A flame-retardant phase change microcapsule and its preparation method, lithium battery separator and battery

Abstract

This invention discloses a flame-retardant phase change microcapsule, its preparation method, a lithium battery separator, and a battery. The microcapsules use polycaprolactone as the wall material and encapsulate a core material composed of a phase change material and a flame retardant. The average particle size is 100-500 nanometers, and the phase change material has a melting point of 50℃-80℃. The preparation method includes: ball milling the phase change material, immersing it in a solution containing polycaprolactone, a flame retardant, and N-methylpyrrolidone, ultrasonically dispersing and stirring to form a slurry, coating it onto a PTFE membrane, freeze-drying, and then ball milling it again to obtain nanocapsules. These microcapsules can be coated onto the surface of a plasma-treated polypropylene separator to form a flame-retardant coating for use in assembling lithium batteries. When the battery temperature reaches 65℃-80℃, the microcapsules rapidly undergo a phase change to release the flame retardant, effectively preventing thermal runaway without affecting the battery's electrochemical performance. This invention features a simple process, low cost, and environmental friendliness, making it suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of lithium battery safety protection materials technology, specifically to a flame-retardant phase change microcapsule and its preparation method, as well as a lithium battery separator and battery. Background Technology

[0002] Today, lithium batteries are widely used in electronic devices, electric vehicles, and many other fields, showing great promise. However, lithium batteries still face challenges such as insufficient thermal stability, including inadequate safety performance, difficulty in early warning of thermal runaway, and a narrow applicable temperature range. Therefore, developing a safety protection strategy that can protect lithium batteries in the early stages of thermal runaway and reduce the risk of combustion is of great significance for creating safe batteries and expanding the application of lithium batteries in high-energy-density devices.

[0003] Currently, protection against early-stage thermal runaway often employs physical isolation or free radical scavenging mechanisms, involving the direct addition of flame retardants to the electrolyte. However, phosphorus-containing additives can hinder ion transport and reduce ionic conductivity to some extent, thus degrading battery performance. Subsequent researchers encapsulated flame retardants within polymer shells such as polymethyl methacrylate (PMMA), creating commercially available self-extinguishing microcapsule-modified separators. The polymer shell effectively prevents the flame retardant from directly dissolving into the electrolyte; when thermal runaway occurs, the polymer shell melts, releasing the flame retardant.

[0004] However, because the melting point of polymers is much higher than the initial thermal runaway temperature of lithium batteries, in practical applications, the polymer casing often does not have enough time to release the flame retardant before a safety accident occurs. Therefore, there is an urgent need to use a material with high thermal sensitivity and a melting point lower than the thermal runaway temperature of lithium batteries as a protective shell to achieve timely release of the flame retardant. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a flame-retardant phase change microcapsule and its preparation method, as well as a lithium battery separator and battery.

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a flame-retardant phase change microcapsule and its application that can rapidly release flame retardants and block thermal runaway when a lithium battery enters a dangerous temperature, without deteriorating the battery's electrochemical performance.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This application provides a flame-retardant phase change microcapsule for thermal runaway protection of lithium batteries, comprising a core material and a wall material. The core material is a composite core material formed by uniformly mixing a phase change material and a flame retardant. The wall material is a polycaprolactone shell that completely encapsulates the core material. The average particle size of the microcapsule is 100-500 nanometers. The melting point of the phase change material is 50℃-80℃.

[0008] Optionally, the phase change material is paraffin wax, and the flame retardant is triethyl phosphate.

[0009] Secondly, a method for preparing flame-retardant phase change microcapsules is provided, comprising the following steps: S1. The phase change material is subjected to high-energy ball milling to obtain small phase change particles; S2. The phase change particles obtained in step S1 are immersed in a mixed solution of polycaprolactone, flame retardant and N-methylpyrrolidone, and then ultrasonically dispersed and stirred to form a slurry; S3. The slurry is coated onto a PTFE membrane and then freeze-dried to obtain a composite powder; S4. The composite powder is subjected to secondary high-energy ball milling to obtain nanoscale microcapsules.

[0010] Optionally, the high-energy ball milling time in step S1 is 2-4 hours; the volume ratio of polycaprolactone to N-methylpyrrolidone in step S2 is 1:10-1:20, the freeze-drying temperature is -50℃ to -80℃, and the freeze-drying time is 12-24 hours; the second high-energy ball milling time in step S4 is 1-3 hours.

[0011] Thirdly, an application of flame-retardant phase change microcapsules in the modification of lithium battery separators is provided, including the following steps: The microcapsules are dispersed in water to form a slurry, which is then coated onto the surface of a plasma-treated polypropylene membrane. After drying, a flame-retardant modified membrane is obtained.

[0012] Fourthly, a flame-retardant modified separator for lithium batteries is provided, comprising a polypropylene base film and a flame-retardant coating coated on at least one surface of the base film, wherein the flame-retardant coating comprises flame-retardant phase change microcapsules.

[0013] Fifthly, a lithium battery is provided, comprising a flame-retardant modified separator.

[0014] Optionally, the diaphragm has a thermal shrinkage rate of ≤5% at 130°C and an ionic conductivity of ≥1.0×10⁻⁶. -3 The flame retardant release rate in the flame retardant coating is ≥90% when the temperature reaches 65℃-75℃.

[0015] Optionally, the conditions for ultrasonic dispersion in step S2 are: power 300-500W, time 20-40 minutes; stirring speed 800-1200rpm; and the mass ratio of paraffin to triethyl phosphate in the core material is 10:3.2–8.6.

[0016] In a sixth aspect, a lithium battery thermal safety management system is provided, including a lithium battery and a temperature monitoring unit; when the internal temperature of the lithium battery reaches 65-80°C, the flame-retardant phase change microcapsules trigger the release of flame retardants.

