; then adding 2 parts of base film wetting agent (Digo 270) and 5 parts of base film dispersant (triethylhexyl phosphate) to the ball mill jar, adjusting the ball milling speed to 600 rpm, and ball milling for 4 h, then adding 33 parts of base film pore-forming agent (polyethylene glycol, weight average molecular weight of 4000) and continuing ball milling for 3 h to obtain base film slurry, using a casting coating machine to obtain the base film PI, drying it in the drying zone, and then passing it through the hot water tank area at 110℃ in the production line, washing away the pore-forming agent by three hot water soaks to obtain the PI base film.

[0160] Example 2

[0161] This embodiment provides a diaphragm, which, by weight, includes a base membrane and a functional membrane layer disposed on the surface of the base membrane; the base membrane is made of PI base membrane, has a porosity of 40%, and an air permeability of 70s / 100mL; the thickness of the base membrane is 8μm; and the thickness of the functional membrane layer is 5μm.

[0162] The coating slurry for the functional film layer comprises: polymer A2, zirconium oxide with a particle size range of 25~48nm, water, wetting agent (sodium tripolyphosphate), dispersant (sodium silicate), isopropanol and binder (hydroxypropyl methylcellulose) = 25:10:66:0.08:0.3:1:7.

[0163] The preparation method of the coating slurry for the functional film layer includes: mixing a dispersant and water for a first stirring, wherein the rotation speed of the first stirring is 1000 r / min, the revolution speed is 40 r / min, and the time is 70 min; then adding inorganic materials for a second stirring, wherein the rotation speed of the second stirring is 3800 r / min, the revolution speed is 30 r / min, and the time is 30 min; then adding isopropanol, binder, polymer and wetting agent, and subjecting the mixture to vacuum oscillation to form the coating slurry for the functional film layer.

[0164] The preparation method of the base film includes: mixing 350 parts by weight of PI adhesive (brand name DuPont PI-2540) with water (PI adhesive to water ratio of 350:40 g / mL) and sonicating for 90 min, then placing it in a ball mill jar and ball milling for 6 h at 25℃ and 900 rpm; then adding 3 parts of base film wetting agent (Digo 270) and 6 parts of base film dispersant (methylpentanol) to the ball mill jar, adjusting the ball milling speed to 500 rpm, and ball milling for 5 h, then adding 31 parts of base film pore-forming agent (polyethylene glycol, weight average molecular weight of 4000) and continuing ball milling for 4 h to obtain base film slurry, using a casting coating machine to obtain the base film PI, drying it in the drying zone, and then passing it through the hot water tank area at 120℃ in the production line, washing away the pore-forming agent by three hot water soaks to obtain the PI base film.

[0165] Example 3

[0166] This embodiment provides a diaphragm, which, by weight, includes a base membrane and a functional membrane layer disposed on the surface of the base membrane; the base membrane is made of PI base membrane, has a porosity of 50%, and an air permeability of 120s / 100mL; the thickness of the base membrane is 6μm; and the thickness of the functional membrane layer is 2μm.

[0167] The coating slurry for the functional film layer comprises: polymer A3, zirconium oxide with a particle size range of 20~48nm, water, wetting agent (sodium pyrophosphate), dispersant (sodium polyacrylate with a weight average molecular weight of 80000Da), isopropanol, and binder (polyvinyl alcohol with a weight average molecular weight of 120000Da) in a ratio of 15:23:40:0.02:0.1:4:3.

[0168] The preparation method of the coating slurry for the functional film layer includes: mixing a dispersant and water for a first stirring, wherein the rotation speed of the first stirring is 2000 r / min, the revolution speed is 60 r / min, and the time is 46 min; then adding inorganic materials for a second stirring, wherein the rotation speed of the second stirring is 3200 r / min, the revolution speed is 10 r / min, and the time is 50 min; then adding isopropanol, binder, polymer and wetting agent, and subjecting the mixture to vacuum oscillation to form the coating slurry for the functional film layer.

