A lithium-sulfur battery separator, its preparation method and application, lithium-sulfur battery

Abstract

This invention discloses a lithium-sulfur battery separator, its preparation method, and its application, as well as a lithium-sulfur battery. The lithium-sulfur battery separator comprises a substrate layer and a functional layer. A polymer / haloysite nanotube electrospun membrane serves as the substrate insulating layer, and polypyrrole is grown in situ on the surface of the carbonized halloysite nanotube membrane as the functional layer. This invention focuses on the modification of the lithium-sulfur battery electrospun separator, preparing a bilayer composite electrospun membrane. One layer of the electrospun membrane is carbonized, utilizing halloysite nanotubes (HNTs) to adsorb and dissolve polysulfides in the electrolyte, thus improving the "shuttle effect" of polysulfides. Simultaneously, conductive polymer polypyrrole is grown in situ on the HNTs carbonized membrane fibers. Polypyrrole can convert soluble polysulfides into insoluble polysulfides, thereby improving the utilization rate of active materials, effectively suppressing the polysulfide "shuttle effect," and enhancing the electrochemical performance and cycle stability of the lithium-sulfur battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a lithium-sulfur battery separator, its preparation method and application, and lithium-sulfur batteries. Background Technology

[0002] Lithium-sulfur batteries (LSBs) have attracted widespread attention and research due to their advantages such as high theoretical energy density, low cost, and environmental friendliness, making them one of the most promising candidates for next-generation energy storage systems. They boast a high theoretical specific capacity of up to 1675 mAh / g and a high theoretical energy density of 2600 Wh / kg, several times that of lithium-ion batteries (LIBs). Furthermore, sulfur, their active material, offers advantages such as abundant reserves, low cost, environmental friendliness, non-toxicity, and high safety.

[0003] However, the commercialization of lithium-sulfur batteries still faces many challenges, such as the severe polysulfide "shuttle effect," poor electrochemical reversibility, and rapid capacity decay. Among these, soluble lithium polysulfides (Li₂S₂)... x The shuttle effect (x≥4) is the most critical factor in reducing the electrochemical performance and cycle stability of LSB.

[0004] Therefore, solving the problem of severe polysulfide "shuttle effect" in lithium-sulfur batteries and improving their electrochemical performance and cycle stability is of great significance. Summary of the Invention

[0005] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes a lithium-sulfur battery separator, its preparation method and application, and a lithium-sulfur battery, aiming to solve the problem of severe polysulfide "shuttle effect" in current lithium-sulfur batteries and improve the electrochemical performance and cycle stability of lithium-sulfur batteries.

[0006] In a first aspect, the present invention provides a lithium-sulfur battery separator, comprising a substrate layer and a functional layer disposed on the substrate layer; the substrate layer comprises a polymer / haloysite nanotube electrospun film layer; the functional layer comprises a halloysite nanotube carbonized film layer and a polypyrrole film layer grown on the surface of the halloysite nanotube carbonized film layer.

[0007] The lithium-sulfur battery separator according to embodiments of the present invention has at least the following beneficial effects: The present invention proposes an integrated bilayer lithium-sulfur battery separator comprising a substrate layer and a functional layer. A polymer / haloysite nanotube electrospun membrane serves as the substrate insulating layer, and polypyrrole (PPy) is grown on the surface of the carbonized halloysite nanotube (HNTs) film as the functional layer. In the functional layer, conductive polypyrrole adheres to the surface of the carbonized fibers of the halloysite nanotube film, which can convert soluble polysulfides into insoluble polysulfides, thereby improving the utilization rate of active materials. Furthermore, due to its tubular structure and positive internal and negative external nuclear charge state, the silicate mineral halloysite can adsorb and dissolve polysulfides in the electrolyte, weakening the "shuttle effect" of the lithium-sulfur battery and reducing capacity decay during battery cycling. Ultimately, this invention addresses the modification of electrospun separators for lithium-sulfur batteries by preparing a bilayer composite electrospun membrane. One layer, a polymer / haloysite nanotube electrospun membrane, is carbonized. HNTs adsorb and dissolve polysulfides in the electrolyte, mitigating the polysulfide "shuttle effect." Simultaneously, conductive polypyrrole is grown on the carbonized halloysite nanotube membrane surface. Polypyrrole converts soluble polysulfides into insoluble polysulfides, thereby improving the utilization rate of the active material. The lithium-sulfur battery separator of this invention not only improves the utilization rate of the positive electrode active material but also effectively suppresses the polysulfide "shuttle effect," enhancing the electrochemical performance and cycle stability of lithium-sulfur batteries.

[0008] In some embodiments of the present invention, the carbon atom content of the polymer in the polymer / haloite nanotube electrospun film layer is ≥35%.

