Preparation method and application of composite diaphragm with multistage nanometer branch structure

Abstract

The invention provides a preparation method and application of a composite diaphragm with a multistage nanometer branch structure, and belongs to the technical field of lithium metal batteries. A polymer matrix, a first structure inducer and a second structure inducer form a precursor solution, and the precursor solution is subjected to an electrostatic spinning process and in-situ induction to form the multi-stage structure composite diaphragm with a trunk-branch characteristic; the first structure inducer is a quaternary ammonium salt compound, and the second structure inducer is imidazolium ionic liquid. The dual-inducer system endows the lithium metal negative electrode with interface stability and cycle reliability.

Description

Technical Field

[0001] This application relates to a method for preparing and applying a composite separator with a multi-level nanobranch structure, belonging to the field of lithium metal battery technology. Background Technology

[0002] Lithium metal anodes are considered an ideal choice for next-generation high-energy-density batteries due to their extremely high theoretical specific capacity, but their commercialization is limited by safety risks caused by lithium dendrite growth and short cycle life. Traditional polyolefin (PE / PP) separators have poor wettability, insufficient thermal stability, and uniform structure, making it difficult to effectively homogenize lithium-ion flow and suppress dendrites.

[0003] While polymer nanofiber membranes prepared by electrospinning possess high porosity and good electrolyte affinity, their fiber structures are typically quite uniform, making it difficult to effectively control ion migration pathways and local electric field distribution at the microscale. Improvements to electrospun membranes often focus on introducing single structural scales or single functional components, and these methods are frequently complex, making it difficult to simultaneously achieve synergistic optimization of membrane microstructure control and interfacial chemical behavior within the same system.

[0004] How to construct a membrane in situ using a simple process that can both promote rapid ion transport and guide uniform lithium deposition and stabilize the interface remains a technical challenge that urgently needs to be solved. Summary of the Invention

[0005] In view of this, the present invention aims to provide a method for preparing a composite membrane with a multi-level nanobranched structure. Unlike conventional uniform nanofiber membranes, this application utilizes two types of inducing agents, which work together during the evolution of the electrospinning jet, rather than acting independently. This allows for the in-situ construction of a multi-level nanobranched structure that is difficult to form with a single inducing agent system. The ion migration path and local electric field distribution are reconstructed at the microscale, resulting in a uniform distribution of lithium ion flow along the membrane thickness direction.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a composite membrane with a multi-level nanobranched structure is disclosed, comprising a polymer matrix, a first structure inducer, and a second structure inducer to form a precursor solution, which is then subjected to electrospinning to induce the formation of a composite membrane with a multi-level structure featuring a trunk-branch feature in situ; the first structure inducer is a quaternary ammonium salt compound, and the second structure inducer is an imidazole ionic liquid.

[0007] This application employs a dual-inducer system consisting of quaternary ammonium salt and ionic liquid structures. This system generates a physicochemical synergistic effect during the jet evolution stage of single-step electrospinning, enabling the in-situ construction of a homogeneous composite separator with a multi-level nanostructure exhibiting trunk-branch characteristics. The composite separator reconstructs the ion migration pathways and local electric field distribution within the separator at the microscale, significantly altering lithium-ion deposition behavior. Simultaneously, the ionic liquid inducer participates in interfacial film formation, creating a stable inorganic component-enriched SEI film that chemically stabilizes the interface, fundamentally inhibiting lithium dendrite growth. This synergistically enhances the interfacial stability and cycle reliability of the lithium metal anode.

[0008] Furthermore, as a preferred option: The total amount of the first and second structure-inducing agents added is 3-30% of the polymer matrix mass. More preferably: The amount of the first structure inducer added is 12-18% of the polymer matrix mass, with 15% being optimal.

[0009] The amount of the second structure inducer added is 8-12% of the polymer matrix mass, with 10% being optimal.

[0010] The first structure inducer is tetrabutylammonium chloride (TBAC), and the second structure inducer is 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide salt ([EMIM][FSI]).

[0011] The polymer matrix is ​​polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP).

