Ultra-thin coating composite diaphragm for lithium metal battery and preparation method and application thereof

Abstract

The present invention discloses an ultra-thin coating composite diaphragm for lithium metal batteries, and its preparation method and application, which belong to the field of lithium ion battery technology. The preparation steps of the ultra-thin coating composite diaphragm for lithium metal batteries include: uniformly dispersing the two-dimensional oxide in a non-solvent to obtain a non-solvent dispersion containing the two-dimensional oxide, placing the lithium metal battery diaphragm in the non-solvent dispersion containing the two-dimensional oxide for displacement, and drying after the displacement is completed to obtain the ultra-thin coating composite diaphragm for lithium metal batteries. The preparation method provided by the present invention can obtain a diaphragm coating with a nanometer thickness, and ensure that it is not easy to fall off, showing significant technical advantages. The ultra-thin coating can increase the surface porosity, wettability, optimize ion transmission and other properties while ensuring the integrity of the diaphragm structure, as well as having good mechanical strength and ionic conductivity.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to an ultra-thin coating composite diaphragm for lithium metal batteries, a preparation method thereof, and applications thereof. Background Art

[0002] In electrochemical energy storage devices such as lithium batteries and supercapacitors, the separator is a key component, separating the positive and negative electrodes to prevent short circuits while allowing ions to pass freely to maintain the electrochemical reaction. As electrochemical energy storage devices develop towards higher energy density, improved safety, and longer cycle life, the performance requirements for separators are also increasing. While traditional polyolefin separators (such as polyethylene and polypropylene) possess good mechanical strength and chemical stability, their poor thermal stability, low porosity, and insufficient wettability limit their application in harsh conditions such as high temperature and high power.

[0003] To overcome the shortcomings of traditional separators, researchers have developed a variety of composite separator technologies, including coating traditional separators with ultrathin functional layers, in-situ nanomaterial growth, and metal ion-bonded separators. These technologies effectively inhibit lithium dendrite growth, enhance separator thermal stability and electrolyte wettability, while maintaining mechanical strength and ionic conductivity. Common coating materials include inorganic nanoparticles (such as TiO2, Al2O3, and SiO2), polymers (such as PVDF and PMMA), and organic-inorganic composites. These materials are deposited on the separator surface through various coating processes (such as blade coating, spray coating, and dip coating) to form an ultrathin functional layer. However, existing coating composite separator technologies still have certain drawbacks, such as difficulty controlling coating thickness and uniformity, insufficient interfacial bonding strength, process complexity, high cost, and limitations of functional coatings. Specifically, existing technologies struggle to achieve ultrathin nanometer-scale coatings, resulting in increased overall separator thickness and reduced battery energy density. Existing coating processes (such as dip coating and spray coating) are prone to uneven coating thickness and particle agglomeration, affecting the consistent performance of the separator. After coating, the bonding between the nanocoating and the base membrane is weak, making it prone to delamination during battery cycling, leading to separator failure. Furthermore, coating technology requires a multi-step process with high requirements for equipment and process parameters, increasing production costs. While existing coating materials improve certain properties (such as thermal stability), modifying the separator through in-situ growth or ionic bonding often destroys the separator's inherent structural stability, potentially sacrificing other properties (such as ionic conductivity or mechanical strength), affecting the separator's long-term stability and making it difficult to achieve synergistic optimization of multiple properties. Summary of the Invention

[0004] The present invention aims to provide an ultrathin coated composite diaphragm for lithium metal batteries, its preparation method, and its application. By optimizing and improving the coating process and simplifying the production process, not only is the preparation cost significantly reduced, but the production efficiency and overall performance of the diaphragm are also greatly improved. The preparation method provided by the present invention achieves ultrathin and efficient production while ensuring high porosity, uniform pore size distribution, and excellent mechanical properties of the diaphragm. This provides strong technical support for the high energy density and high safety of lithium-ion batteries and has broad market application prospects.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] One of the technical solutions of the present invention is to provide a method for preparing an ultra-thin coating composite diaphragm for a lithium metal battery, comprising the following steps:

[0007] Uniformly dispersing the two-dimensional oxide in a non-solvent to obtain a non-solvent dispersion containing the two-dimensional oxide, placing a lithium metal battery separator in the non-solvent dispersion containing the two-dimensional oxide for displacement, and drying after the displacement is completed to obtain an ultra-thin coating composite separator for a lithium metal battery, wherein the thickness of the ultra-thin coating does not exceed 10 nm;

[0008] The two-dimensional oxide is titanium iron oxide nanosheet and / or titanium dioxide nanosheet, and the chemical formula of the titanium iron oxide nanosheet is Ti 0.7 Fe 0.3 O2;

[0009] The mass ratio of the two-dimensional oxide to the non-solvent is 1:10000-20000.

