A flame-retardant modification method for high-safety lithium batteries

Abstract

This invention discloses a flame-retardant modification method for high-safety lithium batteries. The method includes: performing low-temperature plasma surface activation treatment on a polyolefin separator; preparing a phosphorus-nitrogen-silicon composite flame-retardant sol and coating it onto the activated separator surface; forming a dense and flexible flame-retardant composite coating using a gradient temperature curing process; and assembling and encapsulating the modified separator with positive and negative electrodes and a flame-retardant modified electrolyte. This invention employs a phosphorus-nitrogen-silicon ternary synergistic flame-retardant mechanism combined with electrolyte synergistic modification to construct a solid-liquid dual flame-retardant protection system. Through plasma activation and gradient curing, a strong bond is achieved between the coating and the substrate. This significantly improves the flame-retardant safety performance of lithium batteries while effectively ensuring the battery's electrochemical performance. The process is simple and suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a method for flame-retardant modification of high-safety lithium batteries. Background Technology

[0002] With the rapid development of new energy vehicles, energy storage power stations, and consumer electronics, the market demand for lithium batteries as core energy storage devices continues to grow. However, lithium batteries are prone to thermal runaway under abuse conditions such as overcharging, overheating, and puncture, which can lead to combustion or even explosion. Safety issues have become a key bottleneck restricting the further development of the lithium battery industry.

[0003] Traditional lithium-ion batteries often require high amounts of flame retardants in the electrolyte, significantly reducing electrolyte ionic conductivity and deteriorating battery rate performance and cycle life. Conventional separator coating flame retardant technologies generally suffer from weak adhesion between the coating and the polyolefin substrate, easy detachment and cracking, and high internal resistance. Furthermore, they often employ a single flame retardant mechanism, making it difficult to achieve effective protection under critical thermal runaway conditions. Therefore, developing a simple flame retardant modification method for lithium-ion batteries that combines high flame retardancy with excellent electrochemical performance has become a pressing technical problem in this field. Summary of the Invention

[0004] To overcome the shortcomings of existing technical solutions, this invention provides a flame-retardant modification method for high-safety lithium batteries, which can effectively solve the problems raised in the background technology.

[0005] The technical solution adopted by this invention to solve its technical problem is:

[0006] A flame-retardant modification formula for a high-safety lithium battery includes the following steps:

[0007] Step S1: Low-temperature plasma surface activation treatment is performed on the lithium battery polyolefin separator to remove impurities on the separator surface and construct a micro-rough interface.

[0008] Step S2: Prepare a phosphorus-nitrogen-silicon composite flame retardant sol, and uniformly coat the flame retardant sol onto the activated diaphragm surface to form a uniform flame retardant precursor coating.

[0009] Step S3: The membrane with the precursor coating is cured using a gradient temperature curing process to form a dense and flexible flame-retardant composite coating on the surface of the membrane, thereby obtaining a modified flame-retardant membrane.

[0010] Step S4: The modified flame-retardant separator is assembled and packaged with the positive electrode, negative electrode, and flame-retardant modified electrolyte of the lithium battery to complete the flame-retardant modification of the lithium battery.

[0011] As a further description of the above technical solution, in step S1, the low-temperature plasma activation treatment uses air plasma with a processing power of 80-120W, a processing time of 30-90s, and a processing distance of 10-20mm.

[0012] As a further description of the above technical solution, in step S2, the phosphorus-nitrogen-silicon composite flame retardant sol includes phosphorus-based flame retardant monomers, nitrogen-based flame retardants, silane coupling agents, and anhydrous ethanol, with the mass ratio of each component being 15-25:8-15:3-8:60-75.

[0013] As a further description of the above technical solution, the phosphorus-based flame retardant monomer is dimethyl methylphosphonate, the nitrogen-based flame retardant is melamine cyanurate, and the silane coupling agent is an aminosilane coupling agent.

[0014] As a further description of the above technical solution, in step S2, a microgravure coating method is used for coating, with a coating speed of 5-15m / min and a single coating thickness of 2-5μm.

[0015] As a further description of the above technical solution, in step S3, the gradient temperature curing process is divided into three stages of curing: the first stage temperature is 40-60℃ and the holding time is 10-20 min; the second stage temperature is 70-90℃ and the holding time is 15-25 min; and the third stage temperature is 100-120℃ and the holding time is 5-10 min.