[0009] In some embodiments of the present invention, the polymeric material includes fluorocarbon polymers.

[0010] In some embodiments of the present invention, the polymeric material includes polyvinylidene fluoride (PVDF).

[0011] In some embodiments of the present invention, the polymer / halothite nanotube electrospun film layer is a polyvinylidene fluoride / halothite nanotube (PVDF / HNTs) electrospun film layer. Compared with commercial polyolefin films, the PVDF-containing electrospun film of the present invention has higher porosity, electrolyte absorption capacity, thermal stability, and ionic conductivity.

[0012] In some embodiments of the present invention, halloysite nanotubes (HNTs) have the molecular formula Al2Si2O5(OH)4·2H2O.

[0013] HNTs are natural clays composed of aluminosilicate layers. They are silicate minerals with spherical, plate-like, and tubular structures, with the tubular structure being the most common. The silicates in the molecule form the tubular layers of halloysite nanotubes, which roll inwards in more than twenty layers to form a tubular structure. The inner diameter of the tube is generally 15–100 nm, and the tube length is generally 500–1000 nm. HNTs possess polar functional groups; the Si-O tetrahedra on the outer surface and the Al-O octahedra on the inner surface result in HNTs exhibiting an external negative charge and an internal positive charge, exhibiting good ionic conductivity and adsorption properties. They can adsorb polysulfides, reducing the "shuttle effect" in lithium-sulfur batteries. This invention adds HNTs to an electrospun membrane to adsorb and dissolve polysulfides in the lithium-sulfur battery electrolyte, reducing or even preventing the diffusion and migration of dissolved polysulfides to the negative electrode side of the battery, where they undergo side reactions to generate insulating and insoluble Li₂S₂ and Li₂S, which then cover the negative electrode surface, thereby suppressing the "shuttle effect" in lithium-sulfur batteries and improving battery performance.

[0014] In some embodiments of the present invention, the functional layer includes a halloysite nanotube (HNTs) carbonized film layer. This HNTs carbonized film layer is obtained by high-temperature carbonization of a polymer / haloysite nanotube electrospun film layer. After carbonization, the polymer transforms into a carbon material, forming the HNTs carbonized film layer together with the halloysite nanotubes. Preferably, the HNTs carbonized film layer is obtained by high-temperature carbonization of a polyvinylidene fluoride / haloysite nanotube (PVDF / HNTs) electrospun film layer. The present invention carbonizes a polymer / haloysite nanotube (e.g., PVDF / HNTs) electrospun film at high temperatures, resulting in a film with good conductivity. Simultaneously, the HNTs in the carbonized fiber film can adsorb and dissolve polysulfides in the electrolyte, improving the poor electrochemical reversibility and rapid capacity decay caused by the "shuttle effect" of polysulfides in the battery.

[0015] In some preferred embodiments of the present invention, the conditions for the high-temperature carbonization treatment are: carbonization in a tubular furnace at 800–1000°C for 8–12 hours under a protective gas atmosphere. Preferably, the protective gas is nitrogen. Preferably, carbonization in a tubular furnace at 1000°C for 8 hours.

[0016] In some embodiments of the present invention, the functional layer further includes a polypyrrole film layer grown in situ on the surface of the HNTs carbonized film layer.

[0017] Polypyrrole (PPy) is a heterocyclic conjugated conductive polymer with good air stability. It is a conductive polymer that is easily electrochemically polymerized into films. Polypyrrole possesses conjugated chain oxidation and corresponding anion-doped structures, achieving a conductivity of up to 10⁻⁶. 2 ~10 3With a tensile strength of 50–100 MPa and excellent electrochemical redox reversibility, polypyrrole (PPY) exhibits a high conductivity (S / cm). The conductivity mechanism of PPY is as follows: PPY has a conjugated structure composed of alternating carbon-carbon single and double bonds. The double bonds are composed of σ and π electrons. The σ electrons are fixed and cannot move freely, forming covalent bonds between carbon atoms. The two π electrons in the conjugated double bonds are not fixed to any one carbon atom; they can translocate from one carbon atom to another, tending to extend along the entire molecular chain. That is, the overlap of π electron clouds within the molecule creates a shared energy band, and π electrons are similar to free electrons in metallic conductors. When an electric field is present, the electrons forming the π bonds can move along the molecular chain. Polypyrrole possesses excellent conductivity, electrochemical performance, and the ability to adsorb and utilize polysulfides, inhibiting the dissolution and shuttle effect of polysulfides. Simultaneously, polypyrrole and the highly conductive HNTs carbonized film work synergistically to effectively suppress the "shuttle effect" of polysulfides.

