Composite separator and method for manufacturing the same, secondary battery

Abstract

The application relates to the technical field of batteries, in particular to a composite diaphragm, a preparation method thereof and a secondary battery. The composite diaphragm comprises a base film, a coating layer arranged on at least one side surface of the base film along the thickness direction, and the coating layer comprises a composite material; the composite material comprises polyimide fibers and a solid-state electrolyte coated on at least part of the surface of the polyimide fibers. In the composite diaphragm, the coating layer of the composite diaphragm contains the composite material, the lithium ion flux can be improved, the lithium deposition is uniform and dense, the growth of lithium dendrites is inhibited, the thermal shrinkage performance and the mechanical strength of the diaphragm can be greatly improved, the volume change in the charging and discharging process of the lithium metal negative electrode can be coped with, the use amount of electrolyte can be reduced, and the cycle performance, the rate performance and the safety performance of the battery are improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a composite separator, its preparation method, and a secondary battery. Background Technology

[0002] With the rapid development of electric vehicles, consumer electronics, and other fields, the market demand for high-energy-density batteries continues to rise. However, the energy density of traditional lithium-ion batteries is approaching its theoretical limit (approximately 300-350 Wh / kg), making it difficult to meet future requirements for longer driving range and higher performance. In contrast, lithium metal batteries, with their high-capacity lithium metal anode (theoretical specific capacity of 3860 mAh / g), significantly enhance the energy density potential of batteries and have become a cutting-edge direction for breaking through existing technological bottlenecks. However, their commercialization process has long been constrained by a series of key technological challenges, severely limiting their application in electric vehicles, aerospace, and energy storage systems.

[0003] First, the dendrite growth problem in lithium metal anodes is one of the core challenges. During charging and discharging, the uneven distribution of lithium ion flux leads to highly non-uniform deposition on the lithium metal surface, resulting in abnormal expansion of lithium dendrites. The root cause of this phenomenon lies in the double-layer charge effect formed by lithium ions on the electrode surface, causing excessively high local current density. The uncontrolled growth of dendrites not only punctures the separator and causes internal short circuits but may also trigger violent decomposition reactions in the electrolyte, inducing serious safety risks such as thermal runaway. Second, the insufficient mechanical properties and thermal stability of traditional separators further exacerbate these problems. Conventional polypropylene (PP) separators, due to their limited mechanical strength, cannot effectively block the penetration of lithium dendrites, and their irreversible shrinkage characteristics at high temperatures (such as above 130°C) lead to direct electrode contact, causing short circuits and deteriorating cycle stability. These defects significantly shorten battery life, especially in high-rate charging / discharging or fast-charging scenarios. In addition, the safety hazards of the electrolyte are also a key factor hindering the application of lithium metal batteries. While traditional carbonate electrolytes possess high ionic conductivity, their flammability and continuous side reactions with the lithium metal anode lead to persistent loss of active lithium and reduced coulombic efficiency. More seriously, under conditions of mechanical abuse or high temperatures, the flammability of the electrolyte can easily trigger thermal runaway, causing battery fires or explosions, thus severely limiting their use in applications with stringent safety requirements.

[0004] In view of this, it is necessary to provide a composite separator to solve the problems existing in the above-mentioned lithium batteries, thereby improving the cycle performance, rate performance and safety performance of lithium batteries. Summary of the Invention

[0005] In view of this, the present invention aims to at least partially solve one of the technical problems in the related art. To this end, the present invention provides a composite separator and its preparation method, and a secondary battery, which can alleviate the problems of existing separators, such as difficulty in promoting uniform lithium-ion deposition, poor thermal performance, and unstable interface, and can reduce the amount of electrolyte used, thereby improving the cycle performance, rate performance, and safety performance of the battery. At the same time, it also has the advantages of low preparation cost, simple process, and the ability to meet the technical requirements of industrialization.

[0006] To solve the above-mentioned technical problems, this application is implemented as follows:

[0007] According to one aspect of this application, an embodiment of this application provides a composite separator, the composite separator comprising:

[0008] Base film;

[0009] A coating is disposed on at least one surface of the base film along its thickness direction, the coating comprising a composite material;

[0010] The composite material includes polyimide fibers and a solid electrolyte coating at least a portion of the surface of the polyimide fibers.

[0011] In addition, the composite diaphragm according to this application may also have the following additional technical features:

[0012] In some embodiments, the solid electrolyte in the composite material is completely coated with polyimide fibers, and the coating thickness is 10~100nm.

[0013] In some embodiments, the polyimide fiber has a diameter of 100-500 nm and a length of 10-100 μm.

[0014] In some of these embodiments, the solid electrolyte includes an inorganic solid electrolyte.

[0015] In some embodiments, the inorganic solid electrolyte is an oxide solid electrolyte, preferably at least one of perovskite solid electrolyte, NASICON solid electrolyte, LISICON solid electrolyte or garnet solid electrolyte.

[0016] In some embodiments, the coating thickness is 1~3 μm.

[0017] In some embodiments, the thickness of the base film is 5 to 12 μm.

[0018] In some of these embodiments, the porosity of the base membrane is 35-55%.

[0019] In some embodiments, the base film includes at least one of polypropylene film, polyethylene film, meta-aramid film, polyvinylidene fluoride film, poly(vinylidene fluoride-hexafluoropropylene) film, polyimide film, polyacrylonitrile film, or polyethylene terephthalate film.

[0020] According to another aspect of this application, embodiments of this application provide a method for preparing a composite separator, comprising the following steps:

[0021] Polyamic acid is dissolved in an organic solvent to obtain a spinning solution, and polyamic acid fibers are obtained by electrospinning. After the polyamic acid fibers are thermally imidized, polyimide fibers are obtained.

