Artificial SEI film, preparation method thereof, negative electrode sheet and sodium-ion battery

Abstract

The application belongs to the technical field of sodium ion batteries, and particularly relates to an artificial SEI film, a preparation method thereof, a negative electrode sheet and a sodium ion battery. The preparation method comprises the following steps: uniformly laying a sodium metal layer on a base material; spraying a precursor solution in a spray form to the surface of the sodium metal layer, the precursor solution being used for generating an organic sodium salt layer compound in situ on the surface of the sodium metal layer; roll pressing the sodium metal layer with the precursor solution; vacuum drying the roll-pressed sodium metal layer; and finally obtaining the artificial SEI film on the sodium metal layer. The artificial SEI film prepared by the application is composed of organic sodium salt, and has the dual advantages of low ion diffusion energy barrier of organic material and high stability of inorganic material. The low ion diffusion energy barrier promotes the uniform deposition of sodium ions, improves the rate performance of the battery, and reduces the interface impedance; and the high stability ensures the cycle stability, greatly reduces the side reaction of sodium metal and electrolyte, effectively inhibits the piercing of sodium dendrites, and greatly enhances the safety of the battery.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to an artificial SEI film and its preparation method, a negative electrode sheet, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries have great potential to become the next generation of energy storage technology. The mainstream anode materials are hard carbon (capacity ~300 mAh / g) or alloy materials (such as Sn / Sb-based). However, these materials have inherent defects such as low initial efficiency (<80%) and large volume expansion during cycling (>200%), which severely limit the energy density and lifespan of the battery. In contrast, metallic sodium anodes have extremely high theoretical specific capacity (1166 mAh / g). -1 Sodium is considered an ideal alternative due to its low redox potential (-2.714 V vs. standard hydrogen electrode). However, in practical applications, it faces problems such as dendrite growth and interface instability: First, during cycling, uneven sodium deposition can form dendrites, which can puncture the separator and cause short circuits, often resulting in a cycle life of less than 100 cycles. Second, the high activity of sodium can cause side reactions with the electrolyte, forming a poorly stable and structurally uneven natural solid electrolyte interphase (SEI) film in situ. This film continuously consumes active materials and electrolyte, leading to rapid capacity decay and safety hazards.

[0003] To address the aforementioned issues, constructing artificial SEI films on sodium metal surfaces has become a key strategy. However, existing artificial SEI material systems have several problems: First, while organic SEI films possess good ion conductivity, their stability is insufficient, making it difficult to effectively suppress dendrite growth; second, although inorganic SEI films exhibit high stability, their high ion diffusion barrier limits the rate performance of the battery.

[0004] Therefore, existing artificial SEI material systems typically struggle to simultaneously and ideally balance high ionic conductivity and high stability. Consequently, there is an urgent need for a method to construct SEI films that can better balance or synergistically enhance ionic conductivity and stability.

[0005] It should be noted that this part of the present invention only provides background technology related to the present invention, and does not necessarily constitute prior art or known technology. Summary of the Invention

[0006] This invention provides an artificial SEI film and its preparation method, a negative electrode sheet, and a sodium-ion battery, which at least solves the problem that existing SEI films cannot simultaneously achieve high ionic conductivity and high stability.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing an artificial SEI membrane, comprising the following steps:

[0008] S1. Spread the sodium metal layer evenly on the substrate;

[0009] S2. The precursor liquid is sprayed onto the surface of the sodium metal layer in the form of a spray. The precursor liquid is used to generate organic sodium salt layer compounds in situ on the surface of the sodium metal layer.

[0010] S3. Roll pressing the sodium metal layer with the precursor liquid;

[0011] S4. The sodium metal layer after rolling is vacuum dried to obtain an artificial SEI film on the sodium metal layer.

[0012] Preferably, in step S1, the substrate is aluminum foil or copper foil.

[0013] Preferably, in step S1, the thickness of the substrate is 10μm to 20μm.

[0014] Preferably, in step S1, the thickness of the sodium metal layer is 20 μm to 80 μm.

[0015] Preferably, in step S2, the precursor solution is at least one of anhydrous glacial acetic acid, anhydrous ethanol, anhydrous phenol, anhydrous benzenesulfonic acid, or a liquid ammonia solution of a primary amine.

[0016] Preferably, step S2 specifically includes moving the substrate under a fixed spraying device at a preset speed, the spraying device spraying the precursor liquid onto the surface of the sodium metal layer, and the pressure of the spraying device being 0.1 kPa to 16 kPa.

[0017] Preferably, the preset spraying rate of the substrate is 0.5 m / min to 10 m / min. The preset rate is set according to the reaction rate between the precursor liquid and sodium metal, and the preset rate is positively correlated with the reaction rate.