[0018] In this invention, polypyrrole is grown in situ on carbonized halloysite nanotube carbonized film fibers. Polypyrrole possesses good electrical conductivity, electrochemical properties, and the ability to adsorb and utilize polysulfides, and can suppress polysulfide shuttle. This polypyrrole grows on the fibers without occupying additional space and can be used as a separator to store Li. + In this environment, HNTs provide additional capacity for lithium-sulfur batteries. Furthermore, due to their excellent ionic conductivity, HNTs can synergistically work with polypyrrole to further improve the cycle stability of lithium-sulfur batteries.

[0019] Meanwhile, compared with commercial polyolefin membranes, the electrospun membrane containing PVDF of this invention has higher porosity, electrolyte absorption capacity, thermal stability and ionic conductivity.

[0020] In some embodiments of the present invention, the mass fraction of halloysite nanotubes in the polymer / haloysite nanotube electrospun film layer is 1-20%, preferably 1-5%.

[0021] In some embodiments of the present invention, the halloysite nanotube carbonized film layer contains 1 to 20% by mass, preferably 1 to 5%.

[0022] In some embodiments of the present invention, the mass fraction of halloysite nanotubes in the polyvinylidene fluoride / haloysite nanotube (PVDF / HNTs) electrospun film layer is 1-20%, preferably 1-5%.

[0023] In some embodiments of the present invention, the halloysite nanotube carbonized film layer contains 1 to 20% by mass, preferably 1 to 5%.

[0024] The mass fraction of HNTs in PVDF / HNTs electrospun membranes affects their function. When the mass fraction of HNTs is greater than 5%, the functional layer may pulverize and fall off during cycling as the HNTs percentage increases, thus losing its original function. When the mass fraction of HNTs is less than 1%, the adsorption of polysulfides by HNTs will decrease.

[0025] In some embodiments of the present invention, the thickness of the substrate layer is 8 to 10 μm, for example, it can be 8 μm, 9 μm, or 10 μm.

[0026] In some embodiments of the present invention, the thickness of the functional layer is 8 to 10 μm, for example, it can be 8 μm, 9 μm, or 10 μm.

[0027] A second aspect of the present invention provides a method for preparing the above-mentioned lithium-sulfur battery separator, comprising the steps of:

[0028] S10, Prepare a polymer / haloite nanotube solution;

[0029] S20. Prepare a polymer / haloite nanotube electrospun film layer by electrospinning process.

[0030] S30. The prepared polymer / haloite nanotube electrospun film layer is subjected to high-temperature carbonization treatment to obtain a halloysite nanotube carbonized film layer.

[0031] S40. A polypyrrole film layer is grown in situ on the surface of the halloysite nanotube carbonized film layer to obtain a functional layer.

[0032] S50. Another layer of polymer / haloite nanotube electrospun film is prepared on the functional layer by electrospinning process as a base layer to obtain the lithium-sulfur battery separator.

[0033] The method for preparing a lithium-sulfur battery separator according to embodiments of the present invention has at least the following beneficial effects: The method involves preparing a single-layer polymer / halothite nanotube electrospun membrane via electrospinning, followed by high-temperature carbonization of the polymer / halothite nanotube electrospun membrane. Then, redox-active polypyrrole is grown in situ on the carbonized fibers. Finally, another layer of polymer / halothite nanotube electrospun membrane is electrospun onto this membrane as an insulating layer. Ultimately, the bilayer composite separator prepared by the present invention exhibits high electrochemical performance and excellent adsorption and utilization capacity for polysulfides. Furthermore, the preparation process is simple, the reaction conditions are not harsh, and the time consumption is short. It is also compatible with common commercial battery preparation methods, providing a solution for the commercialization of lithium-sulfur batteries.

[0034] In some embodiments of the present invention, the mass ratio of polymer to halloysite nanotubes in the polymer / haloysite nanotube solution is (99:1) to (80:20); preferably, the mass ratio of polymer to halloysite nanotubes is (99:1) to (95:5); more preferably, the mass ratio of polymer to halloysite nanotubes is about 95:5.

[0035] The mass fraction of halloysite nanotubes (HNTs) in the polymer / haloysite nanotube solution affects the mass fraction of HNTs in the formed electrospun film, thus influencing its function. When the mass fraction of HNTs is >5%, the functional layer may pulverize and detach during cycling, losing its intended function, as the HNTs percentage increases; while when the mass fraction of HNTs is <1%, the adsorption of polysulfides by HNTs decreases. In some embodiments of the present invention, in steps S20 and S50, the thickness of the polymer / haloysite nanotube electrospun film layer prepared by the electrospinning process is 8–10 μm, for example, 8 μm, 9 μm, or 10 μm.

[0036] In some embodiments of the present invention, in step S30, the conditions for the high-temperature carbonization treatment are: carbonization in a tubular furnace at 800–1000°C for 8–12 hours under a protective gas atmosphere. Preferably, the protective gas is nitrogen. Preferably, carbonization in a tubular furnace at 1000°C for 8 hours. The halloysite nanotube carbonized film is obtained by high-temperature carbonization of the polymer / haloysite nanotube electrospun film. After carbonization, the polymer becomes a carbon material, forming an HNTs carbonized film together with the halloysite nanotubes.