[0022] The polyimide fiber and the solid electrolyte element source material are mixed in a first solvent to obtain a precursor solution, and then a hydrothermal reaction is carried out to obtain a solid electrolyte-coated polyimide fiber composite material.

[0023] The solid electrolyte-coated polyimide fiber composite material and the binder are mixed in a second solvent to obtain a coating slurry. The coating slurry is then coated onto a base membrane to obtain a composite membrane.

[0024] In some embodiments, the polyamic acid has a mass percentage of 5-30% in the spinning solution.

[0025] In some embodiments, the polyamic acid fiber thermal imidization treatment is carried out under an inert gas atmosphere at a temperature of 250-350°C for 1-5 hours.

[0026] In some embodiments, the solid electrolyte element source material includes a lithium source material and a second source material.

[0027] In some embodiments, the second source material includes at least two of lanthanum source material, zirconium source material, aluminum source material, titanium source material, phosphorus source material, silicon source material, or tantalum source material.

[0028] In some of these embodiments, the mass ratio of the polyimide fiber, the solid electrolyte element source material, and the first solvent is (4~9):(1~6):(25~100).

[0029] In some of these embodiments, the hydrothermal reaction is carried out at a temperature of 120-200°C, a pressure of 1-1.5 MPa, and a reaction time of 12-48 h.

[0030] In some of these embodiments, the mass ratio of the solid electrolyte-coated polyimide fiber composite material to the binder is (95~99):(1~5).

[0031] In some embodiments, the mass percentage of the solid electrolyte-coated polyimide fiber composite material and the binder in the coating slurry is 15-40%.

[0032] In some embodiments, the adhesive includes at least one of acrylates, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol.

[0033] In some embodiments, the coating slurry is further dried after application.

[0034] In some of these embodiments, the drying is carried out in flowing air at a velocity of 2 to 3 m / s.

[0035] In some embodiments, the drying temperature is 40~60°C and the time is 10~30 min.

[0036] According to another aspect of this application, an embodiment of this application provides a secondary battery including a composite separator, wherein the composite separator is the aforementioned composite separator or a composite separator prepared according to the aforementioned preparation method.

[0037] Implementing the technical solution of the present invention has at least the following beneficial effects:

[0038] In this embodiment, the composite separator provided includes a coating comprising a composite material. The solid electrolyte coating at least partially covers the surface of the polyimide fibers, allowing lithium ions to easily migrate to and accumulate on the surface of the solid electrolyte under the influence of the liquid / solid interface chemical potential. This creates negatively charged vacancies within the solid electrolyte particles, forming a space charge region. This continuous space charge region on the surface of the solid electrolyte particles accelerates the rapid migration of lithium ions, thereby increasing lithium ion flux and improving the battery's cycle performance and rate performance. The uniform coating of the solid electrolyte on the surface of the polyimide fibers not only increases lithium ion flux but also ensures uniform lithium ion flux, resulting in uniform and dense lithium deposition and suppressing lithium dendrite growth. Furthermore, due to the presence of the composite material, the polyimide fibers in the coating possess excellent thermal stability, exhibiting superior thermal stability at high temperatures. They maintain their structure and performance at high temperatures, retaining more than 50% of their original strength at 250°C, and their elongation at break remains essentially unchanged within the range of room temperature to 450°C. Furthermore, the composite material can form a three-dimensional structure on the base membrane surface, significantly improving the thermal shrinkage performance and mechanical strength of the separator, thus mitigating the risk of internal short circuits and enhancing the battery's thermal safety. The excellent mechanical strength of the composite separator also plays a role in coping with the volume changes during the charging and discharging of the lithium metal anode. This further mitigates the risk of internal short circuits. In addition, the three-dimensional structure formed on the base membrane surface by the composite material can further create pores, which improve the separator's liquid retention capacity. This allows for a reduction in the amount of electrolyte injected into the lithium metal battery while maintaining a high lithium-ion flux. Reducing the amount of electrolyte used significantly reduces the safety risks associated with flammable and toxic organic solvents.

[0039] Based on this, the composite separator of this application can improve the lithium-ion flux by including composite materials in the coating of the composite separator, make the lithium deposition uniform and dense to suppress the growth of lithium dendrites, and also significantly improve the thermal shrinkage performance and mechanical strength of the separator, so as to cope with the volume change during the charging and discharging process of lithium metal anode, while also reducing the amount of electrolyte used, thereby improving the cycle performance, rate performance and safety performance of the battery.

[0040] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0041] Figure 1 The image shown is a scanning electron microscope image of the surface of the composite membrane prepared in Example 1 of the present invention.

[0042] Figure 2 The diagram shows the structure of the composite material (solid electrolyte / PI fiber composite material) prepared in Example 1 of the present invention.

[0043] 1-Polyimide fiber;

[0044] 2-Solid electrolyte. Detailed Implementation

[0045] The present application will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0046] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​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 or 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.

[0047] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0048] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0049] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0050] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0051] In related technologies, some separators include a base membrane and a coating disposed on the base membrane. The coating can increase the puncture strength and high-temperature resistance of the separator. However, existing separators still cannot solve the problems of lithium dendrite formation, separator puncture, and electrolyte instability in lithium metal batteries.

[0052] [Composite diaphragm]

[0053] In view of this, embodiments of this application provide a composite membrane, the composite membrane comprising:

[0054] Base film;

[0055] A coating is disposed on at least one side surface of a base film along its thickness direction, the coating comprising a composite material;

[0056] The composite material includes polyimide fibers and a solid electrolyte coating at least a portion of the surface of the polyimide fibers.

