CN115483505B - A lithium metal battery functional diaphragm and its preparation method and application - Google Patents

Abstract

The invention discloses a lithium metal battery functional diaphragm, a preparation method and application thereof, wherein the functional diaphragm is formed by compounding a substrate material, a covalent organic framework, polyvinylidene fluoride and stannous fluoride, and the substrate material can be a polypropylene diaphragm. The preparation method of the composite functional diaphragm comprises the steps of preparing covalent organic frameworks and stannous fluoride, uniformly dispersing the prepared covalent organic frameworks and polyvinylidene fluoride to form slurry, coating the slurry on a substrate material, drying at a proper temperature, dripping a proper amount of stannous fluoride solution, and drying to form the composite functional diaphragm. The functional diaphragm has the function of constructing a stable lithium metal interface, namely, the nitrate ions and stannous fluoride in the functional diaphragm form a solid electrolyte interface layer rich in lithium nitride and lithium tin alloy, so that the stability of the lithium metal battery can be effectively improved.

Description

A lithium metal battery functional diaphragm and its preparation method and application

Technical Field

The invention belongs to the field of lithium metal batteries, and specifically relates to a preparation method and application of a composite functional diaphragm.

Background technique

Lithium metal has the advantages of high energy density (3860mAh/g), low density (0.53g/cm ³ ) and low potential (-3.04V), and lithium metal can match the current mainstream commercial cathode materials such as lithium iron phosphate, lithium cobalt oxide and ternary materials. Therefore, lithium metal batteries are considered to be the next generation high energy density battery system with the most development potential.

However, direct contact between highly reactive lithium metal and electrolyte will form an unstable solid electrolyte interface layer on the surface of lithium metal. During the cycle, the unstable interface layer causes uneven deposition of lithium ions, forming dendritic lithium dendrites. The disordered growth of lithium dendrites can pierce the diaphragm, causing a short circuit and triggering thermal runaway of the battery, leading to safety hazards of combustion or explosion, which has greatly aroused the industry's concerns about the safety of lithium metal batteries. Therefore, constructing a stable solid electrolyte interface layer on the surface of lithium metal anode is an effective strategy to optimize the performance of lithium metal batteries.

Summary of the invention

The purpose of the present invention is to overcome the above-mentioned deficiencies of the prior art and to provide a functional separator for a lithium metal battery, aiming to construct a stable and effective solid electrolyte interface layer for a lithium metal battery to protect the lithium metal.

In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is as follows:

In a first aspect, the present invention provides a functional diaphragm for a lithium metal battery, wherein the functional diaphragm is composited by a base material, polyvinylidene fluoride, a covalent organic framework and nano-stannous fluoride.

The base material is selected from one of a polyethylene diaphragm, a polypropylene diaphragm or a polypropylene-polyethylene-polypropylene three-layer composite diaphragm film.

The mass ratio of the base material, polyvinylidene fluoride, covalent organic framework and nano-stannous fluoride is (30-50): (1-10): (20-40): (1-10).

The mass ratio of the polyvinylidene fluoride to the covalent organic framework is (5-15): (85-95).

The skeleton material of the covalent organic framework is prepared from 2,4,6-triformylphloroglucinol, ethidium bromide and lithium nitrate.

The functional diaphragm provided by the present invention is prepared by coating a coating slurry of a covalent organic framework and polyvinylidene fluoride on a base material, and then dripping a stannous fluoride solution on the surface, so that a stable solid electrolyte interface layer rich in nitride and lithium-tin alloy is formed by nitrate ions in the covalent organic framework and stannous fluoride.

In a second aspect, the present invention provides a method for preparing a functional diaphragm, comprising the following steps:

Step S1: preparing a substrate material;

Step S2: preparing a coating slurry containing a covalent organic framework and stannous fluoride, applying the coating slurry on the surface of the base material, and drying;

Step S3: After drying, the stannous fluoride solution is added dropwise to the surface of the functional diaphragm coated with the slurry, and then dried to obtain a lithium metal battery functional diaphragm.

The base material is selected from one of a polyethylene diaphragm, a polypropylene diaphragm or a polypropylene-polyethylene-polypropylene three-layer composite diaphragm film.

The coating slurry is obtained by dispersing a covalent organic framework and polyvinylidene fluoride in N-methylpyrrolidone, wherein the mass ratio of polyvinylidene fluoride to the covalent organic framework is (5-15): (85-95);

The stannous fluoride solution is prepared by dissolving stannous fluoride in dimethyl sulfoxide.

