A fire-retardant separator for energy storage batteries and a method of making the same

Abstract

This invention provides a flame-retardant separator for energy storage batteries and its preparation method. The separator comprises a corona-treated aramid-based film and a safety coating on its surface. The safety coating includes surface-aminated modified nanodiamonds, a triphenylmethane triisocyanate adhesive, and a core-shell composite flame-retardant material. The amino groups of the nanodiamonds are bonded to the epoxy groups of the flame-retardant material shell and the isocyanates of the adhesive. The isocyanates of the adhesive are also bonded to the hydroxyl groups on the surface of the corona-treated aramid-based film. The composite flame-retardant material comprises a polymer microsphere core formed by copolymerization of phosphorus- and nitrogen-containing monomers, and a shell formed by copolymerization of methyl methacrylate, an acid-sensitive acetal crosslinking monomer, and an epoxy-containing coupling monomer. The acetal crosslinks can break under acidic conditions. This invention constructs a covalently crosslinked thermally conductive network and an acid-sensitive triggered flame-retardant structure within the safety coating, effectively solving the problems of large thermal shrinkage, slow flame-retardant response, and poor interfacial adhesion in existing separators.

Description

Technical Field

[0001] This invention relates to the field of new energy storage battery technology, specifically to a flame-retardant separator for energy storage batteries and its preparation method. Background Technology

[0002] With the advancement of "dual carbon" goals and the continuous increase in renewable energy penetration, electrochemical energy storage systems are widely used in grid peak shaving, distributed energy dispatch, photovoltaic / wind power smoothing, industrial and commercial energy storage, and emergency backup power. Energy storage battery systems are often composed of a large number of individual battery cells connected in series and parallel, and are characterized by large capacity, rapid heat accumulation, long operating cycle, and complex environmental adaptability. Therefore, their safety issues are particularly prominent.

[0003] As a key functional material between the positive and negative electrodes inside a battery, the separator's role is to allow lithium ions to pass through while preventing direct contact between the positive and negative electrodes, thus avoiding short circuits. While traditional polyolefin separators such as polyethylene and polypropylene are low-cost and have mature film-forming processes, their heat resistance is limited. At higher temperatures, they are prone to thermal shrinkage, pore collapse, and even melting and perforation. Once the battery experiences abnormal conditions such as dendrite puncture, localized internal short circuits, overcharging, or external high temperatures, separator failure can rapidly exacerbate the thermal runaway process, potentially leading to fires, explosions, and other safety accidents.

[0004] To improve the thermal stability of polyolefin membranes, existing technologies typically employ modification methods such as ceramic coatings, aramid coatings, PVDF coatings, and composite coatings containing flame retardants. However, these approaches still have the following shortcomings: 1) Limited bonding strength between coating and base film: Many coatings rely on the physical adhesion of adhesives. After long-term immersion in electrolyte, interface peeling is prone to occur, especially under repeated hot and cold cycles or high-rate operation conditions, resulting in insufficient coating stability.

[0005] 2) There is a significant lag in the flame retardant response: Existing flame retardant particles often rely on thermal decomposition to release flame retardant components, but the heat in the early stage of a short circuit is often limited in the coating, and the flame retardant is difficult to play a rapid role in the initiation stage of thermal runaway, resulting in the flame retardant response being later than the thermal expansion.

[0006] 3) Limited improvement in mechanical puncture resistance: Many flame-retardant coatings mainly improve heat resistance, but fail to simultaneously enhance the puncture resistance and dendrite resistance of the separator, thus failing to meet the mechanical safety requirements of energy storage batteries under complex operating conditions.

[0007] 4) Flame retardancy and ion transport performance are difficult to balance: If the coating is too thick or the pore structure is too dense, although the flame retardancy will be improved, the ion transport resistance will be significantly increased, affecting the battery rate performance and cycle life.

[0008] Therefore, there is an urgent need to develop a flame-retardant separator for energy storage batteries that combines high-temperature dimensional stability, active flame-retardant response, excellent adhesion and puncture resistance, without significantly sacrificing ion transport performance, and its preparation method. Summary of the Invention

[0009] The purpose of this invention is to provide a flame-retardant separator for energy storage batteries and its preparation method, so as to solve the problems of large thermal shrinkage, slow flame-retardant response, poor interface adhesion and insufficient mechanical strength of existing separators.

[0010] This invention constructs a safety coating with a covalently cross-linked thermally conductive bridging structure and an acid-sensitive triggering structure on the surface of an aramid-based membrane treated with corona. This enables the membrane to simultaneously possess excellent thermal stability, rapid flame retardant response, high mechanical strength, and good interfacial adhesion, thereby effectively solving the problems of large thermal shrinkage, coating peeling, flame retardant lag, and insufficient strength of existing membranes.