[0037] In some embodiments of the present invention, step S40 specifically includes:

[0038] S41. Mix an appropriate amount of pyrrole monomer solution, ammonium persulfate solution and ferric chloride solution to prepare polypyrrole solution;

[0039] S42. The halloysite nanotube carbonized film is placed in the polypyrrole solution, so that polypyrrole grows in situ on the surface of the halloysite nanotube carbonized film to form a polypyrrole film.

[0040] S43. After standing at room temperature for a predetermined time, the functional layer is obtained.

[0041] In some preferred embodiments of the present invention, in step S43, the mixture is left to stand at room temperature for 12–48 hours; preferably, for 18–30 hours; more preferably, for 24 hours. Whether polypyrrole grows in situ on the surface of the halloysite nanotube carbonized film layer affects the adsorption, conversion, and utilization of polysulfides by the composite membrane. Too short a reaction time is not conducive to the formation of polypyrrole coating on the carbonized fiber surface; while when the reaction reaches its optimal state, increasing the time will cause polypyrrole aggregation and additionally increase the thickness of the membrane, thus reducing its effectiveness.

[0042] In some preferred embodiments of the present invention, after obtaining the functional layer in step S43, excess polypyrrole and excess ions on the surface can be washed with deionized water and placed in a vacuum drying oven at 45°C for 12 hours.

[0043] In a specific embodiment of the present invention, the method for preparing the lithium-sulfur battery separator includes the following steps:

[0044] S100, prepare a polyvinylidene fluoride / haloite nanotube (PVDF / HNTs) solution;

[0045] S200, Prepare polyvinylidene fluoride / halothite nanotubes (PVDF / HNTs) electrospun film layer by electrospinning process;

[0046] S300. The electrospun polyvinylidene fluoride / haloysite nanotubes (PVDF / HNTs) film layer is subjected to high-temperature carbonization treatment to obtain a carbonized halloysite nanotube (HNTs) film layer.

[0047] S400. A polypyrrole film layer is grown in situ on the surface of the halloysite nanotube (HNTs) carbonized film layer to obtain a functional layer.

[0048] S500. Another polyvinylidene fluoride / halothite nanotube (PVDF / HNTs) electrospun film layer is prepared on the functional layer by electrospinning process as a base layer to obtain the lithium-sulfur battery separator.

[0049] Figure 1The diagram illustrates the preparation process of a specific embodiment of the lithium-sulfur battery bilayer composite separator of the present invention. First, a PVDF / HNTs solution is prepared, and a single-layer PVDF / HNTs electrospun membrane is prepared using an electrospinning process. Then, the PVDF / HNTs electrospun membrane is carbonized at a high temperature above 800°C. Subsequently, redox-active polypyrrole is grown in situ on the carbonized fibers to obtain a (PVDF / HNTs carbonized membrane)@polypyrrole monolayer membrane, serving as the functional layer. Finally, another PVDF / HNTs electrospun membrane is electrospun onto this membrane as the insulating layer. Ultimately, the bilayer composite separator prepared by this invention exhibits high electrochemical performance and excellent adsorption and utilization capabilities for polysulfides, providing a solution for the commercialization of lithium-sulfur batteries.

[0050] In some embodiments of the present invention, the mass ratio of PVDF to HNTS in the PVDF / HNTS solution is (99:1) to (80:20); preferably, the mass ratio of PVDF to HNTS is (99:1) to (95:5); more preferably, the mass ratio of PVDF to HNTS is about 95:5.

[0051] The mass fraction of HNTs in the PVDF / HNTs solution affects the mass fraction of HNTs in the formed electrospun membrane, thus influencing its function. When the mass fraction of HNTs is >5%, the functional layer may pulverize and detach during cycling, losing its original function, as the HNTs percentage increases; while when the mass fraction of HNTs is <1%, the adsorption of polysulfides by HNTs decreases.

[0052] In some embodiments of the present invention, in step S100, a PVDF / HNTs solution is prepared using DMF as a solvent, and the mass fraction of the solution is 10-20%, preferably about 13%.

[0053] In some embodiments of the present invention, in steps S200 and S500, the thickness of the polyvinylidene fluoride / haloite nanotube electrospun film layer prepared by electrospinning process is 8 to 10 μm, for example, it can be 8 μm, 9 μm, or 10 μm.

[0054] In some embodiments of the present invention, in step S300, the conditions for the high-temperature carbonization treatment are: carbonization in a tubular furnace at 800–1000°C for 8–12 hours under a protective gas atmosphere. Preferably, the protective gas is nitrogen. Preferably, carbonization in a tubular furnace at 1000°C for 8 hours.