In step S2, the drying temperature is 60°C-80°C, and the drying time is 6-24h; in step S3, the drying temperature is 60°C-80°C, and the drying time is 6-24h.

The preparation method of the covalent organic framework comprises the following steps:

Dissolve 2,4,6-triformylphloroglucinol and ethidium bromide in an organic solvent, add acetic acid, react at 100-130°C under an inert gas atmosphere to obtain a dark red precipitate, wash and dry it, place it in a saturated lithium nitrate solution of methanol and deionized water, stir it for 24-48 hours, wash and filter it, repeat 3-5 times, finally wash it with water and dry it;

The preparation method of the stannous fluoride solution comprises the following steps:

Stannous bromide and ammonium fluoride are dissolved in methanol in a molar ratio of 1:2, mixed at 5-10°C, reacted for 3-7 hours, washed and dried, and then dissolved in dimethyl sulfoxide to obtain the product.

In a third aspect, the present invention provides the use of the above-mentioned lithium metal battery functional membrane or the lithium metal battery functional membrane prepared by the above-mentioned lithium metal battery functional membrane preparation method in lithium metal batteries, especially in improving the cycle performance of lithium metal batteries.

The functional diaphragm was assembled and tested in a battery. During the lithium metal full battery test, the battery cycle life was greatly improved, proving that the functional diaphragm can be used in lithium metal batteries.

Compared with the prior art, the present invention has the following advantages:

(1) The inventors creatively coated a mixed slurry of a covalent organic framework and polyvinylidene fluoride on the base material of the functional diaphragm of a lithium battery, and then dripped a stannous fluoride dispersion, thereby forming a stable solid electrolyte interface layer rich in nitride and lithium-tin alloy through nitrate ions in the covalent organic framework and stannous fluoride, thereby achieving the technical effect of protecting the lithium battery and improving the cycle performance of the lithium battery.

(2) The inventors creatively discovered that adding a covalent organic framework to a lithium battery functional separator can optimize lithium ion conduction and provide lithium nitrate to form a stable interface layer; adding polyvinylidene fluoride can bring about good chemical stability and temperature characteristics, excellent mechanical properties and processability, and has a positive effect on improving the bonding performance, ultimately jointly improving the battery's cycle performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a scanning electron microscope image of the covalent organic framework prepared in Example 1;

FIG2 is a transmission electron micrograph of the covalent organic framework prepared in Example 1;

FIG3 is a scanning electron microscope image of stannous fluoride prepared in Example 1;

FIG4 is a transmission electron micrograph of stannous fluoride prepared in Example 1;

FIG5 is a cycle performance diagram of Example 2;

FIG6 is a cycle performance diagram of Example 3;

Detailed ways

For ease of understanding of the present invention, the present invention will be described more fully below with reference to the embodiments, and preferred embodiments of the present invention are given below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. The purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive. It should be understood that the experimental methods in the following examples that do not specify specific conditions are usually based on conventional conditions or the conditions recommended by the manufacturer. The various commonly used reagents used in the embodiments are all commercially available products.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present invention. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and/or" used herein includes any and all combinations of one or more related listed items.

In a first aspect, this embodiment provides a functional diaphragm for a lithium metal battery, wherein the functional diaphragm is composited by a base material, polyvinylidene fluoride, a covalent organic framework and stannous fluoride, and the base material is a polypropylene diaphragm.

The functional diaphragm provided in this embodiment is a polypropylene diaphragm coated with a coating slurry of a covalent organic framework and polyvinylidene fluoride, and then a stannous fluoride dispersion is added dropwise to form a stable solid electrolyte interface layer rich in nitride and lithium-tin alloy through nitrate ions in the covalent organic framework and stannous fluoride.

Furthermore, the coating slurry is prepared by dispersing polyvinylidene fluoride and a covalent organic framework in N-methylpyrrolidone, and the mass ratio of the polyvinylidene fluoride and the covalent organic framework is (5-15): (85-95). Among them, the covalent organic framework is used to optimize lithium ion conduction and provide lithium nitrate to form a stable interface layer; polyvinylidene fluoride has good chemical stability and temperature characteristics, excellent mechanical properties and processability, and has a positive effect on improving bonding performance. Too much covalent organic framework will result in poor bonding of the slurry, and too little will be unfavorable for forming a stable interface. In this embodiment, the mass ratio of polyvinylidene fluoride and the covalent organic framework is controlled to be (5-15): (85-95), which can form a more suitable coating slurry.