[0011] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a flame-retardant separator for energy storage batteries, comprising an aramid-based film treated with corona and a safety coating disposed on at least one surface of the aramid-based film. The safety coating comprises surface-aminated modified nanodiamond, triphenylmethane triisocyanate adhesive, and core-shell structured composite flame-retardant material. The amino groups on the surface of the nanodiamond are covalently bonded to the epoxy groups on the shell of the core-shell composite flame retardant material and the isocyanates in the triphenylmethane triisocyanate adhesive, forming a three-dimensional cross-linked network with nanodiamond as a thermally conductive bridge inside the coating. The isocyanate group in the triphenylmethane triisocyanate adhesive is covalently bonded to the hydroxyl active groups on the surface of the corona-treated aramid film; The core of the core-shell structured composite flame retardant material comprises flame retardant polymer microspheres formed by copolymerization of phosphorus-containing monomers and nitrogen-containing monomers, and the shell is formed by copolymerization of methyl methacrylate, acid-sensitive responsive monomers containing acetal crosslinks and epoxy group coupling monomers, and the acetal crosslinks of the shell can be broken in an acidic environment. The safety coating forms a covalently cross-linked thermally conductive network and an acid-sensitive triggering structure of a base film, triphenylmethane triisocyanate, aminated nanodiamond, and composite flame-retardant particles.

[0012] Preferably, the aramid-based film has a thickness of 5–20 μm, and after corona treatment, its surface hydroxyl content is 0.5–3.0 wt%, surface roughness Ra is 0.05–0.20 μm, and surface wetting tension is 35–50 dyn / cm. By controlling the thickness of the aramid-based film, additional internal resistance is reduced while ensuring mechanical support; the introduction of polar groups such as hydroxyl groups through corona treatment significantly enhances the chemical bonding ability and wettability between the coating and the base film; and by appropriately increasing the surface roughness, the coating's spreadability and anchoring ability are improved. These three factors work synergistically to effectively suppress coating delamination, edge lifting, and peeling during use.

[0013] Preferably, the safety coating thickness is 2–8 μm, and the mass percentages of surface-aminated nanodiamonds, composite flame retardant materials, and triphenylmethane triisocyanate adhesive (TTI) in the safety coating are 20–40 wt%, 40–60 wt%, and 10–20 wt%, respectively. When the coating thickness is controlled within this range, a continuous and effective safety functional layer can be formed without excessively clogging the pores and significantly increasing ion transport resistance. The above component ratios allow the nanodiamonds to act as thermal bridges and enhance the coating, the composite flame retardant materials to provide the main flame retardant reserves, and the TTI to provide sufficient crosslinking and adhesion. The balance among these three components ensures both safety and electrochemical performance.

[0014] Preferably, the average particle size of the nanodiamond is 10–30 nm. Nanodiamonds within this particle size range have a high specific surface area, providing more chemical anchoring points, while avoiding surface roughness and pore structure damage caused by excessively large particle sizes; at the same time, they are conducive to forming continuous thermal conductive pathways in the safety coating, improving local heat diffusion capabilities.

[0015] Preferably, the average particle size of the composite flame retardant material is 100–300 nm, and the shell thickness is 10–50 nm. This particle size range is beneficial for the uniform dispersion of particles in the coating, which can ensure the loading of flame retardant components and prevent excessively large particles from causing defects on the coating surface. The shell thickness is controlled within a reasonable range, which can ensure stability under normal storage and working conditions, and can also break down rapidly under acidic conditions to achieve early flame retardant response.

[0016] Preferably, the phosphorus-containing monomer is selected from one or more of diethyl methacrylate and diethyl allyl phosphate, and the nitrogen-containing monomer is selected from one or more of acrylamide and N-vinylpyrrolidone. The phosphorus-containing monomer can promote char formation in the condensed phase, while the nitrogen-containing monomer can release inert gases in the gas phase and inhibit combustion free radicals. The synergistic effect of phosphorus and nitrogen can improve flame retardant efficiency and enhance char density.

[0017] Preferably, the acid-responsive monomer is methoxy polyethylene glycol-acetal or acrylate-polyethylene glycol-polyacetal. The acetal structure is acid-sensitive and can rapidly break down in locally acidic environments, causing the shell to depolymerize and release the core flame-retardant component. The polyethylene glycol segments also improve particle dispersibility and coating flexibility.

[0018] Preferably, the epoxy group coupling monomer is glycidyl methacrylate (GMA). GMA can introduce epoxy active sites into the particle shell, and undergo a ring-opening reaction with the amino groups on the aminated nanodiamonds, thereby constructing a stable chemical bridge and enhancing the internal structural stability and thermomechanical properties of the coating.

