A modified diaphragm with flame retardancy and strong mechanical properties and a preparation method thereof

Abstract

The present invention belongs to the technical field of secondary batteries, and in particular relates to a modified diaphragm with flame retardancy and strong mechanical properties and a preparation method thereof. The present invention constructs a diaphragm coating with good flame retardancy and excellent mechanical properties through the interaction between a solid electrolyte, a polymer, and a phosphate flame retardant. The interaction between the three allows a large amount of phosphate flame retardant solvent to remain in the coating, avoiding the reduction of phosphate flame retardant molecules at the negative electrode to form a poor quality SEI layer, without affecting the release of phosphate flame retardants at high temperatures to participate in flame retardancy; in addition, this interaction reduces the crystallinity of the polymer, and the coating with stronger viscosity is not easy to peel off in the battery system, and can improve the tightness of the bonding with the positive and negative electrodes, prevent the diaphragm from wrinkling, and improve the consistency of the battery cell.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular relates to a modified diaphragm with flame retardancy and strong mechanical properties and a preparation method thereof. Background Art

[0002] A rechargeable battery, also known as a rechargeable battery or storage battery, is a battery that can be recharged after discharge to reactivate its active materials and allow continued use. The rechargeable nature of rechargeable batteries and their efficient and stable energy conversion capabilities make them a key technology in modern power applications. They are widely used in applications requiring high power output or long-term continuous power supply, such as electric vehicles, electric bicycles, and energy storage systems.

[0003] The extremely high energy density and integration density of power batteries mean that thermal runaway can cause rapid fire or even explosion. When the internal temperature of the battery rises abnormally, the electrolyte decomposes to produce a large number of hydrogen radicals, which react with the oxygen released by the positive electrode active material, releasing a large amount of heat and triggering a chain reaction, leading to thermal runaway. Adding flame retardants to consume hydrogen radicals and block chain reactions and thermal runaway has been widely proven to be a feasible solution. However, commonly used inexpensive flame retardants such as TEP and TMP have poor reduction resistance and are easily reduced at the negative electrode, causing the SEI to thicken, which in turn affects the battery capacity.

[0004] Chinese patent publication number CN115939661A discloses a process for preparing a sodium ion battery coated diaphragm, which mixes poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) with polyethylene oxide (PEO) and boehmite. Boehmite provides skeleton support to ensure the mechanical strength of the diaphragm, and at the same time forms a three-dimensional porous grid structure with PVDF-HFP and PEO, greatly improving the ionic conductivity, which can improve the first efficiency and rate performance; maleic anhydride is then added, and polar functional groups are introduced on the PVDF-HFP surface through a grafting reaction. Through hydrogen bonding, the adhesion between the diaphragm and the negative electrode is improved, the peel strength is improved, the internal resistance of the battery is reduced, and the cycle performance is improved; finally, a coated diaphragm with high ionic conductivity and good adhesion to the positive and negative electrode sheets is obtained through the above process. However, to improve the ionic conductivity of the separator coating, this method uses expensive PVDF-HFP with a high dielectric constant and compounds it with PEO, resulting in a high overall cost. Furthermore, to improve the separator's adhesion to the positive and negative electrodes, maleic anhydride is introduced, which has a low melting and boiling point. When the battery operates at high temperatures, the melting of maleic anhydride may cause the polymer coating to peel off. Therefore, it can be seen that the separator provided by this patent does not improve the safety performance of the battery. Although PVDF-HFP absorbs heat when it melts at high temperatures, its heat absorption capacity is limited and it cannot absorb heat in the initial stage of battery temperature rise to prevent thermal runaway.

[0005] There is currently no effective and low-cost way to integrate flame retardants such as TEP inside the battery to prevent reduction at the negative electrode without affecting its function. Summary of the Invention

[0006] In view of this, the technical problem to be solved by the present invention is to provide a modified diaphragm with flame retardancy and strong mechanical properties and a preparation method thereof.

