Solid-solid phase change diaphragm material with in-situ thermal management and high-temperature conductivity enhancement functions and preparation method of solid-solid phase change diaphragm material

Abstract

The invention discloses a solid-solid phase change diaphragm material with in-situ thermal management and high-temperature conductivity enhancement functions and a preparation method of the solid-solid phase change diaphragm material, and belongs to the technical field of new energy materials and devices. A polyurethane solid-solid phase change material is used as a functional matrix, polyacrylonitrile is used as a spinning aid, the polyurethane solid-solid phase change material and the polyacrylonitrile are fully mixed in a solvent to form a spinning solution, and then the solid-solid phase change diaphragm material is obtained through electrostatic spinning. The diaphragm material not only can effectively maintain the structural integrity at high temperature, but also can greatly improve the lithium ion conductivity after high-temperature phase change. The diaphragm material also has excellent electrolyte wettability, greatly improves the safety and energy density of a battery system as a battery diaphragm, and has the advantages of simple preparation process, strong controllability, no need of expensive complex equipment, and good large-scale production potential.

Description

Technical Field

[0001] This invention belongs to the field of new energy materials and devices technology, specifically relating to a lithium-ion battery separator, particularly a composite nanofiber separator material with solid-solid phase change temperature control function and enhanced conductivity at high temperatures. Background Technology

[0002] With the increasing demand for fast charging and high energy density in electronic devices and electric vehicles, the thermal safety of lithium-ion batteries has become crucial to their reliability and lifespan. Phase change materials (PCMs), due to their ability to absorb or release large amounts of latent heat at constant temperatures, are considered ideal for battery thermal management. Currently, researchers are exploring the application of PCMs in the cooling systems of battery modules or packs, utilizing their heat storage capabilities to mitigate temperature fluctuations.

[0003] However, existing technologies have the following prominent problems and drawbacks:

[0004] Traditional separators lack in-situ temperature control capabilities: Current commercially available polyolefin (PP / PE) separators are "thermally inert" in terms of thermal management. When the internal temperature of the battery rises abnormally, traditional separators can only passively defend against the heat by closing the ion pathway through a pore-closing mechanism, and cannot actively absorb the accumulated heat. In addition, these separators have poor thermal stability (typically <160℃) and are prone to severe thermal shrinkage at high temperatures, leading to direct contact between the positive and negative electrodes and causing an internal short circuit.

[0005] Traditional thermal management relies on external components: Currently, most mainstream battery thermal management solutions focus on the battery pack level, namely using liquid cooling plates, air cooling systems, or filling the gaps between cells with a large number of phase change capsules. This "external temperature control" method not only has a long path and slow response, but also introduces a large number of inactive components, significantly increasing the size and weight of the battery system, and severely reducing the battery pack's assembly efficiency and overall energy density.

[0006] Bottlenecks in the application of phase change materials: Existing research attempts to introduce phase change materials into the battery for heat absorption and cooling. However, most PCMs (such as paraffin) have extremely poor ion conduction capabilities, and their addition significantly reduces lithium-ion transport efficiency. More seriously, traditional PCMs typically cause a sharp increase in battery internal resistance at high temperatures, limiting the battery's output capacity under high-power conditions.

[0007] Existing technologies for battery thermal management using phase change materials lack a smart membrane material that can both "actively absorb heat and cool down" and "increase instead of decrease" ion transport efficiency at high temperatures (near the phase change point).

[0008] Therefore, there is an urgent need in this field for a phase change membrane material that can simultaneously achieve battery thermal management, lightweighting, and help improve the high-rate performance of batteries. Summary of the Invention

[0009] This invention aims to solve the problem that existing phase change membranes limit the fast charging performance of batteries due to poor ion conductivity, as well as the pain point that traditional membranes cannot actively control temperature. It provides a solid-solid phase change membrane material of polyurethane / polyacrylonitrile composite nanofibers, which can improve transmission efficiency by utilizing waste heat while ensuring the internal thermal safety of the battery.