[0018] Preferably, when the driving fluid is anhydrous ethanol, the preset rate is 0.5 m / min to 1.5 m / min; when the driving fluid is anhydrous glacial acetic acid, the preset rate is 5 m / min to 7 m / min; when the driving fluid is anhydrous phenol, the preset rate is 3 m / min to 5 m / min; when the driving fluid is anhydrous benzenesulfonic acid, the preset rate is 7 m / min to 10 m / min; and when the driving fluid is a liquid ammonia solution of a primary amine, the preset rate is 1 m / min to 3 m / min.

[0019] Preferably, in step S4, the thickness of the artificial SEI film is 1 μm to 10 μm.

[0020] Preferably, in step S4, the vacuum drying temperature is 80℃~120℃ and the time is 12 hours~36 hours.

[0021] Secondly, the present invention provides an artificial SEI membrane prepared by the above-described preparation method.

[0022] Thirdly, the present invention provides a negative electrode sheet, comprising a substrate and a sodium metal layer and the aforementioned artificial SEI film sequentially stacked on the substrate.

[0023] Fourthly, the present invention provides a sodium-ion battery, including the aforementioned negative electrode sheet.

[0024] The beneficial effects of this invention are as follows:

[0025] 1. The artificial SEI membrane prepared by this invention is composed of organic sodium salts (sodium acetate, sodium ethoxide, sodium phenolate, sodium sulfonate, and sodium amine), cleverly combining the dual advantages of low ion diffusion barriers of organic materials and high stability of inorganic materials. Its low ion diffusion barrier promotes uniform sodium ion deposition, improving the rate performance of the battery and reducing interfacial impedance; while its high stability ensures the stability of the SEI membrane during cycling, greatly reducing side reactions between sodium metal and the electrolyte, effectively suppressing sodium dendrite penetration, and significantly enhancing battery safety.

[0026] 2. The preparation method of this invention is simple and has a short process, which can be achieved through continuous spraying for large-scale production and is easy to integrate with existing electrode preparation processes. In addition, the pre-construction of the artificial SEI film greatly reduces the time and energy consumption of the battery formation stage, reduces the consumption of active sodium, and thus reduces the overall production cost.

[0027] 3. The artificial SEI membrane constructed by this invention can maintain structural stability at high temperatures, effectively blocking the continuous side reaction between the electrolyte and sodium metal, delaying the consumption and drying of the electrolyte, and significantly improving the storage safety and service life of the battery.

[0028] 4. This invention allows for convenient control of the chemical composition and physical properties of the artificial SEI film by selecting different precursor solutions, thereby meeting the specific requirements of different battery systems for interface characteristics, and has a wide range of applications.

[0029] 5. The "spray-rolling-vacuum drying" process of this invention forms an organic whole. The spray method enables in-situ uniform generation of the SEI film, the rolling method improves the film's density and adhesion, and the vacuum drying method ensures the final purity and stability of the film. The three steps work together to produce an SEI film that simultaneously achieves high ionic conductivity and high stability. Attached Figure Description

[0030] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 A flowchart illustrating the preparation method of the artificial SEI membrane provided in this embodiment of the invention.

[0032] Explanation of reference numerals in the attached figures:

[0033] 10. Substrate; 20. Sodium metal layer; 30. Spraying equipment; 40. Precursor liquid; 50. Artificial SEI film. Detailed Implementation

[0034] In this invention, unless otherwise stated, directional terms such as "up," "down," "left," and "right" are generally understood in conjunction with the accompanying drawings and the directions shown in actual applications.

[0035] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0036] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0037] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. The terms "optional" and "discretionary" mean that they may or may not be included (or may or may not be present).

[0038] This invention provides a method for preparing an artificial SEI membrane, comprising the following steps:

[0039] S1. Spread the sodium metal layer evenly on the substrate;

[0040] S2. The precursor liquid is sprayed onto the surface of the sodium metal layer in the form of a spray. The precursor liquid is used to generate organic sodium salt layer compounds in situ on the surface of the sodium metal layer.

[0041] S3. Roll pressing the sodium metal layer with the precursor liquid;

[0042] S4. The sodium metal layer after rolling is vacuum dried to obtain an artificial SEI film on the sodium metal layer.

[0043] Preferably, step S1, by uniformly laying a sodium metal layer on the substrate, provides a smooth and consistent sodium metal surface for the subsequent spraying construction of the artificial SEI film, ensuring that the precursor solution reacts with the sodium metal to generate a uniform and dense organic sodium salt layer. This uniformity helps to reduce the sodium ion diffusion barrier, promotes the uniform deposition of sodium ions during cycling, thereby effectively inhibiting dendrite growth, reducing interfacial side reactions, improving the rate performance and cycle stability of the battery, and laying a good foundation for subsequent rolling and drying processes, ensuring the structural integrity and functionality of the artificial SEI film.