[0017] A flame-retardant phase change microcapsule for thermal runaway protection of lithium batteries includes a porous polylactic acid microsphere wall material and a core material loaded inside the porous polylactic acid microspheres. The core material is a composite core material formed by mixing a phase change material and a flame retardant. The average particle size of the microcapsule is 1-10 micrometers. The melting point of the phase change material is 50℃-80℃.

[0018] A method for preparing flame-retardant phase change microcapsules includes the following steps: S1. Dissolve the molten phase change material and flame retardant in hot ethanol to form a mixed solution; S2. Add porous polylactic acid microspheres to the mixed solution and stir at a constant temperature to allow the core material to penetrate into the pores of the microspheres; S3. After vacuum drying, centrifugation, and washing, flame-retardant phase change microcapsules with loaded core material are obtained; S4. The microcapsules are mixed with anhydrous ethanol and a dispersant, and then stirred and ultrasonically dispersed to form a suspension. The suspension is coated on the surface of a plasma-treated polypropylene diaphragm and dried to obtain a flame-retardant modified diaphragm.

[0019] Compared with the prior art, this application has the following beneficial effects: This application provides a flame-retardant phase change microcapsule and its preparation method, as well as a lithium battery separator and battery. Paraffin wax, with a melting point matching the thermal runaway initiation temperature of lithium batteries (60-80℃), is used as the phase change material. Upon reaching the phase change temperature, a solid-liquid phase change occurs, causing volume expansion and rapid release of the encapsulated flame retardant, achieving early intervention. The PCL shell remains stable at the normal operating temperature of the battery, effectively preventing contact between the flame retardant and the electrolyte, thus avoiding the negative impact of the flame retardant on ionic conductivity. Experiments show that the ionic conductivity of the modified separator can still be maintained at 1.0 × 10⁻⁶. -3The microcapsules have a particle size distribution of S / cm or higher. The process utilizes PTFE film formation combined with freeze-drying and ball milling, resulting in a simple workflow that avoids complex emulsion polymerization or interfacial polymerization processes, making it more suitable for large-scale production. PCL raw materials are moderately priced and biodegradable; PCL is a biodegradable aliphatic polyester with good environmental compatibility, aligning with the trend of green chemistry. The prepared microcapsules have a uniform particle size distribution and high encapsulation efficiency. The modified separator exhibits low thermal shrinkage (≤5%) at 130℃, demonstrating excellent thermal dimensional stability. Different phase change materials with varying melting points or different types of flame retardants can be selected to meet the safety requirements of various lithium battery systems. Furthermore, this application also provides a flame-retardant phase change microcapsule based on porous polylactic acid (PLA) wall material. The core material is loaded via solvent permeation, simplifying the preparation process and achieving precise temperature control and rapid flame-retardant response in the early stages of thermal runaway. Moreover, as a biodegradable material, PLA further enhances the product's environmental friendliness. Attached Figure Description

[0020] Figure 1 This is a flowchart of a flame-retardant phase change microcapsule and its preparation method.

[0021] Figure 2 This is a process flow diagram for Example 4.

[0022] Figure 3 This is a diagram showing the results of the heat shrinkage test. Detailed Implementation

[0023] 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.

[0024] Furthermore, in this invention, an element referred to as fixed to or disposed on another element may be directly disposed on the other element, or there may be an intermediate element. When an element is considered to be connected to another element, it may be directly connected to the other element, or there may be an intermediate element present simultaneously. The terms vertical, horizontal, left, right, and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0025] See Figures 1-3 This application provides a flame-retardant phase change microcapsule for thermal runaway protection of lithium batteries, comprising a core material and a wall material. The core material is a composite core material formed by uniformly mixing a phase change material and a flame retardant. The wall material is a polycaprolactone shell that completely encapsulates the core material. The average particle size of the microcapsule is 100-500 nanometers. The melting point of the phase change material is 50℃-80℃.

[0026] In this embodiment, the phase change material is preferably polyethylene glycol (PEG) with a number average molecular weight of 1000-4000, and its phase change enthalpy can reach 180-220 J / g, which can rapidly absorb a large amount of heat in the early stage of thermal runaway of lithium battery to delay the temperature rise. The flame retardant adopts a compound system of tris(2-hydroxyethyl) phosphate (THEP) and aluminum hydroxide, with a mass ratio of 2:1. This compound system significantly improves the flame retardant efficiency through the synergistic effect of gas-phase flame retardancy (decomposition to generate phosphorus-containing free radicals to capture active free radicals in the combustion chain reaction) and condensed-phase flame retardancy (forming a dense carbonized layer to isolate oxygen and heat), and has little impact on the heat storage performance of the phase change material. The mass ratio of phase change material to compound flame retardant in the core material is 4:1. At this ratio, the microcapsules have both excellent heat storage capacity and flame retardant effect. The thickness of the polycaprolactone wall material is controlled at 30-40 nanometers, achieved by adjusting the concentration of the polycaprolactone prepolymer (5%-8% by mass) and the polymerization reaction time (2-3 hours). This wall material exhibits good mechanical strength and resistance to electrolyte corrosion. After immersion in carbonate electrolytes for 72 hours, the wall material shows no swelling or cracking, and the core material leakage rate is less than 0.5%, ensuring the structural stability of the microcapsules throughout the battery's lifespan. Furthermore, the average particle size of 100-500 nanometers allows the microcapsules to be uniformly dispersed in the lithium battery separator coating, neither clogging the separator micropores (pore size 1-5 micrometers) to ensure lithium-ion transport, nor increasing the contact area with heat and accelerating the thermal response speed. In this embodiment, the microcapsules were prepared by interfacial polymerization: the core material mixture (PEG + compound flame retardant) was dispersed in a dichloromethane organic phase containing polycaprolactone prepolymer, and ultrasonically emulsified to form a water-in-oil emulsion (emulsification time 15-20 minutes, power 300W); then, the crosslinking agent hexamethylene diisocyanate (HDI) was added to initiate the polymerization reaction, the reaction temperature was controlled at 60℃, and stirring was continued for 2 hours; after the reaction was completed, the microcapsules were separated by high-speed centrifugation (speed 12000 rpm, time 10 minutes), washed three times with anhydrous ethanol to remove residual solvent and unreacted monomers, and finally vacuum dried (temperature 40℃, time 12 hours) to obtain the target product.

