High-temperature-resistant low-internal-resistance high-strength thermal battery diaphragm and preparation method thereof

Abstract

The invention discloses a high-temperature-resistant low-internal-resistance high-strength thermal battery diaphragm and a preparation method thereof, and belongs to the technical field of electrochemistry. The high-temperature-resistant, low-internal-resistance and high-strength thermal battery diaphragm is formed by compounding a nano magnesium oxide-loaded Si3N4 nanowire skeleton matrix and a solid molten salt electrolyte. The prepared diaphragm shows excellent high-temperature stability and has excellent compressive strength, and stable work under the condition of large external mechanical stress is ensured. In addition, the low internal resistance characteristic of the diaphragm is beneficial to efficient migration of internal ions, so that the energy density of the thermal battery is remarkably improved. And the small thickness of the thermal battery also provides powerful support for the miniaturization design of the thermal battery. According to the innovation, the preparation process of the thermal battery diaphragm is simplified, the comprehensive performance of a thermal battery is remarkably improved, and the thermal battery diaphragm has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical technology, specifically relating to a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator and its preparation method. Background Technology

[0002] A thermal battery is a common type of high-temperature primary battery that uses a solid molten salt as the electrolyte. This electrolyte is a non-conductive solid at room temperature, but melts and becomes highly ionicly conductive when the temperature rises to the operating temperature. Due to its high energy density and reliability in high-temperature environments, it is widely used in aerospace, military equipment, and oil and gas exploration. However, when a thermal battery is in operation, the electrolyte becomes liquid and fluid, leading to a decrease in energy density and, in severe cases, even a short circuit, causing a safety accident. Therefore, it is necessary to add an appropriate amount of binder to suppress the flow of the molten electrolyte and isolate the positive and negative electrodes. Initially, kaolin and SiO2 were added as binders to suppress the flow of the liquid electrolyte. While this was effective to a large extent, excessive amounts, coupled with the fact that binders are non-ionic conductors, significantly increased the thickness of the thermal battery separator and reduced the ion migration capacity within the separator. Furthermore, with the widespread use of lithium alloy anode materials, SiO2 can react with the negative electrode material.

[0003] In recent years, magnesium oxide has become the most commonly used binder for thermal batteries due to its excellent wettability and fixation ability for electrolytes, as well as its simple preparation process and low cost. However, the fabrication of thermal batteries typically involves a pressing process under pressure of 5–25 MPa. To ensure that the formed separator has sufficient mechanical strength to resist the high compressive stress generated during operation, and to effectively isolate the positive and negative electrodes of the battery, the mass fraction of non-ionic conductive magnesium oxide added is usually not less than 30%. This not only increases the thickness of the separator but also reduces its ionic conductivity, severely limiting the working performance of the thermal battery.

[0004] In summary, achieving both excellent mechanical strength and low internal resistance in thermal battery separators at high temperatures is crucial for the fabrication of high-performance thermal batteries. Therefore, effectively adding magnesium oxide without letting its amount or thickness negatively impact the performance of the thermal battery separator has become an urgent technical challenge. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator and its preparation method, so as to solve the technical problem that the high content of added magnesium oxide affects the thickness of the separator, thereby affecting the mechanical strength and electrical performance of the thermal battery separator.

[0006] To achieve the above objectives, the present invention employs the following technical solution: This invention discloses a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator, which is composed of a Si3N4 nanowire framework matrix loaded with nano-magnesium oxide and a solid molten salt electrolyte; wherein: The solid molten salt electrolyte is selected from binary, ternary and quaternary mixed systems of alkaline halides.

[0007] Preferably, the diameter of the Si3N4 nanowire is 20-500 nm; Nano magnesium oxide includes nano magnesium oxide particles or magnesium oxide nano-coatings; The particle size of the nano-magnesium oxide particles is 50-400 nm; the thickness of the magnesium oxide nano-coating is 50-400 nm.

[0008] Preferably, the thickness of the high-temperature resistant, low-internal-resistance, high-strength thermal battery separator is 0.1-0.4 mm.

[0009] Preferably, the high-temperature resistant, low-internal-resistance, high-strength thermal battery separator has a stress of 5-25 MPa when the compressive strain is 5%-10%.

[0010] Preferably, the electrolyte absorption rate of the high-temperature resistant, low-internal-resistance, high-strength thermal battery separator is 3000-7000%; the electrolyte leakage rate is 1.1-2.3% after 30 minutes at the operating temperature, and the radial dimensional change rate is 0.1-0.9%.