[0057] The phrase "the coating is disposed on at least one surface of the base film along its thickness direction" means that the coating can be disposed on one surface of the base film along its thickness direction, or on two surfaces of the base film along its thickness direction. Here, "surface" can be the entire area of ​​the base film or a part of the base film. As in this embodiment, the surface can be the entire area of ​​the base film. This application does not have any particular limitation in this regard, as long as the purpose of this application can be achieved.

[0058] As an example, the base film has two surfaces opposite each other in its own thickness direction, and the coating is disposed on the two opposite surfaces of the base film. It will be understood that in other embodiments, the coating may also be laminated on either of the two surfaces of the base film.

[0059] Of particular note is that in the composite separator of this application embodiment, the coating includes a composite material. The solid electrolyte coating at least partially covers the surface of the polyimide fibers, allowing lithium ions to easily migrate to and accumulate on the surface of the solid electrolyte under the driving force of the liquid / solid interface chemical potential. This creates negatively charged vacancies within the solid electrolyte, forming a space charge region. The continuous space charge region formed on the surface of the solid electrolyte particles accelerates the rapid migration of lithium ions, thereby increasing lithium ion flux and improving the battery's cycle performance and rate performance. The uniform coating of the solid electrolyte on the surface of the polyimide fibers not only increases lithium ion flux but also ensures uniform lithium ion flux, resulting in uniform and dense lithium deposition and suppressing lithium dendrite growth. Furthermore, due to the presence of the composite material in the coating, the polyimide fibers in the composite material possess extremely high thermal stability, exhibiting excellent thermal stability at high temperatures. They can maintain their structure and performance at high temperatures, retaining more than 50% of their original strength at 250°C, and their elongation at break remains essentially unchanged within the range of room temperature to 450°C. Furthermore, since polyimide is a fibrous material, its coating can form a three-dimensional structure, which can significantly improve the thermal shrinkage performance and mechanical strength of the separator, mitigating the risk of internal short circuits and improving the battery's thermal safety. The excellent mechanical strength of the composite separator also plays a role in coping with the volume changes during the charging and discharging of the lithium metal anode. This further mitigates the risk of internal short circuits. In addition, the composite material can form a three-dimensional structure on the base film surface, which can further create pores (pores between fibers). These pores improve the separator's liquid retention performance, thus maintaining a high lithium-ion flux while reducing the amount of electrolyte injected into the lithium metal battery. Reducing the amount of electrolyte significantly reduces the safety risks associated with flammable and toxic organic solvents.

[0060] Based on this, the composite separator of this application can improve the lithium-ion flux by including composite materials in the coating of the composite separator, make the lithium deposition uniform and dense to suppress the growth of lithium dendrites, and also significantly improve the thermal shrinkage performance and mechanical strength of the separator, so as to cope with the volume change during the charging and discharging process of lithium metal anode, while also reducing the amount of electrolyte used, thereby improving the cycle performance, rate performance and safety performance of the battery.

[0061] In some embodiments, the solid electrolyte in the composite material completely coats the polyimide fiber, with a coating thickness of 10-100 nm. As an example, the coating thickness can be 10 nm, 30 nm, 60 nm, 90 nm, 100 nm, etc., or other values ​​within the above range, and is not limited here. The uniform coating of the solid electrolyte on the surface of the polyimide fiber not only increases the lithium-ion flux but also ensures a uniform lithium-ion flux, resulting in uniform and dense lithium deposition and inhibiting lithium dendrite growth. If the coating thickness is too low, the amount of solid electrolyte is too small, leading to an increase in the internal resistance of the battery during charging and discharging, thus reducing the battery's cycle performance. However, when the coating thickness exceeds the above range, the permeability of the composite separator decreases, further affecting lithium-ion transport, which also increases the internal resistance of the battery during charging and discharging, thereby reducing the battery's cycle stability.

[0062] In some embodiments, the polyimide fibers have a diameter of 100-500 nm and a length of 10-100 μm. For example, the diameter of the polyimide fibers can be 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, etc., or other values ​​within the above range, and is not limited here. For example, the length of the polyimide fibers can be 10 μm, 30 μm, 60 μm, 90 μm, 100 μm, etc., or other values ​​within the above range, and is not limited here. If the diameter of the polyimide fibers is too small, it is difficult to form a three-dimensional structure between the fibers in the composite membrane coating; if the diameter of the polyimide fibers is too large, the pore size formed between the fibers in the composite membrane coating is too large, resulting in poor liquid retention performance.

[0063] In some embodiments, the solid electrolyte includes an inorganic solid electrolyte.

[0064] In some embodiments, the inorganic solid electrolyte is an oxide solid electrolyte, preferably at least one of perovskite-type solid electrolyte, NASICON-type solid electrolyte, LISICON-type solid electrolyte, or garnet-type solid electrolyte. As an example, the oxide solid electrolyte may be a perovskite-type solid electrolyte.

[0065] In some embodiments, the coating thickness is 1~3μm. As an example, the coating thickness can be 1μm, 2μm, 3μm, etc., or other values ​​within the above range, and is not limited here. If the coating thickness is less than 1μm, the improvement on the composite separator's ability to suppress lithium dendrite growth, mechanical properties, and thermal properties is limited, and the battery's cycle performance, rate performance, and safety performance are difficult to further guarantee and optimize. If the coating thickness is greater than 5μm, it will affect the electron transport efficiency, increase the battery's internal resistance, and further affect the battery's rate performance.

