Ultrathin coating composite diaphragm for lithium metal battery as well as preparation method and application of ultrathin coating composite diaphragm

Abstract

The invention discloses an ultrathin coating composite diaphragm for a lithium metal battery as well as a preparation method and application of the ultrathin coating composite diaphragm, and belongs to the technical field of lithium ion batteries. The preparation method of the ultrathin coating composite diaphragm for the lithium metal battery comprises the following steps: uniformly dispersing a two-dimensional oxide into a non-solvent to obtain a non-solvent dispersion liquid containing the two-dimensional oxide, placing a lithium metal battery diaphragm into the non-solvent dispersion liquid containing the two-dimensional oxide for replacement, and drying after the replacement is completed to obtain the ultrathin coating composite diaphragm for the lithium metal battery. The ultrathin coating composite diaphragm for the lithium metal battery is obtained. According to the preparation method provided by the invention, the diaphragm coating with nano thickness can be obtained, the diaphragm coating is ensured not to fall off easily, and remarkable technical advantages are shown. The ultrathin coating can ensure that the diaphragm is complete in structure and has good mechanical strength and ionic conductivity while improving the surface porosity and wettability and optimizing the properties such as ion transmission.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to an ultra-thin coated composite separator for lithium metal batteries, a preparation method thereof, and an application thereof. Background Art

[0002] In electrochemical energy storage devices such as lithium batteries and supercapacitors, the separator is one of the key components. Its main function is to isolate the positive and negative electrodes to prevent short circuits, while allowing ions to pass freely to maintain the electrochemical reaction. With the development of electrochemical energy storage devices towards high energy density, high safety, and long cycle life, the performance requirements for separators are also increasing. Although traditional polyolefin separators (such as polyethylene and polypropylene) have good mechanical strength and chemical stability, their poor thermal stability, low porosity, and insufficient wettability limit their application under harsh conditions such as high temperature and high power.

[0003] To overcome the defects of traditional separators, researchers have developed various composite separator technologies, including coating ultra-thin functional layers on the surface of traditional separators, in-situ growth of nanomaterials, and metal ion-bonded separators to prepare composite separators, which can effectively inhibit the growth of lithium dendrites, enhance the thermal stability and electrolyte wettability of the separator, while maintaining its mechanical strength and ionic conductivity. Common coating materials include inorganic nanoparticles (such as TiO2, Al2O3, SiO2), polymers (such as PVDF, PMMA), and organic-inorganic composites. These materials are attached to the surface of the separator through different coating processes (such as doctor blade coating, spraying, dip coating) to form an ultra-thin functional layer. However, the existing coated composite separator technology still has certain defects, such as difficulties in controlling the coating thickness and uniformity, insufficient interfacial bonding strength, process complexity, high cost, and limitations of functional coatings. Specifically, it is difficult to achieve an ultra-thin coating at the nanoscale in the prior art, resulting in an increase in the overall thickness of the separator and a decrease in the energy density of the battery. Existing coating processes (such as dip coating and spraying) are prone to problems such as uneven coating thickness and particle agglomeration, affecting the performance consistency of the separator. The bonding force between the nano-coating and the base film after coating is weak, and it is easy to peel off during the battery cycle, resulting in separator failure. Moreover, the coating technology requires multiple steps and high requirements for equipment and process parameters, increasing the production cost. When improving a certain performance (such as thermal stability) with existing coating materials, modifying the separator by in-situ growth or ion bonding usually destroys the stable structure of the separator itself and may sacrifice other performances (such as ionic conductivity or mechanical strength), affecting the long-term stability of the separator and making it difficult to achieve the synergistic optimization of multiple performances. Summary of the Invention

[0004] The object of the present invention is to provide an ultra-thin coated composite separator for lithium metal batteries, a preparation method thereof and an application. By optimizing and improving the coating process and simplifying the production process, not only the preparation cost is significantly reduced, but also the production efficiency and the comprehensive performance of the separator are greatly improved. The preparation method provided by the present invention realizes ultra-thin and high-efficiency production while ensuring high porosity, uniform pore size distribution and excellent mechanical properties of the separator, 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: providing a preparation method of an ultra-thin coated composite separator for lithium metal batteries, comprising the following steps:

[0007] Uniformly disperse two-dimensional oxide into a non-solvent to obtain a non-solvent dispersion liquid containing two-dimensional oxide, place a lithium metal battery separator in the non-solvent dispersion liquid containing two-dimensional oxide for replacement, and dry after replacement is completed to obtain an ultra-thin coated composite separator for lithium metal batteries, wherein the thickness of the ultra-thin coating does not exceed 10 nm;

[0008] The two-dimensional oxide is titanium iron oxide nanosheets and / or titanium dioxide nanosheets, and the chemical formula of the titanium iron oxide nanosheets 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 inversion process, the carbon-nitrogen bonds in the polymer raw material of the lithium metal battery separator are unstable. When the non-solvent enters the polymer, the two-dimensional oxide dispersed therein is introduced onto the polymer surface. At the same time, the oxygen atoms in the two-dimensional oxide bond with the broken nitrogen atom bonds 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, and 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 ultrasonic treatment.