[0011] Preferably, the binary, ternary, or quaternary mixed system of the alkali halide includes one or more of the following: LiCl-KCl, LiI-KI, LiCl-LiI, LiF-LiCl-LiI, LiCl-LiI-KI, LiF-LiCl-LiBr, LiF-LiBr-KBr, LiCl-LiBr-LiI, LiCl-LiBr-KBr, LiF-LiCl-LiBr-LiI, LiBr-LiI-KI-CsI, and LiCl-LiBr-NaF-KF.

[0012] This invention also discloses a method for preparing the above-mentioned high-temperature resistant, low-internal-resistance, high-strength thermal battery separator, comprising: Si3N4 nanowire framework was prepared by hot pressing and sintering using Si3N4 nanowire foam as raw material. A Si3N4 nanowire framework matrix loaded with nano-magnesium oxide was prepared by introducing magnesium oxide nanoparticles or magnesium oxide nanocoatings onto the Si3N4 nanowire framework using a precursor impregnation pyrolysis method. A high-temperature resistant, low-internal-resistance, high-strength thermal battery separator was prepared by combining a solid molten salt electrolyte with a Si3N4 nanowire framework matrix loaded with nano-magnesium oxide using a melt impregnation method.

[0013] Preferably, the raw material Si3N4 nanowire foam is a three-dimensional porous structure composed of single-crystal α-Si3N4 nanowires, with a porosity exceeding 99% and a bulk density of 1-3 mg / cm³. 3 ; The hot-pressing sintering process for preparing the Si3N4 nanowire framework specifically includes: Si3N4 nanowire foam was hot-pressed and sintered in an oxygen-free atmosphere at a pressure of 0.1-55 MPa, a temperature of 600-1500 ℃, and a time of 1-3 h to obtain a density of 50-1000 mg / cm³. 3 The Si3N4 nanowire framework has a layered structure with an average pore size of 0.14-2 µm.

[0014] Preferably, the precursor used in the precursor impregnation pyrolysis method is one or more of magnesium acetate solution, magnesium nitrate solution and magnesium citrate solution, and the pyrolysis temperature is 350-550 °C.

[0015] Preferably, a melt impregnation method is used, in which a solid molten salt electrolyte is heated to a molten state, and then a Si3N4 nanowire framework containing nano-magnesium oxide is impregnated therein to obtain a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The high-temperature resistant, low-internal-resistance, high-strength thermal battery separator disclosed in this invention is composed of a Si3N4 nanowire framework matrix loaded with nano-magnesium oxide and a solid molten salt electrolyte. On the one hand, thanks to the highly cross-linked nanowire network in the Si3N4 nanowire framework, the framework retains the characteristics of ultra-high porosity and improves the liquid absorption capacity of the framework. On the other hand, the Si3N4 nanowire framework loaded with nano-magnesium oxide effectively improves the wettability of Si3N4 nanowires to the solid molten salt electrolyte, thus possessing strong liquid-locking ability and effectively preventing the leakage of solid molten salt electrolyte at high temperatures. Unlike traditional magnesium oxide thermal battery separators, which suffer from high internal resistance, high thickness, and poor mechanical properties, the high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator of this invention provides excellent mechanical strength and low internal resistance at high temperatures, while significantly reducing the separator thickness. Compared with Si3N4 nanowire frameworks without precursor conversion treatment, the compressive strength is increased by 2-5 times, greatly improving the service reliability of the thermal battery separator. At the same time, the adsorption and fixation capacity of solid molten salt electrolyte is also significantly improved, and the internal resistance is significantly reduced, providing a feasible solution for improving the energy density, reliability, and miniaturization of thermal batteries.

[0017] Furthermore, the selected Si3N4 nanowire framework is a layered porous network structure (porosity of 69-97%) formed by bonding together Si3N4 nanowires with diameters of 20-1000 nm.

[0018] Furthermore, by changing the density of the Si3N4 nanowire framework and then loading nano-magnesium oxide particles onto the nanowire surface through precursor conversion, the degree of cross-linking of the nanowires in the framework and the wettability of the framework to solid molten salt electrolyte can be controlled, thereby controlling the strength and internal impedance of the framework. Furthermore, by controlling the particle size of the magnesium oxide particles loaded by the precursor conversion method, the strength and internal impedance of the final skeleton can also be regulated.