[0016] As a further description of the above technical solution, in step S4, the flame-retardant modified electrolyte is prepared by adding 5-10 wt% fluorophosphate flame-retardant additive to a conventional lithium battery electrolyte.

[0017] As a further description of the above technical solution, the thickness of the flame-retardant composite coating after curing is 3-8μm, the limiting oxygen index is ≥32%, and the coating is firmly bonded to the diaphragm substrate without any peeling or cracking defects.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] The flame-retardant modification method for a high-safety lithium battery of the present invention has at least one of the following beneficial effects during use:

[0020] An integrated process involving low-temperature plasma activation, phosphorus-nitrogen-silicon composite flame-retardant sol coating, gradient temperature curing, and electrolyte synergistic modification significantly improves the safety performance of lithium batteries while effectively ensuring that electrochemical performance remains unaffected. Employing a phosphorus-nitrogen-silicon ternary synergistic flame-retardant mechanism, combined with electrolyte fluorophosphate additives, a solid-liquid dual flame-retardant protection system is constructed, substantially enhancing the flame-retardant performance of the separator and effectively passing safety tests such as needle penetration and hot box tests. The plasma activation and gradient curing processes enable chemical bonding between the flame-retardant coating and the separator substrate, resulting in a strong bond without peeling or cracking, avoiding the significant increase in internal resistance inherent in traditional coating technologies. The coating formulation and process parameters have been optimized, achieving highly efficient flame retardancy without a significant increase in battery internal resistance, and maintaining cycle performance and rate performance comparable to conventional batteries. This invention features a simple process that can be directly implemented in existing lithium battery production lines with limited increase in processing costs, making it suitable for large-scale industrial production. Attached Figure Description

[0021] Figure 1 This is an overall cross-sectional view of a lithium battery for a flame-retardant modification method for a high-safety lithium battery according to the present invention.

[0022] Figure 2 This is a schematic diagram of the process flow for a flame-retardant modification method for a high-safety lithium battery according to the present invention.

[0023] Figure 3 This is a schematic diagram of the cross-sectional structure of the modified separator in the flame-retardant modification method for a high-safety lithium battery according to the present invention.

[0024] Numbering on the map:

[0025] 1. Lithium battery positive electrode; 2. Lithium battery negative electrode; 3. Modified flame-retardant separator; 4. Flame-retardant precursor coating. Detailed Implementation

[0026] 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.

[0027] like Figure 1-3 As shown, this invention provides a flame-retardant modification method for high-safety lithium batteries, comprising the following steps:

[0028] Step S1: Low-temperature plasma surface activation treatment is performed on the lithium battery polyolefin separator to remove impurities on the separator surface and construct a micro-rough interface.

[0029] During low-temperature plasma treatment, high-energy electrons, ions, free radicals, and other active particles bombard the surface of the polyolefin membrane, producing a triple effect:

[0030] Cleaning effect: High-energy particles in the plasma bombard and remove impurities such as residual antistatic agents, processing aids, and oil stains from the diaphragm surface, resulting in a clean surface;

[0031] Etching effect: Active particles perform micro-etching on the membrane surface, forming a nanoscale rough interface with unevenness, which increases surface energy and contact area;

[0032] Grafting effect: The oxygen and nitrogen active groups in the plasma introduce polar functional groups such as hydroxyl, carboxyl, and amino groups onto the membrane surface, improving the wettability and adhesion of the membrane surface.

[0033] Step S2: Prepare a phosphorus-nitrogen-silicon composite flame retardant sol, and uniformly coat the flame retardant sol onto the activated diaphragm surface to form a uniform flame retardant precursor coating 4.

[0034] Phosphorus-based flame retardant monomers decompose upon heating to form polyphosphoric acid, promoting char formation and creating a dense char layer that blocks heat and oxygen. Phosphorus-containing compounds decompose upon heating to produce phosphorus-oxygen reactive free radicals. These free radicals can capture hydrogen and hydroxyl reactive free radicals generated during the combustion chain reaction, thereby halting the continuation of the combustion chain reaction and ultimately terminating the combustion process.