[0007] The present invention provides a modified diaphragm with flame retardancy and strong mechanical properties, comprising: a base film and a coating provided on at least one surface of the base film;

[0008] The coating comprises a polymer, a phosphate flame retardant, a solid electrolyte, a wetting agent and a binder.

[0009] Preferably, the mass ratio of the polymer to the solid electrolyte is 1:(3-5).

[0010] Preferably, the coating is formed by a slurry; the mass ratio of the polymer to the phosphate flame retardant in the slurry is 1:(11-15);

[0011] The mass of the wetting agent is 0.3% to 0.5% of the mass of the slurry;

[0012] The mass of the binder is 3% to 5% of the mass of the solid electrolyte.

[0013] Preferably, the polymer is selected from one or more of vinylidene fluoride-chlorotrifluoroethylene copolymer, polyvinyl fluoride, polyvinylidene fluoride, ethylene-chlorotrifluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer;

[0014] The phosphate flame retardant is selected from one or more of trimethyl phosphate, triethyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, tris(1,3-dichloro-2-propyl) phosphate, diphenyl cresol phosphate and diphenyl monooctyl phosphate;

[0015] The solid electrolyte is selected from a lithium solid electrolyte or a sodium solid electrolyte;

[0016] The lithium solid electrolyte is selected from one or more of lithium lanthanum zirconium oxide solid electrolyte, lithium lanthanum titanium oxide solid electrolyte and lithium titanium aluminum phosphate solid electrolyte;

[0017] The sodium solid electrolyte is selected from Na3AlF6, Na3ZrF6, Na2SiF6, Na 12 Al 12 Si 12 O 48, one or more of NZSP and Na-β / β”-Al2O3;

[0018] The wetting agent is one or more of an acetylenic diol wetting agent and a polyether siloxane wetting agent;

[0019] The binder is selected from one or more of carboxymethyl acrylate, sodium carboxymethyl cellulose, polyacrylamide binders, polyvinyl alcohol binders, and PVDF binders.

[0020] Preferably, the coating has a thickness of 1 to 5 μm.

[0021] Preferably, the base film is selected from polypropylene film or polyethylene film; the particle size of the solid electrolyte is 500-600 nm.

[0022] The present invention also provides a method for preparing the modified diaphragm having flame retardancy and strong mechanical properties, comprising the following steps:

[0023] coating the slurry on at least one surface of the base membrane to obtain a modified separator;

[0024] The slurry includes a polymer, a phosphate flame retardant, a solid electrolyte, a wetting agent and a binder.

[0025] Preferably, the slurry is prepared according to the following method:

[0026] S1) mixing a solid electrolyte with a portion of a phosphate flame retardant to obtain slurry A;

[0027] mixing the polymer with the remaining phosphate flame retardant to obtain slurry B;

[0028] S2) mixing the slurry A, slurry B, a wetting agent and a binder to obtain a slurry.

[0029] Preferably, the solid content of the slurry A is 50% to 60%; the mass ratio of the polymer to the remaining phosphate flame retardant in the slurry B is 1:(8-10).

[0030] The present invention also provides a secondary battery comprising the modified diaphragm having flame retardancy and strong mechanical properties, a positive electrode and a negative electrode.

[0031] The present invention provides a modified diaphragm with flame retardancy and strong mechanical properties, comprising: a base film and a coating disposed on at least one surface of the base film; the coating comprising a polymer, a phosphate flame retardant, a solid electrolyte, a wetting agent, and a binder. Compared to the prior art, the present invention constructs a diaphragm coating with excellent flame retardancy and mechanical properties through the interaction between the solid electrolyte, the polymer, and the phosphate flame retardant. The interaction between the three allows a large amount of phosphate flame retardant solvent to remain in the coating, preventing the phosphate flame retardant molecules from being reduced at the negative electrode, resulting in the formation of a poor-quality SEI layer, while not affecting the release of the phosphate flame retardant to participate in flame retardancy at high temperatures. In addition, this interaction reduces the crystallinity of the polymer, resulting in a more viscous coating that is less likely to peel off in a battery system and improves the tightness of the bonding with the positive and negative electrodes, preventing the diaphragm from wrinkling and improving the consistency of the battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a SEM image of the PVDF and solid electrolyte composite coated diaphragm obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0033] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0034] The present invention provides a modified diaphragm with flame retardancy and strong mechanical properties, comprising: a base film and a coating arranged on at least one surface of the base film; the coating comprises a polymer, a phosphate flame retardant, a solid electrolyte, a wetting agent and a binder.