[0010] During the NIPS phase transition process, the carbon-nitrogen bonds in the polymer raw materials of the lithium metal battery separator are unstable. When the non-solvent enters the polymer, it will introduce the two-dimensional oxides dispersed in it into the polymer surface. At the same time, the oxygen atoms in the two-dimensional oxides bond with the broken nitrogen atoms in the polymer to form a stable composite structure.

[0011] Preferably, the material of the lithium metal battery separator includes polyimide (PI), polybenzimidazole (PBI) or polyetherimide (PEI).

[0012] Preferably, the thickness of the lithium metal battery separator is 24-26 μm, more preferably 25 μm.

[0013] Preferably, the mass ratio of the two-dimensional oxide to the non-solvent is 1:10000.

[0014] Preferably, the method of uniformly dispersing the two-dimensional oxide in the non-solvent is stirring followed by ultrasonication.

[0015] More preferably, the stirring temperature is 20-40° C., the stirring speed is 300-500 rpm, and the stirring time is 1-3 h; the ultrasonic frequency is 80 Hz, and the stirring time is 1 h.

[0016] Most preferably, the stirring temperature is 25° C., the speed is 350 rpm, and the time is 1 hour.

[0017] Preferably, the replacement time is 25 to 35 seconds.

[0018] The second technical solution of the present invention: provides an ultra-thin coating composite diaphragm for lithium metal batteries prepared according to the above-mentioned preparation method of the ultra-thin coating composite diaphragm for lithium metal batteries.

[0019] The third technical solution of the present invention is to provide an application of the above-mentioned ultra-thin coating composite diaphragm for lithium metal batteries in the preparation of lithium metal batteries.

[0020] The beneficial technical effects of the present invention are as follows:

[0021] The preparation method provided by this invention can produce a nanometer-thick membrane coating that is resistant to detachment, demonstrating significant technical advantages. The ultra-thin coating can increase surface porosity, wettability, and optimize ion transport properties while ensuring the integrity of the membrane structure, as well as good mechanical strength and ionic conductivity.

[0022] Compared to traditional coating and in-situ growth techniques, the preparation method provided by this invention achieves higher-precision thickness control, addressing the issues of uneven coating thickness and particle agglomeration common in traditional methods. By optimizing the coating material and coating process, this invention significantly enhances the interfacial adhesion between the coating and the base film, effectively avoiding the risk of coating delamination during battery cycling, thereby improving the long-term stability of the separator.

[0023] Furthermore, this invention simplifies the production process, reduces process complexity and equipment requirements, improves production efficiency, and significantly reduces costs, while achieving the coordinated optimization of multiple performance characteristics. More importantly, the materials and processes employed in this invention are more environmentally friendly, reducing the use of organic solvents, environmental pollution, and operator health risks, providing strong support for the high-performance, low-cost, and environmentally friendly production of lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 These are SEM images of HT-PBI prepared in Example 1 and CT-PBI prepared in Comparative Example 3, wherein A is the SEM image of HT-PBI and B is the SEM image of CT-PBI.

[0025] Figure 2This is a diagram showing the cycling performance of the HT-PBI and blank PBI separators prepared in Example 1 in lithium metal full batteries.

[0026] Figure 3 These are SEM images of the surfaces of the HT-PBI and blank PBI separators prepared in Example 1 after 100 charge and discharge cycles in a lithium metal full battery, where A is the surface SEM image of the HT-PBI and B is the SEM image of the blank PBI separator.

[0027] Figure 4 The lithium symmetric battery assembled with the HT-PBI prepared in Example 1, the CT-PBI prepared in Comparative Example 3 or the blank PBI separator was tested at 1 mA·cm -2 1mAh·cm -2 Overpotential and cycling stability diagram under test conditions.

[0028] Figure 5 The lithium symmetric battery assembled with the HT-PBI prepared in Example 1, the CT-PBI prepared in Comparative Example 3 or the blank PBI separator was tested at 5 mA·cm -2 1mAh·cm -2 Overpotential and cycle stability diagram under high rate test conditions. DETAILED DESCRIPTION

[0029] Various exemplary embodiments of the present invention are now described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terms used in the present invention are only for describing specific embodiments and are not intended to limit the present invention.

[0030] In addition, for numerical ranges in the present invention, it is understood that each intervening value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any stated value or stated range, and any other stated value or intervening value in the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention.

