Janus diaphragm for low-temperature-resistant lithium-sulfur battery as well as preparation method and application of Janus diaphragm

Abstract

The invention discloses a Janus diaphragm for a low-temperature-resistant lithium-sulfur battery as well as a preparation method and application of the Janus diaphragm. The diaphragm comprises a porous diaphragm substrate, polymer layers coated on two sides of the porous diaphragm substrate, and a composite inorganic material layer coated on the surface of the polymer layer on any side, the composite inorganic material layer is formed by compounding defective graphite phase carbon nitride, nickel-based double hydroxide, conductive carbon and a binder. The preparation method comprises the following steps: firstly, coating the surface of a porous diaphragm substrate with a polymer layer by a phase conversion method, then preparing a defect graphite phase carbon nitride / nickel-based double hydroxide composite material by a hydrothermal method, and coating any surface of the diaphragm coated with the polymer layer with the defect graphite phase carbon nitride / nickel-based double hydroxide composite material to form the Janus diaphragm with an asymmetric structure; and the polymer layer is used for promoting rapid migration of lithium ions and improving the interface stability of the lithium negative electrode. According to the Janus diaphragm disclosed by the invention, the reaction kinetics performance of the lithium-sulfur battery in a low-temperature environment is remarkably improved, and high energy density and long cycle stability are realized.

Description

Technical Field

[0001] This invention belongs to the field of lithium metal battery technology, specifically relating to a Janus separator for low-temperature resistant lithium-sulfur batteries, its preparation method, and its application. Background Technology

[0002] Lithium-sulfur batteries are considered a promising high-energy-density battery system due to the high theoretical energy density of sulfur cathodes (up to 2600 Wh / kg), abundant resources, and environmental friendliness. However, this system still faces key challenges: the shuttle effect of soluble polysulfide intermediates leads to irreversible loss of active material and capacity decay; the intrinsic insulating nature of sulfur and its discharge products results in slow electrochemical reaction kinetics and a significant decrease in high-rate performance. These problems are further exacerbated at low temperatures, where the reaction rate constant is significantly reduced, and the sluggish reaction kinetics lead to the accumulation of polysulfides in the electrolyte, which not only intensifies the shuttle effect but also increases battery polarization, resulting in severe performance degradation. Currently, polyolefin separators are commonly used in lithium-sulfur batteries. Although these separators have good chemical stability and mechanical strength, their surface chemical inertness and large pore size cannot effectively block the migration of polysulfides, and they lack the function of promoting polysulfide conversion or stabilizing the lithium anode interface.

[0003] In summary, existing lithium-sulfur battery separators suffer from problems such as limited functionality, inability to simultaneously suppress shuttle effects and improve reaction kinetics, and severe performance degradation, especially at low temperatures. Therefore, developing a novel multifunctional separator that can effectively anchor and catalyze the conversion of polysulfides while ensuring efficient lithium-ion transport and simultaneously improving the stability of the lithium anode interface, especially maintaining high battery performance under harsh low-temperature conditions, is key to promoting the development of lithium-sulfur batteries. Summary of the Invention

[0004] The purpose of this invention is to provide a Janus separator for low-temperature resistant lithium-sulfur batteries, its preparation method, and its application. This method addresses the challenges of ion transport, interface stability, and polysulfide conversion in lithium-sulfur batteries at low temperatures by constructing a functionally complementary bilayer coating on a porous substrate. Applying the prepared Janus separator to low-temperature resistant lithium-sulfur batteries can significantly improve the reaction kinetics performance of lithium-sulfur batteries in low-temperature environments, achieving high energy density and long-cycle stability.

[0005] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows: A Janus separator for low-temperature lithium-sulfur batteries includes a porous separator substrate, polymer layers coated on both sides of the porous separator substrate, and a composite inorganic material layer coated on the surface of either polymer layer. The composite inorganic material layer is composed of defective graphitic carbon nitride, nickel-based double hydroxides, conductive carbon, and a binder. The polymer layer is used to promote rapid lithium-ion migration and improve the interface stability of the lithium metal anode. In the composite inorganic material layer, defective graphitic carbon nitride acts as an electron donor, providing electrons to the active centers of nickel-based double hydroxides, thereby synergistically achieving strong chemisorption and efficient catalytic conversion of polysulfides.

[0006] Preferably, the polymer layer is made of one or more of polymethyl methacrylate, polyetherimide, polyvinylpyrrolidone, and polyetheretherketone; the thickness of each side of the polymer layer is 1 to 10 μm.

[0007] Preferably, the nickel-based double hydroxide contains nickel and a metallic element selected from iron, cobalt, chromium, and aluminum; the conductive carbon is selected from at least one of conductive carbon black, ordered mesoporous carbon materials, and carbon nanotubes; the binder is selected from at least one of polyvinylidene fluoride, polyvinylpyrrolidone, and polyvinyl alcohol; and the thickness of the composite inorganic material layer is 5–25 μm.

