A flame-retardant solid-solid phase change material, its preparation method, and its application.

Abstract

This invention discloses a flame-retardant solid-solid phase change material, prepared by a crosslinking reaction of polyurethane prepolymer, carboxylated chitosan, thermally conductive filler, and flame-retardant additives. The polyurethane prepolymer is prepared by a crosslinking reaction of polyether polyol and isocyanate. This invention, through a two-step chemical crosslinking reaction, transforms solid-liquid phase change polyethylene glycol into a solid-solid phase change material at the molecular level. This fundamentally solves the problem of decreased thermal control performance of solid-liquid phase change polyethylene glycol due to leakage after long-term cycling. Due to the rich covalent crosslinking network and strong hydrogen bonding within the material, the flame-retardant solid-solid phase change material exhibits good mechanical strength and thermally induced flexibility, which helps reduce the contact thermal resistance between the material and the battery, improves the heat transfer efficiency of the battery thermal management system, and allows the material to be applied to battery thermal safety systems, achieving both efficient heat dissipation during conventional thermal management and preventing the spread of thermal runaway.

Description

Technical Field

[0001] This invention relates to the technical field of polymer materials, and in particular to a flame-retardant solid-solid phase change material, its preparation method, and its application. Background Technology

[0002] The rapid development of electric vehicles and electrochemical energy storage systems is of great significance for reducing greenhouse gas emissions and promoting the clean and efficient use of energy. Lithium-ion batteries, due to their inherent advantages such as high energy density, long cycle life, and flexible design, have become the mainstream technology in the field of rechargeable energy storage. However, LIBs are highly sensitive to temperature, a characteristic that poses a challenge to their safety and performance in practical use. Especially under high-rate discharge conditions, complex electrochemical reactions inside the battery can lead to a sharp rise and uneven distribution of module temperature, thus affecting system performance and lifespan. To ensure the reliability of lithium-ion batteries during long-term operation, the battery thermal management system must control their maximum temperature below 50°C and maintain the temperature difference within the module within 5°C to effectively mitigate problems such as capacity decay. More seriously, the continuous generation and accumulation of heat can accelerate the battery aging process and may even induce thermal runaway and explosion. During thermal runaway, the large amount of heat released by the battery in a short period can easily trigger thermal runaway in adjacent batteries. Therefore, a battery thermal management system that combines efficient temperature management and thermal runaway protection is crucial.

[0003] Research on battery thermal management systems under normal operation is quite extensive, including solutions such as air cooling, liquid cooling, composite phase change material (PCT) cooling, or hybrid cooling. Among these, composite PCTs have broad application prospects due to their high energy density, low cost, and lack of external power supply. Unlike active air and liquid cooling, passive battery thermal management systems based on composite PCTs can suppress temperature rise and promote uniform temperature distribution by efficiently absorbing and releasing heat during battery charging and discharging when overheating or overcooling occurs. However, traditional composite PCTs are flammable under high-temperature conditions, and the presence of flammable volatile gases such as hydrocarbons emitted from the battery can even exacerbate the spread of thermal runaway. Furthermore, most existing documents and patents on preventing battery thermal runaway involve designing heat-insulating aerogels. This, in turn, has an adverse effect on thermal management under normal battery operation, easily leading to heat accumulation, resulting in large temperature rises and temperature differences, which is detrimental to the long-term operation of the battery module.

[0004] Current research on flame-retardant composite phase change materials mainly focuses on improving the single performance of the material, which makes it difficult to solve the long-standing contradiction between the incorporation of flame-retardant additives and the deterioration of phase change performance and mechanical properties. Moreover, the large-scale addition of flame-retardant additives inevitably leads to the formation of a rigid polymer skeleton, which inevitably increases the contact thermal resistance between the composite phase change material and the battery in the module, affecting the heat transfer efficiency of the system. Therefore, there is an urgent need to develop a multifunctional flexible flame-retardant phase change material (FRCPCM) that simultaneously possesses high flame retardancy, excellent temperature control performance, stable shape, and flexible installation. This is of great significance for improving the thermal safety performance of battery modules and suppressing the spread of thermal runaway. Summary of the Invention