[0044] Preferably, in step S1, the substrate is aluminum foil or copper foil. Aluminum foil or copper foil is chosen as the substrate because they have good conductivity, flexibility, and compatibility with sodium metal, and can effectively act as current collectors and support the sodium metal layer.

[0045] Preferably, in step S1, the thickness of the substrate is 10μm to 20μm. Controlling the substrate thickness to 10μm to 20μm ensures sufficient stability to prevent deformation or breakage during processing and cycling, while also avoiding increased ineffective weight and internal resistance of the battery due to excessive thickness.

[0046] Preferably, in step S1, the thickness of the sodium metal layer is 20 μm to 80 μm. Setting the thickness of the sodium metal layer to 20 μm to 80 μm can provide sufficient sodium source to ensure battery capacity while avoiding problems such as uneven sodium deposition, intensified dendrite growth, or excessive volume changes during cycling due to excessive thickness, thereby synergistically optimizing the battery's energy density, interface stability, and overall electrochemical performance.

[0047] Preferably, step S2 involves spraying the precursor solution onto the sodium metal layer surface using a spray method. This enables a uniform and controllable in-situ chemical reaction between the precursor solution and the sodium metal, thereby generating a dense and uniform organic sodium salt layer (sodium acetate, sodium ethoxide, sodium phenolate, sodium sulfonate, sodium amine) on the sodium metal surface. This method is not only simple to operate and easy to scale up, but also effectively controls the reaction interface, forming an artificial SEI film with both low ion diffusion barrier and high stability. This effectively promotes uniform sodium ion deposition, inhibits dendrite growth, and significantly reduces side reactions between sodium metal and electrolyte, comprehensively improving the battery's interface stability, cycle life, and safety.

[0048] Preferably, in step S2, the precursor solution is at least one of anhydrous glacial acetic acid, anhydrous ethanol, anhydrous phenol, anhydrous benzenesulfonic acid, or a liquid ammonia solution of a primary amine. These five specific components were chosen as precursor solutions because they all possess the ability to undergo a mild, controllable in-situ chemical reaction with metallic sodium, and the resulting organic sodium salt layer exhibits significant beneficial properties in the battery system. These precursor solutions all contain active hydrogen molecules (-OH, -SO3H, -NH2), enabling them to react efficiently and uniformly with the sodium metal surface to form a dense and stable protective layer.

[0049] More importantly, these five precursor solutions were systematically screened, and their reaction products (organo-sodium salts) not only possess good ionic conductivity and a low sodium ion diffusion barrier, but also exhibit excellent stability. These organic-sodium salt layers can effectively promote rapid and uniform sodium ion conduction, inhibit dendrite formation and growth, and significantly reduce side reactions between sodium metal and the electrolyte, thereby comprehensively improving the cycle life, rate performance, and safety of sodium-ion batteries. Therefore, the design of this precursor solution system not only ensures the controllability and repeatability of the process, but also guarantees the optimization and reliability of the constructed artificial SEI film in terms of composition and function.

[0050] Furthermore, the five precursor solutions selected in this invention, after reacting with sodium metal in situ, respectively generate sodium acetate, sodium ethoxide, sodium phenolate, sodium benzenesulfonate, and sodium amino acid. These five organic sodium salts each possess beneficial properties in the battery system and can complement each other functionally. Specifically, sodium acetate provides a good balance between ionic conductivity and stability, forming a stable substrate; sodium ethoxide, with its strong alkalinity, efficiently passivates the interface, significantly reducing side reactions and lowering impedance; sodium phenolate, with its high mechanical modulus and chemical stability, imparts robust resistance to dendrite penetration to the SEI film; sodium benzenesulfonate, with its strong affinity for sodium ions, greatly promotes ion migration and improves rate performance; and sodium amino acid provides passivation and deep isolation, effectively blocking the electrolyte's corrosion of sodium metal. These properties can be set individually or synergistically integrated through the combination of multiple salts to construct a multifunctional composite artificial SEI film.

[0051] Preferably, in the liquid ammonia solution of the primary amine, the primary amine is monomethylamine. As the simplest primary amine, monomethylamine has a small molecular weight and extremely low steric hindrance. It exhibits excellent diffusion and reaction kinetics in liquid ammonia solvent, and can rapidly undergo a uniform and controllable in-situ reaction with the sodium metal surface to generate a high-purity, dense sodium amide (NaNHCH3) protective layer with good ionic conductivity. This product not only possesses strong alkalinity to achieve efficient passivation of the sodium metal interface and effectively suppress side reactions between sodium and the electrolyte, but its small molecular size also helps to form a denser and thinner SEI film structure. This avoids the problem of increased ion transport resistance caused by the obstruction of reaction sites or excessive product layer thickness of long-chain primary amines. Thus, while ensuring excellent interfacial stability and safety, it also takes into account the efficient migration of sodium ions, thereby improving the overall electrochemical performance of the battery.