[0027] In one specific embodiment, the phase change material is paraffin wax, and the flame retardant is triethyl phosphate.

[0028] In this embodiment, the residual amount of the prepared flame-retardant phase change microcapsules at 350℃ reached 45%, which is about 2.8 times higher than that of pure paraffin. Differential scanning calorimetry (DSC) test showed that its phase change enthalpy was 118 J / g and its phase change temperature range was 55-65℃, which can effectively absorb the excess heat generated during the operation of lithium batteries. The microcapsules were coated on the surface of a polypropylene separator at a mass fraction of 15% (coating thickness of about 2 μm) to form a flame-retardant modified separator. After being assembled into a soft-pack lithium battery, an overcharge test was conducted: when the charging voltage rose to 10V, the highest temperature of the modified separator group battery stabilized at about 80℃, and no safety accidents such as smoke or fire occurred; while the temperature of the blank separator group battery rapidly rose to 275℃ and thermal runaway occurred. In addition, in the 1C rate cycle test, the capacity retention rate of the modified separator group battery after 150 cycles was 90.8%, which was similar to that of the blank group (89.5%), indicating that the microcapsule coating in this embodiment did not have a significant negative impact on the electrochemical performance of the battery.

[0029] Secondly, a method for preparing flame-retardant phase change microcapsules is provided, comprising the following steps: S1. The phase change material is subjected to high-energy ball milling to obtain small phase change particles; S2. The phase change particles obtained in step S1 are immersed in a mixed solution of polycaprolactone, flame retardant and N-methylpyrrolidone, and then ultrasonically dispersed and stirred to form a slurry; S3. The slurry is coated onto a PTFE membrane and then freeze-dried to obtain a composite powder; S4. The composite powder is subjected to secondary high-energy ball milling to obtain nanoscale microcapsules.

[0030] Regarding the parameter ranges for each step in the above method, the applicant has verified its rationality through extensive experiments. For example, when the high-energy ball milling time is less than 2 hours, the particle size of the phase change particles is too large, affecting the subsequent encapsulation efficiency; when the ball milling time is greater than 4 hours, the phase change material is prone to agglomeration, and the particle size distribution becomes wider. When the volume ratio of polycaprolactone to N-methylpyrrolidone is in the range of 1:10-1:20, a stable coating layer can be formed; outside this range, it is difficult to form a continuous shell.

[0031] Freeze-drying temperatures below -80℃ or times less than 12 hours result in incomplete solvent sublimation, leading to insufficient density of the microcapsule shell. Temperatures above -50℃ may cause premature softening and leakage of the core material. Secondary ball milling, controlled within 1-3 hours, yields nanoscale microcapsules with uniform particle size; excessively long milling times will damage the shell structure.

[0032] In this embodiment, in step S1, the phase change material is preferably paraffin wax with a melting point of 60℃-70℃ and a latent heat of phase change of 200-250J / g; the ball-to-material ratio of the high-energy ball mill is 10:1-20:1, the rotation speed is 300-500rpm, and the ball milling time is 2-4 hours, finally obtaining phase change small particles with a particle size of 5-10 micrometers.

[0033] In step S2, the polycaprolactone (PCL) is prepared into a solution with a mass concentration of 5%-10% using N-methylpyrrolidone (NMP) as the solvent; the flame retardant is triethyl phosphate (TEP), and its addition amount is 30%-80% of the mass of the phase change material (i.e., 3-8 mTEP is added for every 10g of phase change particles); the ultrasonic dispersion power is 300-500W, and the time is 20-40 minutes; the stirring speed is 800-1200rpm, and the stirring time is 1-2 hours to ensure uniform dispersion of the slurry; when coating the slurry onto the PTFE film, the thickness is controlled to be 50-100 micrometers using a doctor blade coating method; then freeze-drying is performed at a drying temperature of -50℃ to -80℃ for 12-24 hours to allow the solvent to completely sublimate, finally obtaining a "paraffin-TEP@PCL" composite powder with an encapsulation efficiency ≥85%.

[0034] In step S3, the ball-to-material ratio of the secondary high-energy ball mill is 15:1-25:1, the rotation speed is 400-600 rpm, and the ball milling time is 1-3 hours, finally obtaining flame-retardant phase change microcapsules with an average particle size of 100-500 nanometers and a particle size distribution variation coefficient ≤15%.

[0035] In step S4, the obtained nanoscale microcapsules are soaked in water, and polyvinylpyrrolidone (PVP) with a mass concentration of 0.5%-1% is added as a dispersant. The ultrasonic dispersion power is 200-300W and the time is 15-30 minutes to form a uniform and stable microcapsule dispersion with a mass concentration of 10%-20% for subsequent modification and coating of lithium battery separators.

[0036] In one specific embodiment, the high-energy ball milling time in step S1 is 2-4 hours; the volume ratio of polycaprolactone to N-methylpyrrolidone in step S2 is 1:10-1:20, the freeze-drying temperature is -50℃ to -80℃, and the freeze-drying time is 12-24 hours; the second high-energy ball milling time in step S4 is 1-3 hours.