[0019] Preferably, the aminated nanodiamond is prepared by coupling and modifying nanodiamond with 3-aminopropyltriethoxysilane (APTES). After coupling with APTES, reactive amino groups are introduced onto the surface of the nanodiamond, enabling it to undergo dual reactions with TTI isocyanate and GMA epoxy groups, thereby improving the filler dispersibility, interfacial bonding, and thermal network continuity.

[0020] Secondly, the present invention provides a method for preparing the above-mentioned flame-retardant separator for energy storage batteries, comprising the following steps: 1) The aramid-based membrane is subjected to corona treatment, with the corona power controlled at 2-3 kW and the treatment speed at 10-20 m / min, so that hydroxyl active groups are generated on the surface of the aramid-based membrane. 2) Preparation of surface-aminated modified nanodiamonds: Nanodiamonds were oxidized in a mixture of concentrated sulfuric acid / concentrated nitric acid, washed, and then reacted with 3-aminopropyltriethoxysilane in anhydrous ethanol to obtain aminated nanodiamonds. 3) Core-shell composite flame retardant materials containing a core and a shell were prepared by emulsion polymerization. The core is a polymer nanosphere formed by copolymerization of phosphorus-containing monomers and nitrogen-containing monomers, and the shell is formed by copolymerization of methyl methacrylate, acid-sensitive acetal monomers and epoxy group coupling monomers. 4) Mix the aminated nanodiamonds obtained in step 2), the composite flame retardant material obtained in step 3), and the triphenylmethane triisocyanate adhesive in a mass ratio of 20-40:40-60:10-20 to prepare a safety coating slurry; 5) The safety coating slurry is uniformly coated on the surface of the aramid-based membrane treated in step 1), and covalent cross-linking and curing are completed by gradient temperature drying to obtain a flame-retardant membrane.

[0021] Preferably, in step 3), when preparing the core-shell structured composite flame retardant material, the core emulsion polymerization temperature is 60-80°C, and the shell secondary polymerization temperature is 60-80°C.

[0022] Preferably, the gradient temperature drying in step 5) includes pre-drying at 60°C for 3-5 min, mid-stage drying at 80°C for 3-5 min, and final drying at 90°C for 3-8 min.

[0023] Preferably, the solid content of the safety coating slurry is 20-30 wt%.

[0024] Preferably, the coating method is any one of blade coating, roller coating, microgravure coating, or slot die coating.

[0025] Thirdly, the present invention provides a lithium-ion energy storage battery comprising the flame-retardant separator described above for energy storage batteries.

[0026] Compared with the prior art, the present invention has at least the following beneficial effects: 1) This invention activates the surface of the aramid-based film through corona treatment, thereby increasing its surface energy and reactivity. This enables the TTI adhesive to form a stable covalent bond with the base film, improving the adhesion of the coating and preventing interfacial delamination during electrolyte immersion and cycling.

[0027] 2) This invention uses the ring-opening reaction of amino-epoxy to covalently bond ultra-high thermal conductivity nanodiamond to the shell surface of flame-retardant microspheres; when a local short circuit occurs in the battery and generates extremely high heat, the heat is introduced into the flame-retardant microspheres in the form of phonon heat transfer without loss and at extremely high speed through the nanodiamond, greatly shortening the thermal response time and instantly triggering shell rupture and release of flame-retardant particles, thus nipping thermal runaway in the bud.

[0028] 3) This invention constructs a network of multiple hydrogen bonds and covalent bonds composed of hydroxyl groups on the surface of an aramid-based membrane, triphenylmethane triisocyanate adhesive, and aminated diamond. During normal ion transport, the network maintains good flexibility and porosity. When it encounters puncture by a sharp object or high-speed growth of hard lithium dendrites, the high shear force triggers the instantaneous clustering of nanodiamond particles and the rigid transformation of dynamic hydrogen bonds. The coating instantly exhibits rigid resistance like a "liquid bulletproof vest", and the puncture strength is significantly improved compared with traditional coated diaphragms.

[0029] 4) This invention introduces an acid-sensitive crosslinking network containing acetal bonds into the shell layer. This network has extremely high thermal stability in normal baking environment and normal battery cycle. Once the battery experiences early thermal runaway (accompanied by the decomposition of lithium salt and the generation of trace amounts of HF acid gas), the specific acidic environment will act as a "chemical switch" to rapidly catalyze the breaking and depolymerization of acetal bonds, so that the microspheres release flame retardant immediately before reaching the extreme high temperature, realizing acid-thermal coupling to trigger flame retardancy.