[0169] The preparation method of the base film includes: mixing 250 parts by weight of PI adhesive (brand name DuPont PI-2540) with water (PI adhesive to water ratio of 250:90 g / mL) and sonicating for 60 min, then placing it in a ball mill jar and ball milling for 9 h at 45℃ and 500 rpm; then adding 1 part of base film wetting agent (Digo 270) and 3 parts of base film dispersant (sodium dodecyl sulfate) to the ball mill jar, adjusting the ball milling speed to 700 rpm, and ball milling for 2 h, then adding 35 parts of base film pore-forming agent (polyethylene glycol, weight average molecular weight of 4000) and continuing ball milling for 2 h to obtain base film slurry, using a casting coating machine to obtain the base film PI, drying it in the drying zone, and then passing it through a hot water bath zone at 100℃ on the production line, washing away the pore-forming agent by three hot water soaks to obtain the PI base film.

[0170] Example 4

[0171] This embodiment provides a diaphragm, which is the same as that in Example 1 except that it uses polymer A4, and will not be described again here.

[0172] Example 5

[0173] This embodiment provides a diaphragm, which is the same as that in Example 1 except that it uses polymer A5, and will not be described again here.

[0174] Example 6

[0175] This embodiment provides a diaphragm, which is the same as that in Example 1 except that it uses polymer A6, and will not be described again here.

[0176] Example 7

[0177] This embodiment provides a diaphragm, which is the same as that in Example 1 except that it uses polymer A7, and will not be described again here.

[0178] Example 8

[0179] This embodiment provides a diaphragm, except that the mass fraction of zirconium oxide with a particle size range of 20~30nm is replaced with 25 parts, and the mass fraction of polymer A1 is adapted to be adjusted to 16 parts so that the mass ratio of inorganic material to polymer A1 is 25:16. Otherwise, the diaphragm is the same as in Example 1, and will not be described again here.

[0180] Example 9

[0181] This embodiment provides a diaphragm, except that the mass fraction of zirconium oxide with a particle size range of 20~48nm is replaced with 5 parts, and the mass fraction of polymer A1 is adapted to be adjusted to 30 parts so that the mass ratio of inorganic material to polymer A1 is 30:5. Otherwise, the diaphragm is the same as in Example 2, and will not be described again here.

[0182] Example 10

[0183] This embodiment provides a diaphragm, which is the same as that in Embodiment 1 except that zirconium oxide is not added to the functional membrane layer, and will not be described again here.

[0184] Example 11

[0185] This embodiment provides a diaphragm, which is the same as that in Embodiment 1 except that zirconium oxide is replaced with silicon oxide, and will not be described again here.

[0186] Example 12

[0187] This embodiment provides a diaphragm, which is identical to that in Example 1 except that the ultrasonic dot coating is replaced with comma coating, and will not be repeated here. The specific preparation method of the comma coating includes: stirring the functional film coating slurry until the viscosity stabilizes (2200 mPa·s, 25℃); coating parameters: base film tension controlled at 12N, distance between the comma doctor blade and the base film set at 2.5μm (corresponding to the functional film thickness), coating speed 6m / min; drying process: using a three-stage drying process (40℃→60℃→80℃), total drying time 18min, removing solvent (residual moisture <0.1wt%); post-treatment: after drying, calendering (pressure 8MPa) is performed to ensure tight bonding between the functional film and the base film.

[0188] Comparative Example 1

[0189] This comparative example provides a diaphragm, which is the same as that in Example 1 except that it uses polymer D1, and will not be described again here.

[0190] Comparative Example 2

[0191] This comparative example provides a diaphragm, which is the same as that in Example 1 except that it uses polymer D2, and will not be described again here.

[0192] Comparative Example 3

[0193] This comparative example provides a diaphragm, which is the same as that in Example 1 except that it uses polymer D3, and will not be described again here.

[0194] Comparative Example 4

[0195] This comparative example provides a diaphragm, which is the same as that in Example 1 except that it uses polymer D4, and will not be described again here.

[0196] Comparative Example 5

[0197] This comparative example provides a diaphragm, which is the same as that in Example 1 except that it uses polymer D5, and will not be described again here.