[0052] The reaction equations for the five precursor solutions reacting with sodium are as follows:

[0053] 1. Anhydrous glacial acetic acid (CH3COOH)

[0054] 2CH3COOH+2Na→2CH3COONa+H2↑;

[0055] 2. Anhydrous ethanol (C2H5OH)

[0056] 2C2H5OH+2Na→2C2H5ONa+H2↑;

[0057] 3. Anhydrous phenol (C6H5OH)

[0058] 2C6H5OH+2Na→2C6H5ONa+H2↑;

[0059] 4. Anhydrous benzenesulfonic acid (C6H5SO3H)

[0060] 2C6H5SO3H+2Na→2C6H5SO3Na+H2↑;

[0061] 5. Liquid ammonia solution of primary amines (e.g., R-NH2, where R is an alkyl group).

[0062] 2R-NH2+2Na→2R-NHNa+H2↑ (reaction in liquid ammonia environment).

[0063] Preferably, step S2 specifically includes moving the substrate under a fixed spraying device at a preset rate, the spraying device spraying the precursor liquid onto the surface of the sodium metal layer, and the pressure of the spraying device being 0.1 kPa to 16 kPa, which can be 0.1 kPa, 1 kPa, 2 kPa, 3 kPa, 4 kPa, 5 kPa, 6 kPa, 7 kPa, 8 kPa, 9 kPa, 10 kPa, 11 kPa, 12 kPa, 13 kPa, 14 kPa, 15 kPa, 16 kPa and any value between them. This invention employs a spraying method to apply a precursor solution to the surface of a sodium metal layer, enabling uniform control of the reaction interface, reaction rate, and SEI film. By controlling the pressure of the spraying equipment and the preset rate of the substrate, it ensures that the precursor solution reacts fully but not excessively with the sodium metal surface in the form of tiny, uniform droplets, thereby generating a dense, continuous, and uniformly thick organic sodium salt protective layer. This method effectively avoids problems such as uneven reaction, excessive corrosion of sodium metal, excessive film thickness, or cracking that may occur with methods such as soaking or coating. It also effectively avoids the diffusion limitations and uniformity problems inherent in vapor-phase modification methods.

[0064] Furthermore, compared to conventional SEI construction methods, this invention generates an organic sodium salt layer through an in-situ reaction between a precursor solution and sodium metal, achieving strong adhesion between the SEI film and the substrate, a dense and defect-free microstructure, and a highly tunable chemical composition. This method effectively overcomes common problems in traditional methods such as weak film layer adhesion, loose structure, and single composition. The resulting artificial SEI film exhibits excellent ion conductivity and stability, significantly improving the interfacial stability and overall electrochemical performance of the battery. Simultaneously, this process is easily integrated into continuous production lines, demonstrating promising prospects for large-scale application.

[0065] Preferably, the preset spraying rate of the substrate is 0.5 m / min to 10 m / min, and can be 0.5 m / min, 1 m / min, 1.5 m / min, 2 m / min, 2.5 m / min, 3 m / min, 3.5 m / min, 4 m / min, 4.5 m / min, 5 m / min, 5.5 m / min, 6 m / min, 6.5 m / min, 7 m / min, 7.5 m / min, 8 m / min, 8.5 m / min, 9 m / min, 9.5 m / min, 10 m / min, or any value between them. The purpose is to control the reaction contact time between the precursor solution and the sodium metal surface, thereby achieving effective control over the thickness and uniformity of the SEI film. By optimizing the rate within this range, excessive corrosion of the sodium layer in localized reactions or the formation of an excessively thick, easily cracked, brittle film can be avoided. Insufficient reaction can also prevent uneven coverage and defects in the generated SEI film, thus failing to effectively protect the sodium metal. It can ensure that the precursor solution and sodium metal undergo a full and uniform in-situ reaction to form a dense, complete and moderately thick high-performance artificial SEI film. While ensuring excellent ion conductivity and stability, this rate range is also easy to match with industrial continuous production lines, taking into account both process effect and production efficiency.

[0066] More preferably, the preset rate is set according to the reaction rate between the precursor liquid and sodium metal, and the preset rate is positively correlated with the reaction rate. Establishing a positive correlation between the preset rate and the reaction rate between the precursor liquid and sodium metal aims to match process parameters with chemical reaction kinetics, thereby ensuring that precursor liquids with different reactivity can react with sodium metal at the optimal contact time. For highly reactive precursor liquids (such as anhydrous glacial acetic acid), the reaction time can be shortened by correspondingly increasing the preset rate of the substrate, effectively preventing excessive corrosion of the sodium metal layer and excessive SEI film thickness. For low-reactivity precursor liquids (such as liquid ammonia solution of primary amines), the reaction time is extended by decreasing the preset rate, ensuring the formation of a sufficiently thick and dense SEI film. This dynamic correlation design makes the entire spraying process adaptable and controllable, ensuring the formation of a uniform, stable, and high-performance artificial SEI film regardless of whether a single or composite precursor liquid is used, while also improving the process's versatility and scalability.