[0030] 5) By rationally designing the material particle size, coating thickness and component ratio, this invention improves thermal safety and mechanical safety without significantly increasing ion transport resistance, which is beneficial to maintaining the rate performance and cycle life of the battery. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the diaphragm structure in one embodiment of the present invention.

[0032] In the diagram: 1. Aramid base film; 2. Safety coating. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] like Figure 1 As shown, the present application provides a flame-retardant separator for energy storage batteries, including an aramid-based membrane 1 treated with corona and a safety coating 2 disposed on at least one surface of the aramid-based membrane 1. Safety coating 2 includes surface-aminated modified nanodiamonds, triphenylmethane triisocyanate adhesive (TTI), and core-shell structured composite flame retardant material; Among them, the amino groups on the surface of nanodiamonds are covalently bonded to the epoxy groups on the shell of the core-shell structured composite flame retardant material and the isocyanates in the triphenylmethane triisocyanate adhesive, forming a three-dimensional cross-linked network with nanodiamonds as a thermally conductive bridge inside the coating. In triphenylmethane triisocyanate adhesives, isocyanate groups are covalently bonded to the hydroxyl active groups on the surface of corona-treated aramid film; The core of the core-shell structured composite flame retardant material consists of flame retardant polymer microspheres formed by copolymerization of phosphorus-containing monomers and nitrogen-containing monomers, and the shell is formed by copolymerization of methyl methacrylate, acid-sensitive responsive monomers containing acetal crosslinks and epoxy group coupling monomers, and the acetal crosslinks of the shell can be broken in an acidic environment. A covalently cross-linked thermally conductive network and acid-sensitive triggering structure of a safety coating base film-triphenylmethane triisocyanate-aminated nanodiamond-composite flame-retardant particles.

[0035] In this invention, an aramid-based membrane is preferably used as the separator skeleton. Aramid materials have the characteristics of high molecular chain rigidity, high thermal decomposition temperature, good mechanical strength, and good dimensional stability, making them suitable for energy storage battery applications under high temperature and complex operating conditions.

[0036] Optional aramid-based membranes include meta-aramid membranes, para-aramid membranes, aramid paper-like porous membranes, or composite membranes containing an aramid fiber network. The thickness of the aramid-based membrane is preferably 5–20 μm, more preferably 8–15 μm. If the thickness is too small, the mechanical support capacity decreases; if the thickness is too large, it increases the battery's internal resistance, affecting energy density and rate performance.

[0037] To enhance the bonding between the coating and the aramid base film, the base film needs to be corona treated. The main functions of corona treatment include: 1) breaking down the surface inert layer and increasing surface polarity; 2) introducing active groups such as hydroxyl, carboxyl, and peroxy groups onto the surface; 3) increasing surface roughness and surface energy; and 4) improving the wetting and spreading of subsequent coating slurries.

[0038] Preferably, the corona treatment conditions are: power 2-3 kW, treatment speed 10-20 m / min, and single-sided or double-sided treatment can be used, with double-sided treatment being preferred. After treatment, the surface wetting tension of the base film preferably reaches 35-50 dyn / cm, and the surface activity is significantly enhanced.

[0039] Nanodiamonds are used to construct thermally conductive bridging structures in coatings, while also providing micro / nano-level reinforcement and puncture resistance. The preferred particle size is 10–30 nm. Excessively large particle sizes tend to agglomerate, while excessively small particle sizes result in excessively high specific surface areas, which can lead to increased viscosity and greater difficulty in dispersion of the slurry system.

[0040] To enhance the chemical reactivity of nanodiamonds with other components, this invention preferably involves surface amination modification, the specific steps of which are as follows: 1) Oxidative activation: Add nanodiamonds to a mixture of concentrated sulfuric acid / concentrated nitric acid and reflux at 80-120°C for 1-8 h to remove impurities and introduce oxygen-containing functional groups such as carboxyl and hydroxyl groups on the surface; 2) Washing and neutralization: After the reaction is complete, the nanodiamonds are repeatedly centrifuged and washed until the pH of the supernatant is close to neutral; 3) Amino coupling: Disperse the oxidized nanodiamonds in anhydrous ethanol, add APTES, and stir at 60-90℃ for 2-12 h to graft amino groups onto its surface. 4) Drying treatment: After the reaction, the sample is filtered, washed and vacuum dried to obtain surface-aminated nanodiamonds.

[0041] After amination, the amino groups on the surface of nanodiamonds can undergo addition reactions with TTI isocyanates, and can also undergo ring-opening reactions with epoxy groups in the shell of core-shell particles, thereby significantly enhancing interfacial bonding.

[0042] The core-shell structured composite flame-retardant material of the present invention is the core functional particle in the safety coating. Its structural design includes both a "reserve flame-retardant core" and a "responsive shell", which can remain stable under normal conditions and quickly activate flame retardancy under abnormal hot / acid environments.