[0198] Electrical performance testing

[0199] 1. Preparation of lithium-ion batteries

[0200] (1) Preparation of positive electrode sheet

[0201] The ternary material NCM811 (LiNi0.8Co0.1Mn0.1O2), the binder PVDF (polyvinylidene fluoride), the conductive agent SP (super-P conductive carbon black), and SWCNT (single-walled carbon nanotubes) were mixed and stirred evenly in a mass ratio of 94:3:2.9:0.1 to obtain a positive electrode slurry. The positive electrode slurry was then coated onto aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0202] (2) Preparation of negative electrode sheet

[0203] Silicon-carbon anode material (55wt%), conductive agent SP (Super-P conductive carbon black), SWCNT (single-walled carbon nanotubes), binder PAA (polyacrylic acid) and SBR are mixed and stirred evenly in a mass ratio of 92:2:0.5:3:2.5 to obtain anode slurry. The solid content is controlled at 30wt%. Then, the anode slurry is coated onto a copper foil current collector through a coating process. After vacuum drying and cold pressing, the anode sheet is obtained.

[0204] (3) Selection of electrolyte

[0205] An electrolyte was prepared with a mass ratio of EC:PC:DMC:DEC:FEC of 15:20:25:30:10 and a LiPF6 concentration of 1.0 mol / L, and 0.5% DTD was added as an electrolyte additive.

[0206] (4) Selection of separator membrane

[0207] The diaphragms of the above embodiments and comparative examples were selected.

[0208] (5) Preparation of lithium-ion batteries

[0209] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0210] 2. Performance Testing

[0211] The rate performance of the battery was tested under lithium-ion battery testing conditions. The tests were conducted at room temperature (25℃) using the LAND battery testing system from Wuhan Jinno Electronics Co., Ltd., with the charge / discharge voltage limited to 2.5V~4.2V. The initial coulombic efficiency (first efficiency), cycle performance, and rate performance are shown in Table 1 below.

[0212] (1) First Coulomb efficiency

[0213] At 25°C, the lithium-ion battery was charged at a constant current and constant voltage of 0.33C to 4.2V, allowed to stand for 10 minutes, and then discharged at a constant current of 0.33C to 2.5V, allowed to stand for 10 minutes. The initial coulombic efficiency of the lithium-ion battery was calculated.

[0214] 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%.

[0215] (2) Capacity retention rate after 1000 cycles at room temperature (1C / 2C)

[0216] 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.

[0217] 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.

[0218] (3) Room temperature 6C rate performance - constant current charge ratio

[0219] At 25℃, the lithium-ion battery was discharged at a 1C rate to 2.5V under constant current, allowed to stand for 10 minutes, and then charged at a 6C rate to 4.2V under constant current and constant voltage, with a cutoff current of 0.05C. After a 10-minute stand, the constant current charging capacity Q1 and the total constant current and constant voltage charging capacity Q2 were recorded. The constant current charge ratio at the 6C rate was calculated using 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%.

[0220] (4) Capacity retention rate at room temperature 1C / 8C discharge

[0221] The lithium-ion battery, after capacity gradation, was charged at 25℃ using a 1C rate with constant current and constant voltage to 4.2V, with a cutoff current of 0.05C; it was then allowed to stand for 10 minutes; next, the lithium-ion battery was discharged at a 1C rate with constant current to 2.5V, and its discharge capacity Q1C was recorded as the initial discharge capacity; then, the lithium-ion battery was charged at 25℃ using a 1C rate with constant current and constant voltage to 4.2V, with a cutoff current of 0.05C; it was allowed to stand for 10 minutes; then, the fully charged battery was discharged at an 8C rate with constant current to 2.5V, 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%.

[0222] (4) Thermal shrinkage rate of diaphragm at 180℃ / 30min

[0223] Referring to the standard test method (GB / T36363), the dimensional changes of the diaphragm sample in the free state are measured at the specified temperature (e.g., 180℃) and time (30 min), and the transverse (TD) and longitudinal (MD) thermal shrinkage rates are calculated.

[0224] High-temperature furnace: temperature control accuracy ±1℃, internal atmosphere is air or inert gas (such as N2, selected according to patent requirements).