[0008] Preferably, the porous membrane substrate is a polyolefin membrane, which is selected from polyethylene membrane, polypropylene membrane, and polyethylene / polypropylene composite multilayer membrane, and the thickness of the polyolefin membrane is 10-40 μm.

[0009] To achieve the above-mentioned objective, the present invention also provides a method for preparing the Janus separator for low-temperature resistant lithium-sulfur batteries, comprising the following steps: S1. The polymer layer is formed on the surface of the porous membrane substrate by phase inversion method to obtain a membrane with a polymer layer on both sides. S2. Preparation of defective graphitic carbon nitride / nickel-based double hydroxide composite material: Defective graphitic carbon nitride is dispersed in water, nickel salt and another metal salt are added, mixed evenly, and then an alkaline solution is added. After hydrothermal reaction, cooling, centrifugation, washing and drying, the defective graphitic carbon nitride / nickel-based double hydroxide composite material is obtained. S3. The defective graphitic carbon nitride / nickel-based double hydroxide composite material prepared in step S2 is mixed with conductive carbon and binder in a mass ratio of 6:1:1. N-methylpyrrolidone is added to make a slurry, which is then coated on either side of the membrane with a polymer layer prepared in step S1. After drying, the Janus membrane is obtained.

[0010] Preferably, the concentration of the defective graphitic carbon nitride is 3–20 g / L; the nickel salt is selected from nickel nitrate, nickel sulfate, nickel chloride, and nickel oxalate; the other metal salt is selected from iron salt, cobalt salt, chromium salt, and aluminum salt; the concentrations of the nickel salt and the other metal salt are each independently 0.1–0.4 mol / L; the alkaline solution is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and ammonia solution, with a concentration of 0.8–1.2 mol / L; the pH of the reaction system is adjusted to 9.5–11 by adding the alkaline solution; the hydrothermal reaction temperature is 100–150°C, and the time is 12–24 h.

[0011] Furthermore, in step S1, the phase inversion method specifically involves: dissolving the polymer layer material in an organic solvent to obtain a solution, immersing the porous membrane substrate in the solution for 2 hours, removing it, placing it in deionized water for phase inversion, and then drying it to form the polymer layer.

[0012] Preferably, the organic solvent is selected from one or more of acetone, butanone, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; and the solution concentration is 5~15 g / L.

[0013] To achieve the above-mentioned objectives, this invention also provides the application of the Janus separator prepared by the above method in low-temperature lithium-sulfur batteries.

[0014] Preferably, the lithium-sulfur battery includes a lithium metal anode, a sulfur cathode, an ether electrolyte, and the Janus membrane disposed between the cathode and anode; in the Janus membrane, the polymer layer faces the lithium metal anode, and the composite inorganic material layer faces the sulfur cathode.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) In the composite inorganic material layer designed in this invention, nitrogen atoms in the defective graphitic carbon nitride serve as Lewis base sites, chemically adsorbing polysulfides by forming SN / N-Li bonds. Nickel-based double hydroxides provide abundant catalytic active sites. After the two are combined, electron rearrangement occurs at the heterogeneous interface. The defective graphitic carbon nitride acts as an electron donor, efficiently transferring electrons to the active centers of nickel-based double hydroxides. This effectively solves the problems of poor conductivity and low utilization of catalytic sites in nickel-based double hydroxides, thereby synergistically achieving strong chemical adsorption and efficient catalytic conversion of polysulfides, significantly improving the kinetics of sulfur oxidation-reduction reactions, especially at low temperatures.

[0016] (2) The polymer chains in the polymer layer of the present invention contain functional groups such as carbonyl groups, ester groups, imide groups, and ether groups, which can interact with lithium salts in the electrolyte, facilitating lithium salt dissociation, increasing the concentration of free lithium ions, providing a low-energy-barrier migration channel for lithium ions, and improving ionic conductivity at low temperatures. At the same time, this uniform functional interface layer can guide the uniform deposition of lithium ions, inhibit lithium dendrite growth, and significantly improve the stability of the lithium anode / electrolyte interface.

[0017] (3) This invention, through the asymmetric Janus structure design that "promotes lithium transport on one side and sulfur conversion on the other," synergistically optimizes the lithium-sulfur battery system from multiple levels, including ion conduction, interface stability, and reaction kinetics. Lithium-sulfur batteries using this separator can maintain high ion conduction rates and efficient polysulfide conversion capabilities at low temperatures, thereby achieving excellent high-rate performance, long-cycle stability, and significantly improved low-temperature discharge capacity retention, thus promoting the development of lithium-sulfur batteries in wide-temperature-range, high-energy-density applications. Attached Figure Description