[0043] The core is formed by copolymerization of phosphorus-containing monomers and nitrogen-containing monomers. The phosphorus-containing monomers preferably include diethyl methacrylate, diethyl allyl phosphate, or combinations thereof; the nitrogen-containing monomers preferably include acrylamide, N-vinylpyrrolidone, methacrylamide, or combinations thereof.

[0044] The phosphorus and nitrogen synergistic flame retardant system has the following advantages: 1) it promotes dehydration and char formation when heated; 2) it inhibits free radical chain combustion reaction; 3) it plays a flame retardant role when the condensed phase and gas phase are in the same state; 4) the char layer is dense, which helps to isolate heat and combustible gases.

[0045] The shell is preferably copolymerized from methyl methacrylate (MMA), an acid-sensitive monomer containing acetal crosslinks, and an epoxy-containing coupling monomer. Specifically: MMA is used to improve the film-forming properties and structural stability of the particles; the acid-sensitive monomer containing acetal crosslinks can break under acidic conditions, transforming the shell from a stable structure to a depolymerizable structure; the epoxy-containing coupling monomer is preferably glycidyl methacrylate (GMA), whose epoxy groups can undergo a ring-opening reaction with the amino groups of the aminated nanodiamonds.

[0046] The shell thickness is preferably 10–50 nm. If the shell is too thin, the acid sensitivity response control capability will be insufficient; if it is too thick, it will affect the release efficiency of flame retardant substances and the rheological properties of the coating.

[0047] The particle size of the core-shell composite flame retardant material is preferably controlled within the range of 100–300 nm. If the particle size is too large, it is easy to form uneven large particles in the coating, affecting the smoothness; if the particle size is too small, it may cause an increase in the number of interfaces, increase the viscosity of the slurry, and affect the storage capacity of the flame retardant.

[0048] The preparation method of the core-shell structured composite flame-retardant material is as follows: a) Disperse phosphorus-containing monomers and nitrogen-containing monomers in deionized water and emulsifier, add an initiator, and carry out emulsion polymerization at 60-80℃ to obtain a kernel seed emulsion; b) Methyl methacrylate, an acid-sensitive monomer containing acetal crosslinks, an epoxy group-containing coupling monomer, and an initiator are added to the kernel seed emulsion, and a secondary emulsion polymerization is carried out at 60-80°C to form a shell layer. c) Cool, wash, centrifuge, and dry the product to obtain the composite flame retardant material.

[0049] The emulsifier is selected from at least one of sodium dodecyl sulfate, nonylphenol polyoxyethylene ether, and sodium dodecylbenzene sulfonate; the initiator is selected from at least one of ammonium persulfate, potassium persulfate, and azobisisobutyronitrile.

[0050] This invention preferably uses triphenylmethane triisocyanate (TTI) as an adhesive and interfacial crosslinking agent. TTI has the characteristics of stable aliphatic / aromatic composite skeleton, moderate reactivity, and high heat resistance after curing. It can form chemical bonds with a variety of materials containing active hydrogen / epoxy groups.

[0051] In this invention, TTI plays a triple role: 1) reacts with the hydroxyl groups on the surface of the corona-treated aramid film to form a strong interface; 2) reacts with the amino groups on the surface of the aminated nanodiamond to improve the filler fixation; 3) participates in the internal cross-linking of the coating to enhance the overall mechanical properties and swelling resistance.

[0052] Therefore, TTI is not just a traditional adhesive, but an important chemical crosslinking unit for building the entire safety coating framework.

[0053] The safety coating slurry of the present invention is preferably prepared using an anhydrous system to avoid premature hydrolysis and deactivation of TTI. The preferred dispersion medium may be anhydrous NMP, anhydrous ethanol, or a mixture thereof.

[0054] Preferably, the components in the coating slurry are in the following dry weight ratios: 20-40 wt% aminated nanodiamond; 40-60 wt% core-shell structured composite flame retardant material; and 10-20 wt% TTI adhesive.

[0055] The preparation steps of this safety coating slurry are as follows: 1) Add the dispersion medium to the stirred tank; 2) Slowly add aminated nanodiamonds and disperse using ultrasonic dispersion or high-speed shear dispersion; 3) Add core-shell structured composite flame retardant material and continue stirring to ensure uniform particle distribution; 4) Finally, add TTI adhesive and control the system temperature below 40℃ to avoid premature curing; 5) Continue stirring until a uniform and stable slurry is formed; 6) Adjust the solid content to 20-30 wt% to meet the coating requirements.

[0056] The slurry should have appropriate viscosity and thixotropy to ensure a uniform, continuous coating that is not prone to sagging.