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

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

[0017] Preferably, the replacement time is 25~35 s.

[0018] The second technical solution of the present invention: Provide an ultra-thin coated composite separator for a lithium metal battery prepared by the preparation method of the ultra-thin coated composite separator for a lithium metal battery according to the above.

[0019] The third technical solution of the present invention: Provide an application of the above ultra-thin coated composite separator for a lithium metal battery in the preparation of a lithium metal battery.

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

[0021] Through the preparation method provided by the present invention, a separator coating with a nano-thickness can be obtained, and it is ensured that it is not easy to fall off, showing significant technical advantages. The ultra-thin coating can increase surface porosity, wettability, optimize ion transport and other properties while ensuring the integrity of the separator structure, as well as having good mechanical strength and ionic conductivity.

[0022] Compared with traditional coating and in-situ growth and other technologies, the preparation method provided by the present invention realizes higher-precision thickness control, solves the common problems of uneven coating thickness and particle agglomeration in traditional methods. By optimizing the coating material and coating process, the present invention significantly enhances the interfacial bonding force between the coating and the base film, effectively avoids the risk of coating peeling during the battery cycle, and thus improves the long-term stability of the separator.

[0023] In addition, the present invention also simplifies the production process, reduces the process complexity and equipment requirements, improves the production efficiency and significantly reduces the cost, while realizing the collaborative optimization of multiple properties. More importantly, the materials and processes adopted by the present invention are more environmentally friendly, reduce the use of organic solvents, reduce environmental pollution and operation health risks, and provide strong support for the high-performance, low-cost and environmentally friendly production of lithium-ion batteries. Description of the Drawings

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

[0025] Figure 2Cycling performance graph of the HT-PBI prepared in Example 1 and the blank PBI separator in a lithium metal full cell.

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

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

[0028] Figure 5 Overpotential and cycling stability graph of the lithium symmetric cell assembled with the HT-PBI prepared in Example 1, the CT-PBI prepared in Comparative Example 3, or the blank PBI separator at 5 mA·cm -2 1 mAh·cm -2 under the high-rate test conditions. Detailed implementation manners

[0029] Now, various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation manners of the present invention. It should be understood that the terms used in the present invention are only for describing specific implementation manners and are not used to limit the present invention.

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

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

[0032] Regarding the terms "comprising", "including", "having", "containing", etc. used herein, they are all open-ended terms, meaning including but not limited to.

[0033] The titanium iron oxide nanosheets (Ti used in the examples and comparative examples of the present invention0.7 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 the raw materials according to the molar ratio of K2CO3, TiO2, Fe2O3, and Li2CO3 of 0.2:0.67:0.15:0.01 and mix them. Put the mixed raw materials into a mortar and grind for 30 minutes. Then, place the mixture in a muffle furnace and heat it at 900 °C in an air environment for 1 hour to promote decarbonization of the mixture. After it cools, grind the powder again for 30 minutes, then heat it in an air environment at 1000 °C 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 weight of the final product.

[0035] (2) Prepare H 0.42 Ti 0.67 Fe 0.3 O2 nanosheets by protonation and exfoliation: Dissolve the precursor K 0.4 Ti 0.67 Fe 0.3 Li 0.02 O2 in a HCl solution with a concentration of 1 mol·L -1 and stir continuously for 7 days. Collect most of the precursor H 0.42 Ti 0.67 Fe 0.3 O2 by filtration, and rinse it repeatedly with deionized water. Finally, place the large precursor H 0.42 Ti 0.67 Fe 0.3 O2 in tetrapropylammonium hydroxide (TBAOH) and shake it to exfoliate, so as to obtain titanium iron oxide nanosheets (Ti 0.7 Fe 0.3 O2). Before using the two-dimensional oxide, it also needs to be washed repeatedly with deionized water to remove the residual TBAOH.

[0036] The preparation steps of the PBI separator 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 according to a solid-liquid mass ratio of 1:9, and stir it at a speed of 350 rpm in a magnetic stirrer for 10 days until there are no particles and bubbles in the solution to obtain a PBI solution.

[0038] 2) Pour the PBI solution onto a smooth glass plate, and then use a scraper to scrape a PBI separator with a thickness of about 25 μm on the glass plate. Immerse the scraped separator together with the glass plate in absolute ethanol to displace the solvent, and dry to obtain the PBI separator.