[0037] In this embodiment, the above method is described in detail: S1: Place an appropriate amount of phase change material (preferably paraffin wax with a melting point of 60℃-70℃ and a latent heat of phase change of 200-250J / g) into a high-energy ball mill, set the ball-to-material ratio to 10:1-20:1, the rotation speed to 300-500rpm, and ball mill for 2-4 hours to obtain small paraffin wax phase change particles with a particle size of 5-10 micrometers. S2: The phase change particles obtained in step S1 are immersed in a mixed solution composed of polycaprolactone (PCL), flame retardant triethyl phosphate (TEP), and N-methylpyrrolidone (NMP) (wherein PCL is prepared as a 5%-10% mass concentration solution using NMP as a solvent, and the amount of TEP added is 30%-80% of the mass of the phase change material). The mixture is then ultrasonically dispersed at a power of 300-500W for 20-40 minutes, and then rapidly stirred at 800-1200rpm for 1-2 hours to form a uniform slurry. The slurry is then coated onto a PTFE film using a doctor blade coating method (controlling the thickness to 50-100 micrometers), and subsequently freeze-dried at -50℃ to -80℃ for 12-24 hours to obtain a "paraffin-TEP@PCL" composite powder with an encapsulation efficiency ≥85%. S3: The composite powder obtained in step S2 is placed into a high-energy ball mill for secondary ball milling. The ball-to-material ratio is set to 15:1-25:1, the rotation speed is 400-600 rpm, and the ball milling is carried out for 1-3 hours to obtain nano-scale “TEP-paraffin@PCL” microcapsules with an average particle size of 100-500 nanometers and a particle size distribution variation coefficient ≤15%. S4: Soak the microcapsules obtained in step S3 in water, add 0.5%-1% polyvinylpyrrolidone (PVP) as a dispersant, and perform ultrasonic dispersion at a power of 200-300W for 15-30 minutes to obtain a uniform and stable slurry; coat the slurry onto a lithium battery PP separator that has been treated with air plasma (power 100-200W, air pressure 0.1-0.2MPa, time 30 seconds) using a doctor blade coating method to complete the loading of microcapsules on the separator surface.

[0038] Thirdly, an application of flame-retardant phase change microcapsules in the modification of lithium battery separators is provided, including the following steps: The microcapsules are dispersed in water to form a slurry, which is then coated onto the surface of a plasma-treated polypropylene membrane. After drying, a flame-retardant modified membrane is obtained.

[0039] In this embodiment, the nanoscale “TEP-paraffin@PCL” microcapsules with an average particle size of 100-500 nm and a particle size distribution variation coefficient ≤15% obtained in step S3 are immersed in water, and polyvinylpyrrolidone (PVP) with a mass concentration of 0.5%-1% is added as a dispersant. The mixture is then subjected to ultrasonic dispersion treatment at a power of 200-300 W for 15-30 minutes to obtain a uniform and stable slurry. Subsequently, the slurry is coated onto the surface of a lithium-ion battery polypropylene (PP) separator pretreated with air plasma using a doctor blade coating method. The process parameters for the air plasma treatment are: power 100-200 W, air pressure 0.1-0.2 MPa, and treatment time 30 seconds. After coating and drying, a flame-retardant modified separator with uniformly loaded flame-retardant phase change microcapsules is obtained. This modified separator can be directly used in lithium-ion battery assembly, leveraging the synergistic effect of phase change heat storage and flame retardancy of the microcapsules to improve the thermal stability and safety performance of the battery.

[0040] Fourthly, a flame-retardant modified separator for lithium batteries is provided, comprising a polypropylene base film and a flame-retardant coating coated on at least one surface of the base film, wherein the flame-retardant coating comprises flame-retardant phase change microcapsules.

[0041] In this embodiment, the nanoscale “TEP-paraffin@PCL” microcapsules with an average particle size of 100-500 nm and a particle size distribution variation coefficient ≤15% obtained in step S3 are immersed in water, and polyvinylpyrrolidone (PVP) with a mass concentration of 0.5%-1% is added as a dispersant. The mixture is then subjected to ultrasonic dispersion treatment at a power of 200-300 W for 15-30 minutes to obtain a uniform and stable slurry. Subsequently, the slurry is coated onto the surface of a lithium-ion battery polypropylene (PP) separator pretreated with air plasma using a doctor blade coating method. The process parameters for the air plasma treatment are: power 100-200 W, air pressure 0.1-0.2 MPa, and treatment time 30 seconds. After coating and drying, a flame-retardant modified separator with uniformly loaded flame-retardant phase change microcapsules is obtained. This modified separator can be directly used in lithium-ion battery assembly, leveraging the synergistic effect of phase change heat storage and flame retardancy of the microcapsules to improve the thermal stability and safety performance of the battery.

[0042] Fifthly, a lithium battery is provided, comprising a flame-retardant modified separator.

[0043] In this embodiment, the lithium battery is composed of a positive electrode, a negative electrode, an electrolyte, and the aforementioned flame-retardant modified separator, which are sequentially stacked or wound. The components are assembled into a soft-pack battery cell in an argon-protected glove box. Subsequently, a dimethyl carbonate-ethylene carbonate (DMC-EC) mixed electrolyte containing 5% fluoroethylene carbonate (FEC) additive is injected. The battery undergoes vacuum sealing, formation (0.1C constant current charging to 4.2V followed by constant voltage charging to current ≤0.05C), and aging (45℃ standing for 24 hours) to obtain the finished battery. Performance tests show that after 500 cycles at 1C, the lithium battery retains 92.3% of its capacity, a 14.6% improvement over the control group using an unmodified PP separator. In the overcharge test (charging to 5V), the control group battery experienced open flame combustion within 120 seconds, while the battery in this embodiment only showed slight bulging without combustion. Its safety performance optimization mechanism is as follows: when the internal temperature of the battery rises to the paraffin phase change point (about 62°C), the microcapsule phase change absorbs heat and slows down the temperature rise; if the temperature exceeds the PCL wall material tolerance threshold (about 125°C), the TEP flame retardant is released and reacts with the free radicals generated by the decomposition of the electrolyte to block the combustion chain transmission and effectively suppress thermal runaway.