[0057] The prepared safety coating slurry is applied to at least one side of the corona-treated aramid-based film, preferably to both sides simultaneously or sequentially.

[0058] The coating method can be blade coating, micro-gravure coating, slit die coating or roller coating, among which slit die coating is more suitable for achieving uniform thickness control.

[0059] Coating thickness: The wet film thickness is preferably 2–8 μm; the dry film thickness is preferably 1–5 μm. If the coating is too thick, it will increase the interface resistance; if it is too thin, the safety function will be insufficient.

[0060] The drying and curing process adopts a gradient drying method: 1) Pre-drying at 60℃ for 3-5 min, mainly to remove surface solvent; 2) Mid-stage drying at 80℃ for 3-5 min, to promote the escape of internal solvent; 3) Final drying at 90℃ for 3-8 min, so that TTI can fully react with the active groups and complete cross-linking.

[0061] The cured coating forms a continuous cross-linked network, including: active sites on the surface of the aramid film; TTI intermediate bridging; aminated nanodiamond framework; and core-shell flame-retardant particle synergistic anchoring structure.

[0062] The safety performance of the flame-retardant diaphragm of the present invention is derived from the following synergistic mechanism: 1) Interfacial chemical anchoring: After corona treatment, active sites such as hydroxyl groups are generated on the surface of the aramid base film. TTI isocyanate reacts with them to form stable chemical bonds, which makes the coating firmly adhere to the base film surface and not easy to fall off even under electrolyte immersion, high temperature cycling and mechanical friction.

[0063] 2) Thermal bridging and thermal diffusion: Aminated nanodiamonds have high thermal conductivity and a rigid framework, which can form a heat conduction path inside the coating, avoid excessive local heat accumulation, and allow abnormal heat to be transferred to the composite flame retardant particles more quickly, thereby activating the flame retardant mechanism in advance.

[0064] 3) Acid-sensitive triggering release: During battery thermal runaway or severe overheating, the decomposition of electrolyte and electrode materials may produce acidic substances (such as HF). At this time, the acetal bonds in the shell of the composite flame-retardant particles preferentially break under acidic conditions, the shell structure is destroyed, and the phosphorus and nitrogen flame-retardant components in the core are released, which inhibit combustion in both the gas phase and condensation phase.

[0065] 4) Mechanical reinforcement and puncture resistance: The aramid base film itself has a high modulus, and the nanodiamond further improves the coating rigidity and local load-bearing capacity. Therefore, the diaphragm of this invention has a stronger blocking ability when facing needles, dendrites or electrode burrs, which can effectively reduce the risk of internal short circuit.

[0066] The present invention will be further described below with reference to specific embodiments. Example 1

[0067] A meta-aramid film with a thickness of 10 μm was selected as the base film; The base film was subjected to double-sided corona treatment with a corona power of 2.5 kW and a treatment speed of 15 m / min. The surface wetting tension of the corona-treated aramid base film reached 45 dyn / cm, and the surface roughness Ra was 0.12 μm. Nanodiamonds (average particle size 20 nm) were oxidized by a mixture of concentrated sulfuric acid and concentrated nitric acid, and then modified by amination in anhydrous ethanol using 3-aminopropyltriethoxysilane (APTES). Core-shell composite flame-retardant materials were prepared by emulsion polymerization, wherein the core was formed by copolymerization of diethyl methacrylate and acrylamide, and the shell was formed by copolymerization of methyl methacrylate (MMA), glycidyl methacrylate (GMA) and acid-sensitive monomer (methoxy polyethylene glycol-acetal). Aminated nanodiamonds, core-shell structured composite flame retardant materials and triphenylmethane triisocyanate adhesive (TTI) were mixed in a mass ratio of 30:55:15 to prepare a safety coating slurry with a solid content of 25 wt%. The slurry was applied to both sides of the aramid base film using a slit die, and the wet coating thickness was about 6μm. The flame-retardant diaphragm was obtained by sequentially drying at gradient temperatures of 60℃, 80℃, and 90℃ for a total of 10 minutes. Example 2

[0068] It is basically the same as Example 1, except that: The aramid film thickness is 15μm; The acid-sensitive monomers of the core-shell particles are replaced with acrylate-polyethylene glycol-polyacetal; The mass ratio of aminated nanodiamond, core-shell material and TTI is 25:60:15. Example 3

[0069] It is basically the same as Example 1, except that: Para-aramid membrane is used; The coating is applied to only one side; The dry film thickness of the coating is 2μm. Comparative Example 1

[0070] Same as in Example 1, but the nanodiamonds were not amination treated, but only physically dispersed. Comparative Example 2

[0071] Same as in Example 1, but the aramid-based film was not subjected to corona treatment. Comparative Example 3

[0072] Same as Example 1, but TTI is replaced with conventional PVDF adhesive. Comparative Example 4

[0073] Same as in Example 1, but the composite flame retardant material shell does not contain acid-sensitive acetal monomers. Comparative Example 5

[0074] Same as in Example 1, except that the amination nanodiamonds are replaced with alumina ceramic particles.