[0018] Figure 1 Transmission electron microscope image of the defective graphitic carbon nitride / nickel-iron double hydroxide composite material prepared in Example 1; Figure 2 The ionic conductivity test chart and lithium symmetric battery cycle stability test chart are for the separators prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively, and the commercial polypropylene separator selected in Comparative Example 3: (a) Ionic conductivity test chart; (b) Lithium symmetric battery cycle stability test chart. Figure 3 Electrochemical performance test graphs of lithium-sulfur batteries assembled with the separators prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively, and the commercial polypropylene separator selected in Comparative Example 3, at 25°C: (a) Cycling performance at 1C rate; (b) Charge-discharge curve at 10C rate. Figure 4 The graph shows the cycle performance of lithium-sulfur batteries assembled with the separators prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively, and the commercial polypropylene separator selected in Comparative Example 3, at -20°C and 0.2C rate. Detailed Implementation

[0019] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0020] Unless otherwise specified, all raw materials and reagents used in the following examples are commercially available products with analytical purity or higher. The defective graphitic carbon nitride used was purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., model XFI10, with a purity of 99%.

[0021] Example 1 A Janus separator for low-temperature resistant lithium-sulfur batteries includes a porous separator substrate, polymer layers covering both sides of the porous separator substrate, and a composite inorganic material layer coated on the surface of the polymer layer on either side. The porous membrane substrate is a polypropylene membrane substrate with a thickness of 20 μm; The polymer layer is made of polyvinylpyrrolidone, and the thickness of the polymer layer is 5 μm; The composite inorganic material layer is composed of defective graphitic carbon nitride, nickel-based double hydroxide, conductive carbon, and a binder; the nickel-based double hydroxide contains nickel and iron elements; the conductive carbon is selected from conductive carbon; the binder is selected from polyvinylidene fluoride; the thickness of the composite inorganic material layer is 10 μm. The polymer layer is used to promote lithium-ion migration and improve the stability of the lithium anode interface; the defective graphitic carbon nitride acts as an electron donor to provide electrons to the active center of the nickel-based double hydroxide.

[0022] The preparation method of the above-mentioned low-temperature resistant Janus separator for lithium-sulfur batteries includes the following steps: S1. A polyvinylpyrrolidone layer is formed on the surface of a polypropylene membrane substrate by a phase inversion method to obtain a membrane with a double-sided polyvinylpyrrolidone layer. The specific steps are as follows: polyvinylpyrrolidone is dissolved in N,N-dimethylformamide to obtain a solution with a concentration of 10 g / L; the polypropylene membrane substrate is immersed in the solution for 2 hours, then removed and placed in deionized water for phase inversion, and then dried to form a polyvinylpyrrolidone layer. S2. Preparation of defective graphitic carbon nitride / nickel-based double hydroxide composite material; specifically: 0.5g of defective graphitic carbon nitride is dispersed in 0.1L of water, 3.9g of nickel chloride and 1.6g of ferric chloride are added, so that the concentrations of nickel chloride and ferric chloride in the solution are 0.3mol / L and 0.1mol / L, respectively. After mixing evenly, a 1mol / L sodium carbonate solution is added to adjust the pH of the reaction system to 10; the reaction is carried out hydrothermally at 120℃ for 18h, and after cooling, centrifugation, washing and drying, the defective graphitic carbon nitride / nickel-iron double hydroxide composite material is obtained. S3. The defective graphite phase carbon nitride / nickel-iron double hydroxide composite material obtained in step S2, conductive carbon black and polyvinylidene fluoride are mixed in a mass ratio of 6:1:1, N-methylpyrrolidone is added to make a slurry, and the slurry is coated on any side of the membrane with polyvinylpyrrolidone layer obtained in step S1. After drying, the Janus membrane is obtained.

[0023] The Janus membrane prepared above is used in a low-temperature lithium-sulfur battery, which includes a lithium metal anode, a sulfur cathode, an ether electrolyte, and the Janus membrane disposed between the cathode and anode; in the Janus membrane, the polymer layer faces the lithium metal anode and the composite inorganic material layer faces the sulfur cathode.

[0024] Comparative Example 1 As a control experiment for Example 1, the structure of the diaphragm in Comparative Example 1 is similar to that of Example 1, but the composition of the composite inorganic material layer differs from that of Example 1. Comparative Example 1 directly uses uncomposite defective graphitic carbon nitride as the sole active component of the inorganic material layer. The specific preparation method is as follows: S1. A polyvinylpyrrolidone layer is formed on the surface of a polypropylene membrane substrate by a phase inversion method to obtain a membrane with a double-sided polyvinylpyrrolidone layer. The specific steps are as follows: polyvinylpyrrolidone is dissolved in N,N-dimethylformamide to obtain a solution with a concentration of 10 g / L; a 20 μm thick polypropylene membrane substrate is immersed in the solution for 2 hours, then removed and placed in deionized water for phase inversion, and then dried to form the polymer layer, the polymer layer having a thickness of 5 μm. S2. Defective graphitic carbon nitride, conductive carbon black and polyvinylidene fluoride are mixed in a mass ratio of 6:1:1, and N-methylpyrrolidone is added to make a slurry. The slurry is coated on any side of the membrane with polyvinylidene fluoride layer obtained in step S1 to obtain an inorganic material layer. After drying, the Janus membrane is obtained. The thickness of the inorganic material layer is 10 μm.