[0032] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0033] Titanium ferrite nanosheets (Ti0.7 Fe 0.3 The preparation steps of O2 are as follows:

[0034] (1) First, synthesize the precursor K 0.4 Ti 0.67 Fe 0.3 Li 0.02 O2: Weigh and mix the raw materials according to the molar ratio of K2CO3, TiO2, Fe2O3 and Li2CO3 of 0.2:0.67:0.15:0.01. Place the mixed raw materials in a mortar and grind them for 30 minutes. Then, place the mixture in a muffle furnace and heat it at 900℃ in air for 1 hour to promote decarburization of the mixture. After cooling, grind the powder again for 30 minutes, then heat it in air at 1000℃ for 24 hours, and then cool it to obtain the precursor K 0.4 Ti 0.67 Fe 0.3 Li 0.02 O2, and weigh the final product.

[0035] (2) Preparation of H by protonation and stripping 0.42 Ti 0.67 Fe 0.3 O2 nanosheets: the precursor K 0.4 Ti 0.67 Fe 0.3 Li 0.02 O2 is dissolved in a solution with a concentration of 1 mol·L -1 The precursor H was collected by filtration. 0.42 Ti 0.67 Fe 0.3 O2, and then rinsed repeatedly with deionized water. Finally, the bulk precursor H 0.42 Ti 0.67 Fe 0.3 O2 was placed in tetrapropylammonium hydroxide (TBAOH) and shaken to peel it off, thereby obtaining titanium iron oxide nanosheets (Ti 0.7 Fe 0.3 O2). Before using the two-dimensional oxide, it is necessary to repeatedly wash it with deionized water to remove residual TBAOH.

[0036] The preparation steps of the PBI membrane used in the examples and comparative examples of the present invention are as follows:

[0037] 1) Prepare a solution of PBI powder and N,N-dimethylacetamide (DMAC) solvent at a solid-liquid mass ratio of 1:9. Stir in a magnetic stirrer at 350 rpm for 10 days until the solution is free of particles and bubbles, thereby obtaining a PBI solution.

[0038] 2) Pour the PBI solution onto a smooth glass plate, then use a scraper to scrape a layer of PBI membrane with a thickness of approximately 25 μm onto the glass plate. Immerse the coated membrane and the glass plate in anhydrous ethanol to replace the solvent, and dry to obtain the PBI membrane.

[0039] Example 1

[0040] Preparation of composite diaphragms with ultrathin nanocoatings:

[0041] 1) Titanium iron oxide nanosheets and anhydrous ethanol were mixed at a mass ratio of 1:10,000, stirred in a magnetic stirrer at 350 rpm and 25°C for 1 h, and ultrasonicated (80 Hz) for 1 h to obtain a uniform mixed solution;

[0042] 2) Immerse a glass plate with a 25 μm PBI membrane in the above mixed solution for 30 seconds to cause a phase inversion reaction. After the reaction is completed, a composite membrane with an ultra-thin coating (HT-PBI) is obtained.

[0043] Example 2

[0044] Preparation of composite diaphragms with ultrathin nanocoatings:

[0045] Compared with Example 1, the only difference is that the titanium ferrite nanosheets and anhydrous ethanol are mixed in a mass ratio of 1:20000.

[0046] Example 3

[0047] Compared with Example 1, the only difference is that the titanium ferrite nanosheets are replaced with titanium dioxide nanosheets of equal mass.

[0048] Comparative Example 1

[0049] Preparation of composite diaphragms with ultrathin nanocoatings:

[0050] Compared with Example 1, the only difference is that the titanium ferrite nanosheets and anhydrous ethanol are mixed in a mass ratio of 1:5000.

[0051] Comparative Example 2

[0052] Preparation of composite diaphragms with ultrathin nanocoatings:

[0053] Compared with Example 1, the only difference is that the titanium ferrite nanosheets and anhydrous ethanol are mixed in a mass ratio of 1:3000.

[0054] Comparative Example 3

[0055] Traditional preparation method of composite diaphragm with coating:

[0056] (1) The prepared titanium iron oxide nanosheets (HT) were mixed in a ratio of HT: polyvinylidene fluoride: acetone = 1 mg: 0.1 mg: 2 mL, and stirred in a magnetic stirrer at 350 rpm for 1 h to obtain a coating solution;

[0057] (2) A glass plate with a 25 μm PBI membrane was placed in a coating machine, and a layer of coating with a thickness of about 3 μm was scraped on the surface of the membrane using the above-mentioned coating solution. After drying, a composite membrane with a coating (CT-PBI) was obtained.