[0039] Example 1

[0040] Preparation of a composite separator with an ultra-thin nanocoating:

[0041] 1) Mix titanium ferrite nanosheets and absolute ethanol in a mass ratio of 1:10000, and stir in a magnetic stirrer at a speed of 350 rpm and a temperature of 25 °C for 1 h, and then ultrasonicate (80 Hz) for 1 h to obtain a homogeneous mixed solution;

[0042] 2) Immerse the glass plate with a 25-μm PBI separator in the above mixed solution for 30 s to cause a phase inversion reaction, and obtain a composite separator (HT-PBI) with an ultra-thin coating after the reaction.

[0043] Example 2

[0044] Preparation of a composite separator with an ultra-thin nanocoating:

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

[0046] Example 3

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

[0048] Comparative Example 1

[0049] Preparation of a composite separator with an ultra-thin nanocoating:

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

[0051] Comparative Example 2

[0052] Preparation of a composite separator with an ultra-thin nanocoating:

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

[0054] Comparative Example 3

[0055] Traditional preparation method of a composite separator with a coating:

[0056] (1) Mix the prepared titanium ferrite nanosheets (HT) in the ratio of HT: polyvinylidene fluoride: acetone = 1 mg: 0.1 mg: 2 mL, and stir for 1 h at a speed of 350 rpm in a magnetic stirrer to obtain a coating solution;

[0057] (2) Place the glass plate with a 25-μm PBI separator in a coater, and scrape a coating about 3 μm thick on the surface of the separator with the above coating solution. After drying, a composite separator with a coating (CT-PBI) is obtained.

[0058] The SEM images of HT-PBI prepared in Example 1 and CT-PBI prepared in Comparative Example 3 are shown in Figure 1 , where A is the SEM image of HT-PBI and B is the SEM image of CT-PBI. Figure 1 It shows that the coating thickness of the composite separator prepared by traditional coating is significantly higher. The thickness of the traditional coating will affect ion conduction, resulting in a decrease in properties such as conductivity. The thickness of the ultra-thin two-dimensional oxide coating is nanoscale, which can increase its properties without affecting the separator structure.

[0059] Perform performance tests on the composite separators prepared in Examples 1-3, Comparative Examples 1-3, and the blank PBI separator:

[0060] 1. Ionic conductivity

[0061] Assemble a stainless steel sheet symmetric cell (SS / Separator / SS) to obtain the cell body resistance R b , the test frequency range is 0.1-10 6 Hz, 10 mV AC sine perturbation voltage, and then calculate the ionic conductivity σ according to the following formula. The electrolyte formula used: 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.

[0062]

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

[0064] 2. Electrolyte contact angle

[0065] Conduct according to the provisions of GB / T 30693-2014. The electrolyte formula used: 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 transference number

[0067] According to the Bruce-Vincent method, a Li||Li symmetric cell was used for steady-state polarization until the current reached a steady state. Based on the impedance changes before and after polarization, as well as the initial and steady-state currents, Li was calculated according to the following formula + the 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 voltage pulse of DC polarization (unit: s); R s and R0 refer to the steady-state interfacial resistance (unit: Ω) and the initial interfacial 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 for the ultra-thin coating composite separator prepared by the present invention, compared with the original separator, its electrolyte contact angle is significantly reduced, and the ionic conductivity and lithium ion transfer number are also significantly improved.

[0074] From the data of Examples 1-2 and Comparative Examples 1-2, the amount of nanosheets in the non-solvent affects the surface morphology of the separator. The pores of the coating composite on the separator surface can be regulated by controlling the ratio. An appropriate ratio can maximize the contact angle and ionic conductivity. Compared with Example 1, Example 2 has a smaller amount of nanosheets, so the coating is not evenly coated and the porosity is relatively low, but the performance of the composite separator is still better than that of the blank PBI separator. In Comparative Examples 1-2, the amount of nanosheets is increased, but too high a content leads to too many nanosheets and the coating is too dense, which will cause the pores of the separator to be blocked, affecting the contact angle, conductivity and lithium ion transport performance.

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

[0076] Compared with Example 1, for the conventional coated separator in Comparative Example 3, due to the difficulty in reducing the separator thickness, the pores are blocked, and the coating is not conducive to lithium ion transport, thus degrading the performance of the lithium battery.

[0077] Application Example 1

[0078] The composite separator with an ultra-thin coating (HT-PBI) prepared in Example 1 and the blank PBI separator (control group) were applied to a lithium metal full cell. The full cell material system was lithium iron phosphate-lithium metal, and the cell structure was a round button cell. The standard specific capacity of the lithium iron phosphate full cell was 170 mAh·g -1 ; The battery was tested for cyclic performance at 25 °C at room temperature, and the charge-discharge rate was 1C for both charging and discharging.