[0225] Sample clamps: stainless steel frame or quartz glass plate, ensuring that the sample shrinks freely without restraint.

[0226] A vernier caliper or optical projector with an accuracy of 0.02 mm (suitable for micron-level diaphragms); graph paper or laser rangefinder (for marking initial dimensions).

[0227] Sample preparation:

[0228] Cut size: 100mm × 100mm, 10mm away from the edge of the diaphragm. Marking: Draw a cross on the sample surface and record the initial length L0 in the transverse (TD) and longitudinal (MD) directions (accurate to 0.1mm).

[0229] Test steps

[0230] Pretreatment: The sample was placed in an environment of 23±2℃ and 50±5%RH for 24 hours.

[0231] High-temperature treatment:

[0232] Place the sample flat on the fixture and put it into the center of a high-temperature furnace preheated to 180°C, ensuring that the sample does not touch the furnace wall; after holding the temperature for 30 minutes, quickly remove the fixture and cool it at room temperature for 10 minutes.

[0233] Size measurement:

[0234] Measure the crosshair length L1 of the sample after cooling (measured separately in the TD and MD directions); test 3 parallel samples for each sample and take the average value.

[0235] Data calculation: heat shrinkage rate (%) = (L0-L1) / L0×100%; the transverse (TD) and longitudinal (MD) shrinkage rates need to be recorded separately.

[0236] (5) Cell Thermal Runaway (ARC) Test: Start the ARC adiabatic thermal runaway test (the test sample is heated from room temperature to 45±2℃ in the chamber, and after being left for 90 minutes, the change in the battery temperature rise rate is detected. 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. Maintain the adiabatic environment until the battery thermal runaway occurs; if the temperature rise does not exceed 0.2℃ within 10 minutes (i.e., SHR≤0.02℃ / min), continue to the next step temperature rise test; each temperature step is 5℃, and the steps are repeated on each step. The ARC test temperature range is 45℃~300℃. The self-heating start temperature is T1 (temperature rise rate SHR>0.02℃ / min), and the thermal runaway start temperature is T2 (temperature rise rate SHR>1℃ / min). Wherein SHR is the self-heating rate (SHR).

[0237] (6) Cell heating chamber test 150℃ & 30min: After the single cell is fully charged (the lithium-ion battery after capacity division is charged at 25℃ at a 1C rate to 4.2V with constant current and constant voltage, and the cut-off current is 0.05C), it is placed in a temperature chamber and heated from room temperature to 150±2℃ at a rate of 5℃ / min. The temperature is maintained for 30min and then heating is stopped. The cell is observed for 1 hour to see if an explosion or fire occurs.

[0238] (7) Method for testing the porosity of the diaphragm: Refer to GB / T21650.1-2008 "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Intrusion Porosimetry and Gas Adsorption Method - Part 1: Mercury Intrusion Porosimetry", specific steps: Sample preparation: Cut a 10mm×10mm diaphragm, dry it (80℃, 2h), weigh the mass m1, and measure the thickness d; Mercury intrusion test: Use a mercury intrusion porosimeter (such as Micromeritics AutoPore IV), pressure range 0~414MPa, record the mercury intrusion volume V; Calculation: Porosity P(%)=[V / (S×d-m1 / ρpolymer)]×100%, where S is the sample area, and ρpolymer is 1.1~1.2g / cm 3 .

[0239] The test results of the above embodiments and comparative examples are shown in Table 1.

[0240] Table 1

[0241]

[0242] The following points can be observed from Table 1:

[0243] (1) As can be seen from the comprehensive examples 1 to 3, the porosity of the separator provided by the present invention is in the range of 38 to 42%, which has excellent electrolyte wetting performance. The battery has high thermal stability and does not explode or catch fire when tested in a hot cell box at 150°C for 30 minutes. The appearance is normal. The thermal shrinkage rate of the separator at 180°C for 30 minutes is less than 3.2% (TD) and less than 2.8% (MD). It also improves the adsorption performance of electrolyte and has a constant current charge ratio of 6C at room temperature of more than 77.