[0010] The core of this invention lies in utilizing polyurethane solid-solid phase change materials as the basis for achieving multifunctionality. During the preparation of the electrospinning solution, the in-situ cross-linked polyurethane phase change material is dispersed in a polyacrylonitrile spinning aid at high temperature, ensuring thorough mixing of the phase change material and the spinning aid. Subsequently, a solid-solid phase change membrane is obtained through electrospinning. The spinning aid ensures that the phase change material retains its solid-solid phase change characteristics even above the phase change temperature, thereby preventing leakage of the phase change material that could lead to a short circuit in the battery. It also promotes stable adhesion between the phase change membrane and the positive and negative electrodes and enhances its affinity for the electrolyte.

[0011] Compared to existing solid-liquid phase change composite membranes, the solid-solid phase change membrane material of this invention can effectively maintain structural integrity at high temperatures, and the lithium-ion conductivity can still be significantly improved after high-temperature phase change. Existing phase change membranes usually rely on the melting characteristics of the material for heat absorption, and are prone to leakage after phase change due to their liquid state, requiring additional encapsulation methods. This invention utilizes electrospinning technology to disperse and composite polyurethane solid-solid phase change material with a spinning aid skeleton, exhibiting excellent electrolyte wettability, enhancing the heat resistance and dimensional stability of the membrane, and also utilizing the ether chain thermal activation mechanism at high temperatures to enhance ion transport efficiency.

[0012] The working principle of the solid-solid phase change membrane material of this invention is as follows: the selected polyurethane (PU) is composed of soft segments (polyether) and hard segments (diisocyanate). When the battery temperature rises to the phase change point, the PU soft segments undergo a solid-solid phase change from a crystalline state to a disordered state, absorbing a large amount of latent heat (enthalpy), thereby suppressing the battery temperature rise. Due to the solid-solid phase change, the material remains solid throughout, eliminating the risk of leakage. The PU soft segments are rich in ether oxygen bonds (-O-), exhibiting Lewis basicity, and can react with lithium ions (Li... + Coordination. At room temperature, polymer chain segment movement is restricted; however, at high temperatures (phase transition range), thermal energy "activates" the mobility of the ether chains. The vigorous chain movement reduces the Li... + The dissociation energy, and like a "conveyor belt" accelerates Li + The jumping between chain segments causes the ionic conductivity of the membrane to increase significantly with increasing temperature.

[0013] Specifically, the solid-solid phase change membrane material provided by the present invention is a composite nanofiber material obtained by uniformly blending polyurethane and polyacrylonitrile at the nanoscale. The polyurethane is a solid-solid phase change material that is in-situ crosslinked with polyether soft segments and diisocyanate hard segments. Polyacrylonitrile is a spinning aid. The polyurethane and polyacrylonitrile are fully mixed in a solvent to form a spinning solution, and then the solid-solid phase change membrane material is obtained by electrospinning.

[0014] The solid-solid phase change membrane material of the present invention includes:

[0015] (1) Functional matrix: Polyurethane with solid-solid phase change behavior, accounting for 30-70 wt.% of the total mass of the membrane material, most preferably 50 wt.%. Preferably, the soft segment of the polyurethane is composed of polyethylene glycol, and the hard segment is composed of one or more of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and toluene diisocyanate (TDI). In some embodiments of the present invention, the soft segment of the polyurethane is composed of polyethylene glycol (PEG) with a molecular weight of 6000, and the hard segment is composed of hexamethylene diisocyanate (HDI), with a mass ratio of 1:1.

[0016] (2) Spinning matrix: accounting for 30~70 wt.% of the total weight of solid-solid phase change membrane material, with the most preferred being 50 wt.%. Polyacrylonitrile (PAN) has a high melting point (>300℃) and excellent mechanical strength. When PU undergoes phase change and softening, the nanofiber skeleton formed by PAN plays a supporting role and maintains the dimensional stability of the membrane.

[0017] The final solid-solid phase change membrane material structure shows that PU and PAN are uniformly blended at the nanoscale, with fiber diameters distributed between 500 and 750 nm, constructing a three-dimensional porous network with high porosity.