[0019] This invention discloses a method for preparing a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator. Utilizing a precursor conversion process, nano-magnesium oxide particles are loaded onto the surface of Si3N4 nanowires to improve the wettability of the Si3N4 nanowires to the electrolyte. Simultaneously, the contacting Si3N4 nanowires are fixed in a non-rotational state, resulting in a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator. This preparation method is characterized by high efficiency, simple process, and short preparation cycle, making it suitable for industrial production. Attached Figure Description

[0020] Figure 1The images show the microstructure of the thermal battery separator substrate after the Si3N4 nanowire framework and its precursor conversion method are loaded with nano-magnesium oxide; where (a) is the Si3N4 nanowire framework and (b) is the Si3N4 nanowire framework loaded with nano-magnesium oxide. Figure 2 The images show the microstructure of the thermal battery separator for Si3N4 nanowire framework and the composite electrolyte after loading nano-magnesium oxide using the precursor conversion method; where (a) is the thermal battery separator for Si3N4 nanowire framework and (b) is the thermal battery separator for loading nano-magnesium oxide using Si3N4 nanowire framework. Figure 3 The microstructure of the Si3N4 nanowire framework loaded with magnesium oxide particles of 200 nm and its composite electrolyte thermal battery separator are shown in the figure. (a) shows the Si3N4 nanowire framework loaded with magnesium oxide particles; (b) shows the Si3N4 nanowire framework loaded with magnesium oxide. Figure 4 The images show the microstructure of a Si3N4 nanowire framework loaded with 140 nm thick layered magnesium oxide and its composite electrolyte thermal battery separator; (a) shows the Si3N4 nanowire framework loaded with magnesium oxide particles; and (b) shows the Si3N4 nanowire framework loaded with magnesium oxide and its thermal battery separator. Figure 5 The pulse discharge characteristics and internal impedance of thermal batteries with unloaded magnesium oxide and loaded magnesium oxide separators under the same discharge conditions are given; where (a) represents the pulse discharge characteristics and internal impedance of the thermal battery with unloaded magnesium oxide separator; and (b) represents the pulse discharge characteristics and internal impedance of the thermal battery with loaded magnesium oxide separator. Detailed Implementation

[0021] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. 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 should fall within the scope of protection of the present invention.

[0022] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0023] The present invention will now be described in further detail with reference to the accompanying drawings: First, the present invention provides a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator, which is composed of a nanowire framework composite solid molten salt electrolyte. The nanowires are Si3N4 nanowires loaded with nano-magnesium oxide particles by a precursor conversion method. The nano-magnesium oxide particles act as a "wetting agent" and "binder", which significantly improves the wettability between the solid molten salt electrolyte and the nanowire framework, while also enhancing the mechanical strength of the separator.

[0024] The Si3N4 nanowires have a diameter of 20-500 nm.

[0025] The Si3N4 nanowires are loaded with magnesium oxide nanoparticles with a particle size of 50-400 nm. The magnesium oxide nanoparticles improve the wettability of the framework and the solid molten salt electrolyte, while fixing the nanowires in contact with each other into a non-rotational state.

[0026] The thickness of the high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator is 0.1-0.4 mm.

[0027] The high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator has a stress of 5-25 MPa when the compressive strain is 5-10%.

[0028] The high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator can effectively fix the molten electrolyte at high temperatures. When operating at the operating temperature for 30 minutes, the electrolyte leakage is only 1.1%-2.3%, the radial dimensional change rate is 0.1-0.9%, and with its excellent electrolyte absorption rate (3000~7000%), the internal resistance is only 0.01-0.20Ω.

[0029] The aforementioned high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator is based on the literature. Nano Lett. The methods disclosed in 2023, 23, 1289-1297 include: Using SiO powder as raw material, in a nitrogen atmosphere of 0.6 MPa, at 1450 o Flexible Si3N4 nanowire foam was prepared by C. This Si3N4 nanowire foam has a three-dimensional porous structure composed of single-crystal α-Si3N4 nanowires, with a porosity exceeding 99% and a bulk density of 1-3 mg / cm³. 3 Subsequently, at high temperatures (600-1500) o C) Hot pressing under high pressure (0.1-55 MPa) and oxygen-free environment for 1-3 h yields Si3N4 nanowire framework with a highly cross-linked nanowire network structure.