[0005] The purpose of this invention is to solve the problems in the prior art by proposing a flame-retardant solid-solid phase change material, its preparation method, and its application. This flame-retardant solid-solid phase change material, while ensuring high latent heat, high thermal conductivity, and good mechanical flexibility, also has excellent flame-retardant properties. When applied to battery thermal safety systems, it can simultaneously achieve efficient heat dissipation during conventional thermal management and prevent the spread of thermal runaway. This demonstrates great application potential in improving the thermal safety of electric vehicles and other battery energy storage systems.

[0006] To achieve the above objectives, the present invention proposes a flame-retardant solid-solid phase change material, which is prepared by a crosslinking reaction of polyurethane prepolymer, carboxylated chitosan, thermally conductive filler and flame-retardant additive, wherein the polyurethane prepolymer is prepared by a crosslinking reaction of polyether polyol and isocyanate.

[0007] Preferably, the polyether polyol is polyethylene glycol, which has a molecular weight of 4000 and a latent heat of phase change of 180.8 J / g.

[0008] Preferably, the isocyanate is diphenylmethane diisocyanate.

[0009] Polyethylene glycol is used as the phase change substrate, with a content of 59.73 wt%. Diphenylmethane diisocyanate and carboxylated chitosan are used as crosslinking agents, with a total content of 10.27 wt%. The content of diphenylmethane diisocyanate is 7.43 wt%, and the content of carboxylated chitosan is 2.84 wt%.

[0010] Preferably, the thermally conductive filler is worm-shaped expanded graphite with a size of 100 mesh, a carbon content of 99.5%, and an addition amount of 5 wt%.

[0011] Preferably, the flame retardant additive is a composition of ammonium polyphosphate, melamine phosphate and pentaerythritol phosphate, with an addition amount of 25 wt% and a mass ratio of ammonium polyphosphate:melamine phosphate:pentaerythritol phosphate = 6:3:1.

[0012] The preparation method of the above-mentioned flame-retardant solid-solid phase change material includes the following steps: S1, polyether polyol and isocyanate undergo a cross-linking reaction under the action of a catalyst to form a polyurethane prepolymer; S2. Add carboxylated chitosan, expanded graphite and flame retardant additives to the polyurethane prepolymer and carry out a crosslinking reaction to obtain a flame retardant crosslinking system. S3. After adding glutaraldehyde to the flame-retardant crosslinking system and mixing it evenly, pour the mixed solution into the medium mold and air-dry to cure it to obtain the flame-retardant solid-solid phase change material.

[0013] Preferably, the specific steps of S1 are as follows: polyethylene glycol is placed in a three-necked flask containing solvent and completely melted under a nitrogen atmosphere at 80°C. Then, diphenylmethane diisocyanate is added to the flask, with a molar ratio of polyethylene glycol to diphenylmethane diisocyanate of 1:2. Then, 0.1 wt% of the catalyst dibutyltin dilaurate is slowly added dropwise, and the mixture is stirred for a certain period of time at a speed of 1000 rpm to obtain a polyurethane prepolymer.

[0014] Preferably, the specific steps of S2 are as follows: continue to add carboxylated chitosan, expanded graphite and flame retardant additives to the flask, rotate at 1500 rpm, and continue to react at 80°C for a certain period of time.

[0015] Preferably, the specific steps of S3 are as follows: add an appropriate amount of glutaraldehyde solution, stir rapidly at a higher speed for 5 minutes, and finally pour the mixed solution into a customized silicone mold and cure it in a forced-air drying oven at 80°C for 48 hours to obtain a shape-stable flame-retardant solid-solid phase change material.

[0016] As a preferred application, the flame-retardant solid-solid phase change material prepared by the above method is used in a battery thermal safety system: the flame-retardant solid-solid phase change material is installed in a square lithium iron phosphate battery module.