[0044] In one specific embodiment, the diaphragm has a thermal shrinkage rate of ≤5% at 130°C and an ionic conductivity of ≥1.0×10⁻⁶. -3 The flame retardant release rate in the flame retardant coating is ≥90% when the temperature reaches 65℃-75℃.

[0045] In this embodiment, the flame-retardant modified separator is prepared by uniformly coating the above-mentioned flame-retardant phase change microcapsules onto the surface of a polypropylene (PP) base film, with the microcapsule coating amount controlled to be 18% of the base film mass. The modified separator is then sequentially stacked with an NCM523 positive electrode and an artificial graphite negative electrode and wound into a cylindrical battery cell. The battery cell is assembled in an argon-protected glove box, and then injected with a mixed electrolyte of dimethyl carbonate-ethylene carbonate (DMC-EC, volume ratio 1:1) containing 5% fluoroethylene carbonate (FEC). The finished battery is then obtained through vacuum sealing, formation (0.1C constant current charging to 4.2V, then constant voltage charging to current ≤0.05C), and aging at 45°C for 24 hours.

[0046] Performance characterization of the battery: A 130℃ heat shrinkage test showed that the lateral shrinkage rate of the separator was 3.2%, and the longitudinal shrinkage rate was 2.8%, both meeting the requirement of ≤5%; the ionic conductivity at room temperature was 1.2 × 10⁻⁶. - ³S / cm, higher than 1.0×10 - The threshold of ³S / cm; thermogravimetric analysis results show that when the temperature rises to 70℃, the flame retardant release rate in the flame retardant coating reaches 95%, which meets the index of release rate ≥90% at 65℃~75℃.

[0047] Electrochemical performance tests showed that the battery retained 92.3% of its capacity after 500 cycles at 1C, a 14.6% improvement over the control group without modified PP separator. When overcharged to 5V, the control group battery exhibited open flame combustion within 90 seconds, while the battery in this embodiment only showed slight swelling without combustion or explosion. In further thermal runaway simulation tests, the battery was left to stand at 120°C for 30 minutes without showing any signs of thermal runaway, while the control group battery experienced violent combustion within 15 minutes under the same conditions.

[0048] The core reason for the above performance improvement is that the low thermal shrinkage rate of the separator ensures effective isolation between the positive and negative electrodes during cycling, and the high ionic conductivity promotes rapid charge transfer. When the internal temperature of the battery reaches 65℃~75℃, the flame retardant is released rapidly, capturing the free radicals generated by the decomposition of the electrolyte and blocking the combustion chain reaction. If the temperature continues to rise above the paraffin phase transition point (about 62℃), the paraffin phase transition in the microcapsules absorbs heat, delays the temperature rise, and further inhibits the occurrence of thermal runaway.

[0049] In one specific embodiment, the ultrasonic dispersion conditions in step S2 are: power 300-500W, time 20-40 minutes; stirring speed 800-1200rpm; and the mass-to-volume ratio of paraffin to triethyl phosphate in the core material is 10g:3-8mL.

[0050] In this embodiment, the flame-retardant phase change microcapsules prepared under the above-described ultrasonic dispersion and stirring conditions have an average particle size of 150-250 nm, uniform particle size distribution (PDI < 0.2), and an encapsulation efficiency of over 85%. The microcapsules exhibit good shell density, effectively preventing leakage of the core material during subsequent processing and battery cycling. Simultaneously, their surface possesses certain polar groups, enhancing the interfacial bonding with the PP separator substrate and ensuring the modified separator maintains structural stability during long-term charge-discharge cycles. After loading the microcapsules onto the surface of the PP separator using a dip-coating method, the porosity of the modified separator remains at 40%-45%, showing minimal change compared to the unmodified separator. This ensures the unobstructed ion transport channels and endows the separator with excellent flame-retardant and phase change temperature control properties.

[0051] In a sixth aspect, a lithium battery thermal safety management system is provided, including a lithium battery and a temperature monitoring unit; when the internal temperature of the lithium battery reaches 65-80°C, the flame-retardant phase change microcapsules trigger the release of flame retardants.

[0052] In this embodiment, the temperature monitoring unit employs a distributed thermocouple sensor array, uniformly arranged in the gaps between the lithium battery cells and near the tabs, transmitting temperature data to the control module in real time. When the monitored local temperature rises to a preset threshold, the paraffin core material in the flame-retardant phase change microcapsules rapidly undergoes a solid-liquid phase change, absorbing heat and lowering the ambient temperature. Simultaneously, the phase change expansion causes microcracks in the dense shell, rapidly releasing and diffusing the encapsulated triethyl phosphate flame retardant into the battery interior. The released flame retardant can capture active free radicals generated during thermal runaway, interrupting the chain reaction, and forming a dense carbonized barrier layer on the electrode surface, preventing oxygen from contacting the flammable electrolyte and effectively inhibiting further spread of thermal runaway. Tests using needle penetration, overcharge, and high-temperature short-circuit methods verified that the lithium battery equipped with this thermal safety management system exhibited a 4.2-fold longer thermal runaway trigger time and a more than 60% reduction in peak temperature compared to the control group without the system, with no fires or explosions observed. In addition, the system's sensor array is integrated with the battery structure, which does not occupy additional internal battery space and has less than 2% impact on battery energy density. It can be widely used in power and energy storage lithium batteries for electric vehicles, energy storage power stations and other scenarios.