Furthermore, a stannous fluoride dispersion is dripped onto the surface of the functional diaphragm, and the stannous fluoride solution is prepared by dissolving stannous fluoride in dimethyl sulfoxide.

In a second aspect, an embodiment of the present invention further provides a method for preparing a functional diaphragm for a lithium metal battery, which is prepared by the preparation method described above.

The preferred steps of the method for preparing the functional diaphragm of this embodiment are:

Step S1: preparing a base material, wherein the base material is a polypropylene diaphragm.

Step S2, uniformly dispersing the prepared covalent organic framework and polyvinylidene fluoride in N-methylpyrrolidone to form a slurry, wherein the mass ratio of the polyvinylidene fluoride to the covalent organic framework is (5-15): (85-95), and coating the slurry on a polypropylene diaphragm.

Step S3, after drying, dripping a stannous fluoride solution onto the surface of the diaphragm, wherein the stannous fluoride solution is prepared by dissolving stannous fluoride in dimethyl sulfoxide, and finally drying at 60°C.

On the third aspect, this embodiment provides an application of the lithium metal battery functional membrane according to the first aspect in a lithium metal battery. The functional membrane described in the first aspect is subjected to battery assembly and testing, and the results show that the performance of the lithium metal full battery is effectively improved, proving that the functional membrane can be used in lithium metal batteries.

Optionally, the lithium-ion battery prepared by the battery separator as described above may be a button cell or a soft pack cell.

The present invention has been tested for many times, and some test results are now cited as references to further describe the invention in detail, and the following is a detailed description in conjunction with specific embodiments.

Example 1

Preparation of covalent organic frameworks:

Weigh 2,4,6-triformylphloroglucinol and ethidium bromide (molar ratio 2:3) and disperse them in a mixed solution of 1,4-dioxane and mesitylene (volume ratio 1:1). Add acetic acid of appropriate concentration and react at 120℃ for 72h under the protection of nitrogen. After washing with acetone and tetrahydrofuran, dry at 85℃ for 12h. Then take 1.5g of the obtained precursor and disperse it in a saturated solution of 15mL lithium nitrate in water and methanol (volume ratio 1:1) and continue stirring for 24h, filter and repeat 3 times, finally wash with water three times, and vacuum dry at 120℃ for 12h.

Characterization of Covalent Organic Frameworks:

FIG1 shows a scanning electron microscope image of the covalent organic framework, and FIG2 shows a transmission electron microscope image of the covalent organic framework, indicating that the prepared covalent organic framework is arranged in layers.

Preparation of stannous fluoride:

Weigh ammonium fluoride and stannous bromide and dissolve them in methanol (molar ratio 1:2) respectively. Add the ammonium fluoride solution dropwise into the stannous bromide solution at 10°C. After reacting for 12 hours, centrifuge the methanol for 5 times and dry at 80°C for 12 hours.

FIG3 shows a scanning electron microscope image of stannous fluoride, and FIG4 shows a transmission electron microscope image of stannous fluoride, indicating that the prepared stannous fluoride is nanoparticles.

Preparation of functional membrane:

Step S1: preparing a polypropylene diaphragm as a base material;

Step S2: Weigh 10% polyvinylidene fluoride and 90% covalent organic framework and disperse them in N-methylpyrrolidone, and form a coating slurry under continuous stirring, apply it on the substrate material, and dry it at 60° C. for 12 hours.

Step S3: dissolving stannous fluoride in dimethyl sulfoxide, and then dropping it onto the surface of the dried diaphragm, and finally drying it at 60° C. for 12 hours.

Example 2 Application of functional separator for lithium metal battery: NCM811 as positive electrode material for lithium metal full battery

The lithium metal functional diaphragm prepared in Example 1 was used as a diaphragm for a lithium metal battery, and NCM811 was used as a positive electrode material to assemble a full battery.