[0012] The precursor solution also contains a mixed solvent, which is obtained by mixing N,N-dimethylformamide (DMF) and acetone at a volume ratio of 5~7:3~5 (6:4 being optimal). The precursor solution is prepared as follows: the polymer matrix is ​​dissolved in the mixed solvent to obtain a solution with a mass concentration of 10~16% (13% being optimal), then a first structure inducer and a second structure inducer are added and dispersed evenly to obtain the precursor solution.

[0013] In the electrospinning process: The distance between the spinning needle and the receiving device is 10-20 cm, with 15 cm being preferred.

[0014] The relative humidity of the electrospinning environment is controlled below 30%.

[0015] The thickness of the diaphragm received by the electrospinning is controlled at 20~30μm, with 25μm being preferred.

[0016] The product obtained from electrospinning is dried in a vacuum oven to obtain a composite diaphragm. The drying temperature is 40~80 ℃, preferably 60 ℃. The drying time is 12~36 h, preferably 24 h.

[0017] The composite separator prepared by the above method can be applied to liquid lithium metal batteries. The introduction of the composite separator in this application enables the corresponding lithium battery to maintain stable lithium deposition / stripping behavior over a long period of time during operation and exhibits excellent capacity retention performance under long-term cycling conditions.

[0018] The liquid lithium metal battery preferably includes a lithium metal anode, a high-voltage cathode (such as high-nickel ternary material NCM811), an electrolyte, and a composite separator.

[0019] The liquid lithium metal battery can be a Li|separator|Li symmetric cell or a Li|separator|NCM811 full cell.

[0020] The beneficial effects of this invention are as follows: Through the synergistic induction of TBAC and ionic liquids, a multi-level nanobranched structure is directly formed during electrospinning, greatly increasing the specific surface area and network connectivity, and significantly improving the electrolyte wettability of the separator. The trunk-branch structure reconstructs the ion migration path and local electric field distribution within the separator at the microscale, resulting in a homogenized distribution of lithium ion flow along the separator thickness direction. This regulatory effect cannot be explained by porosity or pore size distribution alone. Simultaneously, a second structure inducer is pre-embedded in the multi-level nanofibers of the separator, preventing it from participating in interfacial reactions as a main electrolyte component. Instead, it participates in the interfacial film formation process in a controlled manner through separator-interface coupling, thereby inducing the formation of a highly stable inorganic component-enriched SEI film. This chemically enhances interfacial stability. This invention eliminates the need for multi-step coating or etching processes required for traditional complex structure construction. It enables the simultaneous in-situ construction and functional integration of multi-level structures during electrospinning without additional post-processing or multi-step structural reconstruction. The process is simple and highly compatible with existing battery production lines. Furthermore, thanks to the high thermal stability of PVDF-HFP and ionic liquid, the present invention has a low thermal shrinkage rate of the separator, and lithium metal batteries using this separator exhibit long cycle life and high capacity retention. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application.

[0022] Figure 1The images show SEM morphology comparisons of different diaphragms in this application. Part (a) is the PVDF-HFP diaphragm of Comparative Example 1, part (b) is the PH-IL diaphragm of Comparative Example 2, part (c) is the PH-T diaphragm of Comparative Example 3, and part (d) is the PH-T-IL diaphragm of Example 1.

[0023] Figure 2 This is a comparison diagram of the electrolyte contact angles of different diaphragms in this application. In the diagram, (a) is the PVDF-HFP diaphragm of Comparative Example 1, (b) is the PH-IL diaphragm of Comparative Example 2, (c) is the PH-T diaphragm of Comparative Example 3, and (d) is the PH-T-IL diaphragm of Example 1.

[0024] Figure 3 1.0 mA / cm 2 Comparison of cycle performance of Li|Li symmetric cells under different conditions.

[0025] Figure 4 This is a comparison chart of the long-cycle performance of Li|NCM811 full cells with different separators at 1C rate.

[0026] Figure 5 The following are SEM morphology comparison images of the PH-T membrane with different TBAC addition amounts in this application. In the figure, (a) represents 3% TBAC, (b) represents 5% TBAC, (c) represents 10% TBAC, and (d) represents 15% TBAC.