[0035] The present invention fixes the phosphate flame retardant through the synergistic effect between the phosphate flame retardant and the solid electrolyte. Taking TEP as the phosphate flame retardant and PVDF as the polymer as an example: TEP, as a cheap flame retardant, can block the free radical chain reaction inside the battery cell and avoid thermal runaway. However, if TEP is directly added to the electrolyte, it will be reduced at the negative electrode, resulting in a thicker SEI, which affects the cycle stability. The present invention first fixes the Li + / Na +It forms a stable solvation structure with TEP molecules to fix the TEP solvent. Secondly, the high-valent metal ions in the solid electrolyte interact with TEP to produce an alkaline liquid environment. During the high-temperature drying process after the diaphragm is sprayed, PVDF is dehydrofluorinated and then cross-linked and gelled, reducing the crystallinity of PVDF to effectively accommodate the TMP solvent. In addition, PVDF contains a large amount of fluorine and is non-flammable. It will melt and absorb heat at high temperatures, and the melted PVDF will block the pores of the diaphragm, avoiding short circuits and thermal runaway. PVDF with poor crystallinity can fix solid electrolyte particles, enhance the tensile strength and puncture strength of the diaphragm, and thus effectively suppress lithium dendrites.

[0036] According to the present invention, the modified diaphragm provided by the present invention includes a base film; the base film can be a base film well known to those skilled in the art without any special restrictions, and is preferably a polypropylene (PP) film or a polyethylene (PE) film in the present invention.

[0037] According to the present invention, at least one surface of the base film is provided with a coating; the coating comprises a polymer, a phosphate flame retardant, a solid electrolyte, a wetting agent and a binder.

[0038] In a specific embodiment provided by the present invention, the polymer is a polymer well known to those skilled in the art and is not particularly limited. In the present invention, it is preferably one or more of vinylidene fluoride-chlorotrifluoroethylene copolymer (VDF-CTFE), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), ethylene-chlorotrifluoroethylene copolymer (ECTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE) and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA); the weight average molecular weight of the polymer is preferably 100,000 to 2 million, more preferably 500,000 to 1.5 million, and even more preferably 800,000 to 1 million.

[0039] In a specific embodiment provided by the present invention, the phosphate flame retardant is a liquid phosphate flame retardant well known to those skilled in the art, and is not particularly limited. In the present invention, it is preferably one or more of trimethyl phosphate (TMP), triethyl phosphate (TEP), tris(2,2,2-trifluoroethyl) phosphate (TFP), tris(1,3-dichloro-2-propyl) phosphate (TDP), diphenyl cresol phosphate (CDP) and diphenyl octyl phosphate (DPOF).

[0040] In a specific embodiment provided by the present invention, the coating is formed by a slurry; the phosphate flame retardant in the slurry also serves as a solvent, and the mass ratio of the polymer to the phosphate flame retardant in the slurry is preferably 1: (11-15), more preferably 1: (12-15), and even more preferably 1: (13-15); in some embodiments provided by the present invention, the mass ratio of the polymer to the phosphate flame retardant in the slurry is specifically 1:14, 1:13 or 1:15.

[0041] In a specific embodiment provided by the present invention, the solid electrolyte can be a solid electrolyte well known to those skilled in the art, without any special restrictions, and is preferably a lithium solid electrolyte or a sodium solid electrolyte in the present invention; the lithium solid electrolyte can be a lithium solid electrolyte well known to those skilled in the art, without any special restrictions, and is preferably one or more of lithium lanthanum zirconium oxide solid electrolyte (LLZO), lithium lanthanum titanium oxide solid electrolyte (LLTO) and lithium aluminum titanium phosphate solid electrolyte (LATP); the sodium solid electrolyte is selected from Na3AlF6, Na3ZrF6, Na2SiF6, Na 12 Al 12 Si 12 O 48 , NZSP and Na-β / β"-Al2O3; the particle size of the solid electrolyte is preferably 500-600 nm; the mass ratio of the polymer to the solid electrolyte is preferably 1:(3-5); in some embodiments provided by the present invention, the mass ratio of the polymer to the solid electrolyte is specifically 1:3, 1:4 or 1:5.