[0066] In some embodiments, the thickness of the base film is 5~12μm. As an example, the thickness of the base film can be 5μm, 7μm, 9μm, 11μm, 12μm, etc., and of course, other values ​​within the above range are also possible and are not limited here. By limiting the base film thickness within the above range, the composite separator meets the mechanical strength requirements (able to resist certain physical stress and dendrite growth) without significantly reducing ion conductivity, maintaining a low internal resistance in the battery, and avoiding a significant negative impact on the battery's charge / discharge efficiency and power output.

[0067] In some embodiments, the porosity of the base membrane is 35-55%. As an example, the porosity of the base membrane can be 35%, 40%, 45%, 50%, 55%, etc., or other values ​​within the above range, which are not limited here. By limiting the porosity of the base membrane within the above range, the composite separator can absorb more electrolyte, thereby improving the uniformity of electrolyte distribution in the battery and contributing to improved ion conductivity and overall electrochemical performance. Simultaneously, with the base membrane porosity within this range, the mechanical strength of the composite separator will not be too low, making it less prone to damage during battery use or assembly.

[0068] In some embodiments, the base film includes at least one selected from polypropylene film, polyethylene film, meta-aramid film, polyvinylidene fluoride film, poly(vinylidene fluoride-hexafluoropropylene) film, polyimide film, polyacrylonitrile film, or polyethylene terephthalate film. As an example, the base film may be a polypropylene film or a polyethylene film.

[0069] In some embodiments, the coating further includes an adhesive; the adhesive includes, but is not limited to, at least one of polyvinylidene fluoride, polyacrylate, polyimide, perfluoroalkoxy (PFA), polytetrafluoroethylene, polyacrylonitrile, or polyvinyl alcohol.

[0070] In this application, the coating may primarily consist of a composite material and an adhesive. The adhesive can be used to enhance the adhesion between the composite material and the base film, thereby improving the bonding effect. The specific type of adhesive is not limited to the substances listed above; other materials that can be used to improve adhesion do not limit the purpose of this invention, and other similar adhesive materials can also be used in this invention.

[0071] [Preparation of Composite Separator]

[0072] Based on the same inventive concept, this application provides a method for preparing a composite separator, comprising the following steps:

[0073] Polyamic acid is dissolved in an organic solvent to obtain a spinning solution, and polyamic acid fibers are obtained by electrospinning. After the polyamic acid fibers are thermally imidized, polyimide fibers are obtained.

[0074] Polyimide fibers and solid electrolyte element source materials are mixed in a first solvent to obtain a precursor solution, and then a hydrothermal reaction is carried out to obtain a solid electrolyte-coated polyimide fiber composite material.

[0075] Solid electrolyte-coated polyimide fiber composite material and binder are mixed in a second solvent to obtain a coating slurry. The coating slurry is then coated onto a base membrane to obtain a composite diaphragm.

[0076] In this application, a composite material consisting of polyimide fibers and a solid electrolyte is used to prepare a coating containing the composite material. After coating both sides of a base membrane, the coating is dried under normal pressure to obtain a composite separator with excellent overall performance. The prepared multifunctional separator coating has a more uniform and stable structure. After being coated on the surface of the lithium battery base membrane, it significantly improves the battery's cycle performance, rate performance, and safety performance. Furthermore, the method provided in this application has low preparation cost and simple process, meeting the technical requirements for industrialization.

[0077] In some embodiments, the mass percentage of polyamic acid in the spinning solution is 5-30%. As an example, the mass percentage of polyamic acid in the spinning solution can be 5%, 15%, 25%, 30%, etc., or other values ​​within the above range, which are not limited here.

[0078] In some embodiments, the organic solvent includes at least one selected from N,N-dimethylacetamide (DMAC), N-methylpyrrolidone (NMP), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). As an example, the organic solvent may be N-methylpyrrolidone (NMP) or N,N-dimethylacetamide (DMAC).

[0079] In some embodiments, the electrospinning step includes: loading the prepared spinning solution into a syringe, connecting it to a high-voltage electrostatic generator, causing the solution to be sprayed out from the nozzle under the action of an electric field to form a fine stream, and collecting the polyamic acid nanofibers on a receiving screen after the solvent evaporates.

[0080] In some embodiments, the polyamic acid fiber thermal imidization treatment is carried out under an inert gas atmosphere at a temperature of 250-350°C for 1-5 hours. As examples, the heat treatment temperature can be 250°C, 300°C, 350°C, etc., or other values ​​within the above range, which are not limited here. As examples, the heat treatment temperature can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, etc., or other values ​​within the above range, which are not limited here.

[0081] In some embodiments, the solid electrolyte element source material includes a lithium source material and a second source material.

[0082] In some embodiments, the second source material includes at least two of lanthanum source material, zirconium source material, aluminum source material, titanium source material, phosphorus source material, silicon source material, or tantalum source material.

[0083] In some embodiments, the lithium source material includes at least one of lithium nitrate, lithium sulfate, lithium oxalate, lithium acetate, lithium carbonate, lithium hydroxide, lithium phosphate, lithium silicate, lithium citrate, and lithium oxide; the lanthanum source material includes at least one of lanthanum nitrate, lanthanum sulfate, lanthanum phosphate, lanthanum hydroxide, lanthanum oxide, and lanthanum acetate; the zirconium source material includes at least one of zirconium nitrate, zirconium sulfate, zirconium carbonate, zirconium oxide, and zirconium oxychloride octahydrate; the aluminum source material includes at least one of aluminum nitrate, aluminum sulfate, aluminum phosphate, aluminum hydroxide, aluminum oxide, and aluminum acetate; the titanium source material includes at least one of tetrabutyl titanate, titanium isopropoxide, titanium dioxide, titanium tetrachloride, titanium sulfate, and titanium nitrate; the phosphorus source material includes at least one of phosphoric acid, lithium phosphate, and diammonium hydrogen phosphate; the silicon source material includes ethyl silicate and / or silicon dioxide; and the tantalum source material includes tantalum nitrate and / or tantalum pentoxide.