[0067] Preferably, the present invention provides preset rate ranges verified by DOE experimental design for different single-component precursor solutions. Specifically, when the precursor solution is anhydrous ethanol, the preset rate is 0.5 m / min to 1.5 m / min; when the precursor solution is anhydrous glacial acetic acid, the preset rate is 5 m / min to 7 m / min; when the precursor solution is anhydrous phenol, the preset rate is 3 m / min to 5 m / min; when the precursor solution is anhydrous benzenesulfonic acid, the preset rate is 7 m / min to 10 m / min; and when the precursor solution is a liquid ammonia solution of a primary amine, the preset rate is 1 m / min to 3 m / min.

[0068] Preferably, even when the precursor solution contains multiple components, the corresponding substrate preset rate can still be confirmed through DOE experimental design. The preset rate of the substrate should be between the preset rates of the multiple components, and the reaction rates of the multiple components of the precursor solution with sodium metal should be relatively similar; otherwise, the faster-reacting components will react rapidly, while the slower-reacting components will be unable to penetrate into the near-reaction layer, resulting in component loss.

[0069] Optionally, when the precursor liquid has multiple components, the preset rate of the substrate corresponding to the multiple component precursor liquid can be calculated by weighting the preset rates of the substrate corresponding to the different single component precursor liquids according to the mass ratio of the multiple components to the total mass of the precursor liquid. This can avoid conducting a large number of experiments for each possible new formulation, thereby improving the efficiency and operability of process development, while ensuring that even for mixed precursor liquids with complex components, a near-optimal preset rate can be calculated.

[0070] Preferably, step S3 involves rolling the sodium metal layer after spraying the precursor solution. This is to prevent the artificial SEI film from becoming too thick after the in-situ reaction, resulting in poor density, excessive electrolyte absorption, and slow sodium ion migration. By applying uniform mechanical pressure, the density, uniformity, and adhesion to the sodium metal substrate of the generated artificial SEI film can be significantly improved. This process effectively eliminates any air bubbles or voids in the film layer, compacts the organic sodium salt particles generated by the reaction, and forms a continuous, defect-free, and tightly structured solid protective layer. This greatly enhances the stability of the artificial SEI film, effectively passivates the negative electrode material, and enables it to more effectively suppress sodium dendrite penetration and block continuous side reactions of the electrolyte. Rolling after spraying also reduces the liquid absorption rate of the artificial SEI film, optimizes the ion transport channels of the film layer, reduces interfacial impedance, and provides a more stable and flat substrate for the subsequent vacuum drying process. Ultimately, this ensures the construction of a high-performance artificial SEI film with both excellent mechanical stability and high-speed ion conduction capability.

[0071] More specifically, step S3 is completed by a rolling equipment (usually a double-roll press). After spraying, the substrate and the sodium metal layer with the precursor liquid are fed between the upper and lower rolls of the double-roll press. Under a preset pressure and gap, the sodium metal layer with the precursor liquid is continuously and uniformly rolled by the upper roll.

[0072] Optionally, the rolling pressure ranges from 0.5 MPa to 5 MPa. This range provides sufficient pressure to compact the organic sodium salt layer, eliminate air bubbles and voids, and form a dense and continuous artificial SEI film, while avoiding excessive pressure that could lead to excessive deformation of the sodium metal layer or damage to the substrate. More preferably, the pressure is from 1 MPa to 3 MPa to optimize process stability while ensuring film density.

[0073] Optionally, the gap between the rollers can range from 20μm to 90μm. In practice, the roller gap needs to be adjusted according to the initial substrate thickness, the initial sodium metal layer thickness, and the desired thickness of the artificial SEI film to ensure that the film layer is uniformly compacted and the thickness is controllable after roller pressing.

[0074] Preferably, the sprayed substrate and the sodium metal layer with the precursor liquid are cut to appropriate sizes before rolling to ensure that the subsequent rolling process can be carried out efficiently, uniformly and stably.

[0075] Preferably, step S4 involves vacuum drying of the rolled sodium metal layer to effectively remove any residual solvents, moisture, and byproducts (such as hydrogen) that may remain after the precursor solution reaction. Simultaneously, heat treatment promotes the final curing and stabilization of the organic sodium salt SEI film. Drying in a vacuum environment effectively avoids oxidation of the sodium metal at high temperatures and damage to the film structure from residual solvents, ensuring the formation of a pure, dense, and chemically stable artificial SEI film. This step not only significantly enhances the bonding force between the artificial SEI film and the sodium metal substrate but also further improves the stability and ionic conductivity uniformity of the film. Ultimately, this gives the artificial SEI film the comprehensive ability to resist dendrite growth, suppress electrolyte side reactions, and maintain interface stability during long-term cycling, thus optimizing the electrochemical performance and safety of the battery. If the coating is dried directly after spraying, unreacted precursor solutions will remain in the SEI film, which is detrimental to the growth of deeper SEI film components.