[0027] Figure 6 The images show the SEM morphology comparison of the PT-IL membrane with different amounts of [EMIM][FSI] added in this application. Part (a) represents 5% [EMIM][FSI], part (b) represents 10% [EMIM][FSI], part (c) represents 15% [EMIM][FSI], and part (d) represents 20% [EMIM][FSI]. Detailed Implementation

[0028] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the technical solutions in the embodiments of this application will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit the technical solutions of this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without creative effort are within the scope of protection of this application.

[0029] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or specifying the number of technical features. "A plurality of" means two or more, unless otherwise explicitly defined.

[0030] In this embodiment, the constant current charge-discharge performance was tested using the Land CT2001 from Wuhan Landian Electronics Co., Ltd.

[0031] Example 1

[0032] This application provides a composite membrane with a dual inducing agent system. The preparation process of the composite membrane is as follows: Step 1, Prepare the spinning solution: S1, weigh PVDF-HFP and add it to a mixed solvent of DMF and acetone. Under heating and stirring conditions, it is fully dissolved. The volume ratio of DMF to acetone is 6:4 and the mass concentration of PVDF-HFP is 13%, thus obtaining a basic solution with electrospinning properties.

[0033] S2, TBAC was added to the base solution and uniformly dispersed in the system by stirring and sonication. Then, [EMIM][FSI] were introduced to allow the two types of inducers to coexist in the solution, yielding the precursor solution. The amount of TBAC added was 15% of the mass of PVDF-HFP, and the amount of [EMIM][FSI] added was 10% of the mass of PVDF-HFP.

[0034] Step 2, electrospinning: The precursor solution is injected into a syringe using a flat-tipped metal needle. The precursor solution is then electrospun, and under the combined effects of electric field stretching and solvent evaporation, two types of structure inducers are induced in situ to form a multi-level nanobranched structure with trunk-branch characteristics during the jet evolution and solidification process.

[0035] In this embodiment, the spinning voltage is 20 kV, the receiving distance is 15 cm, the feeding speed is 0.5 mL / h, and the relative humidity is less than 30%.

[0036] Step 3, Post-processing: The collected fiber membranes were dried at 60°C for 24 hours to obtain a composite membrane with a multi-level nanobranched structure, denoted as membrane PH-T-IL.

[0037] Comparative Example 1

[0038] The setup for this comparative example is the same as that for Example 1, except that no inducing agent was added. The preparation process of this inducing agent-free PVDF-HFP membrane is as follows: Step 1, prepare the spinning solution: Weigh PVDF-HFP and add it to a mixed solvent of DMF and acetone. Under heating and stirring conditions, it is fully dissolved. The volume ratio of DMF to acetone is 6:4, and the mass concentration of PVDF-HFP is 13%, thus obtaining a basic solution with electrospinning properties.

[0039] Step 2, electrospinning: Inject the spinning solution into a syringe using a flat-tipped metal needle. Perform electrospinning on the spinning solution at a spinning voltage of 20 kV, a receiving distance of 15 cm, a feed rate of 0.5 mL / h, and a relative humidity below 30%.

[0040] Step 3, post-processing: The collected fiber membrane is dried at 60℃ for 24 hours to obtain a common electrospun membrane, denoted as membrane PVDF-HFP.

[0041] Comparative Example 2

[0042] The setup for this comparative example is the same as that for Example 1, except that only a single inducing agent [EMIM][FSI] is added in S2, and TBAC is not added, resulting in the PH-IL membrane.

[0043] Comparative Example 3

[0044] The setup for this comparative example is the same as that for Example 1, except that only a single inducing agent (TBAC) is added in S2, and [EMIM][FSI] is not added, resulting in membrane PH-T.

[0045] The basic structural morphology of the diaphragms obtained in Example 1, Comparative Examples 1 to 3 was characterized, and the results are as follows: Figure 1 As shown: The PVDF-HFP membrane has a smooth fiber surface and a uniform structure (see...). Figure 1 (part (a) in the text).