[0084] In some embodiments, the mass ratio of polyimide fiber, solid electrolyte element source material, and first solvent is (4~9):(1~6):(25~100). As an example, the mass ratio of polyimide fiber, solid electrolyte element source material, and first solvent can be 4:1:25, 6:3:50, 9:6:100, etc., and of course, other ratios within the above range are also possible, which are not limited here.

[0085] In this application, the amount of solid electrolyte source material is determined by adding the corresponding solid electrolyte source material according to the molar ratio of each element in the solid electrolyte. As long as the corresponding solid electrolyte can be synthesized, this application does not impose any special restrictions on this.

[0086] In some embodiments, the hydrothermal reaction temperature is 120~200℃, the pressure is 1~1.5MPa, and the reaction time is 12~48h. As examples, the hydrothermal reaction temperature can be 120℃, 140℃, 160℃, 180℃, 200℃, etc., or other values ​​within the above range, which are not limited here; the hydrothermal reaction pressure can be 1MPa, 1.2MPa, 1.5MPa, etc., or other values ​​within the above range, which are not limited here; the hydrothermal reaction time can be 12h, 24h, 36h, 48h, etc., or other values ​​within the above range, which are not limited here.

[0087] In this application, during the hydrothermal reaction, the raw materials undergo a chemical reaction and generate nano-solid electrolytes on the surface of polyimide fibers. This method eliminates the need for a subsequent calcination process and enables uniform crystallization and morphology control of the solid electrolytes.

[0088] A solid electrolyte / polyimide fiber composite material was prepared by in-situ hydrothermal growth. The solid electrolyte grows in situ on the polyimide fiber (i.e., the solid electrolyte is coated on the polyimide surface). The solid electrolyte is uniformly dispersed in the composite material, and when used as a solid coating for a separator, it reduces the impact of uneven mixing and dispersion caused by multiple material types. The solid electrolyte / polyimide fiber composite solid separator can better suppress the growth of lithium dendrites, reducing battery short circuits caused by lithium dendrites piercing the separator, thereby improving battery cycle stability and extending its service life.

[0089] In some embodiments, after the hydrothermal reaction is completed, the process further includes centrifugation, washing, and drying.

[0090] In some embodiments, the solid electrolyte particle size D50 on the surface of the polyimide fiber is 50-70 nm, where D50 refers to 50% of the material having a particle size of 50-70 nm. Solid electrolyte materials with this particle size exhibit good coating effects, provided they are relatively easy to prepare.

[0091] In some embodiments, the mass ratio of the solid electrolyte-coated polyimide fiber composite material to the binder is (95~99):(1~5). As an example, the mass ratio of the solid electrolyte-coated polyimide fiber composite material to the binder can be 95:5, 97:3, 99:1, etc., or other ratios within the above range, which are not limited here.

[0092] In some embodiments, the mass percentage of the solid electrolyte-coated polyimide fiber composite material and the binder in the coating slurry is 15% to 40%. As an example, the mass percentage of the solid electrolyte-coated polyimide fiber composite material and the binder in the coating slurry can be 15%, 25%, 35%, 45%, etc., or other values ​​within the above range, which are not limited here.

[0093] In some embodiments, the adhesive includes at least one of acrylic esters, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAF), or polyvinyl alcohol (PVA). As an example, the adhesive may be PTFE or PVDF.

[0094] In some embodiments, the first solvent and the second solvent each independently comprise water.

[0095] In some embodiments, a drying step is also included after the coating slurry is applied.

[0096] In some embodiments, drying is carried out in flowing air at a velocity of 2 to 3 m / s. As examples, the air velocity can be 2 m / s, 2.5 m / s, 3 m / s, etc., or other values ​​within the above range, which are not limited here.

[0097] In some embodiments, the drying temperature is 40~60℃ and the drying time is 10~30min. As an example, the drying temperature can be 40℃, 50℃, 60℃, etc., or other values ​​within the above range, which are not limited here; the drying time can be 10min, 20min, 30min, etc., or other values ​​within the above range, which are not limited here.

[0098] Based on the same inventive concept, embodiments of this application provide a secondary battery, including a composite separator, wherein the composite separator is the aforementioned composite separator or a composite separator prepared according to the aforementioned preparation method.

[0099] Because this secondary battery includes the composite separator provided in the embodiments of this application, it has superior cycle performance, rate performance and safety performance.

[0100] In this embodiment, the materials and structures of the positive electrode current collector, the conductive agent and the binder in the positive electrode active material layer are not limited, and the positive electrode structure and composition known in the art that can be used in secondary batteries can be selected.

[0101] In this embodiment, the materials and structures of the negative electrode current collector, the conductive agent and the binder in the negative electrode active material layer are not limited, and the positive electrode structure and composition known in the art that can be used in secondary batteries can be selected.

[0102] It should also be noted that the secondary battery of this application does not limit the specific material or type of electrolyte. Any components and types known in the art that can be used in secondary batteries can be selected, as long as the purpose of this application can be achieved.

[0103] Since the battery provided in this embodiment of the invention adopts all the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, and will not be described in detail here.