[0244] (2) In Comparative Example 1, polymer D1 was used and in Comparative Example 2, polymer D2 was used. The effect of copolymerization of the two monomers could not be achieved. In the end, the battery cell was tested at 150℃ for 30 minutes and produced smoke, explosion and fire.

[0245] (3) In Comparative Example 3, the monomer lacks a tertiary amine group (-NR5R6), resulting in the disappearance of lithium-ion transfer sites and weakening of the electronic effect of the conjugated olefin chain, leading to a 40% decrease in olefin chain conjugation and a decrease in electronic conductivity; furthermore, thermal stability and film-forming properties deteriorate, and the thermal shrinkage rate increases. This is because the coordination bond formed between the tertiary amine group in polymer A1 and the formyl group of the second monomer can inhibit molecular chain creep at high temperatures, with a thermal shrinkage rate (TD) of 2.8% at 180℃ / 30min; Comparative Example 3 lacks this effect, the molecular chain is prone to relaxation, and the thermal shrinkage rate (TD) rises to 11.2%, increasing the risk of shell deformation during battery hot box testing (150℃). Moreover, the tertiary amine group can improve the interfacial bonding between the polymer and zirconium oxide (N and Zr). 4+(Coordination); Comparative Example 3 lacks tertiary amine groups, and zirconium oxide is prone to agglomeration (particle size increases from 20~30nm to 50~80nm), resulting in "pinhole" defects in the functional film layer, which leads to shortened battery cycle life and deterioration of safety performance.

[0246] (4) Comparing Example 1 and Comparative Example 4, it can be seen that in polymer A1, the tertiary amine group (-NR5R6) of the first monomer is directly connected to the conjugated olefin chain (-C(CN)=C(NR)-), and the coordination distance with the formyl group of the second monomer is 0.28 nm (the optimal coordination distance). However, in Comparative Example 4, the tertiary amine group is located on the side chain (-CH2-CH2-N(CH3)2), and the olefin chain contains a methyl group (-CH3), which increases the steric hindrance, increases the coordination distance to 0.45 nm, and reduces the coordination efficiency to 30% of that in Example 1, thus reducing the lithium-ion diffusion coefficient. Moreover, the methyl substitution in Comparative Example 4 disrupts the planar conjugated structure of the olefin chain (due to steric repulsion causing olefin chain distortion), reduces the π-π conjugation range by 50%, and decreases the electronic conductivity. Furthermore, in Comparative Example 4, the methyl substitution increased the hydrogen bond distance between the ester group (-COOR) and the acrylamide group (-NHCO-) ​​of the second monomer from 0.29 nm to 0.38 nm, reducing the hydrogen bond density to 60% of that in Comparative Example 1. This weakened the functional film's resistance to silicon expansion and decreased cycle performance. Moreover, the positional shift of the tertiary amine group in Comparative Example 4 prevented it from forming an "NR…CN" electron collaboration with -CN, making -CN easily oxidized and reducing its antioxidant performance. Ultimately, this led to a decrease in the cycle performance of the lithium-ion battery and the occurrence of smoke during cell hot box testing at 150℃ for 30 minutes.

[0247] (5) Comparing Example 1 and Comparative Example 5, it can be seen that there is no "tertiary amine-formyl" lithium transfer channel in Comparative Example 5, and the lithium-ion diffusion coefficient is reduced to 1.4 × 10⁻⁶. -10 m 2 / s; without the conjugation enhancement effect of methoxy groups, the interfacial impedance increases to 120 mΩ·cm 2 Furthermore, in Comparative Example 5, the synergistic effect of antioxidant and hydrogen bonding was weak, resulting in a reduced capacity retention rate after 1000 cycles.