[0076] Specifically, step S4 is completed using a vacuum drying device (usually a vacuum oven), in which the rolled sodium metal layer and the substrate are placed into the vacuum oven, and the vacuum oven continuously dries the rolled sodium metal layer under vacuum.

[0077] Preferably, in step S4, the vacuum drying temperature is 80℃~120℃, and the time is 12 hours~36 hours. This effectively removes residual solvents, moisture, and volatile byproducts (such as hydrogen) after the precursor reaction, while avoiding sodium metal melting, SEI film organic component decomposition, or structural damage due to excessively high temperature or time. This mild yet sufficient drying condition ensures that the organic sodium salt SEI film can gradually solidify and form a stable microstructure, effectively enhancing the film's density, chemical stability, and adhesion to the sodium metal substrate. Ultimately, this allows the artificial SEI film to maintain its complete physical barrier function during battery cycling, significantly inhibiting dendrite growth and side reactions, and improving the battery's interface stability and safety performance.

[0078] Preferably, in step S4, the thickness of the artificial SEI film is 1 μm to 10 μm. The aim is to ensure that the SEI film has sufficient stability to maintain cycle stability while also maintaining a low ion diffusion barrier, thereby synergistically improving the battery's interface stability, cycle life, and fast-charging capability. Optimizing the thickness within this range avoids the problem of an excessively thin film layer, which might fail to completely cover the sodium metal surface, resulting in localized defects and insufficient stability, making it difficult to effectively suppress dendrite penetration and block electrolyte side reactions. It also avoids the possibility that an excessively thick film layer might increase the diffusion resistance of sodium ions, raise the interface impedance, and impair the battery's rate performance and capacity.

[0079] This invention provides an artificial SEI membrane prepared by the above-described method.

[0080] The present invention provides a negative electrode sheet, comprising a substrate and a sodium metal layer and the above-mentioned artificial SEI film sequentially stacked on the substrate.

[0081] The present invention provides a sodium-ion battery, including the above-mentioned negative electrode sheet.

[0082] In addition to the aforementioned negative electrode, the sodium-ion battery provided in this application also includes a positive electrode, an electrolyte, and a separator. The positive electrode, electrolyte, and separator can all be made from conventional sodium-ion battery materials in the art, either commercially available or prepared in-house. Since these are all known technologies, they will not be described in detail here. The assembly of the sodium-ion battery is also a conventional assembly method in the art and will not be described in detail thereafter. The form of the sodium-ion battery provided in this application is not unique; it can be a button cell, or designed as a thin-film, flat-plate, cylindrical, or stacked cell, depending on the requirements.

[0083] The present invention will be described in more detail below through embodiments, which are exemplary and do not limit the scope of protection of the present invention in any way.

[0084] Example 1

[0085] like Figure 1 As shown, a method for preparing an artificial SEI membrane 50 includes the following steps:

[0086] S1. A sodium metal layer 20 with a thickness of 50 μm is uniformly laid on a copper foil substrate 10 with a thickness of 12 μm.

[0087] S2. The substrate 10 is moved at 0.8 m / min below the fixed spraying device 30. The spraying device 30 sprays the precursor liquid 40 onto the surface of the sodium metal layer 20. The pressure of the spraying device 30 is 0.5 kPa. The precursor liquid 40 is anhydrous ethanol.

[0088] S3. The sprayed substrate 10 and the sodium metal layer 20 with the precursor liquid 40 are cut into suitable roll forming dimensions, and the sodium metal layer 20 with the precursor liquid 40 is rolled using a double roll press; the preset rolling pressure is 2MPa, and the rolling gap is set to 45μm.

[0089] S4. The rolled sodium metal layer 20 and the substrate 10 are placed in a vacuum oven and dried in a vacuum oven at 100°C for 24 hours. Finally, an artificial SEI film 50 with a thickness of 3μm is obtained on the sodium metal layer 20.

[0090] A method for preparing a sodium-ion battery includes the following steps:

[0091] (1) Preparation of positive electrode: Sodium iron pyrophosphate, carbon black and PVDF are homogenized in NMP solution in a mass ratio of 94:3:3. The positive electrode slurry is coated on the current collector and then dried, rolled and slit to obtain a positive electrode that can be directly stacked.

[0092] (2) Preparation of negative electrode: including a substrate and a sodium metal layer and an artificial SEI film prepared by the above preparation method.

[0093] (3) Cell manufacturing: The slit positive and negative electrode sheets are stacked on a stacking machine, and the separator is made of three layers of PP / PE / PP material to form a soft-pack cell.