[0104] The following describes the implementation methods of this application. The implementation methods described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the implementation methods, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents, materials, or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0105] Example 1

[0106] Preparation of polyimide (PI) fiber materials by electrospinning:

[0107] (1) Preparation of spinning solution: Polyamic acid (PAA) is dissolved in N,N-dimethylacetamide (DMAC) to prepare a spinning solution with a concentration of 20%.

[0108] (2) Electrospinning process: The prepared spinning solution is loaded into a syringe and connected to a high-voltage electrostatic generator. The solution is sprayed out from the nozzle under the action of electric field force to form a fine stream. After the solvent evaporates, PAA nanofibers are collected on the receiving screen.

[0109] (3) Thermal imidization treatment: The obtained PAA nanofiber membrane was subjected to heat treatment at 250°C for 4 hours under argon protection to convert PAA into PI, resulting in PI nanofibers with a diameter of 200 nm and a length of 30 μm.

[0110] Preparation of in-situ grown solid electrolyte / PI fiber composite materials by hydrothermal reaction:

[0111] (1) Solid electrolyte (LATP:Li 1.3 Al 0.3 Ti 1.7 (PO4)3) Precursor solution preparation: Weigh LiOH, AlNO3, Ti(OCH(CH3)2)4, and H3PO4 materials according to the molar ratio of Li:Al:Ti:P = 1.3:0.3:1.7:3, dissolve them in deionized water, add PI fiber material, and mix to form a homogeneous precursor solution. The mass ratio of polyimide fiber, solid electrolyte source material, and deionized water is 5:3:50.

[0112] (2) Hydrothermal reaction: The precursor solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 150°C and 1.3 MPa for 24 h. During the hydrothermal reaction, the raw materials underwent a chemical reaction and generated LATP solid electrolyte with a particle size D50 of 70 nm (meaning that 50% of the material has a particle size of 70 nm) on the PI surface.

[0113] (3) Product processing: After the reaction, the product was centrifuged, washed, and dried to obtain the in-situ grown LATP / PI fiber composite material with a mass ratio of 30:100 and a LATP coating thickness of 60-80 nm. See the schematic diagram below. Figure 2 .

[0114] Composite membrane preparation:

[0115] The prepared LATP / PI fiber composite material (97%) and polyvinylidene fluoride (3%) were added to water and stirred to disperse, resulting in a slurry with a concentration of 25%. The slurry was coated on the upper and lower layers of a membrane, and after baking for 15 minutes at an oven temperature of 55°C and an air flow rate of 3 m / s, an LATP / PI fiber composite membrane was obtained, with the thickness of both the upper and lower layers being 2 μm. Figure 1 This is a scanning electron microscope (SEM) image of the surface of the composite membrane coating prepared in this embodiment. Figure 1 It can be seen that the LATP / PI fiber composite material is uniformly coated on the surface of the base film, forming a porous three-dimensional structure (the pores are formed between the fibers).

[0116] Example 2

[0117] The difference between Example 2 and Example 1 lies in the hydrothermal reaction conditions. In Example 2, the hydrothermal reaction was specifically performed by transferring the precursor solution to a hydrothermal reactor and conducting a hydrothermal reaction at 200°C and 1.5 MPa for 24 hours. During the hydrothermal reaction, the raw materials underwent a chemical reaction and formed a solid electrolyte LATP with a particle size D50 of 50 nm on the PI surface. The final thickness of the LATP coating layer was 30–60 nm.

[0118] Example 3

[0119] The difference between Example 3 and Example 1 is that the thickness of both the upper and lower coatings in Example 3 is 3 μm.

[0120] The difference between Example 4 and Example 1 is that the solid electrolyte is LLTO, and the LLTO / PI fiber composite material is prepared as follows:

[0121] (1) Solid electrolyte (LLTO:Li 0.33 La 0.56 Preparation of TiO3 precursor solution: Weigh La(NO3)3, TiO2, and LiOH materials according to the molar ratio of Li:Lal:Ti = 0.33:0.56:1, dissolve them in deionized water, add PI fiber material, and mix to form a homogeneous precursor solution. The mass ratio of polyimide fiber, solid electrolyte source material, and deionized water is 5:3:50.

[0122] (2) Hydrothermal reaction: The precursor solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 200°C and 1.3 MPa for 12 h. During the hydrothermal reaction, the raw materials underwent a chemical reaction and generated LLTO solid electrolyte with a particle size D50 of 70 nm (meaning that 50% of the material has a particle size of 70 nm) on the PI surface.

[0123] (3) Product processing: After the reaction is completed, the product is centrifuged, washed and dried to obtain in-situ grown LATP / PI fiber composite material with a mass ratio of 30:100 and a thickness of 60~80nm for the LLTO coating layer.

[0124] Example 5

[0125] The difference between Example 5 and Example 1 is that the polyimide fiber in Example 5 has a diameter of 500 nm and a length of 100 μm.

[0126] Example 6

[0127] The difference between Example 6 and Example 1 is that the polyimide fiber in Example 6 has a diameter of 100 nm and a length of 10 μm.

[0128] Comparative Example 1

[0129] Preparation of PI fiber materials by electrospinning:

[0130] (1) Preparation of spinning solution: Polyamic acid (PAA) is dissolved in N,N-dimethylacetamide (DMAC) to prepare a spinning solution with a concentration of 20%.

[0131] (2) Electrospinning process: The prepared spinning solution is loaded into a syringe and connected to a high-voltage electrostatic generator. The solution is sprayed out from the nozzle under the action of electric field force to form a fine stream. After the solvent evaporates, PAA nanofibers are collected on the receiving screen.