[0248] (6) As can be seen from Examples 1 and 8-10, the zirconium oxide (Zr) in Example 8 4+ Zr is a hard acid that can form "Zr-N" and "Zr-O" coordinate bonds with cyano groups (-CN) and ester groups (-COOR) in polymers (bond energy 25~30kJ / mol); when zirconium oxide is abundant, Zr... 4+ Excessive zirconium oxide will form "multi-point coordination" with the polymer molecular chain, leading to a sharp increase in molecular chain rigidity (a 40% decrease in flexibility) and increased brittleness of the functional film layer; in Example 9, when zirconium oxide was insufficient, Zr 4+The shortcomings are that the density of the "Zr-N / O" coordination bonds with the polymer is reduced to 40% of that in Example 1, which makes it impossible to form a continuous inorganic support framework. The molecular chains are prone to creep at high temperatures, the thermal shrinkage rate (TD) at 180°C / 30min increases, the self-generated heat initiation temperature T1 in the battery ARC test decreases, and the risk of thermal runaway increases. In Example 10, without the addition of zirconium oxide, the performance is further reduced compared to Example 9. This shows that by preferably controlling the content of zirconium oxide particles within a reasonable range, the performance of the separator can be further improved.

[0249] (7) As can be seen from Examples 1 and 11, zirconium oxide is used as inorganic particles in Example 1. Compared with silicon oxide as inorganic particles in Example 9, the cycle performance and thermal stability of Example 1 are better, indicating that the polymer of the present invention can better cooperate with zirconium oxide and improve the overall performance of the membrane.

[0250] (8) As can be seen from Examples 1 and 6, the second monomer in Example 6 lacks a formyl group, and the coordination channel is missing. Lithium ions can only migrate through the polymer chain gaps, and the diffusion coefficient drops to 1.4 × 10⁻⁶. -10 m 2 Below / s, the constant current charging speed during 6C fast charging is lower compared to Example 1; moreover, Example 6 has no methoxy groups, low electron cloud density of the benzene ring, broken conjugated system, and interface impedance reduced from 80mΩ·cm in Example 1. 2 Increased to 120mΩ·cm 2 In Example 1, the battery rate performance is better; and in Example 6, the oxidation resistance and hydrogen bond network are weak, and the capacity retention rate is 78% after 1000 cycles.

[0251] (9) As can be seen from Examples 1 and 7, the second monomer in Example 7 has no chlorine atom, which leads to a decrease in both antioxidant properties and mechanical strength. In addition, there is no formyl group (-CHO), resulting in the breakage of the lithium ion conduction channel. Due to the absence of this coordination channel, lithium ions can only migrate through the gaps between polymer chains, and the constant current charging during 6C fast charging is lower than that in Example 1. Moreover, there is no "Cl-CN" and "CHO-NR" dual synergy in Example 7. In Example 1, the balance between cycle and rate is achieved through the dual synergy of "Cl-CN" antioxidant and "CHO→NR" lithium transfer. In Example 7, there is no such synergy, making it difficult to resist electrolyte oxidation and reducing lithium transfer performance.

[0252] The verification of synergistic effect with electrolyte was performed using the diaphragms in Examples 1-3 and Comparative Examples 1-2. Except that 0.5 wt% DTD was not added to the electrolyte, the test process was the same as described above. The results are shown in Table 2.

[0253] Table 2

[0254]

[0255] As can be seen from Table 2:

[0256] After adding electrolyte additives in Examples 1-3 of this invention, the increase in capacity retention rate and constant current charge ratio at room temperature 1C / 2C for 1000 cycles compared to those without electrolyte additives is greater than the increase in Comparative Examples 1-2 after adding electrolyte additives. This indicates that the present invention uses specific electrolyte additives in combination with the separator to further synergistically improve the cycle performance of lithium-ion batteries.

[0257] The present invention has been illustrated with the above embodiments to illustrate its detailed features, but the present invention is not limited to the above detailed features, that is, it does not mean that the present invention must rely on the above detailed features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the selected technical features, additions of auxiliary technical features, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A diaphragm, characterized in that, The membrane comprises a polymer, wherein the polymer monomers comprise a first monomer and a second monomer; The structural formula of the first monomer is shown in equation (1): Equation (1); The structural formula of R1 is shown in formula (2): Equation (2); Wherein, R3 is a group containing -NR5R6 and a carbon-carbon double bond, and R2, R5 and R6 are each independently a hydrocarbon group; R4 is an oxygen-containing group; and R4 contains an ether group. The second monomer has the structural formula O=CH-Ar-NHCO-CH=CH2, where Ar is a substituted or unsubstituted aromatic ring, the aromatic ring including benzene and / or naphthalene, the substituted aromatic ring being a benzene ring substituted with formyl and / or chlorine, and / or, the substituted aromatic ring being a naphthalene substituted with formyl and / or chlorine.