[0017] Preferably, the flame-retardant solid-solid phase change material is prepared into a cuboid with a thickness of 6 mm and a length and width that are the same as the maximum surface area of ​​the lithium iron phosphate single cell, and sandwiched between every two cells and on the outside of the battery module in a sandwich structure.

[0018] The beneficial effects of this invention are: This invention transforms solid-liquid phase change polyethylene glycol into a solid-solid phase change through a two-step chemical cross-linking reaction and molecular-level structural design. This fundamentally solves the problem of decreased thermal control performance of solid-liquid phase change polyethylene glycol due to leakage after long-term cycling. Due to the rich covalent cross-linking network and strong hydrogen bonding within the material, the flame-retardant solid-solid phase change material exhibits good mechanical strength and thermally induced flexibility, which helps to reduce the contact thermal resistance between the material and the battery and improve the heat transfer efficiency of the battery thermal management system.

[0019] In the second step of the crosslinking reaction, the present invention adds carboxylated chitosan as a crosslinking agent. The surface of carboxylated chitosan has abundant hydroxyl, amino, and carboxyl groups, which is the basis for forming a strong hydrogen bond network. It can strengthen the interfacial interaction with polyethylene glycol, thereby improving the stability and crosslinking density of the three-dimensional network through chemical crosslinking and strong hydrogen bonding.

[0020] The chemically intumescent flame retardant of this invention is composed of ammonium polyphosphate, melamine phosphate, and pentaerythritol phosphate, integrating an "acid source," "gas source," and "carbon source." The worm-like expanded graphite and the "three-in-one" chemically intumescent flame retardant synergistically improve the flame retardant performance and high-temperature stability of the material in both the gas phase and condensed phase. At the same time, this preparation method solves the long-standing contradiction between the large-scale incorporation of flame retardants and the decline in other properties.

[0021] From a synthetic perspective, this invention can achieve the polymerization reaction of materials under mild conditions through a "one-pot" preparation method, which is suitable for large-scale commercial preparation; and the melt obtained from the reaction can be used to prepare flame-retardant solid-solid phase change materials of different shapes by adjusting the mold size according to actual specifications, so as to meet different application scenarios.

[0022] While commercially available thermal insulation aerogels used in battery thermal management systems have shown good effectiveness and advantages in preventing battery thermal runaway, their low thermal conductivity is not conducive to heat dissipation during conventional thermal management, increasing the risk of temperature rise and temperature inhomogeneity in battery modules. The flame-retardant solid-solid phase change material provided by this invention not only balances the temperature distribution of battery modules during conventional thermal management but also effectively suppresses the risk of thermal runaway, providing important technical support for thermal safety solutions for next-generation power battery modules and battery energy storage systems.

[0023] The features and advantages of the present invention will be described in detail through embodiments and in conjunction with the accompanying drawings. Attached Figure Description