[0025] The prepared comparative separator was also used to assemble a lithium-sulfur battery. Its battery structure (lithium metal anode, sulfur cathode, ether electrolyte, and the separator prepared in Comparative Example 1 placed between the cathode and anode) and separator placement direction (polymer coating layer facing the lithium metal anode, and inorganic material layer facing the sulfur cathode) were consistent with those in Example 1.

[0026] Comparative Example 2 As a control experiment for Example 1, the difference between Comparative Example 2 and Example 1 is that the membrane structure does not contain a composite inorganic material layer. The specific preparation method is as follows: A polyvinylpyrrolidone (PVP) layer is formed on the surface of a polypropylene membrane substrate by a phase inversion method, resulting in a membrane double-sided coated with a PPVP layer. The specific steps are as follows: PPVP is dissolved in N,N-dimethylformamide to obtain a solution with a concentration of 10 g / L; a 20 μm thick polypropylene membrane substrate is immersed in the solution for 2 hours, then removed and placed in deionized water for phase inversion, and then dried to form the polymer layer, the polymer layer having a thickness of 5 μm.

[0027] The prepared comparative membrane was also used to assemble a lithium-sulfur battery, and its battery structure (lithium metal anode, sulfur cathode, ether electrolyte, and the membrane prepared in Comparative Example 2 placed between the cathode and anode) was consistent with that of Example 1.

[0028] Comparative Example 3 As a control experiment for Example 1, Comparative Example 3 used a commercial polypropylene separator, model Celgard2400. Comparative Example 3 was also used to assemble a lithium-sulfur battery, and its battery structure (lithium metal anode, sulfur cathode, ether electrolyte, and the commercial polypropylene separator of Comparative Example 3 placed between the cathode and anode) was consistent with that of Example 1.

[0029] Figure 1 This is a transmission electron microscope (TEM) image of the defective graphitic carbon nitride / nickel-iron double hydroxide composite material prepared in Example 1. The sample preparation method is as follows: The composite material powder obtained in Example 1 was dispersed in anhydrous ethanol, ultrasonically treated, and then dropped onto a copper grid specifically designed for TEM. After drying at room temperature, it was used for observation. The testing instrument was a FEI Talos F200XG2 TEM. Figure 1 As shown, layered nickel-iron double hydroxides are clearly observed to grow uniformly on the surface of the defective graphitic carbon nitride support, forming a tight composite structure. This morphological feature indicates that the structural framework of the defective graphitic carbon nitride is maintained during hydrothermal synthesis, while the active component of nickel-iron double hydroxides is successfully loaded in situ. This tight composite structure facilitates the defective graphitic carbon nitride as an electron donor, providing electrons to the active center of nickel-iron double hydroxides, thereby achieving a synergistic effect between the two at the atomic / nanoscale. This provides a key structural basis for subsequent strong chemisorption and efficient catalytic conversion of polysulfides in membrane coatings.

[0030] The ionic conductivity of the membranes prepared in Example 1, Comparative Example 1, and Comparative Example 2, and the commercially available polypropylene membrane used in Comparative Example 3, were tested. A symmetrical battery with stainless steel electrodes was assembled. The ether electrolyte used consisted of 1 mol / L lithium bis(trifluoromethanesulfonyl)imide and 0.2 mol / L lithium nitrate dissolved in a 1:1 volume ratio of 1,3-dioxopentane / 1,2-dimethoxyethane mixed solvent. The tests were conducted on a CHI604E testing system at 25°C. Figure 2The ionic conductivity test results of Example 1 show that the membrane prepared in Example 1 of this invention has a room temperature ionic conductivity as high as 3.9 mS / cm, which is significantly better than that of Comparative Example 1 (2.5 mS / cm), Comparative Example 2 (1.9 mS / cm), and Comparative Example 3 (0.6 mS / cm). This superior performance is attributed to the multilayer synergistic design of the membrane: First, the polyvinylpyrrolidone polymer layer coated on both sides of the polypropylene matrix by phase inversion can effectively coordinate with lithium ions in the electrolyte by polar carbonyl groups in its molecular chain, which significantly reduces the energy barrier for lithium ion transport across the membrane; Second, in the defective graphitic carbon nitride / nickel-iron double hydroxide composite inorganic material layer coated on one side of the membrane, the defective graphitic carbon nitride acts as a carrier to prevent the stacking of nickel-iron double hydroxide and maintain porous ion channels. Moreover, the abundant polar surfaces (such as nitrogen sites and hydroxyl groups) of both provide active sites for activating and transporting lithium ions, further promoting the rapid migration of ions. The synergistic effect of the polyvinylpyrrolidone layer and the composite inorganic material layer enables the membrane of Example 1 to simultaneously optimize the interfacial dynamics and bulk transport path of lithium ions, thereby achieving a significant improvement in ionic conductivity.