[0046] The PH-IL membrane fibers are relatively smooth cylindrical in shape and do not form a multi-level nanobranched structure (see...). Figure 1 (part (b) of the text).

[0047] The fibers of the PH-T diaphragm have a primary branched structure (see...) Figure 1 (part (c) in the text).

[0048] The PH-T-IL membrane exhibits a multi-level nanobranched structure morphology composed of a continuous trunk and branched structures (see...). Figure 1 (d) in the middle.

[0049] The structural differences between PH-IL, PH-T, and PH-T-IL confirm that TBAC is the key to inducing branching, and [EMIM][FSI] can only play its role in inducing more refined branching in the presence of TBAC.

[0050] The wetting properties of the above embodiments and comparative examples were tested, and the results are as follows: Figure 2 As shown: The contact angle of the PVDF-HFP diaphragm is as high as 35.45° (see...) Figure 2 (see part (a) of the diagram), the contact angle of the PH-IL diaphragm is 24.16° (see...). Figure 2 (see part (b) of the text), the contact angle of the PH-T membrane is 11.36° (see...). Figure 2 (c) of the text) while the PH-T-IL membrane can be instantly wetted (see section (c)). Figure 2 (d) in the middle.

[0051] The separators obtained from the above embodiments and comparative examples were used for battery assembly, and their respective electrical performance was tested.

[0052] 1) Symmetrical Cell Testing: The prepared composite separator was assembled and tested using a conventional coin cell structure. The separator isolates the positive and negative electrodes and provides ion transport channels. The battery assembly process was completed in a controlled environment to ensure assembly consistency. A conventional electrolyte suitable for lithium metal battery systems was selected.

[0053] The results are as follows Figure 3 As shown: 1.0 mA / cm 2 Under the same conditions, the Li / Li battery cell using the PH-T-IL separator exhibited stable lithium deposition / stripping cycling for over 1500 hours, with the overpotential remaining stable throughout. In contrast, under the same conditions, the Li / Li battery cell using the PH-T separator in Comparative Example 3 showed an increase in overpotential after approximately 1000 hours, the Li / Li battery cell using the PH-IL separator in Comparative Example 2 showed an increase in overpotential after approximately 800 hours, and the Li / Li battery cell using the PVDF-HFP separator in Comparative Example 1 showed an increase in overpotential after approximately 600 hours.

[0054] The electrochemical performance of the above-mentioned symmetrical cells shows that when only a single structural inducer is introduced, it is difficult to form a stable trunk-branch structure inside the membrane, and its ion migration behavior is still close to that of a uniform nanofiber system. However, the present application, which uses two inducers, can form a multi-level nanobranch structure inside the membrane, exhibiting significantly different interface evolution behavior. This also indirectly confirms that a single inducer system is difficult to produce the expected nanostructure.

[0055] 2) Full cell test: Under the preset voltage range (2.8~4.4V) and rate (1C) conditions, charge and discharge cycle tests are performed on the batteries assembled with each separator.

[0056] The results are as follows Figure 4 As shown, the battery using the PH-T-IL separator has a discharge specific capacity of approximately 163 mAh / g after 300 cycles, with a capacity retention of approximately 92.5%. The battery using the PH-T separator has a discharge specific capacity of approximately 157 mAh / g after 300 cycles, and the battery using the PVDF-HFP separator has a discharge specific capacity of less than 145 mAh / g. The capacity retention rates of the batteries using the PH-T and PVDF-HFP separators are only 84.3% and 80.1%, respectively, which are far lower than the 92.5% reported in this application.

[0057] Based on the microstructure of each separator, it can be inferred that the PH-T-IL separator has a trunk-branch type multi-level nanobranch structure, which plays a regulatory role in ion migration behavior and interface reaction process during battery operation, thereby improving the stability of battery operation. This application achieves the improvement of the bulk electrochemical performance of lithium metal batteries, rather than the expansion of safety functions or the improvement of solid interface contact.

[0058] Example 2

[0059] This embodiment has the same settings as Embodiment 1, except that the amount of TBAC added is replaced by 3%, 5%, 10%, and 20% respectively instead of 15%.