[0024] Figure 1 The graph shows the performance test results of the phase change materials prepared in Comparative Examples 1 and 2 and Examples 1 to 5; in, Figure 1 (a) is the DSC curve; Figure 1 (b) is a comparison diagram of crystallization enthalpy and melting enthalpy; Figure 1 (c) is a comparison chart of crystallization temperature and melting temperature; Figure 1 (d) is a comparison chart of enthalpy efficiency and relative enthalpy efficiency; Figure 1 (e) is a comparison chart of thermal conductivity; Figure 1 (f) is a comparison chart of leakage rates at 80℃; Figure 2 The XRD patterns are of the phase change materials prepared in Comparative Examples 1 and 2 and Examples 1 to 5. Figure 3 The stress-strain curves of the phase change materials prepared in Comparative Examples 1 and 2 and Examples 1 to 5 at 25°C and 60°C are shown. Figure 4 This is the FTIR spectrum of the flame-retardant solid-solid phase change material prepared in Example 5; Figure 5 These are images of the flame-retardant solid-solid phase change material prepared in Example 5 burning under two consecutive 10-second flame exposures; Figure 6 This is a comparison chart of the flame retardant properties of the phase change materials prepared in Comparative Examples 1 and 2 and Examples 1 to 5; Figure 7 This is a schematic diagram showing the temperature and temperature difference of a battery module without any thermal management at discharge rates of 1-4C. Figure 8 This is a schematic diagram showing the temperature and temperature difference of the battery module used in Example 5 at discharge rates of 1-4C. Among them, T max This is the highest temperature of the battery module; T average This represents the average temperature of the battery module. This represents the maximum temperature difference of the battery module; The lateral temperature difference of the battery module; This refers to the longitudinal temperature difference of the battery module. Figure 9 This is a comparison chart showing the effect of applying Example 5 and Comparative Example 2 on preventing battery thermal runaway; In the attached figures, PEG is the phase change matrix material, PU corresponds to the phase change material prepared in Example 1, PUE corresponds to the phase change material prepared in Example 2, PUE-A corresponds to the phase change material prepared in Example 1, PUE-AM1 corresponds to the phase change material prepared in Example 2, PUE-AM2 corresponds to the phase change material prepared in Example 3, PUE-M corresponds to the phase change material prepared in Example 4, PUE-AMP corresponds to the phase change material prepared in Example 5, Melting temperature is the melting point temperature; Crystallization temperature is the crystallization temperature; λ is the enthalpy efficiency, and η is the relative enthalpy efficiency. Detailed Implementation

[0025] Example 1: Flame-retardant solid-solid phase change materials are prepared according to the following steps: Step (1) Place 59.73 g of polyethylene glycol in a three-necked flask containing 200 mL of N,N-dimethylformamide and melt it completely under a nitrogen atmosphere at 80 °C.

[0026] Step (2) Add 7.43g of diphenylmethane diisocyanate to ensure that the molar ratio of polyethylene glycol to diphenylmethane diisocyanate is 1:2. Then slowly add 10g of dibutyltin dilaurate catalyst dropwise at a speed of 1000rpm and mix for 1 hour to obtain polyurethane prepolymer.

[0027] In this process, the isocyanate groups in diphenylmethane diisocyanate react with the hydroxyl groups in polyethylene glycol through a polycondensation reaction to form a terminal isocyanate prepolymer with a long-chain structure.

[0028] In step (3), add 2.84g of carboxylated chitosan, 5g of expanded graphite and 25g of ammonium polyphosphate, and react at 1500rpm for 1.5h at 80℃.

[0029] Step (4) Then add 10 mL of glutaraldehyde solution and stir rapidly at 1500 rpm for 5 min; finally, pour the mixed solution into the customized silicone mold and cure it in an 80℃ drying oven for 48 h to obtain a shape-stable flame-retardant solid-solid phase change material.

[0030] Example 2: In step (3), add 2.84g of carboxylated chitosan, 5g of expanded graphite, 16.67g of ammonium polyphosphate and 8.33g of melamine phosphate, and react at 1500 rpm for 1.5h at 80°C.

[0031] Everything else is the same as in Example 1.

[0032] Example 3: In step (3), add 2.84g of carboxylated chitosan, 5g of expanded graphite, 8.33g of ammonium polyphosphate and 16.67g of melamine phosphate, and react at 1500rpm for 1.5h at 80℃.

[0033] Everything else is the same as in Example 1.

[0034] Example 4: In step (3), add 2.84g of carboxylated chitosan, 5g of expanded graphite and 25g of melamine phosphate, and react at 1500rpm for 1.5h at 80℃.

[0035] Everything else is the same as in Example 1.

[0036] Example 5: In step (3), add 2.84g of carboxylated chitosan, 5g of expanded graphite, 12.5g of ammonium polyphosphate, 7.5g of melamine phosphate and 2.5g of pentaerythritol phosphate, and react at 1500 rpm for 1.5 h at 80°C.

[0037] Everything else is the same as in Example 1.

[0038] Comparative Example 1: Step (1) Add 85.1g of polyethylene glycol.