[0042] In a specific embodiment provided by the present invention, the wetting agent is a wetting agent well known to those skilled in the art and is not particularly limited. In the present invention, it is preferably one or more of acetylene glycol wetting agents and polyether siloxane wetting agents; the mass of the wetting agent is preferably 0.3wt% to 0.5wt% of the slurry mass.

[0043] In a specific embodiment provided by the present invention, the binder is a binder well known to those skilled in the art and is not particularly limited. In the present invention, it is preferably one or more of carboxymethyl acrylate, sodium carboxymethyl cellulose, polyacrylamide binders, polyvinyl alcohol binders and PVDF binders; the mass of the binder is preferably 3wt% to 5wt% of the mass of the solid electrolyte.

[0044] In a specific embodiment provided by the present invention, the thickness of the coating is preferably 1 to 5 μm, more preferably 1 to 4 μm, and even more preferably 2 to 3 μm.

[0045] In a specific embodiment provided by the present invention, coatings are provided on both surfaces of the base film; the thickness of the two coatings is preferably 1 to 5 μm, more preferably 1 to 4 μm, and even more preferably 2 to 3 μm.

[0046] The present invention constructs a diaphragm coating with good flame retardancy and excellent mechanical properties through the interaction between a solid electrolyte, a polymer and a phosphate flame retardant. The interaction between the three allows a large amount of phosphate flame retardant solvent to remain in the coating, avoiding the reduction of phosphate flame retardant molecules at the negative electrode to form a poor-quality SEI layer, while not affecting the release of the phosphate flame retardant at high temperatures to participate in flame retardancy. In addition, the interaction reduces the crystallinity of the polymer, and the coating with stronger adhesion is not easy to peel off in the battery system. It can also improve the tightness of the bonding with the positive and negative electrodes, prevent the diaphragm from wrinkling, and improve the consistency of the battery cell.

[0047] The present invention also provides a method for preparing the above-mentioned modified diaphragm with flame retardancy and strong mechanical properties, comprising the following steps: coating a slurry on at least one surface of a base film to obtain a modified diaphragm; the slurry comprises a polymer, a phosphate flame retardant, a solid electrolyte, a wetting agent and a binder.

[0048] The present invention has no special restrictions on the sources of all raw materials, and they can be commercially available. The types and proportions of the polymer, phosphate flame retardant, solid electrolyte, wetting agent and binder are the same as those described above and will not be repeated here.

[0049] In a specific embodiment provided by the present invention, the slurry is prepared according to the following method: S1) mixing a solid electrolyte with a portion of a phosphate flame retardant to obtain slurry A; mixing a polymer with the remaining phosphate flame retardant to obtain slurry B; S2) mixing the slurry A, slurry B, a wetting agent and a binder to obtain a slurry.

[0050] In a specific embodiment provided by the present invention, the solid content of the slurry A obtained by mixing the solid electrolyte and part of the phosphate flame retardant is preferably 50% to 60%.

[0051] In a specific embodiment provided by the present invention, the mass ratio of the polymer to the remaining phosphate flame retardant is preferably 1:(5-15), more preferably 1:(8-10).

[0052] In a specific embodiment provided by the present invention, the polymer and the remaining phosphate flame retardant are preferably mixed for 10 to 20 hours.

[0053] In a specific embodiment provided by the present invention, the slurry A and slurry B are mixed and then a wetting agent and a binder are added; the mass of the wetting agent is preferably 0.3wt% to 0.5wt% of the mass of the slurry; the mass of the binder is preferably 3wt% to 5wt% of the mass of the slurry.