[0030] This invention discloses a method for preparing a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator, comprising the following steps: Using Si3N4 nanowire foam as raw material, a Si3N4 nanowire framework is formed by hot pressing and sintering. Then, nano-magnesium oxide particles are loaded onto the surface of the nanowires via a precursor conversion method. The loaded nano-magnesium oxide particles can not only improve the wettability between the framework and the electrolyte, but also form nodes at the nanowires in contact, thereby improving the mechanical strength of the membrane.

[0031] Hot pressing sintering process: at high temperature (600-1500) o C) Sintering is carried out in an oxygen-free environment with a pressure of 0.1-55 MPa for 1-3 h to obtain a Si3N4 nanowire framework with a highly cross-linked nanowire network structure.

[0032] Precursor conversion process: In a vacuum environment, the Si3N4 nanowire framework is immersed in a magnesium oxide precursor solution with a concentration of 0.1-500 g / L for 30-60 min, then removed and dried. After complete drying, it is heated at 350-550 °C. o Keep warm at a temperature of C for 4-6 hours.

[0033] Example 1 This embodiment prepared a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator with a thickness of 0.1 mm. With a density of 1 mg / cm³ 3 Using Si3N4 nanowire foam as raw material, at 650 o Hot pressing sintering was performed under anaerobic conditions at a pressure of 0.1 MPa for 2 hours, yielding a product with a density of 50 mg / cm³. 3 The Si3N4 nanowire framework was prepared by placing it in a vacuum impregnation tank containing a 300 g / L magnesium acetate solution, evacuating, and impregnating for 40 min. Afterward, it was removed and impregnated at 80°C. o Dry in an oven at C for 40 minutes. After drying, place in a box oven at 550°C. oA Si3N4 nanowire framework matrix containing nano-magnesium oxide particles was successfully prepared by holding the substrate at C for 4 h. An electrolyte salt was prepared according to a mass ratio of LiF:LiCl:LiBr = 9.6:22:68.4 to obtain a mixed salt. The mixed salt was then heated at 540 °C. o The mixture was kept at C for 1 h to allow the molten salt electrolyte to mix thoroughly. Then, the Si3N4 nanowire framework matrix containing the above-mentioned magnesium oxide nanoparticles was placed at 550 °C. o After being soaked in molten salt electrolyte at C for 1 h for 30 min, the electrolyte was quickly removed and compacted into electrolyte sheets at 540 ℃ and 7 MPa using a hot press. The sheets were then stored in a glove box with a humidity of less than 2% to obtain a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator with a thickness of 0.1 mm.

[0034] like Figure 1 and Figure 2 As shown, after precursor conversion treatment, "solder joints" were successfully formed between some of the contacting nanowires in the Si3N4 nanowire framework. The precursor conversion treatment significantly improved both the wettability with molten salt electrolyte and the compressive strength of the Si3N4 nanowire framework, thereby reducing the internal impedance of the thermal battery separator and improving the reliability of the thermal battery. Figure 5 As shown in Figure (a), the thermal battery constructed with a Si3N4 nanowire framework membrane was initially placed at 550 °C for 15 seconds to allow the electrolyte to completely melt and activate the battery. Within 15-20 seconds, the battery activated at a current of 21.0526 mA·cm⁻¹. -2 The discharge was carried out at a current density of 49.8947 mA·cm⁻¹ during a 16 s pulse. -2 Discharge under pulsed current density, through the formula , in U、 I represents the voltage and current changes during the pulse, respectively, and R represents the battery's internal resistance. The calculated internal resistance of the battery during operation is approximately 0.6760 Ω. Meanwhile, the thermal battery constructed after loading nano-magnesium oxide particles via precursor conversion, such as... Figure 5 As shown in (b), the battery was left to stand at 550°C for the first 15 seconds to allow the electrolyte to completely melt and activate the thermal battery. Within 15–20 seconds, the battery generated 21.0526 mA·cm⁻¹. -2 The discharge was carried out at a current density of 49.8947 mA·cm⁻¹ during a 16-second pulse current. -2 Discharge under pulsed current density, through the formula ,in U、 I represents the voltage and current changes during the pulse, and R represents the battery's internal resistance. It can be calculated that the battery's internal resistance during operation is approximately 0.1872 Ω, indicating a significant reduction in the internal resistance of the hot battery.