[0132] (3) Thermal imidization treatment: The obtained PAA nanofiber membrane was subjected to heat treatment at 250°C for 4 hours under argon protection to convert PAA into PI, resulting in PI nanofibers with a diameter of 200 nm and a length of 30 μm.

[0133] Preparation of solid electrolytes by hydrothermal reaction:

[0134] (1) Solid electrolyte (LATP:Li 1.3 Al 0.3 Ti 1.7 (PO4)3) Precursor solution preparation: Weigh LiOH, AlNO3, Ti(OCH(CH3)2)4, and H3PO4 materials according to the molar ratio of Li:Al:Ti:P = 1.3:0.3:1.7:3, add deionized water to dissolve, and mix to form a homogeneous precursor solution. The mass ratio of the solid electrolyte element source material to deionized water is 3:50.

[0135] (2) Hydrothermal reaction: The precursor solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 150°C and 1.3 MPa for 24 hours to generate a solid electrolyte LATP with a particle size of 70 nm.

[0136] (3) Product processing: After the reaction is completed, the product is centrifuged, washed and dried to obtain LATP material.

[0137] Membrane preparation:

[0138] The prepared LATP and PI fiber composite material was mixed with polyvinylidene fluoride (3%) in water at a ratio of 30:100 (97%) and stirred to obtain a slurry with a concentration of 25%. The slurry was coated on the upper and lower layers of the diaphragm, and after baking for 15 minutes at an oven temperature of 55℃ and an air flow rate of 3m / s, the diaphragm was obtained, with the thickness of the upper and lower layers being 2µm.

[0139] In Comparative Example 1, the solid electrolyte was not grown in situ on the PI fiber, but was directly physically mixed in the slurry used to prepare the diaphragm.

[0140] Comparative Example 2

[0141] Preparation of solid electrolytes by hydrothermal reaction:

[0142] (1) Solid electrolyte (LATP:Li 1.3 Al 0.3 Ti 1.7 (PO4)3) Precursor solution preparation: Weigh LiOH, AlNO3, Ti(OCH(CH3)2)4, and H3PO4 materials according to the molar ratio of Li:Al:Ti:P = 1.3:0.3:1.7:3, add deionized water to dissolve, and mix to form a homogeneous precursor solution. The mass ratio of the solid electrolyte element source material to deionized water is 3:50.

[0143] (2) Hydrothermal reaction: The precursor solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 150°C and 1.3 MPa for 24 hours to generate a solid electrolyte LATP with a particle size of 70 nm.

[0144] (3) Product processing: After the reaction is completed, the product is centrifuged, washed and dried to obtain LATP material.

[0145] Preparation of fibrous materials with solid electrolytes mixed inside PI by electrospinning:

[0146] (1) Preparation of spinning solution: Dissolve polyamic acid (PAA) in N,N-dimethylacetamide (DMAC) to prepare a spinning solution with a concentration of 20%, add 20% LATP to the spinning solution and mix evenly.

[0147] (2) Electrospinning process: The prepared spinning solution is loaded into a syringe and connected to a high-voltage electrostatic generator. The solution is sprayed out from the nozzle under the action of electric field force to form a fine stream. After the solvent evaporates, PAA nanofibers are collected on the receiving screen.

[0148] (3) Thermal imidization treatment: The obtained LATP and PAA mixed nanofiber membrane was subjected to heat treatment at 250°C for 4 hours under argon protection to convert PAA into PI, resulting in nanofibers with a diameter of 200 nm and a length of 30 μm mixed with LATP inside PI.

[0149] Composite membrane preparation:

[0150] The prepared LATP-infused material (97%) mixed inside PI nanofibers was added to water and stirred to disperse it, resulting in a slurry with a concentration of 25%. The slurry was coated on the upper and lower layers of a membrane, and after baking for 15 minutes at an oven temperature of 55°C and an air flow rate of 3 m / s, a solid electrolyte / PI fiber composite membrane was obtained, with the thickness of both the upper and lower layers being 2 μm.

[0151] Comparative Example 2 directly mixes the solid electrolyte inside the PI fiber.

[0152] Performance testing

[0153] The composite membranes prepared according to the above embodiments and comparative examples were tested, as follows:

[0154] 1. Heat Shrinkage Performance Test: Cut the composite diaphragm into 100×100mm samples and draw two cross lines on the center line of the sample. Place the sample in a constant temperature chamber at 200℃ for 1 hour to conduct a heat shrinkage performance test. Record the change in length of the two cross lines, which are the heat shrinkage parameters of the composite solid diaphragm.

[0155] 2. Needle Penetration Strength Test: Cut the composite diaphragm into 100×100mm samples, lay them flat in the fixture and clamp them securely, ensuring the samples are firmly fixed and flat. Set the test speed to 100mm / min. The puncture needle is a steel needle with a diameter of 1.0mm and a tip radius of 0.5mm. At the start of the test, the puncture needle moves to pierce the sample. The equipment displays the force value change in real time and records the puncture force value in the report at the end of the test. This value is the puncture strength parameter of the composite solid diaphragm.

[0156] 3. Ionic conductivity test: Cut the composite separator into circular samples with a diameter of 19 mm (area S), measure the separator thickness L, assemble the separator into a 2032 coin cell, perform EIS testing, and record the internal resistance R data. The formula for calculating ionic conductivity σ is: σ = L / (R * S).

[0157] 4. Battery performance test

[0158] Battery manufacturing

[0159] A bare cell is obtained by stacking a high-nickel ternary positive electrode sheet, a composite separator, and a lithium metal negative electrode sheet. The positive and negative electrode tabs are welded together, the bare cell is assembled into an aluminum-plastic film battery case (or an aluminum battery case / steel battery case / nickel-plated battery case), liquid is injected, encapsulated, left to stand, formed, and tested for capacity, thus obtaining a solid-state lithium metal battery.