[0054] The slurry is coated on at least one surface of the base film to obtain a modified diaphragm. The coating method is any method well known to those skilled in the art and is not particularly limited. In the present invention, spray coating is preferred.

[0055] The present invention also provides a secondary battery comprising the modified diaphragm having flame retardancy and strong mechanical properties, a positive electrode and a negative electrode.

[0056] In a specific embodiment provided by the present invention, the active material of the positive electrode is preferably NCM811.

[0057] In a specific embodiment provided by the present invention, the active material of the negative electrode is preferably graphite.

[0058] In order to further illustrate the present invention, the following describes in detail a modified diaphragm with flame retardancy and strong mechanical properties and a preparation method thereof in combination with embodiments.

[0059] The reagents used in the following examples are all commercially available; the PE base film used in the examples is a wet double-stretched 9 μm isolation membrane with a porosity of 44%; the styrene-butadiene rubber is an SBR emulsion for lithium batteries (purchased from JSR Corporation of Japan); the polyacrylamide binder used in the examples is purchased from Tianjin Saipurui New Energy Technology Co., Ltd., the polyether-modified polysiloxane wetting agent is purchased from Guangdong Sandingjia New Materials Technology Co., Ltd., model SJD9006, and sodium carboxymethyl cellulose is purchased from Sichuan Lanyang Daily Chemical Co., Ltd.

[0060] Example 1

[0061] 1) Preparation of composite PVDF coated diaphragm:

[0062] The LLZO solid electrolyte is dispersed in TEP by a disperser to obtain slurry A, in which the LLZO particle size is 500-600nm and the solid content is 50%. PVDF (molecular weight 1000000) is added to TEP to prepare slurry B, and the stirring time is 10h, wherein the mass ratio of PVDF to TEP is 1:10. The prepared slurry A and slurry B are mixed in a PVDF:LLZO mass ratio of 1:4, and then a binder (polyacrylamide binder) of 5wt% of the LLZO mass and a wetting agent (polyether modified polysiloxane wetting agent) of 0.3wt% of the total mass of the slurry are added. The prepared slurry is applied to the PE base film by spraying, and the coating thickness is controlled to 2μm, and double-sided coating is applied.

[0063] 2) Lithium-ion battery assembly:

[0064] NCM811 positive electrode active material (purchased from Ningbo Rongbai New Energy Technology Co., Ltd.), Super-p (purchased from Tianjin Huacai Chemical Co., Ltd.), and PVDF binder (HSV900 type from Arkema) were added to NMP in a weight ratio of 96:1.5:2.5 and mixed evenly (solid content 72%). The positive electrode slurry was evenly coated on both surfaces of the aluminum foil (single-side surface density of 15.8 mg / cm 2 ), further dried, rolled, and cut to obtain the positive electrode sheet. The negative electrode active material graphite, Super-p, binder styrene-butadiene rubber, and thickener sodium carboxymethyl cellulose were added to the solvent deionized water (solid content 54%) according to the weight ratio of 96.2:0.8:0.8:1.2, and the mixed negative electrode slurry was evenly coated on both surfaces of the copper foil (single surface density 10mg / cm 2 ), and then the negative electrode sheet is obtained through drying, rolling and cutting.

[0065] The diaphragm is dried at 60°C for 48 hours, and the positive electrode, diaphragm, and negative electrode are stacked in order to obtain a battery cell assembly. After packaging, the lithium ion electrolyte (LP30 electrolyte, with 5wt% fluoroethylene carbonate and 2wt% vinylene carbonate added) is injected. After packaging, standing, and forming, the battery cell is obtained. The rated capacity of the soft-pack battery cell is 1Ah and the injection volume is 5g. The battery cell formation procedure is: charge at a current of 0.1C for 2 hours, then charge at a current of 0.2C to a cut-off voltage of 4.2V, and continue constant voltage charging until the current drops to 0.05C. Maintain 45°C and 10Kg / cm during formation. 2 hot pressing conditions.

[0066] To ensure data credibility, three battery cells were assembled in parallel and numbered as Examples 1.1 to 1.3.