[0055] In some embodiments of the present invention, step S400 specifically includes:

[0056] S401. Mix an appropriate amount of pyrrole monomer solution, ammonium persulfate solution and ferric chloride solution to prepare polypyrrole solution;

[0057] S402. The halloysite nanotube carbonized film is placed in the polypyrrole solution, so that the polypyrrole grows in situ on the surface of the halloysite nanotube carbonized film to form a polypyrrole film.

[0058] S403. After standing at room temperature for a predetermined time, the functional layer is obtained.

[0059] In some preferred embodiments of the present invention, in step S403, the mixture is left to stand at room temperature for 12–48 hours; preferably, for 18–30 hours; more preferably, for 24 hours. Whether polypyrrole grows in situ on the surface of the PVDF / HNTs carbonized membrane affects the adsorption, conversion, and utilization of polysulfides by the composite membrane. Too short a reaction time is detrimental to the coating formation of polypyrrole on the carbonized fiber surface; while when the reaction reaches its optimal state, increasing the time will cause polypyrrole aggregation and additional membrane thickness, thus reducing its effectiveness.

[0060] In some preferred embodiments of the present invention, after obtaining the functional layer in step S403, excess polypyrrole and excess ions on the surface can be washed with deionized water and placed in a vacuum drying oven at 45°C for 12 hours.

[0061] In a third aspect, the present invention provides a lithium-sulfur battery comprising a lithium-sulfur battery separator as described above or a lithium-sulfur battery separator prepared by the above preparation method.

[0062] The lithium-sulfur battery according to the embodiments of the present invention has at least the following beneficial effects: The lithium-sulfur battery proposed in the present invention adopts the above-mentioned composite separator, and therefore has at least all the beneficial effects brought about by the technical solutions of the above embodiments, that is, the utilization rate of the positive electrode active material of the battery is higher, the ability to adsorb polysulfides is better, the polysulfide "shuttle effect" can be effectively suppressed, the battery is more stable, the electrochemical performance is better, the specific capacity is higher, and the cycle capacity retention rate is better.

[0063] In this invention, there are no restrictions on the positive electrode, negative electrode, electrolyte, etc. of the assembled battery, and commonly used materials in the field can be reasonably used, which will not be elaborated here.

[0064] In a fifth aspect, the present invention provides the application of the above-described lithium-sulfur battery separator or the lithium-sulfur battery separator prepared by the above-described preparation method in the preparation of energy storage devices, electrical devices or electronic devices. Attached Figure Description

[0065] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein:

[0066] Figure 1 This is a schematic diagram of the preparation process of the lithium-sulfur battery double-layer composite separator in an embodiment of the present invention. Detailed Implementation

[0067] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0068] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0069] In the description of this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. Unless otherwise stated, the various reaction or operation steps may be performed sequentially or not. Preferably, the reaction methods in this invention are performed sequentially.

[0070] Unless otherwise specified in the following examples, the techniques or conditions described in the literature in this field or in accordance with the product instructions shall apply. All reagents or instruments without a specified manufacturer are commercially available conventional products.

[0071] Example 1

[0072] 1. Preparation of the diaphragm

[0073] (1) Using DMF as solvent, a PVDF:HNTS solution with a mass ratio of 95:5 and a mass fraction of 13% was prepared. The solution was stirred at 90℃ for 5 hours to form a homogeneous precursor solution. The solution was then transferred to a 10mL syringe and allowed to stand until no air bubbles were present. The syringe was then fixed to a syringe pump with a moving speed of 20mm / min. A constant voltage of +26kV and -2kV was applied, and nanofibers were sprayed onto a 600r / min roller at a distance of 20cm at a push speed of 0.4mL / h. 1.5mL of spinning solution was injected to spin the outer diaphragm. Tin foil was sandwiched in the middle with the transverse direction consistent with the rotation direction of the roller to prepare a PVDF / HNTs monolayer electrospun membrane with a thickness of about 10μm.

[0074] (2) The single-layer electrospun membrane prepared in step (1) was placed in a tube furnace at 1000℃ for 8 hours under nitrogen protection to prepare a carbonized PVDF / HNTs membrane.

[0075] (3) Weigh 114g of ammonium persulfate and place it in a 500mL beaker to prepare a 1mol / L ammonium persulfate solution A. Stir well and let stand for later use.

[0076] Weigh 81g of ferric chloride powder and place it in a 500mL beaker to prepare a 1mol / L ferric chloride solution B. Stir well and let stand for later use.

[0077] Weigh 33.5g of pyrrole monomer using a syringe and dissolve it in a 500mL beaker to obtain pyrrole solution C. Wrap the solution in aluminum foil to protect it from light and stir until homogeneous. Let it stand until ready for use.