[0039] Step (2) Add 10.65g of diphenylmethane diisocyanate.

[0040] In step (3), only 4.25g of carboxylated chitosan is added.

[0041] Everything else is the same as in Example 1.

[0042] Comparative Example 2: Step (1) Add 80.84g of polyethylene glycol.

[0043] Step (2) Add 10.12g of diphenylmethane diisocyanate.

[0044] Step (3) Add 4.04g of carboxylated chitosan and 5g of expanded graphite.

[0045] Everything else is the same as in Example 1.

[0046] Experimental Section Phase change properties are of great significance for the application of energy storage systems, and latent heat value is a key parameter for evaluating the thermal efficiency of phase change materials.

[0047] The phase change properties, thermal conductivity, and shape stability of the above-mentioned phase change materials were tested, and the results are as follows: Figure 1 As shown.

[0048] The SEM and EDS images of the above phase change materials were tested, and the results are as follows: Figure 2 As shown.

[0049] The stress-strain curves of the above phase change material at 25℃ and 60℃ were tested, and the results are as follows: Figure 3 As shown.

[0050] The melting and crystallization temperatures of Comparative Example 1 ranged from 40.31 to 63.66 °C and from 26.38 to 38.86 °C, respectively. Furthermore, its enthalpy of melting and crystallization reached 180.8 J / g and 176 J / g, respectively. The endothermic / exothermic peak morphologies exhibited by Comparative Example 2 and Examples 1 to 5 during the melting / crystallization process were consistent with those of Example 1. This indicates that the chemical crosslinking reaction of polyethylene glycol and diphenylmethane diisocyanate, along with the physical blending of other components, did not alter the phase transition characteristics of polyethylene glycol. XRD analysis of the diffraction patterns and peak positions also confirmed that the reaction did not change the crystalline morphology of polyethylene glycol.

[0051] With the addition of diphenylmethane diisocyanate and carboxylated chitosan, the latent heat values ​​of each composite phase change material decreased; among them, the relative enthalpy efficiency (η) of the polyurethane precursor was 80.98%. Figure 1 (c) It can be seen that the melting temperature and crystallization temperature of the composite phase change material are both lower than those of polyethylene glycol. This may be due to the three-dimensional network structure of chemical cross-linking and physical blending, which affects the chain movement of polyethylene glycol molecules, thus hindering the phase change process of the composite phase change material to a certain extent and suppressing its heat release. It is worth noting that the (λ, η) coordinate axes of the flame-retardant composite phase change material with added flame retardant are all to the left of the dashed line, such as... Figure 1 (d); This shows that as the amount of flame retardant added λ decreases, the value of η actually increases; this may be because the micro-nano size flame retardant particles provide more heterogeneous nucleation sites for polyethylene glycol molecular chain segments, promoting the formation of a more perfect and stable crystal structure; Among them, the melting enthalpy and crystallization enthalpy of Example 5 are 100.8 J / g and 92.15 J / g, respectively, with a relative enthalpy efficiency of 93.34%, which has excellent energy storage density.

[0052] Comparative Examples 1 and 2 showed low thermal conductivity, both not exceeding 0.29 W / (m·K). Adding 5 wt% expanded graphite effectively improved the thermal conductivity of the composite phase change material. The study also found that adding a flame retardant could further enhance the thermal conductivity of the composite phase change material. The thermal conductivity of Examples 1, 2, 3, 4, and 5 were 1.32, 1.70, 1.34, 1.31, and 1.60 W / (m·K), respectively. This was mainly due to the small-sized spherical flame-retardant particles filled with expanded graphite and polyurethane. The gaps and defects between the ester matrix make the structure more compact and reduce the interfacial resistance. This indicates that there is a synergistic effect between an appropriate amount of expanded graphite and flame retardant, providing an effective way to prepare composite phase change materials with both high thermal conductivity and flame retardancy. In addition, Example 5 has the best anti-leakage performance, with the overall leakage rate controlled within 0.3%, and the mass does not change after 9 hours. This fundamentally solves the problem of thermal performance degradation and module collapse caused by leakage during long-term battery charging and discharging of phase change material modules prepared by physical mixing in the traditional way.