[0094] Example 2

[0095] The preparation methods of the artificial SEI membrane and sodium-ion battery in this embodiment are the same as in Example 1, except that anhydrous glacial acetic acid is selected as the precursor liquid, and the preset rate of the substrate during the spraying stage is 5 m / min.

[0096] Example 3

[0097] The preparation methods of the artificial SEI membrane and sodium-ion battery in this embodiment are the same as in Example 1, except that anhydrous phenol is selected as the precursor liquid and the preset rate of the substrate during the spraying stage is 4.5 m / min.

[0098] Example 4

[0099] The preparation methods of the artificial SEI membrane and sodium-ion battery in this embodiment are the same as in Example 1, except that anhydrous benzenesulfonic acid is selected as the precursor liquid, and the preset rate of the substrate during the spraying stage is 7 m / min.

[0100] Example 5

[0101] The preparation methods of the artificial SEI membrane and sodium-ion battery in this embodiment are the same as in Example 1, except that the precursor liquid is a liquid ammonia solution of methylamine, and the preset rate of the substrate during the spraying stage is 1 m / min.

[0102] Comparative Example 1

[0103] A method for preparing a sodium-ion battery includes the following steps:

[0104] (1) Preparation of positive electrode: Sodium iron pyrophosphate, carbon black and PVDF are homogenized in NMP solution in a mass ratio of 94:3:3. The positive electrode slurry is coated on the current collector and then dried, rolled and slit to obtain a positive electrode that can be directly stacked.

[0105] (2) Preparation of negative electrode: A 50 μm thick sodium metal layer after die cutting is uniformly laid on a 12 μm thick copper foil substrate;

[0106] (3) Cell manufacturing: The slit positive and negative electrode sheets are stacked on a stacking machine, and the separator is made of three layers of PP / PE / PP material to form a soft-pack cell.

[0107] Comparative Example 2

[0108] A method for preparing a sodium-ion battery differs from Example 1 only in that, in step S2, instead of spraying, solid sodium ethoxide is dissolved in anhydrous ethanol to prepare a 5 wt% sodium ethoxide ethanol solution. This solution is then uniformly coated onto the surface of a sodium metal layer using a doctor blade, and the ethanol is allowed to evaporate at room temperature to form a sodium ethoxide coating. Subsequent rolling and vacuum drying steps are the same as in Example 1.

[0109] Comparative Example 3

[0110] A method for preparing a sodium-ion battery differs from Example 1 only in that the rolling operation in step S3 is omitted. After spraying in step S2, vacuum drying in step S4 is performed directly.

[0111] Test case

[0112] The soft-pack battery cells of the above embodiments and comparative examples were uniformly subjected to electrolyte injection, formation, and aging, specifically including:

[0113] After the battery cells were dried at high temperature, the electrolytes used in the examples and comparative examples were injected into the pouch cells. The electrolyte solution was 1.2M NaPF6 in DME. After the electrolyte injection, the sodium-ion batteries underwent initial packaging and surface cleaning to complete the preliminary work, and were left at room temperature for one day. Formation was carried out using a step-by-step formation method. The first step was a formation current of 0.05C, constant current charging for 2 hours. The second step was a formation current of 0.1C, constant current charging until the voltage reached 3.4V. After formation, the batteries underwent an aging treatment at 45°C for one day, and were then cooled to room temperature for final sealing.

[0114] The obtained pouch batteries were subjected to 25°C and 45°C cycling and safety performance tests using the Xinwei charge-discharge testing system. Specific test results are shown in Table 1. The specific test methods included:

[0115] 1. Room temperature cycling performance test

[0116] At 25°C, the sodium-ion batteries obtained in the examples and comparative examples were charged to 3.4V at a constant current and constant voltage of 0.5C, left to stand for 5 minutes, and then discharged to 2.0V at a constant current of 1C. This constitutes one charge / discharge cycle.

[0117] The capacity retention rate (%) of a sodium-ion battery after 1000 cycles = (discharge capacity of the 1000th cycle / initial discharge capacity) × 100%.

[0118] 2. High-temperature cycling performance test at 45℃

[0119] At 45°C, the sodium-ion batteries obtained in the examples and comparative examples were charged to 3.4V at a constant current and constant voltage of 0.5C, left to stand for 5 minutes, and then discharged to 2.0V at a constant current of 1C. This constitutes one charge / discharge cycle.

[0120] The capacity retention rate (%) of a sodium-ion battery after 800 cycles = (discharge capacity of the 800th cycle / initial discharge capacity) × 100%.

[0121] 3. 55℃ High Temperature Storage Test

[0122] At 25°C, the sodium-ion batteries obtained in the examples and comparative examples were charged to 3.4V at a constant current and constant voltage of 0.5C. After standing for 5 minutes, they were discharged to 2.0V at a constant current of 1C. After three cycles, they were fully charged, and the average discharge capacity of the three cycles was taken as C0. The batteries were placed in a 55°C oven for 10 days, then removed and cooled to room temperature. They were then discharged to 2.0V at a constant current of 1C, and the discharge capacity at this time was recorded as C1. The batteries were then fully charged and discharged to 2.0V at a constant current of 1C, and the discharge capacity at this time was recorded as C2.