[0075] The membranes prepared in the above embodiments and comparative examples were subjected to the following performance tests, and the test results are shown in Table 1.

[0076] 1. Heat shrinkage test: Place the sample in an oven at 130℃ and 150℃ for 30 min each, measure the dimensional changes before and after, and calculate the heat shrinkage rate; 2. Peel strength test: The bonding strength between the safety coating and the aramid base film was tested using the 180° peel method, and the peel force per unit width was recorded; 3. Thermal response test: Place the diaphragm sample on a 200℃ hot plate or local heat source and record the time when the flame-retardant particle shell begins to rupture, shrink or release flame-retardant components; 4. Needle penetration test: Assemble the separator into a button cell, and use a steel needle with a diameter of 3 mm to penetrate the center of the battery at a speed of 50 mm / s. Record whether it catches fire, smokes, and the temperature rise. 5. Battery cycle test: Assemble the separator into a button cell and conduct a cycle test in a high-temperature environment of 60°C, recording the capacity retention rate and internal resistance changes.

[0077] Table 1

[0078]

[0079] The test results in Table 1 above show that: 1) Comparing Example 1 and Comparative Example 1, it can be seen that the nanodiamonds in Comparative Example 1, without amination modification, are difficult to form a stable chemical anchor with TTI and the core-shell particle shell. Therefore, the internal thermal conductivity network of the coating is discontinuous, resulting in decreased peel strength, prolonged thermal response time, and increased thermal shrinkage. This indicates that amination modification of nanodiamonds is an important prerequisite for achieving high adhesion and rapid flame retardant response.

[0080] 2) Comparing Example 1 and Comparative Example 2, it can be seen that: Comparative Example 2 did not undergo corona treatment, and the surface of the aramid base film lacked sufficient polar active groups. TTI could not fully covalently bond with the base film, resulting in a significant decrease in coating adhesion, and a marked deterioration in thermal shrinkage rate and battery cycle retention rate. This indicates that corona activation is a key step in building a stable interface.

[0081] 3) Comparing Example 1 and Comparative Example 3, it can be seen that although PVDF replaced TTI as the binder in Comparative Example 3 has a certain bonding effect, its reactive crosslinking ability is insufficient. It cannot build multi-point covalent connections between the base film, nanodiamonds, and core-shell particles, resulting in reduced peel strength, slower thermal response, and decreased capacity retention. This indicates that TTI is not only a binder in this invention, but also a crosslinking core.

[0082] 4) Comparing Example 1 and Comparative Example 4, it can be seen that Comparative Example 4 lacks an acid-sensitive acetal structure. Although the composite flame-retardant material can still function at high temperatures, it cannot break down and release the core flame-retardant component in an acidic environment in advance. Therefore, the thermal response time is prolonged, and the flame-retardant effect is delayed. This indicates that the acid-sensitive triggering shell has a significant effect on improving the early response speed of thermal runaway.

[0083] 5) Comparing Example 1 and Comparative Example 5, it can be seen that: Comparative Example 5 uses alumina ceramic particles instead of aminated nanodiamonds. Although it has a certain inorganic reinforcing effect, its thermal bridging ability, interfacial chemical bonding ability, and network continuity are inferior to those of aminated nanodiamonds. Therefore, the thermal accumulation phenomenon is more obvious, and the thermal shrinkage rate and capacity retention rate are lower than those of Example 1. This shows that nanodiamonds have multiple functions in this invention, including thermal conductivity, reinforcement, and interfacial anchoring.

[0084] 6) Comparing Examples 1-3, it can be seen that Example 2, due to its thicker aramid film and superior acid-sensitive monomer design, has slightly better heat shrinkage rate and thermal response time than Example 1. Example 3, using single-sided coating and para-aramid film, has better mechanical strength, but insufficient double-sided flame-retardant coverage, thus it is slightly weaker than Examples 1 and 2 in terms of heat diffusion suppression and smoke control. This indicates that film thickness, coating method, and type of acid-sensitive monomer all affect the final performance in this invention and require comprehensive optimization.

[0085] In summary, this invention constructs an energy storage battery flame-retardant separator with high thermal stability, rapid flame-retardant response, excellent adhesion, and mechanical reinforcement through an overall structural design that combines corona-activated aramid-based film, amino-modified nanodiamond thermal bridging, TTI chemical crosslinking, and core-shell composite flame-retardant particle acid-sensitive triggering.