[0067] Comparative Example 1

[0068] Comparative Example 1 is the same as the preparation method of the lithium ion battery cell in Example 1, but the separator used is a PE-based film, and three battery cells are assembled in parallel and numbered as Comparative Examples 1.1 to 1.3.

[0069] Comparative Example 2

[0070] Comparative Example 2 has the same method for preparing the lithium-ion battery cell as that in Example 1, but the diaphragm used is a PE diaphragm coated only with a solid electrolyte. The diaphragm coating scheme is as follows: the LLZO solid electrolyte is dispersed in TEP by a disperser, wherein the LLZO particle size is 500-600nm and the solid content is 50wt%. After dispersion, a binder (polyacrylamide binder) of 5wt% of the mass of LLZO and a wetting agent (polyether-modified polysiloxane wetting agent) of 0.3wt% of the total mass of the slurry are added, and the mixture is applied on the PE base film by roller coating, and the coating thickness is controlled to be 2μm.

[0071] To ensure data credibility, three battery cells were assembled in parallel and numbered as comparative examples 2.1 to 2.3.

[0072] Comparative Example 3

[0073] The diaphragm was coated using only PVDF. The coating protocol was as follows: PVDF was added to TEP to form a slurry, which was stirred for 10 hours. The mass ratio of PVDF to TEP was 1:10. 5 wt% of sodium carboxymethyl cellulose and 0.3 wt% of a wetting agent (polyether-modified polysiloxane) were added to the slurry. The slurry was then sprayed onto a PE-based membrane to a coating thickness of 2 μm, with double-sided coating.

[0074] Example 2

[0075] The preparation method of the lithium-ion battery cell is the same as that of Example 1, and the diaphragm coating scheme used is as follows: the LLZO solid electrolyte is dispersed in TEP by a disperser to obtain slurry A, wherein the LLZO particle size is 500-600nm and the solid content is 50%. PVDF (molecular weight of 1000000) is added to TEP to prepare slurry B, and the stirring time is 10 hours, wherein the mass ratio of PVDF to TEP is 1:10. The prepared slurry A and slurry B are mixed in a PVDF:LLZO mass ratio of 1:3, and then a binder (polyacrylamide binder) of 5wt% of the LLZO mass and a wetting agent (polyether-modified polysiloxane wetting agent) of 0.3wt% of the total mass of the slurry are added. The prepared slurry is applied to the PE base film by spraying, and the coating thickness is controlled to 2μm, and double-sided coating is applied.

[0076] To ensure data credibility, three cells were assembled in parallel and numbered as Examples 2.1 to 2.3.

[0077] Example 3

[0078] The preparation method of the lithium-ion battery cell is the same as that of Example 1, and the diaphragm coating scheme used is as follows: the LLZO solid electrolyte is dispersed in TEP by a disperser to obtain slurry A, wherein the LLZO particle size is 500-600nm and the solid content is 50%. PVDF (molecular weight of 1000000) is added to TEP to prepare slurry B, and the stirring time is 10 hours, wherein the mass ratio of PVDF to TEP is 1:10. The prepared slurry A and slurry B are mixed in a PVDF:LLZO mass ratio of 1:5, and then a binder (polyacrylamide binder) of 5wt% of the LLZO mass and a wetting agent (polyether modified polysiloxane wetting agent) of 0.3wt% of the total mass of the slurry are added. The prepared slurry is applied to the PE base film by spraying, and the coating thickness is controlled to 2μm, and double-sided coating is applied.

[0079] To ensure data credibility, three battery cells were assembled in parallel and numbered as Examples 3.1 to 3.3.