[0035] Example 2 This embodiment prepared a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator with a density of 0.4 mm and a thickness of 0.4 mm. With a density of 2 mg / cm³ 3 Using Si3N4 nanowire foam as raw material, at 1300 o Hot pressing sintering was performed under vacuum conditions at a pressure of 25 MPa for 2 hours, yielding a product with a density of 400 mg / cm³. 3 The Si3N4 nanowire framework was prepared by placing it in a vacuum impregnation tank containing a 300 g / L magnesium acetate solution, evacuating, and impregnating for 40 min. Afterward, it was removed and impregnated at 80°C. o Dry in an oven at C for 40 minutes. After drying, place in a box oven at 550°C. o A Si3N4 nanowire framework matrix containing nano-magnesium oxide particles was successfully prepared by holding the substrate at temperature C for 4 h. An electrolyte salt was prepared by mixing the electrolyte salts at a mass ratio of LiCl:KCl = 45:55, and then heated at 550 °C. o The mixture was kept at C for 1 h to allow the molten salt electrolyte to mix thoroughly. Then, the Si3N4 nanowire framework matrix containing the above-mentioned magnesium oxide nanoparticles was placed at 550 °C. o After being immersed in molten salt electrolyte at C for 1 h for 30 min, it was quickly removed and pressed into electrolyte sheets using a hot press at 540 ℃ and 10 MPa. The sheets were then stored in a glove box with a humidity of less than 2%, resulting in a 0.4 mm thick, high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator. Figure 3 As shown, the particle size of the magnesium oxide nanoparticles on the surface of the Si3N4 nanowires is 200 nm.

[0036] Example 3 This embodiment prepared a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator with a thickness of 0.1 mm. With a density of 3 mg / cm³ 3 Using Si3N4 nanowire foam as raw material, at 1300 o Hot pressing sintering was performed under vacuum conditions at a pressure of 55 MPa for 3 hours, yielding a product with a density of 1000 mg / cm³. 3 The Si3N4 nanowire framework was prepared by placing it in a vacuum impregnation tank containing a 300 g / L magnesium acetate solution, evacuating, and impregnating for 40 min. Afterward, it was removed and impregnated at 80°C. oDry in an oven at C for 40 minutes. After drying, place in a box oven at 550°C. o A Si3N4 nanowire framework matrix containing nano-magnesium oxide particles was successfully prepared by holding the substrate at a temperature of C for 4 h. An electrolyte salt was prepared according to a mass ratio of LiF:LiCl:LiBr = 9.6:22:68.4 to obtain a mixed salt. The mixed salt was then heated at 550 °C. o The mixture was kept at C for 1 h to allow the molten salt electrolyte to mix thoroughly. Then, the Si3N4 nanowire framework matrix containing the above-mentioned magnesium oxide nanoparticles was placed at 550 °C. o After being immersed in molten salt electrolyte at C for 1 h for 30 min, it was quickly removed and pressed into electrolyte sheets using a hot press at 540 ℃ and 10 MPa. The sheets were then stored in a glove box with a humidity of less than 2%, resulting in a 0.1 mm thick, high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator. Figure 4 As shown, the thickness of the magnesium oxide layer on the surface of the Si3N4 nanowire is 140 nm.

[0037] Example 4 In this embodiment, a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator with a thickness of 0.6 mm was prepared. With a density of 3 mg / cm³ 3 Using Si3N4 nanowire foam as raw material, at 1300 o Hot pressing sintering was performed under vacuum conditions at a pressure of 55 MPa for 3 hours, yielding a product with a density of 1000 mg / cm³. 3 The Si3N4 nanowire framework was prepared by placing it in a vacuum impregnation tank containing a 300 g / L magnesium nitrate solution, evacuating, and impregnating for 40 min. Afterward, it was removed and impregnated at 80°C. o Dry in an oven at C for 40 minutes. After drying, place in a box oven at 550°C. o A Si3N4 nanowire framework matrix containing nano-magnesium oxide particles was successfully prepared by holding the substrate at a temperature of C for 4 h. An electrolyte salt was prepared according to a mass ratio of LiF:LiBr:KBr = 0.67:53.5:45.83 to obtain a mixed salt. The mixed salt was then heated at 550 °C. o The mixture was kept at C for 1 h to allow the molten salt electrolyte to mix thoroughly. Then, the Si3N4 nanowire framework matrix containing the above-mentioned magnesium oxide nanoparticles was placed at 550 °C. o After being soaked in molten salt electrolyte at C for 1 h for 30 min, the electrolyte was quickly removed and compacted into electrolyte sheets at 540 ℃ and 10 MPa using a hot press. The sheets were then stored in a glove box with a humidity of less than 2% to obtain a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator with a thickness of 0.6 mm.