[0053] Example 4: Preparation of flame-retardant phase change microcapsules based on porous polylactic acid microspheres and their modified membranes This embodiment provides another method for preparing flame-retardant phase change microcapsules and their modified diaphragms, using porous polylactic acid microspheres as the wall material and loading the core material through a solvent evaporation permeation method.

[0054] S1. Core material mixing: Weigh 400 mg of paraffin wax with a melting point of 70-80℃ and 200 mg of triethyl phosphate (TEP) flame retardant, dissolve them in 2 mL of hot ethanol (50-60℃), and stir magnetically until a uniform and transparent mixed solution is formed.

[0055] S2. Permeation Loading: Add 40 mg of porous polylactic acid microspheres to the above mixed solution and stir continuously at a constant temperature of 50°C for 60 minutes to allow the mixed solution of paraffin and TEP to fully permeate into the hollow structure of the porous PLA microspheres.

[0056] S3. Curing and purification: The loaded microspheres were placed in a vacuum oven at 50°C for 30 minutes to enhance adsorption and remove residual solvent; then centrifuged at 8000 rpm for 5 minutes to collect the precipitate; washed three times with cold ethanol to remove the unadsorbed core material on the surface; finally, vacuum dried at room temperature to obtain TEP-paraffin@PLA composite microspheres.

[0057] S4. Preparation of dispersion: The dried TEP-paraffin@PLA microspheres were mixed with anhydrous ethanol at a mass ratio of 8:2, and polyvinylpyrrolidone was added as a dispersant at a mass ratio of 0.5% of the total mass of the system.

[0058] S5. Uniform dispersion: Magnetic stirring at 500 rpm for 30 minutes, followed by ultrasonic treatment at 100-150 W power for 20 minutes to break up microsphere agglomeration and form a uniform and stable suspension.

[0059] S6. Diaphragm Coating: Commercial polypropylene diaphragms are treated with air plasma for 30 seconds to improve surface wettability; the suspension is uniformly coated onto the diaphragm surface using a doctor blade coating method with a doctor blade gap of 50 μm.

[0060] S7. Drying and film formation: The coated diaphragm is placed in a fume hood at room temperature for 30 minutes to allow the ethanol to evaporate initially. Then it is transferred to a 40°C vacuum oven to dry for 2 hours to completely remove residual solvent and obtain a flame-retardant modified PP diaphragm.

[0061] The performance of the flame-retardant modified diaphragm prepared in this embodiment was tested: Heat shrinkage test: When the diaphragm was placed in an oven at 140°C, the control group PP diaphragm shrank drastically to about 8 mm (long axis) of its original size, while the modified diaphragm in this embodiment still maintained a long axis of 16 mm and a short axis of 13 mm, showing significantly better thermal dimensional stability.

[0062] Flame retardant performance: In the overcharge test, the lithium battery using the modified separator of this embodiment triggered the release of flame retardant when the voltage rose to 8.5V and the temperature reached 68°C, without thermal runaway; while the control group battery using pure PP separator burned violently at 5.8V.

[0063] Electrochemical performance: After 200 cycles at 1C rate, the battery with the modified separator of this embodiment retained 91.5% of its capacity, which is basically the same as that of the battery with pure PP separator (90.5%), indicating that the flame-retardant modified separator has no significant negative impact on the long-term cycle performance of the battery.

[0064] In this embodiment, the following auxiliary methods are used for testing and characterization: Microcapsule particle size and distribution: The average particle size and polydispersity index (PDI) were determined using a laser particle size analyzer (such as Malvern Zetasizer) or scanning electron microscope (SEM).

[0065] Phase change performance test: Differential scanning calorimeter (DSC, such as TA Instruments) was used with a heating rate of 5℃ / min in a nitrogen atmosphere to determine the phase change temperature and phase change enthalpy.

[0066] Diaphragm heat shrinkage rate: Cut the diaphragm into 5cm×5cm samples, place them in a 130℃ oven for 1 hour, and measure the dimensional changes after cooling to calculate the shrinkage rate.

[0067] Ionic conductivity test: The AC impedance of the membrane in 1M LiPF6 / EC:DMC (1:1, v / v) electrolyte was measured in the frequency range of 1MHz–0.1Hz using the blocked electrode method, and the ionic conductivity was calculated.

[0068] Flame retardant release rate determination: A hot stage-gas chromatography / mass spectrometry (GC-MS) system was set up, and the temperature was increased by 5℃ / min to detect the characteristic peak of TEP release and calculate the cumulative release rate.

[0069] Battery cycle performance test: At 25°C, charge and discharge cycles were performed at a 1C rate within a voltage range of 3.0–4.2V, and the capacity retention rate was recorded.

[0070] Overcharge test: At 25℃, the battery is overcharged at 3C constant current until the battery voltage reaches 10V or thermal runaway occurs, and the temperature change and safety behavior are recorded.

[0071] Example 1: Preparation of flame-retardant phase change microcapsules Weigh 10.0g of paraffin wax (melting point 62℃) and place it in a high-energy ball mill. Add zirconia grinding balls and ball mill at 400rpm for 3 hours to obtain paraffin wax particles with a particle size of about 1-10μm.

[0072] Prepare the mixed solution: Dissolve 1.0 g PCL (Mw=80000) in 100 mL NMP and stir magnetically until completely dissolved. Then add 5.0 mL triethyl phosphate (TEP) and stir until homogeneous.

[0073] The paraffin particles obtained in step 1 were added to the above mixed solution and ultrasonically dispersed in an ultrasonic cell disruptor at 400W power for 30 minutes. Then, the mixture was mechanically stirred at 25°C and 1000rpm for 2 hours to obtain a uniform slurry.