[0018] This invention also provides a method for preparing the above-mentioned solid-solid phase change membrane material, comprising the following steps:

[0019] Step 1: Polyurethane with a specific solid-solid phase transition temperature (e.g., 40-60℃) is synthesized by in-situ crosslinking polymerization of polyether polyol and diisocyanate.

[0020] Step 2: Dissolve the PU particles synthesized in Step 1 and PAN powder together in a solvent in a predetermined ratio, and stir at 60~80℃ for a period of time to form a uniform and transparent spinning solution.

[0021] Step 3: Using high-voltage electrospinning technology, the spinning solution is stretched into nanofibers and deposited on the receiving end to obtain the solid-solid phase change membrane material.

[0022] Preferably, the solid-solid phase transition temperature of the polyurethane synthesized in step one is in the range of 40°C to 60°C. The in-situ crosslinking polymerization includes: drying the polyether polyol and dissolving it in an organic solvent (such as DMF), adding diisocyanate and stirring to react, and performing prepolymerization at a first temperature (generally 40°C); then heating the system to a second temperature (generally 70°C) and adding a catalyst such as dibutyltin dilaurate (DBTDL), and continuing to stir to complete the chain extension process; finally, pouring the resulting homogeneous solution into a mold, drying it with forced air to remove the solvent and complete the curing process, thereby obtaining the polyurethane.

[0023] Furthermore, the solid content of the spinning solution in step two above is approximately 10-15 wt.%, and the solvent used is an organic solvent such as N,N-dimethylformamide (DMF) or dimethylacetamide.

[0024] Furthermore, the key parameters for high-voltage electrospinning in step three above are set as follows: the positive high voltage applied to the nozzle is +15~+17 kV, the negative high voltage applied to the receiving device is -4~-6 kV, and the distance from the nozzle tip to the receiver is fixed at 20~25 cm.

[0025] The solid-solid phase change membrane material of the present invention can be used as a membrane material for lithium-ion batteries, which greatly improves the safety and energy density of lithium-ion battery systems.

[0026] Compared with the prior art, the present invention has the following significant advantages and positive effects:

[0027] 1. This invention overcomes the technical bottleneck of the trade-off between "high safety" and "high-rate performance": Existing technologies typically sacrifice ion transport efficiency (such as by introducing an inactive flame-retardant layer) for safety. This invention creatively achieves a significant increase in ionic conductivity with increasing temperature by utilizing the "thermal activation" mechanism of the ether oxygen chain in the polyurethane soft segment. Under high-temperature or high-rate fast-charging conditions, the vigorous movement of the polymer chain actively assists in the rapid dissociation and jumping of lithium ions, resulting in a several-fold increase in the ionic conductivity of the separator in the phase change range (e.g., 45-60℃) compared to room temperature. This not only does not increase internal resistance but also meets the low impedance requirement of fast-charging scenarios, fundamentally solving the problem that traditional phase change materials can only absorb heat but cannot utilize it.

[0028] 2. Achieves in-situ "active" thermal buffering and intrinsic safety within the battery: Unlike commercial polyolefin separators that rely solely on closed pores for passive protection, this invention utilizes the solid-solid phase change properties of polyurethane to impart an adjustable phase change enthalpy to the separator. The separator can absorb accumulated heat at the heat source inside the battery immediately, reducing the peak temperature of the cell and delaying thermal runaway. Furthermore, due to the solid-solid phase change mechanism, the material remains solid after absorbing heat, avoiding the risk of leakage and flow at high temperatures that could lead to insulation failure in traditional solid-liquid phase change materials.

[0029] 3. Lightweight Integration and Process Scalability: This invention integrates thermal management functions into a micron-sized separator, achieving functional integration. Compared to traditional external thermal management solutions that rely on liquid cooling plates, thermally conductive adhesives, or phase change capsules, this invention achieves efficient temperature control without increasing the battery pack's volume and weight, significantly improving the system's energy density. Furthermore, the electrospinning technology and raw materials used are from mature industrial systems, resulting in a simple and highly controllable manufacturing process that requires no expensive or complex equipment, demonstrating excellent potential for large-scale production. Attached Figure Description

[0030] Figure 1 Low-magnification scanning electron microscope (SEM) image of the solid-solid phase change membrane material synthesized in the embodiments of the present invention.