[0031] Lithium-ion symmetric batteries were assembled using the separators prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively, and the commercially available polypropylene separator used in Comparative Example 3. The ether-based electrolyte consisted of 1 mol / L lithium bis(trifluoromethanesulfonyl)imide and 0.2 mol / L lithium nitrate dissolved in a 1:1 volume ratio of 1,3-dioxopentane / 1,2-dimethoxyethane mixed solvent. Long-cycle testing was conducted using a Newway battery testing system at a temperature of 25°C and a current density of 1 mA / cm². 2 Deposition amount 1 mAh / cm 2 The result is as follows Figure 2 As shown in b, Comparative Example 3, without the polymer layer, short-circuited after approximately 120 hours of cycling and exhibited significant voltage polarization. This was mainly due to the inert polypropylene surface's inability to effectively regulate lithium-ion flow, leading to uncontrolled lithium dendrite growth and continuous interface deterioration. In contrast, Examples 1, 1, and 2, containing polyvinylpyrrolidone polymer coatings, all demonstrated excellent cycling stability, achieving stable cycling for 500 hours. This improvement is attributed to the abundant carbonyl polar functional groups in the polyvinylpyrrolidone polymer layer, which promote lithium salt dissociation and provide low-barrier lithium-ion migration channels, thereby effectively suppressing lithium dendrite growth and improving the reversibility of lithium deposition / stripping and interface stability. Furthermore, Example 1 introduced a composite inorganic material layer on top of the polyvinylpyrrolidone polymer layer. The defective graphitic carbon nitride, acting as a carrier, effectively suppressed the stacking of nickel-iron double hydroxides, maintaining a stable ion conduction network; its abundant polar sites (such as nitrogen sites and hydroxyl groups) further accelerated the charge transfer kinetics at the interface, thus Example 1 exhibited the lowest polarization voltage.

[0032] Lithium-sulfur batteries were assembled using the separators prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively, and the commercially available polypropylene separator used in Comparative Example 3. The ether-based electrolyte used consisted of 1 mol / L lithium bis(trifluoromethanesulfonyl)imide and 0.2 mol / L lithium nitrate dissolved in a 1:1 volume ratio of 1,3-dioxane / 1,2-dimethoxyethane mixed solvent. The electrochemical performance was tested on the Xinwei Battery Testing System at temperatures ranging from room temperature (25°C) to -20°C.

[0033] Figure 3 'a' represents the electrochemical performance of a lithium-sulfur battery at 25°C and 1C rate. For example... Figure 3 As shown in Figure a, the lithium-sulfur battery assembled in Example 1 had an initial discharge specific capacity of 1110 mAh / g and a capacity retention rate of 77.6% after 300 cycles; the battery assembled in Comparative Example 1 had an initial discharge specific capacity of 890 mAh / g and a capacity retention rate of 65.7% after 300 cycles; the battery assembled in Comparative Example 2 had an initial discharge specific capacity of 781 mAh / g and a capacity retention rate of 70.0% after 300 cycles; and the battery assembled in Comparative Example 3 had an initial discharge specific capacity of 658 mAh / g and a capacity retention rate of 62.3% after 300 cycles. Figure 3 b represents the charge-discharge curve of a lithium-sulfur battery at 25℃ and a high rate of 10C, such as... Figure 3 As shown in b, the discharge specific capacity of the battery assembled in Example 1 was 541 mAh / g, and the voltage polarization was 51 mV; the discharge specific capacity of the battery assembled in Comparative Example 1 was 410 mAh / g, and the voltage polarization was 64 mV; the discharge specific capacity of the battery assembled in Comparative Example 2 was 371 mAh / g, and the voltage polarization was 59 mV; and the discharge specific capacity of the battery assembled in Comparative Example 3 was 225 mAh / g, and the voltage polarization was 67 mV. Compared with Comparative Examples 1, 2, and 3, the lithium-sulfur battery assembled in Example 1 has advantages such as high specific capacity, low decay rate, and small voltage polarization. This is attributed to the structural characteristics of the composite inorganic material layer: the polar defective graphitic carbon nitride can adsorb polysulfides through nitrogen sites and transport them to the nickel catalytic active sites of adjacent nickel-iron hydroxide, thereby achieving rapid conversion, promoting the conversion of long-chain polysulfides into short-chain lithium sulfides, thus promoting the redox reaction kinetics of the lithium-sulfur battery, improving sulfur utilization and reaction reversibility.