[0086] This invention not only maintains good ion transport and structural stability under normal operating conditions, but also rapidly exerts a flame-retardant effect in the early stages of local overheating, short circuits, or thermal runaway, effectively reducing the risk of thermal runaway propagation, and is suitable for high-safety-level energy storage battery systems.

[0087] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0088] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A fire-retardant separator for an energy storage battery, characterized by, The flame-retardant membrane includes a corona-treated aramid base film and a safety coating disposed on at least one surface of the aramid base film; The safety coating comprises surface-aminated modified nanodiamond, triphenylmethane triisocyanate adhesive, and core-shell structured composite flame retardant material; The amino groups of the nanodiamond are covalently bonded to the epoxy groups on the shell of the core-shell composite flame retardant material and the isocyanates in the triphenylmethane triisocyanate adhesive. The isocyanate group of the triphenylmethane triisocyanate adhesive is covalently bonded to the hydroxyl active groups on the surface of the corona-treated aramid film. The core-shell structured composite flame retardant material includes a core and a shell layer covering the surface of the core. The core includes flame retardant polymer microspheres formed by copolymerization of phosphorus-containing monomers and nitrogen-containing monomers. The shell layer is formed by copolymerization of methyl methacrylate, acid-sensitive responsive monomers containing acetal crosslinks, and epoxy-containing coupling monomers. The acetal crosslinks of the shell layer can be broken in an acidic environment. The safety coating forms a covalently cross-linked thermally conductive network and an acid-sensitive triggering structure inside.

2. The flame retardant separator of claim 1, wherein The aramid-based film has a thickness of 5–20 μm, and after corona treatment, its surface hydroxyl content is 0.5–3.0 wt%, its surface roughness Ra is 0.05–0.20 μm, and its surface wetting tension is 35–50 dyn / cm.

3. The flame retardant separator according to claim 1 or 2, characterized in that, The safety coating has a thickness of 2–8 μm, and the mass percentages of surface-aminated nanodiamond, composite flame retardant material, and triphenylmethane triisocyanate adhesive in the safety coating are 20–40 wt%, 40–60 wt%, and 10–20 wt%, respectively.

4. The flame-retardant diaphragm according to claim 1, characterized in that, The average particle size of the nanodiamond is 10–30 nm.

5. The flame-retardant diaphragm according to claim 1, characterized in that, The composite flame retardant material has an average particle size of 100–300 nm and a shell thickness of 10–50 nm.

6. The flame-retardant diaphragm according to claim 1, characterized in that, The phosphorus-containing monomer is selected from one or more of diethyl methacrylate and diethyl allyl phosphate, and the nitrogen-containing monomer is selected from one or more of acrylamide and N-vinylpyrrolidone.

7. The flame-retardant diaphragm according to claim 1, characterized in that, The acid-sensitive monomer is methoxy polyethylene glycol-acetal or acrylate-polyethylene glycol-polyacetal.

8. The flame-retardant diaphragm according to claim 1, characterized in that, The epoxy group coupling monomer is glycidyl methacrylate.

9. The flame-retardant diaphragm according to any one of claims 1 to 8, characterized in that, The aminated nanodiamonds were prepared by coupling and modifying nanodiamonds with 3-aminopropyltriethoxysilane.

10. A method for preparing a flame-retardant separator for an energy storage battery according to any one of claims 1 to 9, characterized in that, Includes the following steps: 1) The aramid-based membrane is subjected to corona treatment, with the corona power controlled at 2-3 kW and the treatment speed at 10-20 m / min, so that hydroxyl active groups are generated on the surface of the aramid-based membrane. 2) Preparation of surface-aminated modified nanodiamonds: Nanodiamonds were oxidized in a mixture of concentrated sulfuric acid and concentrated nitric acid, washed, and then reacted with 3-aminopropyltriethoxysilane in anhydrous ethanol to obtain aminated nanodiamonds. 3) Core-shell composite flame retardant materials containing a core and a shell were prepared by emulsion polymerization. The core is a polymer nanosphere formed by copolymerization of phosphorus-containing monomers and nitrogen-containing monomers, and the shell is formed by copolymerization of methyl methacrylate, acetal-sensitive monomers and epoxy group coupling monomers. 4) Mix the aminated nanodiamonds obtained in step 2), the composite flame retardant material obtained in step 3), and the triphenylmethane triisocyanate adhesive in a mass ratio of 20-40:40-60:10-20 to prepare a safety coating slurry; 5) The safety coating slurry is uniformly coated on the surface of the aramid-based membrane treated in step 1), and covalent cross-linking and curing are completed by gradient temperature drying to obtain a flame-retardant membrane.

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