[0078] Prepare a 1 mol / L hydrochloric acid solution D for later use;

[0079] Using a syringe, draw 20 mL of each of solutions A, B, and C, stir well, pour into a petri dish, and add dilute hydrochloric acid solution D to adjust the pH to 3.

[0080] (4) Place the PVDF / HNTs carbonized film prepared in step (2) into the petri dish in step (3), store the petri dish in the dark, and let it stand at room temperature for 24 hours. A thin layer of polypyrrole will be obtained on the surface of the PVDF / HNTs carbonized film, and the PVDF / HNTs carbonized film @PPy will be obtained. After reacting for 24 hours, take out the film, wash off the excess polypyrrole and excess ions on the surface with deionized water, and place it in a vacuum drying oven at 45℃ for 12 hours.

[0081] (5) Repeat step (1) to fix the PVDF / HNTs carbonized film @PPy obtained in step (4) onto tin foil and then spin another layer of PVDF / HNTs electrospun film; the spinning environment temperature is set to 25℃, relative humidity is 40%, and vacuum drying is carried out at 60℃ for 12h.

[0082] The final result is a double-layer composite membrane with a thickness of about 20 μm, which is then cut into fiber membranes with a diameter of 19 mm for later use.

[0083] 2. Preparation of the positive electrode sheet

[0084] Elemental sulfur, conductive agent superconducting carbon (Super-P), and binder polyvinylidene fluoride (PVDF) are mixed evenly at a mass ratio of 8:1:1 to prepare a lithium-sulfur battery positive electrode slurry with a certain viscosity. The slurry is coated on current collector aluminum foil, dried at 85°C, and then cold-pressed. Then it is stamped and dried at 110°C for 4 hours under vacuum to produce the positive electrode sheet.

[0085] 3. Preparation of electrolyte

[0086] A 1:1, v / v mixed solvent of 1M lithium bis(trifluoromethanesulfonylimide) (LiTFSI) and 0.2M lithium nitrate (LiNO3) in 1,3-dioxolane (DOL) and dimethyl ethylene glycol (DME) is prepared for use.

[0087] 4. Preparation of lithium-ion batteries

[0088] The above-mentioned positive electrode, composite separator, electrolyte and negative electrode (lithium metal sheet) are assembled into a battery. The composite separator is located between the positive electrode and the negative electrode, and the functional layer faces the positive electrode side.

[0089] Example 2

[0090] The only difference from Example 1 is that in step (1) of Example 2, the mass ratio of PVDF:HNTS is 80:20 when preparing the precursor solution, and the rest is the same as in Example 1.

[0091] Example 3

[0092] The only difference from Example 1 is that in step (1) of Example 3, the mass ratio of PVDF:HNTS is 90:10 when preparing the precursor solution, and the rest is the same as in Example 1.

[0093] Example 4

[0094] The only difference from Example 1 is that in step (1) of Example 4, the mass ratio of PVDF:HNTS is 99:1 when preparing the precursor solution, and the rest is the same as in Example 1.

[0095] Example 5

[0096] The only difference from Example 1 is that in step (4) of Example 5, the reaction time of the PVDF / HNTs carbonized film in the pyrrole solution is 12h, and the rest is the same as in Example 1.

[0097] Example 6

[0098] The only difference from Example 1 is that in step (4) of Example 6, the reaction time of the PVDF / HNTs carbonized film in the pyrrole solution is 18h, and the rest is the same as Example 1.

[0099] Example 7

[0100] The only difference from Example 1 is that in step (4) of Example 7, the reaction time of the PVDF / HNTs carbonized film in the pyrrole solution is 30h, and the rest is the same as in Example 1.

[0101] Comparative Example 1

[0102] The difference from Example 1 is that the composite separator consists of a base layer + a base layer, without a functional layer. In preparing the separator, a two-layer PVDF / HNTs monolayer electrospun membrane is directly prepared as the base layer. The rest is the same as in Example 1 and will not be repeated.

[0103] Comparative Example 2

[0104] The difference from Example 1 is that the composite membrane consists of one functional layer and one functional layer, without a substrate layer. In preparing the membrane, a two-layer PVDF / HNTs carbonized film@PPy is directly prepared as the functional layer. The rest is the same as in Example 1 and will not be repeated.

[0105] Comparative Example 3

[0106] The difference from Example 1 is that the functional layer and the substrate layer do not contain HNTs. When preparing the separator, the precursor solution is a PVDF solution, the PVDF monolayer electrospun membrane is used as the substrate layer, and the PVDF carbonized membrane@PPy is used as the functional layer. The rest is the same as in Example 1 and will not be repeated.