[0038] Example 5 In this embodiment, a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator with a thickness of 0.2 mm was prepared. With a density of 2 mg / cm³ 3 Using Si3N4 nanowire foam as raw material, at 1300 o Hot pressing sintering was performed under vacuum conditions at a pressure of 25 MPa for 2 hours, yielding a product with a density of 400 mg / cm³. 3 The Si3N4 nanowire framework was prepared by placing it in a vacuum impregnation tank containing a 200 g / L magnesium citrate solution, evacuating, and impregnating for 40 min. Afterward, it was removed and impregnated at 80°C. o Dry in an oven at C for 40 minutes. After drying, place in a box oven at 550°C. o A Si3N4 nanowire framework matrix containing nano-magnesium oxide particles was successfully prepared by holding the substrate at a temperature of C for 4 h. An electrolyte salt was prepared according to a mass ratio of LiF:LiCl:LiBr:LiI = 4.9:11.2:34.9:49 to obtain a mixed salt. The mixed salt was then heated at 550 °C. o The mixture was kept at 550°C for 1 hour to allow the molten salt electrolyte to mix thoroughly. Then, the Si3N4 nanowire framework matrix containing the above-mentioned magnesium oxide nanoparticles was placed at 550°C. o After being soaked in molten salt electrolyte at C for 1 hour for 30 minutes, the electrolyte was quickly removed and compacted into electrolyte sheets at 540℃ and 10 MPa using a hot press. The sheets were then stored in a glove box with a humidity of less than 2% to obtain a high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator with a thickness of 0.2 mm.

[0039] In summary, this invention utilizes precursor conversion to load 100-400 nm magnesium oxide nanoparticles onto the surface of Si3N4 nanowires. This improves the wetting properties between the nanowire framework and the electrolyte while simultaneously "welding" some of the contacting nanowires together to form non-rotatable sites, thereby enhancing the strength of the thermal battery separator. Table 1 compares the data of this product (using the product obtained in Example 1 as an example) with that of separators prepared using existing pressing processes. It can be seen that the resulting high-temperature resistant, low-internal-resistance, and high-strength thermal battery separator has a thickness of only 0.1-0.4 mm, breaking the limitations imposed by existing separator pressing processes. The stress corresponding to a compressive strain of 5-10% is 5-25 MPa, enabling the separator to withstand the enormous pressure during operation while maintaining a small thickness, thus ensuring stable operation of the thermal battery. Furthermore, this diaphragm can effectively fix the molten electrolyte at high temperatures. When operating at the temperature for 30 minutes, the electrolyte leakage is only 1.1~2.3%, and the radial dimension change rate is 0.1~0.9%, which is an advantage that other diaphragm technologies do not possess. With its excellent electrolyte absorption rate (3000~7000%), the internal resistance is only 0.01~0.20Ω.

[0040] Table 1 Comparison of performance parameters of different membrane materials

[0041] *Data source in the table: The Si3N4 membrane is a product prepared by the method disclosed in this invention. [1]Czajka, Bogdan, et al. "Modification of MgO as an immobilizing agent for molten electrolyte." Journal of Solid State Electrochemistry 18(2014): 2351-2358. [2]GUIDOTTI, RONALD A., FREDERICK W. REINHARDT, and ARTHUR H.ANDAZOLA. Blending study of MgO-based separator materials for thermalbatteries. No. SAND2002-1458. Sandia National Lab.(SNL-NM), Albuquerque, NM(United States); Sandia National Lab.(SNL-CA), Livermore, CA (United States) States), 2002. [3]Redey, Laszlo, Margaret McParland, and Ron Guidotti. "Resistivity measurements of halide-salt / MgO separators for thermal cells." Proceedings of the 34th International Power Sources Symposium. IEEE, 1990. [4]Zhang, Ping, et al. "Synthesis of porous magnesia fibers with enhanced performance as a binder for molten electrolyte." Electrochimica Acta230 (2017): 358-364. [5] Liu, Yizheng. Design and performance study of separator / electrolyte for high-temperature batteries. 2021. Xiamen University, MAthesis. [6] Zhang, Yichi. Preparation of BN fiber composite membrane and its performance in Li-FeS2 / CoS2 thermal battery. 2016. Wuhan University of Technology, MA thesis. [7] Tang Jie, et al. "Development and performance study of boron nitride fiber-based composite separator for thermal batteries." Modern Technical Ceramics 38.3 (2017): 197-203. [8]Xia, Kunyu, et al. "Porous mullite fiber nonwoven separatormaterials containing Nano-MgO particles for the design of thermal batteries." Materials Letters 297 (2021): 130007. The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A high-temperature resistant, low-internal-resistance, high-strength thermal battery separator, characterized in that, It is composed of a Si3N4 nanowire framework matrix loaded with nano-magnesium oxide and a solid molten salt electrolyte; wherein: The solid molten salt electrolyte is selected from binary, ternary and quaternary mixed systems of alkali halides.