[0123] 55℃ high-temperature storage capacity retention rate = (C1 / C0) × 100%;

[0124] 55℃ high temperature storage capacity recovery rate = (C2 / C0) × 100%.

[0125] Table 1

[0126]

[0127] Based on the results in Table 1, it can be concluded that using different precursor solutions (including anhydrous ethanol, anhydrous glacial acetic acid, anhydrous phenol, anhydrous benzenesulfonic acid, and liquid ammonia solution of methylamine) to construct an artificial SEI film on the sodium metal surface in situ via spraying can significantly improve the cycle stability, high-temperature performance, and safety performance of sodium-ion batteries. Specifically, the capacity retention rate exceeds 88% after 1000 cycles at 25°C and exceeds 85% after 800 cycles at 45°C. Furthermore, it exhibits excellent capacity retention and capacity recovery rates in the 55°C high-temperature storage test, which are far superior to Comparative Example 1 without an artificial SEI film. This demonstrates that the method has broad applicability and good process controllability.

[0128] The results of Comparative Example 2 and Example 1 show that the SEI film pre-prepared using the traditional blade coating method has significantly worse performance indicators than the spray in-situ generation method described in this invention. This indicates that the spray method can achieve a more uniform, denser artificial SEI film with a stronger bond to the substrate, while the blade coating method may result in defects in the film layer and a decline in performance due to problems such as slurry agglomeration and poor wettability.

[0129] The results of Comparative Example 3 and Example 1 show that omitting the rolling step results in an artificial SEI film with insufficient density and stability, failing to effectively suppress dendrite growth. Consequently, its cycling performance (especially high-temperature cycling) and interface stability are significantly inferior to the complete process. This demonstrates the crucial role of rolling in improving the density and stability of the SEI film.

[0130] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing an artificial SEI membrane, characterized in that, Includes the following steps: S1. Spread the sodium metal layer evenly on the substrate; S2. The precursor liquid is sprayed onto the surface of the sodium metal layer in the form of a spray, and the precursor liquid is used to generate an organic sodium salt layer compound in situ on the surface of the sodium metal layer. Step S2 specifically includes moving the substrate under a fixed spraying device at a preset speed, the spraying device spraying the precursor liquid onto the surface of the sodium metal layer, and the pressure of the spraying device being 0.1 kPa to 16 kPa; The preset spraying rate of the substrate is 0.5 m / min to 10 m / min. The preset rate is set according to the reaction rate between the precursor liquid and sodium metal, and the preset rate is positively correlated with the reaction rate. In step S2, the precursor solution is at least one of anhydrous glacial acetic acid, anhydrous ethanol, anhydrous phenol, anhydrous benzenesulfonic acid, or a liquid ammonia solution of a primary amine. S3. Roll the sodium metal layer containing the precursor liquid; S4. The sodium metal layer after rolling is vacuum dried, and finally an artificial SEI film is obtained on the sodium metal layer.

2. The method for preparing the artificial SEI membrane according to claim 1, characterized in that, In step S1, the substrate is aluminum foil or copper foil; And / or, in step S1, the thickness of the substrate is 10μm~20μm; And / or, in step S1, the thickness of the sodium metal layer is 20 μm to 80 μm.

3. The method for preparing the artificial SEI membrane according to claim 1, characterized in that, When the precursor solution is anhydrous ethanol, the preset rate is 0.5 m / min to 1.5 m / min; when the precursor solution is anhydrous glacial acetic acid, the preset rate is 5 m / min to 7 m / min; when the precursor solution is anhydrous phenol, the preset rate is 3 m / min to 5 m / min; when the precursor solution is anhydrous benzenesulfonic acid, the preset rate is 7 m / min to 10 m / min; when the precursor solution is a liquid ammonia solution of a primary amine, the preset rate is 1 m / min to 3 m / min.

4. The method for preparing the artificial SEI membrane according to claim 1, characterized in that, In step S4, the thickness of the artificial SEI membrane is 1 μm to 10 μm.

5. The method for preparing the artificial SEI membrane according to claim 1, characterized in that, In step S4, the vacuum drying temperature is 80℃~120℃ and the time is 12 hours~36 hours.

6. An artificial SEI membrane prepared by any one of claims 1 to 5.

7. A negative electrode sheet, characterized in that, It includes a substrate and a sodium metal layer and an artificial SEI film as described in claim 6, which are sequentially stacked on the substrate.

8. A sodium-ion battery, characterized in that, Includes the negative electrode sheet as described in claim 7.

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