[0058] The SEM images of HT-PBI prepared in Example 1 and CT-PBI prepared in Comparative Example 3 are shown in FIG. Figure 1 , where A is the SEM image of HT-PBI and B is the SEM image of CT-PBI. Figure 1 The results show that the thickness of the composite membrane coating is significantly higher than that of the conventional coating. The thickness of the conventional coating affects ion conduction, resulting in a decrease in performance such as conductivity. However, the thickness of the ultra-thin two-dimensional oxide coating is at the nanometer level, which can increase the performance of the membrane without affecting its structure.

[0059] The performance tests were performed on the composite membranes prepared in Examples 1 to 3 and Comparative Examples 1 to 3 and the blank PBI membrane:

[0060] 1. Ionic conductivity

[0061] Assemble the stainless steel sheet symmetrical battery (SS / Separator / SS) to obtain the battery body resistance R b , the test frequency range is 0.1~10 6 Hz, 10mV AC sinusoidal disturbance voltage, and then calculate the ionic conductivity σ according to the following formula. The electrolyte formula used is: 1.0mol / L lithium hexafluorophosphate, and the solvent system is a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1.

[0062]

[0063] In the above formula, d is the thickness of the diaphragm (unit: cm), R b is the volume resistance (unit: KΩ), S is the effective area of ​​the diaphragm (unit: cm 2 ).

[0064] 2. Electrolyte contact angle

[0065] Refer to the provisions of GB / T 30693-2014. The electrolyte formula used is: 1.0 mol / L lithium hexafluorophosphate, and the solvent system is a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1.

[0066] 3. Lithium ion transfer number

[0067] According to the Bruce-Vincent method, a Li||Li symmetric cell is used for steady-state polarization until the current reaches a steady state. The Li is calculated according to the following formula based on the impedance change before and after polarization and the initial and steady-state currents. + Ion transfer number t Li + .

[0068]

[0069] In the above formula, I s and I0 represent the steady-state current (unit: mA) and the initial state current (unit: mA); ΔV is the DC polarization voltage pulse (unit: s); R s and R0 refer to the steady-state interface resistance (unit: Ω) and the initial interface resistance (unit: Ω).

[0070] The results of the performance test are shown in Table 1.

[0071] Table 1

[0072]

[0073] The results in Table 1 show that the ultra-thin coating composite diaphragm prepared by the present invention has a significantly lower electrolyte contact angle, and a significantly improved ion conductivity and lithium ion transfer number compared to the original diaphragm.

[0074] The data from Examples 1-2 and Comparative Examples 1-2 show that the amount of nanosheets in the non-solvent affects the morphology of the membrane surface. The porosity of the coating composited on the membrane surface can be controlled by adjusting the ratio. A suitable ratio can maximize the contact angle and ionic conductivity. Compared with Example 1, Example 2 has a lower nanosheet content, resulting in less uniform coating and relatively low porosity. However, the composite membrane performance is still superior to that of the blank PBI membrane. Comparative Examples 1-2 increase the nanosheet content, but excessive nanosheet content leads to an excessively dense coating, which can clog the membrane pores and affect the contact angle, conductivity, and lithium ion transport performance.

[0075] Compared with Example 3, in Example 1, since the titanium iron oxide nanosheets have negatively charged pores compared with the titanium dioxide nanosheets, the electrolyte affinity is higher than that of the titanium dioxide nanomaterial, and the performance is improved.

[0076] Compared with Example 1, the conventional coated diaphragm of Comparative Example 3 is difficult to reduce in thickness, resulting in pore blockage, and the coating is not conducive to lithium ion transmission, thereby reducing the performance of the lithium battery.

[0077] Application Example 1

[0078] The composite separator with an ultrathin coating (HT-PBI) prepared in Example 1 and a blank PBI separator (control group) were applied to a lithium metal full battery. The full battery material system is lithium iron phosphate-metal lithium, the battery structure is a round button cell, and the standard specific capacity of the lithium iron phosphate full battery is 170 mAh g -1 The battery was subjected to a cycle test at room temperature of 25°C, with a charge and discharge rate of 1C.

[0079] The cycle performance diagram of the two is shown in Figure 2 ,from Figure 2 It can be seen that the discharge specific capacity of the lithium metal full battery using HT-PBI prepared in Example 1 is higher than that of the control group, and after 1000 cycles, the capacity retention rate is 90%. It can be seen that the present invention significantly improves the battery cycle performance by using the ultra-thin coating to modify the diaphragm.

[0080] The SEM images of the surfaces of the HT-PBI and blank PBI separators prepared in Example 1 after 100 charge and discharge cycles in a lithium metal full battery are shown in FIG. Figure 3 , where A is the surface SEM image of HT-PBI and B is the SEM image of the blank PBI membrane.