[0035] Nitrogen-based flame retardants absorb a large amount of heat when heated, lowering the system temperature, releasing non-flammable gases, diluting the concentration of combustible gases, and forming a phosphorus-nitrogen synergistic system with phosphorus-based compounds, generating PN bonds, and enhancing the charring effect.

[0036] The amino group at one end of the silane coupling agent reacts with the polar groups on the membrane surface, while the siloxane at the other end hydrolyzes and forms a chemical bond with the flame retardant. At high temperatures, a silica ceramic phase is generated, enhancing the density and thermal shock resistance of the carbon layer.

[0037] Step S3: The membrane with the precursor coating is cured using a gradient temperature curing process to form a dense and flexible flame-retardant composite coating on the surface of the membrane, thus obtaining the modified flame-retardant membrane 3.

[0038] First stage (40-60℃, 10-20min): The solvent evaporates slowly and uniformly at low temperature, avoiding pinholes and shrinkage cavities caused by rapid evaporation. Sol particles initially aggregate to form a continuous wet film structure.

[0039] The second stage (70-90℃, 15-25min): The cross-linking reaction proceeds, and the hydrolysis and condensation reaction of the silane coupling agent is fully carried out, forming a Si-O-Si three-dimensional network structure. Flame retardant molecules form chemical bonds with the coupling agent, preventing migration and precipitation. The coating and the membrane surface form chemical bonds.

[0040] The third stage (100-120℃, 5-10min): The structure is stabilized, the cross-linked network is fully formed, the coating structure is stable, the residual solvent is completely removed, there is no VOC residue, the internal stress of the coating is released, and cracking is avoided during subsequent use.

[0041] Gradient curing avoids problems such as coating cracking and poor adhesion caused by traditional one-step high-temperature curing, resulting in a dense, flexible, and firmly bonded flame-retardant coating.

[0042] Step S4: The modified flame-retardant separator 3 is assembled and packaged with the lithium battery positive electrode 1, the lithium battery negative electrode 2, and the flame-retardant modified electrolyte to complete the flame-retardant modification of the lithium battery.

[0043] Fluorophosphate flame retardant additives decompose and release fluorine-containing free radicals, which capture active free radicals in the combustion chain reaction and form a fluorine-containing stable SEI film on the electrode surface. This inhibits the high-temperature decomposition of the electrolyte and forms a "solid-liquid" dual flame retardant system with the diaphragm flame retardant coating, which functions in different temperature ranges.

[0044] To further explain, in step S1, the low-temperature plasma activation treatment uses air plasma with a processing power of 80-120W, a processing time of 30-90s, and a processing distance of 10-20mm.

[0045] Power 80-120W: Low power (80W) results in insufficient activation, while power above 120W can easily cause excessive etching or even perforation of the diaphragm. Time 30-90s: Below 30s, functional group introduction is insufficient, while above 90s can easily cause thermal shrinkage of the diaphragm. Distance 10-20mm: Ensures uniform plasma action and avoids excessively high local energy.

[0046] Furthermore, in step S2, the phosphorus-nitrogen-silicon composite flame retardant sol includes phosphorus-based flame retardant monomers, nitrogen-based flame retardants, silane coupling agents, and anhydrous ethanol, with a mass ratio of 15-25:8-15:3-8:60-75 for each component.

[0047] With a mass ratio of 15-25:8-15:3-8: the phosphorus content is sufficient to provide high-efficiency flame retardant activity, the nitrogen content achieves the best synergistic effect, and the silane coupling agent achieves complete coating without excessive residue affecting ion conduction.

[0048] Furthermore, the phosphorus-based flame retardant monomer is dimethyl methylphosphonate, the nitrogen-based flame retardant is melamine cyanurate, and the silane coupling agent is an aminosilane coupling agent.

[0049] To further explain, in step S2, a microgravure coating method is used for coating, with a coating speed of 5-15 m / min and a single coating thickness of 2-5 μm.

[0050] The liquid volume of the micro-gravure roller is precisely controllable, and the coating speed of 5-15m / min ensures uniform coating. A single coating of 2-5μm avoids excessive coating thickness, which may affect the battery's internal resistance.