[0160] Capacity retention test: At room temperature, charge to 4.25V at a current density of 1C and record the charging capacity at this time as the initial charging capacity; then charge at a constant voltage of 4.25V until the current reaches 0.05C cutoff, and then discharge at 0.33C to 2.8V and record the discharge capacity at this time as the initial discharge capacity. Record the capacity retention rate at the 500th, 1000th, and 1480th cycles.

[0161] The capacity retention rates after 500, 1000, and 1480 laps can be calculated using the following formula:

[0162] Capacity retention rate after 500 cycles = Discharge capacity after 500 cycles / Initial discharge capacity * 100%;

[0163] Capacity retention rate after 1000 cycles = Discharge capacity after 1000 cycles / Initial discharge capacity * 100%;

[0164] The capacity retention rate of the 1480th cycle = discharge capacity of the 1480th cycle / initial discharge capacity * 100%.

[0165] The specific test results are shown in Table 1.

[0166] Table 1

[0167]

[0168] Note: In Table 1, MD direction refers to the longitudinal direction, TD direction refers to the transverse direction, and " / " indicates that the battery test was stopped due to low capacity retention and no response data was available.

[0169] As can be seen from the data in Table 1, the composite separator prepared in the examples has excellent shrinkage properties and mechanical strength, which can improve the cycle performance, rate performance and safety performance of the battery.

[0170] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0171] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0172] It should be noted that the terms "and / or" or " / " used herein are merely descriptions of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The singular forms "a," "described," and "the" used in the embodiments of the invention and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0173] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0174] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite separator, characterized by, The composite diaphragm comprises: Base film; A coating is disposed on at least one surface of the base film along its thickness direction, the coating comprising a composite material; The composite material includes polyimide fibers and a solid electrolyte coating the entire surface of the polyimide fibers; The polyimide fiber has a diameter of 100~500nm and a length of 10~100μm; The composite material is obtained by in-situ hydrothermal growth of solid electrolyte element source material on polyimide fiber without calcination. The hydrothermal reaction is carried out at a temperature of 120~200℃, a pressure of 1~1.5MPa, and a reaction time of 12~48h.

2. The composite separator of claim 1, wherein The thickness of the solid electrolyte-coated polyimide fiber in the composite material is 10~100 nm; And / or, the solid electrolyte includes an inorganic solid electrolyte, wherein the inorganic solid electrolyte is an oxide solid electrolyte.

3. The composite separator of claim 2, wherein, The oxide solid electrolyte is at least one of perovskite solid electrolyte, NASICON solid electrolyte, LISICON solid electrolyte, or garnet solid electrolyte.

4. The composite separator of claim 1, wherein The thickness of the coating is 1~3μm; And / or, the thickness of the base film is 5~12μm; And / or, the porosity of the base film is 35-55%; And / or, the base film includes at least one of polypropylene film, polyethylene film, meta-aramid film, polyvinylidene fluoride film, poly(vinylidene fluoride-hexafluoropropylene) film, polyimide film, polyacrylonitrile film or polyethylene terephthalate film.

5. A method of preparing a composite separator, characterized by, Includes the following steps: Polyamic acid is dissolved in an organic solvent to obtain a spinning solution, and polyamic acid fibers are obtained by electrospinning. After the polyamic acid fibers are thermally imidized, polyimide fibers are obtained. The polyimide fiber and the solid electrolyte element source material are mixed in a first solvent to obtain a precursor solution, and then a hydrothermal reaction is carried out to obtain a solid electrolyte-coated polyimide fiber composite material. The solid electrolyte-coated polyimide fiber composite material and the binder are mixed in a second solvent to obtain a coating slurry. The coating slurry is then coated onto a base membrane to obtain a composite membrane.

6. The method of claim 5, wherein the composite separator is prepared by the steps of: The polyamic acid has a mass percentage content of 5-30% in the spinning solution; And / or, the polyamic acid fiber thermal imidization treatment is carried out under an inert gas atmosphere, and the heat treatment temperature is 250~350℃ for 1~5h.

7. The method of claim 5, wherein the composite separator is prepared by the steps of: The solid electrolyte element source material includes a lithium source material and a second source material; The second source material includes at least two of the following: lanthanum source material, zirconium source material, aluminum source material, titanium source material, phosphorus source material, silicon source material, or tantalum source material.

8. The method for preparing the composite diaphragm according to claim 5, characterized in that, The mass ratio of the polyimide fiber, the solid electrolyte element source material, and the first solvent is (4~9):(1~6):(25~100).

9. The method of claim 5, wherein the composite separator is prepared by the steps of: The mass ratio of the solid electrolyte-coated polyimide fiber composite material to the binder is (95~99):(1~5); And / or, the mass percentage of the solid electrolyte-coated polyimide fiber composite material and the binder in the coating slurry is 15-40%; And / or, the adhesive includes at least one of acrylates, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, or polyvinyl alcohol.

10. The method of claim 5, wherein the composite separator is prepared by the steps of: The coating slurry is further dried after the coating process is completed. The drying process is carried out in flowing air at a velocity of 2-3 m / s. The drying temperature is 40~60℃ and the time is 10~30min.

11. A secondary battery comprising a composite separator, characterized by, The composite membrane is the composite membrane according to any one of claims 1 to 4 or the composite membrane prepared by the preparation method according to any one of claims 5 to 10.

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