[0031] Figure 2 High-magnification SEM image of the solid-solid phase change membrane material synthesized in this embodiment of the invention.

[0032] Figure 3 The electrolyte contact angle test results of the solid-solid phase change membrane material synthesized in the embodiments of the present invention are shown in (a) and (b), which are images of the solid-solid phase change membrane SEP-ES synthesized in the embodiments after wetting the electrolyte at the interface for 0s and 0.01s, respectively; (c) and (d) are images of the commercial polypropylene membrane SEP-PP after wetting the electrolyte at the interface for 0s and 20s, respectively.

[0033] Figure 4 Thermogravimetric analysis curve of the solid-solid phase change membrane material synthesized in Example 1 of this invention, wherein SEP-ES represents the solid-solid phase change membrane material, and the numbers following it represent the ratio of polyurethane to polyacrylonitrile, such as SEP-ES-97 representing that the mass ratio of polyurethane to polyacrylonitrile in the solid-solid phase change membrane material is 9:7.

[0034] Figure 5 The DSC curves of solid-solid phase change membrane materials with different mass ratios synthesized in the embodiments of the present invention are shown. The number after the phase change membrane represents the ratio of polyurethane to polyacrylonitrile, such as 97, which represents that the mass ratio of polyurethane to polyacrylonitrile in the solid-solid phase change membrane material is 9:7.

[0035] Figure 6 The enthalpy and phase transition temperature of solid-solid phase change membrane materials with different mass ratios synthesized in the embodiments of the present invention.

[0036] Figure 7 A comparison of the activation energy of the preferred-ratio phase change membrane synthesized in the embodiments of the present invention and a commercial polypropylene membrane as a function of temperature.

[0037] Figure 8 A temperature comparison graph of the preferred ratio phase change membrane synthesized in the embodiments of the present invention and a commercial polypropylene membrane under different charge and discharge rates. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, so that those skilled in the art can have a clearer and more intuitive understanding of the technical objectives, technical solutions, and technical effects of the present invention. However, it should be noted that the scope of the present invention is not limited thereto. Any modifications or equivalent substitutions to the technical solutions of the present invention that do not depart from the spirit and substance of the present invention should be covered within the protection scope of the present invention.

[0039] Example: Preparation and testing of solid-solid phase change membrane materials

[0040] Synthesis of Polyurethane: Polyurethane was synthesized via a traditional two-step solution polymerization method. First, to completely remove adsorbed moisture, polyethylene glycol (PEG 6000) with a molecular weight of 6000 was placed in a vacuum drying oven and dried at 120°C for 2 hours. Then, 5 g of the dried PEG was dissolved in 10 mL of N,N-dimethylformamide (DMF) and continuously magnetically stirred at 40°C until the solution became completely clear and transparent. Next, 600 μL of hexamethylene diisocyanate (HDI) was added to the above solution, and the reaction was maintained at this temperature for 2 hours for prepolymerization. After the reaction was completed, the system temperature was raised to 70°C, and 10 μL of dibutyltin dilaurate (DBTDL) catalyst was added dropwise, with stirring continued for 1 hour to complete the chain extension process. Finally, the resulting homogeneous solution was poured into a polytetrafluoroethylene mold and dried in an 80°C forced-air drying oven for 12 hours to remove the solvent and complete curing, ultimately yielding a milky white pure PU product.

[0041] Preparation of spinning precursor solution: 0.7 g of polyacrylonitrile (PAN) powder was dissolved in 10 mL of DMF and magnetically stirred at room temperature until the solution became clear and transparent. Then, the solution temperature was raised to 60 °C, and synthesized PU was added at a predetermined mass ratio. Stirring was continued at a constant temperature until a homogeneous and transparent spinning precursor solution was formed. To investigate the effect of components on membrane performance, five spinning precursor solutions with different PU / PAN mass ratios were prepared. Detailed formulations are shown in Table 1.