[0034] Figure 4 The electrochemical performance of lithium-sulfur batteries at -20℃ and 0.2C rate is shown. Figure 4As shown, the initial discharge specific capacity of the battery assembled in Example 1 was 795 mAh / g, and the capacity retention rate after 100 cycles was 79.7%; the initial discharge specific capacity of the battery assembled in Comparative Example 1 was 538 mAh / g, and the capacity retention rate after 100 cycles was 71.9%; the initial discharge specific capacity of the battery assembled in Comparative Example 2 was 457 mAh / g, and the capacity retention rate after 100 cycles was 68.6%; the initial discharge specific capacity of the battery assembled in Comparative Example 3 was 296 mAh / g, and the capacity retention rate after 100 cycles was 25.6%. Compared with Comparative Examples 1, 2, and 3, the lithium-sulfur battery assembled in Example 1 has advantages such as high specific capacity and low degradation rate under low temperature conditions. This is attributed to the Janus membrane prepared in Example 1 having a fast ion conduction channel, ensuring efficient ion transport under low temperature conditions. Its excellent interfacial reaction kinetics also effectively suppress the polysulfide shuttle effect and battery polarization at low temperatures, enabling the battery to maintain high discharge capacity and stable cycle performance even at -20℃.

[0035] Example 2 A Janus separator for low-temperature resistant lithium-sulfur batteries includes a porous separator substrate, polymer layers covering both sides of the porous separator substrate, and a composite inorganic material layer coated on the surface of the polymer layer on either side. The porous membrane substrate is a polyethylene membrane substrate with a thickness of 25 μm; The polymer layer is made of polymethyl methacrylate and has a thickness of 3 μm. The composite inorganic material layer is composed of defective graphitic carbon nitride, nickel-based double hydroxide, conductive carbon, and a binder; the nickel-based double hydroxide contains nickel and cobalt elements; the conductive carbon is selected from carbon nanotubes; the binder is selected from polyvinyl alcohol; and the thickness of the composite inorganic material layer is 15 μm. The polymer layer is used to promote lithium-ion migration and improve the stability of the lithium anode interface; the defective graphitic carbon nitride acts as an electron donor to provide electrons to the active center of the nickel-based double hydroxide.

[0036] The preparation method of the above-mentioned low-temperature resistant Janus separator for lithium-sulfur batteries includes the following steps: S1. A polymethyl methacrylate (PMMA) layer is formed on the surface of a polyethylene membrane substrate by a phase inversion method to obtain a membrane with a PMMA layer on both sides. The specific steps are as follows: dissolve PMMA in acetone to obtain a solution with a concentration of 8 g / L; immerse the polyethylene membrane substrate in the solution for 2 hours, take it out and place it in deionized water for phase inversion, and then dry it to form a PMMA layer. S2. Preparation of defective graphitic carbon nitride / nickel-based double hydroxide composite material: Specifically, 1.5g of defective graphitic carbon nitride was dispersed in 0.1L of water, and 4.6g of nickel nitrate and 2.7g of cobalt nitrate were added to make the concentrations of nickel nitrate and cobalt nitrate in the solution 0.25mol / L and 0.15mol / L, respectively. After mixing evenly, ammonia solution with a concentration of 0.9mol / L was added to adjust the pH value of the reaction system to 10.5. The reaction was carried out hydrothermally at 110℃ for 20h. After cooling, centrifugation, washing and drying, the defective graphitic carbon nitride / nickel-cobalt double hydroxide composite material was obtained.

[0037] S3. The defective graphite phase carbon nitride / nickel cobalt double hydroxide composite material obtained in step S2, carbon nanotubes and polyvinyl alcohol are mixed in a mass ratio of 6:1:1, N-methylpyrrolidone is added to make a slurry, which is coated on any side of the membrane with polymethyl methacrylate layer obtained in step S1, and dried to obtain the Janus membrane.

[0038] The Janus membrane prepared above is used in a low-temperature lithium-sulfur battery, which includes a lithium metal anode, a sulfur cathode, an ether electrolyte, and the Janus membrane disposed between the cathode and anode; in the Janus membrane, the polymer layer faces the lithium metal anode and the composite inorganic material layer faces the sulfur cathode.

[0039] The ionic conductivity of the membrane prepared in this embodiment was tested, and a lithium-ion symmetric battery was assembled for long-cycle testing. A lithium-sulfur battery was assembled using the membrane prepared in this embodiment, and the battery's electrochemical performance at room temperature and low temperature was tested. All testing procedures were performed in accordance with Example 1, and the test results are as follows: The membrane prepared in this embodiment has a room temperature ionic conductivity of 3.3 mS / cm, and the lithium symmetric battery can cycle stably for more than 500 hours. This shows that the membrane prepared in this embodiment has high ionic conductivity and excellent lithium anode interface compatibility.