[0081] Figure 3 The results show that after 100 cycles, the HT-PBI composite remains tightly bonded without the use of an adhesive, and observation of the surface morphology reveals that the ultra-thin coating can guide lithium ions through, achieving uniform lithium deposition (the blue portion in the figure represents uniformly deposited lithium). After cycling, lithium accumulates on the surface of the blank PBI separator, forming "lithium balls" that eventually grow into lithium dendrites that pierce the separator and cause a short circuit. In contrast, the separator with the ultra-thin coating still has a smooth surface, enabling the preparation of dendrite-free lithium batteries.

[0082] Application Example 2

[0083] The HT-PBI prepared in Example 1, the CT-PBI prepared in Comparative Example 3 and the blank PBI separator were subjected to lithium symmetric battery charge and discharge tests. The test conditions were 1 mA·cm -2 1mAh·cm -2 , 5mA·cm -2 1mAh·cm -2 Observe the overpotential and cycle life of lithium symmetric batteries.

[0084] Figure 4 The lithium symmetric battery assembled with the HT-PBI prepared in Example 1, the CT-PBI prepared in Comparative Example 3 or the blank PBI separator was tested at 1 mA·cm -2 1mAh·cm -2 Overpotential and cycle stability diagram under test conditions. Figure 4As can be seen in the graph, HT-PBI remains stable after 1500 hours of cycling, with an overpotential of only 10mV. However, the voltage of the conventional coated separator CT-PBI begins to fluctuate after 300-400 hours of cycling, until the overpotential increases and a short circuit occurs. This indicates that the ultra-thin coating-modified separator of the present invention can significantly improve battery stability and cycle life compared to conventional coated separators.

[0085] Figure 5 The lithium symmetric battery assembled with the HT-PBI prepared in Example 1, the CT-PBI prepared in Comparative Example 3 or the blank PBI separator was tested at 5 mA·cm -2 1mAh·cm -2 The overpotential and cycling stability plots under high-rate test conditions show that even at high current densities, the negatively charged surface coating optimizes lithium conduction and regulates lithium deposition, allowing the battery to operate stably for over 2000 hours with a low overpotential of only 40 mV. This demonstrates that compared to traditional coated separators, the ultra-thin coating-modified separator of the present invention significantly improves battery stability and cycle life.

[0086] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing an ultra-thin coating composite diaphragm for a lithium metal battery, characterized in that: The following steps are involved: Uniformly dispersing the two-dimensional oxide in a non-solvent to obtain a non-solvent dispersion containing the two-dimensional oxide, placing a lithium metal battery separator in the non-solvent dispersion containing the two-dimensional oxide for displacement, and drying after the displacement is completed to obtain an ultra-thin coating composite separator for a lithium metal battery, wherein the thickness of the ultra-thin coating does not exceed 10 nm; The two-dimensional oxide is titanium iron oxide nanosheet and / or titanium dioxide nanosheet, and the chemical formula of the titanium iron oxide nanosheet is Ti 0.7 Fe 0.3 O2; The mass ratio of the two-dimensional oxide to the non-solvent is 1:10000-20000; The material of the lithium metal battery separator includes polyimide, polybenzimidazole or polyetherimide.

2. The method for preparing an ultra-thin coating composite diaphragm for a lithium metal battery according to claim 1, wherein: The thickness of the lithium metal battery separator is 24-26 μm.

3. The method for preparing an ultra-thin coating composite diaphragm for a lithium metal battery according to claim 1, wherein: The mass ratio of the two-dimensional oxide to the non-solvent is 1:10000.

4. The method for preparing an ultra-thin coating composite diaphragm for a lithium metal battery according to claim 1, wherein: The method for uniformly dispersing the two-dimensional oxide in the non-solvent is stirring followed by ultrasonication.

5. The method for preparing an ultra-thin coating composite diaphragm for a lithium metal battery according to claim 4, characterized in that: The stirring temperature is 20-40° C., the speed is 300-500 rpm, and the time is 1-3 hours; the ultrasonic frequency is 80 Hz, and the time is 1 hour.

6. The method for preparing an ultra-thin coating composite diaphragm for a lithium metal battery according to claim 1, wherein: The replacement time is 25 to 35 seconds.

7. An ultra-thin coating composite diaphragm for a lithium metal battery prepared according to the method for preparing an ultra-thin coating composite diaphragm for a lithium metal battery according to any one of claims 1 to 6.

8. Use of the ultra-thin coating composite diaphragm for lithium metal batteries according to claim 7 in the preparation of lithium metal batteries.

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