[0042] Table 1. Formulations of spinning precursor solutions with different mass ratios

[0043]

[0044] Preparation of the phase change membrane material: The above-mentioned spinning precursor solution was transferred to a syringe equipped with a stainless steel nozzle for electrospinning. Key parameters during the spinning process were set as follows: a positive high voltage of +15 kV was applied to the nozzle, a negative high voltage of -4 kV was applied to the receiving device, and the distance from the nozzle tip to the receiver was fixed at 20 cm. The solution propulsion speed of the injection pump was controlled at 0.05 cm / min. After the spun nanofiber membrane was peeled off from the receiver, it was placed in a vacuum drying oven to completely remove residual solvent, finally obtaining the solid-solid phase change membrane material.

[0045] Microscopic morphology observation: Low-magnification scanning electron microscope (SEM) images ( Figure 1 The synthesized phase change membrane material exhibits a continuous and uniform three-dimensional network structure, with randomly oriented fibers interwoven to form abundant pores, which is beneficial for electrolyte adsorption; high-magnification SEM images ( Figure 2 Further research revealed that the nanofibers have smooth and round surfaces, no beading defects, and uniform fiber diameter distribution (approximately 600 nm). This indicates that the functional component PU and the backbone material PAN achieved good molecular-level blending during the spinning process. The phase change component was effectively bound in the PAN backbone network, and no phase separation or agglomeration occurred, ensuring the structural stability of the material.

[0046] Electrolyte wettability: The interfacial wetting behavior of the commercial polypropylene separator (SEP-PP) and the SEP-ES solid-solid phase change separator synthesized in this invention was compared by contact angle testing. Figure 3 ).like Figure 3 As shown in (c) and (d), due to the inherent hydrophobic properties and low surface energy of polypropylene, electrolyte droplets are difficult to spread effectively on its surface. After 20 s of droplet residence, the contact angle of SEP-PP remains at 58°, indicating its poor affinity for ether-based electrolytes. In contrast, SEP-ES exhibits excellent hydrophilicity: electrolyte droplets are rapidly absorbed upon contact with the film surface (< 0.01 s), and the contact angle drops to 0°. This superhydrophilic property mainly stems from the synergistic effect of the material's chemical composition and microstructure.

[0047] Thermal stability: The thermal decomposition temperature of each sample was determined using thermogravimetric analysis. For example... Figure 4 As shown, all solid-solid phase change membranes did not exhibit significant mass loss below 200℃, demonstrating excellent thermal stability. The aforementioned decomposition initiation temperatures are far higher than the normal operating temperature range of lithium-ion batteries and the trigger temperature thresholds under most thermal abuse conditions.

[0048] Latent heat storage capacity: Differential scanning calorimetry (DSC) curve ( Figure 5 The results show that the synthesized phase change membrane material exhibits a sharp and single endothermic peak around 56°C during the heating process, corresponding to the solid-solid phase change process of the polyurethane soft segment crystalline region. At the preferred ratio (polyurethane to polyacrylonitrile mass ratio of 7:7), its enthalpy of melting reaches as high as 70.36 J / g, and the phase change temperature is 56°C. Figure 6 This high enthalpy gives the separator a strong latent heat storage capacity, which can effectively buffer thermal shock, while the suitable phase change temperature is right in the upper limit of the battery's normal operation, which is conducive to timely intervention and temperature control in the early stage of overheating.

[0049] Enhanced Activation Energy: The core innovation of this invention lies in solving the problem of poor ion conduction in traditional phase change materials using a thermal activation mechanism, which is confirmed by the Arrhenius curve of ionic conductivity. Unlike commercial polypropylene membranes, whose conductivity increases slowly and linearly with temperature (activation energy 20.07 kJ / mol), the phase change membrane material synthesized in this embodiment exhibits significant nonlinear characteristics. Figure 7 As shown, below the phase transition temperature, the activation energy is relatively high (44.35 kJ / mol) due to segment freezing; however, when the temperature exceeds the phase transition point and enters the high-temperature region, the slope of the curve changes abruptly, and the activation energy drops sharply to 10.24 kJ / mol. This directly proves that the polyurethane soft segment undergoes a phase transition at high temperatures, and the movement of the ether oxygen chain segment is strongly activated by thermal energy. As a highly efficient "ion transport conveyor belt," it significantly reduces the lithium-ion jumping energy barrier, resulting in a substantial increase in ionic conductivity at high temperatures, even surpassing that of commercially available membranes.