[0107] Comparative Example 4

[0108] The difference from Example 1 is that the PVDF / HNTS film in the functional layer is directly grown in situ without carbonization for PPy growth. The rest is the same as in Example 1 and will not be repeated here.

[0109] Comparative Example 5

[0110] The difference from Example 1 is that the functional layer does not undergo in-situ growth of PPy, i.e. it does not contain PPy. The rest is the same as Example 1 and will not be repeated.

[0111] Lithium-sulfur battery performance testing

[0112] Cyclic testing: Assembled (S / separator / Li) button cells were immersed for 24 hours and then charged and discharged 200 times at a constant current density of 1C using a battery testing system from Newwell Electronics Co., Ltd., within a voltage range of 1.7–2.8V. The test results are shown in Table 1.

[0113] Rate testing: Assemble (S / separator / Li) button cells, immerse them for 24 hours, and use the battery testing system of Newwell Electronics Co., Ltd. to conduct 5 charge-discharge cycles at 0.2C, 0.5C, 1C, 2C and 5C rates respectively under a voltage range of 1.7 to 2.8V. The test results are shown in Table 2.

[0114] Table 1. Discharge specific capacity of coin cells at 0.5C for 200 cycles

[0115]

[0116] Table 2. Specific capacity of coin cells at different rate cycles (unit: mAh / g)

[0117]

[0118] As can be seen from Examples 1-4 and Comparative Examples 1 and 3 in Table 1, after 200 cycles at 0.5C, the specific capacity of Example 1 can still reach 1283mAh / g. This is mainly because the tubular HNTs have polar functional groups. The Si-O tetrahedrons on the outer surface and the Al-O octahedrons on the inner surface cause the HNTs to exhibit external negative charge and internal positive charge, which has good adsorption properties. It can adsorb polysulfides dissolved in the electrolyte, reduce or even avoid the diffusion and migration of dissolved polysulfides to the negative electrode side to generate insulating, insoluble Li2S2 and Li2S, which then cover the negative electrode surface, thereby suppressing the "shuttle effect" of lithium-sulfur batteries.

[0119] By comparing the PVDF / HNTs mass fractions in Examples 1-4 in Table 1, it was found that when the mass percentage of HNTs is >5%, the functional layer will pulverize and fall off during the cycle and lose its original function as the HNTs percentage increases; while when the mass percentage of HNTs is <1%, the adsorption of polysulfides by HNTs decreases.

[0120] As shown in Table 1, Examples 1, 5-7, and Comparative Examples 4-5, whether polypyrrole grows in situ on the surface of the PVDF / HNTs carbonized membrane affects the adsorption, conversion, and utilization of polysulfides by the composite membrane. Too short a reaction time is detrimental to the coating formation of polypyrrole on the carbonized fiber surface; while when the reaction reaches its optimal state, increasing the reaction time of the PVDF / HNTs carbonized membrane in the pyrrole solution leads to polypyrrole aggregation and an additional increase in membrane thickness, thus reducing its effectiveness.

[0121] As can be seen from Comparative Examples 4-5 in Table 1, polypyrrole needs to work synergistically with a highly conductive carbonized film to achieve its effective function.

[0122] As can be seen from Comparative Example 2 in Table 1, the functional layer alone cannot serve as a separator because the functional layer alone has electronic conductivity, which would cause a short circuit between the anode and cathode, making charging and discharging impossible.

[0123] As shown in Table 2, Example 1 and Comparative Example 1 have discharge specific capacities of 1586, 1389, 1056, 897 and 643 mAh / g at 0.2C, 0.5C, 1C, 2C and 5C rates, respectively. Compared with Comparative Example 1, the specific capacity increased by ~660 mAh / g at 2C rate, demonstrating good rate performance.

[0124] Polysulfide adsorption capacity test

[0125] Preparation of polysulfide solutions: These solutions were prepared by reacting elemental sulfur and lithium sulfide in a 1:1 molar ratio in a solvent of 1,3-dioxolane (DOL) and dimethyl ethylene glycol ether (DME) (1:1, v / v). The adsorption capacity of different membrane samples for polysulfides was tested (sample acquisition methods are as described in Example 1), as shown in Table 3.

[0126] Table 3. Adsorption capacity of different samples for polysulfides

[0127]

[0128] Table 3 shows that the PVDF membrane alone has no adsorption effect on polysulfides. The PVDF / HNTs membrane and the PVDF / HNTs carbonized membrane changed from their original yellow color to light yellow after 4 h and 12 h, respectively. This is mainly because HNTs played an adsorption role. Compared with the PVDF / HNTs membrane, the carbonized membrane is loose and porous, which is more conducive to solvent penetration and can play a role more quickly. The (PVDF / HNTs carbonized membrane)@PPy gradually turned light yellow after 4 h and the solution color became clear after 12 h, indicating that it had the best absorption of polysulfides, further proving the adsorption effect of HNTs and PPy on polysulfides.