[0060] The results are as follows Figure 5 As shown, when the amount of TBAC added is too low, the induced nanofiber structure is sparse and unevenly distributed; when the amount added is too high, the properties of the spinning solution change and cannot be spun normally.

[0061] Example 3

[0062] This embodiment has the same settings as Embodiment 1, except that the amount of [EMIM] and [FSI] added is replaced by 5%, 15%, and 20% respectively, instead of 10%.

[0063] The results are as follows Figure 6 As shown, when the amount of [EMIM][FSI] added is too low, the induction effect on secondary branches is not obvious, and the multi-level structure of the diaphragm is not perfect; when the amount added is too high, the conductivity of the spinning solution increases significantly, which leads to unstable jet in the electrospinning process, making it difficult to form a continuous and uniform fiber membrane, causing fiber adhesion, uneven pore distribution, and even pore blockage.

[0064] In summary, this invention, based on the concept of "dual inducer synergy + single-step electrospinning in-situ molding," innovatively utilizes electrospinning technology to prepare a composite separator PVDF-HFP. This composite separator possesses an integrated multi-level nanobranched structure, without delamination, interface splicing, or composite stacking, exhibiting strong structural integrity. While increasing the specific surface area, it also enhances the wettability of the electrolyte, enabling instantaneous wetting. Simultaneously, this application optimizes the deposition behavior of the lithium metal anode from both structural guidance and interfacial chemistry dimensions, reconstructing the ion migration path and thus physically homogenizing the ion flow. [EMIM][FSI] participate in the interfacial film formation process, forming a stable inorganic component-enriched SEI film, chemically stabilizing the interface and fundamentally inhibiting the growth of lithium dendrites.

[0065] The above-described embodiments are merely illustrative of several feasible implementations of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention, nor are the embodiments intended to limit the scope of protection in the claims of the present invention. For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention. All equivalent implementations or changes that do not depart from the present invention should be included in the technology of the present invention.

Claims

1. A method for preparing a composite membrane with a multi-level nanobranched structure, characterized in that: A precursor solution is composed of a polymer matrix, a first structure inducer, and a second structure inducer. The precursor solution is then electrospinned to induce the formation of a composite membrane with a multi-level structure featuring a trunk-branch structure in situ. The first structure inducer is a quaternary ammonium salt compound, and the second structure inducer is an imidazole ionic liquid.

2. The method for preparing the composite diaphragm according to claim 1, characterized in that: The total amount of the first and second structure inducers added is 3 to 30% of the polymer matrix mass.

3. The method for preparing a composite membrane with a multi-level nanobranched structure according to claim 2, characterized in that: The first structure inducer is added at 12-18% of the polymer matrix mass, and the second structure inducer is added at 8-12% of the polymer matrix mass.

4. The method for preparing a composite membrane with a multi-level nanobranched structure according to claim 1, characterized in that: The first inducing agent is tetrabutylammonium chloride, and the second inducing agent is 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide salt.

5. The method for preparing a composite membrane with a multi-level nanobranched structure according to claim 1, characterized in that: The polymer matrix is ​​polyvinylidene fluoride-hexafluoropropylene.

6. The method for preparing a composite membrane with a multi-level nanobranched structure according to claim 1, characterized in that: The precursor solution also contains a mixed solvent, which is obtained by mixing DMF and acetone in a volume ratio of 5~7:3~5.

7. The method for preparing a composite membrane with a multi-level nanobranched structure according to claim 6, characterized in that: The polymer matrix is ​​dissolved in a mixed solvent to obtain a solution with a mass concentration of 10-16%. Then, a first structure inducer and a second structure inducer are added and dispersed evenly to obtain a precursor solution.

8. The application of a composite separator prepared by the method of claim 1 in a liquid lithium metal battery.

9. The application according to claim 8, characterized in that: The liquid lithium metal battery includes a lithium metal negative electrode, a high-voltage positive electrode, an electrolyte, and a composite separator.

10. The application according to claim 8, characterized in that: The liquid lithium metal battery is a Li|separator|Li symmetric cell or a Li|separator|NCM811 full cell.

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