2. The high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to claim 1, characterized in that, The diameter of the Si3N4 nanowires is 20-500 nm. Nano magnesium oxide includes nano magnesium oxide particles or magnesium oxide nano-coatings; The particle size of the nano-magnesium oxide particles is 50-400 nm; the thickness of the magnesium oxide nano-coating is 50-400 nm.

3. The high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to claim 1, characterized in that, The thickness of this high-temperature resistant, low-internal-resistance, high-strength thermal battery separator is 0.1-0.4 mm.

4. The high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to claim 1, characterized in that, The high-temperature resistant, low-internal-resistance, high-strength thermal battery separator has a stress of 5-25 MPa when the compressive strain is 5%-10%.

5. The high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to claim 1, characterized in that, The high-temperature resistant, low-internal-resistance, high-strength thermal battery separator has an electrolyte absorption rate of 3000-7000%; after operating at the operating temperature for 30 minutes, the electrolyte leakage is 1.1-2.3%, and the radial dimensional change rate is 0.1-0.9%.

6. The high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to claim 1, characterized in that, The binary, ternary, or quaternary mixed systems of the alkaline halides include one or more of the following: LiCl-KCl, LiI-KI, LiCl-LiI, LiF-LiCl-LiI, LiCl-LiI-KI, LiF-LiCl-LiBr, LiF-LiBr-KBr, LiCl-LiBr-LiI, LiCl-LiBr-KBr, LiF-LiCl-LiBr-LiI, LiBr-LiI-KI-CsI, and LiCl-LiBr-NaF-KF.

7. The method for preparing the high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to any one of claims 1 to 6, characterized in that, include: Si3N4 nanowire framework was prepared by hot pressing and sintering using Si3N4 nanowire foam as raw material. A Si3N4 nanowire framework matrix loaded with nano-magnesium oxide was prepared by introducing magnesium oxide nanoparticles or magnesium oxide nanocoatings onto the Si3N4 nanowire framework using a precursor impregnation pyrolysis method. A high-temperature resistant, low-internal-resistance, high-strength thermal battery separator was prepared by combining a solid molten salt electrolyte with a Si3N4 nanowire framework matrix loaded with nano-magnesium oxide using a melt impregnation method.

8. The method for preparing a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to claim 7, characterized in that, The Si3N4 nanowire foam is a three-dimensional porous structure composed of single-crystal α-Si3N4 nanowires, with a porosity exceeding 99% and a bulk density of 1-3 mg / cm³. 3 ; The hot-pressing sintering process for preparing the Si3N4 nanowire framework specifically includes: Si3N4 nanowire foam was hot-pressed and sintered in an oxygen-free atmosphere at a pressure of 0.1-55 MPa, a temperature of 600-1500℃, and a time of 1-3 h to obtain a density of 50-1000 mg / cm³. 3 The Si3N4 nanowire framework has a layered structure with an average pore size of 0.14-2 µm.

9. The method for preparing a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to claim 7, characterized in that, The precursor used in the precursor impregnation pyrolysis method is one or more of magnesium acetate solution, magnesium nitrate solution and magnesium citrate solution, and the pyrolysis temperature is 350-550 ℃.

10. The method for preparing a high-temperature resistant, low-internal-resistance, high-strength thermal battery separator according to claim 7, characterized in that, A high-temperature resistant, low-internal-resistance, high-strength thermal battery separator was prepared by heating a solid molten salt electrolyte to a molten state and then impregnating it with a Si3N4 nanowire framework containing nano-magnesium oxide.

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