[0080] Example 4

[0081] The preparation method of the lithium-ion battery cell is the same as that of Example 1. The polymer used for coating the diaphragm is PTFE, and the flame retardant solvent is TMP. The scheme is as follows: the LLZO solid electrolyte is dispersed in TMP by a disperser to obtain slurry A, wherein the LLZO particle size is 500-600nm and the solid content is 50%. PTFE (molecular weight of 1000000) is added to TMP to prepare slurry B, and the stirring time is 10h, wherein the mass ratio of PTFE to TMP is 1:10. The prepared slurry A and slurry B are mixed in a PTFE:LLZO mass ratio of 1:4, and then a binder (polyacrylamide binder) of 5wt% of the LLZO mass and a wetting agent (polyether modified polysiloxane wetting agent) of 0.3wt% of the total mass of the slurry are added. The prepared slurry is applied to the PE base film by spraying, and the coating thickness is controlled to be 2μm, and double-sided coating is applied.

[0082] To ensure data credibility, three battery cells were assembled in parallel and numbered as Examples 4.1 to 4.3.

[0083] Battery test instructions:

[0084] The peel strength of the separator is measured according to the method in standard T / CPPIA10-2021.

[0085] Soft-Pack Cell Hot Box Test: Conducted according to the method specified in GB / T31485, hot-pressed soft-pack cells are divided into smaller volumes and placed in a hot box. The temperature is gradually increased at a rate of 5°C / min and held at the set temperature for 30 minutes. During this period, the cells are observed for fire and voltage changes. For cells that do not catch fire or show a significant drop in voltage, the temperature is increased by 10°C after 30 minutes and held for 30 minutes, while further observation is made. The temperature of the first holding stage is 120°C.

[0086] Determination of battery capacity retention: At 25°C, charge the lithium-ion battery at a constant current of 0.2C to 4.2V, then charge at a constant voltage of 4.2V to a current of 0.05C. After 5 minutes, discharge at 0.2C to 2.7V. The measured discharge capacity is recorded as the initial capacity C0. Repeat these steps for the same battery and simultaneously record the battery's discharge capacity C100 after 100 cycles. The battery capacity retention rate after the cycle is 100% × C100 / C0.

[0087] The performance of the composite PVDF coated diaphragms obtained in Examples 1 to 3, the composite PTFE coated diaphragms obtained in Example 4, the coated diaphragms in Comparative Examples 1 to 3, and the lithium ion batteries obtained in Examples 1 to 4 and Comparative Examples 1 to 3 were tested, and the test results are shown in Table 1.

[0088] The PVDF and LLZO composite coating obtained in Example 1 was examined using a scanning electron microscope, and the SEM images were as follows: Figure 1 As shown, the regular small balls are PVDF particles. It can be found that the PVDF particles in some areas have been deformed and fused into one, indicating that the introduced LLZO can reduce the crystallinity of PVDF, thereby improving the liquid retention capacity of the diaphragm and improving the various mechanical properties of the diaphragm.

[0089] Table 1 Performance test results of diaphragm and lithium-ion battery

[0090]

[0091]

[0092] Compared with Comparative Example 1, the PVDF and LLZO composite coating applied in Example 1 reduces the air permeability of the diaphragm, but the reduction is small and will not significantly affect the performance of the battery cell. Compared with Comparative Example 1 using a pure PE diaphragm, the capacity retention rate after 100 cycles is slightly higher. Compared with the LLZO solid electrolyte layer coated in Comparative Example 2, PVDF can effectively bond inorganic particles and significantly improve the peel strength of the coating. Compared with Comparative Example 1, both Examples 1 and Comparative Example 2 containing solid electrolyte coatings exhibit excellent cycle stability, indicating that the lithium solid electrolyte coating can effectively improve the wettability and liquid retention rate of the electrolyte and maintain cycle stability. In addition, the precipitation of lithium ions in the solid electrolyte into the electrolyte can replenish lithium, thereby increasing the cycle capacity.