2. The diaphragm according to claim 1, characterized in that, In the first monomer, R2, R5 and R6 are each independently an alkyl group.

3. The diaphragm according to claim 1, characterized in that, The number of carbon atoms in R2, R5 and R6 are each 1 to 4 independently.

4. The diaphragm according to claim 1, characterized in that, The oxygen atom content of R4 is 1~2.

5. The diaphragm according to claim 1, characterized in that, The number of carbon atoms in R4 is 1 to 3.

6. The diaphragm according to claim 1, characterized in that, R4 is an ethoxy group.

7. The diaphragm according to claim 1, characterized in that, The structural formula of R1 is shown in equation (3): Equation (3).

8. The diaphragm according to claim 1, characterized in that, The O=CH- and -NHCO-CH=CH2 structures are meta, para, or ortho on Ar.

9. The diaphragm according to claim 1, characterized in that, The formyl group and the chlorine are in a meta position on the Ar.

10. The diaphragm according to claim 1, characterized in that, The structural formula of the second monomer is shown in equation (4): Equation (4).

11. The diaphragm according to any one of claims 1 to 10, characterized in that, The molar ratio of the first monomer to the second monomer is 1:(3~6); The weight-average molecular weight M of the polymer w The range is 50,000 to 100,000 Da.

12. The diaphragm according to any one of claims 1 to 10, characterized in that, The diaphragm includes a functional membrane layer, and the polymer is located in the functional membrane layer; the functional membrane layer also includes an inorganic material; wherein the inorganic material includes inorganic oxides.

13. The diaphragm according to claim 12, characterized in that, The inorganic oxide is zirconium oxide.

14. The diaphragm according to claim 13, characterized in that, The zirconium oxide has a particle size range of 20~48 nm.

15. The diaphragm according to claim 12, characterized in that, The mass ratio of inorganic material to polymer in the diaphragm is (10~23):(15~30).

16. The diaphragm according to claim 12, characterized in that, The functional membrane layer also includes a dispersant.

17. The diaphragm according to claim 16, characterized in that, The mass ratio of the dispersant to the polymer is (0.1~0.3):(15~30).

18. The diaphragm according to claim 12, characterized in that, The functional membrane layer also includes a wetting agent.

19. The diaphragm according to claim 18, characterized in that, The mass ratio of the wetting agent to the polymer is (0.02~0.08):(15~30).

20. The diaphragm according to claim 12, characterized in that, The diaphragm includes a base membrane, and the functional membrane layer is disposed on the surface of the base membrane; The base film is made of PI base film.

21. The diaphragm according to claim 20, characterized in that, The porosity of the base membrane is 40-50%.

22. The diaphragm according to claim 20, characterized in that, The air permeability of the base membrane is 70~120s / 100mL.

23. The diaphragm according to claim 20, characterized in that, The thickness of the base film is 6~8μm.

24. The diaphragm according to claim 12, characterized in that, The thickness of the functional film is 2~5μm.

25. The diaphragm according to any one of claims 1 to 10, characterized in that, The porosity of the diaphragm is 35-45%.

26. A lithium-ion battery, characterized in that, The lithium-ion battery includes the separator as described in any one of claims 1 to 25.

27. The lithium-ion battery according to claim 26, characterized in that, The lithium-ion battery includes a negative electrode, a positive electrode, and the separator; wherein the positive electrode includes a ternary material, and / or the negative electrode includes a silicon-carbon negative electrode material.

28. The lithium-ion battery according to claim 27, characterized in that, The lithium-ion battery further includes an electrolyte; wherein the electrolyte includes additives, and the additives include any one or a combination of at least two of DTD, LiPO2F2 or TMSP.

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