[0053] Adding flame retardants not only improves the mechanical strength of the material but also significantly increases the yield strain. Specifically, Examples 2 and 5 exhibit tensile strengths exceeding 5.8 MPa and elongation at break exceeding 22.5%. This is likely due to the presence of phosphorus-oxygen double bonds (P=O) on the surfaces of ammonium polyphosphate, melamine phosphate, and pentaerythritol phosphate, which can form hydrogen bonds with the ether oxygen bonds (-COC-) of PEG, improving the compatibility of the flame retardant with the matrix and preventing aggregation and migration. This indicates strong elasticity and flexibility at room temperature, providing a certain degree of mechanical impact protection for the battery. Preferably, Example 5 performs best at 60°C, with an elastic modulus of 10.94 MPa and an elongation at break of 40.22%. Figure 4 The fifth embodiment demonstrates that after reaching thermal equilibrium at 60°C, it can be freely bent, twisted, and rolled up by triggering a polyethylene glycol phase change. In addition, after the thermo-flexible effect occurs, the fifth embodiment can adhere tightly to the battery surface in a vertical state, with increased adhesion. This unique thermomechanical behavior ensures that it can still provide mechanical protection for power battery equipment or mobile energy storage systems to a certain extent after undergoing a phase change, and has good prospects for thermal management applications in confined spaces.

[0054] like Figure 5 As shown, Example 5 self-extinguished within 1 second after being subjected to two consecutive 10-second flame exposures, without any melting or dripping, achieving the highest V-0 rating of the UL-94 standard. Example 5 exhibits superior flame-retardant and fire-resistant performance, significantly suppressing the release of heat, smoke, and toxic gases, as shown in the six-dimensional radar chart (…). Figure 6 It can be clearly seen in the image that the "three-in-one" chemically expandable flame retardant system composed of ammonium polyphosphate, melamine phosphate, and pentaerythritol phosphate, in synergy with the physically expandable flame retardant EG, has a highly efficient flame retardant effect in releasing inert gases in the gas phase and forming a protective char layer barrier in the condensed phase.

[0055] like Figure 7 and Figure 8 As shown, under 1-4C charge-discharge cycles, the highest temperature of the battery module without phase change materials has exceeded the normal operating range of lithium batteries; while the battery module using phase change materials can effectively control the average temperature and maximum temperature difference within 50℃ and 3.7℃ respectively, still within the safe operating temperature range; it is worth noting that the lateral temperature difference of the battery module ( ) is always greater than the longitudinal temperature difference ( This is because, in the longitudinal direction, the continuous thermal buffer path of Embodiment 5 can effectively absorb heat, and in the transverse direction, due to the high contact thermal resistance, the convective heat transfer with the air is uneven. The above results show that Embodiment 5 has excellent temperature control and temperature uniformity performance, and can continuously absorb and transfer the heat of the battery during the cycle, thereby effectively suppressing the temperature rise and promoting the uniform distribution of heat in the module.

[0056] Figure 9 (a1, a2) shows the temperature and voltage changes of the thermal barrier-free module; at 2944 seconds, the pressure relief valve of the first battery opened due to excessive internal pressure, releasing a large amount of smoke and accompanied by a brief temperature drop (shown by the gray box in the figure); subsequently, the temperature rose at a rate of more than 1℃ / s, triggering thermal runaway of the first battery; at this time, the battery separator dissolved over a large area, a short circuit occurred between the positive and negative electrodes, and the voltage dropped rapidly to 0 V, reaching 285℃ at 3754 seconds; affected by the large amount of heat released in a short time due to the thermal runaway of the first battery, the pressure relief valve of the second battery was pushed open at 3923 seconds, only 169 seconds later; subsequently, thermal runaway was also triggered at 4227 seconds, with the peak temperature reaching 300.4℃.