[0093] The hot box test examines the safety performance of the battery cells. There are two main reasons that can cause battery cell fires. First, the electrolyte vaporizes severely at high temperatures, and when it encounters oxidants, a strong chain reaction will occur, leading to thermal runaway. Second, the melting point of the PE separator is 130-135°C. High temperatures cause the separator to melt, and short-circuiting the positive and negative electrodes will release heat rapidly. The present invention can significantly improve the safety performance of the battery cell through the composite coating. The TEP in the coating increases the ignition temperature by 30°C through its flame retardant effect. In addition, the solid electrolyte particles remaining after the PE base film of the separator shown in Comparative Example 2 melts can prevent the positive and negative electrodes from short-circuiting, increasing the temperature of the battery cell fire and short circuit. However, after losing the support of the base film, the solid electrolyte particles cannot effectively isolate the positive and negative electrodes. By introducing high-temperature resistant PVDF, its integrity can be enhanced, increasing the temperature at which a short circuit occurs.

[0094] Example 4 shows that this method is also applicable to other similar materials and has a positive impact on battery safety and cycle stability.

[0095] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A modified diaphragm with flame retardancy and strong mechanical properties, characterized in that: include: A base film and a coating disposed on at least one surface of the base film; The coating comprises a polymer, a phosphate flame retardant, a solid electrolyte, a wetting agent and a binder; The polymer is selected from one or more of vinylidene fluoride-chlorotrifluoroethylene copolymer, polyvinyl fluoride, polyvinylidene fluoride, ethylene-chlorotrifluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer; The phosphate flame retardant is selected from one or more of trimethyl phosphate, triethyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, tris(1,3-dichloro-2-propyl) phosphate, diphenyl cresol phosphate and diphenyl monooctyl phosphate; The solid electrolyte is selected from a lithium solid electrolyte or a sodium solid electrolyte; The coating is formed by slurry; the mass ratio of the polymer to the phosphate flame retardant in the slurry is 1: (11-15).

2. The modified diaphragm according to claim 1, characterized in that The mass ratio of the polymer to the solid electrolyte is 1:(3-5).

3. The modified diaphragm according to claim 1, characterized in that The mass of the wetting agent is 0.3% to 0.5% of the mass of the slurry; The mass of the binder is 3% to 5% of the mass of the solid electrolyte.

4. The modified diaphragm according to claim 1, characterized in that The lithium solid electrolyte is selected from one or more of lithium lanthanum zirconium oxide solid electrolyte, lithium lanthanum titanium oxide solid electrolyte and lithium titanium aluminum phosphate solid electrolyte; The sodium solid electrolyte is selected from Na3AlF6, Na3ZrF6, Na2SiF6, Na 12 Al 12 Si 12 O 48 , one or more of NZSP and Na-β / β''-Al2O3; The wetting agent is one or more of an acetylenic diol wetting agent and a polyether siloxane wetting agent; The binder is selected from one or more of carboxymethyl acrylate, sodium carboxymethyl cellulose, polyacrylamide binders, polyvinyl alcohol binders, and PVDF binders.

5. The modified diaphragm according to claim 1, characterized in that The thickness of the coating is 1-5 μm.

6. The modified diaphragm according to claim 1, characterized in that The base film is selected from polypropylene film or polyethylene film; the particle size of the solid electrolyte is 500-600 nm.

7. A method for preparing a modified diaphragm having flame retardancy and strong mechanical properties according to any one of claims 1 to 6, characterized in that: The following steps are involved: coating the slurry on at least one surface of the base membrane to obtain a modified separator; The slurry includes a polymer, a phosphate flame retardant, a solid electrolyte, a wetting agent and a binder.

8. The preparation method according to claim 7, characterized in that The slurry was prepared according to the following method: S1) mixing the solid electrolyte with a portion of the phosphate flame retardant to obtain slurry A; mixing the polymer with the remaining phosphate flame retardant to obtain slurry B; S2) Mixing the slurry A, slurry B, a wetting agent and a binder to obtain a slurry.

9. The preparation method according to claim 8, characterized in that The solid content of the slurry A is 50% to 60%; the mass ratio of the polymer to the remaining phosphate flame retardant in the slurry B is 1:(8-10).

10. A secondary battery, characterized in that: It comprises the modified diaphragm with flame retardancy and strong mechanical properties as described in any one of claims 1 to 6, or the modified diaphragm with flame retardancy and strong mechanical properties prepared by the preparation method of any one of claims 7 to 9, a positive electrode and a negative electrode.

Images