[0074] The slurry was evenly coated onto a clean PTFE membrane using a scraper, pre-frozen in a -60℃ ultra-low temperature freezer for 4 hours, and then transferred to a freeze dryer and dried at -50℃ and 10Pa for 20 hours to obtain a light yellow composite powder.

[0075] The composite powder was placed again in a high-energy ball mill and milled at 500 rpm for 2 hours to obtain a light yellow powder, which is the flame-retardant phase change microcapsule. Laser particle size analysis showed that its average particle size was 215 nm and its particle size distribution index (PDI) was 0.21.

[0076] The particle size distribution of the microcapsules was measured using a laser particle size analyzer (Malvern Mastersizer 3000). The results showed an average particle size of 215 nm and a PDI of 0.21. DSC analysis revealed a phase transition peak at 61.5 °C and a phase transition enthalpy of 85.6 J / g. Thermogravimetric analysis (TGA) indicated that the microcapsule residue at 350 °C was 45%, higher than that of pure paraffin (approximately 16%).

[0077] Example 2: Preparation of flame-retardant modified diaphragm The microcapsule powder prepared in Example 1 was dispersed in deionized water at a concentration of 5 wt%, and ultrasonically treated for 30 minutes to obtain a uniform coating slurry.

[0078] Commercial Celgard 2400PP membrane (25μm thick) was treated with an air plasma treatment machine at 100W power for 30 seconds to improve its surface hydrophilicity.

[0079] The above slurry was uniformly coated on one side of the treated PP diaphragm using a doctor blade coating method, with a wet film thickness of 100 μm.

[0080] The coated diaphragm was dried in a 60℃ forced-air drying oven for 2 hours to obtain a modified diaphragm with a flame-retardant coating. The areal density of the coating was measured to be approximately 1.2 mg / cm³. 2 .

[0081] Example 3: Preparation of microcapsules with another core material ratio The steps were the same as in Example 1, except that the amount of paraffin used was 8.0 g, the amount of TEP was 8.0 mL, the amount of PCL was 1.5 g, and the amount of NMP was 120 mL. The final microcapsules had an average particle size of 180 nm.

[0082] Example 4: Preparation of flame-retardant phase change microcapsules and their modified diaphragms based on porous polylactic acid microspheres. Following the steps of Example 4 in the aforementioned specific embodiments, TEP-paraffin@PLA composite microspheres and flame-retardant modified PP diaphragms were prepared.

[0083] Comparative Example 1: Commercial membranes without flame-retardant coating Use Celgard 2400PP membranes directly without any treatment.

[0084] Comparative Example 2: A diaphragm containing PMMA-encapsulated microcapsules.

[0085] Following the method described in the literature, PMMA microcapsules encapsulating TEP were prepared by emulsion polymerization using methyl methacrylate as a monomer, and coated onto a PP membrane in a manner similar to that of Example 2, with a coating surface density comparable to that of Example 2.

[0086] Experimental Examples and Performance Tests Test Example 1: Thermal Performance Test of Microcapsules The microcapsules prepared in Examples 1 and 3 were tested using differential scanning calorimetry (DSC) at a heating rate of 5 °C / min under a nitrogen atmosphere.

[0087] Example 1 Microcapsules: A significant endothermic peak was observed at 61.5℃, corresponding to a melting enthalpy of 85.6 J / g for paraffin, demonstrating that it has good phase change heat storage capacity and a well-defined thermal response temperature.

[0088] Example 3 Microcapsules: The endothermic peak is located at 60.8℃, and the enthalpy of melting is 78.3J / g.

[0089] DSC testing was performed on the TEP-paraffin@PLA composite microspheres prepared in Example 4. The results showed that the phase transition peak was located at 62.3℃ and the phase transition enthalpy was 76.8J / g, which also showed good thermal response characteristics.

[0090] Test Example 2: Basic performance test of diaphragm Heat shrinkage rate: The diaphragms of Example 2, Comparative Example 1 and Comparative Example 2 were cut into 5cm×5cm samples, placed in an oven at 130℃ for 1 hour, and the dimensional changes were measured.

[0091] Example 2: Diaphragm: thermal shrinkage rate 3.2%.

[0092] Comparative Example 1: Diaphragm: Heat shrinkage rate 58.7% (severe melting shrinkage).

[0093] Comparative Example 2: diaphragm: thermal shrinkage rate 45.5%.

[0094] Example 4: diaphragm: thermal shrinkage rate 4.1%.

[0095] Ionic conductivity: The AC impedance of the diaphragm in 1M LiPF6EC / DMC electrolyte was tested in the frequency range of 1MHz-0.1Hz using the blocked electrode method, and the ionic conductivity was calculated.

[0096] Example 2: Diaphragm: 1.28 × 10 -3 S / cm.

[0097] Comparative Example 1: Diaphragm: 1.32 × 10⁻⁶ -3 S / cm.

[0098] Comparative Example 2: Diaphragm: 0.89 × 10 -3 S / cm.

[0099] Example 4: Diaphragm: 1.21 × 10 -3 S / cm.

[0100] The results show that the modified membrane of the present invention has minimal effect on ionic conductivity, while the PMMA microcapsule coating of Comparative Example 2 has a certain degree of obstruction to ion transport.

[0101] Test Example 3: Flame Retardant Release Behavior Test A visualization hot stage experimental setup was constructed. A glass slide coated with microcapsules was placed on the hot stage, and a gas chromatography-mass spectrometry (GC-MS) instrument was connected to detect the released gas. The temperature was increased at a rate of 5 °C / min.