[0040] At 25°C and 1C rate, the lithium-sulfur battery assembled with the separator prepared in this embodiment has an initial discharge specific capacity of 1050 mAh / g and a capacity retention of 73.3% after 300 cycles. At 25°C and 10C rate, the battery assembled with the separator prepared in this embodiment has a specific capacity of 524 mAh / g and a voltage polarization of 54 mV. At -20°C and 0.2C rate, the battery assembled with the separator prepared in this embodiment has an initial discharge specific capacity of 775 mAh / g and a capacity retention of 77.6% after 100 cycles. Therefore, the lithium-sulfur battery assembled with the separator prepared in this embodiment has advantages such as high specific capacity, low decay rate, and small voltage polarization, and exhibits good electrochemical performance under high rate and low temperature conditions.

[0041] Example 3 A Janus separator for low-temperature resistant lithium-sulfur batteries includes a porous separator substrate, polymer layers covering both sides of the porous separator substrate, and a composite inorganic material layer coated on the surface of the polymer layer on either side. The porous membrane substrate is a polyethylene / polypropylene composite multilayer membrane substrate with a thickness of 30 μm; The polymer layer is made of polyetherimide and has a thickness of 8 μm. The composite inorganic material layer is composed of defective graphitic carbon nitride, nickel-based double hydroxide, conductive carbon, and a binder; the nickel-based double hydroxide contains nickel and chromium; the conductive carbon is selected from ordered mesoporous carbon materials; the binder is selected from polyvinylidene fluoride; the thickness of the composite inorganic material layer is 8 μm. The polymer layer is used to promote lithium-ion migration and improve the stability of the lithium anode interface; the defective graphitic carbon nitride acts as an electron donor to provide electrons to the active center of the nickel-based double hydroxide.

[0042] The preparation method of the above-mentioned low-temperature resistant Janus separator for lithium-sulfur batteries includes the following steps: S1. A polyetherimide layer is formed on the surface of a polyethylene / polypropylene composite multilayer membrane substrate by a phase inversion method, resulting in a membrane with a polyetherimide layer on both sides. The specific steps are as follows: polyetherimide is dissolved in N-methylpyrrolidone to obtain a solution with a concentration of 12 g / L; the polyethylene / polypropylene composite multilayer membrane substrate is immersed in the solution for 2 hours, then removed and placed in deionized water for phase inversion, and then dried to form the polyetherimide layer. S2. Preparation of defective graphitic carbon nitride / nickel-based double hydroxide composite material; specifically: 0.8g of defective graphitic carbon nitride is dispersed in 0.1L of water, 3.1g of nickel sulfate and 7.8g of chromium sulfate are added to make the concentrations of nickel sulfate and chromium sulfate in the solution 0.2mol / L respectively. After mixing evenly, a 0.8mol / L potassium hydroxide solution is added to adjust the pH of the reaction system to 11. The reaction is carried out hydrothermally at 140℃ for 14h. After cooling, centrifugation, washing and drying, the defective graphitic carbon nitride / nickel-chromium double hydroxide composite material is obtained. S3. The defective graphite phase carbon nitride / nickel-chromium double hydroxide composite material obtained in step S2, the ordered mesoporous carbon material and polyvinylidene fluoride are mixed at a mass ratio of 6:1:1, N-methylpyrrolidone is added to make a slurry, and the slurry is coated on any side of the membrane with polyetherimide layer obtained in step S1. After drying, the Janus membrane is obtained.

[0043] The Janus membrane prepared above is used in a low-temperature lithium-sulfur battery, which includes a lithium metal anode, a sulfur cathode, an ether electrolyte, and the Janus membrane disposed between the cathode and anode; in the Janus membrane, the polymer layer faces the lithium metal anode and the composite inorganic material layer faces the sulfur cathode.

[0044] The ionic conductivity of the membrane prepared in this embodiment was tested, and a lithium-ion symmetric battery was assembled for long-cycle testing. A lithium-sulfur battery was assembled using the membrane prepared in this embodiment, and the battery's electrochemical performance at room temperature and low temperature was tested. All testing procedures were performed in accordance with Example 1, and the test results are as follows: The membrane prepared in this embodiment has a room temperature ionic conductivity of 3.4 mS / cm, and the lithium symmetric battery can cycle stably for more than 500 hours. This shows that the membrane prepared in this embodiment has high ionic conductivity and excellent lithium anode interface compatibility.

[0045] At 25°C and 1C rate, the lithium-sulfur battery assembled with the separator prepared in this embodiment has an initial discharge specific capacity of 1035 mAh / g and a capacity retention of 71.5% after 300 cycles. At 25°C and 10C rate, the battery assembled with the separator prepared in this embodiment has a specific capacity of 518 mAh / g and a voltage polarization of 57 mV. At -20°C and 0.2C rate, the battery assembled with the separator prepared in this embodiment has an initial discharge specific capacity of 772 mAh / g and a capacity retention of 76.8% after 100 cycles. Therefore, the lithium-sulfur battery assembled with the separator prepared in this embodiment has advantages such as high specific capacity, low decay rate, and small voltage polarization, and exhibits good electrochemical performance under high rate and low temperature conditions.