[0051] To further explain, in step S3, the gradient temperature curing process is divided into three stages of curing: the first stage is at a temperature of 40-60℃ and is held for 10-20 minutes; the second stage is at a temperature of 70-90℃ and is held for 15-25 minutes; and the third stage is at a temperature of 100-120℃ and is held for 5-10 minutes.

[0052] Furthermore, in step S4, the flame-retardant modified electrolyte is prepared by adding 5-10 wt% fluorophosphate flame-retardant additive to a conventional lithium battery electrolyte.

[0053] Flame retardant effect is not obvious below 5wt%, while above 10wt% will significantly reduce electrolyte ionic conductivity and affect battery rate performance.

[0054] 8. The flame-retardant modification method for a high-safety lithium battery according to claim 1, characterized in that the thickness of the flame-retardant composite coating after curing is 3-8 μm, the limiting oxygen index is ≥32%, and the coating is firmly bonded to the separator substrate without peeling or cracking defects.

[0055] Below 3μm: Insufficient flame retardant loading results in limited flame retardant effect. Above 8μm: Increases battery internal resistance, reduces energy density, and affects ion transport. Therefore, 3-8μm represents the optimal balance between flame retardant effect and electrochemical performance.

[0056] Ordinary polyolefin membranes have a LOI of approximately 17-19%, classifying them as flammable materials. LOI ≥ 32% are classified as flame-retardant, meaning they are difficult to sustain combustion even in oxygen-rich environments.

[0057] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for flame-retardant modification of a high-safety lithium battery, characterized in that, Includes the following steps: Step S1: Low-temperature plasma surface activation treatment is performed on the lithium battery polyolefin separator to remove impurities on the separator surface and construct a micro-rough interface. Step S2: Prepare a phosphorus-nitrogen-silicon composite flame retardant sol, and uniformly coat the flame retardant sol onto the activated diaphragm surface to form a uniform flame retardant precursor coating. Step S3: The membrane with the precursor coating is cured using a gradient temperature curing process to form a dense and flexible flame-retardant composite coating on the surface of the membrane, thereby obtaining a modified flame-retardant membrane. Step S4: The modified flame-retardant separator is assembled and packaged with the positive electrode, negative electrode, and flame-retardant modified electrolyte of the lithium battery to complete the flame-retardant modification of the lithium battery.

2. The flame-retardant modification method for a high-safety lithium battery according to claim 1, characterized in that: In step S1, the low-temperature plasma activation treatment uses air plasma with a processing power of 80-120W, a processing time of 30-90s, and a processing distance of 10-20mm.

3. The flame-retardant modification method for a high-safety lithium battery according to claim 1, characterized in that: In step S2, the phosphorus-nitrogen-silicon composite flame retardant sol includes phosphorus-based flame retardant monomers, nitrogen-based flame retardants, silane coupling agents, and anhydrous ethanol, with the mass ratio of each component being 15-25:8-15:3-8:60-75.

4. The flame-retardant modification method for a high-safety lithium battery according to claim 3, characterized in that: The phosphorus-based flame retardant monomer is dimethyl methylphosphonate, the nitrogen-based flame retardant is melamine cyanurate, and the silane coupling agent is an aminosilane coupling agent.

5. The flame-retardant modification method for a high-safety lithium battery according to claim 1, characterized in that: In step S2, coating is performed using a microgravure coating method at a coating speed of 5-15 m / min and a single coating thickness of 2-5 μm.

6. The flame-retardant modification method for a high-safety lithium battery according to claim 1, characterized in that: In step S3, the gradient temperature curing process is divided into three stages of curing: the first stage is at a temperature of 40-60℃ and is held for 10-20 minutes; the second stage is at a temperature of 70-90℃ and is held for 15-25 minutes; and the third stage is at a temperature of 100-120℃ and is held for 5-10 minutes.

7. The flame-retardant modification method for a high-safety lithium battery according to claim 1, characterized in that: In step S4, the flame-retardant modified electrolyte is prepared by adding 5-10 wt% fluorophosphate flame-retardant additive to a conventional lithium battery electrolyte.

8. The flame-retardant modification method for a high-safety lithium battery according to claim 1, characterized in that: After curing, the flame-retardant composite coating has a thickness of 3-8μm, a limiting oxygen index of ≥32%, and the coating is firmly bonded to the diaphragm substrate without any peeling or cracking defects.

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