[0102] Microcapsules used in Example 2: When the temperature of the hot plate reached 65°C, GC-MS detected a significant TEP characteristic peak signal, indicating that the flame retardant began to be released; when the temperature reached 75°C, the cumulative release rate exceeded 95%.

[0103] Comparative Example 2: The PMMA microcapsules used showed a weak TEP signal until the temperature exceeded 150°C, indicating a severe release lag.

[0104] The microcapsules used in Example 4: When the temperature of the hot plate reached 68°C, the characteristic peak of TEP was detected, and when the temperature reached 78°C, the cumulative release rate exceeded 90%.

[0105] Test Example 4: Battery safety performance test With LiNi 0.8 Co 0.1 Mn 0.1 Using O2 as the positive electrode and graphite as the negative electrode, 18650 type lithium-ion batteries with a rated capacity of 2.5Ah were assembled using the separators of Example 2, Comparative Example 1, and Comparative Example 2, respectively.

[0106] Test conditions: At 25℃, the battery was charged at a constant current of 1C to the cutoff voltage of 4.2V, then switched to constant voltage charging until the current dropped to 0.05C, and then overcharged at a constant current of 3C until the battery voltage reached 10V or thermal runaway occurred.

[0107] The battery using the separator in Example 2: When overcharged to 8.5V, the battery surface temperature reached 68°C, triggering an alarm in the BMS. Disassembly and inspection revealed no smoke or fire, the internal structure was intact, and no thermal runaway occurred.

[0108] The battery using the separator in Comparative Example 1: When overcharged to 5.8V, the surface temperature of the battery rose sharply to 142℃, followed by violent swelling, smoke and fire, resulting in thermal runaway.

[0109] The battery using the separator in Comparative Example 2: When overcharged to 7.2V, the battery temperature rose to 155℃, and then thermal runaway occurred. The PMMA microcapsules failed to provide timely flame retardancy.

[0110] The battery using the separator of Example 4: when overcharged to 8.2V, the surface temperature of the battery reached 72°C, but no thermal runaway or fire occurred.

[0111] Test Example 5: Battery electrochemical cycle performance test The three types of batteries were subjected to charge-discharge cycle tests at 25°C with a 1C rate within a voltage range of 3.0-4.2V.

[0112] After 200 cycles, the battery using the separator of Example 2 retained 92.1% of its capacity.

[0113] The battery using the separator of Comparative Example 1 retained 90.5% of its capacity.

[0114] The battery using the separator in Comparative Example 2 had a capacity retention rate of only 83.7%.

[0115] The battery using the separator of Example 4 retained 91.2% of its capacity.

[0116] The results show that the flame-retardant modified separator provided by the present invention significantly improves safety while having almost no negative impact on the long cycle life of the battery.

[0117] 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.

[0118] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A flame-retardant phase change microcapsule for thermal runaway protection of lithium batteries, characterized in that, It includes a core material and a wall material. The core material is a composite core material formed by uniformly mixing a phase change material and a flame retardant. The wall material is a polycaprolactone shell that completely encapsulates the core material. The average particle size of the microcapsules is 100-500 nanometers. The melting point of the phase change material is 50℃-80℃.

2. The flame-retardant phase change microcapsule according to claim 1, characterized in that, The phase change material is paraffin wax, and the flame retardant is triethyl phosphate.

3. A method for preparing flame-retardant phase change microcapsules as described in claim 1 or 2, characterized in that, Includes the following steps: S1. The phase change material is subjected to high-energy ball milling to obtain small phase change particles; S2. The phase change particles obtained in step S1 are immersed in a mixed solution of polycaprolactone, flame retardant and N-methylpyrrolidone, and then ultrasonically dispersed and stirred to form a slurry; S3. The slurry is coated onto a PTFE membrane and then freeze-dried to obtain a composite powder; S4. The composite powder is subjected to secondary high-energy ball milling to obtain nanoscale microcapsules.

4. The method according to claim 3, characterized in that, In step S1, the high-energy ball milling time is 2-4 hours; in step S2, the volume ratio of polycaprolactone to N-methylpyrrolidone is 1:10-1:20, the freeze-drying temperature is -50℃ to -80℃, and the freeze-drying time is 12-24 hours; in step S4, the second high-energy ball milling time is 1-3 hours.

5. The application of the flame-retardant phase change microcapsules as described in claim 1 or 2 in the modification of lithium battery separators, characterized in that, Includes the following steps: The microcapsules are dispersed in water to form a slurry, which is then coated onto the surface of a plasma-treated polypropylene membrane. After drying, a flame-retardant modified membrane is obtained.

6. A flame-retardant modified separator for lithium batteries, characterized in that, It includes a polypropylene base film and a flame-retardant coating coated on at least one side of the base film, the flame-retardant coating comprising flame-retardant phase change microcapsules as described in claim 1 or 2.

7. A lithium battery, characterized in that, It includes the flame-retardant modified diaphragm as described in claim 6.

8. The flame-retardant modified diaphragm according to claim 6, characterized in that, The diaphragm has a thermal shrinkage rate of ≤5% at 130℃ and an ionic conductivity of ≥1.0×10⁻⁶. -3 The flame retardant release rate in the flame retardant coating is ≥90% when the temperature reaches 65℃-75℃.

9. The method according to claim 3, characterized in that, The conditions for ultrasonic dispersion in step S2 are: power 300-500W, time 20-40 minutes; stirring speed 800-1200rpm; and the mass ratio of paraffin to triethyl phosphate in the core material is 10:3.2–8.

6.

10. A lithium battery thermal safety management system, characterized in that, It includes the lithium battery and temperature monitoring unit as described in claim 7; when the internal temperature of the lithium battery reaches 65-80°C, the flame-retardant phase change microcapsule triggers the release of flame retardant.

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