[0050] Battery thermal management performance: The actual thermal management effect of this solid-solid phase change membrane material was verified through battery charge-discharge cycle tests. Figure 8 As shown, the surface temperature of the pouch cell was monitored at 1C, 2C, and 3C rates, and the temperature rise difference between the commercial polypropylene separator and the separator of this embodiment was compared. At low rates, the difference was not significant, but under 3C high-power conditions, the peak temperature of the commercial polypropylene separator battery rapidly rose to over 62°C, approaching the danger zone; while the battery using the solid-solid phase change separator synthesized in this embodiment had its peak temperature controlled at around 55°C at the end of discharge, with a clear "temperature plateau" appearing near 55°C, resulting in a temperature difference of approximately 7°C. This result conclusively proves that the separator of this invention undergoes a solid-solid phase change inside the battery and absorbs latent heat, successfully achieving in-situ "peak-shaving" thermal management and significantly improving battery safety in high-rate fast charging scenarios. In contrast, the commercial polypropylene separator, lacking phase change heat absorption and without a reduction in high-temperature ion conduction activation energy, cannot effectively suppress temperature rise.

Claims

1. A solid-solid phase change membrane material, characterized in that, It is a composite nanofiber material obtained by uniformly blending polyurethane and polyacrylonitrile at the nanoscale. The polyurethane is a solid-solid phase change material that is in-situ crosslinked with polyether soft segments and diisocyanate hard segments. Polyacrylonitrile is used as a spinning aid. The polyurethane and polyacrylonitrile are fully mixed in a solvent to form a spinning solution, and then the solid-solid phase change membrane material is obtained by electrospinning.

2. The solid-solid phase change membrane material as described in claim 1, characterized in that, The polyurethane accounts for 30-70% of the total mass of the solid-solid phase change membrane material.

3. The solid-solid phase change membrane material as described in claim 1, characterized in that, The soft segment of the polyurethane is composed of polyethylene glycol, and the hard segment is composed of one or more of hexamethylene diisocyanate, isophorone diisocyanate, and toluene diisocyanate.

4. The solid-solid phase change membrane material as described in claim 3, characterized in that, The polyurethane's soft segment is composed of polyethylene glycol 6000, and its hard segment is composed of hexamethylene diisocyanate, with a mass ratio of 1:

1.

5. The solid-solid phase change membrane material as described in claim 1, characterized in that, The solid-solid phase change membrane material is a three-dimensional porous network constructed from fibers with diameters distributed between 500 and 750 nm.

6. The method for preparing the solid-solid phase change membrane material according to any one of claims 1 to 5, characterized in that, Includes the following steps: 1) Polyurethanes with specific solid-solid phase transition temperatures are synthesized by in-situ crosslinking polymerization of polyether polyols and diisocyanates; 2) Dissolve the polyurethane synthesized in step 1) and polyacrylonitrile together in a solvent in a predetermined ratio, and stir at 60~80℃ for a period of time to form a uniform and transparent spinning solution. 3) The spinning solution is stretched into nanofibers using high-voltage electrospinning technology and deposited on the receiving end to obtain the solid-solid phase change membrane material.

7. The preparation method according to claim 6, characterized in that, Step 1) The solid-solid phase transition temperature of the synthesized polyurethane is in the range of 40℃~60℃.

8. The preparation method according to claim 6, characterized in that, The solid content of the spinning solution in step 2) is 10~15 wt.%, and the solvent used is one or more of N,N-dimethylformamide and dimethylacetamide.

9. The preparation method according to claim 6, characterized in that, Step 3) The parameters for high-voltage electrospinning are set as follows: the positive high voltage applied to the nozzle is +15~+17 kV, the negative high voltage applied to the receiving device is -4~-6 kV, and the distance from the nozzle tip to the receiver is fixed at 20~25 cm.

10. The application of the solid-solid phase change membrane material according to any one of claims 1 to 5 as a lithium-ion battery separator.

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