[0129] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A lithium-sulfur battery separator, characterized in that, It includes a base layer and a functional layer disposed on the base layer; The substrate layer includes a polymer / haloite nanotube electrospun film layer; The functional layer includes a halloysite nanotube carbonized film layer and a polypyrrole film layer grown on the surface of the halloysite nanotube carbonized film layer, wherein the halloysite nanotube carbonized film layer has an electrospun film structure.

2. The lithium-sulfur battery separator according to claim 1, characterized in that, In the polymer / haloite nanotube electrospun film layer, the carbon atom content of the polymer is ≥35%.

3. The lithium-sulfur battery separator according to claim 2, characterized in that, The polymeric materials include fluorocarbon polymers.

4. The lithium-sulfur battery separator according to claim 3, characterized in that, The polymer includes polyvinylidene fluoride.

5. The lithium-sulfur battery separator according to claim 2, characterized in that, In the polymer / haloite nanotube electrospun film layer, the mass fraction of halloysite nanotubes is 1~20%.

6. The lithium-sulfur battery separator according to claim 5, characterized in that, In the polymer / haloite nanotube electrospun film layer, the mass fraction of halloysite nanotubes is 1~5%.

7. The lithium-sulfur battery separator according to claim 2, characterized in that, In the halloysite nanotube carbonized film layer, the mass fraction of halloysite nanotubes is 1~20%.

8. The lithium-sulfur battery separator according to claim 7, characterized in that, In the halloysite nanotube carbonized film layer, the mass fraction of halloysite nanotubes is 1~5%.

9. The lithium-sulfur battery separator according to claim 1, characterized in that, The thickness of the substrate layer is 8~10μm; And / or, the thickness of the functional layer is 8~10μm.

10. A method for preparing a lithium-sulfur battery separator as described in any one of claims 1-9, characterized in that, Includes the following steps: Preparation of polymer / haloite nanotube solution; Electrospun polymer / haloite nanotube films were prepared by electrospinning process. The polymer / haloite nanotube electrospun film layer is subjected to high-temperature carbonization treatment to obtain a halloysite nanotube carbonized film layer. A polypyrrole film layer was grown in situ on the surface of the halloysite nanotube carbonized film layer to obtain a functional layer. Another polymer / haloite nanotube electrospun film layer is prepared on the functional layer by electrospinning process as a base layer to obtain the lithium-sulfur battery separator.

11. The method for preparing a lithium-sulfur battery separator according to claim 10, characterized in that, In the polymer / haloite nanotube electrospun film layer, the carbon atom content of the polymer is ≥35%.

12. The method for preparing a lithium-sulfur battery separator according to claim 11, characterized in that, The polymeric materials include fluorocarbon polymers.

13. The method for preparing a lithium-sulfur battery separator according to claim 12, characterized in that, The polymer includes polyvinylidene fluoride.

14. The method for preparing a lithium-sulfur battery separator according to claim 11, characterized in that, The thickness of the polymer / haloite nanotube electrospun film is 8~10μm.

15. The method for preparing a lithium-sulfur battery separator according to claim 10, characterized in that, In the polymer / haloite nanotube solution, the mass ratio of polymer to halloysite nanotube is (99:1) to (80:20).

16. The method for preparing a lithium-sulfur battery separator according to claim 15, characterized in that, In the polymer / haloite nanotube solution, the mass ratio of polymer to halloysite nanotube is (99:1) to (95:5).

17. The method for preparing a lithium-sulfur battery according to claim 10, characterized in that, The specific steps for in-situ growing a polypyrrole film on the surface of the halloysite nanotube carbonized film to obtain a functional layer include: A polypyrrole solution was prepared by mixing appropriate amounts of pyrrole monomer solution, ammonium persulfate solution and ferric chloride solution. The halloysite nanotube carbonized film is placed in the polypyrrole solution, so that polypyrrole grows in situ on the surface of the halloysite nanotube carbonized film to form a polypyrrole film. After being left to stand at room temperature for a predetermined time, the functional layer is obtained.

18. The method for preparing a lithium-sulfur battery according to claim 17, characterized in that, Let stand at room temperature for 12-48 hours.

19. The method for preparing a lithium-sulfur battery according to claim 18, characterized in that, Let stand at room temperature for 18-30 hours.

20. A lithium-sulfur battery, characterized in that, This includes the lithium-sulfur battery separator as described in any one of claims 1-9 or the lithium-sulfur battery separator prepared by the preparation method described in any one of claims 10-19.

21. The use of a lithium-sulfur battery separator as described in any one of claims 1-9 or a lithium-sulfur battery separator prepared by the preparation method described in any one of claims 10-19 in the preparation of energy storage devices, electrical devices or electronic devices.

Images