[0079] The cyclic performance diagrams of the two are shown in Figure 2 , from Figure 2 it can be seen that the discharge specific capacity of the lithium metal full cell 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%. Thus, it can be known that the use of the ultra-thin coating modified separator in the present invention significantly improves the cyclic performance of the battery.

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

[0081] Figure 3 It shows that after 100 cycles, the composite of HT-PBI is still tight without using an adhesive, and by observing the surface morphology, it can be found that the ultra-thin coating can guide the passage of lithium ions to achieve uniform lithium deposition (the blue part in the figure is the uniformly deposited lithium). For the blank PBI separator, lithium will accumulate on the surface of the separator after cycling to form "lithium balls", which will eventually grow into lithium dendrites and pierce the separator, causing a short circuit. In contrast, the separator with an ultra-thin coating on the surface still has a smooth surface, and the preparation of a dendrite-free lithium battery can be realized.

[0082] Application Example 2

[0083] Charge-discharge tests were carried out on the HT-PBI prepared in Example 1, the CT-PBI prepared in Comparative Example 3, and the blank PBI separator for lithium symmetric batteries. The test conditions were 1 mA·cm -2 1 mAh·cm -2 , 5 mA·cm -2 1 mAh·cm -2 The overpotential and cycle life of the lithium symmetric battery were observed.

[0084] Figure 4 For the overpotential and cycle stability diagrams of 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 under the test conditions of 1 mA·cm -2 1 mAh·cm -2 From Figure 4It can be seen that HT-PBI remains stable after 1500 h of cycling, and the overpotential is only 10 mV. However, for the traditional coated separator CT-PBI, the voltage starts to fluctuate after 300 - 400 h of cycling, until the overpotential increases and short circuit occurs. It can be seen that compared with the traditional coated separator, the ultra-thin coating modified separator of the present invention can significantly improve the stability and cycle life of the battery.

[0085] Figure 5 Overpotential and cycle stability diagrams of lithium symmetric batteries assembled with HT-PBI prepared in Example 1, CT-PBI prepared in Comparative Example 3, or blank PBI separators at 5 mA·cm -2 1mAh·cm -2 Under the high-rate test conditions, it can be seen that even at high current densities, due to the negatively charged coating on the surface that can optimize lithium conduction and regulate lithium deposition, the battery can still operate stably for more than 2000 h with a low overpotential (only 40 mV). It can be seen that compared with the traditional coated separator, the ultra-thin coating modified separator of the present invention can significantly improve the stability and cycle life of the battery.

[0086] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A preparation method of an ultra-thin coated composite separator for a lithium metal battery, characterized in that, It includes the following steps: Uniformly disperse two-dimensional oxide into a non-solvent to obtain a non-solvent dispersion liquid containing two-dimensional oxide. Place the lithium metal battery separator into the non-solvent dispersion liquid containing two-dimensional oxide for replacement. After the replacement is completed, dry it to obtain an ultra-thin coating composite separator for lithium metal batteries, where the thickness of the ultra-thin coating does not exceed 10 nm; The two-dimensional oxide is titanium ferrite nanosheets and / or titanium dioxide nanosheets, and the chemical formula of the titanium ferrite nanosheets is Ti 0.7 Fe 0.3 O2; The mass ratio of the two-dimensional oxide to the non-solvent is 1:10000 - 20000.

2. The preparation method of the ultra-thin coating composite separator for lithium metal batteries according to claim 1, characterized in that, The material of the lithium metal battery separator includes polyimide, polybenzimidazole or polyetherimide.

3. The preparation method of the ultra-thin coated composite separator for lithium metal batteries according to claim 1, characterized in that, The thickness of the lithium metal battery separator is 24 - 26 μm.

4. The preparation method of the ultra-thin coated composite separator for lithium metal batteries according to claim 1, characterized in that, The mass ratio of the two-dimensional oxide to the non-solvent is 1:10000.

5. The preparation method of the ultra-thin coated composite separator for lithium metal batteries according to claim 1, characterized in that, The method to make the two-dimensional oxide uniformly dispersed in the non-solvent is stirring followed by ultrasonic treatment.

6. The preparation method of the ultra-thin coated composite separator for a lithium metal battery according to claim 5, wherein, The temperature of the stirring is 20 - 40 °C, the speed is 300 - 500 rpm, and the time is 1 - 3 h; the frequency of the ultrasonic treatment is 80 Hz, and the time is 1 h.

7. The preparation method of the ultra-thin coated composite separator for lithium metal batteries according to claim 1, characterized in that The time of the replacement is 25 - 35 s.

8. An ultra-thin coating composite separator for lithium metal batteries prepared by the preparation method of the ultra-thin coating composite separator for lithium metal batteries according to any one of claims 1 - 7.

9. The application of the ultra-thin coating composite separator for lithium metal batteries according to claim 8 in the preparation of lithium metal batteries.

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