[0057] like Figure 9 As shown in (b, b1), the module using PUE-AMP was able to prevent TR propagation; the highest temperature of the first battery rapidly reached 276.8°C after thermal runaway was triggered; while the highest temperature of the adjacent battery slowly rose to a maximum of 112.4°C after a subsequent time interval of approximately 500 seconds, which was 56.8°C lower than that of the PUE module; apart from this, TF and b never exceeded 50°C; and from Figure 9 (b2) It can be seen that the voltage of the second battery never dropped, which also proves that the second battery did not experience thermal runaway.

[0058] The above embodiments are illustrative of the present invention and are not intended to limit the present invention. Any simple modifications to the present invention are within the scope of protection of the present invention.

Claims

1. A flame-retardant solid-solid phase change material, characterized by: It is prepared by crosslinking polyurethane prepolymer, carboxylated chitosan, thermally conductive filler and flame retardant additive, wherein the polyurethane prepolymer is prepared by crosslinking polyether polyol and isocyanate.

2. The fire-retardant solid-solid phase change material of claim 1, wherein: The polyether polyol is polyethylene glycol, with a molecular weight of 4000 and a latent heat of phase change of 180.8 J / g.

3. The fire-retardant solid-solid phase change material of claim 1, wherein: The thermally conductive filler is worm-shaped expanded graphite with a size of 100 mesh and a carbon content of 99.5%.

4. As described in claim 1, characterized in that: The flame retardant additive is a composition of ammonium polyphosphate, melamine phosphate and pentaerythritol phosphate, with the ratio of ammonium polyphosphate:melamine phosphate:pentaerythritol phosphate being 6:3:1 by mass.

5. A method for preparing a flame-retardant solid-solid phase change material, characterized in that, Includes the following steps: S1, polyether polyol and isocyanate undergo a cross-linking reaction under the action of a catalyst to form a polyurethane prepolymer; S2. Add carboxylated chitosan, expanded graphite and flame retardant additives to the polyurethane prepolymer and carry out a crosslinking reaction to obtain a flame retardant crosslinking system. S3. After adding glutaraldehyde to the flame-retardant crosslinking system and mixing it evenly, pour the mixed solution into the medium mold and air-dry to cure it to obtain the flame-retardant solid-solid phase change material.

6. The preparation method according to claim 1, characterized in that, The specific steps of S1 are as follows: polyethylene glycol is placed in a three-necked flask containing solvent and completely melted under a nitrogen atmosphere at 80°C. Then, diphenylmethane diisocyanate is added to the flask, with a molar ratio of polyethylene glycol to diphenylmethane diisocyanate of 1:

2. Then, 0.1 wt% of dibutyltin dilaurate catalyst is slowly added dropwise, and the mixture is stirred for a certain period of time at a speed of 1000 rpm to obtain a polyurethane prepolymer.

7. The preparation method according to claim 1, characterized in that, The specific steps of S2 are as follows: continue to add carboxylated chitosan, expanded graphite and flame retardant additives to the flask, rotate at 1500 rpm, and continue to react at 80°C for a certain period of time.

8. The preparation method according to claim 1, characterized in that, The specific steps of S3 are as follows: add an appropriate amount of glutaraldehyde solution, stir rapidly at 1500 rpm for 5 minutes, and finally pour the mixed solution into a customized silicone mold and cure it in an 80°C forced-air drying oven for 48 hours to obtain a shape-stable flame-retardant solid-solid phase change material.

9. The application of the flame-retardant solid-solid phase change material prepared by any one of claims 5 to 8 in a battery thermal safety system, characterized in that: The flame-retardant solid-solid phase change material is installed in a square lithium iron phosphate battery module.

10. The application as described in claim 9, characterized in that: The flame-retardant solid-solid phase change material is prepared into a cuboid with a thickness of 6 mm and the same length and width as the maximum surface area of ​​a lithium iron phosphate cell, and sandwiched between every two cells and on the outside of the battery module in a sandwich structure.

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