[0046] Although this application has been described in detail in this specification with general descriptions and specific embodiments, some modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of this application are protected by this application.

Claims

1. A Janus separator for low-temperature resistant lithium-sulfur batteries, characterized in that, It includes a porous membrane substrate, polymer layers covering both sides of the porous membrane substrate, and a composite inorganic material layer coated on the surface of the polymer layer on either side; the composite inorganic material layer is composed of defective graphitic carbon nitride, nickel-based double hydroxide, conductive carbon, and a binder. The polymer layer is used to promote lithium-ion migration and improve the stability of the lithium anode interface. The defective graphitic carbon nitride acts as an electron donor, providing electrons to the active center of the nickel-based double hydroxide.

2. The Janus separator for low-temperature lithium-sulfur batteries according to claim 1, characterized in that, The polymer layer is made of one or more of polymethyl methacrylate, polyetherimide, polyvinylpyrrolidone, and polyetheretherketone; the thickness of each side of the polymer layer is 1 to 10 μm.

3. The Janus separator for low-temperature resistant lithium-sulfur batteries according to claim 1, characterized in that, The nickel-based double hydroxide contains nickel and one metallic element selected from iron, cobalt, chromium, and aluminum; the conductive carbon is selected from at least one of conductive carbon black, ordered mesoporous carbon materials, and carbon nanotubes; the binder is selected from at least one of polyvinylidene fluoride, polyvinylpyrrolidone, and polyvinyl alcohol; and the thickness of the composite inorganic material layer is 5–25 μm.

4. The Janus separator for low-temperature resistant lithium-sulfur batteries according to claim 1, characterized in that, The porous membrane substrate is a polyolefin membrane, which is selected from polyethylene membrane, polypropylene membrane, and polyethylene / polypropylene composite multilayer membrane. The thickness of the polyolefin membrane is 10-40 μm.

5. A method for preparing a Janus separator for a low-temperature resistant lithium-sulfur battery according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1. The polymer layer is formed on the surface of the porous membrane substrate by phase inversion method to obtain a membrane with a polymer layer on both sides. S2. Preparation of defective graphitic carbon nitride / nickel-based double hydroxide composite material: Defective graphitic carbon nitride is dispersed in water, nickel salt and another metal salt are added, mixed evenly, and then an alkaline solution is added. After hydrothermal reaction, cooling, centrifugation, washing and drying, the defective graphitic carbon nitride / nickel-based double hydroxide composite material is obtained. S3. The defective graphitic carbon nitride / nickel-based double hydroxide composite material prepared in step S2 is mixed with conductive carbon and binder in a mass ratio of 6:1:

1. N-methylpyrrolidone is added to make a slurry, which is then coated on either side of the membrane with a polymer layer prepared in step S1. After drying, the Janus membrane is obtained.

6. The method for preparing a low-temperature resistant Janus separator for lithium-sulfur batteries according to claim 5, characterized in that, In step S2, the concentration of the defective graphitic carbon nitride is 3–20 g / L; the nickel salt is selected from nickel nitrate, nickel sulfate, nickel chloride, and nickel oxalate; the other metal salt is selected from iron salt, cobalt salt, chromium salt, and aluminum salt; the concentrations of the nickel salt and the other metal salt are each independently 0.1–0.4 mol / L; the alkaline solution is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and ammonia solution, with a concentration of 0.8–1.2 mol / L; the pH of the reaction system is adjusted to 9.5–11 by adding the alkaline solution; the hydrothermal reaction temperature is 100–150 °C, and the time is 12–24 h.

7. The method for preparing a low-temperature resistant Janus separator for lithium-sulfur batteries according to claim 5, characterized in that, In step S1, the phase inversion method specifically involves dissolving the polymer layer material in an organic solvent to obtain a solution, immersing the porous membrane substrate in the solution for 2 hours, removing it, placing it in deionized water for phase inversion, and then drying it to form the polymer layer.

8. The method for preparing a low-temperature resistant Janus separator for lithium-sulfur batteries according to claim 7, characterized in that, The organic solvent is selected from one or more of acetone, butanone, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; the concentration of the solution is 5~15 g / L.

9. The Janus separator prepared by the method according to any one of claims 5 to 8, and the application of the Janus separator according to any one of claims 1 to 4 in a low-temperature lithium-sulfur battery.

10. The application of the Janus separator as described in claim 9 in a low-temperature lithium-sulfur battery, characterized in that, The lithium-sulfur battery includes a lithium metal anode, a sulfur cathode, an ether electrolyte, and a Janus membrane as described in any one of claims 1-4 disposed between the cathode and a cathode; in the Janus membrane, the polymer layer faces the lithium metal anode, and the composite inorganic material layer faces the sulfur cathode.

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