After the functional diaphragm was assembled into a full battery, a charge and discharge test was performed. Figure 5 is a cycle performance diagram of the NCM811 full battery (positive electrode loading: about 4mg/ ^cm2 ; negative electrode loading about 4mAh; electrolyte dosage 15μL). The test temperature was 30℃, and the full battery was cycled 3 times at a rate of 0.3C (1C＝180mAh/g) between 3-4.4V, and then cycled at a rate of 1C. As can be seen from the figure, after 120 cycles, the battery capacity using the polypropylene diaphragm decayed from 187.9mAh/g to 67mAh/g, and the capacity retention rate was only 35.7%; while the battery using the functional diaphragm slowly decayed from 187.9mAh/g to 175.3mAh/g after 200 cycles, and the capacity retention rate was 93.3%. It shows that the functional diaphragm prepared by this method can effectively improve the cycle performance of lithium metal batteries.

Example 3 Application of functional diaphragm for lithium metal battery: lithium metal full battery with lithium iron phosphate as positive electrode material

The lithium metal functional diaphragm prepared in Example 1 was used as a diaphragm for a lithium metal battery, and lithium iron phosphate was used as the positive electrode material to assemble a full battery.

After the functional diaphragm was assembled into a full battery, a charge and discharge test was performed. Figure 6 is a cycle performance diagram of a lithium iron phosphate full battery (positive electrode loading: about 12 mg/ ^cm2 ; negative electrode loading: about 4 mAh; electrolyte dosage 15 μL). The test temperature was 30°C, and the full battery was cycled 3 times at a rate of 0.2C (1C = 170 mAh/g) between 2.2-4.1V, and then cycled at a rate of 1C. As can be seen from the figure, after 120 cycles, the capacity of the battery using the polypropylene diaphragm decayed from 140.9 mAh/g to 23.1 mAh/g, and the capacity retention rate was only 16.4%; while the capacity of the battery using the functional diaphragm slowly decayed from 141.3 mAh/g to 133.9 mAh/g after 200 cycles, and still maintained a high capacity retention rate of 94.8%. This shows that the functional diaphragm prepared by this method can effectively improve the cycle performance of lithium metal batteries.

The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for those of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims. The above-mentioned are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims (5)

1. The functional diaphragm of the lithium metal battery is characterized by being formed by compounding a base material, polyvinylidene fluoride, a covalent organic framework and nano stannous fluoride; the substrate material is selected from one of a polyethylene diaphragm, a polypropylene diaphragm or a polypropylene-polyethylene-polypropylene three-layer composite diaphragm; the mass ratio of the substrate material to the polyvinylidene fluoride to the covalent organic framework to the nano stannous fluoride is (30-50): (1-10): (20-40): (1-10), the mass ratio of polyvinylidene fluoride to covalent organic framework is (5-15): (85-95);

The framework material of the covalent organic framework is prepared from 2,4, 6-trimethyl phloroglucinol, ethidium bromide and lithium nitrate; the preparation method of the functional diaphragm comprises the following steps:

step S1: preparing a base material;

Step S2: preparing a coating slurry containing a covalent organic framework and polyvinylidene fluoride, coating the coating slurry on the surface of a substrate material, and drying;

Step S3: and after drying, dropwise adding stannous fluoride solution to the surface of the functional membrane coated with the slurry, and then drying to obtain the lithium metal battery functional membrane.

2. The lithium metal battery functional separator according to claim 1, wherein the stannous fluoride solution is prepared by dissolving stannous fluoride in dimethyl sulfoxide.

3. The lithium metal battery functional separator according to claim 1, wherein the drying temperature in step S2 is 60 ℃ to 80 ℃ and the drying time is 6 to 24 h; and S3, drying at 60-80 ℃ for 6-24 h.

4. The lithium metal battery functional separator according to claim 1, wherein the preparation method of the covalent organic framework comprises the following steps:

Dissolving 2,4, 6-trimethyl phloroglucinol and ethidium bromide in an organic solvent, adding acetic acid, reacting at 100-130 ℃ under the atmosphere of inert gas to obtain dark red precipitate, washing, drying, placing into saturated lithium nitrate solution of methanol and deionized water, stirring for 24-48 h times, washing, filtering, repeating for 3-5 times, washing with water, and drying to obtain the product;

The preparation method of the stannous fluoride solution comprises the following steps:

Stannous bromide and ammonium fluoride molar ratio 2:1 are dissolved in methanol, mixed at 5-10 ℃, reacted for 3-7 h ℃, washed and dried, and then dissolved in dimethyl sulfoxide to obtain the catalyst.

5. The lithium metal battery functional separator of claim